Ore forming processes in the multi–stage polymetallic Baycheh-Bagh deposit, NW Iran: Insights from mineralogy, fluid inclusions and stable isotopes

The Baycheh-Bagh polymetallic hydrothermal deposit, in the northwestern part of Urumieh Dokhtar magmatic arc of Iran, is hosted by volcanic and pyroclastic rocks. Multiple mineralization stages are recognized in polyphase veins: the first stage is mainly base-metal mineralization with tetrahedrite group minerals and native gold, the second stage mainly includes native Bi, Ni-Co-Fe arsenides and sulfoarsenides and the third one contains sulfosalts and minor sulfides. The host rocks are affected by hydrothermal alteration to variable degree. Quartz, white micas, adularia, chlorite, calcite and kaolinite are the main alteration minerals.
Fluid inclusion studies reveal that first stage formed mainly due to boiling of hotter high salinity (178.4 to 320.4 °C; avg. 17.5 eq. wt% NaCl) magmatic-hydrothermal fluids but minerals of stages II and III probably precipitated by mixing process of fluids with salinities of 9.7 to 17.5 eq. wt% NaCl eq. and temperatures of 107.4 to 215.1 °C.
Evidence suggesting that a reducing agent from a local source (hydrocarbons, pre-existing sulfides or Fe2+-bearing minerals in host rocks) could have played an important role in change of redox condition and precipitation of arsenides and sulfoarsenides during second stage.
Two sphalerite generations of first stage show FeS content variations (1.09 to 7.97 mol%). The large range of δ34S values (−8.2 to 11.3 ‰) in sulfides from different stages and variation in isotope and chemical composition of different generations of hydrothermal carbonates in the study area indicate ore forming fluids are magmatic as well as partly non-magmatic in origin.
These results suggest that first stage mineralization of Baycheh-Bagh polymetallic veins occurs in intermediate sulfidation epithermal environment, while the second and third stages exhibit features of five-element style mineralization.