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Chromite in the Platreef (Bushveld Complex, South Africa): occurrence and evolution of its chemical composition

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Abstract

The regional distribution and chemical composition of massive and disseminated chromitites through a Platreef sequence and along a strike distance of over ∼20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the Bushveld Complex. The chromitite layers and seams of the Platreef form two main chromite-bearing zones: the Upper Chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the Platreef top contact and the Lower Chromitite that is composed of multiple seams in feldspathic harzburgite, pyroxenite and norite close to the bottom contact of the Platreef with footwall. Electron micro-probe analyses reveal that the chemical composition of chromite depends on the host rock type. Norite and pyroxenite host chromite with the highest Cr2O3 content while harzburgite-hosted chromites are Cr and Mg poor. The wide range in chromite compositions is explained by the influence of late-magmatic processes including post-cumulus growth and re-equilibration, interaction with fluid- and sulphide-saturated magmatic liquid and contact metamorphism. Each of these processes is characterised by its own distinct geochemical signature, but generally they lead to a decrease in Mg and Al and an increase in both di- and tri-valent Fe in the chromite. The occurrence of chromitite locally on the different distance from the contact between the upper Platreef sills and the overlying Main Zone magma suggests erosion of the upper Platreef by the Main Zone as it was emplaced. The localisation of chromitites supports an independent development of the northern limb during the Critical Zone emplacement although the chemical composition of chromite and co-existing silicates from ultramafic rocks suggest a Critical Zone affinity with the eastern and western limbs of the Bushveld Complex.

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Acknowledgements

We are grateful to Anglo Platinum and particularly to Gordon Chunnett for support in all aspects of this work and for permission to show graphically the PGE grade through the borehole cores. Much of the core was provided by Anglo Platinum facilitated by Adrian Colloty at the Mogalakwena core yard. The core from Macalacaskop and Turfspruit was provided by Platreef Resources and the core from Zwartfontein in the Akanani project—by Lonmin with support from Jeff White. We are particularly grateful to Christian Reinke from Spectrau, University of Johannesburg for the analytical support. Discussions with Tony Naldrett, Paul Nex and Stefanie Weise were fruitful at all stages of the study. Reviews by S. Mondal and anonymous reviewer as well as the comments of C. Li and B. Lehmann led to significant improvements in the manuscript and are very much appreciated.

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Authors and Affiliations

  1. School of Geosciences, University of the Witwatersrand, Private Bag 3, 2050, Wits, South Africa

    Marina A. Yudovskaya & Judith A. Kinnaird

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  1. Marina A. Yudovskaya
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  2. Judith A. Kinnaird
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Correspondence to Marina A. Yudovskaya.

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Editorial handling: C. Li

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Appendix 1

EMPA results of chromite composition (XLS 139 kb)

Appendix 2

EMPA results of ortho-pyroxene composition (XLS 27 kb)

Appendix 3

EMPA results of olivine composition (XLS 16 kb)

Appendix 4

EMPA results of accessory minerals’ compositions (XLS 11 kb)

Appendix 5

Whole-rock XRF results (XLS 25 kb)

Appendix 6

EMPA results of chromite composition of Wela SS (2008). Petrographic and geochemical analysis of chromites from the Platreef (the Northern Limb of the Bushveld Complex—South Africa). Honours Degree thesis, University of the Witwatersrand (XLS 165 kb)

Table 2

Representative EMPA results (wt.%) and formulae coefficients of the Platreef spinels, olivine and ortho-pyroxene compositions (DOC 241 kb)

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Yudovskaya, M.A., Kinnaird, J.A. Chromite in the Platreef (Bushveld Complex, South Africa): occurrence and evolution of its chemical composition. Miner Deposita 45, 369–391 (2010). https://doi.org/10.1007/s00126-010-0276-9

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