Weathered chromitite layers from the Lower Group (LG) and Middle Group (MG) of the Bushveld Complex in South Africa are mined for chromite but have the potential to be processed for platinum-group elements (PGE) as a by-product. However, recovery rates of PGE from weathered PGE ores are apparently low because of the supergene redistribution of PGE. Thus, understanding the processes that are responsible for PGE mobilization during weathering processes is useful to assess the PGE potential in weathered chromitites.

In this study the distribution of platinum (Pt) and palladium (Pd) in soil samples from the LG and MG chromitites of the Thaba Mine (western Bushveld Complex) is investigated, with a special focus on the influence of organic matter on Pt and Pd mobilization. For this purpose, the soil samples were treated with different chemical digestion methods (aqua regia digestion and total digestion), and the Pt and Pd concentrations in humic and fulvic acids extracted from the soils were determined.

The average total Pt and Pd concentrations of the soil samples (integrated over the whole soil horizons) are up to 62 ng/g and 128 ng/g, respectively, and their concentrations increase towards the topsoil. The humic acid extracts contain up to 0.04 ng/g Pt and up to 6.78 ng/g Pd and the fulvic acid extracts contain up to 0.16 ng/g Pt and up to 2.72 ng/g Pd. Up to 1% of the total Pt present in the soil is bound to fulvic acids, whereas up to 7% Pd is bound to humic acids. Palladium is mobilized equally by humic and fulvic acids, whereas the transport of Pt occurs preferentially by fulvic acids, indicating a higher mobility of Pd compared to Pt under supergene conditions during soil formation.

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