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Cuiaba |
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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 BIF-hosted Cuiabá orogenic Au deposit is located in the northern part of the Quadrilátero Ferrífero district, near Sabará, ~40 km NE of Belo Horizonte, in the State of Minas Gerais, SE Brazil.
(#Location: 19° 51' 56"S, 43° 44' 8"W).
Mainly surficial and shallow mineralisation was worked at Cuiabá by Portuguese prospectors and artisanal miners from ~1740. Between 1877, when the amalgamated mine workings were acquired by the British St. John Del Rey Mining Co., and 1940, the mine only operated intermittently. In 1977, Morro Velho Mineração S.A. (a joint venture between Anglo American, Bozano and Simonsen Bank) took control, and undertook a program of exploration and development, culminating in the reopening of the mine in 1985 as a large scale mechanised operation. In 1996, the company became a wholly owned subsidiary of the Anglo American Group. In 1999, ownership was transferred to the holding company, AngloGold, now AngloGold Ashanti, operating locally as AGA Mineração. As of late 2015 the deposit had produced ~155 tonnes of gold, 103 tonnes of which were mined between 1985 and 2008. Annual production between 2105 and 2018 averaged ~1.2 Mt of ore per annum, with an average grade of 7.96 g/t Au, with by-product Ag and sulphuric acid, at a cut-off grade of 2.82 g/t Au (Kresse et al., 2018).
For details of the regional setting, see the Quadrilátero Ferrífero District Gold - Geological Setting record. See the detailed description of the Rio das Velhas Supergroup within the Nova Lima-Caeté Domain in that record, concentrating on the Nova Lima Goup, in particular the Mestre Caetano Formation in the middle of that group. This outlines the geology of the host sequence at Cuiabá. The geological map on the same link also shows the location of the deposit.
Geology
The lithostratigraphic succession within the deposit area, includes volcanic, volcaniclastic and sedimentary rocks, characterised by greenschist facies mineral assemblages, and structurally controlled by the Cuiabá anticlinal fold (e.g. Vitorino, 2017). Detrital zircons (U-Pb-SHRIMP) yield a minimum age of 2.74 Ga for the volcaniclastic rocks (Schrank and Machado, 1996; Lobato et al. 2001). The succession is subdivided into lower, middle and upper units of the Nova Lima Group, as defined by Ladeira (1980). At Cuiabá, the sequence comprises:
• Lower stratigraphic unit, characterised by a >400 m thick succession of alternating bands of chloritised, mafic volcanic and pelitic rocks, which are altered to carbonate and sericite, as well as interbedded lenses of carbonaceous pelite. The basal mafic rocks are light green and fine-grained, and are both massive and foliated. The principal minerals are amphibole, plagioclase, clinozoisite/epidote, chlorite and quartz. Vieira (1991) described the presence of pillow lavas, and variolites of andesitic composition that have been metamorphosed to an assemblage of albite, epidote, zoisite (clinozoisite), quartz, actinolite, carbonate and chlorite.
• Middle stratigraphic unit, which comprises the 'Algoma-type' Cuiabá BIF (banded iron formation). This BIF is composed of rhythmic alternations of dark, white, and ochre-coloured, mm- to cm-thick bands, rich in carbonaceous matter and fine-grained quartz and carbonate. Its lower section is typically banded and rich in Fe carbonate and carbonaceous matter. The upper parts, separated by a 15 cm thick mafic volcanic unit, is a highly deformed ferruginous chert, also containing carbonaceous matter.
• Upper stratigraphic unit, a succession of carbonate, muscovite and chlorite altered mafic volcanic, volcanoclastic and pelitic rocks. The upper volcanic units have a basaltic compositions, with a higher Fe/Mg ratio compared to the andesitic mafic rocks of the lower unit (Vieira, 1991). Some 500 m of dacitic to rhyolitic volcaniclastic rocks occur at the top of the stratigraphic sequence, representing the most abundant rock type in the deposit area. Quartz and plagioclase phenocrysts are the main components in a light mica-chlorite-quartz-carbonate matrix. Up to 30 m thick mafic dykes crosscut all other rock types.
• These three units are overlain by ~1000 m of volcaniclastic rocks (Vieira 1991; Lobato et al., 1998; Xavier et al., 2000).
The absence of hornblende within this sequence indicates peak metamorphism did not exceed the greenschist-amphibolite facies transition (Spear 1995). The mineral assemblage is characteristic of metamorphic temperatures of 350 to 450°C and is stable over a large pressure range of up to 4 kbars (Spear 1995).
Structure
The rocks at the Cuiabá deposit were subjected to at least three deformation events (Ribeiro-Rodrigues et al., 2007). D1 and D2 were developed under ductile to ductile-brittle conditions under SE-NW directed compressive stress, with NW vergence. The brittle-ductile D3 structures were the result of east-west compressive stress (Ribeiro-Rodrigues et al., 2007). All of the lithological units are overprinted by a pervasive axial planar foliation that is locally mylonitic, with a prominent mineral stretching lineation indicated by the preferred orientation of elongated sericite, carbonates and sulphide minerals. Late, NW-verging, sigmoidal thrust faults reactivated preexisting structures and caused folding, boudinage and rotation of the Cuiabá BIF (Vitorino 2017).
The Cuiabá fold, which structurally controls the ore deposit, is an anticline which has an inverted north limb and a normal sequence on its south flank, marked by the BIF. The 116° trending fold axis plunges from 35° in the upper section to 12° below level 18. The plunge direction controls the geometry of the orebodies. Vial (1980), Toledo (1997), Ribeiro-Rodrigues (1998) and Ribeiro-Rodrigues et al. (2007) interpreted this fold to be a tubular structure, plunging at 30°SE. More recently, data for the nearby Lamego gold deposit, the fold geometry of which is comparable to that of the Cuiabá fold, suggest the structure is a reclined, isoclinal, cylindrical and rootless (sheath) fold (Martins et al., 2016). Kresse et al. (2020) show the structure to have a concentric oval/tear shape on the 11 Level, elongated in an ENE-WSW direction, as defined by the banded iron formation and other key lithologies.
According to AngloGold Ashanti (2021), the apparent intersections of thrust faults with tight isoclinal folds in a ductile environment, tend to control the mineralisation structures.
Mineralisation
The gold orebodies at Cuiabá all have a strong structural control, characterised by shoots with a consistent, down-plunge continuity (Ribeiro Rodrigues et al., 2007). Three different mineralisation styles are recognised: i) stratabound-replacement, commonly of BIF; ii) disseminated, related to hydrothermal alteration in shear zones; and iii) shear zone related, quartz-carbonate-sulphide veins (e.g., Ribeiro Rodrigues et al., 2007, Martins et al., 2016). Each of these styles reflect variations in fluid-rock interaction, and consequently a distinctive sulphide-gold assemblage and composition (Lobato and Vieira, 1998). Six main BIF hosted orebodies, as defined by a >4 g/t Au cutoff, have been delineated, namely: Balancão, Cantagalo, Fonte Grande, Fonte Grande Sul, Serrotinho and Galinheiro. These stratabound-replacement bodies are usually sulphide-rich segments of the Cuiabá BIF, which grade laterally into non-economic or barren iron formation. Associated quartz vein-hosted orebodies are the Veio de Quartzo, Galinheiro Footwall, Serrotinho Footwall and Fonte Grande Sul Footwall. Minor orebodies include Galinheiro Extensão, Galinheiro footwall, Surucu, Canta Galo and Dom Domingos. Mafic volcanic-hosted orebodies such as Viana and Galinheiro Quartzo are associated with the upper mafic unit and were locally mined in the past. In addition, secondary gold mineralisation occurs in hydrothermally altered schists in the footwall of Galinheiro, in hydrothermally altered schists and quartz veins near the footwall of Fonte Grande Sul and Serrotinho, and in close proximity to the hangingwall of Serrotinho.
The Fonte Grande Sul BIF replacement orebody is situated in the SE closure of the Cuiabá fold and is the largest and highest grade shoot in the deposit. It had an estimated reserve of 0.8 Mt @ 8.11 g/t Au with additional resources calculated at 4.91 Mt @ 13.87 g/t Au (AngloGold Ashanti 2018). The thickness of the orebody is relatively constant, varying from 4 to 6 m, although its area and gold grade increase progressively with depth. The mineralised BIF is intensely sulphidised, folded and disrupted by D1 thrust faults, generally containing disseminated recrystallised and massive ore that lacks banding. The host BIF is composed of rhythmic alternations of dark, white and ochre-coloured, mm- to cm-thick bands, rich in carbonaceous material, fine-grained quartz and carbonate, and is sandwiched between footwall and hanging wall mafic volcanic rocks (Xavier et al., 2000; Lobato and Vieira, 1998). The country rocks to the mineralised BIF contain strong sericite, carbonate and chlorite alteration halos, typical of greenschist facies metamorphic conditions. Transitions from sulphide-rich to -poor portions of the BIF are gradational, or are marked by shear zones or quartz veins.
The Veio de Quartzo quartz vein orebody has significant gold grades associated with andesite-hosted quartz rich shear veins and is located between levels 9 and 17 in the southeastern part of the Cuiabá fold hinge. Mineralisation is dominantly controlled by a strike slip, oblique shear zone, with native gold occurring in quartz-rich shear veins, and disseminated sulphide minerals in hydrothermally altered country rocks. Grades may be as high as ~500 g/t Au due to the presence of free coarse gold (Vitorino 2017).
The typical sulphide mineral assemblage within the mineralised Cuiabá BIF is pyrite-pyrrhotite-arsenopyrite with minor chalcopyrite and sphalerite, and gold in equilibrium. Gold is also found as inclusions and intergrowths in pyrite, arsenopyrite and pyrrhotite, varying in size between 10 and 80 µm, occurring in fractures or along sulphide grain boundaries. Pyrite accounts for >90 vol.% of the sulphides, mostly occurring as porous grains that may be zoned, with As-rich centres.
Alteration
The hosts rocks are variably affected by alteration, a reflection of their physical and compositional characteristics, as follows:
• Cuiabá Banded iron formation, which ranges from 1 to as much as 15 m in thickness, and has a characteristic alternating layering of dark (quartz-carbonate-carbonaceous matter) and pale orange (quartz-carbonate-chert) bands. Pyrite, pyrrhotite and arsenopyrite are abundant and in equilibrium, accompanied by gold grains in fractures and along contacts between sulphide grains. Two individual BIF sequences can be distinguished within the deposit, separated by a 15 cm thick mafic volcanic unit. The lower of these is typically banded with abundant Fe-carbonate, and carbonaceous material, and is the principal host to the gold mineralization. The upper BIF includes a strongly deformed ferruginous chert, as well as carbonaceous matter (Lobato et al., 2001).
• Carbonaceous pelite, is only locally altered by sulphide-quartz-carbonate veinlets. It surrounds the Cuiabá BIF, and mainly comprises carbonaceous matter, hydrothermal smoky (Qz1) and granoblastic quartz (Qz2), carbonate minerals, chert layers, chlorite, muscovite and sulphide minerals. Late phase mineralisation includes pyrite, arsenopyrite with minor proportions of pyrrhotite, chalcopyrite and sphalerite. Free gold occurs, both in equilibrium with sulphides and dispersed in the carbonaceous material. There is a well defined lithological boundary between carbonaceous pelites and mafic volcanics, which varies from concordant to slightly deformed. In contrast, the contact between carbonaceous pelite and the Cuiabá BIF is intensely deformed.
• Andesite of the mafic volcanic assemblage is typically overprinted by pervasive alteration to an assemblage of chlorite with albite ±epidote ±zoisite (clinozoisite) ±quartz ±actinolite ±carbonate (Vieira, 1987). In more detail, alteration of these mafic volcanic rocks progresses from a distal assemblage of chlorite, carbonate ±sericite, and sulphide minerals → intermediate alteration, characterised by an assemblage of carbonates, chlorite, quartz, plagioclase and sulphide minerals → proximal quartz, sericite, sulphide minerals and gold.
The absence of hornblende indicates that peak metamorphic conditions did not exceed the greenschist-amphibolite facies transition (Spear, 1995). Sulphide minerals are accessory within the orebodies, and include mainly pyrite, chalcopyrite and sphalerite in equilibrium. They appear dispersed in the matrix, and are commonly associated with carbonate minerals. Gold mineralisation is in equilibrium with pyrite. Lenses of carbonaceous pelite also occur within the lower and upper mafic volcanic units.
Production, Reserves and Resources
Historic production to late 2015 totalled ~155 tonnes of gold (quoted in Kresse et al., 2018).
Measured + Indicated + Inferred Mineral Resources at 31 December, 2015 (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2015), exclusive of Ore Reserves:
18.38 Mt @ 9.59 g/t Au for 176.36 tonnes of contained gold;
Proved + Probable Ore Reserves at 31 December, 2015 (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2015):
4.83 Mt @ 6.55 g/t Au for 31.63 tonnes of contained gold;
Exclusive Mineral Resources in pillars, satellite orebodies, not yet converted to Inferred, etc., at 31 December, 2015 (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2015):
13.73 Mt @ 10.07 g/t Au for 138.2 tonnes of contained gold.
Measured + Indicated + Inferred Mineral Resources at 31 December, 2021 (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2015):
29.08 Mt @ 5.87 g/t Au for 170.68 tonnes of contained gold;
Proved + Probable Ore Reserves at 31 December, 2015 (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2021):
7.89 Mt @ 4.69 g/t Au for 36.97 tonnes of contained gold;
Ore Reserves below infrastructure, at 31 December, 2021 (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2015):
4.29 Mt @ 4.93 g/t Au for 21.16 tonnes of contained gold.
NOTE: Mineral Resources are inclusive of Ore Reserves (but does not include that below infrastructure).
The information in this summary is drawn from Kresse et al. (2018) and Kresse et al. (2020) and the AngloGold Ashanti Mineral Resource and Ore Reserve Report (2015, 2021).
The most recent source geological information used to prepare this decription was dated: 2021.
Record last updated: 4/1/2023
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.
Cuiaba
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Selected References:
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Anonymous 2001 - Morro Velho: in extract from AngloGold web site http://www.anglogold.com/ informationforinvestors/annualreport99/20f/i1-4susa.htm 2p
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de Alvarenga, C.J.S., Cathelineau, M. and Dubessy, J., 1991 - Au-ore deposition-rock deformation-ore fluid chemistry relationships in quartz veins from Cuiba, Brazil: in Ladeira, E.A. (Ed.), Brazil Gold 91 Balkema, Rotterdam, pp 335-338
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Ferraz da Costa, M., Kyle, J.R., Lobato, L.M., Ketcham, R.A., Figueiredo e Silva, R.C. and Fernandes, R.A., 2022 - Orogenic gold ores in three-dimensions: A case study of distinct mineralization styles at the world-class Cuiaba deposit, Brazil, using high-resolution X-ray computed tomography on gold particles: in Ore Geology Reviews v.140, 18p. doi.org/10.1016/j.oregeorev.2021.104584.
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Kresse, C., Lobato, L.M., Figueiredo e Silva, R.C., Hagemann, S.G., Banks, D. and Vitorino, A.L.A., 2020 - Fluid signature of the shear zone-controlled Veio de Quartzo ore body in the world-class BIF-hosted Cuiaba gold deposit, Archaean Rio das Velhas greenstone belt, Brazil: a fluid inclusion study: in Mineralium Deposita v.55, pp. 1441-1466.
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Kresse, C., Lobato, L.M., Hagemann, S.G. and Figueiredo e Silva, R.C., 2018 - Sulfur isotope and metal variations in sulfides in the BIF-hosted orogenic Cuiaba gold deposit, Brazil: Implications for the hydrothermal fluid evolution: in Ore Geology Reviews v.98, pp. 1-27.
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Lobato L M, Santos J O S, McNaughton N J, Fletcher I R and Noce C M, 2007 - U-Pb SHRIMP monazite ages of the giant Morro Velho and Cuiaba gold deposits, Rio das Velhas greenstone belt, Quadrilatero Ferrifero, Minas Gerais, Brazil: in Ore Geology Reviews v32 pp 674-680
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Ribeiro-Rodrigues L C, de Oliveira C G and Friedrich G, 2007 - The Archean BIF-hosted Cuiaba Gold deposit, Quadrilatero Ferrifero, Minas Gerais, Brazil: in Ore Geology Reviews v32 pp 543-570
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Soares, M.B., Selby, D., Robb, L. and Correa Neto, A.V., 2021 - Sulfide Recrystallization and Gold Remobilization During the 2.0 Ga Stage of the Minas Orogeny: Implications for Gold Mineralization in the Quadrilatero Ferrifero Area, Brazi: in Econ. Geol. v.116, pp. 1455-1466.
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Viera F W R, Corbani M, Fonesca J T F, Pereira A, de Oliveira G A I, Clemente P L C 1986 - Excursion to the Cuiba gold mine, Minas Gerais, Brazil: in Thorman C H, Ladeira E A, Schnabel D C (Ed.s), Gold Deposits Related to Greenstone Belts in Brazil - Deposit Modelling Workshop, Part A Excursions pp A79-A86
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Viera F W R, Lisboa L H A, Chaves J L, de Oliveira G A I, Clemente P L C, de Oliveira R L 1986 - Excursion to the Morro Velho gold mine, Minas Gerais, Brazil: in Thorman C H, Ladeira E A, Schnabel D C (Ed.s), Gold Deposits Related to Greenstone Belts in Brazil - Deposit Modelling Workshop, Part A Excursions pp A67-A73
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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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