The fine-scale chemical variation of chromite in the UG-2 chromitite and the distribution of platinum-group elements in sulfides of the Bushveld Complex, South Africa
The fine-scale chemical variation of chromite in the UG-2 chromitite and the distribution of platinum-group elements in sulfides of the Bushveld Complex, South Africa
Previously Published Material: I would like to present the work which is published in the following two papers - last one recently accepted in Mineralium Deposita. Junge, M., Wirth, R, Oberthür, T., Melcher, F., Schreiber, A. (2015; online first at Mineralium Deposita) Mineralogical siting of platinum-group elements in pentlandite from the Bushveld Complex, South Africa and Junge, M., Oberthür, T., Melcher, F., (2014) Cryptic variation of chromite chemistry, platinum-group element and mineral distribition in the UG-2 chromitite - an example of the Karee Mine western Bushveld Complex, South Africa, Economic Geology, v. 109, p. 795-810.
Abstract ID#: 33790
English Abstract:
The UG-2 chromitite of the Bushveld Complex in South Africa contains the world's largest resources of platinum-group elements (PGE). However, only limited work has been conducted on the fine-scale chemical variation of chromite. In the present study one drill core covering the sequence of the UG-2 chromitite in the western Bushveld Complex was studied in detail. The mineral chemistry of chromite is characterized by distinct cryptic variation. From bottom to top each sub-layer is defined by an upward decrease of Mg# combined with increases of Cr# and TiO2, indicating that the UG-2 is more heterogeneously than previously thought. Sulfides (pentlandite, chalcopyrite) - interstitial to chromite grains - are rare, and associated PGM grains are mainly Pt-Fe alloy, laurite and cooperite/braggite. Electron probe microanalysis and laser ablation induced coupled plasma mass spectrometry shows that Pd and Rh contents of pentlandite reach maxima of 2.2 wt% Pd and 3 wt% Rh. However, a direct evidence is lacking on the mineralogical siting of the PGE, e.g. whether they occur in solid solution, as nano- and/or micro-inclusions. Therefore, a combination of focused ion beam and transmission electron microscopy was used which allows to obtain crystal-structural relationships between the host mineral and incorporated trace elements. The PGE in sulfide grains of the Bushveld Complex occur in a number of forms, namely (1) as discrete nano-inclusions of PGM, (2) as patchily distributed solid solution, (3) ordered within the pentlandite crystal structure, substituting for Ni and/or Fe (superlattice), and (4) as homogenous solid solution.