2 - THE SHIELD, PART A - IRON OXIDE COPPER-GOLD DEPOSITS
An overview briefing on the geology and metallogenic setting of the Carajas district was incorporated into the individual visits, rather than as a separate session.
Super Porphyry Cu and Au|
IOCG Deposits - 70 papers|
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The Igarapé Bahia Au-Cu-(REE-U) deposit is located in the Carajás Mineral Province of Para State Brazil, ~60 km west of the Carajás township (or Nucleo Urbano) and ~40 km south of the Salobo deposit.
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(#Location: 6° 1' 57"S, 50° 33' 59"W.
Igarapé Bahia is hosted by the Igarapé Bahia Group, considered to be a lower greenschist facies metamorphosed unit of the Archaean (ca. 2.75 Ga) metavolcano-sedimentary Itacaiúnas Supergroup which comprises two lithological and stratigraphic domains: a lower metavolcanic unit composed of metavolcanic rocks and acid to intermediate volcanoclastics; and an upper clastic-chemical metasedimentary unit and volcanoclastic rocks. The Igarapé Bahia orebodies represents a 100 to 150 m thick gossan-laterite zone from which significant amounts of gold (>60 t) were mined until 2003. Where not outcropping, the primary mineralisation is obscured by a 250 m thick unconformable siliciclastic unit referred as the Aguas Claras Formation.
The copper-gold mineralisation at the Igarapé Bahia deposit is hosted by a hydrothermally altered breccia at the contact between the footwall mafic volcanics, with associated BIF and hyaloclastite, and a dominantly coarse to fine-grained metasedimentary sequence in the hanging wall. The breccia unit is exposed at or near the surface as a semicircular annulus, with a form similar to a ring complex with a diameter of approximately 1.5 km. The mineralised breccia unit occurs as a 2 km long by 30 to 250 m thick series of fault dislocated bodies on the southern, northeastern and northwestern sections of this structure, dipping steeply outwards at ~75°, and is nearly concordant with the metavolcanic-sedimentary wallrocks.
The economically extracted ore at Igarapé Bahia is largely developed as a supergene gossan-laterite enrichment within the 150 to 200 m thick oxide profile. Three orebodies have been mined at this contact, forming a semi-circular trace at the surface namely, Acampamento - dipping at around 75° to the north-east, Furo Trinta to the south-east, and Acampamento Norte to the north-west, forming an outward dipping domal structure in three dimensions.
The oxide zone is characterised by supergene enrichment and hematite, goethite, gibbsite and quartz. This is underlain by a transition zone that may be up to 50 m thick with enriched supergene malachite, cuprite, native copper and goethite and minor amounts of digenite and chalcocite responsible for high grade Cu and Au. This zone is in turn underlain by primary Cu-Au mineralisation, represented by hydrothermal breccias containing chalcopyrite, bornite, carbonate, magnetite and minor molybdenite and pyrite.
Strong hydrothermal alteration of the host sequence produced intense chloritisation, Fe-metasomatism, Cu-sulphidation (chalcopyrite and bornite), carbonatisation, silicification, tourmalinisation and biotitisation in the primary zone.
Gold-copper mineralisation is localised at the commonly brecciated contact between the metavolcanics and the meta-volcaniclastics-metasediments and comprises, magnetite/siderite heterolithic breccias and hydrothermally altered metavolcanics. These rocks are enriched in REE (monazite, allanite, xenotime, bastnäsite and parisite), Mo (molybdenite), U (uraninite), F (fluorite), Cl (ferropyrosmalite) and P (apatite).
The deposit was mined by CVRD/Vale between 1991 and 2002 at a production rate of up to 10 t Au per annum for a total production of 97 t of recovered gold. The remaining reserve in 1998 was quoted as 29 Mt @ 2 g/t Au (Tazava and Oleira, 2000). Reserves in 1999 were quoted by Tallarico et al. (2000) as 18.5 Mt @ 1.97 g/t Au
The Alemão IOCG Au-Cu-(REE-U) deposit is part of the Igarape Bahia mineralised system in the Carajás Mineral Province of Para State Brazil (see the Igarapé Bahia record).
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(#Location: 6° 1' 35"S, 50° 34' 38"W).
Alemão is hosted by the Igarapé Bahia Group, considered to be a lower greenschist facies metamorphosed unit of the Archaean (ca. 2.75 Ga) metavolcano-sedimentary Itacaiúnas Supergroup which comprises two lithological and stratigraphic domains: a lower metavolcanic unit composed of metavolcanic rocks and acid to intermediate volcanoclastics; and an upper clastic-chemical metasedimentary unit and volcanoclastic rocks. The Alemão ore body underlies the far northwestern margin of the Igarapé Bahia deposit, which represents a 100 to 150 m thick gossan-laterite zone from which significant amounts of gold (>60 t) were mined until 2003. Elsewhere it is obscured by a 250 m thick unconformable siliciclastic unit referred as the Aguas Claras Formation.
The copper-gold mineralisation at the Igarapé Bahia/Alemão deposit is hosted by a hydrothermally altered breccia at the contact between the footwall mafic volcanics, with associated BIF and hyaloclastite, and a dominantly coarse to fine-grained metasedimentary sequence in the hanging wall. The breccia unit is exposed at or near the surface as a semicircular annulus, with a form similar to a ring complex with a diameter of approximately 1.5 km. The mineralised breccia unit occurs as a 2 km long by 30 to 250 m thick series of fault dislocated bodies on the southern, eastern and northern sections of this structure, dipping steeply outwards at ~75°, and is nearly concordant with the metavolcanic-sedimentary wallrocks. The Igarapé Bahia deposit is the thick gossan-laterite zone developed within the top 100 to 150 m of the exposed breccia unit.
The Alemão deposit is located immediately to the northwest of this annular zone, occurring as a particularly magnetite-Cu-Au-enriched down-faulted segment of the Acampamento Norte orebody, the northern most orebody of the Igarapé Bahia deposit. It has dimensions of around 500 m in length, 50 to 200 m thick and has been traced down plunge for at least 800 m below the surface, although the top of the deposit is at a depth of approximately 250 m below Aguas Claras Formation sandstone cover.
The Alemão orebody is hosted by a hydrothermally altered breccia at the contact between the footwall mafic volcanics, with associated BIF and hyaloclastite, and a dominantly coarse to fine-grained metasedimentary sequence in the hanging wall. A set of unmetamorphosed 2.75 to 2.65 Ga quartz diorite and 2579±7 Ma dolerite dykes cut the orebodies, the host metavolcano-sedimentary sequence and the overlying clastic metasedimentary sequence of the Áoguas Claras Formation/Rio Fresco Group.
The breccia has gradational contacts with its wallrocks and is made up of polymitic, usually matrix-supported clasts, composed mainly of coarse, angular to rounded basalt, BIF and chert clasts derived from the footwall unit.
The hydrothermal paragenesis is marked by ferric minerals (magnetite and hematite), sulphides (chalcopyrite-pyrite), chlorite, carbonate (siderite, calcite, ankerite) and biotite with minor quartz, tourmaline, fluorite, apatite, uraninite, gold and silver. Sericite and albite are rare. The mineralisation is represented by hydrothermal breccias and hydrothermally altered rocks. These fall within two groups, namely:
i). massive magnetite-chalcopyrite bands and polymict breccias with a matrix of magnetite, chalcopyrite, siderite, chlorite, biotite and amphiboles; and
ii). brecciated hydrothermally altered volcanics with chalcopyrite, bornite, pyrite, chlorite, siderite and ankerite both in the matrix and disseminated in the altered country rock.
Several generations of late mineralised veins crosscut the ore breccia and are composed of variable concentrations of chalcopyrite, pyrite, quartz, calcite, chlorite, and fluorite. The veins commonly display open space - filling textures (e.g., comb)
The total estimated resource in 2001 was 170 Mt @ 1.5% Cu, 0.8 g/t Au. More recently CVRD has quoted a reserve of 161 Mt @ 1.3% Cu, 0.86 g/t Au.
Alemao breccias. Above - Chlorite-carbonate-sulphide breccia. Note the well developed striations are created by the diamond saw blade that cut the surface shown;
Below - magnetite-sulphide breccia. In both images, the smaller scale card graduations are in millimetres.
Images by Mike Porter, 2020, samples collected at Alemao 2001.
The deposit was discovered by Docegeo, exploration arm of CVRD (now Vale) in 1996.
The Cristalino IOCG deposit is located some 40 km to the east of Sossego in a bifurcation of the major regional Carajás Fault in the Carajás district of Para State, Brazil.
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Basement in the area is represented by the Xingu Complex which is >2.86 Ga in age and is composed of a variety of rocks, including the ~3.0 Ga Pium Complex and 2.9 Ga Greenstones. These are overlain by the 2.76 Ga Grão Para Group of volcanics and sediments, cut by the 2.5 Ga Estrella Granite and subsequently by 1.9 Ga granites but is overlain by the un-metamorphosed 2.7 to 2.6 Ga Águas Claras marine sandstones.
Cristalino is hosted by volcanics of the Grão Para Group composed of orange dacite and green andesite with minor basalt and in association with hydrothermally altered and disrupted banded iron formations within this same sequence. These iron formations have been upgraded nearby where they constitute part of the Carajás Iron Resources.
Mineralisation is concentrated in a NW-SE trending, sinsitral transpressive zone of shearing over a drilled length of 2200 m and thickness ranging from 10's of metres to 500 m. The shear zone is several hundreds of metres in width and is a splay of the Carajás Fault. The ore zone is generally brecciated and is found in the volcanics below the iron formation and in the lower sections of the iron formation itself. In general the iron formation forms the upper limit to ore and may have acted as a capping. The hydrothermally altered breccia is composed of 5 to 50% sub-angular to sub-rounded fragments.
Mineralisation is associated with the emplacement of 2.7 Ga diorite to quartz-diorite intrusions into the volcano-sedimentary sequence and iron formation.
There are two styles of mineralisation: (i). 60% of which is crosscutting stockwork veins and veinlets, and (ii). 40% breccia ore where the breccia fragments are surrounded by sulphide veins and a sulphide matrix. Mineralisation is also accompanied by magnetite and associated amphibole alteration. The principal sulphides are chalcopyrite and pyrite in a 2:1 to 3:1 ratio. The Copper was introduced after the magnetite and amphibolite alteration, although the highest grades are associated with the amphibole zones. The iron alteration where it affects the iron formation represents addition, not remobilisation of iron.
Hydrothermal alteration progressed from: (i). early widespread actinolite-albite; to (ii). biotite with scapolite and magnetite; to (iii). amphibole with magnetite as hastingsite, grunerite, actinolite and cummingtonite; to (iv). chlorite with albite, magnetite and hematite; to (v). chlorite and carbonate; to (vi). muscovite and carbonate.
The average 3-5% sulphide mineralisation is associated with the last three overlapping phases of alteration and comprise chalcopyrite, pyrite and lesser arsenopyrite with trace Ni-Co sulphides. The gold is in the pyrite.
Indications of Cu mineralisation were first noted in the area in the late 60's to early 70's. Grid geochemistry and geophysics from 1984-87 led to 2 anomalies being drilled in 1988 with some 13 holes in two prospects. The second phase of work was commenced in 1997-98 with more grid mapping, geochemistry and geophysics, culminating in a drill intersection of 38 m @ 1.4% Cu, 0.25 g/t Au between 76 and 114 m depth.
The resultant approximate resource from the subsequent drilling to 2001 amounted to 500 Mt @ 1.0% Cu, 0.2-0.3 g/t Au. According to CVRD, the reserves amount to 261 Mt @ 0.73% Cu.
The Sossego IOCG deposits are located some 40 km to the south of the Carajás townsite in the state of Para, Brazil. It is approximately 80 km SE of Igarapé Bahia and Alemao, and 40 km west of Cristalino. It is also ~30 km east of the main Carajas Serra Sul iron operation (#Location: 6° 25' 10"S, 50° 3' 55"W).
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For details of the setting of the deposits see the Carajas IOCG Province record.
Mineralisation is hosted along a regional WNW-ESE-striking shear zone that defines the contact between the metavolcano-sedimentary rocks of the ~2.76 Ga Itacaiúnas Supergroup and the basement tonalitic to trondhjemitic gneisses and migmatites of the ~2.8 Ga Xingu Complex.
Both the Itacaiúnas Supergroup and Xingu Complex rocks are intruded by granite, granophyric granite, gabbro intrusions and late dacite porphyry dykes in the deposit area. The ages of these rocks are uncertain, although a Pb-Pb zircon age of 2734±4 Ga for a biotite-hornblende granite and a U-Pb zircon age of 2765±39 Ga for a tonalite-trondhjemite (Sardinha et al., 2004) close to the deposit area, are considered the best estimates. In addition, the Palaeoproterozoic Rio Branco granite intrusion crosscuts the hydrothermally altered volcanic and intrusive rocks in the deposit area. This field relationship, together with the lack of hydrothermal alteration and mineralisation of the granite, indicate it was emplaced much later than the formation of the copper-gold ore at Sossego (Xavier et al., 2010).
An extensive, >20 km2, zone of scapolite alteration representing distal sodic alteration surround the Sossego deposits (Villas et al., 2005; Sousa, 2007). This alteration is characterised by plagioclase replacement and by scapolite in both mafic and felsic protoliths. Scapolite veins, with widths ranging from mms to >1 m, commonly crosscut basement and supracrustal units and have a biotite-scapolite-hastingsite rich halo that grades outward to chlorite.
In the Sossego deposit area, deformation is represented by cm- to metre-wide mylonitic zones, which are regionally crosscut by NE-striking faults. Rocks in the immediate footwalls of these faults are intensely mylonitised with biotite-tourmaline-scapolite and siliceous alteration developed prior to or synchronous with the shearing (Xavier et al., 2012).
The ore is located in two adjacent centres, Sossego Hill (the Sossego-Curral zones) and the larger Sequeirinho (the Pista-Sequeirinho-Baiano zones) which has a length of 1.6 km and thickness of 150 to 200 m in its central section. These two centres are separated by a major high angle fault. The original resource within Sequeirinho and Sossego was distributed ~85 to 15% respectively.
The Sequeirinho orezone lies along a NE striking sinistral fault, associated with a positive magnetic anomaly, and comprises an S-shaped, tabular, subvertical
body. It has been subjected to regional sodic (albite-hematite) alteration, overprinted by sodic-calcic (actinolite-rich) alteration accompanying with the formation of massive magnetite-(apatite) bodies. Both alteration assemblages exhibit ductile to brittle-ductile fabrics and are cut by spatially restricted zones of potassic (biotite and potassium feldspar) alteration that grades outward to chlorite-rich assemblages (Monteiro, et al., 2007).
The Sossego Hill zone is a subcircular, vertical, pipe-like orebody, with a central breccia surrounded by a stockwork of sulphide veins, faults, and shear zones (Morais and Alkmim, 2005; Carvalho, 2009; Domingos, 2009). The orebodies within the zone display only weakly developed early albitic and very poor subsequent calcic-sodic alteration, although they have well-developed potassic alteration assemblages that were formed during brittle deformation that produced breccia bodies. The matrix of the breccias commonly displays coarse mineral infill suggestive of growth into open space (Monteiro, et al., 2007). The potassic alteration assemblages, which mark the onset of mineralisation, grade outward to a widespread zone of chlorite and late hydrolytic (sericite-hematite-quartz)
assemblages crosscut by calcite veins (Carvalho et al., 2005; Monteiro et al., 2008a, ).
The orebodies are commonly brecciated. The Sequeirinho breccias contain rounded fragments of hydrothermal magnetite, actinolite and apatite or rocks with low angularity. The Sossego ore breccias comprise fragments of potassic-altered host rocks (e.g., granophyric granite) with high clast angularity, characteristic
of breccias that underwent minor transport.
The sulphides of both groups of orebodies were initially accompanied by potassic alteration and a subsequent more important assemblage of calcite-quartz-epidote-chlorite. In the Sequeirinho orebodies, sulphides range from undeformed to deformed, while at the Sossego Hill orebodies they are undeformed. Very late stage, weakly mineralised hydrolytic alteration is present in the Sossego Hill orebodies (Monteiro, et al., 2007).
The dominant sulphides are chalcopyrite with subsidiary siegenite and millerite, and minor pyrrhotite and pyrite in the Sequerinho orebodies, although pyrite is relatively abundant in the Sossego Hill bodies.
Chalcopyrite occurs in the breccia matrix associated with pyrite, gold, siegenite, millerite, Pd melonite, hessite, cassiterite, sphalerite, galena, molybdenite, thorianite, and monazite. The resulting Fe-Cu-Au-Co-Ni-Pd- LREE signature characterises the Sossego deposit.
In early 2001 the total resource was quoted as 355 Mt @ 1.1% Cu, 0.28 g/t Au, encompassing a mineable reserve of 219 Mt @ 1.24% Cu, 0.33 g/t Au at a 0.4% Cu cut-off and stripping ratio of 3.3:1 wate:ore.
At the commencement of mining in 2004, reserves were quoted by CVRD as 250 Mt @ 1.0% Cu. Montiero, et al., (2007) published a reserve of 245 Mt @ 1.1% Cu, 0.28 g/t Au.
Mineralisation (gold) was initially discovered by garimperos (prospectors) in 1984 within CVRD concessions. The area was tendered to Phelps Dodge in 1996 and the first major intersections were in early 1997.
In 2001 the project was controlled by Mineracao Serra do Sossego, a 50:50 joint venture between Phelps Dodge do Brasil and CVRD. In 2002 CVRD bought Phelps Dodge's share and commenced mining in 2004 with a nominal capacity of 93 000 tya of Cu in concentrates.
Remaining ore reserves at 31 December 2017 were (Vale 20-F form report to the US SEC, 2017):
Proved Reserves - 110.7 Mt @ 0.68% Cu;
Probable Reserves - 9.4 Mt @ 0.66% Cu;
TOTAL Reserves - 120.1 Mt @ 0.68% Cu, with a recovery range of 90 to 95% of contained Cu.
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).
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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 belong to the supracrustal Igarapé Salobo Group of the Neoarchaean 2.76 to 2.73 Ga Itacaiúnas Supergroup); orthogneisses of the basement Xingu Complex (2950 ±25 to 2857 ±6.7 Ma; U-Pb zircon; Melo et al., 2016) and deformed granitoid gneiss of the 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 include 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 tens of kilometres throughout the district (Siqueira and Costa,1991). However, the mineralised iron-rich rocks 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 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 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, graphite, 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).
Viera et al. (1988) and Souza and Vieira (2000) 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 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.
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; RĂ©quia 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.
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For more information contact: T M (Mike) Porter, of Porter GeoConsultancy (email@example.com)
This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF). While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.
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