Vol 514, No 1 (2024)

Based on the results of mass U–Pb (LA-ICP-MS) dating of detrital zircon from the basal deposits of the Upper Precambrian of the Polar Urals, represented by the Nyarovei series, their lower age limit was established. The upper age boundary of the basal series was obtained from the U–Pb (LA-ICP-MS) dating of zircon from the overlying volcanogenic formations of the Nemyurugan suite. Thus, it has been established that the deposits of the Nyarovey series were formed in a narrow time interval at the end of the Late Riphean about 750–650 million years ago in the Arshinian time. Identification of the series as a Middle Riphean straton is erroneous. The dominant role in the formation of the deposits of the Nyarovey series belonged to the products of erosion of nearby massifs of crystalline rocks, fragments of which are probably polymetamorphic complexes protruding in the Harbei-Marunkeu anticlinorium in the Polar Urals.

Geochemical, geochronological (U–Pb zircons, ID-TIMS) and isotope-geochemical (Sm–Nd) studies of the rocks of the Bambukoy volcano-plutonic association, which form the Zhanok-Bambukoy volcano-tectonic structure within the Anamakit–Muya terrane on the northern flank of the Barguzin–Vitim superterrane of the East Transbaikalian segment of the Central Asian orogenic belt. The association includes volcanic rocks of the Zhanok Suite (dacites and rhyolites mainly), as well as leucocratic and biotite granites of the Bambukoy Complex that cut through them. The granites of this complex host the Mokhovoe tin deposit, which is attributed to the tin-porphyry formation. Subvolcanic rocks of the Zhanok Suite are considered as ore-bearing. The geochemical features of the volcanic rocks of the Zhanok Suite and the granites of the Bambukoy complex bring them closer to S-type granites, and belonging to a single tin-bearing Bambukoy volcanic-plutonic association. The formation of this association is determined by the age interval 834 ± 23–818 ± 7 Ma. The isotopic data point to a source of parental magmas from the rocks of the Bambukoy volcano-plutonic association, formed as a result of mixing of the material of two crustal sources, the mature Early Precambrian and the juvenile Early Baikal. The geochemical data also point to an exclusively crustal source of the rocks of this association. Thus, the Bambukoy tin-bearing volcano-plutonic association was formed in the Neoproterozoic time (Tonian), most likely under lithospheric extension conditions due to a source with a complex and long crustal prehistory.

It is established that faults in the sedimentary cover of the Nansen basin and seismic anomalies of the “flat spot” type associated with the methane accumulation are grouped into three spatial combinations: 1 – synchronized faults and spots, 2 – spots without faults, 3 – faults without spots. They are distributed mainly between linear magnetic anomalies C20 and C12 over negative variations of the lithosphere density with depths up to ~25–30 km and lateral periodicity ~50 km. The genesis of combination 1 is provided by serpentinization of upper mantle rocks in the presence of water that has depth penetration through previously formed tectonic displacements, an increase in the rock volume and local rise of crystalline blocks, leading to the formation of faults of thrust kinematics, crossing the entire sedimentary cover from the acoustic basement to the ocean bottom. Combination 2 consists in the predominance of “flat spots” also of fluid genesis in the absence of faults, which, with a rare seismic observations, may be missed and not appear in the plane of the sections. Combination 3 consists in the presence of faults without “flat spots” with a spatial step of ~ 10 km above the highs of the acoustic basement. In the Bouguer anomalies this combination is manifested over ~80 km depression of ~25 mGal depth, comparable to the gravity depth under the axis of the Gakkel ridge. This is not due to the linear structure of the ridge, but, perhaps, to a single upper mantle plume. From this follows the mechanism of faults formation above it, associated not with serpentinization, but with the rise of the plume body to the surface. Physical modeling of the structure formation during the slowdown of the spreading rate, which took place in the range C20–C12, showed that the amplitude of the relief drops increases greatly. Comparison with a real acoustic basement shows the similarity of its relief in the corresponding time intervals of spreading slowdown with the areas of relief change in the physical model. The increase in the amplitudes of the basement highs is most likely due to the formation of faults that provide the circulation of water necessary for serpentinization.

We consider a tectonothermal model and the evolution of magmatism during the late Paleozoic postcollisional (pre-plume) development stage of the Kara orogen in northern Taimyr, Central Arctic. The model is based on new and published structural, petrologic, geochemical and geochronological data, as well as thermophysical parameters obtained for the Kara orogen that includes great amounts of syncollisional and postcollisional granites formed due to the collision of the Kara microcontinent and the Siberian craton. Based on geological, geochemical and U–Th–Pb isotope data, the granites have been differentiated into syncollisional and postcollisional intrusions formed 315–282 Ma and 264–248 Ma respectively. Our previously published tectonothermal model [1] concerned the syncollisional formation stage of the Kara orogen at 315–282 Ma, during which the emplacement of anatectic granites took place. In this new study, we focus on the evolution of postcollisional magmatism in the orogen at the Permian–Triassic boundary. The existence of multiple bodies of allochthonous granitoids aged 265–248 Ma in the Kara orogen that predate the extensive eruption of the Siberian traps (~250 Ma) motivates us to reconstruct the thermal state and melting mechanisms of the crust on the “pre-plume” stage. To solve this problem, numerical modeling of the thermal, tectonic and magmatic evolution of the Kara orogen’s crust is used alongside geochemical and isotope data reflecting the magmatic sources of the granitoids.

The first data on the composition of rocks and precious metals in the Vasilinovskoye copper-gold-platinoid occurrence, new in the Polar Urals, localized in metagabbro and metapyroxenites of the Late Ordovician Caershor Complex, are presented. It is shown that rocks containing productive gold-platinum mineralization are confined to zones of hydrothermal alteration and are mainly represented by apogabbro and apopyroxenite propylites. The temperature of formation of propylites obtained by pyrite-pyrrhotite and chlorite geothermometers corresponds to albite-calcite-chlorite and epidote-chlorite facies and varies within 200–310 °C. The main ore mass is composed of pyrite, chalcopyrite and magnetite. Pyrrhotite, sphalerite, cobaltite, zinc greenockite, galena, ilmenite, titanite, rutile, cinnabar, native tin, molybdenite, coloradoite, supergene ones – bornite, covellite, minerals of the chalcocite-spioncopite series, malachite, azurite, iron hydroxides are noted. Noble metal mineralization is represented by mercury-bearing gold (including mercuric electrum, mercuric kustelite), gold and silver sulfides and tellurides (sylvanite (AuAgTe₄), petzite (Ag₃AuTe₂), acanthite (Ag₂S), hessite (Ag₂Te), tegamite (Pd₃HgTe₃), minerals of the isomorphic series merenskyite-moncheite (PdTe₂-Pt, Pd)(Te, Bi)₂), stibiopalladinite (Pd₅Sb₂), kotulskite (PdTe), sperrylite (PtAs₂), sopcheite (Ag₄Pd₃Te₄), intergrowths of platinoids and an unknown mineral, with an idealized formula Pd(Sb, Te), possibly a tellurium-rich variety of sudburyite (PdSb). By mineralogical methods, it has been established that precipitation of precious metals occurs after the sample is crushed to a dimension of –0.25 mm, relatively large <0.15 mm gold grains are separated first, then, during the crushing of sulfides, smaller gold flakes and precious metal grains are separated. It is shown that the Vasilinovskoye ore occurrence is promising in terms of gold and platinoids.

The results of a REE ID-ICP-MS,⁸⁶Sr/⁸⁷Sr and¹⁴⁷Sm-¹⁴³Nd AL ID-TIMS study of the isotope systematics of spinel lherzolites from the northern Kraka massif, which is part of the largest (more than 900 km2) lherzolite allochthon thrust over the bathyal and shelf deposits of the passive continental margin, East European platform are presented. As a result, an isochron dependence (MSWD = 0.85) was revealed for the first time, which determines the age of 545±26 Ma and the high value of the initial ratio (¹⁴³Nd/¹⁴⁴Nd)₀ = 0.512390±0.000054, corresponding, within the framework of model representations, to ε_Nd = +8.9. The resulting REE,⁸⁷Sr/⁸⁶Sr, and¹⁴⁷Sm-¹⁴³Nd isotopic signatures indicate the melting of an already depleted protolith, which can be identified as a mantle source, with MORB-like parameters. The isochron age calculated in the framework of the¹⁴⁷Sm-¹⁴³Nd AL ID-TMS study, in combination with the available complex of geological and geochemical data, allows us to state the manifestation of the Late Vendian phase (epoch) of folding and orogeny in the Urals in the interval of 545 ± 26 Ma. Comparison of these data with materials on the geology of Central and Western Europe makes it possible to correlate the Timanid structures formed as a result of this phase of folding with Cadomides, which ultimately, based on global reconstructions of continents for the end of the Proterozoic, will authorize the hypothesis of the existence of the Cadomian orogen on the periphery of Gondwana.

New U-Pb geochronological and geochemical data for granitoids of the Mainitsky terrane of the Koryak folded region are presented. It is shown that granitoids cut the folded-thrust structures of the Mainitsky terrane, which include ophiolites of the Late Paleozoic – Late Mesozoic and volcanogenic-siliceous-terrigenous deposits of the Middle Jurassic – the Turonian. Granitoids correspond to low-alkaline quartz diorites and I-type granodiorites, are part of the Late Cretaceous volcanic-plutonic association and indicate their formation in a convergent margin. The U-Pb ages of zircons are 82–90 million years.

For the first time in podiform chromitites, a mineral of the humite group, titanium–containing hydroxylclinohumite, belonging to natural dense water-containing magnesian silicates (DHMS phases), was found. The find was made in chrome ores of the Paity plateau, located in the northern part of the Voikaro-Synyinsky ultramafic massif of the Polar Urals. Chromitites are deposited in the dunite body, which is located at the contact of apoharzburgite amphibole-olivine-antigorite and amphibole-enstatite-olivine metamorphic rocks containing relics of non-serpentinized harzburgites. According to optical data and chemical composition, the mineral belongs to titanium-containing hydroxylclinohumite, since it contains TiO₂ (up to 5.64 wt.%), in the complete absence of fluoride. The presence of OH-grouping in the mineral is confirmed by the Raman spectrum, which generally corresponds to the standard of hydroxylclinohumite. The results of geothermometry show that this mineral was formed in chromitites of the Polar Urals at a temperature of 668–740º with and, probably, a pressure of 20–25 kbar. It follows from this that the formation (or recrystallization) of chromite ores occurred under high-pressure conditions, i. e. ore occurrences of chromites in the northern part of the Voikaro-Synyinsky massif were formed, apparently, in a suprasubduction environment.

The study of previously collected fossils and new materials on Cambrian mollusks from the Bayangol Formation of Western Mongolia (Zavkhan structural-facies zone) revealed a significant similarity in the taxonomic composition of the mollusk assemblages of Western Mongolia and Southern France (Heraultia Limestone, Marcou Formation, Montagne Noire). In addition to the previously reported four common species (Latouchella korobkovi (Vostokova, 1962), Protoconus orolgainicus (Zhegallo, 1996), Purella tenuis Zhegallo, 1996, and Watsonella crosbyi Grabau, 1900), we have identified 10 more common species for these regions: Auricullina auriculata (Vassiljeva, 1990), Bemella jacutica (Missarzhevsky, 1966), “Calbyella” multicostata Missarzhevsky, 1995, Cambroscutum concameratum Kerber, 1988, Helcionella sp., Merismoconcha tommotica (Zhegallo, 1996), Obtusoconus amplus (Zhegallo, 1982), Prosinuites tripartitus Kerber, 1988, Purella layracensis (Kerber, 1988), and Securiconus sp. We have to reconsider the existing reconstruction of faunal relations of paleobasins on the western frame of Gondwana in terms of substantiating a close faunal exchange with the Zavkhan terrane, which during the Early Cambrian was a part of microcontinents chain located at low latitudes between the Siberian Platform in the west and Gondwana in the east.

A new method for predicting lithofacies, gas fluid and brine zones, zones with abnormally high reservoir pressure, as well as petrophysical properties of rocks using artificial intelligence methods based on a family of new seismic attributes of the RTH method and well drilling data is proposed. The main difference between RTH attributes and traditional ones obtained by migration transformation is their voxel nature and hyperattributive. It turned out that this is a key advantage of the new approach to solving problems of geological forecasting using artificial intelligence methods. The paper presents the results of applying a new method for processing and interpreting modern 3D seismic data, as well as geological forecasting based on it for the area of intense brine occurrence of the Kovykta gas condensate field.

Based on the results of the spectral analysis of the air temperature and ice cover on the East Siberian Sea shelf reconstructed for the last five thousand years, empirical forecast models have been constructed that reflect the “natural scenario” of the dynamics of these climatic parameters during the next centuries. Based on the results of forecast constructions, in the future, the duration of the ice-free period, as well as the temperature of the near-surface air, will tend to increase. The conducted studies allow us to conclude that there is an anthropogenic influence on the temperature regime and ice conditions of the studied region, which manifested itself during the industrial period. Taking into account this factor, it can be assumed that the intensity of the studied natural phenomena will increase at a higher rate than is indicated by model constructions.

The characteristic temporal scales of response of the globally averaged climate model with the carbon cycle to external influences with the analysis of the spectrum of the linearized evolution operator of the corresponding dynamical system are evaluated. The model exhibits response time scales of about 4–6 years (related to the carbon dynamics in vegetation) and in the range of 20–100 years (related to the carbon dynamics in non-humified soil reservoirs). When taking into account the effect of humification in the model reveals the time scale of the response, which is on the order of several millennia. For the closed carbon cycle, a time scale of 10² years is revealed, which characterizes the joint changes in the atmospheric and ocean reservoirs. At high universality of the proposed approach it can be used for a wide range of tasks.

Sediment cores from three lakes in the Boguty depression allowed studying sedimentary records up to ~24 ka old in subaquatic sections up to 4.71 m long. This is the highest (2390–2470 m a. s. l.) and longest achieve of geochronological and paleoecological data for the neighboring arias of Altai, Tuva and Mongolia, which reflects the course of natural processes in the highest and most arid part of the Russian Altai from the Last Glacial Maximum till the present day. Paper reports estimations of the freshwater reservoir effect for the moraine-dammed Upper Boguty lake, which are 290 years for the present time, and about 1200 and 1300 years for the times ~5.7 and ~9.6 ka ago, respectively; chronological benchmarks (16–13 ka ago) for sedimentological transition zone between the late Pleistocene glacial-lacustrine clays and the Holocene biogenic-terrigenous silts, which was determined on the basis of multidisciplinary analysis; the size of glaciers in MIS-2 and in the Younger Dryas, as well as the time of thermokarst lakes formation (no later than 8.7 ka ago) within the upper terminal moraine complex; conformation of wide spreading of tree vegetation in the now treeless Boguty depression in the first third of the Holocene.

We consider the uniqueness conditions of the inverse coefficient problem arising in analytical models of Mercury’s magnetic field.