Physical, Chemical and Mineralogical Evolution of the Tolhuaca Geothermal System, Southern Andes, Chile

The interplay between heat-fluid-rock interaction processes and brittle deformation is fundamental in the development of geothermal systems. However, the feedback mechanisms of such interplay and their effects on the duration and thermal structure of the geothermal system are poorly constrained. The work presented here address this issue by studying the physical, chemical and mineralogical evolution of the active Tolhuaca geothermal system in the Andes of southern Chile. We used temperature measurements in the deep wells and geochemical analyses of borehole fluid samples to constrain present-day fluid conditions. In addition, we used paleo-fluid temperatures and chemistry from microthermometry, and LA-ICP-MS analysis of fluid inclusions taken from wellconstrained parageneses in vein samples retrieved from the Tol-1 borehole core. Complementarily, we investigate the effect that fluid chemistry changes can have on the zonation sulphides by using quantitative micro-analyses of pyrite at grain-scale of samples retrieved from Tol-1. We also analyse the effect of the development of a low-cohesion low-permeability clay cap on the long-term thermal structure of the system by using numerical simulations of heat and fluid flows. These chemical and mineralogical analyses are integrated with a structural study of the geometry and kinematics of the faults and fractures from the Tol-1 borehole. Results show that hydrothermal alteration at Tolhuaca has produced structural and hydrological compartmentalisation of the system by developing an upper, low-permeability and low-cohesion clay-rich zone. In this zone, the formation of extensional fractures is inhibited, contributing to sustaining low-permeability conditions. Numerical simulations of heat and fluid flow support our observations and suggest that the presence of a low permeability clay-cap triplicates the duration of high enthalpy conditions in the deep upflow zone of Tolhuaca. These data reveal that the dynamic interplay between fluid flow, crack-seal processes and hydrothermal alteration are key factors in the evolution of the reservoir at the flank of the Tolhuaca volcano. Our results show that the favourability for developing geothermal resources in the southern Andes is highly dependent on such dynamic interplay.