Manganese-oxide minerals in fractures of the Crater Flat Tuff in drill core USW G-4, Yucca Mountain, Nevada

Abstract

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...

Preliminary description of small block mineralogical features, data report

“…The mineralogy of these fractures is complex, and will result in important, but as yet unquantified, changes in water chemistry, as fluids interact with fracture filling minerals. Our observations of the second generation fracture set mineralogy a.~ consistent with the descriptions of fracture mineralogy described by Carlos et aL(1990Carlos et aL( , 1993, particularly in the occurrence of Ba-bearing mineral phases. Although barite is sparingly soluble (Holland and Mali&, 1979), movement of water from matrix to fi-actures must result in chemical reactions since the matrix mineralogy is dominated by silicates while the fracture mineralogy is dominated by carbbnates and sulfates.…”

“…The association of barium with sulfur in X-ray maps generated with the electron microprobe suggests that the former identification is most likely. Hollandite and barite, however, have both been reported in veins from driIl core (Carlos et al, 1990(Carlos et al, ,1993, and both may thus be present in these Fran Ridge samples. Further work is underway to positively identify the barium-bearing phases.…”

Analysis of fractures in volcanic cores from Pahute Mesa, Nevada Test Site

“…The minerals most often observed as secondary mineral coatings were chalcedony, euhedral quartz, calcite, zeolite, clay, and Fe/Mn oxides (Figure 4-3a). These were also the common fracture-coating minerals observed at Yucca Mountain Core Holes USW G-1 and USW G-2 (Maldonado and Koether, 1983;Carlos, 1990). Similar changes in relative abundances of these minerals were noted at both study areas.…”

Near-field/altered-zone models report

“…The balance between rate of water flow and rate of dissolution will determine whether the water achieves equilibrium with the fracture minerals (Glassley, 1993). The diversity of fracture-lining minerals Carlos et al, 1990Carlos et al, , 1993 suggests that partial equilibrium (i.e., some of the reactions in a system achieve equilibrium while others do not) may persist for some time. The water may eventually reach the boiling front, where the dissolved solids will be deposited on the fracture walls.…”