Fluid geochemistry of hydrothermal systems in the Arica-Parinacota, Tarapacá and Antofagasta regions (northern Chile)

Abstract We investigate the chemical and isotopic composition of water and gas thermal discharges from six hydrothermal systems in the Tarapaca and Antofagasta regions (northern Chile): Surire, Puchuldiza-Tuja, Pampa Lirima, Pampa Apacheta, El Tatio and Torta de Tocorpuri, to determine the chemical–physical conditions at the fluid source. The chemical facies of the thermal discharges vary from Na + –Cl − (El Tatio, Puchuldiza-Tuja and part of Surire where SO 4 2 − -rich waters also occur) to Na + (Ca 2+ )–Cl − (SO 4 2 − ) (Pampa Lirima), Ca 2+ –SO 4 2 − (HCO 3 − ) (Torta de Tocorpuri) and Ca 2+ –SO 4 2 − (Pampa Apacheta). The gas seeps are characterized by the dominance of CO 2 , H 2 S and CH 4 with N 2 /Ar ratios that occasionally are up to 450. Significant amounts of SO 2 and HCl are present at Pampa Apacheta. Water and gas geothermometry suggests that El Tatio and Puchuldiza-Tuja fluid reservoirs have relatively high reservoir equilibrium temperature (up to 270 °C). Gases from Pampa Apacheta apparently equilibrated at unusually high temperature for hydrothermal fluids (up to 350 °C), being likely related to an active magmatic system, as testified by the presence of highly acidic gas compounds such as SO 2 and HCl. On the contrary, low equilibrium temperatures were calculated for the Surire fluids (