Rio Pardo - Rio do Peixe Bravo District, Salinas, Mtansminas, Jiboia

Minas Gerais, Brazil

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The Neoproterozoic Rio Pardo and Rio do Peixe Bravo district iron deposits of Northern Minas Gerais, Brazil, are located ~200 km south of Caetité, 400 km west of the Atlantic coastline and 475 km NNE of the major late Neoarchaean to early Palaeoproterozoic Quadrilatero Ferrifero iron deposits near Belo Horizonte in the south of the same state. Individual deposits include Salinas (Hornbridge), Mtansminas, Jiboia and Gema Verde.

These extensive deposits are associated with iron-rich, glaciogene sedimentary rocks of the Macaúbas Group, a major stratigraphic unit in the northwestern Araçuaí Basin. The iron-rich unit is evident by marked aeromagnetic anomalies that extend over a folded strike length of >250 km.

The Macaúbas Group is interpreted to be an equivalent of the Jacadigo Group, which hosts the similar Corumba/Urucum iron deposits in the Brazilian state of Mato Grosso do Sul, 1500 km to the west. See the Corumba/Urucum record for details.

Regional Setting

The Araçuaí Basin is an extensional rift trough, containing rift and passive margin sedimentary and bimodal magmatic rocks. Minor mafic-ultramafic ophiolitic rocks are found in the centre of the basin. It is bounded to the west and north by an embayment of the São Francisco craton, and to the east by the Congo craton in Africa, with a combined preserved width on both side of the Atlantic Ocean of ~750 km and NNW-SSE length of > 1000 km. The São Francisco and Congo cratons were amalgamated during the Palaeoproterozoic at ~2.17 to 2.09 Ga, and remained joined by the Bahia-Gabon cratonic bridge to the north of the Araçuaí Basin, until the Cretaceous opening of the Atlantic Ocean. Deposition within the Araçuaí Basin, and the Congo Belt in Africa, commenced in the early Neoproterozoic (post 875 Ma and post 912 Ma respectively), with the main deposition related to the Tonian-Ediacaran extensional-rift event. This extension only influenced the continental cratonic bridge in discrete zones, where mafic dykes and alkaline intrusions were emplaced. The Brasiliano orogenic event (~630 to 500 Ma) inverted the precursor basin to form the Araçuaí-West Congo orogen, but affected only the edges of the Bahia-Gabon bridge that survived until the opening of the Atlantic Ocean (Pedrosa-Soares et al., 2008; Da Silva et al., 2008).

Tectonically, the Araçuaí Basin in Brazil can be divided into two distinct domains, an outer zone, adjacent and parallel to the São Francisco craton on the western and northern margins of the basin, characterised respectively by a west- and north-verging fold and thrust belt, with increasing metamorphism from the margin towards the core of the basin (greenschist to amphibolite), and an internal zone of high grade metamorphism and large anatectic, S-type granitoids intruded preceding and during basin inversion. The internal domain is usually west-verging, but is east verging to the east.

The basement to the Araçuaí Basin sequence comprises Archaean to Palaeoproterozoic rocks, and the Mesoproterozoic Espinhaço Supergroup. The Macaúbas Group overlies this basement and varies from a few tens of metres on the basin margin, up to 10 km thick in the main passive margin succession. The stratigraphic subdivision of the group, comprises, from the base (after Pedrosa-Soares et al., 2008):
Duas Barras and laterally equivalent Rio Peixe Bravo Formations within the internal basin - sandstone and arkose, with conglomerate lenses, which lack evidence of glaciation and are fluviatile;
Serra do Catuni Formation and laterally equivalent Nova Aurora Formations within the internal basin - the Serra do Catuni Formation is composed of tillite with minor fluviatile sandstone, varvite and esker deposits (glacio-terrestrial sediments) to the west, and proximal glaciomarine diamictite with lenses of sandstone and pelite to the east. This sequence was thrust over the Neoproterozoic Bambui Group terrestrial cover of the São Francisco craton on the western margin of the basin, indicating the Serra do Catuni Formation is older than 740±22 Ma (the age of the lowermost Bambui Group), while mafic dykes and anorogenic granites suggest deposition at ~906 to 975 Ma (older than Sturtian); The Nova Aurora Formation overlies the Rio Peixe Bravo Formation and comprises a thick glaciomarine sequence of diamictite, containing thick layers of iron formation with a few scattered dropstones, and minor graded sandstones and pelites;
Chapada Acauã Formation - the uppermost and distal diamictite-bearing unit of the Macaúbas Group. The lower section consists of marine debris flows (diamictite and minor clast-supported conglomerate) with lenses of sandstone and pelite, followed by turbiditic graded sandstones and pelites with dropstones, intercalated with diamictite. This diamictite may be Sturtian in age, based on basin-wide correlations. Greenschist facies mafic volcanic rocks, mainly pillowed, tholeiitic, 'within-plate' basalts. The uppermost unit within the formation is a limestone, interbedded with diamictite and pelite, overlain by post-glacial interlayered sandstones and pelites. The upper diamictite in the West Congo Belt (absent in Brazil) may correlate with the ~636 Ma Marinoan glaciation;
Ribeirão da Folha Formation - the distal part of the Macaúbas Group in the passive margin sequence in the internal section of the basin, is diamictite-free, and is divided into two lateral units. The the lower part of the proximal of these units, overlies the uppermost diamictites in the deep-water equivalents of the Chapada Acauã Formation shelf sandstone-pelite unit, and comprises a succession of fine-grained turbidites with thin calc-silicates (after marl) and limestone lenses. The distal unit is locally interleaved with thrust-slices of oceanic meta-mafic and meta-ultramafic ophiolitic rocks (after dolerites and 'ocean-floor' basalts), and is dominated by peraluminous mica-schists with intercalated sulphidic cherts and diopside rocks associated with massive sulphide bodies, graphite schist and oxide, silicate and sulphide-type banded iron formations. This distal unit has been metamorphosed from west to east to garnet, staurolite, kyanite and sillimanite zones. Meta-ultramafic slices interleaved with the distal unit comprise slabs up to 1 km thick and comprise tremolite and talc-anthophyllite schists, and serpentinites with chromite-bearing meta-peridotite layers. These ophiolites, taken to represent oceanic crust flooring the internal part of the basin, have been dated at ~800 Ma;
Capelinha and Salinas Formations - which both unconformably overlie, and have structural contacts with, the Ribeirão da Folha Formation, and respectively comprise proximal sandstone, pelite and detrital iron formation, deposited in a fluviatile environment, and distal greywacke-pelites intercalated with clast-supported conglomerates, locally including abundant volcanic clasts. Both units have been metamorphosed to greenschist to amphibolite facies, and have the characteristics of a sequence deposited in a tectonically active environment. The maximum age of the Salinas Formation is 588 Ma, based on detrital zircons.

The basin inversion magmatism associated with the Brasiliano event, has been divided into four stages (after Pedrosa-Soares et al., 2008):
Pre-inversion 630 to 585 Ma - mainly I-type plutonic batholiths and stocks of tonalite, granodiorite and minor diorite, intruding country-rock that includes migmatised complexes with high-grade 630 to 590 Ma biotite-garnet-cordierite/sillimanite paragneisses of the Nova Venécia Complex;
Syn-inversion 585 to 560 Ma - comprising S-type, garnet±cordierite±sillimanite granites and two-mica granites, commonly with mylonitic textures;
Late-inversion 560 to 530 Ma - composed of S-type garnet- and/or cordierite leucogranite, occurring as veins or small intrusions;
Post-inversion 530 to 490 Ma - representing the collapse of the Araçuaí orogen, with a major shear zone, east-verging folds and antithetic west-dipping major faults. Associated early S-type biotite, muscovite and/or garnet granitoids and later I-type granites, which include charnockitic, enderbitic and mafic fractions form circular, often clustered intrusions, and lack the regional foliation.

Note: Pedrosa-Soares et al. (2008) interpret a setting involving east-west extension and the opening of a continental rift, filled by a rift sedimentary-bimodal volcanic sequence, followed by separation and introduction of limited oceanic crust, and a passive margin sucession. This is consistent with the recorded observations. They then propose east-west compression, accompanied by subduction of the limited oceanic crust to produce a voluminous magmatic (intrusive only) arc and back-arc basin. This subduction-related phase would not appear to be consistent with the high-temperature metamorphism, migmatisation, voluminous anatectic, mainly S-type, granitoids and associated, charnockites, enderbites and gabbroids. It is suggested here that the rifting and resultant crustal thinning (and probable delamination and detachment) led to mantle upwelling, high-temperature metamorphism and anatectic magmatism, followed by compression and basin inversion, which promoted further mantle melting of the thickened lower crust, with the limited oceanic crust from the late rift phase being injected into the thrust package as thick ophiolite wedges, as observed. Hence "inversion" has been substituted for "collision" in the descriptions above.

The combined Araçuaí Basin and the Congo Belt basin was asymmetric. Although both parts of the rift basin include Neoproterozoic glaciogene rocks, the West Congo Belt preserved the eastern, east-verging fold-thrust belt and a thick bi-modal volcanic suite that represents the Early Tonian rift magmatism. The rift magmatism within the African side of the basin persisted from 912 to 875 Ma, and migrated westward over that period. Rift magmatism is largely absent from the Araçuaí Basin, which only contains rift and passive margin sedimentary rocks, including the glaciogene deposits (Pedrosa-Soares et al., 2008).

Rio Pardo and Rio do Peixe Bravo District Geology

The sequence in the Rio Pardo and Rio do Peixe Bravo districts, which are adjacent to the São Francisco Craton, represent an inverted sedimentary basin that underwent low to medium-grade tectonometamorphic-orogenic processes.

The Macaúbas Group is composed of metadiamictites, quartzites, phyllites and quartz schists. The local metadiamictites are interpreted as being related to glaciomarine sedimentary processes, and are intercalated with quartzites and metapellites deposited in a continental rift and passive margin, influenced by extensive inversion related Neoproterozoic magmatism. The Macaúbas Group was concordantly deposited on a basement of quartzitic rocks of the Espinhaço Supergroup, and reaches a few kilometres in thickness, essentially composed of metadiamictites, with a significant vertical and lateral gradation into pure and/or hematitic quartzites.

The Macaúbas Group has been locally subdivided into two distinct lithostratigraphic units, namely the basal Rio Peixe Bravo Formation, and the overlying Nova Aurora Formation. The latter includes the 600 m thick Riacho Poções Member, which is intercalated with the Nova Aurora Formation, and includes local hematite-rich rocks. It mostly comprises greyish diamictites that grade into hematitic, often magnetitic diamictites. Banded hematitic quartzites and quartz phyllites are intercalated with the diamictites, which contain as much as 60% Fe throughout the entire formation.

In the drilled deposits, intersections averaging >15% Fe range in thickness from ~40 to 140 m, with grades of 15 to 30% Fe.

Structured cangas (detrital iron deposits - DID) are developed over the Iron Formation outcrops, but are no more than 20 m thick and are typical of chemical interaction with iron rich groundwaters.

The hematitic quartzites are banded, with quartz-bearing beds (50 to 60% SiO2), rich in sericite, apatite, opaques, chlorite and zircon, alternating with hematite beds (35 to 40% Fe), with magnetite and martite, and ilmenite lamellae. Sedimentary structures are obliterated by metamorphic recrystallisation and foliation, and the sequence is folded with at least two schistosities.

The presence of diamictites in which the matrix is replaced by iron, accompanied by banded quartzites suggests the Riacho Poções Member originally developed in a basin in which both clastic and chemical sedimentation occurred, and further that the circulation of ferruginous fluids continued after chemical and clastic sedimentation and, probably, after diagenesis.

Macaúbas Group glaciomarine sedimentary sequences of the Rio Pardo area also host significant manganese deposits.

Proposed beneficiation is based on a design employed to upgrade lower grade ores in established mines in the Quadrilatero Ferrifero district to the south. This comprises, crushing and grinding to ~0.5 mm, magnetic separation to extract magnetite, desliming to remove ultrafines, flotation to extract hematite-martite to produce pellet and sinter feed. The product is then to be transported to the Atlantic coast by slurry pipeline. The beneficiated product contains 65 to 68% Fe

Mineral resource estimates have been published for some of the deposits of the district, as follows (Brazil Metals Group Ltd, 2012):

  Hornbridge Blocks 7 (Salinas) and 8
      Total measured + indicated + inferred resource - 3.67 Gt @ 20.2% Fe;

  ENRC Jiboia deposit
      Total indicated + inferred resource - 2.86 Gt @ 25.9% Fe;

The most recent source geological information used to prepare this summary was dated: 2013.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

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