Mineralogical characteristics and Pb isotopes of Gongdongchong lead-zinc polymetallic deposit, Dabie orogenic belt

The Gongdongchong Pb-Zn deposit, located on the north border of Dabie orogenic belt, Jinzhai County, Anhui Province, is a representative hydrothermal breccia-type lead-zinc polymetallic deposit in the Dabie Mountain region.The results of microscopic identification, SEM/EDS and EPMA of ore minerals indicate that silver exists mainly in the form of independent silver-bearing minerals, such as polybasite and Ag-tetrahedrite, which occur mainly in galena as micro-inclusions in the granular shape, columnar shape, lath shape and irregular form.The data ²⁰⁶Pb/²⁰⁴Pb (17.804~17.947), ²⁰⁷Pb/²⁰⁴Pb (15.567~15.669) and ²⁰⁸Pb/²⁰⁴Pb (38.496~38.886) of sulfides suggest that the ore-forming metals were derived from crust-mantle mixed source, being closely related to the Yanshanian magmatic activity.Metal ions such as Pb, Zn and Ag which were dissolved from magma migrate as chlorine-complex in the early stage of mineralization.As the temperature, salinity and f_O2 dropped and pH rose, Pb, Zn-chloride-complex was decomposed to form a lead-zinc breccia ore due to the decrease in stability in the ore-forming hydrothermal solution, while the Ag-ions formed sulfur-hydrogen-complex.When the temperature fell, the Ag-sulfur-hydrogen-complex were decomposed and precipitated.Meanwhile, Ag⁺ combined with Sb³⁺, As³⁺, and Cu⁺ to form a large amount of independent silver-bearing minerals.