METASOMATISM AT A GRANITIC PEGMATITE-DUNITE CONTACT IN GALICIA - THE FRANQUEIRA OCCURRENCE OF CHRYSOBERYL (ALEXANDRITE), EMERALD, AND PHENAKITE

The Franqueira deposit is the first documented example of a gem-qualit y chrysoberyl, emerald, and phenakite deposit in western Europe. It is located in the northwestern part of the Iberian Peninsula, in the Gal icia - Tras os Montes zone, which is made up of two domains: (1) schis tose rocks, including mafic-ultramafic overthrusted complexes, and (2) granitic rocks, including peraluminous, heterogeneous, synkinematic t wo-mica granitic rocks. Bodies of granitic pegmatite intrude the schis t and granitic rocks. At Franqueira, a pegmatite body related to the h eterogeneous granites cross-cuts an ultramafic rock of dunitic charact er and associated gabbroic lithologies. In the contact between pegmati te and dunite, a metasomatic zone has been developed in which dunite i s almost completely altered to phlogopite at the contact; distal to th e pegmatite, dunite is altered to tremolite. Adjacent to the dunite, t he metasomatism produced an orthoamphibole (anthophyllite-rich) rim. T he phlogopite-rich metasomatic rocks contain the minerals of gemmologi cal interest, together with apatite. All these metasomatic rocks have a high content of Mg and Cr, which come from the dunite, and of Al, K, Be, and Si, which come from the pegmatite. Three types of fluid inclu sions have been distinguished. The fluid-inclusion populations of emer ald and phenakite are similar. No measurable fluid-inclusions can be f ound in chrysoberyl. Two discontinuous hydrothermal stages have been i dentified. The first stage was characterized by the trapping of two ty pes of aqueous fluid-inclusions with some volatile components. The two types of inclusions may be contemporaneous, which suggests immiscibil ity in the system H2O - NaCl- CH4 - CO2 - other volatile components, H omogenization temperatures range between 318 and 381 degrees C. The ch aracteristics of type-3 inclusions suggest an independent episode of f luid circulation during later tectonic events. The genetic model propo sed involves emplacement of pegmatite and associated mobile elements ( e.g., Be, B, P) into dunite, with subsequent metasomatism of the dunit e into phlogopite and tremolite rocks near the pegmatite body. The for mation of chrysoberyl is probably due to the fact that it shares struc tural features with the olivine; therefore, the growth of chrysoberyl could be favored by epitactic nucleation on olivine relies. Emerald is the latest Be mineral to form; it partially replaces chrysoberyl and phenakite, and could have formed by the reaction chrysoberyl + phenaki te + quartz --> emerald, until quartz is exhausted.