Geochemistry of the J-M (Howland) Reef of the Stillwater Complex, Minneapolis Adit area; I, Sulfide chemistry and sulfide-olivine equilibrium

Abstract

Platinum mineralization in the Stillwater Complex occurs within a complex sequence of olivine-bearing, plagioclase-rich cumulates about 400 m above the base of the Banded zone. The reef consists of a 2-m-thick layer of disseminated sulfides extending over the entire strike length of the complex. In the area of this study the host rock to the reef sulfides shows a lateral transition over a strike length of 700 m from a heterogeneous olivine-plagioclase cumulate (mixed rock) to a plagioclase-olivine cumulate containing a high proportion of postcumulus bronzite, with olivine preserved only as rare resorbed cores on bronzite grains.Platinum-group elements within the reef correlate strongly with abundances of sulfides, which show convincing textural evidence for having originated as immiscible sulfide liquid. Strong correlations are also observed between all six platinum-group elements, Ni, and Cu. Despite the gradation in host-rock mineralogy, the sulfide fraction of the reef displays a constant bulk composition over 700 m of strike length, containing 9.3 percent Ni, 6.9 percent Cu, and approximately 0.7 percent total platinum-group elements (principally Pd). These platinum-group element concentrations are two to three orders of magnitude higher than those found in typical magmatic sulfide deposits and are attributed to equilibration of sulfide liquid with a very large volume of silicate melt.Olivines from the reef show a range of compositions from Fo 71 to Fo 85 , and from 2,400 to 4,500 ppm Ni. A positive correlation is observed between NiO and FeO in olivines from sulfide-bearing samples, indicating Ni-Fe exchange between olivine and sulfides on a local scale, such that Ni contents of olivines were buffered by that of the sulfide phase. A value of 9.70 + or - 1.49 is obtained for the Ni-Fe distribution coefficient between sulfide and olivine, in agreement with values obtained at magmatic temperatures by Boctor (1982).These observations may be interpreted in the framework of the plume model of Campbell et al. (1983). A pulse of replenishing magma enters the Stillwater chamber as a buoyant plume and subsequently spreads to form a layer of hybrid melt. Sulfide liquation and olivine crystallization occur within the plume and the hybrid layer in response to mixing and subsequent cooling. Sulfide liquid attains high platinum-group element concentrations during turbulent ascent and spreading of the plume. Crystallization of olivine from the hybrid layer gives rise to olivine-sulfide boulders, which settle to the floor of the chamber. Reaction between the boulders and the evolved liquid at the bottom of the chamber causes olivine to become more iron rich. Subsequent reequilibration between olivine and sulfide grains gives rise to the observed Ni-Fe distribution.