Sulfosalts record evolution of Cu-Bi-Co-Ni-As mineralization at Baycheh-Bagh deposit, NW Iran

Abstract Minerals of the tetrahedrite group and Bi-bearing sulfosalts and sulfides have been identified in different mineral associations in the polymetallic (Cu-Bi-Co-Ni-As) vein deposit at Baycheh-Bagh, Zanjan Province, NW Iran. The tetrahedrite group members span the complete compositional range from tetrahedrite to tennantite with Fe and less common Zn being the dominant divalent cation. Two generations of tetrahedrite group minerals can be distinguished recording changes in chemical as well as redox conditions of the mineralizing fluid. The first generation (Ia, Ib) includes tennantite-(Fe) succeeded by tetrahedrite-(Zn) to tennantite-(Fe) of variable intermediate composition. Bismuthian tennantite-(Fe) containing up to 1.17 apfu Bi represents the second generation (II) formed during the final hydrothermal stage. Bismuthian tennantite is characterized by primary growth zoning and slight Cu excess; the latter indicating an increase in the oxidation state of the fluid. The compositional variation and zoning of tetrahedrite group minerals reflect multiple pulses of hydrothermal solutions with varying As/Sb content. The significant substitution of Bi for (As + Sb) in the X site in generation II tennantite indicates higher activity of Bi in the late ore fluids. The late-stage Bi-rich assemblages also comprise emplectite, wittichenite, bismuthinite, aikinite and native bismuth. Luzonite is also found with these assemblages. These minerals mainly belong to the final stage of mineralization and formed between ∼200 and 300 °C.