Metalliferous lignite in North Dakota

Thin, impure, lignite beds in a belt across portions of North Dakota and South Dakota are highly enriched in U, Mo, and As. These beds contained on the order of 0.25% U/sub 3/O/sub 8/, and equal amounts of Mo. The metals were leached from overlying volcanic ash, and infiltrated through the lignites with the ground water, where they were precipitated on formed metallo-organic complexes. The belt of metalliferous lignites concides with a major surface drainage divide, where water moves generally downward and laterally.