GEOCHEMISTRY OF THE LITHINIFEROUS VEINS OF THE PEGMATITE-APLITE FIELD OF ALMENDRA-BARCA DE ALVA (NORTHERN PORTUGAL)

The Almendra-Barca de Alva region was known already for the presence of pegmatite-aplite veins with lithium minerals as lepidolite and rare spodumene (Gaspar 1997; Charoy & Noronha 1999). This research work was done in the main vein of this pegmatite-aplite field (an open pit for Ceramic Industry known as Bajoca mine, Almendra). This vein has an extension of 700 meters, with variable thickness between few meters and more than 30 meters. It shows a general orientation N10º with dip variations between 30º and 45º W. It is clearly intrusive in the metasediments of the Centro-Iberian geotectonic zone. There are different types of granites in the vicinity of the pegmatite-aplite field. They cause a metamorphic isograde superimposed to the regional metamorphism that increase in the granites direction. During the field work in the open pit was clearly showed the existence of lithium minerals. They occur as montebrasite-ambligonite and mainly as petalite (beside occurrences of lepidolite in nearby veins of the company concession). The montebrasite-ambligonite appear as accessory mineral, in milimetric crystals. The petalite occurs as microcristaline mineral masses, white in color, or in centimetric crystals with perfect cleavage {001}. It’s an essential mineral in certain parts of the vein, what means that it is considered zoned in terms of lithium, because other parts are almost sterile. Until now it was only possible to analyze 5 Kg samples in each 2 m of a channel sample 30 m long, done in the perpendicular of the main vein. These results are compared with the results to the lepidolite veins of the same area. The results indicate that the lepidolite and petalite-bearing veins have, aproximately the same values in major elements, and in the various analyzed trace elements. In both type of veins, albite is dominant over the K-feldspar (Na2O/K2O>>1.5), especially in the aplitic facies. The peraluminous rate is high (A/CNK)>>1, in both type of veins. Using the K/Rb rate criteria for the fractionation index, the lepidolite-bearing veins seem to be more evolved than the petalite-bearing veins ones. These rare element pegmatites are considered by Cerny classification belonging to the LCT (Li, Cs, Ta) family. They are of the complex type, mainly of the lepidolite subtype meanwhile the main vein of Bajoca mine is classified as petalite subtype.