Evolution of the hydrothermal system at Los Azufres, Mexico, based on petrologic, fluid inclusion and isotopic data

The Los Azufres geothermal reservoir, formed in a fractured, Upper Miocene to Pliocene andesite and basalt base complex, is sealed to the surface by a silicic, mainly rhyolitic sequence of Pleistocene age. Almost the entire sequence is affected by hydrothermal alteration to varying extent. Petrological and fluid inclusion studies confirmed vertical zonation of the reservoir by secondary processes: Hydrothermal alteration processes under low temperature conditions (<170°C) caused argillitization of the shallow zone of the reservoir (depth <500 m). Smectite, zeolite, calcite, and chlorite predominate the mineral assemblage in the shallow zone. epth of 1200–1500 m, the maximum ice melting temperatures (Tmi) values of −0.7 to −4°C and salinities of 6.4 wt% NaCl eq. indicate boiling conditions of the geothermal brine in a vapor-rich zone. Chlorite, calcite, quartz, zeolite, anhydrite, albite, sphene, pyrite, hematite, and illite form the hydrothermal mineral paragenesis with maximum temperatures of 250°C and pressure conditions of 150 bar. Below 1500 m, Tmi reach maximum values of −0.1°C and low salinities of 0.2 wt% NaCl eq. The minerals epidote, amphibole, prehnite, and garnet indicate temperatures above 250°C and pressure conditions between 150 and 200 bar. The measured homogenization temperatures (Th) of fluid inclusions (FI) are consistently higher than the in-situ measured temperatures, which indicates retrograde cooling of the Los Azufres geothermal reservoir since the time of the hydrothermal mineral formation. ng temperature isotherms at the center of the geothermal field (below well Az-9, Az-23, and Az-25) indicate the heating up of the fluids by a shallow magma chamber. Stable isotope data (δ18O, δD) of the geothermal brine indicates mixing processes between meteoric water and a minor magmatic component. Secondary reactions forming sericite may have caused a further positive shift in 18O in the shallow part of the geothermal reservoir.