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Natural Analog Studies at the Nopal I Uranium Deposit, PeAa Blanca District, Chihuahua, Mexico

James D. Prikryl, David A. Pickett, and English C. Pearcy

Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, Texas 78228-05 10 USA

Introduction

Confidence in the behavior of radioactive wastes stored in geologic repositories is enhanced by their comparison with natural geological systems having analogous physiochemical characteristics. Referred to as “natural analogs”, these systems provide a basis for identifying and understanding geologic, hydrologic, and geochemical processes that could affect repository performance over the long time scales relevant to nuclear waste isolation (e.g., 2 10 ka). Because analogs can be used to develop and test models required for estimating long-term repository performance, most countries investigating disposal of radioactive waste in geologic repositories support the study of natural analogs as part of their national programs.

The Nopal I uranium deposit, in the PeBa Blanca District, Chihuahua, Mexico is analogous in many important aspects to the proposed U.S. high-level nuclear waste geologic repository at Yucca Mountain, Nevada (Ildefonse et al., 1990a; Murphy et al., 1991; Murphy and Pearcy, 1992; Pearcy and Murphy, 1992). Analogous features include the desert climate; the Basin and Range tectonic setting; host rocks composed of fractured, silicic, volcanic tuffs underlain by sedimentary carbonates; location above the water table; the oxidizing, bicarbonate-rich geochemical conditions; and the occurrence of uraninite (UOZ+~) as an analog to spent nuclear fuel. Additionally, alteration of uraninite at Nopal I has resulted in formation of a suite of secondary uranium minerals dominated by components common in host rocks of both the Nopal I and Yucca Mountain system (e.g.. Si, Ca, K, Na, and HzO; Pearcy et al., 1994). As a result, the Nopal I deposit presents a unique opportunity to study the products of uraninite alteration over long time periods and under conditions similar to those expected for a Yucca Mountain re- posi tory.

In this chapter, results of natural analog studies at the Nopal I deposit and their use in support of performance assessment modeling for the proposed Yucca

Mountain repository are summarized. Past and present processes at Nopal I that are analogous to those that could affect spent fuel alteration and radionuclide transport at Yucca Mountain include oxidation of uraninite, the resulting transport of U from the sites of original mineralization, and formation of secondary minerals. Consequently, studies at Nopal I have focused on the mechanisms and timing of uraninite alteration and secondary uranium mineral formation, controls on groundwater and secondary mineral composition, and processes affecting elemental migration. The value of these studies is demonstrated in recent performance assessments for the proposed repository at Yucca Mountain (DOE, 1998; TRW, 1998; CNWRA, 1998; NRC, 1999a. 1999b) which all draw on information from Nopal I.

Geologic and Mineralogic Setting

The Nopal I uranium (U) deposit, in the Peiia Blanca mining district, is located along the east-central border of the Sierra Peiia Blanca about 50 km north of Chi- huahua City (Fig. 1). The deposit is part of a west dipping horst block composed of Tertiary volcanic tuffs underlain by carbonate sedimentary rocks. Along with the Nopal I deposit, the Peiia Blanca district contains numerous U deposits and occurrences hosted in Tertiary volcanic tuffs. Uranium deposits in the Peiia Blanca district are associated with hydrothermal alteration at faults and fractures and within breccias (Goodell. 1981; Cardenas-Flores, 1985; George-Aniel et al., 1985; 1991; Leroy et al., 1987; Ildefonse et al.. 1990b; Muller et al., 1990). At present, most of the U mineralization in the district is relatively oxidized; minerals, predominantly uranyl silicates, are most common.

The U deposit at Nopal I consists of a near vertical breccia pipe which extends over a vertical interval of at least 100 m (Fig. 2). The deposit is hosted by heavily fractured tuffs of the Nopal and Coloradas Formations, which are separated by an argillaceous zone interpreted to be an altered vitrophyre. The Nopal Formation (the youngest of the host rocks) has been dated at 43.8 Ma (Alba and Chavez, 1974). Both the Nopal and Coloradas Formations are densely welded, hematitic, rhyolitic tuffs. Comparison of the chemical composition (major oxide abundance) between the Nopal and Coloradas Formations with that of the Topapah Spring Member of the Paintbrush Tuff Formation (the proposed repository horizon at Yucca Mountain) illustrates chemical similarities between the units (Table 1). At depth the Nopal I orebody extends into the Pozos Formation which is largely a limestone conglomerate that formed on the surface of the Cretaceous limestone.

The Nopal I deposit is estimated to have formed about 8 f 5 Ma (Pearcy et al., 1994). After formation and possible hydrothermal alteration, the deposit was lifted above the water table by Basin and Range deformation and exposed at the surface along the eastern face of a horst (Goodell, 1981). In this position, the deposit has been subject to weathering processes common to the desert areas of northern Mexico [i.e., annual rainfall of about 25 cm (US. Department of Com- merce, 1965). occurring in episodic downpours].

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Geologic and Mineralogic Mapping

Mining during the late 1970s and early 1980s exposed the upper portion of the Nopal I deposit on two broad horizontal surfaces with about 10 m vertical separa- tion (Levels +00 and +lo; Fig. 3). Clearing and geologic mapping of the Level +OO and +10 surfaces show that the area of U mineralization is easily visible on the outcrop, roughly elliptical in form, and has maximum horizontal dimensions of about 18 m by 30 m (Fig. 3). Rock within the deposit area and in the surrounding tuff is highly fractured. Most of the fractures are short (c 1 m in length) and tend to occur as groups of sub-parallel breaks (Pearcy et al., 1995). More continuous fractures (i.e., those extending for tens of m) are less common at Nopal I. In general, fractures within or close to the area of visible U mineralization tend to be less continuous.

The limits of U mineralization on Levels +00 and +10 have been further de- fined by a field gamma survey (Fig. 4). Gamma intensity measurements show a roughly annular pattern corresponding to variations in U and non-U mineral occurrence (Pearcy et al., 1995). A contact gamma reading of 1.0 mR/hr is the apparent limiting value for the presence of visible U minerals on the deposit surface. The Level +10 exposure contains an interior zone with low-gamma intensities (C 1.0 mR/hr) which corresponds to an area of strongly hematized tuff with no observable U minerals. An outer ring of relatively high gamma intensities (up to 28 mR/hr) containing abundant, but irregularly distributed, U minerals surrounds this interior zone. Uranium minerals visible on the outcrop generally occur in fractures and consist almost exclusively of uranyl silicates.

The host rock exposed in the cleared areas, Nopal Formation tuff, has been altered in the vicinity of the deposit. Secondary minerals observed in the host rock are clays (variably kaolinite [A12Si205(OH)4] and smectite [(Na,K,Mg, C~)O.~~A~~S~~O~O(OH)~.~H~O], calcite [CaC03], and iron oxides (variably hematite [Fez031 and goethite [a-FeO-OH]). On the Level +10 surface, kaolinite is the predominant clay and tends to increase close to the deposit. On the Level +00 surface, Nopal tuff along the SE boundary of the deposit, corresponding to the downslope direction of the premining surface, is heavily weathered to smectite. The vitrophyre separating the Nopal and Coloradas Formations has also been exposed on Level +OO and is similarly heavily weathered to smectite.

Mineralogic Studies

Petrography and scanning electron microscope analyses indicate formation of the Nopal I deposit by emplacement of U minerals in fractures, cavities, and inter- granular spaces within the brecciated host tuff. The sequence of U mineral formation is relatively simple, consisting of primary uraninite followed by uranyl oxide hydrates followed by uranyl silicates. In addition to secondary U mineral formation, the Nopal I deposit has undergone substantial alteration associated with

U movement within and around the deposit and formation of secondary non-U and U-bearing minerals.

Primary Mineralization

Uraninite, with formulae between U02 and uo2.33, was the first U mineral to be deposited at Nopal I (Pearcy et al., 1994). Uraninite is preserved as micron- to millimeter-sized masses within small volumes (e.g., ~ 0 . 2 5 m2) of strongly silicified, brecciated tuff. The breccia is composed of silicified, angular, tuff fragments cemented by a uraninite-bearing fine crystalline matrix (Fig. 5a). Minerals associated with primary uraninite in the fine matrix are pyrite, kaolinite, and quartz (Fig. 5b). The U deposit is interpreted to have formed from hydrothermal solutions which leached U from volcanic glass in the surrounding tuffs and precipitated a uraninite-pyrite-kaolinite-quartz assemblage in voids and fractures as they moved through a sub-vertical, brecciated zone. The reduced permeability of the silicified breccia likely contributed to the preservation of uraninite at Nopal I by restricting access of altering fluids to pre-existing mineralized zones.

Three texturally distinct forms of uraninite (referred to as “granular”, “euhedral”, and “colloform”) occur at Nopal I (Pearcy et al., 1994). Relations between uraninite textures and chemistry suggest that granular and euhedral uraninite formed early and then were partially dissolved and reprecipitated as colloform uraninite. Texturally, granular and euhedral uraninite are typically intergrown with kaolinite, pyrite, and quartz (Fig. 6a). whereas colloform uraninite is free of discernible primary mineral intergrowths and is generally observed growing from granular uraninite or quartz substrates (Fig. 6a and 6b). Chemically, colloform uraninite is enriched in sulfur and depleted in cations (e.g., Ca, Na, and Pb) when compared to granular and euhedral uraninite.

The textural and chemical evidence is consistent with an alteration scenario in which an acid sulfate solution generated by pyrite oxidation interacted with early- formed euhedral and granular uraninite. Portions of the euhedral and granular uraninite were dissolved, and U may have been transported a short distance (mil- limeters to centimeters) and reprecipitated as colloform uraninite. This process leached Ca, Na, Pb, and other cations from the early-formed uraninite, and precipitated colloform uraninite enriched in sulfur, depleted in cations, and free of kaolinite and pyrite. Concentrations of U and Pb in the late-forming, cation-poor, colloform uraninites yield a chemical U-Pb age of 8 f 5 Ma for the primary min- eralizing event at Nopal I (Pearcy et al., 1994).

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Secondary Mineralization

U Minerals

The initial products of uraninite oxidation at Nopal I are uranyl oxide hydrates that directly replace uraninite and fill voids lined by colloform uraninite (Fig. 6a, 6b, and 7a). The most common uranyl oxide hydrates at Nopal I are the mixed- valence phase ianthinite [ v 4 ' ( ~ 0 z ) 5 ( 0 H ) 1 ~ ~ 3 H ~ O ) r schoepite [UO~HZO], and dehydrated schoepite [UOynHzO; (n<2)]. Becquerelite [Ca(uoz)6o4(oH)6.8Hzo] has been identified by XRD analysis but has not been observed optically at Nopal I. In samples containing multiple uranyl oxide phases, ianthinite is the earliest formed and is followed by schoepite and/or dehydrated schoepite.

Uranyl oxide hydrates are followed paragenetically by uranyl silicates, which are the predominant U phases at Nopal I. Uranyl silicates occur as replacements of earlier formed U minerals and as euhedral crystals within voids and fractures (Fig. 6b and 7a). Uranophane [Ca(U0z)zS i~0~~6H~0] is the most abundant uranyl silicate at Nopal I and occurs throughout the deposit. Its polymorph beta- uranophane is also present in the interior of the deposit. Soddyite [ (U02)2Si04*2HzO], weeksite [K2( U0~)zSi60~~.4H~O], and boltwoodite [KH(UO2)SiO4.1.5Hz0] are also common at Nopal I but have much more limited occurrences than uranophane. Haiweeite [Ca(UOZ)2Si6O1~.5H20] has been detected by XRD in fracture fill in the altered vitrophyre a few meters outside the deposit.

Uranyl silicates are generally complexly intergrown in oxidized samples from Nopal I (Fig. 7b). Weeksite and boltwoodite tend to occur somewhat further from remnant uraninite than uranophane and soddyite, suggesting their formation later in the sequence. However, on a deposit-wide scale, a clear progression from one uranyl silicate to another is not observed. The evidence suggests that the specific uranyl silicate formed in a given area depended on local geochemical conditions (e.g., cation activity) rather than a broad evolution of the oxidizing system. Such local variability is characteristic of natural systems.

With respect to the timing of secondary U mineralization, U-Pb isotopic data on three samples of late-forming uranophane from Nopal I indicate a U phase oxidation event at around 3 Ma (Pickett and Murphy, 1997). Given the uncertainty of the chemical U-Pb age of the precursor uraninite (8 * 5 Ma), it is possible that oxidation followed relatively soon after primary U mineralization.

The composition of the secondary U phases at Nopal I indicate that the fluids, which altered the uraninite, were oxidizing and contained substantial silica, Ca, and K. This composition reflects interaction of the altering fluids with the host tuffs, which have abundant silica and significant amounts of CaO and K2O (see Table 1). Calcium may also have been supplied by limestones, which underlie the volcanic sequence, or by interaction of meteoric fluids with caliche near the surface (Murphy et al., 1991) and/or with abundant calcite veinlets in the deposit area.

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Non-U and U-bearing Minerals

As exposed on the Level +10 of Nopal I, an interior portion of the deposit consists of strongly hematized tuff with no observable U minerals (Fig. 4). Petrography and XRD analyses of this interior zone indicates the occurrence of abundant alunite [KAl3(SO4)2(0H)6] and jarosite [KFe3(S04)2(OH)6] (Leslie et al., 1993). Alteration of pyrite associated with the primary ore assemblage by interaction with oxidizing fluids is a likely mechanism leading to formation of hematite, alunite and jarosite (Bladh, 1982; Nordstrom, 1982). Conditions which favor Fe- oxide and sulfate formation (oxidizing, low pH) are also conditions which tend to mobilize U. Thus, the absence of U minerals in the central part of the deposit likely resulted from mobilization of U associated with pyrite oxidation.

Jarosite has also been observed outside the deposit as a late precipitate in a major fracture which crosscuts the deposit (Pearcy et al.. 1995; Prikryl et al., 1997). The fracture also contains hematite, goethite, and amorphous Fe-oxides with anomalous U concentrations (up to thousands of ppm) but no observable U minerals (Fig. 8a). Petrographic observations indicate that the original assemblage in the fracture was a pyrite-kaolinite association (Fig. 8b). Like the strongly hematized interior zone on Level +lo, the present mineral assemblage in the fracture is most likely the result of chemical weathering of pyrite. The high U contents of the Fe-oxide phases and apparent absence of U minerals within the fracture indicate that U was sequestered during secondary mineral growth or sorbed on mineral surfaces.

Other U-bearing secondary materials found at Nopal I are caliche and opal. Uranium-rich caliche occurs within meters of the deposit in fractures and on surface exposures of tuff which were undisturbed by mining activities. In strongly oxidized fractures within the deposit, U-rich opal coats botryoidal hematite and uranophane (Leslie et al., 1993). Uranium-series data from these materials provide information on the rates and the episodic nature of radionuclide migration (Pickett et al., 2001). Opals have U as high as 10,OOO ppm, while carbonate-rich caliches contain up to 350 ppm U. Most opals appear to be older than 600 ka. Ca- liches, however, have 2?h ages ranging from 18 to 136 ka, clustering around 50 ka. In addition, one opal yielded a 54 ka age (Leslie and Pearcy, 1993). supporting episodic uranium deposition at this time. Deposition rates calculated from these data can be used to evaluate rates used in performance assessments for geologic disposal of nuclear waste (Pickett et al., 2001).

U Transport Studies

Since oxidation of uraninite and formation of secondary U minerals, U evolution at Nopal I has been characterized by mobilization out of the area of visible U mineralization. Examination of the distribution of U in the host tuff at scales ranging from microns to tens of meters indicates that U was transported outside the deposit mainly along fractures (Pearcy and Leslie, 1993; Pearcy et al., 1995; Prikryl et al., 1997; Pickett and Murphy, 1997). Although vertical transport may have been

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dominant, data have been collected primarily in the horizontal direction on the cleared surfaces of the deposit.

Deposit-wide Transport Features

Contact gamma measurements on the cleared surfaces of Levels +00 and +10 indicate the presence of anomalous U (i.e., gamma intensities above local background values of about 0.05 mR/hr) outside the area of visible U mineralization (Fig. 4). Using the continuous 0.15 mR/hr gamma intensity boundary on Level +10 as a basis for comparing U transport distances, there is evidence of relatively little transport to the W (2 to 3 m maximum) and only limited transport to the N (about 20 m). On Level +OO, gamma intensities greater than 0.45 mR/hr extend well beyond the area of visible U mineralization. This extension is greatest to the SE, which corresponds to the downslope direction of the pre-mining surface. Tuff in this area is heavily weathered to clay (mainly smectite). The above observations indicate U mobilization by infiltrating meteoric fluids produced greatest transport in the downslope direction (SE) and the least transport distance in a direction opposite to the slope of the tuffs (i.e., into the hill, to the W).

To the north of the deposit, the distribution of anomalous U concentration corresponds strongly to the locations of major fractures (i.e., fractures with traces extending more than 10 m) (Pearcy et al., 1995). This correspondence suggests that individual major fractures were more important to the long distance transport of U away from the deposit than was the general fracture network composed of thousands of less continuous fractures.

Local Transport Features

Generally Fractured Tuff

Uranium concentrations in generally fractured tuff along a 3-m traverse across the western boundary of the deposit (Fig. 9, traverse A) decrease systematically away from the location of maximum U concentration within the deposit (Fig. loa). The tuff along this traverse is randomly broken on a scale of centimeters. Uranyl silicate minerals (mainly weeksite) occur along fracture surfaces of tuff collected within the deposit. Background U concentrations in the host tuff (about 30 ppm) are reached within 2 m of the deposit (Fig. loa). This observation is consistent with gamma intensity measurements which show little U transport to the W of the deposit.

The decreasing U concentration gradient along the traverse likely resulted from movement of U awa from the deposit. Uranium transport out of the deposit is supported by U1 U ratios along the same traverse which show approximate secular equilibrium (Le., ratios close to unity) within the deposit and an excess of 234U outside the deposit (Pearcy et ai., 1995). Waters tend to be enriched in 234U

234 23J .

because of nuclear surface effects such as alpha recoil. The 234U enrichment outside the deposit suggests that water with an elevated 234U/238U ratio migrated out of the area of visible U mineralization into the surrounding tuff where the 234- enriched U was deposited.

Meso fracture

An East-West (EW) trending mesofracture (i.e., aperture > 1 mm and trace length > 10 m) located between 13 m and 15 m N (Fig. 9, traverse B) is observed to be the longest (about 30 m) and most continuous to intersect the Nopal I deposit. Anomalous U concentrations in bulk infilling material within the fracture have been measured for a distance of 23.2 m outside the deposit (Fig. lob). This distance is comparable to the maximum extent of anomalous U concentrations to the north of the deposit, as determined by contact gamma measurements, which are also interpreted to be a result of U transport along major fracture paths (Pearcy et al., 1995)

Uranium concentrations in bulk fracture-infilling materials from the mesofracture generally decrease with distance from the deposit (Fig. lob). The predominant components of the bulk fracture-infilling are hematite, goethite, and amorphous Fe-oxyhydroxides (Fig. 8a); U minerals have not been detected in the fracture. The U contents of hematite and goethite within the fracture also generally decrease with distance from the deposit and show a similar trend to the U content of the bulk fracture-infilling (Prikryl et al., 1997). These decreasing U concentrations gradients in the absence of U minerals indicate decreasing chemical potential of U with distance outside the deposit, and again suggests transport of U away from the deposit.

Microfractured Tuff

The U content of microfractured (Le.. fractures with apertures c 1 mm and lengths on the order of 1 mm) tuff measured perpendicular to the EW trending mesofracture show systematic variations in U concentration with distance from the fracture and with distance from the edge of the deposit (Fig. 11). Uranium contents in microfractured tuff drop off steeply with distance from the mesofracture indicating that U was transported from the mesofracture into the enclosing microfracture network.

At distances greater than 10 m from the deposit margin, background U contents in microfractured tuff are approached at about 2 cm from the mesofracture (Fig. 11). Within 10 m of the deposit, the distance that anomalous U content extends into microfractured tuff increases (e.g., 10 cm at 6.0 m from the deposit and 20 cm at 0.5 m). The systematic variation in transport distance out into the microfrac- t u rd tuff is consistent with the general decrease in U concentration of the fracture-infilling materials with distance from the edge of the deposit (see Fig. lob).

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Tuff Matrix

Uranium concentration profiles in tuff matrix adjacent to microfractures (Fig. 9, samples 48 and 56) with relatively high U concentrations (e.g., 5.6 x lo5 ppm), largely as uranophane, indicate that a portion of the U within the microfractures was transported out into the tuff matrix (Fig. 12). The extent of excess U in the tuff matrix are comparable (maximum distances of 0.23 to 0.28 mm measured perpendicular to the microfractures). In contrast, measurements of U concentrations in tuff matrix made perpendicular to a portion of the EW mesofracture show essentially no matrix transport of U (Pearcy et al., 1995). Minerals (largely Fe- oxides) filling the mesofracture adjacent to the tuff matrix have an average U concentration of 3.4 x lo3 ppm. The greater transport distance in matrix adjacent to microfractures is likely a function of a greater chemical potential contrast adjacent to the microfractures in which uranophane occurs compared to the mesofracture in which U was sorbed onto, or co-precipitated with, Fe-oxides.

Retardation and Retention of U in Fractures and Matrix

Inventories of U retarded and retained (e.g.. by sorption, precipitation, coprecipitation) along different types of transport paths at Nopal I vary greatly (Pearcy et al., 1995). Uranium distribution patterns along and around the EW trending mesofracture indicate U transport along a two-stage path: (i) transport along the fracture, then (ii) transport out of the fracture into a complex network of microfractures. An estimate of the amount of U retained within the mesofracture (91 ppm U m') is 5 times less than the amount of U retained within the adjacent microfracture network (420 ppm U m') (Pearcy et al., 1995). Essentially no U is sequestered in the unfractured tuff matrix adjacent to the mesofracture. The amount of U transported out of individual microfractures with high U concentrations, largely as uranophane, into the adjacent tuff matrix is only 5% of the U retarded within the microfractures (Pearcy et al., 1995). These estimates indicate that retention of U in the microfracture network and in individual microfractures was more effective in retarding U transport at Nopal I than U retention in mesofractures and tuff matrix.

Isotopic Studies

Uranium-series isotopic data on bulk rocks and fracture-infilling materials from traverses within and outside the Nopal I uranium deposit (Fig. 9) indicate a recent, complex history of U mobilization (Prikryl et al., 1997; Pickett and Murphy, 1997, 2002). Most fracture-infillings and bulk rocks outside the deposit exhibit ' V h - 2MU-238U disequilibrium relationships (i.e., non-unity u4U/23sU and T h l u 4 U activity ratios) (Fig. 13). Inside the deposit, bulk rock samples have equilibrium activity ratios and fracture-infillings show disequilibrium. U-series disequilibrium requires open system U transport during the past several hundred thousand years.

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Disequilibrium relationships between the z3'?h/"8U and 234U/238U ratios require multiple stages of U mobilization. In contrast, more recent U-series data from fracture materials are consistent with a period of enhanced U release at around 400 ka ago, with chiefly closed-system behavior subsequently and only very minor recent mobilization (CRWMS M&0, 2000). However, there is evidence for substantial recent radium mobility from those data, and measurements of (i) radioac- tivity in vegetation growing on Nopal I ore piles and (ii) radium in ore samples illustrate the potential for radium release from U minerals in an unsaturated environment (Leslie et al.. 1999; Leslie and Pickett, 2000).

While the differing conclusions on U mobility from studies of fracture materials have not been reconciled, there is other evidence supporting recent U mobility. As discussed above in the section titled "Non-U and U-bearing Minerals," U-rich opals and caliches record substantial U release within the last 200 ka, with a pro- nounced episode 54 ka ago. This event was marked by water movement and U mobilization in a weathering environment. Recent partial U removal from host altered tuffs is consistent with interpretations of radiation-induced damage in kaolinite (Allard and Muller, 1998).

The U series data imply a complex, episodic history of U mobility, with periods of both U deposition and U removal over the last few thousand years. Uranium transport in infiltrating, oxidizing, meteoric water is the preferred dominant process, owing chiefly to the association between U mobilization and Fe oxide precipitation resulting from oxidation of pyrite (Pearcy et al., 1994; Prikryl et al., 1997). The multistage nature of transport implies shifts in geochemical andor hydrologic conditions since the initial mobilization of U from the deposit. These shifts could be related to geologic (e.g., uplift) andor climatologic changes.

Chemistry of Waters

Information on current U mobilization at Nopal I is provided by the chemistry of unsaturated zone waters (Table 2; Pickett and Murphy, 1999). Perched and seep waters were collected outside the deposit in a 10.7 m deep borehole on Level +10 (BH12) and inside an adit on Level +OO, respectively (Fig. 9). Based on the sample collection locations the waters did not interact with U minerals. The unsaturated zone waters are dominated by calcium bicarbonate. The substantial Ca concentrations in the waters are likely the product of interactions of rainwater with near surface caliche and calcite veinlets in the deposit area.

Geochemical modeling indicates that aqueous U is in the form of uranyl carbonate complexes and that U contents of the waters are below uranyl mineral solubility limits (Pickett and Murphy, 1999). Elevated Si contents in most of the waters result in low calculated solubility limits on dissolved U. Based on limited thermodynamic data, haiweeite and soddyite are the least undersaturated uranyl phases. Therefore, secondary uranyl phases would be predicted to suppress dissolved U concentrations in the Nopal I system.

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The unsaturated zone waters are characterized by 234U enrichment: u 4 U / ? J = 2.2 in perched waters, and 2.8 and 5.1 in seep waters (Table 2). 234U enrichment in the unsaturated waters is consistent with the 234U enrichment observed outside the Nopal I orebody in bulk rock tuff and fracture-filling secondary minerals. A linear correlation between 23%/238U and the reciprocal of U concentration in dis- played by the waters. The higher 234U/238U and lower U content of the seep waters likely reflects the greater influence of alpha recoilldamage-site effects and the lesser influence of U leaching. In perched waters, U leaching becomes more im- gortant due to longer interaction with the host rock and leads to the lower

4U/23sU ratios and higher U contents. The greater influence of recoil-related effects during limited water-rock interaction may also be reflected in the high 2yh/n%I’h ratios in the seep waters.

In a more recent study (CRWMS M&O, 2002), additional water U-series isotopic data support the notion that U may be significantly mobilized in the Nopal I unsaturated zone. Quantitative U-Th isotopic modeling interpreted the data in terms of recoil rate, U dissolution rate, and fluid transit time. New water samples were collected in the dry season (February and March) in contrast to the wet season (September) represented in the Pickett and M hy (1999) data. Seep water results showed higher U concentrations and 10we?U/~~~U in the dry season. This observation was preliminarily interpreted as arising from evaporation. longer fluid residence time, geochemical conditions more favorable for U dissolution, or a combination of these effects. Enhanced dry-season U dissolution may also be reflected in higher U concentrations in nearby saturated zone waters (CRWMS M&O, 2002).

Nopal I Data in Yucca Mountain Performance Assessment

A series of performance assessments for the proposed Yucca Mountain geologic repository have been conducted by the Department of Energy (DOE) and Nuclear Regulatory Commission (NRC). The more recent of these performance assessments (DOE, 1998; TRW, 1998; CNWRA, 1998; NRC, 1999a, 1999b) all recognize and refer to data from natural analog studies at Nopal I. To date, the use and application of analog data from Nopal I in Yucca Mountain performance assessments has been primarily qualitative, providing support for conceptual models (Murphy, 2000; Murphy et al., 2002).

Observations of the mineral products formed by alteration of uraninite at Nopal I have provided the most important information applicable to Yucca Mountain performance assessments. The mineral products and paragenesis at Nopal I are remarkably similar to those observed in long-term laboratory tests of the alteration of uranium dioxide designed to simulate conditions representative of the proposed Yucca Mountain repository (Wronkiewicz and Buck, 1999). Uranium dioxide in the lab tests and the uraninite at Nopal I were progressively altered, first to uranyl

oxide hydrates, then to soddyite and alkali and alkaline earth uranyl silicates, as a result of exposure to oxidizing fluids (Fig. 14).

Minor differences in the alteration phase assemblages appear to have resulted from the availability of cations in the altering fluids. The predominance of uranophane at Nopal I likely reflects the presence of altering fluids enriched in Ca. This possibility is supported by the chemistry of recently collected unsaturated zone waters at Nopal I (Table 2; Pickett and Murphy, 1999). Conversely, the predominance of Na-boltwoodite in the lab tests likely reflects the Na-enriched nature of the leachant used in these tests (Wronkiewicz and Buck, 1999). Regardless of these differences, the general correspondence between the Nopal I mineral paragenesis (on a time scale relevant to nuclear waste isolation) and the laboratory tests (on a time scale of years) provides confidence in a general model of uranium dioxide waste form alteration at a Yucca Mountain repository. Based in part on the paragenetic development of secondary U minerals at Nopal I and in the uranium dioxide lab tests, the uranyl minerals schoepite, soddyite, uranophane, and Na-boltwoodite were included in performance assessment reactive transport simulations conducted by the DOE (DOE, 1998; TRW, 1998).

In performance assessments for the proposed Yucca Mountain repository conducted by the NRC, alternate source term models were implemented based on mineralogic and chemical data from Nopal I (CNWRA, 1998; NRC, 1999a; 1999b). One model was based on the assumption that radionuclides from the spent fuel matrix become incorporated in schoepite (Murphy and Codell, 1999). Radionuclide releases were then modeled to occur in proportion to the solubility limited dissolution of schoepite. For another source term model, Nopal I site information on the age of U minerals and water flow conditions were used to estimate the release rate of matrix components from spent nuclear fuel (Murphy and Codell, 1999). Results from the alternate source term models yielded lower doses than the base case model in the NRC performance assessment analyses, which provides a level of confidence that the base case model results are conservative.

Although direct use of radionuclide transport data from Nopal I is at present absent in Yucca Mountain performance assessments, development of conceptual models could benefit from U transport studies at Nopal I. Radionuclide transport at Nopal I has been recognized as analogous to potential radionuclide transport in the Yucca Mountain environment (Leslie et al., 1993; Pearcy et al., 1995; Prikryl et al., 1997). These studies point to the importance of flow and transport in fractures and the limited significance of matrix diffusion. In addition, the importance of sorptiodcoprecipitation on transition metal oxides as a retardation mechanism in the Yucca Mountain system is supported by the association of U with fracture- filling Fe-oxides and oxyhydroxides at Nopal I (Pearcy et al., 1995; Prikryl et al., 1997). Also, the ubiquity of U isotope disequilibrium in mobilized U at Nopal I has been discussed in terms of its implications for differential radioisotope behavior in a geologic repository (Murphy and Pickett, 2002).

The mechanisms and timing of U mobilization and secondary mineral formation at Nopal I have important implications for attempts to model release and transport processes in the Yucca Mountain system. U-series isotope systematics and chemical age dating of U and U-bearing secondary minerals at Nopal I indi-

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cate an evolutionary history characterized by episodic mobilization and transport. The episodic nature of elemental release and transport are characteristic of natural systems with geologic, hydrologic, and climatic features similar to Nopal I and by analogy to Yucca Mountain. Neglecting the episodic nature of hydrogeochemical processes when modeling radionuclide release and transport rates could lead to underestimates in the amounts of hazardous radionuclides reaching the accessible environment (Murphy, 2000). Therefore, recognition of episodicity of release and transport will be beneficial when conducting future performance assessments at Yucca Mountain.

Summary

Natural analog studies at the Nopal I U deposit reveal a history of U phase evolution and transport in a setting analogous to the proposed Yucca Mountain geologic repository. Following primary U mineralization at 8 f 5 Ma, oxidizing conditions lead to secondary uranyl mineral formation by around 3 Ma. Recently (e.g., in the past several tens of thousand years), the history of the deposit has been marked by episodic U mobilization. Specific episodes of U release associated with open- system mobilization are recorded by U-series mineral age data at around 400 ka and 54 ka. Predictions based on geochemical modeling of present day unsaturated zone waters suggest that secondary uranyl phase formation may suppress dissolved U contents.

Uranium distributions in host tuff and secondary minerals within and surrounding the deposit indicate greater transport along relatively continuous mesofractures (e.g., maximum transport distances > 20 m) than through the general fracture network composed of less continuous microfractures (e.g., maximum transport distances of 2 to 3 m). Sorption and coprecipitation of U on fracture- infilling Fe-oxidedoxyhydroxides generated by pyrite oxidation appears to be the dominant mechanism for retarding U transport outside the deposit. Inventories of U retarded along different transport pathways at Nopal I indicate that the microfracture network and individual microfractures were more effective in retaining U than mesofractures and tuff matrix.

Natural analog data from the Nopal I U deposit has been recognized in performance assessments for the proposed geologic repository at Yucca Mountain. However, direct use of Nopal I data in Yucca Mountain performance assessments has been limited. The correspondence between the uranyl mineral paragenesis at Nopal I and in laboratory studies of synthetic UOz and spent fuel alteration has been the most important observation applicable to performance assessment. With respect to performance issues related to radionuclide transport, data from Nopal I point to the importance of fracture transport as well as retardation on minor phases such as Fe-oxidedoxyhydroxides, the limited significance of matrix diffusion, and the episodic nature of U mobilization and transport.

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Acknowledgments

The authors gratefully acknowledge Philip C. Goodell for bringing the Nopal I deposit to our attention and introducing us to the area. To our knowledge, he was the first person to recognize similarities between Nopal I and the proposed U.S. high-level waste repository at Yucca Mountain. Ignacio Reyes provided information on the geology and mineralogy of the Peiia Blanca District and the Nopal I deposit and carried out field sampling and mapping activities. We wish to thank Linda Veblen for guidance and encouragement during development and execution of activities at Nopal I. Finally, we wish to thank William M. Murphy and Bret W. Leslie for their contributions to this project.

This chapter was prepared to document work performed by the Center for Nu- clear Waste Regulatory Analyses (CNWRA) for the NRC under Contract No. NRC-02-02-012. The activities reported here were performed on behalf of the NRC Office of Nuclear Material Safety and Safeguards, Division of Waste Man- agement and the NRC Office of Nuclear Regulatory Research, Division of Regu- latory Applications. This paper is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC. Reviews by R. Pabalan and B. Sagar are gratefully appreciated.

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bourg, pp. 333-337

Table 1. Comparison of chemical compositions of the Nopal and Coloradas Formations and the Topopah Spring Member of the Paintbrush Tuff Formation (values in weight percent).

Nopal Coloradas Topopah Spring Oxide Formation' Formation' Memberb SiO, 75.24 67.49 74.00

12.75 6.48 1.54 1.07 0.41 0.26 0.15 0.07 0.06 2.60

12.00 4.13 1.99 0.47 4.41 0.24 0.16 0.04 0.11 8.17

12.40 4.00 1.07 3.40 0.66 0.10 0.3 1 1.01 0.08 3.79

Total 100.63 99.2 1 99.82 'Data from Pearcy et al. (1993) bData from Broxton et al. (1986) LO1 = Loss on ignition

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Table 2. Chemistry of unsaturated zone waters from Nopal I. Data from Pickett and Murphy, 1999.

ADIT95-6 ADIT95-9 BH12-04 BH12-05 BH12-11 seep seep perched perched perched

PH 7.32 7.26 7.34 HCO; 8.k-04 2.oe-04 5.7e-03 Cl- 2.7e-05 2.0e-05 1.9e-04 1.9e-04 1.9e-04 F 1.2e-04 1.3e-05 4.k-05 4.7e-05 4.5e-05 NO; 1.9e-05 6.7e-06 5.2e-05 5.3e-05 5.8e-05 PO? <1 .Oe-06 <1 .oe-06 6.k-06 8.k-06 7.9e-06 so: 1.9e-04 1.3e-04 7.5e-05 7.k-05 7.7e-05 A1 <7.k-07 7.8e-07 l.le-06 1.2e-06 1 Se-06 B d.oe-06 d.&-06 3.3e-06 3.4e-06 3.2e-06

Mg 3.3e-06 6.k-06 2.3e-04 2.3e-04 2.3e-04 Mn 5.5e-08 3.7e-05 3.7e-05 3.k-05 2.2e-06 K 3.7e-05 5.3e-05 1.k-04 1 Se-04 1 Se-04 Si 5.2e-04 2.3e-05 6.7e-04 6.7e-04 2.k-04 Na 3.oe-04 4.k-05 8.5e-04 8 .k -04 8.3e-04 U 8.3e-10 3.7e-09 2.k-08 2.k-08 Th 1.9e-12 5.3e-12 1.2e-09 6.7e-10

2 M T h / 2 3 2 T h 15 65 0.21 0.28

Ca 5.3e-04 2.le-04 2.k-03 2.k-03 2.k-03

2 3 4 ~ 1 2 3 8 ~ 5.1 2.8 2.2 2.2

? l l / 2 3 4 U 0.0022 0.0105 0.0018 0.001 1

--.____.--._

0 250 500

Kilometers

TERTIARY VOLCANIC ROCKS

Fig. 1. The Nopal I uranium deposit is located in the Peila Blanca mining district, Chihua- hua, Mexico. Yucca Mountain, Nevada, the proposed site for geologic disposal of high- level nuclear waste in the U.S., is located northwest of the Peila Blanca mining district. The Peila Blanca district and Yucca Mountain are both situated in Tertiary volcanic rocks in a Basin and Range tectonic setting. Figure modified from Pearcy et al., 1994.

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, Nopal I Uranium Deposit 1500

p 1400 m E -

1300 a I I 4stimated Interbasin Potentiometric Surface

0 2W 400 600 8W lo00 12W 1400 1600 1803 2Mx) 2200 2400 2600

$ d 1200 I

METERS SE

Fig. 2. Schematic cross-section of the Nopal I uranium deposit. The deposit is located above the water table and is hosted by silicic tuffs of the Nopal and Coloradas Formations and the basal limestone conglomerate of the Pozos Formation. Figure modified from Pearcy et al., 1994.

Fig. 3. Photographs and map of the Nopal I uranium deposit showing the Level +00 and +10 exposures. The top left photo looks to the southwest; the vertical wall at the left of the photo is about 10 m high and separates the Level +00 and +10 exposures. The plan view map of Nopal I at the top right shows the cleared portions of Level +00 and +10 and the area of visible U mineralization on the cleared outcrop. The contour interval on the map is 2 m, referenced to zero at Level +OO. The bottom photo looks to the south and shows the Level +10 exposure after clearing of loose rock and soil.

V

50 I I 50

40

35

30-

2 5 -

20

15

10

5 -

0 -

- 5

-10

NOPAL I GAMMA

35 - ,--

-

-

-

-

-

-

-

-15 -

Fig. 5. (a) Photograph of rock thin section of uranium ore from Nopal I. The left portion of the section is composed of breccia fragments cemented by a black-colored fine crystalline matrix of uraninite intergrown with pyrite, kaolinite, and quartz. In the right portion of the section, oxidation of the uraninite has led to formation of light-colored uranyl minerals. (b) SEM photomicrograph of uraninite (U) intergrown with kaolinite (K) and quartz (Q). This texture is referred to as “granular” uraninite (Pearcy et al., 1994). Field of view is 0.04 mm across.

Fig. 7. (a) Backscattered electron photomicrograph of a fracture containing dehydrated schoepite (DS) at the margins followed by soddyite (SO) and uranophane (UA). Field of view is 0.6 mm wide. (b) Backscattered electron photomicrograph of intergrown soddyite (SO), uranophane (UA), and weeksite (W) filling a fracture in a highly oxidized sample. Field of view is 0.7 mm wide. At the bottom right of (a) and the top left of (b) original granular uraninite has been replaced by uranophane.

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Fig. 8. Reflected light photomicrographs of Fe minerals filling a fracture at Nopal I. (a) Hematite (H) grows into open space from goethite (G) and amorphous Fe-oxyhydroxide (A) substrates. Jarosite (J) grows from hematite substrates into open voids (V). Field of view is 0.5 mm wide. (b) Goethite (G) partially replaces cubic pyrite which was originally intergrown with kaolinite (K). Open voids (V) left by pyrite dissolution are partially filled by jarosite (J). Field of view is 0.5 mm wide.

Fig. 9. Map of the Level +00 and Level +10 exposures of Nopal I showing locations of sample traverses (lines A-H) and samples (48 and 56) taken for U distribution and/or U- series isotopic analyses. Waters for chemical and isotopic analyses were collected fiorn a borehole on Level +10 (BH-12) and in the Level +00 adit at the projected locations marked by the X’s. Contour interval is 2 m, referenced to zero at Level +OO. Figure modified from Pickett and Murphy, 1997.

V

E 4 - 4000 - E I

I Area of visible U mineralization I

i I 0 . I I I I 0

' ) e

3ooo "I I

I 0 I

Area of visible U mineralization

0 I 1000 0

15 20 o ~ ~ " " " " " " " ' " " ' " " -15 -10 -5 0 5 10

J

Distance EW (rn)

Fig. 10. (a) Uranium content in generally fractured tuff along a 3-m transect across the western margin of the Nopal I deposit (traverse A, Fig. 9). (b) U content in fracture- infilling material along a 30 m long EW trending mesofracture, which crosscuts the deposit on Level +10 (traverse B, Fig. 9). The vertical dashed lines indicate the position of the edge of the area of visible U mineralization. Figures modified from Pearcy et al., 1995.

-

Fig. 11. Plan view drawing showing position of sample transects of microfractured tuff collected perpendicular to the EW mesohcture. NS arrows illustrate the distances of anomalous U concentrations fiom the mesohcture; locations for each traverse are the distance from the edge of the area of visible U mineralization. Uranium concentrations are given as the average for the microfractured areas N and S of the mesofracture for the area between each NS traverse. Note that the NS scale is expanded 10 times compared to the EW scale. Figure modified from Pearcy et al., 1995.

3

0.00 0.10 0.20 0.30 0.40 0.50 0.60

Distance from Microlracture (mm)

1

600

500

400

300

200

100

0 0.00 0.10 0.20 0.30 0.40 0.50 0.60

Distance from Microfracture (mm)

Fig. 12. Uranium concentration profiles in tuff matrix (samples 48 and 56, Fig. 9) adjacent to a microfracture interpreted from alpha impact density measurements. Gray-shaded vertical lines represent the maximum extent of excess U in the matrix. Error bars for distance are constant and correspond to the distance interval counted for each measurement. Alpha impact density errors are 1 sigma uncertainties. Figure modified from Pearcy et al., 1995.

Fig. 13. Uranium-series activity ratio diagram for fracture fillings and altered tuff around the Nopal I ore body. Open symbols are CNWRA decay-counting data (Prikryl et al., 1997; Pickett and Murphy, 1997); representative error bars are shown for two data points. Gray circles are Los Alamos National Laboratory thermal-ionization mass-spectrometric data on 15 samples of fracture-filling materials (CRWMS M&O, 2000); errors are smaller than the symbols. Data falling within the shaded “multistage history” region cannot be explained by single-stage closed-system evolution.

W

3 $j 8

3 @ h

f 5

.g 6

Nopal I Laboratory Tests

Soddyite Soddyite

WeeksitdBoltwoodite

Uranophane (32 - 3.4 Ma) - Na-Boltwocdite

- - Uranophane, Boltwoodite, Sklodowskite -

Uraniferous Fe 0xidesK)xyhydroxides (> 300 tm) and Opal and Caliche (54 ka) -

#

' Synthetic UO, I

I

lanthtnite I.

SchoepitdDehydrated Schoepite Dehydrated Schoepite - II. - Becquerelite I I

Becquerelite, Compreignacite -

natural analog studies at the nopal i uranium deposit, peaa ...the nopal i uranium deposit, in the...

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