Fractional crystallization of anhydrous sulfide liquid in the system Fe-Ni-Cu-S, with application to magmatic sulfide deposits
Phase relations for coexisting monosulfide solid solution [(Fe,Ni)] 1- xS; mss] and sulfide liquid have been determined in the Fe-rich portion of the anhydrous quaternary System Fe-Ni-Cu-S at 52.5 and 50.0 at% S, 850-1050°C and low pressure. Equations for the complex dependence of the mss solidus, sulfide liquidus and distribution coefficients for exchange of Ni/Cu and Fe/Cu on temperature, S content, and proportion of metals have been derived by multiple regression analysis. These equations may be used for geothermometry of pristine magmatic sulfides and prediction of liquid evolution paths, but a limiting factor is uncertainty in the precise bulk composition of natural assemblages, and particularly of S content. Intermediate solid solution [(Cu,Fe)S 1- x; iss] is stable up to 900-950°C in Ni-poor compositions, and a large field of a new ternary (Fe,Cu,Ni)S solid solution intermediate between iss and mss exists at and below 850°C. Disseminated sulfides in komatiites are markedly depleted in Cu, relative to an origin by sulfide liquid immiscibility. The compositions of magmatic sulfide ores from Sudbury, Canada and the Insizwa complex, Transkei are not consistent with derivation of the associated late Cu-rich ores by fractional crystallization of sulfide magma, although the Cu-rich ores of the Noril'sk-Talnakh district, Russia could have evolved through a complex sequence of fractional crystallization events.