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It seems a matter of faith that fluids are responsible for any number of nature's deeds that otherwise have no obvious explanation, a case in point being the petrogenesis of platinum group element (PGE) deposits in layered intrusions. One confusion that persists is that fluids associated with all magmas are similar. They are not, of course. Those associated with acid systems are aqueous, contain high concentrations of dissolved silica and salts and had in many instances a profound influence on metallogenesis. In contrast, the fluids associated with mafic magmas are initially C02-rich and evolve to more water-rich compositions, and their role in redistribution of metals in mafic layered intrusions is uncertain.

In and around many granitoid bodies, such as those that yielded porphyry coppers, both the geology and hydrothermal mineral assemblages lend themselves to theoretical deduction of the nature and petrogenetic role of fluids. However, in complexes such as the Stillwater and Bushveld, key relationships that would allow us to make similar deductions are lacking, and inferences must be based in part on knowledge gained by experiment and analogy to natural systems. Too, the high-temperature, fluid-rock interactions in mafic systems have not been so thoroughly scrutinized.

Sulfur must be included in a discussion of volatiles since it is a component of volcanic gas and sulfides are the major ore minerals. This is particularly so because the stability of sulfide-oxide liquids in equilibrium with silicate melts is sensitive to oxidation state of the system, and oxidation state also influences the

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