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Genesis of the Precambrian Copper-rich Caraiba
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Transcript of Genesis of the Precambrian Copper-rich Caraiba
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Fig. 2. Simplified geology of
part of the Salvador-Curaq/t
Orogen showing the high-grade
terrains of Curaqfi (CT ) and
Jacurici (.IT) and their m afic
ultramafic bodies. Geo logy after
Inda and Barbosa (1978), Gava
et al. (1983) and Mascarenhas et
al. (1984)
indicating that the age of development of the precursor sources of
the two complexes may not have been very different.
Mod els for the araiba mineralisation
T h e Ca r a i b a mi n e ra l i s a t i o n i s b a si ca l l y o f d i s s emi n a t ed
t y p e . T h e m a i n o r e mi n e ra l s - ch a l co p y r i t e an d b o rn i t e
- a r e h o s t ed i n c en t i me t r e t o t en s -o f -me t r e s t h i ck h y p e r -
s then i t i c and nor i t i c bod ies enc losed wi th in the h igh-
g rad e g n e i s s e s . T h e h y p e r s t h en i t e s a r e b y f a r t h e mo s t
Cu - r i ch ro ck s (Cu = 2 -5 w t ; n o r i t e s < 0. 5 w t ) . Ma g -
net i t e , and to l esser ex ten t i lmeni t e , a re the main accessor -
i es i n t h e d i s s emi n a t ed mi n e ra l i s a t i o n , t h e i r ab u n d an c e
corre la t ing pos i t ive ly wi th the main o re minera l s , i . e . , t he
r i ch e r t h e o r e t h e mo re mag n e t i c i t i s (Sa an d Re i n h a rd t
1984) . Veins and ve in le t s o f rem obi l i sed o re a re very
c o m m o n , a n d c o n si s t m o s t l y o f c h a l c o p y r it e a n d b o r n i te
w i t h mi n o r cu b an i t e , mag n e t i t e an d N i - t e l l u r i d e s . So me
v e i n s a r e r i ch e r i n N i an d ca r ry p en t l an d i t e , p y r rh o t i t e ,
mack i n aw i t e , ch a l co p y r i t e an d cu b an i t e . T h es e p a r a -
geneses , a long wi th ch lo r i t e , ep ido te , carbonate , an -
thophyl l i t e and t a l c , ind ica te tha t the ve in su lph ides have
b een r emo b i l i s ed u n d e r amp h i b o l i t e - t o g r een s ch i s t- f ac ie s
m e t a m o r p h i c c o n d it i on s .
T h e o r i g i n o f t h e Cu - b ea r i n g m af i c -u l t r amaf i c r o ck s o f
the Cura~/~ t e r ra in has long been con t rovers i a l . Le inz
(1948) and Schneider (1951) descr ibed the su lph ide-bear -
i n g p y ro x en i te s a s mag m a t i c . L ad e i r a a n d Bro ck es (1 96 9)
a s c r i b ed t h e maf i c -u l t r amaf i c r o ck s t o an i g n eo u s s u i t e
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the sulphides. Mandetta (1982) later extended Linden-
mayer's model to suggest that the Caraiba deposit is
a layered intrus ion composed of cycles of igneous differ-
entiation, with a bottom-to-top sequence of: (i) massive
hypersthenite with minor olivine-pyroxenite, (ii)mela-
norites, (iii) norites and leuconorites with hypersthenite
streaks, and (iv) norite to leuconorite with interleaved
gabbros to gabbronorites, locally banded.
Subsequent field and mineralogical studies on out-
crops as well as drill-cores (Oliveira 1990a,b) failed to
confirm the layered intrusion model of Mandetta (1982).
Instead, crosscutting relationships suggested that the
Caraiba complex may be a multiple sequence of intrusions
of dykes, veins, and breccias. Here, field, mineralogical
and geochemical data are integrated to provide a better
petrogenetic model for the complex and for the min-
eralisation.
Fig. 3. Simplifiedgeologyof part of the Cura~fi high-grade terrain
(modified from Delgado and Souza 1981, and the Caraiba Mine
stare
representing the initial magmatism of a geosynclinal pile.
Suszczynski (1972) suggested that the mafic-ultramafic
rocks resulted from metamorphism of pure- to impure-
carbonates , locally highly magnesian. Delgado and Souza
(1975) presented trace element data for some mafic-ultra-
mafic rocks and concluded that the mean Cr, Ni, Ti, Cu,
Co, and V values found are more compatible with an
igneous origin for these rocks. Townend et al. (1980),
reviewing previous models for Caraiba, proposed an ig-
neous origin, the available data favouring a sequence of
basic to ultrabasic sills metamorphosed at granulite grade.
Figueiredo (1981), noting the similarity between the REE
patterns of an iron-rich hypersthenite from the Caraiba
mine and some regional iron formations, suggested a sedi-
mentary origin for parts of the Caraiba ore-body. Linden-
mayer (1981) described relict cumulus textures in some
gabbros associated with the Cu-bearing norites-hyper-
sthenites and found possible clinopyroxene exsolution
lamellae in hypersthene. On the basis of chemical data and
cross-cutting relationships deduced from drill cores, she
concluded that the ore-hosting rocks are intrusive and
evolved through fractional crystallization of a previously
differentiated Fe-Ti-rich tholeiitic liquid, and that the
hypers thenite-norite-gabbro-anorthosite sequence might
well represent the original igneous stratigraphy. She also
suggested that some anhydrite-bearing marble and calc-
silicate rocks could have supplied sulphur to form part of
ield and p etrographic aspects
As with many igneous complexes in Precambrian high-
grade terrains (cf. Windley et al. 1981), the Caraiba ore-
bearing mafic-ult ramafic rocks still preserve a good pro-
portion of their primary igneous features. Cross-cutting
relationships with the country gneisses, and between the
different rock types within the complex, can be observed
in the Caraiba open pit. These clearly suggest emplace-
ment of the complex as multiple intrusions of norite and
hypersthenite as dykes, veins and igneous breccias. The
associated peridotites and gabbroic rocks are interpreted
as xenoliths.
ypersthenites and norites
These rocks account for more than 94 of the Caraiba
mafic-ultramafic complex, and are found mostly as dykes,
veins, and magmatic breccias. Observations on
9 boreholes suggest the proportion of hypersthenite to
norite is ca. 60:40. The contact between these rock-types is
mostly sharp, but a complete gradation from hyper-
sthenites to melanorites, norites, and to the less abundant
noritic anorthosites may occasionally be observed. Some-
times a homogeneous rock, either hypersthenite or norite,
can be traced for some tens of metres. Centimetre-thick
folded dykes of hypersthenite or norite may sporadically
be found cross-cutting the ore-grade rocks. Similar situ-
ations occur in the southern part of the open-pit where
off'shoots of coarse-grained hypersthenite clearly transect
the metamorphic banding of hornblende-pyroxene
granulite and migmatitic gneiss. The igneous breccias
comprise fragments of norite enveloped by hypersthenite.
This feature, and the offshoots described above, suggest
that some hypersthenites originated from liquids of the
same composition, rather than from another parental
magma.
Field observations on chalcopyrite-bearing hyper-
sthenite dykes from the southern part of the Caraiba open
pit (Oliveira and Lacerda 1993) demonstrate tha t some
hypersthenites have been emplaced synkinematically dur-
ing the most prominent deformational event (F3):
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Fig. 4. Simplifiedgeology of the
Caraiba open-pit with location
of studied outcrops an d
boreholes. Geology after Silva
1985)
1. Dr ag folds in count ry- roc k granuli te-facies gneiss indi-
ca te tha t a s teeply d ipping hypers theni te dyke in t rude d
along a dextral t ransten siona l shear zone Fig. 5a);
2 . A se t o f narrow and para l le l melanori te to hyper-
sthenite dykes cut across the steep fol iat ion of the
migmat i t ic gneiss , whereas another se t , conformable
in ter leaved wi th the gneiss , has been boudinaged and
broken apart , somet imes c lear ly showing a s igmoidal
shape indicat ive of a dextral sense of mo tio n Fig. 5b);
3 . A cm-th ick hypers theni te dyke , a branch of a th icker
and adjacent one, cross-cuts the migmati t ic gneiss fol i-
a t ion and was subsequent ly t ransposed a long the
gneiss fo l ia t ion ; the a d jacent dyke i s nearly conform -
able in the host gneiss and shows anti thet ic sl ip on
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a
W
I I
E
~
9 1
Fig. 5a--c. Cross-sections, Cara iba open-pit, showing evidence of
hypersthenite emplacement controlled by shearing in a) and b) and
concurrent dyking and deformation in c). Grey hypersthenite; sub
parallel lines foliation of migmatitic gneiss
f r ac t u re p l anes , s ugges t i ng dex t r a l d i s p l acem en t
F ig . 5c ) . The t r ans pos ed dyk e con t a i n s hyper s t hene
t r ans ec t ed by b i o t i t e , wh i ch i s a t yp i ca l am ph i bo l i t e -
g rade F 3 m i ne ra l .
The s t ruc t u ra l f ea t u res, des c r i bed by Ol i ve i r a and Lace rd a
1993 ) and s u m m ar i s ed he re , s ugges t tha t a t l eas t s om e o f
t h e C a r a i b a r o c k s m a y h a v e t h e ir e m p l a c e m e n t c o n t r o l l e d
by a r eg i ona l s hea r be l t t ha t r eached s en il it y in t he wan i ng
9 5
8 5
X
O • 7
65
4 5
l ow T - - ~
h i g h T
3 5
0 5
9
. o ~ . + & . w - . ,+ ~ . r + +
0 % ~ 0 o + ~ + 6 + + o
o +
+
H y p e r s t h e n i t e
9
M e l a n o r i t e
o
N o r i t e
I I I ~ I
1 2 3 4
M o l e A I 2 0 3 I n O p x
Fig. 6. Mg-number versus alumina content in orthopyroxenes from
Caraiba norites and hypersthenites. SK Skaergaard trend after
Hoover 1989). Also shown is the low- and high-T trend of the
Josephine peridotite after Dick 1977)
8 5
8O
x
O .
o
_c
s
z
IE
7
65
C o n t a c t C o a r s e
P e r l d o t l t e T r a n s i t i o n H y p e r s t h e n l t e
Fig. 7. Variation of orthopyroxene composition across the contact
between a per idotite xenolith and the host hypersthenite. Sample
380-C from outcrop 380
s t age o f evo l u t i on o f the P a l eop ro t e rozo i c S a l vado r -
Cura~/t orogen.
The hyp er s t hen i te s and no r i te s a r e com pos e d o f hyper -
s thene, p lag ioclase , su lphides , Fe-Ti oxides , ph logopi te
and apa t i t e i n va ry i ng p ropo r t i ons . Under t he m i c ro -
s cope , hyper s t hene and p l ag i oc l as e a re ve ry f r e s h , s om e
s howi ng t r i p l e -j unc t ion con t ac t s i n r e s pons e t o s ubs o l i dus
recrys ta l l i za t ion; but several pr imary features are s t i l l
p res e rved . P lag i oc l as e and h yper s t hene i n t he no r i te s a r e
in propor t ions cons i s ten t wi th eu tect ic crys ta l l i sa t ion . In
t he hyper s t hen i t e s , however , hyper s t hene m ay o f t en con -
t a i n anhed ra l t o s ubh ed ra l i nc l u si ons o f p lag i oc l as e in
op t i ca l con t inu i t y , i m p l y i ng t ha t p l ag i oc l as e m ay be bo t h
a cum ul us o r a pos t cum ul us phas e ; m ore r a re l y p r i s m a t i c
hyper s t hene fo rm s an apa t i t e -hyper s t hene cum ul a t e
o r , t oge t he r w i t h p l ag i oc l as e , s hows c ro s s -bedd i ng l i ke
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i g n e o u s l a m i n a t i o n . W h a t m a y h a v e b e e n t r a p p e d o r
i n t e r st i ti a l no r i t i c li qu i d s a re com m o n l y found wi t h i n t he
hyper s t hen i t e s ; i n t hes e pa t ches , hyper s t hene m ay s how
s ubhed ra l c ro s s - s ec t i on and p l ag i oc l as e is r eve r s e ly zoned .
Mi no r s u l ph i des , ox i des and ph l ogop i t e g ra i n s a l s o occu r
as i nc l u s i ons i n hyper s t hene , t hough m os t a r e t ex t u ra l l y
in ters t i t i a l . They have recrys ta l l i sed to d i f ferent ex tent s
d u r i n g d e f o r m a t i o n / m e t a m o r p h i s m s o th a t i n t e r g ra n u l a r
s u l ph i des and ox i des d i s p lay a l l s tages f rom hav i ng eq u i -
l i b r ium t ex t u res w i t h hyper s t he ne (= annea l ed co n t ac t s )
t o hav i ng c ro s s -cu t t i ng r e l a t ions h i p s ( s ec ondary g rowt h ) .
A l t h o u g h p h l o g o p i t e f r e q u e n t ly s h o w s s e c o n d a r y g r o w t h ,
p r i m ary i n t e r s t i t i a l ph l ogop i t e a round hyper s t hene can be
obs e rved . P h l ogop i t e i s a l s o found as a p r i m ary phas e i n
t he hype r s t hen i t e ve i n (no t ed i n (3) above) a s we l l a s i n t he
pe r i do t i t e -hos t ed hyper s t hen i t e s .
Apa t i t e is a ub i qu i t ous m i ne ra l , be i ng g ranu l a r i n s hape
i n t he hyper s t hen i t e s , where i t m ay r each ove r 30 m o-
da l , and g ranu l a r t o p r i s m a t i c i n no r i te s . P h i l po t t s
(1967) no t ed t ha t h i gh conce n t r a t i ons o f apa t i t e and F e -T i
o x i d e s c o m m o n l y o c c u r w i t h i n a n o r t h o s i t e p l u t o n s a n d
exp l a i ned t he a s s oc i a t i on i n t e rm s o f im m i s c ib l e li qu id s . In
s o m e a n o r t h o s i te s t h e p r o p o r t i o n o x i d e : a p a t i t e i s 2 : 1
wh i ch P h i l po t t s (1967) i n t e rp re t ed i n t e rm s o f eu t ec t i c
c ry s t a l l i za t i on . In t he C ara i ba ano r t hos i t i c rocks , apa t i t e
do es no t s ho w s uch a d i r ec t r e l a t i ons h i p wi t h F e -T i
oxides ; none theless the pe t rogen es i s of P +_ (Fe + T0-r ich
l i qu id s i n m as s i f ano r t hos i t e s r em ai ns en i gm at ic .
Ultramafic xenoli ths
Ul t ram a f i c xeno l i th s a r e loca l l y com m o n , c ropp i ng o u t a s
f i ne -g ra i ned d i s con t i nuous rounded t o ova l s haped ba l l s
i n a g round m a s s o f hyper s t hen i t e and no r i te ; com po s i -
t i ona l l y t hey com pr i s e du n i t e , ha rzbu rg i te , o l iv i ne o r t ho -
py roxen i t e , o l i v i ne -ho rnb l ende o r t hopy roxen i t e and o l i -
v i n e - o r t h o p y r o x e n e h o r n b le n d i t e w i th m i n o r a m o u n t o f
b ro wn t o b l ac k s p i ne l and ph l ogop i t e . I t i s ve ry d i ff i cu l t t o
es t ab l i s h whe t he r t he xeno l i t h s occu r dom i nan t l y a t one
s t r a t i g raph i c l eve l becaus e t hey a re abs en t i n s om e
boreho l es , even i n t hos e d r i l l ed a l ong t he s am e ve r t i ca l
c ro s s - s ec t i on t h rough t he m af i c -u l t r am af i c com pl ex . The
x e n o l i th - g r o u n d m a s s c o n t a c t s a r e s h a r p a n d , f ro m p e t r o -
g raph i c ev i dence , the pe r i do t i t e s c l ea r l y c ry s t a ll i zed be fo re
t he hyper s t hen i t e s and no r i t e s . De t a i l ed exam i na t i on o f
a con t ac t be t we en a pe r i do t i t e xeno l it h and t he g rou nd -
m as s hyper s t hen i t e s hows a f i ne -g ra i ned hyper s t hen i t e
o f f s hoo t i n jec t ed i n t o t he pe r i do t i t e , a s l igh t dec reas e o f
t he hyper s t hen i t e g ra i n s i ze t owards t he pe r i do t i t e , and
e l onga t ed hyper s t henes pe rpend i cu l a r t o t he con t ac t w i t h
t he pe r i do t i t e . Th i s l a s t r em arkab l e f ea t u re i s cons i s t en t
wi t h g row t h o f m i ne ra l s a t a co o l ed s u rf ace . M oreov er , t he
hos t hyper s t hen i t e i s com pos e d o f ph l ogop i t e , apa t i t e and
i n t e r s t i t i a l m agne t i t e , bo rn i t e and cha l copy r i t e , wh i ch
l e av e s n o d o u b t t h a t t h e m a g m a w a s h y d r o u s a n d t h a t
two immiscib le l iqu ids , v iz . s i l i ca te and su lphide-oxide,
coexis ted .
T h e p e r i d o t i te o r t h o p y r o x e n e c o m p o s i t i o n s h o w s a g a p
b e t w e e n 8 - 1 8 E n c o m p a r e d w i t h p y r o x e n e s f r o m
hyper s t hen i t e s and no r i t e s (O l i ve i r a 1990b ; O l i ve i r a and
Ta rney 1994 ), wh i ch aga i n ru l e s aga i n s t de r i v i ng t he l a t t e r
9 0
8 0
i 70
6 0
9 Se t 1 [
5 0
E d g e C e n t r e E d g e
Fig. 8. Compositional variation in reverse zoned plagioclase of
norite 380 1D
Phlogopite Eastonite
100
8O
A
~ . 60
5
9
40
8
20
P h l o g o p i t e
+ -H - +o + 0 ~ o
o So
+ . ~ . o t o r
~ l r
B i o t i t e
+ Norl tes
o Hypersthenites
0
0.44 0.46 0.48 0. 0 0. 2 0.54 0.56
Annite Siderophyllite
AI/ Mg+Fe)
Fig. 9. Composition of biotite phlogopite series in hypersthenites
and norites from the Caraiba mine
f rom t he fo rm er by f r ac t i ona l c ry s t a l l i s a t i on . No t ab l y ,
ca l c ic py roxen e occu r s i n t he m o s t f f ac t i ona t ed pe r i do t i te ,
bu t i s ab s en t i n t he C u- r i ch hyper s t hen i t e s .
abbroic xen ol i ths
X e n o l it h s o f g a b b r o n o r i t e s , g a b b r o s , a m p h i b o l e - g a b b r o s ,
a n d l es s o ft e n a m p h i b o li te s , a r e c o m m o n i n t h e C a r a i b a
m af i c -u l t r am af i c rocks . L i ke t he u l t r am af i c xeno l i t h s t hey
occu r i n cm -s i ze roun ded ova l o r e l onga t e b l ocks . B ecaus e
s om e s i m i l a r l i t ho l og i es occu r i n t he s up rac rus t a l s e -
quence and / o r a s dykes i n t he o r t hogne i s s es G1 and G2 i t
i s potent ia l ly poss ib le that a few xenol i ths could be local .
T h e x e n o li th s a r e c o m p o s e d o f p la g i o c la s e a n d C a - p y r o -
x e n e w i th v a r i e d p r o p o r t i o n s o f a m p h i b o l e , h y p e r s th e n e ,
p h l o g o p i t e a n d o p a q u e s , a n d h a v e g r a n o b l a s t i c , m o r e
ra re l y i n te rg ranu l a r , t ex t u re . In s o m e g abb ro i c xeno l it h s ,
C a-py roxenes a re pa r t i a l l y t o t o t a l l y r ep l aced by ho rn -
b l ende , wh i ch i n t u rn m a y d i s p l ay s ym pl ec t i c i n t e rg rowt hs
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-
4
2
F
C
2
2
N
e
P
a
5
3
2
1
9
8
5
7
0
0
9
1
3
1
1
5
7
1
9
2
0
0
0
1
9
4
6
N
5
1
H P
a
5
7
2
5
1
2
4
8
9
3
3
9
9
6
0
2
1
1
7
1
9
5
0
N 8
3
N
e
P
a
5
3
2
7
8
6
5
9
0
5
9
1
3
1
4
5
5
1
6
2
1
0
1
1
9
4
1
N 8
9
A
h
P
a
5
2
2
6
8
0
6
4
0
4
9
8
3
1
5
5
4
1
5
2
2
0
0
1
9
4
0
N
9
2
H P
o
3
1
4
6
1
1
0
3
1
9
1
3
9
4
9
0
2
5
5
0
5
2
6
0
0
1
3
3
6
1
7
1
5
7
7
F
C
2
7
N
e
P
o
3
0
4
1
1
1
1
7
1
8
0
4
9
2
9
5
2
5
6
0
4
2
5
1
7
3
3
0
1
1
7
1
6
6
7
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9/23
Table
2. Representative analyses of sulphide and oxides from hyper-
sthenites and norites in Drillcores FC2516 and NO 33, Ca raib a mine
Mineral Cpyr Pyrr Born Cpyr
Fe 30.7 59.76 12.64 26.41
Co 0.01 0.04 0.0 0.06
Ni 0.01 0.35 0.02 0.0
Cu 33.8 0.02 60.07 39.59
Zn 0.03 0.03 0.02 0.02
S 34.21 38.77 26.7 32.57
To tal 98.76 98.97 99.44 98.65
Ni /Cu 0.0 15.7 0.0 0.0
Continued
Magn Spin Magn Spin
TiO2 0.33 0.01 0.24 0.04
A12Oa 1.53 53.04 1.22 49.93
CrzO3 5.63 7.51 5.59 7.42
Fe2 03 56.31 - 57.38 0.79
Fe O 29.37 23.64 29.52 23.66
MnO 0.13 0.28 0.12 0.28
MgO 0.07 8.66 0.09 7.86
To tal 93.37 93.14 94.16 89.98
Cation proportions
Ti 0.08 0.002 0.08 0.006
A1 0.587 14.728 0,464 14.486
Cr 1.45 1.399 1.425 1.443
Fe 3+ 13.8 14.0 0.148
Fe 2 + 8.0 4.657 7.96 4.871
Mn 0.036 0.056 0.032 0.059
Mg 0.033 3.041 0.042 2.891
Born
11.60
0.01
0.02
61.15
0.02
26.60
99.40
0.0
Cpyr, Chalcopyrite; Pyrr, pyrrhotite; Born, bornite; Magn, magne-
tite; Spin, spinel
o f h y p e r s t h en e an d p l ag i o c l a s e a t g r a i n b o u n d a r i e s . T h i s
f o r m a t i o n o f a m p h i b o l e a t t h e e x p e n s e o f p y r o x e n e a n d
i t s fu r ther re crys ta l l i sa t ion , sugges t re -equ i l ib r ium of the
h y d ra t ed g ab b ro i c x en o l i t h i c r o ck t o t h e h i g h e r t em-
p e ra t u r e s o f t h e h o s t n o r i t i c -h y p e r s t h en it i c mag m a . T h e
g ab b r o i c x en o l it h s h av e few co u n t e rp a r t s i n t he s u r ro u n d -
ing coun t ry rocks (Ol ive i ra and Tarney 1994) , bu t the i r
d i s ti n c t n eg a ti v e N b an o ma l i e s o n man t l e n o rma l i s ed
d i ag rams co u p l ed w i t h t h e i r f r a c t i o n a t ed RE E p a t t e rn s
sugges t a con t inen ta l in f luence/ inpu t .
359
yroxene
O r t h o p y r o x e n e r a n g e s f r o m En68_54 i n h y p e r s t h en i t e s
through 1~n67.63 i n me l an o r i t e s t o En66_56 i n nor i t es . In
t h e an o r t h o s i t e s t h e r e i s v e ry l i t t l e v a r i a t i o n i n co mp o s i -
t i o n (E n 6 1 -E n 6 2 ) . Co mp ar i n g s amp l e s f r o m d r i l l h o l e s
F C 2 5 1 6 a n d N O 3 3 , o r t h o p y r o x e n e s f r o m t h e f o r m e r a r e
s l igh t ly more i ron r i ch En61.56 c o m p a r e d w i t h En66_6o .
T h e a l u mi n a co n t en t o f p y ro x en es f al ls w i th i n t h e r an g e
3 .6 -1. 1 mo l e , th o s e f ro m t h e h y p e r s t h en i t e s b e i n g mo re
a l u mi n o u s an d l e s s f r a c t i o n a t ed t h an t h o s e f ro m t h e
melanor i t es and nor i t es (Fig . 6 ) . Pyroxenes f rom a l l rocks
fo l l o w a t r en d p e rp en d i cu l a r t o t h e t r en d o b s e rv ed i n
l o w -p re s s u re i g n eo u s i n t ru s i o n (e .g . Sk ae rg aa rd ) b u t p a r -
a l le l t o t h e t r en d d e f i ned b y l o w - t e mp e ra t u r e - h i g h - t em-
p e ra t u r e A l p i n e p e r i d o t it e s an d h i g h -p re s s u re p e r i d o t i te s .
T h e A 1 2 03 c o n t en t o f p y ro x en e s i n c r eas e s w i t h i n c r ea s i n g
t emp e ra t u r e an d p r e s s u re (G reen an d R i n g w o o d 1 9 6 7 ,
G ree n 1 96 9) , h en ce t h es e p a r a me t e r s ma y h av e co n t ro l l ed
A 1 2 0 3 en t e r i n g i n t o o r t h o p y ro x en e . H o w ev e r , t h e h i g h e r
A l zO 3 ab u n d an ces i n p y ro x en es f ro m t h e h y p e r s t h en i t e s
may b e d u e t o o t h e r f a c t o r s . Bo t h h i g h t o t a l an d w a t e r
p r e s s u re i n h i b i t t h e c ry s t a l l iz a t i o n o f p l ag i o c l a s e (Y o d e r
and Ti l l ey 1962) , so i f the hype rs then i t es re prese n t h igher
p r e s s u re mag m as , t h en C a a n d A1 w o u l d b e av a i l ab l e to
en t e r t h e p y ro x en e s t r u c t u r e . H o w ev e r , t h e co ex i st en ce o f
n o r i t e an d h y p e r s t h en i t e mag mas mak es p r e s s u r e d i f f e r -
en ces an u n l i k e l y ex p l an a t i o n . Ca l c i c p y ro x en es o ccu r i n
n o n e o f t h e se ro ck s an d t h e Ca co n t en t o f t h e an a l y s ed
o r t h o p y ro x e n es i s v e ry lo w ( < 0. 60 mo l e ) . I t ap p ea r s
t h a t t h e A 1 20 3 co n t en t o f o r t h o p y ro x en e i s d e t e rmi n ed b y
t h e p r e s en ce o r ab s en ce o f p lag i o c l a se , co u p l ed w i t h
a r e l a ti v e l y A l - ri ch, Ca - p o o r n a t u r e o f t h e p a r en t a l ma g -
mas .
ineral chemistry
Mi n e ra l g r a i n s o f n o r i t e s an d h y p e r s t h en i t e s f r o m
b o reh o l e s F C2 5 1 6 an d N O 3 3 ( see F i g. 4 f o r l o ca t i o n ) w e re
an a l y s ed w i th a J E O L Su p e rp ro b e a t L e i ce s t e r U n i v e r s i t y
t o a s ce r t a i n an y co mp o s i t i o n a l t r en d s t h a t mi g h t s u p p o r t ,
o r o t h e rw i s e , t h e l ay e r ed s i l l s mo d e l o f L i n d en may e r
(1981) and Mandet t a (1982) . These two cores were chosen
b ecau s e t o g e t h e r t h ey co n t a i n r ep r e s en t a t i v e s o f a l l r o ck
t y p es . E x t r a s amp l e s f r o m O u t c ro p 3 8 0 i n t h e o p en -p i t
(Fig . 4 ) he lped c la r i fy the p lag ioc la se and pyro xen e com -
p o s i t i o n a l r e l a t i o n s h i p s i n t h e p e r i d o t i t e -b ea r i n g h y p e r -
s then i t e . Represen ta t ive minera l ana lyses a re g iven in
Table 1.
Fig. 10. Com positional varia tion of pyroxene and plagioclase from
the Caraiba complex compared with island-arc cumulates (Burns
1985), layered intrusions (Wager a nd Brown 1967), massif-type anor -
thosites (Emslie 1985) and the K ope rber g Suite (Mclve r et al. 1983;
Conradie and Schoch 1986)
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1 2
1 0
361
1 0
8
0
o 6
4
.O_2
I -
0 . 4
0 . 3
0
-
= E 0 . 2
0
0 0
0 0 0 0 0 O 0
0 0 0 0 0
0
0 0
e H y p e r s t h e n i t e s
o o N o r i t es
0%*o
0
o 8
0 8 0 o
9 9 ooO 9
I , O0 I
0 0
o% 9~
O O o
9 0 O 0 ~ 0 ~
~ i
~
o o a : i .
, I , 0 ~ I @ , I
9 9 9 0 9
9 9 0 0 O 9
9 o .
0 O ~ 9
0 oO- 9
0 0 0
0 O 0
0 0 0
0 0 9
0
8 o
0 O 8 0 0
O 0 0 0
0 0
b
i
d
O
0 9
0 0
8o o ~ 76 76 76 o
9 9
9 o O o o o O o e~ e
9 9 9 O ~ w
e o .
9 O 0 ~ O
@
O O
0 O 0 O @ O O
9 9 9
I , O' I , 2 1 ,
0
0
0 O 0 0 0
0
O 0 0 O 0 9
o
0 0 ~ @ 9
0 O 0 O 0 u ~ -
9 .' rp
r I O , O 9 I O Q r
I
o e o o d l
o 9 O ~ I a
0 0
0 0 0 0 o O ~ @ 0
o ~
0 0 0 O 0
0
0
0 0
0
0
I I
0 . 4 0 . 5
mg
0 . 1 8~
0
0 . 0
' ' ' ' ' ' ' ' ~
0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 0 . 3 0 . 6 0 . 7
mg
to
0
.1
. 01
1 0
n
z
5
1 0 0 0
,m
Z
1 0 0
10
Fig. 12a-f. Summaryof major and trace element variation versus rag-number (rag*) in hypersthenitesand norites from Caraiba (drillholes
FC2516 and NO33, and outcrops 380 and 365N)
All three types may be variably layered.
In Archaean high-grade metamorphic terranes, the
complexes are layered and can be grouped into a mafic-
ultramafic type and a gabbro-anorthosite type (Windley
et al. 1981). The geotectonic environment of format ion of
these ancient complexes is uncertain, but various authors
have suggested similarity with island-arc (Windley et al.
1981), ocean floor (Weaver et al. 1981, 1982) or cont inental
margin (Srikantappa et al. 1984) environments.
In the Proterozoic, the major ity of the mafic-ultramafic
complexes have been emplaced on to the cratons along
megafractures (Windley 1984). They are represented by
mafic dyke swarms, large layered tabular bodies such as
the Great Dyke of Zimbabwe, and layered complexes such
as the Bushveld in South Africa and the Niquelgmdia in
Brazil. An important type of plutonism characteristic of
the Mesoproterozoic is that represented by massif-type
anor thosites (Herz 1969, Emslie 1985, Ashwal 1988) which
may have been emplaced into the continental crust in
orogenic or anorogenic environments (Emslie 1985). Can
any links be drawn with Caraiba?
In Fig. 10, pairs of coexisting orthopyroxene and pla-
gioclase from the Caraiba hypersthenites and norites are
compared with island-arc cumulates, layered intrusions
and anorthosites of the massif-type. Data for the South
African Koperberg Suite (Schoch and Conradie 1990), the
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362
1 5 0 0
1 0 0 0
C
N
5 O
0
0
N N~
9 Hypersthenites
o Norites
o O
~ 0 0 O 0
I t I J
1
2 0
F e 2 0 3
I
3
4 0 0
b
0 0 ,
o % ~
o o 9
o
q j
9 9 9
o 0 0 ( % ) ~ I I ' .
O O o ~ 9 .
o ~ F e 2 0 3
~ 0 0 o o 9 @
I I f
1 0 2 0 3 0
1 2 0 0
1 0 0 0
8OO
6 0 0 >
4 0 0
2 0 0
0
4 0
O
O,I
O
M .
4O
0
3O
O
o
o o
o O o
1 0 o
0
o
o o
o
o
. . . . . . . . i
1 1 0 0
. o.
9 ~ ' ~ I E "
9
o o 0
O o o
9 @ @
~ o ~
0 0
0 0 O 0
Cr
i i
1 1 1 1 O 1
Fig. 13a-d. Major and trace element variation n hypersthenitesand norites from the Caraiba mine
1 0 0 0 0
1 0 0 0
1 0 0
C r
. . . . . . . 1 0
1 0 0 0 0
closest analogue of Caraiba, are also plotted for compari-
son. Most of the Caraiba rocks plot on the fields for
massif-type anorthosites and the Koperberg Suite. The
orthopyroxene compositions of the Caraiba samples va-
ries less than the plagioclase but no marked positive
correlation is seen like that commonly observed in layered
intrusions. The mineral chemistry of the Caraiba hyper-
sthenites and norites then is similar to that observed in
massif-type anorthosites, and unlike that seen in island-
arc cumulates and tholeiitic layered intrusions. Analogy
between the Koperberg Suite and massif-type anor-
thosites has already been drawn Conradie and Schoch
1986), hence it is useful to test whether models for the
petrogenesis of massif-type anorthosites can be applied to
the Caraiba rocks. First we examine the constraints from
whole rock geochemistry.
G e oc h e m is t r y
Major, trace and rare-earth element analyses of represen-
tative hypersthenites, melanorites and norites from drill-
holes FC2516 and NO33, and outcrops on benches 380,
365N and 395S see Fig. 4 for location) are given in Tables
3a and 3b. It is evident that the typical hypersthenite
or norite rocks from the different localities within the
complex are not significantly different from each other,
and that the chemical variations observed are mostly
controlled by the relative proportions of contained min-
erals. The hypersthenites have higher Fe203, MgO, Cr,
Cu, Mn and Ni, and less A1203, CaO, Na20, Sr and Zr
than the norites. Also, the former are generally more
enriched in Rb, K 20 , Ba, Zn and V, and more depleted in
Nb, TiO2, rare earth elements REE) and P205 than the
latter. Hypersthenites and norites from FC2516 are slight-
ly richer in MgO, Cr, Ni, V, Rb, Ba and K20 than the
equivalent rocks from NO33, reflecting the mineral chem-
istry.
The calculated CIPW norms using Fe203/FeO = 0.15)
of all rocks show very low contents of diopside. Indeed,
the hypersthenites and norites contain significant propor-
tions of normative corundum, indicating saturation with
respect to A1203, and further implying derivation from an
Al-rich magma, as also suggested by the pyroxene com-
positions. The Al-rich characteristic of the Caraiba rocks
is further shown by a plot of A1203
vs
tFeO Fig. 11),
which also emphasises their Fe-rich nature. Plots of
rag
number [MgO/ FeO + MgO) using FezO3/FeO = 0.15]
against major and trace elements Fig. 12), and the posi-
tive correlation between element pairs Zn-Fe, V-Fe, Cr-Fe
and Cr-Ni Fig. 13) reveal that Ni, Cr, MnO, Zn, V and
Fe203 decrease with fractionation, whereas Nb, TiO2,
P2 5
and CaO increase.
Good positive correlation not illustrated here) is
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3 6 3
4 0
3 0
2 0
0
1 0
1 0 0 0
t O 0
1 0
1
0
1
.01
0
0
0
0
0
.-
0
~0
0
4
0
0 0
0
* s
0
0
0
0
0
0
O 0
s
0
* FC2516-Hypersthenites I
9
NO33-Hypersthenites
o FC2516-Nori tes ]
o NO33-Nori tes J
. , , . . , . . , < > = :
o
9 0 ~ 0
O 0
0 0 0
9 0
O 0 0 0
0
0 O
0
0
0
8
~
0
0
0
,I
,I
@
9 *+>* 9 ~ 9 M
~ o*o 00o . .
,01, 0
0
O 0
0
0
@ @
@
o : o
9 o o .
0 0 $ 9 1 4 9
o41, I. e 9
O 9
0 9 O 9 0 0
0
b
1 0 0 0
0
9
d
0
i
f
@
@ O 0
e
9 9
0
@ 9
0 0 @ 0 ~ ' ~ 0 0 " ' ~ 0 0 " "
3
o . ~ ~o r o
o
0 9
0 0
t . ,+ :
S * * r 1 6 2
o+>. ~ . o+O . d ,.
0 0
+,,41
0 Q 0
0
0
o
o 0
o 8
0 0
0
r i i i I
,I
0 0 9 ~ 9
O e ~ 9 0 0 0
0 0 0
0
0 0
o
0 0
~ 0
~
, =
E
0 . 5
F i g . 1 4 a - f . V a r i a t i o n o f r a g -n u m b e r ( m g * ) , m a j o r a n d t r a ce e l e m e n t s i n r e l a ti o n t o c o p p e r c o n c e n t r a t i o n i n s a m p l e s f r o m d r i l l h o l e s F C 2 5 1 6
a n d N O 3 3
observed between e lement pairs Nb-Ti , A1-Ti , K-Ba, Rb-
Ba, Mg-Fe , K-Rb, Ca-Sr and to le ss extent a lso Ti-P. T i
corre lates negat ive ly with V and Zn. Al l these observa-
t ions indicate that Nb is main ly he ld in i lmenite and
poss ib ly a lso in the brown mica ( see h igh Nb value in
biot i t i te 395S-B2 in Table 3 b) . V, Cr and Zn are mo st ly
partit ioned into magnetite, K, R b and Ba into bro wn mica,
and Sr into plagioclase (Ba also in norite plagioclases) .
The increase of T i with increas ing A1 also suggest s that the
amount of i lmenite increases in the most fract ionated
rocks , i. e. the nor i tes. Th e negat ive corre lat ion betw een T i
and V and Zn, on the other hand, indicates that ma gnet i te
is an ear ly fract ionat ing phase a lon g w ith hypersthene and
sulphides . This agrees very we l l with the h igh modal
propo rt ions o f su lphides an d ma gnet i te in hypersthenites .
Foc using o n the behaviour o f copper in the hypersthenites
and nor i tes , p lot s of Cu vs Fe , Ni , Cr , V, K and rag*
(Fig. 14) reveal the following:
1 . The h ighest Cu contents are found in the hyp er-
sthenites as expected from f ie ld and petrograp hic evid-
ence;
2 . T h e C u vs m g* an d C u vs F e 203 p lo t s sh ow t h e N O 33
hypersthenites are geochemica l ly s l ightly more evolv ed
and more Cu-r ich than the ir FC2516 counterparts .
This i s in agreement with the enhanced su lphur so lubi l -
i ty with increas ing Fe content in maf ic magmas, as
determined exper im ental ly (Haugh ton e t a l. 1974) .
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T
e
3
R
a
v
a
y
o
h
h
e
H
y
a
n
e
N
o
,
~
C
o
F
2
F
N
O
3
N
O
3
N
o
2
7
3
3
8
0
9
2
T
H
y
H
y
H
y
H
y
S
O
2
4
5
4
4
3
2
4
7
T
O
2
0
3
0
3
0
8
0
1
A
1
3
4
9
4
1
4
5
2
6
F
2
3
2
2
3
7
1
6
M
n
O
0
3
0
3
0
4
0
4
M
g
O
2
0
2
7
1
6
1
6
C
a
O
0
9
0
6
1
4
7
8
N
a
O
0
9
0
1
0
5
0
2
K
2
0
6
0
8
0
5
0
1
P
O
5
0
0
0
1
0
2
5
2
T
a
9
8
9
7
9
8
9
4
T
e
e
m
e
p
m
)
d
e
m
i
n
b
X
R
F
V
2
3
6
1
C
r
1
1
2
4
N
i
9
1
9
3
C
u
1
1
4
4
Z
4
4
6
3
G
a
2
2
2
1
R
b
2
4
1
6
S
5
1
7
7
Y
1
1
1
1
Z
6
2
4
1
N
b
1
1
3
5
0
8
3
B
3
1
1
5
L
<
2
1
0
4
3
1
6
C
4
8
5
0
2
3
3
N
d
0
6
2
4
8
5
2
T
1
2
2
3
4
8
3
N
i
C
u
0
0
0
0
0
0
0
0
R
e
h
e
e
m
e
d
e
m
i
n
b
C
P
E
3
N
7
B
H
y
4
6
0
4
4
4
2
7
0
3
1
6
1
6
0
3
0
3
0
2
9
5
3
2
1
4
4
2
1
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F
/ ~ o Norites
9 Hypersthenites
A M
Fig. 15. AF M diagram for hypersthenites and nori tes of the Caraiba
com plex. The peridoti tes are show n for compa rison. Fie lds after
Ku no 1968) and Irvine and Baragar 1971)
It had been suggested that the Caraiba complex has the
major element chemistry of tholeiitic suites (Lindenmayer
1981). Fig. 15 shows the AFM diagram for hyperstheni tes
and norites of Caraiba. The rocks clearly plot along
a trend of relatively constant Fe/Mg ratio and increasing
total alkalis, a characteristic of the calc-alkaline series.
The tholeiitic trend obtained by Lindenmayer (1981) re-
sults from the over-emphasis given to peridotites and
gabbroic rocks. Although some hypersthenites and norites
have cumulate textures, which might artificially extend
supposed 'liquid' trends, it is important to note that it
would be difficult to explain the dykes and veins of norites
and hypersthenites, and the mobility of the hypersthenite
groundmass to the breccias, with a liquid component.
Current hypotheses cited to explain the origin of calc-
alkaline suites (or trends), include magneti te fractionation
(Osborn 1959; Gill 1981) and combined assimilation with
fractional crystallization (AFC: DePaolo 1981, Grove and
Baker 1984), while Cawthorn and O'Hara (1976) sugges-
ted that early fractionation of amphibole can generate
calc-alkaline trends. The la tter seems not to be applicable
to the Caraiba norites-hypersthenites as no primary am-
phibole has been observed. The relations between Fe, Ti,
V, and Zn suggest that fractionation involving magnetite
(plus sulphide) could account for the calc-alkaline trend
observed on the AFM diagram. However, the Caraiba
complex is not a normal calc-alkaline suite because
the late 'differentiates' are norites and anorthosites, not
granites.
The mantle normalized multi-element diagrams for the
average compositions of hypersthenite and norites and
various selected samples (Fig. 16) emphasise the strong
mineralogical control on the whole-rock chemistry. Thus,
those hypersthenites and norites with high modal abund-
ances of apatite generally contain high contents of REE,
Y and P (e.g. sample NO33-92.29 in Table 3a and
Fig. 16b, d) confirming that apatite concentrates most of
the REE. Similarly, high modal phlogopitic mica corre-
lates with high contents of Rb, Ba, Ti, Nb and K (e.g.
sample 395S-B2 in Table 3b and Fig. 16b). The coarse-
grained and intrusive hypersthenite of bench 395S (ana-
lyses 395S and 395S-C in Figs. 16a, b) has one of the
lowest modal apatite contents and consequently has low
P and REE abundances on mantle normalized diagrams.
Conversely, it has up to 3 primary phlogopite, reflected
by the high Rb, Ba and K on these diagrams.
In Fig. 16, the plagioclase-poor hypersthenites have
a marked negative Sr anomaly whereas the plagioclase-
rich norites do not. Although Sr partitions strongly into
plagioclase (Philpotts and Schnetzler 1970), it is not easy
to account for the negative Sr anomaly in the hyper-
sthenites unless plagioclase had been involved in their
petrogenesis, for instance through earlier removal of a pla-
gioclase-rich melt from the hyperstheni te source. Both the
hypersthenites and, to a lesser extent, the norites show
marked depletion in Nb relative to the light rare earth
(LREE) and low-field strength elements (LFSE). Such
negative Nb anomalies are characteristic of island-arc
lavas, many continental flood basalts and of the continen-
tal crust i tself(Saunders et al. 1980; Thompson et al. 1983;
Weaver and Tarney 1983,1984). Proterozoic dolerite
dykes commonly show this anomaly (Weaver and Tarney
1983), as do Phanerozoic flood basalts thought to be
derived from the subcontinental lithospheric mantle, such
as the Paran~ from southern Brazil (Mantovani et al.
1985, Hawkesworth et al. 1986). It is reasonable to suspect
then tha t the Caraiba ultramafic/mafic hypersthenites and
norites with this geochemical signature could also be of
sub-continental lithosphere derivation. Rare earth ele-
ments (REE) can be useful petrogenetic indicators in this
regard.
The REE patterns of hypersthenites and norites are
quite fractionated (Fig. 17, Table 3a, b), but variable, with
(La/Yb)N ratios ranging from 2 to 13 in hypersthenites and
from 7 to 27 in norites. The hypersthenites with the
highest and lowest REE abundances are, respectively, the
apatite-rich samples from borehole NO33 (sample 92.29)
and the coarse-grained and intrusive pyroxenite from
bench 395S in the open-pit (sample 395S-C). The hyper-
sthenite dished REE patterns with consistent negative Eu
anomalies (Fig. 17c) would be consistent with removal of
phases such as plagioclase and apatite which normally
concentrate Eu + LREE and MREE respectively. The
norites (Fig. 17a) are more variable regarding the Eu
anomaly, but it is clear that removal of plagioclase from
either the source (i.e. earlier melting episode, as noted
above) or a magma has played a significant role in the
evolution of the Caraiba rocks.
Interesting comparisons can be made with the Koper-
berg Suite in South Africa, and massif-type anorthosites
(Fig. 17b, d). REE pat terns for the Caraiba and the Koper-
berg Suite rocks are much more fractionated than for the
massif-types, especially with respect to the middle- to
light-REE. While this may be because the source for the
Caraiba and Koperberg rocks contained residual phases
with low distr ibution coefficient for these elements, a more
likely explanation is tha t the Caraiba/Koperberg source
had suffered metasomatism that had left it enriched
in more easily fusible LREE- to MREE-rich mineral
phases such as mica and amphibole +_ apatite. The lack of
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8/9/2019 Genesis of the Precambrian Copper-rich Caraiba
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367
5 0 0
2 0 0
1 0 0
5 0
2
g s
2
1
[ a A v e r a g e s o f H y p e r s t h e n i t e s
3 9 5 S
0 . 5 - - 9 3 6 5 N
o 3 8 0
0 . 2 9 N O 3 3
t~ FC2516
0 . 1 I I I I I I I I I I I I
C A v e r a g e s o f N o r i t e s
2 0 0
1 0 0
5 0
2
l O
g
2
1
9 ~ 5 N
0 . 5 -
9 3 8 0
o N 0 3 3
0 . 2 -
[ ] FC2516
0.1 t ~ ~ ~ i i ~ i ~ ~ ~
R b B a K N b L a C e S r N d P Z r T i Y
b R e p r e s e n t a t i v e H y p e r s t h e n i t e - B i o ti ti te
z~ 395S-B2
o 395S-C
o NO33-92.29
d R e p r e s e n t a t i v e N o r i t e s
1 3
- - o N 0 3 3 - 9 3 . 2 6
c= FC2516-265.20
i
R b B a K N b L a C e S r N d P Z r T i Y
F i g . 1 6 a -d . M a n t l e n o r m a l i s e d m u l t i - e l e m e n t d i a g r a m s o f C a r a i b a C o m p l e x r o c k s : a a v e r a g e c o m p o s i t i o n s o f h y p e rs t h e n i te s f r o m d i f f er e n t
l o c a l i ti e s , b s e l e c t e d h y p e r s t h e n i t e s a n d o n e b i o t i t i t e , e a v e r a g e s o f n o r i t e s , a n d d s e l e c t e d n o r i t e s . N o r m a l i s i n g v a l u e s f r o m S u n a n d
M c D o n o u g h 1 9 8 9 )
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369
20
15
10
3 .
0 3
~ 5
o 3
~
Q
5
10
Nori lsk Duluth
I = I ~ I
len et al. (1990). A more recent study of the Koperberg
Suite sulphides by Boer et al. (1994) has shown a weak
negative correlation of mean 034S with mean Cu/S ratios
which is interpreted in terms of sulphur devolatilisation
during the high-grade metamorphism. They also recorded
modest 180 enrichment in the mafic host rocks (range in
61SOrock = + 5.9 to + 8.3 0, compared with a 'normal
mantle' value of ca. + 5.7 ___0.3 0), which was taken to
indicate some crustal contamination of mafic magmas.
The correspondence between Caraiba and the Koperberg
suite is close, despite the ca. 1000 Ma difference in age;
Boer et al. (1994) also favour mantle derivation. Crustal
geochemical or isotopic signatures are present in both
suites, and the ques tion remains as to what processes are
responsible for this.
Fig 18 Sulphur isotope data for mafic complexes and the upper
mantle in comparison with the Caraiba sulphides (data after Kyser
1986; Ohmoto 1986; Von Gehlen et al. 1990)
significant HREE depletion suggests that garnet was not
a residual phase during melting. The Koperberg rocks
have higher abundances of REE but both suites exhibit
similar patterns.
Sulphur i sotopes
Sulphur isotopic compositions of the Caraiba sulphide
minerals were used by Oliveira and Choudhuri (1993) to
evaluate the crustal contribution in the genesis of the
sulphides (cf. Lindenmayer 1981). Fourteen samples of
remobilised and disseminated chalcopyrite or chal-
copyrite + bornite from the Caraiba open-pit were ana-
lysed for sulphur isotopes at the University of Calgary,
Canada. They show 634S in the range - 1.495 to + 0.643
with an average value of - 0.604.
Because the isotopic fractionation factors among all
sulphide species are probably within 0.5 o, magmas for-
med by partial melting of the mantle or rocks crystallized
from such magmas should have 6348 values similar to
those of the parental mantle material (Ohmoto 1986).
Sulphur derived from mantle has an isotopic composition
very similar to meteorite sulphur and it is assumed that
the upper mantle as a whole has 634S = 0 _ 3 0 and
the primitive upper mant le an average of + 0.5 0 (e.g.
Ohmoto 1986; Chaussidon et al. 1989). However, many
mantle derived mafic igneous rocks have 6a*s outside this
range (Ohmoto 1986, and references therein), such as the
Duluth complex (0 to + 17 o, Noril' sk intrusive ( + 6 to
+ 16 o), and the Bushveld complex ( - 9 to -6 o).
Although the mantle might be slightly heterogeneous
(Chaussidon et al. 1989), assimilation of crustal sulphur
has been evoked to explain such anomalous values (e.g.
Naldrett 1981; Ohmoto 1986).
Figure 18 shows the Caraiba sulphur isotope data fall in
the range predicted for the mantle and appear not to have
inherited significant amounts of heavy 3 from supra-
crustal rocks. Moreover, the sulphur isotope ratios from
Caraiba are very similar to sulphide sulphur from the
Okiep copper district, South Africa reported by Von Geh-
D i s c u s s i on
Petrogenetic models for the Caraiba complex are con-
strained by the following observations:
1. Field evidence indicates that the complex consists of
multiple dyke-like intrusions and igneous breccias of
hypersthenite, norite and minor anorthosite with
peridotite and gabbro xenoliths, and tha t it is younger
than most high-grade country-rocks: supracrustals,
banded gneisses, migmatitic gneiss, tonalitic ortho-
gneisses, garnet- and pyroxene granulites. It may be
coeval with the youngest granitic to syenitic sheets.
Several hypersthenites and norites are likely to have
been liquids judging from their mode of occurrence.
2. The peridotitic and gabbroic xenoliths have few
counterparts in the surrounding country rocks and are
not linked to the hypersthenites and norites by frac-
tional crystallization, though may conceivably be re-
lated to them by some other process/processes at some
earlier stage of evolution of the complex;
3. The similarities with massif-type anorthosites suggests
a common petrogenesis.
4. The hypersthenites and norites display a major element
calc-alkaline signature and may be linked theoretically
by fractionation processes involving plagioclase, or-
thopyroxene and magnetite ( + sulphide). Local crus-
tal contamination seems not to have been important.
Field, petrographic and geochemical evidence indi-
cates that there have been several episodes of magma
injection, each having different degrees of enrichment
(e.g. hypersthenites associated with the anorthosites
are richer in incompatible trace elements, Fe and Cu);
5. The primary magma(s) must have had a basic composi-
tion, low calcium content, high Fe /Mg ratio and rela-
tively high alumina; also hydrous and enriched in
Ti, Cu, S, K, REE and P to account for the presence of
primary copper sulphides, ilmenite, phlogopite and
apatite, sometimes in large modal proportion. The
source must have been enriched in incompatible
elements, most likely the subcontinental mantle or
lower crust as suggested by the fractionated REE
(LaN/YbN = 2-27), and by negative Nb anomalies on
mantle-normalized diagrams;
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tend to favour a mantle origin. However, in both these
mecha nisms there is the potential to s ubduct mafic ocean
floor with hydrot herma l copper enrich ment into the deep
crust or upper man tle to create the right source character-
istics for a future Caraiba deposit.
Finally, the Car aiba rocks, especially the hyper-
sthenites may have their emplacement controlled by
a regional shear belt. A coarse-grained hypersthenite
dyke has given a Sm- Nd min eral isochron of
1890 _+ 60 Ma (Oliveira 1990b) which is rema rka bly sim-
ilar to faul t-co ntrol led granites (1889 _+ 64 Ma) report ed
by Padi lha a nd Melo (1991) in the Salvador-Curagfi Oro-
gen south of the Caraiba area. Metasomatism in the
mantle or crust is enhanced within shear zones. It is
possible that the Caraiba Complex may result from re-
mobil isa t ion of metasomatised contaminated mantle or
mafic + sediment lower crust during the waning stage of
evolut ion of the early Proterozo ic Salvador-Curaqfi colli-
sional orogen.
Acknowledoements The Cara iba Mine Ltd., and its staff, are warmly
thanked for field logistic support, and Rob Wilson and Nick Marsh
for laboratory support with microprobe and XRF analyses at
Leicester. The Brazilian CNPq and the British CVCP are gratefully
acknowledged for providing financial support for EPO at Leicester.
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