A fluid inclusion study of the Fosterville Mine: a turbidite-hosted gold field in the Western Lachlan Fold Belt, Victoria, Australia

Gold mineralisation at Fosterville occurs in association with disseminated arsenopyrite and pyrite adjacent to a complex series of quartz/carbonate ve ins along two NNW-trending fault zones. Quartz/carbonate veins, which have formed in dilational fractures within the sulphide ore zone, have delta O-1 8 values between 16.0 and 17.6 parts per thousand and contain the following types of fluid inclusions: Type Ia two-phase, liquid-rich inclusions with less than 10 vol.% vapour and no detectable gases. Type Ib two-phase inclus ions with approximately 10 vol.% CO2-rich vapour, Type II CO2-bearing inclu sions with > 30 vol.% vapour, and Type III liquid H2O and liquid plus vapou r CO2 inclusions with variable H2O/CO2 ratios. Raman microprobe analysis sh owed that Types Ib, II and III contain CO2 with variable amounts of N-2 and CH4 in some inclusions. Type Ia, aqueous inclusions from Stage 4 of the paragenesis, have salinitie s ranging from 5 to 7 wt.% NaCl equivalent whereas the CO2-bearing inclusio ns in the same stage, have average salinities around 3.5-4.0 wt.% NaCl equi valent. The CO2-bearing inclusions exhibited two different types of homogen isation behaviour. Vapour-rich Type II and III inclusions either homogenise d to vapour or showed near critical behaviour by gradual fading of the meni scus with homogenisation temperatures from 234 to 384 degreesC and a mode a t 270 degreesC. Liquid-rich CO2-bearing inclusions homogenised to the liqui d phase with a bimodal distribution. The majority of inclusions homogenised between 243 degreesC and 314 degreesC with a mode around 270 degreesC but a secondary mode occurs around 180 degreesC. This mode corresponds fairly c losely to the mode observed for the aqueous Type la inclusions at 170 degre esC. The variable salinities, coexisting liquid- and vapour-rich inclusions , and the overlap of the homogenisation temperatures suggest that phase sep aration has occurred during vein formation and the resulting depth of miner alisation is estimated to be between 2.6 and 5.7 km. Gold precipitation resulted from the partitioning of H2S into the vapour ph ase during fluid unmixing which was enhanced both by the decreasing pressur e as fluid flowed from the faults and veins into the host rocks and by the addition of N-2 and CH4. Comparisons with available fluid inclusion data fo r the Western Lachlan Fold Belt indicates that the Fosterville gold field f ormed at lower temperatures and at a higher crustal level than the quartz r eef style of gold deposits in the Bendigo-Ballarat Zone. The fluids at Fost erville also contain greater amounts of N-2 and CH4 suggesting that the min eralising fluids penetrated further into the host rocks and reacted either directly with carbonaceous matter or with reduced fluids contained within t he host rocks. Published by Elsevier Science B.V.