Abstract

A striking feature of the Carajás region, Brazil, is the clustering of a variety of different types of Cu-Au deposits. The most abundant in the belt are the >200 million metric tons (Mt) of Fe oxide Cu-Au-(U-REE) deposits, which, despite the variety of host rocks and different orebody morphologies, share a number of diagnostic features, including (1) intense Fe metasomatism leading to the formation of grunerite, fayalite, and/or Fe oxides (magnetite and/or hematite); (2) intense carbonate alteration (mainly siderite); (3) sulfur-poor ore mineralogy (chalcopyrite and bornite); (4) quartz-deficient gangue; (5) extreme low REE enrichment, and (6) enrichment in U and Co. The Igarapé Bahia deposit is perhaps the best documented Fe oxide Cu-Au-(U-REE) deposit of the belt, containing about 219 Mt at 1.4 percent Cu and 0.86 g/t Au. The Cu-Au ore consists of steeply dipping breccia bodies that are hosted by hydrothermally altered metavolcano-sedimentary rocks.

SHRIMP II zircon dating of the host metavolcanic rocks gives a 207Pb/206Pb age of 2748 ± 34 Ma. This suggests a correlation between the Igarapé Bahia volcano-sedimentary sequence and the Grão Pará volcanic rocks, which have published ages of ca. 2.75 Ga. SHRIMP dating of monazite from the matrix of ore-bearing magnetite breccias gives a 207Pb/206Pb age of 2575 ± 12 Ma, confirming the epigenetic nature of the mineralization and placing it ~175 m.y. after accumulation of the host volcano-sedimentary sequence. The 2575 ± 12 Ma SHRIMP age of hydrothermal monazite from the Igarapé Bahia mineralization is indistinguishable from published conventional 207Pb/206Pb ages for zircons from the Archean A-type granites of the Carajás belt, indicating that mineralization processes at Igarapé Bahia were temporally related to these A-type Archean granites. The wide range of highly radiogenic 87Sr/86Sr ratios (0.714–0.755) of carbonates from the Igarapé Bahia deposit suggests multiple crustal sources, consistent with a magmatic-hydrothermal origin. SHRIMP dating of zircon xenocrysts recovered from crosscutting diabase dikes indicates a maximum 207Pb/206Pb age of ~2670 Ma, consistent with field evidence and the age of host rocks, but does not unequivocably constrain the age of the ores.

The styles of hydrothermal alteration, mineralogy, and geochemistry of the Igarapé Bahia ore, as well as published fluid inclusion and stable isotope data, support its classification as a member of the world-class Olympic Dam-type Fe oxide Cu-Au-(U-REE) group of deposits, as previously argued by several authors. The SHRIMP age of 2575 ± 12 Ma for hydrothermal monazite indicates that Igarapé Bahia is an Archean example of this deposit group.

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