The Beauvoir topaz-lepidolite albite granite (Massif Central, France); the disseminated magmatic Sn-Li-Ta-Nb-Be mineralization

Abstract

The Beauvoir topaz lepidolite albite granite (French Massif Central) is the latest intrusion in a Variscan peraluminous granitic complex composed of three units successively emplaced: the concealed La Bosse granite, the Colettes two-mica granite, and the Beauvoir granite. The 900-m-deep drill hole of the Geologie Profonde de la France deep drilling program of the continental crust has provided a continuously cored section of the Beauvoir granite. The exposed section of the Beauvoir granite is presently mined for kaolin in the upper 200 m. It also represents a huge subeconomic disseminated Li, Sn, Ta, Be deposit sharing many characteristics with rare metal-bearing pegmatites. Compared to similar Li-F-rich igneous bodies, the Beauvoir granite is strongly enriched in Sn (200-1,400 ppm), Ta (20-400 ppm), and Be (20-300 ppm). Li is located in lepidolite and amblygonite, Ta and Nb mainly in columbo-tantalite and uranium-rich microlite, and Sn in cassiterite. Be is located in lepidolite crystals and in unidentified mineral(s). The major element composition of the Beauvoir granite is similar to that of Macusani volcanic glasses of Peru and ongonite subvolcanic rocks of Mongolia. The main structural, textural, mineralogical, and geochemical characteristics of the Beauvoir granite can be explained by extreme fractionation of F- and Li-rich magmas and associated fluid phases. Most trace elements show a strong upward enrichment. The high f (sub O 2 ) , low solidus temperature, and high fluorine content of the Beauvoir magma have considerably reduced Sn and W fractionation in the expelled magmatic fluid despite its high chlorine content (25-30 wt % NaCl equiv). However, F- and Li-enriched geochemical halos have developed in the enclosing mica schist in response to the boiling of the magmatic fluid. The relatively deep emplacement of the granite (3 km) has limited the development of hydraulic fracturing and subsequent hydrothermal circulation and mineralization. Thus, despite the very high specialization of the Beauvoir granite, no significant vein-type mineralization is directly related to it. The La Bosse quartz-ferberite stockwork was developed before the emplacement of the Colettes and Beauvoir granites and is related to the earlier concealed La Bosse granite.