The geology and mineralization of acid pegmatities

Abstract

Abstract by author:

Acid pegmatites generally approximate to granites in composition, however a minority may be extremely fractionated and enriched in rare elements. They occur as segregations or small intrusions, in elongate belts, in areas of high grade metamorphism and granite intrusion. A complete range exists from the migmatitic pegmatites of granulite facies metamorphic terranes, through the muscovite and rare-element bearing types of Barrovian and Abukuma terranes respectively, to the rare-element pegmatites derived from the alkaline granites associated with continental rifting. A number of progressive changes are apparent in the tectono-magmatic setting of pegmatites through time. These changes can be related to the evolution of crustal processes from the small scale highly active regimes of the Archaean, through the intracratonic rifting in the Proterozoic to the plate tectonic regimes of the Phanerozoic. A number of similarities are observed between the settings of Archaean and Phanerozoic pegmatite belts. The dominating factor in the generation of rare element pegmatites is the extreme enrichment in water resulting from plagioclase fractionation at constant pressure. Local tectonic conditions are critical in promoting frequent melt segregation and extreme fractionation. The potential for the discovery of large pegmatite hosted rare element deposits is very limited. Most pegmatite hosted deposits of Sn and Ta-Nb are less than 1000 tonnes and are most effectively mined as small scale operations