Abstract 44 - SGEG New Developments in Research for Ore Deposit Exploration Canberra 1997

Geological Society of Australia

ABSTRACTS Number 44

New Developments in Research for Ore Deposit Exploration Third National Conference of the Specialist Group in Economic Geoiogy

Canberra 30-31 January 1997

GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS No. 44 Specialist Group in Economic Geology Conference, Canberra, January 1997

ISSN 0729-01IX

© Geological Society of Australia Incorporated 1997

Copies of this publication may be obtained from the Geological Society of Australia Incorporated, 1203 Wynyard House, 301 George St, Sydney, NSW, Australia 2000 Example citation for papers in this vohmie: Goldfarb, J.K, Nesbitt, B.E. and Newberray, R.J., 1997, Gold vein formation during Cordilleran Orogenesis: a consequence of Metamoiphic Devolatilization, calc-alkaline magmatism, or (and) deep circulation of meteoric fluids. Geological Society of Australia, Abstracts No. 44, pp 30.

GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS Na 44

Specialist Group in Economic Geology Conference, Canberra, January 1997 ACKNOWLEDGEMENTS This conference would not have proceeded in this format without the generous support of^onsors. Major conference q)onsors are: Australian Geological Survey Organisation GPO Box 378 Canberra ACT 2601

AGSO A U S 1 K A L I A .Nl Ct:0<.0CiC\L si/r;v t:> U K C, A N 1 ^ "i : . • N

North Exploration

476 St Kilda Rd Melbourne Vic 3004

Placer Pacific Limited

GPO Box 4315 Sydney NSW 2001

Normandy Exploration Limited

POBox 751 Kent Town SA 5071

NORTH

^LACER PACIFC LIMITED LIMITED

Newcrest Mining Limited

POBox 1367 Milton QLD 4064

Newcrest Mining Also provided ^onsorship for stiident delegates.

The organising committee would also Kke to give special thunVc to the University of Western Australia's Key Centre in Strategic Ore D^osits, and to Professor David Groves and his staff personally, for organising their teaching schedules to enable the key g)eakers to attend the conference.

GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS No. 44 Specialist Group in Economic Geology Conference, Canberra, January 1997

CONFERENCE ORGANISING COMMITTEE Members of the Organising Committee are: Ken Lawrie Terry Memagh David Huston Evgeniy Bastrakov Phillip Ble\Tn Kevin Cassidy Greg Ewers Ken McQueen Roger Skirrow John Walshe DooneWybom

Chauman Secretary Treasurer

MOVEMENT HISTORY OF THE STARRA AND SELWYN HIGH STRAIN ZONES, STARRA OREBODIES, MT ISA INLIER: SIGNIFICANCE FOR MINERALISATION Nicole S. Adshcad-BeU National Key Centre in Economic C5cology, Department of Earth Sciences, James Cook University Townsville QLD, 4811

Starra Au-Cu (7.4Mt @ 3.8g/t Au and 1.9% Cu) orebodies are hosted by the Staveley Formation in magneute ironstones and comprises five small high grade orebodies (222, 244, 251. 257 and 276) The timing of mineralisation is interpreted to be syn-post D4 (D4 equates with regional D3 as observed by workers in the Mt. Isa region) and localised by the development of the Starra high strain zone The associated eastern (barren, hematite rich ironstones) are localised by the development of the Selwvn hieh strain zone. '' ^ The western and eastern ironstones were previously interpreted to be folded by D2. Structural moping ^oth surface and underground) integrated with microstructural analysis of spatially oriented thin sections from rocks composing the high strain zones, eastern and western ironstone lenses and the Starra mine area has provided evidence for several distinct stages of cleavage development (Sj-Se). All spatially oriented thin sections, preserving clear-cut S(/S2 asymmetries, uniformly indicate an F2 antiformal hinge west of the Starra high strain zone and F2 synformal hinge east of the Selwyn high strain zone. Significantly the dominant fabric in the mine region is a composite S2-S4 fabric, rather than the previously interpreted S, fabnc. The mam effect of S4 was to reactivate the previously developed S2 foliation.

Previous workers (Laing et al., 1988) invoked the Starra Shear to explain perceived grade differences between inferred upper greenschist grade Staveley Formation and amphibolite grade Gin Creek Block Beardsmore (1992) interpreted that another shear zone, the Selwyn Shear, occurs along the boundary between the Staveley Formation and the Soldiers Cap Group. Recent work suggests that there is no difference in grade between the Staveley Formation and the Soldiers Cap group. Deformed localised amphibohte grade rocks have been noted within the Staveley Formation and Rotherham (pers.comm.) has inferred that amphibolite facies mineral assemblages exist within the ore zone. The Starra and Selwyn Shears were interpreted to have initiated during Dj and subsequenUy reactivated by later deformation events (Laing et al., 1988). In contrast, recent work suggests the Starra and Selwyn Shears are actually zones which represent an increase in strain during the development of D4 (the terms Starra and Selwyn high strain zones are preferred) rather than shear zones initiated in D,. The shear sense during D4 was west-side-up (dextral looking north).

Previous work (Rotherham pers.comm.) demonstrates that gold mineralisation is associated with the hematisation of magnetite. Preliminary microstructural analysis of samples fi-om the 222, 244, 244-257 orebodies indicates that magnetite has grown syn-post the development of the dominant S4 cleavage and thai hematisation of magnetite has occurred post D4 syn De. In conclusion, the eastern and western ironstones are not folded by D2, but occur as discrete shear controlled (syn D4) north-south striking lenses and mineralisation is not syngenetic in origin and/or localised by D2 folds but is controlled by late syn \yjvjn Dg shear controlled iron metasomatism

REFERENCES Beardsmore, T.J., 1992. Petrogenesis of Mount Dore-style breccia-hosted copper ± gold mineralisation in the Kundala-Selwyn region of northwestern Queensland. Unpublished PhD thesis. James Cook University Queensland, Australia. / Laing. W.P., Rubenach, M.J. & Switzer, C.K., 1988. The Starra gold^opper deposit: syndeformational metamorphic mineralisation in a folded early regional zone of decollement. Geological Society of Australia Abstracts, 21,220. ^ ^ Acknowledgements: Selwyn Mine (ARIMCO) are thanked for logistical support.

THE NIFTY COPPER DEPOSIT - GEOLOGY AND yTRUCTURE Bruce Anderson', Patrick Dare', Ron Berry', and J. Bruce Gemmell' Special Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79C, Hobart, TAS 7001 WMC Resources Ltd (Nifty Copper Operations), PO Box 7001, Qoisters Square, WA 6850

The Nifly copper deposit is a 32 Mt @ 3.2% orebody located approximately 450 km east of Port Hedland, Uesiern Australia in the Prolerozoic Paterson Orogen. The Paterson Orogen is a northwest trending bell of folded and metamorphosed igneous and sedimentary rock (Hickman et al., 1994) consisting of the Rudall Complex, the Yeneena Supergroup and the Tarcunyah Group (Williams & Bagas, in press). Several other major economic mineral deposits occur with the Paterson Orogeny, including the Telfer Au-Cu deposit the unconformity-associated, vein-type Kintyre uranium deposit and the low-grade, disseminated Maroochy'dore Cu deposit.

The Rudall Complex is composed of the oldest rocks (Palaeoproterozoic) in the Paterson Orogen and is comprised of an arenaceous and pelitic succession intruded by pre- and syn-orogenic granitoids (Hickman et al., 1994). The Rudall Complex has been metamorphosed to amphibolite facies and overprinted by a highly variable retrogressive greenschist facies metamorphism (Bagas & Smithies, 1995). Unconformably overlying the Rudall Complex are rocks of the Yeneena Supergroup. The Yeneena Supergroup is comprised of sandstone, shale and carbonates of the Throssell and carbonate dominated Lamil Groups, but the relationship between these two groups is unclear (Bagas & Smithies, 1995). The Throssell Group has been metamorphosed to greenschist facies (Hickman & Clarke, 1994).

The Nifty copper deposit is hosted within the Upper Unit of the Broadhurst Formation of the Throssell Group.

1 he Broadhurst Formation is a succession of carbonaceous shale, turbiditic sandstone and shale beds with minor sandstone, dolomite and limestone units (Hickman et al., 1994). The Nifty copper deposit occurs as a 12 Ml secondary oxide body pre.sently being mined by open cut methods and a larger, unmined primary chalcopyrite-bearing primary body. Primary mineralisation is hosted by silicified cryptalgal textured carbonates of the Nifty Carbonate Member (Norris, 1987) and occurs as vein networks and carbonate replaeement. A prominent bed with abundant evaporite pseudomorphs and anomalous Pb and Zn occurs at the top of the Nifty Carbonate Member and is locally called the Pyrite Marker Bed. Five deformation events are recogni.sed from bench mapping in the open pit and outcrop mapping in the Nifty area. is recognised as a minor recumbent, isoclinal and angular upright folding event with the direction of maximum compression indeterminable. D,., is a regional folding and cleavage development event that occurred in response to NE-SW compression. Folds lend to be doubly plunging with axes trending NW-SE Regional D,, has been defined as the Miles Orogeny (Bagas & Smithies, 1995). Mineralisation at Nifty is confined within a D^ synform. D^, is an event that has not been recognised elsewhere in the Yeneena Supergroup, but at Nifty D^, occurs as an upright folding and faulting event that folds S„. A pooriy developed spaced cleavage is observed in the core of D^, folds. D,, is a complex folding event with axial planes generally horizonatal and trends coaxial to It is interpreted that the D^ folds resulted from stress release. D has been correlated to the Paterson Orogeny (Bagas & Smithies, 1995). A brittle deformation event, DNS, has'also been recognised. This structural synthesis shows that there is significant deformational complexity associated with the Nifty copper deposit.

REFERENCES Bagas. L., & Smithies, R.H., 1995. Geology of the Connaughton 1:100 000 Sheet, Western Australia. Western

Australian Geological Survey Explanatory Notes.

Hickman. A.H. & Qarke, G.L., 1994. Geology of the Broadhurst 1:100 000 Sheet. Western Australian

Geological Survey Explanatory Notes.

Hickman. A.H., Williams, l.R. & Bagas, L., 1994. Proterozoic geology and mineralisation of the Telfer-Rudall

region. Geological Society ofAustralia (WA Division) Excursion Guide 5, 60 p.

Norris, M.S., 1987. Geology of the Nifty arbonate Member, Broadhurst Formation, Paterson Province Western Australia. Unpublished M.Sc. thesis, University of Western Ontario, London, Canada, 295 pp Williams. l.R. & Bagas, L, in press. Geology of the Throssell 1:100 000 sheet, W^tern Australia.

Australian Geological Surwy Explanatory Notes.

ISOTOPE HYDROGEOCHEMISTRY IN EXPLORATION FOR BURIED AND BLIND MINERALIZATION, EYRE PENINSULA, SOUTH AUSTRALLV _AniU S. Andrew' Graham R. Can'. Angela M. Giblin'and David J. Whhford' ^ 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

Buried and blind deposits, with no direct geological or geochemical manifestaUon at surface, are becoming increasingly important targets in Australia. One of the key exploraUon challenges relates to assessing and ranking targets established from geophysical and other remotely sensed sur\'eys. One approach is the detecUon of concealed ore-bodies from the isotopic composition of Pb, S and Sr dissolved in groundwaters Success in this approach will lead to cost effective methods of assessment of anomalies beneath cover; samples being taken from water bores, old dnil holes, natural springs or percussion holes drilled to the water table.

At Menninnie Dam significant Pb-Zn-Ag mineralization is "blind"; the host Hutchison Group metasediments are unconformably overlain by variable thickness of Gawler Range Volcanics. Outcrop is poor and weathering IS deep The exploration problems are typical of thos^ found on the ancient peneplaned Australian surface Further-more, the non-mineralized Gawier Range Volcanics cover adds another level of complexity to explorauon Within the area of drilling, the surface of the water table slopes genUy from north to'south consistent with the regional topographic slope, although local surface drainage varies considerably over the prospect.

The Pb isotope signature of galena in Menninnie Dam mineralization is typical of Proterozoic mineralization plotting close to the crustal growth curA-e and with ^ P b / ^ P b ratios in the range 16.0-16.3. Sulfide minerals (py^po. cpy. gal and sph) from the mineralization have S^'S values in Uie range -2.5 to 5 6%o CDT The predoininant source of sulfur in the surficial em iromnent of the Eyre Peninsula is that derived from marine aerosols An estimate of the sulfur isotope value for regional groundwaters (16-18%o) is derived using regional lacustrine and regolith g>psum samples (18.6%o) and by direct analysis of sulfate in the waters from regional bores and a percussion hole collected in August 1993 range (l6.3-19.5%o).

Groundwaters sampled from within the immediate mineralized environment have Pb isotope ratios (<16 3) that approximate to the well defined galena signatur^ Also, the waters from within the central part of the prospect Fb/

^ ^ ^ ^ ^ ^ distal waters ^ t h Pb ratios (17.3-18.8) indicative of significam mixing with host rock Pb with ^ b / ^ P b ratios >18.

The vaJues for sulfate in saline groundwaters from the prospect area (ll.9-17.5%o) all show some depletion relative to the assumed regional background, with the lowest values occurring in the south. The north to south disinbuuon of 5 S values suggests mixing sulfate derived from weathering sulfides and sulfate-rich regional groundwaters, down the hydromorphic gradient. Sulfate content does not show such variation and is controlled by precipitation and dissoluUon of sulfate minerals. Mass balance calculations show the most '^S-depleted suindes"'''"

Dam (8'^S=11.9%o) have about 35% of their sulfate derived from oxidizing

The major NS fault di% ides the prospect beneath the unconformity into east and west zones. The ""ST/^ST ratio best renec^ this gross basement geology in groundwaters, with significamly higher ''Sr/^Sr in the west of the prospect The overall high 'Sr/«^Sr ratios suggests that the groundwaters mihin the GRV are reHecting the baseinem hthologies and that we are thus seeing through the cover rocks. The relativelv high ^Sr/^Sr raUos in groundwaters from Memiimiie Dam appear to reflect the presence K-rich alteraUon related to mineralization.

Isotopic methods complemem conventional hydrogeochemical/cxplorauon methods and have proved highly L^rrl'rll "" explorauon. Lead and S isotopes provide e i i l y interpretable dired ore indicators; Pb isotopes provide a local target whereas S isotopes h a ^ application in Z r .Trf.r "P kilometres from m i r a i Z r ' T ?

Sr/-Sr raUos in groundwaters, apparenUy r e f l i u n g alteration related to mineralizauon, may also provide a usefiil regional exploration tool.

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exploration technology and isotope methods could be ^ S and Sr isoto^^analyses will have application in regional target defmiUon whereas Pb will have applicaUon in prospect scale^aluation. a ^ t L n t

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CRYPTIC GOLD IN THE HILLGROVE SB-AU DEPOSIT, NSW, WITH IMPLICATIONS FOR METALLURGY AND ORE GENESIS Paul Ashley ^ and Chris Crcagh^ 1. Department of Geology and Geophysics, University of New England, Arraidale, NSW 2351 2. Murchison United NL, Level 20 Jetset Centre, 288 Edward St., Brisbane. Queensland 4000

Many gold mining operations are hampered by the presence of "refractory" gold, i.e. non-particulate gold cr>'ptically held in apparent solid solution in certain common sulphide minerals, especially arsenopyrite and arsenian pyrite. Such gold cannot be extracted by conventional milling, gravity or cyanidation techniques and requires alternative methodologies such as roasting, pressure oxidation and bacterial leaching of ores. This type of problem is relatively common in arsenopyrite-bearing gold ores of mesothermal type, but also appears in certain deposits with epithermal characteristics where arsenian pyrite may be prevalent. At the Hillgrove Sb-Au deposit in northeastern NSW, about 25% of gold produced is contained in an arsenopyrite-pyrite concentrate in which the gold is in cryptic form.

Currently, this concentrate is exported for gold recovery. Milling and processing operations at Hillgrove also result in about 20% gold loss to tailings, in part due to cryptically-held gold in arsenopyrite and pyrite. New England Antimony Mines is presently investigating methods of processing the arsenopyritepyrite concentrate on site with the view to enhancing gold recoveries and reducing costs.

Mineralisation at Hillgrove occurs in steeply-dipping mesothermal veins systems cutting late Palaeozoic metasediments and granite of the New England Orogen. The veins occupy extensive fracture systems displaying dilational sites. Fracture-filling material is dominated by quartz and stibnite, with a little carbonate and traces of gold, electrum and aurostibite; this mineralising event largely post-dates earlier deposition of carbonate, quartz, scheelite, pyrite and arsenopyrite in veins and adjacent altered wallrock.

Arsenopyrite-pyrite concentrates largely derive from the latter style of mineralisation and typically contain 100-200 ppm Au. As particulate gold is essentially absent in the concentrate, it follows that gold is mostly held in cryptic form.

Investigations involving KMn04-H2S04 staining, and electron and proton microprobe analyses have shown that both arsenopyrite and pyrite are compositionally zoned. In arsenopyrite, grain cores are commonly As-deficient and contain up to several wt % Sb. There is some tendency for oscillatory zoning and to somewhat more As-rich rims. Gold concentrations correlate modestly with increased As content and are typically in the range 250-2000 ppm. Arsenopyrite also contains several hundred ppm of Ag and Cu. Pyrite is commonly complexly and oscillatory zoned, and is typically arsenian, with up to 9.4 wt. % As. There is moderate correlation between As content of pyrite and Au content, with the latter being present at up to 220 ppm. Arsenian zones also contain the highest contents of Cu, Ag, Sb and Se in pyrite.

Stibnite concentrates, previously leached by thiourea, generally contain about 30 ppm of residual Au. Although it is possible that minor Au (probably <20 ppm) may be held in stibnite, most is present in rare grains of arsenopyrite, pyrite, aurostibite and encapsulated gold/electrum. The fact that both arsenopyrite and pyrite contain cryptically-held Au in complex zonation (with arsenopyrite containing about 7 times that of pyrite), implies that metallurgical recovery of Au from concentrates of these minerals must require complete mineralogical breakdown. To improve Au recoveries from stibnite concentrate, finer grinding may liberate some of the encapsulated particulate Au and careful control of stibnite flotation quality is necessary to exclude arsenopyrite and pyrite contamination.

The presence of cryptic Au in arsenopyrite and arsenian pyrite at Hillgrove further confirms mineral chemical studies from several other mesothermal gold deposits. Gold substitution in arsenopyrite and pyrite appears to be favoured by high-As zones (perhaps by Au-Fe^"^ (AsS)^' exchange). In pyrite, arsenian zones also contain highest Cu and Sb contents, perhaps involving minor solid solution with a tetrahedrite-type phase. The tendency for strong non-stoichiometry and As-deficiency in Au-bearing arsenopyrite at Hillgrove may indicate solid solution with a FeS2-type phase and together with the fine scale, complexity and sharp compositional changes across zones in arsenopyrite and pyrite probably reflect rapid, non-equilibrium, depositional conditions. Mineralisation may have occurred at -3(X)°200°C, with rapid precipitation facilitated by periodic pressure drops during extensional faulting, vein dilation and possible phase separation in the fluid phase. Precipitation of Au-bearing arsenopyrite and arsenian pyrite was a prelude to the main phase of vein- and breccia-hosted gold and stibnite mineralisation. Their presence in altered wallrock leads to potentially extensive geochemical haloes of Au and As about mineralised structures. This geochemical dispersion could be favourably utilised in underground and surface exploration.

Acknowledgements: Funding for this study came from New England Antimony Mines and a small ARC grant. We thank NEAM for logistic support, and David French, Ken Kinealy and Chris Ryan at CSIRO Division of Exploration and Mining for electron and proton microprobe analyses.

GEOLOGY AND ORIGIN OF THE TALLAWANG MAGNETITE SKARN, GULGONG, NSW Shaun Avshford Robin Offler and Philip K. Seccombe Department of Geolog\', The University of Newcastle, Callaghan NSW 2308

Located north of Gulgong, NSW and a source of high-grade magnetite for coal beneficiation, the Tallawang magnetite skam represents one of the few economic skam deposits of the north-eastern Lachlan Fold Belt. The deposit lies within rocks of the North Hill End Trough, west of the Capertee High and the post-orogenic, Middle Carboniferous Gulgong Granite. Mineralisation has developed in a sequence of felsic and basic metavolcanics and pelitic and calcareous metasediments of Silurian to Devonian age, at the margin of a deformed, sill-like microgranodiorite intrusion. Structurally disrupted and deformed, pod-like bodies of semi-massive magnetite and calc-silicates dip steeply W at the contact of the deformed granodiorite.

Skarn formation developed in two major stages. An anhydrous skam assemblage comprising magnetite, garnet (grossularite-andradite) and hedenbergite formed by contact metamorphism at shallow levels in a continental arc en\ ironment (P-^lKb; T--600' to 650°C). Brecciation of the magnetite preceded a hydrous stage of skam formation invoK ing deposition of further magnetite, accompanied by ferro-pargasite, quartz, apatite and pyrite.

Skam mineralisation is overprinted by regional metamorphism under lower amphibolite facies conditions in the late Devonian-Early Carboniferous. At least two ductile deformation events are recorded at Tallawang. Peak metamorphic conditions were established syn- to post-Dj at P, <3kb and T in the range 530° to 590°C. Prograde metamorphic conditions are inferred from the gamet-biotite geothermometer and assemblages including cordierite-gedrite+chlorite in the metamorphosed skam, cordierite+biotite in metapelites and epidote-^homblende+Ca-plagiocIase in amphibolite.

Veins containing a retrograde metamorphic assemblage of chlorite+epidote+calcite+quartz or actinolite were established under g^eenschist facies conditions, consistent with mean Th data in the temperature range 350° to 365°C for carbonic and aqueous fluid inclusions from vein quartz and epidote. Emplacement of the Early Carboniferous Gulgong Granite has had little effect on the skam deposit and represents the onset of late brittle deformation and minor sulphide mineralisation.

Iron at Tallav^ang is likely to have been mobilised from the volcano-sedimentary sequences. Sulphur isotope compositions for pyrite (6^'S range 8.3 to 9.9 per mil) give a signature typical of a sulphide source derived from crusta! rocks of the Lachlan Fold Belt.

The magnetite skam at Tallawang has been subjected to a complex geological history. The orebody in its present form represents the effects of an early metasomatic event overprinted by multiple stages of deformation over an extended period. The asemblages developed in the rocks indicate an anticlockwise P-T-time path, which is characteristic of magma-controlled metamorphic complexes in which crustal thickening and metamo^hism are iargelN due to the intrusion of magma.

OLD TECHNIQUES - NEW INTERPRETATION EFFECTIVE THREE DIMENSIONAL ANALYSIS OF MINERALISED SYSTEMS John L. Baxter Continental Resource Management, P.O. Box 307, Belmont, Western Australia

MINERALISING SYSTEMS Epigenetic gold mineralisation develops in a wide range of rock types and a wide range of metamorphic and kinematic conditions. Studies throu^ the decades have concentrated on source rock chemistry, fluid chemistr>\ the host rock chemistry, the equilibrium reactions, vein shapes and strain distribution and all have contributed to debate on the formation of economic mineral deposits. This paper contributes to the debate on the geometry of deposits It is clear the location of miner^isation can be diverse i.e. dilational jogs, rheology contrast causing bends in shears, development of intersection riedels, and interaction of shears with rock tvpes.

GEOMETRY OF A MINERALISED STRUCTURE Three dimensional mapping of veins, lodes, fauks, shears and joints in a mineralised environment can produce spectacular fractal patterns across the mapped zone. These planes in space usually have a common feature, they intersect on a common line (or more correctly on a cluster of lines) which plot as a cluster of poles on a stereographic net Shear zones evolve and periodically have catastrophic movement through time and often under changing PT conditions. The resulting geological features have a complex, often apparently non-systematic relationship in the timing of veins, shears and fauks etc.

Lineation studies in the same deposits have produced a range of resuks which have led to fiirther complexity being added to the argument Various lineations will be treated differently including fold hinges, intersections of planes, mineral lineations, stretching lineations, slickensides, oriented fragments etc. A common feature of lineation studies is that a group of lineations (commonly fold hinges and intersection lineations) will be located at the pole of the girdle containing the poles to the various planes in the deposit when plotted on a stereographic net. Equally commonly the stretching lineations and slickensides are oriented in such a way that they plot on the girdle containing the poles to the planes.

In most mineralised systems this geometry is easily observed by careftil mapping of a variety of oriented fabrics in plan and section followed by simple plotting on a stereographic net.

GEOMETRY OF MINERALISATION High grade shoots occur parallel to the intersection lineation in dilational jogs (e.g. Bellevue and Norseman) and veins develop at dilational sites focussed on intersections with faults and shears. In these quartz lodes the differential stress is low and the intermediate stress field is parallel to the intersection lineation. Complex deformation associated with mineralised quartz vein deposks including antitaxial veins, superimposed fauks, irregular folding in the vein are all part of the geometry of the mineralised environment.

Lode deposits, commonly without quartz veins, in the brittle ductile environment have high grade shoots subparallel to the intersection lineation of the lode (e g Youanmi and the Golden Mile). The high grade shoots to develop at or near the intersection of secondary shears, and often these shears develop around undeformed country rock Lode deposits display strong flattening febrics and appear to form in high differential stress environments where the mineral growth can be parallel to the intersection lineation.

Gold mineralisation developed in ductile shear zones rarely contain dilational shoots. Higher grade areas are commonly focussed on the boundaries of boudins and at bends in the shear zone.

MAGMAS, FLUroS, AND TECTONICS: THE EMPEROR STORY Graham C. Be^p', Robert R. Loucks^ David R. Gray\ David A. Foster', Adam J. Kent^ David R. Cooke^ 'WMC Resources Ltd, P.O. Box 91, Belmont, Western Australia, 6104 ^Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601 'Department of Earth Sciences, Monash University, Clayton, Victoria 3168 'Victorian Institute of Earth and Planetary Sciences, School of Earth Science, Latrobe University, Bundoora, Victoria 3083 'Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125 ^Geology Department, University of Tasmania, Hobart, Tasmania

The giant (-9 Moz) intrusion-related Emperor epithermal gold deposit is situated on the margin of the Tavua Caldera, which lies in the centre of the Tavua Volcano, northern Viti Levu, Fiji. Telluride-rich bonanza gold grades formed on narrow post-caldera thrusts ("flatmakes") and reactivated steep faults during a period of regional N-S compression. Onset of this compression choked mantle magma supply to the upper crustal magma chamber, encouraging magmatic fractionation and episodic caldera collapse as the magmatic system waned.

This onset can be linked to a succession of arc-scale events involving collision, subduction flipping, relaxation and associated backarc and intra-arc basin formation, arc migration and growth of an inter-arc transfer zone, and fmally the need to accommodate plate convergence across the new transfer zone (Begg, 1996).

Pre-epithermal metal-poor porphyry-Cu-style alteration occurs associated with steep caldera faults along the caldera margin within the limits of the epithermal deposit. The two events (porphyry-style and epithermal) were essentially synchronous (--3.9 Ma). Temperatures within the porphyry-Cu-style system declined from -350®C to -'250°C by the time of initiation of ore-depositing epithermal conditions, to <170°C by the end of epithermal oredepositing conditions. "Porphyry" fluids were dominantly low salinity (<4 equiv. wt. % NaCl) with a subordinate moderate-high salinity brine (--15 equiv. wt. % NaCl). Epithermal fluids decreased in salinity with time (there is little spatial variation), from about 13.0 equiv. wt. % NaCl to 5.0 equiv. wt. % NaCl. Precipitation of epithermal ores was through a combination of wallrock sulphidation reactions and boiling, reflecting fluid pressure fluctuations arising from "epithermal-fault-valve" behaviour.

Isotopic (Sr, S, O, H) and thermochemical studies (Ahmad et al., 1987; Begg, 1.996) indicate that hydrothermal fluids within both the porphvTy-Cu-st>le and epithermal systems were sulphur-poor, mildly acidic, hybrid magmatic-meteoric fluids.

Sulphur isotopic data also highlight a continuum between the porphyry and epithermal environments, with lighter sulphide isotopic values with time. Despite the spatial, temporal, isotopic, and temperature continuum bet^\•een the tv. o systems, the precious-metal-rich nature of the epithermal fluids, their higher salinities and COj content, indicate that the two systems tapped different fluids. This can be explained if a metal-rich magmatichydrothermal fluid exsolved from the crystallising Tavua Volcano magma chamber were trapped at depth within the upper portions of the chamber (^3.5 km below surface) by lack of vertical permeability due to both horizontal compression and mineral sealing. Slow upward leakage of small amounts of this fluid, and incorporation into a larger volume of freely convecting meteoric water, may have resulted in a fluid capable of producing porphyry-style alteration assemblages in the upflow path. The "switch" to a magmatic-hydrothermal gold-rich epithermal system is linked to a sudden and large increase in fracture permeability generated by "epithermal-fault-valve" behaviour, triggered by lithostatic fluid pressures arising due to near-surface sealing of the porphvT> -Cu-style system upflow zone. Such a mechanism is consistent with observations of large amounts of anhydrite in the porphyry' upflow zone, which may have 'clogged' the fluid pathways, forcing fluids to deviate around the resulting low permeability zone. Such deviations require an increase in fluid pressure in order to drive the fluids laterally (and overcome the effects of friction between the fluid and the wallrocks) over the additional pathlength. To derive this fluid pressure, the upflow column widened and slowed down, thereby dissipating less fluid pressure (ie. less drag against wallrocks) in the upflow below the seal; and effectively transmitting higher fluid pressures to the upper part (from the lower part) of the upflow column.' Once pressures reached near-lithostatic values, fracturing of the intact rock occurred. Under the tectonic conditions prevailing at the time, this favoured generation of gently-dipping thrusts and fractures, and tapping of gold-rich fluids.

REFERENCES Ahmad, M., Solomon, M., & Walshe, J.L., 1987. Mineralogical and geochemical studies of the Emperor gold telluride deposit, Fiji. Economic Geology, v. 82, p. 435-370.

Begg. G.C., 1996. Genesis of the Emperor Gold Deposit, Fiji. Unpub. PhD. thesis, Monash University, Melbourne, Australia

WALLROCK ALTERATION AS AN EXPLORATION TOOL FOR TURBIDITE-HOSTED GOLD DEPOSITS ^ Frank P, Bicrlfin'. Troy Fuller', Kun StOwe'. Dennis C. Ame' and Jenny Besanko'

Minerals Industry Research Institute, University of Ballarat, PO Box 663, Ballarat Vic 3353 Department of Earth Sciences, Monash University, Qayton Vic 3168

The recognition of wallrock alteraUon adjacent to sediment-hosted gold deposits not only places constraints on the chemistry of the fluids responsible for ore deposition and the relative timing of gold mineralisation, it also represent a very useful tool for exploration. However, it has long been the popular view that turbidite-hosted gold deposits, such as those found in Victoria, are characterised by a lack of significant wallrock alteration, either due to low fluid/rock ratios or the emplacement of quartz early in the development of the auriferous veins effectively 'sealing' the unaltered host rocks. This perception generally discouraged detailed studies on the significance and extent of wallrock alteration in gold deposits in Victoria, although detailed investigations have demonstrated that some deposits exhibit characteristic and rather extensive alteration zones. Commonly, the most striking type of alteration consists of a combination of pyrite and arsenopyrite which occur as disseminations in slates and sandstones up to tens of metres away from major fault-hosted lodes, whereas carbonate spotting is often even more extensive. Little detailed work has been done to either geochemically characterise the fluids responsible for ^ e growth of carbonates spots or determine their timing relative to the developement of regional metamorphic fabrics. Other alteration styles recognised in Central Victoria include carbonatisation, (de-) silicification sericitisation, chloritisation and albiUsation.

The wide range of alteration styles directly reflects the liUiological diversity of the hosting composite turbidites.

The orientation of the hosting metasediments relative to auriferous veins, as well as related variations in permeability and porosity account for the irregular width of alteration haloes from a few centimetres to several hundreds of metres. The recognition of wallrock alteration features is further impeded by variations in the degree of reactivity of the hosting sediments, dilution of the reacting volume by quartz, which in turn results in bulk changes in the amount of alteration, and the presence of disseminated syn-sedimentary pyrite An aided complication for the recognition of hydrothermal wallrock alteration patterns at surface are the overprinting effects of chemical weathering and secondary dispersion. Much of Central Victoria is characterised by a dea) regoluh layer in which primary alteration signatures are not easily identified. Thus, the development of geochemicai, petrographic and isotopic criteria for the recognition of hydrothermal alteration must take into account these additional effects.

Previous work has demonstrated that significant shifts in the sulphur isotope composition of pyrite occur in the vicinity of the Wattle Gully Mine, with gold mineralisation associated with 5^'S values near 0 per mil whereas diagenetic pyrite in the distal metasedimentary rocks generally gives values greater than 15 per mil. Arsenopyrite (asp) does not occur as a syn-sedimentary phase around gold deposits in Victoria and the presence of disseminated asp porphyroblasts therefore provides a good estimate of the extent of the sulphide introduction into the hosting metasediments. These asp (+ pyrite) haloes vary between one and approximately 20 metres in width, but there is a wide transitional zone between asp-bearing and asp-free rocks. Hydrothermal alteration of the metasediments has resulted in discolouration and sericitisation, and extensive ankeritic and calcitic carbonate spotting is coimnon within at least 50 meu-es from tiie auriferous veins of investigated deposits. While most major and trace elements correlate with vanations in host rock lithology, a very distinct COj halo extends independently of the host rock luhology. Commonly, CO^ contents range from background values of 0.5 wt% to 8 wt% close to die main reefs. Geochemicai traverses further indicate depletion of NajO and increased values of CO , H O, K O, S, As, and possibly Au, within 8 to 10 metres from the auriferous lodes.

2- 2 • 2 • • The nature of turbidite-hosted gold mineralisation (i.e., size, structure, distribution, ore grades) poses an immense challenge to exploration and the delineation of drilling targets. Nevertheless, the study of mineralogical and chemical changes within turbidites associated with gold mineralisation provides invaluable information regarding ore genesis and represents a potentially powerful exploration tool. In particular, broad bleached zones characterised by carbonate, sulphide and sericite alteration surrounding mineralised zones provide an exploration target of increased magnitude. Significant values to look for are COj concentrations > 3 wt% K O > 3 wt%

N a - O < 1.5 wt^r. H j O > 2 wt9c, S > 0.3 wt% and A s > 50 ppm.

' ^

REGIONAL-SCALE SEAFLOOR HYDROTBERMAL ACnVITY ASSOCIATED WITH FELSIC VOLCANIC ROCKS IN THE EASTERN MANUS BACK-ARC BASIN, PAPUA NEW GUINEA Rj\. fiiMs\ S.D. Scott^ R.D. hcmic\ KA.W.Chx>k\ D.W. BtwMds\ J.B. CJcmnctf. R. Moss^. S.Nckitcl^. J.M. R. Shvpe^ CJ. Ttylcr* and J.CWaten' *CSIROExplonUioo lad Mining, PO Box 136 North Ryde, NSW 2113 'Marine Geology Resevcfa Litoitory, Univwsity of Torooto, Toronto M5S 3B1, Canwla 'CSIRO Manne Research, GPO Box 1538 Hob«t Tasmania 7001 ^HURL. University of Hawaii, 1000 Pope Road, Honohihi, HI 96822, USA ^CODES, Univeraity of Tasnania, GPO Box 252C Hobart Tas 7001 •Geological Survey of Papua New Guinea, PMB POrt Moresby, PNG

In December 1996 the PACMANUS-III cruise of RV FrcmUin discovered a new, very substantial, active hydrolhernial system associated with felsic vc^canic rocks at Susu Kndlls in Ae eastern Manus Basin. The eastern Manus Basin also contains the PACMANUS and DESMOS hydrothermal fields, and is becoming recognised as a regional-scale modan analogue for mineral fields OD land that possess a variety of vdcanichosted polymetallic massive sulfide and related sub-vdcanic ores. Unlike the central Manus Basin, where hack-arc spreading is creating new oceanic crust and where basalt-hosted hydrothermal deposits resanble those of mid-ocean ridges, the eastern Manus Basin is a region undergoing extensional thinning <rf oLder arc crust fcmned during subduction of the Pacific Plate along the Manus-Kilinilau Trench. Concurrent submarine igneous activity in the eastern Manus Basin is related to opposed subduction of the Solomon Sea Plate at the New Britain Trench.

Our photographic surveys and sampling show Susu Knolls to consist of two high-standing feldspar-phyric dacite dcxnes erupted above andesite lavas. Dacites at the aests of both knolls are extensively brecciated and altered, and carry widespread disseminated and stockwork sulfides as well as massive sulfide chimneys, mounds and breccias. A lower ridge collinear with the knolls ("Little Su") has exposures erf both andesite and feldspar-phyric dacite, and fields of massive sulfide diinmeys associated with mounds erf finely laminated to coarsely banded sulfidic sediments with interbedded vdcaniclastic layers. The overall hydrothamal systan as presenUy mapped extends 5 km aaoss Susu Knolls and 50Qm down slope. An intense hydrothmnal plume in the seawater column overlies, and extends some 5 km southeastfi-om,Susu Kndls uiiich are manfled by clouds of bacterial mat. The geanetry of this plume suggests that venting also occurs lower on the slopes (rf Susu Knolls than has so far been discovered. Broadband acoustic emission occurs at South Su. presumably from a high velocity steam or CO2 vent site.

In contrast to the central eruption of viscous, partly crystallized dacite lava domes at Susu, at PAO^IANUS there have been fissure eruptions of veryfluidaphyric lava along a ridge approximately normal to the direction of back-arc extension. Isolated hydrothermal dq)osits occur for some 10 km along the crest erf this ridge, most notably in a 2 km long zone near its highest pcrint. Three active Wack smoker and grey smdcer diimney fields are situated at fissured zones in unaltered dacite lava. A fourth activefieldshows extoisive diflRise ventmg of lower temperature fluids through altered dacite and rhyodacite at a small eruptive centre. We anticipate that "subhalative" massive sulfides He beneath Oiis zone. DESMOS is a small but very active hydrothermal field at the wall of a caldera on a low basaltic andesite volcano. No massive sulfides are so far known at DESMOS. Mineralogical and morphological diaracteristics erf Susu diimneys are similar to those at the PACMANUS site, which is also hosted predominantly by dacites with andesites in the near vicinity. Both sites are notable for the abundance of chalcopyrite. PACMANUS diimneys carry elevated silver and gold, while Susu samples remain to be assayed for precious metals. More so than the chimneys at cither site, which have few fossil analogues, the banded and brecciated deposits found at Susu dosely resemNe ore types typical of many andoit polymetallic VMS ores. Similar material has not yet been observed at PACMANUS. Togetho-, the PACMANUS-DESMOS-Susu KnoUs sites define a hydrothennally active region (rf volcanic edifices and sedimented graben some 4000 km^ in extent Tliis is comparable in scale to important mmwal districts on land, and crfTers the possibility erf estaWishing regional cootrcds on mineralisation. Despite their contrasted volcanological setting, the PACMANUS and Susu hydrothermal fields both occur at bathymetric highs, reflecting maximum magmatic activity. Resolving the rde of magmatic fluids in arc and back-arc hydrothermal settings is the focus of mudi current researdi. The PACMANUS and Susu discoveries provide an opportunity to compare and contrast two felsic-hosted massive sulfide sites in the same general teaonic environment, and thereby to reach more generaUy ^licable oxiclusions regarding are genetic processes.

MAGNETITE-APATITE DEPOSITS AND THEIR RELATIONSHIP TO IRON-OXIDE HOSTED CU-AU MINERALISATION K.L.BlikeaiidP.J.Wmiains National Key Centre in Economic Geology, I^paitmeot of Earth Sciences James Cook Univeroty of North Queensland, TowmviUe, Qld 4811 Magnetite (lapatite, actinolite, clinopyroxene) deposits occur throughout a range of tectonic settings and geological ages exemplified by the Proterozoic Kiruna Iron ores of northern Sweden through to the Mesozoic-Tertiaiy iron oxide systems of Chile. Such deposits show a marked similarity in their overall mineralog>', textural features and geochemistiy and have led to them often being labelled as the "Kiruna type". Whilst the largest of the iron oxide bodies constitute economic ore in their own right, deposits of this aflRnity may also form the host to or be spatially associated with iron oxide-Cu-Au mineralisation (Olympic Dam, South Australia; Ernest Henry, Cloncurry district, NW Queensland).

Magnetite formation is typically associated with an early, widespread sodic alteration event replacing a large component of the original host rocks with albite, which are in turn brecciated, veined or replaced by magnetite. Gangue minerals include apatite, actinolite (Iclinopyroxene) and minor amounts of titanite, the amount of gangue minerals present may vary greatly from one ore body to another and also within a single ore sy stem. Typically, as in the case of Kiruna, the varying gangue contents reflects separate magnetite formation events although the geochemistry and stable isotope characteristics of the magnetites themselves do not suggest any marked differences influidtemperature or composition.

The widespread development of hydraulic breccias, vein infill and replacement textures are characteristic features of this style of iron-oxide (Cu-Au) deposit and demonstrate that most, if not all, magnetite to be the product of precipitation Irom hydrothermal fluids. At Kiruna, where a clear origin for these fluids has been demonstrated, the fluids are seen to be magmatic in origin with temperatures in excess of 450°C and a of ca. 77oo in isotopic equilibrium with the volcanic host rocks. Preliminary stable isotope investigations of the Cloncurry iron oxide hosted Cu-Au deposits which indicate ore stage iron oxides were in equilibrium with a fluid compositions of 8 to 10 7oo (Starra) and 7 to 8 7oo (Osborne) and temperatures of ca. 350°C (Starra) and 500°C (Osborne), reflecting diflferent conditions/fluid compositions during their formation.

Further more, earlier, pre-mineralisation magnetites at Osborne appear to have equilibrated with an isotopically lighter fluid (5^®0 ca. 5 to 67oo) showing that within at least some of these deposits the fluids responsible for diflering paragenetic stages of magnetite either interacted with other fluids or were the product of a completely separate fluid.

Where Cu-Au mineralisation is present it is typically associated with K-silicate alteration. Such mineralisation in the Cloncurry district often display a m ^ e d structural control to fluid localisation and magnetite (± hematite) may form at several different paragenetic stages within an ore body, as seen at Ernest Henry where ore stage magnetite post dates an earlier magnetite-biotite alteration event Also, highly varied fD2 and fS2 conditions during ore-stage Cu-Au mineralisation are inq)lied by differing ore-stage assemblages in the deposits of the Cloncurry district - e.g. pyrrhotite±magnetite (Osborne East and Eloise), p>Tite-magnetite (Ernest Henry and Osborne West), hematite (Starra).

The combined geological characteristics of this suite of iroQ< oxide dominated dqposits suggest suggests similar overall formation processes, the result of high temperature, low sulphidation hydrothermal fluids associated with a distinctive alkali dominated alteration. Such deposits do however tend to di^lay internal variation in their paragenesis in particular with regard to multiple generations of magnetite (± hematite) and varied gangue and sulphide mineralogies. The woik to date does show an overall link between such deposits and ftirther highlights a number of, as of yet, unanswered questions which will be addressed in fiiture studies: namely, are all these deposits strictly m^j^ma/zc-hydrothermal 7; arc aiqr spcdfic magma-type/tectonic settings implicated in their formation ?; is there a ftmdamental difference between that will lead to a Cu-Au mineralised system ?

10

IGNEOUS METALLOGENY: TEMPLATES, PROVINCES AND THE FUTURE P. L. Blevin National Key Centre for Geochemical Evolution and Metallogeny of Continents Department of Geology, Australian National University, Canberra, ACT, 0200 The understanding of magmatic controls on ore element ratios (Cu-Au-Mo-Sn-W-etc; OER) in intrusive-related ore deposits has been significantly advanced in the last few years to the stage where it can be used as a predictive tool. These ratios are dominantly functions of magmatic compositions, process and intensive variable considerations. While the deposit spectrum present in eastern Australia (Au-Cu through Cu-Mo, W, Sn and Mo) correlates with the degree of compositional evolution of associated magmas, it does not correlate with their degree of isotopic evolution. For example, lithophile mineralisation (Sn, Mo, F) in the southern New England Orogen (NEO) is associated with compositionally evolved but isotopically juvenile magmas.

Numerous conceptual advances in recent years have shed new light on some old dogmas. These include: • "Au mineralisation cannot be sourced from felsic magmas". Spatial and genetic relationships, metallogenic associations, and recent experimental evidence (eg. Candela et al., 1996) now demonstrate otherwise. The recognition that Au is an integral part of a diverse range of felsic igneous metallogenies, and is not hostage to Cu, PGEs, mafic rocks, the mantle or some special "magic bullet", has practical implications for where and how Au exploration should be conducted in various terranes.

• "A-types are anhydrous and F-rich relative to other granite types". Halogen data from minerals have established that I-, S- and A-type granites have similar F contents and that F/OH and F/Cl ratios vary as a function of composition. Differences in CI abundance and Cl/OH ratios between I- and S-types, and between different Itype associations are significant - with implications for metal partitioning and transport. A-types are also not anhydrous, indeed volatile exsolution textures are commonly present.

• "Sn is an S-type element". In Australia greater than 70% of Sn production has been sourced from I-types, including all major western Tasmanian Sn systems.

• "Metallogenic provinces = metal specific provinces". OERs in intrusive-related mineral deposits vary with igneous composition. Thus single supersuites may generate a variety deposits. The Moonbi Supersuite in the NEO for example has been a source for Cu, Au, Sn, W, Mo and Bi mineralisation (Blevin & Chappell, 1996). Igneous metallogenic provinces should be regarded as polymetallic.

• "Giant ore deposits require special processes". Experimental and theoretical limitations on magmatic and hydrothermal processes suggest that giant deposits more properly represent systems where "everything went right". Magmas with anomalous metal contents will still be anomalous relative to "normal" magmas if the overall efficiency of metal partitioning between the melt and the exsolving volatile phase is the same in both cases.

The development of igneous metallogenic concepts has lead to the recognition of igneous metallogenic provinces in eastern Australia (Blevin et al., 1996). These provinces define areas of potential for certain elements based on the nature and composition of igneous rock suites. As granites are sourced from the mid to lower crust and mantle, these provinces do not necessarily coincide with upper crustal lithostratigraphic boundaries. Non magmatic-compositional factors are also important in controlling the distribution and preservation of mineralisation in the crust. There is a need to better estimate the pressure (depth) at which crystallisation and volatile exsolution occurred in felsic igneous complexes relative to their current level of exhumation. This will define regions where the epizonal mineralisation window has been preserved within igneous metallogenic provinces. Other advances include: new methods to assess the intrinsic oxidation state of magmas and to recognise the effects of alteration; using element ratios and "proxy" elements (Mo, Re, Ni, Bi) to better understand and track processes in magmas favourable for generating mineralisation; and using textures as a monitor of crystallisation and volatile exsolution mechanisms. The application of these methodologies as a tool for area selection, and their development as a set of field based tools for assessing the metallogenic potential of igneous complexes, provide an intriguing challenge for the future.

REFERENCES Blevin, P. L. and Chappell, B. W., 1996. Intemal evolution and metallogeny of Permo-Triassic high-K granites in the Tenterfield-Stanthorpe region, southern New England Orogen, Australia. CeoL Soc. Aust. Abstracts 43, 94-100.

Blevin, P. L., Chappell, B. W., and Allen C. M., 1996. Intrusive metallogenic provinces in eastern Australia based on granite source and composition. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 281-290.

Candela. P. A., Piccoli, P. M. and WiHiams, T. J., 1996. Preliminary study of gold partitioning in a low-sulfur, high oxygen fugacity melt/volatile phase system. Geol. Soc. Amer. Abs With Programs 28(7), 402.

11

METALLOGEMC IMPLICATIONS OF GRANITE TECTONICS: THE LACHLAN FOLD BELT AS A CASE STUDY Phillip L. Blevin, David J. Ellis, Rosalyn G. Warren National Key Centre for Geochemical Evolution and Metallogeny of Continents Depanmeni of Geology, Australian National University, Canberra. ACT, 0200 T h e L a c h l a n Fold Bell ( L F B ) is a tectonic regime which has not been deeply eroded since it b e c a m e a stable tectonic regime in the D e v o n i a n . T h e oldest units c o m p r i s e extensive Ordovician turbidites and igneous units, which were followed by elongate Silurian basins filled by marine sediments and volcanics, and separated by highs of granite and subaerial felsic volcanics of Silurian to Early Devonian age. Silurian and D e v o n i a n mineralisation in the L F B can be classified into three groups: granite-related deposits of Sn, W , M o , C u and A u ; V H M S (AgP b - Z n - C u - A u ) deposits in the Silurian basins; and "turbidite-hosted" gold-quartz vein deposits. In addition, the Ordovician "basement" to the L F B is host to several large porphyry C u - A u systems, w h i c h h a v e been preserved despite extensive Siluro-Devonian plutonism now exposed at the same erosional level. W e propose that unusual tectonic character of the L F B generally and the unusual association of metallogenic styles present in the L F B can all be explained as a response to a single cycle of crustal overturn driven by granite tectonics.

The cycle of crustal overturn that m a k e s up granite tectonics (Sorgenfrei, 1971; W a r r e n & Ellis, 1996) begins with an influx of hot mantle-derived material into the base of the crust causing widespread anatexis. Felsic melts coalesce at depth and rise into diapiric structures. R e m o v a l of material into the rising diapirs is balanced by d o w n w a r p i n g between the diapirs, creating sag-ducted basins (marginal or rim- synclines). Felsic volcanics may break through to the surface, either ahead of the rising granites or as sills and e r u p t i v e s into the sag-ducted basins. B a s e m e n t beneath the sag-ducted basins will move down into higher temperature regions in a stress field that may c h a n g e f r o m vertical to horizontal, and may begin to melt. At high crustal levels, outward expansion of granite plutons over the rim synclines and underlying crust cause local c o m p r e s s i o n . T h e net e f f e c t is to m o v e older deeply-buried crustal material back towards the surface in the f o r m of granites and in s o m e cases aureoles, while m o v i n g y o u n g e r sediments, and their basement, in the sag-ducted b a s i n s d o w n w a r d s . Granite tectonics results in massive crustal reworking but no crustal thickening and/or mountain building.

T h e model allows for the p e n e c o n t e m p o r a n e o u s production of fluids f r o m diverse sources. T h e s e include: the mantle or d e e p crust, progressive dewatering of the sag-ducted sedimentary pile, m a g m a t i c fluids derived f r o m Igneous intrusions and "local" waters generated in the upper crust by shortening. Seawater incursions into sagducted basins, as well as meteoric water provide additional fluid sources. Sulfur isotope data for the L F B V H M S systems require variable proportions of magmatic and seawater inputs ( S o l o m o n & G r o v e s , 1994), but recycled O r d o v i c i a n Pb is also present (Carr et al., 1995). Sag-ducted Ordovician ore deposits beneath Silurian basins may act as metal sources in this regard. Isotopic and fluid inclusion data for the gold-quartz vein deposits suggest that the auriferous fluids were exotic relative to the immediate environment of the deposit and derived f r o m the d e w a t e r i n g of pelitic s e d i m e n t s at depth during a m p h i b o l i t e grade m e t a m o r p h i s m . Direct m a g m a t i c f l u i d contributions to s o m e shear hosted Au deposits also occurs. T h e s e fluids were channelled into the upper crust where they were structurally trapped, Au precipitation being driven by processes such as cooling, m i x i n g with locall\ deri\ ed fluids or via wallrock reactions. The high temperature stages of granite-related mineralisation are d o m i n a n t l y m a g m a t i c . with ore metal ratios in these s y s t e m s being s i m p l e f u n c t i o n s of m a g m a t i c s o u r c e , process and m a g m a t i c intensive variables.

Granite tectonics p r o v i d e s a unified model linking the production of felsic melts, vertical and lateral crustal m o \ ements. h i g h - T l o w - P metamorphic P-T-t paths, fluid production and circulation, and source and depositional e n \ i r o n m e n t s for significant accumulation and preservation of metals. It explains the o c c u r r e n c e of diverse m i n e r a l i s a t i o n styles within one m e t a l l o g e n i c e p o c h . L a r g e p o r p h y r y C u - A u s y s t e m s in the O r d o v i c i a n volcanics and related intrusives o w e their preservation to granite tectonics, these systems would have been eroded aw a\ if subsequent crustal thickening, mountain building and erosion had occurred in the L F B (Ellis, 1987).

REFERENCES Carr. G. R.. et al., 1995. Precise lead isotope fingerprinting of hydrothermal activity associated with Ordovician to Carboniferous metallogenic events in the Lachlan Fold Belt of New South Wales. Economic Geology 90, 1467-1505.

Ellis. D. J.. 1987. Origin and evolution of granulites in normal and thickened crusts. Geology 15, 167-170.

Sorgenfrei, T., 1971. On the granite problem and the similarity of salt and granite structures. Geologiska Foreningens i Stockholm Forhandlingar 93, 371-435.

Solomon. M., & Groves, D. I.. 1994. The geology and origin of Australia's mineral deposits. Clarendon Press, Oxford 951 p.

Warren. R. G., & Ellis, D. J., 1996. Mantle-underplating, granite tectonics, and metamorphic P-T-t paths. Geology 24, 663-666.

12

SEQUENCE STRATIGRAPHY PROVIDES NEW CONSTRAINTS ON CORRELATIONS BETWEEN THE MT ISA AND MCNAMARA GROUPS OF NORTHERN AUSTRALIA. NABRE Team aJphabeiic authorship list follows Barry Bradshaw. Jan Domagala*. Mart Wnurm, M. Jim Jackson. Andrew Krassay. Jim Uvcn, John Lindsay. Brucc A. McConachic. Rod Page, Terry Sami" Deborah Scott, Peter N. Southgate. Chris Tarlowski and Allan Wells

Australian Geological Survey Organisation. GPO Box 378. Canberra 2601 [pepanmem of Mines and Energy. PO Box 194 Brisbane 4001 "Department of Geological Sciences. Queen's University. Kingston. Ontario K7L 3N6 Canada

An ability to reconstruct basin architecture and sediment geometry at the time of fluid flow is critical to understanding the likely distribution of potential resources in a sedimentary basin. A key objective of the North Ausu-alian Basins Resource Evaluation project (NABRE) is to provide the resource exploration industry with a time series predictive tectonosu-atigraphic framework for Palaeoproterozoic basin evolution in northern Australia.

To achieve this objective NABRE is utilizing concepts of sequence stratigraphy to identify chronosu-atigraphic surfaces within the Mt Isa-McNamara-Fickling-McArthur Groups and correlate between them. In Palaeozoic rocks biostratigraphic, radiometric, chemostratigraphic, and magnetostratigraphic techniques are the principal methods for providing a highly resolved time scale. However, in the Proterozoic where biostratigraphic time control is lacking innovative techniques and methods of dataset integration are required if the age of sedimentary units are to be better consu^ained and more precise correlations achieved.

In Australia traditional correlation techniques have relied upon lithostratigraphic methods of subdivision in combination with SHRIMP zircon radiomeu-ic techniques. Using these techniques it has been possible to provide correlations at the Group level (eg the Mt Isa-McNamara-Fickling-McAnhur Groups are correlated) but examination of recent SHRIMP dating (Page & Sweet in press) indicates that this group spans an interval of some 60 million years and funhermore includes between 6-8kms of section. SHRIMP dates have been used to refine correlations within the Groups and on the basis of these datasets tuffs from the Urquhart Shale in the Mt Isa Group with a date of 1652 ± 7Ma are correlated with tuffs from the lowermost Paradise Creek Formation in the McNamara Group at 1653 ± 7Ma. Results from the 1995 and 1996 NABRE field seasons are providing significant new insights into correlations between the Mt Isa and McNamara Groups. Sequence su-atigraphic interpretation of gamma ray (Fig. 1) and fades datasets collected from outcropping sections near Gunpowder (McNamara Group) and Crystal Creek (Mt Isa Group) has identified several chronostratigraphic surfaces (sequence boundaries and nruiximum flooding surfaces) that can be correlated across the Mount Gordon Arch. The new datasets indicate that the Gunpowder Creek, Paradise Creek and a significant part of the Esperanza Fonnations (McNamara Group) correlate with the Moondarra Siltstone (Ml Isa Group). Poor outcrop of the upper Esperanza Formation near Gunpowder precludes a precise correlation between this Formation and the Breakaway Shale/Native Bee Siltstone. However, current data suggests that these two Mt Isa Group Formations may correlate to either the upper Esperanza Formation or lower pans of the Lady Loretta Formation. Based on these new correlations the Urquhan Shale, which hosts the Mt Isa and Hilton lead-zinc-silver deposits cannot be correlated with the Paradise Creek Formation, but instead possibly correlates with the Lady Loretta Formation 1647 ± 4 Ma. An improved understanding of the chronosu-atigraphic position of ore-bearing strata will assist in consu-aining exploration strategies in the Palaeoproterozoic of northern Australia. A further implication of this work involves the degree of precision that may be achievable through the integration of SHRIMP dating and sequence stratigraphic correlation techniques. When SHRIMP dates are used to constrain sequence interpretations correlations based on sequence startigraphic techniques are able to refine chronostratigraphic surfaces within the error bars of the SHRIMP dating methods.

In the case of the example discussed here such integration may provide accuracy to within 1 Ma. Reference; Page. R.W. and Sweet, I.P. (in press) Geochronology of basin phases in the western Mouni Isa Inlicr, and correlaiion with the McAnhur Basin.

Australian Journal of Earth Sciences.

Figurt 1 Esperanza Formation BarrHoie

Esperanza Watars

Torpedo C^eek Ouartzite mhostratioraphic Bouf^Ury ChronoMratigraphtc Boundary Se SmjutfK^ Boundary MFS Uaiimum Flood^ Surface

13

THE GEOLOGY OF THE STRELLEY GRANITE AND ITS ROLE AS SUBVOLCANIC HEATER TO OVERLYING VMS SYSTEMS Carl Brauhan, Geology Key Centre, University of Western Australia, NEDLANDS W.A. 6907.

The Strelley Granite, in the Pilbara Blcx^k of Western Australia, comprises two major phases; an outer equigranular hornblende granite and an inner, generally porphyritic biotite-homblende granite. Both phases are oxidised but the inner phase is characterised by a greater biotite content and a generally porphyritic texture. They are intruded by microgranite, dolerite and pyroxenite-peridotite bodies. The outer phase comprises a coarse grained base in the west which fines upwards to a fine grained granophyre at the northern, eastern and southeastern margins. The granophyric margin is conformable with overlying volcanic rocks. Granophyric texture, in conjunction with miarolitic cavities and porphyritic texture, indicate that the Strelley Granite is a high level intrusion.

Both granite phases and the overlying volcanic sequence have U-Pb in zircon ages of 3237 ^ 2 Ma. They have REE profiles (L^^/Ybf, of 2.1 - 9.0) and Zr/Y ratios (1.1 - 5.5) indicative of a tholeiitic affinity. Extensive alteration in the volcanic rocks precludes the use of major element geochemistry in establishing a genetic link between the volcanic rocks and the granite. However, field relationships, age dating and immobile element geochemistrv' provide overwhelming evidence that the Strelley Granite is a subvolcanic intrusion into the Strelley sequence.

The granophyric margin and immediately underlying equigranular granite is pervasively chlorite altered. Within this blanket of chlorite alteration there are centres of quartz-sericite and quartz-chlorite-sericite, feldspar destructive alteration. These zones underlie alteration centres in the overlying volcanic rocks related to VMS mineralisation and as such appear to be related to the hydrothermal systems responsible for VMS mineralisation.

Copper-zinc-tin bearing veins, developed in the upper parts of the outer phase, may also be related to the VMS systems. Greisen style alteration is developed about the same structures which host copper-zinc-tin veins, but deeper in the granite where base metal mineralisation is generally absent.

inner phase granite outer phase granite mafic intrusions ^ ^ ^ •

feldspar destructive alteration VMS prospect

5 Kilometers

Fig. 1 Relationship between feldspar destructive alteration and VHMS prospects.

14

REGIONAL ALTERATION SYSTEMS ABOUT VMS MINERALISATION AT PANORAMA, PILBARA BLOCK, WESTERN AUSTRALIA Carl Brauhan ^ & Peter Morant ^ 1. Geology Key Centre, University of Western AustraUa, NEDLANDS W.A. 6907, AUSTRALIA. 2. Sipa Resources limited, 87 Colin Street, WEST PERTH W.A 6005, AUSTRALIA

The informally named Strelley succession, which includes the coeval Strelley Granite, lies in the Pilbara Block of Western Australia. Turbidites overlie the Strelley succession, and VMS mineralisation is developed at several locations along the contact. The sequence dips moderately to the east but is otherwise undeformed, and the metamorphic grade is lower greenschist facies or lower. Outcrop is excellent and the exposure of the alteration system below the VMS deposits is arguably the best in the world.

In the north the volcanic pile comprises andesite overlain-by a narrow dacite unit. This grades into rhyolitic lavas and volcaniclastic rocks in the south. The Strelley Granite is an oxidised hornblende granite with a central core of quartz feldspar porphyry and an equigranular outer phase which fines upwards to an altered granophyric margin against the volcanic rocks.

Three alteration facies are recognised and are common to all units in the volcanic pile and the upper part of the Strelley Granite. (1) Feldspar destructive quartz-chlorite alteration comprises a quartz-chlorite-sericite ^ leucoxene ^ haematite assemblage, with sulfide only developed immediately beneath mineralisation. (2) A sericite-quartzcarbonate-leucoxene ^ albite ^ chlorite ^ sulfide assemblage typifies quartz-sericite alteration. (3) Background alteration is spilitic in mafic to intermediate rocks and keratophyric in felsic rocks and comprises an albite-chloritecarbonate-quartz-pyrite ^ leucoxene ^magnetite assemblage.

Quartz-chlorite alteration, developed towards the base of the volcanic pile throughout much of the belt, is overlain by background alteration, which is overlain in turn by albitic quartz-sericite alteration at the top of the volcanic pile.

However, in areas below mineralisation quartz-chlorite alteration cross cuts the other facies, and under major mineralisation such as the 6.2 Mt Sulphur Springs Zn-Cu deposit, quartz-chlorite alteration is continuous to the top of the volcanic pile. These zones are underlain by feldspar destructive quartz-chlorite and sericite-quartz alteration in the granite.

Regional spilitisation and the layered distribution of alteration facies is consistent with a large influx of sea water into the volcanic pile. This fluid is interpreted to have been returned to the sea floor, via the zones of cross cutting quartz-chlorite alteration, where it precipitated massive sulfide. The Strelley Granite is envisaged as the subvolcanic heater which drove the system, and may have contributed metal to the system via the release of volatiles.

15

NEW DATA AND IDEAS ON ARCHEAN GOLD-BEARING PALEOHYDROTHERMAL SYSTEMS PhUip E. Brown^ and StcfTen G. Hagemann^ ^

' Uni versitx of Wisconsin-Madison, Dcpt of Geology and Geophysics, 1215 W. Dayton St, Madison, WI 53706, USA Technical Uni\ ersit> of Munich, Dept. of Applied Mineralog>' and Geochemistry, Uchtenbergstr.4, 85747 Garching, Gemiany; NOW AT University of Western Australia, Key Center for Strategic Mineral Deposits, Nedlands WA 6907

Recent detailed investigations of Archean lode-gold deposits suggest that examples of this ore type formed throughout the crust from nearly the surface to the root zone of greenstone belts. ConsequenUy. the goldbearing paleohydrothermal systems developed under a wide range of P-T conditions from cpizonal to mesozonal to hvpozonal levels in the crust. Here we examine fluid sources and possible depositional processes at shallow and deep crustal levels using constraints from: (i) the recenUy developed University of WisconsinMadison laser-based ox> gen isotope facility-, (ii) combined gas- and ion-chromatography of fluid inclusions in silicates and sulfides using techniques developed at tlie University of Toronto, and (iii) experimental studies that show significant H2O and H2 diffusion at amphibolite- to granulite-facies conditions.

Laser-based oxTgen isotope micro-analysis on quartz-comb cry stals from gold-rich ore shoots of tlie Wiluna lode-gold deposits allow us. for the first time, to map-out distribution in single (e.g. 1.0 x 0.5 cm) quartz coiiib-cn stals The light 5 O fluid values (as low as -4 per mil) and the large range (up to 6 per mil difference within a single cry stal) suggest an important surface water component in the fluid but also indicate that the comb cry stals formed through complex processes such as boiling and/or fluid mixing. The precise spatial anal> ses of O distribution in growth zones of the quartz combs fiuther allow the interpretation that these processes occurred during the final stages of the complex evolution of the Wiluna paleohydrothermal system.

There is no indication that the boiling or mixing of the fluids induced rapid and widespread precipitation of gold as is know n to happen in lower pressure geothermal environments.

Gas-and ion-chromatography (GC-IC) of fluid inclusions trapped in pyrite, arsenopyrite and stibnite from the Wiluna deposits provide constraints on the precise gas and ion content of the fluids responsible for the formation of these sulfides In addition. GC-IC data on inclusions in quartz that is in apparent textural equilibrium with these sulfides shows that there is locally a distinct fluid disequilibrium between silicate and sulfide assemblages The halogen geochemistry of fluid inclusions documents an evolution of Br/Cl ratios in the Wiluna paleoh>drothernial system and allows comparisons of halogen signatures of Wiluna fluid inclusions w ith present day and Archean ocean waters.

New fluid inclusion data on hypozonal lode-gold deposits such as the Three Mile Hill Griffins Find and Mar\ el Loch deposits in the Yilgarn Craton document the widespread occurrence of apparently primaiy methane-rich inclusions Isochores for these inclusions project to pressure values as little as half of the pressures inferred for the mineralization on petrological grounds. Comparison with new experimental data suggests that post-mineralization modification such as hydrogen and/or HjO diffusion may have selectively effected fluid inclusion populations.

New analytical techniques such as laser-based stable isotope analysis or GC-IC, in addition to new experimental data on sy nthetic fluid inclusions as well as the possibility of analyzing in situ or ultra-small samples (<10 micromole gas), reveals a much higher complexity in the evolution of paleohydrothermal fluids than pre\ iously thought. Given the intricacy of processes and probable multiple fluid sources that supply hy drothermal fluids in a metal-bearing hydrothermal system, it remains questionable whether unifying genetic models tliat call on one particular fluid source (e.g. magmatic or metamorphic fluids) or process, are able to explain the genesis of Archean lode-gold deposits that occur throughout the entire upper crust of the earth.

16

DEOOMPRESSK)NAM)REOQNa^^ SOME GaEM3l\L IMIlICAnC»« FOR Grcspry H. Cameron'. John L.Walshe', Christoph A. Hcinrich^ and V J . Wall^ Australian National University, Canberra ACT 0200, Australia ^ETH Zurich, Switzerland, ^Southwestern Gold Corporation. Spring Hill QLD 4004, Australia

Recent work on the Porgera Gold Deposit has revealed some rather interesting aspects of the ore forming processes which we feel may be relevant to the general understanding of ccrtain types of gold mineralization.

High grade Au mineralization at Porgera appears to be the result of two spatially and temporally distinct mineralization events referred to as stages I and II. Stage I mineralization resulted in the formation of pyrite Fe-rich sphalerite, galena and minor pyrrhotite rich vein sets, spatially associated with intrusions, and hosted in intrusions, the surrounding altered sediment and calcareous black shale. The veins carry 10 to 20 g Au/tonne but due to their scattered nature generally do not constitute high grade ore. The fluid responsible for this mineralization contained about 1 molal NaCl (corrected for CO2 after Richards and Kerrich, 1993), a minimum of 2 molal CO2, yields an average Th of about 2 8 0 T (Richards et al., op. cit.) and was relatively reduced. Stage n mineralization, which constitutes the bulk of the high grade ore, consists of Au ore hosted in banded quartz and quartz-roscoelite-pyrite-barite breccia veins, which are spatially associated with the Roamane Fault. Altered sediments are the main host, although intrusive rocks locally host significant mineralization. Fluid inclusion data from stage n quartz yields a bimodal salinity at 4.2 and 7.8 wt % NaCl equiv., and an average Th of 145'C (Richards et al., op. cit.). The CO2 content of these fluids is extremely variable ranging up to at least 2 molal. High grade ore shoots occur at the intersection of stage I and II veins sets. Quartz and carbonate are intimately intergrown with sulphide but are rarely observed in textural equilibrium with sulphide. We infer that sulphides formed early in stage I and were dissolved during stage II quartz and carbonate precipitation. Munroe and Cox (1996) suggest that the Roamane Fault was not active during stage I, but was the main spatial control upon development of stage 11 mineralization.

We propose that Porgera evolved initially during stage I within the, Pb-Zn rich, moderately distal reaches of a porphyr>' hydrothermal system, within relatively impermeable host rocks, and under moderately high pressure.

Both Au and base metals were complexed as chloride and deposited through passive boiling of a CO2 rich moderately saline fluid at about 280'C. Rupture of the Roamane Fault resulted in pressure drop and associated throttling and adiabatic decompression of the fluid, which prematurely ended the stage I hydrothermal system.

Fluid that originally deposited base metal sulphide and Au in the early veins, immediately began dissolving both, and simultaneously deposited quartz and carbonate as the fluid flowed into low pressures zones along the Roamane Fault. Here the same fluid further decompressed and continued to deposit barren quartz and lesser carbonate. Decompression also permitted deeper sourced, fresh, oxidized magmatic fluid to quickly achieve similar levels within the Roamane Fault and mix with the reduced pregnant ore bearing fluid. Mixing of the two fluids at the confluence of stage I and II structures resulted in oxidation of the reduced Au bearing fluid and precipitation of the high grade Au rich quartz-roscoelite layers. That Au was carried in the reduced (rather than the oxidized) fluid is evidence by the replacement of pyrite by Au within the assemblage quartzroscoelite-pyrite-barite.

Key elements of this model are the CO2 rich (high fluid pressure) composition of the early fluid, the impermeable nature of the host black shale (to maintain fluid pressure) and its calcareous composition (buffer capacity). Decompression resulted in loss of CO2 to the vapour phase which under normal conditions would lead to sulphide deposition due to pH increase. However, stage I fluid was internally buffered with respect to pH (due to dissolved HCO-3, Ca, Fe and Mg) and therefore loss of H2S and H2 to vapour resulted in the dissolution of sulphides and Au respectively. Together, these factors resulted in the remobilization of a significant proportion of the widely distributed stage I Au and its subsequent redeposition (through mixing induced oxidation) in a more concentrated form within a much smaller area.

We propose that the physical parameters necessary for decompression induced reconcentration of Au are commonly achieved in hydrothermal systems. Many of the Carlin Type deposits have similar characteristics (to those at Porgera) which include, calcareous shale host rocks, early widely dispersed disseminated/veinlet hosted mineralization deposited by early CO2 rich fluids and later overprinting more spatially restricted Au rich mineralization associated with arsenical pyrite within a relatively oxidized assemblage. Other examples might be the large Ashanti Gold mine in Ghana (Mumin et al., 1994) and Kelian in Indonesia.

REFERENCES Mumin, A.H., Fleet, M.E., & Chryssoulis, S.L., 1994. Gold mineralization in As-rich mesothermal gold ores of the Bogosu-Prestea mining district of the Ashanti Gold Belt, Ghana: remobilization of "invisible" gold Mineral Deposita 29, 445-460.

Munroe, S.M., & Cox, S.F., 1996. Porgera: Structural Controls on High Grade Gold Deposition. GeoL Soc. Australia, Abstracts 41, 307.

Richards. J.P. & 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.

17

Nd-Sr ISOTOPES AND TRACE ELEMENTS IN SCHEELITES FROM GOLD DEPOSITS AT NORSEMAN AND KAMBALDA, WESTERN AUSTRALIA Majid Ghaderi, Ian H, Campbcil. Malcolm T. McCuUoch, Victoria C. Bennett, J. Michael Paiin, Paul J. Sylvester and Graham E Mortimer

Research School of Earth Scicnccs, The Australian National University, Canberra, ACT 0200

Neodymium and Sr isotop>es in scheelites from gold deposits can be used to trace the pathway of the ore forming fluid(s). Previous Nd isotopic analyses of scheelites from Mount Charlotte gold mine in Kalgoorlie 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, whereas Sr isotopic studies of other deposits suggest a mixed felsic-mafic source. To further test the value of Nd and Sr isotopes as tracers for gold fluids, samples of scheelite have been analysed from several Kambalda deposits that are above the komatiite horizon and also from Norseman mines which are below that horizon.

Norseman scheelites which are from the Mararoa, Crown Reef, O.K. and Bullen gold mines, have e^d 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 yield ENd values between -1.0 and +1.4. These results imply that the Nd in Norseman scheelites have 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 ^^Sr/^^Sr ratios (0.70177 to 0.70399) which is in agreement with an earher 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 and a mafic source which is probably the underlying basalts.

The scheelite samples from mineralised vein systems in Hunt, Revenge, Junction and Victory gold mines at Kambalda have eNd(2630) values between -4.0 and +16.3, indicating komatiites and Lunnon Basalts as the main sources of Nd at Kambalda with small contribution from Paringa Basalts and/or felsic rocks in some parts of Victory mine. The difference in e^d values for these scheelites and Norseman scheelites can be related to the stratigraphic positions of the gold deposits. Unlike the deposits at Norseman, the majority of ore bodies at K^balda 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. The anomalously high e^d values for some of the Kambalda scheelites may indicate that these scheelites have undergone post-crystallisation disturbance. The Kambalda scheelites have a lower and more limited range of initial ° Sr/""Sr ratios (0.70093 to 0.70197) when compared with those from Norseman. These results require most of the Sr in the Kambalda scheelites to be derived from a mafic source.

Rare Earth Element (REE) concentrations in the scheelites have been measured by laser ablation-ICPMS. Chondrite-normalised REE patterns for the scheelites vary smoothly as a function of atomic number and define two main groups: hump-shaped (Typ)e-I) and flat (Type-II). Type-I patterns indicate preferred incorporation of MREE into the scheelite structure and have high LREE abundances. This type of scheelite has high Na contents suggesting that the Na"*" is both size and charge-balanced by the REE^"^. The main type of substitution in these scheelites is thought to be: 2Ca2"^= REE^"^ + Na"^.

Type-II patterns on the other hand, have lower XREE and show no preference for REE^"^ of a particular ionic radius. These scheelites are characterised by strong positive Eu/Eu* and low Na. Substitution of REE^"^ into these scheelites is considered to be vacancy controlled and of the form: ^C??"^ = 2REE^"'' + \)Ca- A few samples show variation from one type of pattern to the other, suggesting the presence of both fluids in the same location.

Neodymium-Sr isotope tracing suggests that at Norseman the ore fluids have sampled Nd and Sr from mainly basaltic rocks with, in some cases a minor contribution from felsic sources. At Kambalda, most of the Nd and Sr has been derived from komatiites and basalts. If the ore fluids have extracted Nd and Sr from predominantly basaltic and komatiitic sources, they may have also extracted gold mainly from these sources. The two types of REE pattern found in the scheelites from these areas imply two types of ore fluid; one Na-rich and the other Na-poor. Sodium-rich scheelites are dominant at Kambalda and the Na-poor type at Norseman.

REFERENCE Mueller, A.G., de Laeter, J.R., & Groves, D.I., 1991. Strontium isotope systematics of hydrothermal minerals from epigenetic Archaean gold deposits in the Yilgam Block, Western Australia. Economic Geologx 86, 780-809.

18

THE ROLE OF THE MANTLE IN LARGE SCALE HYDROTHERMAL PROCESSES - EVTOENCE FROM RADIOGENIC ISOTOPE SYSTEMS Graham R. C m \ and David J. VVTiilford' 'CSIRO Division of Exploration and Mining. PO Box 136 North Rydc NSW 2113 H:SIR0 Division of Petroleum Resources, PO Box 136 North Ryde NSW 2113

Interaction between the Earth's crust and mantle has been fundamental to the evolution of the continents.

Howe\ er, in considering the critical processes and events that combine to form very large hydrothermal ore deposits, the role of the mantle is rarely clear cut. Two important question with implications for formation of > er> large ore deposits are (1) Is mantle involvement direct (derivation of heat and/or fluids, metals ligands from mantle melts) or indirect (rcc>'cling of metals emplaced in the crust in an earlier event)? (2) If there is direct mantle involvement is it critical in the ore forming process, especially for very large deposits, and can we measure its fingerprints for use in exploration? Lead Nd, Sr and Os isotopic compositions of hydrothermal phases provide prime sources of e\idence for mantle involvement in such processes. There is now a considerable volume of Pb, Nd and Sr isotopic data available of modem and ancient systems in and around the Australian plate and we will describe these data from some important tectonic/metallogenic environments.

Although Os isotope fingerprinUng is potentially a veiy precise discriminator between crustal and mantle source, there is to date a dearth of data.

Volcanic rocks in modern island arc emiromnents, display isotopic mixing trends which clearly indicate crustmantle mixing both in the subvolcanic crust and as a result of sediment subduction. In such environments hydrothermal mineral deposits of VHMS and porphyry affiliation have isotopic signatures which are very similar to those of the volcanic rocks and are interpreted to indicate the derivation of metals from the contaminated melts. Whether large deposits are only associated with contaminated rocks is a critical question which remains unanswered for most terrains.

In ancient Australian environments, possible island arc environments crust-mantle isotope mixing trends, although the interpretation of these trends is ambi^ous. For example, Silurian VHMS deposits of the Lachlaii Fold Belt ha>'e Pb isotope ratios which lie on mixing lines between Silurian crustal Pb and mantle Pb witli a signature ver\ similar to Ordovician mafic volcanics of the Molong Arc. Those deposits with the most mantle like Pb lie close to major crustal sutures which juxtapose the Ordovician and Silurian rocks. In this case, the mantle Pb may not have been derived from a mantle melt, but may have been recycled from the Ordovician volcanics either by melt contamination or interaction with the hydrothermal fluids. Porphyry Cu/Au mineralizaUon genetically related to the Ordo^•ician magmatism have Pb, Nd and Sr isotopic compositions indicative of direct derivation of metals from the melting of an enriched lithospheric mantle.

In other metallogenic situations, there is evidence of mantle involvement in some but not all hydrothermal deposits The breccia-pipe Au deposits in NE Queensland ( e.g. Kidston and Mt Le>shon) have crustal Pb isotopic s>stematics, which indicate that dominant source of metals is from within the crust. However, variation in Nd isotope systematics may indicate a late direct or indirect mantle component. Tin systems in Tasmania associated with the Blue Tier Batholith, (Sun and Higgins, 1996) and southern NSW at Ardlethan (Walshe et al, 1995), have dominant crustal Pb, but Nd isotopic systematics which have been interpreted to suggest manUe input at a late stage of the magmatic process. The Proterozoic sediment hosted base metal deposits of northern Australia, all have crustal Pb isotope signatures indicative of derivaUon of metals from the intracrotonic rift sedimentary pile. In contrast, the metamorphosed "Broken Hill - type" base metal deposits such as Broken Hill and Pegmont have distinctively more manUe-like Pb which suggests a fimdamental difference in the metallogenic process between these two very important groups of deposits.

REFERENCES Sun. S -s and Higgins, 1996. N.C. Neodymium and Strontium isotope study of the Blue Tier Batholith, NE J^smania, and its bearing on the origin of Un-bearing alkali feldspar granites. Ore Geology Reviews, 10, pp.

Walshe. J.L., Whitford, D.J. and Sun, S.-s, 1995. The origin of the Wagga trin belt and the Ardlethan porphyry-st> le tin deposits. Centre for Isotope Studies Research Report (Sydney), 1993-94. Pp. 53-55.

Whitford. D.J and Jezek, P.A., 1982. Isotopic constraints on the role of subducted sialic material in Indonesian island-arc magmatism. BuIL GeoL Soc. Amer., 93, pp. 504-513.

19

THE GRANITOID-HOSTED WESTONIA GOLD DEPOSIT, YILGARN CRATON, WESTERN AUSTRALIA: EVIDENCE FOR ARCHEAN LODE GOLD MINERALISATION SYNCHRONOUS WITH THE FINAL STAGES OF CRATONISATION Kevin F. Cassidv^^ and Steffen G. Hagcmann^' ^ ^ Australian Geological Survey Organisation, Canbcira, 2601, Australia ^ Technical Universiiy of Munich. Dept. of Applied Mineralogy and Geochemistry, 85747 Garching. Gernumy ^ Key Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands, 6907, Australia

Lode gold mineralisation in high-grade metainorphic terranes has been the subject of much debate regarding its timing with respect to magmatism, metamorphism and deformation. Tlie Archean granitoidhosied Wesionia lode-gold deposit provides evidence for high temperature gold mineralisation that was synchronous with mid-crustal peak metamorphism, felsic magmatism and the final stages of cratonisation.

I he Wesionia deposit, located within a small greenstone belt in the central part of the Yilgam Craton, is localised in the ca. 2.8 Ga Archean tonalitic Edna May Gneiss. The Westonia greenstone belt comprises a sequence of deformed mafic and ultramafic amphibolites, overlain by metasedimentary schists and paragncisses, and intruded by tlie Edna May ortliogneiss. Peak metamorphic conditions are constrained to 650" to TOO'^C at pressures of «4 kbar. A series of syn- to post-peak metamorphism microgranitoid (U-Pb zircon age: 263718 Ma) and pegmatite (Sm-Nd mineral isochron age: 2640±11 Ma) sills truncate the sequence and provide a minimum age for mineralisation at Westonia (Kent et al., 1996).

Gold mineralisation is localised in a series of quartz-rich veins within a broad shear zone in the Edna May Gneiss and its contact zones with mafic-ultramafic amphibolites. The ore zones comprise (1) Type la lolraiion-paraller veins: narrow variably deformed quartz-bearing veins and zones of intense meiasomaiism oriented parallel to the dominant gneiss fabric, and characterised by recrystallised grajioblasiic textures; (2) Type lb 'foliation-parallel' veins: narrow quartz-diopside-bearing veins typically (sub-)parallcl to the gneissic fabric and characterised by unstrained textures; and (3) Type 2 Quartz Reefs: laniiiiaied to massive thick quartz-rich reefs that form broad antiformal structures which cross-cut the foliation and have a pseudo-pegmatitic texture. Textural relationships suggest Type la veins formed early m the deformation sequence, whereas Type lb veins and Quartz Reefs developed syn- to post-peak metamorphism and late in the deformation sequence.

Metasoinatic assemblages associated with Type la veins are typically of low-variance and comprise one or more of the following minerals: quartz, K-feldspar, plagioclase, biotite, muscovite, siUimanite, cordicritc, amphibole, diopside. Hydrothermal alteration associated with Type lb veins in the Edna May Gneiss and mafic amphibolites comprise a dominant quartz-diopside-titanite±K-feldspar-rich assemblage in veins, a diopside-homblende±plagioclase±K-feldspar assemblage on vein margins, and plagioclase-biotitehornhlcndefK-feldspar assemblage in the wallrock. The Quartz Reefs are composed of quartz with subordinate K-feldspar, plagioclase, biotite, diopside, hornblende, and minor muscovite. Hydrothermal \\allrock alteration associated with the high-grade Quartz Reefs in tlie Edna May Gneiss is limited to development of biotite and pyrrhotite. All vein types and walkock contain pyrrhotite and ilmenite, minor pyrite, chalcopyrite, galena, and trace scheelite, wolframite and molybdenite.

Pieliminar> microlliermometric investigations on fluid inclusions hosted in Type lb Toliation-paraller veins and l ype 2 Quartz Reefs, and spatially related to gold mineralisation, are of three primary types: (1) H.O-NaCl±ha]iie inclusions of moderate salinity (mean 15.2±5.1 [la] eq. wt% NaCl), (2a) HjO-COjNaCltCH. uiclusions of low salinity (mean 5.0±3.3 [la] eq. wt% NaCl), containing a mean of 31±18 ( l a ) imV7( CO.. and molar volumes with a mean of 32±10 ( l a ) c m \ (2b) COj-CH^ inclusions with minor amounts of CH4 (mean 13±5 [la] mol%), and molar volumes with a mean of 52±2 [la] cm , and (3) CH4 inclusions containing minor amounts of CO2 (<10 mol% CO2). Mass balance calculations indicate the addition of Si, K, Rb and S from tlie ore fluid, and an ore element association of Au, Ag, W, Mo, Cu, Pb ± Bi. Mineral equilibria studies on low-variance Type la vein and high-variance Type lb and Type 2 vein assemblages indicate P-T conditions of formation of 650'^±50X at kbar, and 600®±50'^C at «4 kbar, respectively. Quartz-diopside oxygen-isotope geothermometry gives temperatures from 590 to 615°C for Type lb veins in mafic amphibolites. Using the mineralisation temperatures of 600°C and isochores calculated from Type 2 inclusions, pressures of about 4±2 kbar are obtained. Ore and alteration assemblages poorly constrain X(C02) to within the broad range of 0.05 < X(C02) < 0.50, but importantly indicate a value of /O2 such that CO2 would be the dominant carbonic phase in the fluid. A magmatic and/or metamorphic origin for the ore fluid is suggested by stable isotope analyses on quartz, diopside, hornblende and biotite which indicate a ore fluid composition of 7.3 to 8.9%o (n = 19) and a 5D ore fluid composition in the range from -63 to -29%o (n = 8).

In combination, structural, mineralogical, textural, fluid inclusion, stable isotope and thermodynamic studies indicate that mineralisation was synchronous with peak to slightly post-peak metamorphism and likely has a mixture of magmatic and metamorphic fluids. Preliminary interpretations suggest that the major crustal fluid flow responsible for gold mineralisation at Westonia was related to partial melting and intrusion of granitoids and high-grade metamorphism within the more deeply exposed segments of the Yilgam Craton at ca. 2640 Ma.

20

SUPERPOSITION OF LOW SULFIDATION EPITHERMAL Au AND PORPHYRY Cu-Au MINERALISATION AT ACUPAN, PHILIPPINES, BY TRANSPRESSIONAL UPLIFT AND EROSION David R. Cooke Special Research Centre for Ore Deposit Studies (CODES), University of Tasmania, GPO Box 252-79, Hobart, 7001 INTRODUCTION Sillitoe (1989) emphasised the occurrence of zoned mineral districts centred on porphyry Cu-Au deposits in the western Pacific. In the Philippines, both low sulfidation (e.g., Baguio district; Acupan, Antamok, etc.) and high sulfidation epithermal deposits (e.g., Mankayan district; Lepanto) are superimposed onto porphyry systems.

Recent research (e.g. Arribas et al., 1995) has shown that high sulfidation epithermal Au-Cu-As systems can be genetically related to the formation of porphyry-style mineralisation, and can formed above or immediately adjacent to some porphyry systems (eg. Lepanto / Far South East). Relationships between the near-surface (0-1 km depth) low sulfidation environment and porphyry systems (depth of emplacement: «2-5 km) are more problematic, and have received comparatively little attention in recent years. In many areas, low sulfidation epithermal Au-Ag veins and breccias are known to be spatially associated with porphyry deposits. They are generally offset laterally by several kilometres, and are inferred to have formed at least 1 Ma after the cessation of porphyry emplacement. However, at some localities, 'telescoping' or superposition of low sulfidation epithermal deposits directly onto pre-existing porphyry systems has occurred. For example, Sillitoe (1994) proposed that sector collapse of a composite volcano was responsible for the superposition of a giant low sulfidation epithermal gold system onto pre-existing porphyry mineralisation at Lihir, Papua New Guinea. While this mechanism is undoubtedly effective, it is also probably unique to Lihir, and an alternative explanation is required to explain the superposition of the two deposit types in other settings.

BAGUIO DISTRICT Several large low sulfidation epithermal deposits occur in the Baguio mineral district, situated at the southern end of the Central Cordillera in a restraining bend of the major left-lateral strike-slip Philippine Fault system. At the Acupan gold mine, 200 tonnes of gold have been mined from over 460 interconnected epithermal veins and related breccias. The Acupan low sulfidation epithermal gold mineralisation is Pleistocene in age (0.79 Ma; Aoki et. al., 1993), and has overprinted Pliocene (2.4±0.5 Ma; Cooke et al., 1996) porphyry Cu-Au mineralisation at Acupan South. Telescoping of epithermal mineralisation onto the older porphyry system at Acupan was favoured by high uplift rates, related to transpressional uplift of segments of the Central Cordillera, combined with high erosion rates associated with the monsoonal climate. Denudation of the local topography was exacerbated at Acupan by the emplacement and subsequent partial removal of a phreatomagmatic vent/diatreme complex prior to epitherm^ vein formation. Fluid inclusion and stable isotope studies have shown that there was no direct input of magmatichydrothermal fluids from the porphyry system into the low salinity epithermal vein system. Superposition of low and high epithermal mineralisation onto porphyry systems in the Central Cordillera was the result of a combination of rapid transpressional uplift and high erosion rates, and is likely to have been a more common telescoping mechanism in the Western Pacific than catastrophic unroofing of a volcano by sector collapse.

Transpressional uplift probably also favoured the superposition of high sulfidation and porphyry-style mineralisation at Lepanto in the Mankayan district, and may be a key factor in the close spatial occurrence of these two styles of mineralisation.

REFERENCES Aoki, M., Comsti, B.C., Lazo, F.B. and Matsuhisha, Y, 1993, Advanced argillic alteration and geochemistry of alunite in an evolving hydrothermal system at Baguio, Northern Luzon, Philippines* Resource Geology, v.

43, p. 155-164.

Arribas, A., Jr., Hedenquist, J.W.. Itaya, T., Okada, T., Concepci6n, R.A., and Garcia, J.S.. 1995.

Contemporaneous formation of adjacent porphyry and epithermal Cu-Au deposits over 300 ka in northern Luzon, Philippines. Geology, 23, 337-340.

Cooke, D.R., McPhail, 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. 91, pp. 243-272.

Sillitoe, R.H., 1989. Gold deposits in western Pacific island arcs: The magmatic connection Econ. Geol.

Mon., 6: 266-283.

Sillitoe, R.H., 1994. Erosion and collapse of volcanoes: causes of telescoping in intrusion-centred ore deposits.

Geology, v. 22, p. 945-948.

21

LOCALISATION OF ORE DEPOSITION IN FRACTURE-CONTROLLED HYDROTHERMAL SYSTEMS: TOWARDS A PERCOLATION THEORY APPROACH StephenF. Cox'. Jean Braun^ Mark A. Knackstedt'Department of Geology, The University of Newcastle, Callaghan, NSW 2308 'Research School of Eareth Sciences, The Australian National University, Canberra, ACT 0200 ^Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200

Percolation theor>^ provides potentially useful contraints for understanding ore deposit localisation in cases where the architecture of fluid flow in hydrothermal systems is controlled by linked networks of active faults, shear zones and associated fracture systems.

At depth in the Earth's crust, sustained fluid flow requires active deformation to enhance and maintain permeability. The permeability distribution and architecture of flow is influenced by competition between deformation-induced porosity-creation processes and a variety of porosity-destruction processes which are controlled by thermal and hydrothermal processes and stress regimes. Localisation of deformation along faults, shear zones and their associated fracture systems usually leads to flow localisation. Large-scale flow systems develop when faults and shear zones develop sufficient connectivity with each other to create percolation networks.

The length scales of fluid flow are controlled by connectivity among elements of an active fracture network and between potential metal sources and depositional sites. For very low strains, individual fractures are disconnected and fluid flovn is limited by the permeability of the intact rock mass. Increasing strain promotes increasing connectivity among fractures, faults and shear zones. The onset of fluid flux on scales sufficient to link fluid sources and potential ore deposition sites occurs at a percolation threshold which is dependent on factors including strain and fracture geometry. Critical strains required to reach the percolation threshold are low.

Regions of fluid focusing develop around the upsU-eam segments of percolation networks and fluid discharge regions develop in the downstream parts of these systems.

The architecture of fluid flow in networks of faults and shear zones can be understood in terms of the connectivity among elements of the network. Fractures which form a connected pathway across a hydrothermal system are known as backbone elements. These elements support most of the fluid flow in the system. Dangling elements link to backbone elements and feed fluid into, or out of, the backbone. As such they are dead-end structures and carr\ lower fluid flux than the backbone. The backbone and dangling elements together form the accessible population of fractures that localise all the fluid flow if the rock matrix is impermeable. Isolated elements are the fractures that are unconnected to the rest of the fracture system and therefore carry httle fluid flow.

The relative proportions of isolated, dangling and backbone elements change as a function of the total strain or fracture density. For systems just above the percolation threshold, the backbone is a very small proportion of the total fault population and flow is localised on a few structures linking fluid sources and sinks. Particularly in the mid- to upper-crustal seismogenic regime, rapid changes in permeability distribution in fault networks due to episodic fault slip, aftershock sequences and interseismic hydrothermal sealing can lead to rapid changes in flow paths within fault networks. In systems near the percolation threshold, this can lead to rapid changes to the location and architecture of flow backbones. For hydrothermal systems with a much higher density of permeable fractures, fluid flow is distributed over a larger proportion of the fracture population.

Mesothermal, shear-zone-hosted and fault-hosted gold systems in Archaean greenstone terranes provide examples of percolation phenomena in crustal-scale hydrothermal systems. We discuss controls on localisation of such shear-hosted gold deposits in terms of percolation phenomena and illustrate flow patterns and the geometry of fluid focussing and discharge regions around permeable fault systems by the use of two-dimensional finite element models of steady state fluid flow in and around simple, high-permeability fault networks that are embedded in a less permeable medium.

22

ASPECTS OF GOLD MINERALIZATION IN THE HODGKINSON PROVINCE, NORTH QUEENSLAND, AUSTRALIA Brett K. Davis\ Mark Lindsay* and Joao F.M. Hippcitt^ ^Department of Earth Sciences, James Cook University, Townsville, Qld, 4811 'Departamento dc Gedogia, Univcrsidadc Federal de Ouro Preto, Mono do Cnizein), Cep 35400-000, Ouro Prcto. MG. Brazil The Hodgkinson Province is a tract of multiply deformed sediments and volcanics within the Tasman Orogenic Zone, north Queensland, Australia, which has been intruded by several early Permian granite supersuites. A regionally consistent deformation history conq>rises four events of variable intensity, DrD4, with subsequent local ductile fabric formation and faulting. Granitoid en^lacement was coeval with the major D4 crustal shortening event that occurred in response to the Permian-Triassic Huntcr-Bowen Orogeny.

Many areas within the Hodgkinson Province, particularly within the Silurian-Devonian Hodgkinson Formation, have been exploited for gold. Hie majority of this has been alluvial due to the rarity of gold hosted by in-situ veins. This study has focussed on hardrock gold deposits of the West Normanby Goldfield J^proximately 80 km south of Cooktown. Regional structural moping and underground mapping at Maddens Mine have been combined with datafromspatially oriented thin sections to resolve the structural age of gold mineralization.

Gold-bearing veins are hosted by Taylors Fault, a major structure that formed during D2, and infill dilatational jogs that opened during sinistral-normal reactivation of the fault in D4. Veins contain graphitic laminations that formed when fault planes segmented walkocks adjacent to the veins, producing tabular clasts that were tectonically sliced into the reefs. Laminations are commonly stylolitic due to progressive shortening during D4.

Gold particles have preferentially nucleated in zones of relatively coarser-grained quartz adjacent to the shear planes, where shortening strain caused microfracturing and allowed access of fluid. Gold may have been introduced with the quartz, but was redistributed within the reefs and localized along the laminations by the effects of synchronous, progressive deformation.

In summary, we propose that the gold-bearing quartz veins from the West Normanby Goldfield in the northern Hodgkinson Province were emplaced during the D4 event. Regionally, hard-rock gold deposits show close spatial relationships with granite plutons of the areally extensive Whypalla Supersuite or with zones of Hodgkinson Formation that have been metamorphosed to higher grades than the more typical chlorite-grade rocks of the province. The Mount Madden structural dome, which is a macroscale D4 feature in the western Hodgkinson Province, contains mica schists and has historically yielded significant quantities of large gold nuggets and hosted numerous small-scale hardrock workings. Gravity images, metamorphic grade variations and structural relations indicate the presence of shallow subsurface granite here also. Maytown is famous for its historical gold production and is located northwest of the Cannibal Creek Granite. Hostrocks in this area are commonly phyllitic and of higher metamorphic grade than typical Hodgkinson Formation rocks, suggesting the presence of subsurface granite. Another area renowned for nugget production, Campbells Creek, lies immediately adjacent to the Permian Koobaba and Kelly St George Granites in the central eastern portion of the South Palmer River area.

Hard rock mines have also been developed in the vicinity of the Permian Tinaroo Granite at Gold Mine Creek.

The development of the R.B. Mine in Permian Normanby Formation west of Cooktown confirms gold mineralization in the Hodgkinson Province during or after the Permian. Consequently, we suggest that syn-D4 granites were emplaced during a major episode of crustal shortening, in particular those belonging to the Whypalla Supersuite, and were integral to the major gold mineralizing event in the Hodgkinson Province during the Permian, and that the mineralization in the West Normanby Goldfield is a reflection of this.

Acknowlegdements: Logistical support by West Normanby Resources is gratefully acknowledged.

23

NSW METALLIC MINERAL OCCURRENCE DATABASE (METMIN) PetoMDownes Geological SurveyrfNcwSouth Wtla, PO Box 536 St LeooardB, NSW, 2065 The Geological Survey of New South Wales, Department of Mineral Resources, commenced its MetaUogenic Mapping program in the late 1960s. As part of the metallogenic m ^ i n g program, mine data sheets were compiled and published together with metallogenic notes that accompanied each map sheet During the mid 1980s the mine data sheets were con^Hiterised and incorporated in the Dq»rtmcnts land information system (MRLIS). More recently the database has been maintained and extended using Symantec Q&A software however it still refleaed the original mine data sheet structure. Following a review of the existing data structure and allowable terms in late 1995, the structure of the database has been upgraded to meet the needs of users in a GIS environment using Microsoft ACCESS V2 as the compilation software package.

The new data structure (METMIN), consists of two core tables; the site and occurrence data tables, together with eight subsidiary tables containing details on production, reserves, mineralogy and host rocks. The database has been designed for modelling in a GIS environment. Two fundamental changes distinguish this database from previous databases. The database uses the fiill term rather than a concatenated code as the allowable term in any field, and the amount of metal present in a deposit is recorded in a "calculated field" in such a way as to be useable for modelling purposes.

The existing Departmental metallic mineral occurrence datasets are being systematically migrated into the new structure. The migration process consists of taking all existing data in the MRLIS structure, breaking up the majorit} of comma delineated and textfieldsinto searchable relational tables and decoding all binary code and concatenated codefieldsinto their allowable terms and placing those terms into the ai^rc^riate fields.

To date, most of the Lachlan Fold Belt data have been migrated with some initial validation/editing performed.

At present (November, 1996), the remaining data for the Lachlan (Bathurst dataset) together with the published datasets for the southern New England are being migrated and validated. The next stage will involve the migration of the preliminary working datasets for the northern New England region.

The Forbes and Dubbo datasets are currently undergoing updating in conjunction with the NGMA mapping program. The new data will be included in the METMIN database when these projects are complete. Under the D2000 program the Cargelligo and Narrandera datasets will shortly be updated. When completed, this dataset will be included in the statewide database. The data for the Broken HUl region, including the Koonenberry area is currently being validated and will be included, as it becomes available.

The ne\\ database represents a significant departurefi-omearlier databases in that METMIN is more tightly structured than the MRLIS database. The attributes selected have been restricted to those of key in?x)rtance so as to maintain simplicity and care has been taken to separate different attributes and to restrict the list of acceptable values to a small number of generalised terms. Detailed information about the occurrence is placed in unstructured free textfieldswhere appropriate. This will allow the data to be more widely searchable.

The aim of the database is not to replace existing descriptions, rather, it is a compromise between an irniex and a complete data source, designed to allow companies and researchers to quiddy fi^ out what occurrences exist, their characteristics and where to find fimher data about the occurrence. In addition, the database has the capacit> to be a powerful research tool for mineral potential mapping.

It is proposed that the complete (statewide) dataset, including the current versions of the preliminary sets of data for the northern New England and central Lachlan areas, will be available on CD-ROM by June 1997. It should be noted that this dataset will be in general more comprehensive and hence distinct fit)m those partial mineral occurrence datasets released as part of the mineral occurrence information in the D2000 Lachlan areas.

Broken Hill Exploration Initiative, and Resource Audit (Lachlan Fold Belt padcage) data releases.

Acknowledgments: This abstract is published with the permission of the Director-General, NSW Department of Mineral Resources.

24

SECOND GENERATION METALLOGENIC MAPPING OF THE DUBBO 1:250,000 SHEET - A PROGRESS REPORT PelerM.Dawnes Geological Survey oi}i€w South Wiki, PO Box 536 St Leonvdk, NSW, 2065 The first generation geological map of the Dubbo 1:250 000 sheet area was published in the late 1960s. Matson (1975) completed a metallogenic study of the area and identified some 256 mineral occurrences. More recently the Geological Survey of New South Wales (Department of Mineral Resources) and AGSO, under the National (jeoscience Ma^^ing Accord, commenced second generation geological map^ng of the Dubbo sheet in mid 1994 (Pogson et al, in prep). An iqxlate of the mineral occurrence database for the area commenced in mid 1995. This wiU be incorporated with the revised geology and is due to be conq)leted by the end <rf 1997.

The Dubbo area covers the north eastern extension of the Lachlan Fold Belt, a c(Hig>osite Palaeozoic orogenic bell, which occiq>ies most of central New South Wales. Two younger sedimentaiy basins, the Surat Basin and the Sydney Basin, cover the northern and eastern edges, respectively, of the Lachlan Fold Belt. IntheDiAbo map sheet area, the Lachlan Fold Beh consists of a mmbcT of discrete teclono-stratigraphic units. These include the Capertee Zone, a belt of Early Ordovician quartz turbidites and Late Ordovician to Early SUurian mafic volcaniclastics with subordinate mafic igneous rocks. To the east rf the Ciq)ertee Zone are units of the Sydney Basin. To the west of the Capertee Zone is the Hill End Trough, which contains a sequence of Middle Silurian to Middle Devonian turbidite sediments and volcanics. To the west of this is the Molcmg Zone, a sequence of Ordovician to Early Silurian mafic volcanics and sediments. The Cowra Trough lies to the west of the Molong Zone. This is a continuous sequence of Silurian to Early Devonian sediments with scMne volcanics.

The (Juambone-Young Zone lies on the western edge of the sheet area and consists of a pluton belt of Late Silurian and Devonian age. The Dubbo sheet area has been subsequently intruded by a number of granites of Devonian and Carboniferous age. Some of these intrusives are believed to be generators erf* mineralisation.

Significant gold, silver, copper, lead, zinc, and iron occurrences have been found in the Dubbo 1:250 000 map sheet area in the past. In addition, diamonds and sapphires have been recovered as part of past alluvial mining activities and the area hosts a number of inywrtant clay and other industrial mineral deposits. To date over 800 mineral occurrences have been identified and in part documented in the area. Styles of mineralisation include: • Shear hosted gold mineralisation such as the ductility controlled hard rock mineralisation in the Gulgong area, laminated auriferous quartz reefs such as those found at Bodangora and the tuibidite hosted saddle reefs of the Hargraves and northern Hill End CJoldfields,.

• Ordovician hosted porphyry copper-gold mineralisation such as that fouiKl in the Kaiser area.

• Strata-bound Kuroko Type massive sulphide deposits associated with Silurian volcanics such as those found in the Commonwealth and Galwadgere areas.

• Yeoval style copper-gold mineralisation associated with granites of Early Devonian age.

• Lead-zinc, cow)er and magnetite skam type mineralisation such as that found at Leadville and Tallawang.

• Epithermal style silver mineralisation as found in earliest Permian volcanics at the Bowden's deposit.

• Auriferous deep leads such as the Gulgong Deep Leads • Alluvial deposits such as the Macquarie River (Quaternary alluvials which contain a variety of commodities including gold, diamonds and sapphires.

At the end of the current data compilation and field checking ^hase, follow up work on major occurrence styles in the Dubbo area will be imdertaken.

REFERENCES Matson, C.R., 1975. Mine Data Sheets to acconqjany Metallogenic Map DiAbo 1:250 000 Sheet. Geological Survey of New South Wales, Sydney.

Pogson et al, in prep. Dubbo 1:250 000 Geological Map SI/55-4, Geological Survey of New South Wales, Sydne>' / Australian Geological Survey Organisation, Canberra.

Acknowledgments: This abstract is published with the permission of the Director-General, NSW Department of Mineral Resources.

25

ANATOMY OF SEDIMENT HOSTED SEAFLOOR MASSIVE SULFIDE DEPOSITS: PRELIMINARY RESULTS FROM ODP LEG 169 DRILLING IN MIDDLE VALLEY AND ESCANABA TROUGH Rowcna C Duckworth' and the Leg 169 Shipboard Scicntific Party ' National Key Centre in Economic Geology, Dept. Earth Scicnccs, James Cook University, Townsville Queensland 4811

Drilling and downhole geophysical logging through the Bent Hill Massive Sulfide deposit (BHMS) in Middle Valley, N. Juan de Fuca Ridge, provide an unprecedented three dimensional view of a large seafloor hydrothermal deposit. BHMS was revisited during Leg 169 for the chief purposes of assessing the thickness and lateral extent of the mineralization and to determine the nature of the hydrothermal feeder zone underlying the deposit The holes drilled on Leg 169 constrained the minimum extent of BHMS to at least 100m in the N-S direction and 150m in the E-W direction. The thickness at the centre of the mound is a minimum of 103m.

There is an abrupt contact underlying the massive sulfide with a 107m thick sulfide stringer zone developed in sediment which represents the feeder system for the hydrothermal fluids that formed the Bent Hill massive sulfide mound (104-211 mbsf). The upper 49m of the stringer zone is intensely veined by pyrrhotite-isocubanitechalcopvTite-pyrite veins that comprise greater than 10% of the core. The veins are dominantly vertical and as much as 8cm wide. The dominate alteration minerals throughout the stringer zone are quartz and gray chlorite.

The lowermost 10 metres of the sulfide stringer zone is intensely mineralized (10-50% sulfide) and h\drothermall\ altered sediment. Mineralization in this lower copper-rich zone is controlled entirely by sedimentar> features and veining is absent. Sulfide replacement textures mimic original sedimentary features, such as cross laminations in sandy intervals that marked the base of turbidites.

The base of the stringer zone is marked by an abrupt transition (fauh?) to moderately indurated, but only slightly altered and nonmineralized mudstones, siltstones, and sandstones (211-432 mbsf). The zone fi-om 432 to 470 mbsf consists of interlayered diabasic sills and highly indurated turbiditic sediment. The sills are pervasively altered and are locally cut by chlorite-sulfide and quartz-sulfide veins. From 470 mbsf to the bottom of the hole at 500 mbsf drilling recovered only fme-grained hydrothermally altered pillow basalt. The occurrence of pillow flows indicates that the area to the east of Middle Valley is underlain by normal oceanic crust constructed by extrusion of basalt prior to the onset of rapid sedimentation at the ridge axis.

Geochemical composition dowiihole logging, and textural evidence indicate that the massive sulfide formed as a mound-like feature above the sediment. With the exception of a 5 meter thick interval near the top of the mound, the massive sulfide contains no intermixed sedimentary component. The primary sulfide assemblage was dominated b\ pvrrhotite with less abundant isocubanite, chalcopyrite, and minor sphalerite. Late hydrothermal reprecipitation of base metals (zone refming) is responsible for the upward increase in Zn/Cu in the massive sulfide. The deposit consists of mainly pyrite and pyrrhotite, hence, the base metal concentrations are very low, only 12% Zn, 0.5% Cu and 180 pmm Pb. This is in strong contrast to the active ODP deposit, 350m to the south of BHMS, that was drilled for the fist time on Leg 169. In places, 260®C fluids are venting fi-om the top of this deposit. The hole drilled through this deposit revealed a multilayered, stacked massive sulfide lens system, with at least three major zinc-rich bodies at different depths within the sediments. The preliminary analyses indicate an average content of 19.6% Zn, 3.3%Cu and 132 ppm Pb. With the thickness of the sulfide lenses up to 39m, the ODP mound probably represents an economic deposit.

The sulfide mineralisation in Middle Valley is very diflferent to that at Escanaba Trough on Ae Gorda Ridge, which is characteristed by shallow (less than 20m thick), surficial mineralisation without any well developed feeder zones.

26

NEW PERSPECTIVES ON THE MCARTHUR BASIN FROM GIS AND GEOPHYSICS MsLLJM^«dD«vidE C^iilreforOreDepoakaMlExplonrtknSUK^ UiiivcrshyrflMniMU.GPO Box 252-79 HobMt,Tai700l' ^Leamm Geophyao, GPO Box 320D Hobirt, Tts 7001 The McArthur Basin contains a world-class, tolifimn sediment-lioslcd base metal (SSHBM) dqx)sit (HYC), which, because of its relativel) undefonned and unmetamorphosed state, has been widely used as a basts for models of SSHBM oiiaendisation. Many of these aodels poitidale m depositiofi as a i t s ^ of fluid fknv within a rift-sag tectonic oiviroamoit, although basin morptology is not well coastrained by surface napping due to ver> limited exposure of many of the deeper basin units. It is aecessaiy to constrain such models in the third dimension by defining the geometry of basins and the bounding faults which may control fhad migration.

This has been attempted b> integrating geophysical imcrpretation results as a 34) ccmipcment m a metallogenic geographical informjrtion system.

Rigorous quantitative analysis of AGSO regional Bouguer gravity and magnetic data has been employed to investigate the geometry of the McArthur and Ta^^-allah Groups, as well as that erf the underlying 'basemem' units Complemcntar>' interpretation of an interlocking array of 2-D gravity and magnetic pnrfiks was undertaken b\ forward modelling constrained by application of the criteria of Lcaman (1994). The results were used to generate isochore and structure contour maps of all major basin components. These maps reveal the gross morpholog} of the basin fill, its basement and units which, while correlated with exposed rocks, are much more voluminous than hitherto su^)ected from their exposure.

Structure and isochore contours were digitised and gridded using the Arc/Info GIS software, enabling redisplay and analysis of the geophysically derived structural information with respect to other geoscientific data in the GIS such as surface geolog>' and mineral deposit locations. The gross basin units are presented in the poster accompanying this 2rt)stract as a series of 3-D rendered (isometric) views displayed in relation to their outcropping equiv-alents, regional first-order fault structures and various mineral occurrence types.

The McArthur Group is seen to extend well beyond its eastern limit of outcrq) as defined by the Emu Fault.

Units identified as p r o ^ t i v e using concq)tual lithological criteria encoded in the GIS contain all knoy^n stratiform base metal mineralisaUon Such deposits are preferentially located on the periphery of the thickest accumulations of McArthur Group sedimentar>^ rocks. Volcanism in the upper and lower Tawallah Group is much more voluminous than its comparatively low stratigraphic thickness measured in outcrop would suggest.

ON er 15 km of basin fill (including volcanic rocks) is impHed in some areas, but this may vary n^idly over short distances, implying considerable pre-McArthur Group structural development. A number of lineaments \ isible in the isometric images converge at the position of HYC, indicating bounding fault and strike-slip fault activit} at this location during a large portion of basin evolution. A large pile of basal volcanics is inferred to be equivalent to felsic metavolcanics of the Barramundi Orc^eny. Granitoids of varying composition and timing are interpreted to have intruded beneath much of the McArthur/Mount Isa Basin region, but the timing of this ev ent is poorly constrained.

Combination of rigorous geophysical analysis and GlS-supported di^lay and integration with other data sets permits a new view of the McArthur Basin and its mineralization. Isometric views of structure contours and isochores enable easier visualization of relationships between basin geometry, faults and mineral occurrences.

Geologically interpreted major structures mapped at the surface;nay not necessarily have been significant, or possessed their present geometrj', throughout the basin's evolution. Such information must be considered in de\ eloping metallogenic models incorporating tectonic setting and fluid flow.

REFERENCE Leaman, D.E., 1994. Criteria for evaluation of potential field interpretations. First Break 12, 181-191.

Acknowledgements: This work was undertaken as part of AMIRA project P384, which ran from 1992 to mid1995 The support of sponsoring companies and organisations through AMIRA is gratefiilly acknowledged.

The AGSO in particular is thanked for supply of gravit>' and magnetic data, and the provision of an AGSO Postgraduate Research Award to one of us (MLD).

27

NEW GEOLOGICAL INTERPRETATIONS ON THE BIGGENDEN MAGNETITE SKARN, SOUTHEAST QUEENSLAND MansourEdraki and Paul Ashley Department of Geology and Geophysics. University of New England, Armidale. NSW 2351

In recent years, underground mining and diamond drilling by Conmiercial Minerals Ltd at the Biggenden magnetite mine in southeast Queensland have provided an excellent three-dimensional opportunity for observation of the skam-hosted orebody and associated rocks. Detailed mapping of the deposit, together with mineralogical and geochemical investigations, have allowed more refined interpretations to be made on ore genesis, as well as indicate the need for further research.

The Biggenden deposit occurs within a steeply-dipping, N-S striking sequence of deformed turbiditic siltstone/diamictite, limestone and basaltic and andesitic volcanic rocks of the Permian Gympie Group.

Within a few metres to tens of metres of the deposit, the Gympie Group has been intruded by the Triassic Degilbo granite which has produced a hornblende homfels grade metamorphic aureole several hundreds of metres wide. Initial metamorphic assemblages have developed essentially isochemically. The orebody is a steeply-plunging, elongate lens-shaped mass, approximately concordant with the attitude of the enclosing rocks. However, it is associated with local zones offracture/sheardevelopment and has a close spatial relationship with a near-vertical, ?re-activated, N-S striking fault on its western side. The Biggenden mine lies along strike from several other smaller hydrothermal magnetite and base metalbearing deposits adjacent to the contact with the Degilbo pluton, and on the trace of a regional-scale structure evident on a TMI image.

Magnetite ore is part of a prograde skam assemblage, with associated calcite and grandite garnet. This assemblage, together with other prograde skam types rich in garnet, or in diopside-hedenbergite, have replaced prior contact metamorphic assemblages, with replacement of all rock types, including siltstone, volcanics and marble. There is no preference for skam replacement of marble. The orebody contains irregular patches of very coarse grained hydrothermal calcite and at depth is associated with spectacular hydrothermal breccia containing a coarse grained calcite matrix and protolith clasts showing varying replacement by prograde skarn, including by massive magnetite. Prograde skam calcite, gamet and clinopyroxene contain hypersaline fluid inclusions. Development of coarse calcite and prograde skam has involved considerable desilication of the total rock package, with introduction of Ca, Fe, Mn, Mg, Sn, W, Mo, Bi, Cu, Zn and S. There may also have been significant volume reduction. Retrograde skam assemblages are widespread as pervasive and vein replacements and involve development of epidote and Cl-bearing hastingsite, followed by chlorite, calcite, actinolite, quartz and sulphides (commonly pyrite, chalcopyrite, bismuthinite, but with traces of arsenopyrite, molybdenite, cobaltite, sphalerite and stannite). Gold is associated with higher Cu and Bi grades and was previously mined from near-surface, supergene-enriched material. Late stage retrograde alteration is manifest in a nontronite-calcite assemblage. The mineralogical and chemical composition of the orebody imply that it is an Fe skam, but with gradational characteristics into Au and Cu skms (cf. Meinert, 1988; 1992).

The adjacent intrusion is a relatively felsic I-type granitoid (-69% Si02) with a few microgranite dykes.

These rocks show little alteration at the level of exposure, with only a few amphibole veins on fractures.

No pervasive endoskam assemblages are developed. The composition of the intrusion is inconsistent with those reported from Fe or Au skams and together with the lack of significant alteration, raises the possibility that it is not directly responsible for skam generation. However, mineralogical, fluid inclusion, C and O isotopic and trace element characteristics of the orebody all indicate a strong magmatic (granitic) influence in its genesis. That the orebody is Si02-poor and FeCa-rich could imply that granitederived hydrothermal fluids have equilibrated with large volumes of mafic volcanic and carbonate rocks.

REFERENCES Meinert, L.D., 1988. Gold skarn deposits - geology and exploration criteria. Economic Geology Monograph 6, 537-552.

Meinert, L.D., 1992. Skams and skam deposits. Geoscience Canada 19,145-162.

Acknowledgements: Connimercials Minerals Ltd, and especially Geoff Weekes, are thanked for providing data on, and access to, the Biggenden mine. Nick Stephenson has provided much mineralogical help.

28

THE GEOLOGY AND MINERALISATION OF THE WAISOI PORPHYRY COPPER DEPOSITS, NAMOSI,FUL EtidJim', J. Brace Geamdl^ Anthony J. CnmW mi Dwid R. Coakel , Gold Mines ofAiisJrili^PO Box 3«,Tenioni, NSW, 2666.

Tlje Waisoi poiphyiy copper/gold deposits it Namosi. m the Republic of F ^ o o a d ^ 930 buUmb toane st 0.43% copper and 0.14 ppm gold. The host rocks ve strongly fiulted,frMtareda d dtered Totiary votanic rocks. The lower host volcanic unit consists of coherent a d autobrecdatsd b«ltic adesite hvas of ifae Late Ohgocene to Late Miocene (32 to 10 Ma), Wainimala basement (early island are magmarir affinities) whereas the upper host volcanic unit is dominated by Late Miocene («to 5.5 Ma), caio-aBaiiM andesitic dmntt lavas and volcaniclastics of the Namosi Andesite (mature island are m^gmatic affinities). TTiese two nils «« sepmted by a significant time break, widi a mass flow conglomerate nnit (Basal Vtmod Con^omerate) mmkmg dte beginning of the calc-alkaline volcanism. These rocks intruded (at 5.5 - 6.0 Ma) by dadtic qmatz dkrites of possibly similar chemistry to the Namosi Andesite volcanics. Fhii& assodtted wifli flwse kttrusives cased extensive hydrothennal alteration in the Namosi area.

Porphyry-style mineralisation is controlled by lidwlogy » d structure. R^icnal stnictnre controlled Hie location of the mineralising porphyries, the fluid pathways and the porosity/penneability of the host rocks, whereas the hthologies controlled the degree of chemical reaction and the style of mineralisatioa. Hie duninaot sinictures that controlled the emplacement of the poiphyries are northeast- and Borthwest-ferendiBg, with lesser iafhieoce of east-trending zones. East- and north-trending structures tended to occur u d disr^sted the raiBerelBation into a series of block faults.

Petrological, mineralogical, alteration and fluid inclusion investigations, cooibined with field observations of temporal relationships between geological features, have identified evidence for cariy, regional, greenschist fades, burial metamorphism followed by porphyry-related hydrotheimal alteration. TTie hydrofliennal system produced early albitisation, sericitisation, and silicification, prior to introduction of the main mineralising fluids which produced potassic alteration with most of fte copper and gold. This was followed by a cWoritisation (intermediate argillic) event, also with si^ificant copper mineralisaticm. The minenlising event was followed by several late-stage carbonate-rich alteration events.

Although minor chalcopyrite was deposited during die silicification and late-stage carbonate aheration events, most of the copper and gold was introduced and deposited as chalcopyrite a d bomite in coojunctimi wiA die potassic alteration episode. Fluids evolved from highly saline at temperatures in excess of 500°C, to high salinity (+45 wt.% NaCl equivalent) fluids at temperatures above 380°C, to fluids widi lower salinities (10 wt% NaCl equivalent) and temperatures around 300°C. Early weak Cu/Au mineralisation associated with thin, subvertical, northeast-trending, quartz diorite porphyry dykes in the Waisoi West/Waisoi North area, was rranobilised to fwm the Waisoi West quartz vein stodcwork zone by later nordiwest-trending compression. Tectonic relaxation allowed the mjectioB of the main, overpressured, massive porphyries at Waisoi East, Waisoi West and Waisoi North. Fluids from witiiin the intrusives mineralised areas of well-prepared (shattered) Wainimala Aggl<Mnerate a d penneable Basal Namosi Conglomerate. In areas of abundant hydroAwmal fracturi^ a d structural pi^Mration in the Waisoi Nwtii area, the Namosi Andesite also has significant amounts of Cu/Au mineralisation. Elsewhere (where lh«e is little structural preparation) the Namosi Andesite acted as a c^) a d contains only kw-grade, sulKcooomic, disseminated Cu/Au mineralisation. Compositional similarities between the Namosi Andesite a d the mineralising porphyries made the Namosi Andesite chemically unfavourable fw si^ficant Cu/Au dq)osition.

Stronger permeability-controlled mineralisati<Hi in die Wainimala Agglomerate a d Basal Namosi Cwiglomerate is controlled by physico-chemical chmges caused by die differing pr<^)mies between the source a d the host lithologies. Concentration of the mineralising fluids in die earlier brecciated dylces led to Wgher Cu/Au grades in die Waisoi North ( a d Waisoi West?) area.

Uter, subvertical, thin, porphyiy "ring" dykes vnfli weak Cu/Au mineralised haloes occur in Waisoi East.

Acknpwlgdgementf>: The authors acknowledge die support of Placer Exploration Limited for this study a d their permission to present this paper.

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ABSTRACT GEOLOGY OF THE TRTTTON MASSIVE SULPHIDE DEPOSITS J. Mike Fotuty, ExplonHkw Maufer AMCraHa, NoH Padfk Ltd Levd 15,3 Spring Street, Sydney, NSW, 2000

The Tritton polymetallic massive sulphide deposits arc sitiuited 22 kms south west of the Girilambone Copper Mine, some 105 kms ENE of Cobar and 45 kms Dorth-west of Nyngan within Ordovician tower greenschist facies meta-sediments of the Girilambone Group.

Nord Pacific Limited commenced exploration of the Girilambone Copper Mine for leachable copper chalcocite mineralisation in 1989 and esUblished an Ore Reserve of 8 million tonnes at 1.8% copper. In 1991, a Joint Venture was formed with Straits Resources Umited for development of the mine that is currently producing 99.999% pure electrolytically refined copper at the rate of 16,000 tonnes per annum.

As manager of a Joint Venture formed to explore the surrounding district since 1992, Nord has discovered additional leachable Ore Reserves at Girilambone North with 41,000 tonnes of contained copper metal that is currently being mined and which will extend the life of the project to 2003. At Budgerjgar, 800 metres north of Tritton, additional leachable chalcocite resources have been established, and further drilling and feasibility studies are planned.

The Tritton massive sulphide. Upper and Lower Zones are deeper than 180 metres beneath the surface, have no surface outcrop or anomalous geochemical response and were discovered in May 1995 by the use of Sirotem techniques. The Upper Zone has dimensions of 300 metres strike length, 250 metres down dip, true widths up to 24 metres and dips easteriy at 35 degrees. The Lower Zone has a strike length of at least 400 metres, a down dip extension of at least 900 metres and true widths of up to 40 metres. The 60 degree dip in the upper horizons flattens to 33 degrees from 600 to 1000 metres beneath the surface.

The mineralisation of both the Upper and Lower Zones contains central thkkened ribs that pitch to the SSE along a common axis, and the two zones appear to be confined between two SSE trending faults with a pitch similar to the central rib. The extensions beyond these limits has not yet been tested by drilling.

The Budgery gar massive sulphide zone, 800 metres to the north, has a strike extent of 350 metres and a thickness of 30m, and may be the faulted continuation of the Lower Zone which it resembles geologically.

At Tritton, a resource of 9.75 million tonnes at 3.01% Cu, 0.21 g/t Au, 11 g/t Ag - (293,000 tonnes of copper) has been calculated from the results of 117 diamond drill holes of maximum depth 1100 metres.

The Upper Zone is located within an intensely silicified medium grained quartz-greywacke. Copper, gold and silver contents are vertically zoned with the highest values and chakopyrite/pyrite ratios occurring near the top of the Zone (eg 12 metres of 21.2% Cu, 1.5g/t Au, 164 g/t Ag in BDS048 from 252m) and with lower chalcopyrite/pyrite ratios and copper values at greater depths (e.g. 13 metres 5.1% Cu, 0.33 g/t Au, 10 g/t Ag in BDS019 from 315m). Bomite and minor tetrahedrite occur near the top of the Upper Zone only. Minor sphalerite and galena have been identified microscopically.

The Lower Zone is strongly banded with black and green chlorite schist alternating with massive pyrite /chalcopyrite seams. The top of the Lower Zone contains wide, high grade copper intersections (e.g. 23 metres of 6.07% Cu, 0.11 gA Au, 11.9 g/t Ag in BDS055 from 586m). Copper vahies at tower levels generally decrease (e.g. 14 metres of 3.41% Cu, 0.20g/t An, 15 g/t Ag in BDS107 from 880 metres). The hanging wall contact of both zones with the wall rock is extremely sharp, but stringery chalcopyrite mineralisation occurs below the footwall of the massive inlphides of the Lower Zone.

Zinc content is slightly higher in the Lower Zone than in Oe Upper Zone, but Lead vahies are all tow.

Alteration minerals include hematite, yeltow-green chtorite, sericite, black chtorite, siderite within and adjacent to the mineralised zones; and epidote^ chlorite, siderite, magnetite within mafic flows and tuffs.

The mineralisation appears generally conformable with the stratigraphy as indkated by medium grained greywacke sequences and three mafk horizons. The setting ctosely resembles Aat at Girilambone where mineralisation occurs within both intensely sOkified greywacke and within chtoritk schist

30

DETAILED S^^S INVESTIGATION OF THE TAG HYDROTHERMAL MOUND AND STOCKWORK ZONE, 26°N, MID-ATLANTIC RIDGE ^ ^ *

J. Brace GemmeU and Robina Shnpe Exploration Studies. University of Tasmania, GPO Box 252C. Hobait, Tasmania, Amndia 7001

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-AUantic Ridge at 26°N. ITie 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 Wack smoker chimneys emitting fluids up to 360°C and consisting of chalcopynte, pyrite and anhydrite is 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 70 m awayfix)mthe black smoker chimneys.

GDP drilling of the TAG hydrothwmal mound and undcriying stockwoilc zone in 1994revealeda complex intern^ strangraphy consisting of, with increasing depdi, massive pyrite and pyrite breccias ± chalcopynte and sphalente in places, pyrite-anhydrite breccias, pyrite-silica breccias, silicified wallrock breccias aid chlontized basalt breccias. Several stages of quartz pyrite ± chalcopynte veins occur in the stockwoik zone and lower portions of the mound while anhydrite ± pyrite ± chalcopynte veins are common within the central and upper parts of the mound. This is the first time an active black smoker system has been drilled.

A detailed S^-^S investigation of sulphides and sulphates within mound and the underlying stockwork TOne has revealed that tte overall range of sulphide S^^S isfrom0.35 to 10.27 %c, with a mean of 7.20 %c Anhydnte has a mean value of 21.10 %cfroma tight range of 20.55 to 21.56 %c. ITiere are distinct ^erences in values between the different texmral types of pyrite (massive sulphide, breccia clasts disseminations assoc. with alteration, and veins) within the hydrothennal mound and stockwork zone. W massive sulphide and breccia clasts have a similar distribution of S^^S values ( 6 - 8 %o) however the values of the disseminated pyrite associated with the alteration are distinctly heavier (S^'^'S S = 8 - 10 %c) Vein sulphides teve the lightest S^^S values (S^^S = 5 - 7 %o) at TAG. The sulphur isotope values measured at TAG arc, in general, the heaviestreportedfor unsedimented mid-ocean ridge mineralisation.

A sulfur isotope model is proposed to account for the heavy S^^S signature, the distribution of S^S values from the vanous textural styles and the spatial distribution of the fiS^S values both laterally aid vemcally throughout the hydrothennal mound and underlying stockwock zone. The two initial sources of sulfur dunng the Ufe of TAG are seawater sulfate (S^^S = 21 %o) and MORB derived sulfur (S^^S = 0 - 1 %C). Vanauons in d^^'S values at TAG can be explained in a model where totally to partially reduced seawater sulphate of shallow origin mixes with a deep hydrothennal fluid dominated by MORB sulphur aid interacts with previously fonned sulphide and sulphate minerals in the upper parts of the stockwork zone and within the mound. Deep sul>scafloor processes cause the initial hydrothennal fluids entering the TAG system at depth to have a S^^S value of approximately 0 - l%c. This fluid mixes with locally entrained partially reduced seawater in the uppa parts of the sub-seafloor stockwork system (aeated by local hydrothennal convection though the porous and penneable mound and stockwork zone) and creates a modified hydrothennal fluid with a S^^S value of approximately 6 - 7 %o. ITie fluid rises to the seafloor precipitating pyrite in the quartz-pyrite veins (S^S = 6 - 7 in the upper parts of the stockwork aid mixes with cold seawater which causes rapid precipitation of the massive pyrite ± chalcopyrite (S^^S = 6 8 %c) found at the top of the mound. Anhydrite (5^S = 20-21 %c) is fonned from the heating of entrained seawater circulating within the massive sulphide or upper parts of the stockwork zone. During periods of IitUe or no high temperature hydrothennal upflow thennal collapse is accompanied by infiltration of cold seawater through the TAG mound and upper parts of the stockwork. Anhydrite that was within the mound and overlying chimneys dissolves creating clasts andfragmentsof pyrite ± chalcopyrite ± sphalerite within and on top of the mound. When the hydrothennal system resumes high temperature upflow simUar process as ^ s e descnbed above in the initial high temperature phase take place, precipitating massive pyrite ± chalcopynte massive sulphide (S^^S = 6 - 8 %c) present at the top of the mound and the pyrite and quartz veins m the stockwoik zone and ^ydrite ± pyrite ± chalcopyrite veins within the mound (S^^S = 5 - 7 %c). Anhydrite in the veins has S ^ values of 21 %c, whichreflectsrapid heating of entrained seawater that IS mfiltrating the mound. Hydrothennal fluid leaks out from the veins though the poitxis and penneable mound and reacts with^viously fonned sulphides and sulphates causing the hydrothennal fluid to become sLghtly ennched in This fluid is respcmsible for precipitation of disseminated pyrite, the heaviest sulphur isotope values in the TAG system (S^^S = 8 - 10%c), associated with alteration in the stockwork zone and lower portions of the mound.

31

GOLD VEIN FORMATION DURING CORDILLERAN OROGENESIS: A CONSEQUENCE OF METAMORPfflC DEVOLATIUZATION, CALC-ALKALINE MAGMATISM, OR (AND) DEEP CIRCULATION OF METEORIC FLUIDS? Richard J GoMfarh' Rmcr R Nesbitt^, and Riincr J. Ncwbciry'

'U.S. Geological Survey. Box 25046. Denver FWend Center, Denver. CO 80225 USA ^ Dept. of Earth and Atmospheric Scienccs, U. of Alberta. Edmonton. AB T6G 2E3 Canada ' Dept. of Geology and Geophysics. U. of Alaska, Fairbanks, AK 99775 USA

Low-sulfide gold-bearing quartz vein formation was an inherent process in the development of the western margin of the Cordilleran orogen of western North America. Economically productive goldfields formed along an active continental margin of more than 4,000 km in length between 170 and 50 Ma. Stretching from the Sierra Nevada foothills of California to the Nome district in Alaska, the lodes and associated placers, mainly hosted within metamoiphosed accreted terranes, have yielded more than 180 million oz of gold. Most of the high grade lodes were developed in the late 1800's and first half of the 1900's, but in recent years regions such as the Fairbanks district and the Sierra foothills are being reworked for lower grade, high-tonnage ores.

The late Proterozoic through mid-Paleozoic passive margin of western North America was dominated by construction of a miogeoclinal shelf whose strata now mark the boundary with the craton. The Late Devonian and Early Carboniferous onset of continental margin tectonism is marked by the Ellesmerian orogeny to the north and the Antler orogeny to the south. Deformation was dominated by the easterly thrusting of shallow water basinal rocks over autochthonous shelf strata. Additional defomation along the margin continued as part of Permo-Triassic Sonoma orogeny that included thrusting of deeper marine sequences over marginal basins. These mid-Paleozoic through early Mesozoic orogenic events brought the North America plate into contact with offshore island arcs of the Pacific basin.

Notably, though, they are not associated with significant episodes of gold vein formation.

The Jurassic began a time of easteriy-directed subduction of oceanic crust beneath North America.

This initiated the accretion, obduction, and migration of island arc and oceanic terranes along the continental margin. The 130-100 Ma opening of the Canada basin led to development of a continental margin backstop for the growth of Alaska along the northwestern comer of western North America.

Terranes migrating along the northern Cordilleran margin were subsequently accreted along the northern rim of the Pacific basin. Interactions between the Pacific basin plates and the North America plate led to the voluminous magmatism which characterizes much of the Cordilleran orogen. Large regions of allochthonous blocks were heated to greenschist and amphibolite facies conditions and also experienced voluminous gold-depositing fluid flow.

The oldest gold lodes in the North American Cordillera formed in northernmost (Atlin) and central (Stewart Lake) British Columbia at about 170 Ma. Much more significant gold-veining occurred along the length of the entire Cordillera at about 140 Ma and includes the Klondike district of Yukon, the Cassiar and Cariboo districts of British Columbia, the Klamath district of northern California, and the Grass Valley district in the Sierra foothills. The Yukon and British Columbia ore systems are scattered along the Omenica Geanticline, an uplifted region between the accreted toranes and the Paleozoic North American craton. They are especially notable in that they are the only Cordilleran mesothermal vein systems that lack a spatial/temporal association with igneous rocks. Gold veins in other parts of the Sierra foothills, including along the Mother Lode belt, were emplaced between 125110 Ma. These veins are coeval in age with the onset of formation of the Sierra Nevada batholith, located a few tens of kilometers inboard of most of the central California gold districts.

Two regions of the northern Cordillera are characterized by mid-Cretaceous gold vein formation. A broad belt across interior Alaska, including the rich placer fields of Nome and Fairbanks, contains Alaska's oldest gold lodes. Unlike many of the other Cordilleran gold systems, the 110 Ma Nome deposits formed during trench rollback and large-scale extensional beating. Renewed subduction beneath the tectonically thinned crust of interior Alaska was likely associated with ore genesis at 90 Ma in the Fairbanks region. This hydrothermal activity extended eastward of the accreted margin into cratonic rocks of western Yukon. Along the continental margin to the south, simultaneous with this event in interior Alaska and Yukon, collisional orogenesis was associated with ore-forming processes

32

in the Bridge River (B.C.) and Ketchikan (Alaska) districts. These districts developed coevally on the landward and seaward margins of the evolving Coast Range Andean-type magmatic arc.

The youngest gold-bearing veins in metamcMphosed rocks of the western part of Ae orogen developed between 70-50 Ma on both sides and within the southern Alaskan magmatic arc. In die back-arc basin rocks of southwestern Alaska, 70-65 Ma gold-bearing veins indmately associated with felsic to intermediate intrusive bodies are being recognized as new bulk-tonnage precious metal targets. Archosted gold deposits of the same age occur 500 km to Ae cast intibeWillow Creek district within the Talkeetna Mountains batholith of south-central Alaska, In southeastern Alaska, the veins of the Juneau gold belt were emplaced at about 55 Ma along the seaward edge of the Coast batholith. Goldbearing veins throughout the entire Chugach accretionary prism of southern Alaska were generated during 60- to 50 Ma ridge subduction beneath the northern Gulf of Alaska continental margin.

The goldfields of the Cordilleran orogen have many features in conMnon that hint at a conmion oreforming process. The more productive lodes are spatially associated with deep crustal structures that, in some cases, are terrane boundaries. Adjacent to the first-order structures, relatively co!i^)etent igneous rocks and relatively carbonaceous sedimentary rocks typically provide second-order structural or chemical traps, respectively, for ore. Gold:silver ratios of most productive systems range fix)m 1:1 to 10:1. The deposits show great vertical continuity (1-2 km), and geochemical studies indicate that the ore fluids were consistently C02-rich and characterized by isotopically heavy oxygen compositions of 4 to 12 per mil. Low salinities are consistent with the generally limited (<2-5%) sulfide content of the ores. Except for a part of the Canadian Cordillera, plutonism is often coeval with, or only a few million years older than, gold vein emplacement.

Despite these many similarities, the source for the fluids that deposited the Cordilleran veins is controversial. Nesbitt (1991) argues that meteoric water is the predominant fluid type in the mesothermal vein deposits of the Canadian Cordillera Such deposits, inboard of the magmatic arc, possess distinctly low 6D values in inclusions fluids of -90 to -160 %o, which decrease from southwest to northeast. These are explained as indicating that the principal ore-forming fluid was a deeplyconvected, chemically-evolved meteoric water. Results of 6D analyses of vein muscovite from mesothermal veins of the Klondike district confirm the low 5D values of the inclusion fluids.

Extremely D-depleted values also characterize chlorites from gold-bearing veins of the Nome district.

In addition, Nesbitt and Muehlenbachs (1995) argue against the concern that widespread contamination of the primary 5D signal of inclusion fluids by unrelated secondary fluids is the cause of the light isotopic values. They point out that in the Canadian Cordillera, barren metamorphic quartz veins and hydrothermal dolomites both possess high 5Dfj. valuses, inspite of the fact that they have gone through major orogenic events where the 5D signal of the samples could have been contaminated by later fluids. Analyses of noble gas isotopes and halogens within ore stage fluid inclusions from the Alleghany district of the Sierra foothills has provided further support to the meteoric hypothesis (Bohlke et al., 1989). The lack of Early Cretaceous magmatic or metamorphic events in the Klondike district has also been used to support the meteoric hypothesis.

Goldfarb et al. (1993, 1996) have favored ore fluids produced during prograde greenschist-toamphibolite facies metamorphic reactions in the crust. The spatial association between gold and greenschist facies rocks, the volatile chemistry of the ore fluids, and fluid 5D and have been stressed as supporting the metamorphic model. This model assumes that the relatively heavy 5D measurements from the hydrothermal micas best represent the ore fluids. Light mica values from the Nome lodes and a few Canadian districts are argued to be the result of isotopic exchange between hydrothermal micas and meteoric waters subsequent to ore-formation. The extremely broad range in 5D for fluid inclusion waters, and the fact that waters from the least milky and deformed quartz are significantly heavier than meteoric waters, have been pointed out as evidence that much of the 5Df.i. data represent waters trapped in secondary fluid inclusion trails during uplift of the already formed ore systems. Furthermore, a consistently low water:rock regime is a suggested requkement for the narrow range in if fluids were of a meteoric origin. The enormous quartz volumes and association of ores with major structures is argued as inconsistent with such a suggestion.

Newberry et al. (1995) and McCoy et al. (1996) have recently postulated a magmatic fluid source for many of the gold lodes in interior and southwestern Alaska and central Yukon. Many significant gold 33

ores in these regions are essentially identical in age to host plutons and there seems to be a spatial ass^iauon between gold and those plutons with a low primary oxidation state. Lead, sulfijr, Jnd ^ b o n isotopes, with values significantiy different from other Cordilleran gold vein deposits, have been argued to reflect magmatic signatures. The enrichment of these ores in Bi. Te. and Sb; the common occurrence of potassic. albitic. sericitic. and propyUtic alteration assemblages; and the presence of contemporaneous gold-bearing skams and replacement bodies or distal base-metal rich veins in most of the districts have all been argued as being most consistent with magmatic devolatihzauon McCoy et al. indicate country rocks in interior Alaska were metamoiphosed to lowest amphibobte facies >20 m.y. before gold veining and thus were not a possible fluid source. In tac^ the belt of 90 Ma deposits continues eastward into unmetamorphosed rocks of the craton at Dublin Gulch. Geochemical dau from the interior Alaska deposits are intopreted as supporting a model in which C02-rich. gold-bearing fluids are exsolved from highly differentiated grardte magmas Early, pre-gold. high-salinity. magmatic(?) brines have been documented in a few of these systems.

Cert^nly meteoric, metamorphic. and magmatic fluid types were present during orogenesis, but whether specifically one or all three played a role in ore formation remains uncertain. If distinct chemical differences consistently exist between these fluid types, we have yet to clearly define and agree on such In part, this might be the result of the fact that a very similar fluid chemistry could evolve through a variety of processes. It is such a near-neutral pH. moderate temperature, low fluid'Iourcr relatively high HjS synorogenic fluid that is critical for the gold oreregardlessof REFERENCES Bohlke, J.K Kirschbaum, C.. & Irwin, J.. 1989. Simultaneous analyses of noble-gas isotopes and halogens in fluid inclusions in neutron-irradiated quartz veins by use of a laser-microprobe noble-cas mass spectrometer: U.S. Geol. Survey Bull. 1890. 61-88.

Goldf^b, R.J Miller. L.D.. Leach. D.L. & Snee. L.W., 1996. Gold deposits in metamorphic rocks: Econ. Geol. Mon. 9, in press.

Goldfarb, R.J Snee, L.W., & Pickthom. W.J.. 1993. Orogenesis, high-T thermal events, and gold vein tormation within metamorphic rocks of the Alaskan Cordillera: Mineral. Mag.. 57. 375-394. tioA^; R J-. Layer. P.. DiMarchi. JJ.. Bakke. A.. Masterman. J.S.. & Minehane. D.L..

1996. Plutonic related gold deposits of interior Alaska: Econ. Geol. Mon. 9. in press.

Nesbitt 1991. Phanerozoic gold deposits in tectonically active continental margins, in Foster K.F., ed.. Gold metallogeny and exploration: Blackie and Son Ltd, Glasgow. 104-132.

Nesbitt, B^E & Muelenbachs. K.. 1995. Geochemical studies of the origins and effects of synorogenic Vn7 im^ ™ Omineca Crystalline Belt of British Columbia, Canada: Geol. Soc. Amer. Bull., I y 1 Vy.31 UDU. Newberry, R.J., McCoy, D.T., & Brew, D.A., 1995. Plutonic-hosted gold ores in Alaska-Igneous vs metamorphic origins: Resource Geology Special Issue 18,^7-100.

34

RE>OS AND SM-ND EVIDENCE FOR ARCHAEAN LTTHOSPHERIC MANTLE MODIFICAllON BY ORCXa?<ESIS, NORSEMAN, WESTERN AUSHIAUA: IMPUCAITONS FOR DIAMOND EXPLORATION Snian Gfaham'. David D. Lamben'. Simon R. Shee^ and Roger Hamilton' 'VIEPS Department of Eanh Sciences, Monash Univereit> . Clayton. VIC 3168 Stockdaie Prospecting Ltd. P.O. Box 126, South Yarra. VIC 3141 V.M.C. Resources, 191 Great Eastern Highway, Belmont. WA 6430

In 1979, Stockdaie Prospecting Ltd, during routine kimberlite exploration in the Norseman region recovered p^roilmenites from loam samples (Robey ex al., 1989). W.M.C. Resources also identified a dyke of kimberlitic attinity withm the Lady Miller gold mine. Diamond drilling of two picroilmenite anomalies revealed the primaiy picroilmenite source to be olivme-phlogopite melnoite (aillikite ultramafic lamprophyre). The eastern Yilgam granite^-^frn^'^r^s^f^f (3100-2600 Ma) has been subject to rqjeated igneous activity including, calc-alkaline volcanism (2660 Ma), a^kalme syenite emplacement (2600-2300 Ma) and the emplacement of the widespread Widgiemooltha nontic dykes (24W Ma). A thermally reworked section of this Archaean crust abuts an area >200 km x 30 km of metamorphosed Proterozoic crust, of the Albany-Fraser Orogen (AFO). The AFO is characterised by high grade gneisses, granite intrusions and two Proterozoic tectonic events between 1900-1100 Ma. Previous Rb-Sr, Sm-Nd and i^Pb isotopic studies have shown that most activity occurred during the second tectonic event at ca. 1400-1100 Ma. TJus conunent-conunent collision resulted in granulite facies metamorphism of the deep crusu before the emplacement ot the mafic Eraser Complex at 1300 Ma and the post orogenic granites of die Nomalup Complex at 1100 Ma. We have obtained Re-Os md Sm-Nd isotopic and geochemical data from Nwseman melnoite dykes and picroilmenite xenocr>'sts to ^sess the tectonic evolution and diamond exploration potential of this section of Yilgam lithospheric mantle. The dykes have an emplacement age of 850 Ma (Rb-Sr isochron, Robey ei al., 1989; perovskite U-Pb, Heaman, unpubl. data, 1994). Sm-Nd isot^ic data have yielded radiogenic initial e^,. values (+4.4 to +5.4). Re-Os isotopic data also yield radiogenic iniUal '«t)s/'"Os ratios (0.2715 and 0.4377) aiKThigh initial YQ, values (percent deviation from chondntic mantle of the same age) of 120 and 260. High y^^ values may infer contamination with radi^ogeniccr^st. However, low SiO^ and high LILE, LREE and HFSE concentrations preclude this interpretation. The high HFSE abundances also imply that a typical subduction-related geochemical component is absent from the melnoite ^urce regio^ Sm-Nd depleted mantle model ages (TDM) of 1100 Ma and Re-Os asthenospheric mantle model ages Fma) of 1100 and 1600 Ma are within the known active periods of die AFO. Our trace element and Sm-Nd and Re-Os isotopic data are therefore consistent with a low degree partial melt of an enriched lithospheric mantle source.

Ilmenite xenocrysts from the Norseman melnoites have high MgO (7-13%) with low Cr,03 (<1%) compositions (Stockdaie Prospecting, unpul. data.; Robey et al., 1989). Re-Os isotopic data have yielded' a radiogenic ratio of 1.890 corresponding to a high y^, value of 1450 at melnoite emplacement. The isotopic data for the ilmenites yield a T^A mc^el age of 2600 Ma, consistent with lithospheric mantle modification during emplacement of the alkaline syenites or Widgiemooltha dykes.

Isotopic data suppon a regional evolutionary model encompassing late Archaean continental lithospheric mantle metasomatism, which includes ilmenite growth. Either Proterozoic oceanic lithosphere underpladng during continentcontinent collision or conunental lithospheric mantle trapping of Proterozoic asthenospheric melts arc associated with the melnoite source. Isotopic data from igneous rocks within the AFO have been interpreted as reflecting a subduction relied genesis (Nelson et al., 1995). However, the Sm-Nd and Re-Os isotope systematics and geochemistry of the melnoite source may reflect either the melting of dehydrated carbonated lithospheric (eclogitic) mantle or the re-melting xf e x p o n e n t of the lithospheric (also eclogitic) mantle. The ilmenite and melnoite Re-Os model ages (ibUO Ma and 1100 Ma respecuvely) arc significant because they support cratonic lithosphere models permitting the growth of continental lithosphenc mantle through the accretion of younger fertile lithospheric mantle beneath older conunental hthosphere.

Melnoite isotopic and geochemical data are consistent with the storage of a Proterozoic carbonated enriched litiiospheric mantle below metasomatised Archaean continental lithospheric mantle. The low Cr,0., of the picroilmenites is, however, not wnsidered to reflect a typical source associated with diamondiferous areas. Also, high YQS (>50), arc unknown for ^amond transporting rocks (e.g. Pearson, et al.. 1996). We propose that the lithospheric mantle arcfetecture below the Norseman region is such that asthenospheric melts find it difficult to penetrate the undoplated melnoite source mantle. The thermal trigger for emplacement of the Norseman melnoites resulted in continental lithosphenc mantle melting. If there were any Archaean peridotitic or Proterozoic eclogitic diamonds within the Norseman lithosphenc mantle the resulting magma from continental lithospheric mantle melting would be too hot to transport diamon^ to the surface. Answers to the way in which mantle plumes interact with the continental mhosphenc mantle in an area such as Norseman may be found by detailed geophysical and geochemical compilations.

Uur study indicates that regional geologic features such as crustal pre-history and lithospheric fracture zones may affect' the ability of the conunental lithospheric mantle to resist melting. Where these factors allow alkaline ultramafic melts to pass from the asthenosphere through a cool deep lithosphere without lithospheric melting occurring tiwre is a far greater chance of transpOTUng diamond.

REFERENCES NELSON, D R., MYERS, J.S. & NUTMAN, A.P. 1995. Chronology and evolution of the Middle Proterozoic AlbanyFraser Orogen, Western Australia. Australian Journal of Earth Sciences 42: 481 -495.

PEARSON, D . G . , ROGERS, N . W . , IRVING, A J . , SMITH, C . B . & HAWKESWORTH, C J . 1 9 9 6 . S o u r c e regions of

kimberiites and lamproites: Constraints from Re-Os isotopes. Sixth International Kimberlite Conference Extended Abstracts 430-432.

ROBEY. J . V . A . , BRISTOW. J . W . , MARX, M . R . , JOYCE, J., DANCHIN, R . V . AND ARNOT, F .

ultrabasic dikes 382-391.

1989.

Alkaline

near Norseman, Western Australia. Geological Society of Australia, Special Publication 14 •

35

METEORITE IMPACT CRATERING AS AN AGENT IN ORE DEPOSIT FORMATION Lloyd H. Hamilton

School of Natural Resource Scicnccs, QUT, GPO Box 2434 Brisbane, Queensland 4001

Ore deposits are found in relative abundance within large impact craters (Grieve & Masaitis 1994), but effects of impacts far beyond the meteorite craters also act as ore depositing agents producing a wide range of deposits. These effects include magma generation, stimulation of geological plumbing systems (Hamilton 1987), interaction of marine and continental waters, and redox changes.

Although large deposits of iron and nickel are unlikely to be found in large impact structures, the nickel, platinum and chromium at Sudbury and in the Bushyeld Complex formed from magmas related to impact events. The Veredefort Dome in the South African goldfields is responsible for raising a significant portion of the gold-bearing Witwatersrand Group to mineable levels.

Impacts can destroy deposits but more commonly they can generate, or regenerate, deposits after brecciation enhances leaching of metals and provides space for solution movement and deposition of metals. This appears to account for silver-lead-zinc veins in the Siljan Astrobleme (Masaytis 1989).

Apart from sea level changes as agents in ore deposit formation (Hamilton 1987), other widespread changes related to large impacts involve oxidation reactions and acid rain, especially with impacts into ev^orites or limestone. Extensive leaching of rock may have been responsible for bauxite and kaolinite formation at Weipa about the time of the KT boundary. The generation of world-wide bauxite deposits roughly correlates with impact of large meteorite impacts. For sedimentary and diagenetic ore deposits the effects of meteorite impact cratering are much more significant than is generdly realised.

Grieve, R.A., & Masaitis, V.L., 1994. The economic potential of Terrestrial impact craters: International Geology Review 36, 105-151 Hamilton, L.H., 1987. Mineralization related to sea level changes : Pacific Rim Congress, Australasian Institute of Mining and Metallurgy, Proceedings,!, 805-807.

Masaytis,V.L., 1989. The econoniic geology of impact craters:/n/erriarz^^na/Gcoto^/?evievv 57,922-933

36

NEW MINERALISATION AT MT MORGAN, QUEENSLAND: STRATABOUND VHMS DEPOSITS John Hartley' and Alex Taube^

J.S. Hifticy iDd Asiocialcs, PO Box 153, Mission Beach Q, 4854 Mount Morgan Joint Venture, PO Box 72, Mt Morgan, Q, 4714

The Mount Morgan Mine, 36 km southwest of Roddianqrton, Queensland, produced over ei^t millkm cmnces of of gold during its 109-year life span.

Recent exploration drilling by the Mount Morgan Joint Venture (Perilya Mines N.L. and Aumin Techndogy and De\eIopment N.L.) has revealed the presence of stratabound massive sulphide mineralisation in an area centred about 600 m southeast of the old open pit.

Two main zones of mineralisation have been defined..

The "Slag Heap" mineralisation occurs at 100-150 m stratigraphically below the level of the "Bimded Mine Sequence", a complex thinly bedded sequence of jasper, quartz-feldspar porph}T>', volcaniclastic sediments, and Vperites. The mineralisation occurs in two zones separated by 20 m or so of barren rock. The v^pper zone is mainly disseminated p>Tite, and the lower zone is discrete massive to semi-massive pyrite, sometimes with lenses of high-grade zinc mineraUsation. The lower zone grades into a weU-bedded magnetite banded iron formation.

Minor stnnger mineralisation occurs in a siltstone unit beneath the bedded mineralisation. Best values intersected to date include 11 m grading 1.8 g/t Au, 0.1% Cu, 8.3% Zn and 44 gA Ag (PD33). The top of the zone is about 200 m below the surface.

The "Car Park" mineralisation occurs about 90 m below the '"Slag Heap" mineralisation. This body consists mainly of ultra-massi\'e sulphides (>80% p>Tite) with local zones of chalcopyrite and sphalerite. Well-bedded magnetite banded iron formation caps and grades into the massive sulphide. Relatively minor stringer mineralisation occurs beneath the massive sulphide. The best zinc and copper grades occur at the top of the massive sulphide, with zinc abo\'e the copper, and lo^\-grade gold values throughout the body. The best intersections of mineralisation to date include 30.1 m at 2.76% Cu, 0.73% Zn and 0.82 g/t Au (PD31), and 115 m at 0.33 g/t Au (PD30). The sulphide s> stem attains mdths of over 100 m downhole and extends over a strike length of 600 m. The top (rf the zone is about 300 m below the surface.

The mineralisation may represent a faulted-oflF portion of the original Mount Morgan ordbody, but its configuration closer to the mine is notfiillyunderstood.

The discovery substantially supports the thesis that the Mount Morgan ord)ody is a volcanic-hosted massive sulphide deposit. Drilling is continuing.

Acknowledgements: Perilya Mines N.L. for permission to piAlish.

37

HYDROTHERMAL PROCESSES AND METALLOGENESIS: PRESENT UNDERSTANDING AND FUTURE RESEARCH DIRECTIONS Jeffrey W. Hedenquist Mineral Resources Department, Geological Survey of Japan. J-1-3 Higashi. Tsukuba 305, Japan (presently at Institute of Geological and Nuclear Sdeoces. NZ)

Mineral exploration relies principally on observation and interpretation of geological features, although exploration can be more effectively guided by genetic models that are built from field and laboratory data.

Understanding the processes related to the acquisition, transportation and precipitation of metals in hydrothermal systems (e.g.. porphyry and epithennal base- and precious-metal deposits) will improve genetic models, and will identify the "fingerprints" that these processes leave on the rocks. Analytical advances over four decades in mineralogical, geochemical. isotopic and fluid inclusion techniques, coupled with studies of active hydrothwmal systems (geothermal and volcanic), have helped determine the general characteristics and sources of hydrothermal ore fluids (magmatic- to meteoric-water transition in porphyry to epithermal systems) and factors influencing metal precipitation (such as boiling in epithermal systems).

Building on past studies of porphyry systems, and incorporating results from recent work on plutonic and volcanic systems, it is now possible to more clearly identify fundamental processes in the spatial and temporal evolution from the porphyry to high-sulfidation epithermal environment. Based on results fi-om the Far Southeast-Lepanto porphyry-epithermal Cu-Au deposit, PhiUppines. it is clear that potassic and advanced argillic alteration were coupled in their formation, being associated with hypersaline liquid (550 T , 55 wt% NaCl eq.) and low-salinity vapor phases, respectively; separation occurred across the ductile-britUe transition. This coupling is a conclusion that can be applied to most porphyry systems. In the case of the FSE deposit, magma evolution (6 km depth) led to a lower heat flux in the late stage of the system, followed by cooling in the vicinity of the porphyry dikes (2 km depth); this allowed brittle fracturing to occur under hydrostatic conditions.

Late illite (sericitic) alteration was still dominated by magmatic water, although of low salinity (5 wt% NaCl eq.), and this boiling fluid (350 'C) was responsible for quartz-Cu sulfide mineralization in the porphyry environment, followed by enargite-Au mineralization in the supradjacent high-sulfidation epithermal environment, the latter hosted by advanced argillic alteration of the "lithoci^". Thus, mineralization in these porphyry and high-sulfidation epithermal ore bodies was also coupled, and critically linked to the late magmatic fluid input. Meteoric-water involvement was marginal to the porphyry system, and groundwater was entrained progressively within the epithermal system with increasing distance from the magmatic source, during lateral outflow along the Lepanto fault.

Many questions remain, for porphyry and epithermal deposits alike. Some, such as the importance of magmatic components in low-sulfidation epithermal systems, are being addressed at present Increasingly, detailed isotopic study on gangue intimately related to high-grade ore indicates evidence for a large magmatic water component during mineralization. Other questions, including the possibility that the bulk of epithermal ore deposition is episodic and of short duration relative to the life of most hydrothermal systems, will be ellucidated by the use of the new generations of analytical techniques that are constantly appearing. These techniques feature very small sample size, low detection limits and high precision (e.g.. ion and X-ray probes; laser ablation coupled with ICP-MS); they will eventually provide the chemical (plus metal) and isotopic composition of individual fluid inclusions. For example, recent measurements indicate high Cu concentrations in low-salinity, vapor-rich porphyry fluid inclusions, with implications for metal transport into the linked epithermal environment; these results are consistent with evidence from erupting volcanoes of high metal contents in high-pressure vapor. To be relevant, studies employing these techniques must be problem- rather than tool-driven, and thus there must be su-ong cooperation between research groups and the exploration industry to identify problems of merit.

Although such techniques may not always have direct application to exploration, the information gained will continually improve genetic models which will in turn assist exploration efforts.

ApKjipwj^gpmgnt^; My studies of active volcanic systems and hydrothermal ore deposits have been in collaboration with M. Aoki, A. Arribas, Jr., W.F. Giggenbach. E. Izawa, Y. Matsuhisa. H. Shinohara. and staff of Lepanto Consolidated Mining Co.

38

~JS IMPORTANT FOR INDUSTRY! R.W. l^ley Etherid(c Henley WiUiaiBt MGcasCoart ]>e»ldn ACT2M0 Arguably the most conmion phrase in research gram appUcations, what does this throw-«way Uoe actually mean In . to the process of exploration? How will the leseanji actually be applied? What else will it enable the industiy to achieve? Which directions are likely to be most fiuitM and how docs mineral deposit reaearch rdate to more reqtectable geological research?

Inreviewii^these qi^ons, we need to recall how thefieldofappUed geosdeoce has changed over the last thirty yean as It has moved firm an obseivational science based on scarce dau but careful thought, to one of data affluent with an evej- mcreasing dependency on "models". Exactly what is a modd anyway?

How has this change influenced theresearchmethods which we use? What kinds of people are attncted to modem day economic geology research? If economic geology research is to be important for industry, what kinds of questions can we answer now, which we were unable to answer thai? implied research needs to be linked to outcome driven ejqjloration j^ect management. It needs to be tied to that elusive queston of maximising the probability of discoveiy. Shareholders necessarily are seeking more definite returns than the so-called empirical ejq)lonttion methods currently yield.

But on top of all ofthis I believe that we are, as an industiy and as a nation, undermining ourfimireby devaluing alf rfur curiosity in our research funding I think the nurturing of that is the most important task fc

39

THE LAKE SELINA PROSPECT: GRANITE RELATED MINERALISATION WITHIN THE MT. READ VOLCANICS, WESTERN TASMANIA Steven.OPO R. Himtis Geolog)' Dq)ailnient, Univ-cnity of TainiMiii, Box 252C, Hobirt, Tasiunu 7001, Australia The Lake Selina Prospect is a large •'barren'* mineralised hydrothermal system, that contains semi-massive to disseminated magnetite and pyrite and minor basemetal mineralisation. The mineralisation is concentrated into two linear zones, the Eastern Pyrite Zone and the Western Pyrite Zone, and is hosted within volcaniclastics of the Eastern quartz-phyric sequence (EQPS) of the Mt Read Volcanics. At the Lake Selina Prospect the EQPS is a broadly west facing sequence of lithic dominated and crystal-rich volcaniclastics, lavas and intrusive rhyodadtic porphyries The volcaniclastics have been intruded by the Late Cambrian Murchison Granite. The EQPS has been sUongly deformed and folded into a north-south trending f o r m a l structure. The mineralisation and alteration are interpreted to be related to the intrusion of the Murchison Granite, and is centred along the contact between a massive central rhyo-dacitic porphyiy and the enveloping EQPS. The major alteration phases present are potassium feldspar, chlorite, muscovite, silica and carbonate. These alteration phases form three broad zones, based on the presence of the dominant mineral phase present: 1) Kfeldspar zone, 2) Chlorite zone, and 3) Muscovite/quartz zone. The mineralisation is con^)osed predominantly of disseminated and crosscutting veins of magnetite and pyrite and minor chalcopyrite. The magnetite and pyrite mineralisation is associated with the chlorite alteration, although crosscutting pyrite veins can occur with silica alteration.

Sulphur isotope values suggest that the fluids were derived from the Murchison Granite. Lead isotopes have a comparable signature to the major Tasmanian volcanic-hosted massive sulphide deposits of Rosd)eiy, Hellyer and Que River.

The absence of significant base and precious metals ma>te attributable one of four possible mechanisms or a combination of these: • the hydroUiermal fluids had a magmatic origin and were undersaturated with respect to Cu, Pb, Zn, Ag and Au. • the h} drothermal s}'stem was only short lived, • the Murchison Granite was essentially a granite, • the volcaniclastic pile tliat the Murchison Granite intruded into was essentially fluid poor and an efficient convecting hydrothermal s>stem could not have been formed.

During the De^ onian Tabberabberan Orogeny, the deformation resulted in the development of mylonites and shear zones. During this phase of defomiaUon basemetal sulpliides were mobilised and precipitated into pressure fringes growing off pyrite grains.

The mineralisation and alteration at the Lake Selina Prospect is considered to be genetically related to the intrusion of the Murchison Granite, and has closer genetic links to porphyry-style mineralisation than to volcanic-hosted massive sulphide deposits of the Mt. Read Volcanics

40

TRACE ELEMENT AND ISOTOPE GEOCHEMISTRY OF KOMATIITES AS A GUIDE TO NICKEL SULPHIDE MINERALISATION: GROUND MELTING OR HYDROTHERMAL ALTERATION? Mary Jane Ian Caitwright and David Lambot Victorian Institute of Earth and Planetaiy Science, Department of Earth Scienccs, Monuh University. Oayton, VIC 3168

p e Norseman-Wiluna greenstone belt of the Archaean Yilgam Craton contains the largest magmatic nickel sulphide de^sits in Australia. At Kambalda, nickel sulphide mineralisation occurs within trough-like embayments at die interface between the host komatiites of the 2.7 Ga Kambalda Komatiite Formation (KKF) and the underlying Lunnon Basdt. The troughs have been interpreted as primary dejaessions modified by thermal erosion (see summary in Lesher, 1989). The formation of this style of nickel sulphide deposit is dqiendoit on the host lava attaininc su p^de saturation. The addition of crustal sulphur to sulphide-undersaturated komatiites via "ground melting" of sulphidic sediments has been proposed as a mechanism by which sulphide saturation was achieved in these lavas ^sher, 1989). However, to accurately assess the ground melting model of ore formation and to use geochemical data as vectors to ore, these data must be able to see through a number of post-magmatic processes that have affected the KKF, including: (1) seafloor hydrothermal alteration; (2) deformation; (3) greenschist-amphibolite facies metamoiphism; and (4) a ca. 2.6 Ga mesothermal gold event. This is crucial to the interpretation of trace clement and isotopic data as metamorphic/hydrothermal modification of rock geochemistiy can mimic "ground melting".

In order to assess further the ground melting model of ore genesis, we have obtained major element, trace element nd stable isotopic data for a complete range of KKF samples from a single deep drill hole above the Hunt ore shoot focussing on the metamorphism and geochemical alteration of the host komatiites. In particular, the careful' examination of a single komatiite flow approximately 2 metres thick can lead to better understanding of chemical mobility of key trace elements that have been used in the assessment of ground melting, in particular the REE This single flow exhibits well preserved and characteristic komatiite textural relationships, including flow top l»eccia, spinifex textured A-zone and cumulate B-zone. Hie original mineralogy consisted of olivine, chromite and a small amount of interstitial pyroxene and glass, all of which have been replaced by seipentine minerals during seafloor alteration ^d upper greenschist facies metamorphism. Large variations in elements that are incompatible with this original olivine-dominated mineralogy can likely be assumed to be the result of magmatic crustal contamination or post-magmatic element mobility. Our data show that compatible major and minor elements such as Mg and Ni do follow olivine control lines, suggesting limited mobility of these elements, as noted by Lesher (1989) However the REE data reveal large variations in the light REE and Eu. (La/Sm)„ ratios across the single flow from Hunt range from 0.5 to 1.0, greater than the value of 0.2 to 0.3 expected in a high-degree depleted-manUe melt, and Eu exhibits both posiuve and negative anomalies. These enriched LREE data have been inteipreted to reflect ground melting aid mco^oration of crustal materials with (La/Sm).of 3.0-4.0 into the komatiite lava. However, oxygen isotopic data for silicates and carbonates within the same flow have been shifted from primary magmatic 8"0(SM0W) values of 5.0-6.0 to an ^erap value of 8.1 for silicate whole rock, a 6"0(SM0W) of 11.1 for carbonate whole rock, and an average 6 C(PDB) of-7.5. The carbon isotoperesultsindicate fluid from a deep crustal source, consistent with Pb and Os isotopic data from ores and sediments in the pile (Foster et al., 1996; Lambert et al., 1997), while the oxygen values illustrate that the carbonate oxygen is in equilibrium with the shifted silicate oxygen values. In oider to c ^ a shift of this magnitude via ground melting, the addition of a veiy large percentage of crustal material is required, which IS not supported by the REE and radiogenic isotope data. Moreover, these altered values are found in other volcanic units, including the Paringa Basalt, Devon Consols Basalt, and the Tripod Hill Formation, suggesting tiiat large scale COj-bearing fluid flow has occurred throughout the volcanic pile.

This study demonstrates that alteration, in particular by large-scale deep crustal fluids, can be responsible for the geochemical variation seen m the komatiite host rocks. A comparable study by Lahaye et al (1995) also demonstr^s geochemical and isotopic mobility in mineralised komatiite lavas of the Abitibi greenstone belt of Canada. Thus, it may not be necessary to invoke assimilation of sulphidic sediments to explain the nickel sulphide mineralisauon at Kambalda.

REFERENCES Foster, J O., Lambert, D.D., Frick, L.R., & Maas, R. 1996. Re-Os isotopic evidence for genesis of Archaean nickel oresfromuncontaminated komatiites. Nature, 382, 703-706.

Lambert, D.D Foster, J G., Frick, L.R., Hoatson, D.M., & Purvis, A.C., 1997. Application of the Re-Os isotopic system to the study of Precambnan magmatic sulfide deposits of Wcstan Australia: Australian Journal of Earth Sciences, in press.

Lesher, C.M. 1989. Komatiite-associated nickel sulphide deposits, in Whitney, J.A. and Naldrett, A J eds OK Deposition Associated with Magmas. Reviews in Economic Geology, v. 4, Society of Economic Geologist.

Lahaye. Y Amdt, N-. Byerly, G., Chauvel, C., Fourcade, S. & Gruau, G., 1995. The influence of alteration on the trace-element and Nd isotopic compositions of komatiites. Chemical Geology, 126.43-64.

41

ALTERATION HALOS ASSOCIATED WITH STRATIFORM SEDIMENT HOSTED rSEDEX^ PbZn Ag DEPOSITS; DEVELOPMENT OF AN ALTERNATIVE EXPLORATION APPROACH Ross Large and Peter McGoldrick CODES Special Research Centre, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, 7001

The chcmical and inincralogical characterisation of alteration halos in volcanic successions has played an iniponant role in tlie exploration for VHMS and epithermal deposits over the past thirty years. However, little comparable research has been carried out on the nature of alteration surrounding sediment hosted base metal deposits. Our recent studies on deposits in the Proterozoic zinc belt of northern Australia indicates the potential for the development of an alteration vector approach to exploration for stratiform Pb-Zn-Ag deposits in sedimentar) basins.

Sampling and analysis of the host sediments to the Lady Loretta deposit, Lawn Hill Platform, indicates that the stratiform Pb-Zn ore lens is surrounded by an inner sideritic halo, intermediate ankerite halo and outer dolomite halo The sideritic halo exhibits enrichment in Zn. Pb, Ba, Fe, Mn, Sr and Tl and depletion in Ca. Mg and Na The ankeritic halo shows enrichment of Mn and Tl only. Based on whole rock and microprobe anal> ses. a marked increase in MnO content of dolomite and ankerite is defined in the footwall sediments approaclung the ore zone. The study has enabled the development of a series of geochemical criteria which allo\v discrimination of potential host horizons to stratiform Pb-Zn deposits and which can be used as vectors to target deposit locations. Based on the Lady Loretta study, the two key vectors are: Alteration index, defined by AI = 100 [(lOMnO + FeO)/(MgO + NasO + lOMnO + FeO)J and MnO content of dolomite, defined by MuGd = (MnO\\R X 30.41)/CaOv^R. AI increases from 30 to 95 approaclung ore, both along strike and across strike, and MnOo increases from 0.2% to over 1% approaching ore from the footwall side. Zn and Tl values of the sediments are generally more erratic and should be interpreted in conjunction with the AI and MnOo vectors Comparison of the Lady Loretta data with previous geochemical studies at HYC (Corbett et al,, 1975) indicate that the HYC deposit has no associated siderite development but is surrounded by an extensive ankerite halo.

The Mn-carbonate halo is e\en more extensive at HYC, extending up to 23 km.along strike jfirom the deposit.

The geochemical vectors developed at Lady Loretta are found to be directly applicable to HYC and the McArthur Basin sediments. Further sampling of selected drill holes in the McArthur Basin has revealed a relationship between sedimentary facies and alteration geochemistry, which enables the formulation of a genetic model relating sedimentation and exhalative mineralisation.

The results from this research indicate the potential to apply litliogeochemistiy in the search for giant stratiform Pb-Zn-Ag deposits in an analogous way to tlie use of straUgraphic drilling and source rock studies in oil exploration The new approach suggested imolves a two-stage drilling program: (1) Lithogeochemical stratigraphic drilling (LSD) of discrete sub-basins to determine the presence or absence of potential horizons for giant Pb-Zn-Ag deposits. One drill-hole through the cartwnate-shale facies stratigraphy may be sufficient to test a sub-basin of diameter up to 10-20 km. If no horizons witli anomalous alteration index (AI), MuOd or thallium are identified, then the sub-basin may be considered barren of giant Pb-Zn-Ag deposits (2) Lithogeochemical Vector Drilling (LVD). If an horizon or horizons with AI. and/or MnOo are identified in the first LSD drill-hole, follow-up drilling at 2 km spacings may be used to determine the vector toward ore.

Increasing MnOo and AI values, coupled with their relationship to sedimentary facies, will provide information on the placement of step-out and in-fill drilling. This exploration approach "takes off the blinkers" in that emphasis is placed on lithogeochemistr}' and alteration, rather than the assumed "preferred stratigraphy".

REFERENCES Corbett. J.A., Lambert. I B. and Scott, K.M., 1975. Results of analyses of rocks from the McArthur Area.

Northern Territor}: CSIRO Technical Communication No 57. Mineral Research Laboratories, August 1975.

Ackno\\ledgments: Sponsor companies of AMIRA/ARC Collaborative Project P384 (1992-1995) are gratefully ackno\\ ledged for their support of this research.

42

biFFERENTIATION OF TRANSPORTED AND IN-SITU KAOLINTTE XRD AND SPECTRAL REFLECTANCE METHODS David C. Lawic* 'Dqwitmcnt of Geology and Gcophysics, Umvcisity of New England, NSW, 2351

Kaolinite varies in its crystallinity and the variation is related to regolith type and genesis. Kaolinite developed and remaining in situ is highly crystalline, whereas kaolinite that been through a cycle of erosion, transport and deposition, is poorly crystalline. If the crystallinity of kaolinite can be determined, important information is available eg, for determining the location of boundaries between in situ weathered basement and an overlying sequence of transported rock, often an important, but by no means easy geological boundary to I: determine.

J

One method to determine and quantify crystallinity is with the Hinckley Index (HI) (Figure 1). Figure 1 shows a part of an XRD spectrum for two samples. The Figure 1 Hinckley index calculation (HI =(A + C)/(A + B)) for highly crystalline highly crystalline sample comprises quartz, kaolinite (left), HI = 1.2, and poorly crystalline kaolinite (right), HI = 0.15. kaolinite and muscovitic illite (jfrom weathered, in-situ, Proterozoic pelite). The pooriy crystalline, transported rock, comprises fine to medium grained quartz, muscovite and kaolinite. Texturally, it is immature, but mineralogically it is mature. Inspection of the XRD data and calculation of the HI shows there is a difference in crystallinity and the difference is quantifiable.

However, while the calculation of the HI is simple, it is still time consuming, expensive and cannot be carried out on-site. \

2171

Information about kaolinite and its crystallinity, is also present in the SWIR region. All samples run on XRD have also been presented to the PIMA instrument and their JIM \ reflectance spectra in the 1300 - 2500 nm range recorded.

SMI Figure 2 shows part of wavelength range measured by the PIMA for the same samples shown in Figure 1. A general Figure 2 Variation in PIMA spectra for pooriy feature of highly crystalline kaoHnite in the SWIR is the crystaUine kaolinite (left) and highly ciystalline presence of extra absorption features wliich are also more kaolinite (right). Ratio of 2171/2160 is a f(crystallinity). sharply defined. Qystallinity information in the SWIR may also be quantified by, for example, taking the ratio of the reflectance values at different wavelengths. For example, if the reflectance at 2171 nm is ratioed to the reflectance at 2160 nm, the result will be higher for the more crystalline kaolinite (Figure 2).

1" !

I,.

/

i

r

i

74

m

DO

1 / • —

•

I n Cit..

^

4 04

To test the suitability of the PIMA for determining kaolinite crystallinity, a variety of 'parameters' were calculated from the PIMA spectra and plotted against their corresponding HI (Figure 3). If the calculated PIMA parameter is a good measure of crystallinity, it should have a linear relationship to the HI. T^e parameter that most closely follows the HI is the 2171/2160 ratio (as shown in Figure 2). While the relationship is ikX porl^t, it can b ^ seen an increase in the HI is associated with a commensurate increase in the derived PIMA parameter, and that both are able to discriminate between transp(»1ed and in situ kaolinite.

Out

OJ

tJQ

IJ

Figure 3 Plot of Hinckley index against PIMA panmaeter.

Increasing values of both indicate an increase in kaolinite crystallinity

The crystallinity of kaolinite can therefore be quickly, cheaply and rapidly determined with tfie PIMA. Furthermore, Ae crystallinity says much about the kaolinites genesis, and as such, becomes a practical exploration tool.

43

GENESIS OF BASE METAL MINERALISATION IN THE OLARY BLOCK, SOUTH AUSTRALUCONSTRAINTS FROM ISOTOPIC DATA FROM SULPHIDES AND GOSSANS David C. Uwic^ Paul M. Ashlcy\ Rank P Bicricin' and Adam J. R. Kent' Department of Geology and Geophysics. University of New En^and, NSW. 2351 ^ l o g y Department, University of Ballarat. PO Box 663. VIC. 3353 ^vision of Geological and Planetary Scienccs, Caltech. Mail Stop 170-25. Pasadena, CA. USA, 91125

The Palaeoproterozoic Willyama Supergroup of the Olary Block (OB) in South Australia is host to a variety of syn-depositional, structurally controlled and replacement sulphide deposits. Of these, two main styles of mineralisation are recognised: stratiform and stratabound, disseminated to massive Fe. Cu, Zn. As (Co. Mn, Pb, W, U). and epigenetic vein/stockwork Fe, Cu, Au, Co, Ni and U. Syndepositional mineralisation mainly occurs in the Bimba Suite, with disseminated Fe, Zn (Cu, Pb) sulphides associated with marble and calcsilicate rocks.

Much epigenetic mineralisation, with a strong Cu, Fe sulphide (+/- Au) signature, appears to be related to postpeak metamorphic, high temperature, hydrothermal fluid flux. Extensive replacement of the Bimba Suite has occurred, with development of retrograde calcsilicates ('stratabound skam'), as well as overprints on iron formations and albitites. The latter rocks host several zones of low grade vein/stockwork mineralisation.

Preliminary Sm-Nd isotopic data inqjly that calcsilicate replacement zones occurred at - 1580 Ma, immediately following high grade metamorphic events and emplacement of S-type granites.

Bierlein et al (1996) determined the Pb isotopic ratios of galena separates and low-lead sulphidesfroma variety of sulphide deposits from throughout the OB. They concluded the calculated model age of the least radiogenic Pb is in good agreement with the depositional age of the Willyama Supergroup and the Broken Hill Pb-Zn-Ag deposit (- 1.7Ga). In contrast to the extremely homogenous signature of the Broken Hill ore body, the sulphides from the OB have a heterogeneous Pb isotopic signature. Bierlein et aL (1996) concluded this was caused by either the sulphides being the product of a singlefluidpass or they were formed from convective hydrothermal ceils of limited dimensions. Epigenetic mineralisation ranges in age from post peak metamorphic (-1.5Ga) to Delamerian (-0.5Ga), with evidence of a significant event at - 1.1 Ga (Grenvillean Orogeny). Isotopic data, together with integrated petrographic interpretation, indicate the sulphides are not derived from pre-existing mineralisation, but rather were sourcedfroma relatively homogenous crustal reservoir at various times since the mid-Proterozoic (Bierlein etal. 1996).

Surface expressions of sulphide mineralisation in the OB have three major geochemical signatures in gossanous ironstones. These types are: (1) Mn-Zn-Pb-Ba (Bimba syndepositional mineralisation), (2) Cu-Fe-As-Co-MoNi-Au (epigenetic mineralisation) and (3) low base metal ironstones (derived from the weathering of iron sulphide- and calcsilicate-rich rocks). Many ironstones contain elevated U values. Gossanous ironstones derived from the weathering of stratiform/stratabound and epigenetic sulphides have also been analysed for Pb isotope ratios using low cost techniques ($50/saniple. ICP-MS) to determine if they retain their primaiy sulphide isotopic ratios. New data have been acquired from three ironstone prospects. Brooks Dam (type 1), Blue Dam (shows characteristics of types 1 and 2) and Peryhumuck (type 2). When the data werefirstplotted, all points fell on the right hand side of the growdi curve, suggestingMcess radiogenic Pb may have been added. Further investigation revealed a linear relationship between U and As the sulphide data indicate the least radiogenic Pb is of Broken Hill age, it is possible using the assumed age, and U and Pb concentrations, to calculate the amount of radiogenic Pb added. When the excess ^^b/^Pb is removed, all the gossan data are moved closer to the growth curve indicating the correction is valid. Brooks Dam spears to be of Broken Hill-type age confirming its type 1 signature, Blue Dam is similar, but may have been overprinted by epigenetic mineralisation causing the points to plot along the growth curve. Peryhumuck remains a problem. It may have had Pb addedfroma higher \i source, or may have suffered laboratory contamination. The corrected data show the gossans retain their primary isotopic fingerprint and the U in the gossans is a feature of the initial mineralisation. Additionally, low cost, lower precision analyses provide adequate information forfingerprintingweathmd materials REFERENCES Bierlein F.P., Haack U., Forster B. & Plimer I.R. 1996. Lead isotope study on hydrothermal sulphide mineralisation in the Willyama Supergroup, Olary Block, South Australia. Australia Journal of Earth Sciences 43, 177-187.

44

HOST ROCK CONTROL ON LOCALISATION OF THE GIDGINBUNG EPITHERMAL HIGH SULPHIDATION Au DEPOSIT K.C. Li\iTic\ T.P. Mernmgh* & P. EUis^ 'Australian Geological Survey OrganisaUon, GPO Box 378, Canberra, ACT 2601 ^Gold Mines of AurtraUa, PO Box 38, Tcmora, NJ5.W. 2666 The Gidginbung Au mine in central New South Wales is a high sulphidation deposit fliat has produced approximately 10 m.t ore at 2.1 g/t Au. The orebody is hosted in Ordovician volcamclastic Uthologics of the Lachlan Fold Belt, and has been described as a deep level syn-dcformational shear zone-hosted deposit (AUiboae et al 1995). Mming of the deposit beneath the base of oxidation has recently feciHtated a study of host rocks and their relation to structure and alteration, and has led to a revised model for the formation of flie deposit.

Primary lithologies are mappable in the open pit even in s t a s that have mdergone pavisive ovcarprinting alteration. A relatively gently-dipping sequence of interbedded breccias and mudstones is revealed Breccias show marked variations between beds in both the proportion and size of clasts (i^ to 40 metre blocks). Some units display rapid thickness changes, and internal fades can be m^ped by observing dia^es in dast composition; all are polylithic, multi-phase breccias, with clasts dominantly comprismg epiclastic Myologies; no juvenile material was observed. A steeper internal zoning in clast compositions is observed within itkc breccia that is host to the main ore zone. All syn-sedimentary breccias fomied by episodic mass flow in a srt)a^ieous environment, probably in an episodically subsiding basin. Regional geological considerations suggest mi environment that is most likely proximal to a volcanic centre (to Ae north at Rain Hill ?), There has been siibstantial post-depositional injection of the fine ^ i n e d mudstone lithologies into overiying breccia hthologies. This is interpreted as a resuh of pre-lithification compaction rf unconsolidated, over-pressured, water-rich fine-grained mudstones to form mudstone injection breccias.

There is a host rock control on mineralisation and alteration. High Au grades are coincident with siBca-pyrite alteration (Allibone et al., 1995), which is in turn preferentially developed within mudstone-dominated breccias Tlie latter f o m a distinctive wedge that thins to the west and parallels and abuts against a fauh fliat marks the eastern wall of the open pit. This breccia wedge and the principal ore zone are essentially coincident and have a crudely southerly-plunging tubular geometry over a strike length of at least 600 metres. Concentric iteration zoning around the silica-pyrite core suggests decreasing temperature and increasing pH conditions away from a higher temperature, more acidic fluid that was channelled within a central conduit. Hydrothermal fluid flow was from depth to the south of the open pit and obliquely upwards to the north within the breccia package ^ c h may have had favourable permeability and/ or enhanced chemical reactivity. More massive sedimentary grit (breccias) enveloping the deposit may have formed a less chemically reactive and possibly less permeable zone that channelled hydrothermal fluid flow. The latter was accompanied by fracturing of devetoping siKca pyrite alteration zones, however there is no major ductile shear zone developed in the open pit. Deformation is restricted to localised shearing and cataclastic re-brecciation along the margms of larger silicifled dasts and alteration zones. Ductile deformation (a weak slaty cleavage) is only locally developed, defonns ore and alteration, and is restricted to the un-silicified matrix of some breccias. There is brittle cataclasis and veining cf silicifled clasts related to strike-slip deformation in narrow brittle-ductile fault zones.

Fluid inclusion results in quartz fibre veins cutting the silica-pyrite alteration are in good agreement with a previous fluid inclusion study by Allibone et al (1995) with a peak in the homogenisation temperature around 120 - 130 C, and salinities of around 5% NaCl equivalent. Note that neither study has found any evidence fcr boiling. A close association was noted between enargite and barite in boA fibre veins and vuggy quartz and the observation of mainly all-Hquid inclusions in barite suggests temperatures < 50 Since vapour-containing inclusions are quite rare (and are most likely leaked inclusions), tiiese inclusions have not foimed by necking down, and thus are indicative of trapping within the low-temperature, phreatic zone. These results suggest that the massive bante m these samples represents the final parageneric stage of Ae vein system and that the barite was precipitated at low pressuresfi^oma low temperature, low salinity aqueous fluid. It seems unlikely that a large pressure correction is warranted (Allibone et al., 1995), and the range of geological and fluid data point to hydrothermal alteration and mineralisation at shallow crustal levels, not at crustal depths in excess of 4 5 kms as proposed previously.

High sulphidation and porphyry deposits are commonly linked in tibeir formation (eg Lepanto/FSE and Nena/Fneda River deposits), and recognition of fluid migration pathways within the high sulphidation OTvironment can assist m targeting porphyry magmatic fluid sources at depth. At Gidginbung, the hycfroAermal fluids originated at depth, to the south of the current open pit. Although zonation in Au and Cu is commonly observed within high sulphidation systems, the Au-rich character of ore at Gidginbung is attributed more to flie Au-nch, (relative to Cu), sulphide-undersaturated nature of Ordovician-Earliest Silurian magmatism in the LFB. REFERENCES Allibone et al, 1995. Synchronous advanced argilUc alteration and deformation in a shear zone-hosted maematic hydrothermal Au-Ag deposit at the Temora (Gidginbung) Mine, N.S.W. Econ. Geol., v. 90, pp. 1570-1603.

Ackflgwlgd^gnti^ Permission to publishfi-omGold Mines of Australia and the Executive Director of fte Australian Geological Survey Organisation is acknowledged.

45

VHMS DEPOSIT AND PORPHYRY Cu-Au POTENTIAL IN THE EARLY PROTEROZOIC OF SOUTH-CENTRAL HNLAND. J K. C. Uwric^ & D. R. Bowes^ Australian Geological Survey Organisation, GPO Box 378, Cmberni, ACT 2601 ^Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ, Scotiand.

While the prospectivity for VHMS deposits in many Proterozoic volcanic belts has been demonstrated flie potential for mtrusive-related or porphyry Cu-Au deposits in these terrancs has recently been highlighted by reinterpretation of known deposits (eg Kiruna), discovery of large tonnage deposits (eg Aitik), and recognition cf metamorphosed equivalents of porphyry-related alteration systems in high grade metamorphic rocks.

The Early Proterozoic terranes (Svecofennides) of South-Central Finland are characterised by regionally metamorphosed and polyphase deformed meta-volcanic and meta-sedimentary belts intnided by large volumes cf syn-kmematic mtrusions (1.9 to 1.86Ga) mainly of granodioritic to quartz dioritic composition. The Virtasahni Volcanic belt is one of the largest meta-volcanic belts, and consists predominantly of submarine tholeiitic volcanic lavas and sills that have been regionally metamorphosed to granulite facies, retrogressed, and subjected to complex polyphase ductile deformation. The volcanics are sub-alkaUne tholeiites similar to E-type MORBs. Within this belt, several small tonnage Cu-Fe orebodies, including the Hallinmaki deposit, are spatially associated with quartz diontic and granodioritic intrusions, and calc-silicate 'skam' hthologies, leading to initial consideration of the mineralisation as intrusive-related. However, structural, stratigraphic and geochemical studies have revealed a syn-volcanic origin for the Hallinmaki orebody (Uwrie, 1987). The intrusions are synkmematic (mostly syn- to post D2), structurally-localised, primitive, Cu-bearing, relatively oxidised magmas with an extended SiO^ range and calc-alkaline affinity, and are similar in many respects to Tertiary calc-alkaline I-type magmas. They, cross-cut mineralisation which formed pre-granulite facies metamorphism.

Within the Hallinmaki deposit, mineralisation is preserved largely as a cpy-po-py± magnetite assemblage Stratabound ore and subjacent stockwork mineralisation occurs within variably 'skamed' meta-tholeiitic amphibolites and m calc-sihcate 'skam' rocks (probably epidosites originally) that fomi an alteration halo subjacent to the deposit. Extreme element mobility including the HFS (Zr, Nb, P and Ti) and the LRE (La, Ce) elements, is demonstrated in the alteration zones subjacent to the deposit. Deformation partitioning' and restriction of texture-destructive retrograde metasomatism within narrow shear zones enabled regionally developed synvolcanic alteration to be mapped. Meta-volcanic Hthologies beneath the deposit were originally high in Cu abundances but are strongly depleted over large areas beneath the deposit. It is thought that the ^ebody represents the stockwork zone and lower part of a 'typicaP vertically zoned VHMS mound deposit There is an erosional angular unconformity at the top of the orebody, and overlying meta-volcanics are reltively unaltered (not skamed), and have markedly different primaiy magmatic compositions. The stratigraphic top cf the deposit marks a regional hiatus, with a significant change thereafter in regional tcctono-magmatic processes Pillow lavas and limestones indicate a shalow submarine environment.

Economic viablity of the mine was significantly enhanced by structural remobilisation of semi-massive stockAvork and stratabound mineralisation into a series of steeply-plunging high grade ore shoots Ore remobilisation occurred during brittle-ductile transpressional shearing, synchronous with local emplacement cf granodioritic and quartz dionte intrusions. High grade ore shoots of massive and breccia ore fomied in localised zones of extension, particularly where transcurrent movement on brittle-ductile shear zones caused reactivation cf previously formed fold hinges in interbedded Hthologies of marked competency contrast.

Subsequent deformation, metamorphism and remobilisation of ore synchronous with emplacement of synkinematic calc-alkaline intrusive rocks is interpreted to have occurred during the progressive unroofing at a destructive plate margin, and this has led to telescoping of the crustal sequence that is now exposed Exploration success for porphyry systems in similar areas in the Proterozoic will in part depend on targeting areas with suitable intrusive magma chemistry, and recognising areas which were at a relatively high structural levels during intrusive emplacement (not furdier eroded). The telescoped crustal section preserved in the Virtasalmi District represents a lower to mid-crustal section (in the brittle ductile transition zone at greenschist fecies metamorphic conditions) at the time of emplacement of oxidised I-type (calc-aBcaline) Cu-bearing magmas which were too deep for porphyry styles to develop. Within the Early Proterozoic of Finland potential for synkincmatic mtrusive-related Cu deposits may He where crustal scale shear zones have provided access of primitive magmas to higher structural levels in the orogen, and/ or where any supercrustal volcanic equivalents of these magmas are preserved.

REFERENCES Lawrie, K. C., 1987, The origin, nature and tectonic significance of the Hallinmaki Cu deposit Virtasahni Distnct, South-Central Finland. 297p. University of Glasgow (Unpub. PhD thesis).

Acknowledgments Ken Lawrie acknowledges the support of a Glasgow University Postgraduate Scholarship Award. Outokumpu Oy are thanked for access to data and logistic si^port for this study.

46

The Ckmcleristics of iMdigo Gold depodte, Ceatnl Victoria; Gnidet to Ezpionlion XU U and TEUNIS AJ». KWAK , Lflti^ UBNcnHy, B«idoorA,V^ The BencMgo ore field, second lirgest tn Australia, is hotled in die metamorphosed, lower to inkkiie Ordovician Castlemaine Siq>ergroiq) (Cas and VandenBers* 1988) Uffbidite sediments 2-3 km west of the major Whiteiaw fault and 10 km NNE of the middle Devonian Harcxniit Badiolith. On a regional scale, the sedim^ts have been folded into NNW-trending anticlinoria and synclinoda widi a revosal of plunge roughly ooincidmg to a E-W axis bisecting the ore field (Shazpe and MacGeehan,1990). High-grade, folded, laminated veins are intmected by mineralised '^hits' md odjer veins show a varying degree of folding. The sitting and geometry of quartz veins is strongly controlled by bedding, folds and faults. The shale-sandstone interface was a zone of low pressure during the mineralisation with tbe 'ch<Ae' restricting ttie flow of orefluids.On die odiCT hand, the bedding parallel faults extensively developed during fold lock up. Under conditions of hi^flowpressure the bedding parallel veins deposited along the shale-sandstone interface and /or die bedding parallel faults. After the folds locked up. die significani dilation associated widi the develofment of strike faults result in the deposition of extensive quartz veins including tension gash (spurs), massive, brecciation, classic saddle reef and fault veins.

The study of mineralogical and chemical dianges in Bendigo in^ily that an approximately 150m wide primary alteration zone exists around die core of the Nell Wynne Anticline characterised by pyrite, arsem^yrite, chalcopyrite, sphalerite, millerite, tetrahedrite, quartz, carbonate, sCTicite and chlorite. Generally, the caihonate compositions indicate a progressionfromFe-rich, paragenetically early to Fe-poor, paragenetically late. Whether the former or the later carbonate spots are related to hydrothermally intrcxluced COj. Siderite and sideroplesite ' ^ t s ' m commonly displaced by the Sj cleavage in the fold hinge zone. Fe-rich cabonate alteration is relate to tte early-staged pyrite mineralisation which predates Sj. Ankerite alteration has a close relationship to gold-pyrite-arsenopyrite-chalcopyritegalena-sphalerite-boulangerite mineralisation during die vein formation and syn- or postdates Sj. Sericite is phengitic, and chlorite is ripidolite. Carbonate alteration ^spots' related to mineralisation have been attenuated along fold limbs and mineralised veinlets are displaced by plane cleavage, which indicate that alteration and mineralisation pre-date/syndate the continuing cleavage development. However, pyrite aggregates occur along the bedding in black slate and elongated pyrite-quartz-carbonate assemblage 'spots' are distributed along the axial plane cleavage vMch show die alteration and mineralisation postdated the development of the axial plane cleavage. Therefore, mineralisation pre-, syn-, and post-dated folding. The microstructural relations and the mineral composition studies imply diat vein formation and hydrothermal alteration were contemporaneous. There are the same minerals in the vein system as in ' the alteration system except for albite.

8 ^ScDT values in pyrite range fran an average value of L27%c m and nearest veins to 17.95%c 170mfromdie saddle reefs in the Nell Gwynne anticline.Values vary systematically and synunetrically outwardfromtbe core.

Guides to exploration for deposits similar to Bendigo, include: (1) an extensive zone of hydrothermal alteration is needed, probably die larger and more intense the alteration, the larger the deposit Sericite and chlorite alteration extend weU beyond carbonate alteration, the style generally thought of as the most useful. As sericite has K present, radiometric surveys may indicate areas of such alteration. Inqx»tant values of constituenU in altered host rocks are: As > lOOppm, Cu > 4(^pm, Zn > lOOppm, COj > 3%, H.O > 2%, 6-ScDT =-2.58 --I- 1.98%c; (2) a thick shale unit, doubly plunging, near-isoclinal anticlinal and steep chevron structures (dcnnes) facilitate a 'choke' mechanism; (3) a mechanism of 'locking' is envisioned where directional stress can no longer be taken up by folding. Continued directional stress then produces reverse faulting at acute angles to the princpal stress, particularly near the apices of the anticlinal domes, historically the main areas of gold mineralisation; and (4) The association Fe-Cu-Zn-Ni-As-Sb-Au spheroids (frambc»ds?) in die host rocks proximal to gold veins may indicate the former is the source of gold present.

CORRELATIONS OF THE McNAMARA AND MOUNT ISA GROUPS - A SEQUENCE STRATIGRAPHIC FRAMEWORK FOR STRATIFORM PB-ZN-AG DEPOSITS AT MOUNT ISA AND LADY LORETTA Bruce A McConachie. Jan Domagala", M. Jim Jackson, Rod W. Pigc ind Peter N. Southgatc bmcconac@agso.gov.au jjackson@agso.gov.au rpage@agso.gov^u psouthga@agso.gov.iu Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2600 Department of Minerals and Energy , Geological Survey of Queensland, GPO Box 194, Brisbme. 4001, Qkt Austnlia

Many world class, generally stratabound Pb-Zn-Ag deposits have been discovered in the Mount Isa Basin. The basin may host many additional major deposits but the controls on the migration of ore fluids and controls on traps arc only now beginning to be discovered. Sequence stratigraphic studies in AGSO^s NABRE project have provided broad genetic correlations between the lithosu-atigraphic units that host two important deposits, Lady Loretta and Mount Isa. The Mount Isa ore body is contained within the Urquhart Shale in the Mount Isa Group and the Lady Loretta ore body occurs in the Lady Loretta Formation in the lower McNamara Group. As described by Krassay and McConachie (1996) systematic trends commonly correlate with shallowing and deepening environments. Offsets in gamma ray curves (interpreted as significant breaks in Lady Loretta Mount Isa Area sedimentation) can be used to consuiict depositional Area histories. Broad similarities in log patterns and zircon upper McNamara Mount Isa Fault ages suggest the correlations shown in Table 1.

Qu Magazine Shale Based on gamma logs recorded at Crystal Creek and Kennedy Siltstone Esperanza Waterhole, Mount Isa and lower Spear Siltstone McNamara Groups sequence stratigraphic Lady Loretta Urquhart Shale correlations indicate a direct equivalence of stacking 1647±4 Ma 165217 Ma patterns over the lower part of the stratigraphy.

Detailed gamma log patterns from the Lady Loretta Formation and Urquhart Shale also exhibit suikingly similar sequence stacking patterns. Ore deposit settings in both areas occupy lowstand to transgressive system tracts slightly cleaner and presumably more permeable than the confining silistones (Figure 1). Tuffs from the Lady Loretta Formation and Urquhart Shale (Page and Sweet, in press) have been dated at 1647±4 and 1652±7 Ma respectively.

Such a close correlation between Mount Isa and Lady Loretta is indicative of a nnineral system control on stratiform Pb-Zn-Ag ore traps. Similar regional controls are typical of petroleum systems in Phanerozoic basins.

Native Bee Siltstone Breakaway Shale

Moondarra Siltstone Wanina Park Quartzite

Esperanza Fm Paradise Creek Fm Gunpowder Creek Fm Torpedo Creek Quartzite

Table 1 Stratigraphy - Mount laa and Lady Loratta Figure 1 Q m m log or driOhola P148 (Big Synciine) LadyLorvlta Qanvna oounta (oounta par aacond) 0

200 400

REFERENCES KRASSAY A.A. & McCONACHIE, B.A., 1996.

Outcrop and core derived ganuna-ray curves: New insights into lithostratigraphic subdivisions and facies relationships. Geological Societ)' of Australia Abstracts No. 41,239.

PAGE, R.W. and SWEET, LP., in press, Geochronology of basin phases in the western Mount Isa Inlier, and correlation with the McArthur Basin.

Australian Journal of Earth Sciences. i^gknowlcdgemcnt: Geoff Weber of Pan Coniinenul (now Buka Minerals) and John Dunster of CODES are thanked for their assistance at Lady Loretia.

48

MPS MaxiniiroRoodii^ Suftace

S B Sequence boundary

THE ROLE OF ARC MAGMA ALKALINITY IN CU-AU »ffiTALLOGENESIS Brent I.A. Mclnnes GEMOC and CSIRO Exploration & Mining, PC Box 136. N. Rydc, NSW 2113 INTRODUCTION Many of the world's prolific Cu and Au metallogenic regions are associated with alkaline an: nuunnatism Papua New Guinea (Lihir, Porgera, Ok Tedi, Panguna and Mt. Kare), Irian Jaya (Eitsbeg m O Q a ^ ) ^ t i s h Columbia (Galore Creek, Lonaine, Mount MilUgan and Mount PoUey), Fiji ( E a n n r i A u s t r ^ (Goonumbla). ^ l o r a d o (Cripple Creek) and Chile (Maricunga bek). TTie jmtpose of this c o r a o H ^ ^ is to examine the underlying petrological principles that support the notion that there is a direct relationship between the alkalinity of arc magmas and Cu-Au metallogenesis.

SULFUR SOLUBILITY AND DISSOLUTION MECHANISMS IN SILICATE MELTS Sulfur IS a key element m controlling the metal contents of magma becayse of its unique ndoK-depeadem solubility in silicate melts as both sulfiite (SO^'-) and sulfide (S' ). The diagnm below abows ^ a » i > i n ^ effects of die key parameters affecting sulfur solubility: teiiq)«ature, <»ygeo fiigadty and — r - r bulk composition (alkalinity). The vertically oriented Ni-NiO buffer surface approximately separates the Hd&de ($'• » S 0 4 ) and sulfate (SO4 stability fields, and the sub-horizontal cwviplanar smiacet depict ite S concentrations at which sulfide and sulfate saturation occurs, respectively.

calc-aikaline melts

shoshonitic mens

•71

^

1000

ff'i r c ) - 1 6 - 1 4 - 1 2 - 1 0 - 8 -e log f02 The heavy arrows in the left diagram demonstrate the condition where die redox state of a cak-a&aliae m y m . with mitial fO: > NNO becomes isothermally lowered (as a result of magma mixing a s s i m i l a t i o n ^ degassmg) d ^ g transit from its source region to the upper crust. The convereion of sulfite to sulfide via the reaction SO4 => S + O2 will drive the magma into the sulfide stability field, where onder aU cooditions it wll be sulfide-sahirated Because sulfide liquids are dense and have extremely high partition coefficients for Au, Cu and PGEs (Kp = 10^ to 10^; Mclnnes and Evans, 1996, Geol. Soc. Australia Abst. No. 41, 288), the onset of sulfide saturation in an arc magma will quickly lead to the depletion of Au, Cu and PGEs in the nelt during Its transit to the crust. Such depleted intrusions or volcanic suites would be poor Cu-Au exploratioo targets.

-16-14-12-10-8 log

-6

/b2

THE ROLE OF MAGMA ALKALINITY IN PREVENTING SULFIDE SATURATION The petrological importance of alkalinity is that the solubility of sulfate is 2-3 times greater in hieh-K cafcalkaline and shoshonitic melts than calc-alkaline melts under equivalent T - f O i conditions (Ducea et al 1994 Int. Gwl Review 36, 703-714). The metallogenic importance of alkalinity is lhat oxidized, sulfete-rich,"high-k ^c-alkaline and shoshomtic magmas are tnore resistant to redox changes leading to sulfide saturation (see rieht diagram), and are dierefore more eflfective in transporting the metals dissolved within them to the upDer c i ^ t where they will be available for incotporatioo into a poiphyry-type ore dqx»it ^ ^ CONCLUSIONS The observation that Cu-Au deposits at convergent margins are preferentially associated with alkaline src magmas can be explained by die significant role that bulk composition (alkalinity) plays in sulfiir (and metal) solubility. n , e low viscosity and high volatile contents of alkaline magmas also contribute to the^ enhanced feruhty b^ause ^ y facilitate the rapid transport of magma (and metals) fiom souree to upper crust, and thus decrease die dwell time in tiie upper manUe and mid-cnistal areas where redox reactions are Ukdy to o ^

49

LANDSCAPE, R E G O U T H AND ORE DEPOSIT RELATIONSHIPS I N THE COBAR AREA, NSW J Kamgtfig.MgOKyr' and David L. Gibwn^ I ' Bdeonnen. ACT 2616 -CKt, LEME, Australian Geological Survey 0!:gaiusa«jon. PO Box 378 Cubena, ACT 2601 « understanding the chemical and physical expression of ore deposits ,n the ancient and regoUth-dominated Australian landscape. Investigations A u t ^ r r r^t J! improvements in mineral exploration in the Y i l g a m l i o n of Western Australia and tbs work .s now being extended to the more recenUy active and conjlex regoUth-lanSo™ environments of southeastern Australia. « uui mnaionn The Cobar region in northwestern NSW is an area of complex landscape history and contmuous regolith development. It straddks the Canobolas Divide, a low drainage divide which develo^d in the Late M e s o S Early Temaiy during downwarping of the Murray Basin. TTie terrain is marked by very genUe northern slop« towards the Darling River (possibly reflecting an older Eromanga surface) and steejS^ L e i n c i ^ slopes, related to development of the Lachlan River.

"H^IMXI souinem -niere is much evidence of inversion of reUef in the Cobar area. Old quartzose sands and gravels occur on hilltops west, southwest and north of Cobar and in places these have silcrete caps. Some ^oundwater channels have concentrated carbocretes (variable mixtures of predominantly calcite and dolomite) and these are also now inverted as small flat-topped rises in the topography. Poorly consoUdated alluvial sands are preserved on the ^ ^ ^ ^^ dx^nngt, which appears to have significanUy dissected older land surfaces ! r u Much of this incision and some of the earlier topography appears to have been infiUed during changes in regional base levels, probably related to fluctuating levels in the Munay Basin.

The present landscape has a widespreadregoUthcover, but in many areas this is relatively thin « 5 m ) TOcker regolith occurs in the drainage and palaeodrainage. Sedimentary cover thickens towards the Darling River in the northwest and obscures a possible northern extension of the Cobar Basin. Outcrops are mostly of the more siliceous rock units and some of these also show surface siUcification. TTiese are^ are covered or surrounded bv a thm or scattered regolith of abundant lag derived from weathered bedrock, with sheetwash and a significant component of pama (aeolian sand and silt). Older silicified surfaces are also common to the north of Cobar.

Withering profiles are typically up to 20-30 m thick, but oxidation extends to 100 m in places. TTiere is no evidence for the prior existence of a widespread ferricrete (or laterite) cap. Where present, ironstones occur in low parts of the palaeotopography. Concentrations of ferruginous lag are common in areas underlain by the Ampbiheatre &oup or in transported regolith. TTiis ferruginous lag has largely been derived fiom the underlying motUed zone and ferruginous veining in oxidised saprock. Much lag on the present surface is probably of relatively recent origin, although deposits in older drainage chamiels suggest similar lags developed fcoughout ^ e Tertiary. Transported regolith varies from thin veneers of sheetwash to valley fills up to m ^ than 70 in thick. Low areas in the landscape commonly have surface layers of well sorted maghemite-rich lae whereas hills and slopes have mixed lag accumulations which mclude poorly sorted maghemitic lag, irregul^ ferruginous lag, ferruginised lithic lag and lithic fragments. e Most ore deposits discovered in the Cobar region are associated with silicification of the bedrock and as a result outcrop as topographic highs (hills or rises). ITie general style of mineralisation so far discovered is as verticdly extensive, steeply plunging pipe-like ore shoots and veins (e.g. Elura and the Peak) or fracturecontrolled stockworks (e.g McKinnons). Tliis geometry means that most ore bodies have a small surface/water ^ l e mtersecuon and are thus small geochemical targets. Although known ore deposits occur as positive femures in the present and palaeo landscapes, some may have been subsequentiy covered by alluvial and colluvid infilling of the older landscapes. TTiere may also be other styles of mineralisation in the Cobar area which do not have associated intense siUcification and which therefore would have a different topographic expression (e.g. large carbonate-bearing alteration systems along thrusts with low-grade gold mineralisatioiO. •^e long history of weathering, erosion and transport of ferruginous components in the Cobar landscape means that g^hemical dispersion is quite complicated. There are many large geochemical dispersi^haloes particularly in trai«ported ferruginous lag, and one of the major challenges for geochemical exploration is t(i locate the source of the metals for these widespread dispersions.

50

THE MANDAMAH PORPHYRY Ci-Ao PROSPECT FMKCorquodale' (Md Mines of Australia. POBox3«. Tcmon, NSW. 2666.

The Mandamah Proq)ect, recently discovered by Gold Mines of Australia, is located 30km north of Temora. in so^em central New South Wales. It lies within tbe Late Oixloviciu Gidginbung Vokamc-Imrusive c o m ^ which hosts the Gidginbung high sulphidisation gold deposit ivecompjex Region^ wide-spaced drilling over a geophysical/geochemical tvget intersected anomalous Cu-Au miner^isation at Mandamah. FoUow up aircore drilling outlined a 600 x 200m Cu-Au bedrock anomaly (>0 1% Cu and 0. Ippm Au). Tluee diamond holes confirmed die presence of a significant volume of porphyry style CuAu mmeralisation fiom 60 to 240m below the surfiice. Significant intersectioas inchided: LD 194A 92m @ 1 JSpimi Au - 0.32% Cu, LD 196 185.6m @ 0.36ppm Au - 0J% Cu. At M a n d ^ 50m of transported clays and sands overUes a +30m thick partiy erxnied weathering profile developed on diontes. This zone of bleached clays includes a supergene gold zone at the ti«m»orted cover/saprohte mterface and secondary gold and copper enrichment at the base of oxidation.

The primary mineralisation is associated with quartz stockworks and is hosted by hydrothennally altered ouartz diorite porphyries. ' ^ Tie hydrothermal alteration comprises an early event of quaitz-chalcopyrite, quartz-magnetic veins associated with magnetite-K-feldspar-albite wallrock alteration. TTiis principal mineralising event is overprinted by a chlonte-sencite-albite-quartz-pyrite-chalcopyrite assemblage which is cross cut by a structural controUed, massive, quartz-chalcopyrite-pyrite vein assemblage associated widi high grade copper mineralisation. The final event is strong, shear-related, quartz-sericite-pyrite overprinting. Tbese structurally controUed events are associated with defonnation which postdates the earlier "porphyiy" styled mineralised event.

The mineralised zone at Mandamah is open along strike and wifli depth.

51

A MINERAL SYSTEMS APPROACH FOR ASSESSING ORE DEPOSIT PROSPECTIVITY TcfrtnceP. Mermudi'. Divid L Hurtoo^ Kemdh C. Uwric^ Lesley A.I. Wybom^ Kevin F. Cassidy^ Roger G. Skirrow', and Gregory R. Ewers* * Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601

Ever since Lindgren (1933) first classified mineral dqwsits, many schemes have been advanced to sort mineral deposits into groups for which genetic models can be developed. However, most mineral dqposit models incorporate only deposit-scale features. In order to adequately analyse the mineral potential of a qiecific area, it is crucial that the ore deposits are considered as only one part of a complete regional-scale mineral system.

Based on this realisation, hydrothermal mineral systems can be considered to comprise the following major components: (1) energy to drive the system, (2) sources for the fluids, metals and ligands, (3) a migration pathway, (4) a chemical and/or physical trap site, and (5) an outflow zone.

The mineral s}'stems approach de\elopcd by AGSO attempts to identify processes that operate on all scales from regional to local, and to use these processes to define mappable criteria which can be incorporated into Geographic Information Systems (CIS). An automated system for tlie assessing mineral prospectivity of any region can then be developed by combining the mappable criteria with Uie essential ingredients needed to form each particular type of ore deposit.

The origin of hydrothermal fluids in most systems can be ascribed to one of three source emironments: (1) active metamorphic regimes, (2) magmatic provinces, and (3) tectonically active basins. Depending upon crustal level, potential mineralising fluids can be derived hy: (a) magmatic degassing or metamorpliic devolatilisation, (b) circulation of surficial fluids, and (c) expulsion of connate fluids. As these fluids migrate their chemical compositions may evolve through reaction with the host rocks.

While the chemical processes in the source regime and migration pathway determine the capability of tlie fluid to carr>' specific metals, it is the chemical and physical processes in the depositional regime that determine if the fluid will precipitate the metals and form a mineral deposit. As ore metals are commonly undersaturated in hydrotliermal fluids, liighly efficient and rapid processes are required to form economic mineral deposits.

Processes which possess these characteristics include fluid reaction with the host rocks and the mixing of reactive fluids. These first order processes are important in tlie formation of most hydrothermal mineral deposits, but second order processes are quite varied and can produce strikingly different styles of mineral deposits.

The outflow zone is often neglected in many ore deposit studies. However, analysis of the outflowing fluids and their associated alteration may delineate the extent and geometry of the ore-forming hydrothermal system and this can, in turn, provide vectors towards mineralisation.

Mappable criteria can be used to assess processes tliat may have occurred within a mineral i^stem, and knowledge of these processes can be used to predict the characteristics of mineral dqwsits wiUiin tliat environment For instance, magnetite-series granitoid magmas may evolve oxidised fluids from which metals can precipitate by cooling, dilution, pH neutralisation and reduction. The first two processes may occur if the magmatic-hydrothermal fluid mixes with surficial or metamorphic fluids, whereas the latter two processes may occur if the fluid encounters reactive rock types (e.g. carbonate or carbonaceous sediments). Tliis example suggests that ore deposition can occur in a number of different geological environments around a magnetiteseries granitoid and the presence of mappable criteria such as carbonate or carbonaceous rocks, hematitic alteration or regions of demagnetisation may indicate possible sites of ore deposition. In addition, knowledge of specific deposits may be used as evidence for the occurrence of certain processes, which in turn can be used to predict the possible existence of other dqx>sit types.

REFERENCES Lindgren, V., 1933. Mineral Deposits (4th ediUon). McGraw-Hill. New York, 930p.

Acknowledgements: The authors publish with the permission of the Executive Director of Australian Geological Survey Organisation.

52

ALTERATION AND MINERAL IDENTIFICATION W I T H THE LASER RAMAN MICROPROBE Temencc P. Memigh^ md Kenacth C. Lswrie^ ^ Australian Geological Survey Organisation, GPO Box 378 Dnbeni, ACT 2601

The laser Raman microprobe offers a rapid method of identifying mino^ls in sections, lock dups of drill core. Advantages of this method include visual sclcction of each mineral, spot analysis (down to Ifim), Kttle or no sample preparation, and the ability to identify mineral polym<^phs. The Raman i o M q a c is a i n n of vibrational molecular spectroscopy that is very sensitive to crystal structoc, aid heocc it cia be used to ikx^ a wide range of minerals including most silicates, carbonates, sulphates, lAratos, pho^Aates, liydroxides, oxides, and sulphides. The small spot size also makes this a powcrM me&od fcr tfie idmriftcation of very fine grained mixtures of minerals such asfliosecommonly encoimtcred in attoratioft » » e s smrowidiag hydrotihermal mineral deposits.

Traditionally, identification of these fine grained altwation assemblages has rcKed on a combinatioQ of llun section, X-ray diffraction and staining techniques, and more recently, PIMA spectral data. Even wilh 4kse specialised techniques it may still be difficult to imiquely identifyttiecoBgtitii«ts due to die mixing of naierals at the microscopic scale. However, Ae micron-scale resoluticm of Ae laser Raman aaicroprobe p r o v ^ more definitive data to resolve the constituents of such assemblages. Many mmeral am be quickly identified by noting the most intense band in the Raman spectrum. For example, most sulphates display an intense band near 1000 cm'^; carbonates have an intense band near 1100 cm'^; andfeldsparskave an mtese band near 510 an'^ Another advantage of Raman spectroscopy is that it can easily identify varions polymcHphs such as those of TiOj. Since each polymorph has a distinct stability field, identification rf the TiO^ polymcMph preseirt w^bin a particular alteration assemblage enables better definition of the presswe md temperature limits of that ater^ion assemblage. Preliminary studies of alteration assemblages torn porfAyry, and both low and high solphidadon deposits in the Lachlan Fold Belt have demonstrated significant differences in TiO^ phase stability between &ese styles of deposits, and also between aheration types within individual deposits. For example, w^fain some individual porphyry alteration systems anatase, rutile, brookite (and titanite) have been identifi^ however dbe occurrence of specific Ti-phases appears to correlate with the alteration assemblage. Prelkninary studies erf* some Oidovician porphyry Cu-Au occurrences in the Lachlan Fold Belt indicate &at anatase md brookite (bofli previously identified as rutile) are the dominant TiO^ phases in alteration assemblages associated with ttiese deposits. The presence of anatase indicates formation at relatively low temperatures and possible low fli^ (Mathews, 1976). By contrast, TiO^ occurs as ^eletal rutile grains which pseudomc»ph cmfy tftanoni^Mtito me low fuli^dation dcpodts. A »requires fifftfaer i n v e s ^ p ^ into » bkween high and low sn^rfuir systems. However, coupled with observed differences in grain morphology, and characterisation of Ti-phases within individual alteration assemblages, this technique may be of potential use in porphyry and cpifeennal terranes where resistate mineral studies are employed within exploration programs.

Raman spectroscopy may be particularly useful in high-sulphidation epithermal systems wiA vuggy silica cores within which the minerals (including Au) may be difiBcuk to identify due to the friable and vuggy nature cf alteration assemblages and the difficulty in preparing eood-quahty ^ n sections in these kstances. Also, Ibc identification ofadularia is important a s i t is one of toe key means of distinguishing between low and Ugh sulphidation styles of epithermal Since all adularia types have compositions close to Aat of orthoclase, this mineral is extremely difiicuh to identify by o&er analytical techniques but die structural disOTder commonly present in adularia is reflected in its Raman q)ectrum which has fewer peaks and broader bands than that cf orthoclase.

deposits.

The laser Raman microprobe is akin to a coarse sampling microprobe diat does not require analysis in a vacium with a carbon coated sample. The ability to identify minerals in situ avoids flie time consuming sample preparation procedures associated wiA the prej^tion of thin sections CM* with analysis x ^ diffraction, eto.

Furthermore, x-ray methods may not detect minerals present in trace concentmtions but these may be readily identified using the Raman microprobe spot analysis technique. In ore dc^xisit and akeration studies, the technique is particularly powerful when used to rapidly ideirtify minerals in tUn sections in si|>port rftextural and paragenetic information. In ore deposit studies, the technique is also used in fhnd m c i n ^ studies to identify molecular species such as CH4, COj, N j within individual inclusions.

REFERENCES Mathews, A., 1976. The crystallisation of anatase and rutile from amorphous titanium dioxide under hydrothermal conditions. American Mineralogist 61 y 419-424.

Agtogwlgdsmgnts The authors publish with the permission of the Executive Director of the Australian Geotogical Survey Organisation.

53

THE CADU GOLD - COPPER DEPOSIT, NSW NvMcrMt Mioiaf SMT Brisbn* Officc: 2/349 Coronatioo Driv*, MBton, QM. 4064 Ca4U Offic*: c/. SiMtli Oni«t Port Office O n ^ NSW. 2800 OrdtMcian-age volcano-intrusive complexes of the eastern LacWan Fold Belt of New South Wales host several poiphyry-style gold-copper deposits. The largest of these presently is Cadia, which was discovered in late 1992 and contains pubUshed resources of more than 9 million ounces of gold and 1 million tonnes of copper. An extensive halo of sub-economic mineralization extends northwest and southeast of the deposit, significanUy e y e i n g the metal inventory of the porphyry ^ m . Productive mineralization is developed partly within the marginal quartz monzomte porphyry phase of a composite intrusion and partly in shoshonitic volcanic wallrock. Chemical features of the monzonite porphyry suggest affinities with shoshonitic magmatism Intrusion of the monzonite porphyry had a strong northwesterly control, as did development of the predominant vem direction. A m^or westerly-dipping reversefeulthas dislocated the deposit and elevated the northwestern, ^edominanUy monzomte poiphyry-hosted part of the deposit at least several hundreds of metres Development of the Cadia mineralization probably resultedfromlate magmatic-early hydrothermal exsolutions from coohng and crystallization of deeper levels of the presently-exposed monzonite porphyry. Hydrothermal alteration effects within the Cadia system are variably developed. Weak to moderate intensity, pervasive propybtic alteration with minor, weak potassic vein-selvage alteration and late, structurally-controUed phyllic overpnnttng, is typical of the northwestern part of the system, where monzonite poiphyry is the dominant rock type. Late-stage hematite alteration has been superimposed widely, producing a characteristic reddening of much of the altered rock. In the interpreted shallower-level, almost exclusively volcanic-dominated, southeastern part of the system, the intensity of hydrothermal alteration is stronger, and includes a substantial developmern of barren phyllic alteration. Typical of many gold-rich porphyry deposits, magnetite is an important alteration product. Sulphide species and content vary across the deposit. The northwestern part of the deposit is gold-dominant and sulphide-poor, where chalcopyrite is the main copper sulphide, with lesser bormte; total sulphide content is less than one percent, with pyrite occurring in relatively trace founts on average In the southeastern part of the deposit, copper content is increased and gold decreased, along with an assoaated mcrease in molybdenum grade. Sulphide concentrations are considerably enhanced, with stronger development of pyrite and bomite. Cadia is classified as a wallrock porphyry in keeping with the oorDhvrv terminology of Titley. t^f-jj

//

54

KEY FACTORS INFLUENCING FLUID FLOW IN TBE MID CRUST: IMPLICATIONS FOR METALS TRANSPORT AND ORE DEPOSITION hfidiolK H.S. aiver^ Alisoo Ord^. md Brace E. Hobbs^ ^School of Applied Gerfogy, Cuitin University, GPO Box U1987, Pfcith, WA. 6001 ^Aastratoi Geodynamics CRC tnd CSIRO Explondon & Mming, PO Box 437. Nttflands WA 6009

A simple concqmial model for Ac fcwm^ion of vein- or shear-zone hosted ort deposits during deformation and metamorfrfiism involves a pervasive metalsteachingphase, and a choMidized depositicm jAase, with metals derived predominantly from theregionalrock sequence rather than from a localized source. While q)ecific controls on ore deposition arc complex, many workers appeal to a combination of structural and chemical/lithological controls. More difficult to appreciate in most fossil hydrothermal systems is the nature of the mechanistic, temporal, and spatial connecting links between regional mtetals source regions and the local metal deposition site. Studies of both mineralized and unmineralized metamorphic-hydrothermal systems as well as recent numerical modelling allow some advances to be made. At moderate temperatures (upp«- greenschist to upper amphibolite), the degree of channelization of fluid flow is a function of rheological heterogeneities in the rock and the deformation conditions. Shear zones and vein systems are a natural consequence of deformation of heterogeneous rocks at moderate to high strains at these temperatures. Metals leaching of sufficient magnitude to subsequenUy form very large deposits will only be efficient in such channels if a) there is significant fluid/rock interaction within and around the channel, b) the vectors of maximum metals solubility are of approximately the same direction and magnitude throughout the channels, and c) there isregionalconnection of the channels. The prime conditions would be a shear zone netwoik that periodically pumps fluid in one overall direction, across a significant temperature gradient. Such systems are difficult to positively identify, but metalspoor albitic shears or giant quartz-filled fault zones in mineralized regions (e.g. Mt Isa Block) may be good candidates. Only large, geometrically homogeneous channels should permit km-scale mass transfer. 10m- to lOOm-scalefluidchannels are more likely to result in metals deposition or lack of long-distance metals transport, because of the rapid spatial and temporal variations in fluid pressure and chemistry that can occur at channei intersections, chokes, jogs, and lithological boundaries. During metals deposition, this relationship is one of several explanations for why orebodies are commonly found around smaller structures rather than larger ones.

Regional metals extraction sufficient to subsequently form giant ore deposits will be best achieved by pervasive fluid flow in response to broad fluid pressure gradients, particularly in conjunction with thermal gradients. At 350 to 550°C, many metals of economic interest (especially Cu, Au) have prograde solubilities in most fluids.

So, if fluid can travel through a grain-scale networic across large re^ons, locally equilibrate with the rock during fluid-rock interaction, and move from low temperatureregionsto high temperature regions, the changing metalfluid partitioning coefficients across this thermal gradient will allow metals extraction. While this combination of factors may seem difficult to achieve, this style of model has been i^^posed to explain broad isotopic and chemical shifts in the composition of unmineralized metamoiphic rocks in several parts of the world. The models require an interconnected permeability in whichfluidat the upstream parts 'Tcnows" about the temperature and hydraulic head of fluid in the downstream part, perhaps across 5 or more km. If deformation is responsible for the regional hydraulic gradients, then very broad, evenly distributed strains should be apparent, otherwise localization will occur. Such conditions are possible before or soon after major deformation events in rheologically homogeneous rocks. Alternately, fluid flow may be thermally driven. Previous concerns about the viability of convection in overpressured systems have been recently addressed by numerical models (Ord etal.

1997, Zhao et al 1997, this conference) in which there is full coupling of deformation and fluid flow.

Hydrostatic fluid pressure gradients in rocks with near-lithostaticfluidpressures can allow iM-oad interconnection and convection in permeable mid-crustal rocks, and these conditions are ideal for the regional leaching of metals.

A spectrum of deformation- aiKi thermally-drivenregionalflowtypes is possible.

The transition from pervasive to channelized flow is critical fop ore genesis. The ideal combination is a sudden change in temperature accompanied byfluidflowchannelization. These conditions may best be achieved during regional metamorphism that is driven by intrusion, particularly when the thermal peak outlasts the major deformation phase. Two possibilities for defcMmation- andfluidflowlocalization accompanying cooling of latesyntectonic granitoid plutons are 1) in the granite carapace as it cools, superimposing a spatially restricted upwards fluid flow on a regional lateral flow regime (e.g. Kanmantoo copper), and 2) the onset of retrograde deformation, localizing around granite/metasediment interfaces. If there is a short time betweenregionalfluid flow accompanying peak temperatures, and localizedfluidflowacccmpanying earlyretrogression,the change in mineral solubilities may match the change in deformation conditions such that economically significant ore deposits form in the early retrograde part of the thermal evolution of a mineralized terrain. Alternately, if the scale of the fluid flow systems is sufficiently broad, the spatial and temporal variation of heating/cooling cycles in the metamorphic pile may be sufficient to localize metal deposition in those parts of the rocks that are cooling, synchonously with metals extraction from those parts that are heating.

55

HYDROSTATIC FLUID GRADIENTS AND VEIN GEOMETRIES AUaon (M, BtooeE Hobbs & Knwfl MHS CSmOExpk»«kii®dMniig.POBoDc437.NedtaiA, WA6009.AIBTRALIA We begin ^ examining thefluidpressure dismlwti e^nt in the crust and boun(Wl5rrelativelyi i n p ^ ^ We aaniiiie the situation for astatic g^ent, with no regional deformation and with no chemicalreactionsoccurring The established folklore p r ^ that fo fluid-rock systems deeper than a few kilometres in ^ padients equal to the Uthostatic gradient are ubiquitous. Such lithostaticfluidpressure gradients are to ^se from processes such as compaction, a host of pore-volume reduction mechanisms, and ly additicorf fluKls to the system either through devolatilissationreactions,maturation of organic materials or crystallisation offluidnch magmas.

Lithostatic fluid pressure gradients are unstable and evolve continuously towards a hydrostaticfluidpressure ^ e n t Notice that what isreferredtohere as ^orfiento, not the ^ u t e values of thefluidpressures Thus thefluidpressure gradient can be hydrostatic at the same time as the iteolute values of thefluidpressures ar^ clc^ to lithostatic. As an example: in an over-pressuredfluidchamber with a top at 5km depth and a base at 1 5 ^ depth the equilibrium hydrostaticfluidpressure distnlwtion that would evolvefioman initially imposed bthostatic gradient would exceed the Uthostatic pressure by 86MPa at the top of the chamber but wouldte below lithostatic pressure by an identical amount at the base of the chamber.

A ^ i a l pointtoconsider is that the time constant for the evolution of a Uthostatic gradienttoan equiUbrium hydrostatic gradient in the above chamber is small (of the order of 10-1000 years depending on the rock's permeabihty)relativetoother time scalesrelatedtotheformationof an orebo(fy (see Phillips 1991 p82) Thus there must always be strong and long-lived "forcing" isrelaxed,the ^ m decays more or less in^taneously back to an equilibrium hydrostatic gradient.

The present of hydrostaicfluidgradients in overpressured systems dictates the types of vein geometries that can form. One would expect, in such systems, differences betweenregionshigh in the q ^ m (i e shallower) where fluid pressures are super-Iithostatic and regions low in the system (i.e. deeper) where pressures are subhthostatic. For a shortening deformation the change-over point, where fluid pressures are precisely lithostauc, marks the lowest depth in the system where veins can form. No veins are possible below this depth unless they occur in localised shear zones orfiuiltQ^ms. Thus, mapping that q»tial distribution, internal textures and geometiy of vein systemsrevealsconsiderable information on th^ plumbing system which can then te used as a direct exploration guidetopin-point precisely the most promising structural and stratigraphic sites for nuneralisation.

We Resent several examples of layered geological systems undergoing large permanent deformation win which the fluid flow is coupled to the deformation. We examine the distribution of u-Vp and link this to the development of vein systems throughout the deforming system. Fluid flow in these examples is driven by ^adients m hydraulic head which ariseformchanges in pore volume induced by deformation These examples ^ w out the important distinction between the Darcy velocity field which is driven by gradients in the hydraulic head and the pattern of mineralisation which isrelatedtogradients in iht pore pressure as weU as to gradients in the hydraulic head.

The regional geometry of the mineralising q«tems is an essential predictive expioiatioD tool in understanding why minei^s^on is localisedexacUy where itisandnot 500 metersawiQrin another similar structural location. The basic prmaples of thermal convection and deformation drivenfluidflowen2d>le some general understanding of why one site is advantageous for mineralisation rather than another rimiiar ate and enable controls on the size and grade of mineralisationtobe WTirciatwl REFERENCES Philips, O.M. 1991. Flow and reactions in perme^e rocks. Cambri4ge Univeisi^ Pten, Cambridge, pp 285.

56

OXYGEN ISOTOPES IN HYDROTHERMAL SYSTEMS - BEYOND THE "BULL'S-EYE'* ^ Genesis ^ PalinSdences. ANU. Canberra, ACT 0200 Ore Groap. Research School Jof Earth Oxygen isotopes provide evidence for the passage of hydrothermal fluids through rocks, sometimes in the absence of other recognizable mineralogic or chemical records. In addition to spatial patterns of whole-rock 8 ^O alteration, which gauge reactive water-to-rock ratios, oxygen isotopes provide additional quantitative information on ancient hydrothermal systems with important implications fw mineral exploration.

COOLING HISTORIES OF PLUTONS Plutonic igneous rocks are important sources of heat and metals in many types of ore-forming hydrothermal systems. Whereas radiogenic isotopes (Pb, Nd, Sr, Ar) yield information on emplacement ages, plate tectonic setting and cooling rates of these bodies, oxygen isotopes provide additional constraints on cooling rates and the subsequent hydrothermal history of plutonic rocks/ Q Kf PI Bl Am Mt Both the partitioning of oxygen isotopes between 1 12 minerals and the diffusivity of oxygen within minerals are dependent on temperature, and because oxygen is a major constituent of silicate and oxide minerals, data Granite Soiidus can be obtained on plutonic rocks of all compositions. ... ^ a o o x In order to assess the cooling history of a sample, quantitative relations for oxygen isotope partitioning and diffusivity as functions of temperature are Hydrotrwrmal — combined with measurements of the oxygen isotope ^ X -lOOC^/Ma compositions, modal proportions, and grain sizes for ^^ «1 Conductive \ ^ -10®/Mi all constituent minerals. Combining these data with W/R « 0 specified cooling histories, oxygen isotope -L- J 1 1 _i. J 1 . 1 . compositions for all minerals can be calculated and 6 compared to measured data in an iterative process (Fig. 1). Oxygen isotope redistribution between minerals in 'Q - mineral FK3. 1 a closed-system would be indicative of conductive cooling. For samples exhibiting open-system characteristics, hydrothermal cooling at specified water-to-rock ratios can be examined. Amphibole is particularly resistant to diffusive oxygen isotope exchange in either case and, thus, provides an estimate of the original oxygen isotopic composition of a pluton. Reversals in ^^O enrichment between feldspars and amphibole, the magnitude of which is a function of time, cooling rate, and fluid flux, provide unambiguous evidence for hydrothermal cooling.

\1 ^^ "T. ^VX \

:

HYDROTHERMAL FLOW IN FRACTURED ROCK Quantitative relations for coupled transport and reaction demonstrate that oxygen isotopic equilibration of hydrothermal fluid with rock is sensiuve to geometry of flow paths. In the case of simple fracture networks, the T (°C) length-scales of fluid-rock equilibration can vary by many orders of magnitude depending on the fracture 15 - I l l ' / density. Extraction of metals from rocks requires up-T flow extensive reaction with fluids and thus a close approach 10 to oxygen isotope equilibrium between fluid and wall rock is diagnostic of potential metal source regions. down—T On theother hand, fcHination of ores requires efficient fiow transport and focusing of metal-bearing fluids. Oxygen isotq)e disequilibrium between fluids and wall rocks is indicative of such flow paths. Some consequences of these relations for a meteoric water dominated, plutondriven hydrothermal system are illustrated in Fig. 2. ctures / m * High fracture densities lead to extensive fluid-rock -10 - 1 1 1 1 1 1 1 1 i 1! 1 1 1 1 • oxygen isotope exchange in the lowest temperature 10 15 regions and retard the progress of low alteration z (km) FIG. 2 fronts along®up-T flow paths. All , .. , , . rill other uuici factors latiuii being uciii^ equal, lower fracture densities lead to a by-pass of oxygen isotope exchange at low temperatures and more extensive low 5l»0 alteration toward the interior, high temperature regions of the hydrothermal system (i.e. formation of a classic low "bull's-eye"). In both cases, zones of ^^o enrichment are produced along downT flow paths of the hydrothermal system which would be focused above the plutonic heat source

57

APPLICATION OF ROCK GEOCHEMISTRY IN SEARCH FOR VOLCANIC-HOSTED MASSIVE SULPHIDE DEPOSITS, MOUNT READ VOLCANICS, WESTERN TASMANIA Aung Pwa and J.C. van Moort Geology Dqjartmcnt, University of Tasmania GPO Box 252-79, Hobart. TAS 7001 The Mount Read Volcanic belt is one of the world's richest provinces of volcanic-hosted massive sulphide (VHMS) deposits hosting such major massive sulphide deposits as: Mount Lyell, Hercules, Rosebery, Que River and Hellyer. In this study acid insoluble concentrations of elements delineate the geochemical halos associated with VHMS deposits, and identify alteration related to VHMS mineralisation.

Mineralogical alteration such as silicification, sericitisation, chloritisation, carbonitisation, etc. occur in different intensities in the Mount Read Volcanics, but does not show significant differences between VHMS related and unrelated alteration.

There are two t>pes of geochemical halos. Type 1 halo related to wall rock alteration trends NE-SW, and is characterised by enrichment in CI, and possibly K and Rb, and depletion in Al, Ca, Na, Ti and Sr. Type 2 halo related to massive sulphide mineralisation trends NS (parallel to the stratigraphic trend) and is defined by enrichment in Fe, Mn, Ba, Zn, Pb and possibly K, Rb and F. Both halos intersect at Rosebery and Hercules in mineralisation and associated footwall alteration zones. Recognition of geochemical halos has led to formulation of geochemical indices for identification of alteration related to VHMS mineralisation.

Using multiplicative indices and ratios, alteration related to massive sulphide mineralisation is defined by the binary relations between Mn x Ba x F and Ca x Na x Sr, and Mn x Ba and Na x Sr in the Mount Lyell, Hercules, Rosebery, Que River and Hellyer areas. All these deposits display similar geochemical alteration signatures. The alteration related to VHMS mineralisation in the Mount Read Volcanics can be distinguished from alteration unrelated to mineralisation by higher geochemical indices of K x Mn x Ba x Rb, Mn x Ba x F and Mn x Ba. The ratios of pairs of geochemical indices Mn x Ba x F/Ca x Na x Sr and Mn x Ba/Na x Sr spatially define the footwall alteration zones associated with the Rosebery and Hercules deposits and also identify the alteration associated with the Mount Lyell, Que River and Hellyer deposits.

58

lUGH-LEVEL INTRUSIONS IN THE McARTHUR BASIN, NT: DEFORMATION STYLES IN THE HOST STRATIGRAPHY AND METALLOGENICIMPUCATIONS Divid J. Rawliigs CX>DES SRC, Univeisity of Tasmmit, GPO Box 252-79, Hobart, TAS 7001

The Jimbu Gr^'te, n^ich cn^s out to central Arnbem Laod, is an example of a composite high-level microgranite pluion suite e m p l a ^ into aflat-lyingsequence of sandstone and lesser mudstone, carbonate and basalt of the Katherine River Group (McArthur Basin) during the latter sUges of sediment dqwsition.

Rawlings (1996) concltKted that emplaceoKot was facilitated by three distinct mechanisms: • the thick basal sandstone package was updomed by emplacement of a laccolith at its base • overlying lutite units were laterally shortened by outward gravity slide along decollemait surfaces provided by the presence of ev^xmtes • the upper-most sandstone unit was raised vertically above the pluton by way of a monoclinal margin and steep reverse detachment faults.

This resulted in the development of three macroscopic structural elements in the area: • steeply and radially outward dipping domes of lower Katherine River Group sand^ne with granite cores • irregular, short-wavelength fold patterns in the overlying incompetent lutite units • discrete flat-lying blocks of the upper-most sandstone unit with steeply upturned or downtumed margins These collective structures are analogous with those generated during the experiments of Merle and Vendeville (1995) involving shallow intrusions into sedimentary packages.

As a result of these emplacement mechanisms, localised steep-sided palaeotopographic features with peripheral deformation are interpreted to have formed. These would have greatly influenced sedimentation and gravity-driven groundwater flow geometries in the surrounding apron and, subsequently, the redistribution and concentration of Cu and other economic elements by oxidised, saline groundwaters. This process was particulariy important around the Jimbu Granite, as emplacement took place during dewatering of the sediment pile. Hydraulic head would have increased in areas surrounding the rising sub-volcanic structure and groundwater flow would have been channellised by the development of structures in the substratum. Potential mineral and oil structural traps were generated at the periphery of plutons in much the same manner as salt diapirs. In the study area, the stratigraphy exhibits contrasting oxidation state and permeability and is locally structurally juxtaposed, promoting the focus of diverse groundwaters into potential trap sites. Hydrothermal activity, associated with emplacement of the Jimbu Granite, may have led to eariy maturation and generation of hydrocarbon-bearing fluids in the adjacent organic-rich lutite of the McCaw Formation. Owing to the sensitivity of the area, there has been no exploration and no reports of mineralisation to date, but the potential is considered to be high given the conjunction of favourable lithologies, trap-sites and fluid-flow mechanisms during granite emplacement.

A possible example of this metallogenic association is the Redbank area, in the southeast McArthur Basin, where significant Cu mineralisation is hosted in correlative formations, comprising redbeds, basalt and TOCrich lutite, in pipe-like structures interpreted herein to have formed during emplacement of nearby Packsaddle Microgranite plutons. Mineralisation shows a strong association with structure and hydrocarbons, and was interpreted by Wall and Heinrich (1990) as being formed by the interaction of an oxidised sulphate-metal basinal brine deriveed from redbeds and a reduced hydrocarbon-bearing flw'd from the TOC-rich Wollogorang Formation. Jura-style folding and thrusting in this area has been attributed by Rod (1978) to similar decollement and gravity-slide processes as recognised around the Jimbu Granite, but was not thought to relate to plutonism.

Recognition of structures associated with plutonism elsewhere in the McArthur Basin is inhibited by a lack of connecting evidence or, in many areas, by the apparent absence of the actual intrusions. Tectonic processes are traditionally invoked to explain this style of deformation, but it is possible that it may locally relate to concealed plutons. This model provides an avenue for hydrocarbon-play and basemetals exploration in these domains.

REFERENCES Merle, O. and Vendeville, B., 1995. Bulletin of Volcanology, 57,33-43.

Rawlings, DJ., 1996. Proceeding of MIC '96 Conference Abstracts, 108-112.

Rod, E., 1978. Journal of the Geological Society of Australia, 25,89-95.

Wall, VJ. and Heinrich, Cj\., 1990. Proceedings of Mount Isa Inlier Geology Conference.

59

APPLICATIONS OF THE BATHURST 1:250,000 SHEET CIS FOR MINERAL EXPLORATION ,

I2U!£.£aU!!fiod'andOTegMacR>e' AustrahanGeologkalSunwyOrginwaioii, GPOBox378, CMtbena, ACT. 2601 NSWIVpartineotofMiiienlRewurcei, POBoxS36, StLeoovdi, NSW, 2065

m s presentation iUustrales the applicaUon of GIS analysis to the digital geological data package for the Balh^rt 1:250,000 map sheet, lecenUy released the Australian Geological Survey OrganisaUon and New South Wales Department of Mineral Resources. Other datasets used as part of the GIS include mineral occuirence data, airborne magnetics and radiometrics, whole rode geochemistry and stream sediment geochemistiy. Using empirical associations of these digital datasets as weU as conceptual or model driven analysis, areas may be targeted as prospective for different styles of mineralisation. Attempts to quantiftr the im^rtance of associaUons and fuzzy logic modelling. of mineral occunences with other datasets can be made with weights of cviince v^tucucc Rock units can be classified by their dominant Uthology, environment of deposiUon, age, mineralogical and geochemical characterisUcs. For example, to delineate areas prospective for Ordovician copper-gold mineralisauon, a prime target of mineral exploration in recent years, areas of subaerial or shallow volcanics and mtrusives of Ordovician age may be deUneated to suit models of epithermal or porphyry style mineralisauon such as Cadia. Alternatively, areas of marine felsic volcanics or volcanicIasUcs might be highlighted for exploration for VMS style mineralisation such as Lewis Ponds.

Analysis of the whole rock geochemistiy database illustrates the relationship of host rock composition to the dislnbuuon of mineral occurrences. For example, there is a strong association of copper mineralisation in and adjacent to rocks with a high background copper concentration, suggesting a local source for metals in the mineralisation. Likewise, polymetalUc base metal mineralisation lias a tendency to occur in and near rocks with high background lead levels. At a more local scale, stream sediment geochemistiy can be integrated to further delineate geochemically anomalous areas. The influence of structure on mineralisation can be gauged through proximity analysis of mineral occurrences to faults and fold axes. However, it should be realised that GIS IS essentially a two dimensional tool and cannot account for the dip of structures and subsurface relationships.

Combinations of many of the above datasets can form integrated models as an aid to mineral exploration For example, carbonate-bearing rocks near younger fractionated granites containing high levels of base metals, may be delineated for skam mineralisation potential. New theories can be evaluated by adding new constraints to a model For example, a fault controlled fluid pathway may be added to the previous model requiring a certain proximity to a pre-existing feult Alternatively, as it becomes apparent that some associations of datasets may not berelatedto mineralisation thqr can be omitted from the model.

In an area such as the Bathurst 1:250 000 sheet where a large mineral occurrence database of over 1300 occurrences exists, weights of evidence modeUing may be appUed. This technique evaluates purely empirical associations between known mineral occurrences and geological features. Prospective areas are then deUneated by combining tiie associations of many datasets, weighted according to their statistically evaluated importance Using fuzzy logic modelling, conceptual theories can be integrated by altering the weighting of datasets according to a particular ore deposit model. It must be established that datasets are conditionally independent for tiiis type of modelling to be valid. However, it may be difficult to achieve this completely, as some geological datasets such as Uthology and geochemistiy can never be entirely independent

.

OUie Raymond pubUshes with the permission of the Executive Director, AustraUan Geological Surv^ Organisation. Greg MacRae pubUshes with the pomission of the Director-General NSW Department of Mineral Resources.

60

ASPECTS OF THE GEOLOGY AND MINERALISATION OF PROTEROZOIC VHMS SYSTEMS IN KOONGIE PARK, WESTERN AUSTRAUA PttcrSRca Lachlin Resources NL Lcvd 37,100 MiUcr Street NORTH SYDNEY NSW 2060 The Koongie Park Prospect is located 20km south of Halls Creek and is situated within Weston Australia's Kimberley Region. The prospect occurs within the Koongie Parte FwmatiOT of the Lower Proterozoic Hall's Creek Group, which in part makes up the Early Proterozoic basement inlier of the Hall's Credc Province.

Massive sulphide mineralisation occurs in felsic volcaniclastics and sediments, all of which v t deformed by a strong NE fabric that dominates this portion of the Halls Creek Mobile Zone. Radiometric zircon datmg by the Kimberley Mapping Accord partners, GSWA and AGSO, indicate that the Koongie Parte Fonnation is 1843+/2Ma^ Two prospects, Sandiego and Onedin, have massive sulphide mineralisation with significttt associated carbonate, an unusual feature in Australian VHMS systems. Mineralisation ^ botfi prospects occiffs near the contact between the Coolibah Tuff volcaniclastic unit and overlaying Camp Shale.

At Onedin hydothermal activity led to the f(MTnation of a caitonate dcminated stockwork, and possibly a mound, with multiple generations of silica carbonate veining, hwciation, and sphalerite dominated stringer mineralisation. The intense alteration, and vigorous physical processes which p-obably inchided some form of hydraulic jacking, has led to ahnost total replacement of the host sediments and volcaniclastics wift only a few quartz-eye boulders remnant. The carbonate was probably formed through die reaction of organic material (CH4) with anhydrite. K. Orth and A. Hill identified pumice shards preserved within tbtse carbonates and equated them with similar vitriclastic textures seen in footwall carbonate aheration at Rosd)eiy \^ere it is si^ested that the carbonate may have been early and was certainly of sub-sea-floor replacement style.

At Sandiego similar carbonate alteration occurs in the footwall with ahnost total replacOTient of the volcaniclastics, but with only minor stringer sulphide mineralisation. The massive sulphide mineralisation appears to be stratiform with a thickness of associated silicate, carbonate, and oxide facies exhalatives. This is in contrast with Onedin where only minor occurrences of possible exhalative style massive su^>hide have been recorded. The differences in mineralisation styles between Ae two deposits possibly indicates that Onedin formed with greater hydrothermal activity and at shallower water depth. This hypothesis is m part suppontd by the larger alteration halo at Onedin and the larger volume of carbonate.

The deposits have been long recognised to have discrete m ^ i ^ c signatures. Recent woric hasft)cusedon the use of high resolution magnetics for mapping and further exploration. Drilling of discrete magnetic anomalies at the Rockhole prospect has identified two mineralised systems wiA significant accmnulaticMis of oxide facies exhalatives with footwall carbonate/talc alteration similar to Onedin prospect.

ACKNOWLEDGEMENTS Permission for presentation of this paper was kindly granted by Uchlan Resources NL and project joint venture partners Anglo Australian Resources NL. The author also acknowledges many informative discussions with Karin Orth who is currently completing a PHD thesis on the Koongie Parte area at CODES University of Tasmania.

61

FLUID CONDITIONS OF POST-METAMORPHIC IRONSTONES AND MINERALISATION AT THE STARRA An-C« DEPOSIT, CLONCURRY DISTRICT J a d d e F. Rocbeitam National Key Centre for Economic Geology and Department of Earth Scieocet, James CocA Univcnity TcfwnsviUe, Queeniiaixi, 4811.

The Proterozoic Starra ironstone-hosted Au-Cu deposit (7.4Mt @ i.Sg/t Au and 1.9% Cu) is located in the aoncurry District, approximately 160 km southeast of Mount Isa. It is hosted within the amphibolite-grade Staveley Formation of the Maiy Kathleen Group, which has undergone several intense alteration episodes.

The origin of the ironstones and contained mineralisation has been controversial with both syngenetic (Davidson et al. 1989) and epigenetic (Williams 1994, Rotherham et aL 1996) models proposed The ironstones form two well-e35>osed north-trending ridges. Ironstone lenses dip steeply east and plunge north and south. These ironstones are locally known as the western ironstones (magnetite-rid and mineralised) and the eastern hematites (anomolous gold only). Five economically mineralised zones (222, 244,251, 257 and 276) occur along a strike length of 6 km in the western ironstones. The Starra orebodies exhibit high Au:Cu (except 276 which is Cu-ridi) compared to nearby Cu-Au deposits such as Osborne and Eloise. The Starra Shear occurs to the west of the western ironstones and is a north-south striking structural feature of major significance with regard to the ironstones and mineralisation. The ironstones represent replacement bodies that occur within isolated sheared-brecda zones associated with the Starra Shear.

Three paragenetic stages are recognised: 1- Na-Ca metasomatism (quartz-albite-actinolite-scapolite), 2- KFe metasomatism (biotite-quartz-magnetite-hematite) and 3 -mineralisation (sulphates, caldte, hematite, gold, sulphides, chlorite, muscovite). Microthermometric analyses show four dominant types of inclusions are present from stage 2 and stage 3 (Table 1). Type 1 are ahnost pure COj, vapour-ridi inclusions which have high densities (0.72-1.0g/cm ') and represent separation of inmiiscible hypersaline brine and COj-rich components. Type 2 inclusions are dosely assodated with type 1 and are multiphase. (Table 1). Type 3 inclusions are 2-phase, liquid-rich ± 1 solid and type 4 indusions are mult^hase (up to 6 solids). Type 1 and 2 inclusions are hosted within stage 2 quartz associated with the ironstones and type 3 and 4 indusions are hosted within stage 3 quartz associated with the mineralisation. The calculated fluid composition from magnetite-quartz isotope pairs representing stage 2 imply waters of 7.87^0 from hematite-caldte isotope pairs representing stage 3 imply waters of 9^7oo- The data suggest the ironstones formed from a probably magmatic, saline (34-52 wt.% NaQ equiv.) fluid that readied temperatures of 600°C. The later mineralising fluid is also consistant with a dominantly magmatic, saline (30-42 wt.% NaQ equiv.) character and reached temperatures of 360°C. The variation (number and composition) of daughter minerals within the two types of multiphase indusions suggest stage 2 and 3 fluids were diflferent.

s

TVpc 1 - C02 vapour-rich

T A G E 2

Type 2 - Multiphase

Density0.72-1.0g/cm'

345^irC 34.52wt% NaQ equiv. m a f o e t i t e i u ^ ^ cilcite^^^

s TVpc3-2pluBC.

T

A G E 3

Liquid-hch W-l solid S-27wl% N i a equiv.

lype 4-Multiphase/ moltisolid

225.360r 3(M2wt% NaQ equiv.

I®

Table 1 Four types of fluid indusions hosted within quartz from stage 2 (ironstones) and stage 3 (mineralisation).

REFERENCES Davidson, G J., Large, R., Kary, G., & Osborne, R., 1989. The BIF-hosted SUrra and Trough Tank Au-Cu mineralisation: a new stratiform association from the Proterozoic eastern succession, Australia. Economic Geology Monograph 6, 135-150.

Rotherham, J.F., Blake, K.L., Cartwright, I., & Williams, PJ., 1996. Stable isotope evidence for the origin of the Starra Au-Cu deposit, Qoncurry district. Economic Geology Research Unit, Contribution no. 55.

Williams, PJ., 1994. Iron mobility during synmetamorphic alteration in the Sehvyn Range area, NW Queensland: implications for the origin of ironstone-hosted Au-Cu deposits, A/ifiera/ium Deposita, 29,250260.

62

PROBING ORE FORMING FROCESSES

USING THE SCANNING PROTON MIOIOPROBE

C.G. RyanJ, Khin Ztw2, CA. HcinridP. D.N. Jmnesoo^ and E. van Achtcrt)cr^i «GEMOC and CSIRO Ex|>k»ition md Mining, PO Box 136. North Rydc NSW 2113. Australia.

2CX)DES. Geology Department. University of Tasmania. Hobait. TAS 7001. Australia.

3 Deportment Entwissenschaften. ETH Zoitnmi. Zttridi CH-8092. Swkzeriand. ^ Sdiool of Physics. University of Melbourne. Parkvilie VIC 3152, Australia.

T^e large penetration depths and iM^ctable nature of MeV proton trajcctorics has pcnnittcd the development of standardless qirnumive for microanalysis of minerals at ppm levels, trace-element imaging and nonnlestructive analysis of mdividual fluid inclusions, all of w^hich offer particular benefits to orc-fcMmation research.

FLUID INCLUSION ANALYSIS

By modelling Proton Induced X-ray Emission (J^IXE) yields from the complex 3D geometry of an inclusion in its host mineri, and by using beamscanning to control the proton dose distribution across an inclusion, the CSIRO method enables quantitative analysis of fluid inclusions of 5-20 [im in diameter with sensitivities for the ore elements down to 40 ppm in the fluid (Ryan et al., 1995).

Research at the CSIRO is focused on the analysis of ore elements in fluids and experimental studies.

Much of the work is centred on the analysis of hydrothermal fluids associated with copper-gold deposits. A good example is the Kidston granite-related breccia gold-copper deposit in North Queensland, Australia. Brine and vapour inclusions show strong partitioning of Cu into the vapour frfiase and the presence of S in the vapour (Fig. 1). This, and other work, suggests that brine-vapour segregation of trace metals, and transport in the vapour phase, plays an important role in Cu (and Au) deposit formation (Heinrich etal., 1993).

10 TRACE ELEMENT IMAGING X-rav Enerov fkeV) A new method called Dynamic Analysis (DA) devel- Fig. 1 PIXE spectra from vapour (top) and brine (bottom) nuid incluoped at the CSIRO formulates the PDOE analysis sions in quartz from the Kidston copper-gold deposit, Queensland, anproblem as a matrix transform that can directly un- alyzed using a 3 MeV proton beam. Note the strong partitioning of mix elemental components to yield accurate major- Cu into the v ^ u r phase. and trace-element images of spatial distribution in r^-time (Ryan et a/., 1996) This technique permits the imaging of precious metal distribution in sulfides, for example, for basic studies of ore formation and as a tool for mineral processing. Fig. 2 illustrates the capability with an example of ^ace Au distribution in pyrite from the Emperor Mine, Fiji. The image shows a record dfpyrite growth and reveals a det^led correlation of Au with As incorporation in pyrite. Pb (also Mo and Sb) deposition ^ u r s in distinct episodes, and is mutually exclusive to Cu. Au ranges from 180-2000 ppm in these pyrites

r^ipn n ^ ^ ' " " P

e'e'"®"^ distribution in a pyrite grain from the ore-zooc of the Emperor Mine, Fiji extracted using^ the ^ ^ ^ ^ was used. over J ^ of 500 obtained

Bcstcn, J. den, Jamieson, D.N., and Ryan, C O., -Latticc location of gold in natural pyrite oystals", in prep.

Heinrich, C.A., Ryan, C.G., Mcmaugh, T.R, and Eadington. PJ., 1993, Economic Geology 87,1566-1583.

Ryan, C.G., Heinrich, C.A., Van Achterbergh, E., Ballhaus, C, and Memagh, T.P., 1995, NucL Instr Meth. BJ04,182-190 Ryan, C.G., Van Achteibergh, E., Jamieson, D.N., and Churms, CL., 1996, NucL Instr. Meth. 8109/110,154-160.

63

GLEN EVA - AN EPFTHERMAL Aa A Ag DEPOSFT GEOLOGY AND RESOURCE ESTIMATION

x:

MikeSeecFidWriaYwohy' 'Ross MmingN.L.VtDdMOoW Mine, PO Box 242, COHM^^ nVirem Eiplonrtion Ply LUI, 5/25 IU)bert8 Sti^

^

The den Eva deposit (also previously known as Hill 273) is located near Mt Coolon, Mme 300 km by load sooth of To\^Tisville in north Queensland.

Gold (and silver) of economic significance at Gkn Eva wasfirstdiscovered by Dominion in 1993. The outcropping quartz had first been identified as silica sinter by BHP Minerals Ltd's ^k>gists in the mid-1980*s, however their subsequent RAB drilling was unsuccessful in kwating Ae blind mineralisation. Dominion conducted programmes of geological mapping, surface geochemistry and geophysics data acquisition, md several canyaigns of drilling. The min^alisation wasfinallydrilled on 50m 50m ^aced ocntres Dominion announced aresourceof425,000 tonnes @ 5 4 g/t Au uncut in 1995. Ross Mining Ni.'sresourcedefinition drilling was undertaken on a maximum 25m by 25m infill spacing and ccra^)rised Ae drilling of 59 holes fOT 226.55 m of HQ core)fi^omJune 3 to August 8,1996.

The Au & Ag mino-alisation is part of a high level, low su^hidation (adularia-sericite) qpithermal system with a strcmg phyllic alteration envelope Silcrete with a variable diickness to 20 m overlies all other units. Par^ outcropping and shallowl} dipping sinter, with characteristic ''elephant skin'' texture and sometimes well devek)ped,fliin,sibceous laminations or banding, fonns a number of disjointed dialcedonic quartz outxmps. The mineralisation host unit comprises a sequence of texturall>' complex, felsic vokanicflowsand pynoclastics of cakalkaline dacitic to ihyolitic compositioa Fracturing and brecdation of die rocks is common. Blecdatiini is povasive ftrou^ die altered ph)11ic core and is considered to be a product of hydrothennal processes combined continued and q>isodicfiuihing.The brecciation closely miirors Ae >10% quartz vein material per metre estimates. Post mina-alisatkm, moderately poiphjTitic volcanic sills intrude the stratigraphy.

^ 4.

The Au & Ag mineralisation occurs over some 350m strike within a strcmgly altered, hydrothomal and/cH* fault brecciated central core. The core has a funnel Aaped, inwardly flared geometty ••tummg some 80m width at a SOm vertical depth and closing to some 20m width at 125m vertical depth. This core carriesflie3 main and several lesser Au & Ag mineralised lodes and narrow, weakly minwalised (>0.10 g/t Au) quartz-pyrite stodnvork veins. The kxles comprise of p>Tite banded, chalcedonic and brecciated quartz (±adularia) and varyfrom<1 to almost 8m true widdi.

They have moderate (40^ to 50°) dip to the NE in Ae i5)pCT levels (rf die s y s t ^ dq)th. Textures in die lodes and stodcwoik are chakeckxik quartz, coUofonn and d ui^ifoim banded quartz, bladed quartz and brecciated quartz. Gold averages I to 3 microns in size thou^ sooie was rq>oited in pan ooDce^ microns Grades often reach bonanza levels (>100 g/t Au) and the gold occurs as native goU, electrum and telluride.

The native gold is commonly hosted by pyrite, chakxipyrite, qihakrite and quartz. No pynliodte is presmt Silver minerals idoitified are electrum, acandiite and native silvo-. Arsoiic is anomalous oommonty occurring in jmustite and widiin pyrite but does not correUte ck>sely with gold.

Hydrothennal alteration comprises phyllic aheraticm widiin die core of the syston. This consists of die mineral assonblage sibca-pyrite-illite/sericite-leu(x)xene, and grades to a transitional potasac-fdiyilic alteration iqireseoted by sihca-K feklspar-ilhte/sericite-pyrite-feiicoxeQdixiik)^^ SiUcification is wide^read and dissoninated pyrite (1% to 5% by volume) is a c^acteristic part of die alteration assemblages. Propylitic alleratioQ is present pei^i^al to the subvertical phyllic core and is dominated by dikxite-carbonate. The alteration and accoiiq>anying quartz veins are abn9>dy boundtodie N E a strike persislcnt slnic^ Odier parallel structures may be presoit which potentially ofEset die kxks.

A manual resource estimate used parameters erf* minimum Im mtaoepL maxhnnm 2m wick iBtemal dilnfigw Ati^rity of 2.50 g/m^ top cuts of 32.2 ^ An and 93.0 g/t Ag and kmer cm (tf 1.00 gA resuhed in atotalmeasured and indicatedresourceestimate of340,000t@ 5.5 g/t An. 13.1 g/t Ag if^iidi iq)reseiits an in ground metal content of scnne 60,000 ounces of gokl and some 140,000 ounces of silver.

Acknowledpnentf; This presentation is made with die kind pennission ofRoss Mining N.L. The oottt^ uoderstandiug of the Gkn Eva deposit geotegy by previous worims, in particular Adrian Shq>herd, Dave Hewitt and Tony AMoo, tre acknowledged.

64

I I '

M E T A L L O G E N I C EVOLUTION OF THE SOUTHERN SIERRAS PAMPEANAS. ARGENTINA

'Aosto,II.„ Gel.^c.1 Surrey

G. Skltrow'.wi J,ta f. Stai' GFO B., 371. C«U,m,, A^T, A.tr.11. 2MI

dommatcd by the cmplaccmcn. of £«ctionatcd

J ^ T l ^ ^ ^ ' : ^

I ^ ' T r n i f i L n ' , r e l a u o n s h i p s in conjunction with our stable isotope studies has

from -390 to -365 Ma Based on f.. metal,ogcnic ph^e w a s ^ ^ r w ^ h ^ ^ r ^ ' "h r ' T Mesothenna, sLr-hosted A u ^ u q rlz ve^ related Au in the Candelaria dis.ric.\nd high l e v T ^ P b l o ^ a ^ de C6rdoba) are now recoenised tn hav^ • . ^

wolframite veins at Aguas de Ram6n fProv de r/irrinh«rKo

> f

m'

suggests mineialisaUon occuired ^^

cle U, Rioja). shearC>oth Prov.

sulfides ^ m quartz-

l, - ^ onoT/I^rSS^^^s"'

A" (-Ag-Pb-Zn) minenUisalion in ihe U Ctoolina dlsinc. (Prov ae formed» m csssocailon poossic, t ^ e r ^ ^ c wju, s i n dMioooe-Pliocene, i c t mera. , „ ^calcslkaJine S l j : voK^m A i S l .

REFERENCE

65

CONTROI^ ON : ^ R A D E AT THE WARRABARTY CARBONATE-HOSTED Zn-Pb PROSPECT, W.A., WITH SOME IMPLICATIONS FOR THE GRADE OF MVT DEPOSITS Stuan G Smith», CODES Key Centre, University of Tasmania, GPO Box 252-79, Hobart 7001 •Present address, d - North Ud.. cor CUuke and Alluvial Sts, Paritcs, NSW, 2870

TTie Warr^arty prospect is a large subeconomic carbonate-hosted Zn-Pb deposit in the Palaeoproterozoic ^terson Orogen of Western Australia. Detailed study of the paragenesis. textures and geochemistry of the Wa^abarty prospect has led to an understanding of the controls on sphalerite deposition and provided an expiration for the typically low Zn grade of the deposit. TTie conclusions drawn from this work may provide ms.gh^ into the distribution of Zn grade in some carbonate-hosted Zn deposits worldwide. Mineralisation at wairabarty is known only from drilling and occurs predominanUy in breccias and veins witfi lesser disseminated to massive sulphide. The main generation of sphalerite is Fe-poor and typically occurs as thin « 5 mm) rims ^ound breccia fragments or coating vein walls. Sphalerite rarely occludes porosity with several generations of dolomite and minor quartz gangue typically post dating sphalerite. Both sphalerite and dolomite have complex interna^ P^g^nesis and detailed work has documented a series of precipitation-dissolution episodes in both minerals. The Warrabarty mineralisation is Zn dominant with typical 100Zn/(Zn+Pb) values in the range 85-100minor Pb-nch mineralisation occurs within the prospect area but is paragenetically later than the Zii minerahsation and is not discussed further in this contribution. Zinc grades are generally low. typically ranging up to approximately 5% Zn, although parts of the prospect cany grades up to 40 % Zn. TTie low grade Lnes are typified by sphalente coating breccia fragments or vein walls, and show Uttle evidence for large scale carbonate rep acement by sphalerite. In contrast, higher grade zones are characterised by large degrees of carbonate replacement; high grade void-fill sphalerite mineralisation occurs only rarely.

Carbon and oxygen isotope data for gangue dolomites and original host rock dolomites show almost complete overlap on 5 O - 5 C plots indicating that the mineralising fluids did not receive significant input from sources external to the host carbonate package. Furthermore, dolomite gangue compositions show no evidence for fluid mixing trends and it is concluded that a single fluid was responsible for carbonate transport and deposition.

Temperatures estimated from fluid inclusion analyses indicate sphalerite and dolomite deposition occurred in the i ^ g e 150-200°C from highly saline (22-25.5 wt % total salts), complex NaCl-CaCl: (±Mg-Fe-K) bearing brines Thermodynamic modelling indicates that a fluid of this type is capable of transporting lO's to lOO's ppm Zn and r^uced S together at a pH close to dolomite saturation. Under such conditions sphalerite precipiution is ettectively accomplished by increasing pH attending dolomite dissolution, as described by a coupled dolomite dissolution-sphalerite precipitation reaction: 2Zn2+ + 2H2S + CaMg(C03) 2 <=> 2ZnS -h Ca2+ + Mg2+ + 2CO2 +2H2O (1) Precipitation of sphalerite at Warrabarty by reaction 1 is consistent with the documented textures and paragenesis of Ae Warrabarty mineralisauon and is interpreted to indicate that sphalerite precipiution occurred only when Zn-+, HjS-bearing fluids had access to the wall-rock. Large zones of void-fill sphalerite do not occur at Warrabarty, as would be expected if deposition was caused by fluid mixing. Reaction 1 will proceed only if H+ generated is consumed on site and the reaction producte (except sphalerite) are removed by the solution. If H+ is lost to the solution, or the products remain at the dissolution site, the reaction reverses causing dolomite precipitation and sphalente dissolution. Therefore sphalerite precipitation by reaction 1 is a relatively inefficient process, and it is interpreted that this fundamental inefficiency is the cause of the low overall grade of the Warrabarty prospect. Continued progress of reaction 1 to the right is only likely to have occuned in exceptional circumstances, represented at Wanabarty by the rare zones of wholesale carbonate replacement and high Zngrade.

Low grade MVT deposits occur throughout the worid and many (eg M-J District, East Tennessee. 2-4% Zn) are typified by mineralisation which occurs as thin sphalerite coatings on vein walls and breccia fragments; textures similar to those typical of Wanabarty. In contrast, a number of higher grade MVT deposits occur (eg. Nanisivik Canada, 14% Zn; San Vincente. Pern. 10% Zn; Cadjebut. Western Australia, 14% Zn) and typically occur as stratabound tabular massive sulphide bodies. The textural and grade similarities between the low grade MVT class and typical Wanabarty mineralisation suggest that they fonned from a similar, inefficient wall-rock reaction mechanism. The higher grade class of MVT deposits may have fonned from fundamentally diff^erent processes, involving a more efficient depositional mechanism, such as fluid mixing.

Acknowledgments: The financial support of WMC for this project is gratefully acknowledged.

66

RE-OS AND SM-ND ISOT^ ON THE GENESIS OF MAGMATIC ORES IN THE PALAEOPROTEROZOIC PANTON INTRUSION, EAST KIMBERLEY > c J Rcfrccca SprOBlff'. David Umbert' and Dean Hoatson^ Victonan Imutute of E ^ md P l a ^ ^ c e . Depaitmeat of Earth Sdcnces, Mooash University. Oayton. VIC 3168 Australian GetrfopcaJ Sunrey Oiganisation, Cro Box 378, Canberra, ACT 2601 The Halls Creek Mobile Zone (HCMZ) was intruded by at least 46 Palaeoproterozoic mafic-ultnimafic intrusions in STnTS'l'nP" ^ Ma (Page « a/.. 1995; Tnidu & Hoatson. W ^ T w ^ A ^ t ^ . m i n e ^ b o n comdore are observed in the HCMZ: an older and shallower northeast-trending Q PGEm Z SSi ^ ^ Ni-Cu-Co ± PGE comdor. including S l y f •• ^ ^ ^ Sm-Nd isotopic data in order to set better i S t^^n c ^ ^ ^ f ^ ^ formation within the Panton intrusion, an elongate mafic-ultramafic body 11 2.5 km. TTiis intrusion consists of a lower. rhy5m2cdlya y e ^ , ultramafic senes ^ S : -^50 m thick) and an upper mafic series (MS: -900 m thick) and f s K l P w;re f ^^ ^ ^ mineralisation (originally in m a s t i c s u l p w S S X v*^ remobibsed dunng metamoiphism) is concentrated in chromitite seams wWch were foS^S Iwowc/m ^ ^ • ^ ^ A -^20 m below the UMS - MS contact, is the SSdoi 1k ^ ^ PP'" -3.7 ppm Pd, -105 ppb Os). As S e pSitoS ^ IZhZ H throughout Ae entire stratigraphic sequence. parenJal magmas to Ae iniusion may tovJ been sulphide-saturated prior to entermg the magma chamber and thus had entrained sulphide proto-ore. and below the A chromitite scam define a "model 3" isochron (Mclntyre et al., 1966) w th an age of 1850 ± 40 Ma, within error of the SHRIMP U-Pb age, and an initial Vn value of +10 fro™ chondritic asthenospllric m a n T K e ^ ^ e age) s i Nd isotopic data also yield an isochron with an age of 1860 ± 36 Ma and i i initial £Nd value of - 0 7 wWch" much less ^ o g e n i c than depleted mantle at the time of intrusion (eNd = +5). Systemic chsmgwln n i S Vo w ^ ^ ' J . r . t ' . r - t h a t mixing of was not a f ^ t o r i n chromitite genesis. Several interpretations are consistent with the Os and Nd isotonic E crustally-contaminated asthenospheric or lithospheric mantie S t ^ o ^ S ^ S S two^omponent mixing suggests that -15% contamination of a basaltic mantle melt wiA Archaean crust can yield a Yqs value of+10. However, R-factor (= effective mass of s i L ^ ^ t with which a given mass of plphide melt has equilibrated), an important g e o d y n a i S ^ ^ t ^ in m a S t i c ^ systems is not routinely mcluded in isotopic modelling of sulphi^ore gineshi ( F o s ^ T T , ^ c i a l for radiogenic isotopes that have a strong affinity (D » 1) for magmatic s u ^ d e Uquid e . g T X and Pb) Thus, crustal c o n ^ n a t i o n processes are more difficult to assess using Rt-Ch isotows ^ the e ^ f nf c o n ^ n a u o n can be masked by an R-f^tor process in which sulphide melt IjuTlfbSes S ^ e x t c ^ s i ^ e a ^ o S of chon(taUc asthenospheric mantle melt. Calculations based on PGE data from the chiUed m a r e i S ^ m ^ ? ; to high R-f^tors (6000 ioUOOO) for Panton. suggesting that sulphide meU h ^ e S J ^ I v S r S S v S S S l ' S ^ the parental silicate melt, nierefore, the Os isotopic data require cHistal contamination to live S S n f . r " ^ y have been much W g h e T f f S o implying a larger d e ^ of crustal contamination prior to the R-factor process. Tlius. we envisage "multi-steee mc^el in which sulphide-undersaturated basaltic melts assimilated AichaLi lower-middle ^ t S ^ u ^ T c n S ^ to tiie upper crust, promoting sulphide-saturation. During the passage to the Panton n w g ^ c h S ^ r ^ S S sulphide protoK,re scavenged the PGE from the host silicL m e ^ u l t i n g in i o T m S R - f S T S X ^ ^ c h new p u l s ^ f pnmitive magma e ^ the chamber, mixing with resident magma at R-f«tor i n c i S ' t t e TOE tenor (= PGE concentration in 100% sulphide) of the sulphide melt droplets and txiggfred c S t e f S S i ^ km) compared to Ni-Cu-Co bearmg intrusions (20-24 km) in the HCMZ (Tnidu & H o S n 1996) ff T e S n ^ h ^ ' ^ ' i W ^ ' ^ ' ^ ' ' ® " o c c p e d at^pth, the shallow level of final'sulphide a S S L n may help Rm!^'^ ^ sulphides in the Panton and other simil^ intrusions ( e T s S a T e f Bushveld) in which a near-surface and lai^er magma chamber promoted higher R - f S Henci d S h of REFERENCES Foster, J.G., I ^ b e r t , D.D., Frick, L.R., & Maas. R.. 1996. Re-Os isotopic evidence for eenesis of Archaean nickel ores from uncontaminated komatiitcs: Nature. 382,703-706 cvwcnce lor genesis ot Archaean

Agtawwlcdgfmfnfs: Financial assistance and permission to publish were kindly provided by HeUx Resources NL.

67

POROSITY, PERMEABILITY AND PORE PRESSURE VARIATION IN DEFORMING ROCKS fhaodre UlAoa & Abacn Ord Aiub^OimGeodynankzQxjpentiw .

CSIROEjq)loradaiaDdldfcing,POBax437,Nedl^ WA6009. AUSTKAUA Knowledge of the relationships between cnistal porosity, permestoiUty and pore pressure is crucial to our understanding of crustal fluid flow and fluid-nx^ interactions in defiorming rocks. The pressure or steence of a fluid, as well as the magnitude of the pore pressure, stongly influences the mechanisms by vMch crustal rocks deform. In addition, movement of fluid through the crust has a significant influence on heat flow, on the distribution of oxygen and hydrqgen isotopes, and on the development of ore dspoeits.

Mechanisms which changes to porosity may occur include metamoiphic vdume diange, microcraddng with subsequent healing and scaling, dissolution, including pressure solution, and precipitation of solids and poroelastic effects (Upton et at. 1995; Ord & Oliver in press). As porosity changes, so wiU p e r m e ^ t y and pore pressure.

Porosity and/or permeability may decrease with time and the resultant pore pressure will be a product of two interrelated effects; the gradual expulsion of water out of the decreasing pore space and build up of pore pressure within the remaining pore space; the relative inqxwtance of each being a fmcdcm of the permeability.

For porosity reduction rates of greater than 10*" s ^ significant excess pressures are generated for rocks with permeability of c. 5x10"^® m^ The elevated pore pressures m ^ lead to brittle failure and qnsodic loss of fluid from the overpressured regions. Such fractwing. if associated with crack healing or sealing, should slow but not eliminate the process of pore pressure build up with porosity reduction (Walder & Nur, 1984).

Porosity and/or permeabaity may increase with time. This is the likely result of deformation tAitig jjace in a plastically dilatant material (Upton et al 1995, Ord & CMiver in press). Dilatancy, which is defined as the inelastic dilatation relative to the shear strain wtoch occurs during deformation, has been proposed as a mechanism by wiiich fluid flow m ^ be enhanced in rocks of low permestolity.

We present a series of coupled mechanical/fluid flow numerical models designed to investigate the interdependence of crustal porosity, permeability and pore pressure. WE also add the dissolution and precipitation of quartz, with its associated influences on porosity and permesOrility, during the defonnation process. These studies define the general processes that focusfluidflowinto shear zones, &ult systems and the hinges of antiforms.

Understanding the nature of crustal fluid flow and associated processes, such as veinfiwmatiQn,ore deposition, and basin dewatering is inseparable from and understandi^ of the nature of porosity, permesinlity, pore pressure and their variation with time and space. Numerical modelling allows us to investigate the e f l ^ of various models of porosity, permeability and pore pressure regimes on a large number of geological scenarios.

Temporal and spatial variations in porosity, p e r m e ^ t y and pore pressure must play a significant role in determining where and when fluid focusing along shear zxmes will occur, vAien ore deposits will form, wiiere hydrocaibons will migrate and be c^tored and m ^ play a role in the control of wiien and wliere earthquake rupture will occur. Further understanding of the nature of the earth's hydraulics wiU add to the understanding of many crustal processes.

REFERENCES Ore, A & Oliver, N Mechanical controls on fluid flow during r^onal metamonriiism: Scmie numerical models. Journal ofMetamorphic Geology^ In Press Upton, P., Kooks, P. O. & Chamberlain, C.P., 1995, Ptaetration of deformationKlriven meteoric water into ductile rocks: isotopic and motel observations from the Southern Alps, New ^^land. New Zealand Journal of Geology and Geophysics, 38, 353-543.

Walder, J. and Nur, A., 1984, Pdrosity reduction and cnistal pore pressure development Journal of Geophysical Research. 89, 11539-11548.

68

LATE ARCHAEAN DEFORMATION AND GOLD MINERALISATION, THE ARCHAEAN YILGARN CRATON: AN OVERVIEW Julian Veamcombe Veamcombe & Associates Pty Ltd. 14A Bamett St. Fremantle WA 6160

INTRODUCTION The formation of giant hydrothermal ore deposits involves the migration of large volumes of fluid from a source region to a site of deposition via discrete channel-ways in the earth^s crust. For Archaean-lode gold, isotope data which imply a spectrum of fluid sources including deep crustal or mantle components, have been used to suggest that craton-scale shear zones represent the channel-ways between source and final deposition site. In this presentation we test critically this model with a review of shear zone, fault and fracture controls on gold mineralisation in the Archaean Yilgam Craton.

DUCTILE SHEAR ZONES The structure of the Yilgam Craton is dominated by greenstone belt-scale conjugate ductile shear zones, mosdy de\ eloped on granite-greenstone margins and with significant flattening strains. However, these shear zones do not host gold deposits and lack isotopic evidence of the gold mineralising fluids. The shear zones comprise 140®to 160°-trending sinistral and 000°- to 030°-trending dextral zones, in a conjugate pair with the maximum compression direction about 080®, on the obtuse angle during ductile deformation. Previous interpretations of the shear zones, as part of a linked system, as craton-scale terrane boundaries, and as pathways for gold mineralising fluids, ^pear equivocal.

LATE ARCHAEAN FAULTS AND FRACTURES Late Archaean brittle conjugate faulting, with 010®- to 050®-trending dextral faults, and 090®- to 130®-trending sinistral faults has the maximum compression direction remaining about 080®, but on the acute angle. The spatial distribution of economic gold deposits in the Norseman-Wiluna Belt shows the regional shear zone direction of 170®- to 150®-trends to be the strongest control, but with about 045® and 120®-trending relationships as well. Gold mineralisation, is structurally late, although in some cases before peak thermal metamorphism accounting for the high metamorphic grade of some deposits. From deposit- to vein-scale, gold occurs in laminated and otlier schistosity-parallel quartz veins along second and third order ductile shear zones where intersected by brittle cross-faults and fractures, and as high-angle quartz veins along the late Archaean britde conjugate faults.

GEOMECHANICS OF GOLD MINERALISATION The geomechanics of gold mineralisation will be illustrated with spedal reference to Bronzewing gold mine in die Vandal Belt, but the principals are of universal application. Veining and gold mineralisation in the ductile shear zones were a result of reactivation at the time of conjugate faulting. Dilatational shear failure along the conjugate cross-faults occurred with the normal stress On negative. Reactivation of the pre-existing weakness along the ductile shear zones is related to a stress state giving rise to tensional veins parallel to foliation and near perpendicular to the maximum compression o i This is achieved by supralithostatic fluid pressures shifting the Mohr circle to the left (03 and o] are negative) and by a reduction in the differential stress by stress relief.

Thus a sequential model is developed of increasing fluid pressure and decreasing mean stress as conjugate faults develop, dilate and then reactivate pre-existing weaknesses such as the ductile shear zones. This last phase appears to be responsible for the higher gold grades and larger tonnages of gold.

CONCLUSIONS Models for gold mineralisation which imply deep fluid sources and fluid focussing in o-aton-scale shear zones appear equivocal. Fluid flow was focused into sites of low mean stress, determined by the orientation of lithological units and pre-existing shear zones relative to the late Archaean stress directions, in a late Archaean fault-mesh of conjugate fractures. Significantly, the recognition of these mid-crustal processes lends support to the metamorphic model for gold mineralisation.

Acknowledgments: To Great Central Mines Ltd for support and permission to present the results of this research.

69

CONTROLS ON HYDROTHERMAL MINERALISATION: THE BRX PRECIOUS METALS PROSPECT, ARIZONA, USA Susan Veamcombel Lc Furlong^ Robert KerridP, Julian Veamcombel 1 Veamcombe & Assodates Pty Ltd, 14A Bamett Street. Firaiantle WA 6160 2 International Precious Metals Corp. Suite J-1.4625 South. Ash Ave, Tcmpe. AZ 85282. USA 3 Department of Geological Sdences. University of Saskatdiewan. Saskatoon. Canada INTRODUCTION Well known as a porphyry copper province. Arizona is also host to significant hydrothermal mineralisation, although controls on this style of mineralisation are pooriy understood. In this presentation we describe hydrothermal gold with devated platinum group dements (PGEs) in felsic volcanic-hosted quartz vdn stockworks. and pyrite alteration in silty sedimentary rocks from the BRX exploration project, 92km west of Phoenix. The BRX anomalies occur at a break in mountains between the Tertiary southern Little Harquahala Mountains and the Eagletail Mountains.

CHANGES IN VOLCANIC AND TECTONIC STYLE BETWEEN THE SOUTHERN LITTLE HARQUAHALA MOUNTAINS AND EAGLETAIL MOUNTAINS The BRX prospect is located on the southern limit of deformation of the Jurassic fold and thrust deformation of the Maria Belt, in the Basin and Range Province of mid-Totiary extension deformation.

Changes occur between northeast-dipping listric faults with large displacements linked to recumbent detachments favouring exposure of mid-crustal rocks in the Little Harquahala Mountains, and the Eagletail Mountains, in which steeper southwest-dipping partly listric faults dismember the upper crustal rocks, and lack exposure of the mid-crustal rocks. The southern Little Harquahala Mountains comprises locally mylonitic granites unconformably overlain by immature sediments, unconformably capped by a volcanic basalt unit. The Eagletail Mountains comprise spherulitic dadte-ihyolite volcanic rocks and viirophyre. Between the two mountain ranges, a transitional domain comprises intrusive porphyr) with quartzites, a siltstone-sandstone sequence and felsic volcanic roc^s. Within the transitional domain is a transfer zone about 5km wide with intrusive porphyries, structures trending about 060® to 080°, and stockwork mineralisation. The sediment host^ pyrite-related mineralisation and tlie Eagletail stockwork are on the margin of this transfer structure. The T)asement'' granites of the southern Little Harquahala Mountains and intrusive porphyry have identical parent chemistry, and are, therefore, of similar age, about mid-Tertiar> . The REE and extended trace clement chemistry of porphyr), granites and volcanic rocks suggest a genetic and comagmatic relationship. The host volcanic rocks at Stockwork Hill are chemically related to rocks of the Eagletail Mountains.

Gra\ ity and magnetic data strongly support the sub-division of the area into a series of structural domains separated by northeast-trending transfer structures. In addition, dianges in the wavelengtlis of anomalies imply vertical displacements on northwest-trending extension faults.

MINERALISATION Four styles of mineralisation are recognised. These are (i) quartz stockwork mineralisation of mid- to late-Tertiar> age, with hydrothermal brecdation, dialcedony, aiistifomi and codcade quartz veining, and adularia-seridte alteration diaracteristic of dassic low sulfidation-style epithermal systems; (ii) quartzcopper veins commonly in granites, but also in the siltstone-sandstone unit; (iii) gold-PGE in siltstone-sandstone rocks with pyrite and fine microscopic quartz veining; (iv) dluvial gold-PGE mineralisation which represents the prindpal economic resource to date.

TRANSFER ZONES AS A CONTROL ON MINERALISATION Mid-Tertiar> extension in Arizona was directed to about 060®, and transfer zones would be similarly oriented. We are aware of no clear statement in die literature that known tectonic lineaments such as the Holbrook and Bright Angel lines, or that the 060®-trending lines of both porphyry copper and hydrothermal vein-type mineralisation are transfer faults. However, 060® appears to be a very important direction.

CONCLUSIONS The BRX prospect is located between the 160®-tr«Kling southern Little Harquahala Mountains and the 140®-trending Eagletail Mountains, in the Basin and Range Province, Arizona. The style of midTertiar} extension deformation changes between listric faults with large displacements linked to recumbent detachments with exposure of mid-crustal rocks in the southern Little Harquahala Mountains, and, in the Eagletail Mountains, steeper partly listric faults which dismembered the upper cnxstal rocks, but lack exposure of the mid-crustal rocks. Between these two styles of mid-Tertiarj' extension is a transfer zone up to 5km wide comprising structures which trend between about 060® and 080®. Mineralisation at the BRX prospect is located within and on the margins of the U^sfer zone.

Acknowledgments To IPM for support and permission to present the results of this research.

70

PRECIOUS METAL ABUNDANCES IN ORDOVICIAN MAGMAS OF THE BATHURST 1:250K SHEET AREA - GUIDES TO MINERALISATION POTENTIAL

David A. WaUace' and Doooc Wybom ^ ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 'Australian National Universit>', Canberra, ACT 0200

It is well established that late Ordovician magmatism in central New South Wales is the source of world-class porphyry copper-gold mineralisation. The widespread occurrence and the unusual character of this magmatism, established by recent geophysical surveys and mapping programs, further highlighted their precious metal potential. Modelling suggests that high volumes of magma were produced as a result of source metasomatism in a mantle environment which was low in sulphur but rrtained normal abundances of precious metals and copper. These conditions favoured development of large upper-cnistal magma chambers which promoted strong fractional crystallisation and generated highly oxidised K-rich sulphur-undersaturated shoshonitic magmas, characteristically high in incompatible elements and water. Intrusive complexes and volcanics derived from these magmas host important Cu-Au deposits in the Parkes-Narromine and OrangeWellington belts, and the Fifield-Nyngan belt contains Alaskan-type mafic-ultramafic intrusions enriched in platinum. These areas are currently the main focus of e?q)loration activity. During joint AGSO-Geological Survey of New South Wales geological mapping of the Bathurst 1:250 000 Sheet area as part of the National Geoscience Mapping Accord a geochemical study of the of the Ordovician volcanics of the area was undertaken.

Most units are composed of shoshonitic mafic extrusive rocks and derived volcanogenic sedimentary rocks emplaced in the Late Ordovician. They appear to be transitional to and overlie the widespread quartz-rich turbidites of the Early Ordovician Adaminaby Group, Elsewhere in the eastern part of the Lachlan Fold Belt, they correlate with volcanics in the Fifield-Nyngan, Parkes-Narromine belts, and with the Kiandra Volcanics in southern NSW. Nd-isotope systematics on basalt samples yielded eNd values of+5 to 8, typical of Ordo\ ician shoshonite derived from a long-term light-rare-earth-element- (LREE-) depleted mantle. The basalts have high K2O,averaging 2.5wl%, and K20:Na20 ratios averaging 1. The basaltic rocks are also characterised by high LILE concentraUons (Ba up to 5000 ppm) and P2O5 (up to 0.31%), and low high-fieldstrength elements (Ti02 <0.6%, Nb <10 ppm, Zr <50 ppm, Y <15 ppm) and LREEs (La 3-8 ppm; Ce 6-19 ppm) Of parUcular interest are the high MgO basalts and possible ultramafic lavas from the Rockley Volcanics and B>Tig Volcanics.

The Ordovician volcanics have Pt, Pd, and Au abundances (Wybom 1990: BMR Research Newsletter). [Pt + Pd] abundances range up to 20 ppb, while Au ranges from 2-5ppb in mafic volcanics and volcanic derived sedimentary rocks. The use of MgO as an index of fractionation demonstrates that Pd and Au systematically increase with fractionation, and that Pt abundances increase slightly from peridotite to pyroxenite, then fall appreciably with decreasing MgO; thus, Pt and Pd are inversely related during the pyroxenite-basalt stage of fractionation. [Pt + Pd] increases with fractionation to about 20 ppb at MgO -20 per cent, then levels out as compositions become more basaltic.

Pt.Pd: Au ratios derived from interpolated trends in XY plots for various rock compositions in the peridotite-tobasalt assemblage change systematically from 14:5:1 at MgO -35% through to 2:5:1 at MgO -8%. These trends show that the Pd:Au ratio is constant throughout the evoluUon of the suite, and that there is an overall sevenfold decrease in Pt in the system as it becomes less magnesian. PGE levels and ratios in the volcanolithic samples are consistent with those in the primary magmatic rocks with the same MgO suggesting that the sedimentar> and depositional processes which occurred during the history of these rocks had minor significance in mobilising PGE or changing their inherited PGE. The elevated PGE abundances in the Ordmician volcanics in the Bathurst Sheet area, and their pattern of variaUon with fracUonaUon, is consistent with other cvidcnce that they were S-undersaturated during their early history, and thus retained PGEs in their melts. Pt:Pd: Au ratios of these rocks are also consistent with expected magmatic ratios pertaining to S-undersaturated conditions for Ordovician shoshonites elsewhere in the Lachlan Fold Belt (Wybom & Sun 1994: AGSO Research Newsletter 21,7-^). Thefractionationhistory of the mafic-ultramafic rocks suggests that sulphur depletion continued to be a factor in determining PGE and copper concentration throughout the later stages of the evolution of the magmas, which in turn has a major influence on the potential for the development of epigenetic coppcr and gold mineralisation.

71

STRUCTURAL CONTROL OF THE BROWNS CREEK GOLD-COPPER SKARN DEPOSIT, BLAYNEY, NSW Colin Wiikins^ ^ Department of Geology & Geophysics, Sydney University, NSW 2006.

The Browns Creek gold-copper underground mine is located 8km west of Blayney, 220 km west of Sydney in central western NSW on the Bathurst (SI 55-8) 1:250 000 scale geological map. Gold-copper mineralisation occurs within skamed marble and mafic volcanics at their western contact with a major granodiorite intrusion.

Massive garnet, pyroxene and wollastonite dominated skams contain disseminated copper sulphides (chalcopyrite, bomite) and gold. Prior to Hargraves Resources' acquisition of the property in November 1993, the Browns Creek mine had produced in excess of 2.6 million tonnes of ore with 7075 kg of gold, 8000 kg of silver and 96001 of copper recovered. First worked in 1871 the mine had more recently been operated by M. Hickey (minor open cut and underground operation), and then BHP Gold and Newcrest Mining via c ^ n pit mining. The Browns Creek total mineable resource (measured + indicated + inferred) at 30 June 1996 comprised 1.36 million tonnes at 6.10 g/t gold and 0.44% copper. Underground mining to date has been located in the upper 150 m of a southerly plunging ore zone. The mine resource continues to tfie south and at depth and current mine development is progressing into the deeper southern ore zones.

The ore body had an east-west trend in the open pit but thinned towards the base of the pit as it swung into a southerly orientation adjacent to the Mt. David Fault Zone (the eastern margin of the open pit mineralization).

As the ore body thinned with depth, open pit mining ceased and the venture was acquired by Hargraves Resources which undertook a programme of deep exploration drilling between 1993-1994 and successfiilly delineated a new north-south trending orebody, the Cowriga Lode, beneath the previous open pit. The Cowriga Lode, in the Hargraves Resources underground operation, consists of structurally controlled copper-gold bearing skams and gold-rich retrograde skams located between multiple strands of the 4{X)0E Fault Zone which propagated at the contact between granodiorite and the Blayney Volcanics / Cowriga Limestone country rock sequence . The structural complexity of the ore body became apparent when continuous exposures were available for miapping prior to stoping. Banding within mart>les, in the hangingwall to mineralised skams, was not relict bedding but an intense foliation in calcite mylonites developed within the 4(KX)E Fault Zone.

Dextral transcurrent motion on the 40(X)E Fault Zone (equivalent to the Mt. David Fault Zone at surface) produced vertical anastomosing fault duplex systems that controlled the location and formation of metasomatic prograde wollastonite-gamet-pyroxene-plagioclase exoskam in both marble and homfelsed basaltic volcanics.

Fault controlled marble-derived and basalt-derived skarns were the locus for the first stage of copper-gold mineralisation (pyrite-pyrrtiotite-arsenopyrite-chalcopyrite-bomite-chalcocite-gold-tellurides) and retrograde gangue (chlorite-sericite-quartz-calcite-epidote-biotite-homblende) shortly after prograde skam formation as temperatures began to fall. Movement on the 40(X)E Fault Zone, a dextral transcurrent fault system (with gently plunging slickenside lineations on fault surfaces) controlled the prograde and copper-gold skams and then switched to dextral transtensional dip-slip movement (with steep slickenside lineations on fault surfaces) forming extensional duplexes at offsets and overlaps of fault strands. Transtensional movement along duplex fault slices caused brittle failure and the formation of large volumes of extensional vein arrays preferrentially hosted in pyroxene-gamet basalt-derived skam units. These structurally controlled sheeted quartz vein arrays host lower temperature retrograde ore skarn assemblages (quartz-calcite-epidote-prehnite-chlorite-sericite-gold-chalcopyrite-bomite) characterised by high, and locally bonanza, gold grades (up to 40 g/t). Post-mineralization retrogression (calcitequartz-chlorite-sericite), and the intrusion of monzonite sills and dykes are ubiquitous. Prominent east and west dipping low angle conjugate fault sets are the final phase of deformation recognised in the underground mine and substantially increase the structural complexity of the deposit.

Inspection of a geological level plans illustrates the main structural features of the ore body which is developed in Blayney Volcanics and Cowriga Limestone, bounded to tfie west by the Carcoar Granodiorite and to the east by the 4000E Fault Zone. Hydrothermal fluid flow was guided by the permeability of fault netwoite and caused heterogeneous skam formation and mineralisation in alteration zones up to 40 m in width. Skam reaction fronts, for example those affecting marble, are clearly fault controlled. The disposition of Blayney Volcanics, Cowriga Limestone and tlieir skam derivatives are controlled by dextral strike-slip duplexes at a major bend in the irregular granodiorite contact that imposed a fundamental boundary constraint during the propagation of fault strands. The extensional form of the fault duplex also controlled the intmsion of post-ore monzonite bodies that cut and invade sheeted quartz vein ore and contain rotated blocks of ore.

A genetic model for the Browns Creek orebody involves a primary late stage intrusive-related hydrothermal fluid focussed into permeable fault zones that propagated adjacent to pluton margins, with fluid mixing and bimetasomatism during skam formation tending to obscure a purely magmatic signature for the mineralisation.

72

THE POSSIBILITY OF USING PETROLEUM EXPLORATION THERMAL MATURITY TOOLS IN MINERAL EXPLORATION Ronald W.T.Wilkins CSIRO Division of Petroleum Resources, P. O. Box 136, North Ryde, NSW 2113

Organic matter is extremely widespread in sediments and it undergoes irreversible changes under thermal stress that have b«n mtensively studied and extensively applied to thermal maturity assessment in petroleum exploration. Thermal matunty tools are relative geothennometers designed to give mformatlon on the degree of generation of petroleum from organic matter which takes place broadly withm the range 50-150 "C However several of the thermal maturity tools such as those based on the reflectance of vitrinite, and changes in the colom of spores and conodonts, continue to provide palaeothermal information on itKks that have been exposed to temp^atures of up to 500 »C. The calibration of these thermal maturity tools as geothennometers is complicated by differences m rates of alteration due to variation in initial chemical composition of organic matter even within narrowly defined maceral or biological groups. This problem restricts the time range of application of these techniques and m addition there is a particular problem in dealing with pre-land plant organic matter that is from Silurian and older rocks. Nevertheless, several independent calibrations have been accomplished of the vitrmite reflectance geothermometer and the technique has several benefits including simplicity and widely available instrumentation. Although there remains some disagreement about the possible significance of duration of heatmg in organic matter geothermometry (Barker and Pawlewicz, 1994), this should not affect the application of vitrinite reflectance in palaeothermal moping.

The Raman spectra of organic matter can also be used as a thermal maturity tool in petroleum exploration however the major changes in the spectra occur in the region of low grade metamoiphism rather than in the zone of catagenesis. The technique has been calibrated against palaeotemperatures derived from oxygen isotope geothermometry in the range 200 - 450 »C to provide a Raman carbon geothennometer for organic matter in low grade metasediments (Hu and Wilkins, 1992). Tliis geothermometer has the benefit that within this temperature range, ^ e effect of organic matter type is small, thus it appears that it may be applicable not only to land plant macerals but also to the organic matter of Lower Palaeozoic and Proterozoic rocks.

Organic geothennometers may assist in two aspects of a mineral exploration program: (a) A wide variety of ore deposits were formed in environments where temperatures during deposition were elevated with respect to regional temperatures. The evidence of palaeothermal effects may extend far beyond the easily recognisable zone of metasomatic alteration of the host rocks because the hot fluids may be nearly m chemical equilibrium with the rocks tiirough which they pass. Hie thermal effects of the Bramsche Massif, a 5 km deep igneous intrusion in the Lower Saxony Basin of N. W. Germany can be detected from maps of values of thermal maturity indicators derived from organic matter in surface sediments (Bartenstein ct al., 1971). (b)Many models of ore genesis, as well as the possibility of metamoiphic overprinting, infer temperature relationships between ore bodies and host rocks. Although orebody palaeotemperatures can often be determmed from fluid inclusion and isotopic studies, the palaeotemperatures of the host rocks may be poorly known, due to a lack of suitable high resolution geothennometers for the range 200-400 'C Organic matter geotheimometry may answer questions which often remain as to whether measured orebody palaeotemperatures belong to the period of ore fonnation or are a result of metamorphic overprinting.

REFERENCES Barker, C. E. & Pawlewicz, M. J. 1994 Calculation of vitrinite reflectance from thennal histories and peak temperatures. A comparison of methods. In Mukhopadhyay, P. K. and Dow, W. G. eds. Vitrmite ReflectarKe as a Maturity Parameter, pp. 216-229. American Chemical Society, Washington.

Bartenstein, H., Teichmuller, M. & Teichmuller, R. 1971 Die Umwandlung der Organischer Substance im Dach des Bramsche Massivs. Fortschritte in der Geologie von Rheinland und Westfalen 18, 501-538.

Hu, K. & Wilkins, R. W. T. 1992 A laser Raman carbon geothennometer - a new method of detenninine palaeotemperature. Chinese Sciences Bulletin 37,1302-1305.

73

ALTERATION IN BROKEN HILL-TWE DEPOSITS: EVIDENCE FROM CANNINGTON

mineral and metal zonalion and cl the ™sunctive he-Mn-Ca -nch lock types, b) the patterns of (Chapman, 1994; Bodon T ? ; ^ R ^ e t o o K a f l ^ ^ =" "" "'P®" metamorphosed tetetj^lc a d S Ca«"l"8<on is hosted by a other d l T l ^ ^ supergroup") which also contains a number of at Broken Hill in New ^ o X S s S H t ^ l ^ ^ S S l ^ t T ^iant deposit

deposit is essentially epigenetic and coeval with J c u ^ u - f t systems REFERENCES Bodon, S.B., 1996. Paragenetic relationships at the Cannington Ag-Pb-Zn deno«. Mt ico i iQueensland. James Cook University EGRU ConrrituriLse'est^lS^ "" Chapman L.H., 1994. Ag-Pb-Zn mineralizaUon hosted by pyroxene-ovroxenoiH o»r„.. ..

Cannington deposit. Cloncurry district. Queensland.

Annual Meeting Abstracts with Programs, A-379-380 ^ ^ America 1994 Richmond J.M., Chapman. L.H., and Williams. PJ.. 19%. Two phases of garnet alteration at th. r , • .

Strain ? 1993. Gold mineralization in the at the Cannington Ae-Pb-Zn d«.nn«it strain, H.. r.

UnpubIishedBScHonoursthesis.Townsvine,JamesCru.?veL^ ^V^lters. S.G and Bailey. A.. 1996. Geology and mineralisation of the Cannington Ag-Pb-Zn d^no^'t example of Broken Hill type mineralisation in the Eastern SuccessioHf ^ e M o ^ M .

M!^ Queensland. James Cook University EGRU Contributions Seru^l^ ull^^^^ '

74

ORIGIN OF COj-RICH FLUID INCLUSIONS IN SYNOROGENIC VEINS FROM THE EASTERN MOUNT ISA FOLD BELT, AUSTRALIA Guojian Xu Department of Earth Sciences, James Cock Umvcrsity. TownsviUe, Qld. 4811 Fluid inclusions of CO2 without a visible H2O phase are commonly found in the synorogenic veins from the Prolerozoic Eastern Mount Isa Fold Belt Although the local presence of carbonate minerals suggests a COrrich fluid may have been responsible for the vein formation, the occurrence of predominantly pure to nearly pure CO2 inclusions is nevertheless unusual for a hydrothermal fluid systenL Many studies of similar C02-rich fluid inclusions, mainly in metamorphic rocks, proposed preferential leakage of H2O from H2O-CO2 inclusions after entrapment. In this study however, it is proposed that phase separation of low to moderate salinity C02-rich hydrothermal fluids led to the selective entrapment of the CO2. Considering the development of similar C02-rich fluid inclusions in the mineral deposits in this region as documented in literature (such as Dugald River Zn-PbAg and Osborne Cu-Au), the fluid immiscibility, where volatile species such as CO2, CH4 and H2S are lost from the fluid system is capable of triggering ore deposition by increasing the fluid pH. Evidence used to support the presence of fluid immiscibility is mainly derived from the observations of coexisting H20-rich and C02-rich inclusions along the same trail and widely varying CO2 contents for apparently the same generation. In addition, these two unmixed fluids are also found on adjacent fractures vAitrt monophase C02-rich inclusions are closely related to H20-rich inclusions. The unmixing of COrrich saline fluids is further revealed by the intimate association of halite-bearing multiphase inclusions with monophase or two phase CO2rich inclusions in group or along array. r t

The veins investigated in this study are generally parallel to the axial plane cleavage of the regional N-S trending folds. The minerals are predominately quartz, with variable amounts of albite, K-feldspar, calcite, hematite and local presence of garnet, staurolite, andalusite and kyanite. Field evidence and fluid inclusion work indicate that vein formation mostly occurred during a decompressional regime following the peak metamorphism and regional D2 deformation. In quartz grains, at least three episodes of immiscibility events are detected, each documented by the presence of a C02-rich and a H20-rich inclusion generation. Homogenization of liquid plus vapor to liquid occurs between -11 and +30 °C and correspond to CO2 densities of 1.0 to 0.6 g/cm^. In general, there appears to be no correlation between homogenization temperature and inclusion size, but in some cases small inclusions tend to show higher homogenization temperature than large ones. The presence of nearly identical densities in single trails is a common feature in the samples involved, suggesting CO2 inclusions with variable densities must have been trapped at different stages during the retrograde path.

Melting temperatures of solid CO2 range from -56.6 to -60.6 ®C and show a slight asymmetric distribution toward temperatures below the triple point of pure CO2, indicating the presence of minor CH4 along with CO2 in the inclusions, as was further confirmed by Laser Raman spectroscopy. However, a wide range of homogenization temperatures are recorded by pure CO2 inclusions, which implies homogenization temperature is predominately controlled by CO2 fluid density rather than any dissolved species.

The reason for fluid inmiiscibility is possibly a decrease in pressure and temperature as result of crustal uplift. A increase in salinity due to interaction between the migrating fluid and evaporitic metasediments may have aided immiscibility development as there are evaporites in the regional stratigraphic succession. TTiis episodic entrapment of unmixing fluids most likely resulted from the cyclic gravitational collapse during orogenesis characterized by intervening subvertical and subhorizcmtal foliations. The sources of CO2 aie uncertain in this case. However, given that fluid compositions are largely independent of the lithology in the host rocks, a magma-derived or mantle source could be employed to account for the consistent occurrence of C02-rich fluids even though decarbonation reactions would be more favorable for those preserved in carbonate-bearing rocks.

75

Large Variations in Temperature of Hydrothennal Vein Precipitation in the Victory Gold Deposit Kambalda, WA XJiu, J.M. Palin and I.H. Campbell Rcscarch School of Earth Scicnccs. ANU, Canberra, ACT 0200 In several major >^haean greenstone belts (e.g. the Norseman-Wiluna belt of WA and the Abitibi belt <£ Cana^), there is abundant evidence that structure and lithology controUed the distribution of hydrothennal lode gold deposits. However, a question arises as not aU of thefevourablestructural-iithological positions within a deposit host gold In this study, we investigate the role of temperature in controlling fluid gold mineralisation in the Victory gold deposit of WA. © © "" The Victoiy mine is located in the Kambalda-St Ives area of the Norseman-WUuna greenstone belt. The regional geology and mme geology have been previously described by Claric et al.(1989) and Roberts and Elias (1990) Hy^othermal gold mmeralisation is mainly situated in NNW trending faults that splay from the major BoulderUfroy fault system. Gold is hosted by eveiy rock type on the mine scale. For this study, samples cf ftydrothermal vems were collectedfromdie major shear zone associated with gold mineralisation, andouoi^^ analysed for oxygenisotopes.

Temperatures of vein precipitation were calculated by measuring the difference in oxygen isotope compositions between coexistmg mmeral pairs. Vein temperatures derived from the quartz-albite pairs reveal laree relative vanations, ranging from 540°C to 350°C. The higher temperatures (above 430X) arefix)mthe souths part cf the deposit, and the highest temperature (540X) is close to the peak metamorphic temperature (525 ± 25®C) fir the Kambalda area (Bavington, 1979). Most of the lower temperatures come fixm the northern side and M withm the range of temperatures previously estimated for hydrothennal wall rock alteration in this portion of the Victoiy mme (390 ± 40X, Clark et al., 1989). The lowest quartz-albite temperature (350X) is existent with temperatures denvedfromquartz-scheelite pairs.

The values of vein quartz vary from 11.0 to 12.8, with an average of 11.8 ± 0.5(19). These data are consistent with previous work by Golding et al.

(1988). The oxygen isotope compositions of water calculated from vein quartz are in the range = 6 to 10.

The oxygen isotope data indicate that the major mineralised shear zone in the Victory deposit underwent hydrothermal fluid flow over a much larger temperature interval than previous thought.

Although the time - space evolution cf hydrothermal activity along this structure is still not completely understood, the observations suggest a relation between temperature and gold mineralisation.

700

T(»C)

too

• •

Quwtz-AMt

• QuBrtz-MuKxwtia

w OmtZ'Toum^ D A

Quwtz-aolii* OuMtz-Ooiofnili

500

400

SOO

200

10400

10*00 DI«Une»{m) " " o

REFERENCES Bavington, O.A., 1979. Interflow sedimentaiy rocksfromthe Kambalda uhramafic sequence: their eeochemistrv metamorphism and genesis. Ph.D. thesis, ANU (unpubl).

Clark, M.E Cannichael, D.M., Hodgson, C.J. and Fu, M. 1989. Wall-rock alteration, Victoiy Gold Mine Kambalda, Western Australia: Processes and P-T-Xc02 conditions of metasomatism. Economic Geolom Monograph, 6, 445-459.

Golding, S.D., McNaughton, N.J., Barley, M.E.. Groves. D.I., Ho. S.E., Rock, N.M.S. and Turner, J W 1989. Archean carbon and oxygen reservoirs: fteir significance forfluidsources and circulation paflis fbr Aidiean mesothermal gold deposits of the Norseman-WUuna Belt, Western AustraUa. Economic Geology Monograph, 6. p.376-388. ' ' Roberts, D.E. and Elias, M. 1990. Gold deposits of the Kambalda-St Ivesregion.In F.E. Hughes eds. Geolofry of the Mineral Deposits of Australia and Paupa New Guinea, The Australian Institute of Minine a ^ Metallurgy, 1, 479-491, Melbourne. ^ Actoowledgmgnts: The authors acknowledge the support of WMC, and in particular express appreciation to Senior Research Geologist Dr Kim A. A. Hein and the mine staff for guidance with the geology and sampling.

76

I

4

BASE METAL SULFIDES IN THE ULTRAMAFIC CUMULATE FROM THE GREAT SERPENTINTTE BELT, NSW K. Yan^^ and P.K. Scccombc^ ^CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 2113 ^Department of Geology, University of Newcastle, Callaghan, NSW 2308

The Great Serpentinite Belt comprises variably altered and deformed mafic and ultramafic rocks of an ophiolitic association. Lenticular bodies of olivine-chromite ultramafic cumulate rocks, commonly developed at the base of the crustal succession of the ophiolite, contain abundant base metal sulphides present as either multiphase aggregates or individual disseminated grains. Of all lithological units of the ophiolite, the ultramafic cumulate is the richest in sulphides, with the sulfur content up to 1000 ppm.

In the olivine-chromite cumulate, multiphase sulfide aggregates, associated with or without magnetite, are located interstitial to olivine and chromite, and are partly or entirely surrounded by intercumulus plagioclase or clinopyroxene. Sulphide aggregates, often of a rounded or angular shape, range approximately from 0.2 to 0.9 mm in diameter. A cumulus origin for the sulfide aggregates is suggested by their textures, bulk compositions and occurrences, although hydrothermal alteration has variably modified their magmatic nature.

In the sulfide aggregates, the common assemblages are pentlandite + chalcopyrite -f pyrrhotite +/- bomite, pentlandite + chalcopyrite + bornite, pentlandite + chalcopyrite, and pentlandite + bomite. These assemblages of Fe-Cu-Ni composition probably originated from the primary inmiiscible sulfide liquids formed during the crystallisation of olivine and chromite in the parent mafic magma.

Less important are the assemblages consisting of pentlandite + millerite + chalcopyrite, heazlewoodite + Cu-Ni alloy, chalcopyrite + Cu-Ni alloy, chalcopyrite + bomite, and heazlewoodite + millerite. The Ni- or Cu-rich bulk composition for these assemblages probably reflects the fact that these assemblages were significantly altered during serpentinisation. These assemblages, therefore, represent the modified magmatic sulphides.

Individual grains of disseminated sulphides are widespread throughout the mass of the ultramafic cumulate.

They occur in all altered silicate minerals including olivine, plagioclase and clinopyroxene, without a preferred occurrence in certain cumulus or intercumulus phases. The disseminated sulphides are fine grained (much finer than a sulphide aggregate) and comprise a single phase of, in most cases, pentlandite, heazlewoodite or millerite.

Pyrrhotite and chalcopyrite are rarely encountered. In ultramafic cumulates, the disseminated sulphides are less abundant than the aggregated sulphides, and they represent hydrothermal sulphide formation during serpentinisation.

The presence of abundant segregated sulphides in the basal ultramafic cumulate of the cmstal succession of the ophiolite indicates sulfide saturation at an early stage of crystallisation within the cmstal magma chamber. The majority of magmatic sulphides in the intensely altered ultramafic cumulate have probably survived the serpentinisation.

%

77

HIGH-RESOLUTION REFLECTION SEISMIC IN THE HELLYER ORE ENVIRONMENT A N Yeat«\ A McNeiIl\ S Richlrdson^ T J Barton*. B J Dniinmond\ J saic^ md R G Ridurdson' Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2601 Abcrfoylc Resources Lid. Level 31, 525 Collins Street, Mefboume, Vic., 3000 ^asmanian Geological Survey. PO Box 56 Roroy Park, Tas., 7018.

The richly mineralised Cambrian Mount Read VHMS district of Tasmania contains diverse submarine lavas and volcaniclastic rocks (Corbett, 1992). Lenticular, bulging and domal lava forms (Cas, 1992) in basaltic and andesilic lavas of a host unit, the Que-HeUyer Volcanics (QHV) (McPhie & AUen, 1992; Waters & Wallace, 1992) may be imaged on high-resolution reflection seismic sections. Some high-grade mineral deposits in the region, e g classic mounds such as the Hellyer Zn-Pb-Cu (Ag, Au) deposit (Large, 1992a) also have potential for being detected by seismic survejing due to their density contrast (Read, 1989) and form.

A 1.2 km-long high resolution reflection seismic test line was shot over a prospective portion of the Hellyer ore environment, northwest Tasmania, to determine to what extent the seismic method might have potential to identify key described geological features in gently to moderately dipping sequences at depths greater than those currenlly being explored. The work formed a small part of a seismic traverse across the Dundas Trough (Fig 1; Dnimmond et al., 1996). AGSO's Land Seismic Group conducted the sur\'ey and processed the data.

Recording parameters for the high resolution line were: 10m station spacing, 60m shot point interval and a 2kg charge at 5m depth (Barton et al., 1995). Gravity data were acquired at 120m intervals bj' Richardson (1995).

2000

Fig. 1 Location (see Barton et al, 1995). Fig. 2 Results uninterpreted.

Fig. 3 Results interpreted. as/Kss^

Figures 2 and 3 reveal weak flat reflectors at 100-200m. The>^ are considered spurious, perliaps multiples from the base of weathering. Between 200 and 500m, dips accord better with projeaed surface and drillhole values (but note V/H <1 to show reflectors better). Below 500m, geological control in the vicinity is laddng. Beneatli station 1055, prominent reflectors at about 900 and 1150m are interpreted as the top and base of the Que River Shale (QRS) It overlies the QHV whose base is interpreted as the reflector about 1500m below station 1055 (Fig 3). There, interpreted QHV has a notable bulge, witli a small coincident gravity gradient. Reflections are weaker in the bulge, suggesting alteration. Strong reflectors above, at 1150m, suggest high-velocity rocks, e.g. carbonate, dolerite, or ?massive sulphides, a potential target warranting consideration for drilling (cf. Gemmell & Large, 1992, fig. 6). Results are exciting, as new deposits in the region are thought more likely to be found below depths of 300m (Large, 1992b). [Acknowledgements: J Mifsud (artwork); Published with permission].

REFERENCES (Note: titles of several papers have been shor^ined for brevity).

Barton, T.J., et al., 1995. "TASGO" Seismic Survey 1995: Operational Report. AGSO Record 1995/72, 57p.

Cas, R., 1992. Understanding VHMS deposits from submarine volcanism. Economic Geology 87, 511-541.

Corbett, K.D., 1992. Cambrian Mount Read Volcanics. Economic Geology 87, 564-586.

Drummond, B.J., et al. 1996. Cnistal architecture nortliwest Tasmania ..AGSO Research Newsletter 25, 17-19.

Gemmell, J.B., & Large, R.R, 1992. Hellyer stringer system and alteration.. Economic Geology 87, 620-649.

Large, R.R., 1992a. Austalian VHMS deposits: features, styles, genetic models. Economic Geology 87,471-510 Large, R.R., 1992b. Tasmanian exploration .... Tasmanian GeologicalSun'ey Bulletin 70. 118-123.

McPhie, J., & Allen, RL., 1992. Mount Read Volcanicsfeciesarchitecture. Economic Geology 87, 587-596.

Read, J.J., 1989. Seismic reflection investigations of Hellyer orebody... Exploration Geophysics, 20, 159-162.

Richardson, R.G., 1995. Operations rqwrt - gravity... Tasmanian Geological Survey Record 1995/7.

Waters, J.C., & Wallace, D.B., 1992. Que-Hellyer Volcanics, west Tasmania. Economic Geology 87,650-666.

78

u VT AS AN EXPLORATION TOOL Chongbin Zhao, Hans-Berad Mohlhaus, Bruce F. Hnhhs & Alison Ord Australian Geodynamics Cooperative Research Centre and CSIRO Exploration and Mining, PO Box 437, Nedlands, 6009, Australia

general principles that govern the formation of giant ore deposits that owe their origin to

where u is the D^cy fluid velocity, c. is the equilibrium concentration of a mineral of interest in the interstitial fluid T is S Z T L V " ''"" concentrations of other species (including hydro^n S ) i a t Tn^Ti^ reaction of mterest. H e summation is over the r species that influence tiie reaction of interest, thTdot denotes the scalar product of the Darcy fluid velocity with the gradients, V, of temperature, fluid pressure L o h t ^ c a l species, and D is the species diffiisivity. The size and grade of a mineralised deposit is given b y j ' q ^ d t .

Thus, the

ithe T yw^ay ^ Zuiiwhich T t these W diffuse and accumulate ^^ over the time of formation of the deposit.

to Q^ and

^^^^

f " systems undergoing thermal convection. The emphasis is on initial fluid H " magnitude, and realistic geological configurations such as rifted strucS^Ll f^^Z?- H compartments with folded impenneable beds and faults as boundaries. Boundary effects are V ^ ^^ ""derstanding the regional geomedy of the fluid flo^system^S particularly recognising the boundaries of the system, is important as an exploration tool.

Dehydration or magma crystallisation adds high pressure fluids to the base of an established convecting system- rising fluid ' thermal connection can be M e d or S S Id ht;?h" ^^ f v T S o? development of high thermal gradients, and hence of high values of u VT and ofu Vp centrally moverpressured systems. i i l T t ^ f c o n d i t i o n s impose a new thermal regime on an older overpressured system that already has established a jeady state convecuve system. What is important here is an increased vigour of the convecting system resul new distributions of u VT and, m particular, of u Vp. Tlie increased fluid pressuTe gradients at various p a L of t h T S h ^ b e r can now exceed lidiostatic, generating fracturing adjacent to and within the n e w ^ e o u s intrusion. of entire^ different chemistries. We present examples of the plumes and trails of fluids (such as relatively dense metal rich brines) that result from mixing during thennal cSnvertiSi Examples are presented where such plumes or trails intersect with regions of high u VT thus resulting in h ^ V f a v o i S w e sites for mineralisation. Fmally, we specifically add the dissolution and precipitation of quartz. lavourable

t T ^ f y

^ commonly quoted « an important process leading to the formation of a mineralised system, fluid mmerahsation if the fluid flow vectors are directed parallel to isotherms, isobars or iso olme

Z f the J resultant u mfluence " on mmeralisation ^^^^ is zero." " that

« ^'o^gly focusstd but u is normal """uoi to lu VT v i so so //

REFERENCES PHILLIPS, O.M. 1991. Flow and reactions in permeable rocks. Cambridge University Press, Cambridge 285pp.

»

79

Physicochemical Conditions and Aqueous Geocheniistry During Copper-Gold Mineralisation at the Osborne Deposit, Cloncurry District. NW Queensland Taihe ZHOU' and Neil D ADSHEAD^ National Key Centre in Economic Geology and Department of Earth Sciences.

James Cook University. Townsville. QLD 4811

ABSTRACT The Osborne deposit is approximately 195 km to the southeast of Mount Isa and is one of the economically significant examples of epigenetic copper-gold mineralisation in the mid-Proterozoic Cloncurry District. NW Queensland.

The majority of the host rock sequence has undergone upper amphibolite facies regional metamorphism and comprises multiply deformed feldspathic psammite, enigmatic banded ironstone, sillimanite-bearing pelite, Mg-amphibole-bearing schist, metatholelite dykes and a body of metaperidotite.

At least three mineralogically discrete styles of pegmatite sheet intrusion and subordinate lamprophyre dykes post date the regional metamorphism but are variably deformed by. or cross-cut the effects of. a period of relatively late, brittle-ductile deformation that appears to be coeval with the copper-gold mineralisation.

Detailed paragenetic relationships within and between the temporally and spatially different alteration assemblages at Osborne are complex but the secondary phases generated during the thermal event responsible for the deposition of copper and gold can be simplistically subdivided into pre-mineralisation and syn-mineralisation. Assemblages with Fe-hornblende. biotite. talc and ferropyrosmalite (a Cl-bearing Fe-phyllosilicate) typify the earliest hydrothermal alteration associated with the metalliferous hydrothermal event whereas the slightly later and retrograde stage of significant chalcopyrite-gold deposition is characterised by secondary chlorite, carbonate and muscovite. Detailed electron microprobe analyses reveal that the pre-mineralisation Fe-hornblende, biotite and, in particular, ferropyrosmalite (4-6wt.% CI) contain significant concentrations of chlorine. Fluid inclusion, oxygen isotope and thermodpamic studies on the early alteration assemblages indicate they equihbrated with high temperature (^450°C). high salinity («60wt.% NaCl equiv.), moderately oxidised fluids that were unusually Cl-Fe-Na-K-Ca enriched and had a moderate sulphur content (logiS « - 1 ) whereas during the main period of Cu-Au deposition the hydrothermal fluid was cooler ( « 320°C) and less saline (<20-37 wl.% NaCl equiv.).

The decrease in temperature between the period of pre-mineralisation alteration and Cu-Au deposition may simply reflect a cooling of the hydrothermal system but it may also be partly controlled by phase separation. The concomitant decrease in fluid salinity, however, appears to at least partly reflect the locally extensive. Cl-bearing silicate wall rock alteration (indicating the salinity was rock-buffered) because the oxygen isotope data provide little evidence for fluid mixing and dilution. Thermodynamic modelling indicates the effect of the significant temperature decrease and salinity reduction on the metalliferous hydrothermal fluid would have reduced the solubility of copper and gold by more than three orders of magnitude.

Consequently, the changes in these physicochemical parameters are regarded as important in the deposition of economic concentrations of copper and gold at Osborne.

Acknowledgments P.J.Williams. G. Neil Philhps. Guoyi Dong and Graham Taylor are acknowledged for their advice and discussions. Osborne Mine and Placer Exploration Ltd. are also thanked for their permission to present this contribution.

present address; Cooperative Research Centre for Landscape Evolution k Mineral Exploration, c / University of Canberra. PO Box 1. Belconnen ACT 2616 ^ present address; c / - Misima Mmes. PO Box 5418, Cairns QLD 4870

80

NEW PERSPECTIVES ON THE MCARTHUR BASIN FROM GIS AND GEOPHYSICS Mark L- Pyff^tt' and David E. Leaman'-' 'Centre for Ore Deposit and Exploration Studies, University of Tasmania, GPO Box 252-79 Hobart, Tas 7001' 'Leaman Geophysics, GPO Box 320D Hobait, Tas 7001

The McArthur Basin contains a world-class, stratiform sediment-hosted base metal (SSHBM) deposit (HYC), which, because of its relatively undeformed and unmetamorphosed state, has been widely used as a basis for models of SSHBM mineralisation. Many of these models postulate ore deposition as a result of fluid flow within a rift-sag tectonic environment, although basin morphology is not well constrained by surface mapping due to very limited exposure of many of the deeper basin units. It is necessary to consUain such models in the third dimension by defining the geometry of basins and the bounding faults which may control fluid migration.

This has been attempted by integrating geophysical interpretation results as a 3-D component in a metallogenic geographical information system.

Rigorous quantitative analysis of AGSO regional Bouguer gravity and magnetic data has been employed to investigate the geometry of the McArthur and Tawallah Groups, as well as that of the underlying 'basement' units. Complementary interpretation of an interlocking array of 2-D gravity and magnetic profiles was undertaken by forward modelling constrained by application of the criteria of Leaman (1994). The results were used to generate isochore and structure contour maps of all major basin components. These maps reveal the gross morphology of the basin fill, its basement, and units which, while correlated with exposed rocks, are much more voluminous than hitherto suspected from their exposure.

Structure and isochore contours were digitised and gridded using the Arc/Info GIS software, enabling redisplay and analysis of the geophysically derived sU-uctural information with respect to other geoscientific data in the GIS such as surface geology and mineral deposit locations. The gross basin units are presented in the poster accompanying this abstract as a series of 3-D rendered (isometric) views displayed in relation to their outcropping equivalents, regional first-order fault structures and various mineral occurrence types.

The McArthur Group is seen to extend well beyond its eastern limit of outcrop as defined by the Emu Fault.

Units identified as prospective using conceptual lithological criteria encoded in the GIS contain all known stratiform base metal mineralisation. Such deposits are preferentially located on the periphery of the thickest accumulations of McArthur Group sedimentary rocks. Volcanism in the upper and lower Tawallah Group is much more voluminous than its comparatively low stratigraphic thickness measured in outcrop would suggest.

Over 15 km of basin fill (including volcanic rocks) is implied in some areas, but this may vary rapidly over short distances, implying considerable pre-McArthur Group structural development. A number of lineaments visible in the isometric images converge at the position of HYC, indicating bounding fault and strike-slip fault activity at this location during a large portion of basin evolution. A large pile of basal volcanics is inferred to be equivalent to felsic metavolcanics of the Barramundi Orogeny. Granitoids of varying composition and timing are interpreted to have intruded beneath much of the McArthur/Mount Isa Basin region, but the timing of this event is poorly consU-ained.

Combination of rigorous geophysical analysis and GIS-supported display and integration with other data sets permits a new view of the McArthur Basin and its mineralization. Isometric views of structure contours and isochores enable easier visualization of relationships between basin geometry, faults and mineral occurrences.

Geologically interpreted major structures mapped at the surface may not necessarily have been significant, or possessed their present geometry, throughout the basin's evolution. Such information must be considered in developing metallogenic models incorporating tectonic setting and fluid flow.

REFERENCE Leaman, D.E., 1994. Criteria for evaluation of potential field interpretations. First Break 12, 181-191.

Acknowledgements: This work was undertaken as part of AMIRA project P384, which ran from 1992 to mid1995. The support of sponsoring companies and organisations through AMIRA is gratefully acknowledged.

The AGSO in particular is thanked for supply of gravity and magnetic data, and the provision of an AGSO Postgraduate Research Award to one of us (MLD).

THE CHALLENGER GOLD PROSPECT: SOUTH AUSTRALU '

I

Ross Kennedy Resolute Limited, 28 The Esplanade, Perth WA 6000

South Australia's Gawler region promises to be one of Australia's premier gold provinces in the next few years.

Resolute/Dominion tenure (23,400 km^) should contain a number of open cut and underground gold mines due to the number and size of hydrothermal gold centres and high grade nature of mineralisation discovered to date in this region. Based on geophysical/structural interpretation and geochemical results to date, the Joint Venture believes it controls the best ground position in this - Australia's newest gold field.

Less than 2% of the basement Archaean and Proterozoic rocks outcrop in this region, the overwhelming situation being of younger sediment and windblown sand cover.

The aridit}' of this region has led to the formation of calcrete (a carbonate evaporite) in the overburden profile.

Gold from the underl>ing weathered basement also concentrates \\ithin the calcrete. By sampling and anah sing the calcrete regions for gold and other discriminators, anomalous basement has been defined.

Extensive regional calcrete sampling, which is still continuing, has generated numerous large (kilometre scale) gold anomalies. In the Challenger region, 70 such anomalies have been generated of which, to date, 20% have been drill tested. Approximately 56% of the project area still remains to be covered by first pass reconnaissance calcrete sampling. Of the area already sampled, numerous anomalies still remain to be infilled and extended.

Drilling results from the Challenger Prospect (the first prospect tested, and fortuitously concealed beneath onl\ 2m of cover) continue to be highly encouraging. The high grade free milling nature of the shoots defined to date, both in the oxide and primar>' sulphide zone, indicate potential for both open cut and underground mining. Economic mineralisation has yet to be closed off along strike and do^^^l plunge and the distribution of other gold intercepts within a 2 km radius, indicates that Challenger is but one of a number of deposits. Ore grade gold intersections have also been made at a number of other prospects within the Resolute-Dominion tenure. These are at an earlier stage of evaluation than Challenger.

At Challenger, two shoots plunge 30° NNE on the west limb of an antiform in Archaean granulites of the Christie Gneiss. Both are open at depth with the western shoot displaying the strongest gold mineralisation and excellent continuit}' at >100 gxm. Primary mineralisation is sulphur poor and comprises free gold with minor arsenop>Tite and p>Trhotite.

Ex-ploration at this outstanding prospect is accelerating.

I

AUTHORS INDEX Adshead,N D Adshead-Bell, N S Anderson, B Andrew, A S Ame, D C Ashley, P Ayshford, S Barton, T J Baxter, J L Beattie, R D Begg, G C Bierlein F P Bennett, V C Beny,R Besanko J Binns R A Blake, K L Blevin, P L Bowes, D R Bradshaw, B Brauhart, C Braun,J BnDVMi, P E Cameron, G H Campbell, 1 H Canr, G R Cartwright, 1 Cassidy, K F Chapman, L H Cooke, D R Cox, S F Crawford, A J Creagh, C Crook, K A W Dare, P Davis, B K Domagala, J Downes, P M Drummond, B J Duckworlh, R C Duffett Edraki, M Edwards, D W Ellis, D J Ellis, P D Ewers, G R Fogarty, J M Foster, D A Fuller. T Furlong, L Gemmell J B Ghaderi, M Giblin, A M Gibson, D L Goldfarb, R J Graham, S Gray, D R

80 1 2 3 8 4. 28.44 5 78 6 9 7 8,44 18 2 8 9 10 11,12 46 13 14,15 22 16 17 18,76 3.19 41 20,52 74 7,21.29 22 29 4 9 2 23 13,48 24.25 78 26 27 28 9 12 29,45 52 30 7 8 70 2. 9. 29. 31 18 3 50 32 35 7

Hagemann, S G Hamilton, L H Hamilton, R Hartley, J Hedenquist. J W Heinrich, C A Henley. R W Hippertt. J F M Hoatson. D Hobbs, B E Hunns, S R Huston. D L Idnurm, M Jackson. M J Jamieson. D N Jane,M Kent, A J Kenich, R Knackstedt, M A Krassay, A Kwak, T A P Lambert, D D Large, R LavMe, D C Lavsrie, K C Leaman, D E Leven.J y, Xia Lindsay, J Lindsay, M Loucks, R R MacCorquodale, F MacRae, G Mason, R McConachie, B A McCulloch, M T McGoldrick. P Mclnnes. 6 1 A McNeill, A McQueen, K G Memagh, T P Morant. P Mortimer. G E Moss. R Muhlhaus. H B Nekitel. S Nesbitt, B E Newberry, R J Nevscrest Mining Staff Offler.R Oliver. N H S Ord.A Page.RW Palin. J M Parr. J M Pwa. A RavMings. D J

16,20 36 35 37 38 17,63 39 23 67 5 5 . 5 6 79 40 52 13 13 63 41 7.44 70 22 13 47 35. 4 1 . 6 7 42 43,44 45, 46. 52, 53 27 13 47 13.23 13

7

51 60 56 13.48 18 42 49 78 50 45, 52, 53 15 18 9 79 9 32 32 54 5 55 55. 56. 68. 79 13.48 18. 57, 76 9 58 59

Raymond, 0 Rea, P S Richardson, R G Richardson, S Richmond, J Rotherham, J F Ryan, C G Sami, T Scott, D Scott, S D Seccombe, P K Seed, M Sharpe, R Shee, S R Shywolup, W Silic, J Sims, J P Skirrow, R G Smith, S G Southgate, P N Sprouie, R Stuwe, K Sylvester, P J Tarlowski, C Taube, A Taylor, C J Upton, P van Achterbergh, E van Moort, J C Veamcombe, J Veamcombe, S Wall, V J Wallace, D A Walshe, J L Warren, R G Waters, J C Wells, A Whitford, D J Wilkins, C Wilkins, R W T Williams, P J Wybom, D Wybom, L A 1 Xu, G Xu, Y Yang, K Yeates, A N Zaw, K Zhao,C 2hou,T

60 61 78 78 74 62 63 13 13 9 5.77 64 9,31 35 64 78 65 52,65 66 13,48 67 8 18 13 37 9 68 63 58 69,70 70 17 71 17 12 9 13 3,19 72 73 10,74 71 52 75 76 77 78 63 79 80

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