Structural control in the distribution of hydrothermal alteration at surface of an active geothermal system: the Nevados de Chillan Geothermal System as a case study

Abstract

Geothermal energy must be relevant in transitioning from non-renewable to renewable energy sources. Then, understanding geothermal systems through their surface hydrothermal expressions and fluid flow in crustal rocks is essential for resource exploitation and exploration. The Nevados de Chillan Geothermal System (NChGS), located in the Chilean active Nevados de Chillan Volcanic Complex (NChVC), is proposed to be hosted in Miocene fractured granitoid where secondary permeability may favor the circulation of hydrothermal fluids. This work focuses on the surface expression of the NChGS, where an intense hydrothermal alteration can be observed. The surface temperature was measured with a Hanna HI 98509 thermocouple and Fluke TiS45 infrared camera. The temperature distribution was analyzed using IDW interpolation in ArcGIS software. A total of 91 surface hydrothermal alteration samples were identified by XRD and quantified using EVAS and TOPAS software. Fault and fractures (n=36) were measured in the c. 1 km2 sampled area. Results indicate an intense argillic patch-type alteration pattern characterized by chalcedony (Chc), native sulfur (S), alunite (Alu), kaolinite (kln), illite (I), interstratified illite-smectite (I/S), iron oxides (IO), tridymite (Try), Gypsum (Gyp) and hydrated sulfates. In most cases, the alteration is coupled by high temperatures (94-95°C) and water/steam discharges such as fumaroles, vents, hot springs, muds pools, and heated soils. Four directions of fault families were identified: N0-20E/subvertical, N50-80W/subvertical, N55-60E/30E-76W, and N10-25W/20-30E. Kinematic indicators are scarce due to intense alteration; however, these families have been correlated with the regional-scale data. The highest measured temperature and a significant quantity of Chc, S, Alu, Kln, Gyp, and IO are distributed along NNW and WNW directions. In contrast, the presence of I and I/S is only found in reverse fault planes. These results suggest that fluid flow at NChGS is mainly controlled by secondary permeability, particularly subvertical WNW and NNW faults. Also, our results highlight how the distribution of surface hydrothermal alteration could be a window for exploring active geothermal systems. The Chilean ANID Fondecyt Regular #1220729 grant supports this work.