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Pitangui Project, Sao Sebastao |
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Minas Gerais, Brazil |
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Au
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Super Porphyry Cu and Au
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The São Sebastião gold deposit within the Pitangui Project is located ~110 km NW of the city of Belo Horizonte, in Onça do Pitangui County, 13 km east of the town of Onça do Pitangui, 23 km SE of Pitangui and 20 km east of the Turmalina gold deposit, in the northwestern Quadrilátero Ferrífero District of Minas Gerais, Brazil.
(#Location: 19° 44' 4"S, 44° 40' 23"W).
Regional Setting
For details of the regional setting, geology and metallogeny see the Quadrilátero Ferrífero District Gold - Geological Setting record. See the main geological map in the same record for its location.
The Pitangui district is located on the southern tip of the western São Francisco craton, within the northwestern extremity of the Quadrilátero Ferrífero District (Almeida, 1977; Alkmim, 2004; Hasui et al., 2010). The latter is composed of Archaean greenstone belts, Mesoarchaean to Proterozoic granite-gneiss terrains, and Neoarchaean to Palaeoproterozoic supracrustal units (Baltazar and Zucchetti, 2007). The principal greenstone sequence of the Quadrilátero Ferrífero is the Rio das Velhas Greenstone Belt, mainly comprising the Rio Das Velhas Supergroup, subdivided into the Nova Lima and overlying Maquiné groups, tectonically juxtaposed against granite-gneiss terrains as described in the linked record above. The Nova Lima Group mainly comprises mafic volcanic rocks with komatiitic flows at the base, clastic sedimentary units, abundant chemical sedimentary rocks, including iron, quartz-dolomite and quartz-ankerite formations, conglomerate and carbonaceous phyllite. The volcanic rocks have been dated at between 3035 and 2772 Ma (Baltazar and Zucchetti, 2007). The Maquiné Group comprises sandstones, conglomerate and quartz-phyllite, unconformably overlying the Nova Lima Group. These are, in turn, overlain by the Palaeoproterozoic Minas, Mesoproterozoic Espinhaço and Neoproterozoic Sáo Francisco Supergroups that were deposited in passive margin basins during intermittent episodes of rifting (Baltazar and Zucchetti, 2007; Castro and Dardenne, 2000).
The Pitangui Greenstone Belt is a northwest- trending synclinorium interpreted as the northwest extension of the Rio das Velhas Greenstone Belt (Heineck et al., 2003; Romano, 2007; Pinto, 2014). It is bound to the SW by the Mesoarchaean Divinópolis TTG Complex and to the northeast by the Late Meso- to Neoproterozoic Florestal and Pequi TTG plutons and by the Mesoarchaean Belo Horizonte TTG Complex. The belt is covered by the Ediacaran sedimentary rocks of the Bambuí Group to the NW. The central part of the belt, <5 km to the SE of the São Sebastião deposit, is occupied by the smaller Neoarchaean A-type potassic Jaguara granitic stock.
Geology
The Pitangui Greenstone Belt has been divided into lower, middle and upper units. The lower and middle units are broadly correlated with the Nova Lima Group, while the upper unit is a correlate of the Maquiné Group of the Rio das Velhas Greenstone Belt. These comprise:
• The lower unit - mafic and interbedded ultramafic metavolcanic rocks, turbiditic meta-wackes, banded iron formation, clastic and chemical metasedimentary rocks and sericitic to carbonaceous meta-pelite. Metavolcanic mafic and ultramafic rocks are predominant in the lower unit, whilst meta-dacitic volcanic and clastic metasedimentary rocks increase in abundance towards the top. The clastic and chemical meta-sedimentary rocks include pelites, sandstones, BIFs and cherts. Chlorite, talc, sericite and biotite schists are common metamorphic products in this unit. The maximum depositional age for the meta-sandstones is estimated at 2.86 Ga (Soares et al., 2017). The Lower unit, which hosts the deposit, has been divided into five main intervals, as follows:
i). a succession of interbedded meta-sandstones, BIFs and minor meta-pelites;
ii). an ~100 m thick package of interbedded turbiditic meta-sediments, with BIF layers that are collectively referred as the Pimentão BIF;
iii). ultramafic meta-volcanic and carbon-rich meta-pelitic rocks, containing thin meta-chert lenses;
iv). meta-sedimentary rocks containing lenses of carbon-rich meta-pelite at the base, overlain by a 2 to 30 m thick layer of laterally extensive BIF, the Biquinho BIF;
v). an ~5 m thick mafic meta-volcanic band, interstratified with a 3 to 5 m unit of thinly layered BIFs, the Tomate zone. This interval contains thick packages of meta-sandstones interbedded with mafic meta-volcanic rocks.
• The middle unit - composed of clastic meta-sedimentary rocks with minor chemical meta-sedimentary and meta-volcanic rocks, and common interbedded phyllites (both carbonaceous and sericitic), sericite and biotite schists, and micaceous quartzites. The maximum depositional age is inferred to be 2.76 Ga (detrital zircon, U-Pb isotopic dates).
• The upper unit - which is correlated with the Casa Forte Formation of the Maquiné Group, and comprises micaceous quartzite, quartz-schist and polymictic meta-conglomerates. This unit is exposed at surface in the northern sector of the belt.
Structure
Both the granite-gneiss terrains and greenstone belts of the Quadrilátero Ferrífero have been subjected to three main orogenic events, namely:
i). The ~2.75 to 2.65 Ga Rio das Velhas orogeny, producing an overall NE-SW compression (Baltazar and Zucchetti, 2007).
ii). The 2.10 to 1.90 Ga Rhyacian-Orosirian Transamazonian orogeny, resulting in amalgamation of the São Francisco palaeo-continent.
iii). The ~600 to 500 Ma Panafrican-Brasiliano orogeny, with associated east-west thrusting along the easternmost margin of the Quadrilátero Ferrífero (Alkmin and Marshak, 1998; Noce et al., 2007).
Four generations of structures have been recognised in the Rio das Velhas Greenstone Belt, related to these same three regional deformation events (Baltazar and Zucchetti, 2007). D1 resulted in the formation of west- to NW-striking faults and folds, the first stage of a progressive compressional deformation episode D1-D2 during the Rio das Velhas orogeny. The continuation into D2 resulted in SW-verging, NW-striking ductile shear zones and associated overturned tight to isoclinal NW-trending folds. These latter structures are spatially related to gold deposits. D3 resulted in diapiric rising batholiths within metamorphic core complexes during the Rhyacian-Orosirian orogeny, coinciding with the formation of regional-scale open and upright synclines in the Archaean and overlying Palaeoproterozoic supracrustal units. These structures defined the form of the Quadrilátero Ferrífero, including dome complexes of granite-gneiss basement. D4 accompanied the Panafrican-Brasiliano orogeny, which commenced with an initial east-west compression that evolved into transpressional late-orogenic belt-parallel tectonics that affected the eastern margin of Brasil, and the western Quadrilátero Ferrífero.
Locally, within the Pitangui Greenstone Belt, three deformation events have been recognised, in which lithological domains are oriented NW, with variable dips (Heineck et al., 2003; Romano, 2007, Reis et al., 2017). These events are characterised as follows:
• D1 produced isoclinal to tightly recumbent NW-plunging folds, accompanied by bedding-parallel shear zones, and a NW striking S1 foliation that is subparallel to bedding, except in fold hinges. This main penetrative S1 fabric is tangential to the relict S0. S1 is also associated with bedding-tangential shear and thrust fault zones. These structures have been interpreted to define a pervasive D1 deformation phase, related to NE-SW shortening and tectonic transport.
• D2 - open to tight NW-plunging folds that have steep to moderately dipping axial planes that refold earlier isoclinal folds, NW-striking, NE-dipping shear zones, with a poorly developed S2 axial planar and crenulation cleavage. The similarity of style and orientation with D1 structures may imply a single progressive deformational event.
• D3 NE-plunging open folds with an associated, spaced S3, axial planar and crenulation cleavage.
Those structures can be used to define a pervasive D1 deformation phase, related to northeast-southwest shortening and tectonic transport.
The gold deposit at São Sebastião occurs along the northeastern limb of the D2 Jaguara anticline, an open, SW vergent asymmetric fold that plunges NW. The structural framework of the Pitangui greenstone belt has been described as producing a 'dome and basin' fold interference pattern.
The metamorphic character of in the Pitangui Greenstone Belt suggests maximum pressure-temperature conditions within the lower- to medium amphibolite facies (Romano, 2007; Brando Soares et al., 2017; Silva et al., 2018). In the NW, a maximum temperatures ~600°C has been estimated on the basis of a garnet geothermometer (Silva et al., 2018). The bulk of the belt has been subjected to biotite grade regional metamorphism, with sporadic garnet and hornblende, followed by widespread retrogression to chlorite.
Mineralisation
Gold mineralisation at the São Sebastião deposit is hosted in three main stratabound sulphidation zones within several stacked BIF layers in the lower unit of the Pitangui Greenstone Belt, namely the Tomate, Biquinho and Pimentão from top to bottom, as described above. The principal of these is the Biquinho BIF, whilst the smaller Pimentão zone is located within a thrust-fold where the fold-axis plunges gently to the north, with gold mineralisation concentrated in one of its limbs. The Biquinho and Pimentão BIF packages are the more continuous, occurring in intervals ii). and iv). of the lower unit.
Sulphide mineralisation within these zones is most commonly found as disseminations replacing magnetite, although occasional massive sulphide mineralisation can be found with associated quartz-carbonate veins and breccias. The dominant sulphide is pyrrhotite, followed by lesser arsenopyrite, chalcopyrite, pyrite and arsenian-pyrite.
Gold is present in its native form and as electrum. It is free, but fine-grained, occurring as rounded to semi-angular grains with sizes ranging from <5 to 200 µm, usually between 10 and 20 µm. Gold inclusions are commonly found in pyrrhotite crystals from breccia zones and fold hinges, but also occur in pyrrhotite crystals formed after magnetite. Larger 50 to 200 µm gold grains are found between crystals of gangue minerals, whilst rounded, fine-grained, ~10 µm gold inclusions with exsolution features locally occur in pyrite crystals. Free gold, varying from 20 to 80 µm, may fill fractures in arsenopyrite crystals, or as fine-grained 5 to 10 µm inclusions, or, more rarely, on the margins of crystals. Minor invisible gold has been detected in the less common arsenical pyrite crystals.
Whilst the native Au in pyrrhotite formed in dilatation domains (e.g., breccias, fold hinges) during compressional events (Rio das Velhas Orogeny at 2.8 to 2.75 Ga) it is mainly found as inclusions in late- to post-kinematic pyrite and arsenopyrite, as fracture infills in arsenopyrite and in contact with gangue minerals. Electron-microprobe analyses in arsenopyrite reveal a 465 to 560°C precipitation temperature, interpreted from As atomic percentages, and indicate that high-temperature fluids were active after the deformational event.
Four mineralisation styles have been recognised within BIFs at São Sebastião, as follows:
i). Disseminated sulphides, resulting from the replacement of magnetite by sulphides, occurring as tabular masses that follow relict bedding. This style comprises the vast majority of the deposit by volume.
ii). Sulphides in breccia zones, with thicknesses varying from a few centimetres to a few metres that are found throughout the deposit, particularly in fold hinges.
iii). Quartz-carbonate-sulphides veins which contain variable sulphides, and may be spatially associated to breccia zones.
iv). Other sulphidation styles, such as sulphides disseminated in chlorite schists, carbonaceous phyllites and tourmalinites.
Resources
NI 43-101 compliant Mineral Resources as at 31 December, 2021 (IAMGOLD Reserve and Resource Report, 2021) were:
Indicated Mineral Resource - 3.330 Mt @ 4.4 g/t Au, for 14.65 tonnes of contained gold;
Inferred Mineral Resource - 3.559 Mt @ 3.8 g/t Au, for 13.52 tonnes of contained gold.
The bulk of the information in this summary has been drawn from "Mitrofanov, A., Cole, G., Leuangthong, O. and Smith, J., 2020 - Independent Technical Report
for the São Sebastião Gold Deposit, Pitangui Gold Project, Brazil; an NI 43-101 Technical report produced by SRK Consulting (Canada) Inc. for IAMGOLD Corporation, Toronto; 113p."
The most recent source geological information used to prepare this decription was dated: 2020.
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.
Sao Sebastao
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Selected References:
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Brando Soares, M., Correa Neto, A.V., Bertolino, L. C., Alves, F.E.A., Almeida, A.M., Silva, P.H.M., Araujo, R.O., Manduca, L.G., Araujo,I.M.C.P., 2018 - Multistage mineralization at the hypozonal Sao Sebastiao gold deposit, Pitangui greenstone belt, Minas Gerais, Brazil: in Ore Geology Reviews v.102. pp. 618-638.
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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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