Variscan Sb-Au mineralization in Central Brittany (France): A new metallogenic model derived from the Le Semnon district

Numerous Sb-Au deposits are known in the European Variscan belt and are often associated with the late Variscan hydrothermal events linked to postorogenic extension and strike-slip faults, through the whole belt. The central part of the Armorican Massif (Central Brittany) hosts several Sb-Au deposits, such as the Le Semnon deposit. However, little is known about its formation, its structural control or hydrothermal history. In order to provide new constraints on the Sb-Au mineralization framework of the Variscan Armorican Massif, structural, geochronological and chemical analyses have been performed on the Le Semnon Sb-Au deposit. First, we highlight that Sb-Au mineralization consists of fault-fill and flat extensional vein systems resulting from smallscale fault-valve mechanism controlled by fluid overpressure at shallow depth. Indeed, this deposit is characterized by the presence of dolerite dykes which acted as the plumbing system for fluids within impermeable slates. The low permeability of the slates permitted the establishment of local supralithostatic fluid pressure in a predominantly hydrostatic regime. Coupled with new (micro-) textural evidences, a reappraisal of the paragenetic evolution and new physico-chemical data, 40Ar/39Ar and U-Pb analyses show that the Le Semnon Sb-Au mineralization was emplaced around 360 Ma and therefore indicates an early hydrothermal event in the Variscan history of the Armorican Massif. Moreover, absolute dating of mafic magmatism coupled with hydrothermal alteration associated with the deposition of Sb-Au mineralization strongly suggest that the emplacement of the Sb-Au mineralization was coeval with the emplacement of the host rock doleritic dyke. Furthermore, the high temperature recorded in fluid inclusions and the paragenetic evolution suggests an elevated thermal anomaly in Central Brittany. Thus, we propose that the emplacement of the widespread mafic magmatism at ca 360 Ma played a major role in the mobilization of fluid flows and the redistribution of metals in the Armorican crust.