Multiple Modes of Emplacement of Rare Earth Elements in the Fire Clay Coal, Eastern Kentucky: Implications for Utilization

Rare earth elements and yttrium (REY) in coal deposits have attracted much attention in recent years because of their potential economic significance. Based on previous studies (e.g., Seredin and Dai, 2012), four possible modes for emplacement of the REY are considered: terrigenous, tuffaceous, infiltrational, and hydrothermal, with eastern Kentucky’s Pennsylvanian-age Fire Clay coal considered to be a typical example of the tuffaceous mode. The Fire Clay coal owes much of its high REY content to REY-bearing zircon and phosphates in a volcanic-ash-fall tonstein within the coal in many locations. However, some of the original REY elements may have been terrigenous detrital minerals deposited in the peat. Leaching of REY from the tonstein into the surrounding coal and the hydrothermal overprint of mineralizing fluids associated with the northwestwardly movement of the Pine Mountain thrust sheet contributed to the total REY signature in the Fire Clay coal. Each type of REY emplacement has the potential to create different patterns of elemental distribution. Not all coals are going to have the complex history of the Fire Clay coal, but it should be considered that the total history of REY emplacement by multiple mechanisms contributes to both the total REY concentration and the relative distribution of the individual lanthanide elements. In the beneficiation of the mined raw coal feed, certain portions of the mined coal will be removed from the clean product, meaning that the product has a different whole-coal REY total and REY distribution than the mined material.