Home – Home – Jornals – Russian Geology and Geophysics 2020 number 9

Homogenized melt inclusions in olivine were studied in Archean komatiites from the Barberton Greenstone Belt, Weltevreden Formation in South Africa (3.3 Ga), Abitibi Greenstone Belt in Canada (2.72 Ga), and the Belingwe Greenstone Belt in Zimbabwe (2.69 Ga). Contamination of the komatiite melts with crustal material enriched in Rb, Cl, and H2O during the crystallization of olivine is demonstrated. Uncontaminated melts have mantle Rb/Nb ratios but are significantly enriched in Cl and H2O relative to K and Ce, respectively, exhibiting similar incompatibility during crystallization and partial mantle melting. These observations suggest the presence of a chlorine- and water-enriched mantle source before 3.3 Ga. The excess Cl and H2O contents in the komatiites are assumed to result from the interaction of partially molten mantle plumes with the mantle transition zone. The most likely source of Cl and H2O enriching the deep mantle is the oceanic lithosphere that endured a seafloor alteration. We conclude that the recycling of the altered oceanic lithosphere into the mantle, probably via subduction, began in the first billion years of the Earth’s history. Delamination of the Archean crust could not cause transport of chlorine and water into the deep mantle.

We performed geological, geochronological, geochemical, and isotope-geochemical studies of igneous rocks of the Ust’-Ignok gabbrodiorite massif in the Urik-Iya graben of the Siberian craton and summarized the obtained and published data on early Proterozoic mafic igneous rocks in the South Siberian postcollisional magmatic belt. It has been established that the Ust’-Ignok massif is composed of rocks of the continuous series from biotite gabbro via gabbrodiorites and diorites to quartz diorites. U-Pb zircon dating of quartz diorites of the Ust’-Ignok massif yielded an age of 1836 ± 10 Ma, i.e., the massif rocks might have originated at the final stage of the formation of the South Siberian postcollisional magmatic belt. The rocks of the Ust’-Ignok massif are of normal and medium alkalinity. All igneous rocks from gabbro to quartz diorites show distinct negative anomalies of Nb-Ta and Ti in their multielement patterns, and their εNd(T) values vary from +0.3 to -0.9. The geochemical indicator ratios in the gabbroids point to insignificant contamination of their source with continental-crust material and to their formation through the melting of an enriched lithospheric-mantle source. Gabbrodiorites-quartz diorites of the Ust’-Ignok massif resulted, most likely, from the fractional crystallization of gabbroids. Analysis of the geochemical and isotope characteristics of mafic igneous rocks of the South Siberian postcollisional magmatic belt shows that most of them resulted from the melting of the subcontinental lithospheric mantle with suprasubductional geochemical features. This mantle might have formed during subduction processes preceding the formation of the Siberian craton.

Gold-bearing dumps of processed pyrite-polymetallic ores of the Ursk ore field (Novo-Urskoe and Beloklyuchevskoe deposits, Salair Ridge) have been studied. Physicochemical modeling of the gold behavior in wastes allowed us to describe quantitatively the gold precipitation process in different horizons of the dumps. In the upper part of the sulfide-rich section, sulfide minerals undergo intens oxidation accompanied by the dissolution of structural and surface-bound gold. Gold redeposition on the pyrite surface (sorption reduction barrier) as a result of electrochemical processes is accompanied by the formation of heavy-metal sulfates and barite. Under the sorption barrier, there is a loose leaching horizon with high humidity, where gold is present in pore solutions as thiosulfate, hydrosulfide, and hydroxo complexes; its content reaches 68 mg/L. In the middle part of the section, composed of compact siliceous sandstones (hardpan), high-grade (up to 973 ‰) gold forms through the disintegration of thiosulfate complexes, but its content decreases to 10^-6 mg/L (lithologic reduction barrier). No visible gold was found in the lower part of the section (soil bed), but its high contents (up to 0.35-0.42 g/L) might be due to the sorption by organic high-molecular compounds, such as humic acids. The morphology and chemical composition of native gold from the gold-containing dumps of processed pyrite-polymetallic ores have been first studied. It is shown that the gold surface has traces of supergene transformations, e.g., gold nano- and microparticles as sponge overgrowth on the gold or barite particle surface or as newly formed gold phases in Fe, Mn, and Al hydroxide films.

Comprehensive geological and geophysical investigations were conducted to obtain new data on the structure and physical nature of the discharge site of steam hydrothermal fluids by the example of the Pauzhetka geothermal deposit (southern Kamchatka). An isometric concentric zonal structure has been identified within the temperature, geoelectric, magnetic, and gravimetric fields. It spatially correlates with an elevated tectonic block previously detected in the area of the Upper Pauzhetka thermal field. The central part of this structure includes a consolidated rock block composed presumably of quartz-adularia metasomatites formed at the pre-Holocene stage of evolution of the Pauzhetka hydrothermal system. The rocks form a physical heterogeneity within the structure of the aquifer, which greatly contributes to the distribution of flows of ascending thermal, mixed, and meteoric waters beneath the Upper Pauzhetka thermal field. The central area of the isometric concentric zonal structure is outlined by a zone consisting of local anomalies of positive magnetic field values. The wide occurrence of subintrusive bodies (sills, dikes, and extrusion roots) of intermediate to rhyolite composition suggests the magmatic nature of the identified anomalies. The peripheral areas correlate with large discharge sites of high-temperature fluids. Thus, it is demonstrated that the structure of the circulation zones of waters of various types in the area of the Upper Pauzhetka thermal field is governed by the concentric zonal structure of the elevated tectonic block and the distribution of physical heterogeneities, both primary (of magmatic or volcanosedimentary nature) and resulted from the hydrothermal metasomatic alteration of the source rocks.

We present the results of gas-geochemical surveys in the sea surface water layer, water column, and bottom sediments of the Tatar Strait (north of the Sea of Japan) in 2012, 2014, 2015, 2017, and 2018. The distribution of methane fluxes at the water-atmosphere interface is examined, and its relationship with the geologic structure of the Tatar Strait area is discussed. Methane emission has been revealed throughout most of the Tatar Strait area. The most intense methane fluxes at the water-atmosphere interface, up to 482 mol/(km2×day), are observed on the gas-bearing southwestern shelf and on the gas hydrate slope of Sakhalin Island. The high concentrations of methane in seawater and the high contents of methane, hydrogen, and helium in the shelf and slope bottom sediments are probably due to the seismotectonic activity in the region, the presence of gas hydrates, gas concentration zones, gas migration channels, and the regional deep structure. Application of the model for calculation of the flux and impurity transfer fields to the studied water area has shown high methane emission from the sea surface in areas of vertical gas migration from lithospheric sources. The contribution of hydrodynamic factors to the formation of such zones of high methane emission is less than that of geologic factors. The obtained data on methane flux at the water-atmosphere interface for a shallow sea gave a detailed insight into the main gas discharge zones in the southern Tatar Strait.

We present data, collected in 2016, on the concentration of n -alkanes and polycyclic aromatic hydrocarbons in water and bottom sediments as well as the abundance and composition of the cultured microbial community in the area of the oil seepage near Cape Gorevoi Utes. Since its discovery in 2005, the development dynamics of the oil seepage has demonstrated a decrease in the total concentration of normal hydrocarbons and polycyclic aromatic hydrocarbons in oil slicks and bottom sediments, partial degradation of oil entering the water surface, and an increase and subsequent reduction in the number of microorganisms in water and bottom sediments with the maintained structure of the cultured microbial community. From 2006 to 2016, there was a low total concentration and a narrow range of detected concentrations of n -alkanes and polycyclic aromatic hydrocarbons in the water column, which indicates the preservation of water purity in the lake near the oil seepage.

The Anyue gas field is located in the middle part of the Sichuan Basin, SW China. It occurs in the oldest marine carbonate strata and is characterized by the highest degree of thermal evolution and the largest gas reserves in China. A significant breakthrough was made with the Gaoshi-1 risk-taking exploration well deployed in 2011. As of 2015, the proven geologic reserves of natural gas were 657.4 billion m³, and the total gas reserves including proven reserves, controlled reserves, and predicted reserves exceeded 1.5 trillion m³. A total of three sets of gas-bearing strata are developed in the Anyue gas field in a descending sequence: The gas reservoir in the Cambrian Longwangmiao Formation (Ꞓ₁l) is treated as a structure-lithologic gas reservoir; the gas reservoir in Section No. 4 of the Dengying Formation (Z₂dn⁴) is a structure-stratigraphic gas reservoir; and the gas reservoir in Section No. 2 of the Dengying Formation (Z₂dn²) is a structural gas reservoir. A comparative analysis of the gas sources has shown that the gas Cambrian of reservoirs was mainly from mudshale of the lower Cambrian Maidiping and Qiongzhusi formations and the gas of the Dengying Formation reservoirs was from mudshale of the Qiongzhusi Formation and mudstone of Section No. 3 of the Dengying Formation. All gas reservoirs are referred to as reservoirs of dry gas with medium or low contents of sulfur and medium contents of CO₂. Gas reservoirs in Ꞓ₁l are characterized by a large burial depth, a high temperature, and a high pressure, while the gas reservoirs in Z₂dn² and Z₂dn⁴ are characterized by an ultralarge depth, a high temperature, and a normal pressure. The accumulation of gas reservoirs is controlled mainly by two factors. The distribution of hydrocarbon generation centers is controlled by the late Sinian-early Cambrian intracratonic rift, which acts as effective updip sealing conditions for gas reservoirs in the Dengying Formation. The late Sinian-early Cambrian Gaoshiti-Moxi paleouplift experienced a long-term inherited development, which controlled the generation and distribution of three sets of large-scale high-quality reservoirs in Z₂dn⁴, Z₂dn², and the Longwangmiao Formation and the generation of three sets of high-quality reservoir-caprock assemblages. An inherited giant structural trap with a long-term stable development is always a favorable zone for petroleum accumulation.

Magnetostratigraphic data from wells in southern West Siberia that strip the Upper Cretaceous and Cretaceous-Paleogene boundary strata in three areas (three wells in the Omsk Basin, two wells in the Bakchar Iron Basin, and two wells in the southern Kulunda Basin) are used to compile the respective regional magnetic-polarity scale. According to the available biostratigraphic constraints, the deposition spanned the period from Albian to Bartonian. The reported regional polarity scale is based on integrated paleomagnetic and biostratigraphic data from the seven wells and comprises four Upper Cretaceous zones of normal (NK_1-2(al-st) and NK₂mt) and reverse (RK₂km and RK₂mt) polarity corresponding to the C34, C33r, C31r, and C30n Chrons of the global magnetic polarity scale and four Paleogene zones of reverse polarity: R₁E₁zl, R₂E₁t, R ₁E₂t-i(?), and R ₁E₂l-b, with the first two correlating with the C26r and C25r Chrons. Some of the Upper Cretaceous magnetozones enclose thin intervals (microzones) of the opposite polarity. The regional Cretaceous-Paleogene magnetic polarity scale of southern West Siberia reveals several deposition gaps from 6 to 28 Myr long. The magnetostratigraphic data can be used to determine deposition rates and can make reference for local, regional, and global correlations of geologic events given that polarity reversals are of global extent.

We consider results of magnetotelluric and magnetovariational soundings in the period range 10 < T < < 20 000 s in the North Vietnam area. The simple structure of magnetovariational responses is shown, which generally reflects the electrically conductive quasi-two-dimensional structure of the Earth’s crust. Impedance responses form as a superposition of responses of local complex subsurface and quasi-two-dimensional regional deep-seated structures. Separation of local and regional effects made it possible to construct a geoelectrical depth model of the regional tectonosphere, whose main elements are electrically conductive subvertical trans-lithospheric faults and high-resistivity disturbances of the conductive asthenosphere. These elements, favoring the flow of telluric currents induced in deep-seated electrically conductive systems into the sedimentary cover, form zones of abnormal apparent-resistivity curves. The apparent resistivity monotonously increases with increasing the period up to 20 000 s. We called the combination of such elements of the geoelectrical model a ultradeep fluid-fault system (UDFFS). The modeling has established the location of three orthogonally intersecting UDFFS of NE and SE strikes in the North Vietnam area. It shows that conductive (fluid-saturated) translithospheric faults extending to the base of the sedimentary sequence control the location of petroleum fields and ore deposits. A method for separating local and regional magnetotelluric effects is proposed. It permits one to determine reliably the main strikes of a regional two-dimensional structure and the configuration of the apparent-resistivity curves along them.

The East European Platform (EEP) is characterized by a rather weak seismicity. The south of the Komi Republic is the most seismically active territory in the northeast of the EEP. The 1939 Sysola earthquake (grade 7), one of the strongest earthquakes in the European north of the 20th century, occurred there. Many world’s seismic stations recorded an earthquake in the Priluzsky region of the republic on 9 November 2002, but it was not thoroughly studied and is not considered in literature. In this work we recalculate the parameters of the earthquake hypocenter, substantiate its tectonic nature, construct its focal mechanism, and describe its tectonic position. For processing, we used data on 86 seismic phases from 58 stations with epicentral distances from 5.9 to 57°, azimuth angles from 1.5 to 341.7°, and the maximum azimuthal gap of 70°. The following parameters of the hypocenter were obtained: t ₀ = 06 h 47 min 17.9 s, 59.93° N, 49.76° E, R_minor = 7.7 km, R_major = 10.7 km, Az_major = 20°, h = 16 km, and M_S = 3.4. The earthquake is localized in the upper crust and is confined to the zone of the junction of the eastern slope of the Sysola arch and the western flank of the Kirov-Kazhim aulacogen of the Volga-Ural anteclise of the Russian Plate. We have established a strike-slip fault focal mechanism of the earthquake, which corresponds to the latest stress field of the region. The estimated axis parameters (value, azimuth, plunge) are as follows: T = 0.707, 90.0, 0; N = 0, 0, 39.792; and P = -0.707, 180.0, 50.208. The plane parameters (strike, dip, slip) are estimated at 327, 57, -140° for the first plane and 213, 57, -40° for the second plane. The recorded seismic events in the northeast of the Russian Plate indicate that the platform area is not seismically passive. The performed research shows that recent seismotectonic processes are related to the structure and state of the Earth’s crust within the platform.