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Alemão Copper-Gold (-U-REE) Deposit, Carajãs, Brazil
 
by
Petronilia C. Ronze, Anselmo D. V. Soares, Marcos Giovanni S. dos Santos, Célio F. Barreira, Rio Doce Geologia e Mineração SA, DOCEGEO, Brazil.

in - Porter, T.M. (Ed), 2000 - Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide, v. 1, pp 191-202.

ABSTRACT

   The Alemão copper-gold deposit is located within the Carajás Mineral Province of Northern Brazil. It was discovered in 1996 by DOCEGEO using geophysical and geological techniques. Alemão is hosted by the Igarapé Bahia Group, 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 is covered by a 250 m thick unconformable siliciclastic unit referred as the Águas Claras Formation.
   The ore body, which is 500 m in length and 50 to 200 m wide, strikes NE-SW and dips steeply to the NW, being emplaced along the contact between the two stratigraphic domains of the Igarapé Bahia Groups. In the ore zone, the hydrothermal paragenesis is marked by ferric minerals (magnetite-hematite), sulfides (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 mineralization is represented by hydrothermal breccias and "hydrothermalites" classified into two types: (1) the BMS type is composed of massive bands of magnetite and chalcopyrite and by polymitic breccias with a matrix composed of magnetite, chalcopyrite, siderite, chlorite, biotite and amphiboles; (2) the BCLS type breccia which is composed of brecciated hydrothermalized volcanic rocks with chalcopyrite, bornite, pyrite, chlorite, siderite, ankerite, tourmaline and molybdenite in the matrix, as well as dissemination in the rock.
   The geochemical association of Fe-Cu-Au-U-REE in iron rich, heterolithic, hydrothermal breccias at the Alemão Cu-Au Deposit, as well as its possible association with an extensional tectonic set, suggests a correlation with the Olympic Dam type mineralization. The total estimated ore resources based on a krigging method was of 170 Mt @ 1.5% Cu and 0.8 g/t Au.

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