Home – Home – Jornals – Russian Geology and Geophysics 2026 number 2

We present new lithological and biostratigraphic data on stratotype and paleontologically representative marine sedimentary sections of the Malinovka Group (Borlug, Tarlyk and Tamzyrin formations) in the Uyuk area of the region, Shemush-Dag Group (Ayangaty and Adyr-Tash formations) and Chergaky Group (Alavelyk Formation) in the Khemchik area of the region, Chergaky Group (Alavelyk Formation) in the Alash area of the region, Kargy Formation in the Kargy area of the region. The materials on the Sistig-Khem Group (Uza, Ust-Khamsary and Kugar formations) in the Sistig-Khem area of Tyva are considered. We specified the chronostratigraphic position of the Tarlag-Aksy Horizon and justified the identification of a new Borlug Horizon. We found out that the Tyva Ordovician marine sediments are currently located in individual tectonically isolated blocks and are not fragments of a single paleobasin. Based on the analysis of the taxonomic composition of fauna assemblages and trace fossils, we suggest that the Alash-Khemchik, Uyuk and Kargy blocks with outcrops of Ordovician marine sediments of Tyva are paleozoogeographically close to the marine paleobasins of the Gondwana group of continents. At the same time, we note a low migration potential of their fauna assemblages relatively to the possibilities of communication with coeval communities of the Altai Basin and of that of the Siberian Platform and Taimyr. The Sistig-Khem block of Tyva, having effusive-sedimentary sequences of marine genesis, was not only significantly distant from the listed Siberian paleobasins, but was also separated from the group of Alash-Khemchik, Uyuk and Kargy blocks with Ordovician marine sediments.

The article presents new data on the formation conditions of basalt and basaltic andesite of the Kharchinsky Volcano (Central Kamchatka Depression, CKD). It is shown that the liquidus association is represented by olivine (Fo₉₁) and Cr-spinel, which crystallized in oxidized conditions NNO+0.4 - NNO+1.5 at temperatures of 1,110-1,210 °C. The study of melt inclusion in olivine phenocrysts allows us to reconstruct compositions of parental melts for basalt and basaltic andesite. These melts have Mg-high (Mg# ≈ 76), Al-low and Ca-low basic compositions enriched in volatiles. Water content in melts could have reached up to 5.5 wt.%. These melts formed from a peridotite source, sometimes with slight admixture of pyroxenite component. Crystallization of melts could have occurred in several intermediate chambers (up to 1.5, at 5-7 and 11-13 kbar).

Norilsk type intrusions are characterized by unique reserves of disseminated Cu-Ni mineralization localized in picritic and taxitic gabbro-dolerites. Picritic and taxitic gabbro-dolerites are rocks of different genesis, but the mechanism of formation of picritic gabbro-dolerites is still debatable. Most often, they are regarded as a cumulative part of a layered series. In this work we show a well-pronounced geochemical contact between a layered series and picritic gabbro-dolerites. In the section of the latter, we have established reverse geochemical zoning expressed as regular accumulation of major elements, which do not form a single trend of crystallization differentiation with the rocks of the main layered series. The rocks are discrete within the horizon of picritic gabbro-dolerites; we have identified two intervals: the lower one with low Cr contents and stable Eu and Sr anomalies and the upper one with anomalous high chromium contents, reduced LILE contents, and no positive Eu anomaly specific to the lower section of picritic gabbro-dolerites. Based on the known models of formation of reverse zoning in the marginal zones of layered massifs, we present a new genetic scheme for the formation of picritic gabbro-dolerites as products of pulsed intrusion. This scheme implies that the lower part of picritic gabbro-dolerites formed from hybrid magma, and the upper one, from primitive magma as a result of the pulsed filling of the magma chamber. In our opinion, this is the cause of the reverse zoning and the accumulation of chromium in the upper section of picritic gabbro-dolerites.

The Tomtor Complex of ultramafic rocks and carbonatites is located in the north of the Sakha Republic (Yakutia). Unique Sc-Y-Nb-REE ores of the Burannyi site (Tomtor deposit) are cryptogranular and have high contents of Nb₂O₅, REE₂O₃, Y₂O₃, and Sc₂O₃. Thin-layered ore bodies are stratified formations that presumably occur in weathering crust depressions. Based on modeling the shape of the roof and ore body base, as well as the overlying Permian continental and Jurassic marine sediments of the Burannyi site, the structural and morphological features of the ore layer were identified, using the QGis (QGIS.ORG Association, Switzerland) and Micromine (Micromine Pty Ltd., Australia) software packages. High-grade ores of the Burannyi site lie on an irregular surface. In the base of the ore layer there are two isolated depressions, the Northern and the Southern, which in turn are complicated by basins of various sizes. There are ten basins in the Northern Depression and four in the Southern Depression. The basins form linear structures that coincide with faults identified during exploration. Only four basins in the northern part of the site are completely filled with ore matter. Others are partially filled with ore, and the remaining volume is occupied by Permian coarse-grained clastic coal-bearing sediments. In the Southern Depression, the proportion of basins filled with ore material is the lowest; the deepest basin is filled with ore by only 25%. It is assumed that this difference is caused by different times of basin formation. Developing the hypothesis that the ores are sediments of a thermal reservoir, we suggest that the formation of basins at the ore layer base occurred as a result of hydrothermal (phreatic) explosions. The presence of tectonic breccia confirms the occurrence of fast high-pressure processes in the Tomtor Complex.

The morphology, chemical composition, and ore and heavy-mineral associations of gold from placers in the Ursky ore cluster and the less-studied Kasminsko-Chesnokovsky prospective area in northeastern Salair indicate numerous and diverse primary sources, as well as the presence of gold in intermediate reservoirs. Gold with a fineness of 970-1000‰ is predominantly hypergenically altered. The placers have formed primarily from quartz, quartz-carbonate, and quartz-(carbonate)-sulfide veins and stockworks of the main gold mineralization stage, including mineralization superimposed on barite-polymetallic ores with fine and finely dispersed gold in the Ursky ore cluster, and on basic dikes. Another source of gold is metasomatite (carbonate-mica, quartzite, and quartz-albite-epidote rocks with rutile), which either accompanies gold ore bodies or is indirectly associated with them. Differences in the typomorphic properties of the placer gold are due to varying contributions from these primary sources. In the placers of the upper reaches of the Ur and Zvonchikha rivers, the source is likely gold mineralization predominantly superimposed on basic dikes. This gold exhibits high fineness (>910‰) and contains copper impurities (up to 1-6 wt.%), as well as an inclusion of Pd telluride in one instance. Quartzite is another potential source of Au (as observed at the Kopna deposit). In the lower part of the Ur River placer, the fineness of Au and the proportion of copper-bearing gold decrease, while gold with mercury impurities is more frequently encountered. The gold mineralization here is probably superimposed on polymetallic mineralization or localized within rocks of the Pecherkinsky complex. The presence of gold with fineness below 820‰ (as at the Iyunsky deposit) indicates polymetallic gold-bearing mineralization. Sources for the gold in the Chesnokovka and Kurnichikha river placers, besides mineralization associated with basic dikes, include mineralization characterized by gold with a fineness of 820-910‰ (including rather small and fine gold), which typically contains mercury impurities. These could be quartz veins and metasomatites developed in terrigenous rocks of the Suenginsky Formation, known within the area. Inclusions in gold from placers of the Kasminsko-Ursky ore district are represented by minerals from sulfide ores, metasomatites, and weathering crusts. Unlike gold from the Ursky ore cluster, gold from the Kasminsko-Chesnokovsky area contains no inclusions of copper minerals (chalcopyrite, bornite, covellite, and tennantite), which are common in pyrite-polymetallic ores. The typomorphic properties of gold from the Aprelsky deposit reflect multiple sources, including quartz veins, metasomatites in rocks of the Pecherkinsky Formation, and possibly mineralization superimposed on polymetallic ores. The sources of gold for the Khristinovskaya Yama placer are likely diverse and similar to those for the Ur and Zvonchikha river placers. An isoferroplatinum grain from the Khristinovskaya Yama placer can be classified as the Vilyui type.

In the new paradigm of the Russian oil and gas industry, much attention is paid to small hydrocarbon fields. This statement increases the requirements for the accuracy of small-scale anticlinal trap’s resources estimation. Such estimations can be made by probabilistic estimation of resources, taking into account the probabilities of the field existence. The objective of the research is the development of the scientific approach to quantifying the uncertainties associated with the existence and geometric parameters of small-scale anticlinal hydrocarbon traps mapped by modern 3D seismic exploration. The main method of solving this problem is stochastic modeling of structural uncertainties based on errors of structural mapping. The results of the study are summarized as follows. The probability functions of the trap’s area are determined by the intensity and size of the velocity anomalies, as well as the parameters of the trap itself and the nature of its structural environment. The distributions of the structural closure areas can be described by functions with both positive and negative asymmetry. The lognormal distribution is just one of the possible options. For small-scale hydrocarbon traps, their amplitude is comparable to the thickness of a productive reservoir; therefore, a change in the amplitude of the trap determines a change in the weighted average oil-gas-saturated thickness. As a result of the modeling, a positive relationship was established between variations in the anticline traps areas and their amplitudes. Accordingly, this relationship must be taken into account during the probabilistic assessment of the resources of this kind of the hydrocarbon traps. Otherwise, there may be a significant reduction in the range of uncertainty in resource estimates. Stochastic modeling of structural uncertainties is also a method of estimating the probability of the existence of anticlinal traps mapped by seismic exploration.

The paper describes methods for the three-dimensional interpretation of gravitational and magnetic anomalies, taking into account the relief of the Earth’s surface. Parallel algorithms for solving gravimetry and magnetometry forward problems (calculating field values from sources) are programmatically implemented for personal computers with graphics accelerators. Based on these algorithms, methods for solving inverse problems on correctness sets were developed. When modeling, sources of anomalies of arbitrary shape are approximated by a dense regular grid, the elements of which are parallelepipeds. Methods for identifying anomalies and localizing sources in the Earth’s crust, calculating their physical parameters were applied in modeling the structure of the Kraka ultrabasic massifs in the Southern Urals with an area 50 × 80 km². The height of the mountain ranges in this area reaches 1043 m, and the difference in relief heights is more than 500 m.