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: gouveia@unb.br (C. Gouveia de Oliveira);

rventura@unb.br (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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