Distribution and chemistry of fracture-lining minerals at Yucca Mountain, Nevada

Abstract

Portions of this ABSTRACTYucca Mountain, a >I 5km-thick sequence of tuffs and subordinate lavas in southwest Nevada, is being investigated as a potential high-level nuclear waste repository site. Fracturelining minerals have been studied because they may provide information on past fluid transport and because they may act as natural barriers to radionuclide migration within the fractures. Cores from seven drill holes have been studied to determine the distribution and chemistry of minerals lining fractures at Yucca Mountain.Fracture-lining minerals in tuffs of the Paintbrush Group, which is above the static water level at Yucca Mountain, are highly variable in distribution, both vertically and laterally across the mountain, with the zeolites mordenite, heulandite, and stellerite widespread in fractures even though the tuff matrix is generally devitrified and nonzeolitic. Where heulandite occurs as both tabular and prismatic crystals in the same fracture, the two morphologies have different compositions, suggesting multiple episodes of zeolite formation within the fractures. Manganese-oxide minerals within the Paintbrush Group are rancieite and lithiophorite. The silica polymorphs (quartz, tridymite, and cristobalite) generally exist in fractures where they exist in the matrix, suggesting that they formed in the fractures at the same time they formed in the matrix. Fluorite, calcite, and opal occur over tridymite in some lithophysal cavities. Calcite also occurs over zeolites in fractures unrelated to lithophysal cavities and is often the youngest mineral in a given fracture. The clays smectite, palygorskite, and sepiolite are common in fractures in the Paintbrush Group in drill core USW GU-3; smectite is an abundant fracturecoating mineral in all drill cores at Yucca Mountain.In fractures in the Calico Hills Formation and the Crater Flat Group, zeolites generally exist mainly where the matrix is also zeolitic, although mordenite does occur as fracture linings in some devitrified intervals of the Crater Flat Group as well. Clinoptilolite and mordenite occur in fractures in tuffs containing clinoptilolite; analcime is limited to fractures in tuff intervals containing analcime. These data suggest that the formation of fracture-lining zeolites in the Calico Hills Formation and the Crater Flat Group may have been coincident with the original alteration of the tuffs. Manganese-oxide minerals in the Calico Hills Formation and the Crater Flat Group occur principally in devitrified tuff intervals and are mainly cryptomelane/hollandite family minerals, although lithiophorite, todorokite, pyrolusite, rancieite, and aurorite also occur.The calcic compositions of the zeolites and manganese-oxide minerals in the Paintbrush Group and the sodic and potassic compositions of the zeolites and potassic manganese-oxide minerals in the Calico Hills Formation and Crater Flat Group suggest that fluid compositions in the Paintbrush Group differed from those in deeper tuffs. Although matrix and fracture-lining zeolites may have formed un...

Eu(III), Sm(III), Np(V), Pu(V), and Pu(IV) sorption to calcite

“…1950's [3]. Calcite is also found in relatively large quantities in the subsurface of the Nevada Test Site and along the fracture surfaces in the host rock at the proposed Yucca Mountain nuclear waste repository, USA [4].…”

Groundwater Flow Model of Corrective Action Units 101 and 102: Central and Western Pahute Mesa, Nevada Test Site, Nye County, Nevada, Revision 0

“…Such mineralization is well studied for the Yucca Mountain region of the SWNVF (e.g., Levy et al, 1999;Carlos et al, 1995; Appendix B Carlos et al, 1993), and within the Pahute Mesa source region (Benedict et al, 2001). This generally associated hydrothermal alteration, depending on its intensity, can completely fill the permeable portions of faults, converting them to impermeable barriers .…”

Fluid flow and reactive transport around potential nuclear waste emplacement tunnels at Yucca Mountain, Nevada

“…Minerals were assumed to react at equilibrium or under kinetic constraints (Table 2). Initial mineral abundances were derived from XRD measurements on cores (Carey et al 1997) and analyses of fracture surfaces (Carlos et al, 1993;Levy et al, 1998). Amounts of minerals observed, but present in quantities below the detection limit (typically around a percent for XRD), were estimated.…”