Compositional evolution of the tetrahedrite solid solution in porphyry-epithermal system: A case study of the Baimka Cu-Mo-Au trend, Chukchi Peninsula, Russia

Abstract The Peschanka deposit and Nakhodka ore field occurring in the Baimka Cu-Mo-Au porphyry-epithermal trend in the western Chukchi Peninsula, Russia are spatially related to monzonitic rocks of the Early Cretaceous Egdykgych Complex. Tetrahedrite solid solution (Tt s ) was recognized at the deposit and within the ore field in the following assemblages: (1) porphyry stage bornite, chalcopyrite, and molybdenite, (2) transitional (subepithermal) stage sphalerite, galena, chalcopyrite, and As-free pyrite, (3) HS epithermal stage enargite, chalcopyrite and high-fineness native gold, and (4) IS epithermal stage As-rich pyrite, galena, sphalerite, and low-fineness native gold and electrum. The porphyry and subepithermal Tt s crystals are oscillatory zoned because of variable contents of Sb and As. The literature data show that similar zoning has been recognized in the Tt s crystals in the other porphyry Cu-Mo-Au and transitional assemblages and differs from the other type deposits. Therefore, such a zoning is considered to be a guide to these two mineralization types. No zoning was found in Tt s referred to the HS and IS epithermal assemblages within the Baimka trend. The porphyry stage Tt s evolves from Fe-rich tennantite (( sb  = Sb/(Sb + As) below 0.01, fe  = Fe/(Fe + Zn) 0.60–0.80) through oscillatory zoned tennantite enriched in Sb and Zn ( sb 0.19–0.37, fe 0.56–0.66) to tetrahedrite enriched in Zn ( sb 0.51–0.70, fe 0.39–0.66). This trend is caused by the Sb accumulation and increased f S2 . The review of published data shows that such trend is typical of the other porphyry deposit worldwide. Therefore, it is considered to be a guide to distinguish porphyry deposits from the other type deposits containing fahlores and to distinguish porphyry-stage fahlores. The composition of the transitional stage Tt s evolves from Zn-rich tetrahedrite ( sb 0.56–0.82, fe 0.03–0.05) through Zn-rich tennantite ( sb 0.03–0.19, fe 0.11–0.13) followed by Zn-rich oscillatory zoned solid solution ( sb 0.03 to 0.69, fe 0.09 to 0.11) to goldfieldite. This evolution testifies to increased f Te2 to the end of transitional stage. The HS stage Tt s corresponds to Fe-rich tennantite ( sb below 0.05, fe 0.65–1.00) containing high Cu excess (1.43 apfu). The IS stage Tt s evolves from Zn-rich tennantite ( sb  = 0, fe  = 0.44) to Zn-rich tetrahedrite ( sb  = 0.97, fe  = 0.03). The latest Zn-rich tetrahedrite of this assemblage is enriched in Ag (up to 4.1 wt.%) testifying to increasing Ag activity to the end of mineralizing process. The review of literature data shows the similar trend for the transitional and IS Tt s in the other porphyry-epithermal systems. Therefore the Tt s evolution trend is a criterion to separate porphyry, transitional, IS, and HS Tt s .