Metasomatism at a granitic pegmatite-dunite contact in Galicia; the Franqueira occurrence of chrysoberyl (alexandrite), emerald, and phenakite
Metasomatism at a granitic pegmatite-dunite contact in Galicia; the Franqueira occurrence of chrysoberyl (alexandrite), emerald, and phenakite
The Canadian Mineralogist (August 1995) 33, Part 4: 775-792
- alexandrite
- chrysoberyl
- emerald
- Europe
- fluid inclusions
- Galicia Spain
- gems
- granites
- hydrothermal conditions
- Iberian Peninsula
- igneous rocks
- inclusions
- metasomatism
- mineral deposits, genesis
- nesosilicates
- orthosilicates
- oxides
- paragenesis
- pegmatite
- phenakite
- phenakite group
- plutonic rocks
- ring silicates
- silicates
- Southern Europe
- Spain
- Western Europe
- Franqueira Deposit
This deposit of gem-quality chrysoberyl, emerald and phenakite in NW Spain occurs in the Galicia-Tras os Montes zone. At Franqueira, a pegmatite related to the heterogeneous two-mica granites cuts an ultramafic body, and at the contact a metasomatic zone developed in which the dunite is almost completely altered to phlogopite (with an anthophyllite-rich rim); distal to the pegmatite, the dunite is altered to tremolite. The phlogopite-rich metasomatic rocks contain the chrysoberyl, emerald and phenakite, together with apatite. All the metasomatic rocks are relatively enriched in Mg and Cr from the dunite, and in Al, K, Be and Si from the pegmatite. Two discontinuous hydrothermal stages are distinguished, the first stage being characterized by the trapping of two types of aqueous fluid inclusions with T (sub h) 318-381 degrees C; a third type of inclusion suggests an independent episode of fluid circulation during later tectonic events. The formation of chrysoberyl may be due to its structural similarity with olivine, allowing epitactic nucleation on olivine relics. Emerald is the latest Be mineral to form; it partially replaces chrysoberyl and phenakite and may have formed by the reaction chrysoberyl + phenakite + quartz ->> emerald until the quartz was used up.