PARTITIONING OF PLATINUM-GROUP ELEMENTS AND AU IN THE FE-NI-CU-S SYSTEM - EXPERIMENTS ON THE FRACTIONAL CRYSTALLIZATION OF SULFIDE MELT

Partitioning of platinum-group elements (PGE) between sulfide liquid a nd monosulfide solid solution (mss) has been investigated by crystalli zing mss from Fe - Ni - Cu sulfide liquid at 1,000-1,040-degrees-C, us ing bulk compositions and PGE contents typical of magmatic sulfides as sociated with mafic and ultramafic systems. Products were analyzed in situ for PGE and Au using SIMS. Sulfide liquid compositions were more Ni- and Cu-rich than coexisting mss. Liquid/mss partition coefficients are: Os - 0.23 +/- 0.04, Ir - 0.28 +/- 0.11, Ru 0.24 +/-0.05, Rh - 0. 33 +/- 0.06, Pt - 4.8 +/- 0.7, Pd - 4.8 +/- 1.9, Au - 11.4. Partitioni ng of PGE is independent of PGE concentration and Ni content in the co mposition range investigated. Additionally, Henry's law appears to be obeyed up to minor-element contents in the sulfide liquid and mss. Osm ium, Ir, Ru, and Rh are compatible elements in the anhydrous Fe - Ni - Cu - S system, whereas Pt, Pd and Au are incompatible elements. These affinities correspond to the partitions of PGE between massive and Cu -rich magmatic sulfides. However, the detailed precious-metal composit ions of the Cu-rich sulfides of mafic rock systems, disseminated ores of komatiites and Cu-rich assemblage of droplet ore from the Noril'sk- Talnakh deposits are not consistent with those expected for pristine f ractionated sulfide liquids.