Geochemistry and conceptual model of thermal waters from Erciş - Zilan Valley, Eastern Turkey

•The Zilan Valley is a low-temperature, fracture zone system.•In this system the heat is transferred by advection, involving deeply penetrating meteoric waters.•Thermal and mineral waters are NaCL and NaHCO3 type.•Thermal waters are developed from hydrothermal system that have two distinct structures.•A conceptual model was developed for a hydrogeological flow system in the Zilan area. Faults related to the neotectonic stress regime of Eastern Anatolia permit the rise and outflow of thermal waters in the Zilan Valley located 30 km north of the Erciş, Van province. Thermal springs discharge in a volcanic terrain, often in clusters, along faults in two separate areas: In the vicinity of Taşkapı (Şörköy) and Gergili villages, which lie in the north of the area; and in the area of Hasanabdal (Doğancı) thermal springs that lie to the south of the Zilan Valley. Temperatures of the springs range from 20 to 78 °C, with discharge rates of 4−20 L/s. Thermal waters of the Zilan region are of different types, from Na−HCO3 to Na-Cl, with dissolved ion concentrations varying over a wide range between 484–4572 mg/L. Calcite dissolution and silicate hydrolysis are the dominant water-rock interactions. Constituents with minor and trace element concentrations reinforce conclusions drawn from major-ion composition that the major hydrogeochemical processes are: mixing between cold-shallow and hot-deep waters, loss of energy by heat conduction, and steam-heating of perched-aquifers. The cold spring and Zilan Creek waters are of the Ca−HCO3 and Na−HCO3 type respectively, and their dissolved ion concentrations are low. Hydrogeochemical processes for the cold waters involve carbonate dissolution and silicate weathering reactions. Chemical geothermometers are used to estimate the temperature of the deep fluids using the chemical composition of the less modified fluids. They indicate that fluids emerging from volcanic rocks in the Zilan valley reach temperatures of around 150 °C. Thermal waters, generated through conductive heat transfer or the input of geothermal vapor or gases from below, delineate the extent of the geothermal reservoir(s) at depth. Deuterium and oxygen-18 signatures of thermal waters indicate that they are at least partially derived from thermally heated water through a process of 18O enrichment owing to interaction with the marine limestones through which the water passes to the surface springs. Both geochemical and isotope data indicate that the Zilan Valley h