Antas, Pedra Branca
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The Antas and Pedra Branca copper deposits are located within the Carajás Mineral Province in the state of Pará, Brazil. They are 25 km SE and 100 km south of Parauapebas respectively, and are ~45 km apart.
The Carajás Mineral Province lies in the southern section of the Amazon Craton, and is divided into two tectonic blocks, the southern Rio Maria greenstone terrain, and the northern Itacaúnas Shear Belt. In the northern part of the Carajás Mineral Province, the basement rocks are overlain by meta-volcanosedimentary units of the Rio Novo Group and the Itacaúnas Supergroup. The later is composed of the Igarapé Salobo, Igarapé Pojuca, Grão Pará, and Igarapé Bahia Groups, which form the Archean Carajás Basin.
The Itacaúnas Supergroup hosts all the IOCG deposits of the Carajás Mineral Province and is interpreted to have been deposited in a marine rift environment. Metamorphism and deformation has been attributed to the development of the Itacaúnas shear zone and to the Cinzento and Carajás fault systems. Granites, mafic to ultramafic layered complexes, as well as gabbro dykes and sills, intrude the meta-volcanosedimentary sequence of the supergroup.
The Carajás Mineral Province hosts the world's largest known concentration IOCG deposits e.g., Sossego, Salobo, Igarapé Bahia-Alemão, Cristalino, Gameleira, Tarzan and 118, as well as Antas and Pedra Branca. It also hosts the giant Carajás iron deposits.
For more detail see the Carajas IOCG Province record.
The Antas deposit is located near the southern margin of the 2.76 to 2.74 Ga Estrela Granite Complex and is hosted predominantly by mafic metavolcanic rocks, cut by gabbro dykes.
Associated hydrothermal alteration has a moderate to strong zonation from the surrounding unaltered country rock into the more strongly mineralised portions of the deposit, similar to zones recognised at other IOCG deposits in the Carajás, namely: i). early regional sodic albite-scapolite alteration, followed by ii). potassic alteration, predominantly biotite, and iii). calcic alteration, principally actinolite– cummingtonite. The complex stages of sodic, sodic-calcic, potassic, and hydrolytic alteration observed at Antas are generally similar to those described by Monterio and Xavier (2008) from the Sossego-Sequeirinho IOCG system 45 km to the SW.
The main orebody at Antas is oriented NE-SW and was closely reflected by coincident >1000 ppm Cu and electromagnetic geophysical anomalies.
The majority of mineralisation occurs within a steeply dipping body of hydrothermal breccia that contains fragments of massive sulphide and disseminated sulphide minerals, within a matrix of hydrothermal breccia. High grade massive sulphide zones occur near the periphery of the ore zones and contains variable amounts of chalcopyrite and pyrrhotite as dominant minerals, with chalcopyrite the most abundant sulphide, forming a massive aggregate. Sulphide veins are usually not deformed, and are planar.
Medium to high grade mineralisation represents the bulk of the mineralised zones and contains variable amounts of chalcopyrite and pyrrhotite as the dominant sulphides, which frequently form the matrix of hydrothermal breccia’s.
Low grade mineralisation occurs as fine to medium grained disseminations of chalcopyrite and as fracture fill and small blebs. This ore type commonly occurs between narrow medium to high grade and massive sulphide zones.
Estimated JORC compliant Mineral Resources and Ore Reserves at Antas at May 2016 were (Avanco Copper website viewed March, 2018) at a 0.9% Cu cut-off:
Measured resource - 1.96 Mt @ 3.42% Cu, 0.76 g/t Au;
Indicated resource - 1.61 Mt @ 2.23% Cu, 0.42 g/t Au.
Inferred resource - 1.89 Mt @ 1.59% Cu, 0.23 g/t Au.
TOTAL Mineral resource - 5.46 Mt @ 2.8% Cu, 0.48 g/t Au.
including at a 0.65% Cu cut-off:
Proved reserve - 1.23 Mt @ 3.34% Cu, 0.73 g/t Au;
Probable reserve - 1.69 Mt @ 2.16% Cu, 0.47 g/t Au.
Proved stockpile - 0.12 Mt @ 2.26% Cu, 0.53 g/t Au.
TOTAL Mineral resource - 3.04 Mt @ 2.8% Cu, 0.58 g/t Au.
The Pedra Branca copper deposit is located in the southern part of the Carajás Mineral Province, just under 100 km south of Parauapebas and 30 km east of Canaã.
The deposit is divided into the East and West Zone. The main Pedra Branca East zone is an Iron Oxide Copper Gold (IOCG) type deposit located within the Carajás Basement, and is predominantly hosted by diorite and sheared granite. The enclosing country rocks are mostly biotite orthogneisses, with abundant pegmatite dykes occurring in proximity to the ore zones.
The deposit occurs along an east-west striking regional shear zone, which is represented by metres wide mylonitic zones that have been intensely silicified. This shear zone is, in turn, regionally crosscut by north-south, NE and NW striking faults.
The main Pedra Branca East zone plunges, as defined by the 5 metre-percent contour plunges east at ~70°, with a horizontal width of 450 to 200 m, tapering downward from near surface to a depth of ~600 m below surface. This lens has a core of >60 metre-percent.
Two main ore types are identified:
• Semi-massive, high grade breccia matrix ore that comprises a hydrothermal breccia zones with matrix filled by magnetite and chalcopyrite, plus subordinate pyrrhotite and lesers pyrite. The clasts are composed of altered host rock and coarse amphibole. The typical range of assays for this ore type is between 4.0 and 10.0% Cu.
• Disseminated ore, which is generally a low to medium grade and envelops the high-grade zone. It has the same sulphide assemblage, which is also associated with magnetite that occurs as fine disseminations in the host rock, sometimes following the foliation, but also filling veinlets and fractures. The typical assays range from below 0.9% Cu, the cut-off grade and 2.5% Cu. The ore zones are accompanied by proximal potassic alteration.
Estimated JORC compliant Mineral Resources at Pedra Branca East at May 2016 were (Avanco Copper website viewed March, 2018) at a 0.9% Cu cut-off:
Measured + Indicated resource - 7.70 Mt @ 2.8% Cu, 0.7 g/t Au;
Inferred resource - 2.78 Mt @ 2.7% Cu, 0.6 g/t Au.
TOTAL Mineral resource - 10.48 Mt @ 2.8% Cu, 0.7 g/t Au.
This summary is drawn from descriptions on the Avanco Copper Limited website viewed March, 2018.
The most recent source geological information used to prepare this summary was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
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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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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