PETROGRAPHIC CHARACTERIZATION OF THE BORODINO SEAM OF THE IRSHA-BORODINO DEPOSIT OF THE KANSK BROWN-COAL BASIN

V. L. Kokunov

(Presented by the scientific seminar of the Department of Combustible Minerals)

We carried out the macroscopic description of the coals in a trench of the Irsha-Borodino open-pit coal mine, which made it possible to describe the seam from bottom to top, and also to trace directly, along the seam exposed by the trench, the nature of the distribution within it of individual petrographic types and ingredients distinguished by luster and structure—something that cannot be done when studying a coal core. In all, seven bed-by-bed columns of the seam were compiled, spaced 250–600 m apart (Fig. 1).

The bulk of the coal of the Borodino seam is represented, almost 90%, by semibright and semidull varieties, which can be traced fairly clearly over a distance of 2–3 km. Slightly more than 10% consists of dull coals, among which coals with a lumpy macrostructure are distinguished.

In the predominant mass of coal, the material composition of which was later studied in detail under the microscope, two simple ingredients are readily distinguished with the naked eye: fusain and vitrain, which we shall discuss separately, and a third—complex—ingredient: semidull or semibright.

Fusain. In the seam it occurs in the following modifications: in the form of more or less thin interlayers, from barely noticeable streaks to 1 cm thick, and in the form of separate pieces of cubic or parallelepipedal shape, as well as strips with frayed ends. The first modification in a transverse section has the appearance of more or less broad bands or streaks. This is clearly autochthonous fusain. The cell walls of the fusain are completely opaque, black, and rather thin; between them are cell bands 0.04 mm in size. The shape of the cell openings varies depending on the direction of the cut. It may range from square, with slightly rounded corners in cross section, to strongly elongated and with figures of bordered araucaroid pits in tangential sections.

Sometimes a gradual transition of fusain into xylain is observed within a single fragment. The following regularity should be noted with respect to the preservation of the cellular structure of fusain fragments. If fusain

GEOLOGICAL SECTION ALONG THE WORKING WALL OF THE IRSHA-BORODINO OPEN PIT No. 1 AS OF AUGUST 1960

Horizontal scale 1:2000
Vertical scale 1:200

West

East

Legend

1 — matte ash-bearing coal;
2 — clarain type of coal;
3 — kasyanite type of coal;
4 — durain type of coal, including clarain-durain;
5 — fusain-xylain type of coal;
6 — siltstones;
7 — Quaternary loams;
8 — loose sandstones;
9 — coal soot;
10 — carbonaceous siltstones;
11 — sapropelite-humite type of coal.

in the seam occurs in the form of thin interbeds or inclusions, its cellular structure is well preserved. In thick interbeds of fusain, however, a stellate structure is more often observed.

In some cases the amount of fusain in the seam is so considerable that the coal becomes dull fusain coal. The thickness of such beds, which usually wedge out rapidly, reaches 0.5 m and even more.

The second modification of fusain inclusions is, in all probability, allochthonous, associated with the erosion of bogs during rises in water level. It is noteworthy that coal beds with allochthonous fusain regularly alternate with beds without it. In all, five groups of beds with allochthonous fusain are recorded, as shown in Fig. 2.

Thus, during the formation of the Borodino seam, the conditions of coal accumulation changed no fewer than five times.

Vitren. The Borodino seam is to a considerable degree saturated with vitren. Suffice it to say that coals homogeneous in structure account for only 12–17% of the total mass of coal. The rest consists of banded and striated coals, the structure of which is determined mainly by the presence of vitren.

Vitren stands out sharply in the main mass by its darker color, as well as by a stronger luster approaching resinous. In fracture, vitren appears as long (up to 3 m), thin bands (2 cm thick). These are the largest dimensions that we were able to observe. More commonly it occurs in the form of long, thin lenses corresponding to tangential sections of strongly compacted remains of wood, small twigs, and even whole tree trunks. On bedding planes, vitren also appears as bands, sometimes branching and oriented in all possible directions. Endogenous fracturing, not passing into the main mass of the coal, is highly characteristic of vitren lenses.

Fig. 2.

Under the microscope, three varieties are distinguished among vitren: vitren “A,” vitren “B,” and vitren “V.” Vitren “A” is characterized by the absence of visible structure in transmitted light under one nicol and by a weakly expressed pseudostructure, under crossed nicols, of a transverse-banded or slightly wavy type. It is usually represented by large or small bands and lenses with sharp boundaries against the main mass and, sometimes, with serrated ends along the edges of the lenses. Large lenses are cut by vertical and oblique cracks; smaller ones—only by vertical cracks. Vitren “A” grades into structural vitren “B”; in addition, a transition of xylovitren into vitren “A” is observed. Of all the varieties of vitren, vitren “A” occurs in the Borodino seam quite frequently, being quantitatively second only to vitren “V.”

Vitren “B,” which in form does not differ from vitren “A,” necessarily possesses a more or less pronounced cellular structure, revealed under the microscope by differences in the color shades of the cell walls and their cavities or by traces of former cells. The structural vitrains of the Borodino seam are represented by four varieties of cellular structure: lenses of structural vitren from plant fragments with conducting tissues of plants, cortical tissue of plants, plant tissues filled with resin, and structural vitren,

from whose cells only fine strokes or dots remain, most often arranged at some angle to the elongation of the lenses. Vitrain “B” under crossed nicols most often acquires a well-pronounced reticulate pseudostructure that does not coincide with the cell structure; as a result, the cellular structure is, as it were, obscured and ceases to be visible. Structured vitrain is the most rarely encountered variety of vitrain.

The most frequently encountered variety of vitrain is vitrain “B.” These are thin bands and small lenses with indistinct outlines at the boundary with the main mass, characterized by vertical fracturing (the seam is horizontal). In transmitted light, with one nicol, vitrain “B” is usually structureless; it also does not show pseudostructures under crossed nicols. Its color is somewhat darker than that of vitrain “A.”

The remaining constituents of the coals of the Borodino seam are macroscopically indistinguishable; therefore only their microscopic characterization is given.

Inclusions of xylene in the coals are extremely varied: from fragments of irregular shape to broad, relatively long bands of tangential sections of wood; here, as in fusain, bordered pores are sometimes visible. By color, two varieties of xylene are distinguished: xylene proper, with reddish-brown coloration, and xylinofusain, with dark-brown, often black coloration, which occurs comparatively rarely.

Under crossed nicols, xylene does not produce pseudostructures in most dark-colored varieties, but in the lighter varieties it has a reticulate pseudostructure. Quite often mutual transitions are observed from xylene to xylinofusain, or from xylene to xylovitrain, within a single fragment. Xylene is fairly widely distributed in the Borodino seam.

Most widely developed in the Borodino seam, after vitrain, are two varieties of xylovitrain, usually having the same color as the main mass of the coal, though sometimes the color of the xylovitrain is darker: lumpy xylovitrain with semi-filled cell cavities, bordered by a thin cuticle, and xylovitrain with completely filled cell cavities and a very well-pronounced cellular structure. The first variety of xylovitrain has a lens-like habit with rounded or pointed ends and a characteristic slight sinuosity of the boundaries with the main mass. The length of the lenses ranges from 0.5 to 2 mm, and their thickness from 0.04 to 0.2 mm. Vertical fracturing is not characteristic of xylovitrain; under crossed nicols it has a stellate pseudostructure.

The second variety of xylovitrain has a well-pronounced cellular structure, revealed by differences in the color shades of the cell walls and their cavities. For example, orange cavities and red cell walls, or yellow cell walls and red cavities. The predominant cell form is the form characteristic of cork tissues of arboreal vegetation bark. Their shape is brick-like, with dimensions of 0.008–0.032 mm. Sometimes these cells are arranged concentrically, which corresponds to a transverse section of thin twigs. Fragments of bark tissue have the most varied form; they are clearly or indistinctly separated from the main mass and are widely distributed in the Borodino seam. Sometimes there are so many fragments of bark tissue in the coal that these types of coal can be distinguished as independent petrographic subtypes of bark attrital coals. The bark tissues are impregnated to one degree or another with suberin, detected by the yellow coloration of the walls in transmitted light with one nicol.

In the coals of the Borodino seam there occur xylovitrinite fragments with a cell form that at first glance differs from the structure and shape of cork-tissue cells. The form of the cells in such fragments is usually subprismatic, and their arrangement is somewhat irregular. The cell sizes range from 0.03 mm to 0.06 mm, and the spaces between the cells are filled with a yellow suberin substance; moreover, at times the suberin substance even predominates. In such cases the cells have the appearance of sparse reddish-brown bubbles. On careful examination at high magnifications it proved that these “cells” are in fact groups of cells of ordinary cork tissue, as shown in Fig. 3.

Cutinized components of plants. In transmitted light, with one Nicol prism, they are distinguished by a bright yellow coloration, only rarely changing to orange-yellow. With crossed Nicols they are characterized by a darker, grayish coloration, sometimes by wavy extinction. In polished sections of Borodino coals the following are mainly widespread: microspores, cuticles, resin bodies, and algae. Macropore coats occur rarely.

Fig. 3. Sketch of a portion of a polished section, magnified 300 times.

Macrospores are most often observed as fragments ranging in size from 0.15 to 1.5 mm. Their coloration is orange, more rarely bright yellow. Microspores are encountered more frequently. Their sizes range from 0.015 to 0.075 mm. The form of the microspores varies, changing from short thin strokes to small rings. Their color is bright yellow; they occur in the groundmass usually in a dispersed state. Sometimes accumulations of microspores also occur, representing the contents of a sporangium.

In the coals of the Borodino seam the presence of two varieties of cuticle is noted: thin and thick. The thin cuticle most often borders bands and lenses of lumpy xylovitrinite, often forming the sharp-angled bends typical of it, but it also occurs separately, in the groundmass. The second variety of cuticle usually occurs separately in the groundmass and is less widespread. In general, cuticle occurs more rarely than microspores.

The resin bodies of the Borodino seam more often have an ellipsoidal form, with sizes from 0.12–0.5 mm to 0.02–0.04 mm, but rounded or angular forms also occur, as well as linearly elongated and irregular forms. The color of the resin bodies with one Nicol prism in transmitted light is golden-yellow to amber; cracking of the bodies is characteristic.

Algae occur rather frequently in the Borodino seam. Usually these are rounded, ellipsoidal, or irregularly shaped bodies with uneven contours, pale yellow in color with a greenish tint under one Nicol prism. With crossed Nicols the algae acquire a light-gray color, and the mottling of their coloration does not disappear even with crossed Nicols. The sizes of the algae occurring in the Borodino-

in the seam, range from 0.03 to 0.5 mm in cross section; the average size is 0.25 mm.

In addition to the cutinized components listed above, the coals of the Borodino seam contain bodies of unclear nature, having a rounded form and a vitrain-like character. Such bodies occur rarely and usually in the groundmass.

Groundmass. The groundmass in the coals of the Borodino seam is subdivided into the following varieties: transparent xylovitrain groundmass, transparent homogeneous groundmass, and dark, almost non-translucent groundmass.

The xylovitrain groundmass in transmitted light with one nicol is characterized by a heterogeneous lumpy structure, sometimes with relicts of cellular structure, the various lumps differing from one another in shades of color. A finely lumpy xylovitrain mass and a coarsely lumpy one are distinguished; the color of the former is dirty brown, that of the latter reddish brown. Under crossed nicols the xylovitrain groundmass also appears heterogeneous and usually gives a stellate pseudostructure. The lumps composing the xylovitrain groundmass are remnants of strongly decomposed gelified tissues, and this process has gone so far that formed elements can no longer be recognized in the lumps. At the same time, the presence of a xylovitrain groundmass makes it possible to conclude that the regime was unstable at the time of formation of the peat bog, as a result of which the process of gelification was interrupted.

Transparent homogeneous groundmass occurs in the coals much less frequently. In transmitted light its chief feature is homogeneity of color. Its color is usually bright, orange-brown; in this case the degree of density of the coloration most likely depends on the thickness of the thin section. Under crossed nicols it is characterized by the presence of a weakly expressed transverse-banded pseudostructure.

Dark, almost non-translucent groundmass occurs very rarely and differs from fusainized tissues by the presence in it of formed elements and mineral inclusions.

Among the mineral inclusions in the coals of the Borodino seam there are both allochthonous and autochthonous inclusions. The former include quartz grains of silt size, as well as clay minerals—kaolinite and hydromicas. Quartz grains and clay minerals are distributed in the Borodino seam in layers, forming lenses of ash coal up to 0.3 m thick, which indicates the short duration of the stages of influx of terrigenous particles. The authigenic inclusions include carbonate-siderite concretions of rounded form, ranging in size from 0.5 mm to pea size, having a radial-rayed structure, as well as pyrite cubes.

The results of the petrographic study of the Borodino seam show that this seam, with a thickness of 25 to 60 m, from the genetic point of view represents a complexly constructed geological body, separate parts of which differ in genesis of formation. The various petrogenetic types of coal composing the Borodino seam—and by a petrogenetic type of coal we understand such a type of coal as is characterized by a number of features reflecting specific conditions of accumulation, i.e., facies features [1]—are given in Table 1. The indexing of the coal types listed in the table was made from the initial letters of the name (for example, KT—claren type of coal). The subtype number is given accordingly: 1—for an unstable regime, 2—for a stable regime of coal accumulation.

Table 1

Below is a bed-by-bed description of the Borodino seam on the basis of macro- and microscopic studies, from bottom to top.

The first bed is composed of two types of coal; these are mainly coals of type DT, while coal of type KT-1 sometimes appears in the upper part of the bed. The thickness of the bed in the trench section reaches 2 m, but, according to drilling data, as one moves from the section to the southeast the thickness of the bed increases.

The second bed is composed of coaly aleurolite, dark gray, almost black in color, containing lenses of ashy coal up to 2 cm thick and fusain plates 1.1 cm in size. The aleurolite has fine horizontal bedding, emphasized by interlayers of coaly material. According to drilling data, moving southeastward from the trench, the bed of coaly aleurolite, having a thickness of up to 30 cm, thickens, and sandstones also appear in its composition.

The third bed is represented by coal of type KT-1 and has a thickness of 8 m. Within the bed, coals of types KTS and PKTS are recorded in the form of lenses. The thickness of the lenses does not exceed 1 m.

The fourth bed has a lenticular structure and is composed of semilustrous striated-banded coals of the PKTS, KTS, and DT types. The thickness of the bed ranges from 0.5 to 2 m; characteristic transitions are observed from coals of the PKTS type into coals of the KTS type and, farther eastward, into coals of the DT type. The bed is characterized by the presence of allochthonous fusain.

The fifth bed is composed of semilustrous banded coals of type KT-1 and has a thickness of 2 to 3.5 m. On the western side of the trench, the thickness of this bed increases to 7 m as a result of the wedging out of the following, sixth bed and the merging of the fifth bed with the seventh.

The sixth bed is represented by semidull and semilustrous striated-banded coals of types PKTS and FST; it has a lenticular mode of occurrence. It is distributed only in the eastern part of the trench, where its maximum thickness reaches 3 m.

The seventh bed is represented mainly by semidull striated, rarely banded coals of type KT-2, more rarely KT-1, and contains small lenses of coals of types KTS, PKTS, and DT. The thickness of the bed ranges from 9 to 10 m, while the thickness of lenses of coals of other types within it does not exceed 0.7 m.

The eighth bed is composed of semidull striated coals of type PKTS and has a thickness of 2 to 1 m. The coals of this bed can be traced throughout the entire trench.

The ninth bed consists chiefly of semidull, striated-banded, rarely homogeneous dull coals of type KT-1, often with calcareous “kidneys”—small calcareous-sideritic concretions. The thickness of the bed ranges from 2 to 5 m.

The tenth bed is clearly distinguished in the section of the Borodino seam by the dark-gray color and great hardness of the coal, as a result of which it usually protrudes somewhat in the wall of the bench. The thickness of the bed is small and ranges from 0.2 to 0.5 m. It is represented by high-ash coals, dull and homogeneous in structure, belonging to type KDT, sometimes KTS, with a homogeneous groundmass.

The eleventh bed is represented by semidull striated-banded coals of type KT-1, sometimes KT-2 (microspore clarens). This bed contains lenses of coal of type FST up to 1 m thick, but of limited extent. The thickness of the bed ranges from 2 to 5 m.

The twelfth bed is analogous to the tenth and, like the tenth, is distributed only in the eastern half of the trench.

The thirteenth bed is usually composed of semidull homogeneous coals of type KT-2 (microspore or cuticular clarens),

rarely by coals of type KT-1 (in the western part of the trench). The thickness of the bench is usually 2 m, but in the western half of the trench its thickness increases to 6 m, and lenses of coal of the PKTS type and even KTS appear in the bench.

The coal seam studied belongs to the category of seams of very great thickness and is the principal seam of the Irsha-Borodino deposit of the Kansk coal basin, on whose coals it is planned to build a large state district power station [2]. The results of the study of the petrographic structure of the seam confirm the good quality of the Borodino coals and have made it possible to clarify certain questions of their genesis, which, together with an analysis of the spatial distribution of the coal-accumulation facies and certain other features, lead to conclusions about the formation of the seam over a much larger area of the Rybinsk coal-bearing district than is known at present.

REFERENCES

1. Yu. A. Zhemchuzhnikov and A. I. Ginzburg. Fundamentals of Coal Petrography. Publishing House of the Academy of Sciences of the USSR, 1960.

2. N. F. Ryabokon, K. L. Kokhanchik et al. Coals of Krasnoyarsk Krai. Pol. isk. Kr. kr., Publishing House of the Academy of Sciences of the USSR, 1962.