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Occurrence and morphology of pyrite in Bulgarian coals
International Journal of
International
Journal of Coal Geology 29 ( 1996) 273-290
Occurrence and morphology of pyrite in Bulgarian coals Jordan Kortenski a**,Irena Kostova b a University of Mining and Geology, Sojia 1156, Bulgaria h Institute ofAppliedMineralogy, Bulgarian Academy of Science, Sojia 1000, Bulgaria Received 2 November
1994; accepted 30 August 1995
Abstract Coals with different degrees of coalification (ranging from lignite to anthracite) from seven Bulgarian coal basins have been investigated. The forms of pyrite and their distribution have been established. The types found are: massive pyrite, represented by the homogeneous, cluster-like and microconcretionary varieties; framboidal pyrite, appearing in inorganic and bacterial forms; euhedral pyrite, which is either isolated or clustered; anhedral pyrite, in its infilling and replacement varieties; and infiltrational pyrite, as a replacement or infilling mineral. Most of the forms of the euhedral, framboidal and massive pyrite developed during peat deposition. The anhedral replacement pyrite formed in the peat bed during early diagenesis. Infiltrational pyrite filled fractures and cleats formed during the diagenesis, catagenesis and metagenesis. Both similarities and differences with respect to the distribution of the pyrite types have been determined between coals of different ranks from Bulgarian coal basins. These differences are due to: the presence of Fe and S in the rocks adjacent to ancient peat bogs; the activities of ground and surface waters which brought Fe and S into the peat bogs; the geochemical character (pH and Eh) of the peat bogs and the sulphur bacteria development; and the tectonic situation during diagenesis, catagenesis and metagenesis.
1. Introduction This investigation deals with pyrite occurring in the coals of several Bulgarian basins. The purpose of the study is to establish the types of pyrite, the genesis of these forms, and factors affecting their appearance and distribution. Coal samples with different degrees of coalification and different formation conditions were chosen for the investigations. Lignite specimens originated from the Sofia, Belibreg, and Karlovo Basins (Fig. 1). The tectonic * Corresponding
author
0166-5 162/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSD10166-5162(95)00033-X
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Geology 29 (1996) 273-290
D
- Lignite =
- Sub-bituminous
m
- Bituminous
coal coal
- Anthracite Fig. I. Location of the coal basins in Bulgaria studied. 1= Sofia; 2 = Belibreg; 3 = Karlovo; 4 = Pemik; 5 = Balkan; 6= Suhostrel; 7 = Svoge.
setting of these basins is relatively stable, with minor faulting (Kamenov, 1964). The subbituminous coal samples were collected from the Pernik Basin (Fig. 1) , where the coal deposits are slightly folded and faulted. Bituminous coals from the Balkan and Suhostrel Basins and anthracite from the Svoge Basin were also investigated (Fig. 1). The coal deposits from the Balkan and Svoge basins have undergone complicated folding and faulting, including nappe formation in the Balkan Basin. Table 1 shows the age of the coalbearing sediments and their lithostratigraphy, as well as the number of coal beds and their rank.
2. Methods Coal and coal shale samples from all beds of the coal basins were studied. These include: core samples from lignite beds of the Sofia (252 samples) and Karlovo (47 samples) basins, and from anthracite beds of the Svoge Basin (81 samples); channel samples from
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Table I Age and rank of the coal from the basins” Coal basins
Sofia
Age
Rank of coal
Coal beds Total number
Economic
3-s
1-2
Lignite B
Belibreg Karlovo Pernik Balkan
Pontian Dacian Pliocene Pliocene (?) Upper Oligocene Cenomanian
1-5 II 5 3-8
1 I 3 3-8
Suhostrel
Upper Eocene
I
I
Svoge
NamuriarWestfalian
7-17
7-17
Lignite B Lignite B Subbituminous Bituminous Gas-Lean Bituminous Fat-Coking Anthracite-Metaanthracite
” The data are from Siskov ( 1986).
the lignite bed of the Belibreg Basin (38 samples), subbituminous beds of the Pernik Basin (62 samples), all bituminous beds of the Balkan ( 109 samples) and Suhostrel ( 15 samples) basins, and anthracite beds of the Svoge Basin (24 samples). Optical studies were carried under reflected light with dry and oil immersion objectives using a NU-2 Universal microscope. A total of 155 polished sections were studied. Electron microscope (Tesla) observations were performed using a coal replica ( 10 samples). X-ray diffraction analyses by the Debye-Scherrer method of 40 samples were made with a TURM-60 apparatus, using iron anticathodes with manganese filters at 35 kV and 12 mA. A Philips scanning electron microscope with a tungsten cathode and an EDAX 9100 X-ray micro-analyzer was used for X-ray microprobe analysis of 20 samples. Trace elements in 72 pyrite samples were determined by laser microspectral analysis in an LMA apparatus equipped with an O-24 spectrograph. Chemical analysis data on coal ash from 56 samples were applied to the determination of the environmental acidity in peat bogs. For that purpose the acidity diagram of Kortenski ( 1992) was used (Fig. 2). The diagram is based on two parameters: ( 1) the acidity coefficient of the coal ash; and (2) the ash content of the coals. The former parameter is determined as a ratio of the total content of acid oxides (SiO,, A1203, SO3 and P205) to the total content of the alkali oxides (CaO, MgO, Fe203, NazO and K*O). The content of ash is also used as an environmental parameter because its increase leads to a decrease in the content of the humic acids as well as the total acidity of the peat-forming environment.
3. Results and discussion 3.1.
Description
of pyrite forms
A series of papers have been published on the presence of pyrite in coals. Various forms of pyrite have been described by Grady ( 1977), Crelling et al. (1983), Wiese and Fyfe
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10
20
30
40 Ash
50 -
60 70
80 90
Ad,?”
Fig. 2. Diagram of the acidity. C,, = SiOz + A&O? + SO3 + P20,/Ca0 + MgO + Fez03 + 2 = Belibreg; 3 = Karlovo; 4 = Pemik; 5 = Balkan; 6 = Suhostrel; 7 = Svoge.
Na,O + KZO.1 = Sofia;
( 1986)) Frankie and Hower ( 1987), Querol et al. ( 1989), Renton and Bird ( 1991)) among others. The classification used for the description of the pyrite forms is given in Table 2. Framboidal pyrite
The framboidal pyrite represents spherical pyrite particles. The term ‘framboidal’ is widely applied in the literature. However, some authors use also other terminology; for example, ‘cauliflower’ and ‘blebs’, which are mentioned by Wiese and Fyfe ( 1986). The origin of this kind of pyrite has been discussed in a number of papers. Many researchers, especially those studying framboidal pyrite in coals (Kizilstein and Trufanov, 1968; Kizilstein and Minaeva, 1972; Skripchenko and Berberian, 1975) assume this type of pyrite to consist of pyritized sulphur bacteria. Kizilstein (1968) has suggested the possibility of pyritization of other kinds of bacteria which might have coexisted along with the sulphurmetabolizing bacteria and helped the decomposition and assimilation of the plant tissues. Others, investigating mainly framboidal pyrite in ore deposits, reject its bacterial origin and assume pyrite formation from mineral solutions in inorganic material (Ramdohr, 1960; Farrand, 1970; among others). Similar views have been expressed by Wiese and Fyfe ( 1986) concerning framboidal pyrite in an Ohio coal. They assumed part of the pyrite to represent pyritized algal cells or fungal spores. Both opinions have been confirmed experimentally. In the first case, laboratory experiments were performed and the framboidal forms
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Table 2 Classification
of pyrite forms in coals
Place of formation
Time of formation
Morphology
Pyrite forms
Peat bog
Peat genesis
Euhedral
Isolated Clustered Bacterial Inorganic Cluster-like Homogeneous Microconcretional infilling Replacement Infilling
Framboidal
Massive
Coal bed
Early diagenesis
Anhedral
Late diagenesis Catagenesis
lntiltration
Massive
Metagenesis
Euhedral Replacement
obtained in the presence of various kinds of bacteria were compared (Kizilstein and Minaeva, 1972). To prove the second supposition, mineral solutions containing a certain amount of organics in the absence of bacteria were used (Farrand, 1970). In a study of pyrite in coal from the basins of Sofia, Kortenski ( 1984) suggested the presence of both forms. The globules resulting from massive mineralization have been considered to be pyritized bacteria. Renton and Bird (1991) designated this form as completely coalescent framboids. Framboids showing well shaped crystals are likely to be a result of the crystallization of mineral solutions in the organic matter. This form was described by Renton and Bird ( 1991) as discrete framboids and partly coalescent framboids. In this study, the first type is referred to as bacterial with massive mineralization, and the second as inorganic framboidal pyrite with well shaped crystals. Bacterial framboidal pyrite is found in all the coal samples investigated (Table 2). This type is characterized by the, generally, nonuniform mineralization of the globules (Fig. 3). Some of the globules are densely pyritized (Fig. 4a and b), while others exhibit only a partial pyritization (Fig. 4~). The separate framboids are rarely densely rimmed. Bacterial framboidal pyrite preserves the independence of the separate globules even when they form aggregates (Skripchenko and Berberian, 1975) (Fig. 3). Some of the aggregates are pyritized bacterial colonies, as observed for coal from the Svoge Basin. Bacterial framboidal pyrite seldom appears as single bodies (Fig. 4b). More often, it is found as aggregates of smaller or larger globules, irregular or lenticular, or, less frequently, beaded in shape. Bacterial framboidal pyrite is not associated with euhedral pyrite and is often rimmed by clay minerals. In the latter cases, laser spectral analyses show the presence of Si and Al, as also noted by Wiese and Fyfe ( 1986)) Boctor et al. ( 1976) and Wiese et al. ( 1990). Inorganic framboidal pyrite exhibits the relatively uniform mineralization of the globules (Fig. 5). Only in a few cases do the framboids contain solitary crystals (Figs. 4d and 6) and the binding substance (organic matter and clay minerals) prevails. This form of pyrite exhibits a symmetrical, mostly concentric, ordering of the crystals in the globules (Fig. 7). In many of the inorganic framboidal pyrite globules the crystals are densely intergrown and
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Fig. 3. Bacterial fmmboidal pyrite from the Sofia Basin (oil immersion),
developed upon the earlier-formed crystals in the centre of the globules (Fig. 8). In contrast to the bacterial pyrite, inorganic framboidal pyrite is sometimes characterized by coalescence of globules (Figs. 6 and 7). The pyrite crystals in individual framboidal bodies are usually equal in size (Fig. 6). Crystals with different sizes are rarely found (Figs. 7 and 8). As a rule, crystals of larger sizes are in the centre of the spherulites (Fig. 7 - the globule on the right). The crystals are octahedral (Fig. 6)) pentagondodecahedral (Fig. 8) and cubic (Fig. 8). In almost all cases inorganic framboidal pyrite is associated not only with clay minerals but also with isolated crystals or aggregates of euhedral pyrite (Fig. 5). This suggests a similar formation mechanism for the two pyrite types. All coal samples from the Bulgarian basins show a regular spheritic form of framboidal pyrite, with a few exceptions being in samples from the Sofia Basin (Fig. 4b - oval form). Some differences are observed at higher ranks. In the anthracite from the Svoge Basin the expected spherical shape is in places oval, perhaps due to pressure, which is rarely observed. The sizes of framboidal pyrite vary over a wide range: from l-2 pm to 50-70 pm, with some above 100 pm. Laser spectral analysis has shown some characteristic features of framboidal pyrite in the coals investigated (Table 3), which are similar to features found by Vaughan and Craig ( 1978). The absence of Ni and Co should be mentioned (Table 3). Only the framboidal pyrite from the Svoge Basin showed the presence of Cr and Zn, while Be was only present in samples from the Balkan Basin. Pb was absent from the latter two basins. The Pb content in framboidal pyrite from Sofia and Karlovo basins ranged from 0.01% to 0.5%. These elements were found in the Ohio coals by Wiese et al. ( 1990). The unit-cell parameters of framboidal pyrite exceeded the standard parameters (a,, = 5.4723 A) in the coal from the Sofia Basin. This is ascribed to the higher Ba and Pb concentrations (Table 3). X-ray microprobe analysis of framboidal pyrite from the Sofia Basin indicated a S content of 52.3-52.4%, the recalculated formula being Fe,.o$,.9,.
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(a) Bacterial framboidal pyrite from Sofia Basin, SEM. (b) Bacterial framboidal pyrite with oval shape (oil immersion; Sofia Basin). (c) Bacterial framboidal pyrite from Karlovo Basin (oil immersion). (d) Inorganic
Fig. 4.
framboidal
pyrite (oil immersion;
Svoge Basin).
Eulledrul pyrite
Euhedral pyrite represents well shaped pyrite crystals. The term has been used by a number of authors: Grady ( 1977)) Crelling et al. ( 1983)) Wiese and Fyfe ( 1986)) Frankie and Hower ( 1987)) among others. Euhedral pyrite is separated into isolated and clustered varieties (Wiese and Fyfe, 1986; Querol et al., 1989) or isolated anhedra and aggregates of euhedral crystals (Renton and Bird, 1991). Various amounts of this form of pyrite were observed in the coal samples investigated (Table 2). Isolated crystals and clustered euhedral pyrite were observed in humodetrinite, collinite, and vitrodetrinite. They were generally associated with clay minerals (Fig. 9) and sometimes also with inorganic framboidal pyrite (Fig. 5). The aggregates of euhedral pyrite were lenticular (Fig. lob), beaded (Fig. 5)) or irregular in shape (Fig. 1Oa) . The structure of these aggregates is reminiscent of inorganic framboidal pyrite, differing from the latter by the absence of the characteristic spherical shape. In some aggregates, the crystals were situated close to one another and had a relatively uniform distribution (Figs. 9 and lOa), while others contained regions where the binding substance was predominant. Euhedral pyrite was frequently associated with inorganic framboidal pyrite. The separate crystals were dispersed chaotically around the globules or formed aggregates. These aggregates surrounded spherulites of inorganic framboidal pyrite and almost coalesced with them. In most cases the crystals of euhedral pyrite were small in size,
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Fig. 5. Framboidal and euhedrrd pyrite from the Pemik Basin (SEMI.
Fig. 6. Aggregate of inorganic framboidrd pyrite from the Balkan Basin (SEM)
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Fig. 7. Coalescence of two globules of inorganic framboidal pyrite (oil immersion: Sofia Basin).
rangi lng from submicron to l-2 pm. Only coal from the Balkan (Fig. 11) and Svoge (I 3g. IOC) basins contained larger crystals, up to 70-80 pm. The crystal forms are pentagon dodecal hedral, octahedral, or cubic (Figs. 5, 9 and 11). The impurity elements found in the
Fig. 8. Aggregate of inorganic framboidal pyrite with pentagondodecahedral and octahedral crystals from the Balkan Basin ( SEM),
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Table 3 Trace elements in the pyrite forms Element
Content in the pyrite forms (%) FramboidaJ
Be” Ti Cr” Mn co Ni Cu Zn” AS Ba Pb
0.00s 0.01-0.03 0.01-0.03 0.01-0.03
pyrite
Massive pyrite
Euhedral pyritea
Infiltrational
0.01-1.0 0.01-0.03 0.01 0.0-0.01 0.01-0.03
0.3 _
pyritea
_ 0.01-0.03 0.01 0.0-0.03 _
_ 0.001-0.01 0.1
0.005-0.01 _
0.05-o. _
0.01-0.1 0.01-0.5
0.01-0.03
0.0-O. I 0.01
I
0.01 _ _ 0.005 _ _ _
O.OlLO.1
” Only for the coal from the Balkan and Svoge basins
euhedral pyrite are listed in Table 3. Ni, Co, and As were detected only in the euh edral pyrite. The content of Cu is highest in this form and Pb is missing (Table 3). Massive
pyrite
Many authors denote pyrite grains with irregular shapes and different sizes by the term “massive pyrite” (Grady, 1977; Wiese and Fyfe, 1986; among others). Renton and Bird ( 1991) denote this pyrite as “irregular forms”. The massive pyrite is found in all coal
Fig. 9. Clustered euhedral pyrite associated
with kaolinite (SEM; Balkan Basin).
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Fig. IO. (a) Lenticular cluster anhedral pyrite (oil immersion; Sofia Basin). (b) Clustered anhedral pyrite with irregular shape (oil immersion, Sofia Basin),
(c)
Large crystals of euhedral pyrite in the coal from Svoge Basin.
(d) Homogeneous massive pyrite of irregular shape from Sofia Basin.
samples from all basins as irregular grains and sizes ranging from lo-20 pm to 34 mm. The homogeneous massive pyrite is mostly porous (Fig. 10d) , which is due to the inclusion of relict organic matter and clay minerals during the crystallization processes. In these cases, laser spectral analyses indicate the presence of Si and Al. Homogeneous massive pyrite can be lenticular or irregular (Fig. 10d). Aggregates of fine pyrite grains united by clay or organic matter are designated as cluster-like pyrite in this study. In some cases the mineralization of these aggregates leads to coalescence of part of the grains and homogeneous massive pyrite, with a mosaic structure (due to inclusions of the binding substance) being formed. This is illustrated in Fig. 12. The anthracites from the Svoge Basin contain microconcretions with rounded shapes and a zonal structure. The formation of these microconcretions was the result of repeated environmental changes from weakly acid to weakly alkaline character, which led to successivedeposition of massive siderite and massive pyrite. The latter is denoted as microconcretionary in this study. The content of trace elements in massive pyrite differs from that in other pyrite forms (Table 3). The concentration of Ti is especially high ( 1%) in massive pyrite in concretions from the Balkan Basin, whereas in the remaining coals it varies between 0.01% and 0.03%. This element was not detected in the Ohio coals (Wiese et al., 1990). The low Ba content is also noteworthy. The concentration of Mn is low as well. It should be pointed out that the elementary cell dimensions of massive pyrite are smaller than the standard pyrite (a,,= 5.3970 A) for the Sofia coal. Its calculated formula is Fe,.,I?SI.CJX.
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I 1.Euhedral pyrite from the Balkan Basin (SEM).
Fig. 12. Massive
(Pv”‘) and fmmboidal (pi’) pyrite from the Sofia Basin.
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Table 4 Content of the pyrite forms and total sulphur in the coal from the various cord basins Coal basins
Content of the pyrite forms (%) Framboidal
pyrite
Inorganic
Bacterial
Sofia Belibreg Karlovo
2-3 _
Pernik Balkan Suhostrel Svoge
l-2 l-2 2-3
l-2 l-2 l-2
Euhednl pyrite
Massive pyrite
Anhedral pyrite
Intiltrationrd pyrite
Total sulphur content (%)
1-2 l-2 l-2 1-2 2-3 1-2 2-3
45 1-2 1-2 2-3 2-3 2-3 34
<
2.1 2.1 2.5 2.9 3.1 1.8 3.5
Anhedral pJ)rite This term corresponds to pyrite forms whose shape depends on the shape of the plant debris in which they were deposited. There are two varieties of anhedral pyrite, which are treated as independent forms in other publications (Grady, 1977; Crelling et al., 1983; Wiese and Fyfe, 1986). The replacement anhedral pyrite is a result of the mineralization of cell walls and is denoted (Wiese and Fyfe, 1986) as “replacement of plant material” or massive pyrite replacement of organic matter (Querol et al., 1989). The other variety is the infilling anhedral pyrite, filling cell lumens and pores in the plant debris. This kind of pyrite is designated as “massive cell filling” (Grady, 1977), and “filling cell lumens” (Crelling et al., 1983; Wiese and Fyfe, 1986) and “massive pyrite filling cell structures” (Querol et al., 1989). The anhedral pyrite is rarely found in the coal types investigated (Table 4); replacement anhedral pyrite is common. It is deposited in the lumens of textinite, ulminite, fusinite ( Fig. 13)) sclerotinite, and suberinite, taking their shape. In other cases the replacing anhedral pyrite is associated with euhedral pyrite, which crystallizes in the internal part of the lumens (Fig. 13). In addition, pyrite may be deposited in the lumens of previously mineralized plant tissue (most often in sideritized tissues). This was observed in lignites from the Karlovo Basin, in the concretions of bituminous coal from the Balkan Basin (Fig. 14), and in the anthracites from the Svoge Basin. The infilling anhedral pyrite was observed primarily in fusinite. Concretions from the Balkan Basin contain anhedral pyrite formed as a result of the replacement of anhedral siderite. Infiltrational pyrite The term is used for pyrite deposited in fractures and cleats which determine the path of solutions penetrating into a coal bed. Two varieties are distinguished: infilling and replacing infiltrational pyrite. The infilling infiltration pyrite has two varieties: fracture- and cleattilling. This kind of infiltrational pyrite is designated as “massive fracture-filling” (Grady, 1977), “tilling cleat and fractures” (Crelling et al., 1983), “massive pyrite filling cleats” (Querol et al., 1989), and “cleat and fracture fillings” (Renton and Bird, 1991). Infiltrational pyrite is observed rarely in low-rank coal samples (Table 4). The fracture-filling
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Fig. 13. Intilling anhedral pyrite
(f.v”“)
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pyrite in the lumens of fusinite (SEM; Balkan B:asin )
infi Itrational pyrite is deposited in endogenous and exogenous fractures of the coal lay rers, filli .ng them completely (Fig. 15) or partially. The cleat-filling infiltrational pyrite (Fig. 16) is c,haracteristic mainly of the anthracite from the Svoge Basin. These two pyrite varie :ties
Fig. 14. Infilling anhedral pyrite in the lumens of sideritized fusinite from sulphide-carbonate concretions Basin).
(Balkan
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pyrite ( fy’) from the Svoge Basin
cause the replacement of minerals deposited earlier in fractures and cleats. Such a replacement is observed in coal from the Svoge Basin in association with clay minerals and infiltrational siderite (Fig. 16). Fracture- and cleat-filling infiltrational pyrite is usually homogenous and dense, but in some cases organic material or clay minerals particles from fracturing are included in them, as well as relicts from the minerals corroded by them. The pyrite which density-fills the fractures can be characterized as massive, fracture-filling pyrite. Free crystallization of pyrite is observed in some fractures where well shaped crystals
Fig. 16. Cleat-filling
infiltrational
pyrite from the Svoge Basin.
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are formed. This variety can be classified as euhedral fracture-filling pyrite. Replacement infiltrational pyrite can also encrust tissue, as is often observed with concretions from the Balkan Basin, and even fully pyritize them and accept their form. Accessory elements in infiltrational pyrite are Ti, Mn and Cu (Table 3). 3.2. Distribution of pyrite forms from the coals in Bulgarian basins The lignites from the Sofia Basin contain a large amount of pyrite. The prevailing form is that of framboidal bacterial pyrite, inorganic framboidal pyrite also being present (Table 4). Infiltrational fracture-filling pyrite is seen in a few cases. The apparent acidity of the depositional environment, as determined by the acidity diagram (Fig. 2), varies over a wide range (from 3.5 to 7). Framboidal pyrite in coal from Belibreg appears to only be inorganic. Homogeneous massive pyrite is present (Table 4). The environment acidity of the peat bog was relatively constant (pH 6.5-7.5, Fig. 2). The lignites from Karlovo are characterized by the presence of various forms of pyrite (Table 4). Infiltrational pyrite was not observed. The apparent environmental pH values range from 3.5 to 6.5 (Fig. 2). All varieties of pyrite are present in the subbituminous coal from the Pernik Basin (Table 4). Of the infiltrational variety, mainly the fracture-filling pyrite is found, with rare cleatfilling pyrite. The pH in the peat bog varied from 3.5 to 6.5 (Fig. 2). Bituminous coal from the Balkan Basin contains all varieties of pyrite. Infiltrational pyrite is present, with the fracture-filling form being predominant. Massive and euhedral varieties are observed as well (Table 4). The environment pH in the ancient peat bog ranged from 4 to 7 (Fig. 2). Bituminous coal from the Suhostrel deposit contains only a very small amount of pyrite. Inorganic framboidal pyrite was not found and a small amount of bacterial framboidal pyrite was observed. Anhedral pyrite is quite rare and only the infilling variety is observed. The environment in the peat bog was apparently acid (pH 3-5, Fig. 2). Many forms of pyrite are present in the anthracites from the Svoge Basin. The acidity varied between 3.5 and 6.5 (Fig. 2). 3.3. Factors qffecting the occurrence of the various pyrite forms The main factors affecting the distribution of pyrite forms of pyrite are: 1. The presence of Fe and S in the rocks adjacent to the ancient peat bogs; 2. The activities of ground and surface waters, and water streams, which brought Fe and S into the peat bogs; 3. The acidity ( pH), redox potential (Eh), and development of sulphur bacteria; 4. The tectonic conditions and the presence of mineralizing solutions during the lithogenesis. The first two factors are determinant for the penetration of Fe and S in the ancient peat bogs. The pyrite content in the coals from the Suhostrel and Belibreg basins is lower that the other coals (Table 4). This lower pyrite content is a result of the low Fe and S contents in the adjacent rocks of the peat bogs or the insufficient activities of ground and surface
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waters, which brought Fe and S in the peat bogs. Therefore, the S content is lowest in the coals from the Suhostrel and Belibreg basins (Table 4). The association between Fe and S in a pyrite depends on a third factor. pH values of 67.5 are optimal for the sulphur bacteria (Kizilstein, 1968). The low pyrite content in the coal from the Suhostrel Basin is a result of the highest environmental acidity in the ancient peat bog (Fig. 2). In the lignite basins under consideration, the clayey rocks in the roof and underclay served as a barrier and prevented the penetration of mineral solutions into the peats. Only in the Sofia Basin, as a result of isolated faults during early diagenesis, was infiltrational fracturefilling pyrite formed. The tectonics of the Pernik and Suhostrel basins were much more active, and the beds exhibit numerous of tectonic fractures. Mineral solutions have penetrated in these fractures and fracture-filling infiltrational pyrite has been deposited. The coal deposits in the Balkan and in Svoge basins have been repeatedly folded, facilitating the deposition of fracture-filling, cleat-filling, and replacement infiltrational pyrite in the strongly disturbed coal beds.
Acknowledgements The authors wish to thank Prof. Dr. G. Siskov from the University during the preparation of the present paper.
of Sofia for her help
References Boctor. N.Z.. Kullerud, G. and Sweany, J.L., 1976. Sulfide minerals in SeelyvilleCoal III, Chinook mine, Indiana. Miner. Deposita, II: 249-266. Crelling. J.C.. Stuttzman. P.E. and Robinson, P.D.. 1983. Evidence from the Henin No. 6 coal seam for pre- and post-burial differences in the sulfur and mineral content of peat. In: R. Raymond Jr. and M.J. Andrejko (Editors), Mineral Matter in Peat: Its Occurrence, Form and Distribution. Los Alamos National Laboratory, Los Alamos, N.M., pp. I13-121. Fan-and. M., 1970. Framboidal sulphides precipitated synthetically. Mineral. Deposita, 5: 237-247. Frankie. K.A. and Hower, J.C.. 1987. Variation in pyrite size, formand microlithotype association in the Springfield (No. 9) and Herrin (No. 11) coals, Western Kentucky. Int. 1. Coal Geol., 7: 349-364. Grady, W.C.. 1977. Microscopic varieties of pyrite in West Virginia coals. Trans. Sot. Min. Eng. AIME. 262: 268-274. Kamenov. 246.
B.. 1964. Varkhu stratigrafiata
i vaglenosnostta
na paleogena
ot Perniskia basein. Izv. NIGI, I: 233-
Kizilstein. L.Y.. 1968. K voprosu o biogeokhimii sery v uglyakh. Khim. Tverd. Topl., 5: 3-13. Kizilstem. L.Y. and Minaeva. L.C.. 1972. Proiskhozhdenije framboidalnikh form pyrita. Dokl. Acad. Nauk SSSR, 206(i): 1187-l 189. Kirilstein, L.Y. and Trufanov. V.N., 1968. Proiskhozhdenija singenetichnogo pyrita v ugolnikh plastakh. In: Ugienosnije Formatsii i Ugolnije Mestorozhdenija. Nauka, Moscow, pp. 123-127. Kortenski. J.. 1984. Varkhu genezisa i raspredelenieto na pyrita vav vaglishtata ot Sofiiskia pliocenski basein. Annul. Vissh Minno-geol. Inst. Sofia, 30(2): 205-214 (in Bulgarian, with English and Russian abstracts). Kortenski. J.. 1992. Carbonate minerals in Bulgarian coals with different degrees of coalification. Int. J. Coal Geol.. 20: 225-242.
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Querol. X.. Chinenon. S. and Lopez-Soler, A., 1989. Iron sulfide precipitation sequence in Albian coals from the Maestrazgo Basin, Southeastern Iberian Range, Northeastern Spain. Int. J. Coal. Geol., 1 I: 171-189. Ramdohr, P., 1960. Die Erzmineralen und ihre Vetwachsungen. Berlin, 1132 pp. Renton. J.J. and Bird, D.S.. I99 I. Association of coal macerals, sulfur, sulfur species and the iron disuhide minerals in three columns of the Pittsburgh coal. Int. J. Coal Geol., 17: 21-50. Siskov. G., 1986. Kratak geolozhki ocherk za vaglistniten baseini i nahodista v Bulgaria. In: Vagglista i Vagvistni Baseini v Bulgaria. Sofia Univ., pp. 19-32. Skripchenko,
N.S. and Berberian. T.K., 1975. Struktura framboidalnogo
pyrita. GeoI. Rudn. Mestorozhd.,
5: 107-
112. Vaughan, D. and Craig, J., 1978. Mineral Chemistry 410 pp.
of Metal Sulfides. Cambridge
University Press, Cambridge,
Wiese Jr.. R.G. and Fyfe. W.S., 1986. Occurrence of iron sulfides in Ohio coals. Int. J. Coal Geol., 6: 251-276. Wiese Jr., R.G.. Muir. I.Y. and Fyfe, W.S., 1990. Trace-element siting in iron sulfides in Ohio coals determined by secondary ion mass spectrometry (SIMS). Int. J. Coal Geol., 14: 155-174.
International
Journal of Coal Geology 29 ( 1996) 273-290
Occurrence and morphology of pyrite in Bulgarian coals Jordan Kortenski a**,Irena Kostova b a University of Mining and Geology, Sojia 1156, Bulgaria h Institute ofAppliedMineralogy, Bulgarian Academy of Science, Sojia 1000, Bulgaria Received 2 November
1994; accepted 30 August 1995
Abstract Coals with different degrees of coalification (ranging from lignite to anthracite) from seven Bulgarian coal basins have been investigated. The forms of pyrite and their distribution have been established. The types found are: massive pyrite, represented by the homogeneous, cluster-like and microconcretionary varieties; framboidal pyrite, appearing in inorganic and bacterial forms; euhedral pyrite, which is either isolated or clustered; anhedral pyrite, in its infilling and replacement varieties; and infiltrational pyrite, as a replacement or infilling mineral. Most of the forms of the euhedral, framboidal and massive pyrite developed during peat deposition. The anhedral replacement pyrite formed in the peat bed during early diagenesis. Infiltrational pyrite filled fractures and cleats formed during the diagenesis, catagenesis and metagenesis. Both similarities and differences with respect to the distribution of the pyrite types have been determined between coals of different ranks from Bulgarian coal basins. These differences are due to: the presence of Fe and S in the rocks adjacent to ancient peat bogs; the activities of ground and surface waters which brought Fe and S into the peat bogs; the geochemical character (pH and Eh) of the peat bogs and the sulphur bacteria development; and the tectonic situation during diagenesis, catagenesis and metagenesis.
1. Introduction This investigation deals with pyrite occurring in the coals of several Bulgarian basins. The purpose of the study is to establish the types of pyrite, the genesis of these forms, and factors affecting their appearance and distribution. Coal samples with different degrees of coalification and different formation conditions were chosen for the investigations. Lignite specimens originated from the Sofia, Belibreg, and Karlovo Basins (Fig. 1). The tectonic * Corresponding
author
0166-5 162/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSD10166-5162(95)00033-X
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E
N
Geology 29 (1996) 273-290
D
- Lignite =
- Sub-bituminous
m
- Bituminous
coal coal
- Anthracite Fig. I. Location of the coal basins in Bulgaria studied. 1= Sofia; 2 = Belibreg; 3 = Karlovo; 4 = Pemik; 5 = Balkan; 6= Suhostrel; 7 = Svoge.
setting of these basins is relatively stable, with minor faulting (Kamenov, 1964). The subbituminous coal samples were collected from the Pernik Basin (Fig. 1) , where the coal deposits are slightly folded and faulted. Bituminous coals from the Balkan and Suhostrel Basins and anthracite from the Svoge Basin were also investigated (Fig. 1). The coal deposits from the Balkan and Svoge basins have undergone complicated folding and faulting, including nappe formation in the Balkan Basin. Table 1 shows the age of the coalbearing sediments and their lithostratigraphy, as well as the number of coal beds and their rank.
2. Methods Coal and coal shale samples from all beds of the coal basins were studied. These include: core samples from lignite beds of the Sofia (252 samples) and Karlovo (47 samples) basins, and from anthracite beds of the Svoge Basin (81 samples); channel samples from
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215
Table I Age and rank of the coal from the basins” Coal basins
Sofia
Age
Rank of coal
Coal beds Total number
Economic
3-s
1-2
Lignite B
Belibreg Karlovo Pernik Balkan
Pontian Dacian Pliocene Pliocene (?) Upper Oligocene Cenomanian
1-5 II 5 3-8
1 I 3 3-8
Suhostrel
Upper Eocene
I
I
Svoge
NamuriarWestfalian
7-17
7-17
Lignite B Lignite B Subbituminous Bituminous Gas-Lean Bituminous Fat-Coking Anthracite-Metaanthracite
” The data are from Siskov ( 1986).
the lignite bed of the Belibreg Basin (38 samples), subbituminous beds of the Pernik Basin (62 samples), all bituminous beds of the Balkan ( 109 samples) and Suhostrel ( 15 samples) basins, and anthracite beds of the Svoge Basin (24 samples). Optical studies were carried under reflected light with dry and oil immersion objectives using a NU-2 Universal microscope. A total of 155 polished sections were studied. Electron microscope (Tesla) observations were performed using a coal replica ( 10 samples). X-ray diffraction analyses by the Debye-Scherrer method of 40 samples were made with a TURM-60 apparatus, using iron anticathodes with manganese filters at 35 kV and 12 mA. A Philips scanning electron microscope with a tungsten cathode and an EDAX 9100 X-ray micro-analyzer was used for X-ray microprobe analysis of 20 samples. Trace elements in 72 pyrite samples were determined by laser microspectral analysis in an LMA apparatus equipped with an O-24 spectrograph. Chemical analysis data on coal ash from 56 samples were applied to the determination of the environmental acidity in peat bogs. For that purpose the acidity diagram of Kortenski ( 1992) was used (Fig. 2). The diagram is based on two parameters: ( 1) the acidity coefficient of the coal ash; and (2) the ash content of the coals. The former parameter is determined as a ratio of the total content of acid oxides (SiO,, A1203, SO3 and P205) to the total content of the alkali oxides (CaO, MgO, Fe203, NazO and K*O). The content of ash is also used as an environmental parameter because its increase leads to a decrease in the content of the humic acids as well as the total acidity of the peat-forming environment.
3. Results and discussion 3.1.
Description
of pyrite forms
A series of papers have been published on the presence of pyrite in coals. Various forms of pyrite have been described by Grady ( 1977), Crelling et al. (1983), Wiese and Fyfe
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10
20
30
40 Ash
50 -
60 70
80 90
Ad,?”
Fig. 2. Diagram of the acidity. C,, = SiOz + A&O? + SO3 + P20,/Ca0 + MgO + Fez03 + 2 = Belibreg; 3 = Karlovo; 4 = Pemik; 5 = Balkan; 6 = Suhostrel; 7 = Svoge.
Na,O + KZO.1 = Sofia;
( 1986)) Frankie and Hower ( 1987), Querol et al. ( 1989), Renton and Bird ( 1991)) among others. The classification used for the description of the pyrite forms is given in Table 2. Framboidal pyrite
The framboidal pyrite represents spherical pyrite particles. The term ‘framboidal’ is widely applied in the literature. However, some authors use also other terminology; for example, ‘cauliflower’ and ‘blebs’, which are mentioned by Wiese and Fyfe ( 1986). The origin of this kind of pyrite has been discussed in a number of papers. Many researchers, especially those studying framboidal pyrite in coals (Kizilstein and Trufanov, 1968; Kizilstein and Minaeva, 1972; Skripchenko and Berberian, 1975) assume this type of pyrite to consist of pyritized sulphur bacteria. Kizilstein (1968) has suggested the possibility of pyritization of other kinds of bacteria which might have coexisted along with the sulphurmetabolizing bacteria and helped the decomposition and assimilation of the plant tissues. Others, investigating mainly framboidal pyrite in ore deposits, reject its bacterial origin and assume pyrite formation from mineral solutions in inorganic material (Ramdohr, 1960; Farrand, 1970; among others). Similar views have been expressed by Wiese and Fyfe ( 1986) concerning framboidal pyrite in an Ohio coal. They assumed part of the pyrite to represent pyritized algal cells or fungal spores. Both opinions have been confirmed experimentally. In the first case, laboratory experiments were performed and the framboidal forms
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Table 2 Classification
of pyrite forms in coals
Place of formation
Time of formation
Morphology
Pyrite forms
Peat bog
Peat genesis
Euhedral
Isolated Clustered Bacterial Inorganic Cluster-like Homogeneous Microconcretional infilling Replacement Infilling
Framboidal
Massive
Coal bed
Early diagenesis
Anhedral
Late diagenesis Catagenesis
lntiltration
Massive
Metagenesis
Euhedral Replacement
obtained in the presence of various kinds of bacteria were compared (Kizilstein and Minaeva, 1972). To prove the second supposition, mineral solutions containing a certain amount of organics in the absence of bacteria were used (Farrand, 1970). In a study of pyrite in coal from the basins of Sofia, Kortenski ( 1984) suggested the presence of both forms. The globules resulting from massive mineralization have been considered to be pyritized bacteria. Renton and Bird (1991) designated this form as completely coalescent framboids. Framboids showing well shaped crystals are likely to be a result of the crystallization of mineral solutions in the organic matter. This form was described by Renton and Bird ( 1991) as discrete framboids and partly coalescent framboids. In this study, the first type is referred to as bacterial with massive mineralization, and the second as inorganic framboidal pyrite with well shaped crystals. Bacterial framboidal pyrite is found in all the coal samples investigated (Table 2). This type is characterized by the, generally, nonuniform mineralization of the globules (Fig. 3). Some of the globules are densely pyritized (Fig. 4a and b), while others exhibit only a partial pyritization (Fig. 4~). The separate framboids are rarely densely rimmed. Bacterial framboidal pyrite preserves the independence of the separate globules even when they form aggregates (Skripchenko and Berberian, 1975) (Fig. 3). Some of the aggregates are pyritized bacterial colonies, as observed for coal from the Svoge Basin. Bacterial framboidal pyrite seldom appears as single bodies (Fig. 4b). More often, it is found as aggregates of smaller or larger globules, irregular or lenticular, or, less frequently, beaded in shape. Bacterial framboidal pyrite is not associated with euhedral pyrite and is often rimmed by clay minerals. In the latter cases, laser spectral analyses show the presence of Si and Al, as also noted by Wiese and Fyfe ( 1986)) Boctor et al. ( 1976) and Wiese et al. ( 1990). Inorganic framboidal pyrite exhibits the relatively uniform mineralization of the globules (Fig. 5). Only in a few cases do the framboids contain solitary crystals (Figs. 4d and 6) and the binding substance (organic matter and clay minerals) prevails. This form of pyrite exhibits a symmetrical, mostly concentric, ordering of the crystals in the globules (Fig. 7). In many of the inorganic framboidal pyrite globules the crystals are densely intergrown and
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Fig. 3. Bacterial fmmboidal pyrite from the Sofia Basin (oil immersion),
developed upon the earlier-formed crystals in the centre of the globules (Fig. 8). In contrast to the bacterial pyrite, inorganic framboidal pyrite is sometimes characterized by coalescence of globules (Figs. 6 and 7). The pyrite crystals in individual framboidal bodies are usually equal in size (Fig. 6). Crystals with different sizes are rarely found (Figs. 7 and 8). As a rule, crystals of larger sizes are in the centre of the spherulites (Fig. 7 - the globule on the right). The crystals are octahedral (Fig. 6)) pentagondodecahedral (Fig. 8) and cubic (Fig. 8). In almost all cases inorganic framboidal pyrite is associated not only with clay minerals but also with isolated crystals or aggregates of euhedral pyrite (Fig. 5). This suggests a similar formation mechanism for the two pyrite types. All coal samples from the Bulgarian basins show a regular spheritic form of framboidal pyrite, with a few exceptions being in samples from the Sofia Basin (Fig. 4b - oval form). Some differences are observed at higher ranks. In the anthracite from the Svoge Basin the expected spherical shape is in places oval, perhaps due to pressure, which is rarely observed. The sizes of framboidal pyrite vary over a wide range: from l-2 pm to 50-70 pm, with some above 100 pm. Laser spectral analysis has shown some characteristic features of framboidal pyrite in the coals investigated (Table 3), which are similar to features found by Vaughan and Craig ( 1978). The absence of Ni and Co should be mentioned (Table 3). Only the framboidal pyrite from the Svoge Basin showed the presence of Cr and Zn, while Be was only present in samples from the Balkan Basin. Pb was absent from the latter two basins. The Pb content in framboidal pyrite from Sofia and Karlovo basins ranged from 0.01% to 0.5%. These elements were found in the Ohio coals by Wiese et al. ( 1990). The unit-cell parameters of framboidal pyrite exceeded the standard parameters (a,, = 5.4723 A) in the coal from the Sofia Basin. This is ascribed to the higher Ba and Pb concentrations (Table 3). X-ray microprobe analysis of framboidal pyrite from the Sofia Basin indicated a S content of 52.3-52.4%, the recalculated formula being Fe,.o$,.9,.
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(a) Bacterial framboidal pyrite from Sofia Basin, SEM. (b) Bacterial framboidal pyrite with oval shape (oil immersion; Sofia Basin). (c) Bacterial framboidal pyrite from Karlovo Basin (oil immersion). (d) Inorganic
Fig. 4.
framboidal
pyrite (oil immersion;
Svoge Basin).
Eulledrul pyrite
Euhedral pyrite represents well shaped pyrite crystals. The term has been used by a number of authors: Grady ( 1977)) Crelling et al. ( 1983)) Wiese and Fyfe ( 1986)) Frankie and Hower ( 1987)) among others. Euhedral pyrite is separated into isolated and clustered varieties (Wiese and Fyfe, 1986; Querol et al., 1989) or isolated anhedra and aggregates of euhedral crystals (Renton and Bird, 1991). Various amounts of this form of pyrite were observed in the coal samples investigated (Table 2). Isolated crystals and clustered euhedral pyrite were observed in humodetrinite, collinite, and vitrodetrinite. They were generally associated with clay minerals (Fig. 9) and sometimes also with inorganic framboidal pyrite (Fig. 5). The aggregates of euhedral pyrite were lenticular (Fig. lob), beaded (Fig. 5)) or irregular in shape (Fig. 1Oa) . The structure of these aggregates is reminiscent of inorganic framboidal pyrite, differing from the latter by the absence of the characteristic spherical shape. In some aggregates, the crystals were situated close to one another and had a relatively uniform distribution (Figs. 9 and lOa), while others contained regions where the binding substance was predominant. Euhedral pyrite was frequently associated with inorganic framboidal pyrite. The separate crystals were dispersed chaotically around the globules or formed aggregates. These aggregates surrounded spherulites of inorganic framboidal pyrite and almost coalesced with them. In most cases the crystals of euhedral pyrite were small in size,
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Fig. 5. Framboidal and euhedrrd pyrite from the Pemik Basin (SEMI.
Fig. 6. Aggregate of inorganic framboidrd pyrite from the Balkan Basin (SEM)
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Fig. 7. Coalescence of two globules of inorganic framboidal pyrite (oil immersion: Sofia Basin).
rangi lng from submicron to l-2 pm. Only coal from the Balkan (Fig. 11) and Svoge (I 3g. IOC) basins contained larger crystals, up to 70-80 pm. The crystal forms are pentagon dodecal hedral, octahedral, or cubic (Figs. 5, 9 and 11). The impurity elements found in the
Fig. 8. Aggregate of inorganic framboidal pyrite with pentagondodecahedral and octahedral crystals from the Balkan Basin ( SEM),
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Table 3 Trace elements in the pyrite forms Element
Content in the pyrite forms (%) FramboidaJ
Be” Ti Cr” Mn co Ni Cu Zn” AS Ba Pb
0.00s 0.01-0.03 0.01-0.03 0.01-0.03
pyrite
Massive pyrite
Euhedral pyritea
Infiltrational
0.01-1.0 0.01-0.03 0.01 0.0-0.01 0.01-0.03
0.3 _
pyritea
_ 0.01-0.03 0.01 0.0-0.03 _
_ 0.001-0.01 0.1
0.005-0.01 _
0.05-o. _
0.01-0.1 0.01-0.5
0.01-0.03
0.0-O. I 0.01
I
0.01 _ _ 0.005 _ _ _
O.OlLO.1
” Only for the coal from the Balkan and Svoge basins
euhedral pyrite are listed in Table 3. Ni, Co, and As were detected only in the euh edral pyrite. The content of Cu is highest in this form and Pb is missing (Table 3). Massive
pyrite
Many authors denote pyrite grains with irregular shapes and different sizes by the term “massive pyrite” (Grady, 1977; Wiese and Fyfe, 1986; among others). Renton and Bird ( 1991) denote this pyrite as “irregular forms”. The massive pyrite is found in all coal
Fig. 9. Clustered euhedral pyrite associated
with kaolinite (SEM; Balkan Basin).
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Fig. IO. (a) Lenticular cluster anhedral pyrite (oil immersion; Sofia Basin). (b) Clustered anhedral pyrite with irregular shape (oil immersion, Sofia Basin),
(c)
Large crystals of euhedral pyrite in the coal from Svoge Basin.
(d) Homogeneous massive pyrite of irregular shape from Sofia Basin.
samples from all basins as irregular grains and sizes ranging from lo-20 pm to 34 mm. The homogeneous massive pyrite is mostly porous (Fig. 10d) , which is due to the inclusion of relict organic matter and clay minerals during the crystallization processes. In these cases, laser spectral analyses indicate the presence of Si and Al. Homogeneous massive pyrite can be lenticular or irregular (Fig. 10d). Aggregates of fine pyrite grains united by clay or organic matter are designated as cluster-like pyrite in this study. In some cases the mineralization of these aggregates leads to coalescence of part of the grains and homogeneous massive pyrite, with a mosaic structure (due to inclusions of the binding substance) being formed. This is illustrated in Fig. 12. The anthracites from the Svoge Basin contain microconcretions with rounded shapes and a zonal structure. The formation of these microconcretions was the result of repeated environmental changes from weakly acid to weakly alkaline character, which led to successivedeposition of massive siderite and massive pyrite. The latter is denoted as microconcretionary in this study. The content of trace elements in massive pyrite differs from that in other pyrite forms (Table 3). The concentration of Ti is especially high ( 1%) in massive pyrite in concretions from the Balkan Basin, whereas in the remaining coals it varies between 0.01% and 0.03%. This element was not detected in the Ohio coals (Wiese et al., 1990). The low Ba content is also noteworthy. The concentration of Mn is low as well. It should be pointed out that the elementary cell dimensions of massive pyrite are smaller than the standard pyrite (a,,= 5.3970 A) for the Sofia coal. Its calculated formula is Fe,.,I?SI.CJX.
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I 1.Euhedral pyrite from the Balkan Basin (SEM).
Fig. 12. Massive
(Pv”‘) and fmmboidal (pi’) pyrite from the Sofia Basin.
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Table 4 Content of the pyrite forms and total sulphur in the coal from the various cord basins Coal basins
Content of the pyrite forms (%) Framboidal
pyrite
Inorganic
Bacterial
Sofia Belibreg Karlovo
2-3 _
Pernik Balkan Suhostrel Svoge
l-2 l-2 2-3
l-2 l-2 l-2
Euhednl pyrite
Massive pyrite
Anhedral pyrite
Intiltrationrd pyrite
Total sulphur content (%)
1-2 l-2 l-2 1-2 2-3 1-2 2-3
45 1-2 1-2 2-3 2-3 2-3 34
<
2.1 2.1 2.5 2.9 3.1 1.8 3.5
Anhedral pJ)rite This term corresponds to pyrite forms whose shape depends on the shape of the plant debris in which they were deposited. There are two varieties of anhedral pyrite, which are treated as independent forms in other publications (Grady, 1977; Crelling et al., 1983; Wiese and Fyfe, 1986). The replacement anhedral pyrite is a result of the mineralization of cell walls and is denoted (Wiese and Fyfe, 1986) as “replacement of plant material” or massive pyrite replacement of organic matter (Querol et al., 1989). The other variety is the infilling anhedral pyrite, filling cell lumens and pores in the plant debris. This kind of pyrite is designated as “massive cell filling” (Grady, 1977), and “filling cell lumens” (Crelling et al., 1983; Wiese and Fyfe, 1986) and “massive pyrite filling cell structures” (Querol et al., 1989). The anhedral pyrite is rarely found in the coal types investigated (Table 4); replacement anhedral pyrite is common. It is deposited in the lumens of textinite, ulminite, fusinite ( Fig. 13)) sclerotinite, and suberinite, taking their shape. In other cases the replacing anhedral pyrite is associated with euhedral pyrite, which crystallizes in the internal part of the lumens (Fig. 13). In addition, pyrite may be deposited in the lumens of previously mineralized plant tissue (most often in sideritized tissues). This was observed in lignites from the Karlovo Basin, in the concretions of bituminous coal from the Balkan Basin (Fig. 14), and in the anthracites from the Svoge Basin. The infilling anhedral pyrite was observed primarily in fusinite. Concretions from the Balkan Basin contain anhedral pyrite formed as a result of the replacement of anhedral siderite. Infiltrational pyrite The term is used for pyrite deposited in fractures and cleats which determine the path of solutions penetrating into a coal bed. Two varieties are distinguished: infilling and replacing infiltrational pyrite. The infilling infiltration pyrite has two varieties: fracture- and cleattilling. This kind of infiltrational pyrite is designated as “massive fracture-filling” (Grady, 1977), “tilling cleat and fractures” (Crelling et al., 1983), “massive pyrite filling cleats” (Querol et al., 1989), and “cleat and fracture fillings” (Renton and Bird, 1991). Infiltrational pyrite is observed rarely in low-rank coal samples (Table 4). The fracture-filling
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Fig. 13. Intilling anhedral pyrite
(f.v”“)
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pyrite in the lumens of fusinite (SEM; Balkan B:asin )
infi Itrational pyrite is deposited in endogenous and exogenous fractures of the coal lay rers, filli .ng them completely (Fig. 15) or partially. The cleat-filling infiltrational pyrite (Fig. 16) is c,haracteristic mainly of the anthracite from the Svoge Basin. These two pyrite varie :ties
Fig. 14. Infilling anhedral pyrite in the lumens of sideritized fusinite from sulphide-carbonate concretions Basin).
(Balkan
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pyrite ( fy’) from the Svoge Basin
cause the replacement of minerals deposited earlier in fractures and cleats. Such a replacement is observed in coal from the Svoge Basin in association with clay minerals and infiltrational siderite (Fig. 16). Fracture- and cleat-filling infiltrational pyrite is usually homogenous and dense, but in some cases organic material or clay minerals particles from fracturing are included in them, as well as relicts from the minerals corroded by them. The pyrite which density-fills the fractures can be characterized as massive, fracture-filling pyrite. Free crystallization of pyrite is observed in some fractures where well shaped crystals
Fig. 16. Cleat-filling
infiltrational
pyrite from the Svoge Basin.
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are formed. This variety can be classified as euhedral fracture-filling pyrite. Replacement infiltrational pyrite can also encrust tissue, as is often observed with concretions from the Balkan Basin, and even fully pyritize them and accept their form. Accessory elements in infiltrational pyrite are Ti, Mn and Cu (Table 3). 3.2. Distribution of pyrite forms from the coals in Bulgarian basins The lignites from the Sofia Basin contain a large amount of pyrite. The prevailing form is that of framboidal bacterial pyrite, inorganic framboidal pyrite also being present (Table 4). Infiltrational fracture-filling pyrite is seen in a few cases. The apparent acidity of the depositional environment, as determined by the acidity diagram (Fig. 2), varies over a wide range (from 3.5 to 7). Framboidal pyrite in coal from Belibreg appears to only be inorganic. Homogeneous massive pyrite is present (Table 4). The environment acidity of the peat bog was relatively constant (pH 6.5-7.5, Fig. 2). The lignites from Karlovo are characterized by the presence of various forms of pyrite (Table 4). Infiltrational pyrite was not observed. The apparent environmental pH values range from 3.5 to 6.5 (Fig. 2). All varieties of pyrite are present in the subbituminous coal from the Pernik Basin (Table 4). Of the infiltrational variety, mainly the fracture-filling pyrite is found, with rare cleatfilling pyrite. The pH in the peat bog varied from 3.5 to 6.5 (Fig. 2). Bituminous coal from the Balkan Basin contains all varieties of pyrite. Infiltrational pyrite is present, with the fracture-filling form being predominant. Massive and euhedral varieties are observed as well (Table 4). The environment pH in the ancient peat bog ranged from 4 to 7 (Fig. 2). Bituminous coal from the Suhostrel deposit contains only a very small amount of pyrite. Inorganic framboidal pyrite was not found and a small amount of bacterial framboidal pyrite was observed. Anhedral pyrite is quite rare and only the infilling variety is observed. The environment in the peat bog was apparently acid (pH 3-5, Fig. 2). Many forms of pyrite are present in the anthracites from the Svoge Basin. The acidity varied between 3.5 and 6.5 (Fig. 2). 3.3. Factors qffecting the occurrence of the various pyrite forms The main factors affecting the distribution of pyrite forms of pyrite are: 1. The presence of Fe and S in the rocks adjacent to the ancient peat bogs; 2. The activities of ground and surface waters, and water streams, which brought Fe and S into the peat bogs; 3. The acidity ( pH), redox potential (Eh), and development of sulphur bacteria; 4. The tectonic conditions and the presence of mineralizing solutions during the lithogenesis. The first two factors are determinant for the penetration of Fe and S in the ancient peat bogs. The pyrite content in the coals from the Suhostrel and Belibreg basins is lower that the other coals (Table 4). This lower pyrite content is a result of the low Fe and S contents in the adjacent rocks of the peat bogs or the insufficient activities of ground and surface
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waters, which brought Fe and S in the peat bogs. Therefore, the S content is lowest in the coals from the Suhostrel and Belibreg basins (Table 4). The association between Fe and S in a pyrite depends on a third factor. pH values of 67.5 are optimal for the sulphur bacteria (Kizilstein, 1968). The low pyrite content in the coal from the Suhostrel Basin is a result of the highest environmental acidity in the ancient peat bog (Fig. 2). In the lignite basins under consideration, the clayey rocks in the roof and underclay served as a barrier and prevented the penetration of mineral solutions into the peats. Only in the Sofia Basin, as a result of isolated faults during early diagenesis, was infiltrational fracturefilling pyrite formed. The tectonics of the Pernik and Suhostrel basins were much more active, and the beds exhibit numerous of tectonic fractures. Mineral solutions have penetrated in these fractures and fracture-filling infiltrational pyrite has been deposited. The coal deposits in the Balkan and in Svoge basins have been repeatedly folded, facilitating the deposition of fracture-filling, cleat-filling, and replacement infiltrational pyrite in the strongly disturbed coal beds.
Acknowledgements The authors wish to thank Prof. Dr. G. Siskov from the University during the preparation of the present paper.
of Sofia for her help
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