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

Silver-gray tourmaline fibers intergrown with a deep pink elbaite host from the Cruzeiro mine, Minas Gerais, Brazil, provide evidence for the composit ional evolution of magmatic and hydrothermal fluids involved in pegmatite f ormation. Optical and back-scattered electron imaging, together with detail ed microanalysis, establish that the fibers, 0.05-0.3 mm in width, are comp lexly zoned and developed in four distinct generations marked by discrete c ompositions 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 el baite to Li-rich schorl, and the third generation is a yellowish-green "flu or-elbaite". Volumetrically the most abundant, generation-three fibers poik iloblastically replace the earlier generations as well as the host. A fourt h generation of fibrous tourmaline fills fractures that cut all previous ge nerations and the host, but is unrelated to growth of the previous fibers. Compositionally, last generation is indistinguishable from the second-gener ation Li-rich schorl fibers. Textural and compositional discontinuities of each generation record periods of stability followed by reaction(s) in whic h the tourmaline was initially unstable, partially dissolved owing to inter action with fluids, and then redeveloped in response to interactions with e volving orthomagmatic or hydrothermal fluids. The general progression of th e first three generations implies that reacting fluids were generally under going fractionation, becoming successively enriched in Na, Li, Ca, and F du ring late crystallization of the pegmatite. The composition was reset to a Li-rich schorl during fate-phase fracturing. Crystal-chemical constraints s uch as F - X-site vacancy avoidance control part of the compositional varia bility observed. In this multistage tourmaline sample, individual fibers ex hibit the most chemically complex compositions yet recorded, and reflect th e dramatic complexity of fluid evolution involved in their crystallization.