Home – Home – Jornals – Russian Geology and Geophysics 2025 number 7

We present the results of isotope, geochronological, geochemical, and mineralogical studies and mineral composition analyses of melanocratic enclaves in the granites of the Sarkhoi Complex (Kaa-Khem magmatic area, Eastern Tuva). According to geochronological data (zircon U-Pb dating), the age of the enclaves differs from that of the host granites (480 and 450 Ma, respectively). The ε_Nd( T ) values for the enclaves are 3.2, while for the Sarkhoi Complex granites, they range from 0.5 to 1.7. Therefore, the enclaves cannot be considered cumulates from the earlier phase of granite melt crystallization or restites of the Sarkhoi Complex granitoid protolith. Melanocratic enclaves are xenoliths, which, based on their age and isotope characteristics, can be attributed to the early phase of the diorite-tonalite-plagiogranite association of the Early Tannu-Ola Complex. Fragments of mafic rocks were trapped during the ascent of granite melt (ca. 450 Ma) at the middle crustal level ( T of ca. 700 °C, P = 1.9-2.5 kbar), which led to their partial assimilation and compositional alteration through interaction with felsic melt. The xenogenic zircon present in the Sarkhoi Complex granites has an age of 480 Ma close to the age of the xenoliths, which suggests the contribution of Early Ordovician complexes to the formation of a significant volume of magmatic melt at the later stages of regional evolution. Xenoliths of mafic rocks in the granitoids of the Sarkhoi Complex, studied in the eastern part of the Kaa-Khem area, are products of melting of the primitive mantle, thus fundamentally differing from the coeval gabbroid complexes in the western part of the area.

The paper presents new data on the structural position, mineralogy, and geochemistry of ores of the Aliya ore cluster and their technological specifics. The mineralogical and geochemical zoning of the Verkhne-Aliinskoe gold deposit and the entire Aliya ore cluster is considered. We show the subordination of the local and horizontal zoning of mineralization within the ore cluster and the localization of granitoids of the late phase of the Late Jurassic Akatui Complex. A conclusion is drawn about the relationship of mineralization with granitoids within a single ore-magmatic system that existed under unstable shallow-depth P - T - X conditions transitional to subsurface ones. This specifics distinguishes the Verkhne-Aliinskoe deposit from the compositionally similar Sredne-Golgotaiskoe and Sosnovskoe deposits and partly determined the technological resistance of its ores.

This paper presents several opposing points of view on the ore-controlling structure of the large (~2000 t) Sukhoi Log gold deposit, as well as data on a detailed geostructural analysis. The deposit is located in the southern margins of the Siberian Craton in black shale deposits of the Khomolkha Formation (610-600 Ma) on the territory of the Baikal-Patom fold belt. It is revealed that the Baikal-Patom belt is characterized by a complex regional structure resulting from the accretion of the Baikal-Muya belt to the Siberian Craton. The formation of industrial ores of the Sukhoi Log deposit and ore-controlling folding dates back to the Late Ordovician- Early Silurian (~450-420 Ma). The second stage of the development of weakly gold-bearing ores falls on the Early Carboniferous (340-300 Ma). The study demonstrates the absence of an ore-controlling fault zone of the syn-folding stage (450-420 Ma). It is shown that the internal structure of the gold ore zone is comprised of intensely deformed rocks in the anticline core with bedding-plane gold-quartz-pyrite veinlets. It is concluded that the Sukhoi Log deposit is a classic example of disharmonic folding of plastic rocks in the anticline core, where the signs of two stages of structure formation are combined: buckle folding with flow and laminar flow. The present geostructural studies of the deposit and the previously published data suggest that this is an orogenic deposit with metamorphic-metasomatic redistribution of ore matter.

The paper summarizes the results of long-term geochemical studies on the regional geology of the Bodaibo synclinorium as a whole and the local (ore) geology of individual orogenic gold deposits located within the world’s largest Lena metallogenic province. The main attention is focused on the role of organic matter in ore genesis. The paper presents results of a study of the transformation of organic matter of carbonaceous shales (soluble (bitumen) and insoluble carbonaceous matter) during catagenetic and metamorphic transformations. The absence of a significant correlation between insoluble carbon and gold has been demonstrated. This is due to the low sorption capacity of graphitized organic matter relative to its primary (precatagenic) counterpart. Carboxylic esters (lipids) have been identified in bitumens of carbonaceous shales, thus confirming the bacterial and planktonogenic origin of the primary organic matter of sediments. The similar gold contents in bitumen from rocks within the studied deposits and in bitumen from remote rocks indicate the presence of gold in the organic fluid. It is concluded that carbon plays one of the leading roles in the gold mobilization and migration at the stage of catagenesis. However, the hypothesis that organic matter can be a barrier to metal-bearing hydrotherms is not confirmed. Based on the model of the catagenetic formation of oil fields, we show the high probability of the formation of gold mineralization at the Dal’naya Taiga-Zhuya stratigraphic level in the Neoproterozoic section of the Baikal-Patom region.

We carried out palaeontological studies of the Vendian-Cambrian boundary strata from the Leningrad Region in the north of the East European Platform (northern part of the Baltic Basin). A stratigraphic succession of micro- and macrofossil associations has been recognised in the Taitsy-2 drill-core section comprising three Upper Vendian associations: (I) Aataenia reticularis, (II) Vendotaenia antiqua - Incertae sedis Forma 1, and (III) Bicuspidata fusiformis - Vanavarataenia insolita of the Kotlin Regional Stage; and two Lower Cambrian associations: (IV) Ceratophyton sp. - Teophipolia lacerate of the combined Rivne-Lontova Regional Stage, and (V) Archaeodiscina umbonulata - Globosphaeridium cerinum - Skiagia orbiculare of the Dominopol Regional Stage. The early Cambrian taxa Ceratophyton sp., Granomarginata squamacea , Teophipolia lacerata , and Platysolenites antiquissimus first appearing at the base of the Lomonosov Formation and extending their stratigraphic range into the overlying Siverskaya Formation define the position of Precambrian-Cambrian boundary. The Rivne and Lontova regional stages are indistinguishable based on palaeontological data. The cooccurrence of fragmented metazoan fossils and acanthomorph acritarchs in the Lükati Formation expands the palaeontological characteristics of the Dominopol Regional Stage of the European part of Russia. We also conducted isotope-geochronological studies of sandstone-hosted glauconite from the Lower Cambrian Siverskaya Formation that yielded a Rb-Sr date of 457 ± 3 Ma. The glauconite appears to be altered and rejuvenated as suggested by mineralogy and crystallochemistry data, is not suitable for stratigraphic purposes, but provides information on timing and contribution of later geological processes.

We report the results of deep magnetotelluric sounding, conducted for the first time within the Tunka basin system, which belongs to the southwestern flank of the Baikal Rift Zone. Despite the regional scope of the study, electrical conductivity anomalies were identified at virtually all intervals of the geoelectric section constructed to depths of 70 km. In the western part of the Tunka Rift, at the lower lithosphere level, there is an area of rocks that were presumably heated by a mantle plume, which was identified in previous studies in northern Mongolia. The position of the crustal conductive layer, which likely is a mineralized reservoir of planetary scale, is atypical: its thickness increases, and its roof rises towards the central part of the Tunka Basin. In the upper part of the Earth’s crust, subvertical conductive zones have been distinguished, which can be traced from the crustal conductive layer to sedimentary deposits. These zones are associated with fluid migration channels transporting geotherms of mantle origin. A correlation has been noted between these subvertical conductive anomalies and known thermal and/or mineral groundwater outlets, including “Nilova Pustyn”, “Arshan”, and “Zhemchug”. Additionally, one of the subvertical conductive zones in the eastern Tunka Rift, associated with a fluid migration channel, has been detected within sedimentary deposits (or does not reach them) near the Tibel’ti village, where the presence of geothermal waters had not been previously documented.

4D seismic is a series of time laps 3D seismic surveys employed during development of oil and gas fields. Its goal is to register changes in seismic wavefield within target interval caused by hydrocarbon extraction, which allows for optimization of field development plan (i.e. optimizing reservoir drainage, oil well placement, reducing reservoir development uncertainty, etc.). This article provides the first comprehensive analysis of the current state of 4D seismic in Russia, where, unfortunately, there are only a few cases of utilization of this technology, while global experience already includes hundreds of successful examples. Various options for its implementation for deep-sea (offshore) and onshore hydrocarbon exploration and production are considered. However, not numerous, practically all known cases of 4D seismic survey acquisition over Russian hydrocarbon fields are analyzed, and future prospects of this technology in Russia are assessed.

The article, by the example of a layered material which can be reduced to a two-layer equivalent illustrates the differences in the frequency and transient responses of interfacial polarization depending on which source - current or voltage - is used to excite the ground in electrical and EM geophysical prospecting methods. In material science, a voltage source is usually used to study interfacial polarization. In this case, the manifestations of interfacial polarization are known as the Maxwell-Wagner effect. As for the geophysical induced polarization method, it uses a current source to energize the ground, which causes frequency and transient responses of the interfacial polarization to differ from those predicted by the Maxwell-Wagner theory. In principle, using the interfacial polarization frequency or transient responses measured with both current and voltage sources, one can find all properties of the equivalent two-layered model. In practice, however, polarization of grounding electrodes complicates the in-situ studies of the Maxwell-Wagner effect using a grounded source. There is no such a problem in the time domain electromagnetic prospecting method: when the current in the transmitter loop is switched off, a vortex electric field is induced in the ground, which - in terms of the circuit theory - is equivalent to using the voltage source.

Paleointensity determinations of the geomagnetic field in the Proterozoic were obtained using a set of rocks sampled from intrusive bodies in the north of the Siberian platform aged ~1500 Ma. Magnetic and thermomagnetic properties of rocks were studied in detail. X-ray diffraction and electron microscopic analyses were performed. It is shown that single- and small pseudo-single-domain magnetite grains serve as carriers of the characteristic natural remanent magnetization component. Paleointensity B _anc was determined using the Thellier-Coe method with the checkpoint procedure (test heating to lower temperatures) and the Wilson-Burakov method. A total of 22 determinations (34 with duplicates) satisfying modern reliability criteria were obtained for six sites. All of them exhibit low values of the field magnitude and virtual dipole moment. They change within a range of (4.7-17.6) μT and (1.21-3.85) ×10²² Am², respectively, which is approximately four times lower than their mean values in the modern era. Paleointensity determinations for the Proterozoic presented in the international database and the inclination function a ( I ) = 1/(1 + 3cos²(I))^1/2 were subjected to a joint analysis. It is revealed that the data on the diagram ( B _anc, a(I)) are divided into high- and low-paleointensity clusters. At the same time, both clusters indicate a dipole field geometry regardless of intensity. The analysis confirms the hypothesis of the bimodal geomagnetic field generation regime in the Proterozoic, which may indicate the absence of a solid inner core in the early and middle Proterozoic, attributing its formation to a later era (Ediacaran).