Maracas Menchen - Campbell Pit, Gulcari A North (GAN), Novo Amparo North (NAN)
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The Maracás Menchen vanadium-rich titaniferous magnetite deposit cluster is located in southeastern Bahia State, Brazil, ~250 km SW of Salvador and 813 km NE of Brasilia.
The Maracás deposits are located within the Archean São Francisco craton, hosted by the Neoarchaean Rio Jacaré Intrusion, which is situated on the eastern margin of the Palaeoproterozoic Contendas-Mirante supracrustal volcano-sedimentary sequence. The latter forms a large anticlinorium trending approximately north-south, structurally bounded by the Palaeoarchaean Gavião Block to the west. The latter is composed predominantly of tonalite-trondhjemite granodiorite. Both the Rio Jacaré Intrusion and the Contendas-Mirante sequence are bounded by the Neoarchaean Jequié Block to the east, which is largely composed of charnockite and enderbite intrusive rocks with marked calc-alkaline affinities and granulite facies metamorphic rocks. The Contendas-Mirante supracrustal sequence was deformed during the collision between the Gavião and Jequié blocks during the Transamazonian orogeny and now lies along part of the major north-south trending Contendas-Mirante-Jacobina Lineament (Teixeira et al., 2000).
The Rio Jacaré mafic-ultramafic Intrusion, which is largely composed of gabbro, is a linear sheet-like body that strikes almost north-south over a strike length of ~70 km, with an average width of 1.2 km, and a dip of 70°E. Several discrete vanadium-rich titanomagnetite deposits/prospects have been defined along strike from south to north within the intrusive complex, namely the Gulçari A South, Gulçari A (Campbell Pit), Gulçari A North, Gulçari B, São José, Novo Amparo and Novo Amparo North deposits. These deposits are located at a number of stratigraphic position within the Rio Jacaré Intrusion, which is composed of a series cyclic units, and hence are not all related to the same cycle.
The Rio Jacaré Intrusion is structurally bounded by faults to the east and west, with the Jequié block and Contendas-Mirante Sequence respectively. Whole rock dating of rocks from the intrusion includes a Pb/Pb age of 2470 ±72 Ma, a Sm/Nd age of 2800 ±68 Ma, and a zircon age of 2640 ±5 Ma (Brito et al., 2001). It is cut by granitic pegmatite veins that are closely related to a granite intrusion with an age of 1940 ±54 Ma (Brito et al., 2001). The igneous textures and mineralogy of the intrusion have been modified by metamorphism and deformation, with relict minerals being rare, although some igneous textures are preserved, e.g., olivine cumulate textures and layering between pyroxenite and gabbro. Pyroxene in these rock types is now largely altered to hornblende, which is, in turn, frequently replaced by actinolite, tremolite and chlorite. The presence of amphibole and garnet in the gabbro and 'magnetitite' (an igneous rock composed largely of magnetite) in the Rio Jacaré Intrusion indicates amphibolite grade metamorphism.
According to Brito (1983) the stratified Rio Jacaré Intrusion is divided of two zones,
i). a Lower Gabbro-dioritic Zone composed of rocks of the gabbro-diorite transition which are generally grey, massive, mesocratic, medium grained and classified as diorites, gabbros and subordinate anorthosites. This zone contains no known vanadium-rich titaniferous magnetite, and has not been described in detail;
ii). the Upper Stratified Zone that has an average thickness of 600 m and has a transitional basal zone, overlain by layered cycles of gabbros (~80%) varying from leucogabbro to melagabbro, with a lower percentage of pyroxenitic and tonalitic rocks, magnetite-bearing pyroxenite, and bands of stratiform 'magnetitites' that alternate until the top. The pyroxenite consists of thin layers, typically a few centimetres to <1 m in thickness, and which are in many cases associated with the magnetite bodies.
The gabbro is massive, coarse grained, and weakly foliated, whereas the diorite in the Lower Zone is massive and mainly fine grained. The primary igneous mineralogy of the gabbro comprises plagioclase and orthopyroxene as cumulate phases, with interstitial clinopyroxene. Quartz and biotite are only minor phases, whilst apatite and titanite are common accessories. Lenses of magnetite-rich rocks are also found within the Lower zone. The centres of these lenses are composed of massive magnetite (i.e., magnetitite), whilst the outer margins consist of magnetite-bearing pyroxenite with 30 to 70% opaque minerals. The contact relationships with the gabbro are not clear, because these lenses are usually bounded by faults and are poorly exposed.
The Rio Jacaré Intrusion has been mapped in detail for 8 km to the north of Campbell Pit, and for >25 km to the south. The detailed subdivision of the Upper Zone of the intrusion within the deposit area is divided into a number of cyclic units as follows, from the base of the Upper Stratified Zone:
• Transition Zone or TZ - which overlies the Lower Gabbro-dioritic Zone and comprises a 70 to 110 m thick layer of leucogabbro interspersed with 3 to 10 m pyroxenite layers and 1 to 5 m magnetite pyroxenites. It contains modal magnetite that is only weakly mineralised.
• C1 Cycle - which commences with a massive, ~1 to 3 m thick, magnetitite layer that grades upwards into magnetite pyroxenite or pyroxenite with magnetite, and then into a biotite gabbro with sparse disseminated magnetite and thin pyroxenite bands. The SiO2 content increases to the top of the cycle, defining a modal stratification with the crystallisation sequence of the magma being progressively depleted in modal magnetite and enriched in plagioclase.
• C2 Cycle - a thin, up to 5 to 20 m thick, and laterally discontinuous layer, comprising a thin 1 to 3 m thick basal magnetitite, grading into a magnetite pyroxenite and an upper biotite gabbro.
• C3 Cycle , which contains the best development of magnetite mineralisation, including the bulk of the resource in the Campbell Pit. It comprises a lower magnetite-pyroxenite, grading upwards into an ~20 to 40 m thick magnetitite unit, followed by a further magnetite-pyroxenite, a magnetite-gabbro and an upper band of unmineralised gabbro. Magnetitite is interlayered with the magnetite-pyroxenite, but is not laterally continuous, pinching out to the north and to the south. Some of the lithologies of the cycle have a cumulate texture, e.g., where pyroxene crystals and opaque minerals are surrounded by hornblende. This cycle is also characterised by anomalous PGE values, with Pd and Pt values of up to 700 ppb.
• C4 Cycle , comprising a basal magnetite-pyroxenite, overlain by magnetite-gabbro, gabbro and leucogabbro containing disseminated magnetite which exhibit compositional macro-layering. The upper section of this cycle is occupied by a ~3 m thick anorthosite band. Additional magnetite gabbro and magnetite pyroxenite units are developed at the Gulçari A North and São Jose deposits, but not elsewhere. An additional titano-biotite-gabbro is found at the C4 to C5 contact, characterised by well-developed biotite and a mylonitic texture, and is considered the highest occurrence of this lithotype which occurs in all previous cycles, from and including the basal Transition Zone.
• C5 Cycle - predominantly a gabbro enclosing a magnetite-gabbro and a magnetite-pyroxenite band, and an upper pyroxenite overlain by an anorthosite. The magnetite-metagabbro band is one of the main layers at Novo Amparo Norte. At São José, a layer of meta-magnetitite is commonly found in the middle of the cycle. PGE is present in the mineralisation at the base of the cycle.
• C6 Cycle - essentially comprising an ~10 m thick meta-magnetitite layer, grading into magnetite-gabbro, and increasing in plagioclase towards the top. This gradation defines an upward sequence of meta-magnetitite &arr; magnetite-metagabbro &arr; gabbro with magnetite &arr; meta-gabbro &arr; meta-anorthosite. This cycle contains an important resource, characterised by geological continuity and the occurrence of high grades of V2O5 at Novo Amparo Norte and São José.
• C7 Cycle - which grades from the base upward, meta-magnetitites → magnetite-metagabbros → meta-gabbros, with a progressive reduction in magnetite and an increase in plagioclase at the top. The meta-magnetitite and magnetite-metagabbro in this cycle contain 15 to 25% TiO2, whilst the cycle also contains cumulus apatite from immediately above the magnetite-enriched lower portion, within the lower part of the gabbro or magnetite-gabbro unit.
• C8 Cycle - a thick cyclic unit grading from a lower zone of magnetite-gabbro → a ~20 m thick magnetitite → a 2 to 3 m thick mottled anorthosite with clinopyroxene oikocrysts → an ~130 m thick package of gabbro and leucogabbro. The gabbro contains distinctive plagioclase phenocrysts, whilst the base of the sequence grades upwards from magnetite-gabbro to magnetitite in contrast to the typical basal magnetitite unit above a sharp lower contact found in the other cycles. Anomalous copper values of up to 3000 ppm are also noted in this cycle.
• C9 Cycle - a thick cycle commencing with an ~30 m thick magnetite-gabbro at the base, overlain by gabbro, anorthosite and magnetite-leucogabbro. The uppermost section of the cycle is characterised by a 40 to 150 m thick unit of anorthosite which has a cumulus texture and distinctive pyroxene oikocrysts near its upper margin.
• C10 Cycle - which constitutes the uppermost cyclic unit observed at the Rio Jacaré Intrusion. It comprises an ~5 to 20 m thick magnetite-gabbro, grading upwards into gabbro, leucogabbro and anorthosite. The upper contact with the Pé de Serra Gneiss of the Jequié Block is tectonic.
The intrusive 'sequence' is transgressive, with the lowermost four stratigraphic units, namely the Transition Zone and cycles C1, C2 and C3 only found within, and near the Campbell Pit, where the highest preserved is cycle is C7. In contrast, in the northern and southern part of the complex, cycles C4 to C10 are present, with the former overlying a reduced Transition zone. C1 to C4 are less laterally extensive, developed in a thicker, central basal depression within the pre-tilting intrusive chamber. This central zone has been interpreted to represent the feeder zone 'stem' of a mushroom shaped intrusion. In contrast, cycles C5 to C10, which occur in the upper portions of the intrusive complex, are laterally extensive over the entire strike length of the Rio Jacaré Intrusion, forming the upper 'cap' of the mushroom.
Vanadium is hosted within titaniferous magnetite, the major oxide phase present. The next most abundant oxide is ilmenite, which hosts the bulk of the titanium. The magnetite occurs as 0.3 to 2.0 mm long, primary anhedral magmatic crystals that may be partly to wholly martitised. They form a polygonal mosaic, together with ilmenite. The latter is generally found as discrete anhedral magmatic crystals, but may also occur as inclusions within the titaniferous magnetite, where they commonly exhibit exsolution textures. The lower cyclic units, particularly C3 at the Campbell deposit, contain magnetite that has higher V2O5 contents than those from the upper cyclic units. This is consistent across all of the deposits of the intrusion. The massive magnetitites were formed as ilmenite-magnetite heteradcumulates that form 2 cm to 3 m thick layers containing variable amounts of clinopyroxene. These are interlayered with mafic and ultramafic cumulates that are composed of olivine-magnetite cumulates and clinopyroxene-magnetite heteradcumulates, together forming rhythmically micro-layered gabbro, magnetite and magnetite-pyroxenite bands. Fine-grained magnetite also locally occurs as inclusions within silicate grains, the result of alteration of iron-rich silicates, e.g. the uralitisation of pyroxene and serpentinisation of olivine. Other silicates associated with secondary magnetite include augite, plagioclase, hornblende and sparse grains of clinopyroxene, olivine and spinel. Rare olivine and pyroxene grains are observed within the magnetitite, although most are altered to serpentine or chlorite. However, it should be appreciated that the Rio Jacaré Intrusion has been strongly metamorphosed, and as a consequence, the pyroxene compositions observed may reflect metamorphic re-equilibration rather than original magmatic compositions. NOTE: a 'heteradcumulate' is a rock composed entirely of oikocrysts (typically centimetre-sized crystals) enclosing chadacrysts (smaller crystals of a different mineral that crystallized on the liquidus of the parent magma), with little or no trapped interstitial liquid component.
Minor sulphides are present within the magnetitite units, including chalcopyrite and pentlandite with rare pyrite and pyrrhotite, generally accounting for <1% of the rock. Chalcopyrite is the most abundant sulphide, being best developed in rock types containing up to 50% magnetite, where it occurs in association with magnetite or ilmenite, enclosed by amphibole and plagioclase. Pentlandite is much less abundant and occurs within the magnetitite. Minor sphalerite and galena grains are found together in the silicates, associated with the other sulphides, especially in the magnetite-poor rock types. The dominant trace minerals are nickel and cobalt sulphides and arsenides, and cobalt-rich pentlandite. In many cases the arsenides are associated with the sulphides and appear to be alteration products of those sulphides.
Elevated platinum and palladium values are also associated with magnetite-rich zones of the Rio Jacaré Intrusion, and are much greater than those of the surrounding silicate rocks. There are significant correlations between PGM and copper. The most abundant PGM minerals are those that are palladium-rich, particularly bismuthides and antimonides. These minerals accompany interstitial silicates, or are found within silicate inclusions in magnetite and ilmenite grains. They are also associated with pentlandite and, in a few cases, with arsenides. Sperrylite (PtAs2) is the most abundant platinum mineral and is associated with silicates, interstitial to magnetite and ilmenite grains. At sites where the igneous mafic minerals have been altered to amphiboles, sperrylite may be altered to platinum-iron alloys.
Rodriguez et al. (2021) suggest the Cu, Ni and PGM were concentrated in small quantities of sulphide that were co-precipitated with magnetite in the magnetitite layers, and that the platinum minerals were subsequently exsolved in situ from these sulphides. They further suggest, the association of palladium minerals with base metal sulphides and the small variation in the Pt/Pd ratio (of ~4:1) suggests that the PGMs have not been extensively remobilised within the magnetitite. This association is similar to the character of magnetite layers in the Rincón del Tigre (Bolivia), Skaergaard (Greenland) and Stella Complexes (South Africa).
Rocks above the water table, which at Maracás is mostly at a depth of ~30 m, are weathered and oxidised to varying degrees, with deeper oxidation in the vicinity of faults that may provide a conduit for the ingress of meteoric water. Silicate minerals generally weather and break down to clays more rapidly than oxides such as magnetite and ilmenite which are converted to maghemite, hematite, goethite and other iron oxides. The main effect of weathering/oxidation is a potential reduction in vanadium recovery to the magnetite concentrates, as the oxidised products of the latter are not magnetic.
The Mineral Resource estimate as at 16 December, 2021 were (Rodriguez et al., 2021):
Campbell Pit - Measured + Indicated Concentrate Resource - 19.43 Mt @ 1.19% V2O5, 7.89% TiO2;
- Inferred Resource - 5.10 Mt @ 0.92% V2O5, 8.20% TiO2;
GAN deposits - Measured + Indicated Resource - 21.37 Mt @ 0.53% V2O5, 7.87% TiO2;
- Inferred Resource - 4.52 Mt @ 0.64% V2O5, 8.40% TiO2;
NAN deposits - Measured + Indicated Resource - 22.89 Mt @ 0.71% V2O5, 8.74% TiO2;
- Inferred Resource - 5.90 Mt @ 0.67% V2O5, 7.75% TiO2;
TOTAL Maracás Menchen - Measured Resource - 45.95 Mt @ 0.83% V2O5, 8.15% TiO2;
- Indicated Resource - 17.73 Mt @ 0.70% V2O5, 8.37% TiO2;
- Measured + Indicated Resource - 63.69 Mt @ 0.80% V2O5, 8.21% TiO2;
- Inferred Resource - 15.52 Mt @ 0.74% V2O5, 8.09% TiO2;
The estimated magnetic concentrate to be produced from these Mineral Resources include:
- Measured + Indicated Resource - 31.27% Magnetic fraction, 3.08% V2O5, 4.95% TiO2, containing 0.2313 Mt of V2O5 and 1.5501 of Mt TiO2;
- Inferred Resource - 26.68% Magnetic fraction, 2.63% V2O5, 3.98% TiO2, containing 0.047 Mt of V2O5 and 0.4186 Mt of TiO2;
TOTAL Maracás Menchen
- Measured + Indicated Resource - 24.40% Magnetic fraction, 2.46% V2O5, 3.26% TiO2, containing 0.5076 Mt of V2O5 and 5.2312 Mt of TiO2;
- Inferred Resource - 23.27% Magnetic fraction, 2.44% V2O5, 3.13% TiO2, containing 0.1155 Mt of V2O5 and 1.2556 Mt of TiO2.
The information in this summary is principally drawn from "Rodriguez, P.C., Ferreira, G.G., Xavier, F.V.C. and Ferreira, M.S., 2021 - An updated life of mine plan ("LOMP") for Campbell Pit and pre-feasibilty study for NAN and GAN deposits, Maracás Menchen Project, Bahia, Brazil; and NI 43-101 Technical Report Prepared by GE21 Consultoria Mineral for Largo Inc., 474p."
The most recent source geological information used to prepare this summary was dated: 2021.
This description is a summary from published sources, the chief of which are listed below.
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