Home – Home – Jornals – Russian Geology and Geophysics 2025 number 12

The paper presents studies of gneisses and granitoids from three deep boreholes located in the central part of the Nepa-Botuobiya anteclise of the Siberian craton. Based on U-Pb (LA-ICP-MS) geochronological studies of zircon, the age of the granitoid protolith of amphibole-biotite gneiss from Danilovskaya BH-95 has been estimated at 2254 ± 4 Ma, the age of granitoid from Mogdinskaya BH-11, at 1972 ± 9 Ma, and the age of granitoid from Preobrazhenskaya BH-14, at 1981 ± 3 Ma. The protolith of the gneiss from Danilovskaya BH-95 corresponds in composition to granodiorite similar to I -type granite with high contents of high-field strength elements (HFSE). It is characterized by T _Nd(DM) = 2.7 Ga and might have formed as a result of the melting of an Archean crustal source within a certain block. The granitoid from Mogdinskaya BH-11 has features of I -type granites with low contents of HFSE and is characterized by T _Nd(DM) = 2.4 Ga. The granitoid from Preobrazhenskaya BH-14, with T _Nd(DM) = 2.6 Ga, is similar to I -type granites with high contents of HFSE. All the available data and the similar ages (ca. 2.0 Ga) determined earlier for S -type gneiss-granites from one of the Danilovka group boreholes suggest the formation of granitoids with different geochemical characteristics in the central part of the Nepa-Botuobiya anteclise in the period 1.97-2.00 Ga. This was possible in the accretionary orogen including (as evidenced from the isotope characteristics) blocks with Paleoproterozoic and Archean crust. The studied area is a fragment of the Paleoproterozoic Trans-Siberian orogenic belt separating the large Archean Tunguska and Anabar superterranes. The formation of this belt marks the early stage of accretion processes and the beginning of the formation of the Siberian craton in the period 1.95-2.00 Ga.

We report an integrated analysis of new geological and geophysical data from the Lena River delta region to clarify the structural relationships between the Siberian Craton, the Verkhoyansk fold-and-thrust belt, and the Laptev Sea rift system. New geophysical data include the local seismic monitoring (613 earthquakes for the period of 2018-2024) and the magnetotelluric sounding measurements (MTS, 21 points). A joint interpretation of morphostructural studies, seismic tomography, MTS data, and gravity anomalies leads to the following conclusions. Seismic activity is confined to crustal structures of the Verkhoyansk fold-and-thrust belt and the South Laptev rift but has migrating character within this area. Seismic tomography reveals a two-layer crustal structure on the southwestern side of the study area. The upper layer, characterized by higher v_P / v_S ratio, corresponds to the structures of the Verkhoyansk fold-and-thrust belt overthrust onto the margin of the Siberian Craton, which are overlain by structures of the South Laptev rift. The top of the lower layer, with a lower v_P / v_S ratio, dips from southwest to northeast to depths of 15-20 km. This layer corresponds to the Precambrian crystalline basement of the Siberian Craton. This two-layer crustal model is traced beneath the deltaic sediments of the Lena River for approximately 30 km to the northeast. Further it converts into a single-layer structure with high v_P / v_S values. MTS data provide detailed information on the upper crustal structure, consistent with the presence of the South Laptev rift between the Bulkur and Bykov faults, as well as with the presence of the Tumat horst to the northeast of the Bykov fault. Modern fault activity is identified by subvertical low-resistivity anomalies from MTS data (fluid-saturated zones) and zones of earthquake hypocenter concentration from seismological data, as observed for the Bulkur, Nizhne-Lena, Bykov, and Sardakh faults.

The Angara-Kan terrane is located in the southwest of the Siberian Platform and is composed mostly of rocks of the Kan granulite complex and the Yenisey metavolcanosedimentary complex. In this work, we present new data on the metamorphic conditions as well as the composition and age of zircon and monazite of garnet-biotite schists of the Yenisey Complex. The textures, geochemical composition, and wide range of zircon ages indicate that the garnet-biotite schists formed during single-stage metamorphism of terrigenous rocks at P = 7.2-8.2 kbar and T = 700-730 °C. These P - T parameters are similar to those of metavolanic rocks of the Yenisey Complex. The age of the metamorphic generation of zircons and monazites from garnet-biotite schists, 720-730 Ma, is correlated with the time of Neoproterozoic metamorphism of volcanic rocks of the Yenisey Complex. The age spectra of Paleoproterozoic detrital zircons from the garnet-biotite schists show two maxima corresponding to ca. 1.86 and ca. 1.78 Ga. Granulites of the Kan Complex, which underwent two stages of high-grade metamorphism, at 1.89-1.85 and 1.80-1.77 Ga, were probably the source of these zircons. Terrigenous sediments presumably accumulated close in time to the formation of volcanic rocks of the Yenisey Complex, at ca. 1.74 Ga. We assume that most of sedimentary rocks of the Yenisey Group formed after the main orogenic events in the Angara-Kan terrane.

Thermodynamic calculations have been first carried out for the entire series of lanthanides (+Y) to determine the influence of Ca in the system on the formation of rare earth (REE) mineralization during the cooling of a hydrothermal fluid from 500 to 100 °C, affecting the monazite association with a variable amount of calcite. Results show that increasing the calcite introduced into the system and raising the pH of the solution lead to significant changes in the equilibrium mineral assemblage. Increasing the initial amount of calcite in the system is followed by an increase in the stability of parisite and REE-bearing fluorite. The transition from acidic to near-neutral conditions expands the range of parisite formation with a simultaneous decrease in the amount of equilibrium bastnaesite. In acidic fluids (pH = 3 and 4), REE-bearing fluorite forms under low-temperature conditions, whereas it can be stable in the range of 400-500 °C at pH = 6.6. Under near-neutral conditions, residual unconsumed calcite occurs in the equilibrium assemblage. With an increase in calcite introduced into the system under acidic conditions, the concentration of calcium in the equilibrium fluid also rises, with a simultaneous increase in the total equilibrium concentration of lanthanides in it. In this case, calcium-enriched acidic fluids can contribute to the removal of REE, and, conversely, calcium-depleted fluids provide the deposition of lanthanides, as well as an increase in the pH of the ore-forming environment.

This study presents the results of a comprehensive assessment of mercury content in the bottom sediments of four high-altitude lakes on the Ukok Plateau (Altai, Russia): Zerlyukol’-Nur, Krasnoe, Teplyi Klyuch, and Argamdzhi. The aim was to evaluate the mercury contents and mercury fluxes in the sediments as indicators of global atmospheric pollution. The Hg content varies from 28 to 130 ppb, averaging 57.4 ± 21.4 ppb, which corresponds to background values for high-altitude lakes of the Northern Hemisphere. The elevated Hg contents in the upper core layers of three lakes reflect postindustrial contamination, whereas this trend is absent in Teplyi Klyuch, likely because of a thermal endogenous influence. Sedimentation rates of 0.18 cm/yr in Teplyi Klyuch and 0.9 cm/yr in Argamdzhi yield mercury fluxes of 3.9 and 9.1 ng/(cm²∙yr), respectively, comparable to those reported for the southern Himalayas but lower than mercury contents in industrially active regions. The obtained data indicate that the Ukok Plateau remains isolated from direct pollution sources and is significant as a background indicator area for evaluating transboundary mercury transport. This work fills a data gap on Hg distribution in remote high-altitude ecosystems and refines global mercury cycling models.

We consider the structure, depositional conditions, and subsequent development of the Koltogor-Urengoy rift system, which cuts through West Siberia in the meridional direction and has a length of 1925 km. Based on the interpretation of potential fields, the structure of the major rifts and the system of minor grabens leading to them has been defined more precisely. We have established that the Early Triassic rifting greatly influenced the geologic structure of the Mesozoic-Cenozoic sedimentary cover of the West Siberian sedimentary basin and significantly contributed to its petroleum potential. The Koltogor-Urengoy megatrough, a superorder linear depression elongated in the meridional direction and extending across the entire West Siberia, formed above the main graben-rifts of the Koltogor-Urengoy system in the structural plans of the Mesozoic reference levels. In the southern part of the basin, an active subsidence of the Koltogor-Urengoy and Tym graben-rifts in the Mesozoic and Cenozoic predetermined the occurrence of large superrift depression zones within the sedimentary succession of the Pur and Koltogor megatroughs and the Nyurolka and Tym megadepressions, which are the main oil generation zones. The positive structures of orders I and II, main zones of oil and gas accumulation, were formed above the Paleozoic basement uplifts: Severny, Nizhnevartovsk, Alexandrov, and Kaimys uplifts; and Vyngypur, Yetypur, Vasyugan, and Pudino megaswells. In the north of the basin, anticlinal traps, controlling the Aptian-Albian-Cenomanian gas accumulations, formed above the interrift blocks in the relief of the Cretaceous deposits.

The article presents the results of detailed organic geochemistry investigations being part of a comprehensive (lithological, geochemical, petrophysical) study of the Bazhenov Formation in the Tomsk Region applying the Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences methodology developed for organic matter-rich source rock. As the thermal maturity increases from PС₃ to MС₂, the content of “free micro oil” in source rock’s pore spaces has been observed to increase. In the same time, there are modifications to the composition of bitumoids (chloroform source rock extracts) from open and closed pores at both the group and molecular levels. Even “immature” organic matter has been established to show a difference in bitumoids composition from open and closed pores, which increases as it enters the main phase of oil generation (“oil window”).

The paper analyzes the data from the Yamm-Torzhok regional profile intersecting the Ilmen electrical conductivity anomaly along its major axis. This work continues the long-term research on the structure of the Precambrian basement complexes in the western part of the East European Platform conducted by the Department of Geophysics of the Faculty of Geology of Moscow State University and Geoelectromagnetic Research Center of the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (GEMRC IPE RAS). The obtained data combined with the analysis of the potential fields are used to construct a deep geoelectric model. Separately, a model of the structure of the sedimentary cover is presented.