ALBERT

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉The results of lithological, petrographic, and C–O isotope studies of various types of the diagenetic and postdiagenetic carbonate cement in silty mudstones, as well as coeval limestones, in postglacial deposits of the Dal’nyaya Taiga Group (Patom Basin, Siberia) are presented. Evidence of methane generation but not its anaerobic oxidation has been obtained. High δ〈sup〉13〈/sup〉C (up to 14.9‰) values of the earliest carbonate cement generations in mudstones from the Barakun and Ura formations are related to extreme isotopic fractionation during the biogenic methanogenesis. The absence of isotopic signatures of the anaerobic oxidation of methane indicates its isolation during the subsequent reactions, suggesting the accumulation of gas hydrate compounds inside the sedimentary layers. Analysis of the distribution and correlation of δ〈sup〉18〈/sup〉0 and δ〈sup〉13〈/sup〉C in the authigenic and sedimentary carbonates provides new arguments in favor of link of the Ediacaran global δ〈sup〉13〈/sup〉C trend with epochs of the predominant accumulation of gas hydrate hydrocarbon compounds inside the deep-sea sedimentary strata and their subsequent anaerobic oxidation.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉The geothermal and hydrochemical parameters of a large hydrogeothermal system located in the city of Velingrad and its environs (southwestern Bulgaria) are discussed. The article summarizes the results of its research, including those obtained by 2019. Temperature of the discharging thermal waters varies from 37 to 94°C. The hotter mineral waters have SO〈sub〉4〈/sub〉–HCO〈sub〉3〈/sub〉/Na composition with pH ≈ 8.0, while the less hot ones are of HCO〈sub〉3〈/sub〉–SO〈sub〉4〈/sub〉/Na composition with pH up to 9.6. The geothermal and geochemical features of various parts of the system are revealed, indicating an increase in the mineralization of thermal water with increasing temperature.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉The coastal-marine rare metal–titanium placers of the West Siberian megaprovince were formed in a relatively short Eocene–Late Oligocene interval. A similar formation mechanism and commonality of primary sources of ore matter were responsible for the single type and similarity of mineral composition of placers in the megaprovince. Based on the comparison of various characteristics of these placers, common factors determining their industrial value were identified: the structural plan and tectonic movements of the platform basement governed the facies zonation, and lithological and technological characteristics of placer components and host rocks; and postdepositional tectonic processes influenced the position (depth) of placers in the sedimentary cover and the characteristics of ore sands (degree of their secondary supergene alterations). The geological factors, which make it possible to predict mining parameters of placers, technological characteristics of ore sands, quality of mineral concentrates, and geoeconomic indicators of the rare metal–titanium placer deposits, are defined.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉Clay minerals in basalt sills from the northern East Pacific Rise, covered by Upper Pleistocene clayey–sandy–silty sediments (turbidites, hemipelagites, and diatom oozes) sampled from DSDP Holes 477, 478, and 481A, were studied by X-ray methods based on the modeling of diffraction patterns. Trioctahedral smectites formed in thin (0.1–0.5 and 1.8–4 m) fissured sills that are well permeable for water delivered from the water-saturated host sediments heated by the sills. Smectites in basalts are found in the interstices (20–40 vol % in the rock and up to 50–80 vol % in hyalobasalts). They replace olivine and fill cracks and vesicles. Plagioclase and clinopyroxene are generally unaltered. The structure of smectites is characterized by different height of layers depending on the composition of cations in the interlayers and the degree of their hydration. The different-height layers either make up individual smectite phases or alternate in different proportions and with a high degree of segregation in the mixed-layer structures. Under conditions of a prolonged cooling, thick (43 m) sills are distinguished from the thin varieties by the formation of trioctahedral minerals (smectite–chlorite, chlorite, defective chlorite, and talc). They are found mainly in the interstices that make up 3–7 vol % of the rock.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉The study of structural features along with carbon and oxygen isotope compositions made it possible to discriminate lithological rocks in deposits of the Karatau phosphorite basin (Kazakhstan) into the following types: high- and low-grade phosphorites; unaltered and altered dolomites (with and without phosphate) hosting phosphorites of the Chulaktau Formation (lower dolomite and Fe–Mn horizons); dolomites of the underlying and overlapping rocks of the Malokaroi and Dzhanat groups and the Shabakta Formation; calcareous schists, sandstones, as well as secondary calcite. The carbonate material in rocks of the studied deposits is heterogeneous and characterized by wide variations of the δ〈sup〉13〈/sup〉C (–7.6 to 1.6‰ PDB) and δ〈sup〉18〈/sup〉O (15.4 to 27.0‰ SMOW) values. The results obtained suggest that phosphorites and host rocks underwent intense postsedimentary transformations. Isotopic (δ〈sup〉13〈/sup〉C, δ〈sup〉18〈/sup〉O) composition of the carbonate material in rocks of the siliceous and productive (phosphorite) horizons, as well as, to a large extent, the Fe–Mn horizon of the Chulaktau Formation is nonprimary and related to the presence of carbonates of superimposed (secondary) generations during the late diagenesis or catagenesis. In this regard, data on the carbon isotope composition obtained for rocks of the studied Karatau deposits cannot be used for the chemostratigraphic reconstructions.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉In 13 out of 14 samples of carbonate xenoliths from mud volcanic breccias of Azerbaijan, the 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr values (0.70675–0.70823, 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr〈sub〉md〈/sub〉 = 0.7073 ± 0.0005) are within the same limits as in the volcanic waters (Bujakaite et al., 2019). Genetically, they can be associated both with Mesozoic carbonate and saline deposits or with Cenozoic volcanosedimentary strata. Similarity of the Sr isotopic composition with isotopic values in carbonate extracts from terrigenous rocks of the Maikop Formation was noted only in the calcite sample from the Cheildag Volcano (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr = 0.71047). The most interesting results with the lowest Sr ratios were obtained for carbonate fragments from breccias of the Byandovan Volcano. The combination of low δ〈sup〉13〈/sup〉C values (down to –49.2‰), high Sr contents (up to 3108 μg/g) and 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr (0.70675‒0.70690) values typical of the Late Jurassic brings them closer to caprocks of salt domes of Jurassic oil-bearing formations in the western United States.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉Distribution of carbonate sediments of different composition in the Precambrian section is reviewed. It is shown that calcium carbonates prevailed in the Archean–initial Early Proterozoic, and iron carbonates dominated in the banded iron formations. Dolomites and magnesites were more widespread in the Proterozoic. Such distribution suggests an acid reductive geochemical environment in the Archean–Early Proterozoic, which determined the corresponding character of continental weathering: appearance of dissolved forms of Fe and silica that were transported to ancient oceans and deposited, resulting in the formation of ferruginous–quartzites or jaspilites (banded iron formations). Large-scale and almost explosive development of photosynthesizing cyanobacteria at the Archean–Proterozoic boundary stipulated, first, the utilization of carbon dioxide and, correspondingly, the change of acid environment into alkaline, leading to the formation of magnesian carbonates; and, second, to the appearance of free oxygen.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉A complex of lithological-geochemical studies was carried out in rocks of the Upper Jurassic–Lower Cretaceous Bazhenov Formation and their transition zones to host rocks: composition of rocks, ratio of organic carbon and sulfide sulfur (C/S values), and distribution of several redox indicators (degree of pyritization, content of the authigenic uranium, and values of Mn/Al and Mo/Mn ratios). Morphology of pyrite was examined with a scanning electron microscope. Two main types of pyrite were revealed: (a) framboidal pyrite formed during early diagenesis with the participation of bacterial activity; (b) cryptocrystalline pyrite formed during diagenesis (early stage included) and, presumably, mesocatagenesis, i.e., medium substage of catagenesis. It was established that some part of the cryptocrystalline pyrite predated the framboidal variety. Pyritization of rocks with the cryptocrystalline pyrite formation took place at the lower and upper boundaries of the Bazhenov Formation in the over- and underlying rocks, where the low-carbon pyritic rocks are formed. Intense pyritization is also observed in the high-carbon rocks near the top of the Bazhenov Formation, where the pyrite–kerogen rocks are formed. Both low-carbon pyritic and pyrite–kerogen rocks are characterized by C/S 〈 1.5; rocks of the Bazhenov Formation, by C/S 〉 2. The low-carbon pyritic and pyrite–kerogen rocks occur near boundaries of the lithologically different members (mainly biogenic rocks in the Bazhenov Formation and terrigenous varieties in the host rocks) that were deposited under different redox conditions existing in the water column and near the marine paleobasin floor. Boundaries of such members could serve as geochemical redox barriers at late stages of lithogenesis. The pyritic and pyrite–kerogen rocks were likely pyritized during diagenesis because of the deposition of pyrite on the geochemical redox barriers from fluids that contained iron sulfides and migrated from the high-carbon Bazhenov sequence. Presumably, pyritization continued during the catagenesis owing to thermogeochemical processes of the organic matter transformation.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉The Haléo-Danielle Plateau bauxite deposit forms the largest part of the 15 km long Minim-Martap Bauxite ore district in the Adamawa Region of Cameroon. The plateau is covered with thin Upper Pleistocene to Holocene alluvial layer. It covers by laterites and bauxites formed on Tertiary-Quaternary basalts. Gibbsite and boehmite are the aluminum oxide found in the bauxite, while goethite is the main oxyhydroxide iron mineral. Bauxite rock samples from the plateau were collected and investigated with the XRF technique for their chemical composition and parent rocks. Chemical investigation reveals average concentration of the major ore components as: Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉 (43.73%), Fe〈sub〉2〈/sub〉O〈sub〉3〈/sub〉 (24.53%), SiO〈sub〉2〈/sub〉 (2.12%), and TiO〈sub〉2〈/sub〉 (3.54%). Abundant trace and rare elements include Zr (667.25 ppm), V (446.4 ppm), Ce (107.93 ppm), Sr (98.46 ppm), Nb (92.1 ppm), La (58.05 ppm), Ga (55.3 ppm), Ba (53.53 ppm), Nd (37.96 ppm), and ΣREE (245.8%). Calculations of the degree of chemical weathering (Ruxton Ratio) and geochemical coefficient (CIA) indicate that plateau rocks were subjected to intensive weathering, which led to the formation of industrial bauxite deposits. According to the results of processing the chemical composition of bauxites using three independent classification systems, plateau deposits can be classified as ferruginous bauxites. Parent rocks investigation indicates that bauxite was formed due to the decomposition of magnesium-ferrous andesite-basalts under conditions with a near neutral redox potential.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉The Briketno-Zheltukhinskoe U–Mo–Re deposit is located in the southern sector of the Moscow lignite basin (central East European Platform). It is restricted to the paleochannel and deltaic coal-bearing sandy sediments of the Visean Bobrikov horizon lying on the Famennian limestone. They are overlain by the younger Neogene–Quaternary sands and clays. Tectonically, the deposit is ascribed to the southern shoulder of the Pachelma aulacogen crosscut by the SN-trending fault zone. The presence of a thick gray-colored unit of inequigranular moderately and poorly sorted sands containing the coalified plant detritus served as the main lithological-facies factor for the localization of U–Mo–Re mineralization. The Bobrikov sandy formation was accumulated in the fluvial, swampy, onshore deltaic, and shallow-sea lagoonal environments with warm humid climate. A combined syn-epigenetic model is proposed to explain the formation of the U–Mo–Re paleovalley-type deposit. Subeconomic stratabound U (±Mo, Re) mineralization associated with the carbonaceous and clayey horizons was accumulated during sedimentation and diagenesis. During epigenesis, the U–Mo–Re ores were deposited by oxidized groundwater owing to its lateral filtration through highly permeable sand horizons and metal accumulation at the reducing and sorption barriers. The general subhorizontal stratiform structure of the ore deposit is complicated by the subvertical morphological (swell of sand horizon plus strong pyritization) and concentration (Re + Mo column-like maximums) ore shoots. They occur in dome-like structures that disturb the horizontal structure of the host sedimentary unit. It is possible that these domes and ore shoots are the result of hydrothermal-hydraulic processes caused by the activity of deep crustal (likely, mantle as well with allowance for the Re mineralization) fluids.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉Morphology of the Rio Grande Rise and the acoustic structure of different types of deposits in its uppermost sedimentary cover were discussed based on high-resolution seismoacoustic profiling of cruises #32 (2010) and #52 (2016) of R/V 〈em〉Akademik Ioffe〈/em〉. Slopes of the Rio Grande Rise are composed mainly of landslide deposits and gravitites, but contourite sedimentation is possible on its southern slope. Contourite sedimentary waves and, probably, small drifts are identified in the Cruzeiro do Sul Trough at the top of the Rio Grande Rise. Mixed gravitite–contourite sedimentary systems seem to be located at the foot of northern and southern slopes. The downslope density flows and the Antarctic Bottom Water (AABW) contourite current are responsible for the formation of these features.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉New data on the 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratio variations in carbonate rocks of the Dal’nyaya Taiga Group in the Vendian reference section (Ura Uplift, Central Siberia) are presented. The 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr variations correlate with the sequence-stratigraphic scheme reflecting evolution of the paleobasin, δ〈sup〉13〈/sup〉C variations, and paleontological data. Limestones with a high (〉1050 µg/g) Sr concentration, which do not show any signs of alteration of the Sr isotope system, were used for reconstructing the secular 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr variations. Increase in the 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratio from 0.70755 to 0.70823 complicated by subordinate fluctuations was established in seawater during the accumulation of the Dal’nyaya Taiga Group carbonate rocks. Duration of the postglacial deposition of the Dal’nyaya Taiga Group is estimated at no less than 14–15 Ma. Secular variations of the 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratio measured in the Dal’nyaya Taiga Group limestones are similar to changes of this ratio after the Marinoan glacial period. The Sr and C isotope correlations suggest a prolonged hiatus between the Dal’nyaya Taiga and Zhuya groups.〈/p〉 〈/span〉

Description: 〈span〉 〈h3〉Abstract〈/h3〉 〈p〉The article presents data on numerous findings of authigenic K-feldspars in the Lower–Upper Carboniferous carbonate and terrigenous rocks in the southern limb of the Moscow syneclise, including the Oka–Tsna bar. In terms of the composition and crystalline structure, all feldspar morphotypes (idiomorphic monocrystalline fringes on the surface of sandy clastic grains, fine-crystalline aggregates, pseudomorphously replacing organogenic carbonate detritus, and pseudoskeletal crystals) are composed of K-sanidine with the overwhelming predominance of K over Na and extremely low concentrations of Ca, Ba, and Sr. As can be seen from 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dating, the authigenic sanidine mineralization took place during the Permian, i.e., 20–55 Ma later than the accumulation of host Carboniferous rocks. Mineralogical observations, as well as paleofacies and paleogeographic reconstructions, suggest that the formation of authigenic K-feldspars is associated with the gravitation-brine catagenesis in the southern limb of the Moscow syneclise as a result of the seepage of high-mineralization brines from Permian evaporate marine basins into the underlying older rocks.〈/p〉 〈/span〉