Precious metal enrichment in the Platreef,Bushveld Complex,South Africa: evidence from homogenized magmatic sulfide melt inclusions |
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| Authors: | D A Holwell I McDonald I B Butler |
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| Institution: | (1) Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, UK;(2) School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3YE, UK;(3) School of Geosciences, The University of Edinburgh, Grant Institute The King’s Buildings, West Mains Road, Edinburgh, EH9 3JW, UK;(4) ECOSSE (Edinburgh Collaborative of Subsurface Science and Engineering), A Joint Research Institute of the Edinburgh Research Partnership in Engineering and Mathematics, Edinburgh, UK |
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| Abstract: | Magmatic sulfide deposits are the most significant source of platinum-group elements (PGE) in the world. Key to understanding
their genesis is determining the processes and timing of sulfide saturation, metal enrichment and crustal contamination. In
this study, we have identified droplets of magmatic sulfide from the Platreef, South Africa, where droplets of sulfide have
been trapped in the earliest crystallising phase, chromite. Due to their early entrapment at high temperatures, metal concentrations
and ratios that they display are indicative of a very early-stage sulfide liquid in the system, as they will have cooled and
fractionated within an essentially closed system, unlike interstitial blebs that crystallise in an open system as the magma
cools. Analysis of these droplets in an opaque mineral like chromite by LA-ICP-MS is problematic as some of the fractionated
inclusion is necessarily lost during cutting and polishing to initially identify the inclusion. This particularly affects
the ability to representatively sample the most fractionated phases such as gold and platinum minerals. Here, using a novel
technique whereby the inclusions are homogenized and quickly quenched, so that any cutting, polishing and subsequent LA-ICP-MS
analysis samples a truly representative portion of the droplet. This has been used to show that early sulfide liquids in the
Platreef were highly PGE-rich and had Pt/Pd ratios of close to unity that supports genetic models invoking sulfide saturation
and metal enrichment prior to intrusion, with pre-enriched sulfides entrained within the Platreef magma. |
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