Home – Home – Jornals – Russian Geology and Geophysics 2019 number 10

Thermobarogeochemical study of melt inclusions and investigation of clinopyroxenes and amphiboles from effusive rocks of the Uksichan and Ichinsky Volcanoes gave an insight into the parameters of deep-seated melts and the evolution of magmatic systems during the formation of minerals in intermediate chambers. Study of melt inclusions from the Uksichan volcanic rocks made it possible to estimate the pressure during the liquidus crystallization of clinopyroxenes and plagioclases from basaltic magmas and to establish four depth intervals of the formation of these minerals: ~60, 45-30, 27-18, and from 12 km to the subsurface. Comparison of the results of calculation based on melt inclusion data and of the clinopyroxene and amphibole data helped to establish the evolution paths of the P-T parameters of ascending melts of the Uksichan Volcano. The most high-temperature magmas, generated at a depth of ~60 km, are characterized by a successive temperature decrease during their ascent (1320-1240-1200 ºС). Based on the representative data on the compositions of amphiboles from the Uksichan and Ichinsky Volcanoes, we have elucidated the general regularities of the evolution of intermediate and acid magmatic systems, with three depths of crystallization in intermediate chambers. Amphiboles of the Ichinsky Volcano andesites and the Uksichan Volcano latites crystallized at depths of 22.0-18.5 and 18-16 km and at temperatures of 980-930 and 1010-985 ºС, respectively. As melt ascended to a depth of 15.5-11.0 km and a temperature decreased from 945 to 880 ºС, amphiboles of andesites and dacites of both volcanoes were produced. At the final stage (a temperature decrease to 900-810 ºС and ascent of melts to a depth of 3 km), only amphiboles of dacites of both volcanoes crystallized.

The results of this study reveal the chemical heterogeneity of the Yenisei metamorphic complex, which is a series of blocks within the Yenisei Fault of the southern Yenisei Ridge (Angara-Kan block). The Yenisei complex is composed of four metamorphic sequences: amphibolite-marble-paragneiss (volcanic-carbonate-terrigenous), amphibolite-orthogneiss (volcanic), marble-paragneiss (carbonate-terrigenous), and paragneiss (terrigenous). Study of the nature of the protoliths of metamorphic rocks shows that gneisses and schists of sequences I and IV correspond to polymict or arkose sandstones and siltstone-mudstones and can be classified as first-cycle sediments. Garnet-two-mica schists of sequence III correspond in composition to mudstones and show evidence of recycling. Metavolcanic sequence II is composed of andesite-dacite-trachyrhyodacite, leucobasalt-basalt, and basalt-basaltic-andesite-trachyandesite formations. The metasedimentary rocks are 1.2-1.4 times richer in REE and Th than the average PAAS. The high-alumina varieties have high contents of K, Rb, HFSE, Fe, Cr, Ni, and Co. The total similarity of the average trace-element contents in the rocks of the two complexes suggests that the composition of the Kan granulites was inherited by metasedimentary rocks of the Yenisei complex. The U-Pb zircon dates for granite veins cutting gneisses of amphibolite-marble-paragneiss sequence I limit the deposition age to 1.84-1.85 Ga and indicate that these rocks were deposited before the emplacement of postcollisional granites of the Taraka massif and thus predate the major orogenic events within the Angara-Kan block. Therefore, these rocks can be correlated with the lower part of the Urik-Iya graben section. The metamorphic rocks from the lower parts of the sections of the Yenisei complex and the Subluk Group formed during the same rifting phase of sedimentation. The amphibolite-orthogneiss (volcanic) sequence formed in the Angara-Kan block of the Yenisei Ridge during the second stage (1.74 Ga). The volcanic rocks formed in an extension setting and thus can be correlated with the emplacement of within-plate granites of the Taraka massif. In the Sayan area, terrigenous sediments and volcanic rocks of various compositions accumulated at the second stage (1.75-1.70 Ga) during the intracontinental extension. Therefore, there is a good correlation between the ages and geodynamic settings of deposition of late Paleoproterozoic volcanic and volcanosedimentary complexes of the Yenisei Ridge and the Sayan region.

Ore mineralogy of the Kedrovskoe-Irokinda ore field (northern Transbaikalia) has been studied. The ore field comprises ca. 200 quartz veins. Vein 3 and the Kvartsevaya and Serebryakovskaya veins of the Irokinda deposit and the Shamanovskaya, Pineginskaya, Osinovaya, and Barguzinskaya veins of the Kedrovskoe deposit have been described. Quartz-pyrite assemblage (quartz-1, pyrite, pyrrhotite, and marcasite) and quartz-gold-sulfide assemblage (quartz-2, galena, chalcopyrite, sphalerite, electrum, fahlore, Ag tellurides, and sulfosalts of Ag, Cu, Sb, Pb, and Sn) have been revealed. Major ore minerals were investigated by EMPA and LA-ICP-MS. An increase in Ag content in electrum (from 5.5 to 72.4 wt.%) and fahlores (from 5 to 35 wt.%) and in the abundance of Ag minerals during the ore formation has been established. Galena contains impurities of Sb and Ag (thousands of ppm), Se, Cd, Te, and Bi (hundreds of ppm), Cu, Zn, As, and Sn (tens of ppm). It is shown that the Kedrovskoe-Irokinda ore field is a rare type of orogenic deposits with considerable variations in the composition of major ore minerals (electrum, sphalerite, and fahlores), which is explained by the diversity of the host rocks.

The Kighal porphyry Cu-Mo deposit, located at 46°42′36″ N and 38°37′06″ E, occurs 120 km north of Tabriz and 12 km north of Varzeghan in the Arasbaran magmatic belt, NW Iran. The geologic units in the Kighal area are Eocene and Oligocene volcanic rocks associated with Miocene intrusive and subvolcanic rocks. Quartz-monzonite porphyry in the Kighal area hosts porphyry style Cu-Mo mineralization consisting of four alteration zones (potassic, phyllic, argillic, and propylitic) and abundant quartz veinlets. Based on mineralogy and geochemistry data, the intrusive and subvolcanic rocks belong to quartz-diorite, diorite, and granite units with high-K calc-alkaline to calc-alkaline affinities. All samples are enriched in LILE and depleted in HFSE, pointing to a mantle magma source contamination with subducted oceanic-crust material. Three types of fluid inclusions are identified in quartz from different quartz-sulfide veinlets, including multiphase (LVS) and liquid-rich (LV) inclusions in the potassic zone, liquid-rich (LV) and vapor-rich (VL) inclusions in the phyllic zone, and liquid-rich (LV) inclusions in the silicic zone. Microthermometry studies showed that Th in multiphase fluid inclusions in the potassic zone varied from 265 to 450 °C and salinity, from 38 to 59 wt.% NaCl-equiv. These values are higher than those of two-phase fluid inclusions in the phyllic zone (Th of 163 to 466 °C and salinity of 0.3 to 11 wt.% NaCl-equiv.). The calculated δ¹⁸O_H2O and δD_H2O values in biotite, sericite, and quartz from potassic, phyllic, and silicic zones showed that it is predominantly magmatic water with a lesser amount of meteoric water that is responsible for mineralization in the potassic zone. Light δD_H2O values of biotite in the potassic zone suggest a magma degassing process and/or a change in the fluid composition with magmatic water influx to a hydrothermal system.

We present results of research into the mineral composition of the Holocene sediments of Lake Malye Chany of the Chany lake system located in the Baraba steppe and comprising three lakes: Bol’shie Chany, Malye Chany, and Yarkul’, connected by channels. The sediments were studied by XRD, IR and Raman spectroscopy, laser granulometry, analysis of stable 18O and 13C isotopes, elemental analysis (XRF), etc. Mineral analysis has revealed predominant quartz, feldspars, and carbonates and subordinate gypsum, bassanite, pyrite, mica, chlorite, and kaolinite. Mathematical modeling of the XRD spectra of carbonates, using Pearson VII function, made it possible to identify the carbonate phases and determine their quantitative proportions. The obtained high-resolution carbonate record providing information about the stratigraphic distribution of carbonates in the dated section was compared with the available lithological, geochemical, and isotope data. Based on these data, we have reconstructed five stages of the Holocene evolution of the Malye Chany basin. It is shown that the proportions of minerals in the section vary in accordance with the lake level fluctuations in the alternating periods of the Holocene regional arid and humid climate. We compared the mineral compositions of the bottom sediments of Lake Malye Chany and Yarkovsky Pool of Lake Bol’shie Chany. The revealed mineral assemblages reflect the local specifics of the lake system and the influence of natural and climatic factors on the inland sedimentation processes.

Despite the known large natural bitumen accumulations and oil seeps in several kimberlite pipes along the periphery of the Sukhana sedimentary basin, interpreted as direct evidence of petroleum potential, the basin still remains one of the least studied (by geological and geophysical methods) regions of the Siberian Platform. The platform cover of the basin is composed by Riphean, Vendian, and Cambrian clastic (terrigenous) and carbonate deposits reaching 5.5-6 km in thickness in the central part of the basin. The hydrogeological specifics of the basin is largely governed by its location within the northern geocryological zone (Olenek cryoartesian basin) and is expressed as a continuous distribution of permafrost aggraded into the permafrost zone of unique thickness. Direct indicators of ore and gas presence are the East Anabar, Central Olenek, and Siligir-Markha fields of natural bitumen and oil shows in kimberlite pipes of the Daldyn-Alakit region (Udachnaya pipe). The bituminous-carbonate sediments of the Khatyspyt Formation (Vendian, Ediacaran) and the highly carbonaceous carbonate-siliceous-shaly sediments of the Kuonamka Formation (lower-middle Cambrian) are the Sukhana source rock complexes. The geochemically substantiated genetic relationship between the natural bitumen deposits of the East Anabar field and the organic matter of the Vendian Khatyspyt Formation makes it possible to estimate the area of the spread of the latter far to the west, beyond the axial part of the basin. Gammacerane, inherited from the organic matter of the Khatyspyt Formation and ranking as well-preserved and most characteristic biomarker of these bitumens, provides a compelling evidence of their consanguinity. The bitumen and oil of kimberlite pipes in the south of the basin, in the area of reefs of the Siligir-Markha bar, are similar in all geochemical criteria to oils of the Nepa-Botuobiya anteclise. In particular, in primary geochemical characteristics (12- and 13-monomethylalkanes, unique secosteranes, identical carbon isotope composition, etc.) the oils of the Udachnaya pipe are identical to the Irelyakh oils (oil field in the Mirnyi arch). No accumulations of oil or natural bitumen genetically related to the highly carbonaceous Kuonamka Formation have been found within the basin. At the same time, «intraformational» shows of viscous oil, solid bitumen, and allochthonous bitumen (bitumen extracted with chloroform) were documented directly in the sections of the formation, which makes the sedimentary basin a very attractive target for «shale oil» exploration. As for the regional assessment of the petroleum potential of the entire basin, its axial part (Sukhana depression) complicated by local uplifts is of the greatest interest. Both the Khatyspyt and Kuonamka Formations are widespread there, with the thermal maturity of their organic matter corresponding to the oil window. In addition, the regional reservoirs at the Vendian-Cambrian boundary have good petrophysical properties on both the western and the eastern flanks of the basin.

We investigated the molecular and stable isotope compositions of hydrocarbon gases of the Gorevoi Utes oil-gas seep (Lake Baikal) characterized by a simultaneous discharge of oil and gas from the lake floor. It has been found that these hydrocarbon gases are mostly thermogenic methane (δ¹³С-С₁ = -53.9 to -38.8‰; δ¹³С-С₂ = -23.4 to -33.3‰). At the same time, the gases have a minor amount of С₂₊, seldom reaching 10%. The С₁/С₂₊ value varies from 9 to 2700, with the average value being 807, which is atypical of petroleum hydrocarbons. We suggest that degassing of oil leaking to the lake floor affects the molecular composition of such gases. Some secondary processes, primarily molecular fractionation of gas during its migration into the subsurface sediments and anaerobic biodegradation of methane homologues, contribute to the decrease in С₂₊ content.

The potentialitises of studying rock wettability by X-ray core tomography are considered using Visean terrigenous reservoirs of the Solikamsk depression as an example. The studies included comparison of the tomograms of core samples in a dry state and saturated with a sodium iodide solution, which acts as a radiopaque analog of formation water. Differences in impregnation of the core samples, characterizing their wettability, have been established. According to the tomography data, in the hydrophilic samples the solution filled all pores, except for a small portion of the largest ones. In the hydrophobic samples, there was no impregnation of both small and large pores in the central zone. Based on the tomograms, the rocks were divided into groups by the type of wettability: absolutely hydrophobic, with strong signs of hydrophobicity, with signs of hydrophobicity, and hydrophilic. Comparison of the results of tomography with the standard approach showed that the Tulbovich method commonly used for the study area does not fully reflect the rock wettability. Comparison of the results of core tomography and study of thin sections with the results of electrometric logging shows their good agreement. The rocks with absolute hydrophobicity established by tomography have an anomalously high electric resistivity, >1000 Ohm·m, and the rocks with signs of hydrophobicity, >120 Ohm·m. For the hydrophilic intervals, the electric resistivity values are significantly lower, from 17 to 100 Ohm·m. Thin sections of the core samples were studied. Their microscopic analysis has shown a higher content of organic matter in the hydrophobic rocks as compared with the hydrophilic ones. Few exceptions might be due to the study of only local thin sections of the samples. Thus, rock wettability can be monitored by electrical methods, especially lateral logging. The results of the assessment of rock wettability by core study and well logging can be compiled for the exploration of Visean pools of the Solikamsk depression. Geological models constructed with regard to the recognized zones of hydrophilic and hydrophobic reservoirs can be effectively used to optimize the exploration of pools, especially reservoir flooding.