Abstract

The copper ores at Mount Isa (Queensland, Australia) are hosted by the Urquhart Shale, a metamorphosed Proterozoic sequence of dolomitic siltstones and shales. Sulfur isotope data from copper ores and their hosts suggest that the highly pyritic Urquhart Shale provided a major sulfur component to the copper ores and also that variable contributions from an 34 S-enriched source have been added along with copper by the hydrothermal brine. The ratio of sedimentary-diagenetic sulfur to introduced sulfur is poorly constrained because of the probable isotopic heterogeneity of the sedimentary-diagenetic pyrite at the onset of copper mineralization. The isotopic evidence for large-scale incorporation of sedimentary sulfide sulfur into the epigenetic copper ores is consistent with the suggestion that the sulfides in the replaced metasediments were an important ingredient in the chemical trap that contributed to the localization of the copper ores at Mount Isa.

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