V.A. Vanin1,2, A.M. Mazukabzov1, A.E. Budyak2,3 1Institute of the Earth's Crust SB RAS, Irkutsk, Russia 2Irkutsk National Research Technical University, Irkutsk, Russia 3Vinogradov Institute of Geochemistry, Russian Academy of Science, Siberian Branch, Irkutsk, Russia
Keywords: Sukhoi Log deposit, layering, axial-plane cleavage, foliation, linearity, buckling fold, laminar current
The article presents several published opposing points of view on the ore-controlling stage of the large (⁓ 2,000 t) Sukhoi Log gold deposit, as well as data from a detailed geological and structural analysis obtained by the authors. The deposit is located in the southern framing of the Siberian Craton in the black shale deposits of the Khomolkhin suite (610-600 Ma) on the territory of the Baikal-Patom folded belt. The complex regional structure of the Baikal-Patom belt formed as a result of the addition of the Baikal-Muya belt to the Siberian craton is demonstrated. The formation of industrial ores of the Sukhoi Log deposit and ore-controlling folding occurred in the Late Ordovician – Early Silurian (~ 450–420 Ma). The second stage of formation of weakly gold-bearing ores occurred in the early Carboniferous (340‒330 Ma). As a result of the conducted study, the absence of an ore-controlling fault zone of the syn-folded stage (450‒420 Ma) was established. It is shown that the internal structure of the gold ore zone is represented by intensively deformed rocks in the anticline core, where gold-quartz-pyrite veins occur in layers. It is concluded that the Sukhoi Log deposit is a classic example of disharmonious crumpling of plastic rocks in the core of an anticline, where signs of two stages of structure formation are combined: the longitudinal bending of a rock mass with the flow of matter and its (matter) laminar flow. Geological and structural studies of the deposit, together with previously published data, allow us to classify the deposit as an orogenic type, with a metamorphogenic-metasomatic of the redistribution of ore matter.
Yu.P. Ampilov1,2, S.V. Gorbachev1 1Lomonosov Moscow State University, Moscow, Russia 2Oil and Gas Research Institute of the Russian Academy of Sciences, Moscow, Russia
Keywords: seismic monitoring, field development, 4D seismic survey, towed streamers, seafloor stations, reservoir pressure, reservoir flooding, fiber optic systems, seismic inversion, acoustic impedance
4D seismic exploration is a series of consecutive time-separated 3D areal seismic surveys at oil and gas fields under development. Its purpose is to attempt to register changes in productive formations caused by hydrocarbon extraction from the subsoil in the seismic wave field and, on this basis, optimize the scheme for further field development.
This article provides the first comprehensive analysis of the current state of 4D seismic exploration in Russia. Unfortunately, there are only a few cases of using this technology, while global experience already includes hundreds of successful examples. Various options for its implementation at sea and on land are considered. Almost all known cases of 4D seismic exploration application in Russia, which are still few, are given and the future prospects of this method are analyzed.
M.F. Erbajeva1, S.A. Reshetova2,3, B.B. Karasev4, and N.V. Alexeeva1 1Dobretsov Geological Institute, Siberian Branch of the Russian Academy of Sciences, Ulan-Ude, Russia 2Vinogradov Institute of Geochemistry, Siberian Branch of the Russian, Irkutsk, Russia 3Institute of Natural Resources, Ecology and Cryology, Siberian Branch of the Russian Academy of Sciences, Chita, Russia 4A.P. Karpinsky Russian Geological Research Institute, St. Petersburg, Russia
Keywords: biostratigraphy, small mammals, paleovegetation, Late Pleistocene, Eastern Transbaikalia
The current paper presents the results of multidisciplinary studies of the Late Pleistocene deposits exposed at the Nozhyi site of the Eastern Transbaikalia. New data on small mammals have filled gap in the fossil records of the Late Pleistocene faunas of the region. Faunal remains and spore-pollen samples were derived from loessial deposits rare distributed in the region. The rich faunal association and data on paleoflora of one cold stage of the Late Pleistocene were found in the section. The fauna is dominated by taxa inhabited the open steppe landscapes: Ochotona daurica, Marmota sibirica, Lasiopodomys brandti, Cricetulus barabensis and Allactaga saltator. The faunal species composition and the quantitative relationships of taxa as well as the evidence on palaeoflora demonstrate the paleolandscapes around Nozhyi site. Widely distributed forest steppes with open steppe landscapes, herbage meadow and territories with sandy desert were reconstructed for studied faunal locality.
E.Yu. Golubkova1,*, E.A. Kushim1, E.G. Raevskaya2, I.M. Bobrovskiy3, T.S. Zaitseva1, E.G. Dovzhikova4, A.V. Titova1, A.V. Kolesnikov5, D.V. Grazhdankin6 1 Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia 2 A.P. Karpinsky Russian Geological Research Institute, St. Petersburg, Russia 3 Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Potsdam, Germany 4 Ukhta State Technical University, Ukhta, Russia 5 Geological Institute, Russian Academy of Sciences, Moscow, Russia 6 Trofimuk Institute of Petroleum Geology and Geophysics, Novosibirsk, Russia
Keywords: Microfossils, macrofossils, glauconite, Rb–Sr dating, stratigraphy, Vendian, Ediacaran, Cambrian, East European Platform
We carried out paleontological 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 recognized in the section of the Taitsy-2 drillcore 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 Dominopole 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 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 co-occurrence of fragmented Metazoan fossils and acanthomorphic acritarchs in the Lyukati Formation expands the palaeontological characteristics of the Dominopole 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.
V.I. Dzhurik, E.V. Bryzhak, S.P. Serebrennikov, A.N. Shagun, A.Yu. Eskin
Institute of the Earth`s Crust of the Siberian Branch of the Russian Academy of Sciences, Russia, Irkutsk
Keywords: seismic effects, Baikal coast, initial seismicity, earthquakes, seismic microzoning, accelerograms, frequency characteristics, spectra, accelerations, resonant frequencies
Using a complex of geophysical, seismotectonic and engineering-seismological studies and the proposed methodological approaches, an assessment of seismic effects at various levels of seismic hazard zoning of the Central Baikal territories is given. The implemented approaches are substantiated by identifying zones of probable occurrence of large earthquakes and specifying their main indicators necessary for the most justified assignment of initial signals corresponding to the parameters of predicted large earthquakes. The basis for the reliability of the selected approaches and methods, in relation to specifying the initial seismicity, are the manifestations of previously registered relatively large earthquakes in the studied territory and, in the direction of seismic hazard zoning of specific territories on a probabilistic basis, the registered data on the behavior of coastal water-saturated massifs of loose sediments according to local earthquake records are quite reasonably accepted. The implemented approaches using a set of geophysical methods in the studied areas allow improving the methods of forecasting maximum seismic impacts for different soil conditions in order to obtain a set of seismic characteristics for them in the form of accelerograms, acceleration spectra, frequency curves and their parameters required for designing earthquake-resistant structures in the coastal areas of Lake Baikal. The studies carried out in this direction are presented using the example of a site located within the seismically active southeastern part of Central Baikal and on the worst soil conditions in relation to the manifestation of seismicity in its coastal areas.
V.V. Shcherbakova1, A.M. Pasenko2, V.P. Shcherbakov1, G.V. Zhidkov1, N. A. Afinogenova1, A.A. Karimov3 1 Borok Geophysical Observatory, department of Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Yaroslavl obl., pos. Borok, Russia 2О.Yu. Schmidt United Institute of the Physics of the Earth, Russian Academy of Sciences, Moscow, Russia 3Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
Keywords: Proterozoic. bimodal mode of geomagnetic field generation, low paleointensity, Proterozoic, Thellier-Coe and Wilson-Burakov methods, Anabar Uplift, Siberian Platform
Paleointensity determinations of the geomagnetic field in the Proterozoic were performed on a collection of rocks selected from intrusive bodies in the northern part of the Siberian Platform, with an age of approximately ≈ 1500 Ma. The magnetic and thermomagnetic properties of the rocks have been studied in detail, and X-ray diffraction and electron microscopic studies have been performed. It has been shown that the carriers of the characteristic component of the natural remanent magnetization are single- and small pseudo-single-domain magnetite grains. The paleointensity Banc was determined using the Thellier-Coe method with the check-points procedure and the Wilson-Burakov method. For six sites, 22 (34 with doubles) determinations were obtained that meet modern reliability criteria. All of them show low values of the field strength and virtual dipole moment, varying within the range (4.7–17.6) μT and (1.21–3.85)×1022 Am2, respectively, which is about four times lower than their
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average values in the modern epoch. A joint analysis of the paleointensity determinations for the Proterozoic presented in the world paleointensity database and the inclination function a(I) = 1/[1+3cos2(I)]½ was performed. It is shown that the data in the diagram (Banc vs a(I)) fall into two clusters of high and low paleointensity values. At the same time, both clusters testify to the dipole geometry of the field, independent of its intensity. The performed analysis confirms the hypothesis of a bimodal mode of geomagnetic field generation in the Proterozoic, which may indicate the absence of a solid inner core in the Early and Middle Proterozoic, attributing its nucleation to a later time (Ediacaran).
I.V. Karmysheva, V.A. Yakovlev, S.N. Rudnev, E.D. Zyukov, D.V. Semenova
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: Magmatic enclaves, xenolith, I-granite, U-Pb dating, Sm-Nd isotope geochemistry, Kaakhem batholith
The article presents the results of isotopic, geochronological, geochemical and mineralogical studies, as well as mineral composition analyses of melanocratic enclaves within the granites of the Sarkhoy complex (Kaakhem magmatic area, Eastern Tuva). According to geochronological data (zircon, U-Pb), the age of the enclaves differs from that of the host granites (480 MA and 450 MA, respectively). The εNd(T) values for the enclaves are +3.2, while for the Sarkhoy complex granites, εNd(T) ranges from 0.5 to 1.7. This suggests that the enclaves cannot be considered cumulates from an earlier phase of granite melt crystallization or restites of the Sarkhoy complex granitoid protolith. Melanocratic enclaves represent xenoliths that, based on their age and isotopic characteristics, can be attributed to the early phase of the diorite-tonalite-plagiogranite association of the Early Tannuola complex. Entrapment of fragments of mafic rocks occurred during the ascent of the granitic melt (~450 million years ago) at the middle crustal level (T ~ 700°C, P = 1.9–2.5 kbar), leading to their partial assimilation and compositional alteration through interaction with the felsic melt.The presence of xenogenic zircon in the Sarkhoy complex granites, with an age of 480 million years, comparable to the age of the xenoliths, suggests a possible contribution of Early Ordovician complexes to the formation of a significant volume of magmatic melt in later stages of regional evolution. Xenoliths of basic rocks in the granitoids of the Sarkhoy complex, studied in the eastern part of the Kaakhem area, represent products of melting of the primitive mantle, which fundamentally distinguishes them from the coeval gabbroic complexes of the western part of the area.
N.A. Bushenkova, O.A. Kuchay
Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: strongest crustal earthquakes, aftershock and background seismicity, scalar seismic moment, P-wave velocity anomalies field, Central Asian region
The distribution of more than three dozen strongest intraplate crustal earthquakes (with Mw ≥ 6.9) recorded in Central Asia (20° - 55° N, 55° - 120° E) for the 1971-2022 is considered, including the level of their aftershock activity and background seismicity in the field of velocity inhomogeneities of the medium. The analysis of background and aftershock seismicity with mb(isc) ≥ 4.5 (from http://www.isc.ac.uk) was analyzed at the locations of focal zones (3°×3°). Within such zones, the total scalar seismic moment of background seismicity and aftershock sequences were calculated. Background seismicity was estimated for a 50-year time period before and after the main earthquake and its aftershocks. The duration of repeated shocks series is limited to one year. Against the background of the generalized seismotomographic model of Asia (at a depth of ~50 km), crustal foci of 24 strong earthquakes mainly seek toward zones of sign changes in P-wave velocity anomalies, three earthquakes are recorded in the low velocity zones and 4 in the high velocity zones. The largest values of the total scalar seismic moments released during the implementation of background seismicity are located within the negative P-wave velocities anomalies, as well as along the boundaries of sign changes in anomalies. When scalar seismic moments are released during aftershock and background activity, as well as the main event, the prevailing values remain within e+27. Increased values (e+28) are typical for zones of anomalies sign change and the area of positive anomalies of P-wave velocity. Low values of total scalar seismic moments of background and aftershock events are observed in the focal zone of six sites (3°×3°). It has been suggested that the maximum magnitude for the 100-year period (1901-2022) was realized in these zones and the release of accumulated stresses could be carried out exactly through the strongest earthquakes.
Uniaxially compressed granites were subjected to orthogonally directed pointed impact damaging. The shock-induced acoustic emission (AE) was detected by a highly sensitive wide-band piezo-transducer made of PZT ceramics. The AE time series were analyzed in the frequency ranges of 80–200 кГц (conditionally low-frequency zone) and of 300–500 кГц (high-frequency zone). The energy distributions in AE pulses detected in the lower frequency range followed a power law specific to the cooperative microcracking process, while the distributions recorded in the higher frequency zone exhibited an exponential (Poisson-like) function distinctive to random, non-interacting AE-events. The microcrack accumulation in the low-frequency zone was characterized by the b-value based on the Gutenberg-Richter law. At constant impact energy, the b-value grew progressively with the increase of the uniaxial compression of samples from zero to the highest nondestructive value thus indicating the decrease of the contribution of larger cracks. At the compression close to (but not reaching) the ultimate strength, an impact action caused the subthreshold fragmentation (trigger failure) of granite.
Last decades it has been shown that additional information about the structure and composition of rocks can be extracted from time domain induced polarization (IP) studying the transient characteristics of IP in a large time range from 1 ms to 10 s or more. In this paper compare field IP data obtained on the Yasnoe gold deposit (Taimyr Peninsula) with laboratory IP measurements on 37 rock samples from the same deposit. The samples differed in composition, degree of alteration, type of electron-conductive minerals (pyrite, graphite) and their distribution in the rock. For representative samples of the collection mineralogical and micro-computed X-ray tomography (μ-CT) studies were conducted. This allowed to determine the features of the mineral composition and structure of the samples, as well as to obtain a quantitative characteristic of electron-conductive minerals (pyrite and graphite). According to field and laboratory IP data the relaxation time distributions (RTD) have a similar shape for the same types of rocks. Based on the μ-CT data and the Maxwell-Garnett model, the RTD of the samples were calculated. For some of samples found a good agreement between the CT-based and the IP-based RTD. For others to obtain a satisfactory agreement between the CT-based and IP-based RTD it is necessary to assume that a certain number of pyrite inclusions are passivated or have a flattened shape. The revealed contradiction can be explained by the limited applicability of granular models for well-cemented, low-porosity rocks. This demonstrates the necessity to create an IP model of the rock with the inclusions of electron-conductive minerals that incorporates the structural features of low-permeability capillary media.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences, Novosibirsk, Russia
Keywords: mantle, fluid, mantle metasomatism, subduction, deep volatile cycle
At pressures of 3.0 and 5.5 GPa and a temperature of 1200°C, metasomatic reactions involving garnet lherzolite and SiO2-, Al2O3-, and alkali-rich supercritical fluids or melts have been experimentally studied. These fluids and melts could potentially be formed during the processes of dehydration, decarbonation, and melting of metasediments in subduction zones. The reaction of garnet lherzolite with model subduction mobile phase has been shown to lead to changes characteristic of modal metasomatism in the lithospheric mantle. As a result of the reaction with the melt at a pressure of 5.5 GPa, phlogopite-bearing garnet lherzolite is formed, while at a pressure of 3.0 GPa, phlogopite-bearing garnet harzburgite is produced. With an increase in the volatile content, primarily CO2, in the fluid/melt at pressures of 3.0 and 5.5 GPa, the reaction proceeds through intense carbonation of peridotite, leading to the disappearance of olivine and the formation of orthopyroxene and magnesite, resulting in the transformation of lherzolite into carbonated pyroxenite. At a pressure of 3.0 GPa and a temperature of 1200 °C, a carbonate-silicate melt rich in alkalis is formed. Overall, the presence of a significant amount of dissolved CO2
in a SiO2-rich fluid or melt induces metasomatic transformations in peridotite that are very similar to those occurring during its reaction with carbonatite melts. Specifically, at a molar ratio of CO2/(CO2+H2O) < 0.23, phlogopite is formed, while at >0.51, magnesite is produced. Phlogopite and magnesite do not form simultaneously in the products of metasomatic reactions.
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