Microfracturing and fluid mixing in granties: W-(Sn) ore deposition at Vaulry (NW French Massif Central)

The Vaulry W-(Sn) mineralisation, located at the eastern boundary of the Blond rare metal leucogranite, is contained in a set of subvertical quartz veins, locally with muscovite and minor quartz selvages. The sequence of deposition was: (1) milky quartz, predominantly as fracture filling, generally affected by subsequent ductile deformation; (2) hyaline quartz-wolframite cassiterite; (3) minor sulphides. Other sets of quartz veinlets, although generally barren are observed in the Blond massif. Fluid migration at the microscopic scale within the granite and in the vicinity of quartz fractures was constrained by studying the geometry of fluid-inclusion planes and fluid-inclusion chemistry in and outside the mineralised area. Three major sets of subvertical fluid-inclusion planes are recognised: A N050°-060°E set, mostly developed in the veins and in the immediate vicinity, a N1 10°-130°E set, regionally developed in the granite and a N140-160°E set of local extent. As a whole, the density of FIP decreases from the mineralised zones toward the barren part of the pluton, except for the N 140°-160°E set. These are locally abundant around quartz veinlets with similar orientations that form a broad "N-S" band near the Blond locality. Mineralising fluids observed as primary inclusions in cassiterite and in undeformed hyaline quartz are mostly aqueous, with moderate salinity and a minor volatile component, at variance with many other W-(Sn) deposits in the Variscan belt. Ore deposition occurred around 315°C, at an estimated depth of 5.5 km, under hydrostatic to slightly suprahydrostatic pressures. It resulted from fluid mixing, in the central part of a large hydrothermal system, between two end-members: (i) a hot (425-430°C) moderately saline fluid, that contained a diluted volatile component and, although Na-dominated, minor amounts of Li and Ca. The estimated δ¹⁸O indicates that this fluid was completely equilibrated with the tectono-magmatic pile (pseudo-metamorphic fluid). (ii) a "cold" (230°C) low-salinity fluid (evolved meteoric water), that mixed with, and eventually overprinted, the early moderately saline fluid responsible for granite muscovitization at 425-430°C. Later, a second hydrothermal system was initiated by the percolation of heated meteoric water, with very low salinity. The system was by then at least 3.5 km deep and the fluid was heated to 300°C. These characteristics are reminiscent of the ca. 305 Ma episyenitic hydrothermal system known elsewhere in the N Limousin.