Home – Home – Jornals – Russian Geology and Geophysics 2024 number 5

New petrographic, geochemical, and geochronological (LA-ICP-MS) data have been obtained for the Early Proterozoic upper Ilikta Formation of the Sarma Group within the Baikal uplift, a basement inlier of the Siberian craton. The Ilikta Formation consists of polymictic and graywacke sandstones and shales of graywacke siltstone and silty-pelitic mudstone varieties, identified from major-element chemistry and petrography. The mineralogy and chemistry of the samples indicate their origin from a felsic source. The Ilikta Formation rocks eluded the Early Proterozoic regional metamorphism but underwent secondary alteration to different grades associated with early Paleozoic collisional events. The Ilikta Formation was deposited during the 1.91-1.95 Ga interval, as constrained by the ages of regional metamorphism (1.95-1.98 Ga) and granitic intrusion (1.86-1.91 Ga). These estimates are consistent with dating of detrital zircons from the Ilikta Formation sandstone which showed the youngest ages of 1955-1993 Ma. The detrital material for the Ilikta Formation deposition was apparently derived from the igneous and metamorphic rocks of the Early Proterozoic Akitkan orogen, and some amount of material additionally came from the adjacent Aldan superterrane located in the east. The reconstructed geodynamic environment of sedimentation, together with the petrography and chemistry of rocks, allow interpreting the Ilikta Formation of the Sarma Group as molasse accumulated in extension basins after the formation of the Akitkan orogen.

The paper presents the results of studying inclusions and determining the U-Pb age in zircon from the diamond-bearing garnet-pyroxene rock of the Kumdi-Kol metamorphogenic diamond deposit, located in the Kokchetav subduction-collision zone. The distribution of rare earth elements in garnet and clinopyroxene is used as a basis for estimating the equilibrium pressure (5.5 ± 0.3 GPa) and temperature (993 ± 24 ℃), which correspond to the diamond stability field. The composition of mineral inclusions in zircon indicates its formation at both progressive and regressive stages of metamorphism. The concordia diagram shows that zircon figurative points lie on the discordia with an upper intersection at 1953 ± 139 Ma and a lower intersection at 512 ± 4 Ma. The main peak on the graph of the probability density distribution of zircon ages corresponds to an age of 519 Ma. The presence of grossular-almandine garnet inclusions in zircon confirms the previous assumption that the basement rocks of the Kokchetav massif act as protoliths of garnet-pyroxene rocks.

Carbon and oxygen isotope compositions have been determined in (CO₃)-groups of Ca carbonate-silicate minerals (spurrite, tilleyite, and scapolite) and calcite from samples of the Kochumdek contact aureole (East Siberia). The observed small δ¹³C and δ¹⁸O difference between the Kochumdek marbles and their protoliths (Δδ¹³C ≤ 1.5‰ and Δδ¹⁸O ≤ 2.0‰) is consistent with moderate high-temperature metamorphic decarbonation of the protolith (0.1-0.2) and with almost absent metasomatic alteration in the aureole. Significant ¹⁸O depletion was inferred only for vein scapolite (δ¹⁸O from +17.0 to +17.3‰ V-SMOW) and calcite from recrystallized limestone layers (δ¹⁸O from +18.9 to +20.6‰), which is evidence for limited infiltration of magmatic aqueous fluids from the cooling sill into the host sediments. The C and O isotope compositions of (CO₃)-bearing spurrite and tilleyite show lower ¹⁸O depletion and insignificant ¹³C depletion relative to calcite, especially in highest-temperature merwinite marble (Δδ¹⁸O_Spu-Cal = +3.3‰; Δδ¹³C_Spu-Cal = -0.8‰). Gas chromatography-mass spectrometry (GC-MS) provided constraints on the relative percentages of H₂O and CO₂ in the fluid at highest-temperature spurrite-merwinite metamorphism. The fluid phase entrapped in calcite from merwinite marble is rich in CO₂ ( X _CO2 = 0.4-0.6).

The Udokan sedimentary basin is a unique geological structure containing many copperstone deposits and occurrences. A detrital chrome spinel is identified in the metasandstones of the Sakukan Formation at the Udokan and Unkur deposits, which tend to interlayers of natural heavy mineral concentrates. The mineral forms relict inclusions in magnetite crystals. Its composition is distinguished by high concentrations of zinc (up to 11.62 wt.% ZnO) and manganese (up to 6.32 wt.% MnO) with an almost complete absence of magnesium. In comparison with the associated magnetite, chromite is significantly enriched in germanium (up to 666 ppm) and gallium (up to 59.1 ppm). The formation of Zn-chrome spinel occurred during the transformation of detrital chromite against the background of specific chemical processes accompanying the deposition and crystallization of iron gel enriched with adsorbed impurities.

We studied the distribution of gold, silver, and platinum group elements (Pd, Pt, Rh, Ir, and Ru) in two bottom sediment cores of the southern Chukchi Sea. It is shown that the Holocene pelite-silty sediments with an age of up to 4.0 ka BP are significantly enriched in these elements, except for Ru and Rh, relative to their clarkes. Native silver minerals were found in all samples by probe microanalysis, whereas gold minerals were revealed only in the surface layer of the sediment core closest to the Chukchi Sea coast. Multicomponent statistical analysis of the chemical composition and grain size of the sediments and the content of organic matter in them has led to the conclusion about the accumulation of clastogenic and chemogenic forms of precious metals. The abnormally high content of gold (0.3 ppm) in the recent sediments near the Chukchi Peninsula coast might be due to its additional removal from the continent as a result of the placer mining there.

Based on integrated structural and geophysical studies and generalization of geophysical, hydrogeological, mineralogical, geochemical and other data, it is shown that the southern cluster of thermal fields of the Kambalny volcanic ridge (South Kamchatka) is included in the modern hydrothermal system. New data on the structure of the steam hydrotherm discharge area investigated to a depth of about 300 m have been obtained. The structural and hydrodynamic connection between two thermal fields from this group has been established. The Kambalny ridge geothermal system embracing all major clusters of thermal fields is assumed to have a single deep-seated source of heat supply.

Surface waves are the main source of coherent noise in land seismic survey, and their suppression is one of the main stages of common depth point data processing designed to improve the quality of tracking primary reflections on time sections. In practice, noise reduction is carried out using procedures from modern software based on numerical modeling of waveforms. However, they are too resource-intensive and have a large number of subjectively customizable parameters. The known algorithms have a common drawback: either the energy of reflected waves is distorted in an interference zone with a noise wave or the noise suppression quality is unsatisfactory. The current research is aimed at improving the filtering algorithm in a time-frequency domain using the slant Karhunen-Loeve transform in order to overcome these limitations, to increase the accuracy and rate of its software implementation, and also to test it when processing profile field data from land-based 2D seismic surveys. The algorithm is modified by developing a new method for determining static corrections for surface wave hodograph rectification in a time-frequency domain and by the application of preprocessing in which the reflected wave signal is removed preliminarily. These and other modifications ensure faster calculations and improve the quality of surface wave interference suppression. In addition, the slant Karhunen-Loeve transform is accelerated by parallelizing calculations across logical processor cores. In this paper, the algorithm is described in detail, its significant advantage over the standard methods of bandpass filtering and f-k filtering is shown, and the results of processing the field data obtained by the SWANA procedure (Geovation 2.0) and by the slant Karhunen-Loeve transform. The result obtained by the slant Karhunen-Loeve transform is superior to the SWANA procedure in terms of the surface wave filtering quality and has only four adjustable parameters (SWANA has 20 parameters).

The paper discusses the results of the development of a deep learning-based algorithm of the inverse Sumudu transform applied to the problem of on-ground non-stationary electromagnetic sounding. The Sumudu transform has potential for solving forward geoelectric problems in three-dimensional earth models because, unlike using the Laplace or Fourier transform, the Sumudu image of a real function is also a real function. Thus, there is no need to use complex numbers in subsequent calculations, which reduces computational costs and memory requirements in case of successful determination of the Sumudu image of the function. The disadvantages of the approach include the absence of an explicit method for calculating the inverse transform. The inversion can be done by solving the corresponding Fredholm integral equation of the first kind, but this is a poorly conditioned task leading to high requirements for the accuracy of the Sumudu image. The use of modern machine learning techniques can provide a method that is more robust to noise in the input data. This paper describes the process of creating a training dataset and developing a neural network algorithm; we evaluate the accuracy and performance of the obtained solution. The proposed method can contribute to the development of new approaches to physical processes modeling as well as to analysis, processing and interpretation of measured geophysical data.