AN ARCHEAN RE-OS MOLYBDENITE AGE FOR THE GAMELEIRA CU-AU-MO MINERALIZATION, CARAJÁS PROVINCE, BRAZIL

The Gameleira Cu-Au-Mo deposit is a new discovery in the northwestern part of the Late Archean Carajás volcano-sedimentary basin. It lies close to the Pojuca iron oxide Cu-Zn(-Mo) deposit and both are possibly related. Gameleira is characterized by a complex sequence of multiple veining events. Hypogene mineralization at Gameleira includes chalcopyrite plus minor bornite, molybdenite, fluorite, rare pyrite, and uraninite. The host rocks of the deposit belong to the Pojuca Group, which is composed of metavolcanic and volcano-sedimentary rocks, banded iron formation, quartz ±garnet rich schists, meta-arenite and siltstone. The deposit is underlain by a syenitic intrusion. Chalcopyrite intergrown with quartz and biotite from Gameleira gave a d³⁴S_CDT value of 2 ‰. The quartz has a d¹⁸O_SMOW value of 8.8 ‰. Halite-saturated fluid inclusions with homogenization temperatures up to 400ºC have been reported. Assuming these elevated temperatures for the quartz, the oxygen isotope and sulfur isotope data are consistent with a magmatic-hydrothermal fluid component in the ore forming system as previously suggested. Molybdenite from Gameleira yielded a Re-Os age of 2614 ± 14 Ma. This age shows that iron oxide Cu-Au-Mo mineralization at Gameleira is unrelated to widespread 1.8-1.9 Ga anorogenic granite emplacement. The age implies that the ore formation occurred late in the history of the Carajás basin, probably during regional transpression.