Home – Home – Jornals – Russian Geology and Geophysics 2025 number 5

We present new data on the composition and U-Pb (LA-ICP-MS) age of detrital zircons from the Carboniferous sedimentary rocks of the Siberian Platform cover, in which diamonds and accessory minerals were found, namely, the Baeronovka Formation in the southwest and the Tushama and Kata formations at the center of the Siberian Platform. The geochemical characteristics and results of analysis of minerals of the heavy fraction from the Baeronovka Formation indicate a predominantly felsic composition of the provenance rocks. The latter are, most likely, the rocks of the Cis-Sayan uplift of the Siberian Platform basement, including mostly Paleoproterozoic granitoids and volcanic rocks of the South Siberian postcollisional magmatic belt. Smaller amounts of detrital material got into the sedimentation basin of the Baeronovka Formation through the destruction of early Paleozoic rocks at the northern segment of the Central Asian Orogenic Belt. The geochemical characteristics of the terrigenous rocks of the Carboniferous Tushama and Kata formations testify to a felsic composition of the provenance rocks, but the mineral compositions of the heavy fractions indicate different compositions of these rocks. These data and the age of detrital zircons suggest that the Neoproterozoic sedimentary rocks and middle Paleozoic igneous rocks of the Baikal-Patom zone on the southern margin of the Siberian Platform and the Middle Paleozoic igneous rocks of the Vilyui rift and the Yakut diamondiferous province are the main provenances for the Tushama and Kata formations. The established differences in the composition and age of provenance rocks for the Carboniferous sandstones of different areas of the Siberian Platform confirm the earlier conclusions about the existence of several local sedimentary basins within it in the Devonian-Carboniferous. These basins formed after the middle Paleozoic tectonomagmatic activity accompanied by diamondiferous lamproite and kimberlite magmatism.

Three midocean ridges meet in the Bouvet triple-junction region: Mid-Atlantic, Southwest Indian, and American-Antarctic. The triple-junction region is indicated by the Bouvet hotspot magmatism. Available laboratory modeling data are applied to construct a diagram showing the conduit of a thermochemical mantle plume melting from the core-mantle boundary and erupting onto the surface. Morphobathymetric data for the Bouvet Island region are used to obtain the mass flow rate of magmatic melt for the Bouvet plume. Considering the calculated melt flow rate, the thermal power of the Bouvet plume source is N _B = (1.7-2.0) ·10¹⁰ W, and the plume conduit diameter is 9-16 km. Possible evolution of the Bouvet plume is presented on the basis of consideration of its geodynamic regime. The influence of the geodynamic system of asthenospheric convection flows on the ocean floor structure in the Bouvet region is shown. The plume under whose influence Bouvet Island formed is located in the region of the ascending asthenospheric roll flow and locally intensifies it. Transform faults in the Bouvet region were formed under the influence of descending asthenospheric roll flows. The width and the depth of the trough of the Bouvet transform fault are determined by analyzing the flow structure and heat transfer in the asthenosphere in the Bouvet region and with regard to the intensifying effect of the Bouvet plume on the ascending asthenospheric roll flow. The conducted geochemical and thermobarogeochemical studies indicate the decisive role of fluid components in the magmatic systems of the Bouvet hotspot, which are characterized by enrichment in volatiles (H₂, H₂O, and CO₂) and alkalis (primarily potassium) as well as lithophile rare and rare-earth elements (La, Ce, Th, Nb, and Rb). In view of the seismic tomography results, the features of the mantle structure in the triple-junction region are considered. A high-velocity anomaly is identified along the axial zone of the Bouvet transform fault, and the roots of this anomaly in the upper mantle are traced to a depth of 250 km. A low-velocity anomaly is revealed under Bouvet Island, which is traced to about 500-km depths.

The paper considers the age, composition, and genesis of granitoids of the Soktui massif, a petrotype of the Kukul’bei Complex of Mesozoic rare-metal granites in Transbaikalia. The Soktui massif is heterogeneous; it comprises several petrographic varieties: monzogranite-monzoleucogranites and microleucogranites of the major intrusive phase and alaskites, alkali-feldspathic granite porphyry, granodiorites, and quartz syenites of the phase of additional intrusions. According to the obtained U-Pb geochronological data, the ages of all varieties are the same within the analytical error and correspond to the Early Cretaceous. The granitoids are diverse in geochemical characteristics: The monzogranite-monzoleucogranites of the major intrusive phase belong to the rare-metal plumasite type, and the rocks of the phase of additional intrusions show signs of A -type granitoids. The geochemical and isotope characteristics of the rocks confirm the contribution of both continental crustal substrates and deep-seated mantle-derived magmas to their formation. The contents of volatiles in micas and the composition of fluid and melt inclusions indicate the involvement of two types of fluids in the magma generation: reduced chloride (probably mantle) and more oxidized fluoride (presumably of lower-crust origin). All melts were generated at depths of no more than 30 km, and the depth of the massif formation was shallower than 8 km. Based on the obtained data, we propose a model for the formation of the massif rocks.

We have studied the macrocomponent composition of pore solutions in hydrothermal clays forming extended thick strata in the thermal fields of the Pauzhetka geothermal system. Two zones have been identified in the vertical sections of the clay strata, which differ in the physicochemical parameters, composition, and formation conditions of pore solutions. It is shown that the pH of the solutions plays a crucial role in the change of their macrocomponent composition with depth. The conclusion is drawn that the pore solutions resulted from the direct impact of deep-level infiltrating thermal waters on the matrix of hydrothermal clays, which led to the redistribution of elements between the rock and the contact solution. Along with the general regularities, we have established significantly different conditions of formation of pore solutions in the Upper Pauzhetka and East Pauzhetka thermal fields, which is due to the different geologic positions and hydrogeochemical regimes of these fields.

Sulfosalts of the tetrahedrite and enargite groups from the Maletoyvayam epithermal Au-Ag deposit were studied using optical and scanning electron microscopy. We conclude that they were crystallized from acidic magmatic volcanogenic hydrothermal fluids under conditions of elevated oxygen level. Early sulfosalts of the pre-gold stage of the Maletoyvayam deposit, argentotetrahedrite-(Zn,Fe) and tetrahedrite-(Zn,Fe), associated with pyrite, arsenopyrite and galena, evolved with their enrichment in Te, Se and Cu. Increased activity of these elements caused by an increase in the oxidation potential of the environment led to the crystallization of subsequent stibiogoldfieldite, arsenogoldfieldite, and minerals of the enargite group, where the excess Cu content increased as the ore-forming system evolved. Au-bearing minerals are paragenetically associated with sulfosalts of the final stage of this evolution. The crystallization trend of sulfosalts (As→Sb→Te) at the Maletoyvayam deposit is also characteristic of other acid-sulfate type deposits, such as the Ozernovskoe and Prasolovskoe deposits, in contrast to the reverse trend (Te→Sb→As) typical for sulfosalts from adularia-sericite type epithermal Au-Ag deposits.

Active faults under permafrost conditions in the highland part of Gorny Altai (South Chuya and Kubadru fault zones) and the Lena River delta (Primorsky Fault Zone) were studied using the electrical resistivity tomography technique. The method proved to be effective in identifying active fault zones to depths up to the first hundreds of meters under permafrost conditions. However, the presence of ice-rich rocks with a resistivity greater than 100 kΩ·m limits its application because of the screening effect of the insulating unit. The main criterion for identifying active faults in geoelectric sections is the existence of subvertical zones of reduced electrical resistivity against the background of high-resistivity permafrost rocks. This concerns both the young seismic ruptures formed during the 27 September 2003 Chuya earthquake ( M_s = 7.3) in the South Chuya Fault Zone and the older Holocene primary seismic deformations in the zones of the Kubadru and Primorsky faults. At the same time, the electrical resistivity values in the zones of active faults and seismic ruptures are too high to assume their saturation with free water. The decrease in electrical resistivity in such zones relative to the host permafrost frame may be due to (1) intensive fracturing of rocks and sediments; (2) occurrence of finely grated material in the core and damage fault zones, on particles of which adsorbed unfrozen water is concentrated; (3) residual thermal anomalies in the case of modern fault activations, so that negative temperatures have already been restored, but the process of frozen strata aggradation is not complete yet; (4) saturation of the geologic section with sand-silty material as a result of liquefaction and fluidization processes during earthquakes. The revealed regularities can be used not only to confirm the zones of morphologically distinctive segments of active faults but also to search for their buried segments in the permafrost areas typical of seismically active highland and Arctic regions in Russia and worldwide.

The article considers the results of mathematical modeling of transient electromagnetic cross-borehole monitoring data for civil and industrial cryolithozone facilities containing thaw zones (taliks) in their vicinity. A solution to the direct problem is presented based on the Sumudu integral transform and the vector finite-element method for two types of borehole sources: induction coils and a less common electric current line, taking into account the frequency dispersion of the electrical conductivity of permafrost. Three-dimensional numerical modeling of the transient signals is performed in realistic geoelectric models of a gas-producing borehole and a residential building on piles. Based on the modeling results, we have revealed the main features and differences of transient cross-borehole monitoring with coils or a line as the source.