Metallogenesis of the French part of the Variscan orogen. Part II: Time-space relationships between U, Au and SnW ore deposition and geodynamic events — mineralogical and UPb data

Abstract

Mineralogical, geochemical, structural and isotopic (UPb) studies were carried out on representative uranium and gold deposits from the French part of the Variscan orogen. Results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the Variscan terranes, with a particular emphasis on U deposits. The data show that the early metal concentrations or pre-enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition. Thus, the significant geodynamic events which affected the Variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits. UPb geochronological results complemented by a compilation of geochronological data on U and PbZnBaF deposits show that three major stages of U deposition took place in Western Europe in the periods 325–300 Ma, 290–260 Ma and 190–170 Ma. In addition, small uranium mineralization events have been dated at 425 Ma, 90–110 Ma and as Tertiary (about 40 Ma). The Permo-Stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of U concentration. The Jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in FBa (PbZn) deposition throughout Western Europe, especially in the eastern part of the Massif Central. Other stages correspond to subordinate uranium migration within Variscan terranes during earlier or later geodynamic phases: the Silurian metamorphism in Vendée and the Mesozoic to Tertiary tectonic movements related to the Pyrenean or Alpine orogenies in the south eastern part of the Massif Central and in the Alps. Data available on Au and SnW (As, Sb) deposits have been considered for comparison with the U metallogenesis. Stages related to the deposition of such metals display significant similarities with those related to uranium deposition. They occurred mostly during Visean to Permian events related to the intrusion of the late Visean (SnW) and Late Carboniferous (SnW, LiF) granitoids and to the late brittle deformation stages which affected the Hercynian basement in the vicinity of major shear zones (Au, AsSb). However, the significant discrepancies between the physical-chemical conditions of solubilization, transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes.