Paragenesis and evolution of the hydrothermal Bacuri iron oxide-copper-gold deposit, Carajás Province (PA)

The Bacuri copper deposit is located about 9 km east of the world-class Sossego iron oxide-copper-gold deposit in the Carajas Province. It is hosted by the ca. 2.84 Ga Serra Dourada granite, the Bacuri Porphyry, and subordinated gabbro. The Bacuri deposit is situated within the regional-scale, WNW-ESE-trending ductile Canaa shear zone, characterized by early albite, scapolite-magnetite, potassium feldspar and biotite-scapolite-magnetite hydrothermal alteration zones. Copper mineralization was controlled and largely simultaneous to the development of a NE-SW-trending transcurrent fault zone. Within this zone, intense chlorite alteration, silicification and quartz veining overlap previous hydrothermal stages. Copper ore ­(chalcopyrite-pyrite-magnetite) is disseminated and related to veins and breccias. Chalcopyrite occurs along the mylonitic foliation in chlorite alteration zones, in brecciated quartz-(muscovite-fluorite) and undeformed milky quartz veins and in late potassium feldspar-epidote veinlets with open space textures. The relative temporal history of the Bacuri deposit reveals significant changes in physico-chemical parameters during the system evolution. Contribution of hot hypersaline metalliferous fluids was important during early sodic and potassic alteration stages and related to a regional hydrothermal system developed at relatively deep crustal levels. Influx of externally derived diluted fluids within the NE-SW-trending fault zone favoured the late shallow-emplaced chlorite alteration and silicification, and may have triggered the ore deposition due to decrease of temperature and salinity, accompanied by ƒO2 increase and pH decrease. Such significant changes could indicate a hybrid system that evolved in multiple pulses during progressive exhumation.

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