The B26 Cu-Zn-Ag-Au project, Brouillan volcanic complex, Abitibi greenstone belt, part 2: hydrothermal alteration and mineralization

The B26 project is located in the northern Abitibi greenstone belt a few kilometres south of the past-producing Zn-Cu-Ag-Au Selbaie mine. It consists of a transposed Au-rich chalcopyrite stringer zone (Copper zone) overlain by concordant Ag-rich Zn-(Pb) sulphide lenses (Main Zinc zone) and disseminated to discordant sulphide vein- and veinlet-rich zones (Upper Zinc zone). The footwall rocks are affected by a wide and intense semi-conformable alteration halo mainly composed of sericite, chlorite, and quartz assemblages. Fe-Mn carbonate and quartz are common as gangue minerals in the sulphide lenses. The hanging-wall rocks are affected by a proximal K-feldspar-quartz and a more distal albite alteration with Fe-(Mn)-carbonate veins and replacement zones. The mineralized zones and host rocks have undergone major shortening during the main phase of regional deformation, which is responsible for transposition of the Cu-rich footwall stringers, flattening and stretching of the concordant semi-massive to massive sulphide zones, and local remobilization of precious metal-bearing phases into the south-dipping foliation and along the associated southwest-plunging stretching lineation. Gold is directly associated with Cu, and also with Bi, Se, and Sn in the Copper zone as native gold, and Au-rich electrum with native Bi, bismuthinite, laitakarite, ikunolite, stannite, and cassiterite. Silver is present in the Main and Upper Zinc zones as freibergite, native silver, allargentum, acanthite, and pyrargyrite. Deposit-scale metal distribution and primary sulphide assemblages indicate that precious metals introduction was primary, although later local (metre-scale) remobilization occurred in response to deformation and metamorphism. Copper and Zn are decoupled at B26 and result from a strong vertical zonation in the system, suggesting an evolving geological environment with potentially a gradually shallowing seawater level. Shallow-water to subaerial conditions have been documented at the nearby Selbaie deposit, where they are considered responsible for the epithermal-style characteristics of the ore.