Superimposed multi-stage mineralization in the Qingchengzi Pb-Zn ore district, North China: Evidence from geochronology and sulfide geochemistry

The Qingchengzi Pb-Zn polymetallic ore district in northern China records two stages of Pb-Zn mineralization in the Paleoproterozoic and Mesozoic, respectively. The Paleoproterozoic sedimentary-metamorphic stage (stage I) of mineralization was primarily recorded by stratiform ores, and the Mesozoic magmatic-hydrothermal stage (stage II) of mineralization mostly formed vein-type ores. The apatites coexisting with the stage I sulfides yield U-Pb isotopic ages of (1882–1857 Ma), corresponding to the regional metamorphism pervaded in the Paleoproterozoic in the Liaodong region. Zircons from a granite porphyry that is spatially associated with the stage-II mineralization have a U-Pb age of 228.5 ± 5.8 Ma. Stage-I fine-grained sulfides (Py1a and Sp1a) are of SEDEX origin and the fluid is fertile in metals such as Co, Ni, Zn, As, Mn and V. The metals are likely to be sourced from the Liaohe Group, which contains high concentration of those metals. Stage-I recrystallized sulfides (Py1b and Sp1b) show trace elements such as Co, Ni, Cu, As and Au of pyrite and the Cu, Cd, In, Ag, Sb of sphalerite were remobilized out from their crystal lattice under metamorphism. Stage-II sulfides (Py2 and Sp2) are more enriched in Au, As, Cu, Ag, Sb, Pb compared to stage-I sulfides. LA-ICP-MS mapping reveals multigenerational texture of Py2 which features a Cu and Zn enriched core (Py2a) and an overgrowth rim (Py2b) of enrichment in Au, As, Co and Ni. The compositionally distinct core and rim presumably suggest that two episodes of magmatic fluids are involved in the formation of stage-II sulfides. The sedimentary pyrite from Liaohe Group and the stage-I sulfides have δ³⁴S value in the ranges of 10.4–17.04 ‰ and 4.10–9.37 ‰, respectively, suggesting that sulfur was probably derived from seawater sulfate reduction. The sulfur isotopes of stage-II sulfides (δ³⁴S = 4.10–9.02 ‰) reflects a mixing of magmatic sulfur with sulfur leached from meta-volcanic-sedimentary source. This process was associated with the upward migration of magmatic hydrothermal fluids and subsequent fluid-rock interaction, which leached heavy sulfur isotopes along with Cu, Zn, Co, Ni, V metals from wall rocks of mica schist from the Dashiqiao and Gaixian Formations. Integrated mineral assemblage, sulfide texture, trace element and sulfur isotope signatures and geochronological constraints reveal the primary Paleoproterozoic SEDEX mineralization was intensively overprinted by later metamorphism and Mesozoic magmatic hydrothermal events for the Pb-Zn deposits in the Qingchengzi ore district.