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The increasing strategic importance of Li- and Ta-ores during the last decades due to the strong consumption growth for rechargeable batteries and high temperature and corrosion resistant capacitors reactivated the interest of studies in pegmatite fields around the world, because these rocks supply respectively 25% and 100% of the world consumption in these elements. Research on petrogenetic issues and major and accessory mineral chemistry variations in rare element (REL)-pegmatites of the Borborema Pegmatite Province in Northeast Brazil were tested as tools for the diagnosis of the metallogenetic potential of rare metals in individual pegmatites and in the province as a whole along the last dozen of years. The results allowed to establish the nearly isobaric (3.8 kbar) crystallization conditions of the REL-pegmatites between approximately 580 °C (liquidus) and 400 °C (solidus) from a peraluminous melt saturated in an aquo-carbonic medium to low salinity volatile phase and an immiscible peralkaline flux-enriched (H2O, CO2, F, B, Li etc.) melt fraction, based on melt and fluid inclusion studies. Mineral-chemistry data from 30 selected REL-pegmatites in the province allowed to classify three of them as being of the complex-spodumene or -lepidolite subtype in Cerný's classification. Both subtypes are supposed to be potentially fertile, (highly fractionated, and with good chances to bear Li- and Ta-ore concentrations). It was also possible to identify several pegmatitic granite intrusions with textural and lithogeochemical characteristics also found in source granites of REL-pegmatite provinces elsewhere. Preliminary chemical Pb/U/Th geochronological determinations in uraninite and xenotyme crystals of these granites indicate an age of 520 ± 10 Ma and match recently published Ar/Ar in mica and U/Pb ages in columbite-group minerals (CGM) of the REL-pegmatites between 509 and 525 Ma. Mineral-chemistry data from grains of the outer zones of the pegmatites do not allow to distinguish potentially fertile from barren pegmatites. This discrimination is possible only if samples of the inner intermediate zone, replacement pockets or quartz core are used. From the tested minerals trace-element determinations (mainly Li, Al, Ti, Ge, B among 14 tested elements) by LA-ICP-MS technique in quartz seem to be more efficient than the classical approach (of Rb, K, Cs, Ga, Sr Ta) in K-feldspar or micas, due to the susceptibility to hydrothermal or supergene alteration of the latter. Mineral-chemistry variations in CGM, tourmalines, garnet and gahnite turned out to be efficient discriminators but all of them have the disadvantage of an eventual and, if present, random distribution, typical for accessory minerals in pegmatites, not allowing a regular sampling in most cases. Additional tests are recommended to confirm respectively the preliminary results of mineral-chemistry as exploration tools on a larger number of pegmatites and geochronological data to confirm the existence of another, older, synorogenetic generation of REL-pegmatites in the BPP. |
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