Evaluation of past and future alterations in tuff at Yucca Mountain, Nevada, based on the clay mineralogy of drill cores USW G-1, G-2, and G-3

The tuffs at Yucca Mountain in south-central Nevada are being studied by the Yucca Mountain Project (YMP) to determine their suitability for a high-level radioactive waste repository. For predictive purposes, it is important to understand the alteration history of Yucca Mountain and to know how the minerals in Yucca Mountain tuffs respond to changing conditions such as elevated temperatures. The clay mineralogy of these tuffs has been examined using x-ray powder diffraction, and approximation temperatures of alteration have been determined using available clay mineral data and fluid inclusion analyses. Also, several illites from drill holes USW G-1 and G-2 have been dated using K/Ar techniques, yielding ages of about 11 Myr. The clay mineral in Yucca Mountain tuffs are predominantly interstratified illite/smectites, with minor amounts of chloride, kaolinite, and interstratified chlorite/smectite at depth in USW G-1 and G-2. The reactions observed for these illite/smectites are similar to those observed in pelitic rocks. With depths, the illite/smectites transform from random interstratifications (R = 0) through ordered intermediates (R = 1) to illite in USW G-2 and to Kalkberg (R {ge} 3) interstratifications in USW G-1. The illite/smectites in USW G-3 have not significantly transformed. It appears that the illites in deeper rock results from hydrothermal and diagenetic reactions of earlier-formed smectites. These data demonstrate that the rocks at depth in the northern end of Yucca Mountain were significantly altered about 11 Myr ago. Both clay mineralogy and fluid inclusions suggest that the rocks at depth in USW G-2 have been subjected to postdepositional temperatures of at least 275{degree}C, those in USW G-1 have reached 200{degree}C, and USW G-3 rocks probably have not exceeded 100{degree}C. 64 refs., 9 figs., 3 tabs

The importance of zeolites in the potential high-level radioactive waste repository at Yucca Mountain, Nevada

Zeolitic rocks play an important role in retarding the migration of radionuclides that occur in solution as simple cations (Cs, Sr, Ba). However, the interaction of zeolites with complex transuranic species in solution provides little if any advantage over other common silicate minerals. The most important consequences of zeolite occurrences near a high-level radioactive waste repository environment are likely to be their response to thermal loading and their impact on site hydrology. Partial zeolite dehydration during the early thermal pulse from the repository and rehydration as the repository slowly cools can have an important impact on the water budget of a repository in unsaturated rocks, provided that the long-term heating does not result in zeolite destabilization

Detailed mineralogical characterization of the Bullfrog and Tram members USW-G1, with emphasis on clay mineralogy

The detailed mineralogy of the Bullfrog and Tram Members of the Crater Flat Tuff from drill hole USW-G1 has been examined, primarily to characterize fully the amounts and types of clay minerals in the tuffs and the possible effects clay minerals have on rock properties. Results of bulk sample x-ray diffraction analyses agree closely with previous determinations, although slightly higher clay mineral contents were found in this study. X-ray diffraction analysis of fine fractions revealed that the clay minerals in the tuffs are sodium-saturated montmorillonite-beidellites with typical layer charges and no high-charge layers. These smectites are found in virtually all samples of the Bullfrog and Tram, and there is no correlation between the amounts of smectites and the amounts of zeolite, quartz, and feldspar. Smectites are present in both welded and nonwelded horizons and are scarce in some zones with slight-to-absent welding

Smectite dehydration and stability: Applications to radioactive waste isolation at Yucca Mountain, Nevada

Montmorillonite-beidellite smectites are present in amounts up to 50% in the rocks directly underlying the potential high-level radioactive waste repository horizon at Yucca Mountain, Nevada. The thermal reactions of concern include reversible collapse/expansion of the smectite layers due to loss/gain of interlayer water;irreversible collapse due to loss of interlayer water and migration of interlayer cations into the 2:1 silicate layers;irreversible reduction of the osmotic swelling ability through reaction in a steam atmosphere;and inhomogeneous transformation of the smectite into an interstratified illite/smectite. Reversible collapse should be of minor importance because any thermally driven collapse will be reversed when water is introduced and temperatures go down. The amounts of smectite in the potential repository horizon itself are probably insufficient to give rise to rock strength problems due to reversible collapse. The irreversible reduction of somotic selling capacity in a steam environment may be significant in the rocks near the repository horizon. This effect on naturally occurring Na-rich smectites would probably increase permeabilitie shut would also provide for increased cation exchange by the smectite. 60 refs., 9 figs

Thermal stability of zeolitic tuff from Yucca Mountain, Nevada

Thermal models of the proposed repository at Yucca Mountain, Nevada, suggest that rocks near the proposed host rock will experience elevated temperatures for at least 1000 yrs. In order to assess the effects of elevated temperatures on zeolites clinoptilolite and mordenite were investigated using a combination of high-temperature X-ray powder diffraction, thermogravimetric and differential scanning calorimetric analysis, and long-term heating experiments. 13 refs., 7 figs

Mineralogic summary of Yucca Mountain, Nevada

Quantitative x-ray powder diffraction analysis of tuffs and silicic lavas, using matrix-flushing techniques, has been used to obtain a model of three-dimensional mineral distributions at Yucca Mountain, Nevada. This method of analysis is especially useful in tuff, where the most abundant phases are commonly too fine grained for optical determination. The three-dimensional distributions of primary glass and of tridymite are particularly well constrained. Vitric nonwelded glasses occur above and below the welded devitrified Topopah Spring Member, but the glass in the lower nonwelded vitric zone is progressively altered to zeolites to the east where the zone is closer to the static water level. The zeolites clinoptilolite, mordenite, heulandite, and erionite have all been found at Yucca Mountain, but only mordenite and clinoptilolite are abundant and can be mapped between many drill holes and at many depths. Heulandite distribution is also mappable, but only below the densely welded devitrified part of the Topopah Storing Member. Erionite has been confirmed only once, as a fracture coating. There is a fairly continuous smectite-rich interval immediately above the basal vitrophyre of the Topopah Spring Member, but no evidence suggests that the smectites can provide information on the paleogroundwater table. There are at least four mappable zeolitized zones in Yucca Mountain, and the thicker zones tend to coincide with intervals that retained glass following early tuff devitrification. Problems in extrapolation occur where zones of welding pinch out. No phillipsite has been found, and some samples previously reported to contain phillipsite or erionite were reexamined with negative results. The deeper alteration to albite and analcime was not sampled in every drill hole, and the distribution of these phases is difficult to map

Further description of the petrology of the Topopah Spring member of the paintbrush tuff in drill holes UE25A-1 and USW-G1 and of the lithic-rich tuff in USW-G1, Yucca Mountain, Nevada

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

A preliminary comparison of mineral deposits in faults near Yucca Mountain, Nevada, with possible analogs

Several faults near Yucca Mountain, Nevada, contain abundant calcite and opal-CT, with lesser amounts of opal-A and sepiolite or smectite. These secondary minerals are being studied to determine the directions, amounts, and timing of transport involved in their formation. Such information is important for evaluating the future performances of a potential high-level nuclear waste repository beneath Yucca Mountain. This report is a preliminary assessment of how those minerals were formed. Possible analog deposits from known hydrothermal veins, warm springs, cold springs or seeps, soils, and aeolian sands were studied by petrographic and x-ray diffraction methods for comparison with the minerals deposited in the faults; there are major mineralogic differences in all of these environments except in the aeolian sands and in some cold seeps. Preliminary conclusions are that the deposits in the faults and in the sand ramps are closely related, and that the process of deposition did not require upward transport from depth. 35 refs., 25 figs

Inorganic contents of peats

Peat, the precursor of coal, is composed primarily of plant components and secondarily of inorganic matter derived from a variety of sources. The elemental, mineralogic, and petrographic composition of a peat is controlled by a combination of both its botanical and depositional environment. Inorganic contents of peats can vary greatly between geographically separated peat bogs as well as vertially and horizontally within an individual bog. Predicting the form and distribution of inorganic matter in a coal deposit requires understanding the distribution and preservation of inorganic matter in peat-forming environments and diagenetic alterations affecting such material during late-stage peatification and coalification processes. 43 refs., 4 figs., 3 tabs