THE ACHALA GRANITIC COMPLEX AND THE PEGMATITES OF THE SIERRAS-PAMPEANAS (NORTHWEST ARGENTINA) - A STUDY OF DIFFERENTIATION

The metasedimentary sequences of the Sierras Pampeanas (northwest Arge ntina) are intruded by a series of Hercynian granitoids ranging from g ranitic to tonalitic in composition. The most evolved granitoids show alkali granitic characteristics. In the granitoids as well as in the c ountry rocks, pegmatites that are very different in zoning and mineral ogical association are found. The origin of the pegmatites is attribut ed to extreme differentiation by fractional crystallization of a grani tic magma. The economic potential for rare metals is evaluated in geoc hemical discrimination diagrams, showing that some of the pegmatites a re not only highly differentiated in terms of alkalies but are also pr omising targets for small-scale mining of Ta and to some degree for Sn , too.