THE SALOBO IRON OXIDE COPPER-GOLD HYDROTHERMAL SYSTEM, CARAJÁS MINERAL PROVINCE, BRAZIL

The Salobo iron oxide copper-gold deposit (estimated reserves at 789 Mt @ 0.96% Cu, 0.52 g/t Au; Souza & Angelim, 2000), northern Brazil, is hosted by the Archean Salobo Group. It is formed by a sequence of amphibolites, banded iron formations, metagraywackes, and quartzites, deposited in a trondhjemitic basement where a continental rift basin was developed. These rocks were affected by ductile-brittle shear zones between 2581 and 2551 Ma (Machado et al., 1991). Predominantly Fe-K hydrothermal alteration is associated with the ore stage. In host amphibolites, an intense alkali metasomatism is expressed by incipient Na-alteration with superposed K-alteration. K-feldspar, biotite, and oligoclase are the main alteration minerals. A significant increase in the FeO content (up to 35 wt%) accompanied the alkali metasomatism in amphibolites and was marked by the replacement of Ca-amphibole by Fe-Mg amphibole. The Salobo host “iron rocks” and metagraywackes also underwent important high temperature K-Fe alteration. Areas affected by strong K-Fe metasomatism host most of the copper-gold mineralization (Requia and Fontboté, 1999, 2000). Principal ore assemblages are magnetite-bornite-chalcocite and magnetite-bornite-chalcopyrite, with magnetite dominant. The iron oxide copper-gold ore shows elevated concentrations of Ag, U, Co, As, Mo, F, and LREE. Trace and REE geochemistry shows fundamental differences between the magnetite-rich “iron rocks” and the barren banded iron formations. Fluid inclusion data for quartz veins and apatite associated with the copper-gold ore in iron-rich rocks indicate the involvement of high temperature, highly saline fluids in the deposit formation. Sulfur isotope analyses of chalcopyrite and bornite show d 34S values ranging from 0.2‰ to 1.6‰. The calculated d 18O of the ore-stage fluids at 485°C (temperature from fluid inclusions, isotope ratios from magnetite) ranges from 6.6 ‰ to 12.1‰, with a strong mode at 7‰. These stable isotope data and the very saline fluid inclusions suggest an important magmatic component of the fluid. Preliminary results of Re-Os and Pb-Pb geochronology in sulfides (molybdenite, resp. chalcopyrite and bornite) and magnetite indicate an Archean age (around 2550 Ma) for the Salobo mineralization (Requia et al., in prep.).