MIGRATION BEHAVIOR OF NATURALLY-OCCURRING RADIONUCLIDES AT THE NOPAL-I URANIUM DEPOSIT, CHIHUAHUA, MEXICO

Oxidation of pyrite at the Nopal I uranium deposit, Pena Blanca distri ct, Chihuahua, Mexico has resulted in the formation of Fe-oxides/hydro xides. Anomalous U concentrations (i.e. several hundred to several tho usand ppm) measured in goethite, hematite, and amorphous Fe-oxyhydroxi des in a major fracture that crosscuts the deposit and the absence of U minerals in the fracture suggest that U was retained during secondar y mineral growth or sorbed on mineral surfaces. Mobilization and trans port of U away from the deposit is suggested by decreasing U concentra tions in fracture-infilling materials and in goethite and hematite wit h distance from the deposit. Greater than unity U-234/U-238 activity r atios measured in fracture-infilling materials indicate relatively rec ent (< 1 Ma) U uptake from fluids that carried excess U-234. Systemati c decreases in U-234/U-238 activity ratios of fracture materials with distance from the deposit suggest a multistage mobilization process, s uch as remobilization of U from U-234-enriched infill minerals or diff erential or diminished transport of U-bearing solutions containing exc ess U-234. (C) 1997 Elsevier Science B.V.