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Quality variations in the high-sulphur lignite of the Neogene Beypazari Basin, Central Anatolia, Turkey
International
ELSEVIER
Journal ol
International Journal of Coal Geology 27 (1995) 131-151
Quality variations in the high-sulphur lignite of the Neogene Beypazm Basin, Central Anatolia, Turkey M.K.G. Whateley a, E. Tuncah b aDepartment of Geology, University of Leicester, University Road, Leicester, LEI 7RH, UK b Directorate of Mineral Research and Exploration (MTA), Ankara, Turkey
Received 9 May 1994; accepted 23 November 1994
Abstract During the Miocene a number of fault bounded basins developed in Central Anatolia, Turkey. One such basin at Caylrhan, near Beypazan, contains thick, laterally extensive lignite seams. This basin was filled initially with coarse elastic material. Upward fining of the elastics during basin fill, with an increase in the amount of clay and carbon content led to the development of relatively shallow limnic basins in which extensive peat deposits accumulated in low-lying mires. These lignites are characterized by their high sulphur and ash contents (up to 8.2% S and 68.3% ash on an air-dried basis). Studies of the sulphur and ash contents reveal three types of distribution; namely, vertical variation within individual seams, variation between the seams and lateral variation across the basin. In the case of the sulphur content, vertical variation within and between the seams is related to variations in the amount and source of the sulphate in the mire water, and the lateral variation is probably related to structural/topographic control of the mire at the time of formation. Variation in the ash content within the seams is probably the result of depositional processes at the time of mire development. Variation between seams is recognized due to the presence of ubiquitous zeolites in the mineral matter; namely heulandite in the first seam and analcime in the second seam. This is probably the result of variations in the chemistry of the original volcanoclastic or elastic material associated with the lignite or of variations in the chemistry of the circulating fluid. There is a broad east-west lateral variation across the basin in the ash content of the seams, probably resulting from variations in the amount and rate of elastic or volcanoclastic influx into the mire at the time of formation, related to structural/ topographic controls.
1. Introduction The large, Neogene basin centred on Beypazarl, lies approximately 100 km northwest of Ankara in Central Anatoiia (Fig. 1) . The basin is filled with alluvial, lacustrine and volcanosedimentary rocks and the sequence contains economic resources of lignite, bituminous shale and trona. 0 166-5 162/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved XSD10166-S162(94)00023-9
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M.K.G. Whateley, E. Tuncalt /International Journal of Coal Geology 27 (199.5) 131-151
‘.~_~.‘.’
:::.
.j
“”
\,I
ene / El
Teke volcanics
, olorvzoic
Fig. 1. Location of the Beypazarl basin, showing the position of the Qylrhan lignite field and the main rock units in the region (from Whateley and Tuncah, 1995; modified after Ya@urlu et al., 1988).
The Beypazarr basin was first investigated by Kalafatcioglu and Uysalli ( 1964) who studied the stratigraphy, sedimentology and tectonics of the basin and the adjacent area. This was followed with studies by Altinli ( 1977), Saner ( 1979) and Tune ( 1980), who further refined the stratigraphic, sedimentological and tectonic models of the basin. The economic importance of the lignite was recognized because between 1939 and 1954 the basin underwent initial prospecting (Gokmen et al., 1993) and a 1:25,000 scale geological map was produced in 1963 (Gokmen, 1965). Mining of the lignite started in 1966. Extensive drilling took place in 1976, and Narin ( 1980) and Siyako ( 1984) used the results to describe geological aspects of the lignite. The results demonstrated that the Caytrhan and Koyunag 111lignite fields in the Beypazart basin (Fig. 1) contain more than 400 million t of lignite, enough to support the two, 150 MW thermal power stations (TPSs) sited on the Cayuhan lignite field. To alleviate the problem of the high sulphur content, both TPSs are equipped with desulphurization plants. The lignite is extracted by underground methods, which involve both mechanized longwall and modified manual sections. In general, the inorganic constituents of lignite can contribute to numerous technological problems, such as abrasion of mining equipment, boiler fouling and slagging and environmental pollution. Sulphur is one of the main constituents that contributes to the sulphur dioxide emissions and the resulting acid rain problems. Generally, lignite with less than 0.6% S on an air-dried basis would meet the environmental regulations in the USA (Cas-
agrande, 1987). Exploration geologists therefore tend to look for lignite containing the lowest ash and sulphur contents. This discussion examines the lateral and vertical relationships within the upper lignite of the Beypazart basin. These relationships were defined using isopach and quality contour maps as well as seam profiles. Statistical analysis of the relationships between ash content,
M.K.G. Whuteley, E. Tuncalr /International Joumal of Coal Geology 27 (1995) 131-151
133
sulphur content and thickness showed no significant correlation. An appreciation of the spatial distribution of the variables is achieved by visually comparing isopach maps and seam quality maps and sections. These maps and sections provide the means for interpreting the conditions that led to the formation of the mire, the unusual mineral matter (zeolites) and the unusually high sulphur contents in the upper seam.
2. Geological setting The Cayuhan lignite field extends in a NE-SW direction along the northern edge of the Beypazart basin (Fig. 1). The southern and northern margins of the lignite-bearing strata appear to be defined by NE-SW trending faults in places (Fig. 2). In the Cayuhan lignite field the strata are folded along a NESW trending, asymmetric, antiformal axis, with the southern limb steeply dipping to the south. The Miocene volcanosedimentary rocks, currently dated as Middle to Late Miocene (late Serravalian-Messinian), unconformably and disconformably overlie the basement, consisting of Palaeozoic metamorphic schists, which were intruded by granite, Jurassic and Lower Cretaceous limestone and ophiolite, and Upper Cretaceous and Palaeocene elastic sediments. The Miocene sediments (Fig. 3) were deposited in a subsiding half-graben, which developed under the influence of an extensional tectonic regime, with the northern margin acting as the active downthrown side (Yagmurlu et al., 1988). According to Inci ( 1991) these faults appear to have controlled the deposition of Miocene sediments.
Akpinar
Form&on
Hirka Formation Coraklar
+ Y
-~---.-
”
”
.A
”
85000
90000
DIP & Stnke Syncllnal
ax,s
Anticlmal
ax,s
Fault wth downthrow L1gmte mrne OutlIne at ,,gmte tla_,,n
_.;“”
zAYIRF;AN
Formation
95000
Sample sees
100000
Fig. 2. Simplified geology map of the Caylrhan lignite field (from Whateley and Tuncah, 1995; modified after Giikmen et al., 1993). A, B and C mark the location of the underground sites at which the upper lignite seam was sampled.
134
M.K.G. Whuteley, E. Tuncalr/International Journal of Coal Geology 27 (1995) 131-151
AGE
LITHOLOGY
FORMATIONS
claystone, mudstone & gypsum
Upper Miocene
1
~
limestone conglomerate, sandstone
----_-_- Caylrhan Formation .... .... . ... Middle Miocene -
--_-
-
. .
g I * 2 F 2
silicified claystone & limestone, cheri
i a $
shale, bituminous shale, trona & tuff
al E -0 $ a
PreNeogene
claystone, mudstone, fine-grained sandstone
cross-bedded conglomerate, sandstone & mudstone Metamorphics, ophiolites, granites, limestones & elastic sediments
Fig. 3. Schematic stratigraphic section of the Qylrhan basin (from Whateley and Tuncah, 1995; modified after Yagmurlu et al., 1988; Inci, 1991).
Inci ( 1991) described the Miocene sedimentary rocks in terms of three major facies; namely, the stratigraphically lower alluvial facies, the upper alluvial facies and the lacustrine facies. The sediments are divided into eight formations (Fig. 3). The stratigraphically lower alluvial facies, from the base upwards, consists of volcanoclastic conglomerate and sandstone, and green siltstone. These rocks together form the Coraklar Formation (Fig. 3). There are two separate lignite seams in the Cayuhan lignite field. The lower lignite seam was deposited in the lower part of the Coraklar Formation, while the thicker, economically important, upper lignite seam was deposited at the top of the alluvial facies. Whateley and Tuncah ( 1995) suggest that the upper lignite formed in a freshwater, limnic environment characterized by a high rainfall and seasonally high temperatures, which allowed periodic desiccation of the mire to occur. At the northeastern end of the basin, the Coraklar Formation sediments intercalate with the Teke Volcanics. The Coraklar Formation is overlain by the lacustrine facies of the Hirka Formation, with a very sharp contact. This facies consists of carbonate rocks and green claystone, which contain trona and bituminous shale. The sharp change in sediment type and sedimentation style has been put down to a change in climate, from a wet (for lignite) to a hotter and drier (formation of trona) climate. Alkali springs originating in contemporaneous Teke volcanics are also believed to have contributed to the trona brine in the ephemeral lake (Inci, 1991). The climate appears to become increasingly arid until evaporites, including gypsum were deposited in the Kimir Formation (Fig. 3).
M.K.G. Whateley, E. Tuncah /International Journal of Coal Geology 27 (1995) 131-151
The lacustrine facies is which forms the Cayirhan glomerate and fine-grained claystones of the lacustrine
135
overlain by and grades laterally into the upper alluvial facies, and Bozbelen Formations (Fig. 3). These consist of red consandstone beds, which grade laterally into the gypsiferous green facies (Kimir Formation).
3. Character of the lignites Yagmurlu et al. ( 1988) have described the lower lignite seam as being areally restricted and laterally discontinuous, having numerous seam partings and varying from 1.O to 10.95 m in thickness. It was only discovered in 1982 during the development of an underground adit. It is a low quality seam with high ash (52%) and sulphur (3%) contents. This seam is not exploited at present and is not discussed further here. There are up to 150 m of parting sediments between the lower seam and the upper seam. These sediments fine upwards, from sandy braided river channel fill facies above the lower seam, through interbedded sandstone, siltstone and claystone, to shale and carbonaceous shale, deposited in a lacustrine environment, below the upper seam. Whateley and Tuncah ( 1995) describe the upper lignite seam as being laterally extensive and varying from 1 .O to 4.9 m in thickness, averaging 3.0 m. A 1 m thick parting, composed of siltstone with chert nodules, splits the upper seam into two lignite beds, referred to colloquially as the first (TV) and second (Tb) seams (Figs. 4 and 5). Each of these two seams is described separately below. Quality data on the first and second seams were obtained from core samples from 132 coal exploration boreholes drilled in the basin. The first seam was intersected in 13 1 holes and the second seam was intersected in 129 holes. Proximate analyses, total sulphur content and calorific value were determined for each intersection of each seam. In addition, the data base contains the thickness and elevation of each intersection. For this study, a total of 87 samples were collected from three sites in the underground workings, A, B and C (Figs. 2 and 4). The seams were sampled at 10 cm intervals and the macroscopic characteristics of each sample were described in detail. A total of 87 proximate analyses were carried out. Calorific value, combustible sulphur and pyritic sulphur contents, palynological descriptions, petrographic analysis, reflectance values and ash oxide analyses were also determined on each sample (Whateley and Tuncah, 1995). The upper lignite is brownish to black. The first seam is mainly bright with some dull bands. The second seam is generally dull with bright bands. Reflectance measurements (R,,, [ %] ) vary across the area. Samples from site A have an average R_ of 0.34, site B, 0.36 and site C, 0.38. This puts them into the lignite rank. Kavugan ( 1993) has shown that the mean reflectance values of the upper seam increase towards major thrust faults. He suggests that the increase in rank results from accelerated coalification, caused by tectonic factors. The lignite petrography, mineral matter (ash content) and sulphur content are used in the following sections to illustrate the vertical variation within individual seams and variations between the seams. Later sections describe the vertical and lateral variations of seam thickness and quality across the basin.
136
M.K.G. Whateley. E. Tuncalt /Internationd
Journal of Coal Geology 27 (1995) 131-151
Ash
S”lph”r
Fig. 4. Vertical distribution of macerals, air-dried ash and sulphur contents and pyritic sulphur content of the first and second seams as seen at sites A, B and C in the Caylrhan lignite field.
M.K.G. Whateley, E. Tuncall /International Joumal of Coal Geology 27 (1995) 131-151
SW
137
NE C
B
A
Tb
??Bright/banded 69
Dull lignite
? ?Parting
lignite
Ash content
? ?Ash 0
content
Ash content
>40% 20-40% ~20%
Fig. 5. A NE-SW cross-section between the three underground sites A, B and C (Fig. 2). showing the lateral and vertical variation in the macroscopic characteristics and ash contents of the lignite seams in the Caylrhan basin, Central Anatolia, Turkey.
4. Vertical seam variability 4. I. Lignite petrography The petrographic composition of each of the 87 samples from sites A, B and C was determined and the variation of the three main maceral groups (huminite, liptinite and inertinite) was plotted adjacent to a vertical profile of the macroscopic constituents of the seam (Fig. 4). Whateley and Tuncah (1995) found that the total huminite content for the first seam varied between 56 and 86 ~01% and for the second seam between 33 and 83 ~01% (Table 1). In general, the average huminite and inertinite contents of the first seam are higher than that of the second seam (Table 1) , but the reverse occurs in the case of the liptinite. The amount of huminite decreases towards the top of the second seam at sites B and C (Fig. 4)) with a corresponding increase in the mineral matter (ash) content. A negative correlation coefficient of - 0.84 between the ash content and the calorific value confirms this change in quality. Oxidation of the huminite may have occurred as a result of aeration of the peat during the influx of the mineral matter into the mire. The huminite content is much less variable in the first seam (Fig. 4)) a consequence of which is that the first seam is slightly better quality (Table 2). There is a broad correlation between the megascopic characteristics of the lignite and the ash content of the samples (Fig. 4). The dull lignite has a very high ash content ( > 40%)) the bright and dull banded lignite contains between 20 and 40% ash and the bright lignite contains < 20% ash (Fig. 5).
138
M.K.G. Whareley, E. Tuncalr /Intemarional Journal of Coal Geology 27 (1995) 131-151
Table 1 Average maceral content of the 87 samples of the upper lignite seam B and C, in the Caylrhan lignite field, Beyparzari Huminite
from the three undergroundsample sites, A,
Liptinite
Inertinite
Seam
Vol%
Range
Vol%
Range
Vol%
Range
ATv ATb BTv BTb CTv CTb
71.5 68.7 69.7 61.5 70.7 60.2
65-80 36-83 57-86 33-83 61-86 33-80
5.7 6.7 7.3 9.1 5.1 7.0
1-16 &13 O-16 O-15 l-10 2-13
4.5 2.3 4.9 4.9 8.5 5.0
o-5 04 O-23 O-19 O-23 o-19
4.2. Mineral
matter
Mineral matter is the inorganic fraction in coal, made up of a variety of discrete solids of primary or secondary origin. When coal is burnt, the inorganic residue is referred to as ash. It is reported as a percentage of the original air dried mass of the raw coal. This section looks at the mineral matter sensu stricto. Whateley and Tuncah (1995) identified two zeolites, heulandite and analcime, as the major mineral phase in the mineral matter of the as-received, raw lignites. In the first seam, X-ray diffraction (XRD) traces (Fig. 6a) and SEM-EDX analyses indicated the zeolite Table 2 The average proximate analyses, sulphur contents and calorific values of the upper lignite seam in the Cayrrhan lignite field, illustrating lateral variation in quality between the eastern and western areas and vertical variation between the first and second seams throughout the basin Proximate analysis (as received)
Moisture content (~01%) Ash content (~01%) Volatile content (~01%) Fixed carbon (~01%) Total sulphur content (~01%) Calorific value (kcal kg-‘)
Moisture content (~01%) Ash content (~01%) Volatile content (~01%) Fixed carbon (~01%) Total sulphur content (~01%) Calorific value (kcal kg-‘) R Crnaxl(%)
DMMF = dry mineral matter free.
Western area
Eastern area
21.71 34.35 25.67 21.42 (DMMF48.75) 4.04 2557
26.44 25.36 25.92 23.50 (DMMF 48 .76) 2.74 2839
First seam
Second seam
24.69 28.46 26.18 18.10 3.59 2682 0.37
23.99 30.86 24.23 18.91 3.24 2686 0.35
M.K.G. Whateley, E. Tuncalr/Intemational
Journal of Coal Geology 27 (1995) 131-151
139
a
b
t
L 5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
Fig. 6. Typical XRD patterns of the raw lignite. Examples taken from (a) the first (BTv) and b) the second (BTb) seams of the Cayirhan lignite, Beypazan basin, Central Anatolia, Turkey (from Whateley and Tuncah, 1995). H = heulandite; A = analcime; F= plagioclase feldspar; G = gypsum; P = pyrite and Q = quartz. ( [ Ca,Na,] [ A1,Si,Ol,] .6H,O), with some gypsum ( CaSO, . 2H20), pyrite (FeS*) and quartz, and minor amounts of calcite, marcasite and plagioclase feldspar. In the second seam (Fig. 6b) the major mineral phase is the zeolite analcime (Na( AlSi,O,) .H,O), with some gypsum, pyrite and quartz, and minor amounts of calcite, marcasite and plagioclase feldspar. Heulandite and analcime are zeolites that form as secondary minerals derived from the hydrothermal alteration of various aluminosilicates, such as feldspars (usually in volcanic or igneous rocks), or authigenic minerals (usually in sedimentary rocks). Although depthtemperature control on the distribution of zeolites is recognized elsewhere, the vertical zonation of the mineral matter in the lignites in the Beypazarl basin is more probably related to variations in the chemistry of the original volcanoclastic or elastic material in the lignite, or to variations in the chemistry of the circulating fluid (Whateley and Tuncah, 1995). heulandite
4.3. Ash content Figs. 4 and 5 illustrate how the air-dried ash content changes vertically both in and between the first (TV) and second (I%) seams. The average ash content of the second seam is higher (Table 2), and the pattern of ash distribution in the seams differs slightly (Fig.
140
M.K.G. Whuteley, E. Tuncall /International Journal of Coal Geology 27 (1995) 131-151
4). The basal samples of the second seam are relatively high in ash, followed by a general decrease in ash content upwards until, near the top of the seam, there is a marked increase in ash content. In the first seam, the basal samples are also high in ash content and they are followed by a general decrease in ash content upwards until, near the top, the ash content increases again. The top few samples, however, show a decrease in ash content. These patterns suggest that similar controls on the influx of mineral matter into the mire occurred during the deposition of the first and second seams. The best quality lignite (lowest ash content) occurs in the central sections of both seams. The top of the first seam again shows improved lignite quality. 4.4. Sulphur content Pyrite (or marcasite) was seen in the lignites as veinlets, disseminations and framboids, and gypsum was seen as primary plates and very fine secondary needles under the SEM. The total sulphur ( S,) content appears to be uniformly high throughout both seams, although the first seam has a slightly higher average ST (Table 1 and Fig. 4). There is a distinct increase in ST towards the top of the first seam (Fig. 4). This coincides with an increase in the pyritic sulphur (S,,) content (~01%) in the seam (Fig. 4). The second seam generally has a higher S,, content than the first. Sulphur in coal appears in three main forms: organic, pyritic and as sulphate. There are many possible origins for the sulphur in peat; such as marine roof rocks (Horne et al., 1978)) marine influences during deposition (Casagrande et al., 1977)) microbial action and changing pH conditions (Casagrande, 1987). Casagrande ( 1987) believes that virtually all the sulphur in coal can be accounted for during the peat-forming stage, where syngenetic sulphur is incorporated. Hydrogen sulphide, formed from microbial reduction of sulphate, reacts with organic matter to form organic sulphur and reacts with ferrous iron to form pyrite. This reaction appears to proceed at a faster rate when the pH is higher because more ferrous ions are released. Therefore, those areas in a coal-forming environment influenced by higher sulphate content will yield lignites having higher sulphur contents. There is no evidence for marine roof rocks in the Caylrhan basin to explain the high sulphur content. Whateley and Tuncah ( 1995) suggest that alkali springs, originating from the contemporaneous Teke volcanics, may have contributed to the mire water during peat formation. This may have led to elevated sulphate levels, which resulted in lignite with a high syngenetic sulphur content. Adrejko et al. ( 1983) established that minerals, including gypsum ( SgY), may precipitate after prolonged evaporation of saturated interstitial peat waters. Whateley and Tuncalt ( 1995) proposed that the mires in the Cayirhan basin formed in an environment with seasonally high temperatures that probably contributed to the periodic desiccation of the mire. These conditions may have led to the precipitation of S,,, which could contribute to the elevated sulphur content. This may help to explain the higher than normal amounts of S,, in the raw lignites, as seen on the XRD and SEM-EDX traces. Inci ( 199 1) recognized magnesium sulphate, which had precipitated in desiccation cracks in the bituminous shales below the trona in the Hirka Formation, as immediately postdepositional, secondary mineralization. Whateley and Tuncah ( 1995) suggest that circulating, saline, low-temperature hydrothermal solutions in the closed Cayirhan basin may have contributed to the sulphur content by precipitating secondary sulphate in joints and
M.K.G. Whteley, E. Tuncall /International Journal of Coal Geology 27 (1995) 131-151
141
cleats in the lignite. They also suggest that epigenetic S,, could have been introduced to joints and cleats in the lignite by these fluids. In a reducing environment, such as that found in a water-saturated lignite seam, some of the sulphate may have been reduced to pyrite. This would explain the elevated S,, levels in the lignite seams (Fig. 4). The opposite could also occur with the oxidation of the pyrite by circulation of more recent, oxidizing, groundwater. Oxidation/reduction phenomena only redistribute the sulphur species but do not add to the overall sulphur content.
5. Lateral seam variability The first seam was intersected in 129 of the sample sites. Ash content was determined in 122 samples and 129 were analysed for sulphur content. The statistics from the thickness data show possible bimodal populations (Fig. 7a). The data for ash (Fig. 7b) show a nearly gaussian distribution, but the sulphur content (Fig. 7c) shows a distinct bimodal distribution. The seam averages 1.37 m in thickness, with an average ash content of 28.5% and an average sulphur content of 3.6% (Table 2). The second seam was intersected in 133 of the sites sample. Ash content was determined in 122 samples and 128 were analysed for sulphur content. The statistics of the thickness data show a near-gaussian distribution (Fig. 8a), as do the data for the ash content (Fig. 8b). The sulphur content (Fig. 8c) shows a distinct bimodal distribution. The seam thickness averages 1.72 m, with an average ash content of 30.9% and an average sulphur content of 3.3% (Table 2). The structural contour map used in this study (Fig. 9) was drawn using the upper contact of the first seam with the overlying Hirka Formation because this surface is sharp and clearly defined in all boreholes. The strike of the lignite seam at depth can be assumed to be similar to that of the overlying sediments at the surface (Fig. 2). The majority of the sediments show a preferred strike in the NE-SW direction. Where the structural contours show a significant change of orientation, a fault can be inferred (Gribble, 1994; Whateley, 1995). A rapid change in thickness or quality contour lines can also be used to infer the presence of a fault (Whateley, 1995). The most significant change from the NE-SW direction is seen in the south-central part of the basin, where the structural contours swing to a N-S direction. This suggests that a major fault is present which has downthrown the sediments to the southwest, resulting in a deeper part of the basin in the southwest and a structurally elevated region to the northeast. The isopach map of the first seam (Fig. 10a) shows three broad areas of differing average thickness, namely in the west, south-central and northeast, of about 1.0 m, 1.6 m and 1.2 m, respectively. In the extreme north the first seam increases to over 2.4 m in a restricted area. The south-central area is divided by a NNW-SSE trending saddle of thinner lignite, which coincides with the zone in which the structural contours show a significant change in orientation. The isopach map of the second seam (Fig. lob) shows a broad east-west division, with the two sections averaging 1.6 m and 2.2 m, respectively. In the south-central area the second seam increases to over 2.8 m. In this area the isopachs trend in a NW-SE direction, parallel to the zone in which the structural contours show a significant change in orientation. The spatial distribution of the first seam would explain the bimodal distributions
a)
N used:
124
24 F r e
16
q ” e n c
6
Y
2
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
1.371 0.268 0.518 37.779 -0.215 2.641
Minimum: 25th %: Median: 75th %: Maximum:
0.100 0.990 1.405 1.750 2.650
3
Th kness (m)
I
b)
N used:
124
3c
F r e
20
q U
r
e
n
10
C
Y
1 30
L
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
28.465 110.933 10.532 37.002 0.818 3.676
Minimum: 25th %: Median: 75th %: Maximum:
8.510 21.550 26.945 34.900 61.200
60
Ash (%)
c)
N used:
116,
F r e
112
q U e
6
n C
Y
4
Or 0
125
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
3.586 1.411 1.188 33.130 0.129 2.213
Minimum: 25th %: Median: 75th %: Maximum:
1.270 2.635 3.670 4.445 6.170
3 6 Total Sulphur (%)
Fig. 7. Histogram and summary statistics for (a) thickness; the cayuhan lignite field, Central Anatolia, Turkey.
(b) ash; and (c) sulphur contents of the first seam in
a)
N used:
131
30 F I e
20
q ” e ” c
10
Y
0
Mean: Variance: Std Dev: % cv: Skewness: Kurtosis:
1.716 0.329 0.573 33.412 0.425 4.045
Minimum: 25th %: Median: 75th %: Maximum:
0.150 1.306 1.690 2.600 3.550
N used:
122
Ld Thickness(m)
b,
2ov
r e
12
4 U e
.E
n c Y
4
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
30.856 138.989 11.769 38.206 0.647 3.252
Minimum: 25th %: Median: 75th %: Maximum:
11.206 22.550 28.965 38.310 68.330
3 Ash (%)
cl
N used:
16
F r
12
e o U
8
e
n C
Y
4
2
115
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
3.269 0.701 0.837 25.601 0.015 2.747
Minimum: 25th %: Median: 75th %: Maximum:
1.410 2.617 3360 3.923 5.900
4
Total Sulphur (%)
Fig. 8. Histogram and summary statistics for (a) thickness; in the Caynhan lignite field, Central Anatolia, Turkey.
(b) ash; and (c) sulphur contents of the second seam
144
M.K.G. Whteley, E. Tuncalr /International Journal of Coal Geology 27 (1995)131-151
80000
85000
90000
95000
a
100000
Fig. 9. Structural contour map (metres above mean sea level) of the top of the first seam in the Caylrhan lignite field, Central Anatolia, Turkey. Contour interval is 50 m. The coordinate system is metric, with 1000 m intervals. The outline of the lignite field was digitized from the geological map (Fig. 2).
seen in the histogram (Fig. 7a), but the histogram of the second seam (Fig. 8a) has a gaussian distribution. This illustrates the difficulty of relying on statistical evidence for evaluating a deposit: determination of the spatial distribution can indicate significant zonation. The NW-SE structural zone in the south-central area is probably responsible for the increased thickness of both seams in this area. An active synsedimentary fault downthrowing to the southwest and keeping pace with peat growth would allow the development of thicker mires. A similar distribution of the lignite for both seams occurs in the northeastern area, with lignite increasing in thickness from the south to the north. This may be a result of topographic control and/or post-depositional structural control. Thinner mire may have developed over a topographic high, or subsequent exposure of the mire, due to base level change caused by differential uplift, may have led to oxidation of the peat and its subsequent thinning. The isoash map of the first seam (Fig. 1 la) shows a tripartite division, similar to the isopach map, with the western limb averaging 27%, the south-central area averaging 36% and the northeastern area averaging 25% ash content (air-dried basis). The isoash map of the second seam (Fig. 1 lb) shows an east-west division, with western half averaging 40% and the northeastern area averaging 25% ash content (air-dried basis). The histograms of both seams (Figs. 7b and 8b) indicate near-gaussiandistributions, but the spatial distribution in the contour map shows distinctly separate areas of lignite quality. This again illustrates the problem of using non-spatial statistics when investigating a data set. The distribution of the very high ash lignite in both seams in the south-central area points to depositional
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control, probably because of the higher input of mineral matter into the topographically lower area. The ash content of the lignite in the northeastern area, particularly in the second seam, decreases northward. This negative spatial correlation with the seam thickness shows that, in this area, the thinner seams have a higher ash content. This may be as a result of subsequent exposure of the mire, due to base level change caused by differential uplift, leading to oxidation of the peat, its subsequent thinning and the relative increase in the proportion of mineral matter in the lignite.
146
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Journal of Coal Geology 27 (1995) 131-151
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Fig. 11. Spatial distribution of the as-received ash content (percentage) of (a) the first seam and (b) the second seam in the Caylrhan lignite field. Data derived from 130 boreholes and the three underground sample sites, A, B and C. Contours at 3% intervals.
The cross-section (Fig. 5) on which ranges of ash content are plotted shows that there appear to be continuous layers of similar quality lignite across the basin, although the highash zones do grade laterally into medium-ash lignite, that, in turn, grade laterally into lowash lignite. It appears that there were at least two source directions. In Fig. 12 only the high-ash zones are plotted for clarity. It can be seen that the top high-ash band in the first seam (TV) is thickest at site B, thinner at site C and absent at site A. This suggests a source to the west. The basal high-ash zone in the second seam (Tb) is thickest at site C, thin at site A and absent at site B, suggesting a source to the northwest. This suggests that the relatively higher ash content of the dull coal can be attributed to the introduction of elastic material, from either fluvial or volcanic ash fall sources. At this stage it is not possible to
M.K.G. Whteley, E. Tuncalr/Intemational
Journal of Coal Geology 27 (1995) 131-151
147
Fig. 12. A fence diagram drawn between the three underground sites A, B and C (Fig. 2). Only the high-ash zones are plotted for clarity. Lateral variation in the high ash content of the lignite seams shows apparent change in source direction of the mineral matter.
assess the importance of the two processes in increasing the mineral matter in the peat. The location of the zones of high-ash lignite also vary relative to the base of the seam. At site A better quality lignite (low ash content) occurs at the base of both seams (Figs. 5 and 12). This low-ash lignite grades laterally into medium and even high-ash lignite at the base of the lignite in the first seam at site B (Fig. 5). If the high-ash material is the product of one event (a flood or ash fall) there must have been well developed peat (first seam) at site A before any peat formed at site B. The mineral matter must have been deposited on a flat surface, so the inference is that the peat at site A developed in a low-lying mire at the commencement of the first and second seams. The high ash content in all lignite samples (lowest value 8.5%, average ash content 29%) further suggests that all these peats accumulated in low-lying mires. The isosulphur maps of both seams (Fig. 13) also show a threefold division. In the first seam (Fig. 13a), the western limb averages 5.0% and the south-central and northeastern areas average 3.5% total sulphur content (air-dried basis), The latter two areas are separated by a NW-SE trending area of higher sulphur content (4.50/o), which also coincides with the zone in which the structural contours show a significant change in orientation. In the second seam, the western limb averages 3.5%, the central area averages 3.75% and the northeastern area averages 2.75% total sulphur content (air-dried basis). A rapid change in contour values between the western area and the central area also coincides with the zone in which the structural contours show a significant change in orientation. The histograms (Figs. 7c and 8c) do reflect the occurrence of the different populations but not the lateral variability. The alignment of the highest sulphur values with the NW-SE trending structure
148
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Fig. 13. Spatial distribution of the as-received total sulphnr content (percentage) of (a) the first seam and (b) the second seam in the Caytrhan lignite field. Data derived from 130 boreholes and the three underground sample sites, A, B and C. Contours at 0.25% intervals (modified after Whateley and Tuncah, 1995). in the south-central area suggests that there might be structural control on the distribution of the sulphur. Some of the additional sulphur in this area may be of epigenetic origin, introduced along a fracture by hydrothermal solutions.
5’.I. Discussions
of lateral seam variability
One of the geomorphological features that affects the location of coal formation is the topography of the pre-coal surface (McCabe, 1984, 1987, 1991; Whateley and Jordan, 1989). These low-lying mires infill underlying topography and streams and lakes are common within the mires (McCabe, 1991). The through-flow of water increases the mineral
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149
supply and decreases the pH. Both the first and second seams are thickest in the southwestern corner of the basin, which correlates with the deepest and, one can assume, the topographically lowest part of the basin at the time of peat formation. The ash content of both seams is highest in this area, probably because of the higher input of mineral matter into the topographically lower area. Evidence derived from the high ash content bands in the lignite (Figs. 5 and 12) also points to these peats having accumulated in low-lying mires. Evidence from the structural contour maps, as well as the isopach and quality maps, all point to the presence of a NW-SE to N-S trending structural feature in the southwest of the basin. This has resulted in thicker, high-ash, lignite developing in the southwest. This infers structural control on peat formation and waterborne sediment influx (McCabe, 1984; 1987; Moore, 1987; Whateley and Jordan, 1989). The deeper water has resulted in a decrease in syngenetic sulphur in this area, possibly because of the lowered pH (Casagrande, 1987). Some of the additional sulphur in the south-central area may be of epigenetic origin, introduced along a fracture by hydrothermal solutions.
6. Conclusions The upper lignite in the Cayirhan basin formed in a low-lying mire in a freshwater, limnic environment characterized by high rainfall and seasonally high temperatures, which allowed periodic desiccation of the mire to occur. The upper lignite is separated into two seams, the first and second seam, split by a siltstone parting. These lignites are characterized by their high ash and sulphur contents (up to 8.2% S and 68.3% ash on an air-dried basis). The study of the mineral matter and the sulphur contents reveals three types of distribution (Querol et al., 1992), namely: ( 1) vertical variation within individual seams (Figs. 4 and 5) ; (2) variation between the seams (Figs. 4,5 and 12) ; (3) lateral variation across the basin (Figs. 10, 11 and 13). In the case of the ash content, the vertical variation within individual seams is probably the result of depositional processes, and position and stage of development of the low-lying mire relative to the source at the time of the influx of the mineral matter. Variation between the seams is recognized because, although both seams have relatively high ash contents, the first seam has a lower average ash content. This may be a result of a more active depositional setting (elastic or volcanoclastic influx) during the development of the second seam. The mineral matter also varies between the seams, as seen with the presence of ubiquitous zeolites in the mineral matter; namely, heulandite in the first seam and analcime in the second seam. This is probably the result of variations in the chemistry of the original volcanoclastic or elastic material associated with the lignite, or to variations in the chemistry of the circulating fluid. A broad east-west lateral variation across the basin in the ash content of the seams probably results from variations in the amount and rate of influx of elastic or volcanoclastic material into the mire at the time of formation. At this stage it is not possible to assess the importance of the two processes in increasing the mineral matter in the peat. The higher than average ash content in the deeper, southwestern part of the basin may be a result of structural control on the formation of the peat in a low-lying mire. The topographically lower area would attract a higher than average influx of mineral matter.
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The vertical variation of the ST (total sulphur) within, and between, the seams appears to be controlled by external factors. The central role of sulphate and its impact on eventual lignite sulphur content is well documented (Casagrande, 1987). The sulphate may have been derived from the contemporaneous Teke volcanics, resulting in the elevated, syngenetic, ST contents. Increased volcanic activity during the first seam formation may be responsible for its higher ST content. Lateral variation, reflected in the increased ST content in the southwestern half of the basin for both the first and second seams, suggests tectonically controlled topography, producing a deeper part of the basin, possibly because of the lowered pH, which would result in a decrease in the syngenetic pyritic and organic sulphur contents (Casagrande, 1987). Some of the additional sulphur in the area parallel to the NW-SE trending structural zone may be of epigenetic origin, introduced along a fracture by hydrothermal solutions.
Acknowledgements
We would like to thank the Directorate of Mineral Research and Exploration for providing the data, Selami Toprak for the petrographic analyses and Nesrin Tulu and Nevin Gtilgijr for the palynological determinations. Our thanks also to Sue Button for producing the figures for this paper and to Andy Smith and Xavier Querol for their help with the XRD traces. Professor Dutcher and Drs. A.C. Cook and R.J. Gray are thanked for their constructive criticism of the manuscript.
References Admjko, M.J., Cohen, A.D. and Raymond, R., Jr., 1983. Origin of mineral matter in peat. In: R. Raymond Jr. and M.J. Adrejko (Editors), Proc. of Workshop on Mineral Matter in Peat; its occurrence, form and distribution (Sept. 26-30, Los Alamos Noational Laboratory, Los Alamos, N.M.), pp. 3-24. Altinli, I.E., 1977. Geology of the eastern territory of Nallihan (Ankara Province). Istanbul Univ. Sci. Fat. Ser. B, 42( l-2): 29414. Casagrande, D., 1987. Sulphur in peat and coal. In: AC. Scott, (Editor), Coal and Coal-bearing strata: Recent Advances. Geol. Sot. London Spec. Publ., 32: 87-105. Casagrande, D., Siefert, L., Bershiniski, C. and Sutton, N., 1977. Sulfur in peat forming systems of Okefenokee Swamp and Florida Everglades: Origins of sulfur in coals. Geochem. Cosmochim. Acta, 41: 161-167. G&men, V., 1965. Nallihan-Baypaxari (Ankara) civarinda linyit ihtiva eden Neojen sahasinin jeolojisi hakkinda rapor. Directorate of Mineral Research and Exploration Rep. 3802. Giikmen, V., Memikoglu, 0.. Dagli, M., Gz, D. and Tuncah, E., 1993. Ttlrkiye Linyit Envanteri. Directorate of Mineral Research and Exploration, 356 pp. Gribble, P., 1994. Fault interpretation from coal exploration borehole data using Surpac software. In: M.K.G. Whateley and P.K. Harvey (Editors), Mineral Resource Evaluation II - Methods and Case Histories. Geol. Sot. London Spec. Publ. 79: 29-35. Home, J.C., Ferme, J.C., Camccio, F.T. and Baganz, B.P., 1978. Depositional models in coal exploration and mine planning in Appalachian Region. Am. Assoc. Pet. Geol. Bull., 62( 12): 2379-2411. Inci, U., 1991. Miocene alluvial fan-alkaline playa lignite-trona bearing deposits from an inverted basin in Anatolia: sedimentology and tectonic controls on deposition. Sediment. Geol., 71: 73-97. Kalafat9ioglu, A. and Uysalli, H., 1964. Geology of the Beypazan, Nallihan and Seben regions. Bull. Miner. Resour. Explor. Inst. Turkey, 62: 1-12.
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Kavusan, G., 1993. Interrelations between tectonics and vitrinite reflectance of Beypazan Caylrhan lignites. Turkish Geol. Symp. (Ankara), pp. 357-363. McCabe, P.J., 1984. Depositional environments ofcoal and coal-bearing strata. Spec. Publ. lnt. Assoc. Sedimentol., 7: 1342. McCabe, P.J., 1987. Facies studies of coal and coal-bearing strata. In: A.C. Scott (Editor), Coal and Coal-bearing strata: Recent Advances. Geol. Sot. London Spec. Publ., 32: 51-66. McCabe, P.J., 1991. Geology of coal: environments of deposition. In: H.J. Gluskoter, D.D. Rice and R.B. Taylor (Editors), Economic Geology, US. (The Geology of North America, Part 2.) Geol. Sot. Am., Boulder, Colo., pp. 469-482. Moore, P.D., 1987. Ecological and hydrological aspects of peat formation. In: A.C. Scott (Editor), Coal and Coalbearing Strata: Recent Advances. Geol. Sot. London Spec. Pub]., 32: 7-15. Narin, R., 1980. Beypazart, Beysehir lignite deposit in Central Anatolia, Turkey. Bull. Miner. Resour. Explor. Inst. Turkey, 17: 21-50 (in Turkish with English abstract). Querol, X., Salas, R., Pardo, G. and Ardevol, L., 1992. Albian coal-bearing deposits of the lberian Range in northeastern Spain. Geol. Sot. Am. Spec. Pap., 267: 193-207. Saner, S., 1979. Explanation of the development of the western Pontid mountain and adjacent basins, based in plate tectonic theory, northwestern Turkey. Bull. Miner. Resour. Explor. Inst. Turkey, 93: l-20. Siyako, F., 1984. Beypazart (Ankara) Kiimtirlti Neojen Havzasi ve C( cedil)evresinin Jeolojisi. Directorate Miner. Res. Explor. Spec. Rep., 46 pp. Tune, M., 1980. Davutoglan (Beypazart)-Seben (Bolu) arasinda kalan ve Aladag Cay boyunca elan biilgenin stratigrafisi. Ph.D. Thesis, Univ. Ankara. Whateley, M.K.G. and Jordan, G., 1989. Fan delta-lacustrine sedimentation and coal development in the Tertiary Ombilin Basin, W. Sumatra. In: M.K.G. Whateley and K.T. Pickering (Editors), Deltas: Sites and Traps for Fossil Fuels. Geol. Sot. London Spec. Publ., 41: 317-332. Whateley, M.K.G. and Tuncah, E., 1995. The origin and distribution of sulphur in the Neogene Beypazart lignite basin, Central Anatolia, Turkey. In: M.K.G. Whateley and A. Spears (Editors), European Coal Geology. Geol. Sot. London Spec. Pub]., 82: 307-323. Whateley, M.K.G., 1995. Soma Lignite Basin, Turkey. In: A.M. Evans (Editor), An Introduction to Mineral Exploration. Blackwell, Oxford, in press. Yagmurlu, F., Helvaci, C. and lnci, U., 1988. Depositional setting and geometric structure of the Beypazart lignite deposits, Central Anatolia. lnt. J. Coal Geol., 10: 337-360.
ELSEVIER
Journal ol
International Journal of Coal Geology 27 (1995) 131-151
Quality variations in the high-sulphur lignite of the Neogene Beypazm Basin, Central Anatolia, Turkey M.K.G. Whateley a, E. Tuncah b aDepartment of Geology, University of Leicester, University Road, Leicester, LEI 7RH, UK b Directorate of Mineral Research and Exploration (MTA), Ankara, Turkey
Received 9 May 1994; accepted 23 November 1994
Abstract During the Miocene a number of fault bounded basins developed in Central Anatolia, Turkey. One such basin at Caylrhan, near Beypazan, contains thick, laterally extensive lignite seams. This basin was filled initially with coarse elastic material. Upward fining of the elastics during basin fill, with an increase in the amount of clay and carbon content led to the development of relatively shallow limnic basins in which extensive peat deposits accumulated in low-lying mires. These lignites are characterized by their high sulphur and ash contents (up to 8.2% S and 68.3% ash on an air-dried basis). Studies of the sulphur and ash contents reveal three types of distribution; namely, vertical variation within individual seams, variation between the seams and lateral variation across the basin. In the case of the sulphur content, vertical variation within and between the seams is related to variations in the amount and source of the sulphate in the mire water, and the lateral variation is probably related to structural/topographic control of the mire at the time of formation. Variation in the ash content within the seams is probably the result of depositional processes at the time of mire development. Variation between seams is recognized due to the presence of ubiquitous zeolites in the mineral matter; namely heulandite in the first seam and analcime in the second seam. This is probably the result of variations in the chemistry of the original volcanoclastic or elastic material associated with the lignite or of variations in the chemistry of the circulating fluid. There is a broad east-west lateral variation across the basin in the ash content of the seams, probably resulting from variations in the amount and rate of elastic or volcanoclastic influx into the mire at the time of formation, related to structural/ topographic controls.
1. Introduction The large, Neogene basin centred on Beypazarl, lies approximately 100 km northwest of Ankara in Central Anatoiia (Fig. 1) . The basin is filled with alluvial, lacustrine and volcanosedimentary rocks and the sequence contains economic resources of lignite, bituminous shale and trona. 0 166-5 162/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved XSD10166-S162(94)00023-9
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Fig. 1. Location of the Beypazarl basin, showing the position of the Qylrhan lignite field and the main rock units in the region (from Whateley and Tuncah, 1995; modified after Ya@urlu et al., 1988).
The Beypazarr basin was first investigated by Kalafatcioglu and Uysalli ( 1964) who studied the stratigraphy, sedimentology and tectonics of the basin and the adjacent area. This was followed with studies by Altinli ( 1977), Saner ( 1979) and Tune ( 1980), who further refined the stratigraphic, sedimentological and tectonic models of the basin. The economic importance of the lignite was recognized because between 1939 and 1954 the basin underwent initial prospecting (Gokmen et al., 1993) and a 1:25,000 scale geological map was produced in 1963 (Gokmen, 1965). Mining of the lignite started in 1966. Extensive drilling took place in 1976, and Narin ( 1980) and Siyako ( 1984) used the results to describe geological aspects of the lignite. The results demonstrated that the Caytrhan and Koyunag 111lignite fields in the Beypazart basin (Fig. 1) contain more than 400 million t of lignite, enough to support the two, 150 MW thermal power stations (TPSs) sited on the Cayuhan lignite field. To alleviate the problem of the high sulphur content, both TPSs are equipped with desulphurization plants. The lignite is extracted by underground methods, which involve both mechanized longwall and modified manual sections. In general, the inorganic constituents of lignite can contribute to numerous technological problems, such as abrasion of mining equipment, boiler fouling and slagging and environmental pollution. Sulphur is one of the main constituents that contributes to the sulphur dioxide emissions and the resulting acid rain problems. Generally, lignite with less than 0.6% S on an air-dried basis would meet the environmental regulations in the USA (Cas-
agrande, 1987). Exploration geologists therefore tend to look for lignite containing the lowest ash and sulphur contents. This discussion examines the lateral and vertical relationships within the upper lignite of the Beypazart basin. These relationships were defined using isopach and quality contour maps as well as seam profiles. Statistical analysis of the relationships between ash content,
M.K.G. Whuteley, E. Tuncalr /International Joumal of Coal Geology 27 (1995) 131-151
133
sulphur content and thickness showed no significant correlation. An appreciation of the spatial distribution of the variables is achieved by visually comparing isopach maps and seam quality maps and sections. These maps and sections provide the means for interpreting the conditions that led to the formation of the mire, the unusual mineral matter (zeolites) and the unusually high sulphur contents in the upper seam.
2. Geological setting The Cayuhan lignite field extends in a NE-SW direction along the northern edge of the Beypazart basin (Fig. 1). The southern and northern margins of the lignite-bearing strata appear to be defined by NE-SW trending faults in places (Fig. 2). In the Cayuhan lignite field the strata are folded along a NESW trending, asymmetric, antiformal axis, with the southern limb steeply dipping to the south. The Miocene volcanosedimentary rocks, currently dated as Middle to Late Miocene (late Serravalian-Messinian), unconformably and disconformably overlie the basement, consisting of Palaeozoic metamorphic schists, which were intruded by granite, Jurassic and Lower Cretaceous limestone and ophiolite, and Upper Cretaceous and Palaeocene elastic sediments. The Miocene sediments (Fig. 3) were deposited in a subsiding half-graben, which developed under the influence of an extensional tectonic regime, with the northern margin acting as the active downthrown side (Yagmurlu et al., 1988). According to Inci ( 1991) these faults appear to have controlled the deposition of Miocene sediments.
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134
M.K.G. Whuteley, E. Tuncalr/International Journal of Coal Geology 27 (1995) 131-151
AGE
LITHOLOGY
FORMATIONS
claystone, mudstone & gypsum
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1
~
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--_-
-
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Fig. 3. Schematic stratigraphic section of the Qylrhan basin (from Whateley and Tuncah, 1995; modified after Yagmurlu et al., 1988; Inci, 1991).
Inci ( 1991) described the Miocene sedimentary rocks in terms of three major facies; namely, the stratigraphically lower alluvial facies, the upper alluvial facies and the lacustrine facies. The sediments are divided into eight formations (Fig. 3). The stratigraphically lower alluvial facies, from the base upwards, consists of volcanoclastic conglomerate and sandstone, and green siltstone. These rocks together form the Coraklar Formation (Fig. 3). There are two separate lignite seams in the Cayuhan lignite field. The lower lignite seam was deposited in the lower part of the Coraklar Formation, while the thicker, economically important, upper lignite seam was deposited at the top of the alluvial facies. Whateley and Tuncah ( 1995) suggest that the upper lignite formed in a freshwater, limnic environment characterized by a high rainfall and seasonally high temperatures, which allowed periodic desiccation of the mire to occur. At the northeastern end of the basin, the Coraklar Formation sediments intercalate with the Teke Volcanics. The Coraklar Formation is overlain by the lacustrine facies of the Hirka Formation, with a very sharp contact. This facies consists of carbonate rocks and green claystone, which contain trona and bituminous shale. The sharp change in sediment type and sedimentation style has been put down to a change in climate, from a wet (for lignite) to a hotter and drier (formation of trona) climate. Alkali springs originating in contemporaneous Teke volcanics are also believed to have contributed to the trona brine in the ephemeral lake (Inci, 1991). The climate appears to become increasingly arid until evaporites, including gypsum were deposited in the Kimir Formation (Fig. 3).
M.K.G. Whateley, E. Tuncah /International Journal of Coal Geology 27 (1995) 131-151
The lacustrine facies is which forms the Cayirhan glomerate and fine-grained claystones of the lacustrine
135
overlain by and grades laterally into the upper alluvial facies, and Bozbelen Formations (Fig. 3). These consist of red consandstone beds, which grade laterally into the gypsiferous green facies (Kimir Formation).
3. Character of the lignites Yagmurlu et al. ( 1988) have described the lower lignite seam as being areally restricted and laterally discontinuous, having numerous seam partings and varying from 1.O to 10.95 m in thickness. It was only discovered in 1982 during the development of an underground adit. It is a low quality seam with high ash (52%) and sulphur (3%) contents. This seam is not exploited at present and is not discussed further here. There are up to 150 m of parting sediments between the lower seam and the upper seam. These sediments fine upwards, from sandy braided river channel fill facies above the lower seam, through interbedded sandstone, siltstone and claystone, to shale and carbonaceous shale, deposited in a lacustrine environment, below the upper seam. Whateley and Tuncah ( 1995) describe the upper lignite seam as being laterally extensive and varying from 1 .O to 4.9 m in thickness, averaging 3.0 m. A 1 m thick parting, composed of siltstone with chert nodules, splits the upper seam into two lignite beds, referred to colloquially as the first (TV) and second (Tb) seams (Figs. 4 and 5). Each of these two seams is described separately below. Quality data on the first and second seams were obtained from core samples from 132 coal exploration boreholes drilled in the basin. The first seam was intersected in 13 1 holes and the second seam was intersected in 129 holes. Proximate analyses, total sulphur content and calorific value were determined for each intersection of each seam. In addition, the data base contains the thickness and elevation of each intersection. For this study, a total of 87 samples were collected from three sites in the underground workings, A, B and C (Figs. 2 and 4). The seams were sampled at 10 cm intervals and the macroscopic characteristics of each sample were described in detail. A total of 87 proximate analyses were carried out. Calorific value, combustible sulphur and pyritic sulphur contents, palynological descriptions, petrographic analysis, reflectance values and ash oxide analyses were also determined on each sample (Whateley and Tuncah, 1995). The upper lignite is brownish to black. The first seam is mainly bright with some dull bands. The second seam is generally dull with bright bands. Reflectance measurements (R,,, [ %] ) vary across the area. Samples from site A have an average R_ of 0.34, site B, 0.36 and site C, 0.38. This puts them into the lignite rank. Kavugan ( 1993) has shown that the mean reflectance values of the upper seam increase towards major thrust faults. He suggests that the increase in rank results from accelerated coalification, caused by tectonic factors. The lignite petrography, mineral matter (ash content) and sulphur content are used in the following sections to illustrate the vertical variation within individual seams and variations between the seams. Later sections describe the vertical and lateral variations of seam thickness and quality across the basin.
136
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Journal of Coal Geology 27 (1995) 131-151
Ash
S”lph”r
Fig. 4. Vertical distribution of macerals, air-dried ash and sulphur contents and pyritic sulphur content of the first and second seams as seen at sites A, B and C in the Caylrhan lignite field.
M.K.G. Whateley, E. Tuncall /International Joumal of Coal Geology 27 (1995) 131-151
SW
137
NE C
B
A
Tb
??Bright/banded 69
Dull lignite
? ?Parting
lignite
Ash content
? ?Ash 0
content
Ash content
>40% 20-40% ~20%
Fig. 5. A NE-SW cross-section between the three underground sites A, B and C (Fig. 2). showing the lateral and vertical variation in the macroscopic characteristics and ash contents of the lignite seams in the Caylrhan basin, Central Anatolia, Turkey.
4. Vertical seam variability 4. I. Lignite petrography The petrographic composition of each of the 87 samples from sites A, B and C was determined and the variation of the three main maceral groups (huminite, liptinite and inertinite) was plotted adjacent to a vertical profile of the macroscopic constituents of the seam (Fig. 4). Whateley and Tuncah (1995) found that the total huminite content for the first seam varied between 56 and 86 ~01% and for the second seam between 33 and 83 ~01% (Table 1). In general, the average huminite and inertinite contents of the first seam are higher than that of the second seam (Table 1) , but the reverse occurs in the case of the liptinite. The amount of huminite decreases towards the top of the second seam at sites B and C (Fig. 4)) with a corresponding increase in the mineral matter (ash) content. A negative correlation coefficient of - 0.84 between the ash content and the calorific value confirms this change in quality. Oxidation of the huminite may have occurred as a result of aeration of the peat during the influx of the mineral matter into the mire. The huminite content is much less variable in the first seam (Fig. 4)) a consequence of which is that the first seam is slightly better quality (Table 2). There is a broad correlation between the megascopic characteristics of the lignite and the ash content of the samples (Fig. 4). The dull lignite has a very high ash content ( > 40%)) the bright and dull banded lignite contains between 20 and 40% ash and the bright lignite contains < 20% ash (Fig. 5).
138
M.K.G. Whareley, E. Tuncalr /Intemarional Journal of Coal Geology 27 (1995) 131-151
Table 1 Average maceral content of the 87 samples of the upper lignite seam B and C, in the Caylrhan lignite field, Beyparzari Huminite
from the three undergroundsample sites, A,
Liptinite
Inertinite
Seam
Vol%
Range
Vol%
Range
Vol%
Range
ATv ATb BTv BTb CTv CTb
71.5 68.7 69.7 61.5 70.7 60.2
65-80 36-83 57-86 33-83 61-86 33-80
5.7 6.7 7.3 9.1 5.1 7.0
1-16 &13 O-16 O-15 l-10 2-13
4.5 2.3 4.9 4.9 8.5 5.0
o-5 04 O-23 O-19 O-23 o-19
4.2. Mineral
matter
Mineral matter is the inorganic fraction in coal, made up of a variety of discrete solids of primary or secondary origin. When coal is burnt, the inorganic residue is referred to as ash. It is reported as a percentage of the original air dried mass of the raw coal. This section looks at the mineral matter sensu stricto. Whateley and Tuncah (1995) identified two zeolites, heulandite and analcime, as the major mineral phase in the mineral matter of the as-received, raw lignites. In the first seam, X-ray diffraction (XRD) traces (Fig. 6a) and SEM-EDX analyses indicated the zeolite Table 2 The average proximate analyses, sulphur contents and calorific values of the upper lignite seam in the Cayrrhan lignite field, illustrating lateral variation in quality between the eastern and western areas and vertical variation between the first and second seams throughout the basin Proximate analysis (as received)
Moisture content (~01%) Ash content (~01%) Volatile content (~01%) Fixed carbon (~01%) Total sulphur content (~01%) Calorific value (kcal kg-‘)
Moisture content (~01%) Ash content (~01%) Volatile content (~01%) Fixed carbon (~01%) Total sulphur content (~01%) Calorific value (kcal kg-‘) R Crnaxl(%)
DMMF = dry mineral matter free.
Western area
Eastern area
21.71 34.35 25.67 21.42 (DMMF48.75) 4.04 2557
26.44 25.36 25.92 23.50 (DMMF 48 .76) 2.74 2839
First seam
Second seam
24.69 28.46 26.18 18.10 3.59 2682 0.37
23.99 30.86 24.23 18.91 3.24 2686 0.35
M.K.G. Whateley, E. Tuncalr/Intemational
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139
a
b
t
L 5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
Fig. 6. Typical XRD patterns of the raw lignite. Examples taken from (a) the first (BTv) and b) the second (BTb) seams of the Cayirhan lignite, Beypazan basin, Central Anatolia, Turkey (from Whateley and Tuncah, 1995). H = heulandite; A = analcime; F= plagioclase feldspar; G = gypsum; P = pyrite and Q = quartz. ( [ Ca,Na,] [ A1,Si,Ol,] .6H,O), with some gypsum ( CaSO, . 2H20), pyrite (FeS*) and quartz, and minor amounts of calcite, marcasite and plagioclase feldspar. In the second seam (Fig. 6b) the major mineral phase is the zeolite analcime (Na( AlSi,O,) .H,O), with some gypsum, pyrite and quartz, and minor amounts of calcite, marcasite and plagioclase feldspar. Heulandite and analcime are zeolites that form as secondary minerals derived from the hydrothermal alteration of various aluminosilicates, such as feldspars (usually in volcanic or igneous rocks), or authigenic minerals (usually in sedimentary rocks). Although depthtemperature control on the distribution of zeolites is recognized elsewhere, the vertical zonation of the mineral matter in the lignites in the Beypazarl basin is more probably related to variations in the chemistry of the original volcanoclastic or elastic material in the lignite, or to variations in the chemistry of the circulating fluid (Whateley and Tuncah, 1995). heulandite
4.3. Ash content Figs. 4 and 5 illustrate how the air-dried ash content changes vertically both in and between the first (TV) and second (I%) seams. The average ash content of the second seam is higher (Table 2), and the pattern of ash distribution in the seams differs slightly (Fig.
140
M.K.G. Whuteley, E. Tuncall /International Journal of Coal Geology 27 (1995) 131-151
4). The basal samples of the second seam are relatively high in ash, followed by a general decrease in ash content upwards until, near the top of the seam, there is a marked increase in ash content. In the first seam, the basal samples are also high in ash content and they are followed by a general decrease in ash content upwards until, near the top, the ash content increases again. The top few samples, however, show a decrease in ash content. These patterns suggest that similar controls on the influx of mineral matter into the mire occurred during the deposition of the first and second seams. The best quality lignite (lowest ash content) occurs in the central sections of both seams. The top of the first seam again shows improved lignite quality. 4.4. Sulphur content Pyrite (or marcasite) was seen in the lignites as veinlets, disseminations and framboids, and gypsum was seen as primary plates and very fine secondary needles under the SEM. The total sulphur ( S,) content appears to be uniformly high throughout both seams, although the first seam has a slightly higher average ST (Table 1 and Fig. 4). There is a distinct increase in ST towards the top of the first seam (Fig. 4). This coincides with an increase in the pyritic sulphur (S,,) content (~01%) in the seam (Fig. 4). The second seam generally has a higher S,, content than the first. Sulphur in coal appears in three main forms: organic, pyritic and as sulphate. There are many possible origins for the sulphur in peat; such as marine roof rocks (Horne et al., 1978)) marine influences during deposition (Casagrande et al., 1977)) microbial action and changing pH conditions (Casagrande, 1987). Casagrande ( 1987) believes that virtually all the sulphur in coal can be accounted for during the peat-forming stage, where syngenetic sulphur is incorporated. Hydrogen sulphide, formed from microbial reduction of sulphate, reacts with organic matter to form organic sulphur and reacts with ferrous iron to form pyrite. This reaction appears to proceed at a faster rate when the pH is higher because more ferrous ions are released. Therefore, those areas in a coal-forming environment influenced by higher sulphate content will yield lignites having higher sulphur contents. There is no evidence for marine roof rocks in the Caylrhan basin to explain the high sulphur content. Whateley and Tuncah ( 1995) suggest that alkali springs, originating from the contemporaneous Teke volcanics, may have contributed to the mire water during peat formation. This may have led to elevated sulphate levels, which resulted in lignite with a high syngenetic sulphur content. Adrejko et al. ( 1983) established that minerals, including gypsum ( SgY), may precipitate after prolonged evaporation of saturated interstitial peat waters. Whateley and Tuncalt ( 1995) proposed that the mires in the Cayirhan basin formed in an environment with seasonally high temperatures that probably contributed to the periodic desiccation of the mire. These conditions may have led to the precipitation of S,,, which could contribute to the elevated sulphur content. This may help to explain the higher than normal amounts of S,, in the raw lignites, as seen on the XRD and SEM-EDX traces. Inci ( 199 1) recognized magnesium sulphate, which had precipitated in desiccation cracks in the bituminous shales below the trona in the Hirka Formation, as immediately postdepositional, secondary mineralization. Whateley and Tuncah ( 1995) suggest that circulating, saline, low-temperature hydrothermal solutions in the closed Cayirhan basin may have contributed to the sulphur content by precipitating secondary sulphate in joints and
M.K.G. Whteley, E. Tuncall /International Journal of Coal Geology 27 (1995) 131-151
141
cleats in the lignite. They also suggest that epigenetic S,, could have been introduced to joints and cleats in the lignite by these fluids. In a reducing environment, such as that found in a water-saturated lignite seam, some of the sulphate may have been reduced to pyrite. This would explain the elevated S,, levels in the lignite seams (Fig. 4). The opposite could also occur with the oxidation of the pyrite by circulation of more recent, oxidizing, groundwater. Oxidation/reduction phenomena only redistribute the sulphur species but do not add to the overall sulphur content.
5. Lateral seam variability The first seam was intersected in 129 of the sample sites. Ash content was determined in 122 samples and 129 were analysed for sulphur content. The statistics from the thickness data show possible bimodal populations (Fig. 7a). The data for ash (Fig. 7b) show a nearly gaussian distribution, but the sulphur content (Fig. 7c) shows a distinct bimodal distribution. The seam averages 1.37 m in thickness, with an average ash content of 28.5% and an average sulphur content of 3.6% (Table 2). The second seam was intersected in 133 of the sites sample. Ash content was determined in 122 samples and 128 were analysed for sulphur content. The statistics of the thickness data show a near-gaussian distribution (Fig. 8a), as do the data for the ash content (Fig. 8b). The sulphur content (Fig. 8c) shows a distinct bimodal distribution. The seam thickness averages 1.72 m, with an average ash content of 30.9% and an average sulphur content of 3.3% (Table 2). The structural contour map used in this study (Fig. 9) was drawn using the upper contact of the first seam with the overlying Hirka Formation because this surface is sharp and clearly defined in all boreholes. The strike of the lignite seam at depth can be assumed to be similar to that of the overlying sediments at the surface (Fig. 2). The majority of the sediments show a preferred strike in the NE-SW direction. Where the structural contours show a significant change of orientation, a fault can be inferred (Gribble, 1994; Whateley, 1995). A rapid change in thickness or quality contour lines can also be used to infer the presence of a fault (Whateley, 1995). The most significant change from the NE-SW direction is seen in the south-central part of the basin, where the structural contours swing to a N-S direction. This suggests that a major fault is present which has downthrown the sediments to the southwest, resulting in a deeper part of the basin in the southwest and a structurally elevated region to the northeast. The isopach map of the first seam (Fig. 10a) shows three broad areas of differing average thickness, namely in the west, south-central and northeast, of about 1.0 m, 1.6 m and 1.2 m, respectively. In the extreme north the first seam increases to over 2.4 m in a restricted area. The south-central area is divided by a NNW-SSE trending saddle of thinner lignite, which coincides with the zone in which the structural contours show a significant change in orientation. The isopach map of the second seam (Fig. lob) shows a broad east-west division, with the two sections averaging 1.6 m and 2.2 m, respectively. In the south-central area the second seam increases to over 2.8 m. In this area the isopachs trend in a NW-SE direction, parallel to the zone in which the structural contours show a significant change in orientation. The spatial distribution of the first seam would explain the bimodal distributions
a)
N used:
124
24 F r e
16
q ” e n c
6
Y
2
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
1.371 0.268 0.518 37.779 -0.215 2.641
Minimum: 25th %: Median: 75th %: Maximum:
0.100 0.990 1.405 1.750 2.650
3
Th kness (m)
I
b)
N used:
124
3c
F r e
20
q U
r
e
n
10
C
Y
1 30
L
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
28.465 110.933 10.532 37.002 0.818 3.676
Minimum: 25th %: Median: 75th %: Maximum:
8.510 21.550 26.945 34.900 61.200
60
Ash (%)
c)
N used:
116,
F r e
112
q U e
6
n C
Y
4
Or 0
125
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
3.586 1.411 1.188 33.130 0.129 2.213
Minimum: 25th %: Median: 75th %: Maximum:
1.270 2.635 3.670 4.445 6.170
3 6 Total Sulphur (%)
Fig. 7. Histogram and summary statistics for (a) thickness; the cayuhan lignite field, Central Anatolia, Turkey.
(b) ash; and (c) sulphur contents of the first seam in
a)
N used:
131
30 F I e
20
q ” e ” c
10
Y
0
Mean: Variance: Std Dev: % cv: Skewness: Kurtosis:
1.716 0.329 0.573 33.412 0.425 4.045
Minimum: 25th %: Median: 75th %: Maximum:
0.150 1.306 1.690 2.600 3.550
N used:
122
Ld Thickness(m)
b,
2ov
r e
12
4 U e
.E
n c Y
4
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
30.856 138.989 11.769 38.206 0.647 3.252
Minimum: 25th %: Median: 75th %: Maximum:
11.206 22.550 28.965 38.310 68.330
3 Ash (%)
cl
N used:
16
F r
12
e o U
8
e
n C
Y
4
2
115
Mean: Variance: Std Dev: % C.V. Skewness: Kurtosis:
3.269 0.701 0.837 25.601 0.015 2.747
Minimum: 25th %: Median: 75th %: Maximum:
1.410 2.617 3360 3.923 5.900
4
Total Sulphur (%)
Fig. 8. Histogram and summary statistics for (a) thickness; in the Caynhan lignite field, Central Anatolia, Turkey.
(b) ash; and (c) sulphur contents of the second seam
144
M.K.G. Whteley, E. Tuncalr /International Journal of Coal Geology 27 (1995)131-151
80000
85000
90000
95000
a
100000
Fig. 9. Structural contour map (metres above mean sea level) of the top of the first seam in the Caylrhan lignite field, Central Anatolia, Turkey. Contour interval is 50 m. The coordinate system is metric, with 1000 m intervals. The outline of the lignite field was digitized from the geological map (Fig. 2).
seen in the histogram (Fig. 7a), but the histogram of the second seam (Fig. 8a) has a gaussian distribution. This illustrates the difficulty of relying on statistical evidence for evaluating a deposit: determination of the spatial distribution can indicate significant zonation. The NW-SE structural zone in the south-central area is probably responsible for the increased thickness of both seams in this area. An active synsedimentary fault downthrowing to the southwest and keeping pace with peat growth would allow the development of thicker mires. A similar distribution of the lignite for both seams occurs in the northeastern area, with lignite increasing in thickness from the south to the north. This may be a result of topographic control and/or post-depositional structural control. Thinner mire may have developed over a topographic high, or subsequent exposure of the mire, due to base level change caused by differential uplift, may have led to oxidation of the peat and its subsequent thinning. The isoash map of the first seam (Fig. 1 la) shows a tripartite division, similar to the isopach map, with the western limb averaging 27%, the south-central area averaging 36% and the northeastern area averaging 25% ash content (air-dried basis). The isoash map of the second seam (Fig. 1 lb) shows an east-west division, with western half averaging 40% and the northeastern area averaging 25% ash content (air-dried basis). The histograms of both seams (Figs. 7b and 8b) indicate near-gaussiandistributions, but the spatial distribution in the contour map shows distinctly separate areas of lignite quality. This again illustrates the problem of using non-spatial statistics when investigating a data set. The distribution of the very high ash lignite in both seams in the south-central area points to depositional
M.K.G. Whateley, E. Tuncall/Intemational
;A7 ,[,
44000-1 42000
Journal of Coal Geology 27 (1995) 131-151
L12)
145
12
L
40000
a) I
80000
85000
90000
’
95000
100000
95000
100000
’
t
52oooi
\‘.V
50000-l I4
42000-h
80000
AS,
85000
90000
Fig. 10. Isopach map (in me&es) of (a) the first seam and (b) the second seam in the Caylrhan lignite field. Data derived from 130 boreholes and the three underground sample sites, A, B and C. Contour intervals 0.2 m.
control, probably because of the higher input of mineral matter into the topographically lower area. The ash content of the lignite in the northeastern area, particularly in the second seam, decreases northward. This negative spatial correlation with the seam thickness shows that, in this area, the thinner seams have a higher ash content. This may be as a result of subsequent exposure of the mire, due to base level change caused by differential uplift, leading to oxidation of the peat, its subsequent thinning and the relative increase in the proportion of mineral matter in the lignite.
146
M.K.G. Whteley, E. Tuncall/Intemational
Journal of Coal Geology 27 (1995) 131-151
52000-
I
I
80000
I
I, I, 85000
1
I,
I,
I,
,
,
,
,
,
90000
95000
100000
90000
95000
100000
I
I1
52000-
50000-
-f
46000-l
80000
N
m
85000
Fig. 11. Spatial distribution of the as-received ash content (percentage) of (a) the first seam and (b) the second seam in the Caylrhan lignite field. Data derived from 130 boreholes and the three underground sample sites, A, B and C. Contours at 3% intervals.
The cross-section (Fig. 5) on which ranges of ash content are plotted shows that there appear to be continuous layers of similar quality lignite across the basin, although the highash zones do grade laterally into medium-ash lignite, that, in turn, grade laterally into lowash lignite. It appears that there were at least two source directions. In Fig. 12 only the high-ash zones are plotted for clarity. It can be seen that the top high-ash band in the first seam (TV) is thickest at site B, thinner at site C and absent at site A. This suggests a source to the west. The basal high-ash zone in the second seam (Tb) is thickest at site C, thin at site A and absent at site B, suggesting a source to the northwest. This suggests that the relatively higher ash content of the dull coal can be attributed to the introduction of elastic material, from either fluvial or volcanic ash fall sources. At this stage it is not possible to
M.K.G. Whteley, E. Tuncalr/Intemational
Journal of Coal Geology 27 (1995) 131-151
147
Fig. 12. A fence diagram drawn between the three underground sites A, B and C (Fig. 2). Only the high-ash zones are plotted for clarity. Lateral variation in the high ash content of the lignite seams shows apparent change in source direction of the mineral matter.
assess the importance of the two processes in increasing the mineral matter in the peat. The location of the zones of high-ash lignite also vary relative to the base of the seam. At site A better quality lignite (low ash content) occurs at the base of both seams (Figs. 5 and 12). This low-ash lignite grades laterally into medium and even high-ash lignite at the base of the lignite in the first seam at site B (Fig. 5). If the high-ash material is the product of one event (a flood or ash fall) there must have been well developed peat (first seam) at site A before any peat formed at site B. The mineral matter must have been deposited on a flat surface, so the inference is that the peat at site A developed in a low-lying mire at the commencement of the first and second seams. The high ash content in all lignite samples (lowest value 8.5%, average ash content 29%) further suggests that all these peats accumulated in low-lying mires. The isosulphur maps of both seams (Fig. 13) also show a threefold division. In the first seam (Fig. 13a), the western limb averages 5.0% and the south-central and northeastern areas average 3.5% total sulphur content (air-dried basis), The latter two areas are separated by a NW-SE trending area of higher sulphur content (4.50/o), which also coincides with the zone in which the structural contours show a significant change in orientation. In the second seam, the western limb averages 3.5%, the central area averages 3.75% and the northeastern area averages 2.75% total sulphur content (air-dried basis). A rapid change in contour values between the western area and the central area also coincides with the zone in which the structural contours show a significant change in orientation. The histograms (Figs. 7c and 8c) do reflect the occurrence of the different populations but not the lateral variability. The alignment of the highest sulphur values with the NW-SE trending structure
148
85000
80000
90000
500004 A
100000
LA-/T\’
480001 46000-j
95600
/
/
/1
/LY
42000
11I’I
80000
85000
90000
11
95000
1
‘“,‘I
100000
Fig. 13. Spatial distribution of the as-received total sulphnr content (percentage) of (a) the first seam and (b) the second seam in the Caytrhan lignite field. Data derived from 130 boreholes and the three underground sample sites, A, B and C. Contours at 0.25% intervals (modified after Whateley and Tuncah, 1995). in the south-central area suggests that there might be structural control on the distribution of the sulphur. Some of the additional sulphur in this area may be of epigenetic origin, introduced along a fracture by hydrothermal solutions.
5’.I. Discussions
of lateral seam variability
One of the geomorphological features that affects the location of coal formation is the topography of the pre-coal surface (McCabe, 1984, 1987, 1991; Whateley and Jordan, 1989). These low-lying mires infill underlying topography and streams and lakes are common within the mires (McCabe, 1991). The through-flow of water increases the mineral
M.K.G. Whateley, E. Tuncah /International Joumal of Coal Geology 27 (1995) 131-151
149
supply and decreases the pH. Both the first and second seams are thickest in the southwestern corner of the basin, which correlates with the deepest and, one can assume, the topographically lowest part of the basin at the time of peat formation. The ash content of both seams is highest in this area, probably because of the higher input of mineral matter into the topographically lower area. Evidence derived from the high ash content bands in the lignite (Figs. 5 and 12) also points to these peats having accumulated in low-lying mires. Evidence from the structural contour maps, as well as the isopach and quality maps, all point to the presence of a NW-SE to N-S trending structural feature in the southwest of the basin. This has resulted in thicker, high-ash, lignite developing in the southwest. This infers structural control on peat formation and waterborne sediment influx (McCabe, 1984; 1987; Moore, 1987; Whateley and Jordan, 1989). The deeper water has resulted in a decrease in syngenetic sulphur in this area, possibly because of the lowered pH (Casagrande, 1987). Some of the additional sulphur in the south-central area may be of epigenetic origin, introduced along a fracture by hydrothermal solutions.
6. Conclusions The upper lignite in the Cayirhan basin formed in a low-lying mire in a freshwater, limnic environment characterized by high rainfall and seasonally high temperatures, which allowed periodic desiccation of the mire to occur. The upper lignite is separated into two seams, the first and second seam, split by a siltstone parting. These lignites are characterized by their high ash and sulphur contents (up to 8.2% S and 68.3% ash on an air-dried basis). The study of the mineral matter and the sulphur contents reveals three types of distribution (Querol et al., 1992), namely: ( 1) vertical variation within individual seams (Figs. 4 and 5) ; (2) variation between the seams (Figs. 4,5 and 12) ; (3) lateral variation across the basin (Figs. 10, 11 and 13). In the case of the ash content, the vertical variation within individual seams is probably the result of depositional processes, and position and stage of development of the low-lying mire relative to the source at the time of the influx of the mineral matter. Variation between the seams is recognized because, although both seams have relatively high ash contents, the first seam has a lower average ash content. This may be a result of a more active depositional setting (elastic or volcanoclastic influx) during the development of the second seam. The mineral matter also varies between the seams, as seen with the presence of ubiquitous zeolites in the mineral matter; namely, heulandite in the first seam and analcime in the second seam. This is probably the result of variations in the chemistry of the original volcanoclastic or elastic material associated with the lignite, or to variations in the chemistry of the circulating fluid. A broad east-west lateral variation across the basin in the ash content of the seams probably results from variations in the amount and rate of influx of elastic or volcanoclastic material into the mire at the time of formation. At this stage it is not possible to assess the importance of the two processes in increasing the mineral matter in the peat. The higher than average ash content in the deeper, southwestern part of the basin may be a result of structural control on the formation of the peat in a low-lying mire. The topographically lower area would attract a higher than average influx of mineral matter.
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The vertical variation of the ST (total sulphur) within, and between, the seams appears to be controlled by external factors. The central role of sulphate and its impact on eventual lignite sulphur content is well documented (Casagrande, 1987). The sulphate may have been derived from the contemporaneous Teke volcanics, resulting in the elevated, syngenetic, ST contents. Increased volcanic activity during the first seam formation may be responsible for its higher ST content. Lateral variation, reflected in the increased ST content in the southwestern half of the basin for both the first and second seams, suggests tectonically controlled topography, producing a deeper part of the basin, possibly because of the lowered pH, which would result in a decrease in the syngenetic pyritic and organic sulphur contents (Casagrande, 1987). Some of the additional sulphur in the area parallel to the NW-SE trending structural zone may be of epigenetic origin, introduced along a fracture by hydrothermal solutions.
Acknowledgements
We would like to thank the Directorate of Mineral Research and Exploration for providing the data, Selami Toprak for the petrographic analyses and Nesrin Tulu and Nevin Gtilgijr for the palynological determinations. Our thanks also to Sue Button for producing the figures for this paper and to Andy Smith and Xavier Querol for their help with the XRD traces. Professor Dutcher and Drs. A.C. Cook and R.J. Gray are thanked for their constructive criticism of the manuscript.
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