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Boa Esperanca, Tucuma |
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Para, Brazil |
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Cu
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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 Boa Esperança copper deposit of the Tucumã Project is located within the municipality of Tucumã, Pará State, Brazil, 35 km to the southwest of the town of Tucumã, 190 km SW of Parauapebas and ~160 km WSW of the Sossego IOCG deposit (#Location: 6° 51' 42"S, 51° 26' 55"W).
The exploration title encompassing the Boa Esperança copper deposit was initially held by the Brazilian subsidiary of Corporación Nacional del Cobre (Codelco) of Chile. Codelco identified and undertook exploration at Boa Esperança between 2003 and 2006. The resource defined by this work was deemed insufficient to justify Codelco’s continued interest. Consequently, in the second half of 2007, Codelco initiated a competitive bidding process to sell the Boa Esperança copper and cobalt title. Mineração Caraiba S.A. (MCSA) submitted the winning bid and became the legal owner of the mineral rights to the deposit. Subsequently MCSA commenced studies of the deposit with the objective of preparing a Feasibility Study for the Project. In December 2016, the Canadian company, Ero Copper Corp., acquired an ~85.0% interest in MCSA, and by December 2017, through further purchases, held ~99.6%. Following a positive feasibility study that was completed in September 2021, the project was approved in February 2022 and construction commenced in the second quarter of the same year. Construction was completed in the second quarter of 2024, and the first saleable copper concentrate was produced in July 2024 (Ero Copper website viewed September, 2024).
Regional Setting
See the Carajás IOCG Province record for an overview of the broader regional setting. NOTE: Boa Esperança is located ~30 km south of the SW corner of Fig. 3 in that record. As such it is currently an outlier of the Carajas IOCG Province.
The Boa Esperança deposit lies within the older Mesoarchaean core of the Amazonian Craton, represented locally by the 3.0 to 2.85 Ga Xingu and Pium complexes, within what is referred to as the Rio Maria Domain. These are overlain in the northern half of the Carajás District by the 2.76 to 2.73 Ga Itacaúnas Supergroup of the Itacaúnas Shear Belt. These two domains represent two distinct Archaean tectonic terranes. The Rio Maria Domain is characterised by a dome and keel granite-greenstone regime, comprising greenstone belts, intruded by Mesoarchaean tonalite-trondhjemite-granodiorites (TTG) and a variety of younger granitoids, as well as Palaeoproterozoic fissure-controlled volcanic rocks. In contrast, the Neoarchaen 2.8 to 2.5 Ga Itacaúnas Shear Belt has a more linear framework that is characterised by an initial rift-phase, with submarine volcanism and sedimentation, which evolved to a sag-phase characterised by siliciclastic sequences (Teixeira et al., 2021). The Itacaúnas Shear Belt hosts base, precious and ferrous metal deposits associated with granitic rocks, ultramafic intrusions, and metasedimentary sequences.
As detailed above, the Rio Maria Domain is a Mesoarchaean nucleus comprising greenstone belt terrains of the Tucumã and São Félix Groups, TTG suites, and a variety of late Mesoarchean granitoids (Silva et al., 2016). The greenstone sequences comprise both komatiitic and basaltic meta-mafic and minor intermediate lava flows, with intercalated BIF and chert. The 2.98 to 2.86 Ga granitic rocks have been grouped into four sub types (after Almeida et al., 2020) as follows:
• Group I - TTGs sensu stricto, encompassing the Arco Verde, Mogno, Caracol, Marizinha and Ãgua Fria granitoids, with ages that range from 2.96 to 2.86 Ga;
• Group II - sanukitic high-Mg varieties, e.g., the ~2.87 Ga Rio Maria Granodiorite;
• Group III - potassic leucogranites, represented by the 2.86 Ga Xinguara and Mata Surrão plutons; and
• Group IV - high-barium and strontium leucogranites and granodiorites dated at ~2.87 Ga.
The Rio Maria Granodiorite lies within a ductile shear zone, and is limited to both the east and west by Xingu Complex TTGs. It is composed of leuco- to mesocratic, equigranular to porphyritic granites and granodiorites composed of plagioclase, K feldspar, quartz, biotite, hornblende and accessory minerals.
The Boa Esperança copper deposit is hosted by the Boa Esperança granite, a subunit of the Rio Maria granodiorite.
Granitic rocks crosscutting the greenstone sequence are intensely deformed and metamorphosed to amphibolite facies (Macambira and Vale, 1997).
Within the Itacaúnas Shear Belt, the onset of the Neoarchaean Itacaúnas Supergroup rift phase at ~2.76 Ga is marked by the Grão Pará Group, which comprises komatiitic lava flows, lesser felsic volcanic units, and shales and sandstones. The overlying ~2.74 Ga Carajás Formation consists of banded iron formation (BIF) sequences, with rare, minor volcanic units. The succeeding Igarapé Bahia Formation is characterised by mafic lava flows with minor explosive units, and associated shales and BIFs. The Azul Formation was deposited over the Grão Pará Group and marks the transition to a shallower marine environment, composed primarily of shales and siliceous sediments. The Ãguas Claras and Gorotire Formations, deposited discordantly over the Azul Formation, represent the final sedimentary pulse of the sag-phase at ~2.37 to 2.4 Ga, and comprise fluvial sandstones and conglomerates. The entire sequence has been metamorphosed, places to amphibolite facies.
Late Paleoproterozoic ~1.88 Ga A-type granites cross-cut the entire sequence. This magmatism occurs within the Rio Maria Domain, represented by the Seringa, Gradaús, São João, Jamon, Musa, Marajoara, Manda Saia, Bannach and Redenção plutons (Silva et al., 2016), and synchronous intermediate to acid volcanic rocks of the Sobreiro and Santa Rosa Formations, found predominantly as dykes.
Geology
The principal lithologies at the Boa Esperança deposit are as follows:
• Metamorphic Rocks of the Xingu Complex - Strongly banded and silicified light brown to grey gneisses that contain chlorite and epidote and are located in the western segment of the deposit, surrounded by intrusive rocks containing large xenoliths.
• Boa Esperança Granite, with variable compositional and textural types, that hosts the copper mineralisation and intrudes the metamorphic rocks It includes:
- Granite - A granitic body that has been altered and mineralised. Macroscopically, it is a biotite granite with a hypidiomorphic granular texture and an average composition of 40% feldspar, 35% silica, 10% plagioclase 15% and biotite (15%). The biotite has been weakly alteration to chlorite. The granite contains feldspar, quartz and pyrite veins of with epidote selvages. This granite is interpreted to likely be the product of hydrothermal potassic alteration of the surrounding granodiorite, producing a biotite-sericite-K feldspar assemblage.
- Granite Breccia - corresponding to granite with a weak incipient brecciation.
- Coarse-grained Granite - found in the northern segment of the deposit, where there is an intrusive porphyritic granite, with up to 2 cm phenocrysts of zoned feldspar set in a matrix of quartz and feldspar. No contact relationships with the enclosing granitoids have been observed, but it may correspond to a change in composition from the same intrusive event.
- Granite Porphyry - containing phenocrysts of feldspar set in an aphanitic crystalline matrix. Contacts with the enclosing granite range from passive to diffuse.
• Tonalite - an Intrusive that occurs in the western segment of the deposit, interpreted to be part of the host intrusive complex. It includes textural variations, such as being fine grained on the edge of the intrusive body, and variations of the degree of brecciation, as in the tonalite breccia, which exhibits small areas of a strong incipient brecciation.
• Mylonite - found in the north, east and northeast segments of the main deposit area. It is characterised by the occurrence of sericite schists, interstratified with quartzite. and exhibits mild folding, with attitudes ranging from 10 to 165° and dips at between 30 and 80°E. Macroscopically, it is a fine-grained, white to yellowish, foliated, and non-magnetic sericite-quartz mylonite, with large translucent quartz crystals surrounded by clusters of light-yellow sericite. A petrographic report described "a foliated rock of medium to coarse crystallinity, that was intensely sheared and hydrothermally altered, featuring a cataclastic texture with few traces of the original rock texture remaining". The mineralogical composition comprises 60% quartz, 38% sericite, 2% rutile and trace zircon. The quartz occurs as oriented and deformed medium-grained to coarse-grained anhedral crystals displaying strong undulatory extinction in thin section. It also includes granoblastic aggregates, formed following cataclasis and partial recrystallisation. Sericite is found as sheets that form small aggregates surrounding oriented quartz crystals. It locally occurs as prismatic and subhedral pseudomorphs replace feldspar. Rutile is observed as fine prismatic crystals that are included in aggregates of sericite. Zircon occurs as a fine-grained accessory disseminated throughout. The mylonites are interpreted to be the result of local deformation of the granite, or sheared equivalent, which would have occurred prior to the brecciation.
• Mylonitic Granite, also known as foliated granite, in which the foliation is defined by biotite and quartz. Texturally, it is a pale pink, medium-grained rock, similar to the granite, with abundant microcline, quartz, plagioclase and epidote crystals. Microscopically, it is a medium-grained, strongly deformed, sheared and altered granite. It has undergone hydrothermal alteration to an assemblage of microcline, sericite and epidote, which have granular and hypidiomorphic granophyric relict textures. The mineralogical composition is: 30% microcline, 25% quartz,19% sericite, 15% plagioclase, 5% epidote, 3% biotite, 2% sphene, 1% allanite and traces of zirconium, apatite, chlorite, calcite and other opaques. It represents an intermediate phase between the granite and mylonites, in which the deformation is not so severe. This unit is also called foliated granite (GRF).
• Boa Esperança Breccia - which is an open space hydrothermal breccia that was strongly controlled by tectonic shear structures, and has been subsequently mineralised. It has a matrix composed of magnetite, biotite and chalcopyrite engulfing clasts of quartz, granite and pyrite, and was emplaced in two principal pulses. The first pulse of mineralisation introduced quartz-pyrite, which is found in some areas as massive replacement deposits. The composition and degree of foliation of this pulse indicate that it formed in a brittle structural setting. The second pulse introduced magnetite, biotite and copper. Biotite appears to represent the first phase during this pulse, with a texture interpreted as syn-shearing. Magnetite has a massive and only faintly foliated texture, invading the biotite and leaving, in some cases, elongated clasts of biotite. Pyrite and chalcopyrite were deposited on the edges of the breccia fractures, interpreted to possibly suggest that the solutions from this late phase were low in sulphur.
• Dykes, which are primarily dacitic in composition, although some are intermediate to felsic in composition, and are associated with the younger, ~1.8 Ga, Uatumã intrusive/volcanic event.
- Dacitic Dykes - occurring as porphyritic dacite, generally striking at ~110°, similar to the major regional structural grain. Macroscopically, the dykes are dark-coloured, with concentrically zoned feldspar crystals, clear quartz 'eyes' that have resorption recesses, chloritised mafic minerals, chiefly biotite, as well as abundant 1 to 4 cm rounded xenoliths of granite. The matrix is medium- to fine-grained, composed of aggregates of K feldspar, quartz and mafic minerals. Locally, these dykes moderately chloritised and sericitised. In the vicinity of the Boa Esperança hill, these dykes have undergrone supergene alteration to clay, as a result of oxidation of sulphides in the enclosing breccia and granite.
- Intermediate to Felsic Dykes - which are of variable composition and controlled by regional structures.
• Microdiorites, which are found as a series of generally 130 to 150° striking fine granular textured dykes that are of dioritic composition, composed of an aggregate of plagioclase and mafic minerals. The rock is a sub-volcanic lamprophyre dyke with strong hydrothermal alteration to chlorite and saussurite, without deformation, and showing original porphyritic, amygdaloidal, and well-preserved panidiomorphic granular textures.
Mineralisation
The Boa Esperança deposit occurs below a NNE elongated, isolated hill, formed by resistant quartz and magnetite breccias, the host to the copper mineralisation, cutting softer, 2.85 Ga Mesoarchaean biotite granite. Mineralisation consists of a series of brecciated zones, which strike at 60 to 70°, and dip to the SE at 60 to 70°. Individual breccia bodies are ~<20 to >30 m in width and 200 to 800 m in strike length. Below the 'peak' of the hill, there is a greater density of these breccia zones, coalescing over an exposed plan area of ~450 x 350 m, with fingers projecting further out in a ENE and WSW directions. Individual WSW-ENE and WNW-ESE breccia zones are found outside of this central core, spaced at 50 to 200 m. The copper mineralised zone has the geometric shape of a downward tapering cone. Sub-surface, this mineralised zone, as known from drilling in 2021, extends for ~850 m in a WSW-ENE direction, for 640 m NNW-SSE, and is known to a depth of 780 m below surface. It has a steep SE plunge and remains open in that direction. The deposit is hosted in a series of breccias cross-cutting a granite intrusion. These breccias are distributed throughout the ENE-WSW elongated barren pink granite lenses that, in turn, cut the Boa Esperança granite. The breccias are composed of magnetite, quartz, biotite, chalcopyrite and pyrite, with minor molybdenite. The alteration haloes surrounding the breccias are narrow, generally with a width of about a third of the breccia thickness, and are composed of chlorite with epidote and potassium feldspar.
The breccias grade from fractured granite characterised by irregular parallel to sub-parallel mineralised fractures that merge into incipient brecciation and to breccias with clasts that range from a few mm to >5 cm across. These clasts vary from near equi-dimensional to mostly elongated parallel to the main fracture direction, with length to width ratios of 2:1 to 5:1. Sulphides appear to be present as clasts, disrupted veins and blebs within the matrix (from images in Murray et al., 2021).
The Boa Esperança deposit mineralisation has both a primary and secondary zoning. The primary zonation involves a distal pyrite dominated periphery, grading inward towards copper mineralisation in the form of pyrite-chalcopyrite → chalcopyrite-pyrite → chalcopyrite.
The secondary zonation is of supergene origin, and comprises a:
i). discontinuous, barren, sub-horizontal leached lens that crops out on the hill top, and is composed of hematite, goethite and clay minerals. Despite the significant contained sulphide boxworks, the leached zone does not carry economic copper concentrations;
ii). copper oxide zone, which is found immediately beneath the leached zone and adjacent to sulphide brecciasis composed of malachite and copper-bearing clays;
iii). mixed zone of oxides and primary copper sulphides, comprising a variety of oxides, carbonates, secondary supergene sulphides, mainly chalcocite and covellite and primary sulphides, predominantly pyrite and chalcopyrite;
iv). lower zone of supergene copper enrichment comprising 5 to 10 m thick sub-horizontal lenses extending up to 20 to 30 m laterally and composed of primary and secondary sulphides.
Cobalt is geochemically associated with the mineralisation and is concentrated on the surface. However, there is no correlation between copper and cobalt grades within the mineralisation. The cobalt is intimately associated with sulphur and iron, indicating that it is in the lattice of pyrite. Iron is more abundant in the pyrite and chalcopyrite zones, and a higher molybdenum content is found in the chalcopyrite-pyrite mineralisation and in the leached zone.
SRK Consulting (2017) suggest the presence of magnetite within the breccias hosting the copper mineralisation suggests the deposit may be classified as IOCG (Iron Oxide Copper Gold), although, there are other features that vary from the general IOCG deposit type. Among these is the presence of the high-sulphur mineral assemblage characterised by chalcopyrite-pyrite, rather than the low-sulphur copper sulphide chalcopyrite-bornite-chalcocite mineral assemblage characteristic of IOCG deposits, as well as the high quartz content, the absence of extensive pervasive hydrothermal alteration of the host rock, in particular sodic (albite) alteration, and the absence of gold.
Reserves and Resources
Mineral Resources, as at 31 August, 2021 ( Murray et al., 2021) were as follows:
Open pit high grade:
Measured + Indicated Resource - 8.778 Mt @ 2.18% Cu;
Inferred Resource - 0.0405 Mt @ 2.69% Cu;
Open pit low grade:
Measured + Indicated Resource - 38.909 Mt @ 0.57% Cu;
Inferred Resource - 0.5144 Mt @ 0.49% Cu.
Underground high grade:
Inferred Resource - 1.354 Mt @ 2.24% Cu;
Underground low grade:
Inferred Resource - 9.681 Mt @ 0.60% Cu;
Combined TOTAL:
Measured + Indicated Resource - 47.687 Mt @ 0.86% Cu;
Inferred Resource - 11.590 Mt @ 0.80% Cu;
TOTAL Resource - 59.277 Mt @ 0.85% Cu.
Ore Reserves, as at 31 August, 2021 ( Murray et al., 2021) were as follows:
Proved Reserve - 30.674 Mt @ 0.89% Cu;
Probable Resource - 12.378 Mt @ 0.67% Cu;
TOTAL Resource - 43.052 Mt @ 0.83% Cu.
NOTE: Resources are inclusive of Reserves
The information in this summary was largely drawn from: Barbosa, C., Mendonça, R. and Tomi, G., 2017 - Feasibility Study Technical Report Boa Esperança Copper Project, Pará State, Brazil; an NI 43-101 Technical Report, prepared by SRK Consultores do Brasil LTDA., Belo Horizonte, Minas Gerais, Brazil, for Ero Copper Corp. and
Murray, K., Patterson, E.L., Elfen, S., Ré, E.R. and Guzman, C., 2021 - Boa Esperança Project, NI 43-101 Technical Report on Feasibility Study Update, Southern Pará State, Brazil; an NI 43-101 Technical Report, prepared by Ausenco Engineering Canada Inc., Vancouver, Canada; for Ero Copper Corp.
The most recent source geological information used to prepare this decription was dated: 2021.
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.
Boa Esperanca
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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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