Home – Home – Jornals – Russian Geology and Geophysics 2026 number 3

We report results of geostructural, mineralogical, petrographic, geochemical and geochronological (⁴⁰Ar/³⁹Ar) investigations of ophiolites from the northern and southeastern fragments of the Tatarka-Ishimba suture zone of the Yenisei Ridge combined into the Rybnaya-Panimba belt. The ophiolites consist of massive and pillow-lava metabasalt, massive and layered metagabbro and metadolerite, and represent fragments of the upper part of the oceanic crust. They formed in a mid-ocean ridge or marginal sea setting from depleted mantle sources corresponding to components of the N-MORB and E-MORB geochemical types. We propose a model for the tectonic history of emplacement of the ophiolites in the Meso-Neoproterozoic time. At the Meso-Neoproterozoic boundary (Stenian-Tonian), 1051-916 Ma, a thrust system was formed and the oceanic crust fragments (ophiolites) were accreted to the passive margin of the Central Angara terrane (microcontinent). The subsequent convergence and collision of this microcontinent with the Siberian Craton (786-749 Ma) caused the formation of strike-slip/thrust deformation in the rocks of the Tatarka-Ishimba suture zone including the ophiolites. In the Cryogenian time (708-700 Ma), strike-slip and reverse fault deformations occurred in these rocks due to a new episode of tectonic activity caused by the transition between convergent settings - end of the collision and start of the active continental margin.

We analyzed composition of monzonite-syenite intrusive rocks within the Bilikan-Khulamri magmatic zone of the central segment of the Yana-Kolyma Orogenic Belt. These rocks compose small intrusions cutting terrigenous strata of the Inyali-Debin turbidite terrane. The monzonitoids under consideration contain moderate amount of silica and Al₂O₃, relatively high contents of K₂O, Na₂O, MgO; by chemical composition they are similar to latite and very close to alkaline-salic intrusive rocks of the Aldan Igneous Province. Concentrations of major and rare elements indicate that these rocks belong to granitoids of convergent continental margins, including syncollisional and those of volcanic arcs. According to U-Pb dating (SIMS), the age of zircon from quartz syenite is 84.0 Ma. It corresponds to the secondary pulse of magmatic activity of the Okhotsk-Chukotka Volcanic Belt. The specific chemical composition of the monzonitoids close to the shoshonite-latite series is due to their location deep in the back arc of the volcanic belt, at more than 200 km from its axial zone, on the sialic bedrock composed by turbidite terrigenous complexes.

This paper deals with new geological, mineralogical, isotopic-geochemical (major, trace, and rare earth elements, Sm-Nd isotopic data) characteristics of the unique Kalevian (1926-1923 Ma) komatiite-tholeiitic magmatic manifestation for the first time identified in the supracrustal Kaskama Formation from the Inari terrane of the Kola-Norwegian region Fennoscandian Shield. In the massive and porphyritic komatiites, the primary (magmatic) mineral paragenesis represented by olivine 20-40%, orthopyroxene up to 5%, hornblende up to 10%, clinopyroxene 20-40% and plagioclase 20-30% was identified. Liquidus temperatures for olivine and pyroxenes calculated in the COMAGMAT3.73 software suite are in the range of ~ 1500-1200 °C. The komatiites of the Kaskama Formation belong to the Al-nondepleted type and are characterized by low REE _N content (1-2 relative to C1 chondrite), a total concentration ∑REE_avr = 0.15-0.36 ppm, and an unfractionated distribution of REEs, which is a consequence of the generation and evolution of their primary melts outside the field of thermodynamic conditions of garnet stability. The Zr-Y-Nb and Sm-Nd isotopic systematics of the rocks of the komatiite-tholeiitic association indicates the origin of their high-temperature primary melts from a plume source in depleted mantle (ε_Nd( T ) = +3.2 ± 0.3), which is different from the mantle sources of the Jatulian-Ludicovian picrite-komatiite association of the Central Lapland greenstone belt of Finland and the Pechenga intracratonic structure. Considering the amphibolite facies metamorphic conditions of the Kaskama Formation, it can be assumed that the komatiite-tholeiitic association is a deeply eroded (at least 10 km) section of a supracrustal volcan.

The metamorphic rocks in the northeastern Taimyr region are a consequential result of its complex geologic history. Series of metamorphic processes were associated with the formation of lode copper and noble-metal mineralization. The aim of this study is to investigate the type and degree of metamorphism of the host basic volcanic rocks in the ore cluster. We have identified three stages of metamorphic process: The first stage led to the formation of actinolite and sodium plagioclase paragenesis (with its chemical compositions corresponding to temperature of about 400 °C); the second stage resulted in the crystallization of hornblende, plagioclase, spessartine-almandine garnet, biotite, feldspars (oligoclase-andesine and K-Ba types), and various Fe-Ti phases (mineral geothermometers show the highest temperature range of 450-600 °C), and the final stage produced clinochlore, pyrite, and fluorapatite (330-290 °C according to the Al^IV contents in chlorite). We interpret the studied rocks as hornfels based on the highly discordant contours of the metamorphic aureole, the compositions of garnet (spessartine is a predominant component), and the K-Ar ages of biotite (253 ± 5 Ma) and plagioclase (239 ± 8 Ma), which coincide with the time of late Paleozoic-Mesozoic postcollisional granitoid magmatism. The absence of schistosity, the disequilibrium assemblages, and the abundance of volatile-rich minerals (containing F^-, OH^-, and SO₄^2-) also indicate their contact origin. The discovered aureole includes the known Cu-Au-Ag veins and metasomatic occurrences, the age of which is therefore late- or postmetamorphic. We confirm the earlier hypothesis of our colleagues about the impact of a concealed granite massif on the host rocks and consider the high metamorphic grade to be a natural boundary for the ore cluster.

The behavior of Ag in the oxidation zone of the Kyzyk-Chadr gold-sulfide-quartz deposit (Tuva Republic, Russia) is investigated. The primary focus is on the mechanisms of Ag differentiation under supergene conditions, including the formation of secondary minerals and silver incorporation as an isomorphic impurity into other phases. It was established that Ag in the oxidation zone forms chalcogenides (acanthite, petrovskaite, uytenbogaardtite, and jalpaite) and halides (iodargyrite, bromargyrite, and mayersite), which are associated with malachite, chrysocolla, brochantite, quartz, and iron hydroxides. Electron probe microanalysis reveals Ag concentrations in secondary copper sulfides (covellite, chalcocite, yarrowite, etc.) of ≤0.7 wt.%. Optimization of the microanalysis parameters to enhance resolution lowers the detection limit, enabling the determination of low Ag contents in chalcopyrite (≤0.05 wt.%) and bornite (≤0.09 wt.%). The sources of Ag are primary Ag tellurides and selenides, as well as primary and secondary Cu sulfides that decompose during the oxidation. Computer thermodynamic modeling is used to describe the behavior of Fe, Cu, and Ag during the progressive oxidation of primary assemblages (bornite, chalcopyrite, and acanthite). The Eh-pH conditions controlling mineral migration and reprecipitation are calculated, and the element concentrations required for the formation of secondary phases were estimated.

We report results on the age estimates of the Paleogene diatom datums derived from the magnetostratigraphy at the marine Cenozoic stratigraphic section of the Kvachina Bay, West Kamchatka. Based on a direct correlation with the Geomagnetic Polarity Time Scale, it was possible to determine the age of such levels in the interval between Subchron C13r (35.10-33.73 Ma) and Subchron C9r (27.86-27.44 Ma). Of greatest interest are the dates of the first and last occurrences of Lisitzinia ornata (27.71 Ma and 27.54 Ma respectively), and the first occurrence of Rocella gelida (27.49 Ma). This is the first experience of absolute age estimates of Oligocene diatom biohorizons based on magnetostratigraphy not only in Kamchatka sections, but in the North Pacific region in general. A comparison was made between the obtained age estimates and those published in the literature based on the study of deep-sea cores in various regions of the World Ocean.

We have estimated the proportion of sorbed water converted to ice and hydrate during the interaction of wet sand samples with methane by using the nuclear magnetic resonance (NMR) relaxometry method. There is a thin film of strongly bound water in small pores at the rock-fluid interface. This water does not freeze at the subzero temperatures achieved during the experiment; its amount remains constant without regard to the water content in a sample. We have calculated the amounts of strongly bound water and water converted to ice and hydrate for all samples. Based on the established hydrate stoichiometry, we have calculated the amount of methane converted to hydrate. Analysis of the obtained data has revealed the optimal water content in a sample, which results in the greatest amount of hydrate formation.

The unique relationship between induced polarization parameters and the structure of interface and pore space in rocks sustains considerable interest in applied science, driving the continuously expanding range of application of this method. The physicochemical theory of induced polarization in rocks contains significant gaps, which stem from the complexity of the processes generating secondary electric fields within rocks. To date, laboratory investigation results remain the primary source of new knowledge about the mechanisms and processes occurring in rocks under the influence of an electric field. Over the past decade, several new hypotheses, mechanisms, and models of induced polarization in rocks containing electron-conducting mineral inclusions have been proposed; their key features are described in our article. These hypotheses are actively discussed and used for interpreting induced polarization data obtained in laboratory studies and field tests. We present the results of an experimental study of induced polarization at the shungite - pore moisture interface, conducted on a synthetic capillary model of rocks containing an electron-conducting mineral inclusion. Based on voltammetric measurements, the principal characteristics of shungite polarization in an aqueous NaCl solution are established. The magnitude of exchange currents, the dynamics of interfacial polarization at different current densities, and the features of interfacial potential relaxation are estimated. Our study features the first presentation of measurements of electric currents flowing within the model and inside the shungite inclusion - the induced polarization currents. It is proven that the relaxation of the electric field in the model and that of the induced polarization currents exhibit identical dependences. The proportionality between the polarization intensity of the shungite inclusion and the normal current component at the shungite - pore moisture interface is experimentally confirmed. The resulting materials point to the dominant role of the electrode polarization mechanism for electron-conducting mineral inclusions in rocks. A description of the most probable induced polarization mechanism at the mineral inclusion - pore moisture interface in rocks is provided.

This study demonstrates the practical feasibility of high-precision in-flight measurement of magnetic fields from remote radio transmitters, including the use of unmanned aerial vehicles. The proposed method is based on calculating the Euclidean norm of the full complex magnetic field vector, which eliminates any dependence of the result on the orientation of the measurement system. To compensate for primary field instability, normalization is performed using a synchronized signal from a stationary base receiver. Further accuracy is achieved through pre-flight sensor calibration and spatial data smoothing during processing. Test measurements demonstrate good result reproducibility, with a discrepancy of less than 0.5% of the signal level. Moreover, magnetic field anomaly maps obtained at different altitudes show strong correlation both with each other and with archival data from ground-based inductive electromagnetic surveys.