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  O RE S YSTEM S TUDIES 147 ORE SYSTEM STUDIES The Centre for Advanced Studies of Ore Systems (CASOS) is a joint initiative betweenRSES and the ANU Department of Geology (The Faculties). CASOS links togetherresearchers with a common research and training focus on themes dealing with various aspectsof the formation of ore systems.The CASOS mission is:ãTo advance fundamental understanding of processes involved in the genesis of largeand economically significant mineral resource systems on a lithosphere and provincescale.ãIn collaboration with the international minerals industry, to develop and apply newtechniques and new models to drive the development of new minerals explorationconcepts, as well as strategies to direct resource definition and mine development moreeffectively.ãTo provide high level training of graduate students and postdoctoral fellows so thatAustralian scientists are at the international forefront of ore systems research andminerals exploration technology.Expertise at ANU in the development and application of new microanalytical techniques,high-pressure/high-temperature experimental facilities, and modelling capabilities, provideunique opportunities to explore some of the fundamental physical and chemical processesinvolved in ore genesis from metal source, through the transport path, to resource accumulationsites. Ore Systems research at ANU is conducted within six programs:ãOre FluidsãDeformation and Fluid PathwaysãMagmatic ProcessesãStable Isotope StudiesãRadiogenic Isotope SystemsãDiamond SystemsDuring 1999 at RSES, Ore Systems research was conducted across a number of theSchool’s Groups, but principally in Ore Genesis, Petrochemistry and Experimental Petrology,Petrophysics and Geophysical Fluid Dynamics. A major focus of our activities continues to be the relationship between goldmineralisation and the evolution of Archaean, granite-greenstone terrains in the Yilgarn craton of Western Australia. Dr M. Palin and Ms Y. Xu have been using stable isotopes to study thegenesis of the gold deposits in the Kalgoorlie-Kambalda area of Western Australia. It has beenknown for some time that mesothermal gold deposits are commonly hosted by Fe-rich hostrocks, and this has lead to the suggestion that the gold precipitated as a consequence of desulphidation, when sulphur in the ore fluid reacted with Fe in the wall rocks. However manyimportant gold deposits are not hosted by Fe-rich wall rocks. Palin and Xu have found that thesulphides associated with the highest gold grades at Kambalda have the lightest sulphurisotopes. They have shown that this relationship results from oxidation of the ore-forming fluidwhich converts sulphur in the auriferous fluid from sulphide to sulphate, destabilising the gold-carrying sulphide complexes in the fluid and causing gold to precipitate. Destabilisation of thegold-carrying complex and precipitation of gold can therefore occur both as the consequence of reduction or oxidation of sulphur in the ore fluid. It appears that it is the second of thesemechanisms that is the more important, because many of the world’s giant gold deposits,including the Golden Mile, Hemlo, Kirkland Lake and Hollinger-McIntyre, have sulphides withlight sulphur isotopes, indicative of oxidised ore fluids. Oxygen isotopes have been used toshow that formation of the gold deposits also involves mixing between a high-temperature deep-crustal fluid and a low-temperature, presumably oxidised, fluid that is probably seawater or  R ESEARCH S CHOOL OF E ARTH S CIENCES – A NNUAL R EPORT 1999 148meteoric water. The conclusions of Palin and Xu are being further tested at the Golden Mile byMr C. Heath as part of a new PhD project that is being funded through the SPIRT program withKalgoorlie Consolidated Gold Mines. Finally, Dr Loucks has found a relationship betweenArchaean mesothermal gold deposits and anticlines which has lead to the suggestion thatanticlines act as a focussing mechanism for metamorphic fluid flow.Professor S. Cox, in collaboration with WMC Resources Ltd and University of NewcastlePhD student, Mr K. Ruming, is investigating structural and deformational controls on fluid flowassociated with gold mineralisation in faults, shear zones and associated fracture systems at theSt Ives Goldfield, near Kambalda in Western Australia. Fluid-pressure-driven growth of faultsand reaction-weakening in the gold-hosting structures has been found to influence theirlocalisation. Accordingly, current work is focussing on understanding how the distribution of vein-rich and vein-poor mineralisation across the 20 km long, 5 km wide goldfield relates to thedistribution of fluid pressure regimes in the fault/shear controlled hydrothermal system thatgenerated gold mineralisation. A contractional jog on a major shear system is recognised asplaying a key role in nucleating low displacement structures which host some of the majormineralisation at St Ives. The ore-hosting structures are being interpreted as aftershock structures associated with major slip events on adjacent, crustal-scale shear systems.A principal focus of research in the Ore Genesis Group is the development of copper andgold deposits at convergent plate margins. This research program involves three PhDs and DrR. Loucks. Mr B. Rohrlach has been analysing oxygen isotopes in hydrothermally alteredvolcanic samples from the Tampakan stratovolcano, which hosts a major porphyry copper-golddeposit in the Philippines. The isotopic study examines the interaction between magmatichydrothermal fluids and meteoric water, as influenced by volcanic topography on the hydrologyof the ore-forming system, and the relationship of the fluid mixing to ore deposition. Mr B.Setiabudi has analysed samples from Kelian gold mine in Indonesia for major elements and fora wide range of trace elements. The major elements and some of the trace elements have beenaffected by the extreme hydrothermal alteration of the mine area, so samples from the unalteredand petrological similar Magerang area to the northwest were also analysed for comparativepurposes. Trace element ratios, for elements that are normally regarded as immobile inhydrothermal fluids, are the same for the two areas, suggesting that the Magerang data provide agood analog for the Kelian volcanics and can be used to study the chemical evolution of the oreforming system without the complication of alteration. Mr J. Ballard has been studying thegiant Chuquicamata copper deposit in Chile. This year he used the excimer laser (ELA) ICP-MS to date igneous zircons in the three porphyries in the mine and showed that they span aresolvable age range, consistent with the hypothesis that Chuquicamata owes its enormous sizeto three superimposed porphyry systems. He also used variations in the Ce anomaly in zirconsto show that the productive porphyries in the Chuquicamata area crystallised from a moreoxidised magma than the barren intrusions.A feature of the Tampakan, Chuquicamata and Kelian studies is that Y and the heavy rareearth elements decrease with increased magmatic differentiation in the felsic suites associatedwith the ore deposits. This is unexpected because these elements are usually “incompatible” inmajor igneous minerals, and so accumulate in the residual melt as magmatic differentiationproceeds. The unusual depletion trend is attributed to extensive amphibole fractionation andimplies that the ore-producing systems are unusually water-rich. The addition of water to amagma expands the stability field of amphibole at the expense of plagioclase. As aconsequence, it should be possible to use variations in Sr, which partitions strongly intoplagioclase, and Y, which partitions into amphibole, to distinguish ore-bearing and barren felsicsystems. Dr Loucks has shown that the Sr/Y ratio in fertile segments of convergent marginsincreases with fractionation but decreases in barren segments. Furthermore, within ore-bearingsegments, the productive stocks and porphyries have higher Sr/Y ratios than barren intrusions.This study suggests that Sr/Y ratios will be a valuable aid to mining companies in the explorationfor copper and gold deposits at convergent plate margins.Professor Cox, in collaboration with Dr S. Munroe (SRK Consulting), has beenexamining the coupling between stress states and fluid pressure regimes in generating the  O RE S YSTEM S TUDIES 149fault/fracture system which hosts the giant intrusive-related gold deposit at Porgera in PNG.The development of suprahydrostatic fluid pressures, and a change in the stress regime duringthe evolution of this intrusive-related hydrothermal system, is a key factor controlling a changefrom distributed fluid flow to highly localised, fracture-controlled flow. Early distributed fluidflow produced high-tonnage, low grade mineralisation, whereas the later fault-hosted flowregime produced extremely rich mineralisation.Activities in the Petrochemistry Group complement the field-based and analyticalapproaches on convergent margin Cu-Au deposits. PhD student, Mr A. Hack, andDr J. Mavrogenes are conducting an experimental campaign to explore Cu solubility insupercritical and two-phase aqueous solutions, Cu solubility in silicate melts, and the criticalissue of Cu partitioning between silicate melts and co-existing vapour. Significant advances arebeing made using high pressure/high temperature laboratory facilities at RSES and the ANUGeology Department, in conjunction with ELA-ICP-MS techniques. The study is providingfundamental information on the processes leading to the formation of porphyry-type mineraldeposits at convergent plate margins.Using the high temperature laboratory facilities in the Petrochemistry Group,Drs Mavrogenes and H. O’Neill are studying sulphur solubilities in silicate melts undercontrolled fO 2  and fS 2  conditions. The results indicate that FeO has a stronger influence onsulphide capacities in haplobasaltic and basaltic melts than indicated by previous work. Theresults have implications for understanding the genesis of magmatic Ni and platinoid deposits.  High tenor mesothermal gold ores with low δ   34 S  J.M. Palin and Y. Xu Experimental studies conducted previously at the RSES show that gold is present insulphide-saturated hydrothermal solutions at temperatures above 300°C and at near neutral pHprincipally as AuHS(H 2 S) 3 ° or AuHS°. Gold solubility under such conditions is described bythe general reaction:Au + n H 2 S = AuHS(H 2 S) n-1 °  + 1/2 H 2 where n is 1 or 4. Variable and low δ 34 S of pyrite in high tenor ores of the Victory mesothermalgold deposit of Western Australia indicate progressive oxidation of initially reduced ore-formingsolutions. Lowering of the activity of dissolved hydrogen gas, a(H 2 ), increases gold solubilityaccording to the above reaction leading to an apparent contradiction between observation andtheory.Changes in fluid chemistry and sulphur isotope values that accompany carbonation andsulphidation of wall rock can be estimated for incremental reaction of magnetite with CO 2  andH 2 S in a pyrite-saturated fluid according to:Fe 3 O 4  + 3 CO 2  + H 2  = 3 FeCO 3  + H 2 OFe 3 O 4  + 6 H 2 S = 3 FeS 2  + 4 H 2 O + 2H 2 Calculated reaction paths are shown in Figure 1 for two different fluid starting compositions.Solution pH is assumed to remain neutral (near the quartz–K-feldspar–muscovite buffer) in thissimplified model, which is consistent with observed mineral assemblages. The stoichiometry of the carbonation and sulphidation reactions produces changes in H 2  that are the same order of those in CO 2  or H 2 S, although these two species are several orders of magnitude more abundantthan H 2  in the fluids. Thus, shifts in a(H 2 ) are proportionally much larger than those in a(H 2 S)or a(CO 2 ) for each increment of reaction progress. The resulting vertical reaction paths crosscontours of gold solubility at steep angles (Figure 1). In the case of carbonation, gold solubilityincreases as the solution becomes increasingly oxidized until the HSO 4 -   predominance field for  R ESEARCH S CHOOL OF E ARTH S CIENCES – A NNUAL R EPORT 1999 150dissolved sulphur is encountered. From there to the point where the solution reaches hematite ormagnetite saturation, conversion of H 2 S to HSO 4 -  causes a rapid decrease in a(H 2 S) relative toa(H 2 ), which results in flattening of the reaction paths and a precipitous drop in gold solubility.This is because the sulphide complex that carries the gold is destabilised when sulphur in the orefluid is oxidised to sulphate. The largest decreases in a(H 2 S) and a( ∑ Au) occur around theH 2 S/HSO 4-  equal-activity boundary and thus coincide with the point where sulphide sulphurisotope compositions are most strongly shifted to negative values (Figure 1). Wall-rock carbonation thus provides a straightforward explanation for the negative correlation observedbetween gold content and pyrite δ 34 S in the Victory deposit. In contrast, sulphidation of wallrock results in a decrease in gold solubility as the fluid undergoes progressive reduction untilpyrrhotite stability is reached, whereupon further changes in a(H 2 S) and a(H 2 ) are buffered bypyrite-pyrrhotite equilibrium up to magnetite saturation. No sulphur isotope shifts occur duringsulphidation because the fluids remain entirely within the H 2 S predominance field for dissolvedsulphur. PoPy AuHS(H 2 S) 3 Ht AuHS 10 -5 10 -4 10 -3 10 -2 10 -3 10 -2 10 -1 10 0 a(H 2 )a(H 2 S) log a( ∑ Au) = -7-4-6-5   -0.1-1-5-13-17-17.6 400°C1000 bneutral pH 12 Mt H 2 SHSO 4– -3CH 4 CO 2       ∆    3   4    S   H    2    S  –       ∑    S Figure 1:  Reaction paths showing changes of a(H 2 S) and a(H 2 ) in aqueous solution duringwall rock carbonation (solid lines) and sulphidation (dashed lines) at 400°C, 1000 bars andneutral pH from two initial fluid compositions with X(CO 2 ±CH 4 ) = 0.1. Also shown arecontours of total gold solubility (a( ∑ Au)); shifts of δ 34 S H 2 S relative to bulk sulphur insolution; equal activity boundaries for H 2 S/HSO 4 - , CO 2  /CH 4 , and AuHS/AuHS(H 2 S) 3 ; andstability fields for magnetite (Mt), hematite (Ht), pyrite (Py), and pyrrhotite (Po). Unitactivity coefficients assumed for all dissolved species. It is interesting to note that a high proportion of giant and supergiant mesothermal golddeposits worldwide exhibit variably negative sulphur isotope values and extensive carbonatealteration. In the Golden Mile, Hemlo (Canada), Kirkland Lake (Canada), Hollinger (Canada),and McIntyre (Canada) deposits, the sulphur isotope compositions have been interpreted asrecording involvement of oxidized ore-forming fluids. As shown here, fluid oxidationaccompanying carbonation of wall rock magnetite can be an efficient means of goldprecipitation. We speculate that it may play a key role in generating very large mesothermalgold deposits.
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