Isotopic domains controlled by transtensional and transpressional sectors
in the auriferous Diadema shear belt, northern Brazil
Claudinei Gouveia de Oliveira*, Roberto Ventura Santos1
Instituto de Geociencias, Universidade de Brasılia, Brasılia-DF 70910-900, Brazil
Received 30 April 2001; accepted 30 June 2003
Abstract
This paper presents carbon and oxygen isotope data from the auriferous Diadema shear belt, Carajas mineral province, northern Brazil.
This area was affected by large-scale deformation accompanied by metamorphism and carbonate alteration of the metavolcanic rocks.
Carbonate carbon-isotope data ðd13CPDB ¼ 22.2 to 23.6‰, n ¼ 11Þ reveal that the hydrothermally altered zone within the shear zone
has a different isotopic composition than that of the country rock (þ0.4 to 21.7‰, n ¼ 12Þ: Similarly, initial strontium isotope
compositions reveal different isotopic compositions between the country rock of the shear zone (87Sr/86Sr ¼ 0.70454–0.70642) and the
transpressure and transtensional zones (87Sr/86Sr ¼ 0.71286–0.71841), which indicates that the alteration zone resulted from large-scale
fluid flow derived from an external reservoir. In addition to these differences, there are small variations in the d13CPDB values (from 22.8
to 22.0‰) in the shear zone, which may be related to preferential carbonate decomposition (and CO2 devolatilization) in the
transpressional domains. Evidence of carbonate decomposition is supported by calc-silicate rocks, which derived from the reaction
between calcite and silicate minerals.
The hydrothermal alteration and gold mineralization at the Diadema shear belt comprised both fluid infiltration from an external source
and contemporaneous fluid generation in the shear zone. Compared with other gold deposits related to shear zones, the Diadema gold
deposit developed at hypozonal crustal levels in the transition between greenschist and amphibolite metamorphic facies mineralogical
assemblages.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: Carbon isotopes; Gold mineralization; Oxygen isotopes; Shear zone; Structural domains
1. Introduction
Large-scale (.100 km) shear zones are important
pathways for fluid migration and usually are associated
with carbonate deposition and gold mineralization.
Examples of large-scale hydrothermal alteration along
shear zones include the carbonate alteration along fracture
and fault zones commonly observed in lode-gold deposits
from greenstone belt terrains (Kerrich and Fyfe, 1981;
Covine et al., 1984; Kerrich, 1986; Kishida and Kerrich,
1987; Kerrich, 1989; McCaig et al., 1990; Groves and
Forster, 1991; Robert et al., 1991; Dipple and Ferry, 1992;
Groves, 1993; Roberts and Shahan, 1994; Hagemann and
Brown, 1996). Studies that address fluid–source and
fluid–rock interactions in shear zones show that they
display chemical and isotopic compositions that are
different from their host rocks (Oliver et al., 1993;
Wickham et al., 1994; Pili et al., 1997). This difference
indicates that they behave as an open system.
The Diadema shear belt, in northern Brazil, is a regional
structure (.100 km long) that crosscuts the Sapuacaia
greenstone belt and includes several shear zones. In the
main shear zone of the Diadema shear belt, large-scale
deformation, metamorphism, and hydrothermal alteration
were responsible for gold mineralization (Oliveira and
Leonardos, 1990; Oliveira and Santos, 1995; Villas and
Santos, 2001). Oliveira and Santos (1995) show that this
shear zone displays metric scale variations in its miner-
alogical associations, which suggests that conditions grade
from lower greenschist (high Pf and low shear stress) to
amphibolite (low Pf and high shear stress) facies. These
metamorphic conditions are controlled mainly by local
0895-9811/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsames.2003.06.003
Journal of South American Earth Sciences 16 (2003) 513–522
www.elsevier.com/locate/jsames
1 Tel.: þ55-61-347-2078; fax: þ55-61-272-4286.
* Corresponding author. Tel.: þ55-61-348-2830; fax: þ55-61-272-4286.
E-mail addresses: [email protected] (C. Gouveia de Oliveira);
[email protected] (R. Ventura Santos).
variations in the shear stress and fluid pressure (Oliveira,
1993). Because shear zones behave as a dynamic system,
they evolve continuously from transtensional to transpres-
sional domains and vice versa, depending on the shear
stress and Pf :
In this paper, carbon and oxygen isotopes are used to
explicate how these small-scale variations control fluid
migration and fluid–rock interaction in the main shear zone
of the Diadema shear belt. A detailed study addresses the
relationship among local variations in shear stress, fluid
pressure, shear deformation, mineralogical composition,
and carbon and oxygen isotopes in a shear zone.
2. Geology
The Rio Santa Maria granite–greenstone terrain is located
in the southeastern part of the Amazon craton, in an area in
which the monotony of the Amazon forest is broken by a
series of roughly parallel, east–west to N50W ridges (Fig. 1).
These ridges often mark linear domains of strongly deformed
rocks that contrast with the flat-lying, deeply weathered
regions of low deformation (granite–gneissic rocks).
The Diadema shear belt is one such lineament, formed by
the Serra de Diadema, a ridge that lies between the Serra de
Carajas to the north and the Serra das Andorinhas to
the south. Both ridges are topographical expressions of
major shear zones for which intense ductile shearing
processes have taken place, often accompanied by gold
mineralization. The Diadema shear belt, though roughly
coincident with the Sapucaia greenstone belt, is formed by a
complex array of shear zones that crosscut both the
greenstone sequence and the adjoining granite–gneissic
terranes. On a regional scale, the shear zones within the
Diadema shear belt have an anastomosed pattern that
insulates lenses of low deformation from mylonites.
Fig. 2 outlines the local geology of the gold occurrences.
Three main lithological units are distinguished: (1) a
intermediate to basic metavolcanic sequence, (2) a meta-
volcanic acid sequence with minor metasedimentary rocks,
and (3) a basic–ultrabasic layered sequence that crops out to
the north.
The intermediate to basic metavolcanic unit is marked
by deep red soils. Partly decomposed rocks are exposed
along trenches, and fresh rocks are found only in drill
cores. The rocks are strongly foliated and formed by
chlorite, actinolite (or Fe-tschermakite), albite (sometimes
oligoclase or andesine), epidote, quartz, carbonate, sphene,
ilmenite, rutile, magnetite, pyrite, pyrrhotite, and chalco-
pyrite. Other minerals present in these rocks include
Fig. 1. Geologic map of the Rio Santa Maria granite–greenstone terrain showing the E–W Sapucaia greenstone belt (Docegeo, 1988).
C. Gouveia de Oliveira, R. Ventura Santos / Journal of South American Earth Sciences 16 (2003) 513–522514
epidote, oligoclase, calcite, ilmenite, magnetite, and
pyrite. In zones of minor deformation, relicts of the
former pilotaxitic (andesitic) texture are locally preserved.
Chemical analyses fall in the field of andesites and
andesitic basalts (Oliveira, 1993).
The vast majority of the acid metavolcanic rocks are
quartz–sericite mylonitic rocks with varying amounts of
chlorite, albite, and carbonate, as well as minor
magnetite, ilmenite, pyrrhotite, pyrite, and tourmaline.
They commonly form a succession of mylonites with
varying intensities of deformation and recrystallization
and, less commonly, tourmaline-rich veins. Euhedral
quartz, plagioclase phenocrysts, and, more rarely,
K-feldspar are locally preserved in a finely recrystallized
matrix of quartz–sericite–albite–chlorite. Bulk chemical
analyses of those samples that show minimal deformation
fall in the field of rhyolites and dacites (Oliveira, 1993).
Chemical metasedimentary rocks are represented
by banded iron formations composed of quartz, magne-
tite, and Fe-bearing silicates (almandine, grunerite,
biotite, chamosite, cloritoidechloritoid) and carbonates
(siderite, ankerite).
The layered basic–ultrabasic unit comprises cumulate
rocks represented by metamorphosed dunites, peridotites,
pyroxenites, and gabbros. The dunites are represented
chiefly by serpentine–magnetite rocks in which relict
olivine crystals have been preserved in a mesh structure;
the peridotites are formed by a chlorite–talc–actinolite–
serpentine mineral association that envelops relicts of
olivine and uralite; and the pyroxenites are formed
exclusively by actinolite and minor chlorite. The gabbros,
in contrast, present a well-preserved igneous texture formed
by poikilitic plagioclase around cumulate pyroxene. Talc
and carbonate are common in all rock types.
3. Deformation and metamorphism
The rocks in the Diadema shear belt have been subjected
to two distinct deformation events (D1 and D2). The main
shear deformation event D1 was accompanied by progress-
ive or retrogressive mineralogical changes, depending on
the site in the shear zone. Field and petrographic
observations suggest that D1 is associated with gold
mineralization. The deformation event D2 is of minimal
expression. It postdates the mineralization and has been
recorded by flexural folding, crenulation cleavages, and
related kink banding.
The Diadema shear belt is an elongated WNW- to
ESE-trending shear zone that bends to NW–SE in its
Fig. 2. Geological map of the central part of the Sapucaia greenstone belt showing an elongated WNW–ESE—trending shear zone and the location of gold
occurrences.
C. Gouveia de Oliveira, R. Ventura Santos / Journal of South American Earth Sciences 16 (2003) 513–522 515
central portion (Fig. 2). The shear zones are characterized
by deformed, anastomosed, discrete domains that contain
slightly deformed to undeformed zones. The displace-
ment along the shear belt was controlled by a high-angle,
oblique shear zone that resulted from a regional N–S
compression. The deformation event responsible for
these structures was associated with transtensional
zones and an intrusion of granitic rocks accompanied
by gold-bearing hydrothermal fluids (Oliveira and
Leonardos, 1990).
The D1 event in the Diadema shear belt was the result
of metric-scale variations of the degree of strain and path
of deformation (Oliveira and Santos, 1995). The
deformation phases of this event are related to continuous
displacement between adjacent blocks that were affected
by heterogeneous deformation and developed meta-
morphic domains with neither distinct metamorphic
zonation nor mineral isograds. These metamorphic
domains are related to small-scale variations in pressure,
which generates mineralogical associations similar to
those found in regional metamorphic paragenesis. These
metamorphic domains, defined on the basis of miner-
alogical and microstructural features, are more conspic-
uous in the intermediate and basic metavolcanic rocks.
Petrographic studies indicate two metamorphic domains
and a hydrothermal alteration zone: (1) a country rock
domain marked by mineral associations of the greenschist
metamorphic facies, (2) a transpressional domain charac-
terized by sub-amphibolite to amphibolite metamorphic
facies associations, and (3) a transtensional domain
accompanied by hydrothermal alteration in which the
mineral association is compatible with the greenschist
metamorphic facies.
Country rocks of the Diadema shear belt are metamor-
phosed under greenschist facies conditions and occur in
weakly deformed zones in which the original igneous
texture may be preserved. These rocks are characterized by
ripiform and hypidiomorphic relictic plagioclase crystals
immersed in a fine-grained, recrystallized matrix of chlorite,
actinolite, plagioclase, quartz, carbonate, magnetite, and
ilmenite. Lateral variations in modal composition are
common, and deformation leads to a fine-grained mylonite
that displays foliation, as well as quartz and carbonate
segregations.
Metamorphism of intermediate to basic volcanic rocks in
the transpressional domains involves a prograde meta-
morphism from greenschist to sub-amphibolite to amphi-
bolite facies. The transition from paragenesis typical of the
greenschist to sub-amphibolite facies is marked by the
appearance of amphibole (Fe-tschermakite) and a decrease
in the modal abundance of chlorite (clinochlore). Most
metamorphosed intermediate to basic rocks are fine- to
medium-grained and present diffuse foliation and mono-
mineralic calcitic veinlets or veins (Fig. 3A). These rocks
contain poikilitic Fe-tschermakite porphyroblasts in a
matrix of fine-grained quartz, oligoclase, and chlorite
(Fig. 3B). Rocks metamorphosed under amphibolite
facies display a decrease in the modal abundance of
Fe-tschermakite, chlorite, magnetite, and quartz and
an increase in the modal abundance of andesine, epidote,
pyrrhotite, ilmenite, and, in some cases, biotite (Fig. 3C and
D). These rocks have a medium-grained nematoblastic
texture and are crosscut by parallel calcite- and pyrrhotite-
bearing veins.
Intermediate and basic rocks are hydrothermally altered
along shear zones in transtensional domains, and the degree
of alteration varies along the zones. The main petrographic
features of the hydrothermal alteration domains include the
disappearance of Fe-tschermakite (Fig. 3E), oligoclase-
andesine, clinochlore, epidote, magnetite, and pyrrhotite
and an increase in the modal abundance of carbonate,
chlorite (chamosite), albite, sericite, leucoxene, and pyrite
(Fig. 3F). In many cases, tourmaline is an important
accessory mineral.
4. Methods
Isotopic analyses were performed at the University of
Chicago, USA; the Geochemisches Institut der Universitat
Gottingen, Germany; and the University of Brasılia, Brazil.
They were obtained by reacting the samples with 100%
H3PO4 at 258C (McCrea, 1950). The released CO2
was analyzed by mass spectrometry. The oxygen
isotopes were corrected to calcite and dolomite by applying
the fractionation factors of 1.01025 and 1.0111, respectively.
The yields varied between 50 and 95%, depending on the
presence of other minerals (e.g. quartz, sulfides). Samples of
coexisting calcite and dolomite were analyzed according to
the double extraction technique described by Epstein et al.
(1964). To verify interlaboratory calibration, the sample
powders analyzed at the University of Chicago and Institut
der Universitat Gottingen were rerun at the University
of Brasılia. The results of this calibration procedure fell
within 0.3‰.
5. Samples and results
Samples for this study were obtained from drill cores that
crosscut intermediate to basic metavolcanic rocks from the
west zone gold occurrence (Fig. 2). Oxygen and carbon
isotope analyses were obtained from fine-grained, equigra-
nular calcite veinlets parallel to mylonitic foliation and from
coarse-grained ankerite veins that crosscut the foliation. The
oxygen and carbon isotope data of calcite and ankerite from
the Diadema shear belt, which were separated according to
the following metamorphic domains of the shear zone, are
shown in Table 1.
† Country rock domain. Calcite samples of this domain
derive from host rocks metamorphosed under greenschist
C. Gouveia de Oliveira, R. Ventura Santos / Journal of South American Earth Sciences 16 (2003) 513–522516
facies. The samples present diffuse mylonitic foliation
and variable grain size. The d18OSMOW and d13CPDB
values of calcite range between þ9.0 and þ13.2‰ and
between þ0.4 and 21.7‰, respectively.
† Transpressional domain. Rock samples are sharply
different in the metamorphic facies and fabric com-
pared with samples from the country rock domain. In
general, they occur in two distinct metamorphic facies.
Fig. 3. (A) Macroscopic detail of an intermediate to basic metavolcanic rock showing a sin- to tardi-kinematic Fe-tschermakite porphyroblast in a matrix of
fine-grained quartz, oligoclase, and chlorite. (B) Photomicrograph detail of Fig. 3A showing a Fe-tschermakite porphyroblast; the foliation is defined by
ilmenite crystals. The matrix of this rock is quartz, oligoclase, chlinochlore, epidote, biotite, ilmenite, and magnetite. (C) Photomicrograph of an intermediate
to basic metavolcanic rock of amphibolite facies association. This sample displays a medium-grained nematoblastic texture characterized by elongated
plagioclase crystals, epidote, calcite, and ilmenite. (D) Detail from Fig. 3C displaying the paragenetic association of epidote and plagioclase. Other minerals in
this sample are quartz, biotite, and ilmenite. (E) Photomicrograph detail of a stretched Fe-tschermakite porphyroblast replaced by calcite and chlorite. This
feature represents the hydrothermal alteration of the amphibole-bearing rocks. (F) Photomicrograph of a hydrothermally altered rock showing abundant
chlorite (chamosite), albite, sericite, carbonate, leucoxene, and pyrite. Note the transformations of clinochlore to chamosite and plagioclase to sericite, which
characterize the hydrothermal alteration of plagioclase-bearing rocks. Notes: mt, magnetite; ep, epidote; qz, quartz; pl, plagioclase; tsch, tschermakite; ilm,
ilmenite; bi, biotite; cc, calcite; chl, clinochlore; chm, chamosite; ser, sericite; py, pyrite.
C. Gouveia de Oliveira, R. Ventura Santos / Journal of South American Earth Sciences 16 (2003) 513–522 517
Rocks with sub-amphibolite associations are character-
ized by diffuse mylonitic foliation and elongated
Fe-tschermakite porphyroblasts in a matrix of chlorite
(clinochlore), epidote, quartz, biotite, oligoclase,
calcite, ilmenite, magnetite, and lamellar pyrite
(Fig. 3B). In contrast, rocks with amphibolite associ-
ations are characterized by nematoblastic equigranular
rocks that consist mainly of andesine, epidote, biotite,
calcite, pyrrhotite, ilmenite, and lesser chlorite, quartz,
and Fe-tschermakite (Fig. 3C and D). The transition
between the two domains is marked by the progressive
disappearance of Fe-tschermakite porphyroblasts and
an increase in andesine, which underwent extreme
ductile deformation (Fig. 3C). For calcite in these
rocks, d18OSMOW ranges between þ8.9 and þ10.1‰,
and d13CPDB ranges between 22.8 and 22.0‰.
† Transtensional domain. Calcite samples from this
domain are associated with hydrothermally altered
rocks characterized by chamosite-, carbonate-, albite-,
sericite-, quartz-, and pyrite-rich zones. Gold usually
occurs as inclusions in idioblastic pyrite. The calcite
oxygen isotopic composition ranges between þ7.2 and
þ10.1‰, and the carbon isotopic composition ranges
between 23.6 and 22.2‰.
Table 1
Oxygen and carbon isotopic composition of carbonates from the Diadema shear belt
Sample Mineralogy d18OSMOW (‰) d13CPDB (‰) Domain
7WZ16 chl þ tsch þ oli þ qz þ cc þ ep þ ilm þ mt þ9.6 21.7 Country rock (concordant veinlets)
7WZ17 chl þ tsch þ oli þ qz þ cc þ ep þ ilm þ mt þ9.8 21.6
16MZ4 chl þ tsch þ oli þ cc þ qz þ ep þ ilm þ mt þ9.6 21.5
10WZ10 tsch þ chl þ oli þ qz þ cc þ ep þ ilm þ mt þ13.2 21.4
16MZ3 tsch þ chl þ oli þ cc þ qz þ ep þ ilm þ mt þ9.2 21.3
9MZ5 chl þ amp þ alb þ qz þ cc þ mt þ ilm þ10.2 21.2
9MZ3b chl þ cc þ ep þ alb þ qz þ ilm þ mt þ10.5 21.1
2MZ4 amp þ chl þ oli þ qz þ cc þ ilm þ mt þ9.0 21.0
9MZ3a chl þ cc þ ep þ alb þ qz þ ilm þ mt þ11.1 21.0
10MZ2 chl þ amp þ oli þ qz þ cc þ mt þ ilm þ9.8 20.6
10MZ3 amp þ chl þ qz þ alb þ cc þ ilm þ mt þ10.4 20.6
2MZ4 chl þ amp þ oli þ cc þ qz þ ep þ ilm þ mt þ10.8 0.4
7WZ11 cc þ qz þ bi þ10.1 22.8 Transpressional (concordant veinlets)
19WZ2 chl þ tsch þ cc þ qz þ ep þ bi þ ilm þ mt þ9.5 22.7
7WZ3 qz þ bi þ cc þ and þ9.2 22.6
19WZ4b ep þ and þ bi þ cc þ ilm þ po þ9.7 22.5
19WZ4a and þ ep þ cc þ bi þ ilm þ po þ9.8 22.4
19WZ3 bi þ ep þ and þ chl þ cc þ qz þ ilm þ mt þ8.9 22.4
19WZ4c and þ cc þ bi þ ep þ ilm þ po þ9.5 22.3
19WZ2 bi þ ep þ and þ chl þ cc þ qz þ ilm þ mt þ9.1 22.2
19WZ3a ep þ and þ bi þ cc þ qz þ ilm þ mt þ9.3 22.1
19WZ3b ep þ bi þ and þ cc þ qz þ ilm þ mt þ9.3 22.0
19WZ3b ep þ bi þ and þ cc þ qz þ ilm þ mt þ9.3 22.0
11WZ1 qz þ cc þ chm þ ser þ7.2 23.6 Transtensional (concordant veinlets)
7WZ14a qz þ cc þ ser þ chm þ rt þ py þ9.1 23.5
7WZ13b chm þ qz þ cc þ ser þ rt þ py þ7.5 23.1
7WZ14 qz þ cc þ ser þ chm þ rt þ py þ8.7 23.1
7WZ14b qz þ cc þ ser þ chm þ rt þ py þ9.8 23.0
11WZ3 qz þ cc þ chm þ ser þ10.1 23.0
7WZ13a chm þ qz þ cc þ ser þ rt þ py þ8.3 22.9
7WZ12 chm þ qz þ cc þ ep þ rt þ py þ8.2 22.8
6WZ12 chm þ qz þ cc þ ser þ rt þ py þ9.4 22.7
7WZ13c chm þ qz þ cc þ ser þ rt þ py þ8.3 22.6
7WZ12 chm þ qz þ cc þ ser þ rt þ py þ9.3 22.2
19WZ1 Ankerite þ10.0 25.4 Late discordant veins
6WZ2 Ankerite þ12.6 24.2
20WZ20a Ankerite þ11.5 24.1
20W20b Ankerite þ11.7 23.8
5SD8 Ankerite þ13.9 23.6
4SD7 Ankerite þ13.5 23.2
6WZ3 Ankerite þ11.0 23.0
Notes: alb, albite; amp, amphibole; and, andesine; bi, biotite; cc, calcite; chl, clinochlore; chm, chamosite; ep, epidote; ilm, ilmenite; mt, magnetite; oli,
oligoclase; po, pyrrhotite; py, pyrite; qz, quartz; rt, rulite; ser, sericite; tsch, tschermakite.
C. Gouveia de Oliveira, R. Ventura Santos / Journal of South American Earth Sciences 16 (2003) 513–522518
† Late discordant veins. Ankerite occurs mainly in
albite-, ankerite-, muscovite-, pyrite-bearing veins
associated with the hydrothermal alteration. These
veins crosscut the calcite-bearing veinlets and are
related to late hydrothermal activity. Oxygen and carbon
isotopic compositions range between þ10.0 and
þ13.9‰ and between 25.4 and 23.0‰, respectively.
6. Discussion and conclusions
The area under study is characterized by intermediate to
basic volcanic rocks that were metamorphosed predomi-
nantly under greenschist facies and generally lack a strong
mylonitic foliation. These rocks were affected by local
changes in the degree of strain, accompanied by miner-
alogical, chemical, and isotopic modifications.
The structural domains controlled the volume of fluid
infiltration, volatilization, and metamorphic reactions in the
Diadema shear belt. Therefore, carbon and oxygen isotopes
of calcite from the transpressional and transtensional
domains reveal the influence of fluid-related processes
during the shear zone’s evolution. Two possible mechan-
isms may be invoked to explain the decrease in d18OSMOW
and d13CPDB of carbonate: isotope exchange between
rocks and low d18OSMOW2 and d13CPDB2 bearing fluids
(Santos and Clayton, 1995; Gerdes et al., 1995; Losh,
1997) or high temperature devolatilization of calcite, in
which CO2 is isotopically enriched in 13C relative to the
remaining carbonate (Lattanzi et al., 1980; Valley, 1986;
Nabelek, 1987).
Shear zones behave as conduits for fluids of different
sources and chemical compositions. They may have been
subject to different fluid/rock ratios, depending on the
temperature, fluid composition, and isotopic exchange rate.
The difference in carbon isotopic composition in hydro-
thermally altered rocks (22.2 to 23.6‰) relative to the
country rocks may result from large-scale fluid–rock
exchange processes. Because oxygen is much more
abundant than carbon, this process had a greater effect on
the carbon isotopic composition of the rock. This interpret-
ation is supported by initial 87Sr/86Sr isotope ratios in calcite
(Oliveira et al., 1994), which vary from 0.704543–0.706417
in the country rock and from 0.712861–0.718410 in the
hydrothermally altered rocks, that reveal that the fluids were
derived from an external reservoir.
Fluid–rock exchange may explain large-scale variations
of carbon and oxygen isotopes, as might the difference
between country and hydrothermally altered rocks. How-
ever, there also are significant differences in the carbon and
oxygen isotope values in the hydrothermally altered rocks
that seem controlled by deformation mechanisms. For
example, whereas calcite from transpressional domains
has d13CPDB values ranging from 22.8 to 22.0‰ and
d18OSMOW values ranging from þ8.9– þ 10.1‰, calcite
from transtensional domains presents d13CPDB of 23.6 to
22.2‰ and d18OSMOW of þ7.2– þ 10.1‰. Although
small, these differences are significant and may be explained
by the CO2 devolatilization due to carbonate decomposition.
Because, at high temperatures, CO2 is enriched in 18O and13C (both heavy oxygen and carbon isotopes) relative to
calcite (Chacko et al., 1991), devolatilization of CO2 from
carbonate-bearing rocks leads to a decrease in the oxygen
and carbon isotopic composition of the residue.
Fig. 4 shows the calculated trend of carbon and
oxygen isotopic evolution during a Rayleigh devolatiliza-
tion process of CO2, assuming the following parameters:
(1) aCðCO22calciteÞ ¼ 1:0022 (Chacko et al., 1991), (2) initial
d13CPDB and d18OSMOW composition of the carbonate
correspond to the average values of the samples from the
country rock domain, and (3) 40% reactive carbonate.
The devolatilization calculations applied here are based
on the carbon isotopic fractionation between CO2 and
calcite. However, other carbon species (e.g. H2CO3,
HCO32) may have been present in the fluid, which
might also have affected the carbon isotopic fractionation
(Cartwright and Buick, 1995). Fig. 4 shows that the
samples from Diadema fall into discrete domains that
may be explained by the radical changes in conditions in
the shear zone. A similar mechanism has been invoked to
explain carbonate oxygen and carbon isotope variations of
contact metamorphic aureoles (Bowman et al., 1985;
Valley, 1986). Another indication of devolatilization in
Diadema is the presence of amphibolite facies calc-
silicate minerals (epidote and andesine) that formed in
transpressional domains of the shear zone (Fig. 3D), as
well as the presence of CO2-rich inclusions (Oliveira and
Santos, 1995).
According to the crustal continuum model for Archean
lode-gold deposits of the Yilgarn Block (Groves, 1993),
gold deposits range from deep zones, where continuous
ductile deformation takes place, to shallow zones, where
deformation is dominated by pressure-sensitive frictional
sliding and fracturing. Whereas the deep level is dominated
by pervasive devolatilization reactions and high-grade
metamorphism (amphibolite to granulite facies) (Hamilton
and Hodson, 1985; Couture and Guha, 1990; Neumayir
et al., 1993), the shallow level is dominated by channeled
fluid infiltration and low temperature hydrothermal altera-
tion. Deformation, metamorphism, and gold mineralization
in the Diadema shear belt developed at temperatures of
approximately 470 8C and at hypozonal crustal levels
(.12 km; Groves et al., 1998). The temperature was
obtained on the basis of quartz-magnetite oxygen isotope
thermometry, and the hypozonal crustal level was inferred
on the basis of the mechanical behavior of the deformed
plagioclase. According to Tullis (1983); Sibson (1990), this
process occurs at approximately 18 km. In these conditions,
fluid pressure and shear stress varies laterally and along the
shear zone, which leads to sharp changes in deformation and
metamorphism, and then to fluid generation and migration.
Fig. 5 compares the Diadema shear belt with other gold
C. Gouveia de Oliveira, R. Ventura Santos / Journal of South American Earth Sciences 16 (2003) 513–522 519
Fig. 5. Schematic model displaying the relationship among deformation, metamorphism, fluid percolation, and different types of gold deposits in a shear zone.
Whereas the country rock domain was dominated by pervasive fluid percolation, the transpressional and transtensional domains were characterized by
volatilization and channeled fluid infiltration, respectively. The inset shows a model of fluid extraction and migration along a transcrustal shear zone (Ridley,
1993). The definition of hypozonal and mesozonal comes from Groves et al. (1998).
Fig. 4. d13CPDB and d18OSMOW compositions of calcite and ankerite from basic to intermediate metavolcanic rocks. The curve shows the evolution of the
isotopic composition of calcite when affected by a Rayleigh devolatilization process. The curve was calculated assuming the parameters described in the text.
The numbers along the curve represent the fraction of remaining carbonate, and F:R represents the fluid/rock ratio.
C. Gouveia de Oliveira, R. Ventura Santos / Journal of South American Earth Sciences 16 (2003) 513–522520
deposits related to shear zones and shows that Diadema
developed in conditions between greenschist and higher
grade metamorphic terrains.
In contrast to calcite, ankerite is associated with
discordant veins and veinlets. Petrographic observations
indicate that these veins and veinlets were formed after the
main deformation event and associated gold mineralization
and that they contain low temperature minerals (quartz,
albite, muscovite, and pyrite). The carbon and oxygen
isotope results of ankerite reaffirm that they were formed
under different conditions than was calcite.
Acknowledgements
We thank Robert Clayton from the University of
Chicago, USA; J. Hoefs from the Geochemisches Institut
der Universitat Gottingen, Germany; and J.M. Lafon from
Federal University of Para, Brazil, for providing the isotopic
analyses presented here. We also thank the Conselho
Nacional de Desenvolvimento Cientıfico e Tecnologico
(CNPq grant 20.0594/89.3) for providing financial support
for this study.
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