Uranium and Thorium in Paleozoic Aquifers Surrounding the Llano Uplift Area, Central Texas

Yongje Kim, Thomas T. Tieh, Ernest B. Ledger

The Cambrian Hickory sandstones and Cap Mountain sandy limestones constitute important aquifers in the area surrounding the Llano Uplift in central Texas. Ground waters produced from these aquifers, however, have shown Ra and Rn concentrations above the maximum levels established by the EPA. To determine the sources of Ra and Rn, we have examined the abundance, distribution, and nature of occurrence of U and Th in these rocks.

Samples from cored intervals of two test wells in the area have been analyzed by delayed-neutron counting (for U), gamma-ray spectrometry (for Th), and fission-track imaging (for microscopic occurrence of U). The Hickory samples contain an average of 3.6 ppm U, varying from 1 to 8.5 ppm; the Cap Mountain samples an average of 3.7 ppm, from 1 to 6.4 ppm. Th concentrations in these rocks range from 8.4 to 18.6 ppm, averaging 13.9 ppm. The important modes of U occurrence are: 1. in detrital accessory minerals; 2. in thin shaly laminae enriched in phyllosilicates; 3. in authigenic minerals, particularly hydroxy-oxides of iron and clay minerals; and 4. in biogenic materials such as phosphatic brachiopod fragments and bioturbation products in sandstones. U and its decay products can be mobilized from these rocks by ground water from any of these modes of occurrence. However, it appears that the authigenic, or cementing, minerals and the biogenic materials are more susceptable to mobilization.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995