The geothermal resources of the Republic of Djibouti — I: Hydrogeochemistry of the Obock coastal hot springs

This paper examines the hydrochemical features of the waters from the Obock coastal geothermal field. As the submarine waters at the ridge, their chemical and isotope composition shows affinities with sea water–basalt interactions at hydrothermal temperature. Moreover, good linear correlations were obtained between hot springs and of sea water sample points when plotting normalized concentrations of elements to chloride (Mg/Cl, SO4/Cl, K/Cl, Ca/Cl, SiO2/Cl) versus concentration of Li/Cl. This would indicate that two end members exist (seawater and reservoir fluid). After extrapolation of the Mg and SO4 concentrations to zero, the obtained value of SiO2 corresponding to the reservoir fluid end member was used to estimate the reservoir temperature by quartz geothermometers. The obtained temperature of 187 °C is in good agreement with that obtained from multiple mineral equilibrium approach (180–200 °C), cationic geothermometers (172–191 °C) and by the evaluation of isotopic equilibrium between water and sulfate molecule (207 °C). Summarizing all the employed approaches, a mean temperature of 197 ± 10 °C has been estimated. The isotopic δ34S(SO4) signature of the dissolved sulfates in Obock thermal waters confirms that these waters result from the mixture of a hot seawater-derived fluid (absence of sulfates) with cold seawater. However, water isotope data did not exclude the presence of a small contribution from fresh groundwater.