Regional and local anomalies of uranium and thorium in rocks near sandstone-type uranium deposits in Pennsylvania and Colorado

The geochemistry of U and Th in about 500 shales and sandstones from the Catskill Formation in Pennsylvania and the Morrison and adjoining formations in the Uravan mineral belt of Colorado has been investigated to evaluate the magnitude and extent of regional detrital U and Th anomalies, and more local anomalies in the paleoaquifers through which U was transported to form the deposits. Background U and Th are negatively related to grain size and positively related to Al, Ti, and other components of the fine fraction. Weak anomalies in U and Th are accentuated by examination of residuals (deviations) from regressions of U or Th against Al, or U against Th. Some anomalies are accentuated by analysis of extractable U rather than total U. In the Catskill Formation near Jim Thorpe, Pa., the Duncannon-Clarks Ferry Member, which hosts roll-type U ores near Penn Haven Junction, is enriched in both U and Th by 20 to 50%, compared to the underlying Long Run Member which lacks U occurrences; it is also enriched compared to laterally equivalent Duncannon-Clarks Ferry samples 20 to 120 km to the southwest. Zircons from the sandstones show a similar pattern. A U-rich source area apparently was exposed and supplying detritus to the Jim Thorpe area in Duncannon-Clarks Ferry time. The oxidized paleoaquifer through which U reached the roll-type U occurrences contains a fourfold enrichment of U, apparently by adsorption of U from the ground water which formed the rolls. In Colorado, no regional depositional trends, either lateral or vertical, were found in the U and Th content of the host Salt Wash Member, but the entire Morrison Formation may be slightly enriched in U and Th relative to underlying units. The Salt Wash sediments near Uravan contain about twice background amounts of U