| Abstract |
In Aluto Langano geothermal field, the high-temperature recorded in the central part of the field, where wells LA-3, LA-6, LA-9D, and LA-10D are located, with a measured temperature is about 335°C. However, a decrease in underground temperature is observed in the eastern and western parts of the field. The wells that are located in low temperatures, such as LA-4, LA-5, LA-7, and LA-8, show temperature inversion at a depth. From the up-flow zone, the high-temperature geothermal fluid travels along the faults that strike NNE-SSW, which corresponding to the alignment of the Wonji Fault Belt and also that the faults that are aligned in NNE-SSW and NW-SE direction surrounding the main up-flow of the field acts as a hydrological barrier. The hydrothermal alteration minerals in the wells found as a replacement of primary minerals and as fillings of microfractures, veins, and vesicles in the rock units. The sequence of the alteration mineral deposition within the wells ranges from low-temperature to high-temperature minerals. The minerals in the wells range from low-temperature minerals, such as chalcedony, siderite, haematite, and smectite, to moderately high-temperature alteration minerals, such as mixed-layer clay (MLC), illite, quartz, chlorite, epidote, and actinolite. Epidote is a key index mineral related to temperature and fluid composition in geothermal systems, it is found together with quartz, illite, and chlorite. Depends on the distribution of hydrothermal alteration minerals and formation temperature, four alteration zones have been identified. These are the unaltered zone, smectite zone, illite/chlorite zone, and illite/chlorite/epidote zone. |