Geochemical and mineralogical changes in a coal seam due to contact metamorphism, Sydney Basin, New South Wales, Australia

International Journal of Coal Geology, 11 (1989) 105-125

105

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

G e o c h e m i c a l and m i n e r a l o g i c a l c h a n g e s in a coal seam due to contact m e t a m o r p h i s m , S y d n e y Basin, N e w S o u t h Wales, A u s t r a l i a COLIN R. WARD 1, PETER R. WARBROOKE 2 and F. IVOR ROBERTS 1

~Department of Applied Geology, University of New South Wales, P.O. Box I, Kensington 2033, N.S. W., Australia -'BHP Collieries Division, P.O. BOX 171, Belmont 2280, N.S. W., Australia (Received May 3, 1988; revised and accepted November 28, 1988)

ABSTRACT Ward, C.R., Warbrooke, P.R. and Roberts, F.I., 1989. Geochemical and mineralogical changes in a coal seam due to contact metamorphism, Sydney Basin, New South Wales, Australia. Int. J. Coal Geol., 11: 105-125. The mineralogical changes in a bituminous coal seam, thermally altered by a small basaltic intrusion, have been investigated using electronic low-temperature (oxygen plasma) ashing, Xray diffraction and other laboratory techniques. An assemblage dominated by montmorillonite and well-crystallized kaolinite, in the area away from the intrusion, passes laterally to one dominated by illite and poorly crystallized kaolinite, along with abundant dolomite and calcite, at the actual contact point. Geochemical considerations of both the mineral matter and the igneous rock suggest that the changes in the clay fraction were produced by heating to temperatures of between 400 and 650 ° C for a relatively short period, coupled with migration of potassium from the intrusive body into the coal seam. Heating at lower temperatures, although causing an increase in vitrinite reflectance, does not appear to have had a particularly significant effect at all on the clay minerals in the coal seam.

INTRODUCTION

The dramatic changes developed at or close to the contact between coal and relatively small igneous intrusions have been the subject of a number of studies over the years, and many of the features associated with such intrusions into coal seams are described by authors such as Raistrick and Marshall (1939), Stach et al. (1982) and Ward (1984). Even a narrow dyke can drive off the volatile matter and transform the coal into a natural coke or "cinder". Less intense heating, further away from the igneous body, can still give rise to an area of partly devolatilized b u t nonporous "heat-affected coal", while the in-

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© 1989 Elsevier Science Publishers B.V.

106

rusion itself' may also be altered due to interaction of the magma with the coal and its escaping volatile components. Studies of intruded coal have dealt mainly with the thermal effects observed in the organic components (e.g., ,Johnson et al., 1963; Schapiro and Gray, 1966: Kisch and Taylor, 1966), or, in some cases with the effect of the coal on the invading magma (Hamilton, 1968). Studies have also considered the effects on inorganic components such as pyrite (e.g., Stach, 1952), or the generation of' carbonate minerals from the interaction of coal and magma, but little attention has been paid to the effect of igneous intrusions on the clay mineral component.s, which constitute the bulk of the inorganic fraction in many coal seams. STUDY AREA

The present study is concerned with the effects of an analcite dolerite (or teschenite ) dyke intruding the Victoria Tunnel seam of the Permian Newcastle Coal Measures in the northeastern part of the Sydney Basin, New South Wales, Australia. The dyke is exposed in the underground workings of John Darling Colliery at Belmont, approximately 20 km south of Newcastle (Fig. 1 ). It is 3.25 m wide where it intersects the coal seam. The dyke is relatively fresh at the centre, forming a dark green-coloured,

[Hun,e .,ver N Pacific

Oc~n

,o

5

10

!

I

km

Fig. 1. Locality map showing intrusion site.

107 visibly crystalline rock in hand specimen. In thin section it contains essentially unaltered feldspar laths up to 1 mm or so in length, along with prismatic crystals of pyroxene, now replaced mainly by biotite, chlorite and a montmorillonite/chlorite mixed-layer clay mineral. The interstitial areas are dominated by analcite, with fine apatite needles also present. Calcite and a small amount of sericitic mica occur as alteration products, and anatase and iron oxide minerals are distributed throughout the rock mass. At the edge of the intrusion, near the contact with the coal seam, the igneous rock is finer grained and altered to a pale brown colour. Remnants of the original igneous texture can still be identified in thin section, with partly altered plagioclase laths up to 0.5 mm long, and larger, often euhedral phenocrysts, apparently originally of olivine but now replaced by a rim of brown-coloured siderite and a central mosaic of quartz and chalcedony. The majority of the rock, however, consists of an intimate admixture of fine dolomite, siderite and a montmorillonite/chlorite mixed-layer clay mineral similar to that in the unaltered rock mass. Apart from the alteration, the dyke and its chilled margin resemble, very closely, several of the Mesozoic analcite dolerites (teschenites) described by Joplin (1964) at other localities in the Sydney and adjoining Gunnedah Basins. THERMAL EFFECTS IN THE COALSEAM The coal seam is around 3 m thick in the study area. Like the seams around most other dykes in the Newcastle district (Warbrooke, 1985 ), the coal on each side of the intrusion can be divided into the following zones: (a) a zone of dull, massive cinder, close to the contact, with numerous white carbonate veins; (b) a zone of visibly banded cinder, also cut by carbonate veins, with remnants of the original lithotype stratification somewhat folded but still vaguely discernible at a megascopic scale; (c) a zone of heat-affected coal, friable and with deformed stratification close to the cindered zone but without the porosity or other microscopic and macroscopic signs of coking; (d) the surrounding unaffected coal of the Victoria Tunnel seam. Figure 2 shows the distribution of the different zones and the location of the samples taken, from the same ply or horizon with the seam in each case, for the present project. A summary of the analytical properties of the different materials, including the mean maximum vitrinite reflectance, is given in Table 1. The Victoria Tunnel seam in the area studied, outside the influence of the intrusion, is a vitrinie-rich coal with a mean maximum vitrinite reflectance (/~v max) of 0.91%. It has 36.7% volatile matter (dmmf), a little over 15% ash, 0.35% total sulphur (mostly organic) and crucible swelling number of 5. In

1

....

2

til

]

w 02

0

metres


z

o

>

0

0

I:D

¥ c

/

Dirt Band

"j°(

/l\

Dirt B a n d 10 metres omitted

o~ ¢o =3 o

Lo o

ko ~ gm~ o

o

g

o

Fig. 2. Schematic cross section of intrusion showing sample locations. TABLE l Analytical properties of coal samples Sample No.

Distance from dyke edge

Macroscopic description

/~r.ax {% )

(%)

(m) 10907 10906 10905 10585 10586 10587

Dyke centre Dyke edges 0.00 0.35 1.50 2.00

10752

2.25

10588

4.00

10589

26.50

Mineral

matter

Fresh dyke Altered dyke Massive cinder Massive cinder Banded cinder Heat affected coal Heat affected coal Unaffected coal Unaffected coal

Ash (% )

Carbonate CO2

sulphur

3.2 12.9 8.5 4.7 0.68 1.21

O.22 0.49 0.33 0.37 0.47 0.39

(%)

Total

(%)

5.50* 4.30* 2.24* 1.27

47.8 36.4 23.4 16.1

92.1 85.1 40.3 24.8 15.8 15.1

1.02

15.8

15.3

0.12

0.38

0.93

14.1

15.3

0.20

0.34

0.90

17.1

14.2

0.16

0.35

* Semi-coke reflectance

common with other seams from that part of the Newcastle Coal Measures, it is used chiefly as a raw material for metallurgical coke production. Volatile matter and total hydrogen (dmmf) decrease towards the dyke, while ash, total carbon and vitrinite reflectance all steadily increase. Fourier transform infrared spectrometry and carbon-13 NMR spectroscopy (Warbrooke,

109

1985) also show a decrease in alphatic hydrocarbon compounds and an increase in aromaticity as the intrusion is approached. Under the microscope, the material from the heat-affected zone appears to be similar to the coal from the unaffected part of the seam, although the reflectance of the macerals is slightly higher. The material in the cinder zones, however, has a compact coke structure with small pores and thick cell walls that display a fine to medium coke mosaic texture {average domain size 1-2 /xm). This is quite different to the appearance of coke produced artificially in a coke oven, which has large pores supported by thin cell walls showing a very fine, almost isotropic mosaic pattern. The volatile matter and ultimate analysis characteristics of the cindered material indicate that it is a semi-coke, rather than a completely carbonized product. The sequence of metamorphic zones is probably the result of a heat front migrating slowly outwards from the igneous body (Warbrooke, 1985 ). At the outer edge of this heat front the coal became plastic, then as the hotter zones approached it passed through successive phases of fluidity and resolidification analagous to those of an industrial coking operation. Lack of a ready escape for the volatile matter liberated in the process was probably responsible for the deformation of the stratification in the cindered and heat-affected zones, and also, since the material had no space in which to expand, for the dense, compact nature of the coke in the cinder zone. The fact that the coal actually undergoing the coking process had already been significantly preheated, together with the constriction on escape of the volatile components, was probably also responsible in part for the coarser mosaic texture compared to cokes produced under artificial conditions. The reflectance of the coke mosaic material in cindered coal depends on several factors, including the rank of the coal before carbonization, the pressure associated with the seam's depth of burial at the time of carbonization and the temperature at which the carbonization process took place (Stach et al., 1982 ). The final coke temperature and the heating rate may also be important factors. Although the results are not necessarily directly applicable, the maximum reflectance of vitrinite derivatives in the heat-affected and cindered coals of the study area was compared to that of the mosaic material in cokes produced in the laboratory at different temperatures (Chandra, 1965) from coal of similar rank to the unaffected parts of the seam. This comparison (Fig. 3) indicates that cinder development began at a temperature of between 370 and 460 ° C, and that the maximum temperature attained at the contact was in the vicinity of 650 °C. A similar comparison to the work of Johnson et al. (1963), however, involving coal originally of somewhat lower rank, suggests that these temperatures may have been slightly higher, rising up to 750 ° C at the contact point. Kisch and Taylor (1966) indicate that coke mosaic texture begins to form at about 470 ° C, a temperature consistent with its appearance in the cindered

110 8.0 Hoger seam Ashton, Mass. (after Chandra, 1965)

6.0 ~-

/

E

5.50

--

CONTACT

$ I

4.30 c

4.0

--

° CINDER

°L E

i-

x

:~

2.24

2.0

0

200

400

600

800

1.27

~HEAT}..AFFECTED

lO2

JCOAL

1000

Temperature (°C)

Fig. 3. Comparison between vitrinite reflectance in heat-affected and cindered coal of the Victoria Tunnel seam and the reflectance of vitrinite or vitrinite derivatives in coke produced from a coal of similar rank at different temperatures. After Chandra (1965).

coal. Giesler plastometer tests for the Victoria Tunnel seam (Edwards, 1975) typically show an initial softening temperature of 410 ° C, with maximum fluidity developed at 445 °C and resolidification at 465 ° C, temperatures slightly lower but still consistent with the behaviour of the intruded seam. The temperature inferred from Fig. 3 to have been developed at the contact is less than the temperature of 760°C suggested by Kisch and Taylor (1966) at the base of a sill intruding a coal seam in the Bowen Basin of Queensland. Even if the higher temperatures indicated from the work of Johnson et al. (1963) are taken, however, it is still considerably less than that expected to have been present in the magma itself. While there is the possibility that the chilled margin of the dyke provided some degree of insulation between the body of the intrusion and the coal seam, the rate of heat flow in coal is only around 2 cm per hour at 1000 ° C, even when crushed and placed in a commercial coke oven. A mass balance of the cindered material compared to the unaltered coal of the seam shows that the volatile component has been removed, and not simply

111 carbonized with the solid fraction. Detailed examination of the seam and adjacent strata near the dyke, however, show no sign of any remnants of liquid residue, and this suggests that the bulk of the volatiles released form the coal escaped back into the magma itself. Although rare, vesicles filled with oil and tarry products have been found to occur within the igneous intrusion. Analysis by Fourier transform infrared and NMR spectroscopy shows these materials to be rich in aliphatic hydrocarbons, resembling petroleum liquids rather than carbonization products from a coke oven. Warbrooke (1985) suggests that the presence of these materials indicates slow heating rates, together with a natural hydrogenation process, under pressure, before the volatiles escaped. MINERALOGICALINVESTIGATIONS The mineral matter was isolated from the coal samples, at a temperature a little over 100 ° C, using an electronically excited, oxygen-plasma ashing technique (Gluskoter, 1965). The mineralogy of these oxidation residues, along with that of pulverized samples of the igneous rocks, was then investigated by X-ray diffraction methods. As well as the bulk mineralogy of the respective materials, the clay mineralogy of each sample was investigated using oriented aggregates of the < 2Bm fraction. Diffractograms of key samples in each case are given in Fig. 4 and 5. The clay fraction of several samples was also examined as dispersed particles on a metal grid under the transmission electron microscope, and chemical analysis of the igneous materials and the mineral matter, calcined at 1000 ° C to prevent problems with residual carbon in sample preparation, was performed using X-ray fluorescence techniques.

Mineralogical analysis In the area away from the coke, and even within the outer part of the heataffected zone, the coal contains a mineral assemblage made up almost entirely of quartz, kaolinite and a montmorillonitic mixed-layer clay mineral (Fig. 4 ). The sharpness of the (001) peaks on the diffractograms and the presence of fairly well resolved hkl reflection peaks between 4.46 A and 4.13 A in these samples (Hinckley, 1963) indicate that, like most other bituminous coals in eastern Australia (Ward, 1978), at least some of the kaolinite is well crystallized. Much of this material is believed to be essentially authigenic in origin (Ward, 1986 ). Oriented aggregate study of the mixed-layer material in these coals {Fig. 5) shows that it has a basal (001) lattice spacing of between 13 and 15A in a 50% humidity atmosphere, but swells to give a well-defined XRD peak at 17A on saturation with ethylene glycol. The lattice does not collapse readily on heat-

112

10905

D

IML /

Clay

10586

ML

K

I~/K

30

Cobalt K-alpha Radiation

10587 Mo

20

10

Degrees 2 theta

Fig. 4. X-ray diffractograms of mineral matter from selected coal samples. C = calcite; D = dolomite; F-=-feldspar; I = illite; K = kaolinite; M o = montmorillonite; M L = mixed-layer clays; Q = quartz; S = siderite ing, a n d even a f t e r 30 m i n u t e s at 400 ° C a b r o a d d i f f r a c t i o n b a n d at a r o u n d 1213A is still p r e s e n t . H e a t i n g to 5 0 0 ° C for an additional 30 m i n u t e s , however, generally causes t h e bulk of the m a t e r i a l to collapse a n d generate a b r o a d 10A diffraction peak. Because of the resistance of t h e s t r u c t u r e to h e a t i n g it is suggested t h a t t h e m i x e d - l a y e r m a t e r i a l r e p r e s e n t s a m o n t m o r i l l o n i t e with a small a m o u n t of r a n d o m l y i n t e r s t r a t i f i e d chlorite. A l t h o u g h m o r e readily collapsed m o n t m o rillonites are p r e s e n t in o t h e r p a r t s of t h e sequence ( H o l m e s , 1983), a p p a r -

113

Glycol

Atmosphere

400°C

Heated

Saturated

50% Humidity

7.15

7.15

A |

10.1

10.111.1

J

,,5,

i

I

I

1o

I

i

i

i

15

i

Cobalt K - a l p h a

I

Radiation

Degrees

i 2

.

,

,

10 I

•

|

I

I

5 I

I

theta

Fig. 5. Oriented aggregateX-ray diffractogramsof the cla~,fraction ( < 2#m) for mineral matter samples shown in Fig. 4. Numbers indicate d spacings in AngstrSmunits. ently of pyroclastic origin, this heat-resistant form is typical of the clay fraction found in the coals themselves (Ward, in press). It is probably the result of interaction between montmorillonite of more normal character and the organic m a t t e r or the waters of the original peat swamp. Closer to the intrusion, however, the nature of the mineral matter in the coal is distinctly different. The kaolinite crystallinity decreases markedly (Table 2 ), and indeed the hkl peaks cannot be resolved at all near the actual contact point. This is interpreted as representing heating to a temperature where the

114 TABLE '2 Kaolinite crystallinity in coal mineral matter Sample no.

Distance from dyke (m)

Kaolinite crystallinity index ( Hinckley, 1963 )

10905 10585 10586 10587 10752 10588 10589

0 0.35 1.50 2.00 2.25 4.00 26.5

hkl peaks not resolved hkI peaks not resolved 0.41 0.63 0.81 0.81 0.74

crystal structure of kaolinite is disrupted, followed by rehydration under conditions that permitted only poorly crystallized kaolinite (or possibly metahalloysite) to form. The montmorillonitic clay mineral also shows significant changes between this point and the contact surface. As shown in Fig. 5, its (001) XRD peak under air-dried and glycol-saturated conditions is much less intense, and, together with the pattern after heating at 400 ° C, changes progressively in character with increasing metamorphism. At 1.5 m from the dyke, for example, the (001) spacing, although still 12.5 A under air-dried conditions, expands to only 13.2 A with glycol. Much of this material, like the expandable clay in the unaffected coal, still retains a basal spacing of around 12 A after heating at 400 ° C, and only collapses fully to 10 A on heating at higher temperatures. At the contact itself, and to a lesser extent in the sample 0.35 m from the contact, a marked XRD peak at 10 ]k is developed, even under glycol saturation, suggesting the presence of a significant free mica or illite phase. There is, of course, still some expandable material in these samples, but it forms only a broad diffraction zone, particularly in the contact sample, and mostly collapses at 400 °C to a 10 A layered structure.

Infrared analysis Variations in the samples are also indicated by infra-red analysis (Fig. 6) and these variations tend to confirm the X-ray diffraction data. For the wave number range of 4000-3000 cm-1, the sample at the contact with the dyke, as well as the samples 0.35 m and 1.5 m from it, are characterized by absorption bands typical of a mixture of kaolinite and an illitic component. The highintensity band at 3624 cm-1, in particular, is indicative of an illite-type mineral. For the samples taken at distances 2.0 m or more from the dyke, the absorption spectrum is typical of that from a mixture of montmorillonite and kaolinite, resembling a fire clay as depicted by van der Marel and Beutel-

115

10905

,

/

~-~--

f/

t~

10587 /

/ J 3624

35,~1 13624 4000 !

37003625 I I

3400 I

WaveNumber(cm-l~ Fig. 6. Infrared absorption spectra for mineral matter samples. spacher ( 1976 ). The bands at 3624, 3654, 3669 and 3696 c m - 1 in these samples can all be attributed to kaolinite, and the bands at 3400 and 3220 cm-1 to montmorillonite. It appears that the strong band at 3400 c m - 1 (which can be assigned to water molecules ) decreases in relative intensity towards the dyke (Fig. 6). This prob-

2.49

44.78

14.03

16.10 16.83 9.22 5.59 11.01 12.39

AI.,O:~

13.98

16.91 10.80 3.51 2.86 1.22 1.39

Fe~O:~ (Total)

(a) Black Jack Sill, G u n n e d a h (Joplin, 1964).

2.40 2.37 0.28 0.25 0.50 0.50

40.17 38.23 39.26 44.41 54.62 67.09

10907 10906 10905 10585 10586 10587 Teschenite (a)

TiO,2

SiO~

Sample No.

0.14

0.06 0.17 0.10 0.08 0.02 0.02

MnO

9.57

7.45 5.14 6.80 4.32 0.58 0.50

MgO

Chemical composition of dyke rocks a n d coal mineral m a t t e r (wt.%)

TABLE 3

8.12

3.95 6.27 16.18 16.89 3.66 0.38

CaO

3.30

3.22 2.97 1.54 0.97 0.99 1.18

Na._~O

1.77

1.07 0.63 1.09 0.50 0.75 0.64

K,O

0.08 0.37 0.33 0.59 0.44 0.04 -

0.89 0.96 0.02 0.04 0.04 0.07 0.62

P._,O:,

2.19

7.95 t5.48 23.28 23.98 26.64 16.18

100.99

100.25 100.22 101.61 100.48 100.47 100.38

117 ably corresponds to a reduction in abundance of the hydrated mineral component, and thus confirms the transformation of montmorillonite to an illitic component as the dyke is approached.

Electron microscopy Three distinctly different types of material can be recognized when the clay fraction of the coal mineral matter is examined under the transmission electron microscope: (a) hexagonal, plate-like euhedral crystals of kaolinite, up to around 2 ttm in diameter, (b) irregular, wispy particles of montmorillonitic clay, individually up to about 1 ~m in size; and (c) polycrystalline aggregates up to 0.5 ttm in diameter, with a concentric rosette-like structure, closely resembling material identified by Sudo et al. (1981) as spherulitic halloysite or halloysite-allophane masses. The hexagonal kaolinite crystals are probably of authigenic origin, while the montmorillonitic clay, as indicated above, is thought to have been originally introduced to the peat bed as pyroclastic material. The halloysite rosettes have not, however, been previously described in coal seams. Spherulitic halloysite at several localities in Japan is described by Sudo et al. ( 1981 ) as having been formed from alteration of pumice fragments in soil or similar deposits, and such an explanation also appears to be consistent with their occurrence in the Victoria Tunnel seam. Despite the differences in internal structure as revealed by X-ray diffraction, electron microscope studies of the clay fraction from the heat-affected and cindered coals show that the different components retain the same external form as their counterparts in the unaffected portion of the coal seam. Even the halloysite rosettes still retain their intricate polycrystalline structure, up to the actual contact with the igneous body. GEOCHEMISTRY The chemical composition of the mineral matter for each coal sample, as well as of the dyke materials, is given in Table 3. A graphic representation of the relative abundance of each element across the contact zone, based on these data, is also given in Fig. 7. The dyke itself has a very similar composition to analcite dolerites reported elsewhere in the Sydney and Gunnedah Basins (Joplin, 1964), except for a notably low calcium content (Table 3). Aluminium and iron, on the other hand, are slightly more abundant in the fresh dyke of the present study. With the exception of silica, which is by far the most abundant of the coal's inorganic components, the elements studied are all more abundant in the (rel-

118 DYKE

COAL

y"

ToI

,/

25

60~

!

0

~'2o

5o.°f¢) _ E o

q

E

*0

q 15

c E

~ 10 o o u

l

°I ~

5

EE o_

~.5 2E a 2

TiO2

10907

10906

10905

10585

~

10586

0

~

~

•

10587

Fig. 7. Graphical representations of chemical variation across the intrusion and contact zone from data in Table 3.

atively) fresh dyke rock than in the mineral fraction of the unaltered coal seam. The two massive cinder samples, however, have high proportions of CaO and MgO, as well as substantially more FeeO3, apparently due to the abundant carbonates infilling the pore spaces. A certain amount of carbonate (siderite

119

and dolomite) is also present in the altered chilled margin of the dyke rock, while calcite is present, to some extent, in the fresher central portion. Calculations based on the carbonate C02 percentages for the coal samples (Table 1) confirm that most of the additional CaO, MgO and Fe203 in the heat-affected and cindered coals can be explained by occurrence in the carbonate form. If the elements that make up these carbonates are discounted, simply by recalculating the analytical results without including CaO, MgO or Fe203, the different materials would have compositions as indicated in Table 4. A graphic plot of the element distribution across the contact zone in these circumstances is given in Fig. 8. Considered on a Ca-Fe-Mg free basis (interpreted as a "carbonate-free" basis) the chemical changes associated with the transition from dyke to coal are relatively sharp and well-defined. Except for the cindered material at the contact, which has intermediate properties, silica drops from between 80 and 85% in the coal to between 60 and 65% in the dyke rock, while alumina rises from between 10 and 15% to a little 25%. Sodium rises from around 2% in the coal to 5% in the dyke rock, and also has an intermediate value (3%) in the cindered coal at the contact point. Titanium (Ti02) and phosphorus (P2Q), on the other hand, show a sharp transition from low values in the coal to markedly higher values in the igneous body, with no such intermediate values. These trends can be explained by a limited amount of contamination due to incorporation of igneous material in the cinder at the contact point. The contamination was probably confined to the late-forming fraction of the magma, particularly the interstitial fluids rich in sodium-bearing, silica deficient analcite. Titanium and phosphorus were less mobile, however, probably due to their fixation in earlier-formed magmatic minerals, notably titanium-bearing augite, anatase and apatite. Potassium (K20) shows a more complex pattern of distribution, rising from a constant 1% in the coal to slightly more than 2% (Ca-Fe-Mg or "carbonate" free ) in the cinder of the contact zone, then declining to 1% in the altered dyke TABLE4 Chemical composition of calcined dyke rocks and mineral matter recalculated without CaO, Fe203 or MgO Sample no.

Si02 (%)

Ti02 (%)

A1203 (%)

MnO (%)

Na20 (5)

K20 (%)

P20~ (%)

10907 10906 10905 10585 10586 10587

62.9 61.3 76.2 85.7 80.5 81.9

3.8 3.8 0.54 0.48 0.73 0.62

25.3 27.0 17.9 10.8 16.1 15.1

0.09 0.27 0.20 0.16 0.03 0.02

5.0 4.8 3.0 1.9 1.5 1.4

1.7 1.0 2.1 0.97 1.1 0.78

1.4 1.5 0.04 0.07 0.05 0.08

12()

COAL

DYKE •

....

siO~_. . . . . -i,

-i 80 ,~

,i/I/@/"

30

:t 4 •

E

70

~ .5

,

E

4

I

E

251

E

SiO 2

~60 b3

c a_

4

~-. 2 0 O

5O

°

;5 o_

o

~

15

g o_ E o o

g 10

g~ Na20 TiO2

~ K 2 0 10907

10906 10905

10585

N a 2 0

! 10586

10587

Fig. 8. Graphical representation of chemical variation across dyke and contact zone, recalculated to Ca-Fe-Mg free basis (Table 4).

rock before rising again to almost 2% in the fresh igneous material. This is thought, in part, to reflect its fixation in the illite of the coal nearest the contact. Its deficiency in the altered dyke also suggests that the K20 needed for illite formation was drawn in some way from the outer part of the igneous body. Calcium and magnesium, as noted above, are particularly concentrated in and around the contact zone. The rock in the central part of the dyke, on the other hand, is deficient in calcium relative to other analcite dolerites in the region, and thus it is suggested that the calcium occuring in the carbonates was released from the dyke in the course of feldspar alteration. The unaltered coal is very low in calcium, and this precludes, in this instance at least, derivation of the calcium in the carbonate from the coal itself. The percentage of magnesium in the fresh dyke, unlike that of calcium, is

121 roughly comparable to that in other local analcite dolerites. The total proportion of MgO drops in the chilled margin, however, even though abundant dolomite is present. If then rises in the cindered coal of the contact zone, but declines to a very low level in the heat-affected coal and the unaltered parts of the seam. This suggests that the magnesium that formed the carbonates was also derived from the magma, probably in this case from alteration of the mafic minerals in the chilled margin zone. The total iron content of the fresh dyke, expressed as Fe203, is slightly higher than that of other analcite dolerites. Even allowing for dilution by increased calcium, however, its proportion drops significantly in the chilled margin zone, a factor emphasized by the overall light colour of the material in hand specimen. Most of the iron in the chilled margin is probably present as siderite, since biotite is absent and the chlorite found in the fresh part of the dyke is only a minor component of the clay fraction in the altered zone. Volatiles formed on contact with the coal, or possibly solutions formed later in the alteration process, therefore appear to have leached both iron and magnesium from the igneous rock, and relocated them, along with calcium, as carbonates in and around the cindered part of the seam. Manganese, even on a Ca-Fe-Mg free basis (Table 4) is low throughout the dyke and the contact zone. It is, however, markedly higher in the altered dyke and massive cinder near the immediate contact point (Table 3) possibly reflecting an occurrence as part of the carbonate fraction of these materials. Like most of Australia's Permian coals, the Victoria Tunnel seam has a relatively low sulphur content. Most of the sulphur also occurs in organic form. Analysis of the coals and igneous rocks of the present project, prior to mineral matter separation or calcination (Table 1) shows relatively constant values for total sulphur of between 0.3 and 0.4%. Analysis of the mineral matter and the igenous rocks after calcination, however (Table 3), shows relatively low values ( < 0.1% ) in the central part of the dyke and the unaffected part of the seam, but high values (0.4-0.8%) across the bulk of the contact zone. This is thought to reflect at least some transformation of the sulphur to a less volatile form in the course of metamorphism, so that it was retained in the mineral matter or altered igneous rock rather than escaping with the volatile components. Such a process is analogous to the fixation of sulphur by calcium in the ash when coal is heat too rapidly in proximate analysis. Detailed study shows that sulphate sulphur is more abundant (about 0.07% ) relative to pyritic sulphur (0.01%) in the calcium-rich coals of the immediate contact area, and this is also consistent with such a fixation process. No calcium or other sulphate minerals were detected, however, in the respective oxidation residues. DISCUSSION The present study has shown that, like the organic components, the mineral matter of a coal can be significantly altered by the thermal effects of an igneous

122

body intruding into the seam. Although changes in vitrinite reflectance and coal chemistry occur on the outer fringes of the metamorphic aureole, the mineralogical changes, in this instance at least, appear to be confined to those parts oft:he seam where the temperature was raised sufficiently for the reactive macerals to be fluidized and semi-coke development take place. The introduction of carbonate minerals into the pore spaces of the coke was probably a later event, with many of the necessary cation components derived from the igneous body, rather than the inorganic fraction of the coal seam. Well-crystallized kaolinite, a common component of New South Wales coal seams, was broken down to a poorly crystalline material in association with this change, while montmorillonitic clay, typical of the Newcastle Coal Measures sequence, has been progressively altered to an interstratified illite-montmorillonite and finally, near the contact, to illite. Heating of kaolinite in the laboratory at a little over 400°C causes rapid dehydroxylation of the material with the resultant formation of an X-ray amorphous substance known as metakaolin (Roy and Osborn, 1954). This material, however, even if heated to as high as 850 ° C, can usually be reconstituted to kaolinite by relatively mild hydrothermal treatment (Roy and Brindley, 1955 ). Experiments by Hurst and Kunkle ( 1985 ) indicate that the products of the rehydroxylation process depend on factors such as temperature, water pressure, pH and silica content, with components such as pyrophyllite being formed rather than kaolinite at higher temperatures. Since the temperature of cinder development was apparently above that required for formation of metakaolin, it may be expected that the well-crystallized kaolinite in the coal was transformed to metakaolin in the course of the carbonization process. Interaction with water vapour on cooling, however, would be expected to result in the reformation of kaolinite. Experimental autoclave studies by Loughnan and Roberts ( 1981 ) suggest that well-crystallized kaolinite is normally formed by rehydration of metakaolin itself derived from wellcrystallized material, but that poorly crystallized kaolinite may be formed if the rehydration takes place at relatively low temperature. Loughnan and Craig (1960) and Loughnan and Roberts (1981) also describe the formation of fully hydrated halloysite, rather than kaolinite, by rehydration of metakaolin naturally produced by in-situ burning of an adjacent coal seam, and this may have been an intermediate stage of the kaolinite reforming process in cinder development as well. Heating of montmorillonite above 100-200 ° C is well known to result in loss of the interlayer water and collapse of the expandible lattice structure (Grim, 1968), ultimately forming a phyllosilicate with a d(001) spacing, depending on the interlayer cations, of 9.4-10.0 ii,. This basic structure appears to remain essentially intact until it is altered by mineral phase changes at 800-900°C, which in turn allow components such as mullite and cristobalite to form at even higher temperatures.

123 One explanation for the presence of a 10 A clay mineral in the massive cinder near the contact zone is, therefore, that it represents montmorillonite heated to temperatures where the interlayer water cannot be replaced on cooling. Such an explanation, however, is inconsistent with the increase in potassium content associated with the presence of this material in the contact zone. Experimental studies on appropriate mineral residues, moreover, indicate that the montmorillonite from the unaltered part of the seam, heated for a short time to 600 ° C, still retains the ability to adsorb water readily, even from the atmosphere, and display an expandible lattice structure once again on cooling. Many authors, including Weaver and Beck (1971), Hower et al. (1976) and Velde et al. (1986) have described the progressive alteration of montmorillonite through a range of mixed-layer clay minerals to illite with increasing temperature due to deep burial. Other workers, including Steiner (1968), Muffler and White (1969) and Browne and Ellis (1970) also describe similar phenomena at shallower depths under present-day hydrothermal conditions. Correlation with various indices of temperature, including vitrinite reflectance, indicates that illite development in these circumstances begins at around 200 ° C. As well as loss of interlayer water from the montmorillonite, however, the process of illite formation also appears to involve take-up of potassium made available from other parts of the sediment mass. Field evidence for potassium incorporation is discussed by Hower et al. (1976), while studies by Inoue (1983) show a strong tendency for montmorillonites to incorporate potassium in a nonexchangeable form, under laboratory conditions, at temperatures up to at least 300 ° C. Although the pattern of the mineralogical changes noted in the present study is similar to that developed in other metamorphosed or deeply buried sediments, correlation with vitrinite reflectance and other properties of the coal around the contact zone suggests that they did not become irreversible until significantly higher temperatures had been attained. For the final conversion of the montmorillonite to illite, the addition of potassium from an outside source also appears to have been involved. The most likely explanation for the differences in temperature requirements, at this stage, probably lies in the shorter time over which the coal was exposed to elevated temperatures in the contact zone, compared to that expected with burial of similar metamorphic processes. REFERENCES Browne, P.R.L. and Ellis, A.J., 1970. The Ohaki-Broadlandshydrothermalarea, New Zealand mineralogyand relatedgeochemistry.Am. J. Sci., 269: 97-131. Chandra, D., 1965. Use of reflectancein evaluatingtemperatureof carbonizedor thermallymetamorphosedcoal. Fuel (London),44: 1-176. Edwards,G.E., 1975. MarketableresourcesofAustralianCoal.In: A.C.Cook (Editor),Australian Black Coal. Australas. Inst. Min. Metall.,Wollongong,pp. 85-108. Gluskoter, H.J., 1965. Electroniclow temperatureashingof bituminous coal.Fuel, 44: 285-291. Grim, R.E., 1968. ClayMineralogy.McGraw-Hill,New York,NY, 596 pp.

124 Hamilton, L.H., 1968. Interaction of Coal and Magma. M. Sc. thesis, Univ. of New South Wales, (unpubl.). Hinckley, D.N., 1963. Variability in 'crystallinity' values among the kaolin deposits of the Coastal Plain of Georgia and South Carolina. Clays and Clay Minerals. 1lth Conf. Pergamon Press, New York, NY, pp. 229-23,5. Holmes, G.G., 1983. Bentonite and fullers earth in New South Wales. Mineral Resources 45, Geol. Surv. N.S.W., 138 pp. Hower, J. Eslinger, E.V., Hower, M.E. and Perry, E.A., 1976. Mechanism of burial metamorphism in argillaceous sediment: 1. Mineralogical and chemical evidence. Geol. Soc. Am. Bull., 87:725 737. Hurst, V.J. and Kunkle, A.C., 1985. Dehydroxylation, rehydration and stability of kaolinite. Clays Clay Miner., 33(1 ): 1 14. Inoue, A., 1983. Potassium fixation by clay minerals during hydrothermal treatment. Clays Clay Mineral., 31 (2): 81-91. Johnson, V.H., Gray, R.J. and Schapiro, N., 1963. Effect of igneous intrusives on the chemical, physical and optical properties of Somerset coal. Abstracts of Papers, 145th Meeting, Am. Chem. Soc., pp. 110-124. Joplin, G.A., 1964. A Petrography of Australian Igenous Rocks. Angus and Robertson, Sydney, 210 pp. Kisch, H.J. and Taylor, G.H., 1966. Metamorphism and alteration near an intrusive coal contact. Econ. Geol., 61: 343-361. Loughnan, F.C. and Craig, D.C., 1961. A complex interstratified clay mineral in the pottery shale form Marrangaroo, N.S.W. Aust. J. Sci., 23 (11): 379. Loughnan, F.C. and Roberts, F.I., 1981. The natural conversion of ordered kaolinite to halloysite (10A) at Burning Mountain near Wingen, New South Wales. Am. Mineral., 66: 997-1005. Muffler, L.P. and White, D.E., 1969. Active metamorphism of Upper Cenozoic sediments in the Salton Sea geothermal field and the Salton Through, southeastern California. Geol. Soc. Am. Bull., 80:157 182. Pevear, D.R., Williams, V.E. and Mustoe, G.E., 1980. Kaolinite, smectite and K-rectorite in bentonites: relation to coal rank at Tulameen, British Columbia. Clays Clay Miner., 28 (4): 241254. Raistrick, A. and Marshall, C.E., 1939. The Nature and Origin of Coal and Coal Seams. English Universities Press, London, 282 pp. Roy, R. and Brindley, G.W., 1955. Study of hydrothermal reconstitution of the kaolin minerals. Clay Minerals, 4th Nat. Conf. Nat. Acad. Sci., 456, pp. 125-132. Roy, R. and Osborn, E.F., 1954. The system alumina-silica-water. Am. Mineral., 39: 853-885. Schapiro, N. and Gray, R.J., 1966. Physical variations in highly metamorphosed Antarctic coals. Adv. Chem. Set. 55, Am. Chem. Soc., Washington D.C., pp. 196-217. Stach, E., 1952. Mikroskopie nattirlicher Kokse. In: H. Freund (Editor). Handbuch der Mikroskopie in der Technik. Umschau-Verlag, Frankfurt am Main, 2 ( 1 ): 411-422. Stach, E., Mackowsky, M.Th., Teichmtiller, M., Taylor, G.H., Chandra, D. and Teichmtiller, R., 1982. Stach's Textbook of Coal Petrology, 3rd Ed. Gebriider Borntr~iger, Stuttgart, 535 pp. Steiner, A., 1968. Clay minerals in hydrothermally altered rocks at Wairakei, New Zealand. Clays Clay Miner., 16: 193-213. Sudo, T., Shimoda, S., Yotsumoto, H. and Aita, S., 1981. Electron Micrographs of Clay Minerals. Elsevier, Amsterdam, 204 pp. Van Der Marel, H.W. and Beutelspacher, H., 1976. Atlas of Infra-red Spectroscopy of Clay Minerals and their Admixtures. Elsevier, Amsterdam, 396 pp. Velde, B., Suzuki, T. and Nicot, E., 1986. Pressure-temperature-composition of illite/smectite mixed-layer minerals: Niger Delta mudstones and other examples. Clays Clay Miner., 34 (4): 435-441.

125 Warbrooke, P.R., 1986. Thermal metamorphism of coal adjacent dykes in the Newcastle Coalfield. Abstr. 19th Syrup. on Advances in the Geologyof the Sydney Basin, Univ. of Newcastle, N.S.W., pp. 102-105. Ward, C.R., 1978. Mineral matter in Australian bituminous coals. Proc. Australas. Inst. Min. Metall., 267: 7-25. Ward, C.R., 1984. Coal Geology and Coal Technology. Blackwell Scientific Publications, Melbourne, 345 pp. Ward, C.R., 1986. Review of mineral matter in coal. Aust. Coal. Geol., 6: 87-110. Ward, C.R., in press. Distribution and origin of clay minerals in Australian bituminous coals. Proc. llth Int. Congr. on Carboniferous Stratigraphy and Geology,Beijing, Sept. 1987. Weaver, C.E. and Beck, K.C., 1971. Clay-water diagenesis during burial: how mud becomes gneiss. Geol. Soc. Am. Spec. Pap. 134.