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

摘要

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 (0degreesC). and salinities of 0.8 wt% NaCl equivalent. The homogenization temperature (T-h) = 325 +/- 5 degreesC. The boiling zone shows T-h = +/-300 degreesC 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 CO2. Positive clathrate melting temperatures (fusion) with Th = 150 degreesC 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 CO2 (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 (delta(18)O-deltaD) of the geothermal brine indicate mixing between meteoric water and a minor magmatic component. The enrichment in delta(18)O is due to the rock-water interaction at relatively high temperatures. delta(13)C stable isotope data show a magmatic source with a minor meteoric contribution for CO2. The initial isotopic value (delta(34)S(RES) = -2.3%o, which implies a magmatic source. More negative values are observed for shallow pyrite and range from delta(34)S (FeS2) = -4%o to -4.9%o indicating boiling. The same fractionation tendencies are observed for fluids in the reservoir from results for delta(18)O. (C) 2004 Elsevier Ltd. All rights reserved.