Russian Journal of Pacific Geology

This paper reports specific mineralogical and geochemical characteristics of deposits from the local depressions of the Derugin Basin. They were formed in an environment with periodic changes from oxic to anoxic conditions and show evidence for the presence of hydrogen sulfide in bottom waters. The deposits of this type can be considered as a modern model for ancient ore-bearing black shale associations. Compared with typical metalliferous black shale sequences, which are characterized by high contents of organic matter, the sediments described here are depleted in the elements of the organophilic association (Mo, Ni, Cu, Zn, V, and U) but have higher Mn contents.

An algorithm and a program for transformation of gravity anomalies for large territories are presented. They use a spherical shape of the Earth and the geodetic coordinates of measuring points. The calculations employ the “quasi-ellipsoidal” model of V.V. Kavraisky, which allows the transition from the geodetic to the spherical coordinate system with increased accuracy. The algorithm is based on a sourcewise approximation of the field values located at the nodes of a regular grid with the latitude-dependent depths of the equivalent sources. To determine the source masses, the system of linear algebraic equations is solved by the steepest descent gradient method with an accelerated calculation of the step. The results of transformation of the gravitational field in the Bouguer reduction for the Kuril Island arc, the adjacent water and land areas located within 40°–54° N, 142°–162° E and having a total area of about 2.4 mln km2, are presented. A slight difference between the results of recalculating the gravity anomalies into the upper half-space using the “spherical” and “quasi-ellipsoidal” models of the Earth is experimentally established.

The analysis of features of the geological structure of the Adycha gold placer region shows that flat structures of the early collision stage control the setting of the ore fields. The ores are similar to host rocks in the main components, lithofile trace elements, and REEs. The features of the trace-element pattern and indicative ratios agree with the suggested metamorphic origin of the main volume of fluids, their weakly reduced and almost neutral conditions, and sorption rock–fluid interaction. A group of elements, the distribution of which is significantly distinct from those of the host rocks, is determined. The geochemical type of the ore region is caused by Mo, Au, Bi, and Sb, the contents of which are higher in all studied ore objects. Chromium, As, Pb, and Te are present in all ore fields, but not in all objects. The Co–Ni–W–Zn–Cd–Ag, Co–Ni–W–In–Mn–Cu–Sr, and Sn–In–Mn–Cu–Zn–Cd–Sr–Ag associations supplement the geochemical type of the Delyuvial’ny, Lazo, and Sentachan ore fields, respectively. Our studies along with thermobarogeochemical data refine the metamorphic–magmatic model of the formation of the gold–quartz deposits of the Kular–Nera belt and outline approaches for the practical application of these features.

The results of integrated studies in the southwestern part of the Sea of Okhotsk obtained during cruise 74 of the R/V Akademik M.A. Lavrentiev in 2016 are presented. Gas hydrates, carbonate crusts and nodules in bottom sediments and more than 100 gas flares in the water column were recognized. Great attention has been paid to the gas composition of the water column and bottom sediments. Anomalous concentrations of hydrocarbon gases, helium, and hydrogen were revealed in the Kuril Basin–Terpeniya Bay transition zone.

This paper considers the results of the mineralogical and chemical study of sandstones and U–Pb isotope dating of detrital zircons from the Paleozoic–Early Mesozoic deposits of the Laoeling–Grodekovo terrane in western Primorye. It is shown that the sandstones of different age of the terrane differ markedly in their mineralogical and geochemical parameters and, hence, were derived from different sources. The sandstones of the Kordonka Formation are made up of fragments of basic and intermediate volcanic rocks of the oceanic island arc, as well as igneous and sedimentary rock associations of the oceanic island-arc basement. Eroded stable cratons and uplifted blocks representing crystalline basement inliers that are either flanked by rift zones or occurring along transform faults exerted a strong effect on the accumulation of sandstones of the Reshetnikovka, Barabash and Mangugai formations. The main suppliers of detritus were felsic igneous rocks with minor input from ancient sedimentary rocks. The rift margins and the eroded ensialic arc provided clastic material for the rocks of the Talmi Formation. The U–Pb geochronological studies of detrital zircons made it possible to constrain the age and possible position of igneous complexes that supplied clastic material to the sedimentary basins of the terrane. The sandstones were found to contain detrital zircons with concordant ages from 2553 to 205 Ma, which are dominated by Paleozoic zircons divided into several age groups. The zircons of Precambrian and Triassic ages are far fewer. In general, the revealed age ranges of detrital zircons from sandstones of the Laoeling–Grodekovo terrane agree fairly well with the known stages of granitoid magmatism in the eastern part of the Central Asian Orogenic Belt.

Petrophysical characteristics are determined for the rock complexes of the Okhotsk margin within the continent–ocean transition zone. Petrophysical maps showing the major inhomogeneities of the main tectonic elements of the studied area are constructed. Petrophysical inhomogeneities and the corresponding geophysical field anomalies are compared. A relationship between the magnetic field anomalies and subsurface rock complexes is revealed. The gravity field anomalies are related to deep inhomogeneities and are almost independent of the subsurface rock complexes of relatively low thickness.

This paper presents the results of studies of the main factors that are responsible for the formation conditions and localization of oil and gas deposits within the Cenozoic sedimentary cover and Pre-Cenozoic granite basement of the Vietnamese shelf. Our structural–tectonic model of the Cuu Long Basin allows the identification of the formation conditions of regional and local structures with hydrocarbon (HC) deposits, mechanisms of origin of HC traps and reservoirs, the genesis of HCs, including those occurring in the basement rocks, and the possible mechanisms of migration and accumulation of HCs in the basement rocks. Abundant traps are real or potential HC reservoirs in the crystalline basement body. The structural–tectonic processes in the basement resulted in origination and evolution of positive morphostructures (domes and protrusions), whose cores are filled by disintegrated (granulated) basement rocks. For the interpretation of chronothermobaric conditions of origination and evolution of HC sources, formation conditions, and principles of occurrence of oil and gas deposits at the Vietnamese shelf we conducted 3D modeling of the generation and accumulation HC systems using basin modeling in PetroMod program software (Schlumberger, Ltd., United States). The studies of HCs–biomarkers of oils of the deposits of the Cuu Long Basin including those located in the crystalline basement showed similar biomarker parameters of oils and organic matter, which is evidence of the organic nature of the oil deposits of the basement at the Vietnamese shelf.