Sulfur Isotope Variations within the Platreef Ni-Cu-PGE Deposit: Genetic Implications for the Origin of Sulfide Mineralization

Abstract

The Platreef, the world’s third largest platinum group element (PGE) deposit, is a 10–400-m thick pyroxenitic unit at the base of the northern limb of the mafic Bushveld Complex, hosting PGE mineralization in association with base metal sulfides. The sulfide mineralization is thought to have been either largely magmatic in origin, with contamination by assimilation of local floor rocks considered an ore-modifying process, or a product of assimilation of country-rock S and silicic contamination which caused S saturation in the magma, with the contamination being fundamental to the ore-forming process. We have performed an extensive and detailed sulfur isotope study of the Platreef, which indicates that magmatic signatures (δ34S = 0 to +2‰) are preserved in early-formed sulfide droplets within the Platreef pyroxenites in the area from Sandsloot to Witrivier. These values are comparable to sulfide inclusions in diamonds in the nearby Klipspringer kimberlite and are considered to have a primary mantle origin. There is no indication of any significant external S in any of the primary sulfides in the Sandsloot-Witrivier area, however, later sulfides found in calc-silicate floor rocks at Sandsloot and Zwartfontein and in xenoliths of calc-silicate throughout the section indicate an input of country rock S. Anhydrite-bearing horizons in the Malmani dolomites may have exchanged S with magmatic Platreef sulfide during Platreef intrusion, imparting a heavier S isotope signature in the fluid-affected rocks than those in the early sulfides. The Archean basement, although containing minor amounts of sulfide with a negative δ34S value, is not a significant contributor to the S budget of the Platreef. Previous studies have indicated that in areas where the sedimentary floor rocks contain appreciable sulfides, rather than sulfates, such as at Turfspruit, the Platreef sulfides are extensively contaminated with country-rock S. Assimilation of pyrite-bearing shales in the Turfspruit area has locally upgraded the S content of the Platreef and given the basal Platreef sulfides a heavier isotopic signature. Sulfur saturation in the Platreef magma took place before contamination, probably in a staging chamber prior to intrusion. A major pulse of magma entrained the preformed PGE-rich sulfides and was injected to form the Platreef, where assimilation of country-rock sulfides upgraded the S content on a strictly local scale, and hydrothermal leaching introduced S from country-rock sulfates into later stage sulfides, again on a local scale. South of Zwartfontein, it is proposed that the Platreef was intruded into sediments of the Transvaal Supergroup, which formed the floor and the roof of the Platreef. North of Zwartfontein, the Platreef intruded the boundary between the Malmani Supergroup sediments and the Archean basement. This could have potentially mineralized the roof calc-silicates north of Zwartfontein in a manner similar to the floor in the Sandsloot area. The later intrusion of the Main zone magma formed a magmatic unconformity on top of the cold Platreef and entrained large rafts of mineralized calc-silicate for tens of kilometers north of the last footwall outcrop of calc-silicate.