Abstract 41 - 13th AGC Geoscience for the Community Canberra Feb 1996

Geological Society of Australia

ABSTRACTS Number 41

GEOSCIENCE FOR THE COMMUNITY

13th Australian Geological Convention Canberra, 19-23 February 1996

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ISSN 0729-01IX © Geological Society of Australia Incorporated 1996

Copies of this publication may be obtained from the Geological Society of Australia Incorporated, 301 George Street, Sydney, NSW, Australia 2000

Example citation for papers in this volume: Kennard, J.M., Southgate, P.N., and Jackson, M.J., 1996. Sequence analysis, stratal geometries and tectonic-eustatic control of the Late Devonian reef complex, Canning Basin, Western Australia. Geological Society of Australia Abstracts No. 41, 230.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOLOGICAL SOCIETY OF AUSTRALIA INC. President Past President Vice President Hon. Secretary Hon. Treasurer Hon. Administrative Officer Hon. Promotions Officer Councillor of Executive Division

D.I. Groves P.J. Legge D. Denham S.E. Ho P.L. Collins R. Shakesby J. Sheperd M. Freeman

13th AUSTRALIAN GEOLOGICAL CONVENTION ORGANISING COMMITTEE Convention Convenor Secretary/Sponsorship Treasurer Exhibition/Display Scientific Program Posters Excursions Abstract Editor AGSO Symposium Social program GSA Representative Publicity AUSIMM Representative Secretariat - Australian Convention and Travel Services

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Jim Jackson Mike Rickard Peter Wellman John Bain Peter Southgate Neville Exon Ian Sweet Jennie Totterdell Marjorie Muir John Kennard Trevor Powell Larry Harrington David Denham Ian Hodgson Fred Cook Pat Tart

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CONTENTS

Abstracts (alphabetical order of authors) Author Index

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PROVENANCE AND BURIAL HISTORY OF THE NORTHLAND ALLOCHTHON ROCKS, NORTHLAND BASIN NORTH ISLAND, NEW ZEALAND Aadil N.1. Black, P. M.1, Ballance, P. F.1 and Blattner, P.2 Department of Geology, University of Auckland, Auckland, New Zealand " Institute of Geological & Nuclear Sciences, Wellington, New Zealand

Northland Allochthon is a structurally displaced rock unit and is mainly comprised of an originally finingupwards passive margin wedge which accumulated adjacent to northeastern New Zealand between midCretaceous and latest Oligocene. The wedge was obducted on to northern and northern eastern New Zealand at the onset of compressional tectonics about 25 Ma. It was thrusted over the autochthonous, non-marine to shallow marine transgressive sediments (Eocene-Oligocene) and is unconformably overlain by the Waitemata and correlative groups of early Miocene age. In places nappes have been shown to be in re-ordered stratigraphic sequence.

Mean vitrinite reflectance (%Rm), illite crystallinity (IC) and percentage of iliite layers in the illite/smectite interstratified clays from different lithofacies show that the rocks have been exposed to a temperature range of approximately 120°C to 250°C. Oldest sedimentary complex may have been affected by heating to 300°C from intrusion of keratophyre dykes.

The clay mineral sequence observed in the study area is from smectite to illite through illite/smectite interstratified mixed-layered clays. Mixed layer illite/smectite (US) clay from sandstone and shale shows a progressive increase in ordering and number of illite layers with increasing age. Illite crystallinity index (IC) of interstratified illite/smectite mixed-layer clays ranges from O.7OA°20 to 0.30°26. Most of the values are consistent with advanced diagenesis and anchimetamorphism. I/S and %Rm data imply that rocks of Cretaceous age reached about 250°C. The temperature estimated by clay mineralogy is in accordance with the values obtained by vitrinite reflectance measurements. These results show that the diagenesis sequence seems to be that of original burial. It has not been significantly modified by emplacement of Northland Allochthon or by Miocene burial.

A pilot stable isotape analysis of oxygen and carbon in both calcite cements and veins from units within the Allochthon and the lower part of the overlying Miocene Waitemata Group gave valuable information regarding pore fluid compositions during the burial/diagenetic history. It also established the source of carbon in the calcite cements and veins.

The isotape results of calcite veins indicate that the depositing solutions were all probably less saline than seawater, but the wide variation in C values suggests different sources of carbon in the three samples. The strongly negative C values in sandstone concretions indicate origin from oxidation of organic matter, and the big difference from the veins in the Cretaceous rocks supports the o evidence of different source waters. The O values are more negative than the veins in the Cretaceous rocks, suggesting formations at different times when pore solutions were even further from marine composition. Both O and C values indicate a slightly modified seawater source, which support an early diagenetic origin, and a phase of concretion formation separate from the Cretaceous one.

Vitrinite reflectance and clay mineral studies of Allochthon rocks indicate that the original burial metamorphism has not been overprinted during subsequent re-location, re-ordering and re-burial.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ASSESSMENT OF THE HYDROCARBON SHOWS OF THE WESTRALIAN 2 SYSTEM IN THE CURLEW AREA, PETREL SUB-BASIN Kamal Abdelmalek, Marita Bradshaw and Bruce McConachie Australian Geological Survey Organisation (AGSO), GPO Box 378 Canberra, ACT 2600

ABSTRACT Genuine hydrocarbon shows may end up with a discovery, however a false one may lead to a disaster. Good evaluation of hydrocarbon shows is essential for the development of exploration and drilling strategies of hydrocarbon explorers.

Recent discoveries in the Timor Gap Zone ( Elang 1 and 2, Laminaria 1 and Kakatua 1) revealed the possibility of the occurrence of very light oil in the system. Bayu 1 which is a recent gas / condensate discovery in the same area supports the presence of the light hydrocarbon component in the system.

Curlew 1 well was drilled to test the Upper to Middle Jurassic Plover and Flamingo Formations, which falls in the Westralian 2 Petroleum System in the Petrel Sub-basin. This well demonstrated hydrocarbon shows during and after drilling. It recovered, for example, 300 ml of light oil and 0.55 cu ft of gas in one of the six Formation Interval Tests (FIT). The well was plugged and abandoned as a dry hole with a big question mark over the source of this oil, was it true oil from the formation, or diesel contamination from the drilling fluid?

Geochemical analysis of the recently discovered oil is highly recommended. Detailed studies of source facies distribution and other critical controls on the accumulation of hydrocarbons in the Petrel Subbasin should be considered as well.

Here the impact of the lack of hydrocarbons within the Petrel Formation in Curlew 1 significantly alters the strategies for future exploration activity in the area.

Oil shows were encountered only in the sidetrack of the original hole which was drilled with oil based mud and experienced severe mud losses in the formation of interest.

STRUCTURE INTERPRETATION CURLEW-1 O

Normal alkane analysis was done on this oil sample, however it was not conclusive. Sample of the actual diesel oil or the drilling fluid used in this well were not available, so that pattern matching and correlation could not be executed. In addition to that, spersene, bit lube and organic detergents were reported in the daily mud reports.

Formation pressure testing could not be performed due to drilling circumstances. However, thorough investigation of mud and wireline logs suggest a diesel source for the sample. Although there were high gas peaks while drilling and gas was recovered in the FIT, swabbing and surging of the hole had occurred and the gas shows were mostly methane.

Figure 1. Curlew 1 well hydrocarbon show

Structure interpretation and formation juxtaposition suggest the presence of a fault migration conduit for hydrocarbons from a deeper horizon (Fig. 1).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE WHINSTONE BASALT AND LANDSCAPE EVOLUTION IN THE JERANGLE AREA,NSW. Robert S Abell1 and Ian C Roach : 'Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 2 University of Canberra, PO Box 1, Belconnen, ACT 2616

The Whinstone Basalt outcrops as a NW- trending ridge encompassing at least two separate lava flows southeast of Jerangle, approximately 65km SSE of Canberra in the Monaro region of New South Wales. The flow forms a narrow, inverted , terraced relief within a surrounding granitic landscape. The outcrop is approximately 7.5km long, between 0.5 and 1km wide and was at least 100m thick prior to erosion. The Whinstone Basalt rests on silcrete lining a palaeochannel of at least Middle Tertiary age, possibly as old as the Late Cretaceous in keeping with other palaeochannels known from the rest of the Monaro region. The basalt also laps onto Devonian granitoids of the Jerangle Igneous Complex, although contacts with the basement are usually obscured by flanking basaltic colluvium.

The landscape on the Whinstone Basalt is degraded. Removal of the tree cover for sheep and cattle grazing has promoted a surface rubble of basalt, poor soil definition and minor slumping on marginal slopes. A perched water table is evidenced by Lake Whinstone (El. 1200m above sea level) - an apparantly small deflation lake (100m in diameter) sitting in a hollow on the top of the flow. Groundwater leakage (discharge) has initiated a network of secondary (consequent) streams from spring-fed alcoves at the margin and base of the flow.

A small, circular basaltic plug is exposed near Eastwood Station approximately 1km SW of Jerangle along the line of the Narongo Fault, a major NNE trending structure. Mineralogically, both the Eastwood plug and the Whinstone Basalt are feldspar-rich olivine-titian augite basalts which plot in the nepheline hawaiite field according to the CIPW normative classification scheme used for eastern Australian intraplate volcanic rocks. On a Total Alkali-Silica diagram the rocks plot towards the fractionated end of the Monaro Volcanic Province suite, the nearest large intraplate basaltic lava field, which outcrops south of Cooma. Calculated K - Ar ages are Early Miocene, 21.1 ±0.2 Ma for the volcanic plug and 20.3 ± 0.2 Ma for a flow of the Whinstone Basalt, putting the basaltic rocks into the same age range as those of the Snowy Mountains Province. The location of the eruption centre for the Whinstone Basalt is unclear but mapping the base of the basalt onto the 1:25,000 sheet topography suggests a centre in the vicinity of Whinstone Hill (1266m above sea level). While there is no linear relationship between the Eastwood volcanic plug and the Whinstone Basalt the similarity of the chronology, mineralogy and chemistry suggests a coeval magmatic event.

The regional drainage pattern west of the Main Divide (east of the Jerangle area) is pre-basalt (<21 Ma) and is strongly adjusted to aNW- and NS- trending pattern of fractures established by the the Late Palaeozoic. The long term stability of the drainage is also emphasised by tributary systems that have cut back into and across the Narongo Fault. However, mild rejuvenation of this fault may be evidenced by the incised drainage (antecedance) across the Cullarin block; the timing perhaps coincident with the Early Miocene emplacement of the Eastwood volcanic plug.

The Whinstone Basalt was extruded along a headwater channel of Strike-a-Light River. A small alluvial flat north of Jerangle may indicate temporary headwater disruption but the Strike-a-Light River now extends back as a lateral stream partly along the western margin of the flow. The inverted relief of the Whinstone Basalt is now part of a NWtrending watershed separating the Queanbeyan and Bredbo river catchments. Groundwater sapping has also emerged as an important erosional process modifying slope retreat along this part of the watershed.

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MINERALOGICAL STUDY OF GEOCHEMICAL HALOS: APPLICATION TO EXPLORATION OF THE PECHENGA Ni-Cu SULPHIDE DEPOSITS, RUSSIA Marat 2. Abzalov Western Mining Corporation, GPO Box860K, Melbourne, 3001

Ni-Cu sulphide deposits of Pechenga are located in the north-western part of Russia close to border with Norway. The deposits are associated with gabbro-wehrlite intrusions which are hosted by Early Proterozoic sedimentary and volcanic-sedimentary rocks. Ni-Cu sulphide ores occur as sulphide disseminations in ultramafic rocks and also as seams of massive/breccia ores localised in sheared footwalls of the ore-bearing intrusions.

Mineralized intrusions are surrounded by halos of geochemical anomalies of number of elements, eg.Ni, Cu, Co, Pb, Zn, Ag, Au, B, Cr, As, Sb, which are commonly used in exploration as an indicators of Ni-Cu sulphide mineralization. The halos have been formed by hydrothermal-metamorphic alteration of ultramafic rocks remobilizating associated sulphides. The halos mainly confined to the ore-controlling jshear zones.

Indicative elements are accomodated (Table 1) in sulphides, sulpho-arsenides, tellurides and rather less commonly (eg.Cr) in oxide minerals or silicates. Gold occurs as native metal. The minerals are distributed in the host sedimentary sequence as metasomatic disseminations and hydrothermal veinlets of quartz-sulphide composition, with minor carbonates, chlorite, feldspar and oxide minerals.

Table I. Minerals accomodators of indicative elements in the geochemical halos of the Pechenga Ni-Cu deposits. Figures in the cells are highest content (wt.%) of element in the mineral as determined by electron microprobe. Shaded cells denote the main concentrators of the relevant element.

* Significant proportion of chromium is accomodated in silicates. Thus, chlontes contam up to 0.34 wt.% Cr and fuchsite up to 0.84 wt.% Cr. Many of these minerals are distributed regularly in the host rocks, forming the mineralogical and geochemical zoning. The characteristic association in the vicinity to ore-bearing intrusions is pyrrhotite-pentlanditechalcopyrite with gersdorffite, Ni-cobaltite, argentian pentlandite, hessite and gold occurring as minor and accessory phases. The type association of distal zones of hydrothermal halos is pyrrhotite-pyrite-chalcopyrite with accessory arsenopyrite and cobaltite. Minerals occurring throughout the halo often have regular changes in their chemical composition. Thus the content of Ni in pyrrhotite and cobaltite decreases with distance from the ore-bearing intrusions.

Investigation of mineral zoning enables physical-chemical constraints to be put on the conditions of the halo's development and an the evolution of the halo forming hydrothermal system. In particular, replacement of ilmenite by rutile and titanite, followed by replacement of pyrrhotite by pyrite, is indicative of an increasing oxidizing state in the fluids probably accompanied by a pH decrease.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PALAEOMAGNETIC CONSTRAINTS ON THE PALAEOLATITUDES OF SOME OCEANIC TERRANES IN THE NEW ENGLAND OROGEN, EASTERN AUSTRALIA Gary D. A c t o n and Peter G. Flood department of Geology and Geophysics, Univ of New England, Armidale NSW 2351 ODP/Texas A&M University, 1000 Discovery Drive, College Station, TX 77845-9547, U.S.A. 2

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The New England Orogen comprises an amalgamation of Palaeozoic and Mesozoic terranes whose origin, position, and motion relative to the Australian craton are largely unknown. Much of the difficulty in reconstructing the evolution of these terranes, particularly their palaeogeographies, results from poor surface exposures, nonunique geological indicators of the palaeoenvironments of the terranes, and widespread overprinting of their primary magnetizations. Promising geologic and palaeomagnetic results from a Carboniferous seamount terrane near Ashford and a Devonian backarc-rift terrane near Nundle, referred to as the Gamilaroi Terrane, indicate that at least the latter two difficulties may have been overcome. The basalts and limestones obtained from the seamount terrane give well constrained demagnetization lines and planes and, after a bedding correction, give consistent mean inclinations that are near horizontal (Figure 1), suggesting that this terrane formed near the palaeoequator. Such a low palaeolatitude is consistent with that expected for ooilitic limestones, which occur within the terrane. Similarly, pillow basalts from the Gamilaroi Teuane give shallow inclinations after applying a bedding correction. Thus, both terranes appear to be derived from near-equatorial latitudes. An ambiguity exists, however, because the rock exposure consists predominately of steeply dipping units and the in situ palaeomagnetic inclinations are all near vertical.

Thus, the terranes could have been amalgamated to Australia and then remagnetized during an interval when Gondwana was near the south pole. More definitive results will require further sampling from units with a larger range of bedding dips in order to provide a more robust and unique fold test.

Figure 1: Vector demagnetization diagram illustrating the very stable, shallow direction obtained from a Carboniferous limestone sample from near Ashford. The direction changes insignificantly over a range of demagnetization from 120°C to 580°C, even though over 95% of the natural remanent magnetization, likely carried by maghemite or pyrrhotite, is removed by about 350°C. The remaining magnetization with high unblocking temperatures is likely carried by magnetite. Bedding correction (strike = N088°E; dip = 88°SE).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 41 13th Australian Geological Convention, Canberra,, February 1996

PETROGRAPHIC, ELEMENTAL AND ISOTOPIC CRITERIA FOR THE RECOGNITION OF CARBONATE MINERALOGY AND CLIMATES DURING THE JURASSIC (EXAMPLES FROM IRAN AND ENGLAND) Mohammad H. Ariahi and G Prasada Rao Department of Geology, University of Tasmania, Hobart, Tasmania, Australia, 7001

A significant problem in ancient carbonate studies is the recognition of carbonate mineralogy which is related mainly to the seawater temperature and atmospheric PCO2 level. Petrographic features, bivariate plots involving combinations of trace elements and oxygen and carbon isotopes and the determination of isotopic lines of equilibrium for different carbonate mineralogy as a function of temperatures have been used to discriminate carbonate mineralogy. A calcite sea has been postulated for the Jurassic period by some workers, but petrographic and geochemical trends of the late Jurassic Mozduran carbonates of Iran indicate aragonite and mixture of aragonite-calcite mineralogy from the shallowest and relatively deeper parts of the basin respectively.

This interpretation is supported by occuirance of aragonite-calcite mixtures in several large basins during the late Jurassic. The 'Greenhouse Mode' has been suggested for the Jurassic time, but the carbon isotope values and isotopic trends indicate a low CO2 level which corresponds to relatively large temperature fluctuations in a shallow sea.

Sedimentological features indicate that the mid Jurassic carbonates of England were originally calcitic with minor aragonite and no dolomite and evaporites and were deposited at a paleolatitude of 35° N (Marshall and Ashton, 1980). In contrast, the occurrence of algal-rich biota, diverse skeletal and non-skeletal grains, evaporites and early diagenetic dolomites in the shallowest part of the Kopet-Dagh Basin in the Mozduran Formation of Iran are similar to those of modern subtropical shallow marine carbonates and formed at a paleolatitude of 20° N during the late Jurassic. Other petrographic evidence, such as fibrous and isopachous sparry calcite cements, abundant deformed and spalled ooids and shattered micritic envelopes suggest an original aragonite mineralogy. Other samples from relatively deeper parts of the basin contain predominantly calcitic skeletons, such as crinoids, brachiopods, forams and bryozoa and bladed marine calcite cements. These features indicate that carbonatesfromIran are composed of mixtures of aragonite and calcite.

Recently several element versus element plots have been proposed for discrimination between carbonate mineralogy in tropical and temperate environments. Sr and Na covariance plots with respect to Mn and particularly Sr/Na ratios in Mozduran carbonates of Iran indicate aragonite and mixture of aragonite-calcite mineralogy in shallowest and relatively deeper parts of the basin respectively. Sr/Na ratios >1 indicate an original aragonite, while Sr/Na ratios of <1 suggest calcite mineralogy. Inspite of mineralogical change during carbonate diagenesis, the original trace element signature is preserved.

The isotopic criteria proposed in this study involve determination of lines of equilibrium between aragonite and calcite and mixtures of aragonite and calcite with variable temperatures by considering present day average values of 8 ^ 0 of seawater ( 0 ° / o o ) and 8 ^ C of atmospheric C02(-7.2 /oo). The aragonite and aragonite-calcite mixture isotopic equilibrium trend lines pass through the mid Jurassic altered aragonite bulk carbonates, while the calcite equilibrium line passes through the calcitic bulk carbonate and cements of England. The calcite line is also cuts the late mid Jurassic LMC belemnite of Scotland. The micrite, nonskeletal grains and bulk carbonate data of Iran fall mostly above and equilibrium lines with respect to present day atmospheric CO2 levels of -7.20/00. The late Jurassic PCO2 levels calculated from the heaviest 5 ^ C value corresponding to the heaviest 8 ^ 0 value, indicate that the late Jurassic atmospheric 5^C was -6.7%. 8 ^ 0 and 8^C equilibrium lines of an aragonite and aragonite-calcite mixture, using this 8 ^ C value, bisects the Iran carbonate data indicating a lower CO2 level and an aragonite and mixed aragonite-calcite mineralogy. The heaviest 5180 value of the mid Jurassic carbonates of England give a cold seawater temperatures of -8° C, while the heaviest 8 ^ 0 value of LMC belemnite from the late mid Jurassic of Scotland give a temperature of ~13°C similar to modern cool temperate surface seawater temperatures around Tasmania, Australia at a similar latitude. The heaviest 8 ^ 0 value of the late Jurassic marine micritefromIran gives a seawater temperature of 16°C, whereas the heaviest and least altered value of brachiopods correspond to a warm seawater temperature of ~26°C. The temperature fluctuations in the Iranian shallow sea are similar to those in the subtropical Persian Gulf, where temperatures fluctuate in the coastal areasfrom40° in summer to 15° C in winter.

Reference Marshall, J.D., & Ashton, M., 1980. Isotopic and trace element evidence for submarine lithification of hardgrounds in the Jurassic of England. Sedimentology 27,271-289.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE SOURCE OF HYDROTHERMAL FLUIDS RESPONSIBLE FOR CARBONATE ALTERATION, RENISON, TASMANIA, AUSTRALIA Mohammad H. Adabi^Prasada C. Rao ^ d Paul A. Kitto Apartment of Geology, University of Tasmania, Hobart, Tasmania, Australia, 7001 ^Centre for Ore Deposit and Exploration Studies, University of Tasmania, Hobart, Tasmania, Australia, 7001 2

Dolomites and associated clastic sediments of the Late Proterozoic, at Renison in western Tasmania, host significant stratabound replacement tin deposits. These deposits occur within the central region of the Dundas Trough which consists of Proterozoic to early Paleozoic sedimentary rocks. Mineralization resulted from the passage of hydrothermal fluids, sourced from the Devonian Pine Hill Granite, through major faults and fractures, partially to completly replacing three dolomite horizons, and forming a cassiterite-rich pyrrhotite orebody. These cassiterite-bearing sulfide deposits are the largest primary tin deposits in Australia. Petrographic features, major and trace elements, together with 8 ^ 0 and studies, indicate that Renison dolomites have gone through minor to major alteration pest deposition.

The Renison carbonates consist of both dolomitic cements (dolomicrite, dolosparite, vein dolomite) and recrystallized dolomites (dolomicrosparite and coarsely crystalline dolomite). Samples were grouped into less recrystallized and strongly recrystallized carbonates. Dolomites from less recystallized sections have higher Ca, Mg and Na concentrations and heavier 8 ^ 0 and values than strongly recrystallized carbonates. Ca, Mg, Mg/Ca, Sr, Na, Fe and Mn values of the unaltered host dolomites are close to unaltered Proterozoic sedimentary dolomites (PSD) of western Tasmania which formed in anaerobic, temperate, shallow marine environments.

Most altered dolomites contain low concentrations of Ca, Mg, Mg/Ca and high concentrations of Sr, Mn and Fe relative to the unaltered PSD. The covariance trend between 8 ^ 0 and 8 ^ C values, and the variation in 8 ^ C values from -8 to 4% PDB in Renison carbonates are mainly due to alteration by magmatically derived hydrothermal solutions. The 8180 and field of these carbonates overlaps both the magmatic dolomite field for Devonian granite related mineralization and the field for Cambrian hydrothermal carbonates associated with volcanogenic hosted massive sulfide deposits. and 8 l 3 values for carbonates indicate temperatures were around 20°C during early diagenesis, and up to 350°C for magmatically derived hydrothermal fluids. These temperatures are similar to those obtained from fluid inclusion studies andfromcalculations based on S ^ O variations in vein quartz.

Dolomite-fluid 5180 and 8 ^ C fractionation curves that best fit the Renison S ^ O and 8 ^ C carbonate data and fluid inclusion temperatures are for an early stage Devonian magmatic fluid (S^OsMOW = +9%o, S^CpDB = -5%o\ followed by late-stage mixtures of temperate Devonian meteoric water and magmatic fluids. Isotherms on a 8 ^ 0 and 8 ^ C covariance plot illustrate that the isotopic variation in Renison carbonates are the result of changing temperature and fluid/rock interaction. Fluid/rock ratios were predicted to be as high as ~6 (open system) close to the carbonate replacement orebodies, and decreased with declining temperatures away from mineraliszation. Renison siderites (150 to 350°C) were formed as a direct result of early magmatic hydrothermal fluids reacting with sedimentary dolomites, during carbonate replacement mineralization. Altered dolomites (20 to 210°C), however, may have resulted from magmatic hydrothermal fluids or from later nonmagmatic fluids. The extent of late-stage Devonian temperate meteoric water that mixed with magmatic fluids was potentially as high as 65% by volume. Base metal mineralisation was associated with these late-stage fluids. c

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

REGOLITH AND EXPLORATION GEOCHEMISTRY OF THE BEECHMORE BLOCK PARKES NSW 1

S. E. Adamson1 Dr. X. Y. CheirDrR. A. Eggleton1 CRC LEME Australian National University, CANBERRA ACT 0200 2 CRC LEME University of Canberra, BRUCE ACT 2616

INTRODUCTION The Beechmore Block is located adjacent to the North Mining Limited copper / gold porphyry deposit. Due to the limited outcrop in the Beechmore Block and the potential for further discovery in this regolith dominated terrain, the aim of this study is to establish landscape units, soil type and regolith stratigraphy within the context of regional landscape evolution and exploration models.

THE STUDY AREA The Beechmore Block covers an area of 49 square kilometres and is located approximately 37 kilometres north west from the town of Parkes, NSW and immediately west of North's mining leases that cover the Endeavour 22, 26 and 27 mines.

BEDROCK LITHOLOGY Outcrop of Ordovician trachyandesites is restricted to the southern part of the area along a north / south trending ridge of lm to 2m relief. To the north is a smaller circular outlier of Devonian sandstone. An unconformity exists between the gently westward dipping Devonian sandstone that lies along the western third of the Beechmore Block and the older Ordovician volcanics that form the majority of the study area.

GEOMORPHOLOGY This area is essentially of low relief with slight rises and occasional ridges formed by the more resistant Devonian and Ordovician rock types. Vegetation has been cleared for grazing and agriculture with remnant pockets of cypress pine (Callitris sp.) along ridge lines and river red gums (Eucalyptus camaldulensis) along drainage depressions. Drainage channel morphology is braided in some parts and conforms to a dendritic pattern that flows north into the Bogan River system. Soils are commonly well structured red-brown earths. The A horizons have been destroyed by agriculture, leaving well structured clay B horizons. A common feature of the B horizons observed in the Endeavour 22 opencut were the presence of numerous gypsum crystals. Gilgai occurs in soils with a self mulching surface. Where fields have not been cultivated it is readily recognised as a "pock mark" pattern on aerial photographs.

GEOCHEMICAL EXPLORATION Air core holes have been drilled by North Limited to the depth of bedrock refusal. Drill spacing is 400m apart along three traverses in the study area, with the regolith material sampled at 0.5m intervals. Soil textures, colour, sedimentary features, XRD analysis and geochemical assay results have been used to correlate these holes, thus allowing interpretations of the regolith stratigraphy and genesis. To date the preliminary air core drilling has shown that the depth of weathering in the northern part of the study area extends to 45m. This is overlain by a fluvial transported cover sequence containing fragments of bedrock material to a depth of 5m and is consistent with the profiles observed in the Endeavour 22 opencut.

CONCLUSION This study is currently in its preliminary stages although it is hoped that future work will successfully construct a model of the regolith stratigraphy that can be applied to future exploration programmes in the region.

Acknowledgments The author would like to acknowledge the support and assistance of North Limited for allowing this article to be published.

8

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A SEDIMENTOMORPBCEC FRAMEWORK FOR ENVIRONMENTAL ASSESSMENT OF THE INTERTIDAL ECOSYSTEM: A KUWAITI PERSPECTIVE Dhia Al Bakri Orange Agricultural College, The University of Sydney, P.O. Box 883, Orange, NSW 2800

Based on data generated from a study carried out in Kuwait, this paper attempts to demonstrate that the coastal material and related sedimentomorphic processes are the paramount factor influencing the nature and characteristics of the intertidal ecosystem and its benthic macrofaunal community. Data were collected from three tidal levels along 35 transects four times a year for one year. These data were then subjected to rigorous statistical analysis to determine the interrelationship and interaction of the physical, chemical, biological, spatial and temporal variables.

The multivariate and cluster analyses performed on the collected data showed that the composition of the benthic organisms was closely correlated to the sediment type and that the mud, sand and rock substrates supported three distinct biological communities. The seasonal variation in the macrofaunal composition was not statistically significant but differences in transects and tidal levels were significant. The spatial variation in faunal community has been attributed to the variation of sediment types which, in turn, were the result of having different sedimentomorphic processes acting on different sections of the intertidal zone. The physicochemical parameters of the interstitial water and intertidal sediments were found to correlate positively with the mud content of the sediment but were not a determining factor in the overall composition of the intertidal organisms.

The study has revealed that the most significant adverse impacts on the intertidal ecosystem were those associated with land reclamation and dredging activities. These poorly planned and executed operations disturbed the hydrodynamic balance of the affected shores leading to a considerable scouring and siltation in the adjacent coasts. As a result of these processes the original substrates were modified causing migration of certain organisms and loss of original ecosystems. It was concluded that the coastal material and related geomorphic and sedimentological processes are the primary environmental factors controlling the species type, number of taxa and animal density of the macrofauna inhabiting the intertidal zone.

These findings support the conclusion that a good understanding of the sedimentomorphic setting provides a sound basis for conducting effective environmental assessment and monitoring programs of similar coastal ecosystems.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

TREES INFLUENCING FLOW AND SEDIMENTATION IN THE SUB-HUMID, TROPICAL UPPER BURDEKIN RIVER, NORTH QUEENSLAND, AUSTRALIA, Jan Alexander^. Chris R Fielding*, Elissa Newman-Sutherland, Lorraine CampbelP and Victoria Munn* ^Department of Earth Sciences, Cardiff University of Wales, PO Box 914, Cardiff UK ^Department of Earth Sciences, The University of Queensland, Queensland 4072, Australia.

The upper Burdekin River is of moderate sinuosity (c. 1.3) and size (bank-full width 200 - 750 metres; depth c.

25 metres in the study reach near Charters Towers). The discharge measured near the study reach, varies from long periods of little or no surface flow to peak discharges of more than 25500 cubic metres per second (QDPI statistics; P Fiedler, written communication 1993) during major floods. Large volumes of sand and gravel are mobilised during flood events and the distribution of bars and bedforms and their range of scale reflect the Burdekin Rivers extreme discharge fluctuations. The morphology of the river bed, bars and banks are strongly influenced by riparian vegetation, that also influences flood behaviour. In particular Melaleuca argentea (var. fluviatilis) that have grown within the channel of the Burdekin River at low flow conditions withstand very strong currents and strongly influence the pattern of flow and sediment deposition at high and intermediate flow stages.

In the reach of the upper Burdekin River studied in 1994 and 1995, Melaleuca argentea (var. fluviatilis) trees occur on point bar and locally on parts of the outer (cut) bank of bends. These trees are deformed (bent in the down flow direction), have abrasion damage of the bark on their upstream side, have transported flood debris in their branches and sand and gravel (to pebble grade) wedged between branches. All these features support the theory that during flood the trees withstand total submergence and very strong currents (estimates of velocities 6 metres per second and more). Many of the trees are long-lived and must influence flow structure and sedimentation for tens to hundreds of years. The trees are associated with scour and sedimentation patterns similar to those observed for example around bridge stanchions.

The Melaleuca argentea (var. fluviatilis) trees and associated sedimentary features are being used in an attempt to understand the flow behaviour in different flow conditions, while we investigate possible methods for observing the flow conditions directly in these extreme environments. In the study reach the channel pattern changes from a meandering braid-belt at low stage to a low-sinuosity form at high stage when large volumes of water flow directly over the tops of the large meanders. Disturbance of flow by trees encourages low-sinuosity braided stream patterns at intermediate stage.

Observations of Melaleuca argentea (var. fluviatilis) and other tree species surviving extreme floods together contradict the earlier belief of many geologists that vegetation can only survive away from the strongest currents in river channels and that in situ fossil trees within ancient channels must post-date the channel activity.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PLANT-DERIVED CHEMICAL FOSSILS AND THEIR APPLICATIONS TO PETROLEUM EXPLORATION Robert Alexander, Ben van Aarssen and Robert I Kagi Australian Petroleum CRC/Centre for Petroleum and Environmental Organic Geochemistry, Curtin University of Technology, GPO Box U1987 Perth, Western Australia, 6001

The contribution of higher plant material to the main petroleum source rocks in Australian sedimentary basins is well-recognised. Debris from the plants is transported to the site of deposition where the natural products undergo chemical changes during diagenesis. Although a number of processes such as functional group reduction, hydrogenation/dehydrogenation take place the original carbon skeletons of the natural products remain intact and can be recognised when organic extracts of the sediments, or crude oil derived from them, are analysed. Chemical fossils from the plants occur as hydrocarbons that are carried through into the crude oils following generation and migration and can therefore provide valuable information about the source rocks and the plants that contributed to them.

Chemical fossils have applications in oil-source rock correlation. Oil-source rock correlation is based on the chemical fossil fingerprint provided by the plants that occupied the drainage system of the basin during sedimentation. The nature of the plant population at any time depends on the stage of plant evolution and the type of available habitats. Changes in plant habitats caused by tectonic events, eustatic effects and climate instigate alterations in the plant populations. These alterations are reflected in the composition of the plantderived chemical fossils incorporated into the sediments. The chemical fossil fingerprint provided by the plants therefore changes through time in response to these events. The fingerprint obtained from a crude oil can be compared with the profile through time of the sedimentary fingerprint to get a precise age for the sediments that sourced the oil. In this paper we describe the use of plant-derived chemical fossils to correlate Carnarvon Basin crude oils with their source rocks using their chemical fossil fingerprints.

Numerous (>50) aromatic compounds were analysed in a suite of crude oils and sediments and the data subjected to statistical analysis. This treatment enabled three key chemical fossils, each representing a different plant type, to be identified. The habitats of these plant types were deduced from changes in the abundances of the chemical fossils as a result of sea-level changes. For example, flooding can eliminate low lying coastal habitats and effectively increase the distance of the plant source to a given location in the basin. Sedimentary rocks of Jurassic age were characterised in terms of their higher plant derived chemical fossils and compared with the corresponding fingerprint from the crude oils. Using this approach it has been possible to show that sediments with a small age range have been the main contributors to the crude oils.

In a second application of plant-derived chemical fossils we show that it is sometimes possible to identify crude oils that result from mixing of two independent hydrocarbon inputs to the reservoir. Crude oils formed by mixing hydrocarbons from different source rocks can be recognised when the hydrocarbons differ in maturity and each contains a unique, maturity sensitive, chemical fossil. This effect is illustrated by cases where source rocks containing organic matter derived predominantly from marine organisms and those from higher plant sources have contributed hydrocarbons of different maturity to the reservoir.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

FACIES PATTERNS AND VARIABILITY IN INCISED VALLEYS: EXAMPLES FROM THE LATE QUATERNARY George P. Allen. Simon C. Lang & Duncan Lockhart School of Geology, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001.

To a large extent, fades patterns within incised-valley fills reflect the depositional processes and environments which predominate during relative sea level rise and transgression. Studies of Late Quaternary incised valleys in various types of coastal settings have shown that a large degree of facies variability occurs within valley-fills, and that several end-member types of valley-fills can be defined based on the relative strength of wave and tidal processes during transgression.

In fluvial-dominated, low wave and tide settings such as in the Mahakam delta (Indonesia), the valley-fills comprise only two major facies associations: low stand fluvial deposits, and highstand shelf mudstones. In these low energy environments, there is relatively little reworking and erosion of earlier deposits during transgression so that the transgressive systems tract is thin and forms only a relatively minor part of the valley fill. Therefore, when lowstand fluvial aggradation has not entirely filled the valley, the remainder of the valley is filled by regressive highstand marine muds. In this type of setting, the key surface within the valley-fill will be the maximum flooding surface, separating the lowstand fluvial deposits and the highstand marine muds.

In high wave and tide energy coastal environments, extensive sediment erosion and reworking occurs during transgression so that a significant pan of the valley-fill is formed by the transgressive systems tract. These transgressive deposits occur in two phases, each resulting in distinct facies associations and reservoir types.

In micro-and mesotidal environments such as along the south east Queensland coast. The initial phase of transgression results in extensive lagoonal deposits which are then locally eroded and overlain by the landwardmigrating tidal inlet sands. These are in turn truncated by wave erosion and overlain by a wave-deposited lag.

Together, the tidal inlet and shoreface lag deposits comprise the late phase of transgression.

In macrotidal environments such as on the Aquitaine coast in south west France, thick tidal estuarine deposits mark the first phase of transgression, followed by tidal inlet and wave-deposited shoreface lags. In both types of settings, each facies association are separated by distinct stratrigraphic surfaces. These surfaces which record the various phases of transgression are regionally widespread and can be used for regional correlations and reservoir prediction.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996 STRUCTURAL AND TIMING RELATIONSHIPS OF AU-CU MINERALIZATION AT BODDINGTON GOLD MINE, WESTERN AUSTRALIA Andrew Allibone , Jacqueline Windh , Michael Etheridge . David Burton et al., C. Mark Fanning , Richard Wysoczanski Etheridge Henley Williams Geoscience Consultants, P.O. Box 250, West Deakin, ACT 2600, Australia ^orsley Alumina Pty Ltd, Boddington Gold Mine, P.O. Box 48, Boddington WA 6390, Australia Research School of Earth Sciences, Australian National University, G.P.O. Box 4, Canberra, ACT 0200, Australia 1

1

1

2

3

3

3

The Boddington Gold Mine (BGM) is hosted in Archean volcanic, volcaniclastic and shallow-level intrusive rocks that form the northern part of the Saddleback Greenstone Belt (SGB), a fault-bounded sliver of greenstones located in the southwestern corner of the Yilgarn Craton, Western Australia. Total Au content of the BGM (past production plus in situ resource) is 180 tonnes, making the BGM one of the largest Au mines currently operating in Australia.

Geologic mapping combined with radiometric dating indicates five phases of igneous activity during development of the SGB. Basaltic, intermediate and minor felsic volcanism occurred between 2714-2696 Ma, and again at approximately 2675 Ma. Two suites of ultramafic dikes were emplaced, one between 2696-2675 Ma and the other between 2675-2611 Ma. Late granitoid plutons crystallized at 2611 Ma and cut all the other Archean rocks in the SGB. Ductile deformation along three generations of discrete shear zones punctuated the four earlier phases of igneous activity. Foliation development is apparent in rocks dated as >2675 Ma, but is absent from the younger suite of ultramafic dikes and the granitoids, constraining the youngest foliation/ductile shearing event to between 2675 Ma and 2611 Ma. Textural timing relationships indicate that all Au-Cu bearing vein assemblages post-date the youngest regional foliation/ductile shearing event. In the central BGM, sulphide-Au mineralization occurs within fractures and veins associated with small-displacement brittle faults. Mineralized veins crosscut foliated mineral assemblages (biotite-quartz, sericite-quartz, quartz-albite-sericite). Mineralization and associated alteration assemblages are never observed to be foliated, and they show no consistent spatial association with structures associated with the regional ductile deformation event(s), such as shear zones. These crosscutting relationships indicate that the mineralization was introduced into its present sites after the youngest foliation event.

Previous workers at the BGM (e.g. Roth et al., 1990, 1991) have inferred that mineralization is genetically linked to sub-volcanic intrusives emplaced between 2714-2696 Ma, citing similarities between the alteration and ore mineral assemblages developed at the BGM and Phanerozoic porphyry deposits. However, "potassic alteration" described by Roth et al. (1991) is a regional metamorphic assemblage that developed under isochemical conditions. "Phyllic alteration" described by Roth et al. (1991) is a sericitic assemblage localized within discrete ductile shear zones.

The mineralized veins and their alteration haloes overprint the regional metamorphic assemblage and fabrics associated with all three ductile shearing events, the youngest of which occuired between 2675-2611 Ma. AuCu±Mo±W mineralization at the BGM therefore occurred late in the tectonic evolution of the SGB, at least 20 Ma after emplacement of the 2714-2696 Ma host rocks. The mineralization at the BGM is therefore analogous in timing to many other structurally late Au deposits in the Yilgarn Craton, e.g. Mt Magnet, Mt Charlotte, and Wiluna.

Roth, E., Bennett, J.M., Symons, P.M., 1990, in. Ho et al., Geology Department, the University of Western Australia, publication No. 20. pp. 189-196.

Roth, E., Groves, D., Anderson, G., Daley, L., Staley, R., 1991, in Ladeira, E.A. Brazil Gold '91, A.A. Balkema, Rotterdam, pp. 481-488.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ORIGIN OF SULFATE IN SURFACE AND GROUND-WATERS WITHIN THE RANGER URANIUM MINE, NORTHERN TERRITORY AS Andrew1. CA LeGras 2 , RA Akber 2 , AJ Bryce 1 and A J Todd 3 Australian Petroleum CRC, CSIRO Division of Petroleum Resources, PO B o x 136 North Ryde, N S W 2113 Environmental Research Institute of the Supervising Scientist, Locked Bag 2, Jabiru, NT 0886 5 Macquarie University, North Ryde, NSW 2 1 0 9

At many sites near the north wall of the Ranger tailings dam at the Ranger Uranium Mine, elevated concentrations of solutes have been observed in groundwater. These solutes are derived mainly from two sources - leaching from waste rock which comprises the external surface of the dam wall, and seepage from the tailings dam. The relative proportion of each source is significant because the operators of the Ranger mine (Energy Resources of Australia Ltd.(ERA)) are required to return water from its seepage collection system to the tailings dam, regardless of its origin and any tailings-dam effluent that evades the seepage collection system may be more environmentally detrimental and persistent than infiltration water.

A sulfur isotope study of potential sulfur sources, sulfate in surface and subsurface waters has been undertaken over several years to quantify the effects of tailings-dam seepage. Previous estimates of seepage from the tailings dam into the seepage collection system vary between 5 and 61% of the volume of influent to the collector. These are based on mass balance calculations and are subject to errors inherit in complex hydrological models. The alternative approach using the variation in the sulfur isotope composition of sulfate allows a direct measurement of the relative contributions to sulfate concentrations measured in various environments.

The mean 534S in waters from a tailings dam depth-profile samples is 1 8 . 0 % o CDT (534S = 16.7-19.7; n=9) and is the same as the value measured on the sulfur prill (534S = 17.8-18.4, n = 4) between 1984 and 1990. The 534S value of sulfides measured on sulfides from a waste dump is between 2.1 and 2.3%o and well within the range of -5 to +6%o for ore zone vein and vug sulfides at Ranger. Sulfate from water runoff from near Retention Pond (RP) 4 (534S = 5.4) and from a drainage channel from within the wetland waste rock storage and drains into RP1 (534S = 4.7) have sulfur isotope values within the range expected from surficial oxidation of the mine sulfides.

A number of sampling locations for infiltration and groundwater have been constructed by ERA, within the tailings dam wall (lysimeters), and at the base of the dam wall, penetrating the weathered bedrock of the C horizon (foundation bores), to allow monitoring and sampling of infiltration and seepage waters, respectively.

Other groundwaters have been sampled from the seepage collection system, and from bores intersecting the B and C horizons at points more remote from the dam wall (observation bores). The sulfate in waters from lysimeters was anticipated to be derived by weathering of sulfides within the waste rock making up the wall, and the 5^4S values ( 5 . 0 to 7 . 4 % o ) generally support this interpretation. The 534S value of sulfate in waters from foundation bores was anticipated to be dominated by tailings dam-seepage and two samples with 834S values of 34 1 9 . 8 ( 1 0 / 9 2 ) and 1 4 . 9 % o ( 4 / 9 3 ) support this view. The sulfate content and 8 S value of waters sampled from seepage collection trench and sump from the area immediately adjacent to the northern wall of the tailings dam wall for the period 9/92 to 10/94 show a clear seasonal variation in both sulfate concentration and sulfur isotope composition with seepage from the tailings dam predominantly in the Dry season.

Conventional estimation of the amount of seepage from the northern wall of the tailings dam use measurements of sulfate concentration on volumes of water and assume that the sulfate concentration measured on one day represents the monthly average, that it can be related to the monthly volume measured, and that base flow can be modelled. Using the sulfur isotopic data for tailings and waste rock, a straight forward mass balance calculations can be made to estimate the percentage of tailings seepage from the northern wall using only the isotopic composition of the seepage. Precision of this estimate is controlled only by the number of samples measured and even with only 6 samples measured for the period 1992-3 the importance of seasonal variation is evident. In comparing these estimates with those based on isotopic measurements, the conventional approach underestimates tailings seepage during the Dry and overestimates it during the Wet. When considering the relative volumes of water this difference becomes more critical for management of the tailings. On an annual basis, the isotopic approach can be used with volumetric data to calculate percentage tailings seepage of 8% of the total water and 45% of the sulfate.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

APPLICATION OF CHEMOSTRATIGRAPHY TO PETROLEUM EXPLORATION AND FIELD APPRAISAL IN AUSTRALIAN BASINS AS Andrew. DJ Whitford and PJ Hamilton Australian Petroleum CRC, CSIRO Division of Petroleum Resources, PO Box 136 North Ryde, NSW 2113

Chemostratigraphy relies on the variations in major and trace element abundances in sedimentary rocks as a stratigraphic correlation tool. It is particularly relevant in field appraisal where conventional techniques such as seismic and biostratigraphic methods have insufficient resolution.

The geochemistry of sedimentary rocks is a product of their provenance, maturity and diagenetic history.

Elements which do not readily partition into natural waters are strongly indicative of provenance and are unlikely to be mobilised during diagenesis. Such elements include Al, Ga, Ti, Zr, Hf, Y, Sc, REE, Th, Nb, Sn and Be. These elements that can be characterised as immobile are transferred almost quantitatively into clastic sediments and are in very low abundances in chemical sediments such as carbonates and evaporites. Elements which partition strongly into natural waters include most alkali and alkaline earth elements, B and U, may be strongly affected by diagenetic processes and have no simple relationship with provenance.

Sediments with the same provenance should preserve the same relative proportions of immobile elements, irrespective of the degree of dilution or enrichment associated with variations in quartz content or diagenetic effects. On x-y plots the data plot as a linear array projecting to the origin if the elements behave in an immobile manner. Movement along the linear array will be controlled by relative dilution or enrichment related mostly to the amounts of quartz or carbonate in the rocks and may reflect primary or secondary processes. Precipitation of diagenetic rutile, anatase and zircon would result in scatter of the samples. Lines of differing slopes implies a change in provenance. It is important to recognise that the provenance information is retained by the ratios of immobile elements rather than their absolute abundance. Insofar as chemostratigraphic units are defined by provenance, considerable care is required in using absolute abundance data, ratios of immobile elements are probably more robust indicators.

To demonstrate the different behaviour of elements and the influence of both primary and secondary processes, selected major and trace elements may be nonnalized with relative to an immobile element. Many of the geochemical variations correspond with changes in the gamma ray log. The primary variations result from provenance and environment of sedimentation whereas secondary processes reflect diagenesis and fluid flow.

Primary variations indicative of provenance should be the more robust correlation tool on the scale of an exploration permit; they should closely follow time lines. Primary variations indicative of sedimentary environment and may be time transgressive on a basin-wide scale. Likewise secondary variations will tend to follow a particular sedimentary package and therefore be also time transgressive.

Drilling mud is a significant contaminant in some samples. Barium contents exceeding 2000 ppm are considered to be a direct product of mud contamination. Core samples are the least affected by this contamination. Cutting and side wall cores show similar levels of contamination. Major elements are not affected by mud contamination. Other elements effected by mud contamination include Sr and Eu. Two major alteration styles which may affect geochemistry in a way that will effect chemostratigraphic correlations are wholesale secondary carbonation and silicification: • •

Wholesale carbonation (addition of siderite, calcite or dolomite) will decrease Si and Al contents, increase LOI and Fe/Ca/Mg which may result in a sand-rich unit geochemically resembling a shale.

Wholesale silicification will increase Si and decrease all other elements and is much more difficult to recognise. Plots of ratios of immobile elements relative to Si0 2 , show siliceous samples generally have different immobile element ratios and suggests the high silica values reflect primary chemical variation and that they have a different provenance to the shales.

In combination with chemostratigraphy, the carbon isotope variation represents the least ambiguous single point correlation horizon. If the carbon isotope variation is related to a global change in the carbon cycle, then the carbon isotope shift will be a powerful time-constant marker and correlatible within and between basins. If the relationship with palynology can be better resolved it may be useful as an absolute age determinant.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 PERMIAN BRACHIOPODA AND THE SEARCH FOR GONDWANA IN ASIA N, W. Archbold and G.R. Shi School of Aquatic Science and Natural Resources Management, Deakin University, Rusden Campus, Clayton, Victoria. 1368

Permian marine faunas are well known for their provincialism. While different phyla demonstrate different degrees of provincialism, there is broad agreement that water temperature (a reflection in general of prevailing climate) is a critical factor in determining the migration potential and hence distribution of marine organisms. Representatives of the Brachiopoda are present in all Permian marine sedimentary basins and are one of the most highly provincial groups. They demonstrate subtle sensitivity to temperature, even when other factors for migration potential are present (Archbold & Shi, 1995), and significant endemism of genera even within provinces of low temperature and polar conditions (Archbold, 1995).

While this extreme provincialism can seriously hamper regional correlation, as for example between the marine Permian of western and eastern Australia, it can be used with great effect to delineate faunal province boundaries and relocate the now disjunct terranes within Asia back to their pre Permian positions within, or peripheral to, Gondwana. Rift-drift models can also be proposed for individual terranes during the Permian if sequences and faunas are complete enough within the terrane to show change of faunal relationships during the Permian. These faunal changes may be interpreted as reflecting tectonic events in addition to global climatic changes (Shi & Archbold, 1995). We provide a reconstruction below (Figure 1) of the world during the Baigendzhinian-Early Kungurian time slice showing the provincial relationships we have identified.

Recognition of the predrift position of these Asian terranes should assist in the prediction of the potential for the discovery of economic energy and mineral deposits within these now disjunctfragmentsof Gondwana.

1. Marine provincialism of the western Pacific during the Baigendzhinian-Early Kungurian.

Provincial regions: V = Verkolyma, SM = Sino-Mongolian, C = Cathaysian, CM = Cimmerian - Himalayan, CS = Cimmerian - Shan Thai, W = Westralian, A = Austrazean.

REFERENCES Archbold, N.W., 1995. Evolutionary and migration history of Australian Permian brachiopod faunas. In Kuzina, L.F., ed., International Symposium 'Evolution of Permian marine biota', August 1995, Abstracts, pp. 45-47, Paleontologicheskii Institut, Rossiiskaya Akademiya Nauk, Moskva.

Archbold, N.W. and Shi, G.R., 1995. Permian brachiopod faunas of Western Australia : Gondwanan - Asian relationships and Permian climate. Journal ofSoutheast Asian Earth Sciences 3,207-215.

Shi, G.R. and Archbold, N.W., 1995. Permian brachiopod faunal sequence of the Shan-Thai terrane : biostratigraphy, palaeobiogeographical affinities and plate tectonic/palaeoclimatic implications. Journal of Southeast Asian Earth Sciences 3,177-187.

Acknowledgements : Our work is financially supported by the Australian Research Council.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

DYNAMICS OF THE EARTH: HOW THE MACHINE WORKS TODAY Richard J. Arculus Department of Geology, Australian National University, Canberra, ACT 0200

INTRODUCTION For the past 40 years, satellite technology, high-speed and extensive data computation, and (perhaps most encouragingly) a growing level of international cooperation has stimulated major improvements in our ability to monitor the exterior and interior of our planet. Measurement, monitoring, experimentation and tentative prediction of the way this planet works, have led to an appreciation of the intricate and complex linkages that exist between the subsystems of biosphere-hydrosphere-atmosphere-bulk Earth.. Although the pressure for increasing analytical and computational specialisation is immense, major progress in understanding how the Earth works demands interdisciplinary approaches. We are of course far from a comprehensive understanding of the subsystems that form our planet let alone being in a position to have a high degreee of confidence in our predictive capabilities, and our consideration of the way the Earth works today derives'in large part from a longer term view of the evolution of terrestrial dynamic processes.

THE DRIVERS Much of the fascination of the Earth as an object of study derives from its chemical heterogeneity.

Fundamentally, this heterogeneity is controlled by the variable gravitational attraction for chemically distinctive solid-liquid-vapour states of phase. In addition however, some of the most extreme chemical fractionation compared with bulk Earth abundances of the elements is achieved by the surface processes of weathering/sedimentation and biologic agencies. The separation of melt from solid or vapour from melt are examples of the importance of physical separation of chemically distinct phases. Subsequent solidification of separated basaltic melt has some intriguing results. The low density of A1 (in 4-fold coordination) at low pressures (<2 GPa) compared with its high density (in 6-fold coordination) at pressures >2 GPa is a major factor in the negative bouyancy of oceanic lithosphere and the relative bouyancy of melt-depleted mantle.

The solar-derived energy flux controlling much of the dynamic workings of the biosphere-hydrosphereatmosphere subsystems allied with the shedding of heat from the Earth's interior are the primary drivers. We have substantive evidence for the variability of the solar energy flux on time scales that are geologically short (~ 10 4 to 105 years) as well as a more theoretical basis for believing that during the lifetime of the Sun, this flux has increased by about 30%. We know that the shorter-term variability affects biologic productivity and at the least modulates the intensity of "ice-" vs. "hot-house" conditions.

The precision of satellite-dependent global positioning measurements has permitted unequivocal confirmation of the major tenets of the plate tectonic paradigm as well as demonstrating the relative fixity of hot spots. It is only recently that we have begun to appreciate that cooling of the outermost boundary layer (resulting in subduction of the lithosphere) and differential heating of an interior boundary layer (at the core-mantle interface) generating mantle plumes, function as two relatively independent modes of interior heat loss. Higher internal temperatures in the geologic past means that the Phanerozoic style of subduction/hot spot convection modes may not have operated in the Proterozoic/Archean.

THE CONSEQUENCES (& UNKNOWNS) Remarkable sequences of events stem from variability in the driving fluxes. For example, the increased flux of oceanic magmatism - 1 2 0 Ma ago (both plume and spreading ridge), possibly ultimately trigggered by variability in convective characteristics of the outer core, led to increased vapour fluxes (including CO2), global marine high stands and atmospheric/biospheric effects. Redistribution of continents embedded in lithosphere can also affect the external workings of the Earth through creation of climate-modifying mountain belts, and the pattern of seaways controls oceanic circulation patterns.

A model of the way the Earth works, analogous to global atmospheric circulation models, is an attractive goal.

However, we are ignorant of some fundamental fluxes and sizes of reservoirs neccessary for modelling. For example, our knowledge of the 3-D distribution (abundance, phase, mineral host) of H2O in the (subductable) oceanic lithosphere is minimal., and our understanding of H2O fluxes from exosphere to the Earth's interior is primitive. These parameters are needed to understand the evolution of the hydrosphere and the internal properties/behaviour of the mantle - societal welfare as well as intellectual satisfaction is at stake.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996

THE MERSEY VALLEY OIL BOOM OF THE 1920s Carol A. Bacon, Mineral Resources Tasmania, PO Box 56, Rosny Park 7018

Naturally-occurring deposits of petroleum have been used for millennia. The basket which cradled Moses in the bullrushes was waterproofed with "slime and with pitch", while Noah's Ark was "pitched within and without". The builders of Babylon used pitch as a mortar for their fabulous constructions. The Egyptian Pharaoh Thothmes III extracted 1200 kg of bitumen as a tribute from certain cities in Mesopotamia, and a thousand years later Alexander the Great visited a Persian town where he was greeted by people lighting oil sprinkled in the streets. At Baku, in Azerbaijan, pilgrims flocked to the Eternal Fires, fuelled by naturally-occurring gas from petroleum seeps.

By 1920, the use of petroleum was well established and a number of eager entrepreneurial types appeared in Tasmania in search of this very profitable substance. Part of this interest was due to the Commonwealth Government, which had offered a reward of £10,000 in January 1920 for the discovery of a payable oil deposit anywhere in Australia. The oil seekers examined the potential for liquid oil (petroleum), then oil from oil shale, and oil from a coal (pelionite) in the Mt Pelion area at the head of the Mersey Valley.

At the first sniff of economic activity, Government Geologist Loftus Hills was on the trail — ensuring with all the vigour he could muster that all geological finds were recorded accurately and that no speculative statements were made without adequate foundation. From the outset, Hills was fighting a losing battle against the forces of unbridled optimism.

Following articles in The Age and the Zeehan and Dundas Herald, Hills wrote to Acting Secretary for Mines, Pretyman, explaining the difference between albertite and pelionite, stating: "Investigation of the Barn Bluff-Pelion area as a coalfield is a very desirable undertaking, but I consider it my duty to point out that money spent in the hope of locating liquid oil will be surely wasted". Hills requested that those responsible for the articles produce more evidence and continued: "In the absence of such more definite indications I am compelled by a realisation of my duty to the General Public to warn them that on the evidence in our possession there is no hope of any discovery of liquid oil in the Barn Bluff-Pelion area".

Hills' protestations did nothing to stop the flood of newspaper articles, the media being hungry for information on a new money-making venture, with all the glamour brought by that magic word 'oil'. This early 'boom' was the start of the search for liquid oil in Tasmania. Altogether 36 holes have been drilled for oil without encountering confirmed or reproducible oil or gas shows. Some 135 'seeps' of oil or gas have been reported to the Department of Mines, but subsequent investigation has almost invariably revealed that the reputed observations are not indicative of petroleum hydrocarbons.

Pieces of ashphaltum found washed up on coastlines have prompted the formation of a number of syndicates intent on developing such finds.

Small deposits of oil shale in northern Tasmania have been previously investigated as a potential source of hydrocarbons or road bitumen, and the shale has been mined intermittently since the 1860s. During the 1930s a number of experimental retorts were used by different companies to produce a variety of fuels and fuel products, although none of the retorts operated as a commercial success. The oil in the shale is derived from microfossil algal cysts or algal bodies, which release oils when heated. The host sediment is of marine origin. The shale is of Late Carboniferous age and is older than the Mersey and Preolenna Coal Measures, and is not a facies variant thereof. Recent exploration has defined indicated in situ reserves of 40 million tonnes of oil shale, although no commercial applications currently exist for the products which can be produced.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE PRINCIPAL COMPONENTS OF NORTH QUEENSLAND GEOLOGY John H.C. Bain.1 and John J. Draper2 (1) Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 (2) Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001

The 3rd AGC led to a major review of North Queensland geology ("The Geology and Geophysics of Northeastern Australia" - Henderson & Stephenson, 1980). In the nearly two decades since there have been enormous amounts of new geoscience information acquired in North Queensland, especially by AGSO and GSQ, but also by the petroleum and minerals exploration industry and universities. Significant technological developments such as image processing, higher resolution airborne geophysics, geographic information systems (GIS), improved seismic reflection techniques, and ion microprobe (SHRIMP) geochronology, have provided a better understanding of North Queensland geology.

To clearly present and interpret the complex web of interlocking and overprinting elements that make up the featured 450,000 km2 North Queensland geology map and to aid the management of information in relational databases, we have chosen a framework that involves some changes to traditional ways of presenting regional geology. The key elements are geologically meaningful geographic regions (mostly ranging from 10,000 to 50,000 km2) and a synthesis based on the nature and relationships of geological provinces and basins (e.g. Etheridge Province, and Carpentaria Basin).

The geological regions enclose cohesive, albeit sometimes complex geologic assemblages, e.g., the Georgetown Region is generally equivalent to the established but somewhat inappropriate term Georgetown Inlier. A geological region may contain several geological provinces or sedimentary basins, or parts thereof where the province or basin extends beyond the region.

The Georgetown Region for example contains parts of seven Provinces and three Basins. Regions are represented on the map by single polygons, and their boundaries, although roughly coincident with major geological boundaries are generalised (smoothed). This facilitates data management in relational databases and GIS. Basic descriptive geological information is given for each region and the various interregional rock and structural associations are defined in terms of provinces.

Geological provinces are deformed and metamorphosed basins: they represent coherent packages of rock with specific dimensions, including thickness, and age ranges. Similarly sub-provinces are equivalent to sub-basins. Provinces may extend beneath other provinces or basins, e.g., the Proterozoic Etheridge and Savannah Provinces extend westwards from the Georgetown Region beneath the Mesozoic Carpentaria and Eromanga Basins in the Carpentaria Lowlands Region and much of the Carpentaria Basin is concealed beneath the Cainozoic Karumba Basin. Some provinces are discontinuous, and therefore are represented on the map by more than one polygon e.g., the Cainozoic igneous province of eastern Australia. This facilitates accurate depiction of distribution and relationships, analysis of regional variation, and interregional correlations especially in GIS. The provinces can be divided into two categories: 1) strongly deformed and variably metamorphosed sedimentary/igneous sequences, and 2) superimposed igneous rocks that extend beyond the other entities (provinces). Boundaries may be faulted, intrusive or superpositional.

Some provinces are composite with basement and cover components, others are simple.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PLATINUM-GROUP ELEMENTS IN THE MERENSKY REEF. EXPERIMENTAL SOLUBILITIES OF Pt AND Pt IN PYRRHOTITE (Fel-xS) FROM 950 to 450°C UNDER CONTROLLED fS2 AND fH2 Chris Ballhaus , Chris Ryan , Peter Ulmer GEMOC Department of Geology, A.N.U., ACT 0200 CSIRO Exploration and Mining, PO Box 136, North Ryde NSW 2113 Erdwissenschaften, ETH Zentrum, Sonneggstr. 5, CH-8092 Zurich (Switzerland) 1

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In the Merensky Reef of the Bushveld Complex, the platinum-group elements (PGE) mostly occur as discrete phases (sulfides, sulfarsenides, intermetallic compounds, alloys). In terms of total abundance, the PGE fraction in solid solution in base metal sulfides (pyrrhotite, pentlandite, chalcopyrite) is small. Proton microprobe analyses using PIXE indicate that only pentlandite accommodates appreciable PGE (Pd, Ru, Rh) while in pyrrhotite and chalcopyrite the PGE are below detection limit. We calculate that nearly all Pt and at least 50 percent of the Pd in the ore must be concentrated in discrete PGE phases.

In order to constrain the temperatures at which the discrete PGE phases are in exchange equilibrium with the PGE found in solid solution in the base metal sulfides, we have equilibrated synthetic 1C pyrrhotite (the most common Merensky sulfide) with Pt and Pd (the most abundant PGE in Merensky ore), over a wide temperature range under controlled sulfur and hydrogen fugacities (fS2 and fH2). The sulfide-PGE assemblages plus 2 to 3 JJJ H20 are placed in an open sample container made of silica glass (for high fS2) or graphite (for low fS2). Above the sample container is placed a welded Pt capsulefilledwith a strong H2 sink (Fe203, CuO, Pt02, or Ir203).

Both capsules are then enclosed in a larger diameter Pt outer capsule and equilibrated between 950 and 450°C for 24 h to 240 h at 1 GPa in a standard piston cylinder press. As H2 is absorbed by the hydrogen sink during an experiment, the oxygen fugacity (/02) in the outer capsulerisesaccording to the reaction 2H2 + 02 = 2H20, and /S2 will follow, driven by the equilibrium 6FeS + 402 = 2Fe304 + 3S2. Any /S2 sensitive exchange vector that reaches equilibrium before the H2 sink is exhausted can be studied successfully with this technique.

The technique allows to simulate the entire fS2 stabilityfieldof Fel-xS. The results demonstrate that the PGE solubility in base metal sulfides falls strongly with falling temperature and falling fS2. Both Pt and Pd require the presence of vacancies in the Fe sublattice of Fel-xS, in order to dissolve.

Pd appears to replace Fe atoms that have as nearest neighbours at least four vacancies, while Pt substitutes for Fe atoms that have as nearest neighbors at leastfivevacancies. Both Pd and Pt replace Fe on a one to one basis. The highest PGE grades can thus be expected in the most oxidized ores with the highest sulfur fugacities.

If at a given temperature, the PGE only substitute for Fe atoms that are surrounded by a minimum number of vacancies, then the solubility of PGE in Fel-xS must fall exponentially with falling temperature and falling fS2.

Vacancies in the Fe sublattice of Fel-xS become ordered (evenly distributed) with falling temperature to minimize electrostatic energy, and expelled with falling relative fS2. PGE initially dissolved in monosulfide solid solution (mss) at high temperature will be exsolved to a very large extent as the mss recrystallizes to FelxS and pentlandite during cooling, to form their own discrete PGE phases.

We extrapolate our experimental temperature-solubility profiles to PGE levels typically observed in natural pyrrhotites of Merensky-type PGE deposits. On this basis, we suggest that the association of base metal sulfides with discrete PGE phases is a low temperature phenomenon. We conclude that the sulfides of the cooling Merensky reef may have continued to exsolve PGE to temperatures below 100°C.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE LATE ARCHAEAN BONANZA: MAGMATIC, METALLOGENIC AND ENVIRONMENTAL CONSEQUENCES OF A SUPERPLUME? MarkE. Barlev1. Bryan Krapez1, David I. Groves 1 and Rob Kerrich2 Key Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands, WA 6907 department of Geological Sciences, university of Saskatchawan, Saskatoon , Saskatchewan, Canada S7N OWO 1

The Late Archaean (2.78 to 2.5 Ga) contains periods of intense magmatism and prodigious metallogenic provinces of Ni, Fe, Cu-Zn and Au deposits. In particular the 30 million year period 2.72 to 2.69 Ga saw one of the most widespread episodes of ultramafic and mafic volcanism preserved in the rock record. Extensive Assemblages of this age comprising komatiites and komatiitic basalts derived from deep sourced mantle plumes, together with tholeiites (± calc-alkaline volcanics) are preserved on at least seven'widely dispersed cratons.

Intense submarine volcanism resulted in the formation of komatiite-hosted Ni mineralization in extensional plume environments and Cu-Zn sulphide mineralization in extensional arcs, and was accompanied by a marine transgression and flooding of previously exposed continental crust. Elevated hydrothermal activity, peak organic production and widespread anoxia (the latter not reflected in earlier Archaean terranes) in submarine basins are reflected by the widespread occurrence of sulphide-rich carbonaceous sedimentary rocks and banded iron formations. In granitoid-greenstone terranes submarine volcanism was followed by basin closure, accretionary tectonics with the incorporation of metal- and carbon-rich submarine volcanic and sedimentary successions into nascent continental crust, and the extensive granitoid magmatism between 2.69 and 2.65 Ga. These events were accompanied or followed by a major episode of mesothermal gold mineralization (by 2.63 Ga in Australia).

The order and relative timing of tectonic and metallogenic events in the Yilgarn Craton and Superior Provinces between 2.78 and 2.6 Ga parallels that observed during the opening and closure of the marginal basins, which flanked external oceans during the breakup of supercontinents in the early Palaeozoic and Mesozoic. In the Mesozoic rifting of Pangea was accompanied by Ni-Cu mineralization in continental extensional settings above mantle plumes. Growth of the Atlantic (internal ocean) was accompanied by subduction of the Pacific with the development of marginal basins containing VMS mineralization. Increased spreading rates and submarine plateau volcanism during the Cretaceous superplume (-120 to 85 Ma) within the Pacific (external ocean) resulted in fast subduction of increasingly young ocean crust which closed many of these basins (obducting young ocean and marginal basin crust) and corresponded to peak magmatism in oceanic and continental arcs.

This was also accompanied by a global sea-level high, peak organic production on continental shelves and widespread anoxic conditions in marine basins. Most Mesozoic to Recent mesothermal gold mineralization was associated with reorganization of plate margins, and accretion of arcs and plateaus during and following this period.

The tectonic, magmatic and metallogenic evolution of the Yilgarn Craton and Superior Province are interpreted as a global Late-Archaean tectonic cycle, analogous to the first half of a supercontinent cycle from 2.78 Ga, with enhanced marine and continental magmatism between 2.7 and 2.65 Ga recording a superplume event, analogous to the Cretaceous Superplume. Intense magmatism and elevated heat flow in plume, arc and continental environments is mainly responsible for the prodigious metal inventory and extreme environmental conditions (enhanced organic production, widespread anoxia) of the Late Archaean. The concept of a global Late Archaean tectonic and metallogenic cycle is strengthened by comparable and synchronous patterns of tectonic and metallogenic evolution in the granitoid-greenstone terranes of Zimbabwe, India and Baltica, and the coeval evolution of rift and passive margin successions on continental crust in the Pilbara and Kaapvaal Cratons.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EXTENSION AND MINERALIZATION: SOUTHWESTERN NORTH AMERICA Mark D. Barton Department of Geosciences, University of Arizona, Tucson. AZ 85721

Multiple types of economic and subeconomic mineralization are associated with Mesozoic and Tertiary extensional regimes in southwestern North America (SWNA, see Table below). Best known is mineralization related to mid and late Cenozoic extension (<40 Ma), but analogous episodes during the Jurassic to early Cretaceous (175-150 Ma) bear a similar metallogenic signature. This mixture of deposit types contrasts with the more restricted metallogenic patterns associated with primarily compressional tectonics during the Cretaceous and early Tertiaiy.

Processes (fluid sources)

Deposit Types Porphyry Cu(-Mo-Au), Mo(-Cu), Mo(-W), Sn(F) & replacement/skam Fe-Cu, Zn-Cu-Pb-Ag(-Au), W-Zn(-F-Be), F-Be

Second boiling (magmatic), magma-driven convection (meteoric) [extension is incidental except in promoting preservation and compositional diversity]

Epithermal Ag-Au(-Zn-Cu-Pb), Hg(-Au), Au(-Cu-Ag)

Magma-driven convection (dilute meteoric ± magmatic) [extension is incidental]

Carlin Au(-Ag-Sb-As)

Extension-driven convection (dilute meteoric)

Detachment Au-Cu

Extension-driven convection (saline meteoric)

Sediment-hosted Cu(-Ag), Zn-Pb-Ag(-Cu), MnOx(-Ag)

Rift-related circulation (dilute to saline basinal fluids)

Voicanogenic massive sulfide

Magma-driven convection (seawater) [extension favors marine environment]

Fe-oxide(-REE-Cu-Au)

Magma-driven convection (evaporitic) [extension favors closed basins]

Crustal extension affects the diversity, processes, and preservation potential of mineralization. The multiple deposit types formed during extension reflect varied geological processes and environments. Some are directly related to fluid flow induced or enhanced by extension. These include Carlin and detachment type Au systems and rift-related Ag-bearing base metal deposits. Igneous-related mineral deposits include those with an essential magmatic fluid component and those where various external fluids are circulated by magmatic heat.

In both cases, many types form simultaneously which reflects the diversity of magmatic compositions and nearsurface fluids.

The northern and southern parts of the Basin and Range province provide useful examples. In both areas, multiple types of porphyry, skarn, and replacement mineralization formed approximately contemporaneously in both the Jurassic and the mid-Tertiary. These systems range from Cu(-Au-Fe) deposits associated with quartz monzodioritic stocks to W greisens associated with two-mica granites to Mo(-F) porphyries associated with metaluminous topaz rhyolites. Meteoric water with or without magmatic contributions dominated volcanicassociated epithermal precious metal systems. Their occurrence in extensional regimes is apparently largely coincidental, although extensional deformation enhanced permeabilities and caused subsequent disruption.

Where seawater was available, as to the NW in the Jurassic, contemporaneous magmatism produced VMS systems in marine arcs, whereas late Cenozoic sediment-hosted massive sulfide deposits are related to riftrelated seawater circulation in the Gulf of California. Hypersaline fluids generated in closed basins were involved in formation of Fe-oxide(-REE-Cu) occurrrences by igneous-driven fluid circulation in both the Jurassic and Tertiaiy from Nevada through central Mexico. Regionally extensive sodic alteration, particularly in the Jurassic, reflects this circulation. Similar fluids, at lower temperatures, were circulated amagmatically during major mid-Tertiary extension in the southern Basin and Range forming extensive potassic alteration and the oxidized Au(-Cu) detachment type mineralization. In the northern Basin and Range, similar amagmatic circulation of dilute waters through the reduced metasedimentary pile of the Cordilleran miogeocline appears to be responsible for formation of Carlin type Au deposits. In both settings, the thermal energy, increased permeability, and probable topographic effects related to extension appear to be more than sufficient to drive widespread transient fluid flow events. Lastly, a profound consequence of extension in SWNA is its role in the preservation of mineralized portions of the crust. Deposits formed during extension have a higher probability of being preserved that deposits formed during compression, primarily due to reduction in base levels. A simple model for preservation when constrained by geochronology demonstrates the importance of extension in governing the observed Cu porphyry metallogeny of SWNA. Extension not only preserved older (Laramide: 7050 Ma) deposits against subsequent erosion, but smeared many out in the near-surface environment where larger volumes were subject to supergene enrichment. Preservation potential also helps explain the metallogic patterns for other types of deposits, particularly the near-surface varieties.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE ORIGIN OF GOLD MINERALISATION AT LAKE COWAL, ENDEAVOUR 42 (NSW) Evgeniv N Bastrakov , Murray Brooker , John L Walshe *Dept. of Geology, Australian National University, Canberra, ACT 0200 Inversiones North (Chile) Limitada, San Sebastian 2839, Office 702, Las Condes, Santiago 1

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The regional association of Au and Cu mineralisation of the Central-West New South Wales (Lachlan Fold Belt) with the mantle-derived Ordovician magmatism is widely appreciated. Understanding of the genetic nature of this association is of clear importance for further exploration strategy in the region, currently aimed at discoveries of magmatic porphyry-related hydrothermal deposits. However, in many cases it remains obscure.

The low-grade Au vein mineralisation at Lake Cowal (Endeavour 42) exhibits a set of controversial geological features which make the solution of this problem especially challenging. Hosted by intermediate calc-alkaline volcanics intruded by diorites, it is situated immediately next ( 4.5 km north) to a large, low grade porphyry copper prospect (Endeavour 39). This spatial association, coupled with the pattern of metallogenic zoning in the area (Cu,Au Au -» Zn,Au), suggests a genetic link between the two hydrothermal systems. At the same time the gold mineralisation is broadly associated with shear zones and has geometrical relationships and mineralisation styles typical of shear-zone hosted deposits. Gold distribution is controlled by extensional zones formed in response to regional and local stress patterns. According to results of radiogenic dating (Perkins, Econ. Geol., in press), the gold deposition is broadly contemporaneous with the main intrusive event, but might be significantly later and related to the emplacement of geochemically distinct suite of mafic dykes, enriched in incompatible elements of high ionic potential (Nb, Ce, P, Zr, Hf; Nb).

The detailed geological and petrological study shows that the Endeavour 42 deposit was formed by gradually evolving hydrothermal fluids. Large-scale hydrothermal alteration resulted in the development of quartzsericite-ankerite and chlorite-K-feldspar-carbonate metasomatites spatially adjacent to fault zones. At the regional scale the host volcanic sequence was subjected to propylitic alteration which resulted in almost total decomposition of primary magmatic mafics, deanorthitisation of plagioclase, and development of disseminated pyritisation. The main alteration stage was followed by deposition of ore minerals (pyrite, sphalerite, chalcopyrite, galena, and gold) within the fault zones and stockworks of the related dilational veins. The gangue mineralogy reflects the composition of the immediate altered host, and varies from essentially quartzankerite to quartz-calcite-K-feldspar assemblages. However, it does not significantly affect the sulphide mineralogy. Paragenetic analysis of hydrothermal assemblages supported by thermodynamic calculations reveals near-neutral acidity of the hydrothermal fluids.

Deposition of sulphides and gold was accompanied by the gradual change in the oxidation state of the fluids, which resulted in transition from pyrrhotite to hematite-bearing vein assemblages. The bulk of the mineralisation was formed between the PyPoMt and HemMt redox buffers. Fluid inclusion data suggest the formation of mineralisation at temperatures around 150-200°C, by fluids with a salinity range from 3 to 18 wt% NaCl equiv. Strong variations in the salinity could result from the dilution and boiling of a primary deep fluid with NaCl content of about 7-9 wt%. This interpretation is favoured by the temperature-salinity trends and vein textures, but unambiguous evidence of boiling from vapour-rich inclusions is scarce. Isotopic composition of sulphides (5 S = -3 -5 °/ ) reflects the magmatic origin of sulphur and is consistent with relatively reduced (H S > S0 ) conditions of mineralisation.

The REE pattern of the mineralising fluids deduced from composition of vein carbonates is characterised by a positive Eu anomaly, which developed from hydrothermal destruction of magmatic plagioclase of the host volcanic sequence. In turn, isotopic composition of Pb from the gold ore (Carr, Econ. Geol., in press) reflects re-mobilisation of leadfromthe same country rocks.

Thefluidacidity and redox state coupled with geochemical data suggest their buffering by the host rocks in the source region. The observed change in the redox conditions can be attributed to the shift in the redox equilibria involving chlorite, which resulted from gradual temperature decrease.

The Endeavour 42 gold deposit can be interpreted as a shear-hosted convective hydrothermal system, which was affected by the magmatic volatiles (S, C0 ) and NaCl. The mechanisms of gold and base-metal mobilisation involved rock-buffered leaching, and gold and base-metal precipitation involved partially rockbuffered cooling and boiling of mineralising solutions.

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Acknowledgments: ENB acknowledges the financial support by North Limited and Australian Geological Survey Organisation. Publication from the permission of North Limited.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

FLEXURAL ISOSTATIC FORWARD AND REVERSE MODELLING OF BASIN EVOLUTION: APPLICATION TO THE NORTH WEST SHELF K. Baxter1. P.L. Smith1, K.C. Hill2. & AGSO North West Shelf Study Group3 'Australian Geodynamics CRC, CSIRO, 39 Fairway, Nedlands, Perth, WA 6009 2 VIEPS School of Earth Sciences, La Trobe University, Bundoora, VIC 3083 Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2610

Deep seismic transects acquired across the Phanerozoic basins of the North Wet Shelf provide an ideal opportunity to examine the tectonic development of this important hydrocarbon province using forward and reverse modelling techniques. Such modelling is currently underway in the Australian Geodynamics Cooperative Research Centre (AGCRC) and is initially concentrated on key transects across the Vulcan Graben and Petrel Sub-basin.

Modelling involves the iteration between forward and reverse modelling techniques until a "best-fit" model is developed of the syn-rift and post-rift processes. Reverse modelling includes backstripping, sediment decompaction and reverse thermal modelling constrained by rift age and stretching factor, allowing a rewinding of the post-rift basin history to the top of the syn-rift. Forward modelling allows a construction of basin geometry, crustal structure, and subsidence history through time. Deformation incorporates extension by faulting (simple-shear) on planar faults in the brittle upper crust, and by distributed deformation (pure-shear) in the lower crust and lithospheric mantle. Following rifting, the location of footwall uplift and hence major erosional surfaces may be identified. Perturbation of the lithosphere temperature field during extension is included and the re-equilibration of this over time produces post-rift subsidence.

Five transects across the major basins and plateau areas of the North West Shelf are being structurally and flexurally modelled to assess and to test the important basin-forming mechanisms. The results will highlight major similarities and differences in the evolution of the North West Shelf basins, and help direct subsequent modelling studies. Results obtained so far will be displayed and work in progress discussed.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 INTEGRATION OF GEOCHEMICAL MODELING OF HYDROCARBON SISTEMS AND MATHEMATICAL MODELING OF HYDROCARBON RESOURCE STRUCTURE Michail D. Belonin, Valentin K. Shimansky, Sergey V. Sminiov, All-Russia Petroleum Research Exploration Institute (VNIGRI), St. Petersburg, Russia

The basement of VNIGRI geochemical modeling as a part of general basin modeling is a long experimental and theoretical studies of all petroliferous basins of Russia and other former Soviet Union countries. As a result of these studies its were elaborated two independ sistems of geochemical modeling - balance and kinetic, and multivariant model of probability field size distribution of ultimate hydrocarbon resources (expected resource structure). The other result of these activities is VNIGRI computerized geological data base, containing vast and varied geological and geochemical information on all russian sedimentary basins.

These modelings are realized using VNIGRI's integrated numerical system "GEONOM" and computer programs "PLANNING" and "PROFIT".

In March 1995 these sistems of modeling were used for Timan-Pechora Basin hydrocarbon potential revaluation. The most important objectives of this study have been to: - obtain new estimating of the contribution of 7 main oil and gas generation objects (S1-S2, Dl, D2, D3 fl, D3 sm-fm2, PI ar and PI k) to total hydrocarbon potential of Timan-Pechora Province; - determine the hydrocarbon generation intensivity and time.

- predict the expected oil and gas reserve structure and their geographical location.

Results from study include a sets of maps of total organic csrbon distribution, source rock maturity, oil and gas generation and emigration dencity, dynamic and intensivity of the hydrocarbon generation for each objects. It was noticeable the general decrease of oil and gas generation scales from SI-2 to D3 tm-sr, then sharp increase in the Domanic (D3 sm-fm2) and again the decrease to the PI. The field size distribution of undiscovered oil and gas resources in the region shows that it is possible to discover about 550 oil fields by size larger than 1.0 mln. t with the total reserves at 1900 mln. t. The most sizeable discoveries are expected in the northern Timan-Pechora Province.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 FORMATION AND DEFORMATION OF THE CAMBRIAN STANSBURY BASIN - KANMANTOO TROUGH Antonio P. Belperio Mines and Energy South Australia, PO Box 151 Eastwood SA 5063

The Kanmantoo Trough is an Early to Middle Cambrian basin of extremely thick, dominantly terrigenous clastic deposits extending along the eastern Mount Lofty Ranges, through Fleurieu Peninsula, into Kangaroo Island. The trough represents the last of several phases crustal extension in the Adelaide Geosyncline. The Trough is part of a far wider zone of Cambrian sedimentation (Stansbury Basin) that extended from Yorke Peninsula across Gulf St Vincent (a shallow shelf succession), the Fleurieu Peninsula and Kangaroo Island (rift or trough facies) and beneath the Murray Basin into Victoria and New South Wales (back-arc environment). The Stansbury Basin sequences together represent a well-preserved record of the tectono-sedimentaiy setting and subsequent deformation of the palaeo-Pacific margin of Australia in the Early to Middle Cambrian. Biostratigraphic and geochronological data indicate an age range of 540-508 Ma for sedimentation in the Stansbury Basin, with the main phase of thick, rift sedimentation in the Kanmantoo Trough from 526 to 508 Ma. Sedimentation apparently ceased with the sudden onset of contractional deformation (Delamerian Orogeny).

Basement elements and principal basin components can be identified from magnetics, gravity, drillhole and outcrop geology despite deformational overprinting of the Delamerian Orogeny . These elements include : the Gawler Craton the Ardrossan and Spencer Shelves the Kanmantoo Trough basement to the Murray Basin (Padthaway Ridge, Pinnaroo Zone, Glenelg River Zone, Stawell Zone) the Kangaroo Island Shear Zone and Fleurieu Imbricate Thrust Zone separating trough and shelf facies Beneath Gulf St Vincent, seismic profiles and magnetic data indicate approximately 4-5 km of platformal Early to Middle Cambrian sediments shoaling westwards onto the craton. Deformation of the cover sequence is confined to gentle warping and imbricate brittle thrusting and this region has good potential for hydrocarbons and MVT mineralisation. The sequence beneath Gulf St Vincent extends onto northern Kangaroo Island where it includes proximal algal reef talus breccia as well as shallow marine and intertidal clastics. Archaeocyaths in oolitic limestone at the top of the Mount McDonnell Formation have allowed correlation with both southern Fleurieu Peninsula (Fork Tree Limestone) and Yorke Peninsula (upper Kulpara/lower Parara Limestone).

Investigator 1 stratigraphic drillhole sited at the eastern end of Emu Bay, Kangaroo Island, intersected a new suite of Cambrian strata including redbeds, glauconitic sandstone, dolomite and limestone. The drill hole terminated in Lincoln Complex orthogneisses, confirming a new tectonic model which extends shallow Gawler Craton basement south to the Kangaroo Island Shear Zone.

The platformal succession on northern Kangaroo Island is separated from Kanmantoo Group clastics by a major zone of shearing, thrusting and mylonite development. The original listric margin to the rift basin was inverted during the Delamerian Orogeny into a northwesterly directed zone of transpression controlled by the buttressing effect of the southern Gawler Craton margin. North of the Kangaroo Island Shear Zone, brittle imbricate ramping over shallow basement has resulted in thrust repetitions of the Kangaroo Island Group strata between Snelling Beach and Emu Bay. Magnetic modelling, confirmed by stratigraphic drilling, indicates a region of thin-skinned detachment and sliding of cover units over relatively shallow basement. To the south of the shear zone, Kanmantoo Group metasandstones are complexly folded and metamorphosed by three deformational episodes with internal thrusting within the sequence. On Fleurieu Peninsula, the Williamstown-Meadows fault corresponds with the eastern margin of a foundered cratonic block, including upthrust inliers of basement. Thrusting of sedimentary strata onto the cratonic foreland has complicated interpretation of the original palaeogeography.

The Padthaway Ridge, a prominent linear magnetic and gravity feature extending from Naracoorte through Coomandook and Murray Bridge, includes complex juxtaposition of syn-sedimentary and syn to post - orogenic mafic and felsic intrusives and volcanics. The ridge forms an eastern margin to the thick clastic accumulations in the Kanmantoo Trough. Probable distal equivalents to the Kanmantoo including mafic volcanics and intrusives continue to the east at least as far as the Victorian and NSW borders. The mafics include both within-plate-style alkali basalts, consistent with continental rifting and graben development, and dolerites and basalts with MORBlike geochemistry associated with crustal extension.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE OSMIUM ISOTOPIC COMPOSITION OF EARLY ARCHEAN ULTRAMAFIC ROCKS FROM GREENLAND AND THE PILBARA BLOCK, WESTERN AUSTRALIA: IMPLICATIONS FOR MANTLE EVOLUTION AND KOMATIITE GENESIS Vickie C. Bennett Tezer M. Esat and Allen P. Nutman Research School of Earth Sciences, The Australian National Univ., Canberra, ACT, Australia, 0200

The rhenium-osmium isotopic system has the potential to refocus our thinking on mantle evolution by changing the emphasis from continental crust extraction to the role played by mafic and ultramafic reservoirs in determining the Earth's chemical evolution. This is because in contrast to the commonly used systems, Rb-Sr, Sm-Nd and U-Th-Pb, which are dominated by lithophile behaviour and thus most strongly influenced by continental crust formation, Re and Os are chalcophile/siderophile elements. The Re-Os evolution of the mantle has not been significantly affected by the extraction of the crust. Differences in the osmium isotopic composition of mantle reservoirs can be generated only if the reservoirs are of limited volume and do not communicate with the main asthenospheric mantle mass as is the case for the subcontinental lithospheric mantle or, if the mantle is internally differentiated as from the storage of large mafic reservoirs.

In order to refine the osmium isotopic evolution of the mantle, we have initiated Os isotopic studies of wellcharacterised early Archean ultramafic rocks. Our approach is to identify and analyse samples with high Os concentrations (greater than 0.5 ppb) and extremely low Re/Os ratios. In this way the accuracy of the calculated initial ratios is resistant to geological complications such as secondary Re mobility and crustal contamination, as well as being robust to uncertainties stemming from blank corrections and analytical difficulties associated with Re and Os spike calibrations.

The oldest Os isotopic constraints come from early Archean ultramafic rocks from southwest Greenland. These spinel peridotites occur as large (up to km^) exceedingly well-preserved enclaves within tonalitic gneisses; SHRIMP U-Pb dating of cross-cutting gneisses in three areas requires these peridotites to be older than 3.80 Ga. The measured ratios from four samples are the lowest yet identified in terrestrial rocks with ^ O s / ^ O s =0.1024 to 0.1043. The initial compositions are close to estimates based on chondritic meteorite evolution with yOs(3.8Ga) = -0.2 to +1.5, where yOs is the per cent difference in isotopic composition from chondrite evolution. These data provide the best constraints for early terrestrial mantle evolution and require that the upper mantle has been characterised by chondritic Re/Os ratios for at least the last 4 Ga. We have also analysed chromites from a section of 3.45 Ga komatiites in the Warrawoona group within the Pilbara Block, Western Australia. Chromites have high Os concentrations (up to 100 ppb) and very low Re/Os ratios making them resistant to alteration. The measured isotopic compositions of these samples are similar to 3.45 Ga depleted mantle compositions 1 8 7 / 1 8 8 is from 0.1049 to 0.1094). However when corrected for Re decay, the chromitites fall below the mantle evolution curve with initial O s / O s =0.0998 to 0.1049 and yOs(3.45Ga) = -5 to -0.1. Sub-chondritic compositions have also been reported by Shirey and Walker (1994) for late Archean komatiites from the Superior Province. Komatiites have been proposed to be the ancient analogues of modern ocean island basalts (OIB) (e.g. Griffiths and Campbell, 1990), and similarly related to mantle plumes. However, the komatiite Os signatures contrast sharply with OIB compositions in that OIB's are all significantly more radiogenic than modern depleted mantle. For example our data from Hawaiian picritic basalts shows that they are from 2 to 14% more radiogenic than the modern depleted mantle. These precise Os measurements of ca. 3.45 Ga plume material indicate that plume source regions were equal to, or possibly even less radiogenic than the ancient depleted mantle. Neodymium isotope data also reflect a change in source character, with OIB being derived from LREE enriched mantle sources (low £nd values) and komatiites from depleted mantle (high values). This fundamental change in the chemistry of plume-related rocks through time may result from either a change in source regions (depth of generation?), or chemical evolution within the plume source, perhaps due to increasing incorporation and aging of recycled crustal components. Tracking of the Os-Nd characteristics of high temperature magmas (picrites and komatiites) through time and from different areas will enable these possibilities to be distinguished.

References: (

0 s

0 s

187

I88

Griffiths, R.W. and Campbell, I.H., 1990, Stirring and structure in mantle plumes, Earth Planet. Sci. Lett. 99, 66-78.

Shirey, S. and Walker, R. 1994, Re-Os Isotopic Evidence for Evolution of the Source of Ultramafic Magmatism, Archean to Present EOS, 75, 690.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, CanberraFebruary 1996

APPLICATION OF PALYNOFACIES IN THE SEQUENCE STRATIGRAPHIC ANALYSIS OF PETROLEUM SYSTEMS James M. Benson Santos Ltd, 101 Grenfell St, Adelaide 5000 S. Australia

Microscopic kerogen components viewed as particulate organic detritus in palynological preparations are often categorised in terms of the palynofacies concept where the size, shape and composition of the dispersed phytoclasts are described in association with estimates of phytoplankton abundance and diversity and the degree of palynomacera! sorting. Palaeoenvironmental indices are interpreted from different palynofacies types as the composition of a palynomaceral assemblage is the product of a distinctive suite of biological, chemical and physical processes within a particular depositional environment. Consequently, palynofacies studies may be used to interpret subsurface sedimentary environments and complement depositional systems tract interpretations derived from sequence stratigraphic studies. It is strategically significant to correctly identify which type of depositional systems tract is represented within the sequence stratigraphic model proposed for a prospect to successfully delineate play fairways and predict the geometry of reservoir sands.

A scheme of palynofacies types is presented which attempts to relate assemblages of palynomacerals to their position within the succession of depositional systems tracts developed within a depositional sequence, as illustrated with reference to recent sequence stratigraphic studies by SOEKOR of the Cretaceous passive margin rocks underlying the Orange and Bredasdorp Basins, offshore South Africa. Ternary diagrams and star plots are used to display palynofacies trends with depth against well-log sections and seismic facies. Cyclical occurrence of distinctive marine palynofacies characterised by associations of dinoflagellates within massive encystment intervals are useful stratigraphic markers within the South African depositional sequences as they occur within primary and secondary condensed sections developed at the boundaries between component depositional systems tracts.

Intervals of massive microplankton encystment of either polytaxic or oligotaxic composition reflect different ecological strategies of phytoplankton in response to fluctuating nutrient levels and environmental stress controlled by relative sea-level fluctuations. Based on the empirical relationship observed between downlap surfaces and peak phytoplankton ratios, the stratigraphic position of polytaxic dinocyst associations recorded on palynofacies logs indicate the position of primary condensed sections developed between the TST and HST. More specifically, marine transgressive phases associated with development of the MFS result in suppression of progradation at delta fronts, reduced terrestrial input and lower rates of sedimentation within an oligotrophic water column, supporting a diverse, environmentally selective phytoplankton population seaward of the relict shelf break. By way of contrast, oligotaxic dinoflagellate communities form during rapid, opportunistic colonisation of eutrophic water columns genetically linked to transgression of relict shelf systems and the development of low stand turbidite systems. For example, monospecific dinoflagellate associations rapidly colonise ecological niches created by flooding of lowstand coastal systems and adjacent subaerially exposed relict shelf systems indicating the position of the Transgressive Surface within successive depositional sequences in the Orange Basin. Furthermore, successive growth stages within Albian turbidite systems in the central Bredasdorp Basin are identified and correlated using distinctive low diversity dinoflagellate cyst associations developed as secondary condensed sections within "p-wafer" palynofacies. The framework of serial condensed section development is used to interpret the location and vertical succession of depositional systems tracts within the Albian 14A sequence intersected in 11 boreholes extending west to east over a distance of 50km. The 14A Type 1 unconformity is genetically linked to the development of reservoirs within the lowstand turbidite system and is defined by abrupt palynofacies changes between the slurry facies of the basin floor fan and the underlying primary condensed section.

Identification of potential source rocks, reservoirs, seals and stratigraphic traps within petroleum systems is dependent on recognition and mapping of depositional systems tracts. Successful application of sequence stratigraphic models in predicting play fairways is an iterative process and depends on the confirmation of prognosed palaeoenvironments as a prospect is explored by integrating qualitative and quantitative palynofacies data with well log motifs and seismic facies.

Acknowledgements. The management of SOEKOR (Pty) Ltd are thanked for granting permission to present proprietary data referred to in this abstract.

28

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996 TECTONICS AND WORLD-CLASS GOLD DEPOSITS: ADVANCES IN UNDERSTANDING OF DEPOSITS IN THE WESTERN UNITED STATES AND CENTRAL ASIA Byron R. Berger * United States Geological Survey, MS 973, Federal Center, Denver, CO 80225 1

The western Cordillera of the United States is endowed with numerous gold deposits of many different styles, including some sufficiently large to be world-class in the context of Singer (1995; Econ. GeoL). A world-class gold deposit is one with gold reserves and production within the top 10% of values for all styles of gold deposits world-wide. Homestake, Goldstrike, Round Mountain, and Bingham Canyon in the United States and Muruntau in Uzbekistan are examples of world-class gold deposits. The common attributes of these world-class gold deposits are the effective focusing of substantial amounts of fluid into a confined volume and the coupling of fluid-flow and chemical reactions to the mechanics of fault processes. These gold deposits occur within or associated with reactivated, older fault structures and other deformation-related permeabilities.

At the Homestake, South Dakota, meso- to hypozonal gold-bearing quartz vein deposit, premineralization diagenesis and metamorphism resulted in a reactive, carbonaceous iron-rich formation that was later mineralized during fault-related deformation and intrusive activity in the region. At the genetically similar Muruntau deposit, Uzbekistan, premineralization structural fabrics in a carbonaceous iron-rich siliciclastic formation control permeability during syndeformation mineralization along a left-lateral fault system. Faults transverse to earlier faulting caused both a focusing of fluid-flow and additional permeabilities. Synmineralization granitic rocks were emplaced into both fault systems.

Carlin-style gold deposits such as Goldstrike and Carlin are made up of siliceous carbonatereplacement Au-As-Sb-Hg ores. In the Carlin region, a large, reactivated regional-scale northnorthwest-striking shear system served to focus fluid-flow at =140°-±200°C in the vicinity of shallow crustal heat sources. In the Getchell area, gold mineralization is also related to reactivated faults.

Paleozoic and Mesozoic thrusts were partially reactivated as lateral faults concurrent with the emplacement of Late Cretaceous granitic intrusions and reactivated again in the early Tertiary during gold mineralization.

The world-class Comstock Lode epithermal-style Au-Ag deposit formed along an active, regionally extensive fault zone where this zone crosses an underlying, transverse fracture system in the basement. The Comstock fault system bounded a lake-filled basin; the lake being important in providing large volumes of fluid to the thermal system, contributing to the focusing of fluid-flow in the system, and affecting ore-fluid chemistry and deposition mechanisms.

In the Bingham Canyon, Utah, porphyry copper deposit, gold is an important by-product commodity, sufficiently so to make it a world-class gold resource. Geologic relations at Bingham indicate that the deposit was formed in the left-step between two northeast-striking left-lateral faults. The left-stepping occurs at the intersection with a northwest-striking fault zone. A Precambrian, crustal-scale shear zone is in the basement below the deposit. Repeated magma emplacement and magmatichydrothermal fluid-flow were focused into the same structurally restricted zone, resulting in the very large deposit.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

TRENDS IN GEOSCIENCE INFORMATION MANAGEMENT: AUSTRALIAN AND INTERNATIONAL GEOLOGICAL SURVEYS David M.L. Berman Australian Geological Survey Organisation GPO Box 378, Canberra, ACT 2601

ICGSECS - INTERNATIONALLY Many of the national geological surveys participate in an annual exchange of information in the areas of geoscience information management, covering, standards, telecommunications and networks, computer applications, and management issues, through the International Consortium of Geological Surveys for Earth Computer Sciences. Core membership of ICGSECS is comprised of the geological surveys of some 19 countries, including Australia, as well as several associate member countries.

Over the past 10 years of such meetings, much information has been presented. From this series of snapshots, a number of trends are evident in how national surveys manage their information and associated technology, as well as the allied economic and political environment, both within and across these agencies. Common developments are seen in the following areas: HARDWARE & SOFTWARE • moving away from in-house system design and production, towards adapting industry packages • a pragmatic/economic move from employing proprietary systems architectures to open architectures • support for a heterogeneous desktop computing environment • operating systems Unix SVR4, desktop X-Motif and Windows, with some Macintosh • R&D initiatives in visualisation and distributed heterogeneous processing • working with GIS packages, but complaining about lack of 3D/4D geology capability • digital cartography and on-demand publication NETWORKING • TCP/IP ethernet networking in conjunction with industry LAN standards (Novell) • increasing WAN capabilities, ISDN or better • a recent rush towards Internet access and Web home page development STANDARDS • setting and promulgating data models and metadata standards • working towards supporting the sharing of global databases MANAGEMENT • client focus • outsourcing • security • budgetary constraint and organisational survival • organisation approach to technology & information management • involvement in the wider information policy setting business GGDPAC - WITHIN AUSTRALIA In Australia there is a similar forum, termed the Government Geologists Database Policy Advisory Committee.

Across the states and territories, the key convergences are seen in: relational databases, organisation-wide networking, common data models to underpin geoscience databases, commercial packages such as Arc/Info, Oracle, Microsoft and (what were Lotus/Wordperfect) office suites. Specialist scientific software includes some good Australian products such as ER Mapper, Petroseis, Intrepid, as well as more international titles such as Geoquest There is a variety of CAD, visualisation and graphics packages employed, too, but an increasing push to limit the variety because of support costs.

CONCLUSION Beset by a great range of change, a variety of software and hardware approaches to dealing with geoscience information, geological surveys have changed rapidly in the way they have standardised on and delivered technological support. Formal groups such as ICGSECS and GGDPAC have helped a process of benchmarking, learning and natural evolution, by encouraging a flow of comparative information in addition to the more conventional information sources from the IT industry.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SEDIMENT MODIFICATION IN ANCIENT CARBONATE SEQUENCES: PROCESSES AND PRODUCTS Thomas Bemecker . Brian W. Logan * School of Earth Sciences, University of Melbourne, Parkviile, VIC 3052 ^Department of Geology and Geophysics, University of Western Australia, Nedlands, WA 6009 1

2

It is a well accepted fact that carbonates are highly reactive to diagenetic processes, usually as a response to fluid migration. However, from a sedimentological point of view, the effects of pressurisation in the burial realm as well as during tectonism are not widely appreciated. It appears that most ancient carbonate sequences are interpreted according to depositional facies models, which are derived from modern examples. The application cf uniformitarian principles generally neglects post-depositional sediment modification which frequently obscure and overprint primary sedimentary textures.

Carbonate modification is controlled by processes operating during compressional and tensional stress regimes.

Increasing compressional stress is applied during burial coinciding with an increase in lithostatic and internal fluid pressures.

The most common process during compression is pressure solution which, depending on host-rock composition, can produce a variety of sediment textures. Allochem-rich pack- and grainstones respond by stylolitisation and during intense pressure solution these carbonate types can be converted into monomict breccias (stylobreccias), characterised by stylolites surrounding each breccia clast. Micrite-dominated mud- and wackestones respond by the formation of anastomosing solution seams forming a nodular texture. Clearly, ancient nodular limestones do not exclusively represent deep marine facies, but are actually generated completely independant from primary depositional environments. Moreover, it is critical to understand that sediment modification under compressional stress results not only in decreased porosities and permeabilities but also in volume reduction of often enormous proportions.

Stylolitisation and brecciation in carbonates are also produced during compressional tectonism (thrusting), and represent a second generation of sediment-overprint. Brecciation in the vicinity of thrust-faults can be extreme, leading to the formation of larger cavities which are consequently filled with internal sediment. This breccia infill is composed of crushed materials which can be introducedfroma variety of source rocks. Since thrust-faults are ideal fluid conduits, they often promote dolomitization, recrystallisation, silicicfication as well as mineralisation.

A different process of sediment modification relates to tensional stress. Tensional stress in carbonates is represented by the formation of strain cavities, including veins, vugs and interfragment voids in breccias. In terms of fluid migration, dilation is of greatest significance. Dilation describes the process of dislocation during which opposing surfaces are driven apart by a pressurised dilator, normally a fluid or other fluidised materials (slurries). As a result of pressurisation of internal fluids dilation creates cavities in which newly introduced materials precipitate. More frequently, dilation is promoted by injection of external fluids and/or slurries. Both, veins and cavities can be infilled with carbonate precipitates, they may contain fragmented host-rock and more significantly, they may contain hydrocarbons and/or sulphide minerals in economic quantities. Obviously, dilation creates increasing porosities and permeabilites, at least temporarily, and also produces prominent volume gain cf the host rocks.

Ancient carbonate sequences which have been subjected to extensional tectonism are most commonly hosting a wide variety of dilational products. It is therefore evident that the application of sedimentary facies models can only provide very limited guidance when predicting the distribution and quality of porosities and permeabilites.

The identification and understanding of sediment modification processes has profound implications to the concept of carbonate sedimentology. Many ancient carbonate successions enjoyed at least one major period cf deformation during which primary sedimentary textures were overprinted. Accordingly, the calculation cf sedimentation rates, the delineation of sequence boundaries as well as the construction of facies models cannot be carried out by simple comparison with modern examples. In order to achieve valuable results, the study cf ancient carbonates requires the integration of sedimentology, structural geology as well as reservoir geology. It appears also important to continue with experimental studies during which some concepts of chemical engineering (slurry behaviour) are applied.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EPISODIC RIFT-SAG SEQUENCES IN COVER SEQUENCE T H R E E , MOUNT ISA INLIER.

NORTHWESTERN

Peter Betts. Kate Pound and Gordon Lister.

Australian Geodynamic Cooperative Research Centre, VIEPS, Dept. of Earth Sciences, Monash University, Clayton, Vic. 3168

Field mapping within Proterozoic sediments of cover sequence three in the north western part of the Mount Isa Inlier provides structural and sedimentologic evidence for multiple phases of fault movement and growth on NEstriking, NW-dipping normal faults during pre-Isan Orogeny NW-SE extension. Proterozoic sediments of the Mount Isa inlier are generally interpreted to have been deposited during three stages of extensional basin formation. The youngest of these packets (cover sequence three of Blake, 1987) comprises the Bigie Formation, the Fiery Creek Volcanics, the Surprise Creek Formation and the Mount Isa (or McNamara) Group. Our research has concentrated on the Fiery Creek Dome area in the northwest part of the Mount Isa Inlier, where the effects of N-S and E-W shortening associated with the -1600-1500 Ma Isan orogeny are less intense than further to the south, allowing easier evaluation of early extensional structures. The results of this research show a significantly more complex rift history than previously inferred by Blake (1987).

The base of cover sequence three is marked by the regionally extensive Bigie unconformity (O'Dea, PhD thesis in prep). The Bigie unconformity is locally a high-angle erosional surface carved onto cover sequence two sediments. Sediments of the Bigie Formation and Fiery Creek Volcanics also display paraconformable or disconfoimable contacts with underlying cover sequence two sediments. In the western and northern Fieiy Creek Dome region the development of the Bigie unconformity is related to footwall uplift and hangingwall subsidence on NE-striking NW-dipping normal faults, which resulted in SE tilting. The distribution, facies variations and thickness of the Bigie Formation at the base of cover sequence three together with the overlying bimodal Fiery Creek Volcanics was largely technically controlled by syn-depositional NE-striking, steeply (-70°) NW-dipping normal faults. Conglomerates and interbedded sandstones show stratigraphic expansion into NE striking syndepositional faults. Angular discordance within sediments of the hanging walls provides additional evidence for growth against these syn-depositional faults. The Bigie Formation and Fiery Creek Volcanics are therefore interpreted as part of a syn-rift packet. The Surprise Creek Formation (SCF) is conformable on the Fiery Creek Volcanics. The SCF forms part of a fining upwards sequence of shallow marine to fluvial quartzites, quartzofeldspathic sandstones and siltstones interpreted to represent post-rift or 'sag' phase sediments. Sediments of the SCF clearly truncate early NW-dipping normal faults. The SCF is overlain by the Torpedo Creek Quartzite (TCQ) and is interpreted to be part of the SCF. The TCQ is stratigraphically overlain by the Gunpowder Creek Formation (GCF). Stratigraphic wedges of the GCF are identified in the hangingwall of syndepositional faults and mark the onset of renewed syn-depositional faulting on NE-striking, NW-dipping faults.

Stratigraphic wedges are indicated by thickening of GCF sediments in the hanging wall across syn-depositional faults providing evidence for either growth faulting or normal faulting followed by sag phase sedimentation within a starved basin. The absence of any facies changes within the GCF across these NE-striking faults supports their evolution as growth faults during deposition of the GCF. The Mount Oxide Chert stratigraphically overlies the GCF and defines the onset of a new period of sag phase sedimentation. Minor offset (-20 m) across the earlier faults is interpreted to result from later reactivation during shortening.

Overlying carbonates of the McNamara Group (Paradise Creek Formation) provide no evidence for contemporaneous fault activity during their deposition.

U-Pb zircon dating of the Fiery Creek Volcanics yields an age of 1709±3 Ma (Rodney Page, pers. comm.) providing a minimum age for the earliest phase of extension associated with the deposition of Cover Sequence Three. A depositional age of 1653 Ma (Page, 1994) for tuffs interlayered with the carbonates of the Paradise Creek Formation constrains the timing of the final phase of extension. It is unclear whether the two generations of syn-depositional fault activity represent separate phases of fault movement relating to discrete separate rifting events, or whether they are stages within one protracted rifting event.

REFERENCES Blake, D. H. 1987. Geology of the Mount Isa terrain and environs, Queensland and Northern Territory. Bureau of Mineral Resources, Bulletin 225, Canberra.

O'Dea, M.G. (PhD thesis in. prep). Geometry and structural evolution of the Leichhardt River Fault Trough Implications for the development of the Mount Isa terrain, Northwest Queensland, Australia.

Page R.W, Sun Shen-su Sun, & Carr G. 1994. Proterozoic sediment- hosted lead-zinc-silver deposits in northern Australia - U-Pb Zircon and Pb Isotope studies. Geological Society of Australia Abstracts, 37, 334-335.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE USE OF SEDIMENT TRAPS TO DETERMINE THE RELATIVE CONTRIBUTIONS OF MARINE AND SEWAGE ORGANIC MATTER INPUT TO SEDIMENTS ON THE MID-SHELF OFFSHORE SYDNEY. Gary P Bickford Australian Geological Survey Organisation, Environmental Geo science and Groundwater Division. PO Box 378 Canberra ACT 2601.

The effects of sewage discharges through deep ocean outfalls on sedimentation rates of organic matter and impacts on mid-shelf sediments are being determined. This paper addresses the load and composition of both sewage and marine organic matter and describes a series of experiments using sediment traps deployed on the mid-shelf offshore Sydney.

Replicate sediment traps have been deployed at reference locations to the North of the three deep ocean outfalls and are compared with a sewage impacted site to the South of the Malabar deep ocean outfall.

The experimental design is based upon two sites nested within a location. Within each site, sediment traps were positioned 0.05 nm apart; the two sites are 0.5 nm apart at each location. To examine seasonal variability, sediment traps were deployed twice during oceanographic summer (stratified water column) and oceanographic winter (well-mixed water column) regimes. Sediment traps with a height/diameter ratio of 8 were positioned immediately above and 10 meters above the seafloor. The flux of material to the sediments was determined by measuring the dry weight of material collected in each trap. To distinguish the flux of newly arrived material (both of marine and anthropogenic origin) from resuspended sediment a variety of chemical and biological tracers were used. The methods used to distinguish the different sources of material to the traps and identify differences and similarities between marine and sewage organic matter will be discussed. The loads of both marine and sewage organic matter to the sediments will be outlined.

Acknowledgements: The author acknowledges the assistance to the field and laboratory program by Adam Lovell and Carsten Matthai. This work is contributing to the authors PhD at the University of Sydney; the assistance of Professor Peter Davies, Dr Judi Hansen, Dr David Heggie, Dr Gavin Birch and Professor Tony Underwood is also acknowledged.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 41 13th Australian Geological Convention, Canberra, February 1996

THE AMAZING DISAPPEARING 'URANIUM' ANOMALIES - RESULTS OF A MULTI-DATE AIRBORNE GAMMA-RADIOMETRIC SURVEY NEAR WAGGA WAGGA, NSW. Philip N. Bierwirth and Robyn M. Johnston Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

Airborne gamma spectrometry surveys were flown twice, 12 months apart, for a small (7 x 15 km) area near Wagga Wagga in NSW. Comparison of the surveys indicates that the Th and K results are stable. However, some U anomalies identified in the first survey were not reproduced in the second survey and could not be found using a ground spectrometer. The anomalies were particularly interesting since they correlated with an area of high groundwater tables and associated land degradation due to salinisation. One small anomaly located by both surveys was found to be related to clay deposition. The ephemeral anomalies are attributed to the presence of radon gas clouds, which accumulate in valleys under low wind conditions. Broad scale atmospheric measurements of radon during the initial survey show increased concentrations on the days when the anomalous areas were flown. Weather station data show virtually no wind at the time of the first survey and 6 knot breezes during the second survey. Also, sampling and analysis of groundwaters, from the area of the anomalies, detected very high (> 200 BqL*) concentrations of Rn (Dighton pers comm.). In the uranium decay series, Rn is the direct decay product of radium (226Ra). Radium has enhanced solubility in both acid and saline waters (Dickson et al, 1987; Dickson and Herczeg, 1991). In the study area, highly saline waters associated with rising water tables caused by tree clearing may have increased the local 226Ra content. In areas where the groundwater levels are near the surface, Rn produced from Ra exolves into the atmosphere. After about an hour, enough of the gas has decayed to produce measurable quantities of 214Bi which can be detected by an airborne system on wind-less days.

The study has implications with regard to human influence on radon emanation rates in areas where salinisation of groundwater is occurring. Increased salinity and rising water-tables may increase the risk from exposure to indoor radon. Another implication is that it may be feasible to study groundwater levels, in areas of uniform soil permeability, by measuring the concentration of 222Rn at the surface.

These results indicate that uranium channel results from airborne surveys should be interpreted with considerable caution.

REFERENCES Dickson, B.L., Giblin, A.M., & Snelling, A.A., 1987. The source of radium accumulations near sandstone escarpments, Australia. Applied Geochemistry, 2, 385-398.

Dickson, B.L., & Herczeg, A.L., 1992. Deposition of trace elements and radionuclides in the spring zone, Lake Tyrrell, Victoria, Australia. Chemical Geology, 96, 151-166.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention., Canberra, February 1996

MASSIVE SULFIDE MINERALISATION IN FELSIC VOLCANIC ROCKS OF THE EASTERN MANUS BACK-ARC BASIN, WESTERN PACIFIC: ODP PROPOSAL 479 R.A.Binns & S.D.Scott CSIRO Exploration and Mining, North Ryde, NSW 2113, Australia Scotiabank Marine Geology Laboratory, University of Toronto M5S 3B1, Canada 1

2

1

2

A long-standing ODP objective has been to drill into active hydrothermal systems in order to understand better the nature of the subsurface water-rock interactions and to determine their three-dimensional architecture. The true analogues of significant ancient polymetallic sulfide ore bodies have yet to be tested. Within the important 'volcanic-hosted' and related subvolcanic' categories of massive sulfide orebody (eg Noranda, Kidd Creek, Roseberry, Que-Hellyer, Woodlawn and Scuddles), the associated volcanic rocks tend to be distinctly siliceous and the inferred tectonic setting is continental margin (arc, back-arc) rather than oceanic.

The eastern Manus Basin is an 80-100 km wide rifted zone of thinned crust, formed as part of a sinistral pullapart feature between two of the three major transform faults in the Bismarck Sea region. One of these transforms separates the eastern Manus Basin from the more mature central Manus Basin where an equivalent amount of back-arc extension behind the presently active New Britain subduction trench and volcanic arc to the south is accommodated by a combination of organised seafloor spreading, microplate rotation, and crustal rifting. An en echelon series of neovolcanic edifices cuts obliquely across the extension direction of the eastern Manus Basin. Seismic profiles of the eastern Manus Basin show -300 m wedges of presumed late Pleistocene sediment lapping against older growth faults: this is consistent at present sedimentation rates with extensional rotation and crustal thinning having commenced about a million years ago.

The PACMANUS hydrothermal field lies near the crest of a northeast-trending elongate edifice. Higher parts of thisridgeare occupied mainly by jagged outcrops of dacite and some rhyodacite, while the lower reaches are dominated by ropy sheets and lobateflowsof andesite. Collectively, the highly vesicular volcanic rocks define a geochemically-coherent fractionation series with calcic affinity and mild arc-like trace element signature. A relatively shallow differentiated intrusive source of the lavas is implied. Isolated hydrothermal deposits are scattered along the main crestal zone for almost 10 km at around 1700 m depth. Faunal abundances denote intensely active hydrothermal venting in a 1.2 km central zone. The main deposit types are (1) dark manganiferous crusts and mounds underlain by altered lavas and hyaloclastites, and (2) massive sulfide chimneys, some 5-10 m high, with low-lying pediments that probably include both sulfide and oxide deposits.

Hie chimneys are dominated by sphalerite and chalcopyrite, with barite as the principal gangue. Bulk gold values of the massive sulfides are abnormally high (average 15 ppm). Isotopic research on dredged samples indicates a significant magmatic component to the hydrothermal fluids at PACMANUS, constituting, if true, a major difference from mid-ocean ridge environments drilled by ODP so far where seawater-derived fluids predominate. Collection of hydrothermal vent fluids during an October-November 1995 Shinkai-6500 submersible cruise will enable testing of our expectation that considerable subsurface deposition of sulfides is occurring below PACMANUS, including both subvolcanic and possibly intrusive-related styles.

4

The specific aims of drilling at PACMANUS are (1) to define volcanic stratigraphy, alteration, and sulfide mineral deposition underneath the actively venting exhalative hydrothermalfieldon the crest of theridge,(2) to assess the behaviour and sources offluidsin the subsurface plumbing of the system, and (3) to delineate the nature of basement and any influence it exerts on mineralisation style. In achieving these aims, a number of priority global issues for ODP will also be addressed: • 3-D structure of a massive sulfide deposit hosted by felsic volcanic rocks, for comparison with ancient equivalents (LITHP); • Nature of water-rock interactions in felsic igneous rocks, both in focussed and diffuse outflow zones and in recharge areas (LITHP, SGPP); • 3-D architecture of a felsic volcanic edifice underlying a large hydrothermalfield(LITHP); • History of volcanism, hydrothermalism and extension as recorded in nearby sediments, and temporal interrelationships of these (SGPP, LITHP); • Nature of basement (arc crust, metamorphic?), influencing theories of back-arc extension (LITHP, TECP); • History of opening of a pull-apart extensional basin that has become magmatically active, a case of incipient seafloor spreading? (TECP); • Sedimentary processes and their evolution in an enclosed back-arc basin (OHP, SGPP).

35

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

AN INTERACTIVE, GIS-CONTROLLED DATABASE FOR CONTAMINANT INFORMATION ON SEDIMENTS IN NEW SOUTH WALES ESTUARIES Gavin Birch* and Werner Hennecke^ * Environmental Geology Group, The university of Sydney, NSW, 2006. ^ Geography Department, The University of Sydney, NSWr, 2006.

INTRODUCTION Estuaries are not only dynamic and extremely sensitive environments, but they are increasingly becoming an arena of conflicting interests. Estuaries are important and diverse biological nurseries, frequently urbanized by high-valued real estate, often recreational centres, and are commonly industrialized. The four major estuaries which dissect Sydney are partly responsible for the rapid rate of development of this region. Until 1898 all waste was dumped directly into Sydney's estuaries and disposal into these waterways was only controlled in 1972 by the Clean Waterways Act. A comprehensive study of Sydney's estuaries has been conducted over the past four years and a considerable database has been established. Sometimes several types of samples (grabs, boxcorers, cores, diver samples, vibrocorer, etc.) have been taken at each of the approximately 2000 sampling sites that have been occupied in these estuaries to date. Each sampling device may be subsampled many times at specific stratigraphic intervals, or according to different physicochemical conditions, analyses include texture, coarse (>63 um) and fine (< 63 um) fraction size, heavy metals (Cu, Cr, Cd, Co, Fe, Ni, Mn, Pb, Zn), organo contaminants, nutrients as well as a number of other parameters. This raw, or primary data are subsequently manipulated into many subsets of data substantially increasing the size of the matrix to a unmanageable state.

The nature of contaminant data of this type requires that it be plotted and mapped regionally. With additional data continually being added to the inventory, manual updating of regional representations rapidly becomes an impossible task. A GIS-controlled database has therefore been used to handle the considerable contaminant inventory held by the Environmental Geology Group to carry out these routine assignments. However, by georeferencing contaminant data, a range of environmental issues eg. impact assessment, relationships to source, water quality, etc. can be readily assessed.

Initially, environmental impact can be judged by comparing pre-anthropogenic (background) contaminant levels to current values and expressing the results as enrichment over background, these values are subsequently mapped. A seriatim can be calculated for each element and plotted, or a summed seriatim can be calculated for any number of elements to obtain an estimate for overall impact. Although no biological thresholds have been established for Australian environments, guidelines set for North American conditions can be used to obtain an preliminary estimate of areas where some form of biological effect likely to have some adverse biological impact. Maps can thus be produced which crudely ranks areas most likely to be biologically affected. Georeferenced sediment contaminant data can be rapidly compared to other relevant data eg. water quality, soils, geology, geomorphology, water dynamics etc. to determine interrelationships by adding any number of layers of data. Similarly, sediment contaminant data can be related to various types of known sources, water dynamics, sedimentary parameters, etc. to better understand processes of transport and deposition.

36

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 HEAVY METAL DISTRIBUTIONS IN SURFICIAL SEDIMENT IN SYDNEY'S FOUR MAJOR ESTUARIES

1

Gavin Birch1, Stuart Taylor1, Irvine I1. and Carsten Matthai1.

Environmental Geology Group, The University of Sydney, NSW, 2006.

Sydney Australia's largest city (4 million people) is dissected by four major incised estuaries. For almost two centauries these environments have been a convenient receptacle for the disposal of domestic and industrial waste and they have also acted as a cheap source of coolant for some industries.

A comprehensive study of the environmental health of Port Hacking, Botany Bay/Georges River, Parramatta River/Port Jackson and the Hawkesbury River estuaries has resulted in a large inventory of sedimentological and contaminant (heavy metals, organic contaminants and nutrients) data. Consistency in sample acquisition, pretreatment and analysis has resulted in the largest compatible data base for the region. This has allowed comparisons between estuaries to be made and has provided an understanding of the impact of source on sedimentary environment The data are all size-normalized to avoid confounding texturally-driven distributions and to better establish process-related spatial trends. Analysis of the coarse fraction has also enabled whole-sediment assessment, e.g. on biological effects-based criteria. Although the majority of the data are for surficial sediments, microstratigraphically-controlled subsurface data provides additional information on discrete sources and changes in temporal supply.

The environmental 'health' of Sydney's estuaries has been assessed using the above database by employing a number of different approaches. Anthropogenic enrichment has been established by determining background levels for each estuary. Heavy metal data are related to potential biological impact through a 'Biological Effects Index' by comparison with thresholds established elsewhere. A summed contaminant seriatim for multiple elements has established areas of maximum enrichment for all elements analysed. Spatial documentation of these criteria allow comparisons of different sedimentary environments, hydrological systems and estuaries to be made. Utilizing the above criteria the Parramatta River/Port Jackson estuary is the most severely impacted estuary in the Sydney region, followed closely by the Georges River/Botany Bay estuary. Port Hacking, surprisingly, is somewhat more influenced by heavy metal contaminants than is the Hawkesbury River, except for a small number of restricted localities.

37

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PRELIMINARY RESULTS OF CONTAMINANT DYNAMIC STUDIES IN P O R T JACKSON ESTUARY.

1

Gavin Birch1,Stuart Taylor1, Carsten Matthai1, and Mellisa Fong1.

Environmental Geology Group, The University of Sydney, NSW, 2006.

The Environmental Geology Group at the University of Sydney is involved in two projects which examine contaminant dynamics in Port Jackson estuary. The ultimate aim of these studies is to determine the efficiency of the estuary to act as a sink for contaminants. The projects include an examination of contaminant movement in Port Jackson estuary under low flow conditions and a study of the nature and fate of material exported from the estuary during high flow periods.

Six sediment traps were deployed in Iron Cove, an embayment on the southern shore of Port Jackson estuary, over a four month period during summer and over a similar interval in winter. In summer, sedimentation rates are variable and exhibit a consistent cyclical trend throughout the bay. Of the many potential physical (tidal, wind, precipitation, etc.), biogeochemical (bioturbation, etc.) and anthropogenic (trawling, ferry transport, etc.) processes that could be involved in such behaviour, it transpires that sedimentation rates are closely related to tidal resuspension. Resuspension by wind stress in shallow areas during strong north easterly gales and by disturbance of the estuary floor during prawn trawling have also been identified. The heavy metal, organochlorine and pesticide content of the settling particulate matter (SPM) closely reflects the composition of the ambient surficial estuarine sediment. Surficial sediment exhibits a strong declining trend away from the head of the estuary, suggesting that, although there is considerable vertical movement, minimal net lateral contaminant transfer takes place during tidal events.

Winter sedimentation rates are substantially lower than in summer and the cyclical pattern is subdued.

During high flow events sedimentation rates throughout the embayment are substantially reduced and contaminant concentrations are considerably lower than during low flow periods, possibly due to stratification of the water column and dilution by new particulate material.

The physical and chemical characteristics of the water column in the main Port Jackson estuary channel have been investigated during high and low flow periods. For two high flow events in 1995, the development of the fresh water plume was documented at least every second day over a period of approximately ten days. A two to three meter thick, turbid, low salinity plume developed throughout the entire estuary very rapidly and within approximately 24 hours the plume had exited Port Jackson estuary.

The plume remains stable for at least six to eight days before dissipating very rapidly within a day or two. Due to the rapidity with which the plume develops, samples could not be obtained of the SPM associated with the 'first flush' - material commonly considered to contain the highest concentrations of contaminants. Although spatial and temporal variance in heavy metal concentrations are high, values in SPM decrease down the estuary and with time. SPM heavy metal concentrations in the plume are also considerably lower than for ambient surficial sediment in the estuary.

The mass balance implications of these preliminary data are discussed, however, understanding the processes of low salinity contaminant transfer (high flow events) and tidal contaminant flux (low flow periods) from the estuary to the offshore requires considerable more work before the efficiency of this estuary as a sink for contaminants can be estimated.

Acknowledgments. This project is partly funded by the Australian Research Council.

38

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

INTEGRATED STUDIES OF THE PETREL SUB-BASIN (BONAPARTE BASIN), DEFINING THE PALAEOZOIC FOUNDATIONS OF THE NORTH WEST SHELF J.E. Blevin, J.B. ColwelL D.S. Edwards, C.B. Foster, P.J. Jones, J.M. Kennard, R.S. Nicoll, G.W. O'Brien, J.H. Shergold, R.E. Summons, and D.J. Wilson, Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601

The Petrel Sub-basin is a NW-trending Phanerozoic basin underlying the Joseph Bonaparte Gulf in the far north of Western Australia. Exploration of the area began in 1959 and has resulted in the discovery of two large, but as yet undeveloped gas and condensate fields, Petrel (1969) and Tern (1971), in the central offshore basin. A number of oil and gas shows have been recorded throughout Devonian to Permian sediments from the onshore and offshore parts of the southern basin, with the most significant oil accumulations occurring in Carboniferous and Early Permian reservoirs in the Barnett and Turtle wells.

Previous work based on AGSO's deep seismic grid across the area indicates a major phase of extension occurred during the mid-Devonian to Early Carboniferous along a NE-SW azimuth. This phase of basin development resulted in a series of rift compartments with differing polarities offset by NE-trending accommodation zones.

The basin's underlying architecture and accommodation-fill history appear to have had a fundamental influence on later structural reactivation, salt migration and the evolution of petroleum systems within the basin.

In January 1995, the Australian Geological Survey Organisation (AGSO) commenced an integrated study aimed at defining the Palaeozoic structural and stratigraphic evolution of the southern Petrel Sub-basin. Aspects of the study include: 1) modelling of potential field data; 2) megasequence interpretation and mapping using AGSO's grid of deep-seismic (14.0 s TWT) data, supplemented by an infill grid of industry seismic data; 3) analogue (sandbox) modelling to test aspects of the basin's development, in particular, the fault geometries within accommodation zones and the effect of reactivation events on primary structures; 4) a review of the biostratigraphy of key wells and selected stratigraphic intervals; 5) geohistory and subsidence analyses to establish basin phases and accommodation cycles; and, 6) geochemical (biomarker and isotopic) analyses to identify the source of oils recovered in the Barnett and Turtle wells.

The southern Petrel Sub-basin contains a thick (5.0+ s TWT) sequence of Palaeozoic sediments overlain by a relatively thin cover of Triassic to Recent sediment (up to 2.0 s TWT). The absence of a thick Mesozoic and Cainozoic section provides an opportunity to analyse and date the Palaeozoic tectonic events which have affected the entire North West Shelf region. Initial results highlight the different structural styles of the opposing basin margins. The western part of the basin is dominated by a NW-trending large-displacement margin and a series of rotated fault blocks. By contrast, basement (?fault blocks) underlying the eastern margin has a distinct "ramplike" geometry. A series of complex syn-rift packages are preserved on the steep upper part of the ramp located to the south of Billawock-1. These packages may relate to an early phase of extension which occurred prior to the main mid-Devonian to early Carboniferous extensional phase. Preliminary analyses of gravity and seismic data suggest that an offshore, northern extension of the Halls Creek Mobile Zone (Lasseter Shear Zone) has strongly influenced the structural development of the eastern basin margin producing a prominent N-S crosscutting structural trend extending from the Turtle-Barnett region into the southern Malita Graben area. Post-rift sediments (Visean and younger) thicken into the N/NW-trending axis of the basin and into the Malita Graben.

Integration of the study results has identified eight main phases of basin development. Each phase is initiated/terminated by a major tectonic event. These events include: basin initiation in the Early Cambrian (Antrim Plateau Volcanics), ?Early Devonian compression, mid-Devonian initiation of major extension, Early Carboniferous uplift and erosion followed by the start of thermal sag, a reduced rate of thermal subsidence in the Late Carboniferous, Mid-Triassic to Early Jurassic compression and basin inversion, Early Jurassic erosion, and base Tertiary erosion and downcutting.

Geochemical analyses of the Barnett and Turtle oils have shown that they are sourced from an Early Palaeozoic marine shale. Oils within the upper part of the reservoir are extremely biodegraded while those from deeper in the section are pristine. Potential sources in the Late Devonian and early Carboniferous are being analysed to determine oil-source rock correlations.

39

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

USING MAGNETIC SUSCEPTIBILITY METERS TO INTERPRET THE OXIDATION STATE OF GRANITIC ROCKS Phillip L. B levin Geology Department, Australian National University, Canberra, ACT 0200

The ilmenite (ilm)- and magnetite (mt)-series classification provides a useful metallogenic division for granitic rocks because Cu, Mo and Au deposits are typically related to the former, and Sn (±W) deposits to the latter.

This is due to the effect of magma redox on ore element behaviour during fractionation (B levin & Chappell, 1992). Magnetic susceptibility (k) is now a widely used redox indicator in granitic rocks, however values of k do not necessarily correlate with inferred magmatic /O2 conditions because mt contents of granites are also functions of rock composition (i.e. Fe content). In high silica granites very low Fe and Ti contents may prevent mt and ilm crystallisation, regardless of /O2. Some "oxidised" Fe-Ti-S-0 assemblages (haematite, rutile, pyrite) may have bulk k values less than that of "reduced" assemblages such as ilm and pyrrhotite.

Values of k obtained from -1000 slabs of granitic rocks and from hundreds of field measurements have been interpreted in terms of rock chemistry, alteration and mineralogy. S-types have k values typically < 1 x 10 SI. I-types have a wide range of k with -80% having values > 1 x 10~ SI, and -20% having values < 1 x 10 SI. The upper limit for all granites is of the order of -80 x 10" SI. The majority of granites fall into two distinct populations on K-SiC>2 plots (Fig. 1): a low k series having k - 0.5 x 10- SI at the lower silica end, and a high k series generally with k > 2 x 10~ SI. There is a distinct "K-gap" in the intervening range (0.5 to 2 x 10-3 SI). Granites that fall in this gap generally either have Si02 > -72 wt % so that only small quantities of mt can crystallise regardless of /O2, or show evidence for the breakdown of mt to biotite, sphene ± other secondaries. These latter group of "gap" granites also show poor correlations between k, SiC>2 and Fe203/Fe0 indicating that the progress of mt alteration is not related systematically to granite composition.

The effects of whole rock composition on mt content can be accommodated if k values are overlaid onto a Fe203/Fe0 - Si02 diagram. Such plots can be used in the field if the composition of the granite (in terms of SiC>2 or FeO*) is estimated from the percentage of coloured minerals (colour index). Such relationships also agree well with other mineralogical redox indicators. The occurrence of magmatic sphene correlates strongly with k, although the compositional range over which magmatic sphene appears to be stable is relatively limited (62% < Si(>2 < 72%). The occurrence of "salmon-pink" K-feldspars (Kfs) in oxidised felsic granites also correlates well with F^Os/PtO and k.

Kfs in some oxidised I-type suites change colour from 10 white through to pink with increasing Si(>2 although Fe2C>3/FeO does not change. Samples in which the plagioclases are green, and/or the Kfs are brick red, c 10 show poor correlations between Fe2C>3/FeO, k and Kfs colour. In these samples destruction of primary 53 io°0 mt occurred during late stage magmatic or subsolidus COI 1 alteration at high /O2. Mt destruction during carbonate and/or sulfidic alteration also leads to a © 10 marked drop in k. However in many cases whole-rock Fe203/Fe0 values do not change even in samples 10 " containing a few percent secondary carbonate and/or 50 60 70 80 <-2% sulfides. Thus in interpreting the redox history Fig. 1. k vs Si0 for 825Si02 Australian granitic rocks. of moderately altered granitic rocks, Fe2C>3/FeO values may be more robust than k.

These relationships allow for a more meaningful field based interpretation of k data on granitic rocks. These results also provide the basis for a semi-quantitative calculation of equivalent /O2 values for granitic magmas, and a better understanding of late to post magmatic behaviour of granitic rocks in /02-temperature space.

REFERENCE Blevin, P.L., & Chappell, B.W., 1992. The role of magma source, oxidation states and fractionation in determining the granite metallogeny of eastern Australia. Transactions of the Royal Society of Edinburgh: Earth sciences 83,305-316.

Acknowledgment: The support of AMIRA and several companies (project P147B) is gratefully acknowledged.

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40

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention,, Canberra, February 1996

MAGMATIC CONTROLS ON ORE METAL RATIOS ACROSS THE Cu-Mo(Au) "PORPHYRY" SPECTRUM. Phillip L. Blevin1. Phillip A. Candela2 and Bruce W. Chappell1 * Geology Department, Australian National University, Canberra, ACT, 0200 ^Geology Department, University of Maryland, College Park, MD, 20742, U.S.A.

The Cu-Mo(Au) "porphyry" spectrum covers a broad range of deposit types, from Mo-poor porphyry Cu-Au systems through Cu-Mo porphyries, Cu-poor granodiorite Mo systems, and finally Cu-poor Climax-type porphyry Mo systems. The porphyry Cu-Mo and Mo systems of the northern New England Orogen, and Mo(±W±Bi±Au±Sn) mineralisation associated with fractionated K-rich granites in the southern New England Orogen and northern Queensland are also part of this spectrum. The general association of Au-rich porphyry Cu systems with island arc settings, Cu-Mo porphyry systems in continental margins and porphyry Mo systems in continental settings has long been noted, as has the compositional diversity of the related magmas. This diversity indicates that a range of source materials, from mantle to evolved crust, may act as protoliths for porphyry Cu-Mo(Au) magmas extending well beyond the current emphasis on "alkaline" and "shoshonitic" magma associations. There are numerous examples of Cu-Mo(Au) systems, including substantial Au producers, that are associated with low- to medium-K igneous rocks. The Mount Morgan deposit for example is associated with a very low-K tonalite-trondhjemite calc-alkaline suite of arc-like affinity.

1 |llllll 1 fill • II

There is a good general correlation between Cu/Mo ratios in these deposits and the composition of the related magmas which evolve through a variety of processes. Cu/Mo ratios in the magmas also change during fractionation because Cu behaves compatibly relative to Mo (Fig. 1). In detail the magmatic behaviour of both elements is strongly redox dependent The Cu/Mo ratios in the deposits are interpreted to be a function of these relationships, as probably are many other elements (F, Sn, W, Re, Bi, Te etc). Behaviour of gold at the magmatic stage is very poorly understood. There is a strong association between Au io!i l ! ' mineralisation and oxidised magmas. Arguments • jfr • against such a relationship have been based on the assumption that Au is sequestered by magmatic f io1 r 1 • • Sf^ 1 • • magnetite. In this scenario the formation of Au • i11• * • • 1 r deposits requires reduced magmas, or oxidised magmas 5 • •: 10° B in which magnetite crystallisation has been suppressed • by high "alkalinity" (e.g. magnetite + K -> biotite).

-1 10 Recent experimental data demonstrates that 10" 2 10" 1 1 0 ° 101 102 partitioning of Au between magnetite, pyrite and pyrrhotite is similar and that magnetite fractionation will not remove Au from the melt more significantly Figure 1. Cu/Mo ratio as a function of Rb/Sr in the than would the crystallisation of pyrite or pyrrhotite oxidised granites of eastern Australia. Mineralisation (Cygan & Candela, 1995). with high Cu/Mo ore ratios are associated with less fractionated granites (lower Rb/Sr) and vice versa.

Rb/Sr

The compositional spectrum represented by Cu-Mo(Au) magmas suggests that "special" sources may not be critical to the formation of these deposits, and that these magmas probably have metal contents and ratios typical of similar, but unmineralised magmas. Further studies are required to understand the processes which serve to optimise the retention of Au, Cu and Mo within a melt, how they are efficiently partitioned into an exsolving volatile phase, and the way these elements are transported and deposited. Giant deposits probably represent cases where these processes have operated near their maximum efficiency at both the magmatic and hydrothermal stages (Candela & Blevin, 1995).

REFERENCES Candela, P. A., and Blevin, P. L., 1995. Physical and chemical magmatic controls on the size of magmatichydrothermal ore deposits. Giant Ore Deposits Symposium, Toronto.

Cygan, G. L., and Candela, P. A., 1995. Preliminary study of gold partitioning among pyrrhotite, pyrite, magnetite, and chalcopyrite in gold-saturated chloride solutions at 600 to 700°C, 140 MPa (1400 bar). Min Assoc. of Canada Short Course vol 23.

Acknowledgment The support of AMIRA and sponsor companies (project P147B) is gratefully acknowledged.

41

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CONTROLS ON THE DISTRIBUTION AND CHARACTER OF THE INTRUSIVEMETALLOGENIC PROVINCES OF EASTERN AUSTRALIA Phillip. L- Blevin and Bruce W. Chappell Geology Department, Australian National University, Canberra, ACT, 0200

Phanerozoic intrusive-related mineralisation in E. Australia is heterogeneously distributed in the crust both temporally and spatially. Three broad categories can be recognized: Cu-Mo-Au deposits associated with I-type, low- to medium-K granites in the N. New England Orogen; Cu-Au mineralisation associated with low-K to shoshonitic Ordovician magmatism in the central Lachlan Fold Belt; and W, Sn, and Mo-W±Bi±Au mineralisation associated with high-K, felsic, I- and S-type Siluro-Devonian, and Carboniferous to Triassic magmatism in Tasmania, S. New England Orogen, Lachlan Fold Belt, and N. Queensland. Ore element associations are a simple function of the relative oxidation state and degree of fractionation within the associated intrusive suite (Fig. 1; Blevin & Chappell, 1992; 1995). The progression from Cu-Au, W to Mo mineralisation for example can be traced within single supersuites. W and Sn dominate in more reduced suites undergoing fractionation (Fig. 1). Sn may also occur in relatively oxidised suites if the crystallisation of Ti and Fe phases is late or absent prior to the exsolution of hydrothermal fluids. Both Sn and Mo are associated with isotopically primitive granites through to isotopically and chemically highly evolved K-rich granites. The association of Au with granites of intermediate composition has been described by Blevin and Chappell (1992).

These ore element associations, in combination with age, compositional and source features of the associated intrusive suites, allow for the recognition of intrusive-related metallogenic provinces in E. Australia. In general there is a sympathetic evolution within these provinces between their overall metallogenic character and the degree of chemical evolution of the associated intrusive suites. These provinces do not necessarily correlate with tectonostratigraphic boundaries defined by the near-surface geology but rather with the distribution of granite source regions in the deep crust. Provinces defined by S-type granites are more typically confined within the boundaries of teiranes defined by near-surface geology while some provinces defined by I-type granites cut across terrane and even fold belt boundaries. These contrasting distributions reflect that S-type magma sources were located within the mid crust, whereas I-types were derivedfromlower crustal sources whose compositions and origins may not be reflected in the near-surface geology.

Such relationships between magma composition, redox state and ore element associations is good evidence for the magmatic source of ore elements in intrusive-related mineral deposits, and for the production of the observed ore element ratios dominantly through magmatic processes. The association of certain ore element associations with certain tectonic settings is probably a function of these relationships, as tectonic processes affect the range of protolith materials, and heat and volatile fluxes available for magma generation. Whether or not associated economic mineralisation is developed appears to be determined by factors that are not related in a systematic way to magma composition.

However, such relationships provide predictive tools XS.U/OI for assessing the metallogenic potential and character Fig. 1. Plot of whole rock Fe203/Fe0 vs Rb/Sr for of intrusive provinces and for better understanding the granitic rocks and related ore element associations. distribution of intrusive-related mineralisation in the The field encloses -4000 analyses of granites from upper (accessible!) portion of the crust eastern Australia (data of B.W. Chappell).

REFERENCES Blevin, P. L„ & Chappell, B. W., 1992. The role of magma source, oxidation states and fractionation in determining the granite metallogeny of eastern Australia. Transactions of the Royal Society of Edinburgh: Earth sciences S3,305-316.

Blevin, P. L., & Chappell, B. W., 1995. Chemistry, origin and evolution of mineralized granites in the Lachlan Fold Belt, Australia; the metallogeny of I- and S-type granites. Economic Geology, in press.

Acknpwledgment: The support of AMIRA and several companies (project P147B) is gratefully acknowledged.

42

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention,, Canberra,, February 1996

THE COEN REGION - BELT SUPERGROUP CONNECTION IN RODINIA: A NEW TECTONOSTRATIGRAPHIC, GEOCHRONOLOGICAL AND 1SOTOPIC STUDY Richard S. Blewett Lance P. Black, Jan Knutson, Shen-Su Sun and John. H.C. Bain Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601

The Coen Region of north Queensland (Figure 1) includes the Coen and Yambo Inliers which consist of a Mesoproterozoic and Neoproterozoic metasedimentary and meta-igneous basement intruded by Palaeozoic granitoids. Three Proterozoic tectonostratigraphic domains have been recognised in the region, each reflects a new period of intracontinental crustal growth.

The Yambo Province consists of the Yambo and Newberry Metamorphic Groups which are predominantly clastic sedimentary rocks (probably younger than -1640 Ma) that were deformed, metamorphosed and intruded by granitoids and mafic dykes about 1585 Ma. A possible -1575 Ma event is recorded as metamorphic rims of zircon that grew during granulite-grade metamorphism. Whole-rock Sm/Nd isotopic data indicate source regions with a depleted mantle model age (Tem) range between 2190-2110 Ma (Newberry) and 2340-2140 Ma (Yambo).

The Savannah Province consists of the Holroyd, Coen and Edward River Metamorphic Groups. These are also clastic-dominated sediments, which are up to 10 km thick and intruded by mafic sills in the lower part.

Detrital zircon as young as 1590 Ma occurs in the Holroyd Group, which was apparently also intruded by a granitic gneiss at about 1433 Ma. Deposition of Savannah Province sediment therefore is inferred to have occurred between 1590 and 1433 Ma. Also, the Yambo Province was exposed and supplying detritus at least locally, before 1433 Ma. Whole-rock Sm/Nd isotopic data indicate source regions with a Tem age range between 2440-2230 Ma (Holroyd) and 2290-2060 (Coen).

The Iron Range Province in the northern part of the Coen Region consists of the mostly greenschist-grade Sefton Metamorphics which are a mixed clastic/carbonate sequence with some greenstone and BIF. These maybe the most northeasterly Grenville age rocks in Australia as detrital zircons suggest that they are younger than 1133 Ma. Whole-rock Sm/Nd isotopic data indicate a source region with a Tcm age around 2050 Ma. The Meso to Neoproterozoic Belt Supergroup of western North America (Figure 1) was deposited in an intracontinental setting some time after 1576 Ma, with a sediment source area located somewhere to its west (Ross et al. 1992). Sm/Nd isotopic data suggest a relatively constant source (or a source area with homogenous mixing) with Tem age range of 2200-2400 Ma. The Revell Formation (Lower Belt) contains anomalous (for North America) zircons dated at about 1590 Ma (cf. Yambo) and the Buffalo Hump Formation between 12201070 Ma (cf. Sefton).

The lack of a suitable region for these anomalous zircons in western North America, led Ross et al (1992) to conclude that the Belt Supergroup was derived from central and south Australia. They also indicated that the model could change with new data from the Coen and Georgetown regions. Our U/Pb zircon and Sm/Nd isotopic data "fit" the Rodinian model (Figure 1) of Ross et al. (1992), and provide a more easterly source region for the Belt Supergroup than previously proposed.

AUSTRALIA

Figure 1 - Detail of the location of the Coen Region and Belt Supergroup in Australia and Laurentia (North America). Modified Rodinian reconstruction from Ross et al.

(1992).

BELT BASIN

43

ON THE SIGNIFICANCE OF MAJOR EVENTS IN NORTH QUEENSLAND Richard S. Blewett Lance P. Black and Peter Wellman Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601

The north Queensland region (Georgetown, Coen, Yambo and Dargalong Inliers) contains the most easterly exposed Proterozoic rocks in Australia, as well as the northern end of the Palaeozoic Tasman Orogenic Zone (Hodgkinson Province). Many major deformational/metamorphic, magmatic, and depositional events in this region correlate with major events elsewhere in Australia, as well as events in Precambrian Rodinia and Palaeozoic Gondwanaland.

This paper will discuss and correlate a number of these major events and consider their plate tectonic significance. These events are shown in Table 1 and Figure 1.

Table 1 - Major events in north Queensland and their correlatives elsewhere.

Age Ma Event in north Queensland Possible correlations ~350 Deformation, magmatism, basin inversion -400 Deformation, prograde LP-HT Adjacent South China Block rifts from NE metamorphism and S-type intrusion, basin Gondwanaland margin. Intrusion in Lachlan development Fold Belt.

-1130 ?Onset of sedimentation Most NE Grenville age rocks Climax D deformation, LP-HT metamorphism -1550 Deformation, prograde LP-HT metamorphism, S-type intrusion, onset of in Mount Isa sediments/volcanics.

-1575 Deformation, MP-HT granulite LP-HT granulile metamorphism in Arunta Inlier metamorphism, I-type intrusion -1585 Intrusion (mafics and S-type) Gawler Range V., Hiltaba Suite. Maybe source region for Belt Supergroup (North America) 2

Figure 1 - Events in selected provinces of north Queensland

major shortening event minor shortening event mainly shearing event unconformity U/Pb zircon age, detrital age Rb/Sr age, fossil age age range

44

Dominant rock types felsic volcanics clastic sediment carbonate sediment mafic rocks granitoids (I & S-type)

DIGITAL ON-SITE DATA CAPTURE USING AN APPLE NEWTON PALMTOP COMPUTER Richard S. Blewett and Murray S. Ilazell Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601

The Australian Geological Survey Organisation (AGSO) has been developing corporate relational databases for the storage, manipulation and analysis of field geological data (Ryburn et al., 1993). Customised notebooks were developed to assist the field geologist in recording field observations in aflexiblebut structured maimer for ease of data entry (Blewett, 1993).

The cost of entering data on return from thefieldcan be prohibitive, also errors can occur with the double entry and transcription of traditional notebooks. Therefore, AGSO in conjunction with Resource Industries Associates (Melbourne) have developed a digital field notebook on an Apple Newton MessagePad 120. The system allows the geologist to enter data via a keypad on the LCD display, via pull-down menus on the various lookup tables, and by character recognition. A GPS can be actively linked (via infrared) and combined with a raster image of one's basemap(e.g., culture/drainage), allowing an active position on the base map. Waypoint storage leads to the AGSO menu (Figure 1), which enables attributes for the various OZROX tables (SITES, OUTCROPS, ROCKS/LITHDATA, and STRUCTURES) to be entered. Digital sketches can be stored and linked to the site number. Stored sites can be retrieved and edited, allowing common attributes to be copied from previous sites.

The computer is powered by 4 AA batteries (~5 days use), and data is stored on 2MB flash cards. Data is downloaded and the unit can be customised via an RS232 connection to a PC running Windows. Data is then loaded into the OZROX tables using SQLLoader.

An example of two screen views shown at 66% of actual size

Ryburn, R.J., Blewett, R.S., Stuart-Smith, P.G. and Williams, P.R. 1993. Users' Guide to the NGMA Field Database. Australian Geological Survey Organisation Record 1993/49, 54.

Blewett, R.S. 1993. The AGSO Field Geological Note Books - a User's Guide. Australian Geological Survey Organisation Record 1993/46, 31.

45

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE METALLOGENESIS OF THE EASTERN MARGIN OF THE NAMAQUALAND METAMORPHIC COMPLEX, SOUTH AFRICA, AND ITS RELATION TO PLATE TECTONICS Boelema, R Council for Geoscience, P Bag XI12, Pretoria, 0001, South Africa

The Namaqualand Metamorphic Complex forms part of a major Kibaran-aged mobile belt, known as the NamaquaNatal metamorphic belt, which wraps around the southern periphery of the Kaapvaal Craton in South Africa. The area under investigation is located where the Namaqualand Metamorphic Complex is juxtaposed against the southwestern edge of the Kaapvaal Craton and is separated only by the Eburnian-aged Kheis Province. In this area, the eastern margin of the Namaqualand Metamorphic Complex is subdivided into the Bushmanland and Gordonia Subprovinces, the latter being further subdivided into tectonostratigraphic terranes, namely, from east to west; Upington, Areachap and Kakamas terranes. In general, these terranes show a marked "correlation with the various styles of mineralisation in the area.

The Mid Proterozoic supracrustal sequences of the Kheis Province (subjected to Eburnian-aged deformation and metamorphism) and Upington terrane (subjected to Edurnian-aged and Kibaran-aged tectonism) are generally barren of mineralisation within the study area. Differentiation between the Eburnian and Kibaran-aged supracrustal sequences within the Namaqua Province remains enigmatic. The Areachap Terrane, however, is the only truly juvenile Kibaran-aged crust, situated more or less between the Namaqua and Kheis Provinces. This juvenile unit comprises mainly of metavolcanic sequences geochemically resembling low-K tholeiite and calc-alkaline basalt as well as an upper unit showing strong shoshonitic affinities and have been interpreted as a manifestation of arcrelated volcanism during subduction from west to east. The Areachap Terrane is associated with four centres of volcanogenic exhalative massive sulphide deposits of which two have been mined in the past. These deposits are thought to be of the Besshi-type with some displaying Kuroko-type affinities. The supracrustal sequences of the Kakamas Terrane have been extensively intruded by early-, syn- and late-tectonic granitoids which includes the voluminous calc-alkaline, predominantly I-type granitoids of the Keimoes Suite batholith which appears to form a complete granite series from katazonal granites in the west to epizonal granites in the east. The Kakamas Terrane is furthermore characterised by various tectonostratigraphic assemblages juxtaposed against one another.

Mineralisation in the Kakamas Terrane is largely restricted to those related to granitic systems and show a correlation with the tectonostratigraphic assemblages within this terrane. To the east of the Cnydas-Bovenrugzeer Shear vein-type mineralisation is prevalent and is host to numerous Sn-W-F and base metal deposits which indicates a broad zoning around a possible source granite. To the west of the Cnydas-Bovenrugzeer Shear Be, microclineperthite, mica, Ta/Nb mineralisation is related to pegmatites. There is therefore a series of granite related mineralisation from west to east which corresponds to deeper level granites in the west and shallower level granites in the east which correlates well with the observed surface geology.

The Kibaran-aged Namaqua orogeny commenced with the collision of the "Bushmanland microcontinent" against the Kheis-Kaapvaal Craton resulting in the closure of the oceanic basin and subduction of the oceanic plate under the Kaapvaal craton. This created calc-alkaline volcanism in an island arc setting associated with the Areachap Terrane and related Besshi-type massive sulphide deposits. During advanced stages of subduction, marginal basins were formed into which the Wilgenhoutsdrif Group was deposited. Further closure led to the collision of the "Bushmanland microcontinent" against the Kaapvaal Craton and the genesis and intrusion of predominantly I-type granitoids accompanied by extensive thrusting on the underriding foreland. The various levels of thrusting is reflected in the styles of granite related metallogeny with the shallower level vein deposits closer to the suture zone and deeper level pegmatites further away towards the west. A final period of northward directed movement led to the formation of "pull apart" fault-bounded basins into which the Koras Group of rocks were deposited.

46

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CARBONATE SEDIMENT ACCUMULATION ON THE COOL-WATER, OPEN-SHELF, HIGH-ENERGY MARGIN OF SOUTHERN AUSTRALIA THROUGHOUT THE CENOZOIC 1

?

Yvonne Bone1 and Noel P. James^

*Dept. of Geology and Geophysics, University of Adelaide, South Australia, 5005 2

Dept. of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6, Canada

INTRODUCTION Since the rifting of Australia from Antarctica in the Mesozoic, sediments have accumulated on the southern margin of the continent. These sediments have been predominantly carbonate-rich throughout the Cenozoic.

There have been specific periods of higher benthic calcareous biota production and subsequent sediment accumulation, such as the late Eocene, mid-Oligocene to mid-Miocene, Pliocene to Present.

Aims:- The aims of this study on the southern margin of Australia have been to determine the parameters controlling (i) the production of benthic fauna and flora; (ii) the accumulation of resultant sediment particles; (iii) the subsequent preservation of these sediments and (iv) the diagenesis of the resulting limestones.

FAUNAL AND FLORAL PRODUCTION Inter-related physical and chemical parameters such as nutrient level, water depth, photic zone, energy level, substrate type, siliciclastic input and climate(particularly temperature) all play important roles in the ability of biota to establish themselves on the sea-floor and to then flourish. When one of these parameters waxes or wanes, there is a domino effect amongst the others e.g. climate change will affect water depth which will in turn affect both the energy level and the depth of the photic zone. The former will affect the substrate, thereby affecting biota colonisation of the sea-floor. The latter will affect the floral component which will affect the nutrient level and spatial competition for settlement sites. Similar scenarios apply to alteration of any of the other parameters. All of these parameters have fluctuated in this region throughout the Cenozoic, thus allowing the biota to vary from times of high productivity involving high diversity and low individual abundance, to low productivity with localised high abundance and low diversity.

SEDIMENT ACCUMULATION Once the individuals of the successfully established benthic biota die, their remains will accumulate on the seafloor. These remains of the whole organism will fragment into varying sized particles, dependent on the organism's architecture. For example, within an assemblage of bryozoans, a large, erect-robust Adeona sp. colony will break into cobble-size fragments whereas all articulated zooidal colonies will disaggregate into finesand to silt-sized particles.

SEDIMENT PRESERVATION Physical:: The energy level (currents) and binding affect of the living biomass will combine to control whether the accumulating fragments remain in situ. The currents may winnow out the finer fraction and transport it either shorewards or downslope, depending on the site of production and the substrate on which the organism settled e.g. kelp encrusting bryozoans will be passively transported shorewards if the kelp is ripped up off the sea-floor even whilst the colony is still living. High energy levels may cause mass movement of coarse-grained material downslope and thus form submarine canyons, or may sweep it into large sub-aqueous dunes just shorewards of the shelf break area.

Chemical: The stable forms of calcite have the greatest potential for preservation in a cool-water environment.

Therefore, those organisms that make their skeletal elements out of aragonite or high-Mg calcite will start to dissolve or alter relatively rapidly. Nevertheless, the particles within any accumulation of sediments in the area will refelct the living assemblage, which is usually dominated by those organisms that use either calcite or low- to intermediate-Mg calcite, particularly bryozoans, forams and some bivalves.

LIMESTONE FORMATION AND DIAGENESIS The accumulations of sediments will remain loose, soft, friable calcareous sands if the bulk of the components came from organisms that use pure calcite, low- or intermediate-Mg calcite in their skeletal elements. Those accumulations that contain a fair proportion of originally aragonitic and high-Mg calcite constituents will have this material diagenetically re-precipitated as cement, thus creating a hard limestone.

Acknowledgements: We wish to acknowledge CSIRO Division of Oceanography and the Captains and crews of RV Franklin, ARC and NSERC for help and support for this research.

47

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

STABLE ISOTOPES : DO ALL COOL-WATER BRYOZOANS EQUALLY REFLECT THE SAME PARAMETERS? 1

?

Yvonne Bone1 and Noel P. James^

*Dept. of Geology and Geophysics, University of Adelaide, South Australia 2

Dept. of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6, Canada

INTRODUCTION It has become customary to analyse brachiopods for stable isotopes when wishing to determine paleotemperatures and other environmental parameters. This has been due to their apparent secretion of their skeletal elements in isotopic equilibrium with their ambient sea water and the fact that their primary calcitic mineralogy will not be subject to any diagenetic alteration after their death. Unfortunately, many limestone sequences either lack brachiopods completely or they are rare. Bryozoans, however, are common and widespread in all cool-water limestones throughout the Phanerozoic since Ordovician times. It has been proposed that they also secrete their skeletal elements in isotopic equilibrium with their ambient sea water, but few extensive studies have been undertaken.

Aims:- The aims of this study have been to determine whether (1) all bryozoans secrete their skeletal elements in isotopic equilibrium with their ambient sea water; (2) there is any isotopic difference between aragonitic vs calcitic bryozoans; (3) there is any isotopic difference between low-Mg calcitic and intermediate-Mg calcitic forms; (4) if there are consistencies, at what level do they occur - the growth form (i.e. architectural shape classification), the genus level or the species level? (5) water temperature is reflected in the O 1 8 values at the same level as associated brachiopods; (6) can water depth be used as a proxy for bottom water temperature? (7) MgC03 content is correlated with C 1 3 values due to metabolic affects and (8) it is necessary to apply all the currently known fractionation correction factors to the results obtained..

METHODS 150 samples were selected from 18 sites on the Lacepede Shelf from which bottom sediment samples were dredged during cruises aboard CSIRO RV Franklin during the summers of 1989 and 1991. Bottom temperatures were recorded for some of these and other adjacent sites, and accurate depths were recorded for all sites. Individual samples were sorted into growth forms, checked for lack of contamination, cleaned in bleach if needed, analysed by XRD for primary mineralogy and then analysed for stable isotopes. Selection to individual species was made at four of the sites and 45 analyses were performed on these.

RESULTS AND DISCUSSION Not all bryozoans secrete their skeletal elements in isotopic equilibrium with their ambient sea water. There is a significantly larger spread of values than that shown by associated brachiopods, brought about by those that are in some dis-equilibrium. Aragonitic and calcitic forms clump into separate plots, regardless of whether they are presented as corrected or uncorrected values. The shift in the field position is large for aragonitic forms and small for calcitic forms. Uncorrected values lie closer to brachiopod fields. Most growth forms show consistent values, with flat-robust branching showing the closest values to brachiopods in the aragonitic forms, vagrants the least, and articulated branching (mix of low- and intermediate-Mg calcite) and delicate branching (low-Mg calcite) the best, articulated zooidal the worst in the calcitic forms. This correlation improves significantly at the genus level but is not greatly enhanced by going to the species level.. This correlation is mirrored in the relationship seen between O 1 8 values and water depth. Water depth is only an approximate proxy for bottom water temperature due to a marked, spatially irregular summer thermocline development. Neither metabolic processes nor water temperature appear to be responsible for MgC03 content in the calcitic bryozoans.

CONCLUSIONS Some bryozoans can be used for isotopic studies in lieu of brachiopods. Growth forms such as calcitic articulated branching and aragonitic flat, robust branching will give consistent, reliable and accurate results whereas other forms should not be used. Improved reflection of parameters concerning ambient sea water can be obtained by using bryozoans at the genus level from these groups, but little additional accuracy is obtained by using individual species. Primary mineralogy must not have been diagenetically altered. Thus, those forms with low-Mg calcite or a mix of low- and inter-Mg calcite are the preferred recommendation. These are common in the Cenozoic cool-water limestones of Australia.

Acknowledgements:- We wish to acknowledge CSIRO Division of Oceanography and the Captains and crews of RV Franklin, ARC and NSERC for help and support for this research.

48

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A NEW LOOK AT TECTONIC ELEMENTS OF THE SOUTH-EAST INDIAN OCEAN FROM COMBINED ANALYSIS OF SATELLITE ALTIMETRY IMAGES AND REGIONAL SEISMIC DATA 1

Irina Borissova1, Philip A. Symonds1, and John W. Creasey1 Australian Geological Survey Organisation, GPO BOX 378, Canberra, ACT, 2601

The extension and breakup history of Australia and Antarctica, and the following period of very slow seafloor spreading, still present a number of unresolved questions such as: the event timing; the extent and structural style of the extensional terrane; the nature and location of the continent-ocean boundary; the style and continuity of seafloor spreading; and the origin of the Diamantina Zone. An improved understanding of these tectonic events, and the features they produced, will greatly enhance our knowledge of the evolution of the southern Australian and Antarctic margins, and their associated sedimentary basins.

A different approach was adopted in this study to previous investigations of the region. The interpretation of satellite imagery and regional seismic data was integrated with structural elements -of the southern Australian margin, seafloor spreading anomalies and transform faults in order to map the major structural features and provinces in the region. This integration was carried out within a Geographic Information System. New gravity data derived from satellite altimeter measurements (Geosat Mission data and stacked repeat-track data from Geosat, ERS-1 and Topex) were processed to enhance second-order variations of the gravity field. The resulting image reflects the major heterogeneities of the crust with amazing accuracy and detail. All Eltanin, and selected Shell Petrel and AGSO seismic lines were located on the image and analysed. Gravity features and provinces visible on the satellite image were correlated with structural features and basement types identified on the seismic profiles. This approach allowed characterisation of the various fabrics in the gravity image, and enabled the lateral extent of structures, basement types and crustal zones to be mapped through the region. A number of crustal provinces with different basement structure and/or depth were identified, and in many cases these can be associated with particular tectonic origins.

The areal extent and geographic position of previously poorly defined features and tectonic provinces are now much better constrained. Also, features that have been identified in seismic data on the Australian margin can now be reliably correlated with poorly known features on the Antarctic margin. There are two distinct linear gravity provinces adjacent to the continental margins of Australia and Antarctica, and their mirrored arrangement suggests a common origin. The most landward of provinces is associated with very deep and generally flat-lying basement (between magnetic trough and anomaly 34), whereas the oceanward province correlates with deep and generally faulted basement that is associated with an initial (magnetic anomalies 34-20) slow spreading episode and which appears to form a continuation of the Diamantina Zone. There has been lengthy debate about the origin of these provinces on the Australian margin, and they have variously been interpreted as either highly extended continental crust or oceanic crust. The most landward gravity province is probably associated with extended continental crust, whereas the oceanward province was probably formed during a period of very slow sea-floor spreading that created the incipient Diamantina Zone in the west, and mechanically deformed slow-spreading oceanic crust in the east.

We have also attempted to revise plate tectonic reconstructions of Australia and Antarctica by matching similar features in the altimeter image on opposing margins. We have also examined how the various identified crustal provinces evolved during the separation of Australia and Antarctica, and a set of reconstructions has been prepared for the following times: 35.2-35.8 Ma (anomaly 13), 41.3-42.7 Ma (anomaly 18), 63.0-69.4 Ma (anomaly 27-31) and 84-118 Ma (anomaly 34). These reconstructions indicate that early separation between Australian and Antarctica, up to about anomaly 13 time, occurred by westward-propagating seafloor spreading, which resulted in greater opening in the east than in the west.

This study has provided the first consistent tectonic framework between the conjugate Australian and Antarctic margins. This will aid understanding of along strike variations in margin configuration and the style of basin formation. The new tectonic elements map highlights areas of particular interest to further regional tectonic studies of the South-East Indian Ocean.

49

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

TERRESTRIAL SEDIMENT RESPONSE TO CHANGES IN ACCOMMODATION 1

Ron Bovd1, Claus Diessel 1 , Patrick Elliott2 and Brian Zaitlin2 Department of Geology, University of Newcastle, NSW, 2308, Australia 2 PanCanadian Petroleum Limited, Calgary, Alberta, Canada.

Sequence stratigraphy works best in the marine environment. A current research challenge is to extend this approach to terrestrial depositional settings. The key to success in terrestrial sequence stratigraphy is the recognition that, as for marine settings, cyclic variations in accommodation control stratigraphic subdivision, correlation and interpretation. In marine stratigraphy, changes in accommodation are closely linked to changes in relative sea level.

In non-marine stratigraphy the changes in accommodation may be the result of climatic, tectonic or eustatic variation and may not be directly linked to sea level.

Cyclic changes in accommodation result in characteristic depositional patterns in alluvial, lacustrine and organic facies. The critical level controlling sequence generation is the migrating river channel base in alluvial strata. Under low accommodation conditions, channels reoccupy their earlier positions without significant aggradation, resulting in braided and or amalgamated fluvial facies generation. Under higher accommodation conditions, channels are able to aggrade prior to reoccupation and are more likely to preserve channel and floodplain facies under meandering stream conditions. Lacustrine facies are more common during times of highest accommodation.

Terrestrial organic deposits formed from peat are controlled by the position of the groundwater table. To integrate organic deposits into a sequence stratigraphic framework, we have been investigating a range of contrasting depositional settings ranging from rifted margins and cold climate Gondwanan coals from the Sydney Basin of Australia, to subtropical foreland basin coals from the Western Canada Sedimentary Basin. Several well defined examples from these studies have enabled recognition of upward trends in increasing accommodation as identified from a range of sedimentological, stratigraphic and coal compositional data.

Initial conditions of low accommodation are marked by regional surfaces of either erosion or non-deposition. These surfaces are accompanied by soil profiles and root horizons or Glossifungites ichnofacies. Increasing accommodation leads to a better match of subsidence and peat growth and is reflected in brighter coals with increased H/C ratios, lower ash contents and high volatile matter and vitrinite content. Further increases in accommodation lead to flooding of the peat and a consequent decrease in acidity. This enables an increase in microbial activity which is reflected in suppression of vitrinite reflectance, increase in fluorescence and a corresponding increase in sulphur content. In two cases, flooding progresses to transgression of the peat and the deposition of fossiliferous marine roof rocks. Stratigraphic correlations indicate that these examples of upward increase in accommodation can be placed in either the lowstand wedge or transgressive systems tracts.

50

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SEQUENCE STRATIGRAPHY AND BASIN PHASE GEOMETRY OF THE PROTEROZOIC UPPER McNAMARA GROUP, MT. ISA BASIN, NORTHWEST QUEENSLAND. Barrv E. Bradshqw1 Andrew A. Krassay1, Bruce A. McConachie1, Jim H. Leven1, Doug M. Finlayson1, and Jan Domagala2 Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2600 Geological Survey of Queensland, GPO Box 194 Brisbane, Qld 4001

Proterozoic Upper McNamara strata have been investigated on a regional scale within the Mt. Isa Basin to determine the geometries of sedimentary basins, histories of sub-basin sedimentation and subsequent timing and pathways of fluid flow. Traditionally, researchers have relied on lithostratigraphic units for regional correlations of the Upper McNamara Group. However, the accuracy of such correlations are flawed by the time-transgressive nature of facies boundaries. The concept of unconformity-bounded stratigraphic sequences has developed over the past 30 years as a viable alternative to lithostratigraphy for developing chrono-stratigraphic correlations of strata.

The conventional sequence stratigraphic model divides basins into a series of relatively conformable successions of genetically related strata bounded at their tops and bases by unconformities and their correlative conformities.

The geometric arrangement and hierarchy of sequences reflects an interplay of variations in relative sea level versus sediment supply. Regional unconformities and extensive reservoir-quality sandstone bodies tend to form during relative lowstands in sea level when continental shelves are subearially exposed and incised by fluvial systems. Shale-prone maximum flooding surfaces are highly efficient regional seals and form during the maximum rate of relative sea-level rise when continental shelves are drowned and starved of terrestrial sediment input. Application of sequence stratigraphic models can thus improve both regional correlation of stratigraphic units, and mineral exploration plays through its prediction of geometric arrangements of lithofacies.

Detailed sequence stratigraphic analysis of a Comalco subsurface seismic reflection and well-log data set from the Bowthorn region has enabled the Upper McNamara Group to be divided into a hierarchy of second- through fourth- order sequences. In general, the seismic data clearly shows older stratigraphic units from the Upper McNamara Group onlapping out against the Murphy Inlier basement high to the north. Thus, previous lithostratigraphic correlations of the Fickling Group with this formation are incorrect, with the Doomadgee Formation representing proximal deposits from only an upper-most second-order sequence. At least three secondorder or 'super-sequences' have been identified in the Upper McNamara Group based on analysis of the seismic data set. From oldest to youngest, maximum thicknesses of supersequences are: 1.4 km; 1.4 km; and 2.2 km.

Each 'super-sequence' is identified by a major basinward shift in coastal onlap patterns. In Phanerozoic basins, such 2nd-order sequences generally represent major basin events occurring over periods ranging from lOMO2 m.y.

In the case of the Upper McNamara Group, each second order sequence possibly represents regional uplift across the Murphy Inlier and/or major thrust fault zones such as the Elizabeth Creek and Termite Range Fault Zones.

Using sequence stratigraphic principals we predict that major mineralisation zones are likely to occur in association with the maximum flooding of each second-order sequence. Each of the three Upper McNamara 'super-sequences' is in turn composed of smaller-scale third-order sequences. These are generally an order of magnitude smaller in thickness and are inferred to represent regional tectonic and eustatic events over periods of about 10° m.y. (based on similar time ranges of Phanerozoic sequences). On seismic sections, these third-order sequences are the smallest resolvable sequence stratigraphic units. However, detailed analysis of well-log data (gamma-ray, spontaneous-potential, resistivity) allows component fourth-order sequences (10"1 m.y, eustatic cycles) to be identified and tied into the higher-order seismic sequences. This enables a full integration of the relative roles of eustacy and tectonism in the infilling of the Upper McNamara Group. Analysis of the Comalco data set suggests that in both Lower and Upper McNamara strata, basin depocenters have systematically migrated northwards toward the Murphy Inlier. This observation has important consequences for any attempts to correlate mineralisation zones within known lithostratigraphic formations. In the case of the Upper McNamara Group, the arrangement of supersequences in the Bowthorn area suggests that the lower-most Shady Bore Quartzite and part of the Riversleigh Siltstone lithostratigraphic formations may be absent on the Murphy Inlier due to their onlapping-out further south in the Lawn Hill region. Testing of this model is planned with a seismic survey by AGSO in 1996 which aims to tie the existing Comalco seismic grid to outcrop observations from the Lawn Hill region to the south.

51

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NATIONAL RESERVOIR AND BIOSTRATIGRAPHIC DATABASES : EXPLORATION DATABASES AND APPLICATION SYSTEMS John Bradshaw , Bruce Wyatt Clinton Foster Australian Geological Survey Organisation, GPO Box 378 Canberra ACT, 2601 Wyatt & Associates, PO BOX 705, Mawson ACT 2607 1

1

2,

1

2

Since 1992, AGSO has been developing a series of discipline related databases (Oracle) for the Petroleum Industry on a national scale. They are now very mature and are widely distributed amongst the Petroleum Industry through the Australian Petroleum Systems Project The databases are STRATDAT (Biostratigraphy, well picks, events etc), RESFACS (Reservoir, Depositional Environments, Hydrocarbon Shows) and ORGCHEM (Rock-Eval - TOC, VR, HI, VR, TMAX, etc). Although these are valuable datasets for an individual well, a more meaningful use of these data is to search and map them by time series (timeslices, palynological zones, millions of years, etc). As a result AGSO has developed applications that interface between the databases and generate age/depth curves for wells using the biostratigraphic or sequence picks in each well. These applications allow rapid generation of data on a time series basis so that variables in wells from either a permit, petroleum system or entire basin can be compared and contrasted for a specific time interval (e.g. isopach and structure maps with palaeogeography, source and reservoir quality for D.jurassicum or 144.5 - 147 my (Harland)). Thus a high degree of intelligence has been built into the systems so that searches can be generated that have a high degree of geological integrity to produce event charts such as shown below. Examples of the way the databases are now interlinked and products that can be generated from the application systems will be shown "real time" as well as the cuiTent development of the maps and tables into a GIS environment (ArcView).

The shell of the Oracle databases with the datum dictionaries and the Excel data entry dialogue boxes arefreelyavailable to academic and state government agencies so that national standards can be established.

Recent discussions have also been held with international government agencies regarding the implementation of similar systems in provinces such as the North Sea and Alberta, whilst the datasets already exist for the Papuan Basin. Current developments include extending the system to allow for outcrop and type section information, and thus allow time series searching and use of the application systems in non-petroleum datasets.

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52

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 TECTONIC EVENTS AND PETROLEUM SYSTEMS Marita Bradshaw, John Bradshaw, Bruce McConachie, Jacques Sayers and Lynton Spencer Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

Regional tectonic events have a major impact on petroleum systems effecting source rock type, reservoir quality, maturation history and trap formation and destruction. The work done over the past four years in the AGSO-APIRA Australian Petroleum Systems Project has documented a number of these events and their impacts.

Australian sedimentary sequences can be grouped into several broad petroleum supersystems that demonstrate the linkages between coeval basins with similar palaeogeographic and tectonic setting and thus hydrocarbon prospectivity. The Larapintine Supersystem is characterised by lower Palaeozoic marine facies, including carbonates, evaporites and excellent marine source rocks (organic matter type I and II). The Gondwanan Supersystem includes those sequences dominated by the Late Carboniferous/Early Permian Gondwanan glaciation. The source rocks are predominantly terrestrial (organic matter type III) and the reservoir facies are clastic.

The Alice Springs Orogeny has been especially significant in impacting on Australian petroleum systems. The peak of the orogeny in the Carboniferous marks the change from one major depositional regime to another, from the Larapintine to the Gondwanan Supersystem. During this transition, the climate deteriorated from tropical conditions that encouraged reef growth in the Late Devonian, to glaciation in the latest Carboniferous — Early Permian. It has been postulated that there may be a causal relationship between the peak of the orogeny and the following glaciation - the uplift triggering ice accumulation and preservation. The climatic change also corresponded to a 20 degree shift in Australia's latitude southward over 60 million years.

During this time, the sedimentary sequences deposited were transitional between the characteristic Larapintine and Gondwanan facies and bounded by two regional unconformities. A significant structural unconformity of approximate basal Visean age is recognised in several basins and is often taken as the main break associated with the Alice Springs Orogeny. Others consider the major break to be in the Late Carboniferous, immediately preceding the deposition of glacial sediments; and in many basins, these unconformities coalesce. In the Bonaparte and Canning basins, however, large thicknesses of syn-tectonic sediments were deposited; these are petroliferous and appear to have indigenous sources, thus constituting a distinct petroleum supersystem, transitional between the Gondwanan and Larapintine intervals.

The poorly dated Harris Sandstone in the Carnarvon Basin is a possible equivalent of the petroliferous sections in the Canning and Bonaparte basins but it has no source facies. The molasse Mount Eclipse Sandstone in the Ngalia Basin is also coeval with the supersystem which emphasises its syn-tectonic nature.

The Westralian Supersystem links together basins, from the Exmouth Plateau to the Papuan Basin, which share a history of extension and eventual break-up and sea floor spreading in the Late Jurassic to Early Cretaceous. They have a similar stratigraphy of Triassic to Cretaceous reservoirs, Jurassic marine source rocks, Cretaceous regional seal and a thermal blanket of Tertiary carbonates. On Australia's western margin, the early Triassic regional seal facies is taken as the effective top of the Gondwanan Supersystem. In some places an unconformity in the Late Triassic (Fitzroy Movement) further emphasises the break between the Westralian and Gondwanan supersystems.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LAST GLACIAL MAXIMUM PALAEOGEOGRAPHIC MAP FOR THE WESTPAC REGION Marita Bradshaw Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

Palaeogeographic and data maps have been compiled for the Last Glacial Maximum (LGM), some 20 to 15 thousand years ago. The map coverage extends through Australia and east Asia, from the Artie to the Antarctic. The map compilation is a project of IOC-WESTPAC, the Sub-Commission for the Western Pacific, Intergovernmental Oceanographic Commission, a UNESCO agency. The map will be of use in climate change studies, mineral resource exploration and archaeology.

As part of the third WESTPAC Symposium held in Bali, Indonesia during November 1994, a two day workshop was conducted on the WESTPAC Palaeogeographic map. The workshop was attended by representatives from Australia (Marita Bradshaw), China (Professor Wang Pinxian, Tongji University, Shanghai), Indonesia (Dr Wahyoe Hantoro, Centre for Geotechnology, LIPI, Bandung and Dr Otto Ongkosongo, Indonesian Institute of Sciences LIPI), Japan (Dr Shinji Tsukawaki, Kanazawa University, Kanazawa and Professor Hideo Kagami, Josai University, Nakado), Malaysia (Mr Kamaludin bin Hassan, Geological Survey Laboratory, Ipoh), Thailand (Ass. Prof. Suparb Poobrasert, Chulalongkom University, Bangkok), the Netherlands (Dr Saskia Jelgersma, Geological Survey of the Netherlands, Haarlem) and Belgium (Professor Roland Paepe, Belgian Geological Survey). The Russian representative (Dr Sergei Ganzei, Pacific Institute of Geography, Vladivostok) was unable to attend but his maps were also presented at the workshop.

Country representatives presented maps of the LGM for their regions and these were later compiled into a combined map coverage by Professor Wang. Key features of the maps reflect the lower seal level (130m to 150m lower than today) and colder and drier conditions of the LGM. There was an expanded area of permanent sea-ice around Antarctica, larger glacial areas in the South Island of New Zealand, Tasmania, south-east Highlands of Australia and along the central mountain chain of New Guinea. Large areas of exposed shelf surrounded Australia linking it to New Guinea and Tasmania. The pattern of wind directions during the LGM are preserved by the whorl of desert dunes in central Australia.

During the LGM, the Indonesian Archipelago was the Indonesian maritime continent, with the Sunda Shelf exposed and rivers probably flowing both north and south around Borneo. Malaysian studies provide a map of the drowned drainage systems on the Sunda Shelf. The loss of the pool of warm water on the Sunda Shelf would have had a major impact on global climate in the LGM. However, the deep ocean gateway between the emergent Sunda and Sahul shelves would have remained opened. A drier climate is expected with the exposed shelf and colder waters in the Indian Ocean, rainforest may have contracted.

Thailand and much of the Gulf of Thailand was a land area during the LGM. Aeolian sand deposits, possibily related to the LGM or earlier glacial intervals, have been reported from the Khorat Plateau, indicating a previous much drier climate. In China, a key question for reconstruction of the LGM is whether the Yangtze and Yellow rivers reached the sea during this time of aridity. The South China Sea was still open to the Pacific, but closed to the south across the Sunda Shelf.

In Japan, during the LGM seaways existed between Honshu and Hokkaido, and between Japan and Korea, so that circulation could flow in and out of the Sea of Japan. Extensive glaciers existed in Hokkaido and valley glaciers in central Honshu. In the Russian Far East, there were large areas of exposed shelf during the LGM and the ice shelf extended into the Sea of Okhotsk. The land areas were covered by ice cap, aeolian deposits and periglacial features such as patterned ground and permafrost.

At the workshop it was decided that the next map to be compiled by the group will be the Holocene Optimum, around 6,500 to 5,000 years B.P., when sea level was a few metres higher than present in much of the WESTPAC region. This map will have uses in future planning given the trends of global warming and sea level rise.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SOME 1850s GEOLOGICAL SURVEYS, OFFICIAL AND PRIVATE. David Branagan Department of Geology & Geophysics, University of Sydney, NSW 2006

Government-supported surveys were an important part of the Australian scene in the 1850s in all the Australian colonies and were a direct result of the desire to locate mineral deposits, both metallic and nonmetallic. The work of Samuel Stutchbury and Rev. W.B. Clarke in New South Wales and Queensland was wide-ranging and produced important results, particularly in the folded Palaeozoic sequences which hosted many of the gold and base-metal occurrences of the region. However neither surveyor trained younger geologists, and their work was not carried forward for some years.

The Victorian Geological Survey, under its founder A.R.C. Selwyn, had a systematic mapping program, and trained a considerable number of able geologists. Selwyn had to resist considerable political and bureaucratic pressures to maintain the work of the Survey, but was eventually frustrated in his efforts and moved to Canada.

Selwyn also carried out short investigations in both South Australia and Tasmania in the 1850s. Charles Gould undertook larger scale work in Tasmania, but did not complete his proposed "Geology of Tasmania".

A.C. Gregory and J.S. Wilson covered considerable ground in Northwestern Australia during an exploring expedition, Gregory suggesting correlations with rocks in southeast Australia.

The Australian Agricultural Company employed Ferdinand Odernheimer to advise on the company's estates in New South Wales, mainly in the Newcastle and Nundle regions. A related operation was that proposed for the Cordillera Company in the Nundle area by E.A. Baker, but which struck problems, including a shipwreck, local scandal, absconding miners and a reluctant geologist!

In Victorian mining fields John Phillips, George Bruhn and George Ulrich were among experts who provided consulting advice to individual miners and mining companies. This was generally small scale work, related to local structures and mineralisation.

Another independent soul was "Lying Jack" Calvert who helped himself to other peoples rock collections and reports, passing them off as his own on his return to England.

The degree of interaction between government and private geologists varied considerably. Phillips and Ulrich, for instance, both worked for the Victorian Survey, or on contract with it, but others remained independent. However a network of contacts existed so that geological information was spread within the profession, and sometimes more widely through the local press. The short-lived Dicker's Mining Journal played a significant part in the dissemination of information during the 1850s.

However the profession did not sell itself as effectively as it might have. The pressure of fieldwork and long absences from centres of influence gave little opportunity for publicity, and few startling results could be claimed as a result of such work, although the results were of interest and significance for other geologists.

The accumulation of geological knowledge needed considerable time before a framework of stratigraphic and structural knowledge could befirmlyestablished and used as a basis for mineral exploration. This message was not got over to politicians and bureaucrats, who wanted a "quick fix". Surprisingly the private companies seem to have been more understanding of what geologists were supposed to do. The same problems and attitudes continue to haunt us to the present.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 AUSTRALIAN EARTH SCIENCES HISTORY THE BEGINNING OF A BIOGRAPHIC/PHOTOGRAPHIC INDEX David Branagan Department of Geology & Geophysics, University of Sydney, NSW 2006

Remembering the past is not a waste of time. From the work of the pioneers have come the theories we apply, and the practice we adopt to understand the Earth. In many cases sites which are now inaccessible were examined in detail by earlier workers, and their observations recorded with care. For this practical reason alone their work should not be forgotten, and their records should be preserved. Theories that were put forward and rejected for lack of evidence at the time are often found to be surprisingly "modern" and worth examination.

The contribution of Australia's pioneer geologists to both the theory and practice of geology has received little recognition to date. Two interlocking aspects of this problem have been attacked by the Earth Sciences history Group of the Geological Society of Australia. A large data base of geologists and associated workers (Miners, surveyors) has been prepared, taken from a card index prepared by the late Tom Vallance. At this conference a small photographic exhibition has been mounted to indicate the quality of work done by twelve pioneer geologists who worked prior to 1940. This is the beginning of what is hoped to be a continuing and growing database of photographs and information about Australian geologists which will be available to future generations.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

S T R U C T U R A L C O N T R O L S IN T H E FORMATION O F T H E C A W S E NICKEL LATERITE DEPOSIT, W E S T E R N AUSTRALIA. IMPLICATIONS FOR SUPERGENE ORE FORMATION AND EXPLORATION IN DEEPLY W E A T H E R E D TERRAINS. N.W Brand 1 and C.R.M.Hult Cooperative Research Centre for I.andscajx: Hvolulion and Mineral Kxploralion, CSIRO 1 Private Mail Bag. 1*0 Wembley WA 6014, Australia. ^ C >ii study leave from Western Mining Corporation.

ABSTRACT Nickel laterites of Western Australia have formed on stable cratonic platforms, a markedly different environment to the classic nickel laterite environments of South West Pacific and the Caribbean. Resources in Western Australia currently amount to some 455 MT averaging 1.12% Ni, containing approximately 5 MT of nickel metal. In approximately 80% of these resources, the Ni is hosted by silicate minerals (silicate-nontronite type deposits) and in the remainder, it is hosted by Fe and Mn oxides (oxide type deposits). To date, only 12,317T of nickel metal has been extracted from these resources.

The Cawse nickel laterite deposit is an example of a oxide-type nickel laterite and has an indicated resource of 30 MT. at 1.00% Ni. Discovered in November 1993 by Centaur Mining and Exploration, Cawse is situated 50 km north of Kalgoorlie and 20 km south east of the Siberia nickel-cobalt laterite deposits.

The ultramafic rocks of the Cawse region form an almost continuous, sheet-like, unit of volcanic origin extending from Siberia to Ghost Rocks, NE of Menzies. This unit forms the basal komatiitic sequence composed of the Walter Williams Formation and the Siberia Komatiite.

Underlying and overlying the komatiitic sequence is a series of high-Mg basalts intercalated with tholeiitic basalts, layered mafic-ultramafic sills (e.g. Ora Banda Sill, Mt Pleasant Sill) and an upper sequence of epiclastic sedimentary rocks.

Nickel enrichment within the regolith at Cawse is controlled by a primary 240° strike-slip fault, which transgresses all in situ regolith units and perpendicular to the Archaean stratigraphy. Within the regolith, nickel enrichment (> 0.5% Ni) is confined to ferruginous saprolite (15-55 m depth), developed on komatiitic olivine adcumulate and forms steeply-plunging, east-dipping shoots along the talc-phlogopite shear zone. Nickel is also associated with Mn (Co) oxides in sub-horizontal Ni blankets on the boundary of the mottled zone and ferruginous saprolite, in the upper 20m of the regolith. This Mn-Co-Ni enrichment is controlled by a shear zone, and a dolerite dyke acting as a hydromorphic barrier.

To date, no significant Ni sulphides or their weathered chemical indicaters within the regolith have been observed in the Cawse region. Criteria significant for the successful exploration of nickel laterites in Western Australia will be discussed.

Acknowledgments The authors are grateful to Centaur Mining and Exploration for their permission to allow us to submit this abstract to the 13th Australian Geological Convention. NWB is currently enrolled at the University of Western Australia and gratefully acknowledges the continuing support of Western Mining Corporation and the receipt of an APRA(i) award for his postgraduate studies.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 A NEW GENERATION OF LANDSCAPE EVOLUTION MODELS BASED ON IRREGULAR SPATIAL DISCRETIZATION Jean Braurr1 and Malcolm Sambndge 1' 2 * Research School of Earth Sicences ^Centre for Information Science Research Australian National University, Canberra, ACT 0200

We present simulations of large-scale landscape evolution on tectonic time scales obtained from a new numerical model which allows for arbitrary spatial discretization (Figure 1). The new method makes use of efficient algorithms from the field of computational geometry to compute the set of natural neighbours of any irregular distribution of points in the plane. The natural neighbours are used to solve linear geomorphic equations that include erosion/deposition by channelled flow and diffusion. Althought the set of geomorphic equations we have used is rather simple and only applies to landscape evolution on tectonic time scales, it is easily shown that, via our method, any other set of geomorphic equations could be efficiently solved on an irregular spatial representation of the landscape.

The resulting synthetic landscapes (Figure 2) do not suffer from the directional bias introduced by rectangular numerical meshes which forces streams to develop along four preferred directions only. In our model, river streams are "free" to develop in any direction and form complex braided networks.

Figure 1: Irregular spatial discretization used to solve problem of Figure 2

Figure 2: Example of synthetic landscape

Because of its great geometrical flexibility, the algorithm is capable, for the first time, of solving problems involving complex boundaries, radially symmetrical uplift functions and horizontal tectonic transport across strike-slip faults. The algorithm is also ideally suited to solve problems which require large variations in spatial discretization and/or self-adapting meshing. We present a number of examples to illustrate the power of the new algorithm and its advantages over more "classical" models based on regular (rectangular) discretization. We also demonstrate that the synthetic river networks and landscapes generated by the model obey the laws of network composition and have scaling properties similar to those of natural landscapes. We finally show how orographically-controlled precipitation and flexural isostasy may be easily incorporated in the model without sacrificing efficiency.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 CAMBRIAN VOLCANICS AND INTERFLOW SEDIMENTS - COLBINABBIN, VICTORIA - THEIR STRATIGRAPHY, PETROLOGY, GEOCHEMISTRY, AND POTENTIAL AS A SOURCE FOR TURBIDITE-HOSTED GOLD DEPOSITS OF CENTRAL VICTORIA. JMN Broome WRHRamsav , RR Keays , M Hughes , D Arne , S Reeves 1 Western Mining Corporation, Western Australia ^University of Ballarat, PO Box 663, Ballarat Vic 3353 ^Laurentian University, Sudbury, Ontario, Canada ^Consulting Geologist, 1034 Geelong Road, Mount Clear Vic 3350 The University of Melbourne, Parkville Vic 3350 1

2

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5

5

Massive tholeiitic metabasite lavas, pillowed lavas, and interflow sediments of assumed Cambrian age, are exposed at Lake Cooper Quany at the northern extent of the Heathcote Greenstone Belt, Colbinabbin, Central Victoria. The exposed sequence is 275 m thick, dips 65-70° west, faces west and comprises 6 lava flows totalling 268 m thick. The upper portion of flow 5 is pillowed. Three interflow sedimentary units occur in ascending order between flow 1 and flow 2 (2-3 m thick), between flow 3 and flow 4 (2.5 m thick), and between flow 4 and flow 5 (0.2 - 2 m thick). The lavas vary from medium grained intersertal to quench textured olivine absent rocks of tholeiitic MORB affinity with possible incipient back arc affinities (MgO 6.7-8.8 wt%, Ti/V 19.7-22.2, Ti/Zr 100-129, Zr/Y 2.2-3.4 n = 16). Similar comparisons may be made with metatholeiites from Heathcote and Mount Wellington, Phillip Island and Pitfield (Ramsay et al. 1992). Secondary minerals include chlorite, epidote, prehnite, pumpellyite, sphene, carbonate, and quartz.

The interflow sediments can be grouped into three main types namely, massive sulphide-bearing portions with pyrite varying from 40-75%, finely laminated pyritic and carbonaceous units, and banded chert and/or carbonate units with quartz varying from 50-80% and carbonate 20-45%. Individual interflow units tend to exhibit a vertical zonation from more massive sulphidic and laminated carbonaceous shale sulphidic zones to chert and carbonate units. In addition, a laterally extensive silicified metabasite zone (-12 m thick) occurs 5 m below the base of the pillowed sequence and includes patches (up to 150 mm across) of pyrite and chalcopyrite.

The average gold content of the interflow sediments from the quarry is 40 ppb. A similar value was previously obtained for other interflow sediments in the Heathcote igneous belt, as was an average gold value for ?Cambrian metatholeiites and boninites from Victoria of 1.5 ppb (Hamlyn and Keays 1984). Using Pd contents which are normally not affected by low temperature fluids, unless highly oxidising, Hamlyn et al. (1985) proposed that gold contents in boninites both globally and at Heathcote were originally significantly higher than they are at present. This apparent loss of gold probably is reflected in the elevated gold contents of these interflow sediments regardless of whether they are hosted by boninites or N-type MORBS. 5-^S values for pyrite from the three interflow sediments give a range of -17.9 to -18.9°/ . In contrast, sulphides concentrated from three metabasites in the quarry give 5 ^ S values of 0 to -1.2°/ (Broome 1992).

These data suggest that sulphur in the interflow sediments was derived from a primitive source by reducing bacteria. The origin of the metals (Cu, Zn, As, Au) is equivocal, but they could have been derived from volcanic exhalations or by seawater leaching. Assuming a density for the interflow sediments of 3 g/cm^ and 50 % extraction of gold, a total thickness of interflow sediment of 10 m over an area of 20 km by 50 km could theoretically yield 600 tonnes of gold, enough to account for historical production from the Bendigo gold field.

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REFERENCES Broome, J.M.N., 1992. The Cambrian volcanics of Colbinabbin, their stratigraphy, petrology, geochemistry, and potential as an ore forming environment. BSc (Hons) Thesis, University of Ballarat, Ballarat (Unpubl.).

Hamlyn, P.R., Keays, R.R., Cameron, W.E., Crawford, A.J., & Waldron, H.M., 1985. Precious metals in lowTi lavas: implications for metallogenesis and sulphur saturation in primary magmas. Geochimica et Cosmochimica A eta 49, 1797-1811.

Hamlyn, P.R., & Keays, R.R., 1984. 13th Report to AMIRA on the project, "Precious metals and sulphide immiscibility: implications for ore genesis and the importance of "source" rocks for gold deposits".

Ramsay, W.R.H., Stanley, J.M., Hughes, M., Morand, V., & Carroll, R.P., 1992. The Pitfield-Avoca Fault System: the locus of a cryptic-discontinuous greenstone belt in central-western Victoria. Geological Society of Australia Abstracts 32, 233-235.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996

WORLD-CLASS SEDIMENT-HOSTED STRATIFORM COPPER DEPOSITS: CHARACTERISTICS AND GENETIC CONCEPTS Alex C. Brown Dept. of Mineral Eng,, Ecole Polytechnique de Montreal P.O. Box 6079, Sta. "Centre Ville", Montreal, Quebec, Canada H3C 3A7

ECONOMIC IMPORTANCE Sediment-hosted stratiform copper deposits (SCDs) of economic importance are notable for their infrequent occurrence world-wide (principally the Kupferschiefer of north-central Europe; the Central African Copperbelt, Zambian-Zaire; Dzhezkazgan and Udokan, Russia; and White Pine and Spar Lake, USA). Fortunately, they are equally well respected for their large tonnages (100s and 1000s of millions of tonnes, typically grading 1 to 5% Cu), resulting largelyfromgood lateral continuity of ore zones along well-defined stratigraphic units. In modern times, they have accounted for 20-25% of the western world's total annual copper production, 80% of its cobalt, very significant quantifies of silver, and, more rarely, lead. They may contain untouched reserves of gold, uranium, PGEs and REEs. With these economic charateristics, they form a well-deserved target for exploration.

PRINCIPAL CHARACTERISTICS SCDs are known at the deposit scale for the following common features: economic copper zones characteristically widespread (over many square kilometres) along relatively thin (typically measured in metres) distinct stratigraphic units; a predominance offine-grainedsulphide ore minerals (e.g., chalcocite, bornite, chalcopyrite) disseminated in well-layered concentrations along bedding; peneconformable overall cupriferous mineralization (including economic + uneconomic cupriferous zones); cupriferous zones positioned immediately adjacent to and on the reducing side of a redoxcline separating a major thickness of footwall continental redbedsfromextensive greybeds of the ore zones and hangingwall strata; mineralized zones commonly occupying only a minor portion of the greybeds, the remaining hangingwall being unmineralized carbonaceous pyritic black shales or marginal marine/lacustrine carbonates; one or more metals other than copper (e.g., Ag,Co,Pb,Zn,Cd,Mo,Hg) in or adjacent to the cupriferous zone (Ag and/or Co may be very attractively economic); a zoning of copper and associated metals (e.g., lead, zinc) away from the redoxcline according to decreasing sulphophile behaviours (Cu>Pb,Zn>Fe); and low-temperature mineral parageneses trending from early syndiagenetic Fe-sulphide ± sulphates to subsequent post-sedimentary ore-stage sulphides. At the basin scale, SCDs are found at the redbedgreybed redoxcline transition described above, commonly in close association with evaporitic units (signalling basins most commonly formed 10 to 30° N or S of the equator), especially in or associated with continental rift basinsfilledwith redbeds (signalling ages post-dating oxidation of the Earth's atmosphere, i.e., <-2.4 Ga) ± bimodal volcanic strata.

GENETIC CONSIDERATIONS The above characteristics are especially well explained with an overprint depositional model at the deposit scale: copper and other associated post-sedimentary metals soluble in warm oxidized chloride brines hosted by (and probably generated within) the porous redbeds are overprinted on reduced greybeds immediately adjacent to the redoxcline. The reduced host greybeds were prepared for mineralization during sedimentation and early diagenesis with an abundance of pre-ore-stage sulphur, either already reduced sulphur in iron sulphide or sulphur availablefrombiogenic reduction of sulphates in the presence of a reductant such as in situ carbonaceous matter or more ephemeral hydrocarbons. Some sulphur may have been introduced as sulphate with the infiltrating brine.

Disseminatedfine-grainedsulphides precipitated in zoned arrays, with more-cuperiferous sulphides proximal to the redoxcline, less-cupriferous sulphides in more distal portions of the cupriferous zone, and other still lesssulphophile sulphides (e.g., galena, sphalerite) in still more distal positions. Large volumes of metalliferous brines could have been driven across the redoxcline by sediment compaction, by basin water recharge from rift margin highlands, or by thermal recycling resulting from anomalous heat available in rift basins. Direct magmatic involvement appears to have been unnecessary. Continental-marine (or lacustrine) redoxcline transitions form a basin-scale stratigraphic metallotect. In addition, three global-scale metallotects guide explorationists to SCD occurrences: continentalrifts,formed at low paleolatitudes, in post-Archean strata.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 CLIENT-BASED BUSINESS PROCESS RE-ENGINEERING OF GEOLOGICAL SURVEYS: THE TASMANIAN EXPERIENCE. Anthony V. Brown Tasmania Development and Resources, PO Box 56, Rosny Park, Tasmania, 7018

Over the past ten years due to competing client requirements, the growth in economic rationalism, large changes in information handling and computer technology, Australia's Geological Surveys have been forced to change the way in which they do business. In the late 1980's some clients complained that Surveys had lost their way and were even competing with industiy in exploration and universities in research. Changes in the political and economic climate, w hich brought about a review of all activities of Government, also added to the pressure for change.

During this period the role of the Geological Survey was questioned, challenged and debated by a range of clients including industry, research groups and governments. This discussion has taken place in the understanding that the "client" is always right. The difficulty has been the growing view that almost all players are clients. While they all suggest different roles for the Surveys, there was one common requirement: the provision of geoscientific information.

The challenge, then, in restructuring, is to identify the different client groups and their demands and to prioritise them appropriately. It has also been vital to balance the expectation of internal and external clients.

The Tasmanian Geological Survey is no exception. It is going through a slow process of re-organisation to provide modern up-to-date data, in easy to use packages, so that our two main objectives - to promote Tasmanian prospectivity for minerals and petroleum and to improve land use decision making within the State, - are achieved.

The future engine room of the Tasmanian Geological Survey is being built through Project TIGER (Tasmanian Information on Geoscientific and Exploration Resources). Project TIGER requires the development of a computerised system of interrelated data sets which can be manipulated to produce a variety of client-requested end products. The development of this project, coupled with data capture programmes and the ability to interpret the data and produce outputs on demand, defines the parameters for the future role of the Survey.

It will be from this set of inter-related data bases that all future requests from our various clients will be met.

Due to the input requirement of the computer systems being employed and the varying data output requirements of the mineral industry, local government, and the community at large, work practices and data gathering practices are changing and must, in the future, continue to change.

Special initiative projects to increase the mineral prospectivity of areas not presently under exploration are replacing the traditional systematic regional mapping programmes, however, regional mapping programmes are one of the primary data gathering methods employed by Geological Surveys and, as such, need to be maintained as the basic information gathering activity undertaken by Geological Surveys.

The Tasmanian Geological Survey will, in the future, be striving to fulfil clients needs using a five part strategy- - viz: data collection by special initiative programmes; data collection by co-operative projects with the Australian Geological Survey Organisation under the National Geoscience Mapping Accord or Mineral Province Enhancement Programme; storage, maintenance and interpretation of data by inter-related digital data bases; involvement in improving land management decisions by active participation with local government and planning groups; and continuation of financial assistance to the Key Centre for Ore Deposit and Exploration Studies (CODES) at the University of Tasmania.

The end purpose of these strategies is to be able to satisfy our clients so that Tasmania can build a modern geoscientific understanding of the State, which can evolve as new knowledge becomes available, and thus support the minerals industry and government so that we have a sustainable mining industry and are able to make competent and informed land use decisions.

Results of new initiatives and other projects undertaken since re-structuring will be presented.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE STRATIGRAPHIC NAMES DATABASE - COMING OF AGE! Catherine E. Brown, Sonja L. Lenz Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

In July 1995 the Stratigraphic Names Database was the first AGSO database to go live for querying on the Internet. This Oracle database is the national authority on stratigraphic names. Formerly called GEODX, the database went through a major restructure and upgrade in 1995.

The National Register of Stratigraphic Names was set up by AGSO's predecessor, the Bureau of Mineral Resources (BMR) in 1949 to help geoscientists adhere to the (then) newly created Australian Code of Stratigraphic Nomenclature. With a great deal of help from the State Surveys, lists of names and references were produced and published in the 1950s and early 1960s as State Lexicons of stratigraphic names. An update volume was published in 1975. The central register was maintained at BMR. All information went on to index cards until 1979 when the database was computerised, first in an HP Image database called Geodx with a division between published names and reserved names. In the eighties it was one of the first BMR databases to be transferred into Oracle, the corporate relational database management system, more or less maintaining the Image structure. In the early nineties the need arose for a computerised Stratigraphic Lexicon (STRATLEX) as a reference tool for other areas of BMR. This was achieved in 1991 by supplying a subset of names from GEODX which field geologists used and supplemented with any new names needed. Unfortunately, there was no regular update of the lexicon from GEODX which caused as many problems as had been solved.

In 1994 the Register became part of AGSO's National Geoscience Information System (NGIS) Program and the decision was taken to re-unite GEODX and STRATLEX with the aim of making the information on stratigraphic names more useful, more current and more easily available. This was achieved in April 1995 by redesigning the stratigraphic names part of GEODX, and combining all information in the Lexicon and in both the reserved names and published names tables of GEODX in one table called Stratnames. Fields for information such as status, currency, rank, parent unit, overlying and underlying unit were added to this table.

Currently there are more than 28 000 names contained in this new Stratigraphic Names Database covering all status categories from reserved to defined, from informal to obsolete or superseded. The Stratigraphic Lexicon has become a view (about 15 000 names) of all the current names in Stratnames which means that every update there is immediately reflected in STRATLEX. USES AND ACCESS Australia now uses the International Stratigraphic Guide and the National Register continues to serve as a register of stratigraphic names. The database once again functions well as a Stratigraphic Lexicon. Register data may be used by field geologists in selecting suitable unit names for future definition and use. Historical references can be tracked down to assist with the writing of new definitions. When preparing papers, existing names can be checked for currency, spelling and potential confusion with other names before publication.

Besides these traditional uses, the database can also supply stratigraphic information for Geographic Information Systems (GIS). For example, the parent field allows the GIS user to choose the detail appropriate to the map scale, so that regional maps show only Groups or even Supergroups, while detailed maps show all units, right down to Member level.

The Stratigraphic Index unit consists of one full-time person at present who has to index publications, enter new data and maintain the database, as well as respond to queries. The Variations List and other regular reports are sent to all universities and interested individuals, the State Surveys and Stratigraphic Names Committee. Public access to the computer database is available through the geology section of AGSO's World Wide Web server (http://www.agso.gov.au). This access will soon include additional reference information and more flexible searching. AGSO staff have read-access to the database through a menu with data retrieval forms and various reports. Dial-up access to members of the Stratigraphic Names Committee and the State Surveys can be arranged if there is sufficient interest. Of course we still cater for customers without computer access.

62

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

STRATIGRAPHIC NAMES DATABASE - WHERE TO FROM HERE? Catherine E. Brown, Sonja L. Lenz Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

The Information Services Division at AGSO and its Database Development Group are keen for corporate databases to be more widely publicised and used. The National Register of Stratigraphic Names is a longexisting resource now being better used as a standard for other AGSO databases (eg OZROX, OZMIN) and to provide information to GIS projects. This was made possible by a major restructuring in early 1995, including the introduction of a whole gamut of new fields, providing more information on stratigraphic units (see highlighted items, Figure 1). Wherever possible, the values for the new fields were derived from information already stored in unstructured comments fields.

So far, so good, but much more needs to be done to achieve as complete and up-to-date a database as is possible.

Although the database now has a better structure, only one stratigraphic name actually has all the fields filled the Mount Noma Quartzite in Queensland! We need the expert knowledge of all geologists to help fill in the blanks for other units. As well as the essentials, there are several enhancements we would like to make. One is to add information from nearly 1400 existing definition cards, to make this valuable data more accessible. We would also like to add some old but important references from our card file (1949 - 1969 data is only on cards).

The Stratigraphic Index unit's one full-time officer cannot keep up with the indexing of even the highest-priority publications as well as maintaining the database and answering enquiries. We are looking for more efficient ways of data collection and entry and are grateful for any tangible help the geoscience community can provide.

At the same time we are progressively making the data more accessible (eg, through AGSO's WWW server: http://www.agso.gov.au).

12978 Y

Mount N o m a Quartzite QLD Type state: QLD Entered: 01-JAN-88 by GEODX Status: Defined Defined in Geodx reference: 79/19701 First published in Geodx reference: 79/04754 Definition card: Y Stratname category: Unknown or process pending Reuik: Formation, beds Previously known as: Mount N o m a quartzite Modified: 05-SEP-95 by STRATA Comments: Type section follows Weatherly Creek on the east side of Snake Creek Anticline, Cloncurry 1:100,000 map sheet.

Age: Palaeoproterozoic - Palaeoproterozoic Geological province: Mount Isa Inlier Section/drill hole number: 7064 Meucimum thickness (m): 2150 Parent unit: Soldiers Cap Group Overlying unit: Toole Creek Volcanics conformity Underlying unit: Llewellyn Creek Formation conformity

Geodx No. Usage 79/04754 79/19701 82/22378 84/24309 90/26900 J0102/04

Status

P17 Mentioned P601 Defined P81 Briefly described Mentioned Map legend Table 1 P492 Fully described Defined P601

Comments

Name for Weatherly Creek Quartzite.

References from Geodx 90/26900

Beardsmore T.J. Newbery S.P. Laing W.P. 1988 The Maronan Supergroup: An inferred early volcanosedimentary rift sequence in the Mount Isa Inlier, and its implications for ensialic rifting in the Middle Proterozoic of northwest Queensland.

Precambrian Research 40/41 P487-507

Figure 1. Example of range of data available in the three sections of a stratigraphic names report.

63

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CLARENCE RIVER SUPERSUITE: PRIMITIVE I-TYPE GRANITES OF EASTERN AUSTRALIA Colleen J. Brvant. R. J. Arculus, and B. W. Chappell GEMOC, Department of Geology, Australian National Universirty, ACT 0200, AUSTRALIA

The Clarence River Supersuite (CRS) is one of three late Permian I-type granite superstates in the New England Batholith of the southern New England Orogen, eastern Australia. It is composed of eleven small (usually <100 km^) plutons that occur at the northeastern and southern extensions of the batholith, and collectively intrude five technically distinct tectonostratigraphic terranes. The intrusions are compositionally very diverse, ranging from gabbro through to monzogranite, but tonalite, granodiorite and diorite are the most abundant rock types. With the exception of the Omadale Brook and Barrington River plutons (Barrington Tops) which contain only small amounts of hydrous phases, hornblende and biotite are the most abundant ferromagnesian minerals. However, most intrusions do contain significant amounts of clinopyroxene ± orthopyroxene, or their alteration products. The granites have a marked depletion of the high field strength elements relative to other incompatible elements, a characteristic feature of arc-related granites. They are also characterised by relatively low abundances of the alkalis, P, Nb, Ba, LREE, Pb, Th and U, thereby distinguishing them from other I-type supersuites in the New England Batholith. With the exception of one sample from the Dumbudgery Creek Granodiorite, initial Sr/ Sr and S^d y between 0.7031-0.7042 and +6.1-+1.8, respectively, constituting the most primitive isotopic compositions reported for Palaeozoic granites in eastern Australia.

These granites are geochemically distinct from the Devonian I-type granites of the Lachlan Fold Belt (LFB) having lower abundances of K, high field strength elements, particularly Nb (Fig. 1), and LREE, as well as being more isotopically primitive. The broad mineralogical, geochemical and isotopic characteristics of the CRS granites are very similar to the tonalitic association in the American Cordillera, such as the Peninsular Ranges batholith. However, with an age of approximately 250 Ma, they are distinctly older than those granites.

The mineralogical and geochemical characteristics of the granites of the CRS are consistent with their derivation by dehydration-melting of amphibolite in the lower or middle crust at pressures generally less than 0.8 GPa and temperatures of at least 1000° C, leaving a granulitic residue of clinopyroxene, plagioclase and Fe-Ti oxides ± orthopyroxene. Very steep REE patterns and the absence of Eu anomalies in one intrusion, the Duncans Creek Trondjhemite, imply higher pressures of origin (>0.8 GPa).

Hornblende barometry indicates the magmas only became water-saturated at shallow depths (pressures <0.24 GPa).

The CRS granites were derived from relatively young, isotopically primitive, but heterogeneous arc-related materials. At least three distinct isotopic components (A-MORB; B-lower crust; C:-upper crust) are required in the genesis of the CRS granites (Fig. 2), including both more isotopically primitive lower crustal and LILE-enriched and isotopically evolved upper crustal sources. However, the importance of individual components varies from one intrusion to another. The chemical and isotopic heterogeneity appears to be unrelated to the host tectonostratigraphic terrane.

87

86

v a r

Figure 2. Components involved in CRS genesis

Figure 1. CRS: filled circles and LFB:squares

64

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE MAXIMUM PROBABLE TSUNAMI: SOUTH COAST OF NEW SOUTH WALES Edward A. Brvant. Robert W. Young and D.M. Price School of Geoscience, Uni. Wollongong, Northfields Ave., N.S.W., 2522

Tsunami waves produce 4 general categories of depositional and erosional evidence that can be preserved in the geological record (Table 1). Individual items within these categories are signatures that, singly or in combination with each other, uniquely define the impact of tsunami in the coastal landscape. The array of signatures provide directional information about the origin of tsunami in the west Tasman Sea and, when combined with radiocarbon and thermoluminescence dating, indicate that 6 events of varying magnitude have affected the New South Wales coast during the Late Holocene. The source of these tsunami appears to be large submarine slides on the continental slope. The first event probably occuijed concomitantly with the approach of sea-level near modern levels around 7000 BP. The latest event occurred around 800 BP although there is sufficient evidence to suggest that one event may have occurred just before European settlement. The recurrence interval of tsunami is now approaching one event every 600 years.

Table 1 Tsunami Geomorphic Signatures Depositional

rO Sedimentary Deposits

ER0SI0NRL Sand laminae Sand layers with boulder floaters

-© - © Chaotic sediment

Bedrock Sculpturing

l ^ y

Cavitation features

L ( J 7 ) S-forms

mixes

-0

Imbricated boulder stacks

L

Smear deposits

©

r© Geomorphic Forms

"0 -©

Splayed landward tapering sand units

Geomorphic Forms

Sculptured \headland features

- bored whirlpools - canyon drainage channels - rapids, cascades & falls - roche moutonn6es - toothbrush-shaped stacks - fluted promontories

— / ^ N Landscape features

- truncated cliffs - raised platforms - ramps • headlands - eroded barrier remnants

Beach ridges Coastal barriers

- © Carseiand

- flutes - cavettos - transverse troughs - muschelbruche - sichelwannen - potholes

The impact of these tsunami upon the coastal landscape has been profound. Several signatures provide estimates of the magnitude of run-up of these events. The height to which chaotic mixes of sediment (3) and imbricated boulder stacks (4) have been deposited and the height of headlands that have had a smear of clay, sand and shell (5) plastered across them give general estimates of the run-up height. The elevation of eroded landscape features on headlands (12) gives information about the depth and velocity of flow. The presence of sand laminae (1) and splayed sand units within deltaic sediments (6) permit the landward limit of tsunami impact to be determined. This geomorphic evidence indicates that the largest tsunami waves swept sediment across the continental shelf and obtained flow depths of 15-20 m at the coastline with velocities in excess of 10 meters per second. Along cliffs, and especially at Jervis Bay, waves reached elevations of 40-80 m with evidence of flow depths in excess of 15 m. Preliminary evidence on the Shoalhaven delta indicates that waves penetrated 5 km inland for at least one event. This geomorphic evidence suggests that the New South Wales south coast is subject to tsunami waves an order of magnitude greater than that indicated by historic tide gauge records.

65

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 TIMING OF CLEAVAGE DEVELOPMENT IN THE WESTERN LACHLAN FOLD BELT: NEW CONSTRAINTS FROM A r / A r GEOCHRONOLOGY 40

Martin Bucher . David A. Foster and David R. Gray Australian Geodynamics Cooperative Research Centre, School of Earth Sciences, La Trobe University, Bundoora, VIC 3083 Australian Geodynamics Cooperative Research Centre, Dept. of Earth Sciences, Monash University, Clayton, VIC 3168 1

1

39

1

2

2

The geology of the western Lachlan Fold Belt is dominated by a thick and monotonous succession of deformed Lower Palaeozoic submarine fan deposits. The chevron-folded sand- and mudstones form a generally eastvergent fold- and thrust belt which is cut by three prominent west-dipping, relatively deep-rooted zones of intense deformation: Avoca, Heathcote and Mt. Wellington fault zones. In between these major reverse fault zones, deformation occurred in and along thrust sheets which are also west-dipping, probably listric in form and in general shallower in depth.

The pressure and temperature conditions for the generation of phyllitic rocks correspond to a regime of very low- to low-grade metamorphism where recrystallization under high shear stresses leads to ductile deformed rocks. Under these conditions and given a suitable bulk rock chemistry, white micas are major rock-forming minerals which tend to grow, recrystallize and/or - in the case of surviving detrital micas - simply rotate into a newly formed schistosity. As this deformation process generally occurs rapidly, these rock are particularly suitable for dating tectonic movements.

Twenty-five whole rock samples from fault zones, and from within thrust sheets across the western Lachlan Fold Belt, from the Landsborough fault in the west to the Mt. Wellington Fault Zone east of Melbourne have been analyzed using the A r / A r laser step-heating technique. The cleaved rocks consist of very fine grained metamorphic white mica, quartz, minor chlorite, iron oxide and pyrite, and with or without some detrital white mica and plagioclase. The observed assemblage white mica, quartz, chlorite and iron oxide is consistent with lower greenschist facies metamorphic conditions. Whole-rock samples display variably discordant Ar/ Ar age and K/Ca spectra which reflects varying modal amounts of individual rock-forming phases, but is most important for constraining the amount of detrital white mica present. Where recrystallization during deformation was nearly complete, age spectra are relatively concordant with only minor discordances usually at the beginning and at the end of gas release. Gas fractions liberated during intermediate-temperature portions of the experiments define plateau ages with relatively constant (order of magnitude) K/Ca ratios indicating geological meaningful ages which - in this case - dates the growth of the most abundant K-bearing phase (metamorphic white mica) and therefore the age of deformation.

40

39

40

39

The western-most sample, from the Landsborough fault (Stawell Zone) but within the contact aureole of the Mt. Cole granitic suite has an age of 393±3 Ma indicating complete resetting by the intrusion of the granite suite (390-400 Ma). This places a minimum age of deformation into the Early Devonian. Samples from the vicinity of the Avoca Fault (boundary between the Stawell and Bendigo-Ballarat Zones) reveal three mutually indistinguishable plateau ages of 432±2,433±2,433±4 Ma, and a slightly older age of 440±2 Ma, indicating that deformation occurred in the Early Silurian. The older age may indicate that deformation initiated in Late Ordovician time. Samples collected from along the leading edge of the Heathcote fault zone reveal a rather complicated picture with only one sample indicating an Early Silurian age (436+2 Ma) of deformation. Three more samples give total fusion ages of 456±3, 457±3 and 470±2 Ma, respectively, with concordant plateau segments in intermediate -temperature steps of 467+3, 467i3 and 494+3 Ma. These data are interpreted as reflecting mixtures between partly reset detrital mica and metamorphic mica, because of lower deformational strain and/or temperature of metamorphism. Samples from thrust-sheet-bounded faults in the Bendigo-Ballarat Zone are consistent with Early Silurian/Late Ordovician ages found near and within the Avoca fault zone itself.

Samples from the Mt. Wellington fault zone (eastern boundary of the Melbourne Zone) reveal the same age range as discussed above, with those samples having the least proportion of detrital white mica suggesting metamorphism at ~ 410-435 Ma.

66

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE GEOLOGY OF THE PLUTONIC GOLD DEPOSIT, WESTERN AUSTRALIA Walter R Bucknell, Fergus C M Jockel, Raelene J Kellett, Nancy M Vickery, Peter M Buckley Plutonic Resources Limited, Level 37, 100 Miller Street, North Sydney NSW 2060

The Plutonic gold deposit is situated 180km north-northeast of Meekatharra and 240km south of Newman in Western Australia. The outcropping deposit was discovered in 1988 in an area missed by the early prospectors.

Since then several satellite deposits have also been discovered. At the end of 1994 reserves and resources including satellite deposits aggregated 32.6mt @ 5.9 g/t (6.17moz). Cumulative production of lmoz will be achieved by late 1995.

The Plutonic deposit is situated at the southwestern end of the Archean Plutonic Well Greenstone Belt. This belt trends northeast-southwest, has a strike length of 50km and is located in the central part of the Marymia Dome. The Marymia Dome, an Archean basement inlier within the Proterozoic Capricorn Orogen is located between the Archean Pilbara and Yilgarn Cratons and is a Yilgarn-type granitoid dome comprising granitoids, gneissic assemblages and two greenstone belts (Plutonic Well and Baumgarten). The Plutonic Well Greenstone Belt comprises a northeasterly striking, northwesterly dipping homoclinal sequence of mid- to uppergreenschist metamorphosed tholeiitic basalt, ultramafic volcanics, minor felsic volcanics and sediments and localised quartz-feldspar porphyritic stocks and sills. Northeasterly trending Proterozoic dolerite dykes, intrude this sequence.

At the Plutonic deposit, the local sequence comprises a series of east-west striking, gently northerly dipping ultramafic and mafic volcanics with minor shale interbeds. These units are bounded to the south and west by granites. Mineralization, as presently identified, is constrained within a single mafic volcanic unit termed the Mine Mafic.

Within the Main Pit, ore is disposed as a series of stacked, discrete, subparallel, northwesterly striking lodes that dip 45° to the northeast, obliquely to stratigraphy but restricted to the Mine Mafic. Lodes are l-10m thick, with many individual lodes continuous over several hundred metres. This general geometric relationship between lodes and stratigraphy remains consistent for over 1.5km north and northwest of the Main Pit. An apron of laterite oreflankedthe southern topographic expression of the deposit.

Lodes are composed of a shear-foliated assemblage of quaitz-albite-biotite-cWorite-epidote-carbonate-sulphide + tourmaline and scheelite. Sulphides comprise 5-20% of the lodes as arsenopyrite, pyrrhotite and subordinate pyrite. The lodes typically have strongly albitised cores grading outwards to phlogopite/biotite selvedges with subordinate calcite, chlorite and amphibole. Gold occurs predominantly as native gold disseminated throughout the lode material in spatial association with arsenopyrite and pyrrhotite. Up to 20% of gold occurs within arsenopyrite or less commonly pyrite either asfineinclusions or in solid solution. A subordinate style of lode mineralization is characterised by chlorite-carbonate+biotite alteration with a sulphide assemblage dominated by pyrrhotite. Gold within these types of lodes is almost all free-milling.

The lodes represent stacked parallel shears developed within a brittle mafic volcanic layer bounded above and below by extensive layer-parallel shears generated within the more ductile hangingwall and footwall ultramafic rocks and footwall shales. Regional post-mineralization thrusting has overthrust barren greenstones, and the batholithic granite in turn, southeasterly over the Mine Mafic. A prominent subvertical east-northeast striking sinistral strike-slip fault (the MMR Fault) has offset the southern extension of the orebody, and the southern end of the Greenstone Belt 2.5km eastwards where the satellite deposits at Area 4, Perch, Trout, (and probably Salmon) are situated.

67

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LARGE-SCALE VARIATIONS IN DARCY FLUID FLUX AND INTRINSIC PERMEABILITY DURING HIGH-TEMPERATURE RETROGRESSION OF GRANULITES FROM THE REYNOLDS RANGE, CENTRAL AUSTRALIA. Ian S. Buick1, Ian Cartwright2 & Ian S. Williams3 School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083, Australia. ^Department of Earth Sciences, Monash University, Clayton, Vic, 3168, Australia.

•^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.

1

Granulite-facies marbles of the Reynolds Range Group (northern Arunta Block) were metamorphosed to -4-5 kbar and -750-800 ^C at -1594 ± 6 Ma, and high temperatures (>650 °C) were subsequently maintained for -26 Ma (Williams et al, 1996). The granulite-facies rocks were partially retrogressed in narrow (10's- 100's x 1000's metres), strike-parallel zones. Outside these zones the marbles were internally buffered to high XcC>2 a t the M2 peak. The retrograde zones, which are characterized by resetting of mineralogy and oxygen isotope ratios, major element metasomatism, and quartz veining, were channelways for high-temperature (650-700 °C) waterrich fluids These fluids were derived from crystallising melts sourced from underlying granulite-facies metapelites. In the retrograde zones the following rock-types, which were affected by progressively increasing time-integrated fluid fluxes, occur: i) carbonate-bearing marl layers; ii) wollastonite- or clinohumite-bearing marbles; iii) almost monomineralic lenses of coarse-grained clinopyroxene, grandite-rich garnet, clinohumite, or wollastonite; and iv) high-temperature hydrothermal quartz vein systems.

The marl layers appear to have unreset mineralogy and stable isotope compositions and were probably not infiltrated during retrogression. The wollastonite- or clinohumite-bearing marbles are the mineralogically-reset equivalents of peak-M2 calcite+quartz- and dolomite+forsterite-bearing marbles, respectively. In the case of the wollastonite-bearing marbles, wollastonite (<10%) was probably formed by the reaction: Calcite + Quartz = Wollastonite + CO2 (1). The fluid flux necessary to form the wollastonite can be calculated if it is assumed that the infiltrating fluids flowed vertically through the marble unit after crystallising pelite-derived melts ponded at its base. For likely vertical flow paths of 10 to 200m through the marble, mineralogical resetting requires timeintegrated fluid fluxes of 1.6-33 m^/m 2 . The 8 1 8 0(carb) values of these altered marbles have also been lowered by as much as 4%o from typical unreset values. Marbles with the most reset oxygen isotope values (8^0(carb) = 10-1 l%o) are in approximate equilibrium with the hydrothermal quartz vein sets and partially melted metapelites. Oxygen isotope resetting over the same vertical distances yield time-integrated fluid fluxes of 18360 m^/m 2 . The high-variance lenses were probably formed through major element metasomatism. In the case of the wollastonite-rich lenses there is up to 60 vol.% more wollastonite than can be accounted for by reaction (1). It is likely that the additional wollastonite formed through silica metasomatism, which over a 10 to 200m path length requires minimum time-integrated fluid fluxes of -2x10^-4x10^ m^/m 2 . Estimates of timeintegrated fluid fluxes necessary to precipitate the quartz veins based on silica deposition for fluids flowing down temperature gradients of 100-25°C/km at 4 kbar are m Therefore, within the retrograde zones, time-integrated fluid fluxes in different layers or structures varied by 5 to 6 orders of magnitude.

Fluid flow in the retrograde zones occurred for at least 18 Ma (Williamser al ,1996). If fluid flowed through each rocktype for 18 Ma then the time-integrated fluid fluxes correspond to the following Darcy fluid fluxes: unaltered marl layers (-0 m 3 /m 2 /s); mineralogically reset marbles (2.8x10*^-5.8x10"^ m ^ / m ^ / s ) ; h m o s t isotopically reset marbles (3.2xl0" 1 4 -6.3xl0" 1 3 m 3 /m 2 /s); metasomatised marbles (3.5xl0- 1 2 -7.0xl0" n m 3 /m 2 /s); and hydrothermal quartz veins (8.8x10"^"3.5x10"^ m 3 /m 2 /s). For vertical flow, the differences in Darcy fluid flux recorded in the marbles could have been accomplished by differences in intrinsic permeability from 1.5xl0" 23 3.1xl0" 2 2 m 2 (mineralogically reset marbles) to 1.8xl0 2 ®-3.6xl0"^ m 2 (metasomatised marbles). These permeabilities are comparable with, or smaller than, those estimated in other terrains or experimentally determined (typically 10~ 21 to m 2 ). In the retrograde zones, the unaltered marls, altered but not metasomatised marbles, and the metasomatic calcsilicate rocks are interlayered on a centimetre to metre scale parallel to lithological layering and major structures, and record large variations in fluid flux and intrinsic permeability. The large differences in intrinsic permeability may reflect microscale, deformation-controlled variations in microfracture density in the interlayered rocks. However, the largest fluid fluxes occurred through the the hydrothermal quartz vein systems that make up only a small proportion ( « 5 % ) of the exposed rocks.

REFERENCES t

e

Williams, I.S., Buick, I.S., & Cartwright, I, 1996. An extended episode of Mesoproterozoic metamorphic fluid flow, Reynolds Range, central Australia. Journal of Metamorphic Geology 14 (in press).

68

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PRE-SILURIAN ROCKS OF THE BARNARD METAMORPHICS: A BASEMENT? BLOCK ON THE SE MARGIN OF THE HODGKINSON PROVINCE, NORTH QUEENSLAND Robert J. Bultitude , C. Mark Fanning , David C. Champion and Ian D. Rees 1

2

3

1

(1) Geological Survey of Queensland, GPO Box 194, Brisbane, QLD, 4001 (2) Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT, 2601 (3) Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601

The Barnard Metamorphics, containing anomalously high grade metamorphic rocks, are juxtaposed against Hodgkinson Province rocks south of Cairns, in North Queensland. The formation was previously interpreted either as: 1) the more complexly deformed and, locally, higher grade equivalent of the adjacent Hodgkinson Province rocks (and therefore of probable Devonian age) or, 2) part of a possible Precambrian basement assemblage on which the sediments of the Hodgkinson Province were deposited.

The Barnard Metamorphics consist predominantly of multiply deformed phyllite, quartzite, and schistose to gneissic metasedimentary rocks which have been extensively intruded in places by granite. The metamorphic grade ranges mainly from greenschist to upper amphibolite. Previously unrecorded, isolated outcrops of hornblende granulite have also been included in the unit. In contrast, the adjacent Hodgkinson Province rocks have been metamorphosed only as high as greenschist grade.

The contact between the Barnard Metamorphics and the adjacent Hodgkinson Formation is an intense shear zone (the Russell-Mulgrave Shear Zone). Consequently, the present arrangement of units may bear little or no relationship to their original distribution. In an attempt to resolve these uncertainties, two granites from the Barnard Metamorphics were isotopically dated using the U-Pb zircon (SHRIMP) method. These granites yielded crystallisation ages of 459 ± 8 Ma and 485 ± 7 Ma. The older granite (S-type) intrudes supracrustal rocks of the Barnard Metamorphics and in places contains numerous enclaves, up to —10m long and 5m across, of metasedimentary gneiss and rare amphibolite. These enclaves are interpreted to have originally formed part of the country rock assemblage. The younger granite (I-type) forms a small pod in the older granite.

The Ordovician ages provide a younger limit for the age of the enclosing high-grade, supracrustal rocks in the Barnard Metamorphics. They also indicate at least that part of the unit is older than most of the adjoining Hodgkinson Province succession (mainly Silurian-Devonian). The oldest rocks known in the Hodgkinson Province are in the far west where late Ordovician and possibly early Ordovician units have been recently mapped. The 459 ± 8 Ma age for the younger granite is indistinguishable from the 455 ± 5 Ma (SHRIMP) age yielded by a silicic volcanic clast in the late Ordovician Mountain Creek Conglomerate (in the far west of the Hodgkinson Province). A similar age (using SHRIMP) has also been recently obtained from a silicic volcanic clast in the Hodgkinson Formation (mainly Devonian), south of Cooktown, in the far northeast of the Hodgkinson Province.

The data indicate the Barnard Metamorphics represent a discrete assemblage that is older than and unrelated to the Hodgkinson Province succession. The presence of the Ordovician granites in the Barnard Metamorphics and sedimentary rocks containing late Ordovician volcanic detritus in the juxtaposed Hodgkinson Province also indicates magmatic activity was more widespread in the northern part of the Tasman Orogenic Zone in the Ordovician than previously envisaged. The presence of these relatively old rocks may also have significant implications regarding the evolution of the Hodgkinson Province. A s value of -6.0 (at 280 Ma) obtained from a sample of the 455 my old granite in the Barnard Metamorphics is similar to those yielded by some early Permian granites in the eastern Hodgkinson Province. Furthermore, the 485 my old granite in the Barnard Metamorphics is geochemically similar to early Permian S-type granites of the eastern Hodgkinson Province.

The Ordovician granites of the Barnard Metamorphics and at least some of the Permian granites of the eastern Hodgkinson Province, therefore, may have been derived from source rocks of similar age and composition.

Nd

69

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

COASTAL & MARINE ENVIRONMENTAL GEOSCIENCE: PERSPECTIVE & PRIORITIES Robert V. Burne Australian Geological Survey Organisation, PO Box 378, Canberra City, ACT 2601

The oceans are no longer seen as inexhaustible reservoirs of mineral and living resources or as safe dumping grounds for unwanted materials. The world-wide trends of coastal urban development and marine resource exploitation are placing increasing pressures on all aspects of the marine environment. International focus on these issues is provided by UNCLOS, aimed basically at resource exploitation, and Agenda 21 of UNCED which considers the need for the conservation of biodiversity and natural environments. These agreements create an imperative for sound scientific knowledge of coastal and marine systems so that they can be effectively managed on an ecologically sustainable basis. It has been pointed out that terrestrially based ecosystem theory is inadequate to explain the processes in the oceans , and ocean ecosystem management lacks the scientific theory to underpin the development of a rigorous management process. The need to develop general principles to underpin the scientific management of the marine environment provides a context for the development of modem coastal and marine geoscientific research.

The IGBP Project "Land-Ocean Interactions in the Coastal Zone" (LOICZ) provides one blueprint for the design of national research programs to understand the interactive physical, chemical and biological processes of the coastal zone. The project has four foci. Focus 1 concentrates on the effects of changes in external forcing or boundary conditions on coastal fluxes, and includes consideration of : catchment basin dynamics and delivery; atmospheric inputs to the coastal zone; exchanges of energy and matter at the shelf edge; and the development of coupled models for coastal systems. Focus 2 considers coastal biogeomorphology and coastal change, and addresses: the role of ecosystems in determining coastal morphodynamics under varying environmental conditions; coastal biogeomorphological responses to anthropogenic activities; and the reconstruction and prediction of coastal zone evolution as a consequence of global change. Focus 3 pinpoints carbon fluxes and trace gas emissions through: cycling of organic matter within coastal systems; estimation of net fluxes of N 0 and CH in the coastal zone; and estimation of global coastal emissions of dimethylsulphide. Focus 4 examines the economic and social impacts of global change in coastal systems, including: the evolution of coastal systems under different scenarios of global change; the effects of changes to coastal systems on social and economic activities; and the development of improved strategies for the management of coastal resources. The adaptation and adoption of this program as a framework for Australian research is discussed.

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The coastal zone is defined as extending from the coastal plains to the outer edge of the continental shelves, approximately matching the region that has been alternately flooded and exposed during the sea level fluctuations of the Quaternary period. There an increasing awareness that the focus of a national marine program should be extended to include both the onshore - offshore linkages implicit in the LOICZ program, but also to extend consideration beyond the shelf edge to the outer limits of the EEZ. An initial priority for research is the assessment of the spatial extent of ecosystems within the coastal and marine realm, together with an understanding of their homogeneity and spatial stability in terms of processes and composition. Insights from hierarchy theory and landscape ecology provide a theoretical framework for undertaking this assessment. An ecosystem is the interaction between a biological community and its environment. From this viewpoint, geoscientific data provides invaluable information for understanding the environmental distribution of ecosystems at scales above that of habitat and below that of province, as well as the only means of assessing the long term temporal evolution of ecosystems. Principles of facies analysis may be borrowed from sedimentology to structure this study, and allow the integration of data collected at various scales, from individual core samples to satellite mapping.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

IMPLICATIONS OF CAI AND TAI FOR HYDROCARBON EXPLORATION IN TASMANIA Clive Burrett ' and Malcolm Bendall 1 Geology Department, University of Tasmania, Box 252C, Hobart, Australia 7001 2 Great Southland Minerals P/L, 24 Jackson St, Glenorchy, Tasmania, Australia 7010 1 2

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Work on conodonts from the Ordovician Gordon Group from the western half of Tasmania has shown a decreasing west-east gradient in Color Alteration Index (CAI). Westernmost Tasmania has experienced regional metamorphic temperatures of about 300*C but lower values are found in thin thrust sheets at Point Hibbs. Oil and wet-gas window temperatures are found in the Florentine Valley and southern Tasmania. Much of the heating is assumed to be associated with the Tabberrabberan orogeny and granites as overburden thicknesses are probably insufficient to account for the CAI's. The situation is complicated if an extreme thrust stacking hypothesis is adopted. This may explain the low thermal maturation of hydrocarbons extracted from some limestones with high CAI's from the west coast of Tasmania.

Intensive studies of the numerous seeps recorded this century have shown a very good geochemical match with hydrocarbons extracted from Gordon Group carbonates. The Gordon Group carbonates within the oil and gas windows undoubtedly occur at depth beneath a cover of glacimarine Permian siliciclastics in the Tasmanian Basin of central Tasmania. Studies of vitrinite reflectance, palynomorph TAI and organic geochemistry have shown that, contrary to previously widely held belief, much of the Permian Parmeener Supergroup is also in the oil and wet-gas windows.

Suitable reservoirs and seals occur in both the pre-Permian and Permian sequences, sandstones with high poosities and permeabilities occur within the Lower Parmeener Supergroup, and there may be carbonate reservoirs as paleokarst and reefs within the upper part of the Gordon Group. The pervasive Jurassic dolerite, has locally thermally metamorphosed the Parmeener and Gordon, acts as an effective seal throughout much of the Tasmanian Basin but remains an impediment to seismic prospecting.

oi wndow wet gas window

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CHINESE TERRANES IN RODINIA AND GREATER GONDWANA Clive Burrett and Ronald Berry Geology Department, University of Tasmania, Box 252C, Hobart, Australia 7001 Continental reconstructions for the latest Proterozoic-earliest Phanerozoic are based on several lines of evidence, all of which are contentious. There is no a priori geological or philosophical reasoning for favoring one line of evidence over another. All methods suffer from a general lack of even reasonable quality structural work necessary for stratigraphy, paleontology, paleomagnetism and age-dating throughout most of Asia.

It has been clear for over 20 years that China consists of two major Paleozoic terranes (North and South China) sutured along the Qinling orogen during the Triassic (Burrett, 1974). Paleobiogeographic data strongly suggest that the main Chinese terranes and some of the other minor Asian terranes were part of, or very close to, the Australian-Indian sector of Gondwana during the Early Paleozoic from the Early Cambrian to the Late Ordovician (Burrett etal 1990, Burrett and Stait 1985,1987, Laurie and Burrett 1992) and possibly into the Devonian (Long and Burrett, 1989). Both anecdotal and statistically based biogeographic methods provide constraints on the paleopositions of these terranes within Greater Gondwana and therefore probably also within Proterozoic Rodinia. Paleobiogeographic data have the advantage of indicating paleogeographic proximity and of neither being overprinted nor reset by subsequent events; their disadvantage is that they suffer from uneven geographic, stratigraphic, monographic and taxonomic treatments.

Further constraints are provided by geological comparisons between the terranes, but broad-brush comparisons of fold-belts based on variable method, variable quality, variably overprinted radiometric dates are the basis for most Rodinia reconstructions. It is by no means clear what some commonly used stratigraphic data such as Varangian glacial and Cambrian phosphorite sequences are telling us concerning paleogeogeographic proximity.

Asian paleomagnetic data remain sparse and some or all poles may be remagnetizations. However, we will assume that, where there are fold-tests, conglomerate tests or the like, that the paleomagnetic data are according to their published age. With these caveats in mind we provide tentative reconstructions for the major Asian teiranes within Rodinia and Greater Gondwana that satisfy large proportions of the geological, biogeographic and paleomagnetic data sets.

REFERENCES Burrett, C., 1974. Plate Tectonics and the Fusion of Asia. Earth Planetary Science Letters 21, 181-189.

Burrett, C.F. Long, J, & Stait, B. 1990. Early to Middle Palaeozoic biogeography of Asian terranes derived from Gondwana. Geological Society of London Memoir 12, 163-174.

Burrett, C. and Stait, B., 1985. South-East Asia as part of an Early Palaeozoic Gondwanaland - a palaeobiogeographic test of a geotectonic hypothesis. Earth Planetary Sci. Letters 75,184-190.

Burrett, C. & Stait, B., 1987. China and Southeast Asia as a part of the Tethyan margin of Cambro-Ordovician Gondwanaland. In McKenzie, K. (Ed.). Shallow Tethys 2 Balkema Rotterdam, 65-77.

Laurie, J. & Burrett, C. 1992. Biogeographic significance of Ordovician brachiopods from Thailand and Malaysia.

Journal of Paleontology 66(1), 16-23.

Long, J. & Burrett, C. 1989. Fish from the Upper Devonian of the Shan-Thai Terrane indicate proximity to Australia and South China. Geology 17, 811-813

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 HOLOCENE SEA SURFACE TEMPERATURES FOR THE GREAT BARRIER REEF FROM Sr/Ca PALAEOTHERMOMETRY OF CORALS Darvl P. Burrows*. Malcolm T. McCulloch* and John F. Marshall * Research School of Earth Sciences, The Australian National University, CanbeiTa, Australia. ^Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2600.

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Strontium-calcium ratios within the coral genus Porites, provide a proxy record of sea surface temperatures (SSTs). High resolution records (0.25 mm of coral = 1-2 weeks) have been obtained from separate Porites corals from Stanley and Myrmidon Reefs (central Great Barrier Reef) which have C ages ranging from approximately 4000 to 7200 years BP. We present preliminary Sr/Ca thermometry results using thermal ionisation mass spectrometry (±0.05% precision which corresponds to ±0.1 °C SST variation).

A 5060 ± 100 yr BP coral from Stanley Reef, representing 7.5 years of coral growth (95 mm long), grew in waters with a mean summer maximum sea surface temperature (SST) ~1°C cooler than the present, and a mean winter minimum SST ~1.5°C cooler than present. The mean inter-annual variation"is ~5.5°C as compared to ~4.9°C for the present. A moderate sized El Nino event was identified 18 mm from the top of the coral.

A 32 mm section of Porites coral from Myrmidon Reef (6740 ±140 years BP), representing 5.5 years of coral growth, grew in waters with a mean maximum SST of ~27°C and a mean minimum SST of ~23°C. This indicated that the summers were approximately 2°C cooler and the winters were approximately 1°C cooler than the present (Fig. 1). A large and a moderate El Nino were identified in this six year record, and as a consequence the mean SSTs for this short period could conceivably be cooler than the long term average.

Another much long section of Porites coral (195 mm) from Myrmidon Reef, representing 17 years of coral growth, has an estimated age -7200 years BP. The Sr/Ca sea surface temperature record has variable winter minimum temperatures but consistent summer maximum temperatures. The mean summer and winter temperatures are similar to the instrumental record (Figure 1).

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These results suggest slightly cooler or similar SSTs to present in the central section of the Great Barrier Reef for short time periods in the mid-Holocene. However such short palaeotemperature records may not be representative of this entire period. Rapid decade-scale temperature fluctuations may obscure the longer term temperature trends. For example, SSTs in the Great Barrier Reef were -1°C cooler in the 1970s than the 1980s. El Niiios also appear to have been active and of similar magnitude during mid-Holocene, and hence are likely to be a persistent feature of Interglacial periods.

Figure 1.

Comparison of two fossil corals (6740 ± 140 and -7200 years BP) with an insitu instrument SST record (1991-1993) from Myrmidon Reef.

SST Cycle Referenced To Arbitrary Year Scale

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 GOLD MINERALIZATION AT THE PORGERA GOLD MINE, PAPUA NEW GUINEA, IN RESPONSE TO ADIABATIC DECOMPRESSION AND FLUID MIXING ALONG THE ROAMANE FAULT Gregory H. Cameron . John LWalshe , Christoph A. Heinrich and V.J. Wall Australian National University, Canberra ACT 0200, Australia ETH Zurich, Switzerland, MIM Exploration Ply Ltd, Spring Hill QLD 4000, Australia 1

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Porgera is located in the PNG highlands within Cretaceous Papuan Platform sediments, 25 km south of the Lagaip Fault, and is spatially and temporally associated with the 6 Ma alkaline Porgera Intrusive Complex.

Alteration of the host sediments was the earliest hydrothermal event. It is associated with the bleaching of black shales along thin grey pyrite veinlets termed G stringers, and we believe it is equivalent to propylitic alteration of the intrusive phases. This alteration is associated with gains in K, Si, Rb, Ba, Fe * and S; and losses in Na, Ca, Pb, Zn, Fe * and Sr. Local geometry, paragenesis and geochemistry indicate that the fluid responsible for this alteration emanated from the currently exposed intrusives.

Mineralization consists of two stages, both associated with phyllic alteration of intrusive rocks. Stage I vein sets are spatially associated with the intrusions and are continuous from the intrusions through altered sediments to the black shales. Veins in the intrusions contain arsenical pyrite, Fe-rich sphalerite, galena, rare pyrrhotite, quartz and Ca-Mg-Fe-Mn carbonate and commonly show well developed sericitic selvedges. Veins in black shale, away from the intrusives, are dominantly pyritic with carbonate, only subordinate sphalerite and galena, and do not show altered selvedges. Later Stage II mineralization is spatially associated with the Roamane Fault and constitutes high grade gold ore hosted in quartz-roscoelite (KV [AlSi ]0 o[OH] )-pyrite breccia veins. Most of the high grade Stage II mineralization is hosted along and in the footwall of the Roamane Fault where altered sediments are the main ore host, although intrusive rocks locally host significant mineralization. High grade ore shoots occur at the intersection of Stage I and Stage II vein sets. Munroe, (this session) has shown that the Roamane Fault was not active during Stage I, but was the main spatial control upon development of Stage II mineralization.

Quartz and carbonate are intimately intergrown with sulphide but are rarely (if ever) observed in textural equilibrium with sulphide. We infer that sulphides formed early in Stage I and are being dissolved during quartz and carbonate precipitation. Stage II veins show well developed cyclic banding (punctuated by brecciation) with alternating layers of quartz, roscoelite, pyrite, barite and Au with layers of barren quartz.

Stage I sphalerite hosts fluid inclusions with 7-11 wt% NaCl equiv. salinity and TVs between 200 and 320°C, quartz from these veins show similar salinities but homogenize at higher temperatures of up to 360°C and Stage II fluid inclusions contain 4-8 wt% NaCl equiv. and TVs of 120 to 180°C (Richards and Kerrich, 1993). Our study has revealed evidence of CO2 rich fluids indicated by the presence of clathrate and liquid CO2 in inclusions hosted in both Stage I and Stage II barren quartz. High salinity brine inclusions are also observed in quartz (intergrown with magnetite, hematite, epidote, and calcite) from vesicles in fresh diorite.

Bulk 3 S values for Stage I and Stage II pyrite are +3 to +5 per mil and -14 to -10 per mil respectively (Richards and Kerrich, 1993). SHRIMP analysis of Stage II pyrite yielded d^S values ranging from -18 to -14 per mil.

Pyrite from the G stringers returned SHRIMP d*S values of -4 per mil whereas bulk sulphur analyses returned values from -1.2 to 3.7 per mil. While G stringers are not strongly negative they are more negative than the bulk rock values for Stage I sulphides. These data indicate reducing fluids during Stage I, oxidizing fluids during Stage II and an intermediate redox state for the early G stringer fluids.

We suggest that early weakly oxidizing saline fluids were derived from the exposed shallow intrusive complex and were responsible for the formation of the altered sediment as well as propylitic alteration of the intrusions. Lower salinity reduced Stage I fluid may represent evolved magmatic fluid which has equilibrated with black shale during ascent. Sulphide deposition probably resulted from cooling of this fluid under sublithostatic conditions. Rupture of the Roamane Fault rebrecciated and further opened Stage I veins resulting in violent throttling and adiabatic decompression of the Stage I fluid. This resulted in phase separation and loss of CO2 and H2S to the vapour phase, as well as a significant adiabatic temperature drop. Loss of H2S would explain the increase in solubility of base metal sulphides and hence their dissolution. Loss of CO2 and drop in temperature would explain the simultaneous deposition of carbonate and quartz. Decompression induced fluid mixing in dilational sites along the Roamane Fault, of reduced Stage I fluid with fresher CO2-SO2 rich magmatic fluid (vapour?), tapped at depth by footwall splays off the fault, resulted in high grade gold roscoelite rich ore shoots at the intersections of Stage I and II vein sets. This spatial relationship and the occurrence locally of high grade gold, quartz, roscoelite, barite layers, following pyrite is consistent with Au transport in reduced sulphur complexes and its deposition by extreme oxidation of a reduced Au bearing Stage I fluid.

REFERENCES Richards J. P. and Kerrich R. 1993. The Porgera gold mine, Papua New Guinea; magmatic-hydrothermal to epithermal evolution of an alkalic-type precious metal deposit. Econ. Geol. 88, 1017-1052.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

T H E EVOLUTION OF T H E MANTLE'S CHEMICAL STRUCTURE Ian H. Campbell Research School of Earth Sciences, Australian National University, Canberra, ACT 0200

The melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time. The highest-temperature melts (komatiites and picrites) derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates. A survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the Archean-Proterozoic boundary. Archean komatiites have depleted or neutral geochemistry, whereas most Proterozoic and younger picrites have originated from enriched (OIB-type) mantle, although rare depleted plumes persist to the Tertiary (e.g., Gorgona). If plumes originate from a thermal boundary layer near the core-mantle interface, the composition of this layer must have changed from a mixture of depleted and neutral mantle during the Archean to mainly enriched mantle in the Proterozoic. Alternately, if plumes originate from the upper mantle-lower mantle boundary, these conclusions apply to the transition zone. The change in the composition of plumes coincides with a drop in the maximum MgO content of the magmas from >30% in the Archean to <22% in younger magmas, indicating a decrease in plume source temperature. I suggest that the change in the temperature and composition of the hotspot sources reflects a fundamental change in the dominant form of the component of the earth's convection, driven by surface heat loss: from cold plumes or drips breaking away from beneath a stable lithosphere during the pre-Archean to lithospheric subduction in the Archean and post-Archean mantles. The absence of enriched or OIB-type plumes in the Archean is interpreted to reflect the length of time for oceanic crust to become unstable, sink to the core-mantle boundary, heat up, and return to the upper mantle in plumes. The fall in the maximum MgO content of plume-related magmas between the Archean and Proterozoic may be due to the accumulation of a compositional boundary layer above the core that lowers the temperature of the plume source region by partially insulating it from the core. Continental flood basalts and oceanic plateau basalts have less extreme geochemical characteristics than those of the associated komatiites or picrites. If these magmas form by melting in the head of a starting plume that is a mixture of mantle from the plume source and entrained lower mantle, the modern lower mantle must have geochemical and isotopic characteristics closer to chondritic than the depleted or enriched mantle reservoirs.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 MINERAL EXPLORATION IN THE TASMANIDES: Pb ISOTOPE MODELS AS A GUIDE TO PROSPECTIVITY Graham R. Carr . Judith A. Dean , Brian. L. Gulson , Paul. M. Ashley , Michael J. Korsch CSIRO Exploration and Mining, P.O. Box 136 North Ryde 2113 Graduate School of the Environment, Macquarie University, Sydney, 2105 Department of Geology and Geophysics, University of New England, Armidale 2350 1

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A large Pb isotope database on sulfide mineralization in the Palaeozoic and Mesozoic terrains of eastern Australia is now available. By comparing these data to initial Pb isotope ratios of magmatic source rocks models can be developed that relate hydrothermal and tectonic development and provide a precise, predictive tool for mineral exploration. The terrains studied form part of the Tasman Fold Belt System (Tasmanides), which extends over 1200 km from east to west and over 3000 km from north to south.

The proposed Tasmanides model is based closely on a Lachlan Fold Belt model (LFB model) developed recently in CSIRO. This model proposes that in the early Palaeozoic, crustal magmatic rocks and associated hydrothermal events derived Pb from an isotopically homogeneous source with a very long crustal residence time. Mantle Pb was emplaced into the crust during the Ordovician Molong Arc volcanic event. Ordovician metallogenesis, represented principally by a Cu/Au-shoshonite association, was dominated by the mantlederived Pb. However, Silurian and younger hydrothermal events derived Pb principally from crustal sources, although with evidence of variable incorporation of the Ordovician mantle Pb. A plumbotectonic model for the LFB based on "mixing isochrons" between crustal and mantle end-members yields mineralization model ages for the Palaeozoic with a precision of generally better than ±15 Ma. The Early Cambrian Kanmantoo Fold Belt contains stratiform Pb-Ag mineralization with homogeneous Pb isotope ratios with a LFB model age of - 540 Ma, consistent with derivation from a very similar crustal source to that of the LFB. This represents a 10-20 Ma overestimate of the geological age, which is within the overall precision of the model. In contrast, Cambrian mineralization in the Mt Read Volcanics and the Mt Windsor Volcanics give significantly younger LFB model ages, 450-300 Ma and 450-400 Ma respectively. The heterogeneity and radiogenic nature of the Mt Read mineralization indicate different sources and crust-mantle mixing processes to the LFB. The Mt Windsor results suggest a unique radiogenic crustal source, although a younger age for the major hydrothermal activity must also be considered. Elsewhere in the Thompson Fold Belt granites and granite-related mineralization have LFB model ages consistent with geological age. The Pb isotope ratios of galena associated with gold mineralization in the Stawell, Bendigo-Ballarat and Melbourne Zones of the LFB give a range of LFB model ages mainly between 500 and 450 Ma and suggest a primary crustal source, with local mantle Pb addition. These results are consistent with the age range of the host sequences and are interpreted as suggesting primary metal accumulation occurred during basin formation, although remobilization and mixing may have occurred during later magmatic or metamoiphic hydrothermal events.

Mineralization in the Hodgkinson Fold Belt and associated with Devonian and Permo-Carboniferous intrusions in the Georgetown Province has heterogeneous Pb with very old model ages and enriched in P b relative to the LFB crustal curve. Where comparisons are possible, the magmatic rocks have similar initial Pb ratios to the hydrothermal mineralization. This results from mixing of normal Tasmanides crustal Pb with Pb derived from metamorphosed, Proterozoic basement. Devonian to Permian mineralization in the southern New England Orogen (NEO), contains Pb with mixed crust-mantle signatures, and LFB model ages which closely match geological age. Moreover there is a close isotopic similarity between mineralization and associated magmatic units. Fewer data are available in the northern NEO, but it is apparent that an isotopically distinct mantle reservoir is involved.

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The general model can be used in exploration over large areas of the Tasmanides to relate the age of hydrothermal activity to the age of host rocks or potentially fertile magmatic activity and predict those prospects which have the highest probability of representing potentially fertile hydrothermal activity. In terrains where the general model does not to apply, an understanding of the local source rock environment can be used to calibrate a terrain specific model which can be applied to exploration samples.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE HISTORY OF THE ANTARCTIC CIRCUMPOLAR CURRENT (ACC) & THE PACIFIC DEEP WESTERN BOUNDARY CURRENT (DWBC), NEW ZEALAND SECTOR, SOUTHWEST PACIFIC OCEAN R.M. Carter & L. CarteP 1

department of Earth Sciences, James Cook University, Townsville New Zealand Oceanographic Institute, Wellington 2

Forty percent of the world ocean's cold, deep water enters the ocean through the Southwest Pacific Gateway as the Pacific Deep Western Boundary Current. The evolution of this current system, and of the related circum-Antarctic currents, has taken place since 30-25 Ma when plate movements created the first oceanic gaps south of Australia and South America. An excellent stratigraphic record of these events, and of the development of the modern DWBC, occurs in intermediate depth waters east of the New Zealand microcontinental plateau. The DWBC is supplied from the deeper part of the Antarctic Circumpolar Current, and forms the linkage between the ACC and mid-Pacific upwelling deep water. The DWBC passes into the Pacific Ocean east of the Campbell Plateau and Chatham Rise, submerged continental fragments which rifled from their counterparts in Antarctica in the late Cretaceous. The history of the DWBC is therefore contained within the continental margin sediments of the New Zealand plateau.

Sedimentary basins on the east side of New Zealand uniformly contain passive margin rift-drift sediments of Cretaceous through early Oligocene age. Evidence for regional current activity commences in the late Oligocene, represented by phosphatic greensand and limestone deposited above a regional unconformity - the 30 Ma Marshall Paraconformity. The supeijacent limestones, of early Miocene age, contain large-scale cross-bedding indicative of the presence of powerful bottom currents, generated at this time by the opening of the Australian/Antarctic and Drake Passages, and marking the initiation of the palaeo-Circum-Antarctic Current.

The early Miocene limestones pass up into terrigenous silts of Miocene-Pleistocene age which were shed from the developing Alpine Fault plate boundary further west. Seismic evidence demonstrates that these silts were deposited as part of the prograding shelf sequence of eastern South Island, and that substantial parts of them comprise current-drifts deposited in palaeo-water depths of c.300-1,000 during the middle and late Miocene.

Major sediment drifts of Miocene-Recent age are also present in locations adjacent to or beneath the modern DWBC, in water depths of 1,500-4,750 m. These sediment drifts were supplied throughout the Neogene from the uplifting Alpine plate boundary on the western side of New Zealand, via three major submarine channel/trough complexes, from south to north the Solander Channel, the Bounty Channel/Fan, and the Hikurangi Channel/Fan-drift. The Neogene history of activity of the Pacific DWBC, and therefore of the developing flow of deep, cold-water into the world ocean, lies contained within the extensive Miocene-Recent sediment drifts of the eastern New Zealand continental margin.

In order to assess the Neogene history of these deep water current systems, we propose to drill sites in the Southwest Pacific to reconstruct the palaeohydrology and dynamics of the DWBC and related water masses.

The proposed sites comprise a transect across the DWBC between water depths of 400 m and 4,900 m. Only one previous ODP site (594) is located in this large area.

Pacific Gateway drilling will provide the sedimentary sequences needed to study a range of high priority problems in Neogene palaeoceanography, including: (1) development of the DWBC system through time, including the changing hydrology of circum-polar deep water (CDW) being supplied to the Pacific; (2) speed and flux estimates for the CDW into the Pacific through time; (3) past changes in the position of the Sub-Antarctic Front and Antarctic Convergence; (4) history of productivity fluctuations at the Subtropical Convergence; (5) tests for coherence between past changes in circum-Antarctic flow and orbital forcing; (6) tests for glacial-interglacial fluctuations in the "salt conveyor belt" circulation system; (7) changes in the response of deep ocean fans, fan-drifts and drifts to external controls such as climate and sea-level; (8) testing whether the 30 Ma (mid-Oligocene) oceanographic event in the Southwest Pacific was caused by a sea-level change (rise or fall), or by the initiation of the circum-Antarctic circulation system; and (9) establish a sedimentary and geochemical budget for the Eastern New Zealand Sedimentary System (ENZOSS), within which the DWBC forms a major influence.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

STABLE ISOTOPE GEOCHEMISTRY OF CALC-SILICATE BOUDINS, RAUER GROUP, EAST ANTARCTICA: CONSTRAINTS ON THE DURATION OF RETROGRADE METAMORPHIC REACTIONS Ian Cartwright*. Ian S. Buick^, and Simon L. Harley^ * VIEPS, Department of Earth Sciences, Monash University, Clayton, Vic. 3168, Australia ^ VIEPS, School of Earth Sciences, LaTrobe University, Bundoora, Vic. 3083, Australia 3 Grant Institute of Geology, University of Edinburgh, West Mains Rd., Edinburgh EH9 3JW, UK

Documenting the timing and duration of fluid-rock interaction during metamorphism is of paramount importance. While granulite facies metamorphism in most terrains appears to have been largely fluid absent, many terrains experienced fluid-rock interaction following the peak of metamorphism. Here we examine local post-peak metamorphic fluid-rock interaction between calcsilicate boudins and felsic orthogneisses from the Rauer Group, Antarctica. We highlight the importance of decompression following the peak of granulite-facies metamorphism in generating fluids, and estimate the duration over which fluid-producing metamorphic reactions occurred.

The Rauer Group, east Antarctica is a polymetamorphic granulite-facies terrain that contains four distinct lithological associations: 1) felsic to intermediate orthogneisses; 2) layered paragneisses (pelites, semipelites, quartzites, and calcsilicates); 3) homogeneous felsic to intermediate orthogneisses that contain boudins and rafts of marble and Mg-rich metapelites; and 4) layered mafic granulites and minor marbles that often occur as rafts within the orthogneisses of Association 1. Associations 1 and 2 were only metamorphosed by M3 metamorphism (700-900 MPa, 840±40 °C). Following M3, the rocks underwent near isothermal decompression to 200-400 MPa while temperatures remained in excess of 750 °C. In addition to M3, Associations 3 and 4 preserve evidence of undergoing an earlier regional metamorphism (1000-1200 MPa, 1000-1050 °C).

Zoned calcsilicate boudins from the Rauer Group occur enclosed in pelitic paragneisses, and may represent boudinaged marl layers. The boudins are typically 5-25 cm in diameter and are concentrically zoned with wollastonite-rich cores and clinopyroxene-rich rims. This mineralogical zonation is the product of two separate events: 1) infiltration of water-rich fluids prior to the peak of M3; and 2) diffusion between the boudins and the surrounding orthogneisses during M3. Boudin cores have d C(Cc) values as low as -21 %o, which probably resulted from infiltration-driven decarbonation during the period of fluid infiltration. The boudins underwent several reactions during decompression, including: A) Gr + Cc + Qtz /E Wo + Me; B) Gr + Qtz + CO2 /E Wo + Me; C) Me + Qtz /E Wo + An + C0 ; D) Wo + C 0 /E Cc + Qtz; E) Me + Wo + Cc /E Gr + C0 .

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Calcite stable isotope ratios from six boudins from Little Italy Island show the following: 1) a general increase in d C values from core (as low as -17%o) to rim (-9 to -10%o); 2) d 0 values varying much less than d C values; and 3) little correlation between d ^ C and Wt% calcite. The original decarbonation resulted in low d C(Cc) values, while the core-rim d C trends most likely reflect later isotopic diffusion. The d C profiles from boudins are similar to those predicted to result from diffusion of a tracer within a sphere that is surrounded by a well-mixed, homogeneous reservoir. Isotopic diffusion through minerals is extremely slow, and significant C diffusion on geological timescales would probably only have occurred through a grain-boundary fluid. For metamorphic porosities of 10" -10" , the d C profiles could have formed in -800 to 80,000 years. Small (millimetre to centimetre) scale variations in d l 3 values that may have been initially present within the boudins would have been homogenised on much shorter timescales than those required to form the profiles.

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C diffusion was probably fluid hosted. This fluid may have been generated during decompression when the rocks crossed fluid-producing reactions. The preservation of the d C profiles suggest that diffusion only took place over a few thousand years. The calculated timescales may reflect the time over which the metamorphic reactions occurred and reaction-enhanced porosity was present. Even if porosities were very small, 10" -10" , the preservation of the d C profiles requires that an interconnected fluid-filled porosity was present for less than a few million years.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 PUTTING GEOLOGY INTO TOURISM - SOME TIPS AND PRACTICAL EXPERIENCES. John N. Casey and A.E. (Tony) Stephenson 11 McLachlan Crescent, Weetangera ACT 2614 Bureau of Resource Sciences, PO Box El 1 Queen Victoria Terrace, Barton ACT 2600 1

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Ecotourism is nature based tourism that involves education and interpretation of the natural environment and is ecologically sustainable. Geology has a basic role to play in ecotourism, and geologists need to be aware that educated lay people undertaking ecotours are clamouring for well presented explanations of landscape forms, the reasons for the colours of the countryside, and what makes the country 'tick'. Too often the biologist, the botanist, the zoologist and the historian forget that basic to all of this is the geology - the rocks and landforms.

When explaining geology to the interested layman, it is important to forget technical jargon, and instead translate geological terminology and processes into household examples and experiences. For example, 'metamorphism' to a geologist has a particular meaning, but in these educated lay - tours one needs to use 'cooked' or 'pressurised' when the original rock has been changed. The geological time scale is another difficult concept to impart - it needs to be put into everyday language, before you lose your audience. Putting geological history into a 24 hour clock is one useful device. As a rule of thumb, 1 million years equals 19 seconds, so the middle Devonian can be described as being about 10 PM, and the end of the Cretaceous as 11.40 PM. It is a very effective description when the layperson realises that the Great Pyramids of Egypt were built at less than a tenth of a second before midnight in earth-clock terms.

Geologists have a broader view of the countryside, the outback, the landforms, and industry than most other scientists because of their ability to range over a wide spectrum of modern and ancient environments. Their field experiences and their understanding of minerals, oil and water can generate interest in what others might call 'useless country'. Most geologists should have enough experience to identify and relate the common vegetation, the animals and birds, the agriculture, and of course the mineral resources to the country being travelled. Human history is also important in tours and needs to be referred to - who discovered such and such and when and where did the name come from sets the scene for the interrelationship between man and geology.

In this context, any Aboriginal legends about an area are also invariably of interest to the ecotourist. Ideally, backgound information on an area's geology should be included in a brief itinerary for tour participants. It gives the scientific ecotours a more professional approach, and stimulates discussion with participants. Verbal descriptions can easily 'go in one ear and out another', but something in writing gives more time for thought.

We will give just a few examples of well known tourist features which lend themselves to an interesting, informed, illustrated and innovative discussion. The Great Artesian Basin lies below onefifthof Australia, and is the reason why the pastoral industry was able to develop in large parts of western NSW and Qld. The mound springs issuing near Maree produce drinkable 2 million years old water, which has travelled about 2 metres per year through the aquifer from intake beds near the Great Divide. A description of artesian water can include aquifer 'heads', what porosity means (not vast underground caverns, as many laymen think), solution geochemistry (why some water is 'hard', and why GAB springs can cause mounds), and can be tied into human usage and the unique local ecology associated with flowing bore drains. Cradle Mountain is one of the great tourist drawcards in Tasmania, and is a geological and geomorphological masterpiece with four major geological events etched in the majestic mountain: the Precambrian unconformity, the Permian sediments including some coaly layers, the dolerite which was squeezed into 1000 m of Permian-Triassic sediments now removed, and the sculpturing by (Pleistocene) Ice Age glaciers to form the U-shaped valley and lake. The gaps in the MacDonnell Range have their own story, in particular Standley Chasm, the only one not formed by streams cutting through weak zones of faults and joints in the quartzite. Standley Chasm is vertical because the softer dolerite dyke was eroded more easily than the host rock. The Murray River Gorge is a classic geological area. The yellow sandy limestone cliffs are rich in a fascinating variety of marine fossils, evidence of this region being a huge gulf of the Southern Ocean between 30 and 15 million years ago, teeming with life. A second marine incursion about 5 million years ago lasted 'only' a million years, and deposited different rocks with a much sparser fauna. Yet a third marine flooding affected only the then river valley about 3 million years ago, and is characterised by massive fossil oyster beds. Later still, a huge inland lake left deep clay deposits seen in the upper gorge. Depositional processes in the river valley and the cause of the gorge itself are also of interest to the ecotourist - as well as the orchards and wineries! Many major tourist attractions exist for a geological reason, which when properly explained in non-jargon terms can give another dimension to a tourist's enjoyment and education, and broadening their horizons.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GOLD MINERALISATION SYNCHRONOUS WITH THE FINAL STAGES OF CRATONISATION, YILGARN CRATON, WESTERN AUSTRALIA: EVIDENCE FROM Sm-Nd AND U-Pb AGES OF CROSSCUTTING (POST-GOLD) DYKES 1

Kevin F. Cassidv1. Adam J.R. Kent2 and C. Mark Fanning2 Key Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands, 6907, Australia 2 Research School of Earth Sciences, The Australian National University, Canberra, 2601, Australia

An understanding of the timing of gold mineralisation with respect to tectonothennal events, such as magmatism or metamorphism, is crucial to the development of holistic models for the genesis of Archaean lode gold mineralisation in the Yilgarn Craton, Western Australia. Geochronological studies can provide information about the presence or absence of temporal links between mineralisation and tectonothermal events that could be associated with generation of the hydrothermal fluids responsible for mineralisation. To date, isotopic dating of hydrothermal minerals from Archaean lode gold deposits within the Yilgarn Craton of Western Australia has provided limited evidence for a craton-wide episode of gold mineralisation at ca. 2630 Ma (i.e., 2630 ± 1 0 Ma; Groves, 1993 and references therein), albeit based on three published precise U-Pb or Pb-Pb ages.

In well-studied areas such as the highly mineralised Kalgoorlie Terrane, geochronological results show that mineralisation at ca. 2630 Ma occurred some 70 million years after greenstone belt deposition, and up to 30 million years after widespread upper crustal granitoid emplacement and metamorphism, and gold mineralisation is considered to be unrelated to these processes. However, recent geochronological results for the central and southern regions of the Yilgarn Craton, including the Sm-Nd and U-Pb results detailed herein, have revealed that geographically widespread thermal reworking of the lower- mid crust, involving partial melting, metamorphism and granitoid intrusion occurred concurrent with regionally extensive gold mineralisation at ca. 2630 Ma. Intrusive rocks that crosscut gold-mineralised structures are comparatively rare in the Yilgarn Craton, and are restricted to deposits which form at relatively deeper crustal levels (generally at pressure-temperature conditions equivalent to amphibolite facies or greater). In the majority of cases these intrusions are pegmatitic or similar near minimum-melt intrusions, and their restriction to relatively deep crustal levels is probably due to limited vertical mobility of volatile-laden pegmatite melts away from a deep-seated magmatic source.

Samples of garnet-bearing pegmatites which demonstrably crosscut, and thus postdate, gold-bearing structures were obtained from three lode gold deposits: Westonia and Nevoria deposits in the Yellowdine Terrane, and the Scotia deposit at the southern end of the Norseman Terrane. Sm-Nd garnet-albite isochron ages for pegmatite dykes provide minimum age constraints of 2640 ± 1 1 Ma, 2628 ± 10 Ma and 2620 =fc 36 Ma for lode gold mineralisation at the Westonia, Nevoria and Scotia deposits, respectively. A post-gold mineralisation microgranite dyke comagmatic with pegmatites at Westonia has a U-Pb zircon age of 2638 ± 8 Ma. The results are similar to the 2620 ± 6 Ma Pb-Pb wholerock isochron age reported by Bloem et al. (1995) for post-gold pegmatite intrusion at the Corinthia mine, also located within the Yellowdine Terrane In combination with previously published data, the ages are consistent with these deposits forming during a regionally extensive gold mineralisation event at ca. 2630 that involved deposition of substantial reserves of gold over a range of crustal depths corresponding to greenschist to granulite pressure-temperature conditions, consistent with the 'crustal continuum1 model of Groves (1993).

Partial melting and intrusion of granitoids and metamorphism within the more deeply exposed segments of the Yilgarn Craton at ca. 2630 Ma constitutes the last major tectonothermal event currently recognised within the craton (Qiu et al., 1995). Lode gold mineralisation at ca. 2630 Ma was contemporaneous with this widespread tectonothermal episode, which suggests that the major crustal fluid flow responsible for gold mineralisation was probably an intrinsic part of the thermal reworking of the crust at this time. Thus, although gold mineralisation demonstrably postdates greenstone belt deposition and metamorphism and granitoid intrusion within the more shallowly exposed levels of the crust, mineralisation appears to be synchronous with metamorphism and granitoid intrusion at deeper levels during the final stages of cratonisation.

REFERENCES Bloem E.J., McNaughton N.J., Groves D.I. & Ridley J.R., 1995. An indirect lead isotope age determination of gold mineralization at the Corinthia mine, Yilgarn block, Western Australia Australian Journal ofEarth Sciences 42,447-451.

Groves, D.I., 1993. The crustal continuum model for late-Archaean lode-gold deposits of the Yilgarn Block, Western Australia.

Mineralium Deposita 28,366-374.

Qiu, Y.M., McNaughton, NJ. & Groves, D.I., 1995. Lead isotope and SHRIMP zircon age constraints on the timing and sources of late- to post-tectonic granitoids from the central southern Yilgarn Craton, Western Australia. Third Australian Conference on Geochronology, Perth Abstract Volume.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOCHEMICAL AND ISOTOP1C CHARACTERISTICS OF MESOZOIC ALKAL1C INTRUSIONS IN THE CORDILLERA OF BRITISH COLUMBIA: TECTONIC SIGNIFICANCE AND IMPLICATIONS FOR Cu-Au METALLOGENY Kevin F. Cassiriv1 James R. Lang2, Brian A. Lueck 2 , James K. Mortensen2, J. Kelly Russell2 and John F.H. Thompson2 1 Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E2 2 Mineral Deposits Research Unit, University of British Columbia, Vancouver, B.C., Canada V6T 124

Silica-saturated and silica-undersaturated alkalic igneous complexes form a distinct group of Triassic-Jurassic rocks that intrude basement and volcanic arc rocks of the Quesnell and northern Stikine terranes of the Cordillera of western Canada. At that time, amalgamation of Stikinia and Quesnellia and the intervening Cache Creek terrane produced Superterrane I which was accreted onto the ancient continental margin of North America in the Middle Jurassic. Most of the alkalic intrusions were emplaced during a brief interval from 210 to 200 Ma in northern Stikinia and Quesnellia (Mortensen et al., CIM Spec. Vol. 46, in press), although several complexes (e.g., Mt Milligan area, Whiterocks Mtn) in Quesnellia were emplaced during the Middle Jurassic. The older period coincides with a magmatic hiatus between Upper Triassic and Lower Jurassic arc volcanism in both terranes, whereas the younger period coincides with the inferred timing of accretion of Quesnellia onto ancestral North America.

Silica-saturated alkalic complexes are typically multiphase systems dominated by monzonite and diorite, which have only minor normative nepheline or quartz; syenite and pyroxenite are rare. Individual intrusions typically are equigranular to porphyritic with an igneous mineral assemblage comprising augite, biotite, magnetite, plagioclase, K-feldspar and apatite; hornblende and titanite are less common and quartz is rare.

Individual silica-undersaturated intrusions (e.g., Averill, Whiterocks Mtn, Zippa Mtn) typically show welldeveloped compositional zoning from an aegerine-augite pyroxenitic border phase to a central core of K-feldspar megacrystic syenite with intermediate phases dominated by melanocratic syenites and monzonites. Characteristic features include the presence of and/or textures indicative of feldspathoids (nepheline, pseudoleucite), and the igneous mineral assemblage comprising alkali-feldspar, aegerine-augite, biotite, melanite, titanite and apatite.

Hornblende, magnetite and vishnevite-cancrinite are important constituents in some individual intrusions.

Several alkalic complexes contain both silica-saturated and silica-undersaturated igneous rocks (e.g., Mt Polley).

Volcanic rocks of similar compositions occur in both terranes, locally in close proximity to the intrusive complexes. Collectively, the intrusions and associated subalkaline to alkaline volcanic rocks define a characteristic suite of alkaline arc magmas that are comparable to shoshonitic magmatism in active arcs.

Copper-gold mineralisation occurs in several of the alkalic intrusive complexes in Quesnellia and Stikinia. The mineralisation can be classified as porphyry-style, although there are distinct differencesfrom'conventional' calcalkaline porphyry deposits (Lang et al., Geology 23(5), 1995). Both types of alkalic intrusions contain some mineralisation, suggesting that these magmas contained appropriate amounts of metals and ligands as a result of similar melting and fractionation histories.

Radiogenic isotope data indicate that mineralised silica-saturated and silica-undersaturated alkalic intrusions have a restricted range in Sr and Nd isotope composition, are enriched relative to depleted mantle with low 87 Sr/86Sr (0.7034 ± 0.0004) and high 143Nd/144Nd (eNd: +2.7 to +7.9) ratios, suggestive of limited continental basement beneath the terranes. Shoshonitic and high-K volcanic rocks spatially associated with the alkalic intrusive complexes in Quesnellia have e N d values ranging from +5.0 to +7.9 and initial 87Sr/86Sr ratios of 0.7030 to 0.7042 (Smith et al., Can. J. Earth Sci. 32, 1995; this study). Unmineralised silica-undersaturated alkalic intrusions in northern Stikinia (Rugged Mtn, Zippa Mtn) have primitive e Nd (+2.5 to +4.8) and initial 87 Sr/ 86 Sr ratios (0.7036 to 0.7047). In contrast, unmineralised silica-undersaturated alkalic complexes in Quesnellia (Averill, Kruger, Whiterocks Mtn) have variable e Nd (-3.0 to +4.7) and initial 87Sr/86Sr ratios (0.7035 to 0.7046) which suggest that individual phases of the complexes contain a significant component of older continental crust. These intrusions were emplaced during the Middle Jurassic, suggesting that accretion of southern Quesnellia onto ancestral North America had already taken place by this time. In addition, U-Pb ages on xenocrystic zircons indicate that a minor older crustal component exists in some of the alkalic plutons.

The alkalic complexes represent an important group of alkaline rocks which is distinct from the more widely recognised alkalic associations of intraplate and extensional tectonic environments. Preliminary interpretations suggest that the alkalic magmatism and associated Cu-Au mineralisation in the Quesnell and northern Stikine terranes reflects discrete magmatic events related to arc and/or post arc collision events after cessation of active subduction during the late Triassic to middle Jurassic.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, CanberraFebruary 1996

ARCHAEAN GRANITOIDS OF THE NORTHERN EASTERN GOLDFIELDS PROVINCE, YILGARN CRATON, AUSTRALIA: CONSTRAINTS ON CRUSTAL GROWTH David C. Champion1 & J.W. Sheraton1 Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601, Australia.

The Archaean granitoids of the Eastern Goldfields Province in the Yilgarn craton, Western Australia, comprise more than two-thirds of the present exposure and, as such, provide an extensive record of. and place constraints on, the crustal history of the province. Archaean granitoids from the northern Eastern Goldfields Province (NEGP) can be subdivided, on the basis of geochemistry and petrography, into two major groups (High-Ca and Low-Ca), and three minor groups (High Field Strength Element (HFSE)-Enriched, mafic, and syenitic).

The high-Ca group is the most abundant, comprising over 60% of the total granitoids. It consists of variablydeformed leucogranodiorite, trondhjemite and granite, and includes banded, locally migmatic, gneiss. The granitoids are felsic (68-77% Si0 2 ) and, like many Archaean granitoids, have high A1203, Na 2 0 and Sr, and low Y (and, by analogy, HREE). However, they are also characterised by moderate K 2 0, Rb, Pb, Th and U, and appear to lack the more mafic varieties typical of Archaean Tonalite-Trondhjemite-Granodiorite (TTG) suites.

The low-Ca granitoids are predominantly undeformed monzogranite and syenogranite (70-76% Si0 2 ). They make up more than 20% of the NEGP, and differ from the high-Ca granitoids in having lower A1203, CaO, and Na 2 0, but higher total FeO, K 2 0, Rb, Th, U and Pb. The Low-Ca granites possess some A-type characteristics, with moderate to high Zr, Y and LREE. Trace-element trends are consistent with fractionational crystallisation.

The HFSE-enriched and mafic granitoids and the syenites form a minor component (10-20%) of the granitoids of the NEGP, and appear to occur mainly within, or marginal to, the greenstone belts, The HFSE-enriched group comprises very felsic granites (75-77% Si02) that are restricted to a narrow NNW-trending zone east of Leonora. For a given silica content they are higher in Ti0 2 , total FeO and MgO than all other NEGP granitoids, and have high Y, Zr and Ce, but low LILE contents. The A-type syenites (50-68% Si0 2 ) are commonly localised along lineaments. They are clearly distinguished by their high total alkalies, and have high Y. and low Ti0 2 , total FeO, MgO, CaO, and P 2 0 5 relative to other granitoids. The mafic group is lithologically diverse, ranging from diorite, through tonalite and trondhjemite, to granite; all are characterised by their lower, and variable, silica contents (50->70% Si0 2 ). At higher silica levels they tend towards compositions similar to the High-Ca group. The mafic granitoids, like all the granitoids of the NEGP, exhibit negative Nb anomalies.

Published geochronological data (U-Pb zircon ion probe ages: mostly 2.69-2.65 Ga) suggest that all exposed granitoids are contemporaneous with, or postdate, greenstone formation. Although no granitic basement rocks have yet been identified, the presence of inherited zircon in many of the granitoids, plus an inherited age of 2.8 Ga from an High-Ca granitoid (L.P. Black, written comm., 1994), strongly implies the existence of pregreenstone felsic crust. eNd values for the High-Ca group range from -0.6 to +3.3 (19 analyses) with all but two between 0.4 and 2.3. Data for the mafic granitoids and syenites are similar to the High-Ca group, e ^ values for the Low-Ca group show geographical variability, ranging from -3.0 to -4.0 in the west to -0.9 to -0.4 in the central zone to 3.0 in the east. Although the data are limited, s Nd values (3) for the HFSE-enriched group also suggest some geographic zonation: 1.8 to -1.3 from east to west. Two-stage Nd depleted-mantle model ages (allowing for changes in 147Sm/144Nd during melting) are mostly between 2.75 and 3.0 Ga, up to 3.2 Ga in the Low-Ca granites. Model ages for the High-Ca group are consistent with inherited zircon in these granites.

The high-Ca granitoids appear to have been derived at high pressures, within the garnet-stability field, either by partial melting of a mafic to intermediate source coupled with fractionation and/or assimilation, or via remagmatisation of tonalitic precursors similar to those found in Archaean TTG terranes elsewhere. The isotopic and inherited zircon data suggest that this protolith formed less than 250 Ma prior to granite formation. Given the voluminous nature and general isotopic and geochemical homogeneity of the High-Ca group, this earlier crustal growth must have been extensive. Both the low-Ca and the HFSE-enriched granitoids appear to have been derived by lower pressure crustal melting. Modelling suggests that the low-Ca group was derived from a protolith of similar composition to the mafic end-members of the high-Ca group. s Nd data for the Low-Ca group suggests an old crustal source that either youngs considerably to the east or involves a second, younger, component. The data indicate that the 2.69-2.65 Ga magmatic event involved mostly crustal reworking of pre-existing felsic crust of at least 2 ages, although the presence of possibly mantle-derived mafic granitoids and syenites suggests some new crust formation at this time.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

DRAINAGE EVOLUTION IN THE BATHURST REGION, NSW Roslyn Chan, Australian Geological Survey Organisation

The drainage evolution in the Bathurst region is complex and has been influenced by both external regional factors and local factors. These factors relate to passive margin tectonics, intraplate basins, bedrock lithology and structure, and volcanism, and are fundamental in explaining the geomorphic and regolith evolution of this region.

Western tributaries of a northwesterly flowing proto MacQuarie River system have been gradually captured to the west to become the present Lachlan River system. Eastern tributaries have been beheaded along the Great Divide and reversed towards the coast to the southeast. There has also been a migration of some streams towards the southwest due to possible tilting induced by differential sediment loading in the Murray Basin. The formation of the major drainage divides, the Great Divide (between inland and coastal river systems) and Canobolas Divide (between Murray and Darling River systems), the onset of three periods of Tertiary volcanism, and the formation of erosion bowls due to incompetent lithologies have been significant factors in the drainage evolution in the Bathurst region.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 A PALAEOGEOGRAPHIC AND PALAEOCEANOGRAPHIC INTERPRETATION FOR THE "AUSTRALIAN-INDONESIAN GATEWAY" BASED ON LARGER FORAMINIFERIDS George C. Chaproniere Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

It has been realised for some time that the clearance of Australia from Antarctica in the Eocene, together with the opening of the Drake Passage (separating southern America from Antarctica) by the Oligocene, resulted in the establishment of the Circum-Antarctic Current. The initiation of this current had profound implications for global climate, setting the oceanographic and climatic stage for the Neogene. With the continued northward movement of Australia toward Indonesia, the northern seaway separating the two areas gradually became more restricted, again having profound global climatic effects, resulting in large changes in sea level and oceanic circulation. The major drop in sea level during the late Miocene, and the coincident "Messinian" salinity crisis in the Mediterranean, may have resulted from these changes in ocean circulation patterns in the Indian and Pacific Oceans. The changing palaeogeography of this northern seaway can be illustrated by the palaeobiogeography of late Oligocene to late Miocene larger foraminiferids.

Larger neritic foraminiferids are tropical/subtropical in distribution and, due to their relationship with symbiotic algae, inhabit shallow water regions. As a group they are very useful as biogeographic and palaeoenvironmental indicators. Such foraminiferids are able to disperse readily along coastlines or over narrow, shallow seas, but wide, deep seas appear to provide barriers to their dispersal. Larger foraminiferids, similar to those found in northern Australia, were present in Indonesia from the early Oligocene. The presence of these larger foraminiferids in northern Australia, Irian Jaya and Papua New Guinea during the late Oligocene would suggest the close proximity of southeast Asia (Indonesia). However, traditional plate tectonic reconstructions suggest that a wide strait separated the northern Australian and New Guinea region from Indonesia, a hypothesis supported by the distribution of planktic foraminiferids at this time. If this was the case, the question must be asked as to how the larger benthic forms arrived in the northern part of the Australian Plate by the late Oligocene?

Recently, it has been suggested that a number of terranes had docked at various times on the northern margin of New Guinea. The palaeogeographic distribution of larger foraminiferids supports the presence of terranes in the area between southeast Asia and New Guinea, which provided a series of islands or shallow water areas along which these larger foraminiferids were able to disperse to colonise northern Australia and New Guinea.

Throughout the North West Shelf and New Guinean regions there is structural evidence that indicates deformation processes were active from the late Oligocene, possibly reflecting collision of large terranes to the north.

ODP drilling of selected areas of the northwestern margin of Australia, as well as within the Indonesian area, would improve our understanding of the nature and timing of re-activation of pre-existing structures in northwestern Australia, together with the changing palaeoceanography of the region, since the late Oligocene. This has implications for petroleum exploration in northern Australia especially for source rock maturation, trap formation and fluid movement.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ISOTOPIC SOURCING OF LEAD IN THE ILLAWARRA REGION, NSW, AUSTRALIA Bryan E. Chenhall, Massimo Chiaradia*, Brian L. Gulson*, Brian G. Jones and Aviars M. Depers School of Geosciences, University of Wollongong, Wollongong, 2522, Australia *CSIRO, Division of Exploration and Mining, PO Box 136, North Ryde, 2113, Australia

In recent years, scientific and public awareness about the impact of the toxic heavy metal lead on the natural environment has increased significantly and a variety of techniques have been developed to identify and source lead pollution. Traditionally, lead isotopes have been used to place constraints on the age of geological units but they can also potentially be utilised to 'fingerprint' sources of lead pollution. Lead in the atmosphere may be present in particulate emissions from an industrial source or may be in the form of aerosols from vehicle exhaust emissions. The latter may bond to natural or anthropogenic particulate matter and be deposited as fallout. Composition of this particulate matter can be analysed using either short term (sampling over hours or days) or longer term sampling strategies. The aim of the current study is to apply lead isotopes to the identification of sources of lead from fallout and lead in sediments from two water bodies adjacent to a major industrial complex.

The Illawarra region has undergone continued industrial and urban expansion since European settlement and currently is host to the Port Kembla industrial complex. This complex includes a major steelworks (BHP Steel International), a base metal refining plant (Southern Copper), diversified associated secondary industries, and coal loading and grain handling facilities. In the current study airborne particulate matter, known from microscopic analysis to contain a significant quantity of industrially- and/or urban-derived components, was extracted from the roof cavities of houses in the Illawarra region. This sampling protocol was designed to provide a long-term (up to 65 year) record of lead pollution in the region.

Sample contamination (i.e. by lead from lead-based paint) was minimised by collecting samples from unmodified dwellings with tiled roofs, followed by microscopic screening of the samples. All samples were analysed after HN03:HC1 digestion by thermal ionisation mass spectrometry (TIMS) at the Centre for Isotope Studies. TIMS analyses yielded an isotopically-consistent data set with ^Pb/^Pb ratios of 17.01±0.21, irrespective of the dwelling age and total mass of lead present. No consistent pattern of change in isotopic composition with distance from the Port Kembla industrial complex could be determined, but less radiogenic (i.e. 'older') lead was observed in a few dwellings distant from this facility. No single known point source can fully account for the isotopic uniformity shown by the airborne particulate matter, which is believed to represent an 'isotopic cocktail' of Palaeozoic lead (mainly from Southern Copper) and less radiogenic lead from vehicle exhaust emissions, with a possible minor contribution of lead from iron and steel making.

Lead-bearing particulate matter added to the natural environment accumulates in low energy areas such as standing water bodies. The Port Kembla Inner Harbour and Lake Illawarra are both situated close to the industrial complex and the accumulated sediment can be tested for anthropogenic components by considering the lead isotope composition. The Inner Harbour receives direct runoff from the industrial complex, especially the steelworks, whereas Lake Illawarra is a coastal lagoon that currently only receives fallout particulate matter.

Lead in sediments from the Inner Harbour is isotopically distinctive (^Pb/^Pb ca. 18.29) and clearly matches the particulate emission signature from the adjacent iron and steel making operation.

In contrast, sediments in Lake Illawarra contain an isotopically mixed lead signature. Selective extraction techniques, using ammonium acetate (to obtain ion exchangeable metals) and HF digestion of the residue, failed to distinguish potential sources of anthropogenic lead. The isotopic compositions of these extracts were essentially similar to those obtained by standard HN03:HC1 digestion. The study identified four sources of lead in the sediments. Lead from natural sources, such as the local Permian basaltic and coal measure sequences has a typical Palaeozoic signature (^Pb/^Pb ca. 18.70). This composition is recognised in the lower parts of sediment cores which were deposited prior to European settlement in the Illawarra region. Base metal smelting late last century at a site near the northwest margin of Lake Illawarra (Kanahooka) produced a distinctive isotopic signature (^Pb/^Pb ca, 16.20-16.30) attributed to smelting ore from Broken Hill. This source can be traced from the smelting site towards the lagoon, where it becomes obscured by lead from other anthropogenic sources. Lead isotope-depth profiling in Lake Illawarra strata indicated progressive sediment contamination by an isotopically-homogeneous, though non-unique, lead signature from two additional sources. For the portion of the core that has accumulated since the start of industrialisation, mass balance isotopic calculations for the lower portion of this profile suggest progressive contamination by an airborne (and waterborne) source of particulate matter containing both Palaeozoic base metal and vehicle exhaust components. Significantly, the upper 20 cm of sediment in Lake Illawarra preserves a relatively-uniform, mass-independent isotopic signature (^Pb/^Pb ca. 17.60) consistent with this contaminant source. The content of soluble lead increases towards the water-sediment interface also indicating increased anthropogenic loading.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 CHLORINE-36 IN AUSTRALIAN RAINFALL AND PLAYAS AND THE RESIDENCE TIME OF CHLORIDE IN THE AUSTRALIAN LANDSCAPE Allan R. Chivas . Melita D. Keywood , L. Keith Fifield and Garry L. Allan 1

2

3

3

^School of Geosciences, University of Wollongong, Wollongong NSW 2522 Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia; ^Nuclear Physics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200 2

Bulk deposition for a series of 15 rainfall collectors sampled at seasonal (3-monthly) intervals over four years (September 1988-December 1992) has been analysed for and major and trace ions. Several collectors of varying construction were deployed at each site to monitor evaporation (ie. collection under paraffin oil), and to allow for analysis of 834 and The collectors for 36 1 measurement are stainless steel bins with stainless steel funnels, within which a 5-litre glass vessel is situated. Ten sites comprise the southeastern array that forms an arc from the eastern coast at Kioloa to central New South Wales to the south coast near Adelaide. The remaining 5 sites near Birdsville in central Australia sample the driest part of the continent. Minor constituents, particularly PO^-, NO3", NH4 , and S O a r e used to assess 'baseline' conditions, and reject collections with agricultural contamination (soil, fertilizer).

The east coast site (Kioloa, lat. 35.5°S) shows a range of ^Cl/Cl ratios for individual collection periods from 6 x 10" to 115 x 10" with an inverse relationship to total chloride content (13.5 to 1.2 mg/L); and a mean weighted fallout of 35 atoms 36ci/m /s. Inland stations from the southeastern array (34°S) have a mean, but variable fallout of -25 atoms ^Cl/m^/s, whereas those from central Australia (~25°S) have a mean flux of -13 atoms ^Cl/m^/s.

Modern salt lakes (playas) are abundant in the western two-thirds of the continent. Halite from these lakes has 36CI/CI ratios of 28 x 10"^ to 150 x 10"^- The ^Cl/Cl values in lakes from the Lake Eyre drainage basin (coincident with the central rain-collector array) are low even though this is an area where bulk atmospheric deposition has ^^Cl/Cl ratios of 150 to 500 x 10" . In Lake Eyre North, the surface halite from Madigan Gulf has a C1/C1 ratio of 33 ± 4 x 10-15, at 60 cm depth the ratio is 40 ± 13 x and in the probable late Pleistocene (?last glacial maximum) halite (at 130cm depth) the ratio is 35 + 6 x 1 0 " T h e majority of chloride in Lake Eyre has a long residence time and ancient ultimate sources, ie. the constantly redissolving and reprecipitating salt-crust of Lake Eyre and discharge/leakage from the underlying Great Artesian Basin. The meteoric component is barely recognisable in this system. However, a simple test indicating the presence of meteoric chloride in the same general area is provided by surficial halite from a small isolated salt pan, without apparent groundwater discharge, near Lake Eyre. In this case the 3 6 a c i ratio is 117 + 14 x 10" .

S

C

+

15

1

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36

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15

Surficial halite samples from playas across the Yilgarn Block indicate increasing a / C l 3 6

values from southeast to northwest from 30 x 10"^ to 78 x 10"^ with the trend continuing to central Australia where ratios of up to 144 x 10-15 are recorded. The trend of increasing -^Cl/Cl values with increasing distance from the coast mimics, but at lower 36ci/Cl values, modern rainfall collected along a traverse across the northern Yilgarn Block from Leeman via Wiluna to Everard Junction. The rainfall -^Cl/Cl ratios collected seasonally for two years have mean values of -8 x 10"^ at the coast to -400 x lO" ^ t Everard Junction. This correspondence indicates that a principal source of surficial chloride in the western half of the continent is of meteoric origin with an integrated residence time in the landscape of ~1 Ma. Chloride with a significant 'old' component (similar to that of Lake Eyre) is present in other large surficial drainage basins, e.g. Lake Disappointment at the terminus of Savory Creek and also in the Amadeus Basin where chloride from the Bitter Springs Formation is implied to be present in some surficial playas.

Thus surficial chloride in the Australian landscape displays a variety of 'ages' (residence times), although, with the exception of small contributions from bedrock evaporites (e.g. Bitter Springs Formation) in generally limited areas, the ultimate origin is meteoric i.e. atmospheric via delivery to the continent from marine aerosols.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A GIS LINKED TO THE WORLD WIDE WEB PRODUCES CUSTOM GEOSCIENCE MAPS Prame Chopra and Peter Miller Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

The Australian National Geoscience Information System (@ngis) will provide improved links to geoscience information for Australia and its offshore and marine territories. The @ngis initiative will, when fully implemented, involve the collaboration of a wide range of geoscience information custodians in government agencies, universities, research organisations, companies and industry groups. The intention is not to duplicate existing holdings and access mechanisms but rather to build linkages between custodians and those seeking information. The preferred delivery mechanism for @ngis at present is the World Wide Web and the @ngis home page can be found at URL http://www.agso.gov.au/ngis.

The Australian Geological Survey Organisation (AGSO) is coordinating the development of @ngis in consultation with many interested people and organisations. As part of this work AGSO is developing prototype information query and delivery systems for geoscience data on the World Wide Web (WWW). Aspects of these prototypes may be used by other organisations to provide linkages to their information within the @ngis framework. One of the key components that must be provided if geoscience data and information are to be available from @ngis is geological maps. To be really useful, these maps must be user configurable in terms of geographic extent, scale and content. Thus it must be possible for a user to combine a number of geoscience themes such as lithological contacts, faults, geomagnetic contours, and regolith cover with other information such as roads, towns, topography and drainage. Moreover, the user should be able to zoom and pan within the map as it is displayed on-line to specify the scale and geographic extent of the map.

Functionality like this is readily available in a modern geographic information system (GIS) such as the Arc/Info software used by AGSO and many other geoscience organisations in Australia. This software is not however well suited to access through the WWW for a number of reasons: The software is very complex and considerable training is needed to use it effectively. It is very expensive to purchase and when working with very large datasets, it must be hosted on expensive computer hardware. The GIS cannot output in hypertext markup language (html) - the lingua franca of the WWW. Hence remote users would have to be provided with Telnet access to the GIS which has unacceptable security implications for organisations. For these reasons, an interface must be provided between the remote user on the WWW and the GIS. We describe a prototype interface between the WWW and the Arc/Info GIS and demonstrate its use in producing user customisable on-demand "maps of geoscience data held in AGSO's Arc/Info GIS.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

BUILDING AND MANAGING A GEOSCIENCE SITE ON THE WORLD W I D E WEB Prame Chopra and Peter Miller Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

AGSO is using a unique devolved development environment for its World Wide Web (WWW) site (URL http://www.agso.gov.au/) which encourages individual contributions from AGSO staff whilst at the same time guaranteeing the integrity of the on-line system. The procedures used are based on change control management practises like those used in the computer software industry and are akin to the well-known publication paradigm that is familiar to all geoscientists. Material for publication on the WWW server is written by individuals, reviewed by peers and then corrected as necessary by an editor in order to adhere to the "house style." Much of the necessary communication between authors, reviewers and editors uses automatically generated electronic mail (email). This speeds up the whole publication process and also keeps AGSO users of the system informed about newly available and pending developments.

The change control management software used with the AGSO WWW server is known as Aegis. Aegis recognises the individuals involved in the publication of material on the AGSO WWW server as belonging to one or more of the following categories: developers, reviewers, integrators and administrators. Each category has a prescribed set of powers and responsibilities.

Developers are responsible for the creation and development of material to be published on the AGSO WWW server (hereafter called the server). In this way, developers are like the authors in the normal publication paradigm. While developers may collaborate in writing material for the server, ultimately, one developer will be responsible for the submission of the material for publication. This developer can be thought of as the senior author of a normal geoscience publication.

Reviewers are the equivalent of the peer reviewers known so well to anyone used to publishing in the scientific literature. In the AGSO Aegis system, any one of a list of accredited reviewers may review a submitted hypertext manuscript. These manuscripts are either passed by the reviewer or returned to the author/s for revision and email is generated accordingly.

The role of the integrators is to act like the editor of a journal. Thus the integrator's task is to make sure that the material that has passed peer review adheres to the journalistic standards expected. In AGSO's case, this means that the text meets the appropriate standard, that the size and clarity of any images are suitable, and that icons are of the standard size and format.

Administrators are responsible for the operation of the Aegis software and for the granting and revoking of privileges (e.g. for giving a new aspiring author developer privileges, or for removing a reviewer's name from the list when necessary).

Aegis provides each developer with a separate working environment within the AGSO computer system for their WWW authoring work. These environments are separate from the "baseline" which contains the core hypertext markup language files for the on-line system but each includes read-only links to all of the baseline files. In this way, the integrity of the on-line system is guaranteed while at the same time, all developers have access to all of the files. Any of the baseline files can be copied into a development area for editing or augmenting. Aegis then allows a developer to build a complete test system that has all of the functionality of the on-line system plus the specific changes made by the developer. This ability to meld development work with the full system is invaluable during the development and testing of new material for the server.

Developers can work in parallel on projects even if these projects are working with the same baseline files.

Aegis keeps track of any conflicts and ensures that these are resolved. Thus with Aegis, AGSO is able to develop Web pages on its WWW server in a devolved manner. Development work does not have to be channelled through a few individuals with the inevitable bottlenecks that would arise.

The Aegis software for Sun 4 and 5 UNIX computers is publicly available from the AGSO anonymous ftp server (ftp://ftp.agso.gov.au/pub/Aegis).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

DEVONIAN-CARBONIFEROUS-PERMIAN ORGANIC BUILDUPS AS DYNAMIC INDICATORS OF TECTONICS AND SEDIMENTARY DEVELOPMENT IN THE URALS. B. L Chuvashov Institute of Geology and Geochemistry of the Russian Academy of Sciences Ekateringburg, Russia

There are two grandiose Early - Middle Devonian reef belts in the Western and Eastern Urals. The first of them was connected with a rim of carbonate platform. Reefs of the Eastern belt formed near Island Arcs and above collisional uplifting blocks. Frasnian organic buildups of the Eastern Urals were situated along the borders of flysch trough. A border of reef-like rings bound the extensive Late Devonian uplifts, which have been separated by the relatively deep and narrow depressions. This kind of a tectonic relief results from the collision between the Eastern - European platform and the Kasakh continent. The reef building processes were suppressed during the Early and Middle Carboniferous because of high level of the world ocean.

Numerous organic buildups were situated along the Preduralyan Foredeep during the Late Carboniferous - Early Permian. There are two belts of organic buildups. Near-shore rather small organic buildups are traced along the eastern border of the Preduralyan Foredeep.

It was constructed by stromatolites, calcareous algae, paleoaplysinas, bryozoans, brachiopods, tube-forming worms (for Kungurian only).

Another belt of more grandiose reefs there were formed along the western border of the Preduralyan Foredeep, exactly on the boundary between Foredeep and platform. Both groups of organic buildups were separated by a deep water, up to 70 km wide.

The boundaries of the Preduralyan Foredeep during the Middle Carboniferous were variously displaced, along different parts of Foredeep, in a westerly direction. There are some regularities in distribution of organic buildups according to the timing of a widening and displacement of Foredeep axis: 1) an organic buildup could appear if there was no movement (or a stop) in process; 2) thickness and maturity of organic buildups is dependent upon the duration of the break in displacement of Foredeep axis; 3) rapid widening and numerous short-time stops of Foredeep could result in some (up to 12) generations of small organic buildups. By contrast, the result of long-time stop was a very thick (up to 1200 m) reef.

89

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SHRIMP ZIRCON AGE CONTROL OF LATE PALAEOZOIC TIME Jonathan Claou6-Long* and Clinton Foster1 * Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601

Carboniferous and Permian time is the focus of active research using the new methods of ^Ar/^Ar analysis and SHRIMP zircon dating. These techniques are able to date the thin and heavily altered volcanic horizons in marine sequences by selective analysis of preserved igneous minerals (sanidine, zircon), and by separating the age signatures of eruption, inheritance, overprinting, and isotopic leakage. Major revisions of the durations and divisions of Late Palaeozoic sequences are being produced As an example, Figure 1 shows the effect of work now in progress on the ages of Permian stages. The various Permian timescales in use worldwide today are based on data with huge uncertainties; in some cases the stratigraphic range of samples used to define ages within the Permian is anywhere from the Devonian to the Jurassic. All the available published Permian time scales have been estimated by interpolation of this uncertain data set.

The five small filled rectangles in Figure 1 illustrate the precision of new SHRIMP zircon U-Pb ages for volcanic horizons in the classic name-giving Permian sections in the Urals and in China. From this well controlled matching of ages and type biozonation, much more precise timing control becomes possible in Permian biostratigraphy and basins. Besides being more precise, the new time scale is radically different from the published scales in general use. The effects of using the revised ages of biozones include large changes to geohistory modelling in Permian basins.

In related work, isotopic dating is achieving new correlations of biostratigraphically barren sedimentary packages. SHRIMP zircon dating is establishing relationships between Gondwanan faunas of Palaeozoic eastern Australia and wam-climate Tethyan equivalents in Europe, and is correlating the Late Palaeozoic continental glaciation of Gondwana with the European glacio-eustatic coal bearing systems.

TRIASSIC

PERMIAN

CARBON -IFEROUS;

SHRIMP Ages

(1995)

350 300 250 200Ma Figure 1. The error boxes in this diagram show the uncertainty in isotopic age (x-axis) and biostratigraphic range (y-axis) of the data from which the Permian timescale is constructed. Pre-1991, the scale was estimated from data with the huge uncertainties represented by the hollow rectangles. The five small filled rectanges illustrate the precision of new SHRIMP zircon agesfromtype biostratigraphic sections in Russia and China.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

BROWSE BASIN AND KIMBERLEY BLOCK CRUSTAL STRUCTURE DEFINED FROM COINCIDENT REFRACTION AND REFLECTION SEISMIC DATA C.D.N. Collins1, A.G. Goncharov1, P.A. Symonds l and I.S. Lukaszyk1 Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2600

The Australian Geological Survey Organisation (AGSO) acquired 3400 km of deep (16 s TWT) seismic reflection profiles over the Browse Basin on the North West Shelf in 1993. These data generally show excellent structural detail in the sedimentary section down to about 5-7 seconds, and in some areas good reflections down to the crust- mantle boundary. However, in common with reflection profiles world-wide, the deep basement, midto deep crust and crust-mantle boundary is in many places poorly defined, if at all. Moreover, without additional information which relates directly to rock type, such as seismic velocity, the interpretation of many deep reflection boundaries remains speculative. The seismic velocity is also essential to convert the reflection boundaries to depth. Such primary parameters as total thickness of the sedimentary section, variation of crustal thickness, and the type of lithology represented by the deep reflection images (eg. are reflective bodies volcanic?) have major implications for models of the tectonic structure and history of the North West Australian rifted margin.

The reflection method cannot give reliable velocities at depths below a few kilometres. This is primarily because of the relatively short length of the recording spread compared to the depths of the boundaries, the lack of coherent reflectors within basement, and the low energy of vertical reflections from deep boundaries. These limitations can be overcome by recording out to large distances from the shots with portable recording systems.

The reflected energy increases as the angle of incidence on the boundary increases at large distances, and these wide-angle reflections are generally more coherent at the lower frequencies recorded by portable recorders. Also, at sufficiently large distances, refracted energy is recorded which has travelled within the crustal layers providing a direct measure of the seismic velocity within the layers.

During the reflection profiling by AGSO's Rig Seismic, portable seismic recording stations were deployed along the Kimberley coast, on Mitchell Plateau and on Browse Island to record the ship's air-guns. These provided wide-angle and refraction data from a wide range of distances and azimuths; good data were recorded to over 250 km along some lines. The shot-to-recorder geometry is not ideal due to the limited locations available for land stations. However, the reflection profiles were used to constrain the geometry of the basin margins and major basin sequences down to 10-15 km depth. The data were forward modelled by ray-tracing through a starting model. This model was derived from sea bed topography, well data and other geological constraints, and the sedimentary basin geometry from the reflection profiles, combined with a simple plane-layer inversion of the refraction data at each station. The model was then tested for consistency with the vertical profiles by comparison with vertical reflections computed from the model.

The crust of the Kimberley Block is interpreted as having three major layers. The upper layer corresponds to the Kimberley Basin, and has an average velocity of 5.8 km/s. The basement to the Kimberley Basin lies at a depth of about 8 km, and has an average velocity of 6.2 km/s. A series of high apparent-velocity arrivals are probably due to thin high-velocity layers or bodies within the Kimberley basement. These bodies may be seen in the reflection section as high amplitude, laterally discontinuous reflections and diffractions. The lower crust, at a depth of about 17 km, has an average velocity of 6.8 km/s. This increases to about 7.4 km/s beneath the eastern margin of the Browse Basin where it coincides with a zone of westward dipping reflectors. This high velocity zone may correspond to mafic intrusives or underplate associated with the formation of the landward boundary of the basin. The base of the crust, which is about 35 km deep in the east, is marked by a transition zone with high velocities averaging about 7.0 km/s.

The crustal thickness decreases from approximately 35 km under the Kimberley coast to about 23 km under the Browse Basin. Sediment velocities range from 2.5 to 4.8 km/s, and the maximum sediment thickness within the basin may be about 12 km. The underlying crystalline crust is therefore about 11 km thick. If the pre-existing crust was 35 km thick, as in the present day eastern margin of the Basin, thinning by a factor of at least three has occurred during the basin-forming events.

Acknowledgement: This paper is published with the permission of the Executive Director of the Australian Geological Survey Organisation.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PERVASIVE MAGMA TRANSFER THROUGH THE LOWER-MIDDLE CRUST DURING NON-COAXIAL COMPRESSIONAL DEFORMATION: AN ALTERNATIVE TO DYKING WJ. Collins and E.W. Sawyer ^Department of Geology, University of Newcastle, Newcastle, NSW, 2308, Australia.

Sciences de la Terre, University du Quebec, Chicoutimi, Quebec G7H 2B1, Canada.

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Granitic magmas migrated through the middle-lower crust at Mt Hay, via a diverse network of narrow structurally-controlled channelways, during a period of progressive W-SW directed thrusting (Dia-Djj).

They utilized existing folds, boudins and shear zones, or created new channels by magmatic fracture either parallel to layering or, rarely, in irregular arrays. The magmas rose obliquely, parallel to the plunging (5060°) regional elongation direction, which was defined by coaxial folds, boudin necks and a strong elongation lineation, rather than vertically* Magacrystic charnockitic magmas migrated through metre-scale conduits during Di -Dib> but leucosomes were generally restricted to smaller (cm-scale) structures that existed throughout the entire deformation history. Thus, D^/D^^ leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma, thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting.

The upper-middle crust of the Anmatjira-Reynolds Range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at Mt Hay. They are considered to have formed as syntectonic intrusions during W-SW directed thrusting, similar to Mt Hay. Thus, granitic magmas formed near the base of the continental crust are believed by us to have passed through the mid-lower crustal level (25-30 km) exposed at Mt Hay and accumulated, in batholithic proportions, at shallower crustal levels (12-20 km) such as the Anmatjira-Reynolds Range area.

The observations demonstrate that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive, non-coaxial shear deformation associated with W-SW directed thrusting.. Localization of magmas by sequentially-developed, narrow, compressive structures suggests that dilatancy followed successive foliation-forming events, a situation that can occur during steady state deformation if the effective confining pressures are low (Zhang et al. 1994). The abundant evidence for hydraulic fracturing during deformation at Mt Hay and the localization of melts around anisotropics are consistent with local (transient?) high magma pressures. Taking into account the probability that magmas rapidly segregate at low melt fraction from their source (Sawyer, 1994) and the rapid rates of magma migration through conduits (Petford et al., 1993a) relative to strain rates during deformation (Paterson & Tobisch, 1992), we consider that fluid pressure driven dilatancy, possibly associated with a fluctuating magma supply, is likely to generate the apparent magma pressure variations in the deep crustal, structurally-controlled channelways of the Arunta Inlier. A direct implication is that magma chambers will not form in the deep continental crust a

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REFERENCES Paterson, S.R. and Tobisch, O.T., 1992. Rates of processes in magmatic arcs: implications for the timing and nature of pluton emplacement and wall rock deformation. Journal of Structural Geology 14, 291300.

Petford, N., Kerr, R.C. & Lister, J.R., 1993. Dike transport of granitoid magmas. Geology, 21,845-848.

Sawyer, E.W., 1994. Melt segregation in the continental crust Geology, 22,1019-1022.

Zhang, S., Cox, S.F. and Paterson, M.S., 1994. The influence of room temperature deformation on porosity and permeability in calcite aggregates. Journal of Geophysical Research, 99,15761-15775. y

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EARLY DEVONIAN STRATIGRAPHY AND PALAEOGEOGRAPHY OF THE CAPERTEE HIGH, NORTHEASTERN LACHLAN FOLD BELT Gary P. Colquhoun, School of Geosciences, University of Wollongong, Wollongong NSW 2522*

The Capertee High in the Mudgee-Capertee region was a Late Silurian to earliest Middle Devonian shallow marine to subaerial palaeogeographic unit to the E of the Hill End Trough. Rocks of the Capertee High largely occur as structurally complex sequences along with Ordovician basement, thick Late Devonian siliciclastic sequences of the Lambie Group, and Middle Carboniferous granitic intrusions. Flat-lying Permian-Mesozoic sequences overlap the High in the E and N. Broadly, a two-fold subdivision of the Early Devonian sequences associated with the Capertee High is apparent: Platform Sequences were deposited on the main shelf areas of the High and are dominated by the ~4000 m thick Kandos Group. Recent conodont dating of this sequence of fine- to coarse-grained siliciclastics, carbonates, and silicic volcanics indicates it spanned almost all of the Early Devonian. The Kandos Group is most fully exposed in the Rylstone-Cudgegong district where it progressively onlaps the Silurian sequence; however, a correlative of the Pragian Riversdale Volcanics (silicic pyroclastics and volcaniclastics) occurs in the Capertee Valley (Huntingdale Volcanics), and the early Emsian Carwell Creek Formation (shallow marine siliciclastics and carbonates) outcrops almost continuously from the southern Capertee Valley (Myrtle Grove Formation) to northeast of Mudgee. Western Platform Margin Sequences consist of siliciclastic and carbonate mass-flow deposits, hemipelagites and rare silicic to intermediate volcanics deposited in slope and base-of-slope settings adjacent to the western margin of the High. These sequences crop out in the Queens Pinch Group south and east of Mudgee, in the Limekilns Formations at Limekilns, and in the Kingsford Formation near Hford. Most units are well dated by conodonts, allowing correlation with platform sequences; correlation with the units of the eastern Hill End Trough is more difficult as their ages are comparatively poorly constrained. Sedimentation in marginal High sequences is linked to episodes of transgression, regression, erosion and volcanism on the platform areas of the High.

Palaeogeography and Sedimentation: Lochkovian platform sequences are known only from the RylstoneCudgegong area, although very similar fades were deposited 150 km to the S in the Lochkovian Tangerang Formation. The Lochkovian portion of the Kandos Group is a transgressive to regressive shallow marine sequence deposited on a W- to SW-sloping shelf flanked to the E by eroding sedimentary basement and active silicic volcanoes, and passing to the W into deepwater facies along the western margin of the High (Mullamuddy Formation) and in the eastern Hill End Trough (upper Crudine Group). Basal fan deltaic clastics (Warrah Conglomerate) are overlain by mainly biostromal carbonates (Clandulla Limestone) which, in turn, are overlain by the muddy shelf deposits of the Yellowmans Creek Formation. The thick regressive siliciclastic sequences of the storm-dominated Roxburgh Formation overlie the Yellowmans Creek Formation and possibly extend into the earliest Pragian. Facies prograded W during the late Lochkovian regression, culminating in shelf exposure and some erosion in the E. During the Pragian, relative low stands of sea level dominated and shelf areas of the High were largely subaerial, apart from a narrow shallow marine shelf margin. Voluminous silicic pyroclastic volcanism dominated the shelf area and calderas, probably centred southeast of Cudgegong and in the eastern Capertee Valley, were flanked by extensive ignimbrite sheets. Short-lived transgressions on the platform resulted in minor carbonate deposition during volcanically quiet periods. Vast volumes of volcanic detritus were transported to the W and deposited as low-stand fans and aprons along the margin of the Capertee High and in the Hill End Trough.

Transgression in the latest Pragian-earliest Emsian partly eroded, then drowned the existing volcanic topography; a retrograding complex of shallow marine shelf and shoreline siliciclastics and carbonates was then deposited in barrier island to open marine settings in the platform areas. Some silicic volcanoes remained active, perhaps forming islands on the shallow shelf. Concurrent sedimentation in the marginal High sequences was dominated by fossiliferous condensed mudstone sequences. During the mid to late Emsian, at least two major regressions caused erosion of early Emsian platform carbonates and siliciclastics; these were redeposited in mass-flow submarine fans and aprons adjacent to the W margin. Minor silicic volcanism occurred in these marginal High areas at this time. Carbonate deposition on the platform continued into the earliest Eifelian (at Mt Frome) before being replaced by shallow marine siliciclastics of the Boogledie Formation. Deposition was probably terminated in the early Eifelian by uplift associated with the weak Middle Devonian Tabberabberan deformation.

•Present address: Geological Survey of New South Wales, P.O. Box 536, St Leonards NSW 2065 93

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE MUD GEE 1:100,000 SHEET: STRATIGRAPHY, STRUCTURE, AND MINERALISATION Garv Colquhoun , Simone Meakin , Jan Rrynen , John Walkins , Tony Henderson , and Elizabeth Jagodzinski 'Geological Survey of New South Wales, P.O. Box 536, St Leonards NSW 2065 Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2600 1

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The Mudgee 1:100,000 Geological Sheet forms the most southeasterly of the six sheets comprising the Dubbo 1:250,000 Geological Sheet and encompasses the northeastern margin of the exposed Lachlan Fold Belt and the western margin of the Sydney Basin. Compilation of existing geological data and field mapping of the Mudgee 1:100,000 Sheet were undertaken in January to June 1995 by the Geological Survey of New South Wales and AGSO as part of the National Geoscience Mapping Accord. Mapping was aided by airborne magnetic and radiometric dataflownin 1991 at a 400 m line-spacing. The stratigraphy and structure differ significantlyfromthose embodied in the original Dubbo sheet (1st. ed., 1971); salient features of the new map include: Ordovician: major changes to the extent and structure of the Ordovician units, particularly the volcanic units of the Cabonne Group which are currently significant exploration targets for Cu-Au mineralisation. The Lue beds previously considered Silurian - have been recognised as Ordovician (see Fergussonand Colquhoun, this volume) and divided into the Early Ordovician Adaminaby Group (quartz tuibidites) and the Late Ordovician-?Early Silurian Coomber Formation (mafic volcaniclastics, lavas, intrusives and mudstone). The mafic-volcaniclastic Burranah Formation, previously regarded as Early Devonian, contains a Late Ordovician coral fauna in allochthonous limestones and has a similar radiometric signature to the Coomber Formation; it is suspected this unit may have been the source of much of the alluvial gold in the nearby Gulgong goldfields.North of Aarons Pass, the mapped outcrop and structure of the Sofala Volcanics has altered considerably.

Silurian: inclusion and definition of the Tannabutta Group, a Capertee High unit of Late Silurian shallow marine clastics, carbonates and silicic volcanics outcropping between Mudgee and Cudgegong in the west, and Botobolar and Kandos in the east. The silicic volcanics of the Group contain a number of massive sulphide and vein gold prospects. A small area of the Wenlockian Tanwarra Shale (Mumbil Group) has been mapped to the east of the Sofala Volcanics, west of Hford. In the Hill End Trough strata, the turbiditic Chesleigi Formation (Mumbil Group) was subdivided into a lower unit of slate and lithic sandstone and an upper unit of predominantly volcaniclastic sandstone which includes two concordant rhyolite to dacite and coarse volcaniclastic horizons, the upper formerly known as the Nulling Formation.

Devonian: the complex Early Devonian shallow marine to subaerial sequences of the Kandos Group (see Colquhoun, this volume) - mostly undifferentiated on the original sheet - have been mapped in detail, largely following the work of University of Wollongong students and staff. The fault-bounded Early Devonian sequences south of Mudgee (the Queens Pinch Group) remain little changedfromthe original map; however, some constituent units of the Group have been traced northwards to the southern outskirts of Mudgee, and new units are recognised north of Mudgee (Tinja Formation) and near Dford (Kingsford Formation). The stratigraphy and structure of the Crudine Group of the Hill End Trough have been modified: the Lana Formation was found to be a junior synonym of the Turondale Formation, and the Dunmoogin and Guroba Formations are lateral fades equivalents of the Wateibeach Formation. The four-fold subdivision of the Late Devonian Lambie Group remains unchanged from the original sheet; however, recent mapping has increased knowledge of the complex structure to the west of Lue, where an en-echelon fault system cuts the sequence into several openly-folded blocks.

Carboniferous-Permian: Mid Carboniferous granites of the Mudgee Sheet lack the concentric radiometric and magnetic zonation discovered in many plutons of the Bathurst Batholith. Instead the radiometrics highlighted anomalous zones in the Aarons Pass (328 Ma) and Camboon Granites of relative thorium enrichment and depletion separated by sharp, linear contacts; these features are yet to be explained. In addition, magnetic data indicate the Aarons Pass and Havilah Granites have considerable shallow sub-surface extent and an associated network of mostly unexposed dykes. Outcrop of the earliest Permian (292 Ma) Rylstone Volcanics along the edge of the Sydney Basin has been refined and extended. This unit now constitutes an important exploration target following the discovery of a major epithermal vein and disseminated Ag-Pb-Zn deposit in the unit near Lue. Hill End Trough strata are characterised by tight, gently-plunging folds with plunge reversals common over several kilometres. Capertee Zone rocks vary from open to isoclinally folded and contain several generations of reverse and normal faults. One major deformation (D ) affected the sheet area in the Early Carboniferous, resulting in folding, thrusting and cleavage development Evidence for earlier deformation (Dj) is meagre and restricted to steeply plunging folds and lineations in the Ordovician Adaminaby Group. Areas of kinking and folding of S cleavage in the Lue-Havilah area indicate significant D deformation (probably also Early Carboniferous). Major changes in the regional bedding and cleavage trends occur across lineations in the Bocoble and Havilah areas, suggesting significant north-south compression postdating D but un-timed with respect to D .

Disconformities between the Ordovician-Silurian and Silurian-Devonian sequences suggest erosion and exposure at these times, due either to gentle uplift or sea-level fall.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

K-METASOMATISM AND Cu-Zn-Pb DEPLETION IN THE TAWALLAH GROUP VOLCANICS, MCARTHUR BASIN - IMPLICATIONS FOR BASE METAL MINERALISATION David R. Cooke , Stuart Bull , Jamie R. Rogers and Serena Donovan Codes Key Centre, University of Tasmania, GPO Box 252C, Hobart, 7001 RGC Exploration, PO. Box 20, Zeehan, 7469, Tasmania 1

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INTRODUCTION Petrographic and geochemical evidence for regional scale brine migration is preserved in Palaeoproterozoic volcanic units of the Upper Tawallah Group in the McArthur Basin. In the Mallapunyah Dome area, the Settlement Creek Volcanics (SCV) are comprised mostly of massive fine- to medium-grained dolerite sills.

Locally extensive monolithic, pebble- to cobble- sized doleritic breccias are interpreted to represent quenched and/or mechanically brecciated sill margins. The overlying Wollogorang Formation consists of fluvial and lacustrine dolomites, shales and sandstones. Stromatolitic bioherms and locally abundant halite casts indicate an evaporitic depositional environment. Dolerite sills of the Gold Creek Volcanic (GCV) have intruded the top of the Wollogorang Formation at several localities on the flanks of Mallapunyah Dome. Heterolithic volcanic breccias that are stratigraphically equivalent to the Gold Creek Volcanics contain clasts of underlying units set in a terrigenous muddy matrix, and are interpreted as epiclastic debris flow deposits.

PETROGRAPHY SCV and GCV dolerites have been altered to pervasive potassic and chlorite-orthoclase alteration assemblages at Mallapunyah Dome. The two alteration types can be distinguished visually on the basis of colour, and on orthoclase and chlorite content. Potassic altered samples are pink due to abundant hematite-dusted orthoclase.

Chlorite-orthoclase altered samples contain visible chlorite and range in colourfromdark green-black (chloriterich) to green-pink (chlorite + orthoclase rich). Potassic alteration is characterised by orthoclase + quartz ± sericite ± hematite ± dolomite ± anatase ± barite. Three varieties of intense potassic alteration are recognised: (1) texturally destructive vein-related potassic alteration halos; (2) texturally destructive pervasive potassic alteration; and (3) texturally destructive potassic alterationrinds(< 1 cm thick) that surround amygdules.

WHOLE ROCK GEOCHEMISTRY K-metasomatism has resulted in significant compositional changes to the SCV dolerites, which now contain up to 11.8 wt % K 0, 68.4 wt % Si0 and between 0.4 and 23.5 wt % Fe20 , The dolerites have been depleted in MnO, MgO, CaO, Na 0, P 0 , Cr, Ba and all analysed REE except Eu Significant base metal depletion occurred as a result of K-metasomatism (3-12 ppm Cu; 4-18 ppm Zn; 2-6 ppm Pb) compared to orthoclasechlorite-altered samplesfromMallapunyah Dome (6-308 ppm Cu; 27-137 ppm Zn; 3-59 ppm Pb) and Bauhinia Downs (34-42 ppm Cu; 182-366 ppm Zn, 53-151 ppm Pb).

FLUID INCLUSIONS AND STABLE ISOTOPES Two-phase primary fluid inclusions in quartz- and dolomite-filled amygdules and veins enclosed by potassic alteration halos have homogenisation temperatures between 71.3 and 122.9°C (mean: 103.4°C), and final ice melting temperatures between -19.8 and -41.3°C. The presence of primary halite- and hydrocarbon-bearing fluid inclusions are interpreted to indicate that saline evaporite-derived brines were involved in amygdule formation and K-metasomatism. Oxygen and carbon isotopic analyses of dolomite veins and amygdules in the SCV associated with potassic alteration are characterised by heavy 5 0 MOW) values (+12.4 to +28.8%©) and light 8 C ) values (-2.0 to -4.3%©). Calculated 5 0 and 5 C ) values are = -1%© and -7%© respectively, consistent with an evolved meteoric ± seawaterfluidsource.

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i3

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(PDB

(fluid)

(S

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CONCLUSIONS K-metasomatism occurred when diagenetic basinal brines migrated down from the Wollogorang Formation and overlying units into the SCV sometime after sill emplacement. The brines were low temperature, saline and hydrocarbon-bearing. Potassic alteration formed at high water-rock ratios, resulting in leaching of most chemical components from the volcanics, including significant quantities of Cu and Zn. These metalliferous brines may have been important for the formation of base metal mineralisation in the Upper Tawallah and McArthur Groups. Preliminary numerical modelling has indicated that metalliferous brines produced during potassic alteration of the SCV had the potential to produce economic base metal mineralisation, provided an appropriate trap was encountered.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra:, February 1996 TELLURIDE MINERALISATION IN LOW SULFTDATION EPITHERMAL VEINS CONTRIBUTIONS OF MAGMATIC VOLATELES David R. Cooke and Derry C. McPhail Codes Key Centre, University of Tasmania, GPO Box 252C, Hobart, 7001 Department of Earth Sciences, Monash University , Clayton, 3168, Victoria 1

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INTRODUCTION Tellurides are a minor component of many low sulfidation epithermal Au veins in the Southwest Pacific.

Examples include Acupan (Philippines), Tongyoung (Korea), Wild Dog, Kerimenge and Lihir (Papua New Guinea), Lebong Tandai (Indonesia) and several deposits in the Hauraki Goldfield (New Zealand). Gold tellurides are the principal ore minerals at Emperor (Fiji).

TELLURIUM SPECIATION AND DEPOSITIONAL MECHANISMS Results of thermodynamic modelling using new data for aqueous Te species (McPhail, 1995) have important implications for the origin(s) of Au-Ag telluride mineralisation in low sulfidation veins. At temperatures of 250300°C, the predominant aqueous tellurium species in chloride waters in equilibrium with quartz, pyrite, muscovite and/or K-feldspar are predicted to be H Te , HTe% or (in some cases) H Te0 q). Numerical simulations indicate that cooling is the most important depositional process for Au-Ag tellurides, whereas boiling-induced H S loss is the most effective mechanism for electrum precipitation.

VOLATILE TRANSPORT 2

(aq)

2

3(a

2

Concentrations of aqueous tellurium species in chloride waters are predicted to be too low for significant aqueous Te transport within the low sulfidation environment (Cooke et al. 1996). Although little thermodynamic data are available, we also consider gold and silver telluride complexes to be unimportant for Te transport. Significant concentrations of Te are probably introduced into the epithermal environment as gaseous species such as H Te(g) or Te ), with deep-seated magmatic gases rising and condensing into circulating chloride waters, causing increased /(Te^ resulting in telluride precipitation.

Shelton et al. (1990) proposed that an increase in Te2 fugacities at the Tongyoung deposit resulted from the inundation of progressively cooler meteoric fluids, without any contributions of magmatic volatiles. We prefer a magmatic fluid source for Te in epithermal veins, because it is difficult to envisage why progressive meteoric flushing would result in highly localised Te enrichment, especially when telluride solubilities are predicted to decrease by 2-3 orders of magnitude when chloride waters cool from 300° to 250°C. ?

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2(g

Contributions of magmatic waters were probably not significant in most western Pacific low sulfidation deposits, because this would cause increased salinities and enrichment of O in the mineralising fluids, features that have not been noted from fluid inclusion and stable isotope studies. Instead, entrainment of magmatic gases (C0 , H S and Te ) from a deep-seated crystallising K-rich pluton into circulating chloride waters is considered the most likely mechanism for Te enrichment. Condensation of magmatic C0 , H S and Te into the predominantly meteoric chloride waters resulted in local increases of/(jeo>/(H S) ^ / ( C O ^ causing telluride, carbonate and sulfide deposition. This model is consistent with measured magmatic carbon and sulfur isotopic signatures. Acid-generating gases such as S0 and HC1 could not have been introduced in significant quantities to low sulfidation vein systems, because hypogene advanced argillic alteration assemblages are absent ls

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REFERENCES Cooke, D.R., M Phail, D.C., and Bloom, M.S., 1996. Epithermal gold mineralisation, Acupan, Baguio District, Philippines: Geology, mineralisation, alteration and the thermochemical environment of ore deposition.

Econ. Geol. (in press).

M Phail, D.C., 1995. Thermodynamic properties of aqueous tellurium species between 25°C and 350°C. Geochim. et. Cosmochim. Acta, 59: 851-866.

Shelton, K.L., So, C., Haeussler, G.T., Chi, S. and Lee, K., 1990. Geochemical studies of the Tongyoung goldsilver deposits, Republic of Korea: Evidence of meteoric water dominance in a Te-bearing epithermal system. Econ. Geol., 85:1114-1132. c

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

N E O P R O T E R O Z O I C GLACIAL EPISODES IN THE KIMBERLEY N O R T H W E S T E R N AUSTRALIA

REGION,

M. Corkeron1. K. Grey 2 , Z. X. Li1 and C. McA. Powell1 * University of Western Australia, Nedlands, WA, 6907 ^Geological Survey of Western Australia, Plain Street, Perth, 6001

Earth's Neoproterozoic (1000-545 Ma) history is punctuated by several glacial episodes as interpreted from glaciogene strata from several continents. In Australia, two Neoproterozoic glacial intervals have been recognised and described from the Adelaide Geosyncline and central Australia (Amadeus, Georgina, Ngalia, Officer and Savory basins). These are the Sturtian (-700 Ma) and Marinoan (-600 Ma) glaciations.

Neoproterozoic glaciogene successions were first identified in three separate areas in the Kimberley region of NW Australia during BMR regional mapping in the 1960s - East Kimberley-Victoria River, Mt Ramsay and Mt House areas. It is only in the Mt Ramsay area that two glacial intervals are preserved-together as represented by the Landrigan Tillite which is overlain by marine sedimentary rocks, and the Egan Formation which unconformably overlies these lower units. Opposing intraregional correlations made for the Kimberley glaciogene units by Plumb and Gemuts (1976) and Coats and Preiss (1980), both assumed that the two recognised Kimberley glacial episodes correlated with the Sturtian and Marinoan glacial episodes of the Adelaide Geosyncline. Revised correlation (Plumb, 1996) supports the original intraregional correlation of Plumb and Gemuts (1976) and has significant implications on regional correlation with the South Australian glacial episodes (Plumb, 1996).

Further studies of the Kimberley glacial successions are being carried out by the University of Western Australia and the Geological Society of WA. As part of these studies detailed field analysis was carried out on the Egan Formation in the Mt Ramsay area, and results support the intraregional correlation of Plumb and Gemuts (1976), and suggest that the Egan Formation is younger than the previously recognised Marinoan continental glaciation in South Australia. Detailed stratigraphic sections from the Egan Formation in the O'Donnell and Louisa Synclines show a carbonate-dominated unit (including stromatolitic bioherms), with associated diamictite and fluvial sedimentary rocks very distinct from all other glacial units in the Kimberley region. The diamictite has a localised and variable distribution. The carbonates are thick units of thinly laminated dolostone, significantly different from the 'cap' dolomites associated with the other Kimberley glacial units and with the Marinoan 'cap' dolomites of central Australia and the Adelaide Geosyncline. Moreover, preliminary identification of the stromatolite Tungussia julia from the formation indicates a biostratigraphic correlation with the Julie Formation of the Amadeus Basin and upper Wonoka Formation of the Adelaide Geosyncline, implying an episode of glaciation younger than the Yerelina Subgroup tillites of the Marinoan in South Australia.

The Egan Formation has a relatively isolated distribution and variable diamictite content, and therefore can be interpreted as a localised glaciation. However, similar end-Neoproterozoic glacial episodes have been suggested for the Adelaidean succession and have also been reported in other continents. The occurrence of such glacial events near the end of the Neoproterozoic could be significant in defining a third late Neoproterozoic global glacial event.

REFERENCES Coats, R. P. & Preiss, W. V., 1980. Stratigraphic and geochronological reinterpretation of Late Proterozoic glaciogenic sequences in the Kimberley region, Western Australia. Precambrian Research, 13, 253-268.

Plumb, K. A., 1996. Revised correlation of Neoproterozoic glacial successions from the Kimberley region, northwestern Australia. This volume.

Plumb, K. A., & Gemuts I., 1976. Precambrian geology of the Kimberley region, Western Australia.

International Geological Congress, Excursion Guide 44c.

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25th

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

WORLD-WIDE-WEB ACCESS TO INTEGRATED AUSTRALIAN GEODYNAMIC DATA Simon J D Cox Australian Geodynamics Cooperative Research Centre CSIRO Exploration & Mining, PO Box 437 Nedlands, WA 6009

The principal outcomes of the Australian Geodynamics Cooperative Research Centre (AGCRC) will comprise a variety of products, including but not restricted to maps and sections, 3D models, 4D reconstructions, and databases. While many conventional printed and published reports will be prepared from these, it is intended that a significant proportion of the results will be made available in digital form, so that the exploration community will be able to access the information and incorporate it into their own research, including further modelling exercises.

MODES FOR DELIVERY OF DIGITAL DATA For much of the information, it will only make sense to provide the full data in a native format of the software used in its preparation. In these cases, further use of the information will require access to the same specific software packages, which may involve both hardware and licensing considerations. However, it is intended that key or summary results from most of the AGCRC's research will be provided in more generic formats and will be accessible on-line through the World-Wide-Web (WWW). While much of the information will be open to anyone with WWW access, there are also straightforward methods for restricting access to subsets of the data.

The opportunities for innovative provision of information on the WWW have been widely recognised. As well as on-line mixed text and graphics presented in the current generation of HTML hypertext documents, most HTTP browsers also incorporate tools to capture images or text, and FTP file transfer. The emergence of the new VRML standard further enhances the capabilities of the Web to handle 3D information. However, most information on the WWW is organised as files or images which contain predetermined quanta of data. Thus, users can find themselves downloading large files in which most of the data is surplus to their requirements.

Also, indexing is normally fixed through a scheme determined by the custodian, so the relevant data may not be listed where the user expects it, and so may not be easily located. A challenge in the effective use of the WWW by information providers is to provide tools to overcome these limitations.

SELECTION OF DATA USING AN ON-LINE GIS Much of the information resulting from AGCRC research can be effectively presented, or at least indexed, through maps. The first WWW tool to be implemented is a Web-hosted Geographic Information System (GIS), based on the US Army's GRASS system, through a customisation of the GRASSLINKS interface developed at U.C. Berkeley. This allows a theme or set of themes to be selected from a list, for any specified area and resolution. Thus, only the information required is actually presented. A resulting map can be examined as an on-screen image, or downloaded as a PostScript file. As well as map generation, a subset of the capabilities of the GRASS system, including analytical tools, is available using the Forms interface of HTML. Although the GRASS system has some limitations, particularly in its object database management capabilities, its open Unixbased design makes programming a WWW interface straightforward using standard CGI methods.

INDEXING THROUGH METADATA TERMS This GIS method requires that the relevant information be added to the GRASS database on the Web Server running the GRASSLINKS software, and so is still reliant on a custodian's intervention for maintenance.

Alternatively information can be placed on local servers as HTML documents. In this case the format of the information will be determined by the person posting it. Although the central AGCRC Web Server will be aware of the other servers hosting AGCRC results, the custodian of the central AGCRC Web Server may not be explicidy notified of changes and additions, so an automatic method for finding information of interest is needed.

Information describing the content of an HTML document may be embedded in the header block of the HTML source using the <TITLE> and <META> tags. These fields can be used to provide a dynamic index for a WWW search engine, which enables specific information to be rapidly located, with the user constructing a "query" according to their own needs of the data. The principal requirement for effective use of this method is an agreed set of metadata items and a dictionary of terms to be used as key-words and for location. This latter approach is also expected to be incorporated in the @ngis system being coordinated by AGSO. It is intended that the AGCRC Web will be fully compatible and integrated with @ngis.

The AGCRC World-Wide-Web address is URL= http://www.ned.dem.csiro.au/AGCRC/ . All the developments referred to in this paper can or will be accessible from this location.

Acknowledgments: Published by permission of the Director of the Australian Geodynamics Cooperative Research Centre.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

DISCOVERY 2000 — TURBOCHARGING MINERAL EXPLORATION IN NEW SOUTH WALES J N Cramsie, Director Geological Survey of New South Wales, PO Box 536 St Leonards NSW 2065

The Discovery 2000 Program is an important government initiative aimed at providing a major boost to the mineral and petroleum industries in New South Wales. The objective of Discovery 2000 is to promote exploration by providing an outstanding framework of geological and geophysical information. High quality regional aeromagnetic data is the cornerstone of modern regional exploration and is the major component of "Discovery 2000". About 460 000 line kilometres of airborne surveys have been flown so far in support of mineral exploration, with a further 267 000 line kilometres in support of petroleum exploration. The Discovery 2000 Program has been developed and implemented in close consultation with industry.

THE DISCOVERY 2000 MINERALS PROGRAMS The Discovery 2000 minerals program is focusing on acquisition of new data for: • • •

extensions to the known mineralized belts; greenfields areas not previously considered because of poor exposure or thin cover obscuring evidence of mineralization; and The Broken Hill Exploration Initiative, promoting the discovery of new ore deposits in the Broken Hill region to sustain this major regional mining centre. This is a National Geoscience Mapping Accord project.

Another important component of the program is the enhancement of the Department's geoscientific and exploration databases. Particular attention is being paid to drainage geochemistry, exploration drillholes, mineral deposits, gravity surveys, and petrophysical properties. During 1994/95 the Discovery 2000 minerals program targeted four regions — Parkes/Nyngan, Bourke, Koonenberry, and Broken Hill. In 1995/96 the focus will be on the Cargelligo area.

RESULTS OF DISCOVERY 2000 YEAR 1 The Parkes/Nyngan project area extends north from Parkes, covering the Narromine and part of the Nyngan 1:250 000 sheets. The Ordovician shoshonitic volcanics of the Parkes-Narromine Volcanic Belt are broadly distributed through the Narromine sheet and extend north under cover on the Nyngan sheet. This belt hosts the Northparkes mine, and proposed mines at Peak Hill and Lake Cowal. The recently released high resolution airborne geophysics has dramatically improved knowledge of both the nature and extent of this highly attractive package of volcanic rocks.

The Bourke-Brewarrina project focuses on the northern extension of the Darling Basin, which includes rocks of the Cobar Supergroup. This sequence hosts the Elura, CSA and Peak deposits. The recently released high resolution surveys have enabled these prospective sequences to be traced north of Cobar towards Bourke, under a shallow blanket of younger sediments and soils.

The Koonenberry Belt lies to the east and northeast of Broken Hill, runningfromthe Scopes Range through to Tibooburra. The extensive areas of deformed Proterozoic and Palaeozoic rocks that are present in this belt have not been thoroughly studied or explored in the past, partly due to poor outcrop and the relative remoteness of the area.

The Broken Hill project is being conducted as a joint program with the Commonwealth's Australian Geological Survey Organization (AGSO) and Minerals and Energy South Australia (MESA). A major program of mapping by Department of Mineral Resources geologists has provided a framework for interpretation of the very detailed geophysical data which has been provided by AGSO. CARGELLIGO PROJECT High resolution airborne geophysical surveys and gravity are planned for the Cargelligo 1:250 000 sheet and some adjacent areas during 1995/96. Mineralized Lachlan Fold Belt units crop out in the eastern part of the survey region. Siluro-Devonian sediments and volcanics are prospective for base metals and gold, while a tinbearing granite belt extends across the project region.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ENHANCING THE PROSPECTIVITY OF VICTORIA: IDENTIFICATION OF MOUNT READ VOLCANICS CORRELATIVES IN WESTERN V I C T O R I A : A. J. Crawford^. A.G. Donaghy 1 , L.P. Black 2 , P.G. Stuart-Smith 2 , 1: Dcpt of Geology, University of Tasmania, GPO Box 252C, Hobart, TAS 7001 2: AG SO, Divn/of Regional Geology and Minerals, GPO Box 378, ACT 2601

The Middle and Late Cambrian Mount Read Volcanics of western Tasmania host five world-class, Au-rich VHMS deposits, and numerous smaller deposits. New generation detailed geological mapping by K.D. Corbett and colleagues at Mineral Resources Tasmania has provided a new framework for exploration, and the last 10 years has seen 5-10 million dollars spent annually on exploration of the Mount Read Volcanics belt.

The Mount Read Volcanics are a basalt-andesite-dacite-rhyolite suite with affinities varying from medium-K calcalkaline, through high-K calc-alkaline to shoshonitic. Geological and tectonic considerations suggest that they formed shortly after an arc-continent collision -510-515 Ma, during which extensive boninite-bearing ophiolites were emplaced westward onto an east-facing latest Neoproterozoic passive margin. Post-collisional extension and rebound exhumation of underthrust, thinned passive margin crystalline crust formed the Precambrian Tyennan nucleus of Tasmania. Grabens formed along the margin(s) of the rapidly-rising Tyennan block were the focus of strong lithospheric extension, and controlled regional eruption of the Mount Read Volcanics between -503 and 498 Ma. The final phase of Mount Read Volcanics magmatism is represented by the felsic lavas, tuffs and volcaniclastics of the Tyndall Group, which Nd isotopic data (Whitford and Crawford, in prep.) indicate may be largely crusal melts. These may have formed when extension slowed and mafic magmas were forced to pool in the crust, which subsequently partially melted.

Andesites and more felsic rocks of presumed Cambrian age occur in several NNW-trending subparallel and discontinuous belts in western Victoria, termed the Stavely Greenstone Belt. The largest outcrop area of these rocks, forming the Mount Stavely Volcanic Complex (MSVC), plunges beneath the Siluro-Devonian redbeds of the Grampians Ranges to the north, and beneath Mesozoic and Tertiary sediments and volcanics of the Otway Basin and Newer Volcanics to the south. The MSVC is fault-bounded on the west, and probably also on the east, against folded, unfossiliferous quartz-rich flysch of the Glenthompson Beds. New SHRIMP U-Pb dates on zircons separated from several andesites and dacites in the MSVC range from 501±9 Ma to 495±5 Ma. Geochemical studies of best-preserved samples show the MSVC to be an andesite-dacite-rhyolite package with medium- to high-K calc-alkaline affinities, and strong HREE and Y depletion, suggesting affinities with adakitic lavas. Basalts have yet to be recognized. The uppermost unit in the MSVC, the Towanway Tuff, is compositionally distinct from other felsic rocks in the MSVC, and is probably largely a crustal melt, although isoptoic studies on this suite are yet to be done. A significant feature of the MSVC is the presence of long, narrow fault slices of serpentinized ultramafic rocks that are broadly concordant with, though locally crosscutting, the local stratigraphy within the MSVC. Relict chromites in the serpentinites have distinctive very high-Cr, low-Al compositions, and match those from the boninite-bearing ophiolites in western Tasmania and the Heathcote and Mt Wellington Greenstone Belts further east in Victoria. Geophysical modelling suggests that the MSVC ultramafics occur along sole thrusts of west-dipping thrust slices, matching the structural style of the greenstones along the Heathcote and Mt Wellington Greenstone Belts. Importantly, recent drilling has confirmed the presence of boninitic lavas and cumulates northwest of the MSVC and west of the Grampians, at Wartook, and indicates that the boundary between the Lachlan Foldbelt and the Adelaide (Kanmantoo) Foldbelt is clearly significantly further west that the Avoca Fault.

We argue that the striking age- and geochemical-petrological similarities of the MSVC and boninitic ophiolites in western Victoria with the Mount Read Volcanics and adjacent ophiolites in western Tasmania is strong evidence to support a correlation of these Middle and Late Cambrian units across Bass Strait. The presence of slices of ophiolitic serpentinized boninitic cumulates thrust up into the MSVC argues that the MSVC is likely to be a post-collisional magmatic suite, with a similar origin and setting to the Mount Read Volcanics. Based on analogy with the MRV, we suggest that the Stavely Greenstone Belt is a prime exploration target for Cambrian Au-rich VHMS deposits such as the Rosebery, Que River and Hellyer deposits in western Tasmania. Much of the >300km length of this aeromagnetically-defined belt extends NNW-ward beneath the Murray Basin. At the southern end of the belt, the Murray Basin sediments are only 0-20m thick, but they thicken to more than 600m at Mildura, where the belt swings off to a NNE-trend around the southern margin of the Willyama Block. The challenge for explorers will be to geophysically identify the prospective andesite-dacite-rhyolite volcanics beneath the shallow, saltwater-laden sediments of the southern Murray Basin, and to discriminate and filter out the strong magnetic signal imposed by slices of serpentinized, unmineralized ophiolitic rocks.

Acknowledgments: We thank Harry Horvath and Wayne O'Neill (North Ltd, Exploration) for their support and interest in this work.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 COMPOSITIONAL AND OTHER CHARACTERISTICS OF SOME SKARN MAGNETITES A.J. Cross and K.G. McQueen University of Canberra, PO Box 1 Belconnen, ACT 2616 1

1

Magnetite is a common constituent of oxidised contact replacement skarns. It is also a widespread accessory mineral in a variety of rocks and many other different types of ore deposits. Discrimination of skarn magnetite from magnetite derived from other sources is potentially important in skarn exploration, particularly during drainage and soil sampling surveys.

Magnetite can incorporate a wide range of minor and trace elements which partly reflect the conditions of crystallisation/equilibrium and fluid and host rock compositions. Growth conditions, incorporation of other mineral inclusions, exsolution and other replacement phenomena can also give rise to a range of textural characteristics in magnetite.

This study examines the minor element geochemistry and textural characteristics of magnetites in a number of magnetite-bearing skarns from a range of different geological environments. The skarn magnetites generally have relatively high concentrations of Mn and Mg, low Ca and very low Ti Cr and V compared to magnetites from commonly associated igneous rocks. Some also show variable but relatively high concentrations of Zn and Si. Some of these compositional characteristics, for example the high Mn and low Ca, may reflect varying partitioning behaviour of abundant elements during skarn development and replacement of limestone. Other elements such as Zn are probably related to introduction by hydrothermal mineralising fluids. Compositional discrimination of skarn magnetite from magnetite in associated igneous rocks can simply be achieved by using the combination of Mn, V, Mg and Ca contents plotted on ternary diagrams (figure 1).

Skarn magnetites typically show massive or blocky-granular textural characteristics. They lack the ilmenite exsolution lamellae common in magnetite from granitic sources and the acicular or dendritic-skeletatal forms common in volcanic rocks. In some cases skarn magnetites contain incorporated inclusions and penetrating veins and aggregates of other skarn minerals including sulfides such as chalcopyrite, sphalerite, galena and bismuth minerals.

Mn

Mn

[ o ] MtBiggenden skarn magnetite [ A ] Paddy's River skarn magnetite

a On Shan skarn magnetite [ T ] Ertsberg skarn magnetite M

Figure 1. Ternary diagrams showing the compositional characteristics of skarn magnetitesfromthe Mt Biggenden (Queensland), Ma On Shan (Hong Kong), Paddy's River (ACT) and Ertsberg skarns (Irian Jaya). The Labelled fields show compositions for diferent magnetites: A is skarn magnetite, B is felsic volcanic-derived magnetite associated with the Paddy's River skarns (ACT) and C is granite-derived magnetite.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THERMOTECTONIC EVOLUTION AND CRUSTAL LINEAMENT ANALYSIS OF NEW GUINEA Peter V. Crowhurst. Richard D. Kendrick, Kevin C. Hill, David A. Foster, Paul B. O'Sullivan Australian Geodynamics Cooperative Research Centre VIEPS La Trobe University, Bundoora, Vic. 3083

Thermochronological and geophysical techniques are currently being used to delineate the tectonic history of New Guinea. Emphasis is on identifying major fault blocks and their tectonic histories in the context of mineral and hydrocarbon exploration. The results reveal that the Early Miocene was probably a period of rollback of the subducting slab beneath New Guinea which placed the northern margin into extension, creating starved graben and causing regional subsidence. Extension resulted in the exhumation of metamorphic core complexes adjacent to the graben This caused middle crustal rocks to cool rapidly from temperatures >500'C in the New Guinea Mobile Belt The core complexes show rapid cooling from between ~25 and 20 Ma. Continued subduction beneath New Guinea resulted in formation of the Marimuni Arc in the Middle Miocene and the end of extension. In the Late Miocene, collision of the Melanesian Arc caused regional uplift of all basement of northern Papua New Guinea, mainly from 8-5 Ma, causing at least 3-4 km of denudation. The compressional deformation propagated southward causing uplift, denudation and cooling in the Papuan and Irian Jaya Fold Belts starting at -4 Ma and continuing to the present The identificationfromgravity and surface geology of NE-SW trending lineaments at a high angle to the trend of the major tectonic belts (figure 1) is a key feature in interpreting regional deformation. Such lineaments may be deep-seated basement structures (continental transforms) that controlled structure and facies distribution during Mesozoic passive margin rifting and during episodes of Tertiary crustal shortening and extension. These lineaments may have had important influences on the initiation and localisation of magma intrusion, primarily at intersections of lineaments where dilational zones possibly host mineralisation. Differential offset across the major faults may affect hydrocarbon source and reservoir distribution, and trap style, while differential vertical offset results in variable source rock maturity. Pre-existing extensional faults apparently have exerted both a strong influence on the initiation of basement inversion, and acted as lateral ramps to compartmentalise inverted blocks and separate them from areas of thin-skinned thrusting. Thermochronology, coupled with structural studies provide insight into both the timing and significance of these lineaments.

Arafura Sea Arafura Platform

Irian Jaya

Figure 1. Proposed lineaments of south-central New Guinea.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MEGABRECCIA BLOCKS AND CALDERAS IN THE SILURIAN GOOBARRAGANDRA VOLCANICS, SOUTHEASTERN NSW Kelsie A. Dadd Key Centre, Geochemical Evolution and Metallogeny of the Continents, School of Earth Sciences, Macquarie University, NSW 2109 and Department of Applied Geology, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007

Megabreccias made up of blocks of limestone, interbedded limestone and chert, and siltstone which are embedded in tuff have recently been identified in the Silurian Goobarragandra Volcanics. The Volcanics dominantly comprise an extensive uniform succession of crystal-rich dacitic tuffs but include some epiclastic rocks, minor lava flows and mafic and felsic intrusives. The crystal-rich nature of the tuffs, rarely preserved shards, welding textures, perlitic cracking, and a uniform nature over large areas indicate they were emplaced as ash-flow tuffs and were originally substantially welded and glassy and hence the depositional environment was most likely subaerial.

Blocks within the megabreccia have discordant contacts with the surrounding tuff and lack any indication of gradational relationships between blocks and tuff. Thus, large outcrops of massive fossiliferous limestone are surrounded by tuff without sign of mixed rocks or, for example, adjoining reef-flank facies. The age of most blocks is uncertain, but some siltstone-dominant blocks contain Late Ordovician graptolites; an age significantly older than the surrounding tuffs. Although contacts are rarely exposed they indicate that siltstone and dacite do not foim conformable sequences, that the siltstone was lithified and that the dacite was hot when it contacted the siltstone. Siltstone is locally baked at the contact with the tuff and in places is brecciated and intruded by veins of dacite and the tuff is locally chilled against the siltstone.

The blocks within the Goobarragandra Volcanics were previously considered to be conformably interstratified with the tuff and were used to indicate the structure of the volcanic sequence and to place constraints on its environment of deposition. The presence of limestone was taken to indicate a marine depositional environment for at least the adjacent tuff although there is little or no other corroborative evidence.

The term megabreccia is used in volcanology to describe a unit "in which many clasts are larger than 1 m in diameter, and the clastic nature of the deposit is obscure in many individual outcrops" (Lipman, 1976, p. 1398).

Lipman suggested that megabreccia is dominant in the lower part of caldera-fill sequences, is especially thick adjacent to caldera walls, and accumulates during intense stages of ash-flow eruptions. The presence of megabreccia in the Goobarragandra Volcanics suggests that the host tuffs are the fill of a large, eroded caldera, consistent with the inferred great diickness of the ash-flow sheets.

The wide distribution of the ash flow tuffs and the comagmatic Young Granodiorite suggests that the eruptive complex consisted of several calderas and that the Goobarragandra Volcanics include both caldera-fill and outflow deposits. The tuff and the granodiorite, however, are uniform in composition and texture over large areas and these features cannot be used to separate the product of one caldera from another. However megabreccia blocks of a particular lithology tend to be concentrated in discrete areas and are likely derived from a proximal source. Mapping the distribution of block lithologies may lead to delineation of separate caldera structures.

The graptolite-bearing blocks in some megabreccia units indicate that the Goobarragandra Volcanics erupted through a basement which included a Late Ordovician sequence. These blocks are tentatively correlated with the Warbisco Shale of eastern Victoria and southeastern NSW which has a mid-Gisbomian to mid-Bolindian range. This unit probably extended beneath the present outcrop of the Goobarragandra Volcanics and was exposed in the steep caldera walls early in the development of the caldera complex.

REFERENCES Lipman, P.W., 1976. Caldera-collapse breccias in the western San Juan Mountains, Colorado. Geological Society of America Bulletin. 87,1397-1410.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A NEW CONCEPTUAL MODEL FOR SALINITY PROCESSES ON THE EASTERN DUNDAS TABLELANDS, VICTORIA - IMPLICATIONS FOR THE LAND MANAGER. Peter Dahlhaus and Richard MacEwan University of Ballarat, P.O. Box 663 Ballarat Vic 3353

The Dundas Tablelands (DTL), bounded by the Grampians Range, the Glenelg and Wannon Rivers, form the western extreme of the Victorian Central Highlands. The Yarramyljup Fault Zone bisects the tablelands such that the Cambro-Ordovician Glenelg River Complex underlies the western portion, and the Silurian Rocklands Rhyolite and Grampians Group sediments underlie the eastern portion. The DTL comprise an extensive, upwarped, laterised and youthfully dissected palaeoplain. The greater part of the laterite has formed in situ on the Mesozoic-Tertiary planation surface.

To the east of the Yarramyljup Fault Zone the Grampians Group underlies the Rocklands Rhyolite (Simpson & Woodfiil, 1994). The Grampians Group underwent deformation before the sequence of silicic volcanism which produced the ignimbrites, lavas and dykes of the Rocklands Rhyolite. The rhyolite is fractured by well developed columnar jointing which continues through the weathering rock horizon and into the laterite profile.

Detailed studies of the groundwater and salinity have been conducted by: i) Lewis (1985) who suggested a shallow, local groundwater system in the regolith and attributed saline discharge to increased recharge flushing cyclic salt following native vegetation clearing; ii) Jerinic (1993) who proposed a multi-layered groundwater system with a local, fresh water, perched aquifer in the lateritic gravels; a saline pallid zone/weathering zone aquifer; and a less saline aquifer in the unweathered rhyolite; and iii) Woof (1994) who, on the basis of detailed chemical analyses, concluded that although there is a difference in the hydraulic properties of the zones, the groundwater is stored within a single connected aquifer incorporating the lateritic profile and the unweathered rhyolite. Elemental/ionic ratios in the groundwater also indicated that the major source of salt is from the weathering of the rhyolite, and is not cyclic.

We propose a groundwater system in which the Grampians Group sandstone underlying the Rocklands Rhyolite is acting as a semi-confined aquifer, leaking into the overlying rhyolite. Recharge occurs through the Grampians, Black and Dundas Ranges. The groundwater movement may be strongly influenced by granite intrusions underlying the rhyolite obstructing flow through the Grampians Group and feeding into the overlying rhyolite. This would account for the observed areas of higher saline discharge (eg. the Dundas River catchment), the lower salinities in the deeper bores, and the uniformity of the groundwater chemistry. A drilling program has been proposed to test the hypothesis.

We contend that the recharge through the Grampians Group sediments has remained relatively unchanged since European settlement and that clearing the DTL of native vegetation will have decreased soil water use and increased waterlogging and runoff. The hydraulic properties of the regolith are such that excess water will move dominantly by interflow and throughflow thereby increasing the frequency and extent of waterlogging in low lying and break-of-slope areas. Spread of salinity in the DTL is therefore more likely to be caused by waterlogging than by groundwater recharge. Consequently, salinity management in the DTL should be directed by an understanding of waterlogging processes. Appropriate remedial practices are planting of productive high water use pastures, installation of surface and subsoil drainage, and tree planting to intercept throughflow.

Dryland Salinity has always and will always be present in the DTL, the community expectation that it can be removed by planting trees or pastures to control recharge is unjustified.

REFERENCES Jerinic F.L., 1993. A hydrogeological study of dryland salinity in the Bulart region, western Victoria.

B. Ap.Sci.(Hons) thesis. Ballarat University College, Ballarat (unpubl.)

Lewis, M.F., 1985. Factors affecting the development of dryland salinity in a catchment on the Dundas Tableland, western Victoria. Land Protection Division, Department of Conservation, Forests & Lands, Melbourne.

Simpson, C.J. & Woodfull, C.J., 1994. New field evidence resolving the relationship between the Grampians Group and the Rocklands Rhyolite, western Victoria. Australian Journal of Earth Sciences 31, 761-777 Woof, C.P., 1994. Geological and hydrogeological mechanisms of salinisation at Bulart. B.Ap.Sci.(Hons) thesis University of Ballarat, Ballarat (unpubl.)

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NEW EXPLORATION MODELS FOR THE GAWLER CRATON FROM HIGH QUALITY AEROMAGNETIC DATA S.J.Daly1, MC. Fairclough2 and C.M. Fanning3 2department of Mines and Energy, South Australia. P.O. Box

151, Eastwood, SA 5063 Regional Exploration, Mount Edon Gold Mines.P.O. Box 133, Greenwood, WA 6024.

Research School of Earth Sciences, ANU. GPO Box 4 Canberra ACT 2601

High resolution aeromagnetic images of the western Gawler Craton show that it is dominated by a broad curvilinear belt of anastomosing shear zones broadly subparallel to the Karari Fault Zone. Within and adjacent to these major shears are a number of crustal blocks which have different origins and metamorphic histories. Development of Proterozoic shears, and some reactivation of interpreted Archaean precursors, has resulted in recycling and re-equilibration of Archaean and Proterozoic paragneisses of the Gawler Craton.

These shear zones have also acted as foci for multiphase acid and basic intrusives and extrusives with potential Au/Cu and Ni mineralisation and possible remobilisation of Archaean Au. Southeast of the Karari Fault Zone the aeromagnetic images show lateral rotation of complexly folded Archaean gneisses, with ductile both dextral and later major sinistral movements. Higher level brittle fractures within Archaean gneisses are host to the new Challenger Au prospect. It is suspected that the Archaean Au has been remobilised into higher level Proterozoic structures. Extensive development of these fractures indicates the potential for further discoveries.

Further south, prominent aeromagnetic and gravity highs separated* by anastomosing shears, comprise the Fowler Suture Zone (Fairclough el al 1994, MESA digital dataset). The complex aeromagnetic signature of the Fowler Suture Zone presents a dramatic contrast to the magnetically quiet Archaean rocks to the north and east Although largely strike-slip movements are evident in the aeromagnetic images of the Fowler Suture Zone and Karari Fault Zone this is not consistent with outcrop scale observations. Stretching lineations are all steeper than 60-70 0 to the north within sub-vertical foliation planes. Rare S-C fabrics are consistent with an east block up dip-slip movement There is no evidence of a discrete dip-slip overprint or superimposed fabrics. Such observations indicate that sinistral strike-slip movement is locally and spatially only a minor component of the deformation, and that to achieve the enormous displacement apparent in aeromagnetic images, an extreme degree of associated dip-slip movement is required within the Fowler Suture Zone.

Bleeker (1990, Ph.D Thesis, New Brunswick University) describes a similar contradiction between geophysical and geological evidence in the Thompson Nickel Belt, Canada. Detailed structural analysis in that region has revealed that transpressional deformation, with strain partitioning into broad dip-slip and intense narrow strike slip domains, can account for the apparent discrepancy. Geophysical images show many geometric similarities to the Thompson region. It is suggested that transpressional collision can also account for regional structural features apparent in the FSZ with broad basement zones deformed by dip-slip strain accounting for most of the rare outcrop, separated by volumetrically minor (non-outcropping) zones of concentrated strike-slip deformation.

The Fowler Suture Zone consists of Palaeoproterozoic meta-igneous calc-alkaline cumulus gabbro-diorite and tonalite. Ultramafic rocks, pelitic schists and BEF have also been intersected in drillcore. The aeromagnetic geometries of the polyphase mafic and ultramafic rocks indicate that they were emplaced during the development of the Fowler Suture Zone. It is possible that the oldest mafic bodies may have been seafloor sills intruded into the original Proterozoic basin. The sediments and associated mafic bodies and structural setting, as discussed above is very similar to the Thompson Nickel Belt, Manitoba.

The age of the Fowler Suture Zone is constrained between 1730-1540Ma, significantly younger than previously first thought. The younger age implies a post Hiltaba Suite deformational event in the west Gawler Craton. Large multiphase granitoid plutons occur to the southeast of the Fowler Suture Zone and are probably of similar age and lithology to the mildly deformed 1630Ma St Peter Suite. Undeformed plutons have been interpreted as ca.l585Ma Hiltaba Suite, however it is likely that some faulted and foliated granitoids may also be related to this Suite. The Hiltaba Suite and comagmatic Gawler Range Volcanics are the source of Au/Cu anomalism within the Gawler Craton. It is proposed here that these mildly deformed plutons and surrounding fractured basement represent excellent Au/Cu exploration targets.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE MANTLE DYNAMICAL REPERTOIRE, TECTONIC EVOLUTION AND MAGMATISM Geoff Davies Research School of Earth Sciences, Australian National University, Canberra ACT 0200

Calculations of the thermal evolution of the mantle based on purely thermal convection yield very smooth changes, which are difficult to reconcile with the apparently episodic accumulation of continental crust and with observed changes in tectonic style through earth history. The effects of chemistry on the density of the lithosphere, both at the surface and after subduction, can change the dynamics of plates and mantle convection, resulting in a repertoire of mantle behaviour that may explain the observed tectonic evolution. Thus major mantle overturns may have occurred during the Archean and possibly the Proterozoic, with dramatic tectonic and magmatic effects, and plate tectonics may not have worked in its modern form when the mantle was hotter.

The earth's tectonic regime is governed by the way the mantle gets rid of its internal heat. Currently, this is by plate tectonics, which is a form of thermal convection: the plates and their associated "plate-scale" flow are driven by the negative thermal buoyancy of the plates, and the cycle of plate formation at a spreading center, cooling, subduction and reheating in the mantle accounts for most of the heat lost from the mantle. Thus the plate-scale flow is the dominant form of mantle convection.

Because the oceanic crust is less dense than the mantle, the present oceanic lithosphere is buoyant until it is about 15 Ma old. When the mantle was hotter, the oceanic crust was thicker, and the plates would have been buoyant for longer. As well, the hotter mantle would be less viscous and convect faster, so plates would be younger when they reached a subduction zone. At a mantle temperature only about 50°C higher than at present, plates would still be neutrally buoyant when they reached a trench (Fig. 1), on average, and so they would not want to subduct until they had aged further. At higher mantle temperatures, plates could have subducted at a slower rate, but not fast enough to cool the mantle. The viability of subduction might be extended by the transition of basalt to eclogite if plates penetrate to sufficient depth. It is likely in this case that plate tectonics would be more episodic in a hotter mantle, and might not be the dominant heat removal mechanism.

Phase transformations in the mantle transition zone might also induce episodic behaviour by temporarily blocking flow between the upper and lower mantle. Thermal evolution calculations indicate that in this case the upper mantle would cool rapidly and the lower mantle would heat, until the layering became unstable (Fig. 2).

At that point the cooler material would drain into the lower mantle and be replaced by material possibly up to 300°C hotter from depth. This would cause a major tectonic and magmatic convulsion, equivalent to about 100 flood basalt eruptions within roughly 10 Ma. Such events might account for Archean granite-greenstone formation episodes, and might have had major effects on the atmosphere and on the evolution of life.

As internal radioactivity declined, plates would become slower, thicker and more able to penetrate the phase transformation barrier. Calculations indicate that mantle layering would be broken down by the Phanerozoic (Fig. 2). The present regime would then be whole-mantle convection, consistent with geophysical evidence, but with the mantle still close to layering conditions. Local and transient interruptions of flow through the transition zone might cause continuing minor regional breakthroughs, which might explain such events as the Mesozoic "superswell" in the Pacific.

0

1300

1320

1340

1360

1200

1380

Mantle temperature (°C)

Fig. 1. Plate age at subduction and age at neutral buoyancy.

3.61

2.71

1.81

0.91

0.

Age (Ga)

Fig. 2. Comparison of a thermal evolution with episodic layering and overturns and smooth whole-mantle evolution.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996

ASPECTS OF T H E 1994 E R U P T I O N OF RABAUL V O L C A N O Hugh L Davies Geology Department, University of Papua New Guinea, University NCD, Papua New Guinea Fax 675-3260369; email I00353.l55@compuserve.com

An eruption of Tavurvur and Vulcan vents in the Rabaul volcanic complex began on 19 September 1994. The eruption came on very rapidly, with only 27 hours of precursory earthquake activity. It was of small to moderate size by world standards but caused considerable damage to property, especially to structures within Five km downwind of the two vents. Four people were killed as a direct result of the eruption. All were in Rabaul town.

Total damage was estimated at around K200 million (SA 200 million).

Most damage was causd by accumulation of ash on roof tops and by downslope flow of waterborne ash. In addition, areas within 2.5 km of Vulcan were subjected to devastating pyroclastic flows. During and since the eruption damage has been caused by flash floods and mud flows emanating from ash-covered hill slopes during heavy rain. This hazard continues.

45 000 people were evacuated from the town area and nearby villages at the time of the eruption. Through the first weeks of the eruption, 105 000 displaced people received food and shelter in care centres. After three months the numbers in care centres had declined to 45 000 and after nine months (June 1995) all care centres were closed.

The rehabilitation and reconstruction of facilities to serve the Rabaul region will take some years and will involve the transfer of most government services from Rabaul to Kokopo, SE of Rabaul. Light industry and some commercial operations will remain in Rabaul town, notably in the area near the main wharf that escaped serious damage. Restoration of government services alone is estimated to cost K100 million.

In hindsight, the Rabaul disaster was remarkable for the evacuation and relocation of a large number of people in a very short time, and for the very small loss of life - this despite the rapid onset of the eruption and the scale of subsequent devastavation. The successful outcome was due to sound planning for the expected emergency and to a high level of public awareness. When the emergency developed, people knew what to do and where to go.

Awareness, in turn, arose from public education by the authorities and the guidance of those older inhabitants who had lived through the 1937 eruption.

Acknowledgement. I acknowledge with thanks the support given by staff of the Rabaul Volcanological Observatory, the Chairman and members of the East New Britain Provincial Disaster Committee and the UNDP Office in Port Moresby.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PALAEOFLUVIAL CHANNELS IN THE JOSEPH BONAPARTE GULF, OF INTEREST TO DIAMOND EXPLORATION Geoff. L. Deacon Department of Geology & Geophysics, University of Western Australia, WA. 6907

The Joseph Bonaparte Gulf lies offshore to the north of Kununurra in northwestern Australia. It is constrained by rifting which formed the Bonaparte Basin in the Devonian (Gunn, 1988). Considerable thicknesses (up to 2.4 km) of Cainozoic to Recent sediments are present as shelf carbonates and detrital quartz-rich sediments in the offshore Bonaparte.

The Joseph Bonaparte Gulf is of significance to diamond placer exploration as it is fed by large fluvial systems such as the Ord and Victoria Rivers as well as smaller systems such as the Berkeley. All of these systems have been found to contain diamonds, the Ord being significant since the Argyle area lies within its watershed.

Furthermore, the primary diamondiferous pipes in northwestern Australia were all emplaced prior to the Palaeozoic, consequently significant erosion of the primary pipes has taken place, resulting in the transport and deposition of significant quantities of diamonds into the Joseph Bonaparte Gulf.

Alluvial diamond exploration in a region such as the Joseph Bonaparte Gulf is attractive due to the fact that the diamonds are of high quality due to the progressive removal of flawed gems by the rigors of fluvial transport.

Furthermore, the mining of diamonds in the Joseph Bonaparte Gulf is facilitated by both the relatively shallow water depths and the predominantly unconsolidated cover.

Exploration in the Joseph Bonaparte Gulf differs from coastal placer exploration elsewhere in the world in that the sediments believed to host diamonds in the Joseph Bonaparte Gulf are predominantly fluvial. Their presence in a marine setting occurs due to the combination of factors. Firstly, the low offshore slope gradient of the Joseph Bonaparte Gulf extends the present neritic zone (water depths less than 150 m) to well beyond 200 km from the southern shoreline; and secondly, the mean sealevel over the past 140 000 years has been 50 m lower than present. As a result, fluvial systems active during this period have been subsequently inundated by the subsequent high stand resulting in the burial of palaeochannels by marine (and terrestrial) sedimentation. This means that placer deposits formed during the active periods of the fluvial systems are preserved at depths which, due to the relatively shallow water depths of the Joseph Bonaparte Gulf, are readily amenable to offshore mining.

Delineation of buried palaeochannels is accomplished from the interpretation of high resolution magnetics which relies on a degree of concentration of magnetic detrital minerals present in the system. Magnetite, ilmenite pyrrhotite and chromite are all capable of relatively high magnetic susceptibilities and if concentrated in fluvial channels to any degree, enable the recognition of the channels by this method. Depth control is poorly constrained by this technique. However, by interpreting the magnetics in conjunction with shallow, high resolution seismics the relative superpositions of individual channels may be ascertained, and channels considered appropriate for exploration identified.

Stratigraphic interpretation is accomplished using drill hole cuttings which are interpreted in terms of grain size, carbonate stratigraphy and micropalaeontology. Such interpretation allows the coordination of stratigraphy with sealevel and climatic variations to tie in both the fluvial and the marine sediments to the Quaternary history of the gulf as well as documenting temporal environmental variations on a basin wide scale. Carbon dating is also being used to constrain sediment ages.

Once the palaeochannels have been identified, the architecture of the fluvial systems can be interpreted in terms of meander wavelength, channel width etc. to provide information as to the energy of the systems. Potential placer concentration relies on the fluvial architecture which is influenced in turn by slope, climate and sealevel variations as well as local structural controls.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 AN ASSESSMENT OF THE MINERAL POTENTIAL OF CAPE YORK PENINSULA T.J. Denaro and G.R. Ewers department of Minerals and Energy, Geological Survey of Queensland. GPO Box 194 Brisbane, QLD 4001 Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2600 1

2

2

Cape York Peninsula has a rich and diverse mining history dating from the discovery of Au on the Palmer River by William Harm's expedition in 1872. Population centres such as Cooktown, Coen and Weipa owe their establishment to mining. The area has produced 215 million tonnes of bauxite, more than 720 000 tonnes of kaolin, 16.5 million tonnes of silica sand, at least 48 tonnes of Au bullion, about 16 000 tonnes of Sn concentrates, about 6000 tonnes of W concentrates, 18 000 tonnes of Cu, 1 tonne of Ag, and 5 tonnes of Sb concentrates. In 1992/93, the region produced more than $262 million worth of minerals, representing 4.7 per cent of the total mineral production of Queensland and 14.5 per cent of non-fuel minerals. By value, this included all of Queensland's bauxite production, 98 per cent of kaolin, 84 per cent of silica sand, and 0.5 per cent of Au production. The region is the third most important non-fuel mineral producing district in Queensland (after Mount Isa and Charters Towers). Mining is an important source of export income and employment in the area.

This mineral resource assessment was made under the Land Use Program (stage 1) of the Cape York Peninsula Land Use Strategy (CYPLUS), a jointly funded initiative of the Commonwealth and Queensland Govermnents, and will be used in conjunction with other datasets and reports to formulate land use options and policy decisions affecting the future development of Cape York Peninsula. The mineral resource assessment of the region combines knowledge of its geology, geophysics, geochemistry, mineral deposits and occurrences with mineral deposit models and the results of exploration. It has drawn on geoscientific data compiled as part of the Natural Resources Analysis Program (NRAP) of CYPLUS, and those gathered jointly under the National Geoscience Mapping Accord (NGMA) by the Geological Survey of Queensland (GSQ) of the Department of Minerals and Energy, Queensland, and the Australian Geological Survey Organisation (AGSO).

Apart from the significant economic resources of bauxite, kaolin, and silica sand, there is potential for the development of known Au, Sn, W, coal, limestone and heavy mineral resources and for the discovery of additional deposits of these commodities. The area also contains known resources of Fe, Mn, Sb and base metals. There is some potential for the discovery of gemstones (diamonds and sapphires) and petroleum.

The Coen and Yambo Inliers form the central ridge of the Peninsula and comprise Proterozoic metamorphic rocks which are intruded by Siluro-Devonian and Permian granitic rocks. The Coen Inlier contains iron formation-hosted Au deposits, mesothermal Au and Au-Sb veins, porphyry intrusion-hosted Au, epithermal Au, W veins, stratabound massive sulphide deposits, enriched iron formation and alluvial Au, Sn, and heavy minerals. Coal occurs in localised Carboniferous sedimentary rocks in the northern part of the Coen Inlier.

The Cape York - Oriomo Inlier forms the islands of Torres Strait and the adjacent mainland and comprises Carboniferous volcanic rocks which are intruded by granites of the same age. This area contains porphyry intrusion-hosted Au, Sn and W veins, Cu veins, and alluvial Au and Sn deposits.

The Hodgkinson Province forms the southeastern part of the study area and extends from Cape Melville south beyond Cooktown and west to Palmerville. It comprises Ordovician to Devonian metamorphosed sedimentary rocks and small basins of Permian sediments, and has been intruded by Permian granites. Deposit types include mesothermal (slate-belt) Au and Au-Sb veins, chert-hosted Au, volcanogenic massive sulphide deposits, basalthosted Cu deposits, Sn and W veins, Sn greisens, W skarns, limestone, alluvial Au and Sn deposits. Permian basins contain sub-economic coal deposits and Cainozoic dune fields on the east coast contain silica sand.

The Jurassic to Cretaceous Laura Basin extends between Laura and Princess Charlotte Bay, and contains known coal resources, is an important groundwater reservoir, and may contain petroleum. It overlies the Permian to mid?-Triassic Lakefield Basin which could contain coal resources. Cainozoic beach ridge deposits contain heavy minerals and silica sand. The Carpentaria Basin forms the western half of the Peninsula and developed at the same time as the Laura Basin. It is an important groundwater reservoir and may contain petroleum; coal occurs within a small sub-basin, the Olive River Basin. The younger Karumba Basin is superimposed on the Carpentaria Basin and hosts bauxite, kaolin, silica sand and heavy mineral deposits and is a source of groundwater.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE "YARRA AQUIFER" - A CASE OF MIXED URBAN INTERACTIONS Boyd B. Dent Dept. Applied Geology, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007

When Captain James Cook explored Botany Bay in 1770 he drew water from a stream and swamp system behind the dunes of Yarra Bay. This stream is an intermittent one. The presence of surface water was reported, and later this was a key aspect in the determination to found the colony of New South Wales on the northern shore of Botany Bay. Eighteen years later, when Governor Arthur Phillip arrived with the First Fleet - the swamp and its feeding stream were dry. If Cook or the early colonists had had the services of a hydrogeologist they would have found that the "Yarra Aquifer", over which the stream flows, could provide enough groundwater to support the new settlement.

The stream is still present today, the swamps and dunes have been replaced with a municipal landfill and market gardens; the stream's primary spring source has been built over, the stream has been partially piped, deepened and is consequently now an influent one. Its significance appears to be as yet unrecognised, but its presence is such that it provides the primary water source for the market gardens which have occupied this alluvial filled valley since the 1880's at least.

A hydrogeological unit - the "Yarra Aquifer" is described. This phreatic aquifer is composed of sand dune and reworked fluviatile and beach sediments equivalent in time, mode of emplacement and general lithology to those of the Botany Basin. The aquifer occupies a small northern extension of the Botany Basin sediments in an embayment about 80 ha in area, behind the present day Yarra Bay. Except for under Botany Bay itself the unit is hydraulically separate to the Botany Basin, it has a catchment of about 224 ha and annual recharge of 130-140 ML. Recently detailed geological mapping and some subsurface investigations were undertaken in connection with the development of a groundwater supply for Botany Cemetery, which is located along the lower half of the "Yarra Aquifer's northern margin. The new subsurface investigations were confined to the aquifer's northern margin and included augered boreholes to bedrock, geophysical gravity survey, hand augered holed to the water table or bedrock, the establishment of a fully completed water bore, pumping and slug tests. The conditions proved to be inhomogeneous and anisotropic related to the wedging out of the aquifer formation. Hydraulic parameters are represented here by the low values of K = 0.8 m/day, T = 2.08 m2/day, and Sy = 0.04. A water bore established in 1965, at the lowest and deepest section (here 21.4 m) of the aquifer, is reported to have produced flows of 7.578 L/s over a 10 day (?) test.

The "Yarra Aquifer" hosts a spring fed baseflow from the surrounding ridges of Hawkesbury Sandstone. These ridges are predominantly developed for housing. A former oil refinery site, 4 major municipal landfills, Botany Cemetery, market gardens, a penitentiary, some housing, and open space make up a strange mixture of land use patterns on the aquifer's surface. Leachate plumes from the landfills have been detected, the oil refinery has left a legacy of LNAPLs in the groundwater, and the urbanisation is also effecting the hydrogeochemistry.

Another important dimension to the studies undertaken was the groundwater hydrogeochemistry of the Botany Cemetery. This has been considered in detail from the aspects of deriving a groundwater supply for lawn and garden irrigation, pollution and preliminary studies in the decay of human remains interacting with the hydrogeological cycle. Key microbiological indicators of faecal coliforms, faecal streptococci, biochemical oxygen demand (5 days) were determined in the Cemetery-derived waters as representative indicators for human impact pollution. The Cemetery-derived groundwaters are of a good quality - in many respects better than background which is showing some influence of residential occupation (elevated salinity, increased nitrate, sulfate and zinc).

The studies of hydrogeochemical processes related to the recent interment of human remains continue. However, initial results confirm the presence of a low positive redox, very slightly acid, salinity plume close to the remains and that this rapidly attenuates downgradient. Essentially the plume consists of Na. K. Ca. Mg. Fe cations and a complicated array of CI, S0 4 N0 3 N0 2 S0 4 NH4 and P0 4 anions. Only a very few studies of this kind of phenomenon have been reported - Worldwide. A conceptual model of the hydrogeological processes in cemeterics, which should be treated as special kinds of landfills, has been developed.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

COMPUTER MODELLING OF CONTROLS ON MVT MINERALISATION ON THE SOUTHEASTERN LENNARD SHELF, WESTERN AUSTRALIA Cecilia D'Ercole* and Derek Milton^ 1 Key Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia, 6907 2 Department of Geography, University of Western Australia, Nedlands, Western Australia, 6907 Computer-generated metallogenic models are new exploration techniques that can be used in poorly explored or unexplored terrains. There are two main approaches to metallogenic modelling: a conceptual approach and an empirical approach. The former relies heavily on an understanding of both the geological factors required to formulate a specific deposit type and the geology of the area being analysed (eg . Wyborn et al., 1994). The latter model is the more traditional approach, whereby models for predicting mineral potential are based on statistical (commonly multivariate statistical analysis techniques) or heuristic relationships (eg. Bonham-Carter, 1994). Historically, mineral potential mapping, in the form of selection, evaluation and combination of evidence for mineral deposits, was originally undertaken with the aid of a light table with the various maps physically superimposed on one another to determine relationships between anomalies. Geographic information systems (GIS) have improved the efficiency with which mineral potential mapping is carried out and expand the possibilities for data processing and analysis.

The Lennard Shelf forms the northern margin of the Canning Basin in Western Australia, and is characterised by Devonian shallow marine carbonates which are host to several Mississippi Valley-type (MVT) Pb-Zn deposits.

Exploration models for the Lennard Shelf have been based on North American methods. Areas of interest were defined using a combination of geology, geochemistry and geophysics, and these areas were then systematically grid-drilled. This exploration method proved to be effective but slow on the Lennard Shelf. Potentially, a more advanced approach than traditional MVT exploration is the integration of all geoscientific data into a GIS with subsequent interrogation.

The purpose of this paper is to discuss conceptual modelling of MVT mineralisation on the Southeastern Lennard Shelf. This has been achieved by selection of the essential features of MVT deposits, including relationships to their host rocks, which are characteristic of enough known MVT deposits to be used as indicators of mineralisation. Based on the model proposed by Wyborn et al (1994), these features are then categorised into components which comprise a regional-scale MVT mineral system. In this system, each component has been digitally translated into a mappable criterion that is displayed within the GIS, with several criteria integrated into a metallogenic model useful for exploration. These criteria are geologic parameters that affect the favourability for the presence of a MVT deposit and have been categorised into either diagnostic or permissive criteria. Diagnostic criteria are those that are true of all, or nearly all, known MVT deposits and, in most cases, are considered to be required for the presence of a MVT deposit. Permissive criteria are those that are present in enough known MVT deposits that they may be considered to enhance the favourability of a MVT deposit, but their absence does not diminish the possibility.

Most known MVT occurrences on the Southeastern Lennard Shelf are (1) in limestones on the edge of a sedimentary basin, (2) near or within faults and fractures, (3) spatially related to surface and subsurface basement highs, (4) near major basement lineaments, and (5) near areas of anomalously high amounts of base metals within stream sediment and rockchip data. All these criteria are classified as diagnostic criteria. Each criterion which forms part of the MVT regional mineral system have been stored as an individual layer within the GIS.

REFERENCES BONHAM-CARTER, G.F., 1994. Geographic Information Systems for geoscientists: modelling with GIS. Computer Methods in the Geosciences 13. Pergamon Press, Canada, 398p.

WYBORN, L.A.I, GALLAGHER, R. & JAGODZINSKI, E.A., 1994. A conceptual approach to metallogenic modelling using GIS: examples from the Pine Creek Inlier. In Proceedings of a Symposium on Australian Research in Ore Genesis, Australian Mineral Foundation, 15.1-15.5.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 PRE-DAWN THERMAL INFRA-RED IMAGERY: A REGOLITH MAPPING TOOL Donald de Vries CSERO COSSA, GPO Box 3023 Canberra, ACT 2601 Phone: (06) 2167200 Fax: (06) 2167222 Email: ddv@cossa.csiro.au

ABSTRACT Airborne generated images depicting an outline of unique geomorphological feature, otherwise obscure to onground investigations, are generated using thermal infra red (TIR) scanning missions. The processed TIR image identifies temperature anomalies that, in the case of the regolith, are associated with soil moisture, soil aquifers, perched water tables, springs, landslides, structural geology and buried paleo-drainage patterns. The black and white images require photogeological interpretative input to locate* features for field work investigations. TIR imaging in the 8 pm to 14 pm spectrum requires robust digital scanning technology; an efficient flexible modern airborne platform; the skills of pilot and operator; and a carefully compiled planned mission plan that optimises such variables as season, weather and time. The final image is the product of the afore mentioned. High resolution images (3 to 30 metres) are customised by planning the mission speed and altitude of aircraft See Table 1.

An Australian owned commercial TIR scanner is the Daedalus 1268 instrument installed on a Gates Learjet 35A aircraft based in Nowra, NSW. This scanner is the airborne yersion of the LANDSAT TM satellite instrument and features a total of 11 radiometrically calibrated channels including the TIR channel. All bands are co-registered as shown in Table 2. The TIR data from Channel 11 has provided unique data for several geoscientist's missions capturing daytime surface radiation at it's maximum and capturing pre-dawn ambient residual temperature emissions. The quantitative mapping of diurnal heating and cooling ground features has increased the effectiveness of targeting features of prime significance while maintaining effective coverage.

The temperatures measured range from -15°C to +100°C with precision of 0.1°C to 0.5°C. By recording at air temperature and humidity on the ground at several locations and air temperature at aircraft altitude, an atmospherically corrected image of the apparent ground (or water) surface temperatures (radiant surface temperatures) are generated to an accuracy of <0.5°C. The thermal infrared band is unaffected by sunglint and aerosol induced variances as viewed by an airborne platform.

Table 1

Pixel size (metres) 30x30 20 x 15 15x 15 10x 10 7.5 x 7.5 5.0 x 5.0 3.0 x 3.0 3.0 x 3.0

Table 2 Daed aius Band Configluration LANDSAT Band Edge TM (Mm) (Mm) Equivalent VIS Ch 1 0.420 0.450 VIS (Chi) Ch 2 0.450 0.520 VIS (Ch 2) Ch3 0.520 0.600 VIS Ch4 0.605 0.625 VIS (Ch 3) Ch 5 0.630 0.690 VIS Ch 6 0.695 0.750 NIR (Ch 4) Ch 7 0.760 0.900 NIR Ch8 0.910 1.050 Ch9 1.550 1.750 SWIR (Ch 5) Ch 10 2.080 2.350 SWIR (Ch 7) TIR (Ch 6) Ch 11 8.500 13.000

Pixel size options Speed Aircraft (km/hr) AGL (m) 12,000 8,000 6,000 4,000 3,000 2,000 1,200 1,200

745 810 610 405 610 405 490 245

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

VICTORIAN INITIATIVE FOR MINERALS AND PETROLEUM (VIMP) Tom Dickson, General Manager Geological Survey of Victoria Department of Agriculture, Energy and Minerals P O Box 2145 MDC, Fitzroy, Vic 3065

The Victorian Government through the Department of Agriculture, Energy and Minerals is undertaking a 3-year, $16.5 million Victorian Initiative for Minerals and Petroleum (VIMP) to support the state's mineral and petroleum exploration industry. Projects being undertaken as part of the initiative include the acquisition of airborne magnetic and radiometric, gravity and regional seismic data, stratigraphic drilling and geophysical interpretation and geological mapping products.

A number of databases and data compilations are also included to assist in focussing attention on the mineral and petroleum potential of the state. Particular attention is being given to the North West of the state and the Eastern Highlands, where major airborne surveys are to be carried out.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE EFFECTS OF VARIATIONS IN ACCOMMODATION ON COAL PROPERTIES Diessel, C. F. K., Department of Geology, The University of Newcastle, NSW

Amplitude and rate of oscillations of depositional base level determine the manner in which sediments are accommodated. Sediments, such as peat, that are generated at the depositional site reflect the conditions of accommodation more strongly in their composition than clastic sediments whose composition is determined in the source area and during transportation.

The ecologic specificity of peat-forming plants combined with the high sensitivity of biochemical coalification of groundwater-table variations provides coal seams with a detailed record of base-level movements.

Palaeobotanical, maceral, photometric and chemical studies of coal offer therefore an opportunity to assess the position of coal-bearing sediments within a sequence-stratigraphic framework.

In the early lowstand systems tract, peat formation is impeded by the rapid fall of base level and environmental instability including widespread erosion. The conditions of accommodation improve in the late lowstand in incised valleys, on lowstand delta plains and in interfluve settings.

Following the lowstand, relative sea level begins to rise and generates increasing accommodation during the transgressive systems tract. Because of the rising watertable there is little time for oxidative decay. For this reason cell structures tend to be preserved which results in the formation of woody or hemic to fibric peat precursors of vitrinite-rich coals, provided plant growth, peat accumulation and the rise in water table are balanced. A decrease in tissue preservation will occur in marine-influenced seams through increased bacterial activity when the peat's acidity is reduced in contact with sea water. Under cool to temperate conditions, good peat-forming conditions exist at the beginning and towards the end of the transgressive systems tract when the rise in base level is relatively slow. Conversely, tropical peats show their best development when base level rise is fastest half-way through the transgressive systems tract.

As relative sea-level rises begins to decelerate, accommodation decreases during the highstand systems tract. At the beginning of the highstand systems tract, accommodation rates are still positive and of a magnitude that can be matched by temperate-climate-peats. The resulting coals may therefore still be clean, reasonably bright and have a relatively high tissue preservation potential. The raised mires persisting into the late highstand systems tract under an increasing incremental loss of accommodation space suffer frequent exposure of the peat surfaces accompanied by oxidation and partial erosion.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

STRONTIUM AND SULPHUR ISOTOPES AS TRACERS OF INTER-AQUIFER MIXING AND WATER-ROCK INTERACTION, SOUTH-WESTERN MURRAY BASIN Shawan S. Dogramaci1*3. Andrew L. Herczeg~& Yvonne Bone J 1

Centre for Groundwater studies, Private Bag No.2, Glen Osmond SA 5064

^Centre for Groundwater Studies and CSIRO Division of Water Resources, Private Bag No.2, Glen Osmond S A 5064 ^Department of Geology & Geophysics, University of Adelaide, South Australia,5005

INTRODUCTION The Mallee region of South Australia has no surface water drainage, and therefore is entirely dependent on groundwater for irrigation, domestic, stock and municipal use. The regional groundwater system comprises the unconfmed Murray Group aquifer overlying the confined Renmark Group aquifer. Groundwater salinity in the Murray Group aquifer increases along the hydraulic gradient, ranging from less than 1000 mg/1 near the basin margins to more than 20,000 mg/1 near the River Murray. Inter-aquifer mixing is one of the processes affecting the increase of salinity in the Murray Group aquifer. Because hydrochemistry and stable isotopes alone are not sufficient to distinguish waters from the Murray Group and Renmark Group, we explore the possible use of other isotope systems such as strontium (Sr) and sulphur (S) ( 87 Sr/ 86 Sr and d 34 S), to evaluate physical and chemical processes occurring in the Murray Group aquifer system to establish flow systems and inter-aquifer mixing with the Renmark Group.

RESULTS The Sr isotope results indicate that values for the Renmark aquifer are generally more radiogenic (ranging from 0.70926±9 to 0.71092±24) than the corresponding water samples from the Murray aquifer. Concentrations of Sr and 87 Sr / 86 Sr ratios of the Murray Group aquifer samples can be divided into three groups. The majority of the samples fall into Group I, with Sr concentrations between 3 and 28 mmol/1 and Sr isotope ratios between 0.70879.8 and 0.70951±8. The high 87Sr /86Sr ratio in this group (higher than the corresponding host rock) could be due to mixing, where one end member represents groundwater equilibrated with Miocene limestone and the second end member is derived from minerals containing more radiogenic values (>0.7096), such as feldspar or mica. The 87 Sr / 86 Sr ratios of Group II are -0.70867, with higher Sr concentrations than the first group. These groundwaters may have approached equilibrium with the carbonate component of the host rocks. Group III represents two wells at the end of the flow line, near the River Murray. The high 87 Sr / 86 Sr values of 0.70970±4 and 0.71114±22 may be due to upward leakage from the Renmark Group aquifer. The 87 Sr / 86 Sr ratio of these samples are in agreement with the hydraulic data, which indicates the potential for upward leakage from the Renmark aquifer in this region.

The S isotope content of sulphate from most wells within the Murray aquifer, indicates that almost all the sulphate is derived from cyclic salt plus some from the oxidation of organic S ( d 34 S =+6 to +15%©), and that very little sulphate is derived from the oxidation of sulphides. All but two Renmark aquifer samples exhibit d 3 4 S value in the range of +18 to +55 %o, and are significantly enriched relative to the values obtained for the Murray Group. Values for the up gradient part of the Renmark aquifer (i.e. distant from the River), +14.9 and +17.4.

These are in agreement with the hydraulic head distribution. This indicates that there is potential for downward leakage from the Murray aquifer in this area. The high d 34 S value of +26%c for the well at the River end of the flow line from the Murray Group aquifer indicates upward leakage from the Renmark Group aquifer, and is in agreement with results obtained from Sr isotopes.

CONCLUSIONS 1. The results of Sr isotopes demonstrate their utility as an indicator of the source of different water bodies, as well as the nature and extent of water-rock interaction.

2. The d 3 4 S in both aquifer systems are distinct and reflect the source of sulphate in the groundwater, and can be used to estimate the extent of inter- aquifer mixing in some parts of the basin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE EFFECT OF SLAB-DERIVED HYDROUS FLUIDS ON THE OXIDATION STATE OF MANTLE BENEATH THE CASCADE ARC: CONSTRAINTS FROM MOSSBAUER-DETERMINED FE3+/FE2+ IN SPINEL l

David S. Draper1 and Alan D. Brandon2 Key Centre for Geocfaemical Evolution and Metallogeny of Continents (GEMOC) and Centre for Petrologic and Lithospheric Studies (CPLS) School of Earth Sciences, Macquarie University Sydney NSW 2109 Australia department of Terrestrial Magnetism, Carnegie Institution of Washington 5241 Broad Branch Road, NW, Washington DC 20015 USA

Type I spinel peridotite xenoliths from Simcoe Volcano, southern Washington (USA), are from lithospheric mantle approximately 65 km inboard from the axis of the subduction-related Cascade Range. The xenoliths are primarily refractory spinel harzburgite with minor websterite, but show evidence for metasomatism such as the presence of phlogopite and trace element enrichments in clinopyroxene. Oxygen fugacities calculated from contents of Fe372Fe in Simcoe spinels, detennined by Mossbauer spectroscopy, are up to 1.4 log units more oxidizing than the FMQ buffer.

These are among the most oxidized mantle xenoliths reported, with oxygen fugacities substantially higher than those calculated for mantle beneath most of western North America. These results, together with those from amphibole-bearing spinel peridotites from Ichinomegata, Japan, provide evidence that the mantle above subduction zones is more oxidized than is oceanic or ancient cratonic mantle. We suggest that oxidation was accomplished by an agent ranging in composition from solute-rich hydrous fluid to water-bearing silicate melt, and we develop a semiquantitative model relating extent of oxidation, duration of the oxidation process, and proportion of the available water derivedfromsubducting slabs that oxidizes Fe in spinel (Figure 1). This model suggests that such an agent can easily produce the observed extents of oxidation over timescales similar to the typical lifespans of subduction zones. For the Cascade arc, where subduction has taken place for at least 50 Ma, the observed oxidation in the Simcoe peridotites can be achieved by reaction of 2% to 8% of the available water, assuming that 50% of the water flux from subducting slabs is transported into the mantle. These results demonstrate that water can be an efficient oxidizing agent, and that such a mechanism is a viable way to cause large-scale oxygen fugacity heterogeneity in the mantle.

% of Available Water Reacted

Figure 1. Model results showing time required for a given percentage of the available subducted water to produce Maximum and Minimum derees of oxidation as recorded by Simcoe spinels. Shaded region denotes typical ages of subduction zones (20 to 100 Ma) and corresponds to conditions over which the proposed model could produce the observed oxidation signature in subarc mantle.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 EXPERIMENTAL P-T PHASE RELATIONS OF SILICIC, ALKALINE, ALUMINOUS GLASSES TRAPPED IN MANTLE XENOLITHS David S. Draper and Trevor H. Green Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC) and Centre for Petrologic and Lithospheric Studies (CPLS) School of Earth Sciences, Macquarie University Sydney NSW 2109 Australia

Many mantle xenoliths contain silicate glasses, trapped as discrete phases, whose compositions would at first glance seem difficult to account for via trapping of host basaltic liquid, melting of mantle materials (with or without a volatile flux), or melting of typical hydrated mantle (i.e., amphibole- or phlogopite-bearing). These glasses have the following ranges of major-element composition (wt. %): SiCX, 55-65; Ti(X 0.2-2.0; A1203, 18-24; FeO*? 0.5-1.5; MgO, 0.75-1.5; CaO, 2-5; NaX), 3-6; and KjO, 4.5-7.0. The (very few) published trace element data on these glasses show enrichments in incompatible trace elements such as light rare earths and high field-strength elements. Schiano and coworkers (e.g.

Nature 368:621-624) have also identified glasses having very similar bulk compositions but occurring as melt inclusions in xenolith minerals. These two separate investigations are consistent with the view that these compositions represent a type of metasomatic agent (in addition to C02- and H20-rich fluids and carbonate liquids). Accordingly, we have performed a series of experiments on three such liquid compositions in order to identify' the mineral assemblage(s) with which they could coexist. It is our goal to place major-element, phase-equilibrium constraints on this type of mantle metasomatism. Experiments were run at pressures ranging from 1.0 to 3.0 GPa under both anhydrous and COH-fluid-saturated conditions; in the latter, fluid compositions were either Xmo = 1.0 or XH20 =0.5.

Under anhydrous conditions, two of our three studied liquid compositions coexist with mantle-like phases (spinel, Forich olivine, En-rich orthopyroxene, Di-rich clinopyroxene, Py-rich garnet), and one of them shows near-liquidus saturation with olivine, orthopyroxene, and clinopyroxene at P = 1.0 to 1.2 GPa. Under water-saturated conditions, phlogopitic mica is the liquidus phase for all three compositions at all pressures investigated. At fluid saturation with XH2Q =0.5, near-liquidus phase relations are similar to the anhydrous case; however, further beneath the liquidus initially-crystallizing orthopyroxene and/or olivine react with liquid to form phlogopite at pressures up to 2.0 GPa. The near-liquidus anhydrous mineralogy persists over a very large temperature range-approximately 900 to 1100°C-and overlaps the range of temperatures thought to prevail in the upper mantle. At 3.0 GPa, C02 solubility in the melt greatly depresses the liquidus surface-liquidus temperatures at this pressure are -1000-1050°C compared to 1100-1125°C at 2.0 GPa—and near-liquidus mafic phases (garnet, orthopyroxene) give wav to carbonates (magnesite-siderite solid solutions and ferroan dolomite) and kyanite rather than to phlogopite as at lower pressures.

These results are somewhat surprising; most workers would not consider liquids so rich in silica, alumina, and the alkalies to be saturated in mantle minerals. The near-liquidus saturation, however, implies that the studied liquids can coexist with the saturating minerals. Because these minerals are like those typical of depleted mantle, it appears clear that these liquids could easily migrate through such mantle material without undergoing large-scale, bulk compositional change via wallrock reaction. Therefore, they could serve as effective metasomatic agents because they could survive to convey whatever trace elements they might carry from one mantle region to another, rather than be forced to react with mantle minerals or to freeze into immobility. Another implication of the near-liquidus phase relations is that the protolitii(s) for these liquids could be either hydrated mantle material, or possibly an eclogitic assemblage (garnet and ahnninosilicate at 3.0 GPa), but this interpretation is subject to clarification from additional experimentation. Finally, the appearance of carbonates near the high-pressure liquidii for these compositions suggests that it may be possible to find conditions where there is overlap between silicate and carbonate agents of metasomatism.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 INTERPRETATION OF SOUTH AUSTRALIA'S MINING AND GEOLOGICAL HERITAGE GREG J. DREW Mines and Energy South Australia P.O. Box 151 Eastwood, SA 5063

Mining has played, and continues to play, an important role in South Australia's economic development. The first metal mining in Australia commenced near Adelaide in 1841 and was followed soon after by the mining of copper at Kapunda (1844) and Burra (1845). Further copper discoveries of world significance were made at Wallaroo in 1859 and Moonta in 1861. These and numerous other lesser discoveries have made a significant impact on settlement patterns, transport systems and social conditions, and have left an important array of heritage items.

In South Australia, systematic documentation and assessment of sites has led to the conservation and interpretation of a number of ruinous sites by careful selection of priorities, allowing limited resources to be used for the greatest overall benefit. The success of this work is due to a number of factors including the accessible locations and aesthetic qualities of many historic mine sites, the permanence of many structures and buildings (especially Cornish enginehouses), protection by State Heritage legislation and adequate documentation of sites. Interpretive methods have varied from simple self-guided walking and drive trails to fee-charging museums.

Since the early 1980s, a series of Heritage Drives has been established in the major historic mining towns in conjunction with high-quality guide books and appropriate signposting. A network of ten self-guided walking trails has been developed at a variety of mine sites, most within two hours drive of Adelaide. Interpretation has been based on information derived through detailed research into the technological and cultural history of each site. A highlight of this work is the interpretation of remaining evidence by means of historic photos and reconstructions using high-quality, low maintenance and long-life materials. Examples include the Talisker silver-lead mine, Barossa Goldfield, Kapunda Mine and Sleeps Hill stone quarries.

At locations where historic mine sites are associated with nearby towns, reconstructions and adaptions have been completed for use as museums or interpretive centres. Examples include the Burra Mine Museum, Moonta Mines State Heritage Area and Broken Hill - The Living Museum. The Burra Mine Museum has been developed as an open-air museum, with three Cornish enginehouses being the main exhibits. The feature of the museum is Morphetts Enginehouse, which was reconstructed in 1986 and houses an interpretive display on the Cornish beam engine. Drive and walking trails allow self-guided access to various exhibits, lookouts and interpretive sites. At Moonta, a number of sites are linked by a drive trail, including Moonta Mines Museum located in the former Moonta Mines School, tourist railway, Hughes Enginehouse and a typical Cornish miner's cottage. The Living Museum project at Broken Hill uses a combination of museums, interpretive centres, drive and walking trails, and lookouts to provide the visitor with an understanding of the city's unique history and heritage.

Mining heritage and its interpretation now plays an important role in cultural tourism by providing the visitor with authentic evidence and experiences of how former generations worked and lived. This work has resulted in economic benefits to local communities, and increased public awareness of the mining industry.

Following the success of the mining heritage interpretive program, a similar project has commenced for sites of geological interest. Geological sites have been well documented as Geological Monuments by the Geological Society of Australia and several sites have been selected from major tourist regions for interpretation. Two of these programs have been completed — Hallett Cove and Brachina Gorge - A Corridor Through Time. Brachina Gorge in the Flinders Ranges provides an ideal geological drive trail covering about 150 million years of geological time. Interpretation includes a small visitor centre at each end of the trail, rock formation markers and panels along the drive which.describe each formation. More detailed interpretation is provided at selected outcrops which describe features such as stromatolites, the Ediacaran fauna and archaeocyaths. Geological interpretation is planned for the Fleurieu Peninsula, Kangaroo Island and other tourist regions in South Australia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CRUSTAL ARCHITECTURE BENEATH THE DUNDAS TROUGH, TASMANIA AS REVEALED BY DEEP SEISMIC REFLECTION PROFILING Barry J Drummond1. T.J. Barton1, R.J. Korsch1, A.V. Brown2 & D.W. Johnstone1 Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601 2 Tasmania Development & Resources, PO Box 56, Rosny Park, Tasmania 7018

A deep seismic reflection survey recorded on the West Coast of Tasmania early in 1995 was designed to cross most of the major geological structures and boundaries in the region. Environmental and access problems in the mountainous terrain restricted field operations to roads, requiring two traverses. Both traverses were oriented approximately east-west. The western line, Line 1 (95AGS-T1), at a latitude of approximately 42° 45'S, was positioned to image the Arthur Lineament at depth under the Proterozoic Oonah Formation which crops out just east of the Arthur Lineament, and then profile eastwards across the highly mineralised Dundas Trough and Mt Read Volcanics, to finish at the Henty Fault Zone. The second line, Line 2 (95AGS-T2), was offset about 15 km to the north of Line 1, started west of the Henty Fault Zone thus overlapping the geological section covered by Line 1, and continued east onto the northern subsurface extension of theTyennan Block.

The crust-mantle boundary in the region appears as a transition zone over 5 km thick in which strong lower crustal reflectors give way to a non-reflective mantle. The top of the transition zone is about 30-33 km deep.The crust is partitioned into several distinct blocks. The most distinctive reflections can be attributed to the Arthur Lineament which dips at about 30° to the east. The reflections are very strong in the upper 15 km of the crust. In the lower crust, weaker reflections extend across into Line 2. The Arthur Lineament is a major crustal feature which underlies the Oonah Formation, the Dundas Trough, and possibly the western part of the Tyennan Block.

The crust under the Tyennan Block contains numerous strong reflections. This zone extends upwards to within about 10 km of the surface, where it is overlain by several kilometres of rocks which are poorly reflective, consistent with the monotonous, predominantly quartz-rich lithologies found in the Tyennan Block. They in turn are overlain by more reflective rocks of the lower Palaeozoic section.

The boundary between the reflective lower crust and poorly reflective upper crust appears to dip sharply to the west on the eastern side of Line 1 and is possibly truncated against the Arthur Lineament. A major east dipping fault system cuts the surface in the central part of Line 1. This system, and splays off it, form the boundaries of serpentinite belts in the region. Other fault systems in this region appear to be linked with this system. The Henty Fault System dips west from the surface and the Roseberry Fault dips to the east; both faults link with the major fault system at depth.

Reflections from within the Oonah Formation in the west are parallel to the Arthur Lineament, but at a depth of about 8 km on the Arthur Lineament, the beds of the Oonah Formation are terminated abruptly by a west-dipping structure which extends upwards to the east. In the area of the seismic section this structure does not crop out due to termination with the major fault that forms the underside of a zone of ultramafics west of the Dundas Trough.

The Palaeozoic section of the Dundas Trough and Mount Read Volcanics is imaged as a highly folded and faulted succession with variable reflection character. It has a total thickness of at least 4 km on Line 2. However, this may not be representative of the true stratigraphic thickness in the area because shortening on a series of thrust faults is evident on Line 2 beneath Tertiary basalt cover east of the Henty Fault System and north of the Tyennan Block. In places, particularly west of the Dundas Trough, major faults have the same geometry as folds in the sedimentary section that they cut, suggesting a complex history of sedimentation and tectonics.

Line 1 passes near several granites but does not cross any at the surface. Several zones of complex reflection character in the seismic section are attributed to sideswipe of the irregular sides of the granites at depth. Line 2 crosses several low gravity anomalies that are interpreted as buried granites. Two zones in Line 2 with no reflections have been interpreted as the buried granites. Their tops are at about 2 and 4 km, and they appear to be about 5 km thick in the plane of the section. They have about the same width as thickness.

Acknowledgements : The results presented here form part of the TASGO Project, a joint AGSO - Industry, Safety & Mines Tasmania Project undertaken as part of the National Geoscience Mapping Accord. They are presented with permission of the Executive Director of AGSO and the Director Mineral Resources and State Chief Geologist, Industry, Safety & Mines.

119

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MULTIPLE VEIN ARRAYS AND ZONED ALTERATION AT BRONZEWING GOLD DEPOSIT, W.A. Allison.L.Dugdale University of Western Australia , Dept. of Geology & Geophysics, Nedlands, 6907, W.Australia.

The Bronzewing gold deposit is located approximately 350km northwest of Kalgoorlie, in the Yandal Greenstone Belt of the North Eastern Goldfields Province of the Yilgarn Block, Western Australia. The deposit, owned and operated by Great Central Mines N.L, comprises three main ore zones Discovery, Central and Western, with a mineral resource estimate of 3.1 million ounces Au. Operations consist of two open-cut mines accessing the upper portion of Discovery and Central zones, with underground development from the base of Central Pit. The local mine area is dominated by a sequence of differentiated tholeiitic basaltic flows and a pyroxenitic sills, which exhibit regional metamorphism of mid to upper greenschist facies. The tholeittic basalt flows exhibit intense deformation, with the development of anastomosing high strain shear zones surrounded by low strain massive boudins. Oblique slip shearing is developed on the margins of the pyroxenitic sills Porphyritic and granodioritic sills and dykes intrude the tholeiitic sequence within the anatomosing high strain zones.

Ore zones are confined to the high strain zones, and comprise stockwork and breccia quartz vein systems, which exhibit a multiphase fluid injection. At least six types, of quartz veins comprising the mineralised zone have been defined, Table 1. Gold mineralisation is hosted predominantly within the late quartz veins, commonly associated with tellurides (altaite, tetradymite). Siting of the gold is predominantly in calcite and sulphide filled fractures and as free gold, rarely as euhedral crystals, within quartz. Hydrothermal alteration associated with the development of the quartz vein systems is extensive throughout the high strain zones. Vertical zonation of alteration is developed in Discovery zone, where the upper portions of the orebody exhibit muscovite/ankerite/pyrite (Muscovite zone) dominant alteration whereas the lower portion exhibits biotite/chlorite/calcite/pyrrhotite (Biotite zone) alteration. An intermediate alteration zone comprises biotite/chlorite alteration which appears to overprint muscovite/ankerite assemblages. Variations in the petrology of the various quartz veins, together with the changes in alteration type with depth, are suggestive of a physiochemical changes in the ore fluid over time. Either the temperature of the fluid has increased, C02 content decreased, or f02 has decreased. These alternatives are being actively addressed by fluid inclusion studies and carbon/oxygen isotope measurements.

Table 1 Vein Types : Bronzewing Gold Deposit Deposit Width Mineralogy Host Lithology Wall-rock alteration Proximal Distal Basalt V I Discovery l-3cm Qtz, Ep, Cc, Mu/Ep Chi, Py Central Basalt Mu/Ank Bio/Chl/Cc V 2 Discovery <5cm Qtz, Ank, Alb Py Py, Po, Cpy Po/Cpy Mu Porphyry (P) (P) Alb V 3 Discovery 2-7cm Alb, Ank, Qtz Py,Po,Cpy,Au Basalt(B) (B) Mu/Alb/Ank Chi/Bio ?/Cc Basalt(B) (B)Muscovite zone: V 4 Discovery 1cm - Qtz, Cc, Alb Mu/Ank/Chl Chl/Ank Porphyry (P) lm Ank, Tm, Sch Chl/Cc Central Py Py, Po, Cpy, Po/Cpy Biotite zone: Tell, Au Bio/Alb/Cc Chl/Cc Po Py Po/Cpy Intermediate zone: Bio/Chl/Cc Mu/Bio/Ank Bio/Chl/Cc P) Alb Mu Basalt(B) (B) Alb/Ank/Mu V4* Discovery 5cm - Alb,Qtz,Ank, Mu/Ank Porphyry (P) lm Cc, Bio,Po,Py (P) Alb Central Mu/Ank Basalt(B) (B) weak ankerite replacement by calcite 5cm Qtz, Cc, Chi V 5 Discovery Porphyry 10m Tm,Sch, Po, Central Cpy,Au, Tell.

Basalt Qtz Silicification V 6 Discovery 10cmlm Central Acknowledgements: Many thanks to GCM and all the geological staff at Bronzewing for their support.

120

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PALAEOZOIC ALPINE TECTONICS IN CENTRAL AUSTRALIA: REFINEMENT THROUGH MICROSTRUCTURAL AND ISOTOPIC ANALYSIS. W. J. Dunlap Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200

The evolution of the Ruby Gap ductile duplex of central Australia has been examined using structural and microstructural analysis, constraints from rock deformation experiments, results of ^ A t f t ^ A r isotopic analysis, and thermal modelling. The results allow reconstruction of the duplex through time, and help to explain an apparent 65 km of Palaeozoic crustal shortening in the southeastern Arunta Block. The analysis reveals that the duplex formed over an extended period, from about 350 to 270 Ma, and developed through a piggyback, rather than back-break, sequence of imbrication.

The Ruby Gap duplex is an antiformal stack of five thrust sheets that were deformed and superposed under greenschist fades conditions during the mid-Palaeozoic Alice Springs Orogeny. Duplex formation ensued as a consequence of overthrusting of the Amadeus Basin sediments by a basement-cored nappe 6-10 km thick forming the upper plate of the thrust system. Mylonitized Heavitree Quartzite (undeformed prior to thrusting) is the key marker horizon found in each of the thrust sheets of the duplex. Cataclastic deformation of the quartzite postdated mylonitization and was localized along the fault zones bounding the thrust sheets.

The microstructure of the quartzite in the duplex varies from nearly undeformed in the foreland to coarsely recrystallized in the hinterland. Within each thrust sheet, recrystallized grain size of both quartz and cleavageforming white mica increases systematically toward the hinterland. Consequently, estimates of the flow stress decrease smoothly toward the hinterland, with each sheet containing a characteristic stress magnitude. A critical observation is that coarsely recrystallized mylonites have been thrust over finer grained mylonites, indicating that thrusting continued after ductile deformation ceased. This overlap in the apparent magnitude of flow stress has been restored in order to image the geometry of the duplex just prior to the cessation of ductile deformation.

A similar type of restoration has been performed using observations of dislocation creep microstructures. The change in quartzite microstructure across the duplex is consistent with either an increase in temperature or a decrease in strain rate, or both, during duplex formation.

Over fifty mineral separates from the vicinity of the duplex have been dated by the K-Ar and 4 0 / ^ / 3 9 ^ methods. The overall pattern that has emerged is one where the rocks of the upper plate have apparently cooled below the closure windows for argon diffusion in amphiboles and micas between about 400 and 350 Ma. In contrast, cleavage-forming white micas from the mylonitized quartzite within the duplex yield apparent ages between 340 and 310 Ma. The apparent ages of many of the cleavage-forming white micas are believed to represent"deformation ages", that is, ages of crystallization instead of cooling.

The above interpretations are strongly dependent on the thermal history during deformation, a problem which has been addressed through multi-domain analysis of K-feldspar ^ A r / ^ A r diffusion experiments. Seven diffusion experiments have been performed on K-feldspars, enabling the thermal history of the various structural levels of the duplex to be assessed through inversion of the diffusion information. The thermal models all indicate that the ductile deformation occurred in the approximate temperature range of 240 to 330°C. In addition, after ductile deformation in the duplex had ceased and the duplex reached its final geometry (-290 Ma?), relatively rapid cooling of the entire thrust system ensued, between 290 and 270 Ma. The results are consistent with the following evolution for the duplex. The imbrication process probably started between 350 and 340 Ma, with mylonitization occurring at around 330 ± 20 °C. By about 335 Ma ductile deformation was occurring at about 250 °C in the cooler foreland portion of the duplex and at about 300 °C in the hotter hinterland portion of the duplex. Between 335 and 310 Ma the ductilely deforming portion of the duplex passed, from foreland first to hinterland last, through the brittle-ductile transition for wet quartzite, progressively locking in the "deformation ages" in the white micas. When the quartzite passed through the brittle-ductile transition, deformation partitioned onto narrow fault zones, forming late cataclastic textures on fault surfaces and juxtaposing dissimilar ductile microstructures. Subsequent to about 310 Ma, the entire thrust stack cooled to below 240 ± 20 °C while being thrust over undeformed quartzite. Final "rapid" cooling of the thrust system (at about 5-10 °C/Ma) started around 290 Ma., likely due to extensional collapse of the upper plate and concurrent erosional unloading and exhumation. Only a piggyback sequence of imbrication and ductile deformation is consistent with the structural and thermal constraints.

121

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EXTENSIONAL TECTONICS, DIAPIRISM AND FORMATION OF SEQUENCE BOUNDARIES IN NEOPROTEROZOIC SUCCESSIONS OF THE ADELAIDE GEOSYNCLINE, SOUTH AUSTRALIA Ian A. Dyson National Centre for Petroleum Geology and Geophysics, University of Adelaide, Adelaide, SA 5005.

In the Flinders Ranges of South Australia, Neoproterozoic and early Palaeozoic rocks are found within a series of broad, open, dome and basin-shaped folds. This region contains most of the diapiric structures in the Adelaide Geosyncline, many, but not all, coincident with the cores of anticlinal domes. The diapirs were sourcedfromincompetent carbonate, clastic and evaporitic sediments of the Callanna Group at the base of the Neoproterozoic (Adelaidean) succession. Together with the overlying Burra Group, these sediments represent deposition during rifting of the Neoproterozoic supercontinentThe succeeding Umberatana and Wilpena Groups represent sedimentation during passive margin development of the Adelaide Geosyncline. The development of several prominent sequence boundaries within these groups was related to periods of active diapirism, which in turn was related to major extensional events during break-up of the Neoproterozoic supercontinent. The resultant depositional sequences are defined as third-order tectono-eustatic cycles on which were superimposed high-frequency glacio-eustatic cycles. The sequences may be interpreted as a series of accommodation cycles that were formed as a response to active subsidence with subsequent catch-up by sedimentation as the rate of subsidence slowed. Large -scale growth faults occur within the Pualco Tillite and Holowilena Ironstone at the base of the Umberatana Group. They are truncated by a break-up unconformity at the base of the Warcowie Dolomite. This sequence boundary is characterised by the occurrence of olistostromes and large submarine channels. Another break-up unconformity is represented by the base of the Wilcolo Sandstone. The underlying Sandison Subgroup, which includes the ABC Range Quartzite and Brachina Formation, also displays large-scale growth faults. The overlying Bunyeroo Formation represents transgressive sedimentation during a phase of pronounced basin subsidence.

A thin dolostone, the Wearing Dolomite, is developed on the maximumfloodingsurface at the top of the Bunyeroo Formation. It is coincident with kilometre deep canyons that are filled with clastics of the Wonoka Formation. The canyons were formed by retrogradational slumping on the outer shelf during transgression and were possibly initiated by seismic events. Diaprism is associated with 2 main stratigraphic horizons: (1) the Warcowie Dolomite and Tindelpina Shale in the Umberatana Group, and (2) the Wilcolo Sandstone and Wonoka Formation in the Wilpena Group.

Unconformities at the base of these formations, together with diapiric detritus, are most pronounced adjacent to diapirs, e.g. at Moralana and Oraparinna. Here, the Wilyerpa Formation and o Tindelpina Shale pinch out against the diapir, suggesting the strata were truncated during rise of the diapir. The Moralana andOraparinna diapirs o also display faulted flanks adjacent to the crestal graben and are interpreted as reactive diapirs that o STURT1AN began to subside during regional extension. At Moralana Diapir, extensional faults were reactivated as reverse faults during another phase of diapiric uplift, possibly corresponding to sedimentation of the Bunyeroo Formation. Thus, unconformities or sequence boundaries adjacent to t2 g diapirs are related to major extensional events during passive margin development of the Adelaide T0RRENS1AN Geosyncline. Other sequence boundaries in the WILL0URAN Umberatana and Wilpena Groups correspond to lesser extensional events.

ARCHAEAN to MES0PR0TER0Z0IC COMPLEXES MORALANA SUPERGROUP

HAWKER GROUP

EARLY

POUNO

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SUBGROUP

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WILPENA

SANOlSON IUBGROUP

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Wilcolo

Ouortzite

Brochino

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MARINOAN

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Elotino

Formotion

Trezono

Formotion

Enoromo

Etino

Sho»e

2 1< 1 1 O

Formotion

< I

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UJ O <

UMBERATANA o CL Topley Hill F o r m o t i o n CROUP fwockmrowirro Xktamf Mbr. <CR SmKiHb^ Mbr. 2 O 1 I M fMt Cwmaww Cr^yocM Mbr.

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2 z> =>TO

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Worcowie

Dolomite

o Holowileno

Ironstone

=5 CO

OCL U* =>

3

S DC

Z> •=> TO

BURRA GROUP

WARR1NA SUPERGROUP

CALLANNA CROUP

Shale

Wilyerpo

P u o t c o Tillite

SkiUogolee

Dolomite

CURDIMURKA

SUBGROUP

ARKAROOLA

SUBGROUP

Pre-ADELAIDEM

122

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A CASE FOR THE FALLING STAGE SYSTEMS TRACT - THE SIGNIFICANCE OF FORCED REGRESSIVE DEPOSITS IN THE NEOPROTEROZOIC SANDISON SUBGROUP Ian A. Dyson National Centre for Petroleum Geology and Geophysics, University of Adelaide, Adelaide, SA 5005.

In the classic Exxon sequence stratigraphic model, the Type 1 depositional sequence consists of lowstand, transgressive and highstand systems tracts which are schematically tied to specific increments of the eustatic curve. However, an increasing volume of literature suggests that deposition during a fall in relative sea level may be placed into a fourth systems tract between the highstand systems tract (HST) and sequence boundary.

This systems tract has been previously referred to as the falling stage or forced regressive systems tract. Study of the Neoproterozoic Sandison Subgroup at Hallett Cove, Kulpara, Bunyeroo Gorge and Trebilcock Gap in the Adelaide Geosyncline suggests that progradational tidal sand sheets and sharp-based shoreface deposits of the Brachina Formation and ABC Range Quartzite may be assigned to the falling stage systems tract (FSST).

These units represent regressive sedimentation of the Sandison Subgroup. The lower and upper boundaries of the FSST are fixed on the relative sea level curve. However, the increments of the other systems tracts are not fixed and will vary due to subsidence rate and sediment supply. Shoreface sandstone displaying SCS at the base of the Corraberra Sandstone Member of the Brachina Formation corresponds to the base of the FSST. Its upper boundary is the sequence boundary which is defined here as the lowest point of relative sea level. The correlative conformity may be analogous to the downlap surface or disconformity at the top of submarine fans in earlier Exxon models. It passes updip into the subaerial unconformity associated with the sequence boundary.

The Brachina Formation is an upward-sanding succession of interbedded shale and fine-grained sandstone. The sandstone beds display several sedimentary structures associated with storm deposition such as Bouma sequences, HCS, micro-HCS and quasi-planar lamination. At the top of this succession, the Corraberra Sandstone Member consists of several sharp-based shoreface sandstones about 1-2 m thick that display SCS. They are commonly erosive into underlying parasequences that comprise tidal sand sheets. These SCS sandstones are referred to as attached shoreface deposits. Some SCS sandstones are completely encased within shale and are referred to as detached shoreface deposits. The base of each SCS sandstone is a high-frequency sequence boundary. The erosive shoreface deposits are interpreted as forced regressive deposits (FRD's). The Corraberra Sandstone Member is gradationally overlain by parasequences of tidal-dominated sandstones and quartzarenites of the ABC Range Quartzite. Combined with a high sediment supply, progradation of the lower shoreface resulted in a wide, shallow shelf which was conducive to tidal amplification. No submarine fan deposits have been recognised at this stratigraphic level elsewhere in the basin. The Sandison Subgroup is capped by a sequence boundary, corresponding to the base of the Wilcolo Sandstone. It marks the development of broad (c. 10-20 km) incised valleys that attain a thickness of some 25 m in outcrop. They consist of a basal, trough cross-bedded facies of fluvial origin, overlain by swaley cross-stratified shoreface sands. The swaley cross-stratified shoreface sands pass rapidly upward into basinal shale of the Bunyeroo Formation. The Wilcolo Sandstone and Bunyeroo Formation together constitute the Aruhna Subgroup.

In siliciclastic successions dominated by active subsidence and abundant sediment supply such as the Sandison Subgroup, a fall in relative sea level most likely occurred in a series of punctuated events. Such events resulted in deposition of erosive shoreface sands that have been previously referred to as forced regressive deposits. Any fall in relative sea level below the offiap break intiated a subaerial unconformity. The offiap break is defined as the lower limit of the shoreface. However, if the fall in relative sea level was below the shelf edge, lowstand fan deposition was more likely. If the fall in relative sea level was not below the offiap break, the F inflection point would mark the change from progradational to more aggradational shoreface deposits and conform to the Type 2 sequence boundary in Exxon terminology. Such falls were responsible for deposition of attached shoreface deposits in the Corraberra Sandstone Member and Wilcolo Sandstone. Detached shoreface deposits are interpreted to have formed when the fall in relative sea level was below the offiap break but not the shelf edge.

The F inflection point represents the time of most rapid sea level fall and minimum sediment accomodation.

Parasequences of the ABC Range Quartzite are well developed below the F inflection point and are increasingly aggradational in nature as the sequence boundary is approached. The absence of submarine fan deposits within the FSST of the Sandison Subgroup suggests that falls in relative sea level were not below the the shelf edge.

123

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE SIGNIFICANCE OF A LOWSTAND PROGRADEVG WEDGE BOUNDED BY DEEP-WATER CARBONATES OF THE CAMBRIAN MILENDELLA LIMESTONE, KANMANTOO TROUGH, S.A. Ian A. Dyson National Centre for Petroleum Geology and Geophysics, University of Adelaide, Adelaide, SA 5005

The Cambrian Milendella Limestone of the Kanmantoo Group crops out in the Kanmantoo Trough of South Australia. The former Milendella Limestone Member of the Carrickalinga Head Formation is raised here to Formation status to better reflect its regional significance. In the southern Kanmantoo Trough, the Milendella Limestone is about 10-30 m thick. It consists of interlaminated shale and calcium carbonate or white to lightgrey marble that sometimes passes laterally into black shale. In this region, its base is interpreted as a combined sequence boundary/transgressive surface and its upper surface represents a downlap surface for inteibedded shale and sandstone in the highstand systems tract of the Backstairs Passage Formation. However, two carbonate bands, occasionally lenticular, are apparent as the formation is traced northwards. Near the Karinya Syncline, in the northern Kanmantoo Trough, the Milendella Limestone sharply overlies the Carrickalinga Head Formation with apparent conformity. A possible combined transgressive surface/sequence boundaiy is placed in the Heatherdale Shale where black shale sharply overlies a large-scale (-200 m thick) regressive cycle, about 70 m below the level of the first medium to thick-bedded sandstone in the overlying Carrickalinga Head Formation. The basal carbonate band of the Milendella Limestone, about 30 m thick, is gradationally overlain by some 80 m of interbedded grey shale and fine-grained sandstone, referred to here as the Wyeroo Sandstone Member. The Wyeroo Sandstone Member consists of 2 units. The lower displays an upward-sanding trend, interpreted to reflect progradation. The sandstone beds in turn become successively thicker up section and commonly display features associated with storm deposition on the inner to outer shelf (e.g. HCS, quasiplanar lamination and Bouma sequences). This unit is overlain by a thick aggradational packet of dominantly interlaminated to thinly interbedded shale and fine-grained sandstone. It is in turn overlain by another carbonate band about 50 m thick. The two carbonate bands of the Milendella Limestone bound an interpreted lowstand prograding wedge (LPW). The LPW is commonly lenticular and is observed to pass laterally into a single, thick (-70 m) carbonate band which is in turn gradationally overlain by shale at the base of the Backstairs Passage Formation. This interpretation has important implications for petroleum and base metal exploration in the Kanmantoo Trough. Sequence analysis suggests that each carbonate band of the Milendella Limestone was deposited on different increments of the relative sea level curve. The basal carbonate was deposited on a sediment starved hiatal surface at the base of the LPW. A flooding surface could be mistakenly identified at the base of the aggradational unit within the LPW. The base of the uppermost carbonate band within the Milendella Limestone is interpreted as a transgressive surface. A maximum flooding surface is contained within or at the top of this carbonate band. Its apparent lenticularity is due to its backstepping nature as the LPW was transgressed. The LPW of the Milendella Limestone is inferred to pass updip into the incised valley fill on the former shelf. The black shales and two carbonate bands are interpreted to have been deposited on sediment-starved hiatal surfaces under anoxic conditions and are suggested to merge down dip. Carbonate bands and black shale of the Milendella Limestone represent potential source rock and seal for hydrocarbons and a reducing front for mineralisation. TOC analyses include 2-4 % for black shales and 1-2% from the upper and lower carbonate bands. The LPW of the Milendella Limestone is a potential reservoir for hydrocarbons and a host for syn-sedimentary mineralisation where porous sandstone is adjacent to carbonate or black shale. Base metal mineralisation is thought to be both epigenetic and syngenetic in nature. Epigenetic Pb-Zn(-Ag) mineralisation is associated with strike slip faulting, e.g. at the Royal Keyneton Mine where galena occurs in fault zones up to 0.5 m thick. Syngenetic Cu-Pb-Zn mineralisation occurs in sandstone of the LPW where it is adjacent to carbonate and black shale of the Milendella Limestone. Similar mineralisation occurs in the underlying Carrickalinga Head Formation. Here, sandstone is highly gossanous (e.g. Cu 2%, Zn 0.5%, Au 0.3 ppm) in outcrop where its is overlain by black shale. The shale overlies a marine flooding surface that caps a clastic parasequence. Some 100 m below, anomalous base metal and gold values are found in a volcanic suite consisting of andesites and basalts. The volcanics are contained within the Heatherdale Shale. This observation suggests a volcanogenic/exhalative source for base metal mineralisation in the lower Kanmantoo Group. It further suggests that syngenetic mineralisation in the Kanmantoo Group is developed on major bounding surfaces within a sequence stratigraphic framework.

124

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

STORM-DOMINATED SHALLOW MARINE SEDIMENTATION IN THE LOWER DEVONIAN WALHALLA GROUP AT CAPE LIPTRAP, SOUTHERN LACHLAN FOLD BELT Ian A. Dyson National Centre for Geology and Geophysics, University of Adelaide, Adelaide, SA 5005

The Lower Devonian Walhalla Group in the Melbourne Trough of the southern Lachlan Fold Belt consists of shale and thin to very thick-bedded fine-grained sandstone. It has been invariably interpreted as a tuibidite or shallow marine succession. The doubt surrounding the palaeoenvironmental interpretation of the Walhalla Group has arisen because sandstone beds within this succession resemble classical turbidites. The beds typically contain vertical stratification sequences that are identical in important aspects to the Bouma sequence for tuibidites in submarine fans. Until now, there have been no reported occurrences of HCS in the Walhalla Group. Storminfluenced sedimentary structures have been recognised in fine-grained sandstone of the Walhalla Group at Cape Liptrap, 130 km SE of Melbourne. They include hummocky cross-stratification (HCS), quasi-planar lamination (QPL), micro-HCS and Bouma BC sequences. These lithofacies are combined within a number of upward-sanding and hence upward-shoaling cycles. Fundamental cycles are commonly about 10 m thick and are interpreted as parasequences. There is no evidence of sedimentation at or above fairweather wave base. Thin-bedded sandstones display a sharp, flat base. Sole marks are commonly grooves and flutes with occasional gutter marks. Palaeoflow data from flutes suggests current directions towards the northwest and south-southwest. Internally the beds are parallel-laminated at the base passing upwards into ripple cross-lamination. Thinner beds display only the rippled interval. The sandstones are capped by either unidirectional ripples of the linguoid type or interference ripples., interpreted as current or combined flow ripples. Sandstones capped by combined-flow ripples are interpreted as micro-HCS. However, the palaeoflow data from beds capped by combined-flow ripples is east-west. The apparent reversal of sole marks from one bed to another displayed by micro-HCS is interpreted to reflect the reversing nature of oscillatory currents. BC beds and micro-HCS were deposited in relatively deep water in a mid to outer shelf setting or under the influence of weaker storm events They resemble classical turbidites and are interpreted as tempestites. Many sandstone beds at Cape Liptrap display QPL which is a gently undulating, almost planar style of lamination in which individual laminae often downlap onto the base of the sandstone. The uppermosrt interval may consist of three-dimensioanl ripples, rare symmetrical ripples or, instead, pass directly upwards into shale. Some QPL beds are lenticular or capped by undulose surfaces that are erosional in nature., suggesting reworking by oscillatory currents. They commonly range in thickness from 10 to 50 cm, with amalgamated beds up to several metres thick. Flutes marks are directed towards towards the west and southwest. QPL sandstones were deposited under high-energy combined-flow conditions. Downlapping laminae at the base of QPL suggest offhore sediment transport under the influence of unidirectional currents. The undulose upper surface of some QPL beds were formed under conditions of strong oscillatory flow and reflect peak storm conditions. They are interpreted as first-order surfaces. HCS is difficult to identify at Cape Liptrap. Many QPL sandstone beds are erosively overlain by a thin interval of laminae that symmetrically drape and thicken off both sides of an antiform, interpreted as isotropic HCS of the scour-and-drape type. It was formed under the influence of pure or dominantly oscillatory flow.. The antiform is a first-order surface formed by erosion from oscillatory currents. A rare asymmetrical form of HCS consists of laminae that dip predominantly in the same direction. The beds are flat based, but no sole marks were observed. Downlapping laminae at the base of the sandstone are erosively overlain by a thin interval of low angle parallel lamination. The HCS is not capped by ripples, but is sharply overlain by shale. This form of HCS is interpreted as anisotropic HCS. It is in effect a large-scale combined-flow ripple that is both climbing and migratory in its form. Basal laminae downlap towards the west in the interpreted offshore direction under the influence of combined wave surge and unidirectional currents. The strike of rare wave ripples crests on top of QPL indicates the Lower Devonian palaeoshoreline of the Melbourne Trough was oriented approximately north-south, with the interpreted offshore direction towards the west. A palaeowater depth for deposition of QPL at Cape Liptrap was possibly about 100 m, determined from application of Airy wave theory to first-order surfaces capping some beds. Storm-influenced sedimentary structures such as HCS, QPL, micro-HCS and Bouma sequences can be placed in an idealised vertical fades succession which can be used to infer hydrodynamic conditions at the time of deposition In a progradational succession, it reflects the relative strength of unidirectional versus oscillatory currents with depth. Thus, sandstone beds at Cape Liptrap are inferred to have been deposited in an inner to outer shelf setting. There is no need to invoke a turbidite origin for these sandstones as suggested by some palaeogeographic reconstructions. This study suggests that any palaeogeographic reconstruction of the southern Lachlan Fold Belt should take into account sedimentary structures, other than HCS, which are diagnostic of storm deposition. Furthermore, sandstone beds of the Walhalla Group are more likely to be tempestites deposited in a storm-dominated shallow marine environment.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

REGIONAL GEOCHEMISTRY: A CONTINENTAL PERSPECTIVE Alfred J. EGGO CRA Exploration Pty. Limited, Bundoora Private Bag 3, Bundoora MDC, Victoria 3083

Keywords: exploration geochemistry, geographic information systems, image processing, statistical methods.

ABSTRACT The compilation of large and comprehensive regional databases of geochemical and mineralogical information resulting from drainage, rock, laterite, lag and heavy mineral concentrate surveying carried out by CRAE and other organisations over Australia (Figure 1) can be used to provide important knowledge on the geochemical characteristics of large parts of the continent. After corrections have been applied to the raw geochemical data to minimise variations due to incompatible analytical techniques, differing lower limits of detection, changes in lithology and secondary environmental effects image processing techniques can be used to prepare regional geochemical maps. These maps provide a unique overview of relative geochemical abundance levels, regional trends, and anomalous patterns for large parts of the continent. The identification of metal-rich provinces within which economic ore deposits are located is a significant benefit of regional geochemical mapping.

Integration of the rectified geochemical data with geological and metallogenic data, geophysical and remote sensing information and structural information obtained from a high resolution digital terrain model of Australia is best carried out using Geographic Information Systems.

Figure 1 Extent of CRAE and external regional geochemical and mineralogical surveying.

This paper will present some examples of the application of large regional geochemical databases to regional target selection, warn of the pitfalls in their use, and provide examples of their use in research & development such as identification of metal-enriched source rocks for metallogenic modelling.

Acknowledgments The author thanks CRA Exploration Pty. Limited for allowing permission to publish this paper.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE BROKEN HILL ORE BODY: CONCENTRATION OF THE LEAD DURING HIGH GRADE METAMORPHISM K. Ehlers , J.Foster and A.P. Nutman Department of Earth Sciences, Monash University, Victoria 3168 Australia Research School of Earth Sciences, A.N.U., Canberra, A.C.T. 0200 Australia INTRODUCTION The origin of the Pb-Zn-Ag Broken Hill ore body is the source of much debate: What was the source of the metals? When and how were they introduced into the host Willyama Supergroup? How and when were they concentrated into the present ore body? Solutions to these questions include models of volcanosedimentary ± exhalative deposition and concentration to syn-sedimentary accumulation in a pro-grading deltaic fan (e.g. Wright et al., 1987). In this presentation we do not concern ourselves with how the metals originally got into the Willyama Supergroup, but propose that from reappraisal of published Pb-Pb isotopic data combined with U-Pb mineral geochronology that the Pb of the Broken Hill ore'body was concentrated during high grade metamorphism (T>700°C at a depth of >15 km) at ca. 1590 Ma. WHOLE ROCK Pb-Pb DATA Pb is the major element of the ore body, and is thus protected againsts shifts in isotopic composition by later exchange with exterior regions. Thus being able to trace the history of the Pb by isotopic means places a major constraint on the evolution of the ore. As discussed by previous workers (Gulson, 1984; Reynolds, 1971) regression of lode apatite and sulphide Pb-Pb data yields a secondary isochron significantly younger than the depositional age of the host rocks (currently thought to be ca. 1690 Ma, e.g. Page and Laing, 1992).

Our treatment of these data yields an age of 1565 ±22 Ma (MSWD 9.18, n=55). This age is indistinguishable from two stage Pb model ages of Broken Hill lode sulphides, which yield a mean age of 1574 Ma (|i=9.7, indicative of a crustal Pb-source).

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U-Pb MINERAL AGES Monazite and low Th/U metamorphic zircon ages of 1600-1570 Ma have been obtained by several workers (Gulson, 1984; Page and Laing, 1992) on rocks close to the Broken Hill ore body. We have undertaken zircon geochronology on garnetite rocks found in intimate association with the ore body. These rocks have been interpreted as a silicate-rich metasomatic halo to the ore body or as a heavy-mineral rich detrital sediment unit. Using SHRIMP we have found only 1590 Ma metamorphic zircons in the garnetite, which strengthens our interpretation of it as having a metasomatic origin, associated with the ore body.

CONCLUSION The Pb-Pb isotopic data in concert with U-Pb mineral ages are consistent with concentration of Pb (hence other economic metals) at ca. 1574 ±20 Ma. This age coincides with that general accepted for high grade metamorphism in the area. This implies that the Broken Hill ore body formed at ca. 1574 ±20 Ma at a depth of >15 km and at a temperature of >700°C. It is also of note that the ore body lies in the part of the area that preserves evidence of the highest metamorphic grade.

REFERENCES Gulson, B.L., 1984. Uranium-Lead and Lead-Lead investigations of minerals from the Broken Hill lodes and mine sequence rocks. Economic Geology, 79, 476-490.

Page, R.W. & Laing, W.P., 1992. Felsic metavolcanic rocks related to the Broken Hill Pb-Zn-Ag ore body, Australia. Economic Geology, 57,2138-2168.

Reynolds, P.H., 1971. A U-Th-Pb isotope study of rocks and ores from Broken Hill, Australia. Earth and Planetary Science Letters, 12, 215-223.

Wright J.V., Haydon, R.C., & McConachy, G.W., 1987. Sedimentary model for the giant Broken Hill PbZn deposit, Australia Geology, 15, 598-602.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 EVIDENCE FOR ANTIQUITY AND LONGEVITY OF AN AUSTRALIAN CONTINENTAL-SCALE LINEAMENT FRAMEWORK Catherine I. Elliott School of Earth Science, University of Melbourne, Parkville, 3052.

•Present address Minoil Exploration, PO Box 325, Wandin, 3139.

Australia is transected by a network of systematic continental-scale lineaments that are considered to be zones of concentrated, aligned tectonic activity which have apparent continuity over vast distances (Elliott, 1994a). The influence of lineaments on the rock record can be identified in many types of data-sets, and existing data reveals previously undescribed basement influences. Several continental-scale lineaments can be traced offshore with apparent continuity for hundreds to thousands of kilometres, two of which are seen to cross the Tasman Sea in offshore eastern Australia.

Geological and chronological evidence demonstrates that many of the lineaments have been zones of reactivation since at least the Early Proterozoic (-1880 Ma) and that they appear to cross major terrane boundaries. Alternative models for their origin include a) a pre-existing lineament network maintained in an ancient basement underlying the entire continent; b) lateral propagation of crustal-scale structures; c) alignment of genetically unrelated lineaments giving the appearance of continuity.

Australian deep-seismic profiles show that continental-scale lineaments are zones of crustal-scale structure which in some cases transect the crust-mantle boundary. Lineaments demonstrate many faulting styles, e.g. listric extensional (G3), planar moderate-angle thrusts (G21), and sub-vertical thrusts (G17). In some cases the structural style varies laterally along the length of the lineament.

Six continental-scale lineaments were chosen as examples of the Australian lineament framework, the NNW G2, WNW G3, NNE G5, NW G11, NS G12 and WNW G13 gravity corridors. Previous studies by O'Driscoll (1985, 1990) identified a network of continental-scale gravity and geological lineaments including the G2, G3 and G5 corridors. This study has further documented the geological evidence to demonstrate the antiquity and longevity of these, and additional continental-scale structures.

The G3 corridor, for example, coincides with the King Leopold Mobile Zone in northwestern Australia and can be traced as a major basement fault in the aeromagnetic data across to central Australia. In eastern Australia the G3 is seen to have influenced the tectonic and sedimentary history of the Palaeozoic and Mesozoic Eromanga and Sydney basins. The corridor can be traced as a double-edged feature across the Tasman Sea using Seasat/Geosat data and is observed to coincide with the dextral tectonic zone of the Cook Strait between the North and South islands of New Zealand. The G5 corridor coincides with the Halls Creek Mobile Zone in northwestern Australia and can traced in the aeromagnetic data offshore through the Arafura Basin, and in the tectonic and Seasat data across to Iryan Jaya.

The tectonic and depositional history of the Kimberley region in northwestern Australia and in particular the Bonaparte Basin, can be related to the interactive influence of at least five continental-scale lineaments (the G2, G3, G5, Gil and G12) The lineaments have pre-determined sites of basin margin faulting, and reactivation of these basement structures has controlled the subsequent depositional history (Elliott, 1994b).

REFERENCES Elliott, C.I., 1994a Australian lineament tectonics with an emphasis on northwestern Australia. PhD Thesis.

Unpubl University of Melbourne, Australia. 262 pp.

Elliott, C.I., 1994b. Lineament tectonics: An approach to basin analysis and exploration. In Purcell, P. and R., Sedimentary Basins of Western Australia—Conference Proceedings. Petroleum Exploration Society Australia. pp 77-90.

O'Driscoll, E.S.T., 1985. The application of lineament tectonics in the discovery of the Olympic dam Cu-Au-U deposit at Roxby Downs, South Australia. Global tectonics and metallogeny. 3 (1). pp 43-57 O'Driscoll, E.S.T. 1990. Lineament tectonics of Australian ore deposits. In. Geology and mineral deposits of Australia and Papua New Guinea. Aus.LM.M., pp 33-41 Acknowledgments: The author gratefully acknowledges the following companies for their financial support of the Ph.D. study; BHP Petroleum, BHP Minerals, ACS Laboratories, Western Mining Corporation, Acacia Resources and Stockdale Prospecting. Thanks are also due to my supervisors, I.B. Campbell, E.B. Joyce and C.J. Wilson.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 LINEAMENT TECTONICS: AN APPROACH TO EXPLORATION AND REGIONAL TECTONICS Catherine I. Elliott *. Ingrid B. Campbell , Chris, J. L. Wilson and Bernie Joyce School of Earth Science, University of Melbourne., Parkville, 3052. Australia. ^Present address Minoil Exploration., PO Box 325, Wandin, 3139. Australia.

Continental Resources NL., Camberwell, 3124. Australia.

Image processed gravity and aeromagnetic data sets, together with topography, bathymetry and offshore Seasat, reveal a pattern of intersecting continental-scale lineaments that transect the Australian continent. Detailed studies of the structural and depositional histories of the terrains through which they pass have demonstrated their antiquity and complex reactivation histories.

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Results of this study (Elliott, 1994) show that a geographical spatial association exists between lineaments and resources, as suggested by earlier research (O'Driscoll, 1990), and that both lineament intersections and lineament edges are of significance. The studies of Western and northwestern Australia indicate that lineament edges and intersections have been important in the localization of mineral deposits. The intersection zones of continental-scale corridors seem to be the most favourable regions for the deposition of major accumulations.

Within these, it is the second order intersections that pinpoint the potential target area.

Based on the observation that lineaments are zones of energy release (i.e. igneous activity, faulting etc.), and fluid flow, intersections appear to be favoured sites for the formation or entrapment of resource commodities.

This may be because they are zones of intersecting basement trends that have been technically reactivated through time. As such, they are zones of increased fracture porosity and therefore conduits for deep-crustal and mantle-derived fluids, and potential mixing zones for fluids of differing chemistries.

We suggest that lineament edges are zones of preferential mineral emplacement due to their being an interface between the reactivated, often highly structured zones of the lineaments, and adjacent stable cratonic blocks.

Studies of northwestern Australia, for example, show that minerals tended to accumulate along the edges of the major corridors, e.g. G3 and G5. Both of these zones have been shown in this study to have undergone a complex reactivation history in comparison to the terranes on either side.

The West Australian studies show that mineralization is preferentially located within 20 km, or a maximum of 50 km, of a lineament edge and not necessarily within the lineament. The conclusion is drawn that mineralization is probably associated with secondary structures related to the major basement weakness (identified as the continental-scale corridor) and that the secondary structures postdate the main fault zone and behave as dilational zones for preferential fluid flow and mineral entrapment. The geometry of faulting within the corridor and changes in the style and polarity of the structures laterally, will pre-determine the lineament edge of relative economic importance.

Lineament intersection zones provide areas of enhanced favourability with respect to petroleum habitat. They are areas of increased fracture porosity (migration pathways), technically active sites which lead to increased erosion and depositional rates (reservoir formation), areas of higher heat flow (maturation) and zones of enhanced structural activity (trap formation). The location of the major petroleum provinces of Australia's northwest margin at the intersection of the northeast structural trend (Tl) with the northwest continental-scale lineaments, supports this observation. Within the continental-scale intersection, the junction of less well-defined cross-strike discontinuities with the regional structural grain, such as the north-south fracture zones in the Vulcan Sub-basin (O'Brien, 1993), are areas of enhanced prospectivity.

REFERENCES Elliott, C.I., 1994. Australian lineament tectonics with an emphasis on northwestern Australia. PhD Thesis.

Unpubl. University of Melbourne, Australia. 262 pp.

O'Brien, G.W., 1993. Some ideas on the rifting history of the Timor Sea from the integration of deep crustal seismic and other data. Petroleum Exploration Society Australia Journal. 21, pp 95-115.

O'Driscoll, E.S.T. 1990. Lineament tectonics of Australian ore deposits. In. Geology and mineral deposits of Australia and Papua New Guinea. Aus.I.M.Mpp 33-41 Acknowledgments: The author gratefully acknowledges the following companies for their financial support of the Ph.D. study; BHP Petroleum, BHP Minerals, ACS Laboratories, Western Mining Corporation, Acacia Resources and Stockdale Prospecting.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 INTEGRATING REMOTELY SENSED DATA WITH OTHER SPATIAL DATA SETS TO PREDICT AREAS AT RISK FROM SALINITY F.H. Evans , P. Caccetta , and R. Ferdowsian 1

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In order to make the most appropriate use of resources, it is important to know which areas are affected by land degradation such as salinity, waterlogging and wind erosion. Satellite and airborne remotely sensed data may be processed to provide information which can be used for mapping and monitoring land condition.

Landsat TM satellite images from 1987 to 1994 have been used to map salinity in representative catchments in the southwest of Western Australia. Images from two years and information about the position of a site in the landscape have been combined to discriminate between areas that are salt-affected and areas with temporary low productivity due to management and seasonal effects. These salinity maps can be compared through time to identify areas where the land condition is changing.

Expert knowledge about the expression of salinity in the landscape has been used to identify spatial data sets which can be integrated with satellite images to predict areas at risk from salinity. Spatial data sets relating to terrain, geomorphology, and ground water movement can be easily derived from digital elevation models. These data sets have been combined with satellite images to establish a cost-effective method for mapping and predicting areas at risk from salinity.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 I3th Australian Geological Convention, Canberra, February 1996 THE APPLICATION OF REGIONAL OXYGEN ISOTOPE DEPLETION PATTERNS AS AN AREA SELECTION TOOL FOR LOW-SULPHIDATION EPITHERMAL GOLD DEPOSITS G.R. Ewers , D.E. Mackenzie , B.I. Cruikshank , J.H.C. Bain and A.S. Andrew Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2600 CSIRO Division of Petroleum Resources, PO Box 136, North Ryde, NSW 2111 1

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The recognition of regional oxygen isotope depletion patterns in high-level igneous rocks provides a means to discriminate areas which could be prospective for low sulphidation (adularia-sericite type) epithermal gold mineralisation. Unaltered primary igneous rocks typically have a narrow range of 5 O values between about +5 and +12 per mil relative to Standard Mean Ocean Water. However, there has been a recognition that some volcanic and plutonic rocks are regionally 0-depleted relative to "normal" igneous values. The only plausible means of producing this depletion requires the interaction between low- 0 meteoric fluids and rocks at high temperatures, because other fluid sources (e.g. magmatic or metamorphic) will either increase or only slightly decrease "normal" whole-rock igneous values. This meteoric water-rock interaction could occur either at the subsolidus stage during cooling (the most likely scenario), or at the magmatic stage through direct interaction with meteoric fluids or the assimilation of rocks already depleted in 5 0. An association between epithermal mineralisation and regional 0 depletion could be anticipated: low sulpliidation epithermal systems almost invariably involve meteoric water, and hydrothermal systems dominated by meteoric water could be expected in volcanic terrains during the waning stages of igneous activity. ls

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The coincidence of an extensive regional oxygen isotope depletion pattern over most of the northern Drummond Basin with a recently discovered epithermal district (which includes economic deposits at Pajingo and Wirralie and numerous prospects) is consistent with a similar association for younger, world-class epithermal districts in the United States (Ewers & others, 1994). Reconnaissance whole-rock oxygen isotope data for Permo-Carboniferous volcanic rocks in the northern Coen Inlier indicate an area of isotopic depletion that correlates with regional stream sediment geochemical anomalies normally associated with epithermal deposits (Ewers & others, 1995). The data suggest that the northern Coen Inlier is a region of high epithermal potential worthy of more systematic exploration. In contrast, extensive exploration within and around the Featherbed Cauldron Complex has failed to define any significant epithermal mineralisation. Whole-rock oxygen isotope values are predominantly near-normal, and isotopic depletion is confined largely to the Late Carboniferous volcanic rocks in the southern areas of the complex, particularly along caldera margins where major structures provided pathways for fluid circulation (Ewers & others. 1995). The data are consistent with earlier observations that the Late Carboniferous sequence is more closely associated with hydrothermal activity than the Early Permian volcanics, and that meteoric fluids were focussed through these major structures, at least during the waning stages of igneous activity. Regional oxygen isotope depletion patterns have also been identified in the Scardons and Galloway Volcanics on the Red River 1:250 000 sheet area. Preliminary interpretation of these data suggests that depletion in the Scardons Volcanics is associated with major structures and the margins of caldera collapse features, whereas depletion in the Galloway Volcanics is related both to major structures and to hydrothermal systems associated with the late emplacement into the volcanic pile of zoned,fractionatedgranodiorite-granite plutons.

The use of regional oxygen isotope depletion patterns to assess the epithermal gold potential of late Palaeozoic igneous terrains in north Queensland has demonstrated that this association is not simply a Tertiary phenomenon confined to epithermal districts in the southwestern USA. Younger terrains are less likely to be eroded or overprinted by later events and, in this sense, regional oxygen isotope signatures should be more obvious. However, this approach is worth testing elsewhere in geological provinces (possibly much older) that have potential for low-sulphidation epithermal deposits.

REFERENCES Ewers, G.R, Mackenzie, D.E., Wyborn, D., Oversby, B.S., McPhie, J., & Andrew, A.S., 1994. Regional 0 depletions in igneous rocks from the northern Drummond Basin, Queensland, Australia, and their implications for epithermal gold mineralisation. Economic Geology, 89, 662-673.

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Ewers, G.R, Mackenzie, D.E., Cruikshank, B.I., & Andrew, A.S., 1995. Whole-rock regional oxygen isotope depletion patterns as a guide to epithermal gold exploration in north Queensland. AGSO Journal ofAustralian Geology & Geophysics, 15, 395-407.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, CanberraFebruary 1996 A PROPOSAL FOR ODP PALAEOCLIMATIC AND PALAEO-OCEANOGRAPHIC DRILLING IN THE "SOUTHERN GATEWAY" OFF TASMANIA Neville Exon , Peter Hill , George Chaproniere and Greg Whitmore Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 Geology Department, James Cook University, Townsville, Queensland 4811.

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The Cretaceous and Cainozoic sedimentary record of the region south and west of Tasmania is strongly influenced by the history of the breakup of Australia and Antarctica, from the continental stretching and slow seafloor-spreading phase of the Cretaceous and Palaeogene to the fast-spreading phase of the last 45 million years. This record starts with early-rift non-marine sedimentation in the Early Cretaceous, continues with restricted shallow-marine sedimentation in the rifts between the two continents in the Late Cretaceous and Palaeogene, and changes to open marine sedimentation in the Oligocene as Antarctica cleared the South Tasman Rise and the Circum-Antarctic Current first came into play through the formerly closed gateway south of Australia. We are in the process of preparing a submission to the Ocean Drilling Program (ODP) to investigate this area of global palaeoclimatic and plate tectonic significance.

In 1973, Sites 280-283 of Deep Sea Drilling Project Leg 29 allowed the development of a broad, globally significant history of the Cainozoic events in the region. However, the sites were generally located on regional highs in order to minimise the depth of penetration necessary to reach older strata, and hence much of the sequence was cut out by hiatuses. Furthermore, this first scientific drilling in the area was carried out with only occasional cores in the holes, so that detailed resolution of the sediment history was impossible. Nevertheless, Nick Shackleton and Jim Kennett did produce composite oxygen and carbon isotope curves from foraminifera that covered the Late Paleocene to the Pleistocene. These show the general fall of bottom and surface water temperatures through the Cainozoic, with a general fall in the Palaeogene, a rapid fall in the early Oligocene, steady temperatures until the middle Miocene when there was another rapid fall as Antarctic glaciation came into full sway, and then the complex fluctuating fall through into the Pleistocene.

Global palaeoclimatic and palaeo-oceanographic questions still applying to high southern latitudes, such as the number, timing and character of warm intervals in the Paleocene and Eocene, need high-quality continuous cores for their resolution. The Tasmanian region remains a key area to address such questions, and we propose that about six ODP sites be drilled there. One, west of Tasmania, would cover Late Cretaceous history, including the Cretaceous-Tertiary boundary event and the transition from non-marine to restricted marine sedimentation.

Several, with at least one on the South Tasman Rise and another in the deepsea sediment drift to the east, would investigate Palaeogene events including the development of the Circum-Antarctic Current. These and other sites would look at the Neogene deepening of the ocean and temperature fluctuations, and especial emphasis would be put on Quaternary events related to climate change, possible movements of the Sub-Tropical Convergence and the Polar Front, and fluctuations in the carbonate compensation depth. We envisage that penetration in each of three sites would be about 1000m, and in the other three less than 500 m; about half the sites would continue to basement.

The need for ODP drilling is compelling, and there is no alternative way to acquire the necessary information. An extensive gravity coring and dredging campaign has provided ground truthing for about 20 000 km of goodquality seismic profiles. The seismic grid includes 13 600 km of moderately high-resolution data recorded along with swath bathymetry and imagery by R.V. L'Atalante in 1994, with an average spacing of 16 km, and we believe that appropriate and safe sites can be selected on the basis of existing data, although ODP may possibly require further limited site surveys. Most sites would be in water depths of 2000-4500 m, and none would be in water shallower than 800m. The thickness of all the sequences would allow high-resolution biostratigraphy and isotope stratigraphy: more than 1500 m of Late Cretaceous, 2500 m of Palaeogene, and 600 m of Neogene strata are present in places. Much of the Neogene sequence should be drillable with the advanced piston corer, so one could expect excellent recovery and little disturbance. The older sequences would require the use of the extended core barrel or the rotary core barrel, probably leading to poorer recovery and certainly to more disturbance, but the results would remain more than adequate.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

REMOTELY SENSED SEABED IMAGERY AND BATHYMETRY OFF TASMANIA: ITS VALUE FOR FISHERIES AND CONSERVATION Neville Exon , Peter Hill and Tony Koslow Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 CSIRO Division of Fisheries, GPO Box 1538, Hobart, Tasmania 7001 1

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An offshore area of 200 000 km was mapped by the Australian Geological Survey Organisation (AGSO) early in 1994, south and west of Tasmania. The multibeam sonar system of the French research vessel LAtalante was used. It generates 162 narrow sonar beams that fan out across the ship's path, giving a survey width of up to seven times the water depth beneath the vessel. Scientific participation was from AGSO and Australian and French universities The work was part of an ongoing AGSO geoscience program of mapping the Australian continental margin.

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The Australian Southeast Fishery (SEF) has important fishing grounds east, south and west of Tasmania, and large parts of the latter two were mapped during the L'Atalante cruise. The fish of most interest in the fishery are orange roughy and blue grenadier. Both are caught near the bottom, using demersal otter trawls. The 1994 approved total allowable catch (TAC) for orange roughy, the most valuable species in the SEF, was 8,000 tonnes, which was caught with a landed value of approximately $25 million. Orange roughy is caught at midslope depths of about 700-1200 m. In winter it is fished primarily on its spawning ground, a single seamount off northeast Tasmania. In summer, orange roughy is fished from a group of about 35 seamounts off southern Tasmania. The 1994 TAC for blue grenadier was 12 351 tonnes, of which about 3,000 tonnes were caught, worth about $7.1 million. Blue grenadier is fished largely in winter on its spawning ground off western Tasmania, where it aggregates in canyons at depths of 300-600 m.

Before releasing three detailed maps of the two fisheries areas to fishermen, AGSO consulted widely with Commonwealth agencies about conservation questions, not only concerning the fish stocks, but also the unique benthic communities growing on the seamounts. The maps are at 1:100 000 scale with 20 m contours, and are a tremendous advance on the very generalised pre-existing maps of the fishing grounds. Two maps have been produced of the southern orange roughy fishing grounds centred 70 km south of Tasmania: one plain bathymetry, the other with areas of rough sea bed mapped from the acoustic imagery. These cover water depths of 800-3000 m on a slope that generally falls southward at 3-5°, and show that the fishermen's hills are steep-sided volcanoes up to 600 m high, with rocky slopes of 20-30°. About 50 known hills and 20 new ones are now very accurately located, and mapped in the sort of detail which allows fishermen to minimize the risk of damage when trawling over the bottom.

The seamounts contain a highly diverse, characteristic benthic fauna of corals, sponges, fishes, and other organisms. Many of the species are believed to be endemic, but the fauna is largely undescribed. To protect this fauna, the Australian Nature Conservation Agency and the Australian Fisheries Management Authority set aside 370 square kilometres containing some of the newly-mapped hills in an interim Marine Protected Area, one of the first seamount reserves in the world. The area will be protected for three years, while CSIRO investigates the impact of trawling on the seamounts and the conservation value of the interim reserve.

The western area off most of southwest Tasmania is a generally smooth, sedimented slope of about 5°, cut by submarine canyons running downslope to the west-southwest. Fifteen major canyons extend into water shallower than 1500 metres, and most of these (and some other smaller canyons) extend to the edge of the continental shelf at 170-200 m. The canyons, cut into Neogene sediments, are steep-walled and seldom more than 100 m deep.

Those in water shallower than 600 m are important to the blue grenadier fishery, and their accurate location and shape, as revealed in the bathymetric map, are clearly of benefit to fishermen.

A very strong case can be made for more such mapping around Australia's margin, for geological and fisheries purposes, but that depends on a swath-mapping system being available in the country. AGSO, CSIRO and other agencies are looking into the possibility of importing such a system.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

INTEGRATED AEROMAGNETIC, STRUCTURAL AND TECTONIC INTERPRETATION USING GIS. Martin C. Fairclough , Antonio P. Belperio and Susan J. Daly 1

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'Regional Exploration, Mount Edon Gold Mines PO Box 133 Greenwood WA 6024 Mines and Energy South Australia PO Box 151 Eastwood SA 5063 2

Limited basement outcrop on the western Gawler Craton has previously been a major obstacle to delineating potential targets for mineral exploration. To overcome this, Mines and Energy South Australia (MESA) undertook a major airborne magnetic surveys over crucial portions of the Craton. To maximise the benefit of this data, all relevant information required to "ground truth" the aeromagnetics was captured and integrated in digital (GIS) form. Priority was given to geological, geochemical and drillhole data needed to generate an internally consistent, regional bedrock geology interpretation. Much of the data was. sourced from previous exploration company open-file reports, as well as past MESA reports of investigations.

All necessary data sets were captured in digital form and migrated to a common GIS. This formed the basis for interrogation, interpretation and construction of a solid geology map of the region (Belperio, 1993). Finally, the source data and basement interpretation were packaged and publicly released (Belperio, 1995). This digital integration approach has several advantages over traditionally derived tectonic/structural maps: widely variable data types can complement each other such that more information is extracted than if individual themes were assessed separately ("the whole is greater than the sum of the parts"). inclusion of the final basement interpretation with the set of data that was used to generate it gives users an insight into the rationale behind the interpretation and allows an objective opinion on its validity. databases are assessed, verified or modified as they are intensively used for the interpretation and consequently their quality and integrity is substantially improved as a result. spurious geological information can be easily identified when displayed with other data and contradictions resolved. For example outcrop data can be easily revised on the basis of more recently available magnetics to give the most up to date synthesis available. data themes can be queried and saved as separate subsets to remove superfluous information. For example, separate themes of basement outcrops and basement intersecting drillholes only can be generated from detailed geology and drillhole coverages. hardcopy maps can be generated as required at the most useful scale and configuration. geologists can generate interpretations directly on the computer, thereby avoiding drafting delays and allowing easy ongoing modification as ideas progress and evolve.

These techniques were used to generate several data packages for the Gawler Craton including tectonic and metallogenic models and released in their entirety in digital form. This method of integrating data and interpretation has proved extremely popular in the exploration industry and the wider geological research community. New ideas arising directly out of this work (Daly et al.,1995) include a Palaeoproterozoic continental collision margin (the Fowler Suture Zone) and associated new exploration models.

REFERENCES Belperio, A. P., 1993. Integration of Geological data using GIS. Preview August 1993: 18-23.

Belperio, A. P., 1995. South Australian initiatives in GIS integration of geoscience information. 3rd National Conference on the Management of Geoscience Information and Data. AMF, Adelaide 61- 62.

Daly, S. J., Fairclough, M.C., Fanning, C. M. and Rankin, L. R., 1995. Tectonic evolution of the western Gawler Craton: Palaeoproterozoic collision and likely plate margin. Geological Society ofAustralia Abstracts 40: 35-36.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE "MAWSON CONTINENT": ARCHAEAN TO PROTEROZOIC CRUST IN T H E EAST ANTARCTIC SHIELD AND GAWLER CRATON, AUSTRALIA. A CORNERSTONE IN RODINIA AND GONDWANALAND C. Mark Fanning*. David H. Moored Vickie C. Bennett* and Susan J. Daly^ * Research School of Earth Sciences, ANU, CANBERRA, ACT 0200, AUSTRALIA ^Geological Survey of Victoria, PO Box 2145, MDC Fitzroy, Vic 3065, AUSTRALIA ^Geological Survey of South Australia, Box 2506, EASTWOOD, SA 5063, AUSTRALIA

There is now considerable evidence that south-central Australia was not merely adjacent to East Antarctica across the Southern Ocean, but that the Gawler Craton (South Australia) was part of the East Antarctic Shield during the Archaean and Proterozoic. We propose the term Mawson Continent for this once continuous block of Archaean to Palaeoproterozoic crust. Of the exposed portions of Antarctica, the Mawson Continent comprises George V Land, Terre Adelie and Miller Range of the Central Transantarctic Mountains, and the Gawler Craton in southern Australia. Clearly the full extent of the Mawson Continent is masked by the ice shelf, but on the basis of U-Pb zircon studies in the Shackleton Range, similar Archaean and Proterozoic crust is present. Thus the Mawson Continent may extend across Antarctica to Coats Land.

The East Antarctic Shield was thought to be encircled by a Grenville age orogen. U-Pb zircon studies have now shown much of this region to be Pan African in age, either distinct Pan African orogens, for example the Liitzow-Holm Complex (500-550 Ma), or Grenville age orogens reworked during Pan African times, for example the Rayner Complex. This orogenic belt, or series of orogenic belts, marks the western boundary to the Mawson Continent in Antarctica. The presence of Pan African tectonism implies that these orogens were involved not only in the amalgamation of Rodinia, but may also represent collision zones reactivated during the formation of Gondwanaland.

Outboard of these orogens in Antarctica are the Annandagstoppane region in western Dronning Maud Land, the Napier Complex in Enderby Land and the Rauer suspect terrane, Vestfold Hills. These are regions of Archaean material, separated from their main cratonic nuclei during Gondwana break-up. The Annandagstoppane region is believed to be originally part of the Kalahari craton of southern Africa, the Napier Complex was originally connected to southern India, and at this stage the Rauer suspect terrane has an enigmatic association.

Within Australia, high quality aeromagnetic images enable us to delineate the Fowler Suture Zone and Karari Fault Zone as orogenic belts that are part of the north and northwestern margins of the Gawler Craton, and hence the Mawson Continent. SHRIMP U-Pb zircon ages constrain major tectonism in these at about 1650 and 1540 Ma, considerably older than Grenville. The 1650 Ma and 1540 Ma tectonism is significant in that it places the amalgamation and subsequent mobile belt development in the south-central Australian continent much earlier than the previously suggested Grenville age event. We propose that these belts were continuous from the western Gawler Craton into Antarctica, extending through the coast to near Cape Goodenough in Wilkes Land.

The eastern margin of the Mawson Continent was the palaeo-Pacific Ocean. During the late Neoproterozoic, deposition in the Adelaide Fold Belt developed along this margin in Australia, continuous with sedimentation if the Ross Orogen, Antarctica. Whether Laurentia, and China were located outboard of the Adelaide-Ross orogen is debatable. However, it is clear that this was a passive margin during much of the sedimentation in the Adelaide-Ross orogen, developing into a rift setting late in the Neoproterozoic and locally an arc environment during the Cambrian.

In southern Eyre Peninsula there is evidence for a lengthy passive margin history, extending perhaps to the Palaeoproterozoic with continental shelf deposition of the 2000-1850 Ma Hutchison Group. This prolonged eastern passive margin history for the Mawson Continent may be traced through the Proterozoic with a series of "failed-rift" related basins, each with small packages of volcanic and sedimentary material. Finally the onset of the Adelaide-Ross orogen at about 800 Ma culminates with the development of the Pacific.

Thus we propose that the Mawson Continent in east Antarctica and southern Australia was a cornerstone in the development of Rodinia and Gondwanaland. Initially forming in the middle Archaean, with a period of consolidation and orogenesis during the late Archaean to early Palaeoproterozoic, further growth in the Palaeoand Mesoproterozoic, before a stable craton developed after massive late to postectonic magmatism around 1580-1600 Ma. Collision with other Australian cratons occurred between about 1500 and 1550 Ma to amalgamate the southern and western parts of the Australian continent. Successive Grenville and Pan African age belts encircle the Mawson Continent which acted as a stable nucleus for successive terrane amalgamation to Rodinia and Gondwanaland.

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GEOLOGICAL SOCIETY OF A USTRAL1A, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CENOZOIC COOL-WATER CARBONATES OF THE GREAT AUSTRALIAN BIGHT: an ODP proposal to decipher the record of Southern Ocean evolution, sealevel, paleoclimate, and biogenic production David A. Fearv . Noel P. James , Brian McGowran , and Peter L. Smart Australian Geological Survey Organisation, Dept. Geology & Geophys., University of Sydney, Sydney NSW 2006 Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6, Canada Department of Geology & Geophysics, University of Adelaide, Adelaide SA 5005 Department of Geography, University of Bristol, University Road, Bristol BS8 1SS, England 1

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This proposal advocates the drilling of a transect of holes across the southern continental margin of Australia; the largest cool-water carbonate shelf on Earth today. This latitude-parallel shelf along the northern margin of the Southern Ocean contains fundamental geological and paleoceanographic information of global geodynamic, sedimentological, paleobiological, and paleoclimatological importance. The major objectives of this proposal are: 1) to ascertain the way in which a large, high- to mid-latitude shelf carbonate platform evolved throughout the past 65 m.y. in response to oceanographic and biotic change; and 2) to extract information contained in the carbonate sediments detailing global sealevel fluctuations, physical and chemical paleo-ocean dynamics, biotic evolution, hydrology, and diagenesis. Furthermore, because of architectural and compositional similarities with many older Phanerozoic carbonate platforms, the results from the proposed drilling would be of tremendous importance for the actualistic modelling of ancient open platforms and ramps.

Offshore seismic data and limited drillhole information indicate that the margin has been the site of dominantly cool-water carbonate shelf deposition since the Eocene, and show a detailed accretionary histoiy of progradation, erosion, and biogenic mound growth. The subsidence history is relatively simple, resulting in a 1 km-thick Cenozoic section. Onshore exposures provide a basis for predictive analysis of the offshore sequences identified in seismic sections, and confirm that the sediments are predominantly soft, friable, and abundantly fossiliferous.

A shallow shelf to deep continental rise transect of 12 holes is proposed (Fig. 1), located to penetrate inner shelf, outer shelf, upper slope, upper slope terrace, and continental rise settings. The primary drilling objective is a more detailed understanding of global environmental change in high- to mid-latitude settings. Cores from different facies at various depths during a range of geologic periods will yield a detailed anatomy of a Cenozoic cool-water carbonate shelf. The response of this depositional system to inferred sealevel fluctuations will be compared to records from warm-water, rimmed and un-rimmed carbonate platforms in order to test and refine the global sealevel curve, and most importantly to describe the reaction of cool-water carbonate depositional systems to different phases of the sealevel cycle. Biological and chemical paleoenvironmental proxies will be used to decipher a detailed paleoceanographic record, in order to more precisely describe the timing and paleoceanographic effects of the opening of the Tasman Gateway, and the influence of the Leeuwin Current on paleoproductivity over time. The shelf-to-basin transect will also provide high resolution data on the tempo and pattern of biotic evolution in oceanic and neritic environments.

Secondary objectives are directed towards understanding the hydrology of a carbonate platform adjacent to a vast inland karst with sluggish water circulation; and the nature of early burial diagenesis (lithification and dolomitization) in a cold, seawater-dominated system.

South Australian Abyssal Plain

Fig 1. Map of the western Great Australian Bight, showing bathymetry and the 12 proposed ODP drill sites.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 TECTONIC SIGNIFICANCE OF ORDOVICIAN ROCKS OF THE LACHLAN FOLD BELT, SOUTHEASTERN AUSTRALIA Christopher L. Fergusson School of Geosciences, University of Wollongong, Wollongong, NSW 2522

Ordovician strata in eastern Victoria and southeastern New South Wales are divided into a lower unit of monotonous quartz turbidites with some interbedded chert, and an upper unit of graptolitic "black shale" or more accurately a black fissile siliceous mudstone, containing few sandstone beds (VandenBerg & Stewart 1992). Sandstones contain quartz, muscovite, biotite, feldspar and low-grade metamorphic rock fragments consistent with derivation from an uplifted metamorphic-granitic terrain that was most likely the RossDelamerian mountain chain of South Australia and Antarctica. The Delamerian event has a modern analogue in the formation of Taiwan where an island arc has collided with a passive continental margin.

In the Late Cambrian - Early Ordovician, collision occurred between the passive margin, represented by the Adelaide Trough, and an oceanic realm to the east that contained a Cambrian western Pacific-style island arc system that formed basement to the Ordovician turbidite fan. Collision formed the mountain range that provided the source of the Ordovician succession. It is therefore quite- incorrect to refer to the Ordovician quartz turbidite wedge as a passive margin deposit. The tectonic setting of the Ordovician volcanic pile is considered by Fergusson & Colquhoun (this volume).

In eastern Victoria, one of the most important Ordovician sections east of the Melbourne Zone occurs in the upper Howqua River (VandenBerg & Stewart 1992, p. 165). Here, the Cambrian greenstones are steeply dipping and younging towards the east and include 3500 m of boninitic lavas, tholeiitic basalts, associated breccias and sedimentary rocks (Crawford 1988, p. 50-54). Farther east, occur latest Cambrian chert with overlying black shale, chert and a turbidite succession containing distinctive green sandstones.

Contacts between units have been described as locally faulted but the age determinations are consistent with an overall conformable succession from the Cambrian into the Early Ordovician. Farther upstream the green sandstones pass rapidly into a monotonous quartz turbidite succession that contains early Bendigonian graptolites. Work in progress (S. Tye and C.L. Fergusson unpub. data) has established that the green sandstone unit contains a higher proportion of feldspar (mainly orthoclase) and lithic fragments than normally encountered in the Lower Ordovician turbidite unit. The monotonous turbidite unit is overlain by Darriwilian-Gisbornian black shale which in turn is faulted against an unfossiliferous turbidite succession of probable Silurian age. The significance of this section is that it demonstrates: (1) that the oceanic Cambrian succession occurs east of the Melbourne Zone, and (2) that the Cambrian oceanic succession is basement to the Ordovician succession.

In Merrijig Creek southeast of Tabberabbera, the Ordovician succession is multiply deformed but has been shown to consist of the lower unit of monotonous quartz turbidites of Early Ordovician (BendigonianChewtonian) to Late Ordovician (pre-Eastonian) age (Stewart & Fergusson 1988). This lower unit contains an up to 22 m thick interval of black shale and chert, containing late Darriwilian-early Gisbornian fossils.

The upper unit consists of Eastonian black shale (Stewart & Fergusson 1988). Farther west, the Cobbannah Group occurs to the southwest of the Wonnangatta Line and was previously regarded as an Ordovician unit (VandenBerg & Stewart 1992). It is now thought to be Early Silurian, equivalent to the Yalmy Group of the Delegate-Mountain Creek region farther to the east. Although no age diagnostic fossils have been recovered from the Cobbannah Group, the following features of the unit are considered more likely for it to have a younger age: (1) the unit contains no chert as found farther east in the Ordovician Hotham Group, (2) the unit is divisible into a number of subdivisions, with mudstone-rich and mudstone poor formations, in contrast to the monotonous Early Ordovician turbidite unit, (3) burrowing in mudstones is common in contrast to Ordovician units where bioturbation is largely restricted to tracks on bed bases, and (4) an identical unit occurs in the upper Howqua River and is faulted against, and distinct from, the neighbouring Ordovician succession. An Early Silurian age for the Cobbannah Group indicates a more complex palaeogeography than previously recognised. Palaeocurrents in the Cobbannah Group indicate derivation from the west, possibly from the fold-thrust belt developing in western Victoria (BallaratBendigo and Stawell Zones).

REFERENCES Crawford, AJ., 1988. Cambrian. In Douglas J.G. and Ferguson J.A. eds. Geology of Victoria, pp. 37-62.

Geological Society of Australia, Victorian Division.

Stewart, I.R., & Fergusson, CI-., 1988. A Lower to Middle Ordovician age for the Hotham Group, eastern Victoria. Proceedings of the Royal Society of Victoria 100, 15-20.

VandenBerg, A.H.M., & Stewart, I.R., 1992. Ordovician terranes of the southeastern Lachlan Fold Belt: stratigraphy, structure and palaeogeographic reconstruction. Tectonophysics 214, 159-176.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 EARLY PALAEOZOIC QUARTZ TURBEDITE FAN AND VOLCANICLASTIC APRON, MUDGEE DISTRICT, NORTHEASTERN LACHLAN FOLD BELT, NEW SOUTH WALES Christopher L. Fercusson and Gary P. Colquhoun School of Geosciences, University of Wollongong, Wollongong, NSW 2522

Early Palaeozoic rocks of the northern Capertee Zone, in the northeastern Lachlan Fold Belt, consist of two major units: (1) the Early Ordovician Adaminaby Group, that is elsewhere widespread throughout the eastern half of the fold belt and contain a quartz turbidite succession, and (2) the Late Ordovician - Early Silurian Coomber Formation, a predominantly mafic volcaniclastic unit with subordinate mafic igneous rocks, that represents a deep-marine apron of island arc rocks farther west. In the Bara Creek area, the Adaminaby Group is divided into three subunits: member A is a thick succession of thick-bedded, graded sandstones with basal chert and slate; it is overlain by member B with slate and bedded chert of late Darriwilian age; and is in turn overlain by member C with a thin succession of thin-bedded turbidites. The Coomber Formation conformably overlies the Adaminaby Group; the contact is sharp with no interfingering between the two units. Concordantly overlying the Coomber Formation are Late Silurian silicic volcanics, clastics and limestone of the Dungeree Volcanics indicating shallowing of depositional environments either due to uplift or infilling of the basin of deposition.

Ordovician - Early Silurian rocks of the northern Capertee Zone reflect the distal expansion of a volcanic island chain onto a giant terrigenous submarine fan. The quartzose turbidites and radiolarian cherts of the Adaminaby Group indicate a deep-marine environment and they are far removed from the continental source in the Delamerian Fold Belt in western New South Wales and western Victoria. In contrast to areas farther south, Adaminaby Group of the Mudgee district is not overlain by the Gisbornian-Bolindian graptolitic black shale, known in southern New South Wales and eastern Victoria as the Bendoc Group.

The Bendoc Group correlates with the unfossiliferous Coomber Formation which represents the incoming of mafic volcanic detritus from a neighbouring chain of marine volcanoes. The Coomber Formation is correlated with the Cudgegong Volcanics to the southwest that contain Late Ordovician fossils amongst a complex assemblage of lavas, breccias and finer volcaniclastics. In contrast to Ordovician successions of the Molong High farther west, no in situ late Gisbornian to Bolindian carbonate platform occurs in the Capertee Zone. A decrease in abundance of coarse clastics from the Cudgegong Volcanics to the Coomber Formation implies a source to the west; this could be either the Molong High or a closer island chain now covered by younger strata to the west. Alternatively, the two units could be non-contemporaneous due to the lack of accurate biostratigraphic data from the Coomber Formation.

Constraints on the tectonic setting of the Ordovician rocks are obtained from consideration of the setting of Cambrian rocks of southeastern Australia. Cambrian rocks in central Victoria have a western Pacificstyle island arc signature. Scattered Cambrian ophiolites with supra-subduction zone characteristics occur along the Peel Fault and support an east-facing island arc setting for Cambrian rocks of the Lachlan and New England Fold Belts. Ordovician volcanism is considered to reflect a continuation of the same tectonic pattern as for the Cambrian, except that volcanism was on a significantly reduced scale. It is now recognised, from deep-sea drilling, that mafic volcanism extends over a much greater area than just the island arc, with submarine volcanic centres occurring in the both the forearc and backarc regions. For example, the Eocene Izu-Bonin-Mariana system developed rapidly and formed a mafic volcanic belt up to 450 km wide and 3000 km long with crustal thicknesses of 10-20 km. The extent of volcanism can also be highly variable over time with the possibility of synchronous activity on submarine volcanic ridges in addition to frontal and remnant arcs. In the Lachlan Fold Belt, attempts to portray the location of the arc axis and forearc basin are fraught with difficulty given the presently limited extent of exposed mafic volcanic/volcaniclastic rocks. Early Ordovician mafic volcanism is considerably more restricted than that in the Late Ordovician, showing that volcanism has changed dramatically in volume over time, as has been documented for both the Izu-Bonin-Mariana and Tonga island arcs. Regardless of the cause of these variable patterns of volcanism, the duration of intermittent mafic volcanism is consistent with long-lived subduction related to an island arc system. The subduction zone itself has apparently remained anchored throughout the Palaeozoic in the New England Fold Belt.

As for elsewhere in the northeastern Lachlan Fold Belt, only one regional cleavage (S ) is developed and affects all units, including the nearby Late Devonian succession, consistent with an Early Carboniferous timing of the deformation. Folding and thrusting accompanied cleavage development with tectonic transport from west to east. D was imposed on Fj that are indicated by steeply plunging folds (Fj), intersection lineations and some downward-facing F . D cleavage, folds and thrusts are folded in areas affected by significant D deformation resulting in locally subhorizontal S and common kinking of S . Early Carboniferous deformation occurred in a backarc setting and was synchronous with the development of a huge subduction complex in the New England Fold Belt.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SEDIMENTARY RECORD OF THE ONSET OF FORELAND THRUST LOADING: LATE PERMIAN, BOWEN BASIN, QUEENSLAND Christopher R. Fielding. Christopher J. Stephens & Rodney J. Holcombe Department of Earth Sciences, University of Queensland, Qld 4072, Australia

In this paper we document the sedimentary record of the transition from passive, thermal subsidence to flexural, thrust load-driven subsidence at the orogenic margin of the Bowen Basin, a complex extensional to retroarc foreland basin. Spectacular outcrops of coarse clastic deposits containing slump folds and other evidence of mass wasting are interpreted in terms of destabilisation of an offshore marine environment by subsurface thrust propagation.

During the Late Permian, the Bowen Basin was a retroarc foreland basin in response to the propagation of thrust sheets west-southwestward into the basin from a developing orogen to the east. The basin developed a pronounced cross-sectional asymmetry, with an elongate depocentre located adjacent to the eastern (orogenic) margin. The initially marine basin rapidly filled with first-cycle, volcanogenic detritus derived from the rising orogenic mountain range. Spectacular natural exposures in the banks of the Fitzroy River, west of Rockhampton in coastal central Queensland, provide a superb illustration of the onset of foreland thrust-loading close to the original eastern margin of the Bowen Basin. The rocks are referred to the Upper Permian Moah Creek Beds, and are preserved within the Gogango Overfolded Zone. The exposed succession is dominated by thick intervals of rhythmically interbedded siltstones and very fine- to medium-grained sandstones, with at least three discrete packages of pebble to boulder conglomerates and diamictites.

Sandstones and siltstones are typical of middle Permian offshore marine shelf facies across the greater part of the Bowen Basin. Siltstones contain scattered lonestones, Planolites burrows and body fossils of crinoids, bivalves and brachiopods (mostly intact and articulated). Sandstones occur in sharp-based, often graded and gradationally-topped beds which display flat lamination and ripple cross-lamination only in their upper parts.

Bioturbation in the tops of sandstone beds and interbedded siltstones is referrable to Rhizocorallium, Planolites, and less abundant Teichichnits, Palaeophycus and Zoophycos. Plant debris is abundant, ranging in size from macerated fine detritus to large stalks and branches.

The coarse-grained facies, which occur mostly in non-erosively-based beds and may have either sand or silt matrix, are transitional into one another. These facies vary from clast to matrix-supported, in many instances laterally within the same bed. Two populations of clasts are evident: 1) well rounded pebbles and cobbles of basement rock types (mainly sandstone, chert, volcanics and quartz), and 2) angular to rounded pebbles, boulders and large rafts of intraformational sandstone, interlaminated sandstone/siltstone and siltstone. Little internal bedding is evident in these rocks, although a-axis and less common b-axis imbrication of coarse clasts is ubiquitous. Sandstone slabs are in places stacked in an imbricate fashion, or are rarely contorted into overturned folds. Plant debris is abundant throughout coarse-grained facies.

This entire succession is interpreted to have formed in a marine environment. An offshore, relatively quiet shelfal setting is indicated by the preservation of articulated body fossils and the lack of any wave-generated structures. Sharp-bounded sandstones were introduced to this environment by tractional currents which flowed in an offshore (westward) direction: the nature of these beds and their contained structures (including some sole structures) is suggestive of deposition from turbidity currents. Lonestones may have been introduced from floating ice. Coarse-grained facies are superimposed on this background pattern of sedimentation. The disorganised, silt matrix and often matrix-rich nature of the conglomerates and diamictites indicates deposition from sediment gravity flows, further supported by the sheet-like geometry of beds and the lack of basal erosion.

Mass transport mechanisms ranging from plastic, laminar debris (plug) flow, through partly turbulent debris flow (cohesionless debris flow) to high -concentration turbidity flow are interpreted.

Similar facies are preserved in outcrops and stratigraphic drillcores from the southeast part of the basin, in a stratigraphically equivalent unit (Barfield Formation). This interval, which correlates with other major stratigraphic changes, is therefore interpreted to represent a transition from thermal to flexural subsidence in the Bowen Basin. We suggest that the section described represents the initial disruption of the stable marine shelf which characterised the middle Permian thermal subsidence phase of the basin, and that deposition of the massflow conglomerate packages was controlled by early propagation of thrust sheets in the subsurface.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 FACIES AND CYCLICITY OF THE UPPER PERMIAN BAINMEDART COAL MEASURES IN THE NORTHERN PRINCE CHARLES MOUNTAINS, MACROBERTSON LAND, ANTARCTICA Christopher R. Fielding and John A. Webb department of Earth Sciences, University of Queensland, Qld 4072, Australia department of Geology, La Trobe University, Bundoora, Vic 3083, Australia 1

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Many coal-bearing successions display vertical cyclicity of lithofacies, which in recent times has often been interpreted in terms of fluctuating base-level. In this paper, we present details of an alluvial, coal-bearing succession with well-developed cyclicity which cannot be satisfactorily explained by base-level changes, autocyclic mechanisms or tectonic causes. We conclude that the dominant control on cyclicity in this case is climatic.

The Upper Permian Bainmedart Coal Measures form part of the Permo-Triassic Amery Group, which crops out in the Beaver Lake area of the northern Prince Charles Mountains, MacRobertson Land, Antarctica. The exposed strata are believed to have formed in graben or half-graben sub-basins on the western edge of the Lambert Graben, a major failed rift system. Sedimentological analysis has revealed that these rocks formed in alluvial environments in which swiftly-flowing rivers of low sinuosity (represented by Facies A1 and A2) flowed northward down the axis of the basin, and were separated by waterlogged floodbasin and peat-forming wetlands (Facies B1 - B4). A third Facies Association (comprising Facies CI - C3), interpreted as the deposits of lake floor and delta environments, is exclusively developed within a distinctive, fine-grained interval named the Dragon's Teeth Member. The proportion of Association B facies within the succession increases markedly above the level of the Dragon's Teeth Member (at about 300 m above the base of the formation).

Flat, low-angle and undulatory bedding structures preserved within channel deposits are suggestive of sediment deposition in flow conditions which were often critical or supercritical. Presence of massive and chaotic intervals of sandstone further implies some deposition from high-concentration aqueous flows. Alluvial channel bodies show evidence of incision into underlying substrates, both during initiation and at later stages in channel belt construction. The lack of interfingering between channel deposits and coals, and the lack of coal seam splits, suggest that thick peats formed only in areas and at times of minimal clastic sediment supply.

Deposition of the Bainmedart Coal Measures (BCM) was strongly cyclic, repetitive cycles commencing abruptly with a channel sandstone which fines upward into floodbasin facies and then coal. The average compacted thickness of cycles throughout the BCM is 9-10 m, suggesting that the cause of the cyclicity was constant throughout the sequence. Fourier Series analysis of the succession, and other considerations, together suggest that the dominant control on sequence architecture was climatic, related to precessional Milankovitch fluctuations of c. 19 ka periodicity. Cycles began abruptly with the depositional of coarse-grained material in high-energy alluvial channels, which contracted with time in response to changes in water supply (rainfall).

Upper parts of cycles are dominated by finer-grained sediments and then coal, indicative of progressively reduced coarse sediment input. Tectonic processes overprinted this pattern at least once during the period of sediment accumulation, to form the Dragon's Teeth Member.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

DEEP BASIN/BASEMENT FEATURES ACROSS THE CONTINENT-OCEAN MARGIN OF THE OTWAY BASIN D. M. Finlavson. I. Lukaszyk. E. C. Chudyk, and A. M. G. Moore Australian Geological Survey Organisation. GPO Box 378. Canberra A.C.T. 2600

The Otway and Sorell Basins in southeastern Australia include a triangular area of extended continental lithosphere formed during extensional episodes in Cretaceous - Miocene times which ultimately led to the separation of Australia and Antarctica. The extended continental margin, defined largely by onshore gravity and offshore Seasat/Geosat data, is located in a transition region between an area to the west where the locus of continental separation occurred close to the Australian craton, and the area to the east where continental separation failed through the Bass Strait but was instead offset along the Sorell Fault and Tasman Fracture System to the region south of the South Tasman Rise.

WIDE-ANGLE SEISMIC PROFILING During late 1994- early 1995 the Australian Geological Survey Organisation (AGSO) research vessel "Rig Seismic" conducted seismic profiling (about 3500 km) at both near-vertical incidence and at wide-angle incidence across the Otway Basin margin from near-shore to the deep ocean basin. This paper presents results of wide-angle seismic recordings made in the Port Campbell Embayment region which define the velocity structure of the deep basin sediments and the underlying faulted and extended Lachlan Orogen basement rocks.

The wide-angle seismic profiling technique has come to the fore in recent years as an innovative method of investigating deeper structures when used in conjuction with large airgun seismic sources towed routinely behind near-vertical incidence profiling vessels. During the 1994/95 AGSO Otway Basin survey, seismic stations onshore were able to record the "Rig Seismic" airguns at wide-angle offsets greater than 200 km (more than 5000 individual shots on some profiles). This enabled the interpretation of velocity structures throughout the extended crust of the continent-ocean transition to depths of about 30 km. The results provide tight controls on the interpretation of nearvertical incidence seismic profiles, Seasat/Geosat infomation and subsidence/geothermal history data.

BASIN AND UNDERLYING BASEMENT FEATURES There are major structual features along profiles both parallel to the coast and extending to the deep ocean at right angles to the coast. Along an onshore-offshore profile in the Portland area (Fig. 1), the greatest thickness of basin sequences lie 30-60 km offshore at depths down to at least 10 km. There are significant structural features affecting the thickness of basin sequences in the Heywood area (?an extension of the Tartwawp Fault Zone) and just offshore (?the Bridgewater Bay Fault Zone). Palaeozoic upper crustal basement thins from 10-12 km onshore to less than 1 km thick about 120 km offshore. Over the same distance the Moho shallows from about 30 km onshore to about 15 km and then to about 12 km in the deep ocean at the limits of the profile. o

Fig. 1 - Onshore-offshore velocity profile no.700 from Milltown (near Heywood) to the deep ocean. Recorders were located at Milltown and Gorae onshore and 1985 AGSO sonobuoy (SB 13) data constrain interpretation offshore.

141

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

STUDYING EARTH SCIENCE: OPEN LEARNING INITIATIVE AT UNE, ARMIDALE Peter G Flood Department of Geology and Geophysics University of New England Armidale NSW 2351 Australia

The Department of Geology and Geophysics at the University of New England, Armidale, NSW is responsible for the presentation and delivery of several Earth Science units via the Open Learning Mode.

These units include: 1. GEO 11/12 'Earth Revealed', an introductory unit equivalent to First Year Geology, 2. ENV 21 'Environmental Geology', a second year unit which is identical to an external unit offered by the Department; and 3. X4001 'Environmental Geology', a postgraduate unit especially designed for the Graduate Diploma in Natural Resources.

GEO 11/12 was offered in 1993, the other units from 1994. The Open Learning Agency of Australia — UNE contract provides for these units to be offered again in 1995 and 1996. These units have proved extremely successful with more than 200 students successfully completing their studies.

The experience obtained and the lessons learned in dealing with Open Learning students has demonstrated that face-to-face contact between lecturer and student is not necessary when the subject matter is presented via multi media methods including notes, textbooks, TV, audio visual tapes, and the occasional phone call to the lecturer when all other learning media fail. However, the majority of students still express a desire for one-to-one intellectual exchange with the lecturer. The superior students realise that it is teacher/student personal contacts that 'fires the imagination' and enhances problem-solving capabilities as opposed to simply accumulating facts.

142

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra,, February 1996

INTERPRETATION OF THE ORIGIN OF MASSIVE REPLACIVE DOLOMITE WITHIN ATOLLS AND SUBMERGED CARBONATE PLATFORMS: STRONTIUM ISOTOPIC SIGNATURE ODP HOLE 866A, RESOLUTION GUYOT, MID-PACIFIC MOUNTAINS Peter G. Rood University of New England, Department of Geology & Geophysics, Armidale NSW 2351, Australia

Endo-upwelling is a geothermally driven convective process operating within the upper part of the volcanic foundation and overlying carbonate pile, in atolls and guyots. By this process deep oceanic water, rich in CO^ and dissolved nitrates, phosphates and silicates is drawn into the pile, circulates slowly upward through the porous-permeable carbonate interior and emerges at either the reef crest or lagoon on atolls to support the primary productivity of the surficial communities, or towards the interior of the platform surface on guyots.

Continuous operation of the endo-upwelling process requires: a) heat from the volcanic foundation; b) an external impermeable apron on the submerged flanks to confine the convective flow within the pile; and c) a porous cap from which water exiting the plumbing system either returns to the ocean.

At ODP Hole 866A on Resolution Guyot, Mid Pacific Mountains the Sr isotopic signature of massive white-coloured, coarsely crystaline dolomite indicates a considerable time delay of approximately 100 Ma between carbonate deposition and dolomitization. This time delay is determined by compairing the Sr isotopic value of the dolomite and the time that ocean seawater displayed a similar Sr isotopic value. This interpretation of the Sr isotopic values assumes that all of the Sr is viewed as coming from seawater and none from any precursor limestone.

The massive white replacement dolomite from Resolution Guyot possibly provides confirmation of the origin of dolomite by way of thermally-driven convective flow within submerged carbonate platforms. Endo-upwelling seawater probably enters the carbonate pile at some depth, thermally circulates upwards, and produces carbonate dissolution and could conceiveably produce massive dolomite replacement.

Acknowledgements: PGF acknowledges the opportunity provided by the Ocean Drilling Program to participate in Leg 143 Atolls and Guyots I, and financial suport in 1993 from the Australian Research Council. This extended abstract is similar to an article published with co-authors A. Fagerstrom and F. Rougerie in the Journal Sedimentary Geology (vol., 99) in late 1995 as an Expressed note.

143

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE TECTONIC SETTING AND INTERNAL STRUCTURE OF THE CAMBRIAN KANMANTOO BASIN, SOUTHEAST AUSTRALIA Thomas Flottmann1. Peter Haines2, Pat James1 and A.P. Belperio 3 1 Department Geology & Geophysics, The University of Adelaide, Adelaide SA 5005 2Department Applied Geology, The University of South Australia, The Levels, Pooraka SA 5095 3 Mines and Energy South Australia, PO Box 151 Eastwood, SA 5063 INTRODUCTION Understanding of the evolution and potentially related metallogenic processes of many Palaeozoic and older sedimentary basins is often blurred by subsequent orogenic processes and much information may be lost due to erosion of the upper crustal portion of a basin. Geological investigation is in many cases therefore restricted to the analysis of erosional basin-remnants. Consequently significant information regarding the principal basin formation processes ie. tectonic framework, structural evolution, potential sediment source areas etc. are difficult to obtain.

This presentation gives results of an ongoing, multidisciplinary investigation aimed at establishing the tectonic setting and sedimentary and deformational history of the Cambrian Kanmantoo basin, South Australia. This basin forms an eastward thickening sedimentary wedge constituted by dominantly clastic units of the Kanmantoo Group that were rapidly deposited during the Cambrian after 525 Ma and are partly underlain by Neoproterozoic Adelaidean sedimentary rocks. During the Delamerian Orogeny (500 Ma) the Kanmantoo Group rocks were deformed and metamorphosed under greenschist to upper amphibolite facies conditions. The Kanmantoo basin contained several mined base metal deposits and is currently subject to renewed exploration.

APPROACH/METHOD Recent seismic surveys show that the southwestern margin of the Kanmantoo Basin formed by a zone of reactivated growth faults between a Cambrian shelf sequence that overlies the Gawler Craton and the Kanmantoo basin proper. This margin forms the principal boundary between the southeast Australian cratonic province and the Phanerozoic regime of southeastern Australia. The eastern margin of the Kanmantoo sequence remains speculative, but seismic data suggest an eastern basin-limit - 200 km to the southeast.

Cross section balancing and restoration indicate that the accommodation space in which the Kanmantoo Group was deposited is controlled by eastward dipping inverted growth faults across which progressively higher stratigraphic units thicken eastwards.

Aeromagnetic surveys recently flown as part of the South Australian Exploration Initiative allow the tracing of key horizons under Neogene cover particularly in the eastern part of the Kanmantoo basin. The aeromagnetic data form the basis for a three-dimensional restoration of the deformed basin.

The internal basin architecture of the Kanmantoo basin suggests that the Kanmantoo Group was potentially deposited in several sub-basins that were strongly influenced by transcurrent faults at the southern basin margin.

Transcurrent faults are the expression of contrasting orogenic boundary conditions at the Australian and Antarctic segments at the formerly contiguous palaeo-Pacific margin.

A source region for the clastic sedimentaiy sequences is indicated by a zircon provenance study which shows a bimodal peak of zircon ages of around 550-600 Ma and -1100 Ma. This shows that the Kanmantoo Group sediments are not simply produced from recycled outcropping Proterozoic rocks but have an independent source.

RESULTS/IMPLICATIONS The formation of the Kanmantoo basin signifies a fundamental early Phanerozoic restructuring and shift of tectonic processes at the eastern margin of the Australian Craton and its former continuation at the Antarctic craton. The formation, geometry and potentially also the sedimentary infill of the Kanmantoo basin appears intimately linked to the (contrasting) orogenic processes at the palaeo-Pacific margin. The results of this study emphasize the need for a multidisciplinary approach to establish the tectonic and sedimentary framework of this (and potentially other) Palaeozoic sedimentary basins that were subject to substantial orogenic modification.

Understanding the dynamics and mutual influence of basin forming and subsequent orogenic processes appears the most beneficial approach to develop new and innovative leads towards potential economic mineralisations.

144

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOCHEMICAL RESPONSE OF MAGMAS TO MICRO-CONTINENTAL COLLISION IN EASTERN INDONESIA. J. Foden Department of Geology and Geophysics, University of Adelaide

The complexities of geochemical signatures of island arc magmas reflect the involvement of three or more source components. The isolation of the individual influence of each of these has long been a challenge to geoscience. This debate is important because it relates to the role of arcs in continental growth. One of the compositional vectors in arc magmas leads to the development of K^O-rich suites with features which include silica-undersaturation, very high K/Nb, Ba/Nb and Ce/Y, ^ 7 Sr/^Sr values which are moderately low and high 14 ^Nd/ 144 Nd values. These features differ from those of a melt component derived from the subducted sediment- and alteration- enriched upper slab. The latter component is only moderately LIL element-enriched with high 87 Sr/ 86 Sr and 2 0 7 Pb/ 2 0 4 Pb ratios and low Ce/Pb and low 143 Nd/ 144 Nd. This study of early Neogene to modern volcanics in Indonesia from the island of Sulawesi and the eastern Sunda arc directly monitors the effects of secular change on the composition of magmatic rocks during the transitory passage of a unique source into the subduction zone.

The Indo-Australian Plate is converging on the southern and eastern Indonesian arc systems, sweeping dispersed fragments (micro-continents) of Gondwanan continental lithosphere into this subduction zone.

The resultant collisions lead to deformation and uplift and variable subduction of continental crustal and subcontinental lithospheric mantle. Because of crustal buoyancy, the continental lithospheric fragments may become delaminated, the sub-continental mantle lithosphere component becoming isolated and being carried into the mantle under the arc. Because this lithospheric mantle is potentially relatively old, it may be the site of geochemical enrichment and anomalous Sr-, Nd- and Pb -isotopic signatures, thus providing a potential alkaline magma source.

The Indonesian islands of Buton, Sumba and Timor are examples of micro-continental fragments which have undergone collisions with volcanic arcs. These collisions lead to transfer of at least the crustal portion of each fragment to the overriding plate by means of ocean-ward out-stepping of the trench to incorporate the micro-continent in the forearc. The Buton-Sulawesi collision occurred in the Mid Miocene (-19 Ma), and the Sumba collision with the eastern Sunda arc, in the Late Miocene (5 - 10 Ma). Both these collisions resulted in deformation and the rapid transition of magmatic rocks towards potassic, often leucite-bearing highly potassic types. In the east Sunda Arc, the "footprint" of Sumba is defined on and to the north of Sumbawa by potassic volcanism of Pliocene - Modern age. In South western Sulawesi, the potassic spike arose in the Mid Miocene (~15Ma) following the collision of Buton and had decayed by the Late Miocene or Pliocene when normal (sub-alkalic) calc alkaline magmatism was re established.

145

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PROVENANCE O F NEOPROTEROZOIC & EARLY PALAEOZOIC SEDIMENTS EAST AUSTRALIA: IMPLICATIONS FROM Nd ISOTOPE & ZIRCON STUDIES. John Foden Department of Geology and Geophysics, University of Adelaide.

Nd-isotopic compositions of fine-grained Neoproterozoic sedimentary rocks from the Adelaide Fold Belt (AFB) are unlike those of potential Archaean to Palaeoproterozoic source rocks on the nearby Gawler Craton or from the Willyama or Mt Painter inliers (e.g. Turner et al., 1993). The Neoproterozoic rocks have relatively high initial £Nd (mean = -8.7) values and young crustal residence ages (1.98 ± 0.13 Ga) compared with the assumed source terrains which have T

Nd

2.2 - 2.8 Ga. and eNd values at Adelaidean

time in the range -12 to -22. Adelaidean shales also have higher Sm/Nd ratios than average adjacent basement rocks.

The AFB sequence shows systematic temporal variation, with highest sNd values (-5) in the carbonaterich Tapley Hill formation (Sturtian), decreasing gradually towards the end of the Proterozoic followed by an abrupt downward offset to eNd = -12 in the Cambrian Kanmantoo group. However, where the Neoproterozoic sequence is clearly proximal to basement sources, such as near Mt Painter, Nd isotopic compositions are more basement-like, with lower eNd values and older model ages. Likewise, Sturtian and Marinoan glacials are also associated with basement-ward Nd isotopic excursion. Interestingly the coeval Amadeus Basin sequence (Zhao et al, 1992) has Nd isotopic characteristics exactly like those of the AFB. Again the maximal extent of positive Nd-isotopic deviation away from basement (Arunta) sources is in the Tapley Hill equivalent (Aralka Formation & Inindia Beds).

It is often assumed that because of the insolubility of the rare earths in low T hydrous fluids and their very short mean ocean residence time (MORT), the Nd -isotopic composition of clastic sedimentary rocks reflect the mean values of their source rocks. If this is always so then the Nd results from this study indicate an unknown and younger source of the AFB sediments than the nearby basement inliers. It also suggests a significant shift in source region at the onset of Cambrian Kanmantoo sedimentation.

As a direct test of sedimentary provenance, the ages of detrital zircons were determined by the Pb-Pb Kober evaporation method. In spite of the Nd results these appear to support the derivation of the Adelaidean sequences from adjacent basement, but interestingly suggests that a new and very significantly younger terrain with Late Neoproterozoic felsic igneous rocks must become a new source to the Cambrian Kanmantoo group.

The Nd isotope data suggest that the composition of Adelaidean shales migrate furthest away from values typical of adjacent Archaean - Proterozoic basement when rates of clastic influx were lowest (e.g. Tapley Hill formation) implying low rates of erosion, due to low relief or lack of sub aerial exposure (particularly in the Late Sturtian and Early Marinoan). The data document competition between two sources, that of the nearby basement terrains, and the other significantly younger, and / or more primitive (ie more mantlelike) characterised by more radiogenic Nd. The origin of this component is unresolved; options could include the North American Grenville or the Musgraves. The zircon ages from the Neoproterozoic sequences imply these alternatives are unlikely. Interestingly, the Neoproterozoic shales have both Nd isotopic compositions and secular variation like that of contemporary seawater, and an alternative explanation, is one of seawater-sediment exchange. Global seawater during the Sturtian had relatively high eNd values implying a low rate of clastic input and a high rate of hydrothermal flux to the Neoproterozoic ocean waters. The trend through the Vendian to Early Cambrian of much more "crustal", less radiogenic Nd was a response to increased rates of clastic input due to increased plate motion and local and global orogenic activity (Pan African / Petermann Ranges Orogeny/ Delamerian-Ross Orogeny).

The early Palaeozoic flysch which forms the widespread basement to the Lachlan Fold Belt, has a slightly lower model age (1.85 ± 0.04 Ga) and higher £Nd(500) value (-9.9) than the Kanmantoo and is consistent with derivation by erosion of this source together with 5 - 1 0 % Cambro-Ordovician magmatic rock, from the Ross-Delamerian mountain belt.

REFERENCES Turner,S., Foden,J., Sandiford,M. and Bruce,D. 1993. Sm-Nd istopic evidence for the provenance of sediments from the Adelaide Fold Belt and S.E. Australia with implications for episodic crustal addition.

Geochim. Cosmochim. Acta., 56, 921-940.

146

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 COUPLED RE - OS ISOTOPIC AND FLUID DYNAMIC CONSTRAINTS ON THE GENESIS OF ARCHAEAN KOMATIITE ASSOCIATED FE - NI - CU (PGE) DEPOSITS Jeff. G. Foster ' and David. D. Lambert Victorian Institute of Earth and Planetary Science, Department of Earth Sciences, Monash University, Clayton, VIC 3168 2 Western Mining Corporation, Kambalda Nickcl Mines, Kambalda, WA 6442 1 2

1

1

The Re - Os isotopic system is based on the beta decay of Re to the platinum group element 0s. During mantle melting rhenium behaves as an incompatible element (D =0.1), whereas osmium behaves as a compatible element ( D = 5). Osmium is thus preferentially retained in the mantle relative to rhenium.

The process of mantle melting followed by melt extraction and magmatic fractionation results in crustal reservoirs with high Re/Os ratios that, with time, develop radiogenic Os isotopic compositions. Hence, the two reservoirs rapidly develop contrasting osmium isotopic compositions. Furthermore, rhenium and osmium have high sulphide liquid/ silicate liquid partition coefficients and are thus preferentially incorporated in to magmatic sulphide liquids. The Re - Os isotopic system is therefore a unique tracer for components involved in the genesis of magmatic sulphide deposits.

I87

187

Re

0s

residue/melt

residue/melt

We have obtained low - blank Carius tube/ ID - NTIMS Re - Os isotopic data from the Kambalda, Perseverance and Mt Keith deposits in WA, in order to investigate the role of thermal erosion of sulphidic sediments in the genesis of komatiite associated magmatic sulphide deposits. Re - Os isotopic data for massive, matrix and disseminated magmatic sulphides (1% S - 36% S) define an isochron with an age of 2709 ± 30 Ma and a chondritic initial y (percent deviation from chondritic at 2709 Ma) value of +0.03 ±0.12. This age is in excellent agreement with U - Pb zircon ages that range from 2703 ± 6 Ma to 2717 ± 4 Ma for sediments within the komatiite pile. The isochron demonstrates that the Re - Os isotope systematics for these samples have not been disturbed by metamorphism, deformation or gold metallogenesis, and that these deposits are broadly contemporaneous. The magmatic sulphides are also distinctive as they exhibit a restricted range of Re/ 0s < 2. A thin unmineralised aluminium undepleted komatiite flow from the Kambalda komatiite formation (MgO = 29.3%, (La/Sm) = 0.48, £ = +3.0) is isochronous with the ores and has a Re/ 0s = 2.35. In contrast to the unmineralised komatiite and magmatic sulphides, a contact sulphidic sediment has a high Re/ 0s = 106, very radiogenic initial Tqs value of +900 and a model age of 3200 Ma. The early Archaean Os component in the sulphidic sediment is consistent with the presence of zircon xenocrysts within the volcanic pile that yield U - Pb ages up to 3500 Ma. These new data allow us to evaluate the proposition that ore formation occurred via thermal erosion and disaggregation of sulphidic sediment, with the sulphide component retained at the base of the komatiite flow as an immiscible sulphide liquid (ISL). The ore elements including rhenium and osmium have high sulphide liquid/ silicate liquid partition coefficients and are thus scavenged by the ISL from the overlying komatiite liquid. The concentration of the ore elements in the ISL increases as a function of lava flow - through. This process can be modelled in Re - Os isotope space as a function of the mass ratio of komatiite liquid to immiscible sulphide liquid or R factor (equation below), where Cm = common Os concentration in source magma, Csm = common Os concentration in sediment sulphide component, Dn = Nemst partition coefficient for Os, pm = measured isotopic composition of source magma, psm = measured isotopic composition of sulphidic sediment, X = Re decay constant, t = closure age of isotope system and tot = resultant initial osmium isotopic composition.

0s

187

ch

187

Ndi

187

/ 1870s \ \ 1880s /tot

/

((CmDn ( R + 1 ) / ( R + D n ) ) ((CmDn (R + 1) / (R + Dn)) + Csm

/1870s \ \ 1880s/p

/l87Re\( \1880s/

m

188

188

188

e?ltl

p m

((CmDn(R + l ) / ( R + D n ) ) / 1870s\ / 187Re V(ee k l - l j ((CmDn (R + 1) / (R + Dn)) + Csm \1880s/p m 11880s/ psm Assuming osmium is strongly chalcophilic ( D P = 10 ), the model predicts that the osmium content of the ISL rapidly increases with R factor. The radiogenic character of the sediment sulphide component is rapidly diluted by the addition of chondritic Os from the komatiite and lost at R factors in excess of 1000. However, R factors at Kambalda are estimated to range from 100 to 500. This results in predicted y s values that range from +14 to +3, well above our isochron initial y value of +0.03 ± 0.12. These data indicate that ore formation was dominated by uncontaminated aluminium undepleted komatiite liquids and suggests that local contact sulphidic sediments of the type analysed may not be important in ore genesis. This result is consistent with mass balance calculations which show that there is insufficient sulphide missing from the local contact sediments to generate the ores.

1-

S

s u l

h i d e / s i l i c a t e

5

0

0s

147

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ODP LEG 160: EASTERN MEDITERRANEAN SEA E. FRANKEL and Shipboard Scientific Party Leg 160* Department of Applied Geology, University ofTechnology, Sydney, PO Box 123 Broadway, NSW, 2007.

Leg 160 of the Ocean Drilling Program (March - May 19995) was the first of a two-leg program planned to investigate aspects of the tectonic and palaeoceanographic history of the Mediterranean Sea. The first focus of this Leg in the Eastern Mediterranean was on the accretionary and collisional processes associated with the convergent boundary between the African and Eurasian plates. The second major objective was concerned with the origin and palaeooceanographic significance of sapropels, organic-rich layers intercalated in the PlioQuaternary sediments of the Mediterranean.

The tectonic-oriented drilling on Leg 160 addressed several objectives amongst which were, 1) the nature of the collision between the Eratosthenes Seamount and the Cyprus margin, and 2) the origin of mud volcanism on the Mediterranean Ridge.

A transect of four sites across the Eratosthenes Seamount extending from the crestal area to the upper and lower slopes and on to the lower Cyprus margin provided evidence of significant faulting allied with rapid subsidence and sea level fluctuations during the Tertiary These results are interpreted in terms of flexural faulting and collapse related to loading by an overriding Cyprus plate and thus document a process of initial collision that is of fundamental importance to the interpretation of many mountain belts.

The Mediterranean Ridge is interpreted as an accretionary prism that developed during the last 25 million years as a result of northward subduction of the African plate beneath Eurasia. Along the northern margin of the Ridge numerous mud volcanoes are developed. To shed light on the intriguing problem of the origin and nature of these mud volcanoes, several holes were drilled across each of the Napoli and Milano Domes. Material recovered in this exercise revealed that both structures are well over 1 million years, old, and showed that much of the flank and crustal regions of the structures are composed of debris flows. These incorporate clasts of mainly Miocene lithologies that have been transported within the flows. The Milano structure shows evidence of clathrates close to the seafloor and gas (predominantly methane and hydrogen sulphidre) was detected at both locations. Pore-fluid compositions indicate the presence of evaporites below.

In the unconsolidated sediments of the Plio-Quaternary section more than 80 individual sapropels were recovered.

These occur in distinctive packets and time intervals which are separated by oxidised carbonate-rich sediment.

Individual beds in the packets were correlated between holes separated by several hundred meters, and packets of the sapropels could be matched between sites up to several hundred kilometers apart. The sapropels are up to 50 cm thick and many individual beds contain up to 30% by weight of predominantly marine organic carbon. In addition many individuals display unusual magnetic properties probably due to the presence of various iron sulphide species.

Sapropel occurrences mark periods when the Mediterranean catchment area experienced increased humidity and relatively high average temperatures. Once such conditions became established, they profoundly changed processes in the biologically active surface layer and at the seafloor. Our findings indicate that deep-water anoxia coupled with dramatically increased carbon flux were essential prerequisites for sapropel formation in the Eastern Mediterranean, which today is oligotrophia

* K-C. Emeis, A. Robertson, C. Richter, M-M Blanc-Valleron, I. Bouloubassi, H, Brumsack, A. Cramp, G, De Lange, E.Di Stefano,\ R. Flecker, M. Howell, T. Janecek, M-J. Jurado-Rodriguez, A. Kemp, I. Koizumi, A. Kopf, C. Major, J. Mart, D. Pribnow, A. Ribaute. A. Roberts, J. Rollkuttert, T. Sakamoto, S. Spezzafem, S. Staerker, J, Stoner. B. Whiting, J. Woodside.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

WORLD CLASS VOLCANOGENIC MASSIVE SULPHIDE DEPOSITS: LESSONS FROM LAND AND SEA James M. Franklin Geological Survey of Canada, 601 Booth St., Ottawa, Ontario K1A0E8, Canada

Research on modern hydrothermal systems provides quantitative measures of the processes of formation of hydrothermal fluids and of precipitation mechanisms from them. Ancient deposits provide new data on volcanological processes related to the deposits, as well as data on their spatial and temporal compositional variations.

The fundamental control on size of metalliferous hydrothermal systems is the supply of heat to the upper two km of the crust. Heat is usually supplied from a focused source (shallow-level subvolcanic intrusion).

Ambient heat also may have provided enough energy for the reactions necessary to effect metal leaching, especially where heat flow is particularly high. For most systems, the metals were derived through leaching from footwall strata; interstratal fluids may enter subvolcanic intrusions during episodes of magma chamber "cracking", engendering some "magmatic" component. Such episodes of catastrophic invasion of "unmodified" seawater into the intrusions could generate highly altered breccia zones. Direct contributions of metalliferous magmatic fluid may be significant from some felsic systems. Subvolcanic intrusions undergo two stages of alteration: On intrusion emplacement, fluids may rapidly enter and react with their mesostatic portions (e.g. the sills in the Middle Valley) causing albitization and destruction of oxides and sulfides, and releasing metals. During terminal hydrothermal stages, the metalliferous fluid collapsed into the intrusions, forming disseminated and vein sulphides.

Hydrothermal systems that engender giant VMS deposits must discharge through only a few well-focused sites, and be efficiently capped. Reservoir caps may be physical (impermeable sediments or well-sealed submarine ash flows), or may have formed through thermally-induced chemical sealing. Examples of the latter include widespread carbonatization (heating of seawater in < 1500m water environments).

Silicification may also be an important reservoir-sealing process.

Faults that control hydrothermal discharge include rift-margin master faults and caldera ring-fault systems.

Extensional regimes both in mid-ocean and back-arc systems, engender VMS-producing structures. A second method for focusing discharge is the presence of sediment-submerged basement or volcanic highs.

For example, current discharge at Middle Valley seems to be controlled by a small buried seamount that formed coincident with basin-wide magmatism and related heating.

Much sulphide precipitation occurs within sulphide mounds, where conductive cooling, aided and extensively modified by circulation of progressively heated seawater, can extensively modify the zoning and composition of the deposits. At TAG. mound growth is supported by anhydrite precipitation, for example.

On cessation of hydrothermal activity, extensive collapse, forming sulphide breccias, will occur. Circulating seawater also modifies the distribution of base and precious metals. In some environments, sulphide precipitation in footwall sediments and permeable volcanic strata, form "replacements-like bodies.

Ore compositions are a function of the primary fluid compositions and, more importantly, the depositional environment. In fclsic and sediment-dominated systems where buffering to acid pH may occur at any temperature, ore-forming fluids may contain more lead, relative to copper and zinc, than those systems which were basalt-dominated, and formed at high T (>350°C). Critical factors in the precipitation environment include temperature (controlled by boiling, i.e. water depth), oxygen availability , organic activity, and aS (i.e. speciation, important in gold abundance). Temperature may effect up to a lOx variation in gold and silver content of the precipitate (Axial Seamount).

"Distal" transport of hydrothermal fluids or particulates is unimportant in today's oceans, and was significant in either highly saline systems (c.f. Red Sea), or those venting into rcduccd basins. Biological activity may create a broadly anomalous zone of metals around some systems, however.

149

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 GEOCHEMICAL ANOMALIES IN SEA WATER AROUND THE DEEP OCEAN OUTFALLS, SYDNEY David J. Fredericks', David T. Heggie1, Rob Cowdell2. Andy Longmore2, and Caroline Lapworth2 'Australian Geological Survey Organisation (AGSO), GPO Box 378 Canberra, ACT 2600 2 Victorian Fisheries Research Institute fVFRI), PO Box 114. Queensciiff, Vic 3 Dept of Geology and Geophysics, Sydney University, 2006

Continuous geochemical tracer data were collected from the coastal zone in the vicinity of the Sydney deep ocean outfalls during 1994 aboard the research vessel Rig Seismic. The primary objectives of this survey were: 1. to use tracers to investigate the physical distribution and behaviour of outfall plumes; and 2. to investigate biogeochemical alteration of nutrients in the plume.

Data collected using the joint AGSO/VFRI continuous geochemical tracers apparatus included the following.

1. Direct Hydrocarbon Detection (DHD) of light hydrocarbons in a continuous stream of seawater by gas chromatography in the ships on board laboratory; 2. Automated measurement of dissolved nitrite (N02')> nitrate (N032*), ammonia (NH*4), orthophosphate (P0 4 2 ') and silica (SiO*4') by colorimetric methods; 3. Total Hydrocarbons (THC) and chlorophyll a by flow through fluorimetry; and 4. Measurement of turbidity, temperature, salinity, dissolved oxygen and pH in seawater using a Yeo-Kal submerged data logger.

Anomalously high concentrations of methane and ammonia in conjunction with reduced salinities were found in the vicinity of the outfalls. Methane and ammonia are unique indicators of sewage as they have very high concentrations in sewage compared to background levels in coastal waters. Salinity may also be used to trace the distribution of freshwater sewage discharge. Nitrite, nitrate, silica and orthophosphate are less sensitive indicators of sewage input because concentrations of these species are only moderately enhanced in sewage plumes and because they have complex background distributions in the water column.

Sewage dilutions were calculated using a simple two end-member mixing model. Concentrations of methane and ammonia in the seawater end-member were estimated from vertical profile data collected to the north east of the North Head outfall in an area that was considered unaffected by pollutant discharges from the major estuaries and deep ocean outfalls in the Sydney Region. Composition of sewage end-members were based on published nutrient data for Sydney effluent and methane analyses of effluent discharged from Malabar (STP).

Estimates of effluent dilutions made using methane and ammonia are highly correlated and are in good agreement. Minimum dilutions in the immediate vicinity of the outfalls ranged from 185 to 250. Ammonia and methane are sensitive indicators of spatial and temporal variation in plume behaviour. Estimates of plume dilution made using salinity are similar but generally lower, ranging from 120 to 150. The salinity data could not resolve the fine structure of the plume and was not useful at high dilutions because of relatively large measurement uncertainties. Systematic differences in estimates of plume dilution made using methane, ammonia and salinity were probably caused by errors in the estimates of end-members compositions. Crossplots of ammonia and methane data show that each plume has a unique ammonia/methane signature that can be used to distinguish input from different ocean outfalls.

Vertical profiles in the vicinity of the outfalls indicated considerable variability: sewage discharge was identified as dense plume at depth in one profile and immediately below the thermocline in an adjacent profile.

Linear relationships between methane and ammonia in these profiles indicate that water mixing is the major control of water chemistry in the immediate vicinity of the outfalls.

Transect data showed sewage discharge was generally present as a horizontal plume trapped below the thermocline. A mass balance indicated that input of nutrients from ocean outfalls was not sufficient to account for the high levels of dissolved nutrients in the vicinity of the outfalls indicating either nutrient cycling or another high nutrient water source.

Acknowledgments: The authors wish to acknowledge the assistance of G. Bickford, D. Hampton, C. Tindall, J. Stratton, P Davis and C. Monohan, the master and crew of the Rig Seismic in the collection and processing of this data.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

HEAVY MINERAL SAND IN THE SOUTHERN PERTH BASIN, WESTERN AUSTRALIA GEOLOGY, MINING, URBANISATION AND PLANNING M.J. Freeman* *- G. Richards* *, K. Miils^ and G. Rippon< > L. Project Geologist, Dept Minerals and Energy, 100 Plain Street, East Perth, Western Australia 6004; 2. Chief Geologist, Westralian Sands Ltd, Capel, WA 6271; 3. Exploration Manager, Cable Sands (WA) Ptv Ltd, Bunbury, WA 6230; 4. Mine Engineer, RGG Mineral Sands, Capel, WA 6271 1

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Heavy mineral sands (HM) are an important product of WA, with 2.85 Mt of HM produced from 38 Mt mined by four main operators in 1994-5, valued at $475 M. Significant value is added by local processing of the HM. The mined product consists of a range of minerals, dominantly ilmenite, rutile, zircon, leucoxene and monazite, and with many nonsaleable products. The total WA resource of contained HM as reported to the Department of Minerals and Energy (DME) is 143 Mt (measured and indicated), although this gross figure includes both uneconomic and nonsaleable mineral. The total equivalent resource figure for the Swan Coastal Plain south of Perth is 36 Mt The deposits mined occur in the upper parts of the Perth Basin sequence and most are in Cainozoic surficial sediments.

Strand lines (SL) are the main locus of the mineralisation and range between 0-75 m above the modem sea level. There are three major "groupings" of mineralisation referred to as strand lines (Quindalup, Capel and Yoganup) which are locally subdivided. Within each sirandline the grade and mineralogy of the mineralisation varies. The highest grades occur where some feature of the palaeocoastline resulted in focussing of the processes which concentrated the highdensity minerals and where the mineral supply was best. Since exposure, erosion has removed significant parts of the strandlines.

The Cainozoic cover of the Perth Basin consists of a 5-10 km-wide belt of dunes and, inland, a 2-10 km-wide belt of lowlands at the foot of the Darling scarp. Urbanisation and industrial developments are concentrated on the slightly elevated dunes because they have: sandy rather than clayey soils; better drainage; are not subject to seasonal waterlogging or inundation; and, for dwellings, the elevated aspects are aesthetically more attractive.

HM mining is mostly by scraper-dozer or similar equipment to a maximum depth of about 10 m. After removal of the 5%-15% of HM present in the run-of-mine ore, the sand and clay is returned to the void, recontoured, surfaced with the original top-soil and planted to achieve the planned end-of-mine land use. Because of the scale of the ore movement during mining, new land-forms can be created to enhance the area for the future uses. DME and the companies have consistently argued that sequential land use was the preferred option where there was competition between land users, with mining preceding subdivision or other developments.

Significant development pressures have been growing during the past few years in the Perth-Bunbury area, with industrial growth, much being dependent on the mineral industry for its raw material, being followed by urbanisation.

The most attractive areas for these developments are along the foot of the Darling Scarp, which is along the mineralised Yoganup SL. Already some 4.5 Mt of HM has been sterilised under industrial and urban developments in this SL. Proposals for developments which sterilise resources are opposed by DME and the companies, but often by the time the proposal information is released, there is significant momentum which can be very difficult to oppose. Because of the increasing number of people wanting to live in these areas, the planning problems are becoming harder to manage.

It was recognised there was a need for planners to have access to resource information to enable them to consider the presence of ore prior to starting planning for developments. However, it is a very competitive industry and for this and several other reasons it was not possible to release precise ore outlines. An alternative system was required. Initially individual company ore outlines were tompiled onto one base plan and this is kept confidential within DME. Then integrated boundaries were drawn around the ore, leaving a buffer between the boundary and the ore which is narrow (order of 100-200 m) next to towns where development pressures are greatest, and broadening out further away from towns. Each company validated the boundary adjacent to its ore (without access to boundaries of it's competitors) and after several modifications a combined plan was released to them. Exploration areas were also shown. A brief report was compiled as an explanation to guide the planners, but the emphasis will always be on the planners contacting the respective companies or DME if there is a potential for a conflict situation developing.

It is believed the new procedure where competitive companies have co-operated closely with the State mining agency has worked well, allowing release of ore "locations" to planners to prevent unwitting sterilisation and to reduce the incidence of severe confrontations between the mineral industry and the rest of the community.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

RE-OS DATING OF METAMORPHISM IN THE LEWISIAN COMPLEX, NW SCOTLAND Louise R. Frick. David D. Lambert, Ian Cartwright Victorian Institutcof Earth and Planetary Sciences, Department of Earth Sciences, Monash University, Clayton ,VIC 3168

The Lewisian complex of northwest Scotland is an example of a well-preserved section through the lower crust that has a history extending back to the middle Archaean. The complex is divided into three regions based on metamorphic history that are believed to represent remnants of material from different crustal levels. Sm-Nd and Pb-Pb isotopic data for the gneisses from the central Lewisian region suggest their formation from supracrustal sediments and mafic and ultramafic rocks which were incorporated at around 2.92 Ga into a developing tonalitic plutonic complex. This was followed by an extended granulite facies metamorphic event (Badcallian) whose peak at 2.66 Ga is recorded in zircons. The Badcallian was succeeded by the Inverian period of deformation and retrogression which, by definition, predates the earliest intrusion of the Scourie Dyke Suite at -2.4 Ga. The Inverian was succeeded by the Laxfordian deformation which caused further minor retrogression and shear zone activity. The Laxfordian, by definition, postdates the latest intrusion of Scourie Dykes at -2.0 Ga and predates the intrusion of post tectonic pegmatite (ca 1.8 Ga) at Laxford.

The Re-Os isotopic system has been used in Archaean terrains to provide geochronological constraints on the formation of ore deposits (Lambert et al. 1994, Foster et al. 1995) and to investigate the pedogenesis of Archaean greenschist-facies komatiites (Foster et al 1995). It has been suggested that the Re-Os geochronometer may be able to 'see through' crustal metamorphic events due to the refractory nature of osmium and its low abundance in crustal rocks and fluids. This is supported by the obtainment of an 2476 ± 14 Ma isochron for the bronzite picrite Beannach Dyke from Scourie (Frick et al 1995) in agreement with published ages. In order to test the usefulness of the Re-Os isotopic system in high grade metamorphic terrains, we have obtained highquality Carius tube/N-TIMS Re-Os isotopic data for a suite of Archaean tonalitic to basic gneisses from the central granulite region. Concentrations for the gneisses range from 8.5 to 87 ppt common Os and 0.082 to 1.2 ppb Re. The isotopic data define a five-point isochron with a Model 1 Mclntyre regression age of 2053 ± 150 Ma (MSWD = 5), and a radiogenic initial isotopic composition of yos = 536 ± 124.

Recent work by Cameron (1994) has related the depletion of gold in the Lewisian granulites to oxidation of primary sulphide (pyrrhotite) to pyrite and magnetite. The timing of this event was assumed to be Inverian, after the peak Badcallian metamorphism, as the oxidation of pyrrhotite to pyrite cannot be accomplished at the low /O2 values of the granulite facies metamorphism. The oxidation event is likewise presumed to predate the intrusion of the quartz dolerite dykes (2.0 Ga) in that they exhibit primary magmatic sulphide mineralogy and textures. As the sulphides within the gneisses are host to the chalcophile elements osmium and rhenium, it is likely that this recrystallisation effectively 'reset' the Re-Os geochronometer, with the mobilisation of Au and other precious metals from primary sulphide hosts by grain boundary fluids. The isochron age therefore is likely to represent the closure of the sulphide host to rhenium and osmium, after recrystallisation. The radiogenic initial isotopic composition of the isochron eliminates the possibility that the sulphides were derived from the intruding quartz dolerite dykes which have near-chondritic initial osmium isotopic compositions. The inclusion of tonalitic, intermediate and mafic lithologies in the regression suggests a regional rather than a local scale for the event. The isochron age obtained here suggests that the intrusion of the volumetrically significant quartz dolerite dyke suite at ca 2.0 Ga may have provided the thermal trigger for the sulphide recrystallisation in the surrounding gneisses. Alternatively, given the large uncertainty in the age, it may be an expression of late Inverian metamorphic effects which were not pronounced in the central region due to the lack of externally derived fluids. This study illustrates the potential uses of the Re-Os geochronometer as a tool in the dating of metamorphic events in which sulphide phases experience recrystallisation.

REFERENCES CAMERON E.M. 1994. Depletion of gold and LILE in the lower crust: Lewisian Complex, Scotland. Journal of the Geological Society, London 151, 747-754.

FOSTER J.G., LAMBERT D.D., & FRICK L.R. 1995. Re-Os isotopic constraints on the genesis of Late Archaean Komatiite-associated Fe-Cu-Ni (PGE) deposits of Western Australia. In 1995 IGCP Project 336, Petrology and Metallogeny of Volcanic and Intrusive Rocks of the Midcontinent Rift System, Proceedings, 49. Duluth, Minnesota.

FRICK L.R., et al 1995. Pedogenesis of the Scourie Dykes by second-stage melting of Lewisian subcontinental mantle: PGE and Re-Os isotopic evidence. In Precambrian 95- International Conference on Tectonics & Metallogeny of Early/Mid Precambrian Orogenic Belts., Program and Abstracts, 86-87. Montreal, Canada.

LAMBERT D.D., et al 1994. Re-Os and Sm-Nd isotope geochemistry of the Stillwater Complex, Montana: Implications for the pedogenesis of the J-M Reef. Journal of Petrology 35, 1717-1753.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 ROLE OF MAGMAS IN THE ORIGIN OF GRANULITE TERRANES B. Ronald Frost, Department of Geology and Geophysics University of Wyoming, Laramie WY 82071 USA

Granulite terranes are characterized both by high temperatures and by the presence of carbonaceous fluids. It has been postulated that these characteristics can be the result of metamorphism of previously dry rocks, flushing of amphibolite-grade gneisses by C02-rich fluids, or the extraction of water through partial melting. It is likely that in many terranes these three mechanisms all may be part of a single process - the passage of hot, dry, C02-laden melts through the crust. In some instances these dry melts may be of mafic composition, but in other terranes these melts may have crystallized to produce pyroxene-bearing granitic rocks - charnockites.

In the four terranes described below, igneous charnockites are likely to have been the source of fluids and heat for granulite metamorphism. In the Wind River Range of Wyoming (P= 4 kbar) and the Thor Range of Queen Maud Land, Antarctica (P = 5 kbar) the igneous nature of the charnockites is obvious, because the charnockites locally grade into typical hornblende or biotite - bearing granites. In South India (P 5-6 kbar, locally higher), the "type locality" for C02-flushing, the origin of the charnockites is less clear, although the charnockites and granulite metamorphism are contemporaneous with typical, cross-cutting biotite granites found at higher crustal levels (P = ca. 3 kbar). In the Western Aldan Shield the ancient granulites (P = 6 - 7 kbar) are immersed in a sea of charnockite that is unfoliated and interpreted to be igneous in origin.

The Fe/(Fe+Mg) ratio of orthopyroxenes from the charnockites described above range from 0.4 in the Wind River Mountains (a composition typical of calc-alkalic rocks) to more than 0.85 in the Thor Range (a composition distinctive of A-type granites). Charnockites thus can form from a wide range of granite compositions and the factors controlling their origin probably relate to a source of sufficient volumes of CO2 rather than to any specific tectonic environment. The CO2 in the magma is probably of mantle origin and was likely transported into the crust by underplated mafic magmas.

If this model is correct, then: 1) The CO2 responsible for the CC>2-flushing seen in some terranes was probably transported into the crust by melts; it did not stream into the lower crust directly from the mantle. 2) Large portions of granulite terranes may have been originally anhydrous rocks and excessive volumes of carbonaceous fluids are not required to dehydrate them. 3) Most granulite terranes probably were metamorphosed at temperatures well in excess of those recorded in the Fe-Mg exchange thermometers, since the temperatures these thermometers record are locked in after melt has been extracted from the system.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SUBSURFACE A R C H I T E C T U R E OF THE TAG HYDROTHERMAL MOUND, MID-ATLANTIC RIDGE: RESULTS OF OCEAN DRILLING PROGRAM LEG 158 J. Bruce Gemmell1 and the ODP Leg 158 Scientific Party2 ^ODES Key Centre, University of Tasmania, GPO Box 252C, Hobait, TAS 7001 Ocean Drilling Program, Texas A&M University, College Station, Texas, USA 77845

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The scientific objectives of Leg 158 of the Ocean Drilling Program were to investigate the fluid flow, geochemical fluxes and associated alteration and mineralisation, microbiological processes, and the subsurface architecture of the TAG active hydrothermal mound located on a slow-spreading, sediment-free segment of the Mid-Atlantic Ridge. This is the first time an active black smoker system has been drilled.

The active TAG sulphide mound was discovered in 1986 and is located at a water depth of 3,650 m at the base of the eastern wall of the Mid-Atlantic Ridge at 26°N. The mound is distinctly circular, measures 200 m in diameter and rises about 50 m above the seafloor and is the largest, singular sulphide mound yet discovered on the seafloor. A cluster of black smoker chimneys emitting fluids up to 360°C and consisting of chalcopyrite, pyrite and anhydrite is located northwest of the centre of the mound on the top of a 10-15 m high cone. A field of sphalerite dominated white smokers venting fluids from 260 to 300°C is located in the southeast quadrant of the mound approximately 70 m away from the black smoker chimneys. Fluids from the white smokers are zinc-rich, and contain lesser amounts of iron and copper than the black smoker fluids.

Seventeen holes were drilled between September and November 1994 in five areas of the mound, with the deepest penetration of 125 m. Drilling in different areas, including a high-temperature black smoker complex and a lower-temperature white smoker vent field, revealed a multi-stage depositional history and the extent of sub-seafloor mineralisation and alteration. The upper 10-20 m of the TAG mound consist of massive pyrite and pyrite breccias, with significant chalcopyrite and sphalerite in places, which are underlain by an pyrite-anhydrite breccias from about 20 to 30 m below seafloor. With increasing depth, quartz-pyrite mineralisation and quartz veining in the pyrite breccias becomes dominant and represents the top of a quartzpyrite stockwork zone. Quartz-pyrite breccias at the top of the stockwork grade into silicified wallrock breccias below 40 m. Chloritised basalt breccias were sampled at depths greater than 100 m. Recovery of relatively unaltered basalt near the edges of the mound has constrained the extend of the stockwork mineralisation and intense alteration to a pipe-like feature of approximately 80 m diameter.

The complex assemblage of breccias which comprises the bulk of the mound includes clastic sulphide debris, chert breccias, anhydrite cemented pyrite breccias and quartz-pyrite breccias. These lithologies may be the products of multiple episodes of mass-wasting, cementation, hydrothermal reworking, and replacement during the growth of the mound. Repeated episodes of cementation and replacement are responsible for the complex stratigraphy encountered during drilling, the present surface morphology, and the distribution of vents.

Five generations of veining are observed in the stockwork zone and within the mound and indicate multiple generations of hydrothermal fluid flow up through the stringer zone and into the mound. The quartz-pyrite stockwork veins (Stages 1-4) are best developed deep in the footwall and pass upwards into anhydritedominated veins (Stage 5). The chalcopyrite content, within the upper part of the stockwork zone and lower to middle portions of the hydrothermal mound, is largely related to the development of selvages on the late Stage 5 anhydrite veins. Overall, the character and intensity of alteration, the change of mineralogy with depth from quartz to chlorite-dominated alteration assemblages, and the style of stockwork veining are very similar to the Cyprus-type massive sulphide deposits and footwall alteration zones and stringer systems of numerous volcanic-hosted massive sulphide described world-wide.

Continuing research on this unique drill core will provide significant insights into the mechanisms of sulphide precipitation, zone refining processes, and the evolution of major black smoker systems on the seafloor as modern analogs for ancient massive sulphide deposits on land.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 41 13th Australian Geological Convention, Canberra, February 1996

THE IMPORTANCE OF SEDIMENTOLOGICAL ANALYSIS IN ESTABLISHING THIRD-ORDER SEA-LEVEL FLUCTUATIONS IN THE UPPER DEVONIAN REEF COMPLEXES OF THE CANNING BASIN, WESTERN AUSTRALIA. Annette D. George . Phillip E. Playford and C.McA. Powell Dept of Geology & Geophysics, U.W.A., Nedlands 6907 ^Geological Survey of W.A., 100 Plain Street, East Perth 6004 1

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A regional model recently proposed for the Lennard Shelf and Fitzroy Trough in the northern Canning Basin emphasises the significance of the deep-water deposits which reside primarily in the subsurface (Southgate et al., 1993). It provides a sequence-stratigraphic framework in which to investigate and establish the effects and timing of third-order sea-level fluctutations (i.e -1 to several million years duration) on carbonate reef development during the Late Devonian. In this model, the third-order changes in sea-level are superimposed on a second-order transgressive-regressive cycle (-25 m.y.) that corresponds to the GivetianFrasnian (aggradational and back-stepping) and latest Frasnian-Famennian (strongly progradational) phases of reef development long recognised from outcrop studies.

The northern edge of the Canning Basin was a technically active margin, with large amounts of siliciclastic sediment supplied from the adjacent Kimberley landmass. The reef complexes, therefore, developed in a mixed siliciclastic-carbonate depositional system that included siliciclastic fans and deltas, reefrimmed carbonate-dominated platforms (with well developed back-reef, reef-flat and reef-margin environments), carbonate-dominated foreslopes and deep-water basinal fans. These kinds of systems are not well described by current sequence-stratigraphic models based on eustatically controlled passive-margin siliciclastic successions because the models do not give adequate consideration to environmental and sedimentological processes that operate in conjunction with the local and regional tectonics and eustasy.

The Napier Range is composed largely of fore-reef limestones and subordinate sandstones (Napier Formation) that display a remarkably similar stratigraphy along a strike length of at least 80 km (southern Dingo Gap to Barker River areas). Along the range, the fore-reef strata abut the eroded edges of older platform remnants. Lithofacies analysis is used to construct a depositional model in which overlapping slope aprons developed along the base of a steep-walled, high-relief platform margin. Coeval platform rocks have been eroded, and therefore we use the sedimentary style and composition of the slope rocks to interpret the history of contemporaneous reef development during the latest Frasnian to late Famennian (Fig. 1).

The sedimentology of the slope rocks closely reflects the levels of carbonate growth and productivity on the platform. Highly productive times (i.e., when the platform was flooded during relative highstands) led to the oversupply of shallow-water grains, particularly ooids and peloids. This sediment was subsequently delivered to the slope via turbidity currents and possibly also grain flows on the steepest slopes. Collapse of the reef margin also occurred during highstand times, and was particularly evident during strong progradation of the platform. Transgressions were characterised by very slow accumulation rates on the slope, with enhanced reworking, bioturbation and, at times, spectacular and widespread development of deep-water non-fenestral stromatolites. Major influxes of siliciclastic sediment occurred during the late Frasnian (locally forming the base of the exposed slope succession) and Early Famennian, and indicate times of reduced carbonate production on the platform. It is not clear that this reduction can everywhere be attributed to sea-level falls, although the Famennian sandstone is associated, at least in part, with a proposed sub-aerial exposure event Alternation of interpreted transgressive and highstand deposits on the slope (consistent with Southgate et al, 1993) combined with conodont biostratigraphy, indicates at least two Frasnian and at least four Famennian sequences produced by third-order fluctuations (Fig. 1). Only two sequence boundaries appear to coincide with global sea-level falls: at or near the Frasnian-Famennian boundary; and the other in the Early Famennian. Some relative sea-level fluctuations, therefore, may reflect tectonic events. The development of base-of-slope aprons below a near-vertical high-relief platform margin, with coarse talus and debris-flow breccias adjacent to the margin, and the presence of allochthonous reefal blocks and breccias at particular stratigraphic levels, suggests tectonic control on large-scale geometries and facies distribution on the slope.

Our work suggests that regional sedimentological studies coupled with biostatigraphic data are required to produce more robust sequence-stratigraphic interpretations for mixed systems that developed in technically active basins. Integration of all available data for the Lennard Shelf reef complexes will enable the effects of eustasy and tectonism in controlling third-order sea-level fluctuations to be more clearly differentiated.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE USE OF Nd-Sr ISOTOPIC TRACING IN THE STUDY OF ARCHAEAN GOLD DEPOSITS IN THE NORSEMAN-KAMBALDA AREA, WESTERN AUSTRALIA Maiid Ghaderi. Ian H. Campbell, Malcolm T. McCulloch, Victoria C. Bennett and Graham E. Mortimer Research School of Earth Sciences, Australian National University, Canberra, ACT 0200

Different hypotheses for the origin of Archaean granite-greenstone hosted gold deposits predict different sources for the gold. Various rock types from ultramafic and mafic, to felsic rocks have been proposed as sources of ore components. One method of evaluating these hypotheses is the use of Nd and Sr isotope systematics of hydrothermal minerals found in the veins to trace the source of the ore fluids (cf. Farmer & DePaolo, 1987; Mueller et al, 1991). If the isotopic signatures of Nd and Sr are known and are distinctive in each potential source rock, the isotopic compositions of the hydrothermal minerals can be used to determine the source(s) of gold in the ore-forming fluid. Scheelite is a common vein mineral which is spatially associated with gold ores in many of the deposits in the Yilgarn Block. Isotopic analysis of the scheelite can therefore yield important information about the source and nature of the ore-bearing fluids. Scheelite is a favourable mineral for Nd and Sr isotopic tracer studies because it contains high concentrations of these elements. In addition, because of the very low Rb/Sr ratios in scheelite, precise values of initial Sr/ Sr ratios can be obtained.Previous Nd isotopic analyses of scheelites from Mount Charlotte gold mine in Kalgoorlie (Kent et al., in press) give ENd values between +3.5 and +9.0 at 2602 Ma (the inferred age of mineralisation), implying that all or most of the Nd was derived from a komatiitic source. To further test the value of Sr and Nd isotopes in scheelite as a tracer for the ore-forming fluids, the Nd-Sr isotopic compositions of scheelites from a number of gold mines at Norseman and Kambalda have been analysed. In addition, samples of the potential source rocks in those areas have also been analysed for their Sr and Nd isotopes.

Scheelites from Mararoa, Crown Reef, O.K. and Bullen gold mines at Norseman have e ^ values between +0.9 and +3.1, with most of the samples lying between +1.5 and +2.0 at the time of gold mineralisation (2630 Ma).

However, the Nd isotopic results for North Royal mine at Norseman yield £ ^ values between -1.0 and +1.4.

These results imply that the Nd in Norseman scheelites has similar £Nd(2630) values to the underlying basalts and must have come mainly from that source, although a small contribution from the surrounding granitoids and felsic porphyry rocks, especially in the case of North Royal mine, is not precluded. Sr isotopic results for scheelites from Norseman show a wide range of initial ratios (0.70177 to 0.70399) which is in agreement with an earlier study (Mueller et al., 1991) and can be interpreted as indicating the involvement of two different sources for Sr, a felsic source such as the surrounding granitoids or the lower crust that melted to form these granitoids, and a mafic source which is probably the underlying basalts.

The scheelite samples from mineralised vein systems in Revenge, Junction and Victory gold mines at Kambalda have £Nd(2630) values between -4.0 and +9.0, indicating komatiites and Lunnon basalt as the main sources of Nd at Kambalda with small contribution from Paringa basalt and/or felsic rocks in some parts of Victory mine. The difference in values for these scheelites and Norseman scheelites can be related to the stratigraphic positions of the gold deposits. Unlike the deposits at Norseman, the ore bodies at Kambalda occur above the komatiites, and therefore the ore-forming fluids can be expected to have flowed through them. As a consequence, the ascending ore fluids at Kambalda had the opportunity to extract Nd and Sr from the underlying komatiites, whereas at Norseman they did not. A notable exception is the Hunt deposit which underlies the komatiite horizon at Kambalda and has scheelites with similar values to the Norseman deposits. The Kambalda scheelites have lower and a more limited range of initial **'Sr/ Sr ratios (0.70097 to 0.70170) when compared with those from Norseman. These results require most of the Sr in the Kambalda scheelites to be derived from a mafic source.

Current Nd-Sr isotope tracing in the study of Archaean gold deposits at the Norseman and Kambalda areas suggests that at Norseman the ore fluids have sampled Nd and Sr from mainly basaltic rocks and in the case of North Royal mine, a contribution from felsic sources. At Kambalda, most of the Nd and Sr has been derived from within the greenstones, particularly the komatiites. If the ore fluids have extracted Nd and Sr from both granitic and greenstone sources, they may have also extracted gold from both sources.

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REFERENCES

Farmer, G.L., & DePaolo, D.J., 1987. Nd and Sr isotope study of hydrothermally altered granite at San Manuel, Arizona: Implications for element migration paths during the formation of porphyry copper ore deposits.

Economic Geology 82, 1142-1151.

Kent, A.J.R., Campbell, I.H., & McCulloch, M.T., in press. Sm-Nd systematics of hydrothermal scheelite from the Mount Charlotte mine, Kalgoorlie. Economic Geology.

Mueller, A.G., de Laeter, J.R., & Groves, D.I., 1991. Strontium isotope systematics of hydrothermal minerals from epigenetic Archaean gold deposits in the Yilgarn Block, Western Australia. Economic Geology 86, 780809.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ENVIRONMENTAL PROTECTION IN CONNECTION WITH OIL POLLUTION. Irina Gildeeva, All-Russia Petroleum Research Exploration Institute (VNIGRI), St. Petersburg, Russia

Oil and oil products take one of the priority place in environmental pollution. No other pollutants, as if they were dangerous, excluding the products of atomic nucleus division, may be comparied with oil and oil products on occurence scales, pollution source amount and influence degree to all the components of natural system.

The evaluation of pollution scales shows that every year the land surface is contaminated by about 30 mln.t of oil, that is equivalent to loss of one large oil field.

Fight with oil pollution demands the development of functionating system of measures on environmental protection. This system includes oil-ecologic monitoring, averting oil pollution, and cleaning contaminated territory (restoration of natural systems).

The detailed characteristic of each measure indicated above is given.

Analysis of toxity of the harmful substances entering in atmosphere during oil refining is given.

The classification of oil and gas fields on the degree of danger to environment and toxity influence to nature is offered. The construction of new type maps - maps of prognosis of ecological danger (risk) during the development of oil and gas fields is recommended on the basis of this classification.

In this report, among the measures on environmental protection, particular attention is given to the technology of production and refining of high-viscous, high-sulfur (including metal-bearing) oils, which was developed in VNIGRI (Sanct-Petersburg, Russia) by V.V.Gribkov, M.D.Belonin, and D.M.Soskind.

The particular attention is also given to the description of new method of soil cleaning from oil and oil products, on the whole, basing on the application of NAPHTOX biopreparations. This method was offered by the Russian specialists (M.D.Belonin, E.A.Rogozina, R.M.Svechina, and others, 1994).

In conclusion, it is indicated that the success in the environmental protection may be reached only under condition of using the all complex of environmental protection measures.

157

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 ESTABLISHING THE FLUID HISTORY OF THE WOODCUTTER S Pb-Zn-Ag MASSIVE SULPHIDE DEPOSIT BY THE USE OF INCLUSION PETROGRAPHY, CATHODOLUMINESCENCE, AND SULPHUR ISOTOPE AND LASER RAMAN MICROPROBE STUDIES

Alan D. Giles and Brian Marshall Department of Applied Geology, University of Technology - Sydney, PO Box 123 Broadway, NSW 2007

In many massive sulphide deposits, the sulphide mineralization is opaque which hinders the determination of the types and properties of fluids associated with the process of mineralization (if indeed such fluids are still preserved), from the viewpoint of fluid inclusion studies. Woodcutter's Mine is an exception to this, with much of the ore containing Fe-poor (i.e. translucent) sphalerite. Studies of the Woodcutter's ore reveals interesting problems that influence fluid inclusion interpretation; the main problems being ore remobilization and recrystallization, and inclusion decrepitation.

The Woodcutter's Mine exploits a series of Pb-Zn-Ag orebodies, and is classified as a deformed, epidiagenetic massive sulphide deposit. The deposit occurs 84km SSE of Darwin in the Northern Territory of Australia, on the western margin of the Pine Creek Inlier. Most of the mineralization is hosted within carbonaceous slates interspersed with siliciclastic tuff horizons, and dolomitic carbonate referred to as "dololutite". Two main distributions of ore are present. These are: banded massive sulphides as carbonate replacements in the hinge zones of two major anticlines (i.e. the "3" and "5" anticlines), or as vein-type mineralization which partly occupies "3" and "5" faults associated with the anticlines. Mine-, meso-, and micro-scale relationships strongly suggest that the ore was emplaced pre- to early syn- D] during lower greenschist fades metamorphism.

The present study supports fluid-rich entrapment of inclusions that post-date both ore remobilization, recrystallization, and an inclusion decrepitation event based on the findings of a combination of techniques.

Detailed fluid inclusion petrography, combined with cathodoluminescence was used to establish 'primary versus secondary' origin for inclusion populations. The sulphur isotope study (in terms of geothermometry), was used to determine 'isotopic re-equilibration' of co-existing sulphide pairs (i.e. pyrite-galena, pyrite-sphalerite, and sphalerite-galena), in conjunction with microscope observations of ore minerals. The Laser Raman Microprobe was used to determine the composition and variability of the fluids contained in the inclusions. The study is based on samples collected from "3" and "5" systems.

Undisrupted, colloform growth bands of Fe-poor and Fe-rich sphalerite are rarely preserved. More commonly, fragments of growth banded but recrystallized sphalerite are enclosed in massive remobilized sphalerite. Based on fluid inclusion petrography, fluid-type primary inclusions are lacking in the growth banded and recrystallized sphalerite, but various populations of secondary (liquid + vapour) microfracture type inclusions, including decrepitated inclusions, overprint the grain boundaries. Dolomite and quartz are associated with the sphalerite and similarly show dense populations of liquid and liquid + vapour secondary inclusions with variable proportions of H2O ± CO2. Cathodoluminescence (and Potassium Ferrocynanide staining) revealed growth zonation in both quartz and dolomite, with the above populations of inclusions overprinting these zones substantiating a secondary origin for these inclusions. The laser raman study verified the presence of CO2 and H2O together with traces of CH4. values for pyrite, galena, and sphalerite have a broad total scatter from 6.8 to 21.7 °/oo, with individual phase ranges of 9.0 - 21.7 (pyrite), 6.8 - 10.3 (galena), and 9.0 - 11.0 (sphalerite)°/oo, respectively. The pyrite has two ranges (9.0 - 12.7, and 14.8 - 21.7°/oo), indicating two generations are present. Calculations based on sulphide pairs reveal a range of temperatures from 393-737° (pyrite-galena), 314-678° (sphalerite-galena), and 277-957°C (pyrite-sphalerite), respectively. However, linked sulphide samples (i.e. the coarse grained sulphide phases adjoining each other in the sample), have large temperature discrepancies. This is supported by the apparent lack of euhedralism of adjoining sulphide grains. Visible remobilization within the sphalerite may also have contributed to the apparent 'disequilibrium' in terms of temperatures of formation.

Woodcutter's Mine demonstrates the benefits of examining co-existing ore mineral and gangue phases, and reveals the destructive influence of crystal plastic deformation and recovery processes on contained fluid inclusions. It also demonstrates that fluid inclusion studies alone cannot unravel the complicated nature of fluid movement over time and must be used in conjunction with other techniques to derive a fluid evolution in metamorphosed and deformed massive sulphide deposits.

158

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 ANOMALOUS STRUCTURE OF THE MOUNT ISA INLIER CRUST AND CRUST-MANTLE TRANSITION ZONE IN THE GLOBAL CONTEXT, FROM REFRACTION/WIDE-ANGLE SEISMIC EXPERIMENTS Alexev G. Goncharov, Clive D.N. Collins, Bruce R. Goleby and Barry J. Drummond Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

Refraction/wide-angle seismic data were recorded along the Mount Isa transect as a part of a broader geoscience project performed by the Australian Geodynamics Cooperative Research Centre in 1994. They show that the seismic velocity distribution in the crust and crust-mantle transition zone is complicated and varies significantly along the transect (fig.

1). There is no sharp velocity boundary between the crust and mantle but a thick (up to 15 km) transitional zone instead. Low velocity layers and zones of increased velocity gradient are quite common in the crust and crust-mantle transition zone. A high velocity body with seismic velocities higher than 7 km/s occurs in the middle crust at the centre of the transect. There are few areas globally which have seismic features similar to those of Mount Isa. Deep seismic results from other Australian Precambrian terranes Fig.l. Simplified velocity model along the Mount Isa are consistent with the concept of thickened Proterozoic crust transect (velocities in tarts) compared to Archaean crust. Where thickening of the crust occurs in Australia it is totally due to the thickening of the lower high-velocity (more than 7 km/s) crust. Velocity distributions from 0 to around 30 km depth are similar in Australian Precambrian areas. Velocities higher than 7 km/s at depths less than 20 km have never been reported in Australia, making the middle part of the Mount Isa Inlier outstanding, at least on an Australia-wide scale. Velocity models of the lower crust and crust-mantle transition zone in the Baltic Shield vary significantlyfromone area to another and can be subdivided into 3 groups: normal, intermediate and anomalous. They characterize geological evolution of continental crust from thin, low average velocity crust (normal seismic model, fig 2) to thick crust with a large volume of high velocity rocks at the base. There is no direct correlation between the type of the seismic model and the age of rocks outcropping at the surface. For example, the anomalous thick crust is typical for the Ladoga-Bothnian zone in Finland and Velocity, km/s 6.5 7.5 its possible extension in Russia which is not manifested at the surface. The age of rocks at the surface variesfromArchaean to Proterozoic along this feature. A similar crustal structure characterizes the zone of Paleozoic reactivation extending from Oslo in Norway to the Khibiny mountains in Russia. There are only very few Paleozoic rock units outcropping at the surface along this zone.

This implies asynchronous development of the crust at different depth levels.

None of the Baltic Shield regional scale velocity models is close to that of the middle part of the Mount Isa Inlier (fig. 2). Only some structures of the Ukrainian and Baltic Shields which are very local compared to the Mount Isa middle crust seismic anomaly remotely resemble Mount Isa crust in velocity distribution. The Fig.2. Velocity modelsfromthe Korosten andKorsun-Novomirgorodplutons in the Ukrainian Shield and Vyborg Mount Isa transect, middle part (triangles) and from normal crust of massif in the Baltic Shield are among them. The upper crust there has a multithe Baltic Shield (hard lines) layered structure with alternating of high (6.6-6.8 km/s) and low (6.0 km/s) velocityrocks. The high velocity layers do not exceed 5 km in thickness and look as if they were intruded into the low velocity background mass. They most likely correspond to sill intrusives of mafic rocks which were emplaced through subhorizontal weakened zones. The Moho beneath Korosten pluton and Vyborg massif is elevated (which again is not similar to Mount Isa) with an intermediate layer with velocities of 7.7-7.8 km/s immediately above. The Lake Superior and Lake Michigan midcontinental rift system in Canada has some seismic features common to those of the Mount Isa Inlier. High velocities up to 7.0 km/s have been detected in the Canadian Shield middle crust at a depth of 18 km. This is in qualitative agreement with the thick series of magmatic rocks of basaltic composition which have been documented there by geologists. A thick crust-mantle transition zone with complex structure and mixed composition occurs in a depth range of 30-60 km. These features are similar to those observed in Mount Isa and therefore support the model for the early development of the Mount Isa Inlier in an extensional regime.

Regional extension is normally associated with increased heat flow and creation of weakened zones along which melted high velocity rocks from the lower crust may have been emplaced to a higher level in the crust. Thus the extensional regime might have been responsible for the formation of the high velocity anomaly in the middle of the Mount Isa transect.

Acknowledgments: This abstract is released with the permission of the Director of the Australian Geodynamics Cooperative Research Centre.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

TESTING TIMES IN THE TIMOR SEA - MUD ADDITIVES, THE BUG-BEAR OF THE PETROLEUM EXPLORATION GEOLOGIST, AND OTHER COMPLICATIONS. John D. Goner, Hardy Petroleum Limited, 40 Kings Park Road, West Perth, WA 6005.

Exploration for hydrocarbons in the Timor Sea is demanding enough without the complications derived from the use of organic mud additives and other down-hole contaminants. Examples of contamination of geochemical samples abound (e.g. Peters, 1986). The deliberate introduction of additives to combat drilling problems can be accounted for if the fact of the addition is noted in the well completion reports and a geochemical analysis of the additive carried out. Major contamination can be expected in wells where there was a history of battling lost circulation or collapsing borehole.

Any well with an 'A' or 'ST' (sidetrack) designation is suspect.

However, the addition of petroleum-based contaminants is often not reported in the drilling records and little other note is made of other potential sources of contamination (e.g. pipe dope on every joint of drill pipe, testing tools and sidewall coring gun lubricants). Several examples of overt (e.g. Basilisk 1A) and covert (e.g. Rambler 1) contamination from the literature and the author's personal experience in the Timor Sea will be given. While contamination from Middle East carbonate sources and Sumatran (oleanane) derived diesels and petroleum products can be documented, what of contamination from diesels of North West Shelf origin derived from a similar source facies to the expected hydrocarbon product (e.g. from Jabiru or Challis fields)? Possible contamination from Bass Strait-derived oil-based additive or diesel is also documented (Avocet 1 A).

Another source of misinterpretation is the occurrence of migrant hydrocarbons in lithologies traditionally thought of as being impermeable to invasion by hydrocarbons: these rocks are often considered source rocks. Supersaturation of the supposed cap-rock shales and claystones over gascondensate and light oil accumulations, and of shales and claystones interbedded with reservoir sandstones within those accumulations, can lead to the erroneous interpretation of these fine clastics as the source rocks of the accumulated hydrocarbons. An example from an unnamed well in the Timor Sea is provided to show the presence of migrated hydrocarbons from the surrounding and underlying reservoir into claystones, silty and sandy claystones and glauconitic rocks. Given geological time, all such lithologies are permeable, and with light oils and condensates, such permeability is likely to have allowed the ingress of hydrocarbons to the detriment of the source rock assessments, and the confusion of the unwary.

REFERENCES Peters, K.E., 1986. Guidelines for evaluating petroleum source rock using programmed pyrolysis.

American Association of Petroleum Geologists, Bulletin 70, 318-329.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

AGE AND TECTONIC SIGNIFICANCE OF OPHIOLITIC ROCKS FROM THE TUMUT-GUNDAGAI REGION OF NSW Ian T.Graham*, Brenda J.Franklin*, Brian Marshall*, Evan GLeitch* and Mark Fanning^ ^Department of Applied Geology, UTS, PO Box 123 Broadway, NSW 2007 2

PRISE, Research School of Earth Sciences, ANU, Canberra, ACT 0200

Ultramafic and associated rocks from the Tumut-Gundagai region of southern NSW have been widely discussed in tectonic interpretations of the Lachlan Fold Belt. Of the ultramafic rocks within this region, the Coolac Serpentinite Belt has been studied the most and has been variously interpreted as the basal unit of a dismembered ophiolite suite, part of an embryonic ophiolite, a tectonic slice derived from the underlying Cambro-Ordovician basement, and an allochthonous slice translated eastward from the Cambro-Ordovician basement of Victoria. Much of the uncertainty as to the tectonic setting and significance of the Coolac Serpentinite Belt and other serpentinite belts (such as the Wambidgee Serpentinite Belt) in the region results from inadequate age constraints.

The Coolac Serpentinite Belt outcrops continuously for over 56km and is up to 2.5km in width. To the east, it has a complex faulted/intrusive contact with the S-type Young Granodiorite, while to the west it is faulted against a Silurian metavolcanic and metasedimentary sequence or is supposedly intruded by mafic/ultramafic igneous rocks such as the North Mooney Complex. Most of the Coolac Serpentinite Belt is composed of relatively unsheared partially serpentinised harzburgite. The Wambidgee Serpentinite Belt outcrops discontinuously for over 100km and is up to 2km in width. It has a faulted/intrusive contact with the S-type Young Granodiorite or faulted contacts with the Cambro-Ordovician Jindalee Beds. Most of the Wambidgee Serpentinite Belt is composed of schistose metaserpentinite.

As well as other typical ophiolitic rocks, both belts contain small bodies of plagiogranite enclosed within schistose serpentinite. These bodies are considered to be of ophiolitic affinity as they are both chemically and mineralogically similar to leucocratic rocks associated with ophiolites world-wide. A variety of silicic magmatic rocks (e.g. the Young Granodiorite) are widespread adjacent to the serpentinite belts, but none has a similar chemistry to those described here. The plagiogranites bodies are widely distributed throughout both belts and occur as either intrusions or tectonic inclusions. Most of these bodies contain small enclaves of metabasalt and mafic amphibolite and are defined as oceanic plagiogranite. That is, they are greatly depleted in potassium and enriched in sodium.

Zircon concentrates obtained from four of these plagiogranites and one leucogabbro were subjected to U / P b isotopic dating using the SHRIMP II ion probe at the ANU. This revealed that the plagiogranites from the Coolac Serpentinite Belt have a mean age of formation of 401 Ma (s.d = 1.4 Ma) while those from the Wambidgee Serpentinite Belt have a mean age of formation of 400 Ma (s.d = 1.2 Ma). Thus both belts are of essentially the same Early Devonian age rather than Cambro-Ordovician or Silurian as previously postulated. It is significant that the formation ages fall within the age range of 420-390 Ma assigned to a protracted thermal event during which most of the granite bodies were emplaced within the Lachlan Fold Belt.

The dates obtained reassert the existence of middle Palaeozoic oceanic crust in this portion of the Lachlan Fold Belt but the surprisingly young age requires a revised approach to the tectonic setting of ophiolitic generation in this part of the Lachlan Fold Belt. We suggest that the ophiolitic rocks are paraautochthonous remnants marking a small middle Palaeozoic intra-arc basin, analagous to those in the southern Andes where, during the Cretaceous, elongate narrow basins opened and were floored by varying amounts of new mafic crust. This basin development was both preceded and followed by voluminous silicic magmatic activity. We suggest that, in the Late Silurian and Early Devonian, the central section of the Lachlan Fold Belt was the site of rapidly opening and closing intra-arc basins controlled by basin-bounding and intra-basinal faults. The implied crustal instability probably reflected the major thermal event that was concurrently manifested by voluminous emplacements of S- and I-type granitic magmas. Thus, both the development of oceanic crust and emplacement of granitic magma are collectively symptomatic of the same major tectonothermal event.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SOURCE REGIONS FOR NORSEMAN AND LARA MELNOITES, WESTERN AUSTRALIA: T R A C E E L E M E N T AND RB-SR, SM-ND AND RE-OS I S O T O P I C CONSTRAINTS Stuart Graham 1 . David D. Lambert 1 , Simon R. Shee2, Roger Hamilton" and Jeff G. Foster1-4 'VIEPS Department of Earth Sciences. Monash University. Clayton, VIC 3168 2 Stockdale Prospecting, P.O. Box 126, South Yarra, VIC 3141 3 Western Mining Corporation, P.O. Box 71, Kalgoorlie, WA 6430 ^Western Mining Corporation, Kambalda Nickel Mines, Kambalda. WA 6442

We have obtained major, minor, trace element and Rb-Sr, Sm-Nd and Re-Os isotopic data from the Norseman and Lara melnoites (= ultramafic lamprophyres) in order to assess the geochemical and isotopic evolution of the eastern Yilgarn Craton and its subcontinental lithospheric mantle (SCLM) keel. Re and Os concentrations and Os isotopic compositions were determined by isotope dilution N-TIMS following a low-blank, Carius tube digestion/equilibration procedure. The Norseman melnoite dykes (850 Ma) intrude Archaean granites of the Eastern Goldfields Province, close to the Albany-Fraser orogenic belt. The Lara melnoite pipe (2100 Ma) is located in the southern pan of the Officer Basin and has a cover of Permian glacial sediments. Petrographically, the melnoites contain abundant phenocrysts of phlogopite and altered olivine set in a fine-grained matrix of phlogopite, diopside, titano-magnetite, perovskite, apatite and ilmenite set in an interstitial base of serpentine, calcite and probable nepheline. Veins and clots of carbonate are also present. Schorlomitic garnet occurs in the matrix of samples close to the contact with the country rock. As yet, mantle xenoliths have not been found but the Norseman dykes are characterised by abundant ilmenite megacrysts.

Geochemically, the melnoites are potassic (0.5-3.2% K,0) and silica-undersaturated (33-36% SiOo) with high MgO (13.3-17.3%), CaO (12.4-14.7%), and Ti0 2 (0.5-3.2%) and low A1203 (4.5-6.0%) and Na.O (0.5-0.7%) contents. High concentrations of incompatible elements (Sr, Rb, Zr, and Nb) and high (La/Lu)n ratios (Norseman = 35; Lara = 50; Lara carbonate clot = 140) suggest that the enriched trace element geochemistry of the melnoites is a result of metasomatism of the mantle source prior to partial melting. PGE patterns for the Norseman melnoites are strongly fractionated with depletion of the IPGE's (Ir, Os, Ru). The Pd/Ir ratio (-16) is slightly higher than that of kimberlites (~5), but similar to other asthenospheric mantle melts and carbonatites. One melnoite from Norseman has Re and Os concentrations of 0.358 and 0.0698 ppb, respectively and a Re/Os concentration ratio of 5.13. The high Pd/Ir and Re/Os ratios but low Ni and Cr concentrations are consistent with low-degree equilibrium melting in the spinel peridotite stability field (McDonald etal., 1995).

Rb-Sr whole rock isotopic data show unradiogenic initial 87Sr/86Sr ratios for the Norseman melnoites (0.70200.7031) and the carbonate clot removed from the Lara pipe (0.7032). Sm-Nd isotopic data yield radiogenic e Nd values (Norseman = +4.5 to +5.5; Lara = +2; Lara carbonate clot = +0.5). The Nd isotopic data for the Lara carbonate clot is consistent with data from the 2100 Ma Mt Weld carbonatite (Nelson et al., 1988; S N d = +0.5), suggesting a possible genetic relationship. All of these intrusions plot within the field for Group I kimberlites on a 87 Sr/ 86 Sr - e Nd diagram, consistent with derivation from an enriched mantle reservoir within either the asthenosphere or SCLM. Preliminary Re-Os data for one melnoite yield a radiogenic initial 187Os/188Os of 0.31 and an initial y 0s value (percent deviation of initial Os isotopic composition from chondritic asthenospheric mantle of the same age) of +150. High y 0s values may imply contamination with radiogenic crust. However, the low Si0 2 and high LILE, REE and HFSE concentrations (higher than average continental crust) preclude this interpretation for the Norseman melnoites.

Sm-Nd depleted mantle model ages (TDM) for the Norseman melnoites are 1100 Ma whereas the Lara melnoites and the Mt Weld carbonatite have model ages of 2400 Ma, indicating two discrete metasomatic events in the SCLM beneath the Eastern Goldfields Province. The Lara-Mt Weld TDM model age is similar to the crystallisation age of the large Proterozoic mafic dyke swarm throughout the Yilgarn, apparently intruded after a period of north-south crustal extension. In contrast, a Norseman melnoite dyke has internally-consistent Proterozoic Sm-Nd TDM and Re-Os TCHUR model ages (1100 and 1200 Ma, respectively), very similar to the age of the Albany-Fraser orogen (1300-1100 Ma). We postulate that this episode of plate collision may have provided the necessary components, via slab subduction or intra-mantle melt addition, to the SCLM to produce the observed geochemical and isotopic features seen in the Norseman melnoites. Re-melting of old, melt modified mantle (MARID veins?) in the SCLM could explain the high concentrations of LILE, REE, HFSE and Re and the very radiogenic initial Os isotopic composition. The Nd and Os isotopic data suggest that metasomatic enrichment occurred ca. 300 Ma prior to emplacement of melnoites in both localities.

REFERENCES NELSON, D. R., CHIVAS, A. R., CHAPPELL, B. W. & MCCULLOCH, M. T., 1988. Geochemical and isotopic systematics in carbonatites and implications for the evolution of ocean-island sources. Geochim.

Cosmochim. Acta 52, 1-17.

MCDONALD, I., DEWITT, M.J., SMITH, C.V., BIZZI, L.A. & VILJOEN, K.S., 1995. The geochemistry of the platinum-group elements in Brazilian and southern African kimberlites. Geochim. Cosmochim. Acta 59 2883-2904.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 41 13th Australian Geological Convention, Canberra, February 1996

MAPPING COMMUNITY VULNERABILITY Ken Granger Principal Project Manager Queensland Emergency Services GPO Box 1425 Brisbane, QLD4001

There is an increasingly prevalent view amongst emergency managers that disasters or emergencies are the result of the interaction between a hazard phenomenon and a vulnerable community. An increasing amount of attention is consequently being paid to developing a better understanding of communities and the characteristics that make them more or less susceptible to the impact of a hazard event. In Australia this work is being undertaken through an Emergency Management Australia-sponsored working group tasked with developing a 'vulnerability index'. Pilot work in support of that program is being undertaken as part of the Tropical Cyclone Coastal Impact Program (TCCIP) but has significance to the widest range of hazards, especially those of geological origin.

At the centre of this development is the notion that the various factors that influence community vulnerability can be mapped as 'vulnerability surfaces'; these can be directly related to the more familiar 'hazard surfaces' such as storm tide inundation or seismic risk zone maps. Three principal groupings of vulnerability factors or 'dimensions' have been identified: •

the people of the community and their various demographic, social and economic characteristics;

•

the features of the built environment including houses, shops, factories, roads, utilities and so on; and,

•

the organisational infrastructure of the community including groups such as P&Cs, Red Cross, sporting clubs, etc, the administrative structures of local and state government agencies and the organisation and coverage of public safety agencies.

These groupings extend across the four major elements of community needs, namely: •

shelter (housing and other accommodation; clothing; protection, eg alert systems, shelters, mobility)

•

sustenance (food; water; air; energy)

•

security (health; wealth; confidence, eg welfare, public safety, etc services)

•

society (identity; structure; well being; company)

The projected outcome of this effort will be the development of atlases of community vulnerability which can be used at all stages of the emergency management process - prevention, preparedness, response and recovery. To date the bulk of the work has been directed towards the first two of these groups of factors with an emphasis on the Queensland cities of Mackay and Cairns.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

REVISED LACHLAN FOLD BELT O R O G E N I C P A T T E R N S BASED ON A NEW 4 0 A r / 3 9 A r D A T A S E T David R. Gray 1 . David A. Foster 2 & Martin Bucher 2 Australian Geodynamics Cooperative Research Centre department Earth Sciences. Monash University, Melbourne, VIC 3168 2 School of Earth Sciences. La Trobe University, Melbourne, VIC 3083

Initial delineation of an orogenic framework for most orogenic belts was based on the recognition of local and/or regional unconformities in the rock record with the assumption that these reflected orogenic beltwide tectonic events. This traditional approach has been used in the Lachlan Fold Belt with four main events recognised (e.g. Fergusson & Coney, 1992; Glen, 1992): (1) Benambran Event (Late Ordovician-Early Silurian: -440-430 Ma), (2) Bindian Event (Late Silurian: -425-415 Ma)IBowning Event (Silurian-Devonian: -415-390 Ma), (3) Tabberabberan Event (Early Devonian: -380-375 Ma), and (4) Kanimblan Event (Late Devonian Early Carboniferous: -360-340 Ma). This approach does not necessarily allow for either localised or wide ranging, diachronous deformations, and as in other orogenic belts this concept of orogeny is perhaps outmoded. Across much of the fold-belt timing of deformation has only been constrained by the age of the youngest sedimentary rocks and the ages of syn- to post-tectonic granitic intrusions within the respective structural zones. Fold-belt evolution is better linked to a series of tectonothermal events which can be correlated over large parts of foldbelts.

New 4 0 Ar/ 3 9 Ar data on slates from the western Lachlan Fold Belt (see Bucher et al., this volume) has shown that the Late Orodvician-Early Silurian event has much greater importance across the fold belt than was originally thought. Previously, the Late Ordovician/ Silurian deformation was believed to be restricted to the central Lachlan Fold belt (Wagga-Omeo and Tabberabbera zones) (see Coney & Fergusson, 1992, fig. 4).

Cleaved and phyllitic rocks from major fault zones in the western Lachlan Fold Belt (Stawell, Bendigo-Ballarat, and Melbourne Zones) however, indicate mica growth accompanying deformation was important between 410435 Ma (Bucher et al., this volume). Deformation within thrusts-sheets recorded by cleavage in chevron-folded sandstones and slates (Stawell and Bendigo-Ballarat Zones) also took place at this time. This further means that the mid- Devonian orogenic event (!Tabberabberan) must now have reduced significance in the context of the foldbelt proper, and perhaps just reflects 'docking' of the western and central/eastern parts of the fold-belt in the late Early Devonian. The Middle-Devonian deformation was formerly considered to be a paroxysmal, terminal event which affected the whole of fold-belt, leading to accretion of the Lachlan Fold Belt with the evolving Australian craton. Incorporating the new thermochronologic data, deformation fronts and/or zones of deformation can be restricted temporally and spatially to: 1) Late Ordovician-Early Silurian: the most widespread belt of deformation within the Lachlan Fold Belt, coinciding largely with the western and central parts. In the central part it is characterised by eastwest-trending folds and related thrusts, and high temperature/low pressure metamorphism. The western part is dominated by an eastwards migrating, east-vergent fold-thrust belt.

2) Middle Silurian to Early Devonian: main effects preserved as an east-vergent thrust-belt in the eastern Lachlan Fold Belt. The Melbourne Zone forms as a 'piggy-back basin' into the Early Devonian, with sediment derived from cannibilisation of already deformed and structurally thickened turbidites to the west (see Pound et al., 1994).

3)'Middle' Devonian: main effect in the Melbourne Zone of the western Lachlan Fold. Widespread effects occur in the eastern and central belt with weak deformation of volcanic and sedimentary cover sequences, and conjugate strike-slip faulting and reactivation of older strike-slip faults in the Wagga-Omeo metamorphic complex.

4) Early Carboniferous: the final regionally extensive deformation, with the Late Devonian-Early Carboniferous (Lambie facies) cover sequences folded throughout the Lachlan Fold Belt. The main effects were in the northeasternmost part of the eastern Lachlan Fold Belt (see Powell, 1984, fig.222B).

REFERENCES Bucher, M., Foster, D.A., & Gray, D.R. 1996. Timing of cleavage development in the western Lachlan Fold Belt: new contsraints from 40 Ar/ 39 Ar geochronology. Geological Society of Australia, Abstracts (this volume) Fergusson, C.L., & Coney, P.J., 1992. Convergence and intraplate deformation in the Lachlan Fold Belt of southeastern Australia. Tectonophysics, 214, 417-439.

Glen, R.A., 1992. Thrust, extensional and strike-slip tectonics in an evolving Palaeozoic orogen- a structural synthesis of the Lachlan Orogen of southeastern Australia. Tectonophysics, 214, 341-380.

Pound, K., Gray, D.R. & Cas, R.A.F., 1994. Provenance, sedimentologic and stratigraphic relationships of Ordovician sandstones of the Bendigo-Ballarat Zone, Victoria: implications for the Tectonosedimentary evolution of the Lachlan Fold Belt. Geological Society of Australia, Abstracts 37, 353-354.

Powell, C.McA. 1984. Uluru regime. ]n: Phanerozoic Earth History of Australia., Veevers, J.J. (ed.), Oxford Geological Sciences Series 2, pp. 290-337.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 GLOBAL CHANGE: A GEOSCIENCE PERSPECTIVE OF SUSTAINABLE DEVELOPMENT David H. Green Research School of Earth Sciences, The Australian National University, Canberra ACT 0200

The human species, by the combination of its numbers, land-use and materials use, has become a major agent of global change — in atmospheric composition, soil erosion, river-systems and groundwater, vegetation, and the abundance and evolution of species. Population increase, the globalisation of information and knowledge and the global pursuit of social justice will together precipitate a 21st Century in which the emphasis must be on global management. Although accustomed to engineering change, to development and local management, neither scientific nor political leadership has adequately considered the implications of global change. There are already major issues of international significance in which geoscientific information dictates the nature of solutions, in a global management context, but these solutions are not yet backed by political will. Environmental fundamentalism can prevent proper long-term management of human impacts, just as effectively as the pursuit of high economic growth without consideration of environmental consequences.

Geosciences are inevitably associated with mining. Mining, mineral processing and disposal of processing wastes are primary targets for protest as environmentally damaging or as dangerously polluting. The management of inorganic wastes must be placed in the context of natural geological processes and natural variability so that decision on "dilute and disperse" or "containment" strategies are based on sound environmental information. The essential issue is one of responsible use of 'best practice' and the adoption of such practices on a global basis. As the Australian mineral industry increases overseas exploration and development, it is essential that environmental standards be maintained and methods adapted to well-researched local situations.

The concept of global management has achieved acceptance and effective implementation in addressing anthropogenic ozone depletion in the upper atmosphere. The much more complex issue of change in abundances and distribution of greenhouse gases is also being addressed by international agencies and political interest. The scientific basis for management requires understanding of natural variability of atmospheric C 0 . The variation in C0 must be correlated with global and regional temperature changes through Quaternary glacial and interglacial cycles. The geological record contains the required information which may be used to test global climate models derived from increased understanding of ocean/atmosphere interactions and behaviour. The evidence for rapid change and short term "events" in temperature variability, and in relative sea-level must be critically assessed, and cause and effect identified for effective management of anthropogenic pressures.

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Issues such as greenhouse gas sources and sinks, energy sources, nuclear waste management and many other activities contain a strong geoscience component. Effective responses to and management of global change are not possible within national borders. Scientists have responsibilities to present scenarios and seek political commitments that recognise the inadequacies of short term market forces and short term political expediency to respond to the major challenge of the 21st Century — instability and fragility of key global systems caused directly by the numbers and lifestyle of the human species.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, CanberraFebruary 1996

EXPERIMENTAL VERSUS NATURAL TWO-MINERAL PARTITION COEFFICIENTS - A "HIGH-TECH" CONTROVERSY Trevor H. Green GEMOC, School of Earth Sciences Macquarie University NSW 2109

Development of several different microbeam techniques allows in situ analysis of trace elements in natural and synthetic mineral assemblages, which in turn enables determination of two-mineral partitioning behaviour for trace elements (D-values). With the experimental approach, control of compositional and physical variables is possible, but equilibrium needs to be carefully assessed, preferably by full reversal of the trace-element exchange, or at least by a series of experiments over different times to demonstrate experimental duration needed for "consistency" of results. Also in some instances compositions and optimal experimental conditions are different from the natural situation and extrapolation to the "real" geological state should be justified. With the natural mineral approach, physical conditions must be independently assessed, accepting the uncertainties inevitably involved in geothermobarometric determinations. Also a potentially complex history of formation must be unravelled, with the attendant possibility of non-equilibrium. For example, where the formation of amphibole in initially anhydrous Iherzolite is attributed to metasomatism by a trace-element and fluid-rich agent, it may be uncertain whether the trace element content of the pre-existing clinopyroxene equilibrates with the trace element content of the newly formed amphibole. Cases exist where isotopic disequilibrium has been demonstrated between coexisting natural mineral pairs in xenoliths, and this adds to the uncertainty of achieving equilibrium trace element distribution between the same minerals.

Comparison of D-values determined for coexisting amphibole and Ca-clinopyroxene indicates overall good agreement for Sr, Zr, Hf, Y and REE but significant discrepancy for Rb, Ba, Nb and Ta (experimental values for amphibole/clinopyroxene are much lower than natural values). For co-existing Ca-clinopyroxene and garnet, the spread of data is greater and fewer elements can be compared (Rb, Ba, Nb and Ta data are not adequate). However Sr, Y, Zr, Hf and Nd-Lu (of the REE) agree reasonably well, but La and Ce experimental values for clinopyroxene/gamet are much lower than the natural values. These differences for both mineral pairs may be attributed in part to critical compositional differences between synthetic and natural minerals, where compositional factors have a key role in controlling accommodation of trace elements in the mineral structure. For example, the Ti02 content of the synthetic pyroxenes is higher than in the natural pyroxenes, possibly favouring acceptance of Nb, Ta into the pyroxene lattice, and hence lower D-values for Nb, Ta for synthetic amphibole-clinopyroxene pairs, compared with the natural pairs analyzed so far. A second factor is the possibility of minute inclusions rich in trace elements trapped in the natural minerals. These may be sub-microscopic and unavoidable, even by microbeam analysis.

Clearly more work on both experimental and natural systems is needed to resolve the discrepancies, and to realise the full potential for using trace elements to assess petrological processes in the mantle, where knowledge of Dvalue variation as a function of composition, pressure and temperature is essential. Much of the wide scatter of the natural data may be caused by complex compositional variation in the clinopyroxene, garnet and amphibole solid solution series and published studies have started to attempt to correlate variation in D-values with selected compositional parameters. Experimental work has the unique capability of isolating different compositional controls, ultimately leading to more precise parameterization of partitioning behaviour. This will be a worthwhile goal of such studies in the future. The natural mineral approach, dependant as it is on mineral compositions to infer a P-T history, cannot unequivocally resolve physical and compositional controls on trace element partitioning.

Future work on the natural mineral pairs will benefit from concurrent TEM studies, to carefully assess the possible presence of minute fluid inclusions rich in trace elements.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996

SECULAR EVOLUTION OF SUB-CONTINENTAL MANTLE W.L Griffin ' , Suzanne Y. O'Reilly , Dmitri A. Ionov and C.G. Ryan 1. CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113 2. GEMOC, School of Earth Sciences, Macquarie University, Sydney NSW 2109 1 2

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Lithosphere mapping using garnet concentrates from kimberlites and other volcanic rocks has produced images of the thermal state and lithostratigraphy of the lithospheric mantle to depths of 150 to 250 km in >35 localities worldwide. These sections show consistent differences between Archean and younger lithosphere. Archean mantle sections contain depleted garnet harzburgites, typically concentrated between 140 to 180 km depth and interspersed with variably depleted lherzolites; the proportion of harzburgite ranges from 10 to 60%. At shallower levels depleted lherzolites make up >90% of the column, while at greater depths more fertile lherzolites may occur. In Proterozoic mantle sections, harzburgitic rocks are very rare, and the garnets of the dominant lherzolites are on average less depleted in LIL and HFSE elements. There is no consistent difference in the paleogeotherm between the Archean and Proterozoic parts of cratons.

Garnet concentrates from volcanic rocks also record secular changes in lithospheric mantle composition. Lherzolitic garnets from Archean sections have high mean Zr/Y (>5) and low mean Y/Ga (<3), while similar gamets from Phanerozoic areas have low mean Zr/Y (<1) and high mean Y/Ga (>4); gamets from Proterozoic sections have intermediate values. Comparisons with xenolith data and numerical modelling based on partition coefficients indicate that these differences reflect an increase in the average Cpx/Gnt ratio of lithospheric mantle from Archean to Phanerozoic time. Garnet peridotite xenoliths from areas with Phanerozoic tectonothermal ages have, on average, high modal (Cpx+Gnt) as well as high Cpx/Gnt. This reflects a high average bulk (Ca+Al), which is shared by spinel lherzolites from many localities. These xenoliths, like oceanic peridotites, have Mg# vs Mg/Si relationships consistent with an origin as residues from extraction of basaltic melts, at degrees ranging from moderate to very low.

Garnet peridotite xenoliths in kimberlites from Archean areas, by contrast, are extremely depleted in (Ca+Al), have low Cpx/Gnt, and have Mg/Si too low (relative to Mg#) to allow their derivation by extraction of basic or ultrabasic melts (Boyd and Mertzman, 1989). Limited data on xenolith suites from areas with Proterozoic tectonothermal ages show that the peridotites are intermediate in their degree of depletion and Cpx/Gnt, consistent with the more abundant garnet trace-element data.

These observations suggest that major changes have occurred in the processes that have produced subcontinental mantle through time; the lithospheric mantle has become progressively less depleted. Archean continental roots retain the products of processes that have not operated since ca 2.5 Ga, while most Phanerozoic lithospheric mantle appears to have formed by subduction of oceanic material. The intermediate nature of Proterozoic lithospheric mantle worldwide suggests that it reflects transitional processes. These might include both generation of "primary" Proterozoic lithosphere in a different convection regime than at present, with more extensive melting at spreading centres, and tectonic/magma tic reworking of Archean lithospheric mantle.

The older (>1.7 Ga) parts of many cratons have keels or roots with high V , extending to depths of 250-450 km (Polet and Anderson, 1995). This material is believed to be both cooler and compositionally different from that underlying younger cratons and Phanerozoic mobile belts, which have no significant V§ anomalies. A large harzburgitic component will contribute to the V anomaly beneath the >2.5 Ga cratons, but the shallow (<200 km) roots of Early Proterozoic cratons do not contain such rocks; this suggests that moderately depleted lherzolites can provide a V anomaly. What of the deeper parts of the cratonic roots? Where the Taihang Fault Zone disrupts the Archean North China Craton, kimberlites carry garnets with Zr-Y-Ga relations typical of Phanerozoic mantle, and derived from depths of <100 km. This material may have underlain the Archean mantle of the craton, and risen to shallow depths as that mantle was disrupted and displaced. We suggest that the deep cratonic keels consist of relatively fertile peridotite, capable of generating both kimberlites and flood basalts; this material must be significantly cooler than otherwise similar Phanerozoic mantle, to provide a deep seismic velocity anomaly. s

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 SEISMIC STRATIGRAPHY AND QUATERNARY EVOLUTION OF THE SOUTHERN PUMICESTONE PASSAGE ESTUARY, QUEENSLAND. Michelle R. Grosser*. Simon C. Lang , and Mai R. Jones .

'School of Geology, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001.

Queensland Department of Minerals and Energy, 61 Mary Street, Brisbane Queensland 4001.

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At the northern end of Moreton Bay lies the coastal barrier of Bribie Island, a strandplain of prograded Holocene and Pleistocene beachridges. Pumicestone Passage separates Bribie Island from the mainland, and represents a mesotidal, elongate back-barrier lagoon estuary with a tidal inlet at either end. Within the southern reaches of the Passage lies the Elimbah Creek bay-head delta, flanked by the shallow estuaries of Bullock and Ningi Creeks. The morphology of Pumicestone Passage is inherited from early Pleistocene and late Tertiary coastal plain, beach-ridge, tidal delta and fluvial channels. The southern Pumicestone Passage estuary is therefore a classic example of a mixed energy estuary, consisting of a tripartite subdivision including a wave-built barrier/inlet complex, a muddy central basin and a bay head delta. This paper presents the results of recent high resolution seismic profiling along Pumicestone Passage, supported by morphostratigraphic mapping.

SEISMIC STRATIGRAPHY The Quaternary geological history of this estuary was interpreted using seismic stratigraphic and sequence stratigraphic methods. Three evolutionary stages have been identified. Stage 1 is the period between about 28,000 yr BP and 18,000 yr BP when sea level fell to a glacio-eustatic minimum. The Pleistocene terrestrial plain surface became traversed by small incised valleys of the ancestral Bullock, Elimbah and Ningi Creeks. The surface became exposed, oxidised and overconsolidated. During this period, fluvial sands and muds were deposited in the incised paleochannels. The seismic clearly shows a pre-Holocene unit with a sheet like geometry which forms a thin layer over weathered Triassic-Jurassic bedrock, except where incised. Its upper surface has a moderately high amplitude reflector, and generally contains an internal structure of parallel, undulating reflectors. These stratified deposits are interpreted as lowstand fluvial deposits of an immediate post-glacial age.

Stage 2 lasted from about 17 000 yr BP until 6 500 yr BP representing the post-glacial transgression during which sea level rapidly rose. The seismic shows a transgressive unit overlying Triassic-Jurassic bedrock and Pleistocene sediments with a sharp erosional contact. Its upper surface has a high amplitude reflector and there is little identifiable internal structure. Generally, the sediments are dominated by tidal delta and tidal channel sands that have entered the back barrier lagoon from the north. The presence of a Pleistocene proto-Pumicestone Passage on Bribie Island suggests that the Passage has been migrating westwards during the transgression. As the Passage has migrated, it has formed a basal, irregular tidal ravinement surface with the underlying muddy lowstand deposits. Paleochannels filled with estuarine muds and sands in the north, and flood tidal delta sands and estuarine muddy sands in the south. On the landward side of the estuary, wave and tidal energy eroded preHolocene sequences and outcropping bedrock.

Stage 3 is identified as the Holocene 'stillstand' that extended from 6 500 yr BP until the present. The seismic shows the top of this unit has a strong, flat to gently undulating reflector at the sea-floor. The internal structure of this unit generally displays flat lying, moderate amplitude reflectors that downlap onto the underlying transgressive sands. The sediments are characteristically thin (approximately 3 m) and have a greater mud content than the sediments below. They have been interpreted as shallow tidal channel and tidal flat sediments. The reflectors dip gently southward, suggesting that the dominant movement of sediment is from the north.

The sediments of the southern Pumicestone Passage estuary therefore result from three major episodes of deposition controlled by paleotopography, tidal channel migration and sea level fluctuation. These factors are controlling the modern development of the estuary. Urban settlements along the creeks and Pumicestone Passage, are being threatened as shoreline displacement continues. This is revealed by erosion along the southerly shores of the estuary and the expansion of intertidal mangrove zones along the shores.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NEW HORIZONS: ENHANCING WESTERN AUSTRALIA'S PROSPECTIVITY Dr Pietro Guj and William Preston Geological Survey of Western Australia (GSWA)

The business of attracting exploration investment has become very competitive. For Western Australia (WA) to continue to lead, it needs to counteract challenges on three fronts. Firstly, WA is the most affected of all states by the pressure of Mabo on land access. In addition a number of largely under-explored developing countries are implementing attractive regulatory and fiscal packages, which cash in on the increasing globalisation" of exploration and of Australia's mining companies in particular. Thirdly there are vigorous incentive initiatives being introduced by other Australian states to enhance their attractiveness. Ultimately, however, it will be a country's/ state's prospectivity that represents the most powerful incentive to exploration.

It could be argued that WA has been successful in enhancing and promoting its prospectivity, attracting around 55% of the total Australian mineral exploration investment and an estimated 42% of its expenditure on petroleum (1994/95). To sustain such results significant Government mineral ($2.5 million per annum) and petroleum exploration ($2.0 increasing to $3.0 million per annum) incentives have been initiated which, over the last few years, have resulted in a virtual doubling of the geoscientific support that the GSWA provides to industry.

GSWA is, however, conscious of the bias in mineral exploration for gold at the cost of other commodities (77% in 1994/95) and paucity of onshore petroleum exploration. The recent proliferation of gold and offshore hydrocarbon discoveries and above all their low unit costs, while explaining current emphasis, does not lessen longer-term worries about insufficient commodity diversification in exploration effort in this State. Although WA is in the middle of a major development boom in many sectors, with the exception of gold, this is based to a large extent on deposits discovered often some 20 years or more ago. Whilst support of the prize performers and more established or advanced exploration-stage regions or commodity sectors are important, there is a need not to lose focus of the undoubted potential for other commodities often in less explored or logistically more difficult regions of the State.

An assessment of the current balance of the work effort within the GSWA's program has been carried out to determine its effectiveness in enhancing and promoting WA's prospectivity. Of particular significance is the split between primary data collection (such as geological mapping) and information services to support industry and to satisfy community interests such as policy and strategic planning. In particular the balance of effort between 'Greenfields" and established areas of mineral prospectivity brought about by the new Government incentive initiatives, at a 60:40 split is seen by industry and the GSWA as roughly the appropriate mix to achieve the strategic objectives. This has allowed greater emphasis to be placed on greenfields areas for new geological and, particularly, geochemical mapping, with focus on the extensive Proterozoic and Phanerozoic sedimentary basins and metamorphic belts and is the basis of the rationale for the onshore focus of the petroleum exploration initiatives. Recent literature tends to confirm that the bulk of metal is contained in essentially four countries and in the top ten per cent of the largest deposits of each of four major metallogenic types. The main geological settings exist for the range of significant base metal and polymetallic type of orebodies in WA and for the most part they can be found in the State but generally as small or subeconomic deposits. Given these facts it could be validly argued that the current absence of large or even giant economically very significant orebodies, such as top rank SEDEX, MVT and VMS, could be validly argued to be due to insufficient or unsuccessfiil exploration effort and current paucity of geological knowledge rather than lack of prospectivity.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

"BIOAVAILABILITY OF LEAD AND ITS IMPACT ON HUMAN HEALTH 11

Brian L. Gulson and Jeffrey J. Davis Graduate School of the Environment, Macquarie University, Sydney 2109 CSIRO/EM, North Ryde 2113 1,2

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Toxicity of contaminants to animals, especially humans, is dependent on several factors including: chemical form, dosage and "bioavailability". "Bioavailability" is one of the most crucial aspects of toxicity and nutrition and refers to the amount of a substance taken up in the gastrointestinal tract compared with the total amount ingested.

Considerable confusion and misuse of the term "bioavailability" has arisen in the scientific literature. The term has been applied to varying investigations ranging from sequential extractions commonly applied in geochemistry to sophisticated experiments performed by the US EPA whereby swine are fed soil doped with mine waste.

"Bioavailability" is a critical factor in lead poisoning, especially for young children of ages 1 to 4 years who are most atriskbecause of their mouthing activities at this age.

On one hand, there are many examples in North American mining communities where the "bioavailability" of lead is low because of: the lead species, their oxidation products providing a barrier to solution, and encapsulation in insoluble minerals such as pyrite and quartz. Even though soil lead concentrations in some of these communities, such as Aspen in Colorado, may be in the percent range, the blood lead concentrations of the children are very low.

On the other hand, many children in the Broken Hill mining community have elevated blood lead levels.

Scanning electron microscopy and solubility investigations of Broken Hill soil and house dust from vacuum cleaners have shown that the lead species are complex PbFeMnAlCaP oxide materials, are highly "bioavailable" and hence can readily explain the elevated blood leads.

Similarly, soil, house dust and paint from Inner Sydney houses have a "bioavailability" ranging from —40 to 100%.

Use of pharmacokinetic models to predict blood lead levels, especially those promoted for US Superfiind sites, can lead to totally erroneous conclusions if "bioavailability" is not taken into account.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF THE TAPANAPPA FORMATION, SOUTHERN FLEURIEU PENINSULA, SOUTH AUSTRALIA. J.CGum 1 ,J.B.Jago 1 .

Gartrell School of Mining, Metallurgy and Applied Geology, University of South Australia, The Levels, Pooraka, South Australia, 5095.

The Tapanappa Formation is part of the Early Cambrian Kanmantoo Group and hosts many of the known mineral deposits found in the Kanmantoo Group. The type section of the Tapanappa Formation is defined on the southern coast of Fleurieu Peninsula, where there are excellent exposures in rugged coastal cliffs.

SEDIMENTOLOGY The Tapanappa Formation has been stratigraphically logged in detail along its type section. It consists of a series of silty sands and interbedded siltstones. The sandstones are usually massive, with minor parallel bedding and rare cross-bedding; they grade rapidly to very thinly laminated silts and muds of hemi-pelagic origin. Current directions have been difficult to determine as sole marks are rarely-seen. There are rare gravel horizons and channels within the sandy beds. These sedimentary structures indicate deposition from sediment gravity flows in a marine environment, below wave-base. Petrology indicates the sediments were derived from a cratonic source. Sulphide-rich horizons within the hemi-pelagic muds indicate locally anoxic conditions.

Clean sands at the tops of some units, are interpreted to be either storm influenced, or possibly due to grain flow effects. Other clean, crossbedded, lenticular sands are interpreted to be due to the effects of bottom currents triggered by storms or seismic activity.

Three sections through the Tapanappa Formation were compiled, all of which show a prograding trend. The shear/thrust zones separating these sections, were most probably pre-existing extensional structures which were reversed during compression of the basin. The marked differences in the thickness of sandstone beds between sections, indicates that there may have been synsedimentary fault movement SEQUENCE STRATIGRAPHY A sequence stratigraphy model of the Tapanappa Formation has been constructed. On the south coast of Fleurieu Peninsula, the sediments were deposited in a deep marine basin with a ramp margin. Cycles visible within the Tapanappa Formation, vary in scale from centimetres to hundreds of metres. This discussion will be limited to the largest scale which broadly encompasses the entire unit.

The Talisker Calc-siltstone, directly below the Tapanappa Formation, consists predominantly of muds deposited on top of a Type 1 sequence boundary. The sandy, basal Cooalinga Sandstone Member of the Talisker Calc-siltstone represents a Lowstand Systems Tract directly above the sequence boundary. The muds of the Talisker Calc-siltstone represent a Transgressive Systems Tract as the relative sea level rose flooding the basin. The Transgressive Surface or Maximum Flooding Surface occurs towards the top of the Talisker Calcsiltstone but is yet to be conclusively identified. Deposition of the majority of the Tapanappa Formation occurs as the sandy sediments prograde out into the basin once the sediment supply to the basin increases. This deposition becomes retrogradational as the Tunkalilla Formation is approached with more silt present in the Formation. This may reflect a drop in sediment supply due to the cessation of tectonic uplift in the south and west The first major mud of the Tunkalilla Formation may have been deposited on a Type 2 sequence boundary, but evidence for this conclusion is lacking at this point in the basin.

STRUCTURE In its type section, the Tapanappa Formation was originally mapped as being over 10 km in thickness. It is now interpreted to vary from between two to three kilometres. Recent detailed mapping has revealed the formation to be drastically thickened by thrust faulting and complex folding. The type section has been divided into three thrust blocks, all of which have been folded as well. As the majority of this area is covered by Permian and Tertiary sediments, detailed aeromagnetic data (recently flown by MESA), was essential in the interpretation of the Cambrian geology.

This mapping provides a more complex target for exploration in the region, but also provides an opportunity for greater strike exposure of mineralised horizons and structural concentration of pre-existing orebodies.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

AEROMAGNETIC MAPPING OF TRANSFER FAULTS IN SEDIMENTARY BASINS Peter J. Gunn Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia

Aeromagnetic data can provide excellent mappings of transfer faults accommodating basin extension in situations where extensive magnetic markers exist in the area undergoing extension prior to the extension. In such cases, the extension will fracture and/or dislocate the magnetic markers and the transfer faults and the relative movements will be indicated by present day geometries of the magnetic sheets which can be deduced from aeromagnetic data. These conditions occur in the Bonaparte Basin of northwestern Australia and in the Otway and Bass Basins of southeastern Australia.

The Bonaparte Basin of northwesten Australia is a Devonian-Carboniferous rift that underwent differential extension along its length. Aeromagnetic surveys over the southern limits of this rift indicate that the area was originally underlain by an extensive contiguous sheet of Proterozoic sills and volcanics. The justification for this interpretation is based on correlations of magnetic anomalies with outcrop geology, drill hole intersections and detailed computer modelling. Extension and basin formation of the area, during the Palaeozoic, fractured the horizontal magnetic sheet Evidence of the fracture pattern is indicated by a series of parallel linear magnetic discontinuites, trending at 035 degrees, bounding rectangular magnetic anomalies interpreted as being due to fragments of the Proterozoic magnetic sheet. The extensional movements can be deduced by reconstructing the fragments of the magnetic sheet and when this is done it is clear that the extension has occurred along 035 degree transfer faults. The interpreted transfer fault direction is parallel to the major Proterozoic lineament known as the Halls Creek-Fitzmaurice Mobile Zone that marks the southeastern limit of the Bonaparte Basin. Seismic and gravity data from the area support the interpretation.

The Otway Basin of southeastern Australia is a Mesozoic pull-apart basin formed by the separation of the Australian and Antarctic continents. Aeromagnetic data indicates that a large portion of the basin is characterised by an extensive magnetic anomaly that computer modelling indicates to be due to a major subhorizontal magnetic sheet near the base of the Otway Basin sequence. This magnetic sheet is interpreted, on the basis of correlations with known geology, to be due to basic lavas/sills emplaced at the time of the initial crustal thinning in the area before major extension occurred. The magnetic pattern has been interpreted to indicate that, after the initial emplacement of the magnetic sheet, basin extension in the 210 degree direction fractured the magnetic sheet into a series of blocks with the 210 degree being one bounding direction and the direction perpendicular to 210 degrees being the other bounding direction. Differential extension was accommodated on the 210 degree "transfer direction". This interpretation is supported by isopach information that indicates sediment depocentres of appropriate ages have been developed in the voids left by the extension.

Narrow linear magnetic anomalies correspond to the positions many of the interpreted transfer faults.

Computer modelling shows these to be due to accumulations of detrital magnetic minerals in palaeochannels that developed above the transfer faults. Present day canyons overlie some of the interpreted transfer faults.

Evidence for the transfer faults can be recognised on seismic sectionsfromthe area, albeit with difficulty.

The Bass Basin, located between mainland Australia and Tasmania, is a failed rift that is contemporaneous with the Otway Basin. The magnetic field over the basin is dominated by a large intense, ovoid, magnetic anomaly that corresponds spatially to the main depocentre of the basin. Modelling indicates that this anomaly is due to a large intrusive body at a depth of approximately 10 kilometres. The source of the. anomaly is interpreted as a magma chamber/intrusion developed as a result of crustal thinning and depressurising of the mantle. The application of vertical gradient filters suppress the effects of this dominant anomaly and reveal a series of weaker rectangular shaped magnetic anomalies that are interpreted to arise from basic igneous material that has been fractured by extension in the basin. The alignments of terminations of the anomalies, that trend at 210 degrees, are interpreted to indicate transfer faults. Such a system of transfer faults has previously been deduced from seismic data in the area. The aeromagnetic data indicates that the transfer faults have developed along pre-existing discontinuities in the Proterozoic basement that floors the basin. The transfer fault system appears to control the geometry of the major igneous body interpreted to underlie the centre of the basin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE LOCATION AND NATURE OF THE PROTEROZOIC/PALAEOZOIC BOUNDARY IN EASTERN AUSTRALIA Peter J. Gunn Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601

The exact position and nature of the eastern limit of the Proterozoic terranes of central Australia, the "Tasman Line", has been the subject of much conjecture in geological literature. New regional compilations of aeromagnetic and gravity data, together with various specialised enhancements of these data, allows the delineation of this boundary with greater confidence than in the past. A new refinement of the boundary, shown in Figure 1, is based on an integration of these images with current geological knowledge.

The interpretation proposes that the eastern margin of the Proterozoic became a divergent margin with spreading along transfer faults oriented approximately 045 degrees east. Rupture of the margin occurred on trends orthogonal to the transfer faults. An irregular, semi-rectangular, continental margin was created by the breakup which in many respects has a similar geometry to the present western margin of the Australian continent.

Many elements of more recent geology and geomoiphology appear to have been controlled by the transfer fault pattern of the Proterozoic boundary. Such examples are the courses of Diamantina and Darling Rivers, outlines of Mesozoic sedimentary basins, transfer faults within Mesozoic sedimentary basins and terminations of Palaeozoic geological units.

A crucial element of this new interpretation has been the acquisition and integration of aeromagnetic data over offshore areas west and north of Tasmania and over the Offshore Otway basin. These data provide controls on the structures that have been interpreted in these areas.

Figure 1

Interpreted outline of the eastern margin of the Proterozoic in Australia

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THREE-DIMENSIONAL PATTERN OF COAL RANK VARIATION IN THE GUNNEDAH BASIN, NEW SOUTH WALES Lila Gurba and Colin R. Ward Department of Applied Geology, University of New South Wales, Sydney, NSW, 2052.

The Gunnedah Basin, part of the Sydney-Bowen foreland basin system, contains up to 1200 m of marine and non-marine Permian and Triassic sediments. These strata rest unconformably on the Lachlan Fold Belt in the west, and are faulted against the penecontemporaneous New England Orogen in the east. The structure of the Permian coal-bearing sequence in the main part of the basin (the Mullaley Sub-basin) is dominated by a central NW-SE trending synclinal axis, divided in the middle by a basement high.

A comprehensive investigation of the organic maturity (rank) of the Permian coals in the basin has been carried out using penological and chemical parameters, with an emphasis on vitrinite reflectance and inherent (airdried) moisture content. The coal itself is of high-volatile bituminous rank, with a mean maximum vitrinite reflectance of between 0.56 and 1.1% and air-dried moisture that varies from more than 8% to less than 2%. A veiy close correlation has been established between vitrinite reflectance and moisture, and each shows a similar lateral and vertical distribution pattern.

Coal rank is significantly higher in the south-western part of the basin, including the SW limb of the syncline, than in the east near the New England Orogen. There are, however, a number of local departures from the regional trend within the sequence, due to variations in vitrinite type, the presence of igneous bodies and postcoalification fault displacements. The structure of several key horizons within the basin fill and the threedimensional pattern of coal rank variation have both been expressed as a series of surfaces using computerbased modelling techniques. Iso-rank surfaces based on reflectance or moisture intersect the structure contour patterns of the Permian beds, with a relationship indicating mainly post-structure coalification in the east and post-coalification structural development in the western part of the basin.

The regional pattern of coal rank variation in the Gunnedah Basin does not appear, from the present study, to be controlled solely by present or past depths of burial. There is some evidence of contemporaneous uplift associated with coalification in the east, whereas in the west coal rank appears to be controlled by the geometry of the basement. The relation of the rank pattern to the geological structure suggests either a higher geothermal gradient along the western margin of the basin, relative to the east, or a substantial degree of postburial uplift in the west for the Permian coal-bearing succession.

The uplift and erosion my have occurred at different times in the eastern and western parts of the basin. As a result, the effective period of maturation may have been different in each case. Such a difference would have influenced the relationship between maturation gradient and geothermal gradient.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention,, Canberra, February 1996

MANTLE PROVINCIALITY OF THE WESTERN PACIFIC - EVIDENCE FROM PB ISOTOPES David A. Gust 1 , Richard J. Arculus 2 ,and Annie B. Kersting 3 * School of Geology, Queensland University of Technology, QLD 4001 , AUSTRALIA 2 GEMOC, Department of Geology, Australian National University ACT 0200 , AUSTRALIA ^Lawrence Livermore National Laboratory, Livermore, CA 91125, USA

The existence of unequivocal provinciality of MORB geochemistry together with preservation of distinctive long-term isotopic variations entrained in the global hot-spot sample (e.g., Pacific vs. Indian vs.

Atlantic MORB, HIMU, EM 1/2, FOZO etc.) has been established through detailed geochemical and isotopic studies (e.g., Sun and McDonough, 1989; Hauri et al., 1994). The origins of these regionally and isotopically distinctive geochemical characteristics are controversial. Recent studies in the western Pacific and Southern Ocean provide examples of spatial and temporal changes in mantle affinities, apparently linked in part to tectonic developments:- 1. the rift-to-drift transition in the Lau Basin and the collision of the d'Entrecasteaux Ridge with the Vanuatu arc (Pacific-to-Indian MORB affinity in both cases - e.g., Hickey-Vargas et al., 1995; Crawford et aL, 1995); 2. the sharp juxtaposition of Indian and Pacific MORB sources at the Australia-Antarctic discordance (Klein et al., 1988); 3. the separation of the mantle sources by the chain of arcs that extend southward from the Japanese archipelago and divide backarc basins to the west (e.g., Sea of Japan, Shikoku, Parece Vela, Mariana Trough) of Indian MORB affinity from the Pacific domain to the east (Hickey-Vargas et al., 1995).

Establishment of the boundaries between Indian MORB mantle and Pacific MORB mantle is highly dependant upon Pb isotope ratios, with Indian MORB signatures being more enriched in 208p b / 204p b and 207 P b / 2 0 4 P b for similar 2 0 6 P b / 2 0 4 P b than are Pacific MORB leads. Note however, that these enrichments may be generated by a number of different processes including recent contamination with old, continentderived sediment, entrainment of various "plume" components, or interaction with 'aged' subcontinental lithosphere.

Comparison of recently acquired Pb isotopic data for Quaternary basalts from Honshu Japan, with published isotopic values for basalts from the southwestern Pacific, East Asian, and Australian regions stimulates the examination of the ingress of Indian MORB mantle into Pacific MORB mantle with time.

Pb isotopic values of Quaternary Honshu basalts are Indian MORB in character and overlap with Pb isotopic data for similarly aged basalts erupted in the Sea of Japan, Korea, southeastern China, and eastern Australia. While published Pb isotopic data for arc basalts from the Philippines and Sunda arcs are also of Indian MORB affinity, the intra-oceanic arcs between Japan and the Philippines are Pacific in character.

A striking feature of the combined Pb-Sr-Nd isotopic systematics is the provincial constancy of Pb coupled with substantial variations of ^ S r f i ^ S r and 1 4 3 N d / 1 4 4 N d > suggestive of the decoupling of Pb from processes that control the other isotopic systems. We suggest that the variation observed in Pb signatures in Australasian-China-Japan region reflect the interaction of Indian MORB asthenosphere with subcontinental lithosphere thickened and 'aged' during the growth of Pangea/Gondawana. The breakup of Gondawana and dispersal of its fragments in the southwestern Pacific is important in controlling aspects of mantle magmatism in this region..

REFERENCES Crawford, A.J., Briqueu, L., Laporte, C.,& Hasenaka, T., 1995. Coexistence of Indian and Pacific oceanic upper mantle reservoirs beneath the central New Hebrides island arc. Active Margins and Marginal Basins, Geophysical Monograph 88, AGU, 199-217.

Hauri, E.H., Whitehead, J.A., & Hart, S.R., 1994. Fluid dynamic and geochemical aspects of entrainment in mantle plumes. J. Geophys. Res., 99, 24275-24300.

Hickey-Vargas, R., Hergt, J.M., & Spadea, P., 1995. The Indian Ocean-type isotopic signature in western Pacific marginal basins: Origin and significance. Active Margins and Marginal Basins, Geophysical Monograph 88, AGU, 175-197.

Klein, E., Langmuir, CH., Zindler, A., Staudigel, H.., & Hamelin, B., 1988. Isotopic evidence of a mantle convection boundary at the Australian-Antarctic Discordance. Nature, 333, 623-629.

Sun, S-S., & McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts:implications for mantle composition and processes. Magmatism in the Ocean Basins, Geol. Soc. Special Publication, 42, 313-345.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SOURCES OF FLUORIDE IN GROUNDWATER IN NORTH QUEENSLAND, AUSTRALIA M.A. HABERMEHL, J.E. LAU, D. E. MACKENZIE, & P. WELLMAN Australian Geological Survey Organisation GPO Box 378, Canberra, ACT, 2601

Groundwater in some areas of North Queensland shows elevated fluoride contents, which are a problem for domestic and stock water supplies. High fluoride content in otherwise potable, good quality groundwater is also a problem in other parts of Australia, including portions of the arid zone in central Australia, and regions of the Great Artesian Basin. However the sources of the fluoride have not often been identified.

Recent groundwater sampling in Cape York has refined the knowledge of the fluoride levels and their distribution, and confirmed that high values are widespread in the Mesozoic aquifers of the Carpentaria Basin, are less extensive in the overlying Cainozoic aquifers of the Karumba Basin, and are present in some exposed pre-Mesozoic basement rocks. However, even in the Mesozoic aquifers of Cape York Peninsula, there are still insufficient numbers of deep waterbores to define specific haloes and sources.

Regional geological mapping of outcropping Permian-Carboniferous volcanic and intrusive rocks of the North Queensland Igneous Province has shown that some of these rocks, notably those of A-type chemistry, are very fluorine-rich compared to the older basement rocks. Fluorine is present in minerals such as fluorite, topaz, apatite, biotite, and some amphiboles, some of which (notably topaz and fluorite) may be present in fluid or vapour cavities. Vein deposits extremely rich in fluorite are locally concentrated adjacent to some of the A-type volcanic-intrusive complexes. Fluorine content of volcanic rocks is generally less than that in comagmatic intrusive rocks because of loss to the vapour phase during eruption and to circulating fluids during compaction, welding, and subaerial exposure. However, any fluorine present in a volcanic pile, especially that in fluorite, will be more accessible to groundwater than that in intrusive rocks because of the much higher permeability of rocks such as tuffs and unwelded ignimbrites. We therefore consider that Permian-Carboniferous igneous rocks, and A-type volcanic rocks in particular, are the most likely source of fluoride in North Queensland groundwater.

Permian-Carboniferous intrusions and volcanic piles can usually be mapped, if covered, by using gravity and magnetic anomalies. Their characteristic signature is a circular magnetic anomaly superimposed on a gravity low. Circular magnetic anomalies of this type have recently been recognised beneath the Millungera-Savannah Downs fluoride anomaly in the southern Carpentaria Basin. Earlier drilling of this circular structure has revealed the presence of an A-type granite, typical of those found in the North Queensland Igneous Province.

We suggest that the association between high-fluoride groundwater, Permian-Carboniferous igneous rocks, and concealed circular magnetic anomalies is a feature common throughout North Queensland, and it should be possible to reduce fluoride risk by using this understanding in the siting of new waterbores.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 INTEGRATED APPROACH TO THE REINTERPRETATION OF THE CAMBRIAN KANMANTOO GROUP TYPE SECTION, SOUTH AUSTRALIA Peter W. Haines . T. Fldttmann\ J.C. Gum , J.B. Jago & C.G. Gatehouse Department of Applied Geology, The University of South Australia, Hie Levels, Pooraka, South Australia 5095 Department of Geology and Geophysics, The University of Adelaide, South Australia, 5005 Mines and Energy, South Australia, PO Box 151 Eastwood, South Australia, 5063 1

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The Cambrian Kanmantoo Group is a thick, predominantly clastic, metasedimentary succession exposed in the Delamerian Orogen in South Australia. The group outcrops in an arcuate belt from the northeastern Mount Lofty Ranges to Kangaroo Island and generally lies unconformably over, or is in fault contact with older Cambrian and Neoproterozoic rocks. The succession was deposited very quickly in a rapidly subsiding basin just prior to the Cambro-Ordovician Delamerian Orogeny under an apparently different tectonic and provenance regime from that of older rocks in the Adelaide Geosyncline.

The Kanmantoo Group is considered prospective for a number of mineral deposit types, particularly stratiform base metal deposits. As such, a detailed knowledge of the basin from a depositional point of view is essential for effective exploration. However, despite economic potential and close proximity to infrastructure, many aspects of the succession, particularly in terms of basin dynamics, palaeogeography, provenance, sedimentology and regional stratigraphy have remained poorly known. The reasons are structural complexity, metamorphic grade, lack of biostratigraphic control, deficiency of regional marker horizons and poor outcrop. This ongoing study involves a detailed re-evaluation of the Kanmantoo Group using an integrated approach based on aeromagnetic data, balanced section structural techniques and various basin analysis procedures including stratigraphic section measurement, palaeocurrent analyses and sequence stratigraphy. Initial work concentrated on the group type section along the southern coast of Fleurieu Peninsula because this transect provides some of the best and most continuous outcrops and is stratigraphically more complete than transects further north.

Eight formations have been defined in the type section, most of which is east-dipping and east-facing leading some previous workers, who did not fully appreciate the structural complexity, to unrealistic thickness estimates. It is now possible to recognise a number of tectonic repetitions of sequence reducing the effective combined thickness to approximately 7000-8000 m. Balanced section reconstructions and sedimentological evidence suggests that at least some thrusts may be reversed syn-depositional extensional structures.

The Kanmantoo Group is dominated by metasandstone with thin interbeds and more substantial units of pelitic rocks. At the type section most sandstone beds are non-lenticular at outcrop scale, massive and sharp-based, with gradational tops. Tbiey commonly occur in thickening-upward packages. Though well-developed grading and Bouma sequences are rare, these beds are interpreted as sediment gravity flow deposits generally formed below storm wave base. However, two formations, the Backstairs Passage Formation and Middleton Sandstone contain an abundance of tidal cross-bedding and ripples, and minor tidal influence is evident in some other units. Preservation of erosional sole marks beneath sediment gravity flows is rare, but where present indicates palaeocurrents consistently directed toward the north to northeast. This direction is essentially orthogonal (and in places diametrically opposed) to palaeocurrent measurements from tidal crossbeds and climbing-ripple sets.

It is suggested that arenaceous sediments derived mainly from the south or southwest were transported into the basin down north to northeast dipping palaeoslopes (in the vicinity of the type section) but were redistributed around the basin by tidal and contour currents at certain times in the basin history.

New aeromagnetic data, recently released by MESA, is of sufficient detail to provide an effective mapping tool where the Kanmantoo Group is covered or poorly exposed. Efforts have concentrated on calibrating the established stratigraphy and structure with the magnetic data along the type section. The Middleton Sandstone, previously considered to be the youngest exposed formation in the group, is separated from rocks to the west by a sharp magnetic lineament. Field examination of this contact reveals a previously undetected east dipping thrust. Thus the stratigraphic placement of this unit is now in doubt and the possibility that it represents a structural repeat of another formation needs consideration. In terms of sedimentology and aeromagnetic character there are close similarities with the Backstairs Passage Formation. Outcrops elsewhere lack exposed contacts.

In conclusion, integration of a variety of techniques can be effectively utilised in basin studies of structurally complex, metamorphosed successions, thus facilitating exploration for sedimentary ore deposits in such areas.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 METAMORPHIC RECORD OF STRAIN PARTITIONING DURING OBLIQUE CONVERGENCE Martin Hand. Betina Bendall & Mike Sandiford Geology and Geophysics, Adelaide University, Adelaide SA 5005

The scales at which strains are partitioned between normal and transcurrent movements during oblique convergence potentially provide important clues on the nature of the boundary conditions that operated during the development of an oblique orogen. In principle it should be possible to gauge the scale at which strains are partitioned by examining the across strike kinematic record of orogenic belts, however in practise this is sometimes difficult since strain partitioning on a local scale can confuse the broader picture. One approach to investigate the way in which strains are partitioned is to examine across strike variations in the magnitude of exhumation relative to the broad structural divisions within the orogen. Since the magnitude of pressure gradients is a function of the length scale over which erosional parameters vary, baric gradients should provide information on the flow of rocks through an orogen.

In this study we have investigated the across strike variation in metamorphic pressure as a function of distance from a 1 - 2km wide Palaeoproterozoic ductile strike-slip shear zone - the Kalinjala Shear Zone, which is a major NNE-trending subvertical structure in the southeastern Gawler Craton. Thermobarometry on upper amphibolitelow granulite facies assemblages has revealed a highly asymmetric baric section with maximum pressures (8-10 kbar) and temperatures 750°C) located within the shear zone (Fig. 1).

Figure 1: Baric data across the Kalinjala Shear Zone. Pressures stabilise at - 4 kbar away from the shear zone, defining an average baric gradient of 0.16±0.03 kbar/ km" to the east, and >1.3 kbar/km" to the west.

Garnet -hornblende-whole rock Sm-Nd isotopic data suggests the high-P assemblages within the KSZ (1730 ± 20 Ma) are broadly contemporaneous with the lower pressure assemblages to the east (1714 ± 12 Ma).

1

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distance km

This implies that differential exhumation was strongly localised within the region of highest strike slip shear strain and suggests the pressure gradient reflects the domain over which net transcurrent movement became increasingly important. In this case, the across strike length over which pressures remain similar may reflect the domain in which crustal thickening was dominated by normal convergence. Quantifying the length scale over which normal convergence (LN) may have dominated is hampered by lack of outcrop, however existing geochronological data suggests that LN is at least on the order of 70 km meaning that transcurrent motion occurred within a narrow domain relative to normal convergence (<~1:7). Preliminary kinematic observations along the baric gradient to the east are broadly consistent with differential exhumation toward the strike-slip boundary. Although pre-existing structures, (particularly mafic dyke swarms in the this example), complicate the loci strain picture, the total deformation pattern within the lower strain regions away from the region dominated by strike slip motion may be of more value in identifying character of the collision than structural studies in the strike slip zone itself, where preservation of material that passed through the'orogen is minimal.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

HIGH HEAT PRODUCTION, LOW-P HIGH-T METAMORPHISM AND CRUSTAL DEFORMATION Martin Hand. Mike Sandiford & Annette Bingemer Geology and Geophysics, Adelaide University, Adelaide SA 5005

The thermal regimes responsible for low-P high-T metamorphism represent significant departures from our notions of normal continental geotherms. For this reason, the advection of heat, in the form of magmas is generally considered necessary in order to create a low-P high-T thermal regime. As a consequence, low-P high-T metamorphic events are generally interpreted to be short lived and spatially and temporally associated with magmatism.

Although the above framework is probably relevant to many high-T low-P environments, the universal application of the foregoing argument is challenged by several important observations from Australian low-P high-T terranes. Both at Mt Isa and in the northern Arunta Inlier, voluminous magmatism significantly predates (>100 Ma) regional high-T, low-P metamorphism, and in the case of the northern Arunta Inlier, high temperatures (>650°C at 4kbars) apparently persisted for upwards of 15 Ma. A notable feature of these terrains is that the voluminous granites at the current exposure level have much higher (7-9 mWm"^) than average crustal heat production (2-3 mWm' 3 ) and thus cannot be representative of the rest of the crust. This poses the question: to what extent do localised regions of high-heat production contribute to crustal geotherms? In part this question can be answered by observation. For example in the Mt Painter region in the northern Flinders Ranges, isograds associated with Delamerian (~ 500 Ma) LP-HT metamorphism (<530°C at 3 kbar) are arranged concentrically around an antiformal core of Proterozoic basement with anomalous heat production (10-12mWm~3), indicating that localised heat production can make significant contributions to mid-crustal thermal regimes.

We show that if heat production relevant to selected Australian Proterozoic terrains is concentrated at shallow-to mid crustal levels (15-20 km), then for a range of thermal parameters, steady state geotherms may produce conditions required for low-P high-T metamorphism (650-700°C, 4-5 kbar) without significant melting of a refractory lower crust. Importantly, geotherms show a very strong dependence on the depth of the heat-producing layer, with the implication that minor burial 5 km) may be enough to initiate low-P high-T metamorphism.

Similarly, only small amounts of erosion may be required to terminate the event.

Thus retrograde decompressive cooling paths are a logical outcome of this style of heating, and strict adherence to the notion that low-P high-T events are transient is not necessary. Furthermore, provided the high-heat production layer remains intact between erosional cycles, reburial introduces the possibility that later metamorphic events will have similar isograd distributions to earlier events.

The metamorphic geology of the Anmatjira and Reynolds Range region of the northern Arunta Inlier lends some support to this style of crustal heating and provides scope for some interesting speculations. Long lived Proterozoic high-T, low-P metamorphism in the region postdated high-heat producing granites (av. 7.5 mWm"3 at 1600 Ma) by 200 Ma and was terminated by -1 kbar of high-T decompression. Additionally, a later system of shear zones (600°C, 5-6 kbar ) shows a near-identical distribution of isograds to that of the earlier higher-T, lower-P metamorphism, implying a similarly distributed but less intense heat source. The age of these shear zones is unknown, however on the basis of the time dependence of heat production, we speculate they are related to the Alice Springs Orogeny (300-400 Ma), rather than Proterozoic tectonism.

Since lithospheric strength is strongly controlled by the moho temperature, changes in the depth of burial of a high-heat production layer must also exert a strong influence on the both the style, and distribution of deformation in the crust. This effect may be particularly important where an anomalously radiogenic basement underlies an insulating sedimentary succession, such as in the Mt Painter region. Burial of a basement with heat production appropriate to the northern Flinders Ranges (11 mWm" 3 ) by only a few kilometers, may annihilate the mechanical contrast between the basement and cover. We suggest this is the reason that distributed coupled cover-basement deformation characterises the Delamerian Orogeny in the northern Flinders Ranges. In contrast in the central Flinders Ranges, cover and basement were decoupled, as a consequence of lesser burial of basement. An obviously important consequence of these results is that intracontinental deformation may be localised by the presence of discrete high-heat production domains.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra:, February 1996

RHENIUM-OSMIUM ISOTOPIC CONSTRAINTS ON THE TIMING OF MELT EXTRACTION EVENTS AND EVOLUTION OF THE LITHOSPHERIC MANTLE, SOUTHEAST AUSTRALIA Monica Handler. Vickie Bennett and Tezer Esat Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200

The continental lithospheric mantle (CLM) is a significant geochemical reservoir and its age and stability have implications for both tectonic and magmatic processes, yet fundamental aspects of its evolution remain unknown. Several techniques, such as Nd model ages and U-Pb zircon ages, give important constraints on the age of crust formation, however it has proven more difficult to obtain such constraints for the mantle portion of the lithosphere. Most commonly used isotopic systems such as Rb-Sr, Sm-Nd and U-Th-Pb are easily disrupted within the CLM by metasomatic events. In contrast, the Re-Os system offers a method of constraining the age of formation of the CLM as the contrasting behaviour of Os (compatible) and Re (incompatible) in the peridotite/melt system means that they are strongly fractionated from each other by partial melting of the mantle.

Residual peridotite is expected to have significantly greater concentrations of Os than metasomatic melts or fluids, making its Os isotopic composition far less susceptible to disruption by metasomatism. Re-poor residual peridotite may however, be susceptible to subsequent contamination by Re-rich melts or fluids, increasing Re/Os and thus the Os isotopic composition over time.

A suite of well-characterised spinel-peridotite xenoliths from southeast Australia (from Mts Leura, Noorat, Porndon and Shadwell, western Victoria and Mt Gambier, South Australia) exhibit a range of compositions from unmetasomatised lherzolites through cryptically metasomatised to phlogopite and apatite ± amphibole-bearing lherzolites and wehrlites. As such they provide an ideal opportunity to assess the effects of metasomatism on ReOs systematics in the mantle. All samples show evidence of melt-extraction and at least two metasomatic events have affected the suite (Frey and Green, 1974; McDonough and McCulloch, 1987; Yaxley et al., 1991). The apatite-bearing lherzolites and wehrlites studied here record varying degrees of carbonatite metasomatism (Yaxley et al., 1991) with some samples displaying reaction textures indicating interruption of the metasomatism by entrainment of the xenolith. A phlogopite-producing event has affected two of the samples, and has been constrained to < 60 Ma from Sr isotope systematics (McDonough and McCulloch, 1987).

These samples of the CLM have Os abundances of 1.1 to 3.6 ppb and Re abundances of 0.008 to 0.080 ppb, with 187 Re/ 188 0s ranging from 0.02 to 0.14, all well below the chondritic mantle estimate of 187 Re/ 188 0s - 0.4. The measured Os isotopic compositions of six samples (three anhydrous, two apatite-bearing and one phlogopitebearing) range from yOs = -1.1 to -8.9 (where 76s is the percent deviation of the sample from chondritic mantle) with a second phlogopite-bearing sample having an unsupported positive yOs of +2.3 (i.e. more radiogenic than chondritic mantle). Our initial results show no correlation between degree or type of metasomatism and the Os isotopic composition, suggesting that the metasomatic agents did not transport significant Os. The Re/Os values are significantly higher in some, but not all, of the the modally metasomatised samples. This suggests that any Re addition was probably too recent to affect significantly the isotopic composition, which is consistent with the young age constraints for metasomatism. In addition, it suggests that carbonatite metasomatism may not necessarily impart a high Re/Os (e.g. an apatite metasomatised sample has the lowest Re/Os), despite the expected high Re content and high Re/Os of carbonatite melt (e.g. the only analysis of a primitive carbonatite erupted at the surface has a Re content of 0.4 ppb and 187 Re/ 188 Os of 6130; Pearson et al 1995).

Model ages based on the assumption of complete Re extraction from the sample at melting circumvent the potential problem of Re-addition and yield minimum estimates of the timing of melt depletion of the peridotite residue. Os model ages determined from the western Victorian xenoliths, representing CLM from beneath the Stawell zone, range from 0.2 to 1.2 Ga. The range of ages may reflect differing periods of lithospheric growth, or differing degrees of partial melting and/or metasomatic addition of Re. In contrast, a sample analysed from Mt Gambier, more than 170km west of the Victorian localities, yields a significantly older model age of 1.7 Ga. This is consistent with older crust beneath the Delamerian Fold Belt to the west, than beneath the Victorian localities. This difference in model ages between the two regions may represent a major lithospheric boundary, perhaps corresponding to the 'Mortlake discontinuity'. The 1.7 Ga Os model age for the Mt Gambier CLM is also consistent with mid-early Proterozoic crustal ages to the west and north of Mt Gambier.

REFERENCES Frey, F. A. and Green, D. H., 1974. Geochimica et Cosmochimica Acta 38. 1023-1059.

McDonough, W. F. and McCulloch, M. T., 1987. Earth and Planetary Science Letters 86: 327-340.

Pearson, D. G., Shirey, S. B., Carlson, R. W., Boyd, F. R., Pokhilenko, N. P. and Shimizu, N., 1995. Geochimica et Cosmochimica Acta 59: 959-977.

Yaxley, G. M., Crawford, A. J. and Green, D. H., 1991. Earth and Planetary Science Letters 107: 306-317.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE GEOLOGICAL AND STRUCTURAL IMPLICATIONS OF FINE SCALE AIRBORNE GEOPHYSICAL DATA ACQUISITION OVER THE BROKEN HILL / OLARY PROVINCE NEW SOUTH WALES AND SOUTH AUSTRALIA Richard Haren1, George Gibson1, David Maidment1, Peter Gunn2 and Peter Milligan2 1 AGSO Division of Regional Geology and Minerals Canberra ACT 2 AGSO Division of Geophysical Observatories & Mapping Canberra ACT GPO Box 378, Canberra, ACT, 2601, Australia.

ABSTRACT The Broken Hill Exploration Initiative (BHEI) a National Geoscience Mapping Accord (NGMA) project is a collaborative joint venture between the Australian Geological Survey Organisation" (AGSO), the New South Wales Department of Mineral Resources (NSWDMR) and the Department of Minerals and Energy South Australia (MESA). The BHEI is programmed to continue for five years until 1999. The initiative's strategy is to acquire new high resolution geoscientific data that will enable the production of innovative map products and interpretations to assist and focus the minerals and exploration industry and facilitate new ore discoveries in the region. The Broken Hill lode will be exhausted in about 15 years and the strategy is to improve the prospectivity and the exploration expenditure within the region, the focus is on base metals with silver, but other commodities such as copper-gold are being addressed.

To this end AGSO will acquire approximately 290,000 line kilometres of airborne magnetics and gamma-ray spectrometric data over the Broken Hill / Olary Province much at 100 metres line spacing and 60 metres flight height. Our collaborators NSWDMR and MESA have acquired an additional 160,000 line kilometres of airborne data. Detailed geological mapping by the State Geological Surveys is in progress and will result in digital coverages at 1:25 000 and 1:50 000 scale. The new high resolution contiguous airborne geophysical dataset has enabled the BHEI team to interpret magnetics and gamma-ray radiometric data at various scales within the region. Concatenation of the detailed magnetics and outcrop geology has enabled the BHEI team to extend the lithology from areas of outcrop to covered areas. Field measurements that include the magnetic susceptibility of anomalous units and individual structural data collected at "Key Locations" within the region have enabled us to develop a preliminary model of the structural history of the Broken Hill region. The magnetics data can be used to define structural assemblages and stratigraphic horizons, enabling preliminary 3D lithostratigraphic models to be produced. The broad eleven kilometre gravity grid over Australia is being tightened in the Broken Hill region with new gravity data collected on a two by one kilometre grid. Already the NSWDMR have collected detailed gravity at 1440 data points within the region and more data is scheduled for collection in 1996.

Preliminary interpretation of magnetics data at various scales down to 1:25 000 will be assessed along with broader scale magnetic domains and structural elements within the region. An assessment of the complexity of the mapped magnetic units on the Broken Hill and Mt Gipps 1:25 000 map sheet areas will be developed in relation to the ability of the magnetics to map stratigraphic units. The extension of lithostratigraphy onto covered areas at 1:25 000 scale will be discussed with examples. The use of broad scale radiometrics coupled with regolith interpretation to elucidate prospective areas will be examined.

New GIS products and anticipated thematic map products will be described in relation to their relevance as prospectivity tools.

Interpretive work and data capture on the BHEI project area is progressing.

REFERENCES Gunn, P., Haren, R., Milligan, P., Mackey, T., Maidment, D. and Murray, A., (in Press). Geophysical Mapping using the National Airborne and Gravity Datasets: An Example Focussing on Broken Hill. AGSO 50th Anniversary Publication.

Acknowledgments: The authors wish to thank the Executive Director of the Australian Geological Survey Organisation for permission to present this paper.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

TRACE ELEMENTS IN PLANT AND ANIMAL DISEASES: HISTORIC TRIUMPHS IN APPLIED GEOCHEMISTRY AND BIOCHEMISTRY IN AUSTRALIA AND NEW ZEALAND HJ. Harrington Australian National University, Canberra, ACT 0200

In the nineteen-twenties very troublesome and widespread diseases affected cultivated plants and animals in New Zealand and the best-watered coastal parts of Australia but were cured by co-ordinated work. The animal trouble included Coast Disease in south Australia, and Denmark Wasting Disease of sheep and Falling Disease of cattle in Western Australia. In New Zealand there was Bush Sickness in North Island and Morton Mains Sickness in South Island, and goitre in the human population. It is time to celebrate the solutions that were found for those problems.

In bush sickness country there were pastures rich in grasses and clover but sheep on them sickened and died. By contrast the Ninety Mile Desert in South Australia was not really a desert because it has a respectable rainfall (430 mm) but it would not support pastures. There were similar mysteries elsewhere.

Geology was important because the diseases were associated with specific rocks and regoliths. Grange & Taylor of the NZ Geological Survey showed that Bush Sickness was confined to two huge rhyolitic ash falls. In South Australia R. Grenfell Thomas, later Chief of the CSIRO Division of Mineral Chemistry, showed that Coast Disease occurred on calcareous aeolianites. It was Thomas who first suggested that the cause might be a deficiency of cobalt, and Lines, an agricultural chemist cured sheep with 1 mg Co/day, and got additional benefit with Cu, a situation called dui-response. Simultaneously in Western Australia Filmer & Underwood cured Denmark Disease and Falling Disease with Co and Cu. In NZ cobalt then cured Bush Sickness, and iodine cured goitre in the human population. In NZ boron was found to cure corky-pit in apples and was quickly used in Tasmania.

Pine plantations sometimes did poorly in NZ but were saved by Zn which also worked in Australia.

Remedial results were often dramatic with both plants and animals. In the central Victorian highlands application of Mo increased carrying capacity from 1 sheep per hectare to 8, with much better wool. It has been calculated that 1 atom of Mo can bring together 100 million atoms of C in carbohydrates and proteins.

The benefits in plant, animal and human health have not been kept before the public, and the economic rationalists. Indeed parts of the public are so confused that some are opposed to the use of trace elements on the land, referring to them as poisonous chemicals, whereas some spend millions of dollars to buy the same chemicals in bottles in Health and Nature shops.

The truth is that Nature is far from uniform, and that rocks and regoliths vary regionally in chemical composition. Some rocks are toxic for some plants, for WHITC UUSCU OttCASC example serpentinites. Toxicity problems have to be avoided, but trace elements, foill « especially some of the transition elements in the periodic table, are at the very core of ijjjl life processes such as photosynthesis, nitrogen fixation and DNA synthesis.

In a superb paper Joyce (in Nicholas & Eden, 1975) showed that the geochemical surveys that are made in Australia for mineral exploration could have other beneficial uses. This is so much the case that the British Geological Survey is making a detailed geochemical survey of the whole of Britain to provide environmental data.

0*R MOMSOOMAI

REFERENCE Nicholas, DJ.D. & Egan, A.R., editors, 1975. Trace elements in soil-plant-animal systems. Academic Press, London & New York.

182

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE RODINIAN BREAKUP ZONES IN THE TRANSANTARCTIC MOUNTAINS AND SOUTHEASTERN AUSTRALIA ff.T. Harrington h RJ. Korsch and D. Wyborn 2

2

Australian National University, Canberra, ACT 0200 Australian/Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 1

2

A. BASEMENT CORRELATIONS Grenvillean and older: Rocks of Grenvillean age in two tiny nunataks between Shackleton Range and the sea have been correlated with the Grenville Belt (1.3 -1.0 Ga) where it is truncated in southwestern USA. Early to Mid Proterozoic rocks occur in Miller Range in central Transantarctic Mountains but cannot yet be correlated soundly with western Laurentia.

Ross Orogen: This orogen extends the full length of the Transantarctic Mountains but its geology is wonderfully N simple at a first approach. Neoproterozoic turbidite sequences, mainly in the greenshist or lower facies, but also in the amphibolite facies, are similar to the Kanmantoo part of the j ^ V ^ ' ^ V W \ Delamerian Orogen, and in a similar way are intruded by Late Cambrian and Early Ordovician granites (c. 520 - 470 Ma). k p ^ ^ ^ p ^ t ^ ^ ^ M They are overlain by a belt of Lower Cambrian limestones and f j S c r a t o n ^ ; \ Orogen a separate belt of Mid and Upper Cambrian limestones both ^^^^i^orrsne) folded in the Late Cambrian and Early Ordovician Ross \V Orogeny.

(Terrane) Bowers Terrane: A fault-bounded belt 15 to 50 km wide Bowers Terrane Borchgrevink Orogen (Terrane) contains 10 km of primitive arc tholeiites and Mid and Late Cambrian mudstone and limestone, all overlain by 4 km of jRoss Orogen (Terrane) Ordovician fluviatile quartzose sediments. It is correlated with the Grampians Terrane.

Robertson Bay Terrane: About 10 km of Neoproterozoic West H fa;'; ^antarctIC/^S VAntarctica (?), Cambrian and Ordovician quartzose turbidites provide a surprise because they are folded on east-south-east axes quite ^'-southPoi, , oblique to the Ross and Bowers belts. Similar rocks occur in S^ir^'jA* East Marie Byrd Land in West Antarctica. They almost certainly OJw?- Antarctica" Shick!§:on correlate with one of the terranes between the Grampians and Rtngt the east coast of Australia, but which one is not established.

They are intruded by I-type Mid to Late Devonian granites (390 - 360 Ma). No Silurian rocks are known.

B. INTERPRETATIONS AND FUTURE DIRECTIONS There is a Grenvillean 'piercing line' from the southwestern USA to the Shackleton Range but no other sound correlations are yet known from the Transantarctic Mountains to Laurentia. Nevertheless the general Proterozoic connection is accepted because of strong palaeomagnetic data and strong correlations between Laurentia and South America. Harrington et al (1991) proposed that the Delamerian, Grampians and Lachlan belts were deposited in zones formed as Laurentia moved away from Australo-Antarctica from about 750 or 600 Ma to the Early Cambrian, c. 530 Ma, in three extensional episodes alternating with compression and sinistral strike-slip. By the Cambrian the Greater Tasman Zones were at least 700 km wide from the craton to Tamworth, Batemans Bay and Marie Byrd Land. They were wider if the huge Alexander Terrane of western Canada is the eastern side of the Lachlan Belt as suggested by Gehrels & Saleeby (1987). As extension occurred, Laurentia moved north far more than 6,000 km relative to Australio-Antarctica and South America, probably producing diachronous events and displaced terranes. A major problem is whether full opening of the Pacific Ocean occurred in the Cambrian or was delayed until the Devonian. Another is to decide whether part of Laurentia or South China was removed from the Barcoo Embayment.

REFERENCES Gehrels, G.E. & Saleeby, 1987. Geologic framework, tectonic evolution, and displacement history of the Alexander Terrane. Tectonics 6,151 -173.

Harrington, H.J., Korsch, R.J., Rickard, M.J. & Wyborn, D., 1991. Terranes of the Pacific margin of Australia and Antarctica - their development by slow spreading and strike-slip. Proceedings, Fifth International Terranes Conference, University of Chile, Department of Geology, Communicaciones 42, 87 - 92.

U c h I a n

G f a m p i 3 n s T e f f a n e

183

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 CONVENORS* INTRODUCTION TO SESSION B l l ; BREAKUP AND DISPERSAL OF THE RODINIA AND GONDWANA SUPERCONTINENTS: IMPLICATIONS FOR RESOURCE EXPLORATION H-T. Harrington * and CJVIcA. Powell

2

Australian National University, Canberra, ACT 0200 Department of Geology and Geophysics, University of Western Australia, Nedlands, WA 6907 1

2

"Hello Rodinia, goodbye Permanent Pacific" was a remark made in a discussion in 1991.

Rodinia was one supercontinent back in time from Gondwana, two from Laurasia and three from Pangea. The concept of it began to appear about 1981 and has developed rapidly since 1991. The timing of its assembly is debated but was possibly recorded by the Grenville Orogeny between 1.3 and 1.0 Ga. It lasted until about 725 Ma when it began to breakup and disperse as fragments that are now cratonic regions in Australia, Antarctica, North America, South America, and possibly eastern China.

Reconstructions of it around 1991 showed Canada in contact with the Proterozoic margin in eastern Australia (the Tasman Line), and the United States and northern Mexico in contact with the Proterozoic margin of East Antarctica (the Transantarctic Mountains). The North American and northern Mexico regions with a northwest corner of Scotland are known as Laurentia (not Laurasia which was formed later from Laurentia, Baltica and Siberia). The breakup of a Proterozoic supercontinent was discussed in Australia by Lindsay et al in 1987. The placing of the western margin of Canada against eastern Australia had been proposed in the same year in a paper given in Australia by Bell & Jefferson of the Canadian Survey but it attracted little attention. The concept has been developed with speed since 1991 when a series of papers in Geology caused people to note the idea, to be astonished by it and to work intensely on it to test its validity. A simple first approach to the idea is to picture Laurentia occupying the space on the globe between Australia and South America. As Laurentia moved away the Pacific Ocean opened on its western side, and the Iapetus Ocean on its South American side. The two remaining segments of Rodinia, minus Laurentia, were reassembled to form Gondwana.

Session Bll is concentrated on the "Rodinian" breakup margin of Australia and Antarctica and on comparisons with the conjugate margins of China and Laurentia. It is equally concerned with the breakup and dispersal of Australian parts of Gondwana.

Regional topics that are being discussed include the connections between the breakup of Rodinia and the development of the Lachlan Foldbelt in a setting of strike-slip and of alternating episodes of extension and compression. The history of opening and closing of the nascent Pacific Ocean is still being worked out. One argument is that it had opened fully by the Cambrian and another is that it did not open completely until the Early Devonian. There is considerable debate about the movements and affinities of Tasmania in Rodinia, and whether it is a "visiting card" left behind by Laurentia.

Breakup and dispersal patterns have importance in global mineral exploration. For example the Rodinian breakup truncated mineral provinces like those of McArthur River, Mt Isa and Broken Hill, and dispersed them to places not yet identified, although progress has been made in solving the jig-saw puzzle.

The breakup of Australian Gondwana is also relevant to a wide belt of central Asia from Kazakhstan to Vietnam, and to locating extensions of the Gympie province in the region from New Caledonia to the Campbell Plateau, and truncated extensions of the Victorian gold provinces in New Zealand.

Supercontinent studies are providing one of the unifying frameworks for global geoscience, and for analysis of the geological history of Australia and its missing parts.

REFERENCES Bell, R. & Jefferson, C.W., 1987. An phyothesis for an Australia - Canadian connectin in the Late Proterozoic and the birth of the Pacific Ocean. Proceedings, Pacific Rim Congress 1987, Australasian Institute of Mining and Metallurgy, Parkville, Victoria, 39 - 50.

Lindsay, J.F., Korsch, R.F. & Wilford, J.R., 1987. Timing the breakup of the Proterozoic supercontinent: Evidence from Australian intracratonic basins. Geology 15, 1061 - 1064.

184

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PROTEROZOIC COPPER DEPOSITS: THE DISCOVERY OF THE OLYMPIC DAM AND THE NIFTY COPPER DEPOSITS Douglas W Haynes Exploration Division, Western Mining Corporation Ltd., PO Box 157, Preston, Victoria, 3072

In mid 1972, exploration for sediment-hosted copper deposits commenced in the Proterozoic basins of South Australia and Western Australia using a simple exploration conceptual model developed during research at the Geology Department, Australian National University. The research showed that altered continental tholeiite basalts in the Tollu Volcanics, Western Australia, were copper depleted, and depletion was related to alteration of the basalts to albite, hematite, epidote, calcite, chlorite and clay minerals. A key postulate of the simple conceptual model was that copper within ore deposits such as White Pine, the Kupferschiefer, and possibly Mount Isa, was sourced from comparably-altered basalts, as these deposits displayed either a close or a more distant spatial relation to such rocks.

A study of geological maps of Australian Proterozoic basins in mid to late 1972 showed that exploration targets for the copper deposits could be defined using mapped occurrences of the altered basalts. The study showed that principal occurrences of altered basalts were in the Mount Isa Inlier, in the McArthur Basin, in several basins in the Kimberley region, in basins bounding the southern and western margins of the Hamersley Basin, in parts of the Adelaide Geosyncline, and in areas adjacent to the Stuart Shelf on the Gawler Craton.

Exploration programs commenced in the Adelaide Geosyncline, in the Stuart Shelf, and in the basins bounding the southern and western margins of the Hamersley Basin in late 1972. In areas of bedrock exposure, the basalts were inspected to assess their alteration characteristics, and then the sedimentary succession overlying apparently favourably altered basalts was checked for copper-anomalous horizons. This approach lead to the discovery of several occurrences of vein-type copper mineralisation, a stratiform bornite-chalcopyrite occurrence in a laminated quartzite, and several other small occurrences of sediment-hosted copper mineralisation. In 1974, favourable alteration, and copper depletion, was noted in basalts in the Roopena area, on the southern margin of the Stuart Shelf. This observation resulted in a decision to explore gravity and magnetic anomalies on the Stuart Shelf. The anomalies were thought to indicate fault-bound, uplifted blocks of altered basalt, adjacent to local basins hosting copper deposits, beneath a Neoproterozoic cover. Subsequent tests of the anomalies resulted in discovery of the Olympic Dam and Acropolis deposits (Haynes, 1979).

In early 1975, continuing research showed altered basalts were not likely to be source rocks for copper in the sediment-hosted deposits in Zambia and Zaire: rather, hematite-bearing clastic sediments, deposited within continental, arid settings were likely source rocks. A later visit to several of these deposits generated additional supporting data. The exploration conceptual model was consequently modified (Haynes et al. 1993).

Exploration programs were thus reoriented to define occurrence of the inferred source rocks within the Proterozoic and Palaeozoic basins of Australia. Programs using the modified model commenced in the Bangemall Basin, in the McArthur Basin, in the east Kimberley area, and elsewhere. The program in the Bangemall Basin ultimately led to exploration in the nearby Yeneena Basin. The Yeneena Basin exploration program resulted in the discovery of the sediment-hosted Nifty Copper deposit, a stratiform copper occurrence, and a breccia-hosted zinc-lead deposit (Haynes et al. 1993).

Success in the exploration programs resulted from the application of simple source-rock oriented conceptual models, persistence, a flexibility in strategy when the evidence required such, and management support.

REFERENCES Haynes, D. W., 1979. Geological technology in mineral resource evaluation: in Kelsall, D. F., and Woodcock, J. T., eds.Mineral Resources of Australia: Australian Academy of Technological Sciences,Proceedings of the Third Invitation Symposium, p. 73-96.

Haynes, D. W., Brooke, W. J. L., and Mazzoni. P. P.. 1993. Application of conceptual models for sedimenthosted ore deposits in the discovery of the Nifty Copper and adjacent lead-zinc deposits. Yeneena Basin, Western Australia: in Kirkham et al.. eds.. Mineral Deposit Afodeling: Geological Society of Canada, Special Paper 40, p. 75-88.

185

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 SO YOUR GIS TELLS YOU THAT YOUR GPS LOCATED SITES AREN'T WHERE YOU THOUGHT THEY WERE ?

Murray S. Hazell, Matti Peljo and Lesley A.I. Wybom Australian Geological Survey Organisation, GPO Box 378 Canberra City, ACT 2601

Geographical Positioning Systems (GPS) are becoming increasingly popular for accurately locating sample collection sites in the geosciences. Many new users of GPS after hearing of sub metre accuracies possible with sophisticated GPS units fall into the trap of expecting their own GPS to give them similar accuracies at the push of a button. It is only when the point data are integrated with georeferenced digital maps or images within a GIS that users realise their locations were not as accurate as they assumed. The mismatches can be attributed to either 1) absolute errors related to GPS usage and/or 2) how the data are integrated within the GIS. 1) Absolute errors are of two types, systematic and random. Systematic errors are indicated by locations which are consistently offset from their expected positions. The most common cause for this is when the GPS is set to a different datum to the base topographic map on which the positions are plotted. Currently in Australia there are 4 different datums being used: Australian Geodetic Datum (AGD) 66, AGD84, World Geodetic System (WGS) 84 and the Geocentric Datum of Australia (GDA). Unless the GPS is set to the same datum as the base map the error can be as large as 300m (for example between WGS84 and AGD66). Unfortunately, most topographic base maps use either AGD66 or AGD84, whilst most commercial GPS units default to WGS84. To overcome these errors it is advisable to ensure that all GPS units are using the same map datum before beginning a survey. It is also critical that the datum on which the GPS is set, be stored as part of the locational information in any database, as is done in the AGSO OZROX field geology databases. By storing the datum it is possible to then convert all locations to the base map datum. However, even after ensuring that the points and the base maps have the same datum, many points still do not fit into the correct polygons due to random errors which are much harder to correct. Most of the random error is a result of Selective Availability (SA) introduced by the United States Department of Defence to purposely degrade GPS locations. Another source of random error is called Position Dilution Of Precision (PDOP) due to variations in satellite geometry. Satellite geometiy is a measure of the spread of satellites in the sky: the better the spread, the more accurate the position obtained.

However when satellite geometry is poor (ie, all available satellites are grouped in one part of the sky) errors can increase dramatically, sometimes by many hundreds of metres. Fortunately with a full constellation of satellites in place there are only brief periods when satellite geometiy is poor. However if a satellite becomes obscured by some object (e.g. a tree, building or your head) poor satellite geometry could also result.

With SA switched on and using statistical probability to calculate expected errors from a single measurement we can expect: to be within +/- 300m 99% of the time (3 Std Dev) to be within +/- 100m 95% of the time (2 Std Dev) to be within +/- 50m 68% of the time (1 Std Dev) to be within +/- 34m 50% of the time (Circular Error Probability) Several methods exist for obtaining more accurate GPS positions including firstly averaging of a large number of measurements, or secondly using differential GPS by either a Block shift method or Pseudorange correction.

2) Errors in integration of point locations with images or maps in a GIS. Even after considering absolute errors, points may still not plot in the expected position as GPS sites only provide absolute, but not necessarily relative accuracy to geographic features. In contrast, with traditional mapping, because sample points were located on an air photograph or base map, point locations were automatically accurate relative to geographical features. For some projects (e.g detailed geological maps, stream sediment surveys), the relative point location should take precedence over the GPS reading, assuming that the base map is the more accurate: this may not always be the case. While most populated areas have excellent 1:50 000 or 1:25 000 base control, many of the outback areas of Australia have very poor 1:250 000 topographic base maps with errors of hundreds of metres in drainage or roads. In these areas the GPS will have the better accuracy. Therefore, before deciding which site is the most appropriate, the lack of relative accuracy of the GPS should be weighed against the lack of absolute accuracy in the 'registered' topographic base maps used for compiling the data. Finally for building a GIS that integrates point and polygon information (e.g., a geological map!) it is critical that the location method be recorded, as well as an estimate of the accuracy of the location (e.g., 5, 50, or 500 m). If a GPS is used to determine the location, it is also desirable to note relative position to a geographical feature such as a stream intersection or a water bore. These attributes are also stored within the AGSO OZROX field geology databases.

Acknowledgments. This paper is published with the permission of the Executive Director. AGSO. 186

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

WITHOUT STRUCTURED, CLEAN, VALIDATED DATABASES YOUR GIS IS JUST A CAR WITHOUT WHEELS. Murray S. Hazell and Lesley A.I. Wybom Australian Geological Survey Organisation (AGSO), GPO Box 378 Canberra City, ACT 2601

Geographic Information Systems (GIS) are powerful tools for the integration and analysis of geoscientific data.

Maps, images and point data sets can be incorporated within the digital environment of the GIS, turning data into information for solving geological problems. The increasing power of computers and the increasing sophistication of software is enhancing the size of data sets which can be analysed. However not even the most powerful hardware and software can successfully utilise data which has been poorly collected and stored. This is particularly true with digital point databases which can easily take GIS one step past being simply a tool to overlay digital maps. In our experience in the Australian Mineral Systems Project at AGSO, correcting poor data in a point database, prior to integration within a GIS, is one of the most time consuming aspects of creating a GIS. Thus getting the point databases right first will ensure all geological information is available for use within the GIS and that time is not wasted in transforming data into a useable format. Properly structured databases will also allow more effective and sophisticated geoscientific analysis within a GIS. There are five main rules for establishing data bases to be used within a GIS. Rule No 1: Any database should be set up with the end usage of the data in mind.

Rule No 2: Where data about a single sample is stored in a number of databases, all locational information should be stored in a single sites table, which is connected to the tables storing information on say geochemistry, geochronology or petrography. Without this, is highly probable that the location of a single sample will vary between databases.

Rule No. 3: Relational data bases must be used, not spread sheets or text files. In most cases the complex multicomponent nature of geological data means that it is best represented by a relational model.

Rule No 4: Authority (coding) tables are mandatory, and free text fields should be kept to a minimum.

Data in even short free text fields describing only a single attribute can be rendered irretrievable due to spelling mistakes or recalcitrant users mixing the attribute types. To ensure data consistency authority tables should also be used to validate data on entry, thus ensuring that key information is easily retrievable for analysis in the GIS. There is a crying need within the Australian geoscience community to establish national coding systems. If this is not possible then the codes used in any database should be properly documented so that data can be readily translated between organisations. Even when the codes are fully documented, translations are rarely one to one.

Rule No 5: Digital databases should be corporate entities so that everybody within any organisation is digitally collating all data collected, ensuring that the data are described and attributed in the same way, and then properly archived. This rule should also apply to geological departments within universities, very few of which even attempt to centralise, let alone standardise the attribution of their data. Much of the geoscience data collected in universities are stored on an individual's PC: rarely are the data digitally archived and documented.

GIS is fundamentally a spatial system. Any database to be used in a GIS should therefore be set up so that no data what-so-ever about a sample can be entered, unless the proper locational information is entered first. For usage of point data within a GIS, locations of sample sites need to be recorded as geographical coordinates and to the greatest possible degree of precision. For AMG coordinates that means 6 digit eastings and 7 digit northings and for latitude and longitude degrees, minutes and seconds accuracy. It is also advantageous that both the AMG and latitude and longitude are stored enabling the data to be easily used on a local scale using AMGs or across zone boundaries on a province or continental scale using latitude and longitude. For each location, information should be stored about how the location was derived (GPS?, topographic map) and an estimate of the accuracy of the sample location should also be entered. Mandatory fields should be used to ensure that these important data are always collected. Fields which are to be used for mathematical analysis should contain only numeric values. If '<' or 4>4 symbols are used they turn numeric fields into character fields which cannot be mathematically analysed. If required these symbols and likewise any symbols for units of measure should be in a separate field controlled by an authority table to avoid 'gms', cg', 'grams1, etc.

Field data should be collected with coded attributes using either structured notebooks or field computers. Field computers remove the transcription errors and allow validation of data as they are recorded in the field. Data entry into the database is also vastly speeded up. Mandatory, coded attributes encourage more systematic data collection and reduce the data being over synthesised and biased towards the collecting scientists speciality.

Acknowledgments. This paper is published with the permission of the Executive Director, AGSO. 187

GEOLOGICAL SOCIETY OE AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 BIOGEOCHEMICAL ANOMALIES IN THE SEA: INDICATORS OF PROCESSES CONTROLLING SEDIMENT AND WATER QUALITY IN COASTAL ENVIRONMENTS. D.T. Heggie 1 W. Berelson2, G.W. Skyring3, R. Cowdell4, A. Longmore 4 and G. Nicholson 4 1 Australian Geological Survey Organisation GPO Box 378 Canberra, ACT 2601 2. Universtiy of Southern California, Los Angeles, California, USA. 3. Skyring Environment Enterprises, 40 Atherton St Downer, ACT, 2602 4. Victorian Fisheries Research Institute, PO Box 114 Queenscliff, Victoria, 3225.

Coastal environmnets may act as 'conveyor belts' for materials added from the catchment, whereby materials are rapidly transported, with short residence times, to the continental shelf through the estuarine environment. Conversely, environmnets may act as 'traps' for materials added from the catchment, residence times of water and dissolved components are relatively long and internal recycling processes between sediments and water control both sediment and water quality.

Early measurements of nutrients in the water column of Port Phillip Bay (PPB), indicated very low (<1 liM total inorganic nitrogen) and chlorophyl-a concentrations suggesting PPB was an oligotrophy environment. AGSO and VFRI have measured the concentrations and distributions of nutrients, nitrate + nitrite, ammonia, phosphate and silicate in the surface waters of PPB using Continuous Geochemical Tracer technology, whereby seawater is pumped continuously from a submersible towfish (at 7 m water depth) into the ship laboratory. Seawater nutrients were measured by segmented flow-analysis and colourimetric techniques. Nutrient data were gathered at 25 m intervals (horizontally) over the seafloor. Highest concentrations of all measured parameters were found in the Yarra River estuary and lowest concentrations in the PPB. When considered with respect to a simple two-end-member mixing model comprising Yarra River water and Bass Strait water, concentrations of nutrients in PPB indicated different modes of behaviour. Nitrate and ammonia were significantly depleted (negative anomalies) in the bay indicating removal from the water column, probably by primary productivity.

Phosphate appeared to behave conservatively but silicate also showed negative anomalies. The data indicate that PPB is a sink for nitrogen species, while some phosphate and silicate are exported to the Bass Strait.

Benthic flux measurements - the rate of transport of materials across the sediment-seawater interfacewere measured at several locations in PPB with benthic chambers. Net oxygen fluxes indicated that respiration dominated over production on the seafloor. Associated with oxygen depletions, nutrients ammonia, nitrate + nitrite , phosphate and silicate were released from the seafloor. These data collectively indicate the oxidation of organic matter and the liberation of dissolved nutrients to the overlying water from the sediments. A stoichiometric model and mass balances of nutrients identified negative anomalies of nitrogen at most stations suggesting denitrification processses - the reduction of nitrate and liberation of nitrogen as unmetabolizable nitrogen gas - are important in the sediments.

Similarly negative anomalies of phosphate were measured suggesting that some phosphate at most stations is strapped in the sediments.

Radon abundances in chambers and the removal of a chamber 'spike' of cesium, indicate that irrigation of the sediments by burrowing organisms on the seafloor (advective) processes rather than diffusion processes controll the transport of solutes between the overlying waters and sediments. Irrigation processes ventilate the sediments and promote the oxidation of organic matter.

When the water column data are considered with the seafloor data, nitrogen was identified as the limiting nutrient and the coupling of nitrification and denitrification in the sediments is a major control of both water and sediment quality.

Acknowledgments.

We thank the crews of the RV Rig Seismic, and the VFRI vessels RV Melita and RV. Sarda for their assistance in the collection of seawater data and the deployment and recovery of benthic chambers.

Anayses were completed at the VFRI facilities in Queenscliff Victoria.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE WELLINGTON 1:100 000 GEOLOGICAL SHEET: NEW FINDINGS IN STRATIGRAPHY, STRUCTURE AND PLUTONISM G. Tony M. Henderson2, Elisabeth J. Morgan1, Ollie L. Raymond2, Martin M. Scott1. Alice Y. E. Warren1. Doone Wyborn2 1 Geological Survey of New South Wales, P. O. Box 53, Orange NSW 2800 2 Australian Geological Survey Organisation, GPO Box 37S, Canberra ACT 2600

Remapping of the Wellington 1:100 000 Geological Sheet has recently been completed as part of the National Geoscience Mapping Accord on the Lachlan Fold Belt, a joint project between the NSWGS and AGSO. The area covers part of the northern Lachlan Fold Belt, incorporating part of the Molong High, Cowra Trough and Devonian volcanics and intrusives. Substantial improvements have been made to previous geology due to the use of high quality aeromagnetic and radiometric data, flown at 80m flight height and 200m line spacing.

Geophysical images will be a feature of this presentation, as well as the updated Wellington 1:100 000 geology.

The Kabadah Formation, previously the Kabadah Beds, has been redefined to include the Loombah, Bournewood, Yullundiy and Myrangle Formations and Buckinbah Volcanics. The Kabadah Formation occurs in a number of fault repetition slices, underlying the Canowindra Volcanics and has an estimated thickness of 1400m. The formation consists dominantly of mafic volcaniclastic sandstones and siltstones. Early Silurian graptolites have been identified within the formation but the mafic character of the sediments suggests the formation extends into the Ordovician.

The Late Silurian Barnby Hills Shale contains a number of discrete carbonate deposits. The deposits consist of calcareous turbidite sequences, debris flow breccias and limestone mega blocks that represent the sequential slumping and large scale erosion of a Late Silurian carbonate platform, the Narragal Shelf. The material was deposited by a process of gravity-induced, mass transport, into a relatively deep marine basin, possibly related to the Nubrigyn Submarine Valley. The deposits may have taken the form of an extensive submarine fan or repeated debris sheets. A number of autochthonous limestone blocks of Late Ordovician age (J. Farrell pers comm.) in the Eurimbla area may derive from local uplift and erosion of basement rocks.

Detailed subdivision of the complex, ?Early Devonian Yeoval Batholith was greatly aided by the high quality geophysical images. Early gabbroic cumulates grade into quartz diorites and quartz monzodiorites, through to granodiorites by cryptic zoning. This complex is in turn intruded by microgranodiorites and finally by microgranites. The microgranites develop as a vertical sheeted dyke complex which increases in abundance to the north and finally coalesces into a mushroom-like pluton some 12km across. The complex represents a classical high temperature fractionating magma system and provides an excellent example of the way such systems undergo convective fractionation. A zircon geochronological study of the batholith is under way.

A possible volcanic centre has been identified within the Middle Devonian Dulladeny Volcanics, at the site of the Yahoo Peaks composite dome. The centre consists of rhyolite, dacite and andesite lavas and extensive volcanic mass flow breccias. Basalt lava and breccias that underlie rhyolites of the Dulladeny Volcanics are now termed the Fairy Mount Basalt and are probably not part of the Dulladerries but may correlate with the early Devonian Cuga Burga Volcanics. This interpretation has implications for the Mt Aubrey gold deposit which is hosted by these older basalts. The epithermal system responsible for mineralisation at Mt Aubrey may still be related to the overlying Dulladeny Volcanics.

The Lambian Unconformity at the base of the Late Devonian Catombal Group is angularly discordant with faulted Early Ordovician to Early Devonian stratigraphic units. Faulting at a shallow angle to bedding, tilted, uplifted and exposed the stratigraphic units to erosion in the Mid Devonian. An east-directed, west-dipping, imbricate thrust system formed during Early Carboniferous deformation, and produced fault repetition of the predominantly upright west-dipping stratigraphic units. Middle Devonian faults are folded and faulted by the Early Carboniferous deformation.

Acknowledgements: Published with the permission of the Director General, New South Wales Department of Mineral Resources.

189

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 TERTIARY UNITS AND LANDSCAPE DEVELOPMENT IN THE TOWNVILLE-MACKAY HINTERLAND, NORTH QUEENSLAND Robert A. Henderson Department of Earth Sciences, James Cook University, Townsville Q4811

Two extensively developed sediment systems are recognised in the Townsville-Mackay hinterland of north Queensland. The older of these is represented by the Southern Cross Formation and its correlative, the Suttor Formation. These units generally form a flat-lying cap to uplands, and the mesas so formed are characteristic features of the regional physiography. Sediments of this system range to 70m in thickness and includes fluvial and lacustrine facies, with oil-shale, represented at several localities..It overlies what was generally a clean, low-relief pediment surface developed on a wide range of Palaeozoic bedrock. Sandstone and conglomerate intervals are quartz-rich whereas claystone horizons protected from subsequent weathering overprint is characterised by kaolinite clay spectra. These attributes suggest provenance from a well-weathered landscape. Palynological spectra from oil shale samples suggest a pluvial climate supporting Nothofagus forest. The system was overprinted by an episode of deep weathering, with the formation of both silcrete and ferricrete duricrusts in the upper part of its preserved sequence. Duricrust formation is considered to have related to a regime of fluctuating water table and warm climate.

The upland association of this sedimentary system is due to topographic inversion resulting from duricrust erosional resistance. Some sandstone intervals within the succession have experienced a strong diagenetic overprint characterised by quartz overgrowths, the growth of marcasite nodules and the development of clay as pore-space infilling.

The system represented by the Southern Cross and Suttor Formations has traditionally been considered to be of Miocene age. However, at Mount Dalrymple a basaltflowdated at 53Ma overlies an outlier of the Suttor Formation and a silcrete horizon developed within it. The system is therefore likely to be of Palaeocene age and broadly correlative with the oil shale basins of coastal central Queensland and infill of the Duaringa Basin. It relates to the warm global climate of the Early Tertiary prior to the Oligocene thermal decline. The depositional basins in which it accumulated are considered to have been sponsored by rifting of the continental borderland associated with opening of the Coral Sea. Post-Eocene regional-scale erosional stripping has left the system as scattered remnants of what were once much more continuous sediment bodies and the depositional architecture is now obscure.

The second system is represented by the Campaspe Formation which is widely developed on lowland in the northern part of the region. Over 7000 drill holes, almost all of percussion type, have been completed by mineral explorationists to sample bedrock. The system consists of poorly sorted, clay cemented sandstone with minor intervals of siltstone and rare conglomerate. It of is up to 120m thick and represents a sediment volume of some 500km . The general coincidence of the top of the Campaspe Formation with the contemporary landscape has lead to the recognition of the Campaspe Surface, a relict physiographic feature of depositional origin that has been essentially unmodified since its origin. Considerable erosional relief is represented on the floor of the system with incised channels up to 50m deep recognised from thickness data obtained from pattern drilling. The Campaspe Formation shows superpositional relationships with the 3.8 Murrlumbing and 1.35Ma Hann Creek basalt flows and its age is constrained as Pliocene.

3

Sedimentary characteristics of the Campaspe Formation indicate that it related to an unusual sedimentary regime.

Poorly sorted massive sandstone intervals are typical of it with non-erosional bases and matrix-supported textures.

Such attributes are suggestive of debris flow deposits resulting from the rapid entrainment and mass movement of large volumes of sediment. Deposits of this type require special circumstances for their formation which include unconsolidated slope debris containing clay to lower its strength when wet, a lack of vegetation, and high intensity, low frequency rainfall events. An arid climate with extensive stripping of a pre-formed regolith is envisaged as the environmental setting of the Campaspe Formation. The current landscape has equilibrated to a like environmental setting and has a high level of natural stability. The Campaspe Surface has existed with scant modification for over a million years with no recognisable geomorphic or sedimentological record of Pleistocene sea-level fluctuations or substantive climate change.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SM-ND GARNET CHRONOLOGY O F G R A N U L I T E FACIES EVENTS IN PRYDZ BAY AND THE RAUER G R O U P , EAST ANTARCTICA. B.J. Hensen & B. Zhou Department of Applied Geology, University of New South Wales, 2052 Sydney - Australia

Sm-Nd garnet chronology shows that widespread and pervasive granulite facies metamorphism and deformation have affected rocks along the Prydz Bay coast from S0strene Island in the west to the Rauer Group in the east.

Until recently this area, together with the Rayner Complex further west, was regarded as part of an extensive Proterozoic Belt. Our new data provide evidence for the existence of a Paleozoic collisional belt and suggest amalgamation of this part of Gondwana was not completed until the Pan African.

In Prydz Bay two lithological units, one predominantly orthogneiss and the other predominantly metapelite, have been considered as Proterozoic basement and cover respectively. All rocks have undergone granulite facies metamorphism. Peak metamorphic conditions (750-850 °C and 6-8 kbar) are remarkably constant over most of the 125 km extent of outcrop. Rapid post-peak exhumation is indicated by a variety of textures, consistent with decompression at high temperature. Exhumation may have been by extensional collapse, in response to crustal thickening, and the terrane may represent the mid crustal section of a collisional mountain belt.

Garnet -whole rock Sm-Nd age dating shows that the main metamorphic event took place during the Pan-African (7 ages: 488-517 Ma; model ages, T d m = 1.55-2.10 Ga; Hensen and Zhou, 1995). Relics of early Proterozoic (c. 1000 Ma) basement have been identified only on S0strene Island. These rocks were targeted for dating because they contain high pressure (10 kbar) garnet and have two stage decompression textures. The fact that these garnets, with an effective diameter of 1 mm, have retained a memory of both their original high pressure-high temperature origin and of their age of formation, through the 500 Ma granulite event, implies that the closure temperatures for significant Mg-Fe-Ca and and REE isotope diffusion were above 700 degrees C. Our evidence for an important 500 Ma high grade event leads us to suggest that the intrusion age of an aplite reported by Kinny et al. (1993), and used by them to bracket the end of the metamorphism in the Rauer Group, may represent an inherited age, and that the concordant 500 Ma ages for the same sample represent the age of intrusion. Using this interpretation we infer a radically different tectonic scenario in which the Archaean and the Proterozoic rocks of the Rauer Group were not juxtaposed until a Pan African collisional event (Hensen & Zhou, 1995), thus explaining the absence of any Proterozoic overprint on the Archaean rocks reported by Kinny et al.

(1993).

To test this hypothesis samples from the Rauer Group, kindly provided by Dr. S.L. Harley, have been dated. The rocks from Filla Island, with Proterozoic model ages ( T d m = 1.59-1.98 Ga), have Sm-Nd garnet ages (486-502 Ma) identical to those of the rest of Prydz Bay, whereas the Archaean (Tdm = 2.9-3.7 Ga) samples from Long Point have garnet ages of 600 (two determinations on one sample) and 485 Ma (different sample). The 600 Ma ages were obtained on a sample from Long Point for which the P-T estimates by Harley and Fitzsimons (1991) are 1000-1050 °C and 10-12 Kb. For garnet - orthopyroxene pairs to record such high temperature conditions cooling must have been fast. As a consequence the Sm-Nd age is likely to correspond to the event producing the garnet. Therefore these new age dating results lend support to our contention that the last intense granulite facies event in the Rauer Group is of Pan African age. Thus the interleaving of Archaean and Proterozoic protoliths may be the result of a collision of an Archaean Block with a composite terrane consisting of Proterozoic basement, metamorphosed at c. 1000 Ma, and a younger cover sequence, during the Pan African.

Because early Paleozoic ages have also been reported recently from the Liitsow-Holm Bay area and parts of the northern Prince Charles Mountains (our unpublished work), it is tempting to link these through Prydz Bay with outcrops reaching as far a the Denman glacier area and possibly the Leeuwin Block in western Australia into a single Pan African Belt, emanating from a triple junction centred on Sri Lanka. These observations suggest that the amalgamation of East Gondwana may not have been completed until 500 Ma ago.

REFERENCES Hensen, B.J. & Zhou, B. 1995. A Pan-African Granulite Facies Metamorphic Episode In Prydz Bay, Antarctica: Evidence From Sm-Nd Garnet Dating. Australian Journal of Earth Sciences 42,249-258.

Kinny P. D., Black L. P. & Sheraton J. W. 1993. Zircon Ages And The Distribution Of Archaean And Proterozoic Rocks In The Rauer Islands. Antarctic Science 5, 193-206.

Harley, S.L. & Fitzsimons, I.C.W. 1991. Pressure - temperature evolution of metapelitic granulites in a polymetamorphic terrane: the Rauer Group, East Antarctica. Journal of Metamorphic Geology 9, 331-143.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A STRATEGY FOR RESTORATION OF BASINS WITH MULTIPLE D E F O R M A T I O N ; T H E R M A L AND STRUCTURAL ANALYSES IN SE AUSTRALIA. Kevin C. Hill & Gareth T. Cooper Australian Geodynamics Cooperative Research Centre VIEPS School of Earth Sciences, La Trobe University, Melbourne, Australia 3083 ABSTRACT Constructing balanced sections across inverted basins is difficult because of the different directions of tectonic transport during extension and compression and the imprecise estimates of the magnitude and timing of inversion events. Sections oriented within 25° of all transport directions can be restored and result in <10% error in the amount of extension-compression (Hill & Cooper, in press). Extensional faults are commonly oblique to the transport direction, so the inferred extension direction may be verified by dip analysis of the pre-rift section.

Where oblique extensional faults are reactivated during compression, the compressional vector is best determined from the orientation and magnitude of large, curved inversion anticlines. For strongly inverted and eroded anticlines, where Tmax is not at the present day, palaeotemperature gradients determined regionally from borehole vitrinite reflectance and fission track profiles constrain the amount of denudation allowing reconstruction of the original anticline morphology. The timing of inversion and erosion determined by apatite fission track analysis allow the eroded strata to be replaced on the section and the inversion restored to reveal the amounts of extension and compression and the syn-rift basin morphology, which may constrain hydrocarbon migration paths. For the Otway Ranges in SE Australia 45% Early Cretaceous extension followed by 10% midCretaceous compression are inferred.

Hill, K.C. & Cooper, G.T., 1996. A strategy for palinspastic restoration of inversion basins; thermal and structural analyses in SE Australia. In Buchanan P.G. & Nieuwland D.A. (eds), Modern developments in structural interpretation, validation and modelling , Geological Society of London Special Publication No. 99, pp. 99-115 (in press).

192

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE DEVELOPMENT OF A REGOLITH STRATIGRAPHIC FRAMEWORK FOR LANDSCAPE EVOLUTION AND MINERAL EXPLORATION MODELS IN THE BROKEN HILL REGION

1

S.M. Hill 1 . Graham Taylor 2 & Tony Eggleton 1 Co-operative Research Centre for Landscape Evolution and Mineral Exploration, The Department of Geology, The Australian National University, Canberra, ACT, 0200 2 Co-operative Research Centre for Landscape Evolution and Mineral Exploration, School of Resource, Environmental and Heritage Science, The University of Canberra, PO Boxl, Belconnen, ACT, 2616

INTRODUCTION As the search for major mineral deposits has advanced in the Broken Hill region, regolith dominated terrains have become one of the major exploration frontiers. The success of this exploration is dependent upon the development of exploration models based on a sound understanding of regolith genesis and landscape evolution. An understanding of the regolith stratigraphy is an important component of any model when considering the evolution of landscapes and associated regolith materials over time.

DURICRUSTS AND WEATHERING PROFILES Previous studies have found duricrusts and weathering profiles a challenge to place within a stratigraphic framework. Robust genetic models and palaeo-environmental reconstructions are needed to meet this challenge.

As our understanding impjroves it has become apparent that they are extremely complex features, particularly with respect to their spatial distributions and origin. Evidence from the Broken Hill region suggests that these features have been continuously modified through time, making the assignment of a specific age difficult and also a challenge for regional correlations. A relative stratigraphy can be determined for specific localties based on the inter-relationships between regolith materials, and used towards developing regional stratigraphy based models.

PALAEOSURFACES Landscape studies, particularly in Australia, have been beset with many of the concepts associated with palaeosurfaces. In many areas their identification and significance are tenuous. At Broken Hill a series of palaeosurfaces associated with depositional features have been recognised, as well as erosional features most probably also related to palaeosurfaces. The materials that characterise these surfaces, as well as their relationships to other landscape features, potentially provides the evidence to place them within a stratigraphic framework.

SEDIMENTARY RECORD The sedimentary record has the potential to provide a stratigraphic framework for regolith and landscape features. In the Broken Hill region the correlation of regolith and landform related features within the sedmentary record of the adjacent Eromanga, Murray and Lake Eyre Basins is, in many cases, directly applicable to similar materials and associated events on the Broken Hill Block. This is important for the uplifted parts of the Broken Hill Block where regolith materials have been stripped and redeposited locally as well as within the adjacent sedimentary basins. Locally derived fluvial, lacustrine, colluvial and aeolian deposits on the Broken Hill Block also provide evidence for local palaeo-environmental reconstruction.

TECTONISM The Broken Hill Block has experienced major episodes of uplift since the deposition of the Proterozoic sediments and volcanics. The later (Mesozoic and Cainozoic) tectonic events have had a major influence on the regolith and landscape of the region, controlling areas of erosion and deposition , as well as disrupting ancient drainage systems. A series of fault blocks, in most cases defined by a conjugate set of faults with one set trending approximately NW-SE and the other NE-SW define the uplifted parts of the Barrier Ranges and surrounding areas of sedimentary basin subsidence.

CONCLUSION The Broken Hill regolith and landscape are primarily controlled by lithological variations, faulting and tilting, and deposition. Deep weathering, and duricrusts are common across many parts of the Block, although their ages are difficult to determine precisely except that many formed early during the landscape history (perhaps Mesozoic) while others such as calcrete are still forming. A variety of ancient fluvial sediments (some lithologically equivalent to the Early Tertiary Eyre Formation) are also widespread, many of which have been indurated and relief inverted. The Late Tertiary and Quaternary aridity has led to widespread aeolian and alluvial features with a long and complex history.

Acknowledgements: CRAE are thanked for generously providing financial support for this project and also providing permission to publish this abstract and present this work. The New South Wales Government "Discovery 2000" initiative and CRCLEME are also thanked for their support.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra:, February 1996

THE CONTEMPORARY STRESS FIELD OF THE AUSTRALIAN NORTH WEST SHELF: II. IMPLICATIONS FOR HYDROCARBON EXPLORATION AND PRODUCTION Richard R. Hillis 1 and Scott D. Mildren1 * Department of Geology and Geophysics, University Adelaide, SA 5005

As outlined by Mildren & Hillis (this volume), borehole breakouts in the Carnarvon Basin indicate a maximum horizontal stress (Ghmax) orientation of 090°N-100°N. In the Bonaparte Basin, breakouts indicate a ahmax orientation of 055°N-060°N. Although there is local variation, in both basins, regionally, the magnitude of a hmax is similar to that of the vertical or overburden stress (a v ), and greater than the minimum horizontal stress (^hminX ie. the fault condition is on the boundary of normal and strike/slip (a v = Ghmax > ^hmin)- I*1 order to illustrate the application of contemporary stress data to hydrocarbon exploration and production, the implications of these regional stress fields are discussed below. However, in practice, the stress field should be constrained at the scale of the prospect/well.

There has been considerable deviated and horizontal drilling activity in the North West Shelf including the North Rankin A-21 well which had a deviation (from the vertical) of 68° for most of its length and intersected gas-filled sandstone at 3 km depth, 5 km to the north-east of the platform, setting a world horizontal reach record.

Wellbore instability threatens to be the greatest impediment to the realisation of benefits offered by horizontal drilling technology. If a horizontal wellbore is deviated in the Ghmax direction, it is acted upon by the vertical and Ghmin s t r e s s e s - Conversely, if a horizontal wellbore is deviated in the Ghmin direction it is acted upon by the vertical and Ohmax stresses. Given that wellbore instability due to breakout is controlled by the anisotropy of the stresses acting on the wellbore, breakout propensity in a horizontal well is thus influenced by its azimuth.

In the Carnarvon Basin, stress anisotropy around the wellbore, hence breakout propensity is minimised if a horizontal well is drilled in the Ghmin direction. Indeed such a well is subject to only very slight, if any, stress anisotropy and, perhaps counter-intuitively, should be less prone to breakout than a vertical well. In practise, reservoir geometry/engineering considerations will generally determine drilling direction, however, the above approach allows the propensity for breakout in horizontal wells to be predicted, and suitable well planning, for example of mud weights and casing depths, to be undertaken.

Induced, hydraulic fractures, which can be used to stimulate production, open 'against' (at right angles to) the minimum principal stress. Hence in the Carnarvon Basin they tend to be vertical and strike 090°-100°N, and in the Bonaparte Basin, vertical and strike 055°-060°N. Knowledge of the orientation of hydraulic fractures can be applied to planning fractures from a well to intersect missed targets such as reefs, to multiple fracturing of deviated wells in order to maximise reservoir drainage, and to planning the locations of wells to be water flooded and produced in enhanced oil recovery operations.

Highly deviated and horizontal drilling has led to the exploitation of naturally fractured reservoirs, the intergranular porosity of which would not permit their economic exploitation, such as the Austin Chalk of the United States. The Flamingo Group of the Bonaparte Basin is naturally fractured, and presents a potential reservoir in the area, though secondary to various reservoirs with primary porosity. Orthogonal sub-vertical fracture sets are present in the Austin Chalk of Texas, however, only the fracture set perpendicular to Ghmin open and controls the reservoir. Although the orientation of natural fractures need not be related to the contemporary stress field, the above examples support the intuitive hypothesis that for natural fractures to be open in the subsurface, and hence productive, their orientations should be similar to those that induced, hydraulic fractures would follow. Hence in order to maximise intersection with, and thus production from open, natural fractures in the Flamingo Group of the Bonaparte Basin, wells should be deviated in the 145-150°N direction.

Many exploration wells in the Bonaparte Basin have been unsuccessful, intersecting only residual hydrocarbon columns, the former presence of which is indicated by inclusions in reservoir cements. The dynamic sealing capacity of a fault is controlled by the effective normal stress (i.e. difference between normal stress and pore pressure) acting upon it. Hence, faults following the regional north-east/south-west structural trend in the Bonaparte Basin are poorly oriented to be dynamically sealing because they are orthogonal to the 055°-060° Ohmin direction in the basin. Structures controlled by faults with other orientations have recently been targeted in the basin, and the Elang Field is located in a structure controlled by east-west oriented faults.

Acknowledgments: SDM is funded by an AGSO postgraduate research scholarship. The work has been undertaken as part of an Australian Petroleum Cooperative Research Centre project on the stress field of the North West Shelf.

194

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra:, February 1996

PALAEOZOIC BEDROCK GEOLOGY OF NARROMINE & NYNGAN 1:250,000 SHEETS, CENTRAL NSW, INTERPRETED FROM NEW DISCOVERY2000 AEROMAGNETIC DATA David Hilyard Geological Survey of New South Wales, PO Box 536, St Leonards NSW AUSTRALIA 2065

New, high-resolution, digital, airborne geophysical surveys of the Lachlan Fold Belt disclose new data on the bedrock geology of NARROMINE and NYNGAN 1:250,000 sheets in central NSW. Mesozoic - Cainozoic cover is nearly complete across most of the area, and the Palaeozoic geology very poorly known.

These surveys form part of the DISCOVERY 2000 programme undertaken to promote resource exploration in NSW. Significant conclusions from the aeromagnetic interpretation include: • Major structural units continue to strike generally north under cover, as suggested by Scheibner (1993).

• A prominent structural corridor approximately 20 km wide runs west-northwest across the project area at about the boundary between the NARROMINE and NYNGAN sheets. Major structural units are displaced left-laterally across the corridor by up to 10 km. The corridor appears to be a deep-seated, fundamental structure, and is also mapped on the gravity images. The structure is parallel to the Lachlan River Lineament further south (Scheibner and Stevens, 1974). The structure is here called the Nyngan lineament.

• Ordovician volcanics in the Parkes - Goonumbla area form a prominent magnetic high. Similar magnetic highs are present south and west of Narromine, and northeast of Nyngan; we infer that they also represent Ordovician volcanics. Goonumbla Volcanics in the south of NARROMINE host the Northparkes mine gold-copper porphyry system (>1.3 m oz contained Au, >845,000 t contained Cu) and may be source of the more distal Peak Hill Au-quartz vein mineralisation (1.8 Mt @ 2.0 g/t).

• ?Cambro-Ordovician metasediments of the Girilambone Group are magnetically quiet and featureless, and form a belt of poor outcrop and subcrop along the western margin of the project area. Low amplitude, irregular discontinuous magnetic anomalies within the Girilambone group area appear to be refolded mafic schists within the metamorphic sequence.

• An extensive granitic batholith lies south of Nyngan. Multiple intrusive phases are mapped within this intrusive complex. Isotopic dating of the suite suggest a Silurian age; crosscutting, possibly intrusive relationships between the complex and Late Devonian Hervey Group units in the Tullamore Syncline are anomalous. The isotopic data should be critically re-evaluated, however the complex as currently mapped may contain several intrusive suites of different ages.

• The Nyngan batholith is constrained by the Nyngan lineament (cf. Bathurst batholith). A concentration of ?roof pendants in the southern part of the batholith suggests that it is tilted to the south.

• ?Siluro-?Devonian units form the Tullamore Syncline, which strikes north across the centre of the project area. The core of the Tullamore Syncline consists of the magnetically quiet Late Devonian Hervey Group.

Previous aeromagnetic data were unable to resolve the structure of the northern part of the Tullamore Syncline (Agostini, 1984). The new, detailed surveys clearly show the Tullamore Syncline continues northward under cover, with left-lateral displacement along the Nyngan lineament.

• Plutons of the Devonian Yeoval batholith continue north under cover, and are probably displaced sinistrally by the Nanromine lineament. Both magnetic and distinctly non-magnetic phases of the batholith are recognisable, as well as ?hornfelsed rims. Non-magnetic phases may be A-type or S-type granites.

• North of the Yeoval batholith is a contiguous zone mapped as volcanics and shallow intrusives. Radial and ring fractures are present, and the complex appears to be a large caldera, with a resurgent central dome.

Outside the main ring fracture we have mapped moat-fill ignimbrite sheets, which may show nestling subsidiary caldera structures.

• The structural continuity of the caldera sequence with the Devonian Yeoval batholith suggests that the volcanics represent upper levels of the Yeoval intrusives, and may thus also be Devonian in age.

REFERENCES Agostini, A., 1984. NYNGAN 1:250,000 sheet - a preliminary geological interpretation from regional aeromagnetic and gravity data.

Geological Survey of NSW, Quarterly Notes 54, 13-23.

Scheibner, E., 1993. Structural framework of New South Wales. Geological Survey of NSW, Quarterly Notes 93, 1-35.

Scheibner, E. and Stevens, B. P. J., 1974. The Lachlan River Lineament and its relationship to metallic deposits. Geological Survey of NSW, Quarterly Notes 14, 8-18.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOLOGICAL HAZARDS ADDRESSED BY AUSTRALIA'S IDNDR PROGRAM PRACTICAL APPLICATIONS TOWARDS EMERGENCY MANAGEMENT *Mr Alan Hodges, AM 1 Chairman, Australian IDNDR Coordination Committee Emergency Management Australia, PO Box 1020, Dickson ACT 2602

In response to the proclamation by the United Nations of the 1990s as the International Decade for Natural Disaster Reduction (IDNDR), Australia announced its participation on 21 April 1989. The Australian IDNDR Coordination Committee was established in 1990 and our efforts began in earnest. To date 55 projects have been facilitated by the Committee: 37 directed to the national need and 18 related to our neighbours in the developing states of the Pacific Island Countries. The projects have considered nine natural hazards (cyclone, severe weather, storm surge, earthquake, flood, tsunami, landslide, volcano and fire) as well as medical and community awareness themes and media campaigns. All projects have complied with one or more of the IDNDR targets (risk assessment, mitigation planning, warning) and have satisfied the strict Australian criteria of having outcomes with direct practical application to natural disaster reduction for the community.

There has been a clear recognition of the past experiences and future potential effects of geological hazards.

Those directly applicable include a natural hazard map of the SW Pacific quadrant, earthquake zonation mapping of urban areas in Australia, landslide hazard in Papua New Guinea, assessment of tsunami risk, and "Hazard-Wise", a book for schools. Others with indirect association include the meteorological projects of tropical cyclone and storm surge and flood.

This paper will present an overview of the Australian IDNDR program and highlight projects associated with the specific geological hazards. An insight will be provided into the success and effectiveness of such efforts in our aim to reduce potential losses that may be inflicted on the Australian and SW Pacific Islands countries in the event of such natural disasters. Future partnership directions for the geological community and the emergency management community will also be outlined.

REFERENCES Emergency Management Australia, 1994: "Australian IDNDR National Report 1990-1994". Presented to United Nations World Conference on Natural Disaster Reduction, Yokohama, Japan, 23-27 May 1994, 81pp.

Emergency Management Australia, 1995: "IDNDR Briefing Paper" October 1995, 9pp (unpublished).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra:, February 1996 THE TOURISM VALUE OF SCENIC ATTRACTIONS: AWARENESS AND INFORMATION AT LOCAL GOVERNMENT LEVEL IN SE QUEENSLAND Gerhard W. Hofrnann Earth Science Consultant, 23 Fuelling Street, Brisbane QLD 4069

Ecotourism and educational tours to natural environments are increasing in popularity. The Federal Government's National Ecotourism Strategy of 1994 recognises this and attempts to influence the development and management of nature-based tourism in Australia. Among the issues identified are education and marketing.

Education includes the education of tourists through easily accessible and accurate interpretive information to enhance their experience. The marketing of ecotourism appears to be confused by definition problems, lack of market knowledge and product fragmentation.

This paper looks at product knowledge and promotion at the local government level.

A survey of 35 Shires in southeast Queensland was carried out by questionnaire in August 1995 to collect information on the awareness of scenic attractions at local government level, their usefulness in attracting visitors, and the level of information provided to visitors. Twenty-two questionnaires were returned (63%). The survey area includes the non-city Shires to about 250km inland from the coast and north to Maryborough.

Table 1 shows the replies to six major questions.

The majority of Shires in the southeastern corner of the State are aware of their scenic attractions, promote them to visitors, and have ensured that tourist information is available. Half of them expect that ecotourism will become a noticeable contributor to their Shire's economy. Only about a quarter collect statistics on visitor numbers or noticed an increase in visitor numbers following the release of tourist information.

TABLE 1: Local government questionnaire returns Reply to Question Aware of popular places in Shire Promote visits to scenic places Collect visitor number statistics Provide information to visitors Noticed effect of information release Expect economic benefitsfromecotourism

%

77 82 23 82 23 50

Information is available in the form of information sheets, booklets and maps for motorists, in about equal amounts, in just over half of the Shires. Only a minority (14%) have maps for bushwalkers. The main topics are the natural environment (55%) and historical sites (45%). Of the remainder, descriptions of landforms (23%) prevail over animals (14%) and plants (9%). This should give heart to earth scientists. By far most of the information was prepared and printed by the Shire Councils themselves (68%). State government agencies, such as the National Parks Service and Forestry Department, are the next largest information source (36%).

Surprisingly, more Shires have used consultants (14%) than relied on learned societies (9%) for information.

One Shire used the tourism board. Descriptive information outweighs explanatory information three to one.

Most Shires promote their scenic attractions through local tourism boards (73%). Local advertising is used by 36% and national advertising by 23%. Only 14% of Shires promote their attractions through travel agents.

Marketing data, such as visitor numbers and feedback, are virtually absent; only two Shires (9%) seek feedback from visitors on their impressions. Scenic places in a majority of Shires (86%) are used for outdoor activities other than bushwalking. Quite a number of Shires are encouraging private tour operators and deplore the slowness of tourism operators in developing products. The Cooloolah Shire, which includes the Great Sandy Region and Mary River valley, is carrying out an environmental audit to identify areas and interesting geological features suitable for ecotourism. It aims to secure those targets through appropriate conservation strategies.

In conclusion, the survey indicates that Shires in the populated southeast of Queensland are generally well aware of scenic attractions in their administrative territory, although a few on the featureless Darling Downs think that they have none. There is increasing realisation that tourism can provide economic benefits to local government areas. Shires have always been promoting their historical sites, forest parks, camping areas and museums. Now they are also promoting koala habitat tours, whale watching, and scenic geological features. Geological features are recognised as an integral part of scenic sites. There are opportunities for earth scientists to provide descriptive and interpretive information so that authentic quality products can be marketed to domestic and, with an added global perspective, to overseas tourists.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 SEQUENCE STRATIGRAPHY AND PALAEOENVIRONMENTS OF TERTIARY COOL-WATER CARBONATES, ONSHORE GIPPSLAND BASIN, VICTORIA. Guv Holdgate and Stephen Gallagher Department of Earth Sciences, Monash University, Clayton, Victoria 3168 Department of Earth Sciences, University of Melbourne, Parkville, Victoria 3168 1

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This contribution represents an integrated study in the Gippsland Basin combining sequence stratigraphic methodology with detailed biostratigraphic and palaeoenvironmental analyses of a succession of Tertiary cool water carbonates. This multidisciplinary approach quantifies Oligo-Miocene eustatic sea level and climatic fluctuations for a thick sequence of platform cool water carbonates in southern Australia. Data for this study is derived from continuous core bore data tied to a regional network of seismic data in the onshore Gippsland Basin of Victoria. The 16,000 km of the onshore Gippsland Basin contains up to 500 metres of carbonates which are assigned to the Lakes Entrance Formation, Gippsland Limestone Formation and the Wuk Wuk Marl Formation of the Seaspray Group.

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The Seaspray Group has now been further subdivided into several new sequence stratigraphic intervals; four in the Lake Entrance Formation, four in Gippsland Limestone Formation and two in the Wuk Wuk Marl Formation.

The Lower Lakes Entrance Sequence was deposited towards the end of the Lower Oligocene and is poorly represented in the basin. The Middle B and Middle A Lakes Entrance Sequences consist of mid to outer shelf interbedded carbonates and clastics and grade offshore into deeper water marls of Upper Oligocene to Early Miocene age. The Upper Lakes Entrance Sequence corresponds to the high sea level period in the early part of the Early Miocene. Eutrophic upwelling palaeoenvironments are suggested for the marly transgressive systems tracts of the Middle B and the Upper Lake Entrance Sequences based on microfossil evidence. These are overlain by high stand system tract mixed carbonates and clastics which are interpreted to have been formed in warm water oligotrophic conditions.

The base of the Lower Miocene Gippsland Limestone Formation forms a series of regional seismic reflectors that correlate to the high stand systems tract of the sequence chronostratigraphic cycle TB 1.4. Continuations to these high carbonate limestones and interbedded marly limestones represent deposition in cold water mid to outer shelf depths. Near the end of the Early Miocene and the top of the Gippsland Limestone warm shallow oligotrophic conditions appear in the high stand of sequence cycle TB 2.1. The Mid Miocene Wuk Wuk Marl Formation records the first two sequences of the Mid Miocene Climatic Optimum, with an influx of abundant foraminifera of the Orbulina lineage and globorotaliids.

The relative depths of sea level change and sedimentation rates were determined from accommodation calculations of seismic lines, foraminiferal assemblages and durations of sequence chronostratigraphic cycles.

The data suggest water depths vary between 0-200m+ and the eustatic range was similar. Sedimentation rates were highest in the oligotrophic facies.

The palaeoenvironments of the sequences studies vary from terrestrial lignites to outer shelf carbonates over a horizontal distance of five kilometres. This suggest a carbonate ramp with an average slope of 3° existed in the onshore Gippsland Basin during Oligocene and Miocene times.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 SEQUENCE STRATIGRAPHIC ANALYSIS AND THE ORIGINS OF TERTIARY BROWN COAL LITHOTYPES, LATROBE VALLEY, GIPPSLAND BASIN G. R. Holdgate , A. P. Kershaw and I. R. K. Sluiter Department of Earth Sciences, Monash University, Clayton, Vic. 3168, Aust., Department of Geography and Environmental Science, Monash University, Clayton, Vic. 3268, Aust.

Department of Conservation and Natural Resources, 253 Eleventh Street, Mildura, Vic. 3500, Aust.

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The methods of sequence analysis have been applied to the onshore Gippsland Basin and in particular to the Latrobe Valley Group coal measures which include up to five coal seams each exceeding 100m in thickness. The methods appear to provide new depositional concepts tot he evolution of these seams, and the development of coal lithotypes. In the eastern half of the Latrobe Valley evidence for marine transgressions into the coal measures are recorded in most of the interseam sediment splits by the presence of contained foraminifera and dinoflagellates. To the west (inland) these splits pinch out into continuous coal. However, they can be followed westwards as enhanced organic sulphur levels along sharply defined boundaries between light coal lithotypes below and dark coal lithotypes above. The dark lithotype immediately overlying each of these boundaries contains the highest sulphur value and warmer climate pollen assemblages (Sluiter et al, 1995).

Colorimeter and lithotype logging strongly supports an upwards lightening cyclicity to coal colour at 12-20m intervals through the circa 100m thick seams, with cycle boundaries defined at sharp planar to undulose surfaces.

The lightening upward lithotype cycles together with their unique boundary conditions (i.e. enhanced organic sulphur levels, warm climatic indicators and laterally equivalent marine clay splits) are interpreted as parasequences and parasequence boundaries respectively. Each major coal seam can comprise up to five parasequences and is interpreted to represent deposition during an outbuilding high stand systems tract at one of several maximum periods of Tertiary coastal onlap. The top of each major seam shows evidence of truncation (erosion?) on a regional scale and these surfaces are interpreted to represent the sequence boundaries. The major seams are usually conformably underlain by marine clays and extensive aquifer sands, being deposits of the late transgressive systems tracts. The low stands and early parts of the transgressive systems tracts appear not to be represented in the Latrobe Valley due to its (more) basin margin location, but are probably present down-dip in the equivalent marine facies of the Seaspray Group.

Stratigraphic correlation of the sequence boundaries identified in the coal measures to the adjacent, internationally dated marine Seaspray Group, provides a basis for chronostratigraphic correlation of the coal successions to the coastal onlap charts of Haq et al., (1988). From this dating is appears that each major seam is confined to high stands of third order eustatic cycles. It therefore follows that the lithotype cylces (parasequences) that comprise each seam are related to fourth order eustatic cycles. By analogy all the coal cycles may have developed under subtropical conditions as ombrogenous forested peat swamps in a similar manner to the Holocene, though tropical, swamps of Indonesia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EXPLORATION BREAKTHROUGH! : THE SOUTH AUSTRALIAN EXPLORATION INITIATIVE Ric Horn1, Tonv Belperio1 and Pru Freeman1 *Mines and Energy, South Australia, (MESA) PO Box 151 Eastwood, SA 5063

During the 1980's, mineral exploration expenditure in South Australia declined significantly to a point where the state was attracting barely 1% of the national exploration expenditure. The South Australian Exploration Initiative (SAEI) which commenced in June 1992, was conceived as a major new State development initiative to attract a greater share of mineral and petroleum exploration and development. The main strategy of the SAEI built on the traditional role of attracting exploration companies through delivery of technical information but in new and innovative ways. After leading the way in this new phase of data acquisition, South Australia is now realising the success of the SAEI. The cornerstone of the initiative has been the regional aeromagnetic and radiometric surveys, the largest and most intensive publicly funded program in Australia. The total area of South Australia covered by the new surveys since the SAEI commenced is 41% comprising: SAEI 34%, Industry 3% and AGSO 4%. The 1 000 000 line kilometres have been flown at a cost of $10.8 million, including the industry contribution of $1 million.

The high quality SAEI data have been made available to industry for the nominal charge of one cent per line kilometre. These surveys have been backed by major drilling programs (over 20 to date), new geological mapping and interpretations, gravity and seismic surveys and the implementation of our comprehensive GIS; SA_DISPLAY. MESA has also been proactive in the marketing and promotion of the State's potential and developing the "Single Window" concept for MESA to provide coordination and facilitation of Government procedures in major resource developments.

Major SAEI Achievements: Dominion/Resolute "Challenger" gold prospect: a recent highlight for the SAEI has been the announcement by Dominion Mining in partnership with the Resolute Samantha Group of very promising gold intersections. The estimated value of gold in ground is greater than the entire expenditure of the SAEI and has generated renewed exploration commitment to the Gawler Craton.

• Stratigraphic drilling to test basement in the Coomandook area (Upper Southeast, SA) intersected anomalous base metals and gold. The region has been completely covered with EL's and Pasminco is leading the follow up of this new prospectivity.

Mineral exploration in South Australia is now at the highest level since 1986 and is double the expenditure of four years ago. The percentage of South Australia under exploration licence (EL) has increased since the start of the SAEI in 1992 from 12% to 20%. The SA expenditure commitment for 1995-96 now stands at $35 million.

• A direct result of the new surveys has been diamond exploration on the Abminga Block which saw no exploration interest during the early 1990's and hosts over 25 EL's today.

• MESA has developed a world-leading GIS to support exploration and associated land management.

• Collaborative projects with industry and other Government agencies such as the SA Steel & Energy (SASE) Project and the Broken Hill Exploration Initiative (BHEI).

New Opal Mining Legislation, Amended Mining Act. MESA is determined to continue the momentum of the SAEI and plans to announce a new set of initiatives in 1996. The South Australian Government recognises that the success of collaborative programs such as the BHEI and SASE is critical to the future of our state. Key areas to be addressed in future programs are: • An innovative new phase of data acquisition.

• Rapid release and delivery of comprehensive data packages of key geological provinces in flexible formats on a range of media.

• Review/improvement of legislature, policy and administrative processes.

• Application of new or improved exploration techniques and state-of-the-art technology to the acquisition, processing, interpretation and promotion of geoscientific data.

Innovative national and international promotional campaigns to advertise South Australia's prospectivity and infrastructure.

Acknowledgments: This abstract is published with the permission of the Chief Executive Officer, MESA.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

STRUCTURAL GEOMETRY ON THE WESTERN FLANK O F T H E MITAKOODI CULMINATION: INSIGHTS INTO THE WRENCH SYSTEM IN THE EASTERN MOUNT ISA INLIER, NW QUEENSLAND Wanfu Huang and Gordon S. Lister Australian Crustal Research Centre, VIEPS Department of Earth Sciences, Monash University, Vic. 3168, Australia

INTRODUCTION The unique Mitakoodi Culmination is one of the most outstanding structures in the Mount Isa Inlier. The architecture of the Mitakoodi Culmination and its roles in the formation of the regional structural framework are interesting, but have not been properly addressed so far. Detailed structural mapping was carried out in the Roos Mine region south of the Corella River Dam, which revealed interesting structural relations that provide some insights into the orogenic evaluation of this mineral-rich inlier.

NEW RESULTS A major fault, namely the Roos Mine Thrust, was discovered along the western flank of the culmination. It thrusts the Mitakoodi Culmination over the Overhang Jaspilite - Corella Formation. The hanging-wall consists of downward facing Mitakoodi Quartzite that dips to the east at 40°-60°. By contrast, the Overhang Jaspilite Corella Formation in the foot-wall is generally upright. This thrust extends downwards about 10 km, and was clearly imaged by the AGSO-AGCRC's seismic transect. The Roos Mine Thrust was traced to the north and the south over 50 km. The direction of the movement on this thrust is probably from the SSE towards the NNW. The footwall of the Roos Mine Thrust consists of the Overhang Jaspilite and Corella Formation. Five generations of structures were identified in these rocks. The Roos Mine Thrust is part of the D2 structures, and was affected by the north-striking folds (D3 & 4) and strike-slip faults (D4 & 5).

The Pilgrim Fault Zone consists of a series of anastomosing faults, and displays a great linear signature both on the geological and geophysical images. This mapping program revealed: (1) there is no significant difference in stratigraphic sequences and structural styles across this fault zone; (2) the fault zone truncates regional predominant structural grains such as the N-striking folds (D3); and (3) the fault zone is filled up with thermal breccias similar to those within other strike-slip fault zones.

CONCLUDING REMARKS There are differences in deformation and metamorphism styles across the Mount Isa Inlier; the Pilgrim Fault Zone was previously considered as the boundary separating different deformation/metamorphism terrains.

However, the new data from this mapping suggest that the Pilgrim Fault Zone is unlikely to be an early terrain boundary, and it is probably part of the wrench system composed of major strike-slip faults such as the Cloncurry Fault Zone and the Fountain Range Fault. The wrench system may have developed late-post Isan Orogeny. The boundary, if there is one, between the Eastern Successions and the rest of the inlier should be located further to the west.

The Roos Mine Thrust is the most significant structural boundary separating the Mitakoodi Culmination from the Wonga Belt to the west. The Roos Mine Thrust is probably part of a major thrust system. The major sole fault of this thrust system at present may be at 10 km deep. The thrust system has great influence on the development of the structural framework of the Mitakoodi Culmination, and it is the key to understanding the structural relations between the Mitakoodi Culmination and the Tommy Creek Block to the north.

The rocks in the Eastern Successions display a complex deformation history. The strain field may have changed from shortening to wrenching during a progressive orogenic event, which resulted in the formation of major thrusts at the early stages and strike-slip faults during late stages respectively. The strike-slip faults have complicated the structural geometry of the early thrust system, but the structural relations revealed in the Roos Mine region may be a snapshot of the tectonic scenario of the Mt Isa Inlier as a whole.

Acknowledgment This project is jointly sponsored by the AGCRC, BHP Minerals, CRA Exploration Pty Ltd, MIM Exploration Pty Ltd, North Limited, Placer Exploration Ltd and Western Mining Corporation Ltd.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 AESIS COMES OF AGE Geoffrey R Hudson Australian Mineral Foundation 63 Conyngham St, Glenside, SA 5065

AESIS, the national minerals and petroleum bibliographic database achieves its majority this year and more than ever is satisfying the objectives set by its founders of garnering and organising industry documented information into a central, accessable system. AESIS, which now contains in excess of 130,000 indexed references on all aspects of these resource sectors, has played a significant role in maintaining awareness of new information and the systematic management of a legacy of minerals and petroleum documentation.

CONTENT The subject coverage of AESIS spans the wide range of topics of interest to the minerals and petroleum industries and although the core areas of geoscience, minerals and mining engineering and petroleum engineering are a major strength, that is only part of the story. Environment, policy and legislation, occupational health and safety and a multitude of other industry-related topics are also included to make it the largest and most comprehensive database of Australian minerals and petroleum bibliographic information.

The document types included in the database span a broad range from books, journal articles, conference proceedings, maps and other published works to open file and Departmental Reports and Records, and university theses. All citations are assigned a Broad Subject Category Code, and are indexed with subject, and a range of other descriptors including mine/well, tectonic unit, basin name, stratigraphic name, map sheet reference etc. as appropriate. To ensure consistency in indexing, that is essential to reliable, rapid searching, AMF has developed the Australian Thesaurus of Earth Sciences and Related Terms, that has been adopted as the industry standard.

AESIS coverage is comprehensive from 1965 for open file reports, and from 1975 for the published literature, but pre-1975 publications from selected sources continue to be added.

ACCESS Access to AESIS has responded to the developments in information technology going on-line in 1980 and since 1993 has been available on CD-ROM as part of the GEOPAC suite of databases. The change to CD-ROM has heralded a new era of accessibility to the database, free of the concerns and constraints of on-line systems and putting it comfortably within reach of not only information professionals but the end users of the data it contains.

In addition to searches on-line and on the CD-ROM, current awareness of new entries is achieved through two hard-copy publications. AMF Reviews, a monthly publication covers both Australian and international published material and book reviews, while AESIS Quarterly is a comprehensive listing of Australian material, both published and open file that has been added to the database during the previous quarter. Thematic bibliographies can also be readily derived from AESIS, and recent examples include a two volume set Mining and the Environment produced on behalf of the Australian Minerals and Energy Environment Foundation and a volume produced in association with the New Generation Gold Deposits Conference in late 1995 entitled New Generation Gold Exploration - 1990 to 1995.

THE FUTURE The establishment, growth and ongoing development of AESIS is a major achievement and an outstanding example of co-operation between the AMF member companies, who provide the bulk of the funding and the Australian Geological Survey Organisation and State Departments of Mines and Energy, who provide data and funding.

In its first 21 years, AESIS has grown to be an unrivalled source of Australian minerals and petroleum information. The future of AESIS lies in maintaining comprehensive and timely coverage of documented information and increased accessibility through the application of the latest "common usage" information technology. A Windows version of the CD is in the pipeline and access via the Internet is perhaps the next delivery mechanism for a range of products including AMF Reviews and AESIS Quarterly.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

OPTIMIZING RESOURCE ANALYSIS OF STRATIFIED SEQUENCES WITH 3D MODELING, GIS, REMOTE SENSING AND 3D VISUALIZATION J.D. Hughes Geological Survey of Canada, 3303-33 St. N.W., Calgary, Alberta, Canada, T2L 2A7

Stratified sequences are host to many economically important resources, including coal, conventional and nonconventional hydrocarbons, minerals and potable water. The exploration and development of these resources over the past century has provided a rich legacy of geological data representing an investment of many hundreds of millions of dollars. The widespread availability of imagery from sophisticated airborneand satellite-based sensors over the past two decades has provided a powerful new data source for geological analysis. The evolution of computer-based tools for the integration of these data into comprehensive spatial models has provided unprecedented capabilities for optimizing resource extraction and management, understanding geological phenomena controlling resource distribution and quality, and optimizing the exploration for undiscovered resources.

In order to optimize the management, utilization and development of Canada's coal resources and to determine supplies available under various price and end use scenarios, digital databases containing geological and analytical information for coalfields across the country have been developed. These databases now contain informationfrommore than 50,000 boreholes, 20,000 outcrops, 40,000 analyses, and digital data from seismic surveys and mines. Concurrent with the construction of these databases, a suite of computer-based tools for the creation of 3D models suitable for geological and resource analysis has been developed. These include tools for data management and verification; an expert system incorporating geological rules for depositional pinchouts, unconformities, layer merging and faulting common to stratified sequences; programs to analyze economic, coal quality and environmental characteristics of the completed models; 3D visualization systems; and interfaces to third-party Geographic Information (GIS), Computer Aided Design (CAD) and Image Analysis Systems (IAS) to facilitate data transfer to external users, provide additional analytical capabilities, and allow the integration of remotely-sensed datasets including Landsat Thematic Mapper, SPOT, ERS-1 radar and Radarsat. These tools have been applied to the analysis of other stratified sequences including conventional hydrocarbons, coalbed methane and aquifers containing potable water.

Data capture is the single most labour intensive and critical aspect of 3D model development. Relational database technology with data dictionaries and automated population of all possible fields is utilized to minimize operator keystrokes and maximize data integrity. In low- or moderately-deformed sequences, 3D interpolation between data points to the nodes of a regular grid is completely automated utilizing an expert system which simultaneously analyses all layers applying rules controlling layer crossing, depositional pinchouts, and truncation at faults and unconformities, so that the resulting model does not violate any of the primary data. Automated displays generated by the expert system flag possible errors in geological interpretation which must be corrected in the database. In highly deformed sequences a CAD system is utilized as a 3D editor to constrain interpolation based on the geologists' interpretation. Completed models form input to economic analysis modules that define resource quantity and quality available given particular extraction scenarios and economic constraints, and produce maps defining the spatial distribution of resources and critical economic parameters. Completed models also allow the display of lithology distribution related to specific depositional environments to facilitate an understanding of deposit genesis, and provide predictions of borehole intersections at any point. 3D visualization systems provide integrated views of several hundred spatial layers and allow the geologist to interrogate data, define cutting planes and generate new models (eg. datum corrected) in near real time. Datasets generated by the modeling system are interfaced to a GIS, which provides an industry-standard platform for dissemination of data to other users, and many additional analytical and display capabilities. The GIS also provides an integration point for remotely sensed imagery from an image analysis system. The continuing evolution of low-cost hardware and software will make these capabilities accessible to a much broader base of users, and heralds a new era in the way nonrenewable resources are explored for, developed and managed.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

VICTORIAN GOLD: BE: Large gold deposits and granites Martin J Hughes1 and G. Neil Phillips2 and Linda Gregory3 1 1034 Geelong Rd, Mt Clear, 3503, Victoria, Australia Great Central Mines N.L., c/- 1 Coppin St, East Malvern, 3145, Victoria, Australia 3 Geological Survey of Victoria, PO Box 2145, MDC Fitzroy, 3065, Victoria, Australia 1

Over 20 percent of the Lachlan fold belt comprises granites, and this high proportion appears valid for much of the Victorian gold province, including the richly gold-endowed Central Victorian Magmatic Province. These granites include I- and S-,fractionated-and unfractionated-, and mafic- and felsic-types (Chappell et al., 1991).

However, despite their regional abundance granites are extremely poorly represented as host rocks for Victorian gold (Phillips and Hughes, 1995). None of the twenty largest gold producers which account for 75 percent of all gold production are within granite, and of the remaining ca 7000 areas of recorded mineralization only a small proportion are in granite and these are strongly concentrated near granite margins, and typically small.-The largest goldfield hosted by granite is Glen Wills (61 Au) which comprises gold mineralization hosted by a shear zone which cuts a two-mica S-type granite, but is mostly within its aureole. Two of the largest 20 deposits are, in part, hosted within the contact aureole surrounding granite plutons, these being the Wonga mineralization, a subordinate part of the Stawell goldfield, and most of the Maldon goldfield, 130 km NW of Melbourne.

The Maldon goldfield is the 4th largest gold producer in Victoria in terms of its hard-rock production of 58 t Au, almost all of which has comefromN-S steeply-dipping auriferous quartz veins. The field was characterized by high gold grades (average 33 g/t) and repetitive structures (e.g. dilational jogs cutting fold axes) that were mined to 600 m depth. The N-S mineralized structures at Maldon are essentially perpendicular to the main granite contact. They transgress the metamoiphic zones of the contact aureole, including a 2.5 km wide cordierite zone. At least one mineralized zone can be shown to abruptly terminate against the granite in an area of reasonable outcrop where the granite is massive and not affected by the gold-related alteration. This is consistent with old mine records which indicate that granitoid dykes postdate the mineralized quartz veins. The quartz within the mineralized veins varies texturally with distancefromthe granite contact (Gregory, 1994). Primary depositional textures can be observed outside the aureole but become less abundant in the biotite and cordierite zones, concomitant with recovery and primary recrystallization textures (polygonization, decrease in average grain size). Secondary recrystallization (exaggerated grain growth) textures appear in the innermost K-feldspar zone, close to the granite contact, where there is a sympathetic increase in grain size.

Complementing this variation in the quartz, the As-S mineralogy is dominated by arsenopyrite away from the granite, and loellingite plus pyrrhotite in the K-feldspar zone within 1.5 km of the granite. Rock alteration selvages are dominated by muscovite, ankerite and sulphides awayfromthe granite, and biotite selvages proximal to the granite. All these features are compatible with, and strongly suggest, that the gold mineralization event pre-dated the Upper Devonian granite intrusion. Similar relative timing can be demonstrated for quartz veins and Early and Late Devonian granites in the Mt Moliagul, Rheola, Tamagulla, Chiltem and Hillsborough areas, and the same relationship is suspected at Wonga.

It is concluded that granites are not the source of the fluids nor the gold that has made the Victorian gold province so productive; furthermore the granites do not appear to have played a critical role in gold genesis. There is evidence that they provided local enrichments of Bi (e.g. resulting in the mineral maldonite, Ai^Bi, at Maldon), Mo and W to the preexisting gold mineralization over distances of a kilometre at most. Granite intrusion was common immediately prior and subsequent to the gold mineralizing event in Victoria, but was a response to the same thermal event that resulted in gold, rather than implying an essential genetic link between gold and granites.

REFERENCES Chappell, B.W., English, P.M., King, P.L., White, A.J.R., & Wybom, D., 1991. Granites and related rocks of the Lachlan fold belt (1:1 250 000 scale map) Bureau of Mineral Resources, Geology and Geophysics, Canberra, Australia.

Gregory, L.M., 1994. Contact metamorphism and quartz vein textures at Maldon, Victoria: implications for timing of gold mineralisation. Unpubl. BSc Hons thesis, Univ. Ballarat.

Phillips, G.N., & Hughes, M.J., 1995. Victorian gold: a sleeping giant. Society of Economic Geologists Newsletter, 21, 1,9-13'

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE EXE RTVER Sn-Pb-Zn-(Cu) PROSPECT, NW TASMANIA - PHYSICO-CHEMICAL CONTROLS ON ORE DEPOSITION Peta Hughes, David R. Cooke. Paul A. Kitto and Ross R. Large Codes Key Centre, University of Tasmania, GPO Box 252C, Hobart, 7001

INTRODUCTION The Exe River prospect occurs two kilometres east of the Renison Sn mine in western Tasmania, and is part of the North Dundas mineral field (NDMF). Vein mineralisation is hosted by Cambrian shallow marine carbonaceous and calcareous turbiditic sedimentary rocks of the Crimson Creek Formation. Graphitic and pyritic black shales are characteristic, with interbedded lithic sandstone and siltstones, and minor carbonates.

Microgabbros and tholeiite dykes and sills are present within the lower Crimson Creek Formation. The Late Devonian Pine Hill Granite underlies the Exe River prospect, and is the likely source for ore-forming fluids in the NDMF. The sedimentary rocks at Exe River were metamorphosed to a tremolite-actinolite-sericite assemblage during granite emplacement.

At Renison and elsewhere in the NDMF, individual vein systems contain early Sn mineralisation that is crosscut by later base metal mineralisation. Exe River is unique in that Sn mineralisation is confined to one NNWtrending vein (the Fentons, or Tin Vein), and Pb-Zn mineralisation occurs in a second, subparallel vein system (Salmons Vein). The two veins are separated by a third, poorly-developed Cu-vein. The Fentons and Salmons veins extend at least 1 km along strike, are spatially separated by less than 50m, and have average thicknesses of 2-3m.

MINERALISATION The Fentons Vein contains abundant anhedral to subhedral quartz grains, and lesser coarse, euhedral arsenopyrite (Stage 1). Fine grained cassiterite is intimately associated with tourmaline and arsenopyrite, and sulfide contents are low (< 3%). Stage 2 consists of early pyrrhotite replaced by later Cu mineralisation. The Salmons vein contains coarse grained sphalerite, galena and minor chalcopyrite, pyrite, tetrahedrite-tennantite and bornite hosted in euhedral quartz-carbonate gangue (Stages 3-4). Sulfides are more abundant than for the Fentons Vein (typically > 10%). The Salmons Vein contains abundant open space fill textures (crustiform and colloform banding, vugs, etc.) that are consistent with a shallow crustal setting for mineralisation. Wallrock alteration is poorly developed around both vein systems, and is characterised by pervasive silicification.

FLUID INCLUSIONS To date, primary fluid inclusion data has only been obtained from stage 3 and 4 vein material at Exe River.

Quartz-arsenopyrite-cassiterite mineralisation in Fentons Vein probably formed from 300-400°C saline (5-10 eq. wt. % NaCl) magmatic-hydrothermal fluids similar to the Renison deposit. Stage 3 and 4 base metal mineralisation in the Salmons Vein formed at temperatures between 143 and 177°C from 1 to 5 eq. wt % NaCl fluids; muscovite and carbonate daughter minerals have been identified in some fluid inclusions.

Laser Raman spectroscopy has been used to identify non-condensable gases in vapour bubbles at Exe River. In addition to H 0, gas bubbles of primary fluid inclusions hosted by stage 3 quartz contain detectable CH and N (3:1 ratio). With time, the mineralising fluids became less reduced, with late Stage 4 fluid inclusions containing abundant C0 , carbonate daughter minerals and low amounts of CH4.

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CONCLUSIONS The abundance of methane and presence of muscovite is consistent with mineralisation forming from reduced, acidic, saline magmatic-hydrothermal fluids released from the crystallising Pine Hill Granite. The physical setting of the adjacent veins sets can be explained by the existence of parallel fault sets, only one of which (Fentons Vein) formed during the early stages of granite emplacement, when high-T Sn-rich magmatichydrothermal fluids were released. Salmons Vein opened during later britde deformation events, when late stage base metal-rich fluids were tapped from the granite.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 A RE-EXAMINATION OF 8 l s O AND 5D VARIATIONS AT THE KIDD CREEK VHMS DEPOSIT, CANADA: GENESIS OF ORE FORMING FLUIDS

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David L. Huston* and Bruce E. Taylor Geological Survey of Canada, 601 Booth St., Ottawa, Ontario K1A 0E8, CANADA Present address: Australian Geological Organization, G.P.O. Box 378, Canberra, ACT 2601

Since publication of preliminary of oxygen isotope results (Beaty and Taylor, 1982), the Kidd Creek deposit has been considered unusual for volcanic-hosted massive sulphide (VHMS) deposits. Initial results suggested that the proximal alteration zone had higher 8 1 8 0 values than the surrounding less altered rocks, a relationship opposite to other VHMS deposits. Later studies (Beaty et al., 1988) suggested that this initial result was incorrect and that the proximal alteration zone in fact had lower 5 1 8 0 values than the surrounding rocks. Beaty et al. (1988) argued that the Kidd Creek ore fluid had high 6 1 8 0 values of 6-9%o and was generated by evaporation in a closed basin.

This study has been undertaken as a follow up of Beaty et al.'s results to document whole rock S 1 8 0 and 6D variations about the orebody, to re-assess ore fluid 8 1 8 0 and 5D values, and to determine the origin of fluids.

RESULTS OF WHOLE ROCK ANALYSES 8 1 8 0 mapping in rhyolitic rocks around the Kidd Creek deposit indicates that the orebodies are associated with a zone of relatively low (<12%o) 8 1 8 0 values that extends up to 200 m laterally from ore and that broadly corresponds to a zone of low Na20 values. A carapace of higher (>13%o) values occurs 300-500 m statigraphically above, and extends 1.5 km from ore. A zone of lower 8 1 8 0 values cuts the carapace above the uppermost orebody. Higher 8 ^ 0 zones also occur in massive rhyolite bodies footwall to the orebodies.

Oxygen isotope variations record three hydrothermal events: early, low temperature (<200°C) silicification that produced high 8 1 8 0 rocks (e.g. massive rhyolite bodies), a high temperature (>300°C) ore-forming event that produced ore-related low 8 1 8 0 rocks, and a low temperature, post-ore event that produced the high 8 1 8 0 carapace.

RESULTS OF MINERAL SEPARATE ANALYSES With one exception, chlorite associated with chalcopyrite stringers has 8 1 8 0 values of 2.7 to 4.1 %o and 8D values of -47 to -41 %o (n = 4). If these values record ore-related high temperature (300-350°C) alteration, the ore fluid had a 8 1 8 0 value of 3.8 ± 0.5%o and a 8D value of -8 ± 5%o. By contrast, chlorite associated with metamorphic quartz veins has 8 1 8 0 values of 5.7 to 5.8%o and 8D values of -59 to -45 (n = 3). Quartz-chlorite 8 1 8 0 pairs indicate metamorphic temperatures of 360-400°C, which are consistent with the incipient presence of biotite. The metamorphic fluids had S 1 8 0 values of 6.7 to 7.0%c, and 8D values of -17 ± 8%o.

THE ORIGIN OF KIDD CREEK ORE FORMING FLUIDS Although this study suggests lower ore fluid 8 1 8 0 values than that of Beaty et al. (1988), 8 l 8 0 values of 4%o still require special processes to generate the Kidd Creek hydrothermal fluids. As suggested by Beaty et al.

(1988), these processes could include: (1) seawater evaporation in a closed basin, (2) isotopic exchange of the fluid with 18 0-rich country rocks, (3) boiling of the fluid, (4) incorporation of metamorphic water, and (5) incorporation of magmatic water. Evaporation and boiling can be exluded as these processes can also increase 8D values, which is inconsistent with the inferred ore fluid 8D values. Incorporation of metamorphic fluid is unlikely given the seafloor geologic setting of VHMS deposit. The most likely origin of the Kidd Creek ore forming fluids is exchange with 180-rich country rocks and/or incorporation of up to 20% magmatic water.

REFERENCES Beaty, D.W., & Taylor, H.P., Jr., 1982. Some petrogenetic and oxygen isotope relationships in the Amulet mine, Noranda, Quebec, and their bearing on the origin of Archean massive sulfide deposits. Economic Geology 77, 95-108.

Beaty, D.W., Taylor, H.P., Jr., & Coad, P.R., 1988. An oxygen isotope study of the Kidd Creek, Ontario, volcanogenic massive sulfide deposit: evidence for a high 1 8 0 ore fluid. Economic Geology 83, 1-17.

Acknowledgments: This study is part of a cooperative research agreement between Falconbridge Limited and the Geological Survey of Canada, and has been supported by the Minerals Program of the joint federal-provincial Northern Ontario Development Agreement (NODA).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 A CORELATION BETWEEN ZINC PRODUCTIVITY AND QUARTZ PHENOCRYST S 0 VALUES IN COGENETIC FELSIC ROCKS IN VHMS DISTRICTS: DOCUMENTATION, GENETIC SIGNIFICANCE AND EXPLORATION APPLICATION 1 8

David L. Huston . Bruce E. Taylor, Don H. Watanabe and Wouter Bleckcr Geological Survey of Canada, 601 Booth Sl, Ottawa, Ontario K1A 0E8, CANADA Present address: Australian Geological Survey Organization, G.P.O. Box 378, Canberra, ACT 2601 1

Figure 1 illustrates a correlation between zinc productivity of volcanic-hosted massive sulphide (VHMS) deposits and 6 ^ 0 values of quartz phenocrysts in cogenetic rhyolitic rocks. In contrast, no correlation exists between copper productivity and quartz phenocryst 8 0 data. Phenocryst-magma exchange determines initial quartz phenocryst 5 0 values. This value can shift during alteration and metamorphic recrystallization. These three processes may contribute to, or affect, correlation between quartz phenocryst 5 0 values and zinc productivity.

1 8

1 8

1 8

13

r

Figure 1. Diagram showing correlation between 5 ^ 0 ' o f quartz phenocrysts in least altered, cogenetic rhyolites and zinc productivty of VHMS districts. The correlation coefficient was calculated from median values (indicated by arrows). The diagram includes data from the Kidd Creek (KiC), Mattagami (M), Noranda (N), Jerome (J), Mt Windsor (MW), Bathurst (B), Buttle Lake (BL), West Shasta (WS), and Iberian Pyrite Belt (IPB) districts. Data are from the literature and from analyses at the Geological Survey of Canada.

° Zn (Mt) HYPOTHESIS I: METAMORPHIC RECRYSTALLIZATION 1 0 1

1

2

Extensive recrystallization during metamorphism may reset the S 0 values of quartz phenocrysts. However, most phenocryst 5 0 values (Fig. 1) are lower than the whole rock values, indicating re-equilibration of feldspar, but not quartz. Recrystallization is not likely to be a process affecting the correlation.

HYPOTHESIS II: HYDROTHERMAL ALTERATION AND RECRYSTALLIZATION Quartz phenocryst in silicified samples commonly show overgowths, and Si02 contents correlate with quartz phenocryst 5 0 values in the Mt Windsor district. These data suggest that alteration, particularly silicification or desilication, could affect the correlation. To minimize the effect of alteration, phenocryst analyses used to construct Figure 1 came from samples with volatile-free Si02 contents between 70 and 78.5% and Na20 contents mainly above 1%. This suggests that if hydrothermal alteration causes the correlation, the alteration is "cryptic" in that it is not be easily recognizable using major or trace element concentrations.

HYPOTHESIS III: DEPTH OF MAGMA INTRUSION 1 8

1 8

1 8

Magma 8 O values, as inferred from quartz phenocryst S 0 values, indicate magma provenance. High values indicate magma derivation by partial melting of water-rich, high 6 ^ 0 sedimentary rocks or hydrothermally altered basaltic crust. Lower 5 0 values indicate derivation from water-poor, igneous sources. Magma water contents influence the depth of magma intrusion: water-rich, high 8 0 magmas tend to crystallize at greater depths than water-poor magmas. Deeper intrusion of magma could produce larger covection cells, and, hence, larger deposits, leading to the observed correlation.

EXPLORATION IMPLICATIONS lg

1 8

1 S

1 8

Irrespective of the origin of the correlation between quartz phenocryst 8 ^ 0 and zinc productivity, it can be used as a tool in area selection during exploration. For an underexplored district, quartz phenocryst 6 0 data can be used as one guide in assessing the potential for VHMS deposits.

18

Acknowledgments: This study is part of a cooperative research agreement between Falconbridge Limited and the Geological Survey of Canada, and has been supported by the Minerals Program of the joint federal-provincial Northern Ontario Development Agreement (NODA).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ORIGIN OF THE TRIASSIC COAL-BEARING SEQUENCE, LEIGH CREEK, SOUTH AUSTRALIA 1

Adrian C. Hutton1, Andrew J. Mandile2 and Mohinudeen M. Faiz3 University of Wollongong, Northfields Avenue, Wollongong, NSW 2522 2 Minerals Processing Research (Mineral Services), Mount Isa Mines, Mt Isa, QLD, 4825 3 CSIRO Petroleum-APCRC, PO Box 136, North Ryde, NSW 2113

The Triassic Leigh Creek coals were deposited in the Copley Basin (Lobe A), Telford Basin (Lobe B) and two contiguous northern subbasins (Lobes C and D). The coal sequences dip steeply at 10 to 50® towards the centre of the basins. Three coaly units occur, the Lower Series comprising numerous thin seams that have been divided into two groups, the Main Series containing the Q seam which is 14 to 20 m thick and the Upper Series which contains 7 coaly units which are 0.5 to 5 m thick. The coals have an in situ moisture of 27 to 37% and ash yields ranging from 14 to 24%. Pyrite contents are generally less than 1%.

Mining necessitates removal of the overburden and interburden which, historically, has either been stockpiled or used as backfill. The interburden comprises claystone and carbonaceous claystonemudstone, with variable organic contents, and hard lenses and thin layers of sideritic claystone. The coals have been exploited for electricity generation since 1944 and more recently some of the interburden with a high organic content has been considered as a potential oil shale.

The depositional environments of coals seams are interpreted either from the petrographic composition of the coals or the nature of the enclosing clastic units. However, few studies have used the organic composition and the mineral composition of the coal to interpret environments of deposition. Reasons for this are varied but one of the more important reasons is the lack of quantitative mineralogy. Until recently the mineralogy of coals was calculated from ash yields after combustion or determined by X-ray diffraction (XRD) techniques after the organic matter has been removed by oxidation or low temperature physical disaggregation techniques. In both cases, the techniques are time consuming and the potential for modification of mineral structure exists.

Quantitative XRD techniques are now refined and both the mineralogy and organic content of an organicrich rock can be determined from a single analysis. Given that the Leigh Creek deposit contains a range of rock types, it is an excellent sequence for a pilot study using quantitative XRD analysis.

Samples were selected from drill hole 8210C which intersected two of the coaly intervals in Lobe B. To obtain quantitative mineralogical data, samples were ground in a McCrone micronising mill and the powders routinely scanned. The organic content of each sample was determined from the XRD trace by the method given in Mandile and Hutton (1995) and these data were input into the SIROQUANT computer program which uses the Rietvelt method to determine the percentages of each mineral.

Reflected white light and fluorescence mode microscopy was used to determine the maceral composition of the samples.

The coals are dominated by vitrinite with up to 10% liptinite, mostly sporinite, and minor inertinite. The mineralogy is dominantly kaolinite and quartz with localised siderite. The organic matter in the interburden comprises mostly inertinite with vitrinite and liptinite (mostly sporinite and minor alginite derived from Botryococcus). Hie mineralogy is predominantly quartz, kaolinite and illite with siderite abundant in the sideritic lenses and layers.

From the petrographic and mineralogical data, it is inferred that the Triassic Leigh Creek sequence was deposited in an environment that alternated between peat mires (coal) and relatively shallow lakes, continually supplied with clastic detritus.

REFERENCE Mandile, A J . and Hutton, A.C. 1995. Quantitative X-ray diffraction analysis of mineral and organic phases in organic-rich rocks. International Journal of Coal Geology, (in press).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GRENVILLIAN AGE MAGMATIC AND METAMORPHIC EVENT IN THE CAPE RIVER AREA, NORTH QUEENSLAND: SIGNIFICANCE FOR LATE MESOPROTEROZOIC C O N T I N E N T A L RECONSTRUCTIONS Laurie Hutton *, C. Mark Fanning^ & Paul Garrad* * Geological Survey of Queensland, GPO Box 194, Brisbane, Qld 4001. ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200.

INTRODUCTION The SWEAT hypothesis places Laurentia off the east coast of Australia and Antarctica during the late Precambrian. It proposes that an all encompassing Grenville age orogen extends from the type area in Canada, encircling the East Antarctic Shield, before passing through the Albany Fraser belt, central Australia and finally somewhere through eastern Australia towards the western side of the Wopmay orogen, back in Laurentia. Whilst inherited zircon components in granites of the Lachlan Fold Belt and age distributions of detrital zircon for Late Precambrian to Silurian sediments in Australia record a significant proportion of 1000-1300 Ma material, outcrop of Grenville age rocks has not previously been documented in eastern Australia.

THE CAPE RIVER M E T A M O R P H I C S The Cape River Metamorphics are a sequence of quartzite, biotite schist, amphibolite and calc-silicate rocks in the Pentland area of north Queensland, about 200 km southwest of Townsville. The rocks were previously thought to be either Late Precambrian, or Cambrian to Ordovician in age. The Cape River Metamorphics range from lower to upper amphibolite grade. Recent mapping has recognised a gradation from medium to high grade metamorphics, through a zone of partial anatexis, to a gneissic, syntectonic, S-Type granite, locally intruded by hornblende bearing I-Type granites. Biotite schist crops out through much of the area, passing into cordierite sillimanite schist and garnet diopside granofels close to zones of partial anatexis. Calc-silicate and amphibolite enclaves, which are common to the S-Type granites, are probably serived from the adjacent Cape River Metamorphics.

GRANITOIDS Two types of granitoid intrude the Cape River Metamorphics; an S-Type cordierite biotite granodiorite derived from partial melting of the Cape River Metamorphics, and an I-Type hornblende-biotite melanocratic granite.

Both granites are thought to be syntectonic. However, the hornblende-biotite granite crosscuts the cleaved cordierite-biotite granodiorite, with evidence for a protracted series of deformation and intrusive events. In places the two granite types are technically "mixed" giving rise to complex lithologies.

ZIRCON DATING SHRIMP U-Pb zircon analyses have been made for a hornblende-biotite granite, a sample of the "mixed" rock noted above. Not surprisingly the zircons form a complex population, comprising a more dominant group of simple euhedral elongate grains that are clear and structurally simple. These are interpreted to have a magmatic origin. Also present are more equant, multifaceted grains that are interpreted as having formed during a high grade metamorphism. The SHRIMP U-Pb analyses form two near to concordant clusters, one at 1105 ± 15 Ma and another at 1238 ± 38 Ma. The euhedral, magmatic grains tend to give ages of about 1105 Ma and we interpret this to be the crystallisation age of the hornblende-biotite granite. The metamorphic zircon morphologies are often slightly older and we believe that these are inherited from the late Mesoproterozoic source of the granite, a metamorphic basement formed during the initial stages of a complex Grenville age event. There is no evidence for a Palaeozoic zircon component in this rock.

SIGNIFICANCE FOR CONTINENTAL

RECONSTRUCTIONS

The discovery of Grenville age granite in eastern Australia has important implications for the SWEAT hypothesis. Previously, rocks of this age were known from the Musgrave Block in central Australia and the Albany-Fraser belt in Western Australia. The present data extends the inferred Grenville belt some 800-1000 km to the east and now makes it possible to assess in detail Rodinian reconstructions. The new data agrees well with the original SWEAT hypothesis, though passes the problem of identifying Grenville age material back into Laurentia, west of the Wopmay orogen. The U-Pb results for the granites from the Cape River metamorphics provide a new source for the many detrital and inherited zircon grains of this age in sediments and granites of the Lachlan Fold Belt.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PROTEROZOIC BASEMENT TO THE LOLWORTH RAVENSWOOD PROVINCE, NORTH QUEENSLAND: EVIDENCE FROM RADIOGENIC ISOTOPE SYSTEMATICS FOR THE REEDY SPRINGS, LOLWORTH AND RAVENSWOOD BATHOLITHS Laurie Hutton*. Ian Rienks , C. Mark Fanning & David Gust^ ^Geological Survey of Queensland, GPO Box 194, Brisbane, Qld 4001. ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200. ^School of Geology, Queensland University of Technology, Brisbane, Qld 4000.

1

2

INTRODUCTION An extensive and diverse period of igneous activity is recorded in the Middle Silurian to Early Devonian of north Queensland. Granites are abundant in the Lolworth-Ravenswood Province south and south-west of Townsville, with three distinct batholiths recognised in this province; namely the Reedy Springs Batholith in the west, the Lolworth Batholith in the centre and the Ravenswood Batholith in the east. A comparison of the radiogenic isotope characteristics for these three batholiths gives an insight into their source(s) and by inference the basement underlying the Lolworth-Ravenswood Province.

GRANITE TYPES Silurian to Devonian diorite to granite occur in the Ravenswood Batholith. Previous work suggested that these granites were derived from partial melting of basic intrusives at mid crustal depths. The Lolworth Batholith comprises felsic two-mica granite intruded by abundant garnet leucogranite dykes and sheets. These are also interpreted as resulting from crustal anatexis, with probably mixed I- and S-type sources. The Reedy Springs Batholith is seen to comprise both metaluminous and peraluminous granites.

ISOTOPIC RESULTS SHRIMP U-Pb zircon and Rb-Sr total rock-mica ages for the granites range from 425-382 Ma for the Ravenswood Batholith, 414-382 Ma for the Lolworth Batholith, and 400-410 Ma for the Reedy Springs Batholith. The latter have zircon populations that are dominated by inherited zircon components which yield ages of 1500-1550 Ma with a minor component at about 2000 Ma. Initial Nd isotope ratios (sNd(t))> calculated at the magmatic SHRIMP U-Pb zircon crystallisation ages for the Ravenswood Batholith, range from -0.3 to -1.9, that is within uncertainty of, or slightly more enriched than the chondritic uniform reservoir (CHUR).

Depleted Mantle model ages (TDM) vary from about 1010-1020 Ma, whilst the corresponding initial Sr/ Sr ratios are 0.7039-0.7051. In the Lolworth Batholith, SNd(t) is significantly more enriched and varies from -7.7 to -10.8. Adjacent to the Lolworth Batholith, mixed I and S type Late Mesoproterozoic granites have SNd(t) of -3.4. However, at 400 Ma (our magmatic age for crystallisation of the Lolworth Batholith) SNd(t) is calculated to be -10.6, similar to the more enriched values calculated for the Lolworth Batholith granites. Two-mica granites from the Reedy Springs Batholith have SNd(t) between -14.0 and -15.3, and, together with U-Pb zircon inheritance ages noted above, document the presence of a significant proportion of Proterozoic crust in the source of these magmas.

87

86

SIGNIFICANCE FOR BASEMENT RECONSTRUCTIONS The results for the Reedy Springs Batholith are similar to those previously reported for the Georgetown Province to the north, across the Broken River Province. In Georgetown, U-Pb zircon studies constrain magmatic events at 1500-1550 Ma and -420 Ma, with inheritance of 2200-2000 Ma material. In the Reedy Springs and Lolworth Batholiths, magmatic ages are 382-414 Ma, with inherited zircons at about 1500-1553 Ma and 2000-2150 Ma. Whilst the Late Silurian to Early Devonian granites from Georgetown and Reedy Springs/Lolworth are of a different chemical and mineralogical character, the isotopic signatures for the two Provinces can be used to imply similar protoliths. In the Lolworth Batholith, U-Pb zircon studies record the presence of a "Grenvillian" (about 1100 Ma) magmatic event, not seen in Georgetown. From the 6Nd(t) f° these Late Mesoproterozoic granite, calculated at 400 Ma, it can be proposed that this material is a possible source for granites of the Lolworth Batholith. Similarly, the Sm-Nd data for the Ravenswood Batholith can be used to imply that Grenvillian age material is also a possible source for the Ordovician and Silurian to Devonian granites. The isotopic data thus points to the preponderance of Proterozoic continental crust underlying the western Ravenswood-Lolworth Province, with subsequent reworking during the Tasman Orogen. Some of this Proterozoic crust has now been positively identified as being "Grenvillian" in age. Significantly, the geochemical signatures indicate that the source rocks, and/or intrusive processes are different in each of the three Ravenswood-Lolworth batholiths. r

210

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra; February 1996

PROLONGED RODINIAN LINK BETWEEN NORTH AMERICA AND AUSTRALIA Mart Idnurm1 and John Giddings1 Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2600

In the past five years the focus on global reconstructions has been largely on the Neoproterozoic configuration of Rodinia, especially the North America - Antarctica - Australia link, with less attention to earlier periods. Now evidence from palaeomagnetism of northern Australia gives a glimpse of earlier times. Combined with palaeomagnetic data from North America, this evidence suggests that the Rodinian North America - Australia link lasted veiy much longer than may have been suspected.

The Australian palaeomagnetic data are based on a study of 2700 oriented samples from the McArthur Basin. A stratigraphically controlled sequence of ten primary poles from the southeastern part of the basin defines an apparent polar wander path (APWP) segment of —120° angular length and a distinctive shape. This segment is reinforced by ten overprint poles from the southeastern and northwestern parts of the basin. U-Pb zircon dates at 1713 ± 6 and 1640 ± 7 Ma provide age calibration points within the segment and suggest that it spans ~100 Ma. Comparable palaeomagnetic data to those of Australia are available from North America where twentytwo mainly overprint poles define the time-equivalent APWP segment. Age calibration for this segment comprises two approximate dates: the first at 1700-1650 Ma is from early diagenetic apatites, the second at 1620 ± 20 Ma is a K-Ar cooling age. Most of the North American data are from eastern Canada and Greenland, data from the latter being transferred to the mainland American reference frame using the rotation parameters of Bullard et al.

The Australian and North American APWPs are similar, as shown in Figure 1A where the North American path is rotated to superpose on the Australian path. Both segments have south-pointing loops, and although the Australian loop appears deeper, this could be due to the lack of data at the apex of the North American loop. The same rotation that superposes the two APWPs (117° clockwise rotation about an Euler pole at 100°E, 38°N) transfers the western margin of Proterozoic North America to the Tasman line (Fig. IB) in agreement with the southwest North America - Eastern Antarctica (SWEAT) hypothesis. The reconstruction places North America farther north relative to Australia than the early SWEAT reconstructions (e.g. Moores), but agrees with subsequent reconstructions based on the matching of lineaments, trends of mafic dyke swarms, isotopic mapping of basements, and identification of sediment source regions. The fit suggests that the North Australian Craton and the North American cratonic assemblage had been contiguous already by 1700 Ma.

Figure 1. A. North American APWP (medium shading) superposed by rotation on time-equivalent Australian APWP (light shading). Striations show less well defined path segments. Age estimates on the North American and Australian APWPs are in bold and italics respectively. B. North America after the same rotation that superposes the APWPs, with Australia shown at its present location.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

DEFORMATION HISTORY OF AN AREA IN THE EAST-CENTRAL LACHLAN FOLD BELT, NSW: IMPLICATIONS FOR THE DEFORMATION HISTORY OF THE GILMORE FAULT ZONE Ian Ingpen North Limited, PO Box 57, Melrose Park, SA, 5039

The first deformation (Dj) identified within the study area is largely evident within the Wagga Zone and within or proximal to the Parkes Thrust and the Gilmore Fault Zone in the Girilambone-Bogan Gate Zone. It is constrained to Late Ordovician age rock units and is probably equivalent to the"Benambran Orogeny" of Early Silurian age.

The deformation is characterised by isoclinal to tight, predominantly steeply NW plunging Fj folds in the Wagga Zone and a moderately NW plunging intersection lineation, within the poorly outcropping rocks of the Jingerangle Formation. A possibly axial planar, slaty cleavage (S^ is developed within the F! folds. This foliation corresponds to that in the oldest volcanics and sedimentary rocks within the Gilmore Fault Zone, as well as a stylolitic cleavage evident in the Jingerangle and Cotton Formations of the eastern Girilambone-Bogan Gate Zone. This early deformation may be considered to be a NE-SW compression involving predominantly dextral strike-slip displacement within the Gilmore Fault Zone accompanied by folding within the Wagga Zone and strike-slip movement along the Parkes Thrust with shortening adjacent to this structure.

A fairly pervasive deformation (D2) affected the entire study area including rock units of ages up to Early Devonian. This deformation is manifested within the Gilmore Fault Zone and to the east of the fault zone as predominantly steeply inclined, approximately N to S, gently doubly plunging to sub-horizontal folding at various scales with a very steeply, ENE to ESE dipping, axial planar cleavage. The folding and related cleavage within the eastern margin of the Girilambone-Bogan Gate Zone and in the Parkes-Tumut Zone have both been strongly controlled by the orientation of the Parkes Thrust and the structure possibly representing the northern extension of the Mooney Mooney Fault in the east. This deformation has been interpreted as equating the Siluro-Devonian deformational event, known as the "Bowning Orogeny".

Possibly synchronous with the formation of the axial planar cleavage, the Booberoi Fault, within the Gilmore Fault Zone, developed a strong, steeply dipping foliation containing shear bands; milky quartz veins localised along these shear bands; and sub-vertically elongated pebbles within the conglomeratic unit occupying the fault.

The sub-vertically elongated pebbles indicate that the movement upon the Booberoi Fault was dip-slip with a reverse movement component. Additionally, conjugate dextral and sinistral fault sets within the margin of the Wagga Zone and in the Gilmore Fault Zone have been inferred to have developed in response to movement on the fault zone associated with an approximately latitudinal principal compression, which is probably equivalent to the D2 deformation.

The third deformation (D3) evident within the components of the Gilmore Fault Zone; adjacent to the Parkes Thrust; and throughout the Parkes-Tumut Zone is represented by open to tight, steeply inclined to upright, moderately to steeply, E and N plunging folds and kinks. The N trending folds are possibly oriented in this direction due to their proximity to the Gilmore Fault Zone and the Parkes Thrust, respectively. These regional F3 folds contain a probable axial planar, steeply SSW dipping cleavage identified in areas of slightly higher metamorphic grade of the Wagga Metamorphics. Dextral displacement upon small, latitudinal and steeply dipping kink band boundary structures is probably also affiliated with the D 3 deformation. The D 3 deformation has been interpreted to be a NNE to SSW oriented compression with associated dextral transpression affecting the Gilmore Fault Zone and undefined strike-slip movement upon the Parkes Thrust. The movement upon the Gilmore Fault Zone may be related to similar movements documented on the bounding faults of the Cobar Basin in the late Early Devonian.

A possible fourth deformation (D4) is characterised by tight to open, gently to predominantly steeply inclined, gently SW plunging folds within the undifferentiated Ordovician sedimentary rocks and the Jingerangle Formation to the east of the Gilmore Fault Zone. A probably axial planar, steeply NW dipping crenulation cleavage has been observed in these rocks. This D4 deformation has been interpreted as involving a spatially restricted, NW-SE oriented compression of post Early Devonian age.

The macroscopic, broad, open, upright fold structures within the Late Devonian freshwater successions of the Hervey Group comprise manifestations of the fold belt wide latitudinal compressional deformation (D5), which probably took place during the Early Carboniferous and has been referred to as the "Kanimblan Orogeny".

212

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 MINERALISATION AND METALLOGENIC ZONATION IN THE NORTH DUNDAS MINERAL FIELD, WESTERN TASMANIA Penny Innes, Paul A. Kitto. David R. Cooke and Ross R. Large CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, 7001

INTRODUCTION The North Dundas Mineral Field (NDMF), situated approximately 4 km south of the Renison Mine in western Tasmania, has been a historic producer of Sn, Pb, Zn, Cu and Ag since the late 1800's. Vein mineralisation is hosted by metamorphosed and altered sediments of the Cambrian Dundas Group, a deep water turbidite package of coarse, polymict conglomerates and graded, crystal-rich sandstones, with thick intervening packages of laminated siltstone and black, pyritic and carbonaceous shales. Allochthonous thrust sheets of Cambrian low Ti basalts and gabbros (the Serpentine Hill Complex) are exposed to the north of the NDMF. The ultramafics were technically emplaced within the Dundas Group during the Early Cambrian. The Cambrian rocks have been deformed during the Tabberrabberan Orogeny in the Middle Devonian. The NDMF is underlain by a large Devonian granitic pluton (the Pine Hill Granite) which is exposed to the north-west of the field area. Granite emplacement occurred syn- to post-compressional tectonism in the Late Devonian - Early Carboniferous.

ALTERATION AND VEIN PARAGENESIS A complex network of structurally-controlled veins overlie the granite pluton in the NDMF, with Sn and base metal ore occurring in planar NW- to NNE-trending veins and faults. Detailed paragenetic studies on samples collected from eleven Sn and base metal prospects have identified at least three different regional alteration phases and four hypogene stages of vein mineralisation, all post-dating hornblende-hornfels contact metamorphism. The earliest alteration event noted in the country rock immediate to, and within the granite, is intense greisenisation. This resulted in the development of proximal tin greisens characterised by abundant tourmaline. This was closely followed by, perhaps coeval with, intense regional tourmalisation of the surrounding country rock. Brittle deformation of the tourmalinised sediments resulted in the formation of the Stage 1 vein assemblages. Stage 1 is characterised by low total sulfide concentrations, and a simple mineralogy of euhedral tourmaline (syntaxial growth), quartz, arsenopyrite and minor cassiterite. This stage formed from high temperature (270 - 400°C) saline magmatic-hydrothermal fluids (10.5 - 12.0 eq. wt. % NaCl), and extends up to 700m from the granite contact. Stage 1 veins were hydraulically fractured and infilled by Stage 2A Sn-rich pyrrhotite + tremolite/actinolite. Stage 2A veins cross-cut Stage 1 mineralisation in the fault-controlled lodes.

Stage 2 also occurs as a replacement assemblage in the gabbros and dolerites throughout the region. The pyrrhotite observed in Stage 2A veins is often replaced by chalcopyrite and galena, which are associated with sphalerite, siderite, Ag-rich tetrahedrite and stannite (Stage 2B). Stannite occurs as inclusions within chalcopyrite grains that have replaced pyrrhotite and formed by local remobilisation of Stage 2A Sn mineralisation. Stage 2B was precipitated from lower temperature (180 - 260°C) saline magmatic-hydrothermal fluids (10.0 - 12.0 eq. wt. % NaCl). The Cu and Pb-Zn zones associated with Stage 2B veins extend out from the granite contact to distances of 1000m and 1200m respectively. Stage 3 veins are barren, and consist of quartz and calcite gangue, with pyrite and marcasite. Stage 3 mineralisation is cross-cut by Stage 4 veins, which are characterised by a complex assemblage of Sn-, Sb- and Pb-sulfosalts, including boulangerite, jamesonite, Snrich tetrahedrite and stannite. Primary fluid inclusion material was not obtained from Stages 3 and 4 mineralisation, which extends at least 1400m from the granite contact.

CONCLUSIONS In the North Dundas Mineral Field, the observed metallogenic zonation out from the granite contact is: Proximal As Sn -> Cu (Zn - Ag - Pb) (Pb - Sn - Sb) Distal Sn and base metal mineralisation is genetically related to the emplacement of the Pine Hill Granite, with episodic release of magmatic-hydrothermal fluids during Late Devonian brittle fracturing events. Early contact metamorphism and tourmalinisation sealed up existing porosity (both primary and secondary) proximal to the granite. Episodic brittle fracturing of pre-existing faults was caused either by changes in the volume of the magma chamber on the release of volatile fluids or by hydraulic fracturing due to high fluid pressures. Each period of brittle failure appears to be associated with the deposition of a different mineral assemblage, suggesting that the physico-chemical character of each of the fluids changed with time. Stages 1 and 2A are interpreted to have been deposited by rapidly cooling fluids as they ascended through much cooler country rock.

Subsequent fluids appear to have deposited their respective metals by a combination of decreasing fluid temperatures and water-rock interaction, resulting in the selective replacement of pre-existing assemblages.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THERMAL STATE OF THE LITHOSPHERE BENEATH MONGOLIA AND SOUTHERN BAIKAL AREA: IMPLICATIONS FOR LITHOSPHERIC STRUCTURE AND MANTLE DYNAMICS IN CENTRAL ASIA 1

!

Dmitri.A. Ionov , Suzanne Y. O'Reilly 1 and William L. Griffin 1 ' 2 Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, NSW 2109 2 CSIRO Exploration and Mining, Box 136, North Ryde, NSW 2113

A suite of garnet-spinel lherzolites, garnet websterites and garnet granulites (27 samples) from the ShavarynTsaram eruption centre in the Pliocene-Pleistocene Tariat volcanic field, northern Hangai Mountains, central Mongolia yields pressure (P) and temperature (T) information for the upper mantle and lower crust in the region.

The P-T data obtained using the Ca-in-opx thermometer of Brey & Kohler (1990) and barometer of Nickel & Green (1985) define a geotherm that passes through 12 kbar at 850°C and 20 kbar at 1220°C. T and P estimates for composite xenoliths (garnet pyroxenite veins in garnet-spinel and spinel lherzolites) are consistent for each xenolith and agree with phase transition boundaries for lherzolitic and pyroxenitic systems. T values calculated for 30 spinel lherzolite xenoliths range from 850°C to 1070°C but -75% of these fall into a narrow interval of 895±20°C. Projection of the cut-off temperature for spinel lherzolites (870°C) onto the geotherm (assuming that the lowest T lherzolite represents the uppermost mantle) defines pressure at the crust-mantle boundary of 12-13 kbar indicating a rather high crustal thickness of about 45 km. This value is significantly lower than estimates (50-60 km) obtained by Zorin et al. (1990) from interpretation of gravity data.

Garnet and garnet-spinel lherzolite xenoliths also occur in a Paleocene picritic tuff deposit and in a few PliocenePleistocene basanitic volcanoes on the Vitim Highland some 200 km east of Lake Baikal (Siberia) and 1100 km north east of the Tariat region. Garnet pyroxenites and granulites are very rare or absent in these occurrences. The thermobarometric data obtained for the lherzolite xenoliths show rather narrow P-T arrays (small depth range) and therefore do not define a full geotherm locus. However, these data indicate significant differences between thermal regimes recorded by xenoliths from the Paleocene picritic tuff and the younger basanites. The P-T data for xenoliths from the younger basanites plot on the high-P-T segment of the Tariat geotherm suggesting a similar thermal state of the lithosphere in the corresponding depth range in these two regions in the late Cenozoic.

However, the xenoliths from the older (Paleocene) picritic tuff yield T estimates that are about 100°C lower at the same P suggesting that the lithospheric mantle beneath Vitim has been heated since the Paleocene. The heating may be related to the alkali basaltic volcanism on the Vitim Highland that largely took place in the Miocene.

Large volcanic fields made up of Miocene and less common Pliocene-Pleistocene lavas occur in the Hamar-Daban Range south of Lake Baikal bordering on northern Mongolia and about halfway between the Vitim and Tariat areas. Garnet-bearing xenoliths appropriate for P-T estimates are not available in the area. Spinel peridotite xenoliths from young (1-6 Ma) basaltic rocks in northern Hamar-Daban represent a narrow T range of 980±30°C; most spinel lherzolites from the southern part of Hamar-Daban (Bartoy) also fall into this range but some extend to higher T values (up to 1150°C). If the cut-off T values for spinel lherzolites from Hamar-Daban (~950°C) are projected onto the Tariat geotherm they yield the depth to the crust-mantle boundary (-15 kbar, ie -55 km) that is much higher that estimates from regional geophysical data (40-45 km). It is possible that the uppermost part of the mantle has not been sampled, but this is unlikely because many Hamar-Daban xenoliths contain accessory feldspar. Alternatively, the geotherm in this region may be different, ie temperatures at the crust-mantle boundary (CMB) could be higher in southern Baikal area (Hamar-Daban) than in central Mongolia. A possible explanation is that voluminous underplating of basaltic magma at the CMB took place beneath the southern Baikal area in the Miocene, but not in the Tariat area where volcanic activity began in the Pleistocene.

The geotherm inferred for central Mongolia (Tariat) converges with eastern Australia geotherm at high pressures (-20 kbar) but yields significantly lower T values (~100°C) near the CMB. The CMB depth inferred for central Mongolia (45 km) is also much deeper than the average eastern Australia CMB (O'Reilly & Griffin, 1995). These differences in the thermal state of the lithosphere are consistent with the apparently more intense magmatic activity in eastern Australia in the Mesozoic-Cenozoic accompanied by under- and overplating at the CMB. Our data also indicate that the late Cenozoic volcanic activity in the Baikal area has resulted in some lithospheric heating.

The uppermost mantle sampled by peridotite xenoliths in the Tariat, Hamar-Daban and Vitim areas has fertile compositions (enriched in basaltic components) providing a geochemical contrast with more depleted and more magnesian mantle beneath the Siberian Platform to the north. The combination of the non-refractory compositions and the moderately high geothermal gradient (relative to stable platforms) is consistent with anomalously low seismic velocities right below the Moho in the region.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE COMPOSITION OF THE EARTH'S MANTLE: INSIGHTS FROM LABORATORY MEASUREMENTS OF SEISMIC WAVE SPEEDS Ian Jackson and Sally Rigden Research School of Earth Sciences, Australian National University, Canberra, ACT 0200

It has been long been recognised that both pressure-induced phase transformations and chemical layering may need to be invoked to explain the structure of the Earth's mantle revealed by seismology. During the past decade, phenomenal advances have been made in seismological definition of the structure of the transition zone, and in detailed experimental determination of the relevant phase equilibria and of the physical properties of the key highpressure minerals. Accordingly, it is now possible to specify much more tightly the variation of seismic wave speeds and density with depth to be expected for a model mantle of uniform chemical composition. Comparison with seismological models can then be used to to identify any discrepancies attributable to chemical layering.

This new synthesis will focus primarily on the behaviour of the seismic parameter <J>(z) = Vp2(z) - (4/3)V$2(z) = K(P,T)/p(P,T), K and p being respectively the bulk modulus (or incompressibility) and density. In this approach, the seismological information of highest quality is emphasised (wave speeds rather than density); however, the resulting function can be modelled through an equation-of-state describing the pressure-temperature dependence of density and incompressibility, thereby avoiding ambiguities associated with the temperature dependence of the shear modulus.

For the best estimates of the seismic parameter for the transition zone, recent measurements of the pressure dependence of the elastic wave speeds can be combined with analysis of P-V-T data through an appropriate thermal equation-of-state. This analysis will be used to address the seismologically observable velocity and impedance contrasts for the 410 and 520 km discontinuities, and also in discussing the magnitude of velocitydepth gradients in the transition zone.

Analysis of newly available P-V-T data for Mg-silicate perovskite with an appropriate thermal equation-of-state yields much tighter constraints on both thermal expansivity a and its pressure dependence (3a/3P)j = (9Kj/3T)p/Kx^ than previously available, confirming unexceptional values for each of these parameters. There is ne also an indication that the temperature dependence of Ks's= (9K$/9P)s m a y gligible (Jackson and Rigden, 1995). These new insights have been built into the modelling of the seismic properties of the adiabatically decompressed lower mantle treated as a perovskite + magnesiowiistite mixture. In addition, properties of this mixture under lower mantle P-T conditions have been computed for direct comparison with seismological models. These approaches indicate that both <|)(z) and p(z) are well matched by a composition comparable with that of the upper mantle, along with a plausible potential temperature of - 1600 K. Significant silica enrichment of the lower mantle would require a substantially higher potential temperature and a mid-mantle thermal boundary layer for which there is no seismological evidence.

It is concluded that within the residual uncertainties, the available seismological, penological and mineral physics data are consistent with the notion that the mantle (with the possible exception of the lowermost D" layer) is grossly uniform in chemical composition throughout. The further testing of this simple but resilient model requires experimental data which will shorten the inevitable extrapolations in temperature, pressure and frequency from laboratory experiments to the conditions of deep mantle seismic wave propagation.

REFERENCES Jackson, I., & Rigden, S.M. Thermoelastic properties of deep mantle phases: constraints from the analysis of PV-T data. Phys. Earth Planet. Interiors (in press).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NEW TECHNIQUES/CONCEPTS FOR CHOOSING THE RIGHT GROUND FOR SEDIMENTHOSTED MINERALISATION - LOWER McNAMARA GROUP, LAWN HELL PLATFORM Michael J Jackson. PN Southgate, AAKrassay, BA McConachie, AT Wells and DL Scott Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601

Natural radioactivity measurements, at 0.5m intervals, collected from outcropping sections with portable gamma-ray spectrometres, in concert with correlations based on sequence stratigraphic concepts, are providing exciting new insights into basin architecture in the Proterozoic of northern Australia. These new techniques and concepts enhance the identification of features such as stratal geometries, basin shape and palaeogeography - factors that are crucial for locating potential seal and reservoir facies and likely fluid pathways. This new approach will prove more successful than traditional lithostratigraphic methods, both in assessment of prospectivity of successions or areas and, more importantly, in defining areas of basins that are likely to contain specific stratigraphic targets, for example thick black shale facies for "sedex" deposits.

Published information, derived from mapping in the late 1970's (Mt Oxide-Mammoth Mines-Lawn Hill 1:100 000 sheets), describes the lower half of the McNamara Group as consisting of five conformable formations with a cumulative thickness of 1400m. Each formation is dominated by a characteristic lithology - Torpedo Creek by cross-bedded quartzite, Gunpowder Creek by fine sandstone and siltstone, Paradise Creek by stromatolotic dolostone and Esperanza by "spectacular" stromatolitic chert (Figure 1). In the Explanatory Notes the depositional environments suggested for this purportedly "concordant and conformable" succession are predominandy lacustrine, lagoonal and very shallow-water marginal shelf.

A significantly different perspective on the evolution of this stratigraphic package and the depositional environments has been provided by our sequence stratigraphic studies. The field gamma log, especially, provides the clearest indication of 1) stratigraphic breaks, i.e. sequence boundaries, 2) periods of major inundation and condensation, i.e. maximum flooding surfaces and, 3) the size and shape of major transgressive and regressive episodes. Five major depositional sequences (A to E, Figure 1) can be identified, each bounded by a regionally traceable sequence boundary (sb). These subdivisions rarely correspond to the lithostratigraphic subdivisions. Erosional relief of at least a few metres is evident on most sb's locally, and regionally some show tens of metres relief. Each sequence commonly starts with an abrupt flooding that results in a thin transgressive series of backstepping parasequences. These are capped by shaley, generally poorly outcropping, maximum flooding or condensed intervals; e.g. black carbonaceous shale in middle of sequence B (Figure 1), which contains economic copper mineralisation. Stacked prograding parasequences (each a few metres thick) constitute the upper, regressive part of each sequence. In A these are coarsening- and thickening-up lithic sandstones; in C they are shallow sub-tidal stromatolitic carbonate cycles; whilst in D they are slumped, finer grained ?mid-shelf dolostones. Although there are some shallow water intertidal-supratidal facies as described in the explanatory notes many of the stromatolites occur in subtidal and even deeper water facies and there are also thick intervals of rocks deposited below wave-base. The recognition of key surfaces such as sequence boundaries and maximum flooding surfaces, within what is described as a concordant and conformable succession, will provide a series of chronostratigraphic surfaces that may well be traceable throughout the Mt Isa-Lawn Hill area. If so, this new sequence stratigraphic approach promises to establish a better stratigraphic framework for exploration for stratiform mineralisation throughout the whole of northern Australia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CHEMICAL PARAMETERS AFFECTING GROUNDWATER QUALITY AND ENVIRONMENTAL HEALTH IN CENTRAL AUSTRALIA Gerry Jacobson, John Wischusen and J.E/Libbie' Lau Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

Chemical analyses of 900 groundwater samples from water bores in the Papunya-Kintore region, southwest Northern Territory have been scanned and related to aquifer geology. In this region groundwater is used to supply aboriginal communities and outstations. The sources are mainly shallow aquifers in Cainozoic basins or in fractured bedrock of the Ngalia Basin and Arunta Complex. In these situations groundwater is used untreated, with minimal chlorination. The region is arid, and recharge to the aquifers is low in the modem climatic regime.

The salinity of groundwaters is a major constraint on present and future development in this region, and a high proportion of water bores are beyond accepted drinking water standards of 1500 mg/L Total Dissolved Solids.

Furthermore, many groundwaters in the region that have relatively low salinity (less than 1500 mg/L), do contain deleterious elements, including nitrate, fluoride, heavy metals and radionuclides. Nitrate concentrations are above 45 mg/L in about half the water bores, and range up to 360 mg/L. High nitrate concentrations are associated with shallow aquifers of various types; the nitrate is concentrated by biological processes in the soil, and flushed through to the water table. Fluoride concentrations are greater than 1.5 mg/L in about one-third of the water bores in this region, and range up to 8.4 mg/L. Some of the highest fluoride concentrations are associated with granitic rocks of the Arunta Complex. Limited data on heavy metal and radionuclide concentrations in the regional groundwaters are presently being supplemented with a program of sampling and analysis.

In the context of present-day aboriginal health, the problems generally associated with poor water quality may be masked by a spectrum of major health problems. Nevertheless there is an obvious need for research and development of low-cost water treatment schemes suitable for arid-zone communities.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ASSESSMENT OF UNDISCOVERED MINERAL RESOURCES

S Jaireth, Y Miezitis and IB Lambert Mineral Resources Branch, Bureau of Resource Sciences, PO Box El 1, Queen Victoria Terrace, Parkes ACT 2600

Informed land-use decisions require an assessment of both the known mineral resources and the mineral potential (that is the mineral resources yet to be found). This applies particularly to areas where future land access for mineral exploration, mining and even for geoscientific investigation could become restricted or disallowed.

This paper reviews the methods being considered by the Mineral Resources Branch of the Bureau of Resource Sciences (BRS) for qualitative and quantitative assessment of mineral potential. Since Allais first conducted resource assessment in the Algerian Sahara in 1957, various types of statistical and probabilistic models have been applied to assess mineral potential and quantify undiscovered mineral resources.

In qualitative assessment the target area is subjectively classified into regions of high, moderate, low and unknown potential. Qualitative methods are combined with statistical and numerical models to generate quantitative estimates. Some of the more common quantifying tools are: numerical testing of a mineral occurrence, determining density of deposit occurrences and the target counting method.

The advancement in image processing techniques, computer hardware and relational databases have led to the development of Geographic Information Systems (GIS) which can be effective in conducting quantitative assessment of undiscovered mineral resources. It needs to be stressed, however, that GIS is a technique of rapid integration of various types of data and its effective visualization. Its effectiveness as a tool of quantitative assessment of undiscovered mineral resources will depend on the support of various types of statistical applications such as Boolean logic models, Fuzzy logic methods and Bayesian methods. The choice of appropriate numerical models will in turn depend upon the aim and the amount of geological information on the target area.

In the past BRS has used qualitative methods for the evaluation of mineral resource potential but there has been an increasing demand from Commonwealth agencies for quantifying undiscovered mineral resources which enable economist to place dollar values on alternative land-uses. Qualitative methods were used for the northeast Queensland wet tropics and the southwest Tasmanian world heritage areas. In the Shoalwater Bay and Lake Eyre drainage basin areas qualitative methods were used for most minerals. Quantitative assessments were applied for selected mineral commodities in both of these areas where it was possible to base such assessments on geologically analogous regions known to contain mineral resources in specific mineral deposit types.

BRS is investigating the development of a GIS-based database of geologic-tectonic (metallogenic) provinces in Australia to support mineral potential assessment, building on GIS in use/being developed for other purposes.

The database would be supported by other mineral databases such as MINLOC, MINRES, OZMIN and will have access to information on mineral occurrences, mineral resources, local and global grade-tonnage models for selected type of deposits, density of mineral deposit occurrence, density of mineralization, and indices reflecting differences and similarities with metallogenic provinces outside Australia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CIRCULATION STRENGTH OF A WESTERN BOUNDARY CURRENT THROUGH THE LATE CENOZOIC (ODP DRILLING PROPOSAL: EAST AUSTRALIAN CURRENT) Dr Chris Jenkins, Ocean Sciences Institute, University of Sydney (chrisj@extro.ucc.su.oz.au)

Western Boundary Undercurrents (WBC) function as the principal surface ocean circulation systems transporting heat from the equatorial zones poleward. The most significant WBC systems are the Gulf Stream, Kuroshio, Argentine Current, Agulhas Current and East Australian Current (EAC). As some of the largest and most energetic circulation systems in the ocean, they have very significant effects on ocean basin and continental margin sedimentation, plankton and fish distributions, and regional and global climate. Only the Antarctic Circumpolar Current has a larger total transport and kinetic energy.

Western Boundary Currents also pose problems for paleobigeographic research: they are large systems for transport of equatorial planktic species over half a hemisphere, ie., to 40 latitude.

Since strong dynamic effectsfromWBC - including the East Australian Current - are known to extend to continental slope and abyssal seafloor depths, the WBC record can be examined in ocean sediments in both the bathyal and abyssal environments. Those dynamic effects - *benthic storms - are barotropic waves propagatingfromringsand meanders in the WBC, create highly variable seafloor currents of up to 40cm/sec at abyssal depths and >100cm/sec on continental slopes.

Ocean drilling at sites under the EAC could acquire high-quality data on the paleoceanography of Western Boundary Currents. Sedimentologic, stratigraphic, magnetic anisotropy and core-image data on the prevalence of erosive bottom currents (above the 20cm/sec threshold and variable in direction) through the Oligocene to Recent would yield a record of past WBC energetics. Western Boundary Currents transport equatorial waters and their planktic/nektic biota poleward to latitudes 40°. Similar analysis would also reveal the degree and extent of warm water transport by this WBC system. Such measures of the dynamical (current erosion) and water mass (biota, isotopes) transport of a WBC would be compared with regional and global climate records to gauge what linkages (even feedbacks) exist between climate and WBC activity.

Seismic fades analysis of the sediments along the continental margin has shown that where the EAC is a south-flowing jet (i.e., north of 32-33 °S) a scoured trough and then a linear abyssal sediment drift lie offshore of (by 150km) and parallel to the foot of the continental slope. The 100-300m high, 30-90km wide and 600km long drift has been named 'Kennedy Drift'. In contrast, south of 32-33° and beneath the eddy field of the EAC, the abyssal plain is flat and fades are spread uniformly over distances of hundreds of kilometres from the continental foot of slope. Hie difference in fades organization is attributed to the changefromwell-organized patterns of deepflowunder the stable EAC jet, as opposed to the unpredictable deep dynamic effects beneath the eddy field - unpredictable in geographic and temporal occurrence of the eddies and their associated abyssal 'benthic storms .

0

1

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

USING GEOLOGICAL DATA TO PREDICT INDOOR RADON HAZARD Robyn Johnston Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601

Radon (222Rn) is a naturally occurring radioactive gas, a decay product of uranium (2:>8U) and its daughter226Ra. Up to 10% of lung cancer incidence internationally is attributed to exposure to high levels of radon in indoor air.

Many countries have set standards for radon in indoor air: in Australia, the National Health and Medical Research Council recommends that remedial action be taken for buildings where the Rn concentration is above 200 Bq m'\ The primary factors controlling radon levels in indoor air are geological: the distribution of uranium (and radium) in underlying rocks and soils, which determines the amount of Rn available, and hydrogeological factors which affect the transport of Rn to the surface, such as soil depth and permeability, presence of fault zones, depth to watertable and groundwater movement. Building construction (the extent of ventilation, presence of a basement) also affects the entry and retention of radon in houses, but at the regional scale geolog'ical controls dominate.

On this basis, geological data can be used to predict regions at risk from high indoor radon. The uranium (radium) concentration in surface materials measured using airborne gamma spectrometry is the single geological parameter which correlates best with indoor radon levels, but the predictive accuracy of the correlations is poor and may be improved by taking into account other factors. Although the factors controlling radon distribution are known, their interactions are complex and the relative contribution of each is not well established. As a result, studies in the USA and Europe using a range of predictive factors have combined available data empirically to produce a "radon hazard index" by combining relative rankings for the various contributing factors (Akerblom 1993, Gundersen et al 1992).

Using this approach, maps of relative radon potenial have been compiled for Australia at a continental scale and at regional scale for pilot areas at Bathurst, NSW and Collie, WA. Existing geological, geochemical and geophysical (airborne gamma spectrometry) data were analysed and combined using ASSESS, a GIS-based risk assessment software package which produces an index of radon risk by combining rankings for a number of different factors (Bowyer and Veitch 1994).

Radon hazard maps derived from geological information offer a rational basis for planning radon surveys in Australia, but cannot be validated from existing Rn measurements which are sparse and often poorly located (to shire or postcode district). It is not apparent from this study that a composite radon index offers any substantial advantages over the use of airborne gamma spectrometry alone for preliminary assessment of radon hazard at the regional level - in fact, combination of data types may obscure small but significant features. However, the relative effectiveness of predictive methods cannot be properly assessed without accurately located radon measurements for comparison.

REFERENCES Akerblom, G. (1994) Ground radon monitoring procedures in Sweden. Geoscientist 4 (4): 21-27.

Bowyer, J. and Veitch, S. (1994) ASSESS: a system for selecting suitable sites for land use. Proceedings OZRI 8, Hobart 1994.

Gundersen, L. C. S.; Schumann, R. R.; Otton, J. K.; Dubiel, R. F.; Owen, D. E.; Dickinson, K. A.(1992) Geology of radon in the United States. In: Gates, A. E.; Gundersen, L. C. S., Editors. Geologic controls on radon.

Boulder, Colorado: Geological Society of America Special Paper 271:1-16.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PHYSICOCHEMICAL CONTROLS OF POLLUTION IN SEDIMENTS FROM LAKE ILLAWARRA, NSW, AUSTRALIA Brian G. Jones, Bryan E. Chenhall, Marrack Payne and Mark Murrie School of Geosciences, University of Wollongong, Wollongong, 2522, Australia

A coastal lagoon (Lake Illawarra) is currently the focus of environmental concern because of urban and industrial pollution and high rates of sedimentation caused by rapid urban expansion. The present study represents an appraisal of physicochemical conditions within the lagoonal sediments, and their relationship to the release and retention of pollutants. All sample sites were cored to provide a record of both preindustrial and polluted sequences, together with on-site determination of pH, redox potential and temperature. The physicochemical conditions and biogenic processes favour trace metal retention in the sediment and the distribution of zinc, lead and copper in the top 20 cm of the cores is proportional to the quantity of fine-grained material. Thus these fine-grained sediments act as both a sink and source for metals and nutrients, and their properties are crucial in an assessment of the status of the waterbody.

Under the oxic alkaline conditions in the upper layer of the lagoonal sediment, iron and manganese form oxy-hydroxide precipitates that have the potential to adsorb trace metals. The redox boundary occurs down to depths of 37 cm in the sandy sediments but is generally less than 5 cm deep in muddy substrates. Below this boundary, the redox potential is negative and the iron and manganese are reduced to the soluble ferrous and manganous valance states. Trace metals that were adsorbed on the oxic precipitates are released during this process. In the anoxic sediment, diagenesis via sulphate-reducing bacteria leads to the formation of framboidal pyrite and base metals are incorporated into the lattice of the pyrite through co-precipitation and adsorption reactions. Continuous deposition of sediment causes a vertical progression of this redox boundary.

A narrowing of the slightly alkaline pH range down-core is attributed to the near complete lack of oxygen, which would stabilise the oxidation-reduction potential, and subsequently stabilise pH, due to relationships between pH and redox potential.

A temperature anomaly (up to 2°C above the temperature of surrounding sediment) occurs at depths of 1525 cm in the lagoonal sediment. It is probably an indicator of biochemically mediated processes, such as the exothermic reaction caused by sulphate-reducing bacteria and associated pyrite formation. Bacteria have developed a number of ways of neutralising the potential toxicity of base metals and incorporating them into their cell structure. Increases in temperature and microbial activity reduce oxygen levels in the interstitial waters, resulting in mobilisation of phosphorus from ferric oxide and hydroxide complexes.

Phosphorus concentrations in the oxic layer (801±182 ppm) are higher than those just below the redox boundary (64Q±163 ppm). This difference has been attributed to potentially mobile phosphorus and the calculated Redfield ratio ranges from 0.47 to 4.02 in the sand-dominated sediments and from 6.05 to 9.09 in the deeper water, muddy substrates. This implies nitrogen limitation for the system as indicated by the prevailing pH and redox conditions which favour denitrification processes within the sediment. The quantity of bio-available phosphorus is dependant on the aerobic state of the sediment and overlying water.

During a eutrophic episode, the lower part of the water column and upper sediment layer become anoxic which causes break-down of iron oxy-hydroxide compounds and desorption of PC^3-. This released phosphorus adds to the already prolific algal growth, creating further reductions in oxygen to the water column.

In anoxic alkaline conditions trace metals are immobilised due to adsorption onto clays and organic matter, and incorporation into the crystal lattice of sulphide minerals. Trace metals, effectively trapped within the sediment, can be chemically re-mobilised by changes in the pH and redox potential. Disturbances to the sediment stratification, where anoxic sediments are moved into an aerobic environment, can cause the rapid oxidation of the diagenetic pyrite, resulting in the release of Fe2* and S042", reducing pH and releasing trace metals into the water column. This process must be considered when planning disposal of dredged sediment onto the lagoon foreshore.

Redox potential and pH in the sediment have been identified as crucial to the mechanisms of metal and nutrient distribution and mobility within Lake Illawarra. The anoxic alkaline conditions within the muddy lagoonal sediment immobilises zinc, lead and copper and favours the use of these trace metals as indicators of anthropogenic input. Although the low concentrations of potentially toxic metals present in Lake Illawarra sediments pose few short term environmental concerns, an understanding of the physicochemical conditions in estuarine systems is very important for their future management on both local and global scales.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 CONSTRAINTS ON THE EARLY PERMIAN AND LATE CARBONIFEROUS OF THE NORTHERN NEW ENGLAND FOLD BELT FROM THE CAMBOON VOLCANICS AND THE TORSDALE BEDS Jones. J.A., Stephens, C.J. and Ewart, A. Department of Earth Sciences, University of Queensland, QLD 4072

The Early Permian Camboon Volcanics and the Carboniferous Torsdale Beds are poorly understood components of the New England Fold Belt. Located on the eastern margin of the Bowen Basin in Central Queensland, the Camboon Volcanics form a north-south trending belt from Cracow Homestead in the south to the town of Banana in the north. The Torsdale Beds are unconformably overlain by the Camboon Volcanics and crop out to the east (ie away from the basin) of the Camboon Volcanics.

Tectonic models proposed for the New England Fold Belt during the mid- to late Carboniferous and Early Permian commonly consider two discrete events of active arc volcanism to be represented within the stratigraphic record; the Devonian-Carboniferous Connors-Auburn Arc, located along the Connors-Auburn Arch and an Early Permian arc (the Camboon Volcanic Arc) which developed over the older arc. The Torsdale Beds and the Camboon Volcanics respectively have been cited as evidence for the existence of these two distinct events. There has, however, been little work and few age constraints on either of these two sequences.

The Camboon Volcanics are predominantly lavas and ignimbrites of intermediate and silicic composition with minor mafic volcanics and associated volcaniclastic material. They are extensively intruded by rhyolitic dykes, dioritic intrusions and rare basaltic and andesitic dykes. The lower part of the Camboon Volcanics is predominantly composed of silicic ignimbrites and minor andesitic basaltic andesite and basalt lavas while the upper part is dominated by a package of volcaniclastic rocks and small, laterally restricted andesitic lavas.

Regional mapping suggests that the upper andesitic sequence overlies the lower bimodal package with what may be an intraformational unconformity. K/Ar dating of andesitic lavas from the top of the Camboon Volcanics has constrained the upper age of the Camboon Andesite to 277±4 Ma. The Torsdale Beds comprise silicic ignimbrites, volcaniclastic rocks and rhyolitic lavas. These lithologies are intruded by late Carboniferous granites of the Auburn Arch. The age for the Torsdale Beds in this area is considered to be Early Carboniferous based on a previous K/Ar date of 343 Ma from an inlier in Jurassic Sandstone to the south. This age for the Torsdale Beds has been cited as support for an early to midCarboniferous volcanic event that corresponds with the development of a forearc basin and accretionaiy complex. New K/Ar dating of an ignimbrite near the top of the Torsdale Beds within this area, however, has yielded an age of 31 l±5Ma. Recent work, including new dating by various researchers on the eastern margin of the Bowen Basin, has suggested that the peak of magmatic activity for a late Carboniferous event in the Connors-Auburn Arch is around 305 Ma; consistent with the distribution of existing K/Ar ages on granites in the Auburn Arch that principally range from -320-290 Ma. The 311±5 Ma age for the top of the Torsdale Beds at Cracow infers that 1) the Torsdale Beds in this area are probably comagmatic with the granites of the Auburn Arch, and thus part of a magmatic cycle of Late Carboniferous age, and 2) that any time break between the end of the Late Carboniferous volcanism and the Camboon Volcanics was probably small. This break must have been, nonetheless, of sufficient magnitude so as to expose the tops of high level granitoids exposed at the Torsdale/Camboon contact. The significance of the 343 Ma age for the Torsdale Beds near Cracow is difficult to gauge because of the isolated nature of the exposure from which the original sample was obtained.

Geochemical data from the Torsdale Beds and Camboon Volcanics, and from studies on granites in the Connors Arch, show a distinct calc-alkaline affinity, suggesting that there is good reason to presume the existence of a continental arc along the line of the Connors-Auburn Arch during the Late Carboniferous and Early Permian. This period, nonetheless, appears to record a broad cycle of magmatic activity that was interrupted by, probably short, tectonic movements that produced the observed unconformities. This interpretation suggests that there is, however, little evidence preserved in the igneous rock sequences of the northern New England Fold Belt for the existence of the Late Devonian-Early Carboniferous Connors-Auburn Volcanic Arc.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A COMPUTER PROGRAM FOR VISUALISATION AND INTERACTIVE INTERPRETATION OF GEOLOGICAL HISTORY Leonie E. A. Jones1 and Ray Stace2 School of Geosciences, University of Wollongong, Northfields Ave, Wollongong NSW 2522 Educational Media Services, University of Wollongong, Northfields Ave, Wollongong NSW 2522

A computer program has been developed for interactive interpretation of geological history from geological maps and cross-sections. The program is intended for use at the first year undergraduate level with the aims of: (1) helping students visualise gradual geological processes on a large scale; (2) developing the logical reasoning ability of students; (3) helping students acquire an essential geological skill; and (4) making the learning process more interesting and satisfying. MACROMEDIA Director was chosen as the authoring package for developing the program because of its movie facility and cross-platform capabilities, making the finished product suitable for use on either Apple/Macintosh computers or IBM-PC compatibles.

The program consists of two parts - a tutorial and a set of exercises. The tutorial leads the user through the basic concepts and illustrates the sequence of geological events reponsible for particular configurations of rocks on maps and cross-sections. A key feature of the instructional design of the program is that it is graphic-driven rather than text-driven, with ample provision for demonstration of concepts using animations and movies in full colour with three-dimensional perspective. Relevant text is supplied, but is secondary to the graphics. The set of interactive exercises is designed to test the user's ability to apply the concepts to various geological maps and cross-sections and to provide reinforcement of these concepts.

The concepts illustrated by computer animations in the tutorial include the principles of original horizontality, superposition, inclusions and cross-cutting relationships. Deformation events such as folding and faulting are demonstrated, as are the ideas of uplift and erosion followed by subsidence and further deposition to produce unconformities. Additional features are the use of colour, appropriate rock symbols, length and time scales, 3-D perspective block diagrams, and options for further information such as viewing the appearance of features on geological maps.

Problems were encountered in transferring some of these concepts to the computer screen. An original goal was to provide a visualisation of geological processes via animated images, rather than the words and static pictures of conventional textbooks. However, creating an animated image makes a specific statement about the type of process involved, whereas the details of these processes may not be fully understood. Such an example is the exact intrusion mechanism of granitic plutons. Thus the various animations were designed according to the criterion of conveying simply and effectively the essence of a process without sacrificing accuracy.

Another goal was to help students visualise large-scale processes occurring gradually over geological time.

The time-scale problem was solved by providing a moving bar-graph giving time elapsed in millions of years. Horizontal and vertical length scales proved more of a problem. The stratigraphic principles and deformation processes are illustrated with reference to a 1:1 block diagram, which is essentially a threedimensional window into the Earth's sub-surface. It then proved difficult to put this block into the largerscale perspective of the regional setting which, perforce, must be displayed with considerable vertical exaggeration. A solution was finally reached by either panning back from the block diagram or by zooming into the regional diagram and at the same time changing the labelled scales. Due to the difficulty of simultaneous display of the local and regional pictures, some of the principles were illustrated by the technique of moving objects from out of frame into the block diagram, for example for the principle of inclusions of older rocks and the intrusion of plutons.

The advantages of this program over conventional teaching methods are the scope for visualisation of geological processes using full colour, attractive graphics and movies, and for interactive testing and reinforcement of the concepts.

Acknowledgements: This program was developed with the support of a National Teaching Development Grant from CAUT (Committee for the Advancement of University Teaching). Computer graphics and animations were produced by Educational Media Services at the University of Wollongong.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 REGOLITH-LANDSCAPE MAPPING IN CENTRAL AND WESTERN VICTORIA: BEDROCK, GRAVELS, LAVA FLOWS AND DEEP LEADS E. B. Joyce School of Earth Sciences, The University of Melbourne, Parkville, VIC 3052

EARLY STUDIES Regolith mapping of Western Victoria began with a study by Oilier and Joyce (1986). An account of the regolith of the adjacent highlands area has been given by Joyce (1992). Recently the Geological Survey of Victoria (GSV) and the Australian Geological Survey Organisation (AGSO), under the Mapping Accord, have geologically mapped five of the six 1:100,000 sheet areas which make up the Ballarat 1:250,000 sheet area, but no further regolith mapping was undertaken.

THE STUDY AREA The Ballarat area is part of the West Victorian Uplands (WVU), the continuation westwards of the Victorian Highlands. In the WVU extensive fluvial gravels and sands on ridge tops and valley sides, and in deep leads under lava flows, range in age from early to late Tertiary. Phases of late Tertiaiy-Quaternaiy Newer Volcanic activity caused derangement of drainage, formation of lakes and swamps, shifting of divides, burial of deep lead gravels and gold, and establishment of new drainage lines.

Regolith has developed on a bedrock of Palaeozoic sediments and granites and includes deep kaolinitic profiles.

Ferricrete and silcrete are found in weathering profiles formed on Tertiary gravels and sands. Soils and regolith of differing thickness and clay type occur on lava flows, often closely related to the age of the flow. Alluvial and colluvial deposits have youthful soils and often include a surface deposit of Post-Settlement Alluvium resulting from European land use practice over the last 150 years.

CURRENT WORK The southern part of the Ballarat sheet includes the Western Victorian volcanic plains. A regolith map at 1:100,000 scale for the northern part of the plains has been compiled from recent Honours project mapping at the University of Melbourne. Work has also been carried out in the WVU during field regolith courses for M.Sc students, and a pilot study of the Creswick 1:100,000 sheet area was undertaken early in 1995 as part of a VIEPS post-graduate course.

The Creswick 1:100,000 sheet area contains all the major units of the Ballarat region, with an extensive sequence of Newer Volcanic activity, and major gold-bearing deep lead systems. In 1996 the sheet will be mapped by Honours students at the University of Melbourne, after which the GSV will carry out a geological mapping program.

The latest techniques applied in the study of the Creswick sheet area include interpretation of satellite imagery, air-borne radiometrics and magnetics. Landsat imagery gives a broader view than that of air photos, and shows aspects of lithology, structure, and broad geomorphic regions, but often it is the vegetation and land use which are most evident. In contrast, in studies undertaken so far, air-borne radiometrics and magnetics have provided new insights into the sub-surface extent of granitic intrusions, helped locate the buried edges of lava fields, allowed subdivision of flows by their magnetic signature and by the soil types formed on the flows, and clearly indicated the upstream source of alluvial deposits.

The Creswick project provides an unusual opportunity to carry out detailed regolith field mapping using modern techniques in advance of a related program of geological mapping.

REFERENCES Joyce, E.B., 1992. The West Victorian Uplands: origin and history. Earth Surface Processes and Landforms 17, 407-418.

Oilier, C.D. & Joyce, E.B., 1986. Regolith terrain units of the Hamilton 1:1 000 000 sheet area, Western Victoria. BMR. Record 1986/33.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 A NEW METHODOLOGY FOR ASSESSING THE VALUE OF GEOLOGICAL HERITAGE IN AUSTRALIA E. B. Joyce School of Earth Sciences, The University of Melbourne, Parkville, VIC 3052

INTRODUCTION Methodologies for classifying and assessing the value of geological heritage features have been developed in Australia over the past 20 years, initially by Divisional Subcommittees of the Geological Society of Australia in South Australia and Queensland, and later in Victoria and other Divisions. As a result of two workshops held in 1982 and 1984, a report on sites of International and National significance in Australia was sponsored by the AHC and this report reviewed the methodologies used up to that time by Divisional GSA Subcommittees. Some further reviews of Australian work are given in papers at two recent international conferences, held at Digne in France in 1991 and at Malvern U.K. in 1993 (see for instance McBriar and Hasenohr 1994).

AUSTRALIAN METHODOLOGIES FOR GEOLOGICAL HERITAGE ASSESSMENT An approach to studying geological heritage has been described by Joyce (1994) and can be summarised as the IDEM concept—Identification, Documentation, Evaluation and Management. A methodology must be concerned with the Identification of sites or features and their Documentation, preparatoiy to an Evaluation (or assessment) of significance. A common way to begin this task is to classify sites by geological or geomorphological type.

The Australian Heritage Commission has developed several schemes over time to classify geological sites submitted to the Register of the National Estate. An example of another methodology is UNESCO's World Heritage assessment of natural sites, which is used in Australian nominations to World Heritage. Further development of this methodology as a specifically geological approach was undertaken by the UNESCO World Heritage Working Group on Geological Sites at a meeting in Paris in February 1992 attended by the author.

Methodologies for assessing geomorphological sites have been developed recently in the Forests, and Parks and Wildlife groups, of the Tasmanian government. In Victoria a number of consultant studies on both geological and geomorphological sites have been carried out by Rosengren, including his most recent report on "Eruption Points of the Newer Volcanics Province of Victoria". Future approaches must also take account of the comprehensive regional assessment process being developed by the Australian Heritage Commission.

THE NEW METHODOLOGY STUDY In 1993 the AHC provided a grant of $27,000 to the Standing Committee for Geological Heritage of the GSA Inc. to provide a report on "Development of a classification system for geological, palaeontological and geomorphological sites for reviewing the Register of the National Estate". In spite of the title of the commissioned report, the study will propose a methodology which not only covers classification of sites, but also includes documentation and evaluation (assessment).

The initial development of the methodology was carried out by several consultants. A draft report was discussed at a Workshop held in Canberra in December 1994 which was attended by representatives of each Divisional GSA Subcommittee and AHC officers. A final report detailing the recommended methodology is now being finalised.

REFERENCES McBriar, E. Maud & Hasenohr, Pamela, 1994. Australian initiatives in earth science conservation.

Proceedings of the 1st International Symposium on the Conservation of Our Geological Heritage, Digne les Bains, 11-16 June 1991. Memoires de la Societe geologique de France, n.s.165, 75-79.

Joyce, E.B. 1994. Assessing the significance of geological heritage sites: from the local level to world heritage. Proceedings of the 1st International Symposium on the Conservation of Our Geological Heritage, Digne les Bains, 11-16 June 1991. Memoires de la Societe geologique de France, n.s.165, 37-43.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 FINDING DISASTER INFORMATION ON THE INTERNET: CAN SPEED AND ACCURACY BE GUARANTEED? E. B. Joyce School of Earth Sciences, The University of Melbourne, Parkville, VIC 3052

Case studies of some recent disasters show that information is slow to appear on the Internet and is often incomplete or inaccurate. Examples include the Mt Ruapehu volcano eruption of September 1995, and the Sumatran earthquake of the 7th October 1995.

Those who seek disaster information include such diverse groups as scientists, academics, school teachers, university and school students, journalists, tourists seeking reassurance about safe travel, and relatives and friends of those living or travelling in the disaster area.

Some Internet systems which might be expected to provide timely and accurate information on disasters include EPIX, GLO-DISNET and Australia's @ngis. However, as will be demonstrated, these systems do not provide the sort of information needed.

The GD-PRIME® system was set up by the author in June 1995 as a contribution to a Workshop on "Harnessing the Communication Revolution: Creation of a Global Information Network" held in Beijing during the Pacific Science Congress. GD-PRIME® is a set of Web pages which attempt to provide the user with rapid access to the best information available at the time of the enquiry. The pages do not contain the information, but provide access to information which is held elsewhere on the Internet. The GD-PRIME® pages fall into the concept of metadata i.e. data about data - see for example recent discussions by Paul Shelley and Dave Johnson on the Web in 1995.

Using both recorded and current (interactive) pages from the Internet this paper will illustrate the need for such a metadata system on global disasters, and show how the GD-PRIME® Index Pages will have been developed up to the time of the AGC conference in Februaiy 1996.

REFERENCES Joyce, E. B. 1995. Disaster Information on the Internet: the proposed GD-PRIME® system. INCEDE Newsletter (in press).

GD-PRIME® Index Page 1995 URL

http://www.science.unimelb.edu.au/GD-PRIME/

Paul Shelley and Dave Johnson 1995.

URL http://spirit.com.au/earthware/Papers/AMF95/Shelley&Johnson.htmlat

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 PRIMITIVE GLASSES FROM THE MACQUARIE ISLAND (SW PACIFIC) OPHIOLITE: AN EXTENDED RANGE OF MORE PARENTAL COMPOSITIONS AND IMPLICATIONS FOR MANTLE MELTING Vadim Kamenetskv. Anthony J. Crawford, Rick Varne and Ruth Lanyon Department of Geology, University of Tasmania, GPO Box 252C, Hobart 7001, Australia

Macquarie Island is a subaerial exposure of part of the Macquarie Ridge, which marks the tectonic boundary between the Indian-Australian and Pacific plates. The island is an ophiolite complex composed of intrusive rocks (restite and cumulate peridotites, gabbros, sheeted dolerite dike complexes) and volcanic rocks (massive and pillow basalt lavas, picrites, breccias, hyaloclastites) and associated sediments (Griffin & Varne, 1980). The age of the ophiolite complex constrained from extrapolation of magnetic anomalies and from micropaleontological data is between Late Oligocene and Middle Miocene, and K/Ar and Ar/Ar ages for lavas fall between 9 and 12 Ma. Glasses from pillow rims and hyaloclastites, and microphenocrysts of olivine included in glass, were analyzed for major elements with an electron microprobe, and H2O contents of glasses were determined by FTIR spectroscopy. Trace elements in glasses were analyzed by ICP-MS. Selected glass samples were analyzed for Sr, Nd and Pb isotopes. An outstanding compositional feature of the glasses is their broad range of K 0 (0.1-1.8) and the strong positive covariation of K 0 with other incompatible elements (e.g. Ti0 0.95-2.1%; Na 0 2.34.3%; P 0 0.08-0.7%; H 0 0.2-1.4%; La 7-50ppm). Glasses have been divided into two compositional groups according to their K20-Mg# relationships (Figure); The primitive glasses have high Mg# (63-69 mol%) and host olivine microphenocrysts (FO88-90)- Fractionated glasses have lower Mg# (58-66) and more evolved olivines (Fog3-88)- The highest-K primitive 68 glasses have (La/Yb) =14.5, whereas 0 the lowest-K glasses have (La/Yb) =2-3. r Furthermore, the extent of enrichment in % 64 | - o Qs O incompatible elements correlates Glasses 60 OO positively with isotopic ratios of Sr prim itive ( Sr/ Sr=0.70255-0.70275) and Pb 0 206 204 1 g .951 -19.493; ^ P b / ^ P b 56 L ' J• ' 1 1 -OL. evolved l 0.0 0.5 1.0 1.5 2.0 = 15.528-15.589; =38.523q 38.979), and negatively with Nd/ Nd (0.51310-0.51304).

Projections of these primitive glass compositions into the basalt tetrahedron suggest that all were produced in the pressure range 15 to 8 kbar, with the high-K melts generated at the higher pressure end of this spectrum.

Calculated temperatures of partial melting across the compositional range are 1245-1255°C based on anhydrous olivine liquidus temperatures and the slope of the olivine liquidus for anhydrous basaltic systems, but the significant H 0 contents of the higher-K melts may indicate partial melting temperatures some 10-40°C lower.

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The unusual major element composition of Macquarie Is. enriched melts (low FeO < 1% and high A1 0 >18%) and their relatively low temperatures of generation (<1250°C) rule out the involvement of a hotspot OIB-type mantle as an end-member source. We argue that the compositional range of primitive basaltic glasses of Macquarie Is. was produced by fractional melting of suboceanic mantle during its adiabatic upwelling from 15 to -8 kb. Low degrees of melting and segregation of small melt fractions are favoured by low melting temperatures.

The MORB compositional spectrum range from ultra-depleted melts (Sobolev & Shimizu, 1993) through NMORB to E-MORB. Our data extend the enriched end of this spectrum to even lower-degree partial melts. For the case of Macquarie Is., the latter formed, segregated, and were erupted without mixing with other higherdegree melt fractions, thus retaining their initial highly enriched geochemical signature.

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REFERENCES Griffin, B.J., & Varne, R., 1980. The Macquarie Island ophiolite complex: Mid-Tertiary oceanic lithosphere from a major ocean basin. Chemical Geology, 30, 285-308.

Sobolev, A.V., & Shimizu, N., 1993. Ultra-depleted primary melt included in an olivine from the Mid-Atlantic Ridge. Nature, 363, 151-154.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THERMAL PULSES AND BARROVIAN METAMORPHISM 1

0

1

Sue Keav1. Gordon Lister" and Bill Compston * Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 "Department of Earth Sciences, Monash University, Clayton, Victoria 3168

Are rocks metamorphosed over long (>10Ma) or short (<lMa) time-scales? To test whether Barrovian metamorphism can be induced by transient thermal pulses associated with magmatism, the ANU's SHRIMP ion microprobe is being used to differentiate the timing of mineral growth events. These can then be related to the tectonic history of a terrane. To achieve sufficient age resolution to distinguish short-lived thermal pulses a relatively young (15Ma) metamorphic terrane is being investigated. This is because the uncertainty in age estimates decreases in absolute terms as age decreases. Single grains of U-Pb bearing accessory minerals (zircon, sphene and monazite) are analysed either directly from polished sections or mounted in resin.

The metamorphic core complex (MCC) of Naxos (central Aegean, Greece) is recognised as having undergone at least two major Alpine metamorphic episodes: a Paleo-Eocene HPLT event (Ml)-(40-50Ma) and a Miocene MPHT event (M2) (15-20Ma). The island forms a N-S elongate structural dome exposing a series of apparent isograd surfaces representing the Barrovian (M2) sequence of progressive amphibolite facies metamorphism.

Metamorphic grade decreases with distance from the migmatite core of the island. A number of geochronological studies, utilising K-Ar, Rb-Sr and ^ A r - ^ A r faxing techniques, have been conducted on the island. The general trend in ages is one of younging towards the high grade core. This has been interpreted as resulting from mixing between populations of M2 minerals from the core and Ml minerals preserved in the SE of the island. These studies have largely relied on mineral separates and have not utilised single crystal dating techniques and so mixing of different age populations is a reasonable assumption, but can you determine exactly how many age populations are involved?

U-Pb dating of sphene from Naxos suggests there may have been at least three separate mineral growth events, possibly related to thermal pulses, producing sphenes of different ages. The first occurred after peak Ml HPLT metamorphism during the Early Oligocene (~30-40Ma) and is preserved only in the SE corner of the island which has experienced only minor M2 metamorphism. This was followed by growth of very early Miocene sphene preserved closer to the core (~22-26Ma), while within the migmatite core mid-Miocene sphene has grown (1015Ma). These results are all from pooling the ages of individual sphenes analysed using the within grain analysis capability of SHRIMP. Thus the transition from old to young ages approaching the core recorded by other dating techniques may not be the result of simple mixing between two separate populations of minerals, but mixing of several populations. The only other explanation for the intermediate aged sample is that the age estimate has been affected by partial Pb loss or U gain. If this had occurred then the sample would be unlikely to yield the same age from different grains whereas each of the sphenes gives a consistent age.

SHRIMP U-Pb dating of thin (<30^m) zircon rims, interpreted to have grown during the migmatisation of the core, yield older ages than sphene from the core (17-20Ma). This difference in age could be interpreted either as the result of slow cooling (sphene becoming closed to diffusion of Pb and U at a lower T than zircon), or resetting of sphene ages related to the intrusion of S-type granites in the area, or new growth of sphene associated with some thermal pulse (possibly the intrusion of S-type granites). Zircon and monazite U-Pb ages from some S-types intruding the core suggest they were emplaced syn- to post-tectonically between 10-15Ma, while a clearly post-tectonic I-type intrusion yields zircon and sphene ages also within this age range.

There is clearly a very close temporal relationship between magmatism and metamorphic mineral growth in the Naxos core which is quite distinct from the age of migmatisation recorded by new zircon growth in core leucosomes. Away from the core there are few Miocene intrusives and the Barrovian overprint is not as strong so older ages are preserved but these remain difficult to interpret without invoking a number of separate episodes of metamorphic mineral growth. The different age groups could be explained in terms of separate thermal pulses affecting the island resulting in new minerals growing at distinct times. Mixing of two age populations can not explain the existence of intermediate ages derived from single crystals, it is more likely these ages represent separate growth or possibly resetting events.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE GENESIS OF GLAUCONY IN THE JAN JUC FORMATION, TORQUAY BASIN, SOUTHEASTERN AUSTRALIA. Jonathan C. Kelly and John A. Webb School of Earth Science, Latrobe University, Bundoora, Victoria 3083 The Oligo-Miocene Jan Juc Formation occurs in the Torquay Basin of southern Victoria, and outcrops in coastal cliffs at Bird Rock, southwest of Torquay. The Jan Juc Formation comprises friable, fine to very fine skeletal packestones and wackestones, interbedded with thin highly indurated skeletal grainstones.

Authigenic glauconitic minerals are disseminated throughout the Jan Juc formation, but are richly concentrated in several distinct horizons. The glaucony is characterised by sand sized green pellets which are predominantly faecal pellet replacements and internal moulds from a variety of bioclasts.

The glauconitic and non-glauconitic lithologies of the Jan Juc Formation display strong textural and compositional similarities which suggests that they accumulated in similar depositional milieu. The sorting, grain size, clay content and preservation state of constituent fossils indicate deposition in a relatively deep low energy regime. The faunal assemblage is consistent with cool mid to outer-shelf conditions.

The glauconitic lithologies are texturally and mineralogically less mature than the non-glauconitic lithologies which suggests that they accumulated under relatively quiescent conditions in deeper water environments, further from the palaeoshoreline. The paucity of terrigenous framework grains and abundance of pelagic bioclasts in the glauconitic lithologies corroborates this hypothesis.

The glauconitic strata of the Jan Juc Formation probably reflect periods of retarded sedimentation associated with marine flooding events. Low net sedimentation appears essential to the glauconitisation process as it allows sediments to remain in the appropriate physico-chemical regime sufficiently long for the complex glauconitic structures to form.

Textural features such as diffuse glauconitic patches within the matrix, detrital inclusions in glaucony and gradational boundaries between pelletal glaucony and interstitial matrix suggest that detrital matrix material is the progenitor of autochthonous glaucony. Positive correlations between the proportions of argillaceous matrix and the abundance and size of glauconitic aggregates support this genetic link.

Bulk chemical data indicate that the glauconitic sediments contain more Fe, Al, K and Si than the nonglauconitic rocks; however these components are proportional to matrix volumes, regardless of the degree to which glaucony has developed. It is therefore reasonable to assume that the sediments have remained closed with respect to these elements and that the chemical constituents of glaucony are predominantly matrix derived.

The matrix of the glauconitic lithologies primarily consists of smectite and subordinate kaolinite. These minerals constitute a viable source of silica and alumina for the glauconitisation process, but contain little or no potassium. In fact no significant potassium bearing phase has been identified in the Jan Juc Formation. It is therefore postulated that marine derived interstitial fluids are the source of the potassium in glaucony. The iron in glaucony is probably sourced from fine grained ferric oxides/oxyhydroxides associated with the clay fraction.

Oxygen isotope data for the clay and glauconitic fractions of a glauconitic horizon which occurs at the boundary between the Jan Juc and overlying Puebla Formations indicate that the clay fraction formed in association with marine waters, whilst the glaucony formed in slightly more depleted waters. The marine signature of the clay fraction probably reflects recrystallisation of detrital clays at or near to the sediment-water interface. The depleted signature of the glaucony is consistent with its development from argillaceous matrix material through a series of clay dissolution-crystallisation reactions in a closed or isochemicam system.

Rb-Sr isotopic data for the same suite of samples indicate that the clay and glauconitic minerals formed at approximately the same time (~25Ma), in a system which has remained closed since the time of their formation.

The initial Sr ratios confirm that the system was initially in equilibrium with marine water.

The development of a closed geochemical system is probably fundamental to the glauconitisation process as it restricts the equilibrating effects of diffusion and allows appropriate physico-chemical conditions to be established at or near the interface between the sediment pile and the open marine environment. Thus argillaceous matrix not only supplies the essential chemical components of glaucony, it also restricts permeability and facilitates the development of the closed system necessary for glauconitisation.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SEQUENCE ANALYSIS, STRATAL GEOMETRIES AND TECTONIC-EUSTATIC CONTROL O F THE LATE DEVONIAN REEF COMPLEX, CANNING BASIN, WESTERN AUSTRALIA John M. Kennard, Peter N. Southgate and M. Jim Jackson Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 Australia

Integrated sequence analysis of seismic and well data, together with preliminary outcrop studies and subsidence analysis, provide a new understanding of stratal geometries, carbonate-siliciclastic relationships, and regional development of the Late Devonian reef complex of the Canning Basin. The reef complex forms a major transgressive-regressive cycle that was initiated by crustal extention and rifting in the Givetian. Reef growth was terminated in the late Famennian at the peak of the regressive half-cycle which marks the onset of a phase of slower thermal subsidence and deposition of an extensive mixed carbonate-clastic ramp succession. Cooler water conditions associated with accelerated southward movement of the Australian plate and global transgression in the Tournaisian, probably also contributed to the demise of reef growth at this time.

Ten "Vail type" depositional sequences have been mapped in the subsurface on the Lennard Shelf, and are tentatively correlated with seven sequences recognised in outcrop on the basis of comparable stratal stacking patterns and conodont biostratigraphy. The Givetian - late Frasnian transgressive half-cycle (Pillara reef complex) is characterised by stromatoporoid-coral-cyanobacterial reef assemblages. This half-cycle comprises at least four sequences which are dominated by thick transgressive deposits and successively back-stepping, platform margins. The oldest sequence onlaps Proterozoic basement, and forms a low relief carbonate bank without a well defined reef margin. The overlying sequences are characterised by high relief, upright, and successively back-stepping, platform margins. Drowned pinnacle reefs and atolls, which are rooted on the basal carbonate banks and draped and buried by transgressive and highstand shales, are beautifully exposed in outcropping portions of the exhumed reef tract. Higher-order cyclicity is commonly well developed in transgressive and highstand back-reef facies within many of the Pillara sequences.

Several tectonic pulses recorded by uplift, fault block tilting and erosion herald the onset of the latest Frasnian Famennian regressive half-cycle (Nullara reef complex), and resulted in major basin-ward shifts of platformal facies. The Nullara reef complex is characterised by cyanobacterial reefs devoid of reef-building stromatoporoids and corals. Six subsurface sequences are recognised within this half-cycle, but to date only 3 have been recognised in outcrop. The sequences are characterised by marked reciprocal sedimentation; thick lowstand siliciclastic-rich deposits in the basin, and transgressive and highstand carbonates on the adjacent platform. These sequences display a pattern of successively advancing platform margins, and seismic geometries define lowstand basin-floor fans, slope fans and prograding complexes. In outcrop in proximal settings, this regressive half-cycle is marked by the spectacular influx of boulder conglomerates that partially fill valleys incised into the underlying Pillara platform. The basal boundary of the oldest and youngest outcropping sequence is locally delineated by a marked platform-margin unconformity with 50-100 m of erosional relief. In proximal basinal settings, lowstand deposits comprise coarse-grained turbiditic sandstones overlain by mixed carbonate-siliciclastic grain flow deposits and debris flow breccias with clasts and huge allochthonous blocks derived from the collapse of the exposed platform margin of the underlying sequence. These deposits have distinct lensoidal or fan-like geometries and are interpreted as proximal slope fan deposits.

Conodont biostratigraphy suggests that the 10 depositional sequences each have a duration of about 1-2 Ma, and they are believed to record third-order sea-level oscillations. The transgressive Pillara and regressive Nullara half cycles record a longer term (second-order) tectonic cycle initiated by crustal extension and rapid subsidence (transgressive half-cycle), followed by pulses of uplift, fault block tilting and waning thermal sag (regressive halfcycle).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 STABLE CI AND CHLORINE-36 IN ATMOSPHERIC PRECIPITATION FROM AUSTRALIA Melita Kevwood , Allan Chivas^* L. Keith Fifield and Richard Cresswell * RSES, Australian National University, Canberra 0200 ^ School of Geosciences, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522 RSPhysSE, Australian National University, Canberra 0200 1

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Temporal and spatial variations of major-element and C1 chemistry in rainfall across Australia has been assessed. Bulk precipitation samples were collected from two arrays over two years at three-monthly intervals: the WE array (10 sites) extended in a west to east direction from the coast of Western Australia south of Geraldton, inland to Warburton in Central Australia, and the SN array (8 sites), extended in a south to north direction from Port Lincoln in South Australia to Kakadu in the Northern Territory.

The major-element chemistry shows the main influence on the composition of precipitation in remote areas of Australia to be mixing between seawater and continental sources, with the seawater end-member being evident at coastal localities, and at non-coastal sites in association with favourable synoptic conditions, such as cold frontal activity in southern and western Australia during winter, and monsoonal activity in northern Australia during summer. The continentally-derived end-member is most likely composed of resuspended soil/dust material, including salt lake and calcareous dune components. In the south of the SN array where agriculture is intense this continental source variably includes a fertiliser component. The chemistry of precipitation across Australia is also affected by an acid-base balance factor, the components of which are derived from natural sources such as biogenic emissions, biomass burning and lightning flash production. The nature of the collection program (i.e. samples are exposed to the atmosphere from the time of deposition to the time of sample retrieval) means that biodegradation is also a feature of precipitation chemistry.

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Chlorine-36 is a cosmogenic isotope with a half-life of 301,000 years. This, combined with the hydrophilic nature of CI, makes C1 useful as a hydrological tracer, relying upon predicted models of C1 and stable CI fallout to calculate C1/C1 ratios for recharge to hydrological systems. The resultsfromthis investigation agree with the general shape of the latitude-dependent predicted C1 fallout curve of Lai and Peters (1967), but suggest the curve underestimates the rate of fallout. A revised mean fallout for the southern hemisphere of 15.4 C1 atoms/m /s is suggested from this work.

The spatial pattern of stable CI deposition to the Australian continent is for decreasing depositionfromsouth to north, reflecting the importance of the southern marine airmasses in the supply of CI. The northern tropical airmasses play a less significant part in this process. The relationship between CI deposition and distance from the coast can be explained in terms of a double exponential decay, suggesting that two processes control this relationship. These processes may reflect the enhanced mixing of airmasses moving inland and/or differences in the rate of removal of CI aerosol and CI gasfrommarine and continental airmasses.

While stable CI concentrations in precipitation display a general exponential decrease with distance, the nature of this relationship is geographically variable. The long-term average predictions of C1 fallout rates used to predict the input ratios of C1/C1 in hydrological investigations should be increased by a factor of 1.4 for the southern hemisphere, and CI concentrations in precipitation should be investigated for each study by local direct measurements, a process that is simple and inexpensive.

The mean fallout calculated from this work is three times lower than has been measured for precipitation in the northern hemisphere. The lower southern hemisphere fallout rates reflect the lower rates of transfer of stratospheric air to the troposphere in the southern hemisphere, which resultsfromthe less dynamic nature of the lower stratosphere in the southern hemisphere. The mean global C1 fallout that incorporates measurements from the northern hemisphere with the results of this work is calculated to be 25-35 atoms/m /s. This value is 23 times greater than predicted by Lai and Peters (1967), indicating that the cross section for the cosmic ray production of C1 is underestimated in their paper.

Lai, D. and Peters, B., 1967. Cosmic ray produced radioactivity on the Earth. Handbuch Der Physik, v. 46, p.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra:, February 1996 EVOLUTION OF THE RENISON HYDROTHERMAL SYSTEM, WESTERN TASMANIA Paul A. Kitto. David R. Cooke and Ross R. Large CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart. 7001

INTRODUCTION The Renison Sn Mine is located 12 km east of Zeehan in western Tasmania, and has an identified mineral resource totalling 9.5 mt at 1.4 % Sn. Total Sn recovery since the commencement of large scale underground mining operations in the 1960's is over 115,000 t. Renison is hosted by subaerial to shallow marine, Late Precambrian to Early Cambrian dolomitic and clastic sediments within the Early Palaeozoic Dundas Trough.

The deposit occurs on the north-east limb of a broad, south-east plunging Devonian anticline which constitutes a major fault-bounded horst, formed by the forceful emplacement of the underlying Devonian Pine Hill Granite.

Kinematic indicators on mineralised faults indicate four phases of brittle deformation (Devonian to Tertiaiy), based on style and relative ages of fault striations. The Federal-Bassett Fault, with up "to 700m of normal-dextral dip-slip movement, resulted from initial brittle deformation associated with the forceful emplacement of the Pine Hill Granite.

MINERAL PARAGENESIS AND MICROTHERMOMETRIC RESULTS Deposition of a high temperature Stage 1 oxide-silicate vein assemblage (qz-asp-cass) was associated with initial faulting. Fluid inclusion homogenisation temperatures for this stage range from >400°C at the base of the Federal-Bassett Fault (3000 m beneath the Devonian palaeosurface), to 300°C near the top of the mine workings.

These early NaCl-KCl-H 0 brines had average salinities between -8 and 12 eq. wt. % NaCl, and fluid pressures of 250 bars (hydrostatic). CH4 was detected by Raman spectroscopy in the vapour phase of primary fluid inclusions throughout the Federal-Bassett Fault. Alternatively, CO 2 vapour was only detected in CH 4-bearing fluid inclusions (CO2/CH4 = 0.5) in the upper sections of the Federal-Bassett Fault where dolomite horizons are intersected. 8 0 f l values of 9 %o and 8 S f l j values between 5 and 6%o characterise the entire fault for Stage 1 mineralisation.

Dextral wrench reactivation and dilation of the Federal-Bassett Fault was associated with a Stage 2 sulfide vein assemblage (po-cass-qz-flu-stan-cpy±asp and minor base metals) which produced stratabound carbonate replacement orebodies that characterise the Renison deposit. Mineral deposition in the Federal-Bassett Fault occurred over a temperature range from <350°C, immediately above the Pine Hill Granite, to ~200°C at the top of the mine workings. The deep-level NaCl-KCl-H 0-rich brines evolved to CaCl -MgCl -NaCl-H20-rich fluids during fluid-rock reactions with carbonates in the upper mine levels. Salinities averaged between 8 and 12 eq. wt. % NaCl throughout the Stage 2 sulphide mineralisation. CH4 was detected by Raman spectroscopy in the vapour phase of primary Stage 2 fluid inclusions throughout the Federal-Bassett Fault. Contoured tin values and homogenisation temperatures from fluid inclusions clearly outline two high temperature tin-rich dilational jogs on the Federal-Bassett Fault, as do variations in 5 S r a ] values. 6 S values remained constant at ~5%o throughout the sulphide stage.

Minor uneconomic Stage 3 base metal veins (rhod-gn-sph-qz), associated with minor fault reactivations, overprint the earlier vein stages, as do Stage 4 vug-fill carbonate-quartz veins (qz-carb±flu±py). Stage 3 and 4 veins were associated with low temperature (150° to 200°C), bimodal salinity (<2 and -10 eq. wt. % NaCl), NaCl-KCl-H20 brines. Raman spectroscopy detected CH4 in the vapour phase of primary fluid inclusions associated with Stage 3. However, variable C 0 , CH4 and N was detected in the vapour phase of secondary fluid inclusions from Stage 4. The only fluid inclusion evidence for phase separation at Renison occurs in the late stage veins. S S values (~5%o) remained constant within the Federal-Bassett Fault during base metal mineralisation.

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CONCLUSIONS Fluid inclusion microthermometry, Raman spectroscopy and stable isotope studies indicate that Sn mineralisation at Renison was associated with NaCl-KCl-H 0-rich (CH4-bearing) magmatic-hydrothermal fluids derived from the underlying Pine Hill Granite. Minor uneconomic late stage mineralisation occurred from NaCl-KCl-H 0-rich brines that formed via mixing of contemporary meteoric groundwaters derived from Nbearing sediments with (CH4-bearing) magmatic-hydrothermal fluids.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE RELATIONSHIP OF GROUNDWATER AND OTHER WATER SUPPLIES TO RESOURCE MANAGEMENT AND PUBLIC HEALTH IN URBAN AND RURAL AREAS Michael J. Knight National Centre for Groundwater Management,University of Technology, Sydney P. O. Box 123, Broadway, NSW, 2007

ABSTRACT Water has always played a significant role in the lives of people. As far back as 2000BC the Minoan culture had piped water in houses. In urbanised Rome, with its million people, sophisticated spring sourced supply systems developed and then died with the empire, only to be rediscovered later. But it was the Industrial Revolution commencing in the eighteenth century that ushered in major paradigm shifts in use and altitudes towards water. Rapid and concentrated urbanisation brought problems of expanded demands for drinking supplies, waste management and disease. The strategy of using water from local streams, groundwater-fed springs and village wells collapsed under the onslaughts of rising urban demands -and pollution due to poor waste disposal practices.

In England, public health crises peaks, related to water-borne typhoid and the three major cholera outbreaks occurred in the late eighteenth and early nineteenth century respectively. This occurred in a totally privatised water managed system that operated in UK for 240 years between 1609-1848. After 1848, government directed technological engineering and institutional responses were successful in solving the problem. It is generally accepted that the putting of water into pipe networks both for a clean drinking supply, as well as using it as a transport medium for removal of human and other wastes, played a significant role in lowering death rates due to waterbome diseases such as cholera and typhoid. This was especially so across the boundary between the nineteenth and twentieth centuries. Medicine then contributed further to increases in lifespan and in reducing the impact of disease. Today similar principles apply. The recent (1995) World Bank report titled Toward Sustainable Management of Water Resources by I. Serageldin (1995) indicates that there can be up to 76% reduction in illness when major water and sanitation improvements occur in developing countries. Groundwater plays a major role in this.

Sydney has seen some radical shifts in water use thinking too. Supplies began with the groundwater sourced Tank Stream that became inadequate and polluted. Groundwater-fed Centennial Park lake and Lachlan lakes in the Botany Sands aquifer followed. When demand exceeded this supply the move to dams and major pipe systems followed.

Water management, technology and thinking were relatively stable in the twentieth century up to the mid late 1970s. The management paradigms were essentially extensions of European strategies with the minor adaptions due to climate and hydrogeology.

During the 1970s and 1980s in Australia, it was realised increasingly that a knowledge groundwater and hydrogeological processes were critical to pollution prevention, the development of sound waste management and the problems of salinity. Many millions of dollars have been both saved and generated as a consequence.

This is especially in relation to domestic waste management and the disposal of aluminium refinery waste.

Major institutional changes in public sector water management in Australia began in this period as well.

Upheveals and change have now reached all states in Australia with various approaches being followed. Market thinking, corporatisation, privatisation, internationalisation, downsizing and environmental pressures are all playing their role in this paradigm shift. One casualty of this turmoil is the progressive erosion of the public sector skillbase and loss of corporate memory. These may become serious issues should a public health crisis occur such as a water borne disease. Such crises have arisen over recent times.

We are on the threshold of significant change in how we use and manage surface water and groundwater, both as a supply and a waste transporter in Urban environments especially. Substantial replacement of the pipe system will be needed in 25 to 30 years time and this will cost billions of dollars. The importance of groundwater resources and the value of subsurface space in general is beginning to be realised. Ownership and property rights of the space will probably need legal resolution.

The competition for water between irrigation needs and environmental requirements in Australia and overseas will continue to be an issue in rural areas. This will be especially heightened by the rising demand for irrigation produced food as the world's population grows Internationally, Australia is in a very strong position to export both the expertise and technologies that will arise out of the process described. This is provided that an effective, cohesive skill base is maintained. Rapid urbanisation and industrialisation in the emerging S.E. Asian countries to our north are currently producing considerable demands for water management skills and infrastructure development. This trend is expected to grow. Environmental public health crises and shortages will help drive the trends.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE EVOLUTION OF THE EARTH: LIFE AND ENVIRONMENTS THROUGH GEOLOGICAL TIME Andrew H. Knoll Botanical Museum, Harvard University, Cambridge MA 02138, USA

Within the Solar System, Earth stands out as the biological planet, a distinction it has held for nearly four billion years. Today, an almost bewildering diversity of plants, animals, and microorganisms contributes to ecosystem function, and each species appears to be closely attuned to its physical and biological environment. Comparative biology and paleontology, however, show that modern ecosystems are of relatively recent origin. Communities dominated by mammals, flowering plants, or scleractinian corals characterise only the latest two percent of life's long evolutionary history. Similarly, a terrestrial surface dominated by widely dispersed continents, oxygen-rich atmosphere and oceans, and large polar ice caps is of relatively recent vintage. Life and environment both have dynamic histories, and the geological record strongly indicate that the two have evolved in concert.

The basic pattern of energy flow and material cycling through ecosystems was established early in the Archaean Eon, when anaerobic prokaryotes evolved complementary metabolic pathways in which the product of one reaction became the substrate for another. The descendants of these early anaerobes still underpin ecosystem function. Opportunities for biological evolution expanded near the beginning and, again, at the end of the ensuing Proterozoic Eon, as technically driven biogeochemical changes increased the oxygen content of the atmosphere.

Coupled late Neoproterozoic tectonic, biogeochemical, climatic, and biological changes provide a particularly vivid example of coevolving life and environment.

Proterozoic environmental changes are linked to the expansion of biological diversity, but short-term perturbations in Phanerozoic environments correlate instead with extinction. Paleoceanographic events characteristic of the later Neoproterozoic were reprised near the end of the Permian Period, this time precipitating the extinction of most marine species.

One practical implication of all this is that geological models based closely on current conditions of life and environment may have a limited power to explain the sedimentary rock record. Evaluation of ancient successions, whether it be for basic or applied reasons, may require that we take a "bottom up" approach in which sedimentary rocks are viewed as the evolved products of a prior geological history rather than simply accepting a strictly uniformitarian "top down" view governed by modern ecosystems.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 PROVENANCE OF PROTEROZOIC M E T A M O R P H I C ROCKS OF THE NORTHEAST Q U E E N S L A N D INL1ERS Janice Knutson . Ian W. Withnall , Shen-su Sun , Richard S. Blewctt , Robert J. Bultitude Lance P. Black . Ian D. Rees (1) Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 (2) Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001 (1)

(2)

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Meso and Neoproterozoic metamorphic rocks crop out extensively in the Coen, Yambo, Dargalong and Georgetown Inliers of North Queensland. The Coen Inlier consists of the Sefton Metamorphics, Coen Metamorphic Group, Holroyd Group, Edward River Metamorphic Group and Newberry Metamorphic Group; the Yambo Inlier consists of the Yambo Metamorphic Group; the Dargalong Inlier consists of the Dargalong and McDevitt Metamorphics; the Georgetown Inlier consists of the Etheridge Group. These units were likely deposited in shallow-water, largely intracratonic basins.

Quartzo-feldspathic rocks dominate. Calc-silicate rocks are prominent in the lowermost part of the Etheridge Group, less common in the Sefton and Coen Metamorphics, and rare or absent in the other metamorphic units.

Similarly, graphitic rocks are common in the Sefton Metamorphics and upper part of the Etheridge Group, but less common in the other units. Felsic volcanic rocks are rare; in contrast, metadolerite and amphibolile are widespread. Metamorphic grade ranges from sub-greenschist to granulite facies. There appears to be an overall trend of increasing metamorphic grade from west to east, and in at least some cases, a temporal link between low- to medium-pressure, high-temperature regional metamorphism and both basaltic (tholeiitic) and granitic magmatism.

Enrichment in Si0 , Ba, Th, Zr, Y, La, Ce and Nd, and depletion in Ti0 , P 0 , Sr, V, Cr, Co, Ni, and Cu, relative to post-Archaean average shale, and large Eu anomalies, suggest the sediments were largely derived from rocks of granitic composition. Increasing chemical indices of alteration (CIA) indicate that the degree of maturity/weathering of the pelitic and psammitic metasediments increases from Yambo and Newberry, through Coen to Etheridge and Holroyd metamorphic units.

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All metasedimentary rocks have REE patterns similar to post-Archaean average shale, although there are variations in total enrichment. The metagranitic rocks mostly have REE patterns similar to those of the metasediments.

Depleted mantle Sm-Nd model ages (T ) are consistent with varying inputs of Neoarchaean, Palaeoproterozoic and younger crustal material, and an overall northward and eastward younging of average source rock age from 2590-2550 Ma for Etheridge; 2540 Ma for McDevitt; 2440-2230 Ma for Holroyd; 23402140 Ma for Yambo, 2190-2110 for Newberry, 2290-2060 for Coen and 2221-2050 Ma for Sefton. Contrary to this trend is a depositional age for the Yambo sediments of >1580, which compares with an apparently younger <1560 depositonal age for the Etheridge sediments.

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The northward decrease in T M is also reflected in zircon U-Pb (SHRIMP) inheritance trends. Excluding rare, possibly tuffaceous, units in the Etheridge Group, there is an apparent northward decrease in minimum inheritance ages (in metasedimentary, metagranitic and granitic rocks) from Georgetown (-1770 to 2700), Yambo (-1640 to 2700), Newberry (-1600 to 2700), Holroyd (-1590 to 3300), Coen (-1200 to 2600), to Sefton (-1150 to 2700).

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A comparison of zircon inheritance, indicates that the metamorphic units studied in northeast Queensland contain elements that could have been derived from the Mount Isa Inlier. or from a similar sequence. Zircons from the Holroyd, Coen and Sefton metamorphic rocks also indicate inputs from terrains that could include Broken Hill and Yambo, as well as unidentified younger sources.

Preliminary data suggest that the source rocks and depositional environments of the northeast Queensland inliers are not greatly dissimilar to those of the Mount Isa and Broken Hill sequences. One difference is that, with the exception of the Croydon Volcanics in the Georgetown Inlier, there is no direct evidence of felsic volcanism in the northast Queensland sequences, even though there was extensive granitoid magmatism in the Georgetown, Dargalong and Yambo Inliers in the Mesoproterozoic.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 Chesapeake Bay Impact Structure, Virginia, USA: Impact in a Shallow Continental Shelf Environment Christian Koeberl , C. Wylie Poag , Wolf Uwe Reimold , and Dion Brandt 1

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institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria U.S. Geological Survey, Quissett Campus, Woods Hole, MA 02543, USA department of Geology, University of the Witwatersrand, Johannesburg 2050, South Africa 2

The recently discovered Chesapeake Bay structure (centered at 37°16.5' N and 76°0.7 W) is a complex peak-ring feature buried 300-500 m beneath lower Chesapeake Bay, its surrounding peninsulas, and the adjacent inner continental shelf (Poag et al., 1994). The outline of the structure was determined on the basis of multichannel seismic-reflection profiles transecting the bay and nearly 60 bore holes drilled inside and outside the crater rim. The seismic profiles define the outer rim of the structure, which is 90 km in diameter. A flat-floored, 300-1200-m-deep, annular trough separates the outer rim from an irregular, low-relief peak ring, with a 30-km-wide inner basin in the center. The pre-formational coastal plain rocks consist of a seaward-thickening wedge (300-1200 m thick) of mainly Lower Cretaceous to upper Eocene, poorly lithified, sedimentary rocks, which is underlain by a crystalline basement complex comprising granitic and metasedimentary rocks of Proterozoic to Paleozoic age. The structure is partly filled with a unit termed the Exmore breccia, mainly composed of autochthonous sedimentary clasts in a sandy matrix, but also containing millimeter- to centimeter-sized basement clasts. The age of the structure has been estimated at 35.5 Ma, based on micropaleontological studies of the breccia and correlation with near-by impact deposits. Simple Bouguer gravity measurements from southeastern Virginia were combined with free-air gravity measurements from the inner continental shelf to produce a gravity anomaly map. The map shows a prominent circular negative anomaly (-20 to -28 mGal), which is not associated with any pluton or rift basin, has an outline nearly identical to the inner basin of the Chesapeake Bay structure (as defined by seismic profiles), and coincides with the thickest part of the breccia deposit. The gravity signature of the Chesapeake Bay structure is in agreement with observations from other complex impact craters (the Ries crater in Germany). To confirm an impact origin, evidence of shock metamorphism is necessary. High pressures (>100 GPa) and temperatures (>1000°C) are uniquely associated with impact cratering. 65 samples from the Exmore, Windmill Point, Kiptopeke, and Newport News cores, which have penetrated into the Exmore breccia, were studied. Clear evidence for shock metamorphism was found in 14 Exmore core breccia samples from 372.0 to 415.6 m depth.

Samples exhibit characteristic microfracture patterns, which indicate shock pressures of 5 to 10 GPa, shock mosaicism in quartz, and abundant occurrences of shocked quartz K-feldspar, alkali feldspar, and plagioclase grains with bona fide PDFs in individual quartz grains and crystals from granitic fragments.

PDFs in shocked quartz were found (Koeberl et al., 1995) in up to 6 intersecting sets of planes corresponding to specific crystallographic orientations, with the shock-characteristic orientations (0001), {1013}, {1012}, {1122}, {1011}, {1121}, and {5161}, (c, o>, tt, r,z, s, and x, respectively), being predominant. The relative frequencies of the crystallographic orientations observedin the Exmore breccia indicate shock pressures of at least 20, and up to about 30 GPa, and provide confirming evidence that the Chesapeake Bay structure is of impact origin. The late Eocene age of the Chesapeake Bay structure and its geographical location have led to the suggestion that it might represent the source of the North American tektites (1). Tektites are natural glasses found in four Cenozoic strewn fields on Earth (North American, Central European, Ivory Coast, and Australasian). The geochemical composition of some basement clasts found within the breccia is very similar to that of North American tektites. Also, Rb-Sr and Sm-Nd isotope studies of North American tektites indicate that their source rocks are likely to be derived from the Appalachian orogeny, in agreement with lithologies present beneath Chesapeake Bay. >

REFERENCES

Koeberl, C, Reimold, W.U., Brandt, D. & Poag, C.W., 1995. Chesapeake Bay crater, Virginia: Confirmation of impact origin. Meteoritics 30, 528-529.

Poag, C.W., Powars, D.S., Poppe, L.J., & Mixon, R.B., 1994. Meteoroid mayhem in Ole Virginny: Source of the North American tektite strewn field. Geology 22, 691-694.

Acknowledgements: This research was supported by the Austrian FWF, Project PQ8794-GEO (to CK).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

METAMORPHISM AND THE MECHANICS OF COLLISIONAL OROGENS P.O.Koons Dept of Geology, Univ. of Otago, Box 56, Dunedin, N. Z.

The mechanics of a collisional orogen are determined by the bounding geometry of the orogen, relative plate velocities, erosion rates and the rheology of the deforming mass. We have investigated many of the former conditions, but the relationship of realistic, dynamic rheology to t h e behaviour of an orogen has largely been ignored. Most information on lithospheric rheologies has generally been drawn from seismological or experimental studies, yet there exists a vast and largely ignored source of information derived from careful metamorphic petrology. In this paper, I shall first produce a theory of metamorphism based upon petrological observations of disequilibrium assemblages and then examine the implications of this theory to large scale orogen mechanics.

The process of metamorphism can be defined within a volume limited by t h e characteristic times for diffusion (=td), reaction (=t r ) and deformation (=t 8 ) (Figure la). Nonlinear coupling among all three processes gives rise to chaotic mixing which is generally necessary to permit equilibration within the time frames of orogen formation. The characteristic times of each individual process alone differ greatly from the time scale of the fully coupled process. Rock packets t h a t mix efficiently along transient, chaotic paths equilibrate; those t h a t follow a steady s t a t e p a t h do not (Figure lb). Metamorphism through mixing leads to a strain-dependent rheology which can then be used in mechanical modelling of an orogen. Applying the straindependent rheology to two- and three-dimensional mechanical modelling of collisional orogens permits the description of metamorphism in terms of velocity fields. Velocity characterisation of metamorphism yields predictions on t h e timing and location of metamorphism within a deforming orogen as well as t h e position and rate of fluid production and migration. The coupling of rheology to orogen mechanics through metamorphism means t h a t reaction behaviour within the orogen should be visible in observations made on actively deforming orogens. I shall discuss the implications of a fully coupled rheology/mechanics to fluid generation and flow, melt generation, geodetic strain patterns and the growth and dimension of the topographic surface.

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TIMING AND LOCATION OF METAMORPHISM IN A 2D OROGEN

Figure l a ) R E D space outlined by the characterisitic times of diffusion, reaction and deformation, lb) example of strain concentrations within a 2D orogen where metamorphism occurs in the high strain zones (lighter areas) and not in t h e lower strain regions. (|i=chemical potential, D=diffusion coefficient, R=production rate, v=mass velocity.)

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 LATE CAINOZOIC LANDFORM MAPPING OF THE WESTERN MURRAY BASIN, SOUTHEASTERN AUSTRALIA, USING NIGHT-TIME THERMAL DATA. l A. KOTSONIS Abstract Night-time thermal imagery derived from the NOAA-AVHRR and the recently launched ERS-1-ATSR instruments were explored to map Late Cainozoic landforms in the western Murray Basin, southeastern Australia. Although the surface is mantled by Quaternary dunefields, night-time imagery revealed numerous subdued NNW trending curvilinear coastal ridges of the Loxton-Parilla Sands, and structural ridges and troughs developed on the sands that confined the Plio-Pleistocene Lake Bungunnia and influence the present drainage systems (Figure 1). The use of night-time derived imagery has yielded results that correlate with known coastal ridges mapped using conventional aerial photography. However, the imagery has revealed many new and recently unknown ridges, and provides an interesting and potentially valuable yet inexpensive tool for geomorphic mapping.

The expression of the above features is the product of the interplay between albedo and emissivity of the surface materials, and cold air ponding within depressions in the landscape. The application of this tool, primarily used in the past for meteorological observations, has far reaching applications for regional geomorphic mapping of subtle landscape features. /

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Figure 1: Landforms interpreted from night-time thermal imagery of the western Murray Basin, southeastern Australia, including sequence of coastal ridges of the Loxton-Parilla Sands.

Present Address: School of Earth Sciences, University of Melbourne, Parkville, 3052.

238

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 OUTCROP- AND CORE-DERIVED GAMMA-RAY CURVES: NEW INSIGHTS INTO LITHOSTRATIGRAPHIC SUBDIVISIONS AND FACIES RELATIONSHIPS Andrew A. Krassay and Bruce A. McConachie Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

Measurement of natural radioactivities of sedimentary rocks in field-based studies yields qualitative and quantitative results which can be of great importance in stratigraphic correlation and re-evaluation of facies descriptions and lithostratigraphic subdivisions. The data are compiled into vertical profiles (curves) of closely spaced readings showing variations in natural radioactivity with depth (through a cored interval or across strike through an outcrop section, Figure 1). Gamma-ray curves commonly show systematic vertical variations that are related to grain size, stacking patterns of sedimentary cycles, and depositional geometries. Total count gamma-ray curves are particularly useful for defining sequence stratigraphic surfaces (sequence boundaries, flooding surfaces). Spectral gamma-ray curves provide U/Th/K assay values, information on compositional trends, and can be used to highlight zones of mineralisation, alteration, and diagenesis.

Gamma-ray curves are generated relatively easily in the field by marking a stratigraphic section at regular intervals (e.g. 0.5 metres) with a staff or tape, and then using a portable gamma-ray spectrometer to take shortperiod readings at each station. Such gamma-ray curves show relative rather than absolute changes in natural radioactivity. However, results are reproducible within experimental error and 'events' on gamma-ray curves can be confidently correlated between sections over significant distances. Similar gamma-ray counts generated from core are qualitative, but by periodically measuring standards of known radioactivity these raw results can be converted into quantitative counts, and also into % potassium, and gamma-equivalent contents of uranium (ppm) and thorium (ppm). A large proportion of the total count (total gamma-ray emissions, TOT) for each sample is typically due to naturally radioactive potassium (K), particularly potassium contained in clays. In general, but with some reservations, counts in the TOT and K channels can be equated to clay content and hence to grain size. Vertical variations in TOT reflect the systematic variation in grain size through a stratigraphic interval, and highlight the nature and magnitude of stratigraphic surfaces and depositional cycles.

Abrupt changes in the total count are commonly related to sharp lithological breaks associated with unconformities and sequence boundaries (Figure 1, 123m). Peaks in gamma-ray curves commonly relate to shale-prone, organic-rich intervals associated with maximum transgression and flooding within a sedimentary sequence (Figure 1, 80-90m). Condensed sections commonly have anomalously high uranium and thorium contents due to the association of these elements with organic matter, and also due to the formation of mineralised hardgrounds in the zone of maximumflooding;suchfine-grainedzones may also act as regional seals to mineralisation. Similarly, alteration halos related to mineralisation and the passage of mineralising brines through a sedimentary section commonly involve mineralogical changes which may be manifested as anomalously high or low potassium contents in gamma-ray curves.

Gamma-ray curves are ideal for highlighting subtle but important trends in mineralogy and grain size that reflect the overall driving mechanisms for sedimentation (e.g. relative sea level, sediment supply) and changes in these parameters over time. These trends are often difficult to distinguish through traditional facies descriptions of outcrops, particularly in fine grained rocks, because they involve incremental changes in rock properties; what appears as a relatively uniform lithostratigraphic unit in the field may often be divided into a series of stacked sedimentary cycles of different orders through the use of gamma-ray curves. These curves provide excellent detail for stratigraphic correlation between outcrop sections and cored stratigraphic intervals at all scales. A series of such curves across a region can be used to reconstruct the regional stratigraphy, to place individual 'events' on any curve into a regional context, and to provide a test of previous lithostratigraphic correlation schemes.

These techniques are now being applied successfully to Proterozoic rocks across a wide area of northern Australia in AGSO's NABRE project 239

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 TECTONICALLY CONTROLLED TRANSGRESSIYE AND REGRESSIVE DESPOSITIONAL SYSTEMS TRACTS IN AN OBLIQUE-SLBP MOBILE ZONE, BROKEN RIVER PROVINCE, TASMAN OROGEN. Simon C. Lang & Christopher. R. Fielding School of Geology, Queensland University of Technology, Brisbane, Queensland, Australia.

Department of Earth Sciences, The University of Queensland, St Lucia, Queensland, Australia.

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The Broken River Province (BRP) of the Tasman Orogen, northeastern Australia contains a thick wedge of Late Devonian (Frasnian-Famennian) non-marine and marine sedimentary rocks (Bulgeri Formation, up to 3600m).

This paper concerns the lower half of this succession. At the base of the succession (Rockfields Member), lies a thin (50m) retrogradational stack of fine-grained, transgressive coastal facies immediately overlying a regional unconformity marked by incised fluvial channel deposits. The transgressive succession is overlain by a thick (750m) aggradational stack of fine-grained alluvial facies. Overlying the Rockfields Member by a slight unconformity is a thick (700m) stack of reddened conglomeratic alluvial facies (mostly Stopem Blockem Conglomerate Member). The lower part of the Bulgeri Formation is therefore an overall coarsening-upward succession with a distinctive fine-grained lower phase and an coarse-grained upper phase. This paper outlines the facies and facies sequence in terms of transgressive and regressive system tracts within a major tectonic cyclothem.

During the Late Devonian, sedimentation in the BRP was profoundly influenced by a major east-west oriented oblique-slip mobile zone (Clarke River Fault Zone). This fundamental crustal structure separated an uplifted Early Palaeozoic and Pre-Cambrian cratonic basement in the south (Lolworth Block) from a rapidly subsiding basin to the north in which the Bulgeri Formation accumulated (Bundock Basin). Furthermore, sedimentation in the basin was also influenced by a rising source area to the east due to the progressive development of a foldthrust belt affecting the pre-Late Devonian strata of the BRP east of a north-south oriented fault system (Gray Creek Fault). The thrusting was the result of the Australian craton colliding with and overriding the pre-Late Devonian sedimentary basins of the Tasman Orogen (including the eastern BRP), and was part of a continentwide tectonic movement. The Bulgeri Formation was therefore deposited in a complex hinterland-type foreland basin that lay to the north and west of the two major fault systems, and therefore a combination of strike slip and foreland tectonics appear to have been the dominant factor influencing facies and sequence development.

The succession unconformably overlies carbonates and clastics that formed part of an Early to Middle Devonian succession (Broken River Group).

Facies analysis of the lower part of the Rockfields Member has recognised a variety of architectural elements and lithofacies typical of a transition from coastal to alluvial depositional environments. The lowermost 50 m of the succession (basal conglomerates, overlain by clean sandstones interbedded with grey to reddened siltstone and fine sandstone, containing marine fossils in places towards the northeastern exposures) overlies an extensive, incised erosional surface. A depositional model is proposed that interprets the lowermost succession as an incised valley system that was partially filled during a transgression with coastal plain and mixed fluvial-, waveand tide-influenced estuarine, shoreline, and shallow marine sediments to the north. Several transgressive pulses can be recognised above the regional unconformity, resulting in a overall retrogradational stack of coastal facies being preserved above the incised fluvial deposits. Following the peak of the transgression, a downward shift in base-level occurred that resulting in a thick, non-marine succession (mainly fine- to medium-grained sandstones interbedded with variegated and reddened siltstone and reworked tuffs) interpreted as alluvial sediments deposited in broad, sandy, low-sinuosity channels and semi-permanent floodplain lakes as sediment supply matched subsidence. This succession is unconformably overlain by a thick regressive succession of reddened, fluvial conglomerates and sandstones sourced from the fold-bell to the east. These are interbedded with non-reddened fluvial sandstones sourced from the Lolworth Block south of the Clarke River Fault Zone.

Both are interbedded with reddened fine-grained palaeosol deposits characterised by abundant calcrete nodules which developed on interfluves. Several unconformities can be recognised within this upper succession, indicating successive uplift and fluvial downcutting events most probably driven by tectonic events.

The thin basal transgressive coastal succession and the overlying thick regressive alluvial succession therefore represent a major tectonic cyclothem with a lower, relatively fine-grained transgressive systems tract and an upper, relatively coarse-grained regressive depositional systems tract. Late Devonian oblique-slip movement (probably sinistral) along the Clarke River Fault Zone at the southern limit of the hinterland-type foreland basin is thought to have been the fundamental control on sequence development rather than eustacy.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

IMPORTANCE OF SCALE AND PROCESS IN CHOOSING THE "RIGHT GROUND" Ross R Large Centre for Ore Deposit & Exploration Studies University of Tasmania, GPO Box 252C. Hoban, Tasmania, 7001

Greenfields area selection begins at the continental scale reducing to the province and district scale before focussing on the prospect scale. Modern tectonic models form the basis for selection on the continental scale as different ore types are found in different tectonic environments. However, as exploration is focussed down to the district and prospect level, other key geological, geophysical or geochemical elements become important. For example, the scale change in an exploration program for stratiform sediment hosted Pb-ZnAg deposits may be: • • •

Continental - sedimentary rift basin Province - third order basin District - coincidence of marginal growth faults and carbonaceous shale/dolomite sequences Prospect - geochemical or geophysical anomaly related to deposit halo.

An exploration program for Palaeozoic VHMS deposits may use the following targeting sequence for ground selection: • • •

Continental - back-arc basin Province - submarine volcanic belt District - mixed felsic/mafic/sediment package ± syn volcanic structures Prospect - exhalites, alteration, isotope halos

Continual updating of ore deposit models through in-house and external research is a key element in the development and modification of ground selection sequences. Incorrect application of a sequence through misinterpretation of key criteria can easily lead to periods of unsuccessful exploration. For example, in the VHMS sequence above, misidentification of the volcanic environment can lead to sustained exploration in a subaerial volcanic sequence where the target deposits are lacking. However, although the "right model" is being applied to the "wrong ground", success may still be achieved, as an epithermal gold deposit or porphyry copper deposit may be discovered in the selected subaerial volcanic package. Examples of selecting the "right ground" for the "wrong reason" are common in exploration case histories and indicate a need for continual reassessment of exploration models and strategies throughout a given program.

9

In parallel with, or as an alternative to, the area selection scale-sequence, is the application of the source—>transport-*trap approach to picking the "right ground". Douglas Haynes will elaborate on the successful application of this approach for Proterozoic copper deposits in a later talk in this session.

Continental and province area selection using this approach depends on identification of the potential source rocks (eg. mafic tholeiitic volcanic pile for copper deposits or illmenite series granites for tin deposits) while the district and prospect area selection depends on identification of the fluid transport structures and metal trap lithologies.

Chemical processes promoting metal transport and metal deposition become vitally important in the prediction and identification of trap lithologies, and the development of computer-based studies on fluid-rock chemical models have come a long way in recent years, with significant applications for exploration. These developments suggest that a new generation of maps I will call redox maps and pH maps, will be used in conjunction with aeromagnetic, geochemical and geological maps to provide a new dimension to future area selection at the district and prospect scale.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE BROKEN HILL OREBODIES RECONSIDERED. David F Larsen and Anthony E Webster Fasminco Mining-Broken Hill, PO B o x 460, Broken Hill, NSW 2880

INTRODUCTION The formation of Pasminco Ltd. in 1988 has provided Pasminco Mining geologists with the best opportunity to reinterpret the geology of the whole Line of Lode including the Broken Hill (BH) orebodies since the work of the Central Geological Survey in the 1930's. The aim of this paper is to demonstrate that there is a consistent and well defined stratigraphy which is remarkably continuous throughout the Pasminco Mining leases, over a strike length of 25 kilometres, despite the effects of three major deformations and high grade metamorphism. The stratigraphy includes the package of up to 10 separate stratiform mineralised horizons that make up the BH lead-silver-zinc deposit.

STRATIGRAPHY The stratigraphy of the southern end of the orebodies and Southern Leases has been well described by previous workers. North Mine and Pasminco geologists have now shown that despite significant textural variations within most rock types due to differing structural and metamorphic conditions, the overall stratigraphy is continuous through to the Northern Leases. Many distinctive marker units and packages of characteristic lithologies can be traced from south to north with great confidence (e.g.

Units 4.7, 4.6, 4.5). Furthermore two important marker units - the Unit 4.4 amphibolite and Granite Gneiss (Unit 3.10) can be used as stratigraphic markers on the scale of the mine leases, regardless of the current debates on their mode of origin.

The BH orebodies occur within a distinctive stratigraphic package of lithologies (Unit 4.7) that can be subdivided into three major units. The regionally extensive basal unit is dominated by spotted psammopelite/Potosi gneiss rock types and hosts C Lode-style mineralisation throughout the leases (including the Flying Doctor mineralisation on the Northern Leases). The majority of the Zinc Lodes (B Lode, A Lode, Southern A Lode, 1 Lens, Southern 1 Lens) occur at characteristic stratigraphic levels within the second major unit which is composed of a thick garnet quartzite sequence and is restricted to the southern part of the deposit. Lead Lodes (2 and 3 Lens) and their encompassing clastic metasediments comprise the third component of the orebody sequence but generally show no clear relationship to major lode rock occurrences, although at the North Mine they are surrounded by a well developed alteration zone which may reflect metasomatic alteration during high grade deformation .

Each mineralised horizon (with the exception of C Lode) is internally stratified and layered. Although there are numerous local discordances, each clearly shows a consistent, conformable relationship with the surrounding strata and occurs in a characteristic stratigraphic position within the Mine Sequence.

The orebodies, associated lode rocks and other elements of the mine stratigraphy preserve their primary disposition and remain conformable throughout the Line of Lode despite the intense deformation.

STRUCTURE Recent work suggests that the main folds affecting the orebodies formed at granulite grade and are partly modified by stratigraphically controlled belts of attenuation. These folds are of the same generation as the regional macroscopic folds such as the Hangingwall Synform, and are not a later (retrograde) generation. No clear evidence has been found for the existence of the BH Antiform .

CONCLUSION The merger of the mines operating at either end of the BH orebodies has allowed access to the vast databases produced by detailed geological mapping and diamond drill core logging over several decades. This has provided a framework for the recent work which shows that the major stratigraphic units that comprise the BH Line of Lode, including the orebodies themselves, are continuous over a strike length of at least 25 kilometres with no major structural breaks.

242

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 FORMATION OF A LARGE GOLD DEPOSIT IN THE INNER CONTACT AUREOLE OF THE TABLETOP GRANITE, CULLEN MINERAL FIELD, PINE CREEK INLIER, N.T.: MULTIPLE PRECIPITATION CONTROLS. K. C. Lawrie.

Australian Geological Survey Organisation. GPO Box 378, Canberra, ACT 2601.

Most gold deposits within the Cullen Mineral Field in die Proterozoic Pine Creek Inlier, N.T., are structurally controlled mesothermal-plutonic quartz vein deposits or disseminated ironstone-bearing deposits that lie within the aureoles of granitoid bodies. The Enterprise mine is a large 40t gold deposit located within the inner aureole (<lkm) of the Tabletop Granite. The latter comprises several intrusive phases including highly fractionated leucogranites. The main Enterprise deposit and numerous smaller deposits located nearby are hosted principally within meta-greywackes and shales. The origin of the gold is controversial, with a magmatic origin implied on the basis of regional metal zoning patterns and 'magmatic' signatures of sulphides in auriferous veins.

The Tabletop Granite truncates and is not deformed by the pre-existing regionally pervasive NW-trending upright folds and associated cleavage, and the contact metamorphic assemblages overprint the regional greenschist facies assemblages. Auriferous veins are localised within and adjacent to a number of subparallel, NNE-trending brittleductile shear zones which cross-cut and off-set subparallel anticlines. Maximum quartz vein development and gold mineralisation is proximal to the intrusion within the inner contact aureole.

Mapping of open pit exposures has revealed a complex vein history that results from progressive reactivation of the principal shear zone, and associated mineralised splays. Multi-stage parageneses have been elucidated within each splay, and several distinctive fluid components recognised. Kriged gold assay data defines a sub-verticallyplunging pipe of high grade gold mineralisation which is localised where the brittle-ductile shear zone cross-cuts the hinge zone and western limb of the pre-existing Enterprise anticline. The hinge zone of this anticline plunges at <10° to the SE except where intersected by the shear zone. Saddle reef, leg and spur quartz veins formed at an early stage in the evolution of the deposit, with veins localised by reverse movement on the main shear zone at sites where bedding geometries are at a high angle to shearing. Vein formation is not lithology dependent in this zone due to the extensive silicification and potassic (k-feldspar) metasomatism at an early stage in the paragenesis. Late-stage gold-rich veins are shear-zone parallel and steeply-dipping curviplanar veins that are developed in localised transtensional offsets of the anticline axis.

Studies of vein parageneses clearly indicate that throughout reactivation of the shear zone, fluids of different origins that were undergoing variable degrees of mixing, were tapped. Dominant fluid types are high temperature, high to hypersaline 'magmatic' fluids, and lower temperature, low salinityH 2 0-C0 2 -CH 4 " late stage metamorphic' fluids, with a progression to a dominance of lower temperature fluids with time and distance away from intrusive bodies. Gold precipitation appears to be contemperaneous with fluid mixing and boiling, but with redox reactions important in some high grade zones. Gold Is precipitated at several stages in a complex paragenesis. However field, penological and microstructural relationships indicate that most of the gold in thelarge economic pod in the Enterprise Mine occurs late in the mineralisation paragenesis, coincident with transcuirent reactivation of the shear zone. 'Magmatic' fluids tapped at this time had evolved to lower sulphur activities, as indicated by a change from a pyrite-arsenopyrite dominated sulphide assemblage to a pyrrhotite-loellingite assemblage. This important stage in mineralisation is also marked by a distinctive Au-Bi± Te and base metals (Cu>Pb+Zn) signature and extensive retrogressive chloritisation of pre-existing hornfels and higher temperature potassic wallrock alteration assemblages.

Old gold workings to the SE of Enterprise, in the outer aureole (l-2kms from contact), are more restricted in vein development, ore grades and tonnage. These deposits are characterised by relatively simple vein geometries and parageneses, widi highest grades restricted to points of intersection between the principal shear zones and related gently-dipping spur veins. Like Enterprise, veins are best developed where shear zones cross-cut anticlinal hinge zones and on the western limbs of anticlines where gaping is favoured. However vein density is much less intense dian in the adjacent Enterprise deposit, and because the host rocks have not been indurated or silicified to the same extent as those within the aureole, veins are only well developed in more competent lithologies. The salinity and temperature of the 'granitic' fluid drops off markedly outside the inner aureole zone, and the dominant precipitation controls in the more distal deposits appear to be lithology-controlled redox reactions and fluid mixing.

Whether the gold itself is sourced directly from magmas locally or not, the Tabletop Granite played an important role in localising gold at Enterprise through providing a high temperature oxidising fluid which mixed with a lower temperature more reduced metamorphic fluid. This led to destabilisation of gold complexes through boiling, and mixing/redox reactions. Ground preparation is also an important factor in determining fracture character and vein density.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE FUTURE O F OIL AND COAL: TAKING BEARINGS IN THE GREENHOUSE IN A POST BRENT SPAR WORLD Jeremy Leggett Greenpeace International

A greenhouse-related environmental driving-force seems set to emerge in the capital markets. High-level concern that an enhanced greenhouse threatens insurance markets and investments is emerging in the insurance industry in Europe and Japan, and to a lesser extent the USA. This concern, and attendant dialogue, has begun to proliferate into the banks and pension funds. The first recommendations that investments be switched away from fossil fuels, as a means of risk-abatement at source, have begun to appear in the financial-sector. Signs of an awakening public concern over global warming are also emerging. A suspicious recent catalogue of climatic extremes continues to build, edging global warming up the list of the publics' environmental concerns, where it has for a long time barely figured. Meanwhile, the Brent Spar and French nuclear testing episodes point to the dangers of underestimating the publics' capacity for sudden involvement where they are given a chance to act on issues which they are concerned about.

A greenhouse-related environmental driving-force in the capital markets in the years ahead will severely compound other already serious environment-related financial problems blighting the oil and coal industry's access-to-capital radar screen. For oil, these involve the prospect of major liability payouts, public-relations disasters, and huge infrastructure-upgrade costs arising from routine operations, in particular as a result of the disgraceful state of the aging tanker fleet and the equally parlous condition of the pipelines for delivering oil to market overland from the frontier provinces.

In the face of all this, in the years to come, energy companies choosing to stay irrevocably wedded to the unsustainable status quo must raise many hundreds of billions. Will they be able to? Would it not be better, for companies and individual geoscientists alike, to begin positioning now for the increasingly inevitable solarenergy revolution?

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ODP LEG 156: A THIRD PROBE INTO THE BARBADOS ACCRETIONARY PRISM Evan C. Leitch Department of Applied Geology, University of Technology, Sydney. PO Box 123 Broadway, NSW. 2007.

Leg 156 of the Ocean Drilling Program (May - July 1994) was the third ODP cruise to the North Barbados Ridge, the accretionary prism developed at the convergent plate boundary where Late Cretaceous Atlantic Ocean crust is subducted beneath the Caribbean plate. Aims of the Leg included (i) investigation of abnormal fluid pressures within the prism as previously indicated on Leg 78A (the 'inadvertent packer experiment1) and as predicted by thrust fault theory, (ii) determination of the relationship between seismic reflection amplitude anomalies along the decollement zone and fluid pressure, (iii) establishment of the composition and hence origin of fluids within the prism, (iv) documentation of core-scale structures across the decollement zone and comparison of structures in sections above and below this interval, and (v) deployment of long term monitoring systems within the prism. Major results are summarised below.

Logging-while-drilling, deployed for the first time on Leg 156, and drill-string packer measurements, both indicated that the decollement at ODP Site 948 is associated with high fluid pressures, probably at least 90% of lithostatic pressure. The logs show the presence of thin intervals (0.5 to 2 metres) of low density within the decollement zone that suggest dilation and possibly hydrofracturing. Because of the limited period over which the packer measurements could be carried out their interpretation is ambiguous but this ambiguity should be resolved once long-term measurements are retrieved.

Drill sites were selected in part to test the hypothesis that negative polarity seismic reflections represent high porosity zones and zones of high fluid pressure. This idea failed to predict the high pressures indicated at Site 948 where the reflection polarity is positive and hence low porosity and pressure were expected. A possible explanation lies in the width of the high-pressured intervals, which may be below the limit of seismic resolution.

Ship-board analyses indicate that fluids in the subduction complex at Site 948 have dissolved Ca 4 to 5 times higher, and dissolved Mg 30 - 60% lower, than seawater. Up to 18% seawater dilution is indicated by the chloride content of some samples. These compositional changes are a product of low temperature mineral transformations, probably mainly the conversion of smectite to illite that occurs at temperatures of about 40® C provided potassium is available.

The decollement zone separating off-scraped from subducted sediment was cored with good recovery at Site 948. It is manifest as a 35 m interval in which deformational structures, notably scaly fabric, shear zones, and stratal disruption, occur much more commonly than elsewhere. A prominent lithological boundary is found within the zone, the upper part comprising brown siliceous pelagic and hemipelagic sediment, and the lower part interbedded grey-green turbiditic and hemipelagic deposits. The lithological contact is a zone of week strain but the boundary appears to mark a plasticity contrast, with material above the contact having a blocky appearance, with more widely spaced and discrete fractures, whereas that has well-defined zones of scaly fabric and stratal disruption. The dip of bedding changes across the decollement, with inclinations seldom greater than 200 below but ranging widely up to the vertical above. The decollement is also associated with a distinct change in the orientation of the principal axes of the ellipse depicting the anisotropy of magnetic susceptibility.

The change indicates mechanical decoupling at the top of the decollement, the orientation above this level suggesting sub-horizontal shortening of the off-scraped sediment mass in the direction of the convergence vector, whereas that further downhole is consistent with vertical shortening (compaction).

Temperature and pressure sensor strings with borehole seal (CORK) assembly were deployed at Sites 948 and 949 and in addition a mechanical continuous fluid sampler driven by an osmotic pump was attached to the string at Site 949. Neither operation was trouble-free but it is hoped that pressure and temperature can be monitored and fluid samples collected for two years.

Acknowledgments: The results summarised above are the products of the joint efforts of the ODP Leg 156 Scientific Party: J. Ashi, P. Blum, F. Filice, A. Fisher, D. Goldberg, B. Housen, M. J. Jurado, M. Kastner, P. Labaume, T. Laier, E. C. Leitch, A. J. Maltman, A. Meyer, G. F. Moore, J. C. Moore, Y. Ogawa, S. Peacock, A. Rabaute, T. H. Shipley, T. H. Steiger, H. J. Tobin, M. B. Underwood, Y. Xu, H. Yin and Y. Zheng.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EARLY PALAEOZOIC CONVERGENT MARGIN ELEMENTS IN THE NEW ENGLAND FOLD BELT AND THE INCEPTION OF THE PACIFIC 'RING OF FIRE' 1

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Evan C Leitch1 and Peter A Cawood^ a p a r t m e n t of Applied Geologv, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007, Australia Department of Applied Geology, Curtin University

Neoproterozoic rifting of Australia-Antarctica from Laurasia marked the inception of the proto-Pacific Ocean.

The earliest manifestation of destruction of the lithosphere of this ocean is preserved in the southern New England Fold Belt where elements of a Cambrian-Early Ordovician convergent margin are found.

Basinal sedimentary rocks, comprising debris flow conglomerates, turbidite sandstones and hemipelagic siliceous siltstone and thin ash fall tuff that form a sequence up to 1700 metres thick, are the oldest exposed strata in the eastern part of the Tamworth Belt. Low in the sequence limestone boulders contain a Middle Cambrian fauna including trilobites, brachiopods and molluscs. Middle Cambrian paraconodonts occur in situ some c.900 metres up sequence. There is limited palaeontological evidence that suggests sedimentation continued into the Early Ordovician, but by the Late Ordovician shallow water limestones were accumulating above the basinal rocks, and there is no indication of continuing nearby volcanic activity. The basinal rocks accumulated below storm wave base off a magmatic arc that was situated to the (present day) west The presence of Cambrian magmatic arc rocks is indicated by volcaniclastic debris found in the Cambrian (?)Early Ordovician sedimentary rocks. The clastic debris includes abundant plagioclase, augite and volcanic lithic clasts. The proportions of components, mineral chemistry and the major and trace element composition of clasts indicate that this detritus was derived from a low-K tholeiitic series that ranged in composition from basalt to rhyolite and in which andesite was probably most abundant. Ash fall tuffs in the basinal sequence indicate that volcanic activity was contemporaneous with sedimentation.

Ophiolitic rocks of supra-subduction zone affinity occur along the Peel - Manning Fault System which lies immediately east of the basinal rocks. SHRIMP zircon ages of about 535 Ma indicate that these rocks are of Early Cambrian age. Based on their regional relationships, and the relationships of similar rocks in less disrupted regions, for example the Great Valley (California) and the Maitai terrane (South Island, New Zealand), they are interpreted as basement to the basinal sequence.

Blueschist and eclogite blocks occur in serpentinite-matrix melange at a number of localities in New England.

The eclogites are of MORB compositions and they and the enclosing serpentinite were derived from subducted oceanic lithosphere, the eclogite reulting from the high pressure metamorphism of mafic dykes that were boudinaged during serpentinisation of the enclosing ultramafic rocks.

Phengite, which occurs closely associated with glaucophane or crossite in late metamorphic veins in some blocks, has yielded K/Ar closure ages of about 470 Ma that are interpreted as dating exhumation of the rocks, probably at the end of an episode of subduction.

Original relationships between the magmatic arc - arc-flank basin, ophiolitic arc basement rocks and highgrade metamorphic blocks are not directly demonstrable but their present distribution suggests development in an east-facing arc. No accretionary complex of Early Paleozoic age is preserved in the fold belt, probably because, as with many modern intraoceanic active margins, subduction accretion did not accompany convergence.

We conclude that the lithosphere of the proto-Pacific was being subducted by about 535 Ma, and that this interval of subduction continued for 60-70 my. Subsequent episodes of convergence, over the periods about 400 - 300 Ma and 250 - 220 Ma, added to the New England Fold Belt supporting the contention that once a subduction zone is initiated it is likely to be the site of convergence over an extended period. More recent activity related to essentially the same plate boundary can be recognised further east. The New England rocks thus mark the inception of the Pacific 'ring of fire1. It is notable that despite a history of convergence extending for over 500 my there has not been a major continental collision associated with this plate boundary, attesting to the profound fecundity of the spreading system within the Pacific Ocean and its precursors.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NABRE SEISMIC - PROVIDING THE THIRD DIMENSION J.H. Leven, D.M. Finlayson and NABRE Team Australian Geological Survey Organisation, GPO Box 378 Canberra ACT 2601

Much of the known mineral wealth of northern Australia is contained in Proterozoic basins in the Mt Isa McArthur River region. The North Australian Basins Resource Evaluation (NABRE) project aims to develop a time-series predictive framework for Palaeoproterozoic and Mesoproterozoic basin evolution in this region.

An essential part of the NABRE project is the use of seismic techniques to image the third dimension, enabling the outcrop geology and existing well and seismic data to be integrated into a model for the basin development in this region. We are proposing to acquire around 225 km of regional reflection seismic data and a coincident wide-angle seismic profile on the Lawn Hill Platform crossing the Murphy Inlier into the McArthur Basin in northwestern Queensland. The reflection seismic will provide the detailed stratigraphy and structure and the wide-angle seismic will provide velocity (compositional) and structural information.

AIMS OF THE PROPOSED NABRE REFLECTION SEISMIC PROFILING: • Correlate between the Mt Isa and McArthur Basins over the Murphy Inlier, and investigate the geometry of the sequences which lap onto the inlier • Investigate the geometry and timing of movement on key faults to test existing basin models • Provide the depth dimension on regional profiles to understand the shape and number of basin containers and enable integration of existing data for the generation of a new more comprehensive model for basin development.

AIMS FOR THE PROPOSED NABRE WIDE-ANGLE SEISMIC PROFILING: • Define velocities and structure within the Proterozoic basin sequences and within the underlying Archaean - Early Proterozoic basement • Complement the near-normal incidence reflection seismic profiling and enable a more robust interpretation beyond the depth of drillholes and outcrop correlation • Provide tight constraints on gravity interpretations within the region.

138 00 OOE 17 00 OOS

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The targets of wide-angle seismic profiling HtSPiFfcoos will be the definition of velocity architecture within the upper crust to depths of about 20 km and the nature of adjacent major faults.

Wide-angle seismic profiling will also be conducted to investigate lower crustal architecture (possible mineralisation source regions) of the Lawn Hill Platform and Murphy Inlier down to Moho depths.

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A prognostic cross-section along the proposed seismic line has been constructed from existing seismic and published geological maps. This cross-section highlights the stratigraphic and structural issues that will be addressed by information from the third dimension provided by the regional seismic program.

Figure 1. Line location of the proposed NABRE reflection profile (in the thick line) planned for 1996. The wide-angle seismic is planned to be coincident with the main NNW-SSE reflection traverse, and extend 50 km farther north and south.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

M A K I N G GEOSCIENCE EXCITING, RELEVANT AND T E A C H A B L E - A G S O GEOSCIENCE EDUCATION Gary B. Lewis Australian Geological Survey Organisation, GPO Box 378. Canberra. ACT 2601

Current research shows that while most sciences are on the increase within our Australian school community, geoscience is in rapid decline. Some states no longer offer senior secondary earth science while "geology" is an endangered species in others.

While the reasons for this demise are complex, one solution can be found in the fact that the majority of science teachers at both secondary and primary levels, have little or no geoscience in their own academic background. Their perception of geoscience is that it is only about the classification of rocks and minerals which has no relevance to their students. The solution to this is simple - show teachers the true nature of geoscience. Show them that it is exciting (volcanic eruptions, earthquakes, grinding plates), relevant (provides evidence for the climate debate, influences the environmental landscape and provides answers for environmental problems) and teachable (fits into the national and state curricula, can be presented as simple concepts with simple models).

The Australian Geological Survey Organisation's (AGSO) Geoscience Education Program, which has operated over the last two years, aims to do precisely this. The program consists of a number of teacher professional development sessions and teacher resources which highlight an aspect of geoscience. Teaching notes and classroom activities are provided that allow non-geoscientific, and is some cases non-scientific, teachers to introduce interesting geoscience experiences into their teaching. Topics include Plate Tectonics.

Volcanoes, Earthquakes, Climate Change, Time and Life, Silicate Chemistry and The Science of Gold.

Since its introduction the program has run sessions for over 425 teachers and sold 1600 booklets, which teachers estimate they will use with over 200,000 students.

By raising teachers' awareness and increasing their enthusiasm for geoscience, we can ensure that a broader section of our community, through students and their families, will have an appreciation of the relevance of geoscience to both the economic and environmental future of Australia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, CanberraFebruary 1996 ROLES OF THE MAJOR EAST ASIAN CRATONIC BLOCKS IN THE ASSEMBLY AND BREAKUP OF SUPERCONTINENT RODINIA Z.X.Li , L.Zhang ' and C.McA. Powell Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA 6907, Australia Department of Geology, China University of Geosciences, Beijing 100083, China The Tarim, North China, and South China Blocks of East Asia have records of the Grenville-age orogenies which led to the formation of Neoproterozoic supercontinent Rodinia, and the rifting events that led to the breakup of Rodinia during late Neoproterozoic. They also share biostratigraphic affinities with the Australian margin of Gondwanaland for at least parts of the Early to mid-Palaeozoic. Nevertheless, there are significant differences between them: (1) Grenville-age orogens were well developed in the South China Block, but are seen only along the very southern margins of the North China and Tarim Blocks, and are shown as disconformities in other parts of the latter two blocks; (2) Three glacial intervals, two in the Lower Sinian, and one in the upper Sinian, have been recorded in the Tarim Block, but only the lower two were developed in the South China Block, and the top one in the North China Block; (3) Lower Cambrian mafic volcanic and tuffaceous succession were developed only along the northeastern margin of the Tarim Block. We present here a working hypothesis which puts the Proterozoic-Early Palaeozoic geology of the three East Asian cratonic blocks in the context of the evolution of the Rodinia and Gondwanaland supercontinents.

1

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SOUTH CHINA: PART OF THE MISSING LINK IN THE SWEAT HYPOTHESIS? We proposed that the South China Block could have been part of the missing link between Australia and Laurentia in the early Neoproterozoic Rodinia reconstruction. The sutures currently within and around the South China Block recorded part of the plate aggregation which led to the formation of Rodinia at ~1 Ga. Prior to this, the Cathaysia Block could have been part of an early Mesoproterozoic metamorphic belt in western Laurentia which provided sedimentary detritus to the Belt basin. Such a reconstruction is consistent with the similarities between the early-Neoproterozoic tectonostratigraphy of the South China Block, the Adelaide "Geosyncline" in Australia, and the western margin of Laurentia. Stable platform marginal successions were deposited in all three regions during the Late Sinian (-700-545 Ma), except in parts of the southeastern China where rifting continued into the Palaeozoic. The South China Block does not have the second glacial event that was recorded in both Australia (Marinoan glaciation) and in western Laurentia (Ice Brook glaciation), nor does it show any effect of the Paterson-Petermann and Delamerian Orogenies of Australia. It thus may have been separated from the Australian and Laurentian cratons by the Late Sinian.

TARIM BLOCK: ONCE CONNECTED TO THE K3MBERLEY REGION? Basin history of northwestern Australia indicates that a continental block broke away from outboard northwestern Australia at -545 Ma. The Palaeoproterozoic metamorphic basement, Meso- to Neoproterozoic tectonostratigraphic record (including three glacial intervals), and the Early Cambrian mafic volcanic successions of the Tarim Block make it comparable with the Kimberley region of northwestern Australia. We suggest that the Tarim Block could be the continental block that broke-away. The Antrim Plateau volcanics in northwestern Australia and the Bonaparte Basin, and the mafic volcanics in the northeastern Tarim Block, were probably related to such a breakup event.

NORTH CHINA BLOCK: ONCE CONNECTED TO SIBERIA? The Proterozoic tectonostratigraphy of the North China Block resembles that of Siberia. The Changcheng and Jixian Systems of North China can be correlated with the Riphean 1 and 2 of Siberia Craton, respectively.

Shallow-marine successions were developed on both cratons during the early Neoproterozoic (-1000-850 Ma), following which there was a major uplift. Major sequences and sequence boundaries during late Neoproterozoic (-800-540 Ma) can also be correlated. The Sinian successions in western Henan Province of North China are particularly similar to those in northern and eastern Greenland, where only Vendian glacial deposits were developed. These tectonostratigraphic similarities lead us to suggest that the North China Block may have been joined with Laurentia and Siberia during almost the entire late-Palaeo- to Neoproterozoic interval. A Grenvillian subduction zone probably extended from east and northeast Laurentia to the southern margin of the North China Block in a arc-trench setting, which caused only minor tectonic disturbance to the craton.

Rodinia broke up in late Neoproterozoic. Fragments of it collided on the other side of the globe before midCambrian to form the supercontinent Gondwanaland. After being separated from the larger cratons towards the end of the Neoproterozoic, all three East Asian blocks drifted close to the Australian margin of Gondwanaland from Cambrian until Devonian.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

COMPUTER SIMULATION OF THE INFLUENCE OF BASIN PHYSIOGRAPHY ON THE INITIAL TIMING AND DURATION OF CONDENSED SECTION DEPOSITION Trent C. K. Liang , Kevu Liu , Lincoln Paters on , Chris, SL G. C. Kendall 1 Department of Geology, The Australian National University, Canberra, ACT 0200, Australia 2 Division of Petroleum Resources, CSIRO, PO Box 3000, Glen Waverley, VIC 3150, Australia 3 Department of Earth Sciences, University of South Carolina, Columbia, SC 29208, USA

INTRODUCTION Sequence stratigraphic analysis commonly involves the correlation of condensed sections which are widely regarded as chronostratigraphic markers. Condensed sections represent periods of very slow deposition due to sediment starvation and have generally been correlated with the maximum flooding surface or associated with the highest relative water depths, inferred sometime between the maximum rate of base level rise and the highest point of base level. However, similar sediment starvation could conceivably be produced by a rapid landward shift in the shoreline over vast distances involving a relatively lower rate of base level rise or a relatively smallerrisein the base level over a very low-gradient surface. Condensed sections need not always correspond to a period between the maximum rate of base levelriseand the highest point of base level but in theory could correlate with the initial transgressive surface. The initial timing and the duration of condensed sections can therefore be expected to be influenced by the basin physiography.

Stratal geometries are influenced by base level, subsidence or uplift, sediment supply and basin physiography.

Of these, base level, subsidence or uplift, and sediment supply are widely documented factors affecting the timing of condensed section deposition. However, the significance of the influence of basin physiography on condensed section deposition has commonly not been emphasised. Apartfromthe influence on the amount of sediment supplied to the basin, basin physiography influences the rate of transgression during a rise in base level and is likely to be an important controlling factor of the amount of sediment supplied to the depocentre.

COMPUTER SIMULATIONS The above scenarios have been illustrated by computer simulations using SEDPAK, an empirical forward modelling program that simulates the sedimentary fill of a basin in two dimensions by considering base level variations, tectonic movement (subsidence/uplift), sediment accumulation and the initial and evolving basin profiles. The computer program allows us to systematically quantify the variations of the initial timing and duration of condensed section deposition for various examples. The influence of physiography can be observed by applying consistent values for base level, subsidence and sediment supply to differing profiles. Simulations can predict the variable timing of sediment starvation for profiles with contrasting characteristics such as shelf elevation, shelf width and gradients. Even at different locations within the same basin the initiation and duration of condensed section deposition can be significantly different depending on the physiography. Those condensed sections that are initiated earlier might be expected to represent a greater period of time.

CONCLUSIONS Basin physiography is a factor that can have a profound influence on the timing of condensed section deposition, together with base level, subsidence or uplift, and sediment supply. Condensed section deposition, by definition, is not represented by an instant in time and may correspond to relatively long lengths of time on the rising limb of a base level curve. Although they can make up a relatively thin proportion of a selected stratigraphic interval, they can represent deposition over a relatively large portion of a base level cycle, and therefore contribute to potential error in chronostratigraphic correlations. Thisfindinghas further cast doubt on the reliability of the conventional approach of global correlation of strata, by comparing with global sea-level curves derived from sequence stratigraphic analysis using condensed sections as eustatic sea-level and chronostratigraphic markers. It has significant implications for sequence stratigraphic analysis of intracratonic basins and broad shelf settings which, in contrast with passive margins and foreland basins, are generally characterised by low-gradient basin floors.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MODELLING ARGON DIFFUSION USING THE MacARGON PROGRAM

Gordon S. Lister Victorian Institute of Earth and Planetary Sciences.

Department of Earth Sciences.

Monash University, Melbourne 3 J68, Australia

The MacArgon computer program was developed to allow simulation of the effect of arbitrary P-T-t histories on solid state diffusion in minerals, assuming specific P-T-t histories as input. Other input parameters include experimentally derived argon diffusion parameters for different minerals. We assume loss of argon occurs via solid state diffusion from a single diffusion domain, and an essentially numerical approach was chosen for the computation path. To use the MacArgon program the diffusion domain parameters must be specified and then a P-T-t history must be chosen. The simulation is then set in progress, and runs to completion. The results can then be analysed using the MacSpectrometer, and various options allow output and/or further analysis of the data produced. The program will be available running on a Powerbook with an overhead display, and the participants will be able to work with the program to model their own apparent age spectra.

Using the MacArgon program and its associated MacSpectrometer we have applied solid state volume diffusion theory to simulate the effect of specified P-T-t paths on 40 Ar/ 39 Ar apparent age spectra. Argon diffusion in a mineral grain can be predicted using the Arrhenius relationship for diffusivity (D): D = D0 exp[- (Q+P v )/RT] as long as the following parameters are known: Q, the activation energy for diffusion; v the activation volume; D 0 the frequency factor; the geometry of the diffusion domain; and JJ, the appropriate radius or half-thickness of the diffusion domain.

The following conclusions have been reached (see paper by Lister & Baldwin, 1995, Tectonophysics, in press): (1) the concept of a "closure temperature" is relevant only to a P-T-t history which involves cooling from temperatures at which the mineral is unable to accumulate a significant concentration of radiogenic argon.

(2) there is no single closure temperature above which a mineral will degas abruptly. The rate of argon loss depends on the duration and magnitude of a period of elevated temperature. A blocking temperature can be defined above which a mineral grain will lose a significant fraction of its accumulated argon during a heating experiment of known duration.

(3) there is a theoretical maximum age that can be retained by a mineral at a particular pressure and temperature, assuming these conditions are applied for a sufficiently long period of time to allow the establishment of a quasi-steady state 40 Ar* concentration gradient.

(4) a mineral can retain an age greater than its theoretical maximum age at a particular temperature and pressure if these conditions are only transiently maintained (i.e., these are not the ambient conditions).

(5) increase in pressure at depth might significantly increase retentivity of different minerals.

(6) mineral grains held at high ambient temperature will display apparent age spectra that are similar in form to those affected by an episode of partial loss, or slow cooling. However these plateaux ages reflect neither the time of closure, nor the time at which a thermal pulse caused an episode of argon partial loss.

Any constraints offered by our analysis are limited by our present understanding of the systematics of argon loss in K-bearing minerals, and by uncertainties posed by the practical limitations of analytical techniques (such as those caused by the dehydroxylation of mica in vacuo or uncertainties in relation to excess argon). More laboratory experiments are needed to specify diffusion parameters.

This presentation is based on the MacArgon program, written by G.S. Lister. The results outlined above are part of a publication "Modelling the effect of arbitrary P-T-t histories on argon diffusion in minerals using the MacArgon program for the Apple Macintosh" by Gordon Lister and Suzanne Baldwin to be published in Tectonophysics (currently in press).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

M O D E L L I N G ARGON SPECTRA FROM THE ISLAND O F SIFNOS, AEGEAN S E A , G R E E C E Gordon Lister and Adamandia Raouzaios

Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences. MonashUniversity, 40

Melbourne 3 J 68, Australia

39

The MacArgon program has been used to model Ar/ Ar apparent age spectra for white micas from the island of Sifnos, Cyclades, Greece. Different P-T-t paths have been proposed for the Sifnos rocks by previous authors.

We have used these P-T-t paths as input to the MacArgon program to predict the (theoretical) 4 0 Ar/ 3 9 Ar apparent age spectra that would result.

First we consider the classic Alpine P-T loop. This involves high pressure metamorphism followed by isothermal decompression until Miocene greenschist facies retrogression takes place. An approximately isothermal drop in temperature takes place from 500°C to 450°C while decompression takes place from 14 kbar to 5 kbar. With such a P-T-t history, closure in the mica grains takes place only after Miocene greenschist retrogression is complete. Relatively young apparent ages are obtained from the simulations, because mica held at ~450°C for several million years will retain no memory of the older high pressure event. Old apparent ages could not be retained using such a P-T-t loop. Thus the classic Alpine P-T path proposed by previous workers is inconsistent with the geochronological data.

To avoid this difficulty subsequent workers {e.g., Wijbrans and co-workers) suggested a significant modification of the P-T trajectories that must be followed by the Sifnos rocks. This involves a thermal excursion during the Miocene, which is not felt by the well-preserved high pressure rocks. These continue to slowly cool. However, to explain the data from the high-pressure rocks on Sifnos we will show that P-T-t paths input into the MacArgon program have to involve rapid cooling from about -40-42 Ma. Otherwise significant partial loss of argon is observed in the simulated spectra. Hence we can also eliminate P-T-t paths that imply slow cooling of the high pressure rocks.

There is a discrepancy between the apparent age spectra that^have been predicted using MacArgon simulations based on published P-T-t paths in comparison with the apparent age spectra that have actually been measured on Sifnos. The next step therefore was to use the MacArgon program to constrain the range of P-T-t paths that are capable of producing apparent age spectra that match the measured spectra. To do this we adopted a parametric approach. Parametric inversion is accomplished in the following way. Two P-T-t paths are chosen that are substantially different, one from the other, in some essential ingredient. These are end-members, and we consider a range of P-T-t paths that vary from one end-member to the other. The computer works out each possible P-T-t path by performing a linear combination of the two paths specified above as end members. The computation begins with one end member P-T-t path (£=0.0), and then increments the linear combination factor, until the final calculation is performed for the other end member P-T-t path (£=1.0). This is described by variation of the linear combination factor, in the range 0.0-1.0. We can then vary the parameters that describe a P-T-t history until we obtain reasonable matching of the apparent age spectra p r o d u c e d in the MacSpectrometer in comparison with the actual measured apparent age spectra. In this way we define a range of parameters that produces apparent age spectra that are compatible with those actually measured from Sifnos.

Flat apparent age spectra can be replicated by MacArgon simulations based on the diffusion parameters for phlogopite if the eclogite-blueschist domain on Sifnos cooled rapidly from peak temperatures (~500°C, 14 kbar) to <~350°C at rates >~50°C/MYr. Cooling must continue after the period of rapid temperature decrease otherwise significant argon loss will take place. MacArgon simulations suggest that cooling in the greenschist domain on Sifnos started at - 3 2 Ma at rates of ~50°C/MYr and rapid cooling continued until temperatures were <~350°C. The simulations also imply that the greenschist domain has not been significantly affected by a Miocene thermal pulse. Significant partial loss would be observed in the measured apparent age spectra if temperatures had exceeded ~350°C for more than ~2 MYr. This contribution is based on material included in a paper "The tectonic significance of a porphyroblastic blueschist facies overprint during Alpine orogenesis: Sifnos, Aegean Sea, Greece" by Gordon Lister and Adamandia Raouzaios. This has been submitted for publication to the Journal of Structural Geology and is currently in review.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberray February 1996

S E D I M E N T O L O G Y , STRATIGRAPHY AND SEQUENCE STRATIGRAPHY O F T H E NEWCASTLE COAL MEASURES, SYDNEY BASIN, AUSTRALIA Little, M.1. Boyd, R.1, Brunton, J.2, Diessel, C.1, Ives, M.\ Rigby, R.4 and Tobin, C.5 1. Department of Geology, University of Newcastle, NSW 2308 2. NSW Department of Mineral Resources, Newcastle, NSW 2285 3. Powercoal, Lake Macquarie, NSW 2283 4. Newcastle Wallsend Coal Company, Cessnock, NSW 2325 5. Oceanic Coal, Teralba, NSW 2284

The non-marine Newcastle Coal Measures (up to 400m thick) were deposited during the Late Permian in the Sydney Basin, a technically active foreland basin. The Newcastle Coal Measures consist of alluvial conglomerate, sandstone, siltstone, shale, tuff and coal (16 informally named coal seams). Stratigraphic review of the Newcastle Coal Measures has been conducted by correlation of coal and tuff horizons across a framework grid of 173 boreholes over 540km2. The correlations confirm that five tuff horizons, the Nobbys Tuff, Warners Bay Tuff, Awaba Tuff and two unnamed tuffs, are present on a regional basis. These regional tuffs, in addition to thinner tuffs that occur within individual coals, permit high resolution lithostratigraphic correlation within the non-marine strata.

Conglomerate/sandstone bodies are interpreted as fluvial deposits which upwardly change in style from meandering to braided. The bodies are multi-channel, vertically stacked units that upwardly thicken and coarsen in grain size, reflecting the progradation of the Newcastle Coal Measures. These in-channel bodies are up to 100m thick and show minimal erosion of underlying coal seams.

Sequence stratigraphy has so far not proved a useful tool in dividing and correlating the Newcastle Coal Measures.

Conglomerate/sandstone bodies do not infill incised valleys and do not truncate coal seams or regional tuffs and therefore are not sequence boundaries. Major flooding surfaces have not been identified and correlations 80km down dip do not connect with marine horizons. Technically influenced sediment supply is the dominant control on sedimentation. Rapid subsidence combined with an overwhelming volume of supplied sediment is responsible for the lack of sequence boundaries and marine flooding surfaces. The entire non-marine Newcastle Coal Measures have the characteristics of a highstand systems tract.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996 Greenstone sequences and structural evolution of the Yakabindie-Leinster area in the northern Eastern Goldfields, WA Songfa Liu ' , Tim J. Griffin Geological Survey of Western Australia, 100 Plain St, East Perth, WA 6004 "Australian Geological Survey Organisation, GPO Box 378, ACT 2600 1 2

1

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Located in the northern Eastern Goldfields of the Archaean Yilgarn Craton, the Yakabindie-Leinster area comprises part of the Mt Keith-Perseverance, Agnew and the newly recognized Yakabindie greenstone belts.

The Yakabindie greenstone belt comprises a layered, metamorphosed intrusive sequence of the Kathleen Valley Gabbro, overlain by the metamorphosed massive tholeiitic Mt Goode Basalt. The Agnew greenstone belt comprises a lower sequence of metamorphosed ultramafic, mafic, felsic volcanic and sedimentary rocks in the Agnew and Leinster Anticlines that is overlain by an upper sequence of metabasalt, metagabbro and metasedimentary rocks in the Mt White Syncline. The western part of the Mt Keith - Perseverance belt is similar to the sequence in the Mt White Syncline. Metamorphosed ultramafic, mafic, felsic volcanic and sedimentary rocks in the Perseverance - McDonough Lookout area extend northwards to the Mt Keith area. The Jones Creek Conglomerate represents a late greenstone sequence and is restricted to a narrow, fault-bounded zone west of the main Mt Keith - Perseverance and Agnew greenstone belts.

Four deformation events are recognized in the granite-greenstones in the Yakabindie-Leinster area. An S! foliation is associated with the flattening of basalt pillows in the Mt Goode Basalt, and D also produced inclined east-plunging folds in the rocks of the Mt Keith-Perseverance belt 1 km east-northeast of Mt Sir Samuel. Major compression occurred during D , continued into D , producing the north-northwest linear structures, greenstone belt trends and regional folds, and generally shaped the crustal architecture. We interpret the contacts between greenstone and granitoid in the Yakabindie area and in the area west of Perseverance mine as thrusts. D produced some normal faults, fractures and subhorizontal crenulations. A major phase of regional metamorphism occurred during D , and the metamorphism peaked late during or after-D . Petrographic evidence suggests there was also a metamorphic event prior to D . Granitoid intrusion occurred throughout the deformation and metamorphic history in the Yakabindie-Leinster area.

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Eleven Mile Fault

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ARCHAEAN CALC-ALKALINE VOLCANISM IN THE MCPHEE DOME AND KELLY BELT, EASTERN PILBARA

2

Stefanie E. Loader1, Mark E. Barley 2 and Neal J. McNaughton 2 *CRA Exploration Pty Ltd, Wynyard Street, Belmont, WA 6104 Key Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands, WA 6907

The Pilbara Craton is a piece of ancient continental crust in North Western Australia. Subaerial to submarine, calc-alkaline, intermediate to silicic volcanic rocks and coeval granitoids are integral components of the early Archaean (> 3.0 Ga) granitoid-greenstone terrane in the eastern Pilbara Craton. Consequently the pedogenesis and setting of these volcanic rocks has important implications for models for the tectonic evolution of Archaean continental crust. SHRIMP and conventional U-Pb in zircon dating indicates that the calc-alkaline volcanic sequences range in age from 3.52 Ga in the Lalla Rookh and Pilgangoora Belts, through 3.47 to 3.45 Ga in the Marble Bar Belt and North Pole Dome. Granitoids of these ages are also common within adjacent batholiths.

Extensive and well exposed successions of mafic to intermediate calc-alkaline volcanic rocks crop out in the McPhee Dome and Kelly Belt south of Marble Bar. New SHRIMP dating of intermediate lavas in the McPhee Dome (3.43 Ga) and Kelly Belt (3.42 Ga) indicates that volcanism in these belts was younger than that near Marble Bar and further north and that early Archaean calc-alkaline volcanism in the eastern Pilbara spans 100 myr. This means that simple lithostratigraphic correlation of felsic volcanic successions in eastern Pilbara greenstone belts is invalid.

Calc-alkaline volcanic rocks in the McPhee Dome and Kelly Belt overlie submarine tholeiitic basalts and range from plagioclase-phyric basalt through andesite to dacite. Basalts and andesites are pillowed or massive flows with intermediate lavas interbedded with hyaloclastite and volcaniclastic sediments. Tops of successions show evidence for reworking in shallow-water environments adjacent to partly emergent volcanoes, and submarine hydrothermal Cu-Zn sulphide mineralization is developed sporadically (e.g. Copper Gorge). Most lavas are pervasively hydrothermally altered and commonly carbonate-rich., although relatively unaltered domains are preserved locally. Trace element contents of the least altered lavas are typical of calc-alkaline volcanic rocks of all ages with enriched light REE, mostly unfractionated heavy REE (one dacite from Copper Gorge is depleted in heavy REE) with either no Eu anomaly, or a small negative Eu anomaly, on chondrite normalised diagrams, and negative Ta, Nb, and Ti anomalies on MORB normalized trace element diagrams. These features reflect varied interaction of melts of primitive mafic or intermediate crust with mantle-derived basaltic magmas via ACF processes. There is no isotopic evidence for a significantly older sialic component in the magma source. This petrogenetic scenario is similar to that in many modern volcanic arcs, although the dominance of intermediate lavas and coeval granitoids implies that melting of older mafic rocks, either at a subduction zone or the base of the crust, was more extensive than is common in modern arcs built on submarine rocks. The tectonic explanation of the age distribution of calc-alkaline volcanic successions in the eastern Pilbara is uncertain, but one possibility is that calc-alkaline arc magmatism migrated south with time as the early Pilbara continent grew.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 HIGH RESOLUTION SEISMIC STRATIGRAPHY OF LATE QUATERNARY BEDROCK-CONTROLLED INCISED VALLEY FILL, SOUTHERN MORETON BAY Duncan A. Lockhan. Simon C. Lang and George P. Allen School of Geology, Queensland University of Technology, GPO Box 2434, Brisbane, 4001 Australia

Moreton Bay forms an 80 km long back-barrier lagoon system up to 25km wide opening to the Pacific Ocean towards the north. This study focuses on the southern extremity of Moreton Bay where the lagoon narrows to less than 10km and is affected by a relatively large fluvial influx from the adjacent Logan River. Continuous seismic profiling around Southern Moreton Bay using a Uniboom system shows a series of incised valleys that traversed a coastal plain towards palaeo-shorelines lying to the east of the present barrier island system during the glacio-eustatic lowstand experienced during the late Quaternary.

These valleys (up to 35m deep and up to 2km between interfluves) represent the bedrock-incised palaeo-Logan River system. The high resolution seismic profiles show a consistent pattern of fill that can be related to distinct phases of relative sea-level and sedimentary processes. The seismic stratigraphic and stratal architecture in these profiles show that the incised valleys in Southern Moreton Bay generally exhibit four-phases of sedimentary fill, characterised by distinctive erosion surfaces and seismic reflectors.

Phase 1 overlies a major erosion surface and consists of laterally accreting deposits onlapping onto the lowermost part of the incised valley walls and migrating towards the thalweg. These are interpreted as onechannel thick fluvial lowstand point-bar deposits up to 4-5m thick and 2km wide. Several deep auger holes on the coastal plain confirm the presence of quartzite gravel fining up into peat at depths of between 29 to 36m.

The lack of significant aggradation implies that sea-level was constant during this period.

Phase 2 consists of a vertically aggrading point-bar system building and continuing on from the earlier deposits without significant truncation and onlapping the incised valley walls. These deposits are interpreted as an early transgressive succession consisting of heterolithic estuarine point-bar sediments characterised by well developed lateral accretion reflectors with strong amplitude contrast. This phase is confined within the broader interfluves of the existing incised valleys. Sediments intersected on the coastal plain by auger drilling which are considered as representative of this phase are dominated by sandy mud and mud. These sediments are characteristically olive-green to black in colour with the minor presence of shells.

Phase 3 consists of an aggradational stack characterised by poorly defined reflectors that overlie a prominent laterally extensive erosion surface which represents a tidal ravinement surface when the tidal lagoon transgressed the estuaries. Within Phase 3 several erosion surfaces can be recognised and these may also represent tidal ravinement surfaces within an overall late transgressive tidal dominated lagoon. These surfaces commonly mimic the present day tidal channels in form, dimension and location. In many example their locations appear to be determined by the pre-existing location of major bedrock incised valleys. Phase 3 sediments are not confined to the incised valley walls and represent a change from relatively restricted estuarine conditions into a more open shallow lagoonal environment similar to the conditions experienced in the present day lagoon. The majority of sediment for this phase was most likely supplied by the existence of a large flood-dominant tide delta situated between a developing barrier island (South Stradbroke Island) and the (Pliestocene) dune-island barrier of North Stradbroke Island, with a minor although significant fluvial input from the Logan fluvial system.

Reflectors within this phase are distinctly chaotic and weakly developed allowing them to be easily distinguished from the high amplitude reflectors of the underlying Phase 2 sediments. Auger drilling on the coastal plain confirms the existence of muddy and clean marine sands within this phase.

Phase 4 sediments are confined to the landward edge of the lagoon, the bayhead delta islands of the Logan River and the mangrove colonised tidal flats of the bedrock islands within the lagoon. The sediments overlie a ravinement surface that truncates most of the underlying succession. Shallow auger drilling on the bayhead delta islands of the Logan River confirms the existence of at least 5m of fluvially dominated sand, silt and mud. The maximum flooding surface for the Holocene transgression is represented by a diachronous surface between Phases 3 and 4 and can be recognised on some seismic profiles as a downlap surface. Phase 4 therefore represents the regressive infilling of the lagoon during the present highstand.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra., February 1996

Terminal Proterozoic Reorganization of Biogeochemical Cycles Graham A. Logan , J.M. Hayes , Glenn B. Hieshiema , and Roger E. Summons 12

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'CSIRO Division of Petroleum Research, PO Box 136, North Ryde, NSW 2113.

Australian Geological Survey Organization, PO Box 378, Canberra, ACT 2601.

Biogeochemical Labs., Indiana University, Bloomington, Indiana 47405, USA. Exxon Production Research Company, PO Box 2189, Houston, Texas 77252, USA. 3

4

The Terminal Proterozoic is a time of immense change for metazoan life. The impact of the radiation of animals with a gut has had a profound effect on the environment and may have been fundamental in the creation of a stable environment for colonisation. Organic matter has been studied in Proterozoic formations indicating that there were significant differences in the C isotopic compositions of certain organic compounds relative to associated kerogen compared to Phanerozoic material. These differences can be explained if heterotrophic activity was more intense in the Proterozoic. Such a situation could occur if the organic matter was extensively reworked by heterotrophic bacteria. Faecal pellets sink rapidly through the water column compared to smaller, less dense algal debris. As this debris slowly sank it was extensively degraded, by the time it reached the sediment only the most resistant material was left. Thus the isotopic relationships in the Proterozoic can be explained by a combination of inputs from resistant algal debris and lipids derived from late stage hetrotorphic bacteria. The evolution of faecal pellets lead to a rapid transport of organic matter out of the water column and into the sediment, thus leading to enhanced preservation of a stronger algal signal and a decrease in the contribution from heterotrophic bacterial sources.

I3

The evolution of faecal pellet transport also had a profound effect on the chemistry of the water column. When slow sinking dominated the water column degradation occurred near the surface and lead to consumption of oxygen instu. Only after a portion of the organic matter was removed from the surface waters could photosynthetically produced oxygen become available for metazoan activity.

Bacterial sulfate reduction would have also been very important in the water column in the degradation of slow sinking algal debris and a dynamic biological redox buffer may have existed ensuring that the deep waters were consistently anaerobic through out the Proterozoic. After faecal pellet transport became significant the redox buffer would have been broken and the environment stabilised for further metazoan activity. This may also be a key event in the increased abundance of phosphorites in the Terminal Proterozoic and into the early Cambrian.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EXPERIMENTAL SOLUBILITY OF GOLD IN SULPHIDIC BRINE TO 625 °C AND 4 KBAR MEASURED IN SYNTHETIC FLUID INCLUSIONS BY ICPMS-ULTEMA Robert R. Loucks and William Hibberson Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200

Experimental data on gold speciation and solubility in sulphidic chloride brines have been limited to T < 350 °C and P < 165 bars (Hayashi & Ohmoto, 1991). We extend the experimental database to 625 °C and 3700 bars and derive refined thermodynamic properties of the relevant aqueous species. The reaction studied is Au + 3/2 H2O + 1/4 FeS + 7/8 FeS2 = 3/8 Fe304 + HAu(HS$ (aq> The /S2, fH^ and pH buffer assemblages pyrrhotite + pyrite + magnetite + water (Py-Po-Mt-W) and muscovite + orthoclase + quartz (Mu-Or-Qz) were loaded in a gold foil capsule, together with -0.06 ml of 1-molal KC1 solution spiked with 7.5 ppm Th and 44 ppm U. U and Th are insoluble in the buffer minerals and quantitatively retained in the fluid. The sealed capsule was run in a cold-seal vessel 10 days at 625 ± 3 °C and 3.7 ± 0.2 kbar. During the run, a quartz rind containing primary fluid inclusions up to 20 |im in diameter was precipitated as an overgrowth upon the starting quartz pieces.

A new analytical instrumentation system—the Ultraviolet Laser Trace Element Micro-Analyzer (ULTEMA)— has been developed in the Research School of Earth Sciences, in a major initiative to devise innovative analytical tools, experimental techniques, and theoretical appproaches to underpin our expanding programme of basic research in hydrotheimal geochemistry. The ULTEMA uses a 193-nm wavelength excimer laser microbeam for photo-chemical ablation of solids and fluid inclusions for quantitative analysis of their elemental composition by inductively-coupled-plasma mass spectrometry (ICPMS). Fluid inclusions synthesized in the gold-solubility experiment were opened by the ULTEMA to measure dissolved Au, employing the U and Th as internal standards, and using a synthetic glass standard to calibrate the relative ionization efficiencies of Au, U, and Th in the ICPMS (Fig. 1). Using -10 cm as the partial molar volume of HAu(HS)2 (derived from Renders & Seward, 1989) and using mineral and water molar volumes (SUPCRT 92), we evaluate (3 log m^ /dP)j = -A r Vf /2.303 RT = 0.609 /kbar at 250 °C and = 0.354/kbar at 625 °C. Our value log m =-2.56 (540 ppm Au) at 3.7 kbar predicts log m = - 3.46 at 625 °C and Tsat" on the critical isochore of 1 m KC1.

Hayashi & Ohmoto (1991) showed that at the /S2 and /H2 of the Py-Po-Mt-W buffer, and acidic pH (Mu-Or-Qz buffer), the HAu(HS) 2 complex accounts for gold solubility at 250-350 °C in vapour-saturated brines of 0-3 molal alkali chloride. Fig. 2 shows their data at 250 °C (.08 ppb), 300 (12 ppb), & 350 (12 ppb). HAu(HS)2 also accounts well for the gold solubility (500 ppb) at 450 °C and 500 bars measured by Gibert et al. (1993) in 0.5 molal chloride brine buffered by Py-Po-Mt-W and Mu-Or-Qz. (Rytuba & Dickson (1977) found Au solubility (1.5 ppm) in sulphidic brine is independent of salinity at 500 °C, 0.4-lm NaCl). These 4 data points and our "Psat" value at 625 °C lie on a straight line, so the dissolution reaction's ArCp~ 0, and A Sf and A Hf are constants assessable by linear regression. Using y (HAu(HS)2 ) 1 . 5 and 8feS = 0.52, regression gives log K t = 4 . 5 6 - 7.178 ( l & I T ) , ArSf = 20.8 cal/mol-K and A H f = +32,845 cal. Using thermodynamic properties of minerals and H2O from SUPCRT 92, we derive the following standard partial molar properties of HAu(HS)| (aq): S 2 9 8 K = 58.8 cal/mol-K, H298K = -11,283 cal/mol, Cp 298K = 35.7 cal/mol-K. Together with V 2 9 8 K = - 1 0 cm /mole, these may be used to evaluate gold solubility as the HAu(HS)2 complex over a wide range of T and P relevant to genesis of porphyry Au-Cu and slate-belt and greenstone-belt gold lodes.

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Figure 1 „

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Figure 2

Fluid Inclusions

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Hydrotheimal Gold Solubility 350°C

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 APPLICATIONS OF THE NEW SIGMA TECHNIQUE FOR MINERAL EXPLORATION IN EPITHERMAL- AND PORPHYRY-TYPE HYDROTHERMAL SYSTEMS Robert R. Loucks. J. Michael Palin, J. Michael Shelley and John A. Mavrogenes Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200

The Research School of Earth Sciences has developed innovative instrumentation for chemically analyzing gaseous components (H2O, CO2, CH4 H2S, SO2, N2, NH3, H2, etc.) in selected singleal fluid inclusions. The SIGMA (Selected Inclusion Gas Micro-Analyzer) is useful for chemically characterizing hydrotheimal fluids, such as comparing fluids from productive and barren veins or paragenetic stages. SIGMA also permits precise determinations of palaeo-fluid pressure and fluid-pressure gradients. These can be used for (1) geobarometric estimation of drilling depth to orebodies; and for (2) palinspastic restorations, e.g., geobarometric evaluation of vertical displacement on post-mineralization faults, or of post-ore erosion, or of regional tilting or relative uplift.

TECHNIQUE A 20-watt Nd-YAG laser supplies a continuous-wave, infrared beam focussable to -It) |im diameter. Quartz and most other colourless minerals do not absorb significantly at the laser's 1064-nm wavelength, but water absorbs the beam energy as heat, which permits a targeted, single aqueous fluid inclusion to be gradually heated until it thermally decrepitates, releasing its volatile components into the high-vacuum sample chamber. The gases are transmitted to a fast-scanning quadrupole mass spectrometer to precisely measure molar proportions of the gaseous species in the inclusion. Calibration is relative to pure gas standards and certified gas mixtures, as well as relative to our custom-made fluid inclusions synthesized with known gas concentrations.

Many ore deposits precipitated from fluids that were boiling, condensing, effervescing, or otherwise immiscible.

Examples include most epithermal precious-metal deposits, porphyry-type copper-gold-molybdenum deposits, tin-tungsten lodes, and many greenstone- or slate-belt gold lodes. If fluid inclusions sampled immiscible fluids, then the hydrothermal fluid's vapour-saturation pressure was equal to the total geologic fluid pressure. We select for analysis fluid inclusions that trapped only the aqueous liquid phase of the immiscible fluids. We recalculate the measured mole ratios of each gas/H20 as partial pressure of each gas, using microthermometry data and appropriate Henry's Law coefficients for vapour/liquid partitioning. Summation of the partial pressures gives the hydrothermal fluid's geologic palaeo-pressure: PH20 C02 CH4 + H2S + S 0 2 + — = Ptotal If applied to a suite of coeval samples collected over a range of elevations, linear regression of the samples' relative depths versus their saturated vapour pressures gives the hydrodynamic pressure gradient in the sampled interval.

EPITHERMAL-VEIN FLUID-PRESSURE GRADIENTS A series of pilot studies applied the SIGMA technique of fluid-pressure geobarometry to three technically undisturbed Tertiary epithermal precious-metal vein systems in Mexico and New Mexico. Quartz-hosted fluid inclusions that had trapped only the liquid phase of boiling solutions were selectively opened individually by laser. SIGMA vapour-pressure geobarometry afforded an average precision of ±3-4% (1 s.d.) of the total fluid pressure. Analysis of sample suites spanning vertical intervals of 280 to 725 metres in various epithermal veins permitted reconstruction of hydrodynamic (not hydrostatic) vertical fluid-pressure gradients with an average precision of ± 6 millibars/metre. The mean vertical gradients varied amongst the veins from 8.7 to 9.1 bars per 100 m. These values resemble reported gradients measured down-hole in active meteoric-water-dominated hydrothermal systems drilled for geothermal power production; in those, the in situ gradients in boiling fluid columns are typically in the range of 8.5 to 9.3 bars/lOOm on volcanic plateaus with subdued topography.

EXPLORATION APPLICATIONS SIGMA vapour-pressure geobarometry could be used to estimate drilling depth below present outcrop to a palaeo-temperature interval that is deemed likely to correspond to the ore-forming interval of the hydrothermal temperature gradient. For example, if a sample of barren vein quartz collected from outcrop had trapped boiling fluids at a mean temperature of 160 °C, and if analysis of inclusions that trapped the liquid phase showed that it contained 0.2 molal CO2 and 0.5 molal (Na,K)Cl, then P H 0 + PCO2 = 35 bars at 160 °C and -66.4 bars at 250 °C. If the explorationist recalls that epithermal vein ore precipitation typically occurs over about a 50 °C interval somewhere within the 200-300 °C range, and if the explorationist assumes a vertical gradient of 0.9 bar/metre in the vapour-saturated fluid column, based on accumulated experience cited above, then the estimated depth from outcrop to the 250 °C palaeo-isotherm is (66.4 - 35.0) / 0.09 = 349 metres.

An analytical precision of ±3% on a total pressure of 50 bars (common for fluids boiling at -200-250 °C) is ±1.5 bars. In a typical vertical gradient of -0.09 bar/metre, a pressure change of 1.5 bars occurs in a depth interval of 1.5/ 0.09 = 17 metres, so the vertical displacement on a post-ore fault could be determined with an uncertainty of - ±17 m at the 80% confidence level by measuring the present elevations and fossil vapour-saturation pressures of five samples on each side of the fault. This procedure may be useful for locating displaced orebodies. >

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SABRE, NABRE AND PROTEROZOIC MINERAL SYSTEMS IN NORTHERN AUSTRALIA Tom S. Loutit and Peter N. Southgate Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

The aim of the Northern Australain Basin Resource Evaluation (NABRE) Project is to understand the timing of the movement of metal-rich fluids and hydrocarbons and their migration pathways within Proterozoic sedimentary basins. Sedimentary basins cover a large proportion of Australia, act as major plumbing systems for fluids that are derived from the crust or deeper and/or from within basins, provide "traps" for mineral deposits and petroleum accumulation and provide the cover that hides these economic targets providing the challenge for explorers. The challenge undertaken by the NABRE Project is to peer through this cover and provide information that will help define the character of a series of mineral and petroleum plays in time and space especially under shallow cover. Because of the difficulties of correlating and defining the age of Proterozoic strata and unraveling the structural complexity of the area the NABRE project has had to apply new techniques and concepts to constrain the evolution of the Proterozoic Basins of Northern Australia. The early stages of the project have concentrated on the relatively unstructured and unmetamorphosed strata in north western Queensland and eastern Northern Territory.

The approach involves the application of petroleum basin analysis techniques. Basin analysis requires five major steps aimed at providing a tectonostratigraphic time-series framework within which to define petroleum and mineral fluid systems as a precursor to prospect definition and evaluation. The advantage of concentrating on the discrete components of fluid systems is to focus research on to the key questions required to understand the geological processes that produce an economic mineral deposit or hydrocarbon accumulation.The approach is to systematically define or constrain the "source" of metals, brines and hydrocarbons, the timing of generation or movement of the fluids, the "migration" pathways of the fluids, the location and timing of "trap" formation and finally the distribution and quality of the "seal" and "reservoir". In the case of a mineral system the "seal" is probably more important in constraining migration paths that metal-rich fluids may take in contrast to the role of an impermeable rock that forms a seal over a petroleum trap.

Thus one of the main objectives of basin analysis is to define the structural and stratigraphic framework at the time of fluid movement so that the migration pathways and sites of potential "traps" can be predicted. The first step is to document the timing of tectonic events and elements that may influence the development of the basin under investigation. The second step is to establish a first-pass structural and stratigraphic framework that defines the sediment accommodation space during each phase of development of the basin. The geometry, characteristics and distribution of the sediment fill are defined during the third step in preparation for a fourth step that involves the prediction of the distribution, character and timing of "play elements" within each basin phase to define areas of higher economic potential in the basin. The fifth step is to define "plays" from combinations of play elements and evaluate the potential of each of these to better define prospects. The methodology is incorporated in AGSO's Systematic Approach to Basin Resource Evaluation (SABRE). The NABRE project has utilised the methodology during the past year, primarily concentrating on steps 1 and 2 with some emphasis on 3 in certain areas.

The initial results of the project are very encouraging, especially because the application of techniques such as gamma ray spectrometry, sequence stratigraphy, chemostratigraphy, Apparent Polar Wander Path (APWP) analysis and SHRIMP dating have provided new insights into the structural and stratigraphic framework of a large areas of Northern Australia. Initial results suggest that the new stratigraphic framework may open a number of new mineral "plays"

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LABORATORY STUDIES ON SEISMIC PROPERTIES O F THERMALLY STRESSED CRUSTAL ROCKS AND THEIR IN-SITU TRANSPORT PROPERTIES Cao Lu and Ian Jackson Petrophysics Group, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200

Seismic methods have been widely used in many geophysical applications such as the monitoring of enhanced oil recovery processes in hydrocarbon reservoirs, the delineation of geothermal reservoirs, and the rock characterisation at potential nuclear waste disposal sites. The petrophysical basis for these applications is the variation of seismic properties of rocks under subsurface conditions of varying effective pressure and temperature.

Thermally induced cracks within the rocks are envisaged to have strong effects on their seismic properties and transport properties. Most of our knowledge about the seismic properties of rocks has come from laboratory measurements with ultrasonic wave techniques, while very few data are available directly from measurements at seismic frequencies. More insight into the seismic properties of (fluid-saturated) rocks under conditions of pressure and temperature is thus needed as well as understanding of the implications for transport properties.

Novel locally developed torsional forced-oscillation attenuation apparatus allows laboratory experiments to be conducted within regime of linear, amplitude-independent mechanical behaviour, and at seismic frequencies which are many orders of magnitude lower than those of the widely used ultrasonic wave propagation methods. The capability of the apparatus has recently been extended to include control of pore-fluid pressure of the rock specimen. With such instrumental capability, we are able to study experimentally the acoustic properties of fluidsaturated rocks at seismic frequencies and also their transport properties.

Experiments were conducted on two rock types: a fine grained granite (aplite, Delegate, NSW) with porosity of 3% and a pure quartzite (Strachan, Tasmania) with almost zero porosity. Shear modulus (G) and associated internal friction (Q"1) were measured on Delegate aplite and Strachan quartzite under the conditions of confining pressure (P c = 50 to 300 MPa) and argon pore-fluid pressure (Pf = 0 to 150 MPa), during multiple thermal cycles (T = 25 to 700°C). The in-situ transport properties of the rock specimens were evaluated with the transient-flow method by imposing an increment of pore pressure at the up-stream reservoir and monitoring the pore pressure decay with time. The in-situ crack porosity of the specimen was also measured by pore-fluid volumometry The experimental results are summarised as follows: (a) Variation of shear modulus during thermal cycling shows effects of thermal cracking under different confining pressures. The shear modulus-temperature curves showed that the onset temperature for thermal cracking, analysed with the elastic theory of fracture mechanics, increases with confining pressure. Higher crack density is thermally generated under lower effective pressure. The results will be combined with the in situ transport properties and crack porosity measurements for more detailed interpretation. (b) The measurements on shear modulus for both rocks are reversible during thermal cycling under confining pressure. Reproducible G(T) under a constant effective pressure suggests the thermal cracks are 'elastic' during thermal cycling, and that confining pressure is sufficient to suppress the thermal cracks generated by thermal stresses. (c) At room temperature, Q"1 is generally low in both rocks with or without argon pore fluid. Regardless of confining pressure, Q"1 increases only slightly with increasing temperature below 400°C, but rises dramatically with increasing temperature above 400°C. Q"1 has much less sensitivity to the effective pressure than to temperature. The onset of internal friction at about 400°C shows that the rock departs significantly from elastic behaviour at high temperature presumably as defects such as dislocations become more mobile. (d) At room temperature, the shear modulus of Delegate aplite follows an effective-pressure law ie G=G(P e ff), where P e ff=P c -n Pf with n=1.5. G=G(Peff) means that crack density must also follow the effective pressure law.

Effective-pressure law may still be valid for shear modulus at high temperature, but will be influenced by the change of micro-structure within the rock specimen. The difference between Gdry and G sa turated w a s found to be significant in thermally cycled Delegate aplite specimens, contrasting to the previous observations at ultrasonic frequencies and the general belief that pore-fluid has little effect on shear wave velocities of fluid-saturated rocks. (e) Anomalously low shear moduli were found at the quartz a-(3 transition temperature, which increases with confining pressure.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

K-Ar AGES OF THE METAMORPHIC AUREOLE OF THE PAPUAN OPHIOLITE, SOUTHEASTERN PAPUA NEW GUINEA W.Y. Lus1.1. McDougall2 and H.L. Davies1 department of Geology, University of Papua New Guinea, PO Box 414 University, NCD, Papua New Guinea 2 Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia

Numerical ages measured by the K-Ar isotopic method on nine hornblende samples from hornblende granulite and amphibolite rocks of the Papuan ophiolite aureole, southeastern Papua New Guinea, range from 57.2 ± 0.6 Ma to 65.4 ± 0.7 Ma with a mean of 61.2 ± 2.6 Ma. These ages are interpreted as broadly reflecting the time of crystallization and cooling in the Paleocene of the aureole rocks during and subsequent to emplacement of the ophiolite on the southeastern Papua New Guinea continental crust. It is not clear whether the spread in apparent age is because of variable excess argon in the hornblendes or whether there was an extended interval over which emplacement and recrystallization of the ophiolite occurred. Nevertheless, an age of 65 ± 1 Ma is a maximum age for the emplacement of the Papuan ophiolite, and 57 ± 1 Ma may be regarded as a minimum age for emplacement and cooling.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE NATURE OF GLAUCONITIC SMECTITE Chi Ma and Tony Eggleton Cooperative Research Centre for Landscape Evolution & Mineral Exploration, C/- Department of Geology, Australian National University, Canberra ACT 0200

The heterogeneous mineralogical and chemical composition of 'glauconite' has been recognized for some time, and this has led to many attempts to develop a common understanding of the meaning of the term 'glauconite'.

This goal has not been completely achieved, and Odin and Matter (1981, Sedimentology 28, 611-641) introduced the general term 'glaucony' for all morphological forms, and the terms 'glauconitic smectite' and 'glauconite mica' as end members of the glauconitic mineral family.

Glauconitic minerals occurring at 30 metres depth in unweathered marine sediments of the Rolling Downs Formation at Weipa, far north Queensland were found to consist mainly of 'glauconitic smectite' - an end member which has not been well documented. This study seeks to reveal the structural and chemical nature of the glauconitic smectite using high-resolution transmission electron microscopy (HRTEM), analytical electron microscopy (AEM) and scanning electron microscopy (SEM).

The glauconitic smectites were identified in high-resolution TEM images as having straight 12-A layers with few defects, whereas other forms of smectite were texturally characterized by having wavy 10- to 12-A layers with a high density of edge dislocations, and illite by its having relatively defect-free straight 10-A layers. A few 7-A layers were found as ordered interstratifications with the 12-A layers of glauconitic smectite, which showed both 19-A and 12-A periodicities at certain focus condition.

Based on analyses by AEM and energy dispersive spectroscopy (EDS) in SEM, the glauconitic smectite has the formula (K,Ca) (Fe,Al,Mg)y[Si4. Al ]Oio(OH)4nH20, where x varies from 0.4 to 1.2, y from 2.0 to 2.6, z from 0.1 to 0.7. The smectite is di-octahedral and has a high-Fe content (ranging from 0.5 to 1.7 in above formula). It differs from nontronite by having high A1 contents in both tetrahedral and octahedral sites (from 1.4 to 0.7 in the above formula).

There are two major morphologies of glauconitic minerals in Weipa. One is of tabular or granular form; the other is film-like, coating grains and ocurring in matrix and fissures. Tabular glauconitic smectites formed by replacing detrital micas (particularly biotites); they show chemical variations and often mix with low-Fe smectite in one grain, whereas film-like glauconitic minerals, believed to be of authigenic origin, have less chemical variation. Morphological criteria alone are not a reliable means for the characterization of either the mineralogical or chemical nature of glauconitic minerals. It is suggested that the film-like glauconitic smectites result primarily from authigenic crystal growth that begins with the precipitation of a poorly crystallized Fe-rich smectite that then evolves into a better crystallized phase. If additional precipitation occurs, the previously formed glauconitic smectite evolves under appropriate chemical conditions into disordered 10-A glauconitic mica. z

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The glauconitic smectites, which formed in a reducing environment (co-existing with authigenic pyrites), become unstable towards the surface due to weathering and transform to low-Fe smectite, kaolinite, and iron oxyhydroxides.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOLOGIC INTERPRETATION OF THE MOUNT ISA SEISMIC TRANSECT

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Tvler MacCreadv 1 *, Bruce R. Goleby 2 *, Gordon S. Lister1* and Barry J. Drammond2* 1 Department of Earth Sciences, Monash University, Clayton, Victoria 3168 Australian Geological Survey Organisation (AGSO), GPO Box 378, Canberra, ACT 2601 Australian Geodynamics Cooperative Research Centre (AGCRC)

In 1994 the AGCRC conducted a multidisciplinary transect across the Mount Isa terrain in northwest Queensland. Detailed mapping carried out along the transect has been combined with previous work in the area to provide a geologic interpretation of the top ten kilometers based on seismic reflection data from the transect.

Preliminary results and implications of this interpretation are presented.

The east-west orientation of the seismic line makes it ideal for imaging structures related to middle Proterozoic shortening in the area. The seismic data demonstrates that in spite of the upright nature of the folding and the steep dip of units at the surface, the deeper controlling structures are often low-angle thrusts.

HIGHLIGHTS OF THE TRANSECT * The Roos Mine Thrust (Huang 1994) on the west edge of the Mitakoodi folds links to a major subhorizontal detachment 6 km below the surface. The geometry of the Mitakoodi folds is governed by the ramp-flat geometry of the underlying fault system. * The Cloncurry Fault is imaged as a zone dipping approximately 40° to the east merging into a subhorizontal detachment 7 km below the surface. This detachment may be linked with the detachment beneath the Mitakoodi folds and form part of a major west-vergent system that decouples the supracrustal rocks from the crystalline basement below. * Subsurface structure in the area of the Mount Isa Fault is constrained by the reflection data. An east-rooted post Mount Isa Group rifting event is hypothesized on the basis of younger on older relationships and the complicated pre-shortening geometry in the region. The reflection data shows the likely location of a normal fault associated with this rifting event. It is imaged as an east dipping zone of angular discordance in reflections beneath the Leichhardt River Fault Trough. * Two fluid pathways are imaged in the data. In the Marimo area a highly reflective zone dips 50° east, cutting across folds, and projecting to the surface along a trend of Cu-Au mineralisation. West of the Mount Isa Fault a similar reflective zone dips 40° west beneath the Sybella Granite. This zone projects to the surface in line with the Adelheid Fault which cuts the upright folds in the region. In both cases these zones persist over 6 km below the present surface, and both apparently link with strongly metasomatised and mineralised regions at the surface. * The geometry of steep strike-slip fault systems like the May Downs Fault and Pilgrim-Fountain Range system are constrained by continuity of reflectors. These systems can be traced through discontinuities in the reflection data to depths of 10-15 km where they truncate against more continuous reflective zones.

DISCUSSION The transect interpretation provides a coherent picture of the shortening geometries of the Mount Isa terrain.

Throughout the eastern fold belt there is a consistent history of west-vergent, low-angle thrust faulting overprinted by upright folding. Vergence direction changes near the western edge of the Kalkadoon-Leichhardt Block. To the west, shortening is characterised by east-vergent structures, and early stages of shortening involve upright folding whereas later stages of shortening occur on steep reverse faults.

The seismic data also helps define a network of thrust and strike-slip faults that cut the ductile fabrics and juxtapose blocks of different metamorphic grade. These structures cut into the crystalline basement rocks and offset the more ductile thin-skinned deformation event. Together, these faults link in a hierarchical network similar to the late stage fault network seen in the regional map pattern. Some of these structures, such as in the Marimo and Mount Isa areas, are guides for metasomatism. Identification of these structures in the seismic data helps define the three dimensional geometry of fluid circulation systems related to mineralisation.

REFERENCE Huang, W., 1994. Structural and stratigraphic relations on the western flank of the Mitakoodi Culmination: a case study in the Roos Mine area south of the Corella Dam, Eastern Mount Isa Inlier, NW Queensland.

Australian Crustal Research Centre Technical Publication 21, 28p.

Acknowledgments: BRG and BJD publish with the permission of the executive director of AGSO. This abstract is released with the permission of the Director of the AGCRC.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE TOWNSVILLE-MORNINGTON ISLAND IGNEOUS BELT: A NEW PERSPECTIVE ON THE CARBONIFEROUS-PERMIAN MAGMATISM OF NORTH QUEENSLAND, AND IMPLICATIONS FOR METALLOGENY. D.E. Mackenzie, P. Wellman and D.C. Champion Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601.

The North Queensland Igneous Province (NQIP), which consists of mainly felsic granitoids and caldera-related volcanic rocks, extends throughout much of eastern north Queensland and contains most of the region's known mineralisation. Interpretation of regional and new detailed geophysical data, combined with recent geological mapping by AGSO and the Geological Survey of Queensland, have shown the NQIP to be much more extensive and complex than was previously appreciated.

The Townsville-Mornington Island Igneous Belt (TMIIB) is a zone of Carboniferous-Permian igneous rocks that extends from Townsville 300 km northwest then 600 km west to Mornington Island. Where exposed (solid line in the figure below), the belt is composed of abundant I-type granitoids, caldera-collapse structures and related volcanic rocks. It is characterised geophysically in this area by a broad magnetic high, probably due mainly to the abundant I-type intrusive rocks, with prominent superimposed circular magnetic anomalies that outline volcanic-intrusive complexes (black areas in figure). The full extent of the belt is inferred (dashed line in figure) from under-cover and offshore continuation of this anomaly pattern. Similar circular anomalies indicate that Carboniferous-Permian caldera-collapse structures and intrusive bodies are scattered beneath cover throughout western Cape York Peninsula and to the south of the TMIIB, and form a broad cluster in western Torres Strait.

Another characteristic of the T M I I B (notably in the west) is a gravity low, due to accumulations of felsic igneous rocks, that is coincident with the broad magnetic high. The westerly trending segment of the TMIIB also coincides with a discontinuity separating Mesoproterozoic basement terrains of different structural grain, and is considered to represent a major crustal break "welded" by Carboniferous-Permian magmatism.

Most known mineralisation in north Queensland is related to Carboniferous-Permian I-type rocks - Sn with strongly fractionated reduced rocks, and W-Mo, Au ± Sb, Cu, and Pb-Zn-Ag with less fractionated oxidised rocks; most economically significant deposits within the TMIIB are located in or adjacent to high-level intrusive bodies and/or caldera collapse-related ring fracture and/or ring-dyke systems. Tin deposits in the Cooktown region to the northeast of the TMIIB are related to reduced, fractionated Permian S-type granites (stippled in figure below). The TMIIB also includes two major belts of Early Permian A-type igneous rocks which have long been regarded as having low or nil prospectivity. However, some gold and uranium mineralisation is associated with them, and evidence is emerging to suggest that they may be more generally prospective than previously recognised.

High-level intrusive rocks and caldera-related volcanic rocks of both A and I-type are extensively developed beneath thin regolith and Mesozoic cover in Red River 1:250 000 sheet area, immediately to the west of the exposed TMIIB. In the light of their obvious prospectivity where exposed, we suggest that the CarboniferousPermian igneous rocks are also worthy of exploration attention where they are concealed. Local magnetic anomalies, such as (1) small "bullseyes", possibly due to high-level, oxidised, fractionated stocks and/or deeper, small mafic intrusions, and (2) broader, cross-cutting lows, possibly due to hydrothermal alteration, are two of the most obvious exploration targets.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ANALOGUE 3D SANDBOX M O D E L L I N G -- A USEFUL TECHNIQUE FOR PREDICTION O F D E F O R M A T I O N IN EXTENSIONAL SEDIMENTARY BASINS Tariq Mahmood1 and Nick Lemon1 'National Centre for Petroleum Geology and Geophysics Adelaide University SA 5005, Australia.

Scaled models of tectonic structures are often used to show allowable geometries and to illustrate the effects of progressive deformation. Analogue sandbox modelling is a tried and proven technique for simulating extensional fault structures. In this study a series of 3D models has been produced in which the nature and orientation of side ramps have been varied to model structures typical of the North West Shelf. These models illustrate the distribution and deformation of sediments in extensional terranes. However, they do not incorporate compaction, thermal or isostatic effects that may be significant in such environments. This technique has the ability to simulate the geometry and kinematics of extensional structures in a complex area. It has proven particularly useful for understanding the structures developed in regions where the sediments have been subject to more than one period of extension, each with a different orientation. The models are valuable for guiding or validating seismic interpretation, particularly in areas of poor data quality.

Interpretations for a number of seismic lines along the main detachments in the Carnarvon Basin were depth converted and the shape of the detachment was built into a scaled 3D model box. The box was loaded with coloured sand in thicknesses equivalent to the known stratigraphy of the area and extended by amounts equivalent to that indicated on the depth converted sections. The basic aim of this work was to provide control on fault linkages and the structure of key horizons in technically complex areas with relatively sparse seismic coverage, and to verify existing interpretations in such areas. These models provide interpretive templates for the evolution of extensional fault structures and show the orientation of faults which may logically be linked in a widely spaced 2D survey.

The North West Shelf of Western Australia has been subject to two main extensional stresses during the sedimentation history of the Carnarvon Basin. Jurassic extension was directed to the northwest while later Cretaceous extension was directed towards the west. This resulted in a number of areas where early faults were either reactivated by extension in a different direction or interacted with later faults by providing a side ramp against which the later faults partly terminated. The interaction of extensional detachments against pre-existing faults has not been described previously in the literature.

Both 2D and 3D models were made, as both forward models and as reconstructions of field examples based on seismic interpretation. The forward models investigated hangingwall deformation and fault patterns in areas where a listric extensional fault is modified by the presence of an existing fault at an angle to the currently active detachment. This geometry was chosen to mirror the two different extensional regimes evident on the North West Shelf. The field examples involved constructing models of the detachment surface from seismic interpretation, and deforming them to check the accuracy of the initial interpretation.

Tracing of particle trajectories is very instructive in understanding how the deformation in the hangingwall develops. Experiments were performed to investigate gradual displacement of marked particles and sequences of faulting occurring during deformation. These models clearly demonstrate sequential fault evolution and illustrate fault reactivation and nucleation as deformation proceeds. The particles move parallel to the detachment, although their displacement diminishes up through the sediment pile. The particle trajectories can be used as templates and placed over a scaled geological section to interpret deformation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

KINEMATICS OF THE KIMBERLEY ARC: LINEAR INDICATORS OF MOVEMENT DIRECTION OF ROCKS AND THEIR TECTONIC SIGNIFICANCE FOR OROCLINE DEVELOPMENT V. M. Mares Department of Earth Sciences, James Cook University, Townsville, Qld, 4811

The use of lineations as movement indicators in rocks is problematic as the relationship between lineations and the direction of applied stress is uncertain. Moreover, in multiply deformed rocks lineations produced by earlier events have generally been reoriented during subsequent deformation such that they can no longer be used as reliable indicators for the early deformation history.

Lineations defining the intersections of early foliations (foliation intersection axes or FIAs), preserved via inclusion trails in porphyroblasts, are aligned perpendicular to the applied stress direction. Thus they provide more reliable indicators of movement in rocks even after these lineations have been reoriented or destroyed in the matrix. FIAs are therefore a valuable tool in the reconstruction of the early tectonic history of large regions.

The Kimberley Province of far north Western Australia is bordered along its southwestern, southern, and eastern margins by an oroclinal arc, the early Proterozoic King Leopold and Halls Creek Orogens (and the Fitzmaurice Mobile Zone in the Northern Territory), that appears to be continuous for its entire exposed length from Derby in the west to Darwin in the north. The rocks in this oroclinal arc have a long history of deformation, the early part of which may be concomitant with the Barramundi orogeny (1890 to 1870 Ma).

A combination of field mapping and microstructural analysis of over 1200 spatially oriented thin sections of samples from six areas around the Kimberley oroclinal arc suggests that the arc has undergone at least ten stages of progressive ductile deformation, D to Di , during the early Proterozoic. These events are associated with compression and uplift producing near-vertical foliations (D D , D , D and D ), alternating with collapse of the overthickened rock mass producing near-horizontal foliations (D , D , D , D , and D ).

Foliation intersection axes (FIAs), obtained from inclusion trails within garnet, andalusite, and staurolite porphyroblasts from 48 of the samples show a trend of 135° for deformations 2 through 7, suggesting that both, orogens were deformed during the same NE-SW-directed events.

History of lower Proterozoic deformation: A possibly pre-Barramundi event, D , imposed a steep foliation upon the rocks of the proto-Australian continent. A remnant of this early event may be the megascopic fold outlined by the Mueller Ranges exposed in the southern part of the Kimberley arc. All other evidence of a D\ foliation in the matrix has been obliterated. Collapse of this early orogen created a shallow foliation.

The earliest microstructural evidence for deformation is a shallow foliation preserved within an early garnet phase from the west Kimberley. There followed a prolonged period of NE-SW-directed compression. Early garnet porphyroblasts from the King Leopold Orogen record seven events that resulted in alternating steep and shallow foliations, all of which have the same trend of foliation intersection axes of 135°. Deformation began in the west and is characterized by high pressure and high temperature mineral assemblages (kyanite, sillimanite, staurolite, garnet) whereas in the east deformation is marked by high temperature mineral assemblages (sillimanite, garnet). This may be due to the western limb of the arc representing a lower level of the crust than the eastern limb. The lowest temperature and pressure assemblage (andalusite, garnet) is present in the southern fold which may represent the highest level of the early Proterozoic crust in this region. Initiation of several of the large shear zones, now preserved in the Halls Creek Orogen, may date back to early pulses of the Barramundi orogeny. Ductile reorientation of the Halls Creek Orogen into its present attitude occurred during a younger, post-Barramundi, event (D ). x

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Tectonic implications: The Barramundi orogeny was initiated by the collision of the Proterozoic Australian plate with another plate to the northeast of it, resulting in a NW-SE-oriented system of fold belts and shear zones/thrust faults. A NW-SE-directed post-Barramundi event ductilely reoriented parts of this fold belt into the arcuate orogen which persists to this day. The present-day Kimberley arc may represent only a small part of a much larger system of anastomosing orogenic belts that existed in proto-Northern Australia at the end of the lower Proterozoic.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MEKRELATiaVSHIPS BETWEEN EPISODIC REGIONAL ALTERATION AND GRANITES OF THE MOUNT ANGELAY PLUTON, CLONCTJRRY DISTRICT, NOR3HWEST QUEENSLAND. Geordie Mark National Key Centre in Economic Geology, Department of Earth Sciences James Cook University, Townsville, Qld 4811.

The Cloncurry district is a major metallogenic province, contains a number of Cu-Au deposits, and possesses many close affinities with other Cu-Au provinces (e.g. Stuart Shelf, Australia; Kiruna Sweden; and Copiapo district, Chile). Granites have for a long time been suggested as being involved in the metallogenesis, but as yet their role is unresolved.

Two of the most prominent rock packages within the district are a voluminous granitic mass, the Williams Batholith, the youngest batholith in the Mount Isa Inlier (ca 1500 Ma), and an extensive (100's km ) package of comprehensively metasomatically reconstituted calc-silicate, metasediment and granitoid. The metasomatically altered rock package (Doherty Formation) and granites of the Williams Batholith are spatially related, in the vicinity of the Cloncurry Fault (Fig 1). The area immediately surrounding the Mount Angelay Pluton is one of the most critical areas in the district for the studying the interrelationships between fluids causing metasomatism and granite intrusion, because all of the crucial geological features (e.g. granitoids, metasomatically altered rock and a fault zone) are within such a compact area(Fig. 1). |

[ Cover • Kfs 4- Qtz alteration f \ ] Bt Granodlorite 3 Alt Monzogranlte j Na-Ca-Fe metasomatism! ] Bt Monzogranlte ] Na-Ca-Fe metasomatism j Qtz Diorite j HW Diorite Qtz Monzonlte Alt Qtz Diorite Albltite Calc-sllicate

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Figure 1. Geology at the southern tail of the Mount Angelay Pluton.

All of the granites in the area of Figure 1 were intruded after the peak of metamorphism and regional D deformation and before, during and after regional D deformation. Granites at the southern margin of the Mount Angelay Pluton are heterogeneous I-type, magnetite-bearing and range in composition from hornblende quartz monzonites to biotite ± hornblende monzogranites and granodiorites. Rapakivi textured K-feldspars are unique to areas of mixing and mingling between early foliated felsic granite and mafic hornblende diorite, and occur in both igneous rock types.

Regional alteration around granites of the Mount Angelay Pluton was episodic and progressedfrompervasive Na-Fe (Ab-Mt) alteration to localised multiple Na-Ca-Fe (Ab-Act-Mt-Tit) brecciation, and finally to fracture controlled epithermal-style Fe-Ca (Ep-Act-Qtz-Chl) and Fe~K (Kfs-Hm-Qtz) veining. The first three main alteration phases recognised around the Mount Angelay Pluton are temporally and spatially associated to a preceding intrusive phase, whereas the latefracturecontrolled epithermal-style veins are not associated with the intrusion of any igneous phase. The temporal and spatial relationship between the first three intrusions and their successive alteration phases, and the consistent albitic nature of the feldspar in each of three alteration phases, suggests that granite emplacement may have been the driving force for up-temperature fluid convection.

The curious synchroneity between the intrusion of the Williams Batholith, regional metasomatism, regional D deformation and Cu-Au mineralisation, strongly implies that a relationship exists between the intrusion of a heat source, fluid movement and mineralisation through and into structural weaknesses and sinks in the crust Acknowledgments: This project was funded by the National Key Centre in Economic Geology at James Cook University. I would like to thank Pat Williams and Julie Richmond for their valued comments on the abstract.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOLOGICAL EDUCATION IN THE COMMUNITY THROUGH NATIONAL PARKS INTERPRETATION Gabor Markovics School of Aquatic Science and Natural Resources Management, Deakin Universitv-Rusden Campus. 662 Blackburn Road, Clayton.

Victoria, 3168

National parks play an important role in educating the community about our environment and the Earth. In the north American continent the importance of this is seen by: • the presence of courses in geology that use national parks to help teach the concepts and processes of the Earth, and • the interpretation of many natural heritage aspects in parks and the presentation of this information to the community in a variety of ways, and • the establishment of co-operation between various government agencies in producing geological educational material for the community at large.

Many aspects about the geology of national parks in Australia are reasonably well known and have been documented by a variety of authors in many different publications.

With few exceptions, most of the information has escaped interpretation at the general community level, but this is, to some extent being readdressed . The best example of this is the 1992 AGSO publication on Uluru (Ayer's Rock). A very successful booklet that drew upon existing knowledge and data and presented it in a reasonably palatable format for the public at large. This particular publication is also a good example of the type of inter-govemment agency cooperation so often seen in Canada and the USA. Using examples, this paper illustrates how the interpretation of the geological heritage of national parks can play a key role in educating the community and presents an outline for the publication of a book on "The Geology of Australian Parks" that may help towards this role.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 EVOLUTION OF A MID-PALAEOPROTEROZOIC FORELAND BASIN, SOUTHERN MARGIN OF THE HAMERSLEY PROVINCE, PILBARA, W.A. D. McB. Martin. R. C. Horwitz, Z. X. Li, C. McA. Powell, C. Twist and M. Worth Department of Geology and Geophysics, University of Western Australia, Nedlands 6907, W.A.

During the late Archaean and early-Palaeoproterozoic the Hamersley Province evolved from a rifted continental margin and shallow marine shelf (Fortescue Group) to a south- and west-facing submerged platform (Hamersley Group). The minimum age of the platform is constrained by the age of the 2440 Ma Woongarra Volcanics at the top of the Hamersley Group. The overlying Turee Creek and Wyloo Groups have previously been interpreted as a continuation of the platform margin succession that was overthrust during collision between the Yilgarn and Pilbara Cratons in the late Palaeoproterozoic. However, palinspastic reconstructions and basin analysis indicate that these strata were deposited in a west-northwest trending foreland basin with an uplifted hinterland to the southwest. This foreland basin has been named the McGrath Trough, and contains the Turee Creek and lower Wyloo Groups.

Structural remnants of the west-northwest trending foreland basin are preserved in the keels of the Duck Creek, Hardey, Brockman, Turner, and Turee Creek Synclines, as well as isolated outcrops in the Pannawonica area. The southern (hinterland) margin is defined by uplift, folding, and the generation of intrabasinal unconformities. This margin lies along the Paraburdoo Hinge Zone. There is no evidence of erosion of the Fortescue and Hamersley Groups along the northern (foreland) margin during basin evolution. The majority of the basin fill is derived from the hinterland margin, and displays compositional characteristics indicative of increasing supply of banded iron formation and jaspilite detritus through erosion of the Hamersley Group. Evolution of this fill is characterised by an upward shallowing from deep-water banded iron formation and fine-grained turbidites to deltaic, shallow marine, and fluvial sedimentary rocks.

The foreland basin succession is best preserved in the Hardey Syncline, where at least three unconformitybounded units have been identified. However, evidence from other areas suggests that there may be additional unconformities in the Turee Creek Group. Unconformities were developed in response to folding and uplift of the Turee Creek and lower Wyloo Groups proximal to the fold-and-thrust belt. Pre-Wyloo Group folding decreases in intensity towards the foreland, and consequently the succession tends towards paraconformity in the Duck Creek and Turee Creek Synclines.

The lithostratigraphic base of the Turee Creek Group is at the top of the Boolgeeda Iron Formation of the Hamersley Group. However, the Boolgeeda Iron Formation is the facies equivalent of the Turee Creek Group and was deposited during flooding of the shallow submarine to subaerial Woongarra Volcanics at the onset of foreland basin subsidence. The apparent truncation of east-trending folds within the Turee Creek Group by the unconformably overlying Wyloo Group (Beasley River Quartzite) in the Hardey Syncline has been used by some authors to define the beginning of a new phase of extension and basin opening. However, the Beasley River Quartzite is coaxially folded about an east-west axis, and sedimentary provenance of the lower Wyloo Group indicates that uplift of the southwestern margin of the basin persisted until at least the basal Cheela Springs Basalt (Numanna Member). This member represents the demonstrable upper limit of the foreland basin succession.

The cross-sectional geometry, relationships between unconformity-bounded units, and sedimentary provenance studies indicate that the McGrath Trough represents a foreland basin developed on the southwestern margin of the Hamersley Province some time after 2.44 Ga. The duration of this foreland basin can be shown to extend into the lower Wyloo Group, and pre-dates the 1843 Ma June Hill Volcanics in the overlying Ashburton Trough.

Deposition of the Cheela Springs Basalt may represent the onset of regional extension and the establishment of a southwest-facing passive margin related to the opening of the Ashburton Trough. This interpretation differs significantly from the current passive margin and foreland basin models for the Turee Creek and lower Wyloo Groups.

ACKNOWLEDGMENTS This research summarises work conducted over the past six years, and has been funded by Hamersley Iron, Robe River Iron Associates, and the University of Western Australia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 SPATIAL AND TEMPORAL TRENDS OF HEAVY METALS IN SETTLING ORGANIC PARTICULATE MATTER NEAR A DEEPWATER OCEAN OUTFALL OFF SYDNEY, NSW Carsten Matthai . Gary P. Bickford and Gavin F. Birch Environmental Geology Group, The University of Sydney, NSW 2006 Australian Geological Survey Organization, Canberra, ACT 2601 1

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A sediment trap programme was instigated on the continental shelf adjacent Sydney to assess the effect of effluent disposal on trace element concentrations in setding organic particulate matter. The potential for a metallic 'signature' for the anthropogenic input was investigated, as was the organic and inorganic phase trace element association. The amount of resuspension in the collected mixture of marine and sewage organic particulate matter and inter- and intraseasonal variability of trace element heavy metals were also investigated.

Trace element concentrations of settled organic particulate matter are a function of a number of sources, including naturally occurring marine organic particulate matter and iesuspended seafloor substrate sedimentary material, as well as sewage-derived settling organic particulate matter.

Settling particulate matter was collected south of the Malabar deepwater ocean outfall (Impacted Site) and at locations off Long Reef and Broken Bay (Reference Sites) on the central NSW continental shelf. Twelve sediment traps were arranged in a nested sampling design and deployed for a period of approximately one week on 10 occasions in January (2 deployments), March (2 deployments), July (3 deployments) and September (3 deployments). Sample locations were chosen to reflect small scale variability, with two replicates deployed 0.05 nautical miles and 0.5 nautical miles apart. The Reference Sites at Long Reef, 12 nautical miles north of the Malabar outfall, comprised sandy sediments (70-80 metres water depth) and muddy sands (80-90 metres water depth). The second Reference Site location off Broken Bay (12 nautical miles north of Long Reef) provides an equidistant reference location to compare particulate matter concentrations over similar spatial scales to the Malabar and Long Reef sample locations.

A box corer was used to collect undisturbed surficial sediment samples to depths of between 10 and 30 cm at the same locations as the sediment trap sample locations. The sediments were analysed in regular downcore intervals to examine a possible trace element trend with depth in the sediment. Setded organic particulate matter and sediment samples woe analysed for 9 trace elements (Cu, Pb, Zn, Cr, Co, Ni, Ag, Fe, Mn) and 3 major elements (Al, K, Mg) using HN0 -H 0 microwave digestion USEPA method 3050A. 3

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The observed trace metal and major element concentrations of the collected mixture of settled sewage organic particulate matter, marine particulate matter and resuspended sediment are highly variable on both spatial and temporal scales, with preliminary results indicating that only Ag displays elevated concentrations at the Impacted Site off Malabar for the sediment trap deployments in March. The determined trace element concentration ranges are similar to concentrations found in average riverine particulate matter, with the exception of Ag which displays concentrations of more than an order of magnitude higher. This elevated concentration may make Ag a suitable indicator element for anthropogenic contamination.

Acknowledgments: This study was funded by Sydney Water, AGSO and an ARC grant.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

REGIONAL TRACE ELEMENT GEOCHEMISTRY OF SURFICIAL SEDIMENTS FROM THE CENTRAL NSW CONTINENTAL SHELF 1

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Carsten Matthai and Gavin F. Birch 'Environmental Geology Group, The University of Sydney, NSW 2006

A regional programme was conducted to sample surficial sediment on the central NSW continental margin to assess their geochemical characteristics on a size-normalized (<63p,m, 63jim-2mm) basis.

Trace element analyses included Cu, Pb, Zn, Cr, Co, Ni, Fe and Mn, as well as total organic carbon (TOC), CaC0 3 , and grain size. A major objective of this study was to determine spatial distributions of anthropogenically derived heavy metals in the fine fraction of surficial sediments. The coarse fraction trace element distribution is an indicator for natural trace element accumulation processes resulting from coprecipitation with Fe and Mn oxides, oxyhydroxides and authigenic minerals.

The spatial distribution of the two respective fractions displays two opposing concentration gradients, with the fine fraction concentrations generally decreasing offshore and with increasing distance from the urban conurbations of Sydney, Wollongong and Newcastle. Coarse fraction heavy metal concentrations generally increase with water depth. This distribution pattern gives rise to the assumption that the anthropogenic heavy metal contribution is associated with the fine fraction sediment components.

Interelement correlations within the fine fraction are most pronounced for Cu, Pb, Zn, Cr and TOC, possibly indicating a common anthropogenic source. An evaluation of background heavy metal concentrations in the sediment fine fraction suggests a substantial enrichment in Cu, Pb, Zn and Cr near urban centres. The trace metal spatial distribution pattern supports longshore, rather than accross-shelf, regional contaminant transport. This dispersion is probably controlled by the East Australian Current (EAC).

X-ray mineralogical and binocular microscopy on the coarse fraction of the outer shelf sediments shows they comprise a substantial amount of authigenic phosphorite minerals and display abundant Fe-oxide coatings. The elevated trace metal concentrations in these sediments are most likely related to the natural accumulation processes associated with authigenic mineralization.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 SYNTHESIS AND ANALYSIS OF C-H-O FLUIDS UNDER UPPER MANTLE P, T CONDITIONS Sergei Matveev * , Kay Fricke^ , and Chris Ballhaus

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*RSES, Australian National University , ACT 0200 , AUSTRALIA ^ Department of Mineralogy, University of Hannover, 30167, GERMANY GEMOC, Department of Geology , Australian National University , ACT 0200 , AUSTRALIA

We have synthesized graphite-saturated C-H-O-bearing fluids at 1000°C and 0.8, 2.4, and 5 GPa in gold capsules in piston cylinder and belt apparati. Our purpose is to test the performance of existing equations of state (EoS) for complex C-H-O fluids under pressure (P) and temperature (T) conditions prevailing in the upper mantle. Fluids are generated by in-situ combustion of organic and inorganic compounds with known H/O ratios, i.e. H2O, phthalic acid, benzoic acid, stearic acid, and anthracene. To counter the effect of hydrogen loss and oxidation, / h 2 internally buffered by metal-oxide and carbide-oxide buffers (SiC, IW, WCWO, and CoO). Typical run times are 4 (CH4-rich fluids) and 24 fo 48 h (H20-rich fluids).

Following an experiment, capsules are pierced at 150°C, and the fluid is released into the helium stream of a gas chromatograph, and analyzed for H2O, CO2, CO, CH4, C2H6, and H2, to better than 0.5 mole % absolute.

Results of the 2.4 GPa experiments are as follows:Bulk H/O ratios range from 1.79 (H2O-CO2) to 1330 (CH4-H2±H 0 with SiC). Maximum H 0 content of a C-saturated fluid at 2.4 GPa and 1000°C, given by C + 2H 0 = CH4 + 0 and C + 2H20 = C 0 + 2H is 95±1 mole %. The H2O content of a fluid in equilibrium with the IW buffer is about 10 to 12 mole %. H2 contents range from 0.5 mole % at the H2O maximum (H/O = 2), to 6 ± 0.5 mole % in pure C-H fluids buffered at SiC. Molar H2/C2H6 is constant within error and independent of the fluid bulk H/O ratio.

Experiments with variable quenching rates, plus the observation that the measured log K of the equilibrium 2CH4 = C2H6 + H2 is independent of fluid bulk composition, suggest that fluids are easily quenchable at 1000°C. The best fit is given by the Modified Redlich Kwong EoS of Holloway (1977), assuming nonideal mixing. However, ideal mixing models also give reasonable results. Nonideality effects evident in endmember equilibria such as C + 2H2 = CH4 and 2C + 3H2 = C2H6 tend to cancel each other in complex mixtures.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 MONAZITE U-Pb AGES: DEFINING THE ALICE SPRINGS OROGENY IN THE HARTS RANGE, ARUNTA INLIER Jo Mawbv, Peter Kinny*, John Foden Department of Geology and Geophysics, University of Adelaide, North Terrace, Adelaide, 5005 department of Applied Physics, Curtin University of Technology, GPO Box U1987, Perth, 6001

Despite an increase in sophistication of chronological techniques, it is still difficult to resolve the timing of peak metamorphism in terranes that have undergone multiple events. As the complexity of terrane histories (in particular high-T terranes) becomes apparent, the ability of conventional (IDTIMS) mineral chronometry {e.g.

Rb-Sr, Sm-Nd silicate mineral isochrons) to resolve peak-metamorphic ages, is increasingly called into question.

One shortcoming is that the closure temperatures (T ) of the analysed minerals often are significantly lower than the metamorphic maxima, in which case cooling from the latest thermal event may be superimposed on an earlier higher temperature isotopic composition. In principle, given an a priori knowledge of the equilibration volume of isotopic systems at different times in the terrane history, it should be possible to gain information about the complete metamorphic history. In practise, however, equilibration volumes are difficult to assess, as the degree of isotopic resetting during subsequent events must be quantified. Ideally, an approach is required using mineral systems which have T close to, or above the metamorphic maxima, in addition to a spatially resolved technique where isotopic disequilibria on the grain scale can be identified. c

c

One approach that encompasses both a high degree of spatial resolution and high T (> 700°C) is SHRIMP analyses of metamorphic monazite. For many medium to high grade metasediments monazite clearly has the potential to record crystallisation ages that correspond to prograde and/or near-peak metamorphic conditions. The Harts Range in the eastern Arunta Inlier is an example of a terrane where constraining the age of upper amphibolite facies metamorphism using conventional mineral chronology has been elusive. Preliminary Sm/Nd mineral data show an array of ages loosely defining both Palaeozoic (Alice Springs Orogeny) and Proterozoic metamorphism (e.g. Foden etal. 1995). In an attempt to better define the Harts Range chronology, this study analysed 30 monazite grains using SHRIMP from a meta-pelitic unit in the Entia Dome and the meta-pelitic Irindina Gneiss of the Harts Range Group. c

y

Flat-lying, regionally exposed metamorphic rocks in the Harts Range comprise a structurally lower unit, the Entia Dome, which consists of felsic-mafic orthogneiss and minor metasediments, overlain by the Harts Range Group, comprising mafic gneiss and the Irindina Gneiss. The migmatitic Irindina Gneiss contains a P-T sensitive assemblage of lineated biotite-sillimanite which encloses garnet-plagioclase and abundant monazite.

Peak conditions for this assemblage are ~ 670 ± 25°C and 6.5 kbars. Monazite in the Entia Dome occurs in pelitic gneiss both as a matrix phase and as inclusions in peak-foliation-defming phases that formed at 640670°C and 7-9 kbars. As peak-T for these rocks did not exceed the inferred temperatures for diffusion of Pb in monazite, the U-Pb ages measured here using SHRIMP are interpreted as recording monazite crystallisation, rather than a T . c

Results indicate that regional deformation and upper amphibolite facies metamorphism in the Harts Range were associated with Palaeozoic orogenesis. This finding has implications for the style and distribution of crustal strain during the Alice Springs Orogeny. In the central and western parts of the Arunta Inlier, Alice Springs structures are defined as major but discrete, moderately dipping shear zones that penetrate the lower crust. In contrast, in the Harts Range in the eastern Arunta Inlier, Alice Springs structures are diffuse and defined by regional flat lying mid-lower crustal foliations associated with elevated temperatures. Addressing the reasons for this broad transition in structural style from west to east may be an important component in understanding the internal dynamics of the Alice Springs Orogeny.

ACKNOWLEDGMENTS The use of SHRIMP was made possible by the generous financial contribution from PNC Exploration (Australia) Pty Ltd. The SHRIMP II facility at Curtin University, Perth, is supported by the Australian Research Council.

REFERENCES FODEN J., MAWBY J., KELLY S. & BRUCE D. 1995. Metamorphic events in the eastern Arunta Inlier, Part 2: Nd-Sr-Ar isotopic constraints. Precambrian Research 71, 207-228.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NABRE: POTENTIAL FIELD INTERPRETATION, PHASE 1 Christopher K. Mawer NABRE Project Team, AGSO, GPO Box 378, Canberra, ACT 2600

Phase 1 of the North Australian Basins Resource Evaluation (NABRE) project involves identification of principal regional tectonic elements and establishment of a tectonostratigraphic framework for the basement to the North Australian Middle Proterozoic sedimentary basins. The area of the study is large—it covers some 12° of latitude (from the north coast of NT to the Arunta Complex) and 12° of longitude (from the Western Succession of the Mount Isa Block to the Hall's Creek Mobile Zone). The study involves the determination of size, shape, and distribution of basement blocks and bounding structures, the nature, kinematics and evolution of tectonic elements in the basement, the relative and absolute timing of tectonic events recorded in the basement, and the influence of basement architecture on basin evolution. Compiled and interpreted regional potential field data sets (specifically, the first generations of AGSO's regional gravity and airborne magnetics data sets), available isotopic age dating, and reliable kinematic information from both field studies and potential field interpretations have been integrated. The potential field interpretations are also being linked to interpretations of other remotely-sensed data (such as SPOT and Landsat TM imagery), a series of proposed regional seismic reflection/refraction traverses, new palaeomagnetic and isotopic data, and an extensive field programme.

A series of airborne magnetic and gravity images will be presented, with accompanying structural interpretations.

These will be compared to AGSO's new Geophysical Basement Elements Map (Shaw et al, 1995), and existing regional geological maps. The potential field interpretations have focussed on identification of probable regionalscale fault zones. If these features are Proterozoic in age, they would have controlled or strongly influenced the localisation, spacing, and geometry of coeval basins and their sedimentary lithostratigraphic assemblages, and acted as fluid conduits for mineralising solutions.

The new potential field interpretations have been compared with existing tectonic models of the region (such as Etheridge and Wall, 1994, and references therein) to test the predictions these make. The interpretations show good agreement with the geometrical elements of the Etheridge and co-worker's model, in that there are two interpreted orthogonal sets of fault zones with regional extent. The interpreted fault zones trend roughly NE-SW and NW-SE, and have been correlated with the Leichhardt extensional event at about 1820-1780Ma (Etheridge and Wall, 1994); the NE-trending fault zones in this model are transfer structures, and the NW-trending fault zones of normal sense. Another set of interpreted fault zones trend roughly NNW-SSE and ENE-WSW, and have been correlated with the McArthur extensional event at about 1730-1680Ma (Etheridge and Wall, 1994); the NNWtrending fault zones in this model are transfer structures, with the ENE-trending fault zones being normal sense.

It is probable that at least some of these fault zones would be favourably oriented for reactivation during Late Proterozoic and Palaeozoic tectonic events. If so, fault offsets should exhibit predictable and consistent kinematic senses. Later known tectonic events include the Isan compressional event at about 1650-1620Ma, a possible Grenvillean compressional event at roughly llOOMa, and the middle Palaeozoic Alice Springs compressional event. The potential field interpretations suggest present-day horizontal components of offset on major fault zones can be explained by an overall N-S shortening {i.e. NE-trending fault zones show sinistral apparent displacements and NW-trending fault zones show dextral apparent displacements). The apparent presentday displacements are not consistent with any known major Proterozoic tectonic event, but are consistent with regional Alice Springs-age displacements. Observations of post-Proterozoic movement on several of the major fault zones within the NABRE area support the interpretation that the Alice Springs compressional event extended well into north Australia. If so, there are significant implications for resource exploration , in the area, and for models of Proterozoic and Palaeozoic tectonic evolution of the region.

REFERENCES Etheridge, M.A., and Wall, V.J., 1994. Tectonic and structural evolution of the Australian Proterozoic.

Geological Society of Australia, Abstracts 37, 102-103.

Shaw, R., Gunn, P., Wellman, P., Whitaker, A., Tarlowski, C., and Morse, M., 1995. Geophysical Basement Elements Map of Australia: implications for crustal tectonism in central and southern Australia. Preview, 57, 133.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NEW TECTONIC MAP AND DATA SET FOR THE LACHLAN OROGEN AND TASMAN FOLD BELT SYSTEM Chris K. Mawer and H. Fred Doutch Australian Geodynamics Cooperative Research Centre, PO Box 378, Canberra, ACT 2001

After more than a decade of planning, interpretation, compilation, drafting and editing, a new stratotectonic data set covering the entire Lachlan Orogen and surrounding elements of the Tasman Fold Belt system, as well as the Proterozoic rocks which form the western structural boundary of Palaeozoic eastern Australia, is nearly complete.

Final completion of the data set will be a significant addition to the general understanding of the geodynamic history of Eastern Australia. This data set will be made available to interested parties in several formats. Initially, a multi-coloured printed map will be produced at a scale of 1:2,500,000, covering the southern half of this very large region (from Fraser Island to southern Tasmania). A final draft print of this southern sheet will be presented at the 13th AGC. A northern sheet (essentially the whole of Queensland) is also nearing its final stages of compilation. As all drafting and compilation has been done digitally, the data set will be translated into Maplnfo® and ARC/INFO® formats soon after the printed map appears. This means that this important stratotectonic data set is easily extensible, and can be readily customised to individual users' needs. Already, several new databases are being compiled for the southern region {e.g. isotopic ages of various types) and synthesis layers extracted from the existing data set {e.g. regional distribution of interpreted comagmatic igneous packages). These will be offered as add-ons to the basic data set as they are completed.

Compilation of the data set, originally conceived as a standard printed map, grew from a suggestion at the Government Geologist's Conference in 1982. The map contents were determined by a Concepts Committee comprising representatives of the BMR (now AGSO), the State Geological Surveys of New South Wales, Queensland, Victoria, Tasmania and South Australia, several universities, and nominees of the Geological Society of Australia's Specialist Group in Tectonics and Structural Geology. Physical compilation has been by the five State Surveys, with BMR/AGSO being responsible for final digital compilation, map design and production. The Australian Geodynamics Cooperative Research Centre, through its project Geodynamics and Metallogeny of the Lachlan Orogen, has supplied critical staff and financial resources, and moral support, towards the end stages of the project. The data set will be used by this project as a starting point for preparation of a 4-D digital map of the Lachlan Orogen and surrounding regions.

The data set presents the Tasman Fold Belt System as eleven Stratotectonic Units that reflect its tectonic evolution from Precambrian time to the Mesozoic. Each Unit is essentially an assemblage of lithofacies units that, taken together, represent a major phase in the tectonic evolution of the System. The Units are bounded by major discontinuities such as angular unconformities, and thus do not necessarily have fixed time boundaries. A notable feature of the data set is the depiction of sedimentary depositional environment by shades of Unit colour (pale to darker indicating respectively continental, shallow marine, and deep marine environments), and of provenance by pattern (quartzose, volcaniclastic, volcanic, etc.). Modifications of Units by metamorphism and igneous intrusion are also indicated by patterns. Boundaries of cover rock sequences younger than the Tasman Fold Belt System are shown by coloured bands. The overall development of the Tasman Fold Belt System is summarised in a space-time diagram, and there is also a Structural Zones Locality Map within the data set to allow ready comparison with widely-recognised structural subdivisions of the System.

This is the first compilation of geoscientific data of this magnitude for eastern Australia. It will form an excellent basis for regional studies in geodynamic evolution, sedimentary basin development, igneous province formation, and patterns of variation of metamorphic grade. It will also be invaluable for regional resources exploration studies.

Acknowledgments: After more than a decade, there are very many geoscientists, cartographers, and administrators spread across Australia and overseas who have contributed in some way, great or small, to the realisation of this project. Without their often-considerable commitments of time and energy, the data set and products to be developed from it would never have been available. All contributors thank the past and present Directors of the Geological Surveys of New South Wales, Queensland, Victoria, Tasmania and South Australia, and the past and present Executive Directors of AGSO, for their support, patience, and intellectual contributions towards the completion of this large, complex, and valuable data set.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

AGES AND RELATIONSHIPS OF DEVONIAN ROCKS FROM THE SOUTHERN NEW ENGLAND FOLD BELT

2

Ruth Mawson 1 , John A. Talent 1 , and Evan C. Leitch 2 1 School of Earth Sciences, Macquarie University, NSW 2109 Department of Applied Geology, University of Technology, Sydney, Broadway NSW 2007

Devonian sequences of the Tamworth Belt provide a record of the history of an arc-related basin and the now obscured contemporary magmatic arc. Although biostratigraphically significant conodont faunas had earlier been extracted from limestone masses within these sequences, age determinations had been hindered by the sparse and fragmentary character of many faunas, and the significance of the suggested ages rendered uncertain by the possibility that many limestone bodies are allochthonous. We summarise here recent investigations of Devonian and adjacent strata, involving conodont age determinations and geological mapping.

Significant findings include: (i) The presence of autochthonous Cambrian units unconformably underlying the Devonian sequence has been confirmed (Stewart 1995). ii) Around Copes Creek, where many type sections are located, the Devonian sequence, previously considered to extend to the base of the Devonian, only includes late Emsian and younger rocks (Furey-Greig 1994). (iii) Strata of similar late Emsian age, but contrasting, deeper water facies, occur east of Copes Creek, separated from those of (ii) by the major Sandy Creek Fault. (iv) The oldest autochthonous Devonian rocks so far identified in the Tamworth Belt occur in the Willow Tree Creek district east of the Namoi Fault (Leitch et al. in review). The sequence comprises a lower limestone unit from which Late Lochkovian (pesavis Zone) conodonts have been extracted, a coarse shallow marine volcaniclastic unit, and an upper limestone unit overlain by the Yarrimie Formation. (v) Major limestones bodies intercalated in the lower part of the Yarrimie Formation between the Namoi and Attunga faults can be allocated to two stratigraphic units: the late Emsian Sulcor Limestone Member (Mawson & Talent unpub. data) and the Eifelian (but not earliest Eifelian) to early Givetian Moore Creek Limestone Member (Mawson & Talent 1994; Klyza 1995). Stratigraphically higher Yarrimie rocks have yielded late Frasnian conodonts elsewhere in this fault block (McMinn 1982). (vi) The widespread basaltic rocks of the Nundle region conformably overlie latest Emsian rocks and are succeeded by the Yarrimie Formation (Mawson et al. 1995). (vii) Autochthonous Pragian and early Emsian limestone lenses (Dongal 1995) and associated volcaniclastic rocks in the upper Manning River region equate with a similar lithological assemblage further north. (viii) The Devonian lithostratigraphic succession changes across major faults and anticlines. Although these structures were last active during late Permian orogenesis, they reflect features that exerted control over Devonian depositional patterns.

REFERENCES Dongal G.M.S. 1995 Early Devonian (Pragian and early Emsian) fauna from the eastern Tamworth Terrane, New South Wales. Mem. Austral. Assoc. Palaeonts 18,131-142.

Furey-Greig T. 1995 The "Nemingha* and 'Loomberah' limestones (Early Devonian: Emsian) of the Nemingha Nundle area, northern New South Wales: conodont data and inferred environments. Cour. Forsch.-Inst.

Senckenberg 182,217-233.

Klyza J.S. 1995 Middle Devonian conodonts from the Moore Creek Limestone Member, east of Attunga, New South Wales. Mem. Austral. Assoc. Palaeonts 18, pp 121-130.

Leitch E.C., Cawood P.A. & Mawson R. in review Palaeozoic rocks of the Willow Tree Creek district, northeastern New South Wales: sequence, structure, age and regional significance.

Mawson R. & Talent J.A. 1994 The Tamworth Group (mid-Devonian) at Attunga, New South Wales: conodont data and inferred age. Cour. Forsch.-Inst. Senckenberg 168: 37-59.

Mawson R. Talent J.A. & Furey-Greig T. 1995 Coincident conodont faunas (late Emsian) from the Yarrol and Tamworth belts of northern NSW and central Queensland. Cour. Forsch.-Inst. Senckenberg 182,421-455.

McMinn, A. 1982 The age and facies distribution of the Yarrimie and Baldwin Formations in the Manilla district In New England Geology, pp. 113-120, University of New England, Armidale.

Stewart, 1.1995 Cambrian age for the Pipeclay Creek Formation, Tamworth Belt, northern New South Wales.

Cour. Forsch.-Inst. Senckenberg 182: pp. 565-566.

277

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 GEOLOGY AND TOURISM Wolf Mayer Faculty of Applied Science, University- of Canberra, Box 1 Belconnen ACT 2616

Tourism, both in Australia and elsewhere, has over recent years experienced a marked trend in the preference by tourists to visit areas of the natural, rather than the man-made environment, where they can experience nature in its many and varied manifestations.

Australia is particularly well suited to meet this growing demand for nature tours, given the wide variety and the wealth of natural features of this continent. The tourist potential of Australia's rich and diverse fauna and flora, and also of some of its most spectacular landforms, has long been recognised and exploited. However, tourists are rarely informed of the fascinating history of geological features. The geological landscape serves, more often than not, merely as an attractive backdrop to the more familiar and more readily understood aspects of the natural world.

The beautiful and often unique geological landforms and the abundance and wide range of the attractive mineral and fossil occurrences in this continent, should provide extensive opportunities for a worthwhile expansion in geotourism to cater for the ever growing number of both Australian and overseas tourists.

Geological content can be made to play a more prominent part in tourist itineraries in both general and specific ways. Tourists would, no doubt, appreciate more geological background in the tour information provided for popular destinations such as Kakadu National Park and the Great Barrier Reef, which offer the flora and fauna of these regions as their main attractions. Tours of a specific geological nature, some of which are already operating, are likely to attract an increasing number of tourists and deserve to be strongly promoted, particularly in the expanding overseas market.

Such geotours may include visits to major mining districts such as the Hamersley and Pilbara region, the Kimberley area, the Hunter Valley or Queensland coal mining districts, etc. Of considerable interest would be tours for tourists interested in fossicking for precious and semi-precious minerals, gemstones and fossils, eg the goldfields of Victoria, sapphires in New South Wales and opals in South Australia, and various fossil localities. The stones used in major buildings, eg Parliament House in Canberra, also attract the attention of tourists. Geotours of wine-growing districts to explain the relationship of source rock, soil type and wine variety have met with the approval of tourists.

It is argued here that for geotourism to expand it is essential that easy to understand information is made readily available to the visitor. This is best provided in small, compact, but well-illustrated guidebooks, which cover regions of particular interest to tourists or a specific geological aspect or feature. The Geological Society of Australia may wish to consider producing a series of geological guides for the general public. These could be sold through tourist organisations and tour operators and at the various tourist localities.

Given that a very large number of tourists visiting Australia come from Japan, some geological guides of major attractions, such as the Great Barrier Reef, for example, should be available in Japanese. In order to promote geotours overseas, there is also a need for geological tour guides who speak Asian and European languages Involvement in geotourism provides an opportunity to earth scientists to offer a useful service to the community, to interpret aspects of the earth to the layperson, and to make the nature of Australia better known to its own people and to visitors from overseas.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MULTI-STAGE EVOLUTION OF AUSTRALIAN SUBCONTINENTAL MANTLE: RE-OS ISOTOPIC CONSTRAINTS FROM VICTORIAN MANTLE XENOLITHS Jannene S. McBride. David D. Lambert and Ian A. Nicholls Victorian Institute of Earth and Planetary Sciences, Department of Eaith Sciences, Monash University. Clayton, VIC 3168

We have analysed ten samples from a suite of spinel peridotite xenoliths recovered from alkali basalts of the Anakies and Mt Porndon, Newer Volcanics Province, western Victoria for Re and Os concentrations and Os isotopic composition by isotope dilution N-TIMS following a low-blank, Carius tube digestion/equilibration procedure. These subcontinental lithospheric mantle (SCLM) xenoliths were chosen because they display a negative correlation between whole rock concentrations of compatible elements (MgO, Ni) and moderately incompatible elements (V, Sc, Al), suggesting that they represent residues from variable degrees of partial melting as well as mixtures of refractory harzburgites and picritic or basaltic melts.

All of the spinel lherzolite-harzburgite xenoliths analysed from Mt Porndon have high Os concentrations and low Re/Os ratios (0.93-3.72 ppb and 0.0066-0.0701, respectively), within the range of refractory, melt-depleted peridotite xenoliths from the Archaean Kaapvaal Craton, southern Africa and have yos" values (percent deviation of measured Os isotopic composition from chondritic asthenospheric mantle) ranging from -0.7 (spinel lherzolite) to -4.3 (harzburgite). Os isotopic composition is positively correlated with A1203 and CaO. The late Proterozoic Re-Os model age for one harzburgite (TRD = 760 Ma; TCHUR = 900 Ma) is broadly consistent with 1000 Ma U-Pb ages for one population of zircon xenocrysts in granites of the Lachlan fold belt (Chen & Williams, 1990). However, the Re-Os isotopic data suggest that the oldest analysed portions of the SCLM in this region may be significantly younger than some components of the overlying Proterozoic continental crust, based on whole rock Nd TDM model "ages (1300-2300 Ma; Eberz et al., 1990) and U-Pb ages for a second population of zircon xenocrysts in the granites (1150-3350 Ma). Several xenoliths also have Re-Os model ages of 300-700 Ma which are broadly consistent with 300-600 Ma Sr-Nd isotopic model mixing ages of Griffin" et al. (1988) for southeastern Australian mantle xenoliths and a younger zircon population from the granites (420650 Ma). The correlations between the Re-Os model ages in the xenoliths and the zircon age population data from the overlying crust suggest that the SCLM beneath southeastern Australia most likely formed during episodic periods of coupled crust-mantle growth. The preservation of Proterozoic Re-Os TCHUR model ages in a volcanically active region also suggests that Cainozoic basaltic volcanism (Older and Newer Volcanics) associated with separation of the southeastern Australian margin from Antarctica and the Lord Howe Rise has not completely reworked the base of the continental lithosphere.

Re-Os isotopic analyses of two spinel lherzolites from the Anakies have yielded near-chondritic Re/Os ratios (0.0645 to 0.0710) and yos values (+1 to +2), consistent with the derivation of these xenoliths from recently accreted SCLM, potentially a result of conductive cooling of the asthenosphere during modern lithospheric extension. Re-Os isotopic data for another fertile spinel lherzolite from the Anakies document the presence of enriched components within the SCLM or "perisphere". A clinopyroxene banded and fertile lherzolite (A13) has given both very radiogenic (+8.5 to+13) as well as sub-chondritic (-1.8) Yos values, suggesting that banding may be the result of intermingling/mixing of old, refractory harzburgite with ancient melts (basalts or picrites).

Thus, the high Yqs value of lherzolite A13 is not supported by measured Re/Os, demonstrating that the source of this xenolith has experienced a multi-stage history of enrichment/depletion. Ancient subduction-related recycling of Re-rich oceanic crust into the upper mantle or intra-mantle melt metasomatism have both been invoked to explain similar enriched Os isotopic compositions reported for some HIMU ocean-island basalts (Hauri & Hart, 1993). Modelling suggests that enrichment of the SCLM with basaltic melt can yield the required high y 0s value of the most radiogenic lherzolite if melt addition occurred at ca. 500-600 Ma. This time period is consistent with a TCHUR model age of 560 Ma for a garnet metapyroxenite from Lake Bullenmerri, suggesting that basaltic melts traversed the SCLM during the Cambrian and may have been temporally associated with greenstone belt development in southeastern Australia.

REFERENCES ANDERSON, D. 1994. The sublithospheric mantle as the source of continental flood basalts; the case against the continental lithosphere and plume head reservoirs. Earth Planet. Sci. Lett. 123, 269-280.

CHEN, Y.D. & WILLIAMS, I.S. 1990. Zircon inheritance in mafic inclusions from Bega Batholith granites, southeastern Australia: an ion microprobe study. J. Geophys. Res. 95, 17787-17796.

EBERZ G., NICHOLLS I., MAAS R., MCCULLOCH M. & WHTfiFORD D. 1990. The Nd- and Sr-isotopic composition of I-type microgranitoid enclaves and their host rocks from the Swifts Creek Pluton, southeastern Australia. Chem. Geol. 85, 119-134.

GRIFFIN W., O'REILLY S. & STABEL A. 1988. Mantle metasomatism beneath western Victoria, Australia: II. Isotopic geochemistry of Cr-diopside lherzolites and Al-augite pyroxenites. Geochim. Cosmochim. Acta 52, 449-459.

HAURI, E. & HART, S. 1993. Re-Os isotope systematics of HIMU and EMII oceanic island basalts from the south Pacific ocean. Earth Planet. Sci. Lett. 114, 353-371.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SURFICAL SEDIMENTATION AND COASTAL EVOLUTION OF THE GODWIN BEACHSANDSTONE POINT AREA, SOUTHEAST QUEENSLAND. Shaiene T. McClure and Simon C. Lang School of Geology, Queensland University of Technology, GPO Box 2434, Brisbane, QLD, 4001.

The northern part of Moreton Bay is dominated by Deception Bay and the surrounding Bribie Island, Pumicestone Passage, and the North Entrance tidal delta. The Godwin Beach-Sandstone Point area is located in the north of Deception Bay between the Caboolture River and Burpengary Creek, and the southern end of Pumicestone Passage. Northern Deception Bay area consists of a strip of unconsolidated Pleistocene and Holocene coastal plain deposits that overlie the early Jurassic Landsborough Sandstone. The area experiences a microtidal to mesotidal tidal range and the prevailing winds are from the south-east.

SURFICAL SEDIMENTATION Extensive sampling of the intertidal and subtidal surfical sediments has delineated six different lithofacies that consist of sand, mud and biogenic carbonate. Two major sources of sediment have been defined and include (1) mature quartz sand from the active transport system of the North Entrance tidal delta and (2) prodelta mud from the Caboolture River and Burpengary Creek. Two lithofacies zones dominate the area; a sand lithofacies which is located offshore from Sandstone Point, and further to the south-west a mud lithofacies that has a large biogenic carbonate component. The intertidal and subtidal lithofacies therefore represent a zone of transition from mature marine quartz sand to prodelta mud, and their distribution is controlled by the early Jurassic Landsborough Sandstone headland at Sandstone Point and the prevailing south-east winds.

COASTAL EVOLUTION A beach ridge-tidal flat system has developed in the Godwin Beach-Sandstone Point area since the end of the post-glacial marine transgression (6,000-6,500 yrs BP) as a response to the morphodynamic conditions of the area. Drilling data indicates the Holocene coastal plain is a 'stillstand' progradational wedge that downlaps onto transgressive sand and mud and displays similar lithofacies to the ones presently operating in the nearshore zone.

The Holocene coastal plain is over 2 km wide and is dominated by a beach-ridge strand plain comprised of two beach ridge systems separated by 750 m of coastal lowlands and swamps. The most landward system, Beach Ridge System 1, developed not long after the end of the post glacial marine transgression. The lateral discontinuity of beach ridges in this system indicates that their deposition has been influenced by a small outcrop of Landsborough Sandstone located in the middle of the system. Beach Ridge System 2 developed in the late Holocene (> 1,500 yrs BP) and consists of a large landward ridge and several smaller seaward ridges. This system is laterally continuous from Godwin Beach through to the Caboolture River (8 km). Between the two systems are coastal lowlands and swamps consisting of supratidal and upper intertidal deposits.

The separation of the beach ridge systems may have been caused by three different processes (1) a slight sea level fall during the mid to late Holocene (up to 0.7 m based on raised upper intertidal sediments), (2) the formation of a barrier built up from the south-west inhibiting the development of beach deposits behind, or (3) a drop in sand supply between the development of the two systems. A modern analog for the development of a barrier or spit is now observed near the mouth of an unnamed tidal creek, where a beach spit has cut-off sand supply and supratidal and upper intertidal conditions now prevail behind it. There is clear evidence that a fall in sea level has occurred in the area, however, it is difficult to access the influence of the other two processes.

Sequential aerial photographs over the last 53 years show the area has experienced short term coastal changes that include encroachment of supratidal environments by intertidal mangroves and beach erosion. These changes may indicate a slight rise in sea level. Urban development along the northern Deception Bay coastline could be threatened if these coastal changes continue.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

DEATH OF THE GUNPOWDER CREEK FORMATION INTERMEDIATE QUARTZITE (PMWq) MAMMOTH MINES 100 000 SHEET, MOUNT ISA BASIN Bruce A. McConachie. Deborah L. Scon, Alan T. Wells, Peter N. Southgate NABRE Project, Australian Geological Survey Organisation

The Gunpowder Creek Formation intermediate quartzite (Pmw ) has been mapped (L. Hutton, G.M. Derrick and J. Gallagher, Geology of the Mammoth Mines 100 000 region, 1980, Fig. 1) as a quartzose sandstone member occurring within the Gunpowder Creek Formation north of Redie Creek. This is the only location where the unit was recognised. q

As part of regional sequence stratigraphic analyses of the Surprise Creek Formation (Pra), Toipedo Creek Quartzite (Pmp) and Gunpowder Formation (Pmw), the Gunpowder Formation intermediate quartzite was traversed and gamma-ray logged. Gamma ray and geological logging of vertical sequences in the Gunpowder Formation at Barr Hole (25 km N), Esperanza Waterhole (10 km NE) and east of Crocodile Waterhole (8 km SE) revealed significant differences between the mapped Gunpowder Formation below the intermediate quartzite at Redie Creek, and typical Gunpowder Formation elsewhere. In particular, the sequence packaging was strikingly different. The thick prograding units below the Gunpowder Formation intermediate quartzite were far better developed than elsewhere in the Gunpowder Formation and, in our estimation had more similarities to sequences logged in the Surprise Creek Formation.

Remapping from 1: 25 000 scale colour aerial photos and geological traverses revealed a previously unrecognised, late stage, east over west thrust fault striking north-south. A breccia zone striking 170° (at 03 19 280 E, 78 07 363 N, Fig. 1) and cropping out for several hundred metres, plus changes in formation dips, support the thrust fault interpretation. In our interpretation the present mapped Gunpowder intermediate quartzite in the Redie Creek area (Fig. 1) is actually a repeat of the Toipedo Creek Formation. In addition, from aerial photo interpretation, an unconformity typical of the base of the Toipedo Creek Quartzite is recognisable beneath the intermediate quartzite.

The economic implications of this reinterpretation may be locally significant. The Big Bend copper mine occurs at the southern end of the exposed thrust on Gunpowder Creek where the fault plane is vertical, striking due north-south. Although the thrust fault exhibits relays and offsets it can be projected north to a previously recognised north-south structure which displaces the distinctive Mount Oxide Chert (Pmo) in a manner consistent with the thrust, fault inteipretation. The size and continuity of this thrust fault and its known association with high grade copper mineralisation, suggest that new copper exploration potential may exist in the area north of Redie Creek. w - w \ „„,

03 10 ooo E

77 05 000 N

Figure 1. Detail of the Redie Creek area from the Mammoth Mines 100 000 Geological Sheet 281

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SEAWATER N E U T R A L I S E D BAUXITE R E F I N E R Y W A S T E S : AN E N V I R O N M E N T A L H A Z A R D OR A POTENTIALLY VALUABLE R E S O U R C E

1

David McConchie 1 , Peter Saenger1 and Richard Fawkes 2 Centre for Coastal Management, southern Cross University, P.O. Box 5125, East Lismore, NSW 2480 2 Queensland Alumina, Parson's Point, Gladstone, Queensland 4680

Management of caustic red mud wastes at the Queensland Alumina Refinery uses an innovative approach involving neutralising the red mud with seawater, allowing the solids to settle in a red mud dam and discharging surplus water (pH = 8.6) to a nearby tidal inlet. This paper presents the findings of studies undertaken to assess the environmental acceptability of this waste management strategy.

The environmental survey involved examination of benthic biota in over 400 grab samples and analysis of 16 trace elements in over 800 water, sediment, mangrove tissue and marine fauna samples; bioavailable nutrients were examined in 14 water and 12 sediment samples, pH, Eh and salinity were determined for all water samples and Eh, texture and mineralogy were determined for all sediments. Samples were collected from: transects adjacent to the red mud and ash pond outfalls, 6 additional transects in South Trees Inlet and the Boyne River estuary, 3 control transects in Wild Cattle Creek, 11 sites in the waste ponds and 13 other sites near the refinery.

Although alumina refineries produce caustic waste water, no rise in water pH was detected near the pond outfalls; water pH actually decreases slightly in South Trees Inlet as distance from the open sea increases, due to the natural input of organic acids. The A1 content of water samples collected near the pond outfalls is slightly elevated due to the presence of colloidal Al-hydroxides that pass through the 0.45p.m membrane used to filter the water samples. The concentrations of all other trace elements in the water samples (except those from the waste ponds) were near or below world average values for inshore marine waters and showed no detectable rise near the outfalls; the concentrations of some trace elements were lower in discharge waters than in seawater in the inlet.

Whole sediment analytical data reveal small rises in the concentration of most trace elements near the pond outfalls. However, when the trace element concentration data are normalised against sediment grainsize or against the proportion of potentially metal binding minerals in the sdiment, the apparent rise near the pond outfalls largely disappears and, with the exception of Al, no change in the concentration of any trace element in the sediment can be clearly linked to pond water discharges; the Al is present as fine Al-hydroxide particles that accumulated in pools where current velocities are low. Trace elements in mangrove tissues showed no concentration trends that could be linked to discharges from the ponds. The trace metal concentrations in marine fauna samples (including oysters collected from the outfalls and butterfish caught in the ash pond) are below the limits set for human consumption in Australia and are typical of concentrations recorded across much of northern Australia. The pond outfalls were not associated with a less diverse benthic biota and no reduction in the Gleason Diversity Index, (used to indicate pollution) was detected near either outfall. A close relationship between sediment texture and species diversity accounts for most of the inter-site variation in species diversity.

The seawater neutralised red mud is dominated by particles with a very high surface area / volume ratio and a high charge / mass ratio. Consequently, it has a high trace element binding capacity (>1000meq/kg dry mud at pH values >6.5) and the ability to strip trace elements from water in contact with it. Most of the environmentally significant trace elements are present as adsorbed species bound to poorly crystalline Al- and Feoxyhydroxides or as constituents of hydroxide, oxide, oxyhydroxide or carbonate minerals. The red mud also has a high acid neutralisation capacity, due to an abundance of amorphous and finely crystalline mineral phases that form weak bases, and compacted red mud has a very low permeability. A high tidal range and >60% flushing of the inlet in each tidal cycle minimise the impact of pond water discharge, but the main reason for the low environmental impact involves trace element trapping by the fine grained oxyhydroxides in the red mud.

As a result of its chemical and physical properties, the seawater neutralised red mud may be useful in environmental protection and rehabilitation programmes, including: a) neutralising acid mine wastes and sulphidic ore tailings, b) blending with acid sulphate soil material to prevent the formation acidic leachates and to enhance the agricultural value of the soil by increasing its cation exchange capacity, and c) as a lining for refuse tip cells to slow leachate release, neutralise any acid leachates, and trap trace metals. There are also other possible uses particularly if the mud can be prepared as porous pellets.

282

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A PALEOSEISMOLOGY CASE STUDY: THE LAKE EDGAR FAULT SCARP IN TASMANIA K McCuel. B Boreham^, R Van Dissent G Gibson4, V Jensen^ and B McKavanagh^ * Australian Geological Survey Organisation, GFO Box 378, Canberra ACT 2601 "Department of Applied Physics, Central Queensland University, Rockhampton Qld, 4700 ^Institute of Geological and Nuclear Sciences, lower hutt New Zealand ^"Seismology Research Centre, RMIT, Bundoora Vic 3083 ^Department of Geology, University of Tasmania, Hobart Tas 7001

There have been 20 large earthquakes of magnitude 6 or more in Australia in the last 100 years and all five since 1968 have ruptured the surface. If their epicentres are randomly distributed, then the probability of one occurring within 100 km of one of the eight capital cities in any 100 year interval is about 40%. This crude estimate probably overestimates the risk for eastern Australian cities and underestimates it for Adelaide and Perth, both of which have been shaken by a magnitude 6 + earthquake within 100 km in the last 100 years. Australian communities are more at risk than is often acknowledged.

To reduce the risk we need to determine any spatial pattern (do they correlate with the geological age or structure of the host crust, heat flow, topography or the distribution of the many small earthquakes located in the last few decades). We also need to improve our determination of the long-term recurrence rate of large earthquakes and be able to estimate the magnitude of the largest earthquake to which our cities might be exposed Paleoseismology, the study of prehistoric fault scarps, gives us one method to determine the long term recurrence rate, the distribution of large earthquakes that have generated surface faults, and an estimate of a maximum credible earthquake.

Several apparently Recent fault scarps have been mapped in Australia during routine geological mapping. One of the most prominent scarps and most important from a risk viewpoint is the Lake Edgar Fault in Tasmania. This fault scarp in southwest Tasmania is in the button grass of the Huon Plains and was notable because it had created two sag ponds, Lake Edgar and the original lake Pedder. It was originally considered to be a normal fault of Pleistocene or even post glacial age. It passes through one abutment of Edgar dam and is within 4 km of Scotts Peak dam which together impound the waters of the new Lake Pedder. With no experienced paleoseismologists in Australia, RVD was invited to join an expedition to study the fault, to determine the age of faulting and whether there had been more than one episode of faulting, and to pass on practical knowledge to other members of the group.

The north-south trending scarp is within the boundary of the Southwest National Park so no trenching was allowed. We discovered a i m wide trench dug to a depth of 2 m in the 1960s by Tasmanian Hydroelectric Authority geologists. The scarp cuts an alluvial fan which descends from the mountains to the east and formed at the end of the last period of glaciation. The trench wall cut through a spongy fibrous peat and the uniformly thick sandy gravel layer and exposed the underlying silty fine sand suspected to be in-situ weathered rock. The trench was overgrown and partially filled with water but the walls were still intact. It extended 80 m west of and perpendicular to the scarp and was eminently suitable for our purposes.

The walls of the trench through the 2 m high scarp were cleaned down with spades and trowels and a stringline grid established on the south wall. Classic faulting features became clear as the face was cleaned including vertical gravel filled tension cracks in the upthrown block, lenses of sand presumed to be caused by liquefaction, and the former gravel fan surface underthrust beneath the weathered rock layer. Like the more recent faults in Australia this was clearly a thrust fault.

The second finding was the fact that the fault offset the post glacial alluvial fan which narrowed the age range to post glacial ie younger than 10 000 years. Thirdly the existence of step-dipping fine sandy laminae near the scarp and near horizontal laminae away from the scarp are indicative of an earlier faulting event which is also indicated by the amount and variability of the uplift.

The three surveyed cross sections show typical dip-slip faulting, near the trench, 70 m north of the trench and 500 m south of the trench. Measured vertical throws were 2.5m, 4.5 m, and 6.5 m respectively.

Unfortunately the C14 dating was inconclusive, the peat was too young for dating and the weathered in-situ sand more than 39 600 years - too old for C14 dating. Further work needs to be done on the Lake Edgar scarp: map the fault, and use other techniques such as pollen analysis and possibly TL dating to further constrain the age of the most recent faulting event.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EVIDENCE FROM THE GEOLOGIC RECORD FOR CATASTROPHIC SEA LEVEL RISE EVENTS AND IMPLICATIONS FOR A GREENHOUSE MODIFIED EARTH Malcolm T. McCulloch Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia.

One of the likely consequences of increased concentrations of atmospheric greenhouse gases and warmer global temperatures is that sea-level will rise. This is due to the combined effects of direct thermal expansion of the oceans together with enhanced melting of glaciers and release of grounded ice sheets. On short time-scales of tens to hundreds of years it is generally believed (IPCC IS 92) that thermal expansion effects will dominate, resulting in an -30 cm to 40 cm rise in sea-level during the next century. These estimates are however uncertain as they depend sensitively on the latitudinal distribution of heating as well as the behaviour of the mixed ocean layer above the thermocline. Of far greater uncertainty however is the response of the two major ice sheets in the Antarctic and Greenland to global wanning. The growth or decay of an ice sheet represents a balance between the amount of snowfall accumulated, especially in winter and extent of summer melting. In addition there are strong feedbacks, with for example initially small increases in sea-level releasing partially grounded ice sheets which in turn results in further increases in sea-level.

The overall contribution of glacial melting to sea-level rise is presently thought to be relatively small (IPCC IS 92), being -10 cm to 15 cm in the next century. The combined estimates of contributions from thermal expansion of the oceans together with glacial melting give a total rate of sea-level rise of - 5 mm/yr or -50 cm for the next century. This latter estimate does not however take into account the possibility of catastrophic melting of glaciers and the release of grounded ice sheets such as in the west Antarctic.

In order to better understand the potential for extremely rapid increases in sea-level due to global warming it is instructive to examine the recent geological record. During the last -140 ka there have been two main phases of rapid sea-level rise accompanying the change in global climate from glacial to interglacial conditions. The last deglaciation from 18 ka BP to 6 ka BP resulted in eustatic sea-level rising by -130 metres to reach present-day heights.

Radiometric dating, combined with careful studies of coral genera distributions has been used to constrain the rate, magnitude and timing of glacio-eustatic sea-level changes. Recent detailed studies of the elevations and ages of drowned Acropora palmata reefs from the Caribbean by Blanchon and Shaw (1995) show significiant discontinuities in the rate of sea-level rise. These discontinuities at 14.2 ka, 11.5 ka and 7.6 ka indicate catastrophic sea-level rise events of >45mm/yr compared to long-term average rates of -10 mm/yr. The duration of these events is relatively short, <200±50 yrs, corresponding to rises in sea-level of from 13.5 to -6.5 metres respectively (Blanchon and Shaw, 1995).

During the penultimate deglaciation at -126 ka, an even larger catastrophic sea-level rise event has been identified at the Huon Peninsula, Papua New Guinea. This is a unique locality as rapid uplift has exposed a well defined sequence of coral terraces that preserve a quantitative sea-level record. The last interglacial terrace (reef VII) which formed when sea-level was only several metres higher than present-day, forms a distinctive broad terrace, which is now uplifted to heights of >200 metres. At -80 m below the top of this main last interglacial reef VH, a cave has been found that provides a window through the younger overlying reef VI (-100 ka) to the base of reef VH. At this locality it is thus possible to directly determine the rate of sea-level rise during the penultimate deglaciation. Precise 234u-230Th dating of exceptionally well preserved Porites found on the cave floor, yield well defined U-Th ages of 126±1 ka and an initial 8 234 U(i) = 151±2, the same as modern seawater. Detailed Sr/Ca analyses at near weekly intervals from the large Porites head indicates ocean temperatures of -20±1 °C compared to present-day temperatures of 27±2°C. This cooler sea surface temperature for the penultimate deglaciation is indicative of colder tropical climtes during low sealevel stands. These observations imply that during the early part of the last interglacial period, (ie at -126 ka) sea-level increased rapidly by at least -80 meters. The duration of this last interglacial sea-level rise event is currently not well constrained at Huon Peninsula, but U-Th ages from equivalent last interglacial corals from stable sites indicates that it must be within the analytical uncertainty of the U-Th age measurements (<1000 yrs). This implies a rate of sea-level rise of >80mm/yr for a duration of <1000 years at 126 ka.

What are the implications of these observations from the geologic record, for pulses of extremely rapid rise in sealevel? These catastrophic sea-level rise events are a factor of xlO to x20 faster than current estimates of sea-level rise due to global warming with business as usual scenarios. The possibility of 5 cm to 10 cm per year increases in sealevel during the next century cannot therefore be discounted if the Earth again steps into a deglaciation mode. We are perhaps fortuneate that for the past -6000 yrs we have been in an interglacial period with constant eustatic sea-level, and summer insolation in the northern hemisphere, the driver of glacial-interglacial changes, has been decreasing.

REFERENCES Blanchon P., &Shaw J., 1995. Reef drowning during the last deglaciation: Evidence for catastrophic sea-level rise and icesheet collapse. Geology, 23,4-8.

Climate Change 1992, the Supplementary Report to the IPCC Scientific Assessment, eds Houghton J.T., Callander B.A, &Varney S.K.

284

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NEW FIELD AND Nd-Pb ISOTOPIC CONSTRAINTS ON THE EARLY EVOLUTION OF THE MOUNT ISA BLOCK Graeme D. McDonald. Kenneth D. Collerson and J. Immo Wendt Department of Earth Sciences, University of Queensland, St Lucia, QLD, 4072

The early-to-mid Proterozoic Mount Isa Block (MIB) in NW Queensland is a regionally extensive and economically important component of the North Australian craton. Understanding the crustal evolution of the MIB has geodynamic and metallogenic consquences that are important for the development of exploration models in the area. New field observations have allowed the establishment of a relative chronology between early gneissic units in the Kalkadoon - Leichhardt Belt (KLB). These relationships combined with Pb and Nd isotopic data, indicate the presence of Archaean and Palaeoproterozoic crust. They support a model for early assembly and stabilization of the MTB that involved lateral accretion processes similar to those responsible for the generation of early Proterozoic crust in other continents.

The northern and central part of the KLB is dominated by the following major lithological units: Leichhardt Volcanics (LV), Kalkadoon Granite (KG), Ewen Granite (EG) and the Argylla Volcanics (AV). A relatively minor unit, the Kurbayia Migmatite (Blake and Page, 1988), interpreted to be largely derived from metasedimentary protoliths, was recognised in the central and southern part of the belt. Recent field mapping SE of Mount Isa indicates that units mapped as "Kurbayia Migmatite" and "Kalkadoon Granite" are significantly more complex than previously acknowledged and preserve a complex lithological and structural record of crustal evolution. These compositely layered gneisses, which are dominantly derived from igneous protoliths, are now termed the Black Angel Gneiss Complex (BAGC). They are cut by sheets of Kalkadoon granite with simple L-S fabrics. These granites also contain gneissic xenoliths. Such relationships provide clear evidence for the relative stratigraphic position of the gneiss complex within the MIB. Three distinct groups of orthogneisses have been identified in areas of low finite strain within the BAGC. The oldest component comprises compositely layered tonalitic, trondhjemitic and granodioritic gneisses that contain sheets of agmatitic metagabbro. These older units are cut by mafic dykes. Younger gneissic units which post-date the dykes include fine grained layered granitic gneisses and units of tonalitic gneiss with simple fabrics. Layering is typically defined in these rocks by flattened cross-cutting pegmatitic and leucocratic veins. Sheets of weakly deformed, relatively homogeneous granodiorite containing rafts of older amphibolite form a distinctive late component of the gneiss complex. The gneiss complex also contains a number of mafic and felsic metavolcanic belts. These rocks are poorly exposed and may be regionally more extensive in subcrop.

BAGC orthogneisses range in eNd(O) from -24 to -33 and yield depleted mantle model ages (TDM) between 2.4 and 2.78 Ga. The oldest TDM model ages, i.e., 2.78 and 2.75 Ga; are given by a metavolcanic supracrustal and a leucogabbro. The younger felsic gneisses (with weak composite or simple fabrics) range in eNd(O) from -25.3 to -29.4 and have T^M model ages between 2.45 and 2.57 Ga. LV's display a restricted range in eNd(O) -23.7 to -26.7. eNd(1.86 Ga) [the time of formation of the LV suite] range between -2.7 and -3.9. KG and EG also lie within this range. The LV, KG and EG's exhibit TDM model ages between 2.42 Ga and 2.66 Ga. eNd(1.86 Ga) in the oldest BAGC samples range from -4.6 to -6. LV, KG and EG melts probably incorporated some crust of this composition. The AV's define two groups: Gp. 1 exhibits eNd(O) in the same range as the LV, KG and EG; whereas, Gp. 2 are less evolved and correspond to eNd(O) of -17 to -19. eNd(1.77 Ga) for the AV's range from -1 to -1.1. Gp. 1 AV's have TDM model ages similar to LV, KG and EG ages, however Gp.2 AV's are younger (TDM 2.34 to 2.38 Ga). This suggest that the Gp. 2 AV's were not formed by melting of BAGC, LV, KG and EG crust. Nd isotopic data together with field evidence for a tectonic contact between AV and LV-KG crust suggests a model that involved lateral accretion and assembly of crust at a continental margin. This is also supported by major and trace element data for felsic and mafic rocks that show strong arc signatures in PMN plots e.g., distinct negative Nb, Sr, P, and Ti spikes.

The BAGC represents a late Archaean to Palaeoproterozoic component in the KLB of the MIB. BAGC felsic gneisses and leachates define a Pb-Pb secondary isochron corresponding to an age of 1726±46 Ma. From field relationships and Nd isotopic data it is clear that this "age" is younger than the formation age of these rocks. The 1726 Ma "event" may reflect U mobilization and re-equilibration of the Pb-system during a thermal/tectonic event associated with eruption or emplacement of the Eastern Creek Volcanics.

REFERENCES Blake, D.H., & Page, R.W., 1988. Early Proterozoic migmatitic basement in the Kalkadoon-Leichhardt belt of the Mount Isa Inlier, northwestern Queensland. BMR J. Aust. Geology & Geophysics, 10, 323-328.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PRIMORDIAL SOLAR NOBLE GAS COMPONENT IN THE EARTH: CONSEQUENCES FOR THE ORIGIN AND EVOLUTION OF THE EARTH AND ITS ATMOSPHERE Ian McDougall and Masahiko Honda Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200

Noble gas geochemistry has provided much useful and unique information concerning the origin and evolution of the Earth and its differentiation into core, mantle, crust and atmosphere/hydrosphere. The atmosphere of the Earth is the major repository of the noble gases, reflecting a high degree of outgassing of the planet.

Nevertheless, analysis of mantle-derived samples clearly shows that outgassing of noble gases from the Earth continues, albeit at a very low rate compared with that during the early history of the Earth.

There is a concordance of evidence from noble gas geochemistry and from other isotopic systems that the crust of the Earth, the atmosphere and hydrosphere have been formed by differentiation from the upper mantle, probably that part of the mantle above about the 600 km seismic discontinuity. This residual upper mantle is considered to be the source of MORB magmas, which exhibit extremely high 40 Ar/ 36 Ar ratios thought to be on average > 20,000. Such high 40 Ar/ 36 Ar ratios provide compelling evidence for profound early degassing of this part of the mantle, certainly within the earliest 500 Ma of Earth history. The presence of high 129 Xe/ 130 Xe ratios in MORBs, compared with atmospheric ratios, indicates that separation of the bulk of the atmosphere from the mantle by degassing is likely to have occurred within the first 100 Ma or so of Earth history. The remarkable coherence and homogeneity of the MORB source in the upper mantle is further reinforced by the concordance of helium and neon isotopic data, and this is also reflected in other isotopic systems. Thus, the upper mantle source for MORB magmas is an important noble gas reservoir, characterized by having been extensively degassed at an early stage in Earth history, related largely to the formation of the crust and atmosphere.

The source for plume-related lavas (OIBs etc) has contrasting noble gas characteristics, indicative of a much less degassed mantle reservoir, generally identified as the region below the depleted, degassed mantle. Whether this volumetrically large part of the mantle is homogeneous or not remains a moot point, as does the question as to how much recycling owing to subduction processes has occurred. This relatively undegassed part of the mantle commonly has been postulated to have a noble gas composition quite similar to that of the present-day atmosphere. This simple proposition needs to be modified as isotopic compositions of neon and argon in particular in OIBs commonly differ significantly from atmospheric.

Neon isotopic compositions found in noble gases released from OIBs and MORBs differ in systematic ways from atmospheric and each other. They show remarkable correlations with helium isotopic compositions. The correlated neon and helium isotopic compositions provide powerful evidence in favour of the notion of a primordial solar component within the Earth, now a widely accepted proposition. However, if the whole Earth started with a solar composition, the derivation of the present Earth's atmosphere from the Earth by simple outgassing processes poses considerable problems. This dilemma may be resolved by postulating that elemental and isotopic fractionation occurred in the atmosphere as a consequence of hydbrodynamic escape processes associated with intense ultraviolet radiation from the Sun and/or Moon formation owing to impact of a Mars-sized body with the Earth. Another possible scenario is that material accreting at a late stage in Earth formation had a more planetary (meteoritic) noble gas composition, so that the atmosphere may be a mixture of solar and planetary components. The helium and neon data are consistent with this hypothesis, and further work, especially on the heavier noble gases, argon, krypton and xenon, on mantle-derived samples, should help to distinguish between the possibilities. Studies of this kind ultimately should lead to a fuller understanding of the Earth's origin and evolution, as well as the better delineation of the kinds and compositions of the noble gas reservoirs in the mantle.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GOLD IN AUSTRALIAN LATE PALAEOPROTEROZOIC STRATIFORM SEDIMENT-HOSTED ZINC-LEAD-SILVER DEPOSITS Peter McGoldrick 1 . Reid Keays 2 , Rob Smith 3 , Gordon McOrist 4 and John Fardy 4 * CODES/Geology, University of Tasmania, PO Box 252C, Hobart, Tas., 7001 ^Faculty of Science and Engineering, Laurentian University, Ramsey Lake Rd, Sudbury, Ontario, P3E 2C6, Canada ^Western Mining Corporation Ltd., Olympic Dam Operations, PO Box 150, Roxby Downs, S.A., 5725 4 ANSTO, Environmental Science Program, Lucas Heights Research Labs, New Illawarra Rd, Lucas Heights, NSW, 2234

The gold contents of ores, pyritic and barren host sediments, and some more distal barren host rocks from the stratiform sediment-hosted (SSH) deposits at Mount Isa, Lady Loretta and HYC deposits have been measured by radiochemical neutron activation analysis.

Zinc-lead-silver ores contain, on average, 2.0 parts per billion (ppb), 2.2 ppb and 5.1 ppb gold, at Mount Isa, Lady Loretta and HYC, respectively. Pyritic shales and siltstones contain 2.1 ppb, 0.9-ppb and 6.2 ppb gold, and barren (low sulfide) shales and siltstones contain 0.6 ppb, 0.4 ppb and 1.7 ppb gold. These are remarkably low gold levels, both in terms of absolute gold tenor and the proportion of gold compared to base metals in the ores.

For instance, Shaw et al., (1976) estimated that "average continental crust" contains 1.8 ppb gold and 52000 ppb zinc (zinc:gold ratio of 29,000:1). Unmineralised fine grained clastic sedimentary rocks contain between 1 and 5 ppb gold (Glasson and Keays, 1978; Crocket and Kuo, 1979) and between 40,000 and 200,000 ppb zinc (zinc:gold ratios between 8000 and 200,000:1). By contrast, ores from the three stratiform sediment-hosted deposits have zinc:gold ratios are between 1 million and 50 million.

The low gold tenor in the SSH deposits can be explained in two ways. Either, the ore forming fluid carried very little gold (and had a base metal:gold ratio » typical crustal ratios), or, the fluid did contain some gold but this gold was not precipitated at the site of base metal sulfide precipitation. The former explanation is preferred because in the latter case, special pleading is required to either, very efficiently remove gold from solution before the base metals, or, to keep the gold in solution during base metal sulfide precipitation. Neither scenario is considered geologically or geochemically likely.

A base metal sulfide ore-forming fluid containing very little gold that is at, or near, saturation in base metals and gold, has important implications for the physico-chemical character of the mineralising fluid in Australian SSH deposits. In order for saline hydrothermal fluids to have low gold solubilities they must be either cool and oxidised, or very reduced. Cool, oxidised saline fluids are excellent base metal solvents, whereas reduced fluids must be hot ( » 200°C) in order to carry base metals. There is no direct evidence (e.g. fluid inclusion data) for the temperature of formation of Australian Palaeoproterozoic SSH zinc-lead-silver deposits, but, the lack of obvious hydrothermal alteration or feeders to the deposits, and the shallow sedimentary setting of some deposits precludes high-temperature fluids. Hence, cool («200°C) and relatively oxidised (S04^"»H2S+HS") are the likely mineralising fluid in the northern Australian deposits. This conclusion has important implications for the ore deposit models used in exploration for this important class of deposit.

REFERENCES CROCKET, J. H., & KUO, H. Y. 1979. Sources for gold, palladuim and iridium in deep-sea sediments.

Geochimica et Cosmochimica Acta 43, 831-845.

GLASSON, M. J., & KEAYS, R. R. 1978. Gold mobilisation during cleavage development in sedimentary rocks from the auriferous slate belt of Central Victoria, Australia: Some important boundary conditions. Economic Geology 73, 496-511.

SHAW, D. M., DOSTAL, J., & KEAYS, R. R. 1976. Additional estimates of continental surface Precambrian shield composition in Canada. Geochimica et Cosmochimica Acta 40, 73-83.

Acknowledgements: This work was made possible through ARGC, ARC and AINSE grants to RRK over many years, and AINSE Grant 93/044 to PMcG. MIM Ltd and Pancontinental Mining provided access to the Mount Isa, HYC and Lady Loretta deposits.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

Cu- AND Pd-RICH IMMISCIBLE SULPHIDE LIQUIDS IN SUBMARINE SHOSHONITIC LAVAS FROM THE TABAR-LIHIR-TANGA-FENI ISLAND ARC, PAPUA NEW GUINEA Brent LA. Mclnnes and Noreen J. Evans CSIRO Division of Exploration and Mining, PO Box 136, N. Ryde, NSW 2113 INTRODUCTION High-K calc-alkaline basalts (shoshonites) were dredged from 1400 m water depths on the eastern flank of the Tanga volcano, Tabar-Lihir-Tanga-Feni island arc, east of New Ireland, Papua New Guinea. The glassy margins of the vesicular flows contain spherical Cu-rich sulphide globules with diameters ranging from 15 to 60 |Lim (average 40 (im). The homogeneous globules have not crystallized sulphide or oxide minerals, indicating that they are quenched immiscible Cu-Fe-O-S liquids trapped as the basaltic melt became sulphide saturated during eruption. Although such phenomenon occur in mid-ocean ridge basalts, this is the first report of immiscible sulphide liquids in calc-alkaline basalts and their occurrence is relevant to the processes of formation of porphyry Cu-Au deposits in arc settings.

CONDITIONS OF FORMATION The basalt contains 23% augite, 3% plagioclase, 2% magnetite and 0.1% sulphide globules in a vesicular glassy groundmass. The whole rock composition is: Si02 49%, Ti02 0.7%, AI2O3 14.5%, Fe 2 03 4.8%, FeO 4.6%, MgO 5.0%, CaO 9.0%, Na20 3.7%, K 2 0 3.3% and 87 ppm Cu. The liquidus temperature and oxygen fugacity (/O2) of the basalt were calculated to be 1160°C and 10' 4 - 8 bars (ANNO = +3.0 relative to nickel-nickel oxide buffer curve (NNO)). Under these highly oxidizing conditions, at least 95% of the dissolved sulfur is present as sulfate, and the remainder as sulphide. Degassing of S 0 2 during eruption lowered the JO2 of the silicate melt to ANNO = +0.8, where it became saturated in sulphide and exsolved an immiscible sulphide liquid. The quenched matrix glass contains 750 ppm S, therefore, prior to degassing the magma must have contained approximately 4000 ppm S. METAL ABUNDANCES AND K D ' S There are no experimental determinations of sulphide-silicate partition coefficients (KQ) for high fO2 calc-alkaline compositions. The abundance of Cu in the sulphide globules and the KQ CU a r e high compared to those of MORB (60000-130000 ppm and 1400, respectively). The sulphide Pd content is also anomalously high compared to MORB sulphides (15-30 ppm). The solubility of Pd in silicate melts is increased at high fC>2 which may account for its elevated concentration in the Tanga basalt at the time of sulphide saturation. The Pd content of the silicate glass is less than the PIXE detection limit (llppm) and, therefore, the Kj-) p^ reported here is four orders of magnitude too low. Likewise, the Kd for Ni, Ag, Te and Se should be considered absolute minimums.

Table 1. Composition of quenched sulphide and silicate liquid in Tanga shoshonitic basalt.

Zn Ni Pd Te S Fe Cu Pb Ag Se 239 1400 1577 60 319000 220000 432100 200 93 Sulphide 155 4 4 4 4 4 4 6 4 4 n 4 750 56 <27 <11 31000 23.5 16 <13 Silicate <50 <3.5 7.2 18400 4.3 88 >58 >5 >15 >1.9 >44 ^Dsul-sil ±0.4 ±1.6 ±19 ±490 Notes: S was determined by electron microprobe and all other elements by!PIXE. Al 1 analyses reported in ppm.

Italicized numbers are the limit of the detection. Values for Mo, Ru and R1l were below the detection limit of 10 ppm, and Au, Pt and Ir were below the detection limit of 50 ppm. We appi eciate the assistance of Chris Ryan and Tin Tin Win with the PEXE and Paul Carpenter with the electron microprobe analyses.

IMPLICATIONS FOR ORE GENESIS PROCESSES AT CONVERGENT MARGINS The data presented here demonstrate that oxidized, shoshonitic melts of the Tabar-Lihir-Tanga-Feni arc transport significantly more Cu, Pd and S from the mantle to the crust than magmas emplaced at mid-ocean ridges. The Tanga basalts did not reach sulphide saturation until at least 85% degassing of SO2 and, therefore, Cu-depletion will not occur until late in the eruptive history of similiar calc-alkaline magmas. The degassing history of the arc magmas is, therefore, a critical process in ore genesis. Rapid degassing during a volcanic eruption, will lead to the formation of immiscible sulphide liquids which will extract Cu, Pd and probably Au, leaving a metal-poor residual magma. If degassing is relatively slow, Cu, Pd and Au will not be fractionated from the magma, and will be available for incorporation in a porphyry-type ore deposit. Partial sector collapse of the Luise volcanic caldera is a possible method of degassing a magma chamber relatively slowly, and may explain the richness of the Ladolam Au-Cu deposit on Lihir island.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LEICHHARDTS LEGACY: A HISTORY OF EARLY COAL EXPLORATION IN THE GOSFORD - SWANSEA AREA OF NSW GregMcNilfr UNSWDept ofApplied Geology, Sydney 2052 NSW The 19th century history of coal exploration in the Gosford - Swansea area, then known as the Brisbane Water District, is a record of efforts wasted because of a simple geological misconception: that the conglomerates of the Newcastle Coal Measures were lateral equivalents of the Hawkesbury Sandstone.

This misconception was first given currency in the published letters and journals of Ludwig Leichhardt, though it probably predated his arrival in the colony. In November 1842, after spending some weeks visiting cliff sections in the Newcastle area, he walkedfromthere to East Gosford via the western side of Lake Macquarie, returning along the coast (where he passed close to the outcrop of the Great Northern Seam at Catherine Hill Bay, but failed to see it). On his outward journey he called in at the Reverend Lancelot Threlkeld's mine at Ebenezer (now within the Threlkeld Reserve at Coal Point, southwest of Toronto) and later followed an exposed coal seam - either the Great Northern or Fassifem - for some distance south along the shore of Lake Macquarie. This, and the perceived similarity between the massive sandstone phases of the Teralba Conglomerate overlying the seam with the Hawkesbury Sandstone, convinced him that the same coalbeds might be encountered at relatively shallow depth between Toronto and Sydney.

The Ebenezer mine closed in 1850, after only ten years in operation, because of the cost of transshipping coal across a rock bar at Swansea Heads. The latter spot is also known as Reid's Mistake, in memory of Certain Reid, who landed here in 1800, collected a cargo of coal from the Pilot Seam and returned to Sydney, thinking that he had visited the mouth of the Hunter River! Several other mines operated briefly in the 1860s from seam outcrops around the shores of Lake Macquarie, including one at Point Morisset (southwest of Swansea). All failed because of the same cost disadvantage, relative to the Newcastle mines, that had defeated Threlkeld Shaft sinking for coal at Tascott, southwest of Gosford, was reported in 1857 and it is likely that other attempts were made to reach coal seams in the Brisbane Water District around this time. However, the earliest recorded coal drilling in the Brisbane Water District was the Mangrove Creek Bore (147m deep, 1878). This was soon followed by Alison's Mullet Creek Bore (408m) in 1879. Both holes were located on the shore of Broken Bay, close to sealevel, to take advantage of sea transport should a mine eventuate. Neither came close to the coal measures, the uppermost seam in the area being at a depth of 700m This was established during the drilling of Windeyert Hawkesbury River Bore (1908-10) near the Mullet Creek Bore site, neatly confirming an 1880 prediction by the then Examiner of Coalfields, Mr J Mackenzie, that coal would be struck at depths between 600 and 800m.

The construction of the Hawkesbury River to Newcastle railway in the 1880s simultaneously provided both a potential market for coal and the first practical means of transporting it from inland mines. The First drillholes to actually intersect coal seams, Alison's Nol (Wallarah Creek) and No2 (Wyong) Bores were both put down in 1882, close to the newly-built railway. Other historic drillholes in the Brisbane Water District include the Wyee Bore (1890, depth 289 m), Bungaree Norah Bore (1907, 348 m) and the Lake MunmorahNos 1 and 2 Bores (1908-9, 122 m and 121 m). Ironically enough, none of these bores resulted in a mine being built, although in 1914 Mr AH Warner offered the underground mining rights to 1500 ha close to the Wyong Bore site, on condition that the Sydney City Council erect a power station. The offer was declined (for reasons unknown) and colliery development in the district is therefore largely a postwar development Furthermore, the only significant prewar mine - Wallarah Colliery (1889) - owed nothing to the railways, but depended instead on sea transport This began in 1873 as the New Wallsend Mine, the workings within the Great Northern Seam being driven inwards from the seam outcrop below the present wharf at Catherine Hill Bay. This mine operated for only three years. The Wallarah Coal Company, which succeeded the New Wallsend Company, successively operated Five collieries (named the A to E pits) in the Middle Camp and Mine Camp areas north of Catherine Hill Bay up to 1963. Their present-day successors are Wallarah and Moonee Collieries, the latter due to be reequipped as a longwall mine. Future development of longwall mines in the Wyong-Yarramalong area is also likely during the next decade.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

yHpmk'k? EVIDENCE OF GROUNDWATER ORIGIN FOR SILCRETES OF THE STUART RANGE, SOUTH AUSTRALIA GonegMcNtfly and Ian Wilson UNSW Dept of Applied Geology, Sydney 2052 NSW Roads and Traffic Authority, Paikes 2870NSW 1

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The Stuart Range is a low north-south escarpment which runs through Coober Pedy SA, separating a duricrust-capped plateau (the Cordillo Surface) in the west from the lowlands of the Lake Eyre drainage basin. The Arckaringa Hills are outliers of this plateau, located about 15km to the east and separated from it by the valley of Arckaringa Creekf. The observations reported here were made during fieldworic in the Evelyn Downs - Arckaringa - Wintinna area, lOO-lSOkm north of Coober Pedy, during 1994-95. The work concentrated on an area to the north and northwest of Arckaringa homestead The dissected breakaways of the Stuart Range and the nearby Arckaringa Hills expose a sequence of strongly altered (kaolinised, silicified and ferruginised) Early Cretaceous rocks along the western margins of the Eromanga Basin. These include - in ascending order - the Bulldog Shale, Cooriakina Sandstone and Oodnadatta Formation This gently eastward-dipping sequence is about 150m thick and acts as a semi-confining layer for the underlying Algebuckina Sandstone and Cadna Owie Formation, which together constitute the main aquifer system in this part of the basin, It is truncated by the nearhorizontal Cordillo Surface, of Early Tertiary age, which is predominantly porcellanous and ferruginous, but includes patchy silcrete. A sinuous double line of mesas, the Mirackina Paleochannel (MPC) lies consistently about 20m below and to the east of the Stuart Range breakaways. The silcretes capping the MPC are more pervasive and younger than those of the Stuart Range, possibly Late Miocene.

During mapping of the MPC, a number of features were observed which indicate a groundwater origin for both the Stuart Range porcellanites and the MPC silcretes. Siliceous vein fillings were seen to connect with horizontal porcellanite bands and in places these layers have been exposed by erosion to form benches in the land surface. The most conspicuous such feature marks the Cooriakina Sandstone / Oodnadatta Formation interface along the flanks of the Arckaringa Hills, suggesting that these bands develop along permeability barriers. Stone piles l-2m across and 0.5m high are very numerous across this surface and resemble modern small mound springs at Lake Eyre South. These stone piles occur in lines up to 300m long and especially at the intersections of vein-filled joints. A few of the larger mounds are several metres across and crater-like, suggesting that the sinter carapace has colkpsed following cessation of groundwater flow. The silcrete wedges capping the MPC mesas are also strongly indicative of groundwater discharge fans located along the margins of a drying braided stream channel (McNally and Wilson, 1996). Sporadic occurrences of grey billy and terazzo silcrete on the Cordillo Surface may be due to this younger silicification.

The presence of supposed fossil mound springs, feeder veins to porcellanite layers along bedding planes and other groundwater discharge features suggests that artesian flow was as prevalent along the margins of the Eromanga Basin during the Tertiary as it is today. However, the piezometric surface which accompanied this discharge must have been about 100m higher than the present sub-artesian groundwater level. The distribution of duricrust deposits resulting from this groundwater movement has been greatly influenced by joint orientation and by the presence of permeability barriers (shale aquicludes) within the Early Cretaceous sequence.

REFERENCE Barnes, LC. & Pitt, G.M., 1976. The'Mirackina Conglomerate. Quarterly Geological Notes JP, SA Geological Survey.

McNally, G.H. & Wilson, LR, 1996. Silcretes of the Mirackina Palaeochannel, Arckaringa SA. AQSO Journal of Australian Geology and Geophysics (in press).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EARLY GEOLOGICAL EXPLORATION OF THE MONARO 1

K G. McQueen1 and G.M. Taylor1 University of Canberra, PO Box 1 Belconnen, ACT 2616

The Monaro is a tableland area of southern New South Wales between the Snowy Mountains and the Great Escarpment. Its southern boundary is the Victorian border and it extends north to around Bredbo-Adaminaby.

Since first explored and settled by Europeans the region has been a focus of geological and geomorphological interest. The name Monaro derives from an Aboriginal word of obscure meaning, although it possibly means "women's breasts" in reference to the small lumpy hills on the Monaro Volcanic Province. There is uncertain evidence of Aboriginal occupation as far back as 25,000 years ago; the most positive date comes from a burial site near Cooma and is about 8,000 years BP. Little is known of the Aboriginal interpretation of the area.

Captain Mark John Currie, Brigade-Major John Ovens and Joseph Wild were the first Europeans to explore the Monaro. Between the 1st and 3rd of June 1823 they rode south from Limestone Plains (now Canberra) and on June 4th exited fine forested lands with rocky outcrop and lofty ranges onto "downy" country. From some aborigines they learned that the clear country was called "Manaroo" and was very extensive. The party rode for two more days and reached the Numeralla River. Squatters moved to the area in the late 1820's and by 1840 the Monaro was formally established as a "Squattage District".

Dr John Lhotsky, a Polish born naturalist, travelled through the Monaro and explored the eastern side of the Australian Alps in 1834. As well as investigating the natural features of the region Lhotsky hoped to find valuable minerals. He noted numerous rock and mineral occurrences and was the first to recognise and ascribe an old age (Primary or Primitive) to the schists and gneisses around Cooma. He also described in detail the mineral spring at Rock Rat Creek in the basalt terrain south of Cooma.

Paul Edmond de Strzelecki skirted the northwest margin of the Monaro in early 1840 on his famous expedition to the Australian Alps. He made numerous geological observations and recorded argillites, siliceous slates and "sienite" belonging to the First Epoch of his geological subdivision, in the area west of the Murrumbidgee River. He also recognised signs of deformation and uplift of the older rocks around the alps. Some of his ideas and interpretations may have been influenced by the earlier work of Lhotsky. Strzelecki described the view of the southern "Moneiro" from the top of Mt Kosciusko but did not visit this region.

The Reverend W.B. Clarke made the first comprehensive survey of the geology of the Monaro over the summer of 1851-52 during his "researches in the southern goldfields". Clarke described the open basalt plains and recognised that the 'trap' had infilled an old deeply dissected landscape. He also recorded two distinct granite types with a marked distribution. Other discoveries included coal within the basalt, fossil wood beneath basalt and gently dipping conglomerates, sandstones and shales which he considered Carboniferous. He made the first observations of silcretes in the area and found "precious opal". Clarke's main purpose was to locate payable gold deposits and he found numerous auriferous indications over much of the Monaro. On January 3rd 1852 he reported the discovery of gold near "Quedong" and in February confirmed the occurrence of gold at this site and described fossiliferous limestones which he assigned a Silurian age. He also discovered signs of copper and lead mineralisation at Quidong, copper near Boco and went on to find gold on the Bendoc River and Maharatta Creek. Clarke's observations led to alluvial gold mining in the Bendoc-Delegate area and attempts in the 1860's to mine copper and lead ores at Quidong. His prediction that the Muniong (Snowy) Range would yield commercial quantities of gold was fulfilled in 1859 with discovery of the Kiandra deep leads. Signs of gold at other localities led to later reef mining, e.g. in the Cowra Creek goldfield east of Bredbo.

In 1896 the New South Wales Border Prospecting Party led by J.E. Carne examined areas in the southern Monaro and along the N.S.W.-Victorian border. Carne described the physical features of the area, the soils and the early Palaeozoic rocks, including fossils from the "Quidong beds". He commented on folding and cleavage development in the Ordovician metasedimentary rocks (grouped as Early Silurian at this time) noted their contact metamorphism around granite and speculated on the nature of the boundary between the Early and Late Silurian sequences. He ascribed a Late Devonian age to red beds previously suggested as Carboniferous by Clarke, and briefly described the intrusive igneous rocks. Carne also observed "drift pebbles under the basalt sheet near Bombala" apparently the first recognition of early Tertiary sediments.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 GOLD MINERALISATION AT MOUNT CHARLOTTE: EVIDENCE FOR FLUID OXIDATION FROM FLUID INCLUSIONS 1 I

Terrence P. Memagh Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

The Mount Charlotte gold mine is located in the Golden Mile district of the Norseman-Wiluna Greenstone Belt of Western Australia and is situated within the western limb of the Kalgoorlie syncline.Veining and mineralisation is largely restricted to Unit 8 of the Golden Mile Dolerite and forms pipe-like to irregularly shaped orebodies situated predominantly within the footwalls of the major oblique faults. Vein minerals include coarsely-crystalline to massive quartz, with minor euhedral to anhedral scheelite, ankerile and sulphide grains, and late calcite and chlorite. The host rocks adjacent to veins are commonly overgrown by early selvages containing subhedral albite, ankerite, pyrite ± pyrrhotite, scheelite and/or quartz.

Previous studies (cf. Clout et al., 1990) have identified three types of alteration haloes surrounding the mineralised veins based on differences in the sequence of mineralogical changes occurring with increasing distance from the veins. Type 3 alteration dominates in the core of the stockwork and at deeper mine levels.

Type 2 alteration haloes occur peripheral to Type 3 and dominate at intermediate mine levels. Type 1 alteration haloes occur along the periphery of the orebodies and dominate at the highest levels of the mine. This study uses fluid inclusions to determine the temperature and fluid chemistry associated with each type of alteration.

"Texturally early" fluid inclusions associated with Type 1 alteration consist mostly of 3 phase aqueous inclusions containing liquid + vapour C0 . These inclusions have salinities of less than 4 wt. % NaCl equivalent and typically homogenise between 200 and 320 °C. Both the microtliermometry and the laser Raman microprobe analyses confirmed that these inclusions contained only water and C0 and no CH was detected.

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The highest temperature inclusions associated with Type 2 alteration consist mainly of liquid- and vapour-rich, two phase aqueous inclusions. Most C0 , vapour-rich inclusions decrepitated before homogenisation but liquidrich inclusions homogenised between 120 and 280 °C. Microthermometry showed depressed C0 final melting temperatures and Raman microprobe analyses revealed that the vapour phase of these inclusions contained up to 50 mol. % CH but the ratio varied considerably from one inclusion to another.

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Type 3 alteration contains two distinct types of fluid inclusions. The most abundant population consists of coexisting liquid- and vapour-rich, two phase aqueous inclusions. These inclusions contained C0 and CH„ in similar quantities to those observed in Type 2 alteration. Nearly all vapour-rich and some liquid-rich inclusions decrepitated before homogenisation with the remaining liquid-rich inclusions homogenising al temperatures up to 440°C. The second fluid inclusion population consists of aqueous inclusions containing approx. 10 vol. % vapour and homogenised at slightly lower temperatures. Raman microprobe analyses of these inclusions showed that the vapour phase contained only CH .

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These observations show that Type 3 alteration is associated with the highest temperature fluids and also the most CH -rich inclusions. Type 2 alteration formed at slightly lower temperatures but also trapped immiscible fluids containing intermediate mixtures of C0 and CH4. Type 1 alteration occurs at the lowest temperatures and appears to be associated with C0 -rich fluids. These lateral and vertical zonation patterns indicate that mineralisation was accompanied by cooling and gradual oxidation of the ore-bearing fluid as it reacted with the host rocks. Gold precipitation at Mount Charlotte most likely occurred by a combination of desulphidation during phase separation and oxidation of an initially moderately reduced fluid.

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REFERENCES Clout, J.M.F., Cleghorn, J.H., & Eaton, P.C., 1990. Geology of the Kalgoorlie gold field, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed. Hughes, F.E.), The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 411-431.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 IGCP LECTURE GONDWANA DISPERSION AND ASIAN ACCRETION Ian Metcalfe Department of Geology & Geophysics, University of New England, ARMIDALE NSW 2351

IGCP Project 321, Gondwana dispersion and Asian accretion (1991-95), is led by Ren Jishun (China), with CoLeaders Shigeki Hada (Japan), Jacques Charvet (France) and Ian Metcalfe (Australia). IGCP 321 is a successor project to the successful IGCP Project 224, Pre-Jurassic geologic evolution of eastern continental margin of Asia. IGCP 321 has been principally aimed at studying the break-up of Gondwana and the rift - drift amalgamation/accretion history of the various ex-Gondwana continental blocks and fragments that now constitute East and Southeast Asia. Particular emphasis has been given to identifying the specific origins of the various Asian terrenes, the timings of their rifting and separation from Gondwana and the ages of their suturing (amalgamation and accretion) to each other and to Asia. By its nature, the project is a multi disciplinary one which has involved integrated studies including palaeontology, tectonics, stratigraphy & sedimentology, geochronology, geophysics (especially palaeomagnetism), magmatism, metamorphism, and geochemistry. The project has enjoyed the active involvement of hundreds of geoscientists from sixteen countries and has led to important international collaboration. Active Australian involvement has been substantial with a current Australian Working Group of 25. It was also an Australian, Clive Burrett (Tasmania), who was instrumental in developing the original proposal for IGCP 321. Five international symposia and field excursions/workshops have been held in Yunnan Province, China (1991), Japan (1992), Malaysia (1993), Korea (1994) and Vietnam (1995). Australia has provided major contributions to IGCP 321 and in particular through palaeobiogeography, radiolarian biostratigraphy, palaeomagnetism, terrane analysis and tectonics.

Comparative studies of the tectono-stratigraphy, palaeontology, and palaeomagnetism of the various continental terranes of East and Southeast Asia suggests that they were all derived directly or indirectly from Gondwana.

Asian continental terranes that are placed on the India-Australian margin of Gondwana in the Early Palaeozoic are Tarim (here regarded to include the Kunlun and Ala Shan terranes), Qaidam, Indochina (which includes East Malaya and the Qamdo-Simao block of western China), North China, South China (amalgamated Yangtze and SE South China blocks), Sibumasu, Qiangtang, Lhasa, Kurosegawa, NW and SE Hainan, West Burma and the Woyla terranes.

These terranes were successively rifted from the margin of this "Greater Gondwana" during the Palaeozoic and Mesozoic, prior to the final break-up of Gondwana. Three episodes of continental rifting and terrane separation occurred on the northern margin of Gondwana, in the Devonian (North China, South China, Indochina, Qaidam, Tarim and NW and SE Hainan terranes); Early-Middle Permian (The Cimmerian continent including the Sibumasu and Qiangtang terranes); and Late Triassic to Late Jurassic (Lhasa, West Burma and Woyla terranes).

The northwards drift of these terranes was accompanied by the opening and closing of three successive oceans, the Palaeo-Tethys, Meso-Tethys and Ceno-Tethys. The timings of rifting and separation of these terranes corresponds to major structural events, basin formation and volcanism in NW Australia and on the northern Gondwana margin in general. Radiolarian biostratigraphic and geochemical work on ribbon-bedded cherts together with studies of melange indicate that the Lancangjiang, Changning-Menglian, Nan-Uttaradit and RaubBentong sutures represent the main branch of the Palaeo-Tethys in East Asia which opened in the Mid-Late Devonian and closed in the Permo-Triassic.

Amalgamation and accretion of the various Asian terranes occurred progressively between the Late Devonian and the Cretaceous, beginning with the intra-Tethyan amalgamation of South China and Indochina (to form Cathaysialand) in the Late Devonian-Early Carboniferous which was then followed by the accretion of the Tarim, Kunlun, Qaidam and Ala Shan terranes to Kazakhstan/Siberia in the Permian. Suturing of Sibumasu and Qiangtang to Cathaysialand and amalgamation of this super-terrane with North China occurred in the Permian Triassic, and accretion to Laurasia was completed by Late Triassic-Early Jurassic times. The highly disrupted Kurosegawa terrane of Japan, possibly derived from Australian Gondwana, accreted to Japanese Eurasia in the Late Jurassic. The Lhasa, West Burma and Woyla terranes, which rifted from NW Australian Gondwana in the Late Triassic to Late Jurassic were accreted to proto-Southeast Asia in the Cretaceous. The South West Borneo and Semitau terranes were derived from the South China/Indochina margin by the opening of a marginal basin in the Cretaceous which was subsequendy destroyed by southwards subduction during theriftingof the Reed BankDangerous Grounds terrane from South China when the South China Sea opened.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MICROTREMOR SURVEY AND MICROZONATION OF LAUNCESTON, AUSTRALIA Marion Michael-Leiba ^ & Vagn Jensen^ Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^ Geology Department, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001 1

During the period 1884-1994, the city of Launceston in northern Tasmania has been damaged by five earthquakes with epicentres up to 200 km away in the Tasman Sea off the northeastern tip of Tasmania. As this damage was thought to be due to amplified site response (Michael-Leiba and Gaull, 1989; Michael-Leiba and Jensen, 1993), the Launceston City Council requested AGSO to prepare a zoning map of Launceston with zones related to the requirements of Australian Standard AS 1170.4-1993 (Standards Australia, 1993). To accomplish this, AGSO carried out a microtremor survey of Launceston. Recordings were made at 53 sites on sediment and three on dolerite, and analysed using the Nakamura (1989) technique. A detailed gravity survey (Leaman, 1994) revealed a complex geology involving at least two deep NNW-SSE trending valleys filled with variably consolidated sediments.

Ratios of the spectra of the horizontal and vertical components of microtremors suggest amplified responses of Tertiary and Quaternary sediments at 46 of the 53 sediment sites at periods ranging from 0.1 more than 1 second. The three dolerite sites showed no site resonance. Sediments in the deep valleys showed amplification at periods around 0.7 to greater than 1 second, which would be expected to most affect medium and high rise buildings. Ceilings of two churches in this zone have been damaged by earthquakes. Natural periods of 0.1-0.5 seconds were measured on Quaternary and Tertiary sediments overlying shallow dolerite basement in the eastern part of the Launceston Central Business District. The buildings which would be most affected by site resonance in this area would be low or mediumrisebuildings. There have been several reported cases of damage to these sorts of structures in this area in earthquakes.

At five out of six sites south of the Tamar River, where microtremor measurements were near to Leaman's (1994) modelled cross sections, measured natural periods overlapped the theoretical range of periods calculated assuming a uniform elastic layer on arigidbase. At the sixth site, the lowest measured period was 13% longer than the upper limit calculated. This could be because the sediment thickness is greater than and/or the S wave velocity lower than expected, or the geology deviated too greatly from the simple model. Also, on the 5-10 metre thick sediments of theriverfloodplain in the old railway yard, anomalously long variable natural periods of 0.3 or 0.4 to more than 1 second were measured, relative to the 0.1-0.2 second periods calculated. This may be due to the non-uniform nature of the sediments, which in some parts are mixed with wood and/or water saturated. Any building development should take this into account.

Based on the microtremor measurements, the soils map (S. Forsyth, 1995) and the gravity results (Leaman, 1994), we have prepared a 1:10 000 scale zoning map of Launceston. This shows the site factor in AS 1170.41993 to be used for low, medium or high rise buildings in various parts of the city. The zones are consistent with the microtremor measurements and with most reports of damage in earthquakes. However, they are generalised and do not take into account very small scale variations in geology, or topographic or seismic wave focussing effects. The zoning maps should provide a reasonable regional indication of site response in another earthquake off northeastern Tasmania (Launceston has been damaged by earthquakes there in 1884, 1885,1892, 1929 and 1946) or in a western Tasmanian earthquake. We recommend that site studies be carried out prior to the erection of a new structure on a site where no specific information is available.

REFERENCES Forsyth, S., 1995. Launceston geodata project. Industry, Safety and Mines Division, Tasmania Development and Resources.

Leaman, D.E., 1994. Assessment of gravity survey, city of Launceston, for Launceston City Corporation seismic zonation study. Unpublished.

Michael-Leiba, M.O. and Gaull, B.A., 1989. Probabilistic earthquake risk maps of Tasmania BMR Journal of Australian Geology & Geophysics 11,81-87.

Michael-Leiba, M. and Jensen, V., 1993. The West Tasman Sea (Flinders Island) earthquake of 14 September 1946. BMR Journal ofAustralian Geology & Geophysics 13, 369-372.

Standards Australia, 1995. Minimum design loads on structures. Part 4: Earthquake loads. Australian Standard AS 1170.4-1993.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

RAISING COMMUNITY AWARENESS OF THE GEOSCIENCES John Mignone Department of Mines and Energy, South Australia

During the latter part of 1992 The South Australian Exploration Initiative was implemented. Funded by the state government, it was designed to stimulated economic interest in the South Australian mineral and energy scene.

By providing exploration companies with new, high quality data in digital and hard copy image form, using state of the art airborne geophysical sensing the project was set in motion.

With a large part of the state about to see exploration aircraft flying a mere 80 metres above the ground, it was decided to establish an Education and Community Consultation Service. The aim was to facilitate access to geoscientific knowledge in an entertaining and easily understood form, for community members ranging from Reception age students through to business sector adults and retired social club members and interest groups.

At the heart of the issue was the de-jargonising and demystification of the geoscience and processes of the project.

In practice the service has been welcomed and thus very successfully completing its 3rd year of operation.

Facets of the Education and Community Liaison Service include: School Visits: To give talks with demonstrations, using small scale interactive models and hand held geophysical sensing equipment illustrating the processes and concepts of modern exploration.

This is offered around the state.

The schools are given teaching resource materials and information on exploration and a range of other topics.

The opportunity for subsequent visits is extended, which can be for Talks, Practical workshops or guided Excursions.

Similar talks and workshops are held at the Gifted and Talented Children's Association.

Community Groups: Liaison and presentations are done for various social and professional groups requiring information on issues of relevance to their situation.

Career Information: Booths with information at Career and Skills Expos.

CSIRO Student Research Projects: Where students carry out a 20 hour research project with a MESA mentor.

Work Experience: Senior high school students are hosted.

Curriculum Resource: Designing and writing resources for ready use in the classroom, including paper masters, video, and CD rom forms.

Liaison: Between various industry and geoscientific institutions.

ASEG: Annual Conference and Exhibition Schools Session.

295

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE CONTEMPORARY STRESS FIELD OF THE AUSTRALIAN NORTH WEST SHELF: L IMPLICATIONS FOR ACTIVE FAULTING Scott D. Mildren and Richard R. Hillis department of Geology and Geophysics, University of Adelaide, SA 5005 1

1

The Australian North West Continental Shelf is located immediately south of the convergent north-eastern boundary of the Indo Australian Plate. The Carnarvon Basin at the Western end of the North West Shelf, lies south of the zone of subduction of oceanic Indo-Australian Plate beneath the Sunda Arc. The Bonaparte Basin, at the eastern end of the North West Shelf, lies south of the zone of collision between the Australian continent and the Banda Arc. 625 borehole breakouts covering a cumulative length of 7.7 km in 42 wells in the Carnarvon Basin indicate a maximum horizontal stress (Ghmx) orientation of 090°N-100°N. 616 borehole breakouts over 6.8 km in 46 wells in the Bonaparte Basin indicate a Oh^ orientation of 055°N-060°N. Together with "extant data from the World Stress Map, these results indicate that the first-order Ohmax orientation of the northern Australian margin is consistently oriented approximately 050°N-060°N over 2000 km from New Guinea westwards through the Bonaparte Basin to the onshore Canning Basin (between the Bonaparte and Carnarvon Basins). Westwards from the onshore Canning Basin, Ohmax orientation swings approximately 40° to 090°N-100°N in the Carnarvon Basin, over 700 km.

The 050°-060°N stress orientation of much of the northern Australian margin is probably controlled by collision in the New Guinea orogen which is approximately orthogonal to the Ohmax direction. The rotation of the horizontal stress orientation along the North West Shelf is due to the changes in force generated along the heterogeneous convergent margin of the Indo Australian Plate to the north of the North West Shelf.

Shear stress is maximised on conjugate planes at approximately 45° to the maximum compressive stress.

Therefore, the maximum horizontal shear component is oriented 050°N and 140°N in the Carnarvon Basin and approximately 010°N and 110°N in the Bonaparte Basin. Active faults striking in these directions are most susceptible to reactivation with a component of strike-slip motion.

In order to fully constrain the nature of contemporary fault movement, stress magnitude data are also required: • normal extensional faulting occurs where a > Ohmax > Ohmin; • strike-slip faulting occurs where Ohmax > o > Ohm^; and • reverse, compressional faulting occurs where Ohmax > Ohmin > o . v

v

v

Provided that the vertical stress (o ) is lithostatic, its magnitude at depth, z, can be determined by integrating density log data from hydrocarbon exploration wells according to the relationship: v

<T = \ p{z)gdz., V

Z Q

where p(z) is the density function of a rock at depth z and g is the acceleration due to gravity. Horizontal stress magnitudes can be determined by monitoring the mud weight required to induce a hydraulic fracture in the wellbore wall. Stress magnitudes calculated thus indicate that, although there is local variation, the fault condition in both the Carnarvon and Bonaparte Basins is approximately on the boundary between extension and strike-slip (ie. o = o ^ > Ohmin). v

Hillis and Mildren (this volume) discuss the implications of the stress field for hydrocarbon exploration and production related issues.

Acknowledgments: SDM is funded by an AGSO postgraduate research scholarship. The work has been undertaken as part of an Australian Petroleum Cooperative Research Centre project on the stress field of the North West Shelf.

296

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LOW 5 1 8 0 SIGNATURES IN GRANULITES FROM THE HARTS RANGE, CENTRAL AUSTRALIA: EVIDENCE FOR EARLY METEORIC FLUID FLOW. Jodie Miller. Ian Cartwright Monash University, Wellington Rd, Clayton, 3150

Fluids play an integral role in metamorphism, being important agents for heat and chemical transfer, for partial melting, and for controlling the stability of mineral assemblages. Hence, determining the timing and mechanism of fluid flow is essential to our understanding of the evolution of metamorphic terrains. Stable isotopes are useful tools for documenting fluid-rock histories as they can delineate patterns of fluid flow and constrain fluid sources.

In the Mallee Bore area, eastern Arunta Inlier, a zone of anomalously low S 1 8 0 values provides evidence for contact metamorphism that occurred early in the metamorphic evolution of the region, resulting in isotopic exchange of the presursor rocks with surficial waters. Subsequent granulite facies metamorphism has overprinted all textural evidence for this early metamorphic event.

The Mallee Bore area is located on the northern margin of the Harts Range, approximately 30 km from the Entia Dome, and is dominated by garnetiferous mafic orthogneisses and pelitic paragneisses of likely Proterozoic age.

Three episodes of metamorphic activity have been identified on the basis of textural and mineralogical evidence.

Combining geothermometry and geobarometry estimates with data from recent melting experiments indicates that peak metamorphism occurred at around 820-850 °C and 8-11 kbar. Large sillimanite-grade shear zones developed subsequent to regional metamorphism under conditions of -700-730 °C and -4.4 - 6.4 kbar. Garnet porphyroblasts within the shear zones were locally retrogressed after the cessation of shearing, probably when the rocks had cooled sufficiently for fluids to start exsolving from crystallizing partial melts (ca. 600-650 °C).

Other granulite terranes record retrogression at similar temperatures, suggesting that retrogression as a result of the influx of fluids derived from the crystallization of partial melts may be a common phenomenon in partiallymelted high-grade rocks.

Within the Mallee Bore area a zone of ^O-depletion on a scale of several kilometers has been identified.

Outside the zone of depletion, paragneisses and orthogneisses have S ^ O values in the range 10 to 12%c, 5.5 to 9.0%c, respectively. These values are within the normal ranges for these rock types. Within the zone of depletion, the 8 ^ 0 values of the orthogneisses and paragneisses are 3±0.5%o and 4±1.5%o, indicating a depletion of up to 10%c compared with their unaltered equivalents. The pattern of depletion is not compositionally or structurally controlled and appears to be centred around an outcrop of orthoamphibole gneisses. These low 8 ^ 0 values cannot have been achieved via metamorphic devolatilization or partial melting and most likley reflect isotopic exchange with infiltrating fluids.

The absence of significant post-peak metamorphic retrogression within the granulite facies lithologies suggests that S 1 8 0 values were reset prior to, or during, granulite-facies metamorphism. Estimates of the S 1 8 0 of the infiltrating fluid suggests that it is considerably lower than the S ^ O of those fluids typically associated with crustal or mantle lithologies. On this basis the most likley source for this low ^ O fluid is meteoric or oceanic waters.Since the anomalously low 8 ^ 0 values indicate interaction with surficial waters, it is unlikely that the alteration of oxygen isotopes took place during granulite facies metamorphism, which is thought to have occurred at depths of around 30kms. This is primarily because it would be difficult for surficial fluids that would initially have been under hydrostatic pressure, to permeate down to this level in the crust where fluids are under lithostatic pressure. Instead, resetting of oxygen isotope ratios probably occurred early in the metamorphic history of the area, while the rocks where still at shallow crustal levels. This type of shallow level isotopic alteration may have been induced via hydrothermal cells generated by the intrusion of early granites. Such a phenomenon has been previously noted around other high level intrusives throughout the world. Contact metamorphism may have occurred between 1820-1770Ma, which was a period of widespread granite emplacement in the Arunta Block. Meteoric fluid flow around granites of this age has been documented elsewhere in the Arunta Inlier. The preservation of oxygen isotope signatures related to contact metamorphism through subsequent regional metamorphism suggests that regional metamorphism in the Harts Range involved relatively little fluid-rock interaction. This is similar to the Reynolds Range (northern Arunta Inlier) where regional metamorphism again involved little fluid flow.

As many metamorphic terrains have undergone multiple epsiodes of metamorphism, investigation of fluid-rock interaction using stable isotopes can aid in differentiating between such events, and prevent textural, mineralogical and isotopic features being incorrectly ascribed to a single metamorphic episode. The documentation of early metamorphic events in complex multiply metamorphosed terrains is therefore essential for the correct interpretation of P-T paths and consequently for unravelling the tectonic and metamorphic histories of polyphase terrains.

297

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

FOSSIL SUBDUCTION IN T H E EASTERN LACHLAN FOLD BELT: SIGNIFICANCE AND TECTONIC IMPLICATIONS J.M. Miller & D.R. Gray VIEPS Department of Earth Sciences, Monash University, Melbourne, Vic, 3168, Australia.

A Palaeozoic subduction zone within the Lachlan Fold Belt of eastern Australia is preserved as an imbricated stack of turbidite, chert, mafic volcanics and chaotic block-in-matrix melange along the south coast of N.S.W. Like other subduction accretionary complexes (e.g. Kodiak Complex, Aleutians and Shimanto Complex, Japan) biostratigraphic data (Bischoff and Prendergast, 1987; Stewart & Glen, 1992) require a complex imbricated sequence. Evidence for subduction relates to the presence of "classic" chaotic block-in-matrix melange, broken formation along high strain zones associated with large scale imbrication and underplating, scaly cleavages, initial "soft sediment" deformation, and structural complexity associated with a marked deformation front.

Subduction accretion or underplating in the Narooma accretionary complex shows a strain dependent transition from an 'inland' belt of chevron-folding cut by reverse faults, to a 'coastal' belt with an early bedding-parallel fabric, isoclinal-recumbent folding, poly-deformation and 'stripy' cleavage (differentiated layering). Within the coastal structural belt a "stacking order" is clearly established at Narooma and Batemans Bay. Uppermost in the imbricate stack is an Early Ordovician to Late Ordovician turbidite sequence. At Murruna Point and Narooma this turbidite is separated from the underlying chert and volcanic sequence by a high strain zone containing broken formation. Pressure solution, dilational veining and boudinage is extensive within the high strain zones.

At Narooma there is an age inversion across this tectonic contact - Early Ordovician turbidite overlying Late Cambrian to Late Ordovician chert. The slaty cleavage at a low angle to bedding within the coastal belt turbidite is a direct result of this imbrication. At Batemans Bay the contact between the two units is conformable and Late Cambrian to Early Ordovician in age (Etheridge et ai, 1973; Bischoff and Prendergast, 1987). The subduction complex is comprised of sediment that was in situ prior to subduction and does not appear to contain accreted exotic terranes.

TECTONIC

SIGNIFICANCE

The presence of a fossil subduction zone on the south coast of NSW requires a 'subduction-related magmatic arc' model to explain continental accretion for eastern Australia. Such a model can explain the magmatic activity, magmatic underplating of the lower crust, the high T-low P metamorphism, and in particular the tectonic vergence of the structural belts that make up the central and eastern Lachlan Fold belt. Thrust-belts develop on the leading edges of actively driving plates where they verge toward, and against the motion of these plates. The angle of subduction controls the development of magmatic activity, with roll-back controlling the migration of plutonism and deformation. Geological complexities are due to: 1) changes from a convergent to a transform margin (normal to oblique convergence), 2) changes in subduction zone dip, and 3) variations in the distribution of continental and oceanic crust. Key tectonic elements in a revised tectonic scenario include: 1) Late Ordovician-Earlv Silurian "arc" (Wagga-Omeo Metamorphic belt) characterised by high T/ low P metamorphism and bounded by linked strike-slip faults and thrust faults.

2) Accretionary complex/forearc belt or a back-arc thrust belt?: Tabberabbera zone which consists of a simply folded turbidite sequence transitional into a 2km wide in situ-derived tectonic melange/broken formation of the Wonnangatta Fault (Fergusson & VandenBerg, 1990).

3^ Early Silurian-Mid Devonian thrust-belt (Delegate-Bungonia thrust-belt: Fergusson & VandenBerg, 1990) transitional into a subduction complex (Powell, 1983, fig. 11; Bischoff & Prendergast, 1987).

REFERENCES Bischoff, G.C.O., & Prendergast, E.I., 1987. Newly discovered Middle and Late Cambrian fossils from the Wagonga Beds of N.S.W, Australia. Neues Jahrrbuch fur Geologie und Palaontologie 175, 39-64.

Etheridge, M.A., Ransom, D.M., Williams, P.F., & Wilson, C.J.L., 1973. Structural evidence of the age of folded rocks on the South Coast of N.S.W. J ournal Geological Society of Australia 19, 465-470.

Fergusson, C.L., & VandenBerg, A.H.M., 1990. Middle Palaeozoic thrusting in the eastern Lachlan Fold Belt, southeastern Australia. Journal of Structural Geology 12, 577-589.

Powell, C. McA., 1983. Tectonic relationship between the Late Ordovician and late Silurian palaeogeographies of southeastern Australia. Journal of the Geological Society of Australia 30, 353-373.

Stewart, I. A. and Glen, R., 1992. New Cambrian and Early Ordovician ages from the New South Wales south coast. Quarterly Notes N.S.W. Geological Survey 85, 1-6.

298

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

"BRISQUAKE 93" - A REAL-TIME EARTHQUAKE RESPONSE EXERCISE FOR BRISBANE ON 12 OCTOBER 1993 Pam Millican , Jack Rynn and Norm Sprenger 1

2

3

Queensland Emergency Service, GPO Box 1425, Brisbane, QLD 4001 Centre for Earthquake Research in Australia, PO Box 276, Indooroopilly QLD 4068 Retired, Queensland Police Service, 85 Peach Street, Greenslopes, QLD 4120

2

3

An earthquake in any major Australian City will result in community disruption and property destruction. One just has to remember the 28 December 1989 Newcastle earthquake. Such a scenario has been considered for the City of Brisbane and surroundings. To better prepare counter disaster agencies in Southeast Queensland, a "real-time" earthquake response exercise - BRISQUAKE 93 - was developed and conducted in Brisbane on 12 October 1993. This paper aims to provide an overview of the preparation, conduct and results of BRISQUAKE 93.

The aim of BRISQUAKE 93 was to assess preparedness and identify deficiencies of the Brisbane Disaster District to deal with generic issues involved in the immediate aftermath of an earthquake event. These included awareness, command, control and coordination, search and rescue, transport, disruption to essential utilities, engineering issues and community health and welfare.

The exercise was based on the lessons from the 1989 Newcastle earthquake, relevant overseas earthquakes which impacted on major western-style urban communities (such as the 1989 Loma Prieta earthquake in California), and the practical application of outcomes of the Australian IDNDR project on earthquake zonation mapping of Southeast Queensland (SEQZON). Planning was undertaken over several months and included seminars and pre-briefings with the total involvement of all relevant agencies, so exercising a multidisciplinary .approach to a potential disaster. This combined hazardspecific information, practical response issues and specialised local geophysical and community knowledge to develop the response. One major aspect, new to the emergency services authorities, was an emphasis on the geological controls to potential damage.

BRISQUAKE 93 ran over fourteen (14) hours which involved a rostered change of staff.

The scenario was for a Richter magnitude ML 6.0 earthquake occurring at 6.00am, centred about 30 km from the CBD in Moreton Bay, and an aftershock of ML 4.5 at 1.30pm. 85 personnel from 15 agencies (Queensland State Government; Local Government Authorities of Brisbane City Council, Redland Shire Council and Pine Rivers Shire Council; private sector; non-government organisations) were activated. Details of their involvement, the exercise serials, results and highlighted practical problems will be presented.

299

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE COPPER-SILVER SOLID SOLUTION OF MARSHITE, MIERSITE AND IODARGYRITE FROM BROKEN HELL, NEW SOUTH WALES, AUSTRALIA P.W. MELLSTEED Faculty of Applied Science, University of Canberra, PO Box 1, Belconnen, A.C.T. 2616 Australia

The iodine-bearing minerals marshite, miersite and iodargyrite have all been reported from various locations within the secondary zone of the Broken Hill orebody. Spencer (1898) originally proposed that isomorphous replacement takes place between the cubic minerals miersite and marshite. Seventy-three years later, Barclay and Jones (1971a) also proposed that a complete solid solution series may exist between pure cubic copper iodide and silver iodide. The limited compositional data calculated from a wet chemical analysis reported by Prior (1902) could only be used to determine a possible formula of 4AgI.CuI for miersite. The formula for marshite, Cul, was also calculated from one wet chemical analysis, Spencer (1902). Barclay and Jones (1971a) examined silver iodide minerals from Broken Hill by X-ray diffraction and determined all except two to be iodargyrite, pure hexagonal Agl. The two exceptions, contained 3.3 and 11.4 atomic weight % Cu respectfully and were described as miersite.

Analysis of 18 representative specimens from Broken Hill by the author reveal that the composition of crystals with cubic structure range form 100% Cu in the copper rich field (marshite) to 0% mol% Cu in the silver rich field (miersite). Silver replaces copper to maintain a metal to iodide ratio 1:1.

Unit cell parameters of specimens measured provide evidence for sub grouping into a cubic sphalerite structured phase incorporating marshite and miersite. Iodargyrite is incorporated into a hexagonal wurtzite structured phase. The marshite subgroup has a composition in which the copper to silver ratio is greater than 1:1. The miersite subgroup has a composition in which the copper to silver ratio is less than 1:1.The unit cell lattice constant parameter in the sphalerite phase changes linearly with Cu:Ag without change in structure, obeying Vegard's Law. Multi component crystals of miersite contain cubic twins and hexagonal intergrowths.

Solid solution is continuous in the cubic sphalerite structured members, regardless of phase intergrowth.

Twinning has been instrumental in morphological variation.

Thermal analyses by Nolting (1964) concludes that the miscibility of the synthetic cubic sphalerite structured salts of Cul-Agl range over the intire miscibility field at least up to 92.5mol% silver iodide. No miscibility gap is established between wurtzite structured Agl and sphalerite structured Cul. Thermal analyses by Davis and Adams concludes that above 80° C and below 146° C the metastable synthetic sphalerite structure transforms spontaneously and irreversibly to the hexagonal wurtzite structure. A densitometer analysis of a two phase crystal by the author defines the compositional limits of the Ag rich wurtzite structure phase boundary arbitrarily at 90 mol % Agl. Halogens are present in the Broken Hill ore body as chalcophile elements (having a strong affinity for sulphur).

A specimen of oxidised galena from Blackwoods Open Cut contains 925 ppm silver, 346 ppm chlorine, 96 ppm iodine and 54 ppm bromine. Pockets of silver halide specimens present within the specimen indicate local enrichment and precipitation. Marshite miersite and iodargyrite may have crystallised from Cu^+ and Ag + ions enriched in I" ions derived from the oxidation of metal sulphides.

REFERENCES Barclay, C.J., Jones J.B., 1971a. The Broken Hill Silver Halides, Journal of the Geological Society of Australia, 18, 149-157.

Davis, B.L., Adams, L.H., 1964. High Pressure Polymorphs in the Silver Iodide Phase Diagram. Science, 146, 519-521.

Nolting, J 1964. Thermodynamic investigation in the system of silver and copper iodide halogen. Report of the Bunsen Society for Physical Chemistry. 68, 932-939.

Prior, G.T. 1902. The identity of Kilbrickenite with geochronite and analysis of miersite, marshite and copper pyrites. Mineral Magazine, 13, 186-190.

Spencer, L. J. 1898. Marshite a cubic modification of native silver iodide, Nature 1898, 57, p574.

Spencer, L.J. 1902. Marshite, miersite and iodyrite from Broken Hill, New South Wales. Journal for Crystallography and Mineralogy 35,452-467.

300

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NEW EXPLORATION STRATEGY FOR THE VLAMING SUB-BASIN OF THE PERTH BASIN l

Shige Miyazaki1 and Steve J. Cadman1 Bureau of Resource Sciences, PO Box E l l , Queen Victoria Terrace, Parkes, ACT 2600

The offshore Vlaming Sub-basin is a major Mesozoic depocentre lying within the Perth Basin in Western Australia (Fig. 1). Bounded by N-S trending faults between Pre-Cambrian blocks, the sub-basin formed as a rift basin associated with the breakup of Gondwana. Early Neocomian faulting, related to this rifting event, is responsible for the structural grain presently observed in the sub-basin. Faulting was followed by regional uplift, which resulted in a major Neocomian Unconformity.

A total of 16 wells have been drilled in the sub-basin, resulting in the discovery of a non-commercial oil accumulation within a thin, lower Neocomian sandstone. Many of the wells were designed to target structures associated with palaeo-highs on the Neocomian Unconformity. Our dry-hole analysis shows that these plays are vulnerable to leakage through cap rocks of poor quality.

Our new study has revealed that the relatively-unexplored Upper and Middle Jurassic Yarragadee Formation is favourably placed to entrap liquid hydrocarbons (Figs 2 & 3). The upper part of the Yarragadee Formation contains highly permeable and porous sandstones even at great depths (Fig. 4). Source rocks of the Formation are within the oil generation window and contain 4% of total organic carbon (TOC) on average (Fig. 5).

Significantly, seismic surveys sparsely cover the inshore part of the sub-basin (Fig. 6), where the Formation lies at relatively shallow depths and appears to be relatively unfaulted.

Fig. 1 Perth Basin Southern Part

Fig. 2 -> (p#n

Vasse Fault j .

%

j,

J

Bandaminna Fault

Ylgam Block Darling Fault 100 KM

Dunsborough Fault

140

90

Fig. 3

CORE P L U G POROSITY

%

Fig. 4

' IPdrmelfaL Fm LU

10 wells

3000

- Wambro Gp « Parmelia Fm • Yarragadee Fm - Cadda Fm & older

X 0 0 Jfa rraga<%e

- Coolyena Gp & younger > Warntoro Gp 4 Pamela Fm • Yarragadee Fm • Cadda Fm & older

16 walls

Fig. 5 Fig. 6

^ivv-

TOTAL ORGANIC CARBON wt%

301

Wapet, 1975 Wapet, 1982 Esso, 1982 .

BP, 1982 Esso, 1983 BP, 1983

e/^

jPZKTH

S e i s m i c Lines Arc?y

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE END PALAEOZOIC MASS EXTINCTION: ISOTOPE GEOCHEMISTRY CONSTRAINTS Ric Morante Australian Petroleum CRC, CSIRO Division of Petroleum Resources, PO Box 136, North Ryde, NSW 2113

The end Permian mass extinction event is the largest in the Phanerozoic. An auxiliary marker of this biostratigraphical system boundary is a global negative shift in 8 1 3 C values in carbonate and organic carbon. This negative shift is of the order of up to 10%o in organic carbon from Australian marine and nonmarine sedimentary basins enabling their time calibration to the geological timescale. The size of the negative shift in5 13 C values suggests that either a huge amount of organic carbon was oxidised or that a catastrophic release of methane from hydrates occurred about the Permian-Triassic boundary. In either case a greenhouse effect of enormous proportions would have ensued. This greenhouse world almost certainly contributed to the dramatic mass extinction event at the Permian-Triassic boundary.

Another major geochemical event in the Permian is the rapid decline in inferred 87 Sr/ 86 Sr seawater values measured in well preserved carbonate sediments through the Early and Mid Permian which is followed by a rapid rise in 8 7 Sr/ 8 6 Sr seawater values during the Late Permian (post Guadalupian). This fall, inflection and rapid rise m seawater values is found in unaltered brachiopod shell calcite from a composite section mainly from the Bowen Basin. The minimum in the 8 7 Sr/ 8 6 Sr seawater curve is found around 300 m down section from the negative shift in 5 1 3 C values and is the first demonstration of the relative order of these two events. This indicates the change in environmental conditions leading to the inflection in 87 Sr/ 86 Sr values was not immediately associated with the end Permian mass extinction. However, it does not preclude the conditions associated with producing the rapid rise in 8 7 Sr/ 8 6 Sr seawater values from being part of a sequence of events that might have contributed to the mass extinction.

The rise in the 8 7 Sr/ 8 6 Sr seawater values indicates a dominance of crustal inputs over hydrothermal inputs to the world ocean during the Late Permian leading up to the Permian-Triassic boundary. This suggests either a decline in hydrothermal activity at the mid ocean ridge (MOR) or and increase in continental weathering. A decline in MOR activity is supported by the general decline in sealevel predicted from eustatic models. A decline in MOR activity could be expected to produce an overall deepening of the ocean basins as the MOR profile contracted.

The location of the inflection point in the 87 Sr/ 86 Sr seawater curve hundreds of metres below the PermianTriassic boundary as correlated by the negative shift inS*3C values suggests ample time for a decline in MOR activity to result in an overall deepening of the ocean basins leading up to the Permian-Triassic boundary.

Methane hydrates are found on continental shelves today and potentially are a massive reservoir of highly 13 C-depleted carbon. Suitable conditions for the preservation of extensive methane hydrate deposits probably existed during the Permian. A possible scenario whereby breakdown of hydrate reservoirs may have contributed to the end Permian extinction follows. Methane hydrates are stable under high pressures and low temperatures. A fall in sealevel resulting from a deepening ocean basin which may be indicated by the Late Permian higher 87 Sr/ 8 6 Sr seawater values that suggest increased erosion from continents and decreased hydrothermal activity would have decreased the pressure on the shelf hydrate reservoirs resulting in their breakdown. The release of that methane may have resulted in a greenhouse effect and a higher global temperature. Any rise in temperature may have further destabilised the hydrate reservoirs providing positive feedback and eventually resulting in catastrophic methane release producing even higher temperatures.

Warmer surface ocean temperatures, the loss of thermal gradient between the poles and equator and an enhanced weathering regime may have produced dysaerobic surface water conditions that contributed to marine extinctions during the latest Permian. A lowered sealevel in a Pangea dominated Earth would have resulted in decreased habitat for marine shelf organisms. This may be reflected in the paucity of carbonate reefs during the Early Triassic. A changed global climate may have led to terrestrial floral and faunal mass extinctions associated with ecosystem collapse on land. Since there is no clear evidence of glacial ice in the latest Permian anywhere in the world negative feedback mechanisms that would normally inhibit catastrophic methane release as temperatures rose may have failed because the negative feedback relies upon deglaciation to produce a rise in sealevel and the consequent increase in pressure leading to increased stabilisation of methane hydrates.

Acknpwledgement?: Isotopic work has been undertaken under the auspices of the Centre for Isotope Studies with financial assistance from an ARC Special Investigator Award to Professor John Veevers.

302

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE KIDSTON GOLD DEPOSIT, QUEENSLAND: A PISTON-CYLINDER MODEL FOR GOLD MINERALISATION IN A PORPHYRY MO SYSTEM Gregg Morn son Mike Seed^, Renato Bobis^ and Frank Tullemans^ ^Klondike Exploration Services, 7 Mary St, Townsville, Q 4810 ^Ross Mining, 189 Coronation Drive, Milton, Q 4064 Kidston Gold Mines, PO Box 7020, Cairns, Q 4871 3

Recent drilling beyond 1300m in the Kidston breccia pipe has demonstrated that gold mineralisation is confined to the upper parts of a zoned polymetallic porphyry system with a Mo (W, Cu) core. A review of the geometry, paragenesis, zoning and fluid evolution for the whole system suggests gold mineralisation is a natural part of the magmatic-hydrothermal evolution, but gold ore formation is a fortuitous combination of fluid confinement, refinement and modification.

The Kidston system is centred on a cylindrical hydrothermal breccia pipe that is 1100 x 900m at surface and at least 1300m deep. The walls of the pipe are regional scale faults and intrusive-related fracture cone sheets that were active during pipe excavation. The distribution of collapse breccia and large blocks in the upper part of the pipe suggests it was enclosed by a roof of unbrecciated country rock that effectively confined later stage ore fluids within the pipe.

The hydrothermal system responsible for brecciation and mineralisation is centred on a nest of porphyry plugs and dikes, part of which is preserved outside the southwest wall of the pipe, part as crosscutting dikes and the rest distributed as clasts in the breccia. There are five main stages of porphyry each with its own potassic-phyllic alteration and variable development of hydrothermal breccia and mineralisation: 1. Early banen felsite dikes.

2. Heterogeneous rhyolite plugs with tourmaline breccia and Cu, Mo stockworks.

3. Normal porphyry with main stage breccia.

4. Crowded porphyry with main stage polymetallic mineralisation.

5. Post mineralisation crowded porphyry and andesite dikes.

The main stage mineralisation is related to the latest, most refined batch of magmatic fluid that leaked from the interior of the porphyry nest and was trapped within the breccia pipe.

The main stage mineralisation is represented by two sets of sheeted veins and by cavity fill that originates from excavation and replacement of the matrix of main stage breccia. It extends throughout the pipe and has vertical zoning that is best defined by metal ratios. The shallowest zone is barren, followed down by Au 4- basemetals, Zn + Pb + Cu + As, Cu + Zn, Mo + Cu, Mo + W and Mo. This is a classical, but incomplete pattern typical of a vertical thermal gradient in a polymetallic porphyry system with a Mo core.

The gold orebodies (140 tonnes contained Au) are confined to the upper 250m of the pipe. They are sandwiched between the pipe roof and a disc-shaped sill of porphyry that extends across the pipe and is connected via a central dike to the core of the nested porphyry plugs. The ore envelope has a bowl shape defined by sheet vein sets that are parallel to the pipe margin and to the flooring sill. Cavities carrying additional grade are localised inside "bend zones" defined by the intersection of the sheet vein sets. Textures in the ore are consistent with purely extensional fracturing in the sheet vein zones and generation of new secondary cavities in the bend zones. The ore is within the Au-basemetal zone of the overall metal zoning pattern, but has its own internal zoning down from carbonate-pyrite, Au-basemetal sulfides-quartz-carbonate, quartz-pyrrhotite to epidote-biotite-magnetite-pyrrhotite.

This is related to phase separation, cooling and gas condensation in an essentially closed system.

The overall model for gold mineralisation requires 1. Progressive refinement of magmatic fluid related to successive batches of magma tapped off an underlying zoned magma chamber.

2. Confinement of the final fluid aliquot within the cylindrical closed breccia pipe and thermal zoning of the trapped fluid concentrating gold and basemetals in the upper part of the cylinder.

3. Emplacement in the upper part of the cylinder of piston-like sills and dikes that heated and overpressured the trapped gold-bearing fluid.

4. Collapse of the underlying magma chamber and withdrawal of the piston leading to pressure drop, phase separation and gold mineralisation in the upper part of the cylinder.

303

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

TIMESCALES; AGSO, HARLAND, DNAG, AND OTHERS: COMPARISONS AND COMMENTS Graham Moss and Clinton Foster Australian Geological Survey Organisation, Box 378, Canberra ACT 2601

For three decades Harland and others, for example, have evolved a geochronological template that has been described as a transient time scale by the authors but has become a reference for industry and research. Why has this occurred? Perhaps the demand for a numeric timescale for use in geologic modelling, coupled with a readily useable format of wall-chart and single volume has made Harland's work one of the most applied in exploration.

Against this geological timescale, based upon a series of standardized reference points and a chronostratigraphic consensus, workers have matched events ranging from climatic shifts to apparent (and real) episodes of accelerated extinction of taxa to the age of source rocks in petroleum provinces.

The development of a reliable geochronological reference requires a multidisciplinary approach.

Biostratigraphers must collaborate with specialists in isotope geochemistry, magnetostratigraphy, sedimentology and others, to reach accord. The AGSO timescale has evolved in such a way via the Phanerozoic Timescales Calibration and Development Project; it is a composite of AGSO research, contemporary published data and SHRIMP dating. Here it is compared to Harland, and other timescales (DNAG, Forster & Warrington, Haq & Van Esinga), in order to highlight obvious disparities between the various scales. The method serves to focus attention at specific intervals, and ask why do the differences occur? Combined, there are some 4,000 datums with Harland and AGSO ages in the STRATDAT Oracle based database at AGSO, with around 650 common datums with allocated ages.

Plots indicate that generally the timescales increasingly diverge with geologic age. Comparisons between timescales show product-moment correlation coefficients (between arrays of identical microplankton, foraminifera and nannofossil datums) of r > 0.99 for the Cainozoic to the Triassic. Despite this degree of correlation, Harland diverges from the AGSO scale at five obvious intervals arranged in increasing similarity: the mid Carboniferous to end of the Permian differs to the greatest degree; the mid Triassic to late Early Jurassic; the mid Jurassic to mid Cretaceous; early Middle Eocene to Miocene; and the Early Eocene.

Correspondence is closest in the Neogene. We discuss possible reasons for these differences and conclude that the AGSO Timescale provides the most reliable numeric set for the Phanerozoic.

AGSO time in Ma

Figure 1 - Harland against AGSO time from microplankton, nannofossil and foraminiferal biozone datums in STRATDAT. Significant differences in timescales highlighted by spikes in curve; N=666.

304

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 ECO-BIOSTRATIGRAPHIC PATTERNS IN NERITIC FORAMINIFERA FROM GRAPHIC CORRELATION AND MULTIVARIATE ANALYSIS: THE IMPACT OF OLIGOCENE GLACIATIONS IN SOUTHERN AUSTRALIA Graham Moss and Brian McGowran 1. Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601.

2. Department of Geology and Geophysics, The University of Adelaide, Adelaide, South Australia 5005.

1

2

The later Eocene to the Oligocene is a critical interval in Cainozoic Earth history. Accelerated AustraliaAntarctic separation and the thermal isolation of Antarctica drove global climatic transformation and the establishment of a permanent Antarctic ice-cap, glacioeustasy, and thermohaline-dominated oceanic circulation patterns. Benthic foraminiferal clades in neritic environments responded to oceanic changes in a series of hierarchal steps.

Fluctuations in infaunal and epifaunal abundances were more marked in the late Eocene than in the early Oligocene, particularly in the 'more restricted' Blanche Point facies in the St Vincent Basin. They appear to respond strongly to sea level change at 3rd order to parasequence timescales. Supplemented graphic correlation (SGC) and cluster analysis of Oligocene assemblages suggest that faunas also responded to lower-frequency, ie. second order cycles. In this category are the more substantial, non-reversible, taxic change at the EoceneOligocene boundary and at late early Oligocene regression followed by the late Oligocene transgressive phase.

Rapid changes in the infaunal:epifaunal ratios mark sequence boundaries and correspond to key glaciations Oil, Oi2 and Mil from the late Eocene to the Oligocene. The results appear to chronicle reversible 'short-term' local and perhaps palaeoenvironmental shifts (more autocyclic), whereas episodes of taxic turnover appear to correspond to more regional exogenic change (allocyclic). Species survivorship curves, derived from SGC, show that the early Oligocene glaciation Oil had a significant impact in the Otway Basin, leaving survivors that were gradually replaced and later joined by warmer-water species through Oi2 and Oi2a in the Oligocene.

Turnover patterns are reflected in ordination plots (Figures a-d), derived from samples from all basins subjected to cluster analysis, that show increasing dissimilarity between assemblages from the early to late Oligocene.

This study from disparate neritic environments (comparatively 'restricted' and more 'open marine') argues that patterns of change in benthic foraminiferal assemblages are a sensitive record of both sequence stratigraphic cycles and global climatic change; these may be signals of processes operating at differing timescales.

Quantitative techniques, such as graphic correlation and multivariate analysis when coupled with the excellent fossil record of neritic foraminifera, can help to identify patterns of change in assemblages and stratal relationships on continental margins at several timescales, and provide a strong test of the putative correlative tool, sequence stratigraphy.

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Figures a-d Eigenvectors derived from cluster analysis of some 250 samples from Oligocene sections in the St Vincent, Murray and Otway Basins. Patterns show increasing assemblage dissimilarity (distance between points) from the early Oligocene to the Miocene. Two dimensional plots produced using detrended correspondence analysis.

305

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

Northern Territory Geological Survey Program C A Mulder Northern Territory Geological Survey GPO Box 2901, Darwin, NT 0801

The Northern Territory Geological Survey (NTGS) significantly increased in 1995 its efforts in regional geological mapping and in airborne geophysical surveys. The regional geological mapping projects which it is presently engaged in, in East Arnhem Land, are being conducted in collaboration with the Australian Geological Survey Organisation (AGSO) under the auspices of the National Geoscience Mapping Accord (NGMA).

Significant stratigraphic ^interpretations have led to a better understanding of the geological evolution of part of the McArthur Basin. Four out of the eight 1: 250 000 scale mapsheets have been scheduled for publication in the 1996/97 financial year.

The NTGS and AGSO are also engaged in the North Australian Basins Resource Evaluation study aimed at a better understanding of the geological evolution of the Proterozoic basins and their associated basements so that a more informed assessment can be made of their mineral and hydrocarbon potential.

Systematic regional geological mapping under the NGMA is further being canied out in the Musgrave Block covering the areas of the Ayers Rock and the Petennann Ranges 1: 250 000 mapsheets.

First edition geological maps for the Flynn and Tennant Creek 1: 100 000 sheets accompanied by explanatory notes were published late in 1995 and preparations for the publication of a second edition of the 1: 250 000 mapsheet for Tennant Creek are well advanced. Geological mapping has resulted in substantial advances in the understanding of the geology of the Tennant Creek Block. Some fundamental revisions to the stratigraphy and relative timing of deformation, intrusive and extrusive activity, and known mineralisation allow new insights into the nature of the so-called Tennant Creek style, 'ironstone-related' Cu-Au-Bi mineralisation. Virtually all of the Tennant Creek Inlier will have been covered by recent first or second edition mapping, once the Helen Springs sheet is published in 1996/97.

A GIS package for the Pine Creek Geosyncline comprising geological, geophysical, and mineral deposit data sets, complemented by 3 atlases portraying the geology, geophysics, and geochemistry of the study area, was published in 1994/95 by the AGSO and NTGS. The NT Geological Survey had released 8 data packages in the Mineral Deposit Data Series by late-1995. Another four packages will be released not later than by the end of the first quarter of 1996, thereby completing coverage of the entire land area of the Territory. These data packages comprise an overview of the regional geology and the exploration history of the area covered, a description of the most important deposits, and Mine Data Sheets containing extensive and detailed information on mineral deposits.

In depth metallogenic studies over particular mapsheet areas as well as commodity studies for gold, uranium, diamonds, bauxite, and a few other minerals are currently under consideration. The Metalliferous Branch of the Geological Survey is also contributing geochemical data of granitic rocks to a program initiated by the AGSO to assess the metallogenic potential of Australian Proterozoic granites, the results of which will be publisheds in GISpackages and in hard-copy atlases.

The Geological Survey not only continued its commitment to airborne magnetic and radiometric surveys, it also increased its efforts in this area by flying the Limbunya Survey (8 x 1: 100 000 sheets) as a prelude to regional geological mapping in addition to the scheduled Mt Peake Survey (9 x 1: 100 000 sheets). Hie latter survey provides crirical information in a prospective area located between the gold-rich Tanami-Granites and Tennant Creek mineral provinces mainly covered by sand and lacking significant rock outcrops.

306

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PORGERA:

STRUCTURAL CONTROLS ON HIGH GRADE GOLD DEPOSITION

Stuart M. Munroe1 and Stephen F. Cox2 l Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Department of Geology, The University of Newcastle, University Drive, Callaghan, NSW 2308

2

The Porgera gold deposit is a world-class intrusive-related, vein and fault-hosted resource which is situated in the western highlands of Papua New Guinea. Gold production since opening in late 1990 has been in excess of 140 Mt (5 Moz). Currently Porgera has a reserve of 140 Mt at 4.4 g/t. The Porgera gold deposit is spatially and temporally associated with Late Miocene (6.0 Ma) mafic alkaline stocks which form the Porgera intrusive complex. Folding and thrusting of shale and limestone in the Porgera area occurred from approximately 10 Ma (in response to arc-continent collision to the north) and was complete before emplacement of the Porgera intrusive complex at 6.0 Ma. Intrusive-related hydrothermal alteration and mineralisation occuued between 6.0 Ma and 5.4 Ma. The depth of emplacement of the currently exposed intrusives is approximately 3 km, based on the deformed thickness of the overlying sediments. Approximately circular air-photograph lineations and drainage patterns and a circular magnetic anomaly associated with the intrusive complex indicate that the currently exposed area represents a deep magmatic system.

Two main stages of mineralisation at Porgera are distinct in their paragenesis and structural association.

Stage I mineralisation consists of compositionally banded veins of pyrite, sphalerite, galena, quartz and calcite with lesser amounts of chalcopyrite, marcasite, arsenopyrite, friebergite, tetrahedrite and native gold. Pyrite is the most common mineral in these veins and native gold occurs as microscopic or sub-microscopic inclusions within the pyrite resulting in gold grades up to 20 g/t within the vein. Most of the stage I veins were deposited within extension fractures, up to 0.5 m in width and 100 m in length, which formed in the presence of high fluid pressures. The stage I veins are hosted by the intrusives or occur in the sediment within 100 m of the intrusive contact. Stage I veins are often sub-vertical striking NNE or sub-horizontal. The bimodal orientation of the sulphide veins in any one area of the mine suggests flipping of the minimum principal stress (o3) direction from sub-horizontal WNW to sub-vertical. The transient change to a sub-vertical minimum principal stress (c3) direction may be a result of magma withdrawal at depth and cauldron subsidence.

Stage II mineralisation consists of quartz, pyrite, roscoelite (vanadium rich sericite) and lesser native gold, tellurides, apatite and barite with rare marcasite, chalcopyrite, tetrahedrite, sphalerite, galena, magnetite and haematite. Quartz dominates the mineralogy of these veins and is barren. Native gold occurs within pyrite or is closely associated with pyrite or roscoelite mineralisation. The gold grades within these veins may be up to hundreds of grams per tonne. Stage II mineralisation is spatially associated with the Roamane Fault Zone, which occurs in the south-western part of the intrusive complex. The Roamane Fault Zone strikes ENE and dips steeply SSE. The Roamane Fault Zone underwent normal displacement during stage II mineralisation and later underwent oblique normal - dextral to dextral movement. Most of the stage II mineralisation occurs as the matrix to a 2 - 5 m thick fault breccia and cataclasite zone which occurs in the immediate footwall to the principal displacement zone of the Roamane Fault. Stage II mineralisation also occurs in E-W striking subsidiary splay faults and adjacent to stage I veins which are near the fault zone. Episodic slip and accompanying changes in permeability on the Roamane Fault Zone and associated vertical fracture networks in the footwall of the structure, influenced stage II mineralisation by promoting episodic changes in fluid pressure and influencing the dynamics of fluid - rock and fluid - fluid interactions around the fault zone.

It is suggested that the Roamane Fault Zone could not have been present during the stage I mineralisation without having some influence on the hydrothermal fluid movement. Furthermore, it seems that the formation of the Roamane Fault Zone marks the transition between stage I (sulphide vein) mineralisation and stage II (sulphide - silicate) mineralisation. The transition from a contractional tectonic environment during stage I mineralisation to a transcurrent (and locally extensional) tectonic environment may be a result of crustal relaxation following arc - continent collision. The transition from shortening to extension at a late stage in the evolution of the Porgera intrusive complex has been critical in the formation of a suitable structural environment for high-grade (stage II) mineralisation.

Acknowledgments: We would like to thank Vic Wall, Greg Cameron, John Walshe, Chris Heinrich and Greg Hall for useful discussions, and the Porgera Joint Venture for their support and permission to publish this work.

307

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE ARCHAEAN BASEMENT OF PAPUA NEW GUINEA: EVIDENCE FROM THE PORGERA INTRUSIVE COMPLEX Stuart M. Munroe and Ian S. Williams Research School of Earth Sciences, Australian National University, Canberra, ACT 0200

Zircon mineral separates from mafic, alkaline stocks of the Porgera intrusive complex have been dated by the UPb method using SHRIMP (Sensitive High Resolution Ion MicroProbe) II at the Australian National University. Zircons from feldspar porphyry and hornblende diorite returned ages of 6.06 ± 0.21 Ma and 5.87 ± 0.15 respectively (95% confidence) which is interpreted as the magmatic cooling age of the intrusives. The hornblende diorite sample is similar to intrusives dated by Richards & McDougall (1990) using the K-Ar method which returned an overall age of 6.0 ± 0.3 Ma (2a). These results suggest that there is no significant age difference between the feldspar porphyry (which has not been dated previously) and other intrusive rock types at Porgera.

Seven zircons from the hornblende diorite sample returned ages of 2624 ±16 Ma, 2766 ± 10 Ma and 3669 ± 26 Ma (all 2a). It is suggested that these Archaean age zircons are inherited xenocrysts, carried by the hornblende diorite from the basement during ascent of the magma from a mantle source. A mantle source for the intrusives is suggested by the Lead ( Pb/ Pb » 18.66, Pb/ Pb - 15.56, Pb/ Pb - 38.55), Strontium ( Sr/ Sr0.7035) andNeodymium (e = +6) isotopes of Richards et al. 1990. The age of one population of Archaean zircons from Porgera is similar to a population of zircons recovered near the D'Entrecasteaux Islands (Baldwin et al. 1992) which returned an age of 2782 ± 10 Ma (2a).

206

204

207

204

208

204

87

86

Nd

The Archaean zircons have a very different chemical composition from the young zircons. The U and Th contents are mostly below 400 ppm, however the Th/U ratios are generally above average and range up to 3.1.

The medium to high U and Th contents and the preservation of prismatic crystal forms suggests that the Archaean zircons have not been reworked through repeated sedimentary cycles. Sedimentary reworking rounds the crystal form and preferentially destroys the higher U and Th grains, as it is these grains which sustain the greatest lattice damage over time. Since the Archaean zircons from Porgera are not rounded and have moderate to high U and Th contents we can eliminate the deformed shallow marine (country rock) sediments as a possible source for the inherited zircon. The Archaean zircons show incipient dissolution and have brightly luminescent rims on SEM cathodoluminescence images as do many of the younger zircons from both the hornblende diorite and feldspar porphyry. There is however, no young magmatic overgrowths on the Archaean zircons, suggesting that they were incorporated into the melt at a late stage without causing any discernible change in the melt isotopic composition. Had the zircons been at magmatic temperatures for an extended period, the lattice damage would have annealed. We are left with the conclusion that the Archaean zircon was inherited from an igneous or immature sedimentary basement rock that the magma passed through during its ascent. It is highly unlikely that they reflect the presence of an Archaean crustal component in the magma source region.

REFERENCES Baldwin, S. L., Ireland, T. R. and Rudowski, R., 1992. 2.8 Ga zircons in Plio-Pleistocene sediments from the Trobriand Basin, Solomon Sea. EOS Transactions of the American Geophysical Union 73, 573.

Richards, J. P. & McDougall, I., 1990. Geochronology of the Porgera gold deposit, Papua New Guinea: Resolving the effects of excess argon on K-Ar and ^Ar/^Ar age estimates for magmatism and mineralisation.

Geochimica et Cosmochimica Acta 54, 1397-1415.

Richards, J. P., Chappell, B. W. and McCulloch, M. T., 1990. Intraplate-type magmatism in a continent-islandarc collision zone: Porgera intrusive complex, Papua New Guinea. Geology 18, 958-961.

Acknowledgments: We would like to thank the Porgera Joint Venture for their support and permission to publish this work. Greg Cameron provided the hornblende diorite sample.

308

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ORGANIC GEOCHEMISTRY OF OILS AND OIL-PRONE SEDIMENTS OF THE PHILIPPINES

2

Andrew Murray1. Chao-Shing Lee1, Evelyn Cortez2, Evelyn Reyes2 and Craig Schiefelbein3.

Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia Energy Research Laboratory, Department of Energy, Merrit Rd., Fort Bonifacio, Makati, Metro Manila, Philippines 3 Geomark Research Inc., 9748 Whithorn, Houston Texas 77095, USA

Interest in the hydrocarbon prospectivity of the Philippines is growing rapidly due, in part, to clear analogies between the region's petroleum systems and the highly productive deltaic and lacustrine provinces of Indonesia, Malaysia, Vietnam and China. Despite this, only a few papers deal with the petroleum geochemistry of the Philippines and all but one of these are limited to the North Palawan Basin. Numerous oil and gas seeps occur elsewhere in the Philippines, notably in the Visayan and SE Luzon Basins. Technically, the Palawan Basin is related to the South China Sea system which developed along the Chinese continental margin, while the Visayan and SE Luzon Basins are associated with the Philippine Fault system. Statistical analysis of biomarker and isotopic data for Philippine oils reveals lacustrine, mixed lacustrine/resinitic, marine clastic and several marine carbonate sub-families.

This paper summarises the bulk, molecular and isotope geochemistry of oils from the Visayan, SE Luzon and North Palawan Basins and the implications for source rock depositional setting. Integration of new biomarker and isotopic tools is revealing details of the environments under which the source rocks accumulated and hitherto unrecognised secondary effects on the oils themselves.

The only commercial hydrocarbon production in the Philippines is from the Palawan Basin where two main oil families are recognisable. One group arises from a clastic marine deltaic source rock and the other from marine carbonates with minor terrestrial influence. The unusually heavy isotope composition of the latter group infers a generally restricted depositional environment and/or some contribution from colonial/brackish water algae.

Although these oils have been assumed to arise from the Miocene Nido Limestone, we believe an older syn- or early post-rift source is more likely.

In the SE Luzon Basin, oils generated from coals and/or carbonaceous shales are rich in resinous, terrestrial, organic material and were deposited in oxic or sub-oxic conditions, probably in a lower delta plain or inner neritic fluvio-deltaic setting. These hydrocarbons are isotopically correlated with submarine gas seeps in the Ragay Gulf. The hydrocarbon-prone sediments in the Philippines are predominantly of Late Oligocene - Early Miocene age. Lacustrine sediments of Late Oligocene - Miocene age and high generative potential have reached maturity in the Visayan Basin and on Mindoro Island and thick coal bearing sequences of Late Oligocene age are also present in the Visayan Basin. Kinetic maturation parameters enable accurate prediction of generation temperatures and also assist oil-source correlation. At least, 28 oil/gas shows and seeps have been noted in the onshore Luzon Basin and a variety of large traps are visible on seismic, particularly in the Early Miocene carbonate reefal buildups. Widespread Q -C4 anomalies in the Ragay Gulf confirm the presence of mature sources from which hydrocarbons are currently migrating to the surface. Maturation modelling from the wells and sediment depocentres on the seismic profiles indicates that the Late Oligocene - Early Miocene sediments have entered the oil window and expelled as early as Miocene and probably during the Pliocene. The timing of oil expulsion is thus favourable for charging reservoirs in the uppermost reef structures. Thus, although data is limited, good quality non-marine source rocks appear to be more widely distributed than marine source rocks in the Philippines.

309

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention., Canberra, February 1996

ROCK-MAGNETIC SIGNATURE OF GAS HYDRATES IN A C C R E T I O N A R Y PRISM SEDIMENTS

Robert J. Musgrave* and Bernard A. Housen^ * School of Earth Sciences, La Trobe University, Bundoora, VIC 3083, Australia ^Institute for Rock Magnetism, University of Minnesota, 387 Shepherd Laboratories, 100 Union St. SE, Minneapolis, MN 55455, USA

Gas hydrates are solid clathrates of gases (most commonly methane, but also higher alkanes, CO2, or H2S), which form at low temperatures and elevated pressures. Sediments from two Ocean Drilling Program Leg 146 sites from the Cascadia margin of western North America have magnetic properties indicating diagenesis of magnetic minerals associated with the presence of gas hydrates. Two indices combining coercivity, remanence, and susceptibility parameters, DJ H (= {J r s/Js}/{H cr /H c }) and D§ (= {Jrs/k}/Hcr,), are diagnostic of these changes. At Site 892, Dg values are distinctly higher and more scattered above the-bottom simulating seismic reflector (BSR), which marks the base of the hydrate stability zone. Within the hydrate stability zone at Site 892, DJH shows two trends: an increase from about 50 metres below seafloor (mbsf) to the BSR at 73 mbsf, corresponding to an expected increase in hydrate concentration near the BSR; and a second increase upwards from 50 mbsf to peak values at less than 21 mbsf, associated with hydrate recovered in cores above 19 mbsf. At Site 889/890 D J H increases downhole to about 285 mbsf, substantially below the BSR at 225 mbsf. This trend at Site 889/890 is consistent with an interpretation based on pore-water geochemistry (low CI") and bottomwater temperature that a "fossil gas hydrate zone" extended downwards to about 295 mbsf during the last glacial.

The observed changes in the two rock magnetic indices can be attributed to steps in the reduction series from magnetite through single-domain greigite (Fe3S4) to pyrite (or to overgrowth of single-domain greigite to multidomain size). Diagenetic growth of magnetic iron sulphides (greigite and/or pyrrhotite) has been reported in other accretionary wedge sediments. Thermal demagnetization of multi-component isothermal remanent magnetization (mIRM) indicates the presence of a low-coercivity magnetic mineral with an unblocking temperature (Tub) between 310° and 350°C. High Jrs/k ratios suggest that the low-coercivity, low-unblockingtemperature mineral is predominantly greigite rather than pyrrhotite. A low- to medium-coercivity mineral with Tub ~ 580°C - magnetite - is also present in varying amounts.

Hydrate apparently controls the presence of greigite by incorporating H2S, shown to be present as a hydrate phase together with methane in hydrate recovered at Site 892. Release of H2S below the base of the hydrate layer allows overgrowth of greigite grains to multidomain size or the conversion of some of the greigite to pyrite.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

Ni-Cu-PGE DEPOSITS OF THE NORIL'SK REGION AND OTHER WORLD-CLASS NICKEL SULFIDE DEPOSITS A.J. Naldrett Department of Geology, University of Toronto, Toronto, ON, M5S 3B1, Canada Magmatic Ni-Cu sulfide deposits form as the result of the segregation and concentration of droplets of liquid sulfide from mafic magma, and the partitioning of chalcophile elements into these from the silicate melt. The composition of the deposit is a function of the composition of the silicate magma (Le. the magma type involved), the relative amount of magma with which the sulfides have reached equilibrium (the "R factor" in the case of bulk equilibration; the "N value" in the case of progressive interaction), and the extent to which the sulfide magma has equilibrated. Segregation of liquid sulfide is not part of the normal cooling and crystallisation of mafic magma. Such deposits occur essentially exclusively at the base of their associated igneous bodies, which implies that the magmas involved were saturated in sulfide, and carrying excess sulfide at the time of their final emplacement. The high PGE content (1-10 ppb Pt, Pd) of most basaltic magma other than MORB implies that these magmas are not sulfide saturated before emplacement. Something has to happen to a magma prior to emplacement to cause sulfide saturation. Sulfide saturation of a magma is not enough in itself to produce a massive concentration of magmatic sulfide. The appropriate physical environment is required to concentrate sulfides from a large mass of magma into one place.

The deposits of the Norilsk region have developed within flat, elongate bodies (15x2x0.2 km) that intrude argillites, evaporites and coal measures, adjacent to a major, trans-crustal fault and immediately below the centre of a 3.5 km-thick volcanic basin. An anticlinal axis that transects the axis of the basin at a high angle has brought these intrusions to surface to give rise to the two major ore junctions, Norilsk and Talnakh. Studies of the overlying basalts have shown that basalts forming a 600 m-thick sequence have lost 75% of their Cu and Ni and more than 90% of their PGE. Basalts above this show a gradual recovery in their chalcophile element concentrations to reach "normal" values 500 m above the highly depleted zone. Two groups of mineralised bodies have been identified as correlative with these basalts; the poorly mineralised Lower Talnakh-type bodies, which resemble the highly depleted basalts, and the ore-bearing Norilsk-type intrusions which correlate with the overlying, essentially undepleted basalts. The high proportion of sulfide (2-10 wt%) associated with the Norilsktype intrusions, the high PGE content of the ores, the extensive metamorphic aureole (100-400 m around the bodies), and the heavy sulftir isotopic composition of the ores (+8 to +12 §Hs) are explicable if the ore-bearing bodies are exit conduits from high level intrusions, along which magma has flowed en route to surface. The Lower Talnakh bodies are interpreted as intrusions along which magma flow stopped earlier than along those of the Norilsk type. The first magma to enter the high level intrusion reacted with much evaporitic sulfur, at a low "R" value and thus gave rise to sulfides with low metal tenors.

Successive flow of magma through the system progressively enriched the sulfides in the conduits, losing progressively less of their chalcophile metals, and thus accounting for the upward increase in metals in successive lava flows. The flow direction along the conduits is shown by the direction in which the tenor of disseminated sulfides decreases. Sulfides have settled from the moving magma to form separate injections of liquid sulfide, up to 3.5x1.5x0.05 km in size. These have fractionated as intrusion progressed, with a consequent increase in Cu, Pt, Pd and Au, and the formation of ore containing >20wt% Cu and > 20 ppm Pt and Pd. The Jinchuan deposit of north central China occurs within a 6 km-long dyke-like body of peridotite. The compositions of olivine within the dyke, the igneous rocks themselves, and the ore are all inconsistent with derivation of the body from ultramafic magma, as originally supposed, and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt. Flow of magma into the intrusion has resulted in olivine and sulfide being retained within this keel. The Voisey Bay deposit lies within a 30-100 m-thick sheet of troctolite in which the lower part consists of a breccia within which lenses of highly mineralised troctolite, and in places massive sulfide, have developed. It is interpreted as a flat-lying part of a feeder for an adjacent intrusion Recent reflection seismic studies at Sudbury have shown the Sudbury Igneous Complex (SIC) to have been much more extensive than originally supposed. Nd and Sr isotopic studies on rocks of the SIC and Re-Os studies on the ores have indicated the incorporation of much country rock gneiss within the SIC. Debate has centred around whether the SIC is entirely an impact melt, or the consequence of the mixing of primary magma with 50 or more wt% impact melt; the most recent evidence favours the latter hypothesis. Fractional crystallisation of contact ores, with migration of the Cu-Pt-Pd-Au rich residual liquid into structures in the footwall are believed to be responsible for the presence of massive veins more than 1 km beneath the basal contact of the SIC. There is a growing body of evidence that "dry" contact ore (i.e. contact ore that has lost much of its fractionated liquid) indicates a favourable environment in which to find footwall veins.

When most major Ni-Cu sulfide deposits, including those at Kambalda, W.A., are viewed in the light of studies at Norilsk, three factors become apparent, (i) the concentration of sulfides in channels or conduits through which much magma has flowed (feeder conduits for intrusions are much more prospective targets for exploration than the base of the intrusions themselves); (ii) the interaction of the source magma with country rocks, either leading to the incorporation of sulfur, or the felsification of the magma in question, and (iii) fractional crystallisation of sulfide liquid giving rise to Cu-rich ores which may be far removed from the "source" ore.

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GEOLOGICAL SOCIETY OF A USTRAL1A, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

TACKLING THE REMEDIATION OF A CENTURY OF MINING IMPACT AT MOUNT LYELL, TASMANIA Stewart Needham 1 and Warren Jones 2 Office of the Supervising Scientist, 40 Blackall Street, Canberra ACT 2601 2 Department of Environment and Land Management, 160 Collins Street, Hobart Tas 7000 1

Mount Lyell, on Tasmania's west coast, has been mined for copper and gold for over a hundred years. Past mining practices were conducted in a way which resulted in environmental impacts no longer considered acceptable in Australia today: tailings and acid water were discharged directly into the river system, and surrounding hillsides were denuded of vegetation and stripped of topsoil through a combination of timber felling, sulphur dioxide emissions from smelting, and wild fires.

The discharge of copper-rich sulphidic tailings, slag and acid drainage has had a severe effect on 50 km of the Queen and King Rivers, virtually eliminating aquatic life and f i l i n g streamside vegetation A 25 ha delta containing 100 million tonnes of tailings has accumulated at the mouth of the* King River where it enters Macquarie Harbour. Unwanted infrastructure, including railway lines, bridges, product dumps and wharves have been left to decay.

The Tasmanian Government has recently approved redevelopment of the Prince Lyell mine by Copper Mines, of Tasmania (CMT). Due to the extent of previous environmental degradation and on-going pollution, an Act of Parliament was necessary to make it clear that CMT would not be liable for the effects of the "pre-existing and ongoing pollution**. Hence, the responsibility for remediating the vast majority of the impacts due to past mining rests with the Tasmanian Government The Commonwealth and Tasmanian Governments have formed a partnership to develop a practical strategy for cleaning up the environmental damage at Mount Lyell. The $2 million program, jointly managed by the Commonwealth Supervising Scientist and the Tasmanian Department of Environment and Land Management, is being undertaken over 18 months, and comprises a series of projects (Table 1) which: • consolidate and evaluate existing information • fill gaps in the information base • develop and trial options for remediation, and • where possible, within the budget and time frame, implement final remediation measures. quantify acid drainage sources

options to reduce acid drainage acid drainage reduction trials from the mine effects of fluvial processes on modelling of river chemistry chemistry of tailings banks tailings banks biological and chemical monitoring toxicity of copper to marine toxicity of copper to farmed fish of rivers organisms rehabilitation of Strahan foreshores chemistry of Macquarie Harbour biological recovery of Macquarie sediments Harbour options/trials to remediate tailings physical/chemical modelling of integrated remediation strategy banks and delta Macquarie Harbour Table 1. Projects in the Mount Lyell Rehabilitation Research arid Demonstration Program The program will end in mid-1996, and will present to government a strategy for remediating the historic and on-going environmental damage. This will include a prioritised series of technically feasible, costed remedial works, together with a prediction of the improvements which will accrue from the works.

Because of the scale of the problem and the potentially conflicting interests it has been important to engage all of the stakeholders, including the local community, the tourism, mining and fishery industries, and government agencies covering parks, environment, forestry, marine and hydro-electricity. Environmental quality objectives were determined through consultation, and regular meetings and newsletters are produced to keep everyone informed. Where possible, broad community involvement is being fostered, including employment and training opportunities through the "Working Nation" program and monitoring of field trials by school children.

Most of the 15 projects are being undertaken by leading experts from throughout Australia chosen through a competitive tendering process. The emphasis in project design and execution is on practical and pragmatic solutions to problems rather than further research. It is an example of how a strong partnership arrangement can successfully work together to produce real results, and is a role model for addressing mining and other contaminated sites both in Australia and abroad.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 REVISED LATE PALAEOZOIC TECTONICS OF NORTH QUEENSLAND John. E. Netherv'. Mike Barr 'Nedex Pry. Ltd., 1 Eastern Street, Chillagoe, Q. 4871 Mike Barr & Associates Pty. Ltd., 422 Kamarunga Road, Redlynch, Q 4870 :

:

Substantial revision of the late Palaeozoic tectonic model for the area encompassing the Hodgkinson Province, and adjacent Georgetown Block, is proposed. The major suture between the middle Proterozoic and Palaeozoic, the Palmerville Fault system, and the adjacent Ordovician to middle Devonian sedimentary sequence, were recognised in the 1980s as a major imbricate thrust package. Late Devonian to mid Carboniferous uniaxial compression, with an E oriented principal stress axis, was invoked to explain the steeply W dipping faulted wedges, and subsidiary NE trending dextral, and SE trending sinistral slip faults, along the N trending section of the Palmerville thrust package.

ENE to E oriented oroclinal fold axes due to a N oriented principal stress axis were invoked to explain a major convoluted linking structure, involving the Palmerville, Burdekin, and Clark River faults, and the Ordovician to mid Devonian sedimentary package. This structure was named the Big Bend Megafold.

The current model differs significantly from these, by attributing the development of the Hodgkinson Basin, its subsequent deformation, and the distribution and timing of middle Carboniferous to early Permian magmatic activity in the region, to regional wrench tectonics, involving several complete reversals in the sense of wrench, and both transtensional and transpressional episodes. Two distinct episodes of thrusting in the Carboniferous are recognised.

The latter episode, only recently recognised, still retains very shallow dipping thrust planes, is evident over a large area extending some 300 kilometres from Cape Melville to Chillagoe, and postdated the first of three PermoCarboniferous magmatic and mineralising episodes.

The Hodgkinson Basin probably developed as an intracratonic rift, analogous to the Gulf of California, due to NE to NNE oriented sinistral transtension wrench, in the period Ordovician to middle Devonian.

An initial thrusting episode circa late Devonian to early Carboniferous (360 - 340Ma) developed an elongate Sshaped thrust, the Palmerville system, which nucleated along reactivated earlier structures. This episode was a result of a complete reversal of the sense of NNE oriented wrench from sinistral transtension to dextral transpression.

Deformation waned and a stress lull prevailed, inducing detachment, oversteepening of the imbricate thrust package, and horst and graben development. Irregular wedges of immature poorly sorted fluvial sediments and piedmont fan conglomerates, collectively named the Quadroy Conglomerate, and to the south the equivalent Gilberton Formation, accumulated with a sporadic distribution. This was followed by intrusion of the I-type O'Brien's Creek Supersuite, and extrusion of related ignimbrites, circa 325 to 310ma. Tin mineralisation was associated with this supersuite, together with the early development of major porphyry systems such as the Phase 1 skarn of the Red Dome deposit.

A major stress field reversal occurred, circa 315ma, to develop NNE oriented sinistral transpression. This produced ENE oriented oroclinal axes, and associated ENE to ESE oriented reverse faults, SE oriented dextral slip in the Chillagoe area, and shallow dipping thrust duplexes. Steep dips and asymmetric folding in volcanics in the Chillagoe Lynd River, Newcastle Range and Broken River areas are attributed to this event. This deformation in the volcanics was previously interpreted as due to extreme sag, during magma withdrawal.

The regional sinistral couple waned, circa 3 lOma, and a transitional stress lull period saw the diapiric rise of Almaden Supersuite and Ootann Supersuite fractionated I-type plutons, circa 305ma to 300ma. Phase 2 prograde skarn developed in the Red Dome alteration system, at Chillagoe, frequently nucleated by shallow dipping thrust planes.

NW oriented dextral transtension developed circa 300Ma, and is interpreted as an arcuate extension to the coeval N oriented dextral transtensional bounding suture of the New England Orogen, between central Queensland and northern NSW. Lags Supersuite A-type magma commenced intrusion as small high level plugs, circa 290ma, often utilising the same conduits as the previous I-type intrusives. Phreatic diatremes, and sinters, and associated epithermal advanced argillic alteration overprinted earlier phases at Red Dome and elsewhere in the Chillagoe district. Major eruption of the Featherbed Volcanics followed soon after, circa 290ma to 280ma.

The structural history, multiple intrusive phases, and timing of various events, is demonstrated in microcosm in the Red Dome, Mungana, and other deposits in the Chillagoe district, as overprinting structures and alteration phases.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

" M A N T L E - L I K E " COMPONENTS IN FELSIC C O M P L E X E S O F THE LACHLAN FOLD BELT Ian Nicholls , Steve Bagaric and Alison Dean Department of Earth Sciences, Monash University, Clayton 3168, Australia

Recent studies of Silurian-Devonian granitic and felsic volcanic rocks of the Lachlan Fold Belt have presented Sr-Nd-Pb isotopic evidence for involvement of "mantle-like" source components, representing end-members of apparent mixing trajectories to contrasted "crustal" components. Most LFB felsic complexes dominated by hornblende-bearing metaluminous ("I-type") rocks include candidates for representatives of mantle-like components. These typically take the form of mafic-intermediate (gabbroic-tonalitic) minor plutons, basalticandesitic dykes and microgranitoid (micro-tonalitic to micro-granodioritic) enclaves within more felsic host rocks. Recent studies of the petrography and trace element and isotope geochemistry of these more mafic rock types and their hosts have provided evidence for the nature of processes of granitic magma evolution and emplacement, and also for the characteristics of source components associated with LFB magmatism.

The 405 Ma Swifts Creek hornblende granodiorite pluton, eastern Victoria, contains up to 40 vol. % of microtonalitic-microgranodioritic enclaves, with diameters of up to tens of metres. These are typically rounded and often have fine-grained, scalloped margins - features most compatible with emplacement as globules of a hot fluid crystal-poor magma into a partially crystallized felsic host magma rich in coarse quartz and feldspar crystals. "Trails" of the latter crystals within some enclaves provide strong evidence for physical mingling and incomplete chemical mixing between enclave and host magmas at levels near that of final emplacement.

However, strongly megacrystic and megacryst-poor enclaves occur together, suggsting that mingling/mixing also occurred at deeper levels These interpretations are supported by gradations between the trace element and isotopic compositions of enclaves and host, and also the observation that together they define apparent twocomponent mixing trajectories between "mantle-like" and "crustal" end-members. Enclaves which lack megacrysts (initial 8 7 Sr/ 8 6 Sr - 0.7060; z ^ - -3.5) plot near possible "mantle-like" end-members. Mixing models most compatible with both trace element and isotope systematics, involving "crustal" end-members with S7s r /86sr > 0.710; eNd ~ -9.0, indicate a mantle-like end-member with ^ S r / ^ S r ~ 0.705; £Nd ~ -2-0 which has significantly higher incompatible trace element abundances than "depleted mantle" at -400 Ma. A component with isotopic and trace element compositions similar to "depleted mantle" requires mixing with unrealistically large proportions of "crustal" components to meet trace element abundance constraints.

Some of the features of the Swifts Creek Pluton are also present in coastal exposures of the 400 Ma "I-type" Bega Batholith. Near Rame Head, eastern Victoria, several generations of highly abundant micro-tonalitic to syeno-dioritic enclaves are present within a granodioritic host. These enclaves are clearly related to common metre-scale dykes, some of which are composite, containing mingled tonalitic and leucogranitic phases.

The 385 Ma Mt. Stirling-Mt. Buller complex, east-central Victoria, includes the major hornblende-bearing Mt. Stirling Granodiorite (which contains abundant relatively potassic micro-tonalitic to syeno-dioritic enclaves) and, in the nearby Howqua River valley, several less potassic dioritic stocks with gabbroic cumulate zones.

Below the summit of Mt. Buller, near the roof of the pluton, a "Complex Zone" of interfingering between the Mt. Stirling Granodiorite and a large pod of a dark fine-grained phase showing strong geochemical similarities to the enclaves is exposed. The summit area also includes several apparently hybrid micro-granodioritic to leucogranitic zones, with variable proportions of plagioclase phenocyrsts. Sr- and Nd-isotopic data for samples from the largest dioritic stock, the syeno-diorites of the "Complex Zone" and similar enclaves, and the porphyritic "hybrid" zones show a narrow range of isotopically primitive compositions (initial ^ S I / ^ S I = 0.7038-0.7045; £Nd ~ +4.0-4.3). These compositions are isotopically more mantle-like than those of some tonalitic representatives of the Bega Batholith (initial 87Sr/86Sr~0.7041; £Nd ~ +3.0) and they are similar to "depleted mantle" components at -385 Ma. The host Mt. Stirling Granodiorite is only slightly more radiogenic (87Sr/86Sr -0.7043; e N d - +2.6).

The reconnaissance geochemical data available raise the possibility that the entire Mt. Stirling-Mt. Buller gabbro-leucogranite suite was directly related by crystal fractionation to parental basaltic-andesitic magmas with strong mantle-like and "arc-like" signatures, with significant crustal input (probably due to crustal assimilation/fractional crystallization) appearing only in the large-volume Mt. Stirling Granodiorite. The more mafic members of this suite may therefore give a strong indication of the geochemical characteristics of mantlederived magmas which provided both heat and chemical constituents to drive LFB magmatism, minimally modified by interaction with igneous or metasedimentary crustal material or its partial melts during transport and magma mingling/mixing. Further trace element, isotopic and related model-age data are required to assess and develop this interpretation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996

T H E S I G N I F I C A N C E O F M I C R O G R A N I T O I D ENCLAVES AND M A F I C DYKES IN S O M E L A C H L A N F O L D BELT FELSIC C O M P L E X E S Ian Nicholls , Marlina Elburg , Alison Dean and Steve Bagaric Department of Earth Sciences. Monash University, Clayton 3168, Australia

A number of petrographic/geochemical studies of microgranitoid enclaves in both "I-type" and "S-type" granitic complexes (and felsic volcanic equivalents) of the Lachlan Fold Belt have been carried out by Monash University Honours and PhD students since the mid 1980s. The major aims of these studies have been: 1) 2) 3)

To assess alternative origins for microgranitoid enclaves as source restite, magmatic cumulates or magma globules Where available evidence supports an origin for enclaves as magma globules, to assess the roles of magma mingling and mixing in the geochemical evolution and emplacement of their host(s) To assess the possible wider role of mafic magmas in crustal magmatism, as agents for the transfer of both heat and chemical constituents from the mantle into the crust, and to assess the degree of "mantle" input into Lachlan Fold Belt crustal magmatic systems

The 405 Ma Swifts Creek hornblende granodiorite pluton, eastern Victoria, contains very abundant microtonalitic enclaves which are typically rounded and often have fine-grained, scalloped margins - features most compatible with emplacement as globules of a hot fluid crystal-poor magma into a partially crystallized felsic host magma rich in coarse quartz and feldspar crystals. "Trails" of the latter crystals within some enclaves provide strong evidence that physical mingling and incomplete chemical mixing between enclave and host magmas continued to levels of final emplacement. Enclaves which lack megacrysts (initial 8 7 Sr/ 8 6 Sr - 0.7060; - -3.5) show isotopic characterists significantly more "mantle-like" than the host, supporting an origin for enclaves from primitive magmas from the deep crust or shallow mantle. The SCP is spatially and temporally associated with swarms of basaltic to andesitic dykes, some of which also have relatively primitive isotopic characteristics initial 87 Sr/86Sr = 0.705-0.706 and e ^ d = and which are probably related to widespread mantle magmatism associated with LFB crustal magmatism.

Many of the features of the Swifts Creek Pluton are also present in coastal exposures of the 400 Ma "I-type" Bega Batholith. Near Rame Head, eastern Victoria, several generations of highly abundant micro-tonalitic to syeno-dioritic enclaves are present within a granodioritic host. These enclaves are clearly related to common metre-scale mafic-intermediate dykes, which are again found throughout the region.

The 385 Ma Mt. Stirling-Mt. Buller complex, east-central Victoria, includes the Mt. Stirling Granodiorite, which contains abundant relatively potassic micro-tonalitic to syeno-dioritic enclaves. Near the summit of Mt. Buller, a large pod of micro-tonalite shows strong geochemical similarities to the enclaves. The mafic-intermediate rock types show a narrow range of isotopically primitive compositions (initial 87Sr/86Sr = 0.7038-0.7045; s ^ d ~ +4.0-4.3). The host Mt. Stirling Granodiorite is only slightly more radiogenic (87Sr/ 86 Sr -0.7043; e ^ d ~ +2.6). The available data suggest that the entire Mt. Stirling-Mt. Buller gabbro-leucogranite suite was directly related, mainly by crystal fractionation, to parental basaltic-andesitic magmas with strong mantle and "arc-like" signatures, with significant crustal input (probably due to assimilation/fractional crystallization) affecting only the large-volume Mt. Stirling Granodiorite.

Microgranitoid enclaves are also present in many Lachlan Fold Belt "S-type" granitic and felsic complexes in Victoria. These enclaves are typically much less abundant and smaller than equivalents in "I-type" complexes, and they less commonly provide clear evidence from their form and spatial relationships for a dominant role for magma mingling/mixing in their origin. However, in all Victorian S-type complexes studied to date, these enclaves, when megacryst-free, are isotopically significantly more mantle-like than their hosts. Complexes studied include the 410 Ma Deddick Granodiorite (a member of the Bullenbalong Suite of the Kosciusko Batholith), the 395 Ma Wilson's Promontory complex and the 375 Ma Violet Town Volcanics ignimbritic complex. In the case of the Wilson's Promontory complex, a recent SHRIMP ion microprobe study of the ages of zircons in microgranitoid enclaves and host granites lends strong support to an origin for the enclaves as magma globules. In both the Deddick Granodiorite and the rhyodacitic ignimbrites of the Violet Town Volcanics tonalitic enclaves contain magnesian orthopyroxenes (in the Violet Town Volcanics these pyroxenes have high Ni and Cr contents), suggesting an origin from mafic magmas of mantle derivation.

Mafic rock types associated with Lachlan Fold Belt felsic complexes of both "I-type" and "S-type" affinities are believed to give a strong indication of the geochemical characteristics of mantle-derived magmas which provided both heat and chemical constituents to drive LFB magmatism. At least in the case of "I-type" complexes, mafic magmas may be minimally modified by interaction with igneous or metasedimentary crustal material or its partial melts during transport and magma mingling/mixing.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February. 1996 PRELIMINARY MONAZITE AGES OF SECOND GENERATION MYLONITES, PRINCE CHARLES MOUNTAINS, EAST ANTARCTICA Geoffrey T, Nichols and Albert Fahey'GEMOC School of Earth Sciences, Macquarie University NSW 2109, AUSTRALIA California Institute of Technology, Division of Geological and Planetary Sciences, Mail Code 170-25, Pasadena CA 91125, USA 1,2

Three generations of mylonites (MY) associated with crustal thickening and subsequent thinning events have been identified recently in the northern Prince Charles Mountains (nPCM), east Antarctica. First generation mylonites (MYi) predate folding and have been annealed during peak metamorphic conditions; these subhorizontal structures may represent early thrusts. Second generation mylonites (MY2) developed at -5 kbar and 700°C, metamorphic conditions that post-date the peak conditions in this region. MY2 preserve pristine mylonitic, to ultra-mylonitic textures, truncate F2-3 folds and consistently display NW-up movement on NESW trending, steeply dipping surfaces. These structures may be responsible for rapid uplift in the nPCM soon after peak metamoiphic conditions were attained, and thus may account for the shallow predominantly cooling trajectories of P-T paths in this region. Third generation mylonites (MY3) truncate MY2 and are associated with lower grade metamorphic assemblages.

Here we describe new geochronology studies undertaken with an oxygen-sourced Cameca 3f ion-probe at Caltech.

Rather than apply mineral separation techniques which inherently erase textural information, we analysed monazite grains directly in thin section. We were thus able to distinguish data from monazites within centimetre wide shears, from monazite ages determined for granoblastic areas of the rock, largely unaffected by MY2 deformation (Figure la). Our preliminary data total 19 analyses from a single thin-section of an MY2 shear zone (N° 77154). The sample from Wall Peak in the south-western nPCM, is a garnet-orthopyroxene-plagioclasequartz gneiss which underwent mylonitic deformation at -5 kbar and 700°C. Th /Pb ^ isotopes were reduced to ages together with separate analyses of monazites of known age measured as a standard. As the measured abundance of P b ^ was very low, no correction was applied for 'common lead .

Figure lb displays ages of monazites plotted with one sigma errors which are a compound error accounting for possible errors in counting statistics, variability of the standard monazite, and also account for the uncertainty of the age of the standard. Figure lb displays two groups of data, with the older ages measured from groundmass monazites (with an average of 930±28 Ma), and younger monazites analysed within the MY2 averaging 800±16 Ma. As expected the older groundmass monazites display greater variability; one monazite with an age of -1100 Ma may represent an inheritance age. The two data groups remain statistically independent to the 2.3 sigma level; thus there is <5% probability that they represent data from the same age. The preliminary data suggest that either the nPCM remained at near peak metamorphic conditions for some 130 Ma before uplift along MY2, or that MY2 are related to a second tectonic episode at 800 Ma. 232

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Figure 1 (a) (b) Preliminary monazite ages from a MY Wall Peak (nPCM) Thin section schematic 2

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A NEW ANALYTICAL TECHNIQUE FOR MEASURING ELEMENT PARTITIONING BETWEEN EXPERIMENTAL VAPOUR AND MELT Geoffrey T. Nichols.Trevor H. Green, Norm Pearson and Ashwini Sharma GEMOC, School of Earth Sciences, Macquarie University NSW 2109

Most recent petrogenetic models for magma genesis in subduction zone environments attribute the characteristic geochemical properties of these magmas to the involvement of a water-rich fluid derived from the subducted slab, with or without an additional sediment derived silicate-melt. In order to constrain the geochemical attributes of such a water-rich fluid it is necessary to determine mineral-vapour and vapour-melt trace-element partitioning behaviour at high pressure. Usually mineral-melt partition coefficients and mineral-vapour values have been obtained independently - either derived experimentally or from natural rocks - and the desired melt-vapour partition values are produced by calculation. Here we describe experiments and a new analytical technique that enable the direct measurement of melt-vapour partition values.

A series of experiments was performed over a temperature range of 1020-1200°C, at 25 kbar, on a basanite+H20 starting composition to determine melt-vapour partitioning values. The basanite glass was enriched in La, Ce, Nd, Sm, Lu, Ba, Sr, Y, Zr, Hf, Nb, Ta, Th, U, Cs, Sn, Rb and Zn, totalling 1.045 wt%. Experiments were conducted using a 12.7 mm apparatus with AgPd capsules. Capsules were packed with an equal weight of H2O and basanite, and run in talc-Pyrex pressure cells. Experiments produced glass with minor clinopyroxene (depending on temperature), vapour and fluid. The vapour was analysed using LAM-ICPMS (Laser Ablation Microprobe-Inductively Coupled Mass Spectrometry). The LAM consists of a frequency quadrupled Nd-YAG laser, operating at 266 nm (UV). The laser beam is focussed through a petrographic microscope, producing a spot size of - 3 0 |im at a power of mJ/pulse. A video camera allows viewing of the ablation process. The laser was focussed on to the polished capsule-tail housed within a sealed sample-chamber, and an argon carrier gas transported emitted matter through to a Perkin-Elmer 5100 (ICPMS); the laser was used to drill through the AgPd capsule to release the vapour + liquid, at which point the laser was turned off. Data acquisition was monitored in a real-time graphics display, and the data were reduced by subtracting background counts from peak, and accounting for machine drift using four glass standard analyses. The vapour signals were transient, typically with peaks lasting <40 seconds. Experimental glasses, exposed in polished sections of capsules, were analysed under the same conditions and were standardized using an internal standard measured by an electron-microprobe to give absolute concentrations. As standardisation procedures for vapours are extraordinarily difficult we have not yet been successful in obtaining absolute concentrations of elements in the vapour. In order to compare our results with other data we divide the vapour composition by the bulk, and normalize to Primitive Mantle (PM) values of Sr or Lu (Figure 1). Experimental vapour appears to selectively transport Rb, Ba, Nb, Sr and Zr while La behaves incompatibly with respect to the vapour. Thus this vapour could cause significant changes to Nb/Ta and Zr/Hf in both melt residues, and to mantle regions trapping this metasomatic vapour.

Figure 1. Enriched basanite+H 2 0, 25 kbar 1100°C, 24 hrs

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CAMBRIAN AND ORDOVICIAN BIOSTRATIGRAPHY OF THE ARAFURA BASIN, NORTHERN AUSTRALIA Robert S.Nicoll . John H. Shergold , John R. Laurie and G.C.O. Bischoff Australian Geological Survey Organisation, P. O. Box 378, Canberra, ACT 2601 School of Earth Sciences, Macquarie University, NSW 2109 1

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Cambrian and Ordovician sediments occur in the Goulburn Graben of the offshore Arafura Basin in Petrofina Arafura #1, Esso Tasman #1 and Esso Torres #1, drilled in 1983, and in Petrofina Goulburn #1, drilled in 1986. The stratigraphic profile is most complete in Arafura #1 which penetrated 1892 m of Cambrian and Ordovician rocks. Tasman apart, these wells have yielded Early Ordovician conodonts. Additionally, terminal Cambrian conodonts and Middle Cambrian phosphatic brachiopods and phosphatised trilobites and Problematica have been obtained from Arafura #1.

Four formations constitute the Cambrian-Ordovician Goulburn Group. Near the bottom of Arafura #1 (3126-3596 m) the Jigaimara Formation, previously recognised in outcrops on Elcho, Howard and Banyon Islands, is inferred to rest disconformably or unconformably on the Neoproterozoic Elcho Island Formation, the uppermost unit of the Wessel Group, with which is was once included. The Jigaimara Formation contains an early Middle Cambrian trilobite fauna comprising eodiscoid, agnostoid and oryctocephalid trilobites, acrotheline and acrotretid phosphatic brachiopods and chancelloriid spicules. An age no younger than early Floran, and most likely Ordian/early Templetonian is indicated. Rocks of this age represent the initial Cambrian transgression in all of the northern Australian Cambrian basins. Correlatives of the Jigaimara Formation in the Daly, McArthur, Wiso and Georgina Basins are frequently phosphatic.

The overlying Naningbura Formation (new name) is 1128 m thick in Arafura #1, (1998-3126 m) and is identified in all four wells. In Torres #1, a cordylodid conodont occurs in the uppermost part of the unit, suggesting a latest Cambrian (Datsonian) or earliest Ordovician (Warendan) age at this level.

The Naningbura Formation resembles the Jinduckin Formation of the Daly Basin in its apparent long stratigraphic range and thickness.

The conformably overlying Milingimbi Formation contains conodonts indicative a latest Warendan (Early Ordovician) age similar to that of the Ooloo Limestone of the Daly Basin and Pacoota Sandstone sequence 3 of the Amadeus Basin. The unit is absent in Tasman #1, probably removed by pre Late Devonian erosion. The youngest Ordovician Formation, the Mooroongga Formation, is also truncated by the late Devonian unconformity in Arafura #1 and Goulburn #1. It contains Bendigonian (early Arenig) conodonts very similar to those of the Emanuel and Gap Creek Formations of the Canning Basin.

Ramifications of the Arafura Basin biostratigraphy are mainly stratigraphic and palaeogeographic, with implications for interbasinal correlation and dating of source and reservoir rocks and megasequence stratigraphic packages.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 ENVIRONMENTAL DEBATES: GEOLOGISTS CAN CONTRIBUTE MUCH-NEEDED INTELLECTUAL RIGOUR Brian J. O'Brien FTSE 12 Calthnoss Rd, Floreat Park, WA 6014

Geologists have professional knowledge, skills, intellectual and practical attitudes that are sorely needed in many environmental and social issues. A few examples are discussed.

In the uncertainties about Hindmarsh Island and Aboriginal sacred rights, for example, geologists might usefully contribute at least one fact, that it is a nonsense to talk of topographical features being sacred for the past 40 000 years, when until about 8 000 years ago the sea level was some 120 metres lower and the "mouth" of the Murray River was out past Kangaroo Island. The scene is still dynamically changing.

The whole Greenhouse debate might have been more rational and with less false emotional hype if, at the start, geologists had been more vocal about the history of the planet and their familiarity with much greater fluctuations in global temperatures and sea levels. Ice cores show that some 10 700 years ago, there were temperature changes of about 7°C in 50 years, a much greater and more rapid change than the 2°C or so now suggested by the year 2100 from greenhouse. Most species of today's flora and fauna evolved before 10 700 BP, and obviously survived such warming as well as previous ones of up to 13°C in a decade. Therefore the greenhouse focus is not an "environmental" issue but simply a "people" problem. The climate of the past 8 000 years has been unusually stable, and in that time homo sapiens developed agriculture and settled cities. Now, to the extent greenhouse fears are valid, the species must adapt to change.

To discuss the rate at which we adapted in the past, I introduce a new unit of geological time, the Canberra, being the length of time since Federal Parliament was located in Canberra and a sheep station was transformed into this modern city. A Canberra equals approximately the Biblical three score years and ten.

Greenhouse is "forecast" to cause sea level rise of about 40 cm in the next Canberra. Yet for the past one hundred Canberras, El Nino caused such changes every 3 to 8 years. Further, whereas alarmist greenhouse fears were of coral islands drowning, in fact healthy coral islands will grow larger to keep pace with rising sea levels.

Geologists have known for years that Australia's great tourist attraction, the Great Barrier Reef, was born 8 000 years ago when the sea level rose at twice the forecast greenhouse rate.

A final comment on greenhouse. There is global concern because of contributions mainly from fossil fuels. The doomsayers assume no significant use of alternative energy sources, fearing nuclear waste. Yet less than one Canberra ago, in 1945, the phrase "nuclear power" meant a bomb, not electricity. And less than a third of a Canberra ago, photovoltaics was a laboratory issue, because the satellites that drove later technologies had not yet been launched. Has mankind's inventiveness necessarily disappeared?

Geological know-how can give perspective to other environmental fears. Many environmental arguments have an unstated assumption taken as revealed truth, that nothing should be allowed to change the present-day environment because it is "natural" and "best". Geologists know dynamic changes are always at work as wind and water alter land and waterways, and that what is "natural" today is only a snapshot in time. Geologists are in touch with the earth and reality, unlike folk called Balmain basket weavers by Mr Keating.

The Federal Government considers that environmental strategies are somehow best if uniform across the nation.

Geologists make a living from the vast natural differences across the continent. I discuss "East-West effects", differences between the long-settled and the frontier States which require diversity in strategies. Similarly, geologists know Australia has 10% of World mineral resources on only 5% of the land with 0.3% of the population. It is the only developed country still classified as having massive biological diversity. It is unique.

So it is foolish to copy-cat global environmental strategies that neglect Australia's individuality.

Furthermore, geologists understand risk and risk management, and the motto of "Who dares, wins". By contrast, environmentalism and now governments embrace an imported version of the "precautionary principle", using lack of certain knowledge of an outcome as a reason for stopping development.

Sloppy thinking is rife in environmentalism and getting worse. Geologists could help reverse the trend.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 DEPOSITS OF THE LAMBERT GLACIER IN PRYDZ BAY- A RECORD OF CENOZOIC CLIMATE CHANGE IN ANTARCTICA P.E. O'Brien 1 and P.T.Harris2 'Antarctic CRC and Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601.

"•Antarctic CRC and Australian Geological Survey Organisation, University of Tasmania, GPO 252c, Hobart, Tasmania, 7001.

The Antarctic Ice Sheet is one of the planets major features and plays a major role in global climate and sea level change. Understanding its reaction to past climate change is an important control on climatic models. The Lambert Glacier is the world's largest, draining nearly 20% of the East Antarctic Ice Sheet through the Lambert Graben, ending in the Amery Ice Shelf in Prydz Bay. Drilling, sediment coring and seismic data have been used to investigate Deposits and landforms left by the advance and retreat of the Lambert Glacier. They provide evidence for the earliest Cenozoic glacigene sediments in Antarctica and provide a complex picture of Neogene ice fluctuations. Pre-Miocene glacial sediments accumulated by vertical aggradation beneath advanced ice whereas Miocene and younger sediments accumulated largely in prograding wedges formed at the shelf edge. Erosion surfaces within the prograding wedges represent periods when the ice did not reach the shelf edge so the upper slope was sediment-starved.

Several major episodes of upper slope erosion took place during the part of the Pliocene in which the ice sheet may have shrunk significantly.

Flat-lying, compact subglacial tills are important beneath shallow banks on the outer shelf but in a large, cross-shelf valley, the Prydz Channel, softer muds and diamictons are present, probably formed by rain out caused by basal melting of the ice shelf. These sediments were deformed during ice re-advances.

The inner shelf in Prydz Bay is deep because it is the area of maximum basal shear stress beneath the ice when the glacier extends to the shelf edge. The sea floor is blanketed by sandy to muddy diatom ooze overlying glaciomarine muds and diamictons. The glaciomarine facies formed during the last retreat of the glacier from the shelf edge and the ooze facies after the development of fully marine conditions in the Bay. Grounding line moraines mark former positions of the Lambert Glacier grounding line and that of the Publication Ice Shelf. One set of moraines formed at the boundary of two ice streams within the Lambert Glacier as it retreated from its last excursion to the shelf edge, the other indicates the position of grounding lines during a more recent, less extensive ice advance, possibly the Last Glacial Maximum. Dating of these Pleistocene deposits will depend on fortuitous preservation of organic matter or the right facies for Thermoluminescence dating.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ANTARCTICA: PRYDZ BAY, CONTINENTAL SHELF, SLOPE AND RISE P.E. O'Brien and G. Leitchenkov Antarctic CRC and Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601.

VNIIOkeangeologia, Antarctic Branch, Maklina 1, St Petersburg, Russia, 190121.

1

2

2

The Antarctic Ice Sheet is a key component of the world's climatic system and has a major influence on global sea levels. To test models of its behaviour it is necessary to examine its fluctuations during episodes of climate change. It is still not clear whether the current ice sheet will grow or diminish with global warming and there is controversy over the stability of the East Antarctic Ice Sheet during the Pliocene. The continental shelf and slope are the major repositories of information on ice sheet behaviour, in the form of deposits laid down by the ice and from the water column, erosional landforms where the ice has advanced across the shelf, and depositional landforms, usually formed during ice retreat.

Prydz Bay lies on the East Antarctic coastline between 068°E and 078°E. It is the downstream end of the Amery Ice Shelf - Lambert Glacier ice drainage system, which drains about 22% of the East Antarctic ice sheet. The convergent flow of the Lambert Glacier it responds to fluctuations of the interior of the East Antarctic ice sheet which are then reflected in the sediments of Prydz Bay. During Cenozoic glacial episodes, the Lambert Glacier advanced to the shelf edge, building a large trough mouth fan on the continental slope on the western side of Prydz Bay. Reasonably complete records of glacial history are contained in trough mouth fans because these fans are built of sediment removed from the shelf during major ice advances. During interglacials, trough mouth fans are subjected to normal slope processes so they may receive biogenic sediments, be sediment starved, or be reworked by oceanic currents and ice shelf water. Major slumping may also take place. In the case of the Prydz Channel Trough Mouth Fan, existing seismic shows less evidence of large-scale sediment gravity flows than similar fans. Gravity cores collected from the Prydz Channel Fan contain beds of sediment with diatoms and planktonic foraminifera suggesting that this fan receives a drape of biogenic sediment during interglacials. Thus, the Prydz Channel Fan could contain the most complete sedimentary record of any trough mouth fan on the Antarctic margin.

PROPOSED DRILLING Large bodies of hemipelagic sediments have been detected on the continental rise off Prydz Bay. Seismic facies within these deposits include well stratified mounded and parallel units and therefore should contain continuous sections of distal turbidites and pelagic oozes. These drifts have the potential to provide a continuous record of Southern Ocean conditions for the Neogene which can be linked to the adjacent record of glaciation. Drilling conditions should be good in these deposits. Approximately half the drilling time would be allocated to obtaining several long cores from these deposits.

We propose three sites in progressively shallower water on the Prydz Channel Fan and in fan sediments on the shelf to investigate the Plio-Pleistocene history of area. Near the toe of the fan there is a relatively thin PlioPleistocene section can be drilled through to the basal reflector of the fan sedimentary package This surface is an undulating erosion surface, suggesting an episode of major slope erosion. This will date the onset of major Trough Mouth Fan growth in the Pliocene. The shallowest part of the fan below the depth of iceberg ploughing is likely to encounter the thickest section of Pleistocene sediments. Gravity cores from the upper slope contain glacimarine clays probably deposited from meltwater plumes. This facies contains scattered foraminifera and can potentially dated by thermoluminescence. A hole in this location should intersect a major downlap surface of probable Pleistocene age. This surface marks a significant change in sequence geometry, suggesting a major change in ice behaviour.

On the western side of Prydz Bay, the oldest glacial sequences are closer to the sea floor and are probably more complete than on the eastern side of the bay. A holes situated in Prydz Channel could drill through the oldest glacial sequence down to preglacial unconformity to date onset of Prydz Bay glaciation. An additional site slightly oceanward of first, within Prydz Channel, would recover late Oligocene to middle Miocene prograding sequences, which likely corresponds to second significant phase of glaciation and advance of ice sheet onto the Prydz Bay Shelf.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EXTENSIONAL GEOMETRIES WITHIN THE LEICHHARDT RIVER FAULT TROUGH O F THE MOUNT ISA TERRAIN, NW QUEENSLAND, AUSTRALIA Mark G. O'Pea and Gordon S. Lister Australian Geodynamics Cooperative Research Centre, VIEPS, Department of Earth Sciences, Monash University, Clayton, Victoria 3168

Prior to the Isan Orogeny, the Leichhardt River Fault Trough of the Mount Isa terrain underwent a long and complex extensional history that involved repeated episodes of rifting, post-rift subsidence and associated depositional phases. Evidence for this history is contained within the rock types deposited, along with their lithological and depositional associations. These associations, combined with the geometries of sediment packages, enable an interpretation of the positions and orientation of original extensional faults within the now shortened Leichhardt River Fault Trough.

Over the past 25 years researchers have ignored the structural implications of this rifting history in their interpretations of local and regional geometries. Consequently, all complex field relations were interpreted to have originated during, rather than prior to, the compressional Isan Orogeny. This oversight was partly due to the fact that the rifting geometry has, until recently, been poorly understood and, therefore, easily ignored. It was also due to the assumption of a layer-cake stratigraphy and the failure to identify and appreciate the geometrical consequences of folding and faulting thick unconformity-bounded rock sequences.

In the Leichhardt River Fault Trough, strata of the lower Haslingden Group were deposited and extruded within the N-S oriented Leichhardt Rift, during significant phase of continental rifting and inferred E-W extension.

Half-graben of the Leichhardt Rift are estimated to have been up to 60 km wide and controlled by the development of widely-spaced, crustal-scale listric border faults of alternating dip direction.

Following the development of the Leichhardt Rift, the Leichhardt River Fault Trough underwent a renewed period of basaltic volcanism and clastic sedimentation during a period of N-S extension. This resulted in the development of southward tapering N-tilted half-graben comprising syn-rift strata ranging from the Pickwick Metabasalt to the Whitworth Quartzite. N-S extension within the Leichhardt River Fault Trough ceased during the deposition of the Lochness Formation. This unit, along with the overlying Quilalar Formation, were deposited during a period of regional post-rift subsidence.

Two angular unconformities developed in the interval between the deposition of the Quilalar Formation and the deposition of the Surprise Creek Formation attesting to two periods of rift-related tilting, uplift and erosion.

Deposition of Quilalar Formation rocks was intemipted by a period of tilting and erosion resulting in the development of the Bigie Unconformity. This erosional surface was overlain at approximately 1710 Ma by conglomeratic rift-related sediments and bimodal volcanics of the Bigie Formation and Fiery Creek Volcanics (Cover Sequence 3). Following the extrusion of the Fiery Creek Volcanics, fault blocks underwent another period of tilting, uplift and erosion resulting in the development of the Surprise Creek Unconformity and the complete bevelling of Cover Sequence 2 and Cover Sequence 3 rocks. This erosional surface was overlain by rift-related sheet sands of the Surprise Creek Formation, and dolomitic sag-phase sediments of the Mount Isa Group (Cover Sequence 4).

The stratal geometries of fault blocks within the Leichhardt River Fault Trough are asymmetric in cross section, exhibiting a pronounced rotational thickening of Cover Sequences 3 and 4 rocks towards the east and a tapering of these units towards the west. Both the Bigie Unconformity and the Surprise Creek Unconformity cut down section along the western edges of fault blocks, suggesting that differential uplift and rotation were responsible for their development. Both unconformities are interpreted to mark pronounced erosion surfaces at the crests of approximately E-tilted fault blocks. The degree of angularity with respect to underlying rocks, across both unconformities, decreases towards the eastern edges of fault blocks where all strata of Cover Sequences 2, 3 and 4 are paraconformable. Thus, strata of Cover Sequences 3 and 4 are interpreted to have been deposited in the hangingwalls of rotating tilt blocks, the width of which were between 15 and 40 km. Following the deposition of the Warrina Park Quartzite, accommodation space remaining in the depocentre of these tilt blocks was passively infilled with post-rift dolomitic sediments of the Mount Isa Group.

There is abundant indirect evidence throughout the Leichhardt River Fault Trough suggesting that numerous blocks were established as fault-bounded entities during the rifting history and continued to°act as coherent structural blocks during the Isan Orogeny. Prior to regional shortening, for example, rocks within the Crystal Creek Block, June Hill Block and Horse's Head Block, were separated from surrounding rocks by faults, and allowed to deform independently of each other. Many of the faults which presently bound these large blocks may run close to original normal faults.

The ca. 200 Ma rifting history of the Leichhardt River Fault Trough was characterised by distinct modes of extension ranging from early listric detachment-style faulting to that of later domino-style faulting. This history resulted in a complex extensional architecture which formed the geometrical template upon which Isan Orogeny compressional structures were superimposed. Understanding the interaction between extensional fault architecture, stratigraphic geometry and regional shortening is critical to interpreting the structural evolution of the Leichhardt River Fault Trough and the Mount Isa terrain as a whole.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

INSIGHTS INTO THE PROVENANCE AND T E C T O N O T H E R M A L H I S T O R Y O F T H E LACHLAN FOLD BELT - RB-SR AND SM-ND I S O T O P I C C H A R A C T E R I S T I C S O F PALAEOZOIC SEDIMENTARY ROCKS F R O M V I C T O R I A G. J. O'Halloran and R.A.F. Cas Department of Earth Sciences, Monash University, Clayton, VIC 3168

Many fundamental questions remain unanswered as to the nature of the basins in which the Cambrian to Early Devonian marine sequences of the Lachlan Fold Belt (LFB) accumulated. Were they: i) predominantly forelandtype depressions evolving adjacent to the Adelaide Fold Belt, ii) back-arc rift-related basins, or iii) more closely associated with convergent arcs, developing as fore-arc or intra-arc basins? The inability to unequivocally identify arc assemblages has continually frustrated attempts to reconstruct the palaeotectonic setting of these Palaeozoic marine basins. Previous investigations have focussed largely on the nature of the quartz-rich turbidite successions, which have proven to contain little information which is diagnostic of either provenance or tectonic setting. Further uncertainties as to the tectonic history of the LFB arise when the timing of deformation is taken into account (see Bucher et al, this volume). This paper addresses some of the questions raised above by investigating the Rb-Sr and Sm-Nd isotopic characteristics of fine-grained sedimentary rocks (ranging from Cambrian to Devonian in age) from the southern LFB. 1. Rb-Sr data The Rb-Sr data for the older group of samples define a latest Ordovician isochron age. Linear arrays formed by sedimentary rocks on Rb-Sr evolution diagrams can be interpreted as either (1) a provenance age (often modified) of the detrital component contributing to that series of rocks, or (2) an age of metamorphic/diagenetic reequilibration. Apart from the fact that the age recorded is either younger than, or close to, the stratigraphic age of the rocks analysed, an absolute provenance age appears to be discounted by the Nd data (see below). As such, the age given by the Rb-Sr data is suggested here to record an age of metamorphism/deformation. This has major implications for the tectonothermal history of the LFB, in that it recognises a previously undocumented late Ordovician event in the southern LFB. Until now, reconstructions of the tectonstratigraphic history of the LFB have assumed that only one major deformational event affected the Melbourne, Bendigo/Ballarat, and Stawell Zones of the LFB. 2. Sm-Nd data The Sm-Nd data for the LFB succession are remarkably uniform, with 8 nd (Tst) values generally between -8.5 and -11.5. By considering the isotopic evolution of various surrounding basement provinces, constraints may be placed on the provenance of the marine basins of the LFB. The Sm-Nd data for the LFB samples are best explained by a Late Proterozoic, Adelaide Fold Belt-derived source.

The results presented here provide insights into the tectonostratigraphic setting of the precursor marine basins of the LFB. Cambro-Ordovician thrusting and crustal thickening in the Adelaide Fold Belt would have provided an obvious, contemporaneous source for vast turbidite fans supplying the marine basins of the LFB, presumably developing in an adjacent foreland depression. With regards to the Ordovician metamorphic episode in the central and western LFB (as recorded by the Rb-Sr data), the question arises as to whether this episode may represent an ongoing continuation of Delamerian orogenic activity, or perhaps some final, westernmost expression of it.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

COAL EXPLORATION AND SEAM CORRELATION IN THE MORANBAH COAL MEASURES BOWEN BASIN K W O'Reilly, R Buzacott Riverside Centre 123 Eagle Street Brisbane Qld 4001

Over the past 25 years the late Permian Moranbah Coal Measures/German Creek Formation coal deposits on the western side of the northern Bowen Basin have been one of the great coking coal producing areas of the World.

With mines now extending over 200km of strike length from North Goonyella in the north to Gordonstone in the south, annual production of export coking coal is approaching 50Mt/annum; and with committed and projected underground developments at Gordonstone West, Crinum, Moranbah South, Moranbah North, Goonyella and Wards Well, annual production is likely to be 65 to 70Mt/annum by 2010.

Exploration and development of the Moranbah/German Creek sequence has been largely confined to the western side of the basin because of the simple structure, flat dips and coking rank. Exploration on the eastern side of the basin in the early 1970's indicated that Moranbah Coal Measures equivalents (Hail Creek Beds) were steeply dipping (-45°) and of anthracitic rank. Since the focus at that time was on exploration for coking coal, the eastern side of the basin was ignored for 15 years.

Since 1988 BHP Mitsui Coal Pty Ltd has been actively exploring the Moranbah Coal Measures at Nebo West on the eastern side of the Bowen Basin between the Peak Downs Highway and the Nebo-Collinsville road, to define resources of anthracite in an area of complex structure, characterised by steep dips (20 - 70°) and multiple folding. The nature of the deposit has necessitated a different approach to exploration, primarily in the proximity of adjacent drill holes and a heavier than usual reliance on downhole geophysics for seam correlation from hole to hole.

During the course of exploration, 17 significant coal seams from 1.5 to 7m true thickness (several of which are splits of thicker seams) have been recognised and named; but until recently no serious attempt had been made to correlate seams at Nebo West with the productive seams on the western side of the basin.

The main productive or potentially productive coal seams on the western side of the basin are the Goonyella Upper / Q, Goonyella Middle / Harrow Creek / Tieri; and Goonyella Lower / Dysart / German Creek seams.

Correlation between these sequences has been aided by the presence of two widespread marker horizons; the P Tuff, a 1 to 2m thick, cream to light grey unit, characterised by abundant bronze mica; and the Goonyella Middle Tonstein, a thin (2 - 3cm) reddish-brown bed composed of vermicular kaolinite.

During exploration carried out over the past two years the P tuff has been positively identified at Nebo West and as a result it has been possible, using the location of seams relative to the P tuff and characteristic geophysical signatures, to tentatively correlate seams at Nebo West with economic seams in the Goonyella - Peak Downs area.

The correlation of seams between the western and eastern side of the Bowen Basin has potential applications in future sequence stratigraphic studies of the Upper Permian succession in the Northern Bowen Basin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention., Canberra, February 1996

TERTIARY LITHOSPHERE EROSION IN EASTERN CHINA Suzanne Y. O'Reilly1, Xisheng Xu 1 ' 2 , Andi Zhang 3 and W.L Griffin 4 ' 1 1. GEMOC, School of Earth Sciences, Macguarie University, Sydney, NSW 2109 2. Dept. of Geology, Nanjing University, Nanjing, China 3. Ministry of Geology, Beijing, China 4. CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113

The occurrence of Paleozoic (450-490 Ma; Ordovician) kimberlites in Shandong Province, eastern China, and of Tertiary to Recent xenolith-bearing basalts in the same region, provides an opportunity to quantitatively evaluate the thermal state and lithostratigraphy of the subcontinental mantle in two time slices separated by significant tectonic activity. The results indicate the removal of ca 180 km of Archean cratonic lithosphere, and its replacement by a more fertile Phanerozoic lithosphere.

Lithosphere mapping by means of garnet and chromite concentrates from kimberlites has provided a detailed picture of the Paleozoic mantle section beneath Shandong Province, in the central part of the North China Craton. The Paleozoic paleogeotherm lay near a 40 mW/m2 conductive model up to temperatures near 1200°C, corresponding to a depth of 180-190 km.

The degree of melt-related metasomatism increases markedly at depths >180 km, and is associated with pronounced heating; both are interpreted as due to the intrusion of asthenosphere-derived magmas. This level is taken as the lithosphere-asthenosphere boundary, implying a lithosphere thickness on the order of 180-190 km. The Pleistocene lithospheric mantle consisted dominantly of lherzolite (based on the geochemical signature of the garnets); the proportion of harzburgitic rocks reaches 40-50% between 130-170 km, and drops off markedly above and below this zone.

The lherzolitic rocks, as defined by their garnet compositions, have low cpx/gnt ratios and are strongly metasomatised with introduction of phlogopite at depths >140 km.

The xenoliths in the Tertiary basalts have been studied in detail at the Middle Pleistocene Nushan volcano, north of Nanjing and ca 300 km south of the Shandong kimberlites; Nushan is thought to lie within the southern boundary of the North China Craton. The volcano contains abundant xenoliths of spinel peridotite, and smaller numbers of garnet- and garnet-spinel lherzolites and pyroxenites. P-T estimates for the garnet-bearing rocks place most of them at depths of 50-70 km, and define a geotherm very close to that derived for Tertiary eastern Australia. The geotherm gives much higher temperatures at shallow depths than standard conductive models, implying advective heat transport by intruding magmas. This geotherm would intersect the mantle adiabat at depths of 100±10 km, defining the top of the asthenosphere and the base of the lithosphere.

This is equivalent within error to the depth to the regional seismic low-velocity zone (LVZ) in this region. The depth to the present-day LVZ beneath the Shandong kimberlite province is similar, though poorly defined.

Since the geophysically defined LVZ has been identified to coincide with the base of the lithosphere in Pleistocene time we can extrapolate that the lithosphere beneath the Shandong kimberlites today is <100 km thick, compared to 180-190 km in Ordovician time. The contrast implies a lithosphere thinning of 80-90 km since the Paleozoic. The most likely time for this thinning is during regional extension in the early Tertiary.

Spinel lherzolites from Nushan are fertile to mildly depleted, and their Mg#-Mg/Si relations are consistent with an origin as residues from basalt extraction at low P (Boyd and Mertzman, 1989); they probably represent underplated oceanic mantle. They are distinct in these respects from the peridotite xenoliths in kimberlites from Archean areas, which are strongly depleted and have high opx/olivine ratios inconsistent with an origin by basalt extraction. The shallow mantle beneath Nushan therefore cannot be a remnant of thinned Archean lithosphere. If such lithosphere did exist beneath this edge of the North China Craton, then all 180-190 km of it has been technically displaced by mantle typical of oceanic areas.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LATE MESOZOIC AND CENOZOIC THERMOTECTONIC EVOLUTION OF TASMANIA Andrea J. O'Sullivan, Paul B. O'Sullivan. and Kevin C. Hill Australian Geodynamics Cooperative Research Center School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083, Australia

Previous studies of the rifted margins of southeastern mainland Australia have suggested that denudation along the margins was caused by the rifting and separation of Australia from Antarctica (-120-90 Ma) and the Lord Howe Rise (-90-60 Ma). These studies have proposed that continental rifting was accompanied by at least 1.53.0 km of uplift and erosion along the Tasman Sea and part of the Bass Strait coasts, with negligible uplift -100 km inland. However, recent fission track studies of the continental margin in New South Wales suggest very different results, that rifting in that region resulted in less than 1 km of denudation along the coast and km-scale denudation of the eastern highlands located inland from the coast.

In an effort to constrain its' Mesozoic and Cenozoic thermal and tectonic history as well as a model for the extensional tectonics of southeastern Australia, a growing data base of apatite fission track analyses from exposed sedimentary and granitic rocks in Tasmania is being generated. Tasmania located at the southeastern tip of Australia, is the ideal site to study the complexities associated with the continental extension tectonics of southeastern Australia as it has rifted margins on all sides. These include the Early Cretaceous failed rift between Antarctica (and Tasmania) to the north, the middle Cretaceous rift between Antarctic and Australia located along the west coast, and the middle Cretaceous to Paleocene rift between New Zealand and Australia along the east coast. This particular study is focussed on the thermotectonic history of northern Tasmania in response to the continental rifting along its' eastern and western margins.

Previous fission track results from the central plateau region of Tasmania indicated that the central part of the island had not been greatly affected by Mesozoic to Cenozoic rifting, although the Permo-Triassic sediments may have been partially reset by earlier Jurassic dolerite emplacement. However, new results presented here from northern Tasmania (including the Flinders Island region) indicate at least two regional episodes of cooling have affected the area since the Mesozoic, during the middle Cretaceous and during the Paleocene to Eocene. We believe that cooling during both of these events was directly related to continental extension along the margins.

Furthermore, there is evidence for a third, more localized episode of rapid cooling during the Late Tertiary, possibly in response to Miocene tectonics in the Bass Basin region.

The first episode of rapid cooling is recorded throughout much of the highlands of eastern Tasmania and occurred during the middle Cretaceous between -90-110 Ma. We believe that cooling was in response to km-scale denudation following the onset of continental extension in the Tasman Sea at -96 Ma. Uplift and erosion at this time resulted in km-scale denudation over much of the southeastern highlands of Australia and may have been caused by underplating inward of the rift. The second episode of rapid cooling occurred during the middle Paleocene to Early Eocene between -60-50 Ma. Evidence for this event is restricted to regions within -20-30 km of both the northeastern and northwestern coastlines, and throughout the Flinders Island region. We believe that rifting between Antarctica and Australia along the western coast of Tasmania during the Early Cenozoic resulted in higher than normal heat flow and that the apatite fission track data is recording the subsequent cooling as rifting progressed beyond the region. However, it is not clear at this time what is responsible for the cooling ages along the eastern coast of Tasmania and this will be addressed with future studies. A third episode of localized cooling is recorded by many of the samples from the Flinders Island region as well as a few samples from northwest Tasmania. These samples characteristically contain a large proportion of very young singlegrain ages less than -15 Ma suggesting that the rocks have recently experienced -30-40°C cooling. Since the evidence for this event seems to be limited to the far north of the state, we propose that northern Tasmania experienced isolated fault reactivation related to recognized Miocene tectonics along the southern margin of the Australian continent.

In conclusion, data from northern Tasmania indicate at least two episodes of rapid cooling have affected the island since the Mesozoic. Ages of 90-110 Ma recorded from the highlands suggest that km-scale denudation occurred, possibly in response to rifting along the eastern margin of the island. Ages of -50-60 Ma recorded from both the northeastern and northwestern coastlines suggest that either km-scale denudation, or regional relaxation of geotherms occurred along both margins during the Paleocene to Eocene. We believe that high heat associated with rifting between Antarctica and Australia along the western coast of Tasmanian during the Early Tertiary was responsible for the results from the northwest, but it is not clear at this time how the eastern coast responded to this event. Isolated indications of the third cooling event may be due to localized fault reactivation, possibly related to the same Miocene tectonics recorded in the Otway and Bass basins.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MESOZOIC AND CENOZOIC THERMOTECTONIC HISTORY OF THE LACHLAN FOLD BELT, AUSTRALIA Paul B. O'Sullivan. Barry P. Kohn, Dave A. Foster, and Andrew J.W. Gleadow Australian Geodynamics Cooperative Research Center School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083, Australia The Lachlan Fold Belt present in New South Wales comprises the southern part of the eastern Australian Tasmanides and is bounded to the west by the Mesozoic to Cenozoic Murray-Darling Basin and to the east by the Permian to Jurassic Sydney Basin and the Tasman Sea. The literature abounds with references describing the timing and the tectonic development of the Lachlan Fold Belt. Most have proposed that deformation responsible for the regional shortening (estimated up to -60%) and formation of structures throughout the fold belt occurred along a convergent margin setting throughout the Early Silurian to middle Carboniferous. The presence of "seemingly undeformed" Late Carboniferous granites, eg. the Bathurst Batholith within the eastern Lachlan Fold Belt, suggest that all deformation ceased prior to their intrusion. The subsequent post-Carboniferous thermotectonic history of the Lachlan is largely unconstrained due to the lack of any low-temperature geochronological data or adequate geologic control.

In an effort to constrain the post Middle Carboniferous thermal and tectonic history of the Lachlan Fold Belt, a growing data base of apatite fission track analyses from exposed sedimentary and granitic rocks is being generated. The fission track results suggest that the rocks within the fold belt have experienced two distinct episodes of rapid km-scale denudation since the middle Carboniferous, subsequent to when the deformation within the fold belt is believed to have ceased.

Data presented here suggest that the first episode of denudation occurred throughout much of the northern Lachlan Fold Belt in New South Wales during the Early Triassic at ~245±10 Ma. This event is possibly in response to the Hunter-Bowen Orogeny which is known to have affected the New England Fold Belt to the north and the Sydney-Bowen Basin to the east at approximately the same time, but the effects of which have previously not been recognized within the Lachlan Fold Belt. The relationship of denudation to structural style is not clear as denudation could have occurred in response to compressional reactivation of earlier structures or to more regional rock uplift. However, data from the Bathurst Batholith in the eastern Lachlan Fold Belt suggest that it experienced an episode of compressional deformation during the early Mesozoic resulting in the repetition of paleo-isothermal surfaces recorded by the fission track data.

The second episode occurred during the middle Cretaceous at 95±5 Ma, possibly in response to the onset of continental extension in the Tasman Sea at -96 Ma. Uplift at this time resulted in km-scale denudation over much of the southeastern highlands of Australia and may have been caused by underplating inward of the rift.

These results suggest that the Lachlan Fold Belt has remained technically active long after the last recognized deformational event in the region during the middle Carboniferous. Dating of apatite samples has provided important constraints on the timing and amount of denudation in response to Mesozoic and Cenozoic uplift and erosion. This information provides previously lacking details on the post-middle Carboniferous cooling history of the region and suggests how that history may be interpreted in terms of specific deformational events.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

STRUCTURAL AND STRATIGRAPHIC RELATIONSHIPS OF THE PADBURY GROUP, GLENGARRY BASIN, WESTERN AUSTRALIA - IMPLICATIONS FOR TECTONIC HISTORY Sandra A. Occhipinti, Cees Swager and Franco Pirajno Geological Survey of Western Australia, 100 Plain St East Perth 6004

The Palaeoproterozoic Glengarry Basin lies in the southern part of the Capricorn Orogen, a major zone of deformed, low- to high-grade metamoiphic rocks and granitoid intrusions formed during continental crastal collision between the Pilbara and Yilgarn cratons about 2000 - 1700 Ma. Preliminary work suggests the Glengarry Basin may be subdivided into four sub-basins, with different stratigraphic and structural histories.

The Padbury sub-basin contains the Padbury Group, the highest stratigraphic unit of the Glengarry Basin, and is separated from the remainder of the Glengarry Basin, by faults. The presence of these faults can be identified from regional cut-outs in the stratigraphy of the Bryah-Padbury Group successions, shear zones between the Padbury Group and the Narracoota Formation of the Bryah sub-basin, and from aeromagnetic images. Many of the faults are thought to be thrusts which developed early in the deformation history, implying that the Padbury Group is largely para-autochthonous. These faults have been folded and reactivated through time, further complicating the structural and stratigraphic analysis of the area.

The Padbury Group is subdivided, in ascending order, into: Labouchere Formation, Beatty Park formation, Wilthorpe Formation, Robinson Range Formation, and Millidie Creek Formation. The Wilthorpe Formation and the Beatty Park formation are lateral eqivalents. The Beatty Park formation's lower contact with the underlying Labouchere Formation appears conformable and its upper contact with the Robinson Range Formation appears both conformable and gradational, with chert, minor banded iron formation, and mafic epiclastic rocks intercalated with sericitic shales typical of the Robinson Range Formation. The Wilthorpe Formations contact with the underlying Labouchere, and overlying Robinson Range Formation appears gradational. The Millidie Creek Formation appears to conformably overlie the Robinson Range Formation.

Previous workers recognised that the Padbury Group consists of both siliciclastic (Labouchere and Wilthorpe Formations) and chemical sedimentary rocks (Robinson Range and Millidie Creek Formations). Our work has shown that these deposits are intercalated with minor mafic lavas and mafic epiclastic rocks. Fine-grained mafic epiclastic rocks occur within the Millidie Creek Formation, and coarse and fine-grained mafic epiclastics within the Beatty Park formation, are at the same stratigraphic position as the quartz-pebble Wilthorpe Formation. The mafic epiclastics of the Beatty Park formation may reflect local derivation from a nearby, uplifted fault block of mafic volcanics of the Narracoota Formation.

Martin (1994) suggested that the Padbury Group was deposited in a foreland basin developed on the northern margin of the Yilgarn Craton. However, he did not account for the presence of mafic volcanics within the basin succession. Possible explanations for the presence of these mafic volcanics include: • they represent a structural window into the underlying Narracoota Formation • they are the remnants of a thrust sheet of Narracoota Formation • they represent syn-depositional mafic volcanism in the Padbury Group It is apparent that faults now observed within the Padbury sub-basin developed early in the deformation history, and in some cases may have been coeval with the deposition of the Padbury Group. Reactivation and deformation of these faults and shear zones may have provided pathways for fluid movement and possibly the formation of epigenetic mineral deposits. Examples of epigenetic mineral deposits in the area are the Fortnum gold mine, in the Padbury sub-basin, and the Peak Hill gold mining area, in the Bryah sub-basin.

REFERENCES Martin, D.M., 1994. Sedimentology, sequence stratigraphy, and tectonic setting of a Palaeoproterozoic turbidite complex, Lower Padbury Group, Western Australia:f7^pwfe/. PhD thesis,The University of Western Australia, 194p.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOCHEMICAL VARIATION WITHIN THE EARLY PROTEROZOIC SALLY DOWNS TONALITE PLUTON FROM THE HALLS CREEK OROGEN, W.A. Masatsugu Ogasawara Geological Survey of Japan, Tsukuba, 305, Japan Granitoids are one of the major components of the continental crust. Therefore, for the understanding of the evolution of the continental crust, it is important to know processes for the generation, ascent, and emplacement of the granitoid magma. A variety of granitoids are found in the Early Proterozoic Halls Creek Orogen, Kimberley region, in northern Western Australia. In addition to a large volume of high K2O granitoids which are typical of the granitoids in north Australian Proterozoic provinces, the Halls Creek Orogen contains tonalitic-trondjhemitic rocks, indicating some differences of tectonic setting in the orogen from that of the other north Australian Proterozoic provinces. A small tonalite pluton, the Sally Downs Tonalite pluton, has been selected for detailed penological and geochemical study to examine geochemical process during emplacement, and to provide constraints for the geochemical processes of ascent and generation of tonalitic magma. It occurs south of the Ord River, near the Sally Downs bore. The pluton is 12km long, and 2.3 to 6km wide with a northnortheasterly prolongation. The central portion of the pluton bulges westward (Fig. 1). Contacts of the pluton tend to be concordant with foliation in the country rock and in the pluton. Numerous xenoliths and screens of country rock are present in the pluton, suggesting that the present outcrop is near the irregularly shaped top of the pluton. The pluton is characterized by the ubiquitous presence of mafic microgranular enclaves which are generally fine grained and quartz dioritic in composition.

Compositionally the pluton varies from hornblende-rich tonalite to hornblende-free biotite tonalite. The hornblende-free biotite tonalite is commonly found in the west of the pluton. Although there is a large scale compositional variation in the pluton, the tonalite is homogeneous on an outcrop scale, except for compositional layering. The tonalite is primarily composed of plagioclase, quartz, biotite, and hornblende.

Accessory minerals are epidote, sphene, apatite, zircon, allanite, and opaque minerals.

A total of 146 samples have been analyzed for their geochemical composition. Systematic sampling (one sample from each intersection of a 600m grid over the pluton) was conducted to obtain areal geochemical variation within the pluton. Large variation in Si02 contents is found in the samples from Sally Downs Tonalite, ranging from 55.7 to 68.8%, but the majority of the SiC>2 contents fall in the range from 59 to 62% (mean value of Si02 contents is 61.2%). The tonalite is metaluminous and is characterized by high Na20 and low K2O contents. Trace element data indicate the following characteristics: (1) high Sr (>400 ppm) and very low Rb/Sr ratios; (2) enriched LREE, depleted HREE, and no Eu anomaly; and (3) low Y (<30 ppm). The major and trace element data suggest that the Sally Downs Tonalite has typical geochemical characteristics of the high Al-type tonalite of Barker (1979).

Areal geochemical variation maps of the Sally Downs Tonalite pluton are constructed for major and trace elements. The SiC>2 map (Fig. 1) indicates that high values are present in the western central part and in areas along the western and northern margins of the pluton. The high SiC>2 areas are considered to represent the final crystallisation part of the tonalitic magma. Other areal geochemical variation maps also support this interpretation. The results suggest fractional crystallization from a primary basic tonalitic magma could be a possible process for control the geochemical variation found in the Sally Downs Tonlalite pluton.

Low Y and high Sr contents of the Sally Downs Tonalite suggest that the estimated primary tonalite magma was formed by partial melting leaving a residue consisting of garnet and possibly pyroxenes, but not plagioclase. The primary basic tonalitic magma must be derived by dehydration partial melting of amphibolite under relatively high pressure. It is inferred that the primary tonalitic magma has been generated by partial melting of subducted oceanic crust.

Fig. 1. Area! variation of Si02 in the Sally Downs Tonalite pluton This model requires convergent tectonics in the Halls GNHrGreat Northern Highway, SDBiSally Downs Bore C r e e k Orogen at the end of the Early Proterozoic.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

H. FILHOL'S "GEOLOGY OF CAMPBELL ISLAND" (1885) Robin L. Oliver, University of Adelaide, South Australia 5005

Fihol's account of the geology of Campbell Island was part of the scientific report of the "Mission de l'isle Campbell", which was part of the Academie des Sciences, Paris, Commission du Passage de Venus, in 1874.

A summaiy of these geological observations is as follows.

Basement schists with abundant quartz veins were ascribed to the Upper Silurian and correlated with similar lithologies considered to be of this age in New Zealand. The presence of silver, copper, antimony and gold in the quartz veins of the New Zealand rocks led Filhol to follow up the reported occurrence (by earlier seafarers?) of minerals on the island and closely examine the Campbell Island schists for favourable indications of these, but without success.

After the Silurian, as recounted by Filhol, the basement was raised above sea level and remained emerged till the Cretaceous or Eocene, then there was unconformably deposited, terrestrially (?), on the schists a 2-3 m thick, sandstone formation containing some pyrite. Following this, the schist and sandstone were submerged to a great depth below the sea and there accumulated a fine-grained, thin-bedded limestone with abundant forams; the only macro fossil seen was a single fragment of Pentacrinus. The limestone forms cliffs 50 m high and as the bedding is nearly horizontal the limestone is at least that thickness. Age possibilities, according to Filhol, are U. Cretaceous, U. Eocence or L. Miocene.

The limestone is covered by the products of numerous volcanic eruptions which were thought to have been from several points of emission, during the Pliocene. The initial ejectamenta were marine deposits of palagonite tuffs and cinder aglomerates containing numerous mollusc fragments, and which attained a thickness of some 30 metres.

Vulcanicity continued in the form of a succession of andesitic and basaltic lava flows which attained a total thicknesss.of some 400 metres. Mineral constituents of the lavas seen under the microscope and described by Filhol include labradoritic and anorthitic feldaspar, pyroxene and p£ridot(olivine), all abundantly phenocrystic. Numerous basaltic dykes with similar mineralogy cut the lavas and manifest the final stages of volcanic activity.

Filhol makes reference to the power of marine erosion eating into the west coast and the liklihood of the island being divided into two rock mases by this process.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE COLOUR OF CHARNOCKITES - REVISITED Robin L. Oliver University of Adelaide, South Australia 5005

The typical dark colour of the supposedly high metamorphic grade charnockite is due to the dark colour of the quartz and feldspar. It has been suggested that this is due to a network of ferruginous veins cutting the quartz and feldspar. The iron can be leached out by hydrochloric acid and the same effect can be produced by weathering conditions of which rainfall is a component.

Previous workers have identified such veinlets as chlorite in charnockites in southern India and considered the veinlets mineralogically the same as an alteration product of orthopyroxene in the same rock. In other samples orthoamphibole has been diagnosed as the alteration product of the orthopyroxene.

A hydrated mineral phase manifesting the breakdown of metamorphic orthopyroxene within "arrested charnockite" from Kurunegala, Sri Lanka, is considered due to a lowering of temperature following the charnockitisation reaction, and facilitated by the retention of some H2O in the CO2 dominant interstitial fluid following the loss of most of the H2O into associated partial melt.

The major-element chemical composition of the retrograde product of orthopyroxene in "arrested charnockite" from Kurunegala, plus the composition of gedrite, are shown in Figure 1. The analyses are all fairly similar.

Analyses of chlorite? veins cutting feldspar in charnockite from near Trivandrum, south India, and Kabbaldurga, southern India, are shown in Figure 2. The difference between these "chlorite" analyses and that of the alteration product of orthopyroxene (Fig 1) is not great, suggestive of possible mineralogical similarity, and perhaps of ciystallisation under similar conditions.

If this is so it follows that the formation of the chlorite? veins causing the characteristic dark colour of the charnockites is a retrograde development, synchronous with the orthopyroxene breakdown, due to cooling.

The dark colour of the charnockites (acid and intermediate), due to the dark colour of the quartz and feldspar, is generally regarded as an essential criterion of the definition of the term but its significance with regard to charnockite genesis is enigmatic. plus gvdritt (0. R ft Z.)

Fig 1 chlorlt* veins Trivnndrum end Kabbaldurga

Fig 2 331

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 POSSIBLE ODP TARGET: SCOTT PLATEAU, FAR EASTERN INDIAN OCEAN. Bradley Opdyke* and Michael Bird , * Department of Geology, The Australian National University, Canberra ACT, 0200; ^Research School of Earth Sciences, The Australian National University, Canberra ACT, 0200 Monitoring the response of the "throughflow" of water from the Pacific into the Indian Ocean to glacialinterglacial climate changes is ranked as a high priority for many international climate programs. Suitable coring sites for tracing changes in throughflow are not common. Desirable characteristics of potential sites are: relatively shallow water, from the lysocline to approximately 2000m; high productivity, so that high resolution records may be obtained; and a location that is affected by changing the rate of water flow through the Indonesian archipelago. We believe that the Scott Plateau fits these criteria.

The Scott Plateau is located on the western edge of the Timor Sea, approximately between 11 and 14 S and 119 and 124 E, there are a number of relatively shallow (between 2000 - 3000m in depth) plateaus and basins. R.V. Rig Seismic obtained two cores in the area (Leg 130 GC1 and GC2), at 2400m and the other at 2000m. Local sedimentation rates vary from 5 cm/ka to 10 cm/ka. These cores represent sedimentary records 70-100,000 years long. Carbonate sediment in these cores is remarkably well preserved. High sedimentation rates are enhanced by wind driven divergence associated with the southeast trade winds which blow perpendicular to the northwest shelf of the Australian continent and possibly local divergence when the ITCZ straddles the region.

Changing productivity in the deeper core is recorded in the density record of the core, and clearly delineates isotope stages 1, 2, and 3, presumably due to the change in radiolarian abundance. Stable isotopic results indicate that accumulation rates double during glacial phases of climate. 5 ^ 0 data from these cores are virtually indistinguishable from comparable stable isotope records from the Banda Sea. Perturbations in § C records show possible links to high resolution data from the Atlantic Ocean. The location is not only desirable for relatively high resolution marine climate records but monitoring aeolian fluxes from the northern portion of the Australian continent. #

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The Scott Plateau has been accumulating pelagic carbonate sediments since the Cretaceous. This location is therefore strategically positioned to record, on a longer timescale, the evolution of the "Indonesian Gateway", and its closure. We believe this area has great potential for future ocean drilling. Ocean drilling within Indonesian waters is of great interest to Earth scientists, but has been stymied by political problems. The Timor sea sites (Scott Plateau, Wilson Spur, Lombok Ridge and Roti Basin , Hinz et al., 1978) are in Australian waters politically, but are eastern Indonesian geographically, where the drilling ship has free access.

REFERENCES Hinz et al., 1978 BMR J. Aust. Geol. Geophys., 3, 319

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996

PHYSICAL REQUIREMENTS OF A TRAP? Alison Ord1 ^SIRO Exploration & Mining, PO Box 437, Nedlands, WA 6009

The histories of physical conditions leading to the formation of mineral deposits may be counter-intuitive. We describe some examples which should cause us to think more deeply about the processes involved in the formation of world-class ore deposits.

The concept of fluid sinks being regions of continually low mean stress and fluid pressure is appealing but not necessarily always correct. Mandl (1988) shows a simple example (Fig. II, 10-1) in which fluid flows UP the fluid pressure gradient rather than down. This is a result of gravity being included in the system; fluid flows normal to contours of hydraulic head and down the gradient of head, where the hydraulic head is the height to which the pore pressure at a point of interest will support a fluid column plus the height of that point above some arbitrarily selected datum elevation. This distinction is of utmost importance in the crust.

We show here some scenarios which demonstrate both intuitive and non- or even counter-intuitive situations.

For example, the intersection of transverse zones of focussed fluid flow form a trap below a lower permeability layer. The focusing of fluid flow along these zones is variable in space and time so that much of the surrounding region may form a source at some time during the deformation history, and therefore be scavenged for minerals, a potentially more economic scenario than if just the one region is the source. There are also distinct zones within which significant fluid mixing occurs, including mixing of meteoric with deeply derived waters.

The continuity in time of regions as 'source' or 'sink' may be explored through plotting histories of total fluid flux into or out of a volume of material, histories of hydraulic head, pore pressure, mean effective stress, or any portion of the stress or strain tensors; or even plotting histories of these parameters against each other for comparison. In this manner, one may find that a region which appears to be a sink at some moment may have formed a source at some earlier stage of the deformation.

We have the opportunity to explore qualitatively and quantitatively why various hypotheses for the formation of world-class ore deposits are or are not fundamentally useful.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996

EXPOSED LATE-ARCHAEAN BASEMENT TERRAINS IN THE GRANITES-TANAMI REGION, NORTHERN TERRITORY

R.W. Page1. S.-S. Sun1, D.H. Blake1, D.R. Edgecombe2, D.P. Pearcey3

1 Division of Regional Geology & Minerals, AGSO, GPO Box 378, Canberra, A.C.T. 2601 ^Dominion Mining Limited, PO Box 255, Eastwood, S.A. 5063 3 PNC Exploration (Australia) Pty. Ltd., 26 Lyall Street, South Perth, W.A. 6151

Two late-Archaean granitic gneiss terrains, close analogues of the Rum Jungle and Nanambu Complexes (in the Pine Creek Inlier), have now been identified in The Granites-Tanami region, Northern Territory investigated as part of the Kimberley-Arunta NGMA project. Zircon U-Pb geochronology, together with Nd isotope model ages and whole-rock geochemistry, demonstrate the antiquity of these largely concealed terrains. The few small exposures are vestiges of Archaean basement upon which subsequent Palaeoproterozoic basins were developed.

Geological setting The Billabong complex (informal name) is east of The Granites, and its banded granitic (quartz-feldsparbiotite) gneisses correspond to a distinctive 'mottled' magnetic pattern that contrasts with linear magnetic patterns of adjacent Palaeoproterozoic Mount Charles Beds. This is evident from imaged aeromagnetic data for The Granites-Tanami region acquired by AGSO in 1993.

The Browns Range Dome straddles the NT-WA border northwest of Tanami, and about 250 km northwest of the Billabong complex. At its southern margin it includes scattered exposures of basement gneiss, granite, and arkosic and conglomeratic metasedimentary rocks.

Geochemistry, Sm-Nd, and U-Pb SHRIMP systematics The Billabong complex granitic gneiss and the Browns Range Dome granites are close in composition to some late-Archaean I-type granites in the Nanambu Complex of the Pine Creek Inlier. In addition, they have Nd TDM model ages (indicating average crustal residence time) of 2516 to 3297 Ma, indicative of a major contribution from Archaean source rocks.

Zircon U-Pb SHRIMP data reveal a complex late-Archaean history for granitic gneiss of the Billabong complex. The interpreted age for the protolith, 2514±3 Ma, is derived from discrete igneous zircon grains as well as rims surrounding inherited grains. This is the best estimate for the igneous age of the granitic gneiss, indicating that it is part of a crustal domain formed in the late Archaean. Inherited zircon suites at about 2550 Ma and 2530±4 Ma are interpreted as relicts / xenocrysts from lower crustal source regions.

Zircon U-Pb data for grey gneiss and leucogranite from the Browns Range Dome are also complex, and reflect a protracted Archaean history (at -3400 Ma, 3140 Ma, 3040 Ma, and 2700 Ma) for the source rocks. The leucogranite data indicate a protolith age of 2510±22 Ma, indistinguishable from 2504±4 Ma, the age of the main zircon population in the grey gneiss. Major recycling and metamorphism of this late Archaean crust took place in the Palaeoproterozoic.

Metasediments in the Browns Range Dome contain detrital zircons ranging in age from 2507±22 Ma to 3460 Ma. Clear age groupings at 3050±30 Ma, 3140±10 Ma, and -3410 Ma mimic ages for inherited zircon in the nearby grey gneiss and in younger leucogranite that intrudes the arkose. There is an additional detrital population at -3270 Ma. These data reinforce the view that rocks as old as early-mid Archaean might be components of unexposed lower crust of northern Australia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 GEOCHEMISTRY, TECTONICS AND THE PERIDOTITES OF THE NORTHEASTERN SOLOMON ISLANDS Ian J. Parkinson R.. J. Arculus K E. McPherson , R. A. Duncan , and R. L. Stanton *GEMOC, Department of Geology, Australian National Universirty , ACT 0200 , AUSTRALIA College of Oceanography, Oregon State University, Corvallis, OR 97331, USA ^Department of Geology & Geophysics, University of New England, Armidale, NSW 2351, AUSTRALIA 1

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The Solomon Islands have a complicated history of subduction zone polarity reversals, oceanic plateau collision and obduction, and active arc magmatism. In the Paleogene, subduction occurred of the Pacific Plate westward beneath an eastward-facing arc ("the "Vitiaz" arc-trench system (VATS)). During the Neogene, collision, docking and obduction of the Ontong Java Plateau (OJP) occurred along the Solomons portion of this arc-trench system, triggering polarity reversal. The OJP is one of the largest flood basalt outpourings known, and at least part of this magmatic pulse occurred during the so-called "Cretaceous Super-Plume" event. The Deep Sea and Ocean Drilling Projects have recovered two major age groupings of pillow basalts (-120 and 90 Ma) from relatively shallow penetrations at geographically widely distributed locations on the Plateau. In the past few years, international collaborative efforts together with the Solomons Survey have targeted the northeastern Solomons (e.g., Malaita, Makira, and Santa Isabel) as a region where potentially more extensive (spatial and temporal) sequences of the OJP might be sampled compared with the drilling recovery. The collision event also seems to have exhumed deeper portions (e.g., plutonic and upper mantle) of the OJP and the old forearc of the VATS. Pillowed basalt slices of the Ontong Java Plateau (OJP) are exposed in extensive river sections on the northeastern flanks of the island of Santa Isabel (SI). New ^Axfi^Ai results for the basalts give ages of 122 Ma and 90 Ma consistent with the two major groups recovered by ocean drilling of the OJP. On SI, the older basalts outcrop southwest of the younger sequence. Within the 2 major age-groupings however, we recognise 4 geochemical groups compared with 3 groupings recognised in the drilling recovery - these occur as discrete stratigraphic groups in the field. For the drilled recovery, a relatively tight grouping of £Nd and S r / S r exists (+4 to +6, and 0.7035 - 0.7042 respectively, J. Mahoney pers. comm.) which are less and more radiogenic than Pacific MORB respectively. Preliminary data for the samples from SI range to slightly higher £Nd of ~ +7. On Vitora island off the southeastern tip of SI, a sample of the plutonic sequences of the Plateau are exposed as troctolites (~Fo8i, An75) and gabbros.

A wide range of ages have been determined for basalts outcropping on the southwestern flank of SI including - 63 and 45 Ma sequences, and transitional MORB-backarc pillow basalts with an age of 34 Ma at Tanabusu along the west coast of SI. A NW-SE-striking major fault running most of the length of SI (Kaipito-Korigole Fault) is clearly a terrane boundary.

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Interleaved with the volcanic sequences on SI are fault-bound slices of ultramafic rocks, some of which are virtually unserpentinised. Fresh ultramafic blocks also occur in fault-bounded serpentine diapirs on San Jorge (off the south coast of SI), analogous to occurrences in the Izu-Bonin-Mariana forearc. Whole rock trace element and /O2 calculations indicate that individual fault slices contain ultramafic rocks with affinities to variously mid-ocean ridge, island arc, or plume-related settings. A melt-metasomatised abyssal peridotite thrust sheet outcrops on southeastern Choiseul (Siruka Ultramafics) to the northwest of SI. We conclude that the mantle section of the OJP and fragments of a subarc mantle - possibly formed in a forearc setting during the pre-Pliocene, westerly-directed subduction of the Pacific Plate - outcrop on SI and San Jorge.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GULF OF CARPENTARIA: A NEW TARGET FOR EXPLORATION IN A STACKED BASIN REGION 1

Virginia L. Passmore1, Tun U Maung1, A1R. G. Gray2, and Peter Wellman3 Bureau of Mineral Resources, PO Box E l l , Queen Victoria Terrace, Parkes, ACT 2600 department of Minerals and Energy, 61 Mary St, Brisbane, Qld 4001 3 Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2600

The eastern part of Australia is known to have several Palaeozoic and Mesozoic stacked basins. They are primarily recognised onshore where most exploration has been undertaken. Recent reinterpretation of seismic in the shallow water, sparsely explored, frontier area of the Gulf of Carpentaria has identified a stacked basin province in the eastern part of the gulf, and delineated a new infrabasin below the Jurassic-Cretaceous Carpentaria Basin.

The Bamaga Basin is the lowermost basin in the stacked basin sequence in the eastern Gulf of Carpentaria. It is an untested and unexplored asymmetrical sag basin that extends northward across the international boundary with Papua New Guinea and Irian Jaya, does not appear to extend onto Cape York. The southern part of the Bamaga Basin, on Australia's side of the boundary, covers over 30,000 sq km. The basin contains overs 1.8 seconds of gently folded and faulted sedimentary rocks. The age and lithology of the basin sequence is uncertain due to the absence of well data and apparent high velocities for the sedimentary section. A likely Palaeozoic and possible Triassic age is inferred from pre-Jurassic sedimentary sequences in nearby basins.

Clastic and carbonate rocks with possible source and reservoir potential is also inferred from data on surrounding basins. The basin contains large untested structural closures and potential stratigraphic pinchout plays that merit further exploration.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996 AERIAL MONITORING OF RADIOACTIVE CONTAMINATION IN EASTERN EUROPE. R. Pavlik and G. Street World Geoscience Corporation

The ENMOS spectrometer was developed with special focus on resolution of spectra in low energies under 1 MeV. This is provided by precise control of spectrum linearity and special anticoincidence facility as well, both running in real time using advanced transputer technology. There was flown a small area about 4 x 8 km near Domasin with line spacing 200 m and terrain clearance of 60 m. The recorded spectra show very good resolution in low energy part with readable peaks of Csl37 at 662 KeV and Bi214 at 609 KeV as well.

In 1992 and 1993 a number of surveys were flown in the Czech and Slovak Republic to map the extent of radioactive contamination from Uranium mines and Nuclear Power Stations.

In the Plounice River Basin of the Czech Republic a survey was flown to follow surface radiation contamination of the uranium mine tailings and the river basin south and west of a major disused uranium mine.

In the Slovak Republic an area around a major nuclear reactor was flown to map the extent of contamination following a nuclear accident.

In 1993 test flights over an area near Domasin in the Czech Republic were flown in order to prove capability of airborne spectrometer and navigational equipment ENMOS for monitoring of fall-out deposition of Cs 137 as well as for testing new technology of data processing using mathematically derived response functions of ENMOS equipment. This new methodology was developed by World Geoscience subsidiary Picodas Prague in co-operation with the Faculty of Nuclear Science in Prague.

A set of approximately 5500 spectra was processed by the method, which utilises a set of detector response functions computed for various radioisotopes and for various altitudes of detector above terrain. The Monte Carlo code for calculation of the energy deposition spectra was used for this purpose. The model follows the random walks of primary photons, all secondary photons, electrons and positrons. Photoefect, Compton and coherent scattering, pair production, positron annihilation and bremsstrahlung production were simulated.

Activities of radioisotopes was processed from measured spectra and detector response functions by the least square and iterative deconvolution methods. In addition to the set of spectra measured on lines there were also sets of spectra measured on one place. These set of 13 spectra were used for comparison of measured and theoretical spectra.

The method of detector calibration based on the mathematical model of experimental setup and on computer simulation of particle transport has been proven to be very flexible, accurate, and in general, exceeding the old method based on experimental calibration using concrete pads. The calculated detector response functions fit well with the measured spectra in the high energy region. In the low energy region, the differences are not sometimes negligible, but the resulting systematic error is not too high.

The results of these surveys show the ability of the ENMOS system to map low levels of contamination and to map changes in contamination due to natural or man-made activities.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LASER ABLATION ICPMS: APPLICATIONS TO DIAMOND EXPLORATION N.J. Pearson1. M. Norman1, A. Sharma1, W.L. Griffin 1 ' 2 1. GEMOC, School of Earth Sciences, Macquarie University NSW 2109 2. CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 2113

Recent advances in understanding pedogenesis as dynamic, process-related phenomena have been achieved by the application of microbeam techniques to the determination of trace elements and isotopic ratios. Just as the high spatial resolution of the electron microprobe revolutionized mineralogy and contributed to the development of quantitative petrology, using major and minor elements, so the coupling of LAM (Laser Ablation Microprobe) to an ICPMS (Induced Coupled Plasma Mass Spectrometer) is now poised to do the same with trace elements.

The development of the LAM-ICPMS as a powerful analytical technique in geochemistry is comparatively recent. Most simply, a focussed laser beam is used to ablate small amounts of solid material which are transported into an ICPMS for trace element and/or isotope analysis. Early laser ablation systems were designed primarily as solid sampling devices, designed to overcome the problems of getting rocks and minerals into solution. Recognition of the micro-analytical potential of the technique has been accompanied by rapid changes in laser technology, so that spot sizes of less than 10 jum are now achievable, depending on laser wavelength, energy and sample type. The sensitivity of the ICPMS is an additional factor that governs the minimum spot size for quantitative analysis.

The laser ablation system at Macquarie University was designed and installed by Drs Simon Jackson and Henry Longerich of Memorial University, Newfoundland. This system includes a Continuum Surelite 1-20 Q-switched Nd-YAG laser with a fundamental wavelength of 1064 nm (IR) and frequency doubling crystals which produce 532 nm (visible) and 266 nm (UV) wavelengths. Ablation yield is improved for minerals with low abundances of transition elements using the frequency quadrupled UV wavelength because of higher absorption. The frequency range of the laser is 1-20 Hz (laser pulses per second) and maximum energy is approximately 7 mJ per pulse for 266 nm at 20 Hz. Typical operating conditions for the quantitative analysis of silicate minerals and glasses involve energies of 0.1 to 1 mJ per pulse, producing pit diameters of 20 to 50 \im. For grain mounts of effectively infinite thickness, ablation times of 2-3 minutes are achievable at low repetition rates (< 5 Hz), before the analyte signal intensity decreases due to defocussing of the laser.

The ICPMS is a Perkin-Elmer ELAN 5100 and in our experience the instrument operating conditions for laser ablation analysis are comparable with those for analysis by solution nebulization. Parameters such as nebulizer gas flow and RF power are varied to optimize sensitivity, background intensities and oxide production. Routine analyses are performed with up to 30 analyte masses: this is not the maximum limit on the number of masses for an analysis but because the analyte signal is transient, precision will be reduced for larger numbers of analyte masses.

Correction of the raw data to give quantitative results involves a calibration procedure that requires (1) an external standard (such as the NIST glasses) for relative element sensitivities and (2) an internal standard to correct for the ablation yield. The internal standard is used to normalize the isotope ratios of the unknown and standard to absolute abundances. Typically this is a major element, such as Ca, Ti, Fe or Mg which can be determined by electron microprobe or some other independent technique. Based on these operating conditions and calibration procedure, detection limits of 0.1 to 0.01 |ig/g are achievable. Longterm precision of <5% has been demonstrated through replicate analyses of standards and homogeneous individual samples.

Proton-microprobe analysis of trace elements in garnets is now widely used in diamond exploration worldwide.

Analysis of megacryst garnets and concentrate garnets derived from diamond exploration show excellent reproducibility of the proton-microprobe data, while the LAM analysis adds significant new information, including rare earth elements and a number of other useful elements such as Sr, Sc, V, Co and Hf. The usefulness of this extra information in the exploration context is only beginning to be explored, but several significant observations can be made. In particular, the shapes of garnet REE patterns carry information about metasomatic processes related to diamond preservation, and particular types of patterns have thus far only been observed in garnets from diamondiferous kimberlites.

The LAM-ICPMS represents a rapid, cost-effective method for in-situ trace element analysis. Compared to ion beam instruments it requires a smaller total capital investment (approx. $0.5 million versus up to $2 million) and can produce data more rapidly and for a wider range of elements. This added information content will require a major interpretive effort, but will enhance the usefulness of trace element data to explorationists.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 INTEGRATED POTENTIAL FIELD DATA SETS ON THE NORTH WEST SHELF: A FRAMEWORK FOR BASIN ANALYSIS P. Petkovic . H.MJ. Slagg & J.B. Willcox Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 1

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Since the early 1970s, a large quantity of magnetic and gravity data have been acquired on the continental margin of northwest Australia by the Australian Geological Survey Organisation (AGSO, formerly the Bureau of Mineral Resources) and in the deep ocean basins by sundry Australian and international institutes. To date, these data have not seriously been used to set a framework for integrated basin analysis on the North West Shelf due to the disparate nature of the platforms and equipment used to acquire the data and the variety of techniques used in their processing. The data are particularly well-suited to studies of the fabric of the basement undeipinning the basins (using gridded data) and to analysis of cnistal profiles by modelling 2dimensional data along AGSO's many deep-seismic profiles through the region.

In 1994, AGSO's Marine, Petroleum and Sedimentary Resources Division initiated a project to integrate the different data sets (including gravity data over the adjacent onshore area, and satellite gravity data offshore) and produce gridded data sets that can be used to provide a framework for a major study of the sedimentary basins of the North West Shelf. The processing was carried out by Ark Geophysics Ltd. The area covered by the compilation extended from 8°-24°S and 110°-138°E and the processed data were supplied on a 2.5 x 2.5 km grid. Processing included mistie correction and computation of magnetic, free-air, and Bouguer anomalies.

Interpretation of the potential field data (particularly free-air and Bouguer anomalies) is being approached in two ways. Firstly, analysis of the grided data can provide valuable information on the structural grain and the character of the basement underpinning and compartmentalising the Phanerozoic basins and the intervening, more stable crustal blocks. For instance: • Major gravity trends reflecting mobile belts (eg the Halls Creek Mobile Zone) can be seen to be continuous with major features that have controlled the boundaries of sedimentary basins (eg the Petrel Sub-basin).

The interpretation of these trends enhances our understanding of how the linked basin systems of northwest Australia have evolved through the Phanerozoic.

• Recognition of gravity provinces, characterised by a distinctive 'grain', can permit the extent of major crustal blocks to be assessed, even where they are deeply buried beneath sedimentary basins. For example, gravity data suggest that the Precambrian Kimberley Basin terrane may underlie much of the Browse Basin. This contrasts with the sharp boundary between the Carnarvon Basin and the Pilbara Block, which indicates that basin development was localised by the presence of a major crustal suture.

The second interpretive approach involves two-dimensional modelling of potential field data along selected AGSO deep-seismic transects across the continental margin. This modelling is being used to develop a understanding of the variation of crustal structure across the margin. This can then be used as an input to geodynamic modelling (being carried out by the Australian Geodynamics Cooperative Research Centre), and hence improve our understanding of margin formation processes. An example of this can be taken from the Enderby Terrace - Dampier Sub-basin - Rankin Platform, where modelling of the gravity, deep-seismic, and sparse velocity data indicate that, firstly, the Carnarvon Basin was formed by a major episode of lower crustal thinning and, secondly, the development of the prime hydrocarbon source depocentre in the sub-basin has been controlled by the location of the most radical crustal thinning.

Detailed analysis of the new potential field data sets, when integrated with the interpretation of deep-seismic data and soon-to-be acquired crustal velocity data, is expected to lead to significant new insights into margin formation models and our understanding of the exploration potential of the North West Shelf in the future.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 AREA SELECTION USING GEOLOGY G. Neil Phillips Great Central Mines, c/- 1 Coppin St, East Malvern, 3145, Vic, Australia

Optimal area selection: the most efficient exploration programme immediately has a reduced probability of success, unless it incorporates optimal area selection. Area selection operates on many scales including country (often a nongeological decision), province (influences include commodity, community, and availability of tenure), district, tenement and target Geology plays a major part in district, tenement and target selection, and the process is necessarily iterative.

Geological input into area selection can be discussed in terms of two conceptual end-members: matching features already known at existing deposits by using descriptive models, and genetic modelling to generate those factors which should be favourable based upon more theoretical concepts. Exploration will be greatly enhanced if both these endmembers can be applied.

Within a mineralized province, matching features can be very effective (whether for the right or wrong reasons) in finding further ore deposits, and has been very effective in many programmes in the past. One of its limitations, however, is that all deposits are different, and there is an infinite number of characteristics that can be used in a matching process: as a result there is always an explicit or implicit element of genetic modelling that is used to determine which features of the descriptive model deserve emphasis, and which are accidental or peripheral. The choice of which features to "match" is paradigm-dependent and exposed to error if the paradigm hasflaws.This may not be too serious whilst looking for repeats within a known province, but is especially relevant when extrapolating to new provinces. Two examples can be drawnfromArchaean gold.

The Yandal belt of Western Australia is an Archaean greenstone belt with several new gold mines including Bronzewing and Jundee. The search by Great Central Mines for further deposits in this belt relies upon a general model for gold formation that dictates and constrains what factors are emphasized during exploration. This genetic model is developed from the concept of an auriferousfluidfroma deep source that dissolves gold as a sulphur complex, channels the fluid through favourable structural geometries, and deposits gold through processes includingfluid- wallrock interaction and immiscibility (developedfromPhillips and Groves, 1983, and later adaptations). Areas of anomalous gold and alteration provide indicators of where thefluidhas been, structural studies highlight favourable areas for channeling, and lithology plays a key role in the depositional process. Since the basic model was developed in the early 1980's, the considerable success finding greenstone gold deposits in the Yilgarn Block and comparable terrains overseas, gives considerable confidence that this model is an effective basis for exploration.

Witwatersrand-type gold remains an elusive but highly sought after target for any major explorer. For such an important ore deposit type, the lack of success outside the Witwatersrand Basin is very evident. For much of its history, the source of the Witwatersrand goldfields has been linked to Archaean greenstone belts because of the high gold productivity of greenstone belts and the presumed appropriate age of mineralization. In the mid-1980's, it was noted during mineralogical studies that the Witwatersrand reefs and some granites surrounding the Witwatersrand Basin had remarkably similar uraninite, gold, pyrite, and "flyspeck" carbon nodules (i.e. based on both chemical and morphological characteristics). The ruling paradigm said that the Witwatersrand reefs were formed by placer processes, so these granites became the source for the Witwatersrand gold and related components (i.e. the HAGS model: hydrothermally altered granite source). In turn, the concept of "the golden arc" developed, and exploration was directed towards granites, and certain arcs. It subsequently transpired that the alteration of the granites (i.e. formation of the minimally goldenriched HAGS) took place well after deposition of the Witwatersrand sediments. Although the HAGS model has lost some credibility, the observations leading to the HAGS model of similar ore mineralogy still appear valid. A fruitful approach for exploration might be to challenge the paradigm that led researchersfromthe valid observations, to the HAGS model, and search for a new paradigm that better serves the interests of Witwatersrand-type gold exploration worldwide. It is suggested that the next "Witwatersrand" deposit will have several features in common with Kalgoorlie, Castlemaine and Carlin.

In modern management jargon, "doing the thingright"versus "doing therightthing" has its exploration equivalent in "exploring therightway" versus "exploring in therightplace". Exploring therightway, but on the wrong ground after focusing on the wrong parameters perhaps because of the wrong paradigm, is not the optimum formula for success.

Phillips, G.N. & Groves, D.I., 1993. The nature of Archaean gold-bearing fluids as deduced from gold deposits of Western Australia. Journal of the Geological Society of Australia 30,25-40.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

VICTORIAN GOLD: Is Global perspective G. Neil Phillips1 and Martin J Hughes2 Great Central Mines N.L., d - 1 Coppin St, East Malvern, 3145, Victoria, Australia 2 1034 Geelong Rd, Mt Clear, 3503, Victoria, Australia 1

World gold production can be subdivided into gold-only deposits (and their placer derivatives), and deposits with significant base metals and/or silver. Most of the latter would be uneconomic as gold deposits and rely on accompanying co- or by-products to be minable. Gold-only deposits account for most of the world's gold production and include Archaean greenstone gold deposits, the Witwatersrand deposits of South Africa (acknowledging by-product uranium in several mines), slate belt gold deposits and some of the deposits loosely referred to as "epithermaT. A number of important similarities are emerging between these gold-only provinces (Phillips and Powell, 1993), especially regarding deeper crustal characteristics such as fluid type and thermal perturbation. The Victorian gold province provides an important link with the other province types, and is perhaps the best example of a well-documented slate-belt gold province.

These two end member metal associations of gold-only and gold with co/by-products can be easily rationalized in terms of gold aqueous geochemistry: the soft - soft preference of Au1+ and the harder character of Au3+ favour complexing with sulphur and with chlorine, respectively. The relatively hard character of base metals and silver means these elements are preferentially transported as CI complexes, rather than as S complexes. Except in special environments, all other stable complexing agents with gold are generally in too low concentration to be critical in the formation of major primary gold deposits (e.g. As, Sb, OH, CN). The sulphur complexing in the gold-only deposits places some limitation upon viable depositional processes. Apart from processes that are relatively independent of host rocks such as boiling, immiscibility andfluidmixing, the two outstanding means of causing destabilization of the gold - sulphur complex are desulphidation and reduction: these are most easily brought about by interaction involving Fe-rich and carbonaceous wallrocks, respectively.

Archaean greenstone gold deposits highlight the importance of Fe in the gold depositional process with Fe-rich host rocks being strongly favoured in some but not all districts, especially amongst the large to giant gold deposits. The largest Archaean greenstone gold deposits, including Kalgoorlie, Timmins, Kirkland Lake, Champion and Morro Velho, are all in Fe-rich rocks. Carbonaceous host rocks are well represented in some greenstone gold deposits such as Sheba Fairview, Owl Creek and the Slave province, and in bonanza shoots such as the Oroya shoot at Kalgoorlie, but as host rocks are distinctly subordinate to Fe-rich rocks. The Archaean Witwatersrand gold comes, almost exclusively, from settings with C (carbon seams and particles) and/or Fe (pyrite or pyixhotite) to reinforce the Fe and C relationship established for Archaean greenstone deposits.

The higher crustal levels inferred for "epithermaT deposits are exactly where the host rock relationship is expected to be less clear due to the potential for mixing processes involving other fluid types, including meteoric, seawater and/or magmatic waters. As predicted from gold geochemistry, there is a correlation between more saline ore-forming fluids, and elevated levels of silver and/or base metals in these systems. Furthermore, recent studies identifying a deep, low salinity fluid in the Carlin deposits consolidates the importance of these links between gold-only provinces.

The Victorian "slate belt" gold deposits provide a different perspective on exactly the same chemical controls as witnessed in the greenstone belts. The major primary Victorian gold deposits are mostly in or near carbonaceous rocks (Bendigo, Ballarat, Castlemaine) although some occur in Fe-rich mafic rocks (Walhalla, Woods Point), and together account for at least 75 percent of total gold production from the Victorian gold province. The fluids in Victoria are similar to those in greenstone belts being low salinity (hence low Ag and base metals), dominant, with reduced sulphur. Structural control is a feature of all primary gold deposits in Victoria, but the dominant types of hosting structures contrast with those in greenstone gold deposits reflecting the incompetent nature of carbonaceous host rocks, and competent nature of the Fe-rich host rocks (relative to their respective enclosing sequences).

H J O - C O J

REFERENCE Phillips, G.N., & Powell, R., 1993. Link between gold provinces. Economic Geology, 88, 1084-1098.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GLOBAL CHANGE: THE FACE OF THE EARTH Kevin T. Pickering Department of Geological Sciences, University College London, Gower Street, London, WC1E 6BT, U.K.

The greatest scientific impact in the Earth Sciences during the 20th Century has been due to the development of the theory of plate tectonics and the appreciation of high-frequency climate change, particularly related to Milankovitch theory - the pace-maker for fluctuations between glacials and interglacials during time intervals when the spatial distribution of the continents is appropriate to significant ice-cap growth. Both theories remain central to any understanding of global climate change, but current interest in global climate change is increasingly focussed on sub-Milankovitch-scale, decadal-, century- and kiloyear- scale variation, periodic vs. event processes. These paradigms, hypotheses and theories embody the importance of internal and external forcing mechanisms in controlling global climate change. Naturally, a critical and robust appreciation of such processes, their magnitudes and rates, is not only a worthwhile intellectual endeavour for its own sake but provides extremely important data for quantitative modelling and prediction of climate change, and the consequences, within a temporal frame that will affect human activity even within our lifespan. This talk is an overview of the controls on the magnitude and rates of global climate change, and the broad implications of such changes for the face of the Earth - the sedimentary environments from subaerial to the deep oceans.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MAFIC VOLCANISM IN THE PALAEPROTEROZOIC GLENGARRY BASIN, WESTERN AUSTRALIA, AND IMPLICATIONS FOR ITS TECTONIC EVOLUTION Franco Pir^jno and Richard Davy Geological Survey of Western Australia, 100 Plain street, East Perth WA 6004

The Palaeoproterozoic Glengarry Basin lies within the southern part of the Capricorn Orogen (2200-1800Ma), on the northern margin of the Archaean Yilgarn craton. On the basis of recent field work by the GSWA the Basin has been subdivided into a number of domains that contain different stratigraphic subgroups. These domains, called the Windplain sag basin, Mooloogool rift, Bryah rift, and the Padbury basin, were formed in response to E-W sinistral transpressive movements which resulted in the development of pull-apart sub-basins, related to oblique convergence of the Pilbara and Yilgarn Cratons (Capricorn Orogeny). For convenience, the term Glengarry Basin has been retained to represent all of the domains outlined above.

Voluminous mafic extrusive and intrusive rocks characterise the Bryah and Mooloogool rift sequences. In the Bryah sequence volcanic and subvolcanic rocks form part of the Narracoota Formation. Now consisting of mafic schist (with Mg# of 59) and hyaloclastite metabasalt (Mg# of 56), they are subalkaline Fe-Mg-rich tholeiites with mixed MORB to oceanic island chemical signatures. Ultramafic schists have high Mg contents (Mg# 81.6) and have been interpreted to represent subvolcanic cumulates. The Narracoota mafic extrusives are characterised by low REE abundances and flat chondrite-normalised patterns with weak Eu anomalies, possibly reflecting depleted asthenospheric mantle sources. A postulated volcanic centre, west of Narracoota homestead, is coincident with a positive Bouguer gravity anomaly.

Mafic volcanic and subvolcanic rocks in the Mooloogool rift sequence belong to the Killara Formation. A number of subparallel linear E-W-trending structures, indicated on the aeromagnetic imagery, correlate with E-W-trending microgabbroic dykes, which may have fed the outpouring of the mafic volcanics. A positive Bouguer anomaly coincides with the Killara mafic extrusive and intrusive rocks. At one site, a diamond drillhole intersected an undisturbed sequence of mafic pillow lavas and microgabbroic sills from a depth of 319 m to the end of the hole at of 503 m. These lavas are overlain by flat-lying black shales of the Maraloou Formation. Near the Killara homestead, diamond drill holes intersected up to 200 m of mafic extrusives and gabbroic sills. In one hole, in a 90 m-thick section, a total of 15 individual lava flows have been recognised, suggesting a high rate of eruption. The Killara extrusives are aphyric and undeformed, and either unmetamorphosed or only affected by low grade greenschist facies metamorphism.

The Killara mafic rocks have subalkaline-tholeiitic to calc-alkaline-tholeiitic compositions, characterised by low REE abundances with a slight LREE enrichment and positive Eu anomalies. Discriminant chemical diagrams for the Killara volcanics suggest a mixed tectonic environment ranging from oceanic island to a continental setting.

The geochemical characteristics of the mafic volcanics of the Glengarry Basin (Narracoota and Killara formations) suggest that rifting of continental crust took place, as mentioned above, in response to transpressive movements during the Capricorn orogeny. In the Bryah rift basin, continental rifting advanced to form oceanic crust and oceanic islands, whereas in the east (Mooloogool rift sub-basin) crustal thinning and stretching produced only small areas of oceanic crust. A possibly analogous present-day tectonic setting is the Gulf of California; a similar situation in the geological record could be that of the Matchless belt of the Damara orogen in Namibia. If these analogies are correct, it may be inferred that volcanogenic Cu-Zn sulphide deposits of the Besshi type may be associated with the Killara formations, whereas MOR-related sulphide deposits may occur in the Narracoota Formation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

REVISED CORRELATION OF NEOPROTEROZOIC GLACIAL SUCCESSIONS FROM THE KIMBERLEY REGION, NORTHWESTERN AUSTRALIA Kenneth A. Plumb Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

Several glacial events are identified worldwide within the Neoproterozoic. Two, the Sturtian (-700 Ma) and Marinoan 600 Ma), have traditionally been recognised from the Adelaide Geosyncline in Australia. The glacial event at -600 Ma is assuming global significance in respect to defining a new chronostratigraphic unit of system status for the Terminal Proterozoic.

Separate stratigraphic nomenclatures were assigned to Neoproterozoic glaciogenic successions in the East Kimberley, Mount Ramsay, and Mount House areas during BMR/GSWA regional mapping of the Kimberley region during the 1960's. Two glacial intervals are preserved in the Mount Ramsay area: the Landrigan Tillite at the base of the Kuniandi Group, and the Egan Formation at the base of the Louisa Downs Group, unconformably above the Kuniandi Group. These were assumed to correlate with the Sturtian and Marinoan glaciations of the classical Adelaidean succession of South Australia (e.g., Plumb and Gemuts, 1976). On the local scale, the Landrigan Tillite/Kuniandi Group succession was correlated with the Moonlight Valley Tillite/Duerdin Group of the East Kimberley, and with the Walsh Tillite/Mount House Group of the Mount House area. The Egan Formation/Louisa Downs Group succession was in turn correlated with the Albert Edward Group of the East Kimberley. Subsequently, Coats and Preiss (1980) reinterpreted these relationships. They raised the Walsh and Moonlight Valley Tillites to correlate with the Egan Formation ("Marinoan"), leaving only the Landrigan Tillite to correlate with the Sturtian; most authors have since accepted Coats and Preiss' correlations. However, recent more-detailed Kimberley-wide observations by the author demonstrate that the original BMR/GSWA intraregional correlations are in fact correct and have significantly revised regional correlations with South Australia and elsewhere.

The overall successions of the Kuniandi, Duerdin and Mount House Groups are, indeed, very similar in quite fine detail throughout. Of particular significance in the context of Coats and Preiss' thesis is the regional extent and similarity of the tillites and their striated pavements, and the virtually identical laminated 'cap dolomites' above them, which are very similar to classical Marinoan 'cap dolomites' of South Australia. Together with near-identical Rb-Sr total-rock shale ages for the Duerdin/Mount House Groups and the early Marinoan of South Australia, this now makes correlation of all these sequences with the early Marinoan the preferred option. This leaves no Sturtian equivalent in the Kimberley. The corollary of this is that the Egan Formation is a younger glacial event in Australia.

The overall successions of the Louisa Downs Group (above Egan Formation) and Albert Edward Group (above Mount Forster Sandstone) are in turn similar to each other and distinct from the underlying groups. The Egan Formation shows a complex interfingering of carbonates, diamictite and fluvioglacial sediments quite unlike that of any other glacial succession in the Kimberley, and the carbonate facies are quite different to that of the 'cap dolomites' of the older tillites. The glaciogene sediments are clearly of limited extent and may simply be interpreted as a local mountain glaciation. Interpretation of the Egan glaciation as a later Neoproterozoic event is enhanced by the preliminary identification and interregional correlation of the stromatolite Tungussia julia from the Egan Formation (Corkeron et al., 1996). Similar local glacial episodes have been suggested from near the end of the Neoproterozoic on several other continents. The new results from the Kimberley rocks may be significant in confirming a new global glacial event near the end of the Neoproterozoic.

REFERENCES Coats, R.P. and Preiss, W.V., 1980. Stratigraphic and geochronological reinterpretation of Late Proterozoic glaciogenioc sequences in the Kimberley region, Western Australia. Precambrian Research, 13, 253-268.

Corkeron, M., Grey, K., Li, Z.X., and Powell, C. McA., 1996. Glacial episodes of the Neoproterozoic in the Kimberley region, northwestern Australia. This volume.

Plumb, K.A. and Gemuts, I., 1976. Precambrian geology of the Kimberley region, Western Australia. 25th International Geological Congress, Excursion Guide 44C.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ENHANCED UNDERSTANDING OF THE McARTHUR BASIN, NORTHERN AUSTRALIA RESULTS FROM NGMA MAPPING IN ARNHEM LAND 1

K.A. Plumb1, B.A. Pietsch2, and R.W. Page1 Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 2 Northern Territory Geological Survey, GPO Box 2901, Darwin, NT 0801

Second-generation mapping of Arnhem Land, N.T. by AGSO and NTGS, under the auspices of the NGMA, is enhancing regional understanding and resource assessment of the mid-Proterozoic McArthur Basin of northern Australia.

New data on stratigraphic relationships, supported by precise SHRIMP U-Pb zircon dating, have revised stratigraphic correlations between the northern (Arnhem Land) and southern (McArthur district) McArthur Basin (Pietsch et al., 1994). High-grade metamorphism of 'basement' rocks (-1870 Ma), and late-tectonic granites intruding them ( - 1835 Ma) are confirmed as being part of the Barramundi Orogeny. Younger 'basement' volcanics, at -1810 Ma, defines a maximum limit to the base of the overlying McArthur Basin. Bimodal igneous and siliciclastic sedimentary rocks in eastern Arnhem Land, previously considered to be 'basement', are now part of the McArthur Basin (Donydji-Spencer Creek Groups) and correlate with Katherine River Group of western Arnhem Land and Tawallah Group of the McArthur district; igneous suites at the top of all three groups have a common age -1710 Ma. The conformably-overlying, siliciclastic Parsons Range Group represents a local rift sequence within eastern Arnhem Land, equivalent to a period of uplift and erosion separating the Tawallah and McArthur Groups farther south. Similar lithostratigraphic successions suggest sequence-stratigraphic correlation between the carbonate-rich Balma-Habgood Groups (Walker Trough) and McArthur Group (Batten Trough) (Haines, 1994), but tuffs define an age range for the upper part of the Balma Group (-1620-1600 Ma) which is significantly younger than the lithostratigraphic equivalents in the McArthur Group (-1640-1625 Ma).

The McArthur Group and equivalents are confined to the Batten and Walker Troughs (Rifts); the correlative Vizard Group wedges out onto the Urapunga Tectonic Ridge. In contrast, the unconformably-overlying Nathan and Mount Rigg Group sag basin may be identified throughout the McArthur Basin. Following a major inversion event, the siliclastic Roper Group was deposited in a younger sag basin, of distinctly different distribution and tectonic setting to older elements of the McArthur Basin.

New airborne geophysical datasets enhance insights into the concealed geology of Arnhem Land: 1) 2-D modelling constrains the extent and form of the Walker Trough; a concealed eastern rift margin, several kilometres east of where previously thought, has major implications for base metal exploration; 2) McArthur Basin and basement structures may be extrapolated beneath the Neoproterozoic Arafura Basin; 3) Extensive, unexposed dyke swarms extend throughout Arnhem Land.

Five principal deformational events may be identified (Plumb, 1994): The earliest, the Tawallah-Katherine River-Donydji Group basin-wide rifting, was controlled by E-W orthogonal, linked extensional faults; Dj is assigned to the McArthur-Balma Group rifting event, involving NW-SE oblique pull-apart of the Batten and Walker Troughs; D2 refers to basin-wide, locally intense, WNW-ESE shortening between deposition of the Nathan and Roper Groups; D3 basin-wide shortening post-dates the Roper Group, related to ENE to NE-directed compression distinct to that of D2; D4 relates to late north-trending dextral faulting.

REFERENCES Haines, P.W., 1994. The Balma and Habgood Groups, northern McArthur Basin, Northern Territory: stratigraphy and correlations with tMcArthur Group. Proceedings of 1994 Annual Conference, Darwin, 5-9 August 1994, Australasian Institute of Mining and Metallurgy, 147-152.

Pietsch, B.A., Plumb, K.A., Page, R.W., Haines, P.W., Rawlings, D.J., and Sweet, LP., 1994. A revised stratigraphic framework for the McArthur Basin, NT. Proceedings of 1994 Annual Conference, Darwin, 5-9 August 1994, Australasian Institute of Mining and Metallurgy, 135-138.

Plumb, K.A., 1994. Structural evolution of the McArthur Basin, NT. Proceedings of 1994 Annual Conference, Darwin, 5-9 August 1994, Australasian Institute of Mining and Metallurgy, 139-145.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 SHIFTING GROUND, ROCK SOLID OR DEAD BORING PUBLIC PERCEPTIONS OF GEOSCIENCE Peter Pockley SciComm (Science Communication Pty Ltd) 25 Avenue Road, Glebe, Sydney, NSW 2037 Phone: (02) 660 6363; Fax: (02) 660 6239 Internet: scicomm@ozemail.com.au

The way the public perceives geoscience is, in the main, a product of the way geoscientists projects themselves and their subject. And, images of geoscientists are only variations of the images of scientists as a whole in the community.

Scientists ignore their responsibility to communicate with the community at the peril of their science and of the public acceptance of its support as a matter of national priority. To do so effectively requires scientists, in the first instance, to become self-aware and then to be active expositors.

Rosslyn Haynes of the University of NSW has shown in her seminal book, From Faust to Strangelove: Representations of the Scientist in Western Literature, how deep rooted are the stereotypes, persisting over centuries as they feed on misconceptions about science. Myth prevails over reality. Whenever something scientific seems to go wrong, one-liners appear in the media about modern-day equivalents of 'Doctors' Faustus, Frankenstein, Jekyll and Strangelove. While the figures arefictional,they are portrayed as archetypes of scientists - dangerous, unreliable, authoritarian, secretive, impersonal and amoral.

As a subset, three images of geoscience are apparent: Shifting ground: The valid notion of a dynamic Earth has been caricatured as: "These scientists are always disagreeing with each other. With all their research and expensive equipment, they can't even predict earthquakes and volcanoes. Therefore, they just don't know what they're talking about and can't be trusted".

Rock solid: On human time scales the ground seems immutable and immoveable. People who go around knocking off bits of rock and looking at them under microscopes are obsessive and uninteresting.

Dead boring: People who perforate the Earth with drills are not interested in sustaining dwindling resources.

Their mentality is purely profit-driven through the simple process offind-it,rip-it-outand ship-it-overseas.

Scientists, individually and as groups, have to work hard to present more realistic and fairer views than these.

They need not only to be conscious of how they are perceived by others but also be well informed about the latent, and increasingly vigorous, opposition to science which is emerging.

In a supposedly well educated age, anti-science, or counter-science, is a troubling and perplexing phenomenon which, far from going away with the fairies, is growing in support among the populace and even among intellectuals from other disciplines in academia.

Geoscientists find themselves at the centre of this stand-off war of words when questions about the origins of the Earth, the Universe and humanity - and their timescales - are raised by creationists and anti-evolutionists seeking to influence science education.

This paper will analyse the state of play and propose a course of self-help action. These include: • Inviting journalists and general writers to be Science Writers in Residence, including taking them on field trips to geologically important locations.

• Promoting high quality communications by professional geoscientists and, separately, by media reporters.

• Relating your work to the wider realms of science and human affairs and talk about it at every opportunity.

• Identifying and expounding values of long-term basic research, never assuming these to be self-evident.

• Investigating the claims of the anti- and counter-scientists and assertively argue why they are wrong.

• Training yourselves and your students in critical thinking, writing and speaking.

• Providing career advancement for generalists and communicatorsfromwithin the profession.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE OXIDATIVE SOLUBILITY OF PETROLEUM IN FORMATION WATERS: IMPLICATIONS FOR HYDROTHERMAL TRANSPORT, DIAGENESIS, AND DEVELOPMENT OF SECONDARY POROSITY IN RESERVOIR ROCKS Vitalii A. Pokrovskii Departement Erdwissenschaften NO, ETH Zentrum, Sonneggstr. 5, Zurich, CH-8092, Switzerland

Water can oxidize petroleum. Chemical interaction of the two agents at the oil-water interface in sedimentary basins at elevated temperatures and pressures may cause (A) hydrolytic disproportionate of heavy hydrocarbons in crude oil to form light hydrocarbons (HELGESON et al., 1993), (B) oxidative dissolution of hydrocarbons to form carbon dioxide, n-carboxylic acids, aromatics, and other aqueous organic species of intermediate oxidation states, and (C) mineral dissolution and/or precipitation in response to evolution of redox conditions, partial C0 2 pressure, and the pH of the aqueous phase.

To achieve a better understanding of the chemical interaction of liquid hydrocarbons with their aqueous and mineralogic environments in sedimentary basins, aqueous solubilities of n-alkane (hexane to eicosane) components of crude oil were calculated in the system C-O-H-Cl-Na-Ca-Mg-Fe as a function of fluid composition and oxygen fugacity at temperatures of 100, 120, and 200°C and pressures ranging from 100 to 1000 bar. The calculations were carried out by a Gibbs free energy minimization technique assuming that carbonate and carboxylate species in oil-field waters are in metastable equilibrium with one another, and that normal alkanes in crude oil with carbon numbers (n) greater than -6-15 (depending on the activity of aqueous C0 2 ) may be in metastable equilibrium with both heavier hydrocarbons in the crude oil and oxygen-bearing carbon species in oil-field waters (HELGESON, 1991). A representative spectrum of inorganic and organic aqueous species was taken into account in the calculations. Thermodynamic properties of aqueous species at elevated temperatures and pressures were computed from HKF equations of state (SHOCK and HELGESON, 1988,1990; JOHNSON et al.,1992), but those for liquid hydrocarbons were generated from PFGC equation of state (MAJEED and WAGNER, 1986). The solubility and stability relations in the system of interest are depicted in a series of comprehensive "temperature - bulk composition", "pressure - bulk composition", and "log^/O^g) - bulk composition" phase diagrams.

The calculations indicate that in the presence of carbonates, the oxidative solubilities of n-alkane components of crude oil may be several orders of magnitude greater than their stoichiometric solubilities.

For example, the oxidative solubility in the aqueous phase of nonane (C9H2o) with an activity of 0.1 in crude oil coexisting with calcite at the oil-water interface increases dramatically with increasing log ./02(g) at 120°C and 400 bar from a stoichiometric solubility of -0.26 ppm to an oxidative solubility of -320 ppm at log./02(g) = -56, which is typical of the fugacity of oxygen in hydrocarbon reservoirs at 120°C and 400 bar.

Predicted equilibrium concentrations of C02(aq> and carboxylic acids in idealized oil-field waters saturated with normal alkanes in crude oil are generally consistent with analytical concentrations reported by KHARAKA et al. (1977) for formation waters recovered from productive wells in the Texas Gulf Coast (USA).

The results of this study leave little doubt that the oxidative dissolution in the aqueous phase of hydrocarbon species in crude oil may cause, depending on pH, dissolution or precipitation of carbonate minerals such as calcite, dolomite, and siderite, which is consistent with stable isotope data reported in the literature. Because oil-field waters are usually acidic, production of carbonic and carboxylic acids by the oxidative dissolution of petroleum in hydrocarbon reservoirs generally favors mineral dissolution. These observations have significant implications with respect to the development of secondary porosity during the maturation of crude oil in dynamic basins in which extensive fluid flow occurs and oil and formation waters are in pervasive contact.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention,, Canberra, February 1996

LIGHT HYDROCARBONS AS AN EXPLORATION TOOL FOR MISSISSIPPI VALLEY TYPE Pb - Zn - Ag DEPOSITS IN THE FLINDERS RANGES, SOUTH AUSTRALIA Paul Polito and Yvonne Bone Dept. Geology and Geophysics, University of Adelaide, South Australia

It is being increasingly realised that many mineral deposits in rocks as old as Proterozoic and even Archean in age have hydrocarbon gases associated with them. Surveys in Europe and Australia, have shown that there are major variations in the hydrocarbon gas content of rocks closely related to mineralisation. European research focused upon using the light hydrocarbons as a potential base metal pathfinder in mineral exploration, as the hydrocarbons are more widespread throughout the stratigraphy than the base metals themselves. It has been concluded that the use of light hydrocarbons as an exploration tool have as much potential in Australia on its arid and deeply weathered terrain as it does in Europe where weathering and oxidation is considerably less.

Following the successful application of the technique in 1993 in the Moorowie Mine and Donkey Bore areas, 4 prospects in the Central Flinders Ranges, S.A., were selected by Mines and Energy South Australia (MESA) to test the methods' reliability, reproducibility and logistical implications under Australian conditions. Three prospects (Linda, Eric and Ediacara), host known sub-economic Pb/Zn/Ag mineralisation and the fourth, Tea Cosy Reef, is considered a possible host prospect with a mapped reef and prepared ground the idealised target.

At the Eric Prospect, 47 samples were collected over an area of 1700 m x 1500 m. It was found that methane values varied greatly (0.15 ^il/gm - 32.6 j-il/gm) with an anomalous high developed toward the NE corner of the prospect. No methane anomaly was detected above a 1% Pb anomaly in the SW corner of the prospect. Initial interpretation failed to explain the low hydrocarbon signature around the Pb anomaly in the south of the prospect when hydrocarbon haloes were distinctive around mineralised horizons in the north.

The Ediacara Prospect had 58 samples collected from four traverses. Results showed methane variations (0.09 jLil/gm - 8.10 jil/gm) to be smaller than those at the mineralised Eric and Linda Prospects. However, this was expected from a dolomitised basin. Methane lows were recorded immediately above the known and worked mineralised areas. High anomalies were recorded < 100m east of the 31.8 million ton deposit.

The Tea Cosy Reef prospect yielded no anomalies from 23 samples collected. Values ranged from 0.13 |il/gm to 1.95 jul/gm, over an area covering approximately 1000m x 850m.

The 51 samples from the Linda Prospect gave methane values that varied greatly ranging from 0.25 |il/gm to 51.85 jil/gm, making the Linda anomalies the largest. The methane high is not related to the known surface mineralisation, but is most prominent within an intertonguing lense of the Mern-Merna Formation. The mineralised area anomalous in Pb, Zn and Ag failed to produce an anomaly greater than 3 fil/gm.

It is thought that light hydrocarbon anomalies appearing at the surface and associated with primary mineralisation, are released from the interface between the fresh rock and the oxidised - weathered rock zone.

These gases migrate slowly, via micropores and microfractures to the surface. It is not mandatory to know the method whereby the C1-C5 hydrocarbons migrate to the surface, but it is important to realise that this migration does occur.

Methane values associated with heavier hydrocarbons (ie, the C3 - C5 hydrocarbons), are assumed to arise from a thermogenic source. The presence of ethane and minor amounts of C3 - C5 hydrocarbons in samples, which give the highest methane values, indicates that the hydrocarbons are not entirely produced by bacteria. Rather, they suggest a petroleum source, in this case that associated with MVT deposits.

CONCLUSION No single geophysical technique is reliable for detecting carbonate hosted Pb - Zn mineralisation. The advantages of this method include its ability to detect buried mineralisation so that it can be used for extending known deposits, defining trends within a mineralised area or for grass roots exploration. Every deposit, both in Europe and Australia, upon which this technique has been tested, has given anomalous levels of hydrocarbons close to known mineralisation.

Acknowledgements: The senior author thanks Mines and Energy South Australia for funding this project.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

T W O PROTEROZOIC METAMORPHIC EVENTS IN GRANULITES F R O M T H E WINDMILL ISLANDS, EAST ANTARCTICA

1

N'.J. Post 1 . P. D. Kinny2 & B J. Hensen1 Dept. of Applied Geology, University of New South Wales, Sydney 2052 2 Dept. of Physics, Curtin University of Technology, Perth 6001

The Windmill Islands and adjacent mainland peninsulas consist of a multiply deformed supracrustal sequence, intruded by minor porphyritic granites and an extensive suite of chamockites. From north to south the metamorphic grade changes from upper amphibolite to granulite facies (Blight & Oliver 1977). Four significant phases of deformation (D1-D4) have been recognised in the region. Di and D2 are penetrative events associated with high grade metamorphism and isoclinal folding, D3 is an open folding event and D4 is associated with retrograde shearing. Heating of the terrane outlasted the last main phase of deformation (D2) and resulted in the widespread annealing of Di and D2 fabrics.

Amphibolite facies gneisses to the north are characterised by biotite-sillimanite-plagioclase-cordierite-quartz and rare garnet in pelitic lithologies and by biotite-hornblende in mafic rocks. By contrast, in the southern granulite facies rocks, garnet is widespread in pelitic gneiss and clinopyroxene-orthopyroxene is common in mafic gneiss.

Across the terrane garnet grains commonly contain sillimanite and biotite inclusions. Dumortierite bearing aplites in amphibolite facies paragneiss and rare kornerupine-grandidierite bearing post D2 partial melts in the higher grade rocks attest to locally high boron activity. In the higher grade gneiss garnet is commonly rimmed by secondary cordierite ± orthopyroxene or plagioclase-orthopyroxene. Peak metamorphic conditions for the granulite facies rocks, derived from garnet-orthopyroxene-plagioclase gneiss are 700 ± 50° C at 4-5 kbar.

New insitu zircon and monazite SHRIMP geochronology supports the conventional SHRIMP data of Post et al.

1995 and is consistent with two major phases of metamorphism, an upper amphibolite facies event (Ml) at c. 1400-1310 Ma followed by a granulite facies overprint (M2) c.1210-1180 Ma. Preliminary Sm/Nd geochronology suggests that the M2 heating may have continued at least until 1180 Ma. The development of a strong north-south foliation in earlier phases of the Ford Island granite suggests that the granite intruded pre to syn- D3. The final deformed phase of the Ford Island granite was emplaced at c.l 170 Ma. Hie last major phase of igneous activity in die area is marked by the emplacement of the Ardery charnockite at c.l 160 Ma. Charnockitic intrusions dominate the southern half of the area and crop out as small nunataks as far south as Snyder Rocks, 120 Km from the Windmill Islands. Along the charnockite-country rock interface, partial melting of pelitic lithologies has resulted in the formation of extensive orthopyroxene-cordierite restite. The dry charnockitic magmas probably formed in the lower crust during M2 and intruded to their present level by the end of M2. A pervasive east-west fabric parallels the margins of these intrusions and probably represents an igneous flow foliation rather than a tectonic fabric. Partial exhumation of the terrane after charnockite emplacement is suggested by the intrusion of rare gabbro dykes containing olivine-plagioclase. The final phase of deformation, D4 resulted in the development of brittle shear zones and faults. Subvertical stretching lineations and shear sense indicators suggest thrusting during D4. Intrusion of Post D4 aplite dykes at c.l 138 Ma marks the end of igneous intrusion during this orogenic cycle. Reactivation, or simply continued movement along D4 shears and faults may account for the final exhumation of the terrane. Abundant, unmetamorphosed post D4 dolerite dykes may be associated with the break up Gondwana in tlie late Mesozoic.

The geochronological similarities of this terrane with the Albany - Fraser complex of western Australia (Nelson et aL 1994) suggests that the peak of metamorphism in the Windmill Islands may have been associated with the collision of the east Antarctic Shield and the Archean Yilgarn Craton c. 1300-1180 Ma. Neither SHRIMP U-Pb nor preliminary Sm-Nd geochronology show any evidence for Pan African tectonism in the Windmill Islands.

REFERENCES Blight,D.F., & Oliver, R.L., 1977. The Metamorphic Geology of the Windmill Islands, Antarctica: A Preliminary Account. J. Geological Society of Australia, 24, 239-262.

Nelson, D.R., Myers, J.S., & Nutman, A.P., 1995. Timing of events in the mid-Proterozoic Albany - Fraser Orogen Western Australia, and implications for Gondwana correlations. In Press.

Post, N.J., Kinny, P.D., & Hensen, B.J., 1995. Geochronlogical Constraints on the Tecto no thermal History of the Windmill Islands, East Antarctica. VII International Symposium on Antarctic Earth Sciences, Abstracts 12, 313314.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

T H E S T R U C T U R E S O F T H E EAST L I M B O F T H E M I T A K O O D I C U L M I N A T I O N E A S T E R N M O U N T ISA I N L I E R Warren A. Potma Australian Geodynamics Cooperative Research Centre. Monash University. Clayton. VIC 3168

The Mitakoodi Culmination is a unique tectonic block within the Eastern Succession of the Mount Isa Inlier, NW Queensland. A well preserved Proterozoic stratigraphy of Cover Sequence Two rocks has enabled a detailed interpretation of the block's structural history which is locally controlled by early extension-related structures.

The Culmination comprises a broad regional anticlinorium (fig. 1) which is interpreted to have formed during a major shortening event (locally known as D?) during the Isan Orogeny. Early syn-depositional extension structures are preserved within the southern part of the east limb of the Culmination, and directly influence strain partitioning associated with the later shortening. These half-graben style normal faults have hangingwall to the SE displacements of up to several kilometers. A localised angular unconformity at the base of the Overhang Jaspilite which is associated with one of these normal faults, has enabled the timing of extension to be constrained at between 1766± 23 Ma and 1720±7 Ma (existing U-Pb dates).

The Mitakoodi Culmination exhibits an abrupt structural transition on its east limb. The southern section is significantly shortened by large scale tight upright folds, while to the north, apart from the regional Duck Creek Anticline, the area is unfolded. This structural boundary coincides with the most northerly extent of the ENE striking normal faults which appear to have acted as a structural backstop for this later shortening.

A significant bedding parallel detachment above the Overhang Jaspilite formed a major fluid conduit during the Isan Orogeny and is now preserved as an intensely silicified zoned at the top of the Overhang Jaspilite on the eastern limb of the Culmination. This major detachment separates the Overhang Jaspilite from the Marimo Slates and appears to have been active, with a top block to N W sense of movement, when the Mitakoodi Culmination was forming.

The structural morphology of the east limb of the Mitakoodi Culmination results from strain partitioning during shortening of units against the competent structural backstops formed by early half grabens.

Ballara/Corella River FaultZone

Figure 1. The Mitakoodi Culmination

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

BREAKUP AND DISPERSAL OF THE RODINIA SUPERCONTINENT: IMPLICATIONS FOR RESOURCE EXPLORATION C. McA. Powell Department of Geology and Geophysics, University of Western Australia, Nedlands, WA 6907

Final assembly of the Rodinia supercontinent took place during the late Mesoproterozoic, as recorded by the Grenvillian-aged (1300 to 1100 Ma) orogenic belts which can be tracedfromnortheastern Laurentia through west Antarctica and thence through India and Antarctica into the Albany, Fraser and Musgrave belts in Australia. The eastern margin of Australia in the Rodinia supercontinent lay along the Tasman Line, recently defined so clearly

by AGSO aeromagnetic and gravity compilations. In the Rodinia supercontinent, Laurentia lay to the east of Australia, possibly with South China in between (Li et al. 1995).

By the early Mesoproterozoic, Australia had three broad, relatively stable regions, the North Australia, the Pilbara-Yilgarn and the Gawler cratons, with active orogenesis along the eastern margins of the North Australia and Gawler cratons. This northerly-trending orogenic belt contained the Mt Isa and- Broken Hill metallogenic provinces, and formed through a complex series of basin openings and closures in the 1.7 to 1.4 Ga interval, during which world-class base-metal deposits were formed. Precambrian Australia was probably largely assembled in its present configuration by 1.4 Ga, so that the Grenvillian orogenesis in Australia was more one of reactivation of zones of crustal weakness than closure of wide oceanic basins. By 1.07 Ga, movement on the Albany-Fraser-Musgrave belt had ceased, and mafic dykes were intruded in central Australia.

Palaeomagnetic data indicate that the Rodinian configuration endured until at least 725 Ma, but that shortly thereafter Laurentia separated from the Australia-Antarctic margin. In the Adelaidean Geosyncline, the time of the breakup is interpreted to coincide with the end of the Sturtian glaciation around 700 Ma, after which a broad sag-phase transgression occurred (Powell et al 1994). Before Rodinian breakup, the interior of Australia was covered by a broad epicontinental basin. Rifting and volcanic activity in northwesterly- and northerly-trending basins along what was to become the eastern margin of Neoproterozoic Australia occurred from -830 Ma until breakup. The NW-trending Gairdner dyke swam was emplaced during this early extension.

During breakup the extension direction was oriented NE-SW, implying that the rectilinear margin of Precambrian comprises a series of NE-trending transform and NW-trending rift margins. Analysis of the directions preserved suggests that opening was about a rotation pole at 126°E, 7°S, approximately 25° from the Adelaidean region where the rift margin is best preserved. The preservation of a wide rift margin in South Australia, which formed the base on which younger Adelaidean sediments were deposited, indicates that the South Australian segment of the Rodinian margin had lower-plate geometry.

Mid-Neoproterozoic breakup along the Tasman Line truncated the Mesoproterozoic metallogenic zones, which raises the question of where missing pieces are. An 800-km extension of the Mt Isa Belt is projected to have been lost along the northeastern South Australian rift margin, and a further 600 to 1000 km was lost along the rift segment south of Broken Hill. At least three possibilities for where thesefragmentsare can be considered: the first is that the they are located in the conjugate margin of Laurentia, most likely to be in western Canada; the second possibility is that they are located in South China; the third is that they lie in a ribbon of thinned continental crust (c.f. the present Lord Howe Rise) now embedded in a Phanerozoic fold belt.

If the conjugate margin lies in western Canada, the missing pieces could lie buried beneath Phanerozoic deposits of the Rocky Mountains. In South China, the conjugate margin could be concealed under the widespread younger Yangtze carbonate platform. If the metallogenic zones are part of a thinned continental ribbon, they could lie in the basement to the Delamerian Fold Belt, having first separated from Australia in the late Neoproterozoic and later having been brought back during the Late Cambrian contractional phase. The NWtrending magnetic anomalies and tectonic fabric beneath the Murray Basin in western NSW and Victoria, southeastern South Australia, and possibly Tasmania, hold some interest in this regard.

REFERENCES LI, Z.X., Zhang, L. and Powell, C. McA., 1995. South China in Rodinia: Part of the missing link between Australia-East Antarctic and Laurentia? Geology 23,407-410.

Powell, C. McA., Preiss, W.V., Gatehouse, C.G., Krapez, B. and Li, Z. X., South Australian record of a Rodinian epicontinental basin and its mid-Neoproterozoic breakup to form the Palaeo-Pacific Ocean.

Tectonophysics 237, 113-140.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 RECORD OF THE BREAKUP OF RODINIA IN SOUTHEASTERN AUSTRALIA Wolfgang V. Preiss 'Department of Mines and Energy, Box 151 Eastwood, South Australia, 5063.

1

Southeastern Australia's Proterozoic record is consistent with the hypothesis of assembly and subsequent breakup of the Neoproterozoic supercontinent Rodinia, in which Australia and adjacent Antarctica were in contact with North America until the late Neoproterozoic. Assembly of the Precambrian shield was complete by the end of the late Mesoproterozoic Albany-Fraser-Musgrave orogenic belt, which may have linked with similar Grenvilleage orogens in Antarctica and North America and may represent a major continental collision. The Gawler Craton, part of the South Australian basement that escaped Neoproterozoic and Palaeozoic tectonism, is composed of the late Archaean high-grade Sleaford and Mulgathing Complexes, with Palaeoproterozoic to Mesoproterozoic orogenic reworking and eastward-accreting events. In the southeast Gawler Craton, the 1.851.65 Ga Kimban Orogeny deformed and metamorphosed 1.9-1.85 Ga platform sediments (Hutchison Group) overlying Archaean rocks on Eyre Peninsula and involved deep-seated synorogenic magmatism. The thick, dominantly felsic Gawler Range Volcanics (1.59 Ga) were extruded over deeply eroded metamorphics on the central Gawler Craton, accompanied by local clastic sedimentation, and slightly younger anorogenic granites were intruded extensively on the craton. The giant 1.58 Ga granite-breccia hosted Olympic Dam Cu-U-Au orebody is located on a NNW-trending basement weakness, the G2 structural corridor.

On northeastern Eyre Peninsula, folded but little-altered 1.74 Ga coarse sediments and felsic volcanics, and metamorphosed distal equivalents containing copper mineralisation at Wallaroo-Moonta (northern Yorke Peninsula), may represent the western edge of a basin complex evolving further east, in which the -1.7 Ga Willyama Supergroup was deposited. This basin involved intracontinental rifts, such as that in which the giant syn-sedimentary Broken Hill base metal orebody was formed in association with mafic and felsic volcanism, and may have been part of a long mobile belt extending at least from Adelaide to Mount Isa. The Willyama Supergroup was deformed and metamorphosed by the 1.6-1.55 Ga Olarian Orogeny, broadly coeval with the Isan Orogeny to the north. Remnants of this orogen are preserved as inliers in the Neoproterozoic-Cambrian Adelaide Geosyncline, beneath which it is the dominant basement. The NE-trending Paralana and Anabama crustal shear zones were eastablished and became important controls in subsequent Neoproterozoic rifting and Delamerian deformation.

Earliest rifting in the Adelaide Geosyncline was in response to NE-SW crustal extension, (cf NW-trending Gairdner Dyke Swarm on Gawler Craton). Willouran (-850-780 Ma) basalt and evaporitic clastic and carbonate sediments were deposited within the G2 corridor and NW-trending grabens in the Flinders Ranges; the Paralana and Anabama shear zones were reactivated as transfer faults. Torrensian (-780-700 Ma) E-W extension established the N-S Torrens Hinge Zone, marking the eastern margin of unattenuated Precambrian crust, and formed a fault-bounded N-S trough within the western Adelaide Geosyncline. In the Sturtian (-700-650 Ma), renewed NE-SW extension, accompanied by widespread glaciation and ?global drop in sea level, produced the NW-SE trending Baratta, E-W trending Yudnamutana and NNW-SSE trending Torrowangee troughs, rimming the Curnamona Province, a circular remnant of the Olarian Orogen. Following Sturtian glaciation, marine transgression encroached onto the eastern Gawler Craton for the first time, reflecting sag-phase deposition probably related to the separation of Australia-Antarctica from North America. Marinoan deposition (-650-540 Ma) continued this sag phase, with a number of upward-shallowing cycles punctuated by eustatically and technically controlled erosional boundaries. Marinoan glaciation was probably centred well to the north (glaciated pavements in the Kimberleys (WA), compared to periglacial features in SA at palaeolatitude -5° N). Kilometre-deep submarine canyons of the Wonoka Formation may relate to extensional events to the north and east. A basinwide hiatus (?global sea-level fall) was followed in the Early Cambrian by renewed transgression and deposition of carbonates in platform and ramp settings. In the late Early Cambrian, renewed rifting in the south led to mafic volcanism and formed the NE-trending Kanmantoo Trough. This feature, oblique to previous rifts, may be related to formation of a marginal sea in response to earliest plate convergence further east (cf oceanic island arcs in Victorian Cambrian greenstone belts). Delamerian deformation started in the south at -510 Ma with NWdirected thrusting toward the southeast corner of the Gawler Craton; these early structures were overprinted by N-S trending upright folds that extend northward to the intracratonic fold belts of the Flinders Ranges.

The Rodinia hypothesis may be tested by comparing ages of basement provinces and NE-trending interpreted transfer faults in Australia with North America. The zone of continental separation truncated older orogenic belts that host major ore deposits such as Wallaroo-Moonta, Broken Hill and Mount Isa. 352

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE EVOLUTION OF PETROLEUM FLUIDS FROM SOURCE-ROCK TO TRAP James Preston Senior Petroleum Geologist, BHP Petroleum Pty. Ltd., 120 Collins St., Melbourne, Vic.

A source rock most commonly consists of an argillaceous sediment into which organic matter was introduced at the time of deposition. As the sediment undergoes early burial and diagenesis, the organic matter is converted to kerogen. With further burial, to temperatures exceeding 100°C, this kerogen is progressively converted to hydrocarbons. The initial chemical composition of these fluids is determined by the origin and nature of the organic matter, its preservation state (the extent to which it has escaped oxidation), and the degree of burial of the organic-rich rock (and hence the extent of its thermal maturation).

Hydrocarbons occupy a volume up to 50% greater than the kerogen from which they are generated. This creates increasing volume imbalance within the source rock, and a critical point is "reached when the increasing pore-fluid pressures cause the claystone to fail by internal micro-fracturing. Hydrocarbons are then expelled into an adjacent zone (or drain) of more normally pressured, porous sediment. This expulsion process is known as primary migration.

Primary migration directly influences the composition of the mobile fluids. Those fluids which are not expelled are relatively enriched in heavier compounds such as resins and asphaltenes, whereas the expelled fluids are relatively enriched in saturated hydrocarbons. This is a geochromatographic effect based on the sizes of various hydrocarbon molecules relative to the effective pore diameters in the source claystone. The effect is more significant with increasing burial, as compaction increases and pore diameters diminish.

If a claystone is thick, rapidly buried, and ineffectively de-watered during diagenesis, generated hydrocarbons will add to the high pore-fluid pressures created by the trapped connate waters, and a zone of overpressuring results. Under such circumstances, primary migration of hydrocarbons is either severely inhibited or totally precluded. In areas such as the Barrow Sub-basin of offshore Western Australia, overpressuring appears to have directly influenced the distribution of gas and oil accumulations; reservoirs within overpressured zones appear to be dominated by gas because of inhibitions to liquid migration. Faulting, however, locally enhances liquid migration from, and through, overpressured zones; this is evidenced by the close correlation of oil accumulations with fault-related trap sites in the eastern Barrow Sub-basin. The Eugene Island field, offshore Louisiana, USA, provides a further example of fault-migration of hydrocarbons from overpressured shales.

Having been expelled from the source rock, the hydrocarbons move through more permeable and porous, waterwet carrier beds, ultimately to the reservoir/trap site. This is known as secondary migration. The main driving force of the hydrocarbons is buoyancy; however, water-flow may reinforce buoyancy effects. Hydrocarbons will migrate through porous beds as long as the buoyancy forces are sufficient to overcome the capillary pressures.

Migration becomes more efficient by saturation-networking of the hydrocarbons within the carrier bed. The gross compositions of migrating hydrocarbons may be further modified in transit by geochromatographic effects, as well as solubility and mineral-reaction effects. As hydrocarbons rise up carrier beds to lower formation pressures, single-phase fluids may convert to liquids and gases, each with a different buoyancy and correspondingly different migration efficiency.

When a body of hydrocarbons encounters a low-porosity/high-capillary pressure bed, it will either form an obstacle to them, or, if it is appropriately structured, trap them. The relative distribution of oil and gas in the reservoir depends on a number of factors, such as the supply of oil or gas (source- and maturity-dependent), reservoir temperature and pressure (determining phase behaviour), oil type (light oil can dissolve more gas than heavy oil), and the relative permeability of the seal (possible differential movement of gas versus oil). However, the composition of trapped hydrocarbons can be modified within the reservoir by processes such as thermal cracking, de-asphalting, and (bio)degradation. Further compositional changes can occur by selective losses by leakage (tertiary migration or dysmigration), for example, in the Skua Field in the Timor Sea province of offshore N.W. Australia,. This may occur by rupture of the seal by faulting, inducing separation/migration effects such as the conversion of single-phase fluids into two-phase systems by pressure release, preferential migration of vapours, and reversion to two phases in shallower trapping positions by retrograde condensation.

353

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ORIGIN OF HIGH K ANDESITES IN TARANAKI NEW ZEALAND

Richard C.Price1, Robert B.Stewart2 and Ian E.M.Smith3

1

School of Eaith Sciences, La Trobe University, Bundoora, Victoria, Australia, 3083.

2

3

Department of Soil Sciences, Massey, Palmerston North, New Zealand.

Department of Geology, University of Auckland, Auckland, New Zealand.

Andesitic volcanism of Quaternary age in Taranaki defines a lineament that extends over a length of 30 km from Paritutu in the north west to Mt Taranaki (Egmont) in the south-east. Volcanism was initiated at Paritutu around 2Ma BP and migrated SE along the lineament. Activity commenced at Egmont Volcano, the youngest of the Taranaki volcanoes around 120 ka BP. The lineament is orthogonal to the axis of the Taupo Volcanic Zone (TVZ) and Egmont Volcano is located approximately 180 km above the present day Wadati-Benioff Zone.

Compared to the andesites of Mt Ruapehu in the TVZ, young (<10 ka) Egmont andesites are relatively potassic.

The early magmatic history of Egmont Volcano, pre-dating the construction of the present day cone, is recorded in the clast assemblages of laharic deposits within the ring plain of the volcano. Data from this succession show that the strongly potassic character of Egmont eruptives has developed relatively recently. Progressively older units are less potassic and the earliest eruptives were much more similar to andesitic volcanics of Ruapehu than is the case for the youngest volcanics.

Data available for Pouakai Volcano, which lies to the north west of Egmont along the lineament, indicate that most of the eroded remnant of this large volcano is similar to the older (relatively lower K) eruptives of Egmont, however, higher K material similar to the younger Egmont eruptives has also been recovered from laharic deposits associated with Pouakai Volcano. TTie data are interpreted to indicate that much of the eruptive history of the Taranaki volcanoes was dominated by andesitic magmas not dissimilar to those that characterise TVZ andesitic volcanoes to the east. The potassic character is manifested most strongly only in the later stages of construction of the Taranaki volcanoes.

Primitive magmas at Egmont were hydrous high magnesian basalts. Early fractionation of olivine and chromite and later olivine, clinopyroxene, titanmagnetite, and orthopyroxene produced high-Al basalts . P-T conditions encountered by the evolved magmas at the base of the crust were such that the amphibole stability field was intersected and reaction between both anhydrous mafic cumulates and wall rocks crystallised amphibole, buffering the melt composition to basaltic andesite. Tapping of these melts to higher levels brought them away from the amphibole stability field, resulting in decompressive melting of amphibole and inconguent melting of any amphibole in the lower to mid-crustal wall rocks. The K20-rich liquids from the incongruent melting were a major source of potassium in the Egmont high-K andesites.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SPATIAL DISPLAY O F P E T R O L E U M ACCUMULATIONS FOR RESOURCE ASSESSMENT STUDY Sandv Radke. Vicki Passlow, Steve LePoidevin Bureau of Resource Sciences, PO Box El 1 Queen Victoria Terrace Parkes, ACT 2600

Information from two existing relational databases were utilised to develop a specialised dataset for petroleum accumulations in onshore and offshore Australia. The PEDIN database contains petroleum data for over 9000 wells including basic data for all wells drilled in Australia and more detailed data such as formation tops, downhole temperature and seismic horizon intersections for wells drilled under Commonwealth legislation. The Australian Petroleum Accumulations (APA) database supports the APA publication series and contains information on petroleum fields including area of closure, trap types, reservoir and seal characteristics, recovery and production status.

When the need arose in BRS to display regions of petroleum occurrences, noting areas of current production, past production and possible future production, the two database were linked to produce .a spatial and attribute set permitting display of oil, gas and oil and gas accumulations. Well locations from the PEDIN database are used to define the field/accumulation areas and the locations are ranked according to the commercial status of the field to which they belong. Attribute data including well name, operator, well status, total depth date, field name, commercial status of the field, fluid types and geologic basin are used to categorise, assign symbology, display and interrogate the database. The development process necessitated intensive data checking between the two source datasets to ensure that PEDIN well data were consistent with APA data and that APA classifications were up to date and accurate. Oracle Data Browser and ESRI's Arc View software were utilised to produce a spatial database for use in resource assessment studies and providing policy advice.

355

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE PITFIELD METABASITES - A NEWLY RECOGNISED ?CAMBRIAN FAULT-EMPLACED IGNEOUS ZONE AND IMPLICATIONS FOR THE EVOLUTION OF THE SOUTHERN LACHLAN FOLD BELT. WRH Ramsay , VJ Morand , M Hughes ^University of Ballarat, P 0 Box 663, Ballarat Vic 3353, Australia Consulting Geologist, 1034 Geelong Road, Mt Clear, Ballarat Vic 3350, Australia 1

2

1

2

The Lachlan Fold Belt (Cambrian-Carboniferous) comprises part of the larger Tasman Orogen of eastern Australia. Within the southern portion of the fold belt occur a number of essentially north-south trending, structurally controlled zones containing lenses and belts of metabasite and lesser amounts of metaperidotite of probable Cambrian age.

Structural studies in western central Victoria (Ramsay et al., 1992; Morand et al., 1995) have delineated a further structurally controlled zone containing massive to foliated metabasite and metaperidotite. This zone comprises the southern extension of the Avoca Fault Zone, which separates the Stawell Structural Zone from the Bendigo-Ballarat Structural Zone.

Texturally, the altered igneous rocks range from quench and fine grained plagioclase-clinopyroxene lavas to olivine-rich cumulates, now strongly serpentinised. Alteration minerals include albite, chlorite, epidote, sphene, quartz, carbonate, ± actinolite (mg 67-69, AI2O3 wt% 1.6-2.3). Chromites from the cumulate metaperidotites are characterised by 29-36 wt% CxjOy AI2O3 contents define two populations, those with low A1 0 (2.7-5.3 wt% - Cr/(Cr + Al) - 0.8) and those with higher A1 0 (17 wt% - Cr/(Cr + Al) ~ 0.6).

2

3

2

3

Geochemical data suggest that the lavas are tholeiitic basalts with MgO 7.4-8.6 wt%, moderate Ti0 (0.981.18 wt%), and variable large-ion-lithophile element chemistry. Immobile elemental ratios (Ti/Zr, Zr/Y, and Ti/V) are closely comparable to modern N-type mid ocean ridge tholeiites. Similar comparisons may also be made with metatholeiites from Heathcote and Mount Wellington (Crawford and Keays, 1987) and Phillip Island (Henry and Birch, 1992). Rare earth element analyses demonstrate a flat pattern enriched some 16 times chondritic for the elements La-Gd. The similarity in normalised REE patterns for the Pitfield metabasites and metatholeiites from Mount Wellington and Heathcote, coupled with primary mineralogy, major, minor, and immobile trace element ratios are strongly suggestive that the parental magmas and tectonic setting for these three volcanic belts, now separated by up to 200 km, were similar. The tectonic setting is likely to have been that of an ocean ridge to incipient back arc region.

2

It is proposed that the Pitfield Metabasites were emplaced along a steeply west-dipping thrust fault with easterly directed transport. This fault zone has been traced northwards to Avoca for over 100 km. On the basis of a variety of criteria listed by Morand et al. (1995) we support the concept that the source of the rocks is a midcrustal layer located beneath the Stawell Structural Zone. Thus a similar structural and lithological style (turbidites with upright folds overlying a metabasite layer which has been thrust eastwards) characterises the Melbourne, Bendigo-Ballarat, and Stawell structural zones. Consequently we regard the western margin of the Lachlan Fold Belt to lie west of the Avoca Fault Zone and possibly west of the Stawell Structural Zone.

Crawford AJ, Keays RR, 1987: Pedogenesis of Victorian Cambrian tholeiites and implications for the origin of associated boninites, Journal ofPetrology 28, 1075-1109.

Henry DA, Birch WD, 1992: Cambrian greenstone on Phillip Island, Victoria, Australian Journal of Earth Sciences 39, 567-575.

Morand VJ, Ramsay WRH, Hughes M, and Stanley JM, in 1995: The southern Avoca Fault Zone: site of a newly identified "greenstone" belt in western Victoria, Australian Journal of Earth Sciences 42, 133143.

Ramsay WRH, Stanley JM, Hughes M, Morand V, Carroll RP, 1992: The Pitfield-Avoca Fault System: the locus of a cryptic-discontinuous greenstone belt in central-western Victoria. Geological Society of Australia. Abstract 32, pp 233-235.

356

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 RECOGNITION OF CLIMATE AND CO2 LEVELS FROM CARBONATE MINERALOGY AND OXYGEN AND CARBON ISOTOPES C.Prasada Rao Department of Geology, University of Tasmania, Hobart, Tas. 7001

Mineralogy of Recent tropical (Australia and Persian Gulf), temperate (Tasmania) and polar (Antarctica) carbonates varies with seawater temperatures. Aragonite is the predominant mineral in tropics, whereas high-Mg, intermediateMg and low-Mg calcites dominate in non-tropical shallow carbonates. The amount of Mg in calcite depends on both seawater temperature and CO2 levels. 5180 and compositions of Recent tropical, temperate and polar carbonates are distinct due to aragonite and calcite contents, seawater temperatures, salinity and variation of 5 ^ C in atmosphere and seawater. The present study proposes 8 ^ 0 and criteria that involve the determination of equilibrium lines for aragonite and calcite as a function of temperatures by considering Recent average 5 ^ 0 values of seawater and 5 ^ C values of atmospheric CO2 and seawater.

Ancient carbonates form during the Greenhouse mode and Icehouse mode and these can be detected from original carbonate mineralogy and 5 ^ 0 and composition. Equilibrium 5 ^ 0 and lines for aragonite and calcite were established as a function of temperatures by considering Tertiary and Ordovician average 5 ^ 0 values of seawater and values of atmospheric CO2 and seawater to differentiate aragonite from calcite in Tertiary and Ordovician. This isotopic criteria provides consistently valid results for 5 ^ 0 and 5 ^ C values from unaltered and altered bulk carbonates, skeletal and non-skeletal grains, sparry calcite and micrite. South Australian Tertiary calcitic carbonate isotope field falls on modern temperate isotope field and deviates to lighter values due to meteoric diagenesis. Worldwide Ordovician temperatures obtained from brachiopod 5 ^ 0 values ranged from 3 to 28°C with calcite forming in non-tropical regions and aragonite and high-Mg calcite forming in tropical Tasmania (Fig.).

To extend these isotopic criteria to other geologic ages require currently available 8 ^ 0 composition of seawater and 513c of very shallow marine (<10m) carbonate in equilibrium with atmospheric CO2 of the period.

Application of this carbonate mineralogy and isotopic criteria for Tertiary and Ordovician illustrates extensive occurrence of non-tropical carbonates.

LATE ORDOVICIAN ALTERED ARAGONITIC Micrite & sparry calcite, Gordon Limestone, Tasmania

SHIFTED TEMPERATE HMC Bulk carbonate

SHIFTED i TEMPERATE Brachiopods

I atm. C 0 2 = -7.2%.

5 w= -5%»

ORDOVICIAN Brachiopods (Wadleigh & Veizer, 1992)

LATE ORDOVICIAN ALTERED ARAGONITIC Bulk carbonate, a Brachiopods.

Tasmania Late Ordovician, -4 Tasmania NON-TROPICAL

TROPICAL 5w= -5%.

29.5

25

20.5

16

11.5

357

2.5 °C

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

OLIGOCENE EXHUMATION AND METAMORPHISM O F ECLOGITE-BLUESCHISTS FROM THE ISLAND SIFNOS, CYCLADES, GREECE. Adamandia Raouzaios 1 . Gordon S. Lister 1 and David A. Foster 2 V E P S , Department of Earth Sciences, Monash University, Clayton 3168, Australia.

2 VTEPS, School of Earth Sciences, La Trobe University, Bundoora 3083, Australia.

The 40Ar/39Ar geochronology technique is being used to constrain the ages of different episodes of phengite growth and recrystallization from the polydeformed and metamorphosed sequences of eclogite-blueschists exposed on the island of Sifnos, Greece. Earlier stages of this study focussed on fabric and microstructural analysis in order to ascertain the relative timing of the various deformations and metamorphic events. Equipped with this detailed history we are currently attempting to constrain the absolute timing of the deformation events and episodes of metamorphic overprint which have effected the rocks during their exhumation.

The island of Sifnos in the western Cyclades contains two Alpine high pressure-low temperature (HP-LT) tectonic slices of the Cycladic Blueschist Belt which are separated by a detachment fault. Both upper and lower plates are believed to have experienced similar peak metamorphic conditions (Ml: ~500°C, 15 kbar) during the Eocene collision between the Apulian microplate and Eurasia. The fabric and microstructural analysis combined with field observations enables confident correlation of events between upper and lower plate.

In the upper plate, or the eclogite-blueschist domain (EBD), M l mineral assemblages such as jadeite + quartz and omphacite + garnet are generally well preserved. Lower pressure overprinting of M l mineral assemblages occurs only locally and is often associated with later deformation. Therefore the previous 40 Ar/ 39 Ar age estimates from the EBD of -40-42 Ma are interpreted as representing cooling of the sequence after the peak metamorphism. Two ductile deformations affected the EBD during its exhumation. We infer that these occurred at pressures below 13 kbar due to the breakdown of the assemblage jadeite + quartz to albite in zones of D3 and D4 shear. Microstructural observations from zones of intense D3 deformation in the EBD indicate new growth and recrystallization of white-mica in S3 crenulations and D3 shear bands. This study has attempted to constrain the age of the D3 deformation by measuring 40 Ar/ 39 Ar age spectra of single phengite grains from samples taken from zones of intense D3 shear. Two samples from different bands of phengite schist from Vroulidia Bay have been dated and both suggest that D3 occurred at approximately 36 Ma. In the lower plate of Sifnos, or the greenschist domain (GSD), the HP/LT assemblages have been overprinted by a pervasive, post D3, epidote-blueschist to greenschist facies metamorphism, M3, at conditions estimated at ~450°C and 8-10 kbar. Microstructural observations from the GSD show that M3 minerals overprint S3 fabrics which have undergone decussate recrystallization during M3. We find that all white mica either grew or recrystallized during M3 and any 40Ar/39Ar age estimates cannot represent earlier events. Three phengite samples have been dated from the lower plate. The ages obtained range from 28-34 Ma and do not appear to reflect the ages of the microstructures. For example a plateau age of 30.0 ± 0.3 Ma was obtained from a sample in which phengitic mica had decussately recrystallized after F4 folding, whereas a younger estimate of steadily decreasing ages from 29-26 Ma was obtained from a sample in which no observable effects of D4 overprint M3 mineral growth. At this stage we can only constrain the age of the D4 deformation and the greenschist facies overprint which predominates the lower plate of Sifnos to prior to 30 Ma. The ages obtained from the GSD may represent cooling of the sequence subsequent to both M3 and D4. The hypothesis we tentatively propose is that the ages represent cooling during uplift along the detachment fault which separate the EBD from the GSD, indicating that the detachment operated during Oligocene times. The next stage of 40 Ar/39Ar dating will address this hypothesis.

Thus our preliminary results have identified two Oligocene deformations, D3 (-36 Ma) and D4 (prior to 30 Ma) which had previously been interpreted as representing Eocene collision and Miocene extension respectively.

In addition the age of the greenschist facies overprint, which predominates the lower plate of Sifnos, can be constrained to pre 30 Ma. This identifies an earlier phase of medium pressure metamorphism in the western Cyclades which cannot be readily correlated with the regional, syn-extensional Miocene Barrovian overprint of the central Cyclades. The NE trending mineral lineation associated with D3 represents the kinematics of the early stages of exhumation , which led to the instability of jadeite in zones which experience D3 deformation.

The latest ductile deformation D4, with a NW directed extension direction, operated 10 Ma prior to regional extension in the central Cyclades. Detachment faulting appears to have commenced after D4, possibly during the period 34-28 Ma. These results imply a more complex history of deformation and high pressure metamorphism than has been recognized previously, with the onset of extensional tectonism taking place during the Oligocene. We suggest that during regional Miocene extension the blueschist sequences of the western Cyclades were already in the upper levels of the crust, and in the western Cyclades, the Miocene extensional epoch of the central Cyclades has had little impact.

358

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE DULLADERRY VOLCANICS - MIDDLE DEVONIAN A-TYPE VOLCANICS FROM CENTRAL-WEST NEW SOUTH WALES. Oliver L. Raymond Australian Geological Survey Organisation GPO Box 378, Canberra, ACT, 2601

The middle Devonian Dulladerry Volcanics is a complex of A-type, predominantly rhyolitic ignimbrites, lavas and breccias. They form a NNW-trending belt some 90 km long and 10 - 20 km wide, east of Parkes, NSW. The volcanics unconformably overlie the Siluro-Devonian sedimentary and volcanic sequence of the Cowra Trough, and are in turn disconformably(?) overlain by the terrestrial sediments of the late Devonian Hervey Group. The volcanics probably overlie most of the granites of the early Devonian Yeoval Complex, although some intrusive contacts have been documented. Regional aeromagnetic data and the occurrence of several small mafic intrusive inliers in the volcanics suggest that a significant part of the Yeoval Complex is buried beneath a thin skin of Dulladerry Volcanics.

The volcanics were deposited in a sub-aerial, probably continental rift setting and are similar in age and composition to the Eden - Comerong - Yalwal Volcanic Zone on the NSW south coast. A U-Pb zircon age of 376±4 Ma coincides with the age of phyllolepid fish fossils found at two locations. The basal units of the volcanics are predominantly coarse grained epiclastic breccias and finer volcaniclastic sediments, intercalated with rhyolite lavas. These are overlain by widespread shallowly dipping ignimbrite sheets, which comprise the bulk of the outcrop of the volcanics, and lesser lavas. The Curumbenya Ignimbrite, which may contain several cooling units in its thickness of at least 200 metres, comprises over 70% of the exposed volcanics. Minor basalt and rare andesite units also occur. Multiple eruptive centres are inferred, now preserved as eroded intrusive or just extrusive rhyolite ± dacite dome complexes.

The volcanics exhibit a distinctive A-type chemistry (high Zr, Y, Nb, REE, Ga), indicating that the volcanics are not comagmatic with the adjacent Yeoval Granite Complex or Eugowra Granite, which show typical I-type fractionation trends. An indicator of some fractionation in the Dulladerry Volcanics is provided by the fairly low abundance (average 230 ppm) and wide range (< 5 to 600 ppm) of Ba in the felsic volcanics. In addition, small granite bodies associated with the volcanics have been observed with extreme A-type geochemical traits, suggestive of extended fractionation or evolution of a mineralising fluid on at least a local scale.

Fractionation is unusual for A-type volcanics, and this combined with their generally water-undersaturated character means that A-type volcanics and granites are not usually regarded as prospective for mineralisation (Whalen et al., 1987). However, the analogous Eden - Comerong - Yalwal Volcanic Zone contains several epithermal gold fields and it is this style of mineralisation around eruptive centres which provides the most prospective targets in the Dulladerry Volcanics. Quartz ± sericite ± chlorite alteration occurs in the Glen Isla and Red Hill areas, with epithermal sinter recognised at Glen Isla (O'Sullivan, 1992). Copper and minor gold mineralisation occurs at the contact of rhyolite and andesite at Reedy Creek, but is not of epithermal origin.

Epithermal gold veins occur in basalt and andesite adjacent to rhyolites of the Dulladerry Volcanics at Mt Aubrey (Hopf, 1992). Recent regional mapping, however, suggests that the intermediate to mafic volcanics are older than the felsic Dulladerry Volcanics. These basalts, called the Fairy Mount Basalt, have been tentatively correlated with the early Devonian Cuga Burga Volcanics. However, the hydrothermal system may still be related to the younger Dulladerry volcanism.

References Hopf, S., 1992. Host rock geochemistry and alteration in the Palaeozoic Mount Aubrey epithermal gold deposit, NSW. Geological Society of Australia, Abstracts 32, 79-80.

O'Sullivan, R., 1992. The geology and genesis of a Devonian epithermal system in the Tomingley area. BSc (Hons) thesis, University of Newcastle, Newcastle (unpubl.)

Whalen, J.B., Currie, K.L. & Chappell, B.W., 1987. A-type granites: geochemical characteristics, discrimination and pedogenesis. Contributions to Mineralogy and Petrology, 95, 407-419.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

FISSION-TRACK ANALYSIS OF THE CENTRAL EASTERN MARGIN OF AUSTRALIA: A RECORD OF THE NORTHWARD PROPAGATION OF THE TASMAN RIFT SYSTEM? Asaf Raza1. Kevin C. Hill1, Andrew J.W. Gleadow1, Roderick W. Brown2, Russell J. Korsch3 and Barry P. Kohn1 * Australian Geodynamics Cooperative Research Centre, School of Earth Sciences, La Trobe University, Melbourne, Vic. 3083 2 School of Earth Sciences, La Trobe University, Melbourne, Vic . 3083 3 Australian Geodynamics Cooperative Research Centre. AGSO, GPO Box 378, Canberra, ACT 2600

The most pronounced tectonic episode affecting the eastern continental margin of Australia during the Mesozoic was the rifting of the Lord Howe Rise/New Zealand continental fragment from Australia and subsequent opening of the Tasman Sea. A widely cited plate tectonic model for this separation envisages the simultaneous initiation of sea floor spreading along the full length of the east Australian margin at ca. 96 Ma (KQZ) (eg Veevers et al., 1991, Fig. 6; Veevers, 1986, Fig. 132), and its continuation until the end of the Palaeocene ca. 54 Ma (A24).

In order to accommodate the geometry of the observed sea floor magnetic lineations within the Tasman Sea (eg Weissel and Hayes, 1977) this model requires asymmetric spreading, with the most rapid rates of oceanic crust formation occurring on the Lord Howe Rise/New Zealand plate.

North of the Gilbert fracture zone (GFZ) the magnetic lineations are oriented NNW-SSE. These are oblique to the present central eastern margin by 10-15°, such that the age of the oldest identified anomalies abutting this section of the margin young progressively towards the north: from anomaly A33 (79.1 Ma) at Point Hicks (38° S) to anomaly A24 (53.7 Ma) at -33° S. South of the GFZ there is an angular difference of 10-15° between the younger anomalies and anomaly 33, which is aligned roughly parallel to the continental margin at 40° S (adjacent to Bass Strait).

Other models have proposed separate stages of sea floor spreading within the southern and northern Tasman Sea basins, and envisage a northward propagating rift system leading to the formation of the northern Tasman Sea (eg Shaw, 1990).

New apatite fission track data from the central eastern Australian continental margin (southern New England Orogen and the adjacent Gunnedah and Bowen basins to the west) provides further independent evidence for the propagating rift model These data record two discreet episodes of crustal cooling: thefirst,between 100-80 Ma, affected both the margin and the interior basins, while the second, occurred at ca. 60 Ma, and was restricted to a <50 km wide zone along the present coastline.

We propose that the earlier, regional cooling at 100-80 Ma was caused by a widespread denudation episode across the study area which ranged from a maximum of - 3 km at some places near the present day coastline to a few hundred metres further inland. This phase of denudation was initiated by intracontinental tectonism driven by the major changes in the tectonic configuration of the Australian plate at this time (ie -95 Ma). These include initial rifting between Australia and Antarctica, and the reorganisation of the ridge system within the south east Indian Ocean. The younger episode of cooling at ca. 60 Ma is roughly coeval with the onset of sea floor spreading at the latitude (-32-29° S) of the central eastern Australian margin. We suggest that this coincidence in timing, the restricted nature of this later episode (to <50 km from the coast) and the geometry of magnetic lineations are best explained by a northward propagating rift system.

References Shaw, RX>., 1990. Development of the Tasman Sea and easternmost Australian continental margin - a review.

AGSO Bulletin 232, 53-66.

Veevers, J J., 1986. (Eds.) Phanerozoic Earth History ofAustralia (Second Edition). Clarendon Press, Oxford.

1 - 418.

Veevers, JJ., Powell, C. McA. & Roots, S.R., 1991. Review of seafloor spreading around Australia. L Synthesis of the pattern of spreading. Australian Journal of Earth Sciences, 38, 373-389.

Weissel, J JC. & Hayes, DJE., 1977. Evolution of the Tasman Sea reappraised. Earth and Planetary Sciences Letters 36, 77-84.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996

STRATIGRAPHY AND STRUCTURE OF THE SOUTHERN TERMINATION OF THE BROKEN HILL OREBODY Lachlan G. Re id Pasminco Mining, PO Box 460, Broken Hill, NSW 2880

SUMMARY Consistent stratigraphic and structural packages have been recognised at the southern termination of the Broken Hill Orebody and this is providing a uniform framework for exploration. Rock units within the Thackaringa, Broken Hill and Sundown Groups are recognised, however focus has been on the internal stratigraphy and structure of the Hores Gneiss which hosts the Broken Hill Orebody.

INTRODUCTION Surface drilling in the early 90's from Section 175 (1.1km south of the Broken Hill Orebody) intersected lodey rocks and thin mineralised pods/stringers within Hores Gneiss. In 1993 exploration diamond drilling commenced at the most southerly underground access on Section 160 (0.8km south' of the orebody) to define the projected mineralisation and lode positions between Sections 175 and the orebody (Section 120).

Subsequent drilling on Section 150 intersected geological packages consistent with those found from Sections 175 to 120.

STRATIGRAPHY A consistent sequence of metasedimentary packages is present to the south of the orebody on Section 160. The Pamell Formation is represented by a variably thick amphibolite and Potosi Gneiss sequence. These rocks are overlain by Freyers Metasediments which at the top consist of a blue quartz lodey psammite with minor sulphides. This stratigraphic position is equivalent to the recently discovered Potosi Orebody ( 9km north east).

Distinctive stratigraphic units are well developed within the Hores Gneiss. At the base an augen feldspar pelite with very thin discontinuous BIF zones is present. This is overlain by a garnet spotted blue quartz lodey psammopelite with blebby blue quartz pegmatite segregations. This is the best lode position recognised and is stratigraphically equivalent to C Lode. Mineralisation occurs as thin stringers and pods (<2m) of sphalerite and pyrrhotite but were found to be discontinuous along dip (sectional drill spacing -50m). High grade sulphides are best developed in blebby blue quartz pegmatite segregations near the base of Hores Gneiss. Downdip from this position Potosi Gneiss is well developed and represents most of the Hores Gneiss.

The middle portion of Hores Gneiss is dominated by numerous pelitic and psammopelitic metasedimentary packages with quartzofeldspathic segregations. The top of Hores Gneiss is characterised by intercalated pelitic metasediments and thick Potosi Gneiss. Where the metasediments are more siliceous, contacts with Potosi Gneiss are commonly gradational.

STRUCTURE The Broken Hill orebody is complexly deformed with a major synform and antiform pairing known as the Western Antiform and the Eastern Synform. To date these folds have not been recognised within Hores Gneiss on Sections 175, 160 and 150. On Section 120 only fold flexures are present rather than well developed folds.

The Hores Gneiss is bisected by a major retrograde shear known as the Main Shear. This is a NE-SW trending sericite rich, highly foliated schist zone with an indicated west block up shearing sense. Updip the Main Shear is a single 10-30m thick zone dipping 80° west and downdip it appears to splay into a series of thin (<l-10m) shears varying in dip from 80° east to 75° west.

West of the Main Shear the Hores Gneiss displays only minor thickness variations, however to the east considerably thickness variations are observed within Potosi Gneiss (20-150+m).

CONCLUSIONS Recognition of discrete lithologies through the Broken Hill rock sequence has provided a uniform approach to understanding the stratigraphy and structure at the southern termination of the orebody.

Acknowledgments: Pasminco Mining for permission to publish and present this paper. Helpful discussion provided by Dave Larsen.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PRECIPITATION TEMPERATURES AND ORIGIN OF QUARTZ CEMENT AND ITS INFLUENCE ON T H E TIRRAWARRA SANDSTONE RESERVOIR QUALITY, SOUTHERN COOPER BASIN, SOUTH AUSTRALIA Mohammad R. Rezaee and Peter R. Tingate National Centre for Petroleum Geology & Geophysics University of Adelaide, GPO Box 498, South Australia 5001.

ABSTRACT Quartz cement is the most abundant authigenic mineral in the fluvio-deltaic Tirrawarra Sandstone. The amount of quartz cement ranges from approximately zero to 19 percent and is controlled by the original sandstone composition. Quartz cement plays an important role in controlling TirTawarra Sandstone reservoir quality and its origin has been examined. Fluid inclusions, electron microprobe and cathodoluminescence (CL) examinations of quartz cement revealed a multi-stage quartz overgrowth with different precipitation temperatures and possible different silica sources. CL observations indicate up to six stages of quartz cement in some samples. The stages of quartz cement can be classified into three zones: an internal zone of brown cements (Zl), a zone of bright blue middle cements (72) and an external zone of brown cements (Z3). Bitumen\dead oil are entrapped between Z2 and Z3, indicating that Z3 formed after oil migration commenced. Homogenisation temperature measurements from fluid inclusions within quartz overgrowths indicate that quartz cement precipitated over a temperature range of 65 to 130°C. Zl quartz cement formed at temperatures between 65 and 80°C, and Z2 cement was precipitated between 80°C and I00°C. Z3 quartz cement formed later at temperatures mostly around 130°C. Electron microprobe analysis of quartz cement shows a consistent variation in A1 between each cement zone.

The average A12OI content for Zl, Z2 and Z3 is respectively 0 . 0 2 6 , 0 . 0 6 and 0 . 0 0 7 wt%. Precipitation temperatures and the A1 content has been used to help identify the sources of silica for different zones of cement.

A likely source of silica for Zl quartz cement is early alteration of feldspar grains. This source is consistent with pore-water oxygen isotope enrichment during coeval siderite precipitation. The Z2 quartz cement has the highest A1 content which appears to be the likely cause of the bright blue CL colour. The source of silica for this cement is likely to be from late dissolution of feldspar grains by Al-rich low pH fluids generated during kerogen maturation. Late quartz cement (Z3) has the lowest A1 content which is similar to detrital quartz grains.

Considering the temperature of precipitation, very low A1 content and occurrence of Z3 cement in facies prone to stylolitisation, the silica source for the cement is likely to have been derived from dissolution of detrital quartz at stylolites and grain contacts.

The best reservoir lithologies are composed of sandstones containing mainly Zl and Z2 quartz cements. They are typically well sorted, quartz-rich sandstones that formed in aeolian and point bar environments and have relatively higher porosity (14-17%). Reservoir lithologies which have ail zones of quartz cement have a high proportion of intergranular volume (IGV) but low porosity. Although these sandstones are clean, they are sandwiched between facies that are prone to stylolitisation and provide silica for their Z3 cements. Parts of the reservoir which contain no quartz cement or only a small amount of Z3 have a very low IGV and no visual porosity, and it is these finer-grained sandstones that display extensive stylolisation.

Acknowledgment: The authors wish to acknowledge SANTOS Ltd (operator on behalf of the Cooper Basin consortium) for permission to present this work.

362

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

RODINIA BREAKUP AND A LOWER-PLATE MARGIN MODEL AS A SOLUTION TO THE LACHLAN OROGEN ENIGMA M.J. Rickard Geology Department, Australian National University, Canberra ACT 0200.

The Lachlan foldbelt is unusual in that, in spite of its width (800km) there is no miogeocline, no foreland fold belt, no large-scale overthrusting, no clear arc-forearc-trench arrangement, no age polarity across the abundant granites or across folded zones. Moreover there is no molasse foredeep or evidence for uplift to mountain chain as is common with crustal thickening in subduction-related orogens.

Tectonic interpretation must focus on two enigmas: i) The deposition of Siluro-Devonian terrestrial-shallow water felsic volcanics on top of thick Ordovician oceanic turbidites, and ii) The lack of an acceptable subduction scenario.

Petrological and geochemical evidence does not support subduction; Ordovician volcanics are of intra-plate origin, and the lack of tonalites and gabbros, the K-rich nature of the granodiorites and the abundance of S-types are features unlike subduction-derived granitoids. Geochronology and geochemistry demand variable sedimentary and igneous sources of Proterozoic age.

A solution is presented in terms of the 'SWEAT' hypothesis and asymmetric break up. It is suggested that the Lachlan developed as a complex lower-plate margin after break up of Proterozoic Rodinia. As western North America moved away from Australia in the Cambrian, the crust was sliced by an extensional fault system that cut out sequentially Proterozoic sediments then the lower (igneous underplated) crust; this transected junction explains the present S-I line. The thinned and listrically block-faulted Proterozoic crust was covered first by fault-located ophiolitic volcanic edifices, then in the Ordovician by a wide thermal sag basin that was filled with extensive turbidite sequences prograding over the margin as a continental-rise prism and with some local intraplate volcanic extrusions. This 4-5km thick flysch sheet returned the crust to about normal thickness over the upwarped lithosphere resulting from the lower-plate geometry. Compression, perhaps caused by sea-floor spreading changes, and limited A-type subduction, began to thicken the deposits so that high-temperature, lowpressure facies metamorphism accompanied folding in the Late Ordovician and Mid Silurian. Melting of the buried Proterozoic metasediments allowed formation of S-type granites and their intrusion in Late Silurian and

by Mid Devonian I-type granites from a deeper igneous source. Mid-Devonian deformation reversed movements on major faults and imbricated the upper crust.

Because there was no addition of melt from the mantle there was no net increase in volume and therefore no uplifted mountain chain or foredeep developed. The Silurian and Devonian volcanics and even the Late Devonian molasse were deposited at or close to sea level.

Thus the Lachlan orogen developed on a passive margin, essentially without subduction. Deformation and magma generation were essentially of intra-plate nature. Major subduction did not commence until the Late Palaeozoic to develop the New England-Yarrol orogen, although it extended across into the Lachlan to cause the major Mid-Devonian folding. This explains why the Lachlan orogen has so many anomalous features, more similar to the Australian Proterozoic ensialic orogenic belts than Cordilleran orogens.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 THE CANOWINDRA FISH FOSSILS - A CASE STUDY Dr Alex Ritchie and Monica Yeung The Australian Museum, 6 College St, Sydney NSW 2000 Gondwana Dreaming Tours, PO Box 3017 Weston ACT 2611 1

2

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With the discovery of a world class fish fossil site nearby, Canowindra, in central western New South Wales, has become the focus of a unique cooperative venture, involving the local community, palaeontologists and a tour operator. Canowindra's unique Devonian fossilfishdeposit was first discovered by chance during council roadworks in 1956. It was relocated in 1993 by Dr Alex Ritchie, from the Australian Museum, after years of unsuccessful attempts. To locate the fossils again heavy earth moving equipment was required, and funding for this simply was not available until Alex gave a talk to the local Rotary Club late in 1992. As a result of this talk the local Council made a 22 tonne excavator and driver available for a major dig, because the tourism potential of such a fossil find near Canowindra did not fail to get their attention.

The main dig took place in July 1993, with the help of the local community members. The amount of fossil material recovered was overwhelming: nearly one hundred pallets full of rock slabs, each weighing around one tonne. This created new problems. Storage and workshop facilities and a fork lift were required. The slabs themselves needed to be cleaned up and latex casts made so that palaeontologists could work on the material, and all this required some major funding. Fortunately, local interest was awakened, especially after Alex proposed that the fossils should stay at Canowindra to form the central attraction of a world class "Age of Fishes" museum. This museum will incorporate workshops and laboratory facilities, and a Research and Learning Centre where students and scientists can undertake research projects. An "Age of Fishes" Committee was set up by about a dozen committed individuals of the town. These people found a temporary home for the fossils under the grandstand at the local showground. It is there that lay-people and scientists have now been working side by side for over two years, cleaning up the rocks containing the fossils and making casts.

Funding for the project has comefromindividual, community and corporate sponsorship, and from participants in the monthly fossil digs organised by Gondwana Dreaming Tours. Groups of about 20 paying volunteers are presently spending weekends at Canowindra, learning about Devonian fossil fish, the environment they lived in and how the fish became fossilised. The volunteers work on the original fossil covered slabs, cleaning and casting them, supervised by scientists. Hands-on learning continues at a recently discovered second fish fossil site in the area, where the volunteers get a chance to dig up fossils in a disused quarry. Over 500 people have so far participated in these digs, which appeal to people of all ages and from all walks of life. The scientists answer any questions on the fossils with a minimal use of scientific jargon, and most of the paying volunteers learn a great deal about fossil fish in the course of a weekend, whilst thoroughly enjoying their experience, which is enhanced by country hospitality, excellent food and interesting company. This is evidenced by the many letters of thanks we receive, as well as the number of clients who return for another weekend fossil dig at Canowindra. The digs provide much needed funds for palaeontological research, a substantial contribution into a building fund for the future Age of Fishes Museum, and ongoing project funding. The groups are made up mostly of lay-people, but have included internationally renowned palaeontologists form all over the world as well as several groups of students, ranging from primary to post graduate level. The level of instruction is geared specifically to each group, from casual visitors to school and university students and fossil specialists.

The Canowindra Fish Fossils attract a lot of public interest with the number of casual visitors steadily increasing. The recently opened temporary Resource and Learning Centre is a small step towards the goal of building the "Age of Fishes" Museum. It allows tourists and groups of students to see some of the fossil material, to hear live or taped lectures about the fossils, watch videos, or to use the interactive computer software and the library.

The Age of Fishes Museum Project is an excellent example of the successful interaction and cooperation of a local community with palaeontologists and a tour operator for mutual benefit. It is a classic example of Matrix Ecotourism (Yeung et al, in press). Ecotourism is nature based tourism that involves education and interpretation of the natural environment and is ecologically sustainable.

REFERENCE Yeung, M., Stephenson, A.E., Ritchie, A., and Connor, F., in press. The Canowindra fish fossils - matrix ecotourism at work. Proceedings 1995 EEA National Ecotourism Conference, Alice Springs, NT 1995, Ecotourism and Nature-Based Tourism: Taking the next steps.

364

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996 EVIDENCE FOR MODAL MANTLE METASOMATISM IN THE MONARO VOLCANIC PROVINCE, SOUTHEASTERN NSW, AUSTRALIA. Ian C. Roach * & Suzanne M. Edgecombe^ ^Centre for Australian Regolith Studies, University of Canberra, PO Box 1, Belconnen ACT 2616 ^Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601

Xenoliths from, the Cr-diopside and Al-augite suites (O'Reilly 1989) are present within alkali basaltic volcanic plugs of the Monaro Volcanic Province, southeastern New South Wales (Roach et al. 1994). Mantle xenoliths are mostly coarse-grained equigranular to weakly-foliated Cr-spinel lherzolite and harzburgite with rare wehrlite, websterite, amphibolite, orthopyroxenite and dunite (Edgecombe 1992; Roach et al. 1994). Xenoliths display a range of petrographic evidence for mantle metasomatism within the Province.

Xenoliths are normally <5 cm diameter and highly rounded although at Amphibole Hill (Roach et al. 1994) some angular xenoliths of about 15 cm maximum length were recently noted. Knots "of microcrystalline olivine, clinopyroxene and opaques mark resorbed xenoliths in coarser-grained plug rocks. Xenocrysts consist of material from disaggregated mantle and crustal xenoliths: ragged olivine xenocrysts often display a halo of microscopic opaques; clinopyroxene xenocrysts may have a halo of titanian augite in optical continuity; orthopyroxene xenocrysts always have a halo of microscopic olivine, clinopyroxene and opaques; and Cr-spinels are generally unaltered.

Evidence for mantle metasomatism consists of: 1. Pargasite in textural equilibrium in spinel-absent lherzolite xenoliths.

2. Medium-grained equant websterite xenoliths.

3. Medium-grained equant to weakly-foliated amphibolite xenoliths.

4. Equant two-pyroxene plagioclase granulite xenoliths.

5. Thin pyroxenite veins in spinel lherzolite xenoliths.

6. Phlogopite in a foliated wehrlite vein in a spinel-absent lherzolite xenolith.

The evidence is interpreted as being the products of ancient pre-eruptive and more modern syn-eruptive metasomatic events. Pargasite in textural equilibrium with lherzolite indicates complete recrystallisation of the rock after the infiltration of a thin hydrous metasomatic fluid. The websterite, amphibolite and granulite xenoliths are mostly in textural equilibrium (equant) indicating a considerable lapse of time between a more vigorous metasomatic event from which they were crystallised and their eruption. These rocks are interpreted as having crystallised within veins in the upper mantle/lower crust. Thin pyroxenite veins in spinel lherzolite xenoliths appear to represent the extremities of the larger metasomatic veins. Weak foliation in some of the xenoliths, depicted by variable extinction in olivine and alignment of amphibole crystals, indicates some preemption shearing within the upper mantle/lower crust. Phlogopite in a foliated wehrlite vein within spinel lherzolite appears to be the product of syn-eruptive melt infiltration and addition of water from a hydrous melt.

References Edgecombe, S.M., 1992. The geochemistry and geological setting of a suite of spinel bearing xenoliths from the Monaro Volcanic Province, southeastern New South Wales. Honours thesis, Australian National University (unpubl.).

Roach, I.C., McQueen, K.G. & Brown, M.C., 1994. Physical and Penological Characteristics of Basaltic Eruption Sites in the Monaro Volcanic Province, Southeastern New South Wales, Australia. AGSO Journal 15(3), 381-394.

O'Reilly, S.Y., 1989. Xenolith Types, Distribution and Transport. In Johnson, R.W. ed. Intraplate Volcanism in eastern Australia and New Zealand, pp 249-253. Cambridge University Press, Melbourne.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE REGIONAL STRUCTURE OF NORTHEAST TASMANIA - A GEOPHYSICAL PERSPECTIVE 1

M.J.Roach1 and D.E.Leaman2 Centre for Ore Deposit and Exploration Studies, The University of Tasmania. P.O. Box 252C, Hobart, Tasmania 7001.

2 Leaman Geophysics, P.O. Box 320D, Hobart, Tasmania 7001.

The lower Palaeozoic sedimentary rocks of eastern Tasmania, the Mathinna Group, differ significantly in character and structural style from sequences of the same age in western Tasmania. The boundary between the two regions is entirely concealed beneath a cover of post-Devonian rocks in the Tamar Valley and northern Midlands. A number of structural models have been proposed for the region but the lack of outcrop in critical areas prevents direct geological assessment. Regional gravity and magnetic data were used to assess the distribution of major rock units in the subsurface and place constraints on the proposed structural models.

The gravity field in northeast Tasmania is dominated by a broad, strongly negative residual Bouguer anomaly in the east which generally coincides with the outcrop of the Blue Tier and Scottsdale Batholiths. Modelling suggests a thickness of at least 8 km of granitic rocks in this area. The steep western margin of this anomaly marks the boundary between crust dominated by low density granitic rocks to the east and denser granodioritic and Mathinna Group rocks to the west. The maximum inferred thickness of Mathinna Group rocks is ~4 km.

The magnetic field is characterised by widespread high amplitude, high frequency anomalies due to Jurassic dolerite and Tertiary basalt. The Mathinna Group rocks and Devonian granitoids are mostly non-magnetic. A high amplitude north-south trending anomaly just to the west of Beaconsfield results from a thrust slice of ultramafic rocks emplaced within a Cambrian sedimentary sequence on the eastern margin of the Precambrian Badger Head Block. To the east of the Tamar River the broad, elliptical "West Sandy Cape Anomaly" extends to the east across the margin of the Scottsdale Batholith. Modelling suggests that this anomaly results from a strongly magnetic, shallowly east-dipping body at a depth of approximately 5 km (Figure 1). This feature is inferred to be a thrust slice of Cambrian ultramafic rocks which underlies the Mathinna Group and marks a major regional detachment surface.

Figure 1. Two dimensional gravity and magnetic model across northeast Tasmania, The gravity and magnetic datasets are complimentary and provide a view of the regional structure of northeast Tasmania which supports a thin-skinned tectonic model involving west-directed thrusting of the Mathinna Group and Cambro-Ordovician sequences over siliceous Precambrian basement. This interpretation has important implications for regional tectonic models of the geological development of Tasmania.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE WITWATERSRAND GOLD-URANIUM DEPOSITS Laurence J. Robb Department of Geology, University of the Witwatersrand, Johannesburg 2050, South Africa

Discovered 110 years ago and mined continuously ever since, the Witwatersrand Basin remains the most prolific gold producer in the history of mankind. A fortuitous combination of events, not often, if ever, repeated in the course of geological history, resulted in the formation of a series of deposits which collectively have produced some 45000 tons of gold metal and over 150 000 tons of uranium concentrate.

GEOLOGICAL FRAMEWORK The Witwatersrand Basin formed over a period of 360 Ma between 3074 and 2714 Ma. Pulses of sedimentation within the sequence and its precursors were episodic, occurring between 3086-3074 Ma (Dominion Group), 2970-2914 Ma (West Rand Group) and 2894-2714 Ma (Central Rand Group). Detritus was derived from a mixed granite-greenstone source which was continuously evolving during the extended period of sediment deposition. At least two periods of greenstone belt formation are now recognized in the Witwatersrand Basin hinterland: the first comprises Barberton-type greenstone belts and granitoids which are >3100 Ma old and represent the basement on top of which the basin was deposited, and the second consists of the Murchison greenstone belt as well as the greenstone belt-like Kraaipan Formation and associated granitoids, which formed at 3100-2900 Ma. Subsequent granitoid plutonism in the basin hinterland continued throughout the deposition of the upper part of the sequence, coinciding with hiatuses in sedimentation and stimulating further episodes of deposition. Many of the provenance granitoids are characterized by hydrothermal alteration, are geochemically anomalous with respect to U and to a lesser extent Au, and may represent viable source rocks for palaeoplacer mineralization. The Murchison and related greenstone belts, with their contained gold mineralization, formed just prior to deposition of the auriferous Central Rand Group, an event which may have had major implications for the fertility of the source area and the concentration of gold into the upper portion of the basin.

Technically, the Witwatersrand Basin evolved in response to processes occurring within a Wilson cycle, the latter associated with the encroachment and ultimate collision of the Zimbabwe and Kaapvaal cratons. The mechanics of this prolonged period of orogenesis are still poorly understood, although it is widely held that early (Dominion) sedimentation and volcanism was related to intracontinental rifting, with much of the subsequent sequence developed in a compression-related foreland basin. Witwatersrand deposition was terminated by outpouring of the Ventersdorp flood basalts and the development of extensional syn-rift volcanism and sedimentation. Subsequent deformation and metamorphism of the Witwatersrand Basin occurred at ca. 2500, 2300 and 2000 Ma. The first two events coincided with the progressive loading of the basin by the Transvaal cover sequences, whereas the last reflects major resetting and deformation related to intrusion of the Bushveld Complex and the Vredefort catastrophism.

MINERALIZATION Gold and uranium mineralization is concentrated in conglomerates of the Central Rand Group and is represented by a complex paragenetic sequence initiated by early accumulation of detrital heavy minerals.

Allogenic gold, uraninite and pyrite, as well as many other phases, were concentrated by a variety of hydrodynamic dispersion processes along unconformities,fluvialchannelways, cross-bed forsets and deflation surfaces.This was followed by three authigenic stages of mineralization related to metam orphic fluid circulation. An early event of authigenic pyrite formation at 2500 Ma was followed at 2300 Ma by maturation of organic material, fluxing of hydrocarbon-bearing fluids through the basin and the radiolytic fixation of bitumen around detrital uraninite. This was followed, at around 2000 Ma, by peak metamorphism which was accompanied by the widespread precipitation of gold and the formation of a variety of secondary sulphides.

Several generations of post-depositional fluid have been recognized, including one associated with hydrocarbon maturation in which substantial concentrations of CO2, N^ and both liquid/gaseous and solid hydrocarbons are included. Physicochemical conditions during fluid circulation were such that metal solubilities were low and precipitation mechanisms very effective, resulting in the superimposition of both primary and secondary mineralization. Authigenic gold occurs in close proximity to allogenic pyrite, authigenic fahlores and bitumen, and has been precipitated by micro-scale electrochemical processes and redox reactions. Although the modified placer hypothesis continues to dominate local thinking, a major challenge still facing Witwatersrand metallogenesis is the question of whether authigenic gold was introduced into the basin as a juvenile component or simply remobilized in situ.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

WORLD-CLASS ARCHEAN HYDROTHERMAL GOLD DEPOSITS: A CANADIAN PERSPECTIVE Francois Robert and K. Howard Poulsen Geological Survey of Canada, 601 Booth St, Ottawa, Canada, K1A 0E8

World-class Archean hydrothermal gold deposits (i.e. >100t Au) in Canada occur mostly in greenstone belts of the Archean Superior and Slave Cratons, with a few deposits in clastic sedimentary sequences. In greenstone belts, the districts that host these deposits share a number of recurring geological and structural characteristics, similar to those in other Archean cratons around the world. The districts and deposits typically occur at or near lithospheric fault zones marking boundaries between lithologically contrasting domains within greenstone belts or along their margins. The districts have a common structural history involving Di "thin-skin" and D "thick-skin" shortening, evolving into D (D ) transcurrent deformation, with deposition of unconformable fluvial-alluvial sediments between D and D . Gold districts in clastic sedimentary sequences also have a complex structural evolution mainly resulting from crustal shortening. Districts in both environments have undergone greenschist to lower amphibolite grade metamorphism. As emphasized in recent years by several researchers, Archean hydrothermal gold deposits.around the world also share a number of recurring characteristics, including common associations with shear zones, relatively simple ore mineralogy, generally elevated gold to silver ratios, and involvement of C02-bearing fluids. These similarities have prompted development of unifying genetic models for lode gold deposits.

A closer examination of the geological characteristics of world-class hydrothermal gold deposits in Canada reveals a significant diversity in styles of mineralization that cannot be accommodated by a single model. The different styles (Table 1) include, in approximate order of decreasing abundance: quartz-carbonate veins related to shear zones and folds (Dome, Kerr Addison, Sigma-Lamaque, Kirkland Lake Main Break?), zones of disseminated sulphides ± stockworks around intrusions (Hemlo, Malartic), massive sulphide lenses (Home, Bousquet #2, ±Lupin), intrusion-hosted sulphiderich veins, stockworks and disseminated sulphides (Doyon, Troilus), and caibonateiquartz veins (Campbell-Dickenson).

A similar diversity of styles of mineralization is noted when considering the entire population of deposits, but a number of world-class deposits combine more than one style of mineralization: quartz-carbonate veins + stockwoik sulphides at Hollinger-Mclntyre, and quartz veins + massive sulphide layers at Lupin.

Table 1: Summary of selected attributes of different styles of Canadian Archean hydrothermal gold deposits, 2

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Style of Mineralization quartz -carbonate veins (<10% sulphides) disseminated sulphide zones ± stockworks massive sulphide lenses

Metal Signature Au>Ag, W, B ±Te, Mo Au>Ag, As, Te ±Hg, Mo, Sb Ag>Au, Cu, Zn

sulphide-rich veins, stockworks Ag:Au variable, Cu± Mo,Zn and disseminated sulphides Au>Ag, As, Hg, carbonate veins with quartz W replacements and breccias

Alteration Minerals carb-ser-py ± alb, apy (1) Kfp-mu-sil-ky (2) ab-carb-sei±Kfp (1) ser-chlo or (2) and-ky-ser (1) ser-chlo (2) Kfp, biot, ser ser-biot-caib-silica

Associations brittle-ductile shear zones and folds tonalitic to syenitic stocks and dykes sequences with felsic volcanics synvolcanic diorite-tonalite intrusions regional synvolcanic alteration zone

3

Canadian Examples Dome, Ken Addison Sigma-Lamaque (1) Hemlo (2) Malartic (1) Home (2) Bousquet #2 (1) Doyon (2) Troilus Campbell Red LakeA.H. White

The diversity of styles of mineralization is reflected in differences in composition of ore, hydrothermal alteration, and lithological or structural associations (Table 1), factors which likely require different types of fluids involved in the formation of these deposits. Furthermore, the different styles of mineralization have formed at different stages in the evolution of their host terranes. Massive sulphide deposits (auriferous VMS), sulphide-rich veins and some intrusionhosted stockwork deposits (porphyry?) predate Di; most shear-zone-related quartz-carbonate veins have formed during D2, whereas several disseminated sulphide ± stockwork deposits, spatially associated with small felsic intrusions and pre-D3 fluvial-alluvial sedimentary rocks, formed between D2 and D3. In addition, there is evidence for their formation in different crustal environments: pre-Di deposits and several circa-D3 disseminated sulphide ± stockwork deposits have formed at relatively shallow (<5 km) crustal depths, at stages of construction of volcano-plutonic edifices. In contrast, syn-D2 quartz-carbonate veins clearly formed in deeper environments (>5 km), at times of deformation of such edifices.

Although these different styles of hydrothermal mineralization demand the existence of more than one distinct genetic type of deposit, they commonly occur within the same gold districts or along the same fault zones. This indicates that gold deposits, formed at different crustal levels, at different times, and by different processes, have been juxtaposed by successive episodes of burial, uplift and deformation that are focussed in certain areas. This allows forrecyclingof the gold from one stage to the next, and for overprinting of different styles of mineralization, perhaps one key to formation of world-class gold deposits. Canadian world-class examples are known for nearly all of the deposit types identified here, suggesting that multiple sets of exploration criteria and models will better serve future exploration than will unifying models.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 MARKETING GEOSCIENCE - MATCHING PRODUCT WITH MARKET AT THE RIGHT PLACE Angus M Robinson General Manager, The Warren Centre for Advanced Engineering, Sydney University. NSW 2006 1

1

With the closure in September 1995 of Sydney s Earth Exchange museum (formerly the Geological and Mining Museum) following a life of barely four years as Australia's first attempt to market geoscience in a purposebuilt geoscience museum, it is timely to review the challenges facing the geoscience profession in reaching the 'hearts and minds' of Australians and the nation's future scientists and technologists.

Geoscience, or more traditionally geology, is the study of the earth and the physical materials from which it is built (hence the rationale for the name. The Earth Exchange). However, it is recognised that the study of and interest in geoscience appears to be diminishing in Australian secondaiy schools. Because of competition from more exciting careers in the information and service sectors of the economy, students are not actively pursuing careers in geoscience neither in scientific research or industry. Unlike the engineering profession, the geoscience community is not making any concerted attempt to improve the public awareness of the role of geoscience in society, in industry and in the environment; in this latter area, it has been recognised from recent 'foresighting* studies that Australia should realise its strengths in environmental management and market these skills to developing countries. Interest and support from the geoscience profession in past secondaiy education program proposals such as the Geoscience Awareness Program has been dismal. Despite the actual lifestyle and skills of the modern working geoscientist, it is suggested that the public (read market) perception of a field geoscientist is one of a "g-pick" wielding rock doctor'. This hardly matches the reality of a highly skilled professional in contact with the world through laptop computers, telecommunication linkages and global positioning satellites! w

By developing the museum themes of formation and transformation, the vision of The Earth Exchange museum was to facilitate the exchange of public knowledge and understanding about the importance to Australia of geoscience and of the minerals and energy industries. With industry financial support, the museum's communication objectives were expected to publicise and strengthen the Museum's role as an authoritative advice and information centre. However, to attract the 'market', the 'product' needed to offer education through entertainment, a strategy which did not appear to meet the expectations of all the museum's stakeholders.

Moreover, to establish a presence as a significant earth science centre, the museum appeared unable to raise sufficient revenue or gain adequate industry backing to develop specialised 'exploratory' exhibits.

Before enthusiasts elsewhere in Australia embark upon a similar endeavour, it is relevant to consider whether any new museum project in Australia will be able to adequately market serious geoscience to the Australian community in a fiscal environment which demands both commercial viability and the need to meet the demands of industry backers. On the other hand, a commercial theme park operator may be able to offer geoscience-themed 'activities' which will be short on serious content but long on entertainment, a strategy which would seemingly meet market needs. But will this be the product which meets the expectations of geoscientists? Perhaps, in defining a 'place' for the 'product', geoscientists need to agree on what sort of location can meet both the expectations of all stakeholders, including the geoscience profession, as well as the perceived needs of the 'market'.

The Warren Centre for Advanced Engineering has argued that although there are several science or technology centres in Australia, they are generally science based, have little industry and engineering, and no national economic context, and have less secondary school/professional focus than desirable. The Warren Centre is proposing an alternative public venue, the City ofIndustry, as the desired model for promoting the development within Australia of a vibrant industry sector based on high value-added, technology-based goods and services.

Further development of this concept may identify strategies and processes which are more effective in raising community awareness of the importance of geoscience to the environmental and economic future of Australia.

Notwithstanding these developments, the emergence of new entertainment technologies eg virtual reality and the transition of 'piped' entertainment and education to the home environment may cause the education role of museums or other public venues to be substantially diminished. It is therefore appropriate to consider alternative strategies for reaching the home and (school) market of the 21st century, particularly in a globalised environment where this task may be delegated, by default, to communicators /educators from other countries.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE SEDIMENTARY RECORD AS A HISTORY OF TECTONIC EVENTS: AN EXAMPLE FROM THE CANNING BASIN, WESTERN AUSTRALIA Karen K. Romine, M.J. Jackson, J.M. Kennard, R. Shaw and Peter N. Southgate Australian Geological Survey Organization, GPO Box 378, Canberra, ACT 2601

Every sedimentary basin holds within it the record of all events that have affected it, however remote. The tectonic history of a basin is the dominant influence on its accommodation history, which in turn determines the stratal geometry, stratal stacking patterns, and, to a lesser extent, the palaeogeographic distribution of the lithofacies. Careful analysis and interpretation of the stratigraphic record in a basin will provide the basis for determination of the phases of a basin's history and the linkage to specific tectonic events.

In the Canning Basin, the Samphire Marsh Extensional Event initiated a phase of subsidence in the Late Cambrian-Early Ordovician that lasted approximately 70 million years. This phase of the basin's history was terminated in the Silurian-Early Devonian by a regional uplift event, the Prices Creek Movement. The sedimentary record during this period illustrates classic features of an extensional basin phase, beginning with the early deposition of marginal marine clastics (Nambeet Formation) during the "syn-rift" period. Thermal cooling, rapid rates of tectonic subsidence and subsequent marine transgression resulted in the deposition of marine carbonates (Willara Formation) followed by deeper marine shales and organic-rich rocks (Goldwyer Formation) at peak transgression. As subsidence rates decreased to a minimum, a period of regression began, commencing with the deposition of shallow-water carbonates (Nita Formation), followed by facies associated with increasing desiccation and culminating in the deposition of salt (Bongabinni, Minjoo, Nibil and Mallowa Salt formations).

During this regressive depositional phase, tectonic subsidence was low to nil, and accommodation (space for sediments to be deposited) was created largely by fluctuations in eustasy. The depositional cycles in the Mallowa Salt record repeated, high-frequency floodings of a very shallow, low relief basin during the initial 2nd-order rise in sea level following the eustatic lowstand that triggered this period of salt accumulation. Each flooding is followed by desiccation, salt accumulation and the deposition of evaporitic, mudflat facies at the end of each cycle (Cathro et al., 1992), just prior to the next eustatic rise. At the peak transgression in sea level, salt deposition was terminated as the basin flooded and a dolomite marker bed was deposited throughout the basin.

The age of this marker bed is earliest Silurian and occurs near the base of the Sahara Formation.

The Silurian was a time of eustatic highstand. In the Canning Basin, the deposition of dominantly siltstone with infrequent, thin sandstone beds (Sahara Formation) suggests a distal position in a shallow, epeiric sea. During the Early to Middle Silurian, a tectonic movement began that terminated the Ordovician-Silurian phase of the basin's history. An initial phase of uplift/tilting of the northern half of the basin began in the Early-Middle Silurian and continued until Late Silurian, indicated by thinning of the Sahara Formation to the north across the Broome Platform. A second phase of uplift began at the end of the Silurian, concurrent with falling sea level, resulting in subaerial exposure, erosion and truncation of the older strata. An angular unconformity was formed, overlain by flat-lying Devonian (Emsian-Eiffelian) aeolian and sabkha deposits. These events or phases of the same event, the Prices Creek Movement, are coeval with the Rodingan and Pertnjara Movements in the Amadeus Basin which also produced an angular unconformity overlain by Early to Middle Devonian sediments. The coincidence in age of these western and central Australian events with Middle Silurian to Early Devonian transtensional movement along the palaeo-Pacific margin in the east (Tasman Line) suggests that they are related, intra-plate deformational events accompanying global plate reorganization.

REFERENCES Cathro, D.L., Warren, J.K., & Williams, G.E., 1992. Halite salterns in the Canning Basin, Western Australia: a sedimentological analysis of drill core from the Ordovician-Silurian Mallowa Salt. Sedimentology 39, 983-1002.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SELLING GEOLOGY TO THE COMMUNITY W. David Roots Australian Academic Tours Pty Limited, 27/7 Bungan Street. Mona Vale NSW

2103

Most Australians consider geology' to be just a catalogue of crystals, minerals and rocks, or worse, as a form of landscape rape and pillage. The word itself is obscure to many, who fail to read it as Earth Science. Geologists need to modify the factors that have produced this bad press, encouraging existing trends that support change.

(1) In contrast to the National Parks USA, our National Parks and Wildlife Service does little to advertise or explain the geology of their parks. Yet many east coast parks are on volcanoes, or other geological structures, which remained rough country until declared National Parks. NP&WS handouts on these parks contain little or inaccurate geological information (eg: Diamond Head in Crowdy Bay NP is a small ash volcano described as a lava flow). This lack of information may occur because NP&WS recruit mainly biology graduates who, knowing little geology, avoid mentioning the subject. The many thousands who visit national parks are susceptible to learning about natural phenomena, and are as open to geology as they are to biology. We need to author the geological content of NP&WS handouts, while educating their staff, and win the hearts and minds of National Parks visitors.

(2) Australia has few base level geological tourist books. In the USA, in contrast, the Roadside Geology series is available for most states, selling at all National Parks information stalls. Regional geological interpretations also sell, aimed at both adults and children. Books exist also on the geology of most scenic locations (The Grand Canyon, Yellowstone, Bivce, Zion, etc.). We have none of these written at tourist level. What is wrong with us? There are good careers in writing such books! In the USA, geology is important, and user-friendly. I even have a book on Yellowstone National Park written in Doctor Zuess style for children. This book is accurate, full of simple drawings, and quite suitable for adults! What have we done, or not done, to produce this contrast in community acceptance? My geologically-biased educational-tour company sends information to 90,000 people each year. We show them how to use geology to read scenery, and to explain current topics and ideas. Many of our travellers pass on to university study.

We employ geologists as tour leaders, and as well-informed advocates of their science they play their part in raising the status of geologists. Part of our brief is the production of books and information, and giving support to geology teachers, continuing education groups, and National Parks staff.

(3) Geology is dying in the Australian schools. In 1995, just 450 students attempted the HSC geology exam, but 16500 attempted biology! A 1:40 ratio. As many biology classes as there are individual geology students. Why? Students and parents believe most geologists are unemployed! Ask parents how many HSC biology students get jobs and they go silent. Some University geologists advise high school students to take mathematics and physics to HSC level, not geology. "We will teach you all the geology you need"! Imagine the effect of this advice on the morale of career geologyteachers? Acting on such advice, students achieve a high equity in Mathematics and Physics, and so enrol at University in these subjects, not in Geology! Their interest has been captured by the exposure to these subjects, so they forget Geology. Can you imagine a University physicist telling an aspiring Einstein to study geology to HSC level? After all, " we can teach you all the Physics you need". No way!

If a geology class drops below the minimum class size, that class dies. Loose that class, and the school may drop the subject totally. We need to support enthusiastic geology school teachers to enthuse students, after which we can take them on to a career. In a year or so, HSC Geology may cease, unless we support its teaching. Support your local High Schools by offering lectures on geological lifestyle, help with excursions, or whatever. They will love you for offering.

(4) We need to remind the community of the importance of geology for understanding our planet, and preventing environmental disasters. We need to relate geology to conservation issues, presenting it as the science that can provide the answers untrammelled by political position.

Many conservation thinkers now see farming (large area, small wealth/ha) as much more damaging to the general environment than mining (small area, big wealth/ha). Many such people also see eco/educational tourism as the cleanest (and largest) of future industries, requiring the preservation of pristine environments to be viable. Australia is in a good position to concentrate on these two industries, and geologists are well trained for both.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MAGMA MIXING AND THE ORIGIN OF S- AND I-TYPE GRANITES A. G. Rossiter and C. M. Gray School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083

The dominant mechanism responsible for the linear trends shown by chemical variation diagrams for the granites of southeastern Australia remains controversial: the main possibilities are restite unmixing, crystal fractionation and magma mixing (including assimilation). Within-suite Rb/Sr isotopic studies can evaluate the importance of such processes in granite pedogenesis and an assessment of the isotopic variation within southeastern Australian granite suites (particularly S-type ones) is long overdue.

Some S-type suites (e.g. Bullenbalong, Dalgety, Marysville) show a considerable increase in initial 87Sr/86Sr with increasing Si02. Crystal fractionation cannot explain isotopic variation of the magnitude observed given the Rb87/Sr86 ratios of the rocks and the likely lifetimes of the intrusive systems. The restite-unmixing model can explain such variation within a suite only if the melt had higher initial 87 Sr/ 86 Sr than the restite. Given that quartzofeldspathic rocks with relatively low Rb^/Sr 86 (and thus relatively low 87Sr/86Sr at the time of melting) would be more likely to melt than pelitic rocks with higher Rb^/Sr 8 6 such a scenario is most improbable. In any case melt and restite would be expected to equilibrate isotopically and all rocks of the suite should have the same initial 87Sr/86Sr following restite unmixing. It appears, therefore, that magma mixing must be invoked to explain the isotopic variation in at least some S-type suites.

Some I-type suites (e.g. Tara, Delegate, Buckleys Lake, Jindabyne, Moruya, Mt Cole) show little variation in initial 87 Sr/ 86 Sr and restite unmixing, crystal fractionation, mixing followed by isotopic homogenisation, and mixing of isotopically similar endmembers are all plausible explanations for the chemical diversity within them.

The Currowong and Finister Suites, however, show significant increases in initial 87 Sr/ 86 Sr with increasing SK>2. Such isotopic variation is too large to be compatible with crystal fractionation and the restite-unmixing model struggles to explain it as there is the requirement that melts derived from source rocks of different Rb^/Sr 86 homogenise and restite derived from different source rocks homogenises (to generate a chemically coherent suite) but isotopic ratios do not equilibrate. Magma mixing is a much more likely scenario.

It seems, then, that mixing of isotopically evolved (crustal) and isotopically more primitive (mantle?) material is necessary to explain the isotopic compositions of some S-type suites and this process may have played a role in the pedogenesis of most southeastern Australian granites. Confirmation that magma mixing is the dominant mechanism controlling chemical variation within suites is found in the presence of mafic microgranular enclaves.

Interestingly these are more siliceous in S-types than I-types. It may be that only evolved (often shoshonitic) mantle magmas are buoyant enough to reach crustal levels where there are abundant sedimentary rocks and so cause the melting responsible for S-type granites (Fig. 1). More primitive mantle magmas appear to pond at deeper levels where igneous rocks dominate and only I-type granites can result when anatexis occurs (Hg. 1).

The zoning seen in the plagioclase of some granites further emphasises the importance of magma mixing.

Kbar

Hg. 1: Genetic model for the S- and I-type granites of southeastern Australia. That the source rocks are of moreor-less Cambrian age is implied by zircon geochronology. BPSS = Boggy Plain Supersuite.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SORRY MATE, YOUR SPREADSHEET IS NOT UP TO IT R.J. Ryburn Australian Geological Survey Organisation. GPO Box 378, Canberra, ACT 2601

Summary - Armed with a PC costing a few thousand dollars, and software ranging from spreadsheets to geographic information systems, modern geologists are equipped to tackle any data processing task they can think of... or are they? The need to manage data efficiently, store it non-redundantly and securely and to provide access to many people often dictates that a relational database system must be used. Full relational databases are commonly beyond the resources of the individual or project and are better implemented corporately.

The rise of the PC and sophisticated PC software has lulled many geologists into thinking that all data management tasks can be accomplished by the individual on his or her PC. And up to a point they are right.

These days, almost any task can be done on a PC, but not by one person. Many jobs have got too big for one person to handle, requiring a team approach with good communications between team members. This applies particularly to government organisations like AGSO and to mineral and petroleum exploration companies.

Some geoscientists insist that all you need is a spreadsheet program to manage data, and for some tasks a spreadsheet is entirely appropriate. But for many wider purposes it is not. Take, for example, a mineral deposits database and what is required to adequately model this with a spreadsheet. Typically you can have a row for each deposit and columns for attributes like host rocks, deposit types and commodities. This works fine for a limited set of deposits, but as you expand the scope to encompass all types of deposit over the continent you are forced to add many more columns until the spreadsheet becomes wide and unwieldy. You are likely to need columns for 100 commodities, or more. A partial solution to this problem is to give each deposit a number and put commodities into a second spreadsheet with a separate row for each commodity, with grades, units, quantities, etc. You might do the same for host rocks, to allow for many different hosts rocks in the one deposit.

However, what you have now created is a relational database, and a spreadsheet is no longer the appropriate tool.

Problems arise when spreadsheets are pushed beyond their inherent limitations. Users are attracted to the spreadsheet's polished interface and easy intuitive operation, and are often dissatisfied by the less friendly face of a relational database. They forget that thousands of man-hours have been invested in the user interface of spreadsheet software, whereas the user interfaces (e.g., screen forms) for databases are one-off constructions, built to constrained budgets and timetables. User friendliness comes at a price.

Many geoscience data management tasks are beyond the capabilities of a spreadsheet. They require relational database management systems to be able to model data correctly, share the data, eliminate redundancy, provide simultaneous update access to many users, validate data, enforce referential integrity, apply 'business' rules, ensure security, backup data automatically and allow real-time access from the World Wide Web. The resources required to set up full-blown relational databases are usually beyond individual users, and commonly outside projects budgets as well. For this reason, relational databases are best built at the corporate level, in close collaboration with the end users. They should also be kept as general-purpose as possible. For specific projects or products, subsets of the corporate database may be downloaded to spreadsheets and other PC software tools.

The traditional refractory interfaces to corporate databases are yielding to client/server technology and Windows software development tools. However, attractive graphical screens with pull-down lists, dialogue boxes, radio buttons, etc, take longer to develop and cost more than their character-based predecessors - typically two to three times as much. They also require different skills to develop, with liberal doses of design artistry and user psychology. Costs are rapidly declining, though, as the tools get better and the developers more experienced.

Another trend is a move away from expensive UNIX systems, requiring the support of UNIX 'gurus', to cheaper PC-based networks that are easier to build and maintain.

373

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EARTHQUAKE ZONATION MAPPING OF URBAN AREAS IN AUSTRALIA AND GEOLOGICAL CONTROLS TO DAMAGE Jack Rvnn1. Ted Brennan1, Peter Hughes1, Ian Pedersen1 and Harold Stuart1 Centre for Earthquake Research in Australia, PO Box 276, Indooroopilly, Queensland 4068 Earthquakes and geology - they are intimately linked, not only in their causal relationships but also in terms of their effects on loss of human life and property. While many of the earthquakes in Australia since European settlement in 1788 have caused some form of damage to property, it was the 28 December 1989 ML 5.6 Newcastle earthquake that instilled the stark reality of the vulnerability of Australia's urban areas to earthquake. Within this, the aspect of geology was a significant element.

Much has been made of the damage to Newcastle and surrounding areas as clear evidence of the "effect" of earthquake wave ground motion on all elements of the built environment. But what is the "cause" of such damage, which is then manifested in its consequential socio-economic effects? The key is the "geology factor". Two broadbased categories are considered : (a) earthquake source zones and mechanisms (at depth); (b) geological controls to damage (at the surface).

With this 1989 Newcastle earthquake as catalyst, a project of earthquake zonation mapping of urban areas of Australia was initiated under the Emergency Management Australia (EMA) Australian IDNDR Coordination Committee. The significance lies in a concept to reduce potential losses in future earthquakes that may strike in such urban areas. The methodology involves a multidisciplinary approach integrating the earth sciences, built environment, socio-economics and disaster planning. This addresses the risk in terms of both the hazard and the vulnerability. The earthquake zonation maps define potential earthquake ground motions (peak ground accelerations and intensities) and categorise potential damage therefrom to all classes of buildings and to surface and underground infrastructure. Such outcomes have practical applications in earthquake mitigation through building codes, land-use planning, rescue and response, and emergency management (planning, training, exercises).

This paper will emphasise the key element of geology in this zonation mapping procedure, in defining potential earthquake sources and in assessing potential damage to the built environment. Consideration of the aspects of alluvial deposits, amplification, liquefaction, lateral spreading and landslides, in terms of the lessons given by the Newcastle and selected earthquakes, are paramount.

These geological factors are interpreted through the earthquake zonation maps and associated commentaries which then provide the relevant and necessary information for emergency management/disaster planning towards mitigation of the earthquake hazard.

374

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

BUILDING MODELS FROM IRREGULARLY DISTRIBUTED DATA IN 2-D A N D 3-D Malcolm Sambridge1'2, Jean Braun 1 , and Herbert McQueen1, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200.

2 Centre for Information Science Research, Australian National University, Canberra, ACT 0200.

1

Most geophysical field data (typography, gravity, magnetic anomalies, etc.) are collected at irregularly spaced locations, whereas visualisation tools and processing software commonly assume that geophysical observables are known at the corners of a regular geographical grid. It is therefore a common procedure in geophysical data analysis to interpolate a field, known at a series of irregularly positioned points, onto a regular mesh. A number of approaches exist for this task. Most involve some sort of smoothing or averaging of the data, e.g a minimum curvature fit. Smoothing can help to reduce the effect of errors in the observables but throws away potentially important information in the original data.

Natural Neighbour interpolation is a technique from the field of computational geometry which avoids all smoothing of the data but can reconstruct a scalar field from any irregular distribution of points in two or more dimensions. This method is based on the Delaunay triangles (2-D), or tetrahedra (3-D). The approach has a useful combination of properties. Firstly, the interpolated value fits the data exactly at the original data points. Secondly, the interpolation is strictly local, i.e. every point is only influenced by its natural neighbour data sites. Thirdly, both first and second spatial derivatives of the interpolated function are thought to be continuous everywhere, except at the positions of the original data. These properties mean that with natural neighbour interpolation one contracts a well defined surface through any 2-D data set which is both locally responsive to changes in the data, and is also smooth between the nodes.

With the advent of new efficient algorithms for calculating Delaunay triangulations and natural neighbour co-ordinates it is now practical to perform natural neighbour interpolation on irregular data-sets containing millions of points. This presentation will cover the basic theory of natural neighbours and show our recent applications in the re-gridding of topographic and other geophysical data-sets. Many other areas of geophysics rely on methods of parameterisation and interpolation, including numerical modelling (e.g. of mantle convection, or crustal deformation) and Seismic Tomography. Examples of how natural neighbour interpolation can be used in these areas will also be presented.

The authors have developed an interactive X-graphics software package which implements Natural Neighbour interpolation of irregularly distributed 2-D datasets and is available upon request. A world wide web page describing this work in more detail may be found at the URL h t t p : webgeod. aim. edu. au/geodynamics/nn/nri .html

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SHRIMP U-Pb DATING OF 2.95 TO 3.0 Ga INTERMEDIATE TO SILICIC ROCKS IN THE SOUTHERN YELLOWDINE TERRANE, YILGARN CRATON Mark D. Savage, Mark E. Barley & Neal J. McNaughton Key Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, The University of Western Australia, Nedlands 6907.

The ages of metavolcanic and sedimentary (greenstone) successions in the Yellowdine Terrane (formerly Southern Cross Province) of the Archaean Yilgarn Craton are poorly constrained. The Ravensthorpe Range and West River greenstone belts are typical of greenstone belts in the southern Yellowdine Terrane, they comprise a succession of metamorphosed intermediate to silicic volcanic rocks overlain by a succession of metamorphosed sandstones, banded iron formations and shales, these are in turn overlain by metamorphosed komatiites and basalts and a second succession of metasedimentary and silicic volcanic rocks. The lower intermediate to silicic volcanic and sedimentary successions are intruded by an ovoid tonalite to granodiorite batholith which separates the two greenstone belts. Both greenstone belts are deformed, metamorphosed (upper greenschist to amphibolite facies), and are surrounded by 2.68 to 2.63 Ga, deformed and post-tectonic graitoids which are dominantly biotite-monzogranites. Mineralization at Ravensthorpe includes structurally controlled Au-Cu and komatiitehosted Ni sulphides.

The lower succession of intermediate metavolcanic rocks in both the Ravensthorpe and West River greenstone belts have yielded consistent SHRIMP U-Pb in zircon ages of close to 2.99 Ga with some older xenocrysts. The Ravensthorpe tonalite gives a SHRIMP U-Pb in zircon age of 2966 ± 1 2 Ma. This age is close to the 2958 ± 4 Ma age obtained from a felsic volcanic rock from the upper silicic volcanic and sedimentary succession by Nelson (1995). The lower intermediate to silicic volcanic succession and the tonalite-granodiorite batholoith are similar in composition to Archaean tonalite-trondhjemite-granodiorite (TTG) suites world-wide. If the stratigraphy of die Ravensthorpe greenstone belts can by correlated with similar lithologies in the Forrestania greenstone belt to the north, these dates confirm the existence of an extensive > 2.95 Ga intermediate to silicic volcanic (TTG) province and associated sedimentary succession in the southern Yellowdine Terrane and also constrains the age of komatiitic volcanism and associated Ni sulphide mineralization in this terrane to the period 3.0 to 2.95 Ga. Tonalitic rocks such as those at Ravensthorpe are also possible protoliths for voluminous 2.68 to 2.63 Ga granitoid magmatism.

REFERENCES Nelson, D.R., 1995. Compilation of SHRIMP U-Pb in zircon geochronology data, 1994. Geological Survey of Western Australia, Record 1995/3, 244pp.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 COMBINED RIFT/PULL-APART MECHANISM VERSUS PURE RIFT MECHANISM FOR THE FORMATION OF THE CARNARVON BASIN DEPOCENTRES... EVIDENCE FROM THE WEST ROEBUCK BASIN, CARNARVON BASIN AND EXMOUTH PLATFORM. Jacques Savers , Lvnton Spencer .

Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601 l

2

A right lateral pull apart originating couple and a rift system are proposed as the primary and secondaiy mechanisms for the formation of the Carnarvon Basin depocentres. The strain ellipsoid(s) for each mechanism is subjectively estimated to represent 65% (pull apart) and 35% (rift) of total deformation intensity. The strain ellipsoids are relevant for the continental plate which includes the Exmouth Platform, Carnarvon Basin and western Roebuck Basin. The importance of each mechanism varies between individual structural provinces, hence within individual basins, the dominance of one component has influenced interpreters to choose incorrectly one or the other mechanism as being the regional mechanism. It is proposed that both mechanisms are operating with various degrees of intensity and expression throughout individual structural provinces.

The major basin bounding faults of the Exmouth Platform and Carnarvon Basin, such as the east Exmouth continental fracture zone, developed during the top Norian to basal Rhaetian. This structural phase is qualitatively seen as representing 25% of the total extension during Pangia breakup phases (Norian to Sinemurian) prior to the main extensional phase at the top Triassic (70 % of total extension). Substantial rotation of Triassic blocks occurred and the Carnarvon Basin depocentres were initiated at the top Triassic.

Some mild reactivation (5% of total extension) at the top Sinemurian is probable and would have terminated the Pangia breakup phase.

Following the termination of the Pangia breakup phase, a new stress field was established on the Exmouth Platform and Carnarvon Basin. This stress field incorporated components of both a right lateral originating couple and a rift system. Both stress fields acted separately and intermittently depending on the stress field variations within the Greater India and Argoland continents. Alternatively both stress fields may have combined to form an oblique trans-tensional system. The Exmouth Platform, west Roebuck and Carnarvon Basins were exposed to this stress field from the top Pliensbachian to the top Callovian. The cause of this stressfieldwas the development of the pre-breakup phases culminating in the formation of the Argo Abyssal Plain. The Exmouth Sub-basin, Victoria, Cossigny and Beagle Troughs developed as part of this trans-tensional system. Wrench structures are mapped on both the northern and southern flanks of the Exmouth Sub-basin, Exmouth Platform, northern Lewis Trough, Beagle Sub-basin and areas of the western Roebuck Basin. Most of the above structural elements abut on the northern side of the earlier formed east Exmouth continental fracture zone, that includes the continental extension of the Argo Abyssal Plain transform, basin bounding faults to the west of the Bedout High, North Turtle Wrench Zone, basin bounding faults on the southern margin of the Cossigny and Beagle Troughs, northern side of the Rankin Trend linking up to the Alpha - Hilda Arch and Yardie Hinge fault.

This is a different concept to the more widely used pure rift model for the initiation and development of the Carnarvon Basin during the Late Triassic and Jurassic or the model that suggest depocentres were initiated during the Palaeozoic.

Figure 1: Conceptional diagram of strain ellipsoids between the top Pliensbachian to top Callovian.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOCHEMICAL EVOLUTION OF MAGMATISM AND DEFORMATION IN THE PALAEOPROTEROZOIC LINCOLN COMPLEX, SOUTH AUSTRALIA; INSIGHTS INTO OROGENESIS AND TECTONISM DURING THE PROTEROZOIC Bruce F. Schaefer Department of Geology and Geophysics, University of Adelaide, Adelaide, 5005

Many Proterozoic terrains remain enigmatic within the plate tectonic paradigm, particularly with respect to spatial and temporal variations in geochemistry of magmatism and styles of metamorphism (eg, Etheridge et al (1987), Wyborn et al (1992) and references therein). This study represents an attempt to integrate geochemical, metamorphic and structural information in an under investigated Palaeoproterozoic terrain on the southern Gawler Craton, South Australia, with a view to comparing and contrasting tectonic styles and settings with other terrains of similar age.

The Lincoln Complex encompasses magmatism associated with the Kimban Orogeny over the period 1850-1700 Ma. This incorporates a phase of voluminous high temperature felsic magmatism (the Donington Suite) at 18501840 Ma, repeated mafic dyke emplacement events in varying orientations between -1830 and 1720 Ma, when peak metamorphism took place. Orogenesis appears to have ceased by 1700 Ma, with the onset of extensive Sand subordinate I-type magmatism associated with the Moody Suite.

Of particular interest within the Kimban Orogen is the opportunity to observe juxtaposed crustal levels of the same orogenic belt across the Kalinjala Shear Zone (KSZ), which separates amphibolite facies metasediments of the Hutchison Group from transitional granulite facies Lincoln Complex. By systematically integrating geochemical and metamorphic information across the orogen, it is possible to begin to place some constraints on the style of tectonism.

Such constraints include the: • geochemical evolution of the Donington Suite, offering insights into mid to lower crustal processes and their surface manifestations during terminal Hutchison Group sedimentation from 1850-1840 Ma. • repeated thermal perturbations in the form of multiple emplacement events of geochemically discrete mafic dyke suites of the Tournefort Dyke Swarm. Crosscutting relationships and variations in dyke orientation possibly record the evolution of far field stresses during the Kimban Orogeny.

• spatial distribution of metamorphic grade with respect to the KSZ: a distinct across-strike baric gradient exists; with pressures decreasing rapidly from the KSZ to the east.

• temporal and geochemical evolution of the post-orogenic Moody Suite granitoids from dominantly S-type to small volumes of I-type, offering insights into the thermal state of the lithosphere immediately postdating the Kimban Orogeny.

While the Kimban shows marked similarities in timing and styles of magmatism elsewhere in the Proterozoic record, (eg, McArthur Basin, Mt Isa Inlier), it also preserves features often associated with modern collisional orogens.

REFERENCES Etheridge, M.A., Rutland, R.W.R and Wyborn, L.A.I., 1987. Orogenesis and tectonic process in the early to middle Proterozoic of northern Australia. In: Proterozoic Lithospheric Evolution, Am. Geophys. Union Geodyn.

Ser., 17, p. 131-147.

Wyborn, L.A.I., Wyborn, D., Warren, R.G. and Drummond B.J., 1992. Proterozoic granite types in Australia: implications for lower crust composition, structure and evolution. Trans. Roy. Soc. Edin. Earth Sc. 83, p. 201209.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 STRATAL GROWTH AND BASIN PHASE GEOMETRIES IN THE PROTEROZOIC OF NORTH AUSTRALIA: PRELIMINARY INVESTIGATIONS Deborah L. Scott . PG Betts , JR Rogers , BE Bradshaw , MJ Jackson , BA McConachie , PN Southgate Australian Geological Suvery Organisation, GPO Box 378, Canberra, ACT, 2601 Australian Geodynamics CRC, Dept. of Earth Sci., Monash Univ., Clayton, Victoria, 3168 Centre for Ore Deposits and Exploration Studies, Univ. of Tasmania, GPO 252C, Hobart, Tasmania 7001 1

3

2

3

1

1

1

1

Structural and geodynamical analyses in areas of complex, multi-phase deformation relies heavily on stratigraphic data for information on timing, deformation style and isostatic compensation. Traditional mapping in the Proterozoic of north Australia assumed "layer-cake" depositional geometries of lithostratigraphic units. Modern basin analyses clearly establishes that depositional geometries are most commonly not of this configuration and lithostratigraphic units are frequently diachronous or time-transgressive. Structurally reasonable cross-sections are impossible to construct without a clear knowledge of depositional geometries. Stratal geometries may be technically induced, for example wedge "growth" in extensional rift basins. Or, stratal growth may be due to geodynamic compensation and climatic changes. The sequence stratigraphic interpretation of stratal geometries relies heavily on the construction of "dip" profiles assumed to be perpendicular to the basin margin. The method was spawned by geometries revealed in seismic data primarily from "passive" margins and thus does not specifically account for lateral variability and/or tectonic setting. Distribution of later "sag" basin phases and the arrangement of their sequences is determined by the underlying structural template of technically active basin forming phases, so that a positive feedback mechanism exists between the sequence stratigraphic and structural interpretations. Unravelling the tectonic history of north Australia requires recognition of the various types of stratal geometries. When correlated with bends in the APWP, identification of tectonic growth in stratal geometries provides constraints on kinematic interpretations and proposed structural or tectonic histories, including basin phase geometries and fluid flow models. As part of an integrated basin analyses (NABRE, AGSO) of a crustal area of over 1.3 million km , with preserved basin stratigraphy spanning 400 million years, three likely stratigraphic candidates for tectonic growth geometries were investigated: 2

•The interpreted "rift" mega-sequence in south-dipping reflections in seismic data located on the southern margin of the Murphy Tectonic Ridge consists of three to five lower order sequences of semi-transparent reflections separated by high-amplitude "train-track" reflections. Although the mega-sequence is offset by north-dipping normal faults,riftgeometries and growth wedges are rarely recognized, but onlap surfaces within the package are apparent. Minor tectonic growth may occur in the largely eroded uppermost few reflections, but on footwalls this uppermost sequence onlaps its underlying surface. Thus, the basinward growth of the mega-sequence is most probably not due to extensional tectonics, but instead may represent a ramp margin. The mega-sequence overlies a series of north-dipping reflections of considerable thickness which may represent an earlier basin-forming phase.

•The deposition of texturally and compositionally immature sandstones, polymictic conglomerates and bimodal volcanics of the Bigie Formation and Fiery Creek Volcanics is interpreted to mark the onset of a rifting event in the northern Mount Isa Basin. Recent field studies to the S W of the Fiery Creek Dome in the Seymour River block (Mt. Oxide 1:100 000) suggest the Bigie Formation was deposited in an alluvial channel and interfluve system, rather than as syn-tectonic wedges. The majority of the tectonic growth in the Seymour River block is due to the development of conglomerate and sand wedges in the upper Fiery Creek Volcanics and Surprise Creek Formation. The stratigraphic wedges thicken towards the southeast against northwest-dipping normal faults. A period of renewed fault control on sedimentation is suggested by map patterns indicating thickening of Gunpowder Creek Formation across northwest-dipping faults. This depositional and structural history is consistent with observations further north in the Police Creek block.

•Field studies in the Batten and Tawallah Ranges in the Central McArthur Basin confirms the apparent growth to the north in map patterns of the Rosie Creek Member of the Sly Creek Sandstone and the Aquarium/McDermott Formations. The stratal geometry and scale of this growth is consistent with an extensional tilt block interpretation. At the northern end of the Batten Ranges opposing younging directions of these units is best explained by an inversion event after their deposition. Different styles of deformation of units above and below this basin phase support the inversion interpretation. A spectacular field example of a normal fault "fanglomerate" in the Masterton Sandstone documents renewed extension (relaxation) which utilized E-W trending normal faults.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EVOLUTION OF THE NORTHERN MOLONG STRUCTURAL ZONE AND ITS MARGINS, LACHLAN FOLD BELT, NSW Martin M. Scott Geological Survey of New South Wales, PO Box 53 Orange, NSW 2800

The Orange and Molong 1:100 000 geological sheets have recently been mapped by the GSNSW and AGSO as part of the Lachlan Fold Belt project of the National Geoscience Mapping Accord. An east-west transect shows the northern Molong Structural Zone to comprise thrust repeats of the Ordovician to Devonian stratigraphy, and is not, as previously interpreted, an anticlinorial zone. The evolution, relationships between, and deformation of the Silurian to Early Devonian morphotectonic features, the Cowra Trough, Molong High and Hill End Trough, is synthesised.

Early Silurian deformation produced different structural configurations. Sedimentation in the Cowra Trough was apparently continuous through the Silurian and Early Devonian. On the Molong High, there are disconformities and low angle unconformities between Late Ordovician Cheesemans Creek Formation and Oakdale Formation basalt-latite volcanics, volcaniclastics and siltstones, the Early Silurian Boree Creek Limestone, and limestone-siltstone units of the late Early-Late Silurian Mumbil Group. On the eastern margin of the Molong High, the basal conglomerate of the Late Silurian Anson Formation has a low angle unconformity with the Late Ordovician-Early Silurian Byng Volcanics and Oakdale Formation.

The earliest cleavage (SI) dips shallowly NNE in serpentinised Late Ordovician-Early Silurian ultramafics of the Lucknow Fault and adjacent to the shallow NNE-dipping Big Bald Hill Fault. The Big Bald Hill Fault has Late Silurian Anson Formation in the hangingwall, thrust over Late Ordovician-Early Silurian Oakdale Formation and ultramafics in the footwall. Younger over older relationships on the Big Bald Hill Fault, and the Black Bulga Fault beneath the Mount Bulga deposit, suggests reactivated thrust displacement along earlier north-dipping normal faults. Steep east-dipping faults such as the Clifton Grove Fault and Godolphin Fault, which also thrust younger over older rocks, and suggest basin inversion along the margin of the Molong High and Hill End Trough.

The low angle unconformity at the base of the Anson Formation indicates the Byng Volcanics and Oakdale Formation were tilted to be dipping south, possibly by Early Silurian north-dipping listric normal faults. On the eastern margin of the Molong High there is a dramatic thickness increase in units of the Mumbil Group, and a change from limestone to rhyolite-pyritic siltstone dominated facies. An Early Silurian scenario for the eastern margin of the Molong High, is that steep east-dipping bounding normal faults in combination with shallow north-dipping listric normal faults produced a volcanic rift basin. The rift basin is host to rhyolitic volcanism and stratabound mineralisation in the Anson Formation and Mullions Range Volcanics, and pyriticcarbonaceous siltstone of the Barnby Hill Shale.

The Lambian Unconformity at the base of the Catombal Group has a low angle with Early Devonian Cuga Burga Volcanics and Garra Formation. Fault tilting and considerable erosion during the Middle Devonian, thinned and removed Garra Formation from beneath the Catombal Group along the eastern limb of the Hospital Hill Syncline.

East-west compression during Early Carboniferous deformation produced an east-directed west-dipping imbricate thrust system in the Cowra Trough and along the central axis of the Molong High, with a single weak cleavage and prehnite-pumpellyite grade regional metamorphism. Compression along the eastern margin of the Molong High produced a west-directed east-dipping thrust system, reactivating the early east-dipping and north-dipping normal faults that formed during the opening of the Hill End Trough. The eastern margin of the Molong High has two strong east-dipping cleavages, and chlorite to biotite grade regional metamorphism. The style of deformation and associated metamorphic grade during east-west compression has been influenced by the buttress of competent Ordovician mafic volcanics, and the orientation of early normal faults formed on the margin of the Molong High and Hill End Trough.

Acknowledgements: Published with the permission of the Director General, New South Wales Department of Mineral Resoucres.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THINKING LIKE A GEOLOGIST: THE CULTURE OF GEOLOGY THE MAWSON LECTURE, FEBRUARY 1996 George Seddon Centre for Studies in Australian Literature University of Western Australia, Nedlands, W.A., 6907

My concern in this discussion is not with Geology as a useful profession, but with Geology as a vital part of our culture, and as education. Science as a whole is a major part of contemporary culture. Geology plays a crucial role in both the scientific and the popular culture: indeed, it has transformed the way ordinary people look at the world over the last two centuries. Much of this transformation is understood in terms of information: information about the age of the earth, about the disposition of the continents, about the evolution of living things. My concern is less with information than with mental attitudes, and my central question is this: how does a solid grounding in the discipline of geology affect the cast of mind, the way one thinks and perceives or behaves — or, to put it differently, what is the culture of Geology? Hence the title.

The discussion that follows is exploratory and introspective. I have approached the question by attempting to reflect on the way in which a training in Geology has influenced my own thinking and enriched my understanding of the world, especially in acquiring a sense of place, which is basic to all sound environmental planning, and important to us individually as a part of feeling at home in any specific environment. The case for the educational value of Geology as distinct from its practical value as a profession has not, in my experience, often been fully articulated in the last half century, although it was in earlier years, especially in Britain. A more reflective approach to the subject could enrich the Australian educational system, and our individual self-understanding, both as Australians and as human beings. It might also sometimes make some practising geologists, better practising geologists.

My reflections have been grouped under six main headings: — — — — — —

the bias towards pragmatism 'getting down to bedrock1 the sense of time the use of multiple working hypotheses the training in distinctive habits of observation the nature of 'the universal' in Geology.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PHOSPHORUS BEHAVIOUR IN I- AND S-TYPE GRANITIC MAGMAS: EXAMPLES FROM THE LACHLAN FOLD BELT, AUSTRALIA Lian-Kun Sha and Bruce W. Chappell Department of Geology, Australian National University, ACT 0200, Australia

The factors controlling the geochemical behaviour of phosphorus in I- and S-type granites from the Lachlan Fold Belt, Australia have been investigated. Silica content, peraluminousity and temperature have been found to be the three most important variables of these factors. P2Os shows three variation patterns with silica contents in the granites: (1) downward concave trend, (2) linear trend and (3) triangular pattern. The first two trends demonstrate a decrease with increasing silica contents in all I-type and Ca-rich S-type granites, and this is generally consistent with previous experimental results (e.g. Harrison and Watson, 1984). In the lower silica region, the downward concavity of P 2 0 5 versus Si0 2 trend observed in the granites is in contrast to the upward concavity shown in the experiments; whereas in higher silica region, P 2 0 5 in these granites shows much stronger decrease with silica compared to the experimental data. These differences are ascribed to fractionation in the granitic magmas. The triangular pattern (Wagga-type pattern), which shows a triangular scatter of P 2 0 5 against Si0 2 , is typical of some peraluminous S-granites having higher alkalis but much lower Ca contents at a given silica content and peraluminousity. Thus, the alkalinity index, AI=(Na20+K20)/(Na20+K20+Ca0) (molecular ratio), is considered to be another important parameter affecting the P behaviour. In Ca-rich metaluminous and peraluminous melts (usually AI<0.72), most of the phosphorus enters into apatites and other phosphates such as monazite and xenotime; in Ca-poor but alkali-rich peraluminous melts, a large portion of P is incorporated in K-feldspar and sodic plagioclase rather than phosphate minerals. P concentrations vary from over 3400 ppm to less than 100 ppm in feldspars, and generally show a systematic increase with increasing Ab contents in plagioclase. Varying An contents of plagioclase and differing ratios of plagioclase relative to K-feldspar result in the triangular Waggatype pattern.

The aluminum saturation index (ASI) has a profound influence on P contents in granites. P shows markedly different geochemical behaviour in metaluminous and peraluminous melts. In peraluminous (ASI>1) I- and Stype granites, P shows an increase with increasing ASI values. In metaluminous melts (ASI<1) on the other hand, we found that increasing ASI values do not enhance but significantly lower P solubilities, with subaluminous granites (ASI=1) having the lowest P concentrations. Thesefindingsare consistent with our recent experimental results on P solubilities in haplogranite systems.

In addition, other composional variables may affect P solubilities in granitic magmas. Fe 2 03 contents of metaluminous and slightly peraluminous I-type granites decrease with increasing ASI, but remain relatively constant in peraluminous S-type magmas. P 2 0 5 increases with increasing Fe 2 0 3 for more oxidised, more metaluminous I-type granitic magmas, but no obvious correlations are found in peraluminous S-type, and more reduced, less metaluminous and slightly peraluminous I-type granitic magmas. In oxidised metaluminous melts, Fe3* may act as a network former with tetrahedral coordination, and consequently it tends to lower P solubilities because both tetrahedral P5+ and Fe3+ compete for charge-balancing cations. In reduced, more aluminous melts, low Fe3+/Fe2+ ratio and higher Al3+ concentration make Fe3+ more likely become a network modifier with octahedral coordination, and therefore ferric iron is less likely to affect P solubility.

Ti0 2 shows a monotonic decrease with increasing ASI values in I-type metaluminous magmas, suggesting that Ti4* probably acts as a network former. In peraluminous S-type granites, Ti02 first demonstrates a decrease with ASI when Ti0 2 is lower (<0.4-0.45 WL%), but increases with ASI when Ti0 2 is higher (Ti02>0.4-0.45 wt.%).

Therefore, the structural role of Ti0 2 in the melts may be dual. In low-Ti peraluminous melts, Ti4* is thought to exist in tetrahedral coordination, probably as discrete titanate units, while in high-Ti metaluminous melts, complexing between Al3+ and Ti4* may be possible.

It is concluded that the solubility of phosphorus in aluminosilicate melts depends on multiple variables, not only on silica content, ASI and temperature, but is also a function of other compositional variables, particularly the tetrahedrally coordinated network formers such as Fe3+, Ti4*, B3+, Ga3+, Ge4* and so on. Furthermore, the influence of network modifiers which are the essential components of P-bearing phases could not be neglected.

Experiments are under way in order to elucidate the influence of these components over the P solubility in granitic magmas.

REFERENCES Harrison, T. M. & Watson, E. B., 1984. The behaviour of apatite during crustal anatexis: Equilibrium and kinetic considerations. Geochim. Cosmochim. Acta 48,1467-1477.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A FRAMEWORK OF APTIAN-MAASTRICHTIAN CALCAREOUS NANNOFOSSIL ZONATION FOR NORTH W E S T AUSTRALIA Samir Shafik Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

A calcareous nannofossil zonation applicable to most of the Aptian-Maastrichtian successions in the Australian North West region is outlined (Table 1).

This zonation is based on examination of mainly core material from a number of both offshore and onshore wells, earlier work on outcrops (e.g. Shafik, 1990), and on unpublished reports. Some of the bioevents defining zonal or subzonal boundaries are similar to those used outside the Australian region (e.g. Sissingh, 1977; Perch-Neilsen, 1985).

Correlation with other zonations, including the informal KCN zones (Rexilius, in unpublished well completion reports), has also been attempted, seemingly successfully.

Australia: implications for palaeolatitudes and pole positions for Australia.

AGSO Research Newsletter, 19, 15.

Sissingh, W., 1977. Biostratigraphy of Cretaceous calcareous nannoplankton. Geologie en Mijnbouw, 56, 37-50.

Table 1 ZONES/S JBZOM ES

Defining & addtional EVENTS Cretaceous species

Mlcula prinsU Mlcula murus Uthraphtdttes quadratus

Most of the stage boundaries coincide with zonal or subzonal bioevents. The Campanian/Maastrichtian boundary does not coincide with a zonal boundary, and is approximated by an additional bioevent, the sharp decline in the abundance of Aspidolithus parcus constricus within the uppermost part of Zone KC-VI. The timescale used is that of AGSO (Burger & Shafik, 1996).

Mlcul* praamurus

' Micula prlnsll 'Caratollthoidas kamptnari ' Mlcula murus * Uthraphldttas quadratus * Uthraphrtos praatpjadratus * Ralnhardtitas lavts

Arkhangalsklalla spacllJata Patrorhabdus eopulatus Ralnhardtltas levis Quadrum trtfJdum

* Tranoitthus orlonatus + A. parcus constrlctus * £. aximhts; *-R- anthophorus ' Rafnhardtltas levls * Quadrum trftfdum

Quadrum slssinghll

' Potrorhabdus eopulatus * Quadrum slsslnghH

Caratollthoidas aculaus

During the Albian and Senonian most of North West Australia was within the Extratropical Nannoprovince (Shafik, 1993), where warm- and cool-water species coexisted. In the southern parts of this province (most of the Carnarvon Basin), finer resolution than shown in Table 1 is possible, because cool-water species (e.g. Nephrolithus frequens in the Maastrichtian) are abundant enough to be used as additional biostratigraphic tools.

' Caratoftthokhs aculous + Bukryastar hayil

Aspldoltthus parcus constrlctus

* M. furcatus;* A. parcus partus * Caratollthotdas vartoaakll * AspkkMhus parcus constrlctus

Aspidoltthus parcus parcus Calcutta* obscurus Luclanorhabus cayauxll Mtcula concava Ralnhardtltas anthophorus

' Bukryastar hayil * Aspidolithus parcus parcus ' CaJculltas obscurus ' Lucianorbabus cayauxll * Micul* concava ' Rainhardtttas anthophorus

Mlcul* staurophora

References

' Uthastrtnus grttlll ' Mlcula staurophora

Marthastarttas furcatus

Burger, D. & Shafik, S., 1996. Cretaceous (Chart 9). In Young, C.G. & Laurie, J. (Eds.), An Oxford Australian Phanerozoic Timescale.

University Press, 160-174.

Elffailtthus aximhts Quadrum gartnari Coroltthlon slgnum CoroHltMon kannedyl Mlcrorhabdulus dacoratus

Perch-Nielsen, K., 1985. Mesozoic calcareous nannofossils. In Bolli, H.M., Saunders, J.B. & Perch-Nielsen, K. (Eds.), Plankton Stratigraphy.

Cambridge Earth Science Series, Cambridge University Press, 329-426.

' Uthastrtnus saptanarius ' Marthastaritas furcatus * Efffalltthus axlntkts ' Quadrum gartnari * Corolllthlon kannedyl Mlcrostaurus chlasOus * Mlcrorhabdulus dacoratus

Elffailtthus turrisalffalli

* Corolllthlon kannadyf * small EHMmhus; * H. albiensis * EtftaUlthus turrisalffalli * small Elffafllthus ( E sp.1 or P-N 79)

Tranoitthus orlonatus

* Axopodorhabdus albianus ' Tranoitthus orlonatus

Shafik, S., 1990. Late Cretaceous nannofossil biostratigraphy and biogeography of the Australian western margin. BMR Report 295, 164 pp.

Prmdlscosphaara columnata Eprollthus ftoratls Rhagodlscus angustus

Shafik, S., 1993. Albian and Maastrichtian nannofloral biogeographic provinces in Western

XXIX

Ruclanoltthus Irregularis

' First (upsection) appearance.

383

' Hayasltas alblansis * Pradlscosphaara columnata ' Eprollthus floralts ' Rhagodlscus angustus • Last (upsection) appearance.

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CALCAREOUS NANNOFOSSILS IN THE COTE DTVORE-GHANA MARGINAL RIDGE, ATLANTIC OCEAN, ODP LEG 159 1

Samir Shafik1 and ODP Leg 159 Shipboard Scientific Party Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

ODP Leg 159 (January-February, 1995) drilled 13 holes at four locations (Sites 959-962) in the Equatorial Atlantic, off Ghana. The sites targeted different, but intimately related, parts of the Cote dTvore-Ghana Marginal Ridge (CIGMR) and its associated minor ridges - to document the different stages in the evolution of this major physiographic feature. The CIGMR lies at the eastern extention of the Romanche Fracture Zone, and separates the Deep Ivorian Basin (DIB) and the Gulf of Guinea Abyssal Plain. The CIGMR was formed as a consequence of a series of events which started early in the Cretaceous by rifting of the northern South Atlantic, followed by active transform motion initially between the African and Brazilian plates, and subsequently between the evolving, extenstional DIB and newly created, southerly bordering oceanic crust. Thermal subsidence and Cainozoic sealevel changes dominated the final stage in the evolution of the CIGMR- At one time, the CIGMR may have acted as a dam to the sedimentation in the DIB. Site 959 is located on a small plateau extending from the CIGMR, on the southern shoulder of the DIB, and the nearby Site 960 is on the crest of the CIGMR; both sites are in <2110m water depth. A 1158.9m-thick sedimentary sequence was sampled at Site 959: a nearly complete Neogene calcareous microfossil-bearing section; a Palaeogene section with intervals totally barren of calcareous microfossils; and a long barren section (below the Late Paleocene CP7 Zone) presumably Cretaceous in age, with occasional calcareous nannofossils.

Foraminifera are lacking in most of the Palaeogene and older sediments at this site. Nannofossils in the Cretaceous section of Site 959, seemingly representing isolated marine occurrences, indicate the late Santonian CC16, early Coniacian CC13, early Turonian CC11, and late Albian CC9 Zones. At Site 960, a 448m-thick sedimentary sequence was sampled: a discontinous, calcareous microfossil-bearing section — Pleistocene to Early Eocene in age; and a (~ 270m) thick, mostly barren section - with nannofossils indicating Coniacian (CC14 Zone) age at its top, and late Turonian (CC12 Zone) age lower down. The Coniacian and Lower Eocene sediments are separated by a hardground in a short core. The Neogene section at this site is less than half the thickness of its counterpart at Site 959, but also contains calcareous microfossils.

Sites 961 and 962 are in waters 3300 and 4650m deep respectively: Site 961 is on the southern exposed tip of the CIGMR, and Site 962 is on a minor ridge to the southwest. A section of 370m was cored at Site 961. The top half of this section yielded a discontinuous, Pleistocene to Early Eocene succession of calcareous microfossil assemblages, but the lower half is almost barren, except for two horizons with a single nannofossil species, the robust Watznaueria barnesae; this species is known to range from Bajocian to Maastrichtian. At Site 962, a thin Pleistocene-Lower Miocene section (-65m thick) overlies a very thick upper Albian section (>320m thick).

Calcareous microfossils occur in only the upper half of the Pleistocene-Lower Miocene section, and thoughout the Albian, all of the latter belonging to the CC9 Zone.

The nannofossil distribution at Sites 959-962 seems to fit the tectonic model - advanced by-Leg 159 scientists for the evolution of the CIGMR, and shows some resemblance with the situation in southern Australia. The presence of late Albian nannofossils at Sites 959 and 962 is probably related to a global sealevel rise, and the two horizons with Watznaueria barnesae at Site 961 are probably coeval with this sealevel rise, diversity being restricted by environmental conditions. The Turonian and Coniacian nannofossil-bearing horizons at Site 960 are indicative of bursts of strong marine influences in otherwise marginal marine environments during the younger Cretaceous. These bursts are a result of subsidence or rapid rise in sealevel. The Turonian horizon probably marks the first sign of significant cooling and subsidence of the CIGMR, after the passage of the active oceanic ridge. The Upper Cretaceous record at Site 959 is consistent with a subsiding CIGMR. The Cainozoic record on the CIGMR, mostly with successions of calcareous and/or siliceous microfossils, was deposited during the passive margin stage (continuing thermal decay, sealevel changes, etc.) in the development of the CIGMR. In southern Australia, the tectonic fabric of the region comprising the continental margin southwest of Tasmania is more complicated than that of the region comprising CIGMR, but some resemblance can still be discerned.

Moreover, the nannofossil distribution in the Upper Cretaceous - Palaeogene section, along most of the southern margin of Australia, shows a similar pattern to that shown at the CIGMR sites: isolated occurrences of nannofossil (representing subsidences or sealevel rises) within a thick barren (mostly marginal marine) section; and an overlying nannofossil-rich section (indicative of continuing thermal decay, sealevel changes, etc.).

384

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CALCAREOUS NANNOFOSSILS AND MAGNETIC LINEATIONS IN THE EASTERN INDIAN OCEAN AROUND CHRISTMAS ISLAND: NEW EVIDENCE T O THE TECTONIC EVOLUTION OF THE NORTH WEST AUSTRALIAN REGION Samir Shafik, Chao-Shing Lee and Irina Borissova Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

In 1992, AGSO (then BMR) conducted a regional survey around Christmas Island, Indian Ocean, within the Australian Extended Economic Zone in an attempt to better understand the seabed morphology, sediment thickness and offshore mineral resources in the area. During the survey, multichannel seismic, magnetic, gravity, bathymetry and geological data were acquired on board the R/V Rig Seismic. Coring and dredging were successful, particularly along several seamounts in the Vening-Meinesz Seamount Chain (VMSC), SW of Christmas Island. Volcanic rocks (basalt, trachyte, hyaloclastite, and volcaniclastics) and carbonates (mainly shallow-water and pelagic limestones) were collected.

Calcareous nannofossils in the carbonates indicate Late Cretaceous and Tertiary ages. The oldest in situ nannofossil assemblage gives a late Campanian age, and a reworked component in a Maastrichtian assemblage suggests a lower Campanian source sediment. These assemblages suggest that since the Late Cretaceous, the VMSC has maintained a generally similar geographic position relative to Australia i.e. at latitudes to the north of the Carnarvon Basin. The Campanian and Maastrichtian assemblages resembled those in Papua New Guinea, lacking the cool-water species which flourished along the western margin of Australia in the Perth Basin and farther to the south. Eocene and Oligocene nannofossil assemblages in the carbonates on the VMSC also suggest a position to the north of Australia.

The Campanian, Maastrichtian and Palaeogene nannofossil assemblages further indicate that the carbonates on the VMSC were deposited under shallow-water conditions, i.e. in marginal marine environments akin to epicontinental sea and shelf. Other fossils in these carbonates support the shallow-water deposition.

The new composited magnetic lineations around Christmas Island indicate two trends, separated along the vicinity of 105° E: a roughly E-W trend in the west, and a NE-SW trend in the east. The boundary line is interpreted to be parallel to the Ninetyeast Ridge and the Investigator Fracture Zone (95° E) ~ implying a similar tectonism.

The magnetic age is consistent with the nannofossil evidence. West of 105° E where the VMSC is located, the EW magnetic lineations are identified as representing Chrons 30-34, Maastrichtian to a minimum age of Santonian; this is supported by a K-Ar date of the basaltic crust at the nearby DSDP Site 211 of Campanian. East of 105° E, the NE-SW magnetic lineations are interpreted as belonging to the magnetic anomaly sequence M0M7, of Barremian to Hauterivian age. This older episode is similar to the magnetic expression in the ArgoGascoyne Abyssal Plain. The oceanic crust in the region around the VMSC including Christmas Island may have once belonged to two different plates which were offset along the interpreted 105° E boundary during the separation of Australia and India. These plates were formed by different spreading ridges which have long been consumed in the Java Trench.

The oceanic crust around the VMSC must have maintained its geographic position relative to the Australian plate since the Late Cretaceous for its seamounts to contain Campanian, Maastrichtian and Palaeogene nannofossil assemblages similar to those in Papua New Guinea.

Most of the VMSC may have originated during an interval of a minimum age of Santonian-Campanian. The morphology of the seamounts, and the nature of the rocks dredged from them attest to their volcanic origin. The initial built-ups must have been enormous by late Campanian, for the top of seamounts to reach shallow-water depths allowing nannoplankton species indicative of marginal marine conditions to flourish. Volcanic activity continued/resumed during the Maastrichtian, virtually maintaining and probably increasing the area of shallowwater environments, allowing the establishment of communities of large foraminiferids and other shallow-water organisms. Deposition of shallow-water carbonate and possible renewed volcanic activities occurred during the Eocene and later in the Oligocene. Bathymetric trends in the VMSC, being difficult to discern, suggest widespread Late Cretaceous volcanic activity.

385

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LITHOSTRATIGRAPHY, SEDIMENTOLOGY AND STRUCTURE OF THE EARLY PERMIAN MANNING GROUP IN THE CURRICABARK DISTRICT, SOUTHERN NEW ENGLAND Tim Sharp Department of Applied Geology, UTS, PO Box 123 Broadway N.S.W 2007.

Recent study of early Permian Manning Group rocks in the Curricabark district, southern New England Fold Belt, has resulted in a revised lithostratigraphic framework, a coherent scheme for the depositional history of the rocks, and a detailed structural interpretation.

The Manning Group unconformably overlies accretionary complex rocks along its northern contact and comprises an upward fining marine sequence. The lowest strata are locally derived coarse clastics. They are overlain by terrigenous rocks ranging from conglomerate to siltstone and including characteristic paraconglomerates, which were increasingly derived from Carboniferous rocks to the south (Sharp et al. 1995).

Distinctive allodapic limestones occur high in the sequence, which totals about 900 m.

Three lithofacies associations occur in upward stratigraphic succession: (i) oligomictic conglomerate and sandstone deposited from traction currents (LA 1), (ii) coarse redeposited rocks mainly paraconglomerate and sandstone (LA 2) and, (iii) interbedded turbidite sandstone and limestone, and siltstone (LA 3). LA 1 comprises near source rocks deposited in a shallow marine environment. The remainder of the sequence indicates a deeper basinal setting, below storm wave base and at the foot of a slope of sufficient scale to allow generation of fully developed turbidity currents and to allow mass flows to accumulate enough energy to spread widely over the depositional surface. LA 2 indicates random deposition of debris flows and high density turbidity currents within an environment otherwise swept by waxing and waning traction currents. Within LA 3 alternating cycles of allodapic limestone and terrigenous turbidites are explained by alternations of sea level highstands and lowstands.

Lowstands allowed coarse sediment to gain direct access to the slope environment thus generating terrigenous turbidites. Highstands reduced terrigenous clastic input and allowed bioclastic carbonate accumulation on an unstable shelf from which limestone turbidites were generated.

The Manning Group rocks and their basement are involved in folds with half wavelengths of approximately 150 to 1000 metres and moderate plunges. Steeply dipping rather sinuous faults that commonly separate adjacent folds are interpreted as thrusts active during folding. These faults and folds have been truncated by the Curricabark Fault Zone which can be mapped over a distance exceeding 50 km and on a regional scale. However, in the Curricabark district it follows an east-west course. Offset of earlier faults and associated ultramafics indicate sinistral movement on the structure and the change in strike gives rise to a releasing bend. The rocks underwent anticlockwise rotation as they moved around this bend and as a result the more westerly folds have a near east-west trend. The releasing character of the bend was also important in allowing the upward movement of serpentinised ultramafic rocks, with the emplacement of a mass across the fault trace and more extensive rise of masses further northwest. I suggest that uplift of the rocks on the eastern flank of this rising mass contributed significantly to the local moderate east and southeast plunge of folds in the early Permian rocks. The displacement of the ophiolitic rocks (which mark the Peel Fault System) by at least 4 km indicates that the majority of post-early Permian strike-slip movement within the Curricabark district did not occur along the Peel Fault, but on the obliquely cross-cutting Curricabark Fault Zone.

REFERENCES Sharp, T. R., Leitch, E. C. & Skilbeck, C. G. 1995. The northeastern margin of the Sydney Basin in the early Permian: a palaeogeographic conundrum. In Diessel C. F. K. & Boyd R. L. eds.28th Newcastle symposium on advances in the study of the Sydney Basin, pp. 78-85. Department of Geology, The University of Newcastle.

386

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention,, Canberra, February 1996

THE NABBERU KIMBERLITE PROVINCE, WESTERN AUSTRALIA : EXPLORATION, PETROLOGY AND STRUCTURAL SETTING Simon R. Shee'. Stuart C. Vercoe2, Bruce A. Wyatt1, Alan N. Campbell3, Elizabeth A. Colgan 4 , Peter H. Hwang5 and Brett D. Merritt6 1 Stockdale Prospecting Limited, PO Box 126, South Yarra, VIC 3141 2 Stockdale Prospecting Limited, 21 Ballantyne Road, Kewdale, Perth, WA 6105 r> Anglo American Corporation of SA, PO Box 61587, Marshalltown, South Africa 2107 4 77 Kingsway, Landsdale, Perth, WA 6065 5 96 Davy Street, Booragoon, Perth, WA 6154 6 Unit 6,27 Talbot Avenue, Como, Perth, WA 6152

The Nabberu 01 kimberlite is a small (~1.5ha), elongated body which intrudes Archaean granitoid rocks of the Marymia Dome, 150km north of Wiluna in the north-eastern Murchison region of Western Australia. The discovery, petrology and structural setting of this body and several other smaller kimberlites and lamprophyres in the vicinity form the subject of this paper. The discovery of ultramafic lamprophyres near Bulljah Pool, Nabberu Basin in 1986 by Western Mining Corporation aroused the interest of other diamond exploration companies. Stockdale Prospecting Limited (SPL) initiated reconnaissance sampling programmes over the Nabberu Basin in 1987. Stream sediment sample results from this programme indicated a wide distribution of low interest spinel grains derived from unrelated mafic - ultramafic rock types. However, one sample taken from a stream towards the north-western edge of the reconnaissance area yielded an abundance of chromiumrich spinels, having mineral chemistries typical of kimberlitic source rocks. Follow-up sampling in 1989 confirmed the presence of these high interest spinels in the drainage referred to as Eastern Creek in the Marymia area. In 1990 an airborne geophysical survey (780 sq. km) was flown over exploration licences held by SPL at that time. A total of 52 aeromagnetic anomalies were followed up by ground magnetics, heavy mineral and geochemical sampling, and where warranted, by drilling. In addition TM and airphoto anomalies were selected, checked on the ground and sampled. Work on one magnetic anomaly labeled MA47 led to the discovery of Nabberu 01, a diamondiferous kimberlite which forms a small outcrop of weathered, yellow porphyrinic rock.

Discovery of Nabberu 02 - 05 followed soon thereafter. In 1991 geotechnical work on Nabberu 01 included orientation heavy mineral and geochemical sampling, geological mapping, structural analysis, diamond core drilling, SIROTEM traverses, a gravity survey and limited bulk sampling. Additional kimberlites and related rocks located in 1991 and 1992 bring the total to twelve, four of which are diamondiferous but subeconomic.

These bodies, all of which are small (< 2ha), comprise part of a new kimberlite province.

Igneous intrusions located within the study area include various lamprophyric rocks associated with kimberlite bodies. The lamprophyres and kimberlites are highly altered, even in drill core. The kimberlites have textures ranging from mylonite, particularly near the margins of the bodies, through to unsheared hypabyssal facies macrocrystic kimberlite near their centres. The shearing has produced a schistosity overprinting the original textures and is clearly evident in drill core material. In thin section, the least deformed kimberlites contain altered olivine macrocrysts and phenocrysts set in a matrix of altered monticellite (pseudomorphed by quartz), relict laths of mica (probably originally phlogopite), altered perovskite, apatite, sphene, rutile and chromiumrich spinels. The groundmass contains abundant secondary iron oxides and is typically altered to talc, serpentine, or quartz. Major and trace element analyses of the chromium-rich spinel xenocrysts from the Nabberu kimberlites have Cr-Ni-Ti-Al-Ga contents and relationships which strongly suggest that they are kimberlitic. Some chromites have low Mg and high Zn contents which are attributed to low temperature metamorphic equilibration which could possibly have occurred during the deformation events which sheared the kimberlites. The majority of the smaller intrusions are clearly not kimberlite and are classified as lamprophyres. Extensive weathering masks most of the primary mineralogy, particularly within the groundmass and, as such, specific classifications are not possible. The lamprophyres are typically fine grained, pandiomorphic rocks which consist mainly of altered mica ranging from phenocrysts (-1mm) down to microphenocrystic groundmass laths. Olivine is typically rare or absent. Primary feldspar is present in a few samples. Chromium spinel xenocrysts are rare or absent in the lamprophyric rocks.

Geological mapping indicates that emplacement and shearing of the intrusions was controlled by two major deformation events and by at least three minor events. Structural relationships indicate emplacement ages of 1900Ma and 1900-1700Ma for the kimberlite and lamprophyric rocks respectively, making these some of the oldest kimberlites ever discovered. Pervasive alteration has prevented radiometric dating of the emplacement ages of these intrusions.

387

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

INNOVATIVE GEOSCIENTIFIC EDUCATION: ILLUSTRATION THROUGH TWO MAJOR INITIATIVES Ms Julie M. Shepherd Kformerly 2)^ M s V i c k y D 0 dds 2 . Mr Derek Longhurst3, Prof. David Groves1, Dr Susan Ho 4 1 Key Centre, Department of Geology & Geophysics, The University of Western Australia, Nedlands WA 6907 2 Scitech Discovery Centre, PO Box 1155 West Perth WA 6872 3 CVA Film and Television, 1141 Hay Street West Perth WA 6005 4 Orebusters Pty Ltd, 4 Handley Close Leeming WA 6149

THE GREAT AUSTRALIAN TREASURE HUNT The Great Australian Treasure Hunt, a major national touring exhibition, was designed and built in less than 12 months by staff at Scitech Discovery Centre, Western Australia's hands-on science and technology centre. The exhibition consists of 26 highly interactive exhibits designed to lead visitors to an increased understanding of the science, technology, and economic and social impact of Australia's mining and energy industry. The $1 million required to build The Great Australian Treasure Hunt was raised from 36 sponsors from the mining industry.

The objective of The Great Australian Treasure Hunt is to provide a range of interactive experiences that convey the main principles of the following themes: Characteristics and formation of mineral and energy resources; Earth processes and deposits; Exploration; Mining; Environmental issues relating to mining; Extraction and processing; Use of minerals and energy resources and economic benefits. The exhibits are designed to appeal to a wide range of ages, backgrounds and interest levels. Some concepts are treated with computer interactive exhibits, while others are made easier to understand for younger visitors by using large colourful simulations.

All exhibits are accompanied by information panels which give clear operating instructions and an explanation of the principles demonstrated by the exhibits.

A range of school-based programs have been developed to accompany the exhibition. These programs target school groups from Years 2 to 12. Resource packages are designed to help teachers link the exhibition experience to classroom curricula in a variety of subject areas.

The exhibition premiered in Perth in July 1994, where it was visited by over 100 000 people. It is currently on a national tour of selected science centres and museums, including the Australian Museum in Sydney, the Brisbane Science Centre, and Sovereign Hill in Ballarat.

THE MAKING OF AUSTRALIA Past and present Executives of the Geological Society of Australia consider the year 2000 to be an opportune time to launch a major Society educational product. It is agreed that the most appropriate product would use the latest technology, showcase Australian geology, and have wide appeal. Provided sufficient funding is secured, the product will be an interactive CD-ROM entitled "The Making of Australia" which is planned to be a uniquely Australian production with world-wide appeal.

The proposed CD-ROM has the dual role of education and entertainment, and will be aimed at teenagers through to adults. The objective is to engross users in an interesting and exciting adventure, while educating them about geology and the geological beauty of Australia. At the time of writing, this project is in the very early stages of development, and so the final content and game play is subject to change.

However, it is planned that users will embark on a quest to explore Australian landscapes by constructing their own pathways through virtual environments. The CD-ROM will focus directly on the 'detective work' of the geoscientific community and the range of techniques used to extract and evaluate the clues contained in the landscape. As a result, it will provide school children with a better appreciation of geological processes and careers in geoscience. The CD-ROM will focus on some of Australia's key landscapes and mineral deposits.

The CD-ROM will have direct relevance to the school curriculum in a number of areas. It is planned to provide a comprehensive learning environment suitable for the classroom, while being sufficiently compelling for home use as a 'scientific detective game'.

388

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THERMAL MATURITY INDICATORS COMPARED Neil Sherwood and Nigel Russell CSIRO Petroleum and Australian Petroleum Cooperative Research Centre, PO Box 136 North Ryde, NSW 2113

Petroleum is primarily generated through the progressive heating of organic matter (OM) as sedimentary rocks are buried within the Earth's crust. Generation is mainly controlled by heat flux, depth of burial, and amounts and types of OM (macerals) present. Assessing the degree to which source rocks have been heated (thermal maturity) gives insights into the amounts generated and timing of generation. Timing is important because potential reservoirs, seals and traps must form prior to generation and expulsion if petroleum is to accumulate.

Consideration of all these factors is termed a petroleum systems approach.

A wide range of methods are available for thermal maturity measurement and a thorough understanding of the relationships between the various thermal maturity indicators is important for petroleum exploration geologists.

The analytical tools used for thermal maturity determination can be divided into six broad categories: • optical, including maceral (phytoclast) and zooclast reflectance/fluorescence, conodont alteration index (CAI) and spore and pollen colouration, i.e. thermal alteration index (TAI) and spore colouration index (SCI); two of the most commonly used indicators are vitrinite reflectance (VR) and TAI, • pyrolysis, including temperature of maximum rate of hydrocarbon pyrolysate production (e.g. Rock-Eval Tmax) and r a t i ° o f free' (naturally generated) hydrocarbons, to hydrocarbons pyrolysed from OM (e.g. RockEval 'production index'-PI), • extract chemistry, including ratios of various types of hydrocarbons in the extractable OM (e.g. biomarker ratios), • novel, including fluorescence alteration of multiple macerals (FAMM), apatite fission track analysis (AFTA), fluid inclusion homogenisation temperature (Th) and thermoluminescence, • non-destructive spectroscopy, including infra-red, nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy and • mineral reactions, including illite-smectite transformation.

Each method has advantages as well as limitations. For example, most optical methods are rapid, non-destructive and inexpensive, but are subjective in part and may present problems where OM is unsuitable for measurement.

Pyrolysis and extract chemistry provide indicators that are quantitative and objective, but are destructive, and are generally affected by OM type as well as thermal maturation. In addition, chemical methods are complicated by giving indications based on the bulk, rather than specific, OM type; different OM types behave differently with maturation. An appreciation of these factors is critical to petroleum explorationists, both for selecting the method to be used and in assessing the analytical results. Emphasis should be placed upon selecting the most appropriate indicators for the specific situation; using more than one indicator is strongly advocated.

Thermal maturity data, along with other variables such as heat flux, porosity and thermal conductivity, are critical inputs for thermal maturation and petroleum generation modelling, to calculate amounts and timing of generation. Application of thermal maturity data to the assessment of petroleum generation is complicated by variation in kinetics (chemical reaction rates in relation to heating factors) for various OM transformations.

These differences exist between the various thermal maturity indicators and between the various OM types that are converted into petroleum. For example, difficulties exist in using simple conversion tables to convert one indicator to another (e.g. VR to TAI), for calibrating a burial and thermal history model and modelling petroleum generation. These conversions are commonly done because a perceived need exists to express thermal maturity in terms of a number of "standard" parameters, such as VR, Tmax, and TAI. Where possible, thermal maturity indicators should be modelled kinetically. Considerable scope exists to more fully incorporate the different kinetics for the various indicators into the available basin modelling software packages.

Although the most appropriate use for many thermal maturity indicators, such as VR and Tmax, is for calibration of burial and thermal history models which, in turn, provide paleotemperature input to drive kinetic petroleum generation models, their use as an approximate guide to the thermal maturity of OM for petroleum generation is not precluded. Some thermal maturity indicators, such as Th and AFTA, provide a direct measure of paleotemperature and can be used to constrain paleotemperature trajectories determined by kinetic modelling.

389

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 GEOCHEMICAL STUDIES ON LATERITIC PROFILES OVERLYING THE ULTRAMAFIC INTRUSIONS, FIFIELD, NSW Bielin Shi and Ian R. Plimer School of Earth Sciences, The University of Melbourne, Parkville 3052

The latcrites overlying the ultramafic complexes in the Fifield region are exceptionally well-developed and wellpreserved weathering profiles. Field, textural and geochemical data all support a chemical weathered origin for the profiles and compatible with meteoric and ground water origins. Weathering of the thick ultramafic rocks under a tropical climate conditions, possibly in the Miocene, with low pH (< 5.7) solutions led to enrichment of Fe, Al, Ti, Mn, V, P, U, Ga and Sc upward profiles and much removal of Ca and Mg from upper part of both weathering profiles, and to the large part removal of Cr, Co, Ni, Zn and REE. Due to the changes of oxidation and reduction and/or fluctuation of the ground water table, removed elements in the top of the profiles concentrated in the 'transition zone' between the oxidation and reduction zones. This makes a striking figure of element distributions along the profiles. Assuming that all elements are soluble to some degree, a Eh level for both profiles can be deduced using the data of Stumm and Morgan (1981). The FIR 375 weathering suite has a Eh level between 7.5 (immobile behaviour of Fe) and 8.3 (immobility of V); the BCR 7 alteration suite has a Eh value > 10.5 (no addition of Cr). These redox level estimates are related to the locations of the profiles and water process in the rock suites.

Rare earth elements, especially LREE, are enriched during the weathering process. Negative Ce-anomalies and negative Eu-anomalies are noticeable features in the chondrite-normalised patterns. However, Eu/Eu* ratios keep unchanged along the studied profile, while Ce/Ce* ratios show positive anomalous in the ferruginous zone and extremely negative anomalous in the saprolitic zone. This feature can be explained that other REE and Ce have a different rate of dissolution and transportation in the weathering conditions. The oxidation of Ce and stabilisation of Ce species during the weathering processes lead to Ce fixation as insoluble Ce (probably cerianite, CeO^ in the ferruginous zone. Consequently, the aqueous solution generated during this weathering process is selectively depleted in Ce and enriched in other REE, and hence displays a negative Ce anomaly in the saprolitic zone. Since Ce oxidation in nature typically occurs in low-temperature environments and is characterised by slow reaction kinetics (Moffet, 1990), the process involved may best be described as weathering or pacdogenesis. Meteoric water percolating through a lateritic crust developed on the top of weathered ultramafic rocks mobilised trivalent REE and phosphate, transported the REE as phosphato-REE complexes in solution, and penetrated along cracks and microfractures into the otherwise fresh rocks underneath. Because the stability of phosphato-REE complexes increases from LREE to HREE, such a solution should be expected to preferentially loose LREE during migration, subsequently leading to LREE depleted (REE)^ patterns (Bau and Moller, 1992).

The absence of this feature from the REE phase present in the lower part of the profile may therefore indicate short migration distances, giving further support to the hypothesis that the REE originate from local supergene solutions. Positive Ce-anomalies indicate sites of oxidation of Ce in connection to mobilisation of REE (i.e. "source"). Negative Ce-anomalies imply sorption/precipitation of REE from Ce-depleted groundwater (i.e. "sink").

3+

4+

4+

3+

3+

In the Fifield region, significant Pt enrichment has occurred in the weathering zone overlying the intrusions.

Laterite weathering of a non-mineralised bedrock seems to develop a residual enrichment 5 - 1 0 times over the primary values. Pt is the dominant PGE present. Pt and Ir enrichments occur at 10 to 25 m depth, and behave different from Au and Pd. Most of the Pt and Ir peaks at Fifield occur at the top of the profile in the upper part of the ferruginous zone. The compositions and distributions of trace elements in the lateritic profiles indicate that supergene enrichment is a significant process for the Fifield area, but substantial addition of downward-residual material added to the system. PGE with most major elements are enriched by both residual or "volume loss" and chemical weathering processes. Pt, Pd and Au are enriched by a multi-stage process, possibly due to residual and supergene processes. It is suggested that the primary rocks have first undergone a loss of mass and volume during lateritisation, which lead to residual concentration of PGE in the lateritic soil. During the lateritisation, the ongoing loss of volume, the change in oxidation state in the ferruginous zone, and lateritic soil horizons may have remobilised the noble metals. Therefore, the elevated PGE and Au mineralisation has evolved through multiple stages ranging from high to low temperature hydromagmatic, with weathering and lateritic enrichment followed by residual erosion and placer accumulation in alluvial channels. The last stage has furnished the largest quantity of Pt and Au from the area.

REFERENCES Bau, M. and MOller, P., 1992. Rare earth element fractionation in metamorphogenic hydrothermal calcite magnesite and siderite. Mineral. Petrol., 45: 231-246.

Moffet, J.W., 1990. Microbially mediated cerium oxidation in seawater. Nature (London), 345:421-423.

Stumm, W. and Morgan, J. J., 1981. Aquatic Chemistry, John Wiley and Sons, New York, N. Y., 780 pp.

390

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE IMPACT RECORD OF AUSTRALIA Eugene M. Shoemaker and Carolyn S. Shoemaker, Lowell Observatory and U.S. Geological Survey, Flagstaff, AZ 86001, U.S.A.

The Australian craton, one of the stablest segments of continental crust on Earth, preserves the world's best known record of Proterozoic impact structures and also of small Quaternary meteorite craters. Six impact structures of known or probable Proterozoic age have been identified in Australia: 1) the Teague Ring structure in central Western Australia, 30 km in diameter and possibly about 1630 Ma in age; 2) the Spider structure in northern Western Australia, 11 km wide, 13 km long, and probably Middle to Late Proterozoic in age, 3) the Kelly West structure in central Northern Territory, possibly between 8 and 20 km in diameter and Late Proterozoic in age, 4) the Strangways structure in northern Northern Territory, possibly 40 km in diameter and about 1000 Ma in age, 5) the Lawn Hill structure in northwestern Queensland, 20 km in diameter and probably of Late Proterozoic age, and 6) the Acraman structure, probably about 35 to 40 km in diameter and about 600 Ma in age.

Discovery of impact structures in exposed gently to mildly deformed stratified Proterozoic rocks in Australia is estimated to be complete at diameters equal to or greater than 20 km. From the four known structures with diameter >20 km, the estimated area of exposed Proterozoic strata (-1.15 x 106km2), and the estimated mean time of exposure (-970 Myr), the cratering rate is estimated at (3.6 ± 1.8) x lO^km'V 1 - This estimate for production of craters >20 km in Proterozoic time is consistent with the mean cratering rate for the last 3.2 Gyr on the Moon and somewhat below the estimated present cratering rate on Earth.

The 20-km cratering rate in the late Phanerozoic estimated from the North American and EoEuropean cratons is 5.6 ± 2.8 x lO'^km'V"1- B y extrapolation from impact structures >10 km diameter in the central United States, the late Phanerozoic 20-km cratering rate is estimated at 6.3 ± 3.2 x lO'^km'V"1- These d a t a suggest, though they do not prove, that the cratering rate has increased late in geologic time, perhaps by a factor of about two. They are consistent with estimates of the present cratering rate based on astronomical surveys of Earth-crossing asteroids and comets, where comet impacts contribute about half the crater production. An increase in the comet flux in late geologic time could readily account for an increase in the cratering rate.

At the pesent rate of bombardment, impact craters >20 km in diameter are formed on Earth about once per 400,000 years. It has been estimated that impacts of this magnitude can produce short-term global changes of climate with potential catastrophic effects on crop production. Evidence from the recent impact of comet Shoemaker-Levy 9 on Jupiter suggests that catastrophic global effects may accompany even smaller and more frequent comet impacts.

The record of Phanerozoic impacts in Australia, in contrast to those of Proterozoic age, is very incomplete except for the Quaternary. This circumstance is largely due to the extensive cover of sandsheets and alluvial deposits over the Phanerozoic basins on the craton. However, one-third of the sites of the world's known small craters with associated meteorites, all of Quaternary age, are found in Australia. They include the 875-m (av.) Wolfe Creek Crater (age 300 ka) of Western Australia, the 170-m Box Hole Crater (25 ka) of Northern Territory, the 70-m Veevers Crater (-20 ka) of Western Australia, a dozen craters in the Henbury crater field (-4 ka) in Northern Territory, and the 24-m Dalgaranga Crater (-1-2 ka?) in Western Australia. Analysis of the Australian record suggests that impact of iron meteorites produces craters >1.2 km in diameter on the land areas of Earth about once per 50,000 years, a result consistent with the 50,000 year age of Meteor Crater, Arizona, the largest known meteorite crater. If the ratio of irons to other meteoroids is no smaller than 2%, as suggested by observed meteorite falls and meteoric fireball data, then >50-m diameter stony bodies (comparable to the 1908 Tunguska meteoroid) probably strike Earth no more often than about once per 200 years. Damage expected from impact of a 50-m stony asteroid is similar to that from the explosion of a 10 Mt nuclear weapon at optimal height.

However, the hazards from the collision of small asteroids, which cause only local effects, are very much lower than the hazards of globally threatening large bodies.

391

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 CEMENTATION IN COOL-WATER SUBTIDAL CARBONATE CYCLES: THE TERTIARY PORT VINCENT LIMESTONE, ST. VINCENT BASIN, SOUTH AUSTRALIA Basim Shubber , Yvonne Bone , Brian McGowran , and Noel James department of Geology and Geophysics, University of Adelaide, South Australia 5005, Australia. ^Department of Geological Sciences, Queen's University, Kingston, Ont. K7L 3N6, Canada.

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INTRODUCTION Cool-water carbonates form on platforms constrained by bottom water temperatures below approximately 20°C. Mid to high latitude unrimmed shelves and deep water tropical slopes are typical cool-water carbonate settings.

Geologic setting:- St. Vincent Basin is a structural intracratonic graben. It was formed during the Middle Eocene as a consequence of sudden and rapid separation between Australia and Antarctica. The Port Vincent Limestone is intermittently exposed for 50km in coastal cliffs on the basin's western margin. It is a bryozoan rich calcarenite, composed of several metre-scale subtidal cycles, each capped by a hardground.

Aims:- To unravel the depositional-diagenetic sequences of the Port Vincent Limestone, and to address the relationship between diagenetic processes and the cool-water depositional model.

CEMENTATION Cement is limited and precipitated in three diagenetic environments: the seafloor, shallow-burial, and meteoric.

Seafloor cementation was synsedimentary, affecting each depositional cycle separately; whereas later shallowburial and meteoric cementation affected the entire succession. Synsedimentary cement precipitated as thin rinds of isopachous spar around echinoid grains and calcareous algae. These cements are non-luminescent, iron-poor, and are now low-Mg calcite. They reflect precipitation from oxidizing marine water on or just below the seafloor. At the top of each depositional-cycle, microbioclastic internal-micrite partially infills inter- and intragranular pores, and also overlies the aforementioned cement. This diagenetic sequence is seen in each successive depositional cycle. After shallow burial, a zoned bright-luminescent cement (Mn= 18680, Fe=670 ppm) precipitated in available pore space around the synsedimentary cement. This cement heals fractures that cut across pre-cemented echinoid fragments. The cement indicates reducing conditions under shallow burial (<100 m). Later, the rocks were exposed to the meteoric realm, where they underwent minor dissolution, and the formation of two cement fabrics, both of which occur in rare amounts. The first is a non-luminescent overgrowth (Mn=0-160, Fe=0-430 ppm), consisting of several thin dull to bright-luminescent zones (Mn<560, Fe<490 ppm).

The latter reflects precipitation during short periods of diminished freshwater supply, possibly induced by climatic fluctuations. The second is a blocky (nonequicrystalline and equicrystalline) non to dull-luminescent mosaic (Mn=0-100, Fe=0-180 ppm), with thin bright to dull-luminescent intercrystalline boundaries. The Fe content may be derived from the clay-rich internal micrite, or the overlying Plio-Pleistocene ferruginous Ardrossan Clay and Sandrock. We conclude that the nonequicrystalline mosaic precipitated within an active fresh water phreatic zone (with high original crystal nucleation), whereas the equicrystalline mosaic precipitated within a relatively stagnant fresh water phreatic zone. A low nucleation rate is expected at this late stage, as pore waters become depleted in CaC03.

Source of CaCOr.- Cool-water carbonates have low diagenetic potential and yield little CaC03 for cementation.

Marine destructive dissolution appears to be the major source for initial lithification. Mild chemical compaction and meteoric dissolution are additional sources for cement.

CONCLUSIONS Initial lithification of the Port Vincent Limestone occurred on the seafloor by synsedimentary marine cement and internal micrite. This took place during and at the end of each depositional cycle. Later, after deposition of all cycles ended, the rocks were cemented by shallow burial cements. Meteoric cements were precipitated during the final subaerial exposure. Sources for cement were essentially autochthonous, and all cements are now lowMg calcite. Except for hardgrounds, the bulk of the rocks remained friable and highly porous because only minor mineral-controlled diagenesis occurred. This is the usual diagenetic pathway for widespread cool-water carbonates, contrasting with that followed by warm-water carbonates. The cool-water model may serve for interpreting ancient calcitic facies, and can be applied towards directing the course of exploration for hydrocarbons and economic ore deposits.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SHALLOW BURIAL DOLOMITIZATION IN THE TERTIARY COOL-WATER PORT VINCENT LIMESTONE, ST. VINCENT BASIN, SOUTH AUSTRALIA Basim Shubber1, Yvonne Bone1, Noel James2, and Brian McGowran1 department of Geology and Geophysics, University of Adelaide, South Australia 5005, Australia, department of Geological Sciences, Queen's University, Kingston, Ont. K7L 3N6, Canada.

INTRODUCTION The Oligocene-Miocene Port Vincent Limestone is intermittently exposed for 50 km along coastal cliffs on the western margin of South Australia's St. Vincent Basin. The Limestone has a cool-water skeletal association. It is a bryozoan dominated calcarenite, locally enriched in foraminifers, echinoids, coralline algae, bivalves, gastropods, and brachiopods.

Aims:- This study documents the presence of Tertiary dolomite in the Port Vincent Limestone. An attempt to address the style, composition, mode of origin, and diagenetic history of these dolomites is intended herein.

Methodology employs chemical staining, transmitted light and cathodoluminescence petrography (CL), X-ray diffraction and fluorescence, scanning electron microscopy, elemental mapping and dry chemical analysis using electron microprobe.

DOLOMITIZATION Dolomitization is local and restricted to a narrow area, extending for approximately 200m south of Port Vincent.

The Limestone is variably altered to dolostone and dolomitic limestone. Contact-rhomb porphyrotopic fabrics (sucrosic) completely or partially replace original matrix and most grains. Individual rhombs are euhedral and occasionally subhedral, with the majority ranging in size between 50-100 \xm. Isotopic and geochemical data suggest that dolomitization occurred during mid-Cenozoic, after shallow burial (<200 m), from fluctuating mixtures of marine waters. The dolomite is non-stoichiometric, composed of 41.6 mole % MgC03, 53 mole % CaC03, 5.4 mole % FeC03, and has elevated Sr, Al, and Na concentrations. Almost all crystals show several distinct contrasting deep red, dull to bright luminescent concentric zones which correspond to variable compositional ratios. Boundaries between adjacent zones are straight and sharp, and sectoral zonations are poorly developed, suggesting no dissolution took place during crystal growth. Scattered rhombs appear to contain microdissolution voids as illustrated by SEM photomicrographs and back-scattered images. Dissolution is fabric selective, affecting crystal centres and iron-poor concentric zones. These thin rhomb-shaped intracrystalline voids are empty or partially filled with iron oxide. Traces of calcite precipitated in a few intracrystalline micropores, in a two-step dedolomitization process (cf. James et al., 1993). The style of formation and subsequent dissolution of these dolomites strongly correlates with other dolomitized Tertiary coolwater carbonates elsewhere in Southern Australia (e.g. the Gambier Limestone, James et al., 1993).

CONCLUSIONS Dolomitization in the Port Vincent Limestone occurred under shallow burial from marine water. Compositional zonation in these dolomites reflects varying marine/diluted marine pore water ratios. Dissolution was fabricselective caused by fresh water input, possibly before the Miocene/Pliocene hiatus. The resemblance of these dolomites to the Gambier Limestone dolomites suggests that the marine-freshwater controlled alterations in these distant successions represent regional sea-level fluctuations.

REFERENCES James, N.P., Bone, Y., and Kyser, T.K., 1993, Shallow burial dolomitization and dedolomitization of MidCenozoic, cool-water, calcitic, deep-shelf limestones, Southern Australia: Journal of Sedimentary Petrology, v. 63, no. 3, p. 528-538.

Acknowledgement: This research was made possible by an Australian Overseas Postgraduate Research Scholarship, and a University of Adelaide scholarship to B. Shubber. Support was also available from an Australian Research Council grant (BS, YB, BMcG). Dr. T. Kurtis Kyser from the University of Saskatchewan (now at Queens University) performed isotope analysis.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 41 13th Australian Geological Convention, Canberra, February 1996

CHARACTERISTICS AND CONTROLS OF THE LARGEST PORPHYRY COPPER-GOLD AND EPITHERMAL GOLD DEPOSITS IN THE CIRCUM-PACIFIC REGION Richard H. Sillitoe 27 West Hill Park, Highgate Village, London N6 6ND, England

Most of the world's giant porphyry copper-gold and epithermal gold deposits occur in arc terranes around the Pacificrim.Eight porphyry copper-gold deposits and 12 epithermal gold±silver deposits attain giant status, defined here to require >200 tonnes of contained gold. The deposits are divided roughly equally between the American Cordillera and western Pacific island arcs and, therefore, may be underlain by either relatively thick cratonic or primitive crust. Six of the 20 deposits are related genetically to alkalic or shoshonitic igneous rocks, a surprisingly large percentage given the restricted distribution and volume of such rocks in the region.

The giant porphyry copper-gold deposits display similar geological characteristics. All occur in and around vertically extensive cylindrical porphyry stocks that appear to be eroded quite deeply. Two deposits, however, retain basal remnants of overlying advanced argillic lithocaps. Copper and gold are present with pyrite-poor Ksilicate alteration which in six deposits, including those with the highest gold contents, is exceptionally rich in hydrothermal magnetite.

The epithermal gold deposits are more varied geologically. Five of them comprise or include bonanza gold accumulations, whereas the rest are low-grade deposits. Two-thirds of the deposits are of low-sulphidation type, the others high sulphidation. Telescoping of ore types or stages characterizes five of the deposits. Two of the low-sulphidation deposits are rich in base metals, whereas most of the others are poor in all sulphides. Four of the deposits are associated with maar-diatreme systems, which are also present as late additions to four of the giant porphyry copper-gold deposits. Two of the largest low-sulphidation epithermal deposits are hosted by poorly welded ignimbrite as parts of ash-flow calderas. Five deposits preserve features indicating proximity to a paleosurface.

No unique regional (metallogenic) or deposit-scale feature, or combination of features, accounts satisfactorily for the formation of all giant copper-gold and gold deposits. Nor do such deposits differ appreciably, except in metal contents, from their smaller brethren. The greater metal contents of the giants are taken to imply larger throughputs of copper- and/or gold-bearing fluids and, in the case of the bonanza deposits, perhaps to exceptionally gold-rich fluids.

In both the porphyry and epithermal environments, rapid decompression of crystallizing magma chambers, leading to accelerated release of metal-rich magmatic fluids, is proposed as a means of achieving enhanced fluid throughputs. Decompression may be caused by either broad-scale rapid tectonic uplift and concomitant erosion or localized gravitational sector collapse of overlying volcanic edifices. Rapid tectonic uplift may be linked to episodic compressive events in arc segments or to arc-arc or arc-continent collisions, as in New Guinea during the Plio-Pleistocene. Rapid unroofing of active hydrothermal systems may facilitate maar-diatreme generation and telescoping. Telescoping facilitates immobilization of metals precipitated previously, a process that may lead to either increases or decreases in copper and gold grades.

Under the hydrostatic conditions prevailing in the epithermal environment, unexpected situations encountered by ascending metal-bearing fluids may lead to increased gold depositioa Among these, marked contrasts in effective wallrock permeability, leading to abrupt changes in several physico-chemical parameters, is preeminent and may be inferred for at least half of the giant epithermal gold deposits. In contrast, large copper and gold concentrations in the porphyry environment, at the hydrostatic-lithostatic interface, seem to be favoured by relatively impermeable wallrocks, such as limestone or andesite flows, and the resultant vertical focusing and perhaps refluxing of fluids within stocks.

Sixteen of these giant porphyry copper-gold and epithermal gold deposits were discovered since the mid-1950s, seven of them by re-examination of old mining districts or known mineral occurrences. Geochemistry played a major role in nine of the discoveries and geophysics in two. Conceptual geology is perceived to have been instrumental in only one, perhaps two, of the discoveries. The challenge in the years ahead is to utilize more effectively the geological characteristics and controls of giant deposits in circum-Pacific exploration programmes. More attention to petrochemical provinces, physical volcanology, tectonic and uplift regimes and host-rock permeability may help to achieve this goal.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A COMPARISON OF SILURIAN CONODONT FAUNAS FROM THE GRAVEYARD CREEK AND CAMEL CREEK SUBPROVINCES, NORTH QUEENSLAND. Andrew Simpson Department of Earth Sciences, University of Queensland, 4072

Silurian conodont faunas have been recovered from a range of autochthonous and allochthonous stratigraphic units within the Graveyard Creek Subprovince. The allochthonous nature of some units is indicated by sedimentological evidence and, less frequently, the occurrence of biostratigraphically incompatible conodont faunas according to presently available range-data. This suggests the cannibalisation of limestones of different ages upslope.

Isolated Early Silurian faunas were recovered from the Quinton and Poley Cow formations of the Graveyard Creek Group. The Jack Formation, the uppermost unit of the Graveyard Creek Group in the Graveyard Creek Subprovince, has yielded autochthonous sequences of Wenlock to Ludlow conodont faunas in the type section at the Jack Hills Gorge. At the Broken River Crossing, the formation has yielded a sequence of Ludlow to Lochkovian conodonts. To the north in the Arch Creek-lower Jesseys Creek area, carbonates presently construed as Jack Formation equivalents yield faunas with a mixed Ludlow-Pridoli aspect indicating allochtheniety.

The Camel Creek Subprovince is an oroclinal region of predominantly Siluro-Devonian turbidites to the east of the Graveyard Creek Subprovince. Despite all these carbonates being allochthonous, there is a broad biostratigraphic coherence with the oldest units consisting of an admixture of Ashgill with minor Llandovery elements low in the Perry Creek Formation and late Llandovery-earliest Wenlock conodonts higher in the formation (Sloan et al., 1995). Silurian faunas in the Camel Creek Subprovince were predominantly sourced from autochthonous units of the Graveyard Creek Group.

REFERENCE Sloan, T.R., Talent, J.A., Mawson, R., Simpson, A.J., Brock, G.A., Engelbretsen, M.J. Jell, J.S., Aung, A.K., Pfaffenritter, G , Trotter, J. & Withnall, I.W. 1995. Conodont data from Silurian-Middle Devonian carbonate fans, debris flows, allochthonous blocks and adjacent autochthonous platform margins: Broken River and Camel Creek areas, north Queensland, Australia. Courier Forschungsinstitut Senckenberg 182,1-77.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MIDDLE PALAEOZOIC OROGENIC EVENTS IN THE SOUTHERN LACHLAN FOLD BELT: AGE CONSTRAINTS FROM CONODONT DATA Andrew Simpsonfl^ & John Talent(2) 1 Department of Earth Sciences, University of Queensland, 4072 2 Centre for Ecostratigraphy and Paleobiology, Macquarie University, 2109

The southern region of the Lachlan Fold Belt includes a structurally complex area of Ordovician to Early Devonian sedimentary rocks, volcanics, intrusives and metamorphics outcropping in the head-waters of the Indi, Buchan and Tambo rivers in eastern Victoria and adjacent parts of New South Wales. Numerous lenticular carbonate units outcrop in a triangular fault-bounded region refered to as the Cowombat Rift (sensu Vandenberg et al.9 1984) or Limestone Creek region. These were previously considered to have .a generalised Late Silurian age (Allen, 1991). These have been interpreted as postdating a thick sequence of volcanics, the Thorkidaan Volcanics, associated with the Benambran Orogeny and construed as extruded during the Late Silurian, interleaved with marine sediments. This scenario is pivotal to interpretation of the region as a rifted margin during Late Silurian times.

Recent sampling of these carbonates for conodonts (Simpson & Talent, 1995) has demonstrated that the marine sequences span a time-interval extending from the early Llandovery through to the Pridoli. Llandovery to Wenlock units include limestones of the Reedy Creek area in the south and McCartys limestone in the north.

Ludlow to Pridoli sequences are preserved at Cowombat Flat and Native Dog Plain. The most complete sequence is preserved in the Claire Creek-Stoney Creek area where two carbonate units are separated by a pelitic interval. The lower unit apparently spans the Llandovery-Wenlock boundary; the upper unit spans the Wenlock-Ludlow boundary. In all the sections sampled including the chronologically substantial Claire CreekStoney Creek region no volcanic units were encountered.

The rift margin hypothesis may be tenable, but, if so, would apply to a much narrower time-interval (or timeintervals) pre-dating the early Llandovery or post-dating a horizon somewhere in the Pridoli. The conodont data (Simpson & Talent, 1995) permits a more accurate chronology for orogenic events and demonstrates the need to reconsider tectonic concepts for this part of the Lachlan Fold Belt during the Silurian.

A small number of other lenticular carbonates are still to be tested for conodonts; these may yield further age constraints on the timing of the Benambran Orogeny, and onset of the Bowning Orogeny. Mineralised areas in the Limestone Creek region and especially south-east of it have been grouped with supposedly Late Silurian strata of the Cowombat Formation (Allen, 1991), but there is no palaeontological evidence for this allocation.

Detailed conodont analysis is required for that area and for the numerous tracts of similar, chronologically poorly understood Silurian carbonates to the north in the Tantangarra-Brindabella and Queanbeyan-Cooma area of southern New South Wales.

REFERENCES Allen, R.L., 1991. Limestone Creek area 1:50,000 geological map, Second edition. Geological Survey of Victoria, Melbourne.

Simpson, A.J. & Talent, J.A., 1995. Silurian conodonts from the headwaters of the Indi (upper Murray) and Buchan rivers, southeastern Australia, and their implications. Courier Forschungsinstitut Senckenberg 182, 79215.

Vandenberg, A.H.M., Bolgcr, P.F. & O'Shca, P.J., 1984. Geology and mineral exploration of the Limestone Creek area, northeast Victoria. Department of Mines and Energy, Victoria, Geological Survey Report 72, 59pp.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE MOUNT ELIZABETH CALDERA: A RESURGENT VOLCANIC COMPLEX IN EASTERN VICTORIA Carol J. Simpson PO Box 38S, Wiliiamstown, VIC 3016

The Early Devonian Mount Elizabeth Caldera is a discrete entity that occurs within the Tabberabbera Zone of the Lachlan Fold Belt in East Gippsland, Victoria. It is located approximately 10 km from the western margin of the Early Devonian Snowy River Volcanics, which is largely confined within the fault-bounded Buchan Rift.

A likely genetic relationship between the Mount Elizabeth Caldera and the nearby extensive belt of Snowy River Volcanics is suggested by the occurrence in both of a distinctive biotite-bearing, crystal-rich quartz ignimbrite.

An Early Devonian age is indicated for the Mount Elizabeth Caldera by stratigraphic relationships and limited isotopic dating of the granitic ring pluton.

The Mount Elizabeth Caldera is a relatively small feature with a diameter of 13.5 km and consists of a central volcanic pile, surrounded by an almost complete, circular ring pluton and associated rhyolite dykes. The volcanic pile is dominated by the rhyolitic Fainting Range Ignimbrite which consists of multiple flow units of a black, mainly densely welded rock that contains sparse feldspar crystals, abundant pumice clasts and scattered lithic fragments. A second rhyolitic ignimbrite (Slater Ignimbrite) occurs close to the top of the preserved succession as a thin horizon that has been repeated by concentric faults and offset by radial faults. It is indistinguishable from crystal-rich, biotite-bearing welded ignimbrites within the Snowy River Volcanics (Mount Tara Subgroup) and is interpreted as an outflow sheet erupted during a hiatus in the eruption of the Fainting Range Ignimbrite. The volcanic succession also contains small rhyolite lavas/domes and dykes and mainly lacustrine sedimentary rocks. The ring pluton (Saint Patricks Creek Granite) occurs as two arcuate bodies, linked in the northeast by a coarsely porphyritic dyke. The centre of the caldera is occupied by the Mount Elizabeth Granodiorite that represents the resurgent phase of caldera development.

Structurally, the Mount Elizabeth Caldera is complex and although the precise thickness of the volcanic pile cannot be measured due to faulting, it is estimated to exceed 1500 m. Unroofing of the resurgent pluton within the caldera indicates substantial erosion of the original sequence and present levels of exposure preserve only the central subsided block of volcanic rocks. The central volcanic pile, ring pluton and narrow ring dykes are bounded by near-vertical ring faults as indicated by fracturing, brecciation and silicification of Ordovician and caldera rocks. Blocks of Ordovician rock slumped from the steep caldera wall are also locally conspicuous. A number of radial faults influence the outline of the central volcanic pile and offset lithologies within it. Most of the volcanic pile is outward dipping reflecting caldera resurgence, however, inward dipping rocks in the strongly faulted northeast sector record downsagging of the volcanic sequence during caldera collapse.

The earliest evidence of volcanism at Mount Elizabeth was the emplacement of a swarm of east-west oriented rhyolite dykes in Ordovician sediments and Silurian granite. The Mount Elizabeth Caldera formed in response to the eruption of the Fainting Range Ignimbrite, although the precise timing is unclear. The preferred model is one involving eruption of a large proportion of the ignimbrite within a slowly subsiding depression in the magma chamber roof rocks, with breaks in the eruption allowing limited reworking of material in small lakes and the emplacement of the Slater Ignimbrite and lava domes. Evidence for breaks in ignimbrite eruption together with the occurrence of the Slater Ignimbrite as a conformable horizon and the absence of early megabreccias favour major caldera collapse late in the eruption of the Fainting Range Ignimbrite. Collapse along an inner ring fault dropped the central block of volcanics well below the level of the caldera rim, and collapse on a series of faults led to sequence repetition in the northeast sector of the caldera. The embayed southeast margin of the volcanic pile suggests that caldera collapse was asymmetrical and hinged on this side.

The Saint Patricks Creek Granite was emplaced along the outerringfaults, along with a series of concentric and cross-cutting dykes. Ring pluton intrusion and subsequent erosion has removed ignimbrite between the inner ring fault and the caldera wall, except for tiny slivers caught in the margin of the ring pluton. Updoming of the central volcanic pile in response to the emplacement of the Mount Elizabeth Granodiorite was mainly accommodated by the caldera collapse ring faults. The resurgent pluton displays textures indicative of a high level of emplacement but geochemically, it is distinct from the earlier phase of activity.

Limited exploration for base and precious metal mineralisation in the Mount Elizabeth Caldera has, to date, reported negligible values of gold and other indicator elements. Favourable sites for future exploration should focus on the ring and radial faults, areas of lacustrine sediments and the resurgent pluton.

Acknowledgments: This work was carried out during preparation of the Mount Elizabeth 1:50 000 Geological Map Report and the Geological Survey of Victoria is acknowledged for their permission to publish this abstract.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ERUPTION HISTORY OF THE ROCKLANDS RHYOLITE, WESTERN VICTORIA Carol J. Simpson PO Box 388, Williamstown, VIC 3016

The Rocklands Rhyolite is a latest Silurian to Early Devonian sequence in western Victoria that is dominated by large volume, mostly densely welded ignimbrites but also includes lavas, sedimentary rocks and dykes.

These rocks occur as a 65 km long belt adjacent to, or conformably to locally unconformably overlying, the western margin of the Silurian Grampians Group which is a sequence of predominantly shallow marine sedimentary rocks preserved within a pull-apart basin. Granitic rocks that are the co-magmatic equivalents of the Rocklands Rhyolite intrude the Grampians Group in the Grampians Ranges. A small patch of granite occurs within the Rocklands Rhyolite and aeromagnetic data suggests the presence of larger bodies of granite at depth.

The Rocklands Rhyolite covers an area of approximately 2000 km 2 and is a horizontal sequence with an exposed thickness of 250 m. The northern and southern limits of the Rocklands Rhyolite are covered by Tertiaiy and Quaternary rocks of the Murray Basin and the Quaternary Newer Volcanics, respectively. There has been no previous attempt to sub-divide or reconstruct the volcanic history of the Rocklands Rhyolite. Surface exposure is poor due to a thick Tertiary laterite cover and resistant lithologies such as welded ignimbrite and lava are preferentially preserved. The small preserved proportion of the original sequence and the apparent absence of syn-volcanic structures and source vents resulted in difficulties in interpreting the palaeovolcanology.

Despite the lack of exposure, however, distinctive units have been recognised and correlated over a large area.

The oldest preserved rocks in the Rocklands Rhyolite are small volume quartz latite lavas that are mainly observed as large blocks at the base of the oldest ignimbrite. The bulk of the stratigraphic succession consists of six welded ignimbrites composed of single to multiple flow units with a sheet-like geometry. Evidence indicating proximity to a source vent/caldera, such as the occurrence of large lithic breccias, has been found in only one of these ignimbrite sheets. A second possible small volcanic centre close to the northern extent of the Rocklands Rhyolite consists of thinly bedded accretionary lapilli-bearing tuffs, interpreted as pyroclastic surge deposits, and an associated non-welded ignimbrite. The ignimbrite sheets are interspersed with thin discontinuous lenses offluvialconglomerate and sandstone and rhyolitic lava flows of limited extent. The lavas include feldspathic, quartz-feldspar porphyritic and aphyric varieties and have a higher aspect ratio than the ignimbrites. Larger areas of enigmatic rocks which have previously been regarded as flow banded lavas are spatially closely associated with two ignimbrites. The southern portion of the Rocklands Rhyolite has been intruded by abundant, massive toflowbanded rhyolitic dykes that are geochemically similar to the host rocks, and by mafic dykes of unknown age.

The Rocklands Rhyolite ignimbrites are all alkali rhyolites in composition and contain varying proportions of quartz, plagioclase and K-feldspar crystals. The ignimbrites have been distinguished on the basis of crystal size and abundance, lithic content and the proportion offlattenedpumice clasts. All ignimbrites retain clear evidence of an explosive eruptive origin, particularly close to the base of flow units and the six welded ignimbrite sheets display many features indicative of high temperature emplacement. The mineralogy is anhydrous, some outcrops are columnar jointed but more convincingly, welding is generally intense throughout the flow thickness and grades upwards into rheomorphic textures. The foliation defined by flattened pumice clasts in some exposures is extremely attenuated to resemble continuous flow banding that typifies lavas. The pumice foliations are chaotic in orientation within ignimbrites that are demonstrably horizontal units. These features indicate that the ignimbrites were sufficiently hot to undergo welding of the pyroclasts and behave as a viscous coherentflowin the final stages of emplacement.

Two large occurrences of lava-like rocks associated with the ignimbrites in the central and northern parts of the Rocklands Rhyolite display many characteristics typical of rhyolitic lavas, such as flow banding and folding.

The extent of these rocks (up to 600 km2), however, is unusually large for such viscous compositions. Rarely preserved fragmental microtextures and a gradationalfieldrelationship between basal ignimbritic textures and the overlying, mineralogically similar lava-like rocks are more consistent with interpretation of the lava-like rocks as the upper intensely rheomorphic part of ignimbritic flows. These rocks have undergone non-particulate flow in the final stages of emplacement which led to complete obliteration of pyroclastic features.

The Rocklands Rhyolite is, therefore, of considerable interest as it represents one of only a few documented examples of voluminous rheomorphic ignimbrites in Australia which reflect higher than usual eruption temperatures and/or an unusual style of eruption.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 ALLOCYCLIC CONTROLS ON FORE ARC VOLCANICLASTIC AND OOID BANK SEQUENCE DEVELOPMENT: ROCKHAMPTON GROUP (CARBONIFEROUS), YARROL BASIN, QUEENSLAND. Glenn Simpson & Simon C. Lang 1. Geological Survey of Queensland, Department of Minerals & Energy, Qld. GPO Box 194 Brisbane, 4001.

2. School of Geology, Queensland University of Technology, GPO Box 2434, Brisbane, Qld, 4001.

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Sedimentation on mixed siliciclastic-carbonate, marine shelves at active margin, forearc basin settings is controlled by an interplay between eustacy, tectonism, and sediment supply. The variable influence of these allocyclic controls were determined for the mixed siliciclastic - carbonate, forearc basin sequences of the Early Carboniferous Rockhampton Group of the Yarrol Basin. The study area, 50km west of Rockhampton, was analysed using modern techniques in facies analysis and sequence stratigraphy to demonstrate that oversupply of volcaniclastic sediment from the arc to the west is at least as important as eustatic and local tectonic influences in controlling sequence architecture.

Six facies associations were interpreted: Gravity Flow, Coastal Gravelly Channel, Sandy Shelf, Muddy Shelf, Bioturbated Muddy Shelf, and Ooid Bank Associations. Stacking of these associations and their relationship to mappable unconformities led to the identification of six unconformity-bounded sequences containing Lowstand, Transgressive, and Highstand Systems Tracts (LST, TST, HST).

SEQUENCE 1 formed during the mid-Tournaisian and involved a rapid transgression resulting in the deposition of oolitic limestone beds which appear to onlap onto the underlying unconformable surface. The onset of the transgression marks the end of tectonic uplift in the area and a rise in global sea level.

SEQUENCE 2 developed in the Late Tournaisian to Early Visean and is characterised by aggradational, retrogradational, and progradational stacking of siliciclastic and carbonate facies which record the response to eustatic sea level change. Aggradationally stacked Ooid Bank Association deposits characterise the late TST to early HST with carbonate production peaking when flooding was at its maximum. Progradationally stacked volcaniclastic sandstones in the HST reflect an increase in the vocaniclastic input which terminated carbonate development.

SEQUENCE 3 formed in the Early Visean and contains deposits of the Coastal Gravelly Channel Association, Ooid Bank Association, and Sandy and Muddy Shelf Association. The TST developed in response to a decrease in the volcaniclastic influx to the shelf. Carbonate production commenced during the TST, peaked when the flooding reached its maximum. The subsequent progradation of a HST with a high volcaniclastic content occurred in response to an increase in volcaniclastic input and carbonate production was terminated in the early HST when the volcaniclastic input became excessive .

SEQUENCE 4 developed in the Late Visean and is characterised by its large thickness and lack of thick oolitic limestone deposits. The sequence formed in response to the combined effect of tectonic subsidence and global sea level change. Increased rates of subsidence, though partially offset by global eustatic sea level fall, resulted in local transgression and a retrogradational stacking pattern in the TST. Subsidence waned during the subsequent Highstand Systems Tract and deposition of Bioturbated Muddy Shelf Association commenced on the stable shelf.

SEQUENCES 5 and 6 formed in the Latest Visean and are dominated by oolitic limestones and calcareous sandstones of the Ooid Bank Association. Retrogradational siliciclastic stacking patterns and aggradational carbonate stacking patterns preserved in the sequences reflect regression - transgression cycles associated with global sea level fluctuations.

Overall, siliciclastic sedimentation during the Early Carboniferous was episodic but dominated during the LST, early TST, and HST. The changes in siliciclastic supply were induced by fluctuations in volcaniclastic input, tectonic subsidence, and eustatic sea level change. Carbonate production was also episodic resulting in oolitic limestone development related to changes in relative sea level and the amount of volcaniclastic input on the shelf.

The development of sequences in the Rockhampton Group, therefore, is a result of the combined effect of eustacy, tectonism and volcaniclastic input and underlines the profound effect of volcanism on sediment supply and patterns of sedimentation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SOME WAYS TO REDUCE THE GEOLOGIC RISK OF FINDING WORLD-CLASS DEPOSITS: KUROKO DEPOSITS, AN EXAMPLE Donald A. Singer* * United States Geological Survey, MS 984, 345 Middlefield Road, Menlo Park, C A 94025

Reducing the risk (i.e.. the uncertainty) of finding world-class deposits involves choosing the right ground. A successful strategy requires identification of the sources of risk and incorporating them in the decision-making process so that controllable risk can be reduced. Among the possible sources of risk are: variation of deposit sizes among deposit types, variation in deposit sizes within types due to local or regional differences in geologic settings, variation in economic returns by type, and discovery chances, given existence of deposits. One way to compare the desirability of different deposit types is to determine the chance that a deposit of a given type might be world-class.

World-class mineral deposits, defined as the upper 10 percent of all deposits in terms of contained metal or value, are the primary exploration targets of many mining firms. About 40 percent of porphyry copper deposits are large enough to be considered world-class, as are 36 percent of Mississippi Valley Zn-Pb districts, and 8 percent of kuroko massive sulfide deposits. These chances must be weighed against the profitability of different types, abilities of the searchers, number of undiscovered deposits that might exist, areas available for search, and regional or local differences within types.

One might choose to search for Mississippi Valley Zn-Pb districts or kuroko deposits of which about 90 percent of known deposits or districts have been economic, before searching for the 50 percent economic porphyry copper deposits. If the number of known deposits is proportional to the number of deposits yet to be discovered, then one would search for kuroko deposits with over 500 deposits, rather than the 10 times more scarce Mississippi Valley Zn-Pb districts, or the half as common porphyry copper deposits. An example using kuroko massive sulfide deposits, serves to demonstrate how regional or local differences within a type can be identified and used in an exploration strategy.

Massive sulfide deposits associated with felsic to intermediate submarine volcanic rocks are herein classed as kuroko deposits. Adjacent mineralization within 500m is combined into one deposit. Tonnages of 424 kuroko deposits from around the world along with the published rock compositions and rock textures up to 500m above and 500m beneath these deposits are the data for this analysis. Compositions were grouped into six broad classes as were textures. Because of the significant correlation between tonnage of ore and contained metal, tonnage is a reasonable surrogate for metal in defining world-class deposits. In order to determine whether the presence of any of these variables is useful in predicting tonnage and identifying environments of large deposits, an analysis of variance method was used on the 424 deposits with the logarithm of tonnage as the dependent variable. The only significant effect for rocks above kuroko deposits is an increased tonnage where the hanging wall rocks are sedimentary. Kuroko deposit tonnages also significantly increase where there are sedimentary or rhyolitic composition footwall rocks. Tonnages significantly decrease where the footwall rocks are flows. The presence of pyroclastics and tuffs in the footwall strata is not related to tonnages of kuroko deposits. Dacitic to rhyolitic breccias or porphyries under kuroko deposits are marginally significant predictors of increased tonnage; however, they are typically localized near the ore and therefore not necessarily useful in reducing the risk of finding worldclass kuroko deposits. Thus, although only 8 percent of kuroko deposits are world-class, the explorationist can significantly improve these odds to about 15 percent by looking in special geologic settings such as those containing sediments and rhyolitic composition rocks.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SHRIMP ION PROBE PROVENANCE STUDIES OF HEAVY DETRITAL MINERALS IN COASTAL SANDS AND SEDIMENTARY ROCKS OF EAST AUSTRALIA Keith Sirconibe & W. Compston Research School of Earth Sciences, Australian National University, CANBERRA ACT 0200

INTRODUCTION In east Australia, the origin of the heavy minerals in the coastal sands have been discussed by various workers, primarily based on mineralogical studies. A lack of correlation between river and coastal sand mineralogy was noticed, leading to a suspicion of a distal Proterozoic/Palaeozoic protosource with intermediate repositories for the heavy minerals along the east Australia coastline. SHRIMP is used here to compile age population datasets of detrital samples from east Australia for provenance study. The age populations from zircon and monazite analyses are plotted and deconvolved to indicate matches with potential provenance sources.

METHODS The two principal minerals used for SHRIMP analysis in this study are zircon and monazite. The ion probe analysis and data assessment of zircon is well established, and that of monazite is becoming routine. Samples were collected from sites along the east Australia coast. Inland sites were also sampled in the Murray Basin, Snowy River and Sydney Basin. Zircon and monazite were extracted, prepared and analysed using standard techniques.

Ideally, at least 60 randomly chosen individual grains are measured to statistically ensure that there is only a 5% chance of missing any component population with a frequency of one grain in 20 or less in the total population.

Ages are calculated using standard data reduction methodology for zircon, with minor modifications for monazite.

From these ages and their errors a variable kernel probability density estimation can be plotted assuming that each individual analysis is gaussian and encompasses an equal area in the plot. The age probability density estimations provide a useful visual reference to discern possible component sub-populations. The finite mixture modelling approach is used to further deconvolve component age groups and their relative proportions.

APPLICATION AND RESULTS The coastal samples show strong dominance of a Late Proterozoic/Early Cambrian population which is termed the Pacific Gondwana igneous component. This is considered an 'exotic' contribution to the east Australia orogenic belt, and is a common feature of other southwest Pacific sediments. The protosource for this population remains undetermined. The dominance of the Pacific Gondwana component varies on a regional scale with components potentially sourced from Lachlan and New England Fold Belt. The presence of the Lachlan component is stronger in the south at Mallacoota, and the New England component is stronger in the north at North Stradbroke Island. The dominance of the Pacific Gondwana component is also seen almost exclusively in the monazite ages, suggesting little significant contribution from younger sources.

Inland samples indicate a range of provenances. In the Murray Basin, there appears to be no significant link to nearby potential sources, notably the Broken Hill region. Sediment from the Snowy River resembles the region it flows through with a Lachlan Fold Belt age distribution. The Sydney Basin samples confirm theories based on paleodirections about provenance, with the Hawkesbury Sandstone also being dominated by the Pacific Gondwana component.

CONCLUSION The ability of the SHRIMP to rapidly generate relatively large datasets is a powerful technique for the provenance study of sediments in various depositional settings. In east Australia it has confirmed a distal Precambrian protosource of the heavy minerals with a strong exotic Pacific Gondwana component with varying proportions of younger components from the Lachlan and New England Fold belts.

401

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 A SUBDUCTION-RELATED ORIGIN FOR ANORTHOSITES AT AN EARLY PROTEROZOIC CONTINENTAL MARGIN IN CENTRAL AUSTRALIA *

2

Warwick J. Sivell 1' 2 and Malcolm T. McCulloch 2 Faculty of Science and Technology, University of Western Sydney, Kingswood N.S.W. 2747 Research School of Earth Sciences, The Australian National University, Canberra A.C.T. 0200

The composition and tectonic affinity of plagioclase-saturated parental magmas from which anorthosites crystallised and their relation to associated magma suites has been a fundamental problem in understanding the origin of anorthositic rocks. Major anorthosite occurs in the -1.76 Ga Harts Range Metaigneous Complex (HRMC), eastern Arunta Inlier, and it is argued, represents the culmination of an episode of ensialic back-arc basin magmatism at the southern Cordilleran-type margin of the North Australian craton. Magmas from which the anorthosites crystallized (upon emplacement within shallow level supracrustals) were probably derived by high pressure open-system fractionation of early-contaminated (picrite-) norite parent melts (8N<j = -0.6; Zr/Nb = 11-20) in deep crustal staging chambers. A wide range of initial N d / N d ratios (ejsjd = -0.6 to -4.4) for the anorthosites - values much lower than for HRMC tholeiites (e^d up to +8) in the host rift sequence (Figure 1) - as well as high K/P, Ti/V, Eu/Eu*, Sr/Nd, Nb/Y and low Ti/Yb, Rb/Nb and Zr/Nb, imply massive AFCtype interaction of these magmas with pre-2.6 Ga Rb-depleted lower crustal components (including 25-70% crustally derived Nd). This provides indirect evidence for Archaean basement in the eastern Arunta. i43

Fig. 1. Diagram of ej^^ against age showing isotopic evolution of Harts Range Metaigneous Complex (HRMC) anorthosites (thick solid trajectories) and related rocks. Inferred trajectories for possible crustal and mantle sources (Sivell and McCulloch 1991) are shown with Sm/Nd enrichment factors ( / ) indicated. DM - depleted mantle. Dotdashed line is trajectory for norite parental to HRMC anorthosite suite. Thin solid trajectories show evolution of juvenile Archaean crust with mantle extraction ages from 2.2 to 3.0 Ga.

i44

S m / N d

Age (Ga.)

The norites, together with the earlier erupted HRMC Lower Tholeiites (eNd = +8.2 to + 6.9; Zr/Nb = 40 to 55) and Upper Tholeiites (ejsjd = +6.9 to +2.8; Zr/Nb = 25 to 40), all possess geochemical signatures of back-arc basin basalts. Primary melts for the sequentially emplaced Lower Tholeiites, Upper Tholeiites and norites are modelled as 5-10%, 15-20% and 25-30% partial melting respectively of predominantly asthenospheric mantle, with a progressively increasing slab-derived component, during extension behind the early Proterozoic Entia magmatic arc. The pronounced crustal signature of the anorthosites represents a major continuation of the upward stratigraphic trend of enhanced crustal interaction shown by chemical and isotopic features of the Lower and Upper TTioleiites and norites (0-5, 5-20 and 14-40% contamination respectively). It is argued that the common occurrence of anorthosite complexes at boundaries between Archaean and Proterozoic provinces, together with high pressure fractionation histories for their parental magmas and thermal regimes conducive to extensive mantle melting and crustal assimilation, is indicative of an ensialic back-arc setting for anorthosite production.

Asthenospheric upflow in mantle wedge domains can be readily focussed into zones of lithospheric weakness at the boundaries between accreted juvenile Proterozoic crust (magmatic arc complexes) and older cratonic provinces, possibly aided by the existence of deep lithospheric mantle keels beneath old Archaean blocks.

REFERENCES Sivell, W.J., & McCulloch, M.T., 1991.Neodymium isotope evidence for ultra-depleted mantle in the early Proterozoic. Nature 354, 384-86.

402

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 Nd-ISOTOPIC EVOLUTION OF A NASCENT PERMIAN BACK-ARC BASIN - ISLAND ARC SYSTEM IN GYMPIE PROVINCE Warwick J. Si veil and Malcolm T. McCulloch Faculty of Science and Technology, University of Western Sydney, Kingswood N.S.W. 2747 Research School of Earth Sciences, The Australian National University, Canberra, A.C.T. 0200 By the Early Permian, much of the New England orogen was the site of numerous back-arc extensional basins filled with marine mass flow deposits and mafic to silicic volcanic rocks. A detailed study of the Nd-isotope geochemistry of basalts in Gympie Province, southeast Queensland, provides the opportunity to document the progressive interplay among subduction-related mantle sources in the genesis of mantle-derived eruptives in one of these extensional basins and hence contribute to the understanding of processes fundamental to crustal evolution at the Gondwana rim. Mixing of three isotopically distinct mantle components gave rise to the diverse Nd-isotope signatures of early Permian basalt suites from the Amamoor beds, Cedarton Volcanics and Highbury Volcanics in Gympie Province. These include (1) long-term depleted N-MORB source mantle with E^d (270 Ma) = +10 - +12 (DM); (2) OIB-source mantle; and (3) relatively enriched (low E^d ~ +5) subduction-modified mantle (DM + slab-derived component) (Figure 1).

1,2

2

Gympie Province basalts

Fig. 1. Plot of S m / N d versus initial N d / N d for Gympie Province basalts. Error bar shows maximum total procedural analytical uncertainty for Gympie basalt Nd-isotope determinations. MTB - Mariana Trough basalts; MIAB - Mariana island arc basalts; OIB - ocean island basalts; N-MORB - depleted mid-ocean ridge basalts.

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The isotopically most enriched Gympie samples are Highbury Volcanics from the Gympie Group with E^d (270) = +4. The Highbury basalts are island arc tholeiites (IAT). Their relatively low 8Nd values, together with high La/Nb, Ba/Zr, etc., typical of arc basalts, are due to the addition of components from subducted oceanic crust to the mantle wedge overlying an early Permian Benioff zone seaward of the Gondwana rim. Amamoor basalts (SNd = +8 - +10) comprise the isotopically most depleted Gympie rocks which possess similarities to back-arc basin basalts (BABB) of both N- and E-MORB affinity. The highest initial N d / N d ratios occur in those Amamoor basalts formed by high degrees of partial melting of LREE-depleted (N-MORB source) mantle (DM). An isotopically enriched (OIB) component is recognised by a slight but systematic decrease in E^d with decreasing S m / N d in Amamoor basalts with low Al/Ti, Mg/Zr and high Ce/Yb generated at small (<10%) degrees of partial melting (Trend 1). These melts preferentially sampled fine-scale lower-temperature melting mantle heterogeneities during seamount-style back-arc volcanism. Mixing between enriched OIB-source components and subduction-modified (Highbury) mantle gave rise to the intermediate isotopic compositions of the lowest Enj Cedarton basalts (Trend 2B). These were probably erupted at the initiation of back-arc rifting when the influence of subduction-related processes was most pronounced, as suggested by good correlations between SNd * trace element ratios which reflect slab-derived contributions. At higher SNd the curvilinear Cedarton trend veers toward the DM-IAT mixing trend displayed by some Amamoor basalts (Trend 2A). These results imply that as back-arc spreading continued, gradual depletion of the enriched mantle component led to melting of predominantly highly depleted (MORB-like) mantle in the wedge, concomitant with a diminishing subduction-related contribution. Good correlations also exist between initial N d / N d ratios and trace element ratios (e.g. Ti/V, Ti/Y, Zr/Y and Nb/Zr) unaffected by slab-derived components. These imply that the sub-arc (Highbury) mantle was even more depleted in incompatible elements, prior to interaction with slabcontributions, than the proximal back-arc mantle.

143

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 TRANSGRESSIVE DEPOSITIONAL CYCLES IN THE TAMWORTH BELT: THE SEDIMENTOLOGICAL SIGNATURE OF EXTENSIONAL FOREARC BASINS C. Gregory Skilbeck and Evan C. Leitch Department of Applied Geology, University of Technology, Sydney. PO Box 123 Broadway, NSW. 2007.

The Late Devonian - Carboniferous depositional history of the Tamworth Belt of northeast New South Wales, during much of this interval the site of a major forearc basin, does not fit textbook models of cyclic epiclastic sedimentation. The models, which have been derived mainly from studies of passive margin sequences, typically show incremental depositional events or parasequences accumulating by an initial regressive outbuilding followed by transgressive reworking of a thin upper veneer of the deposits after abandonment Such sequences are characteristically upward coarsening, and bounded above and below by thin maximum flooding surfaces and equivalent marine condensed sections. In contrast many Tamworth Belt sequences are upward fining and dominated by sediment deposited during transgressive phases.

We describe two stratigraphic sections from close to the western (magmatic arc) edge of the ancient forearc basin. One section, located northwest of Keepit Dam, encompasses the Tulcumba Sandstone and overlying Namoi Siltstone and is of Tournasian age. The other, 200 km to the south, on the eastern limb of the Myall Syncline, includes in ascending stratigraphic order, the Booti Booti Sandstone, Yagon Siltstone, and the Pacific Palms and Seal Rocks Formations. The Myall sequence is Late Visean to Late Carboniferous in age. The depositional histories of the two sequences are remarkably similar, given their temporal and spatial separation.

Both commence with relatively thin non-marine strata, and are overlain by sandy shorezone deposits dominated by tidal estuarine sandflat accumulations. Succeeding marine deposits indicate rapid deepening to below fairweather wave-base and are dominated by bioturbated siltstone and thin graded sandstone. In both cases the depositional cycle is initially transgressive despite the presence of abundant sediment.

The transgressive succession northwest of Lake Keepit unconformably overlies Late Devonian marine strata (Mandowa Mudstone) and can be divided into five units. Unit 1 comprises well rounded cobble and pebble conglomerate and pebbly calcareous sandstone, which is poorly sorted, sparsely fossiliferous, contains scattered pebbles and shows parallel lamination.. Overlying rocks of unit 2 are pebble grade subrounded conglomerate and epiclastic breccia intercalated with which are andesite flows. These lowermost units are interpreted to be alluvial deposits. Unit 3 is dominated by medium bedded sandstone with much less abundant conglomerate, siltstone and limestone. The sandstone beds show a range of features suggesting deposition in a complex estuarine environment, with individual facies indicative of an inner channelled estuary (medium to large scale cross bedding, commonly with gravel lags on cross beds), outer estuary (planar and festoon-bedded sandstone), shoreline (laterally extensive sandy pebble conglomerate) and shallow marine (planar cross-bedded sandstone, dark siltstone, widespread and commonly intense bioturbation, broken fossil debris). The various facies are repeated throughout a stratigraphic sequence some 180 metres thick, probably due to changes in the relationship between subsidence and sediment supply. Overlying strata of unit 4 consist of medium-bedded heavily bioturbated calcareous siltstone, sandstone and bioclastic and oolitic limestone, indicative of shallow marine conditions. These are succeeded by a thick sequence of dark fossiliferous siltstone, uncommon sandstone and bioclastic limestone signifying off-shore shelf deposition which make up unit 5.

At its western most limit the Myall sequence overlies arc volcanics (Nerong Volcanics). It can also be divided into five units. Unit 1 comprises festoon cross bedded sandstone arranged into cycles up to 5m thick and interpreted as fluvial channel deposits. These are overlain by a large-scale planar cross-bedded sandstone facies which is believed to be of tidal estuarine origin. A thin transgressive pebble lag separates this unit 2 from interstratified fine sandstone and siltstone of Unit 3 which are extensively bioturbated and contain an abundant marine fauna (brachiopods, crinoids) of unit 3. The rocks of Unit 4 are similar rocks to those of unit 3 but are arranged into large-scale slumps and slide blocks. Unit 5, which includes rocks deposited in the deepest water, consists largely of sediment gravity flow deposits of sandstone and conglomerate interstratified with thinly interbedded graded sandstone and siltstone.

Other Tamworth Belt upward fining transgressive depositional cycles include the Silver Gully Formation Yarrimie Formation succession, the Keepit Conglomerate - Mandowa Mudstone succession and the Merlewood Formation succession to the top of the Kindalyn Member. The manifestations of such transgressive cycles in the more distal eastern parts of the basin may be the sudden influxes of coarse sediment found in the long-ranging Goonoo Goonoo Mudstone and its correlatives.

Upward-fining transgressive cycles suggest that relative sea-level rise was accompanied by an increase in gradient. This probably indicates that tectonism, in particular basin margin normal faulting and possibly constructive volcanic activity, were more important controls on sea-level than eustatic influences. Transgressive depositional cycles may be characteristic of forearc basins evolving under extensional stresses.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CONTINENTAL BASEMENT OF THE ALBORAN BASIN (WESTERN MEDITERRANEAN SEA): COMPOSITION AND TECTONIC IMPLICATIONS. C. Gregory Skilbeck1, Juan I. Soto2, John P. Piatt3 and ODP Leg 161 Scientific Party Department of Applied Geology, University of Technology, Sydney. PO Box 123 Broadway, NSW. 2007.

Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, Facultad de Ciencias, Campus Fuentenueva, 18071 Granada, Spain.

3 Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, United .Kingdom.

1

2

The Mediterranean Sea provides an opportunity to study the tectonic controls on many types of arc-related sedimentary basins under the constraint of known relative plate motions. The Alboran Basin presents a paradox because although it has undergone as much as 7 km of subsidence (Comas et al, 1992) since the early Miocene, about 22 million year ago, it lies above a continental collisional orogen which has been active throughout the Tertiary. Today the Alboran Basin is surrounded by an arcuate mountain chain and has clearly formed in an area with crust previously thickened by compressional tectonics (Piatt and Vissers, 1989; Garcia-Duenas et al, 1992).

The nature of the basement beneath this basin is crucial for understanding its mode of formation and estimating the amount of crustal thinning involved.

In the western Alboran Basin, where the crust is currently less than 20 km thick, ODP Leg 161 has proved the continental origin of the basement. The metamorphic rocks drilled at Site 976 closely resemble sequences cropping out in the surrounding mountain chains (the Betics to the north and the Rif to the south, linked by the Gibraltar Arc). Basement rocks recovered consist of high-grade schist and gneiss derived from aluminous sediments, with minor amounts of marble, granite dykes, and migmatitic segregations of granitic material.

Mineral assemblages and textural relations in the high-grade schist and gneiss are characterised by two main stages of metamorphism: an early assemblage of biotite, garnet, staurolite, plagioclase, and rutile (syn- to postthe first deformation, Dl), well developed only in the high-grade schist, and a second assemblage of biotite, sillimanite, plagioclase, and potassium feldspar (syn- to post D-2 deformation). Both assemblages are overprinted by andalusite, potassium feldspar, and minor garnet, together with corundum in the high-grade schist, and cordierite in the gneiss (post-D2 and D-3 events). Migmatitic gneiss containing sillimanite, relict andalusite, cordierite, and potassium feldspar, coexisted with granitic melt. Preliminary PT estimates suggest that the metamorphic evolution followed a decompression path from 7 kb to 2 kb or less, under approximately isothermal conditions with temperature in the range 580°C to 630°C. After decompression, granite and migmatite melts formed at P<3 kb and T>670°C, after andalusite breakdown and within the sillimanite stability field. The combination of exhumation in an extensional tectonic environment and isothermal decompression in deep-seated crustal rocks, and the evidence for high and increasing temperature during and after exhumation, provide new constraints on current models for the origin of the basin involving the removal of the lithospheric mantle below a former collision zone.

The timing of unroofing and collapse of the basement to the Alboran Basin is not yet constrained from the rocks recovered during Leg 161. However, radiometric dates on similar high temperature/low pressure metamorphism in the nearby Betic Cordillera (southern Spain) cluster around the period 18-22 m.y.a. (Zeck et al, 1989, 1992).

At Site 976 the metamorphic rocks are overlain directly by middle Miocene (Serravallian) marine sediments, and if the peak metamorphic temperature was reached during the early Miocene as is the case onshore, it is unlikely that the rocks could have been exhumed by erosion (Comas et al 1995).

REFERENCES Comas, M.J. Garcia-Duenas, V. & Jurado, M.J. 1992. Neogene tectonic evolution of the Alboran Sea from MCS data. Geo-Marine Letters 12, 157-164.

Comas, M.J., Zahn, R., Klaus, A., Piatt, J.P., Soto, J.I., and ODP Leg 161 Scientific Party, 1995. Decompression and HT/LP metamorphism in the exhumed floor of an extensional basin, Alboran Sea, western Mediterranean.

Nature (In press).

Piatt, J.P. and Vissers, R.L.M., 1989. Extensional collapse of thickened continental lithosphere: A working hypothesis for the Alboran Sea and Gibraltar arc. Geology 17, 540-543.

Zeck, H.P., Albat, F., Hansen, B.T., Torres Roldan, R.L., Garcia Casco, A. & Martin Algarra, A., 1989, A 21 ± 2 Ma age for the termination of ductile Alpine deformation in the internal zone of the Betic Cordilleras, south Spain. Tectonophysics 169, 215-220.

Zeck, H.P., Monie, P., Villa, I.M. & Hansen, B.T., 1992. Very high rates of cooling and uplift in the Alpine belt of the Betic Cordilleras, southern Spain. Geology 20, 79-82.

Garcia-Duenas, V., Balanya, J.C. and Martinez-Martinez, J.M., 1992. Miocene extensional detachments in the outcropping basement of the northern Alboran basin (Betics) and their tectonic implications. Geo-Marine Letters 12, 88-95.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE CHANGING ROLE OF GEOSCIENCE TECHNICIAN EDUCATION IN PREPARING GRADUATES FOR CAREERS IN THE MINING INDUSTRY P. Smart Canberra Institute of Technology

Geoscience Technician education was first introduced into Australian TAFE Colleges in Canberra in the mid 1970s. Since then, there has been a steadily growing acceptance by Mining Industry employers of the legitimate technical level role these TAFE geoscience graduates can play - as distinct from that of the geoscientist's traditionally unqualified field assistant. With the rapid implementation of technological change in both mineral exploration and mining, the niche for such graduates is likely to become more widely recognized and the demand for graduates to increase.

Since the late 1980s there has also been a greater recognition of the two-year TAFE Diploma* qualification by universities. Formal articulation arrangements are now in place with many Australian universities so that graduates receive advanced-status equivalent to one years full-time study. This has resulted in the proportion of CIT Geoscience graduates proceeding to university increasing from less than 5% in the 1970s to 50% of graduates in recent years. It has also led to a high degree of success, for many who would not otherwise have gained entry to BSc or BAppSc degree studies. The success of these TAFE graduates is reflected in the academic awards they have achieved at university and in the many who have gained Honours degrees, several Masters degrees and two who are completing PhDs. The very practical yet academically demanding TAFE Geoscience Diploma at the Canberra Institute of Technology is, therefore, continuing to serve two very valuable roles in the Mining Industry. (i)

filling a need for highly skilled specialised technical support staff for geoscientists; and

(ii)

producing a significant number of university trained geoscientists by providing their initial practical training

*

In the 1980s the Geoscience course was accredited as a UG3 Associate Diploma and this has been upgraded to a Diploma in 1996.

406

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 SPATIAL ANALYSIS A N D VISUALISATION O F S T R U C T U R A L D A T A U S I N G A GIS A D A P T E D S T E R E O G R A P H I C PROJECTION P L O T T I N G P R O G R A M (GASP) Andrea B. Smith , Stephen J. Gardoll & David C. Robinson Key Centre for Teaching and Research in Strategic Mineral Deposits, Department of Geology and Geophysics, The University of Western Australia, Nedlands, Western Australia, 6907.

Predicting potentially prospective mineralised areas has been greatly facilitated by Geographic Information Systems (GIS), which have the capacity to integrate disparate geoscientific data sets and aid in the understanding of the spatial controls of ore deposit distribution. These mineral resource assessments are carried out over vast areas using either conceptual models (Knox-Robinson, 1994; Wyborn and Gallagher 1994) or multivariate statistics (Harris, 1984; Bonham-Carter et al., 1990) and often result in maps that delineate prospective areas on the order of l-10km wide and tens of kilometres long. As a consequence, only major exploration companies, government agencies and universities have invested in GIS mineral resource appraisal, as the results are generally not applicable to local-scale exploration programs carried out by the majority of mining companies.

Although large-scale GIS prospectivity appraisals incorporate areas where exploration is active often covering entire tenement areas, it does not identify specific prospective areas within the tenement itself. The breakdown in the effectiveness of these geospatial techniques when applied to more local/mine-scales (l-10km) is a direct consequence of a reduction in the number of spatial features required to do the analyses. To compensate and improve GIS prospectivity techniques at smaller scales, we have created a GIS-adapted stereographic projection program (GASP) which, as the name implies, incorporates all the functionality of a conventional stereoplotting application, with the tools that are available within a GIS. In areas where the dominant control on economic mineral occurrences is structural, it is critical to visualise and relate structural data to other spatial features (eg. the distribution of foliation dip angles to a specific geological contact).

Small scale interactive analysis, query and display of structural data, is now possible with GASP. The program displays structural data (bedding, foliation, lineations etc.) and offers the spatial query tools of proximity, overlay and attribute selection for the plotting of structural data on equal-area or equal-angle stereonets, either as poles to a plane or as great circles. The various results can be distinguished with unique symbols or colours and can be displayed on different stereonets or maps. Due to the live links inherent in the application, the selection of data from the map or stereonet, highlights the same data on other documents. For example, reselecting a group of data on the stereonet highlights the spatial location of these data on the map. This techique, allows the user to identify the structural components of the map area. In addition, the program has GIS gridding functionalities, which can be used to break down the map, and transform the data into a continuous surface. This includes modelling the dip variation, strike variation or a combination of both. GASP enables geologists to interrograte structural data, aiding in the rapid identification of areas of similar structural style and thus in the development of targets for exploration in areas where mineralisation is structurally controlled.

REFERENCES BONHAM-CARTT5R, G. F., AGTERBERG, F. P. & WRIGHT, D. F., 1990. Weights of evidence modelling: a new approach to mapping mineral potential. Geological Survey of Canada Paper, 89-9, 171-183.

HARRIS, D. P., 1984. MineralResourse Appraisal (Clarendon Press: Oxford) KNOX-ROBINSON, C. M., 1994. Archaean lode-gold mineralisation potential of portions of the Yilgarn Block, Western Australia: development and implementation of methodology, for the creation of regional-scale prospectivity map using conventional geological map data and a geographic information system (GIS). Unpubl.

PhD Thesis, The University of Western Australia.

WYBORN, L. A. I. & GALLAGHER, R., 1994. Regional analysis of the mineral potential of geological provences.

OZRI8 Annual Conference Proceedings.

407

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SILICIC MAGMATISM ASSOCIATED WITH THE DEVELOPMENT OF MET AMORPHIC CORE COMPLEXES IN THE D'ENTRECASTEAUX ISLANDS, SOUTHEASTERN PAPUA NEW GUINEA Ian E M Smithl, H L Davies2, E Hegner3 1 Department of Geology, University of Auckland, Private Bag 92019, Auckland, N.Z. 2Department of Geology, University of Papua New Guinea, 3Universitat Tubingen, Geochemie, Tubingen, Germany

The D'Entrecasteaux Islands of southeastern Papua New Guinea are part of the eastern New Guinea mobile belt where convergent plate tectonics have led to the emplacement of ophiolite terrains in the Lower Tertiary as well as to the development of the Papuan andesite arc in the late Cenozoic. In detail the islands straddle a complex plate boundary where an active oceanic spreading system is propagating into continental crust. The tectonic response to this complex setting has been widespread horizontal extension, the development of metamoiphic core complexes and extremely high rates of uplift The magmatic response to the evolving tectonic situation has been the development of a complex pattern of spatially overlapping Pliocene to Recent silicic igneous rock associations which have accompanied the uplift of the metamorphic core complexes.

Silicic magmatism is expressed as plutonic intrusives within the metamorphic core complexes and as rhyolitic volcanics which are concentrated in a nascent caldera structure between the core complexes of the western D'Entrecasteaux Islands. The first plutonic phase is represented by wide spread leucocratic gneiss and gneissic granodiorite forming discordant bodies within the metamorphic complexes. The second (Omara stage) occurs as discrete plutons and transgressive intrusions of essentially undeformed diorite and granodiorite. The third and youngest phase of silicic magmatism has been the eruption of rhyolitic lavas and pyroclastics between the core complexes of Goodenough Island and western Fergusson Island. Mailolo plutonics are associated with the formation and uplift of the core complexes. Omara plutonics were intruded after the main deformation event but prior to the most recent uplift and associated erosion. Rhyolitic volcanism is the most recent form of magmatism.

Each of these silicic magmatic associations is distinct in terms of range of Si02 content, compositional variation trends, alkali element and LIL element ratios, REE abundance patterns and radiogenic isotope ratios. The Omara plutonics show a range in compositionfrommafic diorite to granodiorite, are metaluminous characteristically arctype granitiods; their isotopic abundances show them to be slightly more evolved than andesites and dacites of the Papuan arc. Both the Mailolo plutonics and the rhyolites show overlapping highly silicic compositions but are distinct in all other aspects of their chemical composition. The Mailolo are relatively enriched in Ba and Sr but depleted in Rb and heavy REE; they show scattered 87/86Sr and 143/144Nd ratios consistent with a variable crustal source or crustal assimilation. The contrasting nature of these geochemically distinct but spatially and temporally overlapping silicic magmatic associations suggests that they originated in different sources and encountered different rocks during ascent to their present position.

The mineral assemblage of the metamoiphic basement of the D'Entrecasteaux Islands indicates that prior to late Cenozoic uplift the crust beneath the D'Entrecasteaux Islands could have been >100 km thick. This over thickened crust can be explained as the result of obduction together with undeiplating by magma associated with the Papuan volcanic arc. The P/T paths indicated by mineral assemblages in the metamorphic core complexes show that at least some parts of the metamorphic succession have encountered conditions appropriate to crustal melting. During the formation of metamorphic core complexes, crustal melts will play the role of transferring volumes of material from the lower crust into the mid crust resulting in a thickened middle crust. Magmatism also effects the transfer of heat from lower crustal levels to mid crustal levels where high temperatures and extensional stress field triggers deformation.

The geochemical nature of the silicic rocks is consistent with melting of metaluminous lower crustal rocks coupled in some cases with a variable amount of assimilation by peraluminous pelitic upper crust The key points of this model are the nature of the source of the silicic magmas and the relationship between the silicic intrusives within the lower plate of the metamorphic core complexes and the eruption of silicic magma* at the surface.

The silicic magmatic associations are logically the product of crustal processes and appear to be linked spatially and temporally with the development of the metamorphic core complexes. The sequence of silicic magmas suggests initially small scale relatively deep melting (Mailolo stage), more extensive melting at intermediate depths to produce a range of typically arc-type granitiods and a final relatively shallow melting stage which erupted to the surface. In all cases the source rocks were igneous but represent a mixture of Mesozoic protolith and underplated arc.

408

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOCHRONOLOGY O F ARCHAEAN WEST PILBARA G R A N I T O I D S Julie B. Smith. Mark E. Barley and David I. Groves Key Centre for Teaching and Research in Strategic Mineral Deposits The University of Western Australia

The Pilbara Block in the northwest of Western Australia comprises granitoid-gneiss batholiths which are surrounded by volcanic and sedimentary rocks of low metamorphic grade. Together the batholiths and supracrustal rocks comprise an Archaean granitoid-greenstone terrain. NE-SW trending lineaments divide this Pilbara granitoid-greenstone terrain into five tectonostratigraphic domains (Figure 1). The sequential development of these domains is currently not known with certainty, although ages ranging from 2.85 to >3.5 Ga are represented within them.

Previous workers have placed differing interpretations on the tectonic relationship and timing of events in the East and West Pilbara. Dating of West Pilbara crustal and supracrustal rocks provides a geochronological framework for such tectonic interpretations. The West Pilbara contains three greenstone belts: the Sholl, Roebourne and Whim Creek Belts (Figure 1). The Sholl and Roebourne Belts are separated by the Sholl Shear Zone (Figure 1). The Roebourne Belt has a structurally coherent succession, whereas the succession in the Sholl Belt is structurally complex. A major batholith is associated with each of the belts, the Cherratta Batholith in the Sholl Belt and the Karratha Batholith in the Roebourne Belt. The Whim Creek Belt comprises two supracrustal successions which are both younger than those in the Roebourne Belt and Sholl Belt.

There has been limited precise dating in the West Pilbara. An intrusive ultramafic complex in the Sholl Belt has a SHRIMP U-Pb in zircon magmatic age of 2925±16 Ma (Arndt et al., 1991) and a tuff in the same belt has been dated by conventional U-Pb in zircon methods at 3112±6 Ma (Horwitz and Pidgeon, 1991). Felsic volcanism in the Whim Creek Belt has a SHRIMP U-Pb in zircon age of 2991112 Ma (Barley et al, 1994).

Granitoids in the two most western belts fall in the granite and monzogranite field on QAP ternary diagrams and exhibit varying amounts of strain. Intensities of strain vary across the batholiths. Samples of both granitoids and supracrustal rocks from within the Sholl Shear Zone vary from mylonitic to relatively unstrained rocks. Several granitoids were chosen for U-Pb in zircon SHRIMP analysis to constrain the evolution of the two belts. The granitoid batholith that is technically interleaved with the Roebourne Belt is dated at about 3.2 Ga. In addition, a granitoid intrusion in an ultramafic-gabbroic unit, which has yet to be shown to be temporally associated with the Roebourne succession, is dated at about 3.2 Ga. Granitoid intrusions in the Sholl Belt are dated at about 3.1 Ga and 3.0 Ga. Within the Sholl Shear Zone, a granitoid has a similar age of about 3.0 Ga. This granitoid was mylonitised by shear deformation that clearly post-dates intrusion. The Sholl Shear Zone, and hence the present structural geometry of the West Pilbara, postdates 3.0 Ga, and postdates emplacement of granitoids which span 200 myr from about 3.2 to 3.0 Ga. i 117° Indian Ocean

Supracrustal a Roebourne b Sholl c Whim Creek d Malting e Wodgina

p Marble Bar

f Ord

q Coongan

9 Goldsworthy

r Kelly

h Pilgangoora

s Mosquilo Creek

i SUelley j Tambina

t McPhee Granitoids and Granitoid - gneiss Complexes

1 North Pole

l CHiraua II Caines Welt

m Lalla Rookh

III Slreliey

n Shay Gap

IV Shaw

o Coppin Gap

v Mi Edgar

k Soanesvilie

VI Kurrana Tectonostraiigraphic Domains

1 - 5

Mount Bruce Megasequence Set Prolerozoic, and Phanerozoic rocks Supracrustal belts Granitoids and gneisses

Figure 1: Geological map of the Pilbara Block showing granitoids and supracrustal belts, and tectonostratigraphic domains (after Krapez, 1993).

409

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 GEOLOGICAL CONTROLS ON CONSTRUCTION-ROCK QUALITY IN NORTHEAST NEW SOUTH WALES John V. Smith and Stephen J. Cotter Centre for Coastal Management, Southern Cross University, PO Box 157, Lismore , NSW 2480 Northeast New South Wales is one of the most rapidly developing regions of Australia placing great demands on the availability of construction materials. At the same time, northeast New South Wales is an area where environmental issues are rated highly in the community. In this context accurate and thoughtful geological work is essential to avoid unnecessary material shortages and land-use conflicts.

MAPPING GIS-based detailed geological mapping of the area, with a specific focus on bedrock types and structure, has improved understanding of the local geological controls on construction-rock quality. The main sources of construction-rock products are (1) basalts of the Tertiary Lamington Volcanics which occur on the southern flank of the Tweed Volcano and (2) quartzites of the metamorphosed Neranleigh-Fernvale beds of the Beenleigh Block of the New England Fold Belt. Both these rock types can exhibit extreme variations in rock quality over small distances and detailed investigations are required to extend existing deposits and identify new resources.

ROCK QUALITY The production of rock materials of a consistent quality is a most important and challenging requirement for the many different grades of material required for specific engineering applications. Rock quality involves both (1) the quality of the material as it occurs in the deposit and (2) the potential for changes to occur in the material during excavation, processing or use which will affect quality. In both cases, but particularly the second, the mineralogy of the material is the primary control.

The Lamington Volcanics comprises both tholeiitic and alkaline basalts. The tholeiitic basalts are generally low in olivine and are the favoured material for high quality applications. X-ray diffraction analysis has been used extensively to determine regional and local variations in mineralogy. The distribution of smectite clays has been found to be the primary control on basalt rock quality. The lowest smectite occurrence and the lowest tendency for smectite production is associated with thick (>8m) flows of tholeiitic basalt. Major quarry operations extract this rock whereas smaller operations extract rock from successions of thinner flows which have variable amounts of clay but are more easily extracted and suitable for local unsealed road construction.

This relationship between alteration mineralogy and flow thickness is interpreted as consequence of the glass abundance as volcanic glass is one of the most significant sources of smectite clays in these basalts. Alkali basalts commonly contain deleterious minerals including olivine. Recent mapping of the distribution of basalt types has shown that alkali basalt are less wide-spread than previous mapping indicated which has implications for construction-rock exploration.

Quartzite is prized for use in road construction due to its high strength and mineral stability. The metamorphosed Neranleigh-Fernvale beds comprise interbedded quartzites and slates. The tight folding of these rocks has made selective extraction of quartzite difficult and both quartzite and argillaceous materials are commonly extracted. The proportion of clay acceptable depends on the proposed application of the product.

Recent mapping of the Neranleigh-Fernvale beds has shown the presence of north-south trending subhorizontal folds with a dominant wave-length of about 10km. Smaller parasitic folds control the distribution of quartzite on the scale of quarry operations. Structural analysis of deposits has provided valuable assistance in projecting extensions to existing quartzite resources.

CONCLUSIONS In the present time of simultaneous increasing need for and constraints on the development of constructionrock resources, detailed geological investigations are essential. In particular, the distribution of deleterious mineral constituents, for example, olivine, alkaline minerals, smectite and other clays should be a primary consideration in geological mapping and other investigative processes.

410

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE STRUCTURAL AND DEPOSITIONAL EVOLUTION OF THE OFFSHORE CANNING BASIN. Stuart A Smith Australian Petroleum Co-operative Research Centre. National Centre for Petroleum Geology and Geophysics, Thebarton Campus, The University of Adelaide, SA 5005.

Abstract The offshore Canning Basin (or Roebuck Basin) is a frontier province located on Australia's North West Shelf. It is situated between the Carnarvon and Browse Basins, both of which contain major proven hydrocarbon reserves.

Exploration to date has been sparse, with only thirteen wells being drilled in an area of approximately 180,000 sq km; only two of these wells encountered encouraging hydrocarbon shows. In 1993, the Australian Geological Survey Organisation (AGSO), as part of the Continental Margins Program, obtained 4000 km of high-quality deep seismic data across the basin. These data, which supplement some 11,000 km of conventional seismic data recorded by the Japan National Oil Corporation in 1986-87, are the basis of a study of the structural and stratigraphic evolution of the basin.

The basin encompasses three major depocentres - the Rowley and Bedout Sub-basins and the offshore extension of the Fitzroy Trough. The basin margins are defined by the North Turtle Hinge Zone in the southwest, the Leveque Shelf Scott Plateau in the northeast, and the Argo Abyssal Plain in the northwest. Major intra-basin structural highs include the volcanic-capped Bedout High, which separates the Bedout and Rowley Sub-basins, and the Broome Platform, which separates the Bedout Sub-basin from the Fitzroy Trough.

Sediment ages range from Early Palaeozoic to Recent and reach a maximum thickness in excess of 10 km. Within this section, twelve major unconformities, associated with regional tectonic episodes, can be readily identified and mapped throughout the basin.

The oldest rocks penetrated in the basin are the Early Palaeozoic sediments of the nearshore, WNW-trending Fitzroy Trough. These sediments were originally deposited in a broad intracratonic depression bounded by SW-and NEdipping detachment surfaces which form the flanks of the Broome and Leveque Platforms. This sequence has been extensively faulted and folded, and is separated from the overlying sediments of the Westralian Superbasin by a major erosional unconformity. The Fitzroy Trough is presumed to continue to the west-northwest, beneath the continental margin, where it is obscured by the thick Westralian section. The major sediment accumulations of the offshore Canning Basin comprise the WNW-trending depression of the Bedout Sub-basin in the southwest, and the NEtrending and seawards-thickening wedge of the Rowley Sub-basin which underlies the outer continental shelf and upper slope. These sub-basins, elements of the Westralian Superbasin, contain up to ?10 km of relatively unstructured Permian to Jurassic sediments, with a thin overlying Cretaceous to Tertiary section.

Since the onset of the Westralian extension in the ?Carboniferous, structural deformation has been mild, except at the basin margins and beneath the anomalous Bedout High. The North Turtle Hinge Zone appears to have acted as a crustal-scale accommodation zone to transfer rifting from the nearshore flank of the Carnarvon Basin to the outer margin of the offshore Canning Basin. The resultant lack of restricted Jurassic depocentres in the offshore Canning Basin has probably reduced the possibility of good quality Late Jurassic/Early Cretaceous source rocks accumulating in the offshore Canning Basin and severely downgrades its hydrocarbon prospectivity. The prospectivity of the untested Rowley Sub-basin is the subject of further studies which will use modelling techniques.

Acknowled gements: PhD funding from the Australian Geological Survey Organisation (AGSO) is gratefully acknowledged. Seismic data that form the basis of this study have been supplied by AGSO and the Japan National Oil Corporation.

411

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

HOT DRY ROCK GEOTHERMAL ENERGY FOR AUSTRALIA Malcolm Somerville, Doone Wyborn and Prame Chopra Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

Hot Dry Rock Geothermal Energy (HDR) is a conceptually simple technology. Water is injected into a borehole and circulated through a "reservoir" of hot cracked rock several kilometres below the surface. This water is heated through contact with the rock and is then returned to the surface through a second borehole where it is used to generate electric energy. HDR is an environmentally clean energy source and is based on fairly conventional technologies. To date over US$300 million has been spent on HDR research worldwide. The benefits of this research can be utilised in an Australian HDR program.

The prospects for HDR development in Australia are favourable in terms of the 'scale of the resource, the efficiency with which the resource could be exploited, and the cost of developing it. The factors responsible for these conditions can be summarised as follows. First, the large scale of the resource stems from both the thermal blanketing effect of sedimentary basins which cover nearly half of Australia's land surface, and the abundance, particularly in central Australia, of highly heat-producing basement rocks, which make Australia unique.

Second, the anticipated efficiency of resource exploitation derives from the crustal-shortening stress regime of most of the crust in Australia. This should result in favourable HDR reservoir orientations (horizontal or subhorizontal) and minimise leakage of water from these reservoirs. Third, a relatively low cost of resource development is anticipated because of the ease of drilling through sedimentary basins to basement, and the absence of a need for inclined drilling in basement if, as expected, the reservoir orientation is horizontal or subhorizontal.

To assess Australia's HDR resource, we have compiled a database of bottom-hole temperatures from nearly four thousand boreholes in the Australian region and have linked this dynamically to a GIS. By combining geological, geophysical and geographical datasets in the GIS, we have derived a series of HDR resource maps for Australia. An important result documented with this technology is a significant spatial correlation between high geothermal gradient and low gravity zones within sedimentary basins. We interpret this correlation to be due to the presence of low-density high-heat-producing granites below the basins.

Potential energy reserves for electric power generation ranging from thousands to millions of petajoules are accessible below Australian sedimentary basins. The basins below which we have identified a significant HDR resource are in order of resource size, the Eromanga, McArthur, Otway, Carnarvon, Murray, Perth, Canning, East Queensland and Sydney Basins. The Eromanga Basin has by far the largest resource (83%), conservatively estimated at 19 million petajoules. To put this into perspective, annual energy consumption in Australia is currently about 3000 petajoules.

The calculated HDR resources in favoured granite bodies beneath the Eromanga Basin amount to 2.5 million petajoules, 830 times Australia's current energy consumption. Two regions of granite have outstanding potential,, the Innamincka gravity low in NE South Australia and the Betoota gravity low in SW Queensland, where each gravity low is considered to represent a granite body with a temperature of 300°C at 5km. The resource is contained in over 1000 cubic kilometres of granite in each of these two regions, either of which could supply the energy equivalent of 90 times Australia's annual consumption. At a less remote site, beneath the Sydney Basin south of Muswellbrook, an area of high geothermal gradient corresponds to a gravity low thought to be a buried granite body with an estimated 75 cubic kilometres of HDR resource at an average temperature of 250°C. The results of an economic model suggest that for a wide range of possible well systems and plant scenarios, and with a 15% rate of return, a sales price of below $100/MWh is achievable. This figure is comparable to, or better than the current cost of conventional electricity generation in remote areas of Australia and any future regulatory pressures to reduce carbon emissions would significantly favour use of HDR. A singular aspect of HDR as an alternative to fossil fuel energy is that, like fossil fuel technology, it is based in geotechnology. Conversion from fossil fuel to HDR energy can be effected without massive disruption to geotechnical industry, which is a major sector of the Australian economy.

412

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE NORTH AUSTRALIAN BASINS RESOURCE EVALUATION PROJECT (NABRE): TOWARDS A BASIN FRAMEWORK AND FLUID FLOW MODEL FOR THE PALAEO- AND MESOPROTEROZOIC OF NORTHERN AUSTRALIA Peter N Southgate, Tom Loutit & NABRE Team Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

The North Australian Basins Resource Evaluation (NABRE) project is a new National Geoscience Mapping Accord (NGMA) project with the aim of influencing the economic development of Northern Australia (Victoria River Basin to Mt Isa Basin). It is a cooperative initiative between the Commonwealth Government (AGSO), the Queensland and Northern Territory Governments (DME and NTGS, respectively) and includes collaborative studies with university and mineral industry geoscientists.

NABREs' prime objective is to provide industry with a time-series predictive framework for Palaeoproterozoic and Mesoproterozoic basin evolution in northeni Australia. To achieve this objective a multidisciplinary basin analysis of this part of northern Australia is being conducted. The objectives listed below cover the principal discipline-based areas the project is addressing.

Basement Studies are utilizing airborne magnetics, gravity and remotely sensed (TM, DEM and SPOT), outcrop and seismic datasets to: 1) Define the regional crustal thickness, crustal domain boundaries and the character of the basement including its composition, structure, grain and fabric.

2) Determine the character and timing of inter- & intraplate tectonic events impacting basin evolution.

3) Determine basement controls on basin phase development in the Palaeo- and Mesoproterozoic basins.

Basin Fill Studies are utilizing outcrop, seismic, drill core, well log, potential field and remotely sensed datasets to focus attention on how the basins formed and evolved. They aim to:.

4) Determine the sequence stratigraphic architecture of the Palaeo- and Mesoproterozoic basins.

5) Define the structural architecture of the basins and its relationship to the infra-basinal structural history.

6) Determine the basin phase setting and significance of mafic and felsic volcanic suites in the basins.

7) Establish relational databases for the Palaeo- and Mesoproterozoic basins.

Fluid Flow Studies will concentrate on the distribution and character of key play elements within each of the basin phases. These studies aim to: 8) Determine alteration events associated with igneous rock suites and periods of fluid flow and assess fluid migration pathways. The play elements will be analysed within the sequence stratigraphic framework determined for each basin phase.

During the 1995 field season stratigraphic, structural, geochemical and palaeomagnetic data collection focussed on the McNamara and Fickling Groups in western Queensland and the lower McArthur and Tawallah Groups in the Northern Territory. Sections have been measured with Jacob Staff and Abney Levels and logged with hand-held spectrometers. Natural radioactivity data generated from the spectrometers are providing significant new insights into the sequence stratigraphic architecture of these sediments. Initial results suggest that it may be possible to relate changes in accommodation rates discerned from the sequence stratigraphic studies to events on the APWP curve and thus to tectonic and structural histories. As relationships between tectonic events and basin responses become better understood it will be possible to propose more precise correlations across the north Australian region and hopefully relate periods of fluid flow and mineralisation to distinct tectonic events. Critical sections in the McNamara and Fickling Groups near Mammoth Mines and the Murphy Inlier have been sampled for pilot palaeomagnetic studies and a suite of possible tuff beds has been collected from the Gunpowder, Paradise Creek and Esperanza Formations in an attempt to determine how much geological time is represented by these formations and to constrain sedimentation rates.

In 1996 the project will acquire 250km of seismic in western Queensland. This will extend the 1989-91 Comalco oil exploration seismic grid to the south and permit better correlations with the upper McNamara Group in the Lawn Hill region. A major question being addressed by this survey involves the degree to which depositional geometries apparent on the Comalco dataset can be carried south and applied to the Riversleigh, Termite Range and Lawn Hill Formations.

413

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 PHANEROZOIC TECTONIC A N D DENUDATIONAL HISTORY OF THE MT. ISA INLIER, QLD.

Richard A. Spikings. David A. Foster, Andrew J.W. Gleadow and Barry P. Kohn Australian Geodynamics Cooperative Research Centre, La Trobe University, Bundoora, Vic. 3083.

Apatite fission track data from the central and southern parts of the Mt. Isa Inlier, northwest Queensland reveal two periods of accelerated cooling since the Early Carboniferous. Apatite fission track ages vary from 421±42 Ma to 235±25 Ma and mean track lengths range between 13.55±0.19 microns and 11.76±0.21 microns. These results record a protracted and complex cooling history below 110°C±10°C. The first relatively rapid cooling event occurred during the Mid-Carboniferous in response to intracontinental deformation associated with the Alice Springs Orogeny. The event resulted in >2 km of exhumation across the Inlier.

The large proportion of partly annealed fission tracks suggests that rocks now exposed across the Inlier resided at the top of the apatite partial annealing zone (approximately 60°C - 70°C) following the Mid-Carboniferous cooling. Modelling of the fission track age and length parameters suggests that approximately 30°C - 50°C of cooling occurred over the past 100 Ma. Assuming a geothermal gradient of 25°C/km this corresponds to 1.2 - 2.0 km exhumation. The post MidCretaceous cooling is possibly related to extensional tectonics at the southern and eastern margins of the Australian plate during the Mesozoic and Tertiary and more recent collision at the northern margin of the plate. A field of possible thermal histories for exposed rocks in the Mt. Isa Inlier consistent with the fission track data can be seen in the figure below.

Spatial variation of apatite fission track data within the Inlier indicates the three major structural belts, the Western Fold Belt, Kalkadoon-Leichardt Belt and the Eastern Fold Belt, possess similar thermal histories. This suggests that the main faults bounding the belts have not been reactivated in a vertical sense since 330 Ma. However, adjacent smaller scale fault bounded blocks within the Inlier possess variable cooling histories suggesting that reactivation of favourably orientated minor faults within the Inlier has probably occurred over this time interval.

Cooling associated with post Mid-Cretaceous tectonic activity.

0 50 -

HI 1 25 1 50 I 400

300

200

100

0

Time (Ma)

Figure indicating field of possible thermal histories for the ML Isa Inlier estimated by modelling of the observed apatite fission track parameters.

APAZ: apatite partial annealing zone.

414

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

R E - O S A N D SM-ND I S O T O P I C AND G E O C H E M I C A L C O N S T R A I N T S ON T H E S O U R C E O F T H E O W E N D A L E ALASKAN-TYPE C O M P L E X , F I F I E L D , N S W 1

Rebecca Sproule 1 . David Lambert1 and David Murphy 2 Victorian Institute of Earth and Planetary Science. Department of Earth Sciences, Monash University, Clayton. VIC 3168 2 Helix Resources N.L., 1 Havelock Street, West Perth, WA 6005

A series of poorly exposed Ordovician Alaskan-type intrusions, including the Owendale Complex, occur in the Fifield region of central New South Wales. These intrusions may represent the deep cumulate "feeder systems" for porphyry Cu-Au mineralisation near Parkes (e.g., Goonumbla district: Wyborn, 1988). Considerable controversy exists concerning the role of crustal vs. mantle contributions of metals and fluids in the genesis of giant hydrothermal base and precious metals ore deposits, in particular those that were emplaced into the continental lithosphere. Accurately delineating sources of metals in the hydrothermal systems has important implications for exploration programs. We present new trace element and Sm-Nd and Re-Os isotopic data to determine the magma source region and to evaluate the degree of crustal input in the petrogenesis of the Owendale Complex and its magmatic (Cr, PGE) and hydrothermal (Cu, PGE) ores. The Owendale Complex has a concentric morphology, similar to other Alaskan-type intrusions, and has been divided into an Ultramafic series, an extensive Mafic-Felsic series, and the Marginal series. The latter forms a discontinuous contact between the sedimentary host rocks and rarely between the other two series (Elliott & Martin, 1991). The samples used in this geochemical and isotopic study are from the inner Ultramafic series, and include an olivineclinopyroxene-magnetite cumulate and an amphibole-clinopyroxene cumulate. Both samples have large, late phlogopite grains and hydrothermal mineralisation consisting of native copper, digenite and magnetite in veins, similar to that observed in the Salt Chuck intrusion of Alaska.

Trace element data indicate depletion in Nb (1.3 to 23 ppm) and Zr (13.4 to 105 ppm), strong enrichment in P (394 to 1300 ppm), and modest LREE-enrichment (La/Lun = 1.46 to 4.22) compared to primitive mantle. These data for the cumulates are similar to those for Ordovician Lachan Fold Belt volcanics (e.g., Goonumbla), that are geochemically similar to modern shoshonites (e.g., Lihir and Fiji: Wyborn, 1988). However, some of the LILE (e.g., Rb, Sr, and Ba) appear to show some mobility, possibly due to the effects of a fluid phase.

Preliminary Re and Os concentrations and Os isotopic compositions were determined by isotope dilution NTIMS following a low-blank, Carius tube digestion/equilibration procedure. Our data yield exceptionally high osmium concentrations (4 to 8 ppb), low rhenium concentrations (0.15 to 0.37 ppb), and hence very low Re/Os ratios, consistent with Re-Os data for other ultramafic cumulates. Initial ]87 Os/ 188 Os ratios (calculated for an emplacement age of 445 Ma) are 0.1321 and 0.1308, yielding y 0s values of+2.7 and +4.2 (percent deviation of initial Os isotopic composition from chondritic asthenospheric mantle of the same age). This suggests that the parental magma was derived from a near-chondritic mantle source with minimal crustal input. Wyborn & Sun (1994) postulated that metals in the Goonumbla porphyry Cu-Au deposits were derived from metasomatised lithospheric mantle. Our Re-Os isotopic data would be consistent with this interpretation if the lithospheric mantle source for the Owendale parental magmas was juvenile (i.e., not Proterozoic continental mantle).

However, initial Nd isotopic compositions for these two cumulates are not indicative of an Ordovician unmetasomatised lithospheric or depleted asthenospheric mantle source (eNd = - 1 . 9 to +3.3, compared to ~ +8).

Using a two-component mixing model, the combination of unradiogenic e Nd values and near-chondritic y 0 , values are not explained by contamination of a basaltic magma (Os = 0.075 ppb, yo s = 0, Nd = 6.25 ppm and £Nd = + 8.5) with 1500 Ma crust of the Lachlan Fold Belt (Os = 0.025 ppb, yo s = 1500, Nd = 26 ppm and e Nd = - 1 2 ) . Therefore, we suggest that the magmatic Sm-Nd isotopic composition of these hydrous cumulates was altered by a LREE-enriched fluid phase that had equilibrated with Proterozoic crust (Owendale Sm-Nd TDM model ages = 1.3 to 1.4 Ga). The Re-Os isotopic system was less susceptible to this hydrothermal contamination process, due to the high Os concentration of these ultramafic cumulates compared to crustally-derived fluids.

Thus, the Sm-Nd isotopes reflect hydrothermal alteration while the Re-Os isotopes "see through" this event, inferring that the Owendale Complex was derived from a depleted mantle source with minimal crustal input at the magmatic stage.

REFERENCES ELLIOT, S.J. & MARTIN, A.R 1991. The Owendale Alaskan-Type Intrusive Complex and associated PGE mineralisation, Fifield Belt, New South Wales. In Elliot, S.J. and Martin, A.R. eds. 1991. Geology and mineralisation of the Fifield platinum province, New South Wales. 6th International Platinum Symposium.

Guidebook for the Pre-Symposium Field Excursion Guidebook, pp 12-23. Geological Society of Australia.

WYBORN, D. 1988. Ordovician magmatism, gold mineralisation, and an integrated tectonic model for the Ordovician and Silurian history of the Lachan Fold Belt in NSW. BMR Research Newsletter 8, 13-14.

WYBORN, D. & SUN, S.-S., 1994. Sulphur-undersaturated magmatism - a key factor for generating magmarelated copper-gold deposits. AGSO Research Newsletter 21, 7-8.

Acknowledgements: Financial assistance and permission to publish this paper was kindly provided by Helix Resources NL. R. Maas, L. Frick and J. Foster are thanked for assistance with data acquisition and modelling.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE BREMER BASIN: EVOLUTION FROM REGIONAL TECTONIC CONSIDERATIONS H.M.J. Stagg & J.B. Willcox Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 1

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The Southern Rift System (SRS) extends for some 4000 km along the southern margin of Australia from the Naturaliste Plateau in the west to the South Tasman Rise in the southeast While some elements of this system are geologically well-documented, large tracts are still very much frontier territory. The Bremer Basin, on the continental margin south of Western Australia belongs to this latter category. Although briefly explored in the early 1970s by BMR, Esso, and Shell, the Bremer Basin has largely been ignored since then, and remains one of the most poorly known deep-water basins of the Australian continental margin.

The Bremer Basin underlies the outer continental shelf and slope from south of Albany to southwest of Esperance, and encompasses an area of some 9000 km in water depths of 200-3000 m. Interpretation of magnetic data suggests that it contains at least 10 000 m of sediment Geographically isolated, it lies between the major basins of the GAB to the east and the Perth Basin, to the west; technically and stratigraphically, it probably contains features of both those basins. The nearest stratigraphic control comes from the Jerboa-1 exploration well, some 700 km to the east, which intersected a Berriasian syn-rift section overlain by a post-rift section of Hauterivian to Recent age.

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General tectonic considerations from southern margin development indicate that the Bremer Basin formed during the period of NW- or NNW-oriented continental extension that preceded the breakup of Australia and Antarctica in the Early or mid-Cretaceous. Inteipretation of high-resolution satellite altimeter data suggests that the basin and adjacent margin was also strongly influenced by the Triassic (?and older) extension and subsequent spreading events in the Perth Basin and perhaps also on the North West Shelf. Basement structural trends in the basin indicate an old E-W trending ?Palaeozoic fabric that has been subsequently overprinted by the obliquely oriented ?Jurassic extension and wrenching. The resultant structuring is complex, particularly where at the intersection of Palaeozoic and Mesozoic trends.

The main Bremer Basin depocentre can be broadly divided into four zones; from east to west, these are: • a small, intensely folded syncline (Denmark trough) containing more than 3 s two-way time (twt) of sedimentary fill in water 1000-1300 m deep. The complex bathymetry of this part of the margin appears to reflect canyon development focussed along basement faults; • a zone of complex block-faulting in which throws on the major faults are up to 3 s twt (-3-5 km); folding within the rift-fill indicates some compression due to wrenching; • a zone of relatively mild structuring within a thick sedimentary pile in the central part of the basin underlying a narrow mid-slope terrace; and • a zone of extensive high-angle faulting and wrench structuring in the east of the basin, where some largewavelength high-relief anticlines have been deeply eroded at the crests.

As there is no direct well control, dating of the sedimentary section can only be attempted by comparisons with the Eyre Sub-basin and the Perth Basin, onshore wells, and consideration of the tectonic events that formed the southern margin. The most prominent unconformity is a erosional surface which sharply truncates the underlying ?rift section with more than 1 s twt of sediment being removed at anticline crests. The unconformity has the overall characteristics of the Valanginian breakup unconformity in the Perth Basin and is accordingly assigned that age. With a range of other evidence from the GAB, this strongly suggests that a restricted episode of seafloor spreading or extreme continental extension took place in the western half of the SRS some 30 Ma prior to the generally accepted onset of southern margin spreading in the Cenomanian.

Within the underlying rift section, a prominent horizon is tentatively identified as Late Jurassic or earliest Cretaceous, based on comparison with the GAB Basin. Again, by analogy with the Eyre Sub-basin, the rift section is expected to comprise clasticfluvialand lacustrine deposits, probably derived from exposed basement The post-rift section is extensively channelled and mounded, and is expected to comprise Cretaceous and lower Tertiary paralic to open-marine clastic sediments overlain by a thin veneer of later Tertiary carbonates (possibly the Eocene Plantagenet Group).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MORE IN CONTINENTAL SETTINGS: THE SIGNIFICANCE OF THE PERMAIN ROOKWOOD VOLCANICS TO STUDIES OF THE EVOLUTION OF EASTERN AUSTRALIAN LITHOSPHERE Christopher J. Stephens, Shane J. O'Connell, Rodney J. Holcombe, Christopher R. Fielding, Anthony Ewart and Paul R. Messenger Department of Earth Sciences, The University of Queensland, Qld, 4072, AUSTRALIA.

The occurrence of lavas with MORB-like geochemistry in near-continental settings is restricted to extensional settings where the transition to crustal breakup is complete and new oceanic crust is produced, eg the Red Sea. An analogue in oceanic settings is found in back-arc settings, such as in the Lau and related basins of the southwest Pacific, which display intimate spatial and geochemical association between active arcs, back-arc basins floored by oceanic crust, and the older distended arcs. The Lower Permian Rookwood Volcanics of the New England Fold Belt, in eastern Australia, are unusual in recording a regionally extensive mafic volcanic event developed over continental crust that displays primary MORB-like geochemical characteristics.

The Rookwood Volcanics are a sequence of pillow basalts preserved entirely within a fold-thrust terrain, the Gogango Overfolded Zone, of the northern New England Fold Belt (NEFB). Recent work by the authors argues that the Rookwood Volcanics correlate regionally with the development of isolated sub-basins during the extensional event that initiated the development of the Bowen Basin. These basins are characterised by infilling with lacustrine sediments and the development of both margin-derived and axial fluvial systems, although an increasing marine influence is observed progressively eastward in these sub-basins. A characteristic aspect of the development of the sub-basins is the emplacement of substantial volumes of basaltic volcanics as sills and lavas. The extensional event was followed by a period of thermal sag that was cut short by a westwardadvancing thrust deformation, producing the foreland loading that dominates the subsequent basin evolution.

The Rookwood Volcanics represent the largest preserved area of lavas associated with the extensional phase of the Bowen Basin. Typically, the exposures are of pillow lavas and dolerite, with subordinate disrupted sedimentary horizons, and the volcanic pile is interpreted to have constructed as much by high level intrusion as extrusion. Through a combination of the effects of the thrust deformation and the present physiography, most of the contacts are poorly exposed or are structurally modified. The upper and lower stratigraphic contacts are preserved in only one area, where the volcanic package overlies fluvial and nearshore marine clastic sediments, and is overlain by marine siltstones characteristic of the thermal sag phase of the basin. The volcanic sequence thins along strike where it is interpreted to interfinger entirely with the lower clastic sequence.

The geochemistry of the volcanics is dominated by three end-members. A strong signature of enrichment in K, Cs, Rb, Pb and, to a lesser extent Ba, is interpreted to result from seawater interaction with the volcanics. This signature is commonly found even within thick, unaltered dolerites, indicating significant access of seawater to the volcanic pile. The principal characteristic is a tholeiitic one, with depleted LREE trends characteristic of MOR basalts. The lavas fall predominantly within the N-MORB fields on most trace element discrimination plots, but show a limited spread toward the calc-alkaline field. The calc-alkaline trend varies in degree from enrichment in MREE through to samples showing characteristic steep REE patterns with Eu anomalies, and negative Nb-Ta anomalies on trace element normalised diagrams. Volcanics formed in separate extensional sub-basins to the north and west show a predominantly calc-alkaline character trending toward MORB. The MORB-dominated geochemistry of the Rookwood Volcanics is clearly anomalous for rocks found in continental settings. Our mapping constrains the Rookwood Volcanics to have developed within continental sediments, and regional interpretations regard both the preceding and following tectonic environments to be convergent continental margins. As such, the Rookwood Volcanics may show parallels in their development with the modern oceanic back-arc regions of the southwest Pacific. The implications of the MORB-like geochemistry of these lavas to the geochemical evolution of the crust and lithosphere of the region during the late Palaeozoic are that 1) the sub-continental lithosphere at that time was very thin or absent, such that an evolved lithosphere did not contribute significantly to the sources of the magmas, and 2) that the continental crust underlying this region during the late Palaeozoic had a primitive calc-alkaline geochemistry, a situation that has been proposed previously from our studies of Mesozoic volcanism in the NEFB.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 MAJOR REVISIONS OF THE PROTEROZOIC AND CAMBRIAN GEOLOGICAL HISTORY OF FAR WESTERN NEW SOUTH WALES, IMPLICATIONS FOR RODINIA B.P.J. Stevens , C.M. Fanning and A.J. Crawford Geological Survey of N.S.W., 32 Sulphide St, Broken Hill 2880 Research School of Earth Sciences, ANU, Canberra 0200 Geology Department, University of Tasmania, PO Box 252C, Hobart 7001 1

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The geology of far western New South Wales holds some of the clues for the timing and geometry of the Proterozoic Rodinian supercontinent fragmentation. It has been proposed that Laurentia separated from the east coast of Australia-Antarctica at about 780 Ma ago and that continental crustal blocks, now located in China, may have been present between Australia-Antarctica and Laurentia during the Neoproterozoic. The boundary between the exposed Proterozoic craton and the Tasman Fold Belt System runs through far western NSW. However its position and the late Proterozoic and early Palaeozoic history of the area have been unclear.

New SHRIMP U-Pb zircon age determinations establish the major crustal nature of the NNW trending Nundooka Fault, at the western edge of the Bancannia Trough. The Early Proterozoic Willyama Supergroup and the Adelaidean sequence occur to the west, with no equivalents to the east of this structure. East of the Bancannia Trough, the Ponto beds had been correlated lithologically and structurally with the Willyama Supergroup, but our recent zircon dating shows them to be Cambrian in age. The Kara beds were correlated with the Adelaidean, but it appears likely that the Mt Arrowsmith Volcanics, which are within the Kara beds, are very late Proterozoic, and have no equivalent in the Adelaidean sequence. Some sediments included in the Kara beds are Cambro-Ordovician, but the extent of these is not yet clear. It is possible that the Kara beds are a series of fault slices, of more than one age. Cambrian and Early Ordovician volcanics occur in a number of places east of the Trough, and Middle Cambrian dacite occurs at a depth of 3.3km, in or below the Bancannia Trough, only 8km east of the Nundooka Fault. There are no Cambro-Ordovician volcanics west of the Nundooka Fault.

The oldest sequences that clearly overlap the Nundooka Fault are the Late Devonian to Carboniferous sandstones of the Ravendale Formation and the Nundooka Sandstone. However the Delamerian Orogeny may provide a link. West of the Nundooka Fault, some Rb-Sr and K-Ar isotope systems were reset and pegmatites emplaced in the Middle to Late Cambrian, probably coincident with folding of the Adelaidean sequence. East of the Nundooka Fault, an unconformity is clearly displayed between the Early to Middle Cambrian Teltawongee beds and the Late Cambrian Cupala Creek Formation.

The dominant NNW tectonic trend in the region was first apparent with emplacement of NNW trending Mundi Mundi type granite dykes into the Willyama Supergroup at about 1500 Ma. It further developed as Adelaidean sediments and minor basic volcanics were deposited in NNW trending half-grabens, just before and for some time after the breakaway of Laurentia. During the latest Proterozoic, continental rifting occurred, possibly preceding the breakaway of the continental fragments now located in China. This rift sequence also trends NNW and is represented by the Mt Arrowsmith Volcanics, comprising up to 2 km thickness of submarine, alkaline basalt-trachybasalt-trachyte volcanics.

Currently there is no definite record of the 60 m.y. interval between the Mt Arrowsmith Volcanics and deposition of the Early Cambrian Mt Wright Volcanics. However a conformable contact between sediments assigned to the Late Proterozoic Kara beds and turbidites assigned to the Cambrian Teltawongee beds has been postulated. This may mark the transition from intracratonic rifting to passive margin sedimentation.

The Mt Wright Volcanics and volcanics in the overlying Cymbric Vale Formation are calc-alkaline, and were deposited and extruded as part of a continental arc, or as the result of renewed continental rifting. They could be the earliest products in the region, of subduction of the new Pacific Ocean. During the Middle Cambrian, tholeiitic and minor acid volcanics were deposited in the extensive Ponto beds, acid volcanics were deposited in the area which later became the Bancannia Trough, and acid volcanics were deposited in the Tibooburra area.

Late Middle Cambrian Delamerian deformation left highly deformed Ponto beds and ultramafic rocks abutting older, less deformed Gnalta Group, and further enhanced the NNW tectonic grain of the region.

Acknowledgements This paper is published with the permission of the Director-General, NSW Department of Mineral Resources.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 CORRELATION OF FRAGMENTS OF A DEVONIAN INTRA-OCEANIC ISLAND ARC ACCRETED TO THE NEW ENGLAND OROGEN, EASTERN AUSTRALIA James M. C. Stratford 1 , Jonathan C. Aitchison 2 and Peter G. Flood^ 1 Department of Geology and Geophysics, University of Sydney, Sydney, NSW 2006, Australia ^Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong ^Department of Geology and Geophysics, University of New England, Armidale, NSW 2351, Australia

The Gamilaroi terrane witliin the southern New England orogen of NE NSW is a distinctive ?Silurian to Devonian tectonostratigraphic assemblage of rocks which developed within an intra-oceanic island arc setting (Aitchison and Rood, 1995). It constitutes part of the Tamworth Belt which also includes younger, unconformably overlying continental margin volcanic rocks which developed in association with a different style of plate convergence. Although the terrane has long been thought to represent a forearc basin succession associated with a hidden western magmatic arc our recent work in the Upper Barnard district has revealed the presence of widespread felsic volcanics and sub-volcanic intrusives. These rocks represent portions of the arc itself and lie along the eastern edge of the terrane. A thick succession of volcaniclastic sediments overlies the arc basement. Radiolarians are abundant in the volcaniclastic sediments and can be used for correlation between areas of Gamilaroi terrane outcrop. Lower to Middle Devonian limestones comprise a further minor but distinctive part of the terrane and are important for correlations. Submarine basaltic lavas within the volcaniclastic succession indicate a period of arc-rifting. Upper Devonian coarsegrained arc-derived sediments overlie the Gamilaroi terrane with unconformity.

The Birpai subterrane in the eastern part of the southern New England orogen contains a rock succession similar to that of the Tamworth Belt which lies to its west. As with the Tamworth Belt, basement includes igneous rocks with island arc affinities (in this case ophiolitic rocks of the Yarras Complex). The overlying sedimentary succession includes tuffaceous cherts with radiolarian faunas similar to those of the Gamilaroi terrane. Rare limestones are also present. Thick sections of dolerite and basalt associated with arc-rifting are also present. Coarse-grained volcaniclastic sediments appear to overlie these Gamilaroi terrane equivalents unconformably.

In the northern portion of the New England orogen lithologically similar, age equivalents, of the Gamilaroi terrane occur in a similar structural position to their southern correlatives. The Calliope volcanic assemblage includes felsic arc-related volcanics with a thick sequence of volcaniclastic sediments similar to Gamilaroi terrane sections elsewhere. Significant economic mineralisation (Mt. Morgan orebody) is hosted by these rocks. Similar Kuroko-type mineralisation is commonly associated with arc rifting, and other areas of Gamilaroi terrane may be worthy of investigation. Limestones can be correlated with those of the southern Gamilaroi terrane. Basaltic rocks potentially associated with arc-rifting are also present.

Calliope volcanic assemblage rocks are unconformably overlain by younger volcanic and volcaniclastic rocks associated with continental margin arc volcanism.

All three zones share a common geological history early in their development prior to their accretion to the Gondwana margin and are herein correlated as formerly contiguous fragments of the Gamilaroi terrane.

Felsic intra-oceanic island arc basement rocks are overlain by a thick succession of arc-derived volcaniclastic sediments. The presence of basaltic lithologies is consistent with an interpretation that the arc experienced a phase of rifting. In all areas Gamilaroi terrane rocks are unconformably overlain by younger arc-derived volcaniclastic rocks. The exact location of Gamilaroi terrane accretion to the Gondwana margin and nature of dispersal of terrane fragments remains uncertain.

Reference Aitchison, J.C. & Flood, P.G. 1995. Gamilaroi Terrane: a rifted Devonian intra-oceanic island arc assemblage, NSW, Australia, in Smellie, J. (ed.) Volcanism associated with extension at consuming plate margins Geol. Soc. London Spec. Publ 81: 155-168.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SALTMAP - A SOUND TECHNICAL BASIS FOR CATCHMENT AND FARM PLANNING. G. Street World Geoscience Corporation

Salinisation of agricultural lands is the largest environmental problem in Australia. In Western Australia alone around 9% of once productive land is now salt-affected and this will grow to around 15% in 15 years. The degradation of farming land has spawned the Landcare movement in Australia, a unique development to alert the community on the problems and their solutions. The growth of this movement has created a call for better information. Salt is concentrated in the landscape over geological time scales and the mapping of these salt concentrations (salt storages) requires geological exploration techniques.

SALTMAP. A new airborne electromagnetic system was developed specifically to map conductivity variations in the near surface. The data collected by the system can be directly related to the concentrations of salt in the regolith. When used with other datasets such as satellite images, airphotos, magnetic-data, radiometric data, and topography the SALTMAP conductivity gives an insight into the processes that lead to areas becoming saline.

Remedial measures can then be designed with a sound technical basis.

Surveys using airborne electromagnetics have been carried out in 20 areas prone to salinity using the QUESTEM and SALTMAP systems. At Broomehill in the south west of Western Australia. The farmers have developed farm plans for sustainable agriculture based on the results of a SALTMAP survey.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LANDSCAPE DEVELOPMENT ON PASSIVE CONTINENTAL MARGINS: FROM DENUDATION CHRONOLOGY TO THERMOCHRONOLOGY Michael A. Summerfield, Department of Geography, University of Edinburgh, Edinburgh HE8 9XP, UK

Traditional approaches to the unravelling of long-term landscape development in passive margin and cratonic settings have included the correlation and interpretation of erosion surface remnants, and of associated weathering deposits and other continental sediments. In favourable cases good dating control is available for establishing a denudation chronology, but in many instances age constraints are poor. More recently, the availability of offshore stratigraphic data has allowed better dating control of major erosional episodes, although such information cannot provide details of spatial patterns of onshore denudation. The advent of thermochronology, however, has provided new insights into both temporal and spatial patterns of long-term landscape development through the provision of location-specific denudational histories. In addition, the acknowledgment of the role of flexural isostasy associated with differential denudation across passive margins has helped to explain the persistence of the marginal upwarps which are a common feature of such margins.

The cooling history of the upper few kilometres of the crust derived from apatite fission-track analysis of surface and borehole samples from southern Africa has prompted a revision of the previous denudation chronologies of this classic area based on the work of L.C. King. A sub-continental coverage of approximately 200 apatite fission-track analyses indicates a significant crustal cooling episode associated with the break-up of Gondwana and the creation of the continental margins of present-day southern Africa. This cooling arose primarily from accelerated denudation precipitated by the creation of new base levels for erosion along the newly formed coastlines. Along some sections of the margin this phase of relatively rapid dedudation extended well inland of the present position of the Great Escarpment which separates the dissected coastal fringe of southern Africa from the elevated interior plateau. In the Lesotho Highlands region, a detailed cooling history and denudation chronology derived from borehole samples suggests that the Drakensberg Escarpment, which currently marks the divide between Indian and Atlantic Ocean drainage, may not have been far from its present position by -100 Ma ago and that it has experienced relatively little retreat since then. This conclusion raises important questions about rates of landscape change and the long-term stability of drainage divides.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A MAJOR MAGMATIC EVENT DURING 1050-1080 MA IN CENTRAL AUSTRALIA AND AN EMPLACEMENT AGE FOR THE GILES COMPLEX Shen-su Sun, John W. Sheraton. Andrew Y. Glikson and Alastair J. Stewart Australian Geological Survey Organisation, Canberra. A.C.T., 2601

Emplacement of large layered mafic-utramafic intrusions of the Giles Complex in the western Musgrave Block, central Australia, occurred at deep crustal level after regional granulite facies metamorphism. deformation and generation of syn-metamorphic granites. K-feldspar porphyritic granite and rapakivi granite appear to intrude locally in the southern margin of the Hinckley gabbroic intrusion in the Champ de Mars area. Among the felsic and mafic dykes intruding the Giles Complex a set of dykes classified as type-A mafic dykes show chemical and isotopic affinities to the Giles Complex that indicate they are cogenetic to the Complex. Granophyre in the Bell Rock intrusion has been interpreted as a late magmatic differentiate of the gabbroic intrusion. Thus, the emplacement age of the Giles Complex can be constrained by the zircon U-Pb ages of these igneous rocks.

The Minno augen gneiss, a syn-metamorphic granite from south of the Kalka intrusion near Mount Davies has well-zoned, slender igneous zircons which give a concordant zircon U-Pb SHRIMP age of 1198 + 6 Ma. Inherited zircon cores indicate an age of 1296 + 10 Ma for the source rock. This emplacement age of 1198 ± 6 Ma is consistent with a previous whole rock Rb-Sr isochron age of 1204 + 17 Ma (Gray, 1978). A porphyritic granite south of the Hinckley intrusion contains abundant igneous zircons which give a well-defined age of 1188 + 4 Ma. Minor amounts of inherited zircon give ages up to 1500 Ma. Neodymium isotope data for the porphyritic granite are consistent with melting of basement rocks dated at -1550 Ma and -1300 Ma. Igneous zircons from a granite dyke in the western part of the Hinckley intrusion give a 207Pb/206Pb age of 1052 ±11 Ma. A 'chilled' phase (or a type-A dyke?) of the Hinckley intrusion has a preliminary zircon U-Pb age of 1073 ± 4 Ma (based on six grains only). A felsic dyke previously considered to be connected to the 1188 Ma porphyritic granite also yields a zircon U-Pb age -1070 Ma. Identical emplacement ages for the Smoke Hill Volcanics (1078 + 5 Ma) and the granophyre at Bell Rock (1078 + 3 Ma) can represent a contemporaneous igneous event. Until more zircon U-Pb ages for other intrusions are collected we will regard the well defined age of 1078 ± 3 Ma for the Bell Rock granophyre as the emplacement age of the Giles Complex. This conclusion is supported by the study of two leucogabbro samples from the Wingellina Hills intrusion which yield 3-point (clinopyroxene, plagioclase, and whole rock) Sm-Nd isochrons of 1047 ± 28 Ma and 1077 ± 32 Ma. These samples contain olivine, and have igneous textures showing only minor recrystallisation, suggesting that metamorphic overprinting is minimal and that the ages are close to the emplacement age.

This newly defined widespread magmatic event between ca. 1050-1080 Ma in the Musgrave Block has profound implications for the post 1200 Ma granulite-facies metamorphism. Emplacement of a large amount of high temperature (1200-1300° C) mafic magma into deep crustal levels induced granulite-facies metamorphism and crustal melting resulting in bimodal mafic and felsic igneous activities. The range of initial eNd values (+0.9 to -2.5) and the chemistry of 1050-1080 Ma felsic rocks are consistent with magma generation through a range of processes from crustal melting (with sNd of -2.5 to -4.8 for 1550-1300 Ma felsic rocks at 1070 Ma) to derivation from mafic magmas through extensive assimilation-fractional crystallisation. The 1050-1080 Ma magmatic event in the western Musgrave Block can be correlated with similar event more than 400 km to the east in the Kulgera region, eastern Musgrave Block, and in the southern Arunta Inlier. Emplacement of a dolerite dyke swarm in the Kulgera region and the Stuart dyke swarm near Alice Springs have been estimated at 1076 ± 33 Ma and 1090 ± 32 Ma, respectively by Sm-Nd mineral isochrons (Zhao & McCulloch, 1993). A period of lithospheric extension and generation of tholeiitic magma about 1050-1080 Ma in a very wide area covering the Musgrave Block and southern Arunta Inlier is indicated.

REFERENCES Gray C.M., 1978. Geochronology of granulite facies gneiss in the Western Musgrave Block, central Australia.

Journal of the Geological Society of Australia, 25, 403-414.

Zhao, J.-X. & McCulloch, M.T., 1993. Sm-Nd mineral isochron ages of Neoproterozoic dyke swarms in Australia: evidence for two distinctive events of mafic magmatism and crustal extension. Chemical Geology, 109, 341-354.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

INTEGRATION OF SEDIMENTOLOGY, SEQUENCE STRATIGRAPHY AND VOLCANOLOGY TO RECONSTRUCT CARBONATE SHELF-EDGE TO BASIN ENVIRONMENTS IN THE EARLY PALAEOZOIC WARBURTON BASIN, S.A. Xiaowen Sun1 and Colin G. Gatehouse 2 1

National Centre for Petroleum Geology & Geophysics, the Thebarton Campus, the University of Adelaide, 5005 "South Australian Department of Mines and Energy, P.O. Box 151, 191 Greenhill Road, Eastwood, S.A., 5063.

The Middle to Late Cambrian Kalladeina Formation in the subsurface eastern Warburton Basin, South Australia, contains a thick succession of carbonates interbedded with fine-grained siliciclastics.

Integration of sedimentology, sequence stratigraphy and volcanology has permitted the recognition of carbonate shelf-edge, slope and starved basin depositional environments.

In the Gidgealpa area, six lithofacies are recognised based on cores and cuttings. A black mudstone lithofacies contains argillaceous muddy material, millimetre thick laminations, rich organic matter (TOC 0.4-0.85) and early authigenic pyrite, and lacks infauna, indicating that it was deposited from suspension, well below storm wave base, in an oxygen deficient basinal setting. A nodular/ribbon lime mudstone lithofacies is often thinly interbedded with the black mudstone; its high carbonate mud content, containing rare bioclasts and lacking infauna; these all indicate a similar hemipelagic environment. The other four lithofacies are gravity-relocated carbonate facies derived from both shoal-water and slope carbonates by various resedimentation processes. The carbonate turbidite lithofacies shows typical to irregular Bouma sequences, suggesting gravity driven turbidity current deposits. The thin-bedded fossiliferous rudstone/grainstone lithofacies contains abundant and diverse fossil skeletons and lesser peloids, indicating deposition on the flank of the carbonate biogenic factory. The algal wackestone/packstone lithofacies comprises relatively fresh tubules and rafts of tube-form calcareous algae such as Girvanella and Obruchevella. Their unmicritised nature suggests that they were probably derived from a microbial mound on the slope. The mud-rich bioclastic/peloidal wackestone and floatstone lithofacies may be deposited further down the slope. The clast-supported debrite lithofacies recognised in a 10m core in Gidgealpa 1 comprises two kinds of clasts including a variety of consolidated grainstones of shoal-water origin and also semiconsolidated bioclastic-peioidal wackestone/ribbon lime mudstone derived from the slope. The in-situ hemipelagic argillaceous mudstone and nodular lime mudstone punctuated by both slope and shoal-water derived carbonates suggest typical slope apron to basin sedimentation. Thin basaltic lava layers, abundant lava breccias, and ash-sized turbidites are interbedded with these slope apron carbonate deposits in Gidgealpa 1. The predominance of plagioclase laths and vesicular groundmass, with quenching, and their geochemical characteristics, indicate a submarine effusive basic lava source.

Because the basic material is not found in the shoal-water carbonates, the lava flow may be derived from seaward. The homogeneity and episodic nature of these basaltic rocks suggests that they are probably related to periodic reactivation of a deep-seated fracturing zone or renewed rifting event. This is consistent with the reconstruction of a slope apron to basin setting.

Oolitic and cortoid/oncoidal grainstones resedimented into the slope apron environment indicate a typical high energy winnowed edge sand shoal sedimentary environment. Vertical facies successions interpreted from wireline logs and cuttings in the Gidgealpa area suggest gradually upward shallowing sedimentation trends, and also possible slope failures and gravity-controlled mass wasting. Therefore a prograding shelf margin can be predicted from the well data. This is supported by sequence stratigraphic interpretation of seismic data that reveals a mounded carbonate buildup adjacent to a slope profile in the region.

Sigmoid/oblique clinoforms indicate a prograding bank margin. The integration of sedimentofogy and sequence stratigraphy facilitates a confident interpretation of carbonate sedimentation and evolution in the Warburton Basin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MIDDLE CAMBRIAN TO EARLY ORDOVICIAN BIOSTRATIGRAPHY AND APPLICATION TO SEQUENCE STRATIGRAPHY, EASTERN WARBURTON BASIN, SOUTH AUSTRALIA 1

Xiaowen Sun 1 and Jim B. Jago 1

2

National Centre for Petroleum Geology & Geophysics, Thebarton Campus, University of Adelaide, SA 5005 2

Department of Applied Geology, University of South Australia, The Levels, SA 5095

The Early Palaeozoic Warburton Basin underlies the gas and oil producing Cooper and Eromanga Basins. The stratigraphy of the basin is poorly known due to the absence of outcrops, limited core intersection and poor seismic data. The Middle Cambrian to Early Ordovician Kalladeina Formation is more than 1700 metres thick and contains oomoldic and vuggy dolomites that are potential carbonate reservoir rocks. Cores from several key wells of the Kalladeina Formation contain trilobites, conodonts and other fossils. The identifications of many fossil specimens collected and identified by Daily in the 1960-70s were summarized by Gatehouse (1986). After additional collection, a systematic palaeontological study on more than one thousand specimens has resulted in the establishment of eight Cambrian and three Early Ordovician faunal assemblages (FA), ranging from early Middle Cambrian (Late Templetonian/Floran) to Early Ordovician ("Arenigian"). These faunal assemblages can be correlated to the well-established northern Australian biostratigraphic assemblages.

In the Middle Cambrian the Pentagnostus / Galahetes FA correlates with the T. gibbus to A. atavus Zones; Penarosa I Fuchouia fecunda FA with the A. atavus to early E. opimus Zones; the Doryagnostus I T. (Aotagnostus) - Amphoton / Lisania FA with the Doryagnostus deltoides to Goniagnostus nathorsti Zones; Valenagnostus / Agnostus FA with the P. agra and H. arepo Zones and the Lejopyge laevigata / L. armata FA. with the H. arepo Zone. In the Late Cambrian the Ammagnostus / Clavagnostus FA correlates with the Glyptagnostus stolidotus Zone; the Eugonocare / Proceratopyge FA possibly with the Glyptagnostus reticulatus and Proceratopyge cryptica Zones; the Oncagnostus (Strictagnostus) / Neoagnostus FA possibly with the Peichiashania to Hapsidocare Zones; the Cordylodus proavus FA corresponds to the Cordylodus proavus Zone.

The Yosimuraspis / Asaphellus FA has an early Ordovician (Tremadoc) age, possibly correlating with the Cordylodus lindstromi Zone and the Cordylodus angulatus FA correlates with the Cordylodus angulatus Zone.

The Drepanoistodus / Eoneoprioniodus fauna of Cooper (1986) has an early "Arenig" age, possibly correlated with the Paroistodus proteus Zone. The above provides a basis for interbasinal and intercontinental correlation.

The refined biostratigraphy provides a foundation for sequence stratigraphy in the basin. It serves as a guideline for seismic mapping and wire-line log correlation across technically deformed regions in which the Warburton Basin sequence is largely undrilled. The biostratigraphic study by Daily (1966) revealed the repetition of trilobite assemblages that proved the existence of a thrust fault in Gidgealpa I. Recognition of repeated sections is vital for interpretion of the vertical sequences in wells where the available seismic information is inadequate.

The precise correlations between key wells allow the reliable establishment of lateral and vertical facies associations that are fundamental building blocks of systems tracts and sequences. The absence of some trilobite zones indicates time breaks, which define sequence boundaries. The integration of biostratigraphy and sequence stratigraphy also enables correlation of sea-level change on both local and global scales. A major transgressive event during the early Middle Cambrian in the Warburton Basin can be correlated with one in the Georgina Basin and probably Australia wide. This example demonstrates the important role of biostratigraphy in sequence stratigraphy, especially in poorly explored frontier basins lacking adequate well and seismic data.

REFERENCES Cooper, B.J., 1986 A record of Ordovician conodonts from the Warburton Basin, South Australia. Quarterly Geological Notes, Geological Survey of South Australia 100, 8-14.

Daily, B., 1966. Appendix 3 - Part B. In Delhi-Santos Gidgealpa No.l well, South Australia. Bureau of Mineral Resources, Australian Petroleum Search Subsidy Acts Publication 73, 95-111.

Gatehouse, C.G., 1986. The Geology of the Warburton Basin in South Australia. Australian Journal of Earth Science 33, 161-180.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MAGMATISM BELOW HARRINGTON VOLCANO, NSW: BASED ON BASALT AND ZIRCON MEGACRYST DATING F. Lin Sutherland and C. Mark Fanning Australian Museum, 6 College Street, Sydney, NSW, 2000 PRISE. Australian National University, GPO Box 4, Canberra, ACT, 2600 1

2

1

2

The Barrington basalt shield is usually considered a c.54Ma volcano, perhaps with some younger activity.

New age determinations for basalts and zircon megacrysts shed from the shield show a complex structure, built by repeated magmatism from 60-4Ma (Table 1). This raises questions about the magmatic processes.

BASALT AND ZIRCON RELATIONSHIPS Flows in the northeast shield with an age of 59Ma mark early undersaturated activity preceding the 54Ma tholeiitic and undersaturated activity (Table 2). They include primitive basanites (Mg#69-72) and marginally evolved ankaramite (Mg#66). The 59Ma lavas show higher Sr/Rb (37-68), Sr/Ba (2.1-2.4) and lower Zr/Nb(2.6-3.5) than a 54Ma basanite(Mg#66; Sr/Rb 25, Ba/Rb 1.5, Zr/Nb 3.8). Basaltic flow episodes alternate with zircon-bearing eruptions, but dwindle after the 59-36?Ma basaltic peaks (Table 1).

Subvolcanic zircon formation then magmatic eruption every l-13Ma suggests resurgent processes within the mantle. Both colourless zircons (U>80-1400 ppm, Th>20-1200 ppm, Th/U 0.2-0.6) and coloured zircons (U> 35-150 ppm, Th> 10-70 ppm, Th/U 0.3-1.3) can form together, but colourless zircons dominate 3545Ma activity and coloured zircons 55-60Ma activity. Precise Pb-U isotope dating of younger zircons (< 40Ma; FT dating) indicate they are reset older zircons, with a statistically strong formation age at 45 ± IMa. Zircon crystallisation poses an enigma. The Barrington basalts show little evolution towards fractionated end magmas capable of crystallising zircon. REE show a wide range in absolute abundances between colourless and coloured zircons, but have similar chondrite-nonnalised patterns with a positive Ce anomaly, rising HREE and no obvious Eu depletion. Thus, fractionation from evolved basaltic parents is unlikely, even though a magma chamber exists below the basalt shield at 4-13 km depth.

MAGMATIC RELATIONSHIPS The basalt/zircon ages suggest periodic generation of more voluminous mantle melts (basalts), each framed by introductions of low volume felsic melts/fluids (zircons) that are unrelated to basalt fractionation (no Eu anomaly). High Si, A1 and alkali glass found in the mantle xenoliths and produced in experimental high PT runs on peridotite compositions suggest the zircon may come from such melts (attributed to minor partial melting of amphibolised mantle). Repeated magmatism below Barrington volcano may reflect lithospheric motion over asthenospheric magma cells generated by Tasman spreading rifts.

TABLE 1 Comparative zircon and basalt (WR) ages (in decreasing Ma), Barrington volcano 36-37?

43-44 54-55 59 61 Basalt K-Ar 27-28 18? 4-5 44-45 43-44 38 55-58 69?

Zircon FT 35-36?

41-42?

50? 45 56 59-60?

Zircon Pb-U TABLE 2 Comparative CIPW Norms (wt%) and trace elements (ppm), 59Ma and 54Ma basalts 4 2 1 Element 3 4 3 2 1 Norm 640 500 360 520 Ba 5.1 4.8 8.6 9.4 Or 32 15 24 20 Rb 03 12.8 15.1 142 Ab 1350 1040 880 800 Sr 18.4 225 27.8 20.4 An Zr 170 210 240 165 15.8 82 2.4 20.4 Ne 68 68 44 66 Nb 292 20.8 18.1 18.7 Di 40 28 22 70 La 19.0 202 19.9 17.1 Ol 90 58 46 Ce 125 3.5 35 3.4 3.7 Mt 200 150 250 Cr 350 3.7 43 5.1 4.4 n Ni 140 80 115 170 3.0 23 1.7 1.6 Ap 59Ma; 1. basanite (Mg#72), 2. basanite (Mg#69), 3. ankaramite (Mg#66). 54Ma; 4. basanite (Mg#66).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

VOLCANIC DISASTER SCENARIOS SYDNEY AND MELBOURNE REGIONS, EASTERN AUSTRALIA F. Lin Sutherland1 and K.A. Hollis2 Australian Museum, 6 College Street, Sydney, NSW, 2000 Trentham Natural History Museum, 33 Park Street Trentham. Vic, 3458

2

Extensive past volcanism along eastern Australia, may include aboriginal witness of eruptions in the youngest volcanic fields around Mt Gambier, SA, Camperdown, Vic and Atherton, Qld. With burgeoning settlement, some of Australia's large cities lie within 70 km from volcanic fields, where dating reveals repeated volcanism over intervals around 10* to 107 years. Taken with some trends involving discrete but consistent migratory eruptions in the last million years, this raises potential for future eruption near a major population centre. To demonstrate futuristic, but realistic, scenarios of such volcanic eruptions near densely populated areas, two scenarios affecting Sydney and Melbourne are outlined.

SYDNEY OUTSKIRTS ERUPTION This describes a small eruption in high summer in the Blue Mountains, west of Sydney. After earth tremors along the Kurrajong fault, a cinder cone erupts igniting bush fires, showing Sydney with volcanic ash carried by easterly winds, and disabling Richmond air base. Lava streams from the cone flow into Grose River and encroach on Richmond, Windsor and Penrith townships, which are evacuated. After eruption, thick lava dams the drainage forming a large lake extending deep into the Blue Mountains. Debate arises among authorities and residents, whether to restore the original landscape, by massive excavation of the lava, or use the lake as a water supply or develop it for sport and tourism.

MELBOURNE HARBOUR ERUPTION This describes effects from a prolonged fissure eruption in Port Phillip Bay, between Geelong and Melbourne harbours. Initial phreatic explosions off Bellarine Peninsula disrupt shipping lanes into Geelong and cause evacuations along the peninsula. An emergent cone erupts lava into the bay, building out deltas of flow foot breccias and threatening to close Corio Bay. Emergency teams pump huge volumes of water on the cone to forestall this cut off. Another vent emerges in Hobsons Bay, covering inner city and suburbs with ash. Lava flows overrun the foreshore and Albert Park and a tongue of lava travels over Princes Bridge into downtown Melbourne before eruption ceases. A civic debate ensues over whether the covered land should be restored by major earth works or should be retained as natural features, for visitors to admire and citizens to respect.

POTENTIAL FOR ERUPTION The potential for a future volcanic disaster scenario differs in these two regions. The Melbourne scenario is much more likely, based on the average repose between eruptions in the newer volcanic field of Victoria (around 13,000 years). The Sydney scenario seems unlikely, because of inactivity since the Late Miocene.

However, recent results from the SKIPPY project, Australian National University, suggest a strong thermal anomaly in the underlying mantle here, supporting a previous proposal for a hotspot based on migratory volcanic events further north.

REFERENCES Sutherland, F l . 1995. The Volcanic Earth. UNSW Press, Sydney

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

EXTENDED EVOLUTION OF HARRINGTON VOLCANO, NSW A LONG TERM, REPEATED SUPPLY OF ALLUVIAL GEMSTONES F. Lin Sutherland1, R.E. Pogson1 and G. Webb1 Australian Museum, 6 College Street, Sydney, NSW. 2000

The Barrington shield volcano erupted repeatedly over 55 and possibly 65 million years. Initial 59 Ma growth filled east flowing drainage before major 55-54 Ma growth filled western valleys. The combined basalts centred on Barrington Tops imposed radial drainage, partly deflected in the northwest into the Hunter Valley by adjacent shield activity. Subsequent basaltic episodes around 44-43 Ma, 37-36 Ma (and post-28 Ma, Gloucester Tops) further modified drainages into the present radiation now entrenched in the Palaeozoic basement.

Resistant gem minerals appear in present alluvial deposits and deep leads e.g. ruby, sapphire and zircon. Bulk samples show up to 8 wt% zircon and 0.7 wt% corundum, with near-equal ruby and sapphire. Crystals show magmatic corrosion and finds high in the flow sequence suggest basaltic sources. Worn crystals mark washing and recycling.

GEMSTONE/BASALT RELATIONSHIPS Zircons erupted in bursts throughout Barrington volcanism (fission track dating), probably largely in pyroclastic activity outside the main basalt buildups (K-Ar dating). Euhedral, distinctive zircons at Gloucester Tops date a late, local 4.5 Ma outburst. Orange-red zircons tend to dominate early activity (60-50 Ma), pale to colourless zircons early-mid activity (50-40 Ma), red-brown zircons mid activity (40-20 Ma) and yellow-brown, high-U zircons later activity (<20 Ma), but colours are only partly age specific.

Eruptive ages for gem corundums are more problematical, as the mineral is not easily dated. Three types of corundum appear in alluvials: (1) (2) (3)

pink to red corundum (ruby suite) van coloured, little-zoned sapphires (pastel sapphire suite) blue-green, strongly zoned, larger sapphires (typical E. Australia sapphire suite)

Rubies accompanied by early formed zircons wash between the 59 and 54 Ma flow sequences (Tomalla) and also associate with 4.5 Ma young zircons (Gloucester Tops), suggesting long term ruby supply into drainages. Unusual, corroded and reacted aggregates of co-existing ruby, sapphire, sapphirine and spinel at Gloucester Tops represent a xenolithic source rock for the ruby and pastel sapphire suites. The blue-green zoned sapphires may relate to one or more zircon eruptions.

GEOMORPHIC EVOLUTION AND GEM EVALUATION The 59 Ma eastern valley fill, capped by a distinctive basanite flow, descends from 1500m to a base below 1040m elevation and the disruption twinned the drainage to from the lateral Dilgry and Cobark Rivers. A volcanic neck (Moppy Lookout) transgresses through the old lavas, showing local centres outside the main vent area also contributed to shield construction. Realisation that Barrington volcano has a complex, long drawn out eruptive and erosional history is a key to understanding the sources, distribution and redistribution of gem minerals within this multi-formed structure. Only detailed exploration can assess the gemstone potential throughout the volcanic sequence, but National Park and intended wilderness areas make full evaluation difficult.

REFERENCE Sutherland, F.L. & Coenraads, R.R., 1996. An unusual ruby-sapphire-sapphirine spinel assemblage form the Tertiary Barrington volcanic province, NSW, Australia. Mineralogical Magazine 60, in press.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE GENESIS OF Pb-Zn-Cu-Ag ORE-FORMING

FLUIDS

Dimitri A. Sverjensky Department of Earth and Planetary Sciences The Johns Hopkins University, Baltimore, M D 21218

Warm Na-Ca-Cl fluids in sedimentary basins are the precursors of the potential-ore-forming fluids thought to be involved in the formation of many types of sediment-hosted base-metal sulfide ore deposits. The sedimentary basinal fluids can become potential ore-forming fluids if they acquire significant quantities of elements such as Pb, Zn, Cu or Ag. It is proposed that these elements are acquired through water-rock interactions associated with the migration of the basinal fluids. Starting with a single initial basinal fluid composition, and the low oxidation state typical of many oil-field type brines, the interaction with a variety of different principle aquifer lithologies can produce a wide variety of potential ore-forming fluids. All of these fluids will contain some finite amount of Pb, Zn, Cu or Ag (and many other metals). Nevertheless, migration of basinal fluids through different aquifer types will produce fluids with dramatically different oreforming potentials. For example, carbonate or quartz sandstone or anhydrite+red-bed aquifers can result in the chemical evolution of fluids with the potential to form Zn-, or Pb- or Cu-rich ore deposits, respectively.

The chemical evolution from basinal fluids to potential ore-forming fluids via the acquisition of elements such as Pb, Zn, Cu or Ag involves water-rock interactions governed principally by two types of chemical reactions during the migration of basinal fluids: (1) Precipitation-dissolution reactions, and (2) adsoiptiondesorption reactions. Theoretical modeling of precipitation-dissolution reactions has been facilitated by the application of chemical reaction path calculations. These calculations rest on a very wide range of experimentally and theoretically derived thermodynamic properties of minerals, aqueous species (both inorganic and organic), and gases. It has long been established that such calculations permit specific description of alteration reactions involving the major silicate, carbonate, sulfide and oxide minerals in aquifer units. In addition, by taking account of linear free energy correlations amongst oxides and silicates, explicit provision can be made for the equilibrium partitioning of trace metals such as Zn between alteration minerals such as chlorite and aqueous solution.

In contrast, remarkably little has been done to include the consequences of adsoiption-desorption reactions in consideration of the water-rock interactions governing the evolution of potential ore-forming fluids. The main reason for this is that a general quantitative model for the prediction of adsoiption-desoiption reactions involving metals has not been available for application to the quantitative modeling of the water-rock interactions in aquifers. The need for such a model should be clear. The acquisition of a metal such as Cu by migrating fluids in sedimentary aquifers is unlikely to be influenced by Cu-bearing minerals in which Cu is a major element. Instead, one possible source of Cu may be detrital mafic minerals which can be irreversibly destroyed, either partially or completely, during the migration of fluids through aquifers. The Cu released to the fluids may be strongly complexed in the fluid, depending on the redox state, temperature, and chloride content of the fluid. However, if the mafic minerals are altered to fine-grained iron minerals such as goethite, significant amounts of the Cu may be adsorbed onto the goethite and not released to the migrating fluid. Such reactions depend strongly on the pH and chemical compositions of the fluids.

Alternatively, metals such as Cu may be acquired directly by migrating fluids from the iron oxide coatings on many types of detrital grains in sedimentary aquifer units. Whether or not such desorption reactions occur will depend again on the fluid chemistry, temperature, pH and redox conditions. For example, under redox conditions appropriate to the existence of significant amounts of Cu + and Ag + in the fluids, which form strong complexes with chloride even at surflcial temperatures, it can be expected that partitioning of Cu and Ag will be strongly favored into the aqueous chloride-bearing fluids relative to Pb and Zn. All of these reactions will also be a strong function of the surface areas of the iron oxide phases. As goethite ages to hematite, which may have a significantly smaller surface area, desorption of base metals will be favored.

Consequently, the evolution of potential ore-forming fluids in sedimentary aquifers may be strongly affected by the age of the aquifer at the time of fluid migration. Current research is directed towards the construction of a general quantitative model of metal adsorption/desorption with predictive capabilities that can be applied to problems of geochemical interest such as the water-rock interactions in sedimentary aquifer units that influence the chemical evolution of ore-forming fluids.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOSCIENCE IN TOURISM: AGSO's ULURU EXPERIENCE Ian Sweet 1 and Ian Crick1 Australian Geological Survey Organisation, GPO Box 378. Canberra, ACT, 2601

Uluru & Kata Tjuta: a geological history was AGSO's first major venture into a new type of geological product - a public education booklet aimed at a non-specialist market. Many opportunities exist for the production of such books by AGSO, the state geological surveys, the geological societies, and indeed, by individuals. Essential ingredients for such publications are: that they be written in simple but unpatronising language; that they be educational and interesting; and that they be attractively produced, as they have to compete with a plethora of beautiful (but often uninformative) souvenir/coffee table books.

Publication of the Uluru booklet was facilitated because AGSO, in 1989, had set up a project called the "Geology of National Parks". The project provides the opportunity to use the expertise resident in the organisation to target major geological features, and to produce material of interest to the general public.

The writing style and level of complexity should be appropriate to the target audience. But the target audience at Uluru was expected to include not only those who had some understanding of geology, but those to whom geology was akin to a foreign language. We therefore set out to design a book which would attract the widest possible audience through uncomplicated presentation and highly attractive appearance. The booklet had to use simple, but technically correct, language, be visually attractive, as it would have to compete with a wide range of glossy "picture books", but not contain so much detail that the casual browser would be put off before buying.

Have we succeeded? We have relied on book reviews as the best guide. These have been favourable, and in some cases highly complimentary. No such booklet is perfect though, and we need to re-examine the presentation at the time of reprinting. Other booklets may seek to target a different audience. The booklet by Duggan & Knutson on the Warrumbungles, for instance, is more technical in its presentation because it targets mainly students. Nevertheless, it is readable and understandable by any interested lay-person.

These publications are an essential element in, and should benefit from, the interest in so-called ecotourism.

Although the Uluru booklet concentrates on geology and geomorphology, it briefly covers European exploration as well. Other booklets, such as one in preparation on the Bungle Bungles, will cover topics such as flora and fauna as well. The style and content of the Uluru booklet was devised in AGSO, but we collaborated with the Australian National Parks and Wildlife Service (now the Australian Nature Conservation Agency - ANCA), whose views were taken into consideration. ANCA jointly manage Uluru National Park with the local Aboriginal people (Anungu), and this arrangement ensured that Anungu wishes were taken into account at all stages. Aboriginal interpretations of landforms might be appropriate in some cases, although it is generally felt that such interpretations conflict with "western science" based interpretations. It is easy to adopt the attitude that we are presenting the "real facts", but we need to be sensitive to the feelings of Aboriginal Australians in any venture we undertake, and to recognise that their beliefs are strongly held.

Wide distribution of booklets is desirable, and books on geology need to be seen along side those on birds, plants, marsupials, and so on, not only at the parks themselves, but wherever such material will reach an interested audience. The presence of geological publications will help break down the notion in the minds of many that geology is an exploitative science, rather than the historical and environmental science we know it to be. Tourism has grown enormously in Australia in the past decade, and its growth is projected to continue. We need to recognise that tourists are our customers, or potential customers, and that they deserve to be provided for.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

VOLCANIC EVOLUTION OF THE WESTERN AUSTRALIAN CONTINENTAL MARGIN Philip A. Svmonds1, S. Planke2, James B. Col well1 and Anthony J. Crawford3 Australian Geological Survey Organisation, GPO Box 378, Canberra, A.C.T. 2601 Department of Geology, University of Oslo, P. 0 . Box 1047, Blindem, 0316 Oslo, Norway 3 Geology Department, University of Tasmania, Hobart, Tasmania 7001 1

2

New deep-seismic reflection data, combined with existing seismic and well data, has revealed the presence of an extensive volcanic province along the western Australian margin. Much of this province appears to be related to Late Jurassic (Oxfordian) break-up of the Argo Abyssal Plain in the north, and Early Cretaceous (Valanginian) break-up of the Gascoyne, Cuvier and Perth Abyssal Plains to the south. Middle to Late Jurassic subaerial basalt flows, up to -500 m thick, are penetrated by several wells in the Browse Basin, and can be correlated to large areas with disruptive, strong, reflectors, typical of basalt, on the Scott Plateau to the west. Extensive "sill-like" intrusions are mapped in Triassic and older section on the south-central Exmouth Plateau. "Basalt-type" reflectors and eroded volcanic build-ups are identified near the seaward corners of the plateau, and a wedge of seaward dipping reflectors, interpreted as flood basalts, occurs on part of its western margin. Adjacent to the Cuvier Abyssal Plain and beneath the Wallaby Saddle, extensive wedges of seaward dipping reflectors occur, and in places are terminated by a mounded feature interpreted as shallow marine hyaloclastites. The dipping sequences appear similar to those drilled by the Ocean Drilling Program (ODP) on the Rockall and V0ring Plateaux, where they are thought to be the result of voluminous magmatism around the time of continental breakup..

The Wallaby Plateau is underlain by a "basement" containing complex dipping seismic reflector sequences similar to those of large (oceanic, plume-related) igneous provinces (LIPs), such as the Kerguelen and Ontong Java Plateaux. ODP drilling at both the latter locations has confirmed that these sequences are composed of interbedded, subaerial basalt flows and volcaniclastic sediments. Dredging of the margins and central part of the Wallaby Plateau obtained altered tholeiitic basalts, and basaltic conglomerates and sandstones, and supports a volcanic origin for the entire feature. Immobile element compositions of two Wallaby Plateau basalts provide strong support for a correlation with those from the Naturaliste and southernmost Kerguelen plateaux, and the eastern Broken Ridge. All these features have been suggested to have formed by plumehead-related volcanism, and all involved, to some degree, Gondwanan sub-continental lithospheric mantle in their pedogenesis. However, both the basaltic marginal plateaux (the Wallaby and probably the Naturaliste plateaux), and related eastern Indian Ocean LIPs, all appear to be aged from 120-100 Ma, and therefore significantly post-date breakup and accretion of the earliest oceanic crust along the west Australian margin (155-130 Ma). Thus, the plumehead assumed responsible for the formation of all of these features is not necessarily implicated in the rifting and eventual breakup of this margin as proposed by some workers.

It is now clear that the western Australian margin has experienced significant episodes of volcanism, that it takes on different forms along different segments of the margin, and that it appears to consist of two main types.

Triassic to Early Cretaceous volcanism lies within a zone that parallels the margin, and includes areas of seaward-dipping reflectors, flows, sills and possible underplating. This type of magmatism may have been controlled by dynamic rift/breakup processes and not necessarily by proximity to a plume. The second type is associated with large, irregular volcanic buildups, such as the Wallaby and probably Naturaliste Plateaux, that appear to have formed some 10-20 million years after the 130 Ma breakup of the Gascoyne, Cuvier and Perth Abyssal Plains. These features have many of the characteristics of other eastern Indian Ocean LIPs that are distant from continental margins, such as Kergulelen Plateau and Broken Ridge, and are probably the result of voluminous outpourings of plume-related magmas onto pre-existing, relatively young, oceanic crust. It is possible to explain the temporally and spatially complex arrangement of volcanic features along the western Australian margin, and in the adjacent eastern Indian Ocean, by a combination of non-plume, dynamic, rift-related volcanism leading up to and including breakup, and plume-related, plateau-forming, post-breakup volcanism.

Improved understanding of the timing and nature of the various tectono-magmatic events that have affected the western Australian margin has important implications for its thermal evolution and uplift/subsidence history.

These in turn influence the depositional style and sedimentary facies characteristics, and the thermal maturation history of related sedimentary basins. Also, the identification of easily eroded, shallow marine volcanics associated with some magmatic events has implications for volume estimates of extrusives, and regional climatic and environmental effects resulting from vigorous, explosive volcanism.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra,, February 1996

S O L I D - S T A T E V I S C O E L A S T I C RELAXATION AND THE S E I S M I C STRUCTURE OF THE UPPER MANTLE Ben-hua Tan. Ian Jackson and John Fitz Gerald Research School of Earth Sciences, The Australian National University, Canberra , ACT 0200

A layer of relatively low velocity and high attenuation, and a high degree of lateral variability are persistent features of seismological models for the Earth's upper mantle. A detailed understanding of the physical origin of these observed velocity and attenuation anomalies in the upper mantle, which provides insight into the Earth's thermochemical and stress state, is the goal of related experimental research.

Theoretical considerations and previous laboratory measurements of internal friction on relevant geological materials suggest that high subsolidus temperature may be sufficient to explain most of the observed low and variable velocity and high attenuation in the upper mantle. Development of novel experimental apparatus at ANU (Jackson & Paterson, 1993) has finally enabled laboratory measurements of internal friction to be conducted under conditions directly relevant to seismic wave propagation in the deep crust and upper mantle, and it has been demonstrated that under high subsolidus temperature, an olivine-rich rock" (Aheim Dunite) exhibits substantial internal friction and velocity dispersion (Jackson et al., 1992). In order to understand the mechanisms responsible for the observed viscoelastic behaviour, careful control of the microstructure of specimen is prerequisite. To this end, synthetic olivine polycrystals have been prepared by hot-pressing of powder produced from hand-picked olivine megacrystals (San Carlos, Arizona). Microstructural examination with light and electron microscopy indicates a reasonably equilibrated texture, fine grain size (5um to 50um), low porosity (<1%), and low impurity content (<1%) in the specimens. Torsional forced oscillation and creep tests have been conducted on the specimens under 300 MPa pressure and high temperature (to 1000°C) within the linear regime and the seismic frequency band.

At room temperature, the behaviour approaches the elastic ideal, featuring high, frequency-independent shear modulus and negligible internal friction (<0.001). As temperature increases, the specimen deviates progressively from the elastic ideal: internal friction and associated reduction of shear modulus increase monotonically with temperature, and no strain-energy absorption peak is identified. High attenuation (>0.01) and associated strong velocity dispersion in the frequency range (1-10*2 Hz) have been observed under high subsolidus temperature (>900°C), consistent with previous experimental results.

Further experiments on synthetic ultramafic rocks of variable grain size and dislocation density are under way to investigate the solid-state mechanisms probably responsible for most of the seismic attenuation and velocity dispersion in the upper mantle.

REFERENCES Jackson, I., Paterson, M. S., 1993. A high pressure, high temperature apparatus for studies of seismic wave dispersion and attenuation. PAGEOPH, vol. 141, No 2/3/4, 445-466.

Jackson, I., Paterson, M. S., & Fitz Gerald, J., 1992. Seismic wave dispersion and attenuation in Aheim dunite: an experimental study. Geophysical Journal International, 108, 517-534.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NEW VERSIONS OF THE MAGNETIC MAP OF AUSTRALIA C. Tarlowski, P.J. Gunn and T. Mackey Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

The first Australia wide compilation of a digital grid merging all significant publicly available aeromagnetic surveys was prepared by Tarlowski et al. (1992) who subsequently published a 1:5 000 000 colour image of this compilation (Tarlowski et al., 1993). The digital grid of these data and the image have found widespread usage by government bodies, research organisations and exploration companies. Such compilations however can be continually updated by the addition of new data as it is acquired and the purpose of this paper is to present the second edition of this map as published by Tarlowski et al. (1995). As well as including significant additions of new data, this new version of the map has been corrected for a long wavelength warp that was introduced into the original map during the merging of its various constituent surveys. The paper also presents the results of various transformation processes that have been applied to the new data set which reveal feature not obvious in the total magnetic image of Tarlowski et al. (1995).

Vertical gradient (first vertical derivative ) images enhance fine detail by suppressing long wavelength components and by resolving interfering effects of adjacent anomalies. The colour and grayscale vertical gradient image of the total magnetic intensity have worked well in these aspects and show significant detail of the structure and lithologic units not obvious in the total magnetic intensity map. The images display evidence of geological fabric providing internal detail to many of the features of the total magnetic intensity which manifest themselves as semi-uniform contiguous anomalies. It should be noted that the colour and greyscale images complement each other. While in some respects the grayscale image allows recognition of finer detail, the colour image provides a superior indication of amplitude.

Versions of the total magnetic image continued up to distances of 250 metres and 1000 metres respectively, i.e. images of the total magnetic intensity data that would have been produced if the surveys of the original compilation had been flown at heights of 250 metres and 1000 metres higher than the surveys of the original compilation produces a smoothing of the total magnetic intensity according to a precise relationship. In physical terms, the effects of smaller, narrower and thinner magnetic bodies progressively disappear relative to the effects of larger magnetic bodies with considerable depth extents as the continuation distance is increased. As a result, upward continuation maps often give superior indications of the main tectonic and crustal blocks in an area. Such representations can simplify regional interpretations.

The majority of rock units in the Earth's continental crust are magnetised by induction in the Earth's magnetic field. A result of this phenomenon is that a body that causes an anomaly directly over itself at the Earth's magnetic poles where the Earth's field is vertical, causes progressively more asymmetric anomalies as the inclination of the Earth's magnetic field approaches 45 degrees which is the approximate inclination of the Earth's field over much of northern Australia. As asymmetric anomalies are difficult to relate to source geometry, "reduction to the pole processes are often used to transform fields observed in areas of low magnetic latitude to what they would be if observed at the magnetic pole. The reduction to the pole image simplifies magnetic interpretation in northern Australia.

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References Tarlowski, C., Milligan, P.R. & Mackey, T., 1995. Magnetic Anomaly Map of Australia (Second Ed.), scale 1:5 000 000, Australian Geological Survey Organisation, Canberra Tarlowski, C., Simonis, F., & Milligan, P., 1993. Magnetic Anomaly of Australia, scale 1:5 000 000, Australian Geological Survey Organisation, Canberra.

Tarlowski, C., Simonis, F., Whitaker, A. and Milligan 1992 The magnetic anomaly map of Australia.

Exploration Geophysics, 23, 339-342.

433

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 PROGRESSIVE INTERACTION BETWEEN LATE DEVONIAN MAFIC-INTERMEDIATE AND PERALUMINOUS GRANITOID MAGMAS IN THE MEGUMA LITHOTECTONIC ZONE OF NOVA SCOTIA, CANADA Marcus C. Tate and D. Barrie Clarke School of Earth Sciences, Macquarie University. Sydney, New South Wales 2109 department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5 1

:

The Meguma Lithotectonic Zone of southwestern Nova Scotia represents the most outboard terrane in the Canadian segment of the Appalachian Orogen. Crustal thickening during terrane accretion of the Acadian Orogeny at circa 410 Ma, preceded the epizonal emplacement of syn- to late-tectonic granitoid batholiths and plutons between 385 and 372 Ma. The granitoid intrusions consist predominantly of biotite granodiorite and two-mica monzogranite, and most lithologies have peraluminous compositions (molar A/CNK = 1.1-1.3). Ten currently known Late Devonian mafic and intermediate intrusions have similar ages (385-370 Ma) to the granitoids, suggesting that the emplacement of mantle-derived magmas into the-lower and middle crust influenced granitoid melt generation during and after terrane accretion. Well-exposed synplutonic intrusions into two of the granitoid plutons also indicate that spatially intimate mafic-intermediate magmas were chemically modified by. and interacted with, the granitoid melts. Comparison of the field relationships and geochemical characteristics of lithologies at the synplutonic intrusion sites identifies a typical hybridisation sequence for mingled and mixed systems in orogenic granitoids.

Marginal tonalite of the Port Mouton Pluton contains two synplutonic lamprophyre bodies: (1) a circa 5 m wide, arcuate kersantite-spessartite dyke crops out within a 2 m wide carapace of unusually coarse-grained tonalite that contains rare chilled lamprophyric pillows; and (2) a 2 m thick subhorizontal sheet of kersantite occurs surrounded by abundant lamprophyric pillows, all of which have crenulate contacts and chilled margins.

A granodiorite unit of the Shelburne Pluton also contains a >100 m wide, irregular diorite body that grades into a heterogeneous, hornblende-bearing tonalite over circa 50 m. All of the mafic-intermediate intrusions have similar modal mineral assemblages that consist of actinolitic hornblende (40-52%), biotite (Mg# -0.7; 3752%), plagioclase (An _ ; 14-28%), quartz (3-7%), and K-feldspar (<3%). Compared to contemporaneous mafic bodies that intrude metasedimentaiy country rocks, they have shoshonitic concentrations of K 0 (4.2-4.4 wt.%) and high values for P 0 (0.8-0.9 wt.%), Ba (817-1920 ppm), Sr (221-2567 ppm), Y (19-25 ppm), Zr (134-421 ppm), LREE (La/Lu = 317), and Sr/ Sr t = 3 76 (>0.7055). These features show the best development in the small lamprophyric bodies and apparently result from hybridisation with granitoid magmas.

36

52

2

2

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86

Similar modal mineral assemblages, in conjunction with unique geochemical characteristics, suggest that the synplutonic intrusions record the progressive effects of interaction between mafic-intermediate and peraluminous granitoid magmas. The dyke in the Port Mouton Pluton apparently formed by rapid cooling and crystallisation of lamprophyre immediately after intrusion, which heated the host tonalite and formed coarse grain sizes. Residual magma probably remained in the Port Mouton Pluton sheet after pillow formation, allowing more advanced chemical diffusion of alkalies, incompatible trace elements, and Sr isotopes during magma mingling. The large size of the synplutonic diorite probably allowed the most complete interaction to occur. Shelburne Pluton tonalite contains hornblende-biotite synneusis textures, quartz-hornblende ocelli, patchily zoned calcic plagioclase, and fractured accessory minerals, suggesting that it formed by dioritegranodiorite magma mixing. Somewhat linear compositional variation of most oxides and elements (r >0.924) and relatively low A/CNK values (0.9-1.2) permit the tonalite to be a mixture of -83% diorite and -17% granodiorite. Curved trends for AI2O3, P2O5, Sr, Cr, and Ni, and a non-hyperbolic distribution of initial Sr-Nd isotopic values, suggest mixing by incompatible element diffusion followed by crystal exchange, rather than by liquid blending.

Acknowledgments: A Natural Sciences and Engineering Research Council of Canada (NSERC) research grant awarded to D. B. Clarke and a Macquarie University Research Fellowship (MURF) awarded to M.C. Tate funded this contribution.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ACTIVE CONTINENTAL EXTENSION IN THE WESTERN WOODLARK BASIN 1

B. Taylor1, J. Mutter2, R.Binns3. H. Davies 4, & R. Rogerson5 School of Ocean and Earth Science and Technology (SOEST), University of Hawaii, Honolulu, Hawaii 96822, USA 2 Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA 3 CSIRO Exploration and Mining, North Ryde, NSW 2113, Australia 4 Department of Geology, University of PNG, Port Moresby, Papua New Guinea 5 Department of Mining and Petroleum, Geological Survey Division, Port Moresby, Papua New Guinea

The lateral variation from active continental rifting to seafloor spreading within a small region makes the western Woodlark basin an attractive area to investigate the mechanics of lithospheric extension. Earthquake source parameters and seismic reflection data indicate that low-angle (10-25°) normal faulting is active in the region of incipient continental separation. The seismic reflector correlated to the low-angle fault has an emergent segment along the northern flank of Moresby Seamount, a crustal block that dredging and geophysics indicate may be a metamorphic core complex. Basement fault blocks overlain by only minor ponded sediments characterise the southern margin whereas the northern margin (above the low-angle fault) has a down-flexed re-rift sedimentary basin and basement sequence unconformably onlapped by syn-rift sediments and cut by high-angle faults.

We propose to drill a transect of sites across this asymmetric incipient conjugate margin pain ACE-la on the down-flexed northern margin, ACE-2a/4/5 through the hanging wall, the low-angle normal fault zone, and into the footwall, and ACE-3a near the crest of Moresby Seamount The primary objectives at these sites are to: L Test the interpretation that the reflector emergent on the northern flank of Moresby Seamount is a lowangle normal detachment and that the seamount is a lower plate metamorphic core complex. An alternative testable interpretation is that the reflector is a fault bounding a tilted upper plate block.

II. Initially characterise, and subsequently monitor, the in situ properties (stress, permeability, temperature, physical properties, fluid pressure and composition) of an active low-angle fault zone, preferably at several depths.

HI. (a) Determine the differences in in situ properties between the low-angle fault and surrounding crust, including inactive structures of similar geometry, (b) test whether the fault evolved from high angle, high stress, to low angle, low stress, and hence (c) constrain the essential parameters of low-angle faulting.

IV. Determine the vertical motion history of both the down-flexed upper plate (by backstripping the biostratigraphy and regionalising the well data using seismic stratigraphy) and the (unroofed?) lower plate (by P-T-t and petrofabric studies), and hence estimate the timing and amount of extension prior to spreading (for which only regional estimates exist).

435

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 ENVIRONMENTAL IMPACTS OF EXPLORATION MINING AND MINERAL PROCESSING Graham F. Taylor CSIRO Minesite Rehabilitation Research, PMB 2, Glen Osmond SA 5064

For Australia to maintain the high level of export income from its mineral resources, exploration, mining and mineral processing must be allowed to continue at its present or even greater level. The same is true for many nations of SE Asia and Oceania. Inevitably, such activities will impact upon the environment. The mineral industry has gone a long way to comply with new regulatory guidelines and to meet community expectations and is collaborating with the Australian Environment Protection Authority to produce 'Best practice environmental management in mining'. At the same time industry, government instrumentalities, tertiary institutions and consultants are conducting research to ensure that the environmental impact is even further reduced.

There is no doubt that most pollution is derived from past exploration, mining and mineral processing, when existing knowledge and regulations were minimal and community expectations and concerns less. Such sites and pollution require extensive and costly remediation and rehabilitation. Examples from exploration activities are seismic lines, cut grids for geochemical sampling, drilling pads, access roads and plastic bags filled with drill samples.

By far the greatest impact is from abandoned and older mines such as Brukunga, SA, Captains Flat, NSW, Rum Jungle, NT, Mt. Morgan, Qld. and Mt. Lyell, Tas. The common pollutant at each of these mines is acid mine drainage (AMD) and secondary dispersion of toxic elements derived from waste rock, low grade ore, tailings and mine adits. Other contaminants are dust, unstable landforms, chemicals (e.g. cyanide), radioactivity,fibrousparticles (asbestos), hydrocarbons and siltation of waterways. Present large-scale open-cut and strip mining of coal, bauxite and heavy mineral sands disrupt the regolith profile and make rehabilitation (revegetation) difficult. Mineral processing at Port Pirie, SA, Cockle Creek, NSW, Mt. Lyell, Tas, Rum Jungle, NT, Broken Hill, NSW, have had considerable impact on the surrounding countryside.

The geoscientist has a reactive role in the amelioration of contaminants and rehabilitation of such sites including characterization of contaminants, containment of waste products, stabilization of constructed landforms and soil characterization for sustainable ecosystem reconstruction. With the opening of new mines in Australia, attention has shifted to pre-operational design of environmental strategies. In the future, the geoscientist will use data from the exploration and pre-mining feasibility drilling programs to develop environmental strategies as an integral part of mine planning. At the same time, exploration geochemical data will provide the baseline (background) values for environmental monitoring.

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE GEOLOGICAL INFORMATION CONTENT WITHIN POLARIMETRIC RADAR IMAGES G R Taylor1, R D Hewson 1 and A Mah1 department of Applied Geology, School of Mines, UNSW

Multi-frequency, polarimetric radar imagery has been acquired over several Australian test sites by the AIRSAR aircraft system in 1993 and the SIR-C/X-SAR space shuttle system in 1994. This radar imagery contains styles of information not acquired by other remote sensing systems and therefore provides a useful increment to the geological data base. Radar backscatter is a function of both frequency and surface roughness, thus multifrequency images illustrate variations in surface roughness that can be either a consequence of primary lithology or weathering history. The polarization response of a ground surface is both a function of the predominant scattering mechanisms and the surface dielectric properties. Dielectric properties of rocks and soils vary according to density, conductivity, moisture content and salinity. Interferometry data acquired simultaneously with the polarimetric radar provides a high resolution Digital Terrain Model (DTM) of the ground surface. The DTM contains much geological information in its own right, but is also a useful base with which to integrate other remotely sensed and geophysical data sets. It is particularly useful for allowing the user to map, from remotely sensed imagery, three dimensional aspects of geology.

We describe the use of polarimetric radar and interferometry for mapping sedimentary and metamorphic rocks at Flowers Gap, north of Broken Hill. The determination of soil dielectric properties by inversion of radar backscatter models and the use of this for mapping salinity is described for a site in the Murray Darling Basin.

Lastly we demonstrate the mapping of geological structures in three dimensions from integrated radar and interferometry data from the Palm Valley region of central Australia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

APPLICATION OF NITROGEN AGGREGATION AND CATHODOLUMINESCENCE METHODS IN THE STUDY OF THE THERMAL, DEFORMATION AND GROWTH HISTORY OF DIAMOND Wavne R. Taylor Research School of Earth Sciences, Australian National University, Canberra ACT 0200

Many diamonds exhibit complex multistage growth, dissolution, and deformation histories that provide a record of processes that occurred in the lithospheric mantle over the residence time of diamond. In pipe diamonds, this period, which is given the symbol t may extend from a few million years, or less, up to as much as 3.2 Ga, so that studies of diamond populations can potentially provide information over a long period of lithosphere evolution. It is possible to extract detailed information from diamonds because, unlike oxide and silicate minerals, they are not recrystallized during ordinary geological events. Two methods are useful in the characterization of diamond: these are cathodoluminescence (CL) and infrared microspectroscopy (IRMS), both of which are sensitive to lattice defects associated mainly with nitrogen impurities. Most diamonds contain some substitutional nitrogen impurities (usually in the range 10 to 3000 atomic ppm) which were acquired at the time of diamond growth; these diamonds are termed type. I. Nitrogen-poor diamonds containing <10 atomic ppm nitrogen, are termed type n, and amongst pipe diamonds, are much less common than type I stones. However, many type I diamonds show a heterogeneous internal distribution of nitrogen and may contain zones or regions of type 13 material.

In the CL method, visible and near-IR fluorescence occurs under electron bombardment with an intensity in the blue region of the spectrum that is roughly proportional to the concentration of aggregated nitrogen defects present. In sectioned diamonds or in strongly resorbed rough diamonds, internal growth zonation and deformation features (e.g. slip lamallae due to plastic deformation) are revealed by CL imagery using a luminoscope for low resolution studies or SEM with a CL detector for high resolution studies (Taylor et al., 1995; Griffin et al., 1995). The defect centres responsible for the luminescence may be identified by CL spectroscopy at liquid nitrogen temperatures. In rough diamonds, CL is useful in the characterization of surficial radiation (a-particle) damage which is particularly evident in diamonds that have been subject to long-term recycling histories in the alluvial environment In the IRMS method, the nitrogen content of diamond and the proportions of the principal substitutional nitrogen defect centres can be determined from their characteristic IR absorption spectra. Because different growth zones in diamond may only be a few tens of microns across, the determination of nitrogen defect distribution requires a microsampling accessory, such as an IR microscope, with a spatial resolution down to about 20 (im. The proportions of nitrogen centres reveals information on the thermal history of the diamond in the mantle. This is possible because the nitrogen centres aggregate from singly substituted centres in type lb diamond, to nitrogen pairs in type IaA diamond, and finally to a tetrahedral arrangement of nitrogen atoms about a vacancy in type IaB diamonds. The rate of nitrogen aggregation is proportional to temperature, r^, and total nitrogen content but is most sensitive to temperature. The kinetics of lb —» IaA and IaA IaB aggregation have been experimentally determined so that quantitative information on diamond thermal history can be obtained (e.g. Taylor et al., 1995). Many diamonds from kimberlite pipes are type IaAB stones with aggregation states that decreasefromcore torimsuggesting growth and residence under conditions of falling temperature (~1300°C to ~1000°C). However, some rare cubic diamonds and fibrous overgrowths on older diamonds, may contain a type lb component suggesting they formed within a few million years of kimberlite eruption, possiblyfromproto-kimberlite magmas that permeated and crystallized within the lithosphere.

Application of these techniques to the characterization of diamonds from various kimberlite pipes, alluvial sources, and non-conventional diamond sources, such as ultra-high P metamoiphic terranes, will be discussed.

REFERENCES Griffin, B J., Bulanova, G. & Taylor, W.R. (1995) CL and FTIR mapping of nitrogen content and hydrogen distribution in a diamond from the Mir pipe - constraints on growth history. Extended Abstracts, 6th International Kimberlite Conference, 191-193, Russian Academy of Sciences, Novosibirsk.

Taylor, W.R., Bulanova, G. & Milledge, HJ. (1995) Quantitative nitrogen aggregation study of some Yakutian diamonds: constraints on the growth ,thermal, and deformation history of peridotitic and eclogitic diamonds. Extended Abstracts, 6th International Kimberlite Conference, 608-610, Russian Academy of Sciences, Novosibirsk.

M R y

438

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13 th Australian Geological Convention, Canberra, February 1996 METHODS FOR UNDERSTANDING POORLY EXPOSED BASEMENT TERRAINS: A CASE STUDY IN THE WESTERN GAWLER CRATON Jonathan Teasdale Department of Geology and Geophysics, University ofAdelaide, Adelaide, 5005

The Gawler Craton remains one of the least understood Precambrian terrains in Australia, mainly due to a paucity of continuous outcrop. In the western Gawler Craton, sporadic outcrops and drill hole intersections yield a wide variety of rock types which were previously difficult to correlate. The recent availability of high resolution aeromagnetic data produced under the South Australian Exploration Initiative (SAEI) by Mines and Energy, South Australia (MESA) provides an excellent tool with which regional syntheses and correlations can be made. This dataset provides a detailed insight into the geology of the region at scales up to 1:100,000; an excellent framework on which to develop a regional understanding.

It is important to define observable geological features that can be correlated with the aeromagnetic data. Three important "observables" have been defined in this study: • Magnetic Susceptibility: By measuring the magnetic susceptibility of observed lithologies, one can correlate those lithologies with remote-sensed magnetic signatures.

• Metamorphic Grade: If the structural continuity of metamorphically useful lithologies can be proven using the aeromagnetic data, then it is reasonable to assume that the metamorphic effects are regional, not local. By defining pressure-temperature trajectories for these lithologies, it may be possible to evaluate the tectonothermal history of juxtaposed crustal blocks.

• Fault Kinematics: Kinematic observations are important in explaining the juxtapostion of geologically contrasting subdomains along major shear zones.

The western Gawler Craton is dissected by numerous crustal scale shear zones which often juxtapose subdomains of significantly different magnetic character. The shear zones have previously been described as intracratonic, separating either different levels or lateral equivalents of the same continental crust (e.g. Rankin et al., 1989; Drexel et al., 1993). Integrated geological and geophysical evidence increasingly supports the notion that the fault-bounded domains may have undergone significantly different tectonothermal histories. This implies that lateral tectonic accretion of unrelated crustal segments may have occurred; a hypothesis at odds with an intracratonic setting.

One such shear zone is the Karari Fault Zone, which forms a discrete, northwest trending shear zone over 300km long. Investigations into mid-Proterozoic (Drexel et al., 1993) paragneiss at Ooldea, immediately to the west of the Karari Fault Zone in the Nawa Subdomain have yielded spectacular, very high grade ferruginous granulites in various drill-cores. Diagnostic mineral assemblages and reaction textures include sapphirine-quartz, spinelquartz, corundum-quartz, sillimanite-orthopyroxene-quartz±(sapphirine-cordierite), and the probable former existence of osumilite. These assemblages indicate peak metamorphic conditions in excess of 900°C and 8kb, followed by near-isobaric cooling. The Ooldea granulites lie in a set of regionally continuous, east-west trending magnetic highs (probably caused by banded iron formations) which are truncated to the east by the Karari Fault Zone. About 30km east of the Karari Fault Zone in the Christie Subdomain, supposedly Archaean garnet-cordierite-sillimanite paragneiss near Lake Ifould (King, 1951) preserve evidence for peak metamorphism at significantly lower temperatures and pressures, suggesting that the two domains juxtaposed by the Karari Fault Zone have undergone significantly different tectonothermal histories.

References Drexel, J.F., Preiss, W.V. & Parker, A.J. (eds.), 1993, The Geology of South Australia: Volume 1, The Precambrian. State Print, Adelaide.

King, D., 1951, Geology of the Pidinga Area. Trans. Roy. Soc. S. Aust74 (1), 25-44.

Rankin, L.R., Martin, A.R. & Parker, A.J., 1989, Early Proterozoic History of the Karari Fault Zone, northwest Gawler Craton, South Australia. Australian Journal of Earth Sciences, 36, 123-133.

Acknowledgments Mines and Energy, South Australia (MESA) generously supplied geophysical datasets. Equinox Resources N.L. provided logistic support during fieldwork.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PRACTICAL CRITERIA TO DISCRIMINATE LACUSTRINE VS TERRIGENOUS PETROLEUMS: EXAMPLES FROM SOUTHEAST ASIA. H.L. ten Haven TOTAL. Scientific and Technical Centre. 78470 Saint Remy les Chevreuse, France

The palaeodepositional environments of Tertiary graben/half graben rift systems in Southeast Asia are in general considered to have followed the sequence of a lacustrine facies, followed by a deltaic/fluviatile, and ending with a marine facies. Terrigenous source rocks are indeed ubiquitously encountered in these basins, whereas lacustrine source rocks are almost invariably invoked to exist, albeit proof by drilling is frequently lacking. Reports of marine source rocks are scarce, perhaps because the environmental conditions were not conducive for source rock development (Robinson, 1987). Given the enormous oil potential of the proven lacustrine petroleum system in the Central Sumatra Basin, Indonesia, it is not surprising that exploration geologists are most interested to find firm evidence for a lacustrine play.

In the absence of rock samples, geochemical analyses of petroleums (oils and condensates) provides the only way to decipher whether a lacustrine source rock has been deposited, with the prerequisite that the geochemical characteristics of lacustrine petroleums differ significantly from those derived from a terrigenous source. Many geochemical criteria for this discriminative purpose have been proposed in the literature (Peters & Moldowan, 1993). In general the application of these criteria is straightforward for unbiodegraded oils, but becomes problematic for biodegraded oils, and almost virtually impossible for condensates. This latter class of petroleums poses a problem as biomarkers, one of the discriminative tools applied, are quite often below the limit of detection. Further fractionation effects occurring during migration and phase separation have a major impact on many classical source geochemical parameters, such as the stable carbon isotopic signature and the pristane/phytane ratio (Dzou & Hughes, 1994). Thus there is a clear need for reliable geochemical parameters.

Parameters which can be of help in these cases are the so-called Mango light hydrocarbon parameters (Mango, 1987; ten Haven, 1995). These parameters are derived from the distribution of selected C 7 compounds and can be calculated from whole oil gas chromatographic data without any pre treatment. These parameters do not seem to be affected by fractionation processes. Analysis of a great number of oils and condensates from two single fields, one in the Malay Basin the other in the Con Son Basin, shows that the Mango parameters provide a rapid and cheap tool to discriminate lacustrine from terrigenous petroleums. Further indications for mixing from these two sources is found. In both these basins indirect evidence for the presence of a lacustrine source is thus for the first time established.

REFERENCES Dzou, L.I.P., & Hughes, W.B., 1993. Geochemistry of oils and condensates, K field, offshore Taiwan: a case study in migration fractionation. Organic Geochemistry, 20, 437-462. ten Haven, H.L., 1995. Applications and limitations of Mango's light hydrocarbon parameters in petroleum correlation studies. In Organic Geochemistry: Developments and Applications to Energy, Climate, Environment and Human History (eds. J.O. Grimalt & C. Dorronsoro), 1075-1077.

Mango, F., 1990. The origin of light hydrocarbons in petroleum: A kinitic test of the steady state catalytic hypothesis. Geochimica Cosmochimica Acta, 54, 1315-1323 Peters, K.E. & Moldowan, J.M., 1993. The Biomarker Guide. Prentice Hall.

Robinson, K.M.,1987. An overview of source rocks and oils in Indonesia. Proceedings Indonesian Petreoleum Association, Sixteenth Annual Convention, 97-122.

Acknowledgments: This abstract is published with permission of TOTAL.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

HYDROGEOLOGY OF THE ORD RIVER PALAEOCHANNEL Steven J.Tickell1 and Richard Nixon2 Power and Water Authority, GPO BOX 1096 Darwin NT 0801 Geological Survey of Western Australia 100 Plain St, East Perth, WA 6004 1

2

A palaeochannel system associated with former courses of the Ord and Keep Rivers occurs beneath the alluvial plains between Kununuixa in northwest Western Australia and the Keep River in the Northern Territory. The palaeo-Ord River flowed to the northwest beneath the Weaber and Keep River Plains where it was joined by a palaeo-Keep River tributary. It then discharged into the Joseph Bonaparte Gulf an unknown distance seaward of the present day coastline.

The presence of this sand filled palaeochannel has important hydrogeological implications in relation to the proposed extension of the Ord River Irrigation Area to the Weaber and Keep River Plains. The Northern Territory and Western Australian governments are investigating the feasibility of extending the Ord River Irrigation Area. As a first step a hydrogeological study was undertaken to determine if irrigation would induce salinity problems or waterlogging and if so, would these be managable. The palaeochannel was recognised and mapped during this study. It's influence on groundwater recharge and water quality are described here as are the implications for the area's irrigation potential.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

T E C T O N I C EVENTS AFFECTING THE THERMAL HISTORY O F T H E W E S T E R N MARGIN OF THE EROMANGA BASIN. Peter R.Tingate*. Elinor M. Alexander^, Bernd H. Michaelsen^, Ian R. Duddy4 and Cedric M. Griffiths1 *NCPGG, Thebarton Campus, University of Adelaide, SA, 5005. ^Mines and Energy South Australia, PO Box 151, Eastwood, SA, 5063. ^Geology and Geophysics, University of Adelaide, SA, 5005. ^Geotrack International, Earth Sciences Building, University of Melbourne, Parkville, Vic., 3052.

Apatite fission track analysis (AFTA) and vitrinite reflectance data has been collected from several wells neighbouring the Eringa Trough as part of an ARC Collaborative project between the NCPGG and MESA supported by Geotrack. The Eringa Trough, located on the western margin of the Eromanga Basin, consists of up to 2500 m of Permian to Mesozoic section which overlies Palaeozoic units equivalent to the Amadeus Basin.

Wells currently undergoing AFTA interpretation are AFMECO Cur-3 & -5, Mt Hammersley-1, Purni-1 and Mt Crispe-1.

Preliminary results from the wells indicate that all Cambro-Ordovician samples from AFMECO Cur-3 & -5, Mt Crispe-1 and Mt Hammersley-1 have experienced temperatures > 100 °C prior to cooling between 400 and 300 Ma. This cooling is correlated with uplift and erosion during the final stages of the Alice Springs Orogeny.

Purni-1 is the easternmost well analysed and has the most complete Eromanga Basin sequence. Samples from the Winton Formation and Algebuckina Sandstone in Purni-1 are currently close to maximum temperature.

Permian and Mesozoic AFTA samples to the west of Purni-1 all show evidence of experiencing temperatures approximately 30-40 °C higher in the last 150 Ma. Vitrinite data has been collated from open-file sources and new data has been generated for this study. A vitrinite reflectance map for the region has been produced using Permian and Mesozoic samples that are currently close to 500 m depth. Two zones of vitrinite values occur: values are less than 0.4% to the east of 136° longitude and to the west range mainly between 0.4 and 0.5 %. This pattern of reflectance values indicates that the Permian and Mesozoic section to the east of 136° have experienced higher temperatures in the past.

The AFTA and reflectance data show that elevated palaeotemperatures have occurred over a large part of the western Eromanga Basin margin. Likely causes for these elevated palaeotemperatures are burial prior to uplift and erosion of Cretaceous section and heating related to hot water moving through Mesozoic aquifers. There are difficulties in distinguishing the relative importance of the two heating mechanisms but significant erosion of Cretaceous section in the Late Cretaceous- Early Tertiary and later in the mid Tertiary is likely. The thermal history data collected and analysed so far suggest that the top of the oil window is closer to the surface in the vicinity of the Eringa Trough than previously thought making shallow targets more prospective. Work is continuing to further delineate the timing and mechanisms for the elevated temperatures observed in the Permian and Mesozoic section.

Evidence of significant relatively young cooling is not confined to the vicinity of the Eringa Trough. AFTA outcrop and well data from the Arunta and Musgrave Blocks, Amadeus and Officer Basins also indicate cooling of ~ 40 °C during the last 100 Ma. This data suggests that the Eromanga Basin was part of a larger basin which originally extended further west and the location of the present western margin is the result of Late Cretaceous and Tertiary uplift and erosion.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

AN ADAPTIVE PHASE DETECTOR FOR AN ON-LINE BROAD-BAND SEISMOGRAM Tong Cheng and Brian L.N. Kennett Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia

ABSTRACT An adaptive phase detector is designed to detect phases on an on-line broad-band seismogram. Most conventional phase-detectors use energy measures to trigger on the onsets of seismic phases. These detectors can only detect phases whose signal to noise ratio is higher than one. Because they are usually frequencysensitive, these detectors are not suitable for the on-line phase detection of a broad-band seismogram.

A number of different waveform processing techniques are combined to produce automatic phase-detection. At the first stage, the detector uses a complexity measure to detect the approximate position for a phase package.

The original seismogram segment which contains the detected phase package is filtered into a high-frequency trace and a low frequency trace using adaptive procedures based on local frequency-content. Finally, phases are detected on each of the filtered traces by using an adaptive STA/LTA detector which combines energy measures on one or more components of ground motion with a local frequency analysis.

Experiments show that this phase detector has several advantages.

. .

The complexity measure can detect waveform variation both in amplitude (or energy) and in frequency content so that it can detect some high-frequency phases whose signal to noise ratio is less than one (as low as 1/5).

This detector is made adaptive to the local frequency content so that it can be used for an on-line broad-band seismogram.

Furthermore, by using two adaptive filters, this phase detector can detect some major interfering phases.

443

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRA CTS No. 41 13th Australian Geological Convention, Canberra, February 1996 REGOLITH GEOCHEMISTRY AND MINERALOGY OVER THE NORTHPARKES Cu-Au DEPOSITS, NSW *E.K.Tonui: Tony Eggleton* and Graham Taylor^ CRC for Landscape Evolution and Mineral Exploration, ANU, ACT 0200 2 CRC for Landscape Evolution and Mineral Exploration, UCAN, Belconnen, ACT 2605 1

INTRODUCTION The Northparkes mine is located at Goonumbla which is about 28km to the West of Parkes in Central NSW. Regionally, it occurs within the NE portion of the structural subdivision of the Lachlan Fold Belt System known as the Bogan gate synclinorial zone. Open-cut mining at the Northparkes deposits has exposed deeply weathered regolith profiles, which have been used in this study to investigate geochemical dispersion and mineralogy over the Cu-Au mineralisation. Mining commenced on the E27 deposit in September 1993 and subsequently on E22 in December the same year. RAB drill holes have also been used to augment the data obtained from these two deposits in order to facilitate a broad interpretation of regolith and associated landscape features.

GEOLOGY AND LANDFORMS The Cu-Au mineralisation is hosted by a sequence of late Ordovician to early Silurian volcanics, intrusives and sediments. The host rock is a latite (K-rich trachyandesite) which is closely associated with shoshonites (rocks with high A I 2 O 3 : SiO ratios). The dominant intrusive which has given rise to the mineralisation is a quartz monzonite porphyry.

The topography of the area is generally subdued, with the highest point, the 'Secrets' hill, having an elevation of 390m above the sea level. A number of hills stand out as parallel ridges in the landscape fringed by flat valleys below.

REGOLITH FEATURES AND MINERALOGY The weathering front dips gently from the least weathered (20-25m) sections to the west to the more deeply weathered (45-50m) sections to the East for both deposits. The deposits show more or less similar regolith features. The profiles have been subdivided into three major types ie. andesite type, monzonite type and the more 'complex types formed by weathering of transported sediments to the Eastern part The andesite and monzonite types are residual and have shallow depths of weathering (20-30m). They are composed of thin (l-2m) soil horizons, saprolite, saprock, sulphide zones and fresh rock. The soils are reddish brown with minor amounts of carbonates, gypsum and bedrock fragments. The saprolite over the two is rich in kaolin, soft and highly friable with fractures which have been infilled by iron oxides (mostly goethite) and manganese coatings or aggregates.

1

The regolith materials developed in the more deeply weathered sections are complex. In E22 the soils horizons are deep (upto 5m in some parts) and have a high content of carbonates and gypsum. Pseudomottling within the clays marks the transition to the mottled zone, the upper sections of which are rich in nodules while the lower section consists of grey clay (kaolin) with an earthy appearence. In between the mottled zone is a narrow band(35m) of dolomite coated silica nodules. The base of the unit attains a dull grey to dark appearence in transition with the saprolite below. This zone is marked by dark Mn(?) nodules with iron oxide coatings in a matrix of pink clay and minor detrital quartz fragments. This is typical of palaeodrainage environments or palaeotopographic lows.

In E27, the silica nodule zones are restricted in occurrence and tend to be diffuse within the earthy clay matrix.

Two cones of depression marked by the daik Mn (?) aggregates within the mottled clay are suggestive of palaeoredox fronts. The base of the unit and contact with the saprolite is not as well defined as in E22 suggesting that the material could have evolvedfrompalaeo-watertable fluctuations during arid climatic phases.

GEOCHEMISTRY Initial results indicate that most of the elements mainly Si, Cr, Zr, Nb, Fe, Ba, Mg, Ca and Ti tend to be depleted in the saprolite. Ca, Ba, Mg and Sr are reconcentrated within the soils as carbonates. Cr and Si are also enriched in the soils and mottled zones. A1 and Fe are concentrated in the mottled zones as clays mainly kaolin.

Cu is closely associated with Zn, As and Mn within the saprolite and soil horizons.

Acknowledgements: I wish to thank North Exploration for their permission to publish this material and for thencontinued support.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

SEQUENCE STRATIGRAPHIC AND STRUCTURAL EVOLUTION OF THE BOWEN AND SURAT BASINS IN NORTHERN NEW SOUTH WALES Jennifer M. Totterdell. Andrew A. Krassay and Russell J. Korsch Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

The Sedimentary Basins of Eastern Australia project commenced in 1990 as one of the first generation of National Geoscience Mapping Accord projects. The project has been undertaken by the Australian Geological Survey Organisation in collaboration with the Geological Survey of Queensland and the Geological Survey of the New South Wales.

The aim of the project is to enhance the knowledge of, and develop models for, the origin and evolution of the Bowen, Gunnedah and Surat basins in eastern Australia and to relate these models to potential hydrocarbon occurrences as a basis for the future exploration and assessment of resources. The main strategy followed to achieve these goals was to build a chronostratigraphic and structural framework for the Bowen, Gunnedah and Surat basins in Queensland and NSW through the sequence stratigraphic and structural interpretation of seismic and well data.

The Bowen Basin is the northernmost unit of the Bo wen-Gunnedah-Sydney basin system. During its development, the basin was bounded to the east by the active New England Orogen, and to the west by a relatively stable craton of Early-mid-Palaeozoic rocks. In Queensland, the basin contains up to 10km of terrestrial and shallow marine, largely clastic sediments, along with substantial deposits of black coal. The succession in northern New South Wales is much thinner and less complete. In New South Wales, the basin is bounded to the east by the Goondiwindi Fault and onlaps basement to the west. The succession is continuous with the Gunnedah Basin to the south.

The Bowen Basin was initiated in the Early Permian due to back-arc extension associated with subduction east of the continental-margin Camboon Volcanic Arc. This was followed by a period of subsidence driven by thermal relaxation. From the mid-Permian, contractional events in the New England Orogen resulted in the onset of foreland basin conditions. From the Late Permian to the Middle Triassic, foreland loading resulted in accelerated subsidence on the eastern side of the basin. During this period, many of the earlier extension-related faults were reactivated as thrusts. This episode of shortening culminated in the Middle Triassic. Extensive erosion and peneplanation during the Middle to Late Triassic was followed by deposition of the Surat Basin succession in a gently subsiding intracratonic sag during the Early Jurassic to Early Cretaceous.

In northern New South Wales, prolonged erosional episodes caused by contractional events have left a fragmented record of the sedimentary succession, especially that of the Bowen Basin. Nevertheless, four depositional supersequences, each bounded by major unconformities, have been recognised within the Bowen and Surat basin successions. The thick, dominantly fluvial sediments of Supersequence 1 were deposited in the eastern part of the basin during the Early Permian in fluvial and peat swamp environments that were repeatedly inundated by coarse alluvial fan material. A contractional event following deposition of this succession resulted in mild deformation and some erosion of the section. Following this, an increase in accommodation led to the deposition of the mid-Permian shallow marine to deltaic sediments of Supersequence 2. A contractional event and regional tilting in the Early Triassic resulted in the erosion of Late Permian-earliest Triassic sediments from most of the area prior to the deposition of the Middle Triassic fluvio-lacustrine, possibly coastal plain, sediments of Supersequence 3. The most significant tectonic event in the area was the Middle-Late Triassic contractional event. This event deformed the entire Bowen Basin succession and led to major contractional movements on the Goondiwindi Fault. Extensive erosion and peneplanation occurred prior to the intrusion of mafic igneous rocks throughout much of the area in the Late Triassic-Early Jurassic and the deposition of the dominantly non-marine Supersequence 4 in the Early Jurassic-Early Cretaceous.

Given the variable quality and distribution of seismic data in northern NSW, the sequence stratigraphic interpretation of well log data has proven to be a valuable method for analysing and correlating the sedimentary succession, both marine and non-marine, and building a chronostratigraphic framework. This framework, together with our understanding of the structural history of the area, provides a basis for further investigation of the burial history of the basins and assessment of the distribution and timing of play elements.

445

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra:, February 1996 EVOLUTION OF COASTAL SANDY DEPOSITS IN CENTRAL VIETNAM IN RELATION TO THE OSCILLATION OF SEA LEVEL DURING THE QUATERNARY Tran Nghi Faculty of Geology, Hanoi University, 90 Nguyen Trai Street, Hanoi, Vietnam

The original source of sand for the coastal sandy deposits in Central Vietnam was from the Truongson Mountains. The sand was transported to the sea by high energy river currents during flood periods. Sand transport was especially important during major Quaternary low stands of sea level (regressive phases) between the early Pleistocene and the middle to late Pleistocene.

An initial system of coastal sandy bars was formed during the early Pleistocene and is preserved on preQuaternary uplift zones along the coastal margin of Vietnam.

A second system of sandy bars was formed along the coast in the mid to late Quaternary. The marine sand deposits were initially associated with lagoonal facies but most deposition occurred in the nearshore marine environment. This later generation of sandy bars and marine sandy dune bedforms was mainly deposited as a result of storm wave activity in the nearshore zone.

The role of wind in shaping the coastal sand deposits in Vietnam has been particularly important from Quaternary IV " to present. It has eroded the original nearshore marine sand deposits, and has modified the system of coastal lagoons, sandy bars and sandy terraces.

2

3

Coastal placer deposits in Central Vietnam were formed during transgressive phases which eroded the coastal region and produced enrichment of heavy minerals in the shoreface zone as a result of wave action. The most important placer deposits consist of ilmenite, zircon and monazite and are related to the late Pleistocene and middle Holocene transgressions. Equivalent modern placer deposits are also present on the tidal flats and are related to modern marine erosion.

446

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

U-SERIES ISOTOPIC DISEQUILIBRIA IN DESTRUCTIVE PLATE MARGIN MAGMAS: COMPARISON OF THE TONGA-KERMADEC AND LESSER ANTILLES ARCS Simon Turner and Chris Hawkesworth Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, England

High precision TIMS U-Th isotope data have been obtained along with other isotope and geochemical data on some 100 Recent lavas from the Tonga-Kermadec, Lesser Antilles, Kamchatka, Philippine and Indonesian island arc systems. These data provide insights into the rates and processes of magma generation at destructive plate margins and comparisons of inter- and intra-arc variations. This presentation will compare and contrast results from the highly depleted Tonga-Kermadec arc, where the slab fluid signature is best developed, with the Lesser Antilles where large along-arc variations from depleted, tholeiitic to enriched alkaline lavas are interpreted to reflect an increasing contribution from subducted sediment incorporated in the mantle wedge.

TONGA-KERMADEC A series of 25 lavas have been analysed spanning the Tonga-Kermadec island arc and from the back arc island of Niua fo'ou. Excepting the back arc island, the lavas show a very restricted range in Sr, Nd and Pb isotopes. In contrast, Ba/Th ratios span a large range from 800-400 in Tonga to 400-100 in Kermadec over which 8 7 Sr/ 8 6 Sr only varies from 0.7034-0.7037. Combined, the Tonga-Kermadec data lie at the high Ba/Th, low 8 7 Sr/ 8 6 Sr end of a more global array which has been interpreted to have been dominated by two-component mixing between slab-derived fluids and subducted sediment (low Ba/Th, high 87 Sr/ 86 Sr). U-Th isotope data preserve extreme Uexcess in many of the lavas but some are closer to equilibrium and others exhibit Th excess. In detail, 230 Th/ 2 3 2 Th) ( 230 T h / 238u) v a r i e s f r o m 0.8-0.5 and ( from 1.2-2.4 in the northern Tonga islands of Tafahi, Fonualei, Late and Tofua. Moving south along the arc into the Kermadecs, lavas from the Raoul group of islands and Macauley island lie much closer to U-Th equilibrium or show minor Th-excess with ( 2 3 0 Th/ 2 3 8 U) 1.1-0.8 and lower ( 230 Th/ 232 Th) (1.0-1.2). L'Esperance, at the southernmost end of the arc preserves greater Th-excess with ( 2 3 0 Th/ 2 3 8 U) = 1.2 and ( 2 3 0 Th/ 2 3 2 Th) = 1.1. Finally, lavas from the back arc island of Niua fo'ou 230 238 preserve consistent Th-excess with ( Th/ U) = 1.2-1.5 and ( 2 3 0 Th/ 2 3 2 Th) = 1.0-1.3. The high Ba/Th and U-excess in the north in Tonga reflect dominance of the fluid signature relative to a highly depleted mantle wedge. Further south along the arc, the increasingly less depleted mantle wedge obscures the fluid signal leading to lower Ba/Th and higher ( 2 3 0 Th/ 2 3 8 U). No subduction component is seen in the back arc where Ba/Th and (230 Th/ 232 Th) ratios and moderate Th-excess are similar to those of MORB. LESSER ANTILLES A suite of 28 lavas which span the length of the Lesser Antilles island arc vary from low K, tholeiitic lavas with low 8 7 Sr/ 8 6 Sr in the north, to high K, calc-alkaline to alkalic lavas with higher 87 Sr/ 8 ^Sr in the south. The northern lavas are characterised by ( 2 3 0 Th/ 2 3 8 U) < 1 whereas those from the south tend towards secular equilibrium and occasionally have ( 230 Th/ 238 U) > 1. A three component model is required for magma genesis in the Lesser Antilles. Sediments from the subducting slab melt and variably enrich the mantle wedge while the subducting oceanic crust dehydrates, releasing fluids enriched in U, Ba, Rb, K, but with low Th and 8 7 Sr/ 8 6 Sr, which promote partial melting in the wedge. In the northern parts of the arc, the addition of U by these fluids overprints any increases in Th/U that might have been caused by partial melting, and results in lavas with ( 2 3 0 Th/ 2 3 8 U) < 1. Further south, the fluid signature is swamped by partial melts of the subducted sediments, and the ( 2 3 0 Th/ 2 3 8 U) ratios tend towards 1. Assuming that ( 2 3 0 Th/ 2 3 8 U) ratios < 1 reflect the preferential mobility of U in aqueous fluids, the bulk of the LIL element abundances, and the elevated 86 Sr/ 86 Sr ratios in the calc-alkaline and alkalic lavas, were not transported by subduction zone fluids. Rather, Ta/Yb increases with 87 Sr/ 8 6 Sr southwards along the arc and this is inferred to reflect increasing enrichment of the mantle wedge through the addition of partial melts of subducted sediment. A two-component mixing model is developed to calculate ( 230 Th/ 232 Th) ratios at the time of formation of the measured U/Th. The time required for radioactive decay from these calculated ratios to the measured ( 230 Th/ 232 Th) ratios provides an upper limit on the transport time for the relatively high U/Th fluid component through the wedge of 130 000 yr. If the reported -55 000 yr U-Th mineral isochrons from Soufriere on St.Vincent reflect typical magma chamber residence times for this island arc, this leaves -75 000 y for transport from the slab to the crust. This implies an ascent rate of - 1 m yr" which would appeartotoI.require flow through the mantleE.wedge. * Acknowledgements Smith,channelled T. Worthington, R. Macdonald, Heath, P. van Calsteren & N. Rogers

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GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

TECTONIC SIGNIFICANCE OF THE TALATERANG AND SHOALHAVEN GROUPS OF THE SOUTHERN SYDNEY BASIN Stuart C. Tye University of Wollongong, Wollongong NSW 2522

The sedimentary succession which is the focus of this study consists of the basal Talaterang and Shoalhaven Groups at the southernmost extremity of the Sydney Basin, lying between the towns of Nowra and Durras on the NSW South Coast. As a result of the study carried out by Tye et al (in press) the stratigraphy of the southern Sydney Basin has been redefined. The Clyde Coal Measures and Wasp Head Formation have been assigned to the basal Talaterang Group and the Yadboro and Tallong Conglomerates, previously called Talaterang Group, have been assigned to die Shoalhaven Group. The exposed succession of the southern Sydney Basin is interpreted as originating from the cratonic margin of an embryonic foreland basin. The work presented herein builds upon this previous work and provides more insight into the tectonic development of the southern Sydney Basin during the Early Permian.

The Talaterang Group was interpreted by Tye et al (in press) as deposited during an early extensional phase of basin development which has been well documented elsewhere in the Sydney-Bowen Basin. The Clyde Coal Measures contains a mud-dominated alluvial succession which drained axially, northward in inland subbasins.

The base of the Shoalhaven Group marks a major change in sedimentation within the basin. High-energy braidplain systems (Yadboro and Tallong Conglomerates) were initiated from the west. These system fed coastal and shelfal deposits in the east (Yarrunga Coal Measures and Pebbley Beach Formation). This change marks the onset of the second thermal subsidence or embryonic foreland stage. Continued subsidence of the basin led to the deposition of the nearshore and shelfal deposits of the Snapper Point Formation.

The Snapper Point Formation consists of five parasequences which combine to form an aggradational parasequence set. These parasequences are probably the result of fourth order eustatic sea level changes.

Facies in the Snapper Point Formation indicate that the nearshore zone was wave-dominated. This is suggested by the pervasive presence of sedimentary structures such as hummocky and swaley crossstratification. At the top of the final parasequence, however, there is a major facies change. Here, the facies are dominated by planar and trough cross-bedding and structures indicating wave deposition are less common.

This facies succession is interpreted as being deposited by northward moving strong longshore currents. The cause of this facies change may be the uplift and loading along the developing orogen. Emergence of the orogen above sea level would produce a constricted seaway, conducive for the development of strong tidal circulatory currents. At this time an open seaway probably existed within and along the length of the SydneyBowen Basin as indicated by the presence of marine facies in correlative formations in the northern Sydney Basin, Gunnedah Basin and Bowen Basin. This change also marks a major shift in the direction of sediment dispersal within the basin from transverse drainage to longitudinal drainage which is typical of more mature foreland basins. The foreland loading resulted in rapid subsidence, transgression and the deposition of the Wandrawandian Siltstone which is interpreted as a middle to outer shelf deposit. A tuffaceous layer within the Wandrawandian Siltstone also indicates that the orogen was probably emergent and active during this period.

A period of tectonic quiescence followed this foreland loading event and resulted in a regressive succession, culminating in the deposition of the Nowra Sandstone. Progradation of this clastic wedge may have been promoted by uplift and erosion of a forebulge to the west of the basin that was associated with the foreland loading event. The ravinement surface (Purnoo Conglomerate Member) within the Nowra Sandstone, marks the base of a suceeding transgressive sequence which culminated in the deposition of the Berry Siltstone. This transgression may be the result of rapid subsidence associated with a second foreland accretion event.

The foreland loading event which resulted in the transgression at the base of the Wandrawandian Siltstone indicates the onset of the Hunter-Bowen Orogeny which came to a climax during the Late Permian.

REFERENCES Tye, S.C., Fielding, C.R., & Jones, B.G., in press. Stratigraphy and sedimentology of the Talaterang and Shoalhaven Groups in the southernmost Sydney Basin. Australian Journal of Earth Sciences.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 CYCLICITY WITHIN THE EARLY PERMIAN PEBBLEY BEACH FORMATION OF THE SOUTHERN SYDNEY BASIN Stuart C. Tye and Kerrie L. Bann University of Wollongong, Wollongong NSW 2522

The Pebbley Beach Formation forms part of the Early Permian Shoalhaven Group in the southern Sydney Basin. Hie formation is a coastal-shelfal succession which is stratigraphically equivalent to the Yadboro and Tallong Conglomerates in the western part of the basin (Tye et al 1994). Detailed sedimentological and ichnological analysis of the exposed formation along the south coast of NSW in the vicinity of Durras has revealed a series of cyclical sequences which were probably the result of fourth order glacio-eustatic events. Deposition of the formation occurred at the tail end of the Gondwanan glacial period which climaxed during the Late Carboniferous.

The exposed coastal section comprises four cyclical successions which were in deposited in response to the alternation between two prevailing climatic conditions. Cycle thickness range from approximately 10 to 23 m. The cycles comprise two different types of facies associations. The two uppermost cycles form parasequences and contain evidence that sea level change accompanied climatic change. These parasequences are regressive half-sequences bounded by flooding or deepening surfaces.

Ichnofabrics and infaunal community tiering has been used to increase the resolution of depositional environment interpretation. This technique has been particularly useful in discriminating between facies in lower offshore to shoreface environments and in recognising potential hiatal surfaces.

The two lowest cycles of the exposed formation comprise two facies types. The base of each sequence consists of intensely bioturbated sandy siltstone which is interpreted as a lower offshore deposit, laid down close to or just below storm wave base. This interval is overlain by a facies consisting of interbedded diamictite and massive siltstone. This facies is interpreted as being deposited in a nearshore marine environment affected by seasonal and possibly perennial ice cover. Internally, these two cycles reflect changes from interglacial to glacial climatic conditions. The boundary between the two cycles is defined by a coarse sandy interval which may be a transgressive deposit.

The two uppermost cycles crop out at Point Upright and Mill Point and form parasequences. The base of each parasequence is marked by a transgressive ravinement surface which represents a deepening event. The surface which forms the boundary between the tidal facies of the uppermost parasequence and the offshore facies of the underlying parasequence is marked by an omission surface which is capped by a coarse sandstone horizon, containing articulated and disarticulated Eurydesma shells. This interval is interpreted as a condensed transgressive systems tract and may even contain the maximum flooding surface of the parasequence. The overlying interval is identical to that of the lower cycles, comprising lower offshore siltstone facies. This lower unit grades upwards into hummocky cross-stratified sandstone facies, interpreted as a shoreface facies. This, in turn, is overlain by a sequence of tide-influenced estuarine facies interpreted as being deposited within a wave-dominated estuarine environment. Five different facies have been delineated within this interval and are as follows: (a) intertidal channels, (b) tidal flats, (c) lagoon, (d) washover/flood tidal delta and (e) tidal inlet. Structures indicative of tidal action such as combined flow ripple lamination structures, heterolithic stratification, mud drapes, tidal bundles and bidirectional palaeocurrent distributions, are present within this sequence. This tide-dominated interval also contains glendonites which are a good indicator of cold climatic conditions. This regressive sequence comprising the offshore facies through to estuarine facies is interpreted as a highstand systems tract. Lowstand systems tracts are absent from the parasequences.

The cyclicity within the Pebbley Beach Formation, which has been outlined above, probably reflects glacio-eustatic generated sequences. The lack of age control within the sequence deems it extremely difficult to determine the periodicity of the cycles. Cycles with Milankovitch periodicity have been identified elsewhere in the Permian and would seem to be an explanation for the cycles within this formation.

REFERENCES Tye, S.C., Fielding, C.R., & Jones, B.G., in press. Stratigraphy and sedimentology of the Talaterang and Shoalhaiven Groups in the southernmost Sydney Basin. Australian Journal of Earth Sciences.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

P A L A E O P R O T E R O Z O I C DEFORMATION, METAMORPHISM AND IGNEOUS INTRUSION IN T H E C E N T R A L ZONE OF T H E LAMBOO C O M P L E X , H A L L S C R E E K OROGEN I. M. Tyler1 and R. W. Page 2 1

Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth, Western Australia, 6004 2 Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601

Large-scale fold interference patterns within the >1863 Ma mafic volcanic and metasedimentary rocks of the Tickalara Metamorphics provide the basis for the recognition of a consistent sequence of deformational and metamorphic events throughout the central zone of the Palaeoproterozoic Lamboo Complex in the Halls Creek Orogen. SHRIMP U-Pb zircon ages, together with geochemical data from felsic igneous rocks, indicate that these events belong to two distinct orogenies: the 1865 - 1850 Ma Hooper Orogeny; and the 1830 - 1800 Ma Halls Creek Orogeny.

The Hooper Orogeny affected only the Tickalara Metamophics. A minimum age for deposition is provided by an age of 1863±3 Ma for the Rose Bore Granite, a well foliated to gneissose, layer-parallel sheet-like intrusion, that also provides a maximum age for the earliest layer-parallel deformation, Dj. M2 metamorphism accompanied D2 and was low- to medium-pressure with grade increasing from greenschist facies in the south, to granulite facies in the northwest. The Panton layered mafic-ultramafic intrusion, dated at 1856±2 Ma, locally truncates tight to isoclinal D2 folding. It was intruded across M2 isograds, with medium-grade rocks exposed along its northwestern contact, while syn-D2 migmatitic rocks are exposed adjacent to its southeastern margin.

The attainment of peak conditions during M2 occurred later in the higher grade rocks to the north where zircons from an anatectic leucosome in granulite facies migmatitic gneiss give a SHRIMP age of 1852±2 Ma, consistent with ages o f - 1 8 5 0 Ma from pre- to syn-D2 monzogranite and tonalite sheet-like intrusions.

The intrusion of both mafic and felsic magma into the Tickalara Metamorphics occurred throughout the Hooper Orogeny, and was coincident with the development of the Bow River batholith in the adjacent western zone of the Lamboo Complex. Geochemical data suggest that the early sheet-like felsic igneous intrusions in the central zone were the products of subduction (Sheppard et al.5 1995). The mafic and felsic magmatism and accompanying metamorphism may reflect igneous underplating beneath a magmatic arc developed at an active plate margin. Subduction of oceanic crust was to the northwest beneath the Kimberley Craton.

The Halls Creek Orogeny affected the Tickalara Metamorphics, and the - 1 8 4 3 Ma felsic and mafic volcanics, and associated metasedimentary rocks of the Koongie Park Formation exposed in the southern part of the central zone. Both units were intruded by layered mafic-ultramafic bodies, and by felsic and mafic plutonic bodies of the Sally Downs batholith between 1830 - 1800 Ma. The - 1 8 3 0 Ma Mabel Downs Tonalite is well foliated and was intruded pre- to syn-D3, with accompanying medium-grade metamorphism (M3) increasing in grade towards the intrusion, and overprinting M2 assemblages. Kyanite overprinting sillimanite in metasedimentary rocks between the Mabel Downs Tonalite and the Halls Creek Fault, may have grown at this time, accompanying ductile shearing associated with thrusting. Deformation produced tight to isoclinal refolding of D2 structures. D3 fabrics were overprinted by migmatitic hornfelsic rocks within the contact aureole of the - 1 8 3 0 Ma Mcintosh intrusion, and widespread contact metamorphism occurred also within and adjacent to intrusions of the Sally Downs batholith. The Sally Downs Tonalite (1821 ±4 Ma) post-dated D3, providing a minimum age for this event, while D4, which produced large-scale, tight refolding of D3 structures, post-dated this intrusion and younger felsic bodies ( 1 8 1 0 - 1800 Ma) that intruded the Koongie Park Formation. The Halls Creek Orogeny represents the final collision between the Kimberley Craton and the North Australian Craton to the southeast.

REFERENCES Sheppard, S., Griffin, T.J., & Tyler, I.M., 1995. Geochemistry of felsic igneous rocks from the southern Halls Creek Orogen. Western Australia Geological Survey, Record 1995/4, 81 pp.

450

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CONTROLS ON GOLD MINERALISATION AT T H E GRANITES, TANAMI DESERT, NORTHERN T E R R I T O R Y Rick. Valenta and Vic Wall M.I.M. Exploration, GPO Box 1042 Brisbane, Qld 4001

Structurally-controlled gold mineralisation at the Granites occurs in a zone of anomalously high strain and high temperature metamorphism related to granites intruding a regionally metamorphosed package of pre-Barramundi Orogeny pelites, iron formations and volcanics. The host rocks form part of a stratigraphic package which is regionally extensive and broadly correlatable over much of northern Australia.

Gold mineralisation occurs mainly within hornblende hornfels facies iron formations and feuomagnesian schists in a broadly pelitedominated part of the stratigraphy. Most mineralisation occurs within the Host Unit, a stratigraphically distinctive iron formation with a consistently mappable internal stratigraphy.

The earliest structural element at the Granites is a layer parallel foliation with minor subvertically plunging minor folds and a strong subvertical stretching lineation. This is overprinted by one high grade event of minor folding and later minor retrograde folding. These structural elements are folded around the margins of the Granites Granite, which itself is foliated only at the margins.

Mineralisation in the area of the Granites defines layer-parallel bodies which are elongate parallel to the strong subvertical stretching lineation. On a smaller scale, mineralisation occurs mainly in and around layer parallel veins which are usually most closely spaced in the upper part of the Host Unit, near a shear zone at the contact between the Host Unit and the hangingwall metapelites. Higher grades of gold generally correlate with zones of: a) thicker Host Unit; b) higher vein density; c) higher sulphide content; and d) strongest calcic metasomatism.

Mineralised veins mainly formed synchronously with the S t layer parallel foliation.

Pseudomorphous replacement of metamorphic minerals in alteration rims indicates that mineralisation did not predate metamorphism. Mineralisation has been largely overprinted by later deformation, implying that the present geometry of mineralisation is different to the likely original geometry.

Primary fluid inclusions show trapping temperatures of 450°C to 550°C, and define both halite-saturated and halite-undersaturated populations. Oxygen isotope investigations suggest a fluid S 18 0 of 8 to 10%c, and 813C of 5 to -9%o. These observations are consistent with a mixed magmatic-metamorphic source for mineralising fluids.

The dominant local structural control in the Granites area appears to have been dilation on layering related to competency contrasts between the Host Unit and surrounding stratigraphy within a layer parallel shear zone which now shows a subvertical stretching lineation. The dominant chemical control appears to have been the potential for sulphidation of the Fe-rich host rocks, though the redox potential of surrounding graphitic units may also have played a role in localisation. In summary, gold mineralisation at the Granites is structuralcontrolled and chemically-localised during hornblende-hornfels facies thermal metamorphism. The kinematics of the formation and disruption of the early high strain zone, the mechanical contrast between the Host Unit and its pelitic envelope, and the chemical characteristics of the meta-iron formations are the key factors in ore genesis.

451

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

FLANK UPLIFT AND TOPOGRAPHY ADJACENT TO THE CENTRAL BAIKAL RIFT (SE SIBERIA): A TEST OF KINEMATIC MODELS FOR CONTINENTAL EXTENSION. Peter van der Beek1*2 institute of Earth Sciences, Vrije Universiteit, Amsterdam 2 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Rift flanks adjoining deep and narrow continental rifts are supported by flexural isostasy, as shown by the pattern of isostatic residual gravity anomalies associated with them. Models for flexural rift flank uplift differ in their kinematic description of lithospheric deformation, with respect to the role of simple-shear versus pure-shear and the importance of brittle versus elastic upper crustal deformation. We have tested different kinematic models of continental extension by comparing their predictions of rift flank topography and crustal structure with observation from the Baikal rift (SE Siberia), one of the largest neotectonic intracontinental rifts in the world. The Baikal rift is characterized by prominent rift flank topography on both sides of the lake basin. The flanks reach similar elevations but differ in their structure; the footwall flank being tilted away from the basin, whereas the basin-ward part of the hangingwall flank tilts towards the basin center. Geomorphological and sedimentological observations suggest that rift flank uplift commenced in the Pliocene. We present fission track data that indicate that very little erosion affected the flanks since rifting started. The major phase of denudation in the Baikal area coincided with Early Cretaceous orogeny related to closure of the Mongol-Okhotsk ocean. Thus, the present-day topography reflects rift-related tectonic uplift.

Pure-shear 'necking' and pure-shear/simple-shear detachment models of extension predict the topographic and Bouguer gravity anomaly patterns observed along a profile across the central Baikal rift equally well.

A mid-crustal (20 km) depth of necking and/or a mid-lower crustal (20-30 km) detachment level are predicted; best-fit elastic thicknesses are in the range 30-50 km. In contrast, predictions from a flexural cantilever model with low (<10 km) elastic thickness require significant (> 4 km) erosion of the footwall flank to fit the observations, which is incompatible with the fission track data. The inferred elastic thickness and depths of necking and detachment for the Baikal rift agree well with results from coherence studies of Bouguer gravity and topography, as well as with the rheology of the lithosphere underneath the rift, as deduced from thermal, seismic refraction and seismological observations.

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GEOLOGICAL SOCIETY OF A JJSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MAPPING SEISMIC STRUCTURE IN THE AUSTRALIAN LITHOSPHERE THE SKIPPY PROJECT Rob D. van der Hilst, Alet Zielhuis and RHan L.N. Kennett Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia

ABSTRACT The Seismology Group at the Research School of Earth Sciences is currently two-thirds of the way through a field-based project one of whose aspects is to map the three-dimensional shear wavespeed structure in the lithosphere and underlying mantle beneath the Australasian region. This study exploits the favourable configuration of the seismicity in the earthquake belts surrounding the Australian continent to build up a model of seismic structure using shear-wave tomography for records from a mobile array of field recorders. A set of up to 12 portable high-fidelity seismic recorders with a broad frequency range sensor are deployed for up to 6 months at a time, before being moved to a new location. Full continental coverage should be achieved at the end of 1996 after three and a half years of field deployments.

The first group of stations were deployed in Queensland, and the second deployment covered most of the rest of eastern Australia. Three-dimensional images have now been constructed from the data recorded at these arrays using waveform inversion for the shear-wave portion of the seismogram including the large amplitude surface waves. The resulting images show substantial variations in seismic structure in the Australian lithosphere. A zone of relatively low shear wavespeed occurs in the top 200 km beneath the Queensland coast which may well be associated with the Quaternary volcanism. The Mt. Isa block has a deep expression with high velocities extending to at least 200 km rounded by a relatively sharp contrast to the east. The southern New England fold belt is also marked by high seismic wavespeeds at depth which separate the low velocities along the Queensland coast from the major zone of low wavespeeds in the southern Tasman Sea (Figure 1).

130E -315m/s

140E

150E

160E

170E +315m/s

Im Vs.reference = 4.50 km/seC

Figure 1: Variation in shear-wave speed beneath the Australian region at a depth of 140 km from tomographic inversion of broad-band seismograms.

The patterns in the lithospheric structure can be related in part to the surface expression of the Tasman line separating Precambrian and Phanerozoic Australia but there is significant medium scale structure superimposed in the major trends.

The quality of the lithospheric images is improving as additional data is analysed and these seismic results provide a unique insight into the underpinnings of the Australian continent and the relation of regional features such as mineral belts to structures in the mantle beneath.

453

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 EARLY DEVONIAN MEGA-BRECCIAS IN A SILICICLASTIC TURBIDITIC SUCCESSION: IMPLICATIONS FOR THE DEPOSITIONAL SETTING OF SEDIMENTARY BASINS IN THE NEW ENGLAND FOLD BELT. Ken rick Van Noord

School of Geology, Queensland University of Technology, GPO Box 2434, Brisbane, Qld, 4001.

The Silverwood Group is a Silurian? to Early Devonian tectonostratigraphic terrane (Keinjan terrane) that lies within the northern part of the southern New England Orogen. Located approximately 140km southwest of Brisbane, Queensland, the Silverwood Group is a deformed volcano-sedimentary succession of calc-alkaline character. The Silverwood Group is postulated to have formed as an island arc-forearc basin succession within an oceanic-continental convergent plate margin during the late Palaeozoic era. The Group is located on the western hinge-point of a tight, regional antiform, which has placed the shallow basin and volcanic arc to the east of the Carboniferous accretionary wedge (Texas Beds). Unfolding of the antiform places the Group to the west of the Texas Beds, in a similar geographic position to the Gamilaroi terrane, with which it has been correlated.

The Silverwood Group is distincive because younging directions for the sequence are in an opposite sense to that expected if it were part of the Gamilaroi terrane.

Contrary to published work, the Silverwood Group is dominantly undeformed and split into blocks of coherent stratigraphy separated by thrust faults. Detailed mapping within one of these blocks has resolved the facies & facies architecture for part of the Silverwood Group (Rosenthal Formation) and outlined several inconsistencies in the model of formation for the sequence. The purpose of this paper is to detail some of the very coarse grained siliciclastic sedimentary facies present within the Rosenthal Formation and comment on the probable depositional setting in which these facies accumulated. An enigmatic feature of the Rosenthal Formation is the presence of a very thick mega-breccia conformably overlain by very thick, coarse siliciclastic turbidites and massive sandstones. A local unconformity marks the base of the mega-conglomerate which comprises large angular blocks (>100m long and over 50m wide) of matrix supported, synsedimentary deformed laminated chert, siltstone and massive sandstone in a matrix of poorly sorted, tabular pebble-cobble clasts. The unit has a lensoidal map expression and is over 300m thick at its widest, and up to 5km long. It is overlain to the east by a sequence of interbedded, very thickly bedded classical turbidites with well sorted pebble-cobble conglomerate bases grading up over a 5 to 10m interval into massive, well sorted medium grained sandstone and subsequently into planar laminated mudstone.

The succession of very thickly bedded turbidites is punctuated by lenses of massive, well sorted medium to coarse grained litharenite facies between 250 to 300m thick, which form lensoidal map expressions between 2 to 3km wide in sympathy with the western mega-conglomerate. These units lens out along strike and become reduced in their lateral continuity up-sequence. This facies is thought to be an amalgamation of several very thick sandstone units deposited in the thalweg of submarine channels in an overall aggradational cycle. Where the sandstones lens out along strike, they give way to planar laminated very fine sand and siltstone, which form thick sections of slumped accumulations to the northeast of the amalgamated sands. The depositional environment suggested by this very coarse set of facies is a submarine-fan canyon in which the western-most mega-breccia forms the mass transport complex and base of the fan. The sharp and irregular unconformable contact over monotonous, thick, fine grained turbidites at the base of the unit suggests its initiation may have been triggered by sudden catastrophic movements within the nearby arc, with continued deposition possibly resulting from retrograde slumping of the canyon head. Subsequent facies such as the coarse turbidites are channel-filling units, which are punctuated by the massive aggrading thick sands. These facies appear to form the channel-fill succession for a channel-levee system, in which slump overbank facies are exposed laterally to the northeast.

What is interesting about the sequence is the anomalous thickness versus width (along strike) of the channel-fill succession. Compared to modern fans, a total channel-levee width in the order of over 1000km would be required to achieve the thickness of channel-fill present within the Rosenthal Formation. Such thicknesses of sediment are not achieved in present-day fans, which are up to 500m thick and 250km wide. The vertical and lateral arrangement of the facies is suggestive of one channel-levee complex. If the succession forms part of a forearc basin, then the basin was extremely short lived with peculiar depositional conditions, or else large parts of the mid and outer fan successions are missing and shingling of the channel-levee systems was absent. Clearly a forearc basin model cannot adequately explain all the facies and facies architecture within the Rosenthal Formation, therefore intra-arc and/or backarc basin models may be more appropriate depositional scenarios.

454

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE NEOPROTEROZOIC TO CAMBRIAN NAMA BASIN, SOUTHERN AFRICA, AND ITS PETROLEUM PROSPECTIVITY M. R. Walter and R. Swart Rix & Walter P/L, Murramarang Road, Bawley Point, NSW 2539 and School of Earth Sciences, Macquarie University, NSW 2109 NAMCOR, Private Bag 13196, Windhoek, Namibia 1

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The Nama Basin of southern Africa (Namibia, Botswana and South Africa) contains up to 10km or more of Neoproterozoic and Cambrian sediment. On the basis of the dating of the Gannakouriep dyke swarm most, at least, of the succession seems to be younger than 717±llMa. The Nama Group with its included soft-bodied metazoan fossils and Cloudina was deposited in a foreland basin setting resulting from the closing of the Damara Orogen to the north and the Adamastor Ocean to the west The Proterozoic-Cambrian boundary lies within the group. Beneath the Nama Group are poorly known sub-basins that are imaged in deep seismic records in Botswana and which in southern Namibia include two glacial units.

There is renewed interest in the petroleum prospectivity of the basin. Away from the tectonised northern and western margins of the basin the thermal maturity of the basin is moderate, apparently with much local variation resulting from the intrusion of mafic sills and dykes during the Mesozoic. There are numerous petroleum shows spread over a distance of about 300km., including abundant bitumen in joints in the sandstone of the Fish River Sub-group, oil in a fault breccia and in fluid inclusions in quartz crystals, bitumen and "cokelike" material from the Fish River Sub-group in Vreda-1, "bituminous material" in a water borehole, and a reported gas blowout in Berseba-1 in 1928.

To date sampling of potential source rocks is very limited, being concentrated on the unsuccessful exploration wells Maseltlhen Pan-1, Tses-1 and Vreda-1. In Botswana the Nama Basin (Nosop Sub-basin) contains at least 10km of sediment (at least 9km of which is Proterozoic-Cambrian). Almost none of this is known to outcrop and only 3km of the Proterozoic has been penetrated by the drill. There is a minimum of 7km of untested section. Much of this must extend into South Africa and Namibia.

The Nama Basin is the same age and is comparable technically with regions elsewhere that contain major petroleum occurrences and fields. New stratigraphic data allow relatively precise correlations with the petroliferous Neoproterozoic successions of Oman, Siberia and China. There are many similarities with the Centralian Superbasin of Australia. Equivalent successions also occur in the Etosha Basin north of the Damara orogen in Namibia and Angola, in the Passarge Basin of Botswana, and in the Sao Francisco Basin in Brazil.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE NEOPROTEROZOIC PALAEOGEOGRAPHY OF AUSTRALIA M. R. Walter* and J. J. Veevers* -^School of Earth Sciences, Macquarie University, NSW 2109

The Australian continent is a large segment of the Earth's crust and on it Neoproterozoic rocks are particularly abundant, making it a good place to elucidate the history of this eon. Within a framework of lithostratigraphy, sequence analysis and tectonic analysis, isotope chemostratigraphy and acritarch biostratigraphy allow analysis the Neoproterozoic stratigraphy of the continent in terms of four supersequences. Finer subdivision is possible in the Ediacarian ("Terminal Proterozoic").

We have selected eight time intervals for which to portray the palaeogeography of the Australian continent during the Neoproterozoic. Several intervals are temporally well resolved, and truly represent "moments" in geological history, while others are no more than a crude first attempt to portray the palaeogeography of the continent, because the studies required to allow precise correlations between basins are at an early stage. W. V. Preiss has attempted a much more detailed analysis of the palaeogeography of the Adelaide Basin ("Geosyncline") and we have adopted his interpretations for that region.

In Time Slice 1A (~800Ma) a sand sheet wedges out against several internal basement highs and against the granitic basement in the southern Georgina Basin. At the locations of such wedge-outs carbonates are interbedded with the sands. It is a mixed fluvial and shallow marine succession. Time Slice IB (~800Ma, the upper part of Supersequence 1) is characterised by interbedded stromatolitic carbonates and evaporites including halite and anhydrite. These were deposited in peritidal to very shallow marine settings. There is local mafic volcanism.

Time Slice 2A (~700Ma) is characterised by the Sturtian glaciation, the deposits of which are limited in extent.

Time Slice 2B (~700Ma) is assumed to encompass a major eustatic rise in sea level following de-glaciation, resulting in widespread deposition of a thick succession of silt and mud. A shallow epeiric sea is envisaged. The succession shallows up and in places there are peritidal carbonates and sands.

Time Slice 3A (~600Ma) is defined by the second (Marinoan) glaciation. Again it is assumed that all the glacial sediments are approximately coeval. Diamictites are not known from the Georgina Basin, nor certainly from the Officer Basin, and elsewhere are patchy in their distribution. Arkoses, conglomerates and arkosic sands are more widespread, and are interpreted as glacial outwash deposits. A major eustatic rise in sea level (Time Slice 3Bf ~600Ma) followed de-glaciation, resulting in the deposition of a thick succession of silt and shale. A shallow epeiric sea is envisaged. The Musgrave Block soon became emergent and shed coarse sediment to the north and south. The supersequence shallows up to peritidal carbonates including extensive ooid and intraclast shoals; evaporites occur in the southern Georgina Basin (Time Slice 3C, ~580Ma).

Time Slice 4 (~580-550Ma) began with a flooding event during which turbiditic sands were deposited locally, and coarse sediment was again shed from the Musgrave Block. Most of the southern part of the superbasin and the central Amadeus Basin may have been emergent. Fluvial and shallow marine deltaic complexes developed extensively in the north. There was extensive basaltic flood volcanism in the western Officer Basin and in northern Australia (though the ages of these units are poorly known). What seems to have been a eustatic fall in sea level occurred near the time of the Proterozoic-Cambrian boundary, exposing most of the superbasin to subaerial conditions. Fluvial and shallow marine deltaic conditions were re-established in the Early Cambrian.

Laurentia, which adjoined Australia on the east, shared with Australia a) a mafic volcanic event at 800-780Ma, b) an early glacial event, poorly dated at 700Ma, c) a late glacial event, at about 600Ma and d) continental breakup in the earliest Cambrian (544Ma). All except the 800Ma event can be related to supercontinent tectonics.

456

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

FROM BROKEN HILL TO CANNINGTON -KEY ELEMENTS IN THE DISCOVERY OF A NEW BROKEN HILL TYPE AG-PB-ZN DEPOSIT IN THE EASTERN SUCCESSION OF THE MOUNT ISA INLIER Stephen Walters BHP Minerals, 152 Wharf Street, Brisbane 4000

The Broken Hill Main Lode represents a premier example of a world class base metal deposit, with an estimated pre-mining and erosion geological resource in the order of >250 Mt grading around 10% Pb, 8% Zn and 150 g/t Ag. However, despite over 110 years of continuous mining, exploration and active research, there have been no significant new economic discoveries in the Broken Hill Block.

The Cannington Ag-Pb-Zn deposit was discovered by BHP Minerals in 1990, and shows strong similarities with the Broken Hill Main Lode. Cannington is located in the Eastern Succession of the Mount Isa Inlier, and is concealed beneath 10-60 metres of recent and Cretaceous cover. The current economic resource is 45 Mt averaging 13.40/o Pb, 5.60/o Zn and 640 g/t Ag, and when full underground production commences, Cannington will be one of the worlds largest Ag producers. The discovery of Cannington can be directly attributed to area selection and exploration models developed mainly in the Broken Hill Block during the early 1980's, and the eventual successful outcome of these strategies in the Eastern Succession represents the results of almost a decade of systematic application.

The Broken Hill and Cannington deposits are hosted by migmatitic quartzo-feldspathic gneisses and in both cases, mineralisation has undergone a long and complex prograde and retrograde metamorphic, metasomatic and structural history. Despite over 100 years of debate and research, there is still no general consensus on genetic models. Unlike other more widely recognised and better defined base metal classifications, the concept of a distinct 'Broken Hill Type' is a contentious issue.

Mineralisation at Broken Hill and Cannington is associated with a diverse and unusual package of Fe-Ca-Mn-Si rich 'skam-like' mineralogies, which includes bustamite, pyroxmangite, carbonate, knebelite, fluorite and hedenbergite. A wide variety of prograde and retrograde Fe-Mn garnet rich rocks are intimately related to mineralisation, and also occur as part of an incipient but large scale garnet-sillimanite-K feldspar bearing alteration halo in the host migmatitic gneisses. Laterally extensive 'exhalite markers', define prospective regional horizons and include lensoid quartz-gahnite units and sub-ordinate quartz-magnetite BIF's. The Cannington deposit is also associated with significant magnetite-rich ore zones.

A key component in developing an empirical 'BHT' exploration model, was the provision of detailed regional mapping of the Broken Hill Block by the New South Wales Geological Survey in the early 1980's. This provided a coherent lithostratigraphic framework for understanding the distribution of mineralisation and defining regional geophysical signatures, and was the impetus for a number of in-house research projects.

An empirical exploration model was developed on the basis of these studies, and applied systematically to all prospective Proterozoic terranes in Australia. A key component of this strategy given the complex and unusual lithoiogies associated with mineralisation, was a re-examination of all known prospects and mineral occurrences in these terranes. Resistate phases related to these unusual mineralogies, was also a useful regional evaluation tool. This reconnaissance phase quickly identified the Soldiers Cap Group in the eastern Mount Isa Inlier, as rating a top priority. Due to tenement pressure in the mid-1980's, initial exploration was focused on the poorly outcropping and under cover extensions of these prospective sequences.

Although the Broken Hill Main Lode is non-magnetic, initial discoveries of large but sub-economic BHT systems in the Soldiers Cap terrane, indicated a strong association with significant magnetite rich zones. High quality regional aeromagnetics was the key exploration technique in guiding systematic exploration in area of thick Cretaceous cover, both in the definition of prospective lithostratigraphy and as a direct targeting tool.

457

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

AGE CONSTRAIN ON THE SUPRACRUSTAL SEQUENCE IN THE MURCHISON PROVINCE,YILGARN BLOCK — A NEW INSIGHT FROM THE SHRIMP ZIRCON DATING O. Wang. I. H. Campbell and L. Schi0tte* Research School of Earth Sciences, Australian National University, Canberra, ACT 0200

The supracrustal sequence in the Murchison Province, the Murchison Supergroup, consists of two groups, the lower Luke Creek and the overlying Mount Farmer Group. The Luke Creek Group, which is thought to be continuous throughout the province, consists of four formations. From the bottom to the top, these are the Murrouli Basalt, and the Golconda, Gabanintha and Windaning Formations (Watkins & Hickman, 1990). This group is thought to be about 10 km thick and consists mainly of tholeiitic and high-Mg basalts, BIF, with minor komatiites and felsic volcanic rocks. The Mount Farmer Group, which is interpreted to be up to 5 km thick, consists of mainly high-Mg and tholeiitic basalts, felsic volcanic and sedimentary rocks.

Sixteen supracrustal rocks from the Murchison Province have been dated: four from the Yalgoo-Singleton greenstone belt and twelve from the Meekatharra-Wydgee greenstone belt. Three samples from the Gabanintha Formation in the Yalgoo-Singleton greenstone belt give an age of ca. 2.95 Ga in agreement with previous age determinations (Watkins & Hickman, 1990). A ca. 2.8 Ga age has been dated for the Windaning Formation which overlies the Gabanintha Formation, in the Yalgoo-Singleton greenstone belt.

Six samples of the Windaning Formation from the Meekatharra-Wydgee greenstone belt also give ca. 2.8 Ga ages. Furthermore, six samples from the Gabanintha Formation in this greenstone belt, which according to Watkins & Hickman (1990) underlies the Windaning Formation, have ages of less than 2.8 Ga. What have been mapped as the Gabanintha Formation in the Meekatharra-Wydgee greenstone belt can not be correlated with the Gabanintha Formation in the Yalgoo-Singleton greenstone belt. There appear to be serious problems with the stratigraphy of the Murchison Province as proposed by Watkins & Hickman (1990). Our data show that there are at least three greenstone sequences in the Murchison with ages of approximately 2.7, 2.8 and 2.95 Ga, not two as had previously been thought. The tectonic and stratigraphic relationships between these sequences are not yet certain.

REFERENCE Watkins, K.P., & Hickman, A.H., 1990. Geological evolution and mineralization of the Murchison Province Western Australia. Geological Survey of Western Australia Bulletin 137, 267 pp.

* Deceased

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

RECONSTRUCTION OF THE LAST 300 KY FIRE HISTORY OF THE AUSTRALIAN CONTINENT FROM A DEEP SEA CORE OFF NW AUSTRALIA Xuan Wang*. Michael Bird^. Sander van der Kaars*, Peter Kershaw*, Paul Bishop* 1 Department of Geography and Environmental Science, Monash University, Clayton 3168, Victoria 2 Research School of Earth Science, Australian National University

The marine piston core G6-4, recovered from a water depth of 3500 m south of the Sumba Island off NW Australia, has been analysed for elemental carbon (EC) derived from the terrestrial biomass burning. A new technique developed at the Australian National University[l] allows us to analyse elemental carbon accumulation rates and carbon isotopic compositions in as little as 4-5 gram dry sedimentfromdeep sea cores. A 300 ky fire history for Australia and the adjacent islandsfromthe core site has been reconstructed. Thefirerecord presented here is the first of its kind in this area.

Core G6-4 is situated in the monsoonal belt, on the down-wind dust path from the Australian continent. The pollen record from the same core indicates that approximately 80% of the pollen was derivedfromAustralia [2].

The elemental carbon content is generally correlated with the dust content and the grassland pollen percentage.

The biomass burning record inferredfromthe EC accumulation rate therefore primarily reflects the fire history in Australia, which may have been closely associated with Australian continental aridity.

The abrupt increase in the EC accumulation rate at the top part of the core may be the result of the anthropogenic burning in Australia. The exact age of this human impact reported in the top part of the core is not yet known. Clarification of this by the oxygen isotopic chronology and AMS C14 is planned. Burning in the adjacent forest-dominated islands may be indicated by EC with very low 813C values (-27 to -26 per mil).

These low values seem to be particularly associated withwarm phases when continental erosion waned and atmospheric circulation weakened.

The variations in the carbon isotopic compositions of the EC suggest that the types of burnt vegetation followed a cyclical pattern: a cold phase always started with the burning of C3-dominated (woodland) vegetation and ended with burning of C4-dominated (grassland) vegetation, the reverse characterised warm phases. The Hq/12q ratios of the EC in both the warm and cold phases increased towards the top of the core, indicating a gradual shift to more C4-dominated (grassland) vegetations overall, except where anthropogenic burning and local input occurred.

This trend suggests an increasing aridity in the Australian continent over the last 300 ky. This agrees with the grassland expansion recorded in the pollen diagram.

REFERENCES: [1] Bird, M. 1995, Techniques for Elemental Carbon isolation from sediments, in preparation.

[2] Van der Kaars, W. A., 1991, Palynology of eastern Indonesian piston-cores: A Late Quaternary vegetational and climatic record for Australia, Palaeogeography, Paleoclimatology, Palaeoecology, 85:239-302.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

DRAFT TECTONO-STRATIGRAPHIC MAP OF THE ARUNTA BLOCK, CENTRAL AUSTRALIA R.G. Warren Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

Regional mapping by BMR/AGSO and NTGS has now covered about 2/3 of the Arunta Block. This provides regional information 011 the distribution of lithologies in the exposed part of the block. Geochemical data, mainly for felsic igneous rocks, is spread unevenly across the Block, and is supplemented by a very limited number of age determinations available from the literature. By assessing all available information a draft map at 1:500 000 and accompanying text has been prepared.

The Arunta Block is not a tectonic entity. The NE Arunta Block, approximately the area east of Anningie and north of the Delny-Mount Sainthill structure is best considered as the SW margin of the Tennant Creek Inlier.

The extreme E may be SW Mount Isa Inlier. Older basement may be exposed in the NW. A subduction-style environment may have existed in the period ending at about 1880 Ma, along the southern margin of what is now known as the Arunta Block. However, models based on granite tectonics (outcome of plume tectonics, see Warren & Ellis, this volume) apply to a substantial part of the development of the Arunta Block.

A series of granite suites has been delineated, using the geochemistry of the granites, linked to an appreciation of their distribution and setting. Some key suites (e.g., the Marshall Granite and related volcanics, the Barrow Creek Granite) have no age data, and some areas, especially the extreme east and west have no data. However, by extrapolating such geochemical and age data as are available, the evolution of the Arunta Block has been resolved into tectonostratigraphic packages developed in discreet time-slices. Inserts show the development in each time slice. The text reviews the supporting evidence for this summary (without references):TIME SLICE ENDING CIRCA 1880 Ma Turbidites in the NW Arunta Block and S-type igneous activity at 1880Ma. Volcanics, sediments, and intrusions with subduction-related signatures in the SE. These probably extend W in the Southern Province. Minor Cu, Au in the SE, possibly Cu in the Glen Helen area.

TIME SLICE 1880-1850 Ma Shallow water sediments in the NE Arunta Block: Bullion Schists, Delmore, Cackleberry Metamorphics, probably Alcoota-Mt Johnson and Marshall Bar districts. The Alarinjela Granite Complex extending from NW of Barrow Creek to W of Jervois. Perhaps sediments and granites in extreme E. TIME SLICE 1850-1820 Ma Granite intrusion at 1820 Ma:-Mt Stafford Beds Harverson Suite in NW. Perhaps Barrow Ck, Wooodgreen, Mt Ida, Marshall Granites, Mascotte Gneiss in NE, Dobbie Suite extreme E. TIME SLICE 1820-1760 Ma Major period of crustal growth: sediments in N and NW Hatches Creek Group and equivalents, Napperby, ?Barrow Ck Granites, Bonya Metamorphics, Jervois Granite, ?Jinka Granite.

Strangways and Narwietooma Metamorphic Complexs, reworking in SE, VMS Jervois, Strangways Ranges.

TIME SLICE 1760-1730 Ma Harts Range Group, 1730 Ma granites, possibly volcanics in southern zone TIME SLICE 1730-1650 Ma Granites, ?volcanics in Glen Helen Metamorphics, possibly crustal growth to W. TIME SLICE 1650-1580 Ma Crustal growth in south-central area: Ivvupataka Metamorphic Complex, fractionated unfractionated granites. Intrusions in NW, ? igneous activity in W Arunta Block.

TIME SLICE 1200-1000 Ma Mordor Igneous Complex, Teapot Granitoid,pegmatites ?granites in west: Kintore, Mount Webb (?related to Giles-Tollu igneous activity in the Musgrave Block).

MAJOR REWORKING : Aileron. Redbank, Wallaby Knob deformed zones are shown as overprints.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

METAMORPHIC PTt PATHS AND GRANITE TECTONICS R.G.Warren and D.J. Ellis Geology Department, Australian National University, Canberra ACT 0200

The vertical and horizontal reworking of the crust known as granite tectonics is initiated by the influx of hot mantle-derived mafic magma into the lower crust. Bouyant granite melts form in the lower crust, coalesce through lateral migration, and rise into the upper crust. In regions between rising granite, cooler, denser material sinks to replace the granite, and at the surface, rim synclines develop. These are filled by material eroding off the region above the rising granites and by co-magmatic volcanics. The tectonic regime is best described from the Lachlan Fold Belt (e.g., Chappell et al. 1987) which might well serve as the reference area.

Experimental modelling (Ramberg 1982) of gravity-tectonics (encompassing both salt tectonics and granite tectonics) can be used to show vertical and horizontal movements in the crust as granite magma coalesces and ascends (Fig. 1), and from these, PTt paths for regions within the crust can be derived (Fig. 2). As granite rises, Zone B under the rim synclines moves down and heats up, producing an anticlockwise PTt metamorphic path. At depth, Zone B merges with, or is dragged into Zone D, where migmatites and granulites form by dehydration melting and contribute to the granites. Migration of melt towards the regions of granite upwelling creates the horizontal fabrics which indicate extreme extension and are characteristic of most granulite terrains. Experimental modelling shows that Zone C, adjacent to the rising granite, may be dragged upwards, and together with refractory material in the granite, experience pressure-decrease at near-peak temperature (clockwise PTt path). At upper crustal levels (Zone A), heat from the granite produces andalusite- to sillimanite-bearing assemblages, and lateral spreading of the granite causes local compression and folding of the rim synclines. Granite tectonics is driven by the need to minimize gravitational potential energy, so involves little uplift and no mountain-building. The crustal upheaval ends in near-isostatic equilibrium, and all zones cool from peak temperatures isobarically. Later, unrelated, tectonic activity must occur if deeper crustal levels are to be exposed.

REFERENCES Chappell, B.W., White, A.J.R. & Wyborn, D., 1987. The importance of residual source material in granite pedogenesis. Journal of Petrology 28, 111 1-1138.

Ramberg, H., 1982. GRAVITY, DEFORMATION AND THE EARTH'S CRUST (2nd edition) Academic Press, London 452 pp.

T] Figure 1. Stages in gravity tectonics (adapted from Ramberg, 1982), showing changes in crustal profiles during upwards migration of granite. Letter symbols, A,B,C and D trace changes in depth for specific regions. Note the extension (thinning) in region D.

Temperature Figure 2. PTt paths deduced for the regions in Figure 1. The dashed line shows the steadystate geothermal gradient before and after decay of the perturbation from a major influx of heat into the lower crust (mantle plume).

In the one tectonic episode, counterclockwise and clockwise PTt paths form.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PALYNOSTRATIGRAPHICAL INVESTIGATION OF NEOGENE SEDIMENTS, RELATED TO TECTONISM, PAPUAN FORELAND BASIN, NEW GUINEA. Helen K. Wateihouse Department of Geology and Geophysics, University of Western Australia, Nedlands, W.A., 6907

Palynostratigraphical investigation has been undertaken of uppermost Miocene and Lower Pliocene marine sequences of a 2373 metre sedimentary section of the Puri Anticline in the Papuan Foreland Basin, New Guinea. Reworking of Cretaceous and Palaeogene calcareous nannofossils has been recognised (pers. comm.

R.W. Howe, 1995) which reflects the stratigraphic units being exposed and eroded during that time in the New Guinea Highlands. In addition, variations in foraminiferal assemblages have allowed recognition of four cycles of bathymetric change from bathyal to neritic (Haig & Medd, 1996).

Reworked palynomorphs, in particular dinoflagellate cysts, are currently under investigation. These indicate significant reworking of sediments of various ages ranging at least as far back as Cenomanian times, derived from the rapidly rising New Guinea Highlands. The relationship of this reworking to the bathymetric cycles will be presented.

Ultraviolet (UVB) fluorescence is emitted due to both the original (biochemical) fluorescence of organic molecules and any derived (thermochemical) fluorescence due to thermal maturation. In palynological samples, biochemical fluorescence can be extremely useful in the recognition of reworking, as the shorter wavelengths of emitted light are gradually lost over time, leaving characteristic fluorescence spectra for particular times in the past, as far back as the Cretaceous, when almost no biochemical fluorescence remains.

In the samples studied, both the palynomorphs and other organic particles display a wide variety of UV colours , indicating a complicated pattern of reworking episodes. In addition, some palynomorphs display fluorescence colours contrary to those expected for their ages, possibly indicating thermochemical fluorescence.

In addition to the investigation of reworking, contemporaneous palynomorphs and palynofacies assemblages are under investigation. Pollen and spore assemblages can be used in palaeoenvironmental reconstructions to interpret climatic variations and geographic and altitudinal positions of the plants from which they were derived. Palynofacies analysis is the study of all the organic particles present in a palynological sample. It is used in palaeoenvironmental reconstructions and can provide information on processes such as bottom water oxygenation, terrestrial runoff, climate changes, and characteristics of terrestrial vegetation. Absolute particle abundances are employed (by "spiking" the samples with exotic spores) in addition to the usual relative abundances. This increases the amount of information available from which to make interpretations and can provide valuable insight into the study of processes whose effects could not otherwise be isolated from one another. For example, the influences of marine and terrestrial processes on the resulting palynological assemblage can be studied separately.

These contemporaneous fossils are derived from the land flora of the rapidly rising New Guinea Highlands, and display considerable variations between samples through the section. Variations in the parameters mentioned may reflect changes in altitude of the orogenic belt, and therefore provide important data to complement the study of the reworked palynomorphs.

REFERENCES Haig, D.W. & Medd, D., 1996. Latest Miocene to early Pliocene bathymetric cycles related to tectonism, Puri Anticline, Papuan Basin, New Guinea. Australian Journal of Earth Sciences (in press).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention., Canberra, February 1996

Q U A N T I T A T I V E PALYNOFACIES ANALYSIS O F JURASSIC CLIMATIC CYCLES Helen K. Waterhouse University of Southampton, U.K. (current address: Department of Geology and Geophysics, University of Western Australia, Nedlands, W.A., 6907)

It was recognised long ago that variations in the Earth's orbit cause near-regular oscillations in the amount and distribution of solar radiation, or insolation, received on Earth and hence lead to climatically controlled environmental changes which may be preserved in the sedimentary record - "Milankovitch Cycles". These cycles may manifest themselves in a variety of ways, including cyclicity in sediment deposition, fossil abundances in sediments, oxygen isotope data, evaporite deposition, aeolian grain size, banded iron formations, and many other geologically preservable characteristics.

Palynofacies analysis is the study of all the particulate organic matter contained in a sediment, including marine plankton and the pollen, spores, cuticle and wood iragments from terrestrial vegetation. It can provide considerable useful palaeoenvironmental information. The use of both relative and absolute palynofacies particle abundances provides differing but complementary data. This allows the distribution of different groups of palynofacies particles with different provenances to be studied in isolation, thereby obtaining information about the separate reactions of the marine and terrestrial environment to any single overall control. High-resolution sampling techniques have allowed detailed interpretations of the palynofacies and, hence, palaeoenvironmental variations within cycles to be made. This is a necessity if cycles are to be interpreted in terms of palaeoenvironmental changes, and is a method which has been little used in the study of such cycles.

A study of four Jurassic sedimentary sequences in southern England and northwest France has revealed that the palynofacies of the sediments varies in a cyclic manner, often with wavelengths equivalent to, and in phase with, any lithological cyclicity present. In addition, the palynofacies records cyclicity in an apparently uniform sedimentary sequence. Palynofacies cycles have also been found, in a cyclic sequence, that are out of phase with the sedimentary cycles.

The palaeoclimatic and palaeoecological characteristics of the cycles interpreted using palynofacies characteristics, and the attempts at estimating the cycle durations, have allowed the main palynofacies cycles to be interpreted as the palaeoclimatic record of orbital forcing by the Obliquity (41ka) cycle. In addition, secondary cycles were identified in all sections at approximately half the duration of the major cycles. These have been interpreted, by the palynofacies characteristics as well as the wavelength ratio between the two cycles, as the effects of the Precessional (21ka) orbital cycle.

It is thought that the sampled locations were in such a palaeolatitudinal position during the Jurassic that they were under the influence of both the Obliquity and Precessional cycles, which normally have their greatest influence in high and low latitudes respectively. It is further proposed that, in the strati graphic sections studied, the Obliquity cycle had its greatest influence on the marine environment while the Precessional cycle mainly affected the terrestrial environment.

In addition to any insights which Milankovitch Theory may provide into the workings of climate, it has been suggested that the record of orbitally forced cycles in ancient sediments may allow dating of metronomic precision. High-resolution, quantitative palynofacies analysis provides a potentially very useful method of investigating the climatic controls on possibly orbitally forced sedimentary sequences.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 HYDRODYNAMIC RESPONSE OF A CLAY-TILL AQUITARD TO CHANGING POTENTIOMETRIC CONDITIONS IN AN UNDERLYING WATER-SUPPLY AQUIFER: IMPLICATIONS FOR SOLUTE AND CONTAMINANT TRANSPORT Tamie R. Weaver , John A. Cherry , and Shaun K. Frape School of Earth Sciences, University of Melbourne, Parkvilie, VIC 3052 Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada 1

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In many groundwater flow systems, aquitards are relied upon to protect water-supply aquifers from contamination. In the study area in southwestern Ontario, Canada, a 1-3 m thick glacial water-supply aquifer underlies a 40 m thick sequence of clay-rich glacial till. A waste disposal facility has been constructed in the upper 6 m of the clay-till aquitard. The clay-till has a very low hydraulic conductivity (5xlO' mIs) and acts as an aquitard. It has effectively isolated the underlying aquifer from the surface since its depositions about 10 ka.

The upper 6 m of the clay till is weathered and fractured; however, below about 12 m depth fractures are absent and solute transport is controlled predominantly by difiiision (Desaulniers et al., 1981). At present, hydraulic gradients are downward through most of the clay-till aquitard, however, prior to development of the region for water and petroleum supply in the mid-19th century, hydraulic gradients across the entire glacial sequence were probably upward from the underlying bedrock formations (Weaver et al., 1995).

In this research, we identify the evolution of potentiometric conditions in the glacial aquifer that occurred in response to the development of the aquifer and underlying bedrock formations for groundwater supply and petroleum production. We use these data to determine the time frame over which changes in hydraulic head in the aquifer propagated through the overlying clay-till aquitard. Hydraulic-diffiisivity modelling indicates that the initial depressurisation of the glacial aquifer would have propagated through the overlying clay till within several decades. The downward hydraulic gradients present in the till today are, therefore, a recent phenomenon, occurring in the early- to mid-20th century. Since the 1970s, surface water pipelines have replaced groundwater as the major water supply, and hydraulic head in the aquifer has increased with declining groundwater extraction. The higher heads in the aquifer have already begun to propagate into the overlying clay-aquitard, producing hydraulic disequilibrium conditions. Evidence for this is seen in the lower aquitard where vertical hydraulic gradients reverse from upward to downward.

10

The upward hydraulic gradients that characterised the groundwater flow regime before development are likely to have controlled the distribution of natural solutes in the aquitard. However, it is critical to recognise that, even is such low-permeability systems, hydraulic gradients can be reversed on time frames of only decades. If any fractures or open or leaky boreholes were to penetrate the aquitard matrix, the rapidly changing vertical hydraulic gradients would control the direction and rate of solute and contaminant transport between these units along these features. Therefore, it is critical to understand the hydrodynamic response of aquitards to changing potentiometric conditions in adjacent aquifers. If hydraulic gradients are reversed, the direction of contaminant migration along fractures and boreholes will also be reversed. This has the potential to lead to very different scenarios for contaminant migration than would be predicted if hydrodynamic responses of aquitards to adjacent aquifers are ignored.

REFERENCES Desaulniers, D.E., J. A. Cherry and Fritz, P., 1981, Origin, age and movement of pore water in argillaceous quaternaiy deposits at four sites in southwestern Ontario: Journal of Hydrology, v. 50, p. 231-257.

Weaver, T.R., S.K. Frape and J.A. Cherry, 1995, Recent cross-formation fluid flow and mixing in the Michigan Basin, southwestern Ontario: Bulletin, Geological Society of America, v. 107, pp.697-707.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A MASS BALANCE STUDY O F ACID MINE DRAINAGE AT QUEENSTOWN, W E S T E R N TASMANIA. 1

9

John A. Webb 1 and Christian Garland^ * School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083.

-Aquahealth, University of Tasmania, Hobart, Tasmania 7001

Prior to 1994 (when a new company took over operations), the copper mine at Queenstown was a major source of acid mine drainage (AMD), which flowed directly into the Queen and then King Rivers. The AMD was derived from waste rock dumps, abandoned and flooded workings, and infiltration into the active underground mine through an abandoned open cut lying directly above.

Rainfall in the area is high (>2.5m per year), and frequent storms mean that stream flow variability is also high.

To calculate the average flow rates and metal concentrations in the stream water, the median was used. At least 4 years of monthly data were available for most sites, and some stations had continuously monitored flow meters.

Because not all sources of AMD leaving the mine site were monitored, a mass balance approach was used to estimate the relative contributions from the sources listed above. These calculations showed that dewatering of the active mine together with one set of waste dumps were responsible for the bulk of the metals in the AMD (particularly Cu).

When this study was carried out (mid- 1994), tailings from the ore processing were released into the Queen River (this practice has now ceased, and a tailings dam is in operation). The mass balance calculations showed that the tailings removed considerable amounts of metals from solution, probably via absorption onto ferric hydroxide coatings on the tailings grains. The tailings also partially neutralised the acidity, probably through dissolution of siderite and perhaps silicates.

Correlation of metal concentrations with stream flow showed that in streams close to the mine site, some metal levels (Cu, Fe, Zn, Al) were negatively correlated with flow. Presumably this was due to the rapid run off following rainfall, which diluted the AMD leaving the mine site.

Acknowledgements. This study is published with the permission of the new operators of the Queenstown mine, Copper Mines of Tasmania, and Gutteridge, Haskins and Davey Pty Ltd.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 LANDSCAPE EVOLUTION OF THE SOUTHEASTERN HIGHLANDS, AUSTRALIA John A. Webb and Paul B. O'Sullivan School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 Australian Geodynamics Cooperative Research Center School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 In the mid-Devonian the Tabberabberan Orogeny uplifted a large region in the southeastern corner of Australia.

These highlands were denuded through time, particularly during the Early Triassic in response to the HunterBowen Orogeny. However, isostatic rebound accompanying erosional unloading meant that in the Early Cretaceous there was still substantial relief in this area, as shown by the very coarse-grained Early Cretaceous gravels flanking the highlands.

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In the mid-Cretaceous (-95 Ma) the eastern margin of Australia changed from a convergent to a divergent plate boundary. This was accompanied by uplift of the highlands belt, from Eastern Tasmania to Northeastern Queensland. The uplift may have been the result of adjustments in the deep convection flow patterns beneath the margin, or igneous underplating. No matter the cause, uplift of the Eastern Highlands at this time increased the gradients of the westward flowing drainage systems, denuding the highlands (as shown by apatite fission track data) and delivering a thick Late Cretaceous sediment package to the offshore Ceduna Depocenter.

Rifting between southeastern Australia and the Lord Howe Rise began in the south -80 Ma ago, and progressively extended northward to the Qld-NSW border -50 Ma ago. The rifting created the coastal plain and escarpment flanking the Southeastern Highlands, as a result of down-to-the-east displacement along high angle east-dipping normal faults. A strong gravity gradient closely parallels the coastal escarpment along the eastern side of the Southeastern Highlands, and probably represents the onshore limit of crustal attenuation associated with the rifting.

Therefore, we propose that the escarpment of the Southeastern Highlands is a fault scarp, and therefore the coastal plain was not formed by scarp retreat. Tertiary headward erosion of streams flowing towards the coast has dissected the scarp to varying degrees, but it is still essentially straight.

The landscape has been stable on a large scale throughout the Tertiary, as is clearly demonstrated by very low rates of Tertiary denudation in the highlands (shown also by the fission track data), Early Tertiary basalts draped across the scarp, and Cretaceous lavas and mid-Tertiary basalts and marine sediments on the coastal plain.

However, throughout the Tertiary relatively small scale tectonism has affected the Southeastern Highlands, particularly in eastern Victoria. This tectonism may have been concentrated in the Miocene, when the Gippsland Basin records a substantial increase in sedimentation rate. The Tertiary faulting resulted in drainage disruption, river incision and limited displacement along major faults. The area is still active, as shown by recent earthquakes and displacement of apparently Quaternary gravels.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

MAJOR PROVINCES OF THE UPPER CRUST IN NORTHEAST AUSTRALIA FROM GRAVITY AND MAGNETIC ANOMALIES Peter Wellman Australian Geological Survey Organisation, P.O. Box 378 Canberra ACT 2601

Gravity and magnetic images have been made of the land and sea part of the northeast Australian continental area. The gravity map uses Bouguer anomalies for the land area, and free air anomalies from satellite geoid observations released 1995 for the sea area. The magnetic map has been compiled by digitising magnetic surveys for petroleum exploration over the sea area, and combining these with the existing Australian Geological Survey Organisation magnetic grid for the land area.

The resultant gravity and magnetic images depict the geophysical provinces making up the upper crust of the Australian continent (of Mesoproterozoic and Cambrian-Devonian ages), and extensive intra-plate igneous activity during the Carboniferous-Permian. Some of the new features are described below.

The most prominent new feature is an inferred boundary between basement provinces extending from Torres Strait 700 km southwest to northern Arnhem Land, approximately along the boundary between the Gulf of Carpentaria and Arafura Sea. The feature is an elongate, asymmetric, broad gravity high, with minor lows to the south. Magnetic anomalies are elongate and parallel to the boundary. Arafura Basin sediments occur north of the boundaiy, and they lap out near the boundary. The upper crust northwest of the boundary is interpreted to be younger than that to the southeast. This feature terminates at what appears to be a triple point in northern Arnhem Land, with the Batton Trough to the south, and a major basement gravity anomaly extending north.

Gravity and magnetic anomalies over the western and central parts of the Gulf of Carpentaria are generally small-amplitude, irregular, and not very distinctive. Geophysical domain boundaries may occur on the western and eastern margins of a gravity high covering the western part of the Gulf, and 50 km north of the SSW margin of the Gulf at the northern extent of northwest trending anomalies associated with the Murphy Tectonic Ridge. Two major ?mafic intrusives occur on the southwestern margin of the Gulf, one over the Sir Edward Pellew Islands, and one 70 km to the north. These intrusives uplift a widespread mid-crustal reflector, have a large gravity anomaly, and one has a large magnetic anomaly.

The 700 km long Mount Isa Geophysical Domain has a northern end near the coast of the Gulf of Carpentaria.

Collinear to the north is a major crustal feature extending along the eastern margin of the Gulf of Carpentaria the Kerr Weer Geophysical Domain. It is 700 km long, 150 km wide, and consists of a central broad gravity and magnetic low, with marginal, discontinuous, higher amplitude, gravity and magnetic highs.

The south and east margin of the Proterozoic crust is a band 50-100 km wide with characteristic gravity and magnetic anomalies. Within this band the Proterozoic crust has be modified by Phanerozoic intrusion and deformation.

There appears to be a concentration of Carboniferous-Permian igneous activity along a 1000 km long, 50 km wide, Townsville-Mornington Island Igneous Belt, and a 300 by 400 km area including Cape Weymouth and Torres Strait. However isolated activity is very widespread east of about 139°E. In this broad area the activity shows no apparent correlation with the Carboniferous-Permian continental margin. However the magnetic high of the Townsville-Mornington Island Igneous Belt is nearly continuous with the magnetic highs of the northern part of the New England Fold Belt, which are due to igneous activity along the adjacent convergent margin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra:, February 1996 SEQUENCE STRATIGRAPHY OF THE FICKLING GROUP, NORTHERN LAWN HILL PLATFORM A.T.Wells & R.W.Page 1

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'Australian Geological Survey Organisation,GPO Box 378, Canberra City, ACT,2601 A detailed lithological section was measured through the Fickling Group on the south flank of the Murphy Inlier from the top of the Peters Creek Volcanics. Closely spaced spectrometer readings over the better outcrops, combined with the lithological log, has provided an innovative approach to the interpretation of the sequence stratigraphy of the Group. The objective was to obtain ti better understanding of the depositional evolution of the northern Lawn Hill Platform and contribute to the regional sequence stratigraphic synthesis of the Mount Isa Basin.

The Fickling Group, a mixed quartz arenite, lutite and carbonate succession occurs in a strongly faulted, but gently folded inlier; the Group reaches a thickness in excess of 700 m, and was deposited on a stable platform (the Murphy Inlier) at the northern extremity of the Lawn Hill Platform. The age span of the Group is constrained by recently determined U-Pb SHRIMP ages from two of its formations, the Mount Les Siltstone (1640+/-7Ma), and the Doomadgee Formation (1613 +/- 5Ma). The Peters Creek Volcanics, Ptp2, (1726 +/- 2Ma ) unconformably underlie the Fickling Group, and thus deposition of the Group, together with intervening unconformities and periods of non-deposition, may span a period of up to 120 million years.

The Fish River Formation (-320m thick) is divided into four major sequences (A-D, Fig.l). An initial fluvial and alluvial fan sequence (A in Fig. 1, ~ 65m thick), deposited as an incised valley fill, shows a gradual upwards decrease in T log values indicating an increase in maturity of the sediments. This was a result of progressive erosion of the Peters Creek Volcanic provenance and consequent decrease in labile components. The intervals (3438,41-43m) showing relatively high T log readings correspond to coarse conglomerates rich in rhyolite clasts, and probably represent tongues of alluvial fan deposits. The succeeding sequence B (-65-214m, Fig.l) comprises an early aggradational marine succession of predominantly quartz arenite(~100m thick), which is characterised on the T logs by several fining up cycles. The stacked quartz sand intervals are probably upper shore face, with succeeding finer sediments formed by small cyclic changes of sea level and deposited in possibly tidal flats to subtidal environments. The top of each cycle is, in places, marked by stromatolites, all of which are terminated by small scale erosion surfaces. The upper part of sequence B is composed of a fining up unit of shale (~163-207m) formed during a major marine transgression, and showing a maximum flooding surface at about 205m. The upper 7 m of sandstone is interpreted as an incomplete, shallow marine, progradational package that marks the top of the sequence. A thin siltstone /shale unit above sequence B marks the commencement of the early, transgressive, part of a new sequence, C, the major part of which has been eroded beneath a major unconformity. The unconformity can be traced 15 km to the east where sequence D directly overlies sequence B. The overlying stacked quartz sandstones of sequence D (~100m thick) show a comparatively featureless motif and indicate a return to a predominantly aggradational upper shore face environment; an intermediate zone(~234-264m) is composed of higher energy conglomeratic sands bracketed by thin breccia zones with stromatolites and erosional bases. It may have originated as a tidal current sand body.

The remainder of the Fickling Group(~400m thick) is poorly exposed but unconformities in the succession indicate at least four sequences. An unconformity in the basal part of the Doomadgee Formation probably marks a sequence boundary. The sequence comprises shale of the Mount Les Siltstone deposited in a major marine transgression, followed by progradational sands of the basal Doomadgee Formation. Geochronological evidence suggests that the unconformity at the top of this sequence may represent a depositional break of 25 to 30 million years. An unconformity is probably present between the upper two units of the Doomadgee Formation.

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Fig.l

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This study has established a basic sequence stratigraphy for the Fish River Formation which can be extended and combined with other detailed sections in other parts of the Lawn Hill Platform to provide aframeworkfor sedimentary analysis in the Mount Isa Basin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOSCIENCE AND THE DESIGN OF EFFLUENT DISPOSAL SYSTEMS 1

Joe Whitehead1 and Phillip Geary2 Environmental Division, Department of Community Programmes, University of Newcastle, NSW 2308 2 Department of Geography, University of Newcastle, NSW 2308

Domestic wastewater disposal in unsewered communities contributes to a number of water management problems throughout the country. The poor performance of on-site systems for effluent disposal results from an inadequate understanding of the key characteristics of soils and site assessment, and the lack of suitable design guidelines which recognise the importance of land capability criteria for satisfactory wastewater disposal.

In September 1994 a new Australian Standard (AS 1547) dealing with disposal systems for effluent from small premises was released. This standard emphasised the importance of basic land capability criteria and other environmental considerations in locating on-site wastewater disposal systems.

A number of aspects of the Standard are relevant to geoscientists and geotechnical engineers who may now become involved in work which had previously been the preserve of Environmental Health Officers and Plumbers and Drainers. Ad hoc development of locally applied guidelines had resulted in great variability of approach from one Local Government Authority to another. Rules of thumb prevailed and were commonly handed down over the years. A number of surveys have indicated a high incidence of poor performance or system failure, commonly for lack of a scientific approach and a clear understanding of soil and site characteristics.

The new approach to design goes away from an ad hoc approach and is consistent with approaches taken elsewhere, for example in the USA and New Zealand, where, in many areas, a significantly higher proportion of the population live in unsewered premises.

Detailed site assessment is now required with site investigation involving geomorphic mapping and soils determination based on scientific principles. Soil textural classification is determined by field or laboratory methods and soil depth and permeability determined. Long Term Acceptance Rate (LTAR) for absorption systems and Design Irrigation Rate (DIR) for spray irrigation systems, based on permeability, are used to determine appropriate effluent loading rates. Consideration must be given to the risk of prejudicing groundwater supplies. Seasonal changes in groundwater level and absorption capacity must be taken into consideration as must climatic effects on evaporation and transpiration and the effect of seepage and surface water from higher levels on the disposal area.

There has been shown to be a poor correlation between permeability determined by a percolation test and by direct measurement using a permeameter, and considerable variation in measured permeability for soils which are nominally of the same type. Evidence shows that even with the approach of the new Standard an alarmingly high failure rate, commonly in more than 50% of systems, occurs.

There is a clear need for geoscientists with soils experience to be engaged in the determination of loading rates. Elsewhere, particularly in the USA, tables drawn up on the basis of soil textural classification are used to determine loading rates.

Design of disposal areas lends itself to a performance based assessment rather than a prescriptive approach due to the inherent variability in soils. Already the 1994 Standard, which takes such a prescriptive approach, is being revised as it has proved unsatisfactory. The new approach will be performance based, with systems being designed to avoid elements of unsatisfactory performance. A system will be deemed satisfactory if it is designed on the basis of site assessment and soil determination so as not to fail.

This new approach creates employment opportunities for geoscientists with the appropriate training in the areas of site investigation, disposal area and lot sizing, planning and in the provision of expert evidence and advice.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 GEOSCIENCE AND THE DESIGN OF EFFLUENT DISPOSAL SYSTEMS Joe Whitehead and Phillip Geary Environmental Division, Department of Community Programmes, University of Newcastle, NSW 2308 Department of Geography, University of Newcastle, NSW 2308 1

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Domestic wastewater disposal in unsewered communities contributes to a number of water management problems throughout the country. The poor performance of on-site systems for effluent disposal results from an inadequate understanding of the key characteristics of soils and site assessment, and the lack of suitable design guidelines which recognise the importance of land capability criteria for satisfactory wastewater disposal.

In September 1994 a new Australian Standard (AS 1547) dealing with disposal systems for effluent from small premises was released. This standard emphasised the importance of basic land capability criteria and other environmental considerations in locating on-site wastewater disposal systems.

A number of aspects of the Standard are relevant to geoscientists and geotechnical engineers who may now become involved in work which had previously been the preserve of Environmental Health Officers and Plumbers and Drainers. Ad hoc development of locally applied guidelines had resulted in great variability of approach from one Local Government Authority to another. Rules of thumb prevailed and were commonly handed down over the years. A number of surveys have indicated a high incidence of poor performance or system failure, commonly for lack of a scientific approach and a clear understanding of soil and site characteristics.

The new approach to design goes away from an ad hoc approach and is consistent with approaches taken elsewhere, for example in the USA and New Zealand, where, in many areas, a significantly higher proportion of the population live in unsewered premises.

Detailed site assessment is now required with site investigation involving geomoiphic mapping and soils determination based on scientific principles. Soil textural classification is determined by field or laboratory methods and soil depth and permeability determined. Long Term Acceptance Rate (LTAR) for absorption systems and Design Irrigation Rate (DDI) for spray irrigation systems, based on permeability, are used to determine appropriate effluent loading rates. Consideration must be given to the risk of prejudicing groundwater supplies. Seasonal changes in groundwater level and absorption capacity must be taken into consideration as must climatic effects on evaporation and transpiration and the effect of seepage and surface water from higher levels on the disposal area.

There has been shown to be a poor correlation between permeability determined by a percolation test and by direct measurement using a permeameter, and considerable variation in measured permeability for soils which are nominally of the same type. Evidence shows that even with the approach of the new Standard an alarmingly high failure rate, commonly in more than 50% of systems, occurs.

There is a clear need for geoscientists with soils experience to be engaged in the determination of loading rates. Elsewhere, particularly in the USA, tables drawn up on the basis of soil textural classification are used to determine loading rates.

Design of disposal areas lends itself to a performance based assessment rather than a prescriptive approach due to the inherent variability in soils. Already the 1994 Standard, which takes such a prescriptive approach, is being revised as it has proved unsatisfactory. The new approach will be performance based, with systems being designed to avoid elements of unsatisfactory performance. A system will be deemed satisfactory if it is designed on the basis of site assessment and soil determination so as not to fail.

This new approach creates employment opportunities for geoscientists with the appropriate training in the areas of site investigation, disposal area and lot sizing, planning and in the provision of expert evidence and advice.

469

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996 STRONTIUM ISOTOPE STRATIGRAPHY AND MASS EXTINCTION EVENTS: SOME PALAEOZOIC EXAMPLES David J. Whitford . Anita S. Andrew , P. Joe Hamilton , Ruth Mawson , Ric Morante' and John A. Talent' Australian Petroleum CRC, CSIRO Division of Petroleum Resources, PO Box 136, North Ryde, NSW 2113 CSIRO Division of Exploration and Mining, PO Box 136, North Ryde, NSW 2113 SchooI of Earth Sciences, Macquarie University, NSW 2109 1

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Major faunal extinction events in the Palaeozoic have been increasingly well recognized in the palaeontological record. Many of these events are also manifested by isotopic shifts in carbon, oxygen and strontium in the host rocks. The coincident isotopic anomalies not only provide information about the nature and cause of such events but also have the potential to provide precise time markers useful for chronostratigraphic correlation on a global scale.

The isotopic composition of strontium in seawater reflects the balance of inputs into the oceans from the mantle and crust. Mantle inputs are dominated by mid-ocean ridge processes whereas the crustal input reflects mainly riverine fluxes. Because the residence time of Sr in seawater is long compared to the oceanic mixing time, the oceans are well mixed with respect to Sr/ Sr. The uniformity of Sr isotopic composition of seawater at any one time, combined with its variation over time due to changes in the mantle- and crustally-derived fluxes, combine to make it a powerful tool for chronostratigraphic analysis. The isotopic composition of seawater through geological time has been inferred from the isotopic composition of Sr measured in well-dated limestones and other marine precipitates. Of critical importance in applying Sr isotope chronostratigraphy is the assumption that primary isotopic compositions have not been modified during diagenesis or other secondary processes.

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Talent et al. (1993) have distinguished nine and possibly as many as twelve events from the earliest Silurian to the Early Carboniferous that appear to have been global or near-global in extent. Different C isotope responses between the Silurian and Devonian events suggest fundamentally different environmental responses to the events which might reflect different causes (Andrew et al., 1994). From systematically collected limestone sections, some of the extinction events are characterized by anomalous Sr isotopic behaviour in whole-rock samples. Anomalous Sr/ Sr ratios often correlate broadly with offsets observed in 8 C and 5 0 profiles.

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The Sr isotope responses vary between different events and there is little consistent variation. Several sections are characterized by Sr/ Sr ratios higher than those reported in published seawater curves which may reflect either diagenetic alteration and / or a non-marine component in the primary isotopic signatures. In the latest Fammenian Hangenberg event, Sr/ Sr ratios increase sharply over the extinction event in several European sections, in one case mimicking the fine detail of the profile. The correlation between Sr and C isotopic composition is consistent with the limestones recording an abrupt but short-lived change in ocean chemistry.

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The minimum Sr/ Sr value on seawater curves in the Phanerozoic occurs during the late Permian. In Australia, that minimum is preserved in unaltered brachiopod shell material derived from the Bowen Basin.

The rapid decline in Sr/ Sr and equally rapid rise indicates major changes in ocean chemistry leading up to the end of* the Palaeozoic era. The dramatic changes in the Sr/ Sr value suggest significant alteration in continental and hydrothermal fluxes preceding the Permian/Triassic boundary mass extinction.

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REFERENCES Andrew, A.S., Hamilton, P.J., Mawson, R., Talent, J.A. and Whitford, D.J., 1994. Isotopic correlation tools in the mid-Palaeozoic and their relation to extinction events. APEA Journal 34,268-277.

Talent, J.A., Mawson, R., Andrew, A.S., Hamilton, P.J. and Whitford, D.J., 1993. Middle Palaeozoic extinction events: faunal and isotopic data. Palaeogeography, Palaeoclimatology, Palaeoecology 104, 139-152.

Acknowledgments: Isotopic work has been undertaken under the auspices of the Centre for Isotope Studies with financial support from ARC and CSIRO - Macquarie University Research Grants.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

AREA SELECTION USING REGIONAL GEOPHYSICAL DATASETS Tom H. Whiting BHP Minerals Exploration, PO Box 425 Spring Hill, Qld 4004

Exploration success is about being first with the right geological insights or techniques, in the right place. The rapid evaluation of prospectivity from a province down to prospect scale is an important element of this.

Historically this has involved the recognition of potential ore environments from geological maps and the cost effective exploration of these environments by appropriate techniques to discover ore. The techniques used both to map and detect mineralisation depend on the nature and maturity of the terrane. In Australia, the use of high resolution regional geophysical techniques are becoming increasingly important as exploration in traditional outcropping mineral provinces becomes more mature. This is true at both regional and prospect scales as extensive prospective terrane is covered by younger cover sequences. These provinces are consequently inadequately mapped, poorly understood and difficult to explore. Yet modern exploration now routinely and systematically explores these terranes to depths of up to 200 to 300 metres.

Regional geophysical datasets now play a key role in the process of how prospective ore environments are selected under cover and subsequently explored. There are common misconceptions as to how various discoveries under cover have been made. You will often hear simplistic comments to the effect that Cannington was a magnetic discovery or Olympic Dam a coincident gravity-magnetic discovery. The importance of conceptual geological thinking in both the area selection process and subsequent target identification and discrimination is commonly overlooked.

Over the past ten years we have seen great advances in the use of regional aeromagnetic data. Surveys have become larger and more detailed, giving us much more information over more extensive areas. Image processing techniques have dramatically improved visual display so that geologists can intuitively relate to the geological information contained in data allowing better interpretation. In areas of poor exposure these datasets offer by far the most coherent and detailed geological picture available. The challenge for geologists and geophysicists is to extract the maximum amount of geological information from these datasets. Once extracted, as with most geological maps, we can use our knowledge of ore genesis to predict possible styles of and sites for mineralisation. The final product of full interpretation of a geophysical image is a geological map that is both spatial and temporal. Rock types are identified, their distribution mapped, and a stratigraphy is constructed. The magnetic stratigraphy allows recognition of structures, that deform and control the distribution of rock packages. The relative timing of geological events can commonly be determined. By extracting information from aeromagnetic data we can use the geological framework obtained, along with concepts of mineralising processes and environments, to recognise prospective host rocks, structures, and geophysical signatures that can be directly or indirectly related to ore.

Different types of orebodies are associated with different structural, lithological and tectonic settings. We can assess the likely rock types that cause the responses we see in geophysical images through our knowledge of the relationship between magnetic mineral pedogenesis and rock magnetism. We also clearly map the expression of geological structures in aeromagnetic images. The mixture of these two things together, rock type (magnetic stratigraphy) and structural style (tectonic history), show gross patterns in aeromagnetic images. These patterns differ and are quite characteristic of different tectonic settings. This is a reflection of the tectonic processes operating at regional or continental scale and can be a guide to the age, mineralization and therefore the prospectivity of a province.

BHP was one of the first to develop the technology and expertise to fully interpret these datasets and successfully applied it to under cover exploration in the Eastern Succession of the Mt Isa Block. This contributed to the discovery of the Cannington deposit. We are, however, still at an early stage in developing our skills in geological mapping and mineral exploration of concealed terrains. The evolution of the exploration process is such that the boundaries between disciplines such as geology and geophysics are blurring (as they have in the petroleum industry) and to be successful in the future will depend on a multidisciplinary approach.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

INTEGRATION, RESIDUAL ANALYSIS AND MODELLING OF GEOSCIENTIFIC DATA USING GIS. John R. Wilford. Alun Hoggett and Dmitar Butrovski Australian Geological Survey Organisation. GPO Box 378 Canberra, ACT 2600

GIS has become a widespread tool for storing and manipulating spatial datasets. The results from integration and analysis of datasets from the Ebagoola 1:250 000 sheet area in North Queensland are used to demonstrate how GIS can be used as an interpretational and modelling tool for geological data. Datasets over Ebagoola were collected as part of the National Geoscience Mapping Accord (NGMA) and include: bedrock geology, regolith-landforms, rock chemistry, regolith chemistry, stream sediment chemistry, mineral deposits, airborne magnetics and gammaray spectrometries, drainage patterns, Landsat TM and digital elevation model (DEM).

Discrete (i.e. polygon, line and point) and continuous data (i.e. images) types can be readily combined or superimposed to generate integrated products for interpretation. For example, combining stream sediment data with regolith, bedrock geology, magnetics and gamma-ray spectrometries allows the stream sediment values to be interpreted in a weathering, geochemical and structural framework. This type of synergy can lead to new insights that would otherwise not be gained from looking at the datasets in isolation. Visualisation techniques including: colour and textural enhancements, symbols, perspective viewing and shadowing aid in the interpretation and understanding of complex relationships between different but commonly inter-related datasets.

Residual analysis involves subtracting one dataset from another or subtracting a modelled response from the original data. Residual techniques are useful in removing unwanted information and highlighting subtle features or anomalies in the data. For example subtracting average bedrock gold (Au) concentrations from stream sediment Au values show which catchments have elevated Au values compared with background bedrock concentrations.

Another residual analysis technique involves calculating mean image values within polygons of each attribute value (geology or regolith) and then subtracting the mean values from a corresponding imaged dataset (eg. gamma-ray spectrometries and magnetics). A residual image of airborne gamma-ray spectrometries using this technique highlights gamma-ray values which are either below or above the mean image value for polygons with the same attribute value.

Variations in the residual reflcct subtle differences in regolith and geology which are not apparent in the original image. These variations can be used to update die regolith and geological maps by further subdividing the polygons where necessary. Edge effects in die residual image typically correspond to mismatches between die polygon and gridded data and can be used to check and edit the polygon boundaries.

Modelling techniques can be used to generate prospectivity maps which highlight areas favourable to particular types of mineralisation or in mapping particular lidiologies or regolith types. Modelling is best done in the raster domain where a variety of techniques including; conceptual, generic and statistical modelling approaches can be implemented. The conceptual approach uses mineral deposit models to develop a set of criteria which can be used in the GIS to delineate a particular type of mineralisation. Generic modelling builds a selection set by characterising features from different thematic layers in die GIS at or near to known deposits and then locates other regions which meet the selection set. Statistical modelling is based on probability functions and typically incorporates conditional rules and fuzzy logic to generate prospectivity maps. We have used a generic approach on the Ebagoola 1:250 000 map sheet to delineate areas favourable for Au mineralisation based on geological, regolith, stream sediment geochemistry and geophysical datasets.

GIS provides an effective tool for integrating and analysing spatial data, and can be used to generate new derivative or residual images and maps. Modelling techniques using these derivative or residual datasets togedier with odier data can be used to generate prospectivity maps and test various geological models.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CRITICAL TECTONIC EVENTS IN THE DEVELOPMENT OF THE NORTH WEST SHELF J.B. Willcox and AGSO North West Shelf Study Group Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

An interpretation of AGSO's network of deep (largely 16 second) reflection seismic data from the North West Shelf, is an excellent tool to demonstrate the inter-relationship of the structural elements throughout the region, including in many areas the fabric of the Precambrian basement, and deep architecture of the Phanerozoic basins that comprise the Westralian Superbasin. Basin modelling and refraction controls on the crustal structure are being used to place constraints on the mode of basin evolution.

A number of critical tectonic events can be identified within basinal elements, which usually divide the sedimentary fill into well-defined tectono-stratigraphic units (or seismic megasequences).

Inter-basin correlations, based on seismic character ties, common geometrical and facies characteristics, and ties to petroleum exploration wells, indicate that many of these megasequences can be recognised throughout most of the North West Shelf. Thus, it would appear that development and reactivation of the North West Shelf basins has occurred in response to the same set of critical tectonic events, although the nature of that response has been dependent on the location and orientation of individual basins within the margin and on the relative movements of the underlying basement fabric. These events, which are probably the local manifestation of plate-wide or global events, are critical because of their impact on the distribution of organic-rich rocks, hydrocarbon migration, and trap formation and integrity.

The most important events influencing North West Shelf basin formation, deformation, and destruction are: • Early Cambrian volcanism (Antrim Plateau volcanics), marking the onset of intra-cratonic sedimentation throughout northern Australia; • Early to early Middle Devonian rifting of the outer margin of the northern margin of eastern Gondwanaland and development of the Lasseter Shear separating a compressional environment in central Australia and an extensional environment in western Australia; • Late Devonian - Early Carboniferous extension, on a NE azimuth, creating the Fitzroy Trough and Petrel Sub-basin, connected by a linked system of shear zones (Lasseter Shear Zone/ Halls Creek Mobile Zone in the east, and the 'North West Shelf Megashear' (NWSM) near the modern-day shelf-break to the northwest); • Mid-Carboniferous - Early Permian extension and thinning on a 7NNW-SSE azimuth, beginning at peak Alice Springs Orogeny time; this was largely concentrated northwest of the NWSM, initiating the Westralian Superbasin; separation of the Sibumasu Block from Australia and development of a continental margin setting; • Late Triassic - Early Jurassic transpressional reactivation, in response to northerly or north-northwesterly stresses. This resulted in relative movements within accommodation compartments in the Westralian Superbasin, and give rise to Early - Late Jurassic depocentres for source-rock and reservoir accumulation, juxtaposed with uplifted blocks such as the Rankin Platform; • Callovian and Valanginian margin breakup episodes, with the separation of 4Argo Land' and Greater India associated with extensive magmatism, and resulting in regional thermal subsidence and the development of margin-wide unconformities of variable significance.

• Collision along the northern margin of the Australian craton, commencing in the mid-Oligocene, and continuing through to the present day. This event produced ENE-WSW oriented compression, particularly in the north, and had major repercussions for hydrocarbon trap integrity.

Other tectonic events in the Late Permian (Bedout Movement), Middle Triassic (Ladinian), Late Jurassic-Early Cretaceous, Cenomanian, and Eocene, while not as readily identified margin-wide in the seismic data, have also had major effects on structuring, the development of regional unconformities, and the location and preservation of hydrocarbon accumulations on the North West Shelf. Identification and mapping of these events and the related seismic sequences throughout the largely under-explored basins in the region provides an important predictive tool for petroleum search. It could, for example, help focus exploration efforts into localities where the critical events have, by analogy, led to the juxtaposition of a source rock with reservoir and trap.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE NEWCASTLE EARTHQUAKE DATABASE: LESSONS FOR COMMUNITIES CONCERNED WITH NATURAL DISASTERS 1

Chris Williams1 and Aiita Lewis2 Library Manager, Newcastle Region Library, PO Box 489 Newcastle NSW 2300 Project Librarian, Newcastle Region Library, same address

Abstract The 1989 Newcastle earthquake shattered the myth that disastrous earthquakes do not occur in Australia. The event caused death and destruction - 13 people died, more than 100 people sustained injuries and over 10,000 buildings in Newcastle suffered modest to substantial damage with insurance pay outs exceeding the billion dollar mark.

The impact of the earthquake touched many facets of life and generated a mass of documentation as well as visual and oral records. These are all included in the Earthquake Database, which now consists of 3000 entries of published records, unpublished reports, photographs, oral history interviews in audio and video formats and primary data. More records are being added as they become available. The subjects covered in the database include earthquake engineering, seismology, geology, disaster management, insurance and other related topics.

Included also are human interest stories, the psychological, social and economic impact as well as literaiy works inspired by the event.

This information resource is an invaluable contribution towards earthquake and natural hazard mitigation. It provides lessons that are useful to other communities in Australia. The Newcastle Region Library is making the database and its textural resources available in full text electronically to users. It intends in the next few years to develop a multimedia CD-ROM that will enable users to access the textural information, view the images and listen to the audio recordings.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE FOURTH WAVE OF REGIONAL MAPPING - WHAT WELL IT BE? Neil Williams Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2600

Regional geological mapping is undertaken for many reasons. These include the pursuit of basic geoscientific knowledge, as well as the meeting of societal needs such as the discovery of mineral and petroleum resources, natural resource assessment, land-use planning, land management, and the mitigation of geological hazards such as earthquakes, volcanic eruptions, and landslides. > In Australia, prime responsibility for regional geological mapping rests at the national level with the Australian Geological Survey Organisation, and at the State/Territory level with the Geological Surveys of the six States and the Northern Territory.

Since geological mapping began in Australia in the mid nineteenth century, there have been three successive waves of activity, each of which has built on the previous wave, and each of which has been based on increasingly complex technology.

The first was the "boots and hammer" wave which began in the 1800's. This wave involved extensive field mapping which was often localised and detailed, with regional maps emerging through the slow and tedious manual compilation of local maps and data.

The second was the "air photo" wave that was pioneered by C.T. Madigan and W.G. Woolnough in the 1930's and which grew to prominence and matured after World War II when the Commonwealth and States began systematic regional mapping at a scale of 4 miles to the inch (later changed to 1:250 000). The air photo wave greatly enhanced the efficiency and effectiveness of "boots and hammer" mapping by providing field geologists with user-friendly base maps and a powerful tool for delineating regional-scale geological structure.

The third wave is the "geophysics" wave that began in the 1980's and is the basis of most current regional mapping, particularly that being conducted as part of the National Geoscience Mapping Accord. The foundation of this wave comprises seismic and airborne magnetic and radiometric data that are providing unprecedented structural and lithological information about both the surface and the subsurface.

As the third wave matures, the question of what will constitute the fourth wave of regional mapping in Australia grows in importance. Some argue that, like the preceding waves, the fourth wave will be centred on a single technology like, for example, airborne electromagnetics, airborne gravity, or spaceborne imaging spectrometry.

On the other hand, others argue that the fourth wave will be characterised by geographic information systems and interpretative models that facilitate the integration and interpretation of many different kinds of data.

History tells us that whatever underpins the fourth wave, it will be an approach that: focuses on the key fundamentals of user requirements stimulates innovation, and removes geological ambiguity

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

A DOWN-CUTTING THRUST IN THE MT USEFUL SLATE BELT OF EASTERN VICTORIA C.E. Wilbman. M. Hendrickx, A.H.M. VandenBerg, R.A. Cayiey & D. Taylor (Geological Survey of Victoria)

The Mount Useful Slate Belt (MUSB) is a northwest trending belt of deformed Lower Palaeozoic volcanic and sedimentary rocks in the southern part of the Lachlan Fold Belt. It can be divided into two major sectors. In the western zone (10 km wide), Ordovician to Silurian sediments are folded about regional first order anticlinoria and synclinoria (half-wavelengths up to 3 km) and tight second order folds with half-wavelengths averaging about 300 m. In the eastern zone (4 km wide), Cambrian volcanics form an antiformal thrust stack exposed as a series of thrust windows through the two youngest sedimentary units (Serpentine Creek Sandstone and Murderers Hill Siltstone).

The structural history of the MUSB involves the early development of stratigraphically controlled detachment faults such as the Thomas and Fullarton faults. These form the boundaries between three major thrust sheets, each with a slightly different structural history. The Thomas Fault developed first, at or near the base of the sedimentary sequence but was subsequently truncated by the higher-level and younger Fullarton Fault Here, the Fullarton Fault has juxtaposed the base of the Serpentine Creek Sandstone over the Cambrian volcanics and has truncated the Thomas Fault and displaced the Mount Easton Shale, Lazarini Siltstone and Donnellys Creek Siltstone.

In the eastern zone, the geometry of the Fullarton Fault, adjacent folds and S-C fabrics is consistent with east-* directed low angle thrusting. The fault is gently folded indicating fault lock-up occurred before regional foldshortening was completed. Above the fault, the Serpentine Creek Sandstone is either folded into asymetric folds with an enveloping surface parallel to the fault, or it forms cleaved sections of homoclinally dipping sandstone beds that have remained parallel to the fault. Stratigraphic displacement along the fault becomes less apparent away from the Cambrian thrust windows as the detachment becomes sub-parallel to stratigraphy, both in the hangingwall and footwall.

It is inferred that major horizontal detachment occurred first along the Thomas Fault, and within the underlying volcanic sequence. This event was probably characterised by fault imbrication in the lower parts of the sedimentary sequence and the development of an antiformal thrust stack in the Cambrian volcanics.

Subsequent lock-up of the Thomas Fault was probably caused by steepening of the fault adjacent to and above the volcanics along the eastern margin, as continued duplexing within the volcanics increased the amplitude of the antiformal stack. This allowed the transfer of fault shortening to a higher stratigraphic and structural level, that is, along the Fullarton Fault The Fullarton Fault appears to have propagated from west to east as a horizontal thrust, with its orientation apparently more strongly influenced by the competent, thick bedded Serpentine Creek Sandstone in the hangingwall than the underlying more pelitic siltstone. One result of this stratigraphic control is that the Fullarton Fault was able to cut down through much of the underlying stratigraphy where it was west-dipping along the western margin of the Cambrian antiformal thrust stack, while the fault and the stratigraphy above it remained essentially horizontal. The arrival of the Fullarton Fault at the more competent antiformal stack of Cambrian volcanics probably initiated the lockup of the fault, and heralded in the last phase of deformation, the progressive development of folding as the major means of shortening together with some accommodation faults (e.g. Frog Hollow Fault).

Stratigraphy MHS: Murderers Hill Siltstone (Upper Silurian) SCS: Serpentine Creek Sandstone (Upper Silurian) DCS: Donnellys Creek Siltstone (Lower Silurian) IS: Lazarini Spur Siltstone (Lower Silurian)

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MES: Mount Eastone Shale (Upper Ordovician)

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ELEMENTAL CHARACTERISTICS OF TEMPERATE CARBONATES: AN EXAMPLE BASED ON NEW ZEALAND CENOZOIC LIMESTONES Peter R. Winefield*, Campbell S. Nelson and A. Peter W. Hodder Department of Earth Sciences, University of Waikato, Private Bag 3105, Hamilton NEW ZEALAND •Present adress: Centre for Ore Deposit and Exploration Studies, University of Tasmania, GPO Box 252C, Hobart TAS 7001

The use of elemental chemistry for distinguishing temperate and tropical carbonate facies is in its infancy.

Apart from several Tasmanian examples (e.g. Rao, 1991), there is only a relatively limited elemental database for temperate carbonate deposits and none for the widespread occurrences of non-tropical New Zealand limestones of Cenozoic age. This study involved the analysis of nearly 600 samples of limestones from both the North and South Island of New Zealand. The results give average values of Mg, 5450ppm; Fe, 2620ppm; Na, 1450ppm; Sr, 550ppm; and Mn, 510ppm.

As the chemical composition of a limestone generally reflects the physicochemical conditions pertaining at the time of formation and its subsequent diagenesis (Moore, 1989), it should be possible to differentiate nontropical New Zealand limestones from tropical carbonates and to give some insight into diagenetic processes which have influenced their geochemical composition. The New Zealand data indicate that Na, Fe and Mn contents are higher, while the Mg and Sr concentrations are relatively lower than what is typical in reported values of tropical carbonates, whether ancient or modern. Combinations of element-element plots can thus be used to discriminate tropical from temperate carbonates. The Na versus Sr pot is shown to be of particular value in such discrimination which is consistent with the temperature coefficient of equilibrium constants calculated for carbonate dissolution.

It is possible to assess the influence of burial, marine and meteoric diagenetic processes on individual limestone formations from the slopes of regression lines drawn through data plotted on various element-element relationships. For example, Sr and Mn values in particular can be influenced by meteoric diagenetic processes.

Mn has a relatively higher concentration in meteoric water compared to seawater and Sr vice versa. Therefore the relationship of each to the other allows inferences to be made as the influence of meteoric diagenetic processes on carbonates sediments (Brand and Veizer, 1980). Results suggest that the dominant diagenetic characteristic of most Oligocene-Miocene limestones is related to burial or burial/marine processes while strong meteoric signals are also evident in many Plio-Pliestocene limestones of eastern North Island. These observations are consistent with isotopic evidence documented for these carbonate occurrences (Nelson and Smith, in press).

REFERENCES Brand, U. and Veizer, J., 1980. Chemical diagenesis of a multicomponent carbonate system -I: trace elements. Journal of Sedimentary Petrology 51, 1219-1236.

Moore, C.H., 1989. Carbonate Diagenesis and Porosity. Developments in Sedimentology 46. Elsevier, Amsterdam.

Nelson, C.S., and Smith, A.M., in press. Stable oxygen and carbon isotope compositional fields for skeletal and diagenetic components in New Zealand Cenozoic non-tropical carbonate sediments and limestones: a synthesis and review. New Zealand Journal of Geology and Geophysics.

Rao, C.P., 1991. Geochemical differences between subtropical (Ordovician), temperate (Recent and Pliestocene) and sub-polar (Permian) carbonates, Tasmania, Australia. Carbonates and Evaporites 6, 83-106

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

GEOMAP 2005: AN INVESTMENT IN QUEENSLAND'S FUTURE I. W. WithnalL J. J. Draper & M. R. Daly Geological Survey of Queensland, GPO Box 194, Brisbane, Qld 4001

GEOMAP 2005 is a 12-year, $56 million program designed to update the geoscientific knowledge of Queensland, assess the mineral and energy potential of the State, and promote its prospectivity. GEOMAP 2005 is designed to modernise Queensland's geoscientific and resource information in the potentially mineralised areas of the State over the period 1993-2005. In addition, the Queensland Government is implementing a $3.5 million, two year (to June 1996) initiative to acquire high quality airborne geophysical data for the central Queensland coastal region from Ayr to Maryborough. Further geophysical data acquisition initiatives are under consideration.

Existing geoscientific investigations, including those of the National Geoscience Mapping Accord (NGMA)) were incorporated into GEOMAP 2005. NGMA projects undertaken in Queensland comprise: the North Queensland Project which ends in 1996; the Sedimentary Basins of Eastern Australia (SBEA) Project which ended in 1995; and the North Australian Basins Resource Evaluation (NABRE) Project which commenced in 1995 and ends its Queensland phase in 1997.

The northern New England Fold Belt, especially the Yarrol Province which hosts the giant Mount Morgan copper-gold deposit, was identified as the highest priority for mineral resource related investigations. The first new investigations, beginning in 1993, focussed on this region.

The Geoscience and Resources Data Base (GRDB) Project, phase two of the Mineral and Energy Location and Information Network (MERLIN), is a critical element of GEOMAP 2005. This project entails the digital capture of graphic data from existing geoscience maps and exploration permits and the development of digital map production. Existing "stand alone" attribute data bases have been redesigned as relational data bases. Access to information on Queensland's geology and mineral and energy resources to assist exploration and government decision making will be greatly facilitated by this integration.

In north Queensland, all geological mapping planned has been completed, as has mineral occurrence mapping.

Gravity coverage of the Mount Isa area has also been updated. In central Queensland, investigations of the Anakie Province have been completed and comprehensive geological, geophysical and mineral occurrence data made available in hard copy and digital forms. A final synthesis report has been published. In central coastal Queensland, Stage 1 of AIRDATA acquired nearly 144 000 line kilometres of magnetic and radiometric data over an area between Rockhampton and Maryborough. Existing geological and mineral occurrence information on the Yarrol, Auburn, Gogango and Coastal Provinces is being compiled in a Project Geographic Information System (GIS) to enable airborne geophysical data to be integrated with existing data and satellite imagery to produce high quality geological interpretations prior to field checking. Compilation of maps and reports for the Gympie and D'Aguilar Provinces is in progress. These mineral province studies have provided an improved understanding of the geology of the areas, and have highlighted the prospectivity. The NGMA Sedimentary Basins of Eastern Australia (SBEA) Project, in its final reporting stage, has provided an assessment of the petroleum source, migration and reservoir potential of the southern Bowen and overlying Surat Basin, and significantly updated digital data bases.

The second cycle of GEOMAP 2005 (1996-1999) focuses on the geological provinces of coastal central Queensland. During 1995-96, Stage 2 of the AIRDATA Project will acquire 125 000 line kilometres of airborne magnetic and radiometric data over an area from Ayr to Rockhampton. Completion of reporting in north Queensland and southeast Queensland are the other primary targets. Investigations of the Connors-AuburnGogango Provinces in central coastal Queensland are likely to be part of a new NGMA Project. Future initiatives are being considered in respect of the Drummond, Galilee, Cooper and Eromanga Basins.

Other initiatives which are assisting to highlight the prospectivity of Queensland include Q-Therm ($1.7 million over four years to mid 1998) which will provide increased opportunities for exploration and development of Queensland's thermal coals, and the Pacific Resource Information Centre (PRINCE) which is a major computerised facility for the storage, processing and interpretation of existing and future exploration data to be commissioned in February/March 1996 at a cost of $3.6 million.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

NEOPROTEROZOIC TO EARLY PALAEOZOIC ROCKS OF THE ANAKIE INLIER I.W. Withnall1, C.L Fergusson2 and T.J. Green2 1. Geological Survey of Queensland, GPO Box 194, Brisbane, Qld 4001 2. Dept of Geology, University ofWollongong, PO Box 1144, Wollongong, NSW 2500

The Anakie Inlier in central Queensland may represent basement rocks of the Thomson Fold Belt, that underlies the Drummond, Adavale, Cooper, Galilee, and Eromanga Basins to the west. From 1990 to 1992, the GSQ undertook fieldwork (supported by an airborne geophysical survey) in the southern part of the Anakie Inlier. A concurrent study by the University of Wollongong involved detailed structural mapping and isotopic dating. A detailed report and 1:100 000 coloured maps have been published (Withnall & others, 1995).

The main unit, the Anakie Metamorphic Group, now divided into six formations, includes quartzite, pelitic to psammitic schist, greenstone, amphibolite and minor seipentinite. Near Clermont, -greenstone or mafic schist probably represent tectonised tholeiitic basaltic lavas or sills. In the Rubyvale area, foliated metagabbro and laminated amphibolite are less fractionated and similar to alkaline basalts, being enriched in HFS elements.

The metamorphism accompanying D] and D2 graded southwestwards from greenschist (biotite zone) to lower amphibolite fades. Three major deformations and subsequent minor folding events have affected the Anakie Metamorphic Group. In the metapelites the main foliation is generally a layer differentiated S2 crenulation cleavage that was originally flat-lying. In most places it was steepened by later, upright, northeast- and east-trending, folds. No younging sense can be demonstrated, but the foliation and lithological layering dip westwards overall. D2 structures show mylonitic features, and stretching and intersection lineations suggest transport towards the east-northeast Muscovite-biotite granites intruding the Anakie Metamorphic Group near Rubyvale, are also strongly foliated to mylonitic.

The original age of the Anakie Metamorphic Group is uncertain. K-Ar dating suggests that the rocks were deformed and metamorphosed at around 500 Ma, corresponding with the Delamerian Orogeny that affected the Kanmantoo and Adelaide Fold Belts and the Willyama and Wonaminta Blocks in the Cambrian to Early Ordovician. It is also known from the Transantarctic Mountains, where it is known as the Ross Orogeny. The Delamerian age from the Anakie Metamorphic Group suggests that it, and potential correlatives farther north in the Lolworth-Ravenswood Province, the Barnard Metamorphics, and possibly even rocks in Cape York, are the continuation of an orogen that may have extended almost the entire length of eastern Gondwanaland.

All sedimentary structures have been obliterated, so environments of deposition are difficult to determine. The abundance of quartzite suggests a cratonic provenance. The tholeiitic basalts and alkaline metagabbro and metadolerite sills suggest an extensional setting, possibly on a passive margin during the breakup of Rodinia in the Neoproterozoic.

The Late Ordovician Fork Lagoons beds consist of quartzose arenite, mudstone, aphyric basalt, minor mafic to felsic volcaniclastic rocks, and local limestone. They are locally strongly contact metamorphosed by the Devonian Retreat and Taroborah Batholiths. The mudstones generally have a single slaty cleavage, and the arenites have at best a widely spaced, anastomosing dissolution cleavage. The arenite beds are locally disrupted and phacoidal. The cleavage is steeply dipping to vertical.

The boundary between the Anakie Metamorphic Group and Fork Lagoons beds is a steeply dipping fault zone, which may be a thrust The relationships along this contact are similar to those in the Broken River Province, where the Wairuna Formation and Late Ordovician Carriers Well Formation are juxtaposed along the Burdekin and Gray Creek Faults against Precambrian rocks of the Georgetown Province. It is possible that the eastern edge of the Anakie Inlier is a continuation of this fault system.

REFERENCE Withnall, I.W., Blake, P.R., Crouch, S.B.S., Tenison Woods, K , Grimes, K.G., Hayward, M.A., Lam, J.S., Garrad, P. & Rees, I.D. 1995. Geology of the southern part of the Anakie Inlier, central Queensland. Queensland. Queensland Geology, 7,245p.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra., February 1996 A MAGMATIC ORIGIN FOR LATE-STAGE SERICITE-ALUNITE ALTERATION AT THE ENDEAVOUR 48 CU-AU PORPHYRY DEPOSIT, GOONUMBLA, NSW. Rohan C.Wolfe . David R. Cooke , Bruce Hooper & Paul S. Heithersay CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart 7001 Inversiones North (Chile) Ltd, c/o 476 St Kilda Road, Melbourne 3004 North Exploration, GPO Box 1903R, Melbourne 3001 1

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DEPOSIT GEOLOGY The 20 Mt Endeavour 48 Cu-Au deposit is one of four known Cu-Au porphyry orebodies in the Ordovician Goonumbla Volcanic Complex of central western NSW. The deposit is partially hosted by volcaniclastic sandstones, coarse volcaniclastic breccias and blocky trachy-andesite lavas (shoshonites), which are intruded by a weakly mineralised biotite-quartz monzonite (BQM) at depth. Quartz-monzonite porphyry (QMP) was released from the biotite-quartz monzonite in at least two stages, forming an enlongate composite stock that extends 600 m above the BQM. Cu-Au mineralisation is intimately associated with the QMP. Late stage fractionation of the biotite-quartz monzonite melt resulted in the emplacement of small pockets of alkali-feldspar granite within the crystalising BQM. Based on whole rock geochemistry, the intrusions are co-magmatic and are related to the shoshonitic lavas of the host sequence.

ALTERATION Four major stages of hydrothermal alteration and related veins are recognised. Extensive pre-mineralisation selectively pervasive biotite and magnetite alteration and associated magnetite veinlets (Stage 1A veinlets) were the first major hydrothermal event. This was followed by vein-veinlet orthoclase alteration and related quartzbornite-anhydrite stockwork veins (Stages 2A & B). Intrusion of the main QMP stock was closely associated with quartz-sulphide-carbonate mineralisation (Stage 3) and related vein-veinlet sericite alteration. In the deeper parts of the system, irregular quartz veins and hybrid monzonite-hydrothermal quartz 'vein-dykes' indicate high temperature volatile exsolution from the quartz-monzonite porphyry magma. Vein mineralisation was terminated at the end of Stage 3 by pervasive sericite-alunite alteration with associated disseminated chalcocite-bornite mineralisation and associated As-Sb enrichment. This alteration is confined to within the QMP stock, flaring out at the base of the system into the BQM body. Stage 4 (post-mineralisation) consists of carbonate-pyrite-sericite alteration associated with fault zones, extensive selectivity pervasive carbonate alteration and abundant carbonategypsum veins.

Host rock lithology is the main control on alteration at Endeavour 48. Early Stage 1 and 2 potassic alteration assemblages are best preserved in volcaniclastic sandstones, while Stage 3 sericite-hematite alteration is extensively developed within lava-dominated units, resulting in a series of poorly defined alteration 'zones'. These zones are the end-product of several stages of overprinting vein-controlled alteration styles, preserving early alteration styles only in relict kernels and in the distal sections of the units.

ISOTOPE GEOCHEMISTRY Calculated 8 0-8D values of the pervasive sericite-alunite alteration (average 8 0 3so°c = 9.83%o and average SD 350°C = -52 %o, 3 sericite samples) are typical of primary magmatic water. The 8 0-8D values indicate that late-stage magmatic-hydrothermal fluids from the crystallising BQM stock are responsible for sericite-alunite alteration of the QMP stock, in contrast to conventional meteoric water models for late stage sericitic alteration in porphyry systems.

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SUMMARY The porphyry stocks of the Goonumbla district represents an important sub-class of Cu-Au porphyry mineralisation, where volatiles and magma are released into the host rock, forming satellite deposits above the biotite-quartz monzonite. The magmatic-hydrothermal signature of late stage sericite-alunite alteration, dominance of potassic vein-veinlet alteration, limited propylitic alteration, and intimate association between volatile exsolution and magmatism in the deep vein dykes and irregular quartz veins suggest that Endeavour 48 is a magmatic-hydrothermal dominated porphyry system, with minimal contribution from meteoric fluids.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE IMPORTANCE O F PALYNOLOGY AND PALYNOFACIES IN GEOCHEMICAL AND THERMAL MATURATION MODELING Gordon D. Wood Amoco Production Company, P. 0. Box 3092 Houston, TX. USA, 77449

Palynology and palynofacies can impact exploration and exploitation studies beyond their value in correlation and recognition of depositional environments. Cognizance of palynological reworking offers a unique perspective not accessible using vitrinite reflectance and traditional geochemical analyses (e.g., Rock-Eval. Elemental Analysis). Unified, these techniques can provide critical information germane to paleoecology, thermal history and hydrocarbon convertibility. Case history examples illustrating the importance of palynological reworking are discussed using case histories from South America, Pakistan and Myanmar.

A multiple well study using core/cuttings samples from the Chaco Basin of Argentina were used to discern prospective hydrocarbon areas. Vitrinite reflectance measurements of several samples recorded bimodal organic populations indicative of massive reworking. Palynological analyses revealed recycled Devonian palynomorphs (e.g., Navifusa bacilla, Stellinium octoaster, Emphanisporites rotatus) were a major component of the assemblage. This reworked assemblage necessitated reassessment of exploration potential because the Devonian organic material was not convertible to liquid hydrocarbons, a facet not identifiable using only source rock (wt%) analyses.

Fifteen samples from an approximately 4.5 foot thick outcrop of the Socene Jatta Gypsum of the Robat area, Pakistan, were examined for palynology, vitrinite reflectance and a variety of geochemical parameters. Samples with high TOC values (up to 30%) possessed elevated hydrogen indices and were predominately amorphous kerogen. Samples with the lowest TOC's had low hydrogen indices and were dominated by structured or mixed kerogen. Vitrinite reflectance measurements indicated a significant amount of reworking evident in the low TOC samples and this is corroborated by palynological analysis. Indigenous forms (e.g., Tiliapollenites spp., Retitricolpites sp., Muratodinium fimbriatum, Polysphaeridium zoharyi, Homotryblium tenuispinosum) are minor components in comparison to recycled Jurassic palynomorphs (e.g., Callialasporites dampieri, IGonyaulacysta jurassica, Scrinodinium crystallinum, Nannoceratopsis pellucida, Omatia montgomery).

The Upper Cretaceous through Plio-Pleistocene from the Chindwin Basin, Myanmar, was^also afforded a multidisciplinary approach. Palynological examination indicates that the oldest rocks in the Chindwin contained abundant reworked Permian-Triassic. The oldest Tertiary included both recycled Cretaceous and Permo-Triassic organic microfossils. The youngest Tertiary yielded recycled Paleogene forms. These data were used to postulate prospective hydrocarbon and sediment sources areas.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

AN OVERVIEW OF TEMPORAL GEOCHEMICAL CHANGES IN AUSTRALIAN IGNEOUS ROCKS: IMPLICATIONS FOR CRUSTAL EVOLUTION AND METALLOGENY. L.A.I Wvborn, Australian Geological Survey Organisation (AGSO), GPO Box 378, Canberra, ACT, 2601

Igneous penologists in Australia can be broadly divided into two main schools: those who magnify the similarities in compositions throughout time, and those who emphasise the differences. Often using the same data, the former school emphasise the similarity of many ancient igneous suites to those of modern subduction zones, whilst the latter school accept that there is a progressive evolution in igneous compositions with time as a result of changing thermal conditions in the mantle, and the growth and evolution of the continental crust.

ARE THERE CONTINENT-WIDE CHANGES IN IGNEOUS ROCKS AND IN METALLOGENY ?

The ultimate source region of the majority of Australian Archaean granites had garnet, but not plagioclase stable. However, a large garnet-bearing layer cannot be distinguished in the lower crust beneath Archaean terranes. Many trace elements and isotopic compositions of Archaean granites are similar to those of modern subduction-related analogues. However, most late Archaean granites have Si02 > 70 wt%, suggesting that they cannot be direct derivatives from the mantle. K20, Th and U are also significantly higher in late Archaean granites. The ultimate source region of many Proterozoic and early Palaeozoic granites had plagioclase but not garnet stable. The source region for these granites is possibly visible in an intermediate velocity lower crustal layer. Proterozoic and lower Palaeozoic granites have Si02 > 65 wt% and are dominated by I-granodiorite types, inferring that they also cannot be direct derivatives from the mantle. Proterozoic granites are also enriched in K20, Th and U; Palaeozoic granites less so. The size of Palaeozoic plutons is also smaller. In upper Palaeozoic and younger compositions, garnet becomes increasing important in the ultimate source region of granites, and the granites become more mafic, with M-types and I-(tonalite) types being more prominent: these granites are also less enriched in K20, Th and U. Mafic magmas also show a progressive change with komatiites and high-Mg basalts relatively common in the Archaean, whilst alkaline rocks become more important with time. Fe-enriched continental tholeiites are prominent in the Proterozoic, and their appearance is coeval with increasing complexity in the composition of the middle crust. Australian ore deposit types show an appropriate evolution with time with the dominance of Ni, Cr and Au in the Archaean, through the dominance of rift-related, sediment-hosted Pb-Zn in the Proterozoic, with Sn and VHMS deposits becoming more important in the Palaeozoic. Porphyry-style magmatism does not become important until the Palaeozoic, at about the same time the magmas become more 'subduction like' in composition.

ARE THERE PROVINCE-WIDE CHANGES IN IGNEOUS ROCKS AND IN METALLOGENY ?

Within most provinces there is also a progressive evolution in magma types and once a particular granite type is in the upper crust it is not repeated in the same region. The earliest felsic magmas are usually unmineralised S-or I-type restite-rich melts which are followed in time by fractionated I-or S-types which can have associated Au, Sn and base metals. In some provinces the next magma type is the high fluorine incompatible element enriched melts (A-types) which may have Mo and W. The final major gasp is usually relatively anhydrous, oxidised fractionated I-type melts which have Au, Cu and some base metal mineralisation. The progression of felsic compositions seems to reflect an increase in temperature and depth of melting and also a decrease in degree of partial melting in the source region. Mafic magmas also change in time from early depleted more Mg-rich mafic magmas, through continental tholeiites to more alkaline compositions, reflecting a decrease in the degree and increase in the depth of partial melting with time in the mantle. Once felsic magmas have progressed through all these types then the lower crust must be relatively dehydrated. Further large scale crustal melting events will only occur in areas of major reactivation where new material has been added to the lower crust and/or extremely high temperatures have been reached. This is exemplified in the East Australian Tertiary Volcanic event where felsic magmas occur in the less evolved New England Fold Belt, in QLD northern NSW and but not with the more evolved Lachlan Fold Belt in southern NSW and Victoria.

SUMMARY: This summaiy documents evolving changes in Australian igneous rocks throughout time which have implications for crustal growth and metallogeny. I emphasise however, that these changes documented here relate to Australia only. A similar regional scale compilation of global data has not yet been attempted.

Acknowledgments: This paper is published with the permission of the Executive Director of AGSO.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ENVIRONMENTAL GEOCHEMISTRY IN THE SOUTHERN KAKADU REGION AND THE "SICKNESS COUNTRY" CONCEPT: THE NEED FOR BASELINE GEOCHEMICAL STUDIES L.A.I Wvbom1, E.A. Jagodzinski1, G. Jacobsen1 and R.S. Needham2 Australian Geological Survey Organisation (AGSO), GPO Box 378, Canberra, ACT, 2601 formerly AGSO, now Office of the Supervising Scientist, Tourism House, 42 Blackall Street, Barton, ACT, 2600.

Compilations of geochemical data in the southern Kakadu region centred around the upper reaches of the South Alligator Valley show that there are well above average crustal concentrations of certain elements such as uranium, thorium, lead, mercury, radon and fluorine which may be considered deleterious to human health.

The average crustal level of uranium is 2.8 ppm, and granites and felsic volcanics in the region have concentrations well above this, with values up to 30 ppm being common. Uranium is also commonly concentrated within shear zones or in alteration zones, and can have concentrations up to 140 ppm, and sediments in streams draining the granites are also markedly enriched in uranium. The average crustal concentration of thorium is 10.7 ppm. High values of up to 90 ppm thorium are common in the granites and felsic volcanics in the region. Stream sediments draining the granites are also high with values up to 527 ppm being recorded. Thorium is depleted in the uranium deposits along the South Alligator Valley and their associated alteration zones, whilst high values of up to 1187 ppm have been recorded in areas that are topographically above. High concentrations of thorium occur on the mature landforms, in some cases associated with laterites. The average crustal concentration of arsenic is 1.5 ppm and high concentrations are commonly associated with secondary ores, with values of to 1300 ppm. Arsenic is also high in some of the stream sediments where values up to 15 ppm are not unusual. Arsenic is above crustal average (up to 20 ppm) in the Koolpin Formation, particularly carbonaceous shales. High lead can occur locally associated with some of the uranium deposits; and lead prospects are also located in the region. Both mercury and radon were historically used as an exploration pathfinder element for exploration. Concentrations of mercury up to 800 ppm have been recorded in some of the South Alligator deposits, particularly in the secondary ores, whereas the average crustal concentration is 0.05 ppb. Radon measuring devices were also used by companies in the region exploring for uranium. Old company reports document several radon anomalies on the Scinto Plateau. Geochemical data on fluorine are sparse. However, visible fluorite is abundant in the high uranium granites and is also widespread in the metamorphosed volcanics and sediments southwest of Coronation Hill.

Although no systematic surveys of groundwater have been carried out in the region, there are several indicators that in some areas groundwater may be naturally contaminated. In a recent survey of eleven water bores in the Coronation Hill region, four bores were found to have element concentrations above standards set by the NHMRC for iron, zinc and radionuclide concentrations. One bore with high natural radionuclide concentrations was sited nearly 1 km away from known mineralisation. Two prospects in the region were interpreted to result from precipitation of uranium from spring waters, and water sampling has been tested as a possible indicator of uranium mineralisation in the South Alligator Valley THE "SICKNESS COUNTRY".

A large part of the "sickness country" coincides with the region containing localised areas of unusually high natural levels of thorium, uranium, arsenic, mercury, fluorine and radon. Contamination of spring and groundwaters by these elements could be a cause of illness. Other geologically related causes of illness could be the use of ochres rich in uranium, lead, arsenic, and mercury as pigments for painting, and perhaps it is not coincidental that several significant painting sites occur near uranium prospects and mines including Sleisbeck, Christmas Creek and Palette.

Acknowledgments: This paper is published with the permission of the Executive Directors of AGSO and the Supervising Scientist.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ARCHAEAN GEOLOGY OF THE NORTHERN PART OF THE EASTERN GOLDFIELDS YILGARN CRATON, WESTERN AUSTRALIA S. Wvche , T. R. FaiTell , T. J. Griffin , R. L. Langford , S. F. Liu ' , A. J. Stewart , J. M Westaway and A. J. Whitaker Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004 Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 1

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Archaean greenstone sequences in the northern part of the Eastern Goldfields contain a similar range of rock types but there are significant differences in character between individual greenstone belts. The structural and metamorphic history is broadly similar to that described for greenstones to the south. As in other parts of the Eastern Goldfields, all greenstones are metamorphosed but common preservation of primary features allows recognition of protoliths.

The western greenstone sequence, which extends north from Agnew to Wiluna, contains abundant mafic and ultramafic rocks as well as felsic volcanic and volcaniclastic sedimentary rocks, chert, shale and sandstone.

There is a greater volume of ultramafic rocks and individual ultramafic units are thicker than in the greenstone sequences to the east. The komatiites which host the major nickel deposits at Perseverance, Yakabindie, Mount Keith and Honeymoon Well probably represent a single komatiite event, possibly involving a series of komatiite outpourings over a relatively short period of time. These ultramafic units have been correlated with those in adjacent greenstone sequences and with the similar thick, nickel-rich ultramafic units in the Kalgoorlie region to the south. However, field evidence such as geochronological data is limited A late component of the greenstone sequence, the Jones Creek Conglomerate, rests unconformably on volcanic-rich greenstones and early granitoid rocks in a setting like that of similar conglomerates to the south.

The central greenstone sequence, commonly known as the Yandal greenstone belt, is divided into two sections separated by an attenuated zone north of the Bronzewing mine. This zone may represent a change in facies or a structural discontinuity between two sequences. The southern part of the Yandal belt is dominated by the calcalkaline Spring Well felsic volcanic complex. Subordinate mafic, ultramafic and metasedimentary rocks become more significant to the north. The western side of the belt is marked by a prominent unit of banded iron-formation with associated sedimentary, mafic and ultramafic rocks in the north and prominent cherts with a similar association in the south.

The Dingo Range greenstone belt contains a mafic-ultramafic sequence with chert interbeds. Greenstone sequences to the northeast are largely concealed beneath Proterozoic sedimentary rocks of the Earaheedy Group.

The eastern greenstone sequence, which extends southfromthe Duketon area to Laverton, contains a range of rock types including chert, banded iron-formation, shale, sandstone, conglomerate, felsic volcanic and volcaniclastic rocks, and mafic and ultramafic sequences with no one variety dominating the overall succession.

Conglomerate units with granitoid clasts may be equivalent to the Jones Creek Conglomerate.

The most readily identifiable structural elements in all the greenstone belts resulted from east-northeastdirected shortening. Early upright folding was followed by further upright folding accompanied by the development of regional north- to northwest-trending ductile shear zones, similar to D and D in the Kalgoorlie region. However, it is not clear whether a widely recognized earlier fabric was produced by a thrusting event like that identified to the south in the Kalgoorlie-Kambalda area. There is complex interleaving of granites and greenstones with local development of high-strain zones along a number of the granitegreenstone contacts. All the greenstone sequences are metamorphosed, typically in the greenschist facies, but local areas of amphibolite facies metamorphism are common near granite-greenstone contacts. Peak metamorphism probably coincided with the emplacement of large monzogranite bodies during or shortly after the upright folding event. Later granitoid intrusions ranging in composition from syenite to tonalite crosscut all the major structures and therefore must be younger than the regional shear zones.

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Gold mineralization occurs in all the greenstone sequences. It is dominantly structurally controlled, and probably formed very late in the evolution of the Archaean granite-greenstone terrain.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, CanberraFebruary 1996

ISOTOPIC RESETTING O F ZIRCON IN HIGH TEMPERATURE GRANULITE FACIES XENOLITHS FROM THE TRANSANTARCTIC MOUNTAINS R.J. Wvsoczanski 1 . J.H. Berg^ C.M. Fanning1 1

Research School of Earth Sciences, Australian National University, Canberra ACT 0200, ACT - Australia ^Department of Geology, Northern Illinois University, DeKalb, IL 60115 - U.S.A

A suite of two-pyroxene granulitic xenoliths have been entrained by alkaline magmas in the Foster Crater region of the Koettlitz glacier, Transantarctic Mountains. The xenoliths range from basaltic to andesitic in composition with major and trace element data consistent with the suite being coeval. The large range in composition, notably silica (46.6 - 62.4 wt. %), Mg # (70-40) and radiogenic isotope compositions ( 8 7 Sr/ 8 6 Sr = 0.70405 0.71413; 1 4 3 N d / l 4 4 N d = 0.51200 - 0.51287) is the result of the formation of the suite by assimilation of charnockitic crustal material in basaltic melts sourced from the mantle. Nd model ages ranging from 1.13 to 3.1 Ga (typically 1.49-2.19 Ga) may be used to imply that the assimilated material-was late Archean or early Proterozoic in age.

SHRIMP U-Pb zircon ages were determined from two of the xenoliths to constrain the timing of events affecting the lower crust in this region. A number of zircon populations yielded concordant ages of c. 500 Ma, c. 459 Ma, c. 352 Ma, and c. 304 Ma, with all save the youngest population present in both samples. Thin rims that were not analysed in the other sample may be of this age. Three grains older than the c 509 Ma population gave concordant ages of latest pre-Cambrian to earliest Cambrian.

All the age populations include grains that are high in U and Th (< 3725 ppm U, and <1900 ppm Th), and vary greatly in Th/U (0.16 to > 1.0) and in Pb content (18 - 251 ppm). Grains are elongate, and show growth structures under cathodoluminescence imaging, consistent with magmatic growth. In the three younger populations, zircons also occur as overgrowths on older grains. All 'new growth' grains are concordant and show no signs of alteration or radiogenic Pb loss.

Low U and low Th (<50 ppm U and < 30 ppm Th) zircons also occur in both xenoliths. These grains have restricted Th/U ratios (0.27 - 0.55, but typically < 0.40), and have very low Pb contents (1-3 ppm Pb). They are rounded in shape and homogenous under cathodoluminescence imaging, consistent with growth during metamorphism. They are similar in appearance and in chemistry, and seem to represent the same population, however they yield discordant ages within error of the c. 509 Ma, c. 459 Ma, and c. 352 Ma populations defined by the concordant zircon populations.

The c. 509 Ma age corresponds with the Ross Orogeny; a major event which affected much of the Antarctic margin and was widespread throughout the Transantarctic Mountains. This age is also the oldest age in which metamorphic zircon appears. The presence of magmatic zircon growth at this time suggests that the metamorphic grains were formed prior to the Ross Orogeny (and not during it). Nd isotope modelling indicates a late Archean or early Proterozoic component in the lower crust, however we have no record for zircon of this age in the xenoliths. This is consistent with a major event (identified as the Ross Orogeny) resetting pre-existing zircons, with very few surviving (the high U and Th late pre-Cambrian, early Cambrian zircons). The magnitude of the Ross Orogeny is further evidenced by a granite xenolith emplaced above the lower crust in this region, which has been dated at c. 543 Ma. The granite is sintered, having been partially melted by an event dated at c.

503 Ma which was contemporary with the Ross Orogeny. The Ross Orogeny therefore was an extremely powerful event at mid-lower crustal levels, capable of causing partial melting and altering crustal compositions.

The geothermal gradient in the crust in this region, as defined by the xenolith suite, is extremely high (50100°C/km in the upper crust, with a temperature of approximately 1000°C in the lower crust). Conditions during a major orogenic event (i.e. the Ross Orogeny) would have at least been similar to, if not greater than this. The high temperature and pressure of the lower crust, combined with the dry environment, must have been above the closure temperature for these zircons, allowing either Pb to be removed from the crystal into the melt, and/or U and Th in the melt to exchange with the crystal, resetting the isotopic composition of the zircon. Further magmatic events at c. 449 Ma and c. 352 Ma reset some of these grains for a second or third time, but failed to reset all, suggesting that these events were not as intense. The latter events probably occurred by melt infiltration through the crust with little or no partial melting, thus preserving any zircon that did not come in contact with the melt. The implications of this work is that in favourable conditions, such as those described above, the closure temperature of zircon can be exceeded, removing any remnant of ancient events. Care must therefore be taken to firmly establish whether such an event may have occurred in high temperature/pressure rocks to properly interpret any age events, particularly from single crystal analyses or bulk rock isotopic compositions.

485

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

ORIGIN OF GLASS IN MANTLE XENOLITHS FROM SOUTH-EASTERN AUSTRALIA Gregory M. Yaxlev. David H. Green and Trevor J. Falloon Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200.

Preservation of "andesitic" glass trapped as micron-scale inclusions in primary phases in mantle-derived peridotite xenoliths from a range of tectonic and geographic localities, has recently led Schiano and Clocchiatti (1994) to postulate a ubiquitous high SiC>2 metasomatic melt, migrating in the lithosphere. Furthermore, recent high pressure partial melting experiments (Baker et al. 1995) have suggested that low-degree equilibrium partial melts of fertile peridotite may have compositions very similar to the inclusions of Schiano and Clocchiati (1994). This similarity has therefore led to the implication that natural melts preserved as glass in xenoliths are actual samples of low degree, equilibrium partial melts of the mantle (Kinzler and Langmuir 1995).

The glass inclusions are compositionally indistiguishable from glass also found as coarse patches and veins in mantle-derived peridotite xenoliths from western Victoria and many other localities. We use textural and compositional evidence to demonstrate that these glass patches and veins are in no way related to equilibrium partial melting of the mantle, but simply result from partial or complete breakdown of amphibole ± phlogopite during heating and decompressional events related to the magmatic processes which ultimately erupted the xenoliths.

Our xenoliths are spinel ± apatite ± amphibole ± phlogopite-bearing wehrlites, lherzolites and harzburgites, hosted by the Newer Volcanics, and which have been metasomatised by ephemeral carbonatite melt(s) (Yaxley et al. 1991). Glass occurs predominantly in vesicular pools up to several millimeters across. In a few samples, large pools contain clearly partially melted relict amphibole or phlogopite, but hydrous silicate phases are absent in the majority of cases. Glass pools invariably contain complex assemblages of secondary microphenocrysts of olivine, cpx and spinel. Glass/opx contacts (absent in the wehrlites) are invariably lined with fine-grained ol+cpx, which is extensively replacing the opx. The large size of many of the glassy pools, and evidence of reaction with surrounding phases indicate disequilibrium between glass and host assemblage.

Glass chemistry is variable, but is characterised by high SiC>2, AI2O3 and alkalis, and low CaO, MgO and FeO. Compositions vary systematically with the primary phase assemblage in contact with the glass. For example, glasses in contact with relict amphibole ± phlogopite are silica-undersaturated, whereas those in contact with opx are Qz-normative, and relatively low in AI2O3. Glass compositional variation is consistent with controls relating to fractionation of the observed ol+cpx+sp microphenocryst assemblage, and to reaction with primary orthopyroxene. Established phase relations in the simple systems MASK and MASN suggest that these glasses could not be in equilibrium with mantle-like assemblages under any conditions of PT and volatile content. This lack of chemical equilibrium is consistent with the evident textural disequilibrium.

We therefore suggest that the melts which quenched to preserve these glasses resulted from partial or complete breakdown of lithospheric amphibole ± phlogopite during heating associated with adjacent intrusion of magmas related to Newer volcanic activity. Textural disequilibrium preserved in the glass patches suggests entrainment in the host magma occurred very rapidly after the melting event. Subsequent reaction with primary opx and crystallization of secondary ol+cpx+sp modified the melt compositions to those observed. This model accounts for the chemical and textural disequilibrium between the glasses and the surrounding primary peridotite phases.

Devolatilization of the melts during decompression is evident in their vesicular nature, and has forced melt along grain boundaries and fractures in primary phases. Rapid healing could therefore result in formation of melt inclusions, indicating that glasses with an identical compositional range preserved as inclusions in worldwide xenoliths are unrelated to either metasomatism or equilibrium partial melting of the lithosphere.

REFERENCES Baker, M.B, Hirschmann, M.M., Ghiorso, M.S., & Stolper, E.M., 1995. Compositions of near-solidus peridotite melts from experiments and thermodynamic calculations. Nature 375,308-311.

Kinzler, R.J., & Langmuir, C.H., 1995. Minute mantle melts. Nature 375, 274-275.

Schiano, P., & Clocchiatti, R., 1994. Worldwide occurrence of silica-rich melts in sub-continental and suboceanic mantle minerals. Nature 368,621-624.

Yaxley, G.M., Crawford, AJ., & Green, D.H., 1991. Evidence for carbonatite metasomatism in spinel peridotite xenoliths from western Victoria, Australia. Earth Planet. ScL Lett. 707,305-317.

486

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, CanberraFebruary 1996 PETROLOGY OF IGNEOUS CHARNOCKltES, MAWSON, ANTARCTICA: GRANITIC SOURCE PROVINCES AS INDICATORS OF POSSIBLE TERRAIN BOUNDARIES David N. Young . David J. Ellis , Jiau-xin Zhao and Malcolm T. McCulloch Northern Territory Geological Survey, PO Box 2901, Darwin NT 0801 Geology Department, Australian National University, Canberra ACT 0200 Research School of Earth Sciences, Australian National University, Canberra ACT 0200 1

2

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Charnockitic granites are exposed over about 5,000 km at the Mawson Coast in Antarctica. The batholith comprises a large number of different plutons and has a pronounced asymmetry in whole-rock major and trace elements and Sr-Nd isotopic geochemistry. Three zones are recognised across the batholith: the central zone rocks have significantly higher initial Sr/* Sr ratios (0.7205 to 0.7334), lower initial £ values (-8.8 to -11.1) and older Nd depleted model ages (2.03 to 2.16 Ga) than those from the western and eastern zones (0.7076 to 0.7263; -4.0 to -8.4; 1.68 to 1.96 Ga). There is unlikely to have been a significant mantle input: rather, the charnockites in each zone reflect the crustal source rocks they were melted from. It is possible that the spatial penological differences reflect source provinces of different age, present in the middle to lower crust, that were assembled by plate tectonic accretionary processes as ancient as the Palaeoproterozoic.

2

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Nd

Charnockites in the western zone have higher and more variable Sr, Na 0 and Sr/Ba and lower Ti0 , Th, Rb/ Sr and K/Na than those in the central and eastern zones. The western zone has more charnockites that are depleted in heavy rare earth elements: the magmas are thought to have equilibrated with restite garnet during high-pressure crustal melting, probably in orogenically thickened crust.

2

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Apart from the geographic variations, the charnockites, which range from 53 - 74 % Si0 , have two main geochemically distinctive types, named low-Ti and high-H, that occur throughout the batholith and have similar isotopic signatures. The high-Ti group has higher Ti0 , P 0 , K 0, FeO, Na 0+K 0, Zr, Nb, Y, La, Ce, Rb, Rb/Sr and Ga/Al, and lower MgO, CaO, A1 0 , Na 0, Mg#, Cr and Ni than the low-Ti group. The high-Ti group is a typical "C-type" magma, using the terminology of Kilpatrick and Ellis (1992) for igneous charnockitic granites and volcanic rocks, whereas the low-Ti group is unusual and has no recognised equivalents in the literature. Only the low-Ti group has HREE-depletions but both high-Ti and low-Ti types were derived from granulite-facies rocks of intermediate composition that had not previously had a melt removed.

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U-Pb zircon dating has found an igneous age of -970 Ma for the charnockites (Young and Black, 1991), which were emplaced following two initial deformations that affected the basement granulite gneiss. A third deformation later deformed the charnockites, producing retrograde garnet and biotite in many rocks at around 6 kbar (garnet-orthopyroxene-plagioclase-quartz geobarometry). The deformation however did not totally obliterate mineralogical evidence for high igneous temperatures (>1,000° C), such as antiperthite crystals with ternary bulk compositions, and coarsely-exsolved pyroxenes that represent inverted pigeonite. Field evidence was also found for high magmatic temperatures, such as large xenolithic blocks of country rock gneiss that have extensively melted rims in contact with charnockite.

REFERENCES Kilpatrick, J.A. & Ellis, D.J., 1992. C-type magmas: igneous charnockites and their extrusive equivalents. In Brown, P.E. and Chappell, B.W. eds. The Second Hutton Symposium on the Origin of Granites and Related Rocks. Transactions of the Royal Society of Edinburgh: Earth Sciences 83, 155-164.

Young, D.N. & Black, L.P., 1991. U-Pb zircon dating of Proterozoic igneous charnockites from the Mawson Coast, East Antarctica. Antarctic Science 3, 205-216.

487

GEOLOGICAL SOCIETY OF A USTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

LOWER CAMBRIAN SEQUENCE STRATIGRAPHY IN THE ARROWIE AND STANSBURY BASINS, SOUTH AUSTRALIA Zang, Wen-long

(Department of Mines & Energy, South Australia)

Lower Cambrian sediments in the Arrowie and Stansbuiy Basins were deposited on a rift continental shelf which is bounded in the west by the Gawler Craton. Md-Neoproterozoic continental breakup and following Marinoan glacio-eustatic transgression resulted in the deposition of three successive second-order sequences during the Ediacarian (sensu Cloud and Glaessner, 1982) and Early Cambrian in the Adelaide Geosyncline and overlying Arrowie Basin; followed by renewed rifting, reflecting the latest Neoproterozoic - Early Cambrian continental separation that formed the western margin of Laurentia (Powell et al.9 1994). The Ediacarian and Lower Cambrian sequences were mainly formed on the passive margin of palaeo-Pacific Ocean. Five sedimentary cycles can be recognised in the Lower Cambrian successionfromthe Arrowie and western Stansbuiy Basins and their boundaries are related to tectonic movements and relative sea level change. The lowest £ 1.0 sequence (G = Cambrian) contains a deltaic succession (Uratanna Formation) which was deposited in depressions and incised channels within the Ediacara fauna-bearing Rawnsley Quartzite in the central Arrowie Basin; the sequence consists of a lowstand wedge and submarine channel deposit, which passes transgressively into prodelta/mid-shelf deep water siltstone and grades upwards into highstand deltafrontand shoreface siltstone and sandstone. £ 1.0 is considered to have been deposited in the same tectonic setting as the Ediacarian sequences during a major second-order Neoproterozoic sea level fall, even though the sediments contain Cambrian trace fossils Phycodes, Rusophycus and Sabellidites. The major Cambrian transgression in the Arrowie Basin reached maximum coastal onlap during the deposition of the uppermost Parachilna Formation and lower Woodendinna Dolomite or equivalents. £ 1.1 sedimentation was interrupted by a regional uplift event which can be recognised by field mapping and, in the Stansbury Basin, by seismic interpretation. This event and its depositional unconformity divides the sequence into two sub-sequences, whereas the unconformity becomes conformable into eastern troughs. £ 1.1 A (Tommotian) contains the lowstand to transgressive Parachilna Formation and highstand Woodendinna Dolomite or equivalents, whereas £ 1.1B (Atdabanian) is a shelf limestone succession which contains the earliest archaeocyaths and trilobites in Australia £ 1.2 (Botomian) sequence is bounded at the base by the Flinders Unconformity and extensiveriftsduring deposition resulted in carbonate shelf failure and slope sedimentation (e.g. Mermema Formation). £ 1.3 (late Botomian) deposits accumulated during a major sea-level fall and the sediments are mainly deposited in subsiding troughs and grabens, containing both siliciclastics and carbonates. Faunas and acritarchs are generally abundant in highstand, fair in transgressive but rare in lowstand deposits, due to sea-level change and preservational environments. Seven acritarch assemblages have been recognised in the two basins and can be used for intercontinental correlation. The integrated package of biostratigraphy and unconformity-bounded sequence stratigraphy provides a more precise chart for stratigraphic correlation beyond the basinal scope.

The Arrowie and Stansbury Basins contain potential petroleum resources. Extensive exploration for Cambrian petroleum in South Australia began in late 1950s when waxy paraffinic crude was found in Wilkatana 1 in the southwestern Arrowie Basin. More than twenty exploration wells were drilled in the two basins during the 1960s1980s, but most of them were not on structure because of poor understanding of the tectonic evolution and sedimentation in these regions (Gravestock and Hibburt, 1991). Recent study suggests a complex oil generation and migration history in Cambrian sediments and the existence of potential source rocks in thisregion,whereas karstic Wilkawillina Limestone in the Arrowie Basin and vuggy dolomite of the Rulpara Formation in the western Stansbury Basin could be potential reservoir rocks. Integration of the outcrop and subsurface data, based on the sequence stratigraphy and biostratigraphic approaches to analyse the sedimentary basins, will provide a reliable stratigraphicframeworkfor future exploration in South Australia.

References Cloud, P. & Glaessner, M. F., 1982. The Ediacarian Period and System: Metazoa inherit the earth. Science, 217, 783-792.

Gravestock, D. I. & Hibburt, J. E., 1991. Sequence stratigraphy of the eastern Officer and Arrowie Basin: a framework for Cambrian oil search. Journal of the Australian Petroleum Exploration Association, 31, 177-190.

Powell, C. McA, Preiss, W. V., Gatehouse, C. G., Krapez, B. & Li Z. X., 1994. South Australian record of a Rodinian epicontinental basin and its mid-Neoproterozoic breakup (-700 Ma) to form the Palaeo-Pacific Ocean.

Tectonophysics, 237, 113-140. * This study is undertaken in collaboration with Dr. D. I. Gravestock.

488

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

PETROLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF A DEVONIAN REPLACEMENT ZONE IN THE CAMBRIAN ROSEBERY MASSIVE SULPHIDE DEPOSIT, WESTERN TASMANIA Khin Zaw1, Ross R. Large1 and David L Huston1* CODES Key Centre, Geology Department, University of Tasmania, Hobart, Tasmania 7001, Australia ^Present address: Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1 A, 0E8, Canada

Rosebery deposit is a polymetallic, volcanic-hosted massive sulphide (VHMS) deposit in Cambrian Mt Read Volcanic belt, western Tasmania. The deposit underwent upper greenschist facies regional metamorphism and related deformationn during the Devonian Tabberabberan Orogeny, resulting in folding, shearing, and faulting (thrusting) of the ore lenses. The southend of the Rosebery deposit has undergone metasomatic replacement related to the intrusion of an inferred post-orogenic Devonian granite. The granite does not crop out a Rosebery, but the existence of a shallow granite intrusion below the southend of the orebody has been interpreted from detailed gravity data.

Mineralisation at Rosebery consists of three primary sulphide-sulphate zones and a massive carbonate zone of Cambrian age: a lowermost pyrite-chalcopyrite zone (>4% Cu), an overlying by a sphalerite-galena^yrite zone, and an uppermost massive barite and carbonate zone. Devonian metasomatic mineral assemblages that overprint the south-end of the orebody include three major zones: (1) magnetite-biotite±chalcopyrite, (2) pyrrhotite-pyrite, and (3) tounmline-quartznnagnetite. Other metasomatic minerals such as fluorite, garnet, and helvite are also present in the Devonian assemblages (Khin Zaw et al, 1995a).

Field and textural relationships suggest that replacement of primary lead-zinc sulphide lenses occurred after folding.

Magnetite-biotite^halcopyrite assemblages are confined to the lower levels of the mine, whereas pyrrhotite-pyrite and tourmaline-quartznnagnetite assemblages occur toward the upper part of the orebody. The tourmaline-quartz+magnetite assemblage appeals to have formed late in the replacement process, as evidence by irregular and patchy quartz-tourmaline veins cutting the host rock and other sulfide lenses. Detailed underground examination also indicates that tourmaline-quartz veins demonstrably cut cleavage in the tuffaceous host rocks, suggesting that the tourmaline veins formed after Devonian cleavage development.

Electron microprobe studies indicate that garnets from Rosebery consist predominantly of spessartine (74.0-85.0 mole %) with minor grossularite (3.0-16.0 mole %). Rosebery helvite contains up to 7.0 wt.% Zn. Biotite displays a range of tetrahedral A1IV (1.81-2.7.1) and octahedral AlVI(0.02-2.75), and 100Mg/Mg+Fe+2 values range from 22.0 to 54.0. Garnet, holvite, and biotite from Rosebery are compositionally similar to those from the Devonian granite-related Mt Lindsay and Cleveland W-Sn-F replacement deposits in western Tasmania (Kwak, 1983; Collins, 1983). Schorl is the predominant tourmaline at Rosebery; these are compositionally comparable to tournalines from the Devonian Meredith granite, but are more Fe-rich than volcanogenic tourmalines from the Kidd Creek (Ontario) and Appalachian-Caledonian massive sulphide deposits. This study reveals that tourmaline and other skarn minerals at Rosebery are exclusively of Devonian granitic origin.

The Devonian overprinting processes also resulted in the redistribution of FeS content of sphalerites from the Rosebery deposit (Khin Zaw and Large, 1995).

Microthermometry and laser Raman spectroscopy studies of fluid inclusions in minerals from the Devonian replacement zone indicate that the early biotite-magnetite and pyrrhotite-pyrite assemblages formed from the interaction of moderate to high temperature (> 300°C), saline (>10-25NaCl equiv. wt %), C02-bearing fluid with the original stratiform lead-zinc mineralisation. The later stage tourmaline veining and associated replacement assemblages resulted from lower temperature (< 300°C), less saline (< 300°C) fluid. The high temperature, high salinity and C02-bearing metasomatic fluid is consistent with derivation of this fluid from the Devorian granite below the Rosebery deposit (Khin Zaw et al., 1995b).

References Collins, P.L.F., 1981. The geology and genesis of the Cleveland tin deposit, western Tasmania: Fluid inclusion and stable isotope studios. Econ. Geol. 76, 365-392.

Khin Zaw and Large, R.R., 1995 in press. Petrology and Geochemistry of Sphalerite from the Cambrian VHMS deposits in the Rosbery-Hercules District, Western Tasmania: Implications for Gold Mineralisation and Devonian MetamorphicMetasomatic Processes. Mineralogy & Petrology.

Khin Zaw, Huston, D.L., and Large, R.R, 1995 submitted. A chemical model for the Devonian remobilization process in the Cambrian VHMS Rosebery deposit, western Tasmania: Constraints from metal zonation, fluid inclusions and thermodynamic calculations. Econ. Geol Khin Zaw, Large, R.R, and Huston, D.L., 1995 in review. Petrologic and geochemical significance of a Devonian replacement zone in the Cambrian Rosebery massive sulfide deposit, western Tasmania. Can. Mineral.

Kwak, T.A.P., 1983. The geology and geochemistry of the zoned, Sn-W-P-Be skarns at Mt Lindsay, Tasmania, Australia.

Econ. Geol. 78, 1440-1465

489

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

THE ROLE OF SUBCONTINENTAL LITHOSPHERIC MANTLE IN THE GENERATION OF CONTINENTAL BASALTS: GEOCHEMICAL EVIDENCE FROM LAVA-FIELD PROVINCES IN NEW SOUTH WALES Ming Zhang and Suzanne Y. O'Reilly Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC) School of Earth Sciences, Macquarie University, New South Wales 2109

Basaltic rocks from the lava-field provinces in New South Wales during the Cenozoic times are widespread and their Sr-Nd-Pb isotope ratios generally fall in the compositional space defined by oceanic island basalts (OIBs). This indicates a predominant role of sublithospheric mantle sources in the magma genesis and causes difficulties in recognising the role of subcontinental lithospheric mantle (SCLM) as one of the potential mantle source components for these basalts.

However, detailed geochemical studies based on a recently established comprehensive dataset, in combination with analysis of tectonic histories, reveals that the lower SCLM (dominantly in the garnet peridotite stability field) has played a significant role in the genesis of lava-field basalts in New South Wales. The nature of the SCLM inferred from these basalts is consistent with the tectonic evolution in the Lachlan Foldbelt and the New England Foldbelt and corresponds to the geochemical characteristics of the entrained mantle xenoliths.

In the Dubbo area, the coexisting tholeiites and alkaline basalts erupted contemporaneous with the nearby plume-derived central-volcano provinces. The olivine tholeiites show an OIB-type incompatible element pattern and Sr-Nd isotope ratios similar to those of the inferred plume source for primitive basalts of the central-volcano provinces. Therefore, the enrichment in SrNd isotope ratios, the positive correlation between 1 4 ^Nd/ 1 4 4 Nd and Sm/Nd, and the strong depletions of K and Rb exhibited by the alkaline basalts indicate that an old metasomatised SCLM source must have made substantial contributions in the magmas. The lack of correlations between ^ 7 Sr/^^Sr and Rb/Sr in basaltic rocks from Dubbo basalts may be attributed to either partial melting events shortly before the Dubbo volcanism which change the Rb/Sr ratios in the source or the retention of Rb and K in the source during partial melting by phlogopite. A minor proportion of alkaline basalts from the Southern Highlands province to the south with relatively enriched Sr-Nd isotope compositions similar to the Dubbo basanites may also have been generated in the same way.

The primitive alkaline basalts from the Kandos province were erupted in two episodes separated by about 150 Ma: Jurassic (190 - 170 Ma) and Tertiary (50 - 45 Ma). Basalts from both volcanic episodes have similar OIB-like Sr-Nd isotope compositions (^ 7 Sr/ 86 Sr=0.7032 - 0.7036 and £Nd=+5.0 - +3.5) and incompatible element patterns (except for the prominent depletions in Zr and Hf). However, the similarity in both elemental and isotopic chemistry of the Kandos basalts over a time span of about 140 Ma implies that geochemical signatures of these basalts should be derived from a SCLM source (even through the resultant basalts are geochemically indistinguishable from those plume-derived basalts) because horizontal plate movement would have carried this lithosphere away far from the plume source region.

Decomposition of volatile-bearing minerals such as amphibole, apatite, and phlogopite in the metasomatised uppermost lithospheric mantle and their interaction with ascending basaltic magma are also an important factor in modifying the composition of magmas from sublithospheric sources. In Barrington, the primitive nephelinites and basanites have Sr-Nd isotope ratios similar to the other New South Wales lava-field basalts. However, they have distinctive incompatible element patterns that display strong depletions in Rb, K, Zr, and Hf and enrichments in Th, Nb and Ta. These patterns can be modelled by interaction between a parental magma with OIB-like trace element characteristics and an amphibole- and apatitebearing peridotitic mantle. Both amphibole and apatite broke down at the early stage of the process, modifying incompatible element abundances and patterns, but without changing significantly their Sr-Nd isotope ratios as the late Paleozoic tectonism in the New England Fold Belt may not been able to create time-integrated changes in Sr-Nd isotope systematics within the upper SCLM.

490

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

Sr-Nd ISOTOPIC STUDY OF PRE-CHARNOCKITE METAMORPHIC ROCKS IN MAWSON COAST AREA, EAST ANTARCTICA: IMPLICATIONS FOR PROTEROZOIC TECTONIC EVOLUTION J.-X. ZUAQU>2\ D.J. ELLIS*11, J.W. SHERATON^ & M.T. McCULLOCH^, [1] Dept. of Geology, Australian National University, Canberra, ACT 0200; [2] Dept. of Earth Sciences, University of Queensland, Brisbane, QLD 4072; [3] AGSO, GPO Box 378, Canberra, ACT 2601; [4] RSES, Australian National University, Canberra, ACT 0200.

Sr-Nd isotopic data are reported for pre-charnockite gneisses and metapelites in the Mawson Coast area, East Antarctica, where regional granulite metamorphism occurred about 1000 Ma ago. The results confirm a regional distribution of three technically amalgamated terranes each having distinctive crustal formation histories, which was delineated using geochemical and Sr-Nd evidence from the -970 Ma Mawson charnockites (Young et al., 1995). The data are illustrated in the figure below:

Eastern Zone Charnockites

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Figure: Sr-Nd isotopic data for the Mawson rocks.

As can be seen from the figure, initial Sr & Nd isotopic compositions for pre-charnockite metamorphic rocks from the Central Zone are well separated from those of the Western and Eastern Zones. The fields defined by the metamorphic rocks from the three tectonic zones enclose those defined by the charnockites intruding them, with data for the metamorphic rocks displaying a much large scatter. This suggests that the lower crustal source rocks of the charnockites and the metamorphic country rocks hosting the charnockites have similar crustal formation and evolution histories. Some rocks, mainly metapelites, from the Central Zone, show significantly higher time-integrated 87 Sr/ 86 Sr ratios, up to 0.775. This is due to significantly higher Rb/Sr ratios (up to 5.44) in these rocks. The more uniform nature of the charnockites is probably related to averaging effects during partial melting and charnockite magma generation. However, the lack of more primitive £Nd(970 Ma) and initial ^7Sr/8^Sr in the charnockites relative to the metamorphic country rocks indicates the absence of primitive mantle-derived components in the generation of the charnockite magmas, mitigating against the role of an AFC process involving basaltic magma and the lower crust in the generation of the charnockites. Nevertheless, the AFC process cannot be totally discounted, since the mantle-derived component does not have to be necessarily primitive, especially if it was derived from the underlying lithospheric mantle.

This study, together with those of Young et al. (1995) for the Mawson charnockites, and Zhao et al. (1995) for charnockites from the northern Prince Charles Mountains (PCM), further suggests that the Mawson Coast area, and the northern PCM represent a mosaic of Early-Middle Proterozoic orogens formed between Archean cratons (the Napier complex and southern PCM?). These orogens were reactivated, deformed, metamorphosed and thrust along their boundaries during Middle-Late Proterozoic continental collision (-1,000 Ma). The occurrence of igneous charnockites in the Mawson - PCM areas and the regional distribution of Sr-Nd isotopic signatures in the charnockites and metamorphic country rocks are potentially important markers for precise Gondwana reconstruction.

REFERENCES Young, D.N., Zhao, J.-X., Ellis, D.J. & McCulloch, M.T., 1995. Precambrian Res., in press.

Zhao, J.-X., Ellis, D.J., Kilpatrick, J.A. & McCulloch, M.T., 1995. Precambrian Res., in press.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 41 13th Australian Geological Convention, Canberra, February 1996

CLASSIFICATION OF CRUDE OILS BASED ON GENETIC ORIGIN USING MULTIVARIATE MODELING TECHNIQUES J

John Zumberge1 and Scott Ramos2 GeoMark Research, Inc., 9748 Whithorn Dr., Houston, TX 77095 2 InfoMetrix, Inc., P.O. Box 1528, Woodinville, WA 98072

Reliable classification of crude oils as to origin is the goal of this research and related computer program. The molecular and isotopic compositions of oils from given basins/regions and generated from source rocks with known depositional environments, lithology, age, kerogen type, thermal maturity, or other characteristics have been used to construct a model or "expert system" in which new or unknown samples can be quickly and easily compared. Over five hundred oil samples are included in the present training set (or model) for the purpose of predicting not only general source rock characteristics, but also to best match unknown oils with more specific basin origins. The oils were collected from 57 different basins/regions and have been pre-classified into seven source rock type categories (Table 1). In addition, one of 13 source rock age divisions were assigned to each sample.

Many of the variables used in this initial modeling scheme are Table 1. Source Rock Type based on biomarker ratios (GC/MS SIM). These include five 1. Marine Shale (Type II kerogen) tricyclic terpane ratios: C19/C23, C22/C21, C24/C23, C26/C25, 2. Paralic/Deltaic Marine Shale (Type III) and C24 tetracyclic/C23; eight pentacyclic terpane ratios: 3. Marine Carbonate (Type II/I) 25,28,30-trisnorhopane/(Ts+Tm), 28,30-bisnorhopane/hopane, 4. Evaporitic/Hypersaline Source C29 30-norhopane/hopane, C30 diahopane/hopane, oleanane 5. Coal/Resinitic Terrestrial Source /hopane, C31R homohopane/hopane, gammacerane/C31R 6. Lacustrine, Fresh (Type I/II) homohopane, and C35S/C34S extended hopanes; and 5 sterane 7. Lacustrine, Saline (Type I/II) parameters: steranes/hopanes [sum of 15 steranes divided by the sum of 21 pentacyclic terpanes], normalized percent C27, C28, and C29 regular steranes [as measured by the m/z = 218 peak heights of the corresponding pp 20S components], and C27 rearranged sterane/C27 regular sterane [13p, 17a diacholestane divided by aaa cholestane (20R)]. Two parameters derived from whole crude GC-FID were also used as variables: the pristane/phytane ratio as well as the n-C27/n-C17 normal alkane value.

Although stable carbon isotopes are not used in the source rock type classification scheme, the carbon isotope composition of both the CI5+ aliphatic and aromatic hydrocarbon fractions from silica gel LC are used as variables when attempting a classification by basin/region.

Two different classification techniques were used in preparing the models. K-Nearest Neighbors (KNN) is similar to discriminant analysis, in which the N-dimensional distance between all samples in the data set is calculated (where N is the number of geochemical variables), and the test or unknown sample is classified based on the identity of the oils in the training set nearest to the unknown oil. The other classification technique, termed SIMCA (Soft Independent Modeling of Class Analysis), develops specific principal component models for each different category in the training set The predictions based on SIMCA result in one of three possible outcomes: 1) the unknown sample is properly classified into one of the predefined categories; 2) the sample does not fit any of the categories; or 3) the sample properly fits into more than one category. Confidence limits can also be placed on the model predictions.

When categories are strongly subgrouped or the training set is small, KNN works better. SIMCA assumes some degree of homogeneity within each class, works well with large data sets that are not strongly subgrouped, and predicts whether a sample falls within a category or does not fit any category, in contrast to KNN. In the present study, both KNN and SIMCA were used together. The SIMCA models place qualifier restrictions on the KNN predictions. These "match qualities'' are computed for each prediction result, and reported as Excellent, Good, Marginal, Suspect, or Unable to Identify. Two distinct modeling schemes were developed; the first classifies unknown samples into one of seven source rock categories (Table 1). The second modeling scheme attempts to classify samples as being similar to one of 57 different basin/regions.

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Aadil N* Black PM, Ballance PF, Blattner P Abdelmalek K*, Bradshaw M, McConachie B Abell RS*, Roach I Abzalov MZ* Acton GD, Flood PG* Adabi MH*, Rao CP Adabi MH*, Rao P, Kitto P Adamson SE*, Chen XY, Eggleton RA A1 Bakri D* Alexander J*, Fielding CR, Newman-Sutherland E, Campbell L, Munn V Alexander R*, van Aarssen B, Kagi R Allen GP*, Lang SC, Lockhart D Allibone A*, Windh J, Etheridge M, Burton D et al, Fanning CM, Wysoczanski R Andrew AS*, LeGras CA, Akber RA, Bryce AJ, Todd A J Andrew AS*, Whitford DJ, Hamilton PJ Archbold NW*, Shi GR Arculus RJ Bacon C* Bain JHC*, Draper JJ Ballhaus C, Ryan C, Ulmer P Barley ME*, Krapez B, Groves DI, Kerrich R Barton MD Bastrakov EN*, Brooker M, Walshe JL Baxter K*, Smith PL, Hill KC, AGSO NW Shelf Study Group Belonin MD*, Shimanski VK, Smirnov SV Belperio AP* Bennett VC*, Esat MT, Nutman AP Benson JM* Berger BR* Berman DML* Bernecker T*, Logan BW Betts P*, Pound K, Lister G Bickford GP* Bierwirth PN*, Johnston RM Binns RA*, Scott SD Birch G, Hennecke W* Birch G*, Taylor S, Irvine I, Matthai C Birch G*, Taylor S, Matthai C, Fong M Blevin JE, Colwell JB*, Edwards DS, Foster CB, Jones PJ, Kennard JM, Nicoll RS Blevin PL* Blevin PL*, Candela PA, Chappell BW Blevin PL*, Chappell BW Blewett RS*, Black LP, Knutson J, Sun SS, Bain JHC Blewett RS*, Black LP, Wellman P Blewett RS *, Hazell MS Boelema R* Bone Y* James NP Bone Y*, James NP

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Borissova I*, Symonds PA, Creasey JW Boyd R, Diessel C, Elliott P, Zaitlin B Bradshaw BE*, Krassay AA, McConachie BA, Leven JH, Finlayson DM, Domagala J Bradshaw J*, Wyatt B, Foster C Bradshaw M* Bradshaw J, McConchie B, Sayers J, Spencer L Bradshaw M* Branagan D* Branagan D* Brand NW*, Butt CRM Braun J*, Sambridge M Broome JMN Ramsay WRH*, Keays RR, Hughes M, Arne D, Reeves S Brown AC* Brown AV* Brown CE*, Lenz SL Brown CE*, Lenz SL Bryant CJ*, Arculus RJ, Chappell BW Bryant EA* Young RW, Price DM Bucher M*, Foster DA, Gray DR Bucknell WR*, Jockel FCM, Kellett RJ, Vickery NM, Buckley PM Buick IS *, Cartwright I, Williams IS Bultitude RJ*, Fannings CM, Champion DC, Rees ID Burne RV* Burrett C*, Bendall M Burrett C*, Berry R Burrows DP*, McCulloch MT, Marshall JF Cameron GH*, Walshe JL, Heinrich CA, Wall VJ Campbell IH* Carr GR*, Dean JA, Gulson BL, Ashley PM, KorschMJ Carter RM*, Carter L Cartwright I*, Buick IS, Harley SL Casey JN*, Stephenson AE Cassidy KF*, Kent AJR, Fanning CM Cassidy KF*, Lang JR, Lueck BA, Mortensen JK, Russell JK, Thompson JFH Champion DC*, Sheraton JW Chan R* Chaproniere GC* Chenhall BE*, Chiaradia M, Gulson BL, Jones BG, Depers AM Chivas AR*, Key wood MD, Fifield K, Allan GL Chopra P*, Miller P Chopra P*, Miller P Chuvashov BI* Claoue-Long J*, Foster C Collins CDN*, Goncharov AG, Symonds PA, Lukaszyk IS Collins WJ* Sawyer EW Colqhoun GP* Colqhoun G*, Meakin S, Kryner J, Watkins J, Henderson T, Jagodzinski E Cooke DR*, Bull S, Rogers JR, Donovan S Cooke DR*, McPhail DC Corkeron M*, Grey K, Li ZX, Powell CMcA Cox SJD*

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Cramsie JN* Crawford AN*, Donaghy AG, Black LP, Stuart-Smith PG Cross AJ*, McQueen KG Crowhurst PV*, Kendrick RD, Hill KC, Foster DA, O'Sullivan PB Dadd K* Dahlhaus P* MacEwan R Daly SJ*, Fairclough MC, Fanning CM Davies G* Davies HL* Deacon GL* Denaro TJ, Ewers GR Dent BB* DErcole C*, Milton D deVriesD* Dickson T* Diessel CFK* Dogramaci SS*, Herczeg AL, Bone Y Draper DS *, Brandon AD Draper DS*, Green TH Drew GJ* Drummond BJ*, Barton TJ, Korsch RJ, Brown AV, Johnstone DW Dugdale AL* DunlapWJ* Dyson IA* Dyson IA* Dyson IA* Dyson IA* Eggo AJ* Ehlers K, Foster J, Nutman, A P Elliott CI* Elliott CI*, Campbell IB, Wilson CJ, Joyce B Evans FH, Caccetta P, Ferdowsian R Ewers GR*, MacKenzie D, Cruikshank BI, Bain JHC, Andrew AS Exon N* Hill P, Chaproniere G, Whitmore G Exon N, Hill P*, Koslow T Fairclough MC*, Belperio AP, Daly SJ Fanning CM*, Moore DR, Bennett VC, Daly Sj Feary DA*, James NP, McGowran B, Smart PL Fergusson CL* Fergusson CL*, Colquhoun GP Fielding CR*, Stephens CJ, Holcombe RJ Fielding CR*, Webb JA Finlayson DM*, Lukaszyk I, Chudyk EC, Moore AMG Flood PG* Flood PG* Flottmann T* Haines P, James P, Belperio AP Foden J* Foden J* Foster JG*, Lambert DD Frankel E, Shipboard Scientific Party Leg 160

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Franklin JM* Fredericks DJ, Heggie DT, Cowdell R, Longmore A, Lapworth C Freeman MJ*, Richards G, Mills K, Rippon G Frick LR*, Lambert DD, Cartwright I Frost BR* Gemmell JB* George AD*, Playford PE, Powell CMcA Ghaderi M* Campbell IH, McCulloch MT, Bennett VC, Mortimer GE Gildeeva I* Giles AD*, Marshall B Goncharov AG*, Collins CDN, Goleby BR, Drummond BJ GorterJD* Graham IT*, Franklin BJ, Marhsall B, Leitch EC, Fanning M Graham S*, Lambert DD, Shee SR, Hamilton R, Foster JG Granger K* Gray DR*, Foster DA, Bucher M Green DH* Green TH* Griffin WL*, O'Reilly SY, Ionov DA, Ryan CG Grosser MR*, Lang SC, Jones MR Guj P*, Preston W Gulson BL*, Davis JJ GumJC*, Jago JB Gunn PJ* Gunn PJ* GurbaL*, Ward CR Gust DA*, Arculus RJ, Kersting AB Habermehl MA* Lau JE, MacKenzie DE Wellman P Haines PW*, Flottmann T, Gum JC, Jago JB, Gatehouse CG Hand M*, Bendall B, Sandiford M Hand M*, Sandiford M, Bingemer A Handler M*, Bennett V, Esat T Haren R*, Gibson G, Maidment D, Gunn P, Milligan P Harrington HJ* Harrington HJ*, Korsch RJ, Wyborn D Harrington HJ*, Powell CMcA Haynes DW* Hazell MS *, Peljo M, Wyborn LAI Hazell MS*, Wybom LAI Heggie DT*, Berelson W, Skyring GW, Cowdell R, Longmore A, Nicholson G Henderson GTM, Morgan EJ, Raymond OL, Scott MM*, Warren AYE, Wybom D Henderson RA* Hensen BJ*, Zhou B Hill KC*, Cooper G Hill SM*, Taylor G, Eggleton T Hillis RR*, Mildren SD Hilyard D* Hodges A*, AM Hofmann GW* Holdgate G*, Gallagher S

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Holdgate G*, Kershaw AP, Sluiter IRK Horn R, Belperio T*, Freeman P Huang W* Lister GS Hudson GR* Hughes JD Hughes MJ*, Phillips GN, Gregory L Hughes P, Cooke DR*, Kitto PA, Large RR Huston DL*, Taylor BE Huston DL*, Taylor BE, Watanabe DH, Bleeker W Hutton AC*, Mandile AJ, Faiz MM Hutton L*, Fanning CM, Garrad P Hutton L*, Rienks I, Fannings CM, Gust D Idnurm M, Giddings J* Ingpen I* Innes P, Kitto PA*, Cooke DR, Large RR Ionov D*, O'Reilly SY, Griffin WL Jackson I*, Rigden S Jackson MJ*, Southgate PN, Krassay AA, McConachie BA, Wells AT, Scott DL Jacobson G*, Wischusen J, Lau JE Jaireth S*, Miezitis Y, Lambert IB Jenkins C* Johnston R* Jones BG*, Chenhall BE, Payne M, Murrie M Jones JA*, Stephens CJ, Ewart A Jones LEA*, Stace R Joyce EB* Joyce EB* Joyce EB* Kamenetsky V*, Crawford AJ, Varne R, Lanyon R Keay S*, Lister G, Compston B Kelly JC*, Webb JA Kennard JM*, Southgate PN, Jackson MJ Key wood M*, Chivas A, Fifield LK, Cresswell R Kitto PA*, Cooke DR, Large RR Knight MJ* Knoll AH* Knutson J*, Withnall IW, Sun S, Blewett RS, Bultitude RJ, Black LP, Rees ED Koeberl C*, Poag CW, Reimold WU, Brandt D Koons PO* Kotsonis A* Krassay AA*, McConachie BA Lang SC*, Fielding CR Large RR* Larsen DF*, Webster AE Lawrie KC* Leggett J* Leitch EC* Leitch EC*, Cawood PA Leven JH*, Finlayson DM Lewis GB*

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Li ZX* Zhang L, Powell CMcA Liang TCK, Liu K* Paterson L, Kendall CStGC Lister GS* Lister GS*, Raouzaios A Little M* Boyd R, Brunton J, Diessel C, Ives M, Rigby R, Tobin C Liu S* Griffin TJ Loader SE*, Barley ME, McNaughton NJ, Lockhart DA*, Lang SC, Allen GP Logan GA*, Hayes JM, Hieshiema GB, Summons RE Loucks RR*, Hibberson W Loucks RR*, Palin JM, Shelley JM, Mavrogenes JA Loutit TS *, Southgate PN Lu C*, Jackson I Lus WY*, McDougall I, Davies HL Ma C*, Eggleton T MacCready T*, Goleby BR, Lister GS, Drummond BJ Mackenzie DE*, Wellman P, Champion DC Mahmood T*, Lemon N Mares VM* Mark G* Markovics G* Martin DMcB*, Horwitz RC, Li ZX, Powell CMcA, Twist C, Worth M Matthai C*, Bickford GP, Birch GF, Matthai C*, Birch GF Matveev S*, Fricke K, Ballhaus C Mawby J*, Kinny P, Foden J MawerCK* Mawer CK*, Doutch HF Mawson R*, Talent JA, Leitch EC Mayer W* McBride JS*, Lambert DD, Nicholls IA McClure ST, Lang SC* McConachie BA*, Scott DL, Wells AT, Southgate PN McConchie D*, Saenger P, Fawkes R McCue K*, Boreham B, Van Dissen R, Gibson G, Jensen V, McKavanagh B McCulloch* McDonald GD*, Collerson KD, Wendt JI McDougall I*, Honda M McGoldrick P*, Keays R, Smith R, McOrist G, Fardy J Mclnnes BIA*, Evans NJ McNally G* McNally G*, Wilson I McQueen KG*, Taylor GM Mernagh TP* Metcalfe, I Michael-Leiba M*, Jensen V Mignone J* Mildren SD*, Hillis RR Miller J*, Cartwright I Miller JM*, Gray DR

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Millican P*, Rynn J, Sprenger N Millsteed PW* Miyazaki S*, Cadman SJ Morante R* Morrison G*, Seed M, Bobis R, Tullermans F Moss G*, Foster C Moss G*, McGowran B Mulder CA* Munroe SM*, Cox SF Munroe SM*, Williams IS Murray A*, Lee C-S, Cortez E, Reyes E, Schiefelbein C Mus grave RJ*, Housen BA Naldrett AJ* Needham S *, Jones W Nethery JE*, Barr M Nicholls I*, Bagaric S, Dean A Nicholls I*, Elburg M, Dean A, Bagaric S Nicholls JT*, Fahey A Nichols GT*, Green TH, Pearson N, Sharma A Nicoll RS*, Shergold JH, Laurie JR, Bischoff GCO O'Brien BJ* O'Brien PE*, Harris PT O'Brien PE*, Leitchenkov G O'Dea MG*, Lister GS O'Halloran GJ*, Cas RAF O'Reilly KW*, Buzacott R O'Reilly SY*, Xu X, Zhang A, Griffin W L O'Sullivan AJ, O'Sullivan PB*, Hill KC O'Sullivan PB*, Kohn BP, Foster DA, Gleadow AJW Occhipinti SA*, Swager C, Pirajno F Ogasawara M* Oliver RL* Oliver RL* Opdyke B*, Bird M Ord A* Page RW*, Sun S-S, Blake DH, Edgecombe DR, Pearcey DP Parkinson IJ*, Arculus RJ, McPherson E, Duncan RA, Stanton RL Passmore VL*, Maung TU, Gray A1RG, Wellman P Pavlik R, Street G Pearson NJ*, Norman M, Sharma A, Griffin WL Petkovic P* Stagg HMJ, Willcox JB Phillips GN* Phillips GN*, Hughes MJ Pickering KT Pirajno F*, Davy R Plumb KA* Plumb KA*, Pietsch BA, Page RW Pockley P* Pokrovskii VA* Polito P*, Bone Y

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Post NJ*, Kinny PD, Hensen BJ Potma WA* Powell CMcA* Preiss WV* Preston J* Price RC*, Stewart RB, Smith IEM Radke S *, Passlow V, LePoidevin S Ramsay WRH*, Morand VJ, Hughes M Rao CP* Raouzaios A*, Lister GS, Foster DA Raymond OL* Raza A*, Hill KC, Gleadow AJW, Brown RW, Korsch RJ, Kohn BP Reid LG* Rezaee MR*, Tingate PR Rickard MJ* Ritchie A*, Yeung M Roach IC*, Edgecombe SM Roach MJ*, Leaman DE Robb LJ* Robert F* Poulsen KH Robinson AM* Romine KK*, Jackson MJ, Kennard JM, Shaw R, Southgate PN Roots WD* Rossiter AG*, Gray CM Ryburn RJ* Rynn J*, Brennan T, Hughes P, Pedersen I, Stuart H Sambridge M, Braun J*, McQueen H, Savage MD, Barley ME*, Mcnaughton NJ Sayers J*, Spencer L Schaefer BF* Scott DL*, Betts PG, Rogers JR, Bradshaw BE, Jackson MJ, McConachie BA, Southgate PN Scott MM* SeddonG Sha L-K*, Chappell BW Shafik S* Shafik S*, ODP 159 Shipboard Scientific Party Shafik S, Lee C-S*, Borissova I Sharp T* Shee SR*, Vercoe SC, Wyatt BW, Campbell AN, Colgan EA, Hwang PH, Merritt BD Shepherd JM*, Dodds V, Longhurst D, Groves D, Ho S Sherwood N*, Russell N Shi B*, Plimer IR Shoemaker EM*, Shoemaker CS Shubber B*, Bone Y McGowran B, James N Shubber B*, Bone Y, James N, McGowran B Sillitoe RH* Simpson A* Simpson A*, Talent J Simpson CJ* Simpson CJ*

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Simpson G*, Lang SC Singer DA* Sircombe K*, Compston W Sivell WJ*, McCulloch MT Sivell WJ*, McCulloch MT Skilbeck CG*, Leitch EC Skilbeck CG*, Soto JI, Piatt JP, ODP Leg 161 Scientific Party Smart P* Smith AB*, Gardoll SJ, Robinson DC Smith DEM*, Davies HL, Hegner E Smith JB*, Barley ME, Groves DI Smith JV* Cotter SJ SmithS A* Somerville M, Wyborn D*, Chopra P Southgate PN*, Loutit T, NABRE Team Spikings RA*, Foster DA, Gleadow AJW, Kohn BP Sproule R*, Lambert D, Murphy D Stagg HMJ* Symonds PA Stagg HMJ*, Willcox JB Stephens CJ*, O'Connell SJ, Holcombe RJ, Fielding CR, Ewart A, Messenger PR Stevens BPJ*, Fanning CM, Crawford AJ Stratford JMC, Aitchison JC, Flood PG* Street G Summerfield MA* Sun S-S*, Sheraton JW, Glikson AY, Stewart AJ Sun X*, Gatehouse CG Sun X*, Jago JB Sutherland FL*, Fanning CM Sutherland FL*, Hollis KA Sutherland FL*, Pogson RE, Webb G Sverjensky DA* Sweet I*, Crick I Symonds PA*, Planke S, Colwell JB, Crawford AT Tan B-H*, Jackson I, Fitz Gerald J Tarlowski C, Gunn PJ*, Mackey T Tate MC*, Clarke DB Taylor B, Mutter J, Binns R*, Davies, H, Rogerson R Taylor GF* Taylor GR*, Hewson RD, Mah A Taylor WR* Teasdale J* ten Haven HL* Tickell SJ*, Nixon R Tingate PR*, Alexander EM, Michaelsen BH, Duddy IR, Griffiths CM Tong Cheng*, Kennett BLN Tonui EK*, Eggleton T, Taylor G Totterdell JM*, Krassay AA, Korsch RJ Tran Nghi* Turner S*, Hawkesworth C Tye SC*

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Tye SC*, Bann KL 449 Tyler IM, Page RW* 450 Valenta R*, Wall V 451 Van Der Beek P* 452 453 Van Der Hilst RD, Zielhuis A, Kennett BLN* Van Noord K* 454 Walter MR*, Swart R 455 Walter MR*, Veevers JJ 456 Walters S* 457 Wang Q* Campbell IH, Schiotte L 458 Wang X*, Bird M, Van Der Kaars S, Kershaw P, Bishop P 459 Warren RG* 460 Warren RG* Ellis DJ 461 Waterhouse HK* 462 Waterhouse HK* 463 Weaver TR*, Cherry JA, Frape SK 464 Webb JA*, Garland C 465 Webb JA*, O'Sullivan PB 466 Wellman P* 467 Wells AT*, Page RW 468 Whitehead J*, Geary P 469 Whitford DJ*, Andrew AS Hamilton PJ, Mawson R, Morante R, Talent JA 470 Whiting TH* 471 Wilford JR*, Hoggett A, Butrovski D 472 Willcox JB* 473 Williams C, Lewis A* 474 Williams N 475 Willman CE*, Hendrickx M, Vandenberg AHM, Cayley RA, Taylor D 476 Winefield PR*, Nelson CS, Hodder APW 477 Withnall IW*, Draper JJ, Daly MR 478 Withnall IW*, Fergusson CL, Green TJ 479 Wolfe RC*, Cooke DR, Hooper B, Heithersay PS 480 Wood GD* 481 Wybom LAI* 482 Wyborn LAI*, Jagodzinski EA, Jacobsen G, Needham RS 483 Wyche S, Farrell TR, Griffin TJ, Langford RL, Liu SF, Stewart AJ, Westaway JM, Whitaker AJ 484 Wysoczanski RJ*, Berg JH, Fanning CM 485 Yaxley GM*, Green DH, Falloon TJ 486 Young DN*, Ellis DJ, Zhao J-X, McCulloch MT 487 ZangW-L* 488 Zaw K*, Large RR, Huston DL 489 Zhang M*, O'Reilly SY 490 Zhao J-X*, Ellis DJ, Sheraton JW, McCulloch MT 491 Zumberge J*, Ramos S 492

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