Delvigne, Jean

Groke, M.C.T.

Parisot, Jean-Claude

Horizon / Pleins textes

Sci. Gof., iWém., 72, p. 141 - 148, Strasbourg, I983 
PETROGRAPHICAL ASPECTS OF THE SUPERGENE #EATHERING OF GARNET IN THE "SERRA DOS 
CARAJAS" (PARA, BRASIL) * 
2 J.C. PARISOT', J. DELVIGNE', and M.C.T. GROKE 
1. Mission O.R.S.T.O.M.. I n s t i t u t o  de Geociências, Universidade de São Paulo, 
Caixa Pos ta l  20.899, CEP 01498 SÃ0 PAULO, BRASIL 
2. I n s t i t u t o  de Geociências, Universidade de São Paulo, 
Caixa Pos t a l  20.899, CEP 01498 SÃ0 PAULO, BRASIL - 
ABSTRACT 
The copper mineralization of Salobo 3A is  associated with a sch i s tose  formation 
i n  which numerous f ac i e s  are enriched i n  almandine garnet.  During supergene weathe- 
r ing ,  the  garnet is pseudomorphosed by i r o n  oxi-hydroxides. Three s t ages  a r e  obser- 
ved: 1- Appearance of a corona of amorphous i ron  oxi-hydroxides on t h e  w a l l s  of the  
f i s su res ;  2- Formation of a second concentric inner corona of wel l  c r y s t a l l i z e d  goe- 
t h i t e ;  3- Subs t i tu t ion  of the  inner garnet residue by c ryp toc rys t a l l i ne  o r  amorphous 
i ron  oxi-hydroxides. These transformations a r e  isovolumetric wi th  a p a r t  of t he  i ron  
imported. Near the  sur face ,  the  pseudomorphs of garnet form nodules wi th  a s t i l l  re- 
cognizable boxwork s t ruc ture .  
I. INTRODUCTION 
The copper mineralization of Salobo 3A, located about 550 km SSW of Belem (50°W 
6'5) i n  the  southeastern pa r t  of the Amazonian f o r e s t ,  belongs to  t h e  recent ly  dis- 
covered mineral province of Carajis.  The evaluation work is  being ca r r i ed  out by DO- 
GE0 (Subcompany of the "Companhia Vale do Rio Doce"). Geological and pe t rographica l  
s tud ies  have shown tha t  copper is  associated with a sch is tose  formation i n  which nu- 
merous f a c i e s  a r e  enriched i n  garnet (MARTINS e t  a1.,1982). According t o  these au- 
thors,  t h i s  formation underwent a metamorphism reaching the  amphibolite f ac i e s ,  and 
i n  some poin ts  t he  g ranu l i t e  fac ies ,  followed by a greenschis t  retrometamorphism. 
The weathered cover, about 30m th ick ,  developed a t  the  expense of t h e  sch i s tose  
formation i s  being studied. t o  follow the suc- 
cess ive  s tages  of garnet weathering, from the parent-rock up t o  t h e  sur face .  The 
climate i s  characterized by a long wet season (8 t o  9 months). an annual r a i n f a l l  
r a t e  Af 1 800 mm and an average temperature of 25OC. 
Two p r o f i l e s  were sampled from the f r e sh  rock to  the sur face .  These samples we- 
r e  inves t iga ted  sys temat ica l ly  under the polar iz ing  microscope and some were s tudied  
with a scanning e lec t ron  microscope (SEM) and analyzed with a microprobe. 
II. RESULTS 
P i t s  and d r i l l i n g s  make i t  poss ib le  
A. The parent mater ia l  
The mineralogical composition of the parent-rock va r i e s  according t o  the f ac i e s  
but i n  p r a c t i c a l l y  a l l  the f ac i e s ,  amphibole (generally cuming ton i t e  - grune r i t e  
type),  garnet and b i o t i t e  occur i n  var iab le  proportions.  According t o  the  f ac i e s ,  
magnetite (sometimes very abundant), quartz,  o l iv ine  and sometimes p lag ioc lase  can 
0. R.S.T. O. M. Fonds Doc!"t@re 
3 g g 0 4 4 ~ 3  No : a 
be found. The s t r u c t u r e  of t h i s  rock i s  granoblas t ic  t o  lep idoblas t ic  with a f i n e  t o  
la rge  gra in  s i z e  (MARTINS et al.,  1982). Garnets form i r r e g u l a r  porphyroblasts ran- 
ging from 1 mm t o  2 cm i n  diameter. Microprobe analyses show t h a t  they contain 77 t o  
82% of almandine (Table 1)  and have the following average formula: 
This formula i s  i n  accordance with t h e  almandine composition quoted by DEER e t  al. 
(1962). Often, the  garne ts  present a p o i k i l o b l a s t i c  s t r u c t u r e  with amphibole o r  
quartz inclusions.  Garnets are generally s t rongly  f rac tured  (Fig. 1.1). I n  numerous 
f i s s u r e s  and a t  the  gra in  edges, c h l o r i t e  and green b i o t i t e  ( P l a t e  l . a ) ,  o r ig ina t ing  
from the retromeramorphism phases& observed. 
Table 1: Microprobe and mineralogical analyses of s i x  garnet grains.  
Chemical composition (7.) 
1 2 
Si02 35,58 35,13 
A1203 20,87 21,oo 
Fe0 36,90 36,23 
?ln0 3,35 3,44 
NgO 2,05 2,36 
Ca0 1,21 1.55 
99,96 99,71 
Number of ions on the  b a s i s  of 24 
S i  5,852 5,790 
A l  0,148 0,210 
A l  3.889 3,862 
Fe3+ 0,111 0,138 
Fez+ 4.946 4.838 
3 
34,78 
20,94 
35,59 
4,28 
2,05 
1,68 
99,32 
(0 ,  OH) 
5,770 
0,230 
3,857 
0,143 
4.776 
4 
34,84 
20,57 
35,8a 
4,79 
1,70 
1,64 
99,42 
5,800 
0,200 
3,828 
0,172 
4.803 
5 
35.76 
20.71 
37,07 
3,62 
1.62 
1,23 
LOO ,o1 
5,888 
0,112 
3,900 
0,100 
4.987 
6 
35,29 
20.57 
36,15 
3,54 
1,46 
1,29 
9a,30 
5,900 
0,100 
3,945 
0,055 
4,981 
0;584 6,17 0;511 0,424 01399 :l:g: 6,08 Mg 0'505 Mn 0:460 6a12 0,479 0,600 "19 0,674 6'19 0,504 6'11 
Ca 0,213 0,273 0,299 O ,  293 0,216 O ,  230 
Mol. % end members 
Almandin 80,7 78,4 77,2 77,6 81,7 82,O 
Andradite 2,8 395 326 493 2,5 1,4 
Grossular O ,  7 099 1.2 0,4 190 294 
Pyrope 892 935 8 ,3  638 695 6,O 
S p e s s a r t i t e  7 , 6  727 997 10,9 893 892 
B. I n i t i a l  s tages  of garnet weathering 
Garnet weathering s t a r t s  ear ly ,  namely simultaneously with the  amphibole wea- 
thering. I n  the f i r s t  weathering s tage ,  a f i n e  dark oxi-hydroxides, 
with an amorphous aspect,  appears a t  the walls of c h l o r i t e  f i s s u r e s .  The i r o n  oxi- 
hydroxides a l s o  penetrate the garnet f r a c t u r e s ,  giving rise t o  a garnet mosaic (Fig. 
1.2).  
In the next s tage ,  a second corona of well  c r y s t a l l i z e d  goe th i te  forms accor- 
ding t o  a c e n t r i p e t a l  pa t te rn  a t  the  expense of the garnet (P la te  1.b; Fig. 1.3);  
these c r y s t a l l i z a t i o n s  develop on intraminera1 f i s s u r e s  and contact surfaces between 
the garnet and i t s  inc lus ions  (quartz f o r  example). EmRECHTS and STOOPS (1982) des- 
cribed the  same type of weathering f o r  garnets of Cameroon. I n  ou r  case,  however, 
the formation of well  c r y s t a l l i z e d  goe th i te  takes place Indeed, t h i s  s tage  
i s  reached a t  a maximum of 5 meters from the  f r e s h  rock and b i o t i t e s  a r e . s t i l l  i n  
corona of i r o n  
e a r l i e r .  
t h e i r  f i r s t  s tage  of weathering. In t h i s  s tage ,  the weathering of the  f i s s u r a l  chlo- 
rites i n t o  smect i te  begins, even a s  the  i r o n  segregation i n  the  b i o t i t e s  and i n  the 
amphiboles. Our observations d i f f e r  from those of SARAZIN e t  a l .  (1982) who observed 
a l s o  i n  Cameroon, a garnet pseudomorph cons is t ing  of amorphous o r  poorly c r y s t a l l i -  
zed i r o n  oxí-hydroxides; a t  the  ex- 
pense of these  amorphous oxi-hydroxides. 
w e l l  c r y s t a l l i z e d  goe th i te  appears only later, 
/ ' I  
Fig. 1 : Schematic representa t ion  of garnet weathering 
C. Subsequent s tages  of garnet weathering 
Two evolutions,  which seem independent of the f a c i e s ,  can be observed. 
I n  the  f i r s t  case, which is  r a r e ,  one observes a congruent d i sso lu t ion  of t h e  
garnet nucleus: a boxwork s t r u c t u r e  i s  formed (Pla tes  1.c and 2.a). I n  the boxwork 
c a v i t i e s ,  g i b b s i t e  o r  goe th i te  c r y s t a l l i t e s  may appear, these  
c a v i t i e s  a r e  f i l l e d  by amorphous o r  poorly c r y s t a l l i z e d  i ron  oxi-hydroxides (P la te  
1.d; Fig. 1.4).  This weathering evolution i s  i n  agreement with the  observations of 
EMBRECHTS and STOOPS (1982) i n  Cameroon. 
which i s  more frequent,  a second corona of i r o n  oxi-hydro- 
xides appears and the garnet nucleus. Under 
the o p t i c a l  microscope, t h i s  material  seems amorphous o r  poorly c r y s t a l l i z e d  ( P l a t e  
1.c).  SEM observation shows t h a t  i t  cons is t s  of c ryptocrys ta l l ine  i r o n  oxi-hydroxide 
(P la te  2.b).  This second corona increases i n  a cent r ipe ta l  pa t te rn  a t  the expense of 
the garnet,  t o  f i l l  completely, or  nearly completely, the volume i n i t i a l l y  occupied 
by the silicate (Pla te  1.E). SEX observation shows t h a t  the porosity is more and 
more important from periphery t o  center (P la te  2.c; Fig. 1.5).  A t  t h i s  s tage ,  the 
p h y l l o s i l i c a t e s  weathering is  more advanced. 
I n  f i s s u r e s ,  c h l o r i t e s  a r e  nearly completely transformed i n t o  smectite and kao- 
153 
but more frequently,  
I n  the second case,  
between the well c r y s t a l l i z e d  goe th i te  
-- 
l i n i t e .  A s  f o r  b i o t i t e .  the i ron  segregation phase is  more advanced and one observes 
i n t o  i ron  
oxi-hydroxides and in to  smectite.  Once more, the  r ap id i ty  of the  garnet weathering 
is shown, s ince  only 10 meters above the f r e sh  rock is present.  
There is  pseudomorphose of the garnet by i r o n  oxi-hydroxides. S t ruc tures  t h a t  have 
been formed remain s t a b l e  up to  the top of the  profile,where they become rounded and 
form i ron  nodules. In  these  nodules, typ ica l  s t ruc tu re  described above i s  s t i l l  re- 
cognizable, although the  wel l  c rys t a l l i zed  goe th i te  shows a less massive aspect (see 
p l a t e  2.d). I n  the  supe r f i c i a l  horizons, we never f ind  the boxwork s t ruc tu re  with 
cen t r a l  voids. Therefore one supposes t h a t  the  voids a r e  f i l l e d  by i ron  oxi-hydroxi- 
des as described by EMBXECHTS and STOOPS (1982). 
&the beginning of i t s  ex fo l i a t ion .  Amphiboles a r e  completely transformed 
no res idua l  garnet 
Table 2: Microprobe analyses of goe th i t e  end i ron  oxi-hydroxides (X) 
Ext 7 Med 8 Ned 9 MedlO Medll In t lZ  I n t l 3  I n t l 4  
Fe203 75.58 76,16 74.33 75.13 71.88 73.85 72.09 69.25 
ALZO3 6.80 6.58 6.04 6.96 7.25 8.97 7.62 6.80 
SiO2 3.93 3.56 3.80 4.00 3.36 6.53 3.53 4.70 
T iop  0.00 0.00 0.00 0.00 0.15 0.00 0.00 0.00 
XgO 0.00 0.00 0.00 0.00 0.27 0.00 0.27 0.00 
Z 86,31 86,29 84.17 86.09 82.91. 89.35 83.51 80.75 
H20 13.69 13.71 15.83 13.91 17.09 10.65 16.49 19.25 
Ext = External amorphous corona 
>led = Medium corona of w e l l  c rys t a l l i zed  goe th i te  
l n t  = In t e rna l  poorly c rys t a l l i zed  i ron  oxi-hydroxides . 
H 2 0  = Values a r e  calculated by d i f fe rence  
(eventually including P and Cu) 
Microprobe analyses of d i f f e ren t  i ron  oxi-hydroxide coronas do not show impor- 
t an t  chemical d i f fe rences  (Table 2).  Indepedently of t h e i r  pos i t i on  and t h e i r  degree 
of c rys t a l l i za t ion ,  coronas present concentrations of about 7% Alzo3,  4X Si0 and 
73% Fe203. The spec t re  obtained with the  microprobe shows a l so  t r ace  of coppzr even 
a s  a poorly defined pick which may be due t o  phosphorus. Its o r ig in  a p a t i t e  
present i n  these formations. The phosphorus f ixa t ion  by i ron  oxi-hydroxides i s  des- 
cribed by numerous authors (CABRERA e t  a l . ,  1977; RUSSEL et a l . ,  1974). 
I I I .  DISCUSSION AND CONCLUSION 
A l l  the  transformations described are isovolumetric (Fig. 1). Knowing the  
rage  concentrations of elements i n  garnet and i n  goethite,  and using a minimum 
s i t y  fo r  these minerals,  we can ca lcu la te  the percentage of gain and loss 
element during the garnet weathering as follows: 
may be 
ave- 
den- 
for each 
. . -  r r  
V.d.C x 100 ; as  p = p.c and P = V.d : p =loo. = go - gr 100 
A s  the transformations a r e  isovolumetric, V = V and the formula can be writed: 
gr  go' 
where: 
D = percentage of gain (+) and loss (-) ; 
p = weight of element i n  mineral goe th i t e  (p 
V = volume of mineral goe th i te  (V ) and garnet (ygr) ; 
C = concentration ( X )  of e lemenpin  mineral goe th i te  (Cgo) and garne t  (C ) ; 
(Pgr) ; ) and garnet P =we igh t  of mineral go 
d = dens i ty  of mineral goe th i te  (d ) and garnet (dgr). g r  
go 
If  we take a minimum density of 3.3 f o r  goe th i te  and 4.3 f o r  garnet (FISCHESSER 
w e  f i nd  tha t  the  garnet does not contain enough i ron  t o  form the  same volume 1970), 
of goe th i te  (Table 3). 
Table 3: Estimation of t he  gain and lo s s  of elements 
during the isovolumetric garnet weathering i n  VZ. 
go 'gr 
D 
51.33 1,21 1,69 + 39.67 
0.48 0,12 - 74,47 
0,06 - 91,55 
D = percentage of gain (+) and los s  (-); 
C 
P 
= average concentration of element i n  garnet and goe th i te ;  
= weight of element by un i t  voluine i n  gnrnct and goe th i t e  
gr,  go 
gr' (garnet density = 4.3; goe th i te  density = 3.3). 
As voids do not appear between the  garnet and the goe th i te ,  we  can es t imate  
tha t ,  r e l a t i v e  to  the  garnet i ron ,  a minimum of 39% of the  i ron  is  imported. A s  f o r  
the inner f i l l i n g  of i ron  oxi-hydroxide, w e  ob ta in  a density of 2,36, i f  we consider 
t ha t  only garnet-iron has contributed to  i t s  formation. This value would seem to be 
reasonable for the s ign i f i can t ly  porous c ryptocrys ta l l ine  goe th i te  observed i n  t h e  
center  of the  boxwork c e l l s  (P la te  2.d). However, the  presence of copper and phos- 
phorus i n  the  i ron  oxi-hydroxides suggests t he  poss ib i l i t y  of an external source, a t  
l e a s t  f o r  these elements. 
I n  t h i s  paper we show tha t  garnet weathers very quickly in to  i ron  oxi-hydro- 
xides. Two evolutionary pathways a re  observed: pseudomorphose of the garnet by iron- 
oxi-hydroxides and congruent d i sso lu t ion  of the garnet with formation of a boxwork 
s t ruc tu re .  In both cases the  transformations occur a t  constant volume wi th  ex te rna l  
cont r ibu t ion  of i ron  probably coming f i r s t  from amphiboles and l a t e r  from b i o t i t e s .  
AKNOULEDGMENTS 
Authors aknowledge professors G. BOCQUIER and Ph. ILDEFONSE f o r  allowing the  
use of the  SEM and the microprobe of 
the  Pa r i s  V I 1  Univeristy.  This work was financed by the  Conselho Nacional de Pesqui- 
sa(CNPq). Authors a re  a l so  g ra t e fu l  t o  the Professors G. STOOPS, Algemene Geologie, 
Ri jksunivers i te i t  van Gent, Belgie and D. PROUST, Laboratoire de Pédologie de l'Uni- 
v e r s i t é  de Po i t i e r s ,  France, reviewers, f o r  t he i r  he lpfu l  comments on t he  manuscript 
and to  Professor M. UNGS, I n s t i t u t o  de Geociências, Universidade de São Paulo, Bra- 
s i l ,  €or t r ans l a t ion  of the  o r ig ina l  french manuscript. 
REFERENCES 
CABRERA F., MADRID L., m B A R R I  P .  (1977) - Adsorption of phosphate by var ious  oxi- 
des: theor ica l  treatment of the adsorption envelope. J .Soil  Sci., 28, p.306-313 
DEER N.A., HOIJIE R .A. ,  ZUSSMANN J. (1962) - Rock forming minerals.  Vol 1: Ortho- and 
r ing - s i l i ca t e s ,  London, Longmens, p.77-104. 
EMBRECHTS J. and STOOPS G. (1982) - Elicroscopical aspects of garnet weathering i n  
humid t ropica l  environment. J .  So i l  Sc i . ,  33, p.535-545. 
of the  "Laboratoire des Sciences de la Terre" 
115 
..a 
FISCHESSER R. (1970) - Données des principales espèces minérales. société de l'in- I dustrie minérale, Saint Etienne, 679 p. 
MARTINS L.P.B., SAUERESSIG R. and de MELO VIEIRA, MIA. (1982) - Aspectos petrogrãfi- 
Anais do I simpósio da geo- 
structures 
cos das principais litologias da seqflência Salobo. 
logia da Amazônia, Belém, p.253-262. 
of gibbsite, goethite and phosphated goethite. Nature, 248, p.220-221. 
du fer 
et de l'aluminium en milieu ferrallitique. Geochim. cosgochim. Acta, 46, p.1267 
-1279. 
RUSSEL J.D., PARFITT R.L., FRASER A.R. and FARNER W.C. (1974) - Surface 
SARAZIN G., ILDEFONSE Ph. and MULLER J.P. (1982) - Contrôle de la solubilité 
RESUME 
La minéralisation cuprifère de Salobo 3A est associée 5 une formation schisteu- 
se dont de nombreux faciès sont riches en grenat almandin. Lors de l'altération su- 
perficielle, le grenat est pseudomorphosé en oxy-hydroxydes de fer. Trois étapes ont 
été observées: 1- Apparition d'une couronne d'hydroxydes de fer amorphes au contact 
des fissures périphériques ou internes; 2- Formation d'une seconde couronne, interne 
'a la première, des résidus internes 
de grenat Ces transforma- 
tions sont isovolumétriques avec importation d'une partie du fer. Prês de la surface 
les pseudomorphoses de grenat forment des nodules dont la structure interne cloison- 
née, est encore reconnaissable. 
de goethite bien cristallisée; 3- Remplacement 
par des hydroxydes de fer cryptocristallins ou amorphes. 
P L A T E  I : T H I N  SECTION M I C R O G R A P H S .  
a : Unweathered garnet with chloritic fissures. - plain light. 
b : Well crystallized goethite surrounding a garnet nucleus. Note the garnet relic 
in the goethtite. - plain light. 
c : Boxwork structure of goethite with a garnet remnant (Gr) in the void. Note the 
amorphous corona and the inner well crystallized goethite. - plain 
d : Infilling structure formed by amorphous or poorly crystallized iron oxi-hydro- 
xides. - plain light. 
e : Weathered garnet. Note the poorly crystallized corona (In) that appears between 
the well crystallized goethite (Go) and the garnet (Gr). In the fissure, chlo- 
rite is weathered into smectite. - plain light. 
f : Completely weathered garnets into well crystallized goethite (clear peripheral 
coronas) and dark brown cryptocrystalline iron hydroxides cores. - plain light. 
light. 
I li 
PLATE I I  : SEM PHOTOGRAPHS. 
a : Goethite boxwork s t ruc tu re .  Note the twin layers  of i r o n  hydroxide on both s ides  
b : Formation of c ryptocrys ta l l ine  i ron  oxi-hydroxides ( I N )  between w e l l  c rys ta l l i -  
c : Completely weathered garnet. Note the  increasing porosi ty  from periphery t o  
d : Completely weathered garnet (sample near the surface) .  Note the l e s s  massive as- 
of  t he  or ig ina l  f i s s u r a l  network and the open i n t e r n a l  cav i t i e s .  
zed goe th i te  (GO) and garnet (GR). 
center .  
pect of the well c rys ta l l ized  goethi te  ( G O ) .  


Petrographical aspects of the supergene weathering of garmet in the "Serra dos Carajas" (Para, Brasil)

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