Zoning of platinum group mineral assemblages in the UG2 chromitite determined through in situ SEM-EDS-based image analysis |
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| Authors: | Ronald J Voordouw Jens Gutzmer Nicolas J Beukes |
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| Institution: | (1) Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa;(2) Department of Mineralogy, TU Bergakademie Freiberg, Brennhausgasse 14, 09596 Freiberg Saxony, Germany;(3) Paleoproterozoic Mineralization Research Group, Department of Geology, University of Johannesburg, Auckland Park, South Africa;(4) Council for Geoscience, P.O. Box 900, Pietermaritzburg, 3200, South Africa |
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| Abstract: | In situ scanning electron microscopy–energy dispersive X-ray spectrometry analysis of platinum group minerals (PGM) and base
metal sulfides in the UG2 chromitite shows that this ore body is zoned along at least ∼6 km of strike. The uppermost part
of the UG2 chromitite, referred to as the leader seam, is ∼16 cm thick and has a PGM assemblage that is dominated by PGE arsenides,
sulpho-arsenides, and alloys (∼70 vol.% of all PGM), which are typical secondary PGM assemblages in other segments of UG2.
This is the first time such laterally persistent secondary assemblages have been identified in the UG2 chromitite, as previously,
they were only known to occur adjacent to transgressive fluid-bearing structures (e.g., pipes, faults). The underlying main
seam is thicker (one to nine seams totaling ∼130 cm) and has a PGM assemblage that consists mostly of Pt sulfide, Pt–Pd sulfide,
Pt–Rh–Cu sulfide, laurite, and Fe–Pt alloys (∼85 vol.% of all PGM), typically regarded as primary magmatic constituents of
UG2 chromitite. There are, however, some subtle vertical changes in the PGM assemblages of the main seam that include the
occasional presence of secondary assemblages in the top and bottom parts. The origin of these secondary PGM assemblages is
related to alteration by hydrothermal fluids and/or fluid-rich melts that infiltrated during crystallization of the UG2 and
may possibly have been derived from the UG2 chromitite itself. |
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