Home – Home – Jornals – Russian Geology and Geophysics 2024 number 11

We carried out a detailed geological, geochronological, geochemical, and isotope study of diorites from a dike located in the central part of the Baikal uplift of the Siberian craton. The geochemical and isotope data obtained for diorites of the studied intrusion were compared with coeval mafic and intermediate igneous rocks of the southern part of the Siberian craton. The U-Pb (ID-TIMS) baddeleyite age of 1862 ± 7 Ma has been estimated for diorite from a dike located in the area of the Onguren Village. The obtained data are the first reliable age determination for the Paleoproterozoic mafic and intermediate igneous rocks of the Baikal uplift, which are part of the South Siberian postcollisional magmatic belt. The dike is of NE strike. The dike rocks correspond in chemical composition to diorites and are highly differentiated varieties (mg# = 36.5-37.4). There are no significant variations in the composition of diorites in the marginal and central parts of the dike. The rocks are characterized by low contents of TiO₂, P₂O₅, and Nb, high contents of Th, Zr, Ba, and LREE, and low negative values of ε_Nd( T ) (-5.9 to -6.2). We assume that the diorites formed from the enriched subcontinental lithospheric mantle. The obtained geochemical and isotope data show similar negative values of ε_Nd( T ) from -4.3 to -11.6 for most of Paleoproterozoic mafic and intermediate igneous rocks of the South Siberian postcollisional magmatic belt within the Aldan Shield, the Baikal uplift, and the Irkut block of the Sharyzhalgai uplift. These rocks correspond in geochemical features to rocks resulted from the melting of subduction-modified lithospheric mantle sources in the postcollisional extension setting at the final Paleoproterozoic stage of formation of the Siberian craton.

The paper presents data on the age and composition of phlogopite from dikes of ultramafic lamprophyres (aillikites) of the Zima Complex located within the Urik-Iya graben in the East Sayan region (southern margin of the Siberian craton). Samples for study were taken from postcarbonatite dikes of the Bol’shaya Tagna intrusion, Bushkanai dike, and Yuzhnaya volcanic pipe. The studied samples consist of olivine macrocrysts and fully crystallized groundmass composed mainly of phlogopite, perovskite, minerals of spinel and apatite groups, carbonates, and other minerals. Phlogopite is a typical groundmass mineral in the ultramafic lamprophyres of the Zima alkaline ultramafic carbonatite complex. It was also found in polycrystalline inclusions in olivines and in polyphase inclusions in chromites. In the studied samples, the groundmass phlogopite has tabular zoned crystals of greatly varying size and composition. The phlogopite grains show mainly a reverse zoning, with the cores composed of ferruginous high-alumina phlogopite. The intricate zoning probably resulted from the mixing of two magmas. The cores of phlogopites of the Zima Complex aillikites (with Al₂O₃ = 14-18 wt.%, FeO = 7-18 wt.%, and TiO₂ = 3-6 wt.%) formed from early portions of a more evolved aillikite melt. Phlogopite with Al₂O₃ = 10-14 wt.%, FeO = 4-10 wt.%, and TiO₂ = 1-2 wt.% crystallized from the parental aillikite magma. The phlogopite age of the Bol’shaya Tagna aillikites is 635 ± 7 Ma (TGK 3). The age of phlogopite from the Yuzhnaya pipe aillikites is 647 ± 7 Ma (BZT 4/21). The two ages are consistent with the time of formation of Neoproterozoic alkaline ultramafic carbonatite complexes of the Siberian craton and of other occurrences of aillikites resulted from the extension of the Rodinia lithosphere.

Au-Ag mineralization occurrences in sphalerite ores of hydrothermal genesis are paradoxical in view of the incompatibility of these elements in sphalerite. The formation of sphalerite with Au and Ag impurities under hydrothermal crystallization of ZnS at 450 ^oC and 1kbar pressure was studied experimentally. Sn impurity was taken as a source of point defects in crystals modelling the interaction of Au and Ag with vacancies. The Ag solubility in low-Fe sphalerite is estimated as 3.8 ± 0.7 µg/g, Au ̶ ≤ 0.6 µg/g. The main forms of Ag and Au occurrence in sphalerite are the inclusions of (Ag, Au) _x S phases with x varies mainly from 1.8 to 2.0, and Au varies from 0.01 to 0.75 a.p.f.u. The primary forms of the elements in ores might be microinclusions (Ag, Au) _1.8-2.1S or close to (Ag, Au)S at higher f _S2. In presence of Sn, solubilities of Au and Ag become higher. The behavior of Au corresponds to the substitution reaction Sn⁴⁺ + Au⁺ + v^- ↔ 2Zn²⁺ in the presence of two types of vacancy defects (v^-) - the “inherent” vacancies dependent on the crystallization conditions and the vacancies accompanying Sn⁴⁺ incorporation. Ag entrance is seemingly more dependent on f _S2 conditions and does not correlate with Sn. The extra vacancies arise because of metastable crystallization under the conditions of oversaturation of growth medium. This is supported by the spherulite morphology of growth products and the admixture of wurtzite ZnS form. The distribution and cocrystallization coefficients show an increasing trend for both precious metals (PM), due to which Au changes from incompatible to the category of highly compatible elements in sphalerite. The geochemical environments favorable for the formation of imperfect mineral crystals are considered. Such crystals are capable to uptake PMs and other incompatible in “ideal” crystal elements because of their interaction with vacancies, both constitutional (inherent to the substance) and non-equilibrium defects, and surficial nano-sized formations (nonautonomous phases). The evolution of these initially “invisible” forms of PM under metamorphic processes and remobilization of ore substance may result in Au and Ag escape and aggregation into microparticles.

This paper describes a detailed mineralogical and geochemical study (using electron probe microanalysis and secondary ion mass spectrometry) of zircon from rare metal pegmatites of the Polmostundra and Kolmozero lithium deposits (Kola Peninsula, Arctic zone, Russia). The zircons studied here have several distinguishing features. Firstly, anomalously high lithium content (up to 327 ppm Li according to secondary ion mass spectrometry data). Secondly, high hafnium content (up to 39.21 wt.% of HfO₂ according to electron probe microanalysis data). Thirdly, low Zr/Hf ratios (from 0.74 to 4.70). Fourthly, high content of volatile components (up to 5.98 wt.% of H₂O, up to 0.65 wt.% of F, and up to 0.12 wt.% of Cl according to secondary ion mass spectrometry data). Finally, higher content of impurity elements, the main of which are Th, Ta, U, Nb, Ca, and P. It is revealed by the results obtained that the rare metal specificity of the composition of highly fractionated pegmatite melts of the Polmostundra and Kolmozero deposits, from which zircons crystallized, are the key indicator of the high degree of its fluid saturation. The extremely low values of the Zr/Hf ratio for zircons can be explained by the combined influence of fractionation of the initial granitic melt with the accumulation of Li in the residual melt at the final stage of the evolution of pegmatites and the role of fluids saturated with volatile elements. Zircon crystallization occurred at the magmatic stage of pegmatite formation, and zircon alterations occurred at the hydrothermal stage. The nature of the rare earth element distribution in zircons from the Polmostundra and Kolmozero deposits indicates that the formation of the central zones and margins of grains took place during the magmatic and the hydrothermal stage, respectively.

Identifying patterns of geographic differentiation of individual groups of marine invertebrate fauna in the geological past is important for understanding their evolutionary history, solving issues and problems of biostratigraphic division, and correlation of deposits. Paleobiogeographic data is also necessary to identify the development stages of biota and the geological history of marine paleobasins and verify paleogeodynamic reconstructions. In this work, considering the latest data on paleontology and biostratigraphy of the Boreal Triassic, the taxonomic composition and distribution of Upper Anisian ammonoids in various regions of the Boreal realm are revised. A correlation of Upper Anisian deposits of Northeast Asia, British Columbia, Canadian Arctic Archipelago, Svalbard, Franz Josef Land, and Khabarovsk Krai was carried out at the zonal level and a basis was obtained for a comparative analysis of coeval ammonoid faunas. As a result of a qualitative and quantitative comparative analysis of ammonoid assemblages for various phases of the Late Anisian age, it was established that in the Late Anisian age, British Columbia was constantly part of the Canadian province of the Boreal realm, and Northeast Asia was part of the Siberian province. At the end of the Late Anisian age (most of the late part of the Frechites nevadanus or Frechites chischa phases), provincial differences were smoothed out and for the first time, all Boreal regions were included in the Siberian province, except the territory of British Columbia. Analysis of the geographical distribution of some groups of ammonoids in the Late Anisian age and changes in the areas of taxa over time made it possible to identify probable migration routes of Longobarditidae ( genus Longobardites ) and to introduce significant changes in the ideas about the centers of origin and migration of some Beyrichitidae.

This paper presents a solution to the problem of searching for cavernous fractured zones and the zones that determine reservoir heterogeneity in Famennian deposits of one of the licensed sections of the Republic of Bashkortostan. The use of an integrated approach to the study of azimuthal horizontal-transverse isotropy of rocks using wellbore data and modern methods for processing areal seismic data for carbonate deposits associated with the development of organogenic structures is described. The results of using the full-azimuth angular migration technology are obtained. The introduction of high-tech approaches at the research site using full-azimuth seismic data processing, spectral decomposition, AVA/AVAZ inversion, and complex analysis of dynamic properties of the wave field in conjunction with well logging and core data makes it possible to identify the distorted zones of the carbonate reservoir associated with changes in the characteristics of the rock skeleton, the presence of a porous-cavernous-fracture void space and lithological replacement zones, as well as secondary processes, thereby reducing the risks associated with subsequent production drilling.

The degree of organic-matter (OM) maturation in Proterozoic, Paleozoic, and Mesozoic deposits in the Anabar-Khatanga saddle has been determined from examination of the reflectance of macerals (mainly vitrinite). The Proterozoic deposits contain only graphite inclusions of uncertain origin. In the Cambrian, Devonian, and Carboniferous strata, OM has transformed predominantly to apocatagenesis grades (АC_2-3). Similarly transformed OM (АC_1-3) is found in the lower Permian Tustakh and Lower Kozhevnikova formations. In some wells and areas, the degree of OM catagenesis is not higher than МC₁²-МC₂ grades (classification after A.E. Kontorovich). In the Upper Kozhevnikova Formation, catagenesis grades are within МC₁¹-АC₁ in the upper Permian and within МC₁¹-МC₃² in the Triassic. In the Jurassic deposits, OM has transformed to early mesocatagenesis grades (МC₁¹-МC₁²) and reached МC₂ at the most deeply buried segments. In the Cretaceous strata, OM is less transformed, corresponding to PC₃-МC₁¹ or, seldom, higher grades of catagenesis. Some complexes in the Permian, Triassic, and Jurassic-Cretaceous deposits with OM of moderate maturity might be petroleum-promising.

Smectite illitization geothermometry has been used to assess the thermal maturity and hydrocarbon generation potential of the late Paleocene to early Eocene Patala Formation in the Upper Indus Basin. X-ray diffraction (XRD) detected illite, muscovite, quartz, kaolinite, chlorite, and calcite. Comparison between air-dried (AD) and ethylene glycol (EG)-solvated XRD patterns reveals the absence of discrete smectite and interstratified illite-smectite (I-Sm). Additionally, authigenic illite-2M₁ indicates that the Patala Formation has entered the late-stage diagenetic zone or the low anchizone, which lies in the R3 illitization zone. Abundant SiO₂ and Al₂O₃ in the Patala clay fraction indicate the substitution of tetrahedral Si⁴⁺ by Al³⁺ within the smectite interlayers. Likewise, the relative abundance of K₂O to CaO and MgO indicates the exchange of K⁺ with Ca²⁺ and Mg²⁺ during smectite illitization. Scanning electron microscopy (SEM) reveals in situ growth of platy illite crystals that form within the R3 zone. Furthermore, the total organic carbon (TOC) of the Patala shale exposed in Tirah suggests a poor to good source rock. The absence of discrete smectite and I-Sm, combined with the detection of discrete illite in the Patala shale, suggests that hydrocarbon might have potentially migrated from the source to the reservoir rock during smectite illitization.

This paper presents results of experimental studies of a thermal field in the barrel of a shut-in (no fluid movement in the casing) well in relation to determining a behind-the-casing upward flow using the method of active thermometry. The studies are carried out using the physical model of a well that is a vertically located steel pipe with a system of externally attached copper tubes simulating a behind-the-casing flow. The pipe contains a local heating section, above which a temperature probe is located to record thermal disturbance from the heating section. The effect of free convection in a fluid on the temperature field in the pipe during and after heating is described. It is revealed that there are high-frequency temperature oscillations on sensors that record the temperature of the inner surface (wall) of the pipe and fluid above the heating area, whose value reaches higher than 2 °C and decreases when the distance to the heating region becomes longer. There is an empirical relationship that relates the time of arrival of the temperature disturbance front associated with free convection and the distance to the pipe heating region. Azimuthal temperature distribution curves on the inner wall of the pipe above the heating section are constructed in the absence and presence of a behind-the-casing flow. Qualitative criteria have been obtained indicating the presence of an azimuthally localized behind-the-casing flow (sector flow) of fluid based on the azimuthal temperature distribution analysis.

Years after the Chuya earthquake of 2003, geological structures adjacent to the focal area of the Chuya earthquake are still seismically active. The Aigulak focal area is one of them, but energetically the most pronounced. Detailed studies have been carried out with the network of stations of the Altai seismological testing site, supple-mented by temporary stations. The region activated in the form of a local and compact structure measuring 10 × 10 km with focal depths from the first 100 m to 20 km. The focal area is not a subsequent activation along the same fault with the Chuya earth-quake, but is located on a subparallel fault in the nodal region with its branching into three faults. The seismic activation of the Aigulak focal area is not an aftershock pro-cess after a major earthquake, but is an activated structure with a dynamically changing seismic process. An intensive process has formed since the earthquake in 2012 with M_L = 6.1 with a gradual decrease in the number of earthquakes, and in 2019 the Aigulak earthquake with M_L = 5.5 occurred with a very strong aftershock process after it. Our results of an area study of earthquake density in the focal zone indicate a change in the regime over time: from chaotic to self-organizing along short faults. We conclude that the focal area has not reached the maximum level of seismic energy release.