The Karchiga copper massive sulfide deposit in the high-grade metamorphosed rocks of the Kurchum Block; geologic structure, formation, and metamorphism (Rudny Altai)

The Karchiga copper massive sulfide deposit is located in the Kurchum block of high-grade metamorphosed rocks. This block is part of the Irtysh shear zone, which belongs to the largest transregional fault in Central Asia. The deposit is associated with the gneiss-amphibolite middle unit of the metamorphic complex, which is distinct in the geochemical fields. The mineralization is spatially and paragenetically related to the amphibolite beds, which are ore-bearing together with terrigenous rocks. The deposit contains two spatially isolated lodes, in which all the discovered commercial reserves concentrate. They conformably overlie the host rocks and are tabular or ribbonlike. The mineralization has a close spatial relationship with Mg-rich anthophyllite-containing rocks. The sulfide ores are disseminated or massive and comprise pyrite, chalcopyrite, pyrrhotite, sphalerite, and magnetite. The ore is of Zn-Cu composition, in which Cu dominates considerably over Zn (average contents 2 and 0.4%, respectively; Cu/(Cu+Zn)=0.83). The ores are rich in Co (up to 0.16%, averaging 0.02%), poor in Au and Ag (0.3 and 7.2 ppm, respectively), and almost free of Pb and Ba. All the rocks and ores experienced epidote-amphibolitic metamorphism. Meanwhile, the ores experienced a recrystallization and partial regeneration, but the initial shape of the lodes remained unchanged. The essentially chalcopyritic ores, the volcaniclastic ore-bearing rocks, and the spatial and genetic relationship of the mineralization with undifferentiated mafic and siliciclastic rocks suggest that this deposit belongs to the Besshi type, formed in a back-arc environment, near large rises. The studies show that Besshi-type Cu-Zn massive sulfide deposits differ from most of the polymetallic (Kuroko-type) deposits in Rudny Altai in the composition of volcanics and geodynamic settings, but belong to the same evolutionary series in this VMS province. Both types of deposits might have formed in the Paleozoic, during the main peak of VMS generation in the Earth's history.