The tungsten-gold veins of Bonnac (French Massif central): new constraints for a Variscan granite-related genesis

In the Brioude-Massiac district (French Massif Central), a network of W-As-Bi-Au quartz veins constitutes the Bonnac deposit, where tungsten is the major economic element, together with high-grade gold (up to 15 g/t Au). The evolution of this mineralization has been divided into 3 stages: i) an early deep-seated wolframite-lollingite stage formed between 12 to 9 km, at up to 400°C, ii) a ductile/brittle deformation stage associated with scheelite and arsenopyrite deposition, with an estimated temperature of 430-380 °C; iii) a late stage controlled by fluid-overpressure at a depth of 7 to 5 Km and a temperature between 266 to 240 °C, marked by micro-fracturing, infilled by native bismuth, bismuthinite, hedleyite, electrum, pyrite and base-metals. Structural analysis, and apatite LA-ICP-MS U/Pb dating, demonstrate a spatial and temporal link between the emplacement of the peraluminous leucogranitic dykes and the Bonnac mineralization. In more details, the mineralization was deposited between 321-316 Ma, during or just after the emplacement of the peraluminous dykes emplaced around 329-315 Ma, suggesting a magmatic-hydrothermal transition for the ore-forming process. In the proposed model, the cooling of a hidden two-mica granitic pluton could have generated a magmatic fluid, and acted as the heat source responsible for fluid flow towards inherited permeability zones. The magmatic fluid was then re-equilibrated at high temperature by fluid-rocks interaction. The sharp changes in temperature, pressure, and sulfide-fugacity generated by a late input of meteoric fluid were responsible for the deposition of the late gold-stage. At the regional scale, the tungsten-gold event is ascribed to an early hydrothermal stage, dissociated from the formation of the antimony event in the district. The leucogranitic dykes and Bonnac quartz veins are controlled by a NW-SE stretching direction, interpreted as an expression of the Serpukhovian-Bashkirian syn-orogenic extension (D4 event of the French Massif Central). These new data provide evidence for an early tungsten and gold metallogenic event in the FMC, prior the “Or300” event. The genetic classification of the Bonnac mineralization is equivocal. The W-As-Bi-Au-quartz veins exhibit the features of both an “orogenic gold” deposit at a relatively deep emplacement level, and an Intrusion-Related-Gold-Deposit (IRGD) type with a spatial-temporal link with the peraluminous intrusion emplacement. We propose that the Bonnac deposits represent an intermediate type between a typical orogenic-gold deposit and an IRGD. We argue that the presence of this type of mineralization, and more generally the IRGD deposits, have been underestimated in the Variscan French Massif Central.