A.V. Degterev1, S.Z. Smirnov2, D.V. Kuz’min2, T.Yu. Timina2, A.Ya. Shevko2, I.R. Nizametdinov2, F.A. Romanyuk1, M.V. Chibisova1 1Institute of Marine Geology and Geophysics, Far Eastern Branch of the Russian Academy of Sciences, Yuzhno-Sakhalinsk, Russia 2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: Pyroclastic deposits, explosive eruptions, pyroclastic flows, tephra, radiocarbon dating, geochemistry, Iturup Island, Kuril Islands, Lvinaya Past caldera
Two large-scale volcanic eruptions occurred in the southern part of Iturup Island (Southern Kurils) in the Late Pleistocene, which resulted in the collapse of the Lvinaya Past caldera (partly flooded later), the largest one in the Kuril Island arc. It is 7 × 9 km wide, with a rim area of ca. 50 km2 and a volume of ca. 25 km3 (including a submarine part of 12.26 km3). Comprehensive geological and geochronological studies have established that these two large-magnitude caldera-forming explosive eruptions (LP-I and LP-II) were separated by a repose period of several hundred years. The age of the first eruption (LP-I) is estimated at ca. 13,500 cal yr BP. The age of the second eruption (LP-II), based on a series of radiocarbon dates, is ca. 12,300 cal yr BP. Both eruptions were of Plinian type and involved the massive ejection of silicic pyroclastic material, which is represented by pyroclastic-flow deposits and tephra. In silica and total alkali contents the pumice from the caldera-forming eruptions corresponds to low-alkali dacites and rhyodacites (SiO2 = 63.4-69.95 wt.%, total alkalies of 3.9-5.5 wt.%), whereas andesitic (SiO2 = 58.3 wt.%, total alkalies of 3 wt.%) and rhyolitic (SiO2 ≈ 74 wt.%, total alkalies of 5.6 wt.%) varieties are scarce. The total volume of erupted material from both events is tentatively estimated at 80-100 km3 (DRE = 35-45 km3), with the LP-II eruption being 30-40% more powerful than the LP-I one. We suggest that the LP-I and LP-II eruptions might have impacted both the regional and global environment.
P.K. Kepezhinskas1, A.I. Khanchuk2, N.V. Berdnikov1, N.V. Potapova1, V.O. Krutikova1 1Yu.A. Kosygin Institute of Tectonics and Geophysics, Far Eastern Branch of the Russian Academy of Sciences, Khabarovsk, Russia 2Far Eastern Geological Institute, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia
Keywords: Subduction, slab tear and break-off, adakites, microminerals, Cu-Au-Ag mineralization, epithermal and porphyry deposits, Kamchatka
The Valovayam and Tymlat adakites formed during the subduction of the Miocene oceanic lithosphere of the Komandorsky Basin beneath Northern Kamchatka, followed by their interaction with subarc mantle wedge peridotites. Postcollisional adakitic dacites from the Bakening Paleovolcano (Central Kamchatka) are related to the destruction and partial melting of the Mesozoic Kronotsky microplate paleoslab under the influence of the hot subslab asthenospheric mantle after the collision of the Kronotsky island arc. Minerals and volcanic glass in Kamchatka adakites contain predominant Cu-Ag-Au alloys and silver chloride microinclusions along with various sulfides, native metals, alloys, oxides, and hydroxycarbonates of chalcophile elements. Microinclusion associations in these adakites are similar to ore mineral assemblages in epithermal and porphyry deposits of the Russian Far East. The Kamchatka adakites show elevated silver and gold contents in comparison with back-arc basin and volcanic-arc lavas. These elements might have been sourced from the metabasites in the oceanic crust and metal-bearing pelagic sediments in the subducted slab. We conclude that adakites associated with the subduction of the young oceanic lithosphere (Northern Kamchatka) and the melting of the old oceanic lithosphere during the slab tear and break-off (Central Kamchatka) can be magmatic precursors of the cu-Au-ag mineralization in the Kamchatka region. We also suggest that adakite-related metallogenic processes are possible at the convergent plate boundaries in other settings, in particular, in the flat slab subduction setting.
N.Yu. Groshev, A.M. Sushchenko, D.A. Gabov, Y.E. Savchenko
Geological Institute, Kola Science Centre, Rusian Academy of Sciences, Apatity, Russia
Keywords: Kotulskite, platinum group minerals, typomorphic mineral, PGE deposits, Kola Peninsula
Kotulskite, PdTe, is the most abundant palladium mineral in platinum-group element (PGE) deposits of the Fedorova-Pana Layered Complex (FPC). This paper presents new data on the noble metal paragenesis and chemical composition of kotulskite from the North PGE Reef at the Peshempakhk target. At this target, the North Reef, extensively explored at the Kievey deposit, extends eastward. Low-sulfide mineralization containing up to 15 g/t PGE + Au is exposed at Peshempakhk but does not form ore bodies at depth. The study aims to identify mineralogical distinctions between the discontinuous mineralization at Peshempakhk and the ore bodies of FPC deposits. A total of 890 grains of platinum-group minerals and Au were studied in polished sections using electron microscopy, including energy-dispersive X-ray microanalysis. The noble metal assemblage at Peshempakhk has the following relative volumetric composition: kotulskite 38%, isomertieite 22%, sperrylite 18%, stibiopalladinite 11%, hollingworthite 3%, and gold 3%. The noble metal paragenesis of the target differs from that of the main FPC deposits, where sulfides and tellurides of PGE predominate, namely braggite, vysotskite, merenskyite, moncheite, and kotulskite. Kotulskite at Peshempakhk averages 8.4 wt.% of Bi content, whereas deposits exhibit a more complete kotulskite-sobolevskite solid solution series with average Bi concentrations between 13.3 and 20.2 wt.%. Additionally, the studied kotulskite includes an antimony variety containing up to 10.3 wt.% Sb. The simultaneous presence of the two kotulskite types points to the lowest-temperature conditions of a magmatic setting. Thus, the noble metal paragenesis and composition of kotulskite from the FPC PGE mineralization are its key typomorphic features. These results can be used to forecast ore zones in similar settings.
I.F. Chayka1, S.Yu. Stepanov 2, A.V. Kozlov3, F.D. Sandalov4, R.S. Palamarchuk2, N.I. Baykov1, V.S. Zhdanova5, V.D. Abramova4 1Korzhinskii Institute of Experimental Mineralogy RAS, Chernogolovka, Russia ivanlab211@gmail.com 2Natural science museum of the Ilmeny Reserve, Southern Ural center for mineralogy and geoecology UB RAS, Miass, Russia 3Saint Petersburg Mining University of Empress Catherine II, Saint-Petersburg, Russia 4Institute of geology of ore deposits, petrography, mineralogy and geochemistry RAS, Moscow, Russia 5Zavaritsky Institute of Geology and Geochemistry UB RAS, Ekaterinburg, Russia
Keywords: skarn, Cu-Fe skarn formation, native gold, gabbro, S isotopes, Ural Platinum Belt
Skarn-type Fe and Cu deposits enriched in Au are widespread within the Tagil–Magnitogorsk megazone of the Ural Fold Belt and are primarily associated with felsic to intermediate intrusions. In the Northern Urals, Fe–Cu skarn-type deposits and ore occurrences are found in the exocontacts of gabbroic phases of the polyphase intrusions of the Ural Platinum Belt. These systems are of particular interest as they represent the terminal (hydrothermal–metasomatic) stage in the fractionation of chalcophile and noble metals within magmatic systems of young island arcs. The metasomatic rocks developed in the exocontact zone of the Knyaspa intrusion, investigated in this study, contain ore-grade concentrations of Fe and Cu (0.5–5 wt%) and Au (0.2–14 g/t), and are classified as belonging to the Cu–Fe skarn formation of the Ural belt. The geological, mineralogical, and geochemical characteristics of this occurrence are largely typical of Cu–Au skarn deposits and are genetically linked with the gabbroic phase of the intrusion, rather than with the dioritic one. The metasomatic rocks formed after andesibasalts of the Pavda Formation. The following sequence of their formation has been reconstructed: (1) amphibole–plagioclase or clinopyroxene–plagioclase hornfelses (hornfels phase); (2 epidote-bearing assemblages with clinopyroxene or garnet (pre-ore skarn stage of the hydrothermal-metasomatic phase); (3) clinopyroxene–actinolite–epidote associations with magnetite and Cu sulfides (ore skarn stage of the hydrothermal-metasomatic phase); (4) largely zeolite assemblages (late hydrothermal stage of the hydrothermal-metasomatic phase) and (5) supergene phase. The estimated temperatures for the formation of primary sulfide mineralization range from 300 to 400 °C. Formation of native gold, which has exclusively broad compositional range in terms of Cu and Ag admixtures, took place at lower temperatures (approximately 100–250 °C) and probably continued during supergene phase Based on δ34S values in sulfides and the geochemistry of chalcophile elements in skarns, the ore elements were probably orthomagmatic. However, the mobilization of S and Cu from the volcanogenic massive sulfide deposits of the Shemur Formation through their assimilation by the intrusion cannot be ruled out, as well. Regardless of the source, the redox state of sulfur was significantly shifted toward oxidized species (S⁶⁺ or S⁴⁺), resulting in unusually low δ³⁴S values (–6 to –4‰) in the sulfides of the metasomatic rocks.
The first experimental data on the relationship between the carbon dioxide content in minerals of the lazurite-type minerals (LTM) and its partial pressure in the gas phase and temperature in the range corresponding to the lazurite formation process at deposits in the Southern Baikal region have been obtained. The content of structural CO2 species was determined by IR- Fourier spectroscopy. It depends more on temperature than on the partial pressure of CO2
and is maximum for cubic lazurites at 500 °C (0.05-0.07 formula units, f.u.), decreasing to 0.01-0.03 f.u. both when the temperature decreases (460 °C) and when it increases (560 °C). A positive dependence of CO2 content on O2 fugacity in the system has been noted. LTM with an orthorhombic structure (vladimirivanovite) retains CO2 less effectively, with its content decreasing from the initial (natural) 0.08 to 0.01-0.02 f.u. in the specified temperature range of 460-560 °C. The experiments with exposure at 560
oC and subsequent cooling to 460 or 360 oC show the lack of retrograde CO2 solubility in LTM under saturation from gas phase. According to data on CO2 content, cubic lazurites with incommensurate 3D modulation of the structure could have formed at a temperature of about 500 °C, a partial CO2 pressure of ~1.4-2.2 bar, and fO2 at the level of the magnetite-hematite buffer. The high CO2 contents (0.15-0.3 f.u.) recorded in some LTMs may not be related to the direct absorption of CO2 from the gas (fluid) phase, but are the product of relatively low-temperature (<400 °C) transformations of carbon forms, leading to the association of CO2 and molecular forms of sulphur. This temperature range and the fugacity of volatile compounds corresponding to such forms of sulphur should be considered as possible conditions for the synthesis or modification of materials based on sodalite, nosaean and LTMs, which are promising as carbon dioxide absorbers or indicators.
The Mid-Atlantic Ridge within Iceland differs significantly in structure from other ridges. It consists of several modern rift zones with different kinematics and internal structure. At the same time, there are also several inactive rift zones, separated by block uplifts. This structural diversity is caused by the thermal influence of the Icelandic plume, which manifests itself in conditions of asymmetric spreading. To identify the conditions for the development of Icelandic rift zones and the features of their structure in connection with the cyclic plume activity, a physical modeling was used. The resulting model reflects the structure and development of Icelandic rift zones over the last 21.5 million years. It was shown that the kinematics and internal structure of rift zones are a consequence of the development of multi-scale overlaps of spreading centers. Between them, block uplifts arise, which in the modern topography are expressed as elevated peninsulas, mainly in the northern part of the island. The sizes of block uplifts depend on the distance between overlapping spreading axes. As the distance decreases, large uplifts are replaced by a series of small en-echelon blocks. The formation of this structural ensemble is the result of periodic increase in plume activity and its eastward displacement relative to the boundary of the lithospheric plates, which is caused by spreading asymmetry. There are two cycles of plume activity with different durations. The period of 7–8 million years reflects the complete cycle of formation and development of overlappings, and the period of 2–3 million years determines the evolution of the rift zone structure within the entire structural ensemble.
D.A. Nosov1, D.K. Dronov2, Y.G. Turbin2, I.S. Sizikov1. 1Institute of Automation and Electrometry of the Siberian Branch of the Russian Academy of Sciences, postal Novosibirsk, Russia
2Arctic and Antarctic Research Institute, St:Petersburg, Russia
The article presents the first experimental work with the GABL-PM absolute ballistic laser gravimeter on the pier, in ports and on board the ice-resistant platform (LSP) in the expedition "North Pole-42" (SP-42). During the tests, the parametric adjustment of the gravimeter was carried out in order to adapt the device for its operation on the LSP without using a gyrostabilized platform. The results of the GABL-PM measurements at the berth in St. Petersburg are shown, which are required as a reference for testing the gravimeter on the ship, as well as for linking variations in gravity acceleration measured on board the LSP with the Chekan-AM relative gravimeter to absolute values. The estimation of the RMS error of the GAB-PM measurement on board the LSP at various pitching parameters in the ports of St. Petersburg and Murmansk, as well as in the early days of the start of the drift of the North Pole-42 expedition, is given. Based on the measurement results, it was concluded that a gravimeter without a gyrostabilized platform with reduced accuracy values, on average up to 0.60 mGal, can be used to carry out measurements after freezing the vessel and when following all the points of a specially developed instruction.
1Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
Keywords: the Cenozoic, volcanism, Baikal Rift, Biryusa block, Siberian craton, pyroxenite mantle source
Based on an analysis of the hypsometric positions of lavas of different ages in the Iya-Uda volcanic field (~8 and ~4 Ma), it has been established that the main phase of intense relief dissection and activation of block movements along the Main Sayan Fault within the Biryusa block occurred in the late Miocene. Geochemical characteristics of the lavas, such as high values of the FCKANTMS parameter (0.46–0.77) [Yang et al., 2019] and the positions of figurative points on the CaO–MgO and TiO₂/Al₂O₃–SiO₂ diagrams, indicate melting of a garnet-pyroxenite mantle source rather than a typical peridotite mantle. The trace element composition of the rocks is consistent with intraplate basalts such as OIB. Variations in the Th/Nb and TiO₂/Yb ratios indicate that for lavas aged 4 Ma, the role of garnet in the melting zone increases, while for magmas aged 8 Ma, the contribution of the lithospheric mantle becomes more significant. The genesis of the enriched pyroxenite component in the lithosphere of the Biryusa block is most likely related to tectonic convergence processes during Late Cenozoic rifting, which led to a change in the volume of the crust and the involvement of lower crustal material into the mantle of the Siberian Craton. Thus, volcanism in the Iya-Uda volcanic field is the result of melting of heterogeneous and enriched lithospheric mantle.
Z.S. Nikiforova1, A.S. Borisenko2, V. L. Sukhoroslov3 1Diamond and Precious Metal Geology Institute of SB RAS, Republic of Sakha (Yakutia), Yakutsk, Russia 2 V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia 3 Zarubezhtsvetmet JSC, Moscow, Russia
Keywords: eolian processes, relief, formation, eolian placers, distribution conditions, eolian gold, ventifacts, deflation, promising areas, Mongolia
The influence of Aeolian processes on the formation of gold-bearing Aeolian placers has not yet been adequately taken into account in Mongolia, although there is well-reasoned evidence in favor of their formation. Based on the identification of Aeolian gold, the conclusion is substantiated that the formation of gold-bearing placers involved not only hydrodynamic, but also Aeolian processes, which were widely manifested in the Quaternary. The presence of Aeolian gold placers in the troughs and basins of the blowout suggests the formation of gold-bearing Aeolian placers. An analysis of the patterns of distribution of Aeolian gold has shown that the formation of Aeolian gold placers on the territory of Mongolia is quite possible – actually Aeolian placers and placers of heterogeneous origin. Actually, Aeolian placers (autochthonous and allochthonous) are formed due to deflation of ore sources or gold–bearing reservoirs, and placers of heterogeneous origin are formed by deflation of previously formed coastal-lake placers or as a result of alternating activity of temporary watercourses and Aeolian processes. The presence of pseudo-ore gold in the lacustrine-alluvial deposits suggested the arrival of gold from gold-bearing conglomerates of the Mesozoic age. So, based on the results of mineralogical studies of placer gold and field observations, it has been proved that for the first time isolated Aeolian gold and pseudorudic gold deposits on the territory of Mongolia, at a qualitatively new level of knowledge, make it possible to establish the genesis of the formation of gold-bearing placers, as well as more correctly predict the location of gold sources and select methods for their search.
The maceral composition of borehole coal cores from the Tyumen and Vasyugan Formations in the southeastern West Siberian megabasin (Tomsk and Novosibirsk regions) was determined using reflected light microscopy. We have identified and described the groups, classes, subclasses, types, and subtypes, with the most typical macerals photographed and their percentage shares determined from some samples. The maceral compositions are shown to be largely identical in the studied coals of the Tyumen and Vasyugan Formations. The revealed variations in the maceral groups for these formations are as follows (%): vitrinite - 27-100 (averaging 77); inertinite - 0-73 (18); liptinite - 0-33 (8) for the Tyumen Formation, and vitrinite - 52-100 (averaging 82); inertinite - 0-44 (14); liptinite - 0-48 (12) for the Vasyugan Fm (%), thus suggesting a close similarity in the facies conditions of the formation of their coals. The higher plant inputs — lignocellulose tissues, and more rarely, lipid components — served as their source material. In this regard, Upper and Middle Jurassic coals have been extremely underexplored within this area.Analysis of the maceral composition of coals, along with lithological studies, contribute to understanding the facies conditions of the formation of coal-bearing strata. These data may also be useful for petroleum producers in developing hydrocarbon deposits for efficient underground coal gasification, to alleviate the tight supply of natural gas.
