Multiple Sulfur Isotopes Reveal a Possible Non-Crustal Source of Sulfur for the Bushveld Province

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). Also informally called the "Far Northern Limb", the WP is a newly described high-grade deposit of platinum group elements (PGE) that is located in the southern margin of the Limpopo Belt. Both deposits are mafic-ultramafic intrusions associated with ~ 2.06 Ga Bushveld magmatism, and are thought to be separate bodies.

We use multiple sulfur isotopes to make a critical assessment of the contribution upper crustal assimilation provides to sulfide mineralization and refinement of our understanding of sources of mass-independently fractionated sulfur (MIF-S) to these intrusions. The sulfur isotope composition of the WP is very similar to the Eastern and Western Limbs of the RLS, with an average Δ³³S of 0.113‰±0.016‰, 1 s.d. (RLS average = 0.137‰±0.025‰, 1 s.d.). The footwall, a sulfur-rich granofels, has Δ³³S equal to 0.030‰.

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.