GEOCHEMICAL EVIDENCE FOR A MULTISTAGE MAGMATIC GENESIS OF TA-SN-LI MINERALIZATION IN THE GRANITE AT BEAUVOIR, FRENCH MASSIF-CENTRAL

The Beauvoir rare metal granite, at the southern edge of the Echassieres 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 phosphorus-rich type of Ta-bearing granites, and geochemical and mineralogical data establish its similarity to pegmatite melts. The Beauvoir granite results built up from two distinct intrusions. Both intrusions are cogenetic and were emplaced within a short time 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 concentration of 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 of the ore genesis are provided by a careful observation of data; interpretations 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 granitic melt by partial melting; it is not known whether the melt was already enriched in Ta, but this is not a prerequisite. Subsequently, deep-seated fractional crystallization produced granitic magma similar in composition (Ta approximate to 9 ppm) to the adjacent Colettes granite, and further fractionation processes led to a melt already strongly enriched in Ta (ca. 40 ppm), strongly peraluminous, and enriched in the depolymerizing elements F and P. The viscosity was therefore very low, allowing easy ascent. During transport, small-scale fluctuations in the viscosity were amplified by upward movement, leading to an F- and P-rich melt preceding the magma batch. This less viscous melt again underwent crystallization while moving through the upper crust, producing an extremely F- and Ta-enriched melt (Ta approximate to 140 ppm). This latter melt became fluid rich by interaction with meteoric water present in the surrounding rocks at shallow levels (Fouillac and Rossi, 1991). The whole magma batch then reached its present location, with the less viscous melts at the top. Crystallization of the most evolved parts occurred in the presence of unmixed water, and fluids allowed a plume of bubble-laden melt to rise farther (Ta approximate to 200 ppm). However, water distribution resulting from the interaction with meteoric fluids was probably heterogeneous, allowing further enrichment in some parts of the melts by crystallization (up to Ta approximate to 300 ppm) before an F-rich brine unmixed and allowed the enriched melt to reach the top of the cupola. The Ta-Sn-Li ore consists of the three latest stages of evolution, but all previous processes contributed to the ore formation. Melt structure is the main control on the ore formation. In this respect, the abundance of F and P and a strongly peraluminous character allow very high fractionation grades, which are necessary for the appearance of a Ta ore. With the exception of a very late unmixing of fluids which allowed some Ta-rich melts to cluster at the top of the cupola (but did not lead to Ta enrichment), the ore-forming processes at Beauvoir were magmatic.