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-degrees-C and low pressure. Equations for the complex depende nce of the mss solidus, sulfide liquidus and distribution coefficients for exchange of Ni/Cu and Fe/Cu on temperature, S content, and propor tion of metals have been derived by multiple regression analysis. Thes e equations may be used for geothermometry of pristine magmatic sulfid es and prediction of liquid evolution paths, but a limiting factor is uncertainty in the precise bulk composition of natural assemblages, an d particularly of S content. Intermediate solid solution [(Cu,Fe)S1-x; iss] is stable up to 900-950-degrees-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-degrees-C. Disseminated su lfides in komatiites are markedly depleted in Cu, relative to an origi n by sulfide liquid immiscibility. The compositions of magmatic sulfid e ores from Sudbury, Canada and the Insizwa complex, Transkei are not consistent with derivation of the associated late Cu-rich ores by frac tional crystallization of sulfide magma, although the Cu-rich ores of the Noril'sk-Talnakh district, Russia could have evolved through a com plex sequence of fractional crystallization events.