The Cordon 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 degrees 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 Cord(m Caulle have He-3/He-4 ratios typical of subduction margins (6-7 R-A) and N-2/Ar ratios of about 40, indicating deep convection of air-saturated groundwater. Fumaroles at the southwestern border have N-2/Ar ratios >300, suggesting the presence of a deep volcanic component. Gas ratios of fumarole discharges yield equilibration temperatures >300 degrees C, whereas those of hot spring waters suggest temperatures of about 160 degrees 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. (C) 2007 CNR. Published by Elsevier Ltd. All rights reserved.