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Mineralogical siting of platinum-group elements in pentlandite from the Bushveld Complex, South Africa

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Abstract

The Bushveld Complex in South Africa hosts the world’s largest resources of platinum-group elements (PGEs), which are mainly mined from three ore bodies, namely the Merensky Reef, the UG-2 chromitite, and the Platreef. In these ores, the PGEs are bimodally distributed, occurring both as discrete platinum-group minerals (PGMs) and hosted by sulfides. The presence of PGEs in sulfides has been demonstrated by electron probe microanalysis, laser ablation induced coupled plasma mass spectrometry, secondary ion mass spectrometry, and particle-induced X-ray emission. However, evidence is lacking on the mineralogical siting of the PGEs, e.g., whether they occur in solid solution, as nano-inclusions, and/or micro-inclusions. Therefore, in the present study, 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 and revealing the physicochemical state of the PGE in sulfides. The present study confirms the existence of micrometer-sized discrete PGMs in the ores. Further, the PGEs occur in a number of forms, namely (1) as discrete nano-inclusions of PGMs, (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. Nanometer-sized PGMs (nPGMs) show no orientation relationship with the host sulfide mineral. Consequently, they are discrete phases, which were trapped within pentlandite during sulfide growth. Heterogeneous and patchy distributions of Rh and Ir within the pentlandite lattice suggest that Rh and Ir were already present within the sulfide liquid. The absence of possible reaction partners (e.g., Bi, As, and Sn) necessary for the formation of discrete PGMs forced Rh and Ir to remain in the crystal lattice of pentlandite and down-temperature exsolution caused patchy distribution patterns of Rh and Ir. High concentrations of Rh and Ir in pentlandite initiate ordering of the randomly distributed PGE in form of nanometer-sized lamellae resulting in the formation of a superlattice. Palladium is homogenously distributed within the pentlandite lattice, even at high Pd concentrations, and in addition also occurs as nPGMs.

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Acknowledgments

Excellent polished sections were prepared by Peter Rendschmidt, SEM studies were kindly supported by Detlef Klosa, and electron probe microanalysis was ably performed by Jerzy Lodziak, Christian Wöhrl, and Simon Goldmann, all at the BGR in Hannover. Many thanks to Helene Brätz, University of Erlangen, for carrying out the LA-ICP-MS analyses. Thanks to Bernd Lehmann for his professional editorial handling. We are very grateful for the thorough and constructive review by Louis J. Cabri, which improved the quality of the presentation.

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  1. Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655, Hannover, Germany

    Malte Junge & Thomas Oberthür

  2. Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Telegrafenberg, 14473, Potsdam, Germany

    Richard Wirth & Anja Schreiber

  3. Institute of Geological Sciences, University of Leoben, Peter Tunner Strasse 5, 8700, Leoben, Austria

    Frank Melcher

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  1. Malte Junge
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Correspondence to Malte Junge.

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Editorial handling: B. Lehmann

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Junge, M., Wirth, R., Oberthür, T. et al. Mineralogical siting of platinum-group elements in pentlandite from the Bushveld Complex, South Africa. Miner Deposita 50, 41–54 (2015). https://doi.org/10.1007/s00126-014-0561-0

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