Multiple sulfur isotopes in granite-hosted sulfides from the Bushveld Igneous Complex

The 2.05 Ga Bushveld Igneous Complex (BIC), the largest mafic intrusion in the world, is host to three Platinum Group Element (PGE) ore horizons with the largest reserves (known as Merensky Reef, Platreef, and UG2 Chromitite) within the Rustenburg Layered Suite (RLS), the layered mafic/ultra-mafic and anorthosite suite of the BIC. Because the PGEs are hosted in sulfide minerals, it is likely that sulfur, and sulfide production, is critical to understanding how and why the PGE reserves formed in the BIC. Other mineralized bodies like the Leeuwport and Zaaiplaats tin mines within the BIC, as well as a suite of coeval felsic volcanics (Rooiberg Group) and plutonic rocks (Lebowa Granite Suite) are also important in understanding sulfur in the BIC. The origins of the sulfur necessary to produce ore bodies of such magnitude are of prime economic and academic importance. Analyzing multiple sulfur isotopes in various magmatic bodies across the Bushveld Complex can help to elucidate the origin of the sulfur.
In this study, we analyze the multiple sulfur isotopic composition (∆33S) of several hydrothermal ore bodies of tin and fluorite, and brecciated pipes in discordant granites that are found throughout the Bushveld. We hypothesize the tin ore fluid mineralization hosted in both the plutonic, felsic volcanic units (Lebowa Granite Suite) and sedimentary host-rock (Transvaal Supergroup) derive sulfur from the same sources as the mafic (Rustenburg Layered Suite) parts of the complex, which would be shown by similar values of surface-derived sulfur (∆33S) in these rocks. The average value of ∆33S in the RLS is 0.114±0.030‰; Δ33S values of 0.251‰ found in the tin mineralizations hosted by granites show an overlap with the other intrusive bodies in the RLS, which indicates a similar source of sulfur for the magmatic complex, while the brecciated pipes, which contain clasts of sedimentary materials, show the most elevated Δ33S (average of 0.368‰) observed in Bushveld-related rocks to date, suggesting that sediments from the Transvaal Supergroup also contributed sulfur to the mineralization.