Contribution of the oldest Paleoproterozoic marine sulfate evaporites to Bushveld Complex Lower Zone mineralization

Abstract In the past two decades, multiple-sulfur isotope systematics has become increasingly used as a tool to differentiate sulfur sources with implications for exploration targeting. The involvement of crustal sulfur in triggering the Bushveld Complex (South Africa) Cu-Ni-PGE (PGE—platinum group element) mineralization of the Platreef intrusion is widely recognized; however, the extent and source of contamination are still debated. High-Mg magma of the Uitloop Lower Zone intrusion contained insufficient amounts of mantle-derived sulfur to reach the sulfide saturation required for mineralization. Sulfide saturation was reached when magma assimilated sulfur as it ascended through sedimentary strata deposited before and during the Great Oxidation Episode (GOE). Magmatic sulfides on Uitloop are 34S-enriched and either lack, or show small, mass-independent sulfur isotope fractionation, whereas sulfides from the Archean sedimentary footwall rocks exhibit the Archean-style multiple sulfur isotope covariations. We suggest that assimilation of the early Paleoproterozoic upper Duitschland Formation sulfate evaporites, corresponding to an ingrowth of a mass-dependently fractionated seawater sulfate reservoir at the early stage of the GOE, led to sulfide saturation and formation of Cu-Ni-PGE Platreef-style sulfide deposits. The GOE thus not only changed the oxidation state of the atmosphere and oceans, and the style of sediment-hosted mineral deposits, but also generated the sedimentary sulfate reservoir that since then served as a prolific sulfur source for magmatic ore deposits.