MODES OF EMPLACEMENT OF RARE EARTH ELEMENTS IN COALS: EXAMPLES FROM THE FIRE CLAY COAL, EASTERN KENTUCKY

While Seredin and Dai (2012) noted four main genetic modes of REY (rare earth elements (REE) + yttrium) enrichment in coals (terrigenous, tuffaceous, infiltrational, and hydrothermal), the importance of organic associations, particularly in low-rank coals, implies that a fifth category must be added. For this investigation, selected suites of lithotype samples from our collections from the early 1990’s were analyzed for REY in order to supplement previously published information. The tuffaceous influence, in the form of the ash-fall tonstein, is well established. Detrital minerals (accounting for the terrigenous influence) such as zircon and phosphates, both REY-bearing, are known to be part of the detrital influx into coal, in general, and into the basal lithotype of the Fire Clay coal. Infiltrational emplacement includes the redistribution of elements due to the leaching of partings, the floor, and roof. Hydrothermal influences are known to be associated with the emplacement of the Pine Mountain thrust sheet but have not been specifically investigated for the Fire Clay coal. The light REE/ heavy REE (LREE/HREE) observed for the (1) upper coal lithotype, the (2) coal immediately above the tonstein, the (3) tonstein, the (4) coal immediately below the tonstein, and the (5) basal lithotype are 4.10, 6.67, 7.98, 7.31, and 6.47, respectively. This indicates a significant variation in the distribution of the individual REE due to both original processes (terrigenous, tuffaceous, organic association) and diagenetic processes (hydrothermal and leaching/infiltration). The time line for all of the processes is diffuse and it is possible that the diagenetic processes were underway prior to the completion of the deposition of the Fire Clay peat.

Seredin, V.V., Dai, S., 2012. Coal deposits as potential alternative sources for lanthanides and yttrium. International Journal of Coal Geology 94, 67-93.