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Geological Society, London, Special Publications
doi: 10.1144/SP402.2
p121-145.
 2014, v.402;Geological Society, London, Special Publications
 
Evandro L. Klein
 
deposition
sources, and mechanisms of Au transport and
Brazil: a review of the physico-chemical properties, 
Ore fluids of orogenic gold deposits of the Gurupi Belt,
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Ore fluids of orogenic gold deposits of the Gurupi Belt, Brazil: a
review of the physico-chemical properties, sources, and
mechanisms of Au transport and deposition
EVANDRO L. KLEIN
CPRM/Geological Survey of Brazil. Av. Dr. Freitas, 3645, Belém, State of Pará,
Brazil, CEP: 66095-110 (e-mail: evandro.klein@cprm.gov.br)
Abstract: The Neoproterozoic Gurupi Belt in northern Brazil developed at the southwestern
margin of the Palaeoproterozoic São Luı́s-West Africa Craton. Orogenic gold deposits of this
belt are hosted in Palaeoproterozoic (2160–2147 Ma) metavolcano-sedimentary and calc-alkaline
granitoid rocks formed in arc and/or back-arc settings during a protracted Rhyacian orogeny
(2240–2080 Ma). These host rock assemblages were tectonically and isotopically reworked
during the Neoproterozoic and represent the reworked margin of the craton, that is, the external
domain of the Neoproterozoic (Brasiliano-Pan African) orogen. The location of the gold deposits
is controlled by the Tentugal shear zone, which represents the tectonic boundary between craton
and the Gurupi Belt, and its subsidiary structures. Gold occurs in veins and in association with
pyrite, and subordinately arsenopyrite and chalcopyrite, in strongly altered and variable deformed
host rocks. Geological characteristics, petrographic, fluid inclusion, and isotopic evidence indicate
near-neutral, reduced aqueous-carbonic metamorphic fluids, with local contributions from host
rocks at the deposit site. Ore deposition occurred at about 300–370 8C and up to 3 kbars in
response to fluid immiscibility and fluid-rock reactions (sulphidation, desulphidation, carbonatiza-
tion, CO2 removal) and local fluid mixing and oxidation.
The Gurupi Belt is a Neoproterozoic mobile belt
developed at the southwestern margin of the São
Luı́s-West African Craton (Fig. 1) during the
Brasiliano-Pan African cycle of orogenesis (see
Klein & Moura 2008 for a review). Gold is the
main known mineral resource in this belt, and the
metal has intermittently been mined by artisanal
miners since the 17th century. There is no official
historical record on the artisanal production, but
unofficial data indicate over 16 t of gold has been
extracted from Chega Tudo, Serrinha, and Cacho-
eira. To date, more than 120 gold occurrences are
reported, in addition to more developed deposits
(Klein & Lopes 2011). The known gold resources
of four deposits are equal to about 120 t Au
(Table 1), which will be mined in open pit. The
opening of the first organized, industrial operation
is planned to occur at Cipoeiro.
In the last decade efforts have been made to
improve the understanding of the geological evol-
ution of the Gurupi Belt (Ribeiro 2002; Klein
et al. 2005b; Teixeira et al. 2007; Klein & Lopes
2009, 2011; Palheta et al. 2009) and of individual
deposits (Torresini 2000; Ribeiro 2002; Yamaguti
& Villas 2003; Klein et al. 2005a, 2006, 2007,
2008b). Results provided constraints on: (1) the
nature and age of the host rocks; (2) the nature of
the hosting structures; (3) the composition of the
ore fluids; and (4) the T–P conditions of ore depo-
sition. An integration of these studies is presented
in this review, which attempts to link geological
processes documented at the provincial scale with
those documented in individual deposits. Special
attention will be given to the circulation of fluids
that have led to the formation of the deposits, espe-
cially the evaluation of the mechanisms of gold
transport and deposition. Other parameters that are
critical for the development of genetic models
have not yet been determined. Such parameters are
mostly dependent on the absolute age of gold depo-
sition, and also include the tectonic setting of ore
formation. A brief discussion of these issues is
addressed throughout the review.
Geological overview
São Luı́s cratonic fragment
Considering that a significant part of the Gurupi
Belt is made of reworked units of the São Luı́s
cratonic fragment (Fig. 1), the geology and evol-
ution of this cratonic fragment is briefly outlined
here (based on Klein et al. 2008a, 2009, 2012;
Palheta et al. 2009). Three main stratigraphic units
crop out in the cratonic area (Fig. 1a). The oldest
rocks known in the São Luı́s cratonic fragment
belong to the Aurizona Group, which is an island
arc-related metavolcano-sedimentary sequence of
2240 + 5 Ma that was intruded by shallow grano-
phyric rocks at 2214 + 3 Ma and by the juvenile,
From: Garofalo, P. S. & Ridley, J. R. (eds) 2014. Gold-Transporting Hydrothermal Fluids in the Earth’s Crust.
Geological Society, London, Special Publications, 402, 121–145.
First published online March 19, 2014, http://dx.doi.org/10.1144/SP402.2
# The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
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Fig. 1. (a) Location of the São Luı́s cratonic fragment and Gurupi Belt (hatched). For the sake of scale, only the major
rock units of the São Luı́s cratonic fragment are considered here. (b) Simplified geological map of the Gurupi Belt
showing the main gold deposits and showings.
E. L. KLEIN122
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subduction-related, metaluminous to slightly pera-
luminous calc-alkaline granitoids developed in an
island arc between 2168 and 2147 Ma. These gran-
itoids form batholiths and stocks that are included
in the Tromaı́ Intrusive Suite, which comprise
mainly tonalites and granodiorites and subordi-
nated quartz-diorite, monzogranite, and syenogra-
nite. Minor units, which do not appear in the scale
used in Figure 1a, include andesites, dacites, felsic
tuffs, and subordinately basic volcanic rocks depos-
ited over the Aurizona Group and the Tromaı́ Suite
at 2164–2160 Ma. These volcanic associations
have metaluminous to peraluminous, high-K calc-
alkaline to tholeiitic signature and have been inter-
preted to have formed in mature or transitional
arc with minor back-arc component and active
continental margin, respectively. Peraluminous,
collision-type granites of c. 2100 Ma belong to the
Tracuateua Intrusive Suite, crop out in the western
portion of the cratonic area and are covered by Pha-
nerozoic sequences. Late- to post-orogenic small
plutons of highly evolved/shoshonitic granite of
2056–2076 Ma and are the youngest rocks known
so far in the São Luı́s cratonic fragment.
Based on rock association, geochronology, and
on geochemical and Nd isotope signatures, these
Palaeoproterozoic associations of the São Luı́s cra-
tonic fragment are interpreted as a section of the
Rhyacian orogen, which records an accretionary
stage at 2240–2150 Ma and a collisional stage at
c. 2100 Ma (Klein et al. 2008a, 2009, 2012). This
scenario correlates with what is described for
similar successions of the Eburnean-Birimian ter-
ranes of the West-African Craton (Klein & Moura
2008 and referencestherein).
Gurupi Belt
The Neoproterozoic Gurupi Belt developed at the
south-southwestern margin of the São Luı́s cratonic
fragment (Fig. 1b). It consists of igneous, sedimen-
tary, and metamorphic rock units of distinct origin
recording a long geological history from 2695 to
549 Ma, comprising Archaean and Palaeoprotero-
zoic basement units, the reworked margin of the
São Luı́s cratonic fragment, pre-orogenic and oro-
genic intrusions, and sedimentary basins of equiv-
ocal origin. (Klein et al. 2005b, 2012; Palheta et al.
2009; Klein & Lopes 2011). These rock units are
mostly NW-SE trending successions that form the
general tectonic orientation in the belt.
To date, the oldest rocks found in the belt belong
to the Igarapé Grande Metatonalite dated at
2594 + 3 Ma (Klein et al. 2005b), which represents
the vestiges of a poorly defined Archean block that
is part of the belt basement (Fig. 1b). Most of the
basement is composed of Palaeoproterozoic units
(Fig. 1b) that have been interpreted as being part
of active continental arc and collision settings
(Klein et al. 2012). The largest basement unit com-
prises the foliated to banded orthogneisses of the
Itapeva Complex of 2167 + 3 Ma , with TDM ages
of 2.22–2.31 Ga and positive 1Nd values (Klein
et al. 2005b). The complex also includes lim-
ited remnants of paragneisses, amphibolite-facies
schists, and basic-ultrabasic rocks (Klein & Lopes
2011). The weakly peraluminous, high-K, calc-
alkaline biotite-bearing Cantão Granodiorite of
2163 + 4 Ma, with TDM model ages of 2.21–
2.92 Ga and 1Nd values of +2.7 to 2 7.1 (Palheta
et al. 2009), and the 2142 + 9 Ma-old, weakly per-
aluminous, biotite- and muscovite-bearing Jonasa
Granodiorite, with TDM model ages of 2.14–
2.40 Ga, and 1Nd values of+3.9 – 0.2, are probably
related to a continental arc setting (Klein et al.
2012). The Jonasa Granodiorite underwent meta-
morphism and deformation at 525 + 20 Ma (Klein
& Moura 2003). Strongly-peraluminous crust-
derived granitic magmatism took place between
2116 and 2079 Ma (Klein et al. 2012 and references
therein) and is represented by several plutons of
biotite- and muscovite-bearing granites and leuco-
granites (Fig. 1). These granites show TDM model
ages of 2.19–2.62 Ga and 1Nd values ranging
from +3.9 to 20.2 (Palheta et al. 2009; Klein
et al. 2012), in addition to chemical differences
that have been interpreted to result from mixtures
of variable amounts of sedimentary and igne-
ous sources of Archaean to Palaeoproterozoic ages
in a collisional setting (Klein et al. 2012).
Table 1. Ore reserves of gold deposits from the Gurupi Belt
Deposit Indicated (Mt) Grade (g/t) Inferred (Mt) Grade (g/t) Au (t) Ref
Cachoeira 12.5 1.11 0.5 1.27 20.68 1
Cipoeiro 49.4 1.17 6.8 1.10 65.30 2
Chega Tudo 20.7 1.00 12.0 0.98 32.51 2
Mina Nova-S nd nd 0.8* 2.30 1.88 3
nd: not determined.
*Unconstrained resources.
Key to references: 1: Nakai-Lajoie and Clow (2011), 2: Machado (2011), 3: Pastana (1995).
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The 2100 + 21 Ma-old metaluminous to slightly
peralkaline granites, quartz-monzonites, and quartz-
syenites of the Anelis Intrusive Suite show charac-
teristics of highly evolved potassic to shoshonitic
post-collisional rocks (Klein et al. 2012). This
suite is spatially associated with the post-orogenic,
2085 + 4 Ma-old (Palheta et al. 2009), weakly per-
aluminous, high-K, calc-alkaline, biotite-bearing
Timbozal Monzogranite (Fig. 1). Both suites show
Nd isotope signature indicating mixed mantle and
crustal sources (Klein et al. 2012).
The reworked margin of the São Luı́s cratonic
fragment includes plutonic and supracrustal rocks
(Fig. 1). The plutonic rocks are the batholiths of
the juvenile calc-alkaline granitoids of the Tromaı́
Intrusive Suite, with remnants of coeval amphibo-
lite, both showing variable degrees of deformation
within the Gurupi Belt, and gabbroic intrusions of
supposed Palaeoproterozoic age (Ubinzal Gabbro)
(Klein et al. 2005b; Klein & Lopes 2011). The pre-
dominant supracrustal sequence is the 2160 Ma old
metavolcano-sedimentary Chega Tudo Formation,
with TDM ages of 2.20–2.34 Ga and positive 1Nd
values (Klein & Moura 2001), seconded by the sili-
ciclastic Igarapé de Areia Formation, which is
younger than 2110 Ma (Teixeira et al. 2007) and
resembles the Tarkwa sedimentary sequence of
Ghana, West Africa (Klein & Lopes 2009).
Vestiges of the Rhyacian metamorphism and
deformation (and collision?) are inferred from the
presence of the peraluminous leucogranites, some
of them having low Th/U zircon ages of 2098–
2092 Ma (Klein et al. 2005b, 2012).
Neoproterozoic units (Fig. 1) comprise very low-
grade to greenschist-facies metasedimentary rocks
younger than 1140 Ma (Gurupi Group), probably
deposited in a passive margin. They also include
a pre-orogenic amphibolite-facies, undersaturated
alkaline intrusion of 732 + 7 Ma (Boca Nova
Nepheline Syenite); and the orogenic, metamor-
phosed Caramujinho Microtonalite of 624 + 16
Ma, with crustal signature, and the peraluminous,
two-mica granite (Ney Peixoto Granite) of 549 +
4 Ma (Klein et al. 2005b, 2012; Palheta et al.
2009; Klein & Lopes 2011). The Neoproterozoic
metamorphism is estimated to have occurred
between 624 and 549 Ma (Klein & Lopes 2011).
Geology of the gold deposits
Structural setting
The orogenic gold deposits of the Gurupi Belt are
mostly, if not all, hosted in structures related to
the Tentugal shear zone (Fig. 1). This is a sinistral
strike-slip sinistral shear zone that consists of a
.100 km-long and 15- to 30 km-wide corridor of
shear zones striking dominantly N408W developed
under ductile and ductile-brittle conditions at the
greenschist facies (Costa et al. 1988; Ribeiro 2002;
Klein & Lopes 2011). The ductile deformation
imparted a schistose and/or mylonitic fabric to the
rocks, depending on their composition, rheology,
and position in relation to the deformation zones.
Most of the strain was accommodated by the less
competent metavolcano-sedimentary rocks of the
Chega Tudo Formation, and to a lesser extent by
the coarse-grained granitoids of the Tromaı́ Intru-
sive Suite and gneisses and coarse-grained schists
of the Itapeva Complex. Ribeiro (2002) described
the transposition of the N408W-striking structures
by north-NW–south-SE-trending ductile structures,
along with the formation of a north–south-striking
fault system and associated splays (e.g. Cipoeiro
region, Fig. 2) that have subsequently been reacti-
vated and displaced by small-scale thrusts and
east–west-oriented strike-slip faults.
In the northwestern portion of the belt, where the
Cachoeira deposit is located, the Chega Tudo For-
mation displays a north-south-trending foliation
that dips at high angles to the SW, with subhorizon-
tal stretching lineations, indicating a transpressional
sinistral strike-slip regime. Brittle–ductile, north–
south-trending strike-slip shear zones dip at high
angles to the west and were reactivated as brittle
faults that produced brecciated zones and were sub-
sequently displaced by east/west- and WNW/ESE-
trending strike-slip faults (Klein et al. 2005a), in
accordance with what was described by Ribeiro
(2002) for the southeastern portion of the belt.
The ore bodies hosted in supracrustal sequences
of the Chega Tudo Formation are mostly parallel to
the regional structure of the host rocks (Fig. 3), but
can also be discordant. At the deposit scale, they are
confined to smaller-scale brittle-ductile structures.
Accordingly, the ore bodies are discontinuous, sub-
vertical, stretched, and mostly have cylindrical to
lens shapes.
At Cipoeiro, deformation is concentrated in dis-
crete shear zones that cut the hosting tonalites. The
least deformed tonalites show preserved igneous
textures, whereas the more deformed and hydrother-
mally altered types show strongly modified texture
and mineralogy, with the original rocks transformed
in fine-grainedmylonites. The ore bodies have
irregular shapes and are hosted in tonalites near
the contact with metasandstones of the Igarapé de
Areia Formation. Stockwork veining and breccia-
tion have been observed at Mina Nova Sul and
Pipira and may represent a shallower, brittle style
of mineralization.
Host rocks
Most of the deposits and prospects of the Gurupi
Belt are hosted in rocks of the 2160 Ma-old
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metavolcano-sedimentary sequence of the Chega
Tudo Formation. A smaller number are hosted by
the calc-alkaline granitoids of the Tromaı́ Intrusive
Suite (2160–2147 Ma), by the Ubinzal Gabbro,
and by the conglomerates and sandstones of the
Igarapé de Areia Formation (Figs 1 & 2, Table 2).
The metavolcano-sedimentary sequence occurs
as subparallel elongated layers from a few tens to
hundreds of meters in thickness. The rocks are vari-
ably foliated and exhibit subvertical schistosity
and/or mylonitic fabric that trend according to the
regional and local tectonic grain of the Gurupi
Belt, that is, mostly NW–SE. The supracrustal rocks
underwent metamorphism under greenschist to
lower amphibolite facies conditions (Yamaguti &
Villas 2003).
At Cachoeira, mafic and intermediate rocks of
the Chega Tudo Formation are the main hosts for
the ore bodies. Klein et al. (2005a) described tuff,
basalt, andesite, chlorite schist with intercalated
ultramafic rocks (chlorite-talc schist), microdiorite,
and carbonaceous phylite as the main host rocks
(Figs 3a & 4a, b). The volcanic rocks show aphyric
to porphyritic texture. Nakai-Lajoie & Clow (2011)
also described sericite schist, dacite, and silexite and
characterized the tuffs as being ash, lapilli, and
clastic tuffs. To the east, the Chega Tudo Formation
makes a tectonic contact with a weakly mineralized
metasedimentary unit that Klein & Lopes (2009)
assigned to the Igarapé de Areia Formation. The
sequence shows a psamitic unit, composed of
arkose (Fig. 4b), greywacke, and possible tuffs,
and a pelitic unit with intercalated siltstones and car-
bonaceous phyllites. The arkose is a medium- to
coarse-grained rock with preserved primary cross
bedding and has intercalations of conglomerate
levels and magnetite bands.
At Chega Tudo mineralization took place
especially in a metavolcanic domain that parallels
the metasedimentary domain (Fig. 3b). The meta-
volcanic rocks belong to the Chega Tudo Formation,
whereas the metasedimentary domain has rocks of
the Chega Tudo and Igarapé de Areia Formations.
The metavolcanic domain comprises dacite por-
phyry as the predominant host rock, and subordi-
nately rhyolite, andesite, and volcaniclastic rocks
(Torresini 2000; Ribeiro 2002; Klein et al. 2008b).
The host rocks show variable degrees of tectonic
deformation (Fig. 4c–f).
In the Serrinha and Montes Áureos deposits
(Figs 2 & 3c, d), the mineralization occurred predo-
minantly in metasedimentary rocks (Ribeiro 2002;
Yamaguti & Villas 2003) of the Chega Tudo For-
mation (Klein et al. 2006). These are magnetite-
bearing quartz-sericite schist, chlorite schist, and
fine- to medium-grained dark-coloured carbon-
aceous schists (Fig. 4g) with interlayered tuffs and
felsic to intermediate metavolcanic rocks.
The host rocks for the gold mineralization at
Cipoeiro comprise variably altered and sheared
calc-alkaline tonalites of the Tromaı́ Intrusive
Suite (Fig. 4h). The tonalites occur in tectonic
Fig. 2. Detailed geological map of the central portion of the Gurupi Belt showing the location of four of the main gold
deposits discussed in the paper. Modified after Ribeiro (2002) and Klein et al. (2006).
GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 125
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Table 2. Geological attributes from selected gold deposits and prospects of the Gurupi Belt
Deposit Cachoeira Chega Tudo Serrinha Montes Áureos Pretos Cipoeiro Mina Nova-S Pipira
Host unit Chega Tudo Fm.
Igarapé de Areia
Fm.
Chega Tudo Fm.
Igarapé de
Areia Fm
Chega Tudo Fm. Chega Tudo Fm. Chega Tudo Fm. Tromaı́ Intrusive
Suite
nd Chega Tudo
Fm.
Host rock Mafic and felsic
(meta)volcanic
rocks, tuffs,
C-phyllite,
metasandstone
Dacite porphyry
(rhyolite,
andesite,
vulcanoclástica
rocks),
metasandstone
C-schist Pelitic and
C-schists,
rhyolite,
dacite
C-schist Tonalite Altered
gabbro(?)
tonalite(?)
Tuffs,
C-pyllite,
Age of the host
rock (Ma)
2160 2160
,2100
2160 2160 nd 2148 2148 nd
Host structure North–South shear
zone
NW–SE shear
zone
NW–SE shear zone NW–SE shear
zone
NW–SE shear zone North–South splays NW–SE shear
zone
NNE–SSW
faults
Structural style Veins,
disseminations
Veins,
disseminations
Veins,
disseminations
Veins Veins,
disseminations
Veins,
disseminations
Stockwork,
brecciated
veins
Stockwork
Hydrothermal
mineralogy
Qtz, Dol, Ser, Chl Qtz, Cal, Ser, Chl Qtz, Cal, Dol Qtz Qtz Qtz, Cal, Ab, Chl, Phe Qtz Qtz, Cal
Ore mineralogy Py, Apy (Gn,
Sp, Ccp)
Py, Apy, Ccp
(Gn, Sp)
Py Apy, Py (Ccp) nd Py nd Py
Elemental
association
Au, Ag, As, Bi Au, Bi, As, Sb, Te nd nd nd nd nd nd
Au grade (g/t) 1.7–4.4 0.98 nd nd nd 1.51 2.30–2.37 nd
Reserves (t Au) 20 26.6 nd nd nd 60.4 1.88 nd
References 1, 2 3 4 5 6 7 6, 8 6
Key to references. 1: Klein et al. (2005a), 2: KLein & Lopes (2009), 3: Klein et al. (2008b), 4: Klein et al. (2006), 5: Yamaguti & Villas (2003), 6: this work, 7: Klein et al. (2007), 8: Pastana (1995).
Mineral abbreviations: Qtz, quartz; Cal, calcite; Dol, dolomite; Chl, chlorite; Ser, sericite; Ep, epidote; Ab, albite; Phe, phengite; Py, pirite; Apy, arsenopyrite; Sp, sphalerite; Ccp, chalcopyrite; Gn, galena
(minerals in brackets are subordinate).
Other abbreviations: nd – not determined or unavailable, Fm: formation.
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contact with a metasedimentary sequence (Figs 2
& 3e) composed of magnetite-rich metasandstone
and metapelite with lenses of metaconglomerate
(Torresini 2000) ascribed to the Igarapé de Areia
Formation (Klein & Lopes 2009, 2011).
Other prospects have been documented only par-
tially; therefore, only qualitative models can be pre-
sented here. The Ubinzal prospect shows gold
mineralization hosted in basic rocks of the Ubinzal
Gabbro unit that occurs within the Chega Tudo For-
mation, but without clear contact relationships
(Klein & Lopes 2011). The basic rocks are predomi-
nantly coarse-grained (Fig. 5a), locally porphyritic,
actinolite-bearing gabbros, and subordinately mon-
zodiorite, monzogabbro, diorite, quartz monzodior-
ite and, possibly, pyroxenite and hornblendite.
Under investigation are prospects associated with
the siliciclastic sequence of the Igarapé de Areia
Fig. 3. Cross sections of gold deposits from the Gurupi Belt. (a) Cachoeira (Klein et al. 2005a). (b) Chega Tudo
(Ribeiro 2002). (c) Serrinha (Klein et al. 2006). (d) Montes Áureos (Yamaguti & Villas 2003). (e) Cipoeiro
(Torresini 2000).
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Fig. 4. Host rocks of gold deposits of the Gurupi belt. (a) Cachoeira: mafic metavolcanic rock crosscut by quartz
veins (white). (b) Cachoeira: sulphidised felsic metavolcanic rock crosscut by a quartz vein (left) and drill core sample
of a sandstone from the Igarapé de Areia Formation crosscut by quartz veinlets (right). (c) Chega Tudo – drill core
sample of an undeformed and unaltered dacite porphyry. (d) Chega Tudo: same dacite of ‘C’ showing transition between
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Formation (Fig. 1). Epigenetic gold mineralization
has been detected at least in the Cachoeira deposit
and in the Sequeiro prospect (Fig. 5b), and hydro-
thermalalteration is observed at Pico 20, Firmino,
and Boa Esperança (Fig. 5c). In addition, the silici-
clastic formation hosts palaeoplacer gold, which is
not a subject of this review.
Mineralization styles and gold siting
Two similar mineralization styles have been
described in all studied deposits of the Gurupi
Belt, and both include quartz-sulphide veins (Bet-
tencourt et al. 1991; Ribeiro 2002; Yamaguti &
Villas 2003; Klein et al. 2005a, 2006, 2007,
2008b; Machado 2011; Nakai-Lajoie & Clow 2011).
Differences reside mainly in the number and dimen-
sions of veins, in the volume of hydrothermally
altered wall rock (Fig. 6), in the form of gold occur-
rence, and, at least in part, in the timing of vein
deposition.
Style (1) is composed of relatively thick (0.5–
5.0 m) tabular to S-shaped quartz veins (Fig. 6a)
and vein sets with variable length (20–200 m along
strike). The quartz veins show some visible gold
(Fig. 7), centimetre-wide alteration envelopes, and
have low (1–3 wt%) sulphide contents. These veins
are predominantly concordant with the regional
structure of the Gurupi Belt, but locally may
present variable angle relationships with respect to
the foliated host rocks. Furthermore, the veins are
interpreted to have been emplaced early in the
structural-hydrothermal history of the deposits
(e.g. Bettencourt et al. 1991; Klein et al. 2005a,
2007), based on mesoscopic and microscopic defor-
mation evidence, such as the laminated charac-
teristic of some veins, the undulose extinction,
deformation lamellae, aggregates of quartz sub-
grains, and the growing of hydrothermal minerals
in pressure shadows of sulphide crystals. In places,
however, cross cutting relationships among the
two styles are equivocal. In this style, gold is predo-
minantly of the free-milling type.
Style (2) comprises thin, up to a few centimetres-
thick, Au-quartz-carbonate-sulphide veinlets and
Au-sulphide stringers and dissemination in larger
volumes of altered host rocks within sets of closely-
spaced shear zones (Fig. 6b). The veinlets may
be both concordant and discordant with respect
to the hosting structure, and the stringers and
disseminations are in general associated with small
fractures (Fig. 4). The veinlets also underwent
ductile and brittle deformation, as suggested by
changes in the thickness of the veinlets, ubiquitous
presence of undulose extinction of quartz, forma-
tion of subgrains, recrystallization, and fracturing.
In addition to free-milling gold, the second style
shows several modes of gold siting (Fig. 8): (i)
deposited in microfractures cutting quartz crystals,
quartz-carbonate veins and veinlets and sulphide
(pyrite, arsenopyrite) grains; (ii) in pyrite-arseno-
pyrite and quartz-arsenopyrite intergranular con-
tacts; (iii) as inclusions of single particles or
composed Au-arsenopyrite (+sphalerite) grains
included in pyrite; and (iv) as trace-element, espe-
cially in pyrite and arsenopyrite. Bismuth, As, Sb,
and Te are also present as trace-elements in
sulphides.
Possibly, a third style of mineralization is pre-
sent in the region. In recent field work, structurally
controlled stockwork veining and brecciation
have been observed in open pits in two prospects
(Mina Nova Sul and especially Pipira; Table 2).
This style might represent shallow portions of the
gold system and will be the subject of future
investigation.
Hydrothermal alteration
As a whole, the gold deposits of the Gurupi Belt
share similar aspects regarding the hydrothermal
alteration in terms of mineralogical composition,
style, and intensity. In addition to gold, the hydro-
thermal assemblages are in general composed of
variable amounts of quartz, chlorite, white mica,
carbonate minerals, minor albite, and sulphide
minerals. Ilmenite and rutile are very subordinate
phases. Variations may occur at the scale of
deposit and are in general related to the intensity
of deformation and veining, and, to a minor
extent, to the type of host rock.
Around the thicker veins of Style 1, the hydro-
thermal haloes tend to attain a few tens of centi-
metres. At Cachoeira, Nakai-Lajoie & Clow
(2011) described moderate to intense wall rock
alteration in the vicinity of quartz-vein systems
and faults that have been interpreted to be the
feeders of the hydrothermal system. In these
haloes, albite, carbonate, pyrite, and arsenopyrite
are the predominant phases. The quartz vein of the
Fig. 4. (Continued) undeformed and deformed zone with stringers of pyrite (arrows). (e) Chega Tudo: drill core samples
with concordant quartz and discordant quartz-calcite veinlets hosted in deformed rhyodacite and dacite (from Klein
et al. 2008b). (f) Chega Tudo: Mylonite showing a bleached sericitic zone with stringer of pyrite (arrow). (g) Serrinha:
carbonaceous schists with veinlets and spots of carbonate (arrows) (from Klein et al. 2006). (h) Cipoeiro: drill core
samples of thick quartz vein (left), altered but texturally preserved tonalite (right), and variably foliated and veined
(arrows) altered tonalites (three drill cores in the middle). Foliation is outlined by the white dashed line (from Klein et al.
2007). Mineral abbreviations: Cal, calcite; Py, pyrite; Qtz, quartz.
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Fig. 5. Host rocks of gold prospects. Drill core samples are 3 cm-wide. (a) Ubinzal: coarse-grained, altered and veined
gabbro. (b) Sequeiro: quartz vein and pyrite (Py) aggregates hosted in coarse-grained sandstone of the Igarapé de Areia
Formation. (c) Boa Esperança: quartz veinlets (top) and veins (middle) and hematitic (Hem) alteration (bottom) in
sandstones from the Igarapé de Areia Formation.
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Mina Velha ore body of the Chega Tudo deposit is
surrounded by a cm-wide halo of white mica. At
Serrinha, only the silicification (quartz vein) is
visible (Fig. 7).
In Style 2, individual alteration zones range from
a few meters to several tens of meters in thickness.
Together, these zones form multiple pods that attain
several hundreds of meters in thickness (Klein
et al. 2005a; Machado 2011). Possibly, multiple
generations of veining and fracturing rendered the
host rocks highly permeable, allowing high fluid
flow and a better development of hydrothermal
alteration (e.g. Ribeiro 2002; Machado 2011; Nakai-
Lajoie & Clow 2011), which characterizes Style 2
mineralization. Furthermore, a number of stages of
deformation, alteration, and gold precipitation have
probably occurred in each deposit, but in many
cases the establishment of the sequence of these
stages has been hampered by deformation and the
overlapping of alteration phases (Klein et al. 2007).
At Cachoeira, Klein et al. (2005a) described four
alteration stages, which here are summarised into
three stages: (i) milky and smoky quartz (+pyrite,
arsenopyrite) veins, that in this paper correspond
to Style 1; (ii) quartz-dolomite-albite-pyrite-arseno-
pyrite (+sericite-chlorite) around small fractures,
in veinlets or pervasively substituting the host
rocks (Style 2); and (iii) post-mineralization calcite
veinlets (Fig. 9a–e).
The Mandiocal target studied by Klein et al.
(2008b) corresponds to the North Zone of the
Chega Tudo deposit. Mineralization in this target
consists of millimetre- to centimetre-thick quartz
(+calcite, +sulphide) veinlets and enclosing
altered host rocks. In addition to gold, the hydrother-
mal assemblage is composed of variable propor-
tions of quartz, chlorite, white mica, calcite, and
sulphide minerals (pyrite and minor chalcopyrite,
sphalerite, and galena). Locally, the hydrothermal
alteration shows an asymmetric zoning at the centi-
metre scale with quartz veinlets surrounded by prox-
imal calcite (+pyrite), an intermediate chlorite-rich
zone composed of chlorite, quartz, calcite, white
mica, and pyrite, and a distal zone composed of
whitemica, quartz, pyrite, and minor calcite (Fig.
9f). There is increase in the calcite contents and
decrease in the white mica content toward the
veinlet (Klein et al. 2008b). In the Main Zone
of Chega Tudo, Machado (2011) described two
alteration zones: (i) a quartz-sericite-pyrite zone,
that contains the gold mineralization; and (ii) a
chlorite-carbonate-epidote zone that straddles the
ore bodies and hosting shear zones limits. In both
the Mandiocal/North and Main zones, gold-bearing
veinlets and Au-pyrite stringers are located mostly
within the foliation planes of the highly strained
host rocks (Figs 4d, f & 9g), but they also cut
across the foliation (Fig. 4e) (Klein et al. 2008b;
Machado 2011).
In the Serrinha deposit only two ,11 m-thick
mineralized zones have been accessed in a previous
study (Klein et al. 2006). Probably because of the
Fig. 6. Sketch of the styles of gold mineralization in the Gurupi Belt. (a) Thick quartz veins and sets of veins and their
hydrothermal haloes. This style generally shows free-milling gold. (b) Sheared and hydrothermally-altered host rocks,
with thin gold-bearing veinlets and disseminated stringers of auriferous pyrite. This style shows predominantly
refractory gold.
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composition of the host rock, a graphite-bearing
schist, and of a relatively small volume of hydro-
thermal fluids, hydrothermal alteration in these
zones is limited to silicification, in the form of
Au-quartz veins, to quartz-carbonate-sulphide vein-
ing (Fig. 9h) and to precipitation of spots of carbon-
ates (Fig. 4g). Calcite and ankerite are the carbonate
mineral in the upper mineralized zone, whilst calcite
and dolomite occur in the lower zone. Pyrite is the
main sulphide mineral, followed by very small crys-
tals and inclusions of arsenopyrite and galena.
Outside the ore zones, pyrrhotite and arsenopyrite
predominate over pyrite.
So far, Cipoeiro is the only important deposit
hosted in tonalites of the Tromaı́ Intrusive Suite
instead of in metavolcano-sedimentary rocks of
the Chega Tudo Formation. Notwithstanding, both
host units present similar mineralization and hydro-
thermal styles. According to Klein et al. (2007), in
the disseminated style, the host shear zones are up
to a few meters wide and the hydrothermal altera-
tion associated with gold mineralization produced
an assemblage consisting of quartz, chlorite, white
mica, calcite, albite, and pyrite. This alteration
assemblage overprinted the primary magmatic
mineralogy. Locally, bleached haloes occur at the
margins of quartz-pyrite veinlets and stringers of
pyrite in sheared zones are common (Fig. 10).
Microprobe analyses (Klein et al. 2007) indicate
that the white mica belongs to the phengitic series,
and that the chlorite polytypes are ripidolite and
picnochlorite, with Fe/(Fe + Mg) ranging from
0.38 to 0.47. Klein & Lopes (2011) observed that
the tonalites of the Tromaı́ Suı́te have greenish
colours far away from Cipoeiro, which is ascribed
to moderate regional chlorite and saussurite altera-
tion of mafic minerals and plagioclase, respectively.
Machado (2011) reported similar alteration assem-
blage, but stressed that gold is also predominantly
associated with the quartz-sericite-pyrite altera-
tion, as described for Chega Tudo. On the other
hand, silicification and the overall alteration of the
host tonalite are more intense and more widespread
than in Chega Tudo. This likely results from a
larger number of closely-spaced discrete shear
zones cutting across the tonalites.
Timing of gold mineralization
The growth of sericite and chlorite within the milo-
nitic foliation, the growth of chlorite and quartz in
the pyrite pressure shadows, and the deposition of
gold within fractures indicate that hydrothermal
alteration is syntectonic to late tectonic. At Cacho-
eira, the evidence for carbonate alteration overprint-
ing the metamorphic minerals and foliation show
that hydrothermal alteration is post-metamorphic
(Fig. 9d).
Ongoing radiogenic isotope dating, however,
shows conflicting results. For instance, a well-
defined isochron constructed with the Pb isotope
compositions of gold particles from the Mina
Velha target (Chega Tudo deposit) revealed an
Archaean model age, whereas the host rock is
2160 Ma old. The 40Ar/39Ar ratios of hydrothermal
white mica from the Chega Tudo deposit gave a
minimum age of c. 800 Ma, which is older than
the pre-orogenic nepheline syenite that approxi-
mately defines the opening of the orogenic basin
Fig. 7. Style 1 quartz vein from Serrinha outlined by dashed lines, with visible gold (arrows; from Klein et al. 2006).
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that became the locus for the development of the
Gurupi Belt (Klein et al. 2005b). The white mica
40Ar/39Ar ratios also show overprinting by a
younger event at 600–623 Ma, which is consistent
with the estimated timing of the Neoproterozoic
metamorphism in the Gurupi Belt. At Cachoeira,
apparently coexisting pyrite and arsenopyrite
grains from ore bodies hosted in metavolcanic
rocks of the Chega Tudo Formation gave
Palaeoproterozoic (�2.0 Ga) and Neoproterozoic
(�0.84–0.59 Ga) model ages, respectively. In turn,
the epigenetic mineralization that took place in
rocks of the Igarapé de Areia Formation must be
younger than 2110 Ma.
Therefore, this major issue needs further con-
straints. The hypotheses envisaged so far include:
(1) mineralization occurred in the Palaeoprotero-
zoic, and remobilization may have taken place in
Fig. 8. (a) Irregular gold particles in a quartz-carbonate vein (Serrinha; Klein et al. 2006). (b) Gold and chalcopyrite
grains in a fracture of a large pyrite crystal (Chega Tudo; Klein et al. 2008b). (c) Gold in sharp contact with large
arsenopyrite crystal (Cachoeira). (d) Rounded inclusion of gold (arrow) in a pyrite crystal (Cachoeira). (e) Composite
gold-arsenopyrite-sphalerite inclusion in pyrite (Cachoeira). Mineral abbreviations: Apy, arsenopyrite; cb, carbonate;
Cpy, chalcopyrite; Py, pyrite; Qtz, quartz; Sp: sphalerite.
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Fig. 9. (a) Quartz vein of Style 1; crosscutting fractures are filled with quartz-calcite veinlets (arrows) (Cachoeira
deposit; from Klein et al. 2005a). (b) Veinlet and (c) pervasive albite-dolomite-pyrite alteration at Cachoeira (Klein
et al. 2005a). (d) Dolomite alteration over chlorite schist (Cachoeira deposit; Klein et al. 2005a). (e) Post-mineralization
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the Neoproterozoic, with Pb isotopes indicating
both Archaean and Palaeoproterozoic sources, in
this case, different deposits might have been
affected to different extents; and (2) mineralization
occurred in the Neoproterozoic from Archaean to
Neoproterozoic Pb (and Au) sources.
The first hypothesis is preferred here, based on
the following constraints and interpretations:
(1) All the deposits and showings (.120) are
hosted in Palaeoproterozoic rocks (c. 2100–
2160 Ma) that represent the reworked margin
of the São Luı́s cratonic fragment (Klein et al.
2005b, 2012; Klein & Lopes 2011).
(2) The predominant Neoproterozoic sequence of
the Gurupi Belt, that is, the Gurupi Group,
likely represents a passive margin association
(Klein & Lopes 2011; Klein et al. 2012),
which is an inappropriate tectonic setting for
orogenic gold, although the inversion of the
tectonism from passive to active margin has
likely occurred during the Neoproterozoic
orogeny.
(3) The metamorphic nature of the ore fluid.
Metamorphic fluids are produced by devolati-
lization and dehydration reactions occurring
during the progressive metamorphism of vol-
canicand sedimentary sequences deposited
in oceanic and accretionary settings (Phillips
& Powell 1993; Kerrick & Caldera 1998;
Yardley & Graham 2002). This is in line with
the orogenic evolution proposed for Palaeo-
proterozoic units of the São Luı́s cratonic
fragment and reworked/basement portions
of the Gurupi Belt (Klein et al. 2008a; Klein
& Lopes 2011).
(4) The commonly observed close association of
gold deposits, granitoids, and thermal events
(e.g. Phillips & Powell 1993) is envisaged only
for the Palaeoproterozoic of the Gurupi Belt.
Fig. 10. (a) Drill core of the host tonalite of the Cipoeiro deposit showing variation in grain size and mineralogy related
to deformation and alteration. From left to right, the undeformed tonalite pass to a finer-grained rock with the same
chlorite-saussurite alteration; bleached halo in the margin of a quartz-pyrite veinlet and stringer of pyrite crosscutting
the tonalite. (b) Photomicrograph of the chloritised and saussuritised portion of the tonalite. (c) Photomicrograph of the
contact zone between the undeformed and sheared tonalite. Chl, chlorite; Hbl, hornblende; Pl, plagioclase; Py, pyrite.
Fig. 9. (Continued) calcite veinlet (white) crosscutting a fine-grained basalt (light grey) showing a concordant stringer
of very fine-grained pyrite-arsenopyrite (dark grey) (Cachoeira deposit; Klein et al. 2005a). (f) Polished slab from a
mineralized intersection of the Mandiocal target (Chega Tudo deposit) showing zoned alteration consisting of a quartz
veinlet surrounded by a proximal calcite-rich zone, intermediate chlorite-rich zone, and distal sericitic zone (Klein et al.
2008b). (g) Deformed quartz veinlet (white) from the Mandiocal target (Chega Tudo deposit), with stringers of
auriferous pyrite and seams of sericite (arrows) defining the foliation (Klein et al. 2008b). (h) Quartz-calcite veinlet at
Serrinha (Klein et al. 2006). Mineral abbreviations: Chl, chlorite; Py, pyrite; Cal, calcite; Dol, dolomite; Qtz, quartz;
Ser, sericite; Ab, albite.
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Table 3. Physico-chemical properties of fluids from selected orogenic gold deposits of the Gurupi Belt
Deposit n Cachoeira n Chega Tudo n Serrinha n Montes Áureos n Cipoeiro
Fluid composition nd CO2–H2O–NaCl 200 (A) CO2–CH4–H2O–
NaCl;
(B) CO2–H2O–NaCl
170 CO2–CH4–N2–
H2O–NaCl
130 CO2–CH4–H2O–
NaCl-MgCl/FeCl2
nd nd
XCO2 (mol %) nd nd 200 (A) 12–22; (B) 11–13 170 18–33 nd nd nd nd
XCH4 (mol%) nd nd 200 (A) ,6; (B) 0 170 ,2 130 ,6 nd nd
XN2 (mol %) nd nd 200 (A) 0; (B) 0 170 ,4 nd nd nd nd
wt % NaCl equiv.
(average)
nd nd 200 (A) 5.8; (B) 1.6–2.5 170 4.5 130 2.8 nd nd
bulk density
(g/cm3)
nd nd 200 (A) 0.83–0.96;
(B) 0.93–0.96
170 0.89–0.94 130 0.69–0.89 nd nd
T (8C) ,20 330–370 200 (A) 340–370;
(B) 330–340
170 310–335 130 296–302 70 305–319
P (MPa) nd nd nd nd 170 130–300 130 130–280 nd nd
log f O2 nd nd 200 (A) 228.7 to 230.5;
(B) 230.5 to 231.5
170 228.5 to 231.0 nd nd nd nd
d18OH2O (‰) 23 +6.2 to +12.4 10 (A) +7.9 to +9.4;
(B) +5.3 to +7.2
7 +6.2 to +8.4 nd nd 11 +2.4 to +5.7
dDH2O (‰) 3 216 to 228 7 (A) 229 to 237;
(B) 212 to 230
3 219 to 280 nd nd 5 220 to 243
d13C carbonate 12 211.5 to 213.6 1 (A) nd; (B) 23.8 6 214.2 to 215.7 nd nd 3 22.1 to 24.2
d13CCO2 (‰) nd 29.5 to 212.7 2 (A) 26.9; (B) 224.1 1 217.6 nd nd 1 210.7
d34S sulphide (‰) 8 +1.9 to +6.4 3 (A) nd; (B) + 0.1 to +0.8 2 22.6 to 27.9 nd nd 2 +1.1 to +1.7
References 1, 6 2 3 4 5
Key to references: 1: Klein et al. (2005a), 2: Klein et al. (2008b), 3: Klein et al. (2006), 4: Yamaguti & Villas (2003), 5: Klein et al. (2007), 6: Klein (unpublished data).
nd: not determined/not available; A–B: two types of fluids at Chega Tudo.
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Nature and sources of ore fluid components
Composition of ore fluids
Detailed fluid inclusion studies have been un-
dertaken at Montes Áureos (Yamaguti & Villas
2003), Serrinha (Klein et al. 2006), and in two
targets (mineralization styles) of the Chega Tudo
deposit (Klein et al. 2008b), a summary of which
is given in Table 3. These studies show that the
entrapped vein fluids of the Gurupi Belt deposits
are of the aqueous-carbonic (H2O–CO2–NaCl),
carbonic (CO2), and aqueous (H2O–NaCl) types
(Fig. 11). The aqueous fluid is absent at Serrinha.
At room temperature the carbonic fluid is made
of single-phase inclusions, while the aqueous and
aqueous-carbonic fluids are made by two-phase
(liquid-vapor, LV) inclusions with variable phase
ratios. Three-phase inclusions are made of an
aqueous liquid, a carbonic liquid, and a vapor
phase and are exclusive of aqueous-carbonic fluid.
The CO2-bearing fluid inclusions form clusters of
randomly distributed (primary) inclusions in the
inner part of quartz crystals, or form large, three-
dimensional trails (Fig. 11). Some aqueous inclu-
sions have the same distribution and coexist with
the CO2-bearing inclusions (Montes Áureos and
Chega Tudo). Most of the aqueous inclusions,
however, form secondary trails and have homogen-
ization temperatures well below the range of temp-
eratures showed by the H2O-CO2-NaCl inclusions
(Table 3).
The petrographic and microthermometric
characteristics of the fluid inclusions described for
the deposits of the Gurupi Belt, include (in one or
more deposits) contemporaneous entrapment of
different fluid types, which is reflected in the coex-
istence of carbonic, aqueous-carbonic, and subordi-
nate aqueous fluid inclusions, large variation in
the phase ratios and in the temperature of homo-
genization of the CO2-bearing phase (i.e. density
variation), homogenization of the coexisting inclu-
sions both to the liquid and vapour phases over the
same range of temperatures, and partitioning of
salts into the aqueous-rich phase (only observed
at Chega Tudo). These characteristics satisfied
at least some of the criteria used by Ramboz et al.
(1982) to identify fluid immiscibility. In addi-
tion, density variations are at least in part due to
fluctuating pressure conditions, as indicated by
microstructural evidence (coexistence of and/or
alternating ductile and brittle features in the
same vein).
Fig. 11. Fluid inclusion assemblages in gold deposits of the Gurupi Belt. (a) Chega Tudo: clusters of aqueous-
carbonic fluid inclusions (black arrows) and aqueous inclusions (white arrow) in quartz. (b) Random and fracture-hosted
aqueous inclusions (Klein et al. 2008b). (c, d) Serrinha: carbonic and aqueous-carbonic fluid inclusions with variable
CO2/H2O ratios (Klein et al. 2006).
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The mineralizing fluid is interpreted to be of
aqueous-carbonic nature and some lines of evidence
outlined previously in this paper give support to
the above interpretation: (1) the fluid inclusion
studies were performed in (sometimes visible)
gold-bearing quartz veins and quartz-carbonate-
sulphide veinlets (Chega Tudo, Serrinha, Montes
Áureos); (2) the aqueous-carbonic fluid is associ-
ated with the highest gold grades in Chega Tudo;
and (3) the aqueous-carbonic fluid is the only fluid
present at Serrinha.
The ore fluid shows subordinate contents of
CH4 and N2 in the carbonic phase and local presence
of MgCl2 and/or FeCl2 in the aqueous phase
(Table 3). The CO2 contents range between 11
and 22 mol% at Chega Tudo, and is quite higher
(18–33 mol%) at Serrinha. No information is avail-
able for the other deposits. The fluids have low
salinity, averaging 1.6–5.8 wt% NaCl equivalent
in different deposits (Yamaguti & Villas 2003;
Klein et al. 2006, 2008b). Only locally, at Montes
Áureos, fluids with salinity up to 15 wt% NaCl
equivalent have been identified (Yamaguti &
Villas 2003).
The aqueous fluid inclusions present in sev-
eral depositshave usually been interpreted not
to be related to the mineralizing event, but to later
infiltration events, partly related to the regional
post-orogenic uplift. This has been based on the
lower homogenization temperatures presented by
the aqueous inclusions when compared to the
CO2-bearing inclusions, on crosscutting relation-
ships, and on compositional differences.
Calculated fluid properties (T–P–fO2–pH)
Fluid inclusion homogenization temperatures,
stable isotope equilibrium, and the chlorite and
arsenopyrite geothermometers indicate formation
temperatures between 296 8C and 370 8C, and pres-
sures between 130 and 300 MPa for the Gurupi Belt
deposits (Fig. 12a, Table 3). These values are in
keeping with T–P estimations in deposits elsewhere
that have similar hydrothermal assemblages and
structural setting (e.g. McCuaig & Kerrich 1998;
Robert & Poulsen 2001).
Oxygen fugacities have been reported for the
Chega Tudo and Serrinha deposits and calculated
for the range of XCO2 of the ore fluids and the
range of estimated temperatures and pressures
(Klein et al. 2006, 2008b). This approach yielded
log f O2 values between 228.5 and 231.5. These
values plot within the pyrite and magnetite stabil-
ity fields, above the CO2-CH4 buffer, and below
the SO2/H2S buffer (Fig. 12b), defining relatively
reduced conditions for the ore fluids. These con-
ditions are also consistent with the measured d34S
values of sulphide minerals (mainly pyrite), the
large predominance of CO2 over CH4 in fluid
inclusions, the frequent coexistence of pyrite and
chlorite in the alteration assemblage, and the
absence of oxidized minerals, such as hematite
Fig. 12. (a) P–T diagram showing the estimated conditions for gold mineralization in deposits of the Gurupi Belt
(references in the text). The dashed line is the solvus for the CO2–H2O–NaCl system (Bowers & Helgeson 1983). The
shaded box limits the metamorphic conditions for the Chega Tudo Formation (Yamaguti & Villas 2003). (b) Solubility
of gold as a function of temperature and oxygen fugacity for the Chega Tudo (light grey) and Serrinha (dark grey)
deposits. Dashed lines are for bisulphide complex Au(HS)2
2, dotted-dashed lines are for chloride complex AuCl2
2. The
heavy solid lines represent the limits of the stability fields of Fe-oxides and sulphides, and the dotted lines represent gas
buffers (adapted from Romberger 1990 and Ohmoto & Goldhaber 1997). Mineral abbreviations: hem, hematite; mag,
magnetite; py, pyrite; po, pyrrhotite.
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and sulphates, in the alteration assemblage (Kerrich
1987; Romberger 1990; Hayashi & Ohmoto 1991;
Ohmoto & Goldhaber 1997). Considering the simi-
larities between the studied deposits, it is likely that
similar conditions apply also for Cachoeira, Montes
Áureos and Cipoeiro.
The absence of K-feldspar and the presence of
white mica and minor albite as alteration minerals,
and the stability of carbonate minerals imply near-
neutral to slightly alkaline conditions for the miner-
alizing fluids, with pH values between 5.0 and 6.2
(Romberger 1990; Mikucki & Ridley 1993).
Sources of ore fluid components
Stable isotope studies in silicate, carbonate, and
sulphide minerals and in water and CO2 from
aqueous-carbonic inclusion fluids have been used
to constrain potential sources for the ore fluids
(Klein et al. 2005a, 2006, 2007, 2008b). Oxygen
and hydrogen isotope compositions have been
calculated for the estimated temperature of ore
formation in each deposit (Table 3 and Fig. 13).
The results gave d18O values between +5.3‰ and
+12.4‰ and dD values ranging from 212‰ to
237‰ in deposits hosted by metavolcano-sedi-
mentary rocks. These values consistently indicate
a metamorphic source for the water present in
inclusion fluids and in equilibrium with hydrother-
mal minerals. Lower fluid d18O values of +2.4 to
+5.7‰ were found in the tonalite-hosted Cipoeiro
deposit (Fig. 13 and Table 3). Although compatible
with a metamorphic source, it is uncertain if the
lower limit of d18O values also implies a limited
contribution of meteoric water. However, an accom-
panying lowering in the dD values would be
expected in the case of influence of meteoric
water, which is not the case. Also enigmatic is one
strongly negative dD value of 280‰ found in
water from aqueous-carbonic inclusions of one
sample of Serrinha, and that falls in the magmatic
range. Klein et al. (2006) considered H2 diffusion
into inclusion cavities induced by deformation as
the best explanation for this low value.
Limited d13C values of CO2 present in fluid
inclusions vary from 26.9‰ to 224.1‰ and car-
bonaceous schists and phyllites that are common
in the stratigraphy of the host metavolcano-
sedimentary Chega Tudo Formation have d13C
values of 223.5‰ to 229.7‰ (Table 3). The
highest values presented by inclusion fluids likely
represent a mantle-derived carbon source. The
lowest values are clearly of organic origin. These
have been interpreted as a contribution from the
Fig. 13. Oxygen and hydrogen isotope compositions of ore fluids of gold deposits from the Gurupi Belt. The fields
of metamorphic and magmatic waters are from Sheppard (1986).
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Fig. 14. Interpreted evolution of the São Luı́s Cratonic Fragment and Gurupi Belt in the Palaeoproterozoic (not to
scale). (a) Accretion phase: intra-oceanic arc construction and reworking, subduction, and calc-alkaline magmatism. (b)
Collision phase: accretion of the Rhyacian juvenile terranes to an Archaean block, crust thickening, metamorphism,
E. L. KLEIN140
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carbonaceous rocks, whereas the intermediate val-
ues are probable mixtures of deep-seated carbon
with organic carbon at the site of ore deposition
(Klein et al. 2005a, 2006, 2008b).
Sulphur isotope composition of sulphide min-
erals show positive d34S values between +0.1‰
and +6.4‰ at Cachoeira, Chega Tudo, and Cipo-
eiro. Only Serrinha shows relatively low d34S
values of 22.6‰ to 27.9‰ (Table 3). These posi-
tive values are interpreted as reflecting both mag-
matic and mantle sources, or an average crustal
sulphur composition. The magmatic input could be
related either to magmatic fluids or to the dissolu-
tion of primary magmatic sulphides (Ohmoto &
Rye 1979; Lambert et al. 1984). The negative val-
ues of Serrinha may be attributed to oxidation of
the fluids and could be produced by a few mechan-
isms, such as the interaction of hydrothermal
fluids with Fe2+-bearing silicates of the host rocks,
immiscible separation of reduced gases, and the
presence of an originally oxidized magmatic
fluid (McCuaig & Kerrich 1998 and references
therein). As discussed by Klein et al. (2006), the
fluid immiscibility and removal of CO2 from the
fluid trapped in fluid inclusions and to form carbon-
ate minerals may have led to oxidation of the
residual fluid at Serrinha and might be responsible
for the negative d34S values.
Mechanisms of gold transport and
deposition
Gold transport and deposition in hydrothermal
systems is governed by fluid composition, tempera-
ture, pressure, pH, oxygen and sulphur fugacity, and
type and amount of dissolved sulphur and other
species (Mikucki & Ridley 1993; Mikucki 1998
and many others). Depending on these parameters,
hydroxide, chloride, and sulphide complexes may
be responsible for gold solubilization and tran-
sport (Loucks & Mavrogenes 1999; Stefánsson &
Seward 2003, 2004; Pokrovski & Dubrovinsky
2011, and references therein). At the physico-
chemical conditions discussed in the previous
section, it is likely that H2S (alternatively, HS
2)
was the predominant sulphur species in the fluid
(Fig. 12b) and that Au(HS)2
2 was probably the domi-
nant gold-transporting complex (Shenberger&
Barnes 1989; Hayashi & Ohmoto 1991; Benning
& Seward 1996). The physico-chemical conditions
are also consistent with the high solubility of gold
and copper as bisulphide complexes (Fig. 12b).
In regard to gold deposition, complex, some-
times conflicting and competing, processes such as
fluid immiscibility under fluctuating pressure con-
ditions, mixing of fluids, and fluid-rock reactions
may occur at a scale of a single deposit. In the
gold deposits of the Gurupi Belt, extensive fluid
immiscibility (phase separation, generally induced
and/or accompanied by pressure fluctuations) and
local fluid mixing have been documented (Yama-
guti & Villas 2003; Klein et al. 2006, 2008b). The
first process is considered to have played a major
role in gold deposition. In addition, several lines
of evidence indicate fluid-rock reactions, which
may have contributed to the destabilization of the
gold transporting complexes at the depositional
sites, through changes in the pH and in the oxygen
fugacities and sulphur and carbonate activities.
These include:
(1) The presence of a large volume of altered
host rocks (the deposits extend for more
than 1 km in length). Although water/rock
ratios have not been quantified, the alteration
reactions occurred in fluid-dominated sys-
tems and under nearly isothermal condi-
tions, as inferred by small variation in the
fluid isotope compositions in each deposit
(Table 3).
(2) Sulphidation of the host rocks, or desulphida-
tion of the ore fluid by reaction between
sulphur and iron-rich minerals, such as mag-
netite, early pyrite and chlorite, produc-
ing predominantly pyrite, and being the main
responsible for the Au-pyrite paragenesis.
This could be produced by the reaction
Au(HS)2
2 + FeO ¼ Au + FeS2 + H2O sugge-
sted for Montes Áureos (Yamaguti & Villas
2003).
(3) The presence of gold in fractures of quartz and
pyrite (Fig. 8).
(4) Reaction between the fluid and carbon-
bearing host rocks, as depicted by the low
d13C values of fluid inclusion CO2, at least
in Chega Tudo and Serrinha (Table 3).
(5) Removal of CO2 from solution by dissociation
(CO2 + H2O ¼ H+ + HCO32) to precipitate
carbonate minerals (e.g. Rimstidt 1997) or
trapped in fluid inclusions.
(6) Local oxidation of the ore fluid (Serrinha).
Simple cooling is not considered to have
been an effective process for gold precipi-
tation in the Gurupi Belt (Yamaguti & Villas
2003; Klein et al. 2005a, 2006, 2007, 2008b).
Fig. 14. (Continued) crustal melting, peraluminous magmatism, and ore formation. (c) Detail of the present-day
external portion of the Gurupi Belt with the location of selected gold deposits and possible fluid and solute sources and
paths (modified from Klein et al. 2005b, 2008b).
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Tectonic setting and integrated
fluid-structural model
The definition of the tectonic setting in which the
orogenic gold deposits of the Gurupi Belt formed
is highly dependent on their absolute timing of for-
mation. The host rocks formed in an island arc or
back-arc tectonic setting at about 2168–2147 Ma
during a Rhyacian orogenic event (Fig. 14a). This
terrane represents today the reworked southern
margin of the São Luı́s cratonic fragment, that is,
it represents the external domain of the Neoprotero-
zoic Brasiliano-Pan African orogen. The main host
structure, the Tentugal Shear Zone, was certainly
active during the Neoproterozoic as a strike-slip
structure and was probably active in the Rhyacian
orogenic event as part of a compressive regime
(Klein & Lopes 2011). Although I cannot discard
a Neoproterozoic metallogenic event, the Rhyacian
scenario is the preferred one, for the reasons
discussed above.
An integrated fluid-structural model for gold
mineralization in the Gurupi Belt operating in
Palaeoproterozoic times (Figs 14 & 15) is based
on the data outlined above and includes the nature
of the hosting rocks and structures, the hydrothermal
assemblage, the structural style of mineralization,
hosting structures, wall rock alteration, mineral
paragenesis, the fluid inclusion properties, and the
stable isotope data. Considering the similarities
between Chega Tudo and the other deposits of the
Gurupi Belt, the model may also account, in
general terms, for the gold genesis in this belt,
which is valid for both Palaeoproterozoic and Neo-
proterozoic times. The key elements of this model
are: (1) a deeply sourced, low-salinity H2O–CO2–
NaCl – bearing fluid of metamorphic origin
migrated through active structures related to the
Tentugal shear zone under fluctuating pressure con-
ditions; (2) hydrothermal alteration of the enclos-
ing rocks and vein precipitation; (3) the local
enrichment of the hydrothermal fluid in CH4 in
response to its interaction with carbon-bearing
rocks of the Chega Tudo Formation; and (4) a
sudden drop in the fluid pressure, probably related
to a fault-valve behaviour within the hosting struc-
tures, causing fluid immiscibility and forcing the
fluid back into the shear zone and accompanying
mixing with the original deeply sourced H2O–
CO2–NaCl fluid and, in places, with an aqueous
fluid of disputable origin (metamorphic and/or
meteoric) or exchanged with metamorphic rocks.
Mixing, however, has only been documented at
Chega Tudo (Klein et al. 2008b). The model is
Fig. 15. Schematic (not to scale) illustration modified from Klein et al. (2008b) with the interpretation of possible
mechanisms for the migration of the fluids responsible for the deposition of orogenic gold in the Chega Tudo deposit.
The interpretation is broadly valid for the other gold deposits. (a) Pre-fault failure with upward and outward migration of
a deeply sourced fluid, wall-rock alteration vein formation, and deposition of free-gold. (b) Post-fault failure with
continuation of the upward migration of the deeply sourced fluid, opening of spaces, and backward migration of the
reacted fluid, with vein formation and precipitation of refractory-gold. Abbreviations: FI, fluid inclusions; cc, carbonate.
E. L. KLEIN142
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consistent with what is described elsewhere for
shear zone-hosted gold deposits (e.g. Cox et al.
1995; McCuaig & Kerrich 1998; Sibson 2001).
Concluding remarks
The epigenetic gold deposits of the Gurupi Belt
form a coherent group that share similar charac-
teristics, in terms of geological attributes and ore
genesis, with orogenic gold deposits found in
Palaeoproterozoic metamorphic terranes of South
America (e.g. Rio Itapicuru Greenstone Belt
(Xavier & Foster 1999); Ipitinga Auriferous District
(Klein & Fuzikawa 2010 and references therein);
West-African Craton (e.g. Birrimian terranes,
Berge 2011 and references therein); and elsewhere
(see reviews in Groves et al. 2003; Goldfarb et al.
2005)). The timing of gold mineralization in these
terranes is relatively well constrained, placing
the formation of gold deposits at about 2100–
2000 Ma, at the end of the long orogenesis. In con-
trast, the absolute timing of gold deposition in the
Gurupi Belt has not yet been determined. This
major issue, which is critical for the establishment
of genetic models, stems from the poly-orogenic
evolution of the Gurupi Belt, that is, the host rocks
of the ore bodies formed during the Rhyacian oro-
genesis that built up the São Luı́s cratonic fragment,
but these rocks, at least in part, were affected by
Neoproterozoic events related to the evolution of
the Gurupi Belt. Tectonic reworking, isotope reset-
ting, and the lack of isotopic equilibrium in places
where petrographic evidence indicates equilibrium
are widespread features.
Furthermore, the different styles of gold mineral-
ization found in all deposits, that is, free-milling
gold in veins, and refractory and free gold in
altered host rocks could indicate different mechan-
isms for gold precipitation and/or differencesin
the extent of fluid-wall rock interaction, but could
also indicate two separate mineralizing events
with distinct ore fluids. The field and research
work done so far have not been able to solve this
problem, and the model presented here implies fun-
damentally that the gold deposits of the Gurupi Belt
formed in the Palaeoproterozoic from Archaean to
younger source rocks but were remobilized later
on, up to the Neoproterozoic.
The author thanks P. Garofalo for the invitation to write
this paper and for insightful comments on the manuscript.
The paper is a contribution to the projects ‘Metalogenia
do Cinturão Gurupi’ (CPRM/Geological Survey of
Brazil) and ‘Geocronologia e modelamento isotópico de
depósitos aurı́feros do Cráton São Luı́s e Cinturão
Gurupi: a busca da relação metalogênese do ouro com a
evolução crustal’ (CNPq, process 306723/2009-3) and to
GEOCIAM – Instituto Nacional de Ciência e Tecnologia
de Geociências da Amazônia. J. Kolb and an anonymous
reviewer are acknowledged for their comments and
suggestions that helped to improve the manuscript.
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