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) requires the presence of vacancies in the Fe sublattice. Palladium appears to replace Fe atoms that have as nearest neighbors at least four vacancies, while Pt substitutes for Fe atoms that have as nearest neighbors at least five vacancies. Both Pd and Pt replace Fe on a one to one basis. The suggestion that, at given temperature, the platinum-group elements (PGEs) only substitute for Fe atoms that are surrounded by a minimum number of vacancies demands that the solubility of PGEs in Fe1-xS fall strongly with decreasing temperature and decreasing f(S2). Vacancies become ordered with de creasing temperature and expelled with decreasing relative f(S2). PGEs initially dissolved in monosulfide solid solution (mss) at high temperature will be exsolved as the mss recrystallizes to Fe1-xS and pentlandite during cooling, to form their own discrete PGE phases. Extrapolation of our experimental temperature-solubility profiles to PGE levels typically observed in natural pyrrhotites of Merensky-type PGE deposits suggests that the association of base metal sulfides with discrete PGE phases is a low temperature phenomenon. We conclude that the sulfides of the Merensky reef may have exsolved PGE to temperatures below 100 degrees C.