This report contains detailed petrographic descriptions of 74 thin sections from drill hole USW G-1 at Yucca Mountain, Nevada. These descriptions are keyed to the distinctions between devitrified, vitrophyre, vitric, and zeolitized intervals below the Topopah Spring Member repository horizon. The petrographic features of the zeolitized intervals down through the Crater Flat tuff, as well as the sorption properties determined from these intervals, suggest that these zeolite occurrences may each have comparable sorptive capability.

The tuffs of Yucca Mountain at the Nevada Test Site are currently under investigation as a possible deep burial site for high-level radioactive waste disposal. One of the main concerns is the effect of oxidizing groundwater on the transport of radionuclides. Rock components that may affect the oxygen content of groundwater include Fe-Ti oxides, Mn oxides, and glasses that contain ferrous iron. Some phenocryst Fe-Ti oxides at Yucca Mountain are in reduced states, whereas groundmass Fe-Ti oxides have been oxidized to hematite, rutile, and pseudobrookite (Fe{sup 3+}-bearing phases) exclusively. Estimates of Fe{sup 2+}-bearing oxides indicate that less than 0.33 vol% phenocrysts is available to act as solid buffering agents of Eh. Of this percentage, significant amounts of Fe-Ti oxides are isolated from effective interaction with groundwater because they occur in densely welded, devitrified tuffs that have low interstitial permeability. Manganese oxides occur primarily along fractures in the ash-flow tuffs. Because the Mn oxides are concentrated along the same pathways (fractures) where transport has occurred in the past, these small volume percentages could act as buffers. However, the oxidation states of actual Mn-oxide phases are high (Mn{sup 4+}), and these minerals have virtually no potential for reducing groundwater Eh. Manganese oxides may even act as oxidizing agents. However, regardless of their poor capabilities as reducing agents, the Mn oxides could be important as sorbents of heavy metals at Yucca Mountain. The lack of accessible, pristine Fe-Ti oxides and the generally high oxidation states of Mn oxides seem to rule out these oxides as Eh buffers of the Yucca Mountain groundwater system. Reduction of ferrous iron within glassy tuffs may have some effect on Eh, but further study is needed. At present it is prudent to assume that minerals and glasses have little or no capacity for reducing oxygen-rich groundwater at Yucca Mountain. 25 refs., 3 figs., 12 tabs

Drill Hole USW GU-3 was cored continuously from the surface to a depth of 2637.0 ft (803.8 m) beneath the central crest of Yucca Mountain. Drill Hole USW G-3 was cored continuously from 2625.4 ft (800.2 m) to 5030.8 ft (1533.4 m) nearby. Studies of the mineralogy and petrology of these core samples concentrate on the products of low-temperature diagenetic alteration; they indicate less alteration, and of lower grade, than is noted in any of the cored drill holes from farther north at Yucca Mountain. Relatively unstable primary phases such as glasses, tridymite, and cristobalite are preserved to greater depth. Clinoptilolite persists to greater depth, and authigenic albite, a relatively high-grade secondary mineral, does not occur. Calcite is rare, and mordenite is virtually absent, except for rare occurrences along fractures in the Crater Flat Tuff. Compositional zonation of zeolites is highly variable and poorly correlated with depth, and a clearly defined smectite-to-illite transition is lacking. Smectite interstratifications, poor in illite, indicate a maximum alteration temperature no greater than 40{sup 0}C at the bottom of USW G-3 (5031 ft or 1533 m). All these features contrast sharply with those of samples from the northern part of Yucca Mountain. In particular, themore » tuff of Calico Hills can not be relied upon as a zeolitized sorptive barrier throughout Yucca Mountain. However, four commonly zeolitized intervals are defined and traced across the exploration block at Yucca Mountain. Analysis of these intervals indicates that equivalent thicknesses of 100% sorptive zeolite range from 24 to 78 m at various localities below any proposed repository in the moderately to densely welded Topopah Spring unit and above the static water level. 26 references, 12 figures, 4 tables.« less

The Topopah Spring Member of the Paintbrush Tuff and the Lithic-rich tuff and two Tertiary volcanic units that occur in cores from drill holes UE25a-1 and USW-G1 at Yucca Mountain, Nevada. Recently they have been suggested as possibly suitable for the permanent storage of high-level radioactive waste. Earlier petrologic characterization of these units is augmented here. The Topopah Spring Member (approximately 350 m thick) has two compound cooling units. The upper, thinner unit is densely welded to vitrophyric. The lower unit ranges from nonwelded to vitrophyric, and its nonwelded base is extensively zeolitized to clinoptilolite and mordenite. Heulandite occurs as fracture fill in the overlying vitrophyric part, but zeolites are absent above that vitrophyre. Here primary devitrification plus vapor-phase crystallization dominate the mineralogy. Vapor-phase effects are especially prominent between the two vitrophyres in both cores and include numerous large lithophysal cavities throughout most of this moderately to densely welded tuff. The Lithic-rich tuff extends from 1203 to 1506 m in the USW-G1 drill core. It is nonwelded to partly welded but is well indurated due to pervasive intergrowths of authigenic minerals. These phases are analcime, albite, alkali feldspar, sericite, chlorite and quartz. The transition from analcime to secondary albite corresponds to Iijima`s zeolite Zone IV boundary, and this boundary appears in USW-G1 at 1326 m. However, analcime remains as a prominent phase through most of the Lithic-rich tuff. Further work is necessary to assess the suitability of either of these horizons for a waste repository. In the Topopah Spring Member, both mechanical and hydrologic properties of thick lithophysal zone must be studied, as well as the complete sequence of fracture fill. For both units, zeolite and clay mineral stabilities need to be investigated.