Primary mineral distribution and secondary mobilization of uranium and thorium in radioactive granites

Abstract

Radioactive granites in the southwestern United States tend to form distinct geochemical provinces, for which the resistant mineral zircon provides useful indices of primary igneous endowments. A large fraction of the uranium and thorium in these granites is contained in trace minerals, stoichiometric for the actinides (e.g. brannerite, coffinite, uranothorite), present at levels up to tens of ppm. Secondary mobilization of uranium in radioactive rocks seems most dependent on the geochemical stability of these rare phases, especially after their modification by radiation damage. U-Th-Pb isotope systematics provide several independent indications of the magnitude and timing of uranium transfers (1) within the granitic systems in what we believe are important preparation processes, and (2) to sites outside the granites including potential secondary uranium ore deposits.