An Analogue Validation Study of Natural Radionuclide Migration in Crystalline Rocks Using Uranium_series Disequilibrium Studies

7/28/2019 An Analogue Validation Study of Natural Radionuclide Migration in Crystalline Rocks Using Uranium_series Disequilibrium Studies

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m o d e l d e r i v a t io n i n v o lv i n g r a d i o n u c l i d e m i -g r a t i o n h a s b e e n e x p e r i m e n t a l l y b a s e d . A no b v i o u s m e a n s o f v a l id a t in g t h e e x p e r i m e n t a ld a t a is t o e x a m i n e n a t u r a l l y o c c u r r i n g r ad i o -n u c l i d e m i g r a ti o n ( o r la c k o f m i g r a t io n )w i t h in t h e b e d r o c k e n v i r o n m e n t o f t h ec h o s e n r e p o s i t o r y. S u c h an a n a l o g u e s t u d ya l lo w s , m o s t i m p o r t a n t l y , a n a p p ra i sa l o fm i g r a t i o n o n t h e s c a le o f g e o l o g ic a l t i m e .

N a t u r a l l y o c c u r r i n g U a n d T h a r e o fp a r t i c u l a r i n t e r e s t i n r a d i o a c t i v e w a s t e d is p o s -a l s in c e t h e y r e p r e s e n t u s e f u l an a l o g u e s o ft h e a c t in i d e s. F u r t h e r m o r e , t h e d a u g h t e rn u c l i d e s i n t h e 2 3 8 U a n d 2 3 2 T h d e c a y c h a i n ss h o w r a n g e s o f c h e m i c a l p r o p e r t i e s a n dh a l f- l iv e s s u c h t h a t t h e d e g r e e o f e q u i li b r i-u m b e t w e e n p a r e n t - - d a u g h t e r p a ir s c an b eu s e d t o p r o v i d e i n f o r m a t i o n a b o u t ro c k - -w a t e r i n t e r a c t i o n s . S u c h s t u d i e s c a n i n d i c a t et h e d e g r e e o f r a d i o n u c l i d e m o b i l i t y a n d c a np r o v i d e i n f o r m a t i o n o n t h e r a t e s o f p r o -c e s s e s o c c u r r i n g o v e r t i m e i n t er v a l s o f u pt o 1 M y r.

A s p a r t o f a j o i n t ly - f u n d e d p r o g r a m m eo f r e s e a rc h b e t w e e n t h e S w e d i s h N u c l e arF u e l a n d W a s te M a n a g e m e n t C o . ( S K B )a n d t h e S w i ss N a t i o n a l e G e n o s s e n s c h a f t

f i i r d i e L a g e r u n g R a d i o a k t i v e r A b f ~ l l e ( N A G -R A ) , d r i l l c o r e l e n g t h s i n t e r s e c t i n g s u i t a b l ew a t e r - c o n d u c t i n g f r a c t u r e z o n e s w e r e c h o s e nf r o m t h r e e d i f f e r e n t h y d r o g e o l o g i c a l e n vi r o n -m e n t s ; t w o in S w i t z e r l a n d ( B S t t s t e in an dG r i m s e l ) a n d o n e i n S w e d e n ( K r h k e m ~ d a ) ,a l l l o c a t e d i n g r a n i t e b e d r o c k .

T h e s a m p l e s r e p r e s e n t p r o f i le s f r o m t h ei n t e r s e c t e d f r a c t u r e f a c e i n t o t h e b e d r o c k .S e c t i o n s c u t f r o m t h e p r o f i l e s h a v e b e e ni n v e s t i g a t e d u s i n g t h e u r a n i u m d e c a y se r ie s( 23sU- - 234U- - : 3 Th- - 226Ra) , i n con j unc t i onw i t h r a r e ~ e a r t h e l e m e n t ( R E E ) a n d F e 2 + /F e 3 a n a ly s e s , a - t r a c k a u t o r a d i o g r a p h y, f is -s i o n 4 r a c k a n a ly s i s a n d d e t a i l e d m i n e r a lo g y.

To t h e e x t e n t t h a t t h e U - s e r ie s d i s e q u i li b -r i u m is a p r o d u c t o f r o c k - - w a t e r in t e ra c -t i o n , t h e s e m e a s u r e m e n t s m a y h e lp to ev a l-u a t e t h e e f f e c t iv e a d d i t i o n o r r e m o v a l o f Uw i t h i n t h e n e a r v i c in i t y o f a n o p e n w a t e r -

b e a r in g f r a c t u r e z o n e . E v i d e n c e o f su c h am i g r a t i o n p r o f i l e o c c u r r in g i n t o t h e b e d r o c ku n d e r a m b i e n t t e m p e r a t u r es , s h o u l d le n do r d e t r a c t s u p p o r t t o m o d e l s i n v o l v i n g f a r -f ie l d r a d i o n u c l i d e r e t a r d a t i o n p r o c e s se s .

2 . U r a n i u m - s e r i e s d i s e q u i l i b r i u m i n r o c k s

S t u d i e s o f t h e U d e c a y s e r i e s h a v e n o wb e c o m e a f a i r l y s t a n d a r d i n v e s t i g a t i v e m e t h -o d w i t h i n t h e f i el d o f g e o c h e m i s t r y. P io n e e r -e d b y C h e r d y n s t e v ( e .g ., 1 9 5 5 , 1 9 7 1 ) t h em e t h o d s h a v e b e e n s u b s e q u e n t l y i m p r o v e d

a n d a p p l ie d b y m a n y w o r k e r s t o a w i d e ra n g eo f g e o l o g i c a l e n v i r o n m e n t s a n d t h e i r r e l a t e dp r o b l e m s ; s ee , f o r e x a m p l e , R o s h o l t ( 1 9 5 9 ,1 9 8 3 ) , T h u r b e r ( 1 9 6 2 ) , R o s h o l t e t a l. ( 1 9 6 3 ,1 9 6 6 ) , K o i d e a n d G o l d b e r g ( 1 9 6 5 ) , K i g h o s h i( 1 9 7 1 ) , K r o n f e l d (1 9 7 4 ) , O s m o n d a n dC o w a r t { 1 9 7 2 , 1 9 7 6 ) , a n d F l e i sc h e r a n dR a a b e ( 1 9 7 8 ) .

I n c l o s e d g e o l o g i c a l s y s t e m s t h e n u c l i d e s238U--234U-- - :3Th a t t a in radioac t ive equi l ibr i -u m a f t e r ~ 1 . 7 M y r. , i .e . t h e r e s p e c t i v e

ac t iv i ty ra t io s 234U/238U, 23Th/234U a nd23 Th /238U a l l equ a l un i t y. H ow ev e r, i f t hes y s t e m s a r e e x p o s e d t o w e a t h e r i n g a n dg r o u n d w a t e r c i rc u l a ti o n , a n d a s s u m i n g t h a t2 3T h is i m m o b i l e u n d e r n o r m a l g r o u n d w a t e rc o n d i t io n s ( L a n g m u i r a n d H e r m a n , 1 9 8 0 ) ,t h e d i f f e r e n t p h y s i c o - c h e m i c a l c o n d i t i o n sa f f e c t i n g 2 3 8 U a n d 2 3 4 U w i l l r e s u l t i n t h e i rf r a c t i o n a t i o n a n d t h u s i s o t o p i c d i s e q u i li b r i-u m . T h e r e s p e c t i v e a c t i v i t y r a t i o s w i l l t h e r e -f o r e b e g r e a t e r o r l es s t h a n u n i t y, d e p e n d i n go n w h e t h e r t h e r e is a n e x c es s o r d e f i c i e n c yo f 2 3 8 U a n d / o r 2 3 4 U . F r a c t i o n a t i o n o f : 34 Ua n d 2 3 8 U a ri se s f r o m t h e r a d i o g e n i ca l lyp r o d u c e d 2 3 4 U d a u g h t e r w h i c h o r ig in a te st h r o u g h t h e t h r e e - s t e p a - a n d ~ - d e c a y f r o mi t s 238U pa r e n t :

238U ~ 2 3 4 T h -~ :34pa -~ :3a U4.51 Gyr. 24.1 day s 1.18 rain.


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I n g e n e r a l, t h e i s o t o p i c v a r i a t i o n o f Uh a s b e e n r e c o g n i z e d a s b e i n g p r o d u c e d b yt w o m e c h a n i s m s : ( 1 ) s e le c t iv e l e a ch i n g o f2 3 4U i t se l f , ( 2 ) t h e d i r e c t a - r e c o i l t r a n s f e ro f a 234U p r ecu r so r ( i .e . 234Th), an d a com -b i n a t i o n o f t h e s e p r o c es s e s . F o r s e l e ct iv el e a ch i n g t o o c c u r , : 3 4U a t o m s m u s t b e m o r es u s c e p t i b l e t o l e a c h i n g a n d r e m o v a l t h a n~3sU a t om s . Th i s is f a c i l i t a t e d b y l a t t i c ed a m a g e r e s u l t i n g f r o m t h e r a d i o a c t i v e d e c a yo f 2 3 ~ U . T h e r e s u l t a n t d a u g h t e r a t o m s w i llb e l o c a t e d i n w e a k l y b o n d e d o r i n t e r s t i t i a ls i te s , a n d o x i d a t i o n o f t h e d a u g h t e r ( i. e.: 34 U ) t o t h e 6 + v a l e n c y s t a t e a s a r e s u l t o ft h e r e c o i l w i t h i n t h e l a t ti c e , w il l r e n d e r i tm o r e v u l n e r a b le t o r e m o v a l b y w a t e r . D i r e c ta - r e c o il o f ~ 3 4 T h c a n o c c u r f r o m n e a r t h es u r f a c e o f a m i n e r a l g r a i n i n t o t h e i n t e r -g r a n u la r p o r e w a t e r , w h e r e u p o n i t is t r a p p e da n d f u r t h e r d e c a y s t o :3 4 U . T h i s r e s u lt s int h e e n r i c h m e n t o f : 34 U i n w a t e r a n d a :3 4 Ud e p l e t i o n i n t h e m i n e r a l ( i .e . s o l i d - t o - s o l u t i o n2 3 4U r e c o i l g a in ) . A n a d d i t i o n a l m e c h a n i s mr e c o g n i z es t h e p r o c e s s o f d a u g h t e r p r o d u c te m p l a c e m e n t f r o m p o r e w a t e r s c o n t a in i n gd i s s o lv e d ~ 3 8 U a n d 2 3 4 U . S u b s e q u e n t d e c a yo f t h e s e r a d i o n u c l i d e s r e s u l t s i n t h e r e c o i l -

i ng nuc l i de s o f : 3 Th , 234Th, 234Pa and 234Ub e i n g a b s o r b e d o r e m b e d d e d i n p a r ti c u la t em a t t e r a t t h e s o l i d - - l i q u i d i n t e r f a c e . A f t e rs u f f i c i e n t g e o l o g i c a l t i m e , t h i s m e c h a n i s mr e s u l t s i n s o l i d s w h i c h a r e e n r i c h e d i n 23 4Uand 23 Th r e l a t i ve t o : 38U ( i . e . so l u t i on t osol id 234U + 23Th reco i l ga in) .

F o r c r y s t a l l i n e r o c k s , s u c h a s t h e s e d e -s c r i b e d i n t h is p a p e r , t h e U - d e c a y s y s t e mh a s b e e n u s e d a s a s e n s i t i v e i n d i c a t o r o fr e c e n t U m o b i l i t y ( Z i e l in s k i e t a l. , 1 9 8 1 ;S c h w a r c z e t a l., 1 9 8 2 ; R o s h o l t , 1 9 8 3 ; G a s -c o y n e , 1 9 8 4 ; S m e l l i e a n d R o s h o l t , 1 9 8 4 ) .H o w r e c e n t l y t h i s m i g r a t i o n h a s o c c u r r e dis d e t e r m i n e d f r o m t h e h a l f- li fe o f th e n u c l i d ei n v o l v e d . I n t h is p r e s e n t s t u d y t h e n u c li d e s: 3 sU, 23 4U, 23 Th and 226Ra have be en m e a -s u r e d . U s i n g t h e d a u g h t e r / p a r e n t a c t i v it yra t ios ~34U/ :3sU, 23Th/234U and : :6Ra/: 3 T h , r a d i o n u c l i d e m i g r a t i o n o v e r t i m e -s c a l e s o f 1 M y r. , 3 0 0 , 0 0 0 y r . a n d 8 0 0 0 y r . ,r e s p e c t i v e ly, c a n b e e v a l u a t e d .

3 . S a m p l e s e l e c t io n a n d a n a ly s is

D r i l lc o r e l e n g t h s o f 4 0 - - 5 5 c m w e r e c h o s e nf r o m t h e r e s p e c t i v e a r e a s ; t h e s e v a r i ed ind i a m e t e r f r o m 5 6 m m ( S w e d i sh s a m p le )t o 7 6 m m ( S w i ss s a m p l e s) . A t h in r o c ks l ic e w a s r e m o v e d a l o n g t h e d r i l l co r e l e n g t hf o r m i n e r a l o g i c a l a n d f i s s i o n - t ra c k s tu d i e s .A p o r t i o n o f t h e c o r e w a s r e t a i n e d f o r r e f-e r e n c e a n d t h e r e m a i n d e r w a s s a w n ( S w is ss a m p l e s ) o r c l e a v e d ( S w e d i s h s a m p l e ) i n t o

1 - c m t h i c k s li c es f o r a n a l y s is . T h e m i n e r a lc o a t in g s s c r a p e d f r o m t h e i n si d e s u r f a c e o ft h e f r a c t u r e z o n e c o n s t i t u t e d a n o t h e r s a m p l e .

F o r U - s e ri es m e a s u r e m e n t s s a m p l e s o f t h e

p o w d e r e d g r a n i t e w e r e s p i k e d w i t h 2 3 2 Ui n e q u i l i b r i u m w i t h ~ 2 8 T h a n d t h e n d i g e s t e dw i t h a q u a r e gi a a n d h y d r o f l u o r i c a c id an dt h e s m a l l r e s i d u e o f i n s o l u b l e m a t e r i a l w a sf u r t h e r d i g e s t ed w i t h n i tr i c a n d h y d r o f l u o r i cac i ds i n a Pa r r h i gh - p r e s s u r e d i ge s t i onb o m b . T h is p r o c e s s w a s r e p e a t e d i f n e c e s s a r yu n t i l t h e r e s i d u e t o t a l l y d i s s o l v e d i n 6 Mh y d r o c h l o r i c a c id . U a n d T h w e r e r e c o v e r e df r o m h a l f o f th e c o m b i n e d a c id le a ch i n g sb y a n i o n - e x c h a n g e t e c h n i q u e s ( B a c o n a n d

R o s h o l t , 1 9 8 2 ) a n d w e r e t h e n e l e c t r o d e p o s i t -e d o n t o s t a i n l e s s - s t e e l p l a n c h e t t e s f o r a -s p e c t r o m e t r y m e a s u r e m e n t s u s i n g S i s u r f a c eb a r r i e r d e t e c t o r s . T h e p r e c i s i o n o f th e r e s u l t sis b e t t e r t h a n + 4 % f o r th e234U/238Uac t i v i t yr a t i o s , b e t t e r t ha n + 7% f o r t he 23OTh/234Uand 23 Th /~32Th a c t i v i t y r a t i o s and + 15%f o r t he 226Ra/23 Th ac t i v i t y r a t i o .

2 ~ 6 R a a n a l y s e s w e r e i n i t i a l l y a t t e m p t e d o nt h e r e m a i n i n g h a l f o f th e a c i d - l e a c h i n g s o l u -t i o n b y t h e s t a n d a r d t e c h n i q u e o f c o l l e c ti o na n d a - c o u n t i n g o f t h e d a u g h t e r 2 22 Ra ( S c o t ta n d M a c K e n z i e , 1 9 8 4 , 1 9 8 5 ) . S i n c e t h e r e isno sp i ke av a i l ab l e t o a c t a s a y i e l d tr ace r i nt hi s m e t h o d , t h e p r o c e ss d e p e n d s u p o n1 0 0 % t r a n s f e r o f 2 26 Ra t o t h e R n e m a n a t i o nv e s s e l . R e p l i c a t e a n a l y s e s a n d a n a l y s e s o fs a m p l e s t o w h i c h a k n o w n q u a n t i t y o f 2 26 Raw a s a d d e d i n d i c a t e d t h a t t h e a b o v e c o n d i t i o nw a s n o t b e i n g sa t is f ie d a n d t h a t l o ss o f2 26 Ra w a s o c c u r r i n g b e f o r e t r a n s f e r t o t h ee m a n a t i o n v e ss el . N o s a t i s f a c t o r y e x p l an a -


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tion is at present available for this loss butatt empt s at 226Ra analysis by this met hodwere abandoned in favour of direct countingof the 609-keV peak in the T-spectrum of226Ra dec ay products in a sub-sample ofthe untreated powdered granite. This peak,prod uce d by 21+Bi decay, can only be usedif 222Rn does not escape to any significantextent from the sample and initial resultsappear to justify this assumption.

REE and total Fe analyses were carriedout on 0.14 samples using instrumentalneutron activation analysis (INAA). Thisinvolved a 6-hr. irradiation in a thermalneu tro n flux of 3.1012 n cm -2 s -I. Gammacounting using co-axial and planar Ge(Li)detectors was performe d at ~ 3 days and~ 3 weeks afte r irradiation (MacKenzie etal., 1983). Fe 2+ was analysed using standardwet chemical techniques and fission-trackdistributions from polished thin sections ofthe granite were recorded on polycarbonateby thermal neutron irradiation (MacKenzieet al., 1984).

4. Geological setting and mineralogy of thedrillcores

4 .1 . Kr~kem~la d r i l l co re(KI: 317.85--318.40 m)

The Kr~kemiila specimen was selected ata depth o f ~ 318 m fro m one of three testholes drilled in the GStemar granite, locatedalong the southeastern coast of Sweden.The Kriikem~ila area within the GStemargranite has been the focus of geologicalinvestigations as part of an earlier programmeconcerning feasibility studies for the disposalof radioactive waste material in crystallinerocks (Scherman et al., 1977). The geology,geochemistry and geochronology of theGStemar granite has been previously de-scribed by Kresten and Chyssler (1976),Aberg (1978) and Smellie and Stuckless(1985).

The sample location level is characterizedby the coarse-grained granite variety which

is macroscopically homogeneous. The chosencore is traversed by a single open fracturezone at 45 to the vertical drillcore length;from geophysical logging investigations (Mag-nusson and Duran, 1984) the fracture zoneis considered water-bearing. This is partlysupported by fracture plane coatings ofhematite and FeOOH-oxides. In addition,the feldspathic content marginal to thefracture zone shows evidence of alterationextending to 2--3 cm from the fractureplane.

The drillcore is representative of theGStemar coarse-grained granite and is ma-croscopically homoge neous. At greater resolu-tion the rock is mineralogically variable

consisting primarily of quartz {15--45 vol.%),plagioclase (30--42 vol.%) and K-feldspar(30--50 vol.%) with subordinate biotite,muscovite and chlorite. The plagioclase(An < 10%) occurs mos tly as euhedral tosubhedral laths, as a matrix constituent, andincluded in K-feldspar. Accessory mineralsinclude interstitial amounts of magnetite,apatite, zircon, fluorite, sphene and monazite;isolated grains of galena, pyrite, and morerarely molybdenite, also occur. The non-

sulphide accessory phases usually occur asintergranular aggregates sporadically dispersedthrou ghout the rock, resulting in considerableinhomogeneity on the scale of a thin section.

Throughout the length of the drillcoresample the granite is weakly but uniformlyaltered; this is generally typical to the depthsof maximum drilling in the massif (Smellieand Stuckless, 1985). In most sections thepartial breakdown of the feldspars to mus-covite/sericite with minor quartz occurs.Magnetite sporadically shows limited altera-tion to hematite, or partial to completepseudomorphing by chlorite. Biotite is some-times partly altered to chlorite, and mus-covite, normally clear and unaltered, hasoccasionally altered to chlorite and lessfrequently to epidote and opaque phases(mostly Fe--Ti-oxides).

The fractured area occupies a width of1 cm and is composed of a series of paral-


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l e l t o sub - pa r a l l e l m i c r o f i s su r e s ea s i l y d i s t i n -g u i sh e d b y F e O O H - o x i d e c o at in g s . T h ec e n t r a l a n d l a rg e s t f r a c t u r e ( m a x . 1 . 5 m mw i d e ) i s a l s o p a r t l y i n f i l l e d w i t h h e m a t i t ea n d s u b o r d i n a t e c h l o r i t e . M a c r o s c o p i c a l l y,t h e i n f l u e n c e o f t h e f r a c tu r e z o n e e x t e n d su p t o 2 c m i n t o t h e h o s t g r a n it e . T h is i sa p p a r e n t a s F e O O H - o x i d e d u st in g s p r e s e n tb o t h i n t e r s t i t i a l l y w i t h i n t h e r o c k m a t r i xa n d i n c l u d e d w i t h i n a l te r e d to p a r t l y a lt e r e dK : f e l d s p a r p h a s e s ; b i o t i t e a n d m a g n e t i t ea r e a l s o m o r e m a r k e d l y a l t e r e d t o c h l o r i t ea n d h e m a t i t e , r e s p e c t i v e l y.

4.2. B6ttstein drillcore ( B O E : 6 1 8 . 3 4 - -6 1 8 . 7 0 m )

T h e B S t t s t e i n a r e a i s l o c a t e d i n c e n t r a l -n o r t h e r n S w i t z e r l a n d ~ 3 0 k m N W o f Z i ir ic ha n d 5 k m f r o m t h e G e r m a n b o r d e r . In v e s ti g a-t i o n s c a rr ie d o u t b y N A G R A b e g a n i n O c t o -b e r 1 9 8 2 a n d a r e s t il l i n p r o g r e s s . A m a j o rp a r t o f t h i s w o r k h a s i n v o l v e d t h e d r i l l i n go f a d e e p b o r e h o l e t o ~ 1 5 0 0 m : t h e fr a c t u r es a m p l e s e l e c t e d f o r t h i s s t u d y h a s b e e nt a k e n f r o m ~ 6 1 8 m d e p t h . T h e g e o l o g y,g e o c h e m i s t r y a n d g e o c h r o n o l o g y o f t h e

i n t e r c e p t e d g r a n it e , w h i c h f o r m s p a r t o f am u c h w i d e r c o m p l e x , a r e s u m m a r i s e d b yN A G R A ( Te ch n i ca l R e p o r t s : N T B84-25a n d N T B 85-01). E s s e n t ia l ly t w o h y d r o -t h e r m a l e v e n t s a f f e c t e d t h e g r a n it e : a ne a r ly - s ta g e c o e v a l w i t h g r a n it e e m p l a c e m e n tw h i c h o c c u r r e d ~ 3 0 0 M y r. a g o , a n d a l a te r( P e r m i a n ? ) e v e n t w h i c h r e s u l t e d i n a l t e r a -t i o n a l o n g f r a c t u r e a n d f a u l t z o n e s .

D r i ll in g i n t h e a r e a e n c o u n t e r e d t h e c r y s t al -l in e b a s e m e n t a t a d e p t h o f ~ 3 0 0 m . D o w nt o t h e f i n a l d e p t h o f 1 5 0 1 m t h e c o r e c o n -s i s t e d o f t h e B S t t s t e i n g r a n i te . T h i s g r a n i t ei s t y p i c a l l y c o a r s e - g r a i n e d , r i c h i n b i o t i t e ,a n d p o r p h y r i t i c i n t e x t u r e a c c o m p a n i e db y l ar g e p h e n o c r y s t s o f K - f e ld s p a r . T h ec h o s e n d r i l l c o r e s e c t i o n w a s s e l e c t e d f r o ma f r a c tu r e i n t e r se c t i o n w h i c h w a s s u s p e c t e da s b e i n g w a t e r - c o n d u c t i n g . T h e f r a c tu r ew a s l o c a te d n e a r t h e c o n t a c t b e t w e e n t h eg r a n it e a n d t h i n i n t r u d e d h o r i z o n s o f p e g -

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m a t i t e ( u p t o 4 0 c m w i d e ) a n d a p l it e ( u pt o 1 . 5 m w i d e ) ; t h e f r a c t u r e o c c u r s i n t h ep e g m a t i t e o n l y a f e w c e n t i m e t r e s f r o m t h eg r a n it e c o n t a c t . I n t e r s e c t i o n o f t h e f r ac t u r ew i t h t h e c o r e a x i s is 3 7 a n d i ts w a t e r - c o n -d u c t i n g p r o p e r t i e s w e r e i n d i c a t e d b y i tso p e n c h a r a c t e r a n d i d i o m o r p h i c q u a r t zf i ll ings . F l u i d l ogg i ng i den t i f i ed a m a j o rw a t e r i n f l o w b e t w e e n 6 1 8 a n d 6 2 1 m , az o n e w h i c h c o m p r i s e s a p o t e n t i a l l y w a t e r -c o n d u c t i n g k a k i r it e h o r i z o n . I n t e r s e c t i o n b e-t w e e n t h e s e t w o h o r i z o n s a t a s h o r t d i s t an c ef r o m t h e b o r e h o l e i s s u s p e c t e d a s b o t he x h i b i t s im i l ar d i p s c o m b i n e d w i t h s t r ik e sp e r p e n d i c u l a r t o e a c h o t h e r .

T h e s e l e c t e d d r i l l c o r e l e n g t h i n v e s t i g a t e d

c o n s i st s o f ~ 7 c m o f p e g m a t i t e i n c o n t a c tw i t h 1 3 c m o f b i o t i t e - r ic h g r a n i t e w h i c h isr e p r e s e n t a t iv e o f t h e B S t t s t e i n p o r p h y r i t i cg r a n i t e v a r i e t y ; t h e g r a n i te a t t h e p e g m a t i t ec o n t a c t h a s b e e n h y d r o t h e r m a l l y a l t e r e dt o t h e e x t e n t o f ~ 8 c m . I n t e r m s o f a v e ra g em o d a l c o n t e n t ( v o l .% ) , t h e g r a n it e c o m -p r i s e s 2 7 % q u a r t z , 2 6 % p l a g i o c l a s e , 3 8 %K - f e l d s p a r a n d 8 % b i o t i t e . T h e K - f e l d s p a r ,w h i c h i s s e e n a s l a r g e p h e n o c r y s t s i n h a n ds p e c i m e n , i s p e r t h i t i c i n t h i n s e c t i o n a n d

t h e p l a g i o cl a se is n o r m a l l y z o n e d w i t h ac o r e o f o l ig o c l a s e (A n 1 2 - : 0 ) a n d a ri m o fa l b it e . S o m e l a te m a g m a t i c m u s c o v i t e h asf o r m e d f r o m t h e b r e a k d o w n o f K - f e ld s p a ra n d b i o t it e , a n d p s e u d o m o r p h s o f p i n it ea f t e r c o r d ie r i t e a r e c o m m o n . A p a t i t e a n dz i r c o n a r e t h e m o s t f r e q u e n t a c c e s s o r i e sp r e s e n t .

T h e p e g m a t i t e c o n t a i n in g t h e f r a c t u r ez o n e is f r e s h , c o a r s e - g r a i n e d , a n d c o n s i s t so f q u a r t z , K - f e ld s p a r a n d a l b i t e , t o g e t h e rw i t h m u s c o v i t e a n d to u r m a l i n e . R e g a r d -i ng t h e f r a c t u r e f a c e s , t h e d o m i n a n t m i n e r a lp h a s e s a r e c a l c i te , f i n e - g r a in e d q u a r t z a n dc l a y s ( m o s t l y i n t e r l a y e r i n g s o f H l it e a n ds m e c t i t e ) a n d s p o r a d i c a p a t i t e . H y d r o t h e r m a la l t e r a t i o n o f t h e g r a n i t e e x t e n d s f o r ~ 8 c mf r o m t h e p e g m a t i t e c o n t a c t . T h i s i s c h a r a c -t e r i z e d b y s t r o n g a r g iU i s a ti o n w i t h i l l it e /s m e c t i t e a s t h e d o m i n a n t a l t e r a t io n p h a s e s;s o m e c a l c i t e is c o n t a i n e d i n K - f e l d s p a r a n d


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f i n e l y d i s p e r s e d F e O O H - o x i d e s g i v e t h er o c k a r e d d i s h t i n g e . A s e a l e d fi s s u r e ( 0 . 5 - -2 . 0 m m w i d e ) c o n t a i n in g s o m e c a l ci te a n dc l a y m i n e r a l s o c c u r s a t 8 c m a n d t h e r e a f t e rt h e g r a n i t e i s l es s a l t e r e d ( a l t h o u g h t h ef e l d s p a r s a r e s t i l l p i g m e n t e d w i t h F e - o x i d e )a n d t h e b i o t i t e c o n t e n t is h i g h er . T h e g r a n it ec o n t a i n s t w o z o n e s ~ 4 c m w i d e c h a r a c te r -i z e d b y a n in c r e a s e d m a f ic c o n t e n t . O n e o ft h e s e z o n e s c o n s t i t u t e s t h e c o n t a c t a r e aw i t h t h e p e g m a t i t e , a n d t h e o t h e r is l o c a t e d~ 8 c m f r o m t h e c o n t a c t .

4.3. Grimsel drillcore ( F L G : 9 4 . 1 2 - - 9 4 . 5 2 m )

T h e G r i m s e l a r e a i s l o c a t e d n e a r t h e G r i m -

s e l P a s s s i t u a t e d i n c e n t r a l - s o u t h S w i t z e r l a n d~ 1 5 - - 2 0 k m f r o m t h e I ta l ia n b o r d e r . T h es it e, w h i c h n o w c o n s t i tu t e s t h e N A G R Au n d e r g r o u n d t e s t l a b o r a t o r y, w a s i n it ia l lyd e v e l o p e d a s a n u n d e r g r o u n d h y d r o e l e c t r i cp o w e r s c h e m e w h i c h h a d tu n n e l e d t h r o u g ht h e J u c h l i s to c k m o u n t a i n c h ai n . In 1 9 7 9a n e x p l o r a t o r y d r i ll in g p r o g r a m m e w a ss ta r te d b y N A G R A w h i c h w a s c o m p l e t e di n 1 9 8 4 . T h e d r i l l c o r e s a m p l e i n v e s t i g a t e dw a s s e l e c t ed f r o m a n e a r - h o r i z o n t a l e x p l o r a -

t i o n b o r e h o l e ( S B80.001:94.12--94.52 m )d r i l l e d i n t o t h e b e d r o c k f r o m t h e a c c e s st u n n e l t o th e p o w e r s ta t i o n : t h e a l t it u d e o ft h e b o r e h o l e l e ve l is 1 7 3 0 m .

T h e g e o l o g y o f t h e r e g io n is s u m m a r i s e di n r e c e n t N A G R A p u b l i c a t i o n s ( N T B81-07a n d N T B 85-34). T h e A a r m a s s i f o f H e r c y n i a na g e d o m i n a t e s t h e a r e a ; i t is al ig n e d i n aS W- - N E d i r e c ti o n w i t h a m a x i m u m e x t e n s i o no f 1 1 5 k m a n d a w i d t h o f 2 3 k m . T h i s m a s s if ,w h i c h i s t h e l a r g e s t i n t h e S w i s s A l p s , i ss u r r o u n d e d b y m e t a s e d i m e n t s ( m o s t l y s ch is ts )a n d is i ts e lf m a i n l y c o m p o s e d o f p l u t o n i cr o c k s o f g r a n i t ic / g r a n o d i o r it i c c o m p o s i t i o n .T h e s e g r a n i to i d b o d i e s w e r e i n t r u d e d d u r i n gt h e H e r c y n i a n o r o g e n y ( ~ 2 8 0 M y r . ). T h eG a s t e r n g r a n it e w a s p r o b a b l y e a r ly H e r c y n i a n( l o c a t e d t o t h e N W ) a n d t h i s w a s l a t e r f o l -l o w e d b y t h e G r i m s e l g r a n i t e / g r a n o d i o r i t e .T h e t e s t s i t e is l o c a t e d w i t h i n t h e C e n t r a l

A a r g r a n it e w h i c h is t h o u g h t t o b e l a t eH e r c y n i a n . S o o n a f te r e m p l a c e m e n t t h e seg r a n i t o i d s w e r e i n t r u d e d f i r s t b y a p l it icd y k e s , a n d t h e n b y a v a r i e ty o f m a f i c la m p r o -p h y r e s . T h e i n t e r f a ce s b e t w e e n t h e s e la t t e rd y k e s a n d t h e A a r g r a n i t e m a i n l y c o n t r o lt h e g r o u n d w a t e r c o n d u c t i v e s y s t e m i n t h eb e d r o c k ( O h s e , 1 9 8 3 ) .

T h e g r a n i t e is t y p i c a l l y g r e y - c o l o u r e d ,f r e sh , m i n e r a lo g i c a ll y h o m o g e n e o u s i n h a n ds p e c i m e n , a n d m e d i u m - t o c o a r s e - g r a i n e di n t ex t u r e ; t he g r an i t e i s a l so cha r ac t e r i s t i c a l -l y g n e i s s o s e . T h e c h o s e n f r a c t u r e i s f r e s ha n d i n t e r s e c t s t h e c o r e a x i s a t 4 0 ; n o a l t e r a -t i o n o n a m a c r o - s c a le w a s o b s e r v e d a n dt h e r e w a s n o e v i d e n c e o f a n y f r a c t u r e f il li n g

m i n e ra l s. I ts w a t e r - c o n d u c t i n g p r o p e r t i e s a r es u s p e c t e d o n t h e b a s is o f a l a rg e r e l ea s e o fg r o u n d w a t e r e n c o u n t e r e d w h i l e p e n e t r a t i n gt he f r ac t u r e zon e du r i ng d r i ll i ng . Th i s in i ti a l-l y r e s u l t e d i n a f l o w o f ~ 2 4 1 m i n . - 1, w h i c hs u b s e q u e n t l y d e c r e a s e d t o a s t e a d y r at ea r o u n d 2 1 m i n . -1 . H o w e v e r , t h e s e l e c t e df r a c t u r e w a s o n l y o n e o f se v e ra l p r e s e n to v e r a s h o r t d i s t an c e ( f r e q u e n c y o f f r a c tu r -i n g in t h i s p a r t o f t h e c o r e i s 1 0 f r a c t u r e sp e r m e t r e ) . T h e a g e o f t h e s e f r a c t u r e s is

u n c e r t a i n , a t l e a s t l a t e A l p i n e t o R e c e n t ;t h e a r e a i s s ti ll c o n s i d e r e d t e c t o n i c a l l y u n -s ta b le a n d t h e c o n t i n u o u s f o r m a t i o n o fn e w f r a c t u r e s / jo i n t s c a n n o t b e e x c l u d e d .

P e t r o g r a p h i c a l a n d m i n e r a l o g i c a l d e s c r i p -t i o n s o f t h e G r i m s e l a r e a a r e g i v e n b y S t a l d e r( 1 9 6 4 ) . T h e a v e r a g e m o d a l c o n t e n t o f t h eg r a n i te c o r e ( e x p r e s s e d i n vo l .% ) i s 3 3 %q u a r t z , 3 4 % K - f e l d s p a r , 2 1 % p l a g i o c la s e ,7 % b i o t it e / c h l o r i t e , 1 - - 2 % m u s c o v i t e , 1 %h o r n b l e n d e a n d 2 % e p i d o t e . T h e a s s o c i at e da c c e s s o r y p h a s e s a r e s p h e n e , o r t h i t e , a p a t i t e ,z i r c o n , c a l c i t e a n d o p a q u e s . T h e f e l d s p a r sh a v e u n d e r g o n e s o m e a l t e r a t io n t o s e r ic i te ,a n d t h e b i o t i t e t o c h l o r i te ; m a g n e t i t e isp a r t l y o x i d i s ed . T h e s e a l t e r a ti o n s a r e p r o d u c t so f t h e c o m p l e x m e t a m o r p h i c h i s to r y o f th ea r e a a n d s h o u l d n o t b e c o n s i d e r e d a s d u e t ol o w - t e m p e r a t u r e r o c k - - w a t e r r e a c t i o n s d u r i n gr e c e n t g e o l o g i c a l t i m e s .


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5 . C h e m i s t r y o f t h e d r i l lc o r e s

5 .1 . Krakemala dr i l l c or e

T h e d r i l l c o r e c o n t e n t s a n d d i s t r i b u t i o n so f U a n d i ts d a u g h t e r d e c a y p r o d u c t s , F ea n d t w o R E E ' s , L a a n d C e , a r e l i st e d inTa b l e I a n d i l l u s t ra t e d i n F i g . 1 . O t h e r e l e -m e n t s a n a l y s e d b u t n o t i l l u s t r a t e d i n c l u d eN a , K , S c , R b , C s , Ta a n d t h e r e m a i n i n gR E E ' s .

T h e U c o n t e n t s r a n g e f r o m 1 2 . 5 1 t o2 3 . 2 6 p p m ( m e a n 1 5 . 6 7 p p m ) a n d t h e T hf r o m 4 6 . 3 7 t o 8 9 . 4 2 p p m ( m e a n 6 7 . 0 4p p m ) ; t h e m e a n T h / U r a t i o is 4 . 3 8 . T h e s eU a n d T h v a l u e s , w h i c h a r e r o u g h l y i n a g r e e -

m e n t w i t h t h e a v e ra g e v a l u es f r o m t h e g r a n it ea s a w h o l e ( S m e l l ie a n d S t u c k l e s s , 1 9 8 5 ) ,a r e s t i l l a n o m a l o u s l y h i g h w h e n c o m p a r e dt o e i t h e r t h e a v e r a g e c o n t e n t s f o r w o r l d -w i d e g r a n i t es { R o g e r s a n d A d a m s , 1 9 6 9 a , b )o r a c o m p i l a t io n o f 2 5 0 0 g r a n it e s m a i n l yf r o m t h e U . S . A . ( S t u c k l e s s a n d V a n Tr u m p ,1 9 8 2 ) . F u r t h e r m o r e , i n c r u s ta l r o c k s t h ea v e ra g e T h / U r a t io is 3 .5 ( R o g e r s a n d A d a m s ,1 9 6 9 a , b ) , i. e. T h c o n c e n t r a t i o n s u s u a l lyc o r r e l a te w i t h U b y a f a c t o r o f 3 . T h u s t h e

h ig h T h / U r a ti o r e c o r d e d f r o m t h e G S t e m a rg r a n i t e w o u l d s u g g e s t a g e n e r a l r e m o v a l o fU , w h i c h h a s a l so b e e n d e m o n s t r a t e d i s o t o p -i c a l l y b y S m e l l i e a n d S t u c k l e s s { 1 9 8 5 ) .

F r o m f i s si o n - tr a c k s t u d i e s o f t h e G 5 t e m a rg r a n i t e ( K r e s t e n a n d C h y s s l e r , 1 9 7 6 ; S m e l l i ea n d S t u c k l e s s, 1 9 8 5 ) m o s t o f th e U is a s-s o c i a t e d w i t h z i r c o n a n d m o n a z i t e w i t hs u b o r d i n a t e a m o u n t s c o n t a i n e d i n s p h e n e ,m a g n e t i t e , h e m a t i t e , a n d i l m e n o r u t i l e - ty p ep h a s e s . S m a l l a m o u n t s a r e a l s o a s s o c i a t e dw i t h s e r i ci te , e p i d o t e a n d F e O O H - o x i d ed i s p e r s i o n s . A l t h o u g h n o d a t a ar e a v a i l ab l ef o r m i n o r T h , t h e d i s tr i b u t i o n o f m o n a z i t e ,w h i c h c a n c o n t a i n u p t o 1 0 w t . % T h an dp e r h a p s a s h i g h a s 2 6 w t . % T h ( F r o n d e le t a l . , 1 9 6 7 ) , a l m o s t c e r t a i n l y c o n t r o l s t h eT h d i s t r i b u t i o n s i n th e G S t e m a r g r a n it e .

T h e d i s t r i b u t i o n o f U a n d T h a l o n g t h ed r i ll c o r e f r o m t h e f r a c t u r e z o n e i s s h o w n inF i g. 1 . A p a r t f r o m t h e f r a c t u r e f a c e w h i c h

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i n d i c a t e s a n i n c r e a s e , t h e U c o n t e n t i s f a i r lyu n i f o r m f o r a d i s ta n c e o f ~ 8 c m i n t o t h eg r a n it e u n t il a b r o a d p e a k o c c u r s o v e r al e n g t h o f 4 c m . I n c o m p a r i s o n , T h i n d i c a t e sa n a n t i p a t h e t i c d e c r e a s e a t t h e f r a c t u r ef a c e , a n d t h e n s h o w s a n e x t r e m e l y i r r eg u l arf l u c t u a t i o n u n t il i t t o o f o r m s a b r o a d - p e a ks y m p a t h e t i c t o t h a t o b s e r v e d f o r U .

O n t h e b a si s o f m i n e r a l o g y, t h e k n o w ng e o c h e m i c a l b e h a v i o u r o f U a n d T h , a n d t h es i m i la r it y o f t h e T h p r o f i le w i t h th o s e o ft h e R E E ' s { d i s c u s se d o n p . 2 4 3 ) , t h e s e p r o f i l e ss u g g e s t t h a t T h h a s r e m a i n e d r e l a t iv e l yi m m o b i l e t h r o u g h o u t t h e h i s to r y o f t h eg r a n i t e , a n d i t s i r r e g u l a r d i s t r i b u t i o n t h u sr e f l e c t s t h e i r r e g u l ar d i s t r i b u t i o n o f t h e

d o m i n a n t T h - b e a r in g m i n e ra l p h a s e , m o n -a z i t e , w h i c h i s o f p r i m a r y o r i g in . A s t h ef i r s t s a m p l e r e p r e s e n t s f r a c t u r e - s u r f a c e s c r a p -i n g s o f m o s t l y h e m a t i t e , c h l o r i t e , c l a y m i n -e ra l a n d F e O O H - o x i d e s , t h e lo w e r T h v a lu er e c o r d e d h e r e i s p e r h a p s n o t s o s u r p r is i n g .Wi t h r e g a r d s t o U , w h i c h i n c o m p a r i s o n i se a s i l y m o b i l i s e d a n d l e a c h e d f r o m g r a n i t ep r o v i d e d t h e s y s t e m h a s b e e n o p e n t o f l u idm o v e m e n t , F i g . 1 s h o w s s o m e i n t er e s ti n gf e a t u r e s . I f t h e g r a n i t e h a d r e m a i n e d a c l o s e d

c h e m i c a l s y s t e m s i n c e c r y s t a l l i s a t i o n , t h e nm o s t o f t h e U w o u l d b e a s so c i at e d w i t hp r i m a r y r e s i s t a n t r a d i o a c t i v e m i n e r a l s ( o f t e nt o g e t h e r w i th T h a n d R E E ) w i t h m o r e m i n o ra m o u n t s a l o n g in t e r g r a n u la r b o u n d a r i e s (Ti e he t a l ., 1 9 8 0 ) . F o r t h e c as e i n q u e s t i o n t h e r e -f o r e , t h e d i s t r i b u t i o n o f m a j o r U w o u l d b ee x p e c t e d t o b e s y m p a t h e t i c t o T h , w h i c hi n tu r n w o u l d b e a r e f l e c ti o n o f t h e m o n -a z it e c o n t e n t i n t h e r o c k , a s s u g g e s te d a b o v e .I n f a c t t h e r e i s a s y m p a t h e t i c i n c r e a s e in Ua n d T h f r o m ~ 8 - - 1 3 c m a lo n g t h e d r i ll c o r ew h i c h s h o w s t h a t t h is p a r t o f t h e r o c k h a sp r o b a b l y r e t a i n e d i ts o r ig i na l U a n d T hs i g n a t u r e , a l t h o u g h t h e i n t e r g r a n u l a r l a b il eU h a s a lm o s t c e r t a i n l y b e e n r e m o v e d . I nc o n t r a s t , t h e a b s e n c e o f a n y s y m p a t h e t i cU - - T h r e l a t i o n s h i p w i t h i n t h e i n i t ia l 8 c mo f t h e d r i l lc o r e i n d i c a t e s t h a t U h a s a t s o m es ta g e b e e n l e a c h e d f r o m e v e n th e m o r er e s i s t a n t r a d i o a c t i v e m i n e r a l p h a s e s p o s -


8/22

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KIA-1 su r facescrap ings 20 .13 0 .54 46,37 2 .07 2 .3 14.8 0 .4 33.9 +

KIA-2 1 12.51 0 .41 57.96 1 .24 4 .6 9 .2 0 .3 16.7 KIA-3 2 13.19 0 .27 74.52 2 .07 5 .6 9 .7 0 .2 10.9 KIA-4 3 12.78 +_0.41 65.8 3 0 .83 5 .2 9 .4 0 .3 9 .0 KIA-5 4 12.92 0 .41 84.4 6 + 2 .07 6 .5 9 .5 0 .3 8 .5 KIA-6 5 13.46 0 .41 61.69 1 .24 4 .6 9 .9 0 .3 8 .7 KIA- 7 6 13.46 0 .27 64.17 1 .24 4 .8 9 .9 0 .2 8 .6 KIA-8 7 12.65 0 .27 69.13 1 .24 5 .5 9 .3 + 0 .2 7 .9 KIA-9 8 14.55 0 .27 48.02 1 .24 3 .3 10.7 0 .2 9 .4 KIA- 10 9 14.42 + 0 .27 69.97 + 1 .24 4 .9 10.6 0 .2 8 .9

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* D e t e r m i n e d b y r a d i o i s o t o p e d i l u ti o n a n d a - s p e c t r o m e t r y.

0 .8 2 .29 0 .51 31.9 1 .0 0 .94 0 .060.3 1 .81 0 .05 14.8 0 .3 0 .89 0 .050.2 1 .12 0 .02 9 .3 0 .3 0 .85 0 .050.3 0 .96 0 .03 9 .1 0 .2 1 .01 0 .050.3 0 .89 0 .04 8 .5 0 .3 1 .00 0 .060.3 0 .88 0 .03 8 .6 0 .2 0 .99 0 .060.2 0 .87 0 .03 8 .5 0 .2 0 .99 0 .050.2 0 .85 0 .03 7 .8 0 .3 0 .99 0 .070.2 0 .88 0 .02 9 .8 0 .3 1 .04 0 .050.2 0 .84 0 .02 9 .3 0 .2 1 .04 0 .050.3 0 .83 0 .03 -- --0 .2 0 .80 0 .02 -- --0 .5 0 .86 0 .03 16.1 0 .6 1 .10 0 .070.4 0 .88 0 .03 11.4 0 .3 0 .91 0 .060.3 0 .82 0 .03 8 .8 0 .2 1 .04 0 .060.2 0 .87 0 .02 11.4 0 .4 0 .94 0 .06

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9 .7 1 .5 1 .0 4 + 0 .1 6 1 8 .0 0 .57 .8 + 1 .2 0 .8 6 0 .1 3 1 5 .9 0 .2

10.4 + 1 .6 1 .22 + 0 .18 20.4 0 .58 .6 1 .3 1 .0 0 0.1 5 1 4 .9 0 .36 .8 1 .0 0 .8 0 0 .1 2 1 5 .5 0 .37 .5 1 .1 0 .9 6 0 .1 4 1 6 .7 + 0 .37 .6 1 .1 0 . 7 8 0 .1 2 11 .6 + 0 .3

8 .6 1 .3 0 . 9 2 0 .1 4 1 6 .9 0 .3

9.6 1.41 0 .8 + 1 .6 0 .6 7 0 .1 0 2 1 .6 + 0 .6

9 .2 1 .4 0 .8 1 0.1 2 2 0 .1 0 .58.7 1 .3 0 .99 0 .15 12.4+- 0 .29.2 1 .4 0 .81 0 .1 2 17.5 + 0 .6


9/22

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Fig. 1. Distribution of uranium and its daughter decay products, thorium, REE (La and Ce), total iron and ironoxide ratios characteristic of the K1 drillcore from Kr~kem~da, Sweden.


10/22

242

s ib l y d u r in g s o m e h y d r o t h e r m a l e v e n t,t h e s u g g e s t io n o f s u c h a p r o c e s s b e in g s up -p o r t e d m i n e r a l o g i c a l l y b y t h e p a r ti a l a l te r a -t i o n o f m o n a z i t e a l o n g t h is p a r t o f t h e c o r e .H o w e v e r , U r e m o v a l is n o t i n d i c a te d f r o mt h e f r a c t u r e s u r f a c e s a m p l e w h i c h in s t e a ds h o w s a m a r k e d i n c r e a s e . T h i s su g g e s t s t h a ti n t h e g r o u n d w a t e r - - fr a c t u r e s y s t e m Ua c c u m u l a t i o n r a t h e r t h a n l e ac h i n g is t a k i n gp l a c e .

S i m i l a r m a r g i n a l i n c r e a s e s i n U h a v e b e e nr e p o r t e d i n a l t e re d a n d f r a c t u r e d s a m p l e so f g r a n it e f r o m t h e E y e - - D a s h w a L a k e sp l u t o n , O n t a r i o , C a n a d a ( K a m i n e n i , 1 9 8 6 ,i n t h i s s p e c i a l i s su e ) , w h i c h h a s b e e n i n t e r-p r e t e d a s p r e f e r e n t i a l s o r p t i o n o r c o p r e c i p i ta -

t i o n o f U w i t h h e m a t i t e .

5.1 .1 . Uranium-series d isequi l ibr ium.P l o t s o ft h e ac t i v i t y r a t i o s 234U/: 38U, : 3 Th /234U and: 26Ra / 23 Th a r e p r e sen t ed i n F i g . 1 . The234U/ 238U r a t i o s i nd i ca t e gene r a l d i s equ i l i b r i -u m w i t h i n a 2 a a n a l y t i c a l e r r o r , w i t h v a l u e sr a ng i ng f r o m a m a x i m u m o f 2 . 2 9 a t t h ef r a c t u r e s u r f a c e t o a m e a n v a l u e o f 0 . 8 6f o r t h e m a j o r l e n g t h o f t h e d r i l l c o r e . I nco n t r a s t , m os t o f t he 23Th /234U and 22~Ra/

23 Th r a t i o s l i e wi t h i n 2a o f s ecu l a r equ i l i b -r i u m .C o n s i d e r i n g t h e m a j o r l e n g t h o f th e d r il l-

c o r e ( i. e. e x c l u d i n g t h e f r a c t u r e e d g e ) t h ei s o t o p e d a t a s h o w d e p l e t i o n s o f b o t h : 34 Uan d 23 Th r e l a t i ve t o 23sU. Th i s i n d i ca t e s as i g n i f i c a n t : 3 4U l o s s ( m o s t l i k e l y b y r e c o i lp r o c e s s e s ) w i t h l i t t l e : 3 8 U l o s s , f o l l o w e db y a re - e q u i li b r a ti o n o f t h e : 3 T h d a u g h t e rw i t h :3 4U . I n o t h e r w o r d s , t h r o u g h o u t t h eg r a n i te r e p r e s e n t e d b y t h e s e s a m p l e s , th el o ss o f 2 34 U ( p r o b a b l y c o n t i n u o u s ) h a s b e e no c c u r r i n g o v e r a t l e a s t 1 M y r. b y s o l i d - t o -s o l u t i o n a - r e c o il p r o c e s s e s .

I n s t r o n g c o n t r a s t , t h e g r a n i t e m a r g i n a lt o t h e f r a c t u r e z o n e is c h a r a c t e r iz e d b ye n r i c h m e n t s o f a ll f o u r i s o t o p e s ( Ta b le I) ;t h e s e f a ll o f f r a p i d l y o v e r a d i s t a n c e o f1 - - 3 c m i n t o t h e g r a n it e . T h e234U/238Ur a t io s i n d i c a t e a n a c c u m u l a t i o n o f 2 34 Uw h i c h m u s t b e o c c u r r i n g s u f f i c i e n t l y r a p i d l y

t o p r e v e n t r e - e q u i l i b r a t i o n o f th e 23 T hd a u g h t e r w i t h : 34 U ; t h is w o u l d a c c o u n t fo rt h e o v e r a l l i s o t o p i c d i s e q u i l i b r i u m o b s e r v e db e t w e e n : 3 4 U a n d 2 3 T h i n t h e t h r e e s a m p l e sc o m p r i s i n g t h e m a r g i n al z o n e t o t h e f r a c t u r e .

T h e q u e s t i o n n o w a r is e s a s t o w h e t h e rt h e s e o b s e r v e d i s o t o p i c v a l u e s a t t h e f r a c -t u r e - g r o u n d w a t e r i n te r f ac e z o n e a re d u et o c o n t i n u o u s i s o t o p i c m o v e m e n t f r o mt h e g r a n i t e t o t h e i n t e r f a c e z o n e , f o l l o w e db y s u b s e q u e n t so r p t i o n a n d / o r i s o t o p icf r a c t i o n a t i o n o n t o t h e f r a c t u r e m i n e r a lso r , i f t h e s e i s o t o p i c p r o d u c t s h a v e b e e ni n t r o d u c e d f r o m t h e g r o u n d w a t e r s t o t h ef r a c t u r e f a c e b y d i r e c t s o r p t i o n o r t h r o u g hd a u g h t e r e m p l a c e m e n t p r o c e s s e s ( i. e. s o l u-

t i on - t o - so l i d : 34U + 23 Th r eco i l ga i n ) a ss u g g e s t e d b y R o s h o l t ( 1 9 8 3 ) . U n f o r t u n a t e l y,i t i s n o t p o s s i b l e t o s a y i f o n e o r a c o m b i n a -t i o n o f t h e s e t w o p r o c e s s e s is r e s p o n s i b l e .I n a n y c a se i t u n d e r l in e s t h e i m p o r t a n c e o ft h e p h y s i c o - c h e m i c a l p r o p e r t i e s o f t h e f ra c -t u r e m i n e r a l s i n r a d i o n u c l i d e r e t a r d a t i o np r o c e s s es . T h e w o r k o f A i r e y ( 1 9 8 6 , in t h isspec i a l i s sue ) c l ea r l y i l l u s t r a t e s t ha t t hes u b s e q u e n t i m m o b i l i ty o f th e r a d i o n u cl id e sa f t e r u p - t a k e f r o m s o l u t i o n w i ll l a r g e ly

d e p e n d o n t h e i s o t o p i c f r a c t i o n a t i o n b e -t w e e n t h e f r a c t u r e z o n e m i n er a ls . I t w a ss h o w n t h a t U a n d T h t e n d t o c o n c e n t r a t ep r i n c i p a l l y o n t h e F e p h a s e s , w h i l s t R as e e m s t o b e m o r e o f t e n i n a s s o c i a ti o n w i t ht h e c l a y - - q u a r t z f r a c t i o n w h e r e i t s a c t iv i t ye x c e e d s t h a t o f it s p a r e n t T h . F u r t h e r m o r e ,r a d i o n u c l i d e s t a k e n u p b y c r y s t a l l i n e F ep h a s e s s u c h a s g o e t h i t e a r e b e l i e v e d t o b ei n a cc e s s ib l e t o r e m o v a l b y s o l u t io n . W o r kb y K a m i n e n i ( 1 9 8 6 , i n th i s s p e c i a l i s su e )a ls o r e c o r d s t h e i m p o r t a n c e o f g o e t h i te t oe x p l a in e n r i c h m e n t s o f T h a l o n g f r a c t u r e dz o n e s l o c a t e d i n t h e E y e - - D a s h a p l u t o n ,O n t a r i o .

U n f o r t u n a t e l y, f r o m K r h k e m l i l a t h e r e i sn o i s o t o p i c a n a l y si s o f th e g r o u n d w a t e ri n c o n t a c t w i t h t h e fr a c t u r e z o n e t h a t m i g h ti n d i c a te w h i c h U d a u g h t e r s a r e b e i n g r e m o v e df r o m o r a d d e d t o t h e s ol id fr a c t u r e s u r fa c e s.G r o u n d w a t e r s a t t h e d e p t h o f t h e d r il lc o r e


11/22

s a m p l e ( ~ 3 1 8 m ) s h o u l d b e r e d u c i n g inc h a r a c t e r. A c c o r d i n g t o s e c o n d a r y U a c -c u m u l a t i o n p r o c e ss e s a s d i sc u s se d b y O g m o n de t a l. ( 1 9 8 3 ) , t h e i s o t o p i c d a t a f r o m t h ef r a c t u r e - - g r o u n d w a t e r i n t e r f a c e s a m p l e in -d i c a t e s a p o s i t i o n f u r t h e s t d o w n , f l o w w h e r e -u p o n t h e g r o u n d w a t e r is s tr o n g ly r e d u c i n ga n d 2 3 4 U e x c e e d s 2 3 8 U , a s d o e s 2 3 T h , it si n - g ro w n d a u g h t e r . I t w o u l d a p p e a r r ea s o n -a b l e t o a s s u m e , t h e r e f o r e , t h a t ~ th e g r o u n d -w a t e r s a t a d e p t h o f ~ 3 5 0 - - 4 0 0 m a t K r h k e-m ~ l a a r e c h a r a c t e r i z e d b y a n e x c e s s o f 2 34 Ua n d a r e s t r o n g l y r e d u c i n g . A l o n g t h e f r a c t u r es u r f a c e s s o r p t i o n a n d / o r c o p r e c i p i t a t i o n o f2 34 U w i t h F e - o x i d e s i s t a k i n g p l a c e ; s o l u t i o n -t o - so l i d 234U + 23 Th r ec o i l ga i n p r o ces se s

m a y a l so b e c o n t r i b u t i n g . T h e c o n t i n u o u sl o s s o f 2 3 4 U s u g g e s t e d f r o m t h e o v e r a l l l e n g t ho f t h e d r i l lc o r e i n d i c a t e s t h a t t h e r e i s a l sos o m e r a d io n u c l i d e m o v e m e n t f r o m t h ef r a c t u r e e d g e t o t h e g r o u n d w a t e r . T h e d e g r e eo f r a d i o n u c l i d e r e t a r d a t i o n a n d r e t e n t i o nf r o m b o t h p r o c e ss e s w ill b e d e p e n d e n t o nt h e p h y s i c o - c h e m i c a l p r o p e r t i e s o f t h e fr a c-t u r e z o n e m i n e r a l s , w h i c h c o n s i s t m o s t l yo f h e m a t i t e, c h l or i te , F e O O H - o x i d e s an dc l a y m i n e r a l s .

5.1.2. Iron oxidation states. Fig . 1 i l lus t ra test h e d i s t r i b u t i o n o f F e a n d t h e v a r i a ti o n o ft h e F e 2 / ( F e 2 + F e 3 ) o x i d a t i o n r a t i o a l o n gt h e d r i l lc o r e . F e s h o w s a s l ig h t l y i r re g u l a rb u t d i s t i n c t i n c re a s e t o w a r d s t h e f r a c t u r ef a c e ; th i s i s a p p a r e n t m i n e r a l o g i c a l l y b y a ni n c r ea s e o f h y d r o x y F e - o x i d e d i s p er s io n s ,t h r o u g h o u t t h e r o c k , to g e t h e r w i t h t h ep r e s e n c e o f c h l o r i te a n d h e m a t i t e a t a n dn e a r t h e f r a c t u r e f a c e . T h e s im i l a r i t y o ft h e F e d i s t r i b u t i o n p a t t e r n , e x c l u d i n g t h ef r a c t u r e f a c e , w i t h t h a t o f T h , i s d u e t o t h ep r e s e n c e o f m a g n e t i t e g r ai n s w h i c h o f t e na c c o m p a n y t h e m o n a z i t e a g g re g a te s in t h eg r an i t e .

A n a t t e m p t t o a s s es s t h e e x t e n t o f a n yo x i d a t i o n e f f e c t s i n t h e r o c k w a s c r u d e l ym a d e u s i n g w h o l e - r o c k F e 2 a n d F e 3* a n a l-y s i s ; t h e d e g r e e o f o x i d a t i o n is e x p r e s s e d a st h e r a t i o F e 2 + / ( F e : + F e 3 ). To w a r d s t h e

243

f r a c t u r e f a c e t h e r e i s s o m e s u g g e s t i o n o fi n c r e as e d o x i d a t i o n .

5.1.3. Rare-earth ( R E E ) distributions. P l o t s o fL a a n d C e ( w h i c h i l l u s t r a t e t h o s e t r e n d sc o m m o n t o a ll t h e R E E ' s a n d Ta ) a r e p r e s e n t -e d i n F i g . 1 . F r o m t h e g e n e r a l s i m i l a r i t y o ft h e e l e m e n t d i s t r i b u t i o n p a t t e r n s w i t h i nt h e d r i ll c o re , a n d i n c o m p a r i s o n w i t h T h( F i g . 1 ) , i t c a n b e c o n c l u d e d t h a t n o o b v i o u sm o b i l i s a t io n o f t h e s e e l e m e n t s h a s t a k e np l a c e , a n d t h a t t h e i r d i s t r i b u t i o n i s d o m i n a n t -l y a r e f l e c ti o n o f t h e o c c u r r e n c e o f m o n -az i t e i n t he g r an i t e .

5.1.4. Minor uranium distributions. T h e

d i s t r i b u t i o n o f m i n o r U {< 1 p p m ) h a s b e e ni n v e s t i g a t e d b y f i s s i o n - t r a c k a n a l y s i s . T h er e s u lt s s h o w t h a t e v e n t h o u g h t h e r e is ab a c k g r o u n d d i s p e r si o n o f m a t r i x U (i .e . Up r e s e n t w i t h i n a n d i n t e r s ti t ia l t o t h e m a i nr o c k - f o r m i n g m i n e r al s) t h r o u g h o u t th e t o t a ll e n g t h o f t h e c o r e e x a m i n e d , t h e r e i s a s ig n if i-c a n t i n c r e a s e i n d e n s i t y a n d p e n e t r a t i o nw i t h i n t h e - 3 - c m w i d e m a r g i n a d j a c e n t t ot h e f r a c t u r e f a c e; t h is c o r r e s p o n d s t o t h eo b s e r v e d u r a n i u m / F e - o x i d e i n c r e a s e d e s c r i b e d

a b o v e . T h e i m p r e g n a t i o n p a t t e r n c o r r e s p o n d st o t h e d i sp e r si o n o f F e O O H - o x i d e s a n dm i n e r a l b r e a k d o w n p r o d u c t s s u c h as s e ri c it e,w h i c h in t u r n c o r r e s p o n d s t o t h e d e g re e o fa l t e r a t io n . A l t e r a t i o n i s p e r v a s i v e b u t i r-r e g u l a r i n e x t e n t d u e t o m i c r o s t r u c t u r a lc o n t r o l s o n p e n e t r a t i o n , w h i c h p r o g r e s se sa l o n g s t r u c t u r a l w e a k n e s s e s s u c h a s f i ss u r e s,i n t e r g r a n u l a r b o u n d a r i e s , a n d c r y s t a l l o g r a p h i cp l a n e s ( m o s t l y a f f e c t i n g t h e f e l d s p a r s ).

5.2. B6ttstein drillcore

T h e d i s t r ib u t i o n s o f U a n d i ts d a u g h t e rd e c a y p r o d u c t s , F e an d t h e t w o R E E ' s L aand Ce , a r e li s t ed i n Tab l e I I and i l l u s t r a t edi n F i g . 2 .

A l t h o u g h n o t p r e s e n te d , h i g h er a m o u n t so f N a a n d l o w e r a m o u n t s o f K a n d R b su p -p o r t t h e m i n e r a l o g i c a l o b s e r v a t i o n s a n dh e l p t o d i st in g u i s h t h e p e g m a t i t e f r o m t h e


12/22

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I s o t o p i c a n d c h e m i c a l a n a l y s e s o f u r a n i u m a n d t h o r i u m f r o m d r i l l c o r e B O E ( 6 1 8 . 3 4 - - 6 1 8 .7 0 m ) , B o t t s te i n , S w i t z e r l a n d

S p e c i m e n D i s t a n c e f r o m U ( * l ) T h ( . 1 ) T h / U 3 3S U 2 S 4U 2 3 4 U/ 2 3S U 2 3 T h 2 3 T h / 23 4 U 2 2 S R a 2 2 6 R a / Z 3 T h 2 3 2 T h

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B O E - 3 3 9 6 s u r f a c es c r a p i n g s * ~ 2 . 4 2 0 . 9 9 0 . 4 1 . 7 8 1 . 8 4 1 . 0 4 0 . 0 4 1 . 7 8

B O E - 1 f r a c t u r e . 3 4 . 2 0 0 . 1 1 0 . 9 5 0 . 1 2 0 . 2 3 . 0 9 0 . 0 8 3 . 1 7 0 . 0 8 1 . 0 3 0 . 0 4 3 . 0 0 0 . 1 2s l i c e 3 . 9 8 0 . 1 1 0 . 7 4 0 . 2 5 0 . 2 2 . 9 3 0 . 0 8 2 . 6 5 0 . 0 8 0 . 9 0 0 . 0 5 2 . 8 2 0 . 1 1

B O E - 2 1 . 5 * 3 3 . 4 1 0 . 0 8 3 . 2 7 0 . 1 2 1 . 0 2 . 5 1 0 . 0 5 2 . 6 0 0 . 0 5 1 . 0 4 0 . 0 3 2 . 6 5 0 . 0 5B O E - 3 3 . 0 * 3 2 . 6 8 0 . 0 5 7 . 3 2 0 . 2 9 2 . 7 1 . 9 7 0 . 0 4 2 . 0 1 -+ 0 . 0 4 1 . 0 2 0 . 0 3 2 . 2 3 0 . 0 8B O E - 4 5 . 0 * 3 2 . 7 1 0 . 0 4 1 2 . 5 8 0 . 2 9 4 . 6 1 . 9 9 + 0 . 0 3 2 . 0 4 0 . 0 3 1 . 0 2 0 . 0 4 2 . 2 3 0 . 0 6B O E - 5 6 . 5 * 4 5 . 7 3 0 . 0 8 3 4 . 6 9 0 . 8 7 6 . 1 4 . 2 1 0 . 0 6 4 . 1 5 0 . 0 6 0 . 9 9 -+ 0 . 0 2 4 . 6 3 + 0 . 1 4B O E - 6 8 . 0 * 4 3 . 5 9 0 . 0 7 2 3 . 2 7 0 . 5 8 6 . 5 2 . 6 4 -+ 0 . 0 5 2 . 6 5 0 . 0 5 1 . 0 0 0 . 0 3 2 . 8 9 0 . 0 8B O E - 7 1 0 . 0 * 4 3 . 6 0 0 . 1 0 2 4 . 6 3 0 . 7 5 6 . 8 2 . 6 5 0 . 0 7 2 . 6 7 0 . 0 7 1 . 0 1 0 . 0 4 3 . 0 1 0 . 1 2B O E - 8 1 2 . 1 * 4 3 . 6 0 + 0 . 1 0 2 1 . 4 4 0 . 6 2 6 . 0 2 . 6 5 0 . 0 7 2 . 6 4 0 . 0 7 1 . 0 0 0 . 0 4 2 . 5 2 0 . 1 0

3 . 4 0 0 . 1 4 2 3 . 5 2 0 . 7 0 6 . 9 2 . 5 0 0 . 1 0 2 . 4 7 0 . 1 0 0 . 9 9 0 . 0 5 2 . 7 8 0 . 0 8B O E - 9 1 3 . 9 * 4 3 . 5 5 + 0 . 0 7 2 5 . 5 0 0 . 7 5 7 . 2 2 . 6 1 0 . 0 5 2 . 6 3 0 . 0 5 1 . 0 1 0 . 0 3 2 . 9 9 0 . 1 2B O E - 1 0 1 5 . 6 * 4 3 . 7 9 0 . 0 5 2 2 . 2 7 0 . 4 1 5 . 9 2 . 7 9 0 . 0 4 2 . 8 2 0 . 0 4 1 . 0 1 + 0 . 0 3 3 . 1 3 0 . 1 0B O E - 1 1 1 7 . 6 * 4 3 . 3 7 0 . 0 7 2 5 . 3 0 0 . 5 0 7 . 5 2 . 4 8 0 . 0 5 2 . 4 6 0 . 0 5 0 . 9 9 0 . 0 3 2 . 7 0 0 . 0 7B O E - 1 2 2 0 . 0 * 4 4 . 0 9 0 . 0 8 2 4 . 1 8 0 . 5 8 5 . 9 3 . 0 1 0 . 0 6 2 . 9 4 0 . 0 6 0 . 9 8 0 . 0 3 3 . 2 9 0 . 1 0

0 . 9 7 0 . 0 4 - - - - 0 . 2 40 . 9 5 _+ 0 . 0 7 2 . 2 0 . 3 0 . 7 3 0 . 1 1 0 . 2 4 0 . 0 31 . 0 6 0 . 0 8 - - - - 0 . 1 8 0 . 0 61 . 0 2 + 0 . 0 4 2 . 2 0 . 3 0 . 8 3 - + 0 . 1 2 0 . 7 9 0 . 0 31 . 1 1 -+ 0 . 0 6 1 . 9 0 . 3 0 . 8 5 0 . 1 3 1 . 7 7 0 . 0 71 . 0 9 0 . 0 5 2 . 1 0 . 3 0 . 9 4 0 . 1 4 3 . 0 4 - + 0 . 0 71 . 1 2 0 . 0 5 4 . 7 -+ 0 . 7 1 . 0 2 + - 0 . 1 5 8 . 3 8 0 . 2 11 . 0 9 0 . 0 5 3 . 0 -+ 0 . 4 1 . 0 4 0 . 1 6 5 . 6 2 - + 0 . 1 41 . 1 3 0 . 0 7 2 . 8 0 . 4 0 . 9 3 - + 0 . 1 4 5 . 9 5 0 . 1 80 . 9 5 0 . 0 7 2 . 7 0 . 4 1 . 0 7 0 . 1 6 5 . 1 8 0 . 1 51 . 1 3 0 . 0 7 - - - - 5 . 6 8 -+ 0 . 1 71 . 1 4 0 . 0 6 2 . 7 _+ 0 . 4 0 . 9 0 0 . 1 4 6 . 1 6 0 . 1 81 . 1 1 0 . 0 5 2 . 6 0 . 4 0 . 8 3 0 . 1 2 5 . 3 8 0 . 1 01 . 1 0 + - 0 . 0 5 2 . 6 0 . 4 0 . 9 6 0 . 1 5 6 . 1 1 0 . 1 21 . 1 2 0 . 0 5 2 . 9 0 . 4 0 . 8 8 0 . 1 3 5 . 8 4 0 . 1 4

, 1 D e t e r m i n e d b y r a d io i s o t o p e d i l u t i o n a n d ~ - s p e c t x o m e t r y ., 2 I v a n o v i c h a n d W i l k i n s ( 1 9 8 4 b ) ., 3 P e g m a t i t e .

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F i g . 2 . D i s t r i b u t i o n o f u r a n i u m a n d i t s d a u g h t e r d e c a y p r o d u c t s , t h o r i u m , R E E ( L a a n d C e ), t o ta l ir o n an d i ro no x i d e r a t i o s c h a r a c t er i s ti c o f t h e B O E d r i ll c o r e fr o m B S t t s t e in , S w i t z e r l a n d .


14/22

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g r a n it e . F r o m Ta b l e I I t h e p e g m a t i t e Uc o n t e n t s r an g e f r o m 2 . 4 2 to 4 . 4 3 p p m a n dt h e T h f r o m 0 . 7 4 to 1 2 . 5 8 p p m ( m e a nT h / U r a t io o f 0 . 9 ) . I n t h e a d j a c e n t g r a n it et h e c o r r e s p o n d i n g r a n g e s a r e 3 . 3 7 - - 5 . 7 3p p m U a n d 2 1 . 4 4 - - 3 4 . 6 9 p p m T h , r e sp e c -t i v el y ( m e a n T h / U r a t io o f 6 . 5 ). C o m p a r e dt o f r e s h , u n f r a c t u r e d g r a n it e p a rt s o f t h es a m e d r i l l c o r e ( N A G R A 85-01), w h i c hr e c o r d s U v a lu e s r a n g in g f r o m 8 . 6 t o 1 0 . 4p p m a n d T h v al ue s f r o m 1 9 .8 t o 2 9 . 3 p p m{ m e a n T h / U r a t io o f 2 . 6 5 ) , t h e s a m p l e d c o r es e c t i o n i n d i c a t e s a s i g n i f i c a n t d e p l e t i o n o fU ; T h a p p e a r s t o h a v e b e e n i m m o b i l e . T h ef r e sh g r a n i te c o m p a r e s f a v o u r a b l y w i t hw o r l d - w i d e a v e r a g e s f o r n o r m a l g r a n i t e s .

A l t h o u g h n o f u r t h e r d a t a ar e a v a il ab l e f o rt h e p e g m a t i t e h o r i z o n s , a p l it ic z o n e s t h r o u g h -o u t t h e d r i ll c o r e a r e c h a r a c t e ri z e d b y e x h i b it -i n g l o w T h ( 7 . 6 1 - - 1 6 . 6 2 p p m ) a n d h ig h U( 1 0 . 8 8 - - 2 9 . 8 8 p p m ) w i t h a m e a n T h / Ur a t i o o f 0 . 9 5 . F i s s i o n - t r a c k a n a l y s e s a r en o t a v a il a bl e , h o w e v e r m o s t o f th e U a n dT h is b e l i e v e d t o b e a s s o c i a t e d w i t h t h e a c -c e s s o r y m i n e r a l c o n t e n t s w h i c h i n c l u d ez i r co n a n d a p a t i te . M i n o r a m o u n t s o f Ua n d T h m a y b e a c c o m m o d a t e d b y t h e m a f ic

c o n s t i t u e n t s s u c h as b i o t i t e / c h l o r i t e , o ra s s o c i a t e d w i t h f i n e ly d i s p e rs e d F e - o x i d e s .T h e d i s t r i b u t i o n o f U a n d T h a l o n g t h e

d r i l l co r e i s i l l u s t r a t ed i n F i g . 2 . Wi t h i n t heg r a n i te p o r t i o n o f t h e d r i ll c o re t h e U c o n t e n tis u n i f o r m l y d i s t r i b u t e d u n t il t h e p e g m a t i t ei s a p p r o a c h e d w h e r e u p o n a s m a l l i n c r e a s ei s o b s e r v e d . I n t h e p e g m a t i t e t h e U d e c r e a s e so n l y t o i n c r e a s e a g a in ~ 3 c m f r o m t h ef r a c t u r e s u r f a c e ; a s m a l l d e c r e a s e o c c u r s a tt h e f r a c t u r e s u r f a c e . T h e T h i s e s s e n t i a l lys y m p a t h e t i c t o t h a t o f U e x c e p t t h a t int h e p e g m a t i te a c o n t i n u o u s d e c r e a s e is o b -s e r v e d .

A s i n f e r r e d f r o m t h e m i n e r a l o g y, t h eg r a n i t e r e p r e s e n t e d b y t h i s p a r t o f t h e d r il l-c o r e h a s b e e n s u b j e c t e d t o h y d r o t h e r m a lt e m p e r a t u r e s i n a ss o c i a t io n w i t h t h e e m p l a c e -m e n t o f t h e a d j a c e n t p e g m a t i t e a n d a p l it eh o r i z o n s . T h i s a p p e a r s t o h a v e r e m o b i l is e da n d r e m o v e d p a r t o f th e a v a i la b l e U in t h e

r o c k , r e s u l t i n g i n l o w b u t u n i f o r m l e v e l sw i t h i n b o t h t h e g r a n i te a n d p e g m a t i te . T h eo b s e r v e d i n c re a s e o f U a n d T h a t t h e g r a n i t e --p e g m a t i te c o n t a c t m a y b e d u e to l i m it edr e m o b i l i s a t i o n a n d c o n c e n t r a t i o n o f b o t he l e m e n t s m a r g i n a l t o th e p e g m a t i t e ; t h i sc o n t a c t z o n e j u s t w i t h i n t h e g r a n i te is a l s oc h a r a c t e r i z e d b y a n i n c r e a s e i n m a f i c m i n -e r al c o n t e n t .

5.2.1. Uranium-series disequilibr ium. P l o t s o ft he ac t i v i t y r a t i o s 234U/ 238U , 23 Th /234Uand 22~Ra/23 Th a r e p r e se n t e d i n F i g . 2 .T h e 2 3 4 U /2 3 8 U r a t i o is u n i t y f o r m o s t o ft h e c o r e l e n g t h a l t h o u g h a s l ig h t i n c r e a s et o a m a x i m u m o f 1 . 0 4 is o b s e r v e d c lo s e

t o t h e f r a c t u r e f a c e . T h e a b o v e d i sc u s s io no f t h e K r ~ k e m ~ l a 2 3 4U / 23 8 U r a t i o d i s t r ib u -t i o n c a n a g a in b e a p p l i e d i n t h is c a s e b u tt h e d e g r e e o f U m o b i l i s a t i o n i n v o l v e d isobv i ous l y l e s s . The 23 Th / 234U r e su l t s f o rB S t t s t e i n e x h i b i t a v a l u e s l i g h t l y g r e a t e r t h a nu n i t y e x c e p t c l o s e t o t h e f r a c tu r e w h e r e -u p o n a s i g n i f ic a n t d e c r e a s e o c c u r s . T h i sp r e s e n t s a n o b v i o u s c o n t r a s t t o t h e K r ~ k e -m ~ l a c o r e a n d s u g g e st s s o l u t i o n t o r o c kt r a n s f e r o f 2 3O Th p r o d u c e d b y t h e d e c a y o f

2 3 4 U i n s o l u t i o n . T h e d i f f e r e n c e b e t w e e nt he Kr ~kem ~l a and BSt t s t e i n 23 Th / 234Ur e s u lt s c o u l d b e p a r t l y e x p l a i n e d b y a m u c hm o r e r a p i d f lu i d t r a n s p o r t t h r o u g h t h eK r ~ k e m ~ l a c o r e t h a n in t h e B S t t s t e in c o r e .I n c o m m o n w i t h K r ~ k em ~ l a , t h e 2 26 R a/2 3 T h r a t i o i s c o n s i d e r e d t o r e p r e s e n t e q u i l i b -r i u m .

5.2.2. Iron oxidation states. Fig . 2 i l lus t ra test h e d i s t r i b u t i o n o f F e a n d t h e v a r i a ti o n o ft h e F e 2 + / ( F e 2 + F e 3 ) o x i d a t i o n s t a t e s .T h e t o t a l F e c o n t e n t s a r e s i g n i f i c a n t l y h i g h e ri n t h e g r a n i t e t h a n t h e p e g m a t i t e ; t h e i rr e s p e c t i v e d i s t r i b u t i o n s a r e re l a t iv e l y h o -m o g e n e o u s . T h e s e u n i f o r m d i s t r i b u t i o n s a r es u p p o r t e d m i n e r a l o g ic a l ly i n t h a t t h e r e isa f a ir ly r e g u l a r d i s p e r s io n o f F e O O H - o x i d e st h r o u g h o u t t h e r o c k m a tr ix .

T h e o x i d e r a t i o s a r e a l m o s t a l l l es s t h a nu n i t y, w h i c h i n d i ca t e s a f a ir ly u n i f o r m d e g r e e


15/22

o f o x i d a t io n t h r o u g h o u t b o t h r o c k t y p es .T h i s i s i n a c c o r d w i t h t h i s p a r t o f t h e B S t t -s t e i n d r i l l c o r e a s a w h o l e . I t h a s b e e n f o u n d(NTB 85-02) t h a t F e 2 m i n e r a ls a r e c o m m o ni n f r a c tu r e s a b o v e 9 5 0 - m d e p t h , b u t a r ee n t ir e ly a b s e n t b e l o w 1 0 5 0 m .

5.2.3. Rare-earth ( R E E ) distributions. Fi g . 2p r e s e n t s d a t a o f L a a n d C e w h i c h i l lu s t ra t et h e m a i n R E E a n d Ta t r en d s e n c o u n t e r e d .B o t h L a a n d C e s h o w f a i r ly u n i f o r m d i s tr i b u -t i o n s w i t h i n t h e g r a n i t e u n t i l ~ 7 c m f r o mt h e p e g m a t i t e c o n t a c t . A t t h i s p o i n t a s y s-t e m a t i c d e c r e a s e o c c u r s a n d t h e n a v e r ym a r k e d s h a r p i n c r e a se j u s t a t t h e c o n t a c t .T h i s is f o l l o w e d b y a g r a d u a l d e c r e a s e t h r o u g h

t h e p e g m a t i t e t o t h e f r a c t u r e e d g e . I n a d d i-t i o n t o b e i n g t y p i c a l o f all t h e m e a s u r e dl a n t h a n i d e s ( in c o n t r a s t S c , C o , R b , B a ,H f a n d Ta s h o w e d s m a l l in c r e as e s ), t h e L aa n d C e p r o f i l e s a r e s i m i l ar t o t h a t o f T h .T h e d a t a s t r o n g l y s u g g e s t t h a t t h e s e in c r e a s e dc o n c e n t r a t i o n s a r e t h e r e s u l t o f l i m i te dr e m o b i l i s a ti o n a n d c o n c e n t r a t i o n d u r i n g h y -d r o t h e r m a l a c t i v it y c o ev a l w i t h t h e i n t r o d u c -t i o n o f t h e p e g m a t i te s .

5.3. Grimsel drillcore

T h e d i s t r i b u t io n s o f U a n d i ts d a u g h t e rd e c a y p r o d u c t s , F e a n d t w o R E E ' s , L a a n dC e , a r e l i s t e d i n Ta b l e I I I a n d i l l u s t r a t e di n F i g . 3 .

M o s t o f t h e t r a c e - e l e m e n t d a t a a r e r e la t iv e -l y h o m o g e n e o u s t h r o u g h o u t t h e l e n g t h o ft h e d r i l l c o r e s p e c i m e n ; t h e r e a r e n o s y s -t e m a t i c t r en d s t h a t c a n n o t b e ex p l a in e d b ym i n e r a l o g i c a l i n c o n s i s t e n c i e s . A s i m i la r p a t -t e r n is s h o w n b y U a n d T h w h i c h v a ry f r o m5 . 2 6 to 9 . 2 5 p p m a n d 1 6 . 0 6 t o 2 7 .5 3 p p m ,r e s p e c t iv e l y ( m e a n T h / U r a t io o f 3 .1 ) . O n l ya f e w p u b l i s h e d a n a l y s e s a r e a v a i la b l e f r o mt h e A a r g r a n it e a s a w h o l e ; t h e s e r e c o r da v e ra g e U a n d T h c o n t e n t s o f 11 a n d 2 7 p p m ,r e s p e c t i v e l y ( T h / U r a t i o o f 2 . 4 ) . S o m e a u t o r a -d i o g r a p h i c s t u d i e s h a v e b e e n c a r r ie d o u to n t h e G r i m s e l g r a n i t e f r o m s p e c i m e n s

247

c o l l e c t e d ~ 1 k m f r o m t h e t e s t- s i te ( B a j o ,1 9 8 0 ) . T h e s e h a v e s h o w n th a t t h e m a i nU - a n d T h - b e a r i n g m i n e r a l p h a s e s a r e al-l a n i t e , t h o r i t e , z i r c o n a n d a p a t i t e ; p o l y c r a s ei s s p o r a d i c a l l y p r e s e n t s o m e t i m e s i n c l u d e di n f e l d s p a r. A l t h o u g h l i m i t e d f i s s i o n - t r a c ks t u d i e s h a v e i n d i c a t e d a n a b s e n c e o f in -t e r s t it i a l U , t h e s l i g h t ly l o w e r T h / U r a t i o sf o r t h e g r a n it e , in c o m p a r i s o n w i t h w o r l d -w i d e a v e r a g e s , s u g g e s t s t h a t s m a l l a d d i t i o n so f U m a y h a v e t a k e n p l a c e .

F r o m F i g . 3 t h e U d i s t r i b u t i o n v a r ie si rr e g u la r ly b e t w e e n ~ 5 a n d ~ 9 p p m a n dt h is is e v e n m o r e a c c e n t u a t e d b y t h e T hw h i c h r a n g e s f r o m 1 6 t o 2 8 p p m . T h i s v a r i a -t i o n i s n o t a f f e c t e d b y t h e f r a c t u r e a n d c a n

b e e x p l a i n e d b y m i n e r al o g ic a l i n h o m o g e n e -i ti e s. T h e r e i s n o o b v i o u s r e la t i o n s h i p b e t w e e nt h e U a n d T h , w h i c h s u g g es ts t h a t t h e Ua n d T h c o n t e n t s a r e c o n t r o l l e d b y r a d io -a c t i v e m i n e r a l s w h i c h a r e d i s t i n c t l y U - o rT h - b e a r i n g . A l t e r n a t i v e l y, i t m a y r e f l e c t ar e d i s t r i b u t io n o f U w i th i n t h e r o c k . I n c o n -t r a s t T h h a s p r o b a b l y b e e n i m m o b i l e a n dhas r e t a i ned i t s o r i g i na l s i gna t u r e .

5.3.1. Uranium-series disequilibr ium. P l o t s o fthe 234U/238U, ZSTh/234U an d ~26Ra/23Tha c t i v i t y r a t i o s a r e p r e s e n t e d i n F i g . 3 a n ds h o w s e c u l ar e q u i l ib r i u m w i t h i n t h e l im i t so f a n a l y t i c a l p r e c i s i o n ( 2 0 ) . T h i s m e a n sa n a b s e n c e o f r o c k - - w a t e r i n t e ra c t i o n w i t h i nt h e i n t e r v a l o f a t le a s t t h e l a s t 1 M y r.

5.3.2. Iron oxidation states. Fi g . 3 p r e sen t st h e d i s t r ib u t i o n o f t o t a l F e a n d t h e F e o x i d a -t i o n r a t i o s a l o n g t h e s a m p l e d d r i l l c o r e . A p -p r o a c h i n g t h e f r a c t u r e f r o m b o t h e n d s o ft h e d r i ll c o re , t h e t o t a l F e c o n t e n t i n c re a s esa n d t h e n d e c r e a s e s m a r g i n a l t o t h e f r a c t u r e .T h e v a r i a t i o n i n t h e t o t a l F e c o n t e n t i s c o n -s i d e r e d t o r e f l e c t t h e g e n e t i c i n h o m o g e n e -i t i e s d u e t o t h e p r i m a r y m i n e r a l o g y r a t h e rt h a n a n y r e d i s t r i b u t i o n d u r i n g r o c k - - w a t e ri n t e r a c t i o n . T h e F e 2 + / ( F e 2 + F e 3 ) o x i d a -t i o n r a t i o s a r e g e n e r a l l y u n i f o r m a l t h o u g ht h e r e m a y b e a s u g g e s t i o n o f s l i g h t l y e n h a n c e do x i d a t i o n a t o n e s i d e o f th e f r a c tu r e z o n e .


16/22

t~ 0

TABLE I

I

o

c

a

c

mic

a

y

o

u

um

Sp

me

Di

a

fom

U

*

f

u

f

(

p

m)

(m)

FLG-1

20

6

6

01

FLG-2A

39

7

1

01

FLG-3A

50

6

9

01

FLG-4A

70

8

3

00

FLG-5A

88

52

01

FLG-6A

1

8

6

4

02

FLG-7A

1

0

9

2

+01

FLG-8A

1

8

6

1

00

FLG-gA

1

6

6

6

01

FLG-2B

04

7

3

-+01

FLG-3B

06

7

6

01

FLG-4B

22

58

00

68

01

FLG-SB

34

67

_02

FLG-6B

48

74

01

FLG-7B

64

69

+

02

FLG-8B

78

63

01

FLG-9B

92

63

01

FLG-1

B

1

8

72

01

a

th

um

fom d

c

FLG (9

1

9

5

m,e

o

o

b

e

SB 80.001 ),

Grms

te

e

Swiz

a

Th

*

ThU 2

U

2

U

2

U/2

U

2

Th

2

Th2

U

2

Ra

2

Ra2

Th

2

h

(p

m)

2

0

06

35

48

00

47

+

00

09

+

00

54

01

11

00

1

2

07

27

52

_

01

52

01

09

-+00

53

02

10

00

2

1

05

33

51

01

51

01

10

00

56

-+01

11

00

2

5

12

33

61

+

00

62

00

10

00

64

03

10

-+00

1

1

+05

33

38

01

37

01

09

00

42

01

11

00

1

9

_

05

29

47

01

46

_

01

09

00

49

01

10

00

2

6

08

24

68

01

67

01

10

00

72

03

10

00

2

2

04

34

45

00

45

00

10

00

49

01

10

00

1

0

08

24

49

01

48

01

09

00

42

02

08

00

2

6

05

35

54

01

54

01

10

00

56

-+01

10

00

2

8

12

34

55

_

01

55

01

10

_

00

58

03

10

00

1

9

05

32

43

00

42

00

10

00

41

01

09

00

2

4

08

32

50

01

52

01

10

00

50

02

09

00

-

-

49

02

49

-+02

09

00

-

10

00

2

6

05

32

54

00

55

00

10

+

00

59

01

10

00

2

1

06

29

51

-+01

50

01

09

00

50

01

10

00

1

9

05

30

46

01

46

01

10

00

43

01

09

00

2

6

07

32

46

01

46

01

09

00

49

01

10

00

2

6

05

31

53

01

53

01

09

00

58

01

11

00

52

08

09

+

01

55

-+01

53

+

08

09

01

46

+

01

51

08

09

-+01

55

+

01

56

08

08

01

66

+

03

44

07

10

01

41

01

55

08

11

01

45

+

01

62

09

08

01

54

02

58

09

11

01

51

01

56

08

13

02

38

02

65

10

11

01

62

01

69

10

11

01

62

03

62

09

15

02

45

01

57

09

11

01

51

02

57

09

09

01

57

01

57

09

11

01

48

01

50

08

11

01

45

01

51

08

10

01

49

01

59

09

10

01

54

01

*Deemin

b

r

o

o

d

u

o

a

s

ome

y


17/22

R( p p ~

2 3 4 L t / 2 3 8 L

A ct i v i t yR a t i o

2 3 0 1 ~ / 2 3 4 ~

Activ i tyR a t i o

2 2 6 R a / 2 3 0 . ~

A c t i v i t yRatio

10

9

B

7

6

1.1

1. 0

0 9

1 2

1,1

1 0

0. 9

1. 6

1 4

1.2

1.0

O 9

G r i m s e lO r i t i c 0 r e F L G : 9 4 . .1 2 - 9 4 .5 2 m }

o~ tx

I ~ T h

I

121

Q

I T

} ; , ' t t ~ ' t , = ~ - ' t z t zf

I

If

}t {tt t ti

t{ t

' 2 7

2 6

. 2 5

24

- 2 3

, 2 2

21

2 0

19

18

17

1G

Th( ppm ]

249

T a t a r Fe( % ) 131, 21. 1

1. 0

0 9

I

F e 2

(F e 2 + , F e3+ )

( p p m )

,o.8 ' "0. 6

1 0 0 -

9 0 -

8 0 .

7 0 .

6 0 "

5 0 -

4 0 -

3 0 -

2 0 -

1 0 "

0

I % , ,

I

[

II

IC e

#2 3 4 5 6 7 ~ 10 11 12 13 14 15 16 17

/F R A C T U R E FA C E

D r i l lc o r e L e n g t h ( c r n )

F i g . 3 . D i s t r ib u t i on o f u r a n i u m a n d i ts d a u g h t e r d e c a y p r o d u c t s , t h o r i u m , R E E ( L a a n d C e ) , t o t a l i r o n a n d i r o no x i d e r a t i o s c h a r ac t e r i st i c o f t h e F L G d r i l lc o r e f r o m G r i m s e l , S w i t z e r l a n d .


18/22

250

5.3.3. Rare-earth (REE) distributions. Fig. 3presents the distributions of La and Ce whichare representative of the REE's as a whole.The patterns are irregular and reflect thedistributions of REE-bearing minerals suchas allanite and polycrase.

6 . S u m m a r y a n d c o n c l u s i o n s

Drillcore sections traversing water-conduct-ing fracture zones have been investigatedfrom the Kr~kem~la area (GStemar granite)in southeast Sweden, the BSttstein area innorth-central Switzerland, and the Grimselarea in south-central Switzerland. The coreshave been examined using the U decay

series together with the analysis of REE's,Fe2/Fe 3 ratios, and othe r element s suchas Na, K, Sc, Co, Rb, Cs, Ba and Ta. Fission-track analyses have been carried out onpolished thin rock sections from the Krhke-m~la granite.

Mineralogical studies of the GStemarsample ( from ~ 380-m depth) have shownthat at some stage the core has been subjectto a weak pervasive alteration throughoutits length. A more intense alteration is evident

within a marginal zone (3--5 cm wide) ad-jacent to the fracture zone. Here there iswidespread alteration of feldspar and mag-netite together with impregnations of Fe-OOH-oxides. The mineral coatings on thefracture face have not been quantitativelydetermined, but appear to be composed ofa mixture of hema tite, chlorite, clay min-erals and FeOOH-oxides.

Of the elements measured, the variationof REE's and Th (together with Ta, Cs andSc) appear t o reflec t the primar y distribu-tion of those radioactive refractor y phasesdominated by monazite. U, on the otherhand, shows evidence of mobilisation andleaching from monazite up to a distanceof 8 cm from the fracture zone; for theremainder of the core length the U distribu-tion is sympath etic with that of Th andthe REE's. Work by Smellie and Stuckless(1985) showed a general loss of U in the

GStemar granite as a result of large-scalehydrothermal alteration processes some 420+ 171 Myr. ago. These processes m ay have contributed to the breakdown and removal ofU from the monazite phases marginal tothe fracture zone.

Isot opic ally the 234U/238U ratio alongthe major length of the drillcore is less thanunity, indicating U leaching. In strong con-trast, the granite marginal to the fracturezone is characterized b y a s ystemat ic in-crease in the 234U/:38U ratio to a value of2.29 at the fracture face, which indicatesan accumulation of U. The 23Th/:34U ratiois unity throughout most of the core witha small decrease being apparent close to

the fracture. The 226Ra/23Th ratio valuesare considered to indicate secular equilib-rium.

The source of U giving rise to the en-hanced concentrations and z34U/:3sU ratiovalues close to the fracture could be eitherU leached from the granite (as indicated bythe 234U/:3sU ratios for the rest of the core),or from U transported in solution withinthe fracture migrating into the water-saturat-ed rock. If the former process is responsible,

then U migration over at least 40 cm is oc-curring whereas a constrai nt of ~ 3 cmcan be applied if the latter process is takingplace. The two processes are not mutuallyexclusive.

The 23Th--234U and 226Ra--23Th equilib-riu m and 234U--238U diseq uilibr ium suggestthat the process involved is either continuousor else indicates a time-scale long enoughto allow re-equilibration of 23Th with 234Uand 226Ra with 23Th bu t shor t en ough t omaint ain 234U disequilibrium with 23sU,i.e. a time-scale of ~ 1 Myr.

The mechanism giving rise to U deposi-tion at and near the fracture could be dueto any or all of: (a) precipitation at a bound-ary betwee n differing physico-chemic al con-ditons; (b) sorption on the fracture fillingminerals; and (c) solution-to-solid recoiltransfer. The degree and retention of radio-nuclide retardation and retention from


19/22

b o t h p r o c es s e s w i ll b e d e p e n d e n t o n th ep h y s i c o - c h e m i c a l p r o p e r t i e s o f t h e f r a c t u r ez o n e m i n e r a ls .

T h e B S t t s t e i n s a m p l e i s i n c o n v e n i e n t l yl o c a t e d n e a r t h e c o n t a c t b e t w e e n a p e g -m a t i t e a n d a p l i t e h o r i z o n . A s a r e s u l t t h eg r a n i te h a s b e e n s u b j e c t e d t o h y d r o t h e r m a lt e m p e r a t u r e s w h i c h a r e r e f l e c te d in t h em i n e r a l o g y a n d t h e c h e m i s t r y. U h a s b e e nm o b i l i s e d a n d d e p l e t e d f r o m t h e e x a m i n e dc o r e w h i c h h a s r e s u l t e d i n a f a i r ly u n i f o r md i s t r i b u t i o n b o t h w i t h i n t h e g r a n i t e a n dp e g m a t i te . I n c o n t r a s t , T h a n d R E E m o b i l is a -t i o n h a s b e e n r e s t r i c t e d t o a 7 - c m z o n e i nt h e g r a n it e a d j a c e n t t o t h e p e g m a t i t e co n -t a c t , w i t h e n r i c h m e n t o f t h e s e e l e m e n t s

a l o n g a z o n e ~ 1 - - 2 c m w i d e a d j a c e n t t ot h e p e g m a t i t e c o n t a c t .

T h e m i n e r a l o g y o f t h e f r a c t u r e z o n e a n dt h e h o s t p e g m a t i te d o n o t i n d ic a te a n yl o w - t e m p e r a t u r e a l t e r a t i o n p r o c e ss e s w h i c hm i g h t h a v e re s u l t e d i n s y s t e m a t i c c o m p o s i -t i o n a l g r a d i e n t s a r o u n d t h e f r a c t u r e z o n e .

T h e 2 34 U/2 38 U r a t i o is u n i t y f o r m o s t o ft h e c o r e l en g t h a l t h o u g h a sl ig h t in c r e as eis O b s e ~ e d c l o se t o t h e f r a c t u r e f a c e ; t h e: 26Ra / 23 Th r a t i o sugges t s s ecu l a r equ i l i b -

r i u m . T h e a b o v e d i s c u s s i o n o f t h e K r ~ ik e m ~ la2 3 4U /: 38 U r a t i o d i s t r i b u t i o n c a n a g a i n b ea p p l i e d i n t h is c a s e b u t t h e d e g r e e o f Um o b i l i s a t i o n i n v o l v e d i s o b v i o u s l y l e s s .The 23 Th / 234U r e su l t s f o r B5 t t s t e i n exh i b i ta v a l u e s l i

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