Regional variations in the chemical and helium–carbon isotope composition of geothermal fluids across Tunisia

Abstract

Tunisia has numerous thermo-mineral springs. Previous studies have shown that their chemical composition and occurrence are strongly influenced by the regional geology. However little work has been done so far to study the isotopic composition of volatiles associated with these geothermal manifestations. Here, we report on the results of an extensive survey of both natural hot springs and production wells across Tunisia, aimed at investigating the spatial distribution of thermal fluids' geochemical characteristics and He–C isotopic composition. The chemistry of the analyzed samples highlights the heterogeneity of the water mineralization processes in Tunisia, as a consequence of the complex geological and tectonic setting of the country. In terms of chemical composition, we are able to conclude, however, that dissolution of halite and gypsum plays a key control on groundwater chemistry. Helium and carbon isotope systematics confirm the prevalently crustal origin of the volatiles interacting with the aquifer systems, consistent with the absence of any recent magmatism. Most samples are characterized by crustal-type helium (3He/4He in the range 0.02 Ra–0.4 Ra) associated with a CO2 predominantly metamorphic in origin (with the exception of the Saharan platform where the carbon content is low and mostly organically-derived). In Eastern Tunisia, however, a few samples have He–C isotope compositions which suggest (at least partial) mantle derivation of the dissolved gas phase : the 3He/4He ratio reaches 2.4 Ra (corresponding to 30% of mantle-derived helium) at the Ain Garci site, a CO2 rich mineral spring located some 30 km south of the city of Zaghouan. This mantle signature is consistent with the fact that the Pelagian Block, to which Eastern Tunisia belongs, has been deeply affected by extensional and transtensional tectonics since the opening of the Tethys, a process which is still ongoing in the Sicily channel (Pantelleria Rift). As a whole however, our results show that the Italian mantle gas anomaly only marginally extends to Northwestern Africa.