Vol 52, No 2 (2017)

The Sr isotope stratigraphy of the biogenic apatite was used to determine the age of pelagic sediments in the Brazil Basin (Station 1541) that contain ferromanganese micronodules, nodules, and coatings on the weathered volcanic rocks. The age of sediments at horizons 0–5 and 86–90 cm was estimated at 24.1 ± 0.2 Ma and 24.8 ± 0.2 Ma, respectively. The average sedimentation rate in the Late Oligocene was about 13 mm/ka. The hydrogenous Fe–Mn nodule on the sediment surface with the Mn/Fe value of 1.05–1.95 was formed at a rate of 1.2–2.4 mm/Ma, which is 1000 times lower than the growth rate of buried nodule (Mn/Fe 0.4) at depth of 83 cm. Diagenesis provoked changes in the mineral composition of the buried nodule (asbolane-buserite partially replaced by goethite), leading to the loss of a part of Mn, Ni, Li, and Tl but accumulation of trace elements linked with iron oxyhydroxides (Ce, Th, Be, As, and V) were retained. The composition of manganese micronodules at two studied depths in sediments evolved in the course of two stages of ore formation: related to the oxic and suboxic diagenesis. The Sr isotopic composition in manganese micronodules from both horizons do not differ from that of dissolved Sr in the ocean water. The ¹⁴³Nd/¹⁴⁴Nd ratio, which reflects the Nd isotopic composition in the paleocean during the micronodule formation, varies in manganese micronodules from different horizons and is constant in different size fractions.

The paper presents the lithological, mineralogical, and geochemical characteristics of the composition, structure, and organic matter of siderite nodules and host mudstones in the Lower Carboniferous (Tournaisian–Visean) siderite-bearing sequence exposed along the Kozhym River on the western slope of the Subpolar Urals. The obtained results revealed that organic matter in the mudstones is dominated by C₁₆ and C₁₈n-alkanes, suggesting a significant microbial activity in the sedimentation environment. The formation of nodules was promoted by the activity of diverse bacterial communities, the abundance of which was related to processes of methanogenesis in bottom sediments owing to gaseous fluid seepages that promote the saturation of sediments with methane and the flourishment of bacterial colonies. Such processes in a marine basin led to the local freshening or some salinization of water and the development of euxinic setting and specific bacteria. Consequently, siderite nodules therein are marked by bacteriomorphic textures and specific authigenic mineralization (framboidal pyrite, sphalerite, galena, sulfoselenides, and tellurides). Bacteria utilized the substrate of the redeposited weathered crust, resulting in a large-scale formation of the nodular siderite.

The paper presents the results of lithological studies of Jurassic rocks in one of the largest basins in the eastern Transbaikal region (Olov depression). These results refined settings of the volcanosedimentary association and indicated that the Olov depression was formed in three stages: stage 1 related to tectonic activation manifested as cataclasis of granitoids in the Transbaikal region and incipience of numerous depressions (Olov included); stage 2 characterized by catastrophic events related to reactivation of tectonic motions, strong volcanism, and intense activity of geysers; and stage 3 marked by termination of volcanic activity and relative tectonic stability of the region that promoted the deposition of rocks of the middle and upper Ukurei subformations in the course of slow synsedimentary subsidence of the depression. Uranium mineralization in the rocks was governed mainly by the following conditions: (1) abundance of cataclased granitoids that delivered terrigenous material to the sedimentation basin; (2) favorable paleogeographic setting for the formation of both sedimentary and postsedimentary hydrothermal U-rich sequences. Field works in the Transbaikal region and analytical results reported in the present paper made it possible to make a videofilm shown in https://youtu.be/UOe9xzSKOEI.