Cotectic precipitation of olivine and sulfide liquid from komatite magma and the origin of komathte-hosted disseminated nickel sulfide mineralization at Mount Keith and Yakabindie, Western Australia

An empirical equation for the relationship of sulfur solubility to silicate magma composition and temperature has been combined with numerical simulations of the fractional crystallization of olivine from komatiite magma to predict the relative proportions of olivine and sulfide liquid which should segregate at equilibrium from a fractionating komatiite. The predicted cotectic ratio of olivine to sulfide is higher and the S content of the cumulate assemblage lower by a factor of about three compared with the peak observed in type II (Mount Keith-type) dunite-hosted disseminated Ni sulfide deposits for the observed range of olivine compositions in the dunites. This implies that a purely "cotectic" precipitation model for this deposit type is incorrect, and that most of the sulfide liquid found in type II deposits is transported and mechanically deposited.