Junge M.,Federal Institute for Geosciences and Natural Resources |
Oberthur T.,Federal Institute for Geosciences and Natural Resources |
Osbahr I.,Helmholtz Institute Freiberg for Resource Technology |
Gutter P.,SYLVANIA Platinum Ltd.
Mineralium Deposita | Year: 2016
The chromitites of the Bushveld Complex in South Africa contain vast resources of platinum-group elements (PGE). However, knowledge of the distribution and the mineralogical siting of the PGE in the lower group (LG) and middle group (MG) chromitite seams of the Bushveld Complex is limited. We studied concentrates from the LG-6 and MG-2 chromitites of the western Bushveld Complex by a variety of microanalytical techniques. The dominant PGM are sulfides, namely laurite, cooperite-braggite, and malanite-cuprorhodsite, followed by PGE-sulfarsenides, sperrylite, and Pt-Fe alloys. Laurite is the most abundant PGM (vol%). The matching sets of PGM present in the LG and MG chromitites of both the western and the eastern Bushveld Complex, and in the UG-2 chromitite, show strong similarities which support the assumption of a characteristic and general chromitite-related PGM assemblage. Palladium and Rh contents in pentlandite are low and erratic although maximum contents of 7730 ppm Pd and 6020 ppm Rh were detected. Rare thiospinels of the polydymite-linnaeite-greigite series have PGE contents of 1430 ppm Pt, 5370 ppm Rh, and 1460 ppm Pd. The various PGE occur in different deportment: Platinum is generally present in the form of discrete PGM (sulfides, arsenides, alloys). Palladium is present as a large variety of discrete PGM and also incorporated in pentlandite. Rhodium forms discrete PGM and is occasionally present in pentlandite. The IPGE (Os, Ir, and Ru) are dominantly incorporated in laurite (often as inclusions in chromite) and also occur as sulfarsenides. © 2016 Springer-Verlag Berlin Heidelberg
Oberthur T.,Bundesanstalt fur Geowissenschaften und Rohstoffe BGR |
Junge M.,Bundesanstalt fur Geowissenschaften und Rohstoffe BGR |
Rudashevsky N.,CNT Laboratories |
de Meyer E.,Sylvania Platinum Ltd. |
Gutter P.,Sylvania Platinum Ltd.
Mineralium Deposita | Year: 2015
The chromitites of the Bushveld Complex in South Africa contain vast resources of platinum-group elements (PGE); however, except for the economic upper group (UG)-2 chromitite seam, information on the distribution of the PGE in the ores and on the mineralogical nature, assemblages, and proportions of platinum-group minerals (PGM) is essentially missing. In the present geochemical and mineralogical study, PGE concentrates originating from the lower group (LG)-6 and middle group (MG)-1/2 chromitites were investigated with the intention to fill this gap of knowledge. Chondrite-normalized PGE patterns of bulk rock and concentrates are characterized by a positive slope from Os to Rh, a slight drop to Pt, and an increase to Pd again. The pronounced similarities of the PGE patterns indicate similar primary processes of PGE concentration in the chromitites, namely “sulfide control” of the PGE mineralization, i.e., co-precipitation of chromite and sulfide. Further, the primary control of PGE concentration in chromitites appears to be dual in character: (i) base-level concentrations of IPGE (up to ∼500 ppb) hosted within chromite and (ii) co-precipitation of chromite and sulfide, the latter containing virtually the entire remaining PGE budget. Sulfides (chalcopyrite, pentlandite, and pyrite; pyrrhotite is largely missing) are scarce within the chromitites and occur mainly interstitial to chromite grains. Pd and Rh contents in pentlandite are low and erratic. Essentially, the whole PGE inventory of the ores occurs in the form of discrete PGM. The PGM are almost always associated with sulfides. The dominant PGM are various Pt-Pd-Rh sulfides (cooperite/braggite [(Pt,Pd)S] and malanite/cuprorhodsite [CuPt2S4]/[CuRh2S4]), laurite [RuS2], the main carrier of the IPGE (Os, Ir, Ru), sulfarsenides [(Rh,Pt,Ir)AsS], sperrylite [PtAs2], Pt-Fe alloys, and a large variety of mainly Pd-rich PGM. The LG and MG chromitites have many characteristics in common and define a general, “typical” PGM spectrum of Bushveld chromitites. This PGM assemblage is characterized by the predominance of PGE-sulfides including elevated proportions of malanite, variable proportions of (sulf) arsenides, and Pt-Fe alloys in conjunction with a paucity of (bismutho)tellurides. The formation of this specific PGM spectrum is related to the distinct chromitite environment and its depositional and post-depositional history, whereby desulfurization reactions have probably played an important role. The LG-6 samples have higher contents of PGE-sulfides, including extraordinary high proportions of malanite but low PGE-arsenide and PGE-sulfarsenide contents compared to the MG-1/2 samples. This indicates a higher availability of arsenic either in the stratigraphically higher MG-1/2 samples (compared to the LG-6) or regionally in the chromitites south of the Steelpoort lineament. © 2015 Springer-Verlag Berlin Heidelberg