Geology and structural controls on areas of alteration of Portezuelo de las Burras (Cordillera Frontal, San Juan, Argentina)

Abstract

The main stratigraphical, petrographical and structural features of the Portezuelo de Las Burras altered area, located in the Cordillera Frontal (San Juan Province, Argentina), are described. The area is composed of the La Puerta Formation (Upper Carboniferous - Lower Permian), thick volcanic and subordinate sedimentary layers of the Choiyoi Group (Permian - Triassic), levels of andesitic lava of Andesita Las Vizcachas (Upper Mesozoic? - Terciary?), more acidic rocks of the Arroyo de las Chinches Formation (Oligocene - Miocene) and andesitic rocks of Entrecordilleras Formation (Upper Miocene - Pleistocene), intruded by the Entrecordilleras stock (Neogene) and smaller dacitic bodies probably related to it. The courses of the Manantiales and Las Burras rivers are located on a main sinistral wrench zone expressed by synthetic and antithetic faults where the area next to the Portezuelo de las Burras constitutes releasing orerstep in a tectonic transfer with dilatance behavior. This structure controlled the circulation and emplacement of the mineral-bearing hydrothermal fluids, which were responsible of the chromatic, mineralogical and geochemical anomalies detected between the Portezuelo de las Burras and Cerro Manrique. The anomalies are related to argillic and siliceous hydrothermal alterations that increase the possibility of there being economic precious-mineral accumulations.