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

Abstract

Hydrothermal alteration at Los Azufres geothermal field is mostly propylitic with a progressive dehydration with depth and temperature increase. Argillic and advanced argillic zones overlie the propylitic zone owing to the activity of gases in the system. The deepest fluid inclusions (proto-fluid) are liquid-rich with low salinity, with NaCl dominant fluid type and ice melting temperatures ( T mi) near zero (0 °C), and salinities of 0.8 wt% NaCl equivalent. The homogenization temperature ( T h) = 325 ± 5 °C. The boiling zone shows T h = ±300 °C and apparent salinities between 1 and 4.9 wt% NaCl equivalent, implying a vaporization process and a very important participation of non-condensable gases (NCGs), mostly CO 2. Positive clathrate melting temperatures (fusion) with T h = 150 °C are observed in the upper part of the geothermal reservoir (from 0 to 700 m depth). These could well be the evidence of a high gas concentration. The current water produced at the geothermal wells is NaCl rich (geothermal brine) and is fully equilibrated with the host rock at temperatures between T = 300 and 340 °C. The hot spring waters are acid-sulfate, indicating that they are derived from meteoric water heated by geothermal steam. The NCGs related to the steam dominant zone are composed mostly of CO 2 (80–98% of all the gases). The gases represent between 2 and 9 wt% of the total mass of the fluid of the reservoir. 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 ( δ 18O– δD) of the geothermal brine indicate mixing between meteoric water and a minor magmatic component. The enrichment in δ 18O is due to the rock–water interaction at relatively high temperatures. δ 13C stable isotope data show a magmatic source with a minor meteoric contribution for CO 2. The initial isotopic value δ 34S RES = −2.3‰, which implies a magmatic source. More negative values are observed for shallow pyrite and range from δ 34S (FeS 2) = −4‰ to −4.9‰, indicating boiling. The same fractionation tendencies are observed for fluids in the reservoir from results for δ 18O.