Scanning electron microscopy (SEM) and its applications

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Transcription

Scanning electron microscopy (SEM) and its applications
-.- - .
Anais do VI1 Congresso Latino-Americano de Geologia, Belém, Par& NW3, V.l
SCANNING ELECTRON MICROSCOPY (SEM) AND ITS APPLICATIONS: DETERMINATION
OF SOLID AND DAUGTHER MINERALS IN FLUID INCLUSIONS FROM SOME
BRAZILIAN EMERALD DEPOSITS
Gaston Giuliani - Departamento de Geocitkias, ORSTOM, Institut Français de Recherche Scientifique pour Je ddveloppement en Coopération, France, CRPG, Centrede Recherches
Pétrographiques et Gécchimiques de Nancy, France
Alain Weisbrod ENSG, Ecole Nationale Sup6rieure de Gedlogie de Nancy, France, CRPG, Centre de
Recherches Pétrogmphiques et Géochimiquede Nancy, France.
-
RESUMO
Informações quimicas e morfológicas, o b t i d a s ao microscópio
e l e t r ô n i c o de v a r r e d u r a (MEV) , com p r é v i a s observações
Ópticas,
permitiram não SÓ o b t e r novos dados gemológicos a r e s p e i t o d a s
esmeraldas b r a s i l e i r a s , m a s sobretudo i d e n t i f i c a r o s m i n e r a i s de
s a t u r a ç ã o - M S - de s u a s i n c l u s õ e s f l u i d a s - I F
Dois t i p o s de j a z i d a
foram estudadas:
a q u e l a s c u j a r e l a ç ã o g e n é t i c a com g r a n i t ó i d e s 6
evidente,
exemplos de Carnaiba e SocotÓ ( B a h i a ) , o u t r a s c u j o s r e l a ç õ e s
com v e i o s p e g m a t i t i c o s são duvidosas, como em S a n t a Terezinha e
I t a b e r a i (Goiás). Nestas j a z i d a s , a esmeralda 6 a s s o c i a d a a zonas de
f l o g o p i t i t o s c u j o zoneamento metassomático r e s u l t a d a p e r c o l a ç ã o de uma
solução em r o c h a s p o r um s i s t e m a de p o r e s f i n o s . ReaçÕes quimicas n a
rocha permitiram a l t e r a ç õ e s na composiçao da solução e a s s i m uma
-.
-
mudança n a s i n c l u s õ e s s ó l i d a s e m i n e r a i s d e s a t u r a ç ã o a p r i s i o n a d o s p e l a
esmeralda. E s t a s reaçÕes metassomaticas e n t r e f l u i d o s r i c o s em Be e
formações b á s i c a s r i c a s em C r , Mg, F e , produzem e m t o d a s as j a z i d a s
e s t u d a d a s a formação de
flogopita,
anfibolio
(tremolita
e/ou
a c t i n o l i t a ) , esmeralda e s u l f e t o s ( p i r i t a , c a l c o p i r i t a e p e n t l a n d i t a ) .
de
As d i f e r e n ç a s e n t r e as v á r i a s j a z i d a s e s t u d a d a s são: In/ nos t i p o s
i n c l u s õ e s f l u i d a s a s s o c i a d a s : esmeraldas de Carnaiba e SocotÓ mostram
geralmente, i n c l u s õ e s t u b u l a r e s com v á r i a s formas ( a g u l h a s , t u b o s ,
c a n a i s hexagonais ou sob a forma de “concha de belemniteIi, enquanto que
e m S a n t a Terezinha e I t a b e r a i , as cavidades de I F são raras e
desenvolvidas a p a r t i r de inclusÕes s ó l i d a s ; 2 2 1 n o s s ó l i d o s e MS: a / o s
x i s t o s carbonatados r i c o s em C r de S a n t a Terezinha, permitiram a
formação e o aprisionamento de dolomita, a n k e r i t a , & f i b o l i o magnesian0
._
e magnésiocromita. b ) SocotÓ e Carnaiba mostram que as esmeraldas
r e l a c i o n a d a s aos v e i o s p e g m a t i t i c o s são r i c a s e m M S sob a forma de s a i s
complexos. O exemplo de SocotÓ 6 bem c l a r o , podendo-se
observar a
evolução do f l u i d o desde o v e i o p e g m a t i t i c 0 ( I ) a t é a ’ s u a zona de
f l o g o p i t i t o (II) a d j a c e n t e :
Em I , o q u a r t z o d a m a t r i z mostrou
minerais de s a t u r a ç ã o t i p o K, C1 i d e n t i f i c a d o s como s i l v i t a e também
sais t i p o C a , C 1 , Fe, t o d o s . a s s o c i a d o s com i n c l u s õ e s f l u i d a s do t i p o S
(i.é. I F p o l i f a s i c a s com s ó l i d o ) .
A esmeralda desenvolvida no I ,
a p r e s e n t a também m i s t u r a s de sais complexos com v á r i a s composições: Fe,
C 1 ; Ca,Cl,Mn,Fe; Fe, C1,Mn; Ca,Cl,Fe,Ba,K.
N a esmeralda s i t u a d a no
- - f l o g o p i t i t o ( I I ) , o s MS são: C a , Cl,Fe,Mn,K; Cl,Mg,Fe;
Cl,Na,Mg;
eles
mostram também a p r e s e n ç a quase c o n s t a n t e de Mg, elemento móvel,
fornecido p e l o s s e r p e n t i n i - t o s durante a p e r c o l a ç ã o do f l u i d o . N o caso
de , S a n t a Terezinha de Goiás e . I t a b e r a i , o s M S são quase a u s e n t e s
mostrando que as s o l u ç õ e s eram d i l u i d a s .
-
-
-
445
INTRODUCTION
-
The i d e n t i f i c a t i o n of s o l i d i n c l u s i o n s as t r a p p e d or as
d a u g h t e r m i n e r a l s i s always of academic i n t e r e s t (Kalyuzhnyi, 1958;
Roedder, 1972, 1984). Roedder (1972) e x p l i c i t e d t h e methodology t o
i d e n t i f y s o l i d m i n e r a l s (s.1) b u t o p t i c a l means' are always ' l i m i t e d by
t h e s i z e , i m b r l c a t e d occurrence and sometimes xenomorphic shape of t h e
m u l t i s o l i d i n c l u s i o n s . Successful determinations a r e possible i n the
c a s e of microchemical, c r i s t a l l o g r a p h i c and o p t i c a l means (Rankin and
Le B a s , 1974; Coveney and K e l l y , .1971). Metzger e t a l .
(1977)
demonstrate t h a t scanning e l e c t r o n microscopy (SEM) p r o v i d e d a powerful
t o o l f o r t h e s t u d y of daughter m i n e r a l s i n f l u i d i n c l u s i o n s and t h e i r
s u c c e s s f u l approach w z s a p p l i e d t o complex m u l t i s o l i d i n c l u s i o n s from
v a r i o u s environments âs c a r b o n a t i t e s . ( N e s b i k t and K e l l y ,
1977) ,
t i n - t u n g s t e n m i n e r a l i z a t i o n s (Kwak and Tan, 1980; C h e i l l e t z , 1984) or
porphyry copper d e p o s i t s (Le B e l , 1979; Wilson e t a l . , 1 9 8 0 ) .
Nature of s o l i d i n c l u s i o n s i n gemstones i s o b v i o u s l y of
p e c u l i a r i n t e r e s t f o r gemologists, r e l a t i v e l y t o t h e q u a l i t y of t h e gem
enhancing t h e v a l u e or d e s v a l u i n g i t ,
specifiying
it
origin,
c h a r a c t e r i z i n g i t geochemical growing environment and d i s c r i m i n i n g it
w i t h s y n t h e s i z e d gems. O p t i c a l , X-ray and e l e c t r o n microprobe a r e t h e
u s u a l gemmological methods a l l o w i n g t o s p e c i f i c a t i o n of
mineral
i n c l u s i o n s i n gems (Sinkankas and Read, 1 9 8 6 ) . Complete d e t e r m i n a t i o n
of t h e s e s o l i d i n c l u s i o n s r e q u i r e s e v i d e n t l y more s o p h i s t i c a t e d method,
i d a t t h e moment, no work w a s done u s i n g SEM meth0.d.
Between a l l t h e gems, emerald i s of first importance i n
B r a z i l and gemmological s t u d i e s on s o l i d i n c l u s i o n s are s t i l l r e a l i z e d
(Schwarz, 1984; Schwarz and Mendes, 1985;e Schwarz, 19861, evidencing
r e p o r t s of u n i d e n t i f i e d daughter or t r a p p e d m i n e r a l s . P r e l i m i n a r y s t u d y
on f l u i d i n c l u s i o n s for some emerald d e p o s i t s from B r a z i l ( G i u l i a n i , i n
p r e p a r a t i o n ) , demonstrate t h a t daughter m i n e r a l s d i s p l a y a remarkable
and c o n s t a n c y amounts w i t h i n - i n c l u s i o n s b u t w i t h d i f f e r e n c e s f o l l o w i n g
t h e type of deposit.
It t h i s p a p e r , we demonstrate - i n s p i t e of some l i m i t a t i o n s
and a m b i g u i t i e s i n t h e d e t e r m i n a t i o n of t h e daughter m i n e r a l s , t h a t
SEM*
technique h a s useful applications n o t s o i n f l u i d inclusion
i n t e r p r e t a t i o n s as determining morphology s i z e , s p a t i a l r e p a r k i t i o n of
f l u i d i n c l u s i o n c a v i t i e s and d i s p l a y i n g a c o m p o s i t i o n a l l i m i t for t h e
o r i g i n a l f l u i d , b u t a l s o i n gemmology p e r m i t i n g some unambiguous
i d e n t i f i c a t i o n of s o l i d i n c l u s i o n s .
EMERALD MINERALIZATIONS O F BRAZIL
-
B G a z i l r e p r e s e n t s t h e most r e p r e s e n t a t i v e world producer i n
volume and i n emerald q u a l i t y . Nowadays, f o u r economic emerald d e p o s i t s
a r e i n p r o d u c t i o n ( F i g . 1): t h e p r o s p e c t i n g p i t s of Carnaiba and SocotÓ
( B a h i a S t a t e ) , S a n t a Terezinha de Goiás (Goiás S t a t e ) and t h e I t a b i r a
mine (Minas Gerais S t a t e ) . I n a d d i t i o n , o t h e r o c c u r r e n c e s are of minor
economic s i g n i f i c a n c e .
Typical
emerald
mineralization
5s
associated
with
b i o t i t e - s c h i s t r o c k s r e s u l t i n g from t h e chemical i n t e r a c t i o n between
g r a n i t o i d i c r o c k s w i t h i n b a s i c and u l t r a b a s i c f o r m a t i o n s : t h e s o c a l l e d
schist
t y p e b e r y l d e p o s i t (Sinkankas and Read, 1986) o r glimmerite
-
i
,.
* t h e s t u d y w a s r e a l i z e d on a Cambridge S t e r e o s c a n 250 SEM, V
=-25
M,
m u l t i c h a n n e l a n a l y s e s PGT System III.
446
' I
(Smirnov, 1977). B r a z i l i a n emerald d e p o s i t s belong t o two types
(Schwarz, 1986): I type: a s s o c i a t i o n mafic-ultramafic r o c k s (or t h e i r
metamorphic d e r i v a t e s ) w i t h p e g m a t i t i c v e i n s ; II type:
absence of
pegmatitic formations.
Four d e p o s i t s were chosen t o i l l u s t r a t e t h e a p p l i c a t i o n of
t h e SEM techniques. The f i r s t s are t h e p r o s p e c t i n g p i t s of SocotÓ and
Carnaiba, t y p i c a l and c l a s s i c emerald t y p e m i n e r a l i z a t i o n s ( t y p e I ),
a l r e a d y d e s c r i b e d by Cassedanne (1985) and Rudowski e t a l . (1987 a , b ) ;
secondly, t h e d e p o s i t of S a n t a T e r e z i n h a de g o i á s (Cassedanne and
S a u ë r , 1984; Costa, 1986) and t h e o c c u r r e n c e of T t a b e r a i ( t y p e II)
(Leinz and Leonardo, 1936) i n which t h e l a c k of p e g m a t i t i c v e i n s
assumes a non elmagmaLice' o r i g i n f o r b e r y l l i u m (Schwarz, 1 9 8 6 ) .
as
ja ;
'to
IbY
he
he
Pd
7)
11
r
m
- -
INCLUSION-BEARING M ~ ~ ~ E R A LFROM
S
EMERALDS OF CARNAÍBA,
TEREZINHA DE GOIÁS AND ITABERAÍ
PROSPECTXNG PITS.
'L
r
.socor6,
SANTA
I
F '
D e p o s i t s from Bahia S t a t e
I
1
i
l
I
\
i
I
L
Emeralds* from C a r n a i b a can b e e a s i l y d i s c r i m i n a t e d from
t h o s e of SocotÓ. G e n e r a l l y , t h e y e x h i b i t a n d hexagonal prism w i t h a
s i z e up t o 1 0 cm l o n g and w i t h a d i a m e t e r v a r y i n g from 0.5 t o 10 cm..
The c r y s t a l s which grown from a b i o t i t i c or f e l d s p a t h i c m a t r i x show
sometimes t h e p i n a c o i d (0001) t e r m i n a t i o n .
Under t h e microscope, t h e s e c t i o n s show t h e p r e s e n c e of a
r e g u l a r coloured zoning developed a l o n g t h e c - a x i s , and a l s o pyramids
and s t r i a e s of growing, a l r e a d y d e s c r i b e d
b y - Schwarz
(1984).
Microscopic and SEM o b s e r v a t i o n s evidenced a g r e a t
fracturation
a f f e c t i n g emerald d u r i n g and a f t e r i 9 f o r m a t i o n , c a u s i n g a diminution
of t h e c r y s t a l t r a n s p a r e n c y . B e s i d e s , r e h e a l i n g of t h e s e f r a c t u r e s ,
whi'ch a r e g e n e r a l l y o r i e n t e d p e r p e n d i c u l a r t o
the
c-axis,
is
m a t e r i a l i z e d by alignment of b i p h a s e d i n c l u s i o n s ( g a s .+ l i q u i d ) . These
secondary i n c l u s i o n s a r e dominant b u t i t i s always p o s s i b l e t o observe
primary f l u i d i n c l u s i o n s k e p t d u r i n g t h e emerald growth; t h e y c o n s i s t
i n t h i n t u b e s ( 5 0 t o 100 pm l o n g ) d i s p o s e d along t h e c - a x i s
and
i n f i l l e d with a l i q u i d and a g a s phase (which volume ( V g ) r e p r e s e n t s 40
t o 80% of t h e t o t a l volume of t h e i n c l u s i o ? ) .
Some of t h e s e primary
i n c l u s i o n s a r e m u l t i p h a s e d , and p r e s e n t d a u g h t e r m i n e r a l s i . e m i n e r a l s
or s o l i d phases which c r y s t a l l i z e d from t h e f l u i d upon c o o l i n g .
U n f o r t u n a t e l y , SEM o b s e r v a t i o n s do n o t reached t h e s e d e s i r a b l e daughter
m i n e r a l i n c l u s i o n s and f u r t h e r i n v e s t i g a t i o n s a r e n e c e s s a r y f o r t h e
knowledge o f - t h e s e components p r e s e n t as d i s s o l v e d c o n s t i t u e n t s i n t h e
f l u i d i n c l u s i o n d u r i n g t h e entrapment. S o l e , p h l o g o p i t e which forms a
- c o n s t a n t p r o t o g e n e t i c o r s y n g e n e t i c i n c l u s i o n w a s observed ( P l a t e 1
Fig. 5 ) .
P r o t o g e n e t i c i n c l u s i o n s or c r y s t a l l i n e s o l i d i n c l u s i o n s a r e
relatively rare,
except
phlogopite.
Schwarz
(1984)
evidenced
t o u r m a l i n e , a l b i t e , molybdenite, l e p i d o c r o c i t e , g o e t h i t e , emerald and
t r e m o l i t e ( ? ) . The s t r a i g h t a s s o c i a t i o n wih molybdenite i s an i m p o r t a n t
f e a t u r e of Carnaiba emerald. A t t h e moment, Carnaiba d i s t r i c t assumes
t h e main b r a z i l i a n p r o d u c t i o n of molybdenum; t h e Mo-mineralization
is
w e l l - e x p r e s s e d n e a r t h e c o n t a c t of t h e g r a n i t e , and t h e Marota
p r o s p e c t i n g p i t became a Mo-mining a r e a : h e r e , r o s e t t e s ,of molybdenite
( c m t o 5 c m wide) a s s o c i a t e d w i t h g r e e n b e r y l are f r e q u e n t . We can n o t e
also emerald i n c l u s i o n s i n t o emerald crystals ( P l a t e 1, f i g . 6 ) .
Emeralds from SocotÓ d i f f e r by t h e s i z e of t h e i r hexagonal
p r i s m s which a r e s m a l l e r (up t o 5 cm l o n g , 0.2 t o 1 cm l a r g e ) , t h e i r
* emerald i n t h e s.1, i n c l u d i n g a l s o g r e e n b e r y l
447
.
.
.
-
t r a n s p a r e n c y r e l a t e d t o a minor f r a c t u r a t i o n and i t s more i n t e n s e g r e e n
e d e r a l d from p h 1 , o g o p i t i t e s (E1) and from
c o l o r a t i o n . For t h i s st-,
a s s o c i a t e d a l b i t i c - p e g m a t o i d v e i n s (E2) were s t u d i e d , i n c l u d i n g . a l s o
t h e h y a l i n e q u a r t z of t h e pegmatoid v e i n s ( Q z ) .
EMERALD - Growth s t r u c t u r e s a r e c h a r ä c t e r i z e d by t h e
following
f ea t u r e s :
growing zones a r e u n d e r l i n e d .by a t y p i c a l zoning marked by a v a r i e t y
of c o l o u r e v o l u i n g from p a l e t o i n t e n s e g r e e n t o n e s . The v a r i a t i o n i n
c o l o u r from e v e r y zone i s n o t p r o g r e s s i v e , and g e n e r a l l y d a r k zones
. e x h i b i t a g r e a t c o n c e n t r a t i o n o f protogenetic.phlogopites..
growing zones of E 1 and E2 a r e . u n d e r l i n e d by primary f l u i d i n c l u s i o n s
which can be' d i v i d e d i n two k i n d s :
- t u b u l a r i n c l u s i o n s a r e t h e commonest and t h e y r a n g e i n form from
huge and hexagonal t u b e s t o l o n g hairs or n e e d l e s . These l a s t form
alignments p a r a l l e l t o t h e c-axis
and t h e s i z e of t h e n e e d l e s
v a r i e s up t o 100 pm. They a r e b i p h a s e d and w i t h sometimes s o l i d and
d a u g h t e r m i n e r a l s ( P l a t e 2, F i g . 2 , P l a t e 1, Fig. 1). The morphology
of t h e c a v i t i e s a r e n o t always r e g u l a r and s w e l l i n g s a r e common.
These f i n e t u b e s a r e o f t e n a s s o c i a t e d t o zones e n c l o s i n g a l a r g e
number of g r e a t e r t u b e s , g i v i n g r i s e t o a diminution of t h e c r y s t a l
t r a n s p a r e n c y and o c c a s i o n i n g t h e s o c a l l e d " r a i n e f f e c t " .
These
t u b e s p r e s e n t a c o n i c a l form l i k e a "belemnit r o s t r e t 1 . w i t h an
e x t e n s i o n up t o 200 pm ( P l a t e 2 , F i g . 3 ) . They can o c c u r a l s o as
c h a n n e l s w i t h sub-hexagonal s e c t i o n s ( P l a t e 2 , Fig. l), w i t h a s i z e
of 100 t o 150 pm and an e x t e n s i o n up t o 1 mm. Under t h e microscope,
t h e y appear brown-dark c o l o u r e d ; SEM a n a l y s e s from t h e w a l l s of
t h e s e c h a n n e l s d i s p l a y o n l y Fe, probab13 i r o n o x i d e s ' o r h y d r o x i d e s ;
f r e q u e n t l y , s y n g e n e t i c p h l o g o p i t e s , . as encountered i n t h e f i n e
t u b u l a r i n c l u s i o n s a r e seen.
A l l t h e t u b u l a r i n c l u s i o n s a r e g e n e r a l l y i n f i l l e d by a two
phased f l u i d ( l i q u k d t. g a s ) w i t h s o l i d i n c l u s i o n s and d a u g h t e r
m i n e r a l s . X-ray s p e c t r a evidenced abundant p h l o g o p i t e ( S i , A l , K y
Fe, Mg components); p e r s i s t e n t -Fe; C 1 peaks s u g g e s t i n g
the
e x i s t e n c e of Fe h y d r o x y - c h l o r i d e s ,
o r Fe c h l o r o c a r b o n a t e s clr
oxy-chlorides, corresponding i n t h i n s e c t i o n t o p r i s m a t i c high
r e l i e f m i n e r a l s w i t h a moderate r e f r i n g e n c e ; i r o n peak a l l o w i n g t h e
p r e s e n c e of Fe o x i d e s or hydroxydes; C 1 , N a (Mg) w i t h an amorphous
form; C a , C 1 , Mn, Fe e l e m e n t s ( P l a t e 1, F i g . 1) a n a l y s e d i n a
rounded pseudo-rhombic c r y s t a l e x p e c t i n g a more complex phase t h a n
CaC12.6H20; e p i d o t e w i t h S i , C a , A l ,
Fe components; emerald
p r e s e n t i n g i t hexagonal p r i s m a t i c h a b i t w i t h an X-ray
analysis
showing S i , A l , Cr,Fe ( P l a t e 2 , F i g . 6 ) .
- i n d e n t e d o r s w e l l e d f l u i d i n c l u s i o n s a r e a l s o f r e q u e n t and p r e s e n t
a s i z e r a n g i n g from 5 t o 30 pm. They are d i s p e r s e d r e g u l a r l y i n ' t h e
h o s t c r y s t a l w i t h sometimes an aligment p a r a l l e l -to t h e c - a x i s .
They correspond t o m u l t i s o l i d i n c l u s i o n s w i t h a l i q u i d , g a s phase
(Vg occupying 40 t o 50% of t h e t o t a l volume of t h e i n c l u s i o n ) .and
d a u g h t e r m i n e r a l s , X-ray s p e c t r a o b t a i n e d w i t h t h e SEM r e v e a l e d t h e
p e r s i s t e n c e of complex s a l t s c o v e r i n g t h e w a l l s of t h e c a v i t i e s or
minute c r y s t a l s . P l a t e 2, F i g u r e 4 shows t h e p r e s e n c e of C 1 , F e , Mn
s a l t s a s s o c i a t e d w i t h p h l o g o p i t e and rhomboedcic c a r b o n a t e s ( C a ,
M n , Fe or C a , M n ) s u g g e s t i n g t h e p r e s e n c e of a n k e r i t e .
I n other
c a s e s , ( P l a t e 1, F i g . 2 ) , t h e f o l l o w i n g elements were analysed: C a ,
C 1 , Fe, Mn, K and a l s o C a , C 1 , Fe, B a , K ; C a and C 1 a r e t h e m a i n
elements o f t h e s e p r i s m a t i c , rounded, pseudocubic or l a m e l l a r s o l i d
s a l t s . Another t y p e o f s a l t w a s encountered p r e s e n t i n g an amorphous
h a b i t and formed w i t h C 1 , Mg and i r o n . O t h e r m o r e common d a u g h t e r
-
448
.
4
p-
minerals a r e a c t i n o l i t e ( P l a t e 2 , Fig.
51, phlogopite a s s o c i a t e d ,
with i r o n c a r b o n a t e s (Ca, Fe) and probably'. magnetite showing an
o c t a e d r a l from (Fe o n l y ) .
P r o t o g e n e t i c i n c l u s i o n s which correspond t o s o l i d p a r t i c l e s
p r e s e n t i n suspension i n t h e f l u i d a r e common and i d e n t i f i e d as
phlogopite, t r e m o l i t e and a c t i n o l i t e . These i n c l u s i o n s a r e formed on
t h e c r y s t a l s u r f a c e s or can have been s e t t l e d o u t of t h e f l u i d .
Generally, they a c t as n u c l e i i f o r t h e development o f t u b u l a r primary
f l u i d i n c l u s i o n s which extend upward along t h e c-axis b e f o r e becoming
bridged by t h e - g r o w t h of t h e c r y s t a l . T h i s b r i d g i n g o v s r w i l l depend
n a t u r a l l y on t h e v e l o c i t y of t h e growing. A consequence of t h i s
phenomena i s . t h a t t h e s e p r o t o g e n e t i c i n c l u s i o n s can b e entrapped as
s o l i d i n c l u s i o n s and b e confused w i t h daughter minerals,
QUARTZ - q u a r t z from emerald-bearing a l b i t i c pegmatoid v e i n s w a s a l s o
i n v e s t i g a t e d . Under t h e microscope,
t h r e e major t y p e s of
fluid
i n c l u s i o n s were observed:
- a m u l t i s o l i d type ( t y p e S ) p r e s e n t i n g a l i q u i d phase > t o t h e vapor
phase (Vg = 20-30% of t h e t o t a l volume of t h e i n c l u s i o n ) , w i t h one or
two daughter m i n e r a l s . The main abundant and p e r s i s t e n t i s s y l v i t e
(K,C1 peaks on SEM) w i t h a pseudocubic h a b i t and a b r i g h t y e l l o w
c o l o u r ; a n c t h e r unknown mineral which w a s a l r e a d y encountered i n t h e
emerald t y p e E p , i s a white coloured p r i s m a t i c s o l i d which X-ray
s p e c t r a evidences s o t h e C 1 peak ( P l a t e 1, F i g . 3 ) . F i n a l l y , mixtures
of complex s a l t s were evidenced such C a , C 1 , Fe ( w i t h C a = C 1 i n
ccmposition): i t forms g e n e r a l l y , a g r e g a t e s widespread i n t h e c a v i t y
( P l a t e 1, F i g . 4 ) .
- a f l u i d i n c l u s i o n t y p e w i t h a l i q u i d T h a s e c t o t h e vapor phase (Vg =
80-90%; type V ) . Types V and S a r e c l o s e l y a s s o c i a t e d and d i s t r i b u t e d
r e g u l a r l y i n t h e m a t r i x or sometimes l o c a l i z e d on t h e same f r a c t u r e s .
a t h i r d type c h a r a c t e r i z e d by a l i q u i d phase r e p r e s e n t i n g 50 t o 60%
of t h e t o t a l volume of t h e i n c l u s i o n ( t y p e L ) which i s s c a t t e r e d
within t h e quartz.
-
DEPOSITS FROM GOIÁS STATE
Emeralds from S a n t a T e r e z i n h a de Goiás a r e l i t t l e s i z e d
hexagonal prisms (3-5 cm l o n g , 0 . 2 t o 1 c m l a r g e ) , w i t h g e n e r a l l y a
p a l e green c o l o u r . Under t h e microscope, i t s show an i r r e g u l a r
d i s t r i b u t i o n of c o l o u r s and a l s o i n s i g n i f i c a n t zoning.
S t r i a e s of
growing can be e a s i l y observed b u t primary t u b u l a r i n c l u s i o n s , as
observed i n Carnaiba and SocotÓ a r e r a r e . SEM method p e r m i t s t o confirm
t h e s c a r c e t y of t h e s e f l u i d i n c l u s i o n s and t h e constancy of s o l i d
inclusions.
Precoce f l u i d i n c l u s i o n s a r e widespread i n t h e c r y s t a l ;
they
a r e small. ( s i z e c 1 0 pm) and v e r y o f t e n c o n t a i n s ò l i d i n c l u s i o n s and
daughter m i n e r a l s . The most abundant s o l i d i n c l u s i o n s a r e c a r b o n a t e ,
s p i n e l and p h l o g o p i t e , as d e s c r i b e d by Schwarz and Mendes (1985):
- carbonates a r e g e n e r a l l y evidenced by t h e i r rhomboedric c l e a v a g e s
( P l a t e 3 , Fig. 1 ) ; X-ray s p e c t r a i n d i c a t e t h e presence of dolomite
( C a , Mg, Fe; C a , Fe,Mg,
Mn) w i t h Fe and Mn s u b s t i t u t i o n s , w i t h
sometimes C a , Fe, Mn compositions a l l o w i n g t o c o n s i d e r t h e m i n e r a l as
ankerite.
- another c o n s t a n t s o l i d i n c l u s i o n ' encountered a l s o as daughter mineral
i s a member of t h e s p i n e l group: t h e magnesiochromite ( P l a t e 3, Fig.
-3 and 4 ) . It i s p r e s e n t i n a l l t h e s t u d i e d emeralds w i t h major l o c a l
c o n c e n t r a t i o n . It g e n e r a l l y e x h i b i t s a subautomorphic shape b u t
sometimes an o c t a e d r a l form. X-ray a n a l y s i s i n d i c a t e C r , Fe, Mg. N o
449
I,
*
alumina w a s d e t e c t e d and t h e s e d a t a a r e i n agreement with those of
Miyata e t a l . (1987) b u t do n o t p r e c l u d e t h e presence of p i c o t i t e , as
d e s c r i b e d by Schwarz and Mendes ( 1 9 8 5 ) .
8
P h l o g o p i t e i s always p r e s e n t D u t more o f t e n as s o l i d i n c l u s i o n
p e r m i t i n g , during c r y s t a l growth, t h e f o r m a t i o n of a f l u i d i n c l u s i m
cavity.
- P y r i t e and a l s o amphibole ( P l a t e 3; F i g . 2) were a l s o observed i n t h e
emeralds.
I t a b e r a i emerald i s l i t t l e s i z e d ( c r y s t a l s up t o 1 cm i n
diameter, 3 t o 4 cm l o n g ) b u t p o s s e s s e s an i n t e n s e dark g r e e n c o l o u r .
The f r a c t u r a t i o n i s n o t i n t e n s e and the' s o l i d i n c l u s i o n s a r e r a r e .
All
t h e s e f e a t u r e s permit t o c o n s i d e r T h i s gem of good q u a l i t y .
SEM
o b s e r v a t i o n s permit t o confirm a l s o t h e r a r e t y of f l u i d i n c l u s i o n s
which a r e s m a l l ( s 10 km) and g e n e r a l l y biphased. Daughter m i n e r a l s a r e
a b s e n t and p r o t o g e n e t i c i n c l u s i o n s a r e l i m i t e d mainly t o p h l o g o p i t e
( P l a t e 3, Fig.
5 ) . SEM evidenced a l s o t h e p r e s e n c e of a Ni-Fe
arsenosulphide ( P l a t e 3, F i g . 6 ) . X-ray s p e c t r a revealed'Fe., N i , A s ,
S
as major components s u g g e s t i n g e i t h e r p e n t l a n d i t e ( o r a r s e n o p y r i t e w i t h
N i s u b s t i t u t i o n ) . The absence of X-ray s p e c t r a s t a n d a r d of p e n t l a n d i t e
do n o t permit u s t o e l i m i n a t e t h i s u n c e r t a i n t y ; However, t h e r a t i o 2
Ni/Fe allows t o p r e c l u d e a r s e n o p y r i t e
'
-
.
CONCLUSIONS
Scanning e l e c t r o n microscopy permited t o
complete
the
gemmological d a t a a l r e a d y o b t a i n e d on emerald from B r a z i l (Cassedanne
and Sauër, 1984; Schwarz, 1984; Schwarz and Mendes, 1985; Schwarz,
19861, b u t c h i e f l y t h e i d e n t i f i c a t i o n o f f l u i d i n c l u s i o n daughter
m i n e r a l s . SEM views allowed a l s o t o v i s u a l i z e t$e morphology and- t h e
C i s t r i b u t i o n of t h e c a v i t i e s of f l u i d i n c l u s i o n s k e p t d u r i n g t h e
emerald grcwth (primary or secondary f l u i d i n c l u s i o n s ) . Thus, t h e
s e m i - q u a n t i t a t i v e a n a l y s i s combined w i t h t h e observed c r i s t a l l o g r a p h i c
forms of the c r y s t a l s w a s adequate t o s p e c i f y or a t l e a s t t o l i m i t t h e
p o s s i b l e m i n e r a l i d e n t i t y . The knowledge o f t h e s e d a u g h t e r m i n e r a l s i s
of first importance because i t can d i s p l a y a compositional l i m i t for
t h e o r i g i n a l f l u i d , a s s o c i a t i n g t h e volumes of t h e daughter c r y s t a l s a t
room temperature 'GO t h e microthermometric f e a t u r e s .
Emerald d e p o s i t s of B r a z i l a r e l i n k e d t o i n f i l t r a t i o n a l
metasomatic p r o c e s s e s ( G i u l i a n i and Couto, 1988). The f o r m a t i o n of
emerald r e q u i r e s t h e c i r c u l a t i o n o f a c i d Be-bearing
f l u i d s enriched
a l s o i n F, C l , B and C O 2 i n t o c h a n n e l s developed i n Fe, Mg, C r r i c h
u l t r a b a s i c r o c k s . The importance of emerald m i n e r a l i z a t i o n w i l l depend
of s e v e r a l f a c t o r s : f l u i d - r o c k i n t e r a c t i o n , width and composition of
t h e u l t r a b a s i c f o r m a t i o n s , time of c i r c u l a t i o n of t h e hydrothermal
system and t h e thermodynamic e v o l u t i o n of t h e a s s o c i a t e d f l u i d phases.
I n a l l t h e c a s e s , emerald i s a s s o c i a t e d t o p h l o g o p i t i t e r i c h
zones; t h e s e metasomatic zoning r e s u l t s from t h e flowing of a s o l u t i o n
i n t o t h o r o c k s and t h e p e r c o l a t i n g by a system of f i n e p o r e s . Chemical
r e a c t i o n s a l l o w change i n t h e composition o f t h e s o l u t i o n and s o , a
change i n -the s o l i d ' i n c l u s i o n s and d a u g h t e r m i n e r a l s k e p t by _emerald.
These metasomatic r e a c t i o n s produced i n a l l t h e s t u d i e d deposit's,
the
formation of a Mg, Cr, r i c h p h l o g o p i t e , Mg amphibole ( t r e m o l i t e and/or
a c t i n o l i t e ) , emerald and s u l p h i d e s ( p y r i t e , c h a l c o p y r i t e , p e n t l a n d i t e )
(Table 1). T h i s c o n s t a n t - a n d i m p o r t a n t f e a t u r e i l l u s t r a t e s p e r f e c t l y
t h e m i g r a t i o n of mobile elements such Mg, Fe, K which combined w i t h
i n e r t ones as T i , C r , A l and C a . Meanwhile, t h i s s t u d y r e v e a l s t h a t
emerald a s s o c i a t e d t o pegmatoids i s q u i t e d i f f e r e n t from emerald h a v i n g .
no e v i d e n t g e n e t i c connexion w i t h p e g m a t i t i c v e i n s ; t h i s d i f f e r e n c e can
be e x p l k c i t e d by t h e n a t u r e . of t h e i n f i l t r a t i n g f l u i d s b u t mainly by
-
450
t h e composition of t h e i n f i l t r a t e d
formations.
The
carbonated
t a l c - s c h i s t s w i t h i n t e r c a l a t e d Mg c a r b o n a t e s from S a n t a T e r e z i n h a de'
Goiás w i l l a l l o w t o have tE f o r m a t i o n of Ca-rich s o l i d i n c l u s i o n s and
daughter m i n e r a l s i n t h e i n f i l t r a t i n g f l u i d such as dolomite,
ankerite
b u t a l s o Mg-amphibole. The p r e s e n c e of magnesiochromite i m p l i e s t h a t
t h e s o l u t i o n w a s at a moment Cr-Mg s a t u r a t e d . Chromium i s known as an
i n e r t element and Mg a mobile
one;
so,
it
signifies
that
emerald-bearing f o r m a t i o n s were r i c h i n C r y i m p l i c a t i o n s confirming t h e
d a t a of C o s t a (1986).
Another t y p i c a l d i f f e r e n c e between t h e two k i n d of d e p o s i t s
i s t h e t y p e of f l u i d i n c l u s i o n s a s s o c i a t e d t o emerald. C a r n a h a and
SocotÓ p r o s p e c t i n g p i t s e x h i b i t r e g u l a r t u b u l a r i n c l u s i o n s w i t h v a r i o u s
forms as n e e d l e s , hexagonal- . c h a n n e l s o r 'Ibelemnit rostrumt1. S a n t a
Terezinha de Goiás and l t a b e r a i p r e s e n t f e w c a v i t i e s of
f€uid
i n c l u s i o n s . It i s a l s o i n t e r e s t i n g t o ' n o t e t h e importance and t h e
v a r i e t y of d a u g h t e r m i n e r a l s f o u n d i n SocotÓ r e l a t i v e l y t o t h e o t h e r
d e p o s i t s . I n f o r t u n a t e l y , t h e d a u g h t e r m i n e r a l s observed i n m u l t i s o l i d
i n c l u s i o n s from Carnaiba were n o t found i n t h i s SEM s t u d y . But f o r
i n s t a n c e , i t a p p e a r s t h a t emerald from pegmatoid v e i n s a r e d a u g h t e r
m i n e r a l s - r i c h w i t h sometimes complex s a l t s . The example of SocotÓ i s
p a r t i c u l a r l y i l l u s t r a t i v e . The e v o l u t i o n of t h e f l u i d can b e f o l l o w e d
s i n c e t h e pegmatoid t o t h e emerald b e a r i n g p h l o g o p i t i t e .
I n th'e
pegmatoid v e i n , t h e q u a r t z m a t r i x e x h i b i t s K y C 1 d a u g h t e r m i n e r a l s
i d e n t i f i e d as s y l v i t e and a l s o C a , C 1 , Fe m i x t u r e s a s s o c i a t e d t o a S
type f l u i d inclusion.
Emerald t y p e E2, developed i n t h e pegmatoid v e i n p r e s e n t s
a l s o complex s a l t m i x t u r e s w i t h v a r i o u s compositions:
Fe, C 1 ; C a ,
Cl,Mn, Fe; Fe,Cl,Mn; Ca,Cl,Fe,Ba,K; th-e f l u i d a p p e a r s Fe, C 1 , C a r i c h .
R e l a t i v e l y t o emerald t y p e E l , t h e s a l t s are always complex
wikh a l s o C a , C 1 , F e , Mn, K m i x t u r e s , b u t a l s o w i t h C 1 , Mg, Fe o r C1,
N a , Mg compositions.
I n E2, t h e p r e s e n c e o f Mg i s more predominant t h a n El and, we
can n o t i c e a l s o t h a t i r o n o x i d e s a r e abundant and p e r s i s t e n t d a u g h t e r
m i n e r a l i n E 1 and E2.
It i s i n t e r e s t i n g t o n o t e t h a t d a u g h t e r m i n e r a l s from E2 t y p e
show t h e p r e s e n c e of Mg. T h i s element i s a h i g h l y mobile and probably
t h e C a , F e , Mn, B a , K - c h l o r i d e s o l u t i o n on e n t e r i n g t h e Mg-rich
Fe
Mn
K s i l i c a t e s and
s e r p e n t i n i t e environment, p r e c i p i t a t e s Mg
produces MgC12 s o l u t i o n s i n exchange of t h e Fec h l o r i d e s used.
Consequently, as t h e f l u i d c i r c u l a t i o n i s l i m i t e d i n t h e c a s e of an
i n f i l t r a t i n g v e i n , normally t h e MgC12-content i n c r e a s e s away from t h e
source.
! Without doubt, microthermometric s t u d i e s w i l l a l l o w u s t o
v e r i f y t h e s e h y p o t h e s i s . On t h e basis of t h e f i r s t m e l t i n g t e m p e r a t u r e
data measured f o r f r o z e n i n c l u s i o n s , and t h e p r e s e n t d a u g h t e r m i n e r a l s
product d e t e r m i n a t i o n s , we c o u l d choose t h e c o r r e c t phase diagram t o
r e p r e s e n t t h e system of t h e f l u i d s ( ì . e CaClpKCl-MgC12-H20).
This
s e l e c t i o n i s p r i m o r d i a l because t h e d e t e r m i n a t i o n of t h e composition of
t h e l i q u i d phase i s always one of t h e source of, e r r o r I n t h e
c a l c u l a t i o n of b u l k composition f o r f l u i d i n c l u s i o n s .
Emerald from pegmatoid v e i n s , as i n t h e c a s e of SocotÓ and
Carnaiba, a p p e a r s r i c h i n d a u g h t e r m i n e r a l s r e v e a l i n g t h a t
the
a s s o c i a t e d f l u i d s a r e r i c h i n salts. For S a n t a Terezinha de Goiás and
I t a b e r a i t h e d a u g h t e r m i n e r a l s a r e q u a s i a b s e n t , and t h i s l a c k . of
p r e c i p i t a t e d p h a s e s from f l u i d i n c l u s i o n s upon c o o l i n g , r e f l e c t s t h a t
t h e s o l u t i o n s were more d i l u t e and n o t s u p e r s a t u r a t e d w i t h s o l u b l e
complex salts.
.
-
451
-
-
.
-
i.
,..
.' . .
.i
1
ACKNOWLEDGEMENTS
F i e l d work w a s c a r r i e d o n t w i t h t h e generous a s s i s t a n c e of
t h ë g e o l o g i s t , Pedro Couto from t h e C.P.R.M./S.G.M.
S a l v a d o r d a Bahla,
t h e g a r i m p e i r o J o s é P a u l o Campos from Campo Formoso ( B a h i a ) , and t h e
P r o f e s s o r B a r r o s J o r g e G. Cravo from t h e u n i v e r s i t y of B r a s i l i a . W e a r e
t h a n k s a l s o t o Kohler A l a i n from t h e U n i v e r s i t y . o f Nancy ( F r a n c e ) which
provided SEM a n a l y s e s .
'-
BIBLIOGRAPHY
-
-
-
CASSEDANNE, J.P.
1985
Au pays d e s émeraudes
Monde e t Minéraux, no
66. p. 16-2.0.
CASSEDANNE, J.P. and SAUER, D.A.
1984
The S a n t a T e r e z i n h a de Goiás
emerald d e p o s i t , G e m s and Gemology, S p r i n g , p. 4-13.
CHEILLETZ,
A.
1984
- Caracteristiques
géochimiques
et
thermobarométriques d e s f l u i d e s a s s o c i é s à l a s c h e e l i t e e t au q u a r t z
d e s m i n é r a l i s a t i o n s de t u n g s t é n e du J b e l Aouam (Maroc C e n t r a l ) .
B u l l . Minéral., 107, p.255-272.
COSTA, S . A . de G. - 1986 - C o r r e l a ç ã o da s e q u ê n c i a e n c a i x a n t e d a s
esmeraldas de S a n t a T e r e z i n h a de Goiás com o s t e r r e n o s do t i p o
g r e e n s t o n e b e l t de C r i x á s e t i p o l o g i a dos d e p ó s i t o s , Anais do XXXIV
Congresso B r a s i l e i r o de Geologia, v., p. 597-614.
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m i n e r a l i n hydrothermal f l u i d i n c l u s i o n s . C o n t r i b . Mineral.
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infiltrational
metasomatic
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Poços
ff
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1987 a - The p r o t e r o z o l c
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452
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--..-__
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_
_
_
I
g r a n i t e massifs of Campo Formoso and Carnaiba (Bahia, B r a z i l ) wd
t h e i r Be,Mo,W m i n e r a l i z a t i o n s . I n t e r . Symp. G r a n i t e s and a s s o c i a t e d
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(Bahia, B r é s i l ) : um exemple de m i n é r a l i s a t i o n s p r o t é r o z o ï q u e s à Be ,
MO e t W dans d e s u l t r a b a s i t e s métasomatisées, C.R. Acad. Sc. P a r i s ,
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Beryllium. Pitman p u b l i c a t i o n s , England.
WILSON, J.W.J.;
KESLER, S.E.; CLOKE, P.Z. and KELLY, W.C.
1980
F l u i d i n c l u s i o n geochemistry of t h e G r a n i s l e and B e l l porphyry
copper ' d e p o s i t s , B r i t i s h Columbia. Econ. Geol., 75, p . 45-61.
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~
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I
-
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I
453
-
-
O'
100 :
200 s
30° S
700w
60° W
50° W
400w
.
Neivo~OZ/BB
. FIGURE 1: Main emerald b r a z i l i a n d e p o s i t s and occurrences. A: d e p o s i t s ;
-
B: occurrences. Ceará S t a t e : 1 - Coqui; 2
Tauá.
B a h i a S t a f e : 3:
S a l i n i n h a ; 4: SocotÓ; 5: Carnaiba; 6: Anagé; 7: Brumado. Minas G e r a i s
State: 8
I t a b i r a ; 9 - Santana dos F e r r o s .
Goiás
s t a t e : 10
I t a b e r a i ; 11 - P i r e n Ó p o l i s ; 1 2 - S a n t a Terezinha de Goiás;
13 - Mara
Rosa; 14 - Porangatu; 1 5 - P e l a Ema. 454
-
-
l
I
'
I
l
,
#
I
-
Plate 1
S o l i d and d a u g h t e r m i n e r a l s i n f l u i d i n c l u s i o n s o f emerald
from SocotÓ ( P h o t o s 1 t o 4) and C a r n a i b a . ( P h o t o s 5 and S), Bahia S t a t e ,
r e v e a l e d by Scanning E l e c t r o n Microscopy (SEM). Photo 1: C a , ' C l , Mn, Fe
s a l t s ( S ) from t u b u l a r f l u i d i n c l u s i o n s .
Photo 2:
daughter minerals
encountered i n s w e l l e d T l u i d i n c l u s i o n s . S = s a l t w i t h C a , C l , . Fe, Mn, K
composition. Photo 3: q u a r t z m a t r i x from t h e emerald-bearing
pegmatoid
v e i n s ; p r i s m a t i c s a l t which X-ray s p e c t r a evidences s o c h l o r i d e ( C l ) .
Photo 4: C a , C 1 , Fe complex s a l t d e p o s i t i o n (S) i n a c a v i t y of f l u i d
i n c l u s i o p from q u a r t z m a t r i x . Photo 5: P r o t o g e n e t i c c r y s t a l s of
p h l o g o p i t e ( P h ) i n c a v i t y of f l u i d i n c l u s i o n s from Carnaiba emerald.
Photo 6: Emerald i n c l u s i o n s ( E ) , i n emerald.
455
I
456
-----
--
Plate 3
Solid and daughter minerals in fluid inclusions of emerald
from Santa Terezinha de Goiás (Photos 1 to 4) and Itaberai (Photos 5
and 61 - Goiás State. Photo 1: solid inclusion with Ca, Mg, Fe
composition revealing the presence of dolomite (Ca). Photo 2: Amphibole
(Am), observed in fluid inclusion cavity. Photo 3: magnesiochromite
with a Cr, Fe, Mg composition as solid and daughter minerals in
emerald. Photo 4: characteristic X-ray image showing the distribution
of chromium in the solids from--the field of photo 3. Photo 5:
Phlogopite (Ph) forming a protogenetic inclusion in emerald. Photo 6:
Association !of pentlandite. (Àss3 and phlogopite (Ph)
as
solid
inclusions.
457
U
SOLID NINEftALS
DAUGtmER l l l N E R l l l ~ O
4
m
M o l y t d ~ ntle a o
Tourmaline.
Albi t e *
Lwioocrocite.
Goe t h l te+
Emrrald a o
Phlogopite.
44
<
z
<
o
-
Phlogopi 1,.
T r ~ m n l Jtv.O
.-
458
!

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