The Casa de Pedra mining complex (Mina Casa de Pedra and Mina do Engenho) property covers an area of 4,701.55 ha and is owned by CSN Mineração S.A.
CSN Mineração was consolidated in 2015 from the merger of the mining assets of CSN (Companhia Siderúrgica Nacional) and Namisa (Nacional Minérios S.A.) and was divided between CSN (87.52%) and a consortium composed of large Asian steelmakers (12.48%).
Compania Siderurgica Nacional S.A. now owns 78.2% of CSN Mineração mines.
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Summary:
The iron oxide deposits of the Casa de Pedra mining complex are located within the Quadrilátero Ferrífero, an area of approximately 7,000 km2 within the San Francisco Craton in the central part of Minas Gerais, Brazil.
The Minas Supergroup includes four groups: Caraça, Itabira, Piracicaba and Sabará. The thickest sequence of banded iron formation (BIF) with economically exploitable high-grade iron oxides belong to the Itabira Group, which consists of itabirites, dolomites and subordinately, metapelites. The itabirites are metamorphosed and strongly oxidized iron formations with discontinuously high iron contents, a more or less lenticular morphology and dimensions ranging from a few decimeters to hundreds of metres (Rosiére and Chemale, 2000).
The iron formations of economic interest in the property are hosted within the Minas Supergroup, Itabira Group and Cauê Formation.
The area of the Casa de Pedra mining complex is characterized by several occurrences of iron oxide, where the main hematitic bodies occur as lenses in the northwest-southeast direction. These lenses are present along a 50 km synformal structure, with a normal flank in a north-south direction to the west and an inverted flank to the east, surrounding the Bação Metamorphic Complex in the south. The Casa de Pedra mining complex is located on the western hinge of the Sinclinal Moeda.
The Casa de Pedra mining complex area has undergone three deformational events which control the attributes between the Main and Western deposits. These two deposits exhibit distinct differences in mineralogy and texture because of the individual structural domains. The Main deposit is composed of granular hematite and magnetite (itabirite) with porosity averaging 20%, compared to the West body which is richer in specularite (specular hematite) and goethite with lower porosities averaging 17%.
Within the Casa de Pedra mining complex, lithologies are defined from oldest to youngest as:
- Chlorite schists (Nova Lima Group – Rio das Velhas Supergroup)
- Quartzites (Caraça Group – Moeda Formation)
- Carbonous phyllites (Grupo Caraça – Batatal Formation)
- Iron formations and manganese rocks (Grupo Itabira – Cauê Formation)
- Dolomites and dolomitic phyllites (Grupo Itabira – Gandarela Formation)
- Sericite schists - (Grupo Piracicaba – Cercadinho Formation)
- Mafic (basic) intrusives, primarily metagabbro
- Tertiary deposits (stratified clays)
- Quaternary cover.
The Cauê Formation is broken into seven distinct units for mining purposes: hematites (>64% Fe), rich itabirites (58–64% Fe), siliceous itabirites, goethitic itabirites, carbonate itabirites and manganese itabirites. These units form the basis for the mineral exploration program and mineral resource estimation domains.
Mineralized zones
At the Casa de Pedra mining complex, enriched iron oxide is present as compact (hard) or soft/friable hematite, semi-compact to compact itabirites, composed of millimetre to centimetre bands of hematite and/or magnetite with interlayered quartz.
The hematites are characterized by elevated levels of iron oxide (>64% Fe) and by having small quartz interstratifications. In most of the deposit they have a friable behaviour.
The itabirites have alternating millimetre to centimetre bands of iron oxide, in general magnetite and hematite with quartz bands. Rich itabirites are characterized by contents ranging from 58% to 64% Fe and have the same characteristics as hematites in general.
The Casa de Pedra mining complex deposits typically formed from weathering processes, however, there is a strong contribution from tectonic and lithological conditions. Commonly, the iron oxide bodies present complex geometries influenced by the diversity of structural and lithological controls. Weathering generated material more enriched in iron and/or with appropriate granulometric characteristics for mineral processing.
Iron oxide enrichment through supergene processes is controlled along northwest-southeast thrust faults that bound the Main and West bodies. Hypogene alteration is evident through the presence of martite (hematite pseudomorphs after magnetite). The Cauê Formation consists of BIF, amphibolitic BIF and dolomitic BIF with subordinate manganese and phyllite-rich zones. Stratigraphically above the Cauê Formation lies the Gandarela Formation consisting of carbonates and phyllitic itabirite.
The geometry of the deformation within the two host deposits varies slightly due to the influence of regional structures. The West body exhibits foliation and bedding in a north-south direction until truncated by the main thrust fault separating the deposits. The more deformed Main body exhibits a northwestsoutheast foliation roughly perpendicular to the highly deformed bedding.