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1 University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, USA (e-mail: hower{at}caer.uky.edu)
2 Kentucky Geological Survey, Lexington, KY 40506, USA
3 Kentucky Geological Survey, Henderson, KY 42420, USA
Scientific editing by Peter Haughton.
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Premovi
et al. (1997), following studies by Zubovi (1966), Maylotte et al. (1981), and Hower et al. (1990b), discussed the origin of vanadium anomalies in the upper few centimetres of the Springfield (No. 9) coal bed (Middle Pennsylvanian Carbondale Formation) in a limited area of the Western Kentucky coal field. The vanadium enrichment is accompanied by high concentrations of other metals, notably Cr, Ni, and Zn. Their conclusions with regard to the origin of the vanadium enrichment and to the geology of the Springfield coal bed conflict with the established geology of the coal.
The enrichment of V in the top few centimetres of the coal bed was established by Zubovi (1966). Hower et al. (1990b) noted this trend at 42 of 44 Western Kentucky sites, although their study sites did not, in many cases, have the vertical resolution of the previous study. Premovi et al. examined the V enrichment in samples from the Providence Mine, southwestern Hopkins County, Kentucky, a site where the upper 12.6 cm lithotype has 10,600 ppm V (our analysis via XRF; reported on ash basis; 11.67% ash).
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et al. state that ‘It is quite unlikely that a source of V in the W. KY No. 9 coal was ordinary shallow (<100 m) swamp waters’ (p. 280) as the levels of metals in modern swamp waters are very low. The Springfield coal is overlain by a black, phosphatic shale, which yields a characteristically high gamma response on subsurface logs. Such shales are characteristic of many Mid-Continent coal-bearing cyclothems. For example, the Colchester coal, another Desmoinesian coal of the Illinois basin, has a high-gamma roof shale with high V values. The shales typically have widespread distributions and are inferred to represent marine transgression of the underlying swamp deposits. The black ‘core’ shales are inferred to have been deposited in waters deep enough (>50 m) to develop a thermocline strong enough to prevent bottom oxygenation (Heckel 1977). They are commonly enriched in organic matter and trace metals including Cd, Cr, Cu, Mo, Ni, Pb, V, and Zn (Heckel 1977; Coveney et al. 1991). Premovi et al., however, reiterate Zubovi’s (1966) argument that the shale overlying the W. KY. No. 9 coal was unlikely to have been the source of the vanadium on the basis that shales of similar thickness occur throughout Illinois without comparable V-enrichment in the underlying coals.
Having dismissed the marine roof shale as a source of the V, Premovi et al. then go on to argue against a hydrothermal origin for the enrichment, stating that the area was too large and the concentration of V in hydrothermal waters too low to account for the enrichment in a single, pulse of mineralization. However, there may have been multiple episodes of hydrothermal fluid circulation, more than compensating for low levels of V in individual episodes. Although they overstated the area of the most intense mineralization, the area of mineralization is not necessarily a critical factor as, for example, a late Palaeozoic episode of K-mineralization and remagnetization in the Central and Southern Appalachians covered a much wider area (Hearn & Sutter 1985; Hearn et al. 1987; Miller & Kent 1988). Oliver (1986, 1992) speculates that the fluid flow in the US mid-continent in the late Palaeozoic spread through a much wider area than simply a corner of the Western Kentucky coal field. Covenay (1979) documented sphalerite enrichment in Pennsylvanian black shales in Missouri and Kansas and Cobb (1981) described sphalerite veins in Illinois coals. In both cases, post-depositional fluid movement through the sediments was determined to be the primary factor in the enrichment. Confinement of the V enrichment to the upper few centimeters of the coal may reflect a role for the overlying black shale as an effective permeability barrier which trapped metals along the permeable coal/impermeable shale contact. The fact that the V anomaly is at top of the coal bed and V concentrations decrease downward into the coal suggests that the shale or the contact is the source and percolation was downward into the coal.
Having dismissed the roof shale and hydrothermal concentration of the V, Premovi et al. then appeal to an ‘. . . external supply of V of the past swamp/peat water . . . derived from either volcanic water or volcanic ash . . . or transfer of V through weathering/leaching of volcanoclastic materials from adjacent land areas’ (p. 280). There is, however, no known volcanism in the vicinity of the southern Illinois Basin at the time of the Springfield swamp. There is also no evidence, despite the geology of Kentucky and much of Illinois having been being mapped at the 1:24 000 scale, of volcanic sediments in the Carbondale Formation, although coals and underclays represent the most landward portions of the sediment record.
Premovi et al. suggest that the lack of direct evidence for volcanism in the coal may reflect a preservational bias as ‘. . . it is, then, the exception rather than the rule to find these fine volcanic materials in the conditions in which they were deposited’ (p. 281). However, the absolute lack of supporting evidence for volcanic activity, direct or otherwise, cannot be presumed to support the hypothesis of volcanism, notwithstanding the tendency for volcanic ashes to be unstable in acid swamp waters. No known suite of volcanic minerals has ever been noted in the Springfield, one of the most extensively studied coals in North America. Numerous coal balls, including Willard’s (1993) study of the Providence Mine occurrence, have been collected and analysed in terms of their palaeobotany, geochemistry and isotopes, and none have found evidence of volcanics (Phillips 1980). Neither is there any indication of a significant event at the end of Springfield peat accumulation which affected the flora of the mire (Phillips 1980), although catastrophic ash falls elsewhere have been documented to result in dramatic changes in flora (Crowley et al. 1994).
With respect to the origin of fusinite in the coal, citing Hower & Wild’s (1982) observation of an upwards increase in fusinite in the coal, Premovi et al. note that ‘. . . it is difficult to escape the conclusion that forest fires swept through the ancient swamp . . . in the later stages of peat accumulation. It is clear, then, that these fires could be readily triggered by volcanic eruptive materials’ (p. 281). The increase in fusinite was significant but, nevertheless, the top of the coal was still dominated by vitrinite. Fusains in the Springfield coal bed are not unique to the upper lithotype, but, in fact, the upper lithotype at the Providence mine (samples provided by Hower to the authors) contained no measurable (>0.1 mm) fusain bands. Measurable fusains do contribute to the total fusinite in parts of the coal bed, but it is a leap of faith to assume that volcanic ash, direct evidence for which is lacking, was the spark setting the fires. Further, the temperature of the ash particles, following an unspecified time from eruption to deposition, may not have been sufficiently high to start fires unless a very local source of the ash and, possibly, a pyroclastic flow is envisaged as the mode of deposition. The Fire Clay coal bed tonstein, a definitive volcanic ash fall (Bohor & Triplehorn 1981; Chesnut 1983; Hess & Lippolt 1986; Rice et al. 1990; Lyons et al. 1992, 1994), does not directly overlie fusain (Hower et al. 1994).
Premovi et al. go on to conclude that the ash was produced by a single volcanic event of less than one year duration during the 104 year (their estimate) period of deposition of the upper 15 cm of the coal. The ash in the peat and on the ‘elevated land sites’ near the swamp was leached in the 9999 years in which volcanism did not occur. If, indeed, volcanism was a local phenomenon at that time (and recall that we know of no collaborating evidence), then the history of currently active volcanic areas would suggest that events would occur with a frequency on the order of 102–103 years. Returning to the question of the origin of fusinite in the upper portion of the coal bed, a single ash fall could not produce multiple fire horizons through the several thousand-year depositional period of the lithotype. The location of nearby ‘elevated land sites’ is also problematical. The Springfield coal extends throughout much of the Illinois Basin, with local channels being the only disruption in the otherwise near-continuous coal bed. The study area is near the southern margin of the coal field, a structural boundary, not an erosional edge of the palaeoswamp.
With respect to the chemistry of the volcanic ash, they ‘. . . tentatively suggest that the V/Cr/Ni enrichment of the HCl/HF soluble fraction of KY 9 is associated with basaltic volcanic activity which characterizes the Middle Carboniferous volcanism of (Eastern) USA’ (p. 283). The coincidence of high concentrations of V, Cr, and Ni in igneous rocks is greatest in the olivine tholeiite and tholeiite end of the igneous series (Carmichael et al. 1974, p. 72). Such magmas are typically found in mid-ocean ridge settings, particularly the olivine tholeiites, or as continental flood basalt fields. Neither would appear to fulfill the hypothesis of an ash fall into the Springfield swamp. More silicic magmas, more prone to explosive eruptions, can differentiate from basaltic magmas with, however, a concomitant differentiation in the V/Cr/Ni chemistry. Carboniferous volcanism, occurring at converging plates margins, both at the Appalachian and at the Ouachita margins, was decidedly not tholeiitic. The nearest volcanoes, perhaps at least 450 km away at the Appalachian and Ouachita margins, would have had to produce catastrophic amounts of ash to have produced a significant ash fall in western Kentucky. The only known igneous rocks in the area are Permian periodotite and lamprophyre dikes as well as diatremes of ultramafic composition associated with the Fluorite district (Nelson & Lumm 1984, pp. 86–87).
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Hower et al. 1990a, 1991). High Cl/high volatile A bituminous coals generally are found south of the Rough Creek fault zone and northwest of the Central faults, coinciding with the area of V enrichment at the top of the coal. High volatile C bituminous coals with negligible Cl, and generally with a lesser enhancement of V, are found outside of the latter region. A source of metals from the marine black shale, perhaps not as high as the Mecca Quarry shale levels noted by Leventhal (1998), possibly mobilized by the passage of thermal waters through the coal and roof shales (see Hannigan & Basu 1998), would appear to be a simple, viable hypothesis based upon observable features of the coal.
Within the context of the latter argument, Premovi et al. do provide evidence which may serve to constrain the timing of the hydrothermal metamorphism of the coal. As they noted, complexing of V and other metals is considerably more efficient at low levels of organic metamorphism. Enrichment of metals in the Springfield coal bed would have been constrained to subbituminous or, more likely, lower ranks. Perhaps the hydrothermal activity served as the impetus for the enhancement of coal rank from lignite or subbituminous to high volatile A bituminous.
In any case, the established tenants of the geology of the Springfield coal bed in Western Kentucky provide a stronger hypothesis for the geochemical anomalies than the volcanic origin proposed by Premovi et al. Hypotheses built upon negative evidence, in this case the absence of the volcanic ash, both in the coal or in any surrounding areas, carry a considerable burden of proof. In the case of the study in question, such proof is absent.
11 December 1998
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