Geothermal potential and origin of natural thermal fluids in the northern Lake Abaya area, Main Ethiopian Rift, East Africa

In this study the occurrence, chemical composition, origin and geothermal significance of thermal springs and fumaroles naturally discharging in the area located north of the Lake Abaya (western margin of the Main Ethiopian Rift, East Africa) are reviewed in relation with recent tectonics. All thermal springs showed a markedly Na-HCO3 composition, consistent in time since 1972, and most of them lied in a narrow range of ?D and ?18O isotopic compositions of meteoric origin. This suggests a main water-rock interaction process in all aquifers interested by the circulation of thermal fluids, and similar elevation of recharging areas. Most springs discharge along the active faults bordering the western edge of the Rift with associated scarce CO2-rich gas phases. The ?13C values of CO2 and the 3He/4He isotopic ratios point to the presence of active degassing from the mantle, as currently occurs in many other areas along the African Rift. The application of geothermometric techniques to both the chemical components in the liquid and the gas phases suggests the presence of a reservoir that equilibrated the fluids at a temperature of about 180 °C. Setting aside the thermal spring discharges, the presence of fumaroles at boiling temperatures and water/mud boiling pools in several places indicates that the geothermal reservoir is positioned at a relatively shallow depth and likely located in the western side of the study area.