Home – Home – Jornals – Russian Geology and Geophysics 2026 number 4

Two large-scale volcanic eruptions occurred in the southern part of Iturup Island (Southern Kurils) in the Late Pleistocene, which resulted in the collapse of the Lvinaya Past caldera (partly flooded later), the largest one in the Kuril Island arc. It is 7 × 9 km wide, with a rim area of ca. 50 km² and a volume of ca. 25 km³ (including a submarine part of 12.26 km³). Comprehensive geological and geochronological studies have established that these two large-magnitude caldera-forming explosive eruptions (LP-I and LP-II) were separated by a repose period of several hundred years. The age of the first eruption (LP-I) is estimated at ca. 13,500 cal yr BP. The age of the second eruption (LP-II), based on a series of radiocarbon dates, is ca. 12,300 cal yr BP. Both eruptions were of Plinian type and involved the massive ejection of silicic pyroclastic material, which is represented by pyroclastic-flow deposits and tephra. In silica and total alkali contents the pumice from the caldera-forming eruptions corresponds to low-alkali dacites and rhyodacites (SiO₂ = 63.4-69.95 wt.%, total alkalies of 3.9-5.5 wt.%), whereas andesitic (SiO₂ = 58.3 wt.%, total alkalies of 3 wt.%) and rhyolitic (SiO₂ ≈ 74 wt.%, total alkalies of 5.6 wt.%) varieties are scarce. The total volume of erupted material from both events is tentatively estimated at 80-100 km³ (DRE = 35-45 km³), with the LP-II eruption being 30-40% more powerful than the LP-I one. We suggest that the LP-I and LP-II eruptions might have impacted both the regional and global environment.

The Valovayam and Tymlat adakites formed during the subduction of the Miocene oceanic lithosphere of the Komandorsky Basin beneath Northern Kamchatka, followed by their interaction with subarc mantle wedge peridotites. Postcollisional adakitic dacites from the Bakening Paleovolcano (Central Kamchatka) are related to the destruction and partial melting of the Mesozoic Kronotsky microplate paleoslab under the influence of the hot subslab asthenospheric mantle after the collision of the Kronotsky island arc. Minerals and volcanic glass in Kamchatka adakites contain predominant Cu-Ag-Au alloys and silver chloride microinclusions along with various sulfides, native metals, alloys, oxides, and hydroxycarbonates of chalcophile elements. Microinclusion associations in these adakites are similar to ore mineral assemblages in epithermal and porphyry deposits of the Russian Far East. The Kamchatka adakites show elevated silver and gold contents in comparison with back-arc basin and volcanic-arc lavas. These elements might have been sourced from the metabasites in the oceanic crust and metal-bearing pelagic sediments in the subducted slab. We conclude that adakites associated with the subduction of the young oceanic lithosphere (Northern Kamchatka) and the melting of the old oceanic lithosphere during the slab tear and break-off (Central Kamchatka) can be magmatic precursors of the cu-Au-ag mineralization in the Kamchatka region. We also suggest that adakite-related metallogenic processes are possible at the convergent plate boundaries in other settings, in particular, in the flat slab subduction setting.

Kotulskite, PdTe, is the most abundant palladium mineral in platinum-group element (PGE) deposits of the Fedorova-Pana Layered Complex (FPC). This paper presents new data on the noble metal paragenesis and chemical composition of kotulskite from the North PGE Reef at the Peshempakhk target. At this target, the North Reef, extensively explored at the Kievey deposit, extends eastward. Low-sulfide mineralization containing up to 15 g/t PGE + Au is exposed at Peshempakhk but does not form ore bodies at depth. The study aims to identify mineralogical distinctions between the discontinuous mineralization at Peshempakhk and the ore bodies of FPC deposits. A total of 890 grains of platinum-group minerals and Au were studied in polished sections using electron microscopy, including energy-dispersive X-ray microanalysis. The noble metal assemblage at Peshempakhk has the following relative volumetric composition: kotulskite 38%, isomertieite 22%, sperrylite 18%, stibiopalladinite 11%, hollingworthite 3%, and gold 3%. The noble metal paragenesis of the target differs from that of the main FPC deposits, where sulfides and tellurides of PGE predominate, namely braggite, vysotskite, merenskyite, moncheite, and kotulskite. Kotulskite at Peshempakhk averages 8.4 wt.% of Bi content, whereas deposits exhibit a more complete kotulskite-sobolevskite solid solution series with average Bi concentrations between 13.3 and 20.2 wt.%. Additionally, the studied kotulskite includes an antimony variety containing up to 10.3 wt.% Sb. The simultaneous presence of the two kotulskite types points to the lowest-temperature conditions of a magmatic setting. Thus, the noble metal paragenesis and composition of kotulskite from the FPC PGE mineralization are its key typomorphic features. These results can be used to forecast ore zones in similar settings.

The organic matter (OM) of the coal-bearing terrigenous strata of the Tunguska Group (C₂-P) and the Khanar Formation (C_2-3) as well as the Devonian carbonate rocks of the North Tunguska oil and gas region (OGR) has a complex geologic history due to strong heating by traps. This explains the reduced values of genetic biomarker parameters, such as n - C ₂₇/ n - C ₁₇ ( n -alkanes), C₂₉/C₂₇ (steranes), and hopanes/tricyclanes, in a number of terrestrial bitumens and the possible change in tricyclane index I _TC as a result of the accumulation/dispersion of migrating low-molecular compounds. The sterane maturity coefficient in most samples has undergone thermal inversion and is unfit for determining the grade of catagenesis. Devonian samples probably contain marine OM (low δ¹³C; high HI in the insoluble residue and kerogen; in the Pr/ n - C ₁₇-Ph/ n - C ₁₈ diagram of kerogen, the H/C_at values lie at the boundary or in the field of type II kerogen; also, low n - C ₂₇/ n - C ₁₇ and C₂₉/C₂₇ values are typical), although the isotope and pyrolytic characteristics of the kerogen of the Manturovka Formation (D₁) are distorted, probably because of high-grade catagenesis. The coal-bearing strata contain terrestrial OM (with a low HI of kerogen; in the Khanar Formation, it is additionally characterized by high n - C ₂₇/ n - C ₁₇, C₂₉/C₂₇ steranes, and hopanes/tricyclanes ratios). The genetic characteristics of the saturated fractions of bitumens ( n - C ₂₇/ n - C ₁₇, C₂₉/C₂₇ steranes, and hopanes/tricyclanes) in several upper Paleozoic samples are significantly distorted because of the catagenetic redistribution of compounds, with a predominance of low-molecular ones. According to the elemental composition of kerogen, a half of the samples of the Khanar Formation and the Tunguska Group can be assigned to type III kerogen, and the other half, to type IV kerogen. Because of trap intrusions, the catagenesis grade of Carboniferous-Permian OM generally increases upsection from MC₂ to apocatagenesis (which is evidenced by the changes in R ^o_Vt, MPI-1 of the aromatic fraction of bitumen, and H/C_at of kerogens and asphaltenes).

This study is concerned with detailed geochemical investigations of organic matter in Carboniferous-Permian source rock from the Anabar-Khatanga oil- and gas-bearing area, using a representative (both by area and by section) collection of core material. The study has first shown that the organic matter of Permian deposits is polygenous throughout the section and is at different levels of thermal maturation (stages MC¹₁-AC). It has exhausted its generative potential in most part of the section. The deposits contain paraautochthonous and allochthonous bitumens (chloroform source rock extracts) in addition to autochthonous (syngenetic) ones, which indicates intense migration processes. There are also bitumens with traces of biodegradation in Early Permian deposits from the Yuzhno-Tigyanskaya and Nordvikskaya wells.

We comprehensively analyzed the influence of nonlinear reservoir processes on well flowrate vs. drawdown for low permeability reservoirs with hard-to-recover (HTR) hydrocarbon reserves. New nonlinear relationships between the flowrate of low permeability reservoirs and reservoir drawdown (indicator curves) were identified. The nonlinearity of the indicator curves is due to the combined effects of nonlinear filtration, technogenic reservoir change, and the dependence of formation damage parameters on drawdown. The applied approach allowed us to find out qualitatively new regularities in the relationship between flowrate and drawdown in low permeability reservoirs. A well productivity analysis revealed hysteresis in the indicator curves and a shift in critical drawdown values when considering both formation damage and filtration nonlinearity. It has been found that the combined effects of nonlinear filtration and damage effects lead to an additional flowrate decline of 25-40%, as compared to separately considering each of these effects. The obtained results are of practical importance for optimizing the development of low permeability reservoirs with HTR reserves and for predicting their productivity.