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

Abstract

Gold mineralisation at Fosterville occurs in association with disseminated arsenopyrite and pyrite adjacent to a complex series of quartz/carbonate veins along two NNW-trending fault zones. Quartz/carbonate veins, which have formed in dilational fractures within the sulphide ore zone, have δ 18O values between 16.0 and 17.6‰ 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 inclusions with approximately 10 vol.% CO 2-rich vapour, Type II CO 2-bearing inclusions with >30 vol.% vapour, and Type III liquid H 2O and liquid plus vapour CO 2 inclusions with variable H 2O/CO 2 ratios. Raman microprobe analysis showed that Types Ib, II and III contain CO 2 with variable amounts of N 2 and CH 4 in some inclusions. Type 1a, aqueous inclusions from Stage 4 of the paragenesis, have salinities ranging from 5 to 7 wt.% NaCl equivalent whereas the CO 2-bearing inclusions in the same stage, have average salinities around 3.5–4.0 wt.% NaCl equivalent. The CO 2-bearing inclusions exhibited two different types of homogenisation behaviour. Vapour-rich Type II and III inclusions either homogenised to vapour or showed near critical behaviour by gradual fading of the meniscus with homogenisation temperatures from 234 to 384°C and a mode at 270°C. Liquid-rich CO 2-bearing inclusions homogenised to the liquid phase with a bimodal distribution. The majority of inclusions homogenised between 243°C and 314°C with a mode around 270°C but a secondary mode occurs around 180°C. This mode corresponds fairly closely to the mode observed for the aqueous Type 1a inclusions at 170°C. The variable salinities, coexisting liquid- and vapour-rich inclusions, and the overlap of the homogenisation temperatures suggest that phase separation has occurred during vein formation and the resulting depth of mineralisation is estimated to be between 2.6 and 5.7 km. Gold precipitation resulted from the partitioning of H 2S into the vapour phase during fluid unmixing which was enhanced both by the decreasing pressure as fluid flowed from the faults and veins into the host rocks and by the addition of N 2 and CH 4. Comparisons with available fluid inclusion data for the Western Lachlan Fold Belt indicates that the Fosterville gold field formed at lower temperatures and at a higher crustal level than the quartz reef style of gold deposits in the Bendigo–Ballarat Zone. The fluids at Fosterville also contain greater amounts of N 2 and CH 4 suggesting that the mineralising fluids penetrated further into the host rocks and reacted either directly with carbonaceous matter or with reduced fluids contained within the host rocks.