Research subject. Northern part of the Sakmara zone of the Southern Urals, represented by the Krakinsko-Mednogorsk paleovolcanic belt, including the Mednogorsk ore district and the Blavinsko-Komsomolskoye ore field. Aim. To reconstruct the paleogeodynamic settings during the Cambrian–Early Devonian in the Sakmara zone. Methods. An analysis of geological, petrogeochemical, structural-tectonical, and paleovolcanological data obtained by the authors and those reported in literature using conventional and new petrogeochemical diagrams. Results. The following stages of tectono-magmatic evolution in the Sakmara zone were distinguished: (1) Cambrian–Early Ordovician, continental rifting; (2) Early–Middle Ordovician, oceanic spreading; (3) Early Silurian, suboceanic rifting, serpentinite protrusions, edaphogenic breccias, siliceous rocks (starting a subduction zone); (4) Late Silurian–Early Devonian, island-arc basalt-rhyolite volcanism and sulfide deposits, volcanism of the shoshonitic and alkaline series, formation of the rear island arc. The Sakmara zone features no analogues of boninite volcanism (D1e2) typical of the West Magnitogorsk zone, which indicates the absence of signs of the volcanogenic strata of the Voznesensk–Prisakmar zone being pushed into the Sakmara structural zone. Conclusions. (1) In the Cambrian–Early Devonian, volcanogenic and volcanogenic-sedimentary formations and strata of the Sakmara zone formed a megacycle, starting with the processes of continental and oceanic rifting, which led to the subduction process and generation of the frontal and rear island arcs. (2) The volcanism in the Sakmara and Voznesensk–Prisakmar zones developed autonomously, without tectonic transfers from east to west. (3) The partial coincidence of the formation age of the Chanchar (D1lh–e1) complex of the Sakmara zone and the Turin complex (S2p–D1lh) of the Tagil zone suggests the presence of a reduced continuation of the western wing of the Tagil zone in the Sakmara zone.

Research subject. Sedimentary basins within the Mississippian and Early Pennsylvanian (Middle Carboniferous of Russia) rift (sensu lato) structures on the eastern slope of the Southern and Middle Urals. Methods. An analysis of lithological characteristics of sedimentary rocks and relationships with volcanic complexes in order to clarify the evolution of rifting basins. Results. During the Mississippian and Early Pennsylvanian, several large rifting basins were common in this area. The Early Carboniferous rifting basins (Magnitogorsk-Bogdanovskoe graben and Alapaevsk-Kamensk rift) of accretionary margin of the East European craton were characterized by relatively shallow-water (in some places up to coastal-marine and continental) environments, caused by intensive (compensatory) filling of depressions with sedimentary material. In this case, volcanic rocks – those of fissure-type and stratovolcanoes lava flows – also played a significant role. In the Late Visean, the rifting process paused; a relatively shallow shelf basin regime was established throughout the entire area, which persisted until the end of the Early Carboniferous. In the Early Pennsylvanian, due to a hard oblique collision, the rifting process resumed. However, the rifting basins were already smaller in terms of area and most often deeper, whereas the occurring volcanic processes were less intensive. The characteristic feature of these basins was the presence of flysch complexes. The elongated shape of the rifting basins, large thicknesses of sedimentary strata, lateral facies variability, and the abundance of coarse clastic rocks, consisting largely of fragments of directly underlying sediments (Visean, Serpukhovian, Bashkirian), suggest that the Mississippian and Early Pennsylvanian rifts of the Southern and Middle Urals eastern slope had a strike-slip nature, i.e., the rifting basins were pull-apart type depressions.

Research subject. Devonian-Carboniferous boundary beds in the Berchogur Depression (Western Kazakhstan). New data on the fauna of ammonoids, conodonts, and foraminifers from the Berchogur sections were obtained during fieldwork studies in 2018–2019. Aim. Identification of episodes of the Global Hangenberg event (crisis) in the Berchogur section and correlation of the potential markers of the base of the Carboniferous in this section. Materials and Methods. The research was conducted using paleontological and lithological methods. Results. The study of fossil assemblages and rock lithology exposed by new excavations suggests that the markers of the Hangenberg event are present in the section; moreover, they can be correlated with the phases of this event in the sections of Western Europe. A correlation of ammonoid, conodont, and foraminiferal zones is proposed. It is shown that the conodont S. (Eo.) praesulcata in the Berchogur section first appears in Member 2b, and the first appearance of S. (Eo.) sulcata is recorded in Member 3b. Based on the findings of ammonoids of the genus Acutimitoceras, Members 3b and 3c can be correlated with the upper part of the prorsum ammonoid Zone, which corresponds to the level of appearance of S. (Eo.) sulcata. The intervals corresponding to episodes of the global Hangenberg event (pre-crisis, crisis, and partly post-crisis) are provisionally identified in the section. Conclusions. The pre-crisis interval corresponds to the abundance of foraminifers in the Q. kobeitusana substricta Zone and the Upper expansa and lower part of the Lower praesulcata conodont zones. The so-called crisis prelude corresponds to the extinction of the quasiendothyrid foraminiferal community and the upper part of the Lower praesulcata Zone (Member 2a2). The Hangenberg Extinction, or the lower crisis interval, corresponds to the clays of Member 2b in, and to the interval of most of the Middle praesulcata Zone – the lower part of the Upper praesulcata Zone, and to the Hangenberg black shales in Europe. The subsequent regressive episode (Member 3a) corresponds to the upper part of the Upper praesulcata Zone. The members with ammonoids (3c) and conodonts of the sulcata Zone (3b and 3c) and Member 4 correspond to the transgressive phase and are considered equivalent to the Stoсkum Limestone of the Rhenish Massif.

Research subject. An intrusive body of gabbroids breaking through mantle peridotites (dunites and harzburgites) of the Alapaevsky ophiolite massif (Eastern zone of the Middle Urals). Methods. The contents of petrogenic elements were determined by the X-ray fluorescence method; the contents of rare and scattered elements were studied by the ICP-MS method. The age of gabbroids was determined by 147Sm-143Nd ID-TIMS by isotope dating. Results. The gabbro and their containing ultramafic rocks were established to be of almost the same age of about 580 Ма, which indicates their belonging to a single ophiolite association of the Vendian age. At the same time, the gabbroids of the Alapaevsky massif differ sharply from both isotropic and stratified gabbro of the crustal part of the ophiolite section, fragments of which are observed within the Eastern zone of the Middle Urals, in terms of a significantly reduced content of light REE, rare alkalis, barium, uranium and thorium, as well as the absence of Pb maxima on spider diagrams. Conclusions. The established features in the composition and intrusive occurrence of the gabbro of the Alapaevsky massif among mantle ultramafic rocks indicate that the studied rocks cannot be identified with gabbroids of the crustal part of the ophiolite association. This suggests that this rather large intrusion of gabbro may be analogous to small veins and dikes of gabbroids observed among mantle peridotites in a number of ophiolite massifs, such as Voikar-Synyinsky massif in the Polar Urals and the Samail ophiolite in Oman. The distribution specifics of rare elements on the spider diagrams of the gabbroids of the Alapaevsky massif, i.e., the presence of Sr and Ba maxima and Nd and Th minima, as well as the Harzburgite composition of the mantle restite containing the gabbro body under consideration, indicate that the Vendian ophiolite association of the Eastern zone of the Middle Urals formed in a suprasubduction (pre-arc) environment. A model for the formation of large masses of gabbro in the mantle part of the ophiolite section is proposed.

Research subject. Variously-altered tholeiitic basalt xenoliths in ore-hosting rocks from the Rudnogorskoe iron ore deposit, the Angaro-Ilim region of Eastern Siberia. Aim. To identify the sequence of mineral transformations during the formation of magnetite ores. Materials and methods. The mineral composition of weakly-altered and hematitized xenoliths of tholeiitic basalts in skarned rocks and relics of basaltic hyaloand lithoclasts in varying degrees magnetitized volcaniclastites were studied. Minerals were identified using a powder X-ray diffractometer with the determination of the quantitative ratios of mineral phases by SHIMADZU XRD-6000 and DRON-2.0 diffractometers and using the microscopic (Olympus BX51) and electron microscopic (Tescan Vega 3 sbu with an Oxford Instruments Xact energy-dispersive analyzer) and IR spectroscopic (Spectrum One IR Fourier-spectrometer and a Multiscope microscope, PerkinElmer) research methods. Results. In xenoliths of weakly-altered tholeiitic basalts, volcanic glass is smectitized and partially replaced by secondary aggregates of chlorite and carbonate. In hematitized xenoliths, the smectite-hematite mineral association contains skarn epidote and garnet. Smectite aggregates, partially transformed into magnetite mass, are present in the magnetitized volcaniclastites. The studied smectites are classified as saponite according to the obtained values of basal reflections d001 in the range of 14.76–15.23 Å and calculated crystal chemical formulas. The differences in the morphology, chemical composition, and IR spectrometric characteristics of smectites reflect the varying degrees of transformation of the tholeiitic basalts in multi-stage ore-forming processes.

Research subject. Iron oxyhydroxides covering and replacing the chimneys of shimmering water smokers-diffusers of the Rainbow hydrothermal field (MAR). Aim. To identify features of the concentration and associations of chemical elements in varieties of iron oxyhydroxides to recognize patterns of geochemical differentiation under conditions of halmyrolysis of sulfide chimneys-diffusers. Materials and methods. Samples were collected during a dive to a depth of 2300 m using the manual manipulator of the Mir-2 manned vehicle (travel No. 50, research vessel Akademik Mstislav Keldysh, 2005). Varieties of iron ohyhydroxides were identified using electron microscopes (REMMA-202М with LZ-5 Link system, Tescan Vega 3 sbu with an Oxford Instruments X-act energy-dispersive analyzer, and Jeol Superprobe 733 with an EDA Oxford Instruments INCAx-sight) and a powder X-ray diffractometer (SHIMADZU XRD-6000, CuK-α radiation with monochromator). Further, a mass spectrometry with inductively coupled plasma and laser ablation (LA-ICP-MS) analysis was conducted at the South Urals Federal Scientific Center of Mineralogy and Geoecology, Ural Branch of the Russian Academy of Sciences. Results. Microlayered goethite aggregates containing admixtures of barite, calcite, aragonite, native sulfur, covellite, sphalerite, and an X-ray amophoric oxyhydroxide phase of iron cover the shimmering diffusers. Towards the inner parts of the chimney walls, they are replaced by pseudomorphs of lepidocrocite after pyrite and pyrrhotite, and then by radial and bacteriomorphic crustifications of lepidocrocite. The use of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) showed that goethite varieties have the increased contents of Zn and Co associated with other elements of medium-temperature hydrothermal fluids (Cd, Mn, Ni, Ga, Sn, Pb and Sb) in the absence of significant concentrations of a high-temperature hydrothermal association (Se, Bi, Te). The role of elements of seawater association (Mg, Na, K, Sr, U, V, As, Mo, Ni, P, B, W, Cs, REE) decreases from the surface layered goethite aggregates to crustification varieties of lepidocrocite. Different scenarios of accumulation under conditions of sulfide halmyrolysis and precipitation on local reduction barriers are proposed for elements with different valences (U, V, Mo, As, Cr, Eu). It is assumed that some of the microelements (Sr, V, As, P, REE) found in goethite are products of sorption on iron hydroxides or are part of invisible Fe-Ca hydroxyphosphates. Conclusion. The influence of sulfide halmyrolysis on the differentiation of chemical elements has been revealed.

Research subject. Hydrothermal deposits of Golovnin Caldera. Aim. To study the epithermal volcanogenic ore formation. Key points. Until now, there has been a consensus on the exogenous sedimentary (colloidal) genesis of sulfur in volcanic lakes. Our observations and microstructure studies indicate the presence of sulfur melt at the bottom of Kipyaschee Lake. Drops of this melt are carried to the surface of the lake as part of a light gray foam. The significant differences of sulfur spherules in the concentration of sulfide mineralization, in its composition, as well as in the presence or absence of numerous opal inclusions are most simply explained by the capture of droplets in various parts of the sulfur melt and their subsequent movement by a gas stream passing through the melt. Elemental sulfur condensate is formed in bottom sediments as a result of forced cooling of endogenous gas flows by lake water. The main condensation of sulfur occurs here (96% or more of the total potential of fluid sulfur). Residual condensation of sulfur occurs in the aquatic environment. Finely dispersed sulfur condensate in a mixture with water is unstable and breaks down over time with the release of hydrogen sulfide and the formation of sulfurous and sulfuric acids. The activity of bottom hydrotherms and coastal unrest prevents the formation of colloidal sulfur sediment at the bottom of lakes. In the crater depressions at the bottom of the lakes of the Golovnin Caldera, sulfidization of its melt occurs simultaneously with the condensation of sulfur itself. Gravitational deposition of sulfides in the sulfur melt leads to their enrichment of the root parts of crater depressions, where pyrite ore bodies are formed in real time. Terrestrial sulfur deposits, together with the modified rocks overlying them, demonstrate the full profile of endogenous apical oxidation under gas-hydrothermal action: sulfur and sulfur-opal rocks up the section are replaced by gypsum-jarosite rocks and, further, by an “iron hat” of limonite-cemented breccias of the dome mantle. Conclusions. Observations, microstructure studies and molecular chemical modeling indicate the endogenous condensate origin of ore sulfur in the Golovnin Caldera and exclude its exogenous sedimentary genesis.

Research subject. A section of the V ore zone in the Verkhnealiinskoye gold deposit in Transbaikalia. Materials and methods. Samples of host rocks and ores from the core of exploratory wells, which cut the ore zone at depth intervals of 27–34; 57–104; 153–168; 285–293 m, were studied by the methods of chemical analysis, as well as determination of crystal structural perfection, phase α→β transition temperatures, and electrical resistivity of vein quartz. Results and conclusions. An increase in the degree of crystal structural perfection (CSP) of vein quartz in cross sections in the direction from the quartzification zone at the contacts to the axial parts of the vein from 59 to 73 (average 63) in the upper section, from 54 to 79 (average 67.2) in the lower-middle section, and from 86 to 93 (average 89.2) in the section below the wedging of the conductor was established. Over the entire depth range from 27 to 168 m, the average CSP value degree increases from 63 to 89.2. Similarly, although less clearly, phase (α→β) transition temperatures in quartz undergo changes; their connection with the CSP value was revealed. The electrical resistivity of vein quartz increases during the transition from the quartz zone to the quartz vein due to a decrease in the number of inclusions and impurities of hydromica and chlorite. The possibility of using the СSP value in combination with data on the content of gold, silver, and bismuth to assess the level of an erosion section of newly discovered gold manifestations is confirmed, both in individual undercuts by boreholes and in the form of dumps or bedrock outcrops of quartz veins.

Research subject. Minerals of Cu from the Mikheevskoe gold-copper-porphyry deposit in the Southern Urals. Aim. To obtain preliminary data on the isotopic composition of Сu in minerals of various ore types from the largest porphyry deposit of the Urals followed by their interpretation. Methods. The 65Cu/63Cu isotope ratio values were determined using a multicollector inductively coupled plasma mass spectrometer Neptune Plus at the Research Center “Geoanalytik” (IGG of Ural Branch of the Russian Academy of Sciences). In total, 12 samples of three main technological ore types from this deposit were analyzed. Results. The δ65Cu values in the minerals of sulfide, friable, and oxidized ores ranged from –0.36 to +0.25‰ (chalcopyrite, fahlore), from –0.64 to +0.68‰ (bornite, chalcocite, pyrite), and from –2.14 to +0.30‰ (malachite, azurite), respectively. Conclusions: The absence of wide variations in δ65Cu values indicates the formation of the deposit under the conditions of a limited number of stages of hydrothermal ore formation, associated with the evolution of a single hydrothermal system. The obtained data agree well with the values of δ65Cu in porphyry-type deposits from different regions of the world. The conducted study contributes to the current knowledge on behavior of Cu-isotopes in minerals from ore deposits.