Geological and Hydrothermal Alteration Mapping of the Doffen Geothermal Prospect and Adjacent Western Escarpment (Ethiopia)

Geological and surface hydrothermal alteration mapping was conducted at the Doffen Geothermal Prospect located in the extreme southern part of the Afar Depression and the adjacent western escarpment. Mio-Pliocene basement rocks of the rift floor are the flood basalts of the Afar Stratoid Series and the ignimbrite succession of the Nazret Group, both extensively exposed to the northeast and southwest of the study area respectively. During the Plio-Pleistocene a marginal rift basin parallel to the western escarpment accumulated fluvio-lacustrine sediments with intercalated basaltic lava and felsic pyroclastics (Kessem-Kebena Formation). The area along the extensional axis of the rift (Wonji Fault Belt) is affected by intense Quaternary faulting and central rhyolitic and fissural basaltic volcanism. The Doffen Volcanic Complex (DVC) is such a center which produced intermediate and peralkaline rhyolitic lava between 1.7 and 0.2 Ma with an estimated volume of erupted material of about 25 km 3 . The volcanic center is dissected by a series of normal faults which formed a graben and divided the center in two peaks with the northern part of the graben marked by a historical basaltic volcanic eruptions. Active hydrothermal manifestations in the study area occur on the northern and southern parts of the DVC. The manifestations on the northern part of the center are hot springs, hot ground and fumarolic activity with kaolinized ground marked by sulfur mineralization, intimately associated with the recent basaltic aa flows. Fumarolic activity also occurs on the southern flank of the center together with dry mud encrusted rumbling pool on the southern adjoining plain to the center and warm spring activity controlled by the local hydrogeology. Fossil hydrothermal alteration represented by travertine deposits found along the foot of the western escarpment, and also associated with the fluvio-lacustrine sediments attest to the presence of a more widespread hydrothermal activity related to higher water table conditions of the pluvial periods of the Pleistocene. Localities of deep-rooted widespread fossil hydrothermal alteration are found controlled by the tectonic structures that dissect the DVC. Previous gas geochemical study has indicated subsurface temperatures in excess of 250oC in the vicinity of the DVC. XRD mineralogical analyses of hydrothermally altered samples showed the presence of secondary minerals comprising of kaolin, illite, chlorite, smectite, pyrophillite, hematite, pyrite, silica, calcite, anhydrite, allunite, native sulphur, bicarbonate salts and rare aluminosilicates. Hydrothermal mineralogical assemblage in most samples indicates an altering fluid with temperature in excess of 150oC and acidic pH and a noncondensible gases with high H2S / H2 ratio and an affinity to gases from an active volcanic system. The Mio-Pliocene ignimbrite succession, which is widely exposed along the escarpment presumably, makes the reservoir for the prevailing hydrothermal system(s) and hence the western escarpment area requires a focused investigation. It is recommended that a proposed shallow temperature gradient survey consist of at least one drill well at each of the northern and southern part of the DVC to determine the direction of hydrothermal fluid circulation and make plausible isotherms over the magmatic system that determines the thermal budget.