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: The advanced spaceborne thermal emission and reflection radiometer (ASTER) was used to identify different deposits of hydrothermal alteration which indicates that several important lithological groups can be mapped in areas with good exposure by using spectral-matching techniques. Different methods are tested in order to identify and map zones with hydrothermal alteration minerals using the ASTER dataset. These areas are often referred to having large quantities of clay minerals which can be detected using multispectral imagery. Several authors have developed different procedures to map these hydrothermal minerals. Among the simplest ones, band combinations and band ratios have proven to be very useful tools for identifying targets. Lithology indexes point to reinforce the spectral response of this group of minerals working with band products and ratios. So far, these techniques do not need a full image correction. Other methods here employed require further processing of the ASTER scene, especially when spectral data are used. These techniques include spectral angle mapper (SAM) classification and minimum noise fraction (MNF) transforms to segregate noise and reduce computational requirements. Spectral data used in this paper were collected from field samples using SWIR (short wave infrared) reflectance spectroscopy and derived from the scene itself. These mapping methods have been tested in areas of known hydrothermal alteration occurrences, e.g. Los Pelambres, El Pachón and Altar, and in other sector of Santa Cruz region like Carnicerias and La Coipa; all of these at the south westernmost part of San Juan province. The result of this work is here presented as a series of images showing lithology indexes and an expected mineral assembly.

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: El Infiernillo Mining District is located in the San Rafael Block (34°38'20";68°47'37"), province of Mendoza, Argentina. It is a porphyry Cu (Mo) type deposit which is hosted by a volcaniclastic sequence. It develops an alteration halo with a concentric pattern consisting of a central quartz neck surrounded by a potassic alteration zone and outwards a phyllic halo and small polymetallic veins. The potassic halo has a paragenesis of K-feldspar-quartz-(biotite). Three silicification phases are related to this alteration stage, one of them (with temperature between 550° and 390°C and salinity between 35 and 47 %) genetically linked to the copper mineralization. The phyllic alteration consists of quartz-illite-(rutile) with minor albite. The albitization process points out to an early phase of this phyllic stage. The fluid inclusions together with the textures of the quartz neck suggest the shallowing of the active hydrothermal system. The disseminated ore paragenesis consists of bornite, chalcopyrite-molibdenite, pyrite, (galena-sphalerite). The presence of molibdenite-quartz veins indicate the transition from the potassic to the phyllic stage while the pyrite-quartz ones point out to a transition to a fragile deformation regime during which the polymetallic veins, with a paragenesis of chalcopyrite- sphalerite, pyrite, (Ag) tetrahedrite, galena were formed. The petrologic characteristics of the volcaniclastic host rocks reveal that they correspond to the lower section of Choiyoi magmatic cycle. The magmatic and structural controls constrain the age of this ore deposit to the Early-Middle to Late Permian.

Descripción: Located at the transitional southern end (33o S) of the Pampean flat-slab, the Tertiary volcanic belt records the eastward migration of the Andean volcanic arc due to the flattening of the Nazca Plate in Mio-Pliocene times. The tertiary volcanic belt encompasses several very important metallogenetic districts in the San Luis Pampean Ranges. Volcanic rocks and associated ore-deposits crop out following a NW-WNW trending belt at La Carolina, Cañada Honda - Cerros Largos, Cerros del Rosario and El Morro volcanic fields. The available geochronological data indicate that the volcanic activity began early at the western end of the belt and ended ~ 10 Ma later at its eastern border. Cañada Honda district represents the oldest and longest-lived volcanic field of the tertiary volcanic belt. It records eruptive events and related hydrothermal alteration from 12-13 Ma to 7.3 Ma, including lavas and volcaniclastic products. Preliminary structural analysis shows that previous structures have strongly controlled, either directly or indirectly, the emplacement of volcanic rocks and related mineral deposits. Kinematic and strain fabric analyses allow to recognize two volcano-tectonic associations. One of these associations would be related to dextral reactivations of structures parallel to basement foliation. On the other hand, the main one would be linked with the generation of two volcano-tectonic depressions which are aligned in a NW-WNW direction. Their dominant structures trend NW-WNW and reveal sinistral-normal motions, both at local and Tertiary volcanic belt scales.

Descripción: The existence of ammonium anomalies in mineralized deposits in Argentina is presented. These anomalies are given by the substitution of potassium ion by ammonium ion in certain feldspars, clay minerals and sulfates. This substitution doesn't produce physical changes either in the minerals or the host rocks. Ammonium-bearing minerals have been found in several hydrothermally altered metallic deposits. Ammonium ion is specially sensible to SWIR reflectance spectroscopy method, so it became an ideal, easy and valuable tool for ammonium anomalies detection. Although ammonium-bearing minerals are not always associated with metallic ore, they can be used as an exploration guide, being thus a very useful prospective method for metallic deposits in general. © 2006 Asociación Geológica Argentina.