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Russian Geology and Geophysics

2004 year, number 2


S.M. Zhmodik, A.G. Mironov*, L.V. Agafonov, A.S. Zhmodik, A.L. Pavlov†, T.N. Moroz, E.V. Airiyants, Yu.I. Kulikov*, A.A. Borovikov, V.A. Ponomarchuk, and B.B. Damdinov*
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS, 3 prosp. Koptyuga, Novosibirsk, 630090, Russia
* Geological Institute, Siberian Branch of the RAS, 6a ul. Sakh'yanovoi, Ulan Ude, 670047, Russia
Keywords: Carbon, ophiolites, carbonization, ultrabasic rocks, gold, platinum, palladium, PGE minerals, East Sayan
Pages: 210-225


Au-Ag-PGE mineralization has been discovered in carbonized ultrabasic rocks (up to 9.7 wt.% C) from the Ospa-Kitoi ophiolites (East Sayan). Eight types of gold differing in the proportions of Au, Ag, Cu, and Hg have been recognized. PGE minerals are various Pt and Pd compounds and a Pt-Pd-Sn mineral stoichiometrically corresponding to atokite. Ar-Ar dating of muscovites from granite porphyries showed that carbonization proceeded 690 myr ago or even earlier. The isotopic composition of carbon of carbonaceous matter (CM) points to its mantle or crust-mantle source. The CM of carbonized ultrabasic rocks (CUR) includes graphite, graphitoid, shungite, ultradispersed graphite, and amorphous phase (sp3 hybridization). Gas inclusions in carbonized rocks are dominated by methane (60-100 %) and nitrogen, with CM present on the walls of vacuoles. Carbonization of ultrabasic rocks and formation of noble-metal mineralization occurred either synchronously with serpentinization of ultrabasic rocks or later. The latter process produced awaruite widespread in East Sayan ultrabasic rocks. The geodynamic setting in the study area suggests the formation of CUR in ophiolites with the participation of both a mantle source and carbonaceous black-shale deposits as a source of great amounts of carbon. This setting might occur where a subduction zone is associated with a mantle plume or hot spot.