A Model for the Origin of the Platinum-Rich Sulfide Horizons in the Bushveld and Stillwater Complexes

Abstract

The sulfides of the Merensky Reef and the UG–2 chromite seam of the Bushveld Complex and the H.P. Reef of the Stillwater Complex have concentrated platinum group elements (PGE) with remarkable efficiency. Attempts to model these sulfides have exposed two major problems: (i) very high Nernst distribution coefficients (D) are required and (ii) a mass balance problem. The second problem, that of mass balance, stems from the first. If the D value is high, the sulfides must reach equilibrium with a large mass of silicate melt in order to take full advantage of this high D value. In practice R, the silicate/sulfide mass ratio, must be greater than D. It is suggested that the Merensky Reef, UG–2 and H.P. Reef were formed by major new pulses of picritic magma. Factors which control the value of R depend on the injection process and on the mechanism of sulfide liquation. During the early stages of fractional crystallization, when olivine and bronzite are the fractionating phases, the density of the magma in the chamber decreases with increased fractionation. A new pulse of magma entering the chamber will be denser than the magma in the chamber and will spread out across the floor. If the new pulse enters the chamber with sufficient momentum it will initially jet upwards into the overlying magma then fall back on itself when negative buoyancy overcomes the jet's initial momentum. During jetting the new pulse mixes with the host magma to form a hybrid melt which collects at the floor of the chamber, producing a gravity stratified layer with warm primitive magma overlain by cooler, more fractionated magma. On cooling the gravity stratified zone breaks up into a number of double diffusive convection cells. Once plagioclase becomes a cumulus phase the density of a tholeiitic magma increases with increased fractionation. Eventually the density of the magma in the chamber becomes greater than that of the parent magma. A new pulse entering the magma chamber at a stratigraphic level above the density crossover point will rise as a plume to the top of the chamber. During ascent the new pulse will mix turbulently with the host magma to produce a density stratified hybrid layer at the top of the chamber. This time hot primitive magma will overlie cooler, more fractionated magma. Magma mixing is a process that can produce sulfide liquation. The turbulence associated with either jets or plumes can produce a very high R value. The actual value of R is probably controlled by the size of the new pulse; the larger the pulse the higher the R value. If sulfide liquation is temperature-controlled, stratigraphic factors and the relative densities of the two magmas may have an important influence on R. The most favorable condition for producing a high value for R is when a new pulse of hot magma, slightly lighter than the host magma, enters a density stratified chamber. Under these conditions, the new pulse will rise to its own density level, then spread laterally to form a density stratified intermediate layer which will lose heat rapidly to the cooler overlying host magma. If the density difference between the original magmas was small, the density stratification forming within the hybrid layer will destabilize as it cools, allowing any immiscible sulfides which form to equilibrate with a large volume of silicate melt. If the density difference is large, cooling will not destabilize the density stratification and the sulfides will have a low R value.