Laser ablation ICP-MS study of platinum-group elements in sulphides from the Platreef at Turfspruit,northern limb of the Bushveld Complex,South Africa |
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Authors: | D Hutchinson I McDonald |
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Institution: | (1) School of Geosciences, University of the Witwatersrand, Private Bag, Wits 2050, Johannesburg, South Africa;(2) School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3YE, UK;(3) CODES, ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart, Tasmania, 7001, Australia |
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Abstract: | The Platreef unit of the northern Bushveld Complex comprises a diverse package of pyroxenites, peridotites and mafic lithologies
with associated Ni–Cu–platinum-group element (PGE) mineralisation. Base metal sulphides (BMS) are generally more abundant
in the Platreef than in other Bushveld PGE deposits, such as the Merensky Reef and the UG2 chromitite, but the Platreef, though
thicker, has lower overall PGE grades. Despite a commonly held belief that PGEs are closely associated with sulphide mineralisation,
a detailed study by laser ablation ICP-MS (LA-ICP-MS) on a core through the Platreef at Turfspruit suggests that this is not
strictly the case. While a significant proportion of the Pd, Os and Ir were found to be hosted by BMS, Pt, irrespective of
its whole-rock concentration, was not. Only at the top of the Platreef is Pt directly associated with sulphide minerals where
Pt–Pd–(±Sb)–Te–Bi-bearing inclusions were detected in the chalcopyrite portions of large composite sulphides. In contrast,
Pd, Os, and Ir occur in solid solution and as discrete inclusions within the BMS throughout the core. For Os and Ir, this
is usually in the form of Os–Ir alloys, whereas Pd forms a range of Pd–Te–Bi–(Sb) phases. Scanning electron microscope observations
on samples from the top of the core revealed the presence of ≤0.2-mm-long (PtPd)2(Sb,Te,Bi)2 michenerite–maslovite laths within the chalcopyrite portions of large composite sulphides. Additional Pt-bearing minerals,
including sperrylite and geversite, and a number of Pd(–Te–Bi–Sb) minerals were observed in, or close to, the alteration rims
of these sulphides. This textural association was observed throughout the core. Similar platinum-group minerals (PGMs) were
observed within the felsic assemblages composed of quartz, plagioclase, alkali feldspar and clinopyroxene produced by late-stage
felsic melts that permeated the Platreef. Many of these PGMs occur a significant distance away from any sulphide minerals.
We believe these features can all be linked to the introduction of As, Sb, Te and Bi into the magmatic system through assimilation
of sedimentary footwall rocks and xenoliths. Where the degree of contamination was high, all of the Pt and some of the Pd
formed As- and Sb-bearing PGM that were expelled to the edges of the sulphide droplets. Many of these were redistributed where
they came into contact with late-stage felsic melts. Where no felsic melt interactions occurred, the expelled Pt- and Pd-arsenides
and antimonides remained along the margins of the sulphides. At the top of the Platreef, where the effects of contamination
were relatively low, some of the Pt remained within the sulphide liquids. On cooling, this formed the micro-inclusions and
blade-like laths of Pt–Pd–(Sb)–Bi–Te in the chalcopyrite. |
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Keywords: | Platinum-group elements Sulphide mineralisation Laser ablation ICP-MS Bushveld complex Platreef |
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