The multistage genesis of the giant Dongshengmiao Zn-Pb-Cu deposit in western Inner Mongolia, China: Syngenetic stratabound mineralization and metamorphic remobilization

The genesis of the giant Dongshengmiao in the northern margin of the North China Block has been debated since its discovery in the 1950s, because it shows geological and geochemical characteristics with both syngenetic and epigenetic signatures. It has geological settings and sulfur and lead isotopic compositions that are similar with typical SEDEX (sedimentary exhalative) deposit, while the Zn-Pb-Cu mineralization was controlled by shear deformation and metamorphism, showing similarities with orogenic-type deposits. In this contribution, both the syngenetic and epigenetic features of the Dongshengmiao are envisaged, and accounted for in the context of a genetic model with two metallogenic periods. Massive pyrite at the Dongshengmiao was mostly recrystallized during metamorphism, but fine-grained texture was locally preserved, indicating its syngenetic origin. On the contrary, all the Zn-Pb-Cu ores observed in this study show characteristics of epigenetic hydrothermal mineralization that controlled by metamorphism and accompanying shear deformation. The sulfur and lead isotopic compositions of sphalerite and galena indicate that they were in situ remobilized from a syngenetic stratabound source, and the oxygen and hydrogen isotopic ratios of ore-fluid indicate that the large-scale remobilization was assisted by metamorphic fluid. The thermodynamic modeling indicates that the ore-fluid during remobilization has a great potential of transporting Cu. This may account for the abnormally enriched Cu in the remobilized SEDEX deposit. The metamorphic fluid might strip Cu from the fluid source during devolatilization, and overprint it on the Zn-Pb orebodies during remobilization. A secondary flow-through modeling reveals that Zn- and Cu-sulfides would be preferentially redistributed in Fe-rich carbonates during remobilization, as a result of fluid-rock interaction. Conclusively, a multistage genetic model is proposed. During the development of the Proterozoic rift, stratabound Zn-Pb mineralization took place in a SEDEX ore-forming system. The syngenetic sulfides subsequently underwent a large-scale fluid-assisted remobilization during the early Cretaceous metamorphism and thrusting, forming the shear zone-controlled epigenetic orebodies. During the remobilization process, Cu was scavenged from the source of metamorphic fluid, and deposited accompanying remobilized Zn-Pb sulfides. Shear structures and Fe-rich carbonates are ideal sites for redistribution and re-deposition of remobilized sulfide.