Mineral chemistry of (Pt, Pd)-bismuthotelluride minerals in the Platreef at Zwartfontein, Akanani Project, northern Bushveld Complex, South Africa

The (Pt,Pd)-bismuthotelluride minerals at the Akanani prospect show substantial substitution of Te and Bi. Individual grains are often markedly heterogeneous with concentrations of Te and Bi which vary by up to 26 wt.%. They occur as single phase crystals or form composite grains with other platinum-group minerals, and are predominantly associated with (i.e., are in direct contact with) primary and secondary silicate minerals, base-metal sulfides, and magnetite. The Pt-bismuthotelluride minerals are markedly associated with (i.e., tends to occur as composite grains with) Pd-bismuthotelluride minerals, isoferroplatinum, and sperrylite. The Pd-bismuthotelluride minerals are markedly associated with Pt-bismuthotelluride minerals, altaite, hollingworthite, plumbopalladinite, PdSbSn alloy, and sperrylite. The Pt-bismuthotelluride minerals are characterized by high ratios of Te/Bi and form a trend of solid solution from moncheite (PtTe (sub 2) ) towards maslovite (PtTeBi). A few analyses plot close to, or on, the composition of maslovite. The Pd-bismuthotelluride minerals are characterized by a lower ratio of Te/Bi than the Pt-bismuthotelluride minerals. They form a trend of solid solution along the join kotulskite (PdTe)-sobolevskite (PdBi), without merenskyite (PdTe2). There is very limited solid solution of Pt and Pd between the Pt-bismuthotelluride minerals (<5.7 wt.% Pd) and the Pd-bismuthotelluride minerals (<3.3 wt.% Pt), with only four analyses of michenerite (three grains) characterized by elevated levels of Pd (10.3 to 17.1 wt.%). In view of the strong spatial association between the Pt- and Pd-bismuthotelluride minerals at Akanani it is considered likely that the latter were originally in solid solution in a precursor Pd-rich Pt-bismuthotelluride host at temperatures of up to 1150 degrees C, from which they then exsolved during cooling. Further exsolution of Te- or Bi-rich domains in individual Pt- and Pd-bismuthotelluride crystals occurred during late cooling at substantially lower temperatures, and resulted in the observed compositional ranges of moncheite-maslovite and kotulskite-sobolevskite.