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

Platinum-group minerals (PGM) are documented in alluvial sediments in rivers draining the Eastern Bushveld Complex, in South Africa. About 32% of a total of 790 PGM nuggets (50-1600 mu m in size) studied show evidence of chemical and textural modifications. Pt-Fe alloys are most susceptible, and reveal a wide spectrum of textural types, among which a Ni- and a Curich assemblage is most common. In the former, Pt-Fe alloy grains [Pt2.3-2.7Fe] are concentrically rimmed by layers of ferronickelplatinum [Pt2NiFe] and coated by an alloy approximating [Ni(2)ptFe]. In the Cu-rich assemblage, Pt-Fe alloy [Pt2.2-2.6Fe] is concentrically coated by [Pt(Fe,Cu)], i.e., solid solutions of tetraferroplatinum [PtFe] - tulameenite [Pt2CuFe]. A variety of Pd-Pt-Te-Sb-Sn phases, Au-Ag alloys, and base-metal sulfides are included, and small grains of zvyagintsevite [Pd3Pb] occur attached to the outer rim. Pt-Fe alloy may also be replaced by heterogeneous Pt-Fe(Cu,Pd) oxides, which may carry appreciable concentrations of SiO2 and MgO. Cooperite and braggite, and less commonly 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. In rare cases, laurite is replaced by two different types of Ru-Rh-Fe-(Ca-V-Mn) oxides of RuO2 and RuO stoichiometry; subsequently, the PGE oxide phases are transformed into native ruthenium and rhodium. 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 silicate (e.g., smectite, chlorite). The modified grains are genetically attributed to three processes: (1) sulfidation reactions during magmatic-hydrothermal processes affect Pt-Fe alloy, braggite and Rh sulfides, (2) alteration of Pt-Fe alloy to Pt(Fe,Cu,Ni) alloys at moderate temperature in a reducing environment, and (3) oxidation of PGE sulfides in a low-temperature oxidizing environment. Modification and alteration are suggested to have taken place over an extended period of time, possibly under variable climatic and oxidation conditions during the postmagmatic stage, including serpentinization, during weathering of PGE-bearing bedrock, and after placer deposition.