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: These concretions form discrete spherical bodies 2 to 20 cm in sizes, within medium grained massive sandstone (arkosic arenite). Three concentric zones are recognized: a) dark green to black core composed of quartz and feldespar cemented by illite and roscoellite, b) an intermediate pale red zone of hematite-coated quartz, and c) a white, iron-leached outer zone. Edax and ICP analyses reveal high concentrations of vanadium mainly in zones a) and b) (60340 and 2582 ppm respectivily), uranium (237 and 163 ppm), rubidium (252ppm) and copper (178ppm). The concretions are interpreted as formed under eodiagenetic conditions before the development of haematite cement which gave the red colour to the host rock.

Descripción: Small outcrops of biotitic-amphibolic schists and amphibolites found in two localities suggest that the Cushamen Formation is host rock of the Central Patagonian Batholith in Gastre. The characteristics of these rocks were investigated using structural, petrographic, magnetic fabric, and garnet chemical-zonation studies. Garnet is observed as a late product of contact metamorphism. Structural data from the studied area and from other localities in the region are in conflict with models that invoke the occurrence of uniform, large-scale dextral displacements along the Gastre lineament in Mesosoic times. Rather, the data suggest that the Gastre granitoids record heterogeneous low-temperature deformation of likely Late Triassic age. This low-temperature deformation appears as small, localized outcrops, and we suggest that it is related to the accommodation of the successive magma batches that built the Central Patagonian Batholith.