Tectonic evolution of the Cevennes para-autochthonous domain of the Hercynian French Massif Central and its bearing on ore deposits formation
Résumé
The Cevennes area belongs to the para-autochthonous domain of the Hercynian Belt of the French Massif Central. Three lithological series, namely: sandstone-pelite, black micaschist and gneiss-micaschist, are identified. They form an imbrication of five tectonic units which overthrust the unmetamorphosed Viganais Paleozoic units to the south and the gneissic Mamejean Unit to the north. The structural, metamorphic and magmatic evolution of the Cevennes area is characterized by three events, namely: (1) southward shearing coeval to a MP/MT metamorphism dated around 340 Ma; (2) post nappe anatexis (T<750 degrees C, P>5 kb); (3) Namurian (ca 315 Ma) E-W extensional tectonics and plutonism. The structure of the Mt-Lozere-Borne granitic complex is constrained by new AMS and gravimetric data. The plutons are the driving power of the hydrothermal convective circulations responsible for an early deposition of diffuse arsenopyrite in the thermal aureole. Gold bearing sulfides are afterwards concentrated in quartz veins along brittle normal and wrench faults around the granite. Lastly, ore bearing quartz pebbles are sedimented in the Stephanian Ales coal basin.