Caracterización geoquímica y estructural de feldespatos potásicos de algunas pegmatitas de los grupos Balilla y Aurora, distrito pegmatítico Valle Fértil, 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 al­ways 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 in­termediate 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 co­rrelate well with the established patterns of geochemical fractionation for muscovite-bearing and barren rare-element pegma­tites of LCT (Li, Cs, Ta) signature of the Pampean pegmatitic province. (Al, Si) ordering in the tetrahedral sites have been cal­culated from lattice parameters, selected diffraction-peak positions, and translation distances along [110] and [110] directions. The results indicate that the blocky K-feldspar from intermediate zones have a fully ordered structure. The near maximum microcline state suggests conditions of crystallization at low rates of nucleation. Identical ordering values were determined for internal K-felds-par with similar texture and grain-size in other deposits of the Pampean pegmatitic province.