Geochemical survey of the Colpitas-Taapaca volcanic-hydrothermal system, northern Chile

This work presents chemical and isotopic (delta13C-CO2, delta13C-CH4,3He, 4He, 20Ne, 40Ar, 36Ar, delta18O and deltaD) data on fluid discharges fromthe Colpitas-Taapaca volcanic-hydrothermal system, located closeto the Taapaca Volcanic Complex, with the aim to investigate thephysical-chemical conditions of the fluid source and to provide apreliminary evaluation of the geothermic potential of the studyarea. Colpitas thermal springs (to 56 °C) and part of the coldsprings (<=18°C) from this area have a Na+-Cl- composition andTotal Dissolved Solids (TDS) values (from 6,059 to 19,118 mg/L).Putre springs also show a Na+-Cl- composition, TDS values up to7,887 mg/L, and outlet temperatures from 21 to 31 °C. Colpitascold springs, with a Ca2+-SO42- composition and relatively lowTDS values (<=1,350 mg/L), are likely produced by interaction ofshallow water with uprising H2S-rich hydrothermal gases. Thisprocess is likely also controlling the chemistry of Jurase thermalsprings, which have the highest outlet temperatures of the studyarea (up to 68 °C), a Ca2+-SO42- composition and TDS values <=2,355mg/L. Eventually, Las Cuevas springs have temperatures up to 36°C, a Na+-HCO3- composition and low TDS values (<=1,067 mg/L),typical features of springs related to a shallow aquifer. The delta18OH2O and deltaD-H2O values indicate that all waters have a dominantmeteoric origin. Enrichments in 18O and D shown by Colpitasand Putre thermal waters are likely due to steam loss and waterrockinteraction, masking a possible direct steam contributionfrom magmatic degassing. Gas emissions from Colpitas bubblingpools are dominated by CO2, with significant concentrations ofCH4, H2S and H2. The Rc/Ra values (up to 2.04) of Colpitas gasesindicate a significant contribution of magmatic to mantle He,whereas the high CO2/3He ratios, combined with delta13C-CO2 valuesranging from -7.66 to -5.63 %o vs. PDB, imply a dominant crustalCO2 source, mostly involving limestone. Estimated temperaturesbased on the composition of waters and gases from Colpitas areup to 215 °C. Higher temperatures (240 °C) are estimated forPutre thermal waters, although these waters, as well as those fromJurase and Las Cuevas, are too immature for a reliable applicationof geothermometric techniques. Based on the theoretical reservoirtemperature and the measured Cl total output, the thermal energyreleased from Colpitas thermal area is estimated at up to 13.9 Mw.Such results suggest the occurrence of a promising heat source,possibly related to Taapaca volcanic complex, and encourage thedevelopment of future research based on combined geophysical andgeochemical approaches, in order to provide a reliable evaluationof the geothermal potential of the whole area.