Abstract

The crater lake and associated hydrothermal features of Copahue volcano have been studied intensively over the last 20 years (1995–2015). The geochemical and isotopic compositions of the waters provide insights into the processes occurring in the volcanic–hydrothermal system, the crater lake and the thermal springs. Variations in the temperature and chemical composition of the waters reveal fundamental changes in the system that precede and accompany the magmatic and phreatic eruptive events at Copahue. A conceptual model of the summit system was developed involving the intrusion of slivers of magma in the hot acidic hydrothermal cell, the saturation of waters with secondary minerals leading to reduced permeability, the blocking of fluid pathways by liquid sulphur during heating events and the transport of gas from the magma through the ductile–brittle transition into the hydrothermal system. Geophysical data were integrated with the chemical data to provide new insights into the behaviour of the deep magmatic system that feeds the volcano edifice. Multidisciplinary studies were used to identify precursory signals of eruptive activity to give an early warning of pending volcanic hazards. Several geochemical ratios in river water were identified as potential indicators of upcoming volcanic activity, which could be used in co-operation with the community and local authorities to deal with these volcanic hazards.