Lanthanide, yttrium, and zirconium anomalies in the Fire Clay coal bed, Eastern Kentucky

Abstract

The Fire Clay coal bed in the Central Appalachian basin region contains a laterally-persistent tonstein that is found in the coal throughout most of its areal extent. The tonstein contains an array of minerals, including sanidine, β-quartz, anatase and euhedral zircon, that constitutes strong evidence for a volcanic origin of the parting. For this study, five samples of the tonstein and four sets of coal samples underlying the tonstein were collected from five sites in eastern Kentucky. Inductively coupled plasma-mass spectroscopy (ICP-MS) analysis of the tonstein and underlying coal collected from four sites in eastern Kentucky show that although Zr concentrations are high in the tonstein (570–1820 ppm on a coal-ash basis (cab)), they are highest in the coal directly underlying the tonstein (2870–4540 ppm (cab)). A similar enrichment pattern is observed in the concentration of Y plus the sum of the rare earth elements (Y+∑REE): total Y+∑REE concentrations in the five tonstein samples range from 511 to 565 ppm (cab). However, Y+∑REE contents are highest in the coals directly underlying the tonsteins: values range from 1965 to 4198 ppm (cab). Scanning electron microscopy of samples from coal which directly underlies two of the tonstein samples show that REE-rich phosphate, tentatively identified as monazite, commonly infills cracks in clays and cells in clarain and vitrain. Zircon is rare and commonly subhedral. On the basis of coal chemistry and grain morphology, we suggest that volcanic components in the tonstein were leached by ground water. The leachate, rich in Y and REE precipitated as authigenic mineral phases in the underlying coal.