Noble Metal Enrichment Processes in the Merensky Reef, Bushveld Complex

We have analysed sulphides, silicates, and chromites of the Merensky INTRODUCTION Reef for platinum-group elements (PGEs), Re and Au using laser The enrichment of noble metals in layered gabbroic ablation-inductively coupled plasma mass spectrometry and synthetic intrusions to form stratiform Merensky-type ore horizons pyrrhotite standards annealed with known quantities of noble metals. is an extremely efficient yet poorly understood process. Os, Ir and Ru reside in solid solution in pyrrhotite and pentlandite, Metal enrichment must be related to the formation of Rh and part of the Reef ’s Pd in pentlandite, whereas Pt, Au, Re magmatic layering, as the ore horizons form an integral and some Pd form discrete phases. Olivine and chromite, often part of the magmatic stratigraphies of their host insuspected to carry Os, Ir and Ru, are PGE free. All phases analysed trusions. Metal concentration is also related to the prescontain noble metals as discrete micro-inclusions with diameters ence of magmatic sulphides, because the noble metals are either dissolved in base metal sulphides or occur as typically <100 nm. Inclusions in sulphides commonly have the discrete noble mineral phases intergrown with sulphides element combinations Os–Ir–Pt and Pt–Pd–Au. Inclusions in (Vermaak & Hendriks, 1976; Ballhaus & Ryan, 1995; olivine and chromite are dominated by Pt± Au–Pd. Few inclusion Ballhaus & Ulmer, 1995). spectra can be related to discrete noble metal phases, and few There is considerable controversy as to where the inclusions have formed by sub-solidus exsolution. Rather, some metals came from and how metal enrichment worked. PGE inclusions, notably those in olivine and chromite, are earlyFor a stratiform platinum-group element (PGE) deposit magmatic nuggets trapped when their host phases crystallized. We inside the magmatic stratigraphy of a layered intrusion, suggest that the silicate melt layer that preceded the Merensky Reef there are two possibilities: either the metals were derived was PGE oversaturated at early cumulus times. Experiments from the melt column above the ore horizon, or they came combined with available sulphide–silicate partition coefficients sugfrom the magma equivalent to the cumulate package gest that a silicate melt in equilibrium with a sulphide melt underlying the ore. Owing to the large density contrast containing the PGE spectrum of the Merensky ore would indeed be between silicate and sulphide melt, a popular view is that oversaturated with respect to the least soluble noble metals. Sulphide the metals were enriched by gravitational settling of melt apparently played little role in enriching the noble metals in sulphide melt, i.e. out of the magma column above the the Merensky Reef; rather, its role was to immobilize a pre-existing ore (Campbell et al., 1983; Barnes & Naldrett, 1985; in situ stratiform PGE anomaly in the liquid-stratified magma Naldrett et al., 1987). On the other hand, it is well chamber. established that gravitational crystal settling plays little role in producing magmatic layering and magmatic stratigraphies (McBirney & Noyes, 1979; Morse, 1986; Campbell, 1987), and this cannot remain without consequence for stratiform noble metal deposits. A third alternative is noble metal enrichment by late