Noble metal and graphite formation in metamorphic rocks of the Khanka terrane, Far East Russia

Highlight
• For the first time gold and platinum was found in graphite rocks.
• Noble metals are largely related to endogenic processes.
• Noble metals have crystallized from carbon-bearing gaseous fluids.

Noble metal–graphite mineralization has been identified in the Riphean–Cambrian metamorphic complexes of the northern Khanka terrane, Russia. The graphite mineralization is hosted in magmatic and sedimentary rocks metamorphosed under greenschist to granulite facies conditions. This paper provides the results of our study of the Turgenevo–Tamga graphite deposits. This study analyzes the geochemistry of the noble metals with the aim of determining the spatial relationships between noble metals and graphite. The graphitized rocks, analyzed by various geochemical methods, show a wide range of noble metal concentrations (ppm): Pt (0.02–62.13), Au (0.02–26), Ag (0.56–4.41), Pd (0.003–5.67), Ru (0.007–0.2), Rh (0.001–0.74), Ir (0.002–0.55), and Os (0.011–0.09). Crystallization from gas-condensates is indicated by the relationships between the noble metal mineralization and the graphite, and in particular the inhomogeneous distribution of graphite in the rocks, the inhomogeneous distribution of metals in the graphite, the microglobular graphite structures, and the carbon isotopic compositions. Thermal analysis and Raman spectroscopy indicate that some of the graphite formed from the metamorphism of sedimentary biogenic carbonaceous matter. The uneven distribution of noble metals in the rocks, and the compositional variability of the mineralization, implies that the origin of the metals was largely related to endogenic processes involving reduced fluids derived from depth. Our conclusion is that the noble metals and graphite mainly originated from magmatic fluids, but that some material was derived from exogenic and metamorphic sources.