The Richemont rhyolite dyke, Massif Central, France: A subvolcanic equivalent of rare-metal granites

A rhyolitic dyke at Richemont, Haute-Vienne, France, is shown to be the subvolcanic counterpart of rare-metal-bearing granites and pegmatites. Textural relationships provide evidence for a quenched silicate melt with less than 5% phenocrysts, consisting of albite (50%), quartz (20%), K-feldspar (20%), and muscovite (10%). The affinity with pegmatites arises from the ore mineralogy, with "uran-euxenite" and "wolfiamo-ixiolite" as the main rare-metal carriers. An affinity with rare-metal granites arises from the geochemistry. The composition of the rhyolite is quite similar to that of the Beauvoir Ta-Li-bearing granite, corresponding to the high-phosphorus, high-fluorine class of strongly peraluminous leucogranites, enriched in Ta, Nb, Sn, Li, and Be. The melt belongs to the family that typically crystallizes as LCT granitic pegmatites. Analyses of muscovite phenocrysts provide estimates of muscovite - melt partition coefficients, allowing an explanation of some geochemical characteristics of this type of magmatism, e.g., Li, Cs, Ta and Sn increase during differentiation, and patterns of fractionation involve the ore elements W, Nb and Ta. Isolated aggregates of phosphate with inclusions of sulfosalts and of Nb, W, and Sn oxides are interpreted as signaling the onset of silicate - phosphate melt unmixing processes. Microcrystalline facies of the rhyolite are tentatively identified as the result of complete fluid unmixing, allowing a qualitative assessment of element extraction by fluids escaping from high-F melts. Although Sn, Ti, Th, Nb, and Ta are not depleted relative to the melt, other elements are removed, mildly in the case of Rb, Mn, S, Be: and Zn, strongly in the case of U, Li, B, As, F, and Ba,and very strongly in the case of W, Sb, Sr, P, and Ca.