Genesis of late Proterozoic copper mineralization, Copper Claim, South Australia

Baas Becking Geobiol. Lab., Canberra, Australia

Bedded disseminations. No evidence for hydrothermal alteration. The disseminated sulfides have a delta ³⁴ S range of -17.3 to 15.8 per mil, consistent with bacterial reduction of sulfate. Veinlet sulfides have a similar but smaller delta ³⁴ S range, in accord with formation by remobilization of the disseminated sulfides. No significant delta ¹³ C differences between bedded and veinlet carbonates; marked variations in delta ¹⁸ O values, with magnesite values being generally higher than dolomite, and the median value for bedded carbonates being around 23 per mil, some 3 per mil higher than the median value for veinlet carbonates. Low-temperature oxygen isotope equilibration during early diagenetic carbonate reactions. Low-grade regional metamorphism caused minor recrystallization and formation of additional veinlets but did not extensively overprint the diagenetic isotopic trends. Isotopic similarities between the cupriferous and the unmineralized lithologies support introduction of the copper during sedimentation and/or early diagenesis. Veinlet carbonates contain small NaCl-rich fluid inclusions, formed at around 300 degrees C and 4 km depth, during the metamorphism. General trend of increasing mean delta ³⁴ S values with decreasing age of the late Proterozoic strata in the Adelaide geosyncline and Stuart shelf. This trend may be the product of local environmental conditions which resulted in sulfate reduction outweighing sulfate replenishment; alternatively, it could be reflecting global processes which caused major ³⁴ S enrichment of seawater sulfate.--Modified journal abstract.

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