Geochemical methods in geothermal surface exploration

Abstract

Geothermal water is mostly meteoric and oceanic water in origin, but has been altered by interaction with rock and in some cases magmatic fluids. Geothermal waters have been classified with respect to their anion and cation contents into alkalichloride water, acid sulphate water, acid sulphate-chloride water and bicarbonate water. Acid waters are generally unsuitable for elucidation of subsurface properties. Conservative constituents are used for tracing origin and flow of geothermal fluids, stable isotopes (especially 2 H and 18O) along with B and Cl being most important, whereas rock-forming constituents (e.g. SiO2, Na, K, Ca, Mg, CO2, H2) are used to predict subsurface temperatures. Measurements of soil gas concentrations and flux can be useful for delineating fractures or other permeable structures relevant to flow paths in geothermal systems.