Abstract

GAIDO, Maria Fernanda; CEGARRA, Marcelo I  and  ANSELMI, Gabriela. Geological characteristics of the dacitic porphyries and their host rock, Quebrada del Peñasquito, western Precordillera of San Juan. Ser. correl. geol. [online]. 2012, vol.28, n.1, pp.83-90. ISSN 1666-9479.

At Peñasquito creek, in the sierra del Tontal (S 31º 40' 44,5" - W 69º 14' 05"), western San Juan province, at an altitude of 2,953 m.s.n.m, a succession of dacitic dykes of unknown age are recognized, intruding subconcordantly sandstones of the Don Polo Formation of Ordovician age. The sandstones intruded by the dykes present short wavelength symmetrical folding (400 m). The dikes have attitudes of N0º/85ºO-N30º/75ºO, with thicknesses ranging between 3 and 25 m. Microscopically, they present a porfiric texture, composed of quartz, plagioclase, biotite and hematite. A deep alteration (oxidation) confers a yellowish color to the dykes, easily visible by means of satellite imagery. There are numerous folded veins of quartz that accompany the dikes, associated to anticline folds, fractures and shear zones, with sigmoid structures and cinematic indicators with dextral displacement indicators. The dikes may correspond to a Permian-Triasic or Miocene magmatism. The closest dacitic magmatism of Permian age has been recognized to the south of the area, in Pampa de los Pozos and Yalguaraz, with copper mineralization. However, Peñasquito´s dykes have a greater similarity with dacitic dikes described at Carrizal creek, 7 km to the north of the study area. For this reason are assigned to the Miocene magmatism recognized in the region. Exploratory studies are suggested to determine the presence of mineralization.

Keywords : Porphyry; Dacite; Magmatism; Miocene; Peñasquito; Precordillera; San Juan.

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