Platinum-group Elements and Microstructures of Normal Merensky Reef from Impala Platinum Mines, Bushveld Complex

Abstract

The Merensky Reef of the Bushveld Complex contains one of the world’s largest concentrations of platinum-group elements (PGE). We have investigated ‘normal’ reef, its footwall and its hanging wall at Impala Platinum Mines. The Reef is 46 cm thick and consists from bottom to top of leuconorite, anorthosite, chromitite and a very coarse-grained melanorite. The footwall is leuconorite and the hanging wall is melanorite. The only hydrous mineral present is biotite, which amounts to 1%, or less, of the rock. All of the rocks contain 0·1–5% interstitial sulphides (pyrrhotite, pentlandite and chalcopyrite), with the Reef rocks containing the most sulphides (1–5%). Lithophile inter-element ratios suggest that the magma from which the rocks formed was a mixture of the two parental magmas of the Bushveld Complex (a high-Mg basaltic andesite and a tholeiitic basalt). The Reef rocks have low incompatible element contents indicating that they contain 10% or less melt fraction. Nickel, Cu, Se, Ag, Au and the PGE show good correlations with S in the silicate rocks, suggesting control of the abundance of these metals by sulphides. The concentration of the chalcophile elements and PGE in the silicate rocks may be modelled by assuming that the rocks contain sulphide liquid formed in equilibrium with the evolving silicate magma. It is, however, difficult to model the Os, Ir, Ru, Rh and Pt concentrations in the chromitites by sulphide liquid collection alone, as the rocks contain 3–4 times more Os, Ir, Ru, Rh and Pt than the sulphide-collection model would predict. Two possible solutions to this are: (1) platinum-group minerals (PGM) crystallize from the sulphide liquid in the chromitites; (2) PGM crystallize directly from the silicate magma. To model the concentrations of Os, Ir, Ru, Rh and Pt in the chromitites it is necessary to postulate that in addition to the 1% sulphides in the chromitites there is a small quantity (0·005%) of cumulus PGM (laurite, cooperite and malanite) present. Sulphide liquids do crystallize PGM at low fS2. Possibly the sulphide liquid that was trapped between the chromite grains lost some Fe and S by reaction with the chromite and this provoked the crystallization of PGM from the sulphide liquid. Alternatively, the PGM could have crystallized directly from the silicate magma when it became saturated in chromite. A weakness of this model is that at present the exact mechanism of how and why the magma becomes saturated in PGM and chromite synchronously is not understood. A third model for the concentration of PGE in the Reef is that the PGE are collected from the underlying cumulus pile by Cl-rich hydrous fluids and concentrated in the Reef at a reaction front. Although there is ample evidence of compaction and intercumulus melt migration in the Impala rocks, we do not think that the PGE were introduced into the Reef from below, because the rocks underlying the Reef are not depleted in PGE, whereas those overlying the Reef are depleted. This distribution pattern is inconsistent with a model that requires introduction of PGE by intercumulus fluid percolation from below.