Phase Relations in the Fe–Ni–Cu–PGE–S System at Magmatic Temperature and Application to Massive Sulphide Ores of the Sudbury Igneous Complex*

Abstract

Experiments in the Fe–Ni–Cu–S system were performed to identify the role of the metal/S atomic ratio on monosulphide–melt partition coefficients and closed-system fractionation paths. In accord with previous work, D_Cu is ∼0·2 at all temperatures and all metal/S ratios. D_Ni is highly sensitive to temperature and metal/S, and changes from ∼0·6 at high metal/S and high temperature to >2 at low temperature and low metal/S. The temperature at which the cross-over in D_Ni occurs is sensitive to S₂ fugacity. The monosulphide solid solution (mss)–melt partition coefficients of the platinum group elements (D_PGE) are determined with laser-ablation inductively coupled plasma mass spectrometry calibrated on synthetic sulphide standards. At trace element concentration levels, the D_PGE are largely insensitive to metal/S, contrary to previous experiments with PGE concentrations in the percentage range. Pt and Pd are highly incompatible with mss (D < 0·1) whereas Ir, Ru, and Rh are compatible, ranging from >3 to ∼10. The chemical differentiation paths of Fe–Ni–Cu–S sulphide melts experiencing mss fractionation are determined by the metal/S parent melt ratio. Oxidized sulphide melts with metal/S < 1 solidify in the stability field of intermediate solid solution (iss) whereas reduced sulphide melts with metal/S > 1 may fractionate past iss stability. The latter will accumulate Ni together with Cu down to solidus temperature. Toward the end of their fractionation path, they are too depleted in S to crystallize iss. Instead, they will precipitate a copper sulphide with monovalent Cu and presumably solidify at an iss–bornite–millerite eutectic. The dataset is applied to massive sulphide ores of the Sudbury Igneous Complex fractionated with respect to Ni/Cu and (Ni + Cu)/Σmetal ratios. It is shown that the change-over in D_Ni may be used to retrieve parent melt compositions, fractionation temperatures, and magmatic fractionation paths of these deposits.