Research subject. A geochemical study of Cambrian-Ordovician deposits was carried out within the area of the Duderhof dislocations of the platform cover of the southern frame of the Baltic shield.
Materials and methods. The ICP-MS method was used to study 14 samples of sandstones, limestone, phosphorites and black shales, which comprise both weakly dislocated stratified formations (stratified horizons) and strongly deformed formations, as well as secant dome-like and dike-like bodies.
Results. Geochemical studies showed that, in the zones of tectonic disturbances, lower Paleozoic deposits are characterized by high concentrations of a number of trace elements, significantly exceeding the Clark values for sedimentary rocks. This applies to such chemical elements, as Be, Sc, V, Co, Ni, Cu, Mo, Pb, Th and U. At the same time, the concentrations of V and U are higher than those in the developed fields of Estonia and Sweden. An analysis of the cambriansands of the sablin formation showed that the eastern (Archean) part of the Baltic shield could not have been the source of food for their formation. The presence of leucoxene in the sands of the Sablinsky formation, a mineral formed by ilmenite under the influence of high temperatures, and the presence of mechanically unstable minerals in dike-like formations, indicate the influence of deep fluids on the sedimentary cover, whose flows moved along fault zones and carried the above-mentioned chemical elements to the near-surface layers of the earth’s crust. All this points to the endogenous rather than exogenous (glacial) nature of secondary structural-material transformations in the area of the Duderhof dislocations, as well as their genetic relationship with deformations in the Baltic-Mezen shear zone.
Research subject. A study was conducted into the geochemical features of various lithotypes of carbonate rocks (granular and clastic limestones, limestones with planar, planar-columnar and columnar stromatolites) from the upper subformation of the Uk Formation. This formation crowns the Upper Riphean Karatau Group on the Western slope of the Southern Urals (Bashkir meganticlinorium).
Material and methods. The research material was two collections of carbonate rocks (11 and 32 samples), the general geochemical features of which were previously considered in publications in 2019. The main research method was the analysis of the correlation relationships of the main rock-forming components, trace elements and their indicator relationships.
Research results. It was established that all lithotypes are characterized by a significant content of both the main rock-forming components and the trace elements that make up the insoluble residue/fine-grained terrigenous admixture. This suggests that the area of accumulation of carbonate sediments was located near the continent, which served as a source of a significant amount of fine-grained/clay suspension.
Conclusions. A strong positive correlation was observed between K2O and REE in the carbonate rocks of the Upper Uk Subformation, and a strong negative correlation was observed between REE and CaO. This suggests that the REE content in carbonate rocks is controlled mainly by terrigenous/clay admixture. A positive correlation is also observed between REE and Th, Zr, and Al2O3. All this shows that REE in carbonate rocks are mainly present in clay admixtures. The correlation coefficient between Ce/Ce* and (Dy/Sm)N is 0.20, while this coefficient for Eu/Eu* and (Pr/Sm)N equals 0.12. Thus, a definite conclusion about the effect of diagenetic fluids on the distribution of REE cannot be made. The data obtained show that the distribution of REE and Y in our collection of carbonate rocks of the Upper subformation of the Uk Formation of the Southern Urals is controlled mainly by clay admixtures.
Research subject. The composition and isotope systems of zircons and their host granites from the Krutorechensky complex (western part of the Main Uralian fault zone, Northern Urals) were investigated.
Methods. The U-Pb age, trace element contents and Lu-Hf isotopes in the zircons under study were determined by LA-ICP-MS (Ulan-Ude, Ekaterinburg). TIMS was used to determine Sm-Nd isotopes in the rocks.
Results and Discussion. It was shown that the morphology and composition of the main zircon group from granites confirm their magmatic origin and the absence of alteration. It means that the previously defined Vendian-Cambrian (542 Ma) age of the granites remains valid. The relics ancient (1043–122 Ma) cores were probably inherited from Isherim suite rocks. The source of such detrital grains could have been the rocks from the East-European platform basement. Young zircons (400 Ma) differing strongly from others in composition could have been formed around the already existing grains produced by a fluid generated under the action of plume activity. The source for granite melting was mainly of a crust nature: εNd(t) = –6, εHf(t) = –6…–9, initial ratio (87Sr/86Sr)i = 0.796943. In terms of lithology, the source rocks correspond to sandstones with a small admixture of clay components. The obtained information confirms the necessity to further investigate questions concerning the area of distribution and the age of the Sarankhapnorsk suite within the Krutorechensky complex, as well as the position of the eastern boundary of the Isherim block.
Conclusion. The obtained results can be used in geological mapping.
Research subject. The research covered the geological position, material composition and rock age of a comparatively small (about 32 km2) Bystrinskii gabbro massif. This massif underlies a tectonic plate package consisting of serpentinized dunits and harzburgites of the ophiolite association in the Eastern boundary of the Middle Urals.
Methods. The content of petrogenic elements was measured by the Х-ray fluorescence method using an Х-ray multichannel spectrometer CPM-35. The analysis of rare-earth element contents was conducted using a NexION 300S mass-spectrometer. The composition of rock-forming minerals was studied using an Х-ray microanalyzer CamecaSX100. The age of the massif was determined by the 147Sm-143Nd method of isotope dating. Amphibole geobaometers were used to determine the depth of the rock formation.
Results. According to the petro- and geochemical features and composition of the rock-forming minerals, the 147Sm-143Nd isotope age of the rocks was found to be 587 Ma. It was shown that gabbroids in the massif are represented by two petrographic varieties. The predominant type of the rocks is gabbrodolerites, which are similar to the isotropic gabbros of undisturbed ophiolite sections in terms of mineral composition, structure, geochemical features and the depth of formation (not more than 2–3 km). The mapping results showed the massif under study to be the largest among those described thus far. The medium grained gabbroids, which are present in lesser quantities, differ sharply from gabbrodolerites in terms of lower contents of Fe, Ti, both rockforming (K, Na) and rare (Li, Rb, Cs) alkalis, Ba, V, Y, Nb, Zr, Hf and elements of rare-earth group, as well as by significantly higher quantities of Ca, Mg and Cr. The depth of their formation is 10–12 km, which corresponds to the upper mantle.
Conclusions. The obtained information demonstrates that fragments of two levels of the ophiolite section are tectonically aligned in the Bystrinskii massif: relatively shallow isotropic gabbros of the upper part of the ophiolite section and deep gabbros of the mantle part of the ophiolite section.
Research subject. The article is devoted to the study of spessartine-quartz rocks of the Triassic chert formation of the Sikhote-Alin. The research objects involved the outcrops of spessartine-quartz rocks of the Gornaya and Shirokaya Pad areas from Malinovka and Olga ore districts (in the Samarka and Taukha terranes respectively).
Materials and methods. The selection of stone materials was performed manually. The diagnostic of minerals was performed using the microscope in transmitted and reflected light and carried out by the determination of their composion. The polished sections of the minerals were investigated at the FEGI FEB RAS using JXA8100 microanalysers.
Results. Spessartine-quartz and manganese silicate rocks occupy the same position in the section of the Triassic deposits of the Taukha and Samarka terrains. This indicates a synchronous accumulation of sediments (protoliths of these rocks) in a single sedimentary basin. The rocks formed by contact metamorphism in the Late and Early Cretaceous large granitoid massifs. This process is manifested in the presence of a variety of such minerals, as spessartine, members of the ilmenite-pyrophanite series, titanite, monazite, bastnesite, allanite, apatite, zircon, baddeleyite, torianite and others. The rocks under study also include such rare minerals, as cheralite, greyite and a rare variety of fluorine-aluminous titanite.
Conclusion. The occurrence (during contact metamorphism of siliceous-rhodochrosite rocks) of spessartine-quartz or manganesesilicate rocks composed mainly of pyroxenoids, was determined by both the relative amounts of carbonate and clay matter, as well as the concentrations of Ba and alkalis in the initial sediments.
Research subject. This study investigated the mineral association of quartz-wolframite veins of the Bukukinskoye tungsten deposit, with a particular focus on the content of rare-earth elements and their carrier minerals, their chemical composition and the distribution of light and heavy lanthanides.
Materials and methods. The chemical composition of two groups of samples containing different ore minerals, as well as the mineral composition of ores was investigated on 143 samples in thin and polished sections using an AXIO ScopeAI optical polarizing microscope and a LEO 1430 VP scanning electron microscope. Chemical examination of ores was carried out using the ISP MS method.
Results. The study of the chemical composition of group samples determined that the content and distribution of REE, the ratio of light and heavy lanthanides in ores depends on the ratio of wolframite and other ore minerals, associated with fluorite and apatite. The main REE concentrator mineral in the wolframite-sulfide-quartz ores of the Bukukinskoye deposit is monazite-(Ce) containing (wt %) Ce (27.09–31.59) La (13.13–17.67), Nd (5.59–10.26), Pr (up to 2.86), Sm (up to 1.14), Th (up to 6.59).
Conclusion. The obtained results confirmed our earlier findings for the wolframite of the Sherlova Gora that an increase in the concentration coefficient of heavy lanthanides depends on a decrease in their ionic radius. The contents, mineral forms and ratios of REE of the cerium and yttrium groups in the ore of the Bukukinskоyе deposit can be typochemical signs of greisen tungsten deposits, thus being useful for typification of ore formation in geological forecasting and prospecting.
Research subject. The aim was to justify the prospects of uranium deposits in the Tyrminsk area of the Far East, which were briefly studied over 60 years ago. These deposits are highly similar to those located in the Streltsovsk mega uranium district.
Materials and methods. The materials were geological data on uranium deposits in the promising Tyrminsk district and in the largest Streltsovsky district in the Far East. The research method was a comparison of the available information about the geological features of uranium deposits and ore areas as a whole.
Results. The uranium ore regions under study are localized in Mesozoic volcanogenic calderas, within Paleozoic granites. Specific uranium deposits are formed at intersections of multidirectional faults, most frequently in the vicinity of volcanic caldera boundaries. The deposits formed within a contrasting geological section are represented by steep vein-like and stock-like bodies, as well as by gently sloping formation-like bodies under the screens of layers of dense sandstones and siltstones. In addition, deposits are found in favourable layers of conglomerates and effusives breccia. The length and thickness of ore bodies comprise 0.2–1 km and 1–80 m, respectively. Uranium ores belong to the fluorine-molybdenum-uranium type; near-ore rocks are represented by mudstones and hydrosludites. Some fields in the Tyrminsk district are opened by wells to a depth of 80– 160 m from the surface, while the fields in the Streltsovsk district are explored by wells and underground mine workings to a depth of 1.5 km from the surface.
Conclusion. The extent of uranium deposits in the Tyrminsk district is predicted to reach a depth of 500–700 m, with their resources being estimated as “large” based on the high similarity of localities in the areas under study. As the most promising uranium area in the south Far East, the Tyrminsk district is recommended as a priority exploration site for uranium.
Research subject. The high-sulfidation (HS-type) epithermal gold deposits of the Maletoyvayam ore field, which is located in the volcanic belts of the Kamchatka Peninsula.
Materials and methods. The paper presents the results of geophysical and petrophysical studies carried out by “NPP VIRG-Rudgeofizika” (2016–2017 years) in the central part of the Maletoyvayam ore field hosting HS-type gold deposits.
Results. The presented case study shows the potential of geophysical prospecting for HS-type epithermal gold deposits located within altered rock areas. On the basis of the conducted magnetic surveys, fragments of a ring structure broken by faults and the hydrothermally altered bodies within the Neogene volcanogenic-sedimentary sequence of the andesite composition were identified. Using the 3D inversion of magnetic data, the shape, size and structure of the hydrothermally altered bodies were estimated. The electrical survey data allowed the hydrothermally altered areas to be differentiated in terms of composition. As a result, zoning of hydrothermally altered areas was conducted, and ore-bearing sites and gold-bearing bodies of monoquartzites in the plan and cross-section were determined.
Conclusions. Geophysical techniques for prospecting HS-type epithermal gold deposits are effective in distinguishing important gold-related features within the altered zone thereby facilitating identification of specific drill targets.
ISSN 2500-302X (Online)
