GEOCHEMISTRY OF THE PETALITE-BEARING AND SPODUMENE-BEARING VEINS OF THE BARROSO-ALVÃO PEGMATITE-APLITE FIELD (NORTHERN PORTUGAL)

The Barroso-Alvão pegmatite-aplite field (North Portugal) was known until now, for the abundance of spodumene-bearing veins, with rare occurrences of petalite and eucryptite as accessory minerals, beside rare occurrences of lepidolite. Recently, during an Industrial Minerals research project, were discovered veins where petalite is the dominant lithiniferous phase. Every vein is composed of an aplite and pegmatite facies that are mixed in an intricate way. The veins are intruded in metasedimentary rocks, Silurian in age, with an intermediate degree of metamorphism, and belonging to the Galiza-Trás-os-Montes geotectonic Zone. There are different types of granites in the vicinity of the pegmatite-aplite field. The granites are different in their mineralogy (biotitic and two-micas) and in their chronology relative to the last Variscan deformation phase. The pegmatite-aplite veins are deformed and for that reason are younger than the post-tectonic granite. In this work, we compare the geochemistry of the spodumene-bearing veins with the petalite-bearing veins and their significance in terms of classification and evolution. The results indicate that the spodumene and petalite-bearing veins have approximately the same values in major elements, and in the various analyzed trace elements, but are very enriched in Sn and poorer in Fe. Like the spodumene-bearing veins, albite is dominant over the K-feldspar (Na2O/K2O>>0.7), especially in the aplitic facies. The peraluminous rate is very high (A/CNK)>>1.6, as in the spodumene-bearing veins. The major and minor elements values found in the spodumene-bearing veins as in the petalite-bearing veins permitted us to classify both as from the rare-element class, LCT family, complex type, respectively as spodumene and petalite subtypes. This pegmatite-aplite field is in general agreement with the classic concept of rare-element pegmatites as igneous derivatives of fertile granitic intrusions, products of advanced fractionation of pluton-size batches of granitic magmas. It was possible to verify that the values are poor in lithium with the alteration of the pegmatite-aplite vein, causing the lixiviation of the lithium to out of the vein. Using the K/Rb rate for the fractionation index, the petalite-bearing veins seem to be more evolved than the spodumene-bearing veins