Variscan Sb-Au mineralization in Central Brittany (France): A new metallogenic model derived from the Le Semnon district
Résumé
Numerous Sb-Au deposits are known in the European Variscan belt and are often associated with the lateVariscan hydrothermal events linked to postorogenic extension and strike-slip faults, through the whole belt. Thecentral part of the Armorican Massif (Central Brittany) hosts several Sb-Au deposits, such as the Le Semnondeposit. However, little is known about its formation, its structural control or hydrothermal history. In order toprovide 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, wehighlight that Sb-Au mineralization consists of fault-fill and flat extensional vein systems resulting from smallscalefault-valve mechanism controlled by fluid overpressure at shallow depth. Indeed, this deposit is characterizedby the presence of dolerite dykes which acted as the plumbing system for fluids within impermeableslates. The low permeability of the slates permitted the establishment of local supralithostatic fluid pressure in apredominantly hydrostatic regime. Coupled with new (micro-) textural evidences, a reappraisal of the parageneticevolution and new physico-chemical data, 40Ar/39Ar and U-Pb analyses show that the Le Semnon Sb-Aumineralization was emplaced around 360 Ma and therefore indicates an early hydrothermal event in theVariscan history of the Armorican Massif. Moreover, absolute dating of mafic magmatism coupled with hydrothermalalteration associated with the deposition of Sb-Au mineralization strongly suggest that the emplacementof 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 elevatedthermal anomaly in Central Brittany. Thus, we propose that the emplacement of the widespread maficmagmatism at ca. 360 Ma played a major role in the mobilization of fluid flows and the redistribution of metalsin the Armorican crust.
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