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 degrees C at 100 m below the surfa ce, are tied to horst-like basement uplifts along deep-reaching faults . Inorganic carbon dissolved in thermal waters of the region is isotop ically rather heavy, indicating an endogenic origin. In spite of the c onsiderable variation, delta(18) does not allow differentiation of the rmal and non-thermal waters. It does not agree with the isotopic patte rn of recently recharged groundwaters. delta(13)C in fossil spring dep osits of tufa towers and bedded travertines indicates a derivation fro m endogenic CO2 similar to that dissolved in present thermal waters. I n contrast, their delta(18)O is remarkably high, suggesting evaporated basin waters as a possible source.