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.
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The Igarapé Bahia Au-Cu-(REE-U) deposit is located in the Carajás Mineral Province of Para State Brazil.
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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, bastnsite and parisite), Mo (molybdenite), U (uraninite), F (fluorite), Cl (ferropyrosmalite) and P (apatite).
Production has been at a rate of around 10 t Au per annum, with the remaining reserve in 1998 being 29 Mt @ 2 g/t Au. The deposit was and is controlled and operated by CVRD/Vale.
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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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.
The deposit was discovered by Docegeo, exploration arm of CVRD in 1996 and is not in production.
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 deposit is 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.
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Mineralisation is hosted along a regional WNWESE-striking shear zone that defines the contact between the metavolcanosedimentary rocks of the ~2.76 Ga Itacaiúnas Supergroup and tonalitic to trondhjemitic gneisses and migmatites of the ~2.8 Ga Xingu Complex.
The deposit is hosted by granite, granophyric granite, gabbro and felsic metavolcanic rocks representing both suites.
The ore is located in two adjacent centres, Sossego Hill (the SossegoCurral zones) and the larger Sequeirinho (the PistaSequeirinhoBaiano 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 Sequeirinho orebodies have been subjected to regional sodic (albitehematite) alteration, overprinted by sodiccalcic (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 orebodies display only weakly developed early albitic and very poor subsequent calcicsodic 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 sulphides of both groups of orebodies were initially accompanied by potassic alteration and a subsequent more important assemblage of calcitequartzepidotechlorite. 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.
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.
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.
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The deposit is contained in supracrustal rocks of Igarapé Salobo Group of Archaean age (interpreted to be older than the Igarapé Bahia Group that hosts Igarapé Bahia and Alemao), which is composed of iron-rich schists, metagreywackes, amphibolites and quartzites.
This sequence overlies the basement gneisses of the Xingu Complex composed of partially migmatised gneisses. The original stratigraphic relationships with the basement and within host sequence are masked by intense ductile-brittle shear zones and over-thrusting.
The Salobo 3A deposit extends over an area represented by a NW trending strike length of 4 km, 100 to 600 metres in width. Mineralisation has been recognised to depths of 750 metres below the surface.
Copper mineralisation occurs as chalcocite and bornite, with subordinate quantities of chalcopyrite, together with variable proportions of molybdenite, cobaltite, covellite, gold and silver, lodged in schists with variable proportions of magnetite, amphibole, olivine, garnet, biotite, quartz and plagioclase.
The Cu grade is generally proportional to the magnetite content, with the highest grades in +50% magnetite which have >1% Cu, while the bulk of the ore in a schist with 10 to 50% magnetite and containing 0.6 to 1.1% Cu. Other lithologies within the orebody and shear zone have variable iron contents, with Fe silicates (fayalite, grunerite, biotite) predominating and <10% magnetite, as well as lower Cu and Au.
The ore lies within a major brittle-ductile shear zone that has produced irregularly distributed lenticular shaped ore shoots. The host rocks were progressively metamorphosed to pyroxene hornfels facies, at equilibrium temperatures of 750° C, resulting from sinistral transcurrent transpressive shearing accompanied by oblique thrusting.
A first hydrothermal event developed at temperatures between 650 to 550° C causing partial substitution of chalcopyrite by bornite and chalcocite, accompanied by intense K-metasomatism. This was followed by sinistral transcurrent transtensive shear zone formation, causing green schist facies metasomatism, characterised by intense chloritisation and partial substitution of bornite by chalcocite.
The estimated mineral resources prior to 2000 were of the order of 789 Mt with 0.96% Cu and 0.52 g/t Au.
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 project was initially a 50:50 joint venture between CVRD and the Anglo American Group, although CVRD now owns it 100%.
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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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