A conceptual model for the Tufino-Chiles-Cerro Negro (TCCN) geothermal system (Ecuador-Colombia): New insights into geothermal exploration from chemical and isotopic composition of hydrothermal fluids

Abstract

The Tufino-Chiles-Cerro Negro (TCCN), located on the border between Colombia and Ecuador, is one of the most promising geothermal systems in the Andean Northern Volcanic Zone, presenting an estimated geothermal potential of 130-138 MWe associated with two reservoirs located at 500-1000 m, and > 1500 m of depth, respectively. In this work, we present a geochemical conceptual model for TCCN to explore the physicochemical conditions acting on the reservoir, based on a comprehensive dataset of water, gas geochemistry, and stable isotopes of the hydrothermal manifestations of the geothermal system, integrated with previously published data. Four new water samples and two bubbling gas samples were collected from the TCCN area during fieldwork in January 2015. Waters from TCCN, which show temperatures up to 54.7 degrees C, are near-neutral to slightly acidic and have variable TDS values (687 to 3095 mg/L) that generally increase at increasing temperature. All the samples fall far from the chloride-mature waters field, being characterised by SO42- or HCO3- dominated composition, suggesting steam-heated and peripheral water characteristics. The He-3/He-4 ratios (R/Ra) of gas samples range between 2.4 and 4.6, indicating an important mantle He contribution diluted by the addition of radiogenic and atmospheric He-4. Based on their CO2/He-3 and delta C-13-CO2 ratios, gas samples show a carbon origin prevailing from subducted limestone, likely connected to the presence of the subducting Carnegie ridge. A mixing between a thermogenic and biogenic origin for methane is suggested, consistent with the dominant inputs from microbial methanogenesis and thermal maturation of sedimentary organic matter in the TCCN system. The high delta S-34-H2S values of gas (+6.7 parts per thousand and 10.7 parts per thousand vs. V-CDT) and delta S-34-SO4 of water (+19.2 parts per thousand and + 19.3 parts per thousand vs. V -CDT) samples coupled with the high values of S/He-3 indicate that the sulphur signature could be mostly controlled by the incorporation of subducted sulphate. Gas geothermometric estimations in the CO2-H-2 system indicate a temperature between 245 and 250 degrees C for the geothermal reservoir. The gas samples appear to be in disequilibrium concerning the redox conditions related to the FeO/FeO1.5 buffer typical for hydrothermal sys-tems, implying that these samples represent immature vapours. This could be explained by the input of magmatic fluids into the roots of the TCCN geothermal systems, possibly related to the seismic crisis linked to the unrest of the volcanic complex at the end of 2014.