Origin of the anomalous sulfur isotope composition of the Rustenburg Layered Suite (Bushveld Complex), South Africa

Abstract

The 2.06 Ga Bushveld Magmatic Province (BMP) hosts the largest platinum group element (PGE) reserve of the world that occurs mainly as sulfide-rich layers within the Rustenburg Layered Suite (RLS), and also in mineralized layers of the Waterberg Project (WP). Despite extensive studies, many questions remain on the origin and evolution of this large igneous province, and on the source of sulfur that allowed for the extensive PGE mineralization.

This study looks systematically into the multiple sulfur isotope composition of the RLS, finding that all layers show the presence of a mass-independently fractionated sulfur component (Δ33S≠0), which are all distinguishable from the expected Δ33S value of the mantle. The exogenic sulfur reflects contamination by Archean surface-derived material (e.g. sediments, altered oceanic crust). Such contamination can occur in many different stages of the evolution of these intrusions: either by assimilation of wall rock during ascent and emplacement, or in a staging chamber in the lower crust, or by recycling of crustal material in an ancient subduction zone.

The WP, an intrusion related to the BMP that was emplaced off-craton, has a similar sulfur composition to the Main Bushveld Series of the RLS. It is, however, a separate intrusion that crystallized in a separate magma chamber and was emplaced in a different unit than the RLS, which suggests that the contamination of the parental magma occurred at a deeper level, prior to emplacement of magma in the upper crust. Rocks from the Vredefort Dome, used as a proxy for the sulfur composition of the lower crust underneath that region, yield a sulfur composition that cannot account for the composition of the RLS or the WP. Finally, the sub-continental lithospheric mantle has been studied through xenoliths carried by the Premier Kimberlite. These xenoliths, such as what was observed in sulfide inclusions in diamond, also have Δ33S≠0, evidencing that the sub-continental lithospheric mantle may contain recycled sulfur that contributed this sulfur to primitive magmas during the Bushveld magmatic event.