Variations in the nature of the platinum-group minerals in a cross-section through the merensky reef at impala platinum : implications for the mode of formation of the reef

A study of the abundance, size, distribution and composition of platinum-group minerals in samples from a section of the Merensky Reef at Impala Platinum, on the farm Reinkoyalskraal, in the western Bushveld Complex, South Africa, has shown that melanorite. leuconorite and anorthosite contain a PGM assemblage that consists almost exclusively of Pt and Pd bismuthotellurides, predominantly moncheite and merenskyite. In the chromite-rich lithologies, this assemblage of Pt-Pd-Bi telluride PGM is joined by a Pt-Pd-Rh sulfide PGM assemblage of cooperite, braggite and an unnamed Cu-Pt-Rh Sulfide, with laurite and rare Sn-bearing PGM. This additional assemblage tends to be Pd-poor. The PGM are rarely enclosed by chromite. All the PGM are predominantly associated with base-metal sulfides, either as euhedral PGM or laths forming an exsolution texture within the base-metal sulfides. Rhodium, present as the unnamed Cu-Pt- Rh sulfide, is associated with pyrrhotite and pentlandite. Throughout this section of the reef, the PGM are commonly located at the edge of base-metal sulfides adjacent to serpentine, chlorite and amphibole that form on the edges of silicate grains. In the chromite-poor samples, Pt-Pd-Bi tellurides, and their associated base-metal sulfides are located commonly within silicates, including plagioclase and quartz. The chromite-bearing rocks in this section of the Merensky Reef are enriched in Os, It, Ru. Rh and Pt. We test three models for the formation of the PGM: coprecipitation of PGM and chromite. crystallization of PGM from a sulfide liquid, and redistribution of PGE and base metals by hydrous intercumulus fluid. The strong association of PGM with base-metal sulfides suggests that the PGE were collected by an immiscible base-metal sulfide liquid. This liquid crystallized as Mss, with Rh being concentrated in the Mss, and then as Iss. These exsolved to pyrrhotite, pentlandite and chalcopyite and PGM. fit the chromite-rich layer, we note a lack of minerals containing Pd in the PGM assemblage. No one model satisfactorily explains the PGM distribution. Rather, the PGM observed are likely to result from late, low-temperature processes superimposed on the magmatic ones.