IRON OXIDE CU-AU DEPOSITS IN SOUTH AMERICA: CANDELARIA, CHILE, AND SOSSEGO, BRAZIL

Mesozoic and late Archean-early Proterozoic(?) iron oxide Cu-Au deposits occur in South America. The Cretaceous Candelaria deposit (470 Mt at 0.95 % Cu, 0.22 g/t Au, 3.1 g/t Ag) is located in the Chilean Coastal Cordillera south of Copiapó. The orebody is hosted in volcanic and volcaniclastic rocks that underlie Neocomian marine limestones at the western margin of the Atacama back-arc basin. Candelaria lies within the contact metamorphic aureole of the Coastal Batholith, which is exposed about 1 km to the west of the deposit. The magnetite-chalcopyrite-pyrite ±minor sphalerite ±trace molybdenite mineralization is associated mainly with biotite-K-feldspar, amphibole, quartz, ±albite rich alteration assemblages. Mineralization occurred around 115 Ma, coeval with batholith emplacement and regional uplift. The Sossego deposit (355 Mt at 1.1 % Cu, 0.28 g/t Au) is located in the southern part of the Late Archean Carajás volcano-sedimentary basin, Pará Sate, Brazil. The chalcopyrite ±magnetite ±minor pyrite ±trace molybdenite ore is closely associated with amphibole-epidote, albite, apatite, calcite, quartz and ±minor biotite gangue. Pervasively albitized granitic and dioritic-gabbroic intrusive rocks form the hangingwall of the deposit, whereas foliated biotitized volcanic rocks occur in its footwall. Disturbed ⁴⁰Ar/³⁹Ar spectra of amphibole suggest a minimum alteration age of 2.2-2.3 Ga. Despite the differences in age and setting, these two deposits share similarities regarding alteration types, and distribution (zonation) of ore and gangue minerals. Both deposits apparently formed at shallow crustal levels. Their formation is explained by mixing of magmatic with non-magmatic fluids. At Candelaria, a significant magmatic fluid component can be detected, whereas Sossego represents the non-magmatic fluid-dominated portion of an iron oxide Cu-Au system.