Use of isotope techniques to trace the origin of acidic fluids in geothermal systems

Realizing the potential contribution of stable isotopes of the water molecule and those of sulphur compounds in tracing the sources of acidic fluids, especially the sulphate type of acidity in geothermal well discharges, the IAEA organized a Coordinated Research Project (CRP) with the objectives of integrating isotope techniques, in particular the isotopes of sulphur compounds, into (1) identification of the origin of the water component in acidic fluids, (2) identification of the origin of sulphur compounds in acidic fluids and (3) study on mixing of waters from different sources to form acidic fluids. In addition to these objectives, the CRP was also to test isotope geothermometry based on sulphur compounds in geothermal fluids. In this CRP, isotopic composition of oxygen-18 and deuterium in waters sampled from acidic wells indicates that these waters are mixtures of meteoric and magmatic waters. Examples of this type of geothermal systems are Miravalles in Costa Rica, Onikobe in Japan, Los Humeros in Mexico and Several fields in the Philippines. It has further shown that acidic fluids are isotopically heavier in some systems, implying a larger fraction of magmatic inputs. This isotopic evidence confirms the hypothesis that some acidic fluids are un-neutralized magmatic water. The aqueous sulphate in geothermal well discharges of a volcanic geothermal system originates from either oxidation of H2S gas similar to that producing acidic hot springs and ponds at the surface, hydrolysis of magmatic SO2 gas or hydrolysis of native sulphur at shallow depth. Isotope geothermometry based on δ34S (H2S) provides estimates of reservoir temperatures that are compatible to measured temperature values for the acidic wells, but not for neutral pH wells. This suggests that fractionation equilibrium is more easily attained in acidic wells as compared to neutral pH wells. Isotope geothermometer based on δ18O in water and aqueous sulphate was found to be generally not applicable to low temperature (<100 degrees centigrade) formation waters in sedimentary basins, but gave agreeable results for saline thermal waters of marine origin at reservoir temperature of 150 degrees centigrade