Abstract

OYARZABAL, Julio; ROQUET, M. Belén; GALLISKI, Miguel A  and  PERINO, Ernesto. Geochemical and structural characterization of potassic feldspars from pegmatites from Balilla and Aurora Group, Valle Fértil pegmatitic field, San Juan. Rev. Asoc. Geol. Argent. [online]. 2011, vol.68, n.1, pp.5-16. ISSN 0004-4822.

San Guillermo, Santa Amalia, Andacollo and El Chañar muscovite-bearing pegmatites from Balilla and Aurora pegmatite groups, are located between 30°30' and 30°47'S and 67°29' and 67°43'W, in the Sierra de Valle Fértil, 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 N25°-30°W to N25°-45°E predominant strikes, and 30°-45° 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-feldspar with similar texture and grain-size in other deposits of the Pampean pegmatitic province.

Keywords : K-feldspar; Geochemistry; (Al, Si) ordering; Pegmatites; Valle Fértil pegmatite field.

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