Re-distribution of platinum-group elements in supergene ores of the Merensky Reef, Eastern Bushveld Complex, South Africa

The Merensky Reef of the Bushveld Complex, South Africa, is one of the largest resources of platinum-group elements (PGE) worldwide. Near-surface supergene ores of the Merensky Reef contain high grades of PGE and are economical ore bodies. Unfortunately, the recovery rates of these ores are low. This is the first study that investigates the variation of PGE in pristine and supergene samples of the Merensky Reef from the eastern Bushveld.

The whole-rock PGE distribution was investigated via ICP-MS and the platinum-group mineral (PGM) distribution with reflected light microscopy, SEM, and microprobe. Pristine samples reveal that the PGEs occur mainly as discrete PGMs and in solid solution in sulfides (especially Pd in pentlandite). The PGMs in the pristine ore are PGE-sulfides, e.g. cooperite-braggite or laurite; and subordinate PGE-bismuthotellurides and PGE-arsenides. During weathering, sulfur and, to a lower extent, Pd, were removed resulting in a PGE mineralogy that mainly consists of relict PGM, Pt-Fe alloys, and Pt-oxides/hydroxides. Additionally, PGEs (mainly Pd) are hosted by Fe-hydroxides and secondary hydrosilicates (e.g., chlorite or serpentine).

The highly variable distribution in the supergene ore results in very low and thus uneconomic recovery rates. The PGEs in the weathered samples occur only partly as discrete PGMs and therefore the processing is not as effective. The PGEs are also hosted by hydrosilicates and Fe-hydroxide and thus hydrometallurgical or pyrometallurgical processing of the bulk rock is necessary for the extraction of PGE, which is still uneconomic and thus new chemical bulk-leaching methods would need to be developed.