Compositional evolution of the tetrahedrite solid solution in porphyry-epithermal system: A case study of the baimka cu-mo-au trend, chukchi peninsula, russiaстатья
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Аннотация:The Peschanka deposit and Nakhodka ore field occurring in the Baimka Cu-Mo-Au porphyryepithermal
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 highfineness
native gold, and (4) IS epithermal stage As-rich pyrite, galena, sphalerite, and lowfineness
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 Znrich
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
2
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.