Terrigenous Rocks Research Articles

Lower Silurian Terrigenous Rocks of the Laoeling–Grodekovo Terrane, Southern Primorye: Composition and Formation Settings

The original results of a study of the composition of terrigenous rocks of the Early Silurian Kordonka Formation of the Paleozoic–Early Mesozoic Laoeling–Grodekovo Terrane (southern Primorye) are considered. The aim of the study was to interpret the paleogeodynamic setting of the rocks of the formation and to determine the type and the composition of the parental rocks of the source areas on the basis of genetic interpretation of the composition of studied rocks. It was found that, according to mineralogical–geochemical parameters, the rocks of the formation are typical graywackes and petrogeneic (first cycle) rocks, which formed mostly due to mechanical erosion of rocks of the source areas. The rocks exhibit a low degree of maturity and a high rate of burial of clastic material and weak lithodynamic reworking of the parental rocks. The results of a comprehensive study of the rock composition have been interpreted using comparison with that of ancient rocks and present-day sediments accumulated in certain geodynamic settings. Our data indicate that the rocks of the Kordonka Formation accumulated in an oceanic island-arc sedimentary basin. The clastic material was sourced from the arc, which was composed of mafic and intermediate volcanic rocks and igneous and sedimentary rocks of its basement.

ПРОСТРАНСТВЕННО-ВРЕМЕННЫЕ И ГЕНЕТИЧЕСКИЕ СООТНОШЕНИЯ ЗОЛОТОРУДНОЙ И РТУТНОСУРЬМЯНОЙ МИНЕРАЛИЗАЦИЙ НА HG-SB-ЗОЛОТОСОДЕРЖАЩЕМ МЕСТОРОЖДЕНИИ ЧАУВАЙ (КИРГИЗИЯ): ГЕОЛОГИЯ, МИНЕРАЛОГИЯ РУД И ОСОБЕННОСТИ ГИДРОТЕРМАЛЬНО-МЕТАСОМАТИЧЕСКИХ ПРОЦЕССОВ

The Chauvai Hg-Sb deposit is a striking example of combining two contrasting types of mineralization in space: mercury-antimony and gold ones. The article studies the spatial-temporal and genetic relationships of goldore and mercury-antimony mineralization based on a complex of both traditional geological and mineralogicalgeochemical methods, as well as modern instrumental methods for analyzing the mineral composition. Two types of ores with clear structural confinedness have been found at the deposit: a) mercury-antimonic (cinnabarantimonite) ores, associated with jasperoid breccias and manifested exclusively along the tectonic contact of limestone of the Alai section and terrigenous rocks of the Tolubai Formation, and b) gold- sulphide (arsenopyritepyritic) ores, localized in slightly modified carbonate-terrigenous rocks of the Tolubai Formation, overlying the plane of tectonic contact. Ore formation occurred during the following stages: in the late diagenetic, without interruption passing into the catagenetic-hydrothermal, characterized by the formation of gold mineralization, and then in the later hydrothermal-telethermal, characterized by the development of Hg-Sb mineralization. It is established that the main carrying agent of invisible gold (“invisible gold”) in ores is framboidal and idiomorphic pyrite and, especially, its high-arsenic varieties. A set of conducted studies has shown that the gold ore and mercury-antimony mineralization is broken in time and is genetically associated with various hydrothermalmetasomatic processes, and the Chauvai deposit can be classified as a Carlin-like type.

КОРДОНКИНСКИЙ ТЕРРЕЙН – ФРАГМЕНТ РАННЕПАЛЕОЗОЙСКОЙ АКТИВНОЙ ОКРАИНЫ

The Kordonka Formation (lower Silurian) forms a lens-shaped tectonic block up to four km wide, extending southward from the Pogranichny village area (Primorsky Krai) for about 30 km. The formation is composed of argillaceous slate, volcanic-sedimentary rocks with horizons of sandstones, siltstones, conglomerates, and cherts, and also contains flows of basalts and andesite-basalts. The results of lithological-geochemical study of the terrigenous rocks of the formation indicate that the Kordonka Formation was accreted in the outer pre-oceanic part of the paleo-island arc system. The eastern part of the formation is intruded and partially assimilated granitoids of Middle Paleozoic and Late Permian age. The early phase of the granitoids was dated by U-Pb zircone 373.5±1.7 and 383.5±1.6 Ma (Late Devonian). The late phase is characterized by 261.2±1.2, 257.4±0.93 and 261.5±1.6 Ma (Wuchiapingian and Capitanian stages of the Middle and Late Permian). The Devonian granitods first identified in this area may be evidence of the Kordonka block accession to the Bureya-Jiamusi-Khanka orogenic belt consolidated during the Ordovician. The Kordonka terrane thus marks an independent Early-Middle Paleozoic episode in the formation and closure of the Central Asian paleo-ocean.

ПЕРСПЕКТИВЫ НЕФТЕГАЗОНОСНОСТИ И ИЗУЧЕНИЕ КОЛЛЕКТОРСКИХ СВОЙСТВ МЕЛОВЫХ ОТЛОЖЕНИЙ ЕВЛАХ-АГДЖАБЕДИНСКОГО ПРОГИБА

The aim of the study is to substantiate the prospects for the oil and gas content of the Upper Cretaceous effusive and carbonate deposits by studying their reservoir parameters. Previous studies have established that a sedimentary complex, present in the geological structure of the Muradkhanly field, is lithologically represented by clays, siltstones, sands and sandstones, dolomites, limestones, tuffs, Upper Cretaceous porphyrites. Since this lithofacies composition of the section is of great importance as a reservoir rock, volcanic sandy tuffs, mudstones and dense sandstones of the Upper Cretaceous age have been comprehensively investigated throughout the territory. We studied carbonates of this age and found that they have compaction of varying extent and are divided into 3 groups: weakly, moderately and strongly compacted and this, in turn, has considerable effect upon the rock porosity. At the beginning of the study, the reservoir features of Meso-Cenozoic sediments in the Yevlakh-Agdjabedi depression [YAD] were studied mainly on the basis of terrigenous rocks that refer to the Cenozoic. In our studies, Cretaceous deposits were studied, which are represented by effusive, carbonate rocks with secondary reservoir properties. The reservoir properties of chalk deposits in the YAD areas were investigated on the basis of studies of core samples in laboratories. It was found that the reservoir rocks of the area under investigation are volcanic, volcanic-sedimentary and carbonate, that is, they mainly consist of fractured limestones, marls and dolomites. Studies have shown that the oil and gas content of the depression is associated mainly with carbonate and effusive reservoirs. To assess the prospects for the oil-bearing capacity of the YAD, geological sections were studied in terms of the reservoir properties of a number of structures, and on the basis of the obtained core samples data. Trend maps were compiled using the "Surfer" software. Analysis of the maps allows identifying the corresponding zones of carbonate content, permeability, porosity, characterized by high, medium and low values. According to the carbonate readings, the studied territory does not have a strict distribution pattern and, according to the permeability distribution data, it is divided into two zones: the northern zone with average and the southern one with low values. There is, to a certain extent, pattern for variation of the values of carbonate content and permeability, and this can be explained by the secondary transformation of the reservoir properties of carbonate rocks, as a result of which permeability and porosity changed. Porosity distribution, to some extent, is aligned with permeability. Inverse relationship between the values of porosity and permeability can be explained with the filling of voids with cement material in the rock formation environment. Currently, in order to continue exploration and drilling works, comprehensive geological and geophysical studies are being carried out, as a result of which comparison of data obtained will make it possible to assess the prospects of each structure in more detail.

Paleogeodynamic Reconstruction of Cenozoic Sedimentation Settings in the West Sakhalin Terrane Based on the Material Composition of Terrigenous Rocks

The results of studying the material composition of sandy rocks from Cenozoic deposits in the West Sakhalin Terrane are considered. It has been established that sandstones in the terrane correspond to graywackes and petrogenic (“first cycle”) rocks. They are characterized by a low maturity degree of the detrital material formed mainly due to the mechanical destruction of source rocks. Paleogeodynamic interpretation of the obtained data suggests that the Paleocene–Pliocene sedimentation occurred directly along the Asian continent margin in a basin associated with large-scale shear dislocations along transform faults. The feeding zone was mainly composed of the following sources: (i) sialic land composed of the granitic-metamorphic and sedimentary rocks; (ii) mature deeply dissected continental-margin arc, where granitoid batholiths at the base were exposed by erosion. The secondary source was represented by fragments of the Sikhote-Alin accretionary prisms involving ophiolites. The U‒Pb isotopic dating of detrital zircons made it possible to establish the main complexes of granitoids that delivered the clastic material to Cenozoic sedimentation basins in the terrane.

Петрофізичні моделі відкладів менілітової світи олігоценового флішу Карпат і Передкарпатського прогину

Petrophysical and collecting properties of the reservoir rocks of low-porous, low-permeable deposits of the Menilite suite of the Oligocene flysh of the Carpathians and the Precarpathian deep were studied as well as their interconnections and interactions under normal conditions and in conditions that simulated the formation conditions, and their typical geological-geophysical cross-sections were constructed. Creation of identical petrophysical models of reservoir rocks was based on the system integrated approach with which the reservoir rock is considered as a system formed in the geological time and consists of interconnected and interacted elements. Petrophysical investigation of the reservoir rocks as the system includes the studies of the characteristics of their elements, the character of interconnections between them taking the conditions of their occurrence into consideration. On the basis of statistical processing and analysis of the results of laboratory studies of core material, the parametric petrophysical models of “core-core”-type were constructed: statistical dependences between porosity factor, permeability factor, water-saturation, specific weight, parameter of porosity, interval time for arriving acoustic waves and parameter of oil-saturation for atmospheric conditions and effective pressures being comparable to formational ones. Using the method of basic components of factor analysis it was possible to study the influence of geological factors upon geophysical parameters of the reservoir rocks in terrigenous sections and the informative value of geophysical methods while singling-out producing seams in the geological section. Oil- and gas-saturation of the reservoir rocks, their mineralogical composition and porosity of the reservoir rocks have the most influence on the indications of geophysical methods in the boreholes, somewhat lesser: a depth of their occurrence and the thickness of the seams. Producing and water-bearing beds differ in the value of electrical resistance most of all. The influence of lithogeodynamic factors upon the collecting and physical properties of the reservoir rocks was studied. The main geological factors that determine collecting parameters of terrigenous rocks and their physical properties are the following: a mineral composition, a shape, a size of fragmental grains and pores and their mutual position, a type of fluid-saturation, a rate of catagenetic transformations and a thermodynamic state.

Age and Composition of Zircons From the Devonian Petrivske Kimberlite Pipe of the Azov Domain, the Ukrainian Shield

Results of a study of U-Pb and Hf isotope systematics and trace element concentrations in five zircon crystals separated from the Devonian Petrivske kimberlite are reported in the paper. Four zircons have yielded Paleoproterozoic and Archean ages, while one zircon grain gave a Devonian age of 383.6±4.4 Ma (weighted mean 206Pb/238U age). The Precambrian zircons have been derived from terrigenous rocks of the Mykolaivka Suite that is cut by kimberlite, or directly from the Precambrian rock complexes that constitute continental crust in the East Azov. The Devonian zircon crystal has the U-Pb age that corresponds to the age of kimberlite emplacement. It is 14 m.y. younger than zircon megacrysts found in the Novolaspa kimberlite pipe in the same area. In addition, Petrivske zircon is richer in trace elements than its counterparts from the Novolaspa pipe. Petrivske and Novolaspa zircons crystallized from two different proto-kimberlite melts, whereas the process of kimberlite formation was very complex and possibly included several episodes of formation of proto-kimberlite melts, separated by extended (over 10 M.y.) periods of time.

ЕЛЕКТРИЧНІ ТА ПРУЖНІ ПАРАМЕТРИ УЩІЛЬНЕНИХ ТЕРИГЕННИХ ПОРІД КАРБОНУ СХІДНОЇ ЧАСТИНИ ПІВНІЧНОЇ ПРИБОРТОВОЇ ЗОНИ ДДЗ

The main objective of this article is studying of electrical and acoustic parameters of Carbonic period terrigenous rocks of the eastern part of the northern edge of the Dnieper-Donets depression (DDD). It was determined that in atmospheric conditions the resistivity of dry extracted rocks (the specific electrical resistivity of framework of grains) varies from 6.522 kΩ×m to 2.782 MΩ×m (mean 1.033 MΩ×m), for sandstones – from 16.18 kΩ×m to 206.815 MΩ×m (mean 27.973 MΩ×m). The resistivity of rocks saturated with model of reservoir water (salinity M = 180 g/l) varies: for siltstones from 3.3 Ω×m to 26.8 Ω×m (mean 8.5 Ω×m), for sandstones from 2.9 Ω×m to 20.3 m (mean 8.2 Ω×m). In laboratory it was defined that in atmospheric conditions formation resistivity factor varies: for siltstones from 41.2 to 277.9 Ω×m (mean 96.2 Ω×m), for sandstones from 34 Ω×m to 238.5 Ω×m (mean 94.5 Ω×m). Physical modelling of reservoir conditions (temperature 94-126 °Ñ, confining stress 41-55 MPa, salinity M = 180 g/l) showed that the specific electrical resistivity varies: for siltstones from 0.4 Ω×m to 7.7 Ω×m (mean 2.1 Ω×m), and sandstones – from 0.4 Ω×m to 3.5 Ω×m (mean 1.3 Ω×m). Due to the closure of microcracks and the deformation of the pore space, the electrical resistance of rocks increases with increase of pressure. The regression dependence of the formation resistivity enlargement factor with pressure for the studied rocks manifest itself in second order polynomial. It was determined that in reservoir conditions the range of the formation resistivity factor varied: for siltstones from 132 to 2480 Ω×m (mean 562 Ω×m), for sandstones from 81 to 953 Ω×m (mean 339 Ω×m). Laboratory acoustic study of velocities of the P-waves in dry extracted rocks showed that this parameter varies: for siltstones from 2616 m/s to 4706 m/s (average 3940 m/s), for sandstones from 3660 m/s to 4971 m/s (average 4245 m/s). If rocks are saturated with model of reservoir water, velocities have higher values of P-waves: for siltstones from 3596 m/s to 5438 m/s (average 4681 m/s), for sandstones from 4302 m/s to 5463 m/s (average 4942 m/s). It was determined by physical modelling of reservoir conditions that velocities of P-waves vary: for siltstones from 3763 m/s to 6223 m/s (mean 5053 m/s), for sandstones from 4430 m/s to 5752 m/s (mean 5176 m/s). With the increase of pressure, the velocity of P-waves increases as well due to the closure of microcracks and the deformation of the pore space. The dependence of the increase in velocity from pressure is described by second order polynomial. The comprehensive analysis of electrical and acoustic parameters of condensed rocks has been resulted in a set of correlation ratios between reservoir, electric and elastic parameters of studied samples in normal and modeling conditions.

Estimation of Secular Change in the Size of Continents for Understanding Early Crustal Development

The size of continents is an essential parameter to understand the growth of the continental crust and the evolution of the solid Earth because it is subject to tectonism and mantle convection and affects the preservation of the crust. This article reviews the secular change in the size of continents on the early Earth, focusing on terrigenous clastic rocks, especially quartzose sandstones occurring on relatively large continents. The earliest continental crust in the Hadean or early Archean was produced with a width of ∼200–500 km, similar to modern oceanic island arcs along subduction zones or oceanic islands in hot spot regions by mantle plume heating. Through the collision and amalgamation of such primitive continental crusts, continental blocks over 500 km in width and length evolved and appeared by ca. 3.5 Ga. Through further amalgamation, during ca. 3.3–2.5 Ga, the Archean continents emerged with widths and lengths greater than 1,000 km, which were still smaller than those of modern continents. Continents with widths and lengths of nearly 10,000 km have existed since ca. 2.4 Ga (early Proterozoic). Further analyses of the composition and formation mechanism of clastic rocks will help reveal more quantitative secular changes in the sizes of continents.

Archean Metaterrigenous Rocks of the Karelian Province: Geological Position, Geochemistry, Sediment Provenance

The geology and composition of Archean terrigenous sedimentary rocks of the Karelian province (Baltic Shield) are studied in order to establish their positions in the structures of greenstone belts and their role in the provenance composition. It is shown that metaterrigenous rocks from the greenstone belts of the youngest Central Karelian domain, together with intermediate-to-felsic volcanic rocks, compose the main part of the sections, occurring at their bases, and are about 2745 Ma in age. In the belts of the West Karelian domain, metaterrigenous rocks compose the upper parts of the sections, and their age is 2745–2750 Ma. Geological data on the position of metaterrigenous rocks in the belts of the oldest Vodlozero domain also suggest their Neoarchean age. On this basis, it is concluded that metaterrigenous rocks mark the beginning of the Neoarchean stage of crust formation in the Karelian province. According to geochemical data, metagreywackes are predominant among the Archean metaterrigenous rocks of all three domains. At the bases of the sections of some structures, polymictic conglomerates are present with the predominance of tonalite, trondhjemite, and, less often, granodiorite pebbles and with a subordinate amount of igneous rocks of different compositions. Low CaO and Na2O contents and CIA < 55 indicate an insignificant degree of weathering. The Th, Co, Sc, Hf, and La contents indicate a leading role of the TTG-association rocks in the compositions of the sources for all the studied structures. The influence of the basic source was small and affected exclusively Ti, Y, and heavy REE contents in metaterrigenous rocks. The role of komatiites as a source of detrital material was insignificant.

Studying dielectric properties of oil shale

The paper describes the methodology, equipment and results of studying the features of dielectric properties of oil shale in the frequency range for values from values 0.05 to 300 MHz and temperatures for values from 20 to 200°C. It is shown that the dielectric parameters for the studied shale samples are an order of magnitude higher than for terrigenous rocks, and along the layers they are about 1.5 - 4 times higher than across the layers. The dielectric properties of the studied shale are found to undergo dispersion in the range for values from 50 to 125 MHz and have nonlinear temperature dependence.

Geodynamic Emplacement Setting of Late Jurassic Dikes of the Yana–Kolyma Gold Belt, NE Folded Framing of the Siberian Craton: Geochemical, Petrologic, and U–Pb Zircon Data

We present the results of geostructural, mineralogic–petrographic, geochemical, and U–Pb geochronological investigations of mafic, intermediate, and felsic igneous rocks from dikes in the Yana–Kolyma gold belt of the Verkhoyansk–Kolyma folded area (northeastern Asia). The dikes of the Vyun deposit and the Shumniy occurrence intruding Mesozoic terrigenous rocks of the Kular–Nera and Polousniy–Debin terranes were examined in detail. The dikes had diverse mineralogical and petrographic compositions including trachybasalts, andesites, trachyandesites, dacites, and granodiorites. The rocks showed significant similarities in distributions of REE, and their concentrations of most HFSEs were close to the intermediate ones between ocean islands basalts and enriched middle ocean ridge basalts. We propose that the subduction that was ongoing during the collision of the Kolyma–Omolon superterrane with Siberia led to melting in the asthenospheric wedge and in the lithosphere, which formed a mixed source for the dike systems from both an enriched and a depleted mantle source. The U–Pb SHRIMP-II dates obtained for the dikes corresponded to the Late Jurassic interval of 151–145 Ma. We present a geodynamic model for the northeastern margin of the Siberian Craton for the Tithonian age of the Late Jurassic.

Paleomagnetism and Age Correlation of the Mesoproterozoic Rocks of the Udzha and Olenek Uplifts, Northeastern Siberian Platform

Abstract—This paper presents the results of paleomagnetic studies of the Proterozoic sedimentary and igneous rocks composing the structure of sections of the Olenek and Udzha uplifts. Carbonate rocks of the upper unit of Khaipakh formation have been studied within the Olenek uplift, and terrigenous and volcanic-sedimentary rocks of the Udzha and Unguokhtakh formations and the Early Mesoproterozoic igneous complex have been investigated within the Udzha uplift. Within the Olenek uplift, the carbonate rocks of the upper unit of the Khaipakh formation have been studied; within the Udzha uplift–terrigenous and volcanogenic-sedimentary rocks of the Udzha and Unguokhtakh formations, and the Early Mesoproterozoic magmatic complex. The results of the paleomagnetic study (1) indicate that the upper unit of Khaipakh formation of the Olenek uplift and the Udzha formation of the Udzha uplift have different sedimentation age, which contradicts the currently accepted correlation scheme of the Proterozoic sections in the northern Siberian platform; (2) support the existence of two stages of the Proterozoic magmatism in the territory of the Udzha uplift, with a more ancient stage having an age of ~1500 Ma according to the paleomagnetic data; (3) show that from ~1500 to 1110 Ma ago, the Siberian platform was located in the equatorial and subequatorial latitudes.

The Age of the Patom Crater: Dendrochronological and Biogeochemical Investigations

According to data of dendrochronological investigations of larch trees that had grown on the Patom crater (Irkutsk oblast, Bodaibinskii district) and near its base, the initial period of the formation of the crater can be dated back to the late 15th‒early 16th centuries. In 1841‒1842, the trees under investigation recorded in annual rings a catastrophic event which caused a disturbance to their root systems, damage to tree stems, and the formation of compression wood. It is obvious that the event is associated with the time of formation of the late stage ring swell on which sparse larch trees are 93‒101 years of age, and on the central mound emerging upon completion of the formation of the Patom crater the age of the oldest tree is 7 years. Thus dendrochronological investigations confirm a long-lasting and multievent formation of the Patoma crater. In some periods, the deposition cone was evolving with a different intensity, sometime showing an explosive character. X-ray fluorescent analysis revealed a double Sr enrichment of larch wood at the time of significant activity of crater formation (1852‒1859) when plutonic fluids from the igneous source were able to enter the upper horizons of the Earth’s crust at the time of magma intrusion into water-containing rocks. The fluids were enriched with CO2 and transported significant amounts of Sr, as a result of which the 87Sr/86Sr ratios in carbonate and terrigenous rocks assumed higher values in sandstone and schists on the early stage ring swell of the Patoma crater. The findings, coupled with geological data, suggest that the Patom crater emerged as a result of a phreatic explosion that occurred either at the time of magma intrusion into water-containing rocks or as a consequence of the fault and decompression of heated water-containing rocks.

The Sources of the Clastic Material of the Terrigenous Sequences of the Neoarchean and Paleoproterozoic Paleobasins in the Eastern Part of the Fennoscandian Shield Based on Isotope Analysis Data for Detrital Zircons (SIMS, LA-ICP-MS)

Isotope analyses of detrital zircons from the terrigenous sequences of the Neoarchean Uraguba–Titovka greenstone belt and the Paleoproterozoic Pechenga–Varzuga and Vetreny belts, including the basal conglomerates, by the SIMS (SHRIMP-II) and LA-ICP-MS methods were performed for the first time. On the basis of the data obtained, the inception dates of the structures and the ages and lithology of their basement rocks were established, and the origin of the terrigenous rocks was refined. Detrital zircon populations with ages of 2.66–2.73, 2.76–2.79, 2.82–2.87, and 2.98–3.00 Ga are predominant. The sources of these zircons were the rocks of the tonalite–trondhjemite–granodiorite complex, enderbites, high-alumina gneisses, and their metamorphosed varieties, which dominate the present-day erosion surface and have been penetrated by the Kola superdeep borehole. Zircon populations with the age of 3.00–3.70 Ga were also found. Zircon with the age of 3698 ± 8 Ga was found in terrigenous red beds, which mark the global Oxygen Revolution (2.2–2.3 Ga). Data obtained confirm a significant contribution of the Eoarchean and Paleoarchean material to continental crust composition in the region and the different inception dates of the Pechenga and Imandra–Varzuga structures.

Features of the coarse clasts (psephites) distribution in the Upper Quaternary sediments of the northern Barents Sea

The paper presents the characteristics of the coarse clasts (psephites, larger than 1 cm) sampled in the northern part of the Franz Victoria Trough (Barents Sea) during the “Transarktika-2019” expedition. The studied sedimentary section was formed during the transition from the last deglaciation environments to the marine Holocene settings. The amount of psephites in deglacial sediments is much higher than the one in Holocene sediments. The petrographic composition of the psephites, their roundness (according to Waddell and Khabakov classifications) and shape (according to Zingg classification) were studied in detail. It is shown that the majority of psephites is represented by non-rounded or poorly rounded varieties. Isometric and disc-type shapes are predominating among coarse clasts. Isometric psephites prevail in “deglacial sediments”, while disc-shaped, bladed, and rodlike, as a rule, are current in marine Holocene sediments. The petrographic composition of psephites is mostly represented with carbonate (limestone and dolomite) and sandstones. The amount of carbonates increases from the Holocene to the deglacial part of the sediment section. Other rocks found in smaller quantities are represented by quartzites, cherts, shales, basalts, crystalline schists, gneisses, granites, pyrite. During the last deglaciation at the Late Pleistocene the iceberg rafting was the main mechanism of psephites delivery to the sampling points. The major sources of the icebergs were Franz Josef Land and the Belyi — Victoria Islands region. Rocks of Franz Josef Land are represented mainly by Cretaceous gabbro-basalt complex and Triassic sandstones and siltstones. Upper Paleozoic terrigenous and carbonate rocks, as well as Proterozoic metamorphites, occur in the west of the studied area (the area of the Belyi — Victoria Islands). The results of studying the petrographic composition of psephites allow us to conclude that during the last deglaciation icebergs from both sources were heading north towards the Nansen Basin through the Franz Victoria Trough. Both streams of icebergs failed to reach the opposite sides of the Franz Victoria Trough, mixing with each other approximately in the axial part of the trough and leaving it in northern directions.

ЛИТОЛОГИЯ И КОЛЛЕКТОРСКИЕ СВОЙСТВА ДОКЕМБРИЙСКИХ ОТЛОЖЕНИЙ ЛЕНО-АНАБАРСКОЙ НЕФТЕГАЗОНОСНОЙ ОБЛАСТИ

Актуальность исследования. В настоящее время одним из приоритетных направлений в расширении углеводородной базы России стало исследование и освоение её северных окраин. Лено-Анабарская нефтегазоносная область располагается на севере Сибирской платформы и характеризуется очень неравномерной геолого-геофизической изученностью. Наиболее слабо изучены докембрийские отложения, несмотря на их достаточно высокий коллекторский потенциал, а также прямые и косвенные признаки нефтегазоносности. Для более эффективного проведения дальнейших геологоразведочных работ в этом регионе необходимо детальное изучение пробуренных ранее разрезов с выявлением литологических особенностей пород-коллекторов, условий их формирования и распространения. Объект: отложения докембрия, вскрытые глубоким бурением на территории Лено-Анабарской нефтегазоносной области. Цель: на основе анализа состава, строения, обстановок осадконакопления и коллекторских свойств докембрийских отложений Лено-Анабарской нефтегазоносной области выделить перспективные резервуары углеводородов. Методы: построение разрезов докембрийских отложений на основе детального макро- и микроскопического изучения кернового материала и данных ГИС; корреляция разрезов с учетом литологических, палеонтологических и геофизических данных; генетический анализ основных литологических типов пород и расшифровка условий их седиментации; оценка влияния седиментационных и постседиментационных факторов на формирование коллекторских свойств; выделение уровней потенциальных коллекторов. Результаты. Установлено, что докембрийский разрез представлен чередованием пачек карбонатных и терригенных пород, накопление которых происходило в широком спектре обстановок: от дельтовых до сублиторальных при существенном преобладании мелководно-морской седиментации. Выявлено, что в отложениях докембрия присутствуют потенциальные породы-коллекторы, причем наиболее перспективные уровни приурочены к вендской части разреза.

Carbonization of Carbonates and Fractionation of Stable Carbon Isotopes in a Dynamic Slip Zone

Data obtained on carbonates and metaterrigenous rocks of the Devonian complex of the Sultan-Uizdag Range, northwestern Uzbekistan, which are carbonized in local deformation zones, are used in a generalized interpretation of the genesis of carbonaceous matter by means of decomposition of carbonates under the effect of seismogenic movements. The release of free carbon is facilitated by synchronous Fe–Mg metasomatism and by the reducing conditions in the presence of hydrogen. Based on literature data on experimental modeling of carbonization processes at high-speed shear deformations of carbonates in a hydrogen medium, a possible scenario is considered for the generation of hydrogen on the surfaces of quartz dislocations in the presence of water in metaterrigenous rocks in association with carbonates. The variation of carbon isotope composition over the volume of the carbonate and terrigenous rocks is extremely heterogeneous, with the 13C isotope enriched in the fracture zones and 12C enriched in the near-fracture volumes of decompression-damaged rocks. It is hypothesized that deformations can affect the fractionation of stable carbon isotopes. With regard for literature data, it is demonstrated that isotope exchange can proceed between the free carbon and carbonate carbon. Various aspects are discussed of the influence of mineral-phase and structural–textural transformations of carbonate rocks at their thermomechanical decomposition under shearing on the geomechanical behavior of the fracture, manifestations of mechanical instability in it, and causes of frictional weakening.

The Geochemical Features of Ores and Host Rocks of the Malo-Taryn Orogenic Gold Deposit (Verkhoyansk–Kolyma Folded Area, Northeast Russia)

This paper presents the results of a study of the distribution of major, trace, and rare-earth elements in the terrigenous rocks, wall-rock metasomatites, and ore zones of the orogenic Malo-Taryn gold deposit (Verkhoyansk-Kolyma folded area , Northeast Russia). An important role in the formation of metasomatites and ore zones was played by hydromicatization, carbonatization, sulfidization, and silicification, which caused the different mobility of major elements and the predominant development of acidic low-temperature carbonic–calcium metasomatism. An increase in the quartz content in the wall-rock metasomatites and ore zones was accompanied by the removal of most rock-forming elements (K, Na, Al, Fe, Mg, and Тi) and input of Ca and CO2. The ores and metasomatites of the deposit are characterized by the As–Au–Sb–W geochemical specialization at low Bi, Co, Ni, Cr, Cu, and Zn and high Li, Ag, Cd, and Sc concentrations. Their REE distribution patterns in modified rocks are similar to those of the Upper Triassic host rocks, with some tendency to a decreasing REE total content. It was established that REE are removed during the formation of metasomatites and ores. The values of the Eu/Eu* (0.61–0.88) and Ce/Ce* (0.6–1.3) anomalies indicate slightly reducing conditions of ore formation with a limited involvement of oxidized meteoric water.

Pb and Nd Isotopic Data on Granitoids from the Lake Zone, Mongolian and Gobi Altai with Implications for the Crustal Growth of the Central Asian Orogenic Belt

Pb and Nd isotopic composition of granitoids from the Lake Zone, Mongolian and Gobi Altai was analyzed to decipher the continental crustal growth of heterogeneous terranes of the Central Asian orogenic belt. The granitoids were generated in the Late Neoproterozoic–Early Paleozoic juvenile crust of the Lake Zone during island-arc, accretion, and post-accretionary stages at 535–440 Ma. Their Pb and Nd isotopic characteristics are similar and illustrate the dominance of juvenile material in the source of granitoids of all three stages. An insignificant terrigenous contribution in their source provides dispersion of the Pb isotopic composition at a weak effect on the Nd isotopic composition. The Pb and Nd isotopic study of Middle Paleozoic (∼380–355 Ma) synkinematic and Late Paleozoic (∼350–270 Ma) post-kinematic granitoids in the Mongolian and Gobi Altai revealed a significant heterogeneity of their source, which is mainly inherited from terrigenous rocks of the Altai accretionary wedge variably metamorphosed under greenschist to granulite facies conditions. The main source of Altai terrigenous sediments was volcanic rocks from the Lake Zone juvenile crust with subordinate contribution of more mature rocks from the Tuva-Mongolian and Dzabkhan microcontinents, which supplied sediments with highly radiogenic Pb and Nd isotopic composition. The Pb isotopic compositions of the granitoids from the Lake Zone, Mongolian and Gobi Altai, and also the Trans-Altai Gobi (Savatenkov et al., 2016) show that their mantle source has an elevated Th/U ratio compared to the depleted mantle (Kramers and Tolstikhin, 1997). This is a hallmark of the depleted mantle source of the Paleo-Asian province. Juvenile continental crust terrains of the Central Asian orogenic belt (Lake Zone and Trans-Altai Gobi terranes) were formed in various tectonics settings. The island-arc complexes of the Lake Zone developed near the Precambrian Tuva-Mongolian and Dzabkhan microcontinents, which supplied terrigenous material with highly radiogenic Pb to granitoid sources. The Trans-Altai Gobi was a system of ensimatic island arcs separated from the Siberian paleocontinent by a continent-dipping subduction zone, which prevented the influx of ancient terrigenous material from the continent.