Geochemical Prospecting for Thorium and Uranium - Selection of Areas and Radiation Surveys

123 CHAPTER 3 GEOCHEMICAL PROSPECTING FOR THORIUM A N D URANIUM - SELECTION O F AREAS A N D RADIATION SURVEYS

Literature The

literature

on p r o s p e c t i n g for

t h o r i u m a n d u r a n i u m is e x t e n s i v e a n d

t h r o u g h a v a s t p l e t h o r a of p u b l i c a t i o n s t o o n u m e r o u s f o r a n y o n e i n d i v i d u a l

scattered to

peruse.

T h o s e m e n t i o n e d b e l o w a n d in t h e S e l e c t e d B i b l i o g r a p h y a r e o n l y a f e w of t h e p a p e r s a n d b o o k s a v a i l a b l e on t h e s u b j e c t . T h e f o u r v o l u m e s of t h e P r o c e e d i n g s of t h e C o n f e r e n c e s on P e a c e f u l U s e s of A t o m i c E n e r g y h e l d in G e n e v a ( U n i t e d N a t i o n s , 1 9 5 6 , 1 9 5 8 , 1 9 6 5 , 1972) a n d t h e c o n t r i b u t i o n t o t h e First

Conference

by t h e

United

States

Geological

Survey

(U.S. Geol. Surv.

1956)

are

i n v a l u a b l e f o r s u m m a r i e s of t h e m e t h o d s of p r o s p e c t i n g f o r t h o r i u m a n d u r a n i u m d u r i n g t h e p o s t S e c o n d World War p e r i o d t o 1 9 7 2 . P r e v i o u s t o 1958 a n d s h o r t l y t h e r e a f t e r a n u m b e r of p r a c t i c a l b o o k s a n d p a p e r s on p r o s p e c t i n g f o r t h o r i u m countries; among

and uranium

appeared

in

t h e s e m a y b e m e n t i o n e d t h o s e by D e M e n t a n d D a k e ( 1 9 4 8 ) ,

several Davidson

( 1 9 4 9 ) , G e o l o g i c a l S u r v e y of C a n a d a ( 1 9 5 2 ) , F l o r e n c i o ( 1 9 5 2 ) , C o o p e r ( 1 9 5 3 ) , I d r i e s s ( 1 9 5 4 ) , Knoer and Lutjen (1955), Nininger (1956a, 1956b), Broding and R u m m e r f i e l d (1956), G r a n g e (1956), L e n o b l e (1956), R o u b a u l t (1956), C r a w f o r d Atomic

Energy

Wambeke

Commission

(1957), A u s t r a l i a n

and

U.S.

Atomic

and P a o n e (1956), B a r n e s (1956), U.S.

Geological

Energy

Survey

Commission

(1957), (1957),

Kalman Melkov

(1957),

and

Van

Pukhalsky

(1957), L e c o q e t al. (1958), and E c h a g a r a y (1958). In r e c e n t y e a r s a n u m b e r of g e n e r a l p a p e r s , b o o k s , r e v i e w s , a n d b i b l i o g r a p h i e s on p r o s p e c t i n g for t h o r i u m and u r a n i u m h a v e a p p e a r e d .

Those noted during this compilation

i n c l u d e f i v e by t h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y ( 1 9 7 0 , 1 9 7 3 , 1 9 7 6 b , 1 9 7 7 , 1 9 7 8 ) , and

those

by

Cooper

(1953,

1953-58),

Surazhskii

(1960),

Novikov

and

Kapkov

M i c h e l s o n ( 1 9 6 6 ) , Van W a m b e k e ( 1 9 6 7 ) , S a u m a n d Link ( 1 9 6 9 ) , K r a s n i k o v a n d

(1965),

Sochevanov

(1969), G r a m m a k o v e t al. (1969), Bowie e t a l . ( l 9 7 2 ) , F i s c h e r (1974), D a r n l e y e t al. (1975), J o n e s ( 1 9 7 3 a , 1 9 7 3 b , 1975), P r e m o l i ( 1 9 7 6 ) , E l e v a t o r s k i ( 1 9 7 6 , 1 9 7 7 , 1 9 7 8 b , 1 9 7 8 c ,

1979a,

1979b), G a b e l m a n (1977), Bailey and C h i l d e r s (1977), and H a w k e s (1972 - ) . A n u m b e r of r e v i e w s a n d t e x t b o o k s t h a t t o u c h on v a r i o u s a s p e c t s of

geochemical

p r o s p e c t i n g for t h o r i u m a n d u r a n i u m a r e n o w a v a i l a b l e ; n o t e d in t h i s r e v i e w i n c l u d e t h o s e by F a u l e t a l . ( 1 9 5 4 ) , C a n n o n a n d K l e i n h a m p l ( 1 9 5 6 a ) , T b k a r e v a n d S h c h e r b a k o v Nininger (1956b), A l e k s e e v et al.

(1956),

(1959), G i n z b u r g ( i 9 6 0 ) , C a n n o n (1960a, 1960b), Lyubavin

and Ovchinnikov (1961), H a w k e s and Webb (1962), G r i m b e r t and O b e l l i a n n e (1962), Saukov (1963), U n i t e d N a t i o n s (1963, 1970), A d a m s and L o w d e r (1964), M a l y u g a (1965), (1967), C a m e r o n (1967), Boyle and G a r r e t t (1967a), K r e i t e r

Kvalheim

(1968), Z a j a c z k o w s k i

(1968),

C a n n e y (1969), Krasnikov and S o c h e v a n o v (1969), S h i m a (1970), Kogan e t a l . (1971), Boyle

124 and M c G e r r i g l e (1971), Brooks (1972), P e a r l (1973), J o n e s (1973), B r a d s h a w e t al. (1973), G r a n i e r (1973), Levinson (1974, 1980), Siegel (1974), M o n t e r o (1975), Elliott and

Fletcher

(1975), Boyle (1975), S m i t h e t al.

(1976), P o l i k a r p o c h k i n (1976), J o y c e (1976), Butt

Wilding

and

(1977),

Dall'aglio

Dodd

(1978),

(1977), Beus

Kimberley

(1978),

Grigorian

Kuzvart

(1977),

Wenrich-Verbeek et al.

(1978), P e t e r s

(1978),

Hood

and

(1977),

(1978,

1979),

M a r c h a n t (1978), H a m b l e t o n - J o n e s (1978), Telford e t al. (1978), P a r a s n i s (1979), Bowie and Plant

(1979),

Bowie e t al.

(1979),

Bubulis

(1979),

Rose

et al.

(1979),

and

Boyle

(1979b, 1980). a n d a b s t r a c t s on

the

g e o c h e m i s t r y of t h o r i u m a n d u r a n i u m a n d on g e o c h e m i c a l m e t h o d s of p r o s p e c t i n g for

The interested

the

two

elements

in

reader

the

Geologiya (Reference

will find

U.S.S.R.

an a b u n d a n c e

Academy

of

of r e f e r e n c e s

Sciences

(Moscow)

J o u r n a l of G e o l o g y ) a n d R e f e r a t i v n y i

Referativnyi

Zhurnal Khimiya

Zhurnal

(Reference

J o u r n a l of C h e m i s t r y ) ; in N u c l e a r S c i e n c e A b s t r a c t s a n d INIS A t o m i n d e x ( I A E A , V i e n n a ) ; a n d in C h e m i c a l A b s t r a c t s . thorium

and

uranium

A b s t r a c t s of g e o c h e m i c a l a n d g e o l o g i c a l p a p e r s d e a l i n g

deposits

published

in

Russian

in

The

Geology

of

Ore

with

Deposits

( G e o l o g i y a R u d n y k h M e s t o r o z h d e n i y a ) a p p e a r f r o m t i m e t o t i m e in E c o n o m i c G e o l o g y .

•

General P r o s p e c t i n g for m i n e r a l d e p o s i t s by p a t t e r n r e c o g n i t i o n h a s l o n g

b e e n an a c c e p t e d

p r o c e d u r e ; w h a t f o l l o w s , t h e r e f o r e , in t h e t e x t is an e x p o s i t i o n of t h e p r i n c i p a l g e o l o g i c a l , m i n e r a l o g i c a l , g e o c h e m i c a l , a n d r a d i o m e t r i c p a t t e r n s t h a t c h a r a c t e r i z e t h e o c c u r r e n c e of thorium and uranium deposits. biological,

N u m e r o u s p h y s i c a l ( g e o l o g i c , g e o m o r p h i c , and

climatic),

and c h e m i c a l p a r a m e t e r s c o m b i n e t o p r o d u c e a t h o r i u m or u r a n i u m

deposit,

s o m e r e l a t i v e l y o b v i o u s t o t h e i n t e r e s t e d i n v e s t i g a t o r a n d o t h e r s c r y p t i c or only r e v e a l e d after

considerable

research.

In m o s t

cases

the

parameters

(pattern

recognition)

i n t e g r a t e d in t h e m i n d of t h e p r o s p e c t o r a n d w i t h e x p e r i e n c e h e g a i n s a " f e e l for

are ore".

A t t e m p t s h a v e b e e n m a d e in t h e p a s t t o a p p l y m a t h e m a t i c a l c o n c e p t s t o t h e v a g a r i e s of mineralization

(Douglas,

1948) and

in r e c e n t

years

to utilize c o m p u t e r s

to search

and

i n t e g r a t e g e o l o g i c d a t a with a view to r e c o g n i z i n g c e r t a i n d i s c r i m i n a n t s t h a t c a n b e used t o i d e n t i f y s e t s of g e o l o g i c a l , g e o c h e m i c a l , a n d g e o p h y s i c a l d a t a t h a t d i s t i n g u i s h sites (areas, zones) containing deposits from

those that are barren.

between

In t h i s r e s p e c t

the

p a p e r s by B a i a s a n o v e t a l . (1968), H o w a r t h (1973), Silver (1978), H o w a r t h and M a r t i n (1979), D a v i d ( 1 9 7 9 ) , R u n n e l l s (in p r e s s ) , S i n d i n g - L a r s e n e t a l . (in I A E A , 1979b), a n d G a r r e t t e t a l . (1980)

are

of

geomathematics

considerable and

interest

geostatistics

in

understanding

in p a t t e r n

and

classification

applying and

the

pattern

techniques analysis.

of The

s e c o n d f r o m l a s t m e n t i o n e d p u b l i c a t i o n ( I A E A , 1979b) a n d t h e w o r k s h o p r e p o r t by S i l v e r (1978)

also

contain

a

number

of

other

papers

relating

to

the

evaluation

of

r e s o u r c e s of i n t e r e s t t o t h e p r o s p e c t o r s e a r c h i n g for t h o r i u m a n d u r a n i u m d e p o s i t s .

uranium Briggs

a n d P r e s s ( 1 9 7 7 ) h a v e a t t e m p t e d t o t e s t u r a n i u m p r o s p e c t i n g by p a t t e r n r e c o g n i t i o n in a n

125 a r e a o n t h e C o l o r a d o - U t a h b o r d e r u n d e r l a i n by t h e J u r a s s i c M o r r i s o n F o r m a t i o n , a p p a r e n t l y with some success. geologic

and

spectacular. data

They e m p l o y e d essentially geologic t r a i t s ; had they e m p l o y e d

geochemical

parameters

their

success

would

probably

have

They recognized this deficiency and c o m m e n t e d t h a t "when

from

standard

geologic

geophysical surveys are

maps, LANDSAT

imagery,

available, computerised

geochemical,

prospecting

procedures

been

both more

comprehensive

radiometric may

and

provide

a

p o w e r f u l n e w a p p r o a c h t o p r e d i c t i n g n e w a r e a s of c o m m e r c i a l r e s o u r c e p o t e n t i a l " . G e o c h e m i c a l p r o s p e c t i n g f o r t h o r i u m a n d u r a n i u m c a n b e p u r s u e d in t w o w a y s - b y r a d i a t i o n s u r v e y s a n d b y n o r m a l a n a l y t i c a l c h e m i c a l s u r v e y s of g e o l o g i c a l a n d materials.

A c o m b i n a t i o n of b o t h t y p e s of

surveys

biological

is d e s i r a b l e in t h e e x p l o r a t i o n

for

r a d i o a c t i v e d e p o s i t s , a l t h o u g h in s o m e u r a n i f e r o u s a n d t h o r i f e r o u s b e l t s o n e m e t h o d c o m p l e m e n t or s u p p l e m e n t t h e o t h e r . t h e g r i d p r i n c i p l e , on t h e d i r e c t e d principle.

may

S a m p l i n g in b o t h t y p e s of s u r v e y s is c o n d u c t e d w a l k or t r a v e r s e

p r i n c i p l e , or on t h e

random

In t h e f i r s t , r a d i a t i o n c o u n t s or s a m p l e s a r e t a k e n a t s t a n d a r d i n t e r v a l

on

walk grid

p o i n t s in t w o or t h r e e d i m e n s i o n s ; in t h e s e c o n d , c o u n t s or s a m p l e s a r e t a k e n a t s t a n d a r d i n t e r v a l s a l o n g or a c r o s s s o m e t o p o g r a p h i c or g e o l o g i c a l f e a t u r e s u c h a s a s t r e a m n e t or a s p e c i f i c s t r a t u m ; in t h e t h i r d , c o u n t s a r e r e g i s t e r e d o r s a m p l e s a r e s e l e c t e d h e r e a n d t h e r e w i t h no p a r t i c u l a r p a t t e r n . geochemical something

survey

that

T h e l a s t m e t h o d is n o t g e n e r a l l y e m p l o y e d e x c e p t w h e r e

is a n c i l l a r y

to other

e x c i t e s his i n t e r e s t .

t y p e s of s u r v e y s , a n d t h e g e o l o g i s t

In p r a c t i s e

in m o s t

t y p e s of

radiation

the

observes surveys

a

c o n t i n u o u s c o u n t i n g a n d r e c o r d i n g p r o c e d u r e is e m p l o y e d a l o n g t r a v e r s e s , on g r i d l i n e s , d o w n d r i l l h o l e s , a n d so o n .

The

surveys

may

be shoeborne,

muleborne,

camelborne,

c a r b o r n e , s h i p b o r n e , or a i r b o r n e d e p e n d i n g on t h e f a c i l i t i e s a v a i l a b l e , t h e n a t u r e of

the

t e r r a i n , e c o n o m i c s , and o t h e r f a c t o r s .

the

When t h e r a d i a t i o n c o u n t s a r e r e c o r d e d a n d / o r

a n a l y t i c a l r e s u l t s of t h e s a m p l i n g a r e p l o t t e d a n o m a l i e s a r e r e v e a l e d b y c o n t o u r i n g or by i n s p e c t i o n of i n c r e a s e s (or d e c r e a s e s

in c e r t a i n

c a s e s ) in r a d i a t i o n

intensity

or in

the

c o n t e n t s of t h o r i u m or u r a n i u m a n d / o r t h e i r v a r i o u s i n d i c a t o r e l e m e n t s t o w a r d s o m e p o i n t , h o p e f u l l y m a r k i n g t h e l o c a t i o n of an e c o n o m i c d e p o s i t of t h o r i u m a n d / o r u r a n i u m . T h e p l a n n i n g a n d e x e c u t i o n of g e o c h e m i c a l s u r v e y s f o r t h e d i s c o v e r y of t h o r i u m a n d u r a n i u m d e p o s i t s s h o u l d t a k e p l a c e in a n u m b e r of c o n s e c u t i v e s t e p s , t h e s e q u e n c e of w h i c h will d e p e n d of

specific

on t h e f u n d s a v a i l a b l e , t h e t e r r a i n , t h e g e o l o g i c a l s i t u a t i o n , t h e a p p l i c a b i l i t y geochemical

m e t h o d s , a n d so o n .

The ideal sequence

in t h e

planning

and

e x e c u t i o n of g e o c h e m i c a l s u r v e y s is a s f o l l o w s : 1. S e l e c t i o n of a r e a s for p r o s p e c t i n g :

T h i s is e s s e n t i a l l y a l i b r a r y a n d o f f i c e

project

a l t h o u g h n o t i c e of a m i n e r a l s t r i k e in an a r e a m a y d i c t a t e a c q u i s i t i o n of c l a i m s or g r o u n d in w h i c h t o p r o s p e c t in t h e a r e a .

In t h e n o r m a l s e l e c t i o n of a r e a s f a v o u r a b l e

geological

p a r a m e t e r s s h o u l d b e t h e g u i d i n g p r i n c i p l e e s p e c i a l l y t h e p r e s e n c e of t h o r i u m or o c c u r r e n c e s or d e p o s i t s .

uranium

This follows from t h e v e n e r a b l e p r o s p e c t i n g m a x i m "Seek t h o r i u m

126 and u r a n i u m w h e r e t h o r i u m and u r a n i u m h a v e been found". for t h e o c c u r r e n c e of t h o r i u m a n d u r a n i u m

T h e s e l e c t i o n of a r e a s s u i t a b l e

is d i s c u s s e d in s o m e d e t a i l in t h e

following

section. 2 . H a v i n g s e l e c t e d a f a v o u r a b l e a r e a i t s h o u l d b e e x a m i n e d by r e g i o n a l and/or

geochemical

analytical

methods,

particularly

the

analysis

of

radiometric

stream

and

s e d i m e n t s a n d w a t e r s a n d w h e r e p r a c t i c a b l e , t h e a n a l y s i s of c o u n t r y r o c k s , s h e a r m y l o n i t i z e d z o n e s , a n d so o n .

C o n c o m i t a n t p h o t o g e o l o g i c and r e c o n n a i s s a n c e

lake zones,

geological

m a p p i n g a n d m o r e d e t a i l e d g e o l o g i c a l s u r v e y s s e r v e a t t h i s s t a g e t o e v a l u a t e s o m e of t h e g e o c h e m i c a l and r a d i o a c t i v e a n o m a l i e s .

It s h o u l d b e p o i n t e d o u t , h o w e v e r , t h a t

anomalies

s h o u l d n o t b e e l i m i n a t e d a t t h i s s t a g e s o l e l y on g e o l o g i c a l or g e o p h y s i c a l e v i d e n c e . contrary

one should p r o c e e d

to s t a g e 3 and

eliminate

anomalies

only

z o n e s , or

local

on

On t h e

geochemical

evidence. 3.

Having

isolated

favourable

(anomalous)

belts,

areas

follow-up

surveys should be pursued utilizing r a d i o m e t r i c and/or g e o c h e m i c a l analytical m e t h o d s such as

detailed

chemical

lithochemical,

methods.

See

pedochemical,

the

Chapters

hydrochemical,

describing

these

biogeochemical,

methods.

and

atmo-

Detailed

geological

m a p p i n g a n d s u i t a b l e g e o p h y s i c a l s u r v e y s s h o u l d b e c a r r i e d o u t a t t h i s s t a g e for

geological

c o n t r o l , b u t g e o c h e m i c a l a n o m a l i e s m u s t n o t b e e l i m a t e d a t t h i s s t a g e on g e o p h y s i c a l or geological evidence alone. 4.

A focus

employing

the

having

methods

been

obtained

in 3 a b o v e ,

on

a

detailed

suitable geological

zone,

structure,

mapping,

or

stratum

trenching,

close

by grid

d r i l l i n g , a n d d e t a i l e d s a m p l i n g s h o u l d b e p u r s u e d t o d e t e r m i n e t h e w o r t h of t h e a n o m a l y or prospect.

D u r i n g t h i s p h a s e i t is a d v i s a b l e t o c a r r y o u t d e t a i l e d r a d i o m e t r i c , l i t h o c h e m i c a l ,

pedochemical,

hydrochemical,

biogeochemical,

and a t m o c h e m i c a l

methods to t r a c e

any

d i s c o v e r e d m i n e r a l i z a t i o n t o i t s l i m i t s . T h e p r o c e d u r e s for t h i s t y p e of a t t a c k a r e o u t l i n e d in t h e f o l l o w i n g C h a p t e r s . 5 . W h e r e s o m e o r e b o d i e s a r e o u t l i n e d a n d o t h e r s s o u g h t in t h e t e r r a n e , z o n e , s t r a t a , or s t r u c t u r e d e t a i l e d g e o l o g i c a l w o r k in c o n j u n c t i o n w i t h d e t a i l e d a n a l y t i c a l and

radiometric

methods

are

called

for.

Far

too

performed without the requisite geochemical work.

often

only

How often

the

geochemical

geological

work

is

have geologists failed

to

s n a t c h f o r t u n e f r o m a d v e r s i t y by o m i t t i n g t o d o t h e p r o p e r a n a l y s e s of d r i l l c o r e s , s a m p l e s , a n d so o n ?

rock

I s u s p e c t , in f a c t I k n o w , m a n y t i m e s .

6. F i n a l l y , in p h a s e d e x p l o r a t i o n a s s u g g e s t e d a b o v e i t is i m p e r a t i v e t o e v a l u a t e t h e r e s u l t s of e a c h s t e p b e f o r e venture

all

records

should

proceeding be

filed

with for

the next. future

At

t h e e n d of

comparisons

with

the

prospecting

known

areas

of

m i n e r a l i z a t i o n or w i t h t y p e s of d e p o s i t s n o t y e t c h a r a c t e r i z e d . Examples

of

case

histories

of

phased

exploration

utilizing

geological

r a d i o m e t r i c s , a n d g e o c h e m i c a l m e t h o d s a r e f e w in t h e p u b l i s h e d l i t e r a t u r e . during

the

present

compilation

include

those

by D a v i s ( 1 9 7 3 ) , K i n g e t a l .

principles, Three

noted

(1976),

and

127 S m i t h e t a l . ( 1 9 7 6 ) ; o t h e r p a p e r s of i n t e r e s t o n t h e s u b j e c t c a n b e f o u n d in t h e c o m p e n d i a by t h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y , V i e n n a ( 1 9 7 3 , estimating

the

minimum

specifications

for

1976b, 1977).

economically

explorable

A method

for

sandstone-type

u r a n i u m d e p o s i t s is g i v e n in t h e p a p e r by E l l i s ( 1 9 7 9 ) .

Selection of Areas T h o s e s e a r c h i n g for t h o r i u m - a n d / o r u r a n i u m - b e a r i n g d e p o s i t s should s t u d y

carefully

t h e g e o l o g i c a l s e t t i n g s of t h e v a r i o u s t y p e s of d e p o s i t s o u t l i n e d in C h a p t e r 2.

Particular

n o t e s h o u l d a l s o b e t a k e n of t h e m o d e s of o x i d a t i o n of t h e v a r i o u s t y p e s of t h o r i u m

and

uranium deposits briefly described t h e r e . U r a n i u m , l i k e g o l d , is w h e r e y o u find i t , a n d t h e s a m e a p p l i e s t o t h o r i u m .

One should

a l w a y s b e c a r e f u l a b o u t a d o p t i n g r i g i d p r i n c i p l e s in p r o s p e c t i n g e s p e c i a l l y w i t h r e g a r d

to

t h e t y p e s of t e r r a n e s in w h i c h m i n e r a l d e p o s i t s m a y o c c u r .

to

u r a n i u m a n d in s o m e r e s p e c t s t o t h o r i u m a s w e l l .

This a p p l i e s p a r t i c u l a r l y

U r a n i u m has a r e m a r k a b l y high mobility

and may a c t u a l l y b e c o n c e n t r a t e d under a l m o s t all c o n d i t i o n s from t h o s e t h a t prevail a t t h e s u r f a c e t o t h o s e a t g r e a t d e p t h ; t h o r i u m , on t h e o t h e r h a n d is m u c h l e s s m o b i l e h e n c e m o r e r e s t r i c t e d in i t s o c c u r r e n c e .

and

T h e following discussion c o n t a i n s s o m e n o t e s on

m e t a l l o g e n i c f a c t o r s a n d o u t l i n e s b r i e f l y t h e p a r t i c u l a r l y f a v o u r a b l e g e o l o g i c a l t e r r a n e s in which

to

prospect

for

thorium

and

uranium

deposits

employing

geochemical

and/or

radiation surveys. M o s t t h o r i u m a n d u r a n i u m d e p o s i t s o c c u r w i t h i n o r o g e n i c b e l t s o r in t e r r a n e s c l o s e l y associated

with

them.

A general summary

of

the

features

of

these

belts

will

serve,

t h e r e f o r e , t o s e t t h e s t a g e for a m o r e d e t a i l e d d i s c u s s i o n of t h e m e t a l l o g e n e s i s of t h e t w o r a d i o e l e m e n t s a n d p r o v i d e a g u i d e for t h e

selection

of

a r e a s in w h i c h

to prospect

for

t h o r i u m and u r a n i u m . T h e s e d i m e n t a r y a n d i g n e o u s h i s t o r y of o r o g e n i c b e l t s d e v e l o p e d w i t h i n g e o s y n c l i n e s , w h i l e i n v a r i a b l y d i v e r s e a n d c o m p l e x , is i d e a l l y c h a r a c t e r i z e d by a n u m b e r of

successive

stages.

T h e f i r s t is c h a r a c t e r i z e d by a n e a r l y p r e t e c t o n i c t o e a r l y t e c t o n i c m a r i n e b a s i c

effusive

s u i t e of r o c k s ( g r e e n s t o n e s or o p h i o l i t e s ) , a s s o c i a t e d w i t h a g r e y w a c k e s u i t e of

sediments.

Thorium and uranium deposits a r e generally not a s s o c i a t e d with this s t a g e .

next

is m a r k e d

stage

by an a c i d i c s u i t e ( g r a n i t i c

intrusive rocks associated with tectonism,

s u i t e ) of

anatectic

and

are contemporaneous during the

last

rheomorphic

m e t a m o r p h i s m , a n d t h e r i s e of t h e

t h r o u g h t h e geosynclinal pile; t h o r i f e r o u s and u r a n i f e r o u s g r a n i t e s , a p l i t e s , and with the thermal maximum

p h a s e of g r a n i t i c e m p l a c e m e n t

isotherms pegmatites

of t h i s p h a s e , a n d t h e s e a r e

and a f t e r

extensive

The

followed

mylonitization

and

f a u l t i n g by t y p i c a l p i t c h b l e n d e d e p o s i t s , w i t h or w i t h o u t N i - C o a r s e n i d e s a n d s u l p h i d e s , native silver, native

bismuth,

and

so o n .

The geosynclinal

p h a s e of g r a n i t i z a t i o n

and

i n j e c t i o n of g r a n i t i c b o d i e s is f o l l o w e d in s o m e g e o s y n c l i n e s d u r i n g t h e e p e i r o g e n i c ( u p l i f t ) stage

by an i n t e r m e d i a t e

to

acidic

volcanic

stage

during

which

essentially

terrestrial

128 a n d e s i t e , t r a c h y t e , and rhyolite flows, p y r o c l a s t i c s , and shale, s a n d s t o n e s , and l i m e s t o n e s a r e laid d o w n .

In s o m e g e o s y n c l i n e s g r a n i t i c p l u t o n s ( u s u a l l y g r a n o d i o r i t e ) a r e

injected.

F o l l o w i n g f a u l t i n g a n d e x t e n s i v e f r a c t u r i n g of t h e c a l d e r a s a n d p l u t o n s , t y p i c a l p i t c h b l e n d e ( u r a n i n i t e ) d e p o s i t s , c o m m o n l y h i g h in m o l y b d e n u m , a r e d e v e l o p e d .

The extensive

early

g r a n i t i c s t a g e in m o s t g e o s y n c l i n e s is f o l l o w e d b y a l a t e , e s s e n t i a l l y p o s t - t e c t o n i c m o l a s s i c s t a g e d u r i n g w h i c h e x t e n s i v e , m a i n l y t e r r e s t r i a l c o n g l o m e r a t e s a n d s a n d s t o n e s w i t h c o a l or l i g n i t e a r e d e p o s i t e d in i n t e r m o n t a n e b a s i n s a n d d e l t a s .

T h i s s t a g e is

with

volcanic

or

follows

closely

the

intermediate

to

acidic

contemporaneous

stage;

the

typical

t h o r i u m - u r a n i u m d e p o s i t s a r e t h e q u a r t z - p e b b l e c o n g l o m e r a t e d e p o s i t s of P r o t e r o z o i c a g e a n d t h e s a n d s t o n e - t y p e d e p o s i t s of P a l e o z o i c , M e s o z o i c , a n d C e n o z o i c a g e . genic

stage

(laccoliths

of and

geosynclines dikes),

and

is

marked

minor

by

amounts

flood of

basalts,

associated

layered

The

gabbroic

rhyolites

and

postorointrusions

intermediate

e f f u s i v e s ; no e c o n o m i c t h o r i u m or u r a n i u m d e p o s i t s a p p e a r t o b e i n t i m a t e l y a s s o c i a t e d w i t h t h i s s t a g e , a l t h o u g h in s o m e s a n d s t o n e t e r r a n e s c e r t a i n d e p o s i t s m a y h a v e o r i g i n a t e d a s a r e s u l t of t h e h y d r o t h e r m a l a c t i v i t y a s s o c i a t e d w i t h t h e m a g m a t i s m postorogenic

alkali-rich

plutons

(e.g.

Lovozero

Massif,

Kola

of t h i s s t a g e .

Peninsula;

Late

Ilimaussaq

intrusion, Greenland), k i m b e r l i t e s , and c a r b o n a t i t e s c o m m o n l y a s s o c i a t e d with rifting close t h e i g n e o u s h i s t o r y of m o s t d e f o r m e d g e o s y n c l i n e s .

These rocks tend to be enriched

in

t h o r i u m a n d u r a n i u m , b u t o n l y a f e w e c o n o m i c d e p o s i t s o c c u r in or a s s o c i a t e d w i t h t h e m . T h e m e t a l l o g e n i c f a c t o r s b e a r i n g on t h e o r i g i n of t h o r i u m a n d u r a n i u m d e p o s i t s a r e of i n t e r e s t in p r o s p e c t i n g a n d c o n s e q u e n t l y h a v e b e e n m u c h d i s c u s s e d . t h e i n t e r e s t e d r e a d e r should consult t h e views

are

given

in

some

detail

r e m a r k s by Z i e g l e r ( 1 9 7 8 ) .

elsewhere

(Boyle,

1979a) and

are

For a general review The present summarized

writer's in

the

following: T h e s o u r c e of t h e m e t a l s a n d g a n g u e e l e m e n t s in e p i g e n e t i c d e p o s i t s h a s l o n g b e e n t h e s u b j e c t of d e b a t e a m o n g g e o l o g i s t s . s i n c e t h e t i m e of A g r i c o l a .

Four distinct possibilities have been

considered

With r e s p e c t t o t h o r i u m and u r a n i u m t h e s e c a n be c o n v e n i e n t l y

t a b u l a t e d as follows: (1) T h e t h o r i u m a n d u r a n i u m a n d t h e i r a c c o m p a n y i n g e l e m e n t s c a m e f r o m a d e e p s o u r c e in t h e e a r t h :

T h e m a n t l e is o f t e n

regions have been suggested. abyssal.

mentioned

as a possibility,

but even

deeper

An o r i g i n b a s e d o n t h e s e s o u r c e s is c o m m o n l y r e f e r r e d t o a s

T h e t a p h r o g e n i c h y p o t h e s i s of G a b e l m a n ( 1 9 7 7 ) w h e r e b y u r a n i u m p r o v i n c e s a n d

d i s t r i c t s a r e r e l a t e d t o t a p h r o g e n i c s t r u c t u r e s , t h a t is d e e p s e a t e d f r a c t u r e s y s t e m s

that

p e n e t r a t e t h e m a n t l e a n d d r a w off d i f f e r e n t i a t e d v o l a t i l e e l e m e n t s , is o n e of t h e c o n c e p t s of t h e a b y s s a l t h e o r y . A b y s s a l t h e o r i e s p u t t h e r e s p o n s i b l e s o u r c e r o c k s o r m o t h e r s u b s t a n c e s w e l l o u t of r e a c h of a n y m o d e r n t e c h n i q u e s of e x a m i n a t i o n

and analysis; h e n c e t h e y r e m a i n

highly

speculative.

The t h e o r i e s a r e also difficult t o s q u a r e with t h e o b s e r v a t i o n t h a t only c e r t a i n

deep-seated

structures

are uraniferous.

Having regard

to the general

homogeneity

of

129 m a n t l e p r o c e s s e s one would e x p e c t t h a t all such s t r u c t u r e s would c a r r y t h e e l e m e n t , but s u c h is n o t t h e c a s e a s is w e l l k n o w n . (basic and

ultrabasic

rocks)

s e d i m e n t s ( T a b l e 5).

T h e u r a n i u m a n d t h o r i u m c o n t e n t of m a n t l e r o c k s

is low a s c o m p a r e d

As a s o u r c e ,

therefore,

with certain

mantle

rocks

acidic crustal

would

seem

rocks

to

be

and

low

in

from

a

the scale. (2) T h e

thorium

crystallizing

and

magmatic

(e.g. Bilibin,

Page,

and

uranium

and

body:

Basic

others);

more

their

magmas acidic

g r a n o d i o r i t e s and g r a n i t e s a p p e a l to o t h e r s

accompaning are

elements

favoured

magmas

such

(e.g., Emmons,

by

came

some

as those

investigators

that

give

rise

Lindgren, and others).

to

This

s o u r c e is t h e b a s i s of t h e m a g m a t i c h y d r o t h e r m a l t h e o r y . M a g m a s m a y b e of t w o p o s s i b l e o r i g i n s - f u s i o n of t h e l o w e r c r u s t o r u p p e r and

anatexis

appear

to

of s e d i m e n t a r y

be

related

and volcanic piles.

to

the

latter

process;

C e r t a i n t h o r i u m and uranium relationships

to

the

mantle, deposits

former

process

are equivocal. (3) T h e

thorium

and

uranium

and

their

accompanying

elements

came

from

the

c o u n t r y r o c k s t h a t e n c l o s e t h e d e p o s i t s o r f r o m r o c k s t h a t lie a b o v e , b e l o w o r l a t e r a l t o t h e deposits.

T h i s is t h e b a s i s of t h e v a r i o u s s e c r e t i o n ( l a t e r a l , m e t a m o r p h i c , e t c . ) t h e o r i e s .

D i s c u s s i o n a n d s u p p o r t i n g e v i d e n c e for t h i s t h e o r y a r e g i v e n in a n u m b e r of p l a c e s in t h e t e x t of t h i s b o o k .

See a l s o F i g u r e 17 a n d t h e a c c o m p a n y i n g d i s c u s s i o n .

(4) T h e t h o r i u m a n d u r a n i u m a n d t h e i r a s s o c i a t e d e l e m e n t s c a m e f r o m a w e a t h e r e d surface

and

were

carried

into

favourable

structures

by

descending

meteoric

waters:

E x c e p t i n g r o l l f r o n t a n d c e r t a i n o t h e r s a n d s t o n e - t y p e d e p o s i t s t h i s m o d e of o r i g i n c a n , in g e n e r a l , b e d i s m i s s e d f o r m o s t t y p e s of e p i g e n e t i c t h o r i u m a n d u r a n i u m d e p o s i t s . p r o c e s s s e e m s q u i t e i n c a p a b l e of p r o d u c i n g t h e v a r i e d uranium

deposits

that

occur

in

mineralized

t y p e s of e p i g e n e t i c

volcanic

and

sedimentary

F u r t h e r m o r e , t h e p r e s e n c e of m i n e r a l s s u c h a s t h e n i c k e l - c o b a l t

Such a

thorium

and

terranes.

arsenides, the

various

s e l e n i d e s , n a t i v e g o l d , a n d t e l l u r i d e s ; t h e p r i m a r y l a t e r a l a n d v e r t i c a l z o n a t i o n of c e r t a i n m i n e r a l s a n d e l e m e n t s in e p i g e n e t i c t h o r i u m a n d u r a n i u m d e p o s i t s ; t h e m a r k e d in

elemental

precipitation

as

the

mineralization

progressed

in

many

differences areas;

c h a r a c t e r i s t i c w a l l - r o c k a l t e r a t i o n e f f e c t s ; a n d t h e r e m a r k a b l y low s o l u b i l i t y of

the

elements

s u c h a s g o l d , l e a d , a n d t h o r i u m in n o r m a l g r o u n d w a t e r s a l l m i l i t a t e a g a i n s t t h e w e a t h e r i n g h y p o t h e s i s . O n l y in c e r t a i n t y p e s of w e a t h e r e d u r a n i u m d e p o s i t s is s u p e r g e n e u r a n i n i t e a n d pitchblende (uranium black) significant

in e n r i c h i n g t h e o r e s .

In s a n d s t o n e t e r r a n e s t h e

r o l l - t y p e d e p o s i t s a r e e s s e n t i a l l y of t h i s g e n e s i s ; i t is a l s o c o n c e i v a b l e t h a t c e r t a i n t y p e s of s a n d s t o n e - t y p e d e p o s i t s d e r i v e d granitic gypcrete,

rocks, and

rhyolites, similar

uranium from w e a t h e r e d

gneisses,

types

of

surfaces.

and

their uranium from other

supergene

t h e w e a t h e r i n g of

meta-sediments.

deposits

undoubtedly

Uraniferous derived

most

other

exposed calcrete, of

their

130 T h o r i u m and u r a n i u m d e p o s i t s e x h i b i t a s p e c i f i c i t y

with r e s p e c t to geological

t h a t is i m p o r t a n t in t h e s e l e c t i o n of a r e a s in w h i c h t o p r o s p e c t .

age

On a w o r l d - w i d e

basis

t h e r e a r e n o k n o w n e c o n o m i c T h - U d e p o s i t s of u n d o u b t e d A r c h e a n a g e ( B o y l e , 1 9 7 6 b ) , b u t i t m u s t b e p o i n t e d o u t t h a t y o u n g e r d e p o s i t s m a y o c c u r in A r c h e a n t e r r a n e s .

Following

1

t h e A r c h e a n t h e p e a k p e r i o d s of d e p o s i t i o n of t h o r i u m a n d u r a n i u m t o o k p l a c e d u r i n g t h e early,

middle, and

late

P r o t e r o z o i c , the early and l a t e Paleozoic (Hercynian,

Variscan,

Appalachian), t h e Mesozoic (Nevadan, Laramide), and Tertiary ( L a r a m i d e , Alpine).

Many

of t h e e p i g e n e t i c t h o r i u m a n d u r a n i u m d e p o s i t s w e r e f o r m e d d u r i n g or i m m e d i a t e l y

after

t h e o r o g e n i e s in e a r l y , m i d d l e , a n d l a t e P r o t e r o z o i c t i m e a n d a f t e r t h e o t h e r o r o g e n i e s (or t h e i r e q u i v a l e n t s ) n o t e d in t h e b r a c k e t s f o l l o w i n g t h e o t h e r e r a s ; t h e m o d i f i e d p l a c e r s (?) ( q u a r t z - p e b b l e c o n g l o m e r a t e s ) a n d t h e e p i g e n e t i c d e p o s i t s in c o n g l o m e r a t e s a n d s a n d s t o n e s (sandstone-type deposits) cannot be directly related to orogenic processes broadest sense. quartz-pebble

e x c e p t in t h e

B o t h t h e s e t y p e s of d e p o s i t s a r e , h o w e v e r , e s s e n t i a l l y p o s t - o r o g e n i c , conglomerate-type

being

formed

mainly

in

marginal

basins

on

g e o s y n c l i n e s a n d in a u l a c o g e n s , a n d t h e s a n d s t o n e - t y p e b e i n g m a i n l y r e s t r i c t e d orogenic

molassic

facies

developed

on

the

parts

of

massifs

underlying

the

ancient to

sinking

postinter-

m o n t a n e basins. Early (Lower) P r o t e r o z o i c

terranes are particularly

noted

for

their

large

quartz-

p e b b l e c o n g l o m e r a t e u r a n i u m - t h o r i u m d e p o s i t s m a i n l y in t h e C a n a d i a n a n d A f r i c a n S h i e l d s . In a d d i t i o n e a r l y P r o t e r o z o i c a n d m i d d l e a n d l a t e P r o t e r o z o i c t e r r a n e s a r e c h a r a c t e r i z e d by e p i g e n e t i c v e i n - t y p e and i m p r e g n a t i o n - t y p e d e p o s i t s ; metasomatites minerals.

containing

disseminations

of u r a n i n i t e ,

coffinite,

and

other

All P r o t e r o z o i c g r a n i t i c t e r r a n e s c o n t a i n g r a n i t i c , p e g m a t i t i c

Th-U o c c u r r e n c e s , and s o m e c o n t a i n l a r g e d e p o s i t s . deposits

a l s o in s o m e t e r r a n e s by

occur

in

Proterozoic

terranes

(e.g.

and

soda-rich

radioactive migmatitic

Only a few s a n d s t o n e - t y p e u r a n i f e r o u s

Oklo,

Gabon;

Simpson

Islands

and

other

l o c a l i t i e s in t h e E a s t A r m of G r e a t S l a v e L a k e , C a n a d a ) .

T h e A r c h e a n is h e r e c o n s i d e r e d t o e m b r a c e t h o s e r o c k s o l d e r t h a n 2 . 5 b i l l i o n y e a r s . T h i s w o u l d p l a c e t h e u r a n i f e r o u s W i t w a t e r s r a n d q u a r t z - p e b b l e c o n g l o m e r a t e s by c e r t a i n i s o t o p i c a g e d e t e r m i n a t i o n s ( 2 . 7 - 3 . 1 billion y e a r s ) in t h e A r c h e a n . H o w e v e r , t h e s e g r e a t d e p o s i t s l i e u n c o n f o r m a b l y on o l d e r g r e e n s t o n e a n d s e d i m e n t a r y b e l t s ( S w a z i l a n d S y s t e m ) a n d a r e in t h e s a m e s t r a t i g r a p h i c p o s i t i o n a s m a n y o t h e r s u c h d e p o s i t s ( e . g . Blind R i v e r E l l i o t L a k e ) w h i c h a r e of A p h e b i a n or L o w e r P r o t e r o z o i c a g e . It w o u l d s e e m on t h i s basis t h a t t h e a g e d e t e r m i n a t i o n s for t h e W i t w a t e r s r a n d a r e u n r e l i a b l e , t h e s t r a t i g r a p h i c d a t a m o r e s e c u r e , and a L o w e r P r o t e r o z o i c a g e m o r e p r o b a b l e . F u r t h e r s u p p o r t s e e m s e v i d e n t f r o m t h e f a c t t h a t s o m e of t h e a g e d e t e r m i n a t i o n s on T h - U m i n e r a l s in t h e W i t w a t e r s r a n d d e p o s i t s yield ages as young a s 2 billion y e a r s . It s h o u l d a l s o b e m e n t i o n e d t h a t a n u m b e r of t h o r i u m a n d u r a n i u m o c c u r r e n c e s , s o m e c o n t a i n i n g up t o 8 0 0 p p m U, a r e k n o w n in A r c h e a n g r a n i t i c , p e g m a t i t i c , a n d m i g m a t i t i c b o d i e s , e x a m p l e s b e i n g t h o s e in t h e E n g l i s h R i v e r a n d W a b i g o o n S u b p r o v i n c e s of t h e S u p e r i o r P r o v i n c e of t h e C a n a d i a n Shield (Breaks e t al., 1978).

131 Cambro-Ordovician,

Carboniferous,

and

Permian

u r a n i u m d e p o s i t s in m a n y p a r t s of t h e w o r l d . orogenic belts are

marked

terranes

contain

sandstone-type

The H e r c y n i a n (Variscan) and

by v e i n - t y p e , s t o c k w o r k ,

and i m p r e g n a t i o n

t e r r e s t r i a l v o l c a n i c b a s i n s a n d c a l d e r a s a n d in o l d e r

terranes

where

equivalent

deposits often Hercynian

in

granitic

bodies are present. Triassic and Jurassic s e d i m e n t a r y sequences contain s a n d s t o n e - t y p e deposits t h r o u g h ­ o u t t h e w o r l d , a n d C r e t a c e o u s t e r r a n e s a r e s i m i l a r l y e n d o w e d in s o m e p l a c e s . sedimentary

terranes,

particularly

the Tertiary

sedimentary

All C e n o z o i c

sequences, are

notable

for

their sandstone-type and/or supergene chemical p r e c i p i t a t e - t y p e (calcrete) deposits. Nevadan-Laramide,

Alpine,

and

younger

orogenic

belts

(and

their

The

equivalents)

are

c h a r a c t e r i z e d by v e i n - t y p e , s t o c k w o r k , a n d i m p r e g n a t i o n d e p o s i t s in m a n y c o u n t r i e s . of

these

deposits

sedimentary Quaternary

are

terranes

associated intruded

periods were

by

marked

with

volcanic

high

level

basins

granitic

by t h e d e v e l o p m e n t

and

calderas;

bodies.

The

of p l a c e r s

others

late

Many

occur

Tertiary

mainly

of r a r e

in and

earth,

t h o r i u m , a n d z i r c o n i u m m i n e r a l s c o n t a i n i n g s m a l l a m o u n t s of u r a n i u m . W i t h i n t h e b r o a d f r a m e w o r k of t h e e a r t h s o m e 50 p e r c e n t of t h e k n o w n u r a n i u m o r e (mined

and

in

reserves

on

a

tonnage

of

contained

U 3 O 8 basis)

P r e c a m b r i a n r o c k s m a i n l y of L o w e r a n d U p p e r P r o t e r o z o i c a g e .

in

the

world

t h i s p e r c e n t a g e is in q u a r t z - p e b b l e c o n g l o m e r a t e s a n d t h e r e m a i n d e r in g r a n i t i c p e g m a t i t i c , vein, and d i s s e m i n a t e d d e p o s i t s . Proterozoic.

to

Ordovician,

and

calcrete-type

Quaternary Permian,

Quaternary age. rocks; those

deposits

(Recent),

in

with

Triassic-Jurassic,

sedimentary

most

basins

concentrations

Paleocene-Eocene

complex,

ranging in

in

rocks

(Paleogene),

is in

age

of

from

Cambro-

and

Neogene-

T h e d e p o s i t s in m o s t t e r r a n e s a r e g e n e r a l l y o n l y s l i g h t l y y o u n g e r t h a n t h e

in t h e c a l c r e t e - t y p e

deposits are contemporaneous

c e m e n t i n g m a t e r i a l s of t h e u n c o n s o l i d a t e d s e d i m e n t s .

or

nearly

so w i t h

the

T h e o t h e r half of t h e u r a n i u m o r e s

in t h i s c a t e g o r y a r e v e i n , s t o c k w o r k , and d i s s e m i n a t e d t y p e s in r o c k s of v a r i o u s a g e s Cambrian

to

Tertiary.

Most

of

P a l e o z o i c ) and N e v a d a n - L a r a m i d e their equivalents.

in

T h e a g e of t h e s e d e p o s i t s is p r i n c i p a l l y L o w e r

Of t h e r e m a i n i n g 50 p e r c e n t of t h e k n o w n u r a n i u m o r e s a b o u t half

sandstone-type Cambrian

is

S l i g h t l y m o r e t h a n half of

these

deposits

are

related

to

the

and Alpine ( C r e t a c e o u s - T e r t i a r y )

Hercynian

from (Late

orogenic events

and

T h e a b o v e p e r c e n t a g e e s t i m a t e s do n o t i n c l u d e u r a n i u m in o c c u r r e n c e s

s u c h a s b l a c k s h a l e s a n d s c h i s t s , p h o s p h o r i t e s , and o t h e r low g r a d e c o n c e n t r a t i o n s of

the

element.

are

T h e r e s e r v e s in t h e s e

t y p e s of

deposits

are

largely

unknown, but they

c o n s i d e r a b l e , p r o b a b l y g r e a t e r t h a n t h o s e of a l l of t h e d e p o s i t s m e n t i o n e d a b o v e . T h e p e r c e n t a g e d i s t r i b u t i o n of t h o r i u m o r e s is m o r e d i f f i c u l t t o a s s e s s s i n c e f e w d a t a a r e a v a i l a b l e on t h i s s u b j e c t . two

sources - Precambrian

M o s t of t h e p o t e n t i a l r e s e r v e s of t h o r i u m (80 p e r c e n t ) a r e in (Lower

Proterozoic)

quartz-pebble

conglomerates

mainly

E l l i o t L a k e , O n t a r i o , a n d in T e r t i a r y a n d Q u a t e r n a r y ( R e c e n t ) b e a c h , d u n e , r i v e r ,

at

stream,

a n d e l u v i a l p l a c e r s in I n d i a , B r a z i l , a n d A u s t r a l i a , w i t h s m a l l e r r e s e r v e s in U . S . S . R . , U . S . A . ,

132 a n d a n u m b e r of o t h e r c o u n t r i e s in A f r i c a a n d A s i a .

Of t h e 80 p e r c e n t t o t a l p r o b a b l y a b o u t

e q u a l p e r c e n t a g e s of t h e r e s e r v e s a r e in t h e q u a r t z - p e b b l e c o n g l o m e r a t e s modern placers.

Vein-type (South Africa,

and t h e

more

U . S . A . , U . S . S . R . ) a n d d i s s e m i n a t e d d e p o s i t s of

t h o r i u m - b e a r i n g m i n e r a l s in g r a n i t i c c o m p l e x e s , p e g m a t i t e s , p y r o x e n i t e s , c a r b o n a t i t e s , a n d peralkaline syenite complexes (Canada,

Malagasy, South West Africa, Greenland,

Brazil,

U . S . S . R . ) c o m p r i s e t h e r e m a i n i n g 20 p e r c e n t of t h e t h o r i u m r e s e r v e s . C o n c e r n i n g r o c k t y p e s a b o u t 70 p e r c e n t of t h e k n o w n u r a n i u m a n d t h o r i u m

deposits

a r e in s e d i m e n t a r y r o c k s or t h e i r m e t a m o r p h i c e q u i v a l e n t s , p a r t i c u l a r l y t h e c o n g l o m e r a t e s a n d s t o n e - s h a l e s u i t e of t e r r e s t r i a l or n e a r - s h o r e m a r i n e o r i g i n ; d e p o s i t s in c a r b o n a t e r o c k s and e v a p o r i t e s

are relatively

granitic-syenitic

r o c k s of

rare.

both

About

anatectic

15 p e r c e n t and

of

intrusive

the known deposits occur

origin

and

some

in

15 p e r c e n t

v o l c a n i c flows, i g n i m b r i t e s , p y r o c l a s t i c s and a s s o c i a t e d v o l c a n o g e n i c s e d i m e n t s .

in

In m o r e

d e t a i l a b o u t 70 p e r c e n t of t h e u r a n i u m - t h o r i u m d e p o s i t s in s e d i m e n t s a r e in c o n g l o m e r a t e s , sandstones, and

modern placers (gravels and sands), with

c a r b o n a t e r o c k s or t h e i r m e t a m o r p h i c e q u i v a l e n t s . suite

tend

to favour

the

more

alkalic

varieties,

the

remainder

in s h a l e s

T h e d e p o s i t s in t h e and

those

in t h e

and

granitic-syenite

volcanics

exhibit

a

p r e f e r e n c e for r h y o l i t i c , t r a c h y t i c , a n d p h o n o l i t i c t e r r a n e s . F i v e b r o a d s c a l e g e o l o g i c a l e n v i r o n m e n t s h o s t m o s t of t h e k n o w n deposits - geosynclines and their near-shore

marine

volcanic

culminant

centres

(basins

orogenic and

belts; aulacogens; continental

calderas); closed

m o n t a n e s e d i m e n t a r y basins and t r o u g h s ; and deeply gradual uplift.

thorium-uranium

structural

and

and

inter-

w e a t h e r e d young t e r r a n e s undergoing

T h e l a s t c o n t a i n t h e r e c e n t u n c o n s o l i d a t e d p l a c e r s a b o u t w h i c h m o r e will b e

said subsequently. Miogeosynclines types

of

are

favourable

thorium-uranium

environments

deposits -

pyritic

for

the

formation

quartz-pebble

p e g m a t i t i c - m i g m a t i t i c - g r a n i t i c c o m p l e x deposits, and vein d e p o s i t s . in

gently

dipping

molassic-like

facies

of

quartz-pebble

of

three

conglomerate

general deposits,

T h e f i r s t is d e v e l o p e d

conglomerates

and

quartzites

c o n s t i t u t i n g t h e b a s a l , n e a r b a s a l , a n d l o w e r u p p e r s e q u e n c e s l a i d d o w n in m a r g i n a l b a s i n s of m i o g e o s y n c l i n e s d e v e l o p e d

on a n c i e n t

(eugeosynclinal)

r o c k s of

massifs,

an

example

b e i n g t h e v a r i o u s r e e f s of t h e W i t w a t e r s r a n d in S o u t h A f r i c a a n d t h e M a t i n e n d a F o r m a t i o n a t Elliot L a k e , O n t a r i o . deposits

The vein-, s t o c k w o r k - , p e g m a t i t i c - , and g r a n i t i c

in m i o g e o s y n c l i n a l

environments

occur

in m e d i u m

to

complex-type

high g r a d e

metamorphic

sedimentary sequences comprising conglomerates, quartzites, shales, argillites, thin-bedded carbonate

rocks, and a m o d i c u m

of p y r o c l a s t i c s a n d

some volcanics.

t e r r e s t r i a l c o n d i t i o n s of d e p o s i t i o n a r e i n d i c a t e d for m o s t of t h e s e r o c k s . metamorphism

and

tectonic

activity

these

suites

of r o c k s b e c o m e

Near

shore

and

With i n c r e a s i n g

highly

migmatized,

g r a n i t i z e d , a n d i n j e c t e d by a n a t e c t i c g r a n i t i c b o d i e s a n d p e g m a t i t i c m a s s e s ; t h i s c o m p l e x is then sundered

by e x t e n s i v e

associated brecciated zones.

faults

and

mylonitized

zones with numerous subsidiary

During the granitization period thorium-uranium

and

pegmatites

133 and t h o r i u m - u r a n i u m m i g m a t i t e s and m i g m a t i t i c c o m p l e x e s a r e formed; with the late granitization pitchblende

deposits

phase and with faulting

make their

appearance.

contemporaneous

and m y l o n i t i z a t i o n various types

E x a m p l e s of t h e d e p o s i t s in

of

migmatized

t e r r a n e s a r e r e p r e s e n t e d by t h o s e in t h e B a n c r o f t a r e a in O n t a r i o a n d t h e R o s s i n g d e p o s i t in N a m i b i a ; e x a m p l e s in m i o g e o s y n c l i n e s r e l a t e d t o f a u l t s , b r e c c i a z o n e s , a n d m y l o n i t i z e d z o n e s i n c l u d e t h e B e a v e r l o d g e r e g i o n of S a s k a t c h e w a n , t h e E a s t A l l i g a t o r R i v e r r e g i o n of A u s t r a l i a , a n d t h e M a s s i f C e n t r a l of F r a n c e . D e p o s i t s in a u l a c o g e n s a r e v a r i e d , c o m p r i s i n g p y r i t i c q u a r t z - p e b b l e p i t c h b l e n d e veins, and s a n d s t o n e t y p e s .

conglomerates,

E x a m p l e s of a u l a c o g e n s w i t h a v a r i e t y of u r a n i u m

d e p o s i t s i n c l u d e t h e v o l c a n i c a n d t e r r e s t r i a l s e d i m e n t a r y s u i t e s of r o c k s i n t h e E a s t A r m of G r e a t S l a v e L a k e , N o r t h w e s t T e r r i t o r i e s , C a n a d a , b o u n d e d on t h e s o u t h by t h e

MacDonald

F a u l t ; possibly t h e M a t i n e n d a F o r m a t i o n and a s s o c i a t e d s e d i m e n t s and volcanics a t Elliot L a k e a n d Blind R i v e r a n d a r e a s w e s t w a r d , b o u n d e d on t h e s o u t h by a m a j o r sandstone-type formed

along

d e p o s i t s in t h e C a r b o n i f e r o u s major

strike-slip

faults

(Hyde,

Deer

L a k e Basin,

1980); and

Western

various

fault;

the

Newfoundland,

vein-type

deposits

in

u n s p e c i f i e d a r e a s in U . S . S . R . in v o l c a n i c a n d s e d i m e n t a r y s e q u e n c e s t e r m i n a t e d on t h e i r d i p by a x i a l l o n g i t u d i n a l f a u l t s ( M o d n i k o v e t a l . , 1 9 7 9 b ) . U r a n i u m d e p o s i t s i n v o l c a n i c t e r r a n e s o c c u r in t w o e n v i r o n m e n t s - in c o n t i n e n t a l a n d marine

cherts,

and

a s s o c i a t e d ( r e d b e d ) s e d i m e n t s , a n d in or n e a r a n c i e n t c a l d e r a s w i t h r h y o l i t e - a n d e s i t e

near-shore

flow

assemblages, felsic

volcanic

basins

composed

principally

of

pyroclastics,

v o l c a n i c l a s t i c s , c h e r t s , and a s s o c i a t e d t e r r e s t r i a l s e d i m e n t s .

In b o t h

e n v i r o n m e n t s t h e e x t r u s i v e s a n d a s s o c i a t e d v o l c a n i c l a s t i c s a n d s e d i m e n t s a r e u s u a l l y c u t by a c h a r a c t e r i s t i c s u i t e of q u a r t z - f e l d s p a r a n o m a l o u s in t h e i r u r a n i u m c o n t e n t .

and d a c i t e p o r p h y r i e s t h a t a r e o f t e n

by h i g h - l e v e l g r a n i t i c b o d i e s , f r e q u e n t l y of a l k a l i c c h a r a c t e r metamorphic aureoles.

markedly

Some volcanic basins and c a l d e r a s a r e also intruded and with relatively

narrow

T h e u r a n i u m d e p o s i t s a r e v e i n s , s t o c k w o r k s or d i s s e m i n a t e d b o d i e s ,

a n d m a y r a n g e in a g e f r o m P r o t e r o z o i c t o Q u a t e r n a r y .

M a n y of t h e d e p o s i t s e x h i b i t o n l y

primary features, others are deeply leached and enriched.

Silicification and h e m a t i t i z a t i o n

( j a s p e r i z a t i o n ) of b o t h h y p o g e n e a n d s u p e r g e n e o r i g i n is a f e a t u r e of a l l t y p e s of u r a n i u m d e p o s i t s in v o l c a n i c t e r r a n e s .

T h e s u p e r g e n e v a r i e t y of s i l i c i f i c a t i o n is e s p e c i a l l y c o m m o n

in t h e y o u n g e r v o l c a n i c t e r r a n e s , t h e r o c k s in m a n y p l a c e s e x h i b i t i n g v e i n s , s e g r e g a t i o n s , i m p r e g n a t i o n s , r e p l a c e m e n t s , a n d l a y e r s of f i n e g r a i n e d q u a r t z , c h a l c e d o n y , j a s p e r , c h e r t , and opal, all c o m m o n l y r a d i o a c t i v e and t h e last, opal, o f t e n m a r k e d l y f l u o r e s c e n t .

The

o l d e r s e q u e n c e s of v o l c a n i c s a n d t e r r e s t r i a l s e d i m e n t s t e n d t o l i e in w e l l d e f i n e d b a s i n s on c r a t o n s or m a s s i f s w h e r e a s t h e y o u n g e r a r e p r e s e n t e s s e n t i a l l y in c a l d e r a s , o f t e n b u t always near former continental margins. include

the

Territories

Proterozoic with

Echo

numerous

deposits at unspecified

Bay

pitchblende

s i t e s in L a t e

E x a m p l e s of o l d e r v o l c a n i c - s e d i m e n t a r y

Group

in

the

deposits,

Paleozoic

Great and

Bear

various

Lake

basins

Northwest

pitchblende-molybdenite

volcanic-sedimentary

(Kazansky and L a v e r o v , 1977; Modnikov e t al., 1979b).

region,

not

b a s i n s in

U.S.S.R.

E x a m p l e s of t h e y o u n g e r

volcanic

134 centres

hosting

Cenozoic

rocks

commonly

pitchblende

and

of

U.S.A.,

Western

represent

other

include

nearly

largely removed

flat-lying

basic-acidic

deposits are

Japan,

e x t i n c t c a l d e r a s or d e e p - s e a t e d

s u r f a c e p a r t s of w h i c h h a v e b e e n deposits

thorium-uranium

Yugoslavia,

and

widespread

elsewhere.

in

Such

p a r t s of t h e s e f e a t u r e s , t h e by e r o s i o n .

flows,

the

centres near-

T h e h o s t r o c k s of

volcaniclastics,

and

the

terrestrial

s e d i m e n t s ( r e d a n d g r e y b e d s ) , c u t by d i k e s w a r m s a n d s t o c k s , t h e w h o l e c o m p l e x e s p l a c e s being i n t r u d e d by high level g r a n i t i c bodies o f t e n

of m o n z o n i t i c c h a r a c t e r .

u r a n i f e r o u s d e p o s i t s a r e i n v a r i a b l y in f a u l t s , f r a c t u r e s , a n d b r e c c i a p i p e s in z o n e s t h a t extensively examples

propylitized,

are

molybdenite

the

alunitized,

Marysvale

deposits

in

silicified,

uranium

calderas

and

deposits

in

in

unspecified

otherwise Utah

and

areas

highly a

in

altered.

number

the

in The are

Typical

of

pitchblende-

U.S.S.R.

(Modnikov

e t al., 1979a, 1979b). Sandstone-type

deposits

are

the

most

common

uranium

deposits

found

in

closed

s t r u c t u r a l a n d i n t e r m o n t a n e b a s i n s a n d t r o u g h s f i l l e d w i t h a c c u m u l a t i o n s of n e a r l y lying,

gently

warped,

and

moderately

faulted

lenticular

terrestrial

sediments,

flat-

mainly

c o n g l o m e r a t e s , s a n d s t o n e s , s h a l e s , a n d t u f f a c e o u s s h a l e s , d e p o s i t e d on g e n t l y s i n k i n g r i g i d b l o c k s or p r i s m s in t h e m o b i l e b e l t s of t h e e a r t h ' s c r u s t .

Such basins and troughs

g e n e r a l l y r i m m e d or f l a n k e d by m o u n t a i n o u s z o n e s e x h i b i t i n g i n t e n s e d e f o r m a t i o n ,

are

typical

e x a m p l e s b e i n g t h e C o l o r a d o P l a t e a u , t h e Wind R i v e r a n d S h i r l e y B a s i n s in W y o m i n g , t h e Ferghana deposits

B a s i n in U . S . S . R . , constitute

a n d t h e O r d o s B a s i n in C h i n a .

the second

t y p e of u r a n i u m

The

concentrations

calcrete

localized

and

similar

m a i n l y in

the

d r a i n a g e s y s t e m s of c l o s e d s t r u c t u r a l a n d i n t e r m o n t a n e b a s i n s , p a r t i c u l a r l y t h o s e in

the

a r i d a n d s e m i - a r i d r e g i o n s of t h e e a r t h . The

search

migmatites,

and

particularly anatexis,

to

thoriferous

sedimentary

granitic

developed.

for

and/or

uraniferous

migmatite-pegmatite-aplite-granitic

intrusion,

terranes and

their

in

which

zones

associated

granitic complexes

of

high

bodies, should

granitization,

grade

pegmatites, be

confined

migmatization,

metamorphic

facies

are

S u c h c o m p l e x m i g m a t i t i c z o n e s a n d t h e i r a s s o c i a t e d b o d i e s of g r a n i t i c r o c k ,

w h e n h o s t t o g r a n i t i c - t y p e T h - U d e p o s i t s , a r e u s u a l l y m a r k e d by a h i g h e r

than

normal

r a d i o a c t i v e p r o f i l e a n d h e n c e , if r e a s o n a b l y e x p o s e d , c a n g e n e r a l l y b e o u t l i n e d q u i c k l y a n d efficiently the

by r e c o n n a i s s a n c e a i r b o r n e , c a r b o r n e , or s h o e b o r n e r a d i a t i o n s u r v e y s .

granitic

terranes

are

deeply

weathered

or

covered

by

extensive

glacial

Where deposits

reconnaissance hydrochemical (lake sediment, s t r e a m sediment, s t r e a m and spring

water,

a n d s p r i n g p e r c i p i t a t e s ) , a n d / o r s o i l s u r v e y s s e r v e b e s t t o o u t l i n e f a v o u r a b l e a r e a s for m o r e detailed prospecting.

R e c o n n a i s s a n c e b i o g e o c h e m i c a l surveys utilizing p l a n t s and

trees

a r e , l i k e w i s e , e f f e c t i v e in m o s t t e r r a i n s , a n d m a y b e p a r t i c u l a r l y u s e f u l in t h e h o t a r i d a n d s e m i - d e s e r t r e g i o n s of t h e w o r l d .

135 Within g r a n i t i c and a s s o c i a t e d high g r a d e m e t a m o r p h i c t e r r a n e s t h e d e t a i l e d

search

f o r T h - U d e p o s i t s s h o u l d b e f o c u s e d on k n o w n a n d / o r p r o j e c t e d b o r d e r z o n e s a n d c o n t a c t s of

both

anatectic

intrusives;

on z o n e s

and

intrusive

in g r a n i t i c

granitic

bodies;

bodies, contact

on

small

aureoles,

and

medium

migmatites,

sized

and

alkalic

anatectites

m a r k e d by a p r o l i f e r a t i o n of a p l i t i c a n d / o r p e g m a t i t i c p h a s e s ; on a l b i t i z e d , s e r i c i t i z e d , a n d o t h e r p o t a s h m e t a s o m a t i z e d z o n e s in g r a n i t i c r o c k s a n d t h e i r e n c l o s i n g g n e i s s e s , s c h i s t s , and hornfels;

on b i o t i t i z e d , c h l o r i t i z e d , a n d c h l o r i t i z e d b i o t i t i c z o n e s w i t h i n a n d a t

p e r i p h e r y of g r a n i t i c b o d i e s a n d t h e i r a d j a c e n t c o u n t r y r o c k s ; on r e d d e n e d zones;

the

(hematitized)

a n d o n z o n e s of a c t u a l a n d s u s p e c t e d f a u l t i n g , s h e a r i n g , f i s s u r i n g , b r e c c i a t i o n , a n d

mylonitization.

T h e s e a r e i n v a r i a b l y t h e s i t e s t h a t a r e e n r i c h e d in t h o r i u m a n d

uranium

m i n e r a l s a n d a r e b e s t o u t l i n e d b y d e t a i l e d r a d i a t i o n s u r v e y s , soil a n d o v e r b u r d e n s u r v e y s , atmochemical

surveys,

biogeochemical

surveys,

and

where

exposures

permit

by

litho­

c h e m i c a l (rock) surveys. In a n u m b e r of h i g h l y g r a n i t i z e d t e r r a n e s , z o n e s of p r o d u c t i v e u r a n i n i t e ( p i t c h b l e n d e ) d e p o s i t s in p e g m a t i t e s , g r a n i t i c d y k e s , a n d m i g m a t i t e s a r e m a r k e d by a r e g i o n a l h a l o , o f t e n i m p e r f e c t , of s m a l l v e i n s , p e g m a t i t e s , a p l i t e s , a n d g r a n i t i c d i k e s c a r r y i n g s m a l l a m o u n t s of molybdenite, c h a l c o p y r i t e , and fluorite, t h e first mineral being particularly indicative.

A

t y p i c a l e x a m p l e of t h i s p h e n o m e n o n c a n b e o b s e r v e d in t h e B a n c r o f t a n d W i l b e r f o r c e a r e a s in t h e G r e n v i l l e s u b - p r o v i n c e of s o u t h e r n O n t a r i o . T h e i m p o r t a n c e of t w o - m i c a ( b i o t i t e - m u s c o v i t e ) a n d b i o t i t e g r a n i t e s a s a n i n d i c a t o r of t h e p r e s e n c e of u r a n i u m m i n e r a l i z a t i o n h a s b e e n e m p h a s i z e d by a n u m b e r of g e o l o g i s t s , e s p e c i a l l y in F r a n c e , S p a i n , a n d P o r t u g a l .

M o s t of t h e s e g r a n i t i c b o d i e s a r e

high-level

r e l a t i v e l y y o u n g ( a s c o m p a r e d w i t h o t h e r g r a n i t i c m a s s e s in a n a r e a ) i n t r u s i v e b o d i e s .

Such

g r a n i t e s c o m m o n l y h a v e m e t a l l o g e n i c s i g n a t u r e s for t i n , t u n g s t e n , b e r y l l i u m , l i t h i u m , a n d fluorine. granite.

T h e s i g n a t u r e f o r u r a n i u m w o u l d a p p e a r , h o w e v e r , t o d e p e n d on t h e a g e of

the

T h u s , t h e r e a r e n u m e r o u s A r c h e a n t w o m i c a a n d b i o t i t e g r a n i t e s in t h e C a n a d i a n

S h i e l d a n d e l s e w h e r e t h a t do n o t h a v e a s s o c i a t e d u r a n i u m d e p o s i t s b u t m a y h a v e a s s o c i a t e d o c c u r r e n c e s of t h e t w o r a d i o a c t i v e e l e m e n t s o r d e p o s i t s of t h e o t h e r s i g n a t u r e noted.

elements

F r o m this we can say t h a t two mica and b i o t i t e g r a n i t e s younger than A r c h e a n a r e

the most favourable indicator types. Regions containing thoriferous and uraniferous peralkaline syenite bodies, and c o m p l e x e s terranes. overburden

can

be outlined

by

methods

similar

to

those

noted

above

for

massifs, granitic

M o r e d e t a i l e d s u r v e y s for t h e s e b o d i e s s h o u l d i n c l u d e r a d i a t i o n s u r v e y s , soil a n d surveys,

atmochemical

surveys,

surveys where the rocks are exposed.

biogeochemical

surveys,

and

lithochemical

Heavy mineral surveys utilizing panned soils, other

t y p e s of o v e r b u r d e n , a n d s t r e a m s e d i m e n t s a r e c o m m o n l y a l s o e f f e c t i v e in o u t l i n i n g z o n e s in t h e s e r o c k s t h a t a r e e n r i c h e d in z i r c o n i u m , t h o r i u m , u r a n i u m , a n d so o n .

136 Both peralkaline

massifs

and c o m p l e x e s and c a r b o n a t i t e s

tend

to occur

in s t a b l e

c r u s t a l r e g i o n s ( s h i e l d s ) of t h e e a r t h , t h e l a t t e r p a r t i c u l a r l y in or n e a r g r e a t r i f t z o n e s a n d graben structures.

C a r b o n a t i t e s c a n g e n e r a l l y b e p i n - p o i n t e d by r e g i o n a l

reconnaissance

s u r v e y s b a s e d on s t r e a m a n d l a k e s e d i m e n t s , w a t e r s , s p r i n g p r e c i p i t a t e s , s o i l s , a n d t y p e s of o v e r b u r d e n u t i l i z i n g i n d i c a t o r e l e m e n t s s u c h a s N b , B a , a n d F . t h o r i u m and u r a n i u m can f r e q u e n t l y

other

T h o s e e n r i c h e d in

b e o u t l i n e d by r e c o n n a i s s a n c e a i r b o r n e or

carborne

r a d i a t i o n s u r v e y s if t h e o v e r b u r d e n is t h i n ; a l s o by a i r b o r n e m a g n e t o m e t e r s u r v e y s s i n c e s o m e of t h e s e b o d i e s c o n t a i n c o n s i d e r a b l e q u a n t i t i e s of m a g n e t i t e .

T h e w a t e r s of s p r i n g s

a n d s t r e a m s in t h e v i c i n i t y of c a r b o n a t i t e s , e s p e c i a l l y t h o s e of m o r e r e c e n t a g e , b u t a l s o t h o s e of

Paleozoic

abundant C 0

2

and Mesozoic

age, are commonly

characterized

and c h l o r i d e ( C l ) , and alkalis, especially sodium.

and o v e r b u r d e n surveys a r e g e n e r a l l y e f f e c t i v e

by h i g h p H ' s 0^-8),

Detailed stream sediment

in o u t l i n i n g c a r b o n a t i t e

complexes

with

c o n s i d e r a b l e p r e c i s i o n a s s h o w n by t h e w o r k of B l o o m f i e l d e t a l . ( 1 9 7 1 ) in e a s t e r n U g a n d a a n d C F . G l e e s o n ( p e r s . c o m m . , 1975) in w e s t e r n Q u e b e c .

H e a v y m i n e r a l s u r v e y s b a s e d on

p a n n e d c o n c e n t r a t e s a r e , l i k e w i s e , e f f e c t i v e in m o s t p a r t s of t h e w o r l d b o t h for

indicating

t h e g e n e r a l p r e s e n c e of c a r b o n a t i t e s a n d for o u t l i n i n g t h e b o d i e s in d e t a i l . C l o s e s c r u t i n y of a e r o p l a n e

p h o t o g r a p h s of s h i e l d or s t a b l e a r e a s n e a r

suspected rifts and grabens frequently

known

r e v e a l s t h e p r e s e n c e of c a r b o n a t i t e s , t h e

or

distin­

g u i s h i n g c h a r a c t e r i s t i c s b e i n g c i r c u l a r o r o v a l f e a t u r e s of a n e l e v a t e d o r d e p r e s s e d n a t u r e o f t e n m a r k e d by v e r d a n t v e g e t a t i o n .

T h e m o r e l u x u r i o u s v e g e t a t i o n is e v i d e n t l y d u e t o t h e

p r e s e n c e in m o s t c a r b o n a t i t e s a n d t h e i r w e a t h e r e d p r o d u c t s of r e a d i l y a v a i l a b l e p o t a s s i u m , phosphorus, and o t h e r n u t r i e n t s such as r a r e e a r t h s which have a s t i m u l a t i v e g r o w t h on c e r t a i n p l a n t s .

effect

In a d d i t i o n , c e r t a i n s p e c i e s of p l a n t s m a y p r o l i f e r a t e o v e r c a r b o n a t i t e s

c r o w d i n g o u t o t h e r s p e c i e s , t h u s p r o d u c i n g v e g e t a t i v e c o n t r a s t s on a e r o p l a n e p h o t o g r a p h s . Certain

lamprophyres

may

be

radioactive,

contain

significant

amounts

of

t h o r i u m (up t o 50 p p m ) a n d u r a n i u m (up t o 20 p p m ) , a n d m a y b e i n d i c a t i v e of and/or

uranium

mineralization

in

a

district.

In f a c t ,

in m a n y

uraniferous

both

thorium

areas

the

l a m p r o p h y r e s m a y c o n t a i n o r b e t r a n s e c t e d by s m a l l p i t c h b l e n d e ( u r a n i n i t e ) v e i n l e t s e i t h e r w i t h i n t h e b o d i e s , a l o n g t h e i r c o n t a c t s , or in a d j a c e n t r o c k s

(e.g. Massif C e n t r a l , F r a n c e ;

N o r t h S h o r e of L a k e S u p e r i o r , C a n a d a ; B a k e r L a k e B a s i n , N o r t h w e s t T e r r i t o r i e s , C a n a d a ) . Concerning lamprophyres there are two general types - one (minette, vogesite, kersantite) a s s o c i a t e d with and c o m m o n l y younger than high level g r a n i t i c intrusions and t h e (camptonite,

monchiquite, alnoite) associated

with peralkaline

other

s y e n i t e s or

carbonatites.

B o t h t y p e s m a y b e r a d i o a c t i v e ; t h e t h o r i u m a n d u r a n i u m r e s p o n s i b l e for t h e

radioactivity

b e i n g of e i t h e r s y n g e n e t i c or e p i g e n e t i c ( p o s t - l a m p r o p h y r e ) o r i g i n .

S e e a l s o t h e p a p e r s by

E m m o n s e t al. (1953) and Miller (1980). Uraniferous and thoriferous calcite-fluorite-apatite and

bodies

are

mainly

restricted

to

high

grade

veins and similar

metamorphic

terranes

segregations marked

by

p a r a p y r o x e n i t e s and m a r b l e s s u c h a s t h o s e p r e v a i l i n g in t h e G r e n v i l l e P r o v i n c e of C a n a d a ;

137 s o m e of t h e s e b o d i e s a r e , h o w e v e r , p r e s e n t in l e s s m e t a m o r p h o s e d t e r r a n e s b u t s e l d o m f a r from

d i o r i t i c a n d g r a n i t i c i n t r u s i v e s or h i g h l y g r a n i t i z e d

defined

uraniferous

similar

to

those

and/or

thoriferous

described

below

provinces.

for

veins

zones.

Radiation

and

lodes

All o c c u r

within

and g e o c h e m i c a l

can

generally

be

well

methods

applied

in

their search. T h o r i u m - and u r a n i u m - b e a r i n g skarn (including p a r a p y r o x e n i t e s and p a r a a m p h i b o l i t e s ) and c o n t a c t

metamorphic

deposits require

little comment.

These occur

mainly

c o n t a c t a u r e o l e s of g r a n i t i c a n d a l l i e d r o c k s a n d in m e d i u m - a n d h i g h - g r a d e terranes. small

in

the

metamorphic

Many a r e associated with high level i n t r u s i v e - t y p e g r a n i t e stocks, bosses, and

batholiths.

Nearly

all

deposits

are

developed

in

limestones,

dolomites,

c a l c a r e o u s or d o l o m i t i c s a n d s t o n e s or s h a l e s in t h e e n v i r o n m e n t s j u s t n o t e d .

and

Metamor­

p h o s e d s e d i m e n t a r y p i l e s of a r g i l l i t e s , b l a c k s h a l e s , p h o s p h o r i t e s , a n d t h e v a r i o u s c a r b o n a t e r o c k s s e e m t o b e p a r t i c u l a r l y f a v o u r a b l e a s do a l s o p i l e s c o n t a i n i n g m e t a - v o l c a n i c tuffs, and c a r b o n a t e rocks.

rocks,

The t h o u g h t should b e k e p t in mind t h a t t h e b l a c k s h a l e s and

argillites, phosphorites, and t u f f a c e o u s

r o c k s m a y b e t h e s o u r c e of t h e u r a n i u m

and

its

a s s o c i a t e d e l e m e n t s , and t h a t t h e s e w e r e mobilized during m e t a m o r p h i s m and g r a n i t i z a t i o n and

chemically

trapped

in t h e

carbonate

rocks

where

suitable

primary

(porosity)

and

secondary structural conditions prevailed. M a n y of t h e m e t a m o r p h i c mineralization

of

the

skarn

and g r a n i t i z e d

and c o n t a c t

t e r r a n e s of t h e w o r l d a r e s u s c e p t i b l e

metamorphic

types.

Deposits

of

these

to

types

a p p e a r e d f i r s t in e a r l y P r o t e r o z o i c t i m e a n d a r e k n o w n in a l l s u b s e q u e n t a g e s of r o c k s . Among mentioned

the

various

vein,

lode,

stockwork,

in C h a p t e r 2, t w o g e n e r a l u r a n i f e r o u s

and

disseminated

types can

types

of

deposits

be recognized - the

b l e n d e - u r a n i n i t e - c o f f i n i t e t y p e with minor sulphides such as m o l y b d e n i t e and

pitch-

chalcopyrite

and t h e c o m p l e x polymetallic t y p e containing essentially pitchblende, Ni-Co arsenides and sulphides,

Cu-Fe

accompanied

sulphides,

native

silver,

and

native

bismuth.

Both

types

may

be

by a q u a r t z a n d c a r b o n a t e g a n g u e a n d m a y c o n t a i n s e l e n i d e s , g o l d , a n d

v a r i e t y of o t h e r m i n e r a l s .

O n l y o n e t y p e of t h o r i u m - r a r e e a r t h v e i n a n d l o d e - t y p e

a

deposit

is k n o w n - t h e m o n a z i t e - t h o r i t e - t h o r o g u m m i t e t y p e , a l t h o u g h t h o r i u m c o m m o n l y o c c u r s a s a m i n o r e l e m e n t in u r a n i n i t e , b r a n n e r i t e , a n d o t h e r m i n e r a l s in c e r t a i n u r a n i f e r o u s and

l o d e s ; a l s o in c e r t a i n r a r e - e a r t h

bastnaesite, florencite, etc.

mineral veins carrying

minerals such as

veins

allanite,

T h e r e a p p e a r t o b e no g e n e r a l r u l e s g o v e r n i n g t h e l o c a l i z a t i o n

of u r a n i f e r o u s a n d t h o r i f e r o u s v e i n s a n d l o d e s , a f e a t u r e t h a t s h o u l d b e k e p t in m i n d b y t h e prospector.

T h e r e a r e , h o w e v e r , c e r t a i n p a t t e r n s of o c c u r r e n c e of t h e s e d e p o s i t s

that

s e r v e a s g u i d e s f o r t h e i r d i s c o v e r y , n o n e of w h i c h a r e of c o u r s e i n f a l l i b l e . On a s t a t i s t i c a l b a s i s u r a n i u m - t h o r i u m v e i n , l o d e , s t o c k w o r k , a n d d i s s e m i n a t e d

types

of d e p o s i t s s h o w t w o c h a r a c t e r i s t i c m o d e s of o c c u r r e n c e w i t h r e s p e c t t o t h e v a r i o u s t y p e s of c o u n t r y r o c k s . rocks.

M o s t d e p o s i t s o c c u r in m e t a - s e d i m e n t a r y p i l e s a n d a s s o c i a t e d

granitic

S o m e o c c u r in v o l c a n i c t e r r a n e s , m a i n l y in b a s i n s a n d c a l d e r a s f i l l e d w i t h a n d e s i t e -

138 r h y o l i t e f l o w s , v o l c a n i c l a s t i c s , c h e r t s , a n d t e r r e s t r i a l s e d i m e n t s i n t r u d e d by d i k e s w a r m s and g r a n i t i c m a s s e s .

A l i m i t e d n u m b e r o c c u r in r e l a t i v e l y u n m e t a m o r p h o s e d

invariably sandstone-conglomerate

s a n d s t o n e - t y p e and a r e b e s t considered under t h a t c a t e g o r y . the

most

favourable

for

sediments,

sequences; these exhibit a m a r k e d relationship to

uranium-thorium

veins and

the

Of t h e h o s t s e d i m e n t a r y r o c k s

lodes

are

miogeosynclinal

facies,

r a n g i n g in a g e f r o m P r e c a m b r i a n t o T e r t i a r y , c o m p r i s i n g c o n g l o m e r a t e , a r k o s e , s a n d s t o n e , subgreywacke,

shale,

black

pyritic

shale

and

argillite,

thin

chert,

limestone,

and/or

d o l o m i t e beds laid down under s u b - a e r i a l c o n t i n e n t a l c o n d i t i o n s (red beds) a n d / o r shallow to m o d e r a t e l y deep w a t e r marine e n v i r o n m e n t . the favourable

host

rocks are volcaniclastics,

in

a

In t h e v o l c a n i c b a s i n s a n d c a l d e r a s

c h e r t s , and

terrestrial

sediments,

acidic

e x t r u s i v e s , a c i d i c and i n t e r m e d i a t e p o r p h y r i t i c dikes, sills, and small s t o c k s , and a s s o c i a t e d granitic

to m o n z o n i t i c

stocks and b a t h o l i t h s .

T h e s u i t e s of

sedimentary

rocks

in

the

m i o g e o s y n c l i n a l s e q u e n c e s a r e i n v a r i a b l y m e t a m o r p h o s e d t o v a r y i n g d e g r e e s , g r a n i t i z e d in places,

and

intruded

by

granitic

masses;

in

many

regions

great

linear,

curved,

and

i n t r i c a t e l y f o l d e d b a n d s of i n t e n s e l y g r a n i t i z e d r o c k s c o m p r i s i n g m i g m a t i t e s , a n a t e c t i t e s , pegmatitic

zones,

recrystallized

feldspathized

s e d i m e n t s and

containing

uranium-thorium

sequences

the

tectonic

zones,

associated veins,

gneisses,

lodes,

framework

and

volcanics are

is

and

highly

metamorphosed

developed.

disseminated

dominated

by

In n e a r l y all

bodies complex

in

and

regions

miogeosynclinal

folding,

belts

of

m y l o n i t i z a t i o n , and e x t e n s i v e s t e e p l y - d i p p i n g b r e c c i a f a u l t s c u t t i n g s e d i m e n t s , v o l c a n i c s , g r a n i t i z e d zones and g r a n i t i c intrusives and with which a r e a s s o c i a t e d n u m e r o u s subsidiary faults,

fractures,

and

brecciated

zones.

Some

of

the

deposits

extensive faults, but most occupy the subsidiary s t r u c t u r e s .

occur

In t h e

in

or

near

volcanic-sedimentary

t e r r a n e s ( c a l d e r a s a n d b a s i n s ) t h e h o s t r o c k s of t h e d e p o s i t s e x h i b i t o n l y low t o g r a d e s of m e t a m o r p h i s m a n d a r e n e a r l y f l a t - l y i n g or o n l y g e n t l y f o l d e d . localized

mainly

in

faults,

fractures,

volcanic

(breccia)

the

necks,

medium

The deposits are

and

in

r e p l a c e a b l e b e d s in z o n e s t h a t c o m m o n l y e x h i b i t e x t e n s i v e p r o p y l i t i z a t i o n ,

favourable

beresitization,

alunitization and silicification. In P r e c a m b r i a n u r a n i f e r o u s p r o v i n c e s a g o o d i n d i c a t o r of u r a n i u m m i n e r a l i z a t i o n in both

miogeosynclinal

and

volcanic

terranes

is

the

presence

of

large

quartz-hematite

s t o c k w o r k s ( " g i a n t - q u a r t z v e i n s " ) d e v e l o p e d m a i n l y in t h e p r i n c i p a l f a u l t s .

S o m e of t h e s e

a r e a c t u a l l y m i n e r a l i z e d w i t h l a t e p h a s e s of p i t c h b l e n d e a s in t h e B e a r P r o v i n c e , N . W . T . C a n a d a , b u t o t h e r s h a v e n o t b e e n r e o p e n e d a n d h a v e l i t t l e or n o u r a n i u m

mineralization.

A n o t h e r e x a m p l e is t h e e x t e n s i v e b a r r e n q u a r t z v e i n s t y p i f i e d by t h e G r e a t P f a h l r u n s f o r m a n y t e n s of k i l o m e t e r s t r a n s e c t i n g t h e u r a n i f e r o u s E r z g e b i r g e of E u r o p e . a g e of t h e s e v e i n s is u n c e r t a i n , b u t t h e y a r e p r o b a b l y P r e c a m b r i a n .

Giant

which The

quartz-hematite

s t o c k w o r k s a r e g e n e r a l l y n o t d e v e l o p e d in r o c k s y o u n g e r t h a n P r e c a m b r i a n ,

their

place

b e i n g o f t e n t a k e n by e x t e n s i v e z o n e s of s i l i c i f i c a t i o n a n d j a s p e r i z a t i o n , a n d t h e s e m a y b e i n d i c a t i v e of u r a n i u m

mineralization

in s o m e t e r r a n e s .

Other

favourable

i n d i c a t o r s of

v e i n - a n d l o d e - t y p e m i n e r a l i z a t i o n in a l l t y p e s of t e r r a n e s a r e t h e p r e s e n c e of h i g h e r

than

139 normal

radioactive

migmatitic,

aplitic

and

pegmatitic

zones;

higher

than

normal

r a d i o a c t i v e b o r d e r z o n e s in i n t r u s i v e a n d a n a t e c t i c g r a n i t i c b o d i e s ; g r a n i t i c b o d i e s

with

dark ( r a d i o a c t i v e l y d a r k e n e d ) q u a r t z ; r e d d e n e d ( h e m a t i t i z e d ) g r a n i t i c bodies; an a b u n d a n c e of

quartz-feldspar

reddened

porphyry

(hematitized)

uranium

mineral

dikes

or

sills in m e t a - s e d i m e n t a r y

feldspathized

stains

along

zones

joints

in

and

country

shears;

pervasive

o c c u r r e n c e of j a s p e r v e i n s a l o n g f a u l t s , f i s s u r e s , f r a c t u r e d , m y l o n i t i z e d z o n e s ; t h e p r e s e n c e of f l u o r i t e occurrence patches,

in t h e t e r r a n e s of a b n o r m a l l y

and

pegmatite-like

sediments and tuffs.

bodies

and

rocks;

volcanic

secondary

terranes; (supergene)

hematitization

and

the

cataclastic, granulated,

and

in t h e g r a n i t i c b o d i e s a n d in v e i n s ; a n d

the

radioactive

containing

skarn-like segregations,

amphiboles,

apatite,

irregular

fluorite,

etc.

in

D i a t r e m e s a n d a l l t e c t o n i c c o l l a p s e a n d o t h e r t y p e s of b r e c c i a in

s e d i m e n t a r y and volcanic t e r r a n e s should r e c e i v e p a r t i c u l a r

attention

as should also

all

o t h e r porous or s h e a r e d s t r a t a . In a n u m b e r of u r a n i f e r o u s d i s t r i c t s t h e r e is c o m m o n l y a s e q u e n c e of d e p o s i t s oldest to youngest as follows:

P e g m a t i t e s , in p l a c e s w i t h h i g h e r t h a n n o r m a l r a d i o a c t i v i t y ;

s k a r n b o d i e s , in p l a c e s w i t h h i g h e r t h a n n o r m a l r a d i o a c t i v i t y ; h i g h t e m p e r a t u r e feldspar-molybdenite-arsenopyrite-wolframite-scheelite-cassiterite-tourmaline greisens

in

or

near

the

granitic

bodies;

lower

temperature

chalcopyrite-arsenopyrite-molybdenite-sphalerite-galena scheelite

and

cassiterite

from

in

some

chalcopyrite-molybdenite-hematite

districts;

veins

and

quartz-

veins

and

quartz-fluorite-barite-

veins and lodes with

wolframite-

quartz-carbonate-pitchblende-(uraninite)lodes

with

or

without

late

stage

Ni-Co

arsenides and sulphides, n a t i v e silver, and n a t i v e b i s m u t h ; and l a t e q u a r t z - c a r b o n a t e - p y r i t e veins (barren).

When o n e or m o r e of t h e n o n - u r a n i f e r o u s t y p e s of d e p o s i t s n o t e d a b o v e a r e

f o u n d in a n a r e a , e s p e c i a l l y if t h e y h a v e a h i g h e r t h a n n o r m a l r a d i o a c t i v i t y , t h e p r e s e n c e of p i t c h b l e n d e or u r a n i n i t e v e i n s s h o u l d b e s u s p e c t e d .

S i m i l a r i l y , in o t h e r t e r r a n e s m a r k e d by

t h e p r e s e n c e of p i t c h b l e n d e d e p o s i t s w i t h a n a s s o c i a t e d N i - C o - A g - A s - B i m i n e r a l i z a t i o n

a

s e q u e n c e of e v e n t s b e g i n n i n g w i t h t h e f o r m a t i o n of s u l p h i d e s k a r n s o f t e n w i t h a p a t i t e a n d r e l a t e d t o e a r l y p h a s e s of g r a n o d i o r i t e a n d d i o r i t e i n t r u s i o n s a r e c o e v a l w i t h or f o l l o w e d by magnetite-apatite (commonly These

are

granites

succeeded

by

mineralization

and granodiorites,

stockworks arsenide

containing

and

sulphide

rare earth bearing) amphibole

including

pitchblende

events

seemingly

fault-controlled and

assemblage.

Such

P r o t e r o z o i c volcanics, s e d i m e n t s , and

a

related

quartz

chalcopyrite history

veins and i r r e g u l a r

(and

followed of

by

to

younger

carbonate) the

intrusive veins

complex

mineralization

i n t r u s i v e s in t h e B e a r

bodies.

is

present

P r o v i n c e of t h e

and

Ni-Co in

Canadian

S h i e l d in t h e P o r t R a d i u m - C a m s e l l R i v e r r e g i o n , N o r t h w e s t T e r r i t o r i e s ; in t h e B o h e m i a n Massif

of

Central

mineralizations

are

Europe; well

and

elsewhere.

illustrated

in

the

The Bear

sequential Province

details

and

have

of

the

been

various carefully

d o c u m e n t e d by Robinson (1971), B a d h a m ( 1 9 7 5 , 1978), and B a d h a m and M o r t o n (1976).

The

140 origin

and

exact

interrelationships

of

these

particular

mineralization

sequences

are

p r o b l e m a t i c a l , b u t f r o m a p r o s p e c t i n g v i e w p o i n t t h e y a r e of i m p o r t a n c e in t h a t if o n e of t h e p h a s e s is f o u n d , t h e o t h e r s s h o u l d b e s e a r c h e d f o r . T h e s e s p e c i f i c e x a m p l e s i l l u s t r a t e t h e g e n e r a l i t y t h a t c e r t a i n t e r r a n e s a r e m a r k e d by a s e q u e n c e of m e t a m o r p h i c a n d t e c t o n i c , e v e n t s w i t h a s s o c i a t e d

alteration-mineralization

f e a t u r e s t h a t m a y s i g n a l t h e p r e s e n c e of t h o r i u m a n d / o r u r a n i u m o r e b o d i e s . oldest to youngest with brief c o m m e n t s a r e :

These

from

(1) A h o r n f e l s a n d / o r s k a r n s t a g e i n d u c e d in

p e l i t e s and c a r b o n a t e r o c k s by i n t r u d e d high-level d i o r i t e , g r a n o d i o r i t e , and g r a n i t e s t o c k s and small batholiths.

Thorium and u r a n i u m o r e s w i t h t h o r i t e , u r a n i n i t e , c o f f i n i t e , d a v i d i t e ,

e t c . m a y b e d e v e l o p e d in t h e s k a r n r o c k s ; (2) A p o t a s s i u m m e t a s o m a t i c s t a g e d u r i n g w h i c h t h e p l a g i o c l a s e of d i o r i t i c , g r a n o d i o r i t i c , a n d g r a n i t i c s t o c k s , s i l l s , a n d d y k e s is r e p l a c e d by o r t h o c l a s e , p e r t h i t e , or m i c r o c l i n e .

S i m i l a r f e l d s p a r s m a y b e d e v e l o p e d in h o r n f e l s , a n d

minerals such as c o r d i e r i t e , a n d a l u s i t e , and sillimanite may be a l t e r e d to m u s c o v i t e , pinite, etc.

T h o r i u m a n d u r a n i u m d e p o s i t s s i m i l a r t o t h o s e in (3) b e l o w m a y b e f o r m e d ; (3) A

s o d i u m m e t a s o m a t i c s t a g e c h a r a c t e r i z e d by t h e a l b i t i z a t i o n of d i o r i t i c , g r a n o d i o r i t i c , a n d granitic stocks, b r e c c i a t e d p e g m a t i t e s , and tectonically disturbed

z o n e s in g n e i s s e s

and

o t h e r m e t a m o r p h i c r o c k s ; in s o m e t e r r a n e s a l b i t i t e d y k e s a n d s t o c k s a r e d e v e l o p e d d u r i n g this s t a g e . etc.)

Thorium and uranium mineralization (thorite, uraninite, pitchblende,

commonly

takes

place

in

albitized

pegmatites,

in

albitized

coffinite,

brecciated

zones,

m y l o n i t e s , f a u l t s , a n d o t h e r s i m i l a r s i t e s of d i l a t i o n in b o t h t h e g r a n i t i c i n t r u s i v e s country

rocks;

concentrated

(4) An

along

greisenization, minor

late

early

faults

siliceous,

fracture

tourmalinization,

albitization

and/or

hydrous,

zones,

etc.

chloritization,

fluoriferous, and

marked

by

sericitization,

microclinization

sub-stages,

and

boriferous

early

minor

all

and stage

silicification,

pyritization,

essentially

coeval

and and

a t t e n d e d by t h e d e p o s i t i o n of c a s s i t e r i t e , w o l f r a m i t e , ( s c h e e l i t e ) , m o l y b d e n i t e , m a g n e t i t e , (pyrite),

arsenopyrite,

chloritization,

fluorite,

carbonatization,

apatite,

etc.;

argillization,

(5) An

and

early

pyritization

intermediate stage

silicification,

concentrated

along

f a u l t s , f r a c t u r e z o n e s , a n d o t h e r d i l a t a n t f e a t u r e s a n d c o m m o n l y a t t e n d e d by e a r l y p h a s e s m a r k e d by t h e d e p o s i t i o n of q u a r t z , c a r b o n a t e s , p y r i t e , c h a l c o p y r i t e , a r s e n o p y r i t e , e t c . a n d l a t e p h a s e s c h a r a c t e r i z e d b y t h e d e p o s i t i o n of q u a r t z , f l u o r i t e , b a r i t e , c a r b o n a t e s , p y r i t e , g a l e n a , s p h a l e r i t e , s u l p h o s a l t s , e t c . ; (6) A l a t e i n t e r m e d i a t e s i l i c i f i c a t i o n , a r g i l l i z a t i o n , a n d h e m a t i t i z a t i o n s t a g e d e v e l o p e d in f a u l t s , m y l o n i t i z e d

carbonatization, zones, and

other

d i l a t a n t f e a t u r e s a n d a t t e n d e d by a n e a r l y d e p o s i t i o n p h a s e of q u a r t z , c a r b o n a t e s , f l u o r i t e , h e m a t i t e , a n d p i t c h b l e n d e ( u r a n i n i t e ) , a n d a l a t e d e p o s i t i o n p h a s e of q u a r t z ,

carbonates

(calcite, dolomite, siderite), fluorite, barite, ( h e m a t i t e ) , minor pitchblende, native silver, native bismuth, bismuthinite, niccolite, c o b a l t i t e , and o t h e r N i - C o sulphides and arsenides; (7) A l a t e m i n e r a l i z a t i o n s t a g e in l o c a l d i l a t a n t f e a t u r e s c o m p r i s i n g a n e a r l y d e p o s i t i o n of quartz, specular

hematite, carbonates

(calcite), fluorite, barite,

marcasite,

d e p o s i t i o n of m i n o r g a l e n a , s p h a l e r i t e , a n d o t h e r b a s e m e t a l s u l p h i d e s .

and

a

late

M i n o r a m o u n t s of

141 pitchblende

(mainly

redistributed)

may

accompany

this

stage;

and

a r g i l l i z a t i o n s t a g e m a r k e d m a i n l y b y t h e d e v e l o p m e n t of k a o l i n i t e .

(8) A

final

late

Argillization may also

a c c o m p a n y s t a g e s (5), (6), a n d (7). Considerable terranes.

variation

in t h i s i d e a l i z e d

sequence

of e v e n t s

is o b s e r v a b l e in

most

In s o m e , o n e or m o r e of t h e s t a g e s m a y b e w e a k l y d e v e l o p e d o r e n t i r e l y m i s s i n g ;

in o t h e r s s t a g e s (2) a n d (3) m a y r e v e r s e d in t i m e o r o n e o r o t h e r of t h e s e s t a g e s m a y b e attenuated

or

practically

absent.

The

important

consideration

from

a

prospecting

v i e w p o i n t is t h a t if o n e of t h e s t a g e s is p r e s e n t i n a n a r e a , t h e o t h e r s , p a r t i c u l a r l y t h o s e m a r k e d by t h e d e v e l o p m e n t of t h o r i f e r o u s a n d / o r u r a n i f e r o u s m i n e r a l s , s h o u l d b e s o u g h t . Certain

vein

unconformities.

and

lode

deposits

exhibit

a

close

spacial

relationship

to

erosional

In s o m e c a s e s t h i s r e l a t i o n s h i p m a y b e s t r u c t u r a l a n d d u e t o d a m m i n g o r

r e t a r d a t i o n o f m i n e r a l i z i n g f l u i d s ; i n o t h e r c a s e s t h e r e l a t i o n s h i p is p r o b a b l y t h e r e s u l t o f m o r e r e c e n t e r o s i o n of o v e r l y i n g r o c k s , t h u s e x p o s i n g t h e d e p o s i t s w h i c h w e r e from d e e p w e a t h e r i n g and r e m o v a l .

protected

R e g a r d l e s s of w h i c h r e l a t i o n s h i p is t h e c a s e , d e p o s i t s

c a n b e e x p e c t e d b e n e a t h c e r t a i n y o u n g e r c o v e r r o c k s a s w i t n e s s e d by t h e p o s i t i o n of large

Jabiluka

Two

deposit

below

the

Kombolgie

(sandstone)

Formation

in

the

the East

Alligator River region, Northern Territory, Australia, and the Midwest Lake deposit below t h e A t h a b a s c a ( s a n d s t o n e ) F o r m a t i o n in n o r t h e r n S a s k a t c h e w a n . The

philosophy

granitized

terranes

of

prospecting

has long been

for

that

vein the

and

lode

uranium

deposits

and

thorium

in

metamorphic

and

their

and

associated

e l e m e n t s w e r e d e r i v e d f r o m l o c a l g r a n i t e b o d i e s or s o m e o t h e r s u i t a b l e i g n e o u s r o c k magmatic differentiation processes.

by

A l t e r n a t i v e l y , i t s e e m s m o r e p r o b a b l e in m a n y p l a c e s

t h a t t h e h o s t c o u n t r y r o c k s h a v e b e e n t h e p r i n c i p a l s o u r c e of t h e u r a n i u m a n d o t h e r m e t a l s . T h e r e is c o n s i d e r a b l e g e o c h e m i c a l e v i d e n c e t o s u g g e s t t h a t t h e b l a c k g r a p h i t i c a n d p y r i t i c schists, argillites and s l a t e s , phosphorites, shales, and acidic t u f f a c e o u s rocks a r e a d e q u a t e sources

of

uranium

and

thorium.

As t h e s e various rocks w e r e

structurally

deformed,

m e t a m o r p h o s e d , and granitized numerous dilatant zones w e r e formed during t h e late stages of d e f o r m a t i o n a l o n g f a u l t s , s h e a r z o n e s , s h e a r e d b e d s , a n d m y l o n i t i z e d z o n e s .

T h e s e low

p r e s s u r e z o n e s b e c a m e t h e d e p o s i t i o n a l s i t e s for t h e m o r e m o b i l e c o n s t i t u e n t s from country

rocks,

elements. should

including

quartz,

carbonates,

uranium,

thorium

and

their

associated

T h e p h i l o s o p h y in p r o s p e c t i n g f o r t h o r i u m - a n d u r a n i u m - b e a r i n g v e i n s a n d l o d e s

be to

seek

out

areas

containing

the favourable

rocks mentioned

above,

paying

p a r t i c u l a r a t t e n t i o n t o g r a p h i t i c s c h i s t s , p h o s p h a t e - r i c h s h a l e s , a n d a c i d i c t u f f s (in m e t a m o r p h i c t e r r a n e s now graphitic gneisses, schists, m i g m a t i t e s , etc.).

granitized,

favourable areas.

faulted,

and

mylonitized

during

It s h o u l d p e r h a p s b e m e n t i o n e d

the

metamorphic

most

From this point

careful a t t e n t i o n should be d i r e c t e d to z o n e s t h a t w e r e s t r u c t u r a l l y d e f o r m e d , phosed,

the

metamor­

history

in p a s s i n g t h a t t h e g r a p h i t i c

of

the

schists,

shales, and a c i d i c tuffs (and their m e t a m o r p h i c equivalents) a r e usually i n c o m p e t e n t rocks a n d o n l y c o n t a i n t h e d e p o s i t s in c e r t a i n c a s e s .

These rocks probably provided the elements

142 to

the

deposits;

the

more

competent

rocks

such

as

thick-bedded

meta-greywackes,

q u a r t z i t e s , c h e r t s , amphibolites, massive gneisses, hornfels, and local intrusive stocks and d i k e s a r e g e n e r a l l y t h e s i t e s of t h e s t r u c t u r a l t r a p s .

In a f e w t e r r a n e s i t is p o s s i b l e t h a t

g r a p h i t i c and pyritic schists ( i n t e r b e d d e d with c o m p e t e n t q u a r t z i t e , a r k o s e , and a r g i l l i t e , now

gneisses

and

schists)

have

acted

as r e d u c t a n t s

and p r e c i p i t a n t s

of u r a n i u m

from

solutions and diffusion c u r r e n t s . The

complex

polymetallic

type

of

uranium

deposit

containing

hematitic

quartz,

p i t c h b l e n d e , N i - C o a r s e n i d e s a n d s u l p h i d e s , n a t i v e s i l v e r , a n d so on ( t h e s o - c a l l e d

five

e l e m e n t v e i n s , N i - C o - A g - B i - U ) o c c u r in v a r i e d 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 i t is d i f f i c u l t t o f r a m e a p h i l o s o p h y in s e a r c h i n g f o r t h e s e t y p e s of v e i n s .

Furthermore, these

deposits

a p p e a r t o r e p r e s e n t t w o m a j o r , a n d p e r h a p s s e p a r a t e , p u l s e s of m i n e r a l i z a t i o n , n a m e l y a n e a r l y p h a s e m a r k e d by q u a r t z , h e m a t i t e , a n d p i t c h b l e n d e f o l l o w e d by r e o p e n i n g of t h e v e i n s t r u c t u r e s a n d d e p o s i t i o n of a l a t e p h a s e c o m p r i s i n g N i - C o a r s e n i d e s a n d s u l p h i d e s , n a t i v e s i l v e r , n a t i v e b i s m u t h , a n d so o n .

T h i s t w o p u l s e f e a t u r e p r o b a b l y a c c o u n t s for t h e

fact

t h a t c e r t a i n of t h e s e d e p o s i t s , e . g . t h o s e in t h e G r e a t B e a r L a k e r e g i o n of C a n a d a a n d a t J a c h y m o v in C z e c h o s l o v a k i a , w h i c h h a v e a c o m p l e x s t r u c t u r a l h i s t o r y , h a v e b o t h p h a s e s , whereas those at

C o b a l t in C a n a d a

a n d K o n g s b e r g in N o r w a y , w h i c h h a v e a

relatively

s i m p l e s t r u c t u r a l h i s t o r y , l a c k s i g n i f i c a n t q u a n t i t i e s of p i t c h b l e n d e a n d c o n t a i n o n l y

the

l a t e p h a s e of m i n e r a l i z a t i o n .

In a d d i t i o n , t h e v e i n s w i t h b o t h m i n e r a l i z a t i o n p h a s e s

are

g e n e r a l l y l o c a l i z e d in e x t e n s i v e f a u l t a n d f r a c t u r e s y s t e m s c u t t i n g m e t a - s e d i m e n t a r y

and

v o l c a n i c l a s t i c t e r r a n e s i n v a d e d by p o r p h y r y m a s s e s a n d g r a n i t i c b o d i e s , s e t t i n g s a n d

an

origin for uranium similar t o t h a t just d e s c r i b e d for t h e simple p i t c h b l e n d e veins and lodes. On t h e o t h e r hand t h o s e veins w i t h only t h e l a t e N i - C o a r s e n i d e - n a t i v e silver p h a s e o c c u r in r e l a t i v e l y l o c a l s t r u c t u r e s a n d f o r t h i s r e a s o n h a v e b e e n c o n s i d e r e d by t h e w r i t e r t o b e " l a t e d i s t i l l a t e s " , w h i c h d e r i v e d t h e i r c o n s t i t u e n t s l o c a l l y by d i f f u s i o n f r o m f a h l b a n d s a s in the Kongsberg

r e g i o n in N o r w a y

or f r o m

crystallized

diabase

and underlying

sulphide-

bearing g r a p h i t i c bands as a t C o b a l t , O n t a r i o (Boyle, 1968a; Boyle and Dass, 1971a).

The

g i s t of t h i s d i s c u s s i o n i s , t h e r e f o r e , t h a t t h e f i v e e l e m e n t v e i n s a r e n o t u n i v e r s a l i n d i c a t o r s of

the presence

exhaust

all

of u r a n i u m ; w h e n

possibilities

in

his

found

search

in a t e r r a n e , h o w e v e r , for

the

earliest

the prospector

(uraniferous)

should

component

of

the system. C e r t a i n p i t c h b l e n d e a n d o t h e r t y p e s of T h - U d e p o s i t s in v o l c a n i c c e n t r e s ( b a s i n s a n d c a l d e r a s ) m a y h a v e an o r i g i n s o m e w h a t d i f f e r e n t metamorphic

terranes.

As n o t e d

t h a n t h a t j u s t d e s c r i b e d f o r d e p o s i t s in

in p r e v i o u s s e c t i o n s t h e s e d e p o s i t s t e n d

to occur

t e r r e s t r i a l v o l c a n i c s , v o l c a n i c l a s t i c s , a n d s e d i m e n t s l y i n g on o l d e r b a s e m e n t r o c k s .

in

In s o m e

c a s e s t h e s e a r e g r a n i t i z e d a n d m e t a m o r p h o s e d m a s s i f s , in o t h e r s s e d i m e n t a r y s e q u e n c e s of a varied nature.

S p e c i f i c i n d i c a t o r s of t h e p r e s e n c e of m e t a l l i f e r o u s v e i n s a n d l o d e s in

volcanic c e n t r e s a r e late (post volcanic) faults, b r e c c i a t e d

zones and volcanic

p i p e s , a n d e x t e n s i v e z o n e s of p r o p y l i t i z a t i o n , c a r b o n a t i z a t i o n , p y r i t i z a t i o n , silicification,

argillitization,

hematitization,

a n d so o n .

breccia

alunitization,

Veins, segregations, and

small

143 stockworks fluorite

of

are

indicator

opaline and

particularly

fine-grained

indicative

includes a basement

quartz

in s o m e

carrying

calderas

molybdenite, chalcopyrite,

and

volcanic

basins.

A

and

further

of s e d i m e n t s or t h e i r m e t a m o r p h i c e q u i v a l e n t b e l o w

the

v o l c a n i c p i l e s , t h e s e d i m e n t s b e i n g t h e s o u r c e of t h e o r e c o n s t i t u e n t s a n d t h e v o l c a n i c s t h e receptacle

rocks

mineralization,

( F i g u r e 17) particularly

(Boyle,

1979a).

favourable

With

basement

regard

source

to

uranium

rocks

and

include

c r y s t a l l i n e s , m i g m a t i t e s , a n d a n a t e c t i c a n d i n t r u s i v e g r a n i t o i d s ; s e q u e n c e s of black

shales

and

schists;

and

strata

of

terrestrial

conglomerate,

thorium

sedimentary pyritiferous

sandstone,

arkose,

F e w d e f i n i t e c r i t e r i a c a n b e s u g g e s t e d for d i s t i n g u i s h i n g t e r r a n e s l i k e l y t o

contain

and tuff.

t h o r i u m - b e a r i n g v e i n s a n d l o d e s in w h i c h t h o r i t e , b r a n n e r i t e , t h o r o g u m m i t e , a n d m o n a z i t e are the principal economic minerals.

T h e s u g g e s t i o n s n o t e d a b o v e for s e l e c t i n g g e o l o g i c a l

t e r r a n e s likely to c o n t a i n U-Th veins and lodes can be c o n s i d e r e d only as g e n e r a l guides. m o r e s p e c i f i c g u i d e a n d i n d i c a t o r for t h o r i u m - r i c h v e i n s a n d l o d e s is t h e f a c t t h a t

A

these

d e p o s i t s t e n d t o o c c u r in c l u s t e r s in s h e a r z o n e s , f a u l t s , a n d b r e c c i a t e d z o n e s n e a r a l k a l i c intrusives (nepheline syenites, albite syenites, ijolites, shonkinite, etc), particularly swarms, plugs, small

stocks, and

less f r e q u e n t l y

complex

massifs;

alkalic extrusives (phonolite, tinguaite, etc.) may also be favourable.

terranes

containing

B a r i t e - c a r b o n a t e and

f l u o r i t e - c a r b o n a t e v e i n s m a y s i g n a l t h e p r e s e n c e of t h o r i u m - r i c h v e i n s in s o m e more

specific

indicators

are

carbonatite

dikes,

carbonatite

dike

complexes,

terranes;

apatite

t i t a n i u m - b e a r i n g veins with rutile, a n a t a s e , and b r o o k i t e , and veins with Ba-Sr

veins,

minerals,

zirconium minerals and/or rare earth carbonates (bastnaesite, etc). F i n a l l y , i t s h o u l d a l w a y s b e b o r n e in m i n d t h a t a l l t y p e s of u r a n i u m a n d t h o r i u m and

l o d e d e p o s i t s t e n d t o o c c u r in c l u s t e r s w i t h i n d e f i n i t e

areas.

and c o m m o n l y well

If o n e d e p o s i t or o c c u r r e n c e is f o u n d o t h e r s s h o u l d b e s o u g h t w i t h i n t h e

geological and/or tectonic environment.

vein

defined

favourable

O n e s h o u l d a l s o b e a r in m i n d t h a t t h e s m a l l s i z e

of t h e o c c u r r e n c e s b e a r s no r e l a t i o n s h i p t o t h e w o r t h of a n a r e a a s a p o s s i b l e p r o d u c e r of large deposits.

Small o c c u r r e n c e s signal the fact t h a t t h e mineralizing processes have been

a c t i v e , and hidden

large economic

deposits may

structural environments are present.

occur

where

favourable

chemical

and

In t h i s r e s p e c t t h e r e is a n a n a l o g y w i t h a t r o u t p o n d -

m a n y s m a l l fish a n d a f e w l a r g e fish, t h e l a t t e r d i f f i c u l t t o l a n d . In m a n y u r a n i f e r o u s p r o v i n c e s f a v o u r a b l e s t r u c t u r e s c o n t a i n i n g p i t c h b l e n d e v e i n s a n d lodes are marked photographs.

by l i n e a m e n t s

and

curvilineaments

on t h e

ground

and

on

aeroplane

S u c h l i n e a m e n t s c o m m o n l y m a r k d e p r e s s i o n s a n d a r e p r o d u c e d m a i n l y by t h e

d i f f e r e n t i a l w e a t h e r i n g of f a u l t z o n e s , s h e a r z o n e s , m y l o n i t i z e d z o n e s , d i k e c o n t a c t s , t u f f beds, e t c .

t h a t have been c a r b o n a t i z e d , pyritized, and o t h e r w i s e mineralized.

these features

have been

accentuated

by t h e

abraiding

action

of

glaciers.

M a n y of Excellent

e x a m p l e s a r e t h e d e e p l y i n c i s e d d r i f t - f i l l e d v a l l e y s o v e r l y i n g t h e m i n e r a l i z e d f a u l t s of t h e Eldorado Mine, G r e a t Bear Lake, N o r t h w e s t Territories, C a n a d a and t h e linear drift

filled

144 depressions

marking

prominent

faults

in

which,

and

near

which,

occur

p i t c h b l e n d e d e p o s i t s in t h e U r a n i u m C i t y a r e a of S a s k a t c h e w a n , C a n a d a .

a

number

of

It s h o u l d

be

p o i n t e d o u t , h o w e v e r , t h a t n o t a l l u r a n i f e r o u s a n d t h o r i f e r o u s s t r u c t u r e s a r e m a r k e d by depression features.

O n t h e c o n t r a r y , in s o m e u r a n i f e r o u s p r o v i n c e s t h e m i n e r a l i z e d z o n e s

m a y b e h i g h l y s i l i c i f i e d a n d m a y w e a t h e r a s l i n e a r , c u r v e d , or i r r e g u l a r r i d g e s .

F r o m this

discussion it follows t h a t both depressions as well as unusual ridges and o t h e r

elevated

f e a t u r e s s h o u l d b e c a r e f u l l y s c r u t i n i z e d a n d p r o b e d by r a d i a t i o n c o u n t i n g , s o i l s a m p l i n g , overburden drilling and sampling, and diamond drilling. A p l e t h o r a of g u i d e s a n d i n d i c a t o r s of s a n d s t o n e - t y p e d e p o s i t s is a v a i l a b l e p r o s p e c t o r , n o n e of w h i c h e i t h e r s i n g l y or a s a g r o u p a r e i n f a l l i b l e . indicators

include

theories

of

origin

of

the

deposits,

various

c l i m a t i c , and paleohydrologic f e a t u r e s , and m o r e specific

to t h e

These guides

paleogeographic,

and

paleo-

geological, mineralogical

and

geochemical characteristics. C o n s i d e r i n g o r i g i n s of t h e s a n d s t o n e - t y p e d e p o s i t s t h e p r o s p e c t o r s h o u l d c o n s i d e r t h e following in situ

in

p o s s i b i l i t i e s : (1) t h a t the

conglomerates,

e l e m e n t s during diagenesis.

the ores are syngenetic sandstones,

and

shales

and

with

were some

deposited

essentially

rearrangement

of

the

T h e m a r k e d e p i g e n e t i c f e a t u r e s of m o s t of t h e d e p o s i t s , a n d

a g e d e t e r m i n a t i o n s of t h e o r e m i n e r a l s w h i c h s h o w t h e m t o b e y o u n g e r t h a n t h e e n c l o s i n g r o c k s , l a r g e l y d i s c r e d i t t h i s t h e o r y ; (2) t h a t t h e u r a n i u m , v a n a d i u m , a n d t h e i r

associated

e l e m e n t s w e r e l e a c h e d by d e s c e n d i n g , l a t e r a l m i g r a t i n g , or a s c e n d i n g g r o u n d w a t e r s f r o m overlying

or

underlying

volcanic

rocks,

tuffs,

and

pyroclastics

(mainly

of

an

acidic

c h a r a c t e r ) , or h u m a t e , c a r b o n a c e o u s , or p h o s p h a t i c b e d s , a n d d e p o s i t e d in p o r o u s s i t e s in s a n d s t o n e s a s a r e s u l t of v a r i o u s r e d o x r e a c t i o n s .

T h i s t h e o r y h a s m e r i t in a n u m b e r of

s a n d s t o n e a r e a s in t h e w o r l d w h e r e a c l o s e a s s o c i a t i o n of o r e b o d i e s w i t h t u f f a c e o u s s h a l e s , t u f f s , b e n t o n i t e s , p y r o c l a s t i c s , or v o l c a n i c f l o w s is m a r k e d ; (3) t h a t t h e u r a n i u m , v a n a d i u m a n d t h e i r a s s o c i a t e d e l e m e n t s w e r e l e a c h e d by g r o u n d w a t e r s f r o m t h e e n c l o s i n g s e d i m e n t s and/or

from

laterally

meta-sedimentary

situated

crystallines,

or u n d e r l y i n g g r a n i t i c ,

monzonitic,

or s y e n i t i c

terranes,

migmatites, carbonaceous shales, phosphorites, etc.,

u r a n i u m , v a n a d i u m , and o t h e r e l e m e n t s

being p r e c i p i t a t e d

as a result

m a i n l y of

the

redox

r e a c t i o n s ( p r e s e n c e of c o a l i f i e d p l a n t r e m a i n s , H2S, p y r i t e , p e t r o l e u m r e s i d u e s , a n d o t h e r r e d u c t a n t s ) a n d / o r , a d s o r p t i o n a n d ion e x c h a n g e zeolites.

phenomena

involving clay

minerals

and

This t h e o r y f i n d s f a v o u r in J a p a n a n d o t h e r c o u n t r i e s w h e r e u r a n i u m d e p o s i t s in

b a s a l s t r a t a o v e r l i e g r a n i t i c t e r r a n e s a n d in w e s t e r n U . S . A . w h e r e u n d e r l y i n g

Precambrian

c r y s t a l l i n e s a r e p r e s e n t in m a n y a r e a s ; a n d (4) t h a t t h e u r a n i u m a n d i t s a s s o c i a t e d e l e m e n t s w e r e p r e c i p i t a t e d f r o m a s c e n d i n g m a g m a t i c h y d r o t h e r m a l s o l u t i o n s a s a r e s u l t m a i n l y of redox reactions.

An a l t e r n a t i v e is d e p o s i t i o n f r o m h y d r o t h e r m a l f l u i d s d r i v e n o u t of t h e

e n c l o s i n g s e d i m e n t s a n d / o r b a s e m e n t r o c k s a s a r e s u l t of d i a g e n e s i s , m e t a m o r p h i s m , anatectic

processes,

and

volcanic

m a g m a s or r i s i n g p l u t o n s .

The presence

of

deep

certain

d e p o s i t s in d i a t r e m e s a n d in d e e p s e a t e d f a u l t s , c e r t a i n w a l l r o c k a l t e r a t i o n e f f e c t s s u c h a s a l u n i t i z a t i o n a n d a r g i l l i t i z a t i o n in s o m e d e p o s i t s , t h e p r e s e n c e of a s u i t e of r e l a t i v e l y h i g h

145 t e m p e r a t u r e m i n e r a l s such as m o l y b d e n i t e , Ni-Co a r s e n i d e s , and p o l y m e r i z e d h y d r o c a r b o n s , a n d t h e s y n c h r o n o u s a g e s of t h e o r e s w i t h t h e o r o g e n i e s in t h e b e l t s in w h i c h t h e y o c c u r , a n d w i t h i n t r u s i v e s in t h e s e b e l t s , t e n d t o s u p p o r t t h e h y d r o t h e r m a l t h e o r y a c c o r d i n g t o a n u m b e r of a u t h o r i t i e s .

In a l l t h e s e t h e o r i e s t h e m e c h a n i s m of t r a n s p o r t of t h e u r a n i u m w a s

probably as a uranyl c a r b o n a t e c o m p l e x , a s a s u l p h a t e , a s a p h o s p h a t e , as a fluoride, or as an o r g a n o m e t a l l i c c o m p l e x Reduction (U

4 +

of t h e

(chelate

mobile hexavalent

i n v o l v i n g p r o d u c t s of p e t r o l e u m

or h u m i c

u r a n i u m ( U C > 2 ) ion t o t h e i m m o b i l e

matter).

quadrivalent

2+

) ion a n d p r e c i p i t a t i o n m a i n l y a s p i t c h b l e n d e ( u r a n i n i t e ) a n d c o f f i n i t e o r a s a d s o r b e d

a n d / o r f i x e d (ion e x c h a n g e d ) u r a n i u m w a s a c c o m p l i s h e d in m o s t d e p o s i t s b y o n e o r

more

redox reactions with humic m a t t e r (decaying plant remains), petroleum residues (bitumen), methane

and

other

volatile

hydrocarbons,

H2S a r i s i n g f r o m

bacterial reduction

of

s u l p h a t e or d e r i v e d f r o m u n d e r l y i n g o r o v e r l y i n g h y d r o c a r b o n - b e a r i n g r o c k s , r e a c t i o n w i t h pyrite

or

other

reductants,

and

adsorption

and

ion e x c h a n g e

reactions

with

zeolites,

phosphatic m a t e r i a l s , and clay m i n e r a l s . The formation

paleogeographic of t h e

host

and

sediments

paleotopographic of

most

conditions

sandstone-type

existing

deposits

at

were

the

time

of

predominantly

t e r r e s t r i a l , essentially fluviatile, paludal, l a c u s t r i n e , and d e l t a i c ; only r a r e l y l i t t o r a l , n e a r shore m a r i n e , and eolian.

Three principal

s i t e s of s e d i m e n t a t i o n

are

indicated

by

deposits - e x t e n s i v e slowly subsiding i n t r a c r a t o n a l basins with periodic o u t l e t s to

the

epeiric

s e a s or c o n t i n e n t a l s h e l v e s , w i d e i n t e r m o n t a n e v a l l e y s a n d v a l l e y f l a t s w i t h o u t l e t s t o t h e w a t e r s of e p e i r i c s e a s or c o n t i n e n t a l s h e l v e s , a n d g r a b e n s w i t h or w i t h o u t o u t l e t s t o s e a s . In t h e s e t h r e e c a s e s low a l l u v i a l p l a i n s t h r e a d e d by g e n t l y a g g r a d i n g , s l u g g i s h , b r a i d e d , a n d c o a l e s c i n g s t r e a m s a n d r i v e r s h e a d i n g in d i s t a n t v e g e t a t e d

h i g h l a n d s or rising

mountain

m a s s e s w i t h a b u n d a n t a l l u v i a l f a n s a n d f r o m w h e n c e c a m e h e a v y l o a d s of m u d a n d s a n d a r e i n d i c a t e d , a s e t t i n g n o t u n l i k e t h a t of t h e p r e s e n t b a s i n s of t h e A m a z o n a n d G a n g e s R i v e r s . T h e s h i f t i n g s a n d s , a n d l o n g i t u d i n a l , t r a n s v e r s e o r l i n g u o i d , a n d p o i n t or s i d e b a r s a l o n g t h e river channels g a v e rise to braided c h a n n e l s , scours, small depositional basins, and s t r a t h s filled w i t h c o n g l o m e r a t e s a n d c r o s s - b e d d e d s a n d s t o n e s o f t e n i n t e r f i n g e r i n g l a t e r a l l y s i l t s t o n e s and shales, while most producing variegated shales.

of

the

fine

silt and mud s e t t l e d over t h e

with

floodplains

H e r e and t h e r e swales, s w a m p s , and lakes i n t e r r u p t e d

the

c o u r s e of t h e r i v e r s f o r m i n g e n v i r o n m e n t s w h e r e r e d u c e d s e d i m e n t s ( b l a c k a n d g r e y s h a l e s ) and marls w e r e d e p o s i t e d .

Approaching the coasts the rivers debouched onto broad

barren

o r v e g e t a t e d d e l t a s b r a i d e d by i n n u m e r a b l e w a t e r c o u r s e s m e a n d e r i n g h i t h e r a n d t h i t h e r a n d m a r k e d by b a r s , o x - b o w l a k e s , s w a m p s , a n d l a g o o n s . deltas

were,

likewise,

commonly

marked

by b a r s

and

Coastal sections flanking

lagoons.

the

P e r i o d s of a r i d i t y

are

c h a r a c t e r i z e d by d u n e s a n d s a n d t h i n e v a p o r i t e s in s o m e of t h e s e d i m e n t a r y s e c t i o n s .

The

Eh c o n d i t i o n s of s e d i m e n t a t i o n w e r e e s s e n t i a l l y o x i d i z i n g , e s p e c i a l l y d u r i n g s e m i - a r i d

to

a r i d p e r i o d s , w i t h r e d u c i n g e n v i r o n m e n t s in t h e s w a m p s a n d in c e r t a i n l a k e s a n d l a g o o n s . Rapid

burial

sections.

and

isolation

from

oxidation

of

plant

fragments

are

indicated

in

many

F r o m t i m e t o t i m e v o l c a n i s m ( m a i n l y a c i d i c a n d a l k a l i c ) m a r k e d b y o u t b u r s t s of

146 tuffaceous p a r t s of

m a t e r i a l w h i c h s e t t l e d o v e r t h e b a s i n s , a n d o u t p o u r i n g s of l a v a w h i c h

the basins, are recorded

in a n u m b e r

of

the

typical

sections

of

filled

sedimentary

s e q u e n c e s hosting sand s t o n e - t y p e d e p o s i t s . A k n o w l e d g e of s e d i m e n t a t i o n p h e n o m e n a is of c o n s i d e r a b l e i m p o r t a n c e in d e t a i l e d p r o s p e c t i n g f o r o r e b o d i e s in s a n d s t o n e t e r r a n e s , e s p e c i a l l y t h e d e c i p h e r m e n t of directions.

t h e s t r e a m s t h a t d e p o s i t e d t h e s e d i m e n t h a v e f l o w e d , a n d o t h e r o r e b o d i e s lie a t b e n d s or

current

In t h i s r e s p e c t m a n y o r e b o d i e s a r e e l o n g a t e p a r a l l e l t o t h e d i r e c t i o n in w h i c h

just

downstream

from

these

bends.

Elementary

current

direction

meander indicators

i n c l u d e a l i g n m e n t of c l a s t i c p a r t i c l e s ( l i n e a t i o n ) , c r o s s - b e d d i n g i n c l i n a t i o n , c u r r e n t r i p p l e s , and

the

azimuth

of

fossil

logs; o t h e r s

are

enumerated

by S h r o c k

(1948), P o t t e r

and

P e t t i j o h n (1977), and Galloway e t al. (1979). T h e p a l e o c l i m a t i c c o n d i t i o n s i n d i c a t e d in t h e h o s t r o c k s of s a n d s t o n e - t y p e

deposits

w e r e m u c h t h e s a m e in a l l b a s i n s , p r o b a b l y i n c l u d i n g t h o s e of P r o t e r o z o i c a g e a l t h o u g h f e w c l i m a t i c i n d i c a t o r s a r e m a n i f e s t in t h e l a t t e r . coalified

plant

T h e p r e s e n c e of c o a l , l i g n i t e , a n d a b u n d a n t

r e m a i n s in t h e y o u n g s e d i m e n t s

suggests a warm,

moist,

temperate

to

s e m i - t r o p i c a l c l i m a t e o v e r m u c h of t h e b a s i n s ; l o c a l l y a n d p e r i o d i c a l l y h o t a r i d ( d e s e r t ) conditions prevailed although these were generally not prolonged. The paleohydrologic

conditions

associated

with the

meteoric

and/or

hydrothermal

w a t e r s t h a t d e p o s i t e d t h e u r a n i f e r o u s o r e s in s a n d s t o n e t e r r a n e s a r e n o t r e a d i l y a c c u r a t e l y deciphered. solutions

T w o g e n e r a l c o n c e p t s s h o u l d b e c o n s i d e r e d - o n e t h e p h y s i c a l m o v e m e n t of

through

porous

and

permeable

strata

and

the

other

the

diffusion

of

ions,

m o l e c u l e s , and o t h e r dissolved c o n s t i t u e n t s t h r o u g h a s t a g n a n t or s t a n d i n g ground w a t e r or hydrothermal system. the

first

case

the

Conditions may have existed where both processes were active. solutions

moved

essentially

under

the

influence

of

a

physical

In or

h y d r o l o g i c g r a d i e n t ; in t h e s e c o n d t h e i o n s , m o l e c u l e s , e t c . m o v e d u n d e r t h e i n f l u e n c e of a free energy (chemical potential) gradient.

S p a c e p r e c l u d e s a d i s c u s s i o n of t h e d e v e l o p m e n t

of o r e b o d i e s in s a n d s t o n e s by t h e s e c o n d p r o c e s s , b u t t h e i n t e r e s t e d r e a d e r c a n r e f e r t o t h e w r i t e r ' s e x p o s i t i o n of t h e m e c h a n i s m in a n o t h e r p u b l i c a t i o n ( B o y l e , 1 9 6 8 a , p . 182).

With

r e s p e c t to m e t e o r i c w a t e r s t h e w a t e r t a b l e s during s e d i m e n t a t i o n and d i a g e n e t i c p r o c e s s e s w e r e p r o b a b l y c o n s i s t e n t l y h i g h in t h e r o c k s , a s w i t n e s s e d by n o n - o x i d i z e d c o a l i f i e d fragments,

and t h e paleohydrologic g r a d i e n t s

existing

the

in

same

probably different,

rocks;

the

gradients

were

probably similar

associated

b u t by h o w m u c h is c o n j e c t u r a l .

with Careful

to

those

hydrothermal

plant

presently

waters

were

l i q u i d i n c l u s i o n s t u d i e s of

a u t h i g e n i c ( h y d r o t h e r m a l ) q u a r t z a n d o t h e r m i n e r a l s m a y g i v e s o m e i d e a of t h e p r e s s u r e s involved

between

solutions.

points

in

the

paleohydrologic

system

associated

with

hydrothermal

T h e p a t h w a y s f o l l o w e d by t h e m i n e r a l i z i n g w a t e r s o r d i f f u s i n g c o n s t i t u e n t s c a n

o f t e n b e t r a c e d by a n u m b e r of t e c h n i q u e s , s o m e of w h i c h c a n a l s o g i v e a r o u g h i d e a of t h e paleoporosity studies

of

and

redox,

paleopermeability leaching,

and

of t h e r o c k s .

alteration

effects,

The most and

the

useful

techniques

precipitation

of

include

secondary

147 m i n e r a l s e i t h e r in p o r o u s s p a c e s or by r e p l a c e m e n t p r o c e s s e s .

M o s t of t h e r e d o x

effects

a r e of a m i n e r a l o g i c a l n a t u r e a n d i n c l u d e p h e n o m e n a s u c h a s t h e o x i d a t i o n of p y r i t e

and

m a g n e t i t e , o x i d a t i o n of b i t u m i n o u s m a t t e r , r e d u c t i o n of h e m a t i t e , a n d so o n , a l l of w h i c h r e c o r d t h e p a s s a g e of constituents

in a

s o l u t i o n s of

stagnant

water

a n o x i d i z i n g or medium.

The

reducing nature

alteration

or t h e diffusion

effects

manifest

along

of the

p a t h w a y s c o u r s e d by t h e s o l u t i o n s or d i f f u s i n g c o n s t i t u e n t s a r e u s u a l l y d i s t i n c t a n d i n c l u d e t h e a l t e r a t i o n of d e t r i t a l m a g n e t i t e a n d / o r i l m e n i t e t o a g g r e g a t e s of a n a t a s e , r u t i l e , a n d l e u c o x e n e ; t h e s u l p h i d i z a t i o n of m a g n e t i t e , i l m e n i t e , a n d h e m a t i t e

to yield p y r i t e ;

the

p r o f o u n d a l t e r a t i o n of f e l d s p a r s t o g i v e c l a y m i n e r a l s ; t h e i n t r o d u c t i o n of c a r b o n a t e s a n d remobilization

of

silica

producing

overgrowths

on

quartz

grains;

the

development

of

c h l o r i t e , a n d t h e g e n e r a l r e d u c t i o n of h e m a t i t e d u s t , s t a i n s , a n d c o a t i n g s on c l a s t i c g r a i n s t o y i e l d g r e e n i s h a s s e m b l a g e s of c l a y m i n e r a l s a n d o t h e r p r o d u c t s .

Such a l t e r a t i o n

effects

a r e m a n i f e s t by g r e y i s h a n d g r e e n i s h - g r e y b l e a c h e d z o n e s a n d b e d s in o t h e r w i s e r e d d i s h o r buff-coloured

sediments.

The

hydrologic

and

diffusion

pathways

are

also

commonly

m a r k e d b y t h e p r e s e n c e o f v a r i o u s a d d e d c o n s t i t u e n t s a n d m i n e r a l s u s u a l l y o n l y in t r a c e s o r minor a m o u n t s , including

quartz

(chalcedony,

opal,

etc.), carbonates,

gypsum,

b a r i t e , pyrite, base m e t a l sulphides, c a r b o n a t e and F e - M n oxide c o n c r e t i o n s , m a g n e t i t e ( l o w in t i t a n i u m ) , a n d p o l y m e r i z e d p e t r o l e u m p r o d u c t s ( b i t u m e n , ucholite, etc.).

alunite,

authigenic

anthraxolite,

S o m e of t h e p a t h w a y s a r e m a r k e d by h i g h c o n t e n t s of H S a n d a r e f e t i d , 2

o t h e r s c o n t a i n h i g h e r t h a n n o r m a l a m o u n t s of g a s e s s u c h a s C H 4 , H others are slightly m o r e radioactive than the enclosing beds.

2

and He, and still

C o n c e r n i n g t h e l a s t i t is

commonly observed t h a t t h e pathways (greenish-grey leached zones and beds) have a higher t h a n a v e r a g e a m o u n t of s o l u b l e u r a n i u m , a n d o t h e r m e t a l s s u c h a s V, C u , M o , Z n , N i , C o and

As, c o m p a r e d

to

t h e (reddish)

beds unaffected

by t h e p a s s a g e of s o l u t i o n s .

The

p a t h w a y s , u s u a l l y b l e a c h e d or g r e y i s h a n d g r e e n i s h in c o l o u r , a n d w h i c h in m a n y

cases

a p p e a r t o r e p r e s e n t o l d a q u i f e r s , e x h i b i t d i f f e r e n t h a b i t s ; s o m e p a r a l l e l t h e b e d d i n g a n d in many

cases

are

restricted

to

specific

beds

originally

highly

porous,

permeable,

and

c o n t a i n e d w i t h i n i m p e r m e a b l e s h a l e s or s h a l y l i m e s t o n e t h a t f o r m t h e h a n g i n g w a l l s a n d footwalls;

others

cut

obliquely

across

the

stratification;

and

still

others

c h a r a c t e r i s t i c a l l y c r o s s - c u t t i n g marking faults, f r a c t u r e s , and b r e c c i a zones.

are

The hydro-

l o g i c p a t h w a y s m a r k i n g a c c e s s t o r o l l - t y p e o r e b o d i e s a r e of c o n s i d e r a b l e i n t e r e s t a n d h a v e been much studied.

In t h e t y p i c a l c a s e d o w n d i p f r o m t h e s u r f a c e (or c h a n n e l t h a l w e g ) t h e

h y d r o l o g i c or d i f f u s i o n p a t h w a y ( u s u a l l y a p o r o u s b e d e n c l o s e d by i m p e r m e a b l e s t r a t a ) is m a r k e d by o x i d i z e d , g e n e r a l l y c a r b o n a t e f r e e , s a n d s t o n e s w i t h a y e l l o w i s h g r e y t o d u s k y yellow colour

often

with a brownish

to reddish

cast

due to the presence

of

flecks

of

l i m o n i t e a n d / o r h e m a t i t e r e s u l t i n g f r o m t h e o x i d a t i o n of d i s s e m i n a t e d p y r i t e ( F i g u r e 10). M a g n e t i t e m a y b e p r e s e n t in t h i s z o n e in s o m e d e p o s i t s , a n d a n a t a s e a n d l e u c o x e n e usually p r e s e n t .

are

T h i s is f o l l o w e d c o n s e c u t i v e l y d o w n d i p by a l i m o n i t e ( g o e t h i t e ) s t a i n e d

s a n d s t o n e w i t h a y e l l o w i s h b r o w n c a s t , a n a r r o w s e l e n i u m - r i c h b a n d ( b l e a c h e d or c o n t a i n i n g s i d e r i t e in p l a c e s ) , t h e d a r k g r e y t o b l a c k u r a n i n i t e - c o f f i n i t e - p y r i t e

ore roll, and

finally

148 grey

unoxidized

sandstone

containing

disseminated

pyrite,

minor

( M 0 S 2 ? ) , a n d t r a c e t o m i n o r a m o u n t s of U, V, C o , N i , A s , e t c .

carbonate,

jordisite

Some investigators appear

to consider t h e s e redox f e a t u r e s and uranium ore rolls as p r i m a r y p h e n o m e n a , but it s e e m s m o r e p r o b a b l e t o t h i s w r i t e r t h a t t h e y a r e e s s e n t i a l y of s e c o n d a r y ( s u p e r g e n e ) o r i g i n a n d r e p r e s e n t e n r i c h m e n t s of U, Si, V, C o , a n d o t h e r e l e m e n t s s u p e r i m p o s e d on leanly

mineralized

sandstone

beds

or

paleohydrologic

downward moving (surface) ground waters.

channels

of

pre-existing

mineralization

by

T h i s v i e w is s u g g e s t e d by t h e n a t u r e of

the

u r a n i n i t e a n d c o f f i n i t e ( c o m m o n l y s o o t y or in s m a l l s p h e r i c a l or b o y r y o i d a l p a r t i c l e s ) a n d t h e a l t e r a t i o n ( m i n e r a l o g i c a l and e l e m e n t a l ) profiles u p / d i p and d o w n / d i p from

the

rolls,

oxidized

features

similar

uranium-bearing

vein

to

what

deposits.

one

commonly

This c o n c e p t

of

sees

in

vertical

secondary

profiles

enrichment

of

should

ore

receive

d e t a i l e d a t t e n t i o n in t h e f u r t h e r s t u d y of r e d o x or r o l l f r o n t o r e b o d i e s . T h e d i r e c t i o n of flow of m i n e r a l i z i n g s o l u t i o n s or t h e m i g r a t i o n v e c t o r s of

diffusing

c o n s t i t u e n t s i n s t a n d i n g w a t e r in s a n d s t o n e t e r r a n e s a r e g e n e r a l l y n o t e a s y t o e s t a b l i s h .

In

d i p p i n g r o c k s a d o w n - d i p f l o w of g r o u n d w a t e r or m i g r a t i o n ( d i f f u s i o n ) of c o n s t i t u e n t s

is

i n d i c a t e d f r o m t h e k n o w n o r i n f e r r e d h y d r o l o g i c s y s t e m ; t h e s i t u a t i o n in r e l a t i v e l y

flat-

l y i n g c h a n n e l s i s , h o w e v e r , m o r e d i f f i c u l t t o e l u c i d a t e s i n c e p o s t - m i n e r a l i z a t i o n t i l t i n g of t h e s t r a t a m a y h a v e o c c u r r e d , a n d t h e r o u t e of t h e s o l u t i o n s o r d i f f u s i o n c u r r e n t s m a y b e q u i t e devious and e r r a t i c due to b a r r i e r s and o t h e r f a c t o r s .

T h e g e n e r a l o r i e n t a t i o n of f l o w

can

of

often

be

obtained

by

a

study

of

the

elongation

calcite-cemented

sandstone

c o n c r e t i o n s a s M e s c h t e r ( 1 9 5 8 ) h a s s u g g e s t e d , b y e l o n g a t e d s t r e a k s o r z o n e s of c a l c i t e o r silica-cemented

sandstones,

elongated

features

features,

however,

due

by

to the

elongated passage

alteration

of

solutions

give only t h e o r i e n t a t i o n

strips or

or

zones,

diffusing

and not t h e direction

criteria must be sought to solve this problem.

or

by a n y

constituents. of f l o w ,

other These

and

other

S i n c e t h e d i r e c t i o n of f l o w is p e r p e n d i c u l a r

t o t h e i s o b a r i c p l a n e s a n d in t h e d i r e c t i o n of f a l l i n g p r e s s u r e , t h e d e g r e e of f i l l i n g of l i q u i d inclusions

in

minerals

such

as

quartz

and

carbonates

should

reflect

these

pressure

d i f f e r e n c e s a n d i n d i c a t e t h e d i r e c t i o n of f l o w s i n c e t h e t e m p e r a t u r e d i f f e r e n c e s o v e r s h o r t i n t e r v a l s in p a l e o c h a n n e l s w o u l d b e m i n i m a l .

O t h e r p o s s i b l e c l u e s m a y b e g i v e n by

the

s e l e c t i v e e t c h i n g of q u a r t z a n d o t h e r m i n e r a l s on t h e c o u n t e r flow s i d e of c r y s t a l s a n d by the selective

d e p o s i t i o n of

material

on t h e

down

flow

s i d e of

the

c o m p l i c a t i o n s m a y p r e v a i l in t h e i n t e r p r e t a t i o n of t h e r e s u l t s f r o m

minerals.

Serious

liquid inclusions and

o t h e r m e t h o d s of a n a l y s i s of t h e d i r e c t i o n of flow of s o l u t i o n s or d i f f u s i o n c u r r e n t s w h e r e c r o s s s t r a t a l f l o w s a n d m i n g l i n g of s o l u t i o n s f r o m t w o or m o r e c o n d u i t s h a v e

occurred.

Similar p r o b l e m s can be e x p e c t e d w h e r e physical b a r r i e r s and diversions such as faults

are

p r e s e n t in t h e s t r a t i g r a p h i c s e c t i o n s . Some sandstone-type uranium deposits appear to be related to present-day hydrologic conditions and specifically forming

today

sandstones

and

from

to p r e s e n t - d a y w a t e r tables.

ground

waters

conglomerates

or

migrating from

downdip

diffusion

S o m e of t h e s e d e p o s i t s m a y or

laterally

currents

through

pervading

be

permeable

these

rocks.

149 W o o d m a n s e e ( 1 9 5 8 ) h a s d i s c u s s e d t h i s s u b j e c t in c o n s i d e r a b l e d e t a i l a n d h a s g i v e n e x a m p l e s of t h e s p a t i a l r e l a t i o n s h i p of u r a n i u m o r e b o d i e s t o p r e s e n t - d a y w a t e r t a b l e s in t h e C o l o r a d o P l a t e a u , in t h e B l a c k H i l l s , a n d in t h e T e r t i a r y b a s i n s of W y o m i n g a n d C o l o r a d o .

In t h e

C o l o r a d o P l a t e a u he n o t e s t h a t c e r t a i n u r a n i u m d e p o s i t s o c c u r along p r e s e n t - d a y perched

water tables.

shallow

E x a m p l e s of t h e r e l a t i o n s h i p of o r e b o d i e s t o d e e p g r o u n d

c i r c u l a t i o n in p a l e o s t r e a m c h a n n e l s a n d in T e r t i a r y i n t e r m o n t a n e

water

basins are also

given.

T h e i n t e r e s t e d r e a d e r s h o u l d c o n s u l t t h e o r i g i n a l p a p e r for d e t a i l s a n d r e f e r e n c e s .

See

a l s o t h e r e s e a r c h c o l l o q u i u m on g r o u n d w a t e r f l o w s y s t e m s in e x p l o r a t i o n for u r a n i u m

by

Galloway e t al. (1979). R e c e n t c l i m a t o l o g i c a l f a c t o r s s e e m t o h a v e a b e a r i n g on t h e o c c u r r e n c e of e c o n o m i c s a n d s t o n e - t y p e u r a n i u m d e p o s i t s for r e a s o n s t h a t a r e n o t e n t i r e l y c l e a r . deposits occur

in t h e s e m i - a r i d , a r i d , or h o t d e s e r t

N e a r l y a l l of t h e s e

r e g i o n s of t h e e a r t h .

T h e t y p e of

w e a t h e r i n g a n d t h e g r o u n d w a t e r r e g i m e s in t h e s e e n v i r o n m e n t s m a y b e c r i t i c a l especially the latter. the near

surface

factors,

T h u s , t h e q u a n t i t y of m e t e o r i c w a t e r in a r i d r e g i o n s is m i n i m a l , a n d

ground

waters

tend

to

stagnate

and

build

up high

concentrations

u r a n i u m a n d o t h e r e l e m e n t s m a i n l y a s t h e r e s u l t of h i g h e v a p o r a t i o n r a t e s . l e a d s t o d e p o s i t i o n of u r a n i u m .

In t e m p e r a t e r e g i o n s , on t h e o t h e r h a n d , a b u n d a n t

meteoric

w a t e r t e n d s t o flush o u t t h e hydrologic s y s t e m , a f e a t u r e t h a t r e s t r i c t s p r e c i p i t a t i o n uranium.

D e e p l y c i r c u l a t i n g g r o u n d w a t e r s in t e m p e r a t e r e g i o n s m a y , h o w e v e r ,

uranium

in

sandstone

terranes;

from

this

fact

it

follows

that

of

With t i m e this

sandstone

of

precipitate terranes

in

t e m p e r a t e r e g i o n s should b e p r o b e d a t d e p t h for u r a n i u m d e p o s i t s . On a b r o a d r e g i o n a l b a s i s s a n d s t o n e - t y p e u r a n i u m d e p o s i t s a r e c o n f i n e d t o s t r u c t u r a l a n d / o r e r o s i o n a l b a s i n s f i l l e d w i t h c l a s t i c c o n t i n e n t a l s e d i m e n t s in w h i c h t h e r e is c o m m o n l y a c o m p l e m e n t of v o l c a n i c s , m a i n l y t u f f s .

Within t h e basins t h e deposits tend to o c c u r

a l o n g t h e f l a n k s a d j a c e n t t o u p l a n d s (in p l a c e s u p l i f t s ) of m e t a m o r p h i c r o c k s w i t h a m a j o r granitic component. belts

or

trends

M o s t of t h e u r a n i u m d e p o s i t s t e n d t o o c c u r in w e l l d e f i n e d

commonly

separated

by b a r r e n

or

nearly

barren

belts;

mineral

some

of

the

m i n e r a l i z e d t r e n d s a r e e l o n g a t e and r e l a t i v e l y n a r r o w , o t h e r s i r r e g u l a r or sinuous, t h e last b e i n g c h a r a c t e r i s t i c of b e l t s w i t h r o l l - t y p e d e p o s i t s ( F i g u r e 11).

Within t h e

mineralized

b e l t s t h e i n d i v i d u a l d e p o s i t s t e n d t o c l u s t e r a n d a r e s e p a r a t e d by s h o r t s t r e t c h e s of b a r r e n or l e a n l y m i n e r a l i z e d r o c k ( F i g u r e 9 ) . C e r t a i n s e d i m e n t a r y f o r m a t i o n s a r e u s u a l l y f a v o u r e d within t h e basins and

belts, typical examples being

M o r r i s o n F o r m a t i o n of s o u t h w e s t e r n

Colorado, the

t h e d e p o s i t s in t h e U p p e r Triassic Shinarump

Jurassic

Conglomerate

in

U t a h a n d A r i z o n a , t h e T r i a s s i c C h i n l e F o r m a t i o n in U t a h , t h e T e r t i a r y W a s a t c h a n d Wind R i v e r f o r m a t i o n s of C o l o r a d o , U t a h a n d W y o m i n g , a n d t h e L o w e r P e r m i a n f o r m a t i o n s of Europe.

Within t h e s e f o r m a t i o n s m o s t orebodies a r e r e l a t e d to d i s t i n c t

extensive stratigraphic discontinuities.

and

generally

T h e e x a c t c a u s e s of t h e l o c a l i z a t i o n of

sandstone-

t y p e u r a n i u m d e p o s i t s in t h e f o r m of m i n e r a l b e l t s o r t r e n d s , i s , h o w e v e r , r a r e l y e n t i r e l y clear

beyond

porosity

and

the

presence

of

permeability,

generally favourable

favourable hydrologic

sedimentary conditions,

rock and

features

such

erosional

as and

150 o x i d a t i o n / r e d u c t i o n p h e n o m e n a t h a t h a v e led t o e n r i c h m e n t in s o m e b e l t s .

A n u m b e r of

b e l t s o r t r e n d s s e e m t o b e r e l a t e d in a g e n e r a l w a y t o g e n t l e f o l d s o r u p w a r p s , t h e c r e s t s a n d t r o u g h s of w h i c h a r e b r o k e n or f a u l t e d .

In s o m e r e g i o n s t h e r e is a s p a t i a l ( b u t no

particular time) relationship to evaporities; these commonly underlie the uraniferous

belts

a n d in p l a c e s t h e d e p o s i t s m a y e x h i b i t a n a s s o c i a t i o n w i t h s a l t a n t i c l i n e s , s a l t d o m e s , a n d s a l t d i a p e r s ( F i g u r e 12). terranes

may

According

develop under

sabka

to

Rawson

evaporite

(1975) c e r t a i n

conditions,

an

deposits

example

in

sandstone

b e i n g o n e of

the

d e p o s i t s in l i m e s t o n e l a y e r s in t h e J u r a s s i c T o d i l t o F o r m a t i o n n e a r G r a n t s , N e w M e x i c o .

A

relationship

is

to underlying p e t r o l i f e r o u s

and n a t u r a l gaseous ( m e t h a n e ) - b e a r i n g

strata

e v i d e n t in s o m e r e g i o n s ; a s i m i l a r r e l a t i o n s h i p t o p e t r o l i f e r o u s a n d g a s e o u s r e s e r v o i r s a n d f a u l t s y s t e m s a s s o c i a t e d w i t h a n t i c l i n e s , s t r a t i g r a p h i c t r a p s , s a l t d o m e s , a n d so on is a l s o present

in

places

( F i g u r e 12).

In

both

these

cases

H2S

and

CH

rising

H

from

the

p e t r o l i f e r o u s a n d g a s e o u s h o r i z o n s a l o n g f r a c t u r e s a p p a r e n t l y r e d u c e s u r a n y l ion m i g r a t i n g d o w n w a r d o r l a t e r a l l y in m e t e o r i c w a t e r s , p r e c i p i t a t i o n a s u r a n i n i t e ( p i t c h b l e n d e , u r a n i u m b l a c k ) t a k i n g p l a c e in a v a i l a b l e p o r o u s s t r a t a in t h e s a n d s t o n e s a b o v e t h e strata.

petroliferous

M a j o r f a u l t i n g m a y b e a l o c a l i z i n g f a c t o r in s o m e b e l t s , t h e d e p o s i t s b e i n g r e l a t e d

to subsidiary faults, f r a c t u r e s , and broken ground near t h e regional faults.

The relationship

of b e l t s t o m a n i f e s t a t i o n s of i g n e o u s a c t i v i t y is e q u i v o c a l ; in s o m e b e l t s t h e r e m a y b e an affinity

t o i g n e o u s p l u g s , s t o c k s , l a c c o l i t h s , s i l l s , a n d d i k e s ; in o t h e r s a r e l a t i o n s h i p

to

v o l c a n i s m s e e m s e v i d e n t s i n c e t h e d e p o s i t s o c c u r in d i a t r e m e s a n d v o l c a n i c r o c k s ; a n d in still other regions t h e mineralized

b e l t s s e e m r e l a t e d v i a o r i g i n of t h e u r a n i u m a n d i t s

associated

or

elements

summarize:

to

overlying

underlying

tuffs

and

other

volcaniclastics.

To

n e a r l y all f l a t - l y i n g s a n d s t o n e - t y p e o r e b o d i e s a r e l o c a l i z e d j u s t a b o v e s o m e

d i s t i n c t i v e s t r a t i g r a p h i c d i s c o n t i n u i t y usually an u n c o n f o r m i t y ; t h o s e w i t h s t e e p dips a r e generally concentrated

in t e c t o n i c

or v o l c a n i c f e a t u r e s

such as faults, f r a c t u r e

zones,

shear z o n e s , and d i a t r e m e s . T h e l o c a l l i t h o l o g i c a n d t e c t o n i c f e a t u r e s t h a t t h e p r o s p e c t o r s h o u l d s e a r c h for in terranes

likely

to contain

sandstone-type

sandstone-arkose-conglomerate-shale

deposits

sequences

include

with

a

cross-bedded

tuffaceous

and

shale,

lenticular pyroclastic,

a n d / o r v o l c a n i c flow c o m p o n e n t d e p o s i t e d in a n c i e n t c h a n n e l s of b r o a d m e a n d e r i n g r i v e r s , i n i n t e r m i t t a n t l a k e s a n d s w a m p s , in l a g o o n s , on d e l t a s , a n d a l o n g a n c i e n t f l a n k i n g c o a s t a l plains

and

continental

barrier

islands,

environments.

accumulations

and

particularly More specific

in

the

and detailed

sandstone-conglomerate

rocks;

transverse

t o l i n g u o i d , a n d p o i n t or s i d e b a r s ; b a r s

sandstone

lenses

zones

features against in

between include

piedmont

granitic

alluvial

and e m b a y m e n t s

marine

fans;

and

and talus

gneissic

longitudinal,

on t h e

inside

l o o p s , e s p e c i a l l y w h e r e r e m n a n t s of v e g e t a t i o n ( c a r b o n t r a s h ) a n d p y r i t e

accumulated; or

and

pinchouts

basement

meander

fanglomerates

transition

of

have

l e n s e s a n d b e d s of s e d i m e n t a r y b r e c c i a f i l l i n g s c o u r c h a n n e l s ; c h a n n e l " l o w s "

"deeps"; scour

and

channel

f i l l ; l e n s e s of s a n d s t o n e a n d c o n g l o m e r a t e

representing

c h a n n e l , swale, and scour infilling, especially when t h i c k e r t h a n n o r m a l and confined within

151 i m p e r m e a b l e s h a l e s or s h a l y l i m e s t o n e s ; b r a n c h e s , b e n d s , c o a l e s c e n c e s , a n d b a s i n s in t h e p a l e o d r a i n a g e s y s t e m ; f o r s e t d e l t a i c s a n d s t o n e and c o n g l o m e r a t e lenses; aeolian dune sand l e n s e s ; l e n s e s of b e a c h s a n d a n d c o n g l o m e r a t e ; d o w n - d i p , u p - d i p , o r l a t e r a l f a c i e s c h a n g e s and/or

pinchouts,

particularly

transitional

sites

where

channel

sandstones

and

c o n g l o m e r a t e s g r a d e i n t o flood p l a i n s h a l e s or s h a l y s a n d s t o n e s c o n t a i n i n g p l a n t d e b r i s , t h i n lignites,

and/or

petroleum

residues; channel

flanks

where

sandstone

and

conglomerate

l e n s e s g r a d e i n t o o r g a n i c - r i c h s h a l e s , c o a l i f i e d p l a n t z o n e s , or l i g n i t e b e d s ; m u d s t o n e a n d s h a l e s p l i t s ; l e n s e s a n d b e d s of r e p l a c e a b l e , c o m m o n l y c r u m p l e d or o t h e r w i s e

deformed,

l a c u s t r i n e o r n e a r - s h o r e m a r i n e l i m e s t o n e o r l i m y s h a l e ; a n d b e d s of g r e e n i s h a n d g r e y i s h paludal shale with abundant plant remains. wash)

and

containing

petroliferous

abundant

materials

are

particularly

uraniferous sandstone belts.

Basal beds overlying granitic terranes (granite

zeolites,

clay

minerals,

susceptible

and

carbonaceous

to uranium

with

abundant

in

or

some

B e d s w i t h a b u n d a n t f o s s i l b o n e f r a g m e n t s or t h i n l a y e r s a n d

l e n s e s of p h o s p h a t i c ( c o l l o p h a n e ) m a t e r i a l e x h i b i t t h e s a m e p h e n o m e n o n , beds

plant

mineralization

clay

galls,

and

sandstone

c l a y l e n s e s , or c a r b o n a t e a n d F e - M n o x i d e c o n c r e t i o n s

a n d l e n s e s a r e f a v o u r a b l e t o m i n e r a l i z a t i o n in s o m e s e q u e n c e s .

A l l of t h e s e

sedimentary

r e c e p t a c l e s a r e p a r t i c u l a r l y f a v o u r e d a s m i n e r a l i z a t i o n s i t e s w h e n c a p p e d by i m p e r m e a b l e s h a l e s a n d f l o o r e d by s i m i l a r s h a l e s or c r y s t a l l i n e b a s e m e n t r o c k s .

Sandstone

sequences

o v e r l a i n by t u f f a c e o u s b e d s a n d / o r a c i d v o l c a n i c s a p p e a r t o b e p a r t i c u l a r l y f a v o u r a b l e in some belts.

W i t h r e s p e c t t o t h i c k n e s s of s a n d s t o n e m e m b e r s , t h o s e t h a t a r e t h e

a n d h a v e a l e n s e - l i k e m o r p h o l o g y s e e m t o b e f a v o u r e d by m i n e r a l i z a t i o n . not generally apply to the roll-type orebodies.

T e r r a n e s w i t h a s a n d s t o n e / s h a l e r a t i o in t h e

r a n g e 1:1 a p p e a r t o b e t h e m o s t f a v o u r a b l e h o s t s for u r a n i u m m i n e r a l i z a t i o n . t e r r a n e s t h e p r e s e n c e of f a u l t s , f r a c t u r e s , a n d b r e c c i a z o n e s a r e f a v o u r a b l e mineralization,

as

diatremes,

and

explosion

are

structures

and

the

also

collapse

breccias,

various

breccias,

and

dike

sedimentary

thickest

This f e a t u r e d o e s

breccia

and

sill

features

pipes

related

contacts

noted

above

in

to

some

when

In c e r t a i n to

uranium

ring

faults,

belts.

These

mineralized

are

g e n e r a l l y m a r k e d by a r e d u c t i o n of f e r r i c i r o n ( h e m a t i t e ) , a d e v e l o p m e n t of l i g h t c o l o u r e d clay

m i n e r a l s , and c o m m o n l y

an i n t r o d u c t i o n

of c a r b o n a t e s

and sulphur yielding

d i s s e m i n a t e d p y r i t e , t h e w h o l e g i v i n g b e d s , l a y e r s , t o n g u e s , s t r e a k s or e x t e n s i v e

finely

irregular

z o n e s w i t h a g r e y or g r e e n i s h - g r e y c o l o u r a n d / o r b l e a c h e d a p p e a r a n c e in r o c k s t h a t o t h e r w i s e r e g i o n a l l y p e r v a s i v e l y r e d d i s h or b r o w n i s h in c o l o u r ( r e d b e d s ) .

are

Where roll-type

orebodies predominate the near surface altered zones are commonly buff-coloured,

faintly

g r e e n i s h - y e l l o w , or f a i n t l y r e d d i s h w i t h f l e c k s of h e m a t i t e a n d / o r l i m o n i t e m a r k i n g s p o t s of o x i d i z e d p y r i t e ; d o w n d i p or a h e a d of t h e r o l l s t h e r o c k s p r e s e n t t h e g r e y i s h - g r e e n b l e a c h e d appearance noted above. possibility

of

uranium

In a m o r e and

general geological sense,

other

types

of

metallic

good

indications

mineralization

in

of

the

terrestrial

c o n g l o m e r a t e s , s a n d s t o n e s , a n d s h a l e s i n c l u d e t h e p r e s e n c e of p y r o c l a s t i c a n d

tuffaceous

h o r i z o n s a n d l a v a f l o w s , u n d e r l y i n g or o v e r l y i n g b l a c k s h a l e s or p e t r o l i f e r o u s

shales and

limestones, underlying petroliferous

salt domes

and d i a p i r s , and underlying

or

laterally

152 positioned

granitic,

migmatitic,

and

migmatitic-pegmatitic

complexes.

All

of

these

v a r i o u s r o c k s a r e a d e q u a t e s o u r c e s of u r a n i u m , v a n a d i u m , c o p p e r , m o l y b d e n u m , a n d

the

o t h e r e l e m e n t s c o n c e n t r a t e d in t h e s a n d s t o n e - t y p e d e p o s i t s . M a n y of t h e s e d i m e n t a r y , s t r u c t u r a l a n d o t h e r f e a t u r e s n o t e d a b o v e m a y b e r e v e a l e d by drilling

and/or

seismic

refraction

and

reflection

surveys from

which

isopach maps depicting the paleotopographic relief can be c o n s t r u c t e d .

structure Thus, at

and

scales

c o m m e n s u r a t e w i t h t h e s i z e of t h e f e a t u r e s b e i n g s t u d i e d a l a r g e n u m b e r of t h e s e c a n b e portrayed, including the configuration interfingering

of

sandstone

and

a n d t h i c k n e s s of f a v o u r a b l e f o r m a t i o n s ; z o n e s

shale;

facies

changes;

the

contours

of

of

impermeable

b a s e m e n t rocks, footwall rocks and cap rocks; anticlinal, synclinal, and d o m e

structures;

s t r u c t u r a l t e r r a c e s ; o u t l i n e s of b e d s , l e n s e s , c h a n n e l fill, o r e s h o o t s , a n d a l t e r a t i o n z o n e s ; a n d t h e c o n t o u r of t h e g r o u n d w a t e r s u r f a c e .

U s e d in c o n j u n c t i o n w i t h g e o c h e m i c a l

data

f r o m d r i l l c o r e s or c u t t i n g s a n d f r o m a n a l y s e s of d r i l l h o l e g r o u n d w a t e r s t h e s e t y p e s of p l a n s a n d s e c t i o n s a r e i n v a l u a b l e in p r e d i c t i n g t h e p o s s i b l e l o c a t i o n of h i d d e n o r e s h o o t s . Specific

surface

(supergene)

include s m e a r s , c r u s t s , and (hexavalent)

uranium

mineralogical

pulverulent

indicators

m a s s e s of

of

varieties),

the

various

erythrite,

antlerite,

wad

and

pyrite

conglomerates,

(quartz

and

chalcopyrite, selenium,

similar bleached

and

yellow, o r a n g e , and g r e e n

rocks.

commonly

secondary

vanadium

ilsemannite two

by

Among these may be mentioned the

annabergite,

last

marcasite

shales,

minerals

azurite,

(molybdenum

frequently

in

(including

malachite, oxide),

the

U-V

chrysocolla,

jarosite,

pulverulent,

in g r e y i s h , g r e e n i s h ,

and

barite,

chalcocite,

arsenic,

certain circumstances. abundant,

the

and/or

alunite,

bornite,

native

deposits

native

botryoidal,

and

Subsurface (hypogene) mineralogical indicators include the presence

arkose,

opal),

supergene

brochantite,

limonite,

concretionary masses. of d i s s e m i n a t e d

coloured

sphaerocobaltite,

chalcanthite,

sulphur,

sandstone-type

m i n e r a l s a n d a v a r i e t y of m o r e s u b t l e f e a t u r e s n o n e of w h i c h

t h e m s e l v e s a r e i n d i c a t i v e of e c o n o m i c m i n e r a l i z a t i o n . presence

of

the

presence

gypsum,

covellite,

arsenopyrite,

anhydrite,

and b l e a c h e d

carbonates, jordisite,

tennantite-tetrahedrite,

Ni-Co

Hematite and/or

of

arsenides,

and

sandstones,

introduced

galena,

silica

sphalerite,

ferroselite,

Ni-Co-bearing

native

pyrite

l i m o n i t e ( g o e t h i t e ) m a y b e an i n d i c a t o r

in

under

M o s t P r o t e r o z o i c s a n d s t o n e - t y p e d e p o s i t s a r e g e n e r a l l y m a r k e d by radioactive

zones

of h e m a t i t i c

rocks, f a u l t s , and f r a c t u r e s n e a r t h e o r e b o d i e s .

staining and i m p r e g n a t i o n

In y o u n g e r r o c k s t h i s t y p e of

of

wall

pervasive

h e m a t i t i z a t i o n is l e s s f r e q u e n t , a l t h o u g h in s o m e o r e b o d i e s o n e o f t e n o b s e r v e s d u s k y r e d carbonates

impregnated

with

hematite;

hematite

flecks

and

limonitic

disseminations

m a r k i n g t h e o x i d a t i o n of p r e e x i s t i n g p y r i t e a n d / o r s i d e r i t e a l o n g p a l e o h y d r o l o g i c c h a n n e l s frequently amounts

of

indicate detrital

the

presence

magnetite

of

roll-front

and/or

orebodies

ilmenite

may

as n o t e d

indicate

above.

approach

Diminishing

to orebodies

s a n d s t o n e t e r r a n e s , t h e p l a c e of t h e t w o o x i d e s b e i n g t a k e n m a i n l y by p y r i t e a n d titaniferous

residue

minerals

principally

anatase,

rutile, and l e u c o x e n e .

in

various

Carbonaceous

153 materials

including

coalified

plant

fragments

and

petroleum

residues

(bitumen,

a n t h r a x o l i t e , u c h o l i t e ) a r e p a r t i c u l a r l y f a v o u r a b l e m i n e r a l o i d i n d i c a t o r s in a l l

uraniferous

sandstone terranes. T h e p r e c i p i t a t i o n of u r a n i u m in s a n d s t o n e - t y p e d e p o s i t s is c o n t r o l l e d in m a n y c a s e s by t h e

Eh-pH

geochemical

conditions indicators

and are

adsorption of

use

phenomena.

in

prospecting.

In t h i s These

respect include

r e d u c t a n t s such as decaying humic m a t e r i a l s , mainly now coalified

certain the

general

existence

of

plant fragments

l o g s ; t h e o c c u r r e n c e of p e t r o l e u m r e s i d u e s , n o w m a i n l y b i t u m e n a n d s o l i d i f i e d

and

hydrocarbons

s u c h a s u c h o l i t e , a n t h r a x o l i t e a n d o t h e r u r a n o - a s p h a l t i c s u b s t a n c e s ; t h e p r e s e n c e of

lenses

of s a n d s t o n e a n d s h a l e r i c h in z e o l i t e s , m o n t m o r i l l o n i t e , c l a y m i n e r a l s , c h l o r i t e , or f i n e l y d i v i d e d o r g a n i c m a t e r i a l a l l of w h i c h s t r o n g l y a d s o r b a n d a b s o r b u r a n i u m ; a n d t h e p r e s e n c e of f i n e - g r a i n e d p y r i t e , m a r c a s i t e , a n d o t h e r s u l p h i d e s in s h a l e s a n d s a n d s t o n e . coalified

plant

fragments,

thin

lignite

a n t h r a x o l i t e and other p e t r o l e u m

beds,

(generally

the

tops),

A n a l y s e s of

carbonaceous

galls,

r e s i d u e s , p h o s p h a t i c p e b b l e s , fossil b o n e s , z e o l i t i c

and

c l a y m i n e r a l c o n c e n t r a t i o n s , a n d t h i n p h o s p h a t i c s h a l e b e d s or l e n s e s in s a n d s t o n e t e r r a n e s f o r U, V, C u , M o , S e , a n d o t h e r i n d i c a t o r e l e m e n t s ( T a b l e 14) will g e n e r a l l y g i v e a c l u e a s t o w h e t h e r or n o t u r a n i f e r o u s m i n e r a l i z a t i o n h a s b e e n a c t i v e in an a r e a . occurrence

in r e d - b e d s a n d s t o n e - c o n g l o m e r a t e

Similarily

the

s e q u e n c e s of g r e y , g r e e n i s h - g r e y , or

buff

b e d s a n d z o n e s c o n t a i n i n g d i s s e m i n a t e d p y r i t e , a n d s l i g h t l y e n r i c h e d in U, V, C u , M o , S e , and

other

indicator

elements,

is

a

particularly

favourable

manifestation

of

uranium

mineralization processes. A p p r o a c h t o s i t e s of m i n e r a l i z a t i o n

and uranium orebodies across the redox

( f r o n t s or b a r r i e r s ) of s a n d s t o n e t e r r a n e s m a y b e s i g n a l l e d by d e c r e a s e s in t h e

Fe

3 +

zones /Fe

2 +

r a t i o r e c o r d i n g a g e n e r a l r e d u c t i o n of f e r r i c i r o n d u e t o s u l p h i d i z a t i o n a n d o t h e r p r o c e s s e s ; d e c r e a s e in d e t r i t a l m a g n e t i t e , i l m e n i t e , a n d h e m a t i t e c o n t e n t w i t h a p p r o a c h t o o r e is a m a n i f e s t a t i o n of t h e s a m e p r o c e s s e s . mineralization thorium.

T h e U / T h r a t i o g e n e r a l l y i n c r e a s e s t o w a r d s i t e s of

and ore bodies due to the g r e a t e r

of u r a n i u m

as c o m p a r e d

with

S i m i l a r i l y , t h e r a t i o of e a s i l y l e a c h a b l e m e t a l t o t o t a l m e t a l c o n t e n t of t h e r o c k s

may signal approach to mineralized sites. increases

influx

with

approach

frequently noted. an i n c r e a s e

to

ore; with

With r e s p e c t to u r a n i u m this r a t i o c o m m o n l y

other

indicator

elements

a

similar

tendency

is

F i n a l l y , c h a n g e s in t h e c o n t e n t , t y p e , a n d a l t e r a t i o n of c l a y m i n e r a l s a n d

in v a r i o u s

indicator

orebodies are approached.

elements

and

These and o t h e r

in r a d i o a c t i v i t y

lithogeochemical

are

generally

features

noted

as

are discussed

in

g r e a t e r d e t a i l in t h e n e x t C h a p t e r . A

number

of g u i d e s a n d

i n d i c a t o r s of u r a n i f e r o u s

and

thoriferous

quartz-pebble

c o n g l o m e r a t e s a n d q u a r t z i t e s a r e a v a i l a b l e t o t h e p r o s p e c t o r , i n c l u d i n g t h e o r i e s of and

various

paleogeographic,

paleoclimatic,

and

paleohydrologic

features,

and

origin more

154 specific geological, mineralogical, and g e o c h e m i c a l c h a r a c t e r i s t i c s . has affected

Since

metamorphism

m a n y of t h e s e d e p o s i t s t h e p a l e o c l i m a t i c a n d p a l e o h y d r o l o g i c f e a t u r e s

are

o f t e n o b s c u r e d b y e f f e c t s s u c h a s m i n e r a l o g i c a l c h a n g e s , r e c r y s t a l l i z a t i o n , a n d so o n . T h e o r i g i n of u r a n i f e r o u s , t h o r i f e r o u s , a n d a u r i f e r o u s q u a r t z - p e b b l e h a s long b e e n d e b a t e d and still r e m a i n s highly c o n t r o v e r s i a l .

conglomerates

A c o m p r e h e n s i v e a c c o u n t of

t h e v a r i o u s t h e o r i e s of o r i g i n h a s b e e n p u b l i s h e d by t h e w r i t e r ( B o y l e , 1 9 7 9 a ) , a n d o n l y a b r i e f o u t l i n e of t h e m a i n c o n c e p t s w i l l b e a t t e m p t e d h e r e .

Most geologists ( L i e b e n b e r g ,

1 9 5 5 ; R o s c o e a n d S t e a c y , 1 9 5 8 ; R o s c o e , 1 9 6 9 ; R o b e r t s o n , 1 9 7 4 , P r e t o r i u s , 1975) c o n s i d e r t h e d e p o s i t s t o b e m o d i f i e d p l a c e r s , t h a t is t h e g o l d , u r a n i n i t e , b r a n n e r i t e , a n d o t h e r a l l i e d m i n e r a l s a r e e s s e n t i a l l y d e t r i t a l or h a v e s u f f e r e d s o m e r e m o b i l i z a t i o n a n d a s a r e s u l t of d i a g e n e t i c a n d m e t a m o r p h i c p r o c e s s e s .

reconcentration

A few geologists have considered the

d e p o s i t s t o b e of e p i g e n e t i c o r i g i n , t h e g o l d , u r a n i u m , a n d a s s o c i a t e d e l e m e n t s h a v i n g b e e n d e p o s i t e d by h y d r o t h e r m a l s o l u t i o n s ( G r a t o n ,

1 9 3 0 ) or by d e s c e n d i n g h e a t e d b r i n e s

leached the m e t a l s from overlying volcanics and a s s o c i a t e d rocks (Davidson

that

1964-1965).

E v i d e n c e for t h e s e t h r e e c o n c e p t s is u s u a l l y m a n i f e s t in m o s t of t h e d e p o s i t s , t h u s l e a d i n g to

the

violent

Witwatersrand

and

sometimes

acrimonious

deposits, t h e first

controversies

that

of t h e t y p e , w e r e d i s c o v e r e d

have

raged

associated

(alluvial)

origin,

quartzites and

hosting

hence

the

the

gold

and

conditions

uranium

of

their

the

in S o u t h A f r i c a in 1 8 8 6 .

R e g a r d l e s s of w h i c h o r i g i n is a c c e p t e d i t is c e r t a i n t h a t t h e q u a r t z - p e b b l e and

since

deposits

are

accumulation

conglomerates of

are

sedimentary of

particular

i m p o r t a n c e in p r o s p e c t i n g . The

paleogeographic

and

paleotopographic

conditions

existing

at

f o r m a t i o n of t h e q u a r t z - p e b b l e c o n g l o m e r a t e s a n d q u a r t z i t e s w e r e p r o b a b l y terrestrial,

essentially

fluviatile,

deltaic,

and

littoral,

only

rarely

the

time

near-shore

marine,

c o n d i t i o n s n o t u n l i k e t h o s e p o s t u l a t e d for t h e s e d i m e n t a r y h i s t o r y of t h e h o s t r o c k s of sandstone-type deposits.

of

predominantly

the

T h e p r i n c i p a l s i t e s of s e d i m e n t a t i o n i n d i c a t e d by t h e g e o l o g y of

t h e q u a r t z - p e b b l e c o n g l o m e r a t e d e p o s i t s a r e m a r g i n a l b a s i n s of m i o g e o s y n c l i n e s or s l o w l y s i n k i n g e p i c o n t i n e n t a l b a s i n s r e l a t e d t o i n t r a c r a t o n i c d e p r e s s i o n s or g r a b e n - l i k e s t r u c t u r e s (aulacogens).

W i t h i n t h e s e e n v i r o n m e n t s b r o a d a l l u v i a l f a n s on p i e d m o n t s , a l l u v i a l p l a i n s ,

a n d d e l t a s c o u r s e d by w i d e s l u g g i s h r i v e r s a n d s t r e a m s , a n d s k i r t i n g d e e p l y e r o d e d t e r r a n e s g e n e r a l l y of A r c h e a n a g e p r e d o m i n a t e d .

L i t t o r a l or g e n t l y s l o p i n g s h e l f e n v i r o n m e n t s , in

s o m e places flanking deltas, are indicated sandstones,

but these,

economic interest. rounded

indicating

while often

for c e r t a i n q u a r t z - p e b b i e c o n g l o m e r a t e s

mineralized

with

gold

and

uranium,

are

seldom

T h e p e b b l e s in t h e q u a r t z - p e b b l e c o n g l o m e r a t e s a r e w e l l s o r t e d considerable

travel

or

sedimentary

milling;

they

comprise

and of and

mainly

p e g m a t i t i c and vein q u a r t z , the l a t t e r mainly gold-bearing vein q u a r t z according to this w r i t e r ' s o b s e r v a t i o n s on c a t a c l a s i s , i n c l u s i o n s , a n d o t h e r f e a t u r e s of t h e q u a r t z .

The vein

q u a r t z , h o w e v e r , h a s b e e n w e l l m i l l e d a n d is r e l a t i v e l y f r e e of a u t h i g e n i c s u l p h i d e s a n d gold.

Other pebbles include chert,

g r a n i t e , q u a r t z i t e , and hornfels

g e n e r a l l y p r e s e n t o n l y in m i n o r a m o u n t s .

although

these

T h e m a t r i x is e s s e n t i a l l y f i n e - g r a i n e d

are

quartz,

155 feldspar,

sericite,

chlorite,

choritoid,

and

pyrophyllite

in w h i c h

is s e t

much

pyrite

or

h e m a t i t e a n d i l m e n i t e , t h e l a s t t w o m a i n l y in t h e a u r i f e r o u s , n o n - r a d i o a c t i v e d e p o s i t s ( e . g . Tarkwa, Ghana).

T h e p y r i t e m a y b e a l l o g e n i c , a u t h i g e n i c , or b o t h , b u t t o this w r i t e r m o s t

a p p e a r s t o h a v e b e e n g e n e r a t e d a f t e r t h e d e p o s i t i o n of t h e c o n g l o m e r a t e s a n d q u a r t z i t e s . The

nature

and

composition

of

the

pebbles and

the

matrix

suggests

a deeply

eroded

h i n t e r l a n d c o n t a i n i n g n u m e r o u s q u a r t z v e i n s a n d p e g m a t i t e s s e t in a m e t a m o r p h i c v o l c a n i c a n d s e d i m e n t a r y t e r r a n e g r a n i t i z e d or i n j e c t e d

by m i g m a t i t e s a n d g r a n i t i c b o d i e s .

The

t o p o g r a p h y of t h i s t e r r a n e a p p e a r s t o h a v e b e e n m o d e r a t e l y r u g g e d w i t h m a n y v a l l e y s ( n o w c h a n n e l s f i l l e d w i t h c o n g l o m e r a t e ) , j u d g i n g by t h e i r r e g u l a r n a t u r e of t h e u n c o n f o r m i t i e s a t t h e b a s e of s o m e of t h e q u a r t z - p e b b l e c o n g l o m e r a t e s e q u e n c e s .

Stratigraphically

higher

b e d s in m o s t s e q u e n c e s r e c o r d a r e l a t i v e l y u n i f o r m t o p o g r a p h y m a r k e d by c h a n n e l s , s w a l e s , and scours. The associated

paleoclimatic quartzites

conditions

were

laid

under

down

can

which only

the be

e x t e n s i v e w e a t h e r i n g , erosion, and s e d i m e n t a r y milling. p r e s e n c e of c o n s i d e r a b l e a m o u n t s of atmosphere.

quartz-pebble

conjectured.

oxygen, carbon

conglomerates These

rocks

and

suggest

Extensive weathering suggests the dioxide,

and

water

in t h e

paleo-

T h e r e is l i t t l e d o u b t a b o u t t h e l a s t t w o c o m p o u n d s b e i n g p r e s e n t s i n c e t h e

c o n g l o m e r a t e s a n d q u a r t z i t e s w e r e w a t e r - l a i n , a n d v o l c a n i s m in A r c h e a n t i m e s

produced

a b u n d a n t c a r b o n d i o x i d e ( B o y l e , 1 9 7 6 b ) m u c h of w h i c h w a s p r o b a b l y c a r r i e d o v e r i n t o t h e Proterozoic atmosphere to persist through various cyclic processes

to the present

b e i n g a u g m e n t e d f r o m t i m e t o t i m e d u r i n g y o u n g e r p e r i o d s of v o l c a n i s m . of

oxygen

during early P r o t e r o z o i c

investigators

claiming

anoxygenic conditions. iron f o r m a t i o n s

that

the

time

has, however,

quartz-pebble

been

The availability

questioned,

conglomerates

were

day,

a number

laid

down

This a p p e a r s i m p r o b a b l e having r e g a r d t o t h e f a c t t h a t

of

under

hematititc

a r e r e l a t i v e l y a b u n d a n t in t h e A r c h e a n , a n d t h a t m o l e c u l a r o x y g e n

was

p r o b a b l y o n e of t h e g a s e o u s p r o d u c t s of d e g a s s i n g of t h e y o u n g e a r t h , i t s o r i g i n b e i n g d u e t o radiolysis and o t h e r

early geochemical

p r o c e s s e s (Boyle,

1976b).

T h e r e is a

strong

p o s s i b i l i t y t h a t s o m e of t h e q u a r t z - p e b b l e c o n g l o m e r a t e s a n d q u a r t z i t e s of P r o t e r o z o i c a g e r e p r e s e n t w e l l w i n n o w e d a c c u m u l a t i o n s of q u a r t z a n d m i c a s f r o m t i l l s h e e t s , in w h i c h c a s e t h e w e a t h e r i n g and erosion w e r e essentially m e c h a n i c a l .

T h i s is s u g g e s t e d by t h e f a c t

that

t h e A p h e b i a n ( L o w e r P r o t e r o z o i c ) is m a r k e d by g l a c i a l p e r i o d s , t h e t i l l i t e s of t h e G o w g a n d a F o r m a t i o n a n d t h e W i t w a t e r s r a n d ( W i e b o l s , 1955)

being manifestations.

Having regard

to

t h e s e speculations it s e e m s probable t h a t the P r o t e r o z o i c q u a r t z - p e b b l e c o n g l o m e r a t e s and their associated q u a r t z i t e s were deposited during a warming post-glacial period under

an

a t m o s p h e r e w i t h a r e l a t i v e l y high o x y g e n c o n t e n t . R e g a r d l e s s of w h i c h t h e o r y of o r i g i n is e m b r a c e d for t h e u r a n i f e r o u s a n d quartz-pebble

conglomerates

and

quartzites

i t is e v i d e n t

that

these

rocks

thoriferous have

been

c o u r s e d by w a t e r s (or d i f f u s i n g c o n s t i t u e n t s in s t a t i o n e r y w a t e r s ) t h a t f o l l o w e d old r i v e r c h a n n e l s , s w a l e s , and d e e p s c o u r s .

In t h e o p i n i o n of t h i s w r i t e r

these

paleohydrologic

p a t h w a y s a r e m a r k e d by a n u m b e r of m i n e r a l o g i c a l a n d c h e m i c a l f e a t u r e s t h a t a r e e v i d e n t

156 t o t h e n a k e d e y e in s o m e c a s e s a n d t h r o u g h t h e m i c r o s c o p e in o t h e r s .

Of t h e s e , a u t h i g e n i c

p y r i t e is p r o b a b l y t h e m o s t i m p o r t a n t , e s p e c i a l l y t h e r o u n d e d c o l l o f o r m

type, which

many

in

cases

marks

rich

ore

shoots

of

gold

and

U-Th

minerals.

Next

p a r t i c u l a r l y in t h e W i t w a t e r s r a n d , is t h e p r e s e n c e of a u t h i g e n i c or r e m o b i l i z e d g o l d , solid

hydrocarbons

(ucholite,

thucholite),

accompanied

by d i s s e m i n a t i o n s

in

importance,

and

the

scattered

c r y s t a l a g g r e g a t e s of a u t h i g e n i c s u l p h i d e s s u c h a s p y r r h o t i t e , g a l e n a , s p h a l e r i t e , c o b a l t i t e , and n i c c o l i t e .

F u r t h e r i n d i c a t i o n s of t h e p a s s a g e of s o l u t i o n s is m a n i f e s t

in t h e q u a r t z -

p e b b l e c o n g l o m e r a t e s a n d q u a r t z i t e s by a b o v e n o r m a l r a d i o a c t i v e p r o f i l e s , albitization,

and

the

development,

probably

by

remobilization

p r o c e s s e s , of s e r i c i t e , c h l o r i t e , c h l o r i t o i d , a n d p y r o p h y l l i t e .

and

silicification,

recrystallization

All of t h e s e m i n e r a l o g i c a l a n d

c h e m i c a l i n d i c a t o r s r e c o r d t h e p a s s a g e of s o l u t i o n s (or d i f f u s i n g

constituents),

probably

l a r g e l y of d i a g e n i c or m e t a m o r p h i c

the

minerals

(elements) from

origin, t h a t

have winnowed

economic

l a r g e v o l u m e s of c o n g l o m e r a t e s , q u a r t z i t e s , a n d o t h e r r o c k s i n t o l a r g e ,

i r r e g u l a r , a n d o f t e n p a t c h y o r e s h o o t s in c h a n n e l s a n d o t h e r s i t e s w h e r e p e r m e a b i l i t y

and

porosity w e r e initially high. Many

of

the

geological,

mineralogical,

and

geochemical

features

of

interest

in

p r o s p e c t i n g for U-Th q u a r t z - p e b b l e c o n g l o m e r a t e s and q u a r t z i t e s a r e m e n t i o n e d a b o v e and in C h a p t e r

2, a n d o n l y a b r i e f r e s u m e of s o m e of t h e m o r e i m p o r t a n t of t h e s e n e e d

mentioned here.

In s e l e c t i n g f a v o u r a b l e a r e a s i t is g e n e r a l l y d i f f i c u l t

to predict

be

where

t h e s e t y p e s of a u r i f e r o u s , t h o r i f e r o u s , a n d u r a n i f e r o u s d e p o s i t s m a y o c c u r in s e d i m e n t a r y sequences.

M a n y a r e n e a r t h e b a s e of P r o t e r o z o i c s e q u e n c e s l y i n g w i t h g r e a t

unconformity

on h i g h l y f o l d e d A r c h e a n v o l c a n i c , s e d i m e n t a r y , a n d g r a n i t i c t e r r a n e s ; o t h e r s l i e w e l l a b o v e t h e b a s e in s o m e d i s t r i c t s .

S u r f a c e i n d i c a t i o n s of u r a n i f e r o u s , t h o r i f e r o u s , a n d

c o n g l o m e r a t e s a r e c o m m o n l y p r e s e n t in r e g i o n s c o n t a i n i n g t h e s e d e p o s i t s , b e i n g

auriferous manifest

b y l a r g e q u a n t i t i e s of p y r i t e in t h e r o c k s ; e x c e s s q u a n t i t i e s of q u a r t z p e b b l e s o r s a n d in s o i l s , s t r e a m s , a n d r i v e r s ; l a r g e c o n c e n t r a t i o n s of SO4 in g r o u n d a n d s u r f a c e w a t e r s d u e t o o x i d a t i o n of

pyrite; abundant

iron staining

and gossans, both

residual

and

transported;

a n o m a l o u s r a d i o a c t i v i t y in t h e r o c k s , s o i l s , a n d w a t e r s ; a n d t h e p r e s e n c e in t h e t e r r a n e of solid h y d r o c a r b o n s e n r i c h e d in o n e or m o r e of t h o r i u m , u r a n i u m , r a r e e a r t h s , a n d g o l d . The

report

of

the

working

group

on

uranium

in

quartz-pebble

conglomerates

( I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y , V i e n n a , 1 9 7 4 ; F o r m a t i o n of u r a n i u m o r e Proceed.

Symp.

Athens,

p . 707)

is

of

interest

to

those

prospecting

t h o r i f e r o u s , a n d a u r i f e r o u s d e p o s i t s in t h e q u a r t z - p e b b l e c o n g l o m e r a t e s . "Uranium-bearing,

oligomictic,

quartz-pebble

conglomerates

s e d i m e n t s laid d o w n in P r e c a m b r i a n i n t r a c r a t o n i c b a s i n s .

for

deposits;

uraniferous,

They s t a t e : are

epicontinental

M o s t of t h e s e c o n g l o m e r a t e s a r e

f o u n d in t h e s e d i m e n t a r y s e c t i o n p r i o r t o t h e d e v e l o p m e n t of t h e f i r s t l i m e s t o n e s .

None

h a v e b e e n f o u n d of a g e y o u n g e r t h a n t h e f i r s t r e d b e d s a n d w e b e l i e v e t h a t n o n e y o u n g e r will b e f o u n d .

We s u g g e s t t h a t r e p o r t s of y o u n g c o n g l o m e r a t e s of t h i s t y p e , s u c h a s a

C a m b r i a n c o n g l o m e r a t e d e s c r i b e d in S o v i e t l i t e r a t u r e , a r e in e r r o r . "

157 "The deposition

of u r a n i u m - b e a r i n g

closely related to s e d i m e n t a r y features.

m i n e r a l s in t h e k n o w n

conglomerates

appears

T h i s is p a r t i c u l a r l y t r u e in t h e W i t w a t e r s r a n d a n d

a t E l l i o t L a k e , w h e r e t h e p r e s e n c e of u r a n i u m in y o u n g , c r o s s - c u t t i n g c o n g l o m e r a t e

beds

c l e a r l y r e l a t e s t o s c o u r of o l d e r b e d s . " " W h e r e t h e u r a n i f e r o u s m i n e r a l is s t r u c t u r a l l y o r d e r e d t h o r i a n u r a n i n i t e , w e b e l i e v e i t t o b e of d e t r i t a l o r i g i n .

H o w e v e r , p i t c h b l e n d e o c c u r s in s o m e of t h e c o n g l o m e r a t e s a n d , in

our opinion, this must have been deposited from solution even though the uranium has not m o v e d f a r in d i s s o l v e d f o r m . material

normally

In t h e m a i n , s u c h p i t c h b l e n d e is a s s o c i a t e d w i t h c a r b o n a c e o u s

referred

to as thucholite.

The available evidence suggests that

this

m a t e r i a l is of o r g a n i c o r i g i n . " " T h e c o n g l o m e r a t e s d o n o t n o r m a l l y c o n t a i n m a g n e t i t e or i l m e n i t e . hand they always contain a b u n d a n t pyrite.

We a r e f o r c e d

to believe that

o r i g i n a t e s by s u l p h i d i z a t i o n of d e t r i t a l m a g n e t i t e a n d i l m e n i t e .

On t h e

other

most

pyrite

The sulphur

originated

f r o m v o l c a n o g e n i c p r o c e s s e s , a c t i v e a t t h e e a r l y s t a g e s of c r u s t a l e v o l u t i o n , w h i c h p e r t a i n t o t h e t i m e of d e p o s i t i o n of t h e c o n g l o m e r a t e s .

S o m e of t h e p y r i t e s o f o r m e d m a y h a v e

e n t e r e d t h e c o n g l o m e r a t e s in d e t r i t a l f o r m . " "Conglomerates

and

contained

minerals

ultimately

formed

by

collection,

c o n c e n t r a t i o n a n d d i s t r i b u t i o n of d e t r i t u s by m a j o r s t r e a m s y s t e m s . " " T h e f o r m a t i o n of a n e c o n o m i c d e p o s i t r e l a t e s b o t h t o m a t e r i a l a v a i l a b l e a t s o u r c e and

the

degree

concentrate

to

which

the

in t h e u l t i m a t e

sorting

and

conglomerate.

cleaning

process

produces

a

heavy

Known c o n g l o m e r a t e s differ

mineral

w i d e l y in t h i s

r e g a r d so t h a t u r a n i f e r o u s c o n g l o m e r a t e s m a y c o n t a i n a v a r i e t y of o t h e r v a l u a b l e m i n e r a l s . A t p r e s e n t , g o l d is t h e s i g n i f i c a n t e x a m p l e of t h i s . " "Age d e t e r m i n a t i o n s suggest t h a t t h e c o n g l o m e r a t e s form b e t w e e n a b o u t 2800 and 2200 m.y.

m.y.

A s n o t e d e a r l i e r , t h i s is p r i o r t o t h e d e v e l o p m e n t of t h e f i r s t r e d b e d s of t h e

sedimentary section.

P r o s p e c t i o n for such o r e s should u t i l i z e t h i s a s a w o r k i n g h y p o t h e s i s . "

"In t h e o p i n i o n of t h e w o r k i n g g r o u p , t h o s e e n c o u n t e r i n g c o n g l o m e r a t e s of t h i s k i n d should p r e p a r e

heavy

mineral concentrates

from

reasonable

rock volumes

to

facilitate

s t u d y a n d u n d e r s t a n d i n g of t h e o r i g i n of u r a n i u m - b e a r i n g m i n e r a l s a n d o t h e r h e a v y m i n e r a l s in t h e r o c k . there

E x c e p t f o r S o u t h A f r i c a , w h e r e e x t e n s i v e w o r k of t h i s k i n d h a s b e e n d o n e ,

is r e g r e t t a b l y l i t t l e d a t a a v a i l a b l e .

The Working Group r e c o m m e n d s

radiometric

l o g g i n g of a l l d r i l l h o l e s a s t h e R a n d - t y p e t h i n c o n g l o m e r a t e s c a n e a s i l y b e m i s s e d if c o r e r e c o v e r y is p o o r " . T h e w r i t e r w o u l d u r g e c a u t i o n w i t h r e s p e c t t o t h e c o n c l u s i o n s d r a w n a b o u t t h e a g e of the quartz-pebble conglomerates. t h o s e now known.

There may well be much younger representatives

r e g a r d s t h e a u r i f e r o u s t y p e s , a s t h e T a r k w a d e p o s i t s in G h a n a so c l e a r l y t e s t i f y . deposits

are,

than

Finally, t h e s t a t e m e n t a b o u t m a g n e t i t e and i l m e n i t e m a y not be valid as

however,

not

uraniferous

or

thoriferous

as

far

a s is n o w

known.

These The

158 s t a t e m e n t t h a t t h e c o n g l o m e r a t e s f o r m e d e a r l i e r t h a n t h e f i r s t l i m e s t o n e s is d e b a t e a b l e . Archean

rocks

on

which

the

productive

quartz-pebble

conglomerates

were

laid

down

c o n t a i n n o t a b l e a m o u n t s of l i m e s t o n e s in p l a c e s ( B o y l e , 1 9 7 6 b ) . M o r d v i n e t a l . ( 1 9 7 3 ) h a v e i n v e s t i g a t e d t h e s p e c i f i c c h a r a c t e r i s t i c s of

Precambrian

q u a r t z - p e b b l e c o n g l o m e r a t e s in e a s t e r n S i b e r i a a n d h a v e c o n c l u d e d t h e f o l l o w i n g , w h i c h is of i n t e r e s t in p r o s p e c t i n g : (1)

The

auriferous

(and

presumably

uraniferous

and

thoriferous)

conglomerates

are

c o n f i n e d t o t h e m a r g i n s of a n c i e n t s h i e l d s ; (2)

M o s t a r e r e s t r i c t e d t o P r e c a m b r i a n r o c k s f o r m e d in d e p r e s s i o n s t h a t u n d e r w e n t s l o w subsidence;

(3)

All a r e o n l y g e n t l y f o l d e d a n d m e t a m o r p h o s e d ;

(4)

T h e o r e m i n e r a l s a r e c o n f i n e d t o t h e c e m e n t of m o n o m i c t i c q u a r t z i t e s or o l i g o m i c t i c conglomerates with well packed average size pebbles;

(5) (6)

A u r i f e r o u s s e c t i o n s h a v e l a r g e i n t e r v a l s of s e d i m e n t a r y h i a t u s ; The stratification

in t h e a u r i f e r o u s c o n g l o m e r a t e s is b e t t e r d e v e l o p e d t h a n t h a t in

basal conglomerates, indicating their formation as

quartzose terrigenous rocks which

w e r e p r o d u c t s of d e e p c h e m i c a l w e a t h e r i n g ; (7)

Auriferous

conglomerates

occur

in r e g i o n s

marked

by p r e e x i s t i n g e p i g e n i t i c

gold

deposits; and (8)

M a n y of t h e a u r i f e r o u s c o n g l o m e r a t e s o c c u r in s t r a t a e n r i c h e d in i r o n . T h o s e p r o s p e c t i n g for u r a n i f e r o u s e u x i n i c s e d i m e n t s ( b l a c k s h a l e s , b i t u m i n o u s s h a l e s ,

and

black

schists),

sedimentary

phosphates, and

uraniferous

coal and

lignite should

be

g u i d e d p r i m a r i l y by g e o l o g i c a l p r i n c i p l e s a n d w h e r e p o s s i b l e by p a l e o g e o g r a p h i c a l c o n c e p t s . The

environment

of

deposition

of

euxinic

sediments

is

anoxygenic

(reducing)

c h a r a c t e r i z e d by r i c h o r g a n i c a c c u m u l a t i o n s ( s a p r o p e l ) ; t h e s i t e s a r e p a l u d a l , deltaic,

tidal flats, fiords with r e s t r i c t e d

circulation, barred basins, lagoons,

and

lacustrine, back-reef

lagoons, seas with r e s t r i c t e d circulation (e.g. Black Sea), poorly a e r a t e d epicontinental s e a s a n d o c e a n i c c o n t i n e n t a l p l a t f o r m s , a n d so o n . In t h e s e e n v i r o n m e n t s t h e u r a n i u m a n d m a n y c h a l c o p h i l e e l e m e n t s w e r e p r e c i p i t a t e d a n d b o u n d in t h e s a p r o p e l m a i n l y a s a r e s u l t

of

r e d u c t i o n r e a c t i o n s a i d e d in l a r g e p a r t by t h e p r e s e n c e of H S a r i s i n g f r o m t h e b a c t e r i a l 2

d e c o m p o s i t i o n of p r o t e i n a c e o u s s u b s t a n c e s a n d t h e r e d u c t i o n of s u l p h a t e .

Chelation

and

other o r g a n o m e t a l l i c binding processes also played a large p a r t . The

environment

of

marine, mainly the latter. precipitation products

of

phosphate

deposition

of

sedimentary

phosphates

and/or

accumulation

of

phosphatic

a n d c o n s i d e r a b l e a m o u n t s of c a r b o n a c e o u s m a t t e r

sedimentation

marked

may

be

lacustrine

or

The conditions are essentially reducing with abundant c h e m i c a l

by t h e a b s e n c e of

organic

structures

d u r i n g p e r i o d s of v e r y

s e d i m e n t s of a c l a s t i c

or

calcareous

or slow

nature.

159 Glauconitic

sediments

associates.

T h e s i t e s of

(some

uraniferous)

d e p o s i t i o n of

with phosphate

and CO2

cherty

phosphates

generally continental shelving b o t t o m s . charged

and

are

accumulations

lakes

in m i n o r

are

frequent

cases

but

more

T h e r e , cold m a r i n e w a t e r s rising from d e p t h and

u n d e r g o a r i s e in t e m p e r a t u r e

a n d a d e c r e a s e in

the

p a r t i a l p r e s s u r e of C O 2 r e s u l t i n g f i r s t in t h e p r e c i p i t a t i o n of c a r b o n a t e s e d i m e n t s a n d t h e n phosphates as t h e w a t e r s move over t h e shelves.

T h e p r e c i p i t a t i o n is e s s e n t i a l l y c h e m i c a l ,

b u t t h e r e m a i n s of l u x u r i a n t g r o w t h s of p h y t o - p l a n k t o n w h i c h c o n s u m e m u c h a l s o c o n t r i b u t e t o t h e t o t a l a c c u m u l a t i o n of p h o s p h a t i c m a t e r i a l .

phosphorus

These organisms

also

a p p e a r t o s e c r e t e , a d s o r b , or a b s o r b m u c h u r a n i u m w h i c h is u l t i m a t e l y i n c o r p o r a t e d in t h e phosphatic beds. C o a l a n d l i g n i t e a r e of a u t h o c h t h o n o u s or a l l o c h t h o n o u s o r i g i n ; t h e f i r s t d e n o t e s t h e a c c u m u l a t i o n , h u m i f i c a t i o n , a n d c o a l i f i c a t i o n of p l a n t r e m a i n s in situ

m a i n l y in m a r s h e s ,

s w a m p s and bogs; t h e second r e p r e s e n t s t h e a c c u m u l a t i o n , h u m i f i c a t i o n , and of p l a n t r e m a i n s r a f t e d i n t o s w a m p s , l a k e s , a n d d e l t a i c e n v i r o n m e n t s .

coalification

The paleobotanists

t e l l us t h a t t h e c l i m a t i c c o n d i t i o n s under w h i c h t h e c o a l and l i g n i t e flora p r o l i f e r a t e d

were

humid, t e m p e r a t e to sub-tropical, and r e l a t i v e l y uniform over a l m o s t t h e e n t i r e e a r t h . some

cases

humification present-day

uranium

may

have been

accumulated

in t h e

peat

(lignite) swamps

In

during

p r o c e s s e s a s is p r o v e n by a b n o r m a l c o n c e n t r a t i o n s of t h e e l e m e n t in s o m e bogs; more

generally,

however,

uranium

was

adsorbed

and

absorbed

from

g r o u n d w a t e r s a f t e r c o n s o l i d a t i o n of t h e e n c l o s i n g s e d i m e n t s . G e n e r a l i n d i c a t i o n s of u r a n i f e r o u s e u x i n i c s e d i m e n t s a n d c o a l a n d l i g n i t e i n c l u d e t h e favourable s e d i m e n t a r y e n v i r o n m e n t s noted above, a b u n d a n t limonite and jarositic staining, and

residual

and

transported

characterizes these rocks.

gossans

arising

In a d d i t i o n s u r f a c e

from

the

and ground

oxidation

of

pyrite

w a t e r s in r e g i o n s

which

containing

e u x i n i c s e d i m e n t s , c o a l , a n d l i g n i t e a r e g e n e r a l l y g r e a t l y e n r i c h e d in s u l p h a t e a n d m a y b e m o r e r a d i o a c t i v e (due principally to radon) than n o r m a l .

Marine phosphorites are generally

i n d i c a t e d by t h e p r e s e n c e of s i l i c e o u s n o d u l e s of p h o s p h a t e ( c o l l o p h a n e ) in o v e r l y i n g

soils

a n d in s t r e a m a n d r i v e r s e d i m e n t s a n d by h i g h c o n t e n t s of p h o s p h a t e in s u r f a c e a n d g r o u n d w a t e r s , w h i c h a r e c o m m o n l y a l s o m o r e r a d i o a c t i v e ( d u e t o t h e p r e s e n c e of r a d o n ) normal.

Other

t y p e s of p h o s p h o r i t e s

may

a l s o b e so i n d i c a t e d .

Present-day

than

swamps,

m a r s h e s , and bogs n e a r u r a n i f e r o u s euxinic s e d i m e n t s , lignites, and p h o s p h o r i t e s a r e

often

g r e a t l y e n r i c h e d in u r a n i u m a n d i t s r a d i o a c t i v e d a u g h t e r e l e m e n t s c o m p a r e d w i t h t h o s e in s e d i m e n t a r y t e r r a n e s d e v o i d of t h e s e u r a n i f e r o u s r o c k s .

Uraniferous calcretes,

g y p c r e t e s , a n d f e r r i c r e t e s c o m m o n l y m a r k t h e p r e s e n c e of u r a n i u m - b e a r i n g s e d i m e n t s and lignitic s t r a t a ; u r a n i f e r o u s other phosphatic sediments.

phoscretes

silcretes,

carbonaceous

may likewise mark phosporites

and

160 D i r e c t a n d s p e c i f i c i n d i c a t i o n s of u r a n i f e r o u s e u x i n i c s e d i m e n t s , c o a l a n d l i g n i t e , a n d phosphorites

are manifest

by a b n o r m a l

radioactivity.

In t h i s

respect

all

sedimentary

s e c t i o n s , d r i l l c o r e s , a n d d r i l l c u t t i n g s of s e d i m e n t a r y p i l e s s h o u l d b e c a r e f u l l y

monitored

r a d i o m e t r i c a l l y , f o l l o w e d by c o n f i r m a t o r y c h e m i c a l a n a l y s e s , by t h o s e i n t e r e s t e d in f i n d i n g c o n c e n t r a t i o n s of u r a n i u m in c a r b o n a c e o u s s e d i m e n t s , c o a l a n d l i g n i t e , a n d p h o s p h o r i t e s . The

paleogeographic

sites

in

which

the

presently

known

uraniferous

calcrete,

g y p c r e t e , silcrete, f e r r i c r e t e , and other c h e m i c a l p r e c i p i t a t e s have been c o n c e n t r a t e d mainly confined to well developed s t r e a m courses, gulches, arroyos, shallow

are

depressions,

a n d c l o s e d or c e n t r i p e t a l d r a i n a g e b a s i n s f i l l e d w i t h s e m i - c o n s o l i d a t e d g r a v e l s , s e d i m e n t a r y breccia,

sand,

and

other

weathered

debris,

l i m o n i t e , a n d o t h e r s a l t s ( F i g u r e 14).

cemented

by

carbonates,

gypsum,

silica,

M o s t of t h e s e s i t e s h a v e d e v e l o p e d in h o t a r i d a n d

d e s e r t t e r r a i n s w i t h a r e l a t i v e l y m a t u r e t o p o g r a p h y a n d l o w g r a d i e n t s ; s o m e lie on g e n t l y s l o p i n g a r i d p i e d m o n t p l a i n s b e l o w h i g h a n d r u g g e d m o u n t a i n r a n g e s ; a f e w o c c u r in h o t a r i d a n d d e s e r t r e g i o n s in b a s i n s a n d on t h e g e n t l e s l o p e s o r b o t t o m s of g r e a t v a l l e y s r i m m e d b y mountains.

T h o s e on t h e s l o p e s g e n e r a l l y o c c u r a t or n e a r t h e b a s e s of t a l u s a n d a l l u v i a l

fans (fan b a s e c a l c r e t e s ) .

Phoscretes are localized principally over w e a t h e r e d

phosphorites

o r o t h e r p h o s p h a t i c d e p o s i t s o r in t a l u s a n d s l o p e w a s h d o w n h i l l f r o m t h e s e d e p o s i t s . of t h e s i t e s of c o n c e n t r a t i o n a r e g e o l o g i c a l l y y o u n g , g e n e r a l l y Q u a t e r n a r y , m a i n l y Terrains which contain recent

impeded

depressions.

these deposits

drainage

manifest

are

by s a l t

usually c h a r a c t e r i z e d flats

and

salinas

Most

Recent.

by a r e a s of p a l e o

which

occupy

the

and

lowest

The p a l e o c l i m a t i c and p r e s e n t c l i m a t i c conditions under which t h e c a l c r e t e -

t y p e d e p o s i t s a r e c o n c e n t r a t e d a r e g e n e r a l l y t h o s e of h o t a r i d r e g i o n s - low a v e r a g e a n n u a l rainfall,

high

average

annual

temperature,

v e g e t a t i o n , a n d h e n c e low o r g a n i c a c t i v i t y .

low

humidity,

sparse

or

near

absence

M o s t d e p o s i t s a r e in or n o t f a r r e m o v e d

t e r r a n e s c h a r a c t e r i z e d by a b u n d a n t , c o m m o n l y h i g h e r t h a n n o r m a l , r a d i o a c t i v e

of

from

commonly

red stained p e r t h i t i c granitic bodies, some basic rocks, and/or tuffaceous s e d i m e n t s , rocks c o n s i d e r e d by m o s t i n v e s t i g a t o r s t o h a v e p r o v i d e d t h e c o n s t i t u e n t e l e m e n t s s u c h a s U a n d V in t h e d e p o s i t s .

S o m e d e p o s i t s o c c u r on or in t h e v i c i n i t y of p h o s p h o r i t e s ( p h o s c r e t e s ) a n d

uraniferous s e d i m e n t s , particularly black shales and schists. The

paleohydrologic

and

present

hydrologic

conditions

that

characterize

the

u r a n i f e r o u s c a l c r e t e , g y p c r e t e , f e r r i c r e t e , p h o s c r e t e , and o t h e r similar deposits a r e overall relatively

simple

but

in d e t a i l

usually

complex.

The

c a r n o t i t e and/or tyuyamunite, have been precipitated

uraniferous

from

minerals,

slowly moving

generally

groundwaters

c o u r s i n g a q u i f e r s w h i c h f o l l o w d o w n h i l l c h a n n e l s t h r o u g h e l u v i u m a n d t a l u s , old s t r e a m a n d river c o u r s e s , and t h e o t h e r confining depressions noted a b o v e .

The p r e s e n c e and passage

of g r o u n d w a t e r s in a n d t h r o u g h t h e a q u i f e r s is m a r k e d by t h e o c c u r r e n c e of

considerable

a m o u n t s of c a r b o n a t e s ( c a l c i t e , m a g n e s i t e ) a n d g y p s u m ; v a r i a b l e a m o u n t s of

secondary

s i l i c a , c l a y m i n e r a l s , l i m o n i t e , a n d o t h e r s a l t s ; a b n o r m a l r a d i o a c t i v i t y in t h e o l d e r d e p o s i t s a n d i n c r e a s e d a m o u n t s of u r a n i u m in t h e y o u n g d e p o s i t s . g r a d i e n t is or h a s b e e n l o w ; f r e q u e n t l y

In m o s t c a s e s t h e

hydrologic

t h e a q u i f e r s a r e f i l l e d or h a v e b e e n f i l l e d

with

161 nearly stagnant ground w a t e r . have

fluctuated

slowly,

T h e l e v e l s of t h e g r o u n d w a t e r in m a n y of t h e d e p o s i t s i t e s

rising

and

falling

with

the

rate

of f l o w

of

the

groundwater,

a b u n d a n c e of r a i n f a l l , a n d r a t e of e v a p o r a t i o n . G e n e r a l i n d i c a t i o n s of t h e c a l c r e t e - t y p e of u r a n i u m d e p o s i t a r e h o t a r i d t e r r a i n s w i t h sources

of

leachable

uranium

(granites,

p h o s p h o r i t e s ) ; u s u a l l y low f l a n k i n g

rhyolites,

migmatites,

t e r r a n e s w i t h a c o m p l e m e n t of b a s i c r o c k s ; d e p o s i t i o n a l stream

and river

favourable

channels and closed

chemical

saturation

conditions

carbonaceous

highlands c o m p r i s i n g mainly g r a n i t i c and

of

basins (former

precipitation

d u e t o high e v a p o r a t i o n ,

of

s i t e s such as old

or p r e s e n t uranium

high a d s o r p t i o n

debris-filled

s i t e s of a q u i f e r s ) ;

minerals

capacities

including

of c a l c r e t e

super-

activity,

A n u m b e r of c a l c r e t e s a n d s i m i l a r d e p o s i t s a r e

m a r k e d by t h e p r e s e n c e of c l a s t s of s m o k y ( r a d i o a c t i v e l y d a r k e n e d ) q u a r t z , r e d perthite,

biotite,

and

sphene,

an

and

constituents

(carbonates, gypsum, clay minerals, d i a t o m i t e , limonite, phosphates), bacterial and closely controlled redox p a r a m e t e r s .

shales,

migmatitic

indication

of

the

provenance

of

a number

stained of

their

c o n s t i t u e n t s from r a d i o a c t i v e g r a n i t e s , p e g m a t i t e s , gneisses, m i g m a t i t e s , and allied rocks. Mann

(1974,

1976), w h o

has studied

uraniferous

calcretes extensively

in

Western

A u s t r a l i a , c o m m e n t s on t h e v a r i o u s p h y s i c a l a n d c h e m i c a l c h a r a c t e r i s t i c s of t h e s e d e p o s i t s a n d g i v e s s o m e g e n e r a l f e a t u r e s t h a t a r e of i n t e r e s t in p r o s p e c t i n g .

He notes t h a t :

(1)

Not all c a l c r e t e s a r e m i n e r a l i z e d ;

(2)

In a m i n e r a l i z e d c a l c r e t e , t h e m i n e r a l i z e d z o n e m a y b e s u r r o u n d e d by l a r g e a r e a s of barren calcrete;

(3)

T h e z o n e s of m i n e r a l i z a t i o n a r e u s u a l l y c l o s e t o t h e s u r f a c e of t h e c a l c r e t e ;

(4)

T h e z o n e s of m i n e r a l i z a t i o n a r e h o r i z o n t a l , o f t e n d i s c o n t i n u o u s s h e e t s u p t o s e v e r a l m e t r e s in d e p t h , a p p a r e n t l y r e l a t e d t o p r e s e n t or p a s t w a t e r t a b l e s ;

(5)

The

calcrete,

or

calcrete-clay,

containing

the

carnotite

is p o r o u s ,

or

at

least

r e a s o n a b l y p e r m e a b l e t o t h e p a s s a g e of g r o u n d w a t e r a n d g a s e s ; a n d (6)

The c a r n o t i t e

occurs as a thin fine-grained

yellow

d e p o s i t on t h e s u r f a c e of

the

c a l c r e t e or c a l c r e t e - c l a y p a r t i c l e s . T h e g e n e s i s of c a r n o t i t e a t Y e e l i r r i e in W e s t e r n A u s t r a l i a is m o r e c o m p l i c a t e d first

thought

according

to

Mann

and

Deutscher

(1978).

They

envisage

the

than

optimum

c o n d i t i o n s f o r c a r n o t i t e f o r m a t i o n a t a n y p a r t i c u l a r l o c a t i o n t o b e a s p a t i a l c o i n c i d e n c e of t h e following: (1)

C o n s t r i c t i o n of a l a r g e s l o w - m o v i n g v o l u m e of w a t e r

from

an e x t e n s i v e

granitoid

c a t c h m e n t a r e a so t h a t l a r g e a m o u n t s of u r a n i u m a r e a v a i l a b l e ; (2)

P a r t i a l e v a p o r a t i o n of t h e s e w a t e r s in o r d e r t o p r o m o t e h i g h p o t a s s i u m a n d u r a n y l ion concentrations

in

solution,

c a r b o n a t e c o m p l e x e s ; and

the

latter

by

a

decomplexing

of

the

soluble

uranyl

162 (3)

U p w e l l i n g of t h e s e w a t e r s a n d / o r m i x i n g of t h e s e w a t e r s w i t h d e e p e r l y i n g g r o u n d w a t e r s which have had a c c e s s

to a

s o u r c e of

vanadium

(whether

the

source

be

l o c a l i z e d or w i d e s p r e a d ) . M a n n a n d D e u t s c h e r n o t e t h a t f a c t o r s (1) a n d (2) a r e r e g i o n a l c o n t r o l s w h i c h i n d i c a t e t h e s u i t a b i l i t y ( g e o l o g i c a l l y , c l i m a t o l o g i c a l l y , a n d h y d r o l o g i c a l l y ) of t h e n o r t h e r n

half

of

t h e Y i l g a r n B l o c k of W e s t e r n A u s t r a l i a for t h e f o r m a t i o n of t h e Y e e l i r r i e t y p e of c a r n o t i t e mineralization.

F a c t o r (3) is m u c h m o r e d e p e n d e n t on l o c a l i z e d g e o l o g i c a l a n d h y d r o l o g i c a l

c i r c u m s t a n c e s , a n d a s s u c h , p r o b a b l y p l a y s t h e m o s t i m p o r t a n t r o l e in d e t e r m i n i n g e x a c t l o c a t i o n , t o n n a g e , a n d g r a d e of a n y i n d i v i d u a l c a r n o t t i e d e p o s i t w i t h i n t h e

the

calcreted

drainage. P r e m o l i ( 1 9 7 6 ) s t r e s s e s t h e p r i m e i m p o r t a n c e of g e o m o r p h o l o g i c a l ,

climatological,

a n d g e o c h e m i c a l f a c t o r s a n d t h e s u b o r d i n a t e r o l e of g e o l o g y in t h e f o r m a t i o n of and similar uraniferous deposits.

calcrete

H o t s e m i - a r i d c l i m a t i c c o n d i t i o n s in r e g i o n s of e x t e n s i v e

p e n e p l a n a t i o n with i n t e r n a l d r a i n a g e p a t t e r n s having g e n e r a l l y low g r a d i e n t s (2.5 are

most

favourable.

Among the geochemical

factors

m/km)

t h e r o l e of r e d o x p o t e n t i a l s

in

c o n t r o l l i n g t h e p r e c i p i t a t i o n of m i n e r a l s s u c h a s c a r n o t i t e a r e of i m p o r t a n c e . P r e m o l i ( 1 9 7 6 ) a l s o r e c o r d s a n u m b e r of p r o c e d u r e s t h a t a r e e f f e c t i v e in p r o s p e c t i n g f o r c a l c r e t e s a n d s i m i l a r d e p o s i t s in h o t a r i d r e g i o n s .

These are enumerated

here

only;

d e t a i l s s h o u l d b e s o u g h t in t h e o r i g i n a l p a p e r . 1.

ERTS

imagery

and

air photo i n t e r p r e t a t i o n :

S p e c i a l m a p s of c a l c r e t e

drainages

d i v i d e d i n t o g r o u p s of d i f f e r e n t p r i o r i t y s h o u l d b e p r e p a r e d . 2.

Soil m a p s : T h e s e p r o v e m o s t u s e f u l w h e r e a v a i l a b l e .

3.

Airborne reconnaissance

4.

Carborne radiometry

5.

Detailed ground reconnaissance

6.

S y s t e m a t i c drilling and sampling.

7.

H y d r o c h e m i s t r y and b i o g e o c h e m i s t r y : as sampled

in d r i l l e d

Measurements

of

holes,

H y d r o c h e m i s t r y of t h e g r o u n d w a t e r s , m a i n l y

has proved

U, V, p H , E h , a n d

elements are generally remarkably absent. water

a r e , per se,

almost

meaningless;

r e g r e s s i o n a n a l y s e s of u r a n i u m practise. results.

against

t o b e o n e of

conductivity

the

most

useful

should be r o u t i n e .

methods. Pathfinder

A b s o l u t e v a l u e s of u r a n i u m p r e s e n t in t h e in

order

to

define

anomalous

t o t a l s a l i n i t y (or c o n d u c t i v i t y ) is

situations, standard

Biogeochemistry has given s o m e occasionally good, but generally e r r a t i c ,

163 Carlisle

(1978)

and

Carlisle

e t al. (1978)

have

studied

the

varied

c a l c r e t e s i n c l u d i n g t h o s e of p e d o g e n i c , n o n p e d o g e n i c , r e c o n s t i t u t e d ,

occurrence

and d e t r i t a l

of

origin.

F o r t h e u r a n i f e r o u s v a r i e t i e s of g y p c r e t e , d o l o c r e t e , a n d c a l c r e t e t h e y r e c o r d t h e f o l l o w i n g f a v o u r a b i l i t y c r i t e r i a for t h e i r o c c u r r e n c e . (1)

Adequate source terrane, deeply weathered.

(2)

A n o m a l o u s g r o u n d w a t e r u r a n i u m and v a n a d i u m c o n t e n t s .

(3)

L a r g e c a t c h m e n t a r e a w i t h low d r a i n a g e g r a d i e n t s .

(4)

Very limited runoff.

(5)

Absence

of

soil c a r b o n a t e

deposition

or

other

uranium

fixing

processes

in

the

catchment area. (6)

E v i d e n c e of e v a p o r a t i v e c o n c e n t r a t i o n

of u r a n i u m , v a n a d i u m , a n d p o t a s s i u m

down

drainage. (7)

E v i d e n c e of c o n s t r i c t e d , s h a l l o w i n g , or u p w e l l i n g g r o u n d w a t e r flow w i t h i n t h e v a l l e y calcrete area.

(8)

E v i d e n c e of r e c o n c e n t r a t i o n of c a r n o t i t e .

(9)

E v i d e n c e of c h e m i c a l a n d p h y s i c a l s t a b i l i z a t i o n of c a r n o t i t e .

(10)

M o d e r a t e t o low r e l i e f a n d t e c t o n i c s t a b i l i t y of t h e a r e a . U r a n i f e r o u s d e p o s i t s in w e t - s i t e e n v i r o n m e n t s r e p r e s e n t r e l a t i v e l y r e c e n t

chemical

p h e n o m e n a a n d s h o u l d b e s o u g h t in c l o s e d b a s i n s o r d e p r e s s i o n s s u c h a s s a l t p a n s , p l a y a s , a n d s a l i n e l a k e s , or in s i t e s w h e r e c h e m i c a l t r a p s c o n t a i n i n g p e a t , materials retard

and c o n c e n t r a t e

uranium

(and t h o r i u m )

muck

available from

or

phosphatic

the leaching

of

m i g m a t i t e s , g r a n i t i c rocks, acid volcanics, c a r b o n a c e o u s shales and schists, phosphorites, b e d r o c k U - T h d e p o s i t s a n d so o n .

C o n c e r n i n g playas L e a c h e t a l . (1980) r e m a r k t h a t most

of t h e s e b o d i e s h a v e l i t t l e p o t e n t i a l f o r u r a n i u m d e p o s i t s b u t t h a t u n d e r c e r t a i n some playa basins a r e more favourable than o t h e r s . (1) (2)

conditions

These conditions include:

P r e s e n c e of p r e c u r s o r c a r b o n a t e - r i c h s a l i n e l a k e s . P r e s e n c e of u r a n i u m - r i c h s o u r c e r o c k s n e a r b y , i . e . g r a n i t e s a n d c e r t a i n v o l c a n i c s a n d volcaniclastics, especially those interbedded with lacustrine sediments.

(3)

Restricted groundwater discharge into t h e playa.

(4)

Localized precipitation

m e c h a n i s m s o r p h a s e s in t h e p l a y a , i . e . o r g a n i c s ,

s e d i m e n t s , or opaline silica. (5)

Large drainage basin and a d e q u a t e precipitation.

(6)

S t a b i l i t y of a h y d r o l o g i c a l l y c l o s e d b a s i n for a l o n g p e r i o d of t i m e .

reducing

164 Those seeking non-lithified thorium-uranium (mainly m o n a z i t e , thorite, uranothorite, t h o r o u r a n i n i t e , b r a n n e r i t e , t h o r i a n i t e , e t c . ) p l a c e r s should consult t h e s t a n d a r d

references

o n p l a c e r s , p a r t i c u l a r l y t h e w o r k s by G a r d n e r a n d J o h n s o n ( 1 9 3 4 - 3 5 ) , J e n k i n s ( 1 9 6 4 ) , a n d W e l l s ( 1 9 7 3 ) . In a p p l y i n g t h e b a s i c p r i n c i p l e s g o v e r n i n g t h e f o r m a t i o n of p l a c e r s i t m u s t b e c o n s t a n t l y k e p t in m i n d t h a t t h e c l i m a t i c a n d g e o m o r p h o l o g i c a l c o n d i t i o n s w h e n t h e p l a c e r s w e r e f o r m e d in a n y p a r t i c u l a r a r e a w e r e in m o s t c a s e s m u c h d i f f e r e n t obtain

today.

One

must

think

and

construct

models

in

terms

of

than those

that

paleoclimatic

and

p a l e o g e o m o r p h i c c o n d i t i o n s a n d r e c o n s t r u c t t h e h i s t o r y of e r o s i o n a n d d e p o s i t i o n of t h e T h U placers.

C o m m o n l y i t will b e n o t e d t h a t old s t r e a m a n d o t h e r p l a c e r d e p o s i t s b e a r n o

p a r t i c u l a r r e l a t i o n s h i p t o t h e p r e s e n t d a y t o p o g r a p h i c or d r a i n a g e p a t t e r n s . The

occurrence

of

Th-U

placers

in

an

area

is

predicated

on

four

general

c o n d i t i o n s - a s o u r c e for t h e T h - U m i n e r a l s , a m e c h a n i s m for t h e r e l e a s e of t h e s e m i n e r a l s f r o m t h e i r s o u r c e , a c o n c e n t r a t i o n m e c h a n i s m , a n d f a v o u r a b l e c o n d i t i o n s for p r e s e r v a t i o n of t h e d e p o s i t s .

E a c h of t h e s e c o n d i t i o n s c a n b e r e v i e w e d o n l y b r i e f l y h e r e .

T h e s o u r c e of t h e primary

Th-U

minerals

m a y b e o n e or m o r e of t h e f o l l o w i n g :

Th-U deposits; gneisses, schists, m i g m a t i t e s , granites

and

various

c o n t a i n i n g t r a c e t o m i n o r a m o u n t s of T h - U m i n e r a l s ; p o o r l y c o n s o l i d a t e d

various

other

rocks

conglomerates,

s a n d s t o n e s , a n d o t h e r c l a s t i c r o c k s w i t h s m a l l a m o u n t s of r e s i s t a t e T h - U m i n e r a l s ; a n d old placers

derived

from

the

first

two

sources.

Reworking

of

intermediate

and

slightly

e n r i c h e d s o u r c e s in t h e s y s t e m - p r i m a r y s o u r c e - a l l u v i a l p l a c e r - s h o u l d a l w a y s b e k e p t in m i n d w h e n s e a r c h i n g for p l a c e r s . Weathering

is

the

principal

method

of

release

of

thorium-

and

uranium-bearing

minerals from their deposits and host rocks, although glacial action may be effective glacial terrains.

in

T h e e r o s i v e a c t i o n of w i n d is a n i m p o r t a n t a g e n t of l i b e r a t i o n of T h - U

m i n e r a l s in s o m e d e s e r t r e g i o n s . C o n c e n t r a t i o n of m o n a z i t e , t h o r i t e , u r a n o t h o r i t e , a n d o t h e r T h - U m i n e r a l s in p l a c e r s is a c c o m p l i s h e d a l m o s t e n t i r e l y by p h y s i c a l m e a n s .

The principal c o n c e n t r a t i n g agents a r e

g r a v i t y (eluvial p l a c e r s ) , w a t e r (gulch, s t r e a m , r i v e r , flood plain, d e l t a , and b e a c h p l a c e r s ) , wind (eolian placers), and g l a c i e r s (glacial p l a c e r s ) . The

favourable

conditions

for

preservation

of

Th-U

mineral

placers

are:

a b a n d o n m e n t of s t r e a m a n d r i v e r c o u r s e s c o n t a i n i n g p a y s t r e a k s of t h e m i n e r a l s a s a r e s u l t of c h a n g e of c o u r s e o r r a p i d d o w n w a r d c u t t i n g by s t r e a m s a n d r i v e r s ; r e g i o n a l u p l i f t w h i c h mainly affects alluvial deposits especially marine placers raising t h e m above the

active

s u r f ; a n d b u r i a l w h i c h m a y b e by l a n d s l i d e m a t e r i a l s , g l a c i a l m a t e r i a l s , f l o o d p l a i n g r a v e l s , alluvial fans, d e s e r t o u t w a s h d e t r i t u s , or v o l c a n i c flows and t u f f s . T h r o u g h o u t t h e w o r l d a r e a s c o n t a i n i n g n o n - l i t h i f i e d p l a c e r s a r e g e n e r a l l y m a r k e d by a " k i n d l y " t e r r a i n w h i c h is c h a r a c t e r i z e d b y a s u b d u e d t o p o g r a p h y m a r k e d b y b r o a d , terraced

intrenched

valleys and rounded, d e e p l y - w e a t h e r e d

hills c o m m o n l y w i t h

often nearly

165 accordant

summit

levels.

The

paleogradients

approximately 8 m e t r e s per k i l o m e t r e .

have

usually

Few extensive placers

been

were

relatively

formed

in

low,

terrains

m a r k e d by s h a r p a l p i n e f e a t u r e s a n d h i g h g r a d i e n t V - s h a p e d v a l l e y s ; s i m i l a r i l y e x c e s s i v e l y flat t e r r a i n s far from placers.

m o u n t a i n s y s t e m s and their foothills

have yielded few

productive

T h e r o c k s in d i s t r i c t s c o n t a i n i n g T h - U m i n e r a l p l a c e r s a r e i n v a r i a b l y

phosed,

and

intrusive.

there

has

usually

been

considerable

igneous

activity

both

metamor­

extrusive

and

D e e p s e c u l a r w e a t h e r i n g is a f e a t u r e of a l l p l a c e r a r e a s .

I n d i c a t i o n s of n o n - l i t h i f i e d t h o r i u m - u r a n i u m

m i n e r a l p l a c e r s in a n a r e a i n c l u d e

the

f a v o u r a b l e c o n d i t i o n s of f o r m a t i o n of t h e s e d e p o s i t s n o t e d a b o v e , a b n o r m a l q u a n t i t i t e s of m a g n e t i t e , i l m e n i t e , zircon, m o n a z i t e , and o t h e r

placer

minerals

in t h e h e a v y

mineral

s u i t e s of s t r e a m a n d r i v e r s e d i m e n t s , b e a c h s a n d s , e s k e r s , d u n e s a n d s , a n d so o n . p l a c e r s a r e g e n e r a l l y o n l y r e v e a l e d a n d p r o v e n by o v e r b u r d e n d r i l l i n g or d e e p methods

combined

with

heavy

mineral

panning

and

analysis

techniques.

Buried

trenching Down-hole

r a d i o a c t i v e l o g g i n g is a l s o a d v a n t a g e o u s , a n d t h o s e b u r i e d p l a c e r s r i c h in m a g n e t i t e m a y b e r e v e a l e d by m a g n e t o m e t e r s u r v e y s . Fossil p l a c e r s (excluding uraniferous-thoriferous q u a r t z - p e b b l e c o n g l o m e r a t e s ) occur predominantly

in

sedimentary

terranes

s e q u e n c e s of b o t h t h e s e r o c k s .

generally

in

conglomerates

or

n o k n o w n o c c u r r e n c e s of f o s s i l T h - U p l a c e r s in s h a l e s a n d l i m e s t o n e s .

Conglomerates

of m a n y o r i g i n s i n c l u d i n g m a r i n e , l a c u s t r i n e , e s t u a r i n e , d e l t a i c , f l u v i a t i l e and glacial.

quartzites

or

O c c u r r e n c e s in g r e y w a c k e s a r e n o t c o m m o n , a n d t h e r e a r e

Sandstones have similar origins.

are

(fanglomerate),

B o t h of t h e s e c l a s t i c r o c k s g e n e r a l l y

mark

p e r i o d s of d e e p s e c u l a r w e a t h e r i n g w h i c h is f a v o u r a b l e for t h e p r o d u c t i o n of m o n a z i t e a n d other Th-U mineral placers.

M e t a m o r p h i c e q u i v a l e n t s of f o s s i l p l a c e r s , r e p r e s e n t e d

r o c k s such as m o n a z i t i c q u a r t z i t e s , gneisses, and schists and z i r c o n i f e r o u s gneisses

by and

s c h i s t s , o c c u r in a n u m b e r of m e t a m o r p h i c t e r r a n e s . It is n o t p o s s i b l e a priori noted

above

sedimentary

to d e t e r m i n e w h e r e fossil p l a c e r s a r e likely to o c c u r .

belts

should

be

sought

out

sandstones, s e d i m e n t a r y b r e c c i a s and their m e t a m o r p h i c

and

the

various

As

conglomerates,

equivalents examined

carefully

f o r a b n o r m a l r a d i o a c t i v i t y a n d by p a n n i n g a n d c h e m i c a l a n a l y s e s f o r t h e p r e s e n c e of T h - U minerals.

Z o n e s in s a n d s t o n e s a n d c o n g l o m e r a t e s c o n t a i n i n g m a g n e t i t e or i l m e n i t e s h o u l d

receive particular a t t e n t i o n during surveys.

L i k e w i s e , a l l c o n g l o m e r a t e s a n d s a n d s t o n e s , in

f a c t a n y s e d i m e n t a r y r o c k or t u f f c a r r y i n g p y r i t e , p y r r h o t i t e , a n d o t h e r s u l p h i d e s , s h o u l d b e c a r e f u l l y t e s t e d by r a d i o a c t i v e m e t h o d s a n d c h e m i c a l a n a l y s e s f o r t h o r i u m , u r a n i u m ,

rare

e a r t h s , base m e t a l s , gold, and p l a t i n u m . T h e l o c a t i o n of f o s s i l t h o r i u m - u r a n i u m m i n e r a l p l a c e r c o n g l o m e r a t e s , s a n d s t o n e s , a n d t h e i r m e t a m o r p h i c e q u i v a l e n t s in s e d i m e n t a r y s e q u e n c e s is d i f f i c u l t t o p r e d i c t .

Many lie

near

districts.

t h e b a s e of t h e s e q u e n c e s , b u t o t h e r s l i e w e l l a b o v e

I n d i c a t i o n s of f o s s i l present

in r e g i o n s

Th-U

mineral placer

containing

these

types

conglomerates

and

the

b a s e in s o m e

sandstones

of d e p o s i t s , b e i n g o f t e n

are

manifest

commonly by

excess

166 q u a n t i t i e s of m a g n e t i t e , i l m e n i t e , z i r c o n , m o n a z i t e , a n d o t h e r p l a c e r m i n e r a l s in s o i l s a n d stream

sediments.

Those

containing

c h a r a c t e r i z e d by a n a b u n d a n c e of S 0

4

abundant

pyrite

and

pyrrhotite

are

often

in g r o u n d a n d s u r f a c e w a t e r d u e t o t h e w e a t h e r i n g

of t h e i r o n s u l p h i d e s ; a b u n d a n t i r o n s t a i n i n g a n d g o s s a n s , b o t h r e s i d u a l a n d t r a n s p o r t e d , a r e a l s o c o m m o n f e a t u r e s of t h e s e t y p e s of p l a c e r s a n d a l s o of s o m e of t h o s e r i c h in m a g n e t i t e . F o s s i l p l a c e r s e n r i c h e d in t h e T h - U m i n e r a l s h a v e a m a r k e d r a d i o a c t i v e r e s p o n s e , a n d t h o s e containing m a g n e t i t e yield c h a r a c t e r i s t i c m a g n e t o m e t e r

profiles.

Radiation Surveys Both isotopes

thorium

2 3 2

Th,

2 3 i +

and U,

2 3

uranium 5

U, a n d

are 2 3 8

naturally

U

decay

d i s i n t e g r a t i o n t o yield t h e s t a b l e isotopes F i g u r e s 1, 3 a n d 4 ) .

2 0

6

occurring

in a s e r i e s Pb,

2 0

7

radioactive of

P b , and

elements

2 0

8

P b ( T a b l e s 1, 3 a n d 4 a n d

On t h e o t h e r h a n d t h e i n t e n s i t y

of r a d i a t i o n of m a n y of t h e d a u g h t e r e l e m e n t s of t h e t w o p a r e n t s , e s p e c i a l l y 2

6

of

Ra,

Ra,

2 2 8

the

2

2

0

isotopes

methods

beta

T h e r a d i o a c t i v i t y of b o t h t h e i n i t i a l t h o r i u m a n d u r a n i u m i s o t o p e s is

f e e b l e a n d n o t r e a d i l y d e t e c t e d by n o r m a l i n s t r u m e n t a t i o n .

2

whose

s t e p s by a l p h a and

Rn,

2 2 2

Rn,

2 1 2

Bi,

(particularly

in r a d i a t i o n

surveys.

2 1 4

2 1 l +

Bi,

Bi

2 1 0

and

E a c h of

P b , and

2 0

8

T1)

these

2 0

8

T 1 , is m a r k e d .

that

are

isotopes

Th,

0

These

determined

and t h e

2 3

by

various

2 2 4

are

Ra,

some

instrumental

other

daughter

i s o t o p e s of t h e p a r e n t s , t h o r i u m a n d u r a n i u m , h a v e s p e c i f i c e n e r g y s p e c t r u m s t h a t c a n b e resolved with suitable i n s t r u m e n t a t i o n such as multi-channel g a m m a s p e c t r o m e t e r s .

High

r e s o l u t i o n r a d i a t i o n s p e c t r o m e t r y h a s , t h e r e f o r e , g r e a t p r o m i s e in t h e p r e c i s e a n a l y s i s of uranium, 1 4 7

Sm

thorium, and associated

in

studies

dealing

with

radioactive the

elements

isotopic

abundance

d i s e q u i l b r i u m of r a d i o a c t i v e s p e c i e s , a n d so o n . using

such

instrumentation

in s t u d i e s

such as of

4 0

K,

the

8 7

Rb,

1 3 8

radioactive

La,

In f a c t , o n l y a b e g i n n i n g h a s b e e n

dealing

with

the

fundamental

and

elements,

geochemistry

made and

a p p l i e d g e o c h e m i s t r y of u r a n i u m , t h o r i u m , t h e i r d a u g h t e r e l e m e n t s , a n d t h e i r a s s o c i a t e d or indicator

radioactive

elements.

Today,

there

is no d e a r t h of s o p h i s t i c a t e d

radioactive

i n s t r u m e n t a t i o n e i t h e r fully d e v e l o p e d or in t h e p r o c e s s of d e v e l o p m e n t ; w h a t is l a c k i n g is a p r e c i s e k n o w l e d g e of t h e g e o c h e m i s t r y of t h e v a r i o u s r a d i o a c t i v e e l e m e n t s a n d daughter

products.

As n o t e d in C h a p t e r 2, t h e g e o c h e m i s t r y of e a c h of

the

their

daughter

e l e m e n t s of t h o r i u m a n d u r a n i u m d i f f e r s m a r k e d l y f r o m t h a t of t h e i r p a r e n t s , a n d t h i s f a c t m u s t b e t a k e n i n t o a c c o u n t in a l l i n t e r p r e t a t i v e p r o c e d u r e s . The particularly

development uranium

of

and

instrumentation thorium

and

their

for

the

daughter

detection

of

elements,

the and

radio-elements, studies

of

their

d i s t r i b u t i o n , b e g a n in 1896 s h o r t l y a f t e r t h e d i s c o v e r y of t h e p h e n o m e n o n of r a d i o a c t i v i t y in P a r i s by A . H . B e c q u e r e l ( 1 8 9 6 ) , P r o f e s s o r of N a t u r a l P h i l o s o p h y in t h e M u s e e Naturelle.

d'Histoire

M a r i e S k l o d o w s k a C u r i e , t h e f i r s t r a d i o c h e m i s t , b e g a n a n e x t e n d e d s t u d y of

n a t u r a l r a d i o a c t i v i t y r e s u l t i n g in t h e d i s c o v e r y of p o l o n i u m a n d r a d i u m in p i t c h b l e n d e J a c h y m o v , C z e c h o s l o v a k i a (Curie and C u r i e , 1898;

C u r i e e t al., 1898).

from

This w a s followed

167 by e x t e n s i v e s t u d i e s of t h e n a t u r a l r a d i o a c t i v i t y of m i n e r a l s , r o c k s , n a t u r a l w a t e r s , a n d t h e a t m o s p h e r e by m a n y c h e m i s t s , g e o l o g i s t s , a n d p h y s i c i s t s . work were the photographic spinthariscope,

the

last

a

T h e i n s t r u m e n t s u s e d in t h e e a r l y

plate, electroscope, electrometer, most

important

nuclear

research

ionization chamber, instrument

and

developed

by

W. C r o o k e s in 1 9 0 3 a n d e m p l o y e d e x t e n s i v e l y by R u t h e r f o r d a n d o t h e r s ( 1 9 5 1 ) . In 1908 t h e g e i g e r t u b e w a s d e v e l o p e d by H. muller

tube; the

latter

G e i g e r a n d m o d i f i e d in 1 9 2 8 by W. M u l l e r a s t h e g e i g e r -

was adapted

to

field

u s e by G . M . S h r u m

U n i v e r s i t y of B r i t i s h C o l u m b i a in 1932 ( S h r u m a n d S m i t h , 1 9 3 4 ) . surveys

using

a geiger

pitchblende-bearing Canada.

counter

veins

at

were

Labine

carried Point,

out

by

Great

Ridland

Bear

and

R. Smith

The first

( 1 9 4 5 ) in

Lake,

at

the

radioactivity

1939 o v e r

Northwest

the

Territories,

P r i o r t o t h i s ( a b o u t 1930) H.V. E l l s w o r t h of t h e G e o l o g i c a l S u r v e y of C a n a d a u s e d

a n i m p r o v e d v e r s i o n of t h e e l e c t r o s c o p e in t h e f i e l d s u r v e y i n g of u r a n i u m - a n d

thorium-

b e a r i n g p e g m a t i t e s a n d o t h e r r a d i o a c t i v e b o d i e s ( E l l s w o r t h , 1932) a n d l a t e r d e v e l o p e d

a

p o r t a b l e g e i g e r - m u l l e r c o u n t e r for f i e l d u s e in 1 9 3 3 . E l e c t r o n i c s c i n t i l l a t i o n c o u n t e r s b a s e d on t h e p r i n c i p a l of t h e s p i n t h a r i s c o p e , b u t e m p l o y i n g s e n s i t i v e p h o t o m u l t i p l i e r t u b e s , w e r e d e v e l o p e d in 1944 by S . C . C u r r a n a n d W . R . B a k e r ( C u r r a n ,

1953).

This was followed

in

s u c c e e d i n g y e a r s by m u c h r e s e a r c h on l a r g e v o l u m e c r y s t a l s for c o u n t i n g b e t a - p a r t i c l e s a n d gamma

r a d i a t i o n , c u l m i n a t i n g in t h e d e v e l o p m e n t

by R . H o f s t a d t e r

in

1948 of

sodium-

i o d i d e - t h a l l i u m - a c t i v a t e d c r y s t a l s for t h e d e t e c t i o n of g a m m a r a d i a t i o n ( H o f s t a d t e r , 1 9 4 8 ) . T h e s e c r y s t a l s h a v e r e m a i n e d for

many years the most important detector

g a m m a - r a y scintillation spectrometry.

medium

Scintillation c o u n t e r s w e r e first a d a p t e d to

u s e in 1 9 4 9 by G . M . B r o w n e l l ( 1 9 5 0 ) of t h e U n i v e r s i t y of M a n i t o b a , s u r v e y s b e i n g o u t in t h e U r a n i u m C i t y a r e a of n o r t h e r n S a s k a t c h e w a n .

for field

carried

T o d a y , m a n y of t h e s c i n t i l l a t i o n

s p e c t r o m e t e r s h a v e r e a c h e d a h i g h d e g r e e of s o p h i s t i c a t i o n e m p l o y i n g a s m a n y a s

1000

c h a n n e l s f o r t o t a l g a m m a c o u n t i n g a n d s p e c i f i c q u a n t i t a t i v e a n a l y s e s of e l e m e n t s s u c h a s potassium 0 ° K ) , uranium (as

2 1 t f

B i ) , and t h o r i u m ( a s

2 0 8

Tl).

Early radiometric surveys were

m a i n l y s h o e b o r n e ; t o d a y m a n y of t h e s u r v e y s a r e c a r b o r n e , a i r b o r n e , o r s h i p b o r n e . N o a t t e m p t is m a d e h e r e t o d i s c u s s n u c l e a r i n s t r u m e n t a t i o n . should consult thorium

the

technical

exploration

report

(I.A.E.A.,

1974a)

Alekseev, et

al. (1959), Linden and

dealing with

nuclear

laboratories,

and

instruments

in s t a t i o n a r y

on

recommended and

the

Akerblom

and

comprehensive

for

reviews

currently The

r a d i o m e t r i c s u r v e y s a r e b a s e d on t h e d e t e c t i o n

in u s e in t h e

instruments

most

and quantification

of

reader

uranium

and

papers

(1976), Morse (1977), and Bristow

methods

laboratories.

The interested

instrumentation

field,

in

commonly gamma

and by

(1979) mobile used

in

radiation;

a l p h a a n d b e t a c o u n t e r s a r e a l s o a v a i l a b l e , b u t t h e s e h a v e s p e c i a l i z e d a p p l i c a t i o n s in t h e field, e.g.

a l p h a c o u n t i n g of r a d o n .

F o r r a d i o m e t r i c r e p o r t i n g m e t h o d s a n d c a l i b r a t i o n in

u r a n i u m e x p l o r a t i o n s e e t h e r e p o r t by t h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y ( I 9 7 6 d ) . F i v e o t h e r m e t h o d s u t i l i z i n g t h e f e a t u r e s of r a d i a t i o n s h o u l d b e m e n t i o n e d - r a d i o activation

analysis,

fluorescence.

autoradiography,

radiation

track

analysis, radioactive

tracers,

and

R a d i o a c t i v a t i o n a n a l y s i s u t i l i z i n g c e r t a i n n u c l e a r p r o p e r t i e s of t h e i s o t o p e s

168 of t h e e l e m e n t s s o u g h t in t h e s a m p l e , w a s d e v e l o p e d m a i n l y a f t e r t h e S e c o n d W o r l d W a r . Most r a d i o a c t i v a t i o n m e t h o d s utilize t h e r m a l n e u t r o n s as t h e bombarding p a r t i c l e s (neutron activation analysis).

S u c h m e t h o d s h a v e a n e x t r e m e l y h i g h s e n s i t i v i t y in t h e p a r t s

b i l l i o n r a n g e f o r e l e m e n t s s u c h a s g o l d , p l a t i n o i d s , u r a n i u m , a n d so o n . used

extensively

in

basic

geochemical

research

for

many

years

and

per

They have been are

now

finding

i n c r e a s i n g u s e in g e o c h e m i c a l p r o s p e c t i n g , b e i n g p a r t i c u l a r l y a p p l i c a b l e f o r t h e a n a l y s i s of p r a c t i c a l l y a l l g e o l o g i c a l a n d b i o l o g i c a l m a t e r i a l s u t i l i z e d in g e o c h e m i c a l s u r v e y s .

In t h i s

w o r k t h e d e l a y e d n e u t r o n m e t h o d of a n a l y s i s h a s b e e n f o u n d t o b e p a r t i c u l a r l y u s e f u l uranium determinations.

N e u t r o n a c t i v a t i o n m e t h o d s a r e n o w u s e d e x t e n s i v e l y in d o w n -

h o l e p r o b e s in v a r i o u s e x p l o r a t i o n delayed-neutron-activation ( S t e i n m a n , 1975). berylometer

for

drilling p r o g r a m s .

analysis

technique

O n e of

these systems utilizes

employing

2 5 2

Cf

as

a

activator

R a d i o i s o t o p e s h a v e a l s o b e e n h a r n e s s e d for u s e in f i e l d i n s t r u m e n t s ; t h e

which

is b a s e d

on

the

nuclear

reaction, B e , 9

(y, n ) B e , 8

and

uses a

Sb

12H

g a m m a s o u r c e , is u s e f u l in p r o s p e c t i n g f o r d e p o s i t s of b e r y l l i u m m i n e r a l s ( B r o w n e l l , 1 9 5 9 ) . Other sources such as

2 1 0

Po-Be

h a v e b e e n u s e d in c o p p e r s u r v e y s a n d so o n .

Summary

p a p e r s on n e u t r o n a c t i v a t i o n m e t h o d s in g e o c h e m i c a l e x p l o r a t i o n d i r e c t e d t o w a r d a n d g o l d e x p l o r a t i o n i n c l u d e t h o s e by B o w i e e t a l . Plant e t al.

(1976), and a d e t a i l e d

account

of

uranium

(1973b), P l a n t and C o l e m a n (1973), and

nuclear

techniques

in g e o c h e m i s t r y

and

g e o p h y s i c s is p r o v i d e d b y t h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y ( I . A . E . A . , 1 9 7 6 a ) . A u t o r a d i o g r a p h y of p o l i s h e d a n d t h i n s e c t i o n s of r o c k s a n d o r e s h a s b e e n u s e d

for

d e c a d e s t o e s t i m a t e t h e l o c a t i o n a n d q u a n t i t y of t h o r i u m a n d u r a n i u m in s a m p l e s . method

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

emulsion

on f i l m s

The

or p l a t e s

in

c o n t a c t w i t h t h e s a m p l e ; on d e v e l o p m e n t of t h e e m u l s i o n a n e s t i m a t e of t h i s a c t i v i t y c a n b e m a d e f r o m t h e d a r k e n i n g of t h e film or p l a t e , a n d h e n c e t h e c o n t e n t of t h o r i u m uranium can be determined (assuming secular equilibrium). differentiate

between the alpha

activity

due to thorium

and

In g e n e r a l i t is d i f f i c u l t

and t h a t

due to uranium.

a d d i t i o n , l o n g e x p o s u r e t i m e s (10 d a y s ) a r e u s u a l l y r e q u i r e d t o o b t a i n s a t i s f a c t o r y e s p e c i a l l y if t h e s a m p l e s c o n t a i n o n l y l o w c o n t e n t s of t h o r i u m a n d u r a n i u m .

to In

results,

D e t a i l s of t h e

m e t h o d a r e g i v e n in B o w i e ( 1 9 5 1 , 1 9 5 4 ) a n d R a g l a n d ( 1 9 6 4 ) . T h e t h i r d m e t h o d , r a d i a t i o n t r a c k a n a l y s i s , is b a s e d on t h e d a m a g e ( t r a c k s ) p r o d u c e d in s u b s t a n c e s c a u s e d by t h e p a s s a g e of c h a r g e d

particles resulting from induced

p r o c e s s e s or n a t u r a l n u c l e a r d i s i n t e g r a t i o n ( F l e i s c h e r e t a l . , 1 9 7 5 ) .

l o n g h i s t o r y in n u c l e a r r e s e a r c h a n d is p a r t i c u l a r l y u s e f u l in e s t i m a t i n g t h e of u r a n i u m a n d u r a n i u m a n d t h o r i u m u n d e r a p p r o p r i a t e c o n d i t i o n s .

n e u t r o n s s i m i l a r l y e f f e c t t h e s a m e p r o c e s s in s e c t i o n s of r o c k s ) w i t h s u i t a b l e t r a c k neutrons to induce

2 3

5

U t o fission.

detectors

T h and

are

2 3

8

U.

exposed

concentration

O n e t e c h n i q u e is b a s e d

on t h e f a c t s t h a t t h e r m a l m o d e r a t e d o r s l o w e d n e u t r o n s i n d u c e fission in 2 3 2

fission

The m e t h o d has had a

2 3

5

U , and

fast

S a m p l e s ( t h i n or p o l i s h e d first

t o a s o u r c e of t h e r m a l

The d e t e c t o r s a r e then etched to reveal the damaged

a r e a s ( t r a c k s ) , t h e t r a c k s a r e c o u n t e d , a n d t h e c o n c e n t r a t i o n of u r a n i u m is c a l c u l a t e d . thorium

contents

are

sought

the

same

procedure

is e m p l o y e d

but

with

fast

If

neutron

169 i r r a d i a t i o n ; t h e t h o r i u m c o n c e n t r a t i o n c a n b e c a l c u l a t e d by d i f f e r e n c e w h e n t h e T h / U r a t i o is h i g h e n o u g h .

T h e p r i n c i p a l t r a c k d e t e c t o r s u t i l i z e d i n c l u d e h i g h q u a l i t y m u s c o v i t e (for U

c o n t e n t s >10 p p m ) a n d p o l y c a r b o n a t e p l a s t i c ( L e x a n o r M a k r o f o l ) (for U c o n t e n t s < 1 0 p p m ) . S u m m a r i e s of t h e u s e of t h e m e t h o d in u r a n i u m Berzina et (1980).

and thorium

exploration

al. (1967), Wollenberg (1973), Bowie e t al. (1973b), and D u a n e and

A n o t h e r n o v e l i n - f i e l d m e t h o d f o r t h e d e t e r m i n a t i o n of r a d o n by

dielectric

are given

film

under

the

trade

name

"Track

C o r p o r a t i o n of W a l n u t C r e e k , C a l i f o r n i a .

Etch"

has

been

by

Williams

alpha-sensitive

developed

by

Terradex

T h e s e a n d o t h e r t e c h n i q u e s of r a d o n d e t e c t i o n in

g e o c h e m i c a l p r o s p e c t i n g for t h e r a d i o e l e m e n t s , i n c l u d i n g e l e c t r o n i c a l p h a c o u n t e r s ,

are

d i s c u s s e d in s o m e d e t a i l in C h a p t e r 8 ( A t m o c h e m i c a l S u r v e y s ) . Radiotracers

have been

u s e d for

many

s y s t e m s a n d in d y n a m i c g e o l o g i c a l s y s t e m s .

years

to t r a c e

v a r i o u s p r o c e s s e s in l i v i n g

T h e m e t h o d s a r e of i n t e r e s t t o

geochemical

p r o s p e c t o r s since hydrologic r e g i m e s c a n be t r a c e d under c e r t a i n conditions using t r i t i u m tracers

and o t h e r

suitable

isotopes.

Radiotracers

a r e a l s o of c o n s i d e r a b l e

interest

in

f u n d a m e n t a l s t u d i e s i n v o l v i n g d i f f u s i o n p r o c e s s e s in s o i l s a n d r o c k s a n d m e t a l a d s o r p t i o n o n s t r e a m s e d i m e n t s , p e a t , a n d so o n . S u c h s t u d i e s a r e u s e f u l in i n t e r p r e t i n g p a t t e r n s in m e t a l d i s t r i b u t i o n , i n c l u d i n g t h o r i u m a n d u r a n i u m in n a t u r a l e n v i r o n m e n t s . tracer

w o r k in h y d r o l o g y a r e a v a i l a b l e in t w o p u b l i c a t i o n s by t h e

Energy Agency (IAEA, 1967, 1976c).

R e v i e w s of

International

radio­ Atomic

H a n d b o o k s on r a d i o t r a c e r t e c h n i q u e s i n c l u d e t h o s e by

G a r d n e r e t al. (1975) and F e r r o n s k i y (1977). (2537A,

M a n y u r a n i u m s a l t s f l u o r e s c e u n d e r u l t r a v i o l e t l i g h t of s u i t a b l e w a v e l e n g t h s 3660A).

This p r o v i d e s a s u i t a b l e

otherwise escape detection

m e t h o d of d e t e c t i o n

by g e i g e r - m u l l e r

youth and often fine subdivision.

counter

of u r a n i u m

or s c i n t i l l o m e t e r

various

uranium

phosphates,

carbonates,

y e l l o w , g r e e n , or g r e e n i s h - y e l l o w in c o l o u r colour.

The

uranium

carbonates

might

b e c a u s e of

their

Uranium minerals can be classified into those t h a t

strongly fluorescent, weakly fluorescent, and n o n f l u o r e s c e n t . the

minerals that

arsenates,

and e m i t

fluoresce

with

a

In t h e f i r s t c a t e g o r y

sulphates

and

others

a strong yellow-green more

intense

green

are

belong

that

are

fluorescent

colour.

The

nonfluorescent uranium minerals include u r a n i n i t e , p i t c h b l e n d e , t h e silicates and

various

u r a n i u m m i c a s , m o s t of w h i c h a r e b l a c k , b r o w n , or d r a b g r e y in c o l o u r .

certain

Since

s u b s t a n c e s ( e . g . b i t u m e n , p e t r o l e u m ) a n d m i n e r a l s i n t e r f e r e w i t h , i n h i b i t , or m i m i c u r a n i u m fluorescence,

c o r r o b o r a t i n g analyses should always be c a r r i e d o u t to definitely

t h e p r e s e n c e or a b s e n c e of t h e e l e m e n t .

D i s c u s s i o n of

t h e u s e of

ultraviolet

establish light

m i n e r a l o g y a n d p r o s p e c t i n g for u r a n i u m a r e g i v e n in t h e p a p e r s a n d b o o k s by D e (1946),

Przibram

(1946),

Home

(1951),

Bultemann

(1954),

Walenta

in

Ment

(1959),

and

counters

have

b e e n e m p l o y e d e x t e n s i v e l y s i n c e 1950 ( L a n g , 1953) a n d h a v e a s s i s t e d in l o c a t i n g a t

least

Gleason (1972). Ground

radiation

surveys

employing

geiger-muller

and scintillation

80 p e r c e n t of t h e u r a n i u m a n d t h o r i u m d e p o s i t s n o w b e i n g m i n e d o r d e v e l o p e d .

Airborne

170 r a d i a t i o n s u r v e y s e m p l o y i n g s c i n t i l l a t i o n s p e c t r o m e t e r s c a m e i n t o g e n e r a l u s e in t h e 1960's a n d h a v e b e e n i n s t r u m e n t a l in l o c a t i n g a n u m b e r of n e w u r a n i u m - t h o r i u m b e l t s a n d a f e w deposits.

B o t h t y p e s of r a d i a t i o n s u r v e y s a r e s o w e l l k n o w n t h a t n o p o i n t w o u l d b e s e r v e d

in t h i s b o o k surveying

by

by r e p e a t i n g these

the

methods

details.

should

Those

consult

Sinitsyn (1965), B e n n e t t (1971), Cook e t al. (1971), Bowie e t a l .

interested

the

in t h e

various

p a p e r s by B a r a n o v

techniques

(1956), Savosin

of and

(1971), D a r n l e y and G r a s t y (1971), Kogan e t al.

(1972), Miller and Symons (1973), I n t e r n a t i o n a l A t o m i c Energy Agency

(1973, 1979a), D a r n l e y (1973, 1975), S h e r r i n g t o n (1977), Glen and Nelson (1979),

Czubek

(1979), Killeen (1979), G r a s t y (1979), E t t e n s o h n e t a l . (1979), P a r s l o w and Stolz (1979), and C o n a w a y and Killeen (1980).

H e r e , o n l y b r i e f o b s e r v a t i o n s on t h e a p p l i c a t i o n , a d v a n t a g e s ,

a n d l i m i t a t i o n s of t h e m e t h o d s a r e r e c o r d e d . R a d i a t i o n s u r v e y s c a n b e c a r r i e d o u t w i t h g r o s s ( t o t a l ) c o u n t i n s t r u m e n t s or discriminating spectrometers.

In t h e f i r s t c a s e t h e i n s t r u m e n t s a r e g e i g e r - m u l l e r

with

counters

o r s c i n t i l l a t i o n c o u n t e r s t h a t r e c o r d t h e s u m t o t a l of r a d i o a c t i v i t y a r i s i n g f r o m a l l s o u r c e s . These a r e usually small rugged lightweight relatively inexpensive i n s t r u m e n t s t h a t can be h a n d - c a r r i e d o r e a s i l y m o u n t e d in a s m a l l f i x e d w i n g a i r c r a f t o r h e l i c o p t e r , in a c a r , b o a t , or on a h o r s e , c a m e l , or d o n k e y .

Suitably waterproofed

probes coupled to the

various

c o u n t e r s ( i n c l u d i n g s p e c t r o m e t e r s ) c a n b e t r a i l e d a l o n g t h e b o t t o m of r i v e r s , l a k e s , s e a s , a n d o c e a n s or u s e d in b o r e h o l e s .

T h e p r i n c i p a l l i m i t a t i o n of t o t a l c o u n t i n s t r u m e n t s is t h a t

t h e y d o n o t d i s c r i m i n a t e b e t w e e n t h e v a r i o u s s p e c t r a a r i s i n g f r o m t h e r a d i o a c t i v i t i e s of potassium, thorium, uranium, and their daughter e l e m e n t s .

Fitted with appropriate

filters

or w i n d o w s , h o w e v e r , t h e t o t a l c o u n t s c i n t i l l o m e t e r s c a n p r o v i d e r e a s o n a b l e d i s c r i m i n a t i o n of r a d i o a c t i v e s p e c i e s .

This c a n n o t be done with geiger c o u n t e r s which a r e also r e l a t i v e l y

insensitive compared with scintillometers.

Accessory recorders and r e m o t e alarm systems

t o a l e r t t h e o p e r a t o r t o a n o m a l i e s in t h e r a d i a t i o n c o u n t c a n b e f i t t e d t o s o m e t y p e s of scintillometers,

thus giving g r e a t e r

versatility

to the

instruments,

b u t b e c a u s e of

g r e a t e r w e i g h t t h e y a r e g e n e r a l l y o n l y e m p l o y e d in c a r s , b o a t s , a n d s h i p s . spectrometers

are

electronically

sophisticated

instruments

capable

b e t w e e n t h e s p e c t r a of t h e v a r i o u s r a d i o a c t i v e e l e m e n t s , p a r t i c u l a r l y and U ( a s

2 1 1 +

Bi).

require

spectrometers

l f 0

K , Th ( a s

continuous

are

cooling

by

liquid

available - multichannel

nitrogen.

Three

spectrometers,

types

window

8

of

gamma-ray and

T h e s e c o n d t y p e is t h a t g e n e r a l l y u s e d in m o s t r a d i o a c t i v e s u r v e y spectrometers

in h e l i c o p t e r s or s l o w - f l y i n g f i x e d w i n g a i r c r a f t ; t h e y c a n , h o w e v e r ,

with

m o d i f i c a t i o n b e m o u n t e d in c a r s or s h i p s .

Their a d v a n t a g e s a r e t h a t t h e i n s t r u m e n t s

highly

spectral

sensitive,

converted

Tl),

spectrometers,

w o r k , e s p e c i a l l y a i r b o r n e s u r v e y s . B e c a u s e of t h e i r w e i g h t m o s t g a m m a - r a y mounted

2 0

T h e e x t r e m e l y s e n s i t i v e r e s o l u t i o n s p e c t r o m e t e r s u t i l i z i n g solid s t a t e

threshold spectrometers.

are

Gamma-ray discriminating

M o s t of t h e s e i n s t r u m e n t s a r e c o s t l y , c o m p l e x , r e l a t i v e l y d e l i c a t e , a n d of

considerable weight. detectors

of

the

discriminating,

elemental

abundances

and

the

or a b u n d a n c e

data

obtained

either

ratios are relatively

directly

unambiguous.

or

are as The

d i s a d v a n t a g e s a r e t h e c o s t of t h e i n s t r u m e n t a t i o n a n d its m a i n t e n a n c e a n d t h e c o s t of t h e

171 o p e r a t i o n of t h e t r a n s p o r t i n g v e h i c l e s , u s u a l l y a i r c r a f t .

I n t e r p r e t a t i o n of t h e s p e c t r a l d a t a

is d i f f i c u l t , r e q u i r i n g c o n s i d e r a b l e k n o w l e d g e of t h e g e o c h e m i s t r y of p o t a s s i u m , uranium, and

their daughter

elements, a subject

discussed

more

fully

thorium,

in t h e

following

Chapters. Historically, ground radiation surveys w e r e shoe borne and utilized portable counters. employ

T o d a y , b e c a u s e of t h e l o w s e n s i t i v i t y of g e i g e r c o u n t e r s , m o s t g r o u n d

portable

scintillation

total count

and

discriminating

ratemeters

geiger surveys

carried

by

p r o s p e c t o r or m o u n t e d in c a r s , b o a t s , s h i p s , a n d on h o r s e s , c a m e l s , or d o n k e y s .

the

Ground

r a d i a t i o n s u r v e y s m a y b e of a r e c o n n a i s s a n c e o r d e t a i l e d n a t u r e d e p e n d i n g on t h e o b j e c t of the

survey.

Reconnaissance

surveys

are

usually

employed

to

determine

the

general

r a d i o a c t i v e m i n e r a l p o t e n t i a l of a n a r e a a n d w h e r e p o s s i b l e t o o u t l i n e r a d i o a c t i v e m i n e r a l b e l t s o r z o n e s of m i n e r a l i z a t i o n . (commonly

helicopter

T h e r e c o n n a i s s a n c e s u r v e y s a r e p e r f o r m e d by p r o s p e c t o r s

supported)

traversing

stream

networks,

ridges and

spurs,

b o t t o m s a n d r i m s , d r y g u l c h e s , a r r o y o n e t w o r k s , or s o m e o t h e r t o p o g r a p h i c or

valley

geological

f e a t u r e ; by p r o s p e c t o r s in f o u r - w h e e l v e h i c l e s t r a v e r s i n g r o a d a n d t r a c k n e t w o r k s ; o r by p r o s p e c t o r s on b o a t s a n d s h i p s .

T h e s p a c i n g of t h e g r i d s in r e c o n n a i s s a n c e s u r v e y s

d e p e n d e n t o n t h e a v a i l a b i l i t y of s t r e a m n e t w o r k s , r o a d s , t r a c k s , a n d so o n .

are

In g e n e r a l t h e

s p a c i n g s a r e l a r g e (1 t o 5 k m if s p a c e d l i n e s a r e e m p l o y e d ) a n d t h e c o v e r a g e is u s u a l l y o n l y sufficient

t o i n d i c a t e b e l t s or z o n e s t h a t should r e c e i v e d e t a i l e d e x a m i n a t i o n .

Detailed

g r o u n d r a d i a t i o n s u r v e y s a r e u s u a l l y c a r r i e d o u t by p r o s p e c t o r s on f o o t w o r k i n g on a c l o s e g r i d w i t h l i n e s s p a c e d a t 30 m e t r e s or a t m o s t 100 m e t r e s . run to c h e c k

radioactive

anomalies indicated

Specific ground surveys may be

by a i r b o r n e

surveys

and

along

or

across

definite geological f e a t u r e s such as faults, shear zones, mylonitized zones, lineaments, and f a v o u r a b l e f o r m a t i o n s s u s p e c t e d of h a r b o u r i n g r a d i o a c t i v e m i n e r a l c o n c e n t r a t i o n s ; in t h e s a m e c a t e g o r y b e l o n g e x a m i n a t i o n s of old t r e n c h e s , m i n e d u m p s , m i n e w o r k i n g s , a n d d r i l l c o r e and c u t t i n g s .

In a l l c a s e s a c c u r a t e p l a n s a n d s e c t i o n s w i t h a r e c o r d of t h e r a d i o a c t i v e

r e a d i n g s , geology, and c h e m i s t r y should be m a i n t a i n e d .

In t h e f i e l d t h e p r o s p e c t o r s or

s a m p l e r s should e x a m i n e anything t h a t e x c i t e s their curiosity; t h e y should not adhere

to established

grids and should be p r e p a r e d

to examine

any unusual

slavishly geological

f e a t u r e , s e c o n d a r y s t a i n i n g , g o s s a n s , g o p h e r m o u n d s , a n t h i l l s , a n d so on t h a t m a y

come

into t h e i r fields of vision b e t w e e n t h e grid lines. T h e a d v a n t a g e s of g r o u n d r a d i a t i o n s u r v e y s h a r d l y n e e d c o m m e n t .

T h e u n i t c o s t of

i n s t r u m e n t s is in g e n e r a l l o w a n d t h e c o s t of s u r v e y s i s , l i k e w i s e , l o w f o r t h e a m o u n t of data obtained.

T h e p r o s p e c t o r s a r e on t h e g r o u n d and h e n c e c a n e x a m i n e t h e geology and

c o l l e c t s a m p l e s of s t r e a m s e d i m e n t s , s o i l s , r o c k s , v e g e t a t i o n , a n d so on f o r a n a l y s e s w h i l e simultaneously recording the radioactivity. a v a i l a b l e , a n d a p a t t e r n is c o m m o n l y investigations. mountainous

In a d d i t i o n t h e r a d i a t i o n d a t a a r e i m m e d i a t e l y

established

to guide the prospector

in

additional

T e r r a i n is r a r e l y a d i f f i c u l t y e x c e p t p e r h a p s in d e n s e j u n g l e or v e r y h i g h

regions.

O n e of

the disadvantages

of

all ground (and

airborne)

s u r v e y w o r k is t h a t t h e p e n e t r a t i o n of t h e m e t h o d s is e x t r e m e l y l i m i t e d .

radiation

Thus, alpha and

172 b e t a r a d i a t i o n is s c r e e n e d by t h i n f i l m s of p r a c t i c a l l y a n y m a t e r i a l a n d g a m m a r a y s by 25 c m

of

soil

and

about

the

same

depth

of

water.

Uranium-

and

thorium-bearing

s t r u c t u r e s , b e d s , a n d o r e b o d i e s , t h e r e f o r e , h a v e p r a c t i c a l l y n o r a d i o a c t i v e e x p r e s s i o n in d e e p l y w e a t h e r e d or g l a c i a t e d t e r r a i n s .

Finally, most ground radiometric surveys, except

p e r h a p s c a r - b o r n e s u r v e y s in r e g i o n s w i t h a g o o d n e t w o r k of r o a d s or t r a c k s , a r e n o t a l o n e j u s t i f i e d for l a r g e r e g i o n s b e c a u s e of t h e p r o h i b i t i v e c o s t a n d t i m e r e q u i r e d .

Combined

w i t h o t h e r t y p e s of g e o c h e m i c a l r e c o n n a i s s a n c e s u r v e y s , h o w e v e r , r a d i o m e t r i c s u r v e y s a r e justified and indeed i m p e r a t i v e . Prospectors variations

utilizing

ground

in t h e b a c k g r o u n d

overburden

and

elevations).

bedrock

and

radiometric

c o u n t of the

methods

should

regions, a feature

elevation

(cosmic

that

radiation

be constantly

aware

of

varies with the type is m o r e

intense

O t h e r p e r t u r b i n g f a c t o r s i n c l u d e t h o s e r e l a t e d t o t h e g e o m e t r y of

at

of

high

outcrops,

t r e n c h e s , m i n e w o r k i n g s , c o r e s a m p l e s , c u t t i n g s , e t c . , a n d t h e m i g r a t i o n c h a r a c t e r i s t i c s of t h e v a r i o u s r a d i o a c t i v e i s o t o p e s . C o n c e r n i n g t h e l a s t i t is i m p e r a t i v e t o c o n f i r m

anomalies

by m e a n s of c h e m i c a l a n a l y s e s for u r a n i u m a n d / o r t h o r i u m . Airborne

radiation

surveys

have

enjoyed

considerable

favour

since

about

1960.

Initially t h e a i r c r a f t ( h e l i c o p t e r s and slow flying a i r c r a f t ) w e r e f i t t e d w i t h s e n s i t i v e g e i g e r c o u n t e r s and t o t a l c o u n t s c i n t i l l o m e t e r s . employing

as

many

as

1000

precisely determined windows U/Th, U/K,

Th/K,

in a d d i t i o n

channels, 1 + 0

Today, sophisticated g a m m a - r a y are

K , Th ( a s

2 0 8

programmed Tl), U (as

2 1 l +

to

measure

spectrometers,

and

display

from

B i ) , t o t a l c o u n t , and t h e ratios

to t h e cosmic radiation and radon background.

Precise

h e i g h t s ( t e r r a i n c l e a r a n c e ) a r e u s u a l l y c o n t i n u o u s l y m o n i t o r e d by a c o u p l e d r a d a r a l t i m e t e r , a n d p r e c i s e g r o u n d l o c a t i o n s a r e s y n c h r o n i z e d w i t h c o u n t r a t e s by p h o t o g r a p h i c s t r i p f i l m c a m e r a , T V / v i d e o t a p e r e c o r d e r , or r a d a r s c a n n e r .

Certain systems are combined

o t h e r airborne exploration m e t h o d s such as a e r o m a g n e t i c and e l e c t r o m a g n e t i c

with

techniques;

t h u s a s i n g l e a i r c r a f t m a y c a r r y a s p e c t r o m e t e r , m a g n e t o m e t e r , EM a p p a r a t u s , a n d o t h e r airborne equipment as desired to perform multisensor surveys. are

usually

employed

in a i r b o r n e

reconnaissance

radiation

L i n e s p a c i n g of 2 t o 5 k m surveys;

for

more

detailed

s u r v e y s l i n e s p a c i n g of 0.5 t o 1 k m a r e u t i l i z e d . T h e a d v a n t a g e of a i r b o r n e r a d i o m e t r i c s u r v e y s is m a i n l y t h e e a s e w i t h w h i c h l a r g e regions with poor ground a c c e s s and trafficability generally

n o i n t r u s i o n on p r i v a t e

can be covered.

lands and t h e e n v i r o n m e n t

In a d d i t i o n t h e r e

is u n d i s t u r b e d .

a d v a n t a g e is t h a t s e v e r a l a i r b o r n e t e c h n i q u e s s u c h a s m a g n e t o m e t e r a n d surveys (multisensor radiometric

fixed

wing

aircraft

are

most

effective

is

further

electromagnetic

s u r v e y s ) c a n b e c a r r i e d o u t a t t h e s a m e t i m e if d e s i r e d .

surveys utilizing

A

Airborne

in r e l a t i v e l y

flat

t e r r a i n s ; in high r e l i e f a n d m o u n t a i n o u s t e r r a i n s h e l i c o p t e r s c a n b e u t i l i z e d w i t h m o d e r a t e s u c c e s s , a n d in r e g i o n s m a r k e d b y s t e e p c a n y o n w a l l s , d e e p g u l c h e s , a r r o y o s , a n d w a d i s " r i m flying" can

be successfully

employed

a s h a s b e e n d o n e in m a n y p a r t s of t h e

P l a t e a u a n d o t h e r d e e p l y d i s s e c t e d d e s e r t r e g i o n s of t h e w o r l d .

Colorado

173 Airborne major

multi-channel

radioelements

4 0

K,

spectrometers

Th(as

2 0 8

Tl),

are

and

capable

U(as

2 1 l f

Bi),

of

distinguishing

and

the

three

between

ratios

e s p e c i a l l y t h e U / T h r a t i o , a r e f r e q u e n t l y o f t e n m o r e i n d i c a t i v e of d e p o s i t s t h a n m e a s u r e m e n t s of t h e s i n g l e e l e m e n t s .

the

obtained, intensity

G e o c h e m i c a l d a t a are thus provided which

under

o p t i m u m c o n d i t i o n s c a n b e u s e d t o e v a l u a t e t h e t y p e s a n d d i s t r i b u t i o n of r o c k s c o v e r e d b y t h e s u r v e y , t h e p r e s e n c e of p o t a s h - e n r i c h e d z o n e s of a l t e r a t i o n , soil t y p e s , a n d so o n . d a t a a r e a l s o a p p l i c a b l e in m a n y c a s e s t o t h e s e a r c h f o r b a s e m e t a l a n d p r e c i o u s

The metal

d e p o s i t s w h i c h in c e r t a i n c a s e s a r e m a r k e d b y h i g h e r t h a n n o r m a l u r a n i u m c o n t e n t s a n d / o r e x t e n s i v e z o n e s of p o t a s h m e t a s o m a t i s m a n d a l t e r a t i o n . Airborne radiometric surveys are relatively expensive compared with ground surveys, and t h e r e a r e c e r t a i n

limitations dictated

by r u g g e d

and

mountainous

topography

and

c l i m a t i c c o n d i t i o n s ; s o m e of t h e s e c a n , h o w e v e r , b e o v e r c o m e by u t i l i z i n g h e l i c o p t e r s a n d working during o p t i m u m

weather

conditions.

A s in a l l r a d i a t i o n s u r v e y s t h e

results

of

a i r b o r n e r a d i o m e t r i c work r e f l e c t only t h e n e a r s u r f a c e r a d i o a c t i v e r e g i m e ; c o n s e q u e n t l y a i r b o r n e r a d i o m e t r i c s u r v e y s a r e s e v e r e l y r e s t r i c t e d in t h e i r a p p l i c a t i o n o v e r d r i f t , w a t e r , and s n o w - c o v e r e d

terrains, the radiation effects

from

orebodies and

z o n e s b e i n g g e n e r a l l y a t t e n u a t e d or e n t i r e l y s c r e e n e d u n d e r s u c h c o n d i t i o n s .

muskeg,

mineralized Considerable

c a r e must be t a k e n to e n s u r e p r e c i s e h e i g h t ( t e r r a i n c l e a r a n c e ) d a t a , and to apply

the

c o r r e c t b a c k g r o u n d c o r r e c t i o n s a m o n g w h i c h a r e t h e e v a l u a t i o n of t h e r e s p o n s e f r o m

the

2 1 4

B i in t h e a t m o s p h e r e .

Since

2 1 l +

B i a r i s e s f r o m t h e d i s i n t e g r a t i o n of r a d o n (

g a s , a n d s i n c e t h i s g a s is p a r t i c u l a r l y f i c k l e in i t s b e h a v i o u r in t h e a t m o s p h e r e , on

a

host

of

both

atmospheric

and

ground

c o m p e n s a t i n g for t h e v a g a r i e s of a t m o s p h e r i c systems

in

use

atmospheric

have

radon

radiation values. active

and

background

for

which

granitic

variables,

are

the

problem

of

subtracted noise from

both

the

cosmic

automatically

that

obscure

those

due

Some

radiation

from

the

of

and

ground

widely distributed, weakly

to concentrations

a

adequately

bodies, shales, lignites, coal, and o t h e r rocks m a y

anomalies

Rn),

depending

B i is a c r i t i c a l a n d d i f f i c u l t o n e .

determining

Finally, high background

migmatites,

perturbations

instrumentation

2 1 4

2 2 2

radio­ produce

uranium

and

t h o r i u m in b e d s o r v e i n s . Airborne

radiometric

surveys

are

a

reconnaissance

geochemical

tool

and

best

e m p l o y e d a s s u c h , p a r t i c u l a r l y t o o u t l i n e r a d i o a c t i v e b e l t s or z o n e s in w h i c h u r a n i u m thorium deposits may occur.

O n l y in e x c e p t i o n a l c a s e s c a n s u c h s u r v e y s b e e x p e c t e d

i n d i c a t e o r e b o d i e s , s u c h a s w h e r e t h e s e o u t c r o p in a n y t e r r a n e , or w h e r e t h e

and to

orebodies

o c c u r in e x t e n s i v e s a n d s t o n e t e r r a n e s , h a v e a m o d e r a t e l y l a r g e e x p o s u r e , a n d a r e l a t i v e l y high

radioactive

intensity

measured

against

a low r a d i a t i o n

background.

By

contrast,

u r a n i f e r o u s p e g m a t i t e s , m i g m a t i t e s , and g r a n i t i c b o d i e s , p i t c h b l e n d e veins, s t o c k w o r k s , and disseminations, and t h e various t h o r i u m - b e a r i n g

pegmatites,

veins,

and

lodes

o c c u r in m e t a m o r p h i c a n d g r a n i t i z e d t e r r a n e s w i t h a v a r i a b l e a n d u s u a l l y h i g h

generally radiation

174 background,

a

feature

that

commonly

overwhelms

and

masks

anomalies

due

to

point

s o u r c e s s u c h a s t h e d e p o s i t s m e n t i o n e d , e s p e c i a l l y w h e r e t h e y a r e c o v e r e d by o v e r b u r d e n or water as they invariably are.