The Montbelleux intrusion-related Sn-W ore deposit: a witness of an underestimated Cambrian metallogenic event in the Armorican massif (France)?
Résumé
Despite significant outcropping surfaces and favourable geology, pre-Variscan ore deposits remain scarce in France. This is particularly true in the north of the Armorican massif where the Neoproterozoic Cadomian orogeny is well preserved. There, the largest Cadomian terrane, the Fougères unit, is composed of turbiditic sediments affected by a low-grade metamorphism and intruded by granitoids. These metasediments are strongly folded presenting a steeply axial-plane cleavage and both bedding and cleavage strike N40°E. This deformation event is constrained at ca. 560-540 Ma (Ballèvre et al., 2001). Post-dating the folding, granodioritic plutons intrude sediments generating large contact metamorphism aureoles. Intrusions consists of large peraluminous Crd-Bt-granodiorite plutons emplaced at ca 540 Ma as well as scarce and small younger Bt-Ms-leucogranites. Finally, the Fougères unit is affect by a Cambrian extensional event marked by a local NE-SW-trending rifting, associated ash-flow Caldeiras and felsic volcanism (Le Gall, 1995). Close to the town of Fougères, several small Crd-Bt-granites and Bt-Ms-granites intrude the Fougères unit. Numerous occurrences of wolframite and cassiterite are spatially associated with these granites but only one W-Sn ore deposit has been found. This is the Montbelleux W-Sn ore deposit, constituted by at least 2 small and elongated Bt-Ms-granitic sills extended by 5 main quartz lodes (Chauris et al., 1989). The mineralisation, 17000 t Sn+WO3 of measured resources, is hosted by a quartz veins stockwork within the granite and quartz lodes within metasediments. Zircons from the Montbelleux sills have been dated by LA-ICP-MS and provide an age of 538.2 ± 6.1 Ma. Hydrothermal apatite associated with the ore stage, provide an U/Pb age of 524 ± 13 Ma, undistinguishable within error from the age of the sill intrusions. Whole-rock geochemical results show that the Montbelleux deposit belong to ore-bearing peraluminous granites (Nb/Ta).