Gamma -ray characterization of uranium -series nuclides and its application to the study of the Pena Blanca natural analogue site, Chihuahua, Mexico

Virgina Wong, University of Texas at El Paso


Two natural analogue sites located in the Peña Blanca Uranium District, Chihuahua, Mexico were characterized for radionuclide mobility. Analogue I is used to assess the long-term behavior of uranium-series nuclides in a host rock and geochemical environment that is similar to the proposed high-level waste repository at Yucca Mountain, Nevada. Analogue II represents a former dump site to assess short-term radionuclide mobility. Gamma-ray spectroscopy analysis was used to measure radioactivity of the U-series nuclides. ^ Samples analyzed from Analogue I consist of: (1) fracture-infillings associated with different alteration assemblages collected within and outside the breccia pipe from various levels of the deposit and (2) fracture-infillings collected along an east-west trending fracture which intersects the breccia pipe and extends into the host rock. Alteration mineralogy, established via X-ray diffraction analysis, consists of pure kaolinite, a mixture of Fe-oxyhydroxide (goethite, hematite) With inclusions of jarosite and alunite, and carbonates. ^ Results from activity ratios of 230Th/238U versus 226Ra/230Th indicate that majority of the Fe-oxyhydroxides from the breccia zone show a slight disequilibrium with respect to Ra enrichment and U depletion. This observation is modeled as requiring a multiple-event history of U mobility. An amorphous Fe sample distal to the breccia zone shows similar behavior but to a greater extent. This extreme behavior is ascribed to initially low U content and greater late-stage U removal. Two Fe-oxyhydroxide samples from Within the breccia pipe also display multiple-event stages but exhibit both Ra and U leaching. This behavior is shared by Fe-oxyhydroxide samples collected inside and peripheral to the breccia zone from the east-west trending fracture. Finally, three samples, two Fe phase samples outside the breccia zone and a kaolinite inside the breccia zone, show Ra and U enrichment. Also, a distal Fe-oxyhydroxide sample from the east-west fracture shares this mobility trend. The enrichment event is modeled as a single-stage Ra and U precipitation event. ^ Activity results from surface samples collected from Analogue II strengthen the mobility trends documented from Analogue I. Isotopic data reveal leaching of U and Ra from the stockpile into the alluvium and bedrock. Thorium, on the other hand, has not experienced mobility. Lead exhibits variable mobility behavior. ^ Correlation of isotope data with mineralogy indicates that the Fe-oxyhydroxide and clay minerals appear to be excellent scavengers of U. This observation has been documented in other natural analogue studies, for example, the Oklo natural reactor, the Pocos de Caldas Natural Analogue Project, the Koongarra U Deposit, and the Shinkolobwe U Deposit.^

Subject Area

Engineering, Nuclear|Environmental Sciences|Geochemistry

Recommended Citation

Wong, Virgina, "Gamma -ray characterization of uranium -series nuclides and its application to the study of the Pena Blanca natural analogue site, Chihuahua, Mexico" (1998). ETD Collection for University of Texas, El Paso. AAI9919360.