The platinum-group minerals in a drill core taken through the middle group of chromitite layers in the Critical Zone at Marikana in the western Bushveld Complex were found to consist mainly of laurite as inclusions in chromite grains. The platinum-group minerals containing Pt, Pd, and Rh are concentrated in the intercumulus silicates and frequently associated with base-metal sulphides. Up to about 20% of all platinum-group minerals in the investigated chromitite layers contain sub stantial amounts of As. The base-metal sulphides are strongly modified in the postmagmatic stage, which led to a significant loss of Fe and S, in this way concentrating Cu, Ni, and the platinum-group elements by factors of up to 10. Interaction between chromite and base-metal sulphides cannot account for all the Fe lost in chromite-poor samples, and the importance of additional processes is indicated. Inclusions in chromite and orthopyroxene indicate the formation of discrete platinum-group minerals and As-rich phases before the formation of an immiscible sulphide melt. Resorption of earlier formed platinum-group minerals into the immiscible sulphide melt and postmagmatic sulphidation destroyed most of the evidence of the early formed platinum-group minerals.
The morphology and mineral chemistry of gold and associated sulphides at Sheba, Fairview, and New Consort gold mines in the Barberton Greenstone Belt (BGB) identify two main types of mineralization. The first type occurs associated with sulphides (mainly pyrite), either as inclusions (10-30 µm) or as sub-microscopic gold. The second gold type consists of large gold grains (≥ 100µm) within the silicates (mostly quartz).LA-ICP-MS studies reveal that some gold and associated sulphide grains contain high values of Cl, Br, Na, and I.The elemental relationships reflect the different chemistry and precipitation processes of possible source fluids, and identify several episodes of mineralisation in the study area, one of them formed due to a boiling process in a supercritical hydrothermal environment. This paper reports on the compositional characteristics of these gold grains, the significance of the halogen contents, and the implications for possible sources of the gold and associated sulphides.
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