Micromorphology of calcite-silica deposits, Yucca Mountain, Nevada

High-level nuclear waste placed at the proposed Yucca Mountain repository must remain isolated for many thousands of years. Concern remains that calcite-silica vein deposits along fault fractures near the proposed repository are not solely pedogenic, but rather record upwelling of groundwater and thus possible routes for transporting radionuclides to the land surface. The purpose of this study was to provide an additional test for whether the calcite-silica deposits have a groundwater (phreatic) origin or a pedogenic origin. This test involved comparing the micromorphic features of the controversial vein deposits with the micromorphic features of known pedogenic and phreatic deposits. Using a combination of thin sections, scanning electron microscopy, x-ray diffraction, and cathodoluminescence, the following micromorphic features were compared : argillans, calcified microorganisms, detrital grains, aquatic microfossils, calcite crystal size and sorting, and opal content. Based on these comparisons, the controversial vein deposits are more similar to pedogenic deposits than phreatic deposits, and thus appear to be of pedogenic origin. 71 refs., 4 figs., 3 tabs