The Puy-les-Vignes deposit, located in the northwestern part of the French Massif Central, is a remarkable occurrence in the West European Variscan belt of hydrothermal tungsten mineralization associated with a breccia pipe structure. The late stage of the mineralization consists of a mineral paragenesis composed of zircon, xenotime, monazite, Nb-Fe-W rutile, and Nb-Ti-Y-HREE-W-U oxide minerals (hereafter referred to as NTox) within an adularia-tourmaline-chlorite matrix. This study is focused on these rare-metal oxides, which display complex internal textures and uncommon chemical compositions with variable concentrations of Nb, Ti, Y, HREE, and W, not described until now. They are characterized by low microprobe totals (76 to 95%), together with the presence of OH− groups within the crystallographic structure as detected by FTIR spectroscopy, which is interpreted as the result of alteration, such as hydration and/or metamictization. The crystallochemical study shows that these crystals appear as a complex multi-polar solid solution, involving chemical mixing between two groups of binary solid solutions: a first group of anatase-columbite solid solution and a second group of euxenite-(Y)-columbite solid solution. Interpretation of their internal texture and their chemistry suggest that the NTox were formed during multi-phase crystallization in an open system by the mixing of two different hydrothermal fluids: a first fluid (L1) enriched in Ti>Nb, Fe, and W, with the same geochemical signature as the main mineralization, and a second fluid (L2) enriched in Nb>Ti, Fe, Y, REE, and W, with a geochemical signature clearly contrasting with the former and coeval with the crystallization of adularia, xenotime, monazite, zircon, and rutile. This mineral paragenesis is characterized by a P, Y, HREE, Nb > Ta, Ti, Zr, and U geochemical signature, typical of rare-metal peralkaline magmatism, thus suggesting rare-metal mobilization by late hydrothermal fluids with a peralkaline signature, likely derived from an unknown source at depth ( e.g. , NYF pegmatites or related granite), during the late metallogenic stages at Puy-les-Vignes.
Multidisciplinary three-dimensional modelling, involving geophysical, structural and geochemical data, has been used to study the relationships between magmatism, tectonics, fluid circulation and mineralisation in the northern Limousin, and to provide P-T-Z-t paths constrained by the available dating. The ore deposit occurrence displays little spatial relationship with granites emplaced in the 360-320 Ma period, probably because the low global permeability and tectonic regime did not allowed vertical fluid exchanges to be established. In contrast, the change in the tectonic regime induced by the delamination of the lower lithosphere (320-300 Ma), and characterised by the passage to general extension, has played a major metallogenic role. However, the ore deposit processes appear to be specific to each metal. Most of the W-Sn deposits appear to be synchronous with rare metal granites emplacement, at c. 310 Ma, that allowed the focus of fluids of different origins towards the apex of plutons. In contrast, for Au and U, the whole mineralisation process covers several tens of-millions of years. It is controlled by the regional tectonic evolution of the Limousin area during the same period, and especially by a rapid exhumation of the ductile crust which occurred in the 310-300 Ma period.
Leaching of uranium-fertile granites represents a major source of uranium, as uraninite is easily dissolved in oxygenated aqueous solutions. This phenomenon is well documented at surface conditions, but remains poorly documented for granites at depth. In this study, we propose that surface-derived oxidized hydrothermal fluids leached uranium from uraninite in the Questembert peraluminous granite at temperatures greater than ~70° to 160°C. This Variscan synkinematic granite is characterized by widespread and pervasive development of vertical and permeable C-S structures. These structures likely facilitated the infiltration of oxidized hydrothermal fluids from the surface, their circulation at depth, and the subsequent fluid-rock interaction in the granite. Published oxygen isotope data shows that it has undergone subsolidus fluid-rock interaction, dated between 312 and 303 Ma by 40Ar/39Ar analyses on muscovite. These interactions were responsible for the concomitant decrease of the feldspar δ18O values together with uranium leaching. Mass-balance calculations suggest that this hydrothermal event could have liberated several hundred thousand tonnes of uranium from the Questembert granite. The liberated uranium may have been dispersed in Early Permian intramountainous basins and therefore disseminated over large areas. This study emphasizes that the efficiency of uranium leaching may be directly related to the extent of subvertical structure development in granites emplaced along strike-slip shear zones, which allow for downward infiltration of oxidized surface-derived fluids. A specific and systematic sampling is required to better constrain the proposed model.
The main aim of the contribution is to combine data on the consecutive stages of deformation, inflow and migration of palaeofluids and accumulation of mineral filling with uranium traces within the faulted-fractured environment at the Krasnokamensk Area, SE Transbaikalia, Russia. Object of examination is a framework of fault zones transecting the Proterozoic-Paleozoic granitic unit to the extent of northwestern part of uranium-bearing Streltsovskaya caldera of Mesozoic age. Considerations of stress- and permeability-time relationships in faulted-fractured zones were taken with account of stress and strain dependencies within fluid saturated rock massifs at crustal seismogenic level. Stress-time consecution of fault zone permeability was developed using set of fieldwork and lab tests including structural-geological survey, fault slip data analysis, mineral-chemical diagnostics, microstructural observations, and radiographic studies. Practical applications of obtained data for solving uranium mining and environmental issues are indicated in conclusion.
