Geothermal Exploration in the CordÛn Caulle Region, Southern Chile

Abstract

The CordÛn Caulle region (40.5?S) hosts a 15 km long, NW-trending volcanic depression made up of Holocene and historic silicic lavas and pumice deposits overlying Late-Pleistocene basaltic flows. Fumaroles occur at the top of the system (~1500 m.a.s.l.), spatially associated with the edges of the depression. Fumaroles from the northeastern edge (areas of Las Sopas and Los Venados) are interpreted to arise from deeply-convected air-saturated groundwater (N2/Ar ratios of ~ 40), whereas fumaroles from the southwestern edge (area of El Azufral) are believed to be dilute volcanic fumaroles (N2/Ar ratios of ~ 400-500). Deep mixing between the sources of gases of Las Sopas-Los Venados and El Azufral is inferred from the relatively oxidizing conditions prevailing at Las Sopas-Los Venados (RH factor = fH2/fH2O = -3.2), in comparison with the usual and expected redox state of a mature, rock-buffered system (RH = -2.8). Boiling springs occur at the northwest tip of the CordÛn Caulle area (~1000 m.a.s.l.) in close spatial relationship with collapse structures of a 10 km wide, Late-Pleistocene caldera. Boiling springs total an outflow of ~100 l/s and have intriguing chemistry: TDS < 700 mg/l, pH ~ 9, Cl/HCO3 ratios of 0.2 (Cl <30 mg/l), Cl/B ratios of 1, lower Mg (<0.06 mg/l) relative to local meteoric waters (~ 5 mg/l), silica up to 400 mg/l (and widespread silica sinter), and 18O- D values slightly shifted relative to the GMWL. Na-K and pH-corrected silica temperatures in the range of 150-180?C are interpreted to reflect subsurface temperatures of a secondary steam-heated aquifer overlying a main vapor-dominated system. By means of gas geothermometry (H2-Ar, CO-CO2 and CH4-CO2), temperatures greater than 260?C and up to 330?C are estimated for the deep reservoir.