Hydrothermal Alteration, Fluid Evolution, and Re-Os Geochronology of the Grota Funda Iron Oxide Copper-Gold Deposit, Carajas Province (Para State), Brazil

The Crota Funda iron oxide copper-gold (10CG) deposit is situated within a regional WNW-ESE-striking shear zone in the northwestern portion of the Carajas domain, an Archean (3.0-2.55 Ga) segment of the Carajas Province, Brazil. Metavolcano-sedimentary sequences of the Igarape-Pojuca Group, comprising basalt, diabase, gabbro, dacite, and banded iron formations, are the main lithotypes recognized in the deposit area. In this sequence, mafic igneous rocks represent the main hosts to the copper (-gold) mineralization stages. The paragenetic evolution of the Grota Funda hydrothermal system encompasses an early high-temperature calcic-sodic alteration (albite-hastingsite-scapolite), followed by intense Fe metasomatism (magnetite-gruneritealmandine), potassic alteration with biotite, chlorite-quartz-tourmaline assemblage, and late carbonate-quartz veining. Copper (-gold) mineralization stages are spatially and temporally associated with iron-enriched (mineralization I), potassically altered (mineralization II), and chlorite-altered zones (mineralization III). Molybdenite from grunerite-magnetite veins yielded a Re-Os model age of 2530 +/- 60 Ma, which is interpreted as the mineralization I age. The main sulfide ore (mineralization II) predominantly forms breccia bodies characterized by a chalcopyrite-magnetite-sphaleritepyrrhotite assemblage. Chalcopyrite (delta S-34 values (0.9 +/- 0.9 parts per thousand) point to a magmatic source for sulfur, probably leached from the host mafic rocks. Development of early and high-temperature (>500 degrees C) alteration assemblages is attributed to regional circulation of deep-seated hypersaline and metalliferous fluids. Mixing with moderate- to high-salinity (24-29 wt % NaCl + CaCl2 equiv) and cooler fluids may have triggered ore precipitation in the main ore zone of mineralization II, due to a decrease in temperature and Cl- activity. Postore alteration assemblages resulted from considerable temperature and pH decreases