RARE EARTH ELEMENT SOURCES AND MODIFICATION IN A LATE-MIDDLE PENNSYLVANIAN-AGE COAL BED AND ASSOCIATED UNITS, WESTERN PENNSYLVANIA

An investigation of rare earth element (REE) distribution in coal has been carried out in order to address potential occupational health risks experienced by mining and mineral industry workers exposed to REE through a wide variety of exposure modes. The goal of this study is to understand and predict the source of REE in coal units deposited over a relatively extensive area and overlain by a range of depositional conditions. The study was conducted on the late-middle Pennsylvanian Lower Kittanning Coal bed and the adjacent rock units of western Pennsylvania. Channel samples of the coal, the underlying clay (paleosol) unit, and the overlying shale were retrieved over a 170 km east-west range of sampling sites. The depositional environment of the overburden shale in the selected sites is interpreted to range from freshwater to marine.

Whole-coal REE concentrations are enriched relative to chondrites by a factor of 9 to 100 for the light rare earth elements (LREE: La-Nd), and 2 to 38 for the middle-heavy REE (Sm-Lu). No exposure criteria have been established for many REE but the threshold limit value (TLV) for Y is 1 mg/m³. A general correlation between ash content and total REE content suggests that the bulk of the REE are contained in coal mineral matter. The coal overlain by shale deposited in a freshwater environment has lower ash and REE contents than those overlain by marine or brackish water sediments. In general, the REE patterns in the coal samples are similar to average shale, as exemplified by the North American Shale Composite (NASC). In detail, however, most of the coal REE patterns show a slight LREE depletion relative to the immediate shale overburden. A possible explanation is post-depositional redistribution of REE in the coal, provided coal mineral matter is derived from the same source as the overburden. Preliminary neodymium isotope data indicate that the shale and coal mineral matter are derived from similar sources. Slight differences between the coal and overburden REE patterns might also result from mineral fractionation processes during transport or deposition, particularly as the coal mineral matter could have a substantial atmospheric dryfall component.