METASOMATISM AT A GRANITIC PEGMATITE - DUNITE CONTACT IN GALICIA: THE FRANQUEIRA OCCURRENCE OF CHRYSOBERYL (ALEXANDRITE), EMERALD, AND PHENAKITE
AGUSTIN MARTIN-IZARD, ANDRES PANIAGUA and DAMASO MOREIRAS
Departamento de Geología, Universidad de Oviedo, Arias de Velasco s/n, 33005 Oviedo, Spain
ROGELIO D. ACEVEDO
Cirgeo (Conicet) Ramírez de Velasco 847, 1414, Buenos Aires, Argentina
CELIA MARCOS-PASCUAL
Departamento de Geología, Universidad de Oviedo, Arias de Velasco s/n, 33005 Oviedo, Spain
Abstract
The Franqueira deposit is the first documented example of gem- quality chrysoberyl, emerald, and phenakite deposit in western Europe. It is located in the northwestern part of the Iberian Peninsula, in the Galicia - Tras os Montes zone, which is made up of two domains: (1) schistose rocks, including mafic-ultramafic overthrusted complexes, and (2) granitic rocks, including peraluminous, heterogeneous, synkinematic two-mica granitic rocks. Bodies of granitic pegmatite intrude the schist and granitic rocks. At Franqueira, a pegmatite body related to the heterogeneous granites cross-cuts an ultramafic rock of dunitic character and associated gabbroic lithologies. In the contact between pegmatite and dunite, a metasomatic zone has been developed in which dunite is almost completely altered to phlogopite at the contact; distal to the pegmatite, dunite is altered to tremolite. Adjacent to the dunite, the metasomatism produced an orthoamphibole (anthophyllite- rich) rim. The phlogopite metasomatic rocks contain the minerals of gemmological 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 inclusions have been distinguished. The fluid- inclusion populations of emerald and phenakite are similar. No measurable fluid-inclusions can be found in chrysoberyl. Two discontinuous hydrothermal stages have been identified. The first stage was characterized by the trapping of two types of aqueous fluid-inclusions with some volatile components. The two types of inclusions may be contemporaneous, which suggests immiscibility in the system H2O - NaCl - CH4 - CO2 - other volatile components. Homogenization temperatures range between 318 and 381 C. The characteristics of type-3 inclusions suggest an independent episode of fluid circulation during later tectonic events. The genetic model proposed involves emplacement of pegmatite and associated mobile elements (e.g., Be, B, P) into dunite, with subsequent metasomatism of the dunite into phlogopite and tremolite rocks near the pegmatite body. The formation of chrysoberyl is probably due to the fact that it shares structural features with the olivine; therefore, the growth of chrysoberyl could be favoured by epitactic nucleation on olivine relics. Emerald is the latest Be mineral to form; it partially replaces chrysoberyl and phenakite, and could have formed by the reaction chrysoberyl + phenakite + quartz ± emerald, until quartz is exhausted.
Keywords: granitic pegmatite, metasomatism, dunite, emerald, chrysoberyl, "alexandrite", phenakite, genetic model, Galicia, Spain.