Home – Home – Jornals – Russian Geology and Geophysics 2018 number 11

Precambrian cratons cover about 70 % of the total continental area. According to a large volume of geomorphological, geological, paleontological, and other data for the Pliocene and Pleistocene, these cratons have experienced a crustal uplift from 100-200 m to 1000-1500 m, commonly called the recent or Neotectonic uplift. Compression of the Precambrian crust terminated half a billion years ago or earlier, and its uplift could not have been produced by this mechanism. According to the main models of dynamic topography in the mantle, the distribution of displacements at the surface is quite different from that of the Neotectonic movements. According to seismic data, there is no magmatic underplating beneath most of the Precambrian cratons. In most of cratonic areas, the mantle lithosphere is very thick, which makes its recent delamination unlikely. Asthenospheric replacement of the lower part of the mantle lithosphere beneath the Precambrian cratons might have produced only a minor part of their Neotectonic uplifts. Since the above mechanisms cannot explain this phenomenon, the rock expansion in the crustal layer is supposed to be the main cause of the recent uplift of Precambrian cratons. This is supported by the strong lateral nonuniformity of the uplift, which indicates that expansion of rocks took place at a shallow depth. Expansion might have occurred in crustal rocks that emerged from the lower crust into the middle crust with lower pressure and temperature after the denudation of a thick layer of surface rocks. In the dry state, these rocks can remain metastable for a long time. However, rapid metamorphism accompanied by expansion of rocks can be caused by infiltration of hydrous fluids from the mantle. Analysis of phase diagrams for common crustal rocks demonstrates that this mechanism can explain the recent crustal uplift of Precambrian cratons.

Overbank deposits in the Komissarovka River valley consist of alternating silt, clay silt, sand, and soils produced by lacustrine, alluvial, and aeolian deposition and by soil formation. Silt and sand layers in the lower part of the section correlate with the events of Early Holocene transgression and Middle Holocene regression of Lake Khanka. Deposition in the lowermost reaches of the Komissarovka River provides a faithful record of local Holocene landscapes controlled by level changes in Lake Khanka.

We present integrated mineralogical, chemical, and palynological data for Late Pleistocene-Holocene bottom sediments of Lake Arakhlei located in the Beklemishev tectonic basin in the southern Vitim Plateau (central Transbaikalia). The sediment samples were studied by X-ray diffractometry (XRD), Fourier-transform infrared (FTIR) spectroscopy, laser particle sizing, spore-pollen analysis, radiocarbon (¹⁴C AMS) dating, and XRF spectrometry. The cored 128 cm long section of lake sediments consists of two units: one is composed mainly of layered silicates (illite-smectite, illite, chlorite, chlorite-smectite, muscovite, and kaolinite) and organic matter (OM) but no carbonates from 0 to 80 cm and the other contains authigenic Ca-Mg carbonates (up to 30%) of Mg-calcite and excess-Ca dolomite from 80 to 128 cm. The sediments also contain a rare phase of weddellite CaC₂O₄·2H₂O discovered for the first time in Transbaikalian lakes. The evolution of Lake Arakhlei and its drainage basin comprised four stages, with pollen zones that mark the Late Pleistocene and Holocene climate history of the Beklemishev basin. The reconstructed history of Lake Arakhlei for the past ~15,500 years followed general climatic changes in the Northern hemisphere. Thus, integrated research, including detailed analysis of mineral components and spore-pollen assemblages in lake sediments, is a workable tool for studying climatic controls of continental sedimentation.

U-Pb dating of detrital zircons from the sandstones of the Mamakan Formation has been made. Geochemical and isotope parameters of the carbonate deposits of the Yanguda Formation in the Vendian-Cambrian cover of the North Muya continental block have been estimated. It has been established that only the Neoproterozoic (630-915 Ma) rocks of the North Muya block were the provenances of terrigenous material. In the least altered carbonate rocks of the Yanguda Formation, the 87Sr/86Sr ratio is within 0.70814-0.70879 and δ13C varies from -0.4 to + 1.9‰. Comparison of the evaluated isotope parameters with those of carbonate rocks of typical Vendian-Cambrian sections shows that the carbonate deposits of the Yanguda Formation accumulated in the Early Cambrian, about 520 Ma. Sedimentation of the Mamakan and Yanguda Formations took place in the local sedimentary basin in the Vendian-Early Cambrian, in the absence of tectonic activity within the North Muya block. Detrital material that formed during the destruction of the rocks of the Siberian Platform basement and cover was not supplied into the basin.

We consider a hypothesis of the origin of PGE-bearing ultramafic rocks of the Inagli massif (Central Aldan) through fractional crystallization from ultrabasic high-potassium magma. We studied dunites and wherlites of the Inagli massif and olivine lamproites of the Ryabinovy massif, which is also included into the Central Aldan high-potassium magmatic area. The research is focused on the chemistry of Cr-spinels and the phase composition of Cr-spinel-hosted crystallized melt inclusions and their daughter phases. Mainly two methods were used: SEM-EDS (Tescan Mira-3), to establish different phases and their relationships, and EPMA, to obtain precise chemical data on small (2-100 μm) phases. The obtained results show similarity in chromite composition and its evolutionary trends for the Inagli massif ultramafites and Ryabinovy massif lamproites. The same has been established for phlogopite and diopside from crystallized melt inclusions from the rocks of both objects. Based on the results of the study, the conclusion is drawn that the ultramafic core of the Inagli massif resulted from fractional crystallization of high-potassium melt corresponding in composition to low-titanium lamproite. This conclusion is consistent with the previous hypotheses suggesting an ultrabasic high-potassium composition of primary melt for the Inagli ultramafites.

X-ray electron probe microanalysis is a complete mineral analysis technique. Therefore, the sum of obtained concentrations can be used as an indicator of the quality of analysis. There are two kinds of errors of the sum of concentrations. The first is related to the deviation of the sum from the basic value and can be called a difference error. The second is the scattering of results around the mean value and can be called an averaging error. The difference error characterizes the reproducibility of the measurements and equals 0.41% at the current precision level. The averaging error characterizes the repeatability of the measurements and equals 0.23%. The difference error can be used as a quality indicator directly during analysis, while the averaging error can be used after analysis of a batch of specimens. According to the modified three-sigma rule, the acceptable sum values of separate analyses are in the range 98.8%-100.8%. This range is obviously valid only for an ideal model with a basic sum value of 100%. In real analyses, the basic value deviates from 100% because of the presence of elements which are not detected in the analysis and elements with a variable valence. According to the three-sigma rule, the acceptable scattering of the sum of concentrations around the average value is ±0.7%.

The first bryolithochemical studies have been carried out within the Dukat ore field (Balygychan-Sugoi trough, northeastern Russia), in its northeast (Dukat Au-Ag deposit, Chaika site) and on its eastern flank (Au-Ag ore occurrence, Piritovyi site). Sampling was made along primary water streams draining Au-Ag ore zones, because the headstreams localized in the permafrost zone are poor in alluvial sediments or lack them, whereas the stream banks and beds are overgrown with aquatic and semiaquatic mosses. We examined samples of moss cushions with loose sediments (fine silt suspension and sand-silt material). The study has shown that moss cushions are an effective natural trap extracting finely dispersed, ultradisperse, and colloid-dispersed particles, including ore ones, from water stream suspension. The contents of major elements indicating Au-Ag mineralization (Au, Ag, Hg, Sb, As, Pb, and Zn) in the bryolithochemical and lithochemical samples were compared. The highest contents of these elements have been revealed in the bryolithochemical samples. Scintillation analysis shows a predominance of finely dispersed Au and Ag particles. The chemical composition of lithoparticles and probable species of elements indicating mineralization were studied by energy-dispersive X-ray microanalysis. It has been concluded that the bryolithochemical stream sediments are an integral part of the lithochemical ones and can also be effectively used to reveal geochemical anomalies and identify the corresponding ores. It is shown that the bryolithochemical sediment streams bear important information about the chemical composition of primary ores, as they have the same major indicator elements (Au, Ag, Hg, Sb, As, Pb, and Zn) and nearly the same mineral composition (finely dispersed gold and silver, electrum, silver sulfosalts, and simple sulfides). The proposed bryolithochemical research is promising not only for northeastern Russia but also for other areas with similar landscapes, primarily areas where lithochemical and bryolithochemical stream sediments form in the northern subarctic and arctic environments.

The Raigorodok stockwork gold-sulfide-quartz deposit is located in the contact zone of a monzogabbro-diorite massif belonging to the Stepnyak complex (442 Ma). Two main ore formation stages have been established: early, with disseminated gold-bearing pyrite-chalcopyrite mineralization related to skarn, propylite, and K-feldspar formation, and late, with productive mineralization. The late stage was accompanied by the formation of beresite, sericite, and quartz and comprised two substages: (1) quartz-gold-pyrite-chalcopyrite mineralization and (2) quartz-carbonate-gold-polysulfide mineralization. The ⁴⁰Ar/³⁹Ar ages of beresites and sericites are 422.9 ± 4.3 Ma and 416.7 ± 4.3 Ma, respectively, which is 19-22 Myr younger than the age of the host intrusive massif. The deposit ores formed stepwise, at temperatures of 112-335 ºC and fluid salinity of 0.7-21.2 wt.% NaCl equiv. Sulfur for skarns, propylites, beresites, and ores might have been provided by a deep magma chamber, and the carbonaceous shales of the Vendian Sharyk Formation might have been the source of gold. The isotope and geochemical data and the results of fluid inclusion study suggest that Raigorodok is an intrusion-related deposit. At the same time, the deposit has a number of signs of porphyry-epithermal ore-magmatic systems.

We discuss the geologic structure of the Berriasian-Lower Aptian deposits of the Gydan Peninsula. Eight seismic sequences have been distinguished; most of them are associated with Lower Cretaceous regional clinoforms of West Siberia. A correlation of productive beds was based on the stratotype sections of the Yamal and Gydan Peninsulas as well as seismic and well data. A sequence stratigraphic model of the Berriasian-Lower Aptian complex is proposed.

Petroleum prediction in local objects and the applicability of geologic environment parameters in this regard are considered. The proposed guidelines for petroleum potential prediction are based on the analysis of the distribution of typomorphic epigenetic multivalent elements loosely fixed in rocks before and after electric current treatment. The advantage of geoelectrochemical methods, providing data on the chemical composition of above-productive deposits, is demonstrated.

Hydrophysical studies and mathematical modeling of ring structures on the Baikal ice cover have shown that their existence at the stage of ice cover degradation is due to anticyclonic currents. Such currents can be generated as a result of local upwelling, which we associate with the rise of methane hydrates from the top layer of bottom sediments and their dissociation. Analysis of satellite images shows that the radii of ice rings range from 1300 to 2400 m, which is close to the baroclinic Rossby radius of deformation. The measured ice thicknesses in the area of the rings are in agreement with model calculations. Deep water renewal in Lake Baikal can also be associated with the rise of hydrates.

The potential of the load-unload response ratio (LURR) method for medium-term earthquake prediction is studied for Sakhalin Island as an example. An approach to the generation of predicted conditions and assessment of their implementation in real time is considered. The results of a retrospective analysis of other large Sakhalin earthquakes are used for generalization. It is shown that deviations of prediction parameters from specified values are satisfactory for this method. It is recommended that this method be used to compile summaries of medium-term predictions for Sakhalin provided that catalogs should be filled as soon as possible.