GEOCHEMISTRY OF ROCK UNITS AT THE POTENTIAL REPOSITORY LEVEL, YUCCA MOUNTAIN, NEVADA

The compositional variability of the phenocryst-poor member of the 12.8-million-year Topopah Spring Tuff at the potential repository level was assessed by duplicate analysis of 20 core samples from the cross drift at Yucca Mountain, Nevada. Previous analyses of outcrop and core samples of the Topopah Spring Tuff showed that the phenocryst-poor rhyolite, which includes both lithophysal and nonlithophysal zones, is relatively uniform in composition. Analyses of rock samples from the cross drift, the first from the actual potential repository block, also indicate the chemical homogeneity of this unit excluding localized deposits of vapor-phase minerals and low-temperature calcite and opal in fractures, cavities, and faults, The possible influence of vapor-phase minerals and calcite and opal coatings on rock composition at a scale sufficiently large to incorporate these heterogeneously distributed deposits was evaluated and is considered to be relatively minor. Therefore, the composition of the phenocryst-poor member of the Topopah Spring Tuff is considered to be adequately represented by the analyses of samples from the cross drift. The mean composition as represented by the 10 most abundant oxides in weight percent or grams per hundred grams is: SiO{sub 2}, 76.29; Al{sub 2}O{sub 3}, 12.55; FeO, 0.14; Fe{sub 2}O{sub 3}, 0.97; MgO, 0.13; CaO, 0.50; Na{sub 2}O, 3.52; K{sub 2}O, 4.83; TiO{sub 2}, 0.11; and MnO, 0.07.