The Geothermal Systems of the Vallès Fault (NE of Spain): Fracture Network Characterization and Weathering Patterns

The ongoing energetic transition needs critical raw materials such as lithium or other metals and renewable energies, such as low to mid-enthalpy geothermal energy. Geothermal systems, mostly located near plate boundaries, undergo strong structural controls essentially through crustal faults, also known to drain fluids. Furthermore, dissolved metals in geothermal fluids can be exploited as a byproduct in geothermal plants. In the present, study we define spatial variations in metal content and the structural control along the Vallès Fault, as a field analogue of a passive margin domain. The Vallès Fault is a crustal fault that juxtaposes the Miocene detrital basin infill to a syn- and late-Hercynian granitic basement. With a crustal thermal anomaly, a geothermal gradient > 35 °C/km, several geothermal systems of low enthalpy (< 130 °C at 1 km depth) are closely associated to the Fault. These geothermal systems are revealed by several springs with temperatures ranging from 20 to 75 °C. The present work combines hydrogeochemical, structural and petrological approaches to characterize the architecture and the relative chronology of the fracture networks of the sites of hot springs; and the metal content of thermal waters to reconstruct the spatial evolution of the metal content of the geothermal fluids. From the structural and petrological point of view, three main results are highlighted: (1) the footwall of the Vallès Fault is characterized by a network of polyphase fractures affecting the granite basement and constituting an important network of permeability by fracturing; (2) a thick fault zone with cataclastic to ultracataclastic fault rocks constitutes, a priori, a barrier for fluids circulation; and (3) an arkosic Miocene basin infill, affected by secondary faults, constitutes a very good reservoir. The metal content of hot waters is low but significant to establish differences according to their structural position. Tectonic structures and surrounding lithologies seem to be decisive in water compositions.