Complexly zoned fibrous tourmaline, Cruzeiro mine, Minas Gerais, Brazil: A record of evolving magmatic and hydrothermal fluids

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Silver-gray tourmaline fibers intergrown with a deep pink elbaite host from the Cruzeiro mine, Minas Gerais, Brazil, provide evidence for the compositional evolution of magmatic and hydrothermal fluids involved in pegmatite formation. Optical and back-scattered electron imaging, together with detailed microanalysis, establish that the fibers, 0.05-0.3 mm in width, are complexly zoned and developed in four distinct generations marked by discrete compositions and replacement textures. Fiber growth is punctuated by periods of dissolution. The first generation, preserved in the fiber interior, is a dark blue foitite; the blue-gray second generation varies from Fe-rich elbaite to Li-rich schorl, and the third generation is a yellowish-green 'fluor-elbaite'. Volumetrically the most abundant, generation-three fibers poikiloblastically replace the earlier generations as well as the host. A fourth generation of fibrous tourmaline fills fractures that cut all previous generations and the host, but is unrelated to growth of the previous fibers. Compositionally, last generation is indistinguishable from the second-generation Li-rich schorl fibers. Textural and compositional discontinuities of each generation record periods of stability followed by reaction(s) in which the tourmaline was initially unstable, partially dissolved owing to interaction with fluids, and then redeveloped in response to interactions with evolving orthomagmatic or hydrothermal fluids. The general progression of the first three generations implies that reacting fluids were generally undergoing fractionation, becoming successively enriched in Na, Li, Ca, and F during late crystallization of the pegmatite. The composition was reset to a Li-rich schorl during late-phase fracturing. Crystal-chemical constraints such as F - X-site vacancy avoidance control part of the compositional variability observed. In this multistage tourmaline sample, individual fibers exhibit the most chemically complex compositions yet recorded, and reflect the dramatic complexity of fluid evolution involved in their crystallization.

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Canadian Mineralogist

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