Geochemical study of the Sakalol-Harralol geothermal field (Republic of Djibouti): Evidences of a low enthalpy aquifer between Manda-Inakir and Asal rift settings

Eighty-six sodium bicarbonate to sodium chloride hot springs and four water wells in the Tadjourah Region of Djibouti were investigated for major, minor (B, Br, F, Sr, Li) chemistry and isotope composition of water and dissolved components (87Sr/86Sr, 11B/10B, 13C/12C and 14C of DIC, 34S/32S and 18O/16O of sulfate). The deep saline Na-Cl reservoir at 143 °C shows affinity with the shallow geothermal water from the “active” Asal rift. Asal water is a diluted and recycled seawater component with the major cation composition obliterated by equilibration with Stratoid basalt. Locally, the deep reservoir is differentiated in term of recharge, and re-equilibration with rocks and mixing. In particular, two spring groups reveal contributions from evaporites typical of the “passive” graben setting of the Afar. A model on 34S/32S and 18O/16O demonstrates the isotope imprint of magmatic SO2 disproportionation on dissolved and solid sulfate, whose values probably persists in a sedimentary environment without trace of seawater. On the other hand a seawater signature, modified by mixing and secondary fractionation effects, is partially maintained according to the boron isotope composition (up to + 27.4‰). Temperature estimation in low-enthalpy geothermal reservoirs is notoriously difficult, especially where mixing with fluids of differing genesis and/or conduction cooling take place. From a geothermometric point of view, the multi-method approach followed in this study (up-to-date theoretical and thermodynamic equations, ad-hoc silica geothermometers inferred from local rocks, checking of the results on a 18Oαsulfate-water vs. temperature diagram) provides some insights and perspectives on how to tackle the problem.