Home – Home – Jornals – Russian Geology and Geophysics 2021 number 3

The Malyi Orgochor, Orgochor, Birbash, Sukhoi Ridge, Ichketosma, and Tosma uplifts in the eastern Issyk-Kul basin are fault-related anticlinal folds composed of Neogene and Quaternary sediments involved in tectonic movements. The folds have asymmetric transversal profiles, with low-angle southern limbs and steep northern limbs cut by segments of the South Issyk-Kul and Karkara reverse faults reactivated in the late Quaternary. The location and geometry of the two faults, which both show reverse and left-lateral strike slip components, correspond to neotectonic propagation of deformation from the Terskey-Ala-Too Range over almost the whole eastern Issyk-Kul basin. Judging by primary and secondary coseismic surface deformation in the area, the South Issyk-Kul and Karkara faults repeatedly generated large earthquakes ( M ≥ 7, I ≥ 9) in the Late Pleistocene and Holocene. According to trenching results, the historical earthquakes that occurred in the first and 10-11th centuries accommodated motions on the South Issyk-Kul fault. The new seismotectonic and paleoseismicity data from the eastern Issyk-Kul basin provide updates to its seismic potential.

A 48 km long zone of surface deformation produced by the Ms = 7.3 intracontinental earthquake of 2003 in Gorny Altai is studied in its five segments between the Aktru and Irbistu rivers, where ruptures show the greatest offsets and distinct structural patterns. A total of 554 coseismic ruptures of five slip geometry types are analyzed in terms of length, orientation, and relative percentage. The rupture patterns are discussed with reference to previously published evidence and compared with other strike-slip zones worldwide.

We performed a comprehensive study of basanites from the Daur-Hentiyn Range and present the first data on their Sr and Nd isotope and mineral compositions (electron probe microanalysis). The study has shown that the basanites are chemically similar to the volcanic rocks of the South Baikal volcanic area (SBVA ) and are slightly different from oceanic island basalts in higher contents of Ba, Th, La, and Sr. The crystallization temperatures and pressures for the Fo0.85 olivine-melt equilibrium have been estimated, 1186-1137 °C, 1.09-1.06 GPa, and the sequence of mineral crystallization has been established. The obtained Sr and Nd isotope data indicate that the basanitic magmas were generated from material with PREMA and EMI isotope parameters.

We consider the distribution of trace elements and precious metals in sulfides and native gold of the Olimpiada deposit. Analysis of the obtained data provided conclusions about the source of ore substance and the parameters of the ore-forming fluid. We think that the deposit was formed by a medium- to high-temperature fluid with variable salinity and acidity/alkalinity, with chloride complexes dominating. The redox potential changed in the course of ore formation, but the ore mineralization formed under reducing conditions. The early Au-As mineralization might have formed with the major participation of crustal substance, but the supply of substance from another, probably deep-seated, source was also possible. The Au-Sb paragenesis differs significantly in REE and precious metals (primarily PGE) distribution, which might indicate a different source of substance (with a higher portion of a deep-seated component) during the formation of such parageneses. The Au/Ag ratios in sulfides, gold, and ores of the Olimpiada deposit point to the presence of Au and Ag sulfide minerals.

The paper focuses on compositional variations of olivine and chromian spinel in the Monchepluton and Pados-Tundra layered intrusions, which host significant chromitite mineralization. Ore-bearing dunite (with up to 25-30 vol.% Mcr) in the Sopcheozerskoe chromite deposit from the Monchepluton complex, Kola Peninsula, Russia, bears an assemblage of phases with exceptionally high magnesium contents: Fo96 + augite (Mg# = 94) + magnesiochromite, Mcr (Mg# ≈ 65); Mg# = 100·Mg/(Mg + Fe2+ + Mn). However, olivine in the host dunite has normal maximum values of Mg# comparable to those in cumulus olivine from layered intrusions worldwide (Fo≤91-92). The Fo96 phase in the Sopcheozerskoe deposit shows the most primitive composition ever reported from any layered intrusion. Magnesiochromite occurs as unzoned homogeneous euhedral crystals unaffected by subsolidus exchange or metasomatic effects. Olivine in ore-bearing dunite (20-25 vol. % magnesian chromite) from the Pados-Tundra complex attains Fo93, with the Mg# value notably higher than the range (Fo85.5-90.6) in olivine from orthopyroxenite, harzburgite, and dunite within the intrusion. Olivine and chromian spinel in the two complexes behave coherently, with covarying patterns of Mg# and Ni contents in olivine at R = 0.75 ( n = 160) and positive correlation between Mg# in coexisting chromian spinel and olivine grains at R = 0.8 ( n = 150). This behavior indicates that the two phases attained equilibrium during crystallization. It appears unlikely that the extremely high Mg enrichment in olivine (Fo96), as well as in all associated phases of the Monchepluton complex, would result from a subsolidus reaction between olivine and chromian spinel or low-temperature alteration of olivine. We suggest a more realistic explanation that the olivine (+ high-Mg augite)-chromian spinel assemblage crystallized from komatiitic magma under the conditions of progressively increasing oxygen fugacity ( f O2). The high Mg# in the Mcr-chromite-enriched system, above the maximum values common in cumulus olivine from layered intrusions (up to Fo96 against Fo≤91-92), may be caused by shortage of ferrous iron.

We present results of a study of plutonic-rock xenoliths from the Kharchinsky Volcano (Central Kamchatka depression). The studied xenolith collection comprises nine samples of peridotites and clinopyroxenites. The peridotites are identified as wehrlites, dunites, and harzburgites composed of olivine, clino- and orthopyroxenes, amphibole, and chromite in varying amounts. The clinopyroxenites consist mostly of clinopyroxene and often contain subordinate olivine, amphibole, hercynite, and magnetite. The xenoliths have interstitial segregations and veins composed of chlorite, plagioclase, K-feldspar, orthopyroxene, barite, fluorapatite, ilmenite, and, more seldom, anhydrite, phlogopite, and some other minerals. The study has revealed that veinlet minerals sometimes replace primary minerals and form pseudomorphs, thus indicating the metasomatic origin of interstitial and vein mineral assemblages. The thermobarometric calculations for minerals have shown that peridotites formed at ~1140 ºC and ≤10 kbar in the intermediate chambers at the depths from the spinel stability field to the Moho. Interstitial metasomatic alterations of rocks took place at ~400-850 ºC.

In this paper we present a stratigraphic scheme for the subdivision and correlation of the Cambrian deposits in the south of the cis-Yenisei area of West Siberia, which was adopted as a current scheme by the decision of the Interdepartmental Stratigraphic Committee in 2018. This scheme is based on the data from stratigraphic test wells (Lemok-1, Averinskaya-150, Tyiskaya-1, Vostok-1, Vostok-3, Vostok-4, etc.). In the study area, two structure-facies zones were identified: Kas (Lemok-1, Averinskaya-150, Tyiskaya-1, Vostok-4, and Eloguiskaya-1 wells), where sedimentary complexes accumulated in a salt subbasin, and Ket’ (Vostok-1 and Vostok-3 wells), with the deposition in an open sea basin. The boundary between these structure-facies zones is drawn along the inferred N-S-trending barrier reef zone. The rubrication in this paper is compiled in accordance with the requirements of the Stratigraphic Code of Russia for explanatory notes for regional stratigraphic schemes. Local stratigraphic subdivisions (formations, strata) are described and compared with the adjacent Turukhansk-Irkutsk-Olekma facies region of the Siberian Platform.

The least altered limestones of the Ukta and Eselekh formations in the Precambrian section of the Kharaulakh uplift have a minimum ⁸⁷Sr/⁸⁶Sr ratio of 0.70673-0.70715. The lowest ⁸⁷Sr/⁸⁶Sr ratio of the overlying Neleger and Sietachan formations is 0.70791-0.70817. Based on these data, along with the earlier obtained positive δ¹³С values (up to 8 ‰) for the Kharaulakh section, we have estimated the age of the Ukta and Eselekh formations at 800-670 Ma and the age of the Neleger and Sietachan formations at ~640-580 Ma. The Pb-Pb isochron age of the least altered limestones of the Eselekh Formation calculated from eight samples is 720 ± 30 Ma. This age permits us to define the lower part of the Kharaulakh section of the Ukta and Eselekh formations to be the late Tonian of the International Chronostratigraphic Chart or to the Upper Riphean of the General Stratigraphic Scale of Russia. The presence of reliably dated Upper Riphean sediments in the Kharaulakh uplift indicates a more complex structure of the Precambrian sedimentary cover on the Arctic margin of the Siberian Platform than assumed earlier.