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Carajas IOCG Province - Sossego Sequeirinho, Salobo, Cristalino, Igarape Bahia Alemao, Gameleira, Estrela, Grota Funda, Furnas, Alvo 118, Igarape Cinzento Alvo GT46, Aguas Claras, Antas, Pedra Branca, Visconde, Castanha, Bacaba, Jatoba, Bacuri

Para, Brazil

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The Carajás Mineral Province is located ~20 to 120 km west of the north-south trending preserved margin of the southern Amazonian craton in Pará, Brazil, and hosts the world’s largest known concentration of large-tonnage IOCG deposits, including, Sossego - Sequeirinho, Salobo, Cristalino, Igarapé Bahia/Alemão, Gameleira, Estrela, Grota Funda, Furnas and Alvo 118, as well as a number of smaller and satellite deposits such as Igarapé Cinzento, Alvo GT46, Antas and Pedra Branca, Visconde, Castanha, Bacaba, Jatobá and Bacuri (Moreto et al., 2011; Xavier et al., 2010; Requia and Fontboté, 2000; Ronzê et al., 2000; Tazava and Oliveira, 2000; Souza and Vieira, 2000). These deposits are located within the east-southeast trending, 50 to 80 km wide, 150 km long Itacaiúnas Shear Belt (centred on the Carajás fault), where it cuts obliquely across the eastern third of an east-west trending, 400 × 100 km belt of thick Neoarchaean bimodal, but mainly mafic to intermediate volcanism. This belt of volcanism includes chemical and clastic sediments, gabbroic to granitic intrusions, and succeeding clastic sedimentary rocks, and overlies the Mesoarchaean granitoid nucleus of the Amazonian craton.

  The 4.4 million km2 Amazonian craton is divided into two halves by the east-west trending Amazon Basin intracratonic rift that has been active from the late Neoproterozoic to the present, while its eastern margin is marked by the structural boundary with the deformed sediments of the Neoproterozoic Araguaia Belt, part of the Tocantins Orogen that developed during the collision between the Amazonian/West African and São Francisco/ Congo palaeo-continents during the late Neoproterozoic (Fonseca et al., 2004). The Amazonian craton itself is composed of six northwest-trending terranes, each with internally coherent structural and age patterns, and bounded by major structural features. The Itacaiúnas Shear Belt is developed towards the northeastern margin of, and within, the oldest of these terranes, the 400 to 500 km wide, 3.2 to 2.5 Ga Central Amazonian province which is composed of the larger Xingu-Iricoumé and smaller Roraima blocks to the southeast and northwest respectively. The terrane boundary with the 2.25 to 2.05 Ga Maroni-Itacaiúnas province is immediately to the northeast of the Carajás Mineral Province IOCG deposits. This younger province is composed of a series of mobile belts containing metavolcanosedimentary and juvenile calc-alkaline granitic rocks, with large areas occupied by mantle derived Palaeoproterozoic greenstone belts/mafic lavas and tuffs. During the Neoarchaean to Palaeoproterozoic, west Africa and the Amazonian craton were contiguous, and the Maroni-Itacaiúnas province and equivalent Eburnean/Birimian terranes of west Africa separated the Central Amazonian province and the Meso- to Neoarchaean Man and Reguibat Shields of west Africa, the latter of which hosts the ~2.5 Ga Guelb Moghrein IOCG deposit in Mauritania (Kolb et al., 2010, in this volume; Strickland and Martyn, 2002).
  During the Neoarchaean and early Palaeoproterozoic, the southwestern edge of the Central Amazonian province marked the cratonic margin. The 1.98 to 1.81 Ga Ventuari- Tapajós province was accreted to this margin, and is composed of felsic volcanic and gneissic granitic rocks with juvenile isotopic signatures. This terrane is followed to the southwest by the progressively younger 1.78 to 1.55 Ga Rio Negro-Juruena, 1.55 to 1.3 Ga Rondonian-San Ignacio and 1.28 to 0.95 Ga Sunsás provinces (Cordani and Teixeira, 2007).
  The Itacaiúnas Shear Belt involved 2.85 to 2.76 Ga sinistral, transpressive, strike-slip ductile shearing, 2.7 to 2.6 Ga dextral transtension that produced the Carajás and Cinzento strike-slip fault systems, and a sinistral, transpressive regime that evolved at about 2.6 Ga. The basement within this structural zone is composed of tonalitic to trondhjemitic gneiss and migmatite of the Mesoarchaean Xingu and Pium complexes of 3002±14 Ma protoliths, metamorphosed to granulite facies and extensive migmatites at 2861±12 Ma (e.g., Xavier et al., 2010).
Carajas continental setting
Figure 1. The location of the Carajás IOCG Province and the IOCG sensu stricto and other iron oxide-alkali altered ore deposits within the tectonic framework of South America and West Africa. Those of the West African and Amazonian cratons are located towards the margin of Archaean nucleii of the Reguibat Shield (Guelb Moghrein) and the Xingu-Iricoumé block of the Central Amazonian Province of the Amazonian craton in Brazil (Carajás Mineral Province - Sossego, Salobo, Igarapé Bahia, Cristalino and a number of smaller deposits). Note the outline of the ~1.8 Ga large igneous province, a vast sheet of largely felsic volcanic rocks and comagmatic granitoids that may influence the second generation, but smaller deposits of the Carajás Mineral Province. The deposits of the Central Andean Belt in northern Chile and southern Perú, while hosted dominantly by Mesozoic (but also some Palaeozoic) rocks, overlie a thick basement composed largely of exotic terranes of Palaeo-, Meso and possibly Neoproterozoic metamorphics, specifically of the Arequipa (Perú) and Chilenia (Chile) terranes. These older basement blocks are only very locally exposed, being separated and overlain by Neoproterozoic to Tertiary ophiolites, sedimentary sequences and magmatic arcs. However, they influence the controlling structures (e.g., the northern Atacama Fault) and the chemical and physical nature of the crust through which ore related fluids are introduced and circulated, as well as the thickness of underlying subcrustal lithospheric mantle. Details plotted are largely after Cordani and Teixeira (2007), Chew et al. (2010); Ramos (2008); (2004), Petters (1986).
  This older basement is overlain by sequences of metavolcanosedimentary and metamorphosed bimodal basic and felsic (but dominantly basaltic) volcanic rocks, iron formations and clastic sedimentary rocks of the 2.76 to 2.73 Ga Itacaiúnas Supergroup, including the 4 to 6 km thick Grão Pará group that is host to the giant, volcanichosted, 2.7 Ga Carajás banded iron formation iron deposits. The Itacaiúnas Supergroup is composed of a number of such volcanic sequences, some of which are equivalents whereas there are several tens of m.y. difference in age between others. The volcanic rocks of the Itacaiúnas Supergroup have been variously interpreted to refl ect a continental extensional basin environment, or calc-alkaline magmas typical of subduction zones, back-arc basins and continental arcs (see Xavier et al., 2010, this volume, for sources, details and discussion). However, trace element studies suggest crustal contamination of the volcanic rocks (Lobato et al., 2005a), and that they were deposited on attenuated continental crust (Zucchetti, 2007; Zucchetti et al., 2007). The volcanism of the Itacaiúnas Supergroup is part of the major global Neoarchaean pulse of magmatism representing the most intense period of crustal growth in the geological record (Abbott and Isley, 2002).
  This sequence is overlain in turn by poorly deformed platformal siliciclastic and carbonatic sedimentary rocks and minor iron formations of the 2.7 to 2.6 Ga Águas Claras Formation cover sequence. All of these metamorphic, volcanic and sedimentary rocks are intruded by a series of granitoids, including 2.76 to 2.74 Ga syntectonic alkaline granites, 2.76 Ga mafic-ultramafic layered complexes, as well as 2.76 to 2.65 Ga gabbro sills and dykes, 2.70 Ga calc-alkaline monzogranite, 2.65 Ga porphyritic dacitic to rhyolitic rocks, 2.57 Ga A-type granites, 2.51 Ga peralkaline, meta-aluminous granitic rocks, and widespread late 'withinplate' A-type, alkaline to sub-alkaline granites associated with 1.88 to 1.87 Ga Palaeoproterozoic extensional events (e.g., Xavier et al., 2010; Grainger et al., 2008).
  The Carajás Mineral Province is also located within the eastern sector of one of the largest felsic igneous provinces in the world (Grainger et al., 2008). This thick, oval-shaped, 1100 × 1400 km, incised but relatively fl atlying sheet of ~1.9 to ~1.8 (and 1.7?) Ga bimodal (felsic and intermediate) volcanic rocks and co-magmatic A-type granites overlaps both the Archaean Central Amazonian and Palaeoproterozoic Ventuari-Tapajós provinces. It covers over 1.2 million km
2 of the southern Amazonian craton and the southern margins of the northern part of the craton (Schobbenhaus et al., 1995). The ~1.8 Ga A-type granites of the Carajás district are part of the eastern margin of this igneous province (Grainger et al., 2008).
  The Itacaiúnas Supergroup and the overlying sedimentary rocks of the Águas Claras/Rio Fresco Formations cover sequence, remained relatively stable during deposition (as evidenced by the, extensive, thick, finely banded, but lensoid BIFs), interspersed with recurrent structural subsidence and volcanic activity. This was because by the early Neoarchaean the Central Amazonian province of the Amazonian craton was sufficiently rigid (stabilised) to retard the development of the unstable keel and dome granite-greenstone tectonics seen at the same period in cratonic nuclei such as the Yilgarn and Superior. The Archaean succession has subsequently been folded into a large "S-shaped" structure, dislocated near its centre by the Carajás Fault (shear zone), and buttressed by large igneous complexes such as the Mesoarchaean Pium Complex and Neoarchaean granitoids (Rosière et al., 2006).   Teixeira et al. (2009) interpret the Itacaiúnas Supergroup and related magmatic rocks of the Carajás Mineral Province to represent an intracontinental rift zone, with associated areally extensive, tholeiitic basaltic volcanism and anatexis that are the product of inferred mantle underplating. They suggest this mantle induced magmatism was responsible for the two major iron oxide rich mineralising episode within the Carajás Mineral Province: i). the ~2.7 Ga volcanic hosted banded iron formations of the Carajás iron deposits, and ii). the multi-pulse 2.74 and 2.57 Ga IOCG deposits of the province. Both episodes are focused on deep transtensional faults. Teixeira et al. (2009) and Grainger et al. (2008) suggest, the subsequent, spatially overlapping, extensive ~1.8 Ga felsic igneous province resulted in a further phase of IOCG mineralisation and additional deposits in the same province, as outlined below. Teixeira et al. (2009) attribute the inferred underplating and magmatism from 2.75 to 2.5 Ga and at 1.8 Ga to mantle plume activity.
  The bulk of the Carajás Mineral Province IOCG deposits are distributed in two structurally controlled belts, one to the northwest, associated with a structural corridor along the northern margin of the Itacaiúnas Shear Belt (Salobo, Igarapé Cinzento/Alvo GT46, and Gameleira), and the second on its southern margin to the southeast (Sossego, Cristalino and Alvo 118), while Igarapé Bahia/Alemão is located towards the western central part of the cluster of deposits, between the two main groups, possibly fault controlled in the core of a domal structure. The location of all, with the possible exception of Igarapé Bahia/Alemão, is substantially controlled by major, deep-seated faults/shears.
  Despite evidence for a common evolution of the Carajás IOCG deposits, detailed geochronologic data suggest that their formation may possibly be linked to three metallogenic events at ~2.74, 2.57 and ~1.8 Ga. Dating of ore-related minerals has revealed different ages, even in a single deposit (e.g., Igarapé Bahia, Gameleira and Salobo; Réquia et al., 2003; Tallarico et al., 2005; Pimentel et al., 2003). A temporal relationship has been indicated between the widespread Archaean magmatism (~2.74 Ga) and these deposits, particularly at the Cristalino deposit (Huhn et al., 1999), although robust geochronologic data also suggests an important metallogenetic event at 2.57 Ga (Réquia et al., 2003; Tallarico et al., 2005; Grainger et al., 2008). However, except at Salobo, there is a lack of a clear spatial association between IOCG deposits and magmatism of this latter age in the province (Xavier et al., 2010). Moreto et al. (2015) show that the deposits of the Carajás are temporally distributed as follows: ~2.74 Ga - Cristalino, Bacaba, Bacuri and Visconde; ~2.57 Ga - Salobo, Igarapé Bahia, Sossego (Sequeirinho-Pista-Sequeirinho-Baiano); ~1.8 Ga - Alvo 118, Sossego-Curral, Breves, Águas Claras, Gameleira and Estrela.
  Xavier et al. (2010) conclude that, despite the importance of magmatism in providing heat and fluids for the development of extensive hydrothermal systems, the available geochronologic data may not be necessarily correlated to an individual magmatic event. They note that the ages may alternatively be the result of a long-term history of isotopic resetting due to the development and/ or reactivation of Archaean ductile or ductile-brittle shear zones and/or Palaeoproterozoic anorogenic magmatism. Grainger et al. (2008), suggest that the IOCG mineralisation of the Carajás Mineral Province can be subdivided into two groups, namely: i). the larger deposits, which contain 0.2 to 1 Gt @ 0.95 to 1.4% Cu, 0.3 to 0.85 g/t Au, and include Salobo, Igarapé Bahia/Alemão, Cristalino and Sossego. These are dominantly hosted by the lower volcanic sequences and basement gneisses, and occur as pipe- or ring-like, generally breccia bodies that are strongly iron- and LREE-enriched, commonly with anomalous cobalt and uranium, and are quartz- and sulphurpoor, with iron oxides and iron-rich carbonates and/or silicates invariably present. As mentioned above, both 2.74 and 2.57 Ga ages of mineralisation are suggested for those members of this group that have been dated, comparable to the associated granitoids, the latter of which are A-type. ii). A group of smaller, commonly supergene-enriched copper-gold deposits that generally comprise <50 Mt @ <2% Cu, <1 g/t Au in hypogene ore, with enrichment in granitophile elements such as tungsten, tin and bismuth, and which spatially overlap the Archaean IOCG deposits. These include Breves, Águas Claras, Gameleira and Estrela that are predominantly hosted by the upper sedimentary cover sequence, occurring as greisen-to ring-like or stockwork bodies. They generally lack abundant iron-oxides, are quartz-bearing and contain more sulphur-rich copper-iron sulphides. Precise Pb/Pb dating of hydrothermal phosphate of the Breves and Alvo 118 deposits indicate ages of 1872±7 Ma and 1868±7 Ma respectively, comparable with the adjacent young A-type granites and associated dykes which range from 1874±2 Ma to 1883±2 Ma. Age dating also suggests the Sossego Hill (Sossego-Curral) orebodies were formed in the ~1.90 to 1.88 Ga time interval (U-Pb LA-MC-ICPMS hydrothermal monazite and molybdenite Re-Os NTIMS; Moreto et al., 2015) in contrast to the ~2.57 age of the main deposits at Sossego, namely Sequeirinho Pista-Sequeirinho-Baiano zones. It is uncertain whether similar mineralisation occurs within the same 1.8 Ga magmatic complex outside of the Carajás Mineral Province, and if not, why ?
Carajas IOCG Province

Figure 2. The tectonic and geological setting of the Carajás Mineral Province within the southern half of the Xingu-Iricoumé block of the Central Amazonian Cratonic Province. The outline of this figure is shown on Fig. 1. Note the extent of the mafic to intermediate volcanic host sequence of the Itacaiúnas Supergroup and the obliquely crosscutting structures of the Itacaiúnas Shear Belt which influence the location of deposits, the major examples of which as currently known are clustered in the eastern third of the volcanic belt. Note also the extent and distribution of the felsic to intermediate volcanic sheet and coeval A-type granites of the giant ~1.8 Ga large igneous province, and the overlap of the latter into the Carajás Mineral Province (after Schobbenhaus et al., 1995 and Cordani and Teixeira, 2007).

Regional Alteration
  The host volcanic rocks to most large Carajás IOCG deposits also contain the major, stratabound, exhalative BIF (jaspilite) iron accumulations of the Carajás Mineral Province. The latter have undergone several stages of hydrothermal hypogene upgrading during the Neoarchaean and possibly also the Palaeoproterozoic to produce the high grade iron ores of the province (~18 Gt @ >65% Fe; Figueiredo e Silva et al., 2008). Lobato et al. (2005) speculate that the Carajás iron deposits could represent an exhalative end-member of the replacive iron oxide phase of the IOCG system in the province, having probably both involved saline fluids of meteoric as well as those of magmatic-hydrothermal origin. Alternatively, or in addition, both Neoarchaean and late Palaeoproterozoic magmatic-hydrothermal iron oxide-rich fluids responsible for the magnetite of the IOCG deposits may be related to the hypogene upgrading of the BIF ores (Figueiredo e Silva et al., 2008). Hagemann et al. (2005) consider the hypogene enrichment of Carajás BIFs (jaspilites) that produced the high grade iron ores, to be the result of iron-replacement by deep fault sourced, magmatic fluids associated with primitive, mantle-sourced, alkaline to hyperalkaline, mafic magmatic rocks, with an associated, very complex array of both compatible and incompatible elements.
  The earliest alteration surrounding the Carajás IOCG deposits comprises regional sodic (albite-hematite) and actinolite-rich sodic-calcic assemblages, refl ecting high salinity in the early regional hydrothermal fluids. Sodic-calcic alteration was closely controlled by regional ductilebrittle shearing, reflected by preceding mylonitisation and synchronous silicification (Xavier et al., 2010), although the alteration is pervasive, with associated fracture controlled albite. Scapolite and tourmaline are conspicuous within the sodic assemblage in felsic volcanic rocks. Mylonitised metavolcanic rocks are characterised by alternating bands of albite, tourmaline and scapolite. Sodic-calcic assemblages overprint sodic alteration and are dominated by actinolite-albite with accompanying magnetite, calcite, epidote, quartz, titanite, allanite and thorianite, and may have associated massive magnetite bodies, as at Sequeirinho (Monteiro et al., 2007).
  Metal leaching from the host rocks during the sodic and sodic-calcic stages was probably enhanced by the high salinity of fluids, and driven by heat from the intrusive episodes recorded. The geochemical ore signatures of the IOCG mineralisation across the province is variable, and strongly dependent upon the chemistry of the leached host rocks. Actinolite-rich sodic-calcic alteration is commonly associated with large massive magnetite bodies (distinct from the Carajás BIFs) enveloped by apatite-rich actinolitite. Regional shear zones, associated with the known deposits, incorporate extensive biotite-scapolite zones representing early sodic alteration synchronous with shearing (Xavier et al., 2010), over intervals of tens of kilometres and widths of a few hundred metres to a kilometre or more e.g., an area in excess of 20 km
2 of scapolite alteration surrounding the Sossego deposits which lies within a 60 km-long, up to several km wide shear zone, generally following the southeastern margin between the ∼2.76 Ga Itacaiúnas Supergroup and the ∼3.0 Ga Xingu Complex basement, controlling the location of the Alvo 118, Sossego-Sequeirinho and Cristalino deposits (Xavier et al., 2010), but also the smaller/satellite Visconde, Castanha, Bacaba, Jatobá and Bacuri deposits (Moreto et al., 2011). Sodic alteration is an early phase in all of the deposits, dominating at depth, overprinted by other, more localised ore-related assemblages at higher levels, as explained below.

Deposit-scale Alteration and Mineralisation
Early regional sodic and sodic-calcic alteration was followed by a potassic phase, magnetite-(apatite) formation, chloritic, copper-gold mineralisation and hydrolytic alteration centred on the individual ore deposits. As these deposits are developed across the province, at varying crustal levels and local settings, there are corresponding variations in geochemical signature (e.g., tourmaline characterises those in metavolcano-sedimentary units as at Salobo and Igarapé Bahia/Alemão; fayalite, garnet and sillimanite in some hosted in ductile shear zones, including Salobo and Igarapé Cinzento/Alvo GT46; while silica and carbonate are important in brittle-ductile conditions such as Sossego and Alvo 118; Xavier et al., 2010).

Carajas IOCG Province Geology

Figure 3. The geology, structure and mineral deposits of the Carajás Mineral Province. For location, see the outline on Fig. 2 (after Xavier et al., 2010; Rosiere et al., 2006).
  Four types of fluid inclusions have been found in association with the IOCG deposits of the Carajás Mineral Province. These are: i). hypersaline (35 to 70 wt.% NaCl
equiv.), moderate to high temperature (Tht = 250 to 570°C) brines, represented by halite-bearing or multi-solid aqueous inclusions; ii). lower temperature (usually <200°C) aqueous fluids with variable salinity (<5 to 30 wt.% NaCl equiv.) represented by two phase inclusions; iii). low salinity (<6 wt.% NaCl equiv.), moderate temperature (Tdecrep. ≥250°C) aqueous-carbonic (CO2±CH4) fluids; and iv). single phase carbonic (CO2±CH4) fluids (Xavier et al., 2010). Type 1, hot metalliferous, hypersaline brines and lower temperature, low to intermediate salinity type 2 fluids are found at all deposits, while the aqueous-carbonic and carbonic fluids are only present in the west of the province at Salobo, Igarapé Bahia and Gameleira (Réquia and Fontboté, 2001; Ronchi et al., 2001; Dreher, 2004). The lower temperature type 2 fluids predominate in the mineralisation stage and are regarded to represent an influx of surficially derived meteoric water. Ore precipitation was marked by a sharp temperature decrease to <300°C in all IOCG deposits. Copper-gold mineralisation was invariably introduced in the late stages of all of the IOCG systems of the province, generally controlled by subsidiary brittle or brittle-ductile structures. Thus, Xavier et al. (2010) suggest copper deposition might be related to collapse of the early high temperature hydrothermal system, controlled by fluid flow in regional shear zones, and resulting from, or accompanying, the influx of meteoric fluids under hydrostatic, brittle conditions at the brittle-ductile to brittle transition. Stable isotope data do not unambiguously provide evidence for either a magmatic or evaporative /bittern source for type 1 fluids (Xavier et al., 2010). Individual deposits have been emplaced at differing vertical levels, with varying alteration patterns, although it is possible to construct a model based on progressive overlaps between deposits in the southeastern part of the district. At the deepest levels, regional- to districtscale sodic and sodic-calcic (albite-scapolite at ~500°C) alteration predominates, with associated magnetite mineralisation. At the next level up, actinolite appears in association with magnetite mineralisation (also at ~500°C) and sulphides, with the first potassic alteration represented by a relatively narrow biotite fringe to the ore, overprinting the still extensive sodic zone (e.g., Sequeirinho). Higher still, potassic alteration, comprising K feldspar and biotite is progressively more extensively developed at the expense of the earlier, higher temperature sodic assemblage, to become dominant (at ~460°C), where the first muscovite-sericite (hydrolytic) alteration appears (e.g., the Sossego orebody). In the upper levels (e.g., Alvo 118) chloritemuscovite-hematite is dominant at temperatures of <250°C, surrounding potassic remnants (Xavier et al., 2010).
  At Alemão, alteration and mineralisation is characterised by an assemblage, emplaced in the order, magnetite, chlorite (with lesser biotite), sulphides (in multiple pulses, dominantly chalcopyrite with subordinate bornite, lesser molybdenite, pyrite, pyrrhotite and minor galena, digenite and covellite), carbonate veining (mainly siderite, with lesser ankerite and ferruginous dolomite, and calcite) and late silica (with carbonate and local free native gold) (Ronzê et al., 2000).
  The protoliths and alteration of the deepest (>6 km) example at Salobo on the northwestern margin of the province, is uncertain. Interpretations vary from i). an iron formation that was metamorphosed to pyroxene-hornfels facies (Lindenmayer, 1990; Villas and Santos, 2001), or ii). basaltic-andesite and dacite of the Igarapé Salobo Group that have been subjected to extreme iron and potassic alteration at temperatures of >550°C (Lindenmayer, 2003) or iii). intensely ductilely deformed and hydrothermally altered rocks of the basement 2.95 to 2.86 Ga Xingu Complex and the 2.76 Ga Igarapé Gelado suite granitoid gneisses adjacent to the contact with mylonitic quartzites of the 2.76 to 2.73 Ga Itacaiúnas Supergroup, Igarapé Salobo Group volcanosedimentary sequence. Similar, structurally disrupted 'iron formations' are distributed intermittently over tens of kilometres of strike length throughout the district along trend form Salobo 3A (Siqueira and Costa, 1991). The ore zone is associated with a halo of variably magnetite-rich (<10 to >50%) rocks with Mn-almandine, grunerite, Cl-rich hastingsite, fayalite, schorlitic tourmaline, Fe-biotite, allanite and quartz (Réquia et al., 2003; Requia, and Fontboté, 2000). The ore occurs within strongly ironpotassic altered rocks in two main zones: i). massive garnet-biotite-fayalite-grunerite accompanied by generally >50% magnetite, minor graphite and fluorite, and ii). a foliated, granoblastic, almandine-biotite-gruneriteplagioclase- quartz assemblage with 10 to 50% magnetite. There is a direct relationship between copper and iron grades (Viera et al., 1988; Souza and Vieira, 2000).

Structural Control and Brecciation
  Most of the known IOCG deposits of the Carajás Mineral Province are located along, or adjacent to, a regional shear zone that defines the contact between the metavolcano-sedimentary units of the host Itacaiúnas Supergroup and basement rocks of the Xingu Complex as detailed above. The major westnorthwest trending Cinzento shear zone, in the northwestern sector of the province, hosts the Salobo and Igarapé Cinzento/Alvo GT46 deposits. Salobo is entirely within the Cinzento shear zone, emplaced at >6 km depth, under brittle-ductile conditions, resulting in lenticular shaped ore shoots, with little brecciation (Souza and Vieira, 2000).
  In the southeastern sector, an ~60 km-long, up to several kilometres wide, overall east-west-striking shear zone, characterised by biotite-scapolite mylonites, controls the location of the Alvo 118, Sossego-Sequeirinho and Cristalino deposits, together with several other minor occurrences and a number of large, barren massive magnetite bodies (Xavier et al., 2010; Monteiro et al., 2008). In the Sossego-Sequeirinho district, the latter deposit comprises an 'S' shaped, tabular orebody, whose tips are hosted by separate, parallel, sub-vertical eastsoutheasttrending shears in foliated granitoids and schists. These are linked by an offsetting northeast-southwest sinistral fault zone that hosts the bulk of the deposit within mineralised breccias. These two directions are the dominant structural trends in the district (Domingos, 2009). Intense hydrothermal alteration within a few hundred metres into the hanging wall of high angle faults belonging to these two structural trends, surrounds the other ore deposits at Sossego-Sequeirinho. Rocks in the immediate footwall of these same faults have been intensely mylonitised and subjected to biotitetourmaline- scapolite alteration (Monteiro et al., 2008). Within the Carajás Mineral Province there are a number of variations in the occurrence and style of brecciation associated with IOCG mineralisation. No obvious ore related brecciation is recorded at the shear zone hosted, deeply emplaced Salobo deposit (Souza and Vieira, 2000).
  The Igarapé Bahia deposit comprises three orebodies that are spatially disposed within a breccia unit that defines a semicircular shape at the surface, resembling a collapsed ring complex with a diameter of approximately 1.5 km. The breccia unit occurs as a 2 km long by 30 to 250 m thick string of fault dislocated bodies on the southern, northeastern and northwestern sections of this structure, each dipping steeply outwards, nearly concordant with the metavolcanic-sedimentary wall rocks (Tallarico et al., 2005). Each is situated at the interface between metamorphosed sedimentary/volcaniclastic and volcanic rocks, commonly brecciated near the contact with ore. High grade primary copper-gold mineralisation is largely restricted to the breccias, with millimetric to centimetric clasts of varied composition (BIF, meta-volcanic, meta-volcaniclastic and meta-sedimentary rocks) in a hydrothermal matrix containing copper-sulphides (chalcopyrite and bornite with minor molybdenite, digenite and pyrite), magnetite, carbonate (siderite to calcite), fluorite, gold, uraninite, apatite, REE minerals, tourmaline, stilpnomelane and ferropyrosmalite (Tazava, 1999; Tazava and Oliveira, 2000). Much of the mined ore at Igarapé Bahia was strongly leached to form a lateritic gold deposit above the base of oxidation, passing down into refractory primary coppergold sulphide mineralisation. The Alemão deposit is a down-faulted, hypogene magnetite-copper-gold-rich body, that comprises a deeper, down-plunge continuation of the northwestern segment of the Igarapé Bahia mineralised structure, below the base of oxidation. Mineralisation at Alemão is represented by two classes of hydrothermal breccia: i). magnetite-sulphidebreccia ore, composed of both massive bands of magnetite and chalcopyrite, and by polymictic breccias with clasts of volcanics, tuffs and banded iron formation, enclosed within a matrix of magnetite, chalcopyrite, siderite, chlorite, biotite and amphibole; and ii). chlorite-sulphide breccia ore, composed of angular to sub-angular clasts of chloritic, brecciated volcanic rocks with chalcopyrite, bornite, pyrite, chlorite, siderite, ankerite, tourmaline and molybdenite, both within the matrix and disseminated through the rock.
  At Cristalino, copper-gold mineralisation is associated with mafic to felsic volcanic rocks overlain by (the Carajás) iron formation all of which have been locally brecciated. Mineralisation occurs as quartz-carbonate breccias, veinstockworks, and to a lesser degree, as disseminations, and comprises chalcopyrite, pyrite, magnetite, marcasite, bravoite, cobaltite, millerite, vaesite and gold, with subordinate hematite, bornite, covellite, chalcocite, molybdenite and sphalerite (Huhn et al., 1999). At the Sossego-Sequeirinho orebodies, much of the mineralisation is associated with brecciation. At the deeper Sequeirinho deposit, an "S" shaped, tabular breccia is developed in a link fault between parallel two shears (which control the tips of the "S"). This breccia contains rounded fragments of massive magnetite and actinolite within a matrix of hydrothermal minerals. The development of the matrix commenced with coarse-grained actinolite, apatite and magnetite (a later generation to the sodic-calcic phase clasts), followed by epidote, chlorite, quartz, calcite and sulphides. Pyrite is the dominant early sulphide, overgrown by chalcopyrite, which also replaced magnetite. At the more shallowly emplaced Sossego orebody, sulphides are largely restricted to sub-vertical breccia pipes that contain open-vugs. These breccia bodies are sub-circular in plan, with sharp outer contacts, and a halo of radiating stockwork veins. The breccias are dominantly clast supported, with angular to sub-angular blocks (<0.5 to >10 cm) of locally derived host granophyric granite within a matrix of magnetite, actinolite, biotite, apatite, calcite, epidote with lesser pyrite and chalcopyrite (Domingos, 2009; Monteiro et al., 2007; Xavier et al., 2010).

The most recent source geological information used to prepare this summary was dated: 2015.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Bettencourt, J.S., Juliani, C., Xavier, R.P., Monteiro, L.V.S., Bastos Neto, A.C., Klein, E.L., Assis, R.R., Leite Jr., W.B., Moreto, C.P.N., Fernandes, C.M.D. and Pereira, V.P., Vit  2016 - Metallogenetic systems associated with granitoid magmatism in the Amazonian Craton: An overview of the present level of understanding and exploration significance: in    J. of South American Earth Sciences   v.68, pp. 22-49.
Haddad-Martim, P.M., Carranza, E.J.M., de Souza Filho, C.R.,  2018 - The Fractal Nature of Structural Controls on Ore Formation: The Case of the Iron Oxide Copper-Gold Deposits in the Carajas Mineral Province, Brazilian Amazon: in    Econ. Geol.   v.113, pp. 1499-1524.
Moreto, C.P.N., Monteiro, L.V.S., Xavier, R.P., Amaral, W.S., Santos T.J.S. dos, Juliani, C. and Souza Filho C.R.,  2011 - Mesoarchean (3.0 and 2.86 Ga) host rocks of the iron oxide-Cu-Au Bacaba deposit, Carajas Mineral Province: U-Pb geochronology and metallogenetic implications: in    Mineralium Deposita   v.46, pp. 789-811.
Moreto, C.P.N., Monteiro, L.V.S., Xavier, R.P., Creaser, R.A., DuFrane, S.A., Melo, G.H.C., Delinardo da Silva, M.A., Tassinari, C.G. and Sato, K.,  2014 - Timing of multiple hydrothermal events in the iron oxide-copper-gold deposits of the Southern Copper Belt, Carajas Province, Brazil: in    Mineralium Deposita   v.50, pp. 517-546.
Pollard, P.J., Taylor, R.G., Peters, L., Matos, F., Freitas, C., Saboia, L. and Huhn, S.,  2019 - 40Ar-39Ar dating of Archean iron oxide Cu-Au and Paleoproterozoic granite-related Cu-Au deposits in the Carajas Mineral Province, Brazil: implications for genetic models: in    Mineralium Deposita   v.54, pp. 329-346.
Porter T M,  2010 - Current Understanding of Iron Oxide Associated-Alkali Altered Mineralised Systems: Part II, A Review: in Porter T M, (Ed),  2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide   v.3 pp. 33-106
Riehl, W. and Cabral, A.R.,  2018 - Meta-evaporite in the Carajas mineral province, northern Brazil: in    Mineralium Deposita   v.53, pp. 895-902
Toledo, P., Moreto, C.P.N., Xavier, R.P., Gao, J., Matos, J.H. and Melo, G.H.C.,  2019 - Multistage Evolution of the Neoarchean (ca. 2.7 Ga) Igarape Cinzento (GT-46) Iron Oxide Copper-Gold Deposit, Cinzento Shear Zone, Carajas Province, Brazil: in    Econ. Geol.   v.114, pp. 1-34.
Xavier, R.P., Monteiro, L.V.S., Moreto, C.P.N., Pestilho, A.L.S., de Melho, G.H.C., Delinardo da Silva, M.A., Aires, B., Ribeiro, C. and Freitas e Silva, F.H.,  2012 - The Iron Oxide Copper-Gold Systems of the Carajás Mineral Province, Brazil: in    Society of Economic Geologists,   Special Publication 16, Chapter 17, pp. 433-454.


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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