Platinum-group minerals in chromitite xenoliths from the Onverwacht and Tweefontein ultramafic pipes, eastern Bushveld Complex, South Africa

The ultramafic pipes of Onverwacht and Tweefontein, in the eastern part of the Bushveld Complex, in South Africa. contain small xenoliths of chromitite that are believed to originate from the LG6 and MG4 chromitite layers, intersected during the emplacement of the pipes. Platinum-group minerals in the chromitite xenoliths consist of polyphase grains made up of: a) laurite along with unknown Ir-Ni-Fe sulfides and base-metal sulfides, occurring almost exclusively in fresh chromite (type-1 assemblage). b) abundant alloys (isoferroplatinum, ruthenium) and Rh-Pd-Ru arsenides (cherepanovite, ruthenarsenite, rhodarsenide, and palladodymite or palladoarsenide, polymorphs of Pd2As). with accessory laurite and Pd antimonides (stibiopalladanite or sudburyite), included in both fresh chromite and interstitial olivine (type-2 assemblage), and c) relatively large grains of sperrylite and hollingworthite with minor laurite associated with the altered portions of the chromitite host (type-3 assemblage), Only assemblages of type I resemble the PGM observed in undisturbed LG6 and MG4 chromitite layers. in which laurite and minor cooperite are the dominant phases, The paucity of sulfides and the enrichment in alloys and As-rich phases make type-2 and type-3 assemblages more similar to the PGM associations reported from mineralized dunite in the platiniferous pipes, and metasomatized layers of chromitite adjacent to the pipes. These sulfur-poor assemblages of PGM are considered to have formed by metasomatic reaction or the chromitite with the volatile-rich component of the pipes over a range of temperatures, during and after the emplacement of the pipes.