VERTICAL VARIABILITY OF RARE EARTH ELEMENT CONCENTRATIONS WITHIN ENRICHED LIGNITES BENEATH THE RHAME BED, SLOPE FORMATION (PALEOCENE), SOUTHWESTERN NORTH DAKOTA

Rare earth elements (REE) and other critical minerals can be leached by acidic meteoric water infiltration through soil and sediment profiles and captured by underlying organic beds. Most frequently this results in elevated concentrations of REE near tops and bottoms of coal beds and adjacent to tonsteins or partings within coal seams which tend to rapidly decrease toward the interior-portions of coal. The North Dakota Geological Survey has identified two intervals of kaolinized strata, the Rhame bed of the Slope Formation and the Bear Den Member of the Golden Valley Formation, as sources of some of the highest REE enrichment discovered to date, measured as high as 2,792 ppm (dry coal basis, including Y and Sc) in thin lignites (up to 69 cm thick) underlying these intensely weathered strata. Further, multiple depth-profiles of coals underlying the Rhame bed and Bear Den Member have shown elevated REE concentrations throughout the entire thickness of many of these coals. Sample profiles of coals underlying the Rhame bed have been taken with various resolutions, including intervals of 13 cm, 8 cm, and at two locations (sections 97 & 123) with sample interval resolutions of 2.5 cm.

The maximum REE concentration (1,145 ppm) of a 46-cm thick lignite below the Rhame bed (section 97) was measured in a sample collected 5.1 to 7.6 cm below the top of the coal where the ash content of the sample dropped to below 50% of the sample weight from the more clayey organics above it. Ash content of the samples from intervals from 10-41 cm below the top of the coal were within a range of 12-30% with REE concentrations ranging from 446 to 947 ppm and averaging 555 ppm.

In a 48-cm-thick carbonaceous mudstone (section 123) located approximately 2.6 kilometers from section 97, all sampled intervals had ash contents of 86% or greater. The highest concentration (820 ppm) was measured 2.5 to 5.1 cm below the top of the carbonaceous mudstone. From there, concentrations steady dropped to a depth of 23 cm, after which concentrations were relatively stable (303-323 ppm) throughout the bottom 25 cm.