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.

Descripción: Towards the northeast of the Sierra de San Luis several wolfram ore deposits occur. In San Martín district they are divided into three groups: Los Avestruces, La Esperanza and Cerrito Blanco. The geology of this area is formed of the Conlara metamorphic complex composed of schists (with a strike of 300° north), granitoids and pegmatites. In the same area subsaturated chalco-alkaline lamprophyres also appear as subvertical bodies with NW-NE strikes. Toward the boundaries of these rocks, the amount of biotite rises gradually from 80 to 90% developing a biotitite rock due to metasomatic processes and the superimposed deformation. This biotitite is rich in K, Rb and W among other elements. The scheelite mineralization (with quartz, biotite and tourmaline as non ore minerals) occurs in veins that are usually hosted between the lamprophyres and the wall rocks. Scheelite also appears between the schist planes of the biotitite rocks as well as disseminated crystals in quartz aggregates. This mineralization took place from fluids with XH2O: 0.87/0.82 - XCO2: 0.11/0.16 - XNaCl: 0.015/0.013 - (CH4) meanwhile the scheelite, according to the fluid inclusions studies, precipitated from unmixing processes below 320°C and 2.3 kb. Achaliana orogeny and the intrusion of Las Chacras-Piedras Coloradas batholiths (Devonian-Carboniferous) were the responsible for the slight rise of the fluids from the basement, the generation of fault structures through which they rose, the metasomatism and the mineralization.

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: In the southern border of Laguna de Pozuelos Basin, the Pan de Azúcar volcanic complex crops out. It consists of several dacitic volcanic dome centers of ~ 12 Ma. Previous interpretation of seismic lines and a detailed ground magnetic survey indicated the presence of buried intrusive bodies. The determination of the existence of buried bodies has two major implications: 1) these volcanic complexes are closely associated with ore deposits (as part of the Bolivian tin ore polymetallic belt); and 2) the existence of a large ancient caldera (~12 Ma) beneath the central and southern part of the Laguna de Pozuelos Basin, covered by the infilling sediments was previously suggested. The volcanic complexes would represent the final stages of such a calderic magmatic system. In order to confirm the existence of other buried intrusives and/or a buried caldera system, detailed ground magnetic and gravity surveys were carried out. The magnetic map is dominated by positive and negative anomalies in the southern sector of the basin, associated with the dacitic domes. The residual Bouguer anomaly presents a semicircular pattern, having only positive values. We applied the curvature technique to analyse the magnetic and gravity signals and used Euler deconvolution to estimate the depth to the sources. Our results would support the hypothesis of a large caldera buried beneath the Laguna de Pozuelos Basin. The identification of such a large Middle Miocene caldera would bring new insight into the magmatic evolution of the northern Puna.