Physical and stable-isotope evidence for formation of secondary calcite and silica in the unsaturated zone, Yucca Mountain, Nevada

Applied Geochemistry 17 (2002) 735–750 www.elsevier.com/locate/apgeochem

Physical and stable-isotope evidence for formation of secondary calcite and silica in the unsaturated zone, Yucca Mountain, Nevada Joseph F. Whelan*, James B. Paces, Zell E. Peterman US Geological Survey, Denver Federal Center, Box 25046, MS 963, Denver, CO 80225, USA Received 6 December 2000; accepted 30 November 2001 Editorial handling by M. Gascoyne

Abstract Calcite and silica form coatings on fracture footwalls and cavity floors in the welded tuffs at Yucca Mountain, the potential site of a high-level radioactive waste repository. These secondary mineral deposits are heterogeneously distributed in the unsaturated zone (UZ) with fewer than 10% of possible depositional sites mineralized. The paragenetic sequence, compiled from deposits throughout the UZ, consists of an early-stage assemblage of calcite fluorite zeolites that is frequently capped by chalcedony quartz. Intermediate- and late-stage deposits consist largely of calcite, commonly with opal on buried growth layers or outermost crystal faces of the calcite. Coatings on steep-dipping fractures usually are thin (43 mm) with low-relief outer surfaces whereas shallow-dipping fractures and lithophysal cavities typically contain thicker, more coarsely crystalline deposits characterized by unusual thin, tabular calcite blades up to several cms in length. These blades may be capped with knobby or corniced overgrowths of latestage calcite intergrown with opal. The observed textures in the fracture and cavity deposits are consistent with deposition from films of water fingering down fracture footwalls or drawn up faces of growing crystals by surface tension and evaporated at the crystal tips. Fluid inclusion studies have shown that most early-stage and some intermediate-stage calcite formed at temperatures of 35 to 85  C. Calcite deposition during the past several million years appears to have been at temperatures < 30  C. The elevated temperatures indicated by the fluid inclusions are consistent with temperatures estimated from calcite d18O values. Although others have interpreted the elevated temperatures as evidence of hydrothermal activity and flooding of the tuffs of the potential repository, the authors conclude that the temperatures and fluid-inclusion assemblages are consistent with deposition in a UZ environment that experienced prolonged heat input from gradual cooling of nearby plutons. The physical restriction of the deposits (and, therefore, fluid flow) to fracture footwalls and cavity floors and the heterogeneous and limited distribution of the deposits provides compelling evidence that they do not reflect flooding of the thick UZ at Yucca Mountain. The textures and isotopic and chemical compositions of these mineral deposits are consistent with deposition in a UZ setting from meteoric waters percolating downward along fracture flow paths. Published by Elsevier Science Ltd.

1. Introduction Yucca Mountain in southern Nevada is being evaluated as a potential site for the construction of a high-

* Corresponding author. Fax: +1-303-236-4930. E-mail address: [email protected] (J.F. Whelan). 0883-2927/02/$ - see front matter Published by Elsevier Science Ltd. PII: S0883-2927(02)00036-7

level nuclear waste repository. The mountain consists of a thick accumulation of Miocene tuffs of which 500 to 700 m compose the unsaturated zone (UZ). As part of the site characterization investigations, the Exploratory Studies Facility (ESF), a 7.6 m diameter, 7.8 km long, C-shaped tunnel, and the east–west cross drift (EWCD), a smaller, 2.7 km long side tunnel, were constructed in the central part of Yucca Mountain (Fig. 1) to provide

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access to the potential repository rock mass for direct study. Water moving through the UZ in the past formed low-temperature deposits of calcite and silica on fractures and in cavities in the tuffs. The physical, chemical and isotopic characteristics of the deposits provide evidence of the origin, flux, timing and temperature of fluid movement and of the hydrology of the depositional settings. Numerous studies have shown that the secondary calcite and silica in the UZ formed from meteoric water infiltrating through and dissolving components from the carbonate- and silica-rich soils and from the rocks along fracture flow paths (e.g. Szabo and Kyser, 1990; Peterman et al., 1992; Vaniman and Whelan, 1994; Whelan et al., 1994, 1998, 2001; Vaniman and Chipera, 1996;

Marshall et al., 1998; Paces et al., 1998, 2001). An alternative scenario attributed the deposits to upwelling hydrothermal fluids repeatedly rising far above the water table, flooding the UZ, and even discharging at the surface (Hill et al., 1995). This theory was reviewed by a National Research Council (1992) panel and discredited due to lack of compelling supportive evidence. Nonetheless, Dublyansky et al. (1996), on the basis of fluid inclusions in calcite coatings that indicated depositional temperatures of 35 to 85  C, higher than modern ambient temperatures at the ESF level (20– 25  C), again concluded that the deposits formed from hydrothermal waters (see also Hill et al., 1995; Dublyansky and Szymanski, 1996; Stuckless et al., 1998; Hill and Dublyansky, 1999). This paper describes the secondary deposits of calcite and silica found in UZ fracture and cavity settings within the tuffs at Yucca Mountain and establishes a mineral paragenesis based on their petrography and mineralogy. These observations, combined with stable C and O isotopic compositions of calcite, constrain the hydrogeologic conditions of mineral deposition and provide key evidence for evaluating the percolating versus upwelling water depositional models.

2. Hydrogeologic setting of the deposits

Fig. 1. Map showing the location of Yucca Mountain, Nevada, the Exploratory Studies Facility (ESF) and east–west cross drift (EWCD) tunnels, and the generalized surficial geology (Day et al., 1998).

The Timber Mountain caldera complex 5 to 10 km north of Yucca Mountain was active 15 to 11 Ma during a period of extensive Miocene magmatic activity in the SW Nevada volcanic field (Byers et al., 1976; Sawyer et al., 1994). Paintbrush Group tuffs at Yucca Mountain were erupted about 12.8 to 12.7 Ma at temperatures of 700 to 800  C (Lipman, 1971). Although the tuffs probably cooled to sub-boiling temperatures within 103 to 104 a, the shallow magma bodies responsible for these and later eruptions provided a continuing thermal input to the overlying rocks that increased local geothermal gradients for millions of years (Marshall and Whelan, 2000, 2001; Whelan et al., 2001). Low-grade hydrothermal alteration of tuffs below the water table at Yucca Mountain 12 to 9 Ma (Bish and Aronson, 1993; Neymark et al., 2000) and at a number of localities in the region (McKee and Bergquist, 1993) reflects those elevated geothermal gradients. Tuffs in the UZ include the Paintbrush Group and parts of the Calico Hills Formation and Crater Flat Group (see Stuckless and Dudley, 2002). The Paintbrush Group consists of, in descending order, the Tiva Canyon Tuff, mainly a densely welded unit that forms the bedrock surface of most of the mountain; a series of bedded and nonwelded tuffs that includes the Yucca Mountain and Pah Canyon Tuffs; and the densely welded Topopah Spring Tuff with thin nonwelded units at the top and bottom. Montazer and Wilson (1984) and

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Ortiz et al. (1985) proposed an informal hydrostratigraphy for the Paintbrush Group based on hydrogeologic and thermal-mechanical properties. The welded portions of the Tiva Canyon and Topopah Spring Tuffs (the TCw and TSw hydrogeologic units, respectively) have low matrix permeability but high fracture permeability. In contrast, the intervening nonwelded tuffs, collectively termed the Paintbrush nonwelded hydrogeologic unit (PTn), have high matrix porosity and permeability but low fracture permeability (Moyer et al., 1996). The potential repository would be located in the TSw approximately 200 to 300 m below the land surface and 200 to 300 m above the water table (US Department of Energy, 2000).

3. Approach and methods Prior to construction of the ESF and the EWCD, 700 samples were collected from drill core to determine the distribution and origin of secondary calcite and silica from throughout the tuff sequence, including below the modern water table (Whelan and Stuckless, 1992; Whelan et al., 1994). Since 1995,  300 samples have been collected from the ESF and EWCD to help reconstruct the Quaternary paleohydrology of the UZ. (Underground sample locations are defined by stations at 100-m intervals beginning either from the north portal for the ESF or from the entrance to the EWCD or an alcove. For example, sample ESF 64+95 is 6495 m from the north portal.) At the request of the Department of Energy in 1999, the Department of Geoscience at the University of Nevada at Las Vegas (UNLV) and the US Geological Survey (USGS) started detailed studies of fluid inclusions in UZ calcite. These studies were in response to work by Dublyansky et al. (1996), which concluded that the fluid inclusion temperature determinations supported a hydrothermal origin for the UZ calcite. The UNLV and the USGS jointly collected 150 more samples from the ESF and the EWCD specifically for the fluid inclusion research. These samples were protected from temperatures <0 or > 30  C during collection, transport, and petrographic slide preparation, in order to ensure an undisturbed fluid inclusion thermal record. The mineralogy and paragenetic sequence reported here, although focused on the samples collected for fluid inclusion studies, includes observations made from earlier sample collections. The descriptions of mineral textures are based on examination of hand specimens at low magnification aided by short-wave ultraviolet (UV) light, of 100- to 200-mm thick petrographic polished sections at high magnification, and of secondary and back-scattered electron images using a scanning electron microscope (SEM). Some mineral identifications were aided by standard XRD techniques. Cathodoluminescence (CL)

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was used to distinguish mineral zoning and to characterize mineral zoning differences between the paragenetic stages. Calcite was sampled for stable isotope study from hand specimens and polished thick sections using carbide dental burs. Layers as thin as 0.1 mm were milled from mineral faces and channels as narrow as 0.3 mm were excavated from within mineral growth sequences. Carbon dioxide was evolved from calcite for stable isotope analysis by conventional techniques (McCrea, 1950) or from microsamples, by a Finnigan1 (Bremen) Kiel automated carbonate extraction manifold. Stable C and O isotopic compositions of the extracted CO2 were determined on Finnigan MAT 251 or 252 mass spectrometers. Stable isotope compositions are reported as % deviations of the samples from the international standards VPDB (for C) and VSMOW (for O) (Coplen, 1996). One-sigma reproducibility of the d13C and d18O values is typically 40.1 and 40.15%, respectively.

4. Secondary calcite and silica deposits The following descriptions of the calcite and silica deposits are based on hundreds of individual deposits, from both visual examination of the deposits in the tunnels and laboratory study of samples collected from drill core and tunnel exposures. Distribution, morphology, texture, mineralogy, paragenetic sequence, and C and O stable-isotope compositions of deposits in the UZ are described. The observations and conclusions presented here build upon previous studies by Whelan et al. (1994, 1998, 2001) and Paces et al. (2001). 4.1. Distribution of deposits Calcite and silica deposits in the UZ are found predominantly in open fractures and lithophysal cavities in the densely welded tuffs. Lithophysal cavities are cooling features that typically are lined with high-temperature vapor-phase tridymite, cristobalite, quartz, and feldspar. In shallow-dipping fractures and lithophysae, secondary calcite and silica deposits generally are restricted to the fracture and cavity floors. Rarely, calcite, but not secondary silica, may be found on cavity ceilings as scattered patches or small pendants. In steepdipping fractures the deposits generally are restricted to the footwalls although both walls of near-vertical fractures may contain coatings. Mineralized and unmineralized fractures commonly intersect or are in close proximity and only a small number of all fractures contain mineral coatings. Line 1 Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Geological Survey.

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surveys of fracture density (D.L. Barr, US Bureau of Reclamation, written communication, 1996; A.L. Albin and G.L. Eatman, US Bureau of Reclamation, written communication, 1997) and deposit distribution (J.B. Paces, US Geological Survey, written communication, 1998) in the ESF indicate that 6% or less of fractures longer than 1 m are mineralized. Less comprehensive surveys of lithophysae-hosted deposits indicate that 10% or less of the lithophysae contain secondary calcite or silica, except for parts of the north bend area of the ESF, where up to 42% of the lithophysae are mineralized (Marshall et al., 2000). More than 90% of the fractures and cavities exposed in the ESF and EWCD do not contain secondary minerals. 4.2. Deposit morphology, mineralogy, and texture Depth below the land surface and dip of the depositional setting affect the morphology, mineralogy, and texture of the deposits. Within 10 to 15 m of the surface, fractures commonly are coated with mixtures of micritic calcite and opal with minor clays (largely sepiolite). These coatings are texturally, mineralogically, and geochemically similar to pedogenic calcrete in the overlying soils and are consistent with formation from water infiltrating the soil zone and descending along fracture pathways of the UZ (Marshall and Mahan, 1994; Vaniman et al., 1994; Vaniman and Whelan, 1994; Whelan et al., 1994). At greater depths the grain size of the secondary minerals increases markedly and the deposits consist largely of coarse, sparry, intergrowths or freegrowing crystals of calcite with discrete layers or masses of silica minerals (chalcedony, quartz or opal). Opal appearance changes from tannish white, opaque patches in soil zone calcretes to clear, transparent lenses, laminae, or hemispheres at greater depth. In addition to calcite and the silica minerals, the deposits commonly include minor amounts of fluorite, zeolites, and clays (Carlos, 1985). Near-horizontal depositional settings, such as the floors of lithophysal cavities or footwalls of shallowdipping fractures, can contain coarsely crystalline deposits of calcite and silica minerals up to several cm thick. These deposits may be compact and massive but also commonly contain distinctive tall (1 to 3 cm), thin, freestanding calcite blades (Fig. 2a and b) that result in an irregular, hummocky topography. The tips of the blades are often capped with knobby or corniced calcite overgrowths (Figs. 2b and 3a). Calcite or silica do not completely line cavity interiors or form ‘‘bathtub rings’’ of deposits on cavity walls, so there is no evidence of subaqueous deposition. In contrast, secondary mineral coatings on footwalls of steep-dipping fractures (or on both walls in some near-vertical fractures) are thinner, typically only 1 to 5 mm, but up to 2 to 3 cm in the TCw and in shallow-dipping fractures of the TSw. Mostly,

fracture coatings are patchy and discontinuous and uncommon in fractures with apertures less than  2 mm. Calcite forms masses of tightly intergrown crystals, typically with low relief and commonly optically continuous over large areas ( > several cm2). Where steepdipping fractures become more horizontal or intersect subhorizontal fractures, coating morphologies become texturally and mineralogically similar to those of cavity settings (Paces et al., 2001). All minerals display growth layering on mm to sub-mm scales (Figs. 2b, 3a and 4b, c and d). The silica minerals chalcedony, quartz, and opal are widespread. They are most abundant in deposits of the TCw and the upper parts of the TSw and are rare in steep-dipping fractures of the TSw. Chalcedony, opal-A (amorphous), and opal-CT (short-range crystallographic ordering resulting in a mixed cristobalite-tridymite structure) form botryoidal masses, irregular to evenly laminated coatings, or isolated, blister-like, spheroidal grains (Fig. 3b, c and d). Chalcedony and opal were distinguished in this study by the isotropic or faintly birefringent nature of the opal. Although no attempt has been made to distinguish between opal-A and opal-CT, SEM observations indicate that youngest opal surfaces are typically smooth whereas older opal surfaces appear finely bladed and strongly resemble published images of opal-CT (e.g. Florke et al., 1976). This indicates that opal-A may have recrystallized to opal-CT with age. Conversion of opal-CT to chalcedony may be recorded in samples where growth-banded, brownish, and finely fibrous silica appears to be recrystallizing to colorless and more coarsely fibrous chalcedony. Chalcedony is found in the basal parts of deposits and commonly coarsens into druses or sprays of quartz euhedra (Fig. 2c). Opal is commonly intergrown with calcite blades and later calcite overgrowths. Fluorite, although common, is rarely a major phase. In cavity settings, fluorite is found as euhedra, rounded composite crystal aggregates, and anhedral, growthlayered masses (up to 2 mm). It is usually intergrown with or included in coarser grained calcite or chalcedony-quartz but also can predate the earliest calcite as coatings on cavity or fracture surfaces. Zeolites (heulandite, stellerite, clinoptilolite and mordenite) may precede or accompany the earliest calcite as scattered tiny crystals (3/40.2mm), and rarely, may coat buried calcite growth surfaces (Whelan et al., 1994). Although many coatings are firmly attached to the host rock, some are attached loosely. Some fractures contain calcite-cemented breccias consisting of coating fragments that have fallen from higher in the fracture. Some deposits in lithophysal cavities have mm-scale secondary porosity at the tuff contact that commonly is associated with what appear to be embayed and anhedral calcite at the tuff contact (Fig. 2a and d). These textures appear to record corrosion at some time in the past.

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4.3. Paragenetic sequence Examination of the mineralogy, textures, and depositional sequences from hundreds of individual deposits were used to define a generalized paragenetic sequence of early, intermediate, and late stages. Deposits displaying the entire paragenetic sequence are rare. Earlystage silica minerals commonly are missing or weakly developed in the deeper parts of the TSw, as are latestage minerals in the TCw. Nonetheless, the incomplete paragenetic sequences found in many deposits appear to be consistent with this generalized paragenetic sequence. Furthermore, similar depositional histories and a generalized paragenetic sequence for the fracture- and cavity-hosted deposits are supported by the d13C and d18O values, and by the 230Th/U and 235U/207Pb ages of frac-

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ture- and cavity-hosted calcite and silica, both of which display similar long-term trends, ranges, and distributions throughout the UZ (Whelan et al., 1998; Neymark et al., 2002; Paces et al., 2001). 4.3.1. Early stage Calcite is the earliest mineral in many deposits, forming mosaics of anhedral crystals or druses of stubby blades. Tuff detrita up to several mm are common and patches or planes of inclusions [fluid and (or) solid] commonly mark buried growth faces. Some early-stage calcite may show faint CL growth zoning. Early-stage calcite is overlain in many deposits by chalcedony, locally coarsening into drusy quartz (Fig. 2c). Textural relations between chalcedony and calcite vary widely from simple interlayering to chalcedony containing

Fig. 2. Photographs and photomicrographs of secondary calcite (Cal), opal, chalcedony (Cdny), and quartz (Qtz) textures in hand specimen and in thin sections impregnated with blue-dyed epoxy. Large arrows indicate general direction of growth. (a) Coating from a lithophysal cavity floor showing elongated scepter-head calcite blades and local porous zones in the deposit base. From ESF 30+50 m. Scale bar is 1 cm. (b) Intermediate-stage calcite blades, growing on tuff and typical of deposits on cavity floors, capped with corniced overgrowths of clear, sparry late-stage calcite resulting in scepter-like cross-sections. The high porosity at the basal contact and irregular, non-planar, surfaces of some of the calcite blades are common features of these deposits. Sample is from EWCD 10+31 m. Black square is 2 mm wide. (c) Deposit from vapor-phase parting at ESF Alcove 5, 0+28.5 m showing early-stage calcite separated from intermediate- and late-stage calcite by a thick layer of chalcedony and quartz capped with brown, growth-banded opal. Field of view is about 1 cm wide. (d) Approximately 0.5-cm-thick crust-like deposit from a lithophysal cavity at EWCD 3+91 showing patchy calcite on the vapor-phase surface and a loosely attached crust of coarsely crystalline calcite. The high porosity and embayed surfaces of the calcite crystals may indicate dissolution by later solutions. Black square in inset is 2 mm wide.

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irregular, dispersed but optically continuous, masses of calcite, to chalcedony with a distinct rhombic fabric but no calcite. These textures are consistent with replacement of calcite. However, in other samples, calcitechalcedony intergrowths are consistent with concurrentgrowth mechanisms. Most fluorite is present in the early stage as botryoidal to euhedral inclusions in the calcite or silica minerals. The end of the early stage is defined by chalcedony-quartz deposition (Fig. 2c) or by the change of calcite habit from equant intergrowths to upright blades. 4.3.2. Intermediate stage Intermediate-stage deposits consist mostly of calcite and lesser opal. The calcite grows on and may be optically continuous with the early-stage calcite. In subhorizontal settings, intermediate-stage calcite typically forms thin, prismatic to pyramidal, upright blades that

may be one or more cm in height and width but only 1 to 4 mm thick (Fig. 2a and b). This calcite is generally murky due to dense growth zoning traced by planar arrays of inclusions (solid or fluid) on former growth faces and does not luminesce in CL (Fig. 3a and b). Opal is found in intermediate-stage deposits from shallow-dipping fractures and lithophysae as patchy, thin laminar veneer or arrays of isolated hemispheres on buried growth faces. In some coatings, intermediatestage calcite and opal are intimately intergrown in textures that are consistent with either coprecipitation of calcite and opal, or replacement of calcite by opal (Fig. 4a). Rarely, minor fluorite is intergrown with calcite near the early- to intermediate-stage transition. 4.3.3. Late stage In contrast to the inclusion-rich nature of the intermediate-stage calcite, the youngest parts of deposits

Fig. 3. Images showing textures of secondary calcite (Cal) and opal (Op). Large arrows indicate general growth directions. (a) Clear, sparry, late-stage calcite overgrowths on inclusion-rich intermediate-stage, growth-banded calcite blades from ESF 64+95 m. Scale bar is 1 mm. (b) Late-stage calcite with growth zones marked by dark inclusions and scattered opal hemispheres. From EWCD 14+69 m. Scale bar is 0.1 mm. (c) Two periods of deposition of banded, hemispheroidal, late-stage opal (brown followed by clear) on latestage calcite. The contact between calcite and the brown opal is smooth and the calcite appears euhedral, whereas the contact between calcite and the later, clear opal is irregular and may reflect coincident and (or) competitive growth (see Fig. 4b, c and d). Field of view is about 3 mm wide. (d) SEM image of the outer surface of a cavity deposit showing opal hemispheres on and partially engulfed by calcite. From ESF 29+11.2 m. Scale bar is 0.2 mm.

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consist of clear and sparry calcite, locally as free-growing crystals but most commonly as overgrowths on older calcite (Fig. 3a). In shallow-dipping fractures and lithophysae, late-stage calcite often is intergrown with thin layers or spheroids of opal (Fig. 3b, c and d) or caps intermediate-stage blades forming knobby or corniced overgrowths and crystals with scepter-like crosssections (Fig. 2b). Late-stage calcite crystals are typically smooth or striated and sharp-edged (Fig. 3d), though many buried growth faces are marked by dark inclusions or speckled with opal hemispheres (Fig. 3b). Late-stage calcite commonly displays growth banding at 0.01- to 0.1-mm scales under CL; electron microprobe mapping of Mg in late-stage calcite shows growth zoning at a comparable scale (Wilson et al., 2000a). Late-

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stage opal commonly is intergrown with or partially engulfed by late-stage calcite (Fig. 3d). Calcite-opal contacts may be either sharp (Fig. 3c) or complex (Fig. 4b, c and d). In many deposits, the calcite-opal interfaces are irregular and may reflect competitive growth textures formed by concurrent deposition (Fig. 4c). Elsewhere, the calcite-opal contacts are stepped and clearly correlate with growth layering in both minerals (Fig. 4b), possibly indicating cyclic depositional couplets of opal followed by calcite. Rarely, complex intergrowths of calcite and opal are consistent with opal replacing earlier calcite (Fig. 4d) although the calcite-opal contacts are similar to those in Fig. 4c. Deposition of late-stage opal is most common on the tips of calcite blades.

Fig. 4. Images showing textures of secondary calcite (Cal) and opal (Op). Large arrows indicate general direction of growth. (a) Transmitted light photomicrograph of a large, optically continuous, embayed calcite grain with irregular opal intergrowths. From ESF 28+80. Scale bar is 1 mm. (b) Reflected-light photomicrograph of a late-stage opal hemisphere at the tip of a sparry calcite blade. Both minerals show distinct growth layering and smooth, stepped contact relations (small arrow) that correlate with growth layering. The minerals shown may represent more than one million years of deposition based on estimates of late-stage opal growth rate (Paces et al., 2000). Scale bar is 10 mm. (c) Reflected-light photomicrograph of late-stage, growth-layered hemispherical opal (Op) on the side of an intermediate-stage calcite (Cal) blade. Initial growth contacts are smooth and stepped as in (b) (small white arrows), but are irregular and embayed in younger material (small black arrows). Opal may represent 300 to 800 ka of deposition (Paces et al., 2000). Scale bar is 10 mm. (d) Reflected-light photomicrograph showing complex textural relations between opal (Op) and calcite (Cal) in the outer part of a lithophysal cavity deposit from Alcove 5, 0+28.5 m in the ESF. Most of the calcite is optically continuous despite the complex intergrowth of calcite and opal. Opal displays mm-scale growth-layering parallel to calcite contacts (not visible in this image). Scale bar is 10 mm.

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4.4. Calcite d13C and d18O studies The C and O stable-isotopic compositions of secondary calcite and silica minerals from drill core and tunnel samples have been studied extensively (Whelan et al., 1994, 1998, 2001; Moscati and Whelan, 1996; Whelan and Moscati, 1998; Paces et al., 2001). Calcite d13C ( 9.5 to 10.1%) and d18O (3.2 to 23.0%) values display large ranges and a general negative correlation, with d13C values decreasing and d18O values increasing through time (Fig. 5). Early-stage calcite d13C values generally are between 0 and 10% with d18O values of 3.2

to  15%, whereas late-stage calcite d13C values are-4 to-8.5% with d18O values of 16 to 21%. Intermediatestage calcite d13C values typically are between the ranges of early- and late-stage values, but the range of intermediate-stage d18O values is essentially the same as that of the late stage (Fig. 5). The large range of d13C values, as plotted against location in the ESF in Fig. 6, shows that the entire paragenetic sequence is present in mineral coatings throughout the ESF. Radiometric ages (230Th/ U or 235U/207Pb) of opal and chalcedony coupled with the d13C values of associated calcite broadly reflect the different paragenetic stages (Fig. 7). On the basis of these data, the early stage ended between 6 and 8 Ma and the late stage began about 3 to 4 Ma (Whelan and Moscati, 1998).

5. Discussion

Fig. 5. Graph of d13C vs. d18O of ESF calcite. Early, intermediate, and late paragenetic positions determined by petrographic examination (Whelan et al., 1999). In general, d18O values decrease, and temperature and d13C values increase, with age. Calcite with d18O values <10%, found in some early-stage calcite, is consistent with depositional temperatures between 50 and 85  C (see discussion in Section 5).

The distribution and appearance of the calcite and silica deposits in the UZ provide compelling evidence of how they formed. The sparse and heterogeneous distribution of fracture coatings in 6% or less of available fractures, the patchy and discontinuous distribution of coatings within fractures, the gravitational restriction of mineral deposition to fracture footwalls and cavity floors, and the general lack of calcite and silica deposits on cavity walls and ceilings that would be produced if water filled or ponded in the cavities all are consistent with descending percolation of fracture water in a UZ depositional setting. 5.1. Processes of UZ mineral deposition Unsaturated-zone minerals (Whelan et al., 1994; Paces et al., 2001) and fluids (e.g. Thorstensen et al.,

Fig. 6. The d13C values of secondary calcite from fractures (vertical black lines) and cavities (arrow-tipped horizontal red lines) in the ESF. Data-poor zones in the plot generally represent bedded or non-lithophysal welded tuff lithologies in the ESF where secondary minerals are rare.

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1998; Yang et al., 1998) at Yucca Mountain have isotopic and chemical signatures consistent with meteoric water interacting with the overlying soils during infiltration. This water percolates down through the rock mass, dominantly along fracture pathways in the welded tuffs, but also through the porous matrix of the nonwelded tuffs. Although fracture flow has not been observed in the tunnels, either in fresh excavations or in sealed alcoves, UZ fracture flow simulations predict that such flow would be focused into only a few fractures (Anna, 1998a,b; Pruess, 1999). On the basis of numerical simulations that considered fracture-network geometries and fracture-matrix interactions, Pruess (1999) concluded that ‘‘. . .most water seepage in fractured welded units at Yucca Mountain occurs as episodic and ‘‘fast’’ fracture flow in highly localized preferential pathways which wet only a small fraction of total fracture-matrix interface area.’’ Experiments by Tokunaga and Wan (1997) and Su et al. (1999) supported this conclusion and showed that water probably flows down fractures as meandering, fingering films. These models of UZ fracture flow are consistent with the sparse and heterogeneous distribution of calcite and silica deposits. The observation that the deposits are found in larger aperture fractures and cavities and rare in fractures with apertures smaller than 2 mm indicates that open space is an important part of the depositional mechanism.

Fig. 7. The d13C of ESF calcite plotted against 230Th/U or 235 U/207Pb ages determined from closely associated opal or chalcedony by Neymark et al. (2001). The texturally defined boundary between late- and intermediate-stages correlates with the decrease of d13C values between 3 and 4 Ma. The mineralogically defined boundary between intermediate- and earlystages is not as well constrained but probably occurred between 6 and 8 Ma.

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Inorganic precipitation of calcite and silica is driven by the loss of a gas phase: escape of CO2 or evaporation of H2O for calcite, and evaporation of H2O for silica. Larger open spaces and connected fracture flow paths will foster not only separation of gases from the percolating water but facilitate removal of those gases and deposition of calcite and silica. Confined flow paths, such as small-aperture fractures, will be filled more completely by the percolating water film, retarding the removal of the gas phase and inhibiting mineral deposition. As meteoric water infiltrates through the Ca- and Sirich soils at Yucca Mountain, it will acquire pedogenic isotopic and chemical signatures and likely will become saturated with respect to calcite and silica prior to entering the UZ fracture network. As these solutions percolate downward, calcite solubility can be exceeded either by warming in response to the geothermal gradient (CO2 solubility decreases with increasing temperature) or by evaporative concentration. Silica deposition, however, is largely controlled by evaporative concentration of the solutions, partially countered by the increased solubility of silica due to increasing temperature with depth. Water flow is gravity driven and generally will be downward. Gas movement, however, is temperature driven and generally will be upward. Water flow will not necessarily be reflected by mineral deposition except in sites open to escape of the gas phase. The importance of open space and gas-phase separation to calcite and opal deposition in the UZ (Paces et al., 2001) is evident from the greater abundance of silica minerals in fracture coatings of the TCw compared to the TSw. The fracture network in the TCw is open to the surface, facilitating gas loss and evaporation, whereas the TSw is partially isolated from the surface by the nonwelded tuffs of the PTn (Thorstenson et al., 1998). A greater degree of evaporation in the TCw is indicated by late-stage calcite d18O values that show greater effects of evaporative enrichment in the near surface than at greater depths (Fig. 8). The PTn, where percolation is mostly as matrix flow, will greatly retard gas movement, evaporation and 18O enrichment. Below the PTn, calcite d18O values still decrease with depth, but more slowly, at a rate consistent with warming due to the geothermal gradient (Szabo and Kyser, 1990; Whelan et al., 1994). Although mineral formation is driven by evaporation and gas loss, calcite d18O values in the TSw show no clear effects of 18O-enrichment due to evaporation. Evaporative 18O-enrichment is, however, likely to be minimal if the processes of water migration, gas removal, and mineral deposition are slow. In an analogous unsaturated environment at Carlsbad Caverns, New Mexico, only small d18O increases (42%) accompanied evaporation from cave pools in spite of almost

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tenfold increases in water salinity (Ingraham et al., 1991). Apparently, the removal of the gas phase in the caverns is so slow that the system is almost closed and the pool waters remain in isotopic equilibrium with the overall unsaturated hydrologic system through isotopic exchange mediated by the water vapor in the cave atmosphere. Vapor-phase mediated isotopic exchange between water at mineral-depositing sites and that of the general UZ flow system would allow growth of calcite with only minor corresponding d18O value increases. Additional evidence for gas-phase removal from water films in the TSw is interpreted from the differences in coating morphology between steep- and shallow-dipping depositional sites. Downward flow velocities likely are faster in steep-dipping fractures and slower in shallow-dipping fractures and lithophysae. Slower velocities allow an increase in the ratio of gas-phase removal to percolation flux rates. As a result, shallow-dipping fractures and lithophysae may contain thicker, more mineralogically complex coatings than the thinner, calcitedominated coatings found in steep-dipping fracture settings. 5.2. Depositional model A model describing the depositional process for the calcite and silica deposits in the UZ at Yucca Mountain must include mechanisms for distinctive features in morphology, texture, and distribution in the various depositional settings. Descriptions of bladed calcite crystals similar to those observed in the UZ at Yucca Mountain (Fig. 2a and b) were not found in the mineralogical literature. Although calcite is known for its

Fig. 8. Plot of late-stage calcite d18O values against depth below the surface in the ESF, reflecting separate trends in the TCw (squares) and TSw (circles) welded tuffs (see text for explanation).

variety of crystal habits, calcite from the Yucca Mountain UZ displays a number of distinctive textural features that are not apparent in descriptions of other UZ or SZ environments including karstic and hydrothermal settings. Any depositional model must be consistent with the textures of the cavity-hosted deposits, including: (1) freestanding calcite blades that range from several mm to several cm in height and can vary widely in size over cm-scale areas of a single coating, commonly resulting in a highly irregular to hummocky topography; (2) late-stage calcite overgrowths and opal at the tips of blades accompanied by little or no concurrent deposition at the bases of the blades; and (3) possible dissolution of calcite at the base of deposits in many lithophysal cavities (Fig. 2a and d). A depositional model also must be consistent with the sparse and heterogeneous distribution of the deposits in the fracture network and restriction of the deposits to fracture and cavity floors. Percolation may enter shallow-dipping fractures and lithophysal cavities breached by fractures from the fracture network. Many lithophysal cavities containing mineral deposits, however, do not have an obvious connection to the fracture network. Water percolates through the UZ by both matrix and fracture flow. The small percentage (< 10%) of lithophysae containing secondary minerals, however, makes it unlikely that matrix flow was the pathway for mineral depositing fluids. If it were, then most, if not all, lithophysae would contain secondary calcite and silica. The fracture network, perhaps coupled with bedding-plane discontinuities and vapor-phase partings, is, therefore, the most likely fluid source. As the attitude of depositional surfaces changes from vertical or steep-dipping to shallow-dipping or subhorizontal, the deposits grade from even coatings with subdued relief to thicker crusts with tall, freestanding calcite blades. In the more vertical settings, depositing fluids might have moved across the top surfaces of the coatings. In the more horizontal settings, however, the absence of evidence for ponding indicates that the depositing fluids were confined to the base of the coatings. Vapor-phase alteration formed highly porous and permeable selvages around lithophysae (Fig. 9a and b). Vapor-phase partings subparallel to bedding and along which lithophysae commonly are aligned also have altered selvages. As shown in Fig. 10, these selvages may represent hydrologic discontinuities that can focus percolating water to the floors of cavities where it can be wicked up the faces of growing crystals. Focusing of flow through these porous selvages is supported by the lack of evidence of dripping from cavity ceilings (stalactites or stalagmites), even where fractures containing coatings clearly intersect lithophysae ceilings. The UZ gas phase is near saturation with respect to water vapor. Nonetheless, precipitation of calcite and

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silica requires that the percolating fluids lose CO2 and water vapor to the gas phase. The vapor pressure of water varies with the curvature of the water surface (e.g. Ho, 1997). Water will evaporate from films with sharply convex surfaces such as water droplets or where films bend around crystal edges, even if the ambient atmosphere is water-vapor-saturated, and condenses into water films with sharply concave surfaces, such as capillary menisci. This phenomenon provides an explanation for preferential growth of calcite and opal at the tips of blades. Paces et al. (2001) proposed that calcite blades and opal in the UZ precipitated from water films drawn up crystal faces by surface tension. Where such films bend around crystal edges or blade tips, enhanced evaporation will increase salinity and precipitate calcite and

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opal. Focusing of growth at crystal tops over long periods of time likely accounts for the elongated calcite blades with scepter-head overgrowths and the concentration of opal at blade tips (Paces et al., 2001). Cycling of water within the UZ by condensation of water vapor, generated either by warming of descending water or by evaporation during mineral precipitation, provides a likely source of chemically unsaturated solutions to the fracture flow network. As water vapor migrates upward into cooler rocks, it will condense, mix with, and dilute descending percolating water. Similarly, CO2 migrating upward could dissolve into percolating water that it encounters and undersaturate it with respect to calcite. This undersaturation of percolating water could lead to local dissolution of calcite as indicated by the loosely attached coatings in some fractures

Fig. 9. (a) Photomicrograph of the altered tuff margin of a lithophysal cavity. Blue dyed epoxy shows the increase in porosity caused by high-temperature vapor-phase alteration during tuff cooling. The scale bar is 2 mm. (b) Magnified view (50) of the white rectangle in (a) showing the extremely porous nature of the altered tuff.

Fig. 10. Schematic drawing of a vapor-phase parting and lithophysal cavity showing the high-porosity, vapor-phase-altered selvage, and possible fracture flow paths intersecting both the parting, up dip from the cavity, and the cavity. Flow paths are shown as arrows. Secondary calcite and silica are on the floor of the cavity, with a basal zone of enhanced porosity that may reflect later dissolution.

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and the secondary porosity in the basal zones of some lithophysal cavity deposits (Fig. 2a and d). Although the calcite blades in the UZ cavities are unusual, mineral deposition from water films drawn by surface tension is not. Several types of subaerial speleothems have been attributed to water films that were drawn up their outer surfaces by surface tension and evaporated from crystal tips (Hill and Forti, 1997). These forms include a variety of frostwork called anthodite, consisting of radiating, spiky, quill-like sprays of crystals that likely grew from ‘‘thin films moving by capillary action over the surface of needle stalks. . .’’ (Hill and Forti, 1997); growth along the broken rims of cave bubbles ‘‘by capillary uplift and evaporation’’; cave caps, which are ‘‘small (0.5 to 5 mm), hemispherical coatings capping the tops of small, rounded pebbles’’; and subaerial coralloid speleothems that are directly linked to the presence of ‘‘thin films of water’’. In describing the formation of subaerial coralloids, Dawkins (1874) wrote ‘‘. . .each was formed originally on a slight elevation of the general surface, which would cause a greater evaporation of water than the surrounding portions, and therefore be covered with a greater deposit of calcite.’’ 5.3. Upwelling vs. percolation-driven depositional models In contrast to the model of UZ mineral deposition from descending percolation, the large size of calcite crystals, mineral assemblages that include quartz and fluorite, and elevated fluid inclusion temperatures in calcite have been cited as indications of repeated flooding of the UZ tuffs by upwelling hydrothermal fluids (Dublyansky and Szymanski, 1996; Hill and Dublyansky, 1999; Dublyansky et al., 2001). This conceptual model has been maintained in spite of persuasive physical, chemical, and isotopic evidence indicating a meteoric origin for fluids that deposited calcite and silica in Yucca Mountain soil and UZ settings (e.g. Stuckless et al., 1991, 1998; Vaniman et al., 1994; Whelan et al., 1994, 2001; Paces et al., 1998, 2001) and the absence of a credible mechanism to raise the water table hundreds of meters above its present position (National Research Council, 1992; Rojstaczer, 1999). Previous sections of this paper have concluded that the secondary mineral deposits are texturally and mineralogically consistent with deposition from descending percolation. In this section, elevated depositional temperatures from fluid inclusion and d18O studies are attributed to the prolonged thermal input to the UZ from ongoing regional magmatic activity without necessitating hydrothermal fluid inputs (i.e. the potential repository horizon at Yucca Mountain has always been hydrologically unsaturated). Two-phase fluid inclusions in Yucca Mountain UZ calcite with homogenization temperatures (Th) ranging from 35 to

85  C were reported by Dublyansky et al. (1996, 2001). [Fluid inclusions with these temperature ranges were not reported in earlier studies by Bish and Aronson (1993) or Roedder et al. (1994). Such low-temperature fluid inclusions are, however, sensitive to inadvertent heating during drilling, core storage, and sample preparation. Great care was taken in collection and handling during sampling for the recent UNLV and USGS fluid inclusion studies to prevent any thermal disturbance.] Results from recent UNLV and USGS fluid inclusion studies show that about 50% of those samples contain fluid inclusion assemblages indicating elevated temperatures of 30 to  85  C. Detailed petrographic study of these calcite samples indicates that depositional temperatures > 40  C generally are restricted to the early stage, with local evidence of temperatures of 30 to 40  C in the base of the intermediate stage (Whelan et al., 2000, 2001; Wilson et al., 2000b; Wilson and Kline, 2001). No elevated temperature fluid inclusions have been reported from the late-stage calcite, which extends back to 2 to 4 Ma (Whelan et al., 1999; Wilson and Cline, 2001). Depositional temperatures in calcite decreased during the early and intermediate stages of deposition (Marshall and Whelan, 2001; Whelan et al., 2001). Calcite d18O values are consistent with higher depositional temperatures for early-stage calcite and long-term cooling of the rock mass (Whelan et al., 1998, 1999, 2000, 2001). About 85% of intermediate- and late-stage calcite analyses have d18O values of 15 to 20%, whereas 75% of early-stage calcite analyses have d18O values < 15%. In addition, a small number of analyses of early-stage calcite have d18O values < 10%, and as low as 3.4% (Fig. 5). These lower d18O values of early-stage calcite require either deposition from water with much lower d18O values ( 18 to 24% compared to modern ground water d18O values of 11 to 14% reported by Benson and Klieforth in 1989) or at higher temperatures. Assuming equilibrium conditions, water with a d18O value of 13  1% (similar to modern recharge, Yang et al., 1998), would precipitate calcite with d18O values of 3 to 10% at temperatures of 55 to 100  C (Whelan et al., 1999). Although there are no independent estimates of the d18O of past UZ waters, these temperatures are consistent with early-stage depositional temperatures determined by the fluid inclusion studies. Intermediate- and late-stage calcite d18O values decrease with time (Fig. 5) consistent with long-term cooling of the UZ rock mass from early-stage temperatures 550  C to present-day ESF temperatures of 425  C. Warmer depositional temperatures in the past reflect the prolonged thermal input to the UZ from ongoing regional magmatic activity but do not require upwelling of hydrothermal fluids or flooding of the UZ. Yucca Mountain tuffs were erupted between 15 and 11 Ma (Sawyer et al., 1994) from large caldera complexes only

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10 km to the north. Simulations indicate that these Miocene magma chambers would have disturbed local heat-flow regimes on multi-million-year time scales producing elevated UZ temperatures to 6 Ma or younger (Marshall and Whelan, 2000, 2001; Whelan et al., 2001). In addition to depositional temperatures, the fluid inclusion assemblages in the Yucca Mountain UZ calcite provide indirect evidence that calcite formed under UZ conditions. In other UZ environments, calcite deposited as pore-lining cements in near-surface sediments or subaerial speleothems contains characteristic fluid inclusion assemblages consisting of all-liquid inclusions and vapor-rich inclusions with highly variable vapor:liquid ratios. In these environments, the vaporrich inclusions contain atmospheric gases at near-atmospheric pressure and record trapping of both the liquid and gas phases in a UZ setting during mineral formation (Goldstein, 1986; Goldstein and Reynolds, 1994). Calcite fluid inclusion assemblages from the Yucca Mountain UZ display a similar mix of all-liquid and vapor-rich inclusions with highly variable vapor:liquid ratios (Bish and Aronson, 1993; Roedder et al., 1994; Dublyansky et al., 1996, 2001; Whelan et al., 2001) and require only elevated depositional temperatures during the earlier stages of deposition, as predicted from thermal modeling of the rock mass (Marshall and Whelan, 2000, 2001), to be consistent with formation in a UZ setting (Whelan et al., 2001). The distribution of the deposits provides the most compelling evidence against the upwelling, hydrothermal fluid scenario. The sporadic distribution of deposits in the tuffs, with < 10% of fractures and cavities containing secondary calcite or silica; the discontinuous and patchy distribution of coatings within mineralized fractures; fractures containing coatings that are commonly flanked or intersected by barren fractures, and lithophysae containing secondary minerals commonly surrounded by barren cavities all are inconsistent with saturated-zone depositional conditions. In addition, the irregular thicknesses and complex mineral textures of the deposits contrast sharply with the dense, travertine-like coatings typical of saturated-zone deposition. Saturated-zone conditions produced by upwelling and flooding would deposit secondary minerals in all fractures (footwalls and hanging walls) and completely line the interior walls of all cavities connected to the fracture network. Furthermore, the restriction of mineral coatings almost invariably to fracture footwalls and lithophysae floors underscores the control of gravity on descending water films (Paces et al., 1998, 1999, 2001). The scarcity of mineral coatings on cavity ceilings and walls, the absence of mineral high-water marks in lithophysae containing thick deposits of calcite and silica, and the absence of coatings on fracture hanging walls is compelling evidence that water did not fill or even pond in open spaces.

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6. Summary and conclusions Physical, mineralogical, textural, and isotopic data from secondary calcite and silica minerals in the Yucca Mountain UZ are consistent with formation of these deposits in a UZ environment. The deposits consist predominantly of calcite with subordinate silica minerals (chalcedony, quartz, and opal). Fluorite and zeolites are widespread, but not abundant, and generally are restricted to basal positions within the deposits. Coatings on the footwalls of steep-dipping fractures are thinner (1 to 5 mm) and consist of calcite, locally interlayered with silica minerals, whereas coatings on the floors of shallow-dipping fractures and lithophysae are thicker (1 to 4 cm) and consist of blocky to bladed calcite and silica minerals. The coatings display a generalized mineral paragenesis that is divided into early, intermediate, and late stages with boundaries defined by several widely observed mineralogical or textural features. The different stages also are accompanied by significant shifts in d13C values. Combined d13C data from calcite and 235U/207Pb ages from associated opal and chalcedony (Neymark et al., 2002) indicate that the transition between the early and intermediate stages may have occurred 6 to 8 Ma whereas that between the intermediate and late stages occurred 3 to 4 Ma. All minerals in the coatings display mm-scale growth banding. Several lines of evidence indicate that the secondary mineral deposits formed in an unsaturated environment. The deposits are restricted to fracture footwalls and cavity floors indicating descending, gravity-controlled flow of water. They are heterogeneously and sparsely distributed in the UZ (< 10% of possible sites contain secondary minerals) and the absence of evidence for ponding in lithophysal cavities precludes flooding of the depositional sites. Furthermore, the deposits have highly irregular thicknesses and complex mineral textures varying on cm scales rather than forming uniform, dense travertine-like coatings typical of saturated zone deposits. The deposits appear to have formed from downward percolating meteoric water flowing along a limited number of connected fracture pathways. The distribution and textures of the deposits are consistent with formation from thin water films that are unevenly distributed across a fracture surface or drawn up the faces of growing crystals by surface tension. Calcite and silica precipitation require removal of CO2 and (or) water vapor from the depositional site. Water vapor and CO2 loss and, consequently, mineral precipitation, will be greatest where water films curve tightly around crystal edges. This mechanism could account for the large, thin calcite blades and the formation of late-stage calcite overgrowths and opal at the blade tips. Condensation of water vapor, either as reflux from deeper, warmer levels of the UZ or generated during mineral precipitation, provides a possible source of calcite-undersaturated

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water in the UZ. Although depositional mechanisms involving UZ film flow and surface tension are not new, they have not been previously ascribed to UZ secondary minerals in a thick sequence of volcanic rocks. Fluid inclusion homogenization temperatures between 35 and 85  C indicate that some of the calcite formed at temperatures warmer than the modern UZ, stimulating a debate over the possibility of episodic upwelling of warm ground water into the UZ. Petrographic studies of the coatings show that fluid inclusions with elevated homogenization temperatures are common only in early-stage calcite. They are rare in the intermediate-stage calcite and absent in the late-stage calcite. Fluid inclusion homogenization temperatures tend to decrease with decreasing age. Calcite d18O values also indicate higher depositional temperatures for the early-stage calcite and protracted cooling of the rock mass through the early and intermediate stages. Finally, the same calcite that yields elevated homogenization temperatures also contains inclusions with large and variable vapor:liquid ratios that are consistent with heterogeneous trapping of water and air in a UZ environment. Elevated temperatures during the early and intermediate depositional stages were the result of thermal input from large late-Miocene plutons emplaced in the shallow crust just north of the site. Neither the fluid inclusion nor d18O temperature estimates indicate repeated or recent thermal inputs to the Yucca Mountain subsurface. Secondary calcite and silica deposits in the Yucca Mountain UZ can be reconciled only with deposition in a UZ setting and not with an upwelling and flooding scenario because of the distribution of deposits both in the welded tuffs in general and on fracture footwalls and cavity floors in particular. These distributions, plus the mineral textures, isotopic data, and fluid inclusion evidence presented here, are consistent with a meteoric water source that percolated downward through a limited number of UZ fractures. In addition to indicating that the secondary minerals at Yucca Mountain were deposited in a UZ setting, the data also record a gradually declining geothermal gradient over the last 10 million years.

Acknowledgements The US Geological Survey conducted this study from 1990 to 2000, in cooperation with the US Department of Energy, under Interagency Agreement DE-AI08– 97NV12033.

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