δ18O-shift and water/rock ratios of hydrothermally altered rocks at the Los Azufres geothermal field, Mexico

Abstract

Oxygen isotope investigations were carried out on hydrothermally altered rocks, authigenic quartz and calcite of the active geothermal field Los Azufres, Mexico. The δ18O values of the analysed whole rock samples vary from +2.2%0 to +16.7%0. The δ18O range of the analysed authigenic minerals is +3.93%0 to +20.23%0 for the quartz and +3.38%0 to + 12.23%0 for the calcite samples. Although most of the minerals (especially calcite) have isotope ratios close to or in isotopic equilihrium with present thermal or meteoric water at in situ temperatures, some of them may have precipitated from waters significantly enriched in 18O relative to present geothermal fluids. The S18Owhole-rock values and open system water/rock ratios show a temperature-dependent isotope exchange, shifting the rock isotopic composition to heavier δ18O values at temperatures lower than ≈ 90°C, with possible coeval smectite precipitation, and to lighter values, at temperatures higher than 90°C.