Carbonate-terrigenous Rocks Research Articles

FIRST RESULTS OF MAGNETOTELLURIC SOUNDINGS FOR MAPPING THE RIPHEAN SEDIMENTS OF THE BAIKAL FOREDEEP

The article presents the first results of magnetotelluric sounding (MTS) surveys of the Baikal foredeep. This study is part of the project of the Institute of the Earth’s Crust SB RAS, which investigates the geological and geoelectrical structure of the southeastern marginal part of the Siberian platform and updates the model of the Baikal rift zone. An objective is to preliminary assess the mining potential of the Riphean strata in the Baikal foredeep. The relevance of the study centers on the possibility of discovering a new promising oil-gas field in proximity to energy-intensive industrial and agricultural enterprises. The MTS section of the sedimentary cover shows an increased conductivity of its lower part, which is most likely due to highly porous carbonate-terrigenous rocks in the middle – lower part of the ‘Baikal three-member’ structure. The intra-lithospheric conductive layer that hosts most of the earthquake hypocenters of the Baikal rift is investigated.

The Origin of Terraces in Buried Valleys in the Northwest of the East European Plain

Abstract —The paper considers the origin of terraces that often complicate the slopes of abnormally deep buried valleys in the northwest of the East European Plain. The Izborsk–Maly valley (Pskov Region, Russia), which is half filled with Quaternary sediments, was chosen as the object of study. Since the upper part of the valley remains unfilled, it is possible to study in detail both geologic and geomorphologic structures of its terraced slopes being of particular interest in the context of the origin of buried valleys and the role of substrate in it. The four denudation-terrace levels that have been identified on the slopes of the Izborsk–Maly valley at absolute elevations of 53, 56–58, 70–72, and 75–77 m formed during the destruction of the preglacial karst landscape under the influence of Pleistocene glaciations. The formation of the terraces was probably linked with several stages of overdeepening of the valley and its expansion by the glacial erosion processes, which corresponded to at least two stages of the ice sheet advance in this area. The last stage may have been the beginning of the Late Valdai (Weichselian) glaciation or the activation of the edge of an ice sheet during its Luga phase (~15.7 cal kyr BP). Results of the study suggest that the Izborsk–Maly valley formed in subglacial conditions. The degree of glacial erosion was determined primarily by the coherence of Devonian carbonate-terrigenous rocks hosting the valley and the degree of the area transformation by karst processes. The cirque shape of slopes is the evidence of karst processes expression in pre-glacial time. Analysis of literature data shows that the origin of denudation terraces in the buried valleys in the northwest of the East European Plain has been paid little attention. The obtained data contribute to the study of this problem and provide insights into the origin of buried valleys.

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.

FEATURES OF Sr DISTRIBUTION IN ROCKS AND SOILS OF GEOCHEMICAL LANDSCAPES OF THE BLACK SEA COAST OF RUSSIA

The article considers the results of studying the features of Sr distribution in rocks of various ages and composition and soils of landscapes of the Black Sea coast of Russia (in the humus horizon and soil profile). The content of elements in the studied soils is compared with the regional background for the soils of the North Caucasus. A direct relationship between the content of the element in soils and underlying rocks, as well as the influence of technogenesis on the processes of accumulation and removal of the element in the soils of agricultural landscapes, has been revealed. The results of a comprehensive study of the south of Russia became the basis for studying the peculiarities of the distribution of Sr landscapes of the Black Sea coast of Russia. To establish the main parameters of the distribution of chemical elements in rocks and soils of various landscapes and the region as a whole, to identify geochemical features of geographical and technogenic differentiation, to assess the impact of various types of environmental management on changes in the geochemical spectrum of soils, to determine the influence of landscape-forming factors on the migration of chemical elements and the formation of geochemical barriers. The highest concentration of Sr was observed in carbonate-terrigenous rocks of the Paleogene and Cretaceous ages (marls and limestones). In the humus horizon of soils, the average metal concentrations vary in the range from 13.0∙10–3 to 95.0∙10–3 % with a regional clark of 22.0∙10–3 %.

Isotopic composition of lead from dikes and ores of the Vorontsovkoe gold deposit (Northern Urals)

Research subject . The Vorontsovskoe deposit belongs to gold deposits, in which gold is disseminated finely across carbonate-terrigenous rocks (Carlin-type). This study was aimed at identifying the sources of lead in various ore types (in skarn, jasperoid, tuff-sandstone and limestone breccia) and evaluating the role of magmatic rocks (apophises of Auerbakh intrusions and dykes of medium-basic composition) in the mineralization. M aterials and methods . Ore and rock samples for isotopic analysis were collected in the northern quarry of the deposit, as well as were retrieved from exploration holes. The lead isotopic composition was studied in the analytical centre Geoanalytic , Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences, using a mass spectrometer Neptune Plus on the basis of the TL N MC ICP MS method. Results. The lead isotopic composition of ores and igneous rocks indicates mixed mantle-crustal sources. The primary lead isotopic composition of igneous rocks corrected for the age of 400 Ma is characterized by a linear trend with wide variations in 207 Pb/ 204 Pb (15.6824–15.5182) and 206 Pb/ 204 Pb (18.1973–17.7707). The lamprophyre dykes feature the least radiogenic lead located between the evolutionary curves for the mantle and orogen in the Doe–Zartman model. The most radiogenic lead is typical of the granodiorite of the Auerbakh intrusion apophises. The analysis points of diorite porphyrite, gabbro-diorite and diorite dykes are located between the orogen and upper crust curves. The sulphide ores are characterized by lower variations in the primary lead isotopic composition compared to igneous rocks – 207 Pb/ 204 Pb (15.6009–15.5421) and 206 Pb/ 204 Pb (18.0434–17.8404). The most productive ores in tuff-sandstones and limestone breccias are characterized by a significant predominance of the crustal component over the mantle component. Their points in the diagram 207 Pb/ 204 Pb– 206 Pb/ 204 Pb are located between the curves for the orogen and the crustal source with µ 2 ( 238 U/ 204 Pb) = 9.74. Conclusion. Mantle lead in the dykes under study appears to reflect the nature of their parent magmas and may have been partially extracted by melts from the footwall mafic rocks containing pyrite ores. However, crustal lead is likely to have resulted from assimilating host rocks when magmatic melts reached the upper crust level. Crustal lead migrated with the metamorphic fluid formed by the heat action of the Auerbakh intrusion on sulphide-bearing volcano-sedimentary rocks. Mantle lead might have been transported by the fluid flow from a deepseated magmatic chamber.

Brachiopods of the Order Spiriferida at the Devonian–Carboniferous Boundary of Transcaucasia

A distinct change in the taxonomic and ecological structure of assemblages of brachiopods of the order Spiriferida is observed at the Devonian and Carboniferous boundary of Transcaucasia. Representatives of the superfamily Cyrtospiriferoidea dominated in the assemblages in the Late Devonian, and became extinct by the end of the Devonian, while the Early Carboniferous is dominated by the superfamily Spiriferoidea. Cyrtospiriferoids represented a complicated subtype of the anchored ecological type ethologically associated with hard substrates, while the spiriferoids represented the main subtype of the anchor ecological type and the weighted subtype of the free-lying ecological type, replacing each other in ontogeny of brachiopods inhabiting mostly soft substrates. Although the Devonian-Carboniferous boundary in Transcaucasia lies within a continuous succession of lithologically uniform carbonate and carbonate-terrigenous rocks, a change in the diversity and ecological type of spiriferids indicates a change in the density of the substrate. This corresponds to a general global shift from the Devonian calcite sea to an aragonite Carboniferous sea, which affected the density of marine substrates. Thus, the stratigraphic procedure in this case can include the diversity analysis of both the taxonomic and ecological compositions of brachiopods.

The Turonian-Coniacian deposits of the south-west of the Ulyanovsk-Saratov trough

Integrated study of a series of the Turonian and Coniacian complete geological records has revealed the stratigraphic completeness and specified the faunal contents of the deposits. The south and the southwest of the Ulyanovsk-Saratov trough have been shown to comprise the thickest beds of the studied interval represented by carbonate and carbonate-terrigenous rocks. Northwards, in the zone of the Saratov dislocations, the Turonian – Coniacian bodies are peculiar for terrigenous and terrigenous-carbonate compositions. The beds are ubiquitously monotonous, which complicates identifications at the stage and the substage levels. Distributions of the benthic foraminifer assemblages and the data on the finds of cephalopods, inoceramus, echinoderms, brachiopods and siliceous sponges have allowed to distinguish detailed biostratigraphic units at the level of biozones traced within the examined structural units. An attempt has been made to reconstruct the conditions in the marine environment.

Spatial-temporal and genetic relationships between gold and antimony mineralization at gold-sulfide deposits of the Ob-Zaisan folded zone

The Ob-Zaisan folded zone is a fragment of a single structure composed of Paleozoic sedimentary and volcanogenic rocks (mainly black shale), which was formed at the margin of the Siberian continent and features a common set of magmatic complexes and mineral systems. However, there are some differences that determine the specific geological and metallogenic features of the Irtysh-Zaisan and Kolyvan-Tomsk fragments of the Ob-Zaisan folded zone. In the gold deposits of the West Kalba and Kolyvan-Tomsk auriferous belt, the main gold-sulfide mineralization is controlled by zones of shearing and dynamic metamorphism in carbonaceous carbonate-terrigenous rocks. This type of mineralization was formed in tectonic blocks in a compressional setting. Antimony mineralization is characterized by brecciated textures and the vein-like morphology of ore bodies, reflecting extensional tectonics. At some deposits (Zherek, Mirazh, Dalny), Sb mineralization is spatially separated from the main gold-sulfide ores and shows cross-cutting relations to the principal ore-controlling structures. In other gold deposits, stibnite is spatially associated with disseminated gold-sulfide ores and forms mineral assemblages with Ni, Co, Au, Pb, and Fe (Alimbet, Zhanan, Legostaevskoe, Semiluzhenskoe, and Kamenskoe deposits). This study reveals no direct correlation between Au and Sb in gold-sulfide ores of these deposits. SEM analysis indicated the absence of free gold in stibnite veins. However, atomic absorption and electron microprobe analysis indicated the presence of “invisible gold” from a few ppm to several tens of ppm in the stibnite. High gold contents in the gold-sulfide ores overprinted by antimony mineralization (Suzdalskoe, Zhanan, and Legostaevskoe deposits) can be explained by the processes of regeneration and redeposition. The results of microstructural observations, isotope geochronology, studies of mineral assemblages and fluid inclusions in the ores from gold deposits of the Ob-Zaisan folded zone suggest that antimony mineralization was formed at gold-polysulfide stage, which was separated from the early ore pyrite-arsenopyrite stage by a 30 Ma time gap. It can be assumed that the essentially stibnite mineralization was formed at a separate stage and was separated from the gold-polysulfide mineralization by a 7 Ma interval of tectonic activity. Our Ar-Ar data on sericite from ore samples, combined with U-Pb data on zircons from igneous rocks and previous data from the literature show that there were two major stages of ore formation: the Early Permian (282-270 Ma) and the Early Triassic (250-240 Ma). Most researchers suggest that these stages of mineralization can be related to the epochs of intraplate magmatism that led to the formation of the Tarim (280 Ma) and Siberian (250 Ma) large igneous provinces. These global geological events are generally connected with the influence of Tarim and Siberian mantle plumes.

Geochemical aspects of the assimilation of host rocks by basaltic magmas during the formation of Noril’sk Cu-Ni ores

In models for the genesis of the Noril’sk Pt-Cu-Ni ore deposits, much importance is attached to the processes of assimilation of host rocks by basaltic melts. This idea is based on unusual relations between the silicate and sulfide constituents of this type of ore deposits and also on the heavy sulfur isotopic composition of the sulfide ores. The reason for this unusual composition is thought to be the assimilation of anhydrite from the host rocks. However, no other factors able to influence this process have ever been analyzed in the literature. We were the first to thoroughly analyze the inner structure of contact aureoles of the intrusions hosted in various rocks: the Maslovsky intrusion in Early Triassic basalts of the Ivakinsky and Nadezhdinsky formations and the Talnakh intrusion in Devonian anhydrite-bearing carbonate-terrigenous rocks. The distributions of trace elements, the 87Sr/86Sr isotopic ratio, and Sm and Nd isotopes indicate that host rocks were either not assimilated at all, or their effect is perceptible only within a very narrow (1 m) zone in the eastern apophyse in the southern portion of the Maslovsky intrusion. The Sr, Nd, and particularly, Pb isotopic composition indicate that the anhydrite could not be the source of isotopically heavy sulfur for sulfides at Noril’sk deposits. The ores of the Maslovsky and Talnakh intrusions have similar sulfur isotopic composition of their sulfides (the maximum δ34S values of these sulfides reach +10.8 and +14.2‰, respectively), in spite of the significant differences in the rocks hosting these intrusions. Our newly obtained data indicate that assimilation was insignificant and could not affect the origin of the ores.

Supergene processes and uranium ore formation in the Ronneburg ore field, Germany

The Ordovician-Lower Carboniferous sequence of slightly metamorphosed gray carbonate-terrigenous rocks contains the Silurian black cherty shales enriched in carbon (6–9%), pyrite (6–7%), and uranium (∼30 ppm). The uranium ore is localized at the pinch-out of areal and linear zones of the Early Permian supergene (exogenic) oxidation of rocks expressed in reddening (hematitization). U, As, Sb, Cu, Ni, Mo, and Ag have been removed from the oxidized black shales and concentrated in the cementation zone in form of pitchblende and sulfides in wall-rock disseminations and veinlets largely hosted in carbonate-bearing rocks. In the Late Permian, during deposition of the upper Rotliegende and Zechstein, the fractures in the basement were filled with carbonates and sulfates; uranium was partly redeposited along with enrichment in Pb and Zn. Mesozoic and Cenozoic supergene processes altered uranium ore insignificantly.

Genetic suppositions of oil and gas formation and conditions of hydrocarbon deposits in Mesozoic-Cenozoic sediments in the Caspian Sea shelf

Source rocks are present in all the stratigraphic units of the Mesozoic-Cenozoic deposits of the Turkmen shelf in the Caspian Sea. The highest source rock potential is characteristic of I terrigenous and carbonate-terrigenous rocks of Middle Jurassic age, and the Aptian, Paleogene, Neogene and Apsheron stages of the Quaternary system. To reveal the formation conditions of hydrocarbon deposits within Southwestern Turkmenistan, the principle of chemical thermodynamics is primarily used via the calculation of the free energy of gaseous hydrocarbons. This showed that hydrocarbon deposits within the topmost depositional complex (the red-colored strata of the Middle Miocene at the Akchagyl and Apsheron stages) are syngenetic; their formation was due to lateral migration from even-aged deposits from depression zones neighboring the uplifts.

Riphean stromatolitic formations fringing the East European platform

Riphean stromatolitic formations flank the East European epi-Karelian platform only in the east and northeast. They are traceable as long (over 3600 km) relatively narrow belt consisting of two rectilinear segments, one running along the Urals western flank from southern extremity of the Bashkirian meganticlinorium to the Polyudov Ridge and the other one extending from the southern and central Timan to the Kil’din Island and northern Norway. Within the belt there are known stromatolitic formations of all Riphean erathems: the Lower and Middle Riphean stromatolitic buildups are confined to the eastern segment of its southern part only, while the Upper Riphean occur everywhere. Their distribution conformable to large structural elements of the plaform margin being replaced by carbonate-terrigenous rocks almost lacking stromatolites westward and southwestward in the Kama-Belaya aulacogen system and by substantially siliciclastic succession eastward and northeastward. The distribution area of Upper Riphean stromatolitic formations includes the Karatavian stratotype region, where 12 stromatolite beds ranging in age from ≥900 to 620 Ma are established. Many of the beds are traceable along the strike far beyond the stratotype region. Representing relatively small reference units, the beds facilitate reconstruction of distribution dynamics of the Upper Riphean stromatolites. Distribution area of the latter was always parallel to marginal structures of the platform, though being of changeable size, particularly of length. Originated in the stratotype region eastern part, stromatolites first advanced into northeastern areas never crossing boundaries of the Upper Riphean distribution area during the Early Karatavian. In the initial Late Karatavian, they occupied a longest distribution area that was sharply reduced at the end of that period. According to distribution peculiarities in space and with time, the Upper Riphean stromatolitic formations accumulated likely in peripheral areas of an open sea or oceanic basin adjacent to the East European platform, rather than in closed epiplatform basins.

Geological complexes in the margin of the Siberian Craton as indicators of the evolution of a Neoproterozoic Supercontinent

This paper describes the Late Precambrian geologic complexes from the southern margin of the Siberian Craton, associated with the extension epochs. Analysis of the data available suggests that there were two episodes of intracontinental breakup, which resulted in the opening of the ocean (1300-900 and 850-550 million years ago). The time sequence of the rift-related volcanic rocks and terrigenous deposits ! basic dike swarms ! carbonate-terrigenous rocks ! ophiolites and island-arc rocks reflects the successive change of geodynamic environments in the marginal part of the craton. The stage of intracontinental rifting was superseded by the stage of advanced rifting which preceded the continent break and the formation of oceanic crust. This period was followed by two phases of oceanic evolution: a passive phase (sedimentary rocks of the passive margins) and an active phase (island arcs, backarc seas, and the like). Several dierent versions are oered and discussed for the extension processes in the southwestern and southeastern parts of the Siberian Craton is association with the breakup of the Rodinia Continent.