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

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 (Tts) 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 Tts crystals are oscillatory zoned because of variable contents of Sb and As. The literature data show that similar zoning has been recognized in the Tts 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 Tts referred to the HS and IS epithermal assemblages within the Baimka trend. The porphyry stage Tts 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 fS2. 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 Tts 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 fTe2 to the end of transitional stage. The HS stage Tts corresponds to Fe-rich tennantite (sb below 0.05, fe 0.65–1.00) containing high Cuexcess (1.43 apfu). The IS stage Tts 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 Tts in the other porphyry-epithermal systems. Therefore the Tts evolution trend is a criterion to separate porphyry, transitional, IS, and HS Tts.