The δ34S composition of sulfates and sulfides at the Los Humeros geothermal system, Mexico and their application to physicochemical fluid evolution

The δ34S isotopic composition of sulfur was determined in more than 105 pyrite samples found in volcanic formations as well as in the sulfates and sulfides dissolved in the present-day geothermal fluids in the Los Humeros system, Mexico. Analysis of the isotopic values demonstrated that the sulfur compounds of the geothermal system were derived from a magmatic source (δ34SΣS — 1%.). The calculation of the different pH-FO2-(FS2) diagrams showed that the sulfates and sulfides dissolved in the present-day fluids from well H1 do not show chemical equilibrium conditions as was indicated previuosly by Arnold and Gonzalez-P. (1987). The reason for this is that the physicochemical characteristics of the system have been evolving almost continuously as a result of the exploration and exploitation of the thermal fluids from the system. The residence time of the fluids in the geothermal reservoir is now reduced and the chemical and isotopic reactions that occur between fluids and minerals are not carried out completely. Due to the thermodynamic evolution of the fluids, equilibrium among the sulfur phases dissolved in the fluids could not be demonstrated. The δ34S values of pyrite sampled at different depths in the geothermal system display evidence for different isotopic fractionation produced by boiling, fluid mixing, and vapor condensation in meteoric waters. The δ34S values of sulfates in the present-day fluids suggest that these were derived from the oxidation of H2S at relatively shallow depths (< 600 m). In fact, the isotopic compositions of these sulfates trend towards δ34S values of sulfides found in the steam phase.