Vinogradov Institute of Geochemistry

Characterised by heterogeneous microclimatic conditions, a sharp vegetation gradient and a rich archaeological record, Olkhon Island offers a unique setting to investigate past climate change and its potential influence on cultural trajectories in the Lake Baikal Region (LBR). Palynological and geochemical analyses of the uppermost 556 cm section of the 750 cm long, radiocarbon-dated sediment core (Nur-II) from Lake Nurskoye (53°3'N, 106°58'E; 456 m a.s.l.) at the western end of Olkhon Island reveal several long- and short-term climate shifts over the last 8800 years. Forest expansion beginning in the Early Holocene peaked during the second half of the 7th millennium cal BP, followed by a continuous decline culminating around 1500 cal yr BP, when open steppe vegetation reached its maximum extent. This pattern supports the long-debated hypothesis that landscape openness and the associated availability of herbivorous game were key drivers of population size, social practices and subsistence strategies among Mesolithic, Neolithic and Bronze Age hunter-gatherer communities in the region. The maximum expansion of steppe vegetation (ca. 3400–800 cal yr BP) likely attracted Bronze Age pastoral groups who migrated into the area around 3300 cal yr BP, as corroborated by the Nur-II palynological record and archaeozoological data from the LBR. However, the pollen data show no evidence that the mixed foraging-pastoral subsistence was supplemented by crop cultivation prior to the Middle Ages (i.e. 1360 cal yr BP). The pollen-inferred moisture trends on millennial to centennial scales, including the 8.2 and 4.2 ka BP cooling events, show a positive correlation with the intensity of the Asian summer monsoon circulation throughout the study period. This suggests that the region’s hydrology was primarily controlled by a south-easterly circulation regime during the Early–Middle Holocene, while westerly-derived moisture became increasingly important after ca. 7000 cal yr BP. • Detailed palynological and geochemical records from Lake Nurskoye (Olkhon Island). • Major changes in vegetation and lake sediment geochemistry occurred synchronously. • The collapse of the Kitoi culture 6660 cal yr BP followed the maximum expansion of the forests. • The expansion of steppe 3400–800 cal yr BP attracted Bronze and Iron Age pastoralists. • Moisture trends show a positive correlation with easterly and westerly circulation patterns.

The purpose of the study was to investigate the typochemistry of gold-bearing minerals of the silver-gold series at the Dukat gold-silver deposit (northeast Russia). The methods of optical microscopy and X-ray spectral microprobe analysis were used in the research. The minerals were selected from the most common gold-silver ores at the deposit, which have a predominantly silver composition. It was found out that they are represented mainly by gold-bearing native silver and kustelite. Electrum of low fineness is much less common. The fineness of gold-bearing silver varies in the range of 10–99 ‰ (on average 51 ‰), kustelite – in the range of 101–245 ‰ (on average 149 ‰). The studied electrum separations have low fineness – from 274 to 438 ‰ (on average 357 ‰). The following patterns were revealed in the impurity composition. Gold-bearing silver is characterized by the maximum amount of impurity elements: in addition to gold, these are tellurium, copper, antimony, mercury, lead and iron; selenium, sulfur and bismuth were found in some isolated spots. The total content of all impurities (excluding gold) is also maximum – up to 1.35 wt. %. Kustelite has a slightly lower amount of impurities: tellurium, mercury, antimony, copper and lead; bismuth and selenium are noted in isolated spots. The sum of all impurities is less than 1.23 wt.%. Electrum contains the minimum amount of impurity elements: tellurium, mercury, copper and bismuth; less often it also contains arsenic and selenium. The sum of impurity elements does not exceed 0.67 wt. %. The elemental composition of the impurities and their concentration levels in the studied intermetallic compounds generally reflect the geochemical features of gold-silver ores, predominantly silver in composition, which predominate in the upper-medium ore intervals of the Dukat deposit. The identified typochemical features can be used in exploration to determine the ore-formational identity of the discovered mineralization, and also serve as an additional criterion for assessing the erosional truncation of ore zones and predicting mineralization in depth.

Synthesis of published and new data from the Govorov and Kocebu guyots provide geochemical and chronostratigraphic constraints on hydrogenetic cobalt-rich Fe-Mn crusts from the Western Pacific Magellan Seamount Trail (MST). The history of the crusts began about 65–60 Ma, when the relict layer R was deposited in the Campanian—Maastrichtian and Late Paleocene along the shores of guyots. The growth of the old-generation crusts continued in the Late Paleocene—Early Eocene (Layer I-1) and in the Middle—Late Eocene (Layer I-2) in a shallow-water shelf environment. The younger layers formed in the Late Oligocene—Early Miocene (Layer I-2b), Miocene (Layer II), and Pliocene—Pleistocene (Layer III) at depths about the present sea level. The precipitation of Fe and Mn oxyhydroxides from seawater was interrupted by several times, with the longest gap from 38 to 26.5 Ma between the old (R, I-1, and I-2) and young (I-2b, II, and III) layers. Fe and Mn oxyhydroxides in the crusts were affected by two global events of phosphogenesis in the Pacific: Late Eocene—Early Oligocene, from 43 to 39 Ma (Layers R, I-1, I-2) and Late Oligocene—Early Miocene, from 27 to 21 Ma (Layer I-2b). The trace element patterns in different layers of the Co-rich Fe-Mn crusts are grouped using factor analysis of principal components (varimax raw) into four factors: (1) +(all REEs except Ce and La); (2) +(Ce, La, Ba, Mo, Sr, Pb); (3) +(Zr, Hf, Nb, Rb, As)/-Pb; (4) +(U, Th, Co, As, Sb, W)/-Y. The factor score diagrams highlight fields which are especially contrasting for Layers I-1, I-2, and II + III according to factors 2 and 4. Consistent REE and Y variations in Layers I-2b → II → III of the crust from Pallada Guyot correlate with gradual ocean deepening between the Late Oligocene—Early Miocene and Present when the MST guyots were submerging. Large variations in the trace element contents across coeval layers may be due to the hydrodynamics of currents on the guyot surfaces. Furthermore, the geochemistry of the crusts bears effects from repeated episodes of Cenozoic volcanism in the MST region of the Pacific Plate. Higher contents of Nb, Zr, As, Sb, and W in the younger layers II and III may result from large-scale volcanism, including Miocene eruptions of petit-spot volcanoes.

The paper presents the results of U–Pb (CA-ID-TIMS) geochronological studies of columbite-tantalite and zircon from the pegmatite vein of the Aleksandrovskoye ore field, which is part of the East Sayan rare-metal pegmatite belt. As a result of U–Pb geochronological studies of columbite, age estimates of 1825 ± 3 Ma were obtained for the first time for the quartz core of the pegmatite vein and 1817 ± 5 Ma for the zone of intense alteration of amphibolites within its exocontact. The age estimate of 1810 ± 5 Ma was obtained for metamict zircon from the boundary zone of the quartz core. The studies suggest that the age of rare-metal pegmatites of the Aleksandrovskoye ore field in the East Sayan pegmatite belt differs significantly from that of the spatially conjugated granitoids of the Sayan complex (1.86–1.90 Ga).

The paper presents new data on the structural position, mineralogy, and geochemistry of ores of the Aliya ore cluster and their technological specifics. The mineralogical and geochemical zoning of the Verkhne-Aliinskoe gold deposit and the entire Aliya ore cluster is considered. We show the subordination of the local and horizontal zoning of mineralization within the ore cluster and the localization of granitoids of the late phase of the Late Jurassic Akatui Complex. A conclusion is drawn about the relationship of mineralization with granitoids within a single ore-magmatic system that existed under unstable shallow-depth P–T–X conditions transitional to subsurface ones. This specifics distinguishes the Verkhne-Aliinskoe deposit from the compositionally similar Sredne-Golgotaiskoe and Sosnovskoe deposits and partly determined the technological resistance of its ores.

Dating of magmatic rocks from paleo-island arcs of orogenic belts helps to define the precise timing of subduction processes that took place during the formation of the orogen. Within the central (East Trans-Baikal) part of the Mongol-Okhotsk orogenic belt, the Kamensk island-arc terrane is an example of such paleo-island-arc complex. Its intrusive part is included into the Bereinsky complex, represented by a gabbro-diorite-tonalite-plagiogranite series of rocks demonstrating subduction geochemical characteristics. The dating of zircons from acidic rocks of this complex by the U–Pb classical method showed that they were produced in a narrow time interval – 203±1–205±1 Ma, which corresponds to Norian/Rhaetian boundary of the Late Triassic. Taking into account the previously obtained age of the diorites (254±5 Ma), the timing of formation of the entire series of intrusive rocks is about 50 Ma, thus indicating in the Late Permian – Late Triassic the subduction along the northern (in modern coordinates) margin of the Mongol-Okhotsk Paleocean beneath the Siberian paleocontinent. Diorites of the first phase have positive values ɛND(254MA) = 3.2–3.6 (TNd(DM) = 879–994 Ma), and plagiogranites – ɛND(205MA) = 2.3–3.5 (TNd(DM) = 859–1028 Ma), which points to the connection of these rocks with the substance of the depleted mantle source and is consistent with the Sm-Nd isotope characteristics of the juvenile crust of the Central Asian orogenic belt. This study was supported by Russian Science Foundation, grant no. 22-27-00775. The resources of the Shared Use Center for Isotope-Geochemical Research (Vinogradov Institute of Geochemistry, Siberian Branch, Russian Academy of Sciences, Irkutsk) were used in this work.