Variation in platinum group mineral and base metal sulfide assemblages in the Lower Group chromitites of the western Bushveld Complex, South Africa

The Lower Group chromitites of the Bushveld Igneous Complex are mined for chromite as a primary product. The recovery of platinum group elements and base metals (Ni, Cu) as by-products has the potential to add value to the chromite resources. This study focuses on the LG-6 and LG-6A chromitite seams at the Thaba mine located on the western limb of the Bushveld Complex. Platinum group minerals and base-metal sulfides are studied by mineral liberation analysis and electron microprobe analysis to define distinct assemblages and to evaluate the potential for beneficiation. Based on the results two distinct major mineral assemblages are defined: The first assemblage is rich in platinum group element-sulfides, along with variable proportions of malanite/cuprorhodsite and alloys of Fe and Sn. The associated base metal sulfides are dominated by chalcopyrite, pentlandite, but also pyrite and subordinate millerite/violarite. Associated silicates are mainly primary magmatic orthopyroxene and plagioclase. The second assemblage is rich in platinum group element-sulfarsenides and -arsenides and -alloys of Sb and Bi, which are associated with a base metal sulfide assemblage dominated by pentlandite and Co-rich pentlandite. Silicates in this assemblage are marked by an abundance of alteration minerals, such as talc, serpentine and/or carbonates, which are closely associated with the platinum group minerals. Geostatistical evaluation reveals that these two mineral assemblages are not attributable to the origin of samples from different chromitite seams, but document the effects of pervasive hydrothermal alteration. Alteration evidently had similar effects on the different chromitite seams. Occurrence and distribution of these two characteristic assemblages has important implications for beneficiation. Assemblages rich in platinum group element-sulfides associated with base metal sulfides are known to respond well to flotation, different to alteration assemblages rich in arsenides and alloys. The nature of the gangue minerals will also impact platinum group mineral recovery as high phyllosilicate abundances, such as those noted in the alteration assemblage may cause problems during flotation and lead to poor recoveries.