Descripción: The Arroyo La Chilca-Zanjón del Buitre alteration zone, San Rafael Massif, Mendoza. The Arroyo La Chilca-Zanjón del Buitre alteration zone is located in the San Rafael Massif (35°24'8" S; 68°19'25,8" W), Mendoza. Previous works defined preliminary this deposit as a porphyry copper system genetically linked to the gondwanan magmatism. This alteration zone is hosted by a sequence composed of ignimbritic breccias and ignimbrites of trachyandesitic composition with interbedded tuffaceous sandstones and andesitic lavas flows intruded by trachyandesitic-dacitic and andesitic hypabyssal bodies. The lithology and geochemistry of the volcanic sequence confirm the link with the Choiyoi Magmatic Cycle lower section (Lower Permian). The alteration zone consists of a potassic halo with a parageneses of K-feldspar-quartzmagnetite overprinted by phyllic alteration with an assemblage of quartz-muscovite-pyrite-(rutile) and quartz -pyrite veins. A late carbonatization process overprints both the potassic and phyllic alteration. Outside the alteration zone pyrite-chalcopyrite- galena-sphalerite veins with quartz gangue crop out. Metals geochemistry shows moderate Ag and Au anomalies and little Cu, Mo, Pb and Zn anomalies. Statistical analyses allow defining one Cu-Mo mineralizing stage and another Mo mineralizing stage only recognized in Zanjón del Buitre. Moreover, two Zn mineralizing stages, one of them probably genetically related to the Cu stage, were also recognized.

Descripción: The main mineralizations of the Sierra Pintada uranium district, San Rafael Massif, Mendoza, are associated with the Choiyoi volcanic province. In the lower section of this magmatic cycle uranium deposits hosted by epiclastic sandstones are predominant. In the upper section, small vein-type deposits of low economic significance are found instead. During the emplacement of these Permian volcanic and sedimentary sequences two different stress regimes, which conditioned the mineralizing systems, were dominant: transpressional (San Rafael orogenic phase) and transtensional (post-orogenic stage). A strain fabric analyses was performed in an attempt to evaluate the control exerted by the structures in the distribution of the uranium mineralizations during these two stages of deformation. So that, a contribution to a better understanding of the genesis of these deposits will aid to the generation of new exploration guides for uranium in the Choiyoi province. In the case of the deposits related to the lower Choiyoi, three orders of magnitude could be defined for the structural control. The fabric of the structures determining the distribution of these mineralizations reflects the structural grain of the San Rafael orogeny. The deposition of uranium hosted by the upper Choiyoi sequences was directly or indirectly influenced by the structures generated under the post-sanrafaelic stress field.

Descripción: The goal of this paper is to present the petrographic study from outcrop samples of the Central II alteration zone. This area is located in the San Rafael Massif, province of Mendoza (34°54'56" S; 68°37'02" O) and is related to hypoabyssal rhyolitic bodies that intrude a volcanic-volcaniclastic sequence corresponding to the upper section of Choiyoi Group. The alteration zone is represented by pervasive sericitization overprinted by silicic breccias and veins with WNW trend (Az 290° - 300°) and vertical dip. Hydrothermal veins are mainly composed by quartz with primary growth, recrystalization and replacement textures with minor pyrite, chalcopyrite and scarce arsenopyrite. The combination of crustifiform - colloform silica banding, quartz pseudomorphs of bladed calcite and adularia with different morphologies (sub-rhombic, rhombic and pseudo-acicular) is a clear evidence of a low-sulfidation epithermal system with paleozones of upflow boiling.

Descripción: A paleomagnetic study, carried out on 9 sites of the La Flecha Formation (facies Los Sapitos, Upper Cambrian, Marjuman-Steptoean) exposed along the la Angostura creek, Precordillera of La Rioja (29.5°S, 68.7°W), permitted to determine two magnetic components. One is postectonic and of low coercivities and unblocking temperatures (A, Dec: 7.5°, Inc: -40.3°, a95: 10.1°, N: 6), being interpreted as a recent remanence of probable viscous origin. A second, pretectonic, component was isolated in seven sites with opposite polarities and is likely carried by magnetite (B, Dec: 206.9°, Inc: 63.3°, a95: 12.8°, N: 7). The paleomagnetic pole position for this component (Lat: 63.8°S, Long: 244.6°E, dp: 15.9°, dm: 20.2°) suggests that the remanence was acquired by the end of the Permian and it is interpreted as produced by the regional remagnetization associated with the San Rafael orogenic phase found in other units of the Precordillera. An analysis of pole positions, type of magnetizations, polarities and geographic distribution of the remagnetized units suggests a pattern of time-space migration of the remagnetization from west to east, between the Early Permian and the Permian-Triassic. This pattern is consistent with a model of remagnetization by migration of fluids expelled from the orogenic area that reached rocks not involved directly in the deformation. The pole position obtained for La Flecha Formation also suggests the lack of major tectonic rotations in the study area since the Permian. © 2005 Asociación Geológica Argentina.

Descripción: During the Early Permian an extensive deformation belt well-known as Gondwánides developed in the southwest Gondwana margin. A foreland basin has been settled during the Late Carboniferous - Early Permian in the western end of this strip, and is represented by the El Imperial Formation, which crops out in the San Rafael Block. The upper section of this unit records an inversion of the ancient slopes, which has been interpreted as the first evidence of the migration of the orogenic front towards the foreland. Therefore, a paleomagnetic study has been carried out on this unit, searching for elements that allow to constraint the deformation age to evaluate the possible diachronism of it. First paleomagnetic results obtained in EI Imperial Formation, from the lower member in the Represa Agua del Toro and from the upper member in the Puesto La Josefa, are presented and discussed. These rocks have reverse magnetizations carried by hematite, of possible Permian age. Pole positions are discordant with the apparent polar wander path from South America. These differences could be explained because: secular variation was not properly averaged; rotations about vertical axes of the sampling localities; or the combination of these factors. © 2006 Asociación Geológica Argentina.