Modification of detrital platinum-group minerals from the eastern Bushveld Complex, South Africa

The PGM in alluvial sediments in rivers draining the Eastern Bushveld complex are described. About 32% of a total of 790 PGM nuggets (50-1600 mu m in size) show evidence of chemical and textural modifications. The Pt-Fe alloys are the most susceptible and reveal a wide variety of textural types, among which a Ni- and a Cu-rich assemblage is common. In the former, grains of Pt-Fe alloy (Pt (sub 2.3-2.7) Fe) are concentrically rimmed by by layers of ferronickelplatinum (Pt (sub 2) NiFe) and coated by an alloy near (Ni (sub 2) PtFe). In the Cu-rich assemblage, Pt-Fe alloy (Pt (sub 2.2-2.4) Fe) is concentrically coated by [(Pt(Fe,Cu)]. A variety of Pd-Pt-Te-Sb-Sn phases, Au-Ag alloys and base-metal sulphides are included, and small grains of zvyagintsevite (Pd (sub 3) Pb) are attached to the outer rim. The Pt-Fe alloys may also be replaced by heterogeneous Pt-Fe(Cu,Pd) 'oxides', which may carry appreciable amounts of SiO (sub 2) and MgO. Cooperite and braggite + or - sperrylite are coated by porous native platinum, which also penetrates the grains as root-like channels. Monomineralic grains of porous Pt-rich alloy are considered the end product of this alteration. Palladium-rich oxides are observed in some alteration-induced rims, and are associated with Pd-bearing Fe oxide-hydroxide and Pd-bearing Mn-Fe-rich silicates (e.g. smectite, chlorite). The modified grains are genetically attributed to three processes: 1) sulphidation reactions during magmatic-hydrothemal processes affect Pt-Fe alloy, braggite and Rh sulphides, 2) alteration of Pt-Fe alloy to Pt(Fe,Cu,Ni) alloys at moderate temperature in a reducing environment and 3) oxidation of PGE sulphides in a low-temperature oxidizing evironment.