VIGEZZITE AND ASSOCIATED OXIDES OF Nb-Ta FROM EMERALD-BEARING PEGMATITES OF THE VIGEZZO VALLEY, WESTERN ALPS, ITALY

Abstract

Two sequences of crystallization involving oxides of Nb–Ta occurred during the albitization process of three emerald-bearing pegmatites that cut serpentinites at Mount Pizzo Marcio and Alpe Rosso, in Vigezzo Valley, Western Alps, Italy: 1) a higher-temperature sequence, characterized by an alkaline micro-environment [high a (Na + )], allowing the crystallization of Mg-rich columbite, vigezzite, thorian vigezzite and U-rich microlite, and 2) a lower-temperature sequence, characterized by high a (Ca 2+ ), producing replacements of tapiolite-(Fe) and ferrowodginite by microlite, of tantalite-(Mn) by microlite and (later) fersmite, and of columbite-(Mn) by fersmite. Chromium-bearing beryl crystals (emerald) occur along the exocontacts characterized by a green to grey, sheared micaceous unit composed of phlogopite + talc + tremolite. Beryl crystals commonly show a green Cr-rich vitreous rim and a whitish milky core, partially replaced by beryllium silicates such as bavenite, bertrandite and bityite. Tourmaline crystals vary from schorl–dravite to Fe-rich dravite. In the model of evolution envisaged, we propose an anatectic origin of the pegmatite-forming melts during the latest stages of the Alpine tectonometamorphic event. The emplacement of the pegmatitic dikes in the serpentinites was followed by intense circulation of supercritical to hydrothermal fluids enriched in Mg, B and Be, and depleted in F, producing albitization of pegmatites and generating the aforementioned mineral reactions and replacements.