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Útdráttur:
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Aluto Langano geothermal field is located in the Aluto volcanic complex (AVC), within the Central Main Ethiopian Rift System (CMERS). AVC is a silicic peralkaline complex, which is affected by Wonji fault belt (WFB), trending NNESSW. Ten exploratory wells (LA-1 to LA-10D) have been drilled in Aluto Langano geothermal field, with a maximum depth of 2500 m and maximum measured temperature of 335°C. This study focused on sub-surface geology and hydrothermal alteration of wells LA- 9D and LA-10D. The wells were drilled directionally on the top of AVC. Data form neighbouring wells (wells LA-3 to LA-8) are used for comparison purposes. Lithologies that are found in the study wells, identified with the aid of binocular microscopy and petrographic microscopy, are pyroclastics, silicic tuff and breccia, sediments, rhyolite, trachyte, basalt and ignimbrite, while ICP-OES analyses also led to the identification of very scarce and thin layers of trachyandesite, basaltic trachyandesite, trachydacite, basaltic andesite and andesite units. Hydrothermal alteration minerals are found as replacement of primary minerals and as fillings of veins and vesicles. Four alteration zones are recognized in both study wells, and include an unaltered zone, smectite zone, illite/chlorite zone and illite/chlorite/epidote zone. Eight feed zones (with one major and seven minor) were identified in well LA-9D and seven (one major with six minor) feed zones in LA-10D are observed. Most of the feed zones are located in fractured basaltic units. The highest temperature recorded was more than 300°C in well LA-9D below 1560 m depth, and below 1600 m in LA-10D, the recorded temperature indicated is more than 300°C. Both wells appear to be in the upflow zone of the system. |
