- © 2010 Geological Society of South Africa
A petrographic and mineral chemical study was conducted on pyrite from non-conglomeratic sedimentary rocks of the 3.0 to 2.8 Ga Witwatersrand Supergroup. Detailed petrographic analyses revealed the presence of three paragenetic associations, namely detrital, diagenetic and epigenetic pyrite. Detrital pyrite is commonly observed in sandstone and diamictite from proximal shelf deposits. The mineral chemistry of large (0.1 to 0.5 mm in size) and small (<0.05 mm in size) detrital pyrite grains, based on Co, Ni and As contents, suggests a metamorphic and diagenetic origin, respectively. Significantly, a variety of early diagenetic pyrite forms has survived greenschist facies metamorphism; these occur exclusively in carbonaceous mudstones intercalated with fluvial inner shelf sandstones. The high Ni content of early diagenetic pyrite and the occurrence of early diagenetic Cu-Pb sulfides highlight the importance of organic matter and host rock composition. However, in distal to starved outer shelf depositional settings, detrital pyrite is absent and diagenetic pyrite is scarce. Nevertheless, the composition of diagenetic pyrite in distal magnetite-rich lithologies is similarly affected by host rock composition and possibly Fe-Mn reduction (elevated Co and As concentrations). Epigenetic pyrite, pyrrhotite and base-metal sulfides are prevalent in sandstone intercalations near diabase sills and fault zones. The composition of epigenetic pyrite is influenced by the temperature of the fluids and the interaction of the fluids with the host rock, as well as with precursor and associated iron sulfides.
This study finds that local depositional and tectonic settings are instrumental in dictating the distribution, morphology and composition of diagenetic pyrite. In this regard, the formation of metal-rich early diagenetic pyrite is favored in carbonaceous mudstones associated with fluvial regimes in the upper Hospital Hill, Government, Jeppestown, Johannesburg and Turffontein Subgroups, where continental runoff would have supplied nutrients and sulfur species to the shallow water environment. In contrast, the scarcity of diagenetic pyrite in marine-dominated environments of the lower Hospital Hill Subgroup (of which some are carbonaceous) and distal portions of the Government and Jeppestown Subgroups, is consistent with a lack of sulfate in the Archean ocean.