Multiple sulfur isotopes reveal a magmatic origin for the Platreef platinum group element deposit, Bushveld Complex, South Africa

Abstract

The Platreef ore horizon of the Bushveld Complex, South Africa, is the third largest platinum group element ore deposit in the world, but the origin of its ore remains enigmatic. A complex contact relationship between the igneous and footwall rocks of the Bushveld Complex, coupled with evidence for widespread late-stage hydrothermal processing, obscures the original mineralization history of the Platreef. We constrain the parental magmatic origin of the Platreef by exploiting multiple sulfur isotope contrasts across Bushveld Complex contact zones in order to see through the effects of postmineralization hydrothermal activity. We report S isotope measurements made on samples collected along two profi les through the Platreef into underlying metapelitic and metacarbonate footwall rocks. In both profi les, igneous rocks far from the contact have low Δ 33 S values (average Δ 33 S = 0.15‰), whereas metasedimentary rocks far from the contact have high Δ 33 S values (Δ 33 S to 5.04‰) with a smoothly varying profi le between the two end members. The midpoint in both isotope profi les is displaced into the footwall, defi ning a classic advective-dispersive tracer geometry. This geometry is not present in the associated δ 34 S values. The displacement of the Δ 33 S front suggests fl uid transport and advection of S into the country rocks; this was accompanied by back diffusion of the S isotope tracer into the Platreef. The Platreef magma was apparently S saturated prior to emplacement and, counterintuitively, lost S during the formation of the present Platreef ore horizon.