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Zoning of platinum group mineral assemblages in the UG2 chromitite determined through in situ SEM-EDS-based image analysis

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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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Acknowledgments

This project received financial and logistical support from African Rainbow Minerals (ARM), with particular thanks to Jonathan Woolfe and Mike Cowell at Two Rivers Platinum Mine. Bertus Smit and Alet Lamprecht are thanked for their help with MLA work at the University of Johannesburg. Many thanks also to Peter Lightfoot, David Holwell, and an anonymous reviewer for comments that enhanced the balance and presentation of the manuscript. The project was funded by ARM and a National Research Foundation grant to Nic Beukes.

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Authors and Affiliations

  1. Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa

    Ronald J. Voordouw, Jens Gutzmer & Nicolas J. Beukes

  2. Department of Mineralogy, TU Bergakademie Freiberg, Brennhausgasse 14, 09596, Freiberg Saxony, Germany

    Jens Gutzmer

  3. Paleoproterozoic Mineralization Research Group, Department of Geology, University of Johannesburg, Auckland Park, South Africa

    Nicolas J. Beukes

  4. Council for Geoscience, P.O. Box 900, Pietermaritzburg, 3200, South Africa

    Ronald J. Voordouw

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  1. Ronald J. Voordouw
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  2. Jens Gutzmer
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  3. Nicolas J. Beukes
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Correspondence to Ronald J. Voordouw.

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Editorial handling: P. Lightfoot

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Voordouw, R.J., Gutzmer, J. & Beukes, N.J. Zoning of platinum group mineral assemblages in the UG2 chromitite determined through in situ SEM-EDS-based image analysis. Miner Deposita 45, 147–159 (2010). https://doi.org/10.1007/s00126-009-0265-z

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