Platinum-group elements and platinum-group minerals in the weathered chromitities of the Lower and Middle Group chromitites of the Bushveld Complex

The mineralization of economically important platinum group elements (PGE) mainly occurs in mafic-ultramafic intrusions such as the Bushveld Complex. PGE are currently mined from the Merensky Reef, the Platreef and the UG-2 chromitite, which is the only chromitite layer mined for PGE, however, the Lower Group (LG) and Middle Group (MG) chromitites also contain significant PGE concentrations ranging from 0.5 to 9 g/t. In the pristine ores, PGE occur both as discrete platinum-group minerals (PGM) and included within sulfides. Previous studies showed that in the LG-6 and MG-1/2, PGE-sulfides (cooperite-braggite, malanite, laurite) are the dominant PGM, followed by PGE-sulfarsenides, sperrylite and Pt-Fe alloys. The rocks in the Bushveld Complex are strongly weathered down to 10 to 40 m below the surface. Near-surface chromitite seams of the LG and MG of the western Bushveld Complex at the Thaba Mine are studied by electron microprobe and LA-ICP-MS for the distribution of PGE. The analytical conditions employed for silicates were: 20 kV acceleration voltage, 120 nA beam current, focused beam, and up to 360 s measuring time. LA-ICP-MS measurements were carried out on a ThermoScientific Element XR (HR-ICP-MS) coupled with a femtosecond laser ablation system. During weathering, sulfides and PGM are largely destroyed and PGE are mobilized and redistributed in the near-surface ores. Within the near-surface weathered chromitite seams, only relict PGM can be observed, which are observed within silicates or as inclusions in chromite. Former grains of PGM are observed in secondary silicates showing larger shapes but are locally redistributed along small cracks and Pt-Fe-(oxides) are observed as well, arguing for the neoformation of PGM. Secondary silicate minerals in weathered ores show concentrations of Pt and Pd in the 100s-ppm range. A good correlation can be observed with Ni and Pd, arguing for the weathering of pentlandite, which hosts Pd in the pristine PGE ore. Palladium and Ni partly remained and were incorporated into secondary silicates while S is largely lost.