Home – Home – Jornals – Russian Geology and Geophysics 2024 number 12

The montmorillonite samples from shallow parts of the thermal fields within the Mutnovsky hydrothermal steam deposit (the Upper Mutnovsky thermal field and the Dachny site), along with the composition of salts in the clay pore solutions have been studied in the context of ion exchange between them. This aspect represents one of the topical problems due to geothermal clays (montmorillonite) enrichment with elements transferred by steam hydrotherms and concentrating in the near- surface horizons, while montmorillonite as a natural cation exchanger will absorb them, thus acting as a geochemical barrier. The composition of the interlayer cationic complex of montmorillonite samples was studied on isolated clay fractions and from the ion-exchange experiment to determine the salt composition of clay pore solutions and the mineral composition of clay fractions. Geothermally heated soils within the studied thermal fields are dominated by kaolinite-alunite-jarosite assemblage with subordinate amounts of montmorillonite, while montmorillonite prevailing in the mud-water pots is subjected to degradation when they dry out. Crystallization of salts from pore solutions in the near-surface horizons of the studied thermal fields can be exemplified by szomolnokite FeSO₄‧H₂O, metavoltine K₂Na₆Fe²⁺Fe³⁺₆O₂(SO₄)₁₂·18H₂O, leonite K₂Mg(SO₄)₂‧2H₂O, polyhalite K₂Ca₂Mg(SO₄)₄‧2H₂O, mikasaite Fe₂(SO₄)₃, alum and amorphous aluminum sulfate hydrate. Interaction of such solutions with montmorillonite will trigger the cation exchange reactions in the interlayer space of the layered silicate with attendant formation of predominantly Al,Fe-intercalated forms. Alumina hydrates entering the interlayer space of the montmorillonite are reflected by the band at ~ 2500 cm^–1 on the infrared spectra attributed to the water coordinated to aluminum. In addition to aluminum and iron, other cations which are concentrated in the uppermost parts of geothermal clay blankets and penetrate into the interlayer space of montmorillonite are: Li⁺, K⁺, NH₄⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Hg²⁺, etc. As a result, in the thermal field conditions, montmorillonite is intercalated with easily extractable elements, including lithium, mercury, barium and strontium, ammonium, at the level of units (the first tens of grams per ton).

Diagenetically modified carbonate rocks are more common in the rock record. Among these modifications, multiphase dolomitization is the most common process. The Middle Jurassic carbonates of the Samana Suk Formation are extensively altered by the dolomitization process in the Kahi section, Nizampur Basin. The primary objective of this study is to investigate this multiphase dolomitization and to elucidate its possible mechanism.

Field investigation shows both host limestone (oolitic, fossiliferous, and massive) and dolomites. Dolomite bodies are of both beddings: parallel to and crosscutting the bedding planes. Different types of dolomites were recognized on the basis of color contrast as dark gray replacive dolomite, light gray dolomite, brownish dolomite, and yellowish dolomite. Along with the replacive phase, void- and fracture-filling cementing saddle dolomite and cementing calcites are recognized in the field. Petrographic studies show the complex diagenetic history of the Samana Suk Formation from near-surface diagenesis, including micritization, neomorphism, and several varieties of dolomites. These varieties are as follows: RD1 is very fine- to fine-grained dolomite; RD2 is medium- to coarse-grained and anhedral to subhedral dolomite; RD3 is coarse- to very coarse-grained and planar euhedral zoned dolomite; and RD4 is coarse-grained euhedral to subhedral ferroan dolomite. In addition, cementing saddle dolomite SD consists of large crystals with curved faces showing sweeping extinction. Cementing calcite phases are as follows: CC1 is granular mosaic; CC2 is twin; CC3 is fracture-filling; and CC4 is ferroan calcite. The stable isotope values of limestone (δ¹⁸O is –7.13 to –0.73‰ V-PDB, and δ¹³C is –0.05 to 1.32‰ V-PDB) show depletion with respect to the Jurassic marine signature. The values of multiphase dolomites RD1–RD4 and SD (δ¹⁸O is –8.65 to –3.16‰, and δ¹³C is –3.56 to 2.09‰) indicate multiphase dolomitization. The CC1–CC3 values (δ¹⁸O is –11.07 to –8.97‰, and δ¹³C is –2.14 to 0.76‰) indicate highly depleted valu­es of δ¹⁸O, showing hydrothermal origin. From field, petrography, and geochemistry data, it is deduced that a possible source of Mg for hydrothermal dolomites is activation of faults and fractures during active tectonic regime in the area and might be related to activation and reactivation of the Kahi Thrust system.

We present the first results of comprehensive isotope-geochemical studies of mineral radon waters of the Tulinskoe field (Novosibirsk), aimed at identifying their stages of interaction with the host rocks. By geochemical coefficients Ca/Na, Ca/Mg, Ca/Si, Mg/Si, Na/Si, Si/Na, rNa/rCl, and SO₄/Cl, the studied waters are assigned to fracture–vein waters of granitoids. The indices of carbonate mineral saturation of the radon waters show their oversaturation with aragonite, calcite, and dolomite. The waters are also saturated with diaspore, ferrohydrite, gibbsite, and kaolinite, which leads to the deposition of these minerals as secondary phases. In the thermodynamic diagrams, the points of the activities of the radon water components are localized mainly in the stability fields of clay minerals (kaolinite and Na-, Ca-, and Mg-montmorillonites), layered silicates (talc), and zeolites (laumontite). A few points fall in the stability field of silicates (Mg-chlorite). The studied waters of the Tulinskoe field are neutral fresh, with Si = 6.41–9.02 mg/dm³. According to the results of thermodynamic calculations, the radon waters of the Tulinskoe field are in equilibrium with carbonate minerals and hydromicas. Following the classification by S.L. Shvartsev, they are assigned to the Si-Na geochemical type.