PETROGENETIC TYPES OF COALS AND THE STRUCTURE OF A THICK SEAM OF THE KHARANOR LIGNITE DEPOSIT IN EASTERN TRANSBAIKALIA

O. A. Dulzon

(Presented by Professor A. V. Aksarin)

The Kharanor lignite deposit is located in the Kharanor-Turga depression in the southern part of Eastern Transbaikalia. The thick seam under study (Novyi-1a) occurs at a shallow depth, has a thickness of 14–25.50 m, and is currently being mined by the Kharanor open-pit coal mine. The age of the coal-bearing deposits of the Kharanor deposit was determined by V. D. Prenada as Upper Jurassic–Lower Cretaceous [2]. Later, N. F. Karpov and L. P. Nefed’eva regarded these deposits as Lower Cretaceous [4]. The microcomponent composition of the coals of the Novyi-1a seam of the Kharanor deposit is analogous to the Tarbagatai coals of Central Transbaikalia [1, 3].

This paper gives a description of the petrogenetic types of coal, the conditions of formation of the coal seam, and its structure.

The coal deposits of Transbaikalia were subjected to detailed exploration only in recent years and could not be covered by coal-petrographic studies. The structure of the Novyi-1a seam involves mainly microcomponents of three groups: vitrinite, fusinite, and liptinite. In terms of petrographic composition, the Novyi-1a seam is fairly uniform and is composed predominantly of the clarain type of coal and partly of dureno-clarain, clarain-durain, and durain [3].

The clarain type of coal forms under conditions of a stagnant forest swamp with a high groundwater level and is the predominant coal type. The content of clarain coal in the seam amounts to 72.3% of the seam thickness. The Novyi-1a seam is composed predominantly of the clarain type of coal.

The dureno-clarain type of coal is representative of the facies of a forest peat swamp with weak groundwater flow. This type of coal makes up 18.1% of the seam thickness.

The clarain-durain type of coal is representative of the facies of a forest peat swamp with a lowered groundwater level. The clarain-durain type of coal makes up 9.0% of the seam thickness.

The durain type of coal is representative of swampy forests with a low groundwater level. This type of coal composes an insignificant part and amounts to 0.6% of the seam thickness.

The Novyi-1a seam of the Kharanor deposit can be divided into seven petrogenetic coal bands (from bottom to top). Each band begins with coals of the clarain type and ends with fusinite-semifusinite durain or carbonaceous siltstone.

First seam bench. At the base of this bench lie light-gray aleurolite and dark-gray argillite. Above them lies a layer of clarain coal. The main mass has a xylovitrinite structure, in which cellular fragments of gelified and fusinized plant tissue are observed. These include lenses of dark-brown semitellinite with half-closed cellular openings bordered by cutinite; lenses of homogeneous dark-brown tellinite with a vitrinite structure; lenses of suberinite of bark tissue, orange-brown in color; and isolated small bands of black fusinite with a destroyed cellular structure. Among the liptinite elements there are cutinite, isolated accumulations of resinite and microsporinite, uniformly dispersed in the main mass. Depending on the content of gelified and liptinite microcomponents, we distinguish the following petrographic varieties: 1) vitrinite clarain, 2) semitellinite clarain, 3) vitrinite–microsporinite clarain.

Second seam bench. At the base lies a layer of semifusinite–semitellinite dureno-clarain. The main mass of this type of coal has a xylovitrinite structure. Among the gelified cellular microcomponents, irregular fragments of brownish-brown semitellinite with araucaroid-bordered pores are observed. Among the fusinized components there are irregular fragments, bands, and small plates of fine fusinite with a clear cellular, stellate, and completely destroyed structure; broad bands of black-brown semifusinite with a xylovitrinite structure. Among the liptinite elements there are isolated small fragments of cutinite with indistinct denticles and microsporinite. Above lies a layer of clarain coal, consisting of several varieties: 1) vitrinite clarain, 2) semitellinite clarain, 3) semitellinite–semifusinite clarain, 4) semifusinite–microsporinite clarain.

Third seam bench. At the base lies semiphusinite–semitellinite dureno-clarain, passing into clareno-durain. The main mass of the dureno-clarain coal has a xylovitrinite structure, in which fragments of gelified and fusinized plant tissue and liptinite elements are observed. The gelified fragments are represented by bands of brownish-brown semitellinite with transverse and tangential sections, fragments with a xylene structure passing into xylovitrinite. Among the fusinized components there are irregular fragments, plates of black fusinite with a cellular structure passing into stellate, and with a clear cellular structure and annual growth rings. Oval bodies of sclerotia have a reticulate structure, are black in color, and occur as isolated inclusions; they are rarely found in accumulations. Among the liptinite elements there are yellow cutinite with a clear and indistinct dentate structure, isolated fine resinite of various forms and light-orange color, and yellow microsporinite uniformly dispersed in the main mass. The bench is completed by a layer of clarain coal consisting of several petrographic varieties: 1) resinite–cutinite clarain, 2) vitrinite–cutinite clarain, 3) vitrinite–semitellinite clarain, 4) vitrinite clarain, 5) semitellinite clarain, 6) semifusinite clarain.

Fourth seam bench. At the base of the bench lies a dureno-clarain type of coal, having two varieties: fusinite and vitrinite–semifusinite dureno-clarain. The main mass of these types of coal has a dark-brown xylovitrinite structure with black micrinite inclusions. In isolated cases, a transitional microcomponent is observed between the group of gelified and fusinized—

...fusinized elements: dark-brown semifusinite with a xylene, xylovitrain, and stellate structure. Among the fusinized microcomponents there are irregularly shaped fragments and bands of black fusinite with a xylene, xylovitrain, and stellate structure. The pack is completed by a layer of clarain coal composed of several petrographic varieties: 1) vitrinitic clarain, 2) semitellinite-cutinitic clarain, 3) vitrinitic-fusinitic clarain, 4) semifusinitic clarain.

Fifth pack. It is composed, at the base, of semitellinite-semifusinitic dureno-clarain and has a persistent thickness throughout the entire wall of the coal section. In the northwestern and southeastern parts of the section there are lenses of this type of coal. Semitellinite-semifusinitic dureno-clarain has 65.63% groundmass, 29.33% fusinized microcomponents, and 5.04% gelified elements of the seam thickness. As in the preceding pack, clarain coal constitutes the predominant part of this pack. Depending on the composition of the microcomponents, the following petrographic varieties are distinguished among the clarain coals: 1) algotellinite-cutinitic clarain, 2) microsporinitic clarain, 3) semifusinitic clarain, 4) cutinitic clarain, 5) vitrinitic-cutinitic clarain.

Sixth pack. This pack is composed for the most part of clarain-type coal. It is persistent in thickness in the central part of the seam; in the northwestern and southeastern parts of the coal section there occur two small lenses of semifusinitic-cutinitic dureno-clarain, 0.5 to 2.5 m thick. This type of coal has 74.01% groundmass, 3.92% gelified component of the vitrinite group, 18.34% fusinized components, and, among the leiptinite components, 1.78% cutinite and 1.95% microsporinite.

Seventh pack. It is composed, at the base, of fusinite-semitellinitic durain. The coal contains 25.19% groundmass, 6.38% gelified microcomponents, and 68.43% fusinized microcomponents. In the northwestern part of the coal seam a lens of this type of coal is traceable, up to 1 m thick and 8 m long. Clarain coal makes up the greater part of the pack and is represented by several varieties: 1) semitellinite-cutinitic clarain, 2) semifusinitic-microsporinitic clarain, 3) semitellinite-semifusinitic clarain.

In studying the petrographic types of coal and the structure of the thick seam of the Kharanor deposit, the following should be noted.

A common characteristic feature of the coals is the horizontal bedding of gelified, fusinized, and leiptinite plant remains in the central part, and oblique bedding in the marginal parts of the coal basin.

The principal petrographic type of coal composing the thick seam of the Kharanor deposit, and the most persistent in thickness and area, is clarain and, in part, the dureno-clarain type of coal.

A characteristic feature of the groundmass in the clarain and dureno-clarain types of coal of this deposit is its xylovitrain structure.

An essential feature is the autochthonous character of the accumulation of the initial plant material. The type of bogs widespread in the territory of the Kharanor deposit was, for the most part, lowland [2].

References

1. A. V. Aksarin, A. V. Vnukov, O. A. Dulzon. Material composition and petrographic types of coals of the Tarbagatai lignite deposit of Central Transbaikalia. Izv. TPI, vol. 218, 1970.

2. Geology of the USSR, vol. XXVI, Moscow, 1961.

3. O. A. Dulzon. Material composition of the brown coals of the Kharanor deposit. Izv. TPI, vol. 237, 1971.

4. N. F. Karpov, L. P. Nefed’eva. Coal-bearing capacity. In: “History of Upper Cretaceous coal accumulation on the territory of the Buryat ASSR and the southeastern part of the Lena Basin.” Tr. Lab. Geol. Uglya, issue XVIII, Publishing House of the Academy of Sciences of the USSR, 1963.