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Isoferroplatinum-pyrrhotite-troilite intergrowth as evidence of desulfurization in the Merensky Reef at Rustenburg (western Bushveld Complex, South Africa)

Published online by Cambridge University Press:  02 January 2018

Alexander Kawohl  and
Hartwig E. Frimmel
Alexander Kawohl*
Affiliation:
Institute of Geography and Geology, University of Würzburg, Am Hubland D-97074, Würzburg, Germany Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa
Hartwig E. Frimmel
Affiliation:
Institute of Geography and Geology, University of Würzburg, Am Hubland D-97074, Würzburg, Germany Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa
*
*E-mail: alex.kawohl@yahoo.de
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Abstract

Petrographic and mineralogical studies of samples of the Normal (or undisturbed) Merensky Reef from Frank Shaft No.1 at Rustenburg Platinum Mine revealed the presence of a Pt-Fe-alloy, probably isoferroplatinum (58 vol.% of total precious metal minerals), arsenides (21 vol.%), bismuthotellurides (10 vol.%), electrum (9 vol.%) and platinum group element- (PGE-) sulfides and stannides (2 vol.%), associated predominantly with base-metal- and iron-sulfides. A Pt-Fe-alloy-dominated facies has been known for considerable time from potholes and discordant bodies and has been attributed to fluid activity with high fO2 and low fS2. Our petrographic results indicate that the normal thin reef has also undergone hydrothermal alteration. For the first time, the rare mineral troilite (stoichiometric FeS) was found as intergrowths with masses of Pt-Fe-alloy, together with Fe-rich pyrrhotite, secondary hydrous silicates, magnetite and calcite. The observed mineral assemblage and texture is interpreted as the product of partial desulfurization, caused by migrating S-undersaturated fluids, which led to the exsolution of Pt-Fe-alloy from pyrrhotite (Fex–1S) with the latter approaching a stoichiometric composition. Overall our new observations provide convincing support for the importance of metasomatism in the secondary modification of ore mineralogy and textures even in the undisturbed Merensky Reef.

Keywords

Merensky ReefBushveld Complexplatinum-group mineralstroiliteisoferroplatinumPt-Fe-alloydesulfurization
Type
Research Article
Information
Mineralogical Magazine , Volume 80 , Issue 6 , October 2016 , pp. 1041 - 1053
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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