Sedimentologic, petrographic, and sulfur isotope constraints on fine-grained pyrite formation at Mount Isa mine and environs, northwest Queensland, Australia

Fine-grained pyrite is the earliest generation of pyrite and the most abund ant sulfide within the Urquhart Shale at Mount Isa, northwest Queensland. T he pyrite is intimately interbanded with ore-grade Pb-Zn miner alization at the Mount Isa mine but is also abundant north and south of the mine at sev eral stratigraphic horizons within the Urquhart Shale. Detailed sedimentolo gic, petrographic, and sulfur isotope studies of the Urquhart Shale, mostly north of the mine, reveal that the fine-grained pyrite (delta(34)S = -3.3 to +26.3 parts per thousand) formed by thermochemical sulfate reduction dur ing diagenesis. The sulfate source was local sulfate evaporites, pseudo mor phs of which are present throughout the Urquhart Shale (i.e., gypsum, anhyd rite, and barite). Deep-burial diagenetic replacement of these evaporites r esulted in sulfate-bearing ground waters which migrated parallel to bedding . Fine-grained pyrite formed where these fluids infiltrated and then intera cted with carbon-rich laminated siltstones. Comparison of the sulfur isotope systematics of fine-grained pyrite and spa tially associated base metal sulfides from the Mount Isa Pb-Zn and Cu orebo dies indicates a common sulfur source of ultimately marine origin for all s ulfide types. Different sulfur isotope ratio distributions for the various sulfides are the result of contrasting formation mechanisms and/or depositi onal conditions rather than differing sulfur sources. The sulfur isotope sy stematics of the base metal and associated iron sulfide generations are con sistent with mineralization by reduced hydrothermal fluids, perhaps generat ed by bulk reduction of evaporite-sourced sulfate-bearing waters generated deeper in the Mount Isa Group, the sedimentary sequence which contains the Urquhart Shale. The available sulfur isotope data from the Mount Isa orebod ies are consistent with either a chemically and thermally zoned, evolving C u-Pb-Zn system, or discrete Cu and Pb-Zn mineralizing events linked by a co mmon sulfur source.