Ore deposits of the French Massif Central: insight into the metallogenesisof the Variscan collision belt

The French Massif Central (FMC) represents the whole West European Variscan (WEV) belt, in terms of both the geodynamic evolution and the metallic con tent. Thus, a study of the metallogenic evolution of the FMC may elucidate the conditions that allow the mineralisation of a collision belt, since rec ent collision belts, e.g. the Himalayas or the Alps show that mineralisatio n does not necessarily result from the collision process. The Palaeozoic hi story of the FMC is divided into three geodynamic stages unevenly involved from the metallogenic view point. The Eo-Variscan stage (Cambrian to Siluri an) was not important; the Meso-Variscan stage (Devonian-Early Carboniferou s) was of limited importance; and most of the mineralisations formed during the Neo-Variscan stage (Late Carboniferous-Early Permian). In addition, so me more mineralisation was produced during the Mesozoic because of the ther mal reactivation linked with the Alpine orogenies. The Eo-Variscan stage (Cambrian-Silurian) corresponded to the pre-collision history, marked at the WEV belt scale by a fragmentation of the northern G ondwana (immature crust evolved from the Late Proterozoic Cadomian orogeny) , up to the break-up of the crust and the formation of oceanic basins (Camb rian-Ordovician), followed by their resorption by subduction during the Sil urian. In the FMC, no subduction-related magmatism is known (being rare at the WEV belt scale), and consequently subduction-related mineralisation, e. g. porphyry copper, is unknown in the WEV belt. Although some ophiolitic re mnants are known, they never display Cyprus-type VMS deposits, nor massive podiform chromitites. Beside platformal sedimentary deposits on passive mar gins, the only deposits formed during the Eo-Variscan stage were of the SED EX type, linked with the early rifting of the Gondwanian crust. The Meso-Variscan stage (Devonian-Early Carboniferous) corresponded to the collision groper, with the formation of crustal-scale nappe structures and the intrusion of collision-related peraluminous granites. Although these gr anites were enriched in rare metals they did not yield significant hydrothe rmal mineralisation, due to the great depth of their emplacement, as the si milar granites in the Himalayas. However, they were a source of rare metals (in particular, uranium) for later mineralisation events. At the WEV belt scale Devonian distensive events are coeval with the collision. They were r ecorded by the formation of sedimentary basins of limited time and space ex tent, corresponding to the splitting of the continental crust (up to format ion of oceanic domains in many cases), and were characterised by a bi-modal ("spilite-keratophyre") volcanism. These basins formed in transtensional ( or pull-apart) settings along major strike-slip faults, a peculiarity of th e Variscan collision belt (which may conveniently be described as a "strike -slip orogen"). In such basins, many deposits linked with the volcanic ther mal energy were formed: SEDEX deposits of the Meggen-type, iron deposits of the Lahn-Dill-type and VMS base metal deposits, the latter being the only ones known in the FMC (Brevenne deposits). The Neo-Variscan stage corresponded to the ''hypercollision" and was charac terised by a shift from compressional tectonics (late thickening of the cru st during the Sudetian event and long-lasting dextral strike-slip tectonics along NW-SE to NE-SW fault zones) towards extensional tectonics ("basin an d range" of the Late Stephanian-Early Permian), as well as by high heat flo ws, recorded by LP-HT metamorphism, extensive granitisation and granulitisa tion of the lower crust. These characteristics record the development of a lithospheric delamination process. In response to the energetic input relea sed by this process, numerous hy drothermal deposits were formed in the FMC , as well as in the whole WEV belt, during the Neo-Variscan stage. These ar e mainly: (1) high-temperature granite-centered tungsten deposits, mainly a ssociated with cordierite-bearing high level intrusions of Namurian-Westpha lian age; (2) rare metal granites land the associated hydrothermal tin mine ralisations), resulting from fluid-induced low-degree partial melting of th e middle crust in relation with the devolatilisation of the granulitised lo wer crust; (3) shear-zone hosted gold and antimony deposits, related to cru stal-scale hydrothermal circulation, triggered by the transition to extensi onal tectonics at about 300 Ma; and (4) uranium deposition in extensional s ettings related to the Early Permian distension. The Post-Variscan mineralising events recorded the renewal of thermal flows in the lithosphere linked with early Alpine events (mainly the Trias-Lias distension in the Tethyan realm and the middle Cretaceous opening of the Ba y of Biscay in the Pyrenean realm). They resulted in low-enthalpy geotherma l systems, leading to a variety of deposits, mainly: (1) F-Ba districts, re working F and Ba from Late Variscan granites and ignimbrites; (2) a major u ranium deposit (Lodeve), reworking uranium from the Permian Lodeve basin; a nd (3) Zn-Pb districts of the MVT-type. Finally, the mineralisation of the Variscan collision belt is mainly the co nsequence of the Neo-Variscan lithospheric delamination process. By contras t, the absence of such a process in collision belts like the Himalayas or t he Alps is the key of them being devoid of mineralisation. It appears that the mechanical energy released by the collision itself is not sufficient to mobilise and concentrate the trace elements involved in the metallogenic p rocesses.