The geochemistry of host rocks for the proposed repository at Yucca Mountain affects past and future water-rock interaction processes, buffering of water chemistry, and retardation of radionuclides. The proposed repository is in one of the tuff and lava layers deposited from volcanoes to the north, the 12.8-million-year-old Topopah Spring Tuff, underlying the Tiva Canyon Tuff. This tuff changes from a high-silica rhyolite in the lower, crystal-poor part, to a quartz latite in the upper, crystal-rich part.

Analyses of outcrop and core samples of the Topopah Spring Tuff show that the crystal-poor rhyolite is relatively uniform in composition. Analyses of rock samples from the potential repository block underground also indicate the chemical homogeneity of this unit, excluding localized deposits of vapor-phase minerals and low-temperature (secondary) calcite and opal in fractures, cavities, and faults.

Analyses of major oxides and elements and of trace elements will be used for geochemical modeling of rock alteration and secondary mineral deposition during initial heating and later cooling of the proposed repository.

Image of contact between Tertiary-age Tiva Canyon Tuff (crystal rich memeber), and overlying Quaternary-age colluvial deposits in trench MWV-T7. (From F.H. Swan, J.R. Wesling, M.M. Angell, A.P. Thomas, J.W. Whitney, and J.D. Gibson, 2001,Evaluation of the Location and Recency of Faulting Near Prospective Surface Facilities in Midway Valley, Nye County, Nevada.

Image of ashflow tuff in Yucca Mountain area.
Photo by USGS

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