Stable Isotope Composition of Hydrothermally Altered Rocks and Hydrothermal Minerals at the Los Azufres Geothermal Field, Mexico


Abstract: The Los Azufres geothermal field is the second most important geothermal field for electricity production in Mexico, with a total installed capacity of 188 MW. Hydrothermal alteration studies have been an important tool for geothermal exploration and development of the field, but little attention has been given to the geochemical and isotopic characterization of hydrothermal minerals. \delta^1^8O, \delta^2H, and \delta^1^3C systematics at Los Azufres geothermal field were investigated using whole rock samples, as well as hydrothermal minerals separates, obtained from different depths in the wells Az-26 and Az-52. Most \delta^1^8O values reproduce well the present in-situ field temperatures and isotopic composition of geothermal fluids or local meteoric water. Temperature seems to be the most important factor controlling the oxygen isotope composition of reservoir rocks. A vertical correlation with decreasing \delta^1^8O values and increasing temperature is given for both well profiles. Most analyzed calcites have isotope ratios close to or in isotopic equilibrium with present geothermal or meteoric water at in-situ temperatures. A good correlation between lower calcite \delta^1^8O values and high W/R ratios indicate that oxygen isotopic composition of calcite might constitute a tool for identifying areas of high permeability in the geothermal system of Los Azufres. In contrast, the disequilibrium for some quartz samples suggests the presence of reservoir fluids significantly enriched in ^1^8O (\delta^1^8O values about 8%_o higher than those of present geothermal fluids) at the time of quartz deposition.

Keywords: hydrothermal alteration, hydrothermal minerals, oxygen, hydrogen and carbon stable istotopes, geothermal systems, Los Azufres

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