Home – Home – Jornals – Russian Geology and Geophysics 2025 number 10

The Alambay mélange zone is located within the interior of the early Paleozoic Salair Orogen (fold-thrust structure) and is the northern link of the Alambay-Kaim tectonic zone of the Salair Ridge and the Altai Mountains. In paleotectonic terms, the Salair Orogen is part of the Kuznetsk-Altai paleoisland-arc system. The Alambay mélange zone is comprised of two types of mélange: serpentinite and terrigenous. The mélange contains microquartzite, basalt, and limestone blocks. The terrigenous mélange contains a block of garnet amphibolite, and the serpentinite mélange has gabbroid blocks. In terms of geochemical features, gabbroid and garnet amphibolite from the Alambay mélange zone can be attributed to island-arc rock. Chromite from serpentinized hyperbasite in this zone has an increased content of chromium and magnesium, which is also typical for chromite from suprasubduction hyperbasite. The mélange basalt is categorized as an oceanic island basalt and distinguished by a high content of titanium and niobium. There are two morphological varieties of terrigenous mélange. One of them is characterized by the prevalence of inclusions over the matrix and the dominance of limestone and oceanic island basalt within inclusions. The other variety is notable for the prevalence of the matrix over inclusions, and its main type of inclusions is aposilicic microquartzite. The genesis of the Alambay mélanges can be explained using the oceanic plate stratigraphy model [Wakita, 2015]. This model describes the dependence of the composition of subduction mélange on the position of a sole thrust between the subducting plate and the accretionary prism. Within the framework of this model, two varieties of a terrigenous subduction mélange are identified. One of them is a limestone-basalt mélange formed during seamount subduction, and the other one is a sandstone-cherty mélange whose formation corresponds to décollement position along the base of the sedimentary bed of the subducting plate. The formation of the mélange assemblage dates back to 500 Ma and precedes the accumulation of the Late Cambrian-Early Ordovician Green-Violet Group flysch and its analogs, suggesting that the Alambay mélange zone is of Late Cambrian age. The important role of fragments of the island-arc base in the serpentinite and terrigenous subduction mélanges indicates that the Cambrian subduction zone was characterized by tectonic erosion conditions under which the material of the advancing plate was entrained into the subduction zone. The sole thrusts in the subduction zone, along which the material was exhumed into the accretionary wedge, were located along the base of the oceanic sediments, at the base of seamounts, and inside the hanging wall of the subduction zone.

We present new data on high-Ba phlogopites first discovered in kimberlite of the Malokuonapskaya pipe. The studied phlogopites can be divided into two groups: (1) megacrysts up to 2 mm in size with a BaO content of up to 0.58 wt.%, characterized by an increase in the content of this component from core to edge (with direct zoning for BaO), and (2) zoned crystals up to 200 μm in size, containing inclusions of minerals of the kimberlite groundmass, with a BaO content of up to 14.6 wt.% in the cores, regularly decreasing to 1 wt.% toward the edges. The two generations of mica formed in different and unrelated processes. Crystallization of megacrysts took place at the earliest stages of the evolution of kimberlite melt. The slight increase in Ba content from the core to the edges of phenocrysts might be the result of the evolution of kimberlite melt leading to its enrichment in incompatible elements. Phlogopites of group 2 formed at a later stage, together with the groundmass minerals. High Ba contents in these phlogopites are possible in the case of the strong enrichment of the kimberlite melt with light elements. The refertilized lithosphere under the influence of the Siberian traps might have been the source of these elements. ⁴⁰Ar/³⁹Ar dating of the phlogopite yielded an age of 232.1 ± 2.6 Ma.

Using the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) complex, with the involvement of X -ray photoelectron spectroscopy (XPS), we obtained new data on the distribution and the degree of concentration of “invisible” gold on the native surface of arsenopyrite crystals from the Natalka gold deposit (northeastern Russia). The study has shown that these parameters are intimately related to the formation conditions of the deposit. The highest contents of Au are detected on the surface of arsenopyrite crystals formed at the hydrothermal stage of ore generation, and the minimum ones are found on the surface of crystals formed at the early metamorphogenic stage. The extreme variability of the gold distribution profile and contents even within the surface of a single crystal is best pronounced in arsenopyrite of vein and veinlet-vein ores, which is due to the instability of the physicochemical parameters of the processes of hydrothermal ore formation at the later stages of the deposit formation. Basically, these are oxidation processes that lead to a change in the composition of the primary (as-grown) nonautonomous phases and to the redistribution of gold in the surface layer of the crystal, containing highly charged arsenic species, As⁵⁺ and As³⁺. The intricate and long-term metamorphic/magmatic genesis of the giant Natalka gold deposit has been confirmed. The high contents of “invisible” gold in the easily destroyed surface layer of sulfides during enrichment are of significant practical interest for the extraction of gold from “refractory” ores, as they increase the quality and value of the mined raw materials.

The quasi-equilibrium directional crystallization method to model a new type of zoning in the Cu-Fe-Ni-S system was applied. A melt is crystallized with the following composition (in at.%): 14.00 Cu, 30.00 Fe, 4.00 Ni, 51.00 S, and 0.1 each of Pt, Pd, Ag, Au, As, Te, Bi, Pb, Se, and Sn. We use optical and electron microscopy, micro-X-ray spectral analysis, differential thermal analysis, and powder diffraction to study the composition and structure of the ingot. Based on the results of measuring the average solid phase composition, the distribution curves of macrocomponents in the ingot have been constructed in the range of g from 0 to 0.81 ( g is the fraction of crystallized melt). The variations in the melt composition and distribution coefficients between solid phases and the melt in directional crystallization have also been calculated. The obtained sample consists of three primary zones: monosulfide solid solution with the composition ranging from (Fe_0.75Ni_0.10Cu_0.06)_0.91S to (Fe_0.69Ni_0.15Cu_0.09)_0.93S (Mss) in Zone I and two intermediate solid solutions: ~ (Fe_0.57Ni_0.03Cu_0.45)_1.05S (Iss₁) in Zone II and (Fe_0.46Ni_0.06Cu_0.52)_1.05S (Iss₂) in Zone III. The peritectic character of the crystallization of Iss₁ and Iss₂ from the melt has been established. The trajectories of the melt and solid phase compositions and series of conodes have been plotted for Mss and Iss₁ on the phase diagram of the Cu-Fe-Ni-S system. Secondary zonality is described by the following sequence of phases: Pyh 1C + Pyh 3C + Icb + Ccp/Ccp + Iss + Fe-Pn, Sgk/Ccp + Ni-Pn + NiS + Bn (Pyh - hexagonal pyrrhotite, Icb - isocubanite, Ccp - chalcopyrite, Pn - pentlandite, Sgk - sugakiite, Bn - bornite) and belongs to the high-sulfur pyrrhotite + cubanite-chalcopyrite type of ore body zonation.

Since apatite is capable of adsorbing and retaining radionuclides, the experimental removal of uranyl ions (UO₂²⁺) from two Ca-containing technogenic and one model solutions was performed by neutralization with a solution of sodium hydrogen phosphate Na₂HPO₄. The chemical composition of the precipitate was analyzed by X-ray photoelectron spectroscopy (XPS), and its structure was refined by X-ray diffraction (XRD). The formation of hydroxyapatite was confirmed; two samples also contained CaH(PO₄)∙(H₂O)₂ (brushite), in which up to 15% of calcium was substituted by uranium. Study of the uranium species revealed U⁴⁺, U⁵⁺, and U⁶⁺, with U⁵⁺ amounting to 30-35 at.% of the total uranium. The scanning electron microscopy (SEM) analysis did not identify precipitate phases with U > 18%. Uranium was detected in the grain rims, which indicates its adsorption. Thermodynamic computation showed the possible formation of individual uranium phases, such as β-UO_2.333, β-UO₂(OH)₂, and NaUO₂O(OH) (clarkeite), at the measured Eh-pH. The oversaturation of solutions and the spontaneous formation of solid hydroxyapatite and brushite particles lead to a change in the initial Ca/PO₄ ratio; moreover, the P/Ca and Ca/O ratios, as well as the elemental composition in the near-surface layers of three precipitates, are not strictly constant. The newly formed phase contains segregations different in size, the degree of deformation, and sorption surface groups, which is reflected in the mechanism of capture of uranium compounds. The experimental results demonstrate the need for further studies of uranium precipitation during the crystallization of phosphates. The stability of phosphate phases ensures the high reliability of phosphate safety barriers for the uranium extraction facilities. In addition, uranium adsorption by phosphates can be used for remediation of aquifers polluted with radionuclides.

The article summarizes new data on core studies and well logging in the Famennian deposits of the Yugomashevskoye field located in the Republic of Bashkortostan (Russia). Structurally, the subject is categorized as a model of a rimmed platform with characteristic organogenic buildups and associated facies. The identified sedimentation cyclicity, the secondary processes, and the analysis of changes in the relative sea level make it possible to identify the most common types of porosity confined to a particular part of the section and facies zonality. Transgressive and regressive parts of the cycles are characterized by their specific set of lithotypes and characteristic types of void space. It is revealed that the formation of a void space is affected by the duration and intensity of secondary processes: leaching, dolomitization, and claying associated with the subaerial exposure. Moreover, this work substantiates the division of carbonate structures of this region into three types that differ in terms position, morphology, and manifestation of hypergene transformations, as reflected in the differentiation of types of void space of host rocks and their potential in terms of reservoir characteristics.

We present the research results based on experimental data obtained during the expedition of the R/V Akademik Oparin to the Peter the Great Gulf (cruise No. 54, October 2017). Methane fluxes at the water-atmosphere boundary have been calculated for each sampling point using the measured concentrations of dissolved methane in the seawater surface layer and methane in the near-surface atmosphere, temperature, salinity, and wind speed. In all cases, the concentrations of dissolved methane in the seawater surface layer exceed the values in equilibrium with the atmosphere. Methane fluxes from the sea surface vary from 1 to 981 mol/km²·day with an average value of 7.1 ± 4.5 mol/km²·day. The most intense emission has been observed near the mouth of the Tumen River. Gas-saturated sediments with inclusions of subaqueous authigenic minerals have been discovered on the continental slope. The analysis of geological and hydrochemical information has revealed areas of methane influx from bottom sediments. The application of models of current fields and advective impurity transport during the expedition provides the opportunity to determine the prospective zones with increased concentrations of dissolved methane and also to identify for the first time an underwater source of groundwater discharge (presumably the element of the Tumen paleochannel), which affects the distribution of dissolved methane concentrations and salinity in the subsurface waters of the gulf. Comparison of calculated and experimental data on methane transport has shown their spatial coincidence. The calculations have established that tidal currents determine not only the local maxima of methane concentrations and fluxes from the sea surface in certain areas of the gulf but also its transport to the pelagic waters in the western part of the Central Basin of the Sea of Japan.

The problem of an inhomogeneous distribution influence of properties and geometry on the stress-strain state of a 500 km thick block model under the influence of gravity within the framework of the medium behavior of an elastic-plastic model is considered. The calculations were carried out in a 2D formulation of a plain strain state for a rectangular and cylindrical profile, taking into account the curvature of the Earth’s surface. It is shown that anomalies of the stress-strain state and zones of irreversible deformation development are determined primarily by the heterogeneous nature of the structure, the presence of density and rheological anomalies. The geometry of the models significantly affects primarily the stress state in the crust, taking into account the curvature of the lithosphere where the greatest shear strength determines the development of irreversible deformation depending on the heterogeneity of the environment. In this work, numerical modeling was carried out to assess the influence of the geometric factor on the stress-strain state of the lithosphere. The initial geometry of the medium was constructed on the basis of a seismic model along the Craton profile in rectangular and cylindrical coordinates with a length of 3600 km and a depth of 500 km. A comparison of the obtained solutions was carried out. The change in the stress-strain state during the lifting and immersion of a medium block in a cylindrical problem formulation is considered. It is shown that vertical displacements in such a formulation have a noticeable effect on horizontal stresses and can lead to the development of irreversible deformation. It has been established that the deviation of shear stresses from horizontal ones at the daytime surface is negligible in the case of a cylindrical formulation of the problem with successive layer-by-layer build-up of the crust. The maximum deviation is achieved in the middle part of the deformed layer.

Drilling fluid containing a solid dispersed phase penetrates into the surrounding rocks when drilling wells. A colmatation zone may form depending on the size ratio of clay particles (cuttings) and pores, which leads to a significant change in the transport and electrical characteristics of the near-wellbore space. The latter are especially important for reliable interpretation of well logging data, since a directional change in the pore space structure can cause anisotropy of physical properties of the rock. The colmatation process is irreversible and almost irreproducible in laboratory conditions. Its study is based on numerical experiments aimed at analyzing physical characteristics of sedimentary rock samples (sandstones) using their digital models. This article considers the the problem of calculating their effective tensor of specific electrical conductivity at different volumetric saturation of the pore space with clay-based drilling mud, formation water and oil. The effect of the volume content of the indicated phases on the anisotropy of the samples’ specific electrical conductivity is shown using data from non-destructive visualization of their internal structure.

The configuration of acoustic and magnetic fields which provides lateral acoustic oscillations in well conditions in a porous saturated medium is considered. An alternating magnetic field creates an acoustic response at the interface of media with high acoustic amplitude during electroacoustic conversion. The nature of the dependence of amplitudes on the parameters of the medium is described. The possibility of simultaneous determination of three kinetic coefficients (electrical conductivity, acoustoelectric parameter, permeability) is shown.

A significant problem with the magnetotelluric sounding (MTS) method is the influence of local near-surface inhomogeneities that distort MTS data across the entire frequency range. These distortions complicate the analysis and interpretation of data and, ultimately, the acquisition of information about the deep structures being studied. A widely used method for suppressing near-surface distortions is the normalization of MTS curves using spatial low-pass filtering. The aim of this study is to evaluate the effectiveness of this approach, including using weight functions proposed by the authors. A simple geoelectric model of the Earth’s crust containing a conductive sedimentary cover, a high-resistivity basement, and a three-dimensional conductive depression in the basement was compiled. Two model variants with a homogeneous upper part of the section and with multiple local near-surface inhomogeneities were considered. A synthetic magnetotelluric sounding (MTS) dataset was calculated for a system of profiles using three-dimensional modeling. The effect of near-surface distortions is observed from the data of the second version of the model, leading to a shift by level in the MTS amplitude curves. MTS curves were normalized using smoothing filters in a sliding window with different radiuses to suppress the distorting effect of near-surface inhomogeneities. Various weight functions of the filters were used, namely: for distance from the center of the sliding window, for the difference in amplitude from the average in the window and for the difference in the main directions determined by the polar diagrams of the module of the main component of the impedance tensor and the main component of the phase tensor. In general, the experimental results showed a high efficiency of normalization of the main components of the impedance tensor and the telluric tensor, and a lower efficiency of normalization of additional components.