Morro Agudo

Minas Gerais, Brazil

Main commodities: Zn Pb
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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 deposit lies 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, composed of three fold belts: i). the Araguaia Belt to the east of the Amazonia Craton, ii). the Paraguay Belt on the southeastern margin of the Amazonia Craton, and iii). the Brasília Fold Belt 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-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).

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.

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, 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 gray to pink dolomite with algal laminations and dolarenites with stromatolites (Dardenne, 2000). The 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 bounded by a normal fault that strikes at 340 to 350° and dips 75°S, and is interpreted as a syn-sedimentary feeder zone and conduit for ore fluids. The mineralisation is both disseminated and massive along the fault zone and comprises sphalerite, galena and pyrite, associated with quartz and barite (Dardenne and Freitas-Silva, 1999; Misi et al., 2005; Cunha et al., 2007; Monteiro et al., 2007).

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 - 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.

Sulphides are interstitial within the dolarenite, occurring as disseminations, small discontinuous veinlets and remobilisations within the breccias. Grade and thickness are best adjacent to the fault.

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).

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 28/10/2013
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.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
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

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