The main types and mineral composition of bauxites of the Kursk Magnetic Anomaly

A.D. Savko^*, M.Yu. Ovchinnikova^**

Voronezh State University, Voronezh, 394018 Russia

E-mail: ^*savko@geol.vsu.ru, ^**iovchinnikova@geol.vsu.ru

Received April 30, 2020

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DOI: 10.26907/2542-064X.2020.2.274-289

For citation: Savko A.D., Ovchinnikova M.Yu. The main types and mineral composition of bauxites of the Kursk Magnetic Anomaly. Uchenye Zapiski Kazanskogo Universiteta. Seriya Estestvennye Nauki, 2020, vol. 162, no. 2, pp. 274–289. doi: 10.26907/2542-064X.2020.2.274-289. (In Russian)

Abstract

The bauxite-bearing Paleozoic weathering crust of the Kursk Magnetic Anomaly (KMA) is developed along the inter-ore schists of the ferruginous-siliceous formation of the Kursk series. In the banded iron formations, the weathering crust is represented by high-grade iron ores forming a series of deposits with bauxites. They are confined to the ridges of banded iron formations, which tower up in the relief of the Precambrian and are overlain by a thick layer of sediments. The recently obtained rock material studied with the use of precision methods enabled a significant refinement and adjustment of the existing ideas about KMA bauxites. Based on their texture, structure, and mineral composition, the following five main types were singled out: 1 – finely porous substantially boehmite, 2 – macroporous berthierine-boehmite, 3 – stony hydrohematite-berthierine-boehmite, 4 – massive berthierine-kaolinite-boehmite, 5 – pseudo-bean substantially gibbsite and berthierine-gibbsite. Mostly gibbsite varieties gravitate toward the northern part of the Belgorod ore region, while boehmite varieties are more abundantly distributed in its southern part. The main bauxite minerals are boehmite, gibbsite, berthierine, and kaolinite; the minor ones are illite, hematite (hydrohematite), quartz, goethite, chamosite, pyrite, carbonates, and accessories. They formed in several stages. In the pre-Devonian (pre-Eifel) time, an illite weathering crust formed along the inter-ore schists. Since the Givetian time, an illite-kaolinite profile has been developing due to the climate humidization and the emergence of terrestrial vegetation. Laterites with bauxite appeared in the Early Carboniferous and only in the Belgorod region of the KMA, because the latter was not blocked by the sea basin, unlike the rest of the territory. The processes superimposed on the weathering crust led to silicification and carbonation of the uppermost parts of the bauxite section.

Keywords: Kursk Magnetic Anomaly, bauxites, weathering crust, gibbsite, boehmite, berthierine, kaolinite

Figure Captions

Fig. 1. The layout of the deposits of banded iron formations and bauxites. According to [1]. 1 – weathering crusts in the banded iron formations and shales; 2 – exposure of banded iron formations to the Precambrian surface with a thin (up to 10 m) weathering crust. Rocks: 3 – Oskol series; 4 - Stoilenskaya suite of the Kursk series; 5 – Archean; 6 – Shebekinsky syenites; 7 –granites of the Ataman complexes; 8 – tectonic disturbances.

Fig. 2. Varieties of bauxites and their diffraction patterns: a – substantially boehmite microporous bauxite with kaolinite (Melikhovo-Schebekinsky deposit); b – bertierine-kaolinite-boehmite bauxite (Vislovskoye deposit).

Fig. 3. Varieties of bauxites and their diffraction patterns: а – dense boehmite-bertierin-kaolinite banded bauxite with relic texture (Yakovlevskoe deposit); b – brown stony hydrohematite-bertierine-boehmite bauxite (Melikhovo-Shchebekinsky deposit).

Fig. 4. SEM images of bauxites: a – kaolinite-berthierine-boehmite (Yakovlevskoe deposit), b – gibbsite with berthierine relics (Olkhovatskoye deposit). Designations: Bhm – boehmite, Gbs – gibbsite, Brt – berthierine, Kln – kaolinite.

Fig. 5. Forms of berthierine isolation from boehmite bauxites (Yakovlevskoe deposit): a – hypergenic; b – diagenetic.

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