Another PGC International Study Tour
Developed & Managed by Porter GeoConsultancy
Iron 2002-03
Key Iron Deposits of the World
September 2002 & March-April 2003
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Image:   Section of the Carajas N4 mine, Para, Brazil.    
Carajas N4
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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Study Tour Series
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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/resources of the order of 18 Gt with an average grade of 65.4% Fe. All of these are controlled by Vale SA (previously known as Companhia Vale do Rio Doce - CVRD), who operate a series of mines, which together constitute the N4W, N4E and N5 operations on the Serra Norte and the S11D mine in the Serra Sul to the SE. Annual production for the year 2000 was 47.6 Mt grading around 63% Fe, which by 2017 had increased to 169.2 Mt. 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 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 banded iron formation (BIF), referred to as 'jaspilite', 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 to 15%) meta-rhyolitic tuffs and flows. Most of the volcanics have been metamorphosed to a low to medium grade and dip at 55 to 70°. The host Carajás Formation comprises deformed banded jaspilites 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). For a more detailed description of the regional setting, see the Carajás IOCG Province record.

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 jaspilites 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 often more siliceous pods occur within the soft hematite, but also in the transition to the un-enriched jaspilite at depth. The typical un-enriched jaspilite comprises a banded red quartz-hematite rock composed of alternating white to pale red chert with subordinate hematite. The chert comprises crypto- to micro-crystalline quartz with inclusions of cryptocrystalline hematite and lesser martitised magnetite plus occasional sericite. The dark bands of the jaspilite are composed of hematite and martitised magnetite. The un-enriched jaspilite typically carries 15 to 45% Fe, but can range up to 57% Fe with 35 to 65% SiO
2.

Carajas IOCG Province Geology
The geology, structure and mineral deposits of the Carajás Mineral Province (after Xavier et al., 2010; Rosiere et al., 2006; Ferreira Filho et al., 2021 and others).
The protoliths of the high-grade iron ore in the Serra Norte deposits N4E, N4W, N5E, and N5S are the jaspilites, as described above, distributed along, and structurally controlled by, the northern flank of the Carajás fold. High-grade iron mineralisation (>65% Fe) is made up of hard and soft ores. The hard ores can be banded, massive and/or brecciated, and are characterized by hematite-martite and hematite types. The soft ores are very porous, discontinuous and are tabular, friable and banded. The basal contact of high-grade iron ore is defined by a hydrothermally altered basaltic rock mainly composed of chlorite and microplaty hematite (Lobato, et al., 2008).

The jaspilites have been subjected to varying degrees of hydrothermal modification to form iron ores which represent an early hypogene alteration stage. This alteration is zonally distributed around the main ore as follows (after Lobato, et al., 2008):
• The distal expression of this hypogene alteration is mainly characterised by recrystallisation of jasper and the removal of its iron, and the formation of magnetite, which is commonly martitised, overgrowing original microcrystalline hematite and has associated quartz and calcite veins. Two vein breccia types characterise the distal alteration zone: V1a quartz ±sulphide breccias and V1b carbonate ±sulphide breccia veins. The sulphides are pyrite and chalcopyrite.
• The intermediate, peripheral alteration zone was developed synchronously with the main iron ore-forming event, and is characterised by: i). progressive leaching of chert and quartz, producing oxides and vugs; ii). the presence of martite as the dominant oxide following altered jaspilite layers; and iii). partial infill of open spaces by microplaty and/or platy hematite. This intermediate zone is also cut by V2a quartz ±hematite bedding-discordant veins, V2b (discordant, vertical, vug-textured quartz +hematite veins and V3 hematite ±quartz veins that are crosscutting and/or parallel to the jaspilite bedding.
• The proximal alteration zone was also synchronous with the iron ore-forming event, and represents an advanced tage of alteration, and comprises the high-grade iron ore. It is characterised by progressive martitisation, forming anhedral hematite, continued open-space filling by comb-textured euhedral and tabular hematite in veinlets and along banding. This proximal alteration zone contains intense carbonate alteration associated with the high-grade ores, resulting in the production of ore breccias cemented by dolomite. Vein breccias are classified as V4 carbonate (iron cloud)-quartz breccia, and V5 (quartz ±microplaty hematite breccia, both of which occur within in high-grade ore.
Carajas Iron Deposit cross sections
Lobato, et al. (2008) record the following evidence for the influence of hydrothermal fluids. The first evidence for hydrothermal fluids infiltrating the jaspilites is the vein breccia type V1, which contains Ca-Fe rich, high-salinity (up to 29.3 wt.% CaCl
2 equiv.) fluid inclusions in quartz and carbonate with Ttrapping of 209 to 285°C. The next stage of hydrothermal fluid infiltration is characterised by vein type V2, containing medium- to high-salinity Na-Fe-Mg–rich (13.6 to 21.2 wt.% CaCl2 equiv.) and Ca rich fluid inclusions (6.8 to 18.4 wt % CaCl2 equiv.) with Ttrapping of 225 to 275°C and 190 to 295°C, respectively. Vein type V3 is characterised by low- and medium-salinity Ca-(Mg)-Fe-Na–rich inclusions (1.2 to 19.2 wt.% CaCl2 equiv.) with Ttrapping of 195 to 255°C and medium-salinity Na-Mg–rich fluid inclusions (8.9 to 14.4 wt % CaCl2 equiv.) with Ttrapping of 240 to 277°C. Brecciated vein types V4 and V5 have Ca-rich, medium- to high-salinity fluid inclusions in quartz and high-salinity inclusions in carbonate (9.7 to 24.5 and 19.2 to 30.1 wt.% CaCl2 equiv., respectively), both trapped at 237 to 314°C, and low-salinity Na-K-Mg fluid inclusions (0.2 to 7.3 wt.% NaCl equiv.) trapped at 245 to 316°C.

The age of the hypogene alteration and development of high-grade iron ore is considered to be Palaeoproterozoic (Lobato et al., 2005, 2008) and is well correlated with the A-type granites of the Carajás region (e.g., the ~1.8 Ga Serra dos Carajás Granite). Lobato, et al. (2008) also muse that a correlation may exist between the the upgrading of these iron ore deposits and the formation history of hydrothermal magmatic deposits, predominantly rich in Cu and Au (including the extensive IOCG mineralisation), that are distributed throughout the Carajás mineral province, in close proximity to the iron ores, as previously suggested by Lobato et al. (2005).

The ore mined in the Serra Norte comprises three main types:
Hard hematite which is a compact, blue-grey, massive hematite with a metallic lustre, high density and low porosity. Grades range from 65 to 69% Fe. It is the basis of the mines' export lump ore. This ore generally comprises the high grade mineralisation that has been upgraded from the protore jaspilites by Palaeoproterozoic hypogene alteration, but has not been substantially modified by more recent supergene oxidation.
Soft hematite, which is composed of massive to banded hematite, that is occasionally pulverised, and is highly porous, very weak and slightly magnetic, and has average grades of ~65% Fe. It is generally sufficiently friable to be mined without blasting and is the main source of sinter and pellet feed products. Supergene enrichment has upgraded hypogene altered jaspilite that ranged from strongly to weakly altered, as well as being responsible for the soft porous nature of the ore.
Canga, the uppermost unit, that drapes over the deposit and consists of a laterite-saprolite material from weathering of i). the underlying mineralisation (known as 'structural canga', or 'canga ore'), or ii). barren mafic wall rocks (known as Chemical Canga). It is composed of blocks of hematite cemented by hydrated iron oxides (goethite and limonite) and is generally 15 to 20 m thick.

The Carajás BIF/jaspilite unit is evident in the uncleared rain forest as a well demarcated corridor of stunted shrubs and grassland fringed by luxuriant trees over the enclosing mafic wall rocks. The BIFs, canga and their iron ore potential were recognised when a helicopter landed in one of these poorly vegetated clearings at the Serra Arqueada to resupply on July 31 1967. It carried geologist Breno Augusto dos Santos who was working for Cia. Meridional de Mineracao, a subsidiary of United States Steel Corporation, engaged in a regional exploration program looking for manganese. The recognition of the iron formations led to a change in emphasis of the exploration program and the discovery of the major Carajá Serra Norte deposits soon after.

Analyses of the various products in 2010 were, as follows (data provided during mine visit, 2010):
Sinter Feed - 66% Fe, 0.035% P, 1.40% SiO
2, 1.30% Al2O3, 0.65% Mn, with 20% >6.3 mm, 55% >1 mm and 18% <0.15 mm;
Pellet Feed - 65.3% Fe, 0.040% P, 1.40% SiO
2, 1.70% Al2O3, 0.65% Mn, with 5% >0.15 mm, 65% <0.045 mm;
Lump for domestic market - 63.3% Fe, 0.055% P, 2.30% SiO
2, 2.20% Al2O3, 0.95% Mn, with 1% >31.5 mm, 5% >25.0 mm, 12% <6.3 mm.

Reserves/Resources are distributed in a number of deposit groups, the largest of which is the Serra Norte (North Range) with - 6.2 Gt @ 65.8% Fe, 0.038% P, 1.0% SiO
2, 1.05% Al2O3, 0.45% Mn, 0.01% S, 0.02% K2O, 0.03% Na2O and 1.88% LOI. The other reserves include: Serra Sul, (South Range) 35 km to the south - 10.4 Gt @ 66.3% Fe; Serra Leste (East Range) - 400 Mt @ 65.9% Fe; and Serra do São Felix - 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 (Mining Technology website viewed December 2021). No resources estimates appear to have been released for the Serra Arqueada, Serra do Tarzan and Serra Bocaina resources. NOTE: Serra de São Felix is ~85 km west to WNW of the Serra Arqueada resource shown in the SW corner of the map above.

Proved + Probable Ore Reserves at the end of 2017 were as follows (Vale Form 20 Report to NYSE, 2017):
  Serra Norte - 2.1692 Gt @ 66% Fe; which includes N4W, N4E and N5 mines and N1, N2 and N3 deposits not in operation;
  Serra Sul - 4.1953 Gt @ 65.5% Fe; which include the S11C and S11D deposits;
  Serra Leste - 258.1 Mt @ 65.4% Fe;

Proved + Probable Ore Reserves at 31 December 2019 totalled 7.3463 Gt @ 65.9% Fe as follows (Vale Form 20 Report to NYSE, 2019):
  Serra Norte - 2.8237 Gt @ 65.5% Fe; which includes N3, N4W, N4E and N5 mines and N1, N2 project deposits not in operation;
  Serra Sul - 4.1981 Gt @ 66.3% Fe; include the S11C and S11D deposits;
  Serra Leste - 324.5 Mt @ 65.1% Fe;
Drill hole spacing used to classify the Reserves were: 150 x 100 m for Proved Reserves and 200 x 200 m for Probable Reserves.

Carajas Soft hematite ore
Soft hematite ore - porous, oxidised, physically weak mineralisation in the Carajás N4E mine. Image by Mike Porter, 2010.
Carajas partially altered jaspilite
Partially altered jaspilite - intermediate altered, sub-ore grade jaspilite on the margin of the Carajás N4E mine. Image by Mike Porter, 2010.
Carajas canga
Canga - cemented detrital ore overlying the hematite mineralisation of the Carajás N4E mine. Image by Mike Porter, 2010.
Carajas primary jaspilite
Fresh jaspilite - unaltered primary jaspilite in split drill core from below the base of oxidation from the Carajás Serra Norte. Image by Mike Porter, 2010.
Carajas hard hematite ore
Hard hematite - high grade, hypogene hard hematite in split drill core from below the base of oxidation from the Carajás district. Image by Mike Porter, 2010.

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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.
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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.

For detail of the regional setting, geology and stratigraphy of the host sequence and district see the Quadrilatero Ferrifero Iron Province

The deposit is developed on high grade ore within the Cauê Itabirite of the Minas Supergroup Itabira Formation. It is located 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.
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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 operated 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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This tour was designed, developed, organised, managed and escorted by
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.

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