Executive Summary: USGS Open File Report 97-291

Geothermal Potential of the Alid volcanic center, Danakil Depression, Eritrea

The Bottom Line

Results of the studies carried out during this project strongly suggest that an upper crustal magmatic and/or hot plutonic body is present beneath Alid mountain. Compositions of fumarolic gases collected at Alid indicate that the reservoir temperature of a hydrothermal-convection system driven by this heat source is very likely in the range of 250° to 300°C. The geologic structure and tectonic environment of Alid are highly favorable for producing and maintaining substantial fracture permeability beneath the mountain and perhaps even extending north and south somewhat from the mountain's base. The overall temperature and permeability conditions seem so favorable for an electrical grade geothermal resource that exploration drilling to depths of 1.5 to 2 km is recommended. However, before such deep drilling is undertaken, additional tasks are recommended to aid in the selection of specific drill sites and drilling targets. These tasks are:

  1. Obtain new aerial photographs of Alid and surroundings at a scale of about 1:25,000.
  2. Prepare a new topographic map of the area at a scale of 1:25,000 with a contour interval of 5 meters in relatively flat terrain and 10 meters on the steep flanks of Alid.
  3. Prepare a fracture-patter map, using the new aerial photographs as a primary source of information.
  4. Complete an electrical-resistivity survey of Alid and surroundings within Alid graben.
  5. Drill 10 to 15 wells to provide information on temperature gradients and depth to the water table. Nominal target depths in the range of 100 to 200 meters are recommended, using slim-hole diamond-core drilling technology.
  6. Complete a study of carbon dioxide and radon in soil gas to help locate zones of high permeability through which these gases migrate from their deep subsurface hydrothermal source.

The rationale for these tasks is given in the RECOMMENDATIONS section. In addition, information about local meteorological conditions will be essential in order to optimize the design of a power plant. Accordingly, a weather station should be established as soon as possible at Alid to begin to provide a record of temperature, relative humidity, wind velocity and direction, and rainfall.

Summary of Supporting Data

Alid volcanic center rises about 700 meters above the floor of the Danakil Depression, a crustal spreading center that traverses the eastern lowlands of Eritrea. This mountain is a structural dome that formed as a result of local intrusion(s) of silicic magma into the upper crust. Intrusion of this magma domed Precambrian rock and an overlying sequence of late Cenozoic sediments and lava flows. The sediments are fine-grained clastic deposits typical of a shallow inter-tidal environment. Some contain marine fossils. The lava flows include basalt, andesite and rhyolite. Some basalt is pillowed, indicative of emplacement underwater, but most of the lavas were emplaced in a subaerial environment. The domed stratified rocks dip as steeply as 65 degrees on the lower flanks of Alid and as gently as 20 degrees near the top of the mountain. A 2 by 3 kilometer summit area is a depression that apparently formed when the stratified rocks were stretched beyond their elastic limit across the top of the growing dome, and thus broke and collapsed downward and inward to form something akin to a chaotic keystone graben at the apex of the arch. The westernmost part of the summit depression was subsequently enlarged when a post-dome volcanic vent there produced voluminous deposits of rhyolite pumice about 15,000 years ago. This eruption of pumice is evidence that a silicic magma body persisted beneath Alid for some time after it formed the structural dome. The youthfulness of the eruption suggests that this heat source for the hydrothermal system at Alid is still at least partially molten.

By analogy to structural characteristics of doming over crustal intrusions called laccoliths, the steep dips on the flanks of Alid suggest that magma reached to within about 2 to 5 kilometers of the surface. Moreover, miarolitic and granophyric textures in juvenile clasts of granite within the deposits of rhyolite pumice are characteristic of formation at equally shallow depths. These granite clasts are magma that crystallized along the roof of the dome-forming magma body and were then plucked from the roof and incorporated into magma that erupted to produce the pumice deposits.

Interpretation of linear features on aerial photographs of the study area defines a crudely rectilinear pattern over the mountain with many north- to northwest-trending features parallel to the rift spreading axis, and other less prominent features parallel to the east to northeast elongation of Alid. These features apparently reflect tectonic stress, rather than stress that can be associated with a point source that produced the structural dome.

Fumaroles and boiling pools are distributed widely over the northern two thirds of Alid, both at the summit and at lower elevations on the flanks. Through the application of 9 gas geothermometers, the compositions of fumarole gases suggest that their source, an underlying hydrothermal reservoir, has a temperature in the range of about 250 to 300°C. The generally most reliable geothermometer, which utilizes the relative abundances of CH4, CO2, H2S and H2, yields a reservoir temperature of 265°C for gases collected at Ilegedi, the largest and most active of Alid's geothermal manifestations.

The isotopic composition of condensed fumarolic steam is consistent with 220-300°C boiling of ground water that may have come from various sources, including local lowland rain, fossil Red Sea water, or even highland rain water that evaporated significantly before percolating underground. Some gases in the reservoir fluid, particularly CO2, H2, and H2S may be derived, directly or indirectly, by magmatic input from the silicic magma body that likely exists beneath Alid. The results of the present investigation are sufficiently encouraging to justify continued exploration, including drilling at Alid.

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