CONTENT and DESCRIPTIONS OF ORE DEPOSITS
Porter GeoConsultancy, continued its International Study Tour series of professional development courses by visiting a representative selection of the major iron deposits illustrating a range of different ore styles in southern Africa and Brazil as the first of two modules.
- expert classroom workshop in Johannesburg,
Sishen Iron Deposit - Northern Cape, South Africa,
Khumani Iron Deposit - Northern Cape, South Africa,
Brazilian Iron Deposits - expert classroom workshop in Belo Horizonte,
Quadrilatero Ferrifero - field workshop, Minas Gerais, Brazil,
Mariana Iron Mine Complex - eastern Quadrilatero Ferrifero district,
Vargem Grande Iron Complex - western Quadrilatero Ferrifero district,
Carajás Iron Mine Complex & Setting - Pará, northern Brazil,
The tour commenced in Johannesburg, South Africa on the evening of Sunday18 April, 2010 and ended in Carajás, Pará, Brazil on the evening of Friday 30 April. Participants were able to take any 3 or more days, up to the full tour module, as suited their interests or availability.
The main components of the itinerary were:
The details of the tour were as follows:
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Expert Classroom Workshop - The Geology & Iron Deposits of Southern Africa ...................... Monday 19 April, 2010.
The workshop, which was held in Johanensburg, was led by internationally renowned expert Professor Nic Beukes of the University of Johannesburg, and included coverage of the following topics:
South Africa is Africa's leading iron ore producer, with an annual output of around 33 Mt, and 'reserves' of approximately 9 billion tonnes, 45% of which are located in the Northern Cape Province. Production comes from two main areas, the Sishen mine in the Northern Cape Province, accounting for over 24 Mt of high grade 65% Fe ore per annum, and the Thabazimbi Mine in the Northern Province. Thabazimbi generally provides iron to South African domestic smelters at Vanderbijlpark and Newcastle, at a rate of around 2 Mtpa, while almost all production from Sishen is exported through the port of Saldanha north of Cape Town. Other iron mines include Khumani and Beeshoek, both in the Sishen area.
- An overview of the geological and tectonic framework of iron ore mineralisation in Southern Africa, and geological controls and distribution of significant iron deposits within the same region.
- The factors influencing the development of ore grade iron mineralisation in southern Africa and the genesis of the key deposits.
- Descriptions of other significant deposits not to be visited and a comparison with those that are on the itinerary.
Although 700 km apart, Thabazimbi and Sishen are both developed on iron formations in the Palaeoproterozoic Transvaal Sequence, which was deposited between sometime after 2400 Ma and 2100 Ma within an intracratonic basin on the Kaapvaal Craton. The Kaapvaal Craton comprises a 3.4 to 2.8 Ga granite-greenstone basement, similar to the Zimbabwe Craton to the north which is separated by the 2.5 to 2.0 Ga Limpopo Mobile Belt. The granite-greenstone basement terrane is over lain by a series of intracratonic basins, the axes of which migrated north with time from the early "upper" Archaean to the late Palaeoproterozoic Waterberg System to the north.
The oldest of these, the Pongola System is a 10 000 m thick, approx. 3.0 Ga succession comprising a lower basaltic volcanic and lesser quartzite sequence overlain by alternating argillaceous and arenaceous sediments with intercalations of BIF. The overlapping ~9 000 m thick Witwatersrand Supergroup was deposited at between 3075 and 2715 Ma, commencing with early Dominion basaltic lavas, followed by a succession of mainly sandstones and shales with lesser locally gold bearing conglomerates. These were in turn followed by the up to 5000 m thick Ventersdorp Supergroup composed dominantly of basaltic volcanics dated at 2714 Ma.
The up to 12 000 m thick Transvaal Sequence was deposited unconformably on the Ventersdorp Sequence, and occurs in two connected depo-centres, the Transvaal and Griqualand West sub-basins, which define a 1100x350 km, NE-SW elongated area of remaining exposure. This sequence originally covered an area of approx. 500 000 sq. km.
In the Transvaal sub-basin, the Transvaal sequence commences with the up to 2000 m thick Wolkberg (or Buffalo Springs) Group of valley fill basalts and coarse clastics and lesser argillites. These are followed by the few tens to 500 m thick Black Reef Quartzite, which grades up into the around 3000 m thick Chuniespoort Group which comprises the lower up to 2000 m thick Malmani Dolomite, variably composed of dolomite and chert, and the overlying up to 600 m thick Penge Iron Formation.
The Penge Iron Formation is the host to the Thabazimbi iron deposit and is composed of alternating carbonaceous shale and macro-, meso- and micro-banded BIF (quartz- magnetite- hematite- stilpnomelane- riebeckite- minnesotaite- grunerite and ferriferous carbonates). The uppermost member of the Chuniespoort Group is represented by the dolomites, quartzites and shale of the locally preserved Duitschland Formation. The Chuniespoort Group is unconformably overlain by the 7000 m thickness of quartzites, shales and minor basalts of the Pretoria Group and the 2-3000 m of rhyolitic lavas that make up the ~2100 Ma Rooiberg Group.
In the Griqualand West sub-basin, the Transvaal Sequence is represented by the Ghaap Group, which is unconformably overlain by the Postmasburg Group. The Ghaap Group is sub-divided into the lower interbedded silici-clastics and carbonates of the Schmidtsdrif Subgroup followed by the limestones and dolomites of the Campbellrand Subgroup. These are overlain by the Asbesheuwels Subgroup which is sub-divided into the lower Kuruman Iron Formation, composed of interlayered carbonaceous shale and a chert- carbonate- stilpnomelane- magnetite- hematite- greenalite- siderite iron formation, and the upper Griquatown Iron Formation, comprising siderite-hematite and siderite-greenalite lutites. The Asbesheuwels Subgroup is host to the giant Sishen iron deposit. The Ghaap Group is unconformably overlain by the Postmasburg Group, commencing with the thin Makganyene Diamictite, the thick Ongeluk basaltic pillow lavas, followed in turn by the jasper, BIF and sedimentary manganese deposits of the Hotazel Formation, and finally the Mooidraai Dolomite. These are in turn unconfoirmably overlain by the shales and red-bed sanstones of the Olifantshoek Group.
The Transvaal Sequence is unconformably overlain to the north by the extensive thick arkosic arenites of the 2000 to 1700 Ma Waterberg Group.
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The Sishen Iron Deposit & its setting ...................... Tuesday 20 April, 2010.
The Sishen iron ore deposit is situated in the Northern Cape Province of South Africa, 280 km north of Kimberley and 800 km north-east of its export port at Saldanha Bay north of Cape Town, to which it is connected by a 1 m gauge rail line. It is just to the SW of the township of Kathu (#Location: 27° 45' 32"S, 23° 0' 0"E).
The mine is owned and operated by Kumba Resources Limited and produces around 27 Mt pa of 65% Fe hard hematite ore with a high lump proportion, and impurity levels of generally less than 0.05% P, 2.5% SiO2 and 1.2% Al2O3.
The major iron deposits of South Africa, which include Thabazimbi and Sishen that are 700 km apart, are developed on iron formations in the Palaeoproterozoic Transvaal Sequence, which was deposited between ~2.4 and 2.1 Ga within an intracratonic basin on the Kaapvaal Craton. The Kaapvaal Craton comprises a 3.4 to 2.8 Ga granite-greenstone basement, which is separated from the Zimbabwe Craton to the north by the 2.5 to 2.0 Ga Limpopo Mobile Belt. The granite-greenstone basement terrane is overlain by a series of intracratonic basins, the axes of which migrated northward with time from the early Neoarchaean to the late Palaeoproterozoic Waterberg System to the north.
The oldest of these, the Pongola System is a 10 000 m thick, ~3.0 Ga succession comprising a lower basaltic volcanic and lesser quartzite sequence overlain by alternating argillaceous and arenaceous sedimentary rocks with intercalations of BIF. The overlapping ~9000 m thick Witwatersrand Supergroup was deposited at between 3075 and 2715 Ma, commencing with early Dominion basaltic lavas, followed by a succession of mainly sandstones and shales with lesser locally gold bearing conglomerates.These were in turn followed by the up to 5000 m thick Ventersdorp Supergroup composed dominantly of basaltic volcanic rocks dated at 2714 Ma.
The up to 12 000 m thick Transvaal Sequence was deposited unconformably on the Ventersdorp Sequence, and occurs in two connected depo-centres, the Transvaal and Griqualand West sub-basins, which define a 1100 x 350 km, NE-SW elongated area of remaining exposure. This sequence originally covered an area of ~500 000 sq. km.
In the Transvaal sub-basin, the Transvaal sequence commences with the up to 2000 m thick Wolkberg (or Buffalo Springs) Group of valley fill basalts and coarse clastics and lesser argillites. These are followed by the few tens to 500 m thick Black Reef Quartzite, which grades up into the around 3000 m thick Chuniespoort Group that comprises the lower up to 2000 m thick Malmani Dolomite, variably composed of dolomite and chert, and the overlying up to 600 m thick Penge Iron Formation. The Penge Iron Formation is the host to the Thabazimbi iron deposit and is composed of alternating carbonaceous shale and macro-, meso- and micro-banded BIF (quartz- magnetite- hematite- stilpnomelane- riebeckite- minnesotaite- grunerite and ferriferous carbonates). The uppermost member of the Chuniespoort Group is represented by the dolomites, quartzites and shale of the locally preserved Duitschland Formation. The Chuniespoort Group is unconformably overlain by the 7000 m thickness of quartzites, shales and minor basalts of the Pretoria Group and the 2 to 3000 m of rhyolitic lavas that make up the ~2100 Ma Rooiberg Group, marking the stage of emplacement of the Bushveld Complex.
In the Griqualand West sub-basin, the Transvaal Sequence is represented by the Ghaap Group, which is unconformably overlain by the Postmasburg Group. The Ghaap Group is sub-divided into the lower interbedded silici-clastics and carbonates of the Schmidtsdrif Subgroup followed by the limestones and dolomites of the Campbellrand Subgroup. These are overlain by the Asbesheuwels Subgroup which is sub-divided into the lower Kuruman Iron Formation, composed of interlayered carbonaceous shale and a chert-carbonate-stilpnomelane-magnetite-hematite-greenalite-siderite iron formation, and the upper Griquatown Iron Formation, comprising siderite-hematite and siderite-greenalite lutites. The Asbesheuwels Subgroup is host to the giant Sishen iron deposit. The Ghaap Group is unconformably overlain by the Postmasburg Group, commencing with the thin Makganyene Diamictite, the thick Ongeluk basaltic pillow lavas, followed in turn by the jasper, BIF and sedimentary manganese deposits of the Hotazel Formation, and finally the Mooidraai Dolomite. These are in turn unconformably overlain by the shales and red-bed sanstones of the Olifantshoek Group.
The Transvaal Sequence is unconformably overlain to the north by the extensive thick arkosic arenites of the 2000 to 1700 Ma Waterberg Group.
The Sishen deposits are hosted by the Palaeoproterozoic Transvaal Supergroup within the Griqualand West sub-basin. It is situated on the Maremane Dome which is defined by the carbonate sequence of the Transvaal Supergroup, the Campbellrand Subgroup and the overlying host iron formations of the Asbesheuwels Iron Formation. These units dip outwards at less than 10 degrees on the eastern margin of the dome. In the central and western sections of the dome, this part of the sequence is concealed by the unconformably overlying red-bed clastic Gamagara Formation of the Olifantshoek Group.
A unit of ferruginous chert breccia (the Wolhaarkop Breccia) which grades upwards into a distorted banded iron formation (the Manganore Iron Formation) is wedged between the underlying Campbellrand carbonates and the unconformity at the base of the Gamagara Formation. The Wolhaarkop Breccia is matrix supported and consists of unsorted angular chert fragments in a hematite to manganese bearing siliceous matrix.
Some 80 to 90% of the ore at the north-south elongated 12 x 1.5 km Sishen deposit is hosted by the Manganore Iron Formation, which is correlated with the Asbesheuwels Iron Formation, and is found immediately below the unconformity with the overlying clastic Gamagara Formation. Subsequent to deposition, Asbesheuwels Iron Formations are interpreted to have locally slumped onto a palaeo-sinkhole dominated surface developed in the underlying Cambellrand sub-group carbonates to produce the Wolhaarkop Breccia and Manganore Iron Formation during the period of erosion prior to the deposition of the Gamagara Formation.
Silica is believed to have been leached from the slumped and brecciated iron formation by alkaline supergene fluids at the time of slumping, while ferrous ions were oxidised to hematite, and additional transported supergene iron was added. Erosion of the hematite mineralisation and resultant accumulations of hematite pebble conglomerates in alluvial fan environments are preserved as the Doornfontein Conglomerate at the base of the Gamagara formation and account for around 10-20% of the orebody. The Doornfontein Conglomerate is best developed immediately adjacent to pockets of Manganore Iron Formation.
The Manganore Iron Formation is more restricted in areal extent than the Wolhaarkop Breccia. It is composed of 7 zones/bands from the base, namely: Zone 1 - a spotted carbonaceous and dark brown shale with chert pillows and hematite nodules; Zone 2 - hematite micro-banded white chert with interbeds of intercalated cherts and black-brown shale; Zone 3 - chert banded hematite ferhythmites with cycles of hematite-lutite to hematite-microbanded chert, to hematite ribbon, wave and pillow-rhythmite; Zone 4 - hematite rhythmites that represent the bulk of the banded ore; Zones 5 and 6 of hematite-greenalite banded lutites. The first 6 zones were derived from the Kuruman Iron Formation, the lower of the 2 Asbesheuwels units, while a zone 7 composed of hematite lutite with meso-bands of peloidlutite is correlated with the overlying Griquatown Iron Formation. Chert bands within the Manganore Iron Formation are porous, with partially infilling platy hematite. Boundaries of high grade ore cut across primary sedimentary boundaries.
Three types of ore are present in the Manganore Iron Formation, namely:
i). Laminated ore - composed of both thickly laminated alternating massive, porous hematite meso-bands with dull and bright micro-banded 2 to 15 mm thick meso-bands, and of thinly laminated micro-banded hematite comprising very thin microbanded high lustre hematite with equally thin earthy hematite meso-bands;
ii). Massive ore - massive to very poorly bedded, fine grained, porous aggregates of platy hematite;
iii). Breccia ore - of two types which may be developed anywhere in the iron formation, comprising oligomictic hematite rhythmite breccia derived from laminated ores, and oligomictic hematite-lutite breccia.
The Makganyene diamictite and Ongeluk Lava of the Postmasburg Group which unconformably overlies the Ghaap Group and unconformably underlies the Gamagara Formation of the Olifantshoek Group, were subsequently thrust over the deposit and host sequence from the west. Much of the sequence in the region is concealed by around 50 m of Tertiary Kalahari Formation cover.
The 2001 mineral reserve estimate for the Sishen deposit was 877 Mt, and the mineral resource estimate was 1724 Mt. Some 70 km to the south at the Sishen South (or Welgevonden) deposit there is a high quality resource of 259 Mt of high lump ore to be developed by 2006.
In 2005, the proved + probable reserve at Sishen totalled 1021 Mt @ 59.0% Fe, plus a quoted resource of 1957 Mt @ 57.2% Fe.
The proved + probable reserve at Sishen South in the same year was 167 Mt @ 64.2% Fe, plus a resource of 248 Mt @ 65% Fe.
In 2008, the proved + probable reserve at Sishen totalled 956.9 Mt @ 59.6% Fe, plus a quoted resource of 1628 Mt @ 56.4% Fe.
The proved + probable reserve at Sishen South in the same year was 214.1 Mt @ 64.1% Fe, plus a resource of 153.2 Mt @ 64.3% Fe.
Reserves and resources from Kumba Iron Ore website and annual report.
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The Khumani Iron Deposits & their setting ...................... Wednesday 21 April, 2010.
The 10 mtpa Khumani iron ore mine (Assmang Limited), formerly the Bruce, King and Mokaning ("BKM") Project which refers to the farms on which the iron ore resources are located, is situated in the Northern Cape province of South Africa. The iron ore deposits are located adjacent to Kumba Iron Ore's Sishen mine, and approximately 60 km north of Assmang's Beeshoek mine.
As at Sishen, the deposits are hosted within a sequence of Palaeoproterozoic sediments of the Transvaal Supergroup and lie on a limb of the Maremane anticline. The ore is situated along the contact between the Gamagara formation and the underlying Manganore iron formation in the northern part of the regional domal structure. Two ore types are present: i). laminated hematitic ore of the Manganore iron formation, similar to that at Sishen, and ii). conglomerate ore of the Doornfontein Conglomerate member at the base of the Gamagara formation. The distribution of the Manganore iron formation is restricted, only being preserved in pockets above the Wolhaarkop Breccia below the Gamagara unconformity. The basal Doornfontein Conglomerate is best developed where it overlies the Manganore iron formation. The Khumani deposits are characterised by large stratiform bodies and prominent hanging wall outcrops. The down-dip portions of the iron formation are well developed, although in outcrop, the deposits are thin and isolated.
Hematite is the dominant ore mineral, although limonite and specularite are also present. The K2O content of the ore apprears to be directly related to the abundance of muscovite and illite, while the Al2O3 is the result of kaolinite and muscovite in the ore. The ore is depleted in Na and P, with the latter occurring as trace apatite.
Ore is present in two main areas, at Bruce and King, which are approximately 10 km apart. The Bruce orebodies are to be mined from three main and four or five satellite pits, some of which abut the eastern Sishen boundary. The larger Bruce pits will cover areas of from 2000 to 750 m x 750 to 500 m in area with final depths of 95 to 170 m. The King orebody will be mined as part of the expansion to a 16 mtpa rate in two pits, the larger of which will cover an area of 3000x1000 m, exploiting near surface ore to the west, and following it down dip to ultimate depths of 230 m.
Iron ore is mined from a series of open pits and hauled to the primary and secondary crushers, from where, it is transferred by conveyor to stockpiles ahead of the beneficiation plant. The run-of-mine ore will be stockpiled on blending beds in two categories, 'on grade' and 'off grade'. On-grade material will then be washed and screened to produce the final products incorporating tertiary crushing of the oversize material from the screening plant. Off-grade material will also be washed and screened and the oversized material crushed in the tertiary crushers and, thereafter, beneficiated through jigs to remove contaminants. Products will be stockpiled ahead of transport via the 860 km Sishen-Saldanha railway line to the port of Saldanha Bay on South Africa's West Coast from which the iron ore is being exported. Three products are being produced, lumpy, DR lump and fines, with respective grades of 66, 65.5 and 65% Fe.
Total proven + probable reserves as at June 30, 2007, were 444.7 Mt @ 64.7% Fe, 0.14% Mn, 0.25% K2O, 1.77% Al2O3, 0.04% P and 3.8% SiO2.
Total measured + indicated + inferred resources at the same date, were 685.5 Mt @ 64.6% Fe.
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Travelling from the Northwest Cape Province, South Africa, to Belo Horizonte, Minas Gerais, Brazil ...................... Thursday 22 & Friday 23 April, 2010.
Brazilian Iron Deposits - Classroom Workshop ................................ Saturday 24 April, 2010.
A one day workshop will be led by internationally renowned expert Dr Carlos Alberto Rosiére of the Universidade Federal de Minas Gerais in Belo Horizonte providing an outline of the distribution, setting, geology and mineralisation of the major iron producing districts and deposits of Brazil.
Brazil is the world's largest iron ore exporter with an annual production of over 250 Mt, of which over 70% is exported. The majority of this comes 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.
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Quadrilatero Ferrifero Field Workshop ................................ Sunday 25 April, 2010.
A field workshop led by Dr Carlos Alberto Rosiére of the Universidade Federal de Minas Gerais will study the geology and setting of the host successions to the major iron deposits of the Quadrilatero Ferrifero.
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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Mariana Iron Complex ................................ Monday 26 April, 2010.
For safety considerations, the visit to the Itabira complex was replaced by the Mariana complex, which comprises the large Alegria mine that exploits martitic ores, while the nearby Fazendao mine has specularitic highly deformed ores which are very similar to those at the Itabira 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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Vargem Grande Iron Complex ................................ Tuesday 27 April, 2010.
The Vargem Grande Iron Complex, comprising the Tamanduá, Capitão do Mato and Abóboras mines, is located approximately 30 km south of Belo Horizonte, and 80 km southwest of the Ibabira Complex mines in Minas Gerais state, Brazil.
This group of mines extend over a 15 km interval of the north-south trending eastern limb of the Moeda Syncline towards the western margin of the Quadrilátero Ferrífero. Mineralisation is hosted by the Cauê Itabirite at the base of the Itabira Formation of the Minas Supergroup. The iron formation is underlain by the phyllites, cherts, iron formations and quartzites of the Caraca Group, and is overlain by dolomite, limestone, lesser phyllite and further itabarites of the Gandarela Formation; while the overlying core of the Moeda Syncline is occupied by the quartzite, phyllite and dolomite lenses of the Piracicaba Group.
As of December 31, 2005, proved + probable reserves at the Vargem Grande complex mines were as follows:
Tamandua - 99.3 Mt @ 66.5% Fe,
Capitão do Mato - 147.5 Mt @ 66.2% Fe,
Abóboras - 32.2 Mt @ 66.0% Fe.
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Travelling from Belo Horizonte, Minas Gerais, to Carajá, Pará:, Brazil ...................... Wednesday 28 April, 2010.
Carajás Iron Complex ................................ Thursday 29 and Friday 30 April, 2010.
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% SiO2.
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.
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.% CaCl2 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% SiO2, 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% SiO2, 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% SiO2, 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% SiO2, 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.
Soft hematite ore - porous, oxidised, physically weak mineralisation in the Carajás N4E mine. Image by Mike Porter, 2010.
Partially altered jaspilite - intermediate altered, sub-ore grade jaspilite on the margin of the Carajás N4E mine. Image by Mike Porter, 2010.
Canga - cemented detrital ore overlying the hematite mineralisation of the Carajás N4E mine. Image by Mike Porter, 2010.
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.
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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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Most of these sources are listed on the "Tour Literature Collection" available from the Iron 2010 Tour options page.
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