Vol 54, No 5 (2019)

The Sr isotope composition was studied in waters from 22 mud volcanoes in Azerbaijan, where the O and H isotope compositions, as well as concentrations of macro- and microelements, were determined earlier in (Lavrushin et al., 2015). Values of the ⁸⁷Sr/⁸⁶Sr ratio in the Azerbaijanian mud volcanoes range from 0.7067 to 0.7083. Its minimum value was recorded in water of Neftechala Yuzhnaya Volcano located in the southern area (Kura region), where volcanoes are generally characterized by low average values of ⁸⁷Sr/⁸⁶Sr (0.7070 ± 0.0003) and δ¹⁸O (2.0 ± 1.8‰), in contrast to volcanoes located in the northern area (Shemakha‒Gobustan, Apsheron, and Caspian regions) with ⁸⁷Sr/⁸⁶Sr = 0.7076 ± 0.0005 and δ¹⁸O = 4.8 ± 2.5‰. Ratios of the Sr and O isotope compositions of mud volcanic waters in Azerbaijan suggest that isotope characteristics of these waters are related to two major sources: the first source is related mainly to Cretaceous‒Paleogene sedimentary carbonates, whereas the second source dominating in the Kura region bears the signs of isotope exchange with volcanic rocks. The role of Sr dissolved in sea water and of Sr, which could be extracted from the recent terrigenous sediments in the Caspian Sea and the Maikopian terrigenous clayey rocks, is negligible in the Azerbaijanian mud volcanic waters.

The paper analyzes original data obtained on the Atlantis‒Meteor seamount system during Cruise 33 of the R/V Akademik Nikolai Strakhov in the eastern North Atlantic. This system is a volcanic rise formed on the Canary abyssal plate and represents one of the key objects for understanding the geological history of opening of the central segment of the Atlantic Ocean. Basalts, tephrites, and organogenic terrigenous lagoonal marine sediments dredged from the Atlantis, Plato, and Cruiser seamounts are considered. Petrography and compositions of the Atlantis and Cruiser basalts reflect significant differences in settings of their eruptions. Well-crystallized vesicle-free olivine basalts from the Atlantis Seamount were ejected under deep-water conditions. Glassy vesicular basalts of the Cruiser Seamount are typical of shallow subaerial eruptions. Evidence for the accumulation of tuff breccias and tuff gravelstones of the Plato Seamount in subaerial settings are obtained. Tendencies were revealed in the lithogenetic transformations of organogenic‒terrigenous sediments of the Cruiser Seamount, which were subjected to the high-temperature impact of subaerial lava flows. During the volcanosedimentary lithogenesis, the plant lignite-like matter lost its primary structure and was transformed into anisotropic coke with the wide development of fusinite and pyrofusinite. The studied volcanic occurrences are thought to be related to the final (Late Miocene‒Pliocene) volcanic stage in the seamount system, which predated the destruction of the system, its prograde subsidence, and transformation of islands into guyots.

Deposits in the Postmasburg field, South Africa, confined to rocks of the Transvaal Supergroup, occupy a significant place in world resources of the manganese and iron ores. The ore deposits represent karst residual accumulations. Several elements (B, Cr, Ni, Zn, Ge, As, Se, Mo, Ag, Cd, Sb, Te, W, Pb, REE) in them make up an assemblage providing insight into the geochemistry of the processes of ore formation. Among these elements, Mo, As, Ag, and REE are most representative. Molybdenum is marked by the chemosorptional incorporative nature of accumulation in these ores, sometimes with the formation of epictic overgrowths of the ferrimolybdate-type minerals. Arsenic, which is leached from the substrate rocks and accumulated in karst ferromanganese and iron ores, reflects total impact of the dominating iron oxide minerals upon its mobility. The behavior of Ag is controlled by the processes of supergene alteration of Archean–Early Proterozoic carbonate rocks and ores of the banded iron formation (BIF). Comparison of the distribution of rare earth elements (REE) in the karst iron, ferromanganese, and manganese ores, as well as banded iron ores, demonstrates that these elements display similar values of the cerium (C/Ce*) and europium (Eu/Eu*) anomalies, but differ in terms of the HREE and LREE fractionation (typical values: Ce/Ce* = 0.7‒1.0; Eu/Eu* = 0.8‒1.1). Banded iron and associated manganese ores were accumulated in the marginal anoxic-dysoxic marine basin, which was constrained by the continental land. Anoxic and dysoxic conditions were created by an intense hydrothermal activity.