Fluid Inclusion, Hydro-geochemistry and Isotopic Fluid Composition of the Los Azufres Geothermal Field, Central Mexico

Abstract

Hydrothermal alteration at Los Azufres geothermal field is mostly propylitic with a progressive dehydration The temperature increases with depth. Argillic and advanced argillic alteration zones overprint the propylitic zone due to the activity of gases in the system. The deepest fluid inclusions (proto-fluid) are liquid-rich, low salinity fluids dominated by NaCl with calculated salinities around 0.8 wt % NaCl equivalent. The homogenous temperatures (Th) are located around 325 ± 5 C. The boiling zone presents a Th around 300 C and salinities between 1 and 4.9 wt % NaCl equivalent, implying a vaporization process and a very important participation of non-condensable gases (NCG), mostly CO2. The upper part of the geothermal reservoir (from 0 to 700 m depth) presents positive clathrate melting temperatures with Th ? 150 C, which is the evidence of a high gas concentration. The current water produced at the geothermal wells is NaCl rich (geothermal brine) and it is fully equilibrated with the host rock at temperatures between 300?C and 340?C. The hot springs are acid-sulfate, indicating that they are fed by meteoric water heated by geothermal steam. The NCG related to the steam dominant zone are composed mostly by CO2 (80 to 98% of all the gases). The gases represent between 2 to 9 wt % of the total fluid mass of the reservoir.
We interpret the evolution of this system as a deep fluid, mainly water, that boils and circulates through fractures connected to the surface. Boiling is accompanied by a drop of pressure, which in turn favors an increasing steam evolution with the brine ascending towards the surface. During this ascent, the fluid changes from steam-dominated to steam in the shallowest zone, where mixing with perched aquifers and meteoric water occurs. Stable isotope data ( 18Oâ - Dâ) of the geothermal brine indicates the occurrence of mixing between meteoric water and a minor magmatic component. The enrichment in 18Oâ is due to the rock-water interaction at relatively high temperatures.