Geochemistry and Conditions of the Formation of the Ulsk Thermal Spring (Coasts of the Sea of Okhotsk, Khabarovsk Krai)

The report presents original data on the isotope and chemical composition of the nitric thermal, cold underground, and surface natural waters, along with the water-enclosing rocks, of the Ulsk occurrence of thermal mineral waters (coasts of the Sea of Okhotsk, Khabarovsk krai). The data on the concentrations of oxygen and nitrogen isotopes and rare-earth elements, as well as on the volume activity of radon, in the treated underground and surface waters are obtained for the first time. Based on the data of automated monitoring of the physical parameters, the hydrogeological characteristics of the thermal waters are determined and the balneal properties are evaluated. The results obtained show the atmospheric origin of the nitric thermal waters of the Ulsk spring; however, the indicative chemical elements occurring in the considered waters represent the effects of deep-seated high-temperature processes as well. It is found that the waters warmed to 31°C within fractured Paleocene granites do not show significant variations of temperature, and the chemical composition varies exclusively under the interaction with the water-enclosing rocks. The profiles of the distribution of rare-earth elements (REEs) in the thermal waters represent the REE distribution in the water-enclosing granitoids and point to the limitation of the thermal water circulation with the area of the occurrence of the Paleocene granites of the Bekchi–Ul Massif. The temperature of the deep-seated reservoir measured by geothermometers was 80°C or below, which may indicate a depth of water circulation of 1–2 km or less.