Quantitative mineralogy and lithium distribution in the upper part of the Beauvoir granite

Abstract

The albite-lepidolite-topaz granite of Beauvoir is the last intrusion in a peraluminous granitic complex of Variscan age composed of three successively emplaced units: the hidden granite of La Bosse, the two-micas granite of Colettes, and the Beauvoir granite consisting of 3 units B1 to B3. Compared to similar Li-F-rich igneous bodies, the Beauvoir granite is highly enriched in Sn (200-1400 ppm), Ta (20-400 ppm) and Be (20-300 ppm). The B1 unit is composed of abundant albite and lepidolite laths forming a framework filled with globular quartz, K-feldspar, and rare crystals of topaz, Li-phosphates and Sn-Nb-Ta oxides. This work presents a quantitative mineralogical study of the Beauvoir granite (B1 unit) and detailed textural and chemical characterization of the Li-bearing micas from the cores from the PER and EMILI drilling campaigns. The first approcah of this work consisted in estimating the proportion of minerals, and thus determine their abundance by comparing estimates from in situ thin section mapping from micro-XRF and the calculation of mineral phase proportion from whole rock geochemistry on drill cores. The main trend is a "mix", for the fresh rocks, between an albite pole (> 35 %), and a quartz (~25 %) - lepidolite (~15-25 %) pole. The micas are represented, in the fresh facies, by trioctahedral lepidolite while the altered facies shows i) dioctahedral muscovite replacing the feldspars and lepidolite, and ii) lepidolite replaced partially or totallly by muscovite. Trace element composition of the lepidolite show high Li content with average 28000 ppm for lepidolite from the fresh facies and 24000 ppm for those from the altered facies. Compared to lepidolite, muscovite is not a Li-bearing but Sn-rich mineral. Our study shows that hydrothermal greisen alteration has almost no impact on the lithium content of the lepidolite but decrease the population of the lepidolite. The results obtained, combining to drilling data, will lead to a mineralogical model block of the Beauvoir quarry and better understand the magmatic-hydrothermal evolution of the deposit.