The Igarape Bahia Au-Cu(-REE-U) deposit, Carajas mineral province, northern Brazil

The Igarape Bahia Au-Cu-(REE-U) deposit is located in the Carajas Mineral Province - Northern Brazil - and is hosted by an Archaean low-grade metamorphosed volcanosedimentary sequence characterized by metavolcanic rocks of the footwall and metavolcanoclastic/metasedimentary rocks of the hangwall. An intense hydrothermal alteration occurred in this sequence, promoting intense chloritization, Fe-metasomatism, Cu-sulphidation (chalcopyrite and bornite), carbonatization, silicification, tourmalinization and biotitization.

The Cu-Au mineralization is hosted at the contact between meta-volcanic and meta-volcanoclastic/meta-sedimentary rocks and is divided into magnetitic/sideritic heterolithic breccias and hydrothermalized metavolcanic rock, which are enriched in REE (monazite, alanite, xenotime, bastnasite and parisite), Mo (molybdenite), U (uraninite), F (fluorite), CI (ferropyrosmalite) and P (apatite). We believe that saline and F-Cl-rich fluids at high temperatures may have been responsible for carrying the REE, according to fluid inclusion and paragenetic studies.

C and O isotopic data from hydrothermal carbonates and fluid inclusion analyses from carbonate and quartz suggest the mixing of two fluids in both alteration and mineralization processes. Negative values of Ol3C (-9.3 to -5.8%o) indicate the presence of magmatic fluid with high salinity; moreover, the large variation of 8I20 (0.7 to 9.4%o) suggests a mixture between magmatic fluids of high T (higher isotopic values) and meteoric fluids (lower values).

On the basis of chemical and mineralogical composition, isotopic and fluid inclusion data, we propose in this paper a genetic model for the Igarape Bahia deposit, that may be compared to the other intrusion-related hydrothermal Fe-oxide-(Cu-Au-U- REE) deposits.