Home – Home – Jornals – Russian Geology and Geophysics 2020 number 11

The paper provides an insight into the geodynamic history of the Northeast Asian terranes during the Paleozoic, focused on the stratigraphic, paleontological, and sedimentological data indicating that the Precambrian structures of early-middle Paleozoic age were originally part of the passive margin of the Siberian paleocontinent. The geological and paleontological data presented in the foregoing studies have shown the inherited geodynamic regime and synchronous sedimentation and magmatism on the passive continental margin and in most terrains before their separation from the Siberian paleocontinent in the late Paleozoic. The revealed significant differences in evolution between the Okhotsk and Omolon and other terranes give grounds to postulate that they rifted off the paleocontinent even earlier.

Geological, petrological, isotope-geochemical (⁴⁰Ar/³⁹Ar analysis, petrochemical data, and geochemistry of trace elements and REE), and mineralogical researches testify that the formation of the early Paleozoic basalt and picrite complexes of the West Siberian Plate basement was related to the development of the Cambrian subduction zone of the Paleo-Asian Ocean. Using the compositions of clinopyroxenes and amphiboles (and also programs of computational modeling), we have established the Р-Т conditions of formation of early Paleozoic picrite complexes. Crystallization of clinopyroxenes began at significant depths (25-20 km) and at high temperatures (1300-1275 °C). Olivine might have formed at elevated pressures (8-7 kbar) and temperatures of 1540-1490 °C. Amphiboles formed, most likely, at 6.1-4.5 kbar and much lower temperatures, 1105-1060 °C. Petrochemical analysis and data on trace elements and REE in the rocks of the studied early Paleozoic complexes in West Siberia testify to their intricate formation involving magmatic systems with basalt (island-arc and back-arc basins), picrite, and shoshonite (and also WPB type) melt characteristics. Taking into account the similar geochemical characteristics of the early Paleozoic basaltoids and picrites of the West Siberian Plate basement and the Kamchatka volcanics, we suggest that a considerable part of the studied ancient complexes formed by the model implying (as in the case of the Sredinnyi Ridge in Kamchatka) the action of enriched magmatic systems during the development of a destructive window ("slab-window") under rupture of subducted plate on the background of common island-arc magmatism.

The first comprehensive seismotectonic study was performed in the Olenek and Vilyui areas of the Yakutian kimberlite province, which was aimed at elucidating the geodynamic activity of neotectonic structures of the Siberian craton and assessing the environmental hazard of groups of kimberlite fields. Based on the degree of activity and trend of geodynamic processes, we have developed regional principles for the classification of neotectonic structures of the Siberian craton and its fold-thrust framing with the rationale for their differentiation into classes. The active segments of the Verkhoyansk and Baikal-Patom fold-thrust belts are analyzed, which have a dynamic effect on the mode of tectonic deformations of the adjacent sections of the Siberian Platform, where groups of kimberlite fields are localized. The revealed patterns of seismotectonic destruction processes made it possible to establish the intensity and types of the Earth’s crust stresses and strains in the reactivation zones of the marginal sutures of the Siberian craton. Applying mathematical statistics methods permitted a quantitative analysis of the geodynamic parameters of the geologic environment in the Olenek and Vilyui areas of the Yakutian kimberlite province, based on the significant factors responsible for activation of the recent structures as manifestations of a single stress accumulation/discharge process in the Earth’s crust. Using the set of seismotectonic data, we have differentiated the activation zones according to the stress-strain intensity in the Earth’s crust and have assessed the potential environmental hazard of groups of kimberlite fields in the Yakutian diamondiferous province.

A long-term study of the typomorphism of native gold under exogenous conditions gave an insight into its evolution in time and space. The morphology, internal structure, and chemical composition of native gold change depending on the duration of its occurrence under near-surface conditions and on the thermodynamic parameters of the environment. Along with the known facts of gold transformation in the exogenous environment, we consider some of the first identified aspects of the evolution of native gold in weathering crusts and the hydrodynamic, eolian, and anthropogenic environments. Additional attention is given to the transformation of gold in ancient gold-bearing conglomerates under lithostatic pressure and in metamorphic strata depending on the P-T environmental conditions. The paper is based on the results of field work, experiments, and analytical studies of the mineralogy of native gold. The objects of study were gold placers of the eastern Siberian Platform, Tuva, and Mongolia, gold-bearing conglomerates of the Timan Ridge, anthropogenic gold placers of the Yenisei Ridge, kaolinite and laterite weathering crusts of Salair, Kazakhstan, and the Republic of Guinea, and the collections of placer gold from the A.E. Fersman Museum (Moscow), the Central Research Institute of Geological Prospecting for Base and Precious Metals (Moscow), the Moscow Mining Academy, and ZAO VNESHMET (Moscow). In the course of experimental studies, we investigated for the fist time the mechanical transformation of gold particles under the impact of sand-air flow in the eolian conditions and under the lithostatic pressure of the overlying strata on ancient gold-bearing conglomerates. We also used a number of mineralogical and geochemical methods to study the typomorphic features of native gold. The evolution of gold under exogenous conditions depends on the ambient environment. Examination of weathering crust has revealed gold nano- and microparticles resulted from the decomposition of sulfides, tellurides, and other unstable gold-containing compounds. Newly formed gold nano- and microparticles in the form of finest crystals, dendrites, and globules are deposited on the surface of primary endogenous gold. The gold formed in weathering crust is spongy and nodular; the fineness of primary endogenous gold increases, the content of impurity elements in it decreases, and a high-fineness porous shell forms at the edges of the gold particles. In the hydrodynamic environment, placer gold, independently of its form (hemihedral, euhedral, interstitial, etc.), flattens and undergoes a simple deformation, but its chemical composition and internal structure change little; they depend on the stage of ore formation and on the mineragenic type of the gold ore source. We have established that the chemical composition and internal structure of gold change during its long occurrence in the environment and under its repeated redeposition from ancient (Precambrian) to younger (Quaternary) deposits. Based on the obtained results of experimental and mineralogical studies, we have proved that eolian processes change not only the shape of native gold but also its chemical composition and microhardness. In the eolian environment, placer gold of different shapes tends to become a globule with a film-fibrous surface. The change in the shape of gold is accompanied by an increase in its fineness, a decrease in the content of impurity elements, and, as a result, decrease in the gold microhardness. In ancient conglomerates (ancient fossil placers), placer gold subjected to the lithostatic pressure of overlying deposits transforms into pseudo-ore gold. In metamorphic strata with constant temperatures and pressures, gold becomes refined. The identified indicators of placer gold of different exogenous environments make it possible to reconstruct the geologic and geomorphologic conditions of gold placer formation, namely, to determine the genetic type of placers (related to weathering crusts, alluvial, eolian, etc.) and to define the source areas (intermediate or primary sources). This helps to find a more correct technique for the search for gold placer and ore deposits.

The first results on the composition of fluids from native gold and associated pyrite and quartz have been obtained. Despite the small amount of analytical data, these results are of scientific and practical interest. The identified geochemical criteria can be used for the assessment of gold ore shoots and the substantiation of prospecting in the region. The one-act shock-destructive extraction of volatiles from fluid inclusions and their pyrolysis-free gas chromatography-mass spectrometry analysis made it possible to determine the composition of fluids in native gold and in associated pyrite and quartz. Based on these data, we have first shown that fluids in native gold, pyrite, and quartz are a mineral-forming multicomponent system. In addition to water and carbon dioxide, the studied fluid inclusions contain representatives of at least 11 homologous series of organic compounds, including oxygen-free aliphatic and cyclic hydrocarbons (paraffins, olefins, cyclic alkanes and alkenes, arenes, and polycyclic aromatic hydrocarbons), oxygenated hydrocarbons (alcohols, esters, furans, aldehydes, ketones, and carboxylic acids), and nitrogenated, sulfonated, halogenated, and siliconorganic compounds. The portion of hydrocarbons together with S-N-Cl-F-Si compounds reaches 52.0 rel. % in fluid inclusions from native gold, 10.1 rel. % in fluid inclusions from pyrite, and 18.0 rel. % in fluid inclusions from quartz. Gold- transporting gas fluids have reducing properties. Pyrite and quartz contain oxidized water-carbon dioxide fluids with low contents of hydrocarbons and nitrogen-halogenated compounds.

For the first time in the last 35 years, hydrogeochemical data on pre-Jurassic complexes in West Siberia have been generalized. Groundwater and brines of Cl-Na and Cl-HCO₃-Na types with total salinity (TDS) varying from 4 to 330 g/L are found to be widespread in the area under study, with the former type dominating. A detailed analysis of the hydrogeochemical data allowed us to assume the presence of three genetic groups of groundwater and brines in the hydrogeologic section: (1) sedimentogenic, (2) lithogenic (revived) and ancient infiltrogenic, and (3) condensatogenic. An integrated analysis of coefficients r Na/ r Cl, Ca/Cl, (Br/Cl)·10^-3, and (Sr/Cl)·10^-3 and integrated index S of brine metamorphization in the Siberian sedimentary basins has corroborated the fact that groundwater and brines in West Siberia are at the initial stage of metamorphization of their chemical composition. Groundwater and brines in the studied Siberian sedimentary basins show an increase in the degree of metamorphization (catagenetic changes) of their chemical composition in transition from the areas of igneous and metamorphic deposits at the base of the Meso-Cenozoic sedimentary cover of the young West Siberian sedimentary basin to the structures of the ancient Siberian Platform, where ultrastrong Ca-Na and Ca chloride brines are spread.

Quasi-linear sequences of the epicenters of strong earthquakes, identified in many seismically active regions, are phenomenologically viewed as «migrations» of earthquake sources. Following this analogy, an earthquake chain is understood in this paper as a set of seismic events of different energies, leading to a quasi-linear one-way change in the position («migration») of the successive epicenters of shocks on the surface of the lithosphere zone under study. Based on this statement, a formalized method for an azimuthal analysis of seismicity is developed, which allows one to identify and isolate earthquake chains from arrays of seismological data presented in the standard format of a catalog of earthquakes. As this method is tested on a catalog of earthquakes and a catalog of simulation events, all model chains of events and a large number of earthquake chains and simulation events are identified. It is indicated by isolating chains in a random field of simulation events that some of these earthquake chains can be formed by a random spatiotemporal combination of shocks. Migrating seismicity in the lithosphere of the Baikal rift zone (BRZ) is studied by applying the method of identification and isolation of earthquake chains to materials from the «Catalog of Earthquakes of the Baikal region». According to data on 52,700 earthquakes with a representative energy class K _P ≥ 8, occurring in the Baikal Region from 1964 to 2013, there are 2143 earthquake chains identified and isolated within an angular sector of azimuthal analysis q = 10° (±5° from azimuth α to the epicenter). As the angular sector of azimuthal analysis increases to q = 20° (±10°), there is an approximately twofold increase in the number of chains ( M = 4245). As shown by the analysis and comparison of spatiotemporal distribution of earthquakes and earthquake chains, the spatiotemporal and energy distribution of chains of seismic events is formed by earthquake distribution in the BRZ. Beyond this zone, the chains are small in number. The established relationship between the distributions of earthquakes and earthquake chains in space, in time, and by energy indicates that in the epicentral field of seismicity of the BRZ, both «seismicity migration» chains and randomly formed chains («pseudomigration» chains) are identified. The migrating seismicity of the BRZ is studied according to the seismicity statistics by using the criteria developed within the framework of the simulation base model of migrating seismicity to the results obtained. An index of seismicity migration activity (ISMA) that reflects a seismicity migration/pseudomigration ratio at the set level of significance is used to obtain distribution maps over the territory and graphs of changes over the years for this index. The maps show that small areas ISMA > 1 are seen in close proximity to rifting attractor structures (RASs), while seismicity migration in the rest of the BRZ is not statistically obvious. The graphs show that periods ISMA > 1 at three levels of significance are observed three to four years after the activation of RASs, which makes it possible to estimate the phase propagation velocity of a slow deformation perturbation of about 250-300 km/year. It is indicated by the results obtained that the migrating seismicity of the BRZ is directly related to the location and activations of RASs and that the RASs are the sources of local deformation perturbations in the BRZ lithosphere, which, among other things, manifest themselves in the implementation of seismicity migration chains.

The paper deals with the problem of interpreting magnetotelluric data in the differentiated low-conductivity upper part of the section. An observation network often makes it impossible to describe a spatial irregularity spectrum completely; so, using 3D data inversion comes across a significant spatial equivalence of the solution. The effect of irregular conductivity of the upper part of the section at one of the sites is considered by the comprehensively studied near-field transient electromagnetic sounding in dense 3D networks. A technique is proposed for choosing the directions of the minimum gradient of anomaly-causing conductivity and for selecting corresponding quasi-longitudinal curves for estimating the deeper parameters of the model. The advantage of using quasi-longitudinal curves is shown, and the high geological efficiency of the developed technique for interpreting magnetotelluric data is demonstrated at one of the sites of the Siberian Platform.

The successive application of the georadar method to a surface study of media pushes for the creation of a logging georadar. The tool operation conditions in a deep-seated medium with high absorption of electromagnetic waves (in a borehole) require optimization of the georadar technology for logging. Two possible approaches to gpr logging are discussed by the example of various georadars. We analyze the experimental data obtained with differently constructed MALA (Sweden) and GROT 12 (Russia) georadars under the same conditions during an international expedition. Conclusions are drawn that video pulses can be used for gpr logging.