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Abstract

Nearly all mafic igneous bodies exhibit evidence for the presence of volatile-rich melts and fluids. This is true for thin sills, in which localized pods of pegmatitic and gran- ophyric rocks are common, as well as for the layered intrusions. In the Bushveld and Stillwater Complexes, volatile elements, either as a separate vapor or simply dissolved in the melt, were clearly involved in the petrogenesis of the PGE-rich horizons because the constituent rocks are pegmatitic and contain magmatic, volatile-bearing minerals. There is also circumstantial evidence that volatiles were involved in processes of PGE enrichment or redistribution. The specific roles of fluid in these processes are enigmatic because there is a lack of knowledge about the composition of fluid under near-solidus conditions, the ability of fluid to transport PGE and the physical and chemical interactions of fluid with partially molten to completely crystalline cumulates.

A conception of how fluids and volatile-rich melts behave in the environment of a crystallizing layered intrusion must originate from observations of the rocks themselves. The point of this chapter is to bring together such observations. To make this task managable, the description is restricted to the Stillwater and Bushveld Complexes. A reference stratigraphic section of the former is presented in Fig. 9.1, and for the Bushveld reference may be made to von Gruenewaldt et al. (1985). Implicit here is the conception, first expressed by Howland et al. (1936) in the above quotation, that the processes which guided the evolution of the two complexes were essentially

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