Light hydrocarbons as redox and temperature indicators in the geothermal field of El Tatio (northern Chile)

El Tatio (northern Chile), one of the largest geothermal fields of South America, is presently undergoing a new program of geothermal exploration, after the failure of the first exploration phase in the early 1970s. The geochemical features of the fluid discharges characterizing this system mainly consist of boiling pools and fumaroles, and represent the result of a complex mixing process involving 3 main components: (i) hydrothermal; (ii) atmospheric; (iii) magmatic. Chemical reactions involving light hydrocarbons equilibrate at higher temperature than those directly measured in the geothermal wells and calculated on the basis of the composition of the inorganic gas species. This suggests that in the deeper parts of the hydrothermal system temperatures higher than 300 °C may be achieved. Such results can have a strong impact for the evaluation of the potential resources of this geothermal system. Moreover, the chemical characteristics of the organic gas fraction allow the assessment of the chemical–physical conditions governing the geochemical processes acting on geothermal fluids at depth.