Multiple sulfur isotopes reveal a possible non-crustal source of sulfur for the Bushveld Province, southern Africa

The source of sulfur for sulfide mineralization is a major question for the origin of platinum group element deposits such as the Rustenburg Layered Suite (RLS) of the Bushveld Complex and the nearby Waterberg Project (WP; a large palladium-dominant deposit) in southern Africa. Both deposits are mafic-ultramafic intrusions associated with the ca. 2.06 Ga Bushveld magmatism but are hosted in distinct country rocks. This contrast allows a critical assessment of the contribution upper crustal assimilation provides to sulfide mineralization, and refinement of our understanding of sources of mass-independent fractionated sulfur (MIF-S) to these intrusions. The WP has a signature of anomalous sulfur (average Δ33S = 0.113‰ ± 0.016‰, 1 s.d.), similar to the RLS (avgerage Δ33S = 0.137‰ ± 0.025‰, 1 s.d.). There is no evidence for influence of host rock as a source of anomalous sulfur. The lack of a significant variation of Δ33S values within the WP stratigraphy, and the distinct upper continental crust into which the WP magmas would have been emplaced, shows that addition of upper crustal sulfur is not necessary for PGE formation. This suggests that contamination of WP and RLS magmas with a surface-derived component of Archean age occurred at depth, prior to emplacement.