The Crater Flat Tuff is almost entirely below the water table in drill hole USW G-4 at Yucca Mountain, Nevada. Manganese-oxide minerals from the Crater Flat Tuff in USW G-4 were studied using optical, scanning electron microscopic, electron microprobe, and x-ray powder diffraction methods to determine their distribution, mineralogy, and chemistry. Manganese-oxide minerals coat fractures in all three members of the Crater Flat Tuff (Prow Pass, Bullfrog, and Tram), but they are most abundant in fractures in the densely welded devitrified intervals of these members. The coatings are mostly of the cryptomelane/hollandite mineral group, but the chemistry of these coatings varies considerably. Some of the chemical variations, particularly the presence of calcium, sodium, and strontium, can be explained by admixture with todorokite, seen in some X-ray powder diffraction patterns. Other chemical variations, particularly between Ba and Pb, demonstrate that considerable substitution of Pb for Ba occurs in hollandite. Manganese-oxide coatings are common in the 10-m interval that produced 75% of the water pumped from USW G-4 in a flow survey in 1983. Their presence in waterproducing zones suggests that manganese oxides may exert a significant chemical effect on groundwater beneath Yucca Mountain. In particular, the ability of the manganese oxides found at Yucca Mountain to be easily reduced suggests that they may affect the redox conditions of the groundwater and may oxidize dissolved or suspended species. Although the Mn oxides at Yucca Mountain have low cation exchange capacities, these minerals may retard the migration of some radionuclides, particularly the actinides, through scavenging and coprecipitation. I.
I.\ PRODUCTIONYucca Mountain, located on the southwest margin of the Nevada Test Site (Fig. 1), is being studied as a potential repository for high-level nuclear waste. These studies are coordinated under the Yucca Mountain Project (YMP) managed by the Nevada Operations Office of the U.S. Department of Energy. Several holes have been drilled at Yucca Mountain for geologic and hydrologic studies. Drill hole USW G-4, the most recent (1982) cored hole within the proposed repository block, was continuously cored from 22 ft to 3001 ft total depth (TD). The drilling history, core lithology, and geophysical logs of the well are given in Spengler and Chornack (1984). A preliminary study of frequency and orientation of fractures in the core is included in Spengler and Chornack (1984); a more detailed analysis is being performed by the U.S.
Geological Survey with the help of geologists from Fenix and Scisson.No attempt has been made in this study to quantify the relative abundances of different fracture types. Stratigraphic descriptions used in this report are from Spengler and Chornack (1984). A general stratigraphy of the Crater Flat Tuff in USW G-4 based on Spengler and Chornack (1984) is shown in Fig. 2. All the samples from the Crater Flat Tuff are indicated, and the distributions of manganese-oxide coatings an...