The formation of alluvial platinum-group minerals: present knowledge and the way ahead

Abstract

The weathering of platinum-group element (PGE) deposits presents unusual problems, especially in the very active environment of lateritic weathering under tropical conditions. There is clear evidence of the destruction of platinum-group minerals (PGM) to form PGE oxides, or fine intergrowths between relict PGM and iron oxides or hydroxides, as an intermediate stage during weathering. The PGE released by weathering are transported in solution with the more soluble Pd species remaining in solution and travelling further than the less soluble Pt species. The presence of PGM in the laterite differing in mineralogy, mineral assemblage and size from those in the primary rock is difficult to explain, especially when they show secondary textures. Differing interpretations have created controversy. Are alluvial PGM derived unaltered from the primary rock where they are rare and, therefore, not encountered by standard petrographic examination? Is it possible that they could have developed in the laterite by some process that we do not yet fully understand? Some favourable genetic conditions have been outlined and debated. For more than 100 years authors have reported secondary ore textures and recently proposed a biogenic origin. Frank Reith and his co-workers provided evidence of a process involving metallophillic bacteria which, for the first time, demonstrates PGM growth in the laboratory under supergene conditions. Their work shows that a mechanism for supergene growth (‘neoformation’) can occur, which offers a new field of study of the appropriate Eh, pH, $f_{{\rm O}_ 2}$ conditions and organic and bacterial reactions that could permit supergene growth.