Holocene terrace systems and isolated terraces have been identified on the Black Sea coast of the Caucasus, including the northwestern part of the Colchis Lowland. Their composition was studied using a combination of lithological and physicochemical methods. It has been shown that the fine fraction (<0.001 mm) of sediments consists predominantly of newly formed mixed-layer kaolinite‒smectite with a terrigenous admixtures of smectite, kaolinite, micaceous mineral, and occasionally chlorite. Using diffraction pattern modeling for air-dry and ethylene glycol-saturated natural samples, the structural parameters of clay minerals and their quantitative content were determined. No correlation was found between these parameters and the facies settings in which the terrace deposits formed. No correlation was also found between the structural parameters of kaolinite‒smectite and the depth of sediment occurrence or their location within the study area. All samples contain kaolinite‒smectite with a fairly narrow range of the ratio of kaolinite and smectite layers, Wk : Wsm, on average from 0.55 : 0.45 to 0.65 : 0.35. It is shown that the average crystallochemical formula calculated for kaolinite‒smectite with Wk : Wsm = 0.65 : 0.35 exactly corresponds to the dioctahedral structure of the mineral. The decomposition of this formula into smectite and kaolinite components indicates that the 1 : 1 layers in the studied kaolinite‒smectites differ in their crystallochemical structure from true kaolinite ones. A solid-phase mechanism for the formation of kaolinite‒smectites in a smectite matrix is proposed via the removal of fragments of tetrahedral sheets from 2 : 1 layers and adjacent smectite interlayers. The structural evolution of mixed-layer kaolinite-smectites is discussed. The authigenic origin of mixed-layer kaolinite‒smectite minerals in the studied sediments is substantiated.
The facies were studied and the sequence-stratigraphic model of the Zhuya Group in the Ura Uplift (Vendian reference section, southern area of Middle Siberia) was constructed. The whole Zhuya Group represents a single sequence bounded by unconformities. Sediments of the Kullekin member were accumulated in the high-energy coastal-marine environments during sea-level lowstand. The development of transgression initiated a carbonate ramp on the Siberian Craton. The rest of the Zhuya Group above the Kullekin member records the gradual progradation of the ramp and basin compensation of accommodation space during sea-level highstand. The trajectory of the negative C isotope excursion of the Zhuya Group, correlated with the Shuram–Wonoka excursion, follows the relative sea level changes, reaching its nadir close to the maximum of transgression. Elucidation of the reason for the coincidence of these trends requires additional regional and interregional studies. The occurrence of Neoproterozoic zircons in the Kullekin sandstones derived from a northern source reflects accretion of the Baikal–Muya terrane to the Siberian Platform and can be attributed to either pre-Zhuya inversion of the Patom basin (collision) or erosion of synsedimentary tuffs on the Siberian Platform (soft accretion).
Titanium and zirconium are critical metals in many products important to the world economy and technologies. Countries in the former Soviet Union (FSU), such as Russia, Ukraine, and Kazakhstan, have significant reserves of these metals sufficient for the development of own mineral resource base. While the world’s main resources of titanium and zirconium feedstock are concentrated mainly in the beach/marine and eolian dune deposits on the present-day coasts, resources in FSU countries are associated with ancient (Paleozoic, Mesozoic, and Cenozoic) coastal-marine and alluvial placer deposits. Genesis of some placers is discussed. There are two main metallogenic structures in the studied area: East European Megaprovince located within the East European platform (Russia and Ukraine) and West Siberian Megaprovince, which includes the West Siberian Platform (Russia) and North Kazakhstan.
The U–Th–Pb isotope age of detrital zircon populations from sandstones of the Bol’shoi Inzer and Yusha formations of the Burzyan Group (Lower Riphean) of the Yamantau Anticlinorium in the Southern Urals was analyzed. The presence of grains supplied to the sedimentation area from proximal and distal sources was demonstrated. The former likely included the Taratash Complex/Orogen and the Bak 1 complex of the Bakaly granitoid block of the Volga–Ural region, or rock associations similar in age and composition from other regions. The latter include the Volga–Don, Svecofennian, East Sarmatian, and other Early Proterozoic orogens of the East European Craton (Baltica). Considering the intracratonic nature of the Early Riphean sedimentation basin, it has been suggested that some of the detrital material could be delivered from non-Baltica/Siberian sources. Maximum sedimentation age of the Bol’shoi Inzer and Yusha formations has been established. The age of the Bol’shoi Inzer Formation is estimated at 1805 ± 30 Ma (according to the YSG algorithm) and 1870 Ma (according to the YPP algorithm). The Yusha Formation is characterized by the maximum sedimentation age of 1798 ± 10 and 1890 Ma, respectively.
This paper examines cupriferous sandstones and shales hosted in sediments of varying ages (from P2 to PR2) and altered to different degrees (from early catagenesis to the epidote–amphibolite stage). The formation of these rocks is associated with the removal of Cu and other metals by formation waters from the red-colored ore-hosting rocks and their precipitation as sulfides on hydrosulfuric biogeochemical barriers. After their formation, cupriferous sandstones and shales can undergo metamorphic and supergene alterations, which are reflected in the crystallochemical properties of sulfide minerals, the composition of their paragenetic assemblages, the mineral zoning of mineralization, the morphology of ore bodies, and other features. This paper focuses on sulfides of the Cu—S system formed under various conditions. Their composition, structure, physicochemical and thermobaric stability, and occurrence in natural ores are discussed. The composition of copper sulfide ores in cupriferous sandstones, shales, and hypergenesis zones is elucidated. In particular, the paper explains why only nonstoichiometric Cu‒S sulfides (djurleite, geerite, spionkopite, yarrowite) are present in unaltered cupriferous sandstone ores and hypergenesis zones, whereas stoichiometric copper sulfides (chalcocite, digenite, and tetragonal phase) occur in metamorphosed ores. This fact is explained from a physical and chemical perspective. It has been established that each stage of rock formation and transformation is characterized by specific typomorphic minerals of the Cu‒S system and their parageneses that are stable under the conditions of a given stage. With increasing metamorphic grade, the mineral composition of ores and mineral zoning of the deposits change consistently toward simplification due to the gradual disappearance of low-temperature compounds and associations. Indicator minerals of the Cu‒S system, reflecting the thermobaric and chemical conditions of ore formation and transformation, have been identified.
