Clays and other minerals in coal seams of the Moura-Baralaba area, Bowen Basin, Australia

International Journal of Coal Geology 25 (1994) 287-309

ELSEVIER

Clays and other minerals in coal seams of the Moura-Baralaba area, Bowen Basin, Australia C o l i n R. W a r d , P.J. C h r i s t i e Department of Applied Geology, University of New South Wales, PO Box 1, Kensington, N.S. W., 2033, Australia

(Received February 9, 1993; revised version accepted August 6, 1993)

Abstract The clays and other minerals in a succession of Late Permian coals of high-volatile bituminous to semi-anthracite rank have been identified, using low-temperature oxygen plasma ashing and X-ray diffraction, and evaluated to identify the relative roles in mineral matter formation of detrital input, early diagenesis in the peat swamp and late diagenesis associated with rank advance. Although well-ordered kaolinite of probable early diagenetic origin is abundant throughout the succession, the uppermost and lowermost seams of the sequence, regardless of rank, contain relatively abundant illite and/or interstratified illite/ smectite, along with a small but significant proportion of chlorite. These clays are thought to be essentially of detrital origin, washed or blown into the peat deposit in relative abundance during the establishment and subsequent overwhelming of an extensive and longlived swampy environment. Quartz is also abundant in the lower seams of the sequence, especially close to the regional sediment source area. Illite is unusually abundant in the topmost seam in both high- and low-rank parts of the succession, and thus appears to represent detrital input from a particular source material. Although significant changes are reported in the clays of the associated strata due to rank advance, the principal effect of rank advance on the minerals in the coal itself appears to be the development of an ammonium iUite, and possibly some additional fine-grained chlorite, in the semi-anthracite material. Isolation within the organic matter of the coal is thought to have inhibited access for ions such as K ÷, which might otherwise have become involved in metamorphic reactions and given rise to mineralogical changes commonly found in non-coal sedimentary successions.

1. Introduction A considerable b o d y o f knowledge has been built up regarding the organic constituents o f coal (Stach et al., 1982), including the changes that occur in the dif0166-5162/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0166-5162 (93)E0055-P

288

C.R. Ward, P.J. Christie/Int. J. Coal GeoL 25 (1994)287-.~09

(a)

1,

COLLINS.VlLLE

\

/%

ROCKHAMPTON .

B72~12 ~ (b) Moolayember Formation

o ~3

Clematis Sandstone Rewan i

Group o_ (.9

iii {3.

Baralaba Coal Measures I Kaloola Gyranda Formation

Back Creek Group Camboon Andesite

Fig. 1. (a) Locality map of the Bowen Basin. (b) Stratigraphic column of the Moura-Baralaba area.

C.R. Ward, P.J. Christie ~Int. J. Coal Geol. 25 (1994)287-309

289

ferent macerals with rank advance. Techniques such as low-temperature oxygen plasma ashing, in conjunction with X-ray diffraction (Gluskoter, 1965; Rao and Gluskoter, 1973; Ward, 1978), have also helped to provide an understanding of the minerals, including the clay minerals, that occur almost ubiquitously in coal but are not readily identified in optical microscope studies. The processes that form these minerals can include detrital input, either from the same source as the associated non-coal sediments or from special sources such as contemporaneous pyroclastic activity. They can also include alteration of this detrital sediment in the peat swamp to form new minerals, or precipitation of new minerals from dissolved constituents, either in the swamp waters or in the pores of the peat accumulation. As with clay minerals in other rocks (Dunoyer de Segonzac, 1970; Hower et al., 1976 ), the minerals in coal may also be affected by the thermal processes associated with rank advance. The relative roles of detdtal input and early diagenesis as processes of mineral formation have been investigated for a number of different coal deposits, including those of the Illinois Basin of the USA (Rao and Gluskoter, 1973; Ward, 1977 ); the Pennine coalfields of Great Britain (Spears, 1987 ) and the Sydney Basin of Australia (Ward, 1989). Major contrasts have also been pointed out between the mineral matter of low-rank coal, which is dominated by inorganic elements associated with the organic compounds (Miller and Given, 1978; Benson, 1987; Ward, 1991 ), and that of higher rank material, such as bituminous coal, which is dominated by crystalline mineral components. Studies have been carried out on changes in mineral matter in coal associated with contact effects of igneous intrusions (Ward et al., 1989), but despite work on some of the sediments closely associated with coal seams (Srodon, 1979; Pevear et al., 1980), little is known of the mineralogical changes that occur in coal itself in response to the rank advance process. The Late Permian coal seams of the Moura-Baralaba area, in the southeastern corner of the Bowen Basin, Australia (Fig. la), change in rank from high volatile bituminous to semi-anthracite over a strike length of some 70 km (Davis, 1968 ), apparently due to a combination of deeper burial towards an active foreland margin (Mallett et al., 1990) and higher geothermal gradients associated with an adjacent orogen zone (Beeston, 1985 ). Mineralogical changes have been noted in some of the sediments associated with the coal in conjunction with this rank variation (Kisch, 1966; 1968), but no studies have been carried out to date on the mineralogical changes, if any, associated with the rank advance in the actual coal seams. The present study represents an attempt to investigate the variation in mineral matter in the coals across this area, based on a series of samples collected from different seams in active or projected mining operations. It provides an opportunity to assess whether changes in the organic matter due to rank, such as vitrinite reflectance, are necessarily accompanied by mineralogical changes and also to evaluate, from the distribution of minerals in the coals with respect to stratigraphy, rank and location, the relative roles of other processes that might be associated with mineral matter formation.

290

C.R Ward, P.J. Christie/lnt, J. Coal Geol. 25 (1994) 287-309

2. Geologic setting The Bowen Basin is part of a major Permo-Triassic foreland basin system (Murray, 1985; Hobday, 1987), fault-bounded on the eastern side by the contemporaneously active New England orogen and bounded unconformably on the western side by the older, more stable cratonic platform of the Thompson and Lachlan fold belts. The base of the Permian sequence in the Moura-Baralaba area is represented by the Camboon Andesite, a felsic volcanic sequence up to at least 3,000 m thick (Draper, 1985 ). This is overlain by a marine elastic, volcanic and carbonate succession, referred to as the Back Creek Group, and then by the Late Permian coal-bearing strata of the Blackwater Group (Fig. l b). A coal-barren fluvial and red bed succession, the Late Permian to Early Triassic Rewan Group, and in much of the basin the overlying Triassic beds of the Clematis Group and the Moolayember Formation, complete the stratigraphic sequence. The lower part of the Blackwater Group in this area, the non-marine, coalbarren Gyranda Formation, is overlain by a coal-bearing succession up to 450 m thick, usually referred to as the Baralaba Coal Measures (Svenson et al., 1975 ). Revisions to the terminology, however, proposed by Quinn (1985a), describe the coal-bearing sequence in the Moura-Baralaba area as the Baralaba Subgroup. A coal-poor tuffaceous succession, referred to as the Kaloola Member and by Quinn (1985a) as the Kaloola Formation, forms the basal part of the sequence. This is overlain by a major coal-bearing interval correlated by Quinn (1985a) with the extensive Rangal Coal Measures found at the top of the Permian succession in other parts of the basin. The Permo-Triassic strata around Moura have a homoclinal dip towards the west at an angle of between 5 ° and 15 °. However, they are folded into a complex synclinorium structure near Baralaba, 45 km to the north, with dips on the limbs of up to 40 ° . Several local igneous intrusions of probable Cretaceous or early Tertiary age also occur in the coal seams in the Baralaba area, along with a more complex fault pattern (Svenson et al., 1975 ).

3. Deposition of the Baralaba Coal Measures The coal-bearing interval of the Baralaba Coal Measures is thought to have been deposited in an environment that was dominantly paludal (Mallett, 1983; 1985 ), with extensive and long-lived peat-forming swamps interrupted intermittently by elastic sediment influxes producing individual sediment bodies in a range of fluvial depositional systems. After compaction of the peat, this resulted in an anastomosing sequence of up to 10 coal seams in the Moura-Kianga area, splitting, coalescing and in places pinching out around individual lenticular units of coarse to fine elastics (Fig. 2a) that represent interleaved channel, splay, lake and other deposits. The coals in the area south of Moura are worked by a combination of opencut and underground operations in a belt extending some 26 km along the outcrop of

CR. Ward, P.J. Christie ~Int. J. Coal Geol. 25 (1994) 287-309

NIPAN SEAMS NO1

0 I ~

km

291

5 I

KIANGAA

MOODY BOYD

~

4

CAMERON REID DOUBTFUL

~,~

24

.o~"~

~

13~,,~'~

"~ ~

~

_

.ou.~c

[ I

/

"~

DAWSON

DUNSTAN NO9

--

~ ~

MOURAE SF.AM

"~

I WRIGHT

NCRTH

mmmCOOLUM [1.0%

Vltrlnlte Reflectance (Rv max)

m

Fig. 2. (a) Stratigraphic cross-section of the Baralaba Coal Measures in the Moura-Kianga mine area (datum on base of E seam south of fault and base of D seam north of fault), showing seam names (smaller letters) and correlations, rank trends and sample locations (larger letters). After BHP-Utah (1987). (b) Stratigraphic column showing coal seams in the Baralaba area. Compiled from Allied Queensland Coalfields data. The Dawson (DAW) and Coolum (CLM) seams were sampled for the present project.

the coal measures. Individual seams in this area range from 1.5 to 7.0 m thick, with 9 separate mineable horizons recognized (BHP-Utah, 1987 ). The mineable seams are identified by a letter-based system in the north (A, B, C, etc. ) and by a number-based system in the south (Nipan No. l, 2, 3, etc. ), with correlations between the different horizons indicated in Fig. 2a. In the lower part of the Moura sequence (C, D and E seams and their equivalents) the interseam sediments are mainly fluvial sandstones, occurring as lenticular bodies up to 50 m thick and 300 m wide (Mallett et al., 1980). In the upper part, however (A and B seams and their equivalents), the interseam strata are mainly thinly interbedded and intedaminated sandstones, siltstones, shales and claystones of lower energy splay, overbank and lacustrine origin. The upward succession of interseam strata may represent a lateral change in depositional system from seawards to landwards across the peat deposit, with the different lithofacies at each level controlled, at least in part, by the thickness of the individual underlying peat beds (Mallett, 1985 ). Regional studies (Mallett et al., 1980) indicate sediment input from the southwest and a transport direction towards the northeast. Although tufts are not present in the main coal-bearing section (Quinn, 1985a), the interseam sediments of the Rangal equivalents have a predominantly volcanic provenance. The sandstones are dominated by rounded lithic fragments with lesser amounts of quartz

292

('.R. Ward, P.J, Christie ~Int. J. Coal Geol. 25 (1994) 287-309

and often fresh feldspar particles, held together by a fine clay matrix, some authigenic clay (dickite) and a variable amount of calcite and/or siderite cement (Ward et al., 1990). The lutites have a clay fraction consisting of kaolinite, illite and an interstratified illite/smectite (I/S), with the I/S having a typically ordered structure and sometimes a rectorite lattice (Seedsman, 1985 ). The coal-bearing sequence at Baralaba (Fig. 2b) is generally similar to that at Moura, although the complex structure of the intervening area prevents more than a very general correlation of the individual coal seams. Svenson et al. ( 1975 ) have indicated a general equivalence between the A seam at Moura and the Doubtful seam of the Baralaba district.

4. Coal quality The lowest rank coals in the study area are represented by high-volatile bituminous material near Kianga, 25 k m southwest of Moura, where vitrinite reflectance (Rvmax) is around 0.7%. Medium- to low-volatile material, with a reflectance of around 1.1%, occurs at Moura itself (BHP-Utah, 1987) and semianthracites, with vitrinite reflectance values of more than 2.0% (Davis, 1968), are developed at Baralaba, 45 km further to the north. Seams with both high and low overall vitrinite percentages are present. The A seam, at the top of the Moura sequence, is generally dominated by dull lithotypes, but most seams have subsections of relatively bright and interbedded dull and bright material (Svenson et al., 1975; Quinn, 1985b). Typical coking products from the Moura mine have 60-70% vitrinite, 25-35% inertinite, 2-3% liptinite and 2-4% of microscopically recognizable mineral matter (BHP-Utah, 1987). Total sulphur (mostly organic) is typically around 0.4% and the ash percentage is relatively low, depending on the preparation process used.

5. Sampling prod'am Samples of coal from a total of 18 localities within the Moura-Kianga area were supplied by BHP-Utah Coal Ltd for the present project. Each sample represents the washed coal product from a single seam at one particular opencut pit and thus duplicates fairly closely the inherent mineral matter of the different seams at selected points along strike in the sequence. The location and stratigraphic position of these samples are indicated in Fig. 2a. Samples were also taken for the project by Allied Queensland Coalfields Ltd from two of the major seams: the Dawson seam (DAW) and the Coolum seam (CLM) of the same sequence at Baralaba (Fig. 2b ). These were essentially large grab samples selected to represent typical coals that might be mined from these high-rank coal deposits, although no mining is under way at present.

C.R. Ward, P.J. Christie lint. J. Coal Geol. 25 (1994) 287-309

293

6. Analytical program The mineral matter was isolated from each of the coal samples by oxygen plasma (radio frequency) low-temperature ashing, using techniques outlined by Gluskoter (1965). The plasma ash of each sample was analyzed by X-ray powder diffractometry, with the minerals present identified by reference to the JCPDS powder diffraction file. Typical diffractograms are shown in Fig. 3. --

I

I

i

I

I

I

I

I

Q Ct K A

i

c/K

K K'IK~

K Ch

21 K

Q K K

._K

Ct PA PA

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ML

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P,

py PA

KK

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K

~

II o

_ K~

PA /~)

~ 4,

30 I

I

K

,

"r

20I

,

10 I

DEGREES

20 l

Fig. 3. X-ray powder diffractograms of oxygen plasma residues from selected coal samples. A = apatite;

Ch=chlorite; Ct=calcite; D=dolomite; I=illite; K= kaolinite; Q=quartz; PA=potash alum; ML = m i x e d - l a y e r ( i n t e r s t r a t i f i e d ) clay minerals.

26 tt

3.5

3,351

..

i

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3.58

5.18

5.16

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4.26

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3.58

426

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4.98

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4.45 . 4.92

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HEATED 400°C

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10

7.15

/

/

f

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DEGREES 28

10,0 a

10.0

10.3

13,0

Fig. 4. Oriented aggregate X-ray diffractograms of the clay fraction of selected plasma ash residues. (a) Saturated with ethylene glycol. (b) Heated to 400 °C. Figures indicate d spacings in Angstrom units.

...

~

3.43

3.53

3.58

3.58 3.5;

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GLYCOL SATURATED

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(a)

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%

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C.R. Ward, P.J. Christie~Int. J. Coal Geol. 25 (1994)287-309

295

Samples of each plasma ash were also dispersed in water to which a small amount of tetra-sodium pyrophosphate had been added and the clay ( < 2 #m) fraction concentrated by settling. Oriented aggregate mounts of each clay fraction were prepared by the pipette-on-glass-slide technique (Gibbs, 1971 ) and subjected to further XRD analysis after saturation with ethylene glycol, after heating at 400°C for approximately 30 min and after heating at 600°C for a further 30 min. Diffractograms of the clay fractions for selected samples, glycol-treated and heated to 400°C, are given in Fig. 4. Other samples of each coal were ashed at 815 ° C in a ventilated muffle furnace and the chemical composition of the resulting ashes determined by X-ray fluorescence techniques. The results of these ash analyses (Table 1 ) were evaluated using a general purpose normative calculation program for sedimentary materials developed by Cohen and Ward ( 1991 ), in conjunction with the X-ray diffraction data, to provide a semi-quantitative assessment of the components identified in the powder analysis of the mineral matter (Table 2 ). The mineralogy of the clay fraction in each case was evaluated in the process following the method of Griffin (1971). Due to the possible significance of coal rank in interpreting the mineralogical data, vitrinite reflectance values were determined for selected samples. The results of this work are summarised in Table 3. The north-south reflectance variation in the Moura mine area is also indicated from these data in Fig. 2a.

7. Mineralogy of the coal samples The overall ash percentage (at 815 ° C) and the oxygen plasma ash or mineral matter percentages for the Moura samples are both relatively constant (Tables 1 and 2), although higher values are noted in the upper seams of the mine area. This, however, largely reflects the fact that most of the material analyzed was taken from washed coal samples. The two hand-selected samples from Baralaba had moderate to relatively low ash and mineral matter levels. The following minerals were identified in the coal samples: 7.1 Quartz

Quartz is present to at least some extent in all of the samples studied. It appears to be more abundant, relative to other components of the mineral matter, in samples from the southern part of the Moura deposit compared to those from the north, a trend especially discernable in seam A and the probably correlative Nipan No. 3 seam. Quartz is also low, as a proportion of the total mineral matter, in the two Baralaba coal samples. This distribution, combined with independent indications of south-to-north palaeocurrents in the interseam sediments noted above, suggests that the bulk of the quartz may be of detrital origin, washed or blown into the swamps during peat accumulation. 7. 2 Kaolinite

Kaolinite is the dominant mineral in most of the plasma ashes studied. Indeed, it makes up almost the entire clay fraction of the Kianga A and some of the other

Moura Moura A Moura A Moura A Moura A Kianga A Kianga A Nipan 3 Moura B Moura B Moura B Moura B Lower Moura B Lower Moura C Lower Moura C Lower Moura D Upper Moura D Upper Moura D Moura D

Baralaba Dawson Coolum

Coal seam

10 20 21 5 11 22 12 23 13 24 14 25 15 26

4

1 2 3

DAW CLM

10.6 14.1 11.8 16.0 10.1 9.9 13.6 10.8 7.2 7.5 7.7 6.8 7.3 7.1 5.3 5.9 9.8 7.9

4.7 9.5

Sample ASH No. (%)

44.14 37.84 43.65 47.54 50.13 54.43 60.58 47.63 47.45 56.10 60.16 56.83 58.84 50.36 56.31 49.83 60.68 56.67

34.60 45.38

SiO2 (%)

1.50 1.44 1.32 1.36 1.47 1.41 1.22 1.36 1.48 1.22 1.33 1.22 1.10 1.06

1.11

1.70 1.27 1.34

1.51 0.80

TiO2 (%)

Table 1 Location and ash composition of coal samples studied

30.74 21.95 25.43 21.87 29.17 28.80 23.00 26.06 30.01 26.33 29.11 25.85 29.26 27.01 28.70 26.47 25.47 27.47

28.09 29.90

A1203 (%)

3.61 18.81 17.40 8.48 6.53 8.83 4.61 6.23 8.78 11.57 5.57 11.56 5.71 10.68 10.07 15.72 6.29 7.33

7.06 2.03

Fe203 (%)

0.03 0.18 0.14 0.07 0.05 0.02 0.02 0.05 0.04 0.03 0.01 0.03 0.01 0.05 0.03 0.23 0.05 0.02

0.02 0.02

MnO (%)

1.15 2.12 1.37 1.19 1.19 1.43 1.16 1.05 1.86 1.42 0.97 1.73 0.99 1.63 0.93 1.69 0.58 0.98

1.90 1.32

MgO (%)

8.29 8.59 4.49 11.29 5.08 0.99 3.53 9.75 4.74 0.51 0.36 0.24 0.41 3.02 0.42 0.47 0.29 0.47

14.31 10.71

CaO (%)

0.52 0.32 0.44 0.41 0.25 0.27 0.30 0.28 0.33 0.38 0.38 0.37 0.50 0.44 0.49 0.48 0.56 0.75

0.40 0.67

Na20 (%)

2.48 2.26 1.97 1.86 0.47 0.76 2.17 1.13 0.44 1.06 0.98 1.26 1.42 2.12 0.90 2.51 3.77 3.65

0.37 1.22

K20 (O/o)

1.51 1.02 0.55 0.06 1.21 0.54 0.04 1.85 0.14 0.24 0.04 0.03 0.05 0.02 0.02 0.03 0.03 0.02

8.45 4.38

P205 (%)

5.08 4.65 2.20 6.20 2.98 0.07 1.83 3.43 3.90 0.07 0.18 0.07 0.15 1.98 0.07 0.10 0.05 0.08

2.63 2.65

SO3 (%)

99.23 99.01 98.99 100.09 99.74 98.95 98.56 98.33 99.17 99.11 98.99 99.33 98.83 98.55 99.29 98.75 98.87 98.50

99.33 99.05

Total (%)

t~

,c

.~

~. "~

-n

~-,

O~

DAW CLM

1 2 3 4 10 20 21 5 11 22 12 23 13 24 14 25 15 26

Moura Moura A Moura A Moura A Moura A Kianga A Kianga A Nipan 3 Moura B Moura B Moura B Moura B Lower Moura B Lower Moura C Lower Moura C Lower Moura D Upper Moura D Upper Moura D Moura D

Sample No.

Baralaba Dawson Coolum

Coal seam

13.8 19.8 18.2 20.3 15.5 15.2 17.1 16.4 10.7 9.4 9.5 8.6 9.5 9.5 6.9 10.0 13.7 11.7

6.7 13.1

LTA (%)

10

Quartz (%)

5 15 15 20 15 20 30 15 10 25 25 25 25 20 20 20 30 25

Table 2 Mineralogy of oxygen plasma ash residues

50 35 40 30 65 70 35 50 65 50 60 55 55 45 55 35 25 30

45 45

Kaolinite (%)

10 10 10 10 15 5 5

5 10 Trace Trace 5

5 5

Trace

5 5

Chlorite (%)

25 20 25 20 10 5 20 15 10 15 10 10 10 20 15 25 40 40

20 20

(%)

Other clays

5

5 10 10

10 10 5 20 5

5 10

Calcite (%)

Trace 5

5

Dolomite (%)

Trace Trace 5

5 5 [Pyrite 5%] 5

Trace 10 10 5

Siderite (%)

5

5

5 5

20 10

Apatite (%)

[K-Alum] [K-Alum]

Trace

Trace

Trace Trace

Trace Trace Trace Trace Trace

Trace

Bassanite (%)

,o

,o ~-~

~" ~' .~

.~ "~

298

C.R. Ward, P.J. Christie lint. J~ Coal Geol, 25 (1994) 287-309

Table 3 Vitrinite reflectance data for selected coal samples Sample No.

Telocollinite reflectance

All vitrinite reflectance

Range-all vitrinite

DAW CLM 1 2 4 13 21 26

2.18 2.15 1.08 1.02 0.89 0.83 0.81 0.77

2.20 2.14 1.07 1.00 0.87 0.80 0.78 0.76

1.98-2.31 1.94-2.30 0.97-1.14 0.90-1.10 0.77-0.95 0.69-0.90 0.68-0.87 0.66-0.88

(%)

(%)

(%

Analyst H.W. Read

coal seams. The XRD (X-ray diffraction) pattern in most cases indicates that the kaolinite is largely well-ordered, a feature typical of the mineral matter in many other Australian coal seams (Ward, 1978; 1989 ). Some poorly ordered kaolinite, however, is present in the D and D Upper seams near the base of the Moura sequence (samples 15, 25 and 26 ). Evidence from other areas (Ward, 1989) suggests that much of the well-ordered kaolinite is probably authigenic, reconstituted in the waters of the peat swamp or in the pores of the peat deposit. It may represent an alteration residue derived from other clay minerals, or it may have been formed by interaction of Si and AI in solution in association with pH changes and in the presence of organic matter (Ward, 1978; Spears, 1987 ). The poorly ordered kaolinite in the D and part of the D Upper seam, however, is probably of detrital origin; it occurs in samples with a relatively high quartz percentage and in which other clays (see below) are also relatively abundant. 7. 3 Illite Illite, identified by its d (001 ) spacing of 10 A in oriented aggregates saturated with ethylene glycol, is a minor but significant component of the clay fraction of the A seam, the Nipan No. 3 seam and one of the D seam samples. It is only found, however, as a trace component of the mineral matter isolated from the other Moura coal seams. Although common in the associated non-coal shaly sediments, illite is relatively rare in the mineral matter of Australian coals generally (Ward, 1978 ). This may in part reflect filtering out of detrital clay from the waters that penetrated into the coal-forming peat swamps. However, the relative abundance in the coals of another common component of the non-coal sediments, interstratified clay minerals (see below), suggests that any detrital illite may have been altered to form additional interstratified clay minerals, or possibly kaolinite, in the waters associated with the accumulating peat deposit. IUite is most abundant at Moura in coal samples where kaolinite is low and interstratified clays are abundant. From the reasoning outlined above it is, there-

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fore, thought, where present, to be essentially of detrital origin. The significant difference in rank between the illite-bearing samples, and the occurrence of lowiUite coals of intermediate rank between them, appear to rule out the generation of illite from the interstratified clays in the coals by processes associated with rank advance. A mineral with a d spacing of around 10/~, and hence identified as illite, is also noted in the Dawson seam from the Baralaba area. This material gives prominent XRD peaks in both powder mounts (Fig. 3 ) and oriented aggregates (Fig. 4) at 10.3, 5.16 and 3.43 ~, compared to the values respectively closer to 10.0, 5.0 and 3.33/~ that are normally associated with mica and illite (Srodon, 1984), although the peak at 3.43 ~ may also in this case be due to apatite. A d spacing of 10.0 ~ is developed in the material after heating to 600°C. The ash of the coal from which it comes also has a low K20 content (Table 1 ), which is inconsistent with the intensity of this particular XRD peak. The XRD characteristics of this material are similar to those of an ammoniumbearing illite, reported in black shales associated with stratiform sulphide ores by Sterne et al. (1982), and in underclays associated with anthracites and semi-anthracites of northeast Pennsylvania by Juster et al. (1987). They are also comparable to XRD patterns of NH4-bearing illites in a series of anthracite samples from Pennsylvania, mostly higher in rank than the Baralaba materials, described by Daniels and Altaner ( 1993 ). Ammonium illites are generally regarded as having been formed by isomorphous substitution of NH~ for K + in the original mica or iUite structure, either at relatively low temperatures in an environment such as the original peat swamp or at relatively high temperatures in association with low grade metamorphism or rank advance. Daniels and Altaner ( 1993 ), however, suggest that the mineral can also be formed at relatively high temperatures ( >1200 °C) by interaction of kaolinite with nitrogen otherwise occurring in the coal's organic matter. The presence of K-illite in some of the lower-rank coals in the present study appears to confirm that the illite in the Baralaba coals, like that in the anthracites of Pennsylvania, formed in association with rank advance rather than by early diagenesis in the original peat swamp. The mineral at Baralaba could have been developed from either Koillite or kaolinite, since both are present in the lowerrank materials around Moura. IUite is, however, significantly more abundant in the coals of the Moura sequence at approximately the same horizon as the Dawson seam in which the NH4-illite occurs at Baralaba, giving rise to a distribution which possibly reflects a particular type of detrital sediment input. NH4-illite is, moreover, apparently absent from the Coolum seam sample, despite the similarity in rank and the roughly equivalent abundance of kaolinite in both the Dawson and Coolum materials. It is, therefore, suggested that the NH4-bearing material at Baralaba was formed by alteration of a more normal K-inite in association with the increase of the coal to semi-anthracite rank. 7. 4 Chlorite Small amounts of chlorite are present in almost all of the coal samples studied. Only a low intensity (001) peak is typically developed at 14 A in the powder

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XRD pattern (Fig. 3) and the material is more readily identified by its (004) peak at 3.53/k. It can also be identified by these and other peaks in detailed study of the clay fraction in oriented-aggregate samples. Although masked by peaks due to interstratified clay minerals in glycol-saturated slides, a small but consistent XRD peak occurs at 14]k in almost all oriented-aggregate samples after heating to 400 ° and even more so after heating to 600°C. Separate XRD peaks at 3.53/k (004), and in some cases also at 7.05/k (002), both distinguishable from peaks due to kaolinite, are also noted in the oriented aggregates of many of the plasma ash samples (Fig. 4). As with other chlorites (Carroll, 1970; Moore and Reynolds, 1989), however, these tend to be lost on heating to the higher temperatures. The high intensity of the (002) and (004) peaks, relative to the (001 ) and, where present, the (003) peaks, indicates an iron-rich composition (Moore and Reynolds, 1989 ) in this particular instance. Kisch (1966; 1968 ) has reported the occurrence of chlorite-illite assemblages in pelletal claystones (tonsteins), normally expected to be kaolinitic, associated with the coal seams of the Baralaba district. Due to the absence of these minerals from similar materials in the Moura area, he suggested that the chlorite and illite were formed from other clays by metamorphic processes associated with the upper levels of rank advance. However, the presence of chlorite and illite in equal if not greater abundance in the lowest rank coals of the Moura district as well (e.g. Sample 26), where Rvmaxis below 0.8%, suggests that both minerals are more likely in this instance to be of detrital origin, probably derived from a similar source to the mainly volcanogenic interseam strata.

7.5 Interstratified clay minerals A number of different interstratified clay minerals are present in the mineral matter isolated from the coal samples of the study area. A regular illite/smectite, with XRD peaks at d spacings of 28, 13.5 and 9.0/k on glycol saturation (e.g., Sample 26 in Fig. 4), is found in a number of coals from the Moura district, although the 28 A peak and in some cases the 9/~ peak are not always particularly well developed. The structure of this mineral collapses to give a single XRD peak at a little over 10/k after heating to 400 °C and a broad peak at around 10 ~ on heating to 600 ° C. This behaviour suggests the presence of a regularly interstratified illite/smecrite (I/S), with a tendency towards development of a rectorite structure, as suggested by Seedsman (1985). The incomplete collapse to 10 ~ on heating, however, suggests that a small amount of chlorite may also be present in the interstratified lattice in some samples. The mineral is most abundant in the upper and lower seams of the Moura section (A and D seams). It is also present in other seams, but occurs in a less well-ordered form or in smaller concentrations in the mineral matter. It is indicated in the latter cases by a shoulder on the diffractogram at around 30/k and a broad peak at 13.5/~ (often overlapping with the 14/~ chlorite peak) on glycol saturation. Pevear et al. (1980) have described the development of a rectorite mineral from smectite, apparently in association with rank advance, in coal-beating sediments of British Columbia. Rectorite appears in these strata instead of smectite

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where the vitrinite reflectance of the associated coal exceeds about 0.7%. The lowest rank of the coal in the present study is higher than this value and thus a similar process could also have taken place in the Moura area in the course of rank advance. However, the fact that the best ordering occurs in the lowest rank coal and that samples with only slightly higher rank (e.g., Sample 21 ) have more poorly ordered material possibly precludes a metamorphic origin. Smectite without such interstratification is found in and around coal seams of similar rank in the Newcastle district of the Sydney Basin (Ward, 1989; Ward et al., 1989), and thus rank advance to this level in eastern Australia is not necessarily accompanied by such changes in clay mineralogy. The coals with the most abundant and most regularly interstratified material (e.g., Sample 26) also appear to be those most affected by the input of quartz and other detrital material. Although it is possible that some modification may have taken place with rank advance, it is suggested that the regular I/S material in this particular instance is essentially of detrital origin, probably derived from the same volcanic source as the chlorite component. A regularly interstratified clay mineral of slightly different character is present in the plasma ash of the two Baralaba coal samples. Oriented-aggregate diffractograms of this material indicate slightly larger d spacings of 30, 15.2, 9.2 and 7.6 in the Coolum seam sample on glycol saturation, and 32, 15.6, 9.4 and 7.8 ~ in the sample from the Dawson seam. In both cases the clay appears to collapse to a I 0 ~ structure on heating, although the heated material also shows a broad diffraction peak at around 13 ~, which is probably due to the separate chlorite component. The interstratified clay in the Baralaba coals resembles some of the material in shales of the Tomago Coal Measures in the northern Sydney Basin, described by Hamilton (1967) as partly ordered illite/smectite. The glycol-saturated characteristics are also similar to those of corrensite, a regularly interstratified chlorite/ smectite mineral, but the collapse to 10 ~ suggests instead the otherwise similar characteristics of a regularly interstratified vermiculite/smectite (Moore and Reynolds, 1989 ). The interstratified clay in the relatively low-rank coals of the Moura area and that in the highest rank coals at Baralaba are both remarkably regular. The differences that do exist may well reflect relatively minor variations in source materials or peat swamp chemistry. For this reason a detrital origin is preferred for the interstratified clay in both areas, rather than one involving progressive changes of a common parent material with coal seam rank advance. 7. 6 Calcite Calcite is present in the mineral matter of a number of coal samples, particularly those from the upper seams of the Moura sequence. Its presence, together in some instances with apatite (see below), is responsible for the elevated CaO contents (Table 1 ) of the respective (high temperature) ash residues involved. Kemezys and Taylor (1964) suggest that calcite is typically formed in veins and fissure fillings at a relatively late stage in Australian coal seams. Some may owe its origin to expulsion of organically held Ca from the coal with rank advance beyond the subbituminous range (Ward, 1985 ). The preferential occurrence in

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the upper seams at Moura has not been specifically investigated, but may reflect movement of calcium-charged fluids towards the top of the Coal Measures during the diagenesis process. 7. 7 Dolomite A small amount of dolomite is present in some of the A seam samples from Moura; it is abundant in the Dawson seam, and possibly also present in the Coolum seam of the Baralaba district. This mineral has a similar mode of occurrence to calcite in Australian coals (Kemezys and Taylor, 1964) and its distribution in the study area is probably a reflection of similar late diagenetic processes. 7.8 Siderite and Pyrite Small amounts of siderite occur in several seams of the Moura sequence (Table 2 ), especially the A seam, while pyrite is present in detectable amounts in the mineral matter of one of the B seam samples. In the absence of these constituents, however, the Fe203 content indicated by the ash analysis, as well as the MgO in the coals without dolomite, are probably derived from the chlorite and illite/ smectite or vermiculite/smectite components. Siderite is common in low sulphur Australian coals (Kemezys and Taylor, 1964; Ward, 1978 ), typically occurring as spheroidal aggregates of early diagenetic origin. Pyrite is much less abundant in most cases, but where it does occur is usually in coals closely associated with marine strata. The pyrite is thought to form mainly from the bacterial reduction of sulphate-rich waters permeating through the peat bed (Ward, 1978 ). The siderite is believed to be formed, in the absence of significant sulphate, from CO2 released by organic matter decomposition (Botz et al., 1983). 7. 9 Apatite and other phosphate minerals Apatite is an abundant constituent of the mineral matter isolated from the two Baralaba samples, particularly that of the Dawson seam. It is also present, though in lesser amounts, in the mineral matter of several samples from the northern part of the Moura mine area. Phosphorus is an essential component of modern plant tissue, but appears to break down to orthophosphate and be rendered potentially mobile during the decay associated with peat formation (Swain, 1970). Apatite in Australian coal is common as an infilling of cell cavities (Cook, 1962; Kemezys and Taylor, 1964) and thus probably represents an early reprecipitation of the material released under favourable conditions within the peat deposit. The apatite in both Baralaba seams is apparently very fine grained, since it also forms a minor contaminant of the clay ( < 2 #m) fraction isolated for oriented-aggregate study. The reason for its higher concentration in the mineral matter of the coals from the northern part of the field is not clear, but may be related in part to the lower degree of detrital influx in that part of the coal swamp succession. 7.10 Mineral artefacts Traces of bassanite (CaSO4.21H20 ) and potash alum (KAI (SO4) 2" 12H20 ) are present in the plasma ash of many of the coals from the Moura mine area. As indicated by Miller and Given (1978) and Ward (1991), these may represent compounds formed from interaction between organically associated calcium, or

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possibly Ca 2+ ions in the pore water, and the organic sulphur of the coal during the plasma ashing process. However, the occurrence of these materials, or at least of the bassanite, in bituminous coal samples may also reflect the dehydration during ashing of gypsum. Gypsum is itself produced by the interaction of sulphuric acid, derived from pyrite oxidation during storage, with calcium-bearing minerals such as calcite that are abundant in these particular coal samples (of. Ward, 1977 ).

8. Discussion

8.1 Detrital influence From a mineralogical point of view, the coal seams in the Moura mine area can be separated into two groups: (a) the seams with an abundance of illite and/or interstratified illite/smectite in the clay fraction and a lesser but still significant amount of kaolinite; (b) the seams with a clay fraction dominated by well-ordered kaolinite, and with only minor amounts of the other clay minerals. Group a includes the seams at the top and bottom of the sequence (seam A and seam D), where a transition might be expected, in gross terms, between coalforming and non coal-forming depositional conditions. These seams probably represent coal formed with a high rate of detrital mineral influx into the original peat swamp, or at least under conditions where the rate of detrital siliciclastic influx was greater than the rate of biogenic leaching or authigenic precipitation that led to well-ordered kaolinite formation. Quartz and poorly ordered kaolinite are most abundant in the D seam (which is of group a mineralogy) at the bottom of the sequence and in the southern (proximal) part of the D upper horizon. This feature has also been noted in individual seams from other areas by Ward ( 1989 ) and is consistent with high rates ofdetrital input to the depositional environment as peat-forming conditions were becoming established. It is notable that illite is only present in the A seam and the probably correlative Nipan No. 3 seam of the Moura district. This distribution appears to be independent of rank and suggests a detrital origin from a particular type of source materalal. Illite (ammonium illite) is also abundant in the Dawson seam of the Baralaba area, immediately below the Doubtful seam, which is suggested by Svenson et al. ( 1975 ) as equivalent to the A seam horizon. It is not present to a significant extent in the basal seam of either area, the D seam or the Coolum, despite the abundance in both cases of chlorite and interstratified clay components. Coals with group b mineralogy, by contrast, are represented by those seams in the middle of the Moura sequence, the B, B Lower, C and part of the D Upper seam. According to the model presented by Mallett ( 1985 ), widespread and long lasting peat swamps would have been well established as these seams were accumulating, with only short-lived and localised influxes of siliciclastic sediment to

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form the actual interseam strata. Reduced amounts of siliciclastic sediment would therefore have been available to penetrate into the swampy areas at this time, with the abundant organic growth providing more opportunity for the sediment that did penetrate to be altered by the biogenic processes of the peat-forming depositional system.

8.2 Changes with rank advance A transition through progressively more deeply buried sediments, from clays rich in smectite and kaolinite through a series of increasingly illite-rich interstratiffed I/S phases to an assemblage dominated by illite and chlorite, has been described in a number of settings by numerous workers (e.g., Dunoyer de Segonzac, 1970; Hower et al., 1976; Srodon, 1979; Pevear et al., 1980; Chamley, 1989). This transition is ascribed from several lines of evidence to late-stage diagenesis at elevated temperatures, representing essentially the same process as that responsible for coal rank advance. As noted above, Kisch ( 1966; 1968 ) has drawn attention to similar mineralogical differences in sediments associated with some of the high- and low-rank coals in the Moura-Baralaba region and suggested similar metamorphic processes to explain their origin. In particular, he indicates that kaolinite persists as the predominant clay mineral up to low-volatile bituminous rank (89% carbon and 16-17% volatile matter, corresponding approximately to R~ax of ~ 1.1%; Davis, 1978); by slightly higher rank levels (90-91% carbon and 12-15% VM, or Rvm~ of 1.3%) chlorite and illite occur exclusively to the exclusion of kaolinite. With the exception of the differences in the nature of the regularly interstratifled clay component, and the development of NH~- illite in the high-rank materJal, none of these differences have been observed from the present study in the minerals associated with the actual coal itself. Although the mineral appears to change from K-illite to NH4-illite with rank advance, the distribution of illite in the coals of the sequence appears from the samples studied to be controlled more by stratigraphic factors than metamorphic processes. Chlorite is present in coals from the lowest to the highest rank levels in the study area and does not appear from XRD evidence to change in character across this range. Kaolinite, moreover, is present in abundance in the coals throughout the section studied, and shows no indication of destruction even in the high-rank Baralaba coal seams. Srodon (1979) found that chlorite in the sediments associated with coals in the Silesian Basin is partly of detrital and partly of authigenic origin. Its distribution is controlled in part of the section by stratigraphic factors, regardless of rank changes, but outside this interval it appears to develop authigenically from bentonitic material under conditions that correspond to a vitrinite reflectance of about 1.7%. Although the XRD powder patterns from the present study (Fig. 3 ) show no particular changes in chlorite abundance across this rank range, the oriented-aggregate traces (Fig. 4) show stronger peaks at 3.54A, attributable to chlorite, in the mineral matter from the two Baralaba coal seams. This probably reflects an increased proportion of fine-grained chlorite in the higher-rank material,

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a feature which may, but need not necessarily be, the result of additional chlorite generation in the Baralaba coal by similar metamorphic processes. In addition to the above, Srodon (1979) notes a decrease in kaolinite content of the Silesian shales above a vitrinite reflectance of 1.2%. A similar decrease, however, has not been identified by the present study. Srodon (1979) also notes a progressive increase in the proportion of illite in the interstratified I/S component, with a change in the nature of the ordering accompanying this at an Rvm~ of 0.85-0.9%. Reliable identification of illite:smectite ratios, and hence recognition of such changes in the coals of the Moura area, is affected in the present study by the presence of chlorite and illite as separate phases along with the interstratified material, as well as by the effects of peat swamp alteration on the I/S mineral in the main part of the coal deposit. Regular ordering is most apparent in the D seam samples (e.g., Sample 26 in Fig. 4), which in this particular instance have the lowest rank level (Rvn~ = 0.77% ), although some higher rank samples (e.g., Sample 2) also show a well-ordered superlattice structure. Regular ordering is again apparent in the interstratified material of the Baralaba coals but, as noted in the text, the minerals represented appear to be of a slightly different type. An I: S ratio of approximately 1 : 1, giving rise to a superlattice structure in the clay, thus appears to be developed at both ends of the coal rank range in the Moura-Baralaba area. Pevear et al. (1980) note the development of a regular I/S component (Krectorite), associated with well-ordered kaolinite, in a volcaniclastic-rich coalbearing sequence in British Columbia at a vitrinite reflectance level of around 0.70%. The I/S in that area is developed from smectite, which is present instead of the I/S component in sediments associated with coals having Rvn= values down to 0.6%. As noted by Seedsman (1985), if this transition also occurred in the Moura coals, it would have taken place before even the lowest rank coal in the study area reached its present rank level. However, the occurrence of smectite in coals with R~=a~ values of around 0.9% in the Newcastle area of the Sydney Basin (Ward, 1989; Ward et al., 1989) suggests that such a change is not necessarily associated with coal with that particular level of rank advance. The progressive development of more illite-rich I/S with rank advance depends in part on the supply of K + ions to the sediment undergoing metamorphism (Hower et al., 1976). In a study of thermal metamorphism of a smectitebearing seam in the Sydney Basin, Ward et al. (1989) noted that, despite metamorphism to an Rvmax of over 4.0%, illite was only generated where additional K + could be introduced to the coal at the actual contact with the invading dyke rock. For this reason, it might be expected that the minerals in coals with relatively low proportions of total mineral matter, such as those of the present sample suite, will behave essentially as a closed system under metamorphism, preserving for the most part the assemblage developed in the original peat deposit. Mineralogical changes developed by thermal diagenesis in other sediments are, therefore, not necessarily repeated in the mineral matter of the actual coal seams.

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9. Conclusions Despite the change in rank from high-volatile bituminous coal to semi-anthracite across the study area, the assemblages of clay minerals in the coals of the Moura-Baralaba district were controlled largely by a combination of depositional input from the adjacent sediment source area and early diagenesis processes in the original peat swamps. The only notable mineralogical change associated with rank advance appears to have been the development of an ammonium illite, along with some additional chlorite, in the semi-anthracite coals of the Baralaba area. Detrital input produced a relative abundance of illite, interstratified iUite/ smectite and chlorite, along with poorly ordered kaolinite, in the lowermost seams of the sequence as the peat swamps were being established, and again in the uppermost seams as the long-lived and extensive peat-forming environment was progressively overwhelmed by other depositional conditions. Quartz was introduced into the coal-forming system along with this clay during deposition of the lowermost coal seams, particularly near the regional sediment source area. A shortlived but extensive input of illite-rich sediment also occurred, apparently from detrital sources, at the A seam horizon near the top of the coal-bearing sequence. The middle seams in the Moura succession are dominated by well-ordered kaolinite, with only minor proportions of other clay minerals. The kaolinite in this instance is thought to be essentially authigenic. It may represent either precipitation from the peat waters in the virtual absence of siliciclastic input, or it may represent biogenic alteration of the other clay minerals that penetrated into what was then an extensive and well established peat swamp area. The NH4-iUite in the Baralaba semi-anthracites was probably formed by interaction of organic matter and K-iUite in the course of rank advance, particularly since abundant K-illite is found in approximately correlative beds at lower rank levels. Isolation of the minerals within the coal macerals is thought to have prevented access of elements that might otherwise have reacted with the other detrital and authigenic clays to form additional illite and chlorite, at the expense of illite/smectite and kaolinite, at the temperatures associated with semi-anthracite production.

Acknowledgements The work described in this paper was supported by a grant from the Australian Research Council. Thanks are expressed to Grant Quinn, BHP-Utah Coal Limited and Allied Queensland Coalfields Limited for provision of samples, and to David Coffey of the Queensland Department of Minerals and Energy for information on the Baralaba district. Thanks are also expressed to Harold Read for the vitrinite reflectance determinations and to Irene Wainwright and Jaine Steer for assistance with the analytical program. Constructive comments, which greatly

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improved the manuscript, were provided by Professor D.A. Spears, Dr J.W. Beeston and Dr R.C. Neavel.

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