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The platinum-group elements (PGE) ruthenium, osmium, rhodium, iridium, palladium and platinum together with iron, cobalt and nickel form Group VIII of Mendeleev's periodic table. All are strongly siderophile and chalcophile, and their distribution in the crust and upper mantle is largely determined by the distribution of sulfides. In the case of PGE, there are few specific data on how they distribute themselves among sulfides, silicates and other phases or on their solubilities in high-temperature, volatile-rich fluids. In fact, our understanding of how these elements behave in response to igneous differentiation or metamorphism is based on empirical, primarily geochemical, observations. For these reasons, models devised to explain the development of PGE- rich horizons in the layered intrusions are not well constrained.

Their generally siderophile and chalcophile nature should not obscure the fact that in some environments the PGE are volatile. This is clearly demonstrated by the enormous enrichments in iridium (along with gold, silver and certain other trace metals) in Kilauea volcanic gases relative to lavas (Olmez et al., 1986). In the Bushveld Complex, the presence of hydrothermal platiniferous pegmatite pipes (e.g., the Driekop) and the complexities of PGE distribution within the UG-2 chromitite and Merensky Reef imply some control by fluids. Again, lack of data reduce us to little more than speculation on the specific roles of fluids in the genesis of ores in the layered intrusions.

The purpose of this chapter is to assemble all the PGE data relevant to processes occurring in layered complexes. The available data

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