Hydro-geochemical and isotopic fluid evolution of the Los Azufres geothermal field, Central Mexico

The authors interpret the evolution of this system as deep liquid water boiling when ascending through fractures connected to the surface. Boiling is caused by a drop of pressure, which favors an increase in the steam phase within the brine ascending towards the surface. During this ascent, the fluid becomes steam-dominant in the shallowest zone, and mixes with meteoric water in perched aquifers. Stable isotope compositions (δ18O-δD) of the geothermal brine indicate mixing between meteoric water and a minor magmatic component. The enrichment in δ18O is due to the rock-water interaction at relatively high temperatures. δ13C stable isotope data show a magmatic source with a minor meteoric contribution for CO2. The initial isotopic value δ34SRES = −2.3‰, which implies a magmatic source. More negative values are observed for shallow pyrite and range from δ34S (FeS2) = −4‰ to −4.9‰, indicating boiling. The same fractionation tendencies are observed for fluids in the reservoir from results for δ18O.