Gold Shear z

8/12/2019 Gold Shear z

1/11

Mineral. Deposita 25 [Suppl] $4 2-$ 52 (1990)

MINER LIUM

DEPOSIT

9 Springer-Verlag 1990

G o l d o r e s r e l a t e d t o s h e a r z o n e s , W e s t S a n t a C o m b a - F e r v e n z a A r e a

G a l ic i a , N W S p a i n ): A m i n e r a lo g i c a l s t u d y

R Castroviejo

E.T.S. lngenieros de Minas (Universidad Polit6cnica de Madrid), c/ Rio s Rosas, 21 - 28003-Madrid (Spain)

Received: M ay 1989/Accepted: Septem ber 6, 1990

Abstract R e c e n t r e s e a r c h h a s d i s c o v e r e d h i g h - g r a d e A u

o r es in N N E - S S W t r e n d i n g s h ea r z o ne s in m e t a m o r p h i c

p r o t e r o z o i c a n d p a l a e o zo i c t e rr a n e s, s o m e 4 0 k m N W o f

S a n t i a g o d e C o m p o s t e l a ( N W S p a in ) . T h e o r e b o d i e s ar e

b o u n d t o l a t e - s t a g e H e r c y n i a n s t r u c t u r e s , m a i n l y d u e t o

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

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

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

l i t e s , e c log i t e s , a nd g r a n i t e s . O r e m ine r a logy , a l t e r a t i on ,

a n d o r e t e x t u r e s d e f i n e a f r a m e w h o s e m a i n f e a t u r e s a r e

c o m m o n t o a l l p r o s p e c t s i n t h e a r e a . M a i n m i n e r a l s a r e

a r s e n o p y r i t e a n d p y r i t e - a c c o m p a n i e d b y q u a r t z , a d u -

la r ia , se r ic ite , + ( tour ma l ine , ch lor i te , ca rb on a tes , grap h-

i te ), a s m a i n g a n g u e m i n e r a l s - w i t h s u b o r d i n a t e a m o u n t s

o f b o u l a n g e r i t e , b i s m u t h i n i t e , k o b e l l i t e , j a m e s o n i t e , c h a l -

c opy r i t e , m a r c a s i t e , ga l e na , spha l e r i t e , r u t i l e , t i t a n i t e ,

s c he e l i te , be r y l , f luo r i t e , a nd m ino r na t i ve go ld , e l e c t r um ,

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

d e f i n i n g a m e s o - c a t a t h e r m a l p a r a g e n e s i s . D e t a i l e d m i -

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

d e s c r ip t i v e s c h e m e o f t e x t u r a l c l a s s if i c a ti o n f o r t h i s t y p e

o f o r e . M os t o f t he o r e s f i ll ope n spa c e s o r ve in s , s e a l

c r a c k s o r c e m e n t b r e c c i a s ; d i s s e m i n a t e d o r e s w i t h r e -

p l a c e m e n t f e a t u r e s r e l a t e d t o a l t e r a t i o n ( m a i n l y s i li c if ic a -

t i o n , s e r i c i ti z a t io n , a n d a d u l a r i z a t i o n ) a r e a l s o o b s e r v e d .

I n t e n s i v e a n d r e p e a t e d c a t a c la s i s is a c o m m o n f e a t u r e o f

m a n y o r e s, s u g g e s t i n g s u c c es s iv e e v e n t s o f b r i t t l e d e f o r -

m a t i o n , h y d r o t h e r m a l f lo w , a n d o r e p r e c ip i t at i o n . G o l d

m a y b e t ra n s p o r t e d a n d a c c u m u l a t e d i n a n y o f t he s e

e v e n t s , b u t t e n d s t o b e c o n c e n t r a t e d i n l a t e r o n e s . T h e

o r i g i n o f th e g o l d o r e s is e x p l a in e d i n t e r m s o f h y d r o t h e r -

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

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

a l t e c t o n i c f r a m e o f c ru s t a l e v o l u t i o n o f W e s t G a l i c ia .

T h e m i n e r a l o g i c a l a n d t e x t u r a l s t u d y s u g g e s t s s o m e c r it e -

r i a w h i c h w i ll b e o f p ra c t i c a l v a l u e f o r e x p l o r a t i o n a n d f o r

o r e p r o c e s s i n g . O r e g r a d e s c a n b e i m p r o v e d b y f l o t a t i o n

o f a r se n o p y r i te . N o n - c o n v e n t i o n a l m e t h o d s , s u c h a s

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

r e s idue e n r i c he d in go ld .

G o l d o r es h a v e b e en m i n e d i n N W G a l i c ia ( N W S p a in )

s i nc e p r e - R o m a n t i m e s ( F i g . 1 ), b u t m i n i n g a c t i v i t y

c e a s e d a t t h e b e g i n n i n g o f t h i s c e n t u r y . P r e l i m i n a r y r e -

s e a r c h o n o l d (m o s t l y R o m a n ) m i n e s a n d w o r k i n g s i n t h e

l a s t d e c a d e d e m o n s t r a t e d s e v e r al i n t e r e s ti n g p r o s p e c t s i n

H e r c y n i a n m e t a m o r p h i c t e rr a n e s ( R o d r i g u e z 19 84 ), s o m e

o f w h i c h w e r e re c e n t l y d r i ll e d a n d p r o v e d t o c o n t a i n

h i g h - g r a d e o r e s . T h e m i n e s a n d p r o s p e c t s w h i c h h a v e

be e n s tud i e d ( l i s t e d i n Ta b le 2 ) a r e l oc a t e d ( F ig . 2 ) i n a

b r o a d b e l t o f v a r i a b l e w i d t h o f l - 5 k m , w h i c h e x t e n d s

f o r 1 5 k m i n a N N E - S S W d i r e ct i o n b e t w e e n t h e t o w n -

l a n d s o f M a z a r i c o s a n d Z a s i n t h e P r o v in c e o f L a

C o r u f i a .

T h i s i n v e st i g a ti o n f o r m s p a r t o f th e p r o g r a m m e o f

r e s e a r c h w h i c h E N A D I M S A i s c a r r y i n g o u t i n t h e r e -

g i o n , a n d w h i c h a i m s t o d e f i n e a b a s ic f r a m e o f r e fe r e n c e

t o u n d e r s t a n d t h e p ro c e ss o f m e t a l c o n c e n t r a t i o n a n d

e v o l v e m o r e p r e c i se e x p l o r a t i o n c r i t e r ia . T h e p r e s e n t p a -

p e r b a s i c a ll y ad d r e s s e s t h e s e p r o b l e m s f r o m t h e p o i n t o f

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

M e a n o s o r e , w h e r e d r i l l i n g c a r r i e d o u t h a s g i v e n a c ce s s to

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

p le s f r om the o the r oc c u r r e nc e s ( F ig . 2 ) , a s w e l l a s c oun -

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

G e o l o g y

T h e o r e b e l t , w h i c h i s s i t u a t e d t o w a r d s t h e e a s t e r n , m y -

l o n it iz e d , N N E - S S W t r e n d i n g e dg e o f t he a l l o c h t o n o u s

M a l p i c a- T u i U n i t , M T U ( I G M E 1 98 4), f o r m s p a r t o f a

r e g i o n a l g o l d - b e a r i n g m e t a l l o t e c t , b r o a d l y d e f i n e d b y t h e

e a s t e r n m a r g i n o f t h i s un i t ( F ig . 1 ).

T h e d e t a i l e d g e o l o g y o f t h e a r e a u n d e r s t u d y i s t h e

s u b je c t o f a n o n g o i n g i n v es t i g a ti o n b y E N A D I M S A a n d

b y B R G M ( P a g e s a n d C h a m b o l l e i n p r es s ). T h e e s s e n ti a l

f e a t u r e s o f th e d e p o s i t s c o n s i d e r e d a n d t h o s e o f th e i r

g e o l o g i c a l f r a m e w o r k a r e s u m m a r i z e d i n F i g . 2 a n d

Table 2 .

T h e o r e s o c c u r i n v e i n s o r i n i r re g u l a r b r e c c i a b o d i e s

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


2/11

S 43

e N ~ S a n t ia g o d e

C ~ 1 7 6

. . j F i n i s t e r r e

I- E X P L A N A ~ - O - N . . . . .

I ~ . Go ldp ros p ee t ~ r o ld m ine

- -

- v e i n T y p e l o l lu v t a l

~ I ~ I d e t o l l o t e c t

I

P R E ~ R I A N T O I I H e r c y n i o n r o c k s ( in d i f .

~ ; C A R . O N F E R O U S

P RE C. T O E A R L Y ~ M o l p i c o - - T u i U n i t

0 ~ 5 ]O k m IPA LAE O Z O IC

I / F a u l t

Fig. 1 . Sketch map of NW Galic ia , showing

gold prospects and the position of the

al lochtonous M alpica-Tui Unit modified from

IGME 1982)

f o r m e d , o f t h e M T U o r o f t h e b a s e m e n t , t h e S c h i st B e lt

o f C e n t r a l G a l ic i a D o m i n i o E s q u i s t o s o d e G a l i c ia C e n -

t r al , I G M E , 1 9 8 4 ). T h e h o s t r o c k s a r e f re q u e n t l y m y l o n i -

t iz e d m e t a m o r p h i c P r e c a m b r i a n t o D e v o n i a n r o c k s

m i g m a t i t e s , o r t h o - a n d p a r a - g n e i s s e s , m i c a s c h i s t s , a m -

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

T h e r e i s a n e s p e c i a l l y c l o s e s p a t i a l r e l a t i o n s h i p b e -

t w e e n m i n e r a l i z a t i o n a n d l a t e b r e c c i a t io n w i t h h y -

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

i n g t h e m y l o n i t e s a n d t h e g r a n i t i c r o c k s F i g . 2 ). A s s u c h ,

t h e h y d r o t h e r m a l c o n c e n t r a t io n s w e r e g e n e r at e d a f t e r th e

e m p l a c e m e n t o f t h e M T U a n d t h e g ra n it e s. T h e c o u n t r y

r o c k s s h o w t y p i c a l

a l t e r a t i o n s

N o d a l 1 9 8 6) re l a t e d t o th e

h y d r o t h e r m a l i n f i l l . S i l i c i f i c a t i o n , p o t a s s i c a l t e r a t i o n

a d u l a r i z a t i o n ) , a n d s e r i c i t i z a t i o n a r e t h e m o s t w i d e -

s p r e a d F i g s . 4 a , b , c ) ; l o c a l ly , t o u r m a l i n i z a t i o n F i g . 4 d )

a n d c h l o r i t i z a ti o n c a n a c q u i r e i m p o r t a n c e . I n c o n t r a s t t o

o t h e r a u r i f e r o u s d i s tr i ct s , t h e c a r b o n a t e a l t e r a t io n i s g e n -

e r a l ly p o o r l y d e v e l o p e d i n t h e a re a , a l t h o u g h t h e a p p e a r -

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

i n s o m e z o n e s , m i g h t s u g g e s t t h a t s o m e f l u id s w e r e r ic h i n

C O 2 .

T h e m a j o r i t y o f th e p r o s p e c t s r e v e a l a s p a t i al r e l a t i o n

w i t h t h e in t r u s iv e g r a n i t i c r o c k s , a l t h o u g h i n s o m e o f

t h e s e F o u s a s d e V i l a) t hi s re l a t i o n c a n o n l y b e i n f e rr e d

f r o m g e o c h e m i c a l a n d s t r u c tu r a l d a t a P a g 6 s a n d C h a m -

b o l l e , i n p r e s s ) a n d f r o m t h e m i n e r a l o g y , a n d i n o t h e r s

M e a n o s , R i a l ) i t i s n o t e v i d e n t .

Table 1. M ineralogical composition

Gangue

Qua rtz, adularia, sericite, tourm aline, chlorite, accessory

carbonates, scheelite, apatite, beryl, fluorite

Ore

Principal components

arsenopyrite, pyrite

Minor components

boulangerite, bismuthinite, kobellite,

jamesonite, chalcopyrite, marcasite, rutile-sphene, graphite,

galena, sphalerite, pyrrhotite, native gold, electrum, native

bismuth, fahlore

Trace components mackinawite, cubanite, magnetite, miargyrite,

petzite, cosalite

Secondary m inerals

leucoxene, digenite, co vellite, chalcocite,

cerussite, anglesite, limonite, sco rodite, m ansfieldite,

beudantite

Mineralogical textural s tudy of the gold ores

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

s u m m a r i z e d i n T a b l e 1 )

A p a r t f r o m t h e a c c o m p a n y i n g s il ic a te s - c h i e f ly q u a r t z

a n d a d u l a r i a , _ s e r i ci t e , t o u r m a l i n e , c h l o r i t e ) - a n d a c -

c e s s o r y c a r b o n a t e s , s c h e e l it e , e t c . w h i c h c o n s t i t u t e t h e

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

w i t h s u b o r d i n a t e p y r i t e . M i n o r c o m p o n e n t s , l i s te d i n o r -

d e r o f d e c r e a s in g a b u n d a n c e i n T a b l e 1 , a r e s c a r c e in t h e

m a j o r i t y o f th e s a m p l e s a n d a r e u s u a l l y f in e - to v e r y

f i n e - g r a i n e d . I n t h e c a s e o f s u r f a c e s a m p l e s , s c o r o d i t e ,


3/11

S 44

N Y . . " . : l " ~ ~ ~ *

; . . t j t

/

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g , . . .

.

. a + + +

, :A , ~ . / ; , ' /

( / ~ , - 9 I

~ : . " ~ O 7 , : ~ . ' . : 9

; ' q . . . ' . '

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

E X P L A N A T I O N

Zo n e s o f b r e c c iQ t i o n w i t h

h y d r o t h e r m a l

i n f i l [

Zo n e s o f s h e a r

H e r c y n i o n b a s e m e n t ( Sch is ts j Gne isses e n d

M i g m a t i t e s / ' D o m i n i o E sq u i st o s o de G o l i r C e n t r o l )

A I I o c h t o n o u s M a l p i c a - T u i U n i t ( S c h i s t s~

O r t h o - a n d P a r a g n e i s s e s , A m p h ib e l i t e s e n d E c l o g i t e s )

G r a n i t o i d i n t r u s i o n s

S E C T I O N A - A '

W . N W .

/ / \

, ~ : , -~

F r a c t u r e

~ - - - - I n f e r r e d f r a c t u r e

k i t h o l o g i c c o n t a c t

~ I n f e r r e d t h r u s t

~ r 1 7 4 M i n e / p r o s p e c t

E . S E ,

Fi g . 2 . Ge o l og i c a l s ke t c h ma p o f t he

W e s t S a n t a C o m b a - F e r v e n z a a r e a

s h o w i n g t h e l o c a t i o n o f g o l d m i n e s a n d

p r os pe c t s s t ud i e d : 1 Me a nos ; 2 Fous a s

de Vi l a ; 3 Sa n Ma r t i f i o ; 4 L i m i de i r o ;

5 V i l a r c ovo ; 6 A l bo r e s ; 7 R i a l. Age s o f

t h e m e t a m o r p h i c r o c k s a r e P r e c a m b r i a n

t o D e v o n i a n ; g r a n i te s a r e H e r c y n i a n .

M o d i f i e d f ro m P a g 6s a n d C h a m b o l l e

( in press) and No da l (1986) ; sec t ion af ter

I GME ( 1981 , mod i f i e d )

T a b l e 2 . A m i n e r a l o g i c a l c h a r a c t e r iz a t i o n o f t h e o r e s i n t h e W e s t S a n t a C o m b a / F e r v e n z a A r e a

M i n e o r L H o s t r o c k M a i n a l t e r a t io n M a i n o r e m i n e r a l s

p r os pe c t S

M e a no s / I / 3 Me t a s e d i me n t s , o r t hogne i s s e s Se r i c it e , a du l a r i a , qua r t z

S . Ma r t i f i o B , V me t a ba s i t e s , a p l i t e s, o f t e n my l on i t i c

(*)

Fous a s de V i l a 2

( * * ) V

Li mi de i r o 4

(*)

Vi l a r c ovo 5

( * ) ( * * ) v

A l b o r e s 6

(*) V, B

Rial 7

D

M y l o n i t i c b a s e m e n t g n e is s es

n e a r g r a n i t e c o n t a c t

D e f o r m e d g r a n i t e a n d m e t a m o r p h i c

r oc ks

G r a n i t i c m y l o n i t e a n d g r a p h i t i c

g o u g e a l o n g f a u l t

P r o t omyl on i t i c g r a n i t e , me t a s e d i -

m e n t s , a n d m y l o n i t e s

M y l o n i t e s a n d u l t r a m y l o n i t e s

f r om ba s e me n t gne i s s e s

T o u r m a l i n e , q u a r t z ,

ser ic i te , adular ia

Se r i c it e a nd c l a y

( ve r y i n t e ns i ve )

Qua r t z , g r a ph i t e

Q u a r t z , t o u r m a l i n e ,

c h l o r i t e , g r a ph i t e

Ad u l a r i a , s e r ic i te , qua r t z ,

u s u a l l y s y n - m y l o n i t i c

A r s e n o p y r i t e w i t h m i n o r s u l p h i d e s

a nd s u l phos a l t s , na t . Au a nd B i , e l e c t r um

(see Table 1)

Ar s e n opy r i t e , ga l e na , s pha l e r i t e , s c he e li t e,

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

Ar s e nopy r i t e , no v i s i b l e go l d obs e r ve d

( ve r y s c a rc e ou t c r op s )

Spha l e r i t e , ga l e na , a r s e nopy r i t e , na t i ve

gold/e lec t rum, pe tz i te (?)

G o l d - b e a r i n g a r s e n o p y ri t e , m i n o r

ba s e - me t a l s u l ph i de s

Ar s e nopy r i t e , t r a c e s o f py r r ho t i t e ,

s pha l e r i t e , kobe l l i t e , g r a ph i t e

L , Loc a t i on No . F i g . 2 ; S , s t r uc t u r a l t ype B, V, br e c c i a bod i e s , ve i n s ; D , d i s s e m i na t e d my l on i t i c o r e )

( *) A b a n d o n e d R o m a n g o l d m i n e s , ( ** ) m i n e s a c ti v e i n n i n e t e e n t h /t w e n t i e t h c e n t u r y


4/11

I I IC

I l l I I I H I I I I

F i g . 3. T e x t u r a l t y p e s o f a rs e n o p y r i t e i n s h e a r z o n e g o l d o r e s o f t h e

W e s t S a n ta C o m b a - F e r v e n z a a r ea . Black: a r s o n o p y r i t e ; white: sili-

c a t e g a n g u e ; S , t r e n d o f m a i n S - s u r f a c e .

Type I:

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

s u b t y p es : A i d i o m o r p h i c c r y s ta l s a n d a g g r e g a te s ; B - l e n t ic u l a r

a g g r e g a t e s / / S ; C - m i c ro c l a st s ).

Type II:

s t r i n g e r o r f i n e - b a n d e d

o r e s s u b t y p e s: D - b a n d s o f m ic r o c l a st s ; E - c e m e n t i n m i c r o s h e a r s ;

F - f i ll i n g m i c r o f i s s u r e s a n d S - p l a n e s ) . Type I lL m a s s i v e o r e s s u b -

t y p e s : G - f i ss u r e a n d v e i n f i ll ; H - b r e c c i a c e m e n t ; I - c a t a c l a s t i c f il l

o r c e m e n t ) . S c a l e i s o n l y a p p r o x i m a t e v e r y v a r i a b l e g r a i n s iz e )

m ans f i e l d i t e , beudan t i t e , and l i m on i t e t ake t he p l ace o f

a r s enopy r i t e , o f t en r ep l ac i ng i t p s eudom orph i ca l l y .

T h e p a r a g e n e s is r e p r e s e n te d i n T a b l e 1 c o r r e s p o n d s

t o t h e t y p ic a l M e a n o s o c c u r r e n c e , w i t h t h e e x c e p t i o n o f

scheel i t e , bery l , f luor i te and petz i te (?) , observed only in

s a m p l e s f r o m F o u s a s d e V i l a, R i a l o r V i l a r c o v o . I n o t h e r

l oca li ti e s , d i ff e rences i n the p rop o r t i o n o f t he d i s t inc t

c o m p o n e n t s a p p e a r ( T a b l e 2 ); e .g . a g r e a te r a b u n d a n c e o f

ga l ena and s pha l e r i t e i n F ous as de Vi l a o r an abs ence o f

go l d m i ne ra l s i n R i a l .

D e s c r ip t io n a n d t e x t u r a l s t u d y o f t h e o r e s

A v i s ua l exam i na t i on o f d r il l -co re , t r ench -cu t s e t c . , p e r -

m i t s a ca t ego r i za t i on o f the s u l pho -a r s en i ca l o re gene ra l -

l y a s pos t -m y l on i t e and r e l a t ed t o t he l a t e ca t ac l a s i s m en-

t i oned a bove . Two t ypes can be d i s t ingu i s hed : d i s s em i -

na t ed and m as s i ve . Of t en , bo t h ex i s t i n t he s am e s am p l e ,

t h e f o r m e r a s a w a l l - ro c k i m p r e g n a t i o n a n d t h e l a tt e r a s

a f r ac t u re - f i l l and b recc i a cem en t .

A r se nopy r i t e

A s t h e p r i n c ip a l c o m p o n e n t o f t h e o re , t h e a r s e n o p y r i t e

s hows t he m os t cha rac t e r i s t i c t ex t u res . Thes e wi l l be

b r i e f l y ana l yzed , i n o rde r t o cha rac t e r i ze t he o res o f t he

S a n t a C o m b a - F e r v e n z a a r e a , a n d a l s o t o c o n t r i b u t e t o a

t ypo l og i ca l c la s s i f ica t i on s chem e o f s hea r -zone go l d o res .

S 45

M i c r o s c o p i c e x a m i n a t i o n s h o w s th a t , f r o m a g e o m e t -

r i c po i n t o f v iew, t he t ex t u ra l r e l a t i ons h i p s be t w een a r -

s e n o p y r i t e a n d g a n g u e o r h o s t c o r r e s p o n d t o t h e t y p e s :

l d (d i ss em i na t ed ) and 3a ( ve i n - l i ke o r s andw i ch -

t ype ) e s t ab l is hed by Am s t u t z (1960 ). T he l a t t e r has to be

s ubd i v i ded i n t h is cas e i n t o t w o ne w t ypes : ( i) f ine l y band -

ed ( t r ans i ti ona l w i t h l d ) , and ( ii ) m as s i ve . They co r re -

s pond t o t ypes I , I I and I I I ( and 9 s ub - t ypes : A t o I )

r ep res en t ed i n F i g . 3 and de f i ned as fo l l ows :

Type I

( l d , Am s t u t z 1960 ): f i ne ly d i s s em i na t ed ~ tm t o

m m -s i zed a r s enopy r i t e g ra i n s , gene ra l l y i d i om orph i c ,

s om et i m e s fo l l ow i ng t i le s ch i s t o s i t y (F i g . 5 b ) , bu t m ore

o f t en no t (F i g s . 4 b , c) . Of t en r e l a t ed t o hy d ro t h e rm al i n -

f il l s t ruc t u res . Typ i ca l l y pos t -d e fo rm at i on , bu t o ccas i on -

a l ly d e f o r m e d ( F ig . 5 b ) o r e v e n c r u s h e d d o w n t o m i c r o -

c l a s ts w h i ch fo l l ow s hea r -p l anes (F ig . 5 a , t r an s i t i on t o

t ype I I ) .

T y pe H

( t ra n s i ti o n a l I d - 3 a ) , f i n e -b a n d e d : a r s e n o p y r i t e

i n t abu l a r o r banded m i c ros t ruc t u res , e i t he r a s i n f i l l i n

m i c ro f i s s u res and ve i n l e ts (wi d t h s o f ~tm t o m m ) ob l i que

t o t he s ch i s t o s i t y , o r i n s t r i nge r s o r s hea r s , s om et i m es

c rus hed (F ig . 5 a ) . Qu an t i t a t i ve l y o f l i tt le i m por t anc e .

T y pe I I I

(3a , Am s t u t z 1960 ) : a r s enopy r i t e i n t abu l a r

m a s s e s o f m m / d m t h ic k n e s s, w i t h a t e n d e n c y t o f o r m

c o a r s e - g r a in e d i d i o m o r p h i c a g g r e g a te s , i n t e r g r o w n w i t h

o t he r o res and wi t h gangue . Hydro t he rm al i n f i l l t ex t u res

a re dom i nan t . D efo rm at i on i s u s ua l l y s l igh t (F ig s . 5c ;

6c , d ) bu t occas i on a l l y is i n tens e (F i g s. 5d ; 6a , b ) .

The t yp i ca l occu r rence o f the a r s e nopy r i t e is a s pos t ~

m yl on i t i za t i on f i l l o r a s b recc i a cem en t wh i ch can be

a f fec t ed i n va ry i ng deg rees b y l a t e r ep i s odes o f gene ra l l y

b r i t t l e de fo rm at i on . The obs e rved t ex t u res can be i n t e r -

p re t ed a s t he r e s u l t o f a

la te - s tage repe t i t ive ca tac las is

wi t h a s s oc i a t ed hyd ro t he rm al i n f l ux , s o t ha t t he de fo r -

m a t i o n p r o d u c e d e a c h t i m e i n t h e e v o l u t i o n o f th e s t r u c -

t u re can a f f ec t t he m i ne ra l s wh i ch d eve l o ped i n t he p re -

v i o u s e v e n t a n d w h i c h a r e o v e r g r o w n i n t u r n b y t h e n e x t

h y d r o t h e r m a l g e n e r a t i o n . S c a n n i n g e l e c tr o n m i c r o s c o p e

( S E M ) s tu d i e s o f al l th e t y p e s o f a r s e n o p y r i t e d i d n o t

d e t e c t a n y c o m p o s i t io n a l d i f fe r e n c e s b e t w e e n t h e m .

A s r e g a r d s

chronology ,

a ce r t a i n am b i gu i t y i s i n -

ev i t ab l e g iven t he u s ua l o b l i t e ra t i on o f t he ea r l y t ex t u res

o f t he o re , due t o i t s r ec ry s t a l li za t i on du r i ng s ubs e que n t

de fo rm at i ona l even t s . There fo re t he c l a s s i f i ca t i on p re - ,

s yn - and pos t - has t o be r e l a t ed t o t he l a s t de fo rm at i on :

t he l a t e ca t ac l a s t i c s t age . The pos i t i on o f t he B s ub t y pe

i s ques t i on ab l e wi t h r ega rd t o t he t ype o f s ch i s t o s i t y S

and the par t icu lar t ex tural character i s t ics , s ince some la te-

s t age m y l on i t i za t i on i s no t t o be exc l uded ( s ee F i g s . 4d ,

5 b ) . How eve r , i n a s i m p l e f a s h i on , t he fo l l owi ng s ucces -

s i on can be s ugges t ed : (1 ) de fo rm ed o r ca t ac l a s t i c o re

( s ub t ypes C -D- I : pre - la t e cataclas i s ) : F igs . 5a , d ; (2) ore

pa r t i a l l y a f f ec t ed by de fo rm at i on a nd r ec ry s t a l li zed o r

r e - g r o w n

pre - / s y n - la t e

cataclas i s : su bty pe B): F ig . 5b;

( 3) li tt le o r n o t d e f o r m e d o r e ( s u b t y p e s F - A - E - G - H : s y n - /

p o s t -

l a te c ataclas i s ) : F igs . 4b , 5 c .

In

s u m m a r y ,

t he obs e rved t ex t u res i nd i ca t e a s ucces -

s i o n o f c y c l e s o f b r it tl e d e f o r m a t i o n + h y d r o t h e r m a l i n-

f il l, each cyc l e be i ng r ep res en t ed by t he app ea ra nce o f


5/11

S 46

Fig 4a -d . Host rocks of the gold ores.

Photomicrographs with transmitted light,

//N. a Hydrothermal alteration of

ultramy[onitic country rock, Rial.

Earlier, deformed, veinlets of quartz (Q),

and younger microfissures sealed by

adularia

KF),

which also replaces the

original quartz-feldspar-mica matrix.

b Late, undeformed arsenopyrite

phenocrysts

black),

with pressure

shadows with adularia and quartz,

growing on a sericitized plagioclase

fragment, in brecciated plagiogneis. Type

IA, disseminated, ore. Fousas de Vila. c

Typical host rock of gold ores in

Meanos: mylonitic graphite paragneis,

brecciated, silicified, sericitized and

potassically altered, showing abundant

veinlets with hydroth erma l infill, in this

case quartz and adularia

white)

and

arsenopyrite

black).

d Intensely

deformed (proto-mylonitic) early

hydrothermal vein : microclasts of quartz

(Q), scheelite

SCH)

and arsenopyrite

(not seen) in an oriented matrix of

acicular tourmaline (T). Rial

s o me o r a ll o f th e t ex t u r a l s u b t y p es A t o I , in t h e ap p r o x -

i m a t e o r d er I - D - C - B - H - G - E - F - A ( I, D , a n d C , s u p e ri m -

posed on p re-ex i s t ing o res ) .

Pyrite.

Su b o rd i n a t e w i t h r e s p ec t t o a r s en o p y r i t e , a l -

t h o u g h b e i n g i n p l ace s th e p r i n c i p a l co mp o n e n t o f t h e

ore , i t may be a l so scarce o r absen t (Fig . 6a) , I t i s o f t en

o p t i c a l l y an i s o t ro p ic , an d i ts t ex t u re s a r e s i mi l a r t o t h o s e

o f a r se n o p y r i te , s h o w i ng a g r e a t e r a m o u n t o f p o s t- c a t a -

c las t i c fea tu res .

Marcasite. Rel a t i v e l y f r eq u en t a l t h o u g h n o t ab u n d an t , i t

u s u a l l y a cco mp an i e s o r ev en cemen t s l a t e p y r i t e . I t c an

ap p ea r a s s o c i a t ed w i t h a r s en o p y r i t e o r p y r i t e , s o me t i mes

r ep l ac i n g t h em, an d i s a l s o d i s s emi n a t ed i n t h e h o s t r o ck .

O ccas i o n a l b i rd ' s - ey e t ex t u r e s o b s e rv ed i n marc a s i t e

s u g g es t t h a t i t may b e s eco n d a ry t o p y r rh o t i t e .

Sphalerite and galena.

T h es e a r e g en e ra l l y l a t e mi n e ra l s

wi th re spec t to a rsen opy r i t e (Figs . 5 c , 7c) . Of i r regu la r

d i s t r i b u t i o n , t h ey a r e ab u n d an t i n s o me zo n es ( e . g .

Fo u s a s d e V i l a ) b u t u s u a l l y s ca r ce o r ab s en t . SE M an a -

l y se s s h o w t h a t s p h a l e r i t e co n t a i n s ab o u t 7 % Fe an d u p

t o 4 % Cd ( i .e . Fe -Cd - r i ch p r z i b r a mi t e v a r i e t y ) , w hi l e o n -

l y t r a ce s o f A g h a v e b een d e t ec t ed i n t h e g a l en a .

Boulangerite, bismuthinite, kobellite, jameso nite and chal-

copyrite.

These are p resen t in smal l inc lus ions and f i l l ing

mi c ro f i s s u r e s i n a r s en o p y r i t e (F i gs . 6 a , b ) an d p y r i t e

w h i ch t h ey f r eq u en t l y co r ro d e ; r a r e l y i n d i s s emi n a t i o n s


6/11

S 47

Fig. 5. Different stages of deformation affecting he ores (arsenopy-

rite: white;silicate gangue: black). Photomicrographs with reflected

light, /IN. a Disseminated, mylonitic ore in quartz-feldspar-mica

ultramylonite, Ri al. Textural types IC-II D (see text), b Recrystal-

lized mylonitic ore, intergrown with quartz and tourmaline, in my-

or veinlets in the rock. In the case o f the first four miner-

a ls, the very f ine gra in-size has requi red SEM analyses to

confi rm the i r ident ity . Appreciab le quant i t ies of Cu, Pb

and Sb have been detec ted in b ismuthin i te and of Ag in

kobel li te . These minera ls ma y or ma y not occur together

and may a l so be accompan ied by pyrrhoti te, marcasite,

native bismuth and nat ive goldpelec t rum in minute inc lu-

sions or in fine bimineral ic (Fig. 7b) or, rarely, polymi-

neral ic aggregates. Tetrahedri te, miargyri te, cosali te (?)

and petz i te (?) can be present in t race amounts , a l so as

minor inc lusions in arsenopyri te or pyri te .

Gold. This i s present in the na t ive for m in prac t ica l ly

a l l cases . The SEM analyses carr ied out have a lways de-

tec ted s igni f icant quant i t ies of Ag (6- 35 ) , bu t electrum

(> 25 Ag) has only been fou nd in some samples (Vi lar-

covo and Meanos) where the noble meta l tends to be

associated with sulphides.

It is present in blebs o f the ord er of 1 - 5 lxm, less often

up to 50 ~tm and except ional ly up to m i l l imet r ic d imen-

sions . Almost a lways forming inc lusions or microfi ssure

inf i l l s in arsenopyri te , i t somet imes occurs in su lphide

(mostly pyri te) or si l icate minerals.

In de ta i l , the tex tura l re la t ions observed can be char-

ac ter ized by the fo l lowing forms:

1 . Nat ive gold , wi th or wi thout b ismuth carr ie rs (e .g .

Fig. 7b), si l icates (e.g. Figs. 6c, d; 7 a), sulpho salts o r sul-

lonitic quartz tourmalinite with rutile (grey), Fousas de Vila. Tex-

tural Type I B. c Slight brecciation o f quartz-arsenopyrite apy)

vein. Cracks sea led by: (1) galena gn, corrodes apy) and (2) sphaler-

ite (sO. Textural type III (G-H ). Fousas de V ila. d Cataclastic ar-

senopyrite ore, textural type III (I). Meanos

phid es (e.g. Fig. 7c), in generally m assive, type III, ar-

senopyrite: 1 a as minu te inc lusions , somet imes poik i l i t ic

(Figs. 7 a, b); 1 b in micro fissures (Fig. 7 c); lc in blebs

a long c leavage p lanes; I d in ters t it i a l and as in tergran ular

fi lms (Fig. 6d).

2 . Nat ive gold cemen t ing microclas t s of a rsenopyri te in -

c luded in late pyrite.

3. Nat ive gold in silicates (adular ia , qu artz or ch lor i te ), in

genera l c lose to crysta l s of a rsenopyri te (Fig . 7d) .

4. Native gold associated with silicates, cement ing mi-

crofissures in arsenopyrite, w i th o r w i thou t co r ro s ion

phenomena (Figs . 6c , d ) .

The c lose spat ia l re la t ionship observed between nat ive

gold and arsen opyri te (which tends to be ma in ta ine d even

when the noble metal occurs within pyri te or si l icates) is

in genera l independent of the deformat ional charac ter i s-

t ics of the arsenopyri te , bu t no t , i t seems, of types I-II-III

(Fig . 3 ). The grea ter par t o f the gold (group 1 forms) i s

foun d in massive arsenopyr i te of type III ( in whichever of

the subtypes G , H or I ) , and only in except ional cases in

arsenopyr i te of types I and II (especia l ly sub-type B).

Rutile. This i s a common accessory of the ore , a l though

i t i s mor e ab und ant in the host , where it appears d issem-

inated in sm al l tabular c rysta l s . I t i s a lmost a lways a l te red

to leucoxene.


7/11

S 48

Fig

6a -d . Late-stage processes in

massive, type III, arsenopyrite (Meanos):

a, h SEM images; c, d reflected light

photomicrographs, imme rsion,//N.

a Repeated cataclasis affecting the ores.

Crack in arsenopyrite (labelled

APY),

sealed by: (1) pyrite (labelled FE); (2) a

second generation of arsenopyrite; and

(3) bismuthinite (labelled B/).

b Brecciated arsenopyrite welded by

jamesonite (M1) and boulangerite (M o,

M,, Ms). c Native gold

Au)

and silicate

black)

sealing microfissure in

arsenopyrite

white).

d Native gold

Au),

containing 10% Ag, and silicates (quartz,

chlorite,

black)

in crack and interstitial

in arsenopyrite

white).

Slight corrosion

of arsenopyrite by gold

Graphite.

Th is a l so o ccu r s f r eq u en t ly - a l t h o u g h se ld o m

ab u n d an t ly in t h e m in e r a l i z a t io n wi th wh ich i t h a s t o b e

associa ted , no t on ly fo r the spa t ia l re la t ion observed in

han d samples a nd in the f ie ld , bu t a lso fo r the appe aran ce

in minera l ized samples o f spheru l i t ic g raph i te wi th f i -

b r o u s - r ad ia l t ex tu r e s . A l th o u g h th e g r ea t e r p a r t o f t h e

g r ap h i t e m ay b e o f m e tam o r p h ic o r ig in , i t ap p ea r s ce r-

t a in th a t t h e r e i s a h y d r o th e r m a l co n t r ib u t io n r e l a t ed to

the p rocesses which g ive ri se to the min era l iza t ion . The se

co n t r ib u te a l so so m e apatite, tourmaline, etc. , and ( in

some showings) some

scheelite, beryl,

and .f luor i te .

Sequence of crystallization

Table 3 g ives an o verv iew o f the sequence of c rysta l l iza-

t ion of the minera ls a l rea dy descr ibed , in the geo log ic- tec-

to n ic f r am ew o r k o f t h e i r h o s t s t r u c tu r e s.

Th e p r in c ip a l ch r o n o lo g ica l f r am e o f r e f e ren ce is t h e

la te-s tage ca tac las is which permits a d is t inc t ion to be

made be tween pre- , syn- and post - la te ca tac las is o res ,

acco r d in g to t h e b r it t l e d e f o r m a t io n a l e f f ec ts p r o d u ced in

th em . Th e r eg io n a l m e tam o r p h i sm an d th e su b seq u en t

d y n am ic m e tam o r p h i sm ( m y lo n i t i z a t io n ) r e l a t ed to t h e

e m p l a c e m e n t o f t he a l l o c h t o n o u s M T U ( M a l pi c a -T u i

Un i t ) c an co n s t i t u t e a f r am e o f r e f e r en ce f o r t h e p r e - ca t -

ac las t ic o res . However , th is f rame of re ference i s ra re ly

observab le g iven the in tensi ty o f the la te deformat ion in

th e m in e r a l i z ed zo n es . Mo r eo v e r , t h e m y lo n i t i z a t io n in

these makes i t d i f f icu l t to recognize an y prev iou s tex ture .

For th is reason a l l the ear ly o res have been p laced in a

s in g le g r o u p ( m e t am o r p h ic m in e r a l s ) , ch a r ac t e ri zed b y

th e p r e sen ce o f d y n am ic m e tam o r p h i sm . Th ese ea r ly o r e s

are ac tua l ly obs erved in the host roc k (e .g . in the u l t ram y-

lon i tes o f Ria l ) , bu t scarce ly in the wel l -minera l ized

zo n es. M o s t o f t h e o r e can b e th e r e f o r e co n s id e r ed o f t h e

post -metamorphic , la te ca tac las t ic type; the crysta l l iza-

t i o n o f g o ld u su a l ly f o l lo ws th a t o f a r sen o p y r i t e .

Discussion and genesis

Th e sp a t i a l r e l a t io n b e tween th e co n cen t r a t io n s o f g old

an d th e ea s te r n , m y lo n i t i z ed m ar g in o f t he a l lo ch to n o u s

MTU i s n o tewo r th y . Th e m in e r a l i z a t io n o ccu r s i n l a t e


8/11

S 49

F i g .

7a -d . SEM images of earlier (a, b)

and later (e, d) gold ores from Meanos

(a, b, c) and Vilarcovo (d). a Blebs of

native gold

white,

relief, 6.1 Ag

content), with minute adularia inclusions

black), in massive, Type III,

arsenopyrite, b Bimineralic inclusions of

native gold (9.5 Ag) and bismuthinite

in massive arsenopyrite (labelled

A U, BI,

AS, respectively), e M icrofissures in

arsenopyrite, sealed by sphalerite

(labelled

SL,

7.6 Fe) and electrum

(M3: 64.6 Au, 35.4 Ag). d Quartz

vein (Q), with m icrofissures sealed by

electrum (AU-AG: 69.9 Au, 30.1

Ag)

s t r u c tu r e s ( c r o ss - cu t t i n g wi th r e sp ec t t o t h e m y lo n i t i c

sch i s to s i ty ) , ch a r ac t e r i zed b y b r ecc i a t io n , h y d r o th e r m a l

inf il l and a l te ra t ion . A m ore o r less d i rec t spa t ia l re la t ion

to g r an i t i c i n t r u s iv e b o d ie s i s co m m o n , t h o u g h n o t co n -

stan t .

F r o m a

mineralogical

p o in t o f vi ew, t h e re a r e n o tab le

simi lar i ties be twee n a l l the occur re nces in the area , w hose

m a in f ea tu r e s a r e su m m ar ized f o r co m p ar i s o n in Tab le 2.

Th e r e i s a c l ea r a f f in ity in m in e r a lo g ica l co m p o s i t i o n an d

th e o r e s h av e a h y d r o th e r m a l , h y p o g en e - ep ig en e t i c o r i -

g in , i n a b r o ad ly m eso - k a ta th e r m a l r an g e .

F r o m a t ex tura l poin t o f view, there i s a lso a co inc i -

d en ce in t h e ty p o lo g ica l ch a r ac t e r i za t io n o f t h e o r e s

(Fig . 3 ) . These are a lmost a lways post -myloni t iza t ion , a l -

though they can be pre- , syn- o r post - la te -ca tac lasics .

T h e r e a r e a ls o s o m e n o t e w o r t h y

differences

b e tween

the ores o f d i f feren t zones (Table 2 and Fig . 2) :

Min e r a lo g ica l co m p o s i t i o n : T h e r e i s an in c r ea se in t h e

m in e r a lo g ica l v a r i e ty an d in th e co n t en t o f su lp h o sa l t s

an d /o r n a t iv e g o ld f r o m R ia l ( s im p le m in e r a lo g y , d o m i -

n a ted b y a r sen o p y r i t e , K- f e ld sp a r an d q u a r t z , n o g o ld

o b se r v ed a s m in e r a l p h ase ; Tab le2 ) t o Mean o s ( th e

r i ch es t an d m o s t co m p lex o r e s; Tab le 1 ); m o r eo v e r , t h e r e

i s a g r ea t e r ab u n d an ce o f su lp h id es , su ch a s g a l en a an d

sphaler i te (Fig . 5c) , a nd of tou rma l ine (Fig . 5b) in

Fousas de Vi la , where scheel i te , f luor i te and bery l a rc

a lso observed .

Hy d r o t h e r m a l ac t iv i ty : Th i s i n c rea se s in a s im i la r m an -

ner , be ing scarce in the f i r s t loca l i ty (Figs . 4a , d ) and

in tense or ub iqu i tous in the o thers (Fig . 4c) .

- Tex tu re s : Th e r e ar e p r ed o m in an t ly sy n - d e f o r m a t io n a l

( m y lo n i ti c ) , d is sem in a ted o r e s i n t h e u l t r am y lo n i t i c h o s t

a t R ia l (Fig . 5 a ) ; and m assive ores re la ted to repeat ed

ep i so d es o f l a te ca t ac la s i s i n t h e r em a in d e r o f t h e sh o w-

ings and mines (e.g. Figs. 5c, d; 6) .

The overa l l g rouping of s imi lar i t ies and d i f ferences

permits a

synthesis

o f th e ab o v e - m en t io n e d d ep o s i ts t o b e

m a d e w i t hi n t h e f r a m e w o r k o f a c o m m o n m o d e l o f te c -

to n ic an d h y d r o th e r m a l ev o lu t io n . Th i s m o d e l im p l i e s a

t ransi t ion f rom duct i le to b r i t t le condi t ions, wi th succes-

s iv e r eac t iv a t io n s , a cco m p an ied b y h y d r o th e r m a l c i r cu la -

t i o n an d o r e accu m u la t io n , a s i l lu s t r a t ed b y o r e t ex tu r e s .

Th e R ia l o r e s wo u ld r ep r e sen t an ea r ly s t ag e , w i th

d o m in an t m y lo n i t i z a t io n an d sca r ce to i n c ip ien t l a te h y -


9/11

S 50

Table 3. Sequence of crystallization

R e g i o n a l m e t a m o r p h i s m

+ my lon i t iza t i on

Late catac las is

Arsenopyr i te .1

Pyr i te

Mar cas i t e

Bou langer i t e , Jame son i t e

Bismuthin i te , Kobel l i te ,

Nat ive Bismuth

Cha l copyr i t e

l Met amor ph i c

, I mine rals

~ 1 I I I I ~ i i i l i l l F I I I l l q l l l l ~

Hydr o t her ma l m iner a l s

Pre-syn-Late cataclasis

I Post-L. cata.

S u p e r g e n e

miner a l s

G ra p h ite . . . . . . . . . . . . .

N a ti ve G o ld , E le c tr um .~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 9 . ..

Sphaler i te , Pyr rhot i te

G a l e n a

Tet rahedr i te

.i

Miar gy r i t e

Cuban i t e , Mack inaw i t e - - -

Ma gne t i te - -

Rut i le , Sphene

L e u c o x e n e

Quartz, K-Feldspar, Ser ici te

Ch lo r i t e

Tour ma l i ne

C a r b o n a t e

f

Apat i te

Scheel i te , Bery l )

Digeni te, Chalcoci te, Covel l i te

Lim onite, An glesi te, Cerussi te)

in

Scorodi te, Beudant i te ,

1~ M ansfie ldite ,,

0

8

I

i

r

/

in

d r o t h e r m a l i n fl u x. T h e o t h e r e x t r e m e o f t h e p r o c e s s

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

d o m i n a n c e o f la t e c a t ac l a si s c r e a t i o n o f n e w s t r u c t u r e s

r i pe f o r h y d r o t h e r m a l i n fi ll ) a n d a m o r e a b u n d a n t , v a r i ed

a n d d e v e l o p e d h y d r o t h e r m a l i n f lu x w h i c h p r o d u c e s a

g r e a t e r a c c u m u l a t i o n o f go l d. F r o m t h e p o in t o f v ie w o f

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

i n t e r m e d i a t e s t a g e b e t w e e n t h e f o r m e r t w o , a s s u g g e s t e d

b y t h e i n t e n s i ty , a n d v a r i e t y o r d e g r e e o f e v o l u t i o n o f t h e

h y d r o t h e r m a l i n f lu x .


10/11

$5

It seems easy to expla in the s igni f icance of the la te

ca tac las i s a s an aur i fe rous meta l lo tec t . In a duc t i l e

re g i m e , t he r e s t r i c t e d pe rm e a b i l i t y a nd po ros i t y o f t he

m e d i um doe s no t f a c i l i t a t e hyd ro t he rm a l c i r c u l a t i on , bu t

f r a c t u r i ng a nd c re a t i on o f ope n spa c e by b r i tt l e f a il u re

f a v o u r t h e c i r c u la t io n o f s o l u t io n s a n d t h e s u b s e q u e n t

hyd ro t he rm a l i nf il l. The r e pe t i t ion o f e p i sode s o f b re cc i a -

t i on a nd i n fi ll a c c oun t s a s m u c h fo r t he p re se n t t e x t u re o f

t he o re ( c o -e x i s te nc e o f c a t a c la s t i c ge ne ra t i ons a nd i n t a c t

ge ne ra t i ons w h i c h c e m e n t t he m ) a s fo r t he e vo l u t i on o f i ts

pa ra ge ne s i s (g re a t e r va r i e t y a nd m i ne ra l we a l t h i n t he

l a t e r e p i sode s ) . Na t i ve go l d oc c u rs r e l a t e d t o c a t a c l a s t i c

ge ne ra t i ons o r su l phoa r se n i de s bu t a l so t o subse que n t ,

p o s t - d e f o r m a t i o n a l , g e n e r at i o n s, H o w e v e r , h ig h e r g o l d

c on t e n t s a re t o be e xpe c t e d whe re t he o re s a re m a ss i ve

a nd t he l a t e - s t a ge f e a t u re s ( sy n - / pos t - l a t e c a t a cl a s i s) w e l l

r e p re se n t e d .

The p ro c e s se s de sc r i be d a re c ha ra c te r i s t ic o f m a ny

a ur i f e rous ve i n - t ype de pos i t s r e l a t e d t o she a r -z one s i n

m e t a m o r p h i c t e rr a n es , a t y p e w h i c h i s k n o w n w o r l d w i d e .

I t s m os t t yp i c a l oc c u r re nc e i s i n Arc ha e a n g re e ns t one

be l t s . I t i s a l so known , wi t h som e va r i a t i ons , i n

P a l a e oz o i c t e r r a ne s . In sp i t e o f e v i de n t d i f f e re nc e s ( suc h

a s t hose i m p l i e d by t he d i f f e re n t c om p os i t i on o f the c rus t ,

t he s iz e o f the m e t a l c o nc e n t ra t i ons , e t c .) m o s t o f t he

m a i n ge o l og i c a l a nd m i ne ra l og i c a l f e a t u re s de sc r i be d i n

t h i s pa pe r a re a l so found i n t yp i c a l Arc ha e a n l ode go l d

d e p o s i t s , e . g . a s d e f i n e d b y H o d g s o n a n d M a c G e e h a n

(1982) i n the S u pe r i o r P rov i nc e o f t he C a na d i a n S h i e ld ,

or a s sum m arized by Co lvine e t a l. (1984) .

R e c e n t w o r k o n E u r o p e a n g o l d d e p o s it s o f H e r c y n i a n

a g e l e d B o n n e m a i s o n a n d M a r c o u x ( 1 9 8 7 ) a n d T o u r a y

e t a l. ( in p re s s ) t o p rop ose a t ypo l o gy wh i c h c a n a l so be

a pp l i e d t o t he F e rve nz a o re s . The se c a n be de sc r i be d a s

" p o s t - m e t a m o r p h i c t y p e, A u - A s s u b t y p e " i n th e c la s si fi -

c a t i on o f t he l a t te r a u t ho r s . I f Me a n os o re s a re r e fe r re d

t o a s t yp i c a l o f t he a re a , t he y c a n be i nc l ude d i n t he

" m a t u r e - i n t e r m e d i a t e t y p e " a s d e f in e d b y B o n n e m a i s o n

a nd M a rc o ux (1987) . G o l d o re s o f th i s t ype a re ve ry pu re

(90% Au ) a nd be c o m e p rog re s s i ve l y e n r i c he d i n s il ver

( 2 5 - 6 0 % A g ) w i t h l a t e r r e m o b i l i z a t i o n , a c c o r d i n g t o

t he se a u t ho r s .

T h e v a l u e s o b t a i n e d i n F e r v e n z a o r e s ( 6 - 3 5 % A g )

a l so a ppe a r t o c o r re spond ove ra l l t o t h i s c l a s s i f i c a t i on ,

t a k i ng i n t o a c c oun t t ha t , a c c o rd i ng t o t he t e x t u ra l e v i -

de nc e , d i ve r se e p i sode s o f suc c e s s i ve m ob i l i z a t i on a nd

c onc e n t ra t i on a re supe r i m pose d wi t h i n a s i ng l e de pos i t ,

g iv ing r ise to severa l gene ra t ion s o f na t ive go ld . There i s,

t he re fo re , a b roa d a g re e m e n t wi t h t he s e que nc e e s t a b -

l i she d i n t he a re a de sc r i be d : e a r l y a nd we a k m i ne ra l i z a -

t i on in R i a l; m a t u re i n Me a no s ; a l so m a t u re , b u t wi t h

pe rha ps m ore de ve l ope d l a t e r s t a ge s i n F ousa s de Vi l a

a nd Vi l a rc ovo ( r i c he r i n Ag , wi t h nugge t go l d ) .

The re a re s t i l l som e una nswe re d que s t i ons . A l t hough

t he t ype i s we l l known i n i t s e s se n t i a l f e a t u re s (C o l v i ne

e t al . 1984), m an y gen e t ic a spec ts a re s t il l in deb a te , su ch

a s the p re c i s e c onc e n t ra t i on m e c ha n i sm s , t he o r i g i n o f t he

hyd ro t he rm a l f l u id s , a nd t he u l t i m a t e sou rc e o f the go l d .

These wi l l not be d i scussed he re as de ta i led s tudies a re in

p rog re s s a nd know l e dge o f t he o re bo d i e s i s s ti ll i n a p re -

l iminary s ta te .

N e v e r t h e l e s s , t h e m o d e l p r o p o s e d f o r A r c h a e a n d e -

p o s i ts b y H o d g s o n ( 1 98 6 ) p r o v i d e s an a p p r o a c h t h a t c a n

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

a s we ll . The ge ne s i s o f the go l d o re bod i e s , a c c o rd i ng t o

t h i s m ode l , r e l a t e s t o c rus t a l e vo l u t i on t h rough a s e -

q u e n c e o f r if ti n g, s e d i m e n t a ti o n , d e f o r m a t i o n , p o r p h y r y

int rus ion , a l te ra t ion and minera l iza t ion . In th i s sequence ,

g ra n i ti c m a gm a s s e e m t o p l a y a n im p or t a n t ro l e in t he

m ob i l i z a ti on a nd f i nal c onc e n t ra t i on o f go ld , who se u lt i -

m a t e s o u r c e w o u l d b e in t h e m a f i c r o c k s o f t h e G r e e n -

s tone Be l t .

P re se n t i de a s a bou t t he ge o l ogy o f we s t e rn G a l i c i a

a l l ow a c om pa r i son wi t h t h i s m ode l , wh i l e s t i l l spe c u l a -

t i ve . The y i m p l y c on t i ne n t a l r i f t i ng (Le fo r t a nd R i be i ro

1 98 0) an d d e v e l o p m e n t o f an O r d o v i c i a n " p r o t o - o c e a n "

t o t he W e s t ; t he e m pl a c e m e n t o f t he Ma l p i c a -Tu i Un i t

(M TU ) i s e xp l a i ne d , a c c o rd ing l y , by a n e a s t wa rds - t h rus t -

i ng o f t he t h i nne d c rus t a s a r e su l t o f c on t i ne n t a l c o l l is i on

i n De von i a n t i m e s (Or t e ga a nd Gi l 1983) . The p re se n t

b o u n d a r i e s o f t h e M T U a r e m a j o r , s u p e r i m p o s e d n o r m a l

fa u l t s , a nd g ra n i t o i d bod i e s ( s e e F i g . 2 ) whose i n t ru s i on

wa s l a rge l y c on t ro l l e d by t he se f r a c t u re s ( IGME 1984) .

The g ra n i t i c m a gm a s c ou l d a c c oun t fo r t he e x t ra c t i on

a n d m o b i l i z at i o n o f g o l d fr o m M T U a n d b a s e m e n t r o c k s

a t de p t h , a nd fo r i ts r e d i s tr i bu t i on b y hyd ro t he r m a l so l u -

t i ons, r e l a t e d t o l a t e - s ta ge de fo rm a t i on . Th i s e xp l a i n s t he

c h a r a c t er o f th e e a s te r n M T U b o u n d a r y a s a s ig n i fi c an t

go l d m e t a l l o t e c t (F i g . 1 ) . The W e s t bounda ry i s l a rge l y

se a le d , a l ong m ore t ha n 100 km , b y a ba r re n g ra n od i o r i t -

i c body .

E x p l o r a t i o n o r e d r e s s i n g

S o m e o f t h e a b o v e - m e n t i o n e d f e a tu r e s, a d d e d t o t h e

usua l m u l t i - e l e m e n t ge oc he m i s t ry , ge ophys i c s , e t c . , c a n

be use fu l gu i de s fo r exploration in the a rea :

R e l a t i o n t o i m p o r t a n t r e g io n a l s tr u c tu r e s , th r u s t s a n d

faul t s ; ident i f ica t ion of shea r-zones , e spec ia l ly wi th su-

pe r i m pose d a nd re pe a t e d l a t e c a t a c l a s i s a nd hydro t he r -

mal infill.

- P re se nc e o f su l ph ide s , e spe c i al l y m a ss i ve ( t ype I I I ) a r -

s e nopyr i t e wi t h B i su l phosa l t s , a c c om pa n i e d by qua r t z ,

adula r ia , tourmal ine , graphi te , schee l i t e , e tc .

A l t e r a t i o n p h e n o m e n a i n t h e c o u n t r y r o c k s : p o t a s s i c

a l te ra t ion , se r ic i t iza t ion , s i l i c if ica t ion; loca l ly tou rm a-

l ine , chlor i te or ca rbona te a l te ra t ion .

- P re se nc e o f g ra n i ti c i n t ru s i ve s o r dyke s .

The m i ne ra l og i c a l a nd t e x t u ra l c ha ra c t e r i s t i c s obse rve d

a l lo w a l s o s o m e c o n c l u si o n s t o b e d r a w n a b o u t t h e pro

cessing of t he o re s .

F i r s t l y , t he c l o se a s soc i a t i on o f na t i ve go l d wi t h a r -

s e nopyr i t e sugge s t s t ha t t he ove ra l l Au g ra de c a n be i m -

p r o v e d b y p r e c o n c e n t r a t i o n ( f l o ta t io n ) o f t h e s u l p h o - a r -

s e n ide . Th i s shou l d be e a sy i n v i e w o f t he dom i na n t t ype

o f a r s e nop yr i t e a s so c i a t e d w i t h go l d , i .e . m a s s i ve , r e l a-

t i vel y c oa r se -g ra i ne d a r s e nopy r i t e , de sc r i be d a s t yp e I I I

(see textures, Fig. 3).

S e c ond l y , t he f i ne g ra in - s iz e o f t he na t i ve g o l d pose s

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


11/11

S 52

D i r e c t c y a n i d a t i o n w i ll g iv e p o o r r e s u lt s a s t h e g o l d

l o c k e d i n a r s e n o p y r i t e r e m a i n s r e l a t i v e ly i n a c c e s s ib l e t o

t h e r e a g e n t .

F o r t h e s e r e a s o n s i t is a d v i s a b l e t o c o n s i d e r u n c o n -

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

s u c c es s fu l ly in P o r g e r a , P a p u a N e w G u i n e a - o r b a c te r i a l

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

i n g t h e e n v i r o n m e n t a l i m p a c t o f t h e o t he r w i s e n e c e s s a r y

r o a s t i n g o f t h is m a t e r i a l ; n e v e r t h e l e s s t h is i m p a c t c a n a l s o

b e l i m i te d b y t r e a t m e n t o f t h e fu m e s . T h e s e m e t h o d s a r e

d e s c r i b e d b y W e i r a n d B e r e z o w s k y ( 1 9 8 4) , O l l iv i e r (1 9 8 6 )

a n d L e s o i ll e ( 1 9 87 ) , re s p e c t iv e l y . T h e l a s t - m e n t i o n e d o f

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

t h e g o l d o r e s f r o m S a l s i g n e ( F r a n c e ) , a l s o r i c h i n a r -

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

b e i n g t e s t e d . I n e v e r y c a s e t h e o b j e c t i v e is t o o b t a i n a

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

a n a c c e p t a b l e y i e l d .

onc lus ions

G o l d d e p o s i t s o f t h e F e r v e n z a a r e a a r e h y p o g e n e e p i g e-

n e ti c d e p o s i ts o f th e m e s o - k a t a t h e r m a l r a n g e b o u n d t o

l a t e - s ta g e s o f m a i n l y b r it t le d e f o r m a t i o n . T h e y o c c u r i n

s h e a r - z o n e s a ff e c t in g H e r c y n i a n m e t a m o r p h i c t e r ra n e s

a n d i n t r u s i v e g r a n i t i c r o c k s .

M i n e r a l o g y c a n b e l o c a ll y c o m p l e x , b u t o n l y a fe w

m i n e r a l s a re a b u n d a n t ; m a i n l y q u a r t z , a d u l a r ia , a r -

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

b i s m u t h s u l p h o s a l t s , c h l o r i te , g r a p h i t e , s c h e e li t e o r t o u r -

m a l i n e . N a t i v e g o l d o c c u r s u s u a l l y i n f i n e - g r a i n e d , m i -

c r o n - s i z e d p a r t i c le s . I t c o n t a i n s v a r i a b l e a m o u n t s o f si l-

v e r ( 6 - 3 5 % ) a n d t e n d s t o b e a ss o c i a t e d w it h a r s e n o p y r i te

a n d w i t h b i s m u t h o r e s . T h e g e o m e t r i c c l a s s if i c a t io n o f

o r e t e x t u r e s i n F i g. 3, w h i c h h e l p s c o m p a r e t h e d i f fe r e n t

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

u s e f u l f o r t h e t e x t u r a l c h a r a c t e r i z a t i o n o f o r e b o d i e s r e -

l a t e d t o s h e a r z o n e s i n o t h e r d i s t r i c t s a s w e l l. F l o t a t i o n i s

s u g g e s te d a s a n e a s y w a y t o i m p r o v e A u g r a d es ; p r i o r t o

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

w i t h n o n - c o n v e n t i o n a l m e t h o d s , s u c h a s p r e s s u re l e a c h -

i n g o r b a c t e r i a l l e a c h i n g .

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

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

a s s o c i a te d h y d r o t h e r m a l c i r c u la t i o n, t y p ic a l o f v e in - t y p e

g o l d d e p o s i t s i n s h e a r z o n e s , i n A r c h a e a n a s w e ll a s i n

H e r c y n i a n t e r ra n e s . I f c o m p a r e d w i t h t he f o r m e r , t h e r e

e x is t h o w e v e r s o m e n o t e w o r t h y d i f fe r e n c e s ; e. g. d e p o s i t s

o f t h e S u p e r i o r G r e e n s t o n e B e l t ( H o d g s o n 1 98 6 ) a r e la r g e

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

i n te n s iv e c a r b o n a t e a l t e r a t io n a n d a b u n d a n c e o f m a f i c

v o l c a n i c s ( a b o u t 6 5 % ) , i n c o n t r a s t t o s m a l l e r m e t a l c o n -

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

c a r b o n a t e a l t e r a ti o n a n d le ss a b u n d a n t m a f i c v o l c a n ic s in

t h e F e r v e n z a a r e a . T h e r e a r e s o m e c l o se s im i l a r it i es w i t h

o t h e r E u r o p e a n H e r c y n i a n d e p o s i t s , e . g . L e B o u r n e i x

t y p e, a s d e f in e d b y B o n n e m a i s o n a n d M a r c o u x ( 19 87 ).

Acknowh dgem ents.

T h e a u th o r w i sh es to t h a n k E N A D I M S A / I N I

for permission to publish this work; l ikewise, all his colleagues in

that company, for the information and facil i t ies given, especially T.

Noda l , au tho r o f the pet rographic works consu lted , and A. Boni l la ,

for his cri tical discussion of ore-dressing. He also w ishes to thank

Drs. M. Bonnemaison and E. Mareoux (BRGM, Orleans, France) ,

fo r their in form at ion and d iscussions abou t Europe an Herc ynian

gold deposit s . The ass is tance of Prof . C.J . Hodgs on (Qu een 's Uni-

versi ty , K ingston On tar io ) dur ing f ie ldwork in the sum mer o f 1987

is also gratefully acknowledged; the auth or benefit ted very much

from Prof . Hod gso n 's experience and deep knowledge o f Arch aean

gold deposits.

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