Thermobarometry of hydrothermal alteration in the Los Azufres geothermal system (Michoacan, Mexico): Significance of fluid-inclusion data

Abstract

Fluid inclusions were analysed systematically in samples from different wells and depths of the Los Azufres (Mexico) active hydrothermal system. Fluids were studied either in authigenic minerals (primary inclusions) in quartz, calcite or anhydrite, or in magmatic minerals (secondary inclusions in microcracks). The fluids are aqueous and have low salinity, ∼1 eq wt.% NaCl. They were trapped in P- T conditions very close to that fixed by the liquid-vapor equilibrium curve. Fluid pressure is lower than the calculated hydrostatic one assuming a liquid-dominated system, because of the existence of a two-phase vapor-dominated zone at a depth between 500 and 1500 m. Temperature was estimated also by a good number of techniques. Present-day distribution of temperature is given by direct measurements in the wells by Kuster® equipment. Downhole temperature may be estimated using different dissolved species geothermometers and the chemical composition of the produced fluids. The present-day measurements above the downhole give in general lower estimates than those obtained by other means. However, agreement between downhole temperature, chemical geothermometer estimates and fluid-inclusion data is relatively good.