PLATINUM-GROUP ELEMENTS IN THE MERENSKY REEF .2. EXPERIMENTAL SOLUBILITIES OF PLATINUM AND PALLADIUM IN FE1-XS FROM 950 TO 450-DEGREES-C UNDER CONTROLLED F(S2) AND F(H2)

Substitution of Pd and Pt in hexagonal 1 C pyrrhotite (Fe1-xS) require s the presence of vacancies in the Fe sublattice. Palladium appears to replace Fe atoms that have as nearest neighbors at least four vacanci es, while Pt substitutes for Fe atoms that have as nearest neighbors a t least five vacancies. Both Pd and Pt replace Fe on a one to one basi s. The suggestion that, at given temperature, the platinum-group eleme nts (PGEs) only substitute for Fe atoms that are surrounded by a minim um number of vacancies demands that the solubility of PGEs in Fe1-xS f all strongly with decreasing temperature and decreasing f(S2). Vacanci es become ordered with de creasing temperature and expelled with decre asing relative f(S2). PGEs initially dissolved in monosulfide solid so lution (mss) at high temperature will be exsolved as the mss recrystal lizes to Fe1-xS and pentlandite during cooling, to form their own disc rete PGE phases. Extrapolation of our experimental temperature-solubil ity profiles to PGE levels typically observed in natural pyrrhotites o f Merensky-type PGE deposits suggests that the association of base met al sulfides with discrete PGE phases is a low temperature phenomenon. We conclude that the sulfides of the Merensky reef may have exsolved P GE to temperatures below 100 degrees C.