Tracing metallic pre-concentrations in the Limousin ophiolite- derived rocks and Variscan granites (French Massif Central)

International audienceThe French Massif Central (FMC) is part of the West European Variscan belt. The economic and sub-economic ore deposits from the Limousin area are the consequence of intensive fluid-rock interactions related to a lithospheric delamination process that occurred during the Neo-Variscan Period (320-290 Ma). This study focuses on the fingerprints of fluid-rock interactions in the Limousin ophiolite-derived rocks and granitic massifs, in order to trace the source of trace elements transported by fluids involved in the formation of the Variscan ore deposits (including Sn, W, Au and U deposits). A series of compatible and incompatible metallic trace elements (Ag, As, Au, Bi, Li, Co, Cr, Cu, Ga, Ge, Ni, Pb, Sb, Sc, Sn, Ti, V, W) were analysed in a series of basic (metagabbros, basic dykes), ultrabasic (serpentinites) and felsic (granites and gneisses) rocks, as well as in the mineral assemblages, in order to determine their relationships with the Limousin ore deposits. Serpentinites and their mineral assemblages (serpentine, olivine, amphibole, chlorite and spinel) show high concentrations in compatible transition metals (Sc, V, Cr, Co, Ni, Zn, Cu) and low abundances of incompatible metals (Ge, Ga, Mo, Sn, Sb, W). Associated amphibolites (metagabbros and metamorphosed dykes) show high contents of compatible metals, though lower than ultrabasic rocks. Amphibole and, to a lesser extent, titanite, are the main hosts for metallic trace elements. Abundances in incompatible metals are generally low. Amphibole has low Sn and Sb contents, while higher Sn contents were measured in titanite. In a UHP zoisite-eclogite cropping out close to one of the ophiolite-derived massifs, zoisite, rutile, omphacite and retrograde amphibole show high concentrations of incompatible elements (Sn, Sb, W, As, Ga, Ge, Mo and Bi). However, it has been shown that high-temperature and low-temperature hydrothermal alteration that have affected the basic and ultrabasic protoliths is c. 100 Ma older than those that were responsible for the formation of ore deposits. For this reason and as well because of thei