Geothermal activities in the Main Ethiopian Rift: Hydrogeochemical characterization of geothermal waters and geothermometry applications (Dofan-Fantale, Gergede-Sodere, Aluto-Langano)

Ethiopia holds an enormous capacity to generate geothermal energy in the volcano-tectonically active zones of the East African Rift System. In this study, we investigate the potential of three geothermal prospect areas in the Main Ethiopian Rift (Dofan-Fantale, Gergede-Sodere, Aluto-Langano). We examine existing and new data from a water sampling campaign, both of which are evaluated in terms of geochemistry and applicability for the estimation of geothermal subsurface temperatures. Several solute geothermometers, Cl-SO4-HCO3 and Na-K-Mg ternary diagrams as well as silica-enthalpy and chloride-enthalpy mixing models were applied to the prevailing alkaline and Na-HCO3 or Na-Cl-HCO3 dominated waters. Mixing was most pronounced in the individual subregions of Dofan-Fantale and Gergede-Sodere, yet these areas still indicate high heat-generating capabilities. The applied enthalpy-chloride mixing model suggests a subsurface temperature of 190 +/- 20 degrees C for these hot springs. This temperature estimate is in good agreement with those obtained from the Na-K and Na-K-Ca geothermometers (185 +/- 20 degrees C) for both geothermal areas. Additionally, for Gergede-Sodere it agreed well with the silica-enthalpy mixing model hot spring water results with the assumption that steam loss occurs before mixing (170 +/- 20 degrees C). Furthermore, the enthalpy-chloride mixing model refers to reservoir temperatures between 300 degrees C and 370 degrees C for the Aluto-Langano geothermal fluids, which are in the same magnitude than the ones obtained by the silica-enthalpy mixing model for hot springs, if no steam loss occurs before mixing (270 +/- 30 degrees C). In addition, they are comparable with few known data collected in drilled wells in the Aluto-Langano geothermal field (231-282 degrees C for LA-4 and LA-8 as well as 315-363 degrees C for LA-3 and LA-6). (c) 2013 Elsevier Ltd. All rights reserved.