Geology and genesis of the Xiaguan Ag–Pb–Zn orefield in Qinling orogen, Henan province, China: Fluid inclusion and isotope constraints

Abstract

The Xiaguan Ag–Pb–Zn orefield (Neixiang County, Henan Province), hosting the Yindonggou, Zhouzhuang, Yinhulugou and Laozhuang fault-controlled lode deposits, is situated in the Erlangping Terrane, eastern Qinling Orogen. The quartz-sulfide vein mineralization is dominated by main alteration styles of silicic-, sericite-, carbonate-, chlorite- and sulfide alteration. Major Ag-bearing minerals are freibergite, argentite and native Ag. The deposits were formed by a CO2-rich, mesothermal (ca. 250–320°C), low-density and low salinity (<11wt.% NaCl equiv.), Na+–Cl−-type fluid system. Trapping pressures of the carbonic-type fluid inclusions (FIs) decreased from ca. 280–320MPa in the early mineralization stage to ca. 90–92MPa in the late mineralization stage, indicating that the ore-forming depths had become progressively shallower. This further suggests that the metallogenesis may have occurred in a tectonic transition from compression to extension. Geological- and ore fluid characteristics suggest that the Xiaguan Ag–Pb–Zn orefield belongs to orogenic-type systems.The δ18OH2O values change from the Early (E)-stage (7.8–10.8‰), through Middle (M)-stage (6.0–9.4‰) to Late (L)-stage (−1.5–3.3‰), with δD values changing from E-stage −95 to −46‰, through M-stage −82 to −70‰ to L-stage −95 to −82‰. δ13CCO2 values of the ore fluids in the E- and M-stage quartz vary between 0.1‰ and 0.9‰ (average: 0.3‰); δ13CCO2 values of L-stage FIs are −0.2–0.1‰ in quartz and −6.8‰ to −3.5‰ in calcite. The H–O–C isotopic data indicate that the initial ore fluids were sourced from the underthrusted Qinling Group marine carbonates, and were then interacted with the ore-hosting Erlangping Group metasedimentary rocks. Inflow of circulated meteoric water may have dominated the L-stage fluid evolution.Sulfur (δ34S=1.9–8.1‰) and lead isotopic compositions (206Pb/204Pb=18.202–18.446, 207Pb/204Pb=15.567–15.773 and 208Pb/204Pb=38.491–39.089) of sulfides suggest that the ore-forming materials were mainly sourced from the ore-hosting metasedimentary strata. The stepped heating sericite 40Ar/39Ar detection suggests that the mineralization occurred in the Middle Jurassic to Early Cretaceous (ca. 187−124 Ma). Considering the regional tectonic evolution of the Erlangping Terrane, we propose that the Xiaguan Ag–Pb–Zn orefield was formed in a continent–continent collisional tectonic regime, in accordance with the tectonic model for continental collision, metallogeny and fluid flow (CMF).