GEOCHEMICAL FRACTIONATION OF Nb-Ta OXIDES IN Li-BEARING PEGMATITES FROM THE BARROSO-ALVAO PEGMATITE FIELD, NORTHERN PORTUGAL

The Barroso-Alvao granitic pegmatite field, located in northern Portugal, is known for its numerous occurrences of Li-bearing pegmatites, especially spodumene-and petalite-bearing bodies. Recent work on this pegmatite field characterized other types of pegmatites, and complex mineralization and mineral associations. Minerals of the columbite-tantalite group are found in all the LCT (Li-Cs-Ta) pegmatites from Barroso-Alvao: spodumene pegmatites, petalite pegmatites, and lepidolite pegmatites. In the petalite and lepidolite pegmatites, the columbite-tantalite group of minerals are found spatially associated with cassiterite. Cassiterite is not present in the spodumene pegmatites. The columbite-group minerals have Ta/(Ta + Nb) and Mn/(Mn + Fe) values ranging from 0.20 to 0.60, except in the lepidolite pegmatites, where Mn/(Mn + Fe) is close to 1. The fractionation trend starts in the Mn-poor columbite-(Fe) field (spodumene pegmatites) toward Mn- and Ta-enrichment (lepidolite pegmatites). Our results suggest that Mn enrichment is not controlled by silicate or phosphate phases, but instead by the increasing activity of fluorine. The Ta enrichment, on the other hand, may well be controlled by fractional crystallization. We also observe a decrease in the Ti of the columbite-group minerals from the spodumene to the lepidolite pegmatites, which we consider an evolutionary trend.