Nb-Ti-Y-HREE-W-U OXIDE MINERALS WITH UNCOMMON COMPOSITIONS ASSOCIATED WITH THE TUNGSTEN MINERALIZATION IN THE PUY-LES-VIGNES DEPOSIT (MASSIF CENTRAL, FRANCE): EVIDENCE FOR RARE-METAL MOBILIZATION BY LATE HYDROTHERMAL FLUIDS WITH A PERALKALINE SIGNATURE

摘要

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 FM 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 (L-1) enriched in Ti > Nb, Fe, and W, with the same geochemical signature as the main mineralization, and a second fluid (L-2) enriched in Nb > Ti, Fe, Y, HREE, 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.