The Composition, Structure, and Origin of Carbonate Concretions Sampled in the South Kambalnyi Central Thermal Field, Kamchatka

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.


INTRODUCTION
Concretions or concretions (from the Latin concretio meaning condensing, congealing) are understood to be mineral bodies that are formed by material concentrating around rock fragments, organic remains, or other nucleii, and by successive growth of layers (zones) from center to periphery of a mineral aggregate. Concretions are sharply different from the surrounding mineral environment by composition and properties, consequently, they easily remain after rock disintegration, frequently making mineral deposits of the placer type (Geologicheskii slovar …, 1973).
In recent decades special attention has been paid to ferromanganese concretions of sedimentary hydrothermal origin; they are generally enriched in many metals, so can be used as polymetallic mineral deposits (Zhelezo-margantsevye …, 1989;Kolesnik and Astakhova, 2018;Kolesnik and Kolesnik, 2013;Krasnov, 1995;Strakhov, 1974). Mineral concretions of the hydrothermal type proper are of less practical importance, and are less known, probably for that reason (Granina et al., 2010;Manceau et al., 2007). However, they can still supply rich information on the composition and conditions of discharge for metalliferous solutions, phases of mineral generation, and the evolution of the entire geothermal system (Rychagov et al., 2020b).
The present paper is devoted to a description of carbonate concretions found in the South Kambalnyi Central Thermal Field. They were first described as "pseudo-globular aragonite aggregates" (Karpov, 1970). No other publications dealing with these mineral objects are available, in spite of the fact that diverse research has been conducted in thermal fields of the Kambalnyi volcanic mountain range (Belousov et al., 1976;Ogorodova, 1974;Struktura …, 1993). Our interest in these objects is due to the following reasons: 1) the concretions are not monomineral ones, they have a complex chemical composition; 2) they are formed at the base of a hydrothermal clay mass in the hottest area of the thermal field, so can serve as an indicator of localized discharges of an alkaline metalliferous solution by analogy with the East Pauzhetka thermal field (Rychagov et al., 2017b); 3) the textural and structural peculiarities of these concretions are very diverse, so they attract attention as collectible material.

A SHORT DESCRIPTION OF THE SOUTH KAMBALNYI CENTRAL THERMAL FIELD
This thermal field is situated in the southern Kambalnyi mountain range, which is a resurgent tectonomagmatic uplift in the structure of the Quaternary Pauzhetka volcano-tectonic depression (Dolgozhivushchii …, 1980;Struktura …, 1993) (Fig. 1). To the south the mountain range terminates in the active Kambalnyi volcano of basaltic composition (which is situated beyond the lower boundary, see Fig. 1) whose last eruption took place in March-April 2017(Girina et al., 2017Rychagov et al., 2017a). The volcanic range is composed of distinct stratovolcanoes, extrusions, and subvolcanic bodies of Pliocene to Pleistocene age ranging in composition between daciandes-ites and basalts (Belousov, 1978). The prevailing rocks are andesites that commonly show considerable hydrothermal alteration (Frolova et al., 2020). The only rocks that have not been altered are Late Quaternary basalts.
The Kambalnyi volcanic mountain range is characterized by intense geothermal activity (Nekhoroshev, 1959;Struktura …, 1993;Sugrobov, 1979). There are three sets of thermal fields aligned along the axial part of the range at distances of 18-20 km: North, Central, and South Kambalnyi (Belousov et al., 1976). Each set probably represents a large present-day hydrothermal system. It is supposed that "distinct" hydrothermal systems of the Kambalnyi Range may have hydrodynamic connections and a single thermal and water recharge (Rychagov et al., 2017a(Rychagov et al., , 2020a. Special interest attaches to the set of the South Kambalnyi thermal fields situated at shortest distances (7-8 km) from the active volcano. This part of the mountain range includes a complex volcano, Termalnyi (Nekhoroshev, 1959) or North Kambalnyi (Syvorotkin, 1993), dating back to the Middle-Late Pleistocene and which is composed of daciandesites and basaltic andesites. The structure of this volcano is well described in the works of A.S. Nekhoroshev, V.I. Belousov, and V.L. Syvorotkin referred to above. The schematic geological map shown here has this volcano (2) volcanogenic-sedimentary deposits (Q I-II , Pauzhetka suite); (3) ignimbrites (Q II ); (4) magmatic unit of the Koshelev volcanic massif: daciandesites -basaltic andesites in the western part (αζ-αβQ I ), dacites -basalts in the central part (ζ-βQ III ), basaltic andesites in the eastern part (α-βQ IV ); (5) extrusive lava unit (the upper section) (from dacites to basalts) of the Kambalnyi volcanic mountain range (ζ-βQ III ); (6) pumice deposits of dacites and rhyolites (ζQ IV ); (7) subvolcanic and extrusive bodies of intermediate and basic compositions (αN 2 , βN 2 ); (8) same, with a contrasting composition ranging from basalts to dacites (βQ III , ζ-βQ III , ζQ IV ); (9) fields of secondary quartzites (a) and of argillisites (b) in the area of the Tretia Rechka paleo-hydrothermal system; (10) main geothermal anomalies and groups of thermal fields (1 Pervye Goryachie Klyuchi (Pionerlager), 2 Vtorye Goryachie Klyuchi (Pauzhetka hydrothermal system), 3 North Kambalnyi, 4 Central Kambalnyi, 5 South Kambalnyi (enclosed in a square as the object of study in the present paper), 6 Ve; rkhne-Koshelev, 7 Nizhne-Koshelev). in a field of unstratified rocks (from dacites to basalts) at the Kambalnyi volcanic mountain range (marked 5 in Fig. 1). The set of the South Kambalnyi thermal anomalies includes three thermal fields: Dalnee, Tsentralnoe (Central), and Blizhnee. We are concerned with the Central Field here.

SEA OF OKHOTSK
The South Kambalnyi Central Thermal Field (SKC) is confined to a circular morphostructure ≥1 km across (Fig. 2), which seems to be an eroded crater belonging to a cone of Termalnyi Volcano (North Kambalnyi). Basaltic andesite relics only slightly altered hydrothermally extend along the periphery of this morphostructure. The 600 × 600-m thermal field makes the central part of the edifice. Because of active erosion, including that due to geothermal processes acting on the relief, the field surface is intersected, and presents a radially concentric system of lows (brook valleys) and highs (small ridges). The most intense geothermal activity is concentrated over an area of about 150 × 250 m. The thermal occurrences include boiling aqueous and mud-water pots, pulsating springs, steam-gas jets, and steaming soils. The soils are composed of hydrothermal clays. The springs and soils had temperatures of ≤98-100°С at the diurnal surface. Unlike many other thermal anomalies in the Pauzhetka-Kambalnyi-Koshelev area, the SKC is characterized by contrasting physico-chemical conditions in the discharge zone of steam-charged hydrothermal occurrences: the water рН varies between 2.8-3.5 and 7.5-8.0 units, Eh is between +410 and -200. The mineral content reaches 4 g/L, which too is not typical of surface discharges of thermal waters in the area (the total salinity is commonly below 0.8-1.5 g/L). Large pots saturated with carbon dioxide and sulfur-bearing gases, and the pulsating springs in the middle of the field, are dominated by alkaline hydrocarbonate-sulfate ammonium (calcium-ammonium) waters. At the same time, springs with neutral and alkaline solutions alternate with acid springs.
The rocks that compose the thermal field are completely argillized. The hydrothermal clay mass is over 4 m thick and shows the classic zonal structure: one can see zones of sulfuric and carbon dioxide leaching (Rychagov et al., 2020a). The upper sulfuric leaching horizon consists of kaolinite-alunite clays with admixtures of opal, pyrite, Fe and Ti oxides, sulfates of Ca, Fe and other metals. The lower horizon of carbon dioxide leaching is mostly composed of smectite- A group minerals (montmorillonite) and includes sulfides, sulfates, and carbonates of a number of metals (Ogorodova, 1974). The thickness of the clay horizons and layers varies strongly over different areas of the thermal field. A horizon of plastic clay 30 to 130 m has been reached in the top of the carbon dioxide leaching zone; the horizon can serve as an aquifer and a thermal screen, on analogy with the East Pauzhetka thermal field (Rychagov et al., 2017b). Three 2-m wells were drilled in the SKC hydrothermal clay to find oxides of Si, Fe, and Ti, sulfates of Ca, Ba, K, and Al, carbonates of Ca, Fe, and Mn, sulfides of Fe, Hg, As, and Sb (Ogorodova, 1974). Our surveys showed that the deposits in the thermal field have a mineral composition of still greater diversity. We have reviewed geophysical, general geological, mineralogical, and hydrogeochemical data to hypothesize that there is discharge of ascending mineralized alkaline thermal waters that are saturated with carbon dioxide and sulfur-bearing gases in the South Kambalnyi Central Thermal Field (Rychagov et al., 2020a(Rychagov et al., , 2020bStruktura …, 1993;Feofilaktov et al., 2020).
However, these hydrothermal occurrences make local discharges upon the background of a widely abundant acid environment. It is known that the steam-gas jets and condensate waters of thermal solfataric fields are characterized by an abundance of dissolved carbon dioxide and hydrogen sulfide (Kononov, 1983;Pampura, 1985]. In addition, microorganisms are widely abundant at the surface and in the upper horizons of the hydrothermal clay mass (hydrogen, thionic, sulfuric microorganisms and iron bacteria) which exert an active oxidizing effect on sulfur compounds and which catalyze the formation of sulfate ion and sulfuric acid (Zavarzin, 1964;Zavarzin and Karpov, 1982). These studies emphasized the role of sulfate pore solutions in the formation of the sulfuric acid leaching zone in the hydrothermal clay mass and in the transformation of montmorillonite into smectite-kaolinite argillisites as the solutions were seeping into the lower carbon dioxide zone (Eroshchev-Shak, 1992). It thus appears that the discharge of alkaline hydrothermal fluids in the SKC area is not an ordinary phenomenon and is of interest from several different aspects, including min- THE METHOD OF STUDY The method employed for the study of concretions included the following: 1) concretion sampling and sawing the samples into plates (a total of over 30 samples were used of which 15 have been studied); 2) a detailed description of the structure by means of total mineralogical analysis; 3) A study of chemical composition layer by layer using an S4 PIONEER X-ray fluorescence spectrometer; 4) determining the chemical and mineralogical compositions of the layers by microprobe surveys using a VEGA 3 electron scanning microscope equipped with an X-MAX 80 energy dispersive spectrometer; 5) determining the chemical and mineralogical compositions of layers by microprobe surveys using a LEO 1450V scanning electron microscope equipped with an INCA 300 energy dispersive spectrometer. The studies were carried out at the Institute of Volcanology and Seismology (IV&S), Far East Branch (FEB), Russian Academy of Sciences (RAS) and at Faculty of Geology, Moscow State University.

RESULTS OF STUDIES
The concretions make boulders of rounded or flattened shapes 15-20 to >50-70 cm across, and are washed out by brooks from under the base of the thermal ridge (Fig. 3). Most of the aggregates are nearly globular.
The thermal ridge has the shape of a wedge 5 to 20 m in cross section and 50 m long; it is composed of hydrothermal clays and is intensely disintegrating (is falling apart into blocks and slides down) under the action of boiling pots and steam-gas jets. We have not succeeded in finding the in situ positions of concretions, not even by drilling a well and by pitting because of a complex structure of the ridge (among other things, this includes hidden mud-water pots, large cavities, and open joints in the clay mass), high temperatures, and gas presence. We believe that the concretions make a definite horizon (a large lens or zone) at the base of the clay mass in this area of the SKC. No concretions have been detected elsewhere in the thermal field.
The concretions have diverse structures and textures (Fig. 4). Their structure is mostly concentrically zonal (see Figs. 4a,4b). The core of a concretion contains andesite fragments, more frequently completely altered to become a quartz-smectite-chlorite aggregate with numerous pyrite and iron oxide crystals (see Fig. 4c). The middle of a concretion is generally characterized by crustification, colloform-banded, and globular structures, and is black to grey in color (see Figs. 4d,4e). The layers have varying thickness ranging between a few tens of microns and 3-5 cm. The light bands are mostly composed of "pure" calcium carbonate, the darker bands consist of Ca and Mn carbonates with an admixture of other minerals (see Figs. 4f-4h). One also observes small concretions of simple structure: slightly altered andesite fragments are overgrown with two to three carbonate bands (see Fig. 4i); such concretions must be the youngest.
The detailed studies were performed for two large concretions. The one is concretion SKC-Zh2/19 (Fig. 5). It was of great interest because its layers were easily separable from one another, permitting a study of both internal structure and layer surfaces. The groundmass of each layer was calcium carbonate, or aragonite (we did not carry out X-ray diffraction studies, but earlier ones are available (Karpov, 1970)). In addition to aragonite, all layers also include microzones and individual flakes, incrustations, kidney-shaped and globular sinters of a complex mineral composition. One can identify gypsum, barite, manganese carbonates, pyrite and marcasite, siliceous ferrous and ferromanganese aggregates, iron hydroxides, and nitrogen compounds (which are most likely minerals that contain ammonium), and others ( Table 1). The elemental composition of the concretion layers is diverse as well (Table 2). Layer 1a is conspicuous in having increased concentrations of most components and a low value of calcium. For the other layers we see a prevalence of Ca with simultaneous high values of Si, Mn, Mg, P, and S, which provides evidence of the presence of other mineral components in the carbonate groundmass as marked in Table 1. The high value of loss on ignition shows the presence of other volatiles apart from sulfur. The layer surfaces show phosphate films (dominated by F-calcium phosphate), the relief lows are filled with gypsum, siliceous ferrous and siliceous ferromanganese aggregates, smectite in association with gypsum and carbonate (Fig. 6). One notes a constant presence of phosphates, mostly as films or veinlets between zones of carbonate growth. The aragonite crystals have a porous structure and show traces of leaching at fracture surfaces.
Detailed microprobe studies revealed considerable inhomogeneities in the structure and composition of calcium carbonate interbeds (Fig. 7). The carbonate interbeds have a complex chemical composition (there are Si, Fe, Al, Mn, Mg, P etc.) because of the formation of lenses, films and similar segregation forms of other minerals (see Figs. 7a, 7b). The groundmass is a well-crystallized Ca carbonate; it includes particles of smectites, siliceous ferrous and ferromanganese formations (see Fig. 7c). Fragments of microlayers are composed of aggregates of rounded particles having carbonate-phosphate-silicon-alumosilicate composition (see Fig. 7d). Individual globules of the same composition fill in pores in the carbonate groundmass (see Fig. 7e). Gypsum seems to frequently form during the terminal phases of layer formation (see Fig. 7f). The later phases also saw the deposition of Cl, Na, and K salts, which cover the surface of carbonate crystals in the form of overgrowth films and making filiform aggregates between the grains (see Fig. 7h). A wellcrystallized carbonate fills pores in the cryptocrystalline groundmass (see Figs. 7g, i). The surface relief of the outer, ninth, layer is of great interest (Fig. 8): siliceous carbonate flakes make the sand dune sculpture (Lebedev, 1965). L.M. Lebedev carried out several fundamental studies in the mineral generation from colloid compounds of varying composition. He found out that the sand dune sculpture characterizes the spreading of a gel-like material over an inclined surface, with some individual structural elements being isolated from the rest to make a mud-flow and flakes. Each surface fragment like those described above has a zonal microstructure that reflects individual phases in gel crystallization. G.A. Karpov studied the sand dune sculpture of carbonate concretions to hypothesize that the direction of flow and the position of the thermal water source could be reconstructed for the South Kambalnyi Central Thermal Field (Karpov, 1970). There is undoubtedly a localized discharge of hydrothermal solutions in this area of the thermal field, as also supported by geoelectric studies in the SKC (Feofilaktov et al., 2020). However, one cannot unfortunately reconstruct the direction of thermal water flow, unless for the case where concretions are found in situ, which is as good as impossible for this part of the thermal field.
Seven layers were selected in the second concretion SKC-Zh6/19 to study their structure, chemical and mineral compositions (Fig. 9). The layers have distinct colors (from light grey to black-grey) and clear boundaries. The inner zone is a core of carbonate and siliceous carbonate composition (no. 1, see Fig. 9) and contains fragments of a completely argillized rock (no. 2). Between these one finds small siliceous carbonate lenses with large (up to 2-3 mm) crystals of pyrite and probably of other sulfides (nos. 1-2, see Fig. 9). The other layers make a concentrically zonal texture of the concretion. According to electron microscopy, the groundmass in each layer (nos. 3-6) is calcium carbonate (more likely this is aragonite). However, thin films or veinlets between the microlayers, in pores and in interstices between acicular calcium carbonate crystals contain other minerals: rhodochrosite, calcium phosphate, barite, pyrite, and calcium sulfates. The dark color of the layers is due to micro admixtures of Mn, Fe, P, and finely dispersed pyrite. It is of interest to study how the elemental composition varies  (c) specimen for study (the vertical size of the plate is 12-13 cm) and identification numbers of the layers (see Table 1).  (Table 3, Fig. 10). Layers no. 1 and no. 1-2 contain, apart from Ca, also Si, Fe, Mn, P, and S in amounts up to 1-10 wt %. Layer no. 2 stands out because silicon prevails against calcium and because it has high iron values. This "layer" is argillized andesite that has been completely transformed into an aggregate of the following composition: smectite + chlorite + calcium carbonate + opal (chalcedony?) + pyrite (with sphalerite inclusions) + calcium phosphates. The next 4 layers have a uniform chemical composition dominated by Ca. The relatively high concentrations of sulfur can be explained by the presence of pyrite in these layers occurring along aragonite crystals and between its layers. Pyrite also associates with siliceous ferromanganese segregations. It is likely that phosphates are present. These data provide evidence of intense precipitation of many components during the first phases of concretion formation and relatively stable conditions of generation during the terminal phases.

A GEOLOGICAL-GEOCHEMICAL MODEL FOR NODULE FORMATION
A concretion is thus formed by penetration of subalkaline mineralized thermal waters of hydrocarbonate composition saturated with carbon dioxide and with sulfur-bearing gases into open cavities at the base of the hydrothermal clay mass (Fig. 11). The supply of solutions is accompanied by their boiling (degassing) This too is a layer with preliminary boundaries. A sequence of alternating microlayers (1 μm to 2 mm)of carbonate composition and varying in color between light grey and black. The lighter and broad layers are composed of columnar aragonite crystals. The prevailing layers are dark, composed of massive or fine-grained aragonite. Pyrite grains cluster between aragonite crystals. The layer is porous (up to 20% of volume), pores are filled with aragonite, pyrite, barite.
The composition is analogous to that of layer no. 3

5а, 4
The layer groundmass is carbonate. The upper surface of the layer has carbonate composition with segregated incrustations which are found to contain N, F, P, S, Mn, Fe, Al, and Si. Pores contain barite crystals. One encounters pyrite-marcasite sinter features with silica. Iron hydroxides and compounds of other elements make the layer light brown an grey-green.
Pyrite, marcasite, Fe hydroxides, aragonite, barite, nitrogen compounds 5b, 7 The layer groundmass is siliceous carbonate, fine-grained. Brown silky incrustations on the surface are composed of fluorine-bearing Ca phosphates. Fluorine is also encountered in the carbonate groundmass. Fracture surfaces show alternating carbonate layers and phosphate incrustations. There are nitrous compounds of amorphous aspect, ferromanganese ochre, barite crystals. The significant admixtures in the carbonate groundmass and in phosphate incrustations include N, F, Cl, P, S, Na, Mg, Mn, Fe, Al, Si, Sr, Ba.
Mineral composition is analogous to that of layer 5a

6, 2
The layer groundmass is micro-grained siliceous carbonate. The inhomogeneities are composed of aragonite of acicular aspect, accumulations of gypsum crystals, incrustations of F-phosphates of Сa, kidney-shaped aggregates of Mn carbonate, probably rhodochrosite. Microprobe studies induce strong degassing of layer material because of an abundance of volatiles. Qualitative and quantitative determinations were made by E.I. Sandimirova using a VEGA 3 SEM equipped with an X−MAX 80 energy dispersive spectrometer with AZtec software at IV&S FEB RAS under the methodological guidance of V.M. Chubarov. The objects of analysis were visible surfaces of the samples without spray-coating (layers 1, 2, 5a, 5b, 6-9) and polished sections with carbon spraycoating (layers 3 and 4). All samples show strong degassing under microprobe ray, which provides evidence of abundant volatiles in the concretion. Simultaneously, this necessitates the questions marks in this table concerning an accurate detection of mineral phases.

7, 2
Grey-white incrustation composed of an aggregate of lamellar gypsum, acicular aragonite, and silica flakes. On the incrustation surface are kidney-shaped spongy deposits (50-250 μm): a mixture of siliceous phosphate and siliceous carbonate composition with nitrous compounds and significant concentrations of Fe and Mn.

8, 3
An incoherent, ochrous layer. Micro-grained siliceous carbonate groundmass with limonite ochre. Rare impregnations of pyrite and dendrites of iron hydroxides. Scattered impregnation of barite (up to 25 μm) and clusters of lamellar gypsum crystals (?), as well as incrustations of Ca phosphates with an admixture of nitrogen compounds.
Pyrite, limonite, F-phosphates of Ca, aragonite, gypsum (?), barite, nitrogen compounds 9, 7-9 Layer with large sinter scales. The groundmass is micro-grained siliceous carbonate and layered. Grey-whit deposits are gypsum crystals in association with aragonite. The surfaces have a silky lusterincrustations of Ca phosphates. The aragonite surface and the phosphate incrustations contain globular, kidney-shaped, radiate columnar aggregates of rhodochrosite; abundant clusters of barite.
Limonite, phosphates of Ca, aragonite, rhodochrosite, gypsum, barite Layer no., thickness, mm Layer description (from inner part to periphery of concretion) Main minerals   line, base, and rare metals), as well as of phosphorus and nitrogen, at some depth beneath the South Kambalnyi Central Thermal Field. Nitrogen and phosphorus are most likely of inorganic origin as mentioned here above, and are transported toward the diurnal surface as part of volcanic (geothermal) gases of aqueous gas fluids.
The formation of carbonate concretions in the South Kambalnyi Central Thermal Field shows an analogy with the formation of mineral ore associations as veins and zones at the base of the hydrothermal clay mass in the East Pauzhetka thermal field (Rychagov et al., 2017b). The paper just referred to noted the following successive alternation of mineral association (from top to bottom along the section): phosphatealumosilicate-sulfide to siliceous carbonate-sulfide to zeolite-carbonate-sulfide. Accordingly, the physico-chemical conditions of their generation were  . 8. The structure of the surface of the 9th layer in concretion SKC-Zh2/19: "sand dune sculpture", after (Lebedev, 1965).
2 cm changing from low acidic to neutral to alkaline. We thus hypothesize that the occurrence of alkaline conditions in the discharge zone of steam-charged hydrothermal fluids in the East Pauzhetka and South Kambalnyi thermal fields is due to the influence of a metalliferous fluid at depth.

CONCLUSIONS
Carbonate concretions are formed in the discharge zone of alkaline thermal water in the South Kambalnyi Central Thermal Field. The concretions were identified previously and characterized as "pseudo-globular aragonite aggregates" (Karpov, 1970). Our study has shown that these aggregates are not monomineral, we described variations in the composition and structure of concretions and put forward a geological-geochem-ical model to explain the factors that produced the complex chemical and mineral compositions of these aggregates.
The concretions are confined to one of the more active geothermal ridges in the middle of the field and seem to compose a horizon (a large lens, zone. or a similar structure) at the base of the hydrothermal clay mass. The in situ positions of the concretions have not been ascertained because of a great number of steamgas jets and hidden mud-water pots in the clay mass of this area, resulting in large cavities and open joints at the base of the mass. All of these factors made for high gas saturation, increased temperatures, and soil instabilities in the ridge area, making pitting to penetrate down to the base of the clay mass impracticable.
The concretions have complex chemical and mineral compositions and a concentrically zonal structure. In   The carbonate concretions are characterized by diverse structures and textures: superposed upon the background of a concentrically zonal structure common to all globular aggregates one finds colloformbanded, globular, crustification, layered, massive, and radiate structures. The diversity, the definite succession and repetitiveness of the structures favor a multiphase pulsatory character of concretion formation. A more detailed explanation of the conditions and mechanism for concretion formation is proposed in the next section of this paper.
The carbonate concretions in the South Kambalnyi Central Thermal Field are thus formed by penetration of alkaline metalliferous solutions into open cavities at the base of the hydrothermal clay mass, subsequent boiling, cooling, and oversaturation of the solutions relative to the main compositional components, namely, hydrocarbonate, orthosilicic (colloid and dissolved) acid, sulfate, and ammonium. We believe that a source of several metals (alkaline, non-ferrous, and rare metals), as well as of phosphorus and nitrogen, can reside in the interior of the South Kambalnyi Central Thermal Field.

ACKNOWLEDGMENTS
We are deeply grateful to our colleagues at the South Kamchatka-Kurile Expedition, as well as to workers at the Analytical Center of IV&S FEB RAS and at Faculty of Geology, Moscow State University for practical help in our research. This study used equipment acquired for implementing the Development Program of the Moscow State University.

FUNDING
This work was supported by the Russian Foundation for Basic Research, project no. 19-05-00102.

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