Synsedimentary volcanic-ash-derived illite tonsteins in Late Permian coal-bearing formations of southwestern China

International Journal of Coal Geology, 15 (1990) 341-356 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

341

Synsedimentary volcanic-ash-derived illite tonsteins in Late Permian coal-bearing formations of southwestern China Kurt Burger~, Yiping Zhou b and Dazhong Tangb aHalbe H6he 10, F-4300 Essen 1, F.R. Germany bInstitute of Coal Science, Minhang Road 21, Kunming, P.R. of China (Received November 24, 1989; revised and accepted September 19, 1989)

ABSTRACT Burger, K., Zhou, Y. and Tang, D., 1990. Synsedimentaryvolcanic ash-derived illite tonsteins in Late Permian coal-bearing formations of southwestern China. Int. Z Coal Geol., 15:341-356. Ulite claystones (tonsteins) of Late permian coal-bearing formations are well developed and extensively distributed in southwestern China. Over the past decades it has been recognized that they are synsedimentary volcanic ash-falls in origin (altered tuff beds), based on the data derived from the comprehensive investigationson their stratigraphic and geographic distribution, petrographic types, chemical composition, accessory-mineral assemblage, as well as on their morphological characteristics. Recently, mineralogical and petrological identifications and X-ray diffraction analyses on these peculiar tonsteins further suggest that they can be classified mainly into three categories based on their clay-mineralconstitution: kaolinitic, kaolinitic-illitic (transitional type), and illitic. As coal partings in most cases, these different types of tonsteins show a distinct zonal distribution on a regional scale. Their mineral constitution is profoundly modified by the combined effects of many factors during deuterogenic diagenesis, as indicated by the variation in volatile-componentpercentage (VM) of the adjacent coal seams (VM is calculated on a combustible-componentbasis). Those tonsteins intercalated in coal with a VM> 10% are dominated by kaolinite. But they contain increasing amount of illite at the expense of kaolinite when VM of coal decreases from 10% to 8%. When VM declines to less than 8% the clay minerals in these tonsteins are almost entirely illite with some sort of accompanying chloritization. It is clear that, from a regional point of view, the variation in clay-mineral constitution of these tonsteins is in response to the continual and progressive change in geological processes. In addition to the temperature and pressure conditions, the existence of a certain amount of alkali-metal ions and ferrous ions in solution is one of the controlling factors.

INTRODUCTION

It is well known that establishing rightly the stratigraphical frame and the inherent stratigraphical series is a fundamental geological work for recognizing the change of the sedimentary basins and the distributional law of the sedimentary deposits. Determining the relationship of the key horizons which 0166-5162/90/$03.50

© 1990 Elsevier Science Publishers B.V.

342

K BURGER El AI

have clear macrographical characters, which can be easily distinguished from each other, and have a steady lateral distribution, is of special importance in coal geology and exploration. In 1961, more than 10 coal tonsteins were found for the first time in the coal seams of the Xuanwei Formation of Late Permian age in east Yunnan in China. During the past 20 years, researches of this important coal tonstein were made in many fields and it had been put to wide use helping to resolve many geological problems (Zhou, 1975; Zhou et al., 1982 ). In 1981, some high-temperature minerals with a simple composition (mainly fl-quartz, zircon, monazite etc. ) were separated successfully from this intercalated bed and were identified- the morphological characteristics of these minerals were determined by SEM. After comparing with foreign beds of the same kind, it was determined that most of these minerals are altered products of syndepositional acid volcanic ash (Zhou et al., 1982; Zhou and Ren, 1983 ). The clay parting with its peculiar origin and its steadiness and correlatableness on a regional scale further reveals its great value in theory and practice. These results received attenuation in China and in other countries (Burger, 1985 ) and promoted finding of tonsteins in other regions and expanding of their uses (Yang et al., 1986; Feng, 1987 ). In recent years, the authors of this paper, on the basis of the above-mentioned work, have further studied the relationships among the change of clay minerals, chemical compositions, trace elements, accessory minerals in tonsteins and sedimentary conditions, and deuterogenic diagenesis in coal fields of Late Permian age of southwestern China, see Fig. 1 (Zhou et al., 1987, 1988 ). It is found that illite coal tonstein appearing in vast stretches of the sedimentary basin, has a distinct diagenesis binding with deuterogenic diagenesis, and to certain extent shows the trend and the law of deuterogenic alteration of volcanic-ash-altered products. This paper is a preliminary summary of the study.

Fig. 1. Map showingthe study area of report.

ILLITE TONSTEINS IN

LATEPERMIANCOAL-BEARINGFORMATIONSOF SWCHINA

343

COAL RANK AND SAMPLE DISTRIBUTION*

Variation of coal rank is controlled by regional deuterogenic geological processes. In general, the coal ranks in this area show a zonal distribution: the region south of N 27 ° to near-erosional region (approximately west of Shuicheng-Liuzhi-Panxian) is a lower-rank region where the volatile matter (VM) of fine coal is generally 20% or more. Northeast and southeast of these regions, there is a gradual increase in coalification, up to the anthracite stage, with localized regions of VM < 5% of meta-anthracite. In the region near E 107 °, there is a decrease in coalification (Fig. 2 ). In the above-mentioned regions, more than 200 samples were collected from borings and coal mines. All samples were made into thin sections and identified under the microscope, and a part of them were analyzed by X-ray diffraction, and by quantitative determination of chemical composition and trace If

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elements. Separation and examination of accessory minerals were made for a few samples. The main sampling locations are presented on Figure 2. MINERALOGICAL AND PETROGRAPHICAL INVESTIGATIONS OF ILLITE COAL TONSTEIN

X-ray diffraction analyses In order to investigate the variable trend of main clay minerals in tonsteins with changes in coal rank, we have made X-ray diffraction analyses for more than 30 samples in several areas. More typical X-ray diffraction patterns of 7 samples are sorted and presented on Figure 3. In the X-ray diffraction patterns of samples I l, 12 in the bituminous coal region, there are 5 sharp and symmetrical peaks between d ( 0 0 1 ) = 7.15 A and d (002)=3.57 A; there are two groups each including three peaks in d=2.56-2.29 A (Fig. 3). The X-ray pattern is very distinctive. Characteristics of X-ray diffraction pattern show that in samples 1 l, 12, kaolinite with a high degree of order and excellent crystals accounts for most of them. When the coal rank reaches the stage of anthracite (samples 8, 6, 7 ), characteristic peaks of illite begin to appear and their intensity increases gradually. At the same time, there are reflect peaks of mixed layer of montmorillonite-illite. In the region of anthracite and meta-anthracite (samples 9, 10 ), diffractional peaks of kaolinite disappear completely while characteristic peaks of illite are distinctive and symmetrical. An asymmetrical (inclining to lower angle) diffraction peak near 11/~ appears as well, which indicates that it contains more irregular minerals of mixed layer of montmorillonite-illite. The X-ray diffraction pattern shows that there is a small amount of chlorite in samples 9, 10. In order to make a comparison, we have utilized Stadler's (1971) X-ray diffraction pattern of tonstein in the meta-anthracite region (pattern 8 in the bottom of Fig. 3 ) to contrast with it. We infer that there may be a small amount of paragonite formed from metamorphism in samples 9, 10.

Examination of thin sections under microscope The textural type of illite coal tonstein is almost the same as that of kaolinite coal tonstein, only the gel-grain (graupen) type is not seen. Illustrations are explained in the following sections.

Crystalline type (plate 1, 1-3) Sample 6, taken from the coal parting of B4 coal seam of mining area L in Guizhou province, is about 50 m m thick, dark grey, with a fine-grained texture. Its mineral components including illite, kaolinite and a small amount of

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ILLITE TONSTEINS IN LATE PERMIAN COAL-BEARING FORMATIONS OF SW CHINA

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quartz ca. 5% (based on the X-ray diffraction analyses). The sample consists of densely arranged illite crystals and partially illitized kaolinite crystals. Illite with a laminated texture is predominant while some have a pillar (up to 1.5 mm long) and tabular texture. Illitization begins at the striae of the crystal then expands to cover the whole lamina and the crystal. Illite pseudomorphs after biotite and feldspar present a dispersal distribution. Characteristic interference colors can be seen a high-power microscope under crossed nicols. The accessory minerals contain mainly dispersed quartz-crystal fragments and also a tiny amount of sanidine and zircon crystals. Sample 9, derived from the coal parting of B4 coal seam of mining area M in Guizhou province, is 25 mm thick, dark grey in color, with a fine-grained texture. It contains illite, montmorillonite-illite (mixed layer), chlorite and about 5% quartz (based on X-ray diffraction analyses). Microscopic examination shows that pillar and worm illite crystals and their fragments are predominant, biotite pseudomorphs with illite lamina are often seen.

Dense (cryptocrystalline) type (Plate 1, 4) Sample 20, derived from the C1o coal seam of mining area N in Yunnan province, is about 40 mm thick, dark grey, dense. Illite is predominant and there is a small amount ( < 5%) of kaolinite and ( < 5%) quartz (based on Xray diffraction analyses). Microscopic examination shows that it consists mainly of an illitic matrix with strong double refraction. There are dispersed pillar and tabular illite

PLATE 1 Textural types of illite coal tonstein. 1. Crystalline illite coal tonstein; longitudinal and oblique sections of illitic crystals, distinctive polychroism and double refraction, yellow, red, green, blue in polarizational colors; illite aggregates after biotite; dispersal feldspar and crystalline fragments of quartz; brown humic matrix; 3.0 mm long, crossed nicols (sample 6, B4 coal seam of mining area L in Guizhou province). 2. Crystalline illite coal tonstein; brown illitic crystals which are contaminated by organic substance and two biotite pseudomorphs (the above on the fight); dispersed crystalline fragments of quartz (white); 1.44 mm long, 1/4 nicol (sample 7, C9 coal seam of mining area N in Yunnan province). 3. Crystalline illite coal tonstein: typical illitic pillar crystals, rectangular the Basis (001), they show distinct interbedded texture, yellow, red, green, blue of order I to II in interference color, strong double refraction, partial pillar crystals have longitudinal joints; 1.22 mm long, 1/3 nicol (sample 9, B4 coal seam of mining area M in Guizhou province). 4. Dense illite coal tonstein: illitic matrix is predominant contaminated by brown humic substance, with strong double refraction, a small amount of illitic pillar and fine tabular crystals, illitic aggregates of biotite pseudomorphs, occassionally illite after feldspar, dispersed quartz and sanidine crystalline fragments; 1.30 mm long, 1/4 nicol (sample 20, C~o coal seam of mining area N in Yunnan province).

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ILLITE TONSTEINS IN LATE PERMIAN COAL-BEARING FORMATIONS OF SW CHINA

349

crystals with characteristic illitic interference colors. It contains a small amount of kaolinite and illitic aggregates after feldspar and after biotite. The fragments ( < 2/an) having a microcrystalline granular texture are seen in the matrix. Dispersed fragmental quartz as accessory mineral is predominant with a few sanidine crystalline fragments. Chalcedony strips several micrometres wide are also present. Based on the identifications of more thin sections, illites in kaolinite-iUite coal tonstein (transitional) and illite coal tonstein posses two primary forms: ( 1 ) illite crystals - pillar, tabular, worm forms and their fragments; (2) illite aggregates- tabular, fine-tabular biotite pseudomorphs, microcrystalline-granular feldspar pseudomorphs. In addition, the paragenetic association of above-mentioned morphological characteristic kaolinite-illite is also noted. In brief, illite forms which are seen in the area are basically identical with the classification system devised by Schiiller, in Schiiller a n d H o e h n e ( 1956, p. 741 ), for kaolinite coal tonstein of Carboniferous age in Saar area, and they agree completely with the forms of kaolinite coal tonstein in this area (Zhou et al. 1987, 1988 ). This shows that illite is formed by deuterogenic diagenesis of kaolinite. Matrixes Most matrixes are humic matrixes and these contaminated by organic substance are brown to red in color in thin section. It is of interest that the matrixes of illite coal tonstein distributed in the anthracite and meta-anthracite areas and of transitional coal tonstein (samples 6, 20, 22 ) show some secondary textures (Plate 2). Parallel secondary laminatory joints appear in samples 6 and 22, and secondary mosaic joints are seen in sample 22. These secondary textures are not seen in the kaolinitic coal tonstein found in bituminous coal areas. These are similar to the secondary textures found in coal tonstein Hagen

PLATE 2 Secondary matrix textures. 1. Crystalline illite coal tonstein: red-brown matrix contaminated by organic substance, shows parallel laminary joints; 1.27 mm l o n g , / / n i c o l (sample 6, B4 coal seam of mining area L in Guizhou province). 2. Dense iUite Coal tonstein: illite matrix with mosaic secondary joints; 1.15 mm long, 1/2 nicol (sample 20, Cio coal seam of mining area N of Yunnan province). 3. Dense illite coal tonstein: illite matrix with mosaic secondary joints and root organ of plant filled by illitic matrix; 1.30 mm long, 1/4 nicol (sample 20, C~o coal seam of mining area N in Yunnan province). 4. Dense kaolinite-illite coal tonstein: showing parallel secondary laminary joints which cut a cross humic and kaolinitic matrixes; 0.97 mm l o n g , / / n i c o l (sample 22, C13 coal seam of mining area U in Guizhou province).

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ILLITE TONSTEINS IN LATE PERMIAN COAL-BEARING FORMATIONS OF SW CHINA

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1 of anthracite area Nordsee A1 in Germany (Stadler, 1971 ), and they may be the products of deuterogenic structural stresses.

Accessory minerals Crystalline fragments of quartz are predominant with subordinate biotite, chlorite, zircon, dispersed sanidine fragments, fl-quartz, crystalline fragments of feldspar illitized occasionally along the grains (Plate 3). There are tiny berry, ball-like pyrite aggregates in the partial coal tonsteins. The quantities and the assemblage behaviour o f accessory minerals are in general identical with those of kaolinite coal tonstein in this area (Zhou et al., 1982; Zhou and Ren, 1983; Zhou et al., 1987, 1988). CHEMICAL COMPOSITION AND TRACE ELEMENTS

More chemical analyses were done on kaolinite, transitional and illite coal tonsteins (Zhou, 1975; Zhou et al., 1982, 1987, 1988). Further research in this paper shows that the general trend of chemical composition in the transformation from kaolinite to illite coal tonstein, is a slight increase of the ratio SIO2/A1203, on increase in total-alkali quantities and a decrease in textural water. There is no apparent variation in TiO2 (Table 1 ). The analyses of some trace elements show that in the transformation from kaolinite to illite coal tonstein, there are no apparent variations in the con-

PLATE 3 Morphological types of illite crystals and accessory minerals in illite coal tonstein. 1. IUite crystals having interbedded texture of rectangular the (001 ), typical interference color and wormy winding; 0.80 mm l o n g , / / n i c o l (sample 9, B4 coal seam of mining area M in Guizhou province). 2. Aggregates ofillitized biotite-pseudomorph, the section is vertical to the (001 ), strong double refraction, white of order I in interference color; 0.85 mm long, +nicol (sample 6, B4 coal seam of mining area L in Guizhou province). 3. Aggregates of illitized feldspar-pseudomorph, cryptocrystalline texture, typical interference color, strong double refraction; 0.20 mm long, +nicol (sample 20, Cto coal seam of mining area N in Yunnan province). 4. Two crystalline fragments of sanidine, slight brownish matrix; 0.93 mm long, 1/2 nicol (sample 20, C 1ocoal seam of mining area L in Guizhou province). 5. Crystalline fragments of feldspar beginning to illitized in the margin, a illitized feldsparpseudomorph at the top; 0.76 mm long, + nicol (sample 7, C9 coal seam of mining area N in Yunnan province). 6. Fresh sanidinic crystalline fragment with sharp edge; 0.20 mm long, 1/4 nicol (sample 20, Cto coal seam of mining area N in Yunnan province). 7. Illitized biotite pseudomorph, iUitic crystals and sanidinic crystalline fragments, matrix with strong double refraction; 0.45 mm long, 1/2 nicol (sample 20, C~o coal seam of mining area N in Yunnan province).

K, BURGER E'F a.L..

352 TABLE I Chemical composition of coal tonsteins

Kaolinite coal tonsteins Crab (1)

Cryst (14)

Dense tonsteins

Grpt (11)

(5)

SiO2 TiO2 A1203 Fe203 Cr203 MnO MgO CaO SrO BaO

Na20 K20 Rb20 P205 SO3 C1 Pb Zn Cd Cu Ni lgn. loss

Total SiO2 Mol.

41.38 0.66 33.19 2.51

0.78 0.29 0.032 0.024 . 0.681 0.048 0.111 . 0.0039 0.0006 0.0026 20.76 100.47 2.12

41,44 0.18 32,84 4.60

38.68 0.67 32.34 0.55 . . 0.148 0.059 0.007 0.007 . 0.081

0.016 0.40 0.135 0.008 .

. 0.032 . 0.024 4.92*

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. 0.0050 0.0031 0.0062 20.32 104.92 2.14

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44.07 1.25 39.63 1.83 .

26.00 99.60 2.03

. 0.971 0.581 0.025 . 0.324 . 0.083 . 0.0097 . 0.0022 17.00 104.61 1.91

(27)

42,79 1.32 35.41 0.31 . .

0,426 0,171 0.017 0.094 . 0.426 . 0.060 0.026 . 0.0068 0.0271 0.0009 . . 0.0071 14.70 98.74 2.10

(25)

43.94 0.33 39.00 2.35 .

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. 0.104 0.95 . 0.0063 0.0003 0.0024

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41.19 1.17 34.46 0.914

Mean value 43.00 1.02 36.12 1.35

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0.382 0.349 0.016 0.008 .

0.227 0.251 0.016 0.008

0,501 0.338 0,018 0.028

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18.74 100.11 2.05

. 19.00 99.08 2.03

17.35 100.63 2.02

A1203 *S= 1.70 weight %. **Numbers in parentheses represent sample nos. Abbreviations: C r a b = C r y s t a l l i n e tonsteins after biotit; C rys t =C rys t a l l i ne tonstems; G r p t = G r a u p e n tonsteins. Analyst: Dr. H. Pitzner, Geologisches Landesamt Nordrhein-Westfalen, D-4150 Krefeld, Western Germany.

tents of these trace elements. These results are similar to those of previous studies (Zhou et al., 1988 ). The contents of some trace elements approach in general the Clarke value of acid-magmatic rocks, and this reflects to some extent the properties of the source materials (Feng, 1987 ), see also Table 1. SPATIAL DISTRIBUTION

OF LAW OF CATEGORIES

In the bituminous coal areas near the outcropping zone, various textural types of kaolinite coal tonsteins are identified, their distribution is consistent

353

ILLITE TONSTEINSIN LATEPERMIAN COAL-BEARINGFORMATIONSOF SW CHINA

Transition coal tonsteins Cryst

Illite coal tonsteins

Dense tonsteins

Crystalline tonsteins

(8) (28)

Mean value

(6)

(7)

43.60 1.24 33.41 0.604

46.85 0.69 32.71 0.482

43.85 0.24 35.82 0.30

45.27 0.69 35.19 0.46

1.51 0.837 0.035 0.060 0.043 3.610 0.009 . 0.311

0.674 0.294 0.078 0.232 5.150 0.017 . 0.207

0.837 0.388 0.041 0.100 0.014 3.149 0.011

0.410 0.267 0.017 0.067 0.216 1.600 0.008

0.218

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44.42 0.12 35.73 0.32

0.296 0.140 0.0t6 0.025 0.420 0.016 0.321 0.0041

0.0012 17.68 99.50 2.11

(22)

50.01 0.29 32.87 0.52

0.327 0.034 0.009 0.009 0.688 0.009 . 0.138

14.03 98.94 2.58

46.93 0.95 31.84 0.32

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13.68 100.14 2.50

. 13.57 99.08 2.22

0.872 0.008 0.008 0.084

0.0030

0.0073 0.0009

0.0041 -

0.0048 0.0003

. 0.0028

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0.0028

16.41 99.36 2.08

0.537 0.235 0.008 0.050

45.26 0.453 34.78 0.409

0.570 0.353 0.022 0.067 0.755 1.327 0.008 0.003 0.170

.

13.76 99.25 2.43

46.65 0.43 33.35 0.45

Mean value

0.765 0.559 0.043 0.086 2.05 1.51 0.009 0.275

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. 0.0024

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. 0.0040

16.14 99.56 2.18

14.03 100.21 2.38

15.53 99.71 2.21

with the trend of the iso-line of VM; illite coal tonstein is present in the central part of the study area roughly coincident with < 8% VM, its spread directions are near south to north. Kaolinite-illite coal tonstein (transitional type) is located between the illite and kaolinite coal tonsteins (Fig. 4 ). It is clear that the distributional law (Fig. 4 ) of the above-mentioned categories is not occasional. It can now be predicted that some supplements and modifications can be made to the given scheme, but that does not change the general pattern of this distributional form and general trend of the categories.

354

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Illlte c.t.

Fig. 4. The regional distribution of three categories of tonsteins. Profiles in Zhou et al. ( 1988 ). CONCLUSION

A great deal of data shows that various kinds of coal tonsteins are formed by the altered products of synsedimentary volcanic ash-falls. Volcanic glass fragments and crystalline fragments are predominant in the source materials. The assemblage of characteristic accessory minerals and the ratio TiO2/A1203 show that the source materials are acid volcanic ashes. The ratio TiO2/A1203 of some coal tonsteins is very steady in the range of thousands km 2. Euhedral crystals and morphological characteristics of complete high-temperature minerals (fl-quartz, sanidine, zircon, monazite, etc.), the confirmation of tadpole and spindle-like volcanic glass bubbles, and remnant walls of bubbles provide direct, important evidence of altered volcanic origin. When media; crystalline fragments of alumosilicate, i.e. volcanic glass, biotite, feldspar etc. of which volcanic ashes are composed, are almost completely altered into kaolinite, forming the thin and steady coal partings of kaolinite coal tonstein. While accessory minerals such as quartz, zircon, sanidine, etc., which remain steady under this condition, are preserved. But the clay

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minerals of tonsteins are altered with the development of deuterogenic diagenesis. Figure 4 reveals clearly the variational trend of the deuterogenic alteration: the clay minerals of tonsteins are dominated by kaolinite when VM is 10% and more; partial kaolinites are altered into illite and illitic-kaolinite pseudomorphs (transitional tonstein) appear when VM is 8-10%; the clay minerals in these tonsteins are almost entirely illite with some sort of accompaning chloritization and perhaps a small amount of paragonite when VM declines to less than 8%. It is worth noticing that: no apparent variation takes place in rock textures of tonsteins when some tonsteins are under the above-mentioned alteration of the clay minerals, even mineral grains; morphological characteristics in it remain steady, and can be compared with each other. It can be predicted, based on the regional distributional law of tonsteins, that the variation of the clay minerals is a continual and progressive change in geological processes. In addition to temperature and pressure, the existence of a certain amount of alkali-metal ions and ferrous ions in solution is undoubtedly indispensable. ACKNOWLEDGEMENTS

We are greatly indebted to Dai, Henggui, senior engineer in Yunnan Geology Research Institute for granting us access to relevant samples; to Dr. G. Stadlert, Dr. H. Pietzner and Dr. A. Rabitz, Geologisches Landesamt Nordrhein-Westfalen, Krefeld, and Prof. Dr. A. Prashnowsky, Bonn, FRG, for their valuable contribution to X-ray diffraction analysis, X-ray spectrum analysis and biogeochemical analysis of the samples; and to Ren, Youliang, associate professor of East China Normal University, for going over the manuscript and improving the English text.

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