Sealing capacity of clay-cap units above the Cerro Pabellón hidden geothermal system (northern Chile) derived by soil CO2 flux and temperature measurements

Abstract

Cerro Pabellón is a hidden high-enthalpy geothermal system (reservoir temperature up to 250°C) located in the Pampa Apacheta (northern Chile), where the first geothermal power plant in South America is currently operating. A fumarolic field, located on the summit of Apacheta Volcano, about 3.5kmW of Pampa Apacheta, is the only evidence of the presence of a degassing magma connected to the surface. CO2 soil flux and temperatures, on a grid of ~430 points, were measured to understand the absence of any other thermal manifestations. More than 90% of the measured CO2 flux values were below background, estimated at 1.35g·m−2·day−1 whereas the associated soil temperatures (mean value=19.4°C) carried out at ~10cm depth were related, at least partially, to an endogenous, though weak, thermal anomaly. No spatial correlation was observed between soil CO2 flux and temperature measurements, suggesting a minimal role played by the volcano-tectonic structures in the circulation of geothermal fluids. Soil temperature gradients measured on a grid of 85 points in the active fumarolic field (where acid waters and magmatic gases are discharged) revealed the only strong thermal anomaly (temperatures up to ~83°C) of the area. The thick argillic clay-cap (~300m) located above the geothermal reservoir, has a pivotal role in preventing the resurgence of fluids, whereas an efficient interaction between the rising magmatic fluids and the liquid-dominated reservoir favours acidic gas scrubbing processes, thus allowing the ascent of a steam depleted in magmatic compounds. This steam interacts with the shallower, discontinuous aquifer(s) below Pampa Apacheta, with the consequent nearly complete dissolution of CO2 and H2S, and heating of the shallow groundwater. This results in low CO2 fluxes and extended slight thermal anomaly at the surface. Finally, an integrated conceptual model using our results and available geological, mineralogical, geochemical and geophysical data of the Cerro Pabellón hidden geothermal system is presented, with the aim of providing criteria for geothermal exploration of similar high-enthalpy geothermal systems in the Andes that also exhibit a lack of thermal surface features.