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

The Vaulry W-(Sn) mineralisation, located at the eastern boundary of the Bl ond rare metal leucogranite, is contained in a set of subvertical quartz ve ins, locally with muscovite and minor quartz selvages. The sequence of depo sition was: (1) milky quartz, predominantly as fracture filling, generally affected by subsequent ductile deformation; (2) hyaline quartz-wolframite-c assiterite; (3) minor sulphides. Other sets of quartz veinlets, although ge nerally barren are observed in the Blond massif. Fluid migration at the mic roscopic scale within the granite and in the vicinity of quartz fractures w as 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 degrees -060 d egreesE set, mostly developed in the veins and in the immediate vicinity, a N110 degrees -130 degreesE set, regionally developed in the granite and a N140-160 degreesE set of local extent. As a whole, the density of FIP decre ases from the mineralised zones toward the barren part of the pluton, excep t for the N140 degrees -160 degreesE set. These are locally abundant around quartz veinlets with similar orientations that form a broad "N-S" band nea r the Blond locality. Mineralising fluids observed as primary inclusions in cassiterite and in un deformed hyaline quartz are mostly aqueous, with moderate salinity and a mi nor volatile component, at variance with many other W-(Sn) deposits in the Variscan belt. Ore deposition occurred around 315 degreesC, 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 degreesC) moderately s aline fluid, that contained a diluted volatile component and, although Na-d ominated, minor amounts of Li and Ca. The estimated delta O-18 indicates th at this fluid was completely equilibrated with the tectono-magmatic pile (p seudo-metamorphic fluid). (ii) a "cold" (230 degreesC) low-salinity fluid ( evolved meteoric water), that mixed with, and eventually overprinted, the e arly moderately saline fluid responsible for granite muscovitization at 425 -430 degreesC. Later, a second hydrothermal system was initiated by the percolation of hea ted meteoric water, with very low salinity. The system was by then at least 3.5 km deep and the fluid was heated to 300 degreesC. These characteristic s are reminiscent of the ca. 305 Ma episyenitic hydrothermal system known e lsewhere in the N Limousin. (C) 2001 Elsevier Science B.V. All rights reser ved.