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 ( He ratio reaches 2.4 Ra (corresponding to 30% of mantle-derived helium) at the Ain Garci site, a CO 2 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.

C–O Stable Isotopes Geochemistry of Tunisian Nonsulfide Zinc Deposits: A First Look

“…40 • C, which is incompatible with a precipitation related to supergene processes. However, an explanation for high precipitation temperatures may be given if the formation of the Saint Pierre orebody was due to a mineralizing event genetically related to the geothermal activity occurring throughout central and northern Tunisia in connection with the evolution of Quaternary grabens [76], revealed in the Sekarna region through the occurrence of several thermal springs with temperatures around 40 • C and of the local anomalous geothermal gradient of 40 • C/km [77]. Considering this setting, it is likely that the Saint Pierre smithsonites formed from hot groundwaters related to the anomalous geothermal gradient existing in the area, whereas the Saint Eugène smithsonites [47] formed after supergene processes, as the smithsonites occurring in other nonsulfide deposits in Tunisia.…”

Thick‐skinned tectonics within the intracontinental easternmost Atlas foreland‐and‐thrust belt (Tunisia): Meso‐Cenozoic kinematics and implications for regional geodynamics

“…G = La Galite Island, N = Nefza IC = Ichkeul, H = Hairech; PG = Pantelleria Graben, LG = Linosa Graben, MG = Malta Graben. High 3 He/ 4 He indicate mantle‐derived helium anomalies obtained from Fourré et al [].…”

“…This region is considered highly contrasting in terms of subsidence, magmatism, topography, and kinematics compared to the adjacent Central Atlas (Figure b) [ Ellouz , ; Burollet , ]. Although the deep basement is concealed beneath a thick sedimentary succession, observations based on the helium‐carbon isotopic composition revealed crustal‐derived helium and CO 2 in several thermo‐mineral manifestations (Figure ) [ Fourré et al , ]. On the other hand, a regional mantle Helium anomaly is reported by Bent Jedidi, Hammam Zriba, and Ain Garci ( 3 He/ 4 He reaches 2.4 Ra) south of the ZTB [ Fourré et al , ] (Figures and b).…”

“…Although the deep basement is concealed beneath a thick sedimentary succession, observations based on the helium‐carbon isotopic composition revealed crustal‐derived helium and CO 2 in several thermo‐mineral manifestations (Figure ) [ Fourré et al , ]. On the other hand, a regional mantle Helium anomaly is reported by Bent Jedidi, Hammam Zriba, and Ain Garci ( 3 He/ 4 He reaches 2.4 Ra) south of the ZTB [ Fourré et al , ] (Figures and b). These observations support that the basement is significantly affected by deep‐rooted faults that favor the rise of crustal and mantle‐derived elements to shallow levels.…”

An Evaluation of the Long-Term Stagnancy of Porewater in the Neogene Sedimentary Rocks in Northern Japan

“…3 He/ 4 He is a useful indicator of the origin of He [35,[57][58][59]. In the following, 3 He/ 4 He of He produced in both the Koetoi and Wakkanai formations was calculated according to Tolstikhin et al, [60].…”