The purpose of this study was to evaluate radionuclide migration from a uranium-mineralized breccia pipe. The site provides an excellent opportunity to evaluate radionuclide mobility in a geochemical environment similar to that around the proposed high-level waste repository at Yucca Mountain, Nevada. Samples represent fracture-infillings from both within and outside the breccia pipe. Mineral assemblages within the fractures include 1) pure kaolinite, 2) a mixture of ironoxyhydroxides (goethite and hematite) with associated alunite and jarosite, which we refer to as the Fe-mineral assemblage, and 3) carbonates. Uranophane, weeksite, soddyite, and boltwoodite are associated with samples from within the breccia zone.
We obtain radionuclide activities from gamma-ray rather than alpha spectroscopy, and the methodology for these measurements is presented in detail. Plots of 230Th/238U vs. 226Ra/230 Th show three distinct mobility trends. 1) The majority of the Fe-mineral samples from within the breccia pipe yield values between 1.0 and 1.1 for both ratios, 2) Fe-mineral samples from outside the ore zone and a kaolinite from within the ore zone have 230Th/ 238U of 0.58 to 0.83 and 226Ra/230Th of 1.09 to 1.42, and 3) some Fe-mineral samples from within the breccia pipe have values of 1.2 and 0.9 respectively. These data, combined with those from other studies at Peña Blanca suggest that U and Ra are sometimes mobile in the near-surface environment and that multiple episodes of enrichment and leaching are required to explain the trends.