Uraniferous gorceixite in the South Carolina coastal plain (U.S.A.)

Abstract

Gorceixite, a rare Ba—Al phosphate containing 200–2000 ppm U, occurs as thin seams or nodules in Upper Eocene deposits of the Coastal Plain province in Aiken County, South Carolina. At every locality gorceixite appears to be a post-depositional feature. Radiochemical results for various daughter-parent pairs in the 238U decay series show that uranium enrichment occurred at least 1 Ma ago and there has been little uranium migration since. There has been extensive recent redistribution of 226Ra relative to 230Th and 210Po, with 226Ra suddenly migrating from the gorceixite into the overlying sediments within the last one hundred years. The clay-mineral and elemental distributions through a gorceixite outcrop suggest that gorceixite formed as a result of intensive leaching by acidic groundwater during development of a paleo-soil horizon. The sources for the components of gorceixite may have been leached from local sedimentary deposits derived from the barite, monazite and granitic occurrences in the adjacent piedmont rocks. This model provides for: the predominance of gibbsite above and kaolinite below the gorceixite; mechanisms for phosphate fixation and uranium enrichment; depletion of the more soluble elements above the gorceixite; and the thinness and limited areal extent of known occurrences.