No 4 (2023)

The work is devoted to the study of ore minerals of the surface horizon of ore-bearing sediments of the Pobeda hydrothermal cluster. The following methods were used: optical microscopy, scanning electron microscopy, and X-ray spectral microanalysis. It was found that ore minerals are represented by fragments of copper and iron sulfides (isocubanite, chalcopyrite, and pyrite), newly formed iron hydroxides, and atacamite. In addition, barite and edaphogenic material in the form of silicate clasts, sometimes with sulfide inclusions, are present. Structural and morphological types of iron hydroxides are distinguished. On the basis of hydrophysical data, the location of the assumed active hydrothermal source in the area of the Pobeda-3 ore occurrence was updated. The distribution of studied minerals depending on the location relative to active hydrothermal vents is described. A decrease in the size and amount of hydrothermal mineral clasts and edaphogenic material and an increase in the degree of sulfide replacement by iron hydroxides were observed at a distance from the sources. Also, a decrease in the Cu/Fe ratio in the chemical composition of copper and iron sulfides is observed when moving away from the source. An unidentified phase of Cu_3.57‒4.22Fe_1.71‒2.19S_4.99‒5.31 with lamellae of chalcopyrite inclusions was established in the surface horizon of the column at 37L245g.

The article discusses some geochemical characteristics of Upper Vendian and Lower Cambrian clay rocks that form the base of the Moscow syneclise. It is shown that during the several tens of millions of years of geological history we are considering, there was no significant change in the composition of clay rocks. Based on the Zr/Sc and Th/Sc values characteristic of argillites and argillite-like clays, the petrogenic nature of the fine-grained aluminosiliciclastics composing them is substantiated. This conclusion is also confirmed by the values of the CIA/WIP ratio. In general, according to some parameters of their composition, the Upper Vendian and Lower Cambrian clay rocks of the Moscow syneclise are closer to granitoids, according to others, to basic igneous rocks. Thus, compared with the average Late Proterozoic basalts, they contain noticeably more K₂O and Rb, Th, Zr, Hf, Nb, and Ta, while the average Archean granitoids have noticeably lower concentrations of TiO₂, FeO, and MgO, Sc, V, Cr, Co, and Ni. The position of the data points for Upper Vendian and Lower Cambrian clay rocks on the La/Sc–Th/Co, La/Th–Th/Yb, Sc–Th/Sc and other plots confirms what has been said. Chondrite-normalized lanthanide distribution spectra in clay rocks are close to the PAAS lanthanide spectrum. It has been suggested that the transportation of suspended material to the area of sedimentation was carried out by: 1) large rivers with source areas composed of rock complexes of different composition, and 2) rivers that drained provenances composed mainly of sedimentary rocks. The average CIA values inherent in the Upper Vendian and Lower Cambrian clay rocks of the Moscow syneclise are comparable to those typical for the suspended particulate matter of modern large rivers of a humid subtropical and tropical climate, as well as rivers of dry tropical climate regions. The localization of the data points of argillites and argillite-like clays on the SiO₂–(Na₂O + K₂O + MgO + CaO), Al₂O₃–(Na₂O + K₂O + MgO + CaO) and CIA–WIP diagrams suggests that it was the paleoclimate that determined the main characteristics of their composition. The series of α-coefficients characteristic of clay rocks of the Upper Vendian–Lower Cambrian of the Moscow syneclise is quite close to the similar series for fine-grained suspended particulate matter of modern large river systems in southern Africa.

Local factors of formation of coastal-marine rare-metal-titanium placers were analyzed for creation of the search-oriented geological-dynamic model that allows predicting the prospects of rare-metal-titanium placer mineralization. The influence of the factors on the structure, composition, localization of placer concentrations within sedimentation basins, as well as the dynamic interaction of these factors during the formation of productive sediments was investigated. Based on the work of previous researchers who have studied in detail the issues of controlling the formation of these placers at the historical-geological and regional structural-tectonic level, a qualitative model of the formation of placer concentrations on local areas of the rank of districts, clasters, fields and specific placer bodies has been created, which can be the basis for creation of search-orientated model of the deposit and planning exploration and evaluation work. The following factors of the placer formation were studied: hydrodynamic – characterizing the types and mechanisms of placer concentrations (littoral and sublittoral) within the dynamically active coastal zone; lithodynamic – depending on the abrasion-accumulative regime of the coast and the parameters of the long-shore sediment flow; structural–dynamic – determining the possibility of the formation of economically significant productive deposits and their localization; and the factor of transgressive-regressive regime a basin that determines the structure of placer bodies and their transformation at the post-sedimentation stage. Large economically significant rare-metal-titanium placer deposits are formed only in conditions of joint realization of the positive potential of these factors. The developed qualitative model can serve as a basis for the process of digitalization of the forecasting of placer mineralization of this type.