Hydrothermal alteration in a shear zone associated to the Congonhas lineament, Southern Quadrilátero Ferrífero, Minas Gerais

Abstract

Several gold works dating back from the XVII Century are located over the Congonhas Lineament, a NW-trending steep NE-dipping regional structure composed by oblique slip (left-lateral reverse) én-echelon anastomosing shear zones. Hydrothermal alteration was concentrated on ductile to ductile-ruptile shear zones controlled by rheologic contrasts. Carbonatic alteration is widespread, surrounding chloritization and sericitization zones which envelop cores with silica and sulfide alteration. Grunerite and stilplomelane are present in BIFs close to the outer boundaries of the sulfide zones. The sulfide zone has an outer halo where pyrhotite is the dominant sulfide, followed by an intermediate zone with pyrite, chalcopyrite and pyrhotite. At the nuclei, arsenopyrite is the most common sulfide. If the age of the lower (and mineralised) section of the metavolcanossedimentary belt is Siderian, as its upper part, then gold mineralizations at Congonhas Lineament are Proterozoic instead of Archean.