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Antas, Pedra Branca, Santa Lucia
Para, Brazil
Main commodities: Cu Au


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The Antas, Pedra Branca and Santa Lúcia copper deposits are located within the Carajás Mineral Province in the state of Pará, Brazil. Antas and Pedra Branca are 25 km SE and 100 km south of Parauapebas respectively, and are ~45 km apart. Santa Lúcia is ~ 40 km SSE of Antas, 30 km ENE of Pedra Branca, 75 km ESE of Sossego and ~50 km SSE of Parauapebas. Pedra Branca is ~20 km SSE of the Sossego IOCG deposit.

When this summary was written in 2021, these three deposits, and possibly others, are being delineated and/or mined as part of the OZ Minerals Carajás East Hub that involves a central treatment plant at Pedra Branca and a series of satellite mines. Mining at the Antas open pit commenced in 2016, and was completed in June 2021. The first ore from the Pedra Branca underground mine was trucked to the Antas Hub for treatment in 2020, with the tailings being deposited in the exhausted Antas Norte open pit. As of late 2022, the Pedra Branca mine has a capacity of 1 Mt per annum of high grade Cu-Au ore with an 8 year mine life.

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.

NOTE: Whilst the Pedra Branca deposit contains significant magnetite and as such resembles an IOCG deposit, there is apparently considerably less iron oxides at Antas and Santa Lúcia, but a higher proportion of pyrrhotite gangue, possibly suggesting they represent an 'Iron Sulphide Copper-Gold' or ISCG style of mineralisation, as recognised in the Cloncurry IOCG Province in Australia.


Antas

The Antas (or Antas Norte) 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. The Antas deposit is a structurally controlled and spatially related to (WNW-ESE) regional shear zones, specifically the Carajás fault and the Cinzento shear system.

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.

Mineralisation is hosted within hydrothermal brecciated and altered felsic and mafic volcanic units. The bulk of that mineralisation occurs within a steeply dipping body of hydrothermal breccia that contains zones/fragments of massive sulphide and disseminated sulphide minerals, within a matrix of hydrothermal breccia. Mineralisation is interprete to be structurally focussed, resulting in the deposition of high grade (>10% Cu) massive sulphide zones immediately adjacent to or within the interpreted main structural zone. These high-grade massive sulphide zones occur near the periphery of the orebody and contain chalcopyrite, the most abundant sulphide, and pyrrhotite, forming massive aggregates. The proportions of chalcopyrite, pyrrhotite and magnetite are variable. These massive sulphide zones are commonly 2 to 5 m wide and are surrounded by a broader zone of lower grade disseminated mineralisation grading 0.2 to 2.0% Cu.

Medium to high grade mineralisation represents the bulk of the mineralised zones and also 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.

Sulphide veins are usually not deformed, and are planar. The main zone of mineralisation at the Antas Norte mine was defined over a 350 m strike length, 300 m down dip and up to 35 m width. Mineralisation in the eastern part of the pit was offset and truncated by late stage dykes which, during emplacement, have remobilised mineralisation locally along dyke margins.

Domain boundaries for the high-grade, often restricted massive sulphide domains, are distinct and frequently abrupt from a grade perspective from the adjacent low-grade domains. The lower grade disseminated mineralisation to waste boundary has a more gradational nature but can also be quite distinct.

An 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.
    plus 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.



Pedra Branca

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, as defined by the 5 metre-percent contour, plunges east at ~70°, tapering downward. This lens has a core of >60 metre-percent.

The estimated Mineral Resource as of November 2019, dips steeply to the south and extends over a 1700 m along strike, with a 350 m gap between the eastern and western lodes. Plan thickness varies between 130 m in the west, where there are a series of stacked vertical lenses, to 20 m in the east where the orebody presents as a single mineralised zone. Depth below surface to the upper limit of the Mineral Resource is 10 to 20 m with the mineralisation extending vertically for 560 m in the west and 770m in the East (OZ Minerals Release, 2019).

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 lesser 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 (Avanco Copper, 2016). A consistent high-grade zone occurs on the hanging wall margin of the mineralised zone, although its thickness varies. Confidence in the interpreted shape of similar footwall high-grade zones is lower (OZ Minerals Release, 2019).
• 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.

Estimated JORC compliant Mineral Resources at Pedra Branca East and West at 30 June 2022 were (OZ Minerals 2022 Annual Report):

      Measured resource - 1.90 Mt @ 1.6% Cu, 0.48 g/t Au;
      Inferred resource - 8.7 Mt @ 1.7% Cu, 0.44 g/t Au.
      Inferred resource - 7.3 Mt @ 1.4% Cu, 0.36 g/t Au.
      TOTAL Mineral Resource - 17.90 Mt @ 1.6% Cu, 0.71 g/t Au.
    The Mineral Resource is distributed as 13 Mt @ 1.7% Cu, 0.5 g/t Au in the East and 5.5 Mt @ 1.3% Cu, 0.5 g/t Au in the West Lens.
      Proved + Probable Ore Reserves - 4.05 Mt @ 2.0% Cu, 0.53 g/t Au.

The Pedra Branca and Antas summaries are drawn from descriptions on the Avanco Copper Limited website viewed March, 2018; the OZ Minerals Antas North Mineral Resource and Ore Reserve Statement and Explanatory Notes, as at 30 June 2020; and the OZ Minerals Pedra Branca 2019 Mineral Resource Statement and Explanatory Notes, as at 25 March 2019.


Santa Lúcia

The Santa Lúcia copper-gold deposit lies within the southeastern section of the Serra do Rabo trend of the Carajás Mineral Province, occurring along NW-SE splays of the Carajás Fault. However, in some locations there are also east-west and NE-SW structural orientations, possibly related to a large open fold that is evident on satellite, radar and airborne geophysical images. Mineralisation is hosted by a meta volcanic sequence comprising rhyolitic volcanic and sub-volcanic rocks of the Grão Pará Group, and is associated with an alkali granite intrusion. This granitoid trends NW-SE and is generally strongly foliated, and both contains and is bounded by foliation parallel schists. The latter are typically phengite-biotite-quartz or phengite-garnet-biotite schist. The granitoid has been extensively hydrothermally altered, producing a microcline-sericite-biotite overprint. The surrounding country rocks are generally folded banded iron formations. The granitoid is, in turn, intruded by several parallel pegmatites and quartz veins (OZ Minerals, 2021; Hunger et al., 2021).

Intense ductile-brittle deformation generated a prominent mylonitic foliation in some lithotypes, e.g., shales and BIFs, and the development of shear bands, faults, fracture zones and gaps, as seen within the granitoid body. Brittle deformation is evident in the deposit, represented by brittle fracture systems, quartz veinlets and pegmatite dykes (OZ Minerals, 2021).

Pegmatites occur as narrow, NW-SE trending lenses. They are light grey to pinkish grey, and are very coarse with a porphyritic texture, have an isotropic to weak foliation, and contain tourmaline. They are apparently related to the formation of mineralisation, and are intruded in the immediate proximity of the mineralisation, in several places remobilising chalcopyrite, as well as being weakly mineralised themselves. Some hydraulic breccias are associated with the margins of these pegmatites (OZ Minerals, 2021).

Adjacent to the deposit, the host granitoid has undergone intense brittle ductile deformation, manifested as a parallel mylonitic foliation, anastomosed foliation parallel biotite schists, and breccia zones. Hydrothermal alteration proximal to the deposit comprises the regional assemblage, plus a subordinate chlorite-tourmaline overprint (OZ Minerals, 2021). Within the deposit, the paragenetic evolution comprises an early chlorite stage, followed by potassic alteration with microcline, greisenisation (quartz-muscovite-tourmaline), copper-gold ore precipitation, and late sericite and hematite vein formation/fracture infill (Hunger et al., 2021).

Mineralisation occurs as either zones of massive sulphide, typically chalcopyrite-pyrite or chalcopyrite + subordinate pyrrhotite, or as stockwork zones of quartz/sulphide veins, veinlets, stringers and disseminations (OZ Minerals, 2021). This mineralisation is associated with brecciation and has a spatial association with greisen alteration, characterised by the enrichment of light rare earth elements (LREE), Ni, Co and Cr (Hunger et al., 2021).

There are also some erratic developments of massive chalcocite with lesser associated bornite, related to weathering processes, mainly in the transition from the base of the saprolite layer to the top of fresh bedrock, controlled by fractures/faults that influenced water percolation at depth (OZ Minerals, 2021).

The alteration and mineralisation styles, and ore assemblage are very similar to those of the ~1.88 Ga Paleoproterozoic suite of granite-related copper-gold systems of Carajás Mineral Province (e.g., the Breves and Estrela deposits). However, the in situ U-Pb analyses of ore-related monazite returned a weighted average 207Pb/206Pb age of 2688 ±27 Ma, constraining the timing of mineralisation to the Neoarchaean. In addition, tourmaline from the pegmatite and within the ore zones has a range of δ11B values from -3.7 to -0.6‰, indicating a magmatic boron source. Together, these results suggest the Santa Lúcia deposit is the first reduced magmatic-hydrothermal, iron oxide–poor system formed in the Neoarchaean, coeval with the 2.72 to 2.68 Ga metallogenic event that produce most of important IOCG deposits in the Carajás Mineral Province (Hunger et al., 2021).

An estimated maiden JORC compliant Mineral Resources at Santa Lúcia as at 12 July 2021 (OZ Mineral, 2021), at a 0.3% Cu cut-off, were:

      Indicated resource - 0.91 Mt @ 6.1% Cu, 0.97 g/t Au, 9.2 g/t Ag;
      Inferred resource - 4.9 Mt @ 1.3% Cu, 0.24 g/t Au, 3.9 g/t Ag.
      TOTAL Mineral resource - 5.8 Mt @ 2.1% Cu, 0.35 g/t Au, 4.8 g/t Ag.

The Santa Lúcia summary was drawn from the OZ Minerals ASX release of 1 July, 2021 - Santa Lúcia Mineral Resource Statement and Explanatory Notes and Hunger et al., 2021 cited below.

The most recent source geological information used to prepare this decription was dated: 2021.     Record last updated: 15/10/2021
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.


  References & Additional Information
   Selected References:
Hunger, R.B., Melo, G.H.C., Xavier, R.P., Moreto,C.P.N., Talavera, C., Su, Z.-K. and Zhao, X.-F.,  2021 - The Santa Lucia Cu-Au deposit, Carajas Mineral Province, Brazil: a Neoarchean (2.68 Ga) member of the granite-related copper-gold systems of Carajas: in    Mineralium Deposita   v.56, pp. 1521-1542.


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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