Early stages of the Miocene magmatic arc and related hydrothermal alteration at Valle Hermoso, South Central Andes (35°07′S, 70°17′W)

Abstract

For the first time, the eastern flank at Cordón Santa Elena, within the Valle Hermoso district, has been mapped and described in detail. Based on key field observations, petrographic, geochemical and geochronological results, a new Miocene volcanic stratigraphy is proposed for this particular area. The Cordón del Burrero Group (Arroyo del Perdido, Puesto Gutiérrez, Bolchiquito and Santa Elena Formations) has been defined for the ~18-13 Ma timespan, whereas the Huincan Formation has been constrained as younger than ~11 Ma. According to La/Yb ratios, Cordón del Burrero volcanics have been emplaced in a relatively thin crust compared to the Huincan dacites, suggesting that the main phase of crust thickening and tectonic deformation took place at ~12-11 Ma. The studies carried out at Cordón Santa Elena revealed a spatial and genetic relationship between the ~13 Ma dioritic subvolcanic pluton and a widespread calcic-sodic hydrothermal alteration. The areal distribution of the hydrothermal alteration shows a core of calcic-sodic alteration accompanied by propylitic alteration, cut by quartz-pyrite veins spatially associated with phyllic alteration and restricted potassic alteration. This hydrothermal system may be considered as the oldest pulse at this latitude.