Stable isotope studies of the origins of mineralization at Mount Isa. I.

Further support for the view that mineralization at Mount Isa comprises two separate events is provided by ³⁴S/³²S measurement. Isotopic exchange between sulphides in lead-zinc-silver ores appears to have been promoted locally during metamorphism, whereas isotopic disequilibrium persists in the copper ores. These isotopic data are explained by a model in which sedimentary deposition of lead, zinc and silver was succeeded by the post-metamorphic emplacement of copper. Past biological activity is inferred from the occurrence of low concentrations of organic carbon with ¹³C values ranging from –21 to –26 PDB. Carbonate contents, expressed as carbon, vary from <0.1% to 10.9%. The ¹³C and ¹⁸O values for the carbonates are relatively constant at –4.4±1.1 and –17.6±1.1 PDB respectively. These values are interpreted as reflecting isotopic changes induced in original marine carbonates by isotopic exchange during lower greenschist metamorphism.