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Fazenda or Fazenda Brasileiro - Maria Preta |
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Bahia, Brazil |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Fazenda or Fazenda Brasileiro or Maria Preta gold deposit is located at Serrinha in Bahia State, Brazil, some 250 km north-west of Salvador. It falls within the Palaeoproterozoic Rio Itapicuru greenstone belt, near the north-eastern margin of the Archaean to Palaeoproterozoic São Francisco Craton,
The Companhia Vale do Rio Doce (CVRD) had explored the Weber Belt of the Rio Itapicuru greenstone belt since the late 1960s before economic mineralisation was discovered at Fazenda Brasileiro in the late 1970s, and a mine commenced production in 1984. In 2003, the operation was acquired by Yamana Gold Inc. who operated the mine from 2004 to 2014, when it, with other Brazilian mines, was spun off into Brio Gold Inc. In 2018, Leagold Mining Corp. (Leagold) acquired the mine and continued mining. In 2020, Equinox acquired Leagold and has continued production to the present (2022). The Mine is now generally referred to as Fazenda.
The gold deposit is associated with polyphase shearing associated with two regional-scale, sinistral-oblique brittle-ductile shear zones within the Weber Belt, an arcuate, 10 km long, east-west trending, south dipping shear zone that is abruptly folded towards the south near its western extremity. This belt is host to the most significant gold mineralisation of the Palaeoproterozoic Rio Itapicuru greenstone belt, dated at ~2020 Ma in the Orosirian. The Weber Belt also hosts the Barrocas Oeste, Papagaio, Lagoa do Gato and Canto zones, all of which have been in production from small tonnage open pits.
The Weber Belt contains four distinct overturned sequences, which are, from south to north:
• The Riacho do Incó unit, comprising carbonate-chlorite schist with minor intercalated carbonaceous schist, interpreted to represent a basaltic lava protolith;
• The Fazenda Brasileiro unit, which hosts the most important gold concentrations, and is dominated by felsic and mafic schists. This unit is further sub-divided into:
- The graphitic schist, which forms the hanging wall of the main Fazenda ore zone, and due to its lateral persistence and distinctive character, is utilised as a marker horizon;
-The magnetic quartz-chlorite schist, which consists of two major layers of 20 m and 3 m average thickness; part of this unit is at contact with the graphitic schist and hosts the main Fazenda ore shoot, which is the bulk of the mineralisation, which occurs as quartz-albite-sulphide veins;
-The Intermediate Sequence, which is comprised of sericite-chlorite-carbonate schist, which represent less-mafic surrounding basalts; and plagioclase-actinolite schist, derived from weakly altered gabbroic bodies, which show ophitic to subophitic textures, and are distributed through the sericite-chlorite-carbonate schist and sometimes within the quartz-chlorite schist units.
• The Canto unit, which consists of i). fine-grained carbonaceous sedimentary rocks, mainly after pelites and rhythmically banded pelites and psammites, ii). volcanic layers, and iii). an agglomeratic pyroclastic sequence. The pyroclastic sequence is the main host rock for the Canto mineralisation.
• The Abóbora unit in the northern-most part of the Weber Belt is comprised of a thick sequence of basalt flows with local, narrow sedimentary intercalations.
Structural relationships indicate that the primary control of mineralisation was a D1 zone of intense ductile shearing with no major folds, that had an ~8 km east-west extension which developed contemporaneously with granitic diapirism and greenshist grade metamorphism. The main gold lode is interpreted to have formed after the ductile phase, during D2 brittle-ductile dextral transpressive shearing and folding. This deformation produced northward-verging asymmetric folds F2 on all scales. This event folded the pre-existing D1 shear fabrics and produced the ductile-brittle shear zones that are oriented generally east-west, parallel to the axial planes of the folds. D2 appears to have been responsible for the present southerly dip of the entire local succession. Late localised crenulations to open metre-scale folds have refolded the first set of folds but are not important regionally. Late brittle faulting laterally offset mineralisation by up to 100 m.
The deposit is hosted by, or is directly associated with, a highly deformed greenschist facies metamorphosed volcano-sedimentary sequence, occurring along the margin of a mafic (gabbroic) sill at the contact of tholeiitic basalts and a sequence of pelitic schists and psammites intercalated within the volcanic succession. The ore occurs within carbonaceous volcaniclastic rocks, breccia and stockwork systems as hydrothermal quartz-gold shear veins within iron rich chlorite-magnetite schists, and as disseminations in felsic and mafic sub-volcanic bodies. Alteration is pervasive throughout the shear zones and is characterised by carbonatisation and sericitisation of intermediate volcanic, volcaniclastic, and sub-volcanic rocks.
Gold occurs in at least three textural settings: i). as particles attached to sulphide grains; ii). as particles within fractures in sulphide grains; and iii). as particles within fractures in quartz/albite gangue. Gold grains typically contain <5% Ag.
The mineralised sulphide-bearing quartz veins are composed of quartz-albite-carbonate-arsenopyrite-pyrite-gold. There were at least four overlapping periods of veining to produce a complex lode zone with veins at different angles and orientations to the structural fabric. The stacked veins vary in true width from 1.5 to 40 m and horizontal mining widths vary from 3 to 40 m. The regional strike of mineralisation is north-south, while locally the veins are generally arcuate in an east-west trend, and south dipping at 40 to 70°, with a shallow to moderate east plunge. The plunge is quite variable, with some zones plunging westerly. Whilst the bulk of the mineralisation is found in the magnetic quartz-chlorite schist (CLX1), mineralisation is also found at a stratigraphically lower mineralised domain, in the CLX2 and in the Canto unit. Economic mineralisation in these occur as horizontal to sub-horizontal shoots, the locations of which are influenced by a combination of folding and shearing. Shoots range from tens of metres to hundreds of metres in length, and tens of metres in thickness.
Prior to 1984, Reserves originally totalled: 15 Mt @ 8 g/t Au containing ~120 tonnes of gold.
After ~26 years of mining the remaining Ore Reserves and Mineral Resources at 31 December, 2010 (Yamana Gold website) were:
Proved + probable reserve - 1.80 Mt @ 3.04 g/t Au for 5.47 t Au;
Measured + indicated resource - 7.88 Mt @ 1.86 g/t Au for 14.7 t Au;
Inferred resource - 2.02 Mt @ 3.11 g/t Au for 6.3 t Au.
NOTE: Mineral Resources are inclusive of Ore Reserves Total contained gold in resources = 21 tonnes
Ore Reserves and Mineral Resources at 31 December, 2015 (Moore, Michaud and Hampton, 2016 for Brio Gold) were:
Proved Reserves - Underground 4.00 Mt @ 1.94 g/t Au; Open Pit - 0.599 Mt @ 1.94 g/t Au; TOTAL - 4.599 Mt @ 1.94 g/t Au:
Probable Reserves - Underground 1.875 Mt @ 1.74 g/t Au; Open Pit - 0.022 Mt @ 1.79 g/t Au; TOTAL - 1.897 Mt @ 1.74 g/t Au:
Proved + Probable Reserves - 6.496 Mt @ 1.88 g/t Au at a cut-off grade of 1.06 g/t Au underground and 0.40 g/t open pit.
Measured Resources - Underground 1.25 Mt @ 4.10 g/t Au; Open Pit - 0.29 Mt @ 3.24 g/t Au; TOTAL - 1.539 Mt @ 3.86 g/t Au:
Indicated Resources - Underground 0.342 Mt @ 3.23 g/t Au; Open Pit - 0.03 Mt @ 2.32 g/t Au; TOTAL - 0.372 Mt @ 3.15 g/t Au:
Measured + Indicated Resources - Underground 1.592 Mt @ 3.84 g/t Au; Open Pit - 0.32 Mt @ 3.15 g/t Au; TOTAL - 1.912 Mt @ 3.72 g/t Au:
Inferred Resources - Underground 0.84 Mt @ 2.2 g/t Au; Open Pit - 0.18 Mt @ 5.3 g/t Au; TOTAL - 1.02 Mt @ 2.8 g/t Au:
NOTE: Mineral Resources are inclusive of Ore Reserves. Total contained gold in resources = 10 tonnes
Ore Reserves and Mineral Resources at 31 December, 2020 (Araújo, et al., 2016 for Equinox Gold) were:
Probable Reserves - Underground 0.434 Mt @ 1.49 g/t Au; Open Pit - 0.835 Mt @ 0.84 g/t Au; TOTAL - 1.335 Mt @ 1.09 g/t Au:
Proved Reserves - Underground 3.858 Mt @ 1.67 g/t Au; Open Pit - 1.461 Mt @ 1.32 g/t Au; TOTAL - 5.319 Mt @ 1.57 g/t Au:
Proved + Probable Reserves - 6.653 Mt @ 1.47 g/t Au.
Measured + Indicated Resources - Underground 3.426 Mt @ 2.10 g/t Au; Open Pit - 1.741 Mt @ 1.13 g/t Au; TOTAL - 5.167 Mt @ 1.77 g/t Au:
Inferred Resources - Underground 1.72 Mt @ 1.90 g/t Au; Open Pit - 1.563 Mt @ 1.05 g/t Au; TOTAL - 3.282 Mt @ 1.50 g/t Au:
NOTE: Mineral Resources are inclusive of Ore Reserves. Total contained gold in resources = 14 tonnes.
Calculated at a cut-off grade of 1.06 g/t Au underground, and 0.40 g/t open pit.
The information in this summary is largely drawn from: Moore, C.M., Michaud, R.L. and Hampton, A.P., 2016 - Technical report on the Fazenda Brasileiro Mine, Bahia State, Brazil; an NI 43-101 Technical Report, prepared by Roscoe Postle Associates Inc., Toronto, Ontario, Canada, for Brio Gold Inc. of Denver, Colorado, USA, 176p. and
Araújo, F.M., Andrade Filho, H.R., Lipper, G.C., Torresini, C.A. and Cardoso, T.V., 2020 - Technical Report on the Fazenda Brasileiro Gold Mine, Bahia State, Brazil; an NI 43-101 Technical Report, prepared by Equinox Gold Corp. of Vancouver, BC, Canada, 173p.
The most recent source geological information used to prepare this decription was dated: 2021.
Record last updated: 5/7/2022
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.
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Mello E F, Xavier R P, McNaughton N J, Hagemann S G, Fletcher I and Snee L, 2006 - Age constraints on felsic intrusions, metamorphism and gold mineralisation in the Palaeoproterozoic Rio Itapicuru greenstone belt, NE Bahia State, Brazil: in Mineralium Deposita v40 pp 849-866
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Reinhardt M C, Davison I 1990 - Structural and lithologic controls on Gold deposition in the shear zone-hosted Fazenda Brasileiro Mine, Bahia state, northeast Brazil: in Econ. Geol. v85 pp 952-967
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Teixeira J B G, Kishida A, Marimon M P C, Xavier R P, McReath I, 1990 - The Fazenda Brasileiro Gold deposit, Bahia: geology, hydrothermal alteration, and fluid inclusion studies: in Econ. Geol. v85 pp 990-1009
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Teixeira, J.B.G., Misi, A., da Silva, M.G. and Brito, R.S.C., 2019 - Reconstruction of Precambrian terranes of Northeastern Brazil along Cambrian strike-slip faults: a new model of geodynamic evolution and gold metallogeny in the State of Bahia: in Brazilian Journal of Geology v.49, 21p. doi: 10.1590/2317-4889201920190009
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Xavier R P and Foster R P, 1999 - Fluid evolution and chemical controls in the Fazenda Maria Preta (FMP) gold deposit, Rio Itapicuru Greenstone Belt, Bahia, Brazil: in Chemical Geology v.154 pp. 133-154
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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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