MICROLITE AND TANTALITE IN THE LCT GRANITIC PEGMATITES OF LA CANALITA,NAVASFRIAS Sn-W DISTRICT, SALAMANCA, SPAIN

摘要

La Canalita pegmatites in the Navasfrias Sn–W district, in Spain, are a group of Li-rich LCT rare-element pegmatites hosted by metamorphic rocks. It consists of several dikes up to 1 m thick, 16 m in length and of unknown depth that were mined during the 40s and 50s for tin. Here, we document the Ta-rich oxide minerals, such as those of the columbite–tantalite group, tapiolite-(Fe) and microlite. It is possible to distinguish a primary assemblage, formed of extremely Mn-rich columbite and tantalite, and microlite, the latter forming the host of columbite-(Mn) exsolution lamellae in the intermediate parts of the dikes. The influence of metasomatic fluids led to the formation of a lepidolite–albite late unit and the crystallization of a secondary assemblage consisting of tantalite-(Fe), tantalite-(Mn) and tapiolite-(Fe). Uranium-rich microlite (up to 6% wt. UO_2) is present in this assemblage, replacing tantalite. Subsequently, the entrance of hydrothermal fluids, probably mixed with metamorphic fluids, caused a partial greisen-like alteration of the pegmatite bodies and formed a second generation of microlite replacing tapiolite, with high Ca and Ta and low F contents. The lack of a chemically suitable protolith in the surrounding metamorphic rocks, combined with the mineralogical paragenesis containing U-rich microlite, together with the composition of the Sn, Nb and Ta oxide minerals of the pegmatite dikes, suggest that the Cadalso – Casillas de Flores batholith is the most probable source of the melts forming the La Canalita pegmatite dikes.