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Abstract: Determination of Natural Radioactivity in a Wilcox Lignite (Eocene), Rusk County, Texas

Pierrepont M. Bartow, E. B. Ledger

Natural radioactivity was determined in a Wilcox (Eocene) lignite in southern Rusk County, Texas. Natural radioactivity

consists mostly of U-235, U-238, Th-232, and K-40. The mode(s) of occurrence of these isotopes in lignite is as follows. Uranium isotopes may occur in two ways: in heavy mineral crystals carried by streams and deposited with the lignite as it forms, and trapped in the complex molecules of organic matter in the lignite. Thorium occurs only in the heavy minerals in a known concentration ratio with uranium. Potassium occurs in association with the plant remains of the lignite.

Eleven samples were analyzed by gamma-ray spectrophotometry for uranium, thorium, and potassium. Uranium averages 1.18 ppm U and ranges from 0.91 to 1.37 ppm U. Thorium averages 7.32 ppm Th and ranges from 6.30 to 8.87 ppm Th. Potassium averages 0.55 wt. % K and ranges from 0.51 to 0.61 wt. % K. The Th:U ratio of 6.2 is within the range of common heavy minerals like zircon, monazite, and allanite, showing that little organic uranium is present in this lignite. If organic uranium were present, the Th:U ratio would be less than about 4.

Wilcox lignite is lower in uranium than the average United States lignite, and strikingly lower than lignite from south Texas where the sediment enclosing the lignite is a low-level source of uranium. In east Texas, uranium-rich sediments are about 100 km south of the main Wilcox lignites.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994