Lower Pennsylvanian coals in the Warrior field of northwestern Alabama show a distinct local enrichment of metals relative to coals in the Appalachian Basin and averages for U.S. coals. As part of a larger study of these coals, we have conducted detailed electron microprobe studies of pyrite, an important host for metals, especially arsenic (maximum 4.7 wt percent As). Arsenic enrichment is primarily associated with epigenetic pyrite forms. The presence of oscillatory-zoned pyrite, previously found only in epigenetic ore deposits, indicates that hydrothermal fluids permeated these coals, possibly a result of Alleghanian thrust faulting in the southern Appalachians (Goldhaber and others, 1997). Arsenic-rich coals are particularly prevalent in fault zones, consistent with introduction of arsenic by hydrothermal fluids. The results of this study fit an emerging general model for metalliferous coals, involving postcoalification or syncoalification hydrothermal activity.
Examination of results compiled in the U.S. Geological Survey COALQUAL database (Bragg and others, 1998) shows that coals having arsenic contents that exceed the U.S. average for coal (24 ppm) are concentrated in the southern Appalachian region, especially the Black Warrior Basin of northwestern Alabama. Contents of associated elements such as Co, Ni, Cu, Mo, Sb, and Hg also exceed U.S. averages (table 1). The distribution of arsenic-rich coals spans a wide stratigraphic interval (figs. 1, 2). However, arsenic enrichment occurs most commonly in sulfur-rich coals present at depth in Tuscaloosa County. Coals currently being mined have relatively low arsenic contents (Goldhaber and others, 2000).
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Figure 2. Distribution of arsenic in Warrior Basin coals. Note that arsenic enrichment covers a large geographic and stratigraphic range. |
In this study, detailed microprobe/SEM studies were conducted on pyrite, the primary host of arsenic in these coals. Thirteen coal samples were investigated, including material from open-pit and underground mines, core samples, and a prepared commercial coal. The most arsenic-rich coals are found along fault zones, suggesting that arsenic was introduced by migration of fluids along these pathways.
Wavelength-dispersive X-ray mapping of pyrite in Warrior Basin coal samples indicates that arsenic enrichment is episodic and associated with epigenetic pyrite forms. The most arsenic-rich pyrite in the samples investigated (maximum 4.7 wt percent As) is found as discrete subhedral grains, subhedral framboid overgrowths, and subhedral to euhedral oscillatory-zoned pyrite in which the arsenic content varies concentrically over several micrometers (figs. 3-6). The presence of oscillatory-zoned pyrite (fig. 5), previously found only in epigenetic ore deposits (Fleet and others, 1988), indicates that hydrothermal fluids have permeated these coals, possibly associated with Alleghanian thrust faulting in the southern Appalachians (Goldhaber and others, 1997, 2003). Arsenic enrichment is generally lacking in framboidal pyrite, the earliest pyrite form (fig. 4). In cleat pyrite, the latest form, pyrite microcleats include wispy arsenic-rich (1.-1.5 wt percent As) bands that may indicate arsenic remobilization following the main pulse of hydrothermal activity (fig. 6).
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Figure 3. Arsenic content determined by electron microprobe for Warrior Basin framboidal pyrite (71 analyses) versus cleat pyrite (173 analyses). Plot is a compilation for multiple pyrite grains in multiple samples. Cleat pyrite formed late in the coal-forming sequence, and its composition reflects arsenic enrichment in these coals. |
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| Figure 4. Thermal false-color wavelength-dispersive microprobe elemental maps showing distribution of sulfur (left) and arsenic (right) in a cluster of pyrite framboids. Note that arsenic is concentrated in epigenetic framboid overgrowths, whereas the framboid interiors, which formed early in the coal-forming process, contain little or no arsenic. From face channel sample, Newcastle coal bed, Mary Lee coal group, Lost Creek Mine, Walker County, Alabama. Scale bar is 20 mm. | ||
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Figure 5. False-color elemental map and microprobe analysis points showing oscillatory zonation of arsenic in a Warrior Basin pyrite. High-arsenic bands (HS; red), 4.4 to 4.5 wt percent As; medium-arsenic bands (INT; greenish), 3.1 to 3.3 wt percent As; low-arsenic bands (CS; dark blue), 0.3 to 0.4 wt percent As. This texture has previously been found only in hydrothermal ore deposits. This pyrite was collected from a fracture, in a pyritized portion of the Newcastle coal bed, Mary Lee coal group, Lost Creek Mine, Walker County, Alabama. Scale bar is 50 mm. |
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Figure 6 A-C. Elemental maps and microprobe analysis points showing distribution of arsenic in cleat (fracture-filling) pyrite in coal. Left image (A) is a map of iron (pyrite) distribution. Center image (B) is an arsenic map. Right image (C) is an expanded view of a portion of the arsenic map, showing wispy arsenic-enriched microcleats. From core sample of Utley coal group, Tuscaloosa County, Alabama. Scale bar is 500 mm (A, B), and 50 mm (C). |
Textures and compositions of pyrite in Warrior Basin coals provide mineralogical evidence for hydrothermal activity in the basin following coalification. These fluids mineralized cleats that only form after considerable burial and coalification. The mineralizing fluids postdate early diagenetic pyrite forms. These fluids locally enriched the Warrior Basin coals in arsenic and other metals. Arsenic is incorporated into epigenetic pyrite forms, whereas syngenetic framboids show little or no arsenic enrichment. These findings are consistent with the model proposed by Goldhaber et al. (1997, 2003), in which metal enrichment is a consequence of fluids expelled during Alleghanian thrust faulting in the southern Appalachians. Similar models involving postcoalification hydrothermal activity have been proposed for many of the world's metalliferous coals, such as in the Donets Basin, Ukraine; southwestern Guizhou Province, China; and the Pavlovka coal deposit of the Russian Far East (Dvornikov, 1990; Belkin and others, 1998; Seredin and others, 1997).
Bragg, L. J., Oman, J. K., Tewalt, S. J., Oman, C. L., Rega, N. H., Washington, P. M., and Finkelman, R. B., 1998, U.S. Geological Survey Coal Quality (COALQUAL) Database: Version 2.0: U.S. Geological Survey Open File Report 97-134 (CD-ROM).
Belkin, H. E., Warwick, P. D., Zheng, Boashan, Zhou, Daixing, and Finkelman, R. B., 1998, High arsenic coals related to sedimentary rock-hosted gold deposition in southwestern Guizhou Province, People's Republic of China: Proceedings of the Fifteenth Annual International Pittsburgh Coal Conference, 4 p., CD-ROM.
Dvornikov, A. G., 1990, Some patterns of the distribution of cinnabar in coal measures of the Donbas: Doklady Akademii Nauk SSSR, v. 312, no. 5, p. 1218-1222 [translated from Russian].
Fleet, M. E., Maclean, P. J., and Barbier, Jacques, 1988, Oscillatory-zoned As-bearing pyrite from stratabound gold deposits: Evidence for primary mineralization, in Goode, A. D. T., and Bosma, L. I., eds., Bicentennial gold 88, extended abstracts, Geological Society of Australia, Abstracts Series, no. 22-23, p. 241-245.
Goldhaber, M. B., Hatch, J. R., Pashin, J. C., Offield, T. W., and Finkelman, R. B., 1997, Anomalous arsenic and fluorine concentrations in carboniferous coal, Black Warrior Basin, Alabama: Evidence for fluid expulsion during Alleghanian thrusting: Geological Society of America, Abstracts with Programs, v. 29, no. 6, p. A-51.
Goldhaber, M. B., Bigelow, R. C., Hatch, J. R., and Pashin, J. C., 2000, Distribution of a suite of elements including arsenic and mercury in Alabama coal: U.S. Geological Survey MF Map 2333, http://greenwood.cr.usgs.gov/pub/mf-maps/mf-2333/.
Goldhaber, M. B., Lee, R. C., Hatch, J. R., Pashin, J. C., and Treworgy, J., 2003, The role of large-scale fluid-flow in subsurface arsenic enrichment: in Welch, A.H., and Stollenwerk, K.G., eds., Arsenic in ground water: Geochemistry and occurrence: Norwell, Mass., Kluwer Academic Publishers, p. 281-294.
Seredin, V., Evstigneeva, T., and Generalov, M., 1997, Au-PGE mineralization in Cenozoic coal-bearing strata of the Pavlovka deposit, Russian Far East: Mineralogical evidence for a hydrothermal origin, in Papunen (ed.), Mineral Deposits: Rotterdam, Balkema, p. 107-110.
