编号 4 (2025)
Articles
Structure and lithological characteristics of the Upper Jurassic Shalbuzdag reef massif (Northeastern Caucasus)
摘要
The structure of the Upper Jurassic reef complex of the Shalbuzdag mountain range (southern Dagestan) is considered. It was the western segment of the Shahdag zone of the barrier reef, which separated zones with different types of sedimentation: to the north of it was the area of shallow sedimentation of the carbonate platform of the Greater Caucasus, to the south was the Dibrara trough, where thick strata of carbonate and terrigenous flysch accumulated. In terms of the reef massif has an approximately ring-shaped shape with a diameter of about 4 km. There are several large reef structures on its territory, in the center of which there is a biogenic carbonate dome-shaped core surrounded by a plume of sedimentary layers lying relatively steeply. There are also numerous smaller formations, bioherms, ranging in size from a few meters to the first tens of meters. Reef-forming fauna is represented by corals, gastropods, brachiopods, various types of algae, etc. The inter-reef space is filled with sedimentary rocks, which are mainly products of denudation of reef structures. Gradation cyclicity is often observed in these strata. In the structure of the western edge of the massif, according to the ratio of biogenic and sedimentary deposits, at least 3 large impulses in its formation can be distinguished. The factors that most influenced the formation of the Shalbuzdag reef complex should probably be considered 1) climate changes from humid in the Middle Jurassic to arid in the Late Jurassic, 2) proximity to the transition zone between areas with different tectonic development regimes and sedimentation types, 3) sea level fluctuations of various orders.



Clastic rocks of the overlying basement proterozoic sedimentary sequences. Communication 1. Clay rocks, main features of lithogeochemistry
摘要
Analysis of the bulk chemical composition of fine-grained clastic/clay rocks of a number of objects (Ai, Prikamsk and Trekhgornaya formations, Mukun Group and Ust-Ilya Formation, Starorusskaya and Vasileostrovskaya formations, clay rocks of the Lower Vindhyan and Gwalior, Bhima, Athabasca, Libby Creek groups, etc.), participating in the composition of Proterozoic sedimentary sequences unconformably overlying the crystalline basement, showed that their initial mineral composition was close to the composition of most post-Archean clay rocks. The ratio of Zr, Sc and Th in these rocks suggests that they are composed predominantly of weakly recycled material. The share of erosion products of mafic igneous rocks among the sources of their fine-grained aluminosilicoclastics was relatively small. The features of the bulk chemical composition of such clay rocks give reason to believe that the main suppliers of detrital material for them were rock complexes formed in collisional and/or riftogenic settings.



Mineral composition of the productive formation and features of chromites of the Lukoyanovsky placer area (Nizhny Novgorod region)
摘要
As a result of geological exploration within the Lukoyanovsky placer district (Nizhny Novgorod region), increased contents of chrome spinel reaching an industrial level of up to 100 kg/m3 were established in complex coastal-marine rare metal-titanium placers, while their source is not obvious. In addition to chrome spinel, industrial components of placers are ilmenite, zircon and rutile. Morphological features of ore minerals make it possible to assume several sources of ore material transportation, both proximal and distant. Moderately mature mineral composition of the heavy fraction (increased amount of minerals unstable during weathering) indicates that the formation of Permian-Jurassic deposits of this region occurred without the participation (or with weak participation) of chemical weathering crusts. During the mobilization of detrital material in the source zone and during the formation of intermediate reservoirs (Permian and Early Mesozoic complexes of the sedimentary cover of the Ural part of the East European platform), mechanical processes prevailed with a subordinate influence of crust formation. The study of the chemical composition of chrome spinel of the most studied and promising Itmanovskaya placer showed that they are mostly similar to the same-name minerals of the ophiolite formation, as well as concentrically zoned platinum-bearing massifs. It seems that with the general influence on the formation of the sedimentary cover of the central and eastern parts of the East European platform at the Permian-Triassic boundary of the Uralide paleoorogen, the Puchezh-Katunka ring structure, located to the north of the Lukoyanov dome, had a noticeable effect on the composition of the heavy fraction of sediments in the region.



Search-oriented technique of the numerical forecasting of the gold placers: evidence from the Vagran placer district
摘要
As a result of field research, as well as analysis of data from stock reports, two types of placer gold were identified within the Vagran placer cluster of the Northern Urals, which are indicators of primary mineralization of gold-sulfide-quartz and hypogene-hypergene types, which were used as benchmarks for developing a digital system for predicting parameters and localizing primary sources of placer gold. In the present work the typomorphic characteristics of placer gold (size, roundness, sorting and fineness, as well as the content of impurity elements) are formalized (in the form of a quantitative assessment). These data were processed using the methods of multiplicative indicators, linear regression method and a regression tree method using a random forest algorithm, which make it possible to forecast the composition and localization of the primary mineralization with greater confidence and stability than ordinary parameters separately. The data required for such an assessment do not require additional field and highly qualified laboratory studies, they are contained in standard reports on the heavy minerals testing. The study of the correlation matrix allowed to identify characteristic indicators for the primary mineralization of gold-sulfide-quartz and hypogenic-hypergenic types, and to give recommendations for conducting prospecting and exploration in order to identify the primary gold content of the cluster. All three methods used gave consistent results that make it possible to predict the parameters of primary gold content, while the method of calculating multiplicative coefficients gives a more contrasting, pronounced result, seems to be the simplest and most generalized, suitable for any set of consistent data, while other methods require a more in-depth analysis incoming parameters. The proposed method of creating forecast estimates allows to increase efficiency and partially automate the process of determining the prospects for primary mineralization of territories.



New mineralogical and geochemical features of the ore ferromanganese deposits of the Izu-Bonin and Mariana island arcs
摘要
Some features of the ferromanganese deposits of the Izu-Bonin and Mariana Island arcs tested in the 1st (1977) and 5th (1978) flights of the R/V Volcanologist are considered. Two types of genetic deposits have been identified – hydrogenic and low-temperature hydrothermal. The main ore minerals of hydrogenic ferromanganese deposits are poorly crystallized structures with a low degree of ordering Fe-vernadite and Mn-feroxygite, with a smaller amount of goethite and bernessite. Low-temperature hydrothermal deposits consist either primarily of bernessite, as well as vernadite and goethite, or of hematite, goethite and feroxygite. Hydrogenic ferromanganese deposits of the Izu-Bonin and Mariana Island arcs are characterized by a Mn/Fe ratio of 0.84–1.36, for low-temperature hydrothermal deposits of the Izu-Bonin arc it is 6.13–13.9. It was found that the content of Co, Ni and Cu is significantly higher in the crusts of the Mariana arc compared with the crusts of the Izu-Bonin arc. The contents of the remaining cations of heavy and rare metals – Pb, Cd, Ba, Sr and others – in the crusts of both arcs are close to each other. The content of most rare earth metal cations in the hydrogenic deposits of the Izu-Bonin and Mariana arcs is comparable to each other. Low-temperature hydrothermal ferromanganese deposits of the Izu-Bonin arc differ significantly from hydrogenic deposits with a low (by 1–2 orders of magnitude) content of cations of non-ferrous, heavy and rare metals. The contents of rare earth metal cations in low-temperature hydrothermal samples of the Izu-Bonin arc are low and range from 0.24 (Tm, Lu) to 32.35 mcg/g (Y). Among all rare earth metal cations, Y, Ce, Nd are present in the largest amounts in these samples. Among all rare earth metal cations, Y, Ce, and Nd are present in the largest amounts in these samples.


