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Salobo |
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Para, Brazil |
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Cu Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Salobo 3 Alpha IOCG Deposit is located in the Carajás district of Para State, Brazil, and is some 30 km to the north of Igarapé Bahia and Alemao and ~50 km WNW of the major Carajas N4 and 5 iron deposits of the Serra Norte (#Location: 5° 47' 25"S, 50° 32' 2"W).
The Salobo deposit was discovered in 1977. It lies within the WNW-ESE trending Cinzento shear zone, hosted by a package of rocks that includes the Igarapé Salobo Group which is interpreted to be a supracrustal suite that is part of the Neoarchaean 2.76 to 2.73 Ga Itacaiúnas Supergroup; orthogneisses of the Mesoarchaean basement Xingu Complex (2950 ±25 to 2857 ±6.7 Ma; U-Pb zircon; Melo et al., 2016) and deformed granitoid gneiss of the Neoarchaean Igarapé Gelado suite (2763 ±4.4 Ma; U-Pb zircon; Melo et al., 2016). The Igarapé Salobo Group is composed of paragneiss, amphibolites, quartzites, meta-arkoses and iron-rich schist.
For details of the regional and structural setting see the Carajás IOCG Province record.
The Itacaiúnas Supergroup sequence is in tectonic contact with trondhjemitic gneiss of the basement Xingu Complex which has been partially migmatised. The original stratigraphic relationships and contacts with the basement, as well as within the host sequence, are masked by intense ductile-brittle shearing and over-thrusting/reverse faulting. This includes strong deformation within the broad 2.7 Ga Itacaiúnas Shear Belt that caused imbrication, and tectonic layering of supracrustal rocks alternating with basement gneisses at several scales. It forms a broad, tens of kilometres wide, braided zone of steeply-dipping, WNW-ESE trending ductile shearing and high temperature mylonitic fabrics developed under upper amphibolite facies regional metamorphic conditions (e.g. DOCEGEO 1988; Araújo & Maia 1991). Structural indicators imply a regime of predominantly sinistral transpression with partitioning of deformation that produced linked systems of ductile strike-slip and thrust dominated shear zones (Araújo and Maia, 1990; Costa et al., 1994). One of these was the ~2.5 Ga Cinzento ductile shear zone that hosts the Salobo deposit (Machado et al., 1991; Holdsworth and Pinheiro, 2000). These zones of shearing are crosscut by the undeformed A-type, peralkaline to metaluminous (Lindenmayer, 2003) syn-tectonic Old Salobo Granite dated at 2573 ±2 to 2547 ±5.3 Ma (Machado et al., 1991; Melo et al., 2016). The sequence is also cut by the 1.88 Ga anorogenic, metaluminous, isotropic Young Salobo Granite (Lindenmayer, 1990; 1998) which is also recognised at the Salobo deposit. The development of these shear zones resulted in a widespread and penetrative, sub-vertical, northwest-striking mylonitic foliation in the rocks of the Salobo deposit area, with the exception of the Young Salobo granite and late dolerite dykes (Réquia et al., 2003).
The principal lithology in the Salobo deposit area is a biotite-garnet (almandine)-quartz rich rock that has undergone intense iron and potassic hydrothermal alteration at high-temperatures in a ductile regime that has formed a mylonitic rock package containing variable amounts of magnetite, actinolite, grunerite and tourmaline. Lindenmayer (1990) first suggested it was a metagreywacke, but subsequently reinterpreted it to be a hydrothermally altered basaltic-andesite and dacite of the Igarapé Salobo Group. The host package also includes amphibolite to the NE and quartzite to the SW (Xavier et al., 2010). Alternatively, the iron-rich schists have been interpreted to represent sedimentary iron formations within the Igarapé Salobo Group that have been metamorphosed to pyroxene-hornfels facies (e.g., Campo Rodriguez et al., 2019; Xavier et al., 2010; Villas and Santos, 2001; Lindenmayer, 1990). Similar, structurally disrupted 'iron formations' extend intermittently along strike from Salobo over many tens of kilometres throughout the district (Siqueira and Costa,1991). However, the mineralised iron-rich rocks at Salobo differ from the regional iron formations in that they are enriched in Cu, Au, Ag, U, F, Mo, Co and LREE, whereas the banded iron formations (e.g., at Carajás) are depleted in these elements (Réquia and Fontboté, 2000). Melo et al. (2019) interpret the main host unit gneiss, iron-rich schist and structurally overlying amphibolite to be metamorphosed and altered rocks of the Xingu Complex, straddled by the Igarapé Gelado suite, whilst the 2.76 to 2.73 Ga Itacaiúnas Supergroup Igarapé Salobo Group volcanosedimentary sequence is only represented by mylonitic quartzite remnants on the margin of the deposit to the SW (Melo et al., 2016).
The Salobo 3A deposit extends over a NW-SE to WNW-ESE trending strike length of ~4 km and is 100 to 600 m in width. Mineralisation occurs as steeply dipping, irregular, lens-shaped and massive replacement orebodies following the mylonitic foliation, that have been recognised to depths of 750 metres below the surface (Souza and Vieira, 2000).
The deposit occurs as irregularly distributed lenticular shaped ore shoots within the major brittle-ductile WNW-ESE trending Cinzento Shear Zone. The lens-shaped and massive replacement orebodies are parallel to planar S-C structures along the trend of the shear zone, and commonly exhibit plastic flow textures, recrystallisation, mylonitisation and brecciation (Lindenmayer, 1990; Lindenmayer and Teixeira, 1999; Siqueira and Costa, 1991). The host rocks were progressively metamorphosed to pyroxene hornfels facies, at equilibrium temperatures of 750°C and pressures of up to 2 to 3 Kbar (i.e., 7 to 11 km depth), resulting from sinistral transcurrent transpressive shearing accompanied by oblique thrusting/reverse faulting. This metamorphism produced an assemblage with a coarse granoblastic texture, consisting of fayalite, almandine, spessartine, magnetite, hastingsite, chalcopyrite and graphite (Souza and Vieira, 2000).
The structurally-controlled and massive replacement ore bodies are generally 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; Réquia, and Fontboté, 2000). As described below, the ore occurs within strongly iron-potassic altered rocks in two main zones: i). massive garnet-biotite-fayalite-grunerite rock which generally has >50% magnetite with minor graphite and fluorite, and ii). a foliated, granoblastic, almandine-biotite-grunerite-plagioclase-quartz assemblage with 10 to 50% magnetite, and extends into the adjacent biotite-garnet-quartz schists. There is a direct relationship between copper and iron grades (Viera et al., 1988; Souza and Vieira, 2000).
Further to the generalised description in the previous paragraph, the main mineralised core of the deposit has been variously subdivided by different authors. The NI 43-101 Technical Report prepared by Burns et al. for Wheaton Precious Metals and for Vale S.A., dated 31 December 2019, recognises the following lithotypes:
• Biotite Schist - which forms the bulk of the mineralised core of the deposit. It is medium to coarse-grained rock with anastomosed foliation and is characterised by biotite, garnet, quartz, grunerite, magnetite and chlorite. This assemblage is partially replaced by a second generation of biotite and magnetite with chlorite, K feldspar, quartz, hematite and sulphides. Tourmaline, apatite, allanite, graphite and fluorite generally occur throughout the lithotype.
• Magnetite Schist, occurring as branching lenses of massive, foliated and banded rocks, predominantly composed of magnetite, fayalite, grunerite, almandine and secondary biotite. Granoblastic textures with polygonal contacts between magnetite and fayalite are common. The southeast portion of the deposit hosts hastingsite, replaced partially by actinolite, grunerite and sulphide minerals. Fluorite, apatite, graphite and uranium oxides are associated with this assemblage. Within more massive magnetite mineralisation there are small veins and irregular masses of secondary biotite and garnet is completely replaced by magnetite, forming pseudomorphs. Away from the massive magnetite, the magnetite content gradually diminishes, giving way to biotite-garnet schist and/or garnet–grunerite schist. The copper content of magnetite-schist is typically >0.8%.
• Garnet-Grunerite Schist, found as bands and lenses in the central to northeastern sections of the deposit. It has an isotropic texture, with weak dispersed schistosity, and a granoblastic texture. The principal mineral assemblage consists of almandine and cummingtonite-grunerite, with magnetite, hematite, ilmenite, biotite, quartz, chlorite, tourmaline and subordinate allanite. Fluorite and uraninite generally occur in veinlets related to stilpnomelane, calcite and grunerite.
• Feldspar-Chlorite Mylonite, which forms the northwestern margin of the mineralised core. It is principally composed of feldspar, chlorite and quartz with a mylonitic foliation, produced by the orientation of rims of chloritised deformed biotite, hastingsite, elongated quartz and saussuritised plagioclase (K feldspar, epidote and muscovite alteration). Porphyroblastic garnet is partially or totally replaced by chlorite and epidote. Allanite and apatite generally occur throughout this lithology.
• Quartz Mylonite, which forms the southwestern margin of the mineralised core of the deposit. It is grey or white in colour, passing through green to red. Where present, Fe-oxides are medium to fine grained, foliated and composed predominantly of quartz, muscovite, sericite, sillimanite and chlorite. Accessories, such as biotite, feldspar, magnetite, almandine, tourmaline, zircon and allanite are common. The following variations have been differentiated: i). red quartzo-feldspathic rocks composed of K feldspar and quartz and which may be a product of shearing between the gneissic basement and the supracrustal rocks; and ii). chlorite schists, mainly composed of chlorite and quartz, that represent intense hydrothermal alteration. This variant is found near the southwestern border of the deposits, close to important brittle shear zones, which may be interpreted as conduits for hydrothermal fluids.
Viera et al. (1988) and Souza and Vieira (2000) instead, have described the distribution of alteration and mineralisation in terms of five different 'schist types' which together defined the host 'Tres Alpha Formation' of the Igarapé Salobo Group. These occur as compositional lenses characterised by particular mineralogies. They are not stratigraphic units and in general do not display any consistent succession or zoning. They were the main lithotypes distinguished in mapping and drill core logging when the deposit was visited in 1992 (Porter, 1992), and are as follows after (Viera et al., 1988):
• Schist X1 - occurring as discontinuous lenticular zones in the core of the deposit. It is massive and generally coarsely crystalline, with >50% magnetite. Shear banding is sometimes observable. The subordinate minerals are garnet, biotite, fayalite, grunerite, graphite (1 to 1.5%) and fluorite. X1 may contain up to 5% Cu, but generally has >1 to 1.2% Cu.
• Schist X2 - coarse grained, porphyroblastic and foliated, but displaying little compositional banding. It is relatively rare compared to the other schists, and is mainly composed of garnet (2 to 4 mm) and grunerite with <10% magnetite and subordinate biotite and quartz. When very rich in fayalite, X2 may approach the Fe content of X1, but is invariably lower. The main difference to X1 is the magnetite and Cu content. It has the lowest Cu grades of the five schists, usually <0.5% Cu.
• Schist X3 - is both foliated and banded. It has a grano- to lepido-blastic texture and is generally similar in appearance to X1. However the main component minerals are biotite and garnet, with a magnetite content of between 10 and 50%. Subordinate minerals are fayalite, grunerite, quartz and plagioclase. The Cu content is usually 0.5 to 1.1% Cu. This lithotype contains the bulk of the ore in the deposit.
• Schist X4 - is similar in appearance to X3, being medium grained, foliated and banded, with a porphyroblastic texture and <10% magnetite. In addition, the principal component minerals are biotite, garnet and quartz, with variable amounts of grunerite, olivine and plagioclase. The Cu content is usually <0.5% Cu.
• Schist X5 - is foliated, well banded and fine grained and is composed of plagioclase, biotite, quartz and amphiboles, but has no magnetite. Common accessories are garnet and chlorite. This unit generally forms the outer margins of the Tres Alpha Formation, having gradational boundaries with the overlying quartzo-feldspathic rocks of the Cinzento Formation and the underlying Cascata Gneiss. X5 represents zones that underwent greater ductile strain, as a result of both shearing and hydrothermal activity (Souza and Vieira, 2000). This unit always has <0.5% Cu.
Less deformed exposures of these schists yield textures and structures that have been interpreted to resemble volcanic rocks. There is no consistent relationship between each of the schist types, except that X1 is often a core to developments of X3, and the combination of X1 and the more extensive X3 which constitute the orebody, form the core of the overall schist zone. The X1 lenses are generally from a few mm's up to 10 m thick, with a maximum of 30 m on one section, with lateral dimensions of 20 to 500 m, while the combined X3 and X1 may be up to 100 m thick and extend for up to a kilometre. In detail, lenses of X3 also occur within X1 and thin and pinch out along foliation, while others occur in an en echelon pattern. The contact between X1 and the other facies is commonly, but not exclusively, sharp, being generally 1 to 2 mm wide and always parallel to foliation. The transition from X1 to X4 for instance, is marked by a change from mainly magnetite with infrequent grunerite-fayalite to banded fayalite-grunerite with less common 1 to 2 mm magnetite bands. All other contacts are transitional over widths of a few cm's to tens of metres. The spatial discontinuity and lensoid nature observed between the different schist types is interpreted to be the result of intense tectonic dislocation, involving both imbrication and hydrothermal alteration (Souza and Vieira, 2000).
The high magnetite sections of X1 are grey to black, metallic and appear in places to be almost massive magnetite, with well foliated compositional banding, marked in part by variable thin green-yellow fayalite bands. Magnetite occurs as coarse aggregates up to 5 mm across which are usually aligned parallel to foliation. Chalcopyrite is present as up to 2 x 3 mm blobs while more abundant fine 0.25 to 0.5 mm bornite (distinguished from magnetite by its bluish glint) is distributed along foliation planes and to a lesser extent as strings of separated grains filling fractures in a number of directions. In other sections, high grade bornite/chalcocite follows irregular anastomosing fractures. There are also small patches of pale fluorite spread through the schist. The coarse magnetite crystals and aggregates display a texture approaching that of a breccia. In lower magnetite zones of X1 and in X3, greenish yellow fayalite is accompanied by pale greenish-grey grunerite, while garnet is present as oval shaped aggregates up to 1 cm or more long, sometimes as big as 3 x 10 cm. The banding of fayalite and grunerite wraps around the garnets, while in places the larger garnet crystals are commonly shattered with chalcocite and bornite within the cracks. Biotite also develops within the cracks in garnets, while grunerite (an amphibole - Fe7Si8O22•(OH)2) rims the larger fayalites (an olivine - Fe2SiO4). The garnets appear to be both syn- and post-metamorphism, with many being aligned parallel to the foliation, while others cut it. The main garnet is almandine (Fe3Al2(SiO4)3), although it is commonly rimmed by spessartine (Mn3Al2(SiO4)3). In the more heavily sheared intervals, the cupriferous minerals are coarser. Although comparatively rare, 1 to 30 cm thick carbonate veins, are found every 5 to 10 m in core through the ore zone, comprising dolomite-calcite and less commonly, siderite with coarse bornite and chalcocite. There is a close relationship between fluorite, grunerite and fayalite. Fluorite replaces the silicates with associated chalcocite-bornite and lesser chalcopyrite. In general there is no fluorite in massive fayalite, but where fayalite and magnetite are intergrown fluorite is present and follows the foliation. There is also a relationship between magnetite and Cu sulphides. Where magnetite is interbanded with massive fayalite, the associated Cu sulphides are always within the magnetite. In detail, in massive magnetite bands, sulphides are present as rims around the magnetite grains, or as fine stringers in fractures cutting the magnetite. Magnetite is almost invariably coarser than associated chalcocite-bornite. Au is also closely associated with magnetite. There is an apparent variation in the type of Cu sulphide depending upon the gangue. Massive magnetite is characteristically accompanied by chalcocite-bornite, while in interbanded fayalite and magnetite, chalcopyrite predominates. Overall within the orebody, chalcocite and bornite account for 85% of the Cu sulphides, while chalcopyrite comprises the remaining 15%. There is very little pyrite within the deposit, although small inclusions of pyrite with alteration rims and magnetite are evident within some chalcopyrite accumulations, as are pyrrhotite with pentlandite which occur as exsolutions. The information in this paragraph has been drawn from various sources from the reference list below.
Polished sections of the ore reveal a paragenetic succession in which magnetite was deposited in the early oxide stage, accompanied by small amounts of hematite, the silicate minerals fayalite (Fe olivine), biotite, garnet (Fe almandine), some fluorite (which has a close association with fayalite and magnetite), plagioclase and chlorite. Parts of the deposit were relatively reducing during this phase, as is indicated by the presence of graphite. The sulphide stage followed the oxide phase, with Mo deposited early, exhibiting a close association with graphite, which comprises 1 to 1.5% of X1. Re-Os molybdenite dating of the Salobo ore yielded an age of 2576 ±8 Ma (Réquia et al., 2003). The sulphide stage is characterised by the formation of tetragonal chalcopyrite, followed progressively by bornite and finally chalcocite. While these sulphides are the main ore minerals, significant Co, Ni, As, Ag, Au, Mo, F, rare earth elements (REEs), and U are characteristic of the Salobo ore represented in part by the presence of subordinate covellite, molybdenite, cobaltite, safflorite [(Co,Ni,Fe)As2], native gold and silver (Lindenmayer, 1990; Réquia et al., 1995). There are two generations of chalcopyrite, the earlier, pre- bornite development and a late, post chalcocite variety found in veins. In sections of the drill core, chalcopyrite is coarse and spectacular without much apparent accompanying bornite-chalcocite. At the end of the sulphide stage, native gold precipitation occurred in spatial association with cobaltite and safflorite. There is a close relationship between magnetite, Cu and both Au and Ag. In the transition from X1 to X3, the amphibole content does not change substantially. There is however, an antipathetic relationship between magnetite and almandine garnet. Petrographic evidence, such as magnetite cutting rotated garnet and chalcopyrite interstitial to fayalite grains or filling its fractures, indicates that at least some of the mineralisation is post the of peak metamorphism (Réquia and Fontboté, 2000).
Most rocks within the deposit area have been strongly altered, with those least affected, possibly closest to the protoliths but still significantly altered, composed of Ca amphibole ±plagioclase ±quartz ±sericite ±epidote ±chlorite, with or without tourmaline, biotite and K feldspar. The composition of the unaltered rocks is taken to be tholeiitic basalts (Réquia and Fontboté, 2000), based on chemical affinities with the overlying amphibolite interpreted to be of that composition (Lyndenmayer, 1990). These are overprinted by a partially preserved high-temperature calcic-sodic hydrothermal assemblage that includes the amphiboles hastingsite [NaCa2Fe2+4Fe3+(Al2Si6O22)(OH)2] and actinolite [Ca2Mg4.5-2.5Fe0.5-2.5(Si8O22)(OH)2], as well as Ca and Na plagioclase. This phase is marked by rocks with high Na2O contents of up to 4.5 wt.%. Ca is inferred to have preceded Na alteration, as indicated by narrow rims of Na-plagioclase commonly surrounding crystals of Ca-plagioclase. Plagioclase composition ranges from bytownite to sodic oligoclase (Réquia and Fontboté, 2000). Subsequently, silicification, iron-enrichment (almandine-grunerite-magnetite) and tourmaline formation took place. The dominant alteration associated with sulphide mineralisation is potassic, overprinting the Ca-Na phase, characterised by >3.5, up to 4.6 wt.% K2O). It comprises an assemblage that includes K feldspar-quartz ±Ca-amphibole ±cummingtonite ±plagioclase ±sericite ±epidote ±chlorite, with or without biotite, calcite, tourmaline, titanite and kaolinite. The latter assemblage is observed in the central part of the deposit which is also the richest ore zone. Fe-Mg amphibole, represented by cummingtonite, commonly replaces Ca-amphiboles. The local replacement of Mg-hornblende by actinolite is accompanied by epidote, chlorite and quartz formation. Plagioclase crystals, mainly of labradoritic [(Na,Ca)1-2Si3-2O8] composition, are extensively replaced by K feldspar (orthoclase). Biotite dominates in rocks with only minor or no K feldspar, in association with titanite and quartz (Réquia and Fontboté, 2000). This alteration assemblage developed under intense ductile deformation at temperatures between 650 to 550°C, based on the associated mineral assemblages (Lindenmayer, 1990).
The tectonic evolution of the Salobo deposit was complex, including sinistral transpressional ductile shearing with associated thrusts, followed by sinistral, transtensional, brittle shearing (Souza and Vieira, 2000), as follows:
• Sinistral transpressional ductile shearing produced a widespread, NW-SE orientated, sub-vertical, mylonitic foliation and imbrication of the lithological units, and the tectonic layering of strips and lenses of supracrustal rocks alternating with gneisses (Siqueira,1996). The peak of this deformation lies somewhere between 2851 ±4 and 2761±3 Ma (U-Pb, zircon; Machado et al., 1991), but must have been more long lived, or was rejuvenated, as it also apparently affected the 2573 ±2 Ma Old Salobo Granite dated at (Machado et al., 1991). This deformation coincided with the anhydrous metamorphic event that produced the coarse granoblastic textured iron silicate and oxide rich assemblage of ferrosilite [FeSiO3], fayalite, almandine, spessartine and magnetite with hastingsite, chalcopyrite and graphite, characterised by high temperature, low pressure thermal pyroxene hornfels facies metamorphism (750°C; 2 to 3 Kbar; Souza and Vieira, 2000). This high grade metamorphism was followed by the initial calcic-sodic alteration event resulting in the assemblage described previously, which was largely obliterated (Réquia and Fontboté, 2000). Subsequent high temperature potassic alteration led to fluid penetration and hydration of dehydrated minerals, characterised by partial destruction of fayalite, hastingsite and chalcopyrite to produce grunerite, almandine, magnetite, biotite, bornite and chalcocite, as well as the addition of further bornite and chalcocite. This alteration assemblage developed during or in the waning stages of intense ductile deformation at temperatures of between 650 and 550°C (Lindenmayer, 1990). Potassic alteration accompanied the main mineralisation stage with early Mo sulphide mineralisation dated at 2576 ±8 Ma (Re-Os molybdenite; Réquia et al., 2003) and ore samples dated at ~2452 ±14 Ma (U-Pb monazite; Melo et al., 2016). This also temporally corresponds to reactivation of the Cinzento Shear Zone at ~2.5 Ga (Tassinari et al., 2003; Melo et al., 2019), and intrusion of the Old Salobo Granite (2573 ±2 Ma; Réquia et al., 2003). Evidences of this potassic alteration includes growth of grunerite along fayalite cleavage planes; the substitution of fayalite by grunerite plus magnetite; formation of almandine containing inclusions of grunerite; the substitution of chalcopyrite by bornite and chalcocite (Lindenmayer, 1990). The hydrothermal fluid is interpreted to been acidic, weakly oxidising, rich in SiO2 and K+, and also highly saline, given that it introduced Si and K and removed Ca, Mg and Na. The result of the potassic alteration was the enrichment of the host rocks in Fe2+, K, Ce, Th, U and REE (Lindenmayer, 1990).
• Sinistral transtensional brittle faulting was the result of further renewal of displacement on the Cinzento Shear Zone and was particularly evident along the contact of quartzites and gneisses in the SW section of the deposit. It overprinted the earlier deformation with a sub-parallel fabric, dated by Mellito et al. (1998), from magnetite in brecciated iron rocks at 2172±23 Ma (Pb-Pb) and from chloritised gneisses at 2135±21 Ma (Rb-Sr, whole rock). This was accompanied by another hydrothermal event at temperatures of <370°C characterised by the infiltration of Ca-bearing fluids accompanied by intense chloritisation of almandine, biotite and hastingsite within the iron-rich rocks and intense chloritisation in wall rocks. Mineralisation associated with this phase represents the late Riedel shear controlled veining and includes quartz, stilpnomelane, fluorite, allanite, chalcopyrite, molybdenite, cobaltite and gold (Souza and Vieira, 2000) with greenalite-fluorite and uraninite fringes encapsulating fayalite and grunerite, accompanied by partial substitution of bornite by chalcocite (Lindenmayer and Teixeira, 1999; Lindenmayer, 2003). These late veins contain the second generation of chalcopyrite described previously. The fluid introduced during this stage was probably acidic, weakly saline and more oxidising than the high-T fluid in the previous hydrothermal stage (Lindenmayer, 1990).
Campo Rodríguez et al. (2019) recognised three stages of magnetite crystallisation with associated sulphides. Stage I, an inclusion free 'massive cystalline magnetite' with ferrosilite, fayalite, hastingsite and associated bornite as well as chalcopyrite with inclusions of pyrite. This is interpreted to have been emplaced towards the end of the main period of ductile shearing and emplacement of the Igarapé Gelado Suite at 2763 ±4.4 Ma. Stage II is a 'magnetite-bearing breccia', comprising inclusion-rich magnetite surrounded by a chalcopyrite matrix, which in turn, hosts pyrite and pyrrhotite close to Fe-rich magnetite and amphibole mineral grains and fragments. Some pyrite and pyrrhotite esxolutions are replaced by chalcopyrite. Inclusions within the magnetite are micro- to nanometre-scale and randomly distributed, composed by REE, zircon, apatite and Cu-bearing minerals, mainly bornite and chalcopyrite. This stage is also interpreted to have been formed late in the peak metamorphic stage and during emplacement of the Igarapé Gelado Suite. Stage III, 'magnetite schist' is taken to have formed at ~2.5 Ga during reactivation of the Cinzento Shear Zone strike-slip faulting. It has high quantities of inclusion-poor magnetite with an equigranular, granoblastic texture, which follows the schist foliation, and is accompanied by fibrous molybdenite and graphite. δ34S signatures of pyrite, chalcopyrite and pyrrhotite for sulphides associated with stage I and II magnetite range vary from 1.70 to 5.04 (average 2.72‰) and 0.88 to 1.98 (average 1.56‰). The close relationships imply chalcopyrite has inherited δ34S values, at least in part, from the pyrite and pyrrhotite respectively, reflecting reactions between pyrite and an oxidised Cu-rich fluid, which resulted in a replacement of pyrite by chalcopyrite. In addition with the association between high-temperature minerals (i.e., fayalite and ferrosilite) and magnetite-bornite indicate that primary mineralisation included in the stages I and overlapping stage II was formed from the same evolving magmatic fluid at high temperatures. Moreover, the lack of negative or broad Δ33S values, which are close to zero, are interpreted by Campo Rodríguez et al. (2019) to be compatible with high-temperature oxidised-hydrothermal fluids without a contribution from shallow or surficial fluids. Subsequent metamorphism accompanying emplacement of the Old Salobo Granite and reactivation of the Cinzento Shear Zone at ~2.5 Ga generated the stage III magnetite (Campo Rodríguez et al., 2019).
Melo et al. (2019) investigated oxygen and sulphur isotopes to draw similar conclusions, as follows. The iron enrichment at Salobo, occurred at 565 ±50°C, accompanied by hydrothermal fluids with magmatic or metamorphic compositions. This is evidenced by grunerite with δ18OH2O = 7.20 to 8.50‰, δDH2O = -25.33 to -16.01‰;
garnet with δ18OH2O = 7.10 to 9.70‰; and tourmaline with δ18OH2O = 5.07 to 7.37‰, δDH2O = -32.13 to +11.60‰ (Melo et al., 2019). However, the fluid inclusions at Salobo are hypersaline containing 30.6 to 58.4 wt.% NaCl equiv., favouring a magmatic origin in domains where the bulk of metamorphic devolatilisation is restricted to shear zones (Réquia, 1995).
The fluids that are associated with potassic alteration at 565 ±50°C, also have a typical magmatic/metamorphic composition, indicated by biotite with δ18OH2O = 7.23 to 18.03‰, δDH2O = -40.94 to -25.94‰; and
quartz with δ18OH2O = 7.52‰. Similarly, the δ34SV-CDT signatures of chalcopyrite = 0.81 to 1.28‰ and bornite = -0.37 to +1.63‰, which are interpreted to suggest an evolving felsic magmatic sulphur source at Salobo with little or no input from shallower basin or meteoric fluids (Melo et al., 2019). Réquia (1995) showed that fluids at Salobo were rich in H2O-CO2-NaCl-(CaCl2-CH4) with homogenization temperatures up to 485°C. The mineralising hydrothermal fluid is therefore interpreted to have been hypersaline, hot, F-rich, oxidised and carrying Cu, derived from a felsic magma coeval with ductile to brittle deformation at a depth of probably >6 km, presumably precipitating Cu when reduced/neutralised by magnetite.
The uncut geological resource in 2000 was estimated to be:
1926 Mt @ 0.59% Cu, 0.34 g/t Au, 6.07% Fe3O4, 0.16% C, 0.27% S, 0.23% F (Souza and Vieira, 2000).
The estimated mineral resource prior to 2000 was: 746 Mt with 0.93% Cu, 0.56 g/t Au, 9.79% Fe3O4 at a cutoff of 0.6% Cu (Souza and Vieira, 2000).
Following the 2004 feasibility study, CVRD quoted reserves of: 986 Mt @ 0.82% Cu, 0.49 g/t Au at a 0.5% Cu cutoff.
The Salobo I processing plant commenced production in 2012 with a total capacity of 12 Mtpy of ore processed. The open pit mine and mill reached planned capacities of 12 Mtpy of ore processed and 197 000 tpy of copper in concentrates in quarter 4 of 2016 (Vale Annual Report, 2016).
Remaining Ore Reserves at 31 December 2017 were (Vale 20-F form report to the US SEC, 2017):
Proved Reserves - 644.1 Mt @ 0.64% Cu;
Probable Reserves - 549.3 Mt @ 0.57% Cu;
TOTAL Reserves - 1193.4 Mt @ 0.61% Cu, with a recovery range of 80 to 90% of contained Cu.
Remaining Ore Reserves and Mineral Resources at 31 December 2019 were (Wheatstone-Vale NI 43-101 Technical Report, 31 December 2019) at a 0.253% Cu equiv. cutoff:
Proved Reserves - 152.7 Mt @ 0.69% Cu, 0.39 g/t Au;
Stockpile Proved Reserves - 163.4 Mt @ 0.45% Cu, 0.22 g/t Au;
Probable Reserves - 832.4 Mt @ 0.62% Cu, 0.32 g/t Au;
TOTAL Reserves - 1148.4 Mt @ 0.60% Cu, 032 g/t Au.
Measured + Indicated Resources - 193.5 Mt @ 0.61% Cu, 0.31 g/t Au;
Inferred Resources - 176.1 Mt @ 0.59% Cu, 0.29 g/t Au.
NOTE: Reserves are exclusive of Resources.
The most recent source geological information used to prepare this decription was dated: 2010.
Record last updated: 6/8/2018
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.
Salobo
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Amaral, E.V., Farias, N.F., Saueressig, R., Viana filho, A., Andrade, V.L.M., 1988 - Jazida de cobre Salobo 3 A e 4 A, Serra dos Carajas, Para.: in Principais Depositos Minerais Brasileiros-Metais Basicos Nao Ferrosos, Ouro e Aluminio, DNPM, Brazil, v.3, pp. 43-53.
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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.
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Campo Rodriguez, Y.T., Della Giustina, M.E.S., de Oliveira, C.G. and Whitehouse. M.J., 2019 - The giant metamorphosed IOCG Salobo Deposit, Carajas Mineral Province: S isotopic constraints and implications for a multi-stage evolutionary model: in Geociencias e Geopolitica na Amazonia, 16th Simposio de Geologia da Amazonia, 23 a 25 de Setembro de 2019 - Manaus-Amazonas, SBG-NO DEGEO/UFAM CPRM/Manaus, 4p.
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Campo-Rodriguez, Y.T., Schutesky, M.A., de Oliveira, C.G. and Whitehouse, M.J., 2021 - Unveiling the polyphasic evolution of the Neoarchean IOCG Salobo deposit, Carajas Mineral Province, Brazil: Insights from magnetite trace elements and sulfur isotopes: in Ore Geology Reviews v.140, 19p. doi.org/10.1016/j.oregeorev.2021.104572
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Huang, X.-W. and Beaudoin, G., 2019 - Textures and Chemical Compositions of Magnetite from Iron Oxide Copper-Gold (IOCG) and Kiruna-Type Iron Oxide-Apatite (IOA) Deposits and Their Implications for Ore Genesis and Magnetite Classification Schemes: in Econ. Geol. v.114, pp. 953-979.
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Lindenmayer, Z.G. and Teixeira, J.B.G., 1999 - Ore genesis at the Salobo copper deposit, Serra dos Carajas: in Silva, M.G. and Misi, A. (Eds.), 1999 Base Metal Deposits of Brazil CPRM, Brazil pp. 33-43.
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Melo, G.H.C. de, Monteiro, L.V.S., Xavier, R.P., Moreto, C.P.N. and Santiago, E., 2019 - Tracing Fluid Sources for the Salobo and Igarape Bahia Deposits: Implications for the Genesis of the Iron Oxide Copper-Gold Deposits in the Carajas Province, Brazil: in Econ. Geol. v.114, pp. 697-718.
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Melo, G.H.C. de, Monteiro, L.V.S., Xavier, R.P., Moreto, C.P.N., Santiago, E.S.B., Dufrane, S.A., Aires, B. and Santos, A.F.F., 2017 - Temporal evolution of the giant Salobo IOCG deposit, Carajas Province (Brazil): constraints from paragenesis of hydrothermal alteration and U-Pb geochronology: in Mineralium Deposita v.52, pp. 709-732.
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Requia K, Fontbote L 2000 - The Salobo iron oxide copper-gold deposit, Carajas, Northern Brazil: in Porter T M (Ed), 2000 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective PGC Publishing, Adelaide v1 pp 225-236
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Requia K, Stein H, Fontbote L, Chiaradia M 2003 - Re-Os and Pb-Pb geochronology of the Archean Salobo iron oxide copper-gold deposit, Carajas mineral province, northern Brazil: in Mineralium Deposita v38 pp 727-738
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Souza L H, Vieira E A P 2000 - Salobo 3 Alpha deposit: Geology and mineralisation: in Porter T M (Ed), 2000 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective PGC Publishing, Adelaide v1 pp 213-224
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Toledo, P.I.F., Moreto, C.P.N., Meira, V.T., Melo, G.H.C., Hunger, R.B., Monteiro, L.V.S., Garcia-Casco, A., Sanches, J., Lana C.C., Paula. C.C. and Freitas, S., 2023 - Geochronology insights for the Salobo deposit: the role of multiple events in a world-class IOCG deposit: in Geotechnologias e sustentabilidade: A Geologoa ne Ammazonia atual; 23 a 25 de Outibro de 2023 - Santarem, Para; SBG-NO DEGEO/UFOPA - CPRM Para. 17th Simposio de Geologia da Amazonia, Proceedings, 6p.
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Veiga M M, Schorscher H D, Fyfe W S 1991 - Relationship of copper with hydrous ferric oxides: Salobo, Carajas, PA, Brazil: in Ore Geology Reviews 6 pp 245-255
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Vieira, E.A.P., Saueressig, R., Siqueira, J.B., Silva, E.R.P. da, Rego, J.L. do, and Castro, F.C.D. de, 1988 - Caracterizacao geologica da jazida Polimetalica do Salobo 3A reavaliacao: in Provincia Mineral de Carajas, Litoestragrafia e Principais Depositos Minerais, Congresso Brasileiro Geologia, 35, Belem, Anexo aos Anais, pp. 97-111.
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Williams P J, Barton M D, Johnson D A, Fontbote L, de Haller A, Mark G, Oliver N H S and Marschik R, 2005 - Iron oxide copper-gold deposits: Geology, space-time distribution and possible modes of origin: in Hedenquist J W, Thompson J F H, Goldfarb R J and Richards J P (Eds.), 2005 Economic Geology 100th Anniversary Volume, Society of Economic Geologists, Denver, pp 371-405
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Williams, P. J., Kendrick, M.A. and Xavier, R.P., 2010 - Sources of Ore Fluid Components in IOCG Deposits: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide v.3, pp. 107-116.
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Xavier R P, Monteiro L V S, Souza Filho C R, Torresi I, Carvalho E R, Dreher A M, Wiedenbeck M, Trumbull R B, Pestilho A L S and Moreto C P N, 2010 - The Iron Oxide Copper-Gold Deposits of the Carajas Mineral Province, Brazil: an Updated and Critical Review: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide v.3 pp. 285-306
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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.
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Zhu, Z., 2016 - Gold in iron oxide copper-gold deposits: in Ore Geology Reviews v.72, pp. 37-42.
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