Background. The Witwatersrand gold province located in South Africa is the richest in the world. The Witwatersrand deposits are composite, where osmirids and diamonds are mined along with gold, silver and uranium. The genesis of the Witwatersrand deposits is controversial. Most local geologists support the hypothesis of the presence of paleoplacer deposits with subsequent metamorphic transformation of ore-bearing conglomerates. In addition, there are aeolian, hydrothermal-sedimentary, hydrothermal-sedimentary-metamorphic and magmatic models of ore formation.

Aim. To establish the genesis of the Witwatersrand deposits.
Materials and methods. Analysis of published literature and factual data.
Results. Osmirids and diamonds are mined along with gold, silver and uranium from the composite Witwatersrand deposits. Such a set of useful components is not known in any of the deposits of magmatic or hydrothermal genesis. Considering the confinement of useful components to conglomerates, the detrital nature of most of the gold grains and the presence of various accompanying minerals in the heavy fraction, characteristic of igneous rocks of felsic, basic and ultrabasic composition, the hypothesis of the primary alluvial nature of the deposits of the Witwatersrand province looks the most reasonable.
Conclusions. The factual material indicates an alluvial origin of the Witwatersrand deposits with subsequent metamorphic transformation of ore-bearing reefs. The primary alluvial formation of ore-bearing conglomerates is indicated by the confinement of gold and uranium to channel facies with a tendency to accumulate in the basal horizons of the reefs, the presence of rounded gold particles bearing the traces of transportation in alluvial flows, as well as a set of minerals in the heavy fraction of concentrate, characteristic of placers.

Background. Three prospective sedimentary complexes — Aptian-Upper Cretaceous, Paleogene and Neogene — are predicted in the waters of the Eastern Arctic seas. Here, the search for oil and gas is associated with harsh Arctic conditions at sea, as well as with high geological risks and significant expenditures under the conditions of poor knowledge of the region. In this regard, the localisation of prospecting drilling objects and the assessment of the geological risks of deposit discovery should be carried out.
Aim. To assess geological risks and to determine the probability of discovering oil and gas fields, as well as to identify prospective areas for licensing and exploration in the water areas of the Eastern Arctic.
Materials and methods. Structural and heat flow maps along with the results of geochemical analysis, as well as typical terrestrial sections were used as initial materials. Using the method of basin analysis, the modelling of generation-accumulation hydrocarbon systems (GAHS) and the quantitative assessment of its hydrocarbon potential in the Eastern Arctic water area was carried out. The assessment of geological risks and the probability of field discovery was performed using the conventional methodology widely applied by oil companies.
Results. The GAHS modelling using a variation approach showed that, regardless of the kerogen type, with average values of С_org in sediments, potential oil-and-gas source strata (OGSS) were capable of saturating the prospective objects with hydrocarbons. The “OGSS assessment” factor was determined as “encouraging” (0.7). Active geodynamic regime and the manifestation of several folding phases within the study area provided favourable conditions for the formation of anticlinal
traps in sedimentary basins. However, the cap rock quality rating was assessed as “neutral” (0.5). The overall risk for the “Trap assessment” factor was estimated based on the minimum criterion of 0.5.
Conclusion. The most prospective areas recommended for licensing were selected, and the recommendations for further geological exploration work in these areas were given in order to clarify their hydrocarbon potential and reduce geological risks.

Background. The Anadyr depression has a marginal structure. It comprises four sub-regional elements. Disjunctive dislocations are widely developed within the area.
Aim. To reconstruct the paleogeographic and litho-facies conditions of the formation of the Meso-Cenozoic sediments of the Anadyr depression. The reconstruction was performed by evaluating the initial data and developing paleogeographic and litho-facies maps. Data on the conditions of sediment formation in the study area, rich in hydrocarbon and mineral deposits, is essential from a practical standpoint.
Materials and methods. Paleogeographic reconstructions of the Anadyr depression were performed based on an analysis of the sediment thickness and litho-facies composition of the relevant age. To build litho-facies maps, we used the data on the material composition of the Bering Sea sedimentary cover and adjacent areas, including well-drilling data, outcropping descriptions, thickness gradient analysis, seismic study results, etc.
Results. In the course of the work, sedimentary environment maps were created. Litho-facies maps were developed for all four main sedimentary complexes of the Anadyr depression. Based on the generalised geological data and paleoreconstructions, we constructed forecasting schemes of the distribution of barrier formations in the study area.
Conclusions. It was found that the formation of the sedimentary cover of the Anadyr depression took place mainly in shallow-water sedimentary environments. In addition, the depths of paleobasins did not overpass the internal relatively shallow marine shelf. Based on the analysis, two most probable reservoirs were selected. Regional fluid and potential seals were characterised.

Background. The lithogeochemical features of fine-grained detrital rocks (mudstones, shales, and fine-grained siltstones) allow, with a certain degree of success, the main parameters of the formation of sedimentary sequences to be reconstructed. These parameters include (primarily in terms of their REE and Th systematics) the types of river systems supplying thin terrigenous suspension in the sedimentation area: the rivers of the 1st category – large rivers with a catchment area of more than 100,000 km2; 2nd category – rivers feeding on the products of erosion of sedimentary deposits; 3rd category – rivers draining mainly igneous and metamorphic rocks; and 4th category – rivers carrying erosion products of volcanic associations.
Aim. To reveal, based on the analysis of interrelationships between such parameters as (La/Yb)N, Eu/Eu* and the Th content, the types of river systems that fed the Jurassic and Lower Cretaceous deposits of the Shaim oil and gas region (OGR) (Sherkalinsky, Tyumen, Abalak and Mulymya formations) and the region of the North Pokachevsky field of the Shirotnoe Priobye region (Sherkalinsky, Tyumen and Bazhenov formations, Lower Cretaceous deposits).
Materials and methods. The ICP MS data for almost 100 samples of mudstones and fine-grained clayey siltstones were used to analyse the features of distribution of lanthanides and Th in the Jurassic and Lower Cretaceous clayey rocks of the Shaim OGR and the area of the North Pokachevsky deposits. Individual and average composition points for formations, members and layers were plotted on the (La/Yb)N-Eu/Eu*, (La/Yb)N–Th diagrams developed by us with classification areas of the composition of fine suspended material of modern rivers of different categories.
Results and conclusion. The results presented in the article showed that during the formation of the deposits of the Shaim OGR in the Early and Middle Jurassic, erosion affected either mainly sedimentary formations or paleo-catchment areas that were very variegated in their rock composition. In the Late Jurassic, the source area was, most likely, a volcanic province, composed mainly of igneous rocks of the basic composition, and located within the Urals. This conclusion suggested that the transfer of clastic material from the Urals to the Urals part of the West Siberian basin “revived” much earlier than the Hauterivian. The Jurassic-Lower Cretaceous section of the vicinity of the North Pokachevsky field was almost entirely composed of thin aluminosilicaclastics formed due to the erosion of volcanic formations. These volcanic formations were located, as followed from the materials of earlier performed paleogeographic reconstructions, probably within the Altai-Sayan region or Northern Kazakhstan. Thus, the supply of detrital material in the considered territories of the West Siberian basin had a number of significant differences in the Jurassic and early Cretaceous.

Background. Some papers describing the deposits and prospects in the Baimka ore zone (Baimka Trend), the Chukotka Autonomous Region have been recently published. It should be noted that those publications concern with the large Peschanka porphyry copper-molybdenum-gold deposit and Nakhodka ore field. At the same time, some copper prospects were found at the zone flanks and their nature is not clear. There are no data available in the literature that would clarify the mineralogy of hydrothermal alteration and ores at those prospects, including Top and Luchik located at the northern flank. To understand the nature of these prospects, we studied in depth mineralogy of hydrothermal alteration and ores.
Aim. This study aims to describe mineralogy of metasomatic rocks and ores from the Top and Luchik prospects in the northern part of the Baimka zone, Western Chukotka.
Materials and methods. Several dozen polished thin sections were microscopic studied at the Department of Mineralogy of Moscow State University using an optical microscope. The chemical composition of minerals was determined using electron microprobe at the Laboratory of Analytical Techniques of High Spatial Resolution, Department of Petrology and Volcanology, Moscow State University.
Results. It was found that propylitic is the major hydrothermal alteration at the prospects studied here. Metasomatic rocks are cut by mineralized quartz and carbonate-quartz veins. Chalcopyrite and pyrite are the major ore minerals; galena, sphalerite, pyrite enriched in As and Sb, and marcasite are minor; Hg-bearing tetrahedrite-(Zn), native gold with the fineness of ~830, petzite, hessite, and acanthite are rare.
Conclusions. The data obtained suggest that mineralization at the Top and Luchik prospect is attributed to the epithermal intermediate sulfidation type. Considering the occurrence of the epithermal mineralization in the upper part of the hydrothermal column and the porphyry mineralization in the Baimka zone, the porphyry mineralization is possible at depth at the Top and Luchik prospects.

Background. The geodynamic approach is effectively used in regional forecasting and prospecting works. However, its application for local forecasting and prospecting for solid minerals is limited and sometimes impossible. One of the key problems of local forecasting and prospecting for minerals (solid, liquid, gaseous) is the presence of non-standard (flickering) geophysical anomalies, which complicates the interpretation of the results of geophysical surveys performed at different times at different stages of geological exploration. The article is devoted to clarifying the possibility of using geodynamic research in local forecasting and prospecting for solid minerals on the basis of attracting the latest scientific knowledge from the field of studying the spatio-temporal patterns of the development of modern geodynamic processes and their influence on the Earth’s geophysical fields.
Aim. To increase the reliability of interpreting the results of geophysical surveys performed for local forecasting and prospecting for solid minerals.
Materials and methods. The research was carried out on the basis of a comprehensive analysis of literature data, fund materials and the results of many years of the author’s own research in the fields of modern geodynamics and prospecting and exploration geodynamics. The initial data were based on the monitoring data of various Earth’s geophysical fields (deformation, seismic, and surface magnetic).
Results. A geodynamic reason for the appearance of flickering anomalies in the Earth’s geophysical fields (in particular, magnetic) has been established, and a mechanism for their formation under the influence of modern geodynamic processes has been proposed. The possibility of using the geodynamic approach in the prospecting for solid minerals has been expanded, and ways to increasing the efficiency of local searches have been proposed.
Conclusions. The research demonstrates the possibility of using geodynamic studies in local prospecting for solid minerals, which helps to increase the reliability of the results of interpretation of geophysical survey data, and, as a result, to reduce the overall financial and time costs involved with searching for mineral deposits.

Background. Determining the productive deposit thickness is of fundamental importance for assessing the reserves of oil and gas fields. 3D seismic data is used to assess the thickness of seams in the interwell space. However, owing to the limited vertical resolution of seismic data, estimating thicknesses of thin deposits (less than 20 m) is challenging.
Aim. To evaluate different approaches to calculating the thickness of the productive deposits based on seismic data with the purpose of selecting the most optimal approach.
Materials and methods. We compared the results obtained using different approaches to assessing the productive deposit thickness of the Tula-Bobrikovian age in the interwell space, including the convergence method (calculating the thickness for oilwells with no seismic data used), the use of seismic attributes to calculate the “seismic attribute — reservoir thickness” dependency (for attributes, dominant frequency and mono-frequency component at 60 Hz), estimation of the thickness from the seismic signal shape. Cokriging was used to calculate inferred power maps from seismic attribute data and to classify them by waveform. For each of the techniques, the crossvalidation method and calculating the root-mean-square deviation were used as quality criteria.
Results. When assessing the accuracy of thickness map development, the root-mean-square deviation was 12.3 m according to convergence method, 10.2 m — to the dominant frequency attribute, 7.2 m — to the attribute of the monofrequency component at 60 Hz and 6.3 m — to the signal shape classification. The latter method yielded the best results, and the developed thickness map allowed paleo-cut to be traced.
Conclusions. Applying the thickness estimation method based on the seismic signal shape allows the value of the root-mean-square deviation to be reduced by a factor of 2 compared to that of the widely adopted convergence method. This approach permits productive deposits thickness to be more accurately estimated and hydrocarbon reserves to be determined.

Background. Environmental remediation plays an important role in mitigating or eliminating environmental damage. The worldwide experience in organising and regulating ecosystem recovery activity is of interest.
Aim. An extensive comparative analysis of the particularities of legal regulation in the sphere of environmental remediation in foreign countries.
Materials and methods. The study was based on three major criteria: the environmental remediation enshrined in law; the provision of the term “remediation” or its equivalent; identifying the objects of remediation. The analysis was based on the examination of the legal information registers and official government and parliament resources of over 20 countries worldwide. The study covers the period from 1970 to 2019, because since the 1970s, one can speak about the widespread implementation of environmental laws. In individual cases, the first mentioning of the current environmental law was taken as a starting point.
Results. Based on the results of the study, the countries were divided into two categories. The first group was dominated by the vertical environmental legislation system and the second — by the horizontal one. The authors described the features of each group and identified the characteristic features of environmental remediation management. These features include the requirements for remediation plans, action plans enshrined in the law, introduction of special cadastre, importance ranking of remediation objects, etc. Formulating a universal interpretation of the “environmental remediation” term is another significant result of this study.
Conclusions. The study showed that the need for and importance of environmental remediation is recognised worldwide. In the meantime, it is necessary to identify the principles of environmental remediation and to systematise approaches to ecosystem restoration.

  18.06.1939–19.05.2021