Home – Home – Jornals – Russian Geology and Geophysics 2019 number 8

For a multirate approximation, we have determined the dynamics of rock heating by a magmatic-fluid flow in a flat permeable zone cutting the cratonic lithosphere of the Siberian Platform from a magma chamber at a depth of 50 km to the Earth’s surface. This dynamics is compared with the dynamics of infiltration metasomatism in a three-layer lithosphere section: (1) harzburgitic mantle (depth 50-40 km), (2) crystalline basement (39-7 km), whose composition was simulated by the section of rocks hosting the skarn deposits of the Aldan Shield, and (3) platform cover (6-0 km), with its simplified rock compositions specified on the basis of the rock compositions in the southern and northern parts of the trap area of the Siberian Platform. Numerical modeling of the metasomatic transformation of rocks was performed in a multireservoir flow reactor, using the Selektor software. The initial composition of fluids in a magmatic source varied from highly reduced (water-methane) to ordinary (water-acid) (lg p _O2 from -13.0 to -12.0). The obtained balances of the interacting phases show no significant change in the mass of aluminosilicate rocks in the mantle and Earth’s crust sections and a significant loss of their mass under replacement of carbonate and sulfate deposits.

New petrological, isotope-geochronological, and geophysical data reveal two phases of magmatism in the Oortsog mafic-ultramafic intrusion in Western Mongolia. The intrusion consists of (i) rhythmically layered peridotite gabbro (278.7 ± 2.5 Ma) and (ii) poorly differentiated biotite-containing amphibole-olivine gabbro and gabbronorite (272 ± 2 Ma). The inverse upward succession from leucocratic to melanocratic lithologies within each rhythm indicates that the intrusion was tectonically overturned. The earlier rocks (phase 1) have lower contents of major oxides (Na₂O + K₂O, TiO₂, and P₂O₅) than the later ones (phase 2) and show different patterns of incompatible elements. The intrusive rocks of phases 1 and 2 were derived from depleted (positive ε_Nd) and enriched (negative ε_Nd) mantle sources, respectively. The calculated parental melts of both phases belong to picritic (Mg-rich) basaltic magma. More insight into the intrusion structure was gained by mapping magnetic anomalies.

We present results of study of sulfide ore occurrence in highly metamorphosed (granulite facies) Archean rocks of the Siberian craton basement. The host rocks and ore minerals are briefly described, and new data on the multiple sulfur isotope (ґ33S, ґ34S, ”33S) composition of sulfides are presented. Application of high-resolution analytical methods enabling the assessment of the sulfur isotope behavior in situ made it possible to reveal mass-independent fractionation of sulfur isotopes in the rock samples. The isotopic composition of sulfur in the sulfides indicates its inflow fr om several sources, including the ancient Archean atmosphere, wh ere primary sulfur has passed through a cycle of fractionation. Despite the high-gradient metamorphism, the subsequent ultrametamorphic and post-ultrametamorphic transformations accompanied by a change in the primary mineral composition of rocks and by chemogenic fractionation of sulfur, the signature of the sedimentary source of sulfur in sulfide ores has been well preserved. Analysis of the chemical composition of rocks and ore minerals and of sulfur isotopes has led to the conclusion that the studied rocks are metamorphosed Late Archean analogs of black shales and the sulfide mineralization is of stratiform pyrite type.

To solve the problems of regional forecast within the Yana-Kolyma gold ore province, analysis of the spatial distribution of 2140 orogenic ore objects and their gold grades was carried out. Ore objects with >1 ppm Au are mapped on a scale of 1: 2,500,000, and areas of high mineralization density points are outlined in the sequence: general contour-contours around clusters of points with high Au grade-contours around proximal clusters of points-long axes of anisotropic contours. The curves obtained after the interpolation between the axes, with regard to the actual position of the ore objects, are interpreted as intersections of the recent topographic surface with the planes of faults that were active at the time of ore formation (ore-hosting faults). We propose to call the curves intersecting the known deposits “trends” (arched and linear). If no deposits have been revealed, the curves should be called “ore lineaments”, regardless of their curvature. The shape of the general contour around the gold ore occurrences and the distribution of ore objects within this contour permit outlining the Upper Indigirka (UID) and Central Kolyma (CKD) megadistricts. The geometry and spatial position of trends and ore lineaments are compared with the recent structural plan of the area, with geophysical fields, and with the existing ideas of the kinematic types of faults arising under certain geodynamic regimes of evolution of structures in the Yana-Kolyma province. It is suggested that the ore-hosting faults formed successively during the collisional interaction of the passive margin of the Siberian continent with the Kolyma-Omolon superterrane and during the formation of the volcanic arcs of the Okhotsk-Chukchi volcanic belt. Trends and ore lineaments can be correlated with the groups of structures of ore fields formed at the late collisional and subduction (postcollisional) stages of the area evolution. The trends and ore lineaments of NW orientation are attributed to the folded reverse faults and thrusts and to shears of different kinematics. At the postcollisional stage, the ore lineaments and trends of NE orientation formed in the zones of tectonomagmatic activity, subparallel to the strike of volcanic arcs of the Okhotsk-Chukchi volcanogenic belt. The distribution of clusters of gold ore occurrences and “empty” intervals between them along the strike of the recognized structures probably corresponds to the distribution of areas of extension and compression in the plane of the ore-hosting faults. The correlation among the trends, ore lineaments, and ore-hosting faults permits forecasting for the position and approximate size of new ore bodies within the Yana-Kolyma province. Prospective areas for a gold ores prospecting and exploration have been outlined on the extrapolated and interpolated extensions of the trends and at the sites of their intersection.

The Kalguty Mo-W ore-magmatic system (OMS) is a granite batholith (S = 70 km2, V = 12,800 km3 at the recent denudation level, according to geological and geophysical data). This batholith is cut by the East Kalguty belt of rare-metal ongonite-elvan dikes spatially and temporally associated with the East Kalguty quartz-vein-greisen Mo-W deposit. Geological and petrogenetic studies along with published and our new results of geochronological (U/Pb zircon, Re/Os molybdenite, and 40Ar/39Ar biotite and muscovite) dating made it possible to reconstruct the thermochronological history of the Kalguty OMS. Five stages have been recognized: I (215 ± 1 Ma)-formation of granites of major intrusive phase and of Mo-rich mineralization, which is an orebody called the Molybdenum stock; II (206 ± 1 Ma)-formation of leucogranite and intragranitic-pegmatite stocks in the granites of major intrusive phase; III (202 ± 1 Ma)-formation of most of ongonite-elvan dikes composing a dike belt; IV (195 ± 1 Ma)-formation of long ultrarare-metal ongonite-elvan dikes in the central part of the dike belt, which is spatially associated with the W-rich veins of the deposit; and V (181 ± 1 Ma)-formation of thin ongonite-elvan dikes on the periphery of the dike belt. The recognized age stages of the Kalguty Mo-W ore-magmatic system were mathematically tested based on the model of crystallization differentiation and the dynamics of heat and mass transfer in the magma chamber corresponding to the Kalguty granite batholith. The results obtained show that the formation of a granite batholith (215 ± 1 Ma) and a later ongonite-elvan dike complex with Mo-W-rich mineralization (195 ± 1 Ma) can be explained only by a two-level ore-magmatic system with the “upper” granite batholith at a depth of 5-15 km and the “lower” granite chamber at a depth of 20-31 km. The total duration of ore-magmatic processes is 20 Myr (ore production stage) or 30 Myr, if we take into account occasional elvan dikes with poor quartz-fluorite-barite-ferberite mineralization (181 ± 1 Ma) on the periphery of the Kalguty deposit.

The paper presents new lithological, geochronological, and biostratigraphic data on the studied sections of the Middle Valdai interstadial sediments in the sections of two terraces in the basin of the Severnaya Dvina River and on the southern coast of the Kola Region and in its southwestern part. The obtained data are correlated with the data available for other regions of northeastern Europe. The Severnaya Dvina River was found to have flowed into a marine reservoir existed in the White Sea basin throughout the Middle Valdai time corresponding to Marine Isotopic Stage 3 (MIS 3). The alluvial sedimentation in the river catchment area occurred in the environment of base level instability and permanently changing climate. In northeastern Europe, the warmest (optimum) intervals are related to the time spans of 47-43 and 31.3-29.2 cal. ka BP. In general, the available data point to 12 warming and cooling episodes in northeastern Europe.

We discuss the results of a model experiment on cultivation of the microbial community from the area of the Khoboy mud volcano (Akademichesky Ridge) conducted under conditions typical of the hydrocarbon generation zone (80 °C, 5 MPa). The cultivation under conditions of biomass enrichment with the Baikal diatom Synedra acus changed the composition of organic matter. The transformation degree of organic matter in the sediment after the experiment was 16%, whereas the concentration of phenanthrenes relative to methyl-substituted homologues, including retene, decreased, and the concentration of dibenzothiophenes relative to normal alkanes increased. We have identified tri- and monoaromatic steroids, including 17-dismethyl, 23-methyl monoaromatic steroids C₂₇. An increase in the concentration of tri- and monoaromatic steroids in the sediments after the experiment might indicate that the biomass of the Baikal diatom S. acus was destructed, which led to an increase in the steroid concentration. In the control (sterile) sediment, we detected no changes in the composition of organic matter. Representatives of various taxa capable of surviving in anaerobic thermophilic conditions have been identified in the microbial community by molecular genetic methods. Their presence in the surface sediments might be due to the inflow of deep-seated mineralized fluids and breccia from deep-seated sedimentary rocks.

A member and bed-by-bed division of the Upper Vendian formations in the boreholes drilled in the Baikit anteclise and the Angara fold zone in the west of the Siberian Platform has revealed sedimentation gaps above the Danilovka Horizon and above and within the Tira Horizon as well as amplitude erosion of the sediments. Lower beds appear in the above-gap Il’bokich basal argillaceous-dolomite-marl member of the Katanga Formation of the Danilovka Horizon. The thicknesses of all beds gradually increase from north (Baikit anteclise) to south in the Angara fold zone. The upper members of the Tira Horizon did not accumulate below the gap in the north. The Yurubchen Plateau (uplift) was in this area. South of it, the formation thicknesses increase; there are apparently no gaps in the Beryambinskaya area of the Angara fold zone. The Il’bokich member makes up the top of the Moshakovka Formation but is part of the Danilovka Horizon. The Moshakovka Formation spans the top of the Tira Horizon and the bottom of the Danilovka Horizon. In general, stratigraphic bodies in areas with a rapid increase in the tectonic-downwarping amplitudes have a typical geologic structure.