Formation and preservation of polyhalite-containing rocks in the halogen sediments of the Southern Cis-Urals

Research subject. In this research, we set out to investigate polyhalite-containing rocks of the Southern Urals. The purpose of this study was to characterize the patterns of formation and transformation of polyhalite-containing rocks, as well as to identify conditions leading to their destruction.

Materials and methods. We studied available literature and archive materials on the southern Urals, as well as core samples. Samples for thin sections were taken from the core of polyhalite-containing rocks. The sections were manufactured (where possible) in 2 mutually perpendicular cuts relative to the axis of the monolithic core sample. Considering a high solubility of salt minerals (halite) and the possibility of converting other minerals in aqueous media (e.g., polyhalite into gypsum due to the leaching of K and Mg cations herefrom), thin sections were manufactured in anhydrous liquids (kerosene, transformer oil). In order to determine the mineralogical composition, as well as the structural and textural features of the polyhalite-containing rocks under study, a petrographic analysis of the thin sections was carried out optically using a Polam-213 microscope. Geological sections were built using Corel Draw 13. To this end, geophysical data (gamma-ray logging) and lithological composition were studied.

Results. It is found that pelitomorphic sulphate clusters transform into polyhalites followed by their further recrystallization into crystalline accretions. The diagenetic nature of this process is presumed. The location of polyhalite mineralization in the rocks under study confirms the effect of the basin bottom paleorelief on the intensity of polyhalite material accumulation. Conditions determining further destruction of these rocks were formed in local areas under the influence of various geological processes.

Conclusion. The results of our research can be used for predicting zones rich in polyhalites in the southern Cis-Urals.

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