Precious metal enrichment in the Platreef, Bushveld Complex, South Africa: evidence from homogenized magmatic sulfide melt inclusions

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. Keywords Sulfide inclusions Á Platinum-group elements Á Laser-ablation ICP-MS Á Platreef Á Bushveld Complex Communicated by C. Ballhaus.

The Penikat Intrusion, Finland: Geochemistry, Geochronology, and Origin of Platinum–Palladium Reefs

“…They thus suggested that the reefs crystallized from unusually PGE-rich magma derived from a staging chamber. Mitchell & Scoon (2007) had previously argued for reef crystallization from relatively sulphur-rich magmas, and Holwell et al (2010) found sulphide inclusions within chromite, which they argued indicate entrainment of sulphide from a staging chamber. The main problem with the model is that very few of the dykes or sills that could represent the chilled parent magmas to layered intrusions have been found to be enriched in PGE relative to normal basaltic magmas.…”

“…Model 2. The magma from which the PGE reefs crystallized was unusually PGE rich, through assimilation or entrainment of PGE-rich sulphides in a staging chamber (Naldrett et al, 2009;Holwell et al, 2010).…”

Capability of fs-LA-ICP-MS for sulfide analysis in comparison to ns-LA-ICP-MS: reduction of laser induced matrix effects?

“…For suldes usually major elements are used as the internal standard, which may be Zn, 43 Fe, 44,25 S, 45 or Ni. 30 Most suldes do not contain abundant Zn, limiting its use as the internal standard to the analyses of sphalerite and wurtzite.…”

Precious and base metal geochemistry and mineralogy of the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex, South Africa: implications for a multistage emplacement

“…Within the northern limb, Platreef mineralization is present within a 10-400 m thick basal unit, intruded as a series of sills (Kinnaird 2005) that rests directly on Palaeoproterozoic sediments and Archaean gneisses and granites and is overlain by Main Zone gabbronorites. Widespread contamination of the Platreef magma through assimilation of differing floor rocks along its strike length largely accounts for the complexity of the deposit, which formed through the interaction of magmatic, metasomatic, and hydrothermal processes (e.g., Harris and Chaumba 2001;Armitage et al 2002;Manyeruke 2003;Hutchinson and Kinnaird 2005;Kinnaird 2005;Manyeruke et al 2005;Sharman-Harris et al 2005;McDonald 2006, 2007;Holwell et al , 2011Hutchinson and McDonald 2008;McDonald et al 2009;Sharman et al 2013). Gruenewaldt et al (1989).…”