Magma-Vapor Evolution in Granitic Systems of Cornwall, SW England: The Role of Boron

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The granites and associated pegmatites, aplites, and mineralized greisens of Cronwall, southwest England, constitute an archetypal system of post-orogenic, highly differentiated silicic magmatism and associated hydrothermal Sn-W-Cu mineralization. The late-stage evolution of these granites, in particular the temporal and compositional aspects of melt-vapor equilibria, represent a critically important stage of magmatism from both scientific and economic perspectives. Among the volatile or fluxing components in the Cornish granite systems, the role of boron may be the most important and least understood. This proposed work will document the abundance and geologic associations of boron (as tourmaline) in this environment, and the tourmaline will be studied for fluid inclusions that may uniquely reveal the compositions and physical states of fluids generated in the granites and associated veins and host rocks. In addition, analyses of the boron isotopic composition of tourmaline (11B/10B) will help to identify the provenance (source rocks) of the granitic magmas, the extent of mixing and pathways of fluid flow between granites and their host rocks (if multiple reservoirs of boron are found), and with the aid of experimental calibration of isotopic fractionation, the pressures, temperatures, and possibly pH of fluids at various stages of tourmaline crystallization. The results of this developed by the principal investigator to further elucidate the geochemical cycle of boron in magmatic-hydrothermal systems. The results may be generally applicable to similar deposits that are common and widespread in space and time, and will serve as a model for others to follow.