Vol 479, No 2 (2018)

New data are presented on the geology and composition of volcanic and intrusive rocks of the Orochenka caldera, which is located in the western part of the East Sikhote Alin volcanic belt. The SHRIMP and ICP MS age of zircons of volcanic and intrusive rocks, respectively, and the composition of the volcanic rocks allow comparison of these complexes with volcanic rocks of the eastern part of the volcanic structure. New data indicate the period of transition between subduction to transform regimes.

This article presents new data on the age of the largest gold deposits in the southeastern part of Eastern Sayan. The dates have been obtained by Ar–Ar analysis of micas occurring in gold-bearing quartz veins and mineralized zones. The obtained Ar–Ar ages of fuchsite and sericite from the tectonized and mineralized zones of the Zun–Holba deposit (ore body Severnoye-3), range within 353.9–386.4 Ma; a similar result of 352.9 Ma was yielded by Ar–Ar dating of Cr–muscovite from mylonitized listvenite in the veins of the periphery of the Zun–Ospa gold deposit. However, muscovite from the ore-bearing quartz vein of the Pioneer gold–quartz deposit, located near Zun–Holba, has been dated to 421.9 Ma. The obtained new data on isotopic age of the gold–quartz ores and gold–sulphide–quartz deposits allow recognition of the Early Palaeozoic accretion–collision and the Late Palaeozoic shearing stages of formation of gold mineralization in the SE Eastern Sayan.

Forms of occurrence of platinum (sperrilite, moncheite) and palladium (Sb-michenerite, Pd–Bi phase) minerals in intrusive rocks of the Khudolaz differentiated complex have been studied. Platinum minerals were identified in disseminated Cu–Ni sulfide ores from ultramafic olivine–hornblende rocks of the Khudolaz complex, whereas palladium minerals were found in ores from olivine–hornblende gabbroids. The structural arrangement of grains as inclusions in sulfides of the primary magmatic association testifies that they were formed as a result of segregation of platinum group elements, which partitioned into the composition of sulfides during low-temperature mineral formation process at the late-magmatic stage.

U–Pb ID–TIMS zircon analyses of the Dzhigda gabbro–gabbrodiorite Massif (Ilikan block in the southwestern part of the Dzhugdzhur–Stanovoi superterrane) have been carried out. The results demonstrate that the formation of the massif at 244 ± 5 Ma corresponds to one of the stages of formation of the Selenga–Vitim volcano–plutonic belt. The latter stretches along the southeastern margin of the North Asian Craton along its border with the Mongol–Okhotsk fold belt. This indicates that the Selenga–Vitim volcano–plutonic belt along with granitoids and volcanics comprises Permian–Triassic massifs and that this belt is superimposed onto structures of not only the Selenga–Stanovoi terrane but also the Dzhugdzhur–Stanovoi terrane.

Using IR–Fourier spectrometry (FTIR) and simultaneous thermal analysis combined with quadrupole mass spectrometry of thermal decomposition products (STA + QMS), olivines and clinopyroxene from xenolites of spinel and garnet lherzolites contained in kimberlites and alkaline basalts were studied to confirm the occurrence of hydrogen and carbon within the structure of the minerals, as well as to specify the forms of H and C. The presence of hydroxyl ions (OH–) and molecules of crystal hydrate water (H₂Ocryst) along with CO₂, CH, CH₂, and CH₃ groups was detected, which remained within the structures of mantle minerals up to 1300°C (by the data of both techniques). The total water (OH–and H2Ocryst) was the prevailing component of the C–O–H system.

This work describes the geological position, mineral and chemical composition of high-Mg effusive ankaramites occurring as dykes and lava flows. They were found in the mélange zone of the western margin of the Magnitogorsk island arc zone in the Southern Urals. Data on the liquidus association of phenocrysts and on the composition of the matrix of effusives are given. According to the data obtained, the conclusion was drawn that the ankaramites studied can be attributed to the primary island arc melts, which were not subject to essential differentiation. This type of effusives has not been distinguished previously among island arc volcanogenic formations of the Urals. It is shown that ankaramites can be considered to be primary melts parental for dunite–clinopyroxenites–gabbro complexes of Ural–Alaskan type. The occurrence of ankaramites in the Paleozoic island arc formations of the Urals indicates the wehrlite composition of the mantle as the reason for the extremely wide development of wehrlites and clinopyroxenites in different mafic–ultramafic complexes of the Urals.

This work presents the results of geochemical (LA-ICP-MS) study of minerals of peridotites from ophiolite complexes of the Polar Urals to clarify the nature of these formations. The distribution of trace and rare earth elements in clinopyroxenes testifies that there were three types of the mantle substratum, which formed in different geodynamic settings. Two types of primary peridotites were formed upon partial melting of the mantle at different-depth levels in the subduction zone. The first type is represented by lherzolites and diopside harzburgites, formed at partial melting under the spinel facies conditions; the second type is represented by diopside harzburgites, formed under polybaric partial melting under the garnet and spinel facies conditions. In the suprasubduction zone, peridotites experienced fluid-induced partial melting that resulted in crystallization of harzburgites. All types of harzburgites were transformed by ascending melts and fluids (refertilization) and high-temperature hydration with the formation of amphibole. These processes are recorded in variations in the REE spectra of minerals.

An empirical model for the concentration dependence of the Gibbs free energy for solutions of chlorides of alkaline and alkaline earth metals in water is proposed. A simple analytical form of the Gibbs free energy makes it possible to obtain the equations of state for salt solutions that are equally accurate in the entire range of salt concentrations, from dilute solutions to solubility limits. The high accuracy of the thermodynamic description of solutions of high and intermediate concentration is ensured by the presence in the equation for the Gibbs free energy of two terms related to the Margules decomposition of the Gibbs free energy. Our form of the Gibbs free energy also contains a term that reproduces the thermodynamic behavior of solutions of electrolytes, which ensures high accuracy of the proposed model at low salt concentrations in the solution. Using the model, the equations of state for aqueous solutions of NaCl and CaCl₂ at water vapor pressure in the temperature ranges of 423.15 K–573.15 K and 423.15 K–623.15 K were obtained, which corresponds to the parameters of ore-bearing solutions participating in the formation of low-temperature hydrothermal ore deposits.

Hydrochemical analysis of the high-salinity lakes in the Ishim Plain (>250–300 g/L) located at the border with the Northern Kazakhstan uranium ore province is performed. The studies have shown that the main factor of concentration and redistribution of uranium in the lake basins of the Ishim Plain are the processes of intense salt deflation causing sanding of lakes and uranium depletion in the near-surface layer of the bottom deposits. The correlation between the hydroxide forms of uranium binding in the bottom lacustrine deposits of the Ishim Plain and the coffinite composition of the Semizbai deposit makes it possible to consider this province to be promising for the discovery of hydromineral uranium deposits.

To characterize the influence of alkaline metal chlorides on the phase ratios under melting of upper mantle eclogites, the eclogite–CaCO₃–NaCl–KCl system with Н₂О + СО₂-fluid was studied in the experiments under 4 GPa and 1200–1300°C. A low difference in temperatures (<100°C) was registered between the eclogite solidus and liquidus (>1200 and <1300°C, respectively), which is characteristic for the near-eutectic compositions. The phase proportions were peculiar for the absence of any silicate melt over the entire temperature range considered. The carbonate melt coexisted with clinopyroxene and garnet within 1200–1250°C, whereas a carbonate melt exclusively occurred under above-liquidus conditions at 1300°C. The melt quenching resulted in the formation of a multiphase fine-grained mixture of Ca, Na, and K carbonates and chlorides containing microinclusions of clinopyroxene and garnet. The occurrence of a high-calcium carbonate melt in Cl-containing eclogite systems might play a significant role in the mantle metasomatism of subduction zones characterized by the water–alkaline–chloride type of fluids.

The isotope-geochemical characteristics (La_N/Yb_N, Sm/Nd, ^εNd^(t), ²⁰⁷Pb/²⁰⁶Pb, and ⁸⁷Sr/⁸⁶Sr) of ice-rafted sediments in several areas of the Western Arctic (Fram Strait, the Yermak Plateau, the area between Spitsbergen and the North Pole) were studied. A similarity of the above characteristics to the same features of the surface sediments of the Ob and Yenisei estuaries, as well as the mouth areas of the Lena and Indigirka, was found. It was concluded that the main source of sedimentary material in the ice of these regions was the Asian continental margin.

The results describing the ability of contemporary global and regional climate models not only to assess the risk of general trends of changes but also to predict qualitatively new regional effects are presented. In particular, model simulations predicted spatially inhomogeneous changes in the wind and wave conditions in the Arctic basins, which have been confirmed in recent years. According to satellite and reanalysis data, a qualitative transition to the regime predicted by model simulations occurred about a decade ago.

The abilities of an ensemble of present-day global climate models are analyzed during modeling of evolution of the Arctic surface temperature over land including spatial consistency between the modeled and observed data. These results are the basis for probabilistic regional prediction of variation in the surface air temperature for the near future (2021–2040).