Crustal Deformation Effects on the Chemical Evolution of Geothermal Systems: Case Studies from Southern Andes

Abstract

A better understanding of the chemical evolution of fluids in geothermal and hydrothermal systems requires data-basedknowledge regarding the interplay between active tectonics and fluid flow. The Southern Andes volcanic zone is one of thebest natural laboratories to address this issue because of the occurrence of numerous geothermal areas, recent seismic activitygenerated by regional fault systems, and intense volcanic activity. Geothermal systems have been understudied in this area,and limited scientific information exists about the role of local kinematic conditions on fluid flow and mineralization duringthe development and evolution of geothermal reservoirs. In this study, we provide data for a 1:200,000 scale geological andstructural map of the Villarrica–Chihuio area as a setting in which to perform a structural analysis of active geothermal areas.This structural analysis, combined with geochemical modelling of hot spring data, allows the identification of two magmatictectonic-geothermal domains based on fault systems, volcanic activity, and lithologies. The Liquiñe–Ofqui fault system(LOFS) domain encompasses geothermal areas located either along the master or subsidiary faults. These are favourablyorientated for shear and extension, respectively. In the LOFS domain, the geochemistry of hot spring discharges is controlledby interaction with the crystalline basement, and is characterized by low B/Cl conservative element ratios and high pH.In marked contrast, the arc-oblique long-lived fault systems (ALFS) domain includes geothermal occurrences located on theflanks of volcanoes forming WNW-trending alignments; these systems are built over faults that promote the development ofcrustal magma reservoirs. Unlike the first domain, the fluid chemistry of these geothermal discharges is strongly controlledby volcanic host rocks, and is typified by lower pH and higher B/Cl ratios. Reaction path modelling supports our model:chemical evolution of geothermal fluids in the Villarrica–Chihuio area is strongly dependent on structurally controlled mechanismsof heat transfer. Within this framework, heat transfer by conduction is responsible for the LOFS domain, whereasmagmatically enhanced advective transport dominates heat flow in the ALFS domain. Although more studies are neededto constrain the complex interplay between tectonics and fluid flow, results from this study provide new insights towardsefficient exploration strategies of geothermal resources in Southern Chile.