THE GEOLOGY OF CARBONACEOUS DEPOSITS (CARBONIFEROUS) OF THE SOUTH URALS (FEDOROVSKAYA SEQUENCE, SUKHARYSH SYNCLINE)

The geological structure of coal formations of the Sukharysh syncline, which is located in the Kopeysk-Bredy subzone of the Alapaevsk-Adamovka structural-formational zone, was studied. The focus was specifically on the deposits of the Fedorovskaya sequence (C2fd) with outcrops in the sections along the Uvelka River and in few roadside pits. The width of the sequence is about 1000 m. The Middle Carboniferous (Bashkirian) age of the deposits was reliably dated based on the fossil faunal finds. It was shown that shales and aleurolites, which are widespread in the limestones of the Fedorovskaya sequence, belong to the low-carbon type and fall into the field of the siliceous-carbonate unit. The carbonaceous matter is developed in the form of scattered impurities, concentrated in the ultrafine veinlets and laminas, and corresponds to higher kerites subjected to catagenesis and the early stage of green shales facies. The organic carbon content is up to 20%, and, thus, the rock appears to be opaque. Sedimentation process took place in the isolated local shallow-water depressions under the geodynamic conditions of collision. The terrigenous material was formed as a result of the destruction of the underlying rocks of the major and average element composition of the Berezinskaya sequence. The maturity of the deposits consistently increases from the Tugundinskaya to Fedorovskaya sequences.


Introduction
In the world's largest gold deposits, carbonaceous shales often become the oregenerating and -hosting strata and, therefore, hold promise for future research. As for the South Urals, these shales have remained overlooked here for many years, despite their widespread distribution along the structural-formational zones, the widest age interval, and the ability to form in various paleogeographic conditions. Earlier exploration campaigns have found that the Carboniferous carbonaceous shales of the East Urals megazone host a number of commercial gold deposits (Kumakskoe, Svetlinskoe, Kamenskoe, Tetechnoe, etc.), thereby fostering greater research interest in this region [1][2][3][4][5].
This paper is based on the materials obtained from the study of carbonaceous shales of the Fedorovskaya sequence as part of the geological survey [6] and the thematic investigations of the Sykharysh syncline.

Materials and Methods
Thermogravimetric analysis of the carbonaceous rock samples was performed using a Q-1500 derivatograph (analyst T.I. Chernikova, Institute of Geology, Ufa Federal Research Center, Russian Academy of Sciences). The samples were heated in air at a temperature of 20-1000 °С and at a rate of 10 °С/min. Only samples of the least silicified and sulphidized rocks outside the zones of intrusive exocontacts and intensive tectonic reworking were used for analysis to exclude their possible influence and determine the degree of regional metamorphism. Silicate analysis was carried out by the standard methods (analyst S.A. Yagudina, Institute of Geology, Ufa Federal Research Center, Russian Academy of Sciences).

Description of Carbonaceous Shales in the Fedorovskaya Sequence
The Fedorovskaya sequence (C 2 fd) was discovered during the geological survey of the Chelyabinsk area, a northward neighboring region, in 1995-1999 and named after the village of Fedorovka. Its deposits occur unconformably on the Birgil'dinskaya sequence and are represented by organogenic-detrital pelitomorphic gray and dark-gray limestones interbedded with carbonaceous-clayey shales and carbonaceous argillites. The sequence outcrops in the sections along the Uvelka River and in few roadside pits (Fig. 2). The width of the sequence is about 1000 m.
The following rich faunal assemblages pinpoint the Middle Carboniferous age  [6].
Carbonaceous shales and aleurolites of the Fedorovskaya sequence are mostly gray or dark gray in color. Their stratified texture is determined by the presence of interbedded radiolarites and filamentous carbonaceous matter in the ground microcrystalline siliceous mass (Fig. 3, a, b). The clay matter (hydromica) is found in small proportions and forms striated inclusions and short laminas. The rock contains thin (0.5-2.0 mm) radiolarite interbeds with the large detrital accumulations (up to 60%) of radiolarians, sponge spicules (?), and shell fragments, up to 0.1 mm in size (Fig. 3, c, d). Such interlayers are generally porous and characterized by a much lower content of the carbonaceous matter, which is present as a scattered impurity. Phosphorized fragments of dirty brownish ichthyodetritus (?) occur rarely. They are thinly laminated, with the admixed carbonaceous matter (Fig. 3, e). Such fragments often have irregular shape, jagged edges, and sizes of 0.5 to 2.5 mm. The accessory minerals are short-prismatic idiomorphic corroded glauconite crystals (up to 0.1 mm), rarely oxidized pyrite crystals of cubic habit (up to 0.2 mm). Single grains of amphibole, garnet, staurolite, ilmenite, and chromite were recorded in the black sand.
The brown carbonaceous matter is developed in the form of scattered impurities and concentrated as ultrafine veinlets and laminas. In certain areas, its content amounts to 20%, and the rock appears to be opaque. The cavities of organogenic detritus are filled with bitumen greases (Fig. 3, f). The thermal analysis showed that gray shales and aleurolites from the Fedorovskaya sequence have low contents of organic carbon (up to 0.5%). However, the section still displays dark-gray or black interbeds (up to 0.5 m thick) with the C TOC level up to 2.8% (Fig. 2, 4; Table 1). This indicates that the deposits belong to the low-carbon type, which is widespread in the South Urals [7,8]. The exothermic reaction of carbon with oxygen in the shales starts at low temperatures, from 490 °С (550 °С on average), thereby suggesting that the level of carbon metamorphism is low and corresponds to higher kerites subjected to catagenesis and the early stage of green shales facies [9,10].

Petrogeochemistry of the Sukharysh Syncline Rocks
Carbonaceous deposits have proved their worth as reliable sources for reconstructing the sedimentation conditions. The chemical composition of sedimentary rocks can reveal a lot about the original source of clastic elements. The petrogeochemical features of carbonaceous shales and aleurolites in the Tugundinskaya (C 1 tg), Birgil'dinskaya (C 1 bg), and Fedorovskaya (C 2 fd) sequences were studied using the standard classification diagrams (Table 2, Fig. 5).
On the A-S-C diagram [11] (Fig. 5, a), carbonaceous shales and aleurolites of the Tugundinskaya sequence fall into the field of terrigenous-carbonate unit, while those of the Fedorovskaya and Birgil'dinskaya sequences into the siliceous-carbonate unit. The increased S parameter, which is inversely proportional to the amount of terrigenous impurities in the sediment, indicates that the sedimentation process shifts to lower depths or that there are isolated local depressions, where the deposits of the Fedorovskaya sequence accumulated during the Middle Carboniferous.
It is evident from the distribution of the imaging points showing the composition of carbonaceous deposits of the Sukharysh syncline on the DF1-DF2 and F1-F2 diagrams (Fig. 5, b, c) that these rocks settled under the conditions of collision. The sources of terrigenous material supply must have been eroded underlying rocks of the major and average element composition of the Berezinovskaya sequence (C 1 bz).  On the log (SiO 2 / Al 2 O 3 )log (Fe 2 O 3 / K 2 O) diagram [14] (Fig. 5, d), the imaging points of carbonaceous deposits of the Tugundinskaya sequence form a compact area in the fields of shales and ferruginous shales, while the deposits of the Birgil'dinskaya and Fedorovskaya sequences spread over the rest of the fields. Along with the changes in the chemical weathering index CIA = 100Al 2 O 3 /(Al 2 O 3 + CaO + Na 2 O + K 2 O) [15], this testifies that the degree of sediment maturity increases consistently over the time interval.

Conclusions
The study of carbonaceous shales and aleurolites, which are widespread in the limestones of the Fedorovskaya sequence, leads to the following conclusions: 1. These deposits belong to the low-carbon type and fall into the field of the siliceous-carbonate unit.
2. The carbonaceous matter that developed in the form of scattered impurities and concentrated in the ultrafine veinlets and laminas corresponds to higher kerites subjected to catagenesis and the early stage of green shales facies.
3. Sedimentation occurred in the isolated local shallow-water depressions under the geodynamic conditions of collision. The terrigenous material was mainly formed as a result of the destruction of the underlying rocks of the major and average element composition of the Berezinskaya sequence. The maturity of the deposits consistently increases in time from the Tugundinskaya to Fedorovskaya sequences.