For the first time in the Rudny Altai, we propose to distinguish Besshi- and Kuroko-type deposits, previously considered as members of a single massive sulfide-polymetallic group. The ores of the two deposit types differ in mineral composition, with pyrite-pyrrhotite-chalcopyrite ± sphalerite mineralization in the Besshi-type deposits versus pyrite-sphalerite-galena-chalcopyrite-gold mineralization in the Kuroko-type deposits.
The Besshi-type deposits are restricted to the Irtysh terrane and associated with deformed and metamorphosed (greenschist to mid-amphibolite facies) mafic-siliciclastic rock sequences. This contrasts with the well-preserved Kuroko-type deposits, hosted in bimodal volcanic sequences in the Devonian-Carboniferous Rudny Altai magmatic arc.
Our U-Pb, 40Ar/39Ar, and Re-Os isotope dating of sulfides and host rocks of the Besshi-type deposits suggests that they were probably formed during the early Paleozoic in a back-arc rift setting. They were then metamorphosed during Devonian times within the accretionary wedge in front of the Rudny Altai arc, synchronously with formation of the Kuroko-type deposits.