Distribution of uranium and thorium in core samples from the Underground Research Laboratory lease area, southeastern Manitoba, Canada

Chemical Geology, 54 (1986) 97--111 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

97

DISTRIBUTION OF URANIUM AND THORIUM IN CORE SAMPLES FROM THE UNDERGROUND RESEARCH LABORATORY LEASE AREA, SOUTHEASTERN MANITOBA, CANADA

D.C. KAMINENI 1 , C.F. CHUNG 2 , J.J.B. DUGAL 1 and R.B. EJECKAM 1 Geological Survey of Canada, Ottawa, Ont. K I A OE8 (Canada) : Mathematical Applications in Geology Section, Economic Geology and Mineralogy Division, Geological Survey of Canada, Ottawa, Ont. K I A OE8 (Canada) (Accepted for publication June 18, 1985)

Abstract Kamineni, D.C., Chung, C.F., Dugal, J.J.B. and Ejeckam, R.B., 1986. Distribution of uranium and thorium in core samples from the Underground Research Laboratory lease area, southeastern Manitoba, Canada, Chem. Geol., 54: 97--111. Petrographic examination of core samples from seven boreholes in the Underground Research Laboratory lease area of the Lac du Bonnet Batholith revealed that the granitic core can be divided into four sample groups: (1) grey granite; (2) pink granite; (3) deep-red granite; and (4) cream-coloured clay-rich granite. The order of these four groups represents an evolutionary sequence that can be related to the alteration o1" the grey granite. U and Th concentrations were determined for the four groups of granitic core samples. The U concentration in the four groups of core samples showed a distinct variation whereas the Th concentration showed little variation. The distribution pattern of U in the four sample groups can be related to its mobility and concentration under various stages of rock alteration, while the uniform distribution of Th is interpreted to be due to its limited mobility.

l. Introduction A t o m i c E n e r g y o f C a n a d a L i m i t e d is assessing the concept of the disposal of nuclear fuel waste deep underground in plutonic r o c k s in t h e C a n a d i a n S h i e l d . I n t h i s c o n t e x t , a number of plutonic rock bodies, mainly comprising granitic rocks, are being investigated by various geoscientific methods. One

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of the major components of this research prog r a m is t h e c o n s t r u c t i o n o f t h e U n d e r g r o u n d R e s e a r c h L a b o r a t o r y ( U R L ) in t h e L a c d u B o n n e t g r a n i t i c p l u t o n , l o c a t e d in s o u t h e a s t e r n M a n i t o b a ( F i g . 1). B e f o r e e x c a v a t i o n o f the laboratory, several boreholes were drilled to investigate various geological parameters, s u c h as r o c k t y p e , a l t e r a t i o n , a n d f r a c t u r e s and their infillings. Using core samples from

© 1986 Elsevier Science Publishers B.V.

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/ LAKE

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URL

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Fig. 1. L o c a t i o n of the Underground Research Laboratory and the boundaries of Lac du B o n n e t Bathotitb (shaded area) ( m o d i f i e d f r o m M c R i t c h i e and Weber. 197] ).

seven boreholes, the petrography, bulk-rock chemistry and co m pos i t i on of various rockforming minerals were examined in order to characterize the r o ck mass. The results o f preliminary investigations of these features are r e p o r t e d in Kamineni et al. (1984). In this paper, we examine t he distributions o f U and Th in the core samples. Based on these distributions, a n u m b e r o f deductions are made for th e migration and dispersion characteristics o f analogous actinides (e.g., Pu, Am and Np) th at are likely to be present in nuclear fuel waste.

2. General geology o f the Lac du Bonnet pluton The Lac du Bonnet pluton occurs within the Superior Structural Province o f t he Canadian Shield, and yields a Rb--Sr whole-rock age o f ~ 2 . 5 Ga (Penner and Clark, 1971}. The boundaries of the p l u t o n and the location of the URL lease area are shown in Fig. 1. An area o f a b o u t 1.6 km × 3 km has been leased for t he construction of t he laboratory and ot her geoscience research. McRitchie and Weber (1971}, Tammemagi

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et al. (1980), C6rny et al. (1981j, and MeCrank (1984) mapped the Lac du Bonnet pluton and recognized several intrusive phases of granitic rock. Some of these rock types have been identified within the boundaries of the URL lease area by Stone et al. !1984). The granitic map-units at the URL lease area include: (1) pink granite; (2i pink granite with strong foliation; (3) grey granite; (4) xenolithic granite; and (5) pink, coarse and fine-grained granite. 3. Subsurface geology of the U R L lease area Fig. 2 depicts the orientation of the boreholes and the major rock types encountered in a block of rock within the U R L lease area. Except for map-unit (2), all the other (four) units noted on the surface are present in the boreholes, with units (1) and (3) being predominant. Although units (1) and (3) are texturally the same, they can be distinguished easily on the basis of colour, and are believed to represent a primary single phase of granite. The pink-coloured samples invariably contain secondary minerals such as phengite, chlorite, ca]cite and hematite, and consequently the pink colouration in the t o p section of the boreholes is interpreted to be due to hydrothermal alteration (Kamineni et al., 1984). Almost all the fractures recorded in the boreholes and in the xenolithic granite are concentrated in the pink granite, which is interpreted to represent part of the r o o f zone of the pluton. The colouration of the pink granite varies from light to deep shades of red towards fractures. In some cases, the deep-red colouration is replaced by a cream colouration. The cream-coloured zones contain clay minerals and are associated with open fractures. Although the deep-red and cream-coloured varieties of rock appear as zones formed b y the alteration of pink granite, they possess distinct mineralogical and chemical characteristics and, hence, are treated as separate sample groups in this paper.

4. Methods of investigation Samples of core representing the:. various groups of granite were selected for bulk-rock chemical analysis and petrographic examin~ tion. The [J and Th distributions were determined for four groups of granitic samples, representing fresh unaltered granite and various levels of alteration of the same. These groups, arranged in order of increasing alter ation, are: (1) grey granite (fresh}; ~2) pink granite; (3) deep-red granite; and (4) creamcoloured clay-rich granite. Of these four groups, groups (1) and (2) are the most abun. dant. The samples were putverised and analysed by Bondar-Clegg Ltd., Ottawa, Ontario. SiO2, TiO2, A120~, Fe (total}, MnO, MgO. CaO. Na~O, K~O, H20 ÷, H20-, Rb, Ba, and Sr were determined using classical and atomic absorption techniques. FeO was determined by titrimetric methods, and H20* and H~O- were determined by loss on ignition at 600 ° and 100°C, respectively. Fe203 was calculated from the difference between total Fe and FeO. The results are given in Table ]. Th was determined by X-ray fluorescence and U by delayed neutron activation. Five replicate analyses of one pink granite and fe ur replicate analyses of grey granite gave U contents of 4.20 _+ 0.22 ppm ,-- Io1 and 6.53 0.25 ppm (-+ lo), respectively. Four replicate analyses of grey granite gave Th values of 28 ~: 1.0 ppm (± lo). The mean and standard deviation for the concentrations of these elements in all samples analysed is given in Table II 5. Petrographic n o t e s The grey granite is inequigranutar, with phenocrysts of microcline feldspars that show cross-hatched patterns and perthitic texture (Fig. 3A). Plagioclase has well-developed polysynthetic twinning and occurs as smaller grains than the microcline phenocrysts. Quartz is distributed around feldspar phenocrysts and c o m m o n l y exhibits deformation

101

TABLE I M e a n a n d s t a n d a r d d e v i a t i o n of various c h e m i c a l c o n s t i t u e n t s in t h e f o u r g r o u p s o f granitic core samples f r o m t h e U R L b o r e h o l e s (1 to 7) G r e y granite

mean SiO~ TiO 2 A12 03 F% 03 FeO MnO MgO CaO Na20 K20 H20 + H20 Ba Rb Sr

71.87 0.22 14.95 1.23 1.14 0.03 0.41 1.66 4.07 4.85 0.18 . 673 227 279

Pink granite

S.D.

.

2.88 0.14 0.86 0.76 0.35 0.02 0.21 0.50 0.86 1.15 0.10 . 218.2 45.0 56.8

mean 72.20 0.21 14.22 1.62 1.09 0.03 0.48 1.49 3.95 4.90 0.20

S.D. 2.65 0.13 1.58 0.61 0.28 0.02 0.33 0.46 0.83 1.01 0.12

. 583 208 222

180.5 38.6 48.3

Deep-red granite

Cream-coloured granite

mean

mean

72.25 0.22 13.72 2.79 1.00 0.02 0.60 1.05 3.66 5.00 0.25 0.12 563 220 205

S.D. 1.10 0.05 0.90 0.51 0.39 0.01 0.21 0.25 0.75 0.88 0.08 0.05 90.3 33.6 28.3

71.25 0.20 14.79 1.91 0.92 0.02 0.94 0.86 1.85 7.30 0.51 0.15 633 265 136

S.D. 0.85 0.04 0.60 0.37 0.21 0.01 0.30 0.53 0.40 0.55 0.16 0.10 77.5 32.6 36.5

N u m b e r of s a m p l e s : grey granite, 1 5 2 ; p i n k granite, 1 3 6 ; d e e p - r e d granite, 30; c r e a m - c o l o u r e d granite, 22. T h e m i n o r e l e m e n t s Ba, R b a n d Sr are e x p r e s s e d in p p m . T h e m a j o r e l e m e n t s are e x p r e s s e d in wt.%.

T A B L E II Means, s t a n d a r d d e v i a t i o n s a n d o r d e r statistics for u r a n i u m a n d t h o r i u m c o n c e n t r a t i o n s in t h e f o u r g r o u p s of granitic samples (all expressed in p p m ) Uranium

Number of samples Minimum 5% Median 95% Maximum Mean Standard deviation

Thorium

grey granite

pink granite

deep-red granite

creamcoloured granite

grey granite

pink granite

deep-red granite

creamcoloured granite

185 1.1 1.53 5.85 15.78 26.8 6.60

164 1.3 2.2 4.3 9.0 16.8 4.96

20 3.4 3.4 5.8 11.9 12.2 6.67

20 2.5 2.5 8.8 16.2 18.00 9.87

185 7 17 36 59.75 98.00 37.01

164 3 17 38 152.8 211.00 45.63

20 22 22 45 63 108 42.25

20 20 20 40 78 80 42.50

4.31

2.40

2.98

4.42

13.85

36.77

18.62

17.50

l02

bands and polygonization. Biotite, sphene, apatite, and zircon are other minerals noted. Thorite is present as inclusions in the biotite. The pink granite [group (2)] contains various secondary minerals: epidote and carbonate occur in plagioclase, and white mica, re-

sembling sericite, is noted at the gram 0our~.daries and within feldspars. Microprobe anai[ysis of white mica showed significant, amom~ts of FeO (-5%) and MgO (--2%). A ~ompariso~l of the analysis with those of I)c~::.:i~ ,.:~ ~i (1966)' suggests that the white rm,:a is p~t~e~-

103

Fig. 3. A. Photomicrograph of a grey granite. Note the Carlsbad twinning in the microcline ( K f ) grains and slight alteration along the twin lamellae in the plagioclase (Pl) grains ( Q z = quartz). Crossed polars × 10. B. Photomicrograph of pink granite. Note the concentration of alteration products in the plagioclase (Pl) grains ( Q z = quartz; K f = microcline). Crossed polars x 15. C. Photomicrograph of a deep-red granite. Note the preferential concentration of iron oxide in the plagioclase (Pl) grains and microfracture ( M f ) ( Q z = quartz, K f = microcline). Crossed polars × 23. D. Photomicrograph of a cream-coloured clay-rich granite. Note the concentration of clay in the plagioclase (Pl) grains and in a microfracture ( M f ) that transects various mineral grains ( Q z = quartz; K f = microcline). Crossed polars × 15.

104

gite. Chlorite occurs mainly along the (100) cleavage of biotite, and hematite is present in microfractures in quartz and in plagioclase grains. Some of these features are shown in Fig. 3B. The deep-red granite [group (3)] is characterized by numerous microfractures and an abundance of secondary minerals (Fig. 3C). Biotite is highly altered to chlorite and vermiculite, and plagioclase is highly altered to phengite. Hematite, concentrated in plagioclase grains and in microfractures that transect the rock, imparts a reddish colour to the rock samples. Rocks from the cream-coloured clay-rich zones [group (4)] are characterized by the presence of illite. Illite occurs in microfractures (Fig. 3D), which are c o m m o n l y present, and as a replacement of feldspar and biotite. The textural characteristics strongly suggest that the four groups of granitic rocks are comagmatic and may define an evolutionary sequence of alteration. Grey granite represents the least altered pristine rock formed during magmatic crystallization. The pink granite is hydrothermally altered grey granite. Wherever fracturing is intense within the pink granite, biotite and plagioctase have been thoroughly altered to chlorite and phengite, with concomitant precipitation of hematite. The occurrence of phengitic mica as a secondary mineral implies that the temperatures and pressures during alteration were well above surface conditions as, according to Velde (1965), phengitization occurs around 200°C. The secondary minerals in deep-red and cream-coloured clay-rich samples have formed at subsequent rock alteration events, presumably in response to greater fluid activity in the highly fractured zones. Oxygen-isotope analysis of clay minerals gives 5 ' 8 0 values around + 2 1 ° 0 , suggesting their formation at low temperature under chemical weathering conditions (Savin and Epstein, 1970). The formation of clay minerals is accompanied by the leaching of Fe and some other elements, such as Ca and Na

6. Chemical characteristics The four groups of samples, nameiy grey, pink, deep-red and cream-coloured granitic rocks, exhibit certain distinct chemical char° acteris'tics. For example, as shown i~: Table i, the Ca content progressively decreases from the grey to eream-coloured granite. Sr shows the same trend, as expected from ~ts geochemical coherence with Ca ~Goldschmidt, 1954). The cream-coloured granite is enriched in K, presumably due to illitization in these zones. Rb shows a positive correlation with K in all groups, with correlation coefficients c,f up to 0.8 (Kamineni et al., 1984}. In this section, the statistical significance of the distribution of U and Th is examined m detail. But first, the ferrous and ferric iron oxides are considered, because uranium mobility is sensitive to redox conditions (Fyfe, 1979; Boyle, 1982}.

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105

6.1. Ferrous and ferric iron oxides

statistics associated with these "Box--Whisker plots". By examining Fig. 4, it is immediately obvious that the first three groups have distinctly different mean values for the ferric iron (Fe203) but have similar means for the ferrous iron (FeO). Consequently, the oxidation

Fig. 4 shows "Box--Whisker plots" (Tukey, 1977) of ferrous and ferric iron oxides from the grey, pink, deep-red and cream-coloured groups separately, and Table III lists the sample means, standard deviations and ordered T A B L E III

Means, s t a n d a r d deviations and o r d e r statistics for Fe203 and FeO f r o m the f o u r groups o f granite samples (all e x p r e s s e d in wt.%) Fe203

FeO

grey granite

pink granite

deep-red granite

creamcoloured granite

grey granite

pink granite

deep-red granite

creamcoloured granite

Number of samples Minimum 5% Median 95% Maximum Mean

152 0.00 0.19 1.30 2.40 2.71 1.23

136 0.12 0.40 1.81 2.32 2.63 1.62

30 1.83 1.84 2.88 3.48 3.55 2.79

22 1.14 1.15 1.88 2.48 2.52 1.91

152 0.1 0.56 1.12 1.70 1.96 1.14

136 0.32 0.58 1.12 1.51 1.64 1.09

30 0.59 0.62 0.95 1.81 2.08 1.00

22 0.63 0.63 0.89 1.22 1.53 0.92

Standard deviation

0.76

0.61

0.51

0.37

0.35

0.28

0.39

0.21

T A B L E IV R e s u l t s o f F - t e s t s and t-tests o n the o x i d a t i o n ratio F-value

2-Tail probability

P o o l e d variance e s t i m a t e (PVE) t-value

Pink vs. grey 2.71 Pink vs. d e e p - r e d 3.85 Pink vs. creamcoloured 5.63 G r e y vs. deepred 10.42 Grey vs. creamcoloured 15.24 D e e p - r e d vs. creamcoloured 1.46

degrees o f freedom

Separate variance e s t i m a t e (SVE)

2-tail t-value probability 6.11 10.81

253.2 85.9

0 0

7.45

67.9

0

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0

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14.36

149.5

0

0

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11.78

128.7

0

3.32

--

2-tail probability

0 0

0.37

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degrees o f freedom

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50

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--

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106

ratios, Fe:O3/(Fe20~ + FeO) × t 0 0 (Chinner, 1960), are different for the h)ur groups. To investigate the degree of difference, we haw~ applied the statistical t-test, with the h y p o t h esis of equal means for the four groups, a pair at a time. The results are shown in Table IV. In addition to the results of the t-test, Table IV also contains results for F-tests. The F-test was carried out to see whether the variances between various pairs of populations are sufficiently alike. When the hypothesis of equal variances is not rejected (i.e., accepted), the "pooled variance estimate" (PVE) given in the tables are used for the t-tests; however, when the hypothesis of equal variance is rejected (Behrens--Fischer problem; Bickel and Doksum, 1977), one should use the "separate variance estimate" (SVE) of the tables. It must be noted, however, that the "t-value" in the SVE is not distributed as a t-distribution. The degrees of freedom are estimated by "Welch's a p p r o x i m a t i o n " (Bickel and Doksum, 1977). In such cases, the two-tail probability is obtained by interpolation within the t-distribution. At the 97.5% level of confidence, we reject the hypothesis of equal means for all the pairs, with respect to oxidation ratio. The smallest ratio, 0.47, is in the first group, grey granite, and increases to 0.72 in deep-red granite. Evidently, this is a manifestation of rock alteration that presumably t o o k place under oxidizing conditions. In the creamcoloured group of rocks, the oxidation ratio is ~ 0 . 6 7 , which suggests that the clays were formed under somewhat less oxidizing conditions than the deep-red granite. 6.2. Uranium and thorium

For these two elements, 185, 164, 20 and 20 samples (recovered from boreholes URL-1 to -7) representing the four groups of granite, i.e. grey, pink, deep-red and cream-cotoured, were analysed. The "Box--Whisker plots" of U and Th for the four sample groups are

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nrO

Fig. 5. " B o x - - W h i s k e r p l o t " o f u r a n i u m and t h o r i u m c o n c e n t r a t i o n s in grey, pink, d e e p - r e d and creamc o l o u r e d granite samples. T h e plus symbols corres p o n d to m a x i m u m and m i n i m u m values and the dots r e p r e s e n t m e a n values. The horizontal lines within the boxes r e p r e s e n t m e d i a n values.

shown in Fig. 5 and the sample means, standard deviations and ordered statistics associated with the "Box--Whisker p l o t s " axe given in Table II. As illustrated in the plots for U, the sample mean decreases f r o m group ( 1 ) t o (2) and increases from 2 to 4. Because of the skewness and heavy tails shown in the plots, log transformations were applied to the data and then the t-test was performed for: the hypothesis of equal means. The results are shown in Tables V and VI' At the 97.5% level of confidence, except for pairs (1) a n d (3), the hypothesis is rejected. It implies t h a t w e may assume that the means of groups (1) and (3) are equal, but those of groups: ( 1 ) ~ d (2), (2) and (3), and (2) and (4) are distinct. However, as shown in Table VI, the means for Th m a y be assumed to be equal,

107

TABLE V F - t e s t s and t-tests for u r a n i u m in the f o u r groups o f granite samples F-value

2-Tail probability

P o o l e d variance e s t i m a t e (PVE) t-value

Grey vs. pink Grey vs. d e e p - r e d Grey vs. creamcoloured Pink vs. d e e p - r e d Pink vs. creamcoloured D e e p - r e d vs. creamcoloured

Separate variance e s t i m a t e

(SVE)

degrees o f freedom

2-tai! t-value probability

degrees o f freedom

2-tail probability

-0.48

332.7 --

0 --

1.95 2.06

0 0.07

-0.7

-203

1.45 1.06

0.35 0.49

--3.3 --2.9

203 182

1.35

0.32

--6.3

182

1.43

0.45

--2.5

38

--3.53 --

0

- -

- -

- -

0.02

--

--

--

T A B L E VI F - t e s t s , t-tests for t h o r i u m in t h e f o u r groups o f granite samples F-value

2-Tail probability

t-value

Grey vs. pink Grey vs. deepred Grey vs. creamcoloured Pink vs. deepred Pink vs. creamcoloured Deep-red vs. creamcoloured

2.59

0

1.06

degrees o f freedom

(SVE)

2-tail t-value probability

2-tail probability

265.9

0.16

--

--

0.49

--1.47

203

0.14

--

--

--

1.19

0.54

--1.41

203

0.16

--

--

--

2.75

0.01

0.56

33.46

2.18

0.05

--0.35

182

0.73

--

--

--

1.26

0.62

0.03

38

0.98

--

--

--

.

.

the four groups of rock samples rea spectrum ranging from pristine ungranite to highly altered clay-bearing the observed uranium-distribution evidently reflects the behaviour of

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1.4

degrees o f freedom

--

7. G e o l o g i c a l i n t e r p r e t a t i o n Since present altered granite, pattern

S e p a r a t e variance e s t i m a t e

P o o l e d variance e s t i m a t e (PVE)

0.58

this element during magmatic crystallization and subsequent rock alteration. The lower concentration of U in the pink granite (as compared to grey granite) can be attributed to two causes: (1) Assimilation tonalitic and mafic

by the pink granite of xenoliths, whose U con-

] 0~, tent is considerably less (mean of 31 xenoliths is 4 ppm). Whitfield et al. (1959) first proposed this mechanism whereby the assimilation of c o u n t r y rock, with a low (7 content, by granitic rocks, with more U, carl lower the concentration of radioelements in the latter. At the URL lease area, most of the xenoliths occur in the pink granite in the upper sections of the boreholes, which probably represents the roof zone of the batholith. Stone et al. (1984) have reported xenoliths with various degrees of assimilation. The grey granite (with very few xenoliths) has a U c o n t e n t of 6.6 ppm. A 40--50% assimilation of xenoliths with an average U content of 4 ppm (:'an account for the U concentration of ~ 5 ppm in the pink granite. However, the " s u p e r h e a t " required to assimilate great amounts of xenoliths is probably not available in granitic magmas (Bowen, 1928). (2) Migration of uranium. The pink granite is part of the roof zone of the batholith and may have concentrated volatiles, especially in the late stages of magma crystallization. These volatiles would have been responsible for the alteration of silicates, biotite and plagioclase, in particular. Biotite in these samples is comm o n l y chloritized, whereas plagioclase is altered to phengite and epidote. The biotite is altered to phengite and epidote. The biotite hosts small inclusions of zircon and thorite. Uranium present in these inclusions would be mobilized during this alteration process. The solutions causing the alteration appear to have been of oxidizing nature, as indicated by the higher oxidation ratio for Fe in these samples relative to grey granite (see Table I). Uranium, under such conditions, would attain the mobile hexavalent state and thus migrate from its sites of liberation and precipitate in other sites by reduction to the tetravalent state. A process similar to this has also been suggested by other investigators (e.g., Ragland et al., 1967; Rosholt et al., 1973) and has been summarized in Rye and Roy {1978). More recently, Ballantyne and Littlejohn (1982) have used this process to account for the differences in U concentration f o u n d in certain zones of the Surprise Lake Batholith in Brit-

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Fig. 6. Energy-dispersive spectra of hematite (from deep-red granitic core) showing U concentration. Mg, A! and Si are from the matrix ish Columbia, Canada. In groups (2) and (3), i.e. the pink and deep-red granite, respectively the latter is heavily coated with hematite and generally shows higher U concentrations {Table II). Scanning electron microscope examinations, combined with energy-dispersive X-ray analyses, shows that U in the deep-red granite is strongly associated with hematite (Fig. 6). This suggests that the oxidation of bivalent Fe, which is a prerequisite for hematite formation, favoured the concentration of U in this group. The deep-red granite is closely associated with meso- and microfractures. Bivalent Fe is reported (Wintsch, 1981) m be c o m m o n l y released from ferromagnesian minerals (biotite in this case} during grain-size reduction due to brittle fracturing. Wintsch (1981) further suggested that, during deformation, the pH of the pore fluid m a y rise due to surface exchange reactions leading to an increase in fo~ and to the oxidation of released Fe. This would precipitate ferric oxyhydroxides in microfractures and around grain boundaries and, subsequently, hematite would form by recrystallization. During the same process, hexavalent uranium present in the pore fluid could be coprecipitated together with the Fe-hydroxides, as indicated by the following reactions: (UO2) :+ + 2Fe 2+ + 3H20 -~ UO: (s) + Fe(OH)3 (s) + 3H" UO2(CO3)~- + 2Fe 2+ + 3H + -+ UO2 (s) + Fe(OH)3 (s) + 3CO2

109 The cream-coloured clay-rich granite samples [group (4)] contain the highest U content among the four groups (Table II). The origin of the clay-rich granite is interpreted to be due to rock--water interaction in fracture zones under "weathering" conditions (Kamineni et al., 1984). A notable characteristic of the clay-rich granite is its colour, and the absence of deep-red staining, related to hematite precipitation. This suggests that the clay-forming reactions were favourable for retention of the U present in the deep-redcoloured rock matrix, but were capable of removing the Fe from this zone (presumably reducing in nature). From this it can be inferred that U in the illitized zones, i.e. in the clay-rich granite, remained immobile during the formation of clays and may have entered their structures. Recently, Shirvington (1983) suggested that UO~ ÷, which has a charge density between K ÷ and Ca 2÷, may replace K ÷ in the interlayer positions of the illite and montmorillonite series. Another possibility is that part of the U was leached along with the Fe, but was re-extracted much later by clay minerals, via ion-exchange reactions, from the circulating groundwaters. This possibility may be resolved by uranium disequilibrium studies. The t y p e of uranium distribution noted here was f o u n d also in the Eye--Dashwa Lakes pluton near Atikokan, which is another granitic pluton in the Canadian Shield selected for research on nuclear fuel waste disposal in plutonic rocks. Twenty unaltered and 28 altered samples analysed from this pluton gave average U concentrations of 3.8 and 2.5 ppm, respectively. The U concentration reaches as high as 20 ppm in goethite and clay-rich material occurring in open fracture zones. The pattern of uranium distribution described here m a y be more prevalent in granitic rocks than recognized to date. Unlike U, Th shows no pattern of distribution among the four groups of granitic rocks in the URL boreholes. Only in one borehole (URL-2) did the pink granite show an unusually high concentration. This anomalous value

is thought to be primary and related to an abundance of thorite inclusions in the biotite crystals. The uniform distribution of Th in the various groups of granitic rocks is evidently due to the limited mobility of this element during rock alteration. Langmuir and Herman (1980) suggested that dissolved Th is invariably complexed in nature and forms the following inorganic complexes at pH < 4.5: Th(SO4) °, (Th F2)2+; pH 4.5--7.5: Th(HPO4) °, Th(HPO4)2-; pH > 7.5: Th(OH)4. In addition, they suggested that organo-thorocomplexes may predominate in organic-rich environments. Boyle (1982) also noted that none of the thorium complexes mentioned above are particularly stable under natural pH conditions, which vary between 5 and 8, and hence the mobility of this element is greatly restricted during low-temperature rock--water interaction. 8. Summary and conclusions The discolouration of pristine grey granite has been used to establish a sequence of degree and type of alteration in rocks obtained from boreholes at the URL lease area. Differences in the U content among the four groups can be related to this rock alteration. The grey, or least altered granite contains an average of 4 7 ppm U. The pink granite, the hyrdrothermally altered equivalent of the grey granite, contains less U (average ~ 5 ppm) than the grey granite. The lower U content in the pink granite is attributed to the mobilization and migration of this element during alteration of the grey granite. The highest concentrations of U are recorded in the deep-red and cream-coloured clay-rich granites, which usually occur as narrow zones adjacent to fractures within the pink granite. In the deepred granite, U is probably concentrated as a result of the reduction of uranyl ion (present in solution) by the oxidation of bivalent Fe, and is trapped as uraninite on hematite. In the clay-rich granite, U is concentrated by adsorption on the clay minerals. In contrast to U, Th shows little variation

among the four groups of granite, probably because of the limited mobility of this element. The observed distribution pattern of U in the four groups of granitic rock, based on degree of alteration, provides information that is useful for predicting the migration characteristics of certain actinides from a nuclear fuel waste disposal vault. The chemical behaviour of the actinide elements Pu, Np and Am, the most abundant actinides in nuclear fuel waste (Boulton, 1978), is similar to the geochemical behaviour of U (Bagna!l, 1_972).

Acknowledgements Reviews by Drs. R.W. Boyle, J.J. Cramer, I.F. Ermanovics, W.S. Fyfe, M. Gascoyne, T.T. Vandergraaf and S.H. Whitaker improved the quality of this paper. Grateful thanks are due to the personnel of the Word Processing Centre, Geological Survey of Canada, for patiently typing the manuscript.

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