METAMORPHISM OF ORGANIC MATTER IN THE COALS OF TRANSBAIKALIA

O. A. Dulzon

(Presented by Professor A. V. Aksarin)

The problem of the metamorphism of the organic matter of coals and enclosing rocks has attracted the attention of both domestic and foreign researchers. The metamorphism of coals according to the reflectance of vitrinite was determined by Yu. A. Zhemchuzhnikov, L. I. Sarbeeva, I. I. Ammosov, Z. V. Ergolskaya, A. B. Travin, N. I. Babinkova, Tan Syu-i, staff members of the West Siberian Geological Administration, and others [7, 3, 2, 4, 12, 13, 14, 15].

Determination of the degree of coal metamorphism is being introduced ever more widely by coal petrographers in various countries. Determination of the stages of metamorphism is based on the reflectance of a number of coal microcomponents.

The author of the present work studied coals of the thick seams of the Tarbagatai, Chernovskoe, and Kharanorskoe brown-coal deposits of Transbaikalia. Until now, due attention has not been paid to the study of the degree of metamorphism of these coals. In studying coal metamorphism, polished glass was taken as a standard and compared with the vitrin of the polished section under a microscope under identical illumination conditions. The degree of reflectance of the coals of Transbaikalia was measured on a POOS-1 instrument. For determining the stages of metamorphism of the coals of Transbaikalia, the most reliable method proved to be the method of determining the reflectance of vitrin.

When studying coals, it is necessary to pay attention to the following factors that distort the true degree of metamorphism. First, it is necessary to examine and take readings at several points on the standard polished section. We took readings at ten points and then calculated average values. Second, fracturing within the vitrins noticeably affects the readings of the degree of metamorphism. In determining the reflectance of coals, we selected smooth areas of vitrin surfaces without cracks. Third, in order not to make an error and, instead of vitrin, measure the reflectance of semivitrinite or cryptostructural collinite, the polished section was examined at maximum magnifications and then measured. Fourth, in order to prevent error due to oxidation of vitrin, before determining reflectance the polished sections were run over polishing wheels. Fifth, the method of determining the stages of coal metamorphism by liptinite elements can be used only as a concomitant and auxiliary one, but not as the principal one. The performance of experimental work on the study of polished sections made it possible to reveal regularities in the changes in the properties of coals from Transbaikalian deposits.

The brown-coal deposits of Transbaikalia—the Tarbagatai, Chernovskoe, and Kharanorskoe deposits—are associated with Upper Jurassic–Lower Cretaceous coal-bearing strata alternating with coal-free sediments. In the stratigraphic section, a cyclic structure is observed in the coal seams and coaly rocks. Such cyclicity of coal manifestations testifies to repeated changes in the geotectonic and physicogeographic conditions under which plant remains accumulated and were subsequently transformed into coal. These deposits are located in the Khilok, Chitino-Ingodinsk, and Kharanor-Turgin depressions. The coal-bearing sediments of these depressions are several hundred meters thick and are interbedded with coal-free sediments. The depressions of Transbaikalia contain coal-bearing deposits with a large number of coal seams and interbeds, with persistence of the coal-bearing horizons and enclosing rocks. Among the coal seams of the coal-accumulation basins, thick coal seams with simple and complex structure are observed. The thick coal seams studied in the Tarbagatai, Chernovskoe, and Kharanorskoe deposits have a simple structure.

In previous works the author carried out a detailed petrographic study of the coals in order to investigate the material composition, facies variability of the sedimentation basin, and genesis of the coal. In the petrographic study of the coals of the above-named deposits it was established that the predominant coals are clarain, dureno-clarain; partly clarain-durain, durain, and fuseno-xylain coals with interbeds of fusain coal. These coals belong to the humic type with isolated algae. In the peat bog, at the initial stage of coalification of wood, the structure of the tissue becomes less pronounced. The coals studied by us have a xylovitrainous groundmass and fragments of herbaceous and woody vegetation with xylain and xylovitrain structure. In the Tarbagatai and Chernovskoe deposits, fragmentary inclusions are observed, isolated ones with xylain structure, more often with xylovitrain structure, passing into collinitic. The Kharanorskoe deposit has a lower stage of coal metamorphism and contains microcomponents with a distinct xylain and xylovitrain structure. The woody remains have a clear structure and a good degree of preservation. A number of researchers have noted that in the peat bog, at the initial stage of coalification of wood, the structure of the tissue becomes less pronounced. If at the peat stage wood has a xylain structure, then at the brown-coal stage it has a xylovitrain structure, and already at the hard-coal stage it becomes a cryptostructural mass. Such changes in the structure of wood can be explained by the degree of waterlogging of the coal-accumulation basin and the degree of metamorphism. A number of researchers regard analogous phenomena of changes in the structuredness of wood as “vitrenization”—M. Teichmüller—or “collinization”—Van Krevelen [2, 3, 4].

At the first stage of transformation of coal-forming plants, the main role is played by biochemical processes; during oxidation–reduction reactions, the peat mass changes depending on mineral impurities, peat waters, their enrichment with oxygen, etc.

The second stage of alteration of the organic matter of coals corresponds to the brown-coal stage of their geochemical transformations; physicochemical processes proceed at relatively low temperatures and pressures.

At the third stage of transformation of the organic matter of coals, corresponding to the transition to hard coals, the role of physicochemical processes proceeding at higher temperatures and pressures increases. Up to the present time, in the USSR and abroad, quantitative

Table 1

Petrographic composition and stage of metamorphism by lithotype samples from seams of the Kharanor and Turgin-Bukachacha deposits

an assessment of the degree of gelification was not made. The author of the present article counted microcomponents with xylene, xylovitrain, stellate, and hidden structure (Table 1). This determination of plant remains makes it possible to say that the metamorphism of the vitrinite of the coals of the seams changes from top to bottom along the stratigraphic section.

It should be noted that in polished sections the microcomponents of coal have different degrees of metamorphism, which for the Tarbagatai, Chernovskoe, and Kharanorskoe brown-coal deposits lies within the range of 5.07–7.90%. For the Tarbagatai deposit, an insignificant increase in the degree of metamorphism with depth is characteristic, and a substantial increase is characteristic of clarain coals enriched with mineral ore and non-ore impurities.

The process of transformation of peat bogs and the subsequent coalification of organic material took place in intermontane depressions with rapid compaction and burial of coal-bearing seams. Coalification includes the broader process of coal accumulation and transformation of the initial substance. Metamorphism, however, is a narrower process, proceeding with changes in the quality and properties of coals during the transition from brown coals to hard coals and to graphite. The metamorphism of the organic matter of the coals of the thick seams of the above-mentioned deposits increases with depth, which is a characteristic feature of regional metamorphism, i.e., metamorphism that occurred under the influence of the temperature of the Earth’s interior, gradually increasing with the subsidence of sediments. According to reflectance, the coals of the indicated deposits should be assigned to the group of brown coals of ranks B₂–B₃ D: Kharanor coals—B₂, Chernovskie—B₃, Tarbagatai—B₃ D.

LITERATURE

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