Uranium in Precambrian granitic rocks of the St. Francois Mountains, southeastern Missouri, with comments on uranium resource potential

Abstract

Red granites of the St. Francois Mountains are highly radioactive and contain 4 to 34 parts per million (ppm) uranium. The most radioactive is the Graniteville Granite which contains an average of 16.9 ppm U and 42.6 ppm Th. The Butler Hill and Breadtray Granites also contain anomalous amounts, averaging 6.2 and 5.6 ppm U and 23.5 and 20.5 ppm Th respectively. Other Precambrian granitic rocks have normal concentrations of- U and Th. Fission track 'maps' indicate that high concentrations of uranium are associated with magnetite in the red granites; this uranium is presumed to be readily leachable by hydrothermal or supergene solutions. No uranium minerals or ore grade concentrations of uranium were observed in or near the granites, but there-are conceptual reasons for the possible existence of uranium deposits in intragranitic veins and onlapping Cambrian-Ordovician sedimentary rocks. Although the red granites constitute a good potential source of uranium, there is not much evidence for uranium having been mobilized. Identification of features such as lamprophyre dikes and 'episyenite' alteration, or sedimentary rocks containing reductants, would be of value for exploration and would permit more favorable resource appraisal.