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Many hypotheses for the formation of porphyry and skarn deposits and other deposits which are spatially and temporally associated with felsic igneous rocks suggest that the magmatic “volatile constituents” exert strong controls on the quantity, chemistry and style of the mineralization found in these systems. Many workers have summarized the data on the relationships between metallization and parameters such as rock type, ore metal content, and halogen content of the associated pluton(s). More recently, thermodynamic parameters such as fo2 and the fugacities of water, HF, and HCl have been related to the nature of the associated deposits.

In our laboratory, my coworkers and I have examined the partitioning of molybdenum and tungsten between water- saturated, high silica melts and minerals such as magnetite, ilmenite and rutile as a function of fo2 at 800°C and 1 kb (Tacker and Candela, 1987; Bouton et al., 1987; Bouton and Candela, in prep.). In this paper, our experimental results will be combined with previous work on felsic igneous rocks and associated ores in an effort to shed light on the role of fo2 in magmatic-hydrothermal ore genesis. Further, some of the models discussed in Candela (this volume) will be used to suggest links between the volatile constituents of magmas, their source regions, and the ore systems they spawn.

This chapter is not meant to be a review of the work done to date on the igneous petrology of the volatile constituents or their relationship to ore genesis. Rather, I will explore the implications

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