Sulfur saturation and second-stage melts; application to the Bushveld platinum metal deposits

Abstract

A model is presented for the formation of platinum-group element ores by precipitation from sulfur-deficient, platinum-group element-rich, second-stage magmas. Numerous investigations indicate that basaltic magmas originating from undepleted or mildly depleted source regions are sulfur saturated at the time of eruption. Mantle-derived peridotites residual from midocean ridge basalt magma genesis have Pd abundances that are uniformly too high for it to have originated solely from silicate melt retained in the source region at the time of segregation; they imply the presence of an accessory immiscible sulfide melt to host the platinum-group elements. The very high sulfide melt-silicate melt partition coefficients of the platinum-group elements would result in strong fractionation of these metals into the accessory sulfide component during the melting event. The extreme platinum-group element enrichment of this accessory sulfide component is evidenced by the discovery ofplatinoid minerals in residual mantle nodules containing Pd (and Pt) abundances at the low ppb level (Keays et al., 1981).Magmas generated by low to moderate degrees of partial melting of undepleted mantle are therefore sulfur saturated at the time of segregation. These (first-stage) magmas become impoverished in platinum-group elements during the early stages of fractional crystallization because of coprecipitation of an immiscible sulfide phase (e.g., midocean ridge basalt glasses average