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Raposos - Espirito Santo, Espirito West, Mina Grande, Apolinario, Agua Limpa, Tres Vintens, Bom Caminho |
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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 Raposos orogenic gold deposit is located within the Morro Velho string of mines in the north-western section of the Quadrilatero Ferrifero district, some 35 km south-east of Belo Horizonte, Minas Gerais, Brazil (#Location: 19° 58' 8"S, 43° 48' 48"W).
The first mining venture at Raposos began in 1883 when a small company, Ouro Preto Gold Mines, acquired most of the smaller mines around what was to become the Raposos mine. In 1899, these mines were acquired by the British St. John Del Rey Mining Co., but only after 1910 did systematic mining begin. Underground operations took place between 1920 and 1955 and after modernisation restarted in 1961and continued until 1999. The operation was sold in 1975 to Mineração Morro Velho S.A. which was controlled by Anglo American, who subsequently transferred the title to AngloGold Ashanti.
The deposit comprises of 19 separate sulphide orebodies, with resource plan areas of from 100 to 1300 m2, totalling 4000 m2 that are distributed along the 1.6 km strike length of the sequence containing banded iron-formations that host mineralisation. The deposit was known to continue to a depth of 1423 m although when put on care and maintenance in 1999 was being mined between levels 30 and 32, to 1063 m depth, while mine development was proceeding on levels 34 to 36 to a depth of 1183 m. In 2002, the remaining mineral resource was 3.39 Mt @ 6.97 g/t Au (Junqueira et al., 2007, quoting F.W.R. Vieira, pers. comm., 2002). By 1999, ~10 Mt of gold ore had been extracted with in situ grades of between 6.5 and 9.0 g/t Au (Junqueira et al., 2007). The total cumulative production from 1920 to 1955, and 1961 to 1999 has been estimated at 67.14 tonnes of gold (Costa and Rios, 2022).
For details of the regional tectonic, structural and geological setting, see the Quadrilátero Ferrífero District Gold - Geological Setting record and the geological plan therein.
Geology
The Raposos gold deposit is associated with banded iron formation of the Nova Lima Group, within the Rio das Velhas Supergroup as described in the Morro Velho and Cuiabá records. The group comprises a greenstone succession, with ultramafic through mafic to intermediate, and subordinate felsic, volcanic rocks. At Raposos, the lower sections of the group encompasses mafic to ultramafic rocks, with massive, amygdaloidal, variolitic and locally pillowed meta-basalts, intercalated with oxide facies banded iron formation, metachert, and carbonaceous schist. Spinifex-textured peridotitic komatiites and cumulate sills can also include minor banded iron formation. Volcanoclastic rocks comprise pyroclastic dacitic tuffs and agglomeratic horizons with minor lava flows. Volcanogenic and metasedimentary rocks have intercalations of felsic pyroclastic
and epiclastic horizons (greywackes) with minor carbonaceous and carbonate schists. Meta-sedimentary rocks from mixed sources include quartz-mica-
feldspar-chlorite, and carbonaceous schists and metapelites, meta-greywackes and meta-psammites (Junqueira et al., 2007).
Gold mineralisation at Raposos is associated with sheared and hydrothermally altered oxide±carbonate facies banded iron formation (BIF) within the lower Nova Lima Group, and is bounded above and below by envelopes of white mica-bearing schists in the footwall and hanging wall, respectively. These schists were developed after precursor ultramafic and mafic schists (Vieira and Oliveira, 1988), and represent proximal white mica alteration zones.
The sequence that hosts mineralisation at Raposos, as illustrated on level 24 by Junqueira et al. (2007), after Vieira (2000), shows a core of komatiitic ultramafic schists that is at least 3 km long and 150 to 300 m wide, with BIF bands developed on both margins, and within. This core is symmetrically bounded, progressively outward, both to the NE and SW, by tholeiitic metabasalt and then by a thick sequence of metapelite. All of these rocks are intruded by north-south to NE-SW trending dolerite dykes that vary from <50 to >100 m in thickness (Junqueira et al., 2007). The symmetric repetition of the sequence is the result of a sheared D1 anticline that has been folded into a broader 'reverse-Z' shaped structure by the more open D2 deformation, as described below in the Structure and Mineralisation sections.
Structure
Three ductile to brittle deformational events are recognised at Raposos, namely D1, D2 and D3. A fourth suite of structures with spaced cleavages are considered to be a D3 brittle phase (Ladeira et al., 1991; Junqueira, 1997).
D1 includes large-scale, ductile to ductile-brittle shear zones (SZ1), mylonitic foliation, a stretching lineation and intrafolial folds and dislocated grains, typically being largely restricted to the BIF. These SZ1 shears are bounded by hydrothermally altered, blastomylonitic schists, which are 200 m thick and more than 3 km wide. They are the most important structures at Raposos, interpreted to be related to both hydrothermal alteration and gold mineralisation. F1 folds are tight, with east-west hinge lines that parallel the stretching lineation (L1), and for example, at a depth of 1400 m, plunge at from 30 to 12°E and form interference fold patterns. The axial-planar foliation, S1, is parallel to S0 of both BIF and metapelites, and dips at between 25 and 40°. S1 develops a mylonitic or transposition character in highly deformed zones. SZ1 shear zones are roughly parallel to S1 and obliterate S0 in the BIF, which is only preserved as elongated relics within those shear zones (Junqueira et al., 2007).
D2 is represented by north vergent inclined similar folds, that together form a major inclined and overturned fold that defines the shape of the deposit. The NE-striking axial-planar S2 foliation strikes at 30 to 60° with a dip of 45°SE at surface, and 12°SE on level 44, at a depth of 1430 m. It is a transposition foliation in schists, and a spaced cleavage in BIF. The hinge lines are grossly coaxial and parallel mineral and stretching lineations. F2 folds were generated by shearing, as defined by drag folds at the extremities of the Espírito West and Espírito Santo orebodies within the deposit. These shear zones are ductile in schists, to ductile to brittle-ductile in BIF, and are roughly parallel to S2. Dolerite dykes that crosscut the orebodies are also parallel to S2. The dykes are bounded by S2 schistosity along their borders, taken to suggest emplacement during late stage D2 (Junqueira et al., 2007).
D3 occurred in a brittle-ductile regime and generated shear zones SZ3, and spaced and crenulation S3 cleavages, as well as the L3 lineation. SZ3 shear zones trend north-south and dip at 15°E, and are common in both schists and dolerite dykes. A decrease in S2 and L2 dips down plunge with depth suggests a thrust ramp. S-C foliations indicate tectonic transport from east to west. The S3 foliation has an north-south trend and dips between 70 and 90°WSW. Gravity step faults that are parallel to S3 are common and displace the orebodies, with small net slips. The crenulation of S2 and the intersection lineations between S3 and S2 or S1 generate L3. North-south and east-west subvertical, spaced cleavages and joints overprint D3 structures and are considered to represent a late structural phase (Junqueira et al., 2007).
The D2 fold that determines the shape of the deposit has a reverse-'Z' shape, comprising a barren ENE-WSW limb to the SW, folded on its western end to form a NNW trending 1.2 km long limb, which, in turn is folded to form a ~1 km long west trending limb on its northern end. This northern limb is then folded to trend towards the NW. The mineralisation is associated with banded iron formation largely formed on the two margins of a komatiitic ultramafic schist unit on the main ENE-WSW, and the northern east-west limbs of the structure. Another well developed BIF cuts obliquely across the ultramafic schist unit on the ENE-WSW limb (Junqueira et al., 2007).
Mineralisation
Gold mineralisation at Raposos is associated with sulphidised BIF, which is enveloped by white micaceous schists in both the footwall and hanging wall, developed after precursor ultramafic and mafic schists (Vieira and Oliveira, 1988). Undeformed and unaltered Raposos BIF, as seen on level 28, is composed of alternating layers of fine-grained granoblastic siderite, ankerite and rarely, magnetite and quartz. Where sulphides are present, pyrrhotite dominates.
The Raposos sequence is interpreted to have undergone ductile thrusting during D1 with the main mineralisation also being associated with this event.
As such, all of the orebodies are sulphide-bearing SZ1 shear zones focussed within and on the contacts of the BIF, with the exception of the Ouro Preto orebody, which is associated with quartz boudins hosted by talc schist. Mineralisation hosted within BIF are the result of sulphide replacement of magnetite and/or siderite. The gold-bearing zones have a variable sulphide composition, but are laterally continuous, having a pseudo-stratiform appearance. Many are parallel, or slightly oblique to, shear-hosted quartz veins. Gold grades correlate positively with the quantity of sulphide within the host rock. Pyrrhotite, as the dominant sulphide, is followed by lesser arsenopyrite and pyrite. Gold occurs as anhedral grains or films that follow fractures in sulphides. When associated with pyrrhotite, it is relatively coarse, between 50 and 120 µm, and mainly occurs along grain boundaries. Gold inclusions in pyrite are 10 to 50 &micr;m grains, whilst in arsenopyrite or associated with gangue minerals, it is generally finer still at <10 µm. Abundant finer-grained pyrrhotite generally correlates with higher gold grades of up to 60 to 80 g/t Au. Au:Ag ratios range from 8:1 to 5:1 (Junqueira et al., 2007).
The orebodies (ore shoots) vary from ~0.5 to 20 m in thickness, and can be >5000 m in down-plunge extent, parallel to the stretching lineation, and parallel to D1 and D2 fold axes. (AngloGold Ashanti, 2020). The largest orebodies of the deposit, and those with the highest gold grades, are mainly on the northern, east-west trending limb of the major reverse-'Z' shaped D2 fold described above in the Structure section. These are the Espírito West (with a strike length of ~180 m and an average resource plan area of 900 m2 @ 7.56 g/t Au); Espírito Santo (strike length ~180 m and resource plan area of 1500 m2 @ 7.30 g/t Au); N°10 (~75 m long and 450 m2 @ 5.5 g/t Au) and N°11 (also with a strike length of ~75 m) orebodies. The Mina Grande orebody on the eastern limb occupies a local hinge zone, and has the highest grade with 286 m2 averaging 12.8 g/t Au. The other orebodies on the eastern limb are smaller and lower grade, such as Apolinário, Água Limpa, Três Vinténs and Bom Caminho (Junqueira et al., 2007).
The orebodies are characterised by decreasing gold grades down-plunge, as at orebody N°11 from level 24 at 823 m depth; Mina Grande from level 26 at 883 m; and N°10 from level 30. The Espírito Santo orebody has a significant drop in grade from 7.5 to 5.5 g/t Au between levels 32 and 34 at 1123 m depth. In all of these cases, SZ1 shear zones are either absent or rare over the interval of decreasing grade (Junqueira et al., 2007).
On the northern limb of the structure, the high grade orebodies are developed within the BIFs on the southern margin of the ultramafic schist, with the southern wall rocks being mafic and carbonaceous schists of the metabasalts. The structural hanging wall ultramafic schist is derived from peridotitic komatiites, whereas the protolith of the structural footwall metamafic rock was tholeiitic basalt. The least hydrothermally altered ultramafic and mafic rocks are represented by chlorite-actinolite-talc-calcite and epidote-albite-actinolite- chlorite (±calcite) schists, respectively. The quartz-carbonate-muscovite schist with carbonaceous matter within the latter is interpreted to be a metapelite. Dolerite dykes crosscut all of these rocks (Junqueira et al., 2007).
Junqueira et al. (2007) demonstrated a gross zonal hydrothermal alteration pattern surrounding the ore zones on level 28 involved the mafic and ultramafic wall rocks to the BIF. The boundaries between alteration zones vary from knife sharp, to gradational on a cm- to metre-scale, with alteration imposed on the a metamorphic assemblage. This alteration essentially involves chloritisation, carbonisation and sericitisation (white mica). The pre-alteration meta-basalt is composed of albite, epidote and actinolite with lesser Mg/Fe-chlorite, calcite and quartz. The incipient alteration overprinting this assemblage comprises chlorite (with calcite) → a chlorite-muscovite alteration zones → chlorite-albite-carbonate (calcite and ankerite)-quartz schist → advanced alteration that includes a carbonate-albite zone (as carbonate-albite-muscovite-quartz schist) → carbonate-muscovite zone which is characterised by an assemblage of Mg siderite-muscovite-quartz (Junqueira et al., 2007).
The pre-alteration metamorphic mineral assemblage of the ultramafic schists is rarely preserved, the least-altered containing Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. Elsewhere, serpentine, clino- and orthopyroxenes and pseudomorphs after olivine have been described (Vieira 1991). The incipient alteration stage is subdivided into the talc-chlorite (talc-Mg [Fe] chlorite-Fe dolomite-quartz) → chlorite-carbonate zones, the latter comprising chlorite-carbonate-quartz schists with rutile-calcite-quartz. The advanced alteration stage produced the carbonate-albite zone, characterised by an ankerite-albite ±chlorite ±fuchsite-quartz schist → carbonate-muscovite zone, composed of ferroan ankerite-fuchsite-quartz (Junqueira et al., 2007).
Production and Resources
At the closure of operations, prior to being put on care and maintenance in 1999, ~10 Mt of ore had been mined at in situ grades of between 6.5 and 9.0 g/t Au (Junqueira et al., 2007).
The total cumulative production from 1920 to 1955, and 1961 to 1999 has been estimated at 67.14 tonnes of gold (Costa and Rios, 2022).
Remaining Mineral Resource in 2002 was estimated as - 3.39 Mt @ 6.97 g/t Au (Junqueira et al., 2007, quoting F.W.R. Vieira, pers. comm., 2002).
Remaining Mineral Resources as at 31 December 2020 (AngloGold Ashanti Ore Reserves and Mineral Resources Report, 2020) were:
Measured + Indicated + Inferred Mineral Resources - 2.84 Mt @ 6.53 g/t Au for 18.59 tonnes of contained gold.
of this resource, 2.25 Mt @ 6.44 g/t Au or 14.50 tonnes of gold, were Inferred.
As these resources are below infrastructure in the mine which is on care and maintenance, these resources were written off in 2021.
The most recent source geological information used to prepare this decription was dated: 2007.
Record last updated: 27/1/2023
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.
Mina Raposos
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Selected References:
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Junqueira, P.A., Lobato, L.M., Ladeira, E.A. and Simoes, E.J.M., 2007 - Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilatero Ferrifero, Brazil: in Ore Geology Reviews v.32, pp. 629-650.
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Tolbert G E, 1964 - Geology of the Raposos gold mine, Minas Gerais, Brazil : in Econ. Geol. v.59 pp. 775-798
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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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