GEOCHEMISTRY OF THE PAINTBRUSH GROUP, YUCCA MOUNTAIN, SOUTHERN NEVADA

The geochemistry and mineralogy of rocks hosting the proposed nuclear waste repository at Yucca Mountain influence the dissolved ion content of percolating water, and water-rock interaction will be exacerbated if the high-temperature operating mode is selected for the proposed repository. Densely welded and nonwelded tuffs of the Paintbrush Group (Tp) form most of the outcrops on Yucca Mountain and extend to the water table. Within the Tp, the densely welded Tiva Canyon Tuff (Tpc) overlies nonwelded, vitric ash-fall tuffs, which in turn overlie the much thicker, densely welded Topopah Spring Tuff (Tpt) in which the repository will be constructed. The lower three-quarters of both units are crystal-poor rhyolite; the upper members are crystal-rich trachyte in which phenocrysts increase in abundance upward. Duplicate chemical analyses of 20 representative samples of the Tpt rhyolite taken from the subsurface Exploratory Studies Facility emphasize its compositional uniformity with the following mean and ±1 sigma values, in weight percent: SiO₂, 76.29±0.32; Al₂O₃, 12.55±0.14; FeO, 0.14±0.05; Fe₂O₃, 0.97±0.07; MgO, 0.13±0.02; CaO, 0.50±0.03; Na₂O, 3.52±0.11; K₂O, 4.83±0.06; and TiO₂, 0.109±0.004. In comparison, a representative sample of the trachyte in the uppermost part of the crystal-rich member of the Tpt has the following contents, in weight percent: SiO₂, 68.6; Al₂O₃, 14.7; FeO, 0.58; Fe₂O₃, 1.91; MgO, 0.39; CaO, 1.29; Na₂O, 4.23; K₂O, 5.33; and TiO₂, 0.39. These are end-member compositions between which the oxide and element concentrations increase monotonically upward from the top of the rhyolite to the top of the trachyte. The respective rhyolite and trachyte members of the Tpc are similar in composition to those of the Tpt. The nonwelded units between the Tpt and the Tpc are generally intermediate in composition between the upper Tpt trachyte and the lower Tpc rhyolite. Because of their vitric nature and large matrix porosity, these nonwelded bedded tuffs may disproportionately affect the composition of percolating water mobilized by the heat from the nuclear waste.