Multi-layer metallogeny of the maoping carbonate-hosted Pb-Zn deposit, SW China: Constraints from C-O-S-Pb isotopes

The Maoping carbonate-hosted Pb-Zn deposit, located in the Sichuan-Yunnan-Guizhou metallogenic belt (SYGMB), comprises numerous ore bodies hosted in Devonian and Carboniferous Systems. Multi-layer Pb-Zn mineralization (two or more systems) at deposit scale is still rare in the SYGMB and poorly understood. Although & delta;18O values (+9.4 to +12.3%o) of calcite in Devonian System is similar to those (+9.2 to +12.0%o) of calcite in Carboniferous System, & delta;13C values (-1.7 to -0.2%o) of calcite in Devonian System is significantly higher than those (-5.4 to -2.7%o) of calcite in Carboniferous System. Such C-O isotopic characteristics suggest that the source of C was ore-hosting carbonate rocks, whereas the source of O was basement metamorphic rocks. The simulation of C-O isotopes shows that water-rock interaction was the main mechanism for calcite precipitation. The & delta;34S values (+18.3 to +22.7%o) of sulfides in Devonian System are higher than those (+7.1 to +17.5%o) of sulfides in Devonian System. This implies that reduced sulfur in Devonian System originated from Devonian marine sulfate, while those in Carboniferous System originated from Carboniferous marine sulfate. Such S isotopic compositions also show that thermochemical sulfate reduction (TSR) was the dominant mechanism for the reduction of marine sulfate. The isotopic ratios (206Pb/204Pb = 18.680-18.744, 207Pb/204Pb = 15.753-15.764, 208Pb/204Pb = 39.282-39.399) of galena in Devonian System and those (206Pb/204Pb = 18.554-18.581, 207Pb/204Pb = 15.722-15.752, 208Pb/204Pb = 39.018-39.109) of galena in Carboniferous System are similar to those of basement metamorphic rocks and cover carbonate rocks. This indicates that ore-forming metals had mixed sources. Considering the above results, the multi-layer metallogeny is the mixing of metal and sulfatebearing fluids with metal-rich but reduced sulfur-poor fluids at the depositional site where methane or other organic matter reduces sulfate and precipitates sulfides. In the future, A multi-layer exploration strategy for Pb-Zn deposits in the SYGMB should be emphasized.