Image: Section of the Carajas N4 mine, Para, Brazil.
DESCRIPTIONS of ORE DEPOSITS & PROGRAM
The program for this tour included:
MODULE 2 - AFRICA & SOUTH AMERICA
PART B, Brazil - 2 to 10 April 2003
For information on the remainder of the tour, see the
Deposit Descriptions for Module 1, Module 2A
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MODULE 2 - AFRICA & SOUTH AMERICA, PART B - The Iron Deposits of Brazil
In 2002, Brazil was the world's largest iron ore exporter with an annual production of over 200 Mt, of which 158 Mt was exported in 2000. The majority of this came from two regions, the Serra dos Carajás in Para State in the north, and the Quadrilatero Ferrifero further south in Minas Gerais State. The Carajás ores are found within Archaean iron formations, while those in Minas Gerais are hosted by Palaeoproterozoic BIFs. In addition however, high grade, high lump ore is extracted from the Corumba - Urucum district in the state of Mato Grosso do Sul to the west from within Neoproterozoic hosts.
Carajás N4 & N5
The Carajás District contains known reserves of the order of 18 billion tonnes with an average grade of 65.4% Fe. All of these are controlled by the Vale (previously known as Companhia Vale do Rio Doce - CVRD), who currently have four active mines, which constitute the N4 and N5 operations. Annual production for the year 2000 was 47.6 Mt grading around 63% Fe. Because of the high grade, no concentration is undertaken on site at Carajás, although it is beneficiated to produce sinter feed, pellet feed and special fines for direct reduction as well as lump ore. All of this tonnage is transported by single track rail some 890 km to the port of São Luís in the state of Mananhão on the Atlantic coast where a new 6 Mtpa pellet plant is located.
The reserves are distributed in a number of deposit groups, the largest of which is the North Range with - 6200 Mt @ 65.8% Fe, 0.038% P, 1.0% SiO2, 1.05% Al2O3, 0.45% Mn, 0.01% S, 0.02% K2O, 0.03% Na2O and 1.88% LOI. The other reserves include: South Range, 35 km to the south - 10 400 Mt @ 66.3% Fe; East Ridge - 400 Mt @ 65.9% Fe; and South Felix Ridge - 600 Mt @ 62.8% Fe. The current production contains <1% Al2O3, <1% SiO2, <0.03% P2O5 and <0.3 Mn, with about 10% lump and 90% fines.
The Carajás deposits are located in the south-eastern portion of the Amazonian Craton. Basement in this region comprises the 3.0 Ga Pium Complex ortho-granulites and the 2.86 Ga Xingu Complex gneiss and migmatites. These are overlain by the Carajás Basin volcanics and sediments of the 2.76 to 2.6 Ga Itacaiunas Supergroup and the overlying clastic sediments of the Aguas Claras cover sequence. These are variously cut by 2.6 Ga and younger gabbros, 2.53 Ga granites and 1.9-1.8 Ga granitoids.
The Carajás ores are hosted by the Grão Pará Group of the Itacaiunas Supergroup, which is composed of meta-basalts, meta-sediments, ironstones and meta-rhyolites. The ore deposits are hosted by an approximately 300 to 400 m thick banded chert-hematite jaspilite unit that is sandwiched by thick upper and lower volcanic units. The lower volcanic unit, the Parauapebas Formation, is 4000 to 6000 m thick and comprises bi-modal volcanics, dominantly massive, vesicular and porphyritic flows and agglomeratic breccias of meta-basalt, meta-basaltic andesite and meta-trachy-andesites with subordinate (10-15%) met-rhyolitic tuffs and flows. Most of the volcanics have been metamorphosed to a low to medium grade and dip at 55 to 70 degrees. The host Carajás Formation comprises deformed banded iron formations with some interbedded mafic meta-volcanics. The Upper Volcanics are similar to those of the Parauapebas Formation with mixed meta-sediments (fine grained tuffs, tuffaceous siltstones, phyllites, cherts and greywacke).
The volcanic sequence has generally been weathered to a depth of 100 to 150 m, while oxidation is observed to a depth of up to 500 m in the BIFs of the ore zone. The upper 80% of the reserve comprises a soft, friable enriched limonite near surface passing down into hematite to a vertical depth of around 300 m. Hematite rich, but harder and more siliceous pods occur within the soft hematite, but also as a transition to the un-enriched BIF at depth. The typical un-enriched BIF/jaspilite comprises a banded red quartz-hematite rock composed of alternating white chert with subordinate hematite.
At surface the deposit is draped with layer of canga (detritals). The Carajás BIF unit is evident in the uncleared rain forest as a well demarcated corridor of stunted shrubs and grassland fringed by luxuriant trees.
Field Workshop - Quadrilátero Ferrífero
A two day field workshop on the geology and iron formations of the Quadrilátero Ferrífero was led by internationally renowned expert on the district, Professor Eduardo Ladeira.
The Quadrilátero Ferrífero occupies the southern-most part of the São Francisco Craton in Minas Gerais State, Brazil, some 1750 km SSE of the Carajás district. The geology of the Quadrilátero Ferrífero is dominated by Archaean and Proterozoic volcano-sedimentary sequences and Precambrian granitic complexes. The oldest rocks in the district are the 3.38 to 2.9 Ga Archaean banded trondhjemite-tonalite-granodiorite gneiss-migmatite complexes which form the basement to the late Archaean Rio das Velhas Supergroup.
The Rio das Velhas Supergroup is sub-divided into the basal Nova Lima Group which commenced with a succession of komatiitic ultramafic and mafic rocks with BIF intercalations, overlain progressively upwards by three associations, namely: i) a volcanic-chemical and clastic-chemical association composed of tholeiitic and komatiitic basalts with abundant interbedded iron formations or alternating fine grained clastics and iron formations respectively; ii) a volcanic association of felsic pyroclastics, autoclastics and epiclastics; iii) a re-sedimented association of greywacke, quartz-greywacke, sandstones and siltstones. Age dating of volcanic rocks from the Nova Lima group suggest an age of around 2.77 Ga.
The Nova Lima Group is overlain by the Maquiné Group, composed of the lower Palmital and upper Casa Forte Formations which are represented by a shallowing upwards sequence of marine and coastal, then non-marine continental rocks, specifically phyllite, greywacke, quartzites and conglomerates.
The Rio das Velhas Supergroup is discordantly overlain by the mainly Palaeoproterozoic Minas Supergroup quartzites, schists, phyllites, meta-conglomerates, carbonates and iron formations that host the major iron deposits of the district. The Minas Supergroup has been sub-divided into the basal clastic Caraça Group, which is divided into the Moeda Formation quartzite and metaconglomerate (including a Witwatersrand-like metaconglomerate), overlain by metapelitic rocks of the Batatal formations, which is transitional, but punctuated by an erosional unconformity, to the chemical-sedimentary Itabira Group (oxide or carbonate facies banded iron formation with ferruginous phyllite and dolomite), the upper clastic Piracicaba Group (quartzite, phyllite and dolomite lenses) and the overlying Sabará Formation (chlorite schist, phyllite, greywacke, tuff, conglomerate, quartzite and rare itabirite). The age of deposition of this sequence is estimated to be from 2.6 to 2.12 Ga, while an age of 2.42 Ga has been obtained from a dolomite of the Itabira Group.
All of the sequences detailed above are locally overlain by late Palaeoproterozoic and Mesoproterozoic clastic sediments and minor mafic volcanics. Granitoid intrusives appear to have been concentrated in two periods, namely around 2.7 Ga and 2.0 to 2.1 Ga.
The overall structure of the district is characterised by domal granitoids, with thrust faulting and associated isoclinal folds, while the Rio das Velhas and Minas Supergroups are interpreted to have been thrust stacked to the west and north-west. In detail, the basement gneisses and Rio das Velhas Supergroup were subjected to a compressional deformation with tangential thrusting from north to south or SW. A second, Palaeoproterozoic (Trans-Amazonian) compression produced NW striking thrust faults and tight SW-vergent isoclinal folds Metamorphism increases from greenschist facies to the west, to amphibolite and granulite grade in the east. Late extension during the Palaeo- to Mesoproterozoic led to basin formation and the prominent dome and keel architecture of the Quadrilátero Ferrífero. The Neoproterozoic Brasiliano event is evident on the eastern margin of the district produced west-vergent thrust and fault belts. The overall metamorphic grade of the western part of the district is primarily greenschist facies, increasing to amphibolite to granulite grades to the east.
The main iron deposits of the Quadrilátero Ferrífero have been developed within the iron formations of the Minas Supergroup Itabira Group, specifically within the basal unit of that group, the 350+ m thick, 2.58 and 2.42 Ga (Hartmann et al., 2006) Cauê Formation (previously the Tamandua Group), which is composed of itabirite (oxide facies iron formation), dolomitic itabirite and amphibolitic itabirite, with minor phyllite and dolomite. It is overlain by the upper member of the formation, the 600 m thick Gandarela Unit comprising dolomite and minor limestone, dolomitic itabirite, itabirite and dolomitic phyllite.
Itabirite is a term widely used in Brazil to denote a metamorphosed iron formation composed of iron oxides (hematite, magnetite, martite), abundant quartz, very rarely mica and other accessory minerals. It may be schistose or compact. The un-enriched itabirites from the Quadrilátero Ferrífero tend to have little magnetite and comprise principally quartz-hematite, quartz-hematite-carbonate and hematite-carbonate.
Two distinct types of high-grade (>65 wt % Fe) iron ore bodies occur in the Quadrilátero Ferrífero: i). Hard ores composed of hematite, martite, specularite and iron-deficient magnetite (kenomagnetite); ii). Soft, friable ores, distributed as 'alteration halos' around the hard orebodies.
Considerable variations in the structure and textures of the hard iron ores can be observed within the Quadrilátero Ferrífero. A preserved banding and lamination in the thin banded compact hematite ores apparently reflects the original layering and/or the prominent foliation of partially or completely replaced itabirite. Individual deposits vary from almond-shaped and rootless masses to bedded bodies which are both concordant to the main foliation, and to mesoscopic veins and irregular bodies. The ore textures have been grouped into the following styles: i). thin bedded and laminated itabirites, predominantly found in the west and central parts of the district; ii). micaceous, foliated and schistose ores, composed mainly of oriented specularite plates, that are dominant to the east; iii). brecciated mineralisation that is found mainly to the west, and to a lesser degree in the centre; and iv). compact/massive ores which occur as structureless bodies related to the brecciated interval or as isolated bodies in the centre of the district. The bedded and micaceous ores are believed to be the result of synkinematic, acid and oxidised metasomatism under a ductile regime during metamorphism, while the brecciated and massive ores are interpreted to be the result of subsequent, static hydrothermal activity during regional metamorphism in a brittle regime.
Soft high grade orebodies may be powdery, structureless, or have a brecciated structure with relics of the original banding. Huge cavities of several metres diameter may also be present. Soft high-grade ores do not considerably differ in mineral composition from the hard ores except in the case of some discontinuous pockets of powdery blue dust composed of random textured platy hematite that occur in the middle of granoblastic ores. Goethite only occurs at the surface, rapidly decreasing in concentration with depth. Relics of gangue minerals such as quartz dolomite, quartz, chlorite, talc and apatite may be detected.
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Itabira Mine Complex
The Itabira District is located some 80 km to the ENE of Belo Horizonte in Minas Gerais, within an outlier of the Rio das Velhas and Minas Supergroups that are infolded into the surrounded Archaean gneissic complex which separates them from the main Quadrilátero Ferrífero. The operating mines exploit ore developed in the Cauê Itabirite at the base of the Itabira Formation of the Minas Supergroup. This unit is exposed over a continuous strike length of 11 km in a series of synformal and antiformal structures that collectively define a larger synclinorium.
The iron ores of the Itabira district occur both as hard high grade, 67% Fe hematite and as friable lower grade, 45-50% Fe itabirites that must be upgraded. In addition, the orebodies are mantled by canga, (detrital and lateritic material). The hematite ores are interpreted to be due to hypogene enrichment of the itabirites, while the friable ores are the result of supergene leaching of silica and iron enrichment. The geological resource is stated at 1.3 Gt of hematite ore and 2.8 Gt of friable ores.
The Itabira district was the original home of CVRD activities and currently involves a number of their operations, principally Cauê and Conceicão. Production from these mines totalled 39.9 Mt in the year 2000. Part of this output is sold to local steel mills while the bulk is railed 600 km to the company export port near Vitória on the Atlantic coast in the state of Espírito Santo where CVRD also has pellet plants. Cauê has been in production since 1942, while Conceicão commenced operation in 1957. Both are projected to be exhausted in the year 2014. Proven + probable reserves at Cauê in 2000 were 25 Mt @ 51.3% Fe, while at Conceicão there were 338 Mt @ 56.7% Fe, with a further 424 Mt at Dios Córregos grading 59.5% Fe in a number of deposits.
The Itabira district is reported to contain a total of 897 million tonnes of iron ore reserves as of the end of 2002, comprising 401 Mt of hematite and 496 Mt of itabirite. In addition there are 679 Mt of resources made up of 247 Mt of hematite and 432 Mt of itabirites, plus a further 1408 Mts of potential ore.
These lower grade ores have a high percentage of friable itabirite compared to hematite ore and require concentration to achieve shipping grades of 64% Fe. This is done by standard crushing, classification and concentration steps to produce sinter feed, lump ore and pellet feed.
Casa de Pedra Mine
The Casa de Pedra mine is located some 55 km to the south of the centre of Belo Horizonte and 110 km south-west of the Itabira Mine Complex. It is developed on high grade ore within the CauÍ Itabirite of the Minas Supergroup Itabira Formation.
The deposit is found near the southern margin of the tight, complex, thrust bounded and reclined Moeda Synform which has been influenced by at least four deformational events, near where it wraps around the basement Bação Complex and changes trend from north-south to ESE to become the Dom Bosco Synform.
The mine is owned by Cia. Siderurgica Nacional (CSN), the largest integrated steel maker in South America, and is that company's main source of ore for its steel works, with any excess production being taken by CVRD. The mine has a nominal capacity of 12.7 Mt of ore per annum, although in 2001 it produced a total of 7.16 Mt, 25% of which was lump ore, the remainder being mainly sinter feed.
Corumba & Urucum
The Corumba - Urucum district is located in western Mato Grosso do Sul, Brazil, along the border with Bolivia. It hosts extensive manganese rich ironstones of late Neoproterozoic age. The district is some 170 km in length, the greater part of which is in Bolivia. An approximately 30 km strike length of mineralisation is found in Brazil, where the iron formation reaches a thickness of up to 300 m. Resources are stated as being as much as 36 Gt at an average grade of 54% Fe, embracing higher grade enriched zones containing 890 Mt @ 63% Fe.
The host Jacadigo Group is preserved in a series of 8 tabular mountains that rise up to 1000 m out of the swampy lowlands of the Pantanal. Basement is represented by gneisses and amphibolites, with lesser schists and quartzites, cut by 889 to 1200 Ma granitoids. These are unconformably overlain by the Neoproterozoic Jacadigo Group, which comprises a lower 200 m thick Urucum Formation composed of conglomerate and siltstone, followed by mainly black shale and siltstone, passing up into mainly sandstones. All of these rocks have been cemented by carbonates.
The Urucum Formation is followed by the 300 m thick Santa Cruz Formation. The base of this unit is marked by dropstones and the change from carbonate cement to manganese and iron oxides in the host cross bedded sandstone of the 80 m thick lower member. This member carries a basal 0 to 7 m thick manganese rich horizon (Mn1) with concretionary/nodular, detritus-rich and layered massive ores. The upper member is almost exclusively composed of chemical sediments. It commences with a widespread band of layered massive ore that is 0 to 3.5 m thick (Mn2). This is followed by a 70 to 270 m thick monotonous sequence of hematite-jaspilite which covers an area of 120 sq. km. A third 0-2.35 m thick manganese horizon (Mn3) is intercalated with the hematite-jaspilite 40 to 45 m above Mn2, while a fourth such band, Mn4 which is 0-1.4 m thick, is found a further 45 m higher. Mn3 and Mn4 are again layered massive manganese oxide beds. Erratic dropstone from 0.05 to 1 m across are found sporadically throughout the chemical sediments. The exposed Jacadigo Group in the Urucum district is surrounded by the unconformably overlying limestones and dolomites of the late Neoproterozoic Corumbá Group.
The fresh jaspilitic ores of the lower Santa Cruz Formation average 50% Fe, while the supergene enriched sections carry up to 67% Fe.
The higher grade, enriched ores occur in two forms, namely:
i). canga - Fe-hydroxide-cemented breccias of hematite-jaspilite - occurs as a sub-recent weathering crust in places, and
ii). 'colluvial' ore in the form of weathered 0.01 to 10 m blocks of hematite-jaspilite which form debris cones and fans on the flanks of the tabular mountains.
The un-enriched hematite-jaspilites are predominantly composed of micro-crystalline hematite and crypto-crystalline red jasper forming alternating bands on a micro- and macro-scopic scale. In finely laminated ores, spherical and concentric aggregates of hematite and jasper are preserved.
Rio Tinto operates the Corumbá iron ore mine through it's 100% owned subsidiary Mineraçao Corumbaense Reunida, with the output being transported by barge down the major Paraguay River to Argentina. Vale, who also exploits iron and manganese from its nearby Urucum mine, purchased the iron operations from Rio Tinto in 2009.
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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. These are listed in the Literature Collections pages for this tour.
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