We have studied the macrocomponent composition of pore solutions in hydrothermal clays forming extended thick strata in the thermal fields of the Pauzhetka geothermal system. Two zones have been identified in the vertical sections of the clay strata, which differ in the physicochemical parameters, composition, and formation conditions of pore solutions. It is shown that the pH of the solutions plays a crucial role in the change of their macrocomponent composition with depth. The conclusion is drawn that the pore solutions resulted from the direct impact of deep-level infiltrating thermal waters on the matrix of hydrothermal clays, which led to the redistribution of elements between the rock and the contact solution. Along with the general regularities, we have established significantly different conditions of formation of pore solutions in the Upper Pauzhetka and East Pauzhetka thermal fields, which is due to the different geologic positions and hydrogeochemical regimes of these fields.

На основании комплексных структурно-геофизических исследований и обобщения геофизических, гидрогеологических, минералого-геохимических и других данных показано, что термальные поля южной группы Камбального вулканического хребта (Южная Камчатка) входят в состав современной гид­ротермальной системы. Получены новые данные о строении зоны разгрузки парогидротерм до глубины около 300 м. Установлена структурная и гидродинамическая связь двух термальных полей этой группы. Предполагается единое тепловое питание геотермальной системы Камбального хребта, объединяющей все основные группы термальных полей. Based on integrated structural and geophysical studies and generalization of geophysical, hydrogeological, mineralogical, geochemical and other data, it is shown that the southern cluster of thermal fields of the Kambalny volcanic ridge (South Kamchatka) is included in the modern hydrothermal system. New data on the structure of the steam hydrotherm discharge area investigated to a depth of about 300 m have been obtained. The structural and hydrodynamic connection between two thermal fields from this group has been established. The Kambalny ridge geothermal system embracing all major clusters of thermal fields is assumed to have a single deep-seated source of heat supply.

--Based on integrated structural and geophysical studies and generalization of geophysical, hydrogeological, mineralogical, geochemical and other data, it is shown that the southern cluster of thermal fields of the Kambalny volcanic ridge (South Kamchatka) is included in the modern hydrothermal system. New data on the structure of the steam hydrotherm discharge area investigated to a depth of about 300 m have been obtained. The structural and hydrodynamic connection between two thermal fields from this group has been established. The Kambalny Ridge geothermal system embracing all major clusters of thermal fields is assumed to have a single deep-seated source of heat supply.

On the Pauzhetka geothermal field the mineral deposits which are formed, when dumping thermal water of separators of wells, are allocated. Compositions, structure and geochemical properties of this precipitation on pro-deleting and in vertical slits of “raincoats” are studied. It is established that they are put X-ray amorphous mordenite – opal mixes (in the beginning dumping of thermal waters), further precipitation becomes completely siliceous. The zeolitic component of mineral deposits defines their high sorption properties in the relation of Au, Ag, Hg, As, Rb, Sr, Ba, Cs, etc. elements; in a mordenitovy matrix sulfides of iron, silver, copper are formed. It is shown that the mineral deposits which are formed on the day surface of the Pauzhetka geothermal field are the indicator of alkaline mineral- and the ore-forming processes on the lower horizons of the Pauzhetka hydrothermal system.

The features of the transformation of andesites of the East Pauzhetka thermal field under the influence of the argillization process are studied based on a number of samples with different degrees of alteration, which were taken from boreholes and outcrops near the study object. The main patterns of changes in the composition and physical and mechanical properties of andesites have been revealed. A schematic diagram of the structure of the thermal field in section has been compiled; this diagram distinguishes three horizons from bottom to top, showing an increase in the degree of hydrothermal transformation: altered andesites, metasomatic breccias, and hydrothermal clays.

Samples of siliceous rocks of the Southern Kambalny Central Thermal Field (SKC) containing a unique ore mineralization were studied. Optical microscopy, scanning electron microscopy, X-ray microanalysis, X-ray diffraction, ICP-MS, and Raman spectroscopy were used in this study. High concentrations and a wide range of rare and rare-earth elements were found in siliceous rocks. Silicates (quartz, moganite, and opal-crystobalite/tridymite opal), oxides (hematite and anatase), hydroxides (goethite), carbonates (calcite with Fe and Mn impurities), sulfates (barite with Sr impurity and gypsum), sulfides (pyrite, marcasite, chalcopyrite, and chalcocite), phosphates (xenotime-Y, YPO4 with impurities of lanthanides, S, Ca, and As; berlinite, AlPO4 with the V impurity), and apatite were identified. Structures of anatase replacement by quartz often in association with pyrite were identified. The mineralization of SKC siliceous rocks reflects the physicochemical specificity of deep metal-bearing solutions.

Ground temperature survey at three thermal fields of the southern group of the Kambalny volcanic ridge (South Kambalny Near, Central and Far thermal fields) was carried out. Their current state is characterized. The formation of temperature anomalies is determined by the peculiarities of the geological structure of each thermal field.The maps of temperature surveys obtained in 2021 and 1965 are compared. Changes in the conditions of convective heat flow unloading in the geologic structure of thermal fields are shown. The size of temperature fields on all geologic structures of the southern group of the Kambalny volcanic ridge have significantly increased over time. The area of the most warmed areas of thermoanomalies has increased. The maximum ground temperatures increased by 10 °С on average. Some thermoanomalies (boiling and pulsating water boilers and vapor-gas jets) increased their flow rates and (or) temperature. New warmed areas within the thermal anomalies were discovered.

Our multiyear studies have enabled us to make maps showing magnetic anomalies ΔTa for major geothermal systems in the Pauzhetka area of southern Kamchatka. Magnetic fields possess both general characteristics and individual features for each object of study. The Nizhne-Koshelev vapor-dominated geothermal field is identified on the basis of a set of linear negative magnetic anomalies confined to thermal-controlling tectonic faults. The Pauzhetka geothermal field is characterized by an inhomogeneous structure of the anomalous magnetic field ΔTa: the northwestern area has a quiet, slightly negative magnetic field, showing that this part of the field is dominated by lateral spreading of hydrothermal fluids from the upper aquifer; the southeastern part has numerous sign-varying magnetic anomalies of high intensity confined to acid–intermediate subvolcanic bodies. The South Kambalnyi group of thermal fields is characterized by a lower absolute value of magnetic induction Т compared with the Pauzhetka and the Nizhne-Koshelev geothermal fields, showing that the rocks of the Kambalnyi volcanic range are more intensely altered by hydrothermal and metasomatic processes, probably as a result of a long-continued action of convective heat flow.

—A sequence of argillized rocks and hydrothermal clays of the East Pauzhetka thermal field was studied in detail by means of drilling and trenching. We have identified zones composed of unusual mineral associations; their formation conditions are considered. The structure of the base of the hydrothermal-clay mass is shown. The source rocks (brecciated andesites) of the base are altered by hydrothermal–metasomatic processes and are represented by smectite–chlorite–K-feldspar–zeolite–carbonate–siliceous aggregate with sulfides, phosphates, titanium silicates and zircon silicates, and other mineral phases, including rare metals. A conceptual geological and geochemical model for the formation of argillisites and zones with mineral associations has been plotted. It is assumed that modern mineral formation in the East Pauzhetka thermal field inherits an epithermal ore-forming system located in the influence zone of a deep-seated fluid, the derivates of which are outflowing near the surface as alkaline metal-bearing solutions of the Pauzhetka hydrothermal system.

Based on many years of research, maps of magnetic field anomalies ΔTa for large geothermal systems of the Pauzhetsky region of South Kamchatka were constructed. Magnetic fields have both general characteristics and individual features for each research object. In the area of Nizhne-Koshelevsky steam-dominated geothermal field identified a system of linear negative magnetic field anomalies, confined to thermal controlling discontinuous tectonic disturbances. Pauzhetsky geothermal field is characterized by a heterogeneous structure of the anomalous magnetic field ΔTa: NW region is marked by a quiet weakly negative magnetic field, indicating the predominance in this part of the field lateral spreading of hydrotherms from the upper aquifer; SE – a large number of alternating magnetic anomalies of high intensity, confined to subvolcanic bodies of acidic to moderate composition. The South Kambalnaya group of thermal fields is characterized by a lower magnetic induction modulus T as compared to the Pauzhetsky and Lower Koshelevsky geothermal fields, indicating a more intense alteration of the Kambalnaya Ridge rocks by hydrothermal-metasomatic processes, apparently as a result of long-term exposure to convective heat flow.

The Pauzhetka Caldera (27 × 18 km) was formed in the South Kamchatka during the Golygin Ignimbrite eruption (420–440 ka), the largest known eruption in the region in the past 1 Myr. The eruption was preceded by the 3 Ma-old mafic and intermediate volcanism. After the caldera-forming eruption, a variety of products, from basalt to rhyolite, were ejected within the caldera. For understanding the origin of voluminous silicic magmatism in thin mafic South Kamchatka crust, we used geochemical and isotope data. Our research has characterized the major and trace element composition of Golygin ignimbrite, intra-caldera hydrothermally altered deposits, pre-caldera (Mt. Orlinoe Krylo, Mt. Klyuchevskaya) and post-caldera (Kambalny Ridge, Chernye Skaly) eruptive centers. The Sr–Nd isotope composition of the Golygin ignimbrite and some eruptive post-caldera products was investigated. The isotope variations indicate that parental magmas for all rocks of the Pauzhetka area were obtained from a weakly evolved source derived through fluid-assisted melting of a subducted slab. Geochemical data support that the formation of most magmas of the Pauzhetka caldera was mainly controlled by fractional crystallization in the lower to middle crust. MELTS-modelling agrees with geochemical data. The fractional crystallization of Kambalny basalt with 2 wt % H2O at 6 kbar provides the best fit to the observed composition of the Golygin dacite.

The argillized deposits of the East Pauzhetka thermal field in the Pauzhetka hydrothermal system were found to contain a zone of intensive zeolitization which consists of medium to high silica calcium zeolite varieties, namely, laumontite, mordenite, heulandite-Ca, and stilbite-Ca. Of all these, stilbite-Ca has the highest abundance. The zeolites can be identified well enough based both on the relationship between Si and Al, which are part of the zeolite framework, and on the concentrations of non-framework cations (Ca, Mg, Na, K, Sr, and Ba). Zeolites are mostly formed in the lower horizons of hydrothermal clays and in the underlying argillized andesites, as deep alkaline chloride-sodium hydrothermal fluids are discharged and are mixed with condensates of acid (up to neutral) vapor. The resulting sequence of zeolite generation in the shallow zone where alkaline solutions are discharged reflects, to a certain degree, a change of zeolite facies in the deeper horizons of the system: from medium-silica laumontite to high silica stilbite-Ca. We have thereby a general inference as to the regressive directivity of hydrothermal metamorphic processes in the Pauzhetka system structure, from medium-temperature propylites generated during the paleo phase to the present-day low-temperature mudstones.

In this study we present the results of tomography studies for seismic velocity in the area of Kambalny volcano (Southern Kamchatka). After a long repose stage, on March 24, 2017, it produced a strong phreatic eruption, which ejected an ash cloud to the distance of up to 1000 km. We have obtained the first 3D model of seismic velocities beneath the area of Kambalny based on the data recorded by a temporal network of ten seismic stations installed for one year in 2018-2019. The distributions of velocities of the P and S seismic waves, and especially the Vp/Vs ratio, provide the information on the geometry of the plumbing system beneath the volcano in the upper crust down to ~10 km, which makes it possible to build a scenario of preparation and occurrence of the explosive eruption in 2017. We clearly identify an anomaly of high Vp/Vs ratio in the depth interval of 7-10 km, which is interpreted as a magma reservoir responsible for Holocene activity of Kambalny. This reservoir appears to be connected with the volcano edifice by a linear zone of high Vp/Vs ratio, which may represent a system of fractures originated during the eruption in March 2017 and served as a pathway for magma ascent. We propose that the interaction of hot magma with meteoric fluids in shallow layers caused active boiling and steam formation in a closed reservoir below the volcano. After exceeding a critical pressure, the steam escaped to the surface causing an explosive eruption. We also found evidence that geothermal fields located to the north and northwest of Kambalny might be fed from separate deep sources. The area of Kambalny is characterized by strong geothermal activity, most of which is located to the north and to the west of the volcano. The northern geothermal manifestations mostly occur on the northern part of the Kambalny Ridge and in the Pauzhetka depression. We found that the geothermal activity in these areas is likely associated with a deep source, which appears to be isolated from the magma reservoir below Kambalny volcano. A similar isolated anomaly is observed below geothermal fields in the area of the Koshelev volcano to the west, which may indicate that the geothermal activity appears to be independent of the magmatic system of Kambalny volcano, at least for its upper-crustal part. This study was partially supported by the Russian Science Foundation project # 20-17-00075.

На Восточно-Паужетском термальном поле с помощью бурения скважин и проходки шурфов детально изучена толща аргиллизированных пород и гидротермальных глин. Выделены зоны, сложенные минеральными ассоциациями необычного состава, рассмотрены условия их образования. Показано строение основания толщи гидротермальных глин, исходные породы которого (брекчированные андезиты) полностью изменены гидротермально-метасоматическими процессами и представлены смектит-хлорит-калишпат-цеолит-карбонат-кремнистым агрегатом с сульфидами, фосфатами, титанои цирконо-силикатами и др. минеральными фазами, включающими редкие металлы. Построена концептуальная геолого-геохимическая модель формирования аргиллизитов и зон, содержащих минеральные рудные ассоциации. Предполагается, что современное минералообразование на Восточно-Паужетском термальном поле наследует эпитермальную рудообразующую систему, расположенную в зоне влияния глубинного восстановленного флюида, дериваты которого разгружаются вблизи дневной поверхности в форме щелочных металлоносных растворов Паужетской гидротермальной системы.

Carbonate concretions are formed at the base of a sequence of hydrothermal clay in the South Kambalnyi Central Thermal Field situated in the southern part of the Kambalnyi volcanic mountain range, Kamchatka. The concretions have complex chemical and mineral compositions: apart from aragonite which is the main component of each layer, the chemical compounds identified there include oxides of iron and silicon, sulfates of calcium and barium, sulfides of iron and other metals, carbonates of iron and manganese, siliceous ferromanganese formations, nitrogen compounds, and phosphates of calcium and rare metals. The concretions have diverse structures and textures that indicate a multiphase character of formation for these mineral aggregates. It is thought that their formation was due to the discharge of deep-seated alkaline metalliferous solutions in a zone of rock argillization of the South Kambalnyi Central Thermal Field.

The result of ambient noise tomography for the Kambalny Volcano (South Kamchatka), where the first time in the entire history of observations a violent phreatic eruption was observed in March–April 2017, is presented. The results obtained clarify the structure of the upper part of the edifice of the Kambalny Volcano and are consistent with independent data on body waves, as well as with geological information. According to seismotomographic data of the surface waves, low-velocity anomalies are distinguished in the model of the structure of the volcanic edifice. They are asymmetric relative to the volcano cone and are allocated to loose pyroclastic deposits of past eruptions and to deep sources of hydrothermal activity. Perhaps the migration of fluids in these hydrothermal vents to the north and west of the volcano and their interaction with the magma chamber in the upper crust caused the explosive eruption.

Multidisciplinary geophysical surveys have been carried out in the Pauzhetka hydrothermal system area and around in order to identify 10–15-km deep geological structures that exercise thermal control of the system. The results of vertical electrical and electromagnetic sounding, magnetometric and gravity surveys have been integrated, and geological–geophysical data have been generalized, to develop a model of layered block structure for the Pauzhetka hydrothermal system area. We provide an explanation of fragmentation (permeability) in upper crustal horizons in the structure of the present-day hydrothermal system and the geothermal field. A horizon of less dense rocks has been identified at depths of 3–4 to 8–9 km; this horizon can be the source of thermal supply for known and hidden temperature anomalies in the Pauzhetka–Kambalnyi–Koshelev geothermal area in southern Kamchatka.