Platinoid minerals in the gold reefs of the witwatersrand basin

Abstract

From a study of twenty six platinoid mineral concentrates from different gold mines around the Witwatersrand sedimentary basin, seven alloys of osmium and iridium, two alloys of platinum and iron, and five arsenide, sulpharsenide or sulphide, minerals of the platinum group elements were identified and are described. The minerals include osmium, iridosmium, rutheniridosmium, osmiridium, ruthenosmiridium, iridium, rhodiridium, ferroplatinum, rhodoferroplatinum, sperrylite, laurite, one arsenide of osmium and iridium, one complex sulpharsenide of osmium, iridium and ruthenium, and a rhodium sulphide mineral. Where possible, optical properties and quantitative values for chemical composition, reflectivity, microhardness and structure are given for each mineral. The intergrowth textures and shapes exhibited by the platinoid particles are described and discussed. From a grain count study under the electron microprobe of six, selected platinoid mineral concentrates, the distributions in mineralogy and grain size of the particles from different parts of the Witwatersrand basin were determined and are compared. Only minor variations could be detected in both mineralogy and size of the particles across the basin. Four platinoid "mineral grains were found in situ within the reef and these are described and illustrated. Photomicrographs showing a wide variety of grain shapes, intergrowth textures and compositions are provided as an appendix, to supplement the descriptions and discussion in the text of the thesis.