Geochemistry of autochthonous and hypautochthonous siderite-dolomite coal-balls (Foord Seam,...

International Journal of

coal ELSEVIER International Journal of Coal Geology 29 (1996) 199-216

Geochemistry of autochthonous and hypautochthonous siderite-dolomite coal-balls

(Foord Seam, Bolsovian, Upper Carboniferous), Nova Scotia, Canada

Erwin L. Zodrow a,., Paul C. Lyons b, Michael A. Millay c University College of Cape Breton, Department of Earth Sciences, Sydney, Nova Scotia, BIP 6L2, Canada

b U.S. Geological Survey, MS 956National Center, Reston, VA 22092, USA c Ohio University Southern Campus, lronton, OH 45638-2296, USA

Received 24 June 1994; accepted 6 April 1995

Abstract

The 11-13 m thick Foord Seam in the fault-bounded Stellarton Basin, Nova Scotia, is the thickest seam from the Euramerican floral province known to contain coal-balls. In addition to the first discovery of autochthonous coal-balls in the Foord Seam, Nova Scotia, its shale parting also contains hypautochthonous coal-balls with histologically preserved plant structures. The coal-bali discovery helps fill a stratigraphic gap in coal-ball occurrences in the upper Carboniferous (Bolsovian) of Euramerica. The autochthonous and hypautochthonous coal-balls have a similar mineralogical composition and are composed of siderite (81-100%), dolomite-ankerite (0-19%), minor quartz and illite, and trace amounts of 'calcite'. Similar is also their permineralizing mineralogy, which consists of dolomite-ankerite and siderite. Their low pyrite content and carbonate mineralogy, and nonmarine origin, differentiates the Foord Seam coal-balls from other Euramerican coal-ball occurrences.

A preliminary geochemical model, which is based on oxygen and carbon isotopic data, indicates that siderite in both the autochthonous and hypautochthonous coal-balls is of very early diagenetic (nonmarine) origin from ~3C-enriched bicarbonate derived from bacterial methanogenesis of organic matter.

1. Introduct ion

Coal-balls, which are nodular-like rocks indigenous to coal seams, contain histologically preserved, permineralized plant organs or tissue with or without faunal remains (Stopes

* Corresponding Author

0166-5162 / 96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDIO166-5 162( 95 )00008-9

200 E.L. Zodrow et al. / International Journal of Coal Geology 29 (1996) 199-216

Fig. 1. A geological sketch map of the Stellarton Basin showing the location of the Foord Pit and Seam. Map modified from Yen and Ruixiang (1987).

and Watson, 1908; Mamay and Yochelson, 1962, Scott and Rex, 1985; Lyons et al., 1995). They are a very significant source of information on the anatomy and structure of the plants that formed Carboniferous peat and on the nature of Carboniferous plant communities (Taylor and Taylor, 1993). Coal-balls have no known modern analogue, probably owing to a combination of unique palaeoclimatic and geochemical conditions during the Carbon- iferous.

The history of coal-ball discoveries in the Canadian Maritime Provinces began in the early 1950s. In 1952, A.T. Cross (then at West Virginia University) and J.H. Hoskins (University of Cincinnati) found loose coal-balls in a coal mine dump near Stellarton, Nova Scotia (A.T. Cross, oral commun., May, 1992). This discovery was not reported in the literature. Baxter (1960) described coal-balls in the Clifton Formation (New Brunswick) of Bolsovian (ex Westphalian C) age, which, however, were not found in situ. His paper is very brief and not illustrated, but several coal-ball taxa were reported. He also reported that only 1% of the prepared peels show a "variety of reasonably well preserved plant structures". The New Brunswick coal-ball locality described by Baxter could not be relo- cated (Zhifeng Gao, University College of Cape Breton, oral commun., August 1990).

Calder (1979) noted what he described as "sideritic concretions" in the Foord Seam (Foord Pit, Fig. 1), near Stellarton, Nova Scotia. He considered these concretions to be syngenetic with the Foord peat (coal). In 1990, Gao (1992) found coal-balls in scattered waste piles in the Stellarton mining district. These coal-balls have a high pyrite content, are very degraded, and their plant material is poorly preserved (Gao, 1992). Their high pyrite content suggested that they probably did not originate from the low-sulfur Foord Seam, which has an average of 0.5 wt% (weight percent) total sulfur (U.S. Geological Survey,

E.L. Zodrow et al. ~International Journal of Coal Geology 29 (1996) 199-216 201

STELLARTON FM.

500- -

1000 --

200U

THORBURN MBR. (oil shales, thin coal)

COAL BROOK MBR. (oil shales, thin coal)

10

- - Foord 8 - - Cage ~

Third \ >~ - - McGregor ~

New - ~

- -Oi lCoal \ ; 6 /°:

FOORD SEAM

Oil Shale

Bone Coal Coaly Shale

Upper Bench

Parting

Lower Bench

Seat Rock

2B Source: Gibling and others, 1992

2A Fig. 2. Stellarton Formation showing (A) the Albion Member and (B) Foord Seam, Foord Pit, Stellarton Basin, Nova Scotia, Canada.

unpublished data). They probably originated from the high-sulfur seams in the 177 m Foord-McGregor interval below the Foord Seam (Fig. 2A).

In May 1992, P.C. Lyons and A.T. Cross collected 15 siderite nodules from near the parting and upper part of the lower bench of the Foord Seam in the Foord Pit (Fig. 1). Laboratory examination of peels prepared by Lyons and Gao revealed histologically preserved plant matter. Thus, they recognized that some of the 'sideritic concretions' of Calder are coal-balls. This constitutes the discovery of in situ coal-balls in Nova Scotia (Lyons et al., 1993; Zodrow and Lyons, 1993).

In this paper, we recognize both autochthonous and hypautochthonous coal-balls (Fig. 3). The latter are distinguished from the autochthonous coal-balls by the absence of coal, which usually encloses coal-balls, and by their higher clay mineral content (comparatively more AI and Si contents, see Table 2). Examination of about 400 peels from 83 hypautochthonous coal-balls shows distinct coal-ball floral elements (Plate 1 and Plate 2), which are comparable to coal-ball flora typical of late Carboniferous mires (Phillips et al., 1985). The hypautochthonous coal-ball floras are interpreted as representing lower-bench peat that was transported by running water within the Foord coal swamp.


0 cm 2 B

994GF-206A

Fig. 3. (A) Field photograph of a hypautochthonous coal ball (994GF-206), Diaphorodendron sp., flattened stem; position in the Foord pit before collecting. Ish = lower shale; ub = top of lower bench; cross section 1-2 identifies face cut in (B). Foord Seam, bottom of shale parting, Foord Pit. (B) Polished cross section 1-2 of Diaphorodendron sp., showing sclerenchyma patches of cortex ( arrows ), A = flattened vascular cylinder; B = stele.

E.L. Zodrow et al. /International Journal of Coal Geology 29 (1996) 199-216 203

Plate 1. Hypautochthonous coal balls from the shale parting in the Foord Seam, Foord Pit, Stellarton, Nova Scotia.( 1 ) Medullosa sp., peel 992DF-486A-I. Cross section of a vascular cylinder. The ground tissues are not preserved.(2) Stigmaria sp., peel 992GF-557A-2. Cross section of a stigmarian lateral appendage ('rootlet '), showing monarch primary xylem and chrushed cortex.


Plate 2, Hypautochthonous coal balls from the shale parting in the Foord Seam, Foord Pit, Stellarton, Nova Scotia. ( 1 ) Botryopteris tridentata, thin section 993GF-261-1. Cross section of a petiole. (2) Lepidocarpon lomaxii, thin section 992GF-289. Showing reproductive propagule.

E.L. Zodrow et al. / International Journal of Coal Geology 29 (1996) 199-216 205

This paper, which is one of two papers (see Lyons et al., 1995) to discuss in situ coal- balls from a Maritime Coal Basin, presents a preliminary report of geochemical data based on four selected autochthonous coal-balls (two from the upper bench and two from the lower bench) and on two hypautochthonous coal-balls from the shale parting within the Foord Seam. The data are used for a geochemical model which hypothesizes the carbon and oxygen isotopic conditions prevailing when the coal-balls were formed in the low- sulfur Foord Seam (peat).

2. Stratigraphic occurrence and material

The Foord Seam (Fig. 2A) is the uppermost coal seam of the Albion Member of the Stellarton Formation of middle to late B olsovian age (Gibling et al., 1992). It was deposited within an interval containing oil shales of lacustrine origin. This low-sulfur seam is also the thickest and oldest seam mined in the Carboniferous coal basins of Maritime Canada. At our sampling point of the Foord Seam in the Foord Pit (Fig. I ), the seam has an approximate thickness of 11 m: the upper bench is 4.5 m thick, the shale parting is about 1 m, and the lower bench is approximately 5.5 m (Fig. 2b).

In 1992, 1993 and 1994 a total of 392 coal-balls were collected from the two benches and from the shale parting (83 samples) in the Foord Seam at the Foord Pit. This collection constitutes the Foord Seam type and reference collection of the Palaeobotanical Collection of the University College of Cape Breton, and is being curated by Erwin L. Zodrow.

3. Samples and methods

Six coal-ball samples were selected for geochemical analyses. Exact stratigraphic loca- tions for them could not be ascertained but they were collected from the following general stratigraphic positions in the Foord Pit: 1. two coal-balls from the upper bench (accession numbers 992GF-217 and 992GF-218); 2. two coal-balls from the shale parting (992GF-219 and 992GF-220), the latter was

subdivided into 992GF-220A, the coal-ball without its core, and 992GF-220B the core; 3. two coal-balls (992GF-221 and 992GF-222) from the lower bench positions. In Tables 1-5, Fig. 7 and in the text, these are referred to as samples 1-7, respectively.

Prior to analyses, surfaces of the coal-balls were cleaned to remove as much of the encrusting coal as possible, in order to prevent coal-carbon contamination of the organic carbon content (Table 3). Powdered (40-80 g, < - 2 0 0 standard Tyler mesh) samples were prepared for a total of 7 samples (see Tables 1-5 and Fig. 7) for whole-rock elemental and rare earth elements, and carbon and oxygen isotopic analyses. In the powdering process, representative - 140 standard Tyler mesh portions of the 7 samples were split off for use in semiquantitative X-ray diffraction (XRD) mineralogical analyses, Table 1 (Hosterman and Dulong, 1989).

From each coal-ball, two thin and two polished sections were prepared, in addition to 60 sections from other hypautochthonous and autochthonous coal-balls, for examination under petrographic and scanning electron microscopes (SEM) and for microprobe analysis.


Table 1 Semiquantitative X-ray diffraction data a (in weight percent) on coal balls from the Foord Seam, Foord Pit, Stellarton, Nova Scotia, Canada

Sample no. Siderite Dolomite Quartz lUite

Upper bench 1 95 5 0 0 2 99 0 1 0 Shalepaming 3 90 8 2 0 4 81 19 0 0 5 86 7 5 2 Lowerbench 6 89 11 0 0 7 100 0 0 0

aProcedure described by Hosterman and Dulong (1989). Sample numbers 1-7 refer to coal balls numbered 1-6 (see text of section 3).

Table 2 Whole-rock and rare earth element analyses of coal ball samples used in this paper. Part A: Whole-rock analysis (in weight percent )

Sample no. Fe203 CaO MgO A!203 SiO2 TiO2 P205 MnO Na20

Upper bench 1 55.6 3.53 1.87 0.92 1.49 0.05 0.17 0.17 0.10 2 56.0 1.84 1.64 2.99 4.82 0.20 0.14 0.16 0.16 Shale parting 3 53.9 3.69 2.34 2.54 4.73 0.13 0.09 0.16 0.12 4 43.6 7.15 3.84 3.05 5.28 0.14 0.14 0.13 0.11 5 44.2 3.41 2.59 6.24 11.70 0.26 0.16 0.13 0.16 Lower bench 5 56.0 3.40 2.90 0.27 0.90 0.03 0.14 0.35 0.09 7 60.0 2.13 1.53 0.29 0.85 0.02 0.27 0.28 0.12

Sample numbers 1-7 refer to coal balls numbered 1-6 (see text of section 3).

For whole-rock analysis, approximately 1.3 g of each of the 7 samples was heated at 950

C ° for 1 h, fused with 5 g of lithium tetraborate and cast into a 4 cm diameter button for X-

ray fluorescence spectrometry. The instrumental precision of results reported in Table 2 is ___ 0.5 wt%.

Analyses of carbon dioxide, from acid digestion of carbonate, were obtained by coulom-

etry; the detection limit for carbon dioxide gas is 0.01 wt%. The rare earth elements (REE) were determined by inductively coupled plasma combined with mass spectrometry. Detec- tion limits are 0.05 ppm for Eu, Ho and Lu, and 0.1 ppm for the remaining REE.

Standard methods were used for carbon and oxygen isotopic analysis of the bulk samples (H.W. Krueger, Geochron Laboratory Cambridge, Massachusetts, USA, written commun., 1993). The results are reported (Table 5) in parts per mil. The 813C values are referenced to Pee Dee Belemite (PDB) and the ~ 8 0 to Standard Mean Ocean Water (SMOW).


Table 3 Whole-rock and rare earth element analyses of coal ball samples used in this paper. Part B: Organic carbon, sum of whole-rock analysis, carbon dioxide and loss on ignition

Sample no. Organic carbon Sum of whole rock COs Loss on ignition (wt%) (wt%) (wt%) (oxide wt%)

Upper bench 1 3.02 99.3 35.5 35.3 2 1.32 100.3 33.4 32.0 Shale parting 3 1.13 100.5 34.4 32.5 4 3.42 99.5 33.9 35.7 5 1.15 99.5 29.5 29.8 Lower bench 6 2.97 100.1 36.4 36.0 7 2.73 100.4 35.7 34.8


Table 4 Whole-rock and rare earth element analyses of coal ball samples used in this paper. Part C: Rare earth element data (in ppm)

Sample no. La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Upper bench 1

2 Shale parting 3 4 5 Lower bench 6 7

13.7 32.2 3.8 19.0 5.7 1.47 6.9 1.0 5.4 0.95 3.0 0.4 2.0 0.34 17.6 ~ . 2 4.9 24.3 7.8 2.15 9.6 1.4 7.4 1.28 3.8 0.5 2.6 0.42

15.2 32.5 3.5 16.4 5.0 1.21 6.3 0.9 5.0 0.92 2.6 0.3 1.9 0.22 12.2 26.0 3.4 17.2 4.9 1.37 7.0 1.1 6.0 1.15 3.1 0.4 2.5 0.36 12.0 24.8 3.0 15.3 4.4 1.13 5.8 0.8 4.5 0.81 2.5 0.4 2.1 0.28

5.6 13.2 1.6 9.1 2.7 0.74 4.1 0.6 3.2 0.59 1.6 0.2 1.2 0.20 1.8 4.1 0.6 3.3 1.2 0.40 1.5 0.2 1.6 0.32 0.9 0.1 0.8 0.13

Plot of normalized values is shown in Fig. 7. Sample numbers 1-7 refer to coal balls numbered 1-6 (see text of section 3).

For scanning electron microscopy a Joel T3001 model coupled with an energy dispersive X-ray analyzer (EDXA) was used. Some quantitative elemental data on the mineral phases in the samples were obtained by EDXA and a tentative identification was made for 'calcite', which at that time could not be verified by electron microprobe analyses. Dolomite, ferroan dolomite and ankerite were identified from electron microprobe data.

The peel sections were obtained using standard methods (Joy et al., 1956), except that etching time in 20% HC1 was increased to 2-4 min to accommodate the dolomite-ankerite composition, which requires comparatively longer reaction time.

The use of trade names is for descriptive purposes only and does not constitute endorsement of products by the U.S. Geological Survey or the University College of Cape Breton.

208 E.L Zodrow et al. / International Journal of Coal Geology 29 (1996) 199-216

Table 5 Carbon and oxygen isotopic data (per mil) in coal bails, Foord Seam, Foord Pit, Stellarton, Nova Scotia, Canada

Sample no. Siderite Dolomite Calcite 813 C 8180 813C 8t80 813C 8180

Upper bench 1 +9.1 +23.3 +2.9 +20.8 -5.8 +19.0 2 +8.3 +24.7 +3.7 +23.0 +1.8 +22.0 S ~ p a ~ i n g 3 +8.7 +24.7 -0.3 +22.0 -6.3 +20.4 4 +7.3 +20.8 -6.0 +19.1 -11.7 +18.5 5 +5.2 +25.5 -0.5 +23.4 -9.1 +20.2 Lowerbench 6 +10.2 +21.8 +4.0 +20.9 0.0 +19.8 7 +11.3 +24.4 +10.0 +23.0 +9.7 +22.3


4. Analytical results

4.1. Macroscopic observations

The four autochthonous coal-balls (nos. 1,2,6,7) are surrounded by coal. Typically, coal- balls show coal bands, which are up to 9 mm in thickness and are traceable from the enclosing coal into the coal-balls themselves. The coal bands invariably show a pale gray cleat carbonate positioned perpendicular to the coal bands. Microprobe analyses in an autochthonous coal-ball (992GF-244) show the following cationic composition (spot analyses from two cleats):

Fe Mg Ca 0.116 0.378 0.503 0.130 0.368 0.502

The data represent the double carbonate ankerite. It is defined when the Fe -Mg substitution is considerable and cationic iron ranges up to 0.191 with 0.287 Mg and 0.519 Ca (13.56 wt% FeO, 11.38 wt% MgO and 28.70 wt% CaO), as dolomite and ankerite probably form a complete series (Berry and Mason, 1983, p. 339). Coal bands in the samples from the lower bench (nos. 6 and 7) infrequently show disk-shaped pyrite (Fig. 4), approximately 0.4 mm in diameter, which occurs on carbonate (dolomite?) or is found on coal cleat surfaces.

The hypautochthonous coal-balls 3 and 4 (Table 1 ) are dark gray in color and lack coal incrustation. Some of them show coalified 'ribs' , 1-1.5 mm thick, of sigillarian remains which partially cover their surfaces.

4.2. Semiquantitative mineralogical analysis

Semiquantitative X-ray mineralogical analysis of samples 1-7 (Table 1) shows that siderite is the major mineral and that dolomite is a minor component. The absence of quartz


Fig. 4. Scanning electron microphotograph (992GF-221; SEM stub #35) showing disk-shaped pyrite (P) on dolomite (D). Foord Seam, lower bench, Foord Pit. Scale bar = 100/,m.

in the samples from the bottom bench explains their lower SiO 2 content (Table 2). SEM image analysis estimates indicate that less than 3 wt% 'calcite' is present, which is

consistent with the fact that it was not detected by X-ray diffraction techniques (Table 1 ).

4.3. Thin section work and electron microprobe data

Thin sections show that the coal-ball siderite matrix is composed of crystals that range in diameter from 5/xm to 40/zm. Electron microprobe analysis from multiple siderite grains shows that the Fe-Mg-Ca crystal chemistry is close to the siderite end member in the siderite-magnesite-calcite solid-solution series. Data in Lyons et al. (1993) indicate that the average cationic composition of the siderite (n = 48) is:

Fe Mg Ca Mn Sr Ba 0.94 + 0.03 0.02 + 0.02 0.04 4- 0.01 < 0.01 < 0.01 < 0.01

In the siderite matrix vugs as large as 10 mm in diameter are filled with larger dolomite cry stals (Fig. 5 ). Electron microprobe data show that the cationic composition ranges from:

Fe Mg Ca to Fe Mg Ca 0.016 0.454 0.530 0.099 0.366 0.528

with a mean (n =25) of:

Fe Mg Ca 0.067 +0.025 0.410+0.034 0.521 ___0.018


Fig. 5. Scanning electron microscope photograph (992GM-218-1; SEM stub #39) showing rhombohedral dolomite crystals in a vug; tentative mineralogical identification is based on SEM-EDXA analysis (20 kv accelerating voltage). Foord Seam, upper bench, Foord Pit. Scale bar= 10/~m.

The corresponding mean oxide composition is: 5.11 + 1.88 wt% FeO, 17.62 +2.08 wt% MgO, 30.85 _ 1.56 wt% CaO. These data are interpreted as representing dolomite, with Fe 2÷ substituting for Mg 2+ (see Deer et al., 1992, p. 650). Ankerite is also abundantly present as cell-lumen carbonate.

Single or multiple, 1-2 mm a-quartz crystals frequently occupy the center of vugs, which is evidence that quartz was the last to form in the sequence of diagenetic processes.

Petrographic observation, coupled with electron microprobe analyses, show that plant structure (Plates 1 and 2) is permineralized by dolomite-ankerite and by siderite in both autochthonous and hypautochthonous coal-balls (Fig. 6). In some plant structures the cell- lumen carbonate was either dolomite-ankerite or siderite. Petrological studies also indicate that, in the siderite matrix, diagenetic fractures occur which are commonly less than 1 mm wide and are occupied by dolomite-ankerite. A subjective estimate based on thin-section surfaces is that the area percentage of the fractures in terms of permineralized plant structures is approximately 5-20%. Studying cross-cutting relationships in thin sections indicated that some fractures postdate permineralization of the plant structures.

The permineralizing siderite in the samples has the following average cationic composition (n = 7):

Fe Mg Ca 0.929 0.010 0.023

with minor amounts of Na, Mn and Sr. This composition is consistent with the composition of siderite that constitutes the matrix of coal-ball samples, see page 210.


992GF--456AA 700/LI

Fig. 6. Electron microprobe analyses of parts of a transverse section of a woody cylinder of Stigmaria sp., polished section 992GF-456AA. (A) Detail of micrograph of back-scatter mode showing cell lumen replacement by (ferroan) dolomite (gray) and cell wall replacement by siderite (pale grey rims). (B) X-ray map of Fe of the square outlined in (A). Cell lumen and cell wall replacements are more or less of uniform compositions, as indicated by even pale grey/darker gray color tones on the map.


-100 2 10(~ -

5 ~ ~

¢ J -

~ m I I m t J I I I [ t I I I t 11 La Ce Pr Nd PmSmEu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 7. Rare earth element chondrite-normalized plots of sample data (see Table 2). Samples 1-7 refer to coal balls numbered 1--6 (see text of section 3). Note the relative depletion of REE in the lower bench.

Coal-ball plant taxonomy is discussed in detail by Lyons et al. (1995). Coalified remains are abundantly present in the permineralized masses. In the 7 samples they are expressed chemically as organic carbon that ranges from 1 to 3 wt% (Tables 2-4). Pyrite is very rare in the coal-ball samples, which is consistent with the low sulfur content of the Ford Seam coal in the Foord pit (U.S. Geological Survey, unpublished data).

4.4. Whole-rock chemistry and rare earth elements (REE)

The whole-rock chemistry (Table 2) clearly shows the predominance of Fe in all samples. The hypautochthonous coal-balls tend to have a lower Fe content and higher Ca and Mg contents, which together with higher A1, Si and Ti contents correlates with comparatively lower siderite and higher illite contents, respectively.

As noted, in carbonate mineralogy ionic substitution is very extensive (Winchell, 1956). It is assumed that the MnO content (Table 2:0.13--0.35 wt%) in the samples reflects substitution of this element in the carbonate mineral structures and is not rhodochrosite (MnCO3), which was not detected by SEM-EDXA analysis. The organic carbon content of the 7 samples (approximately 1.1-3.4 wt%, Table 3) is believed to be mainly derived from the permineralized and other coalified tissue in the coal-ball samples.

Concentration levels of REE (Table 4; Fig. 7) are highest in the samples from the upper bench, followed by samples from the shale parting; the lower bench samples have the lowest REE concentration. The relative depletion of REE concentration in the lower bench coal- ball samples is a striking feature of REE sample distribution and remains to be confirmed by future sampling and analyses.

Prior to 1992, the Suncor Company reported ironstone concretion bands from exploration drill holes occurring in the Foord Seam. The company reported that these bands had a composition of greater than 95 wt% siderite, minor dolomite, and a quartz content of less than 0.25 wt% (Keith McCandlish, written commun., March 29, 1993). These data are consistent with the data shown in Table 1.


4.5. Stable isotopic analysis o f carbon and oxygen

Isotopic data (Table 5) are different for each of the three carbonate minerals (siderite, dolomite and 'calcite') because, presumably, they were formed at different times during very early diagenesis, as previously noted. This conclusion is based on electron microprobe studies and on observation in thin sections that dolomite-ankerite is secondary, with respect to the siderite.

The isotopic data sets show that the samples from the lower bench are enriched in heavy carbon (~3C), whereas those from the upper bench and the shale parting are relatively enriched in the light carbon (~2C) in all three carbonate minerals. The most consistent set of isotopic data (both carbon and oxygen) is for the siderite, the principal mineral in the coal-balls. The negative 813C values of dolomite and of 'calcite' from the parting samples contrast sharply with the positive 8~3C values from the lower and upper benches (Table 5), which suggests that the carbon in the dolomite and 'calcite' in the shale parting was incorporated at a different time than in the lower and upper benches.

However, isotopic data for dolomite and 'calcite' is somewhat ambiguous because cell- lumina carbonate (Fig. 6) and the carbonate from the diagenetic fracture fillings are mixed in the bulk sample. The consequence of the bulk analyses is that isotopic signatures in these diagenetic carbonates are affected by the 5-20% fracture filling mentioned earlier.

5. Proposed geochemical model for the genesis of Foord Seam coal-balls

We suggest that the most important isotopic data for understanding the genesis of coal- balls in the Foord Seam are those values derived from siderite - - the major mineral in the permineralized mass. The siderite data show a narrow variation in t~3C values (Table 5) and range from + 5.2%0 (core of sample 4) to + 11.3%o (lower bench). The overall trend in carbon isotopic data is a decrease in t~13C, from the lower to the upper bench. In contrast ( unpublished data), the variability of B ~ 3Corg values from 18 coal samples ( data not shown), originating from the upper and lower benches and the shale parting is small: the range is from - 22 to - 25%o and, notably, there is no stratigraphic trend.

Positive values of ~3C are consistent mainly with a nonmarine environment conducive to the formation of siderite (Mozley and Wersin, 1992). In this nonmarine setting, sulfate is considerably lower in concentration than in marine environments and excess iron, which is readily available from Fe-bearing clays, complexes with bicarbonate under reducing conditions to form siderite, rather than pyrite. Pyrite formation is limited by the availability of sulfur to form FeS (or the iron monosulfides). Bacterial methanogenesis results in the preferential use of 12C, which eventually leaves the peat system as methane gas. Therefore, this process enriches the diagenetic pore waters in ~3C-enriched bicarbonate (Faure, 1986), which is available for complexing with Fe to form siderite, in the absence of available H2S to form pyrite. Thus, the trend shown in Table 5 is consistent with increased bacterial methanogenesis in the lower bench of the Foord coal.

Siderite 8180 values vary from + 21.8%o, in a coal-ball from the lower bench, to + 25.5%o, in a coal-ball from the parting (sample 5, Table 5). The tightness of the oxygen isotopic data set, which ranges from + 19.0%o to + 23.4%o for dolomite and 'calcite', is also evident.


The isotopic composition of oxygen in calcite and in other carbonates in equilibrium with water is a function of temperature: higher 3180 values indicate a warmer climate (Faure, 1986). It is probable that the isotopic composition of the pore water in the peat system governs the isotopic composition of the oxygen in the carbonyl ion (CO3)2-, which forms carbonate (DiNiro and Epstein, 1979; Fogel and Cifuentes, 1993). The ~80 values for the carbonate samples shown in Table 5 are consistent with similar values for diagenetic siderite minerals from freshwater lakes and marsh environments and with ~80 values ( + 19%o to + 23%o) from freshwater shell carbonates (Timofeyeva et al., 1976). Values of/StSo for carbonates from coal-balls in the Illinois basin also have a limited dispersion, ranging approximately from -4.5%o to - 10.8%o, which is quite different from the Foord Seam coal-balls, probably reflecting marine or marginal marine influences (Brownlee, 1975), unlike the lacustrine conditions in the Foord mire.

6. Summary and discussion

The 11-13 m thick Foord Seam from the (Acadian) Euramerican floral province is the thickest seam known to contain coal-balls. The siderite in the coal-balls is hypothesized to be derived from ~ 3C-enriched bicarbonate produced in a nonmarine environment by bacterial methanogenesis of organic matter.

The siderite and related carbonate mineralogy of the coal-balls from the Foord Seam is atypical for North America and western Europe, because calcite or dolomite, respectively, is the major carbonate of coal-balls from these areas (see summary by Evans and Amos, 1961; Anderson et al., 1981; Scott and Rex, 1985).

Equally important to Carboniferous studies is the discovery of hypautochthonous coal- balls in the shale parting of the Foord Seam. These contain three-dimensionally preserved plant structures which are being histologically studied.

In the Foord Seam material, the degree of preservation in both autochthonous and hypautochthonous coal-balls is variable and ranges from poor to good, as judged from observations in thin and polished sections. However, during preparation of peel sections, a certain amount of detail is not shown and transferred to peel sections because most plant structures are preserved, in whole or in part, by siderite, which does not readily dissolve in dilute HC1. This process clearly biases histological observation from peel sections and future work requires thin-section rather than peel-section analysis. Both the hypautochthonous and autochthonous coal-balls have similar petrographic and mineralogic characteristics. How- ever, they differ to some degree in geochemical characteristics (clay mineral, iron and REE contents), and samples from the lower bench tend to be enriched in heavy carbon. However, the peel sections of the hypautochthonous coal-balls show plants comparable to those in the autochthonous coal-balls (Lyons et al., 1995). We hypothesize that the coal-balls have a common origin: the hypautochthonous coal-balls represent transported lower-bench peat within the developing Foord Seam mire.

Supporting evidence for a nonmarine origin of the coal-balls from the Foord Seam stems from the observation that the siderite in all 7 samples is close to a pure iron end member (siderite) in the magnesite-siderite-calcite solid-solution series (Mozley and Wersin, 1992):

E.L Zodrow et al. / International Journal of Coal Geology 29 (1996) 199-216 215

Fe Mg Ca Mn Sr Ba 0.94-t-0.03 0 .02+0 .02 0.04-t-0.01 <0.01 <0.01 <0.01

The geochemical evidence for demonstrable nonmarine conditions in the Westphalian strata of the Stellarton Basin is independently supported by terrestrial plant fossils (Dawson, 1868; Bell, 1944; Calder, 1979) and the absence of strata known to contain marine faunas, and the presence of associated lacustrine strata (Gibling et al., 1992).

On the basis of our thin section work, the lack of significant compaction and the geochemical and microprobe data obtained, we suggest that the dolomite-ankerite, siderite and 'calcite', comprising the coal-ball mineralogy, are probably of very early diagenetic origin. Calder's (1979) hypothesis of syngenetic siderite concretions cannot be supported by our data. The carbon isotopic data (Table 5) for these three carbonates are not the same and indicate somewhat variable geochemical conditions, which is consistent with a paragenetic sequence of carbonate mineralization in the coal-balls during very early diagenesis.

Acknowledgements

We acknowledge financial assistance (to E.L. Zodrow from the National Science and Engineering Science Council of Canada, the Nova Scotia Museum, and the University College of Cape Breton. We thank Gary Glass and three anonymous journal reviewers of this paper for their constructive comments and U. Brand, C. Wnuk and E. Spiker for their suggestions for improvement of an early version of this manuscript. We benefited from discussions with K. Gillis and J.H. Calder. K. McCandlish, R.D. Congdon, J.C. Jackson (U.S. Geological Survey) and R. MacKay (Dalhousie University) provided the electron microprobe data on the siderite and the dolomite, respectively, and the former two the X- ray diffraction semiquantitative mineralogical data.

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