GAS- AND LIQUID-PHASE DYNAMICS OF LOW-TEMPERATURE MINERAL FORMATION IN THE UNSATURATED ZONE AT YUCCA MOUNTAIN, NEVADA

Deposits of low-temperature calcite and silica (chalcedony, quartz, and opal) in fractures and lithophysal cavities in the 500- to 700-m-thick unsaturated zone (UZ) at Yucca Mountain were formed by evaporation and (or) CO₂ evolution from meteoric water percolating through the potential high-level radioactive waste repository. The deposits form mm- to cm-thick coatings on fracture footwalls and cavity floors and record slow growth histories (mm/million years or less) over the last 10 million years. The distribution and textures of the deposits provide compelling evidence that calcite and silica precipitate from water films moving across the lower surfaces of air-filled open spaces. Preferential deposition at crystal tips results in delicate, free-growing crystals of bladed calcite capped with calcite overgrowths and opal. Calcite at the bases of these deposits commonly is corroded.

The gas- and liquid-phase dynamics of calcite and silica deposition within a lithophysal cavity are consistent with the mineral morphologies and textures. Preferential deposition at blade tips reflects formation from water films drawn up crystal faces by surface tension. Vapor pressure is increased (and evaporation is enhanced) where water films bend sharply at blade tips. At equilibrium, the gas phase in the cavity responds to this evaporation by condensation of water (or dissolution of CO₂) elsewhere, either within the highly porous walls lining the cavity where water potentials are negative, or in fractures outside the cavity due to cooling. The evaporation-condensation (distillation) cycle is enhanced by differences in thermal conductivity between air and rock that result in slightly steeper thermal gradients within the air-filled cavity than in the surrounding rock mass. If percolation inputs are episodic, refluxing of CO₂-saturated condensate back into percolating water may be sufficient to periodically undersaturate water flow to the floors of cavities with respect to calcite and silica and explain mineral dissolution at the bases of the deposits. Mineral deposition involving gas-liquid interactions and water films is consistent with physical and geochronological observations as well as the chemical and hydrologic constraints in the Yucca Mountain UZ.