Deciphering geochemical variations driven by metasomatism around a lithium pegmatite at the Gonçalo deposit, Portugal: implications for rare-metal exploration in granitic terranes of the European Variscan Belt

Geochemical halos surrounding Li-rich pegmatites represent a potentially effective vectoring tool for the exploration of concealed deposits. To determine the petrochemical processes triggered by fluids derived from evolved pegmatitic melts in granitic host rocks, this work investigates the geochemical anomalies around a lepidolite-rich pegmatite at Gonçalo (Portugal). The study combines EMPA and LA-ICP-MS analyses of rock-forming minerals from the pegmatite and host monzogranite, along with whole-rock geochemistry. Results indicate that fluid exsolution and infiltration into the monzogranite occurred prior to, or coeval with, the onset of pegmatite crystallization. Boron, W, Mo, and As (± Cs, ± Sn) show preferential incorporation into the fluid phase over remaining in the pegmatitic melt, while F, Be, Li, Ta, Rb, Tl, and Ge likely incorporated into metasomatizing fluids due to their strong enrichment in the melt, rather than a preferential incorporation. Greisenization, zinnwalditization and tourmalinization reactions led to the formation of metasomatic Li-rich micas, quartz, apatite, fluorite, rutile, ilmenite, schorl, and sulfides in the variably metasomatized monzogranite. The Li halo in whole-rock extends 4–5 m from the contact and, together with F, B, As, and Cs enrichments, serves as a reliable pathfinder for buried Li-rich pegmatites, detectable up to twice the dyke thickness. Mica chemistry may offer a more sensitive alternative, with Li, Cs, Sn, and Be representing key components of the geochemical fingerprint associated with Li-rich pegmatites. These results may be applicable to the exploration of rare-element pegmatite deposits in granitic terranes, particularly within the Li-metallogenic province of the European Variscides.