Hydrogeochemical controls on thermal springs around Villarrica volcano

Abstract

Villarrica volcano is one of the most extensively studiedactive volcanic systems in the South American Andean range. However, thelinkage between the volcano and its surrounding thermal waters remains poorlyunderstood. In this study, we present a comprehensive geochemical investigationbased on 52 samples of water and 9 dissolved gas samples collected from 18sites during three field campaigns. Integrated analyses of physicochemicalparameters, major ion species, minor elements, stable isotope ratios, and gascompositions have allowed us to identify and differentiate distincthydrofacies. Our findings reveal that water-rock interaction, principallythrough plagioclase dissolution leading to Na enrichment, is the predominantprocess controlling the hydrochemistry. Furthermore, while the aquifers areheated by a deep magmatic reservoir and by the regional geothermal gradient,interactions with ascending hydrothermal gases are critical to the evolvingwater composition. In the southern area, the thermal waters are influenced byinputs of both CO2 and H2S, whereas in the northeastern area waters, only CO2appears to dominate. Structural controls imposed by the Liquiñe-Ofqui FaultSystem and the Andean Transverse Faults play a predominant role in directingthe ascent of gases, and influencing the geochemical signatures of the thermalwaters. This study provides an update about the hydrothermal system related toVillarrica volcano, where new sampling sites were analyzed for the first timeand new processes were recognized, constraining the hydrogeochemical model ofthe geothermal system.