ALLUVIAL AND ELUVIAL PLATINUM-GROUP MINERALS FROM THE BUSHVELD COMPLEX, SOUTH AFRICA

Abstract

The present work provides an initial description of detrital platinum-group minerals (PGM) collected from alluvial sediments of rivers draining the Bushveld Complex, and from eluvial concentrations at the Onverwacht platiniferous pipe. During a field campaign, sediments were sampled at nine localities around the Bushveld Complex, and heavy mineral assemblages were investigated using optical and scanning electron microscopy (SEM) as well as microprobe analysis (EPMA) of PGM. All concentrates from the alluvial samples contain discrete PGM grains with grain sizes in the range from ~50 to 150 μm (maximum 600 μm). The overall PGM proportions are: native Pt, Pt-Pd and Pt-Fe alloys (together 54%), sperrylite (33%), cooperite/braggite (11%), and stibiopalladinite (2%). This PGM assemblage distinctly contrasts to the suite of PGM in the pristine, sulfide-bearing mineralization in the Merensky, UG-2 and Platreef, the assumed sources of the detrital PGM. Specifically, PGE-bismuthotellurides and -sulfarsenides, common in the primary ores, are missing in the assemblage of detrital PGM in the fluvial environment. Nearly all detrital PGM (98%) are Pt minerals, corroborating earlier findings that Pd-dominated PGM are unstable and are dissolved in the supergene environment, and that PGE-bismuthotellurides and -sulfarsenides, common in the PGM assemblages of the pristine ores are unstable during weathering and mechanical transport. The eluvial material collected at Onverwacht contained ca. 150 PGM grains with sizes mainly in the range 100 to 300 μm range (maximum 1.87 mm). The PGM assemblage comprises grains of Pt-Fe alloys (66%), sperrylite (14%), and many rarer PGM including stibiopalladinite, hollingworthite, laurite, PGE arsenides and PGE sulfides. The suite of eluvial PGM observed is similar to the PGM assemblage described previously from the Onverwacht pipe proper, including the type locality minerals genkinite, irarsite and platarsite, as well as some additional and possibly new PGM. Most of the relatively rare PGM detected in the suite of eluvial grains from Onverwacht were also reported in the detrital PGM assemblage from the Moopetsi river, farm Maandagshoek (Oberthur et al., 2004), indicating that many of the latter grains originate from platiniferous pipes and not from the Merensky or UG-2 reefs. Detrital PGM can be expected to be present in rivers draining PGE-bearing layered intrusions, and economic placers may form under particular sedimentological conditions. Therefore, this work also highlights the fact that the nowadays somewhat neglected field methods and basic techniques have their merits and value in mineral exploration, especially if they are combined with modern micro-analytical methods. The systematic recovery of PGM from stream sediments, soils and till should regain wider application in mineral exploration as these tools can provide useful indicators of platinum mineralization.