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Russian Geology and Geophysics

2023 year, number 2

1.
CONDITIONS OF FORMATION AND SOURCES OF MELTS OF EARLY NEOPROTEROZOIC GRANITES IN NORTHERN KUZNETSK ALATAU

S.N. Rudnev1, O.M. Turkina1, D.V. Semenova1, P.A. Serov2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Apatity, Russia
2Geological Institute of the Kola Science Center, Russian Academy of Sciences, Apatity, Russia
Keywords: Early Neoproterozoic granitoids, geochronology, magmatic and xenogenic zircon, isotope geochemistry, Central Asian Orogenic Belt, Kuznetsk Alatau

Abstract >>
We present data on the geochronology, geochemistry, and Nd isotope composition of granitoids of the Gremyachikha and North Gremyachikha massifs (Kundusyul pluton) located in the Neoproterozoic metamorphosed island arc calc-alkalic volcanic rocks of the Talanovka-Bogorodka block in northern Kuznetsk Alatau (Martaiga uplift). The granitoids formed 890-880 Ma as a result of the accretion and collision of Neoproterozoic oceanic/island arc complexes with an unknown block formed by continent-marginal metasedimentary strata. The predominance of a metasedimentary source formed during the erosion of the early Precambrian and Neoproterozoic complexes is confirmed by the wide range of the ages of xenogenic zircons in the granitoids (2800 to 930 Ma) and by variations in the εNd values (-7.8 to -1.0) and model Nd age of the granites (2.20-1.64 Ga).



2.
GRANITOIDS IN THE BOLSHEZEMELSKAYA ZONE OF THE PECHORA BASIN BASEMENT: COMPOSITION AND U-Pb AGE

V.L. Andreichev1, A.A. Soboleva1, E.G. Dovzhikova2, Yu.L. Ronkin3, E.L. Miller4, M.A. Coble4
1N.P. Yushkin Institute of Geology, Komi Science Center, Ural Branch of the Russian Academy of Sciences, Syktyvkar, Russia
2Ukhta State Technical University, Ukhta, Russia
3Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Yekaterinburg, Russia
4Stanford University, Stanford, United States
Keywords: Pechora Basin, Bolshezemel crustal block, basement, granites, zircon, U-Pb age

Abstract >>
We present the U-Pb (secondary-ion mass spectrometry, SHRIMP-RG) zircon ages and geochemical characteristics of granitoids penetrated by two boreholes at a depth of about 4.5 km in the Bolshezemel zone-the northeasternmost block of the Pechora Basin basement. The age of zircon from granites from the 26-East Khar’yaga borehole is 558 ± 6 Ma. These granites are similar in composition and age to syncollisional granites of the Charkayu granite-granodiorite complex of the Pripechora fault (suture) zone (555-544 Ma). Dating of zircon from granodiorite from the 2-Veyak borehole yielded an age of 607 ± 6 Ma. Potassium-rich granodiorites and granosyenites from this borehole have geochemical features of within-plate granites and are correlated with granites (602-595 Ma) of the southeastern part of the Izhma crustal block of the basement, which is supposed to be a submerged part of the northeastern passive margin of the Baltica paleocontinent.



3.
GOLD-RARE-METAL MINERALIZATION OF THE MOKRUNDYA PLACER DEPOSIT (Arctic Siberia, Republic of Sakha (Yakutia))

E.V. Airiyants1, D.K. Belyanin1,2, S.M. Zhmodik1, P.O. Ivanov3, O.N. Kiseleva1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
3Arctic Capital LLC, Yakutsk, Russia
Keywords: Gold-bismuth-telluride mineralization, native gold, maldonite, northeast of the Republic of Sakha

Abstract >>
The Mokrundya alluvial gold placer deposit is located in the Verkhoyansk-Kolyma folded region, within the Tuguchak ore-placer cluster. Using ore and scanning electron microscopy and chemical-analysis methods, we have studied the typomorphic, mineralogical, and geochemical features of gold, minerals of the heavy fraction of the placer, and ore mineralization of the dolerite dike complex stripped in the placer bedrock. The stages of development of the ore-forming system in dolerites have been established. The main productive stages with native gold are associated with the postmagmatic stage: (1) early arsenopyrite-polysulfide stage and (2) gold-bismuth-telluride stage. The gold-bismuth-telluride stage includes bismuth tellurides and sulfotellurides, native gold, bismuth, maldonite AuBi2, and complex intermetallic compounds of Au and Bi. We hypothesize about the sources of placer gold in alluvial deposits, one of which is the ore mineralization of the dolerite dike complex, and the other is associated with the mineralized zones of the Gematitovoe skarn-hydrothermal ore occurrence localized in the upper reaches of the Mokrundya Brook.



4.
CHRONOLOGY OF ALKALINE MAGMATISM AND GOLD MINERALIZATION IN THE CENTRAL ALDAN ORE DISTRICT (southern Yakutia)

I.V. Gaskov1,2, A.S. Borisenko1, I.D. Borisenko1, A.E. Izokh1,2, A.V. Ponomarchuk1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
Keywords: Alkaline magmatism, gold deposits, geochronology, southern Yakutia, Central Asian ore district

Abstract >>
The Central Aldan ore district (CAOD) is unique by the abundance of gold deposits associated with Mesozoic alkaline magmatism. Four types of gold deposits are distinguished here: porphyry gold, gold-sulfide, gold-argillizite-K-feldspar-quartz, and gold-uranium. The available geochronological data on the age of igneous rocks and gold mineralization in the CAOD show that the latter formed in the period 151-120 Ma. These data also agree with the results of U-Pb dating of the El’kon gold-uranium ore-magmatic system (143-125 Ma). Analysis of our geochronological data and previously published ones shows that two stages of magmatism evolution in this region. At the early stage (151-130 Ma), most of alkali syenites, monzonites, and their analogues (sills, stocks, ring intrusions, and volcanic sequences) and ores formed. They are widespread in various deposits and massifs. In the Ryabinovyi massif, this stage is marked by the formation of most of alkaline rocks (Aldan Complex) and ores: aegirine syenites, 151.4 ± 1.9 Ma; pyroxene-K-feldspar pegmatites, 144.8 ± 1.5 Ma; and Au-Cu ores, 137.5 ± 1.7-131.1 ± 16 Ma. The crystallization of amphibole from syenite of the Lunnoe deposit (143.1 ± 2.0 Ma) and clinopyroxenite of the Inagli massif (142.4 ± 2.0 Ma) and the formation of most of the alkaline rocks of its ring framing also took place at this stage. At the Samolazovskoe deposit, this stage is marked by the formation of zircon in pseudoleucite syenite, 135.9 ± 1.9 Ma, and in different syenite porphyry phases, 141.39 ± 0.90-142.4 ± 5.0 and 134.25 ± 0.70-129.9 ± 2.6 Ma, as well as gold-skarn mineralization, 129.9 ± 2.6-134.9 ± 2.8 Ma. The same period included the formation of primary ores at the Kuranakh deposit, 136.2 ± 1.7 Ma, and the deposition of breunnerite mineralization at the Lunnoe deposit, 132.4 ± 1.6 Ma. Early intrusive phases, such as potassic picrites, shonkinites, and lamprophyres, are scarce among the products of this stage of magmatism, which is possibly due to their burial beneath large volumes of later formed alkali syenites and monzanite-syenites. The second stage (128-120 Ma) was distinguished within the Ryabinovyi massif as small intrusions and dikes of olivine lamproites, shonkinite porphyry, minettes, and syenite porphyry. We revealed explosive breccias with an age of 127 Ma at the Samolazovskoe field. Magmatism of this stage was of limited occurrence in the CAOD and did not produce alkali syenites, monzonite-syenites, and ores. At the same time, rocks with an age of 121.1 ± 1.3-115.5 ± 1.6 Ma are widespread in the large (120 km2) Dzheltula ring massif of the Tyrkanda ore district, located east of the Central Aldan region.



5.
NATIVE GOLD OF THE CHUDNOE GOLD-PALLADIUM DEPOSIT (Subpolar Urals, Russia)

S.A. Onishchenko, S.K. Kuznetsov
Institute of Geology of Komi Science Center, Ural Branch of the Russian Academy of Sciences, Syktyvkar, Russia
Keywords: Native gold, Au-Ag-Cu system, exsolution textures, ore zones, rhyolites

Abstract >>
The Chudnoe gold-palladium deposit, located within the Central Ural uplift, is confined to the axial zone of an anticlinal structure complicated by faults. Veinlet mineralization is localized in fractured and brecciated Late Riphean-Vendian rhyolites. Native gold and palladium minerals are concentrated mainly in veinlets of Cr-containing muscovite (fuchsite) and, more seldom, at local sites of rhyolite metasomatism, almost free of sulfides. Using a representative material, we studied the composition and structure of native gold and the regularities of their spatial variations. Native gold has composition Au-Ag-Cu and contains Pd and Hg impurities. The element proportion varies significantly: The content of Au ranges from 65.8 to 92.7 wt.%, the content of Ag varies from 0.4 to 33.8 wt.%, the content of Cu reaches 12.7 wt.%, and the content of Pd is up to 2.9 wt.%. Gold formed as a homogeneous Au-Ag-Cu solid solution at temperatures above 220 °C. The presence of copper in native gold is probably due to the absence of sulfide ion from the ore-forming hydrothermal solution. With decreasing temperature, Au-Ag-Cu solid solution containing more than 1.1-2.5 wt.% Cu disintegrates into two or three phases, forming characteristic plate-lattice and tabular exsolution structures. In the case of two-phase exsolution, the final phases are Au3Cu and Au-Ag solid solution or AuCu and Au-Ag solid solution; and in the case of three-phase exsolution, these are Au3Cu, AuCu, and Au-Ag solid solution. In some cases, the exsolution was accompanied by the recrystallization of gold with the formation of grain intergrowths. The composition of native gold in the deposit varies significantly, showing a discrete character at different sites of the ore zones, which is consistent with the veinlet ore mineralization. At the final stages of mineral formation, native gold was partly replaced by secondary high-fineness gold.



6.
MINERAL FORMATION AT THE EAST PAUZHETKA THERMAL FIELD (South Kamchatka) AS AN INDICATION OF INFLUENCE OF A DEEP-SEATED ALKALINE FLUID AND AN EPITHERMAL ORE-FORMING SYSTEM

S.N. Rychagov1, E.I. Sandimirova1, M.S. Chernov2, O.V. Kravchenko1, A.V. Sergeeva1
1Institute of Volcanology and Seismology, Far Eastern Branch of the Russian Academy of Sciences, Petropavlovsk-Kamchatsky, Russia
2Lomonosov Moscow State University, Moscow, Russia
Keywords: Modern hydrothermal systems, epithermal ore occurrences, deep-seated fluid, alkaline solutions, argillisites, hydrothermal clays, mineral formation

Abstract >>
A sequence of argillized rocks and hydrothermal clays of the East Pauzhetka thermal field was studied in detail by means of drilling and trenching. We have identified zones composed of unusual mineral associations; their formation conditions are considered. The structure of the base of the hydrothermal-clay mass is shown. The source rocks (brecciated andesites) of the base are altered by hydrothermal-metasomatic processes and are represented by smectite-chlorite-K-feldspar-zeolite-carbonate-siliceous aggregate with sulfides, phosphates, titanium silicates and zircon silicates, and other mineral phases, including rare metals. A conceptual geological and geochemical model for the formation of argillisites and zones with mineral associations has been plotted. It is assumed that modern mineral formation in the East Pauzhetka thermal field inherits an epithermal ore-forming system located in the influence zone of a deep-seated fluid, the derivates of which are outflowing near the surface as alkaline metal-bearing solutions of the Pauzhetka hydrothermal system.



7.
POSSIBILITIES OF RIVER SEISMIC SURVEY FOR STUDYING THE STRUCTURE OF THE EARTH’S CRUST AND THE UPPER MANTLE OF THE TERRITORY OF SIBERIA

A.V. Liseikin1, V.S. Seleznev1, V.M. Solov’ev2, A.A. Bryksin1, D.B. Sevost’yanov1, D.V. Krechetov1, I.V. Kokovkin1
1Seismological Division of the Geophysical Survey, Russian Academy of Sciences, Novosibirsk, Russia
2Altai-Sayan Division of the Geophysical Survey, Russian Academy of Sciences, Novosibirsk, Russia
Keywords: River seismic survey, deep structure, CDP-2D method

Abstract >>
A river seismic survey technology based on using pneumatic water sources and autonomous seismic recorders, which are installed on the shore and configured for continuous seismic recording, is developed at the Geophysical Survey of the Russian Academy of Sciences (GS RAS). In recent years, several thousand kilometers of profiles have been worked out by the CDP-2D method on the rivers of East Siberia: Lena, Nizhnyaya Tunguska, and Vitim. Previously, only 6- to 10-second seismograms were used to study the structure of the upper part of the Earth’s crust. At the same time, the deep structure of the Earth’s crust in the vast territories of East Siberia remains poorly understood due to the high cost of research. The river seismic survey data are used on a profile section worked out in the lower reaches of the Lena River to demonstrate the possibility of studying the structure of the Earth’s crust to the full depth, including the Moho discontinuity. For this purpose, the archival seismic records are reprocessed the construction of montages of seismograms of increased duration (up to 23 s). The low-amplitude oscillations of reflected waves from deep boundaries are distinguished due to multiple summation, which is much larger than in conventional seismic surveys. The equipment used in this study has a sufficient dynamic range, and a high magnification is achieved by increasing the binning area. Wind noise reduces the quality of time sections of the upper part of the Earth’s crust, but does not worsen the section at great depths, so this material should not be excluded from processing. The river seismic surveys carried out using the technology developed at the GS RAS on the rivers of East Siberia within about 2700 km contain data that make it possible to build deep sections up to the Moho discontinuity, and this work needs to be done.



8.
CROSS-WELL REFLECTION IMAGING AT THE VERKHNEKAMSKOE POTASSIUM SALT DEPOSIT

A.V. Chugaev, I.A. Sanfirov, M.V. Tarantin
Mining Institute, Ural Branch of the Russian Academy of Sciences, Perm, Russia
Keywords: Seismic well survey, cross-well seismic survey, depth seismic section, reflected waves, cross-well tomography, common depth point, migration of reflected waves

Abstract >>
We propose a technique for the extended processing of cross-well seismic data for reflection imaging. Based on the analysis of the wave field and synthetic modeling, a graph for processing cross-well data is developed to separate the reflections in the presence of the boundary of a sharp change in the velocity of elastic waves. The migration of the reflection waves from the time scale to the depth one is performed by solving a forward problem for each source-receiver pair. As a result, the position of the reflection points is calculated, after which all traces are stacked based on the binning grid. The input information for performing the migration and solving a forward problem is the velocity characteristic of the massif, which is calculated using traveltime tomography on direct body waves obtained from the same data set of cross-well survey and vertical seismic profiling. The resulted depth seismic section has a much higher resolution than that of vertical seismic profiling and ground-based shallow seismic studies. This opens up the possibility of identifying various small-sized natural or technogenic objects in the interwell space. Practical implementation was carried out at the Verkhnekamskoe potassium salt deposit.