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Journal of Petrology Volume 42 Number 10 Pages 1911-1926 2001
© Oxford University Press 2001
Phase Relations in the Fe–Ni–Cu–PGE–S System at Magmatic Temperature and Application to Massive Sulphide Ores of the Sudbury Igneous Complex*
1INSTITUT FÜR MINERALOGIE, UNIVERSITÄT MÜNSTER, CORRENSSTRASSE 24, 48149 MÜNSTER, GERMANY
2GEOLOGY DEPARTMENT, UNIVERSITY OF CAPE TOWN, RONDEBOSCH 7700, SOUTH AFRICA
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, DCu is 
0·2 at all temperatures and all metal/S ratios. DNi 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 DNi occurs is sensitive to S2 fugacity. The monosulphide solid solution (mss)–melt partition coefficients of the platinum group elements (DPGE) are determined with laser-ablation inductively coupled plasma mass spectrometry calibrated on synthetic sulphide standards. At trace element concentration levels, the DPGE 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)/
metal ratios. It is shown that the change-over in DNi may be used to retrieve parent melt compositions, fractionation temperatures, and magmatic fractionation paths of these deposits.
KEY WORDS: Fe–Ni–Cu–S system; partition coefficients; laser-ablation ICP-MS; PGE; liquid immiscibility; Sudbury
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