Home – Home – Jornals – Russian Geology and Geophysics 2019 number 11

We consider the geologic structure, composition, and age of early Paleozoic intrusive rock associations of the Tugrug, Udzur-Hunga, Hatan-Hunga, Mandalt, Bayasgalant, and Dut Uul plutons. The plutons are located among late Neoproterozoic-early Cambrian volcanic and volcanosedimentary strata in the southern part of the island-arc system of the Lake Zone in Western Mongolia. We have recognized plagiogranitoid and gabbroid associations of different petrochemical compositions, ages, and geodynamic settings of formation in these plutons. The results of geochronological studies show that the plagiogranitoid associations in the south of the Lake Zone formed in the period from 531 to 481 Ma. Two major stages of intrusive magmatism in the zone have been established in this time interval: island-arc, 531-517 Ma, and accretion-collision, 504-481 Ma. The plagiogranitoid associations that formed at the island-arc stage are the most widespread, whereas those formed at the accretion-collision stage are much scarcer. The intrusive plutons of all ages are composed predominantly of granitoid associations of the tonalite-trondhjemite series, which are calc-alkalic rocks. Study of xenogenic and inherited zircons from plagiogranitoid associations formed at the island-arc and accretion-collision stages of the regional evolution has shown their age range 664-520 Ma. Four age groups of inherited zircon have been recognized (~664, 570-560, 545-531, and 530-520 Ma), which generally correspond to the stages of island-arc (volcanic and intrusive) and ophiolite magmatism and, most likely, reflect the additional magma-generating sources of parental plagiogranitic melts.

A continuous solid-solution series between the end-members eskolaite (98 wt.% Cr₂O₃) and karelianite (93 wt.% V₂O₃) has been revealed in Cr-V-bearing rocks of the Sludyanka metamorphic complex. Chromium and vanadium oxides crystallized as karelianite-eskolaite minerals during regional high-temperature prograde (granulite facies) metamorphism and participated in the formation of other Cr-V and Cr-V-bearing phases. Ferrian karelianite (up to 12 wt.% Fe₂O₃) and three-component solid solutions Esk_12-50Kar_45-60Hem_6-30 occur in metamorphic rocks that have particular protolith compositions (Fe-Kar) and in later metasomatic rocks ((Cr, V, Fe)₂O₃). Natural eskolaite and karelianite are discussed in terms of paragenesis, and their crystallization conditions are compared with the conditions of their laboratory synthesis.

The mineral composition of ores from the Gornyi occurrence of the Valunistoe epithermal Au-Ag deposit (Chukchi Peninsula) has been studied. It has been found that, in addition to native gold, the Au-Ag mineralization comprises chalcogenides (uytenbogaardtite, petrovskaite, acanthite, naumannite, and cervelleite) and minerals of the pearceite-polybasite series, which occur as microinclusions in fine-grained pyrite. The physicochemical conditions of formation of productive mineral assemblages have been estimated based on the chemical composition of Au and Ag minerals and their relationships with other minerals. It is shown that ores were deposited from weakly acid solutions at the late stages, on the background of a temperature decrease from 350 to 100 ºС, a decrease in the fugacities of sulfur (lg ƒ _S2, from -2 to -23), tellurium (lg ƒ _Te2, from -5 to -27), and selenium (lg ƒ _Se2, from -16.5 to -28), and changes in the redox conditions (lg ƒ _O2 , from -23 to -48).

We present the results of quasi-equilibrium directional crystallization of melt of the composition (mol.%): Fe_31.9Ni_1.7Cu_16.0S_50.4. The produced ingot consisted of the following zones: monosulfide solid solution mss + + isocubanite icb (CuFe₂S₃) + Cu₃Fe₄S₇ // icb + intermediate solid solution iss (~Cu₂Fe₃S₅) + pentlandite // mixture of unidentified decay products of solid solution iss ₁ // chalcopyrite cp + putoranite put // cp + talnakhite tal + bornite bn . The results of chemical analysis of the ingot were used to construct the distribution curves of the components in the solid and to calculate the change in the composition of sulfide melt ( L ) during directional crystallization in four zones of the ingot. Phase reactions involving a melt have been established. It is shown that the Cu-Fe-Ni-S system has a two-phase equilibrium region L + icb *. A similar region was earlier found in the ternary Cu-Fe-S system. A crystallization zone of two intermediate solid solutions iss ₁ and iss ₂, which were earlier obtained by the common method of isothermal annealing and quenching, has been revealed. The peritectic character of crystallization of the phases icb *, iss ₁, and iss ₂ from the melt is established. The results obtained show the fundamental possibility of the existence of a new type of zoning in the formation of copper-nickel low-sulfur sulfide orebodies.

We consider the factors determining the mineral composition of hydrothermal clays in recent thermal anomalies of the Pauzhetka-Kambal’nyi-Koshelevskii region and Ebeko (Paramushir Island) and Mutnovskii Volcanoes. The composition of mineral assemblages is influenced by a number of interdependent factors: the type of discharge (steam-gas jets, heated grounds, and mud water pools), temperature, pH, and Eh of a thermal solution. Mineral assemblages including sulfur, opal, α-quartz, and alunite group minerals form on the heated grounds of the craters of active volcanoes (Ebeko and Mutnovskii). Mud water pools of the craters contain mainly sulfur, opal, and α-quartz. An assemblage of kaolinite, opal, alunite group minerals (in small amounts), and goethite forms in mud water pools with pH = 2-3 in the thermal fields of the Pauzhetka-Kambal’nyi-Koshelevskii region. An assemblage of newly formed kaolinite, smectite, pyrite, marcasite, and, sometimes, opal is specific to mud water pools with pH > 3.5-4.0 At pH > 5, smectite, pyrite, and opal are produced. In the thermal fields of the Pauzhetka-Kambal’nyi-Koshelevskii region, the walls of steam-gas jets that are often flooded with water and dry out are formed by kaolinite, smectite, opal, α-quartz, pyrite, marcasite, and goethite. The walls of steam-gas jets that are not flooded with water for a long time consist of kaolinite, opal, α-quartz, smectite (traces), alunite group minerals, pyrite, and marcasite in close amounts. The heated grounds of the thermal fields of the Pauzhetka-Kambal’nyi-Koshelevskii region are composed of kaolinite, alunite group minerals, opal, and goethite or an assemblage of kaolinite, smectite, opal, pyrite, and marcasite.

We present results of study of the chemical composition of organic matter (C, H, N, and S) from Holocene sections of lake sapropels with undisturbed stratification penetrated by vibratory drilling of the bottom sediments, down to the underlying rocks, of lakes Bol’shie Toroki (1.8 m), Minzelinskoe (5 m), Ochki (4.5 m), Dukhovoe (7 m), and Kotokel’ (6 m). We consider methodological approaches to the identification of the sources and genesis of buried organic matter in marine and lacustrine sediments by a number of organo-geochemical indicators: data of a biological analysis (biostratification based on layer-by-layer determination of organic relics in bottom sediment sections); hydrocarbon biomarkers (molecular composition of normal aliphatic hydrocarbons (n-alkanes), nitrogen compounds of a protein complex, etc.); and C/N ratio reflecting a difference in the biochemical compositions of bioproducers. The results of biological analysis (biostratification) show that planktonogenic sapropel (phyto- and zooplankton, the autochthonous source of organic matter) in Lake Ochki formed for 10,760 years, and sphagnum and hypnum moss were supplied from the bogged shores (allochthonous source of organic matter). In Lake Minzelinskoe, peaty sapropel formed at the stage of a flooded lowland bog (5905 years ago); since 3980 years ago and till the present, macrophytogenic sapropel has formed. Pyrolytic study (RE pyrolysis and Pyr-GC-MS analysis) of bioproducers and sapropel from Lake Ochki has shown that phytoplankton and zooplankton were the main autochthonous source of OM in the lake, which is confirmed by the identified macromolecules of nitrogen compounds of different compositions. The presence of hopanes indicates the contribution of microorganisms to the formation of OM in the sediments. The organic matter of sapropel has a terrigenous component, which is confirmed by the presence of high-molecular odd-numbered n-alkanes, ketones, and methyl esters of fatty acid. The C/N ratios in the stratified sections of macrophytogenic sapropels of Lakes Bol’shie Toroki and Minzelinskoe fall in the range of values specific to higher aquatic and terrestrial vegetation (C/N = 15-18), whereas the C/N ratios in planktonogenic sapropel of Lake Dukhovoe are typical of marine and lake plankton (C/N = 5.7-8.6).

Methods of deriving FeO and TiO₂ contents from Clementine spacecraft data are discussed, and our own approach is developed for deriving the contents from measurements made by the M³ instrument on Chandrayaan-1. The density of lunar soil bedrock is modeled based on the derived FeO and TiO₂ information. The FeO and TiO₂ abundance we derived from the_{M³ data is compared with previous results from the Clementine data and is in good agreement. The FeO abundance data also agree well with the Lunar Prospector data, which were used as an independent source. The previous Clementine and new M³-derived abundances are compared with the laboratory-measured FeO and TiO2 contents in the Apollo and Luna returned samples. The Clementine-derived FeO content was systematically 1-2% lower in all the returned samples than the laboratory measurements. The M³-derived content agrees better with the returned Apollo samples and is within ±2.8% of the laboratory-measured ones. The Clementine-derived TiO2 abundance is systematically 0.1-4% higher than the laboratory measurements of the returned samples. The M³-derived TiO2 content agrees well (±0.6%) with the laboratory measurements of the returned samples, except for the samples with a high TiO2 content. However, caution should be taken when interpreting these results, as the error range needs further study. Unfortunately, no error analysis was provided with the previous Clementine-derived contents.}

Using new computer technologies for the northern part of the South Urals, the structural features of the anomalous magnetic field have been studied, magnetic anomalies from different layers of the Earth’s crust have been identified, and corresponding volume models of the sources of anomalies (the boundaries of basite-ultrabasite massifs, deep-seated belts, and basalt layer) have been constructed. The new data on the structure of the South Urals Earth’s crust make it possible to clarify the position of deep faults and their connection with deep-seated basite-ultrabasite belts. Deep-seated root blocks have been identified for large hyperbasite massifs. Within the Taratash anticlinorium, the constructed models of the sources of magnetic anomalies allow us to conclude that the oldest Taratash complex in the Urals is an elevated part of the deep-seated basite-ultrabasite belt of the East European Platform.

Lateral logging sounding (LLS) is currently the only widely used Russian method of resistivity measurements, sensitive to vertical electrical resistivity in vertical wells. However, interpreting data measured by this method in thin-layered sections is difficult and requires the utilization of resource-intensive numerical simulation algorithms. Today, the development of computational methods and an increase in computer performance allow us to invert LLS data in the class of two-dimensional axisymmetric models. However, in virtue of the large number of difficulties associated with the nonlocal responses of the probes and their asymmetry, this process requires the active participation of a log analyst. One of the first issues is the creation of an initial approximation of the geoelectric model. It consists in splitting the target interval into layers within which the properties of the medium can be considered constant in the vertical direction, since LLS signals have a very complex shape in the intervals of alternation of beds with different resistivities. We propose applying a fully connected convolutional artificial neural network to automatically create sectional layering suitable for constructing the initial approximation of the geoelectric model for two-dimensional LLS data inversion, including vertical resistivity estimation. The neural network was trained and tested on the synthetic and field data measured in West Siberia. Based on the results of the testing, we established the workability of the proposed approach.