Geothermal Geochemistry Based on Alteration Mineralogy and Fluid Inclusion Microthermometry of Hes-Daba (Gagade), Republic of Djibouti

Abstract

The studied area (Hes-Daba) is located in the Gaggade region of the currently active Great East African Rift, which is opening up at the rate of 10-15 mm/year. Hes-Daba block is marked by the presence of several hot springs and fumaroles, besides many hot-spots which are possible relics of hot spring and hydrothermal activity as evidenced by outcrops of silica sinters and bladed chalcedony-calcite veining, besides several sub-parallel zones of deeply altered brownish . This field is considered one of the most valued geothermal area in the Republic of Djibouti, because of surface manifestation such as active natural faults and hydrothermal alteration such as calcite, quartz, and clay. Fluid inclusions of hydrothermal veins from the SE part of the Afar Rift (Djibouti) were studied. Microthermometric measurements were made on quartz collected from the surface, which hosted two phases (liquid+ vapor) inclusions were observed. Mean homogenization temperature ranges from 150 °C to 367 °C and ice-melting temperatures range from −0.05 °C to − 1.14 °C indicating that inclusion solutions consist of 0.1 to 1.9 eq. wt% NaCl. These studies aim estimating formation temperature, evaluating thermal history and thermal structure. Selected samples have been analyzed with XRD to provide direct information on geothermal reservoirs, because geothermal fluids, through their interaction, can alter the composition and properties of rocks. The main alteration minerals found in this area were quartz, calcite, alunite, epidote, hematite and some clay minerals (illite, smectite and chlorite). Therefore, the clay constitutes a transition to a high-temperature environment which is evidenced by high-temperature hydrothermal alteration minerals such as quartz ( more than 180°C), and epidote (~250°C).