Geological and Geothermal Mapping of Butajira Geothermal Prospect, Central Main Ethiopian Rift (Cmer); Ethiopia
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Date
2019-01-02
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Addis Ababa University
Abstract
Butajira geothermal prospect is located in Central Main Ethiopian Rift nearby Western
escarpment associated with SDZFZ and geographically bounded between UTM 427000-
437000E, 882000-891000 N with area coverage of 90 Km2. This is among 23 geothermal
prospects in Ethiopia which has undergone limited geological and geothermal investigations.
The main objective of the study is to assess the existence of promising geothermal system
components at the Butajira geothermal prospect through investigating the geology and
surface geothermal manifestations. To achieve the desired objectives various methodologies
were adopted including review of secondary data, geological field observation, Petrographic
study, and XRD geochemical analysis. As a result, geological and geothermal mapping was
carried out at the scale of 1:50,000, promising sites were identified and a preliminary
conceptual model was presented to related identified geothermal play type.
The main lithological units encompassed in the area are Basaltic Scoria, Basaltic lava flow,
unwelded Tuff, welded Ignimbrite, Phreatomagmatic deposits comprising rock fragments of
trachyte and Diorite as well as alluvial and reworked pyroclastic deposits. Cinder cones,
Crater Lake, faults, lineaments, fractures, and joints were noticed with general fault
orientation NNE-SSW, and NE-SW which dip 600-650 SE. Surface geothermal
manifestations such as hot spring, mud pool, altered ground, and hot drill well and
Abandoned well sites were identified. They prominent distributed in the south-west and
apparently aligned in NE-SW direction around Ashute and Gerbi Ber villages, suggesting
they are manifested along fault planes and at their intersection point. Moreover, high degree
of hydrothermal alterations is vigorous predominantly associated with those structures. The
recognized mineral assemblage consists of Titanite, Actinolite, Chlorite, Rutile, Albite,
Adularia, Biotite, Calcite, Quartz, Stishovite, Cristoblite, Trydimite, and, Sulfur.
A preliminary geothermal conceptual model has been developed which resemble convective
dominated geothermal play type and consistence with CV1b: Extinct Magmatic Play Type.
This accommodates surface manifestations and geological structures and elucidates the
expected geothermal system components like heat source, reservoir, recharge and discharge
area, and an impermeable cap rock. Regarding to the local heat source anomaly that
expressed by numerous surface manifestations two possible scenarios could be put forward
related to the intrusive body. The first one is a series of mantle-derived basaltic dyke
intrusions emplaced at shallow depths. The second one is a series of elongate and
interconnected basaltic sills or magma chambers related to dyke intrusions emplaced beneath
the young basaltic scoria cones and lava flows. Alteration minerals and fluid geothermometer
estimate suggests that the expected reservoir temperature lies between 1500C-2000C
indicating sufficient heat-generating capability and warrant further exploration work to assess
their suitability for energy generation. The source of permeability of the system has
apparently related with tectonic structures while impermeable layer found in upper 1km
depth resulted from hydrothermal alterations. Hence, two promising areas were identified to
execute next exploration phase on Ashute and Gerbi Ber villages.
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Keywords
Geothermal Mapping, Butajira Geothermal Prospect, Central Main Ethiopian Rift (Cmer), Ethiopia