Textural, Chemical, and Mineralogical Characterization of the LCT Bermellar Pegmatite (Salamanca, NW Spain): Genetic Insights

The Sn-Ta-Nb-Li-Be Bermellar Pegmatite belongs to an aplopegmatite field within the Cerralbo-Bermellar area (Salamanca, Spain). Despite its historical small-scale mining past, this area has received little scientific attention. The Bermellar pegmatite is hosted by metasedimentary rocks of the Schist–Metagreywacke Complex in the Central Iberian Zone (CIZ), where many other pegmatite bodies have been interpreted to derive from late Variscan magmatism. Structural relationships indicate a syn-D2 Variscan emplacement, likely associated with the granitoids of the Tormes Dome. Comparisons with other Li-rich aplite–pegmatite dykes in the CIZ, reveals affinities with the Fregeneda–Almendra pegmatite field. Textural, mineralogical and chemical analyses (SEM-EDS and EPMA) have defined an internal zoning of the pegmatite dykes that reflects primary magmatic fractionation. The highly evolved mineral paragenesis of the Bermellar pegmatite (i.e., Li-rich micas, elbaite–rossmanite, montebrasite, and Nb–Ta–Sn oxides), suggests that pegmatite-forming melts would have been significantly enriched in fluxing components, such as P, F, B, and H2O. Textural features such as rhythmic aplite-pegmatite layering parallel to the contacts, graphic intergrowths, unidirectional solidification textures (UST), and internal structure suggest undercooling conditions linked to fluid exsolution (e.g., hydrosaline liquid: HSL) during the magmatic-hydrothermal transitional stage, which promoted Na-metasomatism and significant replacement and overprint textures over the primary mineral paragenesis. Finally, a hydrothermal stage is also evident in local greisenization and kaolinization. All these features support classification of the Bermellar pegmatite within the LCT (Li-Cs-Ta) family and the rare element, Li subclass.