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Vazante |
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Minas Gerais, Brazil |
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Zn
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Super Porphyry Cu and Au
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The Vazante zinc deposit is located in Minas Gerais in Brazil, ~340 km south-east of Brazilia and 500 km WNW of Belo Horizonte (#Location: 17° 57' 44"S, 46° 51' 15"W).
It occurs as a set ore lenses within the same thrust belt on the margin of the Brasilia Mobile Belt with the São Francisco Craton as the stratabound Morro Agudo Zn deposit and the Morro do Ouro Au orebody. Mineralisation is interpreted to be linked to the development of the Vazante Shear Zone, which is ~12 km long, strikes at ~50° and dips 60°NW, and has been suggested to have been a transpressional transcurrent fault that was later reactivated as a normal fault (Pinho, 1990).
Vazante is hosted by grey- to pink-colored dolostone with cyanobacteria mats and bird's-eyes, slate and phyllite of the Lower Pamplona Member of the Neoproterozoic Vazante Group, close to the contact with dark gray dolostone and pyrite-bearing carbonaceous slate and marl of the Upper Morro do Pinheiro Member. The Vazante Group extends over a north-south strike length of up to 250 km in the Brasília Fold Belt, and comprises a thick marine pelitic-dolomitic sequence that includes dolarenites, dolorudites, dololutites/phyllites and stromatolitic dolomites that have undergone lower greenschist facies metamorphism (Dardenne, 2000).
The Vazante Group is divided, from base to top, into seven formations:
Santo Antônio do Bonito Formation, comprising metapelitic rocks with phosphate concentrations and layers of quartzite, diamictite and slate;
Rocinha Formation, dominantly composed of metapelitic rocks, subdivided into a lower rhythmite unit, overlain by dark grey pyritic and
phosphatic slate, which is followed by further rhythmites (Dardenne et al., 1998; Dardenne and Schobbenhaus, 2001);
Lagamar Formation, which represents a meta-psamopelitic unit with basal metaconglomerates, dolomitic breccia, dark grey limestone and stromatolitic bioherms with columnar stromatolites of the Cono-phyton and Jacuto-phyton type (Moeri, 1972; Cloud and Dardenne, 1973)
Serra do Garrote Formation, which comprises a sequence of pyrite-bearing carbonaceous grey slate and quartzite layers (Madalosso and Valle,
1978);
Serra do Poço Verde Formation, which is sub-divided into four members from the base:
• Lower Morro do Pinheiro Member, that is ~500 m thick and is composed of dolostone with cyanobacteria mats, oncolitic dolarenite, intraformational breccias and lenses of dolostone with columnar stromatolites;
• Upper Morro do Pinheiro Member comprising 300 to 500 m of dark grey dolostone with cyanobacteria mats, bird's-eye features, marls, and pyrite-bearing carbonaceous shale;
• Lower Pamplona Member, which is 100 to 200 m thick and hosts the Vazante mineralisation. It is composed of grey to pink micritic cyanobacteria laminated dolostone, grey to green slates, and sericite phyllite;
• Medium Pamplona Member comprising ~400 m of grey to pink dolostone, dolarenite, lamellar breccia, dolostone with columnar stromatolites, and shale lenses;
Morro do Calcário Formation, which corresponds to the Upper Pamplona Member of Rigobello et al. (1988), and is composed
of stromatolitic bioherm and biostrome facies, breccias, oolitic and oncolitic dolarenite and dolorudite. This unit hosts the Morro Agudo, Fagundes and Ambrósia trend Zn-(Pb) deposits;
Lapa Formation comprises a black carbonaceous slate and phyllite, the Serra do Velosinho Member, overlain by phyllite, carbonate-bearing metasiltstone, dolostone and quartzite lenses of the Serra da Lapa Member (Madalosso and Valle, 1978).
The rocks of the Vazante Group are overlain above a major thrust by chlorite-rich calc-phyllite, carbonaceous phyllite, and quartzite of the 1.2 to 0.9 Ga Canastra Group Pimentel et al., 2001) related to the late Brasiliano inversion event at ~630 Ma (Dardenne, 2000).
The Vazante Group has been variously correlated with the 1.2 to 0.9 Ga Paranoá Group or the 0.9 to 0.6 Ga Bambuí Group.
The Paranoá Group commences with a basal para-conglomerate overlain by rhythmites with mudcracks and evaporite layers, marine rhythmites, and quartzites, deposited in a platformal environment, and deeper water pelites alternate with tidal rhythmites and quartzites, storm rhythmites, limestones, and stromatolitic dolomites (Dardenne, 2000). Correlation of the Vazante Group with this unit was based on the similarity of the stromatolites found in both groups (Conophyton Cylindricus Maslov, Moeri, 1972; Conophyton metula Kirichenko; Cloud and Dardenne, 1973).
The Bambuí Group occupies the eastern side of the Brasília Fold Belt and encroaches over large areas of the São Francisco Craton. It comprises basal conglomerate and diamictite (Jequitaí Formation) of glacial origin and a platformal sequence represented by three regressive megacycles characterised by deep marine sequences at the base that grade upwards to shallow platformal, tidal and supratidal facies (Dardenne, 2000). The evidence for the correlation with the Vazante Group are the occurrence of similar diamictite units at the base of both (Dardenne, 2000), and 87Sr/86Sr signals of preserved samples from carbonate and carbonate fluorapatite of both units (Azmy et al., 2001; Misi, 2001).
Metamorphosed mafic dykes are tectonically imbricated into the sequence with associated hydraulic breccias, hydrothermally altering dolostones and slates, and the nonsulfide zinc ore within the Vazante Shear Zone (Monteiro, 1997; Monteiro et al., 1999; Babinski et al., 2005).
In the Vazante area the host sequence is cut by a 150 m thick thrust/shear zone called the Mineira Fault, which generally dips at 50° to the NW and follows the slaty interval that separates the basal dark grey stromalolitic dolomites from the upper pink dolomites of the Vazante Group. This thrust extends from 2 km to the SW of Vazante to 15 km to the NE. It encloses a string of willemite pods and lenses which are found sporadically over 10 km of its length, including the main Vazante deposits. The tectonically controlled non-sulphide zinc ore bodies are mainly by antithetic faults subsidiary to the main shear zone and display pod morphology (Monteiro et al., 2007).
Within this thrust/shear complex there are pods of dolomite which have been brecciated and cemented by silica and dolomite. These range from 1 to 15 m thick, 20 to 100 m long and 50 to 100 m down dip and are surrounded by anastomosing patterns of phyllites forming mylonitic structures. They are concentrated in dilational zones where thrust/shear planes within the main thrust zone depart from being bedding parallel and ramp across the stratigraphy forming sigmoidal or 'S' shaped structures.
The massive non-sulphide mineralisation, which accounts for the bulk of the Vazante ore, comprises coarse-grained colloform or fibrous-radiated willemite (Zn2SiO4), partially replaced by fine-grained willemite, both with strong green cathodoluminescence. The willemite is accompanied by hematite and zincite, with subordinate franklinite and smithsonite. Commonly, quartz, dolomite, barite and apatite are also associated with willemite, but sulphides are generally absent. Monteiro et al. (2006) suggests the predominance of this mineralisation type might also suggest the hydrothermal formation of willemite from a mineralising fluid at high ƒO2/ƒS2. These conditions are consistent with those experimentally predicted by Brugger et al. (2003), leading to the formation of willemite instead of sphalerite, especially at temperatures higher than 150°C.
Small sulfide-rich ore bodies are also present, essentially composed of sphalerite and galena. Sphalerite is dark brown and very homogeneous. It has a strong yellow cathodoluminescence, which could be related to its high Cd (average of 8410 ppm) and low Fe (average of 0.09%) contents (Monteiro et al., 2006). Willemite is also present in the sulphide-rich bodies and is commonly associated with pervasive silicification along the mylonitic foliation, as part of two distinct associations: i). willemite +sphalerite +franklinite ±zincite (without quartz) and ii). willemite +quartz +dolomite +franklinite ±barite ±smithsonite (without sphalerite). These assemblages are taken to suggest willemite was formed from sphalerite and quartz by the reaction: 2 ZnS + 2 SiO2 + 2 O2 → 2 ZnSiO2 + S2, which are interpreted to indicate that ƒS2 and ƒO2 may have played an important role in the stability of this mineral assemblage (Monteiro et al., 1999, 2006).
Hematite and Zn-chlorite are also found in brittle-ductile or brittle structures, generally cutting the previous willemite generations and forming hematite-rich nonsulphide zinc mineralisation, which are ubiquitous in the Vazante deposit. Later siderite and sphalerite veinlets cut the nonsulphide zinc mineralisation, and are considered as related to fluctuations in the ƒO2/ƒS2 conditions (Monteiro, 1997; Monteiro et al., 1999).
The deposit is selectively mined by shallow open pit over a 2.5 km interval and as individual stopes underground. Ore is only found in dolomite breccias formed by thrust/shears cutting two particular dolomitic units. It is unclear whether these host rocks are mineralised outside of the thrust zone.
The cumulative non-sulphide pod reserve + production, was calculated in 1997 as 16 Mt @ 29% Zn. Monteiro et al. (2006) quote the Vazante Willemite as containing 'Measured Reserves' of 28.5 Mt @ 18.3% Zn.
Remaining NI 43-101 compliant Mineral Resources and Ore Reserves at Vazante at 31 December, 2020 (Nexa Resources S.A., Reserves and Resources Report online) were:
Measured + Indicated Resources - 6.28 Mt @ 6.88% Zn, 0.14% Pb, 7.1 g/t Ag;
Inferred Resource - 13.85 Mt @ 6.86% Zn, 0.18% Pb, 9.5 g/t Ag.
Proved + Probable Reserves - 16.68 Mt @ 8.61% Zn, 0.23% Pb, 13.7 g/t Ag.
It is presumed, Resources are additional to Reserves.
The most recent source geological information used to prepare this decription was dated: 2007.
Record last updated: 23/12/2021
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.
Vazante
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Selected References:
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Alvarenga, C.J.S., Oliveira, G.D., Vieira, L.C., Santos, R.V., Baptista, M.C. and Dantas, E.L., 2019 - Carbonate chemostratigraphy of the Vazante Group, Brazil: A probable Tonian age: in Precambrian Research v.331, 20p. doi.org/10.1016/j.precamres.2019.105378.
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Babinski M, Monteiro L V S, Fetter A H, Bettencourt J S and Oliveira T F, 2005 - Isotope geochemistry of the mafic dikes from the Vazante nonsulfide zinc deposit, Brazil: in J. of South American Earth Sciences v18 pp 293-304
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Carvalho M.O., Valeriano C.M., Neto C.C.A., Diniz Oliveira G. and Heilbron, M., 2019 - The Vazante and Canastra groups revisited: Sm-Nd and Sr isotopes - evidence for contribution from Tonian intraplate magmatism during passive margin development along the SW Sao Francisco margin, Brazil: in Brazilian Journal of Geology, 16p. DOI: 10.1590/2317-4889201920180081.
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Cunha, I. de A., Misi, A., Babinski, M. and Iyer, S.S.S., 2007 - Lead isotope constraints on the genesis of Pb-Zn deposits in the Neoproterozoic Vazante Group, Minas Gerais, Brazil: in Gondwana Research v.11, pp. 382-395.
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Dardenne M A, Freitas-Silva F H 1999 - Pb-Zn ore deposits of Bambui and Vazante Groups, in Sao Francisco Craton and Brasilia Fold Belt, Brazil: in Silva M G, Misi A (Ed.), 1999 Base Metal Deposits of Brazil CPRM, Brazil pp 75-83
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Hitzman M W 1997 - Sediment-hosted Zn-Pb and Au deposits in the Proterozoic Paracatu Vazante fold belt, Minas Gerais, Brazil: in http://www.dregs.org/absa997.html 1p
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Hitzman M W, Reynolds N A, Sangster D F, Allen C R, Carman C E 2003 - Classification, genesis, and exploration guides for nonsulfide zinc deposits: in Econ. Geol. v98 pp 685-714
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Monteiro, L.V.S., Bettencourt, J.S., Juliani, C. and Oliveira, T.F., 2007 - Nonsulfide and sulfide-rich zinc mineralizations in the Vazante, Ambrosia and Fagundes deposits, Minas Gerais, Brazil: Mass balance and stable isotope characteristics of the hydrothermal alterations: in Gondwana Research v.11, pp. 362-381.
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Monteiro, L.V.S., Bettencourt, J.V., Juliani, C. and Oliveira, T.F., 2006 - Geology, petrography, and mineral chemistry of the Vazante non-sulfide and Ambrosia and Fagundes sulfide-rich carbonate-hosted Zn-(Pb) deposits, Minas Gerais, Brazil: in Ore Geology Reviews v.28, pp. 201-234.
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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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