GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS NO.37 12th Australian Geological Convention, Perth, September 1994 containing approximately 16±8 mole% C0 2 ,n = 60. The total homogenization temperature of both inclusion types is 260°±50° C at pressures of <0.5 to 1.5 kbar. Stage II fluids may thus have formed through cooling of Stage I type fluids which had earlier undergone phase separation. This explains their lower homogenization temperatures and the lack of a high Xc02 phase in Stage II mineralization. Calculated pressures for both stages of mineralization correspond to paleodepths of <5 km assuming lithostatic fluid pressures. Calculated pH values for Stage I mineralization vary from 5.3 to 5.7, indicating neutral to slightly alkaline ore fluids. Calculated fo 2 values for Stage I ore fluids range from about 10~31 to 10~32-7 bars and correspond to oxidation states roughly 1 to 1.5 log f02 units above the CO2-CH4 redox buffer. Reduced sulphur activities are high, afj2S ranging from 10"17 to 10~2. Calculated fo2 for Stage II gold-stibnite mineralization ranges from 10"39-3 to lO"^0 * bars, slightly above the CO2-CH4 buffer, and aH2S values of the ore fluids are between 10"3-3 to 10~3*8. Results of stable isotope studies on alteration minerals associated with both mineralization stages are as follows: Stage I gold-pyrite-arsenopyrite mineralization: < 5 1 3 C c 0 2 (carb) = -4-3 to -l.l%o (av. -3.0±0.7%o, n = 33), <5180fluid (carb) = 2.8 to 13.3%o (av. 6.1±2.8%o, n = 33), <5Dfluid ( s e r , chl) = -6 to 9%o (av. 0±4%o, n = 9), <5Dfluid incl. = -66 to -37%o (av. -47±13%o, n = 7), and<534SH2S (py)== 02 to 1.5%o (av. 1.0±0.5%o, n = 6). Oresamples containing Group 2 fluid inclusions have<5180fluid (qtz) = -1.2 to 7.0%o (av. 4.0±1.8%o, n = 98) while ore samples containing Group 3 fluid inclusions have<5180flujd (qtz) = -2.8 to 1.6%o (av. -1.2±1.6%o, n = 11). Stage II gold-stibnite mineralization has<5i8Ofluid (qtz) = 2- 1 1 0 6.9_%o(av. 5.5 ±1.0%o, n = 36),<5Dfluid incl. = 57 to -19%o (av. -42 ±17%o, n = 4), and<534SH2S (stib) = -2.1 to -2.6%o (av. -2.4 ±0.3%o, n = 4). Carbon and sulphur isotope signatures are not diagnostic of a particular fluid source, allowing igneous, sedimentary and/or metamorphic sources for carbon and magmatic and/or sedimentary sources for sulphur. In contrast, the combination of enriched D and depleted 1 8 0 relative to magmatic and metamorphic fluids implies the involvement of surface waters in some of Stage I mineralization. In particular, ore samples that contain Group 3 H20-NaCl inclusions are interpreted to have experienced a major input of surface waters. DEPOSITIONAL MODEL A fluid flow model for the Wiluna lode-gold deposits suggests a contemporaneous, dual hydrologic system in which shallow surface water convected in the upper parts of the strike-slip fault system and mixed with pulses of advecting deep-seated magmatic/metamorphic/ ±mantle? fluids. Stage I gold-pyrite-arsenopyrite mineralization is interpreted to have precipitated in equilibrium with a mixture of deep fluids that experienced major dilution by surface fluids. The relatively reduced, slightly alkaline and low-salinity composition of the ore fluid favours gold bi-sulphide complexes as transport mechanism for gold. The close spatial association between gold and the sulphide-rich proximal alteration of Stage I ores argues for wallrock sulphidation as the predominant gold depositional mechanism. The resultant concurrent drop in ore fluid an2S f<02 would cause destabilization of gold-bisulphide complexes and trigger gold precipitation. Phase separation of Stage I ore fluids is characterized by no or only minor sulphide precipitation, and thus the magnitude (extent) of phase separation was evidently insufficient to allow vapor separation to compete with fluid:wallrock reactions in influencing gold solubilities. Stage II gold-stibnite mineralization is interpreted to result from a renewed pulse of deep (magmatic/metamorphic/ ±mantle?) fluids with no or only minor dilution by surface fluids. Given the temperature conditions of stibnite mineralization (260°±50° C) and the lack of evidence for acidic alteration within the stibnite-bearing lodes, stibnite solubility was probably controlled by hydroxothioantimonite species and stibnite precipitation caused by loss of H2S (cf Krupp, 1988). In the absence of conclusive evidence for phase separation during Stage II, simple conductive cooling of the ore fluid probably reduced stibnite solubility, and decreased concentration of H2S, in turn ,might have destabilized complexes such as Au(HS)2" thereby triggering gold precipitation. REFERENCES Angelier, J., 1989. From orientation to magnitudes in paleostress determination using fault slip data. Jour. Structural Geology 11, 37-50. Krupp, R. E., 1988. Solubility of stibnite in hydrogen sulphide solutions, speciation, and equilibrium constants, from 25 to 350° C. Geochimica et Cosmochimica Acta 52, 3005-3015. 161