Proceedings World Geothermal Congress 2005 Antalya, Turkey, 24-29 April 2005 1 Geothermal Exploration in the Cordón Caulle Region, Southern Chile Fabián Sepúlveda 1 , Alfredo Lahsen 1 , Klaus Dorsch 2 , Carlos Palacios 1 and Steffen Bender 2 1 Department of Geology, University of Chile, P.O Box 13518, (21) Santiago, Chile 2 Department of Geo- and Environmental Science, Ludwig-Maximilians University, Luisenstr. 3780333 Munich, Germany fsepulve@cec.uchile.cl Keywords: steam-heated, R H , geochemistry, Chile. ABSTRACT The Cordón 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 (N 2 /Ar ratios of ~ 40), whereas fumaroles from the southwestern edge (area of El Azufral) are believed to be dilute volcanic fumaroles (N 2 /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 (R H factor = fH 2 /fH 2 O = -3.2), in comparison with the usual and expected redox state of a mature, rock-buffered system (R H = -2.8). Boiling springs occur at the northwest tip of the Cordón 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/HCO 3 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 δ 18 O-δD 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 (H 2 -Ar, CO-CO 2 and CH 4 -CO 2 ), temperatures greater than 260°C and up to 330°C are estimated for the deep reservoir. 1. INTRODUCTION Chile represents one of the largest undeveloped geothermal provinces of the world. Geothermal areas in Chile are closely related to Quaternary volcanism. The volcanic- geothermal activity is primarily controlled by the convergence of the Nazca and South-American plates. Active volcanism splits into two main margin-parallel volcanic belts, namely, the Northern Volcanic Zone (NVZ: 17°-28°S) and the Southern Volcanic Zone (SVZ: 33°- 46°S). Detailed geothermal investigations in Chile, including drilling and feasibility studies, date back to the mid 1970’s. At that time, a CORFO-UNDP geothermal program explored northern Chile leading to the identification of several geothermal prospects such as El Tatio, Surire and Puchuldiza (Lahsen and Trujillo, 1975; Lahsen, 1976; Lahsen, 2005, this congress). By the mid 90’s, geothermal exploration was resumed by ENAP (National Oil Company) both in northern and southern Chile. In the SVZ, investigations were focused on the geothermal prospects of Calabozos (35.5°S; Grunder at al., 1987) and Nevados de Chillán (Dixon et al., 1999; Fig. 1). In 1994, a 270 m deep gradient well was drilled in the Nevados de Chillán area encountering wet steam with temperatures of 198°C (Salgado and Raasch, 2002). Later on, a compilation of geochemical data from all the thermal areas of Chile was conducted by the National Geological Survey of Chile (Hauser, 1997; Pérez, 1999), but a thorough interpretation of the structure of the geothermal systems of the SVZ was lacking. The University of Chile initiated a geothermal research program aimed at assessing geothermal resources of southern Chile, giving emphasis to the areas of Nevados de Chillán (36.9°S; Sepúlveda and Lahsen, 2003) and Puyehue-Cordón Caulle (40.5°S; Sepúlveda et al., 2004a). Only in Puyehue-Cordón Caulle and Nevados de Chillán, more than 40 MWt and 100 MWt, respectively, have been reported in association with the surface manifestations (Sepúlveda and Lahsen, 2003; Sepúlveda et al., 2004a). The presence of vigorous fumaroles and large outflows of low- chloride springs was found to be a common feature of Nevados de Chillán and Puyehue-Cordón Caulle. This is distinct from northern Chile were chloride springs are common (e.g. El Tatio, Surire and Puchuldiza; Lahsen, 1976, 1988). Sepúlveda et al. (2004a) presented a conceptual model for the Puyehue-Cordón Caulle geothermal system, distinguishing two main upflows separated by more than 15 km. The northernmost upflow (i.e. Cordón Caulle) was interpreted to be related to a vapor-dominated system overlain by a secondary steam-heated aquifer, with the surface expression of the latter being an outflow of 100 l/s of low-chloride bicarbonate boiling springs. Na-K and silica geothermometers suggested subsurface temperatures of the steam-heated aquifer of about 170-180°C. Equilibration temperatures of the main reservoir, however, remained unconstrained. The southernmost upflow, expressed at surface through a number of hot springs collectively referred to as Puyehue-Aguas Calientes, was found to have a high degree of dilution, hampering a confident application of aqueous geothemometers. In this study, gas chemistry data obtained from fumaroles and hot springs are used to provide a better understanding of the Cordón Caulle geothermal system. 2. GEOLOGICAL SETTING The deep stratigraphy of the Cordón Caulle geothermal system can be inferred from rocks exposures found within and around the perimeter of the geothermal area. Quaternary volcanic rocks are the most widespread, followed by Miocene intrusions, which are found as prominent islands inside neighboring lakes, to the east along regional, intra-arc faults systems, and as accessory fragments of strombolian deposits, which are widely recognized north and south of the Cordón Caulle area,