Nature of graphitization and noble metal mineralization in metamorphic rocks of the northern Khanka Terrane, Primorye

Abstract

Elevated contents of noble metals (NM) have been established in the Riphean-Cambrian graphite-bearing complexes of the northern Khanka Terrane, which metamorphosed under conditions of greenschist to granulite facies. At the previously known graphite deposits of the Turgenevo-Tamga group, NM comprise (ppm): Pt (0.04–62.13), Au (0.021–26), 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). Analyses of graphitized rocks carried out with various methods (IMS, INAA, AAS, AES, fire assay) reveal a wide scatter of the results related to the specifics of sample preparation, in particular, due to a significant loss of NM by thermal oxidation decomposition. Analysis of a low-soluble graphite residue obtained by treatment of graphitized rocks allowed us to establish genetic links between NM mineralization and carbonic alteration of various igneous, granulite- and amphibolitefacies metamorphic rocks, which occur over a vast area. The nonuniform distribution of graphite and NM in rocks, their fine dispersivity, and compositional variability of NM indicate that their origin is related largely to endogenic processes with the participation of deep reduced fluids. In greenschist-facies rocks, fluorine, bromine, and iodine are associated both with ore minerals and graphite, providing evidence for transport of NM by halogene- and carbon-bearing fluids. The inhomogeneous distribution of metals in graphite, microglobular structure, and carbon isotopic composition are the guides for its gas-condensate crystallization. At the same time, thermal analysis and Raman spectroscopy show that graphite formed by metamorphism of carbonaceous matter contained in sedimentary rocks also occurs. It is concluded that the predominant mass of NM is of fluid-magmatic origin with the participation of exogenic and metamorphic sources of metals.