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 s equence is affected by hydrothermal alteration to varying extent. Petrologi cal and fluid inclusion studies confirmed vertical zonation of the reservoi r by secondary processes: Hydrothermal alteration processes under low tempe rature conditions (<170<degrees>C) caused argillitization of the shallow zo ne of the reservoir (depth <500 m). Smectite, zeolite, calcite, and chlorit e predominate the mineral assemblage in the shallow zone. At a depth of 1200-1500 m, the maximum ice melting temperatures (T-mi) valu es of -0.7 to -4<degrees>C and salinities of 6.4 wt% NaCl eq. indicate boil ing conditions of the geothermal brine in a vapor-rich zone. Chlorite, calc ite, quartz, zeolite, anhydrite, albite, sphene, pyrite, hematite, and illi te form the hydrothermal mineral paragenesis with maximum temperatures of 2 50 degreesC and pressure conditions of 150 bar. Below 1500 m, T-mi reach ma ximum values of -0.1 degreesC and low salinities of 0.2 wt% NaCl eq. The mi nerals epidote, amphibole, prehnite, and garnet indicate temperatures above 250 degreesC and pressure conditions between 150 and 200 bar. The measured homogenization temperatures (T-h) Of fluid inclusions (FI) are consistentl y higher than the in-situ measured temperatures, which indicates retrograde cooling of the Los Azufres geothermal reservoir since the time of the hydr othermal mineral formation. Updoming 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 sh allow magma chamber. Stable isotope data (delta O-18, deltaD) of the geothe rmal brine indicates mixing processes between meteoric water and a minor ma gmatic component. Secondary reactions forming sericite may have caused a fu rther positive shift in O-18 in the shallow part of the geothermal reservoi r. (C) 2000 Elsevier Science B.V. All rights reserved.