Abstract
This work presents a review of the geological, hydrogeochemical, and mineralogical information related to the Los Azufres geothermal field, along with new isotopic data from hydrothermal calcite and quartz. The equilibrium state between present-day geothermal fluids and hydrothermal mineral phases in this field was examined using mineralogical, thermochemical, as well as isotopic data. Hydrothermal minerals in Los Azufres are typical for neutral to alkaline, high-temperature thermal fluids, forming an incipient to complete propylitic alteration of the primary rocks. The comparison of activity diagrams of common hydrothermal phases to mineralogical observations at Los Azufres shows that the reservoir rocks tend to approach a state of chemical equilibrium with hydrothermal fluids percolating through them. This tendency to equilibrium has been corroborated by analysis of the oxygen isotopic composition of hydrothermal calcite, whose σ18O values are in equilibrium with present geothermal or meteoric waters. Although 18O/16O ratios from some hydrothermal quartz samples show equilibrium with present-day fluids, others indicate the presence of fluids with higher 18O/16O ratios than modern geothermal water. The oxygen isotopic composition of hydrothermal quartz can thus be used as an indicator of isotopic compositions of fluids at the time of quartz precipitation.
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