Istituto di Geoscienze e Georisorse     
Valori A., Teklemariam M., Gianelli G. Evidence of temperature increase of CO2 bearing fluids from Aluto Langano geothermal field (Ethiopia): a fluid inclusions study of deep wells LA-3 and LA-6. In: European Journal of Mineralogy, vol. 4 pp. 907 - 919. E. Schweizerbart'sche Verlagsbuchhandlung, 1992.
The Aluto-Langano geothermal field is located within one of the most active volcanic aeas of the Main ethiopian Rift system. At Aluto, exploratory drilling has revealed the existence of high temperature (ca. 335C) two-phase fluids. This study is focused on wells LA-3 and LA-6, wich are located on the main upflow zone of the geothermal system. Evidence for the occurrence of high intensity water-rock interaction is given by the presence of minerals such calcite, chlorite, hematite, illite, quartz, sphene, epidote, garnet, prehnite, biotite and actinolite. Seven samples have been selected from drillcores recovered at different depths for petrografic observation, microthermometry and crushing experiments. Primary and essentially secondary fluid inclusions are recognized in authigenic calcite and quartz crystal as well as in igneous quartz crystal of the reservoir rock. Most of the inclusions are two-phase liquid-rich (liquid fraction:0.8-0.9), but single-phase vapour inclusions are also present, expecially in the upper part of the system. A total of 333 temperatures on cooling and 392 temperatures on heating were misurated for liquid-rich (P and S) inclusions. The recognition of first melting indicates a fluid dominated by Na(K)Cl. Melting ice tempertures (Tmi) range from 0.0-1.9C. Clathrate was never observed. Homogenization temperatures (Th) range from 240 to 350C and are generally lower than the measured inhole temperatures at each depth. Both Tmi and Th values show bimodal frequency distributions suggesting that several generations of fluids have circulated early in the system. Crushing experiments and cooling demostrate that important chemical changes have occurred for early hydrothermal fluid circulation to the present day, expecially a regard PCO2. The Tmi- Th distribution defines a diluition trend between low salinity heated ground waters and a saline (about 3% NaCl eq.) high temperature fluid (350C) rising from a deep source. The mineral association calcite, epidote, garnet and/or prenhite is consitent with the initial very low CO2 content. The fluid inclusions study demonstrates heating up of the system from about 250C to the present day temperatures of 320-335C. This heating could be explained by an intake of high enthalpy fluid in the upflow of the system
URL: http://www.schweizerbart.de/j/ejm/
Subject petrography and mineralogy
Aluto-Langano geothermal field

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