Hydrogeology and hydrothermal alteration of the Hanle (Djibouti) geothermal field

Hydrogeochemical and hydrothermal alteration studies were carried out to determine the geothermal potential of Hanle Geothermal Area. Samples of hot spring water, groundwater and steam condensate were analyzed for important parameters such as electrical conductivity (EC), SiO2, Cl, SO4, HCO3, F, Na, K, Ca, Mg, Al, Fe, B, As, D and 18O. The findings indicate that the formation of the water in the Hanle region was influenced by the mixture of heated groundwater and heated fossil seawater or the dissolution of evaporite by heated groundwater. According to the saturation index calculations of the hot waters, geothermal systems in the region generally contain minerals such as dolomite, aragonite and calcite, while different mineral saturations such as alunite, anhydrite, silvite, gypsum and melenterite were found in the Hanle region. However, the reservoir temperature is estimated to be approximately 120 to 125°C. The clay minerals smectite, chlorite and illite, as well as quartz, calcite and alunite, are the main alteration minerals in this region, according to XRD research. Minerals that have undergone hydrothermal alteration are formed according to this composition; some of these minerals are known to occur in particular, stable temperature regimes. The clay minerals represent a transition to a high temperature environment as evidenced by the presence of high temperature hydrothermal alteration minerals such as quartz (>180°C).