Isotopic characteristics of geothermal waters and fossil spring deposits in Mygdonia Basin, Northern Greece

Mygdonia Basin is an active seismotectonic zone. Its regional geology is characterized by step-faulted metamorphosed Paleozoic and Mesozoic sediments partly intruded by igneous rocks. Two geothermal anomalies, heating groundwater to more than 40°C at 100 m below the surface, are tied to horst-like basement uplifts along deep-reaching faults. Inorganic carbon dissolved in thermal waters of the region is isotopically rather heavy, indicating an endogenic origin. In spite of the considerable variation, δ¹⁸ does not allow differentiation of thermal and non-thermal waters. It does not agree with the isotopic pattern of recently recharged groundwaters. δ¹³C in fossil spring deposits of tufa towers and bedded travertines indicates a derivation from endogenic CO₂ similar to that dissolved in present thermal waters. In contrast, their δ¹⁸O is remarkably high, suggesting evaporated basin waters as a possible source.