Noble Metal Enrichment Processes in the Merensky Reef, Bushveld Complex

Abstract

We have analysed sulphides, silicates, and chromites of the Merensky Reef for platinum-group elements (PGEs), Re and Au using laser ablation-inductively coupled plasma mass spectrometry and synthetic pyrrhotite standards annealed with known quantities of noble metals. Os, Ir and Ru reside in solid solution in pyrrhotite and pentlandite, Rh and part of the Reef’s Pd in pentlandite, whereas Pt, Au, Re and some Pd form discrete phases. Olivine and chromite, often suspected to carry Os, Ir and Ru, are PGE free. All phases analysed contain noble metals as discrete micro-inclusions with diameters typically <100 nm. Inclusions in sulphides commonly have the element combinations Os–Ir–Pt and Pt–Pd–Au. Inclusions in olivine and chromite are dominated by Pt ± Au–Pd. Few inclusion spectra can be related to discrete noble metal phases, and few inclusions have formed by sub-solidus exsolution. Rather, some PGE inclusions, notably those in olivine and chromite, are early-magmatic nuggets trapped when their host phases crystallized. We suggest that the silicate melt layer that preceded the Merensky Reef was PGE oversaturated at early cumulus times. Experiments combined with available sulphide–silicate partition coefficients suggest that a silicate melt in equilibrium with a sulphide melt containing the PGE spectrum of the Merensky ore would indeed be oversaturated with respect to the least soluble noble metals. Sulphide melt apparently played little role in enriching the noble metals in the Merensky Reef; rather, its role was to immobilize a pre-existing in situ stratiform PGE anomaly in the liquid-stratified magma chamber.