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

Manuel Inostroza (1,2), Franco Tassi (3,4), José Sepúlveda (3), Francesco Capecchiacci (3), Andrea L. Rizzo (5) & Felipe Aguilera (2,6,7)
(1) Programa de Doctorado en Ciencias, mención Geología, Universidad Católica del Norte, Angamos 0610, Antofagasta, Chile. (2) Núcleo de Investigación en Riesgo Volcánico - Ckelar Volcanes, Universidad Católica del Norte, Angamos 610, Antofagasta, Chile. (3) Dipartimento di Scienze della Terra, Università di Firenze, via G. la Pira 4, Firenze, Italia. (4) CNR-IGG Institute of Geosciences and Earth Resources, via G. la Pira 4, Florence, Italy. (5) Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Palermo, Via Ugo La Malfa 153, 90146, Palermo, Italy. (6) Departamento de Ciencias Geológicas, Universidad Católica del Norte, Angamos 610, Antofagasta, Chile. (7) Centro de Investigación para la Gestión Integrada de Riesgo de Desastres (CIGIDEN), Avenida Vicuña Mackena 4860, Macul, Santiago, Chile. Corresponding author e-mail: manuel.inostroza@alumnos.ucn.cl

This work presents chemical and isotopic (δ¹³C-CO₂, δ¹³C-CH₄, ³He, ⁴He, ²⁰Ne, ⁴⁰Ar, ³⁶Ar, δ¹⁸O and δD) data of fluid discharges from Colpitas-Taapaca volcanic-hydrothermal system, located close to the Taapaca Volcanic Complex, with the aim to investigate the physical-chemical conditions of the fluid source and to provide a preliminary evaluation of the geothermic potential of the study area. Colpitas thermal springs (to 56 °C) and part of the cold springs (≤18°C) from this area have a Na⁺-Cl^- composition and Total Dissolved Solids (TDS) values (from 6,059 to 19,118 mg/L). Putre springs also show a Na ⁺-Cl ^-composition, TDS values up to 7,887 mg/L, and outlet temperatures from 21 to 31 °C. Colpitas cold springs, with a Ca²⁺-SO₄^2- composition and relatively low TDS values (≤1,350 mg/L), are likely produced by interaction of shallow water with uprising H₂S- rich hydrothermal gases. This process is likely also controlling the chemistry of Jurase thermal springs, which have the highest outlet temperatures of the study area (up to 68 °C), a Ca²⁺-SO₄^2- composition and TDS values ≤2,355 mg/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 δ¹⁸O-H₂O and δD-H₂O values indicate that all waters have a dominant meteoric origin. Enrichments in ¹⁸O and D shown by Colpitas and Putre thermal waters are likely due to steam loss and water- rock interaction, masking a possible direct steam contribution from magmatic degassing. Gas emissions from Colpitas bubbling pools are dominated by CO₂, with significant concentrations of CH₄, H₂S and H₂. The Rc/Ra values (up to 2.04) of Colpitas gases indicate a significant contribution of magmatic to mantle He, whereas the high CO₂/ ³He ratios, combined with d ¹³C-CO₂ values ranging from -7.66 to -5.63 ‰ vs. PDB, imply a dominant crustal CO₂ source, mostly involving limestone. Estimated temperatures based on the composition of waters and gases from Colpitas are up to 215 °C. Higher temperatures (240 °C) are estimated for Putre thermal waters, although these waters, as well as those from Jurase and Las Cuevas, are too immature for a reliable application of geothermometric techniques. Based on the theoretical reservoir temperature and the measured Cl total output, the thermal energy released 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 the development of future research based on combined geophysical and geochemical approaches, in order to provide a reliable evaluation of the geothermal potential of the whole area.

Colpitas-Taapaca geothermal system, fluid geochemistry, volcanic-hydrothermal system, geothermal potential.