Okhotsk-Chukotka Volcanic Belt Research Articles

Distinct mechanisms controlling and influencing the supply of clay-sized sediments to the northern shelf of the Sea of Okhotsk since the Last Glacial Maximum

The Sea of Okhotsk is a transitional zone between East Asia and northwestern Pacific Ocean. As a result, its massive thick sediments contain numerous important records of geological past, such as the mechanisms of materials and energy exchanges. However, the scarcity of studies tracing the provenance of clay-sized sediments in the sea has limited the progress of reconstructing the paleoclimate of East Asia. In this study, clay mineral analysis was conducted on 31 surface stations and the LV87–54-1 core. These results were combined with grain-size to provide reliable evidence for distinct mechanisms controlling the influx of clay-sized sediments into the northern shelf of the Sea of Okhotsk since 23 ka. Results of provenance analyses suggested that clay-sized sediments in the northern shelf mainly originated from its northwestern shelf, the Okhotsk–Chukotka volcanic belt, and the Kamchatka Peninsula. Clay-sized sediments of the northern shelf was primarily transported from the northwestern shelf and the Okhotsk–Chukotka volcanic belt after 23 ka, while those analogues from the Kamchatka Peninsula increased after 11.7 ka. Based on variations in clay mineral ratios, sediments from the northwestern shelf were mainly transported by the North Okhotsk Countercurrent, wherein rapid millennial-scale variations were influenced by the high-latitude Arctic Oscillation (from the Last Glacial Maximum to the Last Deglaciation) and the low-latitude East Asian summer monsoon (including the Holocene), respectively. By contrast, the influx of clay-sized sediments from the western Kamchatka Peninsula might be mainly controlled by the intensity of the West Kamchatka Current. From the Last Glacial Maximum to the Last Deglaciation, north and northwest geostrophic winds dominated in winter, and the West Kamchatka Current shifted eastward. Since the Holocene, weakened north and northwest geostrophic winds, but strengthened east winds caused a series of rapid millennial-scale variations in the West Kamchatka Current.

Climates and sea-level controls on chemical weathering in the northern shelf of the Okhotsk Sea in the past 23 ka

The Okhotsk Sea, a subarctic marginal sea with a wide shelf and abundant weathering products, provides ample information about the chemical weathering in the geological past. However, insufficient investigation in the Okhotsk Sea, especially in its shelf, limits our knowledge of the intensity of chemical weathering in high-latitudes. In this study, high-resolution geochemical elements compositions of the core LV87-54-1 were measured to reveal the provenance change in the Northern Okhotsk Sea Shelf since the Last Glacial Maximum (LGM) and to discuss the influential factors of the deep-sea weathering records. The elemental ratios suggested that the sediment provenance remained stable during 23–11.4 ka, mainly transported from the Okhotsk-Chukotka volcanic belt; whereas the sediment input from the Kamchatka Peninsula to the study area has been enhanced since 11.4 ka. During 23–14 ka, the chemical index of alteration (CIA) was observed to align with climatic changes in the Northern Hemisphere. Synchronized climatic deterioration (i.e., cooling and drying) in the Northern Hemisphere mainly contributed to the reduced alteration of the sediments eroded from the Okhotsk-Chukotka volcanic belt. During 14–11.4 ka, the reworking of older weathered sediments caused by sea-level rise overwhelmed climates in controlling silicate weathering, resulting in an increased alteration of sediments. During the Holocene, the core sediment witnessed a lower weathering degree, compared to the LGM–Deglaciation. We attribute this to an increased sediment input from the Kamchatka Peninsula that supplied more weaker weathered sediments to the study area.

The Early Cenozoic Volcanism of the Northern Okhotsk Region System of Grabens and Faults

Abstract —We studied geochemical compositions of the early Paleocene basaltic and andesite dikes associated to linear zones of the Lankovo–Omolon shearing system (Northern Okhotsk region) and basalts of the Evdyreveem volcanic field associated to the Okhotsk–Penzhinsk fault system, and compared them to other synchronous manifestations of basic volcanism: andesibasalts and andesites of the studied earlier Garmanda field, as well as with the Late Cretaceous basalts of the Mygdykit Formation of the Northern Okhotsk region, roofing the Okhotsk–Chukotka volcanic belt. The isotopic composition of Sr and Nd in dikes, the distribution of major and trace elements with the ratios of noncoherent elements indicate the formation of volcanic bodies in the environment of continental margin rifting, which is confirmed by the combination of depleted, intraplate and above subduction geochemical features of their composition. Such behavior of the elements indicates multi-stage processes of the earlier Mesozoic supra-subduction fluid metasomatosis. Melting of an ancient buried Cretaceous slab may explain the appearance of such “above subduction” marks as the Nb–Ta negative anomalies in the studied basaltoids. Andesite dikes are characterized by higher isotope ratios of Nd and lower Sr, with lower absolute concentrations of trace elements and more pronounced anomalies on spider plots.

Orientation Errors of Paleomagnetic Samples When Using a Magnetic Compass and Possible Ways to Overcome Them

A paleomagnetic study of volcanic rocks of the Okhotsk-Chukotka volcanic belt was performed to identify the scale and distribution pattern of orientation errors associated with the use of a magnetic compass, as well as to find how they influence the accuracy of calculated mean directions at both site and regional levels. The impact of these errors on the position of the final paleomagnetic pole and the dispersion of virtual geomagnetic poles, which is a common measure of the amplitude of paleosecular geomagnetic variations, was assessed. The alternative (non-magnetic compass) methods for orienting paleomagnetic samples were analyzed. Their advantages and disadvantages were outlined. A new method for orienting paleomagnetic samples using a theodolite equipped with a laser pointer was proposed. This method has a distinct advantage over previous alternatives as it is not limited by certain critical factors.

Myauchan Ore Field (Northeastern Asia), an Example of Weakly Eroded Ag–Pb–Zn Volcanoplutonic Mineralization

Abstract —The Myauchan ore field located in the Omsukchan zone of the Okhotsk–Chukotka Volcanic Belt is described. The Korennoe and Malyutka Ag–Pb–Zn ore occurrences forming this field are localized at the center of anticlinal fold of NW strike made up of Upper Triassic carbonate-terrigenous deposits intruded by stock-like bodies and dikes of Upper Cretaceous andesite, monzodiorite, granite-porphyry, and rhyolite-porphyry. Sulfide–carbonate–fluorite–quartz veins and vein–veinlet zones with arsenopyrite, pyrite, sphalerite, galena, chalcopyrite, and Ag-tetrahedrite are localized both in intrusions and in terrigenous strata. The field bears As–Sb–Ag–Pb–Au–Bi–Cu–Zn–W mineralization. Study of fluid inclusions and REE and ore element patterns show that the field ores formed from chloride solutions with the participation of surface waters in hydrothermal-magmatic system under epithermal conditions. The ores were deposited from solutions with TDS = 0.5–9.0 wt.% NaCl equiv. in the temperature range 106–287 ºC. The ores containing ca. 70 ppm Ag and up to 1.2% Pb formed through boiling-off of solutions enriched in iron salts. The ores with Ag < 7 ppm and Pb ≤ 25 ppm were generated from diluted hydrothermal solutions with predominant sodium salts and potassium admixture. Mineralization of the Myauchan ore field marks the upper level of the Ag–Pb–Zn ore system similar to the Gol’tsovoe deposit.

Evolution of Aptian and Albian magmatism of Western and Northern Chukotka (Northeast Russia) based on zircon U-Pb geochronology and rock geochemistry

ABSTRACT Across western and northern Chukotka, a region spanning 400 000 km2, we have defined three different tectono-magmatic systems that were active during the Aptian-Albian (123–100 Ma) time span: (1) the Tytylveem post-collisional belt (123–105 Ma); (2) the Chaun province of extension-related magmatism (109–100 Ma); and (3) the earliest eruptive units (106–100 Ma) of the subduction-related Okhotsk-Chukotka volcanic belt (OCVB). The observed tectono-magmatic systems are characterized by their location and by their different chemical and isotopic compositions. The general enrichment of magma sources successively increases from the OCVB to the Tytylveem belt and then to the Chaun province. While the Tytylveem belt was active, the volcanic loci shifted northwest along the strike of the belt, perhaps because of diachronous delamination of the lithosphere of the once present, but extinct, South Anyui ocean. On the scale of the entire region of NE Asia, the tectonic re-arrangement that controlled magmatism in the Early Cretaceous was marked by the extinction of major subduction zones on the periphery of the closing Oimyakon and South Anyui oceanic basins followed by magmatic events likely caused by lithospheric delamination and/or extension. Along the Pacific margin of Asia, subduction-related volcanism was nearly continuous during the entire Cretaceous.

Геохимические особенности Ag-Au рудопроявления Кыплатап (центральная Чукотка)

The article considers the geochemical features of unconventional mineralization in secondary quartzite’s of the Kyplatap ore occurrence, located on the southeastern flank of the volcanic dome structure (VDS) of the same name, complicating the Palyavaam-Pykarvaam volcanotectonic depression (VTD) in the Chaun zone of the Central Chukotka sector of the Okhotsk-Chukotka volcanic Belt (OChVB). The Kyplatap VDS was formed due to the intrusion into the Late Cretaceous Alkaquun volcanites of a large laccolith-like subvolcanic body composed of rhyolite-trachyriolite-trachydacites. Based on the geochemical studies of the Kyplatap ore occurrence, the authors compiled a representative database of geochemical data, including the results of analyses of express-RFA, PKSA and XSA with coordinates and composition of ores and rocks. Correlation analysis of the database showed a positive relationship between Au and Ag, as well as Sb for ore mineralization, and a negative correlation of these elements with the association of the rock group of elements (Ti, Y, Zr, Nb and Yb) that were removed during ore formation. The authors also distinguished the second rock group of elements (K, Ga, Sn, Ba, V, W, Pb, Sc, La and B), characterized by medium and weak correlation of the first group of elements. They calculated the coefficients of enrichment and deficiency of ore-forming elements for the samples taken within the ore zones of the Kyplatap ore occurrence. The extreme enrichment group includes elements Au, Ag, Sb, Bi, As, the average contents of which are 100 times or more, and the minimum concentrations are at least 2-10 times higher than the average contents in the upper crust. The normal enrichment group includes elements S, Sn, Cu, Zn, Mn, Mo and Pb, the average contents of which are 2-10 times, and the maximum concentrations are 5-100 times higher than the average concentrations of the upper crust. The obtained values of the enrichment coefficients indicate a sufficiently intensive metensomatoses and ore formation within the Kyplatap ore occurrence.

Silver Sulfides and Selenides in Ores from Au–Ag Epithermal Deposits of the Okhotsk—Chukotka Volcanic Belt

Silver Sulfides and Selenides in Ores from Au–Ag Epithermal Deposits of the Okhotsk—Chukotka Volcanic Belt

SILVER SULFIDES AND SELENIDES IN ORES FROM Au-Ag EPITHERMAL DEPOSITS OF THE OKHOTSK-CHUKOTKA VOLCANIC BELT

Mineral associations and compositions of sulfoselenides and other silver chalcogenides from epithermal deposits of the Okhotsk-Chukotka volcanic belt (North-East Russia) - Rogovik, Lunnoye, Julietta, Valunistoe, Corrida and Pepenveem have been studied. Different types of relationships between S-naumannite and Se-acanthite in the ores of the studied deposits have been established. It is shown that a general pattern in the sequence of formation of silver chalcogenides is the replacement of S-naumannite and Se-bearing minerals (Se-galena, Se-stephanite) of early parageneses by Se-acanthite or acanthite at later stages. Based on the results of thermodynamic calculations and data on the composition of the main productive mineral associations, the physicochemical parameters (T, ƒS2, ƒSe2, ƒO2, pH) of ore formation at these deposits were reconstructed.

Sr, Nd, and Pb isotope provenance of surface sediments on the East Siberian Arctic Shelf and implications for transport pathways

The East Siberian Arctic Shelf (ESAS) is a crucial area for sea ice formation and exportation in the Arctic Ocean. Precise sedimentary identification in the ESAS is vital to trace sea ice transport. However, tracking sea ice transport is challenging due to the complexity of the sediment sources and transport processes in the ESAS. In this study, we investigated Sr, Nd, and Pb isotopes in residual fractions of recent surface sediments in the ESAS to explore the distribution, sources, and transport pathways of terrigenous materials and track regional sea ice transport. Our results show that approximately 20–30% of the Western Laptev Sea (WLS) sediments were derived from Siberian flood basalt, characterized by a high εNd (ca. -9.0) and a high 87Sr/86Sr ratio (ca. 0.715). Furthermore, most sediments of the Eastern Laptev Sea–Western East Siberian Sea (ELS–WESS) were derived from ancient clastic and metamorphic rocks, with the lowest εNd values (ca. -12.3), highest 87Sr/86Sr ratios (ca. 0.717), and highest 207Pb/206Pb ratios (ca. 0.836). Additionally, sediments in the Eastern East Siberian Sea–Western Chukchi Sea (EESS–WCS) had the highest εNd values (ca. -7.6), lowest 87Sr/86Sr ratios (ca. 0.712), and relatively high 207Pb/206Pb ratios (ca. 0.830) due to the influence of Okhotsk–Chukotka volcanic belt (OCVB) weathering products, which contributed approximately 10–20% of the local sediments. Eastern Chukchi Sea (ECS) sediments showed relatively low εNd values (ca. -9.2), low 87Sr/86Sr ratios (ca. 0.712), and the lowest 207Pb/206Pb ratios (ca. 0.810), which probably influenced by coast erosion inputs from Alaska and Yukon River material transported via Pacific Sea Water. Based on the sediment provenance with Nd isotope, sea ice formed in the WLS and ELS–WESS is predominantly transported by the Trans-Polar Drift (TPD) to the Eurasian Basin and Fram Strait. This study provides insights for further understanding and tracking sea ice transport processes from the ESAS to the Arctic Ocean.

The Kupol Epithermal Au-Ag Vein District, Chukotka, Far Eastern Russia

Abstract The Kupol epithermal Au-Ag vein district is located in the northern part of the Okhotsk-Chukotka volcanic belt, a Late Cretaceous subduction-related continental volcanic arc exposed for >3,000 km along the eastern coast of Russia. High-grade veins are hosted in the Kupol andesite sequence, a 300- to 1,000-m-thick, subhorizontal, layered sequence of andesite flows, sills, and ash tuffs, dated at 97 to 96 Ma (Cenomanian). The Kupol andesite sequence is underlain by mixed mafic-felsic volcanic units plus sedimentary rocks (“older volcanics”) and overlain by a >1-km-thick “upper felsic” sequence of dacitic-rhyolitic tuffs and associated dikes and flow domes, dated at 95 to 85 Ma, with local sequences of fluvio-lacustrine sedimentary rocks. The epithermal veins occupy N-striking, steeply dipping normal faults that cut thick coherent andesite flows and sills in the central-upper part of the Kupol andesite sequence. The district is dominated by the large Kupol vein (180.7 tonnes (t) Au and 1,986 t Ag produced to 2020), hosted by the 5.5-km-long Kupol fault, which accommodates normal, east-side-down displacement of up to 190 m. The Moroshka and Providence veins, 5 km east-southeast of Kupol, occupy shorter faults (1- to 2-km strike) with smaller vertical displacements (to 70 m). The Moroshka vein is dated at 93.5 ± 1.5 Ma (Turonian; 40Ar/39Ar method on adularia), and the timing of vein mineralization here and at Kupol overlaps with the early stage of upper felsic sequence magmatism. Veins contain subhorizontal ore shoots, controlled by the intersection of the steep faults with flat-lying Kupol andesite sequence stratigraphy and by steepening of the faults to a more dilational orientation as the inferred paleosurface is approached. Local structural controls are also evident, reflecting a component of oblique slip on the Kupol fault, with the thickest vein segments at steeply pitching jogs and relays. Main-stage veins grew via repeated encrustation by quartz-chalcedony ± amethyst ± lattice bladed calcite (replaced by quartz), with Au-Ag–bearing crustiform adularia ± clays ± sulfides/sulfosalts/electrum ± chlorite ± hematite bands. The main controls on Au grade are inferred to have been boiling, resulting in sharp vertical limits to high metal grades typical of epithermal veins, coupled with optimal dilation of the vein system where the hosting normal fault steepens near surface with decreasing differential stress. Although much of the displacement on the controlling faults is pre-mineralization in timing, lithified cataclastic breccia, coeval with some vein stages, and vein geometry patterns indicate that some vein development occurred contemporaneously during late normal displacement along the fault system. Waning of the hydrothermal system is marked by late carbonate fill, initially Fe dolomite, then coarse calcite as veins, matrix to vein breccia, and central vein cavity fill. The Kupol district veins have proximal adularia-quartz alteration (haloes meters wide), within an extensive (hundreds of meters in scale) clay alteration halo. Clays are zoned both vertically and laterally with respect to veins, with inner illite-chlorite that was magnetite-destructive (at highest paleotemperature; >220°C), grading outward and upward to illite/interlayered illite-smectite with kaolinite, then to an outer zone (or upper blanket) of smectite, at lowest paleo-temperature (<150°C). The boundary between the illite and smectite zones is interpreted to mark the interaction limit of paleo-hydrothermal systems with cooler groundwater. District-scale pathfinder element zonation correlates with clays, with S-Te-Bi-As in the illite-chlorite core and Sb-Cs-Tl(-As-Li) in the smectite blanket. Pathfinder zonation patterns at Kupol point to a magmatic source at depth or, more likely given the scale of the anomalies, multiple magmatic sources, with the surface clay zonation indicating the extent of coalesced paleo-hydrothermal systems associated with upflow plumes. This is the best-defined alteration record with geochemical signature for a complete district hosting a large, high-grade vein deposit. Early definition of clay and pathfinder element patterns across an entire epithermal district can be carried out at low cost to provide useful constraints on vein targeting.

Amplitude of Secular Geomagnetic Variation in Late Cretaceous Based on Paleomagnetic Studies of the Okhotsk–Chukotka Volcanic Belt from Upper Reaches of Malyi Anyui River, West Chukotka

Amplitude of Secular Geomagnetic Variation in Late Cretaceous Based on Paleomagnetic Studies of the Okhotsk–Chukotka Volcanic Belt from Upper Reaches of Malyi Anyui River, West Chukotka

High‐Latitude Paleointensities During the Cretaceous Normal Superchron From the Okhotsk–Chukotka Volcanic Belt

AbstractWe conducted an absolute paleointensity survey on 74 lava flows from the Okhotsk–Chukotka Volcanic Belt (NE Russia), emplaced 90–83 Ma toward the end of the Cretaceous Normal Superchron (CNS, 121–84 Ma). Relying on preliminary results, we restricted our analysis to eight lava flows (140 Thellier–Coe experiments), two of which also yielded successful Wilson determinations. A detailed analysis of the magneto‐mineralogy—based on X‐ray structural analysis, reflected‐light microscopy, and thermomagnetic curves—indicates that the determinations from solely two lava flows can be fully trusted, with remanence carriers (a) dominated by low‐titanium titanomagnetite, (b) showing unambiguous traces of high‐temperature oxidation, and (c) yielding partial thermoremanent magnetization (pTRM) tails representative of pseudo‐single‐domain grains. Recalculated in terms of geomagnetic dipole strength, our two successful flow‐mean determinations yield virtual dipole moments of 4.76 ± 0.26 × 1022 Am2 (N = 7) and 9.07 ± 0.84 × 1022 Am2 (N = 8). Using an updated version of the paleointensity database for the Cretaceous epoch, we stress that determinations based on nonglassy whole rocks, submarine basaltic glasses, and single crystals are mutually inconsistent, suggesting a separate analysis of their distributions is more appropriate. Despite statistically indistinguishable estimates of dipole strength before and after the onset of the CNS—in accord with recent studies refuting the existence of a strict correlation between chron duration and dipole moment—we found that the distribution of dipole moments during the CNS is slightly bimodal with a leptokurtic dominant mode, thus more inclined to produce outliers and suggesting distinct geomagnetic field behavior during the CNS.

ПЕТРОФИЗИЧЕСКИЕ ОСОБЕННОСТИ ТЕКТОНИЧЕСКИХ СТРУКТУР ЗАПАДНОГО ПРИОХОТЬЯ

Physical properties of rocks from western Priokhotye were studied and petrophysical maps revealing large-scale heterogeneities of tectonic elements of the territory were constructed. Petrophysical heterogeneities and anomalies of respective geophysical fields were compared. A relationship between magnetic anomalies and near-surface rock assemblages is established only for the Mesozoic volcano-plutonic structures of the Okhotsk-Chukotka volcanic belt and partially for the Yudoma-Maya tectono-stratigraphic area. In the ancient blocks of the Aldan-Stanovoy shield and the Uchur-Maya plate, magnetic anomalies are mainly associated with the intensification of tectonic-magmatic processes and to a lesser extent with magnetic properties of specific rocks. Inverse relationships are established between gravity anomalies and rock dense anomalies within the regions of gravity low (Tyrkansky and Yuzhno-Verkhoyansky gravity fields) and the Vilyui syneclise, where domains of gravity high correspond to domains with low density.

Variations of Oxygen Isotopic Composition in Magmas of Okhotsk–Chukotka Volcanic Belt

Variations of Oxygen Isotopic Composition in Magmas of Okhotsk–Chukotka Volcanic Belt

Mineralogy of the Svetloye epithermal district, Okhotsk-Chukotka volcanic belt, and its insights for exploration

The Svetloye epithermal district (SED) is located within the Okhotsk-Chukotka volcanic belt (OCVB), 220 km southwest of Okhotsk. The OCVB, which formed during the Cretaceous with ore potential, is a large marginal volcanic rock belt associated with continental subduction. Svetloye occupies two volcanic edifices of different ages – the northwestern volcanic center (including the Emmy deposit) consists of basaltic-andesite of the Khetanian suite (Coniacian-Santonian time (K2), and the southeastern volcanic center (hosts the Elena, Tamara, Lyudmila, and Larisa deposits) comprises the dacite-rhyolite-leucogranite formation of the Urak suite (Campanian-Maastrichtian time (K2). Distinct host rocks and different levels of erosion in the two edifices of Svetloye led to various altered rocks, mineral assemblages, and diverse types of gold mineralization. The alteration halo of the Emmy deposit contains vuggy residual quartz, with Au-Ag-telluride and Au-Ag mineralization plus other tellurium-bearing minerals (goldfieldite, kawazulite, coloradoite, melonite, altaite, tellurobismuthite, tellurantimony, and native tellurium). Altered rocks of the other deposits are characterized by variable halos, including vuggy residual quartz, alunite, dickite, quartz-alunite-dickite, illite-chlorite, illite, illite–smectite, smectite, quartz-carbonate, and quartz-white mica alteration, hypogene and supergene processes. The hydrothermal (hypogene) and supergene ore processes within the SED occurred in 3 stages: leaching and barren quartz-rutile-pyrite zone with halos of quartz-alunite-dickite-pyrite (or quartz-illite-chlorite-calcite-pyrite), then quartz-pyrite with polysulfides and gold, gold-silver telluride, and finally supergene with 'mustard' gold. The alteration halo, epithermal ore bodies as hydrothermal veins and breccias with vug infill, colloform, and crustiform textures, hypogene sulfide assemblage represented by galena, sphalerite, tennantite-tetraedrite, and chalcopyrite of SED are typical of epithermal styles with an high-intermediate sulfidation signature, possibly indicates a porphyry deposit at depth. The lack of high-sulfidation-state sulfides such as hypogene covellite, enargite, famatinite, luzonite in the residual quartz may be due to their erosion at shallower paleodepth, and/or to their supergene oxidation below the present day surface.

Spatial Distribution and Provenance of Detrital Minerals of Surface Sediment in the Okhotsk Sea

The Okhotsk Sea is a distinctive marginal sea in the northwestern Pacific Ocean, which is characterized by the prevalence of seasonal sea ice in winter. Sediment is sourced from the around region through sea ice transportation, rivers input, and volcanic eruptions. Surface sediments of the Okhotsk Sea shelf vary greatly in grain size, and the sand content is generally high, which is conducive to source-to-sink studies using ice-rafted detritus (IRD),detrital minerals and single mineral geochemistry methods. In this paper, the 63–125 μm grain size fraction was selected for the detrital minerals analysis of surface sediments (top 0–10 cm) from 58 sediment stations and 15 stations samples has been chosen for garnet chemistry. These stations are mainly located in the south central Okhotsk Sea. The distribution and composition of the heavy minerals are influenced by material derived from the Amur River, the north shelf (Okhotsk-Chukotka volcanic belt), Sakhalin Island, the Kamchatka Peninsula, and the Kuril Islands. The detrital mineral results show that hornblende, epidote and garnet are terrigenous material indicators. High contents of fresh hypersthene can be used as an indicator of volcanic eruption materials. And high content of abraded hypersthene can be used as an indicator of Okhotsk-Chukotka volcanic materials. In the northern Okhotsk Sea, the southward moving sea ice produced in Sakhalin Bay collides with the sea ice produced off the east coast of Sakhalin Island, which causes the sea ice to accumulate to the southeast. This results in the deposited ice-rafted debris having a southeastward facing fan shape, and the geochemical analysis of the garnet supports this conclusion. From west to east, the amount of material from Sakhalin Island gradually decreases, while the amount of material from the Amur River and Chukchi-Kamchatka increases.

Золотая минерализация штокового рудного поля (Магаданская область)

The authors briefly characterize the geology and structure of the Shtokovoye ore field attached to the area where the Khurchan-Orotukan zone of tectonic-magmatic activation overlays the structures of the Yana-Kolyma ore-bearing belt. Studied are mineral associations and physicochemical conditions of gold ore bodies, located both in granites and in hornfelsed sedimentary masses. By the main features of its geological structure, ore composition, and physicochemical formation conditions, the Shtokovoye ore field mineralization corresponds to the "depth" group of the gold-rare-metal formation, analogous to the Butarnoye, Basugunyinskiye, Dubach, and Nadezhda occurrences. Its ores are peculiar in the late epithermal mineralization, which is associated with the Okhotsk-Chukotka volcanic belt and overlays the sinaccretional gold-rare-metal mineralization.

New Paleomagnetic Data on Late Cretaceous Chukotka Volcanics: the Chukotka Block Probably Underwent Displacements Relative to the North American and Eurasian Plates after the Formation of the Okhotsk-Chukotka Volcanic Belt?

New Paleomagnetic Data on Late Cretaceous Chukotka Volcanics: the Chukotka Block Probably Underwent Displacements Relative to the North American and Eurasian Plates after the Formation of the Okhotsk-Chukotka Volcanic Belt?

New data concerning the age and composition of intrusion complexes of the northern part of Balygychan-Sugoi rift depression (North-Eastern Russia)

The Balagychan-Sugoi (Omsukchan) riftogeneous depression is described by different authors both as a part of Okhotsk-Chukotka volcanic belt (OCVB) and as a separate structure. The unique Dukat gold and silver deposit and a lot of others ore deposits including rare-metal, such as Arylah, Lunnoe, Askold, Mechta, Tidid, Krasin, are associated with this structure. All mentioned objects are located in the south part of the depression and from this point of view are connected with OCVB structure. Evolution of the Balygychan-Sugoi depression is closely related to the stages of magmatic evolution of the region. In the article, these stages are characterized based on the data form intrusion bodies located in the northern part (Naygain depression). This part is the most outlying from the OCVB and less investigated. For the first time, the U-Pb zircon and baddeleyite dating and the description of petrographic and geochemical composition have been carried out for Mandychan gabbro-diorite and Nayahan gabbro leucogranite intrusion complexes from the northern part of the depression and for Omsukchan leucogranite complex. The Late Jurassic – Early Cretaceous intrusions of the Mandychansk complex dated as 143-139 Ma mark the beginning of the rift depression formation. Intrusion of granitoids of the Nayahan and Omsukchan complexes dated as 84-85 Ma and 81-82 Ma correspondently is related with new stage of magmatic activity, synchronous to OCVB formation. The petrographic and geochemical compositions of these granitoids indicate their belonging to the I-type granites, which are typical for extension environments, whereas leucogranites of the Omsukchan complex have some features corresponding to the A-type granites.