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

We investigate the chemical and isotopic composition of water and gas thermal discharges from six hydrothermal systems in the Tarapacà 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 SO4 2--rich waters also occur) to Na+(Ca2+)-Cl-(SO4 2-) (Pampa Lirima), Ca2+-SO4 2-(HCO3 -) (Torta de Tocorpuri) and Ca2+-SO4 2- (Pampa Apacheta). The gas seeps are characterized by the dominance of CO2, H2S and CH4 with N2/Ar ratios that occasionally are up to 450. Significant amounts of SO2 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 SO2 and HCl. On the contrary, low equilibrium temperatures were calculated for the Surire fluids (b200 °C), suggesting interactions of deep-originated fluids with meteoric water at shallow depth. Shallow, low temperature aquifers mask any signal of deep fluids in the Pampa Lirima and Torta de Tocorcupi thermal discharges. Despite the variations in temperatures, He isotopic compositions and redox conditions of the fluid reservoirs consistently indicate a significant contributionfrom magmatic degassing. Although geophysical studies are necessary to better constrain the geothermal potential of northern Chile, the geothermometric estimations carried out in the present study indicate that El Tatio, Pampa Apacheta, Surire and Puchuldiza-Tuja can be regarded as the most promising areas and deserve more detailed geological investigations.