Enigmatic High-Tenor Rh-, Ru-, Ir-, and Os-Rich Base Metal Sulfide Mineralization Within the Northern Limb of the Bushveld Complex: A Product of Fractionation of a Sulfide Liquid?

Abstract

The Base Metal zone at Sandsloot in the Northern limb of the Bushveld Complex, South Africa, is a highly unusual and high-grade Os-Ir-Ru-Rh, Fe-Ni sulfide-rich horizon hosted within the deep Platreef, below the main platinum group element (PGE) horizon. The Base Metal zone ranges from 5 to 100 meters in thickness and is located up to 150 meters beneath the PGE reef. Base metal sulfide mineralization occurs as disseminated/blebby to semimassive/massive sulfides, with a typical assemblage of ~60/25/15 pyrrhotite/pentlandite/chalcopyrite modal %. The Base Metal zone is characterized by high (Os + Ir + Ru + Rh)/(Pt + Pd) ratios that reflect monosulfide solid solution, primitive mantle-normalized PGE profiles. The PGM assemblage is dominated by laurite (RuS2) (62% by area) and iridium-group platinum group element (IPGE) + Pt arsenosulfides (21% by area). The PGE tenors of the sulfides vary between different textural styles, either reflecting R-factor variations or dilution of tenors by addition of crustal S. Disseminated/blebby sulfides have the highest tenors (up to 153 ppm Pd, 249 ppm Rh, 818 ppm Ru), whereas semimassive/massive sulfides have lower tenors (up to 2.8 ppm Pd, 1.8 ppm Pt, 11 ppm Rh, 17 ppm Ru, 2.2 ppm Os, 3.5 ppm Ir). The PGE geochemistry, IPGE-dominant platinum group metal (PGM) assemblage, abundance of Fe sulfides, and high Ni/Cu ratios are consistent with the Base Metal zone representing the monsulfide solid solution portion of a sulfide liquid formed by fractional crystallization. Furthermore, the Cu + Pt + Pd + Au-poor nature of the Base Metal zone suggests that these metals were removed from the Base Metal zone, and some Cu-rich veins and sections are present around the margins of Ni-Fe sulfide to support this. Increasing Pd/Ir and decreasing Rh/Cu ratios downhole indicate the sulfide liquid fractionated downward. Therefore, a residual Cu-rich liquid, with associated Pt + Pd + Au, likely separated from monosulfide solid solution and was mobilized downward and away from the Base Metal zone. Significantly, the mobilization of a Cu-rich liquid leaves the possibility that an undiscovered Cu + Pt + Pd + Au orebody may exist at depth.