WATER-ROCK INTERACTION PROCESSES IN THE ALUTO LANGANO GEOTHERMAL-FIELD (ETHIOPIA)

Aluto-Langano is a water-dominated geothermal field in the Main Ethiopian rift system. The hydrothermal mineral assemblage is characterized by the presence of illite-montmorillonite (T< 150-degrees-C) and illite-chlorite (150-degrees-C < T < 310-degrees-C). In the hottest part of the wells (up to 335-degrees-C) the mineral assemblage is characterized by the presence of Ca-Al-silicates (epidote, garnet, actinolite). However these phases also occur as relics in wells where lower temperatures are measured. Calcite, quartz and chlorite are widespread minerals in all the wells, both at low and high temperatures. Fluid inclusion data from quartz and calcite of two wells (LA-3 and LA-6) reveal fluids with an apparent salinity of 1.5-2.0 for LA-3 and 0.85-1.36 wt.% NaCl eq. for LA-6. Homogenization temperatures (Th) range from 240-degrees-C to 350-degrees-C. The calculated CO2 concentration in the liquid phase ranges from 0.07 to 0.66 mol. Thermodynamic properties and phase relations of calc-silicates show that the presence or absence of these minerals is mainly controlled by the P(CO2) in the system, even where temperature has played an important role. Fluid inclusion data from two deep wells (LA-3 and LA-6) indicate an increase in temperature and CO2 Content of the geothermal fluid after the trapping of the inclusions. The wells produce an alkali-chloride-bicarbonate water. Enthalpy-chloride and Na/1000-K/100-square-root Mg diagrams indicate that none of the discharge waters can be taken as representative of the deep fluid in full equilibrium with the reservoir rock, and that important mixing processes characterize even the hottest zone of the system. These mixing processes could be responsible for the continuous dynamic evolution of the hydrothermal system, as evidenced by the authigenic mineralogy.