Thermal springs, fumaroles and gas vents of continental Yemen: Their relation with active tectonics, regional hydrology and the country’s geothermal potential

Most thermal springs of continental Yemen (about 65 emergences at 48 sampling sites) and a couple of fumaroles and boiling water pools have been sampled and analyzed for chemical and isotopic composition in the liquid phase and the associated free-gas phase. Whatever the emergence, all the water discharges have an isotopic signature of meteoric origin. Springs seeping out from high altitudes in the central volcanic plateau show a prevalent Na–HCO3-composition, clearly affected by an anomalous flux of deep CO2 deriving from active hydrothermal systems located in the Jurassic Amran Group limestone sequence and/or the Cretaceous Tawilah Group, likely underlying the 2000–3000 m thick volcanic suite. At lower elevations, CO2 also affects the composition of some springs emerging at the borders of the central volcanic plateau. Although mixing to a limited extent with organic CO2 infiltrating together with the meteoric recharge waters cannot be ruled out, all the CO2-rich gas samples have a δ13C–CO2 signature that falls in the range of mantle CO2 (−3 < δ13C < −7‰ V-PDB). The relatively high 3He/4He (1 < R/Ra < 3.2) ratios measured in all the CO2-rich springs and also some mixed N2–CO2 gas vents in the far east Hadramaut region support the presence of mantle magmas and related hydrothermal systems residing at the crust level in several areas of Yemen. This well agrees with the presence of Quaternary basaltic magmatic activity along the Gulf of Aden, as well as inside the central Yemen volcanic plateau. Presently, the thermal springs of Yemen are prevalently used for spas and/or bathing. Nevertheless, liquid- and gas-geothermometry and geological considerations suggest that there are at least three areas (Al Lisi, Al Makhaya and Damt) inside the Yemen volcanic plateau (around Dhamar) that may be promising prospects for the future development of geothermal energy in Yemen. Alternatively, they could be used as a source of energy for small-to-medium scale agriculture and/or industrial purposes. Moreover, most of the thermal water discharges have the chemical characteristics of potable waters and, in an arid country like Yemen, they have to be considered as long-term water supply resources if the country should face water shortages.