Platinum-group element distribution in base-metal sulfides of the UG2, Bushveld Complex, South Africa – a reconnaissance study

The UG2 chromitite of the Bushveld Complex in South Africa contains the world’s largest resources of platinum-group elements (PGE). However, only limited work has been conducted on the distribution of PGE and platinum-group minerals (PGM) within the UG2. In the present study, earlier work on the PGE distribution within the Merensky Reef (Osbahr et al. 2013) is followed up and extended to the UG2. Two drill cores of the UG2 chromitite from the eastern and western Bushveld Complex were studied by whole rock analysis, ore microscopy and SEM/MLA. The PGE, Au and Ag contents of pentlandite, pyrrhotite and chalcopyrite were determined using LA-ICP-MS analysis. Whole-rock analyses of the two profiles reveal highest Pd and Pt concentrations at the top and at the base of the UG2 main seam. Sulfides mostly occur as aggregates of pentlandite, chalcopyrite and rare pyrrhotite and pyrite or as individual grains associated mostly with chromite grains.
In-situ LA-ICP-MS analyses reveal that pentlandite carries distinctly elevated PGE contents. In contrast, pyrrhotite and chalcopyrite contain very low PGE concentrations. Pentlandite shows average maximum values of 350-1000 ppm Pd, 200 ppm Rh, 130-175 ppm Ru, 20 ppm Os and 150 ppm Ir and therefore, is the principal host of Pd and Rh in the studied ores of the UG2.
Mass balance calculations were conducted for samples representing the UG2 main seam of the drill core DT46, eastern Bushveld. Accordingly, pentlandite constantly hosts elevated contents of the whole rock Pd (up to 55 %) and Rh (up to 46 %), and erratic contents of Os (up to 50 %), Ir (2 to 17 %) and Ru (1 - 39 %). PGM investigations support these mass balance results, most of the PGM are Pt-dominant PGM like braggite/cooperite and Pt-Fe alloys or laurite, (carrying elevated concentrations of Os and Ir). Palladium and Rh-bearing PGM are rare.
PGE concentrations and their distribution in BMS in the UG2 largely resemble that of the Merensky Reef, as most of the Pd and Rh are incorporated in pentlandite whereas pyrrhotite, chalcopyrite and pyrite are almost devoid of PGE. Consequently, similar mineralization processes are responsible for the PGE enrichment in the UG2 and in the Merensky Reef.