The geothermal resources of the Republic of Djibouti — II: geochemical study of the Lake Abhe geothermal field

The Lake Abhe Geothermal Field is located in the South-Western region of the Republic of Djibouti, on the border with Ethiopia. The Lake Abhe geothermal system occurs within a rift basin filled with Pliocene-Quaternary volcanic (mainly basalt) and lacustrine sediments. The thermal water in Lake Abhe geothermal field discharges as hot springs at the bottom of hydrothermal carbonate chimneys distributed along the main faults. Hot springs of Lake Abhe geothermal field as well as ground- and surface waters were sampled and major elements, trace elements, and isotopic (18O/16O, 2H/1H, 3H, 34S/32S, 87Sr/86Sr) compositions were analyzed. Hydrochemical features of the hot springs are dissimilar from those of warm waters: the former are mainly Na–Cl dominated whereas the latter were mostly Na–HCO3–Cl–SO4 and Na–HCO3–Cl. The isotopic composition of sulfur and oxygen in dissolved sulfates suggests equilibrium with anhydrite as the major source of sulfates in the thermal waters. Chemical (mainly Na/K and SiO2), isotope (bisulfate- and anhydrite- water), and multiple mineral equilibrium approaches were applied to estimate the reservoir temperature of the hot springs in the Lake Abhe geothermal field. These different geothermometric approaches estimated a temperature range of the deep geothermal reservoir of 120–160 °C. In spite of the relatively wide range, the three different approaches led to a same mean of about 135 °C. The hot spring and warm borehole waters from the southwestern part of the Republic of Djibouti showed a possible mixing with hydrothermal waters from the local rift. The negligible tritium content and the low deuterium values (δ2H < − 10‰) suggest a deeper circulation and an old age for geothermal water, in comparison with surface waters and the local aquifers recharged by modern precipitations (δ2H > − 10‰).