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Sao Bento |
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Minas Gerais, Brazil |
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Au
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The São Bento mine is located in the eastern portion of the Quadrilátero Ferrífero District, ~50 km to the east of the city of Belo Horizonte, Minas Gerais, Brazil.
Gold has been mined intermittently in the São Bento area since 1860, originally as an open-pit operation. The earliest underground activity dates back to 1898, when the British group São Bento Gold States Ltd. began operations. Between 1898 and 1906, 0.211 Mt of ore were mined at an average recovered grade of 9.27 g/t Au for 1.95 tonnes of gold. São Bento Mineração S.A. (the local arm of Eldorado Gold Corporation), commenced mining in 1979 and produced the first bullion in 1987. From 1987 to December 2002, São Bento milled 6 Mt of ore and produced 43.6 tonnes of gold from underground mining of sulphide-bearing iron formation (Martins Pereira et al., 2007). In 2007, the São Bento Mine was acquired from Eldorado Gold Corporation by AngloGold Ashanti and absorbed into their Córrego do Sítio operation. Remaining Ore Reserves at the end of 2002 were 1.79 Mt @ 9.22 g/t of Au, totaling some 16.5 tonnes of gold (Martins Pereira et al., 2007). The total gold endowment (production + Reserves) as known at the end of 2002 was therefore ~62 tonnes of gold.
The geology of the Quadrilátero Ferrífero is dominated by Archaean and Palaeoproterozoic volcano-sedimentary sequences and granitic complexes of the same age. The oldest rocks in the district are Palaeo- to Mesoarchaean TTG gneiss-migmatite complexes which are overlain by the Neorchaean Rio das Velhas Supergroup. For details, see the Quadrilátero Ferrífero District Gold - Geological Setting record. São Bento is developed in the Córrego do Sítio Formation of the Nova Lima Goup, within the Santa Bárbara Domain, as described in that record and in the Córrego do Sítio record.
The Rio das Velhas Supergroup is sub-divided into the basal Quebra Osso Group, a succession of komatiitic ultramafic and mafic rocks with BIF intercalations, overlain by the Nova Lima Group, a middle sequence of schists and phyllites that contain lenses of dolomitic and calcic white marble and by thin beds of iron formation, and finally the upper Maquine Group of quartzites, conglomerates and phyllites. These are discordantly overlain by the Palaeoproterozoic Minas Supergroup quartzites, schists, phyllites, meta-conglomerates, carbonates and iron formations that produce the major iron deposits of the district.
Mineralisation at São Bento is restricted to the fine grained sedimentary rocks, tuffs and chemical sediments of the Nova Lima Group. Four formations have been differentiated on the São Bento property, which from structural bottom to top, are designated the:
• Lower Banded Iron Formation - an oxide, silicate and carbonate facies BIF. The rock is finely banded, and contains quartz, magnetite, chlorite and carbonate;
• Basal Carbonaceous Unit, which is ~650 m thick and consists of dark grey to black, graphitic, generally finely banded schists and phyllites, with quartz, sericite and minor chlorite. It has an average dip of 55°. Barren pyrite nodules occur throughout the graphitic/carbonaceous schists.
• São Bento Banded Iron Formation - which has an average thickness of 120 m. It essentially comprises an oxide-facies BIF, rich in carbonate and clastic components, that, in places, contains primary sulphide. It has mm to cm thick light and dark bands, the result of varying quantities of quartz, carbonates, magnetite and stilpnomelane. The pale, generally cream-coloured layers are dominated by quartz and carbonates, whilst the darker fissile bands contain chlorite and stilpnomelane; and the massive, dark grey layers are composed of magnetite or carbonaceous material and sulphide minerals. Intercalations of fine-grained, pelitic metasedimentary rocks containing carbonaceous, chlorite and mica schists predominate in the lower part of the iron formation. It is divided into the:
- The Lower iron formation which hosts the bulk of the gold mineralisation at São Bento. This section of the iron formation is characterised by lithological heterogeneity, containing oxide- and carbonate-facies BIF and ferruginous chert, with intercalations of fine, clastic metasedimentary rocks which include carbonaceous mica schists, quartz-chlorite schists and phyllites. Un-mineralised BIF is characterised by a compositional layering with alternating quartz-carbonate and magnetite-carbonate bands; the carbonates being dominantly sideritic. This facies grades both laterally and vertically to a distinctive, cream-pinkish, carbonate dominated iron formation, locally containing auriferous sulphides. It is composed of ankerite, calcite, ferroan dolomite and siderite, accompanied by scheelite and albite, with streaks containing variable proportions of magnetite-carbonate rock. The transition from barren to mineralised BIF is sharp and marked by the appearance of sulphides, quartz and additional carbonate. The mineralised Lower iron formation is characterised by Au-bearing, sulphide-rich layers that alternate with carbonate-quartz rich bands, and have associated quartz veins. These horizons strike NE-SW and contain sulphide, carbonate (dominantly ankerite) and cherty bands. Sulphide minerals include arsenopyrite, pyrrhotite and pyrite containing gold in solid solution. There is a close association of Au and sulphides, emphasised by a positive statistical correlation between Au, S and As. Gold grades at São Bento are very constant on all levels.
- The Upper iron formation of the São Bento BIF is of an oxide-facies, with lesser clastic components. It is up to 100 m thick, has a more homogeneous appearance, and has a lower arsenic content of <200 ppm. It has sheared contacts with both the overlying Carrapato unit and underlying Lower iron formation of the São Bento BIF. It is composed of individual 0.2 to 10 cm thick bands of cream carbonate, light and dark grey quartz, and magnetite, and dark green chlorite and stilpnomelane. It locally contains 1 to 2 m thick layers of quartz-sericite-chlorite schist.
• Carrapato unit - with an apparent thickness of >500 m, it consists of a relatively monotonous sequence of light and dark grey
mica schists, and is known as the Hanging wall schists. Discontinuous carbonaceous schists and intercalated carbonaceous quartz-schists are inferred to represent meta-greywackes (Martins Pereira, 1995). The carbonaceous layers may contain stretched nodules of pyrite, as well as carbonate-rich laminae, with which gold may be associated within discontinuous, smoky quartz veins. They contain relatively low, <1%, sulphides, mainly stibnite and arsenopyrite and represent mineralisation similar to that at the Córrego do Sítio deposit, ~5 km to the SW (Lobato et al., 2001), where auriferous quartz-carbonate-sulphide veins and veinlet systems are hosted by pelitic rocks.
Scarce, metamorphosed, green, fine grained mafic dykes to sills are intersected at São Bento, although they are foliated near the contacts with wall rocks. They are composed of relics of green hornblende, actinolite-tremolite, epidote, carbonate, plagioclase and quartz in a matrix composed of mica, chlorite, hornblende and albite, with accessory titanite, apatite, sulphide minerals and magnetite. These dykes cross-cut and displace the ore zones below level 21.
The ore at São Bento is localised within the Lower Iron Formation of the São Bento BIF which has been continuously mined from the surface to a depth of 1100 m. Four mineralised ore horizons occur in the Lower iron formation. Two are located at the lower and upper contacts of the unit respectively, the other two between. These horizons vary from 0.5 to 8.0 m in thickness, with gold being associated with quartz-carbonate-sulphide assemblage. These mineralised horizons are known as the West horizon which occurs at the lower contact with the Basal Carbonaceous Unit; the East horizon which is found at the upper contact. The intervening mineralisation defines the Middle and São Bento horizons. While the West and East horizons are largely stratabound at the margins of the iron formation and ~50 m apart, the São Bento horizons transgresses from the footwall in the SW to the hanging wall in the NE and at depth. The most important of the four are the West and São Bento horizons which are developed over significant strike lengths, whilst the East horizon is less extensive laterally, both along strike and down dip and the Middle horizon has a very restricted extent. In addition, late-stage quartz veins with coarse, barren sulphides and carbonates also occur.
The iron formation and mineralised horizons are disrupted by shearing, and as a consequence, are discontinuous along strike. This dislocation has led to the deposit being divided along strike into three portions, namely the No. 1, No. 2 and No. 3 (also known as the São Bento/Pinta Bem) orebodies. The No. 1 Orebody represent the best developed mineralisation, reaching a thickness of 35 m, displaying at least 6 episodes of gold/sulphide mineralisation. Each orebody contains more than one of the four mineralised horizons, up to all four. Underground mapping shows that the structural features, mineralogical composition and gold grade distribution of the orebodies are the same, inferring that there are no significant variations between the West, Middle, São Bento and East horizons. As the depth increases, differentiation of the four mineralised horizons is no longer possible due to structural complications. Below level 21, the horizons are merged and are referred to as the Main and Secondary horizons (Martins Pereira et al., 2000).
The margins between barren and mineralised BIF in all four horizons are marked by a transition zone of alternating carbonate and quartz bands. This facies is characterised by alternating cream-pink quartz-carbonate and lighter-grey magnetite layers, which are interpreted to represent a carbonate alteration halo, or 'carbonate facies iron formation' (Ladeira and Ferreira, 2000). The gold-sulphide mineralisation may parallel or transect this layering, whilst the layers can be folded and pinched out due to shearing (Martins Pereira et al., 2000). Thick, high-grade gold intersections are the product of tectonic thickening due to intense strain. Such zones are the sheared, inverted and attenuated fold limbs that host the mineralised horizons.
Proximal to mineralised zones, magnetite is replaced by ankerite and/or pyrrhotite, accompanied by arsenopyrite (Martins Pereira, 1995). The composition of the carbonate-dominated host, in decreasing order of abundance, is ankerite, siderite, calcite, ferroan dolomite, muscovite, quartz, chlorite, pyrite, pyrrhotite and arsenopyrite. Locally, the earliest recognised sulphide is synkinematic pyrrhotite, although it may carry fine-grained pyrite inclusions, implying an earlier generation of pyrite. Subhedral arsenopyrite is developed at the expense of pyrrhotite, and both are commonly replaced by arsenical pyrite. Microfractures in pyrite are locally filled with pyrrhotite, gold and rutile, whilst fine-grained chalcopyrite occurs along pyrrhotite borders, or as inclusions within it. Sulphide margins are corroded by carbonate minerals, implying recurrent carbonate alteration. Siderite may occur as inclusions in arsenopyrite, and together with ankerite, in places occur as inclusions in pyrrhotite. Chlorite is found as thin streaks and occurs as inclusions in sulphides. Stilplomelane and muscovite are invariably present, locally accompanied by sphalerite and galena. Other minor phases are albitic plagioclase, magnetite, ilmenite, rutile, titanite, scheelite, sphalerite, covellite, bornite, chalcopyrite and galena (Godoy, 1995; Martins Pereira, 1995; Lobato et al., 1998, 2001).
The distribution of pyrite and pyrrhotite vary throughout the deposit, with arsenopyrite being the most abundant sulphide that has a constant concentration of ~34% of the total sulphides. Pyrrhotite and pyrite are inversely correlated. The amount of pyrrhotite increases at depths below level 21, where deformation becomes more intense, with pyrrhotite ultimately occupying hinge zones of tight folds (Martins Pereira, 1995; Lobato et al., 1998, 2001).
Gold particles occur as follows: i) inclusions in arsenopyrite, pyrrhotite and pyrite; ii) rimming sulphides, particularly near the contacts of pyrrhotite and overgrown arsenopyrite; iii) at the junction of gangue minerals; iv) free within the gangue; and v) in magnetite. The grain size of gold varies from 2 to 125 µm. Approximately 50% of the visible gold occurs as inclusions within or along the margins of arsenopyrite, but is more rarely along boundaries of gangue minerals (Godoy, 1995; Martins Pereira, 1995; Lobato et al., 1998, 2001).
The ore deposits dip at 55 to 60°E and have an average true thickness of 1.4 m. In 1999 540 014 tonnes of ore were milled to produce 3.9 t (0.127 Moz) of gold at an average grade of 8.18 g/t Au. The total cash cost was USD 189 per oz.
The mineable reserves in 2000 were 3.238 Mt @ 8.95 g/t Au for 29 t (0.932 Moz) of gold while the in situ resource was 4.04 Mt @ 10.6 g/t Au for 43 t (1.38 Moz) of Au. Total production to that stage was 35.1 tonnes of Au. The mine was operated by Eldorado Gold Corp., which was 36% owned by Gold Fields Ltd.
As of December 2002, the total Proved + Probable Ore Reserves were 1.79 Mt @ 9.22 g/t of Au, totaling some 16.5 tonnes of gold (Martins Pereira et al., 2007).
In 2007, the São Bento Mine was acquired from Eldorado Gold Corporation by AngloGold Ashanti and absorbed into their Córrego do Sítio operation.
The information in this summary is largely drawn from Martins Pereira et al. (2007).
The most recent source geological information used to prepare this decription was dated: 2007.
Record last updated: 25/1/2023
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.
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Selected References:
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Anonymous 2001 - Eldorado Gold Corp: in extract from Eldorado Gold Corp. web site http://www.eldoradogold.com 5p
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Martins Pereira, S.L., Lobato, L.M., Ferreira, J.E. and Jardim, E.C., 2007 - Nature and origin of the BIF-hosted Sao Bento gold deposit, Quadrilatero Ferrifero, Brazil, with special emphasis on structural controls: in Ore Geology Reviews v.32, pp. 571-595.
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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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