Gas geochemistry of the Cordón Caulle geothermal system, Southern Chile

The Cordón Caulle geothermal system is located in a NW-trending volcano-tectonic depression of the Southern Andean Volcanic Zone of Chile. Outflows of low chloride water were previously interpreted as the surface expression of a shallow steam-heated aquifer, with subsurface temperatures of 150–170 °C. Gas data from fumaroles and hot springs have been used to assess the nature and temperature of the deeper, underlying geothermal reservoir. Fumaroles at the northeastern border of Cordón Caulle have 3He/4He ratios typical of subduction margins (6–7 RA) and N2/Ar ratios of about 40, indicating deep convection of air-saturated groundwater. Fumaroles at the southwestern border have N2/Ar ratios >300, suggesting the presence of a deep volcanic component. Gas ratios of fumarole discharges yield equilibration temperatures >300 °C, whereas those of hot spring waters suggest temperatures of about 160 °C. Based on these data, and comparisons with well documented liquid and vapor-dominated geothermal systems, a model is proposed of a boiling liquid-dominated geothermal system overlain by a secondary steam-heated aquifer.