Paragenesis and evolution of the hydrothermal Bacuri iron oxide – copper – gold deposit , Carajás Province ( PA )

The Bacuri copper deposit is located about 9 km east of the world-class Sossego iron oxide-copper-gold deposit in the Carajas Province. It is hosted by the ca. 2.84 Ga Serra Dourada granite, the Bacuri Porphyry, and subordinated gabbro. The Bacuri deposit is situated within the regional-scale, WNW-ESE-trending ductile Canaa shear zone, characterized by early albite, scapolite-magnetite, potassium feldspar and biotite-scapolite-magnetite hydrothermal alteration zones. Copper mineralization was controlled and largely simultaneous to the development of a NE-SW-trending transcurrent fault zone. Within this zone, intense chlorite alteration, silicification and quartz veining overlap previous hydrothermal stages. Copper ore ­(chalcopyrite-pyrite-magnetite) is disseminated and related to veins and breccias. Chalcopyrite occurs along the mylonitic foliation in chlorite alteration zones, in brecciated quartz-(muscovite-fluorite) and undeformed milky quartz veins and in late potassium feldspar-epidote veinlets with open space textures. The relative temporal history of the Bacuri deposit reveals significant changes in physico-chemical parameters during the system evolution. Contribution of hot hypersaline metalliferous fluids was important during early sodic and potassic alteration stages and related to a regional hydrothermal system developed at relatively deep crustal levels. Influx of externally derived diluted fluids within the NE-SW-trending fault zone favoured the late shallow-emplaced chlorite alteration and silicification, and may have triggered the ore deposition due to decrease of temperature and salinity, accompanied by ƒO2 increase and pH decrease. Such significant changes could indicate a hybrid system that evolved in multiple pulses during progressive exhumation.

The Bacuri copper deposit is located approximately 9 km east from the world-class Sossego IOCG deposit, within an important regional ESE-WNW shear zone, subsidiary to the Canaã shear zone (Pinheiro et al. 2013).Despite its proximity to other important IOCG deposits, geological setting of the Bacuri copper deposit is poorly understood, especially the nature of its host rocks, paragenesis and relationship with a large hydrothermal system in the southern portion of the Carajás Domain.Similar paragenetic evolution (sodic and potassic alteration, magnetite formation, chlorite/epidote alteration, copper mineralization) has been recognized in the Sossego mine (Monteiro et al. 2008a) and in Cristalino, Alvo 118, Castanha, Bacaba, Visconde and Jatobá deposits (Augusto et al. 2008;Ribeiro 2008;Monteiro et al. 2011;Pestilho 2011;Craveiro et al. 2012;Torresi et al. 2012).However, the predominance of specific types of hydrothermal alterations and the distribution of alteration zones, which vary in each deposit, suggests distinct depth of formation (Hitzman et al. 1992;Williams et al. 2005) with variable proportion of magmatic and externally derived components in the paleohydrothermal systems.
In addition, new geochronological data for the Carajás IOCG deposits (ca.2.71 -2.68 Ga and 1. 88 Ga;Moreto 2013) indicate that these deposits were not formed in a single metallogenetic event.Neoarchean and Paleoproterozoic events might be responsible for the complex puzzle of attributes of these important copper-magnetite deposits.Moreover, these data suggest that the Bacuri deposit is an example of a Neoarchean IOCG deposit in Carajás and may be part of a regional and protracted Neoarchean paleohydrothermal system.
This paper introduces the Bacuri deposit and aims to characterize its host rocks, distribution and types of hydrothermal alteration and ore typology and paragenesis, and to compare with the metallogeny of the IOCG deposits in the Carajás Province.

GEOLOGICAL SETTING OF THE CARAJÁS PROVINCE
The Carajás Province, located in the south-eastern part of the Amazonian Craton, is divided into two domains: Rio Maria, in the south, and Carajás, in the north (Vasquez et al. 2008), separated by an E-W regional shear zone (Fig. 1).
The Igarapé Bahia Group, which hosts the Igarapé Bahia/ Alemão deposit, comprises metavolcanic, metapyroclastic and metasedimentary rocks metamorphosed in the greenschist facies, including iron formation.
The Águas Claras Formation overlaps, apparently in angular unconformity, the metavolcanosedimentary sequences of the Itacaiúnas Supergroup, and is composed of metaconglomerates,  metarenites, dolomitic marble, carbonaceous phillyte, and sericite quartzites.Their protoliths have been deposited in fluvial to shallow marine environment (Nogueira 1995), and a minimum age of deposition is given by the crystallization age of mafic dikes intrusive in this formation (2,708 ± 37 Ma; U-Pb in zircon; Mougeot et al. 1996).Mafic-ultramafic layered intrusions represented by the Luanga Complex (ca. 2.76 Ga;Machado et al. 1991) host lateritic nickel, chrome and platinum group element mineralization.
Three main WNW-ESE shear zone systems have been characterized in the Carajás Domain, including Carajás and Cinzento in the north and Canaã in the south (Pinheiros et al. 2013).According to Pinheiro et al. (2013), the widespread E-W foliation, folding and faulting in the Carajás Domain were related to shortening from NNE to SSW.A major set of NE-SW sinistral strike-slip faults, analogous to R-type Riedel structures, were responsible for the sigmoidal "S-shape" of the Carajás Domain (Pinheiro et al. 2013).

IRON OXIDE-COPPER-GOLD DEPOSITS IN THE CARAJÁS PROVINCE: A BRIEF REVIEW
The Carajás Province has the highest concentration of high-tonnage IOCG deposits in the world (Monteiro et al. 2008a), which are located within or close to shear zones in the contact between the metavolcanosedimentary units of the Itacaiúnas Supergroup with the Mesoarchean basement rocks (Moreto et al. 2011).
In general, the Carajás IOCG deposits exhibit similar attributes, such as (i) strong spatial association with WNW-ESE shear zones and with minor subsidiary NE-SW faults; (ii) the presence of hydrothermal breccias; (iii) intense alkaline hydrothermal alteration; (iv) magnetite formation ensued by precipitation of sulphides and (v) a broad range of homogenization temperatures (100 -570°C) and salinities (0 -69 wt.% eq.NaCl) in fluid inclusions, pointing to fluid mixing that allowed the transport and precipitation of metals (Monteiro et al. 2008a;Xavier et al. 2012).Moreover, these deposits have significant contents of LREE (light rare earth elements), P, U, Ni, Co and Pd similar to those described in IOCG deposits in other provinces worldwide (Hitzman et al. 1992;Williams et al. 2005).
The IOCG deposits located in the northern part of the Carajás Domain, close to the contact with the Paleoproterozoic Bacajá Domain (Vasquez et al. 2008), show different host rocks, types and distribution of hydrothermal alteration zones and ore mineralogy in comparison with those of the deposits of the Southern Copper Belt (Moreto 2013).The Salobo deposit, the most important deposit in the north, has zones of intense potassic alteration with biotite and iron enrichment, including almandine, grunerite and fayalite, followed by magnetite formation in high temperatures and ductile conditions (Lindenmayer 2003;Melo et al. 2013).Ore minerals include chalcocite, bornite, chalcopyrite, and gold.
In the Southern Copper Belt (Moreto 2013), the IOCG deposits are located within the Canaã shear zone, a 20-to 14-km wide sinistral WNW-ESE shear zone that has been active in ca.2.7 -2.5 Ga (Pinheiro et al. 2013).These deposits show a paragenetic evolution including sodic, sodic-calcic and potassic alteration, magnetite formation, chlorite-epidote alteration, copper mineralization and late sericite alteration, which is similar to that identified in the Sossego mine (Monteiro et al. 2008a), Cristalino deposit (Ribeiro 2008) and others, such as Castanha, Bacaba, Visconde and Jatobá (Augusto et al. 2008;Monteiro et al. 2011;Pestilho 2011;Craveiro et al. 2012).However, the distribution of the hydrothermal alteration zones varies in each deposit.Alteration patterns similar to those developed in the deeper portions of IOCG systems, with predominance of albite-actinolite and apatite-magnetite associations, have been recognized in the Sequeirinho-Pista orebodies (Sossego mine).Meanwhile, hydrothermal assemblages typical of relatively shallow crustals levels, with predominance of K-feldspar, biotite, chlorite and sericite-hematite, are well developed in the Sossego orebody (Sossego mine) and the Alvo 118 deposit.
The IOCG events have been interpreted as being connected to the widespread events of granite crystallization recognized in the Carajás Domain at ca. 2.76 -2.73 Ga (e.g.Planalto, Plaque, Estrela, Igarapé Gelado) and ca.1.88 Ga (A-type granites).However, a minor magmatic event related to granite formation at ca. 2.57 Ga (e.g.Old Salobo Granite), restricted to the northern sector, was also considered responsible for the establishment of the Salobo and Igarapé-Bahia extensive hydrothermal-magmatic systems (Réquia et al. 2003;Tallarico et al. 2005), but still a matter of debate.
Boron (Xavier et al. 2008) and chlorine (Chiaradia et al. 2006) isotope studies point to the participation of both magmatic and externally derived basinal fluids for the Carajás IOCG systems.Stable isotopes and fluid inclusion studies also indicate fluid mixing involving an externally derived fluid (meteoric water or seawater) in the late stages of the hydrothermal systems evolution (Monteiro et al. 2008a;Xavier et al. 2012).

METHODS
Geological mapping in the vicinity of the Bacuri deposit area and systematic description of samples from five drill holes (BRID 02, 05, 07, 09 and 12) was accomplished, and samples representative of least altered, altered and mineralized lithotypes were collected.Detailed petrographic studies under transmitted and reflected light, and using scanning electron microscopy (SEM) coupled with EDS (energy-dispersive X-ray spectrometer) and cathodoluminescence, were carried out in the Geoscience Institute, University of Campinas, Brazil.

Host rocks
The Serra Dourada granite (Moreto et al. 2011;Feio et al. 2013) is greyish-to-pinkish in colour, medium-to-coarse grained and predominantly isotropic (Fig. 4A) but locally foliated.It has subordinate pegmatitic (Fig. 4B) and micrographic facies and is cut by aplite dikes.Its average composition is represented by 35% quartz, 45% potassium feldspar and 20% plagioclase, as well as minor biotite, and it is classified as a syenogranite.The Bacuri Porphyry is intrusive in the Serra Dourada granite and represents the main host rock in the Bacuri deposit.Its occurrence is restricted to a NE-SW fault, and consequently, it is commonly strongly mylonitized, but locally relicts of igneous textures are recognized (Figs.4C and 4D).In more preserved parts, the Bacuri Porphyry is grey, porphyritic and shows a fine-grained phaneritic matrix composed of quartz (> 50%), potassium feldspar (15%), plagioclase (20%), showing a dacitic composition.Idioblastic-to-subdioblastic phenocrysts of plagioclase (albite) (15%) are up to 3 mm long, show corrosion texture and are mostly fractured.
Gabbro dikes are dark green and have relicts of subophitic texture, defined by plagioclase laths and interstitial poikilitic augite.Late quartz-feldspar porphyry dikes with quartz and potassium feldspar phenocrysts cross-cut hydrothermal alteration zones and are only incipiently altered (Fig. 4E).
The host Serra Dourada granite, the Bacuri Porphyry and gabbro are strongly foliated within the NE-SW subsidiary transcurrent fault.These lithotypes show a NE-SW subvertical mylonitic foliation and evidence significant grain-size reduction in comparison with their least deformed protoliths.Ribbon structures and eye-shaped feldspar porphyroclasts are commonly present in the Bacuri Porphyry.In addition, the fine-grained matrix in these mylonites is predominantly composed of oriented hydrothermal minerals, re-crystallized quartz and intensely strained flattened grains of the pre-existing feldspar.

Hydrothermal alteration stages
Hydrothermal alteration in the Bacuri deposit is mainly controlled by the NE-SW subsidiary transcurrent fault zone, which represents the locus of the copper mineralisation.Outwards the fault zone, protoliths can still be recognized, and hydrothermal alteration mainly depends on the nature of rock (e.g.predominance of hydrothermal biotite in mafic rocks and potassium feldspar in felsic host rocks).In proximal zones, pre-existing features of the host rocks are predominantly obliterated and different protoliths develop similar hydrothermal paragenesis.
Early alteration stages are also mainly identified in the distal portions of the deposit.In proximal zones, overprinting of late pervasive alteration stages and telescoping prevail.However, a complete succession of hydrothermal alteration stages in the Bacuri deposit (Fig. 5) can be revealed by temporal relationships among hydrothermal mineral phases, such as cross-cutting zones, replacement haloes and remnants of previous hydrothermal assemblages within later alteration zones observed in both distal and proximal zones.
In distal zones, the Serra Dourada granite is the predominant host rock.It mainly shows early pervasive sodic alteration with albite and minor scapolite, which replaces the igneous potassium feldspar and plagioclase respectively.Th e sodic alteration is overprinted by potassic alteration with potassium feldspar and biotite.Chlorite alteration is incipient in the distal alteration zones within the Serra Dourada granite and occurs especially in a fi ssural style, selectively replacing biotite crystals.Muscovite formation is late and occurs as external haloes in relation to chalcopyrite-bearing quartz-(feldspar-muscovite) veins.Pervasive silicifi cation and quartz veining are also recognized.Th e latter was accompanied by chalcopyrite and muscovite formation in late stages.In these distal zones, gabbro has also evidence of hydrothermal alteration although subophitic texture is preserved.Scapolite is recognized on the rims of the igneous plagioclase crystals and in veinlets.Augite was successively replaced by hastingsite, biotite and chlorite.
In proximal zones, the main lithotype is represented by the Bacuri Porphyry, but the Serra Dourada granite and gabbro are also recognized.A similar paragenetic sequence to that of the Serra Dourada granite in distal zones is observed; however, hydrothermal alteration is more intense and pervasive, resulting in extremely modifi ed lithotypes.Zones with hydrothermal biotite-scapolite and chlorite are the most predominant.
Late dikes of quartz-feldspar porphyry, even in proximal zones, cross-cut intensely altered rocks but have only epidote veins and narrow haloes with potassic feldspar.Th e types of hydrothermal alteration identifi ed in the Bacuri deposit are detailed below.

EARLY ALBITE ALTERATION
Formation of albite (I) corresponds to the initial stage of sodic hydrothermal alteration of the Bacuri deposit and results in white-to-pinkish colour in altered portions of the Serra Dourada granite (Figs.6A and 6B) and Bacuri Porphyry in distal zones of the deposit (Fig. 6C).Hydrothermal albite (I) with chessboard texture (Fig. 6E) either selectively replaces the igneous potassium feldspar in the Serra Dourada granite or fi lls veinlets.

SCAPOLITE-(MAGNETITE) ALTERATION AND VEINING
The Bacuri deposit records an intense and widespread process of scapolite formation, recognized in the distal and proximal alteration zones.In distal zones, the early scapolite (I) replaces igneous plagioclase along their crystals edges and cleavage planes (Fig. 6F) in the Serra Dourada granite and gabbro and also occurs in the quartz-feldspathic matrix of the Bacuri Porphyry.Veinlets infilled by scapolite (I) are also common.
Towards the central zone of the deposit, scapolite (+magnetite I±quartz) veins up to 10 cm are also observed.In these veins, scapolite (I) crystals are coarse-grained (up to 2-mm long), are fibrous, and show undulose extinction and subgrain formation.Evidence of scapolite replacement by later hydrothermal mineral phases is commonly observed.Network of biotite veinlets cross-cuts scapolite crystals, which are also replaced by potassium feldspar, and thereafter, by fine-grained muscovite.

POTASSIC ALTERATION WITH POTASSIUM FELDSPAR
The potassic alteration stage with potassium feldspar is selective in the Serra Dourada granite and pervasive in the Bacuri Porphyry, giving a reddish colour to the altered rock (Figs.6A, 6B and 6D).In both lithotypes, hydrothermal potassium feldspar (I) replaces igneous microcline and plagioclase, and hydrothermal scapolite.The hydrothermal feldspar is characterized under microscope for examining its turbidity and lack of twinning.Generally, the pervasive potassic alteration zones with potassium feldspar (I) are crosscut by biotite veinlets.Potassium feldspar is replaced by late fine-grained muscovite and/or chlorite as well.

BIOTITE-(SCAPOLITE-MAGNETITE) ALTERATION
Incipient potassic alteration with biotite is recognized in least-altered Serra Dourada granite and Bacuri Porphyry, and is characterized by biotite (I) crystals primarily in veinlets and in the interstices of the igneous quartz-fedspathic matrix of both host rocks.Biotite is also commonly observed in altered gabbro, mainly replacing augite.In proximal zones, biotite-scapolite alteration grades from selective and fissural to pervasive (Fig. 6D) and obliterates the protolith features (Fig. 6G).The mineral assemblage identified in these zones includes biotite (I) + scapolite (III) ± magnetite (II) ± quartz ± allanite ± monazite ± fluorite ± rutile.The altered rocks commonly show mylonitic foliation and are grey-to-brown in colour due to the predominance of biotite (I), but white spots represented by scapolite crystals are very common (Fig. 7A).Mylonitic foliation is defined by the orientation of hydrothermal finegrained biotite crystals and stretched quartz crystals (Fig. 7C), which show undulose extinction and subgrain formation.Scapolite (III) crystals are deformed and commonly have pressure shadows.Network of scapolite veinlets is also recognised in the biotite-scapolite alteration zones.
A second generation of magnetite is broadly found in the scapolite-biotite rich rocks.Magnetite (II) occurs as subdioblastic to idioblastic crystals disseminated in the rock and concordant to the mylonitic foliation.A fair increase in the size of magnetite crystals is accompanied by an increase in the biotite amount.In strongly mylonitized zones, deformed magnetite crystals are variably martitized and occur associated with tabular hematite crystals.

CHLORITE-(EPIDOTE) ALTERATION AND VEINING
Chlorite alteration is the most common and well-developed alteration in the host rocks of the Bacuri deposit and is strongly controlled by the NE-SW fault development.
In the distal zones, chlorite (I) replaces the biotite (I) crystals (Fig. 6D), or it occurs in veinlets and interstices of igneous crystals, in the Serra Dourada granite and mainly in the Bacuri Porphyry (Figs. 7B and 7C).
Towards the orebodies, amount and size of chlorite veinlets increase and the chlorite alteration grades to pervasive.In intensely altered zones, there is a complete replacement of all igneous minerals by aggregates of fine-grained chlorite crystals (Figs.7D, 7E and 7K).Chlorite alteration accompanies the increased intensity of the mylonitisation of the host rocks, resulting in strongly foliated rocks composed mainly of chlorite.
The common mineral association in this stage is represented by chlorite (I) + potassium feldspar (II) + magnetite (III) ± chalcopyrite (I) ± quartz ± epidote-group minerals ± apatite ± tourmaline ± monazite ± titanite.The potassium feldspar (II) was characterized using mainly EDS.It occurs strictly surrounding or near chalcopyrite, magnetite (III) and epidote crystals in chlorite mylonites, and is clear and limpid under transmitted light (Fig. 7E).
Tourmaline occurs in clusters of idioblastic crystals, which are recognized in very limited portions of the rock within the fronts of chlorite alteration and copper mineralisation.Tourmaline has strong pleochroism under transmitted light from dark blue to light blue or greyish, whose composition and optical properties are compatible with schorl (Fig. 7I).
Epidote-group minerals, such as epidote (I), clinozoisite and allanite are specially associated with chlorite, potassium feldspar (II) and chalcopyrite.They are observed disseminated parallel to the foliation and in veinlets.

SILIFICATION AND QUARTZ VEINING
Pervasive silicification and hydrothermal quartz veins cross-cut and are overprinted by the same hydrothermal alteration assemblage, indicating recurrence of silicification and veining.In distal zones of the deposit, silicification is less intense and mostly associated with biotite-scapolite alteration (Fig. 7C).Although quartz veins are recognized in distal zones, fronts of silicification predominate.In the proximal zones to ore, the host rocks are cut by quartz (I) veins and replaced by subdioblastic quartz (I) crystals up to 2 cm long (Figs.8D and 8E) in irregular alteration fronts and pockets.Quartz (I) crystals in silicified zones and in early veins are strongly deformed, showing undulose extinction, subgrain formation and ribbons (Fig. 7L), and the veins are brecciated.Coarse-grained flaky muscovite crystals (Fig. 7M) with kink bands, fluorite and chalcopyrite (II) are common in brecciated quartz veins (Fig. 7G).
Late milky quartz (II) veins (Figs.8B and 8E) with chalcopyrite (III) and minor chlorite commonly cross-cut the chlorite mylonites and brecciated quartz veins of previous generations.Coarse-grained (up to 0.5 cm long) biotite (II) crystals befringe late quartz veinlets.

HyDROLyTIC ALTERATION
Hydrolytic alteration is represented by a large amount of greenish-to-whitish flakes of fine-grained muscovite associated with hematite, which replace previous potassium feldspar, albite and biotite.Close to deformed quartz veins, haloes with coarse-grained muscovite are also observed.

CARBONATE VEINS
Late calcite-(hematite) crystals (Fig. 7H) are observed in veinlets of approximately 2 mm in width that cross-cut zones of previous stages of hydrothermal alteration.Thin calcite veinlets cut even the quartz-muscovite and milky quartz veins (Fig. 7N).

Characterization of the Copper Ore
The orebodies are represented by small lenses, veins and breccia bodies, which are spatially related to a NE-SW-striking transcurrent fault.Copper mineralisation is recognized in (i) chlorite-(epidote) alteration zones, (ii) brecciated silicified zones and deformed quartz-(muscovite-fluorite) veins, (iii) undeformed milky quartz veins and (iv) late zoned potassic feldspar-epidote veinlets.
However, the main orebodies are related to brecciated silicified zones and deformed quartz-(muscovite-fluorite) veins (Figs.8A, 8C and 8D) within zones of intense chlorite alteration.Chalcopyrite (II) and pyrite not only commonly infill network of fractures in deformed quartz and cleavage planes of muscovite, but are also concentrated in massive sulphide zones that replace the brecciated silicified rock.
Chalcopyrite (III) is also observed in late milky quartz veins (Fig. 8E) and in veinlets with cloudy potassium feldspar (III; Figs.8F, 8G and 8H) and epidote (II).The latter show comb and open-space filling textures characterized by crustiform or symmetrical banding.
Malachite [Cu 2 CO 3 (OH)] represents the supergene copper-rich mineral phase, which appears at surface in the chlorite mylonites and in quartz veins.

DISCUSSION
The Bacuri deposit is located within the regional ESE-WNW Canaã shear zone where several IOCG deposits (e.g.Sossego, Cristalino, Bacaba, Castanha, and Jatobá) are also known to exist.However, the deposit is closely related to a NE-SW-trending subsidiary transcurrent fault, which represents a local structural control on copper mineralisation.The Bacuri deposit is hosted by the ca.2.84 Ga Serra Dourada granite (Moreto et al. 2011;Feio et al. 2013), the Bacuri Porphyry and subordinated gabbro dikes.The least-altered Serra Dourada granite is predominant in the distal parts of the deposit, whereas the Bacuri Porphyry represents the main lithotypes within the NE-SW-trending fault.All host rocks are mylonitized and show intense hydrothermal alteration.
Spatial zoning of hydrothermal alteration has been characterized in the Bacuri deposit.In distal zones, a regional halo of incipient sodic (albite and scapolite-magnetite) alteration overprinted by fissural potassic alteration with potassic feldspar or biotite prevails.Hydrothermal alteration in distal zones was predominantly controlled by the nature of   host rocks, indicating relatively low fluid/rock (W/R) ratios.
In proximal zones to ore, within the fault zone, pervasive chlorite alteration, in addition to silicification and quartz veining, is ubiquitous.The nature of host rocks was obliterated and distinct protoliths developed similar hydrothermal paragenesis, which points to high W/R ratios.
The NE-SW subsidiary fault likely played a fundamental role in the formation of the deposit as an effective pathway for fluid circulation, which promotes intense fluid-wall rock interaction.However, if all the hydrothermal alteration stages were related to a single fluid pulse, the expected spatial zoning would reveal a sequence of alteration mineral assemblages that advances outward from the main fluid conduit (Reed 1997).Thus, the latest alteration stages would be found in distal zones.In the Bacuri deposit, however, a more complex sequence of alteration was characterized, implying overprinting of multiple fluid pulses or telescoping.
The characterisation of the relative temporal history of the Bacuri deposit also reveals significant changes in physico-chemical parameters during the system evolution.The early hydrothermal stage, preserved mainly in the Serra Dourada granite in distal parts of the Bacuri deposit, is represented by the incipient sodic alteration, characterized by the replacement of the igneous potassium feldspar or plagioclase by hydrothermal chessboard albite.Sodic alteration also resulted in scapolite-(magnetite) formation in (i) veins and veinlets, in which fibrous scapolite (I) and associated magnetite (I) formed directly from the hydrothermal fluid; and (ii) altered rock (e.g.Serra Dourada granite, Bacuri Porphyry and gabbro), as replacement of previous igneous minerals due to mechanisms of fluid-rock interaction (diffusion, dissolution and re-precipitation).Scapolite (I) crystals in veins show undulose extinction and subgrain formation, evidencing deformation posterior or simultaneous to its formation.Scapolite (I) and (II) in veins and altered rocks has low birefringence under polarized transmitted light, indicating marialitic composition [Na 4 (AlSi 3 O 8 ) 3 (Cl 2 ,CO 3 ,SO 4 )] with low meionite content.Scapolite with marialitic composition is considered as a key mineral indicator of chlorine content in fluids (Vanko & Bishop 1982).In addition, as chlorine is strongly partitioned into fluids in relation to solid phases, chlorine-bearing minerals, such as scapolite, may be a particularly sensitive indicator of fluid-rock interaction and mixing with external fluids (Mora & Valley 1989).The marialite occurrence in the Bacuri deposit reflects temperatures over 500 -400°C (Vanko & Bishop 1982) and buffering of chlorine activity in the early stages of evolution of the hydrothermal system.This could be attributed to regional migration of hypersaline brines without infiltration of diluted fluids, favouring metal (copper, iron, gold) leaching from host rocks and their transport as chlorine complexes.
The precipitation of sodium-bearing minerals (e.g.albite and scapolite) increased the potassium activity in the fluid, allowing the development of zones of potassic alteration with potassium feldspar, which are recognized in the Serra Dourada granite and the Bacuri Porphyry.The precipitation of potassium feldspar was ensued by pervasive and intense biotite (I)-scapolite (III)-magnetite (II) alteration.The presence of scapolite in this mineral association indicates that hypersalinity of fluids still prevailed during the biotite-scapolite-magnetite stage.Fluctuations of fO 2 conditions due to fluid-rock interaction likely controlled the formation of biotite instead of potassium feldspar in this stage.
In the biotite-scapolite-magnetite alteration zones, biotite is oriented parallel to the mylonitic foliation and scapolite crystals are deformed and show pressure shadows.These relationships may suggest that scapolite was formed before the shearing and biotite may have been re-oriented.
Chlorite alteration is the main hydrothermal stage in the Bacuri deposit, overlapping all previous ones.Chlorite alteration is pervasive and resulted in chlorite formation in paragenesis with magnetite (III), epidote (I), apatite, allanite, clinozoisite and monazite, especially in proximal zones close to the orebodies.The overprinting of biotite-scapolite-magnetite zones by the chlorite alteration shows decrease of pH conditions (Fig. 9).Destabilisation of scapolite in this stage may have been favoured by influx of external diluted fluids.
Significant silicification and quartz veining that accompanied and followed the chlorite alteration also suggest significant physico-chemical gradients.As the silica solubility decreases with decreasing salinity, temperature and pH (Fournier 1983), these parameters may have controlled quartz deposition.Silicification and veining were likely strongly influenced by mixing of high-and low-salinity solutions, once mixing promotes significant quartz precipitation (Rimstidt 1997).In early deformed and brecciated quartz-(muscovite-fluorite) veins and hydrolytic alteration, the presence of muscovite, which is a typical mineral formed in acidic conditions (Stringham 1957), also indicates progressive decrease of pH and increase of fO 2 conditions (Fig. 9).
The precipitation of chalcopyrite, the main ore mineral, was late in relation to the succession of the hydrothermal alteration stages characterized in the deposit.The first generation of chalcopyrite accompanied intense chlorite alteration, simultaneous to mylonitization, and silicification.However, expressive formation of chalcopyrite was identified in deformed quartz-(muscovite-fluorite) veins, where chalcopyrite infills fractures.Thus, chalcopyrite formation was mainly related to vein brecciation under brittle conditions.Such conditions also prevailed in undeformed milky quartz veins and in epidote-potassium feldspar veinlets with well-developed open-space filling textures.These textures are typical of ore deposition at temperatures below 250°C, because at higher temperatures and pressures host rocks have fewer voids (Seward & Barnes 1997).
Thus, the paragenetic evolution of the Bacuri deposit points to changes of physico-chemical parameters, such as temperature decrease from 500 to 400°C (scapolite alteration) to below 250°C (late mineralized veins), accompanied by decrease of salinity and pH and increase of fO 2. However, oxidation and pH decrease unlikely caused deposition of metals transported as chloride complexes (Seward & Barnes 1997).Otherwise, decreasing temperature and salinity would be particularly effective for depositing metal in this scenario.
In the Bacuri deposit, the evolution of the hydrothermal system reflects a major change in fluid conditions.Early alteration stages (albite, scapolite-magnetite, and biotite-scapolite-magnetite alteration) developed at high temperatures due to circulation of metalliferous and hypersaline fluids.The onset of ore deposition mineralization was possibly triggered by the mixing of hypersaline, hot, metalliferous fluids and highly channelled diluted and cooler fluids within the fault zone.Transition from ductile-brittle to predominantly brittle conditions, indicated by open-space textures in late veins, could indicate progressive exhumation of the hydrothermal system.The Bacuri deposit shares similarities with other deposits in the Carajás Province.Albite, scapolite and biotite-scapolite alteration were also previously recognized the IOCG deposits located within the regional ESE-WNW Canaã shear zone, in the southern contact between the basement rocks (e.g.2.84 Ga Serra Dourada granite) and the 2.76 -2.73 Ga Itacaiúnas Supergroup, such as the Pista orebody (Sossego deposit; Villas et al. 2005), Serra Dourada area (Sousa 2007), Bacaba, Castanha, Jatobá and Visconde deposits in Carajás (Augusto et al. 2008;Monteiro et al. 2011;Pestilho 2011;Craveiro et al. 2012).Thus, scapolite alteration could be considered particularly notable in the Neoarchean IOCG deposits of the Carajás Province, pointing to regional circulation of hypersaline fluids.In other major provinces that include worldclass IOCG deposits, such as the Fennoscandian Shield, in Sweden (Frietsch et al. 1997;Smith et al. 2009) and Finland (Niiranen et al. 2005), Wernecke Mountains, Yukon, Canada (Hunt et al. 2005) and the district of Cloncurry, Queensland, Australia (Oliver et al. 1992), similar albite, scapolite and biotite-scapolite alteration zones have also been recognized.As predicted in the theoretical model of Hitzman et al. (1992), these alteration stages are typical of hydrothermal systems developed at relatively deep crustal levels.
In the Bacuri deposit, however, chlorite alteration zones and expressive silicification are distinct from those alteration patterns typical of others IOCG deposits in the Carajás Province.These alteration stages may record structurally controlled influx of externally derived fluids in the system coeval with the development of the NE-SW fault, overprinting the deeper regional IOCG hydrothermal system controlled by the regional ESE-WNW Canaã shear zone.

CONCLUSIONS
Detailed characterization of host rocks and hydrothermal alteration patterns carried out in this study provided insights for the evolution of the Bacuri deposit, such as: ■ The Serra Dourada granite, Bacuri Porphyry, and gabbro dikes represent the host rocks of the deposit and are variably mylonitised and hydrothermally altered; ■ The Bacuri deposit is located within the regional ESE-WNW Canaã shear zone, but is specially controlled by a NE-SW subsidiary fault zone; ■ Spatial and temporal zoning in the Bacuri deposit suggest that hydrothermal alteration was developed due to multiple fluid pulses; ■ The hydrothermal alteration sequence of the Bacuri deposit encompasses early incipient sodic (albite) alteration, scapolite-magnetite, pervasive potassic alteration with potassium feldspar, and intense biotite-scapolite-magnetite alteration.These early alteration processes are identified mainly in distal zones and may be intrinsically related to the regional migration of deepseated hypersaline and hot metalliferous fluids within the regional ESE-WNW Canaã shear zone at relatively deep crustal levels.■ Overprinting of proximal chlorite alteration and silicification resulted from significant influx of diluted fluids within the NE-SW subsidiary fault zone, which represented an effective fluid pathway.■ Brecciation and late veins with open-space filling textures indicate progressive exhumation of the hydrothermal system.■ The paragenetic evolution of the Bacuri deposit points to changes of physico-chemical parameters, such as temperature decrease from 500 to 400°C (scapolite alteration) to below 250°C (late mineralized veins), accompanied by decrease of salinity and pH and increase of fO 2. ■ The copper ore occurs disseminated in replacement zones, in small lenses, veins and breccias and is related to (i) chlorite-(epidote) alteration zones or confined in (ii) brecciated quartz-(muscovite-fluorite) veins; (iii) milky quartz veins and (iv) late potassic feldspar-epidote veinlets.
Ore paragenesis includes primarily chalcopyrite, pyrite, magnetite and minor melonite, altaite, galena and cheralite associated with apatite, monazite, epidote, allanite, tourmaline, potassium feldspar, chlorite and andradite.■ The Bacuri deposit evolution points to (i) regional circulation within faults of hot hypersaline fluids that transport metals as chlorinated complexes; (ii) intense fluid-rock interaction resulting in highly modified lithotypes; (iii) mechanisms of ore deposition related to fluid mixing, which possibly triggered copper deposition via temperature decrease and dilution (decrease in a Cl − ).

Figure 1 .
Figure 1.Geological map of the Carajás Domain and adjacent areas showing the distribution of the various IOCG deposits, including the Bacuri deposit (modified from Vasquez et al. 2008).

Figure 2 .
Figure 2. Geological map of the central west part of the Southern Copper Belt, in the Serra Dourada area, showing the location of Bacuri deposit (Moreto 2013).
Figure 3. Schematic geological profiles of the Bacuri deposit, showing host rocks, main hydrothermal alteration zones, and orebodies related with chloritic zones and quartz veins (modified from VALE 2009).

Figure 4 .
Figure 4. (A) Isotropic facies of the Serra Dourada granite in the outskirts of the Bacuri deposit, without any evidence of hydrothermal alteration; (B) Pegmatitic facies of the Serra Dourada granite.(C) Typical texture of the Bacuri Porphyry in the drill cores of Bacuri deposit, showing a quartz-feldspathic matrix with hydrothermal biotite in the interstices.(D) Hydrothermally altered Bacuri Porphyry with pervasive potassic alteration with potassium feldspar (I) and late veinlets of epidote (II) and potassium feldspar (III).(E) Quartz-feldspar porphyry with quartz and potassium feldspar phenocrysts.
Fibrous scapolite (I) is cross-cut by fine-grained scapolite (II) crystals in deformed portions of the veins.All scapolite generations have low birefringence under polarized transmitted light, indicating marialite composition [Na 4 (AlSi 3 O 8 ) 3 (Cl 2 ,CO 3 ,SO 4 )] with low meionite content.Magnetite (I) is strongly related to scapolite alteration and occurs as tiny crystals in the edges and as inclusions in scapolite crystals in veins.

Figure 6 .
Figure 6.(A) Early albite (I) alteration in the Serra Dourada granite overprinted by potassium feldspar (I) alteration.(B) Zones of intense albite alteration in the Serra Dourada granite with veinlets of potassium feldspar (I) and chlorite (I).(C) Narrow zone with chlorite alteration cross-cut by milky quartz-(chlorite II) veinlet in the Serra Dourada granite.(D) Remains of sodic alteration with pinkish albite (I) in the Bacuri Porphyry overprinted by fronts of potassic alteration with biotite, which evolved to chlorite alteration with quartz and chalcopyrite (I).Fine veinlets with potassium feldspar (II) also cross-cut zones with albite (I).(E) Hydrothermal albite (I) showing chessboard texture in the Serra Dourada granite.(F) Selective scapolite alteration along the cleavage planes of plagioclase in the Serra Dourada granite.(G) Scapolite (III)-biotite (I) alteration zone in the Bacuri Porphyry associated with magnetite (II).Photos E-G are polarized light images.

Figure 9 .
Figure 9. Diagram of pH versus fO 2 , a 300°C, 1.5 kbar and aK+ = 0.1, showing the possible evolution of pH and oxygen fugacity of the mineralising fluid (arrow) from the initial stages (reduced conditions) to late stages (oxidized conditions) in paragenesis with the copper ore.