The noble metal distribution in the black shales of the Degdekan gold deposit in Northeast Russia

The mineral and chemical composition of the carbon-bearing rocks of the Late Permian Pionerskaya Formation containing the Degdekan gold deposit has been studied. The bulk contents of Au, Ag, Pt, and Pd in the black shales and their light, sulfide, and electromagnetic fractions were determined by electrothermal atomization. The mineral composition and the phase analysis of the rocks were studied using a scanning electron microscope. Gold is present as fine xenomorphic grains of high fineness with an Fe admixture of up to 4 at %, as well as intergrowths of kustelite and electrum. The Au and Pt contents in the black shales and ores vary in a wide range (g/t): Au 0.01–13.12, Pt 0.001–1.34. The highest Au contents (up to 1748 g/t) were noted in the sulfide fraction. The Pt-bearing phases were not found, whereas a Pt content of about 0.61 wt % was determined using an electron microscope in a carbonaceous matrix. The initial rocks have a steady and low Pt content (less than 0.007 g/t). A stable even Au distribution in the studied rocks was established within 1.14–2.46 g/t. The chemical analysis of the soluble fraction of the carbonaceous matter extracted from the black shales showed the presence of Au 0.375, Ag 3.68, Pt 0.147, and Pd 0.052 g/t. It has been concluded that the carbon-bearing rocks of the Pionerskaya Formation play a resource role in the accumulation of noble metals, whereas economic concentrations of the latters are formed in the course of the superimposed metamorphic-hydrothermal processes.     

Volgian–Early Berriasian Marginal Filter in the West Siberian Marine Basin and Its Influence on Sediment Distribution

Lithology and Mineral Resources - The West Siberian marine basin of the Volgian–initial Berriasian ages is described. It is shown that a marginal filter (according to A.P. Lisitsyn)...     

Biogeochemistry and microfossils of the Upper Jurassic and Lower Cretaceous,Anabar Bay,Laptev Sea

A detailed analysis of organic matter from the Oxfordian-Lower Valanginian interval of the Nordvik section (Anabar Bay) allows the definition of three geochemical horizons (terpane, diasterene, and hopane), which are characterized by specific geochemical compounds and their ratios. These horizons are correlated with several stages in the evolution of microfossils associated with ecological and geochemical changes in sea paleobasin. Our study shows a good correlation among the variation in many geochemical parameters, the composition of microfossil assemblages, and the transgressive-regressive phases of the paleobasin evolution. Moderately shallow-water facies was reconstructed using micropaleontological, palynological, and lithological data from the upper and lower parts of the section, where the terpane and hopane horizons were identified. Both horizons are characterized by a general dominance of hopanes over other polycyclic biomarkers, the presence of compounds with the biological configuration and organic matter of mixed terrestrial-marine origin, the presence of benzohopanes and retene, an indicator of the conifer resins. This is also reflected in the composition of palynological assemblages, which are dominated by terrestrial palynomorphs, with rare conifer pollen grains. Relatively deep-water facies were identified in the middle part of the section (diasterene horizon). A distinctive geochemical feature of this interval is high relative abundances of diasterenes and 4-methyldiasterenes. The lower subhorizon is characterized by the highest values of the pristane/n-C₁₇ ratio and relatively light δ¹³C values of noncarbonate carbon. The aromatic fractions have anomalously high concentrations of methyltrimethyltridecylchromans (MTTCs), which are interpreted to be derived mostly from chlorophyll of prasinophytes. The terpenoid distribution is marked by the presence of neohop-13(18)-enes, which probably originated from the activity of methanotrophic bacteria. The above geochemical parameters indicate high photic zone productivity (which is confirmed by the composition of palynological assemblages with abundant dinocysts and prasinophytes) that favored the accumulation of organic matter in dysaerobic conditions, which periodically occurred in the bottom of paleobasin. The alternation of dysaerobic and low-oxic bottom water conditions is easily reconstructed from the analysis of microbenthic communities. The analysis of biogeochemical, micropaleontological, paleontological, and palynological data on the Upper Volgian-lowermost Berriasian organic-rich shales revealed a very good source rock potential for this part of the section, as indicated by relatively high concentrations of organic matter of mixed bacterial-algal genesis and stagnant conditions during deposition and diagenesis. However, this potential was not realized because of the extremely “mild” thermobaric conditions that existed within the Mesozoic strata of the present-day Anabar Bay. At the same time, our results and analysis of the available data allow an optimistic assessment of the hydrocarbon potential of Jurassic-Lower Cretaceous deposits on the Laptev Sea shelf.     

Mid-Cretaceous Tuor-Yuryakh Section of Kotelnyi Island,New Siberian Islands: How Does the Probable Basement of Sedimentary Cover of the Laptev Sea Look on Land?

The model of geological structure of sedimentary cover of the Laptev Sea accepted by most geologists suggests that the lower seismic complex of the cover begins by the Aptian–Albian sedimentary rocks. They can be studied in natural outcrops of Kotelnyi Island. The section of the Tuor-Yuryakh Trough, which exposes the lower part of the Cretaceous complex, is described in the paper. It is composed of continental coaliferous rocks ~100 m thick. The marking beds divide it into five members, which are traced along the western wall of the trough at the distance up to 3 km. The spore–pollen complexes and plant megafossils indicate that almost the entire visible section of the mid-Cretaceous is Albian. Only its lower part no more than 14 m thick can probably belong to the Aptian. Marine facies with Albian foraminifers were found 15 m above the bottom of the Cretaceous complex. The section of the Cretaceous rocks is underlain by the Lower Jurassic marine clays and siltstones. The foraminifer assemblages of this part of the section are typical of the upper Sinemurian–Pliensbachian and fossil bivalves indicate late Sinemurian age of the host rocks. The hiatus ~70 Ma duration has no expression in the section and this boundary can de facto be substantiated only by microfossils. This vague contact between the Lower Jurassic and mid-Cretaceous rocks does not correspond to geophysical characteristics of the bottom of the lower seismic complex of the cover of the eastern part of the Laptev Sea. The latter is described as the most evident seismic horizon of the section of the cover, suggesting unconformable occurrence of the lower seismic complex on a peneplenized surface of lithified and dislocated rocks. This is mostly similar to the bottom of the Eocene sediments, which were observed on Belkovsky and Kotelnyi islands. The paper discusses possible application of our land results for interpretation of the shelf seismic sections of the Laptev Sea. It is concluded that local reasons are responsible for a vague boundary between the Lower Jurassic and mid-Cretaceous sequences in the section studied. Our observations support ideas on possible Aptian–Albian age of the rocks of the basement of the lower seismic complex; however, it is proposed to use also the previously popular idea on the Eocene age of the lower seismic complex of sedimentary cover of the eastern part of the Laptev Sea as one of the possible working scenarios.     

The Hettangian–Early Pliensbachian Evolution Phase of the Anabar–Lena Sedimentary Basin,Nordvik Facies Region

Lithology and Mineral Resources - New data on the structure and formation conditions of the Anabar–Lena sedimentary basin in the Nordvik facies region at the beginning of the Early Jurassic...     

Jurassic and Cretaceous stratigraphy of the Anabar area (Arctic Siberia,Laptev Sea coast) and the Boreal zonal standard

Recent integrated studies of Mesozoic reference sections of the Anabar area (northern Middle Siberia, Laptev Sea coast) and the reinterpretation of all the previous data on a modern stratigraphic basis permit considerable improvement of the bio- and lithostratigraphic division and facies zoning of Jurassic and Cretaceous sediments in the region. Analysis of abundant paleontological data allows the development or considerable improvement of zonal scales for ammonites, belemnites, bivalves, foraminifers, ostracods, dinocysts, and terrestrial palynomorphs from several Jurassic and Cretaceous intervals. All the zonal scales have been calibrated against one another and against regional ammonite scale. Reference levels of different scales useful for interregional correlation have been defined and substantiated based on the analysis of lateral distribution of fossils in different regions of the Northern Hemisphere. It provides the possibilities to propose and consider parallel zonal scales within the Boreal zonal standard for the Jurassic and Cretaceous periods. A combination of these scales forms an integrated biostratigraphic basis for a detailed division of Boreal-type sediments regardless of the place of their formation and for the comparison with the international stratigraphic standard as far as a possible use of a set of reference levels for correlation.     

Lower–Middle Jurassic foraminiferal and ostracode biostratigraphy of the Barents Sea shelf

The Barents Sea shelf is an attractive target as a prospective large petroleum province. Further development of geological and geophysical exploration in the area requires high-resolution biostratigraphic constraints and update stratigraphic charts. The zonal succession of Lower and Middle Jurassic assemblages of foraminifers and ostracodes of the Barents Sea fits well the division for northern Siberia based on correlated independent Jurassic and Cretaceous zonal scales on all main microfossil groups, of which some scales were suggested as the Boreal Zonal Standard. The stratigraphic range of the Barents Sea microfossil assemblages has been updated through correlation with their counterparts from northern Siberia constrained by ammonite and bivalve data. Joint analysis of foraminiferal and ostracode biostratigraphy and lithostratigraphy of the sections allowed a revision to the stratigraphic position and extent of lithological and seismic units. The discovered similarity in the Lower and Middle Jurassic lithostratigraphy in the sections of the Barents Sea shelf and northern Siberia, along with their almost identical microfossil taxonomy, prompts similarity in the Early and Middle Jurassic deposition and geological histories of the two areas.     

Stratigraphy and ecostratigraphic distribution of foraminiferal morphogroups from the Upper Jurassic of the Makar’yev section (Unzha River,Volga River basin)

Investigation of the Upper Callovian to Lower Kimmeridgian microfossils from the Makar’yev reference section (Unzha River, East European Platform) has been carried out. The section is characterized by ammonite debris and abundant associations of benthic and planktic foraminifers. It is a perfect object for stratigraphic and paleoecological researches. The biostratigraphic distribution of foraminifers from the Makar’yev section allows one to identify standard foraminifera zones of the East European Platform, as well as to upgrade some of them. The analysis of vertical and lateral ammonites and foraminiferal distribution, completed with litho stratigraphy, has precised the stratigraphic volume and position of boundaries of several lithological units.An improved stratigraphic scheme for the Kostroma area of the Moscow Depression is proposed. Analysis of the composition, structure, and dynamic changes of the foraminiferal assemblages has been performed. The morphofunctional analysis of foraminiferal genera has for the first time identified how foraminiferal morphogroups differing in their life style and feeding strategy varied with short-term paleoenvironmental changes. These morphogroup changes allow establishing four ecostratigraphic levels. These paleoecological data have been calibrated along with geochemical factors. They have shown a crisis of foraminiferal association during the Late Oxfordian and Early Kimmeridgian. A similar crisis has also been discovered in the north of Siberia, which may be an argument for its global distribution. The analysis of the taxonomic composition and the density of foraminiferal associations, in parallel with the structure of the association, has revealed a succession of transgressive and regressive events during the Late Callovian-Early Kimmeridgian. It allows the typification of each assemblage in relation with each event and underlines the occurrence of second-order sea-level fluctuations (middle part of the Middle Oxfordian and the earliest Kimmeridgian).     

The first identification of hydrogeochemical indicators of mud volcanic activity

Hydrogeochemical monitoring has been implemented at a mud volcano for the first time in the world. Measurements were carried out during the field season of 2015 at five gryphons of the South Sakhalin mud volcano with various degrees of activity. Statistically significant differences in the chemical compositions of mud-volcano waters from different gryphons are established. These differences are determined by the activity of the gryphons.