PETROGENETIC TYPES OF COALS AND THE STRUCTURE OF SEAM III OF THE CHERNOVSKOYE BROWN-COAL DEPOSIT IN CENTRAL TRANSBAIKALIA

O. A. Dulzon

(Submitted by the Department of Combustible Minerals)

The Chernovskoye brown-coal deposit is located in Central Transbaikalia, 20 kilometers southwest of the city of Chita.

Geologically, the area of the Chernovskoye deposit is an extensive Chitino-Ingodinskaya depression filled with Meso-Cenozoic continental sedimentary deposits. It is similar to the general type of Transbaikal coal-bearing depressions and extends in a northeastern direction from the village of Tanglinskoye to the city of Chita.

The Chitino-Ingodinskaya depression is the largest depression in Central Transbaikalia and is bounded by the Yablonovy Range in the northwest and the Chersky Range in the southeast [4].

The age of the coal-bearing deposits of the Chernovskoye deposit was determined by V. D. Prinada, on the basis of plant remains, as Upper Jurassic–Lower Cretaceous [4]. The coal-bearing deposits of the Chernovskoye deposit are represented chiefly by sandstones with interbeds of conglomerates, clay shales, and coal seams.

The first petrographic description of the coals of seam III of the Chernovskoye deposit was carried out on exploration boreholes by Yu. A. Zhemchuzhnikov [8]. We selected coal samples from seams III and IV of the Chernovskoye brown-coal deposit for the study of the material composition, spectral, palynological, and facial analyses, and stages of metamorphism.

A detailed petrographic description of the material composition of the coals of the Tarbagataiskoye, Chernovskoye, and Kharanorskoye brown-coal deposits was carried out by the author using the new terminology established by GOST 9414—60 [1, 2, 3, 5]. The Chernovskoye coals of seam III have the same material composition and degree of preservation of gelified components as the coals of seam Novyi—1(a) of the Kharanorskoye brown-coal deposit [5]. To study the material composition of the coal-forming plants of the Chernovskoye deposit, 210 samples were examined (in polished thin sections and polished blocks), and the percentage content of microcomponents in seam III was also counted. The counting of microcomponents was carried out on an ISA-type integration stage. Recalculation of the microcomponents in polished thin sections and polished blocks to the content of microcomponents in the coal seam was carried out according to the method of Z. V. Ergolskaya [6].

According to GOST 9414—60, the Chernovskoye coal of seam III is composed predominantly of microcomponents of the following four groups: vitrinite, semivitrinite, fusinite, and leiptinite.

The microcomponents of the vitrinite group are represented by collinites “B” and telinites “A,” “B,” and “V.”

The microcomponents of the semivitrinite group occur in insignificant amounts and are represented by the following varieties: semitelinite and micstinite. This group is combined with the fusinite group.

The microcomponents of the fusinite group have the following varieties: semifusinite, micrinite, fusinite, and sclerotinite.

The microcomponents of the liptinite group are represented by microsporinite, cutinite, resinite, suberinite, and algotelinite. The content of these microcomponents in all types of coal ranges from 0.11 to 10.25%. Algotelinite occurs in polished sections as isolated inclusions with a content of 0.01%, and therefore is not reflected in the properties of humic brown coal. Yu. A. Zhemchuzhnikov and A. I. Ginzburg likewise classified coals with such a low algal content as humic [7].

The microcomponents of the mineral-impurity group in polished sections of Seam III of the Chernovskoye deposit are observed in isolated cases. Mineral inclusions are represented by concretions of spherosiderite measuring 0.12×0.85 mm. The ash of coals of coal Seam III is characterized by the following composition: SiO₂—41.60%, Fe₂O₃—27.90%, MgO—0.42%, Al₂O₃—2.58%, CaO—3.44%, SO₃—5.64%.

The structure of Seam III of the Chernovsky open-pit coal mine is composed mainly of microcomponents of four groups: vitrinite, semivitrinite, fusinite, and liptinite. Seam III is composed of brown coal; in petrographic composition it is rather uniform and consists predominantly of the clarain type of coal and partly of dureno-clarain. The description of the coal types and bands is given from bottom to top along the section.

The clarain type of coal forms under conditions of a stagnant forest swamp with a high groundwater level and is the predominant type of coal. The content of clarain coal in the seam amounts to 94.35% of the seam thickness.

The dureno-clarain type of coal is representative of the facies of a forest boggy swamp with weak through-flow of groundwater. This type of coal makes up 5.65% of the seam thickness.

First band. At the base of this band there occurs a semitelinite-fusinite dureno-clarain 0.5 m thick. Above it lies a clarain coal layer 0.5–1.0 m thick. The main mass has a xylovitrain structure, in which fragments of black fusinite of various shapes with a distinct xylene structure are observed, as well as lenses of homogeneous collinite with transverse shrinkage cracks.

Among the liptinite elements there are isolated inclusions, yellowish-orange in color, of resinite and microsporinite.

The semitelinite-fusinite dureno-clarain has a content of the main mass of 28.50%, collinite 16.00%, telinite 34.94%, fusinite 20.00%, cutinite 0.07%, and microsporinite 0.49%.

The clarain coal layer, depending on the content of gelified fragments and liptinite elements, has the following varieties:

1. Vitrinite clarain.
2. Vitrinite-microsporinite clarain.
3. Fusinite-cutinite clarain.
4. Cutinite-microsporinite clarain.
5. Cutinite clarain.
6. Microsporinite clarain.

Second band. At the base there occurs a thick layer of vitrinite-fusinite clarain with a lens of vitrinite-fusinite dureno-

clarain. This bench is the largest and constitutes 83.60% of the seam thickness.

The vitrinite-fusinite clarain has a dark-brown xylovitrain basic mass, gelified and fusinized components of all possible forms and structures (from xylene to destroyed), and a small amount of liptinite elements represented by isolated microsporinite and resinite. According to the content of gelified and fusinized components, this coal has the following varieties:

1. Vitrinite-fusinite clarain.
2. Fusinite clarain.

A lens of vitrinite-fusinite dureno-clarain has a thickness of 0.5 m and an extent of 250–300 m along the section. The coal is composed of a xylovitrain basic mass, fusinite of all possible structures, bands of semifusinite, telinite, and homogeneous collinites with transverse shrinkage cracks. Among the liptinite components there are isolated inclusions of yellow microsporinite, serrated cutinite, and greenish-golden algotelinite.

The bench is capped by a layer of clarain coal with a content of liptinite elements from 0.11 to 10.25%. Depending on the content of liptinite elements in this bench, the following varieties are distinguished:

1. Vitrinite clarain.
2. Vitrinite-microsporinite clarain.
3. Cutinite-microsporinite clarain.
4. Cutinite clarain.

Third bench. This bench is composed of vitrinite-fusinite clarain, passing into vitrinite-semifusinite dureno-clarain. Both petrogenetic types of coal were described in the preceding benches. The thickness of this bench is 0.5 m.

The coal seam III under study occurs at a shallow depth and is currently being mined by the Chernovsky open-pit coal mine. The coal seam is simple and has a persistent thickness of 6–8 meters.

The author discovered a single interbed of carbonaceous aleurolite, 0.5 m thick, in the southwestern part of the section. This fact indicates a continuous process of accumulation of plant material, which proceeded throughout the entire period of peat-bog accumulation. This phenomenon is very rare in the geology of coal deposits of Transbaikalia. The accumulation of plant material of coal seam III took place against the background of calm, continuous subsidence of the floor of the depression. Peat accumulation corresponded to the subsidence and compensated for it, even with some excess, so that the upper part of the peat bog was subjected to subaerial influences, evidence of which is the abundance of fusain.

The roof and floor deposits of seam III were studied by the author macroscopically from rock hand specimens of the coal section and the mine.

The roof of coal seam III is composed mainly of sandy-clayey rocks, locally passing into clay, in places highly sandy and fairly dense. Some parts of the roof are composed of aleurolite with fusinized remains of woody and stem tissues of plants.

The floor is composed of gray, fine-dispersed clay with small inclusions of detritus and pieces of coal. Microscopic examination showed that these deposits are aleurolite with inclusions of fusinized and gelified plant remains.

At the Chernovskoye deposit, for many coal-bearing horizons and rock beds, an increase in thickness toward the center is observed

depression, which indicates slow downwarping of its floor and intensive coal accumulation. In the Chernovskaya depression there is a certain increase in coal content from south to north. The coal-bearing rocks and coal seams generally have a rather calm mode of occurrence. The angles of dip of the beds in the marginal part of the depression are about 10–15°; toward the center of the depression the beds flatten out and near the central part do not exceed 2–3°.

The coal-bearing deposits of the Chernovskoye deposit contain ten coal seams within their thickness. The structure of most of the coal seams is simple. Some coal seams in the peripheral part of the depression split into benches. The areal persistence of the coal seams is fairly stable; their thicknesses decrease closer to the outcrops at the surface [4].

REFERENCES

1. A. V. Aksarin, A. V. Vnukov, O. A. Dulzon, “Material composition and petrographic types of coals of the Targabatai brown-coal deposit of Central Transbaikalia.” Izv. TPI, vol. 218, Tomsk, 1970.

2. I. I. Ammosov and Tan Syu-i. Stages of Coal Alteration and Paragenetic Relations of Combustible Minerals. Moscow, Publishing House of the Academy of Sciences of the USSR, 1961.

3. N. I. Babinkova, S. A. Musyul. “Characterization of the microcomponent composition of brown coals.” In: Petrogeology of Coals and the Paragenesis of Combustible Minerals. Moscow, Nedra, 1967.

4. Geology of the USSR, vol. XXXVI. Moscow, 1961.

5. O. A. Dulzon. “Petrogenetic types of coals and the structure of the thick seam of the Kharanorskoye brown-coal deposit of Eastern Transbaikalia.” Izv. TPI, vol. 271, 1971.

6. Z. V. Ergolskaya. “A method for petrographic study of a coal seam for determining coal quality.” Materials on the Geology of Western Siberia, No. 59, 1947.

7. Yu. A. Zhemchuzhnikov, A. I. Ginzburg. Fundamentals of Coal Petrology. Moscow, Publishing House of the Academy of Sciences of the USSR, 1960.

8. Yu. A. Zhemchuzhnikov. Materials on the Genesis of the Chernovskoye and Bukachachinskoye Coal Deposits. Moscow–Leningrad, Gosizdat, 1941.