GEOCHEMICAL, ASSESSMENT OF THE NORTHERN TENDAHO RIFT, ETHIOPIA

Abstract

Geochemical data of the hydrothermal manifestation, deep wells and cold water points from the Northern Tendaho Rift, Ethiopia, have been examined and processed in order to have a better understanding of the geothermal system with respect to the nature of the fluids, their maturity, temperature and upflow zone. In the Dubti area, where three deep wells are drilled, the temperature distribution computed from gas compositions and steam fraction could be tied to flow paths of the deep fluid along a fault starting
from an upflow zone which is anticipated to be around the three main fumaroles having an estimated reservoir temperature of about 300?C. A
set of hydrothermal minerals including calcite, anhydrite, pyrite, wairakite, prehnite, Mg-chlorite, albite, epidotes and diopside is in thermodynamic
equilibrium with the geothermal waters of the wells in Dubti and with the waters of hot springs in the Alalobeda hydrothermal area. The equilibration
temperatures computed from water and gas compositions of the wells in Dubti are consistent with the measured values. For the Alalobeda area, a
temperature of about 205?C seems the most reliable. The isotopic composition of carbon and He and the He-Ar-N2 relationship confirm an original magmatic source for the gas. Based on the isotopic composition
of waters from the wells and sampled at the Alalobeda hot springs, the origin of recharge water can be fixed at an elevation of approximately 2500 m in the Etluopian plateau (Western Escarpment).