Hydrogeochemical controls on thermal springs around Villarrica volcano

Abstract

Villarrica volcano is one of the most extensively studied active volcanic systems in the South American Andean range. However, the linkage between the volcano and its surrounding thermal waters remains poorly understood. In this study, we present a comprehensive geochemical investigation based on 52 samples of water and 9 dissolved gas samples collected from 18 sites during three field campaigns. Integrated analyses of physicochemical parameters, major ion species, minor elements, stable isotope ratios, and gas compositions have allowed us to identify and differentiate distinct hydrofacies. Our findings reveal that water-rock interaction, principally through plagioclase dissolution leading to Na enrichment, is the predominant process controlling the hydrochemistry. Furthermore, while the aquifers are heated by a deep magmatic reservoir and by the regional geothermal gradient, interactions with ascending hydrothermal gases are critical to the evolving water composition. In the southern area, the thermal waters are influenced by inputs of both CO₂ and H₂S, whereas in the northeastern area waters, only CO₂ appears to dominate. Structural controls imposed by the Liquiñe-Ofqui Fault System and the Andean Transverse Faults play a predominant role in directing the ascent of gases, and influencing the geochemical signatures of the thermal waters. This study provides an update about the hydrothermal system related to Villarrica volcano, where new sampling sites were analyzed for the first time and new processes were recognized, constraining the hydrogeochemical model of the geothermal system.