Summary:
Vale's Southern System consists of iron deposits located in the Quadrilatero Femfero (Iron Quadrangle), in the south central part of the State of Minas Gerais.
Ore reserves with high ratios of itabirite ore type relative to hematite ore type. Itabirite ore has iron grade of 35-60%.
The Quadrilátero Ferrífero (QF) comprises an area of about 7000 km2 and hosts one of the largest concentrations of lateritic iron-ore deposits in the world. The QF is located in central Minas Gerais state, southeastern region of Brazil (Dorr, 1965). The QF has a quadrangular shape due to the dome and basin structures of the Minas Supergroup (Fig. 1). Most of the iron ore in the QF are hosted in the oxidized, metamorphosed and heterogeneously deformed Banded Iron Formations (BIFs) of the Cauê Formation, the so-called itabirites. The BIFs are altered sedimentary deposits with laminated rocks formed by alternating layers of silica and hematite magnetite, as well as carbonates and iron silicates.
The Quadrilátero Ferrífero is located at the southeastern border of the São Francisco Craton, southeastern Brazil (Fig. 1). According to Alkmim and Marshak (1998) the most important lithostratigraphic units of the Quadrilátero Ferrífero are: (1) the Archean crystalline basement, (2) the Rio das Velhas Supergroup, (3) the Minas Supergroup, (4) the Post-Minas Intrusives and (5) the Itacolomi Group.
The main iron ore types in the Iron Quadrangle are:
Hematite: Hematite represents the high-grade ore type within the iron deposit. The iron content varies from 65 to 67 percent. It is either massive or foliated in nature. The hematite is classified according to its physical and chemical characteristics as Compact Hematite, Friable Hematite, and Goethitic / Argilitic Hematite. Its origin is related to hydrothermal or metasomatic processes.
Itabirite: Itabirite is a term widely used in Brazil to denote a metamorphosed iron formation composed of iron oxides (hematite, magnetite, and martite), abundant quartz, very rarely mica, and other accessory minerals. It may be foliated or compact. The un-enriched (poor) itabirites from the Quadrilatero Ferrffero tend to have little magnetite, and composed principally of quartz-hematite, quartz-hematite-carbonate and hematite-carbonate. Itabirite represents the majority of reserves and resources of the Iron Quadrangle deposits. According to Vale, it was originated by silica leaching and residual iron-oxide enrichment process during post-metamorphic weathering cycles. The iron content averages about 45 percent, and occasionally it may reach up to 60 percent. The Itabirite ore is classified according to its physical and chemical characteristics as Compact Itabirite, Friable Itabirite, Goethitic / Argilitic Itabirite or Ochre.
Canga: Canga ore consists of unconsolidated talus material formed by the weathering of the iron formation. The iron content ranges significantly, with generally high concentration of phosphorous and alumina.
Rolados: A second detrital mineralization type. Consists of a ferruginous or lateritic matrix with fragments and blocks of different materials such as hematite, itabirite, quartz; high Fe grade, with high contaminant levels, such as P and Al203.
Joao Pereira and Alto Bandeira mines are situated within a 40 km long east-west trending limb of the Dorn Bosco syncline of the Iron Formation. The Segredo Mine consists of the Ponto 2, Ponto 3, Area 10 and Segredo deposits. The mine area is surrounded by the western portion of the Dom Bosco syncline, near the Moeda syncline.
The stratigraphic sequence of the region consists of five geological units:
¦ Gneisses of the Achaean crystalline basement;
¦ Phyllites and quartzites of the Rio das Velhas Supergroup;
¦ The Iron Formation, metasediments and orthoquartzites of the Minas Supergroup;
¦ Quartzites and meta-conglomerates of the Itacolomi Group;
¦ Recent cover, including "Canga," laterite and "Rolados."
The crystalline basement rocks are banded gneisses, migmatites, and granites of various compositions, and the metamorphic complexes of Bonfim and Belo Horizonte. The Rio das Velhas Supergroup (from the base to the top) is composed of the Nova Lima group and the Maquine group. The Nova Lima group consists of phyllites, graphitic phyllites, sericite schists, metagreywackes, mafic and ultramafics, Algoma type iron formations, metacherts and dolomites. The bottom of the Maquine group is defined by an erosional unconformity. Basal conglomerates are overlain in turn by massive quartzites, schists, sericite- quartz schist, and phyllites.
The Minas Supergroup consists of the Caraga, Itabira, Piracicaba, and Sahara Groups. The Caraga group is made up of the Moeda and Batatal Formations and occurs in a continuous belt in the western portion of the area in the Serra da Moeda. The Moeda Formation is composed of metaconglomerates, phyllites, and fine grained quartzites with sericite and muscovite. The Batatal Formation is composed of sericitic phyllite, and lesser metachert, iron formation and graphitic phyilite.
The Itabira Group is represented by the Caue and Gandarela Formations. From the base, the Caue Formation is composed of itabirites, dolomitic itabirites, and amphibolitic itabirites, and is approximately 200 m to 400 m thick. The Gandarela Formation forms a gradational contact with the Caue Formation and is composed of dolomitic phyllites, dolomites and phyllites.
The Piracicaba Group of rocks crop out along the entire length of the Serra do Curral, and are represented from the bottom to the top by the Cercadinho, Fecho do Funil, Ta booes, and Barreiro Formations. The Cercadinho Formation is composed of quartzites, ferruginous quartzites, phyllites, and dolomites. The Fecho do Funil Formation is composed of dolomitic phyllites, phyllites, and impure dolomites. The Tabooes Formation consists of fine-grained, equigranular orthoquartzites, whereas the Barreiro Formation is composed of predominantly phyllites and graphitic phyllites.
The Sahara Group crops out to the north of Serra do Curral, and is composed of sericitlc phyllites, chlorite and biotite schists, metagreywackes, quartzites, felsic quartzites, iron formations, itabirites and m eta conglomerates with intercalations of dolomites. The Itacolomi Group is composed of phyllites, silicic phyllites, and quartzites. Mafic intrusives are found in granite-gneiss complexes. The intrusive rocks are in the form of dikes, plugs, and small stocks, and may reach up to 500 m in length. The Itacolomi Group is composed of phyllites, quartzitic phyllites, metaconglomerates, quartzites, and ferruginous quartzites, which are very similar to itabirites.