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Morro Agudo, Ambrosia Sul, Ambrosia Norte, Bonsucesso, Fagundes

Minas Gerais, Brazil

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The Morro Agudo carbonate hosted zinc-lead deposit is located southeast of Paracatu in Minas Gerais, Brazil (#Location: 17° 30' 14"S, 46° 49' 15"W).

The Ambrósia and Fagundes carbonate hosted zinc deposit are located 30 km NNE and 15 km NE of the town of Paracatu, 60 and 45 km north of the sumilar Morro Agudo carbonate hosted zinc deposit, and 120 and 105 km north of the Vazante silicate zinc deposit also in Minas Gerais.

Modern underground mining commenced at the Morro Agudo Mine in 1988. The Ambrósia Norte deposit was discovered in 1973, Ambrósia Sul in 2011, and Bonsucesso in 2014. Mining of the Ambrósia Sul deposit commenced in 2017. During the fourth quarter of 2020, operations at Ambrósia Sul, which was reaching the end of its life of mine, were suspended. Production from Ambrósia Sul between 2018 and 2020 totalled 0.693 Mt @ 3.12% Zn, 0.19% Pb; while production from the Morro Agudo Mine over the same period was 2.39 Mt @ 2.36% Zn, 0.59% Pb (Nexa Resources S.A., 2021).

All three of the deposits lie within the Vazante Group of the Brasilia Fold Belt on the western margin of the São Francisco craton. The Brasília Fold Belt is roughly 1000 km long and is part of the Tocantins Province that is composed of three fold belts: i). the Araguaia Belt in the north, to the east of the Amazonia Craton, ii). the Paraguay Belt on the southeastern margin of the Amazonia Craton, to the west and iii). the Brasília Fold Belt to the south, along the western edge of the São Francisco Craton (Almeida et al., 1981; Pimentel et al., 2001; Valeriano et al., 2008; Rodrigues et al., 2010). The Paraguay and Araguaia fold belts are thrust onto the Amazonia Craton to the west; whilst the central Brasília Fold Belt is thrust east onto the São Francisco Craton (Valeriano et al., 2008).

The Brasília Fold Belt was initiated by continental rifting of the São Francisco Craton at ~1700 Ma, accompanied by the deposition of intracratonic siliciclastic sedimentary and platform carbonate rocks, primarily represented by the Canastra, Paranoá and Vazante groups at ~1200 to 900 Ma (Valeriano et al., 2008; Martins-Neto, 2009), and ended with basin inversion during the Brasiliano Orogeny at ~630 to 610 Ma (Misi et al., 2005; Martins-Neto, 2009). Note however, that of these three, 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. See the Vazante record for more detail of the stratigraphy of the group and the rationale for the varying correlations.

The Vazante Group is located in the NW of Minas Gerais, where it extends for ~250 km north-south, located tectonically below the older ~1200 to 900 Ma Canastra Group phyllites and quartzites, and is thrust over the younger Bambuí Group cratonic carbonate cover to the east (Dardenne, 2000; Pimentel et al., 2001; Monteiro et al., 2007). It is primarily composed of intercalated phyllites and carbonates that have undergone metamorphism up to greenschist facies. The age of the Vazante Group is uncertain and poorly constrained due to the absence of igneous rocks and well constrained detrital zircon dating.


Morro Agudo

The Morro Agudo deposit is hosted by the Morro do Calcário Formation (also known as the Upper Pamplona Member), near the top of the Vazante Group, and is composed of carbonaceous shale, dolarenites, breccias, and stromatolitic bioherm facies (Dardenne, 2000; Slezak, 2012). It overlies the Serra do Poço Verde Formation, that comprises the Upper Morro do Pinheiro Member, a dark grey dolomite with bird's eyes and minor carbonaceous phyllites, the Lower Pamplona Member, a grey to pink laminated dolomites intercalated with green and maroon phyllites, and the Middle Pamplona Member, which consists of light grey to pink dolomite with algal laminations and dolarenites with stromatolites (Dardenne, 2000). The host Morro do Calcário Formation is unconformably overlain by the Lapa Formation, which is composed of black, carbonaceous rhythmites with intercalated slates and phyllites. It also contains sericite-chlorite phyllites, carbonate bearing metasiltstones with dolomite and quartz lenses (Dardenne, 2000; Monteiro et al., 2006).

Mineralisation is hosted by shallow dipping Neoproterozoic dolarenitic breccias which are interbedded with muddy dolomite. The ore is contained within both coarse dolorudites/breccias at the top of the stromatolitic and brecciated dolomite unit and within dolarenites of the overlying intercalated dolarenite and argillaceous dolomite unit.

The mineralisation is controlled by a normal fault that cuts these lithologies, strikes at 340 to 350° and dips 75°S. This structure is interpreted to be a syn-sedimentary 'feeder zone' that acted as a preferential conduit for metalliferous fluids (Dardenne, 1979; Misi et al., 1999; Dardenne, 2000; Cunha et al., 2000). Sulphides are both disseminated and massive along the fault zone and comprises sphalerite, galena and pyrite, with associated quartz and barite (Dardenne and Freitas-Silva, 1999; Misi et al., 2005; Cunha et al., 2007; Monteiro et al., 2007).

The mineralisation is predominantly composed of disseminated fine-grained brown-coloured sphalerite and galena, with subordinate pyrite and galena. These sulphides cement unconsolidated allochemical grains and progressively replace diagenetically coated grains. The sulphides frequently exhibit convoluted or compaction structures. This mineralization style has been considered mainly as syn-diagenetic (Dardenne, 1979; Hitzman, 1997). Colloform sphalerite, galena, and pyrite and coarse-grained, zoned honey-coloured sphalerite represent a late mineralisation stage at the Morro Agudo deposit, and is similar to the main mineralisation stage of the Fagundes deposit.

The Morro Agudo mineralisation is divided into four ore zones that are stratigraphically confined to the Morro do Calcário Formation (Dardenne and Freitas-Silva, 1999; Misi et al., 2005; Cunha et al., 2007), as follows:
i). N orebody - which consists of fine-grained, alternating galena, sphalerite, pyrite and chert beds.
ii). M orebody - has an irregular shape, and is composed of coarse-grained galena and sphalerite (Misi et al., 2005; Cunha et al., 2007).
iii). JKL orebody - made up of massive fine to coarse-grained sphalerite that cements oolitic dolostone beds (Misi et al., 2005).
iv). GHI orebody - mainly coarse-grained sphalerite and galena that cements angular breccia clasts, some of which are derived from the N orebody (Cunha et al., 2007).

Within these orebodies, mineralisation is stratigraphically found in eight 'stacked' lenses, each bounded above and below by fine phyllitic rocks, and elongated parallel and adjacent to the main fault to the SE of the orebody, which is a significant regional structure. Grade and thickness are best adjacent to the fault.

Rubo and Monteiro (2010) identified three episodes of mineralisation:
i). replacement of breccia matrix to form chalcedony with very fine to fine-grained sphalerite and other sulphides, mostly preserving the micritic carbonate particles and giving rise to mineralised dolarenites;
ii). filling of voids, including veins and larger cavities, by fine to coarse-grained sphalerite, pyrite and galena;
iii).  formation of late veins with sulphides cutting the veins that were deposited in the second event.

The total production + resources is estimated at - 17.6 Mt @ 5.1% Zn, 1.5% Pb, 1.6 g/t Ag (USGS online database, accessed in 2013).



Ambrósia Trend

The Ambrósia Trend carbonate hosted zinc deposits include Ambrósia Sul, Ambrósia Norte and Bonsucesso that are located 30 km NNE of the town of Paracatu, 60 km north of the Morro Agudo deposit, and 120 north of the Vazante silicate zinc deposit, also within Minas Gerais, Brazil. They are also hosted by the Upper Pamplona Member of the Morro do Calcário Formation.

Mineralisation is controlled by the reverse Ambrósia fault that strikes at 30° and dips between 60 and 80°SE, and has been displaced by a transverse normal fault system thats trikes at 20°. The zinc mineralisation is hosted by brecciated dolostone that is tectonically imbricated with rhythmic carbonaceous shale, siltstone, marl, argillaceous dolostone and intraformational breccia layers. The latter breccias contain fragments of dolostone, phosphorite and quartz set in a matrix of dolomite, quartz, microcline and plagioclase. Relicts of the original sedimentary and diagenetic textures within clasts of the host dolostone include cyanobacteria mats, stromatolitic structures, irregular fenestrae and bird's-eyes. This dolostone commonly also contains fine layers of detrital minerals, including quartz, plagioclase and authigenic phases, mainly quartz, microcline and euhedral pyrite, set in a dolomitic matrix with associated phosphatic material. The primary zinc mineralisation is composed of veins and veinlets of honey-coloured sphalerite, pyrite, galena, marcasite, and coarse-grained white dolomite. These sulphides mainly fill micro-fractures. The first generation of sphalerite was subjected to mylonitisation, fragmentation, corrosion, dissolution, and remobilisation associated with brittle-ductile deformation. Later, light yellow sphalerite and galena veins commonly cut the mylonitised sphalerite, whilst veins of Fe-rich dolomite, dolomite, quartz, phyllosilicates and apatite are also commonly found in deformed zones. The base-metal vein sulphides overprint both stylolites and tectonic fractures, consistent with an epigenetic origin for the primary ore, which was subsequently mylonitised, while later pulses of mineralising fluids have produced for late sphalerite veining (Monteiro et al., 2007).

In addition to the sulphides, weathering and supergene processes have generated oxides, which include smithsonite and cerussite, and calamine mineralisation that comprises a mixture of hemimorphite [Zn4(Si2O7)(OH)2•H2O) and smithsonite (ZnCO3).

At Ambrósia Norte and Bonsucesso, mineralisation is predominantly vein-like, and is associated with brecciated dolostones of the Morro do Calcário Formation. Mineralisation in these deposits is generally only associated with a single structure although occasionally two or more such faults are evident. At Ambrósia Sul, mineralisation is controlled by hydrothermal breccias.

No separate reserve/resource estimates have been encountered (20 December, 2021), but quoted Measured + Indicated Resources at 31 December, 2020 (Nexa Resources S.A., Reserves and Resources Report online, 2021) for Morro Agudo and Bonsucesso have been stated as:
  17.28 Mt @ 3.25% Zn, 0.64% Pb, plus Inferred Resources of 4.7 Mt @ 3.27% Zn, 0.52% Pb.



Fagundes

The Fagundes carbonate hosted zinc deposit is located 15 km NE of the town of Paracatu, 45 km north of the Morro Agudo zinc deposit, and 105 km north of the Vazante zinc deposit, also within Minas Gerais, Brazil. Fagundes is also hosted by dolostones of the Upper Pamplona Member of the Morro do Calcário Formation, close to the contact with rhythmic graphitic black slate of the Serra do Velosinho Member of the Lapa Formation.

Within the Fagundes area, the Upper Pamplona Member is represented by cyanobacteria-laminated dolostone, dolostone with columnar stromatolite, and dolorudite composed of a dolarenitic matrix enclosing angular or rounded clasts of stromatolitic dolostone, collophane, chert, dolomicrite and dolomicrosparite intraclasts. Mineralisation is stratabound and is mainly composed of rhythmically banded, colloform, or zoned pyrite, honey-coloured sphalerite and galena. The sphalerite commonly has a depositional growth zoning pattern as a result of oscillatory variations in minor (Fe and Cd) and trace (Ge, Cu, Ag) elements (Monteiro et al., 2006). Galena predominantly has infilling or substitution textures, but is also found in veins and veinlets associated with dolomite, pyrite and subordinate sphalerite. The sulphide textures of the stratabound ore indicate mainly sulphide deposition in open spaces within the dolostone host rocks, interpreted to possibly be related to dissolution derived from reaction with the mineralising fluids. Brittle-ductile and brittle processes led to the partial to total obliteration of the originally zoned or colloform sulphide textures, ore brecciation, mylonitisation, and remobilisation. Late light-yellow coloured sphalerite veins and veinlets also cut the mylonitised ore zones. These late veins and veinlets may be related to later pulses of mineralising fluids, representing a hydrothermal-epigenetic mineralisation stage, similar to those described in the Ambrósia zinc deposit (description after Monteiro et al., 2007).

Mineral Resources, as quoted in USGS Open-File Report 2009–1297 after Touahri (1991) were: 3.0 Mt @ 4.5 % Zn.

The most recent source geological information used to prepare this summary was dated: 2007.     Record last updated: 24/12/2021
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.


  References & Additional Information
   Selected References:
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
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
Misi A, Iyer S S, Tassinari C G, Coelho C E S, Kyle J R, Franca-Rocha W J S, Gomes A S R, Cunha I A, Carvalho I G, Filho V M C  1999 - Integrated studies and metallogenic evolution of the Proterozoic sediment-hosted Pb-Zn-Ag sulfide deposits of the Sao Francisco Craton, Brazil: in Silva M G, Misi A (Ed.), 1999 Base Metal Deposits of Brazil CPRM, Brazil    pp 84-91
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.
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.


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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