Abstract

This study investigates whether hydrothermal alteration of metasedimentary rocks associated with orogenic gold mineralization in the Victorian gold province produces systematic changes in oxygen, carbon and sulphur isotope compositions that might provide explorationists with vectors towards ore. The presence of systematic isotopic trends across wallrock alteration haloes in gold deposits in the Stawell, Percydale, Ballarat, Bendigo and Fosterville goldfields clearly supports the interpreted hydrothermal origin of these haloes. General increases in carbonate δ13C towards the ore zones in all deposits record the influx of hydrothermal CO2. Whole-rock silicate and quartz δ18O, carbonate δ18O, and sulphide δ34S shifts equally mirror petrological and geochemical changes that resulted from prolonged and extensive interaction between externally derived hydrothermal fluids and the surrounding siliciclastic wallrocks. With the possible exception of sulphur, however, the isotopic enrichment/depletion trends are subtle and also vary significantly from deposit to deposit. Results of this study and comparisons with published data suggest that the oxygen and carbon isotopic ratios of silicates and carbonates in whole-rock samples within the alteration haloes are buffered by the wallrocks. Hydrothermal sulphides in Victorian orogenic gold deposits are characterized by δ34S values generally in the range −5 to +5‰. However, there is relatively little influx of hydrothermal sulphur beyond c. 5 to 10 m from the vein margins and disseminated pyrite well removed from the lode systems is characterized by either greatly negative or positive δ34S values typically greater than±10‰.

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