New geologic, fluid inclusion and stable isotope studies on the controversial Igarapé Bahia Cu–Au deposit, Carajás Province, Brazil

• Published in: Mineralium deposita 2008-02, Vol.43 (2), p.161-184
• Main Authors: Dreher, Ana M., Xavier, Roberto P., Taylor, Bruce E., Martini, Sérgio L.
• Format: Article
• Language: English
• Subjects: Basalt; Boron; Calcite; Calcium chloride; Carbon dioxide; Carbonates; Chert; Earth and Environmental Science; Earth Sciences; Fluid dynamics; Geochemistry; Geology; Gold; Iron ores; Isotope studies; Isotopes; Leaching; Mineral Resources; Mineralization; Mineralogy; Minerals; Pyrite; Quartz; Salinity; Sedimentary geology; Sedimentary rocks; Sodium chloride; Stable isotopes; Sulfides; Sulfur; Veins (geology); Volcanoes
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-007-0150-6

The Igarapé Bahia Cu–Au deposit in the Carajás Province, Brazil, is hosted by steeply dipping metavolcano-sedimentary rocks of the Igarapé Bahia Group. This group consists of a low greenschist grade unit of the Archean (∼2,750 Ma) Itacaiúnas Supergroup, in which other important Cu–Au and iron ore deposits of the Carajás region are also hosted. The orebody at Igarapé Bahia is a fragmental rock unit situated between chloritized basalt, with associated hyaloclastite, banded iron formation (BIF), and chert in the footwall and mainly coarse- to fine-grained turbidites in the hanging wall. The fragmental rock unit is a nearly concordant, 2 km long and 30–250 m thick orebody made up of heterolithic, usually matrix-supported rocks composed mainly of coarse basalt, BIF, and chert clasts derived from the footwall unit. Mineralization is confined to the fine-grained matrix and comprises disseminated to massive chalcopyrite accompanied by magnetite, gold, U- and light rare earth element (LREE)-minerals, and minor other sulfides like bornite, molybdenite, cobaltite, digenite, and pyrite. Gangue minerals include siderite, chlorite, amphibole, tourmaline, quartz, stilpnomelane, epidote, and apatite. A less important mineralization style at Igarapé Bahia is represented by late quartz–chalcopyrite–calcite veins that crosscut all rocks in the deposit area. Fluid inclusions trapped in a quartz cavity in the ore unit indicate that saline aqueous fluids (5 to 45 wt% NaCl + CaCl 2 equiv), together with carbonic (CO 2  ± CH 4 ) and low-salinity aqueous carbonic (6 wt% NaCl equiv) fluids, were involved in the mineralization process. Carbonates from the fragmental layer have δ 13 C values from −6.7 to −13.4 per mil that indicate their origin from organic and possibly also from magmatic carbon. The δ 34 S values for chalcopyrite range from −1.1 to 5.6 per mil with an outlier at −10.8 per mil, implying that most sulfur is magmatic or leached from magmatic rocks, whereas a limited contribution of reduced and oxydized sulfur is also evident. Oxygen isotopic ratios in magnetite, quartz, and siderite yield calculated temperatures of ∼400°C and δ 18 O-enriched compositions (5 to 16.5 per mil) for the ore-forming fluids that suggest a magmatic input and/or an interaction with 18 O-rich, probably sedimentary rocks. The late veins of the Igarapé Bahia deposit area were formed from saline aqueous fluids (2 to 60 wt% NaCl + CaCl 2 equiv) with δ 18 O fluid compositions around 0 per mil that in