Minerals in fractures of the unsaturated zone from drill core USW G-4, Yucca Mountain, Nye County, Nevada
The mineralogy of fractures in drill core USW G-4, from a depth of nearly 800 ft to the static water level (SWL) at 1770 ft, was examined to determine the sequence of deposition and the identity of minerals that might be natural barriers to radionuclide migration from a nuclear waste repository. Mordenite was found to be present, though not abundant, at the top of the interval sampled (the top of the lower lithophysal zone of the Topopah Spring Member of the Paintbrush Tuff). Heulandite occurs from about 1245 to 1378 ft; below 1378 ft, clinoptilolite rather than heulandite occurs alone or with mordenite. Smectite in fractures is abundant only in the vitrophyre of the Topopah Spring Member of the Paintbrush Tuff and at the top of the Prow Pass member of the Crater Flat Tuff. The unsaturated zone below 800 ft can be divided into three rock types: devitrified, glassy, and zeolitized host rock. Fracture-lining zeolites for each of these three rock types differ in mineralogy and morphology. Similarities, between fracture mineralogy and host-rock alteration in the nonwelded zeolitic units of the Topopah Spring Member suggest that this zone was once below the water table. The difference between microcrystalline ({ge}.01 mm) fracture coatings in the vitric zone and the mostly cryptocrystalline (<<0.01 mm) fracture coatings in the zeolitic zone also suggests that the conditions under which these two types of linings formed were different. Nonwelded glass shards preserved in the host rock above the zeolite-mineral transition in the fractures indicate that the water table was never higher than the lithic-rich base of the Topopah Spring Member in the vicinity of USW G-4. Fracture linings in the zeolitic Topopah Spring Member are clinoptilolite, but the crystal size (0.01 to 0.02 mm) is closer to that of heulandite in fractures of the vitric zone above it than to clinoptilolite in the Tuff of Calico Hills below. 21 refs., 48 figs., 2 tabs.
- Research Organization:
- Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
- DOE Contract Number:
- W-7405-ENG-36
- OSTI ID:
- 59189
- Report Number(s):
- LA-10415-MS; ON: DE85016208
- Resource Relation:
- Other Information: PBD: May 1985
- Country of Publication:
- United States
- Language:
- English
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