Home – Home – Jornals – Russian Geology and Geophysics 2020 number 8

We present results of petrographic, geochemical, and mineralogical studies of apogranite metasomatites associated with sulfide-quartz gold ore veins. The studies show a predominance of muscovite and quartz-muscovite metasomatites. Formation of muscovite metasomatites was accompanied by the accumulation of W, Sc, Zr, Hf, Ga, REE, U, Th, Ta, and Nb and the genesis of new accessory minerals: monazite-(Ce), apatite, zircon, scheelite, W-containing rutile, uraninite, thorianite, cassiterite, etc. Compared with the primary granites, quartz-muscovite metasomatites are richer in Pb, Bi, As, Sb, Co, Ni, Ba, In, Cd, Mo, Te, Ag, and Au (elements of the gold ore assemblage). The high contents of these trace elements are due to abundant galena, fahlores, chalcopyrite, and pyrite among the accessory minerals. Metasomatism of granites was followed by the removal of SiO₂, which was then spent for the formation of quartz veins. We have revealed that the distribution of metasomatites of different types within a dike body affects directly the distribution of sulfide-quartz veins and thus determines the ore content of the dike body fragments.

We studied fluid inclusions, trace elements, and oxygen, carbon, and sulfur isotope ratios in minerals from stockwork sulfide-carbonate-quartz ores of the Nikolaevskoe gold deposit confined to volcanic island arc porphyry intrusions. The study shows that fluid inclusions in quartz were homogenized at 260-200 ºС and those in later formed calcite, at 227-205 ºС. The fluids contain aqueous K-Mg-Na chloride solutions with salinity of 4.1-9.6 wt. % NaCl eq. Raman spectroscopy revealed СO₂ (29-34 mol. %), CH₄ (40-55 mol. %), and N₂ (8-30 mol. %) in the fluids. According to LA-ICP-MS data, quartz has low contents of Al (11.7-102 ppm) and Ti (0.05-0.64 ppm), which indicates its deposition from weakly acid low-alumina fluids at <350 ºС. The REE patterns of calcite show accumulation of heavy lanthanides (La _N /Yb _N = 0.2-0.9), evident of high fluid alkalinity, and negative Ce (0.39-0.82) and positive Eu (1.99-5.25) anomalies. The negative Ce anomalies are due to meteoric water in the fluid and the fluid interaction with limestones. The positive Eu anomalies reflect the high-temperature (>250 ºC) environment that existed before the calcite crystallization. The Y/Ho ratio in calcite (28.7-54.1) suggests that the fluid has magmatic components and components extracted from limestones and contains seawater. The δ¹⁸О_H2O values (3.6 to 5.6 ‰) of the fluid testify to the participation of magmatic and meteoric waters in the ore formation. The δ¹³С_СO2 values (-4.1 to 1.4 ‰) point to carbon of magmatic nature and carbon extracted from limestones. The heavy sulfur isotope composition of pyrite (6.75-9.87 ‰) and arsenopyrite (8.7 ‰) might be due to sulfur supply from the host rocks or to the participation of [SO₄]^2- of seawater in the ore-forming process. According to the results obtained, the Nikolaevskoe gold deposit is an island arc (Au ± Cu)-quartz-sulfide deposit transitional between porphyry and epithermal types.

We have estimated the P-T conditions of formation of basaltoid melts: P = 1.15-1.06 GPa and T = 1379-1293 ºC. Olivine pyroxenites (Ol + Cpx + Grt) are assumed to be a mantle source for nepheline-normative basanitic melts. During transition, the melt trapped mantle xenoliths, which were disintegrated into olivine and augite xenocrysts. A decrease in pressure and temperature led to the crystallization of highly magnesian (Mg# = 86) olivine and diopside and Timgt + Ilm ± Pl phenocrysts. Then, the Ol + Cpx + Timgt + Pl microlite paragenesis formed. Alkaline aluminosilicates (acid plagioclase + nepheline + leucite) were the last to crystallize in the rock interstices. The presence of residual glass indicates that the last stage of crystallization took place under subsurface conditions. The volcanic area of southern Transbaikalia (Khentei Ridge) resulted, most likely, from the mantle plume impact on the lithosphere. The age of this area is estimated at 3.51 Ma. PREMA was the main mantle source for these volcanics, and the contribution of HIMU was strongly subordinate. In geochemical features the studied volcanics correspond to mafic rocks of oceanic islands. They are similar in composition to alkali basalts of the South Baikal volcanic area.

The paper discusses the formation of the chemical composition of water in the sulfate mineral lakes of western Transbaikalia. It is shown that sodium sulfate water in the regional lakes forms in two zones of continental salinization, the Barguzin and Gusinoe Ozero-Orongoi depressions. The average mineralization of the lakes reaches 30.8 g/L, pH = 7.90-8.84, the fractional content of sulfate ion is 55.2-81.8 % eq, and the fractional content of sodium ion is 59.1-82.1 % eq. The paper presents the major-ion and trace-element composition of lake waters and the chemical and mineral composition of bottom sediments. It is shown that groundwater enriched in sodium sulfate plays a major role in the lake formation.

The paper presents results of the pioneering study of equilibria in the water-gas system by the example of Jurassic-Cretaceous deposits of the Yamal-Kara depression located within northern West Siberia and its Arctic regions. Numerical modeling of physicochemical equilibria and evasion-invasion processes in the water-gas system allowed determination of the degree of groundwater saturation with gases and the nature of diffusive redistribution of gases in the media that form at the hydrocarbon deposit-groundwater contact. According to the degree of water saturation with gases ( K_g ), aquifers with poorly (<0.2) to ultimately (0.8-1.0) gas-saturated waters have been established. The revealed increase in the degree of groundwater saturation with gases in sinking producing reservoirs reflects its dependence on their total gas saturation. All waters with a total gas saturation of more than 1.8 L/L become ultimately saturated with gases ( K_g = 1.0), thus theoretically predetermining the formation of hydrocarbon accumulations. Major gas condensate deposits are confined to the zone of gas-saturated waters with K_g from 0.8 to 1.0, while oil accumulations, to waters with lower gas saturation. Based on the established nature of water-gas, we can argue that oil and gas accumulations in Jurassic-Cretaceous reservoirs act as a conservative element of the lithosphere, i.e., its geologic and geochemical “relics”. The surrounding subsurface waters, as a more active constituent of the system, have largely anticipated its geochemical development, which is manifested in the differentiation of the fugacity ratios of individual gases in groundwater and hydrocarbon accumulations. The composition of the latter is therefore subjected to slow directional changes while the equilibrium is established, to usher in the qualitatively new state of the geochemical water-gas system.

A combination of modern analytical methods was used to study terrestrial organic matter from upper Paleozoic rocks in the central part of the Vulyui hemisyneclise (East Siberia), sampled from the depth interval 3370-6458 m penetrated by the Srednevilyuiskaya-27 superdeep well. In this study we used 71 core samples to trace depth variations in the geochemical characteristics of organic matter during middle-late mesocatagenesis and apocatagenesis: organic-carbon content (C_org), Rock-Eval pyrolysis parameters (HI, Т _max), carbon isotope composition, thermal maturity ( R ^o_Vt, %), hydrocarbon type composition (hydrocarbons, resins, and asphaltenes), distributions of n -alkanes (TIC), steranes ( m / z 217), and terpanes ( m / z 191), and standard biomarker indices with increasing degree of thermal maturity. It is shown that during late mesocatagenesis and apocatagenesis, the pyrolysis parameters and biomarker indicators are no longer effective as indicators of organic-matter maturity and genesis.

The work is a study of the geologic structure and hydrocarbon generation potential of Devonian terrigenous strata in the Perm Territory. Prediction for new oil fields is made using quantitative criteria for determining the scale of oil generation by the petroleum source rocks of this petroleum system and the scale of the oil migration. An integrated approach is used, which makes it possible to follow all stages of petroleum generation in the Devonian terrigenous strata. The above petroleum system is the most buried of all commercial ones in the region; it has an intricate geologic structure and is less drilled than others but is considered to be promising for oil resource enhancement. Therefore, a comprehensive assessment with regard to both quantitative criteria and genetic parameters is a necessary step in the future planning of geological surveys. The results obtained during this work point to a low hydrocarbon generation potential of the Devonian terrigenous strata within the study area. This research has first established no genetic relationship between the oils and the organic matter of the Devonian terrigenous strata but has revealed a genetic relationship between these oils and the oils and organic matter of the Upper Devonian-Tournaisian petroleum system. It is shown that oil fields where oil deposits are present in Devonian terrigenous strata usually have reservoirs in the overlying Upper Devonian-Tournaisian petroleum system as well. This phenomenon is most often observed on the flanks of the Kama-Kinel’ depression system. The development of oil fields there gives a high chance of discovering new oil deposits. Taking into account the new data, it is necessary to revise the search criteria and perform geological surveys in the Devonian terrigenous strata, primarily in the areas of explored oil fields where oil is localized in the Upper Devonian-Tournaisian petroleum system.

We consider the ranges of current and voltage values for laterolog tools. It is shown that expansion of the dynamic range of these parameters is possible by mathematical focusing and digital filtering and by fulfillment of the conditions for the effective number of bits and by compliance with the tool requirements. The principles of mathematical focusing are considered by the example of the 5BK tool designed by AO NPF Geofizika (Joint-Stock Company) in Ufa. The digital-filtering calculation and application technique is described. The results of well tests are presented.

Reliable solution of theoretical and applied seismological problems requires the knowledge of natural factors that influence ground motion induced by earthquakes. The effect of seasonal freezing and thawing on the behavior of coseismic ground motion in the Baikal region has been studied using data on local geology, earthquake source parameters, seismogeology, and seismic risk zoning for East Siberia. East Siberia, including the highly seismic Baikal region, is located in a temperate and cold, sharply continental climate, with the mean annual air temperature locally falling below -10 ºC. In this respect, the knowledge of seasonal variations in the ground motion spectra in different seismic-climatic zones of the region is of special importance. We study the dynamic parameters of seismic signals and their variations caused by seasonal thawing and freezing of the ground, using calculated spectra of selected earthquakes that were recorded by 0.5-20 Hz digital seismic stations at a sampling interval (Δ) of 0.01 s. Spectral analysis was applied to three-component records of more than two hundred M = 2.8 ( K = 9-14) earthquakes that occurred in the region for the past twenty years at distances from 32 to 280 km from the stations. The influence of seasonal temperature variations on the frequency responses of coseismic ground motion is discussed for the case of two seismic stations in zones of continuous and sporadic permafrost. The results are complemented by generalized data from other seismic stations located in different permafrost conditions within the Baikal region. The effect of seasonal freezing and thawing turns out to be the most prominent at frequencies above 5-6 Hz and depend on the properties and thermal state of soils beneath the stations. At the same time, they are more prominent in thawing than in freezing curves for any soil, including relatively solid bedrock. The spectral behavior of earthquake-induced ground motion is associated with variations in wave amplitudes, which correlate with seasonal temperature variations. The reported results have implications for geophysical prospecting, seismic-risk zoning, and prediction of shaking intensity of large earthquakes, which require due regard for local permafrost conditions.