Lithology and Mineral Resources最新文献

Complex interpretation of the distribution of δ¹³C and δ¹⁸О values in carbonates, as well as δ³⁴S and ⁸⁷Sr/⁸⁶Sr values, in gypsum from the lower Eifelian Osveya horizon (Belarus) showed that significance of the diagenetic isotope signals is low and the isotope variations can be explained consistently within a sedimentation model. Along with the section intervals with standard marine δ¹³C values, there are negative excursions of the parameter (up to –7…–11‰) marking segments with carbonate–sulfate rocks and smaller sulfate manifestations. The excursions correspond to evaporitic episodes when the basin became shallower and, probably, disintegrated into separate depressions, leading to increase in the role of continental water enriched with soil carbon in the geochemical sedimentation system due to the continental runoff activation and/or shallowing. The waters could enter both from the prolonged exposed land areas where the Eifelian rocks are absent until now and from the periodically existing islands. The oxygen isotope composition does not respond to negative δ¹³C excursions by similar shifts due to the possible absence of significant difference between the δ¹⁸О values of the atmospheric precipitation generating the land water and the seawater at low latitudes where the Belarus region was situated in the Devonian. In 40% of the studied samples, the δ¹⁸О values range from –4 to –2‰, corresponding to the Eifelian “plateau” (approximately –3‰) in the generalized chemostratigraphic curve based on calcite of brachiopod shells from several regions of the world. The δ¹⁸О values higher than –2‰ (44% of samples), probably, are related to water evaporation, whereas values lower than –2‰ (16% of samples), which can decrease to –7.0…–9.5‰ at the peaks of evaporation episodes, are caused by the water heating (sometimes very significant) during the basin shallowing. Activation of the role of continental water accompanying the evaporation episodes is supported by the following fact: the ⁸⁷Sr/⁸⁶Sr ratio in the Osveya gypsum (0.708402–0.708742) is much higher than in the Eifelian seawater according to a global estimate (0.70772), whereas the δ³⁴S value in gypsum (15.5–21.2‰) in more than one-half of the analyzed samples is lower than typical values in the Middle Devonian seawater sulfate (19–20‰).

Local factors of the formation of coastal-marine rare metal–titanium placers were analyzed for creating the prospecting-oriented geological-dynamic model that allows to predict the prospects of rare metal–titanium placer mineralization. Their influence on the structure, composition, localization of placer concentrations within sedimentation basins, and dynamic interaction during the formation of productive sediments was investigated. Based on the works of previous researchers devoted to the historical-geological and regional tectonic factors controlling the formation of these placers, we developed a qualitative model of the formation of placer concentrations in different rank areas (districts, clusters, fields, and specified placer bodies), which can serve as a basis for the creation of prospecting-orientated model of the deposit and planning the exploration and evaluation work. Among the factors controlling the formation of placers, we can identify: hydrodynamic processes characterizing the types and mechanisms of placer concentrations (littoral and sublittoral) within the dynamically active coastal zone; lithodynamic processes depending on the abrasion-accumulative setting of the coast and parameters of the alongshore sediment flow; structural-dynamic processes controlling the formation of economically significant productive deposits and their localization; and the transgressive-regressive setting of a basin that determines the structure of placer bodies and their transformation at the postsedimentary stage. Large economically significant rare metal–titanium placer deposits are only formed under conditions of a joint realization of the positive potential of these factors. The developed qualitative model can serve as a basis for digitizing the forecasting of rare metal–titanium placer mineralization.

The article discusses some geochemical characteristics of Upper Vendian and Lower Cambrian (hereafter, Upper Vendian–Lower Cambrian) clay rocks that make up the base of the Moscow syneclise. It is shown that there was no significant change in the clay rock composition during several tens of million years of the geological history under consideration. Based on the typical Zr/Sc and Th/Sc values, the petrogenic nature of the fine-grained aluminosiliciclastics of mudstones and mudstone-type clays is substantiated. This conclusion is also confirmed by the CIA/WIP values. In general, according to some parameters of their composition, Upper Vendian and Lower Cambrian clay rocks of the Moscow syneclise are closer to granitoids; according to other parameters, to basic igneous rocks. Thus, compared with the average Late Proterozoic basalts, the clay rocks are markedly enriched in K₂O and Rb, Th, Zr, Hf, Nb, and Ta, whereas the average Archean granitoids have notably lower concentrations of TiO₂, FeO, MgO, Sc, V, Cr, Co, and Ni. Positions of data points of the Upper Vendian–Lower Cambrian clay rocks on the La/Sc–Th/Co, La/Th–Th/Yb, Sc–Th/Sc, and other diagrams confirm the above statement. The chondrite-normalized lanthanide distribution in clay rocks is close to the PAAS lanthanide spectrum. It is suggested that the suspended material was transported to the sedimentation area by: (1) large rivers with source areas composed of rock complexes of different composition; (2) rivers that drained provenances composed mainly of sedimentary rocks. The average CIA values inherent in the Upper Vendian–Lower Cambrian clay rocks of the Moscow syneclise are comparable to those typical for the suspended particulate matter (SPM) in modern large rivers of the humid subtropical and tropical climate, as well as in rivers of dry tropical climate regions. Localization of data points of mudstones and mudstone-type clays on the SiO₂–(Na₂O + K₂O + MgO + CaO), Al₂O₃–(Na₂O + K₂O + MgO + CaO), and CIA–WIP diagrams suggests that the main characteristics of their composition was governed by the paleoclimate. The series of α-coefficients characteristic of the Upper Vendian–Lower Cambrian clay rocks in the Moscow syneclise is quite similar to those for the fine-grained SPM of modern large river systems in southern Africa.