Home – Home – Jornals – Russian Geology and Geophysics 2018 number 3

Variations in mercury contents in marine sediments have implications for hydrothermal activity, paleoclimate, depositional environments, and primary bioproduction. Mercury contents reach 148 ppb in hydrogenic ferromanganese crusts on flat-topped seamounts. Such crusts, with up to 4120 ppb Hg, were dredged from the slopes of Seth Guyot in the western Marcus-Wake Seamounts in 1982, during the 13th cruise of RV Vulkanolog. The Seth ferromanganese crusts are of the same origin as hydrogenic Co-rich ferromanganese deposits from seamounts in other oceanic regions. Mercury accumulated in the Cenozoic as Fe-Mn oxyhydroxides in the crusts adsorbed Hg from bottom water. The process was especially rapid during the Pliocene volcanotectonic rejuvenated stage.

The paper deals with geological and geochemical studies of granitoids of the Olenek complex in the Olenek uplift of the basement of the northern Siberian craton. The age of these granitoids was earlier estimated at 2036 ± 11 Ma. The granitoids of the Olenek complex correspond in composition to high-alumina quartz diorites, granites, and leucogranites of the normal petrochemical series. According to geochemical and mineralogical characteristics, the quartz diorites can be assigned to granites of the transitional I-S type, and the granites and leucogranites, to S-type granites. The ɛNd(T) values in the granites of the Olenek complex vary from -0.2 to +1.4, and the Nd model age is 2.4-2.5 Ga. The quartz diorite is characterized by ɛNd(T) = +3.0 and a Nd model age T(DM) = 2.2 Ga. The geochemical characteristics of the granites and leucogranites indicate their formation through the melting of a source of graywacke composition, whereas the quartz diorites resulted, most likely, from the mixing of granitic and basaltic melts. The fact that the granitoids of the Olenek complex intruded the folded rocks of the Eekit Formation but stay virtually undeformed massive bodies suggests that they formed at the postdeformation stage of the regional evolution after the completion of the Paleoproterozoic orogenic events. The intrusion of granitoids marks the completion of the formation of the Early Proterozoic Eekit fold belt on the western (in the recent coordinates) margin of the Birekta terrane of the Olenek superterraine and the final formation of the superterrane structure. At the next stage of magmatism (1.98-1.96 Ga), best pronounced in the uplifts of the basement of the northern Siberian craton, all terranes forming the Anabar and Olenek superterranes assembled into a single structure.

Gold-bearing albite-amphibole-pyroxene rodingites of the Agardag ultramafic massif (southern Tuva, Russia) are confined to the E-W striking serpentinite crush zone. A zone of gold-bearing nephritoids is localized at the contact of rodingites with serpentinites. Optical and scanning electron microscopy, electron probe microanalysis, and fluorescent, chemical, ICP MS, and X-ray phase analyses were applied to study Au-Cu-Ag mineralization in the serpentinites, rodingites, and nephritoids. Copper sulfides, chalcocite and digenite, are present in the serpentinites, whereas gold and silver minerals are absent. Copper impurity is found in antigorite, Cr-spinel, and magnetite (up to 0.1-0.3 wt.%) as well as parkerite (up to 1.2 wt.%) and millerite (up to 7.9 wt.%). A wide variety of native gold and copper minerals has been identified in the rodingites: (1) cuproauride and tetra-auricupride free of or containing silver impurities (0.1 to 1.2 wt.%); (2) electrum of composition Ag_0.50-0.49 Au_0.50-0.51 (650-660‰) intergrown with AuCu, sometimes as exsolution structures; (3) electrum of composition Ag_0.70-0.64Au_0.30-0.36 (440-510‰), with inclusions of AuCu and copper sulfides (geerite and yarrowite); (4) high-fineness gold (750-990‰) as veinlets in electrum; and (5) native copper. The composition of copper sulfides varies from chalcocite to covellite. Submicron inclusions of hessite Ag₂Te were found in chalcocite. The amount of copper, gold, and silver minerals in the nephritoids is much smaller than that in the rodingites. The nephritoids contain chalcocite, electrum of composition Ag_0.64-0.63Au_0.36-0.37 (530-540‰), cuproauride, and tetra-auricupride. The detected hypergene minerals are auricuzite, apachite, brochantite, high-fineness gold, native copper, and cuprite. The sequence of mineral formation in the Agardag ore occurence has been established on the basis of mineral structures and mineral relations in the rodingites and nephritoids. It is proved that Au-Cu-Ag mineralization formed with the participation of Au- and Ag-bearing chloride-free low-sulfur CO2-alkali fluids in reducing conditions.

The problem of comparison of the composition of crystalline rocks with the composition of the constituent minerals of these rocks is considered. It is proposed to present the composition of rocks and the compositions of the constituent minerals in the form of a triangle on the plane. The experience of presentation of compositions in phase diagrams was taken as a basis for the construction. Analysis of the crystallochemical characteristics of clinopyroxenes and garnets has shown that three parameters are enough for depicting the compositions of these and other minerals. For this purpose, similar composition components of rocks and their minerals are summarized in molecular proportions and are plotted on the triangle DO-1/2(R₂O₃)-XO₂, where DO = (MgO + CaO + FeO + MnO + NiO + ...) + 1/4(Na2O + Al2O3) + + 1/4(K2O + Al2O3), 1/2(R2O3) = 1/2(Al2O3 + Fe2O3 + Cr2O3 + ...) - (1/4(Na2O + Al2O3) + 1/4(K2O + Al2O3)), and XO2 = SiO2 + TiO2. The compositions of minerals are expressed as the sums of their components: Ol = Fo + Fa + Lar + Neph + ..., Px = Di + En + Wol + Hed + Hyp + Jd + Aeg + ..., Ga = Pyr + Gross + Alm + Spe + Ski + Knr + Mj + ..., etc. A step-by-step calculation algorithm is proposed, which permits evaluation of the contents of bi- and trivalent iron during probe microanalyses of garnets and pyroxenes. Comparison of the compositions of deep-seated rocks and their minerals shows their good consistency. The proposed schematic projection permits a visual comparison of the compositions of rocks with low contents of carbonates and water, from ultrabasic (e.g., dunites) to acid (e.g., granites) ones.

This study is focused on interpretation of ultrahigh-resolution seismoacoustic data from the northeastern slope of the Ceará Rise (Central Atlantic) acquired using the SES 2000 deep parametric narrow-beam subbottom profiler during cruise 35 of RV Akademik Ioffe in 2011. The geologic nature of most of the detected reflectors is constrained by correlation of the results of seismoacoustic profiling with core data of the Ocean Drilling Program (ODP site 929A-E). Detailed seismostratigraphic study of the Upper Quaternary deposits in the study area has implications for better understanding of the role of gravity flows and bottom currents in sedimentation on the NE slope of the rise for the past 1.2 Myr.

Paleomagnetism and rock magnetism have been studied for the first time in Campanian-Maastrichtian carbonate deposits (Sengilei, Karsun, and Radishchev Formations) from the Bolshevik and Kommunar quarries near the town of Volsk (Saratov area, Volga region). Characteristic components of normal and reverse polarity were detected in samples demagnetized in the alternating field, and positive results of the reversal test were obtained. The virtual geomagnetic pole wander paths calculated for the Sengilei Formation and lower Karsun Formation rocks showed statistical similarity to those for stable Europe of the respective age. The magnetostratigraphic zones revealed in the Bolshevik and Kommunar sections were correlated to chrons 33r, 32n2, 32n1, and 31r of the geomagnetic polarity time scale. The magnetic zones of predominantly reverse and normal polarity detected, though discontinuously, in the low magnetic uppermost Karsun Formation and in the Radishchev Formation correspond to the top of chron 31r and joint chrons 31n and 30n, respectively. Rock magnetic data help identifying deposits rich in cosmogenic metal particles and provide stratigraphic constraints on the Radishchev Formation base.

We studied the distribution of organic carbon in rocks of the Bazhenov Horizon, a unique object of predominantly biogenic sedimentation in the West Siberian sedimentary basin. The contents of organic carbon in the rocks were determined using the data from 4094 core analyses and core-log relationships derived from 48,500 radioactive- and electrical-log measurements. For the Bazhenov and Tutleima Formations, both approaches gave the same results. The average content of organic carbon in the rocks is 7.7%. These data were used to compile a detailed map of the distribution of organic-carbon contents in sedimentary rocks of the basin. It was shown that the average organic-carbon content in the rocks increases from 2-4% on the periphery of the basin to 10-12% in its central, deepest part. The distribution of Corg values in the basin is highly asymmetric. The highest Corg values are observed in the southwestern part of the basin interior, where beds with >10% Corg range in thickness from 5 to 12-15 m. In sections, the highest Corg values are observed in their middle and upper parts, composed predominantly of silicites and mixtites enriched in biogenic silica.

Effects of in-situ stress and joint on permeability of the coal bed depend on orientation relationships between in-situ stress and joint. In-situ stress orientations of Linfen block of southeastern Ordos Basin were determined by systematical measurements of the loess joints. In-situ stress magnitudes were calculated based on well logging and hydraulic fracturing data. Joint orientations of the No.5 coal bed and density distributions of the overlying sandstone bed were investigated. The results show that the NE-oriented maximum horizontal principal stress, approximately parallel to the predominant joint orientation of the coal bed, is favorable to the openness of the predominant joints. The minimum horizontal principal stress, with its orientation perpendicular to the predominant joint orientation, tends to induce the closure of the predominant joints. The stress state of No.5 coal bed is σ_v>σ_H>σ_h, which is favorable for the openness of subvertical joints and an increase in permeability. The permeability of No. 5 coal bed increases obviously with increasing effective vertical stress and effective horizontal maximum stress and decreases with increasing effective horizontal minimum stress. Besides, the permeability of No. 5 coal bed exhibits a good exponential relationship with the density of predominant joint set. Therefore, predominant joints with the orientation of 45° mainly contribute to increases in the permeability of the No. 5 coal bed in Linfen block. Multidimensional analysis show good exponential relationships among the permeability, predominant joint density, and effective stress of the No.5 coal bed.

The problem of causality in low-frequency electrical prospecting with artificial and natural sources is considered. We present numerical examples of the application of dispersion relations to transfer function spectra used in the induced polarization and frequency sounding methods and to the components of telluric, horizontal magnetic, and other magnetotelluric tensors. Approaches to the processing and interpretation of spectra with violated dispersion relations are proposed.

The paper presents the results of a joint inversion of magnetotelluric sounding (MTS) curves taken at several stations without preliminary selection and normalization and distorted by the presence of lateral electrical-conductivity inhomogeneities in the medium. In the calculations, we used synthetic MTS data for a three-dimensional model. Preparation and interpretation of data are carried out by the Trefftz method using a numerical model of the field and MTS curve distortions. To solve the inverse problem and optimize the subsurface model, we used a nonlinear least-squares method and an iterative process with calculation of the sensitivity matrix and its singular decomposition. The target functional is determined by the discrepancies between the model and synthetic experimental apparent-resistivity curves corresponding to the elements of the impedance tensors on the lateral diagonal. The reliability of the reconstructed subsurface model is characterized by the dispersion of the deviations of its parameters from the parameters of the known model used for the preparation of synthetic experimental data. The joint consideration of distorted apparent-resistivity curves at several stations increases the reliability of interpretation results. The obtained solution to the inverse problem is approximate and can be used as a starting model for more complex algorithms and programs.