The platinum-group mineralogy of the Merensky Reef at the Impala platinum mines, Bophuthatswana

Abstract

The platinum-group mineralogy of the Merensky Reef at the Impala platinum mines, South Africa, has been investigated. Polished sections were prepared from pyroxenitic and pegmatoidal reef samples collected underground. Microscope studies were followed up by the identification and analysis of platinum-group minerals by electron microprobe.The most abundant base metal sulfides occurring in the Merensky Reef in this area are: pyrrhotite (41%), pentlandite (37%), chalcopyrite (18%), and pyrite (4%). The core of a typical base metal sulfide bleb consists of pyrrhotite surrounded by a mosaic of pentlandite grains which in turn may be enclosed by a rim of chalcopyrite.The relative volumetric abundances of the platinum-group minerals were determined to be as follows: coopetite (44%), laurite (21%), moncheite (17%), and braggite (12%). Thirteen additional platinum-group minerals comprised the remaining 6 percent. Cooperite and laurite occur preferentially in the chromitite layers, whereas moncheite was most commonly found in the silicate part of the Merensky Reef.Observed textures include the marked tendency of platinum-group minerals to occur with chalcopyrite, the exsolution of laurite and moncheite from pyrrhotite, and stringers of mon-cheite and sperrylite cutting across silicate and base metal sulfide grain boundaries. Pt-Fe alloy commonly forms the core with cooperite as a rim of compound, zoned platinum-group mineral grains.The textural features observed indicate that the platinum-group minerals formed at a relatively late stage in the crystallization history of the Merensky Reef. The preferential association of platinum-group minerals with chalcopyrite is important. Chalcopyrite formed from the Cu-rich sulfide liquid of the original monosulfide solid solution which separated from the silicate magma. It is suggested that the platinum-group elements concentrated in this liquid and that the platinum-group minerals crystallized only after the chalcopyrite, as they occur mostly on the chalcopyrite margins. A part of the platinum-group element fraction remained in the Fe-Ni monosulfide liquid and crystallized as platinum-group minerals associated with pyrrhotite and pentlandite. The remaining platinum-group element fraction, especially Ru and Pd, remained in solid solution in the Fe-Ni monosulfide. The Ru exsolved at lower temperatures as laurite, but the Pd remained in solid solution in the pentlandite. This is concluded from the work of other authors (Ross and Keays, 1979; Watkinson and Dunnington, 1979) and by the paucity of Pd minerals, although Pd is the second most abundant platinum-group element in the Merensky Reef.