Geochemical Survey of low- temperature geothermal resource of the Reşadiye Spa (Tokat, Northern Turkey)

Abstract

The low-temperature geothermal area of Reşadiye is located within the Kelkit Valley in the middle of the Black Sea region of Turkey. The area is covered with widespread travertine deposits which are acquired immediately on deposition from hot spring waters. Thermal waters have temperatures between 42 °C and 49 °C and the discharge rate is 3 L/sec. Total dissolved solids range from 3686 to 4204 mg/L. These thermal waters are of Na-HCO3-Cl type. A geothermal reservoir having probable temperature of about 100° is the source of thermal water for the springs. Before emerging along the margins of exposed carbonate formations through the faults and fractures, the waters interacts sometimes with Na-rich Neogene sediments and/or with B-rich rising geothermal fluids as confirmed by high concentrations of Cl, and, to a greater degree, by the presence of B. The conductive process along with the mixing of thermal waters with shallow groundwaters is the main cooling mechanism. Deuterium and oxygen-18 signatures of thermal groundwater indicate its meteoric origin. Equilibrium states of the Reşadiye thermal waters studied by means of the Na–K–Mg triangular diagram, K–Mg–Ca geoindicator diagram, and finally the activity diagrams in the systems composed of Na2O–CaO–MgO-K2O–Al2O3–SiO2–CO2–H2O phases indicate that the springs and low-temperature geothermal well waters in the area can be classified as shallow or mixed waters which are likely to be equilibrated with muscovit at temperature similar to those calculated from the chemical geothermometers. It was also observed that mineral equilibrium in Reşadiye waters is largely controlled by CO2 concentration. Based on acquired data, a conceptual model of the geothermal system of Reşadiye is proposed.