The primary objective of TRAC-NA research has been to develop an appropriate conceptualization of the geologic system operating in the upper crust at and near Yucca Mountain based on a verifiable understanding of the history of the long-term behavior of this system. Specifically, the major thrust of the research was narrowly concerned with the significance of three lithic facies that are hosted by the tuffs at Yucca Mountain. These are: (1) the metasomatically (alkaline earth) altered tuffs; (2) the AMC (authigenic-matrix-cemented) breccias cements that are enriched in the radiogenic isotope of strontium (⁸⁷Sr) and depleted in the "heavy" isotope of carbon (¹³C); and (3) the veins and slope calcretes that are equivalent (chemically, mineralogically, texturally, and isotopically) to the AMC breccia cements. The fact that these facies are present at Yucca Mountain and that they a) express a common geologic process, b) were formed over the Plio-Quaternary time span, and c) involve veins and calcretes that are less than 100 Ka old is not disputed by scientists involved in studies of site suitability at Yucca Mountain. Further, there is no dispute that the facies are of critical relevance to considerations of the viability and suitability of Yucca Mountain as a high-level nuclear waste repository. What is in dispute, however, is the character of the geologic processes that are responsible for these deposits. In this regard, the conclusion drawn by DOE project personnel is that the controversial facies are pedogenic in origin and are a consequence of infiltration and deposition by rainwater from the topographic surface. By contrast, a conclusion drawn in this study is that arguments in favor of the pedogenic origin lack sound scientific validity, principally because the arguments are based on equivocal evidence, ignore much of the most critical data, and evidence major lapses in elementary logic. Consequently, in view of the geodynamic setting of Yucca Mountain and the inability of pedogenic processes to explain the observations, it was concluded that an epigenetic-hydrothermal alternative needed to be seriously considered.
See Figure 1.
See Figure 2.
See Figure 3.

To unequivocally resolve the interpretive problem involving the origins of the altered tuffs, breccias and calcites at this site, TRAC-NA obtained and analyzed a wide range of data. The objectives were: (1) to determine the evolution of the thermodynamic state of the lithosphere at Yucca Mountain based on the documented history of the late Cenozoic volcanism; (2) to determine the chemical (metasomatic) and mineralogic (propylitic) alteration of the tuffs based on the results of chemical and chronologic analyses of the clinoptilolite-heulandite fraction; (3) to determine the contents of the in-situ grown (hydrothermal) accessory minerals (zircon, apatite, titanite, pyrite, and chalcopyrite) in the altered tuffs, in the AMC breccias and in the calcite/silica veins based on ontogenetic analyses of the separated "heavy" (specific gravity of more than 2.89 g/cm³) concentrates; (4) to investigate the liquid and gas inclusions that were found to be incorporated in crystalline specimens of quartz and calcite from the exploratory tunnel based on measurements of the homogenization temperatures, entrapment pressures (crushing experiments), and chemical analyses of the entrapped fluids and gases; (5) to investigate the stable isotope fractionation effects based on lateral distribution of isotopic signatures of del¹³C and del¹⁸O from the slope calcretes in Crater Flat; (6) to investigate the chronology of the ⁸⁷Sr metasomatism, as well as of the ¹³C depleted and ⁸⁷Sr enriched carbonization based on the record from the USW VH-2 borehole; and (7) to investigate the thermodynamic evolution of the geologic system at Yucca Mountain based on a variety of characteristics of the alteration and epithermal mineralization in the interior of Yucca Mountain.

Collectively, the TRAC-NA findings constitute strong evidence that the area of Yucca Mountain has experienced burst-like eruptions of hydrothermal and gaseous plumes in the vadose zone and at the topographic surface. The eruptions are found to have occurred intermittently over a considerable period from about 8-9 Ma ago up to, essentially, the present. We conclude that the eruptions were triggered by tectonic events, such as earthquakes and/or volcanic intrusions. Given the tectonic setting of Yucca Mountain, which is characterized by recently (geologically speaking) active volcanic centers and by many active faults with an associated geophysical environment that is quite anomalous, there is little doubt that the eruptions will continue to occur and, in fact, could occur at any time in the future.

Specifically, during future local tectonic activity that would include intruding hydrothermal fluids and gases rising along faults and fractures, we conclude that such fluids could inundate the repository and, because the host rock would be at an elevated temperature (above 100°C), steam and super-heated water would be produced and fill the repository, with steam and other gases (particularly carbon dioxide and methane) escaping into the atmosphere. Under these conditions, and in the presence of highly saline fluids, there is little doubt that the waste packages would undergo very rapid corrosive breakdown resulting in the release of radionuclides into the biosphere. The sequence of events to be expected at Yucca Mountain as a consequence of this local tectonic activity are summarized in Figure 4. Thus, a release of radionuclides would be carried into the atmosphere, as well as into the hydrosphere, and spread rapidly. Should this occur, there is little doubt that the effects would be disastrous, both regionally and worldwide.

In addition, it has been recently suggested by Bowman and Venneri, scientists at the Los Alamos National Laboratory, that radionuclides once released from the waste packages and transported by groundwater could accumulate in volume configurations (lenses, or rough spheres) that could result in self-enhancing criticality of the fissile material, producing, in some circumstances, nuclear detonations. The mechanism for release and transport by groundwater would be expected to be activated during periods of tectonic activity.

The expected yield of such detonations ranges from tens of tons of TNT equivalent to hundreds of tons, depending on the size and shape of the accumulations of the fissile material, among other factors. Obviously, if the detonations were to occur, they would lead to further disintegration of the waste packages and eject yet more radionuclides into the atmosphere. While the complete scenario resulting in an explosion may not be highly probable, it appears to be possible and adds yet another risk factor in the disposal of radioactive waste and spent nuclear fuel at Yucca Mountain.

Because tectonic activity is expected to continue for at least hundreds of thousands of years, this series of studies indicates a hazard that cannot be mitigated and reduced to acceptable levels. Therefore, these gas-assisted hydrothermal eruptions entail a hazard of such magnitude that no responsible governmental institution should be willing to consider installation of a radioactive waste and spent nuclear fuel disposal facility at this site. It is therefore evident that a much more geologically stable site must be found.

Summaries of key studies are available by selecting titles given below. Copies of the original reports and papers that provide the detailed analysis, data, and results may be obtained from the Nevada Agency for Nuclear Projects/Nuclear Waste Projects Office by requesting the titles listed below.

• The Thermodynamic Evolution and Present State of the Lithosphere at Yucca Mountain, Nevada
  by J. S. Szymanski and C. B. Archambeau
• Chemical Heterogeneity of the Clinoptilolite-Heulandite Fraction at Yucca Mountain, Nevada: Evidence for Polygenetic, Hydrothermal Alteration
  by D. E. Livingston and J. S. Szymanski
• Hydrochemical Accessory Minerals in Tuffs, Breccias, and Calcite/Opal Veins at Yucca Mountain: Evidence for Plio-Quaternary Hydrothermal Eruptions
  by A. V. Chepizhko, Y. V. Dublyansky, and J. S. Szymanski
• Overview of Calcite/Opal Deposits at or Near the Proposed High-Level Nuclear Waste Site, Yucca Mountain, Nevada, USA: Pedogenic, Hypogene, or Both?
  by C. A. Hill, Y. V. Dublyansky, R. S. Harmon and C. M. Schluter
• Fluid Inclusions in Calcite from the Yucca Mountain Exploratory Tunnel: Evidence for Hydrothermal Origin of the Calcite
  by Y. V. Dublyansky, V. Reutsky, and N. Shugurova
• Stable Isotope Gradients in Slope Calcretes at Yucca Mountain, Nevada: Evidence for the Involvement of Carbonic Gases in the Hydrothermal Discharges
  by J. S. Szymanski and Y. V. Dublyansky
• Sr-, C-, and O- Isotopic Profile from the USW VH-2 Borehole, Crater Flat, Nevada: Evidence for the Intermittancy of the Hydrothermal Discharges and the Plio-Quaternary Age of the ⁸⁷Sr Metasomatism at Bare Mountain
  by J. S. Szymanski , Y. V. Dublyansky, and D. E. Livingston
• Epithermal Mineralization, Alteration, and Spring Deposits at Yucca Mountain, Nevada - Thermodynamic Evolution of the Geologic System
  by J. S. Szymanski
• Carbonate Deposits at Yucca Mountain (Nevada, USA) and the Problem of High-Level Nuclear Waste Repository
  by Y. V. Dublyansky and J. S. Szymanski
• Geohydrological Models and Earthquake Effects at Yucca Mountain, Nevada
  by J. B. Davies and C. B. Archambeau
• Analysis of High-Pressure Fluid Flow in Fractures with Applications to Yucca Mountain, Nevada, Slug Test Data
  by J. B. Davies and C. B. Archambeau

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A large set of petrographic, chronologic, geophysical, and geothermal data has been assembled to assess the potential for the occurrence of hydrothermal eruptions at Yucca Mountain. It is concluded that there is (over the past 10 Ma) a need to augment conductive heat flow by convective and advective modes of heat transport. In other words, the hydrologic system at Yucca Mountain is chronically in a thermally unstable state, and hence, the potential for the occurrence of hydrothermal eruptions over the future 10-100 Ka is fairly high.

The established history of volcanic activity in the region is interpreted to indicate that the balance between conductive and advective components of the total input of heat into the crust from the mantle has been evolving over the past 13 Ma. Although the advective component was steadily decaying over this period, the conductive component was steadily becoming more intense. One effect of these changes is that the present thermal state of the crust is one characterized by substantial gradients of temperature (dT/dz and dT/dx in excess of 40°C/km and 3.2°C/km, respectively). Such gradients virtually assure hydrothermal processes to be active, either continuously or intermittently.

The other effect is that the storage of thermal energy, which could support hydrothermal processes, has changed from convectively recharged, at a time when the advective component was "strong," to conductively recharged, at a later time. The changing character of the recharge mechanism can be expected to have altered the hydrothermal regime at Yucca Mountain, particularly in terms of: (1) diminishing the role of relatively shallow, multi-path, and long-lasting circulations; and (2) enhancing the role of deep-seated, energetic, hydrothermal plumes that can be triggered by tectonic events.

The tuffs at Yucca Mountain are expected to contain epigenetic, hydrothermal minerals (both alteration and vein) with ages spanning a considerable period, from 13 Ma ago up to essentially the present. Further, the minerals are expected to be developed as two chronologically, isotopically, and compositionally distinct facies. The older facies (more than 9 Ma old) is expected to be associated with isotopic and compositional features consistent with the prolonged, but relatively shallow, character of the parental circulations. In contrast, the subsequent facies (less than 8-9 Ma old) is expected to contain isotopic and mineralogic features consistent with the short-lived (burst-like), but deep-seated, character of the parental plumes. This subject relates directly to the considerations of the suitability of Yucca Mountain for the construction of high-level nuclear waste repository. These results suggest that, within its lifetime (about 100 Ka), a repository could be invaded by hydrothermal plumes activated by local tectonic processes, such as earthquakes. Some essential results and findings are given in the two attached figures.
See Figure 1.
See Figure 2.

Key Words: Yucca Mountain, hydrothermal, magmatic processes, heat flow, geodynamics, thermodynamic state.

Close examinations of major element chemical data for the altered tuffs at Yucca Mountain, Nevada, revealed an enormous range of abundances of exchangeable cations (sodium, potassium, calcium, and magnesium). Marked contrasts in the abundances range occur over distances of more than tens of kilometers down to a number of centimeters. Such chemical diversity indicates a complex alteration of the tuffs. The high abundance of alkaline earth elements, relative to the alkali metal initial compositions of the tuffs, indicates some of the alteration to be metasomatic, which means that the altering solutions acquired their chemical character from sources other than the tuffs. It is concluded that the alteration has been caused by hydrothermal metamorphism, rather than by infiltration of meteoric fluids from direct atmospheric precipitation. Thus, the zeolite alteration is neither deuteric nor diagenetic (supergene) in origin, as is believed by the US DOE and its contractors.

It is further concluded that the tuffs at Yucca Mountain host two chronologically and chemically distinct assemblages of the alteration (propylitic) minerals. The older set includes both the smectite/illite series (smectite -› allevardite -› kalkberg -› illite) and the alkali-metal zeolite series (clinoptilolite -› mordenite -› analcime -› albite). The minerals from both of the series carry the same K/Ar ages, ranging from 9.5 to 11.0 Ma. The ages, the pervasive character, and the presence of the orderly interstratified montmorillonite clays all indicate that the older assemblage records hydrothermal circulations supported by convectively recharged heat sources (Timber Mountain Caldera). The younger assemblage is much less pervasive (fracture-based) and includes zeolites from the alkaline earth series (mainly heulandite, with trace erionite, chabazite, stellerite, and laumontite). The corresponding minerals (clinoptilolite-heulandite) carry K/Ar ages ranging from 2.0 to 8.5 Ma. The ages, the fracture-based occurrences, and the absence of the orderly interstratified montmorillonite clays all indicate that the subsequent assemblage records intermittent and short-lived hydrothermal circulations supported by conductively recharged heat sources. This subject relates to the suitability of Yucca Mountain for the construction of a high-level nuclear waste repository. Some essential results and findings are given in the two attached figures.
See Figure 1.
See Figure 2.

Key Words: chemistry - clinoptilolite - heulandite - Yucca Mountain - hydrothermal history.

Accessory minerals were studied in Tertiary tuffs, breccias, and in micritic calcite/opal veins at Yucca Mountain. Five minerals were identified as hydrothermal: zircon, apatite, titanite, pyrite, and magnetite.

All samples contain two types of accessory zircon, relict-allogenic (?) and hydrothermal. The crystals of relict zircon often have complex anatomy (core and shell). They are rounded and reveal indications of mechanical damage (transport). Several metamictic grains were identified. Hydrothermal zircon is represented by well-faced idiomorphic crystals. The crystals often contain solid (geotite, hyrogeotite, zircon), gas-liquid, and all-liquid inclusions. They are often intergrown with one another and/or with other minerals (e.g., magnetite). These fragile aggregates and the presence of undamaged, sub-microscopic growth sculptures on crystal faces indicate in-situ growth.

The overall content of zircon and the content of hydrothermal zircon are both variable. The tuffs contain 1.1 to 4.3 ppm of zircon (20 to 60% hydrothermal). Breccias (tuff fragments and authigenic cement) contain 0.9 to 9.0 ppm of zircon (20 to 80% hydrothermal). The highest content of zircon was found in a micritic calcite-silica vein and calcite-cemented breccia along the Solitario Canyon fault, west of Yucca Mountain: 7.7 and 13.0 ppm (99 and 100% hydrothermal), which is 15 to 25 times greater than the average for the tuffs and mosaic breccias.

The presence of hydrothermal (in-situ formed) zircon in the tuffs and breccias at Yucca Mountain indicates epigenetic hydrothermal alteration. High concentration of hydrothermal zircon in near-surface micritic veins is indicative of the young age of this activity (some veins in the area yield U-series ages as young as 20 Ka). Fluid inclusion studies and fission-track dating are in progress. This subject relates to the suitability of Yucca Mountain to accommodate a high-level nuclear repository. Some examples and findings are given in the next eight figures.
See Figure 1.
See Figure 2.
See Figure 3.
See Figure 4.
See Figure 5.
See Figure 6.
See Figure 7.
See Figure 8.

Key Words: Yucca Mountain - hydrothermal processes - calcite-silica veins - accessory minerals.

Calcite/opal deposits (COD) at Yucca Mountain were studied with respect to their regional and field geology, petrology and petrography, chemistry and isotopic geochemistry, and fluid inclusions. They were also compared with true pedogenic deposits (TPD), groundwater spring deposits (GSD), and calcite vein deposits (CVD) in the subsurface. Some of the data are equivocal and can support either a hypogene or pedogenic origin for these deposits. However, Sr-, C-, and O-isotope, fluid inclusion, and other data favor a hypogene interpretation. A hypothesis that may account for all currently available data is that the COD precipitated from warm, CO₂-rich water that episodically upwelled along faults during the Pleistocene and which, upon reaching the surface, flowed downslope within existing alluvial, colluvial, eluvial, or soil deposits. Being formed near, or on, the topographic surface, the COD acquired characteristics of pedogenic deposits. This subject relates to the suitability of Yucca Mountain as a high-level nuclear waste site.

Key Words: Yucca Mountain - calcite/opal deposits - hypogene - pedogenic.

The calcite filling the fractures within the unsaturated zone of Yucca Mountain is often interpreted as being precipitated from rainwaters that have descended along interconnected fractures carrying dissolved carbonate from the overlying soil environment (e.g., Roedder et al., 1994). This interpretation, however, is at odds with a large body of the data: fluid inclusions, ESR, ¹⁸O in calcite-opal pairs, gradients Dd¹⁸O/Dz in calcites from drill holes (Hill et al., 1995) implying elevated temperatures and geothermal gradients in the Yucca Mountain subsurface in the past.

Previous fluid inclusion studies at Yucca Mountain. Only 7 reliable homogenization temperatures have been obtained by calcite from drill holes (YMP, 1993; depths from 178 to 386 m). The data imply a paleothermal gradient of 150-160°C/km. Inclusions, suitable for instrumental studies were observed by Roedder et al. (1994); no numerical data have been reported, however.

Fluid inclusions in calcite from the exploratory tunnel .

We have observed four major types of inclusions: (1) all-liquid; (2) gas-liquid with low vapor-to-liquid ratio (~0.1 and less); (3) gas-rich with variable, but generally high, gas- to-liquid ratio (~0.4 to 0.8); (4) all-gas (they may contain some water, though it is not always distinguishable optically).

All-liquid and gas-liquid inclusions. These are morphologically similar, often occur as clusters, and may represent the same fluid. Normally, only one out of 150-200 inclusions contains a bubble. Most homogenization temperatures measured in these inclusions fell in the range from 35 to 55°C. Extremely low and consistent homogenization temperatures provide strong evidence against stretching or leakage. All but three inclusions subjected to the freezing experiments (39) contained diluted water; NaCl, Na₂SO₄, and MgSO₄ were identified in several inclusions. It should be noted, that the term "diluted water" depicts water with a concentration of less than approximately 0.5-1.0 wt.% (at such concentrations, freezing rarely yields reliable data due to metastability problems). These concentrations, however, lie well within the "hydrogeological" range of salinities (1 wt.% = 10 g/l).

Three all-liquid inclusions from one sample yielded a concentration of 11 wt.% of MgCl₂. The origin of these fluids is unclear; more data are to be acquired.

Gas-rich and all-gas inclusions. Such inclusions are more rare. They are larger than the typical all-liquid and gas-liquid inclusions and restricted to the earliest generations of calcite (near the contact with the tuff). On crushing, the bubbles of the three gas-rich inclusions decreased in volume by a factor of 30 to 120; hence, they could not have been trapped at atmospheric pressure characteristic of the vadose setting. Neither the gas composition of the bubbles, 80-84 vol.% hydrocarbons (CH₄?), 16-18 vol.% N₂+ noble gases, traces of CO₂, and nil O₂, may be attributed to the vadose-zone underground atmosphere (the semi-quantitative data were obtained by selective absorption volumetric analysis on individual bubbles). Four all-gas inclusions decreased by a factor of 20 to 27 on crushing. Their composition was similar to those of collapsing bubbles from gas-rich inclusions, except three of them contained more CO₂ (10 to 15 vol.%). Another all-gas inclusion showed a slight (1.8) increase on crushing, maintaining gas chemistry typical of the gas-rich inclusions (CH₄ > N₂). Presence of CH₄ (1 to 5 mg per kg of the mineral), as well as traces of heavy and light hydrocarbons was confirmed by of chromatographic analysis.

The gas-rich and all-gas inclusions were trapped from a fluid carrying both dissolved and gaseous methane. An inclusion trapped as one-phase liquid followed the isochore and, once it was cooled enough, the "normal" shrinkage bubble appeared (as the fluid sealed in the inclusion contained dissolved methane, the latter appeared in the shrinkage bubble). The pressure in such an inclusion is less than 1 atmosphere; thus, the bubble decreases on crushing. An inclusion trapped as a two-phase gas + liquid fluid (heterogeneous entrapment) will have a pressure corresponding to the entrapment pressure. Increase of the volume of bubbles on crushing indicates that the latter was higher than 1 atmosphere.

Environment of formation evidenced by fluid inclusion data

The fluid inclusion homogenization temperatures obtained in this study imply that calcites from the exploratory tunnel were deposited from epithermal fluids. The new data are in good agreement with the paleogeothermal gradient inferred from earlier fluid inclusion results given in YMP, 1993.

Large bubbles, occupying 40-80% of vacuoles, have been interpreted earlier as the "air- water-CO₂ phase" (Roedder at al., 1994) trapped during the growth of calcite from films of percolating rainwater in the vadose zone. Our results do not corroborate this interpretation. Instead, they imply heterogeneous entrapment of methane-rich fluids in the saturated zone.

Conclusion

The data obtained in this study are not compatible with the vadose-zone setting. Calcite encountered in association with zeolite, quartz, and opal in the Yucca Mountain exploratory tunnel was formed within a low-temperature hydrothermal (epithermal) system.

References

Hill, C.A., Dublyansky, Y.V., Harmon, R., Schluter, C., 1995, Overview of calcite/opal deposits at or near the proposed high-level nuclear waste site, Yucca Mountain, Nevada: pedogenic, hypogene, or both? Environmental Geology, v. 26, n. 1, pp. 69-88.

Roedder, E., Whelan, J.F., and Vaniman, D.T., 1994, Fluid Inclusion Studies of Calcite Veins from Yucca Mountain, Nevada, Tuffs: Environment of Formation: IHLRWM Proc., 5th. Intern. Conf., Las Vegas, Nevada. pp. 1854-1860.

YMP, 1993, Data released by the Yucca Mountain Site Characterization Project Office in December, 1993.

The calcite/opal deposits at Yucca Mountain were studied with respect to their stable isotope composition and macroscopic textures along two profiles located downslope from some of the local active faults. The profiles range in length up to 3,000 m and are situated on gently inclined surfaces along which topographic relief is fairly small, ranging from less than 100 to a maximum of 140 m. The beginning of each of the profiles coincides with an outcrop of massive calcite/silica veins emplaced along bedrock faults. The veins were found to contain in-situ grown hydrothermal accessory minerals, such as zircon, apatite, magnetite, titanite, and pyrite (Chepizhko et al. (1996)).

Progressive changes in texture and isotopic composition were found to be characteristic of all the profiles. At and near the vein-hosting faults, the deposits are typically M (matrix)-textured. With increasing distance, the proportion of cement diminishes gradually and the deposits become first F (floating)-textured and then GS (grain supported)-textured. Concurrently, the isotopic character of the incorporated carbon and oxygen changes with the increasing distance, and this change is expressed by a gradual enrichment in "heavy" stable isotopes (¹³C and ¹⁸O). The isotopic gradients are a factor of 2.5 - 15.0 "steeper" than the locally established per descensum-pedogenic gradients. The gradients express isotopic enrichment and fractionation effects that commonly are observed in association with many modern, travertine-forming geothermal systems. The progressive isotopic enrichment involves both liquid and solid phases and is attributed to joint action of the temperature-dependent fractionation (for oxygen) and the diffusional enrichment mechanism (for carbon and oxygen). The enrichments express concurrent CO₂-degassing, evaporation, and cooling of the travertine-forming solutions after their discharge at the topographic surface.

Both of the trends strongly argue against per descensum pedogenic origin of the calcite/silica deposits. Along with the occurrence of hydrothermal mineral-bearing veins, these trends indicate the surficial calcite/silica deposits to be of geothermal origin. This subject relates to the suitability of Yucca Mountain to accommodate a high-level nuclear waste repository. Some essential results and findings are given in four attached figures.
See Figure 1.
See Figure 2.
See Figure 3.
See Figure 4.

Key Words: Yucca Mountain - calcite/opal deposits - hydrothermal history.

USW VH-2 is located about 8 km west of Yucca Mountain. This drill hole reveals Crater Flat to be veneered by a thick mantle of debris derived mainly from Bare Mountain. In places, this mantle is about 2,000 ft thick and directly overlies the ash-flow tuffs exposed at Yucca Mountain. In basal parts, the mantle contains volcanic rocks of the Timber Mountain Tuff which are overlain by about 11 Ma old basaltic flows and scorias. Below and above the volcanic rocks, the mantle consists of layers of calcrete, or calclithite, intercalated with layers of loose debris. The cemented layers comprise a significant proportion of the section, 60-70 percent, and range in thickness from less than 1 m to many meters. Abundance and thickness of the layers both increase with depth. The stratigraphic relationships indicate that the cementation occurred intermittently over the past 13 Ma. It is concluded that the cementing agents (calcareous clay and silt) were introduced at the topographic surface by run-off of fluids discharged from faults cutting Bare Mountain.

As expected, the entire section is developed as two isotopically distinct facies. The older facies is equivalent in age to the Timber Mountain Caldera and contains calcite that is depleted in ⁸⁷Sr and enriched in ¹³C. Isotopically and chronologically, the older facies is equivalent to epithermal veins of calcite which occur in the deep (> 800 m) interior of Yucca Mountain. The younger facies is Mio-Pliocene in age (less than 10 Ma) and contains calcareous matter that is enriched in ⁸⁷Sr and depleted in ¹³C, in contrast to the underlying facies. Isotopically, the younger facies is equivalent to Late Quaternary spring deposits from southern Crater Flat and from Amargosa Basin. This facies is also isotopically equivalent to calcite-opal veins that are present in the interior of Yucca Mountain at a depth of less than 1,250 m. The epithermal origin of the veins is indicated by four independent lines of evidence which include homogenization temperatures of fluid inclusions, opal-calcite geothermometry, geothermometry by thermo-luminescence, and rate of decrease in the relative amount of incorporated ¹⁸O as a function of depth (Szymanski, (1996)). This subject relates to the suitability of Yucca Mountain to accommodate a high-level nuclear waste repository. Some essential results and findings are given in the two attached figures.
See Figure 1.
See Figure 2.

Key Words: Yucca Mountain - calcite/opal deposits - hydrothermal history.

A massive set of mineralogic, chemical, isotopic, and chronological data has been assembled to form a coherent picture of hydrothermal invasions, at times assisted by CO₂ pressure, of the tuffs at Yucca Mountain over the past 11 Ma. The data show the tuffs to be hosting two isotopically and chronologically distinct assemblages of epithermal veins, in addition to hosting two chronologically and chemically distinct assemblages of alteration minerals.

The older assemblage includes calcite veins that contain: 1) "heavy" carbon, del¹³C ranging from -2.0 to 5.0 per mil wrt PDB, 2) "light" oxygen, del¹⁸O ranging from 1.0 to 12.0 per mil wrt SMOW, and which 3) are depleted in radiogenic isotope of strontium, ⁸⁷Sr/⁸⁷Sr, about 0.709. The isotopic signatures indicate that the parental fluids exchanged with the Paleozoic carbonates underneath the tuffs. The calcites occur at a depth of more than 850 m and are developed as milky veins and replacement cements of the vein-hosting tuffs. Typically, the veins exhibit elevated concentrations of Sc, Fe, Mn, and a chondrite-normalized REE pattern without any Ce anomaly and little or no negative Eu anomaly. They also contain two-phase inclusions. Thermometric analyses of these inclusions revealed a wide range of homogenization temperatures, from 74 to 260°C. Spatially, the veins are associated with propylitic alteration minerals from the alkali zeolite series and the smectite/illite series. The minerals carry the same K/Ar ages, which range from 9.5 to 11.0 Ma. Thus, these minerals and the associated veins record hydrothermal circulations that were supported by convectively recharged heat sources, specifically the Timber Mountain Caldera. As expected, the circulations were: 1) relatively shallow and insufficient to tap the deep-seated reservoirs of strontium and carbon; and 2) long-lasting, with durations sufficient to produce the orderly interstratified montmorillonite clays. Some of the essential results and findings of this study are given in the three attached figures.
See Figure 1.
See Figure 2.
See Figure 3.

The younger assemblage includes veins that, relative to the older veins, are: 1) depleted in the "heavy" isotope of carbon, del¹³C as low as -10 per mil wrt PDB, 2) enriched in "heavy" isotope of oxygen, del¹⁸O up to 22.0 per mil wrt SMOW, and 3) enriched in radiogenic isotope of strontium, ⁸⁷Sr/⁸⁶Sr, up to about 0.713. The isotopic signatures indicate that the parental fluids were tapping the deep-seated reservoirs of strontium and carbon, which are the Rb-rich rocks of the Pre-Cambrian basement and juvenile carbonic gases and liquids, respectively. Thus, relative to the earlier circulations, the subsequent circulations were spread over a much greater depth range. In contrast to the earlier veins, the later veins carry low concentrations of Sc, Fe, Mn, and display a chondrite-normalized REE pattern with prominent negative Ce and Eu anomalies. Similarly, as in the earlier veins, the subsequent veins were found to contain two-phase inclusions. The homogenization temperatures of these inclusions range from 57 to 227°C. The subsequent veins are present in the surficial tuffs only, at a depth of less than 1,200 m. Further, the veins are characterized by systematic changes in the isotopic character of the incorporated carbon as a function of depth. In this regard, the veins carry del¹³C values that diminish with increasing depth and that reflect the lower diffusional enrichment of the parental fluids for the deeper veins. The restricted depth of occurrence and the isotopic enrichment both indicate that the subsequent veins are recording CO₂-assisted hydrothermal processes. Spatially, the veins are associated with alteration minerals from the alkaline earth zeolite series. The minerals carry K/Ar ages (mixed) ranging from 8.5 to 2.0 Ma. Further, ages of some of the subsequent veins are known independently (based on ¹⁴C, Us/Th, and ESR analyses) and range from more than 400 to as little as 21 Ka. Thus, the veins and the associated alteration minerals record hydrothermal circulations supported by conductively recharged heat sources. As expected, these circulations were: 1) intermittently active over the past 8 Ma; and 2) short-lived, with durations insufficient to produce the orderly interstratified montmorillonite clays.

Key Words: Yucca Mountain - calcite/silica veins - isotopic facies - hydrothermal processes and history - thermodynamic evolution.

Yucca Mountain (located in southern Nevada) has been designated for study as a potential national repository for high-level nuclear waste, the first in the USA. One of the key problems with the geology at the site is the origin of epigenetic calcite-opal veins and slope carbonates. The U.S. Department of Energy defends their hypothesis for the pedogenic origin, which is the dissolution and re-deposition of the wind-blown carbonate dust by percolating rainwater. A competing concept, discussed in this paper, interprets the Yucca Mountain carbonates as hypogene formations.

Carbonate deposits at Yucca Mountain revealed systematic stable-isotopic trends: the d¹⁸O in vein calcite increases from depth toward the surface; and both d¹⁸O and d¹³C in slope deposits increase with increasing distance away from suspected paleo-springs. Such trends are indicative of the cooling, CO₂-degassing, and evaporation of mineral-forming fluids. These trends are not compatible with the pedogenic setting, but are expected in association with hypogene (travertine-forming) systems. Our data implies that during the last 400 to 500 Ka, the groundwater level under Yucca Mountain has experienced multiple large-amplitude (200 to 500 m) upward fluctuations. The geothermal gradient under the mountain increased from 160 to 170°C/km.

The concept of geological disposal requires that the proposed repository would provide an effective geologic barrier against any migration of radionuclides for the 10,000 year period. The presence of geologically young, intermittent hydrothermal activity must be considered as a disqualifying factor for Yucca Mountain as a potential host of the high-level nuclear waste repository.

Key Words: Yucca Mountain - suitability - paleohydrology - calcite silica deposits - hypogene-pedogene.

Yucca Mountain, the proposed site for the high-level nuclear waste repository, is located just to the south of where the present water table begins a sharp rise in elevation. This large hydraulic gradient is a regional feature that extends for over 100 km. Yucca Mountain and vicinity are underlain by faulted and fractured tuffs with hydraulic conductivities controlled by flow through the fractures. Close to and parallel with the region of large hydraulic gradient, and surrounding the core of the Timber Mountain caldera, there is a 10 to 20 kilometer-wide zone containing few faults and thus, most likely, few open fractures. Consequently, this zone should have a relatively low hydraulic conductivity, and this inference is supported by the available conductivity measurements in wells near the large hydraulic gradient. Also, slug injection tests indicate significantly higher pressures for fracture opening in wells located near the large hydraulic gradient compared to the opening pressures in wells further to the south, hence implying that lower extensional stresses prevail to the north with consequently fewer open fractures there. Analytical and numerical modeling shows that such a boundary between media of high and low conductivity can produce the large hydraulic gradient, with the high conductivity medium having a lower elevation of the water table. Further, as fractures can close due to tectonic activity, the conductivity in the Yucca Mountain tuffs can be reduced to a value near that for the hydraulic barrier due to strain release by a moderate earthquake. Under these conditions, simulations show that the elevation of the steady-state water table could rise between 150 and 250 meters at the repository site. This elevation rise is due to the projected shift in the location of the large hydraulic gradient to the south in response to a moderate earthquake, near magnitude 6, along one of the major normal faults adjacent to Yucca Mountain. As the proposed repository would only be 200 to 400 meters above the present water table, this predicted rise in the water table indicates a potential hazard involving water intrusion. Some of the essential results and observations are given in the two attached figures.
See Figure 1.
See Figure 2.

Key Words: Yucca Mountain - water table - earthquakes - geohydrological models - fractures - hazards.

Increases in water pressure, if large enough, can be sufficient to open existing fractures in a fractured rock-water system. Such large water pressure increases are expected during earthquake activity and are also produced artificially in high-pressure hydraulic fracturing and slug injection tests. In an in-situ slug injection test, the slug of water is initially released into a packed-off interval of the borehole where the contained fluid is at ambient pressures. We recognize three separate stages in the behavior of the fracture as the water pressure decreases due to drainage, namely opening, closing, and finally, completely closed. In the first stage, the water will initially drain out along conduits between the faces of the opening fracture. As the fracture opens and grows, it can become long enough to intersect any pre-existing network of open joints and fractures. If this connection occurs, the opened fracture will stop growing and there will be further rapid flow of fluid out along these open conduits. Even if connection to an open conduit does not occur, as the pressure drops, the fracture will stop growing and eventually halt. At the pressure where the fracture tip begins to close, drainage into any open conduits will cease and drainage during fracture closing will thus be by flow into the porous walls of the fracture and borehole. This effect of the conduit being shut off may manifest as a kink in the observed height decay of the slug at this pressure due to the effect of the different drainage terms. As the water pressure decreases, the fracture will continue to close until the water pressure drops below the critical pressure and the fracture becomes completely closed. This total closure occurs at the same pressure as that necessary to re-open the fracture and has been taken as equal to the minimum principal stress. This total closure of the fracture also may manifest as a kink in the observed height decay of the slug, as below this closure pressure the drainage is at the usual low pressures controlled by the permeability of the rock just around the packed-off interval of the borehole. We develop the most general mathematical model for this phenomena using conservation of mass of the water in the borehole-fracture system for each separate pressure stage. This model of fracture behavior, with separate flow stages and pressure regimes, explains the physical basis for the increasingly-used technique in high-pressure injection tests of measuring the so-called re-opening pressure at the sharp change in slope of the rate of change of water height versus height. Analytic and numerical solutions yield the rate at which the water height drops during the slug test, and comparison with high-pressure test data confirms the accuracy of the model. Analysis of data from Yucca Mountain, Nevada, gives values for the fracture opening pressures that indicate the presence of extensional stresses of significant magnitude and of zones of pre-existing open fracture networks in the vicinity of the proposed repository site. Some of the essential results and data are given in the attached figure.
See Figure 1.

Key Words: slug tests - Yucca Mountain - tectonics - fracture flow - flow modeling - in-situ stresses.