PGE geochemistry of the Fengshan porphyry–skarn Cu–Mo deposit, Hubei Province, Eastern China


PGE geochemistry of the Fengshan porphyry–skarn Cu–Mo deposit, Hubei Province, Eastern China

Wang, M.; Gutzmer, J.; Michalak, P. P.; Guo, X.; Xiao, F.; Wang, W.; Liu, K.

The Fengshan Cu–Mo deposit is located in the western part of the Jiurui Cu–Au–Mo district in the Late Mesozoic Middle–Lower Yangtze River Metallogenic Belt (YRMB), Eastern China. The mineralization is spatially associated with the Fengshan granodiorite porphyry stock (149–138 Ma), where two types of ore bodies (porphyry, skarn) occur. The Fengshan deposit is located on the Yangtze Craton, i.e., in an intracontinental extensional environment, a geological setting not considered by previous studies of PGE abundance and distribution in porphyry systems. For the present investigation the PGE geochemistry of fourteen samples of the granodiorite, ore and flotation concentrates were determined by ICP-MS, after preconcentration by the Lead Fire Assay technique from large (30 g) samples. A maximum of 32 ppb Pd and 81.2 ppb Pt is reported for the molybdenum flotation concentrate. Cu, Au, Pt, Pd contents from flotation concentrate samples are almost 1 to 2 orders of magnitude higher than those reported for ore samples; this is especially true for the Pt content. Covariant diagrams of Cu and Mo with noble metals (Au, Ag, Pt and Pd) reveal weak correlation between Cu vs. Au, Ag, and Mo vs. Pt. Also, Au shows a slightly positive relationship with Pt and Pd.

It is apparent that Pd and Pt contents of flotation concentrate from the Fengshan deposit are considerably lower than those reported for porphyry deposits in an island arc setting. However, Au and Ag appear to be elevated in the Fengshan deposit. The intrusion derived from an enriched mantle source, high oxygen fugacity (fO2), and liberation of mantle sulfates during partial melting, are the first steps for PGE enrichment in porphyry Cu deposits. However, the Fengshan granodiorite was most likely generated by partial melting of enriched mantle that was previously metasomatized by slab melts related to an ancient subduction system. Au, Pt, and Pd in potassic alteration zones and/or endocontact zones are transported as an aqueous chloride complex in high temperature, hypersaline fluid. In contrast, Au, Pt and Pd would be transported by bisulfide complex in low temperature, intermediate salinity fluid in phyllic alteration zones and/or exocontact marble. That is distinctly different from the porphyry Cu deposits in an island arc environment, where intrusion derived directly from slab melting and Pd and Pt are transported only as chloride complexes.

Keywords: Platinum group elements; Porphyry–skarn Cu–Mo deposit; Fengshan; Eastern China

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