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Morro Velho - Mina Velha, Mina Grande |
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Minas Gerais, Brazil |
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Super Porphyry Cu and Au
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The Morro Velho mines are located in the town of Nova Lima, ~12 km southeast of the outskirts of Belo Horizonte, in the state of Minas Gerais, Brazil. It lies within the northern part of the Quadrilátero Ferrífero district. For operational reasons the Morro Velho mine was divided into two, the Mina Velha which exploited the upper 600 m of the deposit, and the Mina Grande below that to the final depth of 2453 m
at level 27, reached in 1929.
Gold was first recovered at Morro Velho in 1700 by Domingos da Fonseca Leme. The deposit has been continuously mined (with some periods of limited activity) since 1834, when the British St. John d'El Rey Mining Co. (or São João d'El Rei Mining Company) acquired title to the area and began mechanised operations. Prior to that, the mine is thought to have been worked for ~50 years. The St John d'El Rey company imported British miners, mainly from Cornwall, for their expertise, and until 1879, utilised slaves as labour. Mining started as an open pit, and by 1867 the lode, which in its upper sections plunges at ~45°E, had been followed to a depth of ~335 m. The pit was some 275 m long east-west, and 4.5 to 13.5 m wide. The walls of the stope were held apart by timbers. A fire in 1867 caused serious damage and between 1868 and 1873 production plunged by ~two thirds. During that period two shafts, A and B, were sunk to continue mining which progressed downward from the base of the old open pit with the aid of heavy timbering. In 1886, with mining having reached a depth of 550 m, the mine collapsed, and all operations came stopped. As a result the St. John D'El Rey Mining Co. went into temporary liquidation, but after restructuring, was reopened and again became profitable in the 1890s. Two more, deeper shafts, C and D, had been commenced in 1889, ~460 m east of the open pit, to bottom by 1892 at level 8, 690 m below the surface. Mining continued from these shafts with the aid of drives and crosscuts following the shallowly plunging orebody as it became further from the shafts. In 1897, a new vertical shaft, E shaft, was begun from the end of a drive extended from the base of shaft D. Between 1898 and 1902, mining had followed the plunge of the orebody to a depth of 1042 m below surface. In 1903, a drive from the base of shaft E was commenced and advanced on to level 12, to intersect the trend of the orebody. Shaft F was commenced at the end of that drive in 1908 and bottomed at level 16. Shaft G was commenced from level 16 in 1915 and sunk to level 20 at a depth of 1777 m. This was followed by shaft H from level 20 to 22. The plunge of the deposit at these depths had shallowed to 19°. Between 1922 and 1929, production proceeded down to level 25 at 2171 m, via inclined winzes and shafts I and J. The small, lower grade Northwest ore body was found NW of the main lode on level 23, and between 1931 and 1938 higher grade zones were selectively mined between levels 20 and 27. The grade of the main lode dropped abruptly on level 25, and the Northwest orebody became the principal source of gold. Exploration revealed grades improved in the main lode on level 27, the deepest stoping level reached in the mine. Exploration winzes and tunnels opened up levels 28 and 29 to a depth of 2417 m, but the grade was sub-economic. The deepest point in the mine was reached in 1934, at the bottom of a winze to level 30 at a depth 2453 m. Rock temperatures of ~55°C, as well as the low grade, discouraged deeper mining. In the same year, all development work was suspended in the main lode below level 23, but was continued at deeper levels in orebodies discovered after 1930. After 1936, all development work ceased in the main lode and thereafter virtually all production came from satellite lodes. These included the major South orebody, which was found on level 22 in 1930, close to the main lode, and was traced to higher levels also. In the same year, the small Black orebody was found west of the Main and South ore bodies on levels 11, 12 and 13. In 1931, the markedly different White orebody, a mass of mineralised white quartz, was found to the SE of the main lode on levels 10 to 12. In 1936, whilst sinking the X shaft between levels 13 and 14, the X orebody was found which has been worked between levels 8 and 27. In 1957 a group of investors bought a controlling interest in the company and mining in these orebodies continued. 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. The St. John d'El Rey Mining Co was formally disolved in 1985. 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. In 1997, the deep workings that constituted the Mina Grande mine were abandoned, although mining continued above a depth of 600 m in the Mina Velha mine until it too was closed in 2004. Total estimated production between 1834 and 2003 is 370 tonnes of gold (Costa and Rios, 2022).
Geology
For details of the regional setting, see the Quadrilátero Ferrífero District Gold - Geological Setting record. The Morro Velho deposit is hosted within the 2.77 to 2.68 Ga Nova Lima Goup of the Rio das Velhas Supergroup within the Nova Lima-Caeté Domain, as described in that record. The Nova Lima Group is divided into a number of tectonostratigraphic units within this domain, as follows, from the base (Vial et al., 2007):
Ouro Fino Formation, mainly composed of massive, amygdaloidal and variolitic metabasalts, oxide-facies banded iron-formation (BIF), metachert and carbonaceous schist;
Mestre Caetano Formation, which commences with the lower volcaniclastic and metasedimentary rocks of the Ribeirão Vermelho Member, including pyroclastic dacitic tuffs and agglomeratic units with minor lava flows, followed by or inter-fingering with felsic pyroclastic and epiclastic greywackes, minor carbonaceous schist, and carbonate schist (including the 2.77 to 2.70 Ga lapa seca to be discussed below);
Ribeirão do Brumado Formation, composed of rhythmic depositional cycles of greywacke-argillite, interpreted as turbidites that were re-sedimented from earlier volcaniclastic lithofacies, occurring as meta-psammitic quartz-white mica-chlorite schist; feldspar-quartz schist meta-greywackes and meta-psammitic carbonaceous schist.
Intrusive Magmatism that accompanied or followed the Nova Lima Group included the 2.75 to 2.68 Ga, Neoarchaean Mamona I Magmatic Event, which resulted in the intrusion of voluminous, relatively K-rich magmatic rocks that comprised metaluminous tonalite, grandiorite and calc-alkaline granite; and subsequent peraluminous A-type granitoids. These were followed by the 2.65 to 2.58 Ga Neoarchaean, Mamona II Magmatic Event that produced a suite of post-orogenic, A-type, grey granite dykes and plutons.
The principal lithologies of the Mestre Caetano Formation which hosts the Morro Velho deposit, include plagioclase-white mica-chlorite-quartz schist after feldspathic meta-greywacke, the most common rock type; quartz-white mica-chlorite schist after quartzwacke; plagioclase-chlorite-quartz schist; epidote-chlorite-white mica-quartz-plagioclase schist; quartz-white mica-chlorite-plagioclase schist after meta-tuff; white mica-quartz-chlorite schist after arenaceous siltstone; carbonate-quartz-white mica-chlorite schist after siltstone; and white mica-chlorite-quartz carbonaceous schist (Zucchetti and Baltazar, 1998).
On level 10 of the mine, at a depth of ~865 m below surface, the host sequence is as follows (after Vial et al., 2007), from the base:
• Andesite - a >50 m thickness of andesitic rocks.
• Porphyritic rhyolitic meta-tuff - (Xs), occurs as a pale-grey rock, primarily composed of quartz, white mica and plagioclase, with subordinate epidote and chlorite. Rounded to bipyramidal quartz fragments and plagioclase are studded in the matrix, giving a quartz-eye texture, with the plagioclase, which varies from albite to andesine, occurring as irregular and ovoid shapes. The matrix is mainly fine-grained quartz and white mica with epidote microcrystals, and variable carbonate and chlorite. Titanite is after magneto-ilmenite, and hematite is common and aligned.
• Carbonaceous metapelite (X1) - that is ~80 m thick and is a dark grey schist, primarily composed of quartz, carbonate, chlorite and white mica, containing variable carbonaceous matter, plagioclase and biotite. Compositional layering, interpreted to be bedding, is typical, and is regular, continuous and parallel to lithological contacts. It is defined by the alternation assemblage of carbonate or quartz-carbonate- and mica-bearing layers. Proximal to the contact with mineralised zones, it has a strongly mylonitic texture.
• Felsic dacitic to andesitic tuff, which in the upper levels of the mine is ~15 m thick, known as LSmi or micaceous lapa seca. This unit is composed of four lithofacies, as follows:
- White mica-quartz phyllite (Sep) - characterised by granoblastic to locally lepidoblastic textures, and composed of alternating quartz-carbonate-white mica-chlorite and white mica-chlorite layers. These layers consist of fine-grained quartz plus carbonate, white mica, chlorite and plagioclase, with subordinate fuchsite, albite, zircon, sulphide minerals, leucoxene and hematite. Sep is more common in the upper levels of the mine.
- Chlorite-rich biotite schist (Clx) - which has a greenish colour and is composed of quartz, white mica and carbonate, with subordinate plagioclase, epidote, euhedral tourmaline, sulphides, chloritoid, carbonaceous matter and sparse hematite. It includes alternations of predominantly chlorite/biotite and chloritic/biotitic layers interspersed with quartz, some carbonate and plagioclase, with chlorite+biotite comprising >50 vol.% of the rock.
- White mica-chlorite-carbonate-quartz phyllite (ma) - characterised by a high proportion of well-oriented, platy
minerals, which include white mica and lesser chlorite, and carbonate intermixed with fine grained quartz aggregates and crystals.
- Metatuff (mt) - composed of oriented minerals superimposed on an original, fragmental texture, mainly made up of quartz, plagioclase, white mica, chlorite and carbonate, with subordinate biotite, greenalite and opaque minerals. It is characterised by plagioclase fragments and volcanic quartz, studded through the matrix of fine-grained quartz, carbonate, white mica, chlorite, biotite and locally greenalite.
• Ore - which is 1 to 30 m thick, to be discussed below in the Mineralisation section.
• Carbonatic Lapa Seca (LSca) - which is 8 m thick, and occurs in two forms, which commonly grade laterally into one another:
- Brown Carbonatic Lapa Seca, which is the main host in the underlying ore zone. It is actually a brownish-grey, massive, homogeneous, plagioclase-rich rock with relicts of pre-alteration plagioclase. Plagioclase dominates over quartz and carbonate. The fine-grained matrix is composed of quartz, carbonate and albite to andesine plagioclase. Some carbonate crystals appear to pseudomorph plagioclase, whilst porphyroclastic plagioclase relicts have a volcanic habit and are altered to carbonate, containing fine grained inclusions of opaque microcrystals, white mica and/or chlorite. Plagioclase also contains microcrystalline inclusions of ilmenite, sphene and rutile, carbonates and epidote. This style of lapa seca has a high Al content averaging 9.18% Al2O3.
- Grey carbonatic Lapa Seca, which is predominantly in the hanging wall of the ore zone, and is composed of alternating grey to dark grey, pale-grey and beige carbonate bands, varying from banded to massive to foliated, and locally resembling dolomite. This type is dominated by a fine-grained granoblastic carbonate, quartz and albite matrix, containing white mica and carbonaceous matter, and enclosing porphyroblasts of fine- to medium-grained carbonate and quartz, as both individual grains or in clusters, as well as sulphides and albite. Streaks or discontinuous bands containing aggregates of carbonate ±quartz occur as highly deformed sygmoids. Un-mineralised grey banded lapa seca has a low Al content, averaging 2.91% Al2O3, and generally, geochemically corresponds to a relatively pure limestone and dolomite to silty limestone and dolomite protolith.
Both types of carbonatic lapa seca contain sulphide crystals that are invariably surrounded by vein quartz in pressure shadows that commonly contain chlorite, whilst carbonate and quartz together comprise >60 vol.% of the rock. Carbonate grains, which are elongated parallel to the transposition cleavage, include dolomite, ankerite and siderite, with ferroan dolomite and magnesian siderite predominating. Calcite is absent from all types of lapa seca. Un-mineralised Fe-rich dolomite is indicated to have been altered to siderite-rich lapa seca during the deposition of sulphides in the mineralising process. Where present, white mica and chlorite are disseminated in the matrix as isolated flakes and as films and layers, with associated opaque minerals and carbonaceous matter. Subordinate minerals include tourmaline, leucoxene, rutile, apatite, pyrite, ilmenite, magnetite, stilplomelane, epidote, monazite, xenotime and zircon.
• Metapelite with carbonaceous matter - which is 30 m thick. This unit is mainly composed of X1 schists as described for the carbonaceous metapelites above, but also contains intercalations of rhyolitic meta-tuff Xs, and carbonatic lapa seca LSca, also as described above.
• Porphyritic rhyolitic meta-tuff - ~15 m thick, as described for the same Xs lithology above.
• Carbonaceous metapelite - that is >150 m in thickness, as described as X1 for the similar lithology above, with intercalations of Porphyritic rhyolitic meta-tuff Xs.
At level 25, at a depth of 2170 m, the sequence was as follows (after Vial et al. (2007):
• Carbonaceous metapelitic schist - that is >30 m in thickness, and is as described as X1 above.
• Carbonatic Lapa Seca, 4 m thick, as described above for LSca, with intercalations of andesitic meta-tuff.
• Ore, 20 m thick.
• Carbonatic Lapa Seca, 8 m thick, as described above for LSca, with intercalations of andesitic meta-tuff and lapa seca Clx, ma and mt, but also including biotite-rich chlorite schist (Bix). Bix is similar to Clx, but has more biotite than chlorite and has a brownish colour.
• Ore - which is 1 to 30 m thick, correlated with the main ore zone on level 10.
• Carbonatic Lapa Seca, 12 m thick, as described above for LSca, with intercalations of andesitic meta-tuff.
• Carbonaceous metapelitic schist - that is 16 m thick, and is as described as X1 above.
• Carbonatic Lapa Seca, 15 m thick, as described above for LSca, with intercalations of andesitic meta-tuff.
• Carbonaceous metapelite - that is 14 m thick, as described as X1 for the similar lithology above.
• Porphyritic rhyolitic meta-tuff - ~15 m thick, as described for the same Xs lithology above, correlated with the same lithology near the top of the section on the 10 level.
• Carbonaceous metapelite - that is >20 m in thickness, as described at X1 for the similar lithology above, with intercalations of porphyritic rhyolitic meta-tuff Xs.
Structure
Three phases of folding have been recognised at Morro Velho, interpreted to represent sequential stages of the same progressive deformation. F1 is characterised by tight, overturned, isoclinal folds with large amplitudes. The S1 cleavage, related to this folding, is generally parallel to the original bedding, and as such is only differentiated in fold noses. The fold axes, L1, normally plunge at 47° with an azimuth of 116° on level 10 of Mina Velha. The dip of S1 ranges from 55°SE, at the upper portion of the deposit, to 41°SE at the bottom of the mine (Vial et al., 2007).
F2 is characterised by smaller folds, and is defined by a prominent S2 foliation on both mine- and regional-scales. It is typified by transposition cleavage that is associated with the L2 lineation that parallels the fold axes. The F2 folds are associated with west vergent oblique-slip thrust faults, whose foliation is parallel to the compositional banding of the strata in the fold limbs of the shear-related tight folds (Lobato et al., 2001). These F2 folds are asymmetric, with attenuated and ruptured limbs, and thickened hinge zones. The hinge zones are cut by shears that are parallel to the S2 foliation, which strikes at 34° and dips 47°SE in the upper levels of the mine, to a strike of 36° and dip of 27°SE at greater depths. Fold axes L2 and L1, and their associated lineations, are generally sub-paralle (Vial et al., 2007)l.
The F3 folding is characterised by north-south striking shear zones that parallel the S3 foliation, but only have a small displacement. This phase produced open folds and only buckled the S2 schistosity. Other structures attributed to F3 are only local, producing a spaced fracture cleavage and crenulation lineation L3, which is sub-horizontal and plunges north-south with trends from 0 to 15°N or S. The S3 foliation strikes at 323°, dipping 83°NE at the shallower Mina Velha, and 345° dipping 74°SE at depth in the bottom of Mina Grande (Vial et al., 2007).
The Morro Velho orebodies are mainly controlled by the L1-2 lineation and are elongated, with a spindle and bladed shape. Ladeira (1991) interpreted the lapa seca and the orebodies hosted therein to be distributed in five main tight folds, which are disrupted by NE- to east-striking shear zones. These folds are anticlinal noses related to F1, the best examples of which are developed in the Main, South and X orebodies. They are open folds at level 10 in Mina Velha, and tight folds at level 25 of Mina Grande. The orebodies are thicker in the hinge zones, and attenuated and segmented on fold limbs. Both the folds and orebodies are continuous down plunge for at least 4.8 km with an azimuth of 95° although the plunge changes from 49° at surface → 16° at level 25 → 10° at level 29 in Mina Grande (Vial et al., 2007).
Metamorphism is indicated to have accompanied the early phase of the Mamona I Magmatic Event, between 2.78 and 2.70 Ga, with a mineral paragenesis consistent with greenschist facies metamorphism, as characterised in the mine by the assemblage quartz-chlorite-white mica-dolomite/siderite/ ankerite/calcite-epidote-plagioclase-biotite. Differences in the metamorphic temperature down-section reflect a geothermal gradient of ~13°C/km for burial metamorphism (Vial et al., 2007).
Mineralisation
Four orebodies have been mined at the Morro Velho deposit, the Main, NW, X and South , with the first being by far the most important. Mineralisation within the Main orebody is continuous over a down plunge length of >4.8 km from surface to the lowest level in the mine at a depth below surface of 2453 m. It has a thickness that varies from 2 to 20, averaging 5 m, and a horizontal length of 300 m, and is located in a complex, Z-shaped fold zone related to an F1 folding phase.
The NW orebody lies on an east-west oriented limb, and includes two main sulphide-rich lenses. The north lens is from 0.5 to 6 m thick, with a horizontal length of 525 m. The south lens is 1.0 to 3.5 m thick, with a horizontal length at 300 m. These lenses are known as the D and Quebra Panela orebodies in the Mina Velha and is discontinuous due to shear zone displacement.
The X orebody is a zone massive sulphide at the bottom of the mine, whilst in the upper levels, it occurs as a greenish-grey quartz-carbonate-white mica-chlorite-plagioclase phyllite LSmi lapa seca containing <5 vol.% disseminated sulphides and small quartz veins sub-parallel to the S2 foliation. It is found in hinge and limb zones, in the lower and upper levels of the mine, respectively. At level 25 in Mina Grande, it is up to 20 m thick, averaging 6 m, with a horizontal length of 450 m. At level 10 in Mina Velha, it comprises a series small discontinuous orebodies on the attenuated fold limb.
The South orebody has a 'Z'-shape and averages 4.5 m in thickness over a horizontal length of ~120 m on the 22 level in Mina Grande.
Other smaller orebodies occur in the Mina Velha, named C, F, H, K, M and Gambá which are crosscut and disrupted by shear zones.
Two main gold mineralisation styles have been recognised at Morro Velho, both of which are concentrated along the interface of carbonatic and micaceous lapa seca:
i). dark grey quartz veins, where gold is usually free. These veins are developed parallel to S2, and are composed almost exclusively of quartz, but also containing sparse sulphides as well as the free gold. This style of ore is common in the upper levels of Mina Velha, but is absent at deeper levels. On level 10 of Mina Velha, 25 vol.% of the ore was produced from quartz veins. In contrast, on level 25 of Mina Grande, this had declined to 5 vol.%.
i). sulphide-rich, which is the much more important of the two mineralisation styles, with gold generally being associated with the sulphides. The sulphide concentration varies from sparse disseminations (<5 vol.% sulphides) to massive (>50 vol.%; DeWitt et al., 1996). The massive and disseminated sulphide mineralisation have the same texture, with euhedral sulphide crystals encased in a gangue of predominantly dolomite, Mg- and Fe-rich siderite, and very little quartz. Based on their macroscopic features, sulphide-rich bodies can be further divided into massive, banded and schistose. Sulphide minerals in the massive variety are associated with quartz and carbonate, and lack any form of internal layering. Those that are banded exhibit an alternation of massive sulphide and quartz-carbonate layers. The schistose mineralisation occurs as disseminated sulphide minerals in white mica-quartz phyllite (Sep) and chlorite-rich biotite schist (Clx), with sulphide minerals occurring as aggregates, or scattered and stretched along S2 in chlorite-rich or in quartz-carbonate layers. This schistose type of mineralisation is generally discontinuous, both horizontally and vertically. The proportions of massive, banded and schistose mineralisation at Morro Velho has been estimated at 70, 20 and 5 vol.%, respectively, with schistose and disseminated sulphides being more common in the shallower Mina Velha.
The sulphide assemblage at Morro Velho has been estimated to comprise 74 vol.% pyrrhotite; 17 vol.% arsenopyrite; 8 vol.% pyrite; and 1 vol.% chalcopyrite (Graton and Bjorge, 1929; Ladeira, 1980). Cubanite, sphalerite, galena, tetrahedrite and ullmannite are minor to rare phases. Silicate gangue minerals include white mica, chlorite, biotite, albite, andesine, tourmaline, talc, epidote, clinozoizite, titanite and stilpnomelane, whilst carbonates within the mineralisation are ferroan dolomite, dolomite, ankerite, siderite and calcite. Oxides include magnetite, ilmenite, rutile and leucoxene with other minerals such as apatite, fluorite and scheelite. The dominant gangue is carbonate, whilst quartz, except where locally abundant as quartz-eyes, along with chlorite, white mica and albite are subordinate. Lenses of quartz, or dark grey, cm-scale rounded quartz eyes, are common locally, and vary from 0.3 to 20 cm in length (Graton and Bjorge, 1929). Sulphides replaced dolomite and Fe-poor siderite in the lapa seca, beginning with pyrite and arsenopyrite which generally occur as relatively large, euhedral crystals that are normally non-interlocking crystal aggregates. Pyrrhotite is the dominant sulphide, occurring as fine-grained crystals, predominantly in single or multi-grain thickness layers aligned parallel to the banding in the host. It is interstitial to pyrite and arsenopyrite, and is intergrown with carbonates, albite and quartz. A second, late-stage pyrrhotite, as coarse as 20 mm in diameter, is also present. The pyrrhotite in the massive sulphides is non-magnetic, in contrast to some of the disseminated mineralisation where it is 'quite magnetic'. This is interpreted to imply the latter is monoclinic pyrrhotite. The coarse euhedral pyrite, which is the bulk of that mineral, is coarser grained than the other sulphides, and commonly contains inclusions of pyrrhotite and arsenopyrite. Pyrite is apparently more abundant in the massive sulphides than in disseminated mineralisation, with an overall pyrite/pyrrhotite ratio ranging from 1 to <0.05. The pyrrhotite to pyrite ratio also increases with depth, proportional to the predominance of biotite over chlorite.
Arsenopyrite is the second most abundant sulphide mineral, and where fine grained, i.e., between 5 µm to 1 mm, it replaces pyrrhotite, pyrite and other gangue minerals. In sections of the mine, such as between levels 20 and 25, the mineralisation ranges from medium to coarse-grained, with coarse crystals and aggregates of arsenopyrite distributed irregularly through a finer aggregate of pyrrhotite and gangue minerals. Individual coarse arsenopyrite grains are composed of complex intergrowths and overgrowths of highly variable composition. In these grains, arsenic-rich cores may be overgrown by As-poor rims, although late As-rich margins locally overgrow As-poor cores. This is taken to be indicative of a compositionally variable hydrothermal fluid. Chalcopyrite-rich aggregates that are normally >0.2 mm in diameter are erratically distributed throughout, whilst traces of sphalerite, galena and chalcopyrite may occur as minute inclusions in pyrite and, to a lesser extent, in arsenopyrite.
Gold occurs as fine inclusions in sulphides, mainly pyrite, but also in pyrrhotite or arsenopyrite, where it is found in fractures, cleavage planes or interstices, and also as an intragranular phase. Free gold occurs locally. Pyrite is generally porous, with gold particles that vary from 10 to 80 µm commonly associated with these cavities. Although pyrrhotite and arsenopyrite both host gold, almost half of the total resources at the Morro Velho deposit is within pyrite. Within the massive sulphides, pyrite hosts 40% of the gold in pyrite; 35% in pyrrhotite; 10% in arsenopyrite, with the remaining 5% in cubanite and chalcopyrite (Ladeira, 1980). Gangue minerals, particularlly carbonates, may also contain 20 vol.% of gold inclusions. Whilst fine gold occurs within sulphide minerals, blebs or splinter-shaped grains that vary from 1 to 110 µm in diameter are generally found along boundaries of pyrrhotite, or included in pyrite and arsenopyrite.
Platinum-group-elements (PGE), particularly Pd and Pt, are described at Morro Velho, occurring at concentrations of up to 0.340 g/t Pt and 0.336 g/t Pd (Ladeira, 1980). Whilst no PGE-bearing minerals have been described in the literature, analyses of ore samples indicate elevated concentrations of both Pd and Pt, which are suspected to be associated with ullmannite (NiCoS). The platinum to palladium ratio is typically &gre;8.
The Au:Ag ratio at Morro Velho is 5:1, and there is a positive correlation between Au and As, and to a lesser extent, between Au and Cu. However, copper declines to background values distal to ore more rapidly than does As. Cu in the ore also increases with depth, from ~500 ppm in the upper levels of the mine, to >1000 ppm Cu in the deepest workings. The most common gold-copper association at Morro Velho, is with chalcopyrite-rich rocks which have the highest Au concentrations. Zirconium, Ba, Be and, to a lesser extent Sr, tend to exhibit an inverse relationship to gold compared to that of As and Cu, in that they are relatively depleted proximal to ore zones. Scheelite is abundant at Morro Velho, which is in marked contrast to São Bento (Pereira et al., 2007) and Cuiabá (Ribeiro-Rodrigues, 1998).
Emplacement and alteration
The Morro Velho deposit is developed in close proximity to a major NNW-trending fault zone (Vial et al., 2007). Gold mineralisation in the deposit is hosted by a suite of hydrothermally altered rocks, referred to as lapa seca. These have been developed within a thick carbonaceous phyllite package, containing intercalations of felsic and intermediate volcaniclastic rocks and dolostones. These altered rocks were isoclinally folded and were metamorphosed prior to gold mineralisation, with textural features indicating the sulphide mineralisation post-dates regional peak metamorphism, and that the massive sulphide ore has not subsequently been metamorphosed nor significantly deformed. The orebodies are controlled by pre-existing structures which have a long axis parallel to the local stretching lineation, and have continuity down shallowly plunging fold axes, persisting for at least 4.8 km to a depth of 2450 m. Pb isotope ratios indicate a model age of 2.82 ±0.05 Ga for both sulphide and gold mineralisation. NOTE: This age is from Vial et al. (2007), although Noce et al. (2005) dates meta-volcaniclastic greywacke units low in the Nova Lima Group at 2792 ±11 and 2751 ±9 Ma. Schrank and Machado (1996) dated the carbonate Lapa Seca between 2771 to 2705 Ma.
The lapa seca have been divided into carbonatic, micaceous and quartzose types which respectively reflect zones of i). carbonate ±albite; ii). white mica ±biotite and chlorite; and iii). silica alteration of all rock types hosting this deposit. The carbonatic variety is subdivided into grey and brownish sub-types. The composite alteration envelope defined by the lapa seca has a tabular shape that has been folded to follow the shape of the structure hosting the mineralisation.
A broad zonation of alteration related to mineralisation has been recognised: a). In rocks distal (i.e., >50 m) from ore, epidote and chlorite are characteristic, with lesser white mica, biotite, albite and quartz; calcite is the dominant carbonate and minor pyrite is developed; b). In the intermediate (i.e., <50 to >30 m) zones, there is an increase in micas, particularly white mica, as well as albite and quartz, with chlorite representing a minor, but declining phase, while dolomite/ankerite/siderite increase at the expense of calcite to dominate in the intermediate zone; c). In the proximal and ore zone, chlorite, white mica, biotite and albite decline, quartz is most strongly developed, pyrite increases and pyrrhotite, arsenopyrite and gold appear and are strongly developed.
These observations imply the intermediate zone is represented by a carbonate-white mica-quartz-chlorite phyllite lithofacies; whilst chlorite-white mica phyllite straddles the intermediate to proximal zones. This is consistent with development of the micaceous lapa seca (LSmi) in the intermediate and proximal zones of hydrothermal alteration; and quartzose lapa seca (LSqz) and the brownish carbonatic lapa seca (LSca) representing the proximal and ore zones. The quartzose lapa seca is a well-banded rock resembling metachert, occurring as a quartz-rich matrix containing subordinate intergranular carbonate with intercalated films and lenses of chlorite and/or white mica. It is not an important host rock to gold, only occurring as a sub-economic, banded quartzose rock, mainly in the eastern portion of level 6 at Mina Velha. The proximal zone, dominantly composed of brownish carbonatic and micaceous lapa secas, carries the bulk of the mineralisation and most of the sulphides. These units represent the hydrothermal and sheared products of meta-volcaniclastic/meta-tuffaceous rocks, varying from a dacitic to andesitic and probably basaltic composition, now represented by schists and phyllites. The proximal, gold-bearing sulphide zones are enveloped by the outer grey carbonate-dominated alteration zone (albite-bearing, carbonatic lapa seca) that either contains, or is externally bounded by, zones of white mica ±chlorite and/or albite alteration (LSmi) and locally carries Au-bearing sulphide, mainly pyrite. Grey carbonatic lapa seca mainly occurs in the hanging wall of the mineralisation and does not contain significant ore minerals. It is interpreted to be the product of extreme metasomatic CO2 alteration of a meta-volcaniclastic rock of intermediate composition, probably with intercalations of lenses of more mafic composition.
The lapa seca are interpreted to largely be the results of intense pre-gold carbonate alteration processes, with the gold mineralisation being related to the waning stages of that alteration process. Carbonate gangue textures and compositions show that the ore-forming process significantly altered the precursor rock, specifically both the protolith and earlier stage alteration assemblages to produce the final lapa seca.
High-grade massive sulphide mineralisation occurs below the base of the grey carbonatic lapa seca, at its lower contact with micaceous and brownish lapa secas. The disseminated mineralization and the quartz veins are hosted by micaceous lapa seca.
Despite the overwhelming importance of structural control of orebodies at Morro Velho, geometrical and mineralogical-chemical characteristics of the mineralisation suggest there is also a stratigraphic control by the host rock lithology. Vial et al. (2007) suggest infiltration of mineralising fluid, while controlled by major structures, the fluid limited below the grey carbonatic lapa seca which acted as a impermeable seal.
Production
Total estimated production between 1834 and 2003 is estimated to have totalled 370 tonnes of gold (Costa and Rios, 2022).
In 1999, in the last years of the operation, some 6.1 tonnes of gold were produced at a cash cost of USD 124 per oz and a total production cost of USD 176 per oz. The recovered grade was 7.01 g/t Au.
The most recent source geological information used to prepare this decription was dated: 2007.
Record last updated: 24/1/2023
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.
Morro Velho mine
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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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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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Vial, D.S., DeWitt, E., Lobato, L.M. and Thorman, C.H., 2007 - The geology of the Morro Velho gold deposit in the Archean Rio das Velhas greenstone belt, Quadrilatero Ferrifero, Brazil: in Ore Geology Reviews v.32, pp. 511-542.
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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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