caracteriZació n GeoQuí Mica Y estructural de FeldesPatos PotÁ sicos de alGunas PeGMatitas de los GruPos Balilla Y aurora, distrito PeGMatí tico valle FÉ rtil, san Juan

Geochemical and structural characterization of potassic feldspars from pegmatites from Balilla and Aurora Group, Valle Fertil pegmatitic field, San Juan. San Guillermo, Santa Amalia, Andacollo and El Chanar muscovite-bearing pegmatites from Balilla and Aurora pegmatite groups, are located between 30o30´ and 30o47´S and 67o29´ and 67o43´W, in the Sierra de Valle Fertil, northwestern Argentina. The deposits are hosted in medium to high-grade metamorphic rocks of Barrovian upper amphibolite to granulite facies. The pegmatites are tabular, with N25o-30oW to N25o-45oE predominant strikes, and 30o-45o NE or NW dips. The pegmatites show heterogeneous, complex internal structure with variable textural patterns and mineral modes; the units of primary crystallization are border, wall, intermediate zones and quartz core, discrete muscovite and albite replacement bodies are al ways present. The mineral assemblages are characterized by the predominance of K-feldspar, quartz, plagioclase, muscovite and biotite; the accessory minerals are Grt-Brl-Vrm-Aln-Mnz-Ep. Based on mineralogical, geochemical, structural and para genetic attributes, the pegmatites are mainly classified as belonging to the muscovite (MS) class. The K-feldspar from the in termediate zones shows pink or pale brown color, blocky texture and moderate perthitic character. Their chemical analyses reflect the pegmatite type, geochemical signature and mineralization of the deposits, the K/Rb, K/Cs and Rb/Sr ratios co rrelate well with the established patterns of geochemical fractionation for muscovite-bearing and barren rare-element pegma tites of LCT (Li, Cs, Ta) signature of the Pampean pegmatitic province. (Al, Si) ordering in the tetrahedral sites have been cal culated from lattice parameters, selected diffraction-peak positions, and translation distances along [110] and [110] directions.