I present an empirical parameterization of low-pressure melting relations of monosulfide solid solution in the system Ni-Cu-Fe-S-O and use it to argue that sulfide ores at Sudbury are almost exclusively cumulate in origin. In the model the solid is considered to be composed of four components NiS, CuS, FeS, and {radical dot}S, where {radical dot}S represents substitution of a vacancy for a cation. The liquid solution is considered to be a mixture of cations Ni, Fe, and Cu and anions S and O. The exchange of metals and sulfur between solid and melt is treated as a series of reactions of the form Ml + Sl = MSs, where subscripts l and s denote liquid and solid, respectively, allowing the definition of an exchange coefficient KD = XsMS / (XlM × XlS) where X denotes mole fraction. I have fitted equations of the form log KD = a/T + bXlS + c to the existing database for coexisting sulfide liquid-monosulfide solid solution for each of the liquid components Ni, Cu, and S. The fitted KD expressions have been implemented in a Matlab program to estimate the compositions of coexisting solid and liquid sulfide phases in the system Ni-Cu-Fe-S-O, using the liquidus temperature approximation of Fleet and Pan [Fleet M. E. and Pan Y. (1994) Fractional crystallization of anhydrous sulfide liquid in the system Fe-Ni-Cu-S, with application to magmatic sulfide deposits. Geochim. Cosmochim. Acta 58, 3369-3377], and using a crude approximation to the liquidus surface of magnetite. Ni is found to be incompatible with mss over most of the range of conditions of crystallization of natural sulfide magmas, becoming compatible only at the lowest temperatures and highest Ni and Cu contents attained. Comparison with compositions of sulfide ores from Sudbury demonstrates that many ore compositions are distributed along mixing lines between primary mss and intermediate solid solution (iss) or between high pentlandite (heazlewoodite solid solution; hzss) and iss. The mss-iss trend can be modeled as a mixing line between mss and residual sulfide liquid or between mss and iss cumulates. In the former case, the Ni-rich composition of the mss must reflect continuous equilibration of solids and liquids during cooling to low temperatures close to the sulfide solidus. In the latter case, a significant mass of highly Ni- and Cu-enriched residual sulfide liquid must be interpreted to have left the mss-iss cumulates behind during a late-stage migration process at temperatures well below the solidus temperature of the enclosing silicate rocks. Subsequent crystallization of hzss-iss cumulates from Ni- and Cu-rich residual sulfide liquids produced the second trend between the compositions of pentlandite and chalcopyrite. The balance of evidence favors the hypothesis that most sulfide ores represent cumulate mixtures of iss and mss or of iss and hzss rather than residual Cu-rich sulfide liquids.

Geochimica et Cosmochimica Acta
Department of Earth Sciences

Mungall, J.E. (2007). Crystallization of magmatic sulfides: An empirical model and application to Sudbury ores>. Geochimica et Cosmochimica Acta, 71(11), 2809–2819. doi:10.1016/j.gca.2007.03.026