Abstract
The high-grade gold mineralization at Fazenda Brasileiro occurs in quartz-albite-sulfide veins hosted by iron-rich metagabbros and forms the upper part of a differentiated mafic sill.The veins are enveloped by zones of hydrothermal alteration in which the metamorphic greenschist facies assemblage has been replaced by new mineral associations with a distinct zonation. Based on petrographic criteria these associations are classified into (1) parent rock, (2) zone of incipient alteration, (3) zone of advanced alteration, and (4) vein. The sequence of alteration is characterized by a major decrease in the chlorite content along with increases in albite, carbonates, pyrite, and arsenopyrite.Ore deposition resulted from the interaction between wall rocks and a low-salinity, H 2 O-CO 2 -CH 4 -dominated hydrothermal fluid containing Na, S, and Au, which percolated through permeable zones developed during late-stage deformation. The deposit shows marked overall similarities with other greenschist facies gold deposits, such as Hunt, Sigma, and Kerr-Addison, in regard to the hydrothermal alteration pattern.
