Enrichment Mechanism Of Platinum Group Metals, Bushveld PGE Deposits, South Africa

The Bushveld Complex (South Afica) is the largest known layered intrusion on Earth,contains approximately 80% of the world's resource of platinum group element (PGE).The PGE mineralization mainly occurs as three units: the Merensky reef, Platreef andUG2 chromitite. In this paper, we investigated the distribution of PGE amongchromite, the trapped sulfide inclusions within chomite, and the sulfides interstitial tochromite from the Bushveld Complex. Samples were analyzed with scanning electronmicroscopy (SEM) and EPMA (Electron probe micro-analyzer) to select small areas,and keep a record of the assemblages prior to ablation. The mode of our plasma-massspectrum (LA-ICP-MS) analysis uses scanning ablation and the Iolite software wasused to combine parallel adjacent line-scan into coherent element images. Advancesin the LA-ICP-MS analysis have allowed us to obtain high resolution trace elementmaps of small features, especially for the sulfide inclusion. Comparison of the imageswith the traditional spot analysis, the images can obtain more comprehensiveunderstanding of overall distribution of elements in the complex chromitite samples.The EPMA data of sulfide show that the pentlandite enclosed in the chromite hashigher Ni than the interstitial ones, which suggest that they are modified by the latemagmatic and/or hydrothermal melt/fluids. We found out that the primary pyriteassociation with penlandite takes Os, Ir, Ru and Re; Rhodium is not uniform amongdifferent grains within the same sample; platinum and Bi concentrated outside thesulfides, nothing inside. Chromite from the UG2 chromitite has no detectable HSE insolid solution; pentlandite is the main host of Pd.Inclusions in earliest crystallizing silicate minerals are indicative of a very early-stagesulfide liquid in the system, as they will have cooled and fractionated within anessentially closed system.The comparison of the PGE distributions between theinterstitial sulfide and sulfide inclusions in the same sample provide extremely usefulinformation to understand how PGE were incorporated, how much is from melt andhow much is hydrothermal. EPMA data shows the hornblende in the inclusions ischaracterized by high Mg, and the plagioclase in the inclusions is distinctly more salicthan that in the outside with An6to An21in the inclusions and the outside is An₅₆toAn₇₃.