High hydrothermal activity is concentrated in the crater of Ushishir volcano which represents an almost closed bay connected with the ocean through a narrow and shallow strait. In their composition the thermal waters can be divided into two groups: (1) thermal waters of sea origin of high mineralization altered as a result of high-temperature interaction with rocks and (2) waters of sea origin heated in the near-surface conditions and mixed with fresh meteoric water. Ion and gas geothermometry as well as mixing plots in Na - 1ма_к and CI - coordinates suggest that these thermal vents are fed by steam-water geothermal reservoir with temperature of about 260°C; mineralization of the fluid in equilibrium zone is 23 g/l, C02 pressure being about 4 bar. Calculations of the equilibrium solution composition in the closed "water-andesite" system indicate that the observed Mg concentration could be formed in a wide temperature range but at low, lower than 0,01, mass rock-water ratios. Reequilibration of the solution at temperatures of 170-200°C in the near-surface conditions is most probable. The main discharge takes place primarily in the intersection zone of the ring-shaped and linear faults. Through the ring-shaped fracture mostly gas and steam-heated waters are discharged.

The study of present-day hydrothermal ore-forming process in Uzon caldera (Kamchatka, Russia) is reported. The structure of ore body and the distribution components are described. Parameters of ore-forming process (temperature, pH, Eh and activity of dissolved hydrogen sulfide) are measured with original experimental devices. Parameters of gas and liquid phase flow and heat of hydrothermal fluid are studied in details. Base on this investigation results, the thermodynamic model of ore-forming process in Uzon caldera is designed. This model includes several ore-forming mechanisms: boiling, cooling and mixing and oxidization of hydrothermal solution. The analysis of thermodynamic simulation results of is compared with the data on mineralogical data and concentration of forms of sulfur that has allowed establishing a role of various ore-forming factors in ore deposition. The new model of hydrothermal ore process which explains formation of the full ore structure as consequence of self-development of thermal solutions is offered. Calculated concentration of the dissolved forms of sulphur and ore components are in the good consent with the natural data. Results of the spent works can be useful at research of hydrothermal deposits and modern geothermal systems.

The chemical and isotopic composition of hot springs at the apical part of the active Ebeko volcano (Paramushir Island, Kuril Islands) is characterized based on the data obtained from fieldwork in 2020–2021. Thermal waters discharging from one of the sources of the Kuzminka River are ultra-acid (рН<2) Al-Ca-SO4-Cl type with mineralization up to 5 g/l and temperature up to 70°С. The anionic composition of waters is formed due to the dissolution in groundwater of acidic volcanic gases, partially «purified» in the main reservoir of the volcano's hydrothermal system. The cationic composition of waters, including rare-earth elements, is formed by isochemical dissolution of host rocks in the equivalent of 5 g per 1 liter of water. Differences in the isotopic composition and ratios of macrocomponents (SO4 /Cl, Al+Fe/Ca+Mg/Na+K) of waters of the near the summit  prings and the northwestern slope suggest the presence of different-level aquifers in the hydrothermal system confined to the edifice of the Ebeko volcano.

Data are presented about the chemical and isotopic composition of fumarolic gases and thermal water collected in the crater of the Mutnovskiy volcano from 1963 to 1989. A thermodynamic approach, balance estimates and information on the hydrogeological characteristics of the Mutnovskiy geothermal field and volcanic structure were used in the interpretation of the data. Three groups of fumarolic vents in the craters of the volcano have been active for many years (the Aktivnaya cone, the Verkhneye field and the Donnoye field) and differ in the composition of their gases, the isotopic composition of the water, a activity and He ratios. It was shown that the composition of the fumarolic gases is formed as a result of the interaction between the magmatic fluid, the surface and thermal waters and also the boiling of ultra-acidic brines-the mixed waters that form the local hydrothermal system of the Donnoye field in the crater of the Mutnovskiy volcano.