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Minas Rio Project - Serra do Sapo, Itapanhoacanga, Serro |
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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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The Serra do Sapo iron mine at Conceição do Mato Dentro, along with the Itapanhoacanga and Serro resources, ~10 km north and ~65 km NNE respectively, constitute the Minas-Rio Project. The Serra do Sapo mine is located ~125 km NNE of Belo Horizonte in Minas Gerais, eastern Brazil (#Location: 18° 54' 7"S, 43° 25' 51"W).
The Serra do Sapo mine exploits a continuous thrust bounded section of the banded iron formation of the Upper Palaeoproterozoic Serra do Sapo Formation. The iron formation being mined at the Serra do Sapo operation forms the ~12 km long, NNW-SSE striking Sapo Ridge, which has an average dip of 25°E, and an average thickness of 80 m, although it reaches as much as 350 m in the north. The Sapo Ridge constitutes the northernmost segment of the ~80 km long Serpentina Range, the backbone of which is the Serra do Sapo iron formation.
After mining, a concentrate is produced on site and piped 529 km as a slurry to the iron ore port of Açu in the state of Rio de Janeiro where it is dewatered and pelletised for export. Mining commenced at the Serra do Sapo mine in October 2014. Of the three deposits included within the Minas-Rio Project, only Serra do Sapo is initially being exploited (as at 2023) at a rate of ~20 to 25 Mt or ore per annum.
Regional Setting
The iron formations of the Serra do Sapo Formation were deposited immediately preceding the extensive Late Palaeoproterozoic to Early Neoproterozoic Espinhaço Supergroup. They were emplaced during the initial pulse of the long-lived, episodic extension that formed the rift basin in which both successions were laid down. These iron formations are exposed on the southeastern margin of the Southern Espinhaço Domain. This domain was created when the rocks of the Espinhaço Supergroup, deposited to the east of the São Francisco Craton, were thrust west onto the craton's eastern margin, and were imbricated into numerous superposed thrust slices during the late Neoproterozoic Braziliano Event. The host sequence reflects the early evolution of the Araçuaí Belt/Basin that lies to the east of the São Francisco Craton. This evolution began with i). the spreading episode mentioned above, related to Late Palaeoproterozoic Statherian, post 1990 ±16 Ma rifting (based on detrital zircon dates in quartzites), accompanied by two pulses of felsic volcanism dated at ~1770 and 1711 Ma, ii). the overlapping emplacement of the largely anorogenic alkaline Statherian meta-granites comprising the Borrachudos Suite plutons, part of a long-lived silicic igneous province developed in the interval between ~1750 and 1710 Ma, associated with the Espinhaço taphrogenic event (Magalhães et al., 2018), and iii). the onset of the main Espinhaço intracontinental rifting. Two further Espinhaço rifting and magmatic pulses occurred after these initial stages, namely during the Calymmian 1524 Ma acid magmatism and in the Early Stenian at 1180 Ma.
On the exposed southeastern margin of the Southern Espinhaço Domain, the iron formations of the Serra do Sapo Formation are the upper part of the 50 to 800 m thick Serra da Serpentina Group which is unconformably overlain by the Espinhaço Supergroup. The latter comprises an up to 6000 m thick succession of quartz-rich sandstone, pelite, conglomerate, volcanic rocks and minor carbonate that is only weakly metamorphosed, and in which sedimentary structures and textures are generally well-preserved. The basal unit of the Espinhaço Supergroup, the Serra de São Jośe Group, which overlies the banded iron formations of the Serra do Sapo Formation above an erosional unconformity, also contains banded iron formation. Deposition of these two iron formation bearing suites spans the interval from the late Orosirian to the early Statherian periods, but marks a transition from an epicontinental-epeiric, slowly down-warping sag basin with scant tectonic activity represented by the upper Serra da Serpentina Group, to a tectonically active continental rift-basin during deposition of the Serra de São Jośe Group.
Basement to both succession comprises the underlying Mesoarchaean to early Neoarchaean gneisses belonging to the Guanhães Block. The latter are tectonically reworked equivalents of rocks within the São Francisco Craton.
To the west of the Serpentina Range and the Serra do Sapo mine, the host Serra da Serpentina Group is interpreted to passes laterally into the shallow marine sericitic quartz schists, quartzites and phyllites with lesser iron formations of the Costa Sena Group (e.g., Rolim et al., 2016), which, based on detrital zircons has a minimum age of 2048 ±16 Ma (Machado et al., 1989). This is unconformably overlain by the basal unit of the Espinhaço Supergroup in that area, the 200 m thick Banderinha Formation. The latter comprises a package of pink to reddish quartz-rich sandstones, containing layers and lenses of coarse conglomerate with a maximum depositional age of 1.737 ±11 Ma, based on detrital zircons (Santos et al., 2013). This formation and the overlying regionally distributed <1683 ±11 Ma São João da Chapada Formation are interpreted to be equivalents of the Serra de São Jośe Group described above.
To the east of the Serpentina Range and the Serra do Sapo mine, allochthonous slices of a Late Palaeoproterozoic unit known as the Guanhães Group (e.g., Barrote et al., 2017) are interleaved with the Mesoarchaean to early Neoarchaean gneisses of the Guanhães Block. The Guanhães Group comprises, a lower, up to 50 m thick quartzitic unit, mainly composed of medium to coarse grained saccharoidal quartzites that vary from pure silica to sericitic, arkosic and iron-rich, intercalated with gneiss and schist. These are overlain by medium to fine grained, saccharoidal, banded iron formation that varies from lamellar/specularite to granular and from martite to magnetite. The upper member is similar to the basal unit except for the presence of garnet-rich amphibolite layers close to the contact with the underlying BIF. Based on detrital zircon dates, the Guanhães Group has a maximum age of 2.18 Ga (Barrote et al., 2017) or (1671 ±20 Ma (Silveira Braga et al., 2020; Silveira Braga et al., 2019), suggesting a temporal correlation with either the Serra da Serpentina and/or Serra de São Jośe groups. This implies these two iron formation bearing units originally had a considerable area extent.
Geology of the Host Units
The Serra da Serpentina Group is restricted to the southern half of the eastern margin of the Southern Espinhaço Domain, with preserved thicknesses of from 50 to 800 m. It has a maximum Orosirian depositional age, containing youngest 1990 16 Ma detrital zircons. It is composed of a thickness of from a few to >700 m of fine clastic meta-sediments at the base, overlain by chemical sediments, including banded iron formations (BIFs) that average 80 m in thickness. These sequences comprise the Meloso and Serra do Sapo formations respectively, which together are correlated with the pre-Espinhaço Costa Sena Group. The basal contact of the group with the underlying TTG complexes of the Guanhães Block is sharp, commonly sheared, and masked by highly deformed and hydrothermally altered schistose metabasic rocks and by stretched and fractured syntectonic quartz veins. The Meloso Formation contains quartzite lenses that are metres to several tens of metres thick in its basal section, intercalated with quartz-chlorite-sericite schists. The quartzites are either sericitic or iron- or manganese-rich. The schists become increasingly frequent until they dominate as a characteristic fine, white-greenish (hematite)-quartz-chlorite-sericite schist that may be up to 500 m thick, weather to a pinkish, reddish or yellowish saprolite. In its uppermost sections, metres-thick lenses of black manganese-rich saprolite are discontinuously developed, interpreted to represent the weathering product of manganese-rich carbonate rocks. These uppermost schists grade to a light grey to silver-coloured, finely banded, phyllite with alternating sericitic and quartz layers, and platy hematite crystals commonly occurring in the mica-rich layers. Well-formed late- or post-tectonic magnetite idioblastic crystals up to 5 mm across are also common and characterise the phyllite. They average 5% of the total rock volume, but may locally be as high as 25%. Ubiquitous black and silky graphitic phyllite lenses that average 20 m in thickness occur at the top of the sequence. The upper contact with the Serra do Sapo iron formation is transitional, corresponding to a decrease in the phyllosilicate content.
The Serra do Sapo Formation, the uppermost member of the Serra da Serpentina Group, is predominantly composed of chemical sedimentary rocks, including metamorphosed banded iron formation at the base and discontinuous metadolostone at the top. The banded iron formation, which forms the Sapo ridge along the eastern margin of the Southern Espinhaço Domain, averages 80 m thickness, but may reach up to 350 m in its north. In the Serpentina Range, >10 km to the west, it varies from 80 to 100 m, and from 15 to 35 m along the Morro do Pilar strip to the SW. This banded iron formation is composed of characteristic alternating light to dark grey or bluish black meso- and micro-bands of millimetre- to centimetre-thick quartz-rich and hematite-rich beds,
although primary features may be commonly transposed by the pervasive tectonic foliation. The mineralogical content averages 50.9% quartz, 45.8% hematite, 1.9% white mica and 0.9% magnetite, with accessory chlorite, epidote, cummingtonite, grunerite and rare rutile, allanite, biotite, pyrite, chalcopyrite, apatite and tourmaline. The bulk chemical compositions average 32.5 wt.% Fetotal, 51.8 wt.% SiO2, 0.023 wt.% P, 1.24 wt% Al2O3, and 0.89 wt.% FeO. Sparse lenses of other lithologies include white sericitic quartzites, greyish ferruginous quartzites, hematite-sericite-quartz schists/grey phyllites and metre-thick banded iron-manganese rock. In addition, rare, transgressive, high grade >62 wt.% Fetotallenses of strongly foliated, millimeter-long, specularite crystals are encountered. These are 100 to 300 m long and 5 to 30 m thick, oriented parallel the regional schistosity. To the north, near Itapanhoacanga, up to 80 m thick lenses of metadolostone have been drilled in the upper layers of the Serra do Sapo Formation. These lenses are composed of red to pinkish, fine-grained granoblastic dolomite grains with white quartz and calcite veinlets. The contact with the banded iron formation is gradational via a zone of alternations of both lithologies. In the northern half of the Sapo ridge, the top of the iron formation is locally covered by a 5 to 15 m thick, laterally persistent, clast supported, intraformational metabreccia that is solely composed of 10 to 30 cm, irregular to elongated clasts of iron formations set in a sparse quartzose or ferruginous sandy matrix. This breccia is absent above the meta-dolostones. The top of the Serra do Sapo Formation is separated from the basal Serra de São Jośe Group by an erosional unconformity, overlain by coarse-grained quartzites and rudaceous sediments that contain eroded and reworked BIF and dolomite fragments from the Serra do Sapo Formation.
The Serra da Serpentina Group is interpreted to represent sedimentary deposition within an epicontinental-epeiric, slowly down-warping sag basin with scant tectonic activity.
The younger Serra de São Jośe Group belongs to the basal Espinhaço Supergroup, and is separated from the underlying Serra da Serpentina Group by an erosional unconformity as described above. It marks a significant change in the depositional facies, with the development of a complete cycle of upward-fining sedimentary deposition. It was deposited in a tectonically active continental rift-basin during the early opening of the Espinhaço Trough. It has a total preserved thickness of 30 to 650 m, and stretches along the north-south axis of the rift, where it comprise a complete cycle of transgressive sedimentary deposits, subdivided, from base to top, into the Lapão, Itapanhoacanga and Jacém formations, with the Canjica BIF at the top. The Itapanhoacanga Formation has a maximum depositional age of 1666 ±32 Ma (Statherian), which coincides with the maximum depositional age (i.e., 1683 ±1 Ma) of the overlying, widespread, São João da Chapada Formation (Rolim et al., 2015). The upper quartzite of the Serra de São Jośe Group grades up into the latter formation, which is made up of alluvial sandstones with lenses of quartzite breccias at the base, layers of K-rich volcanic rocks (known as hematite-phyllite) and a succession of coarse-grained sandstones with pelitic intercalations. The volcanic rocks are dated at ~1715 ±2 and 1710 ±12 Ma (Machado et al., 1989; Dussin and Dussin 1995; Chemale et al., 2012). This unit is overlain by the Mato Verde Group in the northern part of the southern Espinhaço range, which also laps onto basement and is separated from the overlying aeolian deposits of the middle Espinhaço sequence by an erosional surface.
The Lapão Formation comprises up to 100 m of meta-conglomerates and coarse- to medium-grained quartzite beds. The metaconglomerates occur as 30 to 50 m thick, poorly sorted, polymictic, clast-supported lenticular beds with rounded to sub-rounded pebble to cobble and boulder clasts of white quartz, micaceous quartzites, grey ferruginous quartzites, dark grey ferruginous meta-chert, banded iron formations, black phyllites, dolostones and rare meta-conglomerates.
The Itapanhoacanga Formation is a suite of thick quartzite packages that has an average thickness of 100 m, locally expanding to a maximum 700 m in the eastern part of the Serra de São Jośe Group to the west of Itapanhoacanga. The quartzites are dominantly impure and sericitic, white and medium- to coarse-grained, containing rare pebbles of rounded white quartz and sporadic flat, low-angle cross-bedding that average of 1 m height and 2 to 3 m in length. Layers of fine quartz-sericite schists that may be tens of centimetres thick are intercalated with the quartzites, as are 2 to 3 m thick lenses of polymictic, matrix supported metaconglomerates. At the top of the unit, the quartzites become finer and grade to the laminated fine quartzites in the
Jacém Formation.
The Jacém Formation comprises monotonous fine- to medium-grained white quartzite with parallel, sub-millimetric sericitic laminations. It that weathers to a yellowish colour. It grades upward into a cover of fine, light grey quartz-sericite schist with are phyllites. These lithotypes cover the banded iron formation layer of the Serra do Sapo Formation on the western slopes of the Sapo Ridge and the Serpentina Range. The upper
contact with the overlying Canjica Formation is transitional, with fine phyllites and quartzites becoming increasingly richer in iron oxides.
The Canjica Formation is discontinuously exposed to the west of Itapanhoacanga in the northern Alves Range and in the
Serpentina Range to the east of the township of Conceição do Mato Dentro. It is 20 to 40 m thick, with drastic lateral changes, but is otherwise similar in character and composition to the Serra do Sapo Formation with millimetre- to centimetre-thick alternating layers of white, fine- to medium-grained quartz and iron oxides, as well as granoblastic and lamellar specularite and accessory magnetite. It also contains rare interbeds and metre thick lenses of sericite schist, grey phyllites and sericitic quartzites.
Structure - The Minas-Rio Project deposits lie along the Serpentina Range, the structure of which is controlled by numerous thrusts and associated folds that developed during the Brasiliano orogeny, segmenting the sequence into as many west-verging imbricated tectonic slices (Herrgesell and Pflug, 1986; Almeida-Abreu et al., 1988; Pedrosa-Soares et al., 2001). Whilst single slices have undergone short displacement, their large number results in a significant reorganisation of the sequence. Two compressional phases are recognised. Phase 1 produced bedding-parallel ductile shear zones that obliterated many of the primary features of the metasedimentary rock, and vary from a few to tens of metres in thickness, with the development of typical mylonitic features including a pervasive foliation, S1, and a distinctive stretching lineation, L1. S-C foliations, sheath folds and rootless intrafolial folds B1 with axes F1 sub-parallel to the stretching lineation are also evident. Cobbles and pebbles in the conglomerates are sub-elliptical to elliptical with axial ratios as high as 100:1. Asymmetric folds, pressure shadows, mantled porphyroclasts and mica-fish are ubiquitous, the vast majority of which indicate a west vergent sense of displacement. Phase 2 is interpreted to be related to foreland thrust faults with west-verging frontal and lateral ramps, tear-faults and asymmetrical propagation folds B2 that comprise an imbricate complexes with with major stratigraphic repetitions, including slices of basement rocks juxtaposed over younger metasedimentary units. This phase also generated east-vergent back thrusts and back folds, B3, but usually only on the hanging wall of frontal ramps from the main thrusts.
A traverse across the Serpentina Range from east to west commences with Meso- to early Neoarchaean gneisses of the Guanhães Block, structurally overlying the Serra de São Jośe Group across a shallow east-dipping thrust. The latter, in turn, overly the main Serra da Serpentina Group of the Serpentina Range, across an erosional unconformity, or structurally across an east dipping thrust plane. Locally, the Serra da Serpentina Group then structurally overlies slivers of the Serra de São Jośe Group. To the SE, both structurally overlie Guanhães Block Meso- to early Neoarchaean gneisses to the west, which further west again, across a broad overturned antiform, are overlain by the Serra da Serpentina and Serra de São Jośe groups. In the northern part of the range, the main Serra da Serpentina Group of the Sapo Ridge structurally overlies an extensive field of felsic volcanic rocks to the west, which in turn are thrust westward over the main Espinhaço Supergroup.
Mineralisation - has been classified as Compact Itabirite, Friable Itabirite, Semi-Friable Itabirite, High Alumina Friable Itabirite, Soft Hematite and Canga - although for Ore Reserve/Resource purposes, it is split into 'Compact Itabirite' and the rest, summarised as 'Friable Itabirite and Hematite' (see below). For Ore Reserve and Mineral Resource purposes after 2014, the Compact Itabirite is referred to as just 'Itabarite', as the Compact Itabirite at Serrado Sapo is softer than that regarded as Compact Itabirite in both the Carajás and Quadrilátero Ferrífero districts.
Silveira Braga et al. (2021) studying iron mineralisation in inferred equivalents of the Serra da Serpentina and Serra de São Jośe groups further east within the Guanhães Block suggest the development of the hypogene ores there involved the following steps:
• Deposition of quartz-hematite iron formation, contemporaneously with the host Orosorian to Statherian Palaeoproterozoic sedimentary successions within an extensive post-Transamazonic intracratonic basin, fertilised by Fe-rich hydrothermal fluids (e.g., Rosière et al., 2019).
• During the Neoproterozoic to early Palaeozoic, Pan-African/Brasiliano Araçuaí-West Congo Orogeny, intense magmatic and metamorphic conditions that reached granulite facies (e.g., Pedrosa Soares et al., 2001) produced large volumes of magmatic and anatectic rocks and magmatic-hydrothermal fluids. These fluids interacted with the iron formations in the course of several discontinuous but long-lived pulses (Silveira Braga et al., 2020) promoting the formation of magnetite mineralisation. These fluids are interpreted to have modified the iron formation protore in three stages:
i). Shear controlled iron enrichment and selective dissolution of quartz by hydrothermal fluids during the syn-collisional and late-collisional stages of the Braziliano Orogeny between 580 and 560 Ma. Hematite crystals grew as fluid assisted, strain-controlled oriented plates, with small scale diffusion and crystallisation (Rosière et al., 2013).
ii). Hot hydrothermal fluids related to the generation and introduction of the widespread anatectic pegmatites which promoted quartz leaching and crystallisation of magnetite. These pegmatites are interpreted to have originated from the the partial melting of the older Guanhães Block and the Palaeoproterozoic Borrachudos Granite intrusions during the late-collision and collapse phases of the Neoproterozoic to Cambrian Brasiliano orogeny between 560 and 490 Ma.
iii). Oxidation of magnetite to kenomagnetite, martite and crystallisation of hematite, the result of oxidation of the high temperature-high salinity magmatic fluids by the influx of low-temperature, low-salinity meteoric waters (Gomes et al., 2018). This fluid mixing in interpreted to have occurred during the final gravitational collapse/delamination phase of the orogeny between ~530 and 490 Ma.
Ore Reserves and Mineral Resources
Pre-mining Ore Reserves and Mineral Resources at a 25% Fe cut-off were as follows (AngloAmerican Annual Report, 2014):
Serro do Sapo
Ore Reserves
Friable Itabirite and Hematite - Probable Reserves - 1.4146 Gt @ 37.9% Fe;
Compact Itabirite - Probable Reserves - 1.3843 Gt @ 30.9% Fe;
Mineral Resources
Friable Itabirite and Hematite - Measured + Indicated Resources - 399.7 Mt @ 37.9% Fe + Inferred Resource - 87.4 Mt @ 36.7% Fe;
Compact Itabirite - Measured + Indicated Resources - 1548.6 Mt @ 30.9% Fe + Inferred Resource - 581.0 Mt @ 31.1% Fe.
Itapanhoacanga
Mineral Resources
Friable Itabirite and Hematite - Measured + Indicated Resources - 148.6 Mt @ 41.1% Fe + Inferred Resource - 114.5 Mt @ 40.4% Fe;
Compact Itabirite - Measured + Indicated Resources - 96.6 Mt @ 34.3% Fe + Inferred Resource - 57.0 Mt @ 34.5% Fe.
Serro
Mineral Resources
Friable Itabirite and Hematite - Measured + Indicated Resources - 92.0 Mt @ 41.2% Fe + Inferred Resource - 32.8 Mt @ 41.0% Fe;
Compact Itabirite - Measured + Indicated Resources - 281.7 Mt @ 32.1% Fe + Inferred Resource - 111.1 Mt @ 34.6% Fe.
Mineral Resources and Ore Reserves at the Serra do Sapo mine at 31 December 2021, cut-off of 25% Fe, were (AngloAmerican Annual Report 2021):
Friable Itabirite and hematite
Measured + Indicated Mineral Resource - 258.6 Mt @ 32.6% Fe + Inferred Resource - 55.8 Mt @ 36.6% Fe;
Proved + Probable Ore Reserve as Saleable concentrate - 682 Mt @ 67.1% Fe;
Itabirite
Measured + Indicated Mineral Resource - 1441 Mt @ 31.0% Fe + Inferred Resource - 442.6 Mt @ 30.9% Fe;
Proved + Probable Ore Reserve as Saleable concentrate - 1033 Mt @ 67.1% Fe.
NOTE: Ore Reserves are additional to Mineral Resources
The most recent source geological information used to prepare this decription was dated: 2021.
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.
Serra do Sapo
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Selected References:
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Barrote, V.R., Rosiere, C.A., Rolim, V.K., Santos, J.O.S. and Mcnaughton, N.J., 2017 - The Proterozoic Guanhaes banded iron formations, Southeastern border of the Sao Francisco Craton, Brazil: evidence of detrital contamination: in Revista do Instituto de Geociencias - USP, v.17, pp. 303-324. doi: 10.11606/issn.2316-9095.v17-352.
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Rolim, V.K., Rosiere, C.A., Santos, J.O.S. and McNaughton, N.J., 2016 - The Orosirian-Statherian banded iron formation-bearing sequences of the southern border of the Espinhaco Range, Southeast Brazil: in J. of South American Earth Sciences, v.65, pp. 43-66. dx.doi.org/10.1016/j.jsames.2015.11.003.
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Silveira Braga, F.C., Rosiere, C.A., Santos, J.O.S., Hagemann, S.G., Danyushevsky, L. and Salles, P.V., 2021 - Geochemical and tectonic constraints on the genesis of iron formation-hosted magnetite-hematite deposits at the Guanhaes Block (Brazil) by contact metasomatism with pegmatite intrusions: in Ore Geology Reviews v.129, 23p. doi.org/10.1016/j.oregeorev.2020.103931.
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