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Alemao |
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Para, Brazil |
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Cu Au
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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 Alemão IOCG Au-Cu-(REE-U) deposit is part of the Igarape Bahia mineralised system in the Carajás Mineral Province of Para State Brazil, ~60 km west of the Carajás township (or Nucleo Urbano) and ~40 km south of the Salobo deposit. (see the Igarapé Bahia record).
(#Location: 6° 1' 35"S, 50° 34' 38"W).
Alemão is hosted by the Igarapé Bahia Group, considered to be a lower greenschist facies metamorphosed unit of the Archaean (ca. 2.75 Ga) metavolcano-sedimentary Itacaiúnas Supergroup which comprises two lithological and stratigraphic domains: a lower metavolcanic unit composed of metavolcanic rocks and acid to intermediate volcanoclastics; and an upper clastic-chemical metasedimentary unit and volcanoclastic rocks. The Alemão ore body underlies the far northwestern margin of the Igarapé Bahia deposit, which represents a 100 to 150 m thick gossan-laterite zone from which significant amounts of gold (>60 t) were mined until 2003. Elsewhere it is obscured by a 250 m thick unconformable siliciclastic unit referred as the Aguas Claras Formation.
The copper-gold mineralisation at the Igarapé Bahia/Alemão deposit is hosted by a hydrothermally altered breccia at the contact between the footwall mafic volcanics, with associated BIF and hyaloclastite, and a dominantly coarse to fine-grained metasedimentary sequence in the hanging wall. The breccia unit is exposed at or near the surface as a semicircular annulus, with a form similar to a ring complex with a diameter of approximately 1.5 km. The mineralised breccia unit occurs as a 2 km long by 30 to 250 m thick series of fault dislocated bodies on the southern, eastern and northern sections of this structure, dipping steeply outwards at ~75°, and is nearly concordant with the metavolcanic-sedimentary wallrocks. The Igarapé Bahia deposit is the thick gossan-laterite zone developed within the top 100 to 150 m of the exposed breccia unit.
The Alemão deposit is located immediately to the northwest of this annular zone, occurring as a particularly magnetite-Cu-Au-enriched down-faulted segment of the Acampamento Norte orebody, the northern most orebody of the Igarapé Bahia deposit. It has dimensions of around 500 m in length, 50 to 200 m thick and has been traced down plunge for at least 800 m below the surface, although the top of the deposit is at a depth of approximately 250 m below Aguas Claras Formation sandstone cover.
The Alemão orebody is hosted by a hydrothermally altered breccia at the contact between the footwall mafic volcanics, with associated BIF and hyaloclastite, and a dominantly coarse to fine-grained metasedimentary sequence in the hanging wall. A set of unmetamorphosed 2.75 to 2.65 Ga quartz diorite and 2579±7 Ma dolerite dykes cut the orebodies, the host metavolcano-sedimentary sequence and the overlying clastic metasedimentary sequence of the Águas Claras Formation/Rio Fresco Group.
The breccia has gradational contacts with its wallrocks and is made up of polymitic, usually matrix-supported clasts, composed mainly of coarse, angular to rounded basalt, BIF and chert clasts derived from the footwall unit.
The hydrothermal paragenesis is marked by ferric minerals (magnetite and hematite), sulphides (chalcopyrite-pyrite), chlorite, carbonate (siderite, calcite, ankerite) and biotite with minor quartz, tourmaline, fluorite, apatite, uraninite, gold and silver. Sericite and albite are rare. The mineralisation is represented by hydrothermal breccias and hydrothermally altered rocks. These fall within two groups, namely:
i). massive magnetite-chalcopyrite bands and polymict breccias with a matrix of magnetite, chalcopyrite, siderite, chlorite, biotite and amphiboles; and
ii). brecciated hydrothermally altered volcanics with chalcopyrite, bornite, pyrite, chlorite, siderite and ankerite both in the matrix and disseminated in the altered country rock.
Several generations of late mineralised veins crosscut the ore breccia and are composed of variable concentrations of chalcopyrite, pyrite, quartz, calcite, chlorite, and fluorite. The veins commonly display open space - filling textures (e.g., comb)
The total estimated resource in 2001 was 170 Mt @ 1.5% Cu, 0.8 g/t Au. More recently CVRD has quoted a reserve of 161 Mt @ 1.3% Cu, 0.86 g/t Au.
Alemao breccias. Above - Chlorite-carbonate-sulphide breccia. Note the well developed striations are created by the diamond saw blade that cut the surface shown;
Below - magnetite-sulphide breccia. In both images, the smaller scale card graduations are in millimetres.
Images by Mike Porter, 2020, samples collected at Alemao 2001.
The deposit was discovered by Docegeo, exploration arm of CVRD (now Vale) in 1996.
The most recent source geological information used to prepare this decription was dated: 2001.
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.
Alemao
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Huang, X.-W. and Beaudoin, G., 2019 - Textures and Chemical Compositions of Magnetite from Iron Oxide Copper-Gold (IOCG) and Kiruna-Type Iron Oxide-Apatite (IOA) Deposits and Their Implications for Ore Genesis and Magnetite Classification Schemes: in Econ. Geol. v.114, pp. 953-979.
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Huang, X-W., Beaudoin, G. and Yang, Y., 2022 - A HR-TEM study on two generations of magnetite from the Alemao IOCG deposit, Carajas, Brazil: Implication for Fe-Cu mineralization: in Ore Geology Reviews v.146, doi.org/10.1016/j.oregeorev.2022.104934.
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Ronze P C, Soares A D V, dos Santos D G V, Barreira D F 2000 - Alemao copper-gold (-U-REE) deposit, Carajas, Brazil: in Porter T M (Ed), 2000 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective PGC Publishing, Adelaide v1 pp. 191-202
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Tallarico, F.H.B., Oliveira, C.G. and Figueiredo, B.R., 2000 - The Igarape Bahia Cu-Au mineralization, Carajas Province: in Revista Brasileira de Geociencias, v.30, pp. 230-233.
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Williams, P. J., Kendrick, M.A. and Xavier, R.P., 2010 - Sources of Ore Fluid Components in IOCG Deposits: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide v.3, pp. 107-116.
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Xavier R P, Monteiro L V S, Souza Filho C R, Torresi I, Carvalho E R, Dreher A M, Wiedenbeck M, Trumbull R B, Pestilho A L S and Moreto C P N, 2010 - The Iron Oxide Copper-Gold Deposits of the Carajas Mineral Province, Brazil: an Updated and Critical Review: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide v.3 pp. 285-306
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Xavier, R.P., Monteiro, L.V.S., Moreto, C.P.N., Pestilho, A.L.S., de Melho, G.H.C., Delinardo da Silva, M.A., Aires, B., Ribeiro, C. and Freitas e Silva, F.H., 2012 - The Iron Oxide Copper-Gold Systems of the Carajás Mineral Province, Brazil: in Society of Economic Geologists, Special Publication 16, Chapter 17, pp. 433-454.
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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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