Istituto di Geoscienze e Georisorse     
Gianelli G., Teklemariam M. Water-rock interaction processes in the Aluto-Langano geothermal field (Ethiopia). In: Journal of Volcanology and Geothermal Research, vol. 56 pp. 429 - 445. Elsevier, 1993.
Aluto-Langano is a water-dominated geothermal field in the Main Ethiopian rift system. The hydrothermal mineral assemblages is characterized by the presence of illite-montmorillonite (T150C) and illite-chlorite (150T310C). In the hottest part of the wells (up to 335C) the mineral asssemblages is characterized by he 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. Fluids 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. Homogenizzation temperatures (Th) range from 240 to 350C. 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 PCO2 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 drapping of the inclusions. The wells produce an alkali-chloride-bicarbonate water. Enthalpy-chloride and Na/1000-K/100vMg 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 process characterize even the hottest zone of the system: these mixing process could be responsible for continuous dynamic evolution of the hydrothermal system, as evidence by authigenic mineralogy
URL: http://https://www.journals.elsevier.com/journal-of-volcanology-and-geothermal-research/
Subject water-rock interactions

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