The metasedimentary sequences of the Sierras Pampeanas (northwest Argentina) are intruded by a series of Hercynian granitoids ranging from granitic to tonalitic in composition. The most evolved granitoids show alkali granitic characteristics. In the granitoids as well as in the country rocks, pegmatites that are very different in zoning and mineralogical association are found. The origin of the pegmatites is attributed to extreme differentiation by fractional crystallization of a granitic magma. The economic potential for rare metals is evaluated in geochemical discrimination diagrams, showing that some of the pegmatites are not only highly differentiated in terms of alkalies but are also promising targets for small-scale mining of Ta and to some degree for Sn, too.