No 3 (2025)

In the geochemical review based on records from cruises of International project of Deep-Sea Drilling and other literature data concerning main lithologic types of the Pacific Pleistocene sediments we presented tables of average arithmetic chemical composition, meanweighted chemical composition, accumulation rates, and mass accumulation rates of chemical components. These tables can be used for comparative analysis with sediments of the same or other stratons in different oceanic basins and also with paleooceanic sediments on the continents. Terrigenous matrix dominates within lithogenic matter. We discovered a close resemblance between chemical composition of hemipelagic clays and volcanic sediments. Peculiarities of hydrothermal sediments are described. Using methods of mathematical statistics, we revealed main geochemical associations and principal factors determinating the chemical composition of studied sediments. Masses of oxides of petrogenic elements and a number of trace elements have been calculated for Pleistocene sediments. We managed to take an idea about average chemical composition of the Pacific Pleistocene.

The results of the lithological and biostratigraphic study of the Ozerninskaya strata, the accumulation of which is associated with the beginning of the early Hercynian stage in the development of the Uda-Vitim structural-formational zone of the Baikal-Vitim folded system and the formation of gold-pyrite-polymetallic ore deposits of the Ozerninsky ore cluster, are considered. The strata is isolated from the Lower Cambrian Oldynda Formation. It is composed predominantly of greywacke arkose and quartz greywacke and includes two substrata. The lower substrata is represented by quartz-feldspar sandstone and siltstone with interlayers and lenses of polymictic conglomerate and gritstone, while the upper substrata is composed of interbedded polymictic sandstone and siltstone, calcareous siltstone and limestone with interlayers of clayey, clayey-siliceous, coal-clayey siltstone and mudstone. The paleontological characteristics of the Ozerninskaya strata determines its stratigraphic belonging to the Lower (Emsian Stage) – Upper (Lower Frasnian Substage) Devonian. The composition, structure, and paleo-biota indicate that the sediments formed in the open shelf sea environment with a significant input of terrigenous component. The lower substrata deposited in the shelf shallow water conditions, with active hydrodynamics, while the upper substrata accumulated in the most submerged parts of the shelf, in areas with stagnant hydrogeological regime.

The material composition of placer (alluvial) dolomite-related nephrite from the Tsipa River in the nephrite-bearing region of the Vitim Highlands has been examined. Initial data on the mineral composition of both the inner and outer zones of alluvial dolomite-related nephrite pebbles have been obtained. Placer nephrite is characterized by a developed staining rim, a small amount of accessory minerals, fine-grained texture, randomly fibrous structure, and elevated alkali content. Chemical composition features of the staining rim formed under exogenous conditions include the development of manganese and iron hydroxides, with a significant increase in Fe³⁺ content and elevated levels of Co and Ba. A genetic relationship has been established between the placer nephrite and its primary source, the nephrite from the Kavoktinskoe deposit. A comparison between primary and placer dolomite-type nephrite has led to the identification of diagnostic characteristics for conducting expert assessments.