Objective The Xitieshan deposit in Qinghai Province is one of the largest lead–zinc (Pb−Zn) deposits in China. Although its host rocks are dominated by mafic volcanic rocks, the ore exhibits a metal assemblage primarily composed of Pb and Zn, with a notable deficiency in copper (Cu) and tin (Sn)—elements typically associated with mafic−hosted mineralization. This compositional mismatch has led to long−standing controversy regarding the deposit’s primary genetic type as Volcanic−hosted Massive Sulfide deposit (VMS) or Sedimentary Exhalative deposit (SEDEX).
Methods This study conducted a comprehensive analysis of ores from the Xitieshan deposit through detailed petrographic observations under the microscope, combined with automated mineralogy using TESCAN Integrated Mineral Analyzer (TIMA) and electron probe microanalysis (EPMA).
Results The main ore types in the stratiform/sub−stratiform orebodies of the Xitieshan deposit are classified into three categories: laminated ores, thin−layered ores, and thick−layered ores, which respectively represent primary syngenetic sedimentary layers, weakly recrystallized layers, and metamorphically reactivated layers. Tin in the laminated ores, representing the exhalative sedimentary stage, is mainly hosted in iron oxides associated with gel−textured goethite. In the thin−layered ores, which reflect weak recrystallization, tin is predominantly hosted in quartz intergrown with pyrite layers, and partially occurs as anhedral cassiterite associated with lattice defects in pyrite. In the thick−layered ores, which have undergone metamorphic reactivation, tin is mainly distributed in late−stage sphalerite and galena micro−veins, with stannite occurring as euhedral grains and cassiterite as anhedral to subhedral grains within galena veins.
Conclusions Based on the regional geological evolution and the compositional variation of the ores, this study proposes a three−stage evolutionary model for the Xitieshan deposit, such as the volcanic exhalation stage, the diagenetic−recrystallization stage, and the metamorphic overprint stage. Due to the complicated formation and subsequent overprinting processes, Sn underwent early−stage exhalative sedimentary enrichment followed by remobilization during a later stage of metamorphism, ultimately resulting in Sn depletion in the current orebodies of the deposit. This study suggests that the intense metamorphic transformation experienced after the formation is the primary reason for the enrichment of Pb–Zn and the concurrent depletion of Sn and other ore−forming elements typically associated with mafic host rocks in the Xitieshan deposit.
DownLoad: