Factors controlling Pt–Pd enrichments in intracontinental extensional environment: Implications from Tongshankou deposit in the Middle–Lower Yangtze River Metallogenic Belt, Eastern China

Abstract

The Tongshankou deposit is located in the southern part of the Edong district in the late Mesozoic Middle–Lower Yangtze River Metallogenic Belt, Eastern China. Alterations related to the porphyry and skarn mineralization are pervasive and intensive. Based on the ore metal contents, four groups of ores/mineralized-rocks are divided: Cu–Mo mineralized rock, Cu–Mo ore, Cu–Mo–W ore and Cu ore. The former two groups are porphyry related, and the latter two groups are skarn related. The average grades of Au, Pt and Pd in the ores are hundreds ppb, ~2.3 ppb, and ~10 ppb, respectively. While they are ~340 ppb, ~29.3 ppb, and ~14.4 ppb in the flotation concentrations, respectively. Considering the similar geological characteristics and tectonic setting (intracontinental extensional environment (IEE)) for the Tongshankou and Fengshan PCDs, they are compared with the PC]Ds that formed in an island arc environment (IAE). Our data show that: (1) Pd and Pt contents of PCDs in IEE are much lower than those of PCDs in IAE; (2) Pd is more abundant than Pt in PCDs. Critical controlling factors for Pt–Pd enrichments in PCDs include: (1) Oxidized, mantle–derived parental magmas, which also explain why PCDs in IAE have higher Pt–Pd contents than PCDs in IEE; (2) Modes of PGE transport and precipitation (as chloride- or bisulfide- complexes) in late-stage hydrothermal fluids; (3) Behavior of sulfur during the magmatic evolution, since PGE and Au could be captured and preserved in magma due to sulfur undersaturation.