Thermal springs associated with the Melilla-Fès-Smaala-Oulmès fault (Morocco): The role of fluid geochemistry in identifying a major active geodynamic structure

• Published in: Applied geochemistry 2024-09, Vol.170, p.106085, Article 106085
• Main Authors: Ntarmouchant, A., Jeddi, E.M., Carvalho, M.R., Bento dos Santos, T.M., Smaili, H., Cotrim, B., Cachapuz, P., Driouch, Y., Mali, B., Ntarmouchant, N., Elabouyi, M., Ferreira da Silva, E.A.
• Format: Article
• Language: English
• Subjects: Eastern rif; Geochemistry; Isotopes; Melilla-fès-smaala-oulmès fault; Rock-water-gas interaction; Thermal waters
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2024.106085

The thermal springs form a hydrothermal system implanted on the active tectonic structure of the Melilla - Fès - Smaala - Oulmès fault (MFSO). This fault forms a multiple play tectonic corridor generally oriented NE-SW parallel to the Nekor fault. It crosses two structural domains, the Meseta, to the South, and the Rif, to the North, forming a highly complex deep shear zone. The thermal waters from these springs have chloride-sodium facies and sodium-calcium bicarbonate facies with emergence temperatures between 24.4 and 53.6 °C. Along this fault, the pH of these waters varies centrifugally, with acidic values in the South and North and neutral values in the intermediate sector. Important information about the origin of thermal waters and the characteristics of the reservoirs is provided by the chemical compositions (chemical elements, isotopes, and gases) of these waters involving rock-water-gas interaction during their circulation in depth and flow to the surface, through different types of reservoirs intersected by the fracture networks associated to the MFSO. These reservoirs correspond to the crystalline basement and its Triassic evaporitic clay, Jurassic carbonate, Cretaceous marl, and Miocene saliferous cover, whose temperatures have been estimated between 100 and 200 °C. The stable isotopic compositions (−8.39 to −5.2‰ for δ18O and −51.2 to 30.1‰ for δ2H) indicate a meteoric origin of these waters, whose recharge areas are located in the Rif and Atlas reliefs. The gas phase present in the waters shows δ13Cco2 (−19.3 ‰ to −4,25 ‰) and 3He/4He (0.203–3.864 Ra) with mantellic signature, implying that the fault is deeply rooted. The mantellic helium content decreases progressively in these waters from SE to NE, following the direction of the MFSO fault, from the emergencies in the Palaeozoic basement to the springs in the secondary and Tertiary formations of the Prerif and Mesorif. This decrease results probably from the mixing with gases derived from the thick secondary and Tertiary sedimentary cover. These waters would also collect these gases during their interactions with these sedimentary formations. Towards the NE, these waters, which rise within the formations of the Mediterranean arc, are also influenced by gases derived from the sediments of volcanic arcs. The MFSO fault, which is permeable due to its fracture network, favours the diffusion of heat and the upwelling of mantle gases, which are collected by the fluids that will interact with the