Geochemical evidence for a multistage magmatic genesis of Ta-Sn-Li mineralization in the granite at Beauvoir, French Massif Central 论文

摘要

The Beauvoir rare metal granite, at the southern edge of the Echassi•res granite massif, French Massif Central, is mineralized by disseminated Ta, Sn, and Li. The granite body formed from an independent melt which was emplaced soon after the main granite facies of the massif. The geochemistry of the Beauvoir granite relates it to the phosphorns-rich type of Ta-bearing ranites, and geochemical and mineralogical data establish its similarity to pegmatite melts. The Beauvoir granite results built up from two distinct intrnsions. Both intrnsions are cogenetic and were emplaced within a short ime interval. A geochemical study of a deep drill hole bored down to 900 m under the surface to the floor of the granite sheet, with selected key zones observed in greater detail, has allowed the recreation step by step of a long process leading to the economic oncentration f Ta, Sn, and Li. The comparison of major element, trace element, and isotopic geochemistry with the petrology makes it possible to unravel the numerous processes contributing to the ore formation. Local events need to be determined before main events can be identified. Constraints on the interpretation ofthe ore genesis are provided by a careful observation f data; interpreta-tions of these observations are strongly backed up by experimental data (e.g., Keppler, 1993). The source rock was most likely enriched in Sn and W at about 10 ppm, producing a slightly enriched