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Rio Pardo - Rio do Peixe Bravo District, Salinas, Mtansminas, Jiboia |
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Minas Gerais, Brazil |
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Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Neoproterozoic Rio Pardo and Rio do Peixe Bravo district iron deposits of Northern Minas Gerais, Brazil, are located ~160 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 (Honbridge), 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. While this extension and deposition was occurred to the south, with the basin opening in a 'nut-cracker' like fashion, hinged to the north, it 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 Formation 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. The Nova Aurora Formation overlies the Rio Peixe Bravo Formation and comprises a thick glaciomarine sequence rich in diamictite and Fe-rich diamictite, including thick layers of iron formation, with a few scattered dropstones, and minor graded sandstones and pelites.
Iron mineralisation is largely confined to the ~600 m thick glaciomarine Riacho Poções Member that is intercalated within the Nova Aurora Formation.
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.
Within the Macaúbas Group, the Matão, Duas Barras and Rio Peixe Bravo formations are pre-glacial, the Serra do Catuni, Nova Aurora and Lower Chapada Acauã formations are glaciogenic, and the Upper Chapada Acauã and Ribeirão da Folha formations are post-glacial (Pedrosa-Soares et al., 2011). This implies the Serra do Catuni / Nova Aurora formations are Cryogenian in age. The pre-glacial and glaciogenic successions record the continental rift to transitional stages of the Macaúbas basin. The post-glacial succession represents proximal and distal passive margin to ocean floor environments (Pedrosa-Soares et al., 2011).
The basin inversion magmatism associated with the Brasiliano event, has been divided into four stages (after Pedrosa-Soares et al., 2008):
Pre-collision 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-collision 585 to 560 Ma - comprising S-type, garnet±cordierite±sillimanite granites and two-mica granites, commonly with mylonitic textures;
Late-collision 560 to 530 Ma - composed of S-type garnet- and/or cordierite leucogranite, occurring as veins or small intrusions;
Post-collision 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 oceanic crust, and a passive margin sucession. They then propose east-west compression and inversion, accompanied by subduction of the oceanic crust to produce a voluminous magmatic, predominantly intrusive arc, but with the significant Rio Doce volcanic arc and back-arc basin sequences. This was followed by a late to post-collisional phase between 560 and 535 Ma, with the break-off of the subduction slab, delamination and detachment of section of the sub-crustal lithospheric mantle below the crystalline core, and renewed asthenospheric upwelling and decompression melting, followed by post-collisional tectonic collapse between 520 and 490 Ma (Pedrosa-Soares et al., 2006, 2008, 2011). The post-collisional stages would be consistent with the the high-temperature metamorphism, migmatisation, voluminous anatectic, mainly S-type, granitoids and associated, charnockites, enderbites and gabbroids that characterise the belt.
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).
Geology and Mineralisation
The sequence in the Rio Pardo and Rio do Peixe Bravo districts, which are east of the São Francisco Craton, represent an inverted sedimentary basin that underwent low to medium-grade tectonometamorphic-orogenic processes, west of the main high grade crystalline core of the Araçuaí Belt/Orogen.
The Macaúbas Group is composed of meta-diamictites, quartzites, phyllites and quartz schists. The local meta-diamictites are interpreted as being related to glacio-marine sedimentary processes, and are intercalated with quartzites and meta-pellites 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 meta-diamictites, 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 Iron mineralisation is largely confined to the ~600 m thick Riacho Poções Member that is intercalated within the Nova Aurora Formation. It mostly comprises greyish diamictites that grade into hematitic, often magnetitic diamictite, with intercalated banded hematitic quartzites and hematitic quartz phyllites. The iron rich diamictite contains 10 to 60 wt.% Fe. The hematitic quartzites are banded with 50 to 60% quartz layers that are rich in sericite, apatite, zircon chlorite and other opaques, alternating with 35 to 40% Fe hematite bands, with magnetite and martite, and ilmenite lamellae. Metamorphic foliation with at least two schistosities and recrystallisation have obliterated any primary sedimentary structures (Revell, 2010, Independent Geologists Report to Lefroy Resources Limited Report on the Rio Pardo Iron Ore Project, submitted to the ASX).
According to Vilela et al. (2021), the matrices of the hematite-rich meta-diamictites comprise 7 to 55 vol.% hematite, 17 to 57 vol.% quartz, 2 to 40 vol.% muscovite, <1 to 30 vol.% carbonate, <1 to 18 vol.% biotite, <1 to 23 vol.% chlorite, <1 to 7 vol.% epidote, 1 to 5 vol.% magnetite, and traces of tourmaline, apatite and zircon. Hematite, mica and stretched quartz define the metamorphic foliation. Clasts are free of iron oxide bearing rocks. Hematite concentration of >50 wt.% Fe occur locally in mylonitic shear zones and tightly crenulated bands where gangue minerals have been removed during deformation, whilst late hydrothermal alteration appears to have produced magnetite-rich meta-diamictite in high-strain zones. Within the diamictites of the Nova Aurora Formation, there is a zonation from base to top, from barren meta-diamictite with quartz-rich matrices and graded-bedded quartzite lenses → meta-diamictite with hematite-rich matrix → metadiamictite a muscovite-rich matrix gradually getting richer in magnetite and sulphide, enclosing meta-pelite lenses.
In the drilled deposits, intersections averaging >15% Fe range in thickness from ~40 to 140 m, with grades of 15 to 30% Fe.
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, replacing the matrix of diamictites.
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. In the Rio Pardo area, cangas appear to have been developed over higher grade iron mineralisation in the underlying bedrock, and as such are an important exploration indicator.
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. Alternatively, Rio Pardo is ~160 km south of Caetité which is on the rail line from Figueirópolis to the Atlantic port of Ilhéus.
Mineral resource estimates have been published for some of the deposits of the district, as follows (Brazil Metals Group Ltd, 2012):
Honbridge 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 decription 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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Peixoto, E., Alkmim, F.F. and Pedrosa-Soares, A.C., 2018 - The Rio Pardo salient, northern Aracuai orogen: an example of a complex basin controlled fold-thrust belt curve: in Brazilian Journal of Geology v.48, pp. 25-49. doi: 10.1590/2317-4889201820170134.
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Vilela, F.T., Pedrosa-Soares, A.C., Babinski, M., Lana, C., Trindade, R.F. and Santos, E., 2021 - Diamictitic iron formation (DIF) deposits of the Neoproterozoic Nova Aurora Iron District (Macaubas Group, Southeast Brazil): in J. of South American Earth Sciences v.112, doi.org/10.1016/j.jsames.2021.103614
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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