RARE EARTH ELEMENT AND CRITICAL MINERAL ENRICHMENTS IN THE TOP AND BOTTOM COALS OF WYOMING’S GREEN RIVER BASIN: A REGIONAL MODE OF OCCURRENCE

Coal from Wyoming’s Green River Basin (GRB) has a significant dip to its numerous strata. It has not been studied for occurrence of the USGS and US-DOE’s 35 critical mineral commodities. Murphy et al. 2018 and Bagdonas et al. 2019 independently found many of these minerals, including rare earths (REY), concentrated in the top meter of North Dakota lignite beds, and both top and bottom meter of sub-bituminous coals from Wyoming’s Powder River Basin. Coals in both those locations are nearly horizontal. This study sought to discover if the same top and bottom enrichments occur in GRB coals.

To answer this question UWyo’s CEGR partnered with the Kemmerer Mine in the GRB. In 2020 a 127-foot drill core was collected through five coal seams. This core was sampled, ashed, and analyzed for whole rock, trace REY, and CM analysis. (N=41 samples)

The analysis showed that the tops (mean: 635ppm REY) and bottoms (mean: 416ppm REY) of the coal seams had concentrations higher than samples in the middle of those seams (mean: 138ppm REY). The shallowest seam had greater total REE than the deeper seams, with the greatest REY concentration (1309 ppm REY) found in a sample just below the shallowest coal’s contact with the overburden. The analysis also revealed a correlation between ash%, LOI, and analyte totals.

The top and bottom enrichments are present in these GRB samples, as past publications showed elsewhere. This could mean that the top and bottom mode of REE enrichment is a regional, or potentially even more widespread, phenomenon. The cause of this enrichment is unknown, but the extra REE in the shallowest top suggests the process is water-infiltration-driven or specific to a coal bog’s first and last years. The ash%-LOI-total correlation indicates that the USGS standard coal analysis procedure results in some volatile coal/minerals passing both the ashing procedure and LOI-furnace, being lost only during ICP analyses.

This study considered an understudied coal, found the same top/bottom meter REE enrichment shared with different basins and coal genesises, and indicates the process causing REE to occur in this way is widespread. Future work should identify the enrichment process, and design tests for selectively mining the meter of greatest-REE coal. Such work could reveal the feasibility of mining what would otherwise be a waste stream.