## Arsenic-Rich Pyrite in Coals of the Warrior Field, Northwestern Alabama, U.S.A.: Mineralogical Evidence for a Hydrothermal Origin
Poster

By Allan Kolker,1 Martin B. Goldhaber,2 Joseph R. Hatch,2 Gregory P. Meeker,2 and Robert P. Koeppen3
1U.S. Geological Survey, Reston, VA 20192
2U.S. Geological Survey, Denver, CO 80225
3U.S. Geological Survey, Seattle, WA 98104

### ABSTRACT

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.

### ARSENIC ENRICHMENT IN WARRIOR BASIN COALS

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).

Table 1. Comparison of average values for Warrior Basin coals with U.S. average values for coal (data from Bragg and others, 1998).
Element | Warrior Basin mean (in ppm) N = 900 | U.S. average (in ppm) N > 7,000
Co | 7.8 | 6.1
Ni | 17 | 14
Cu | 25 | 16
As | 71 | 24
Mo | 6 | 3
Sb | 2.6 | 1.2
Hg | 0.22 | 0.17
Figure 1. Generalized stratigraphy of Lower Pennsylvanian rocks in the Warrior Basin, Alabama, showing sequence of coal-bearing cycles. Vertical dimension is not to scale.

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.

### ARSENIC IN WARRIOR BASIN PYRITE

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).

|  | 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.

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 m m.

|  | 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 m m.

|  | 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 m m (A, B), and 50 m m (C).

### CONCLUSIONS

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).

### REFERENCES CITED

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