Genesis of thermal waters from the Taşkesti-Sarıot geothermal prospect in Mudurnu (Bolu, NW Turkey)

Abstract

The Taşkesti–Sarıot study area is located in northwest Turkey, about 23 km NW of Mudurnu town in the Bolu province. The thermal waters of the Taşkesti–Sarıot were investigated by means of field measurements, chemical analyses and isotope investigations. An area of 20 km2 was geologically mapped around the spa and characteristics of thermal and cold-waters were determined. The Taşkesti-Sarıot thermal waters, (T °C = 63, and flowrate of 1.70 L/sec) emanates along a secondary fault in the North Anatolian Fault Zone (NAFZ). Geothermal water reservoirs are present within fractured limestone, micaschists, diorite, tonalite, serpentinite, gabbropegmatites and graywacke which have the secondary porosity and permeability. Based on the main consitituents, the thermal waters can be classfied as Na(Ca)SO4 types, whereas the cold and stream waters were characterized as a Ca(Mg)HCO3 type waters. Geochemical processes responsible for the genesis of the hydrochemical features of the waters include dissolution, mixing, oxidation and loss of energy by heat conduction. Chemical geothermometers suggest deep temperatures close to 100 °C for the deepest geothermal well (SK-1: 63 °C at the surface) water in the region and thus the Taşkesti-Sarıot geothermal system can be classified as a low temperature resource.These thermal waters are of meteoric origin that circulate and restores heat through the fault zone due to the geothermal gradient and discharge to the surface. The thermal waters are derived from advective flow as indicated by the δ2H and 18O isotope data, which plot on the meteoric water line showing a significant downward shift if compared with a local reference point thus indicating recharge by infiltration derived from a high standing (colder) terrain. The recharge area for the thermal waters is estimated to be nearly 1000 – 1600 m from the northeast part of the study area. Moreover, there is no evidence for isotope exchange between hot waters and reservoir rock. Tritium data confirm the deep origin of the thermal waters. Finally, a conceptual geochemical model of the geothermal system is proposed.