Genesis of the giant Shizhuyuan W–Sn–Mo–Bi–Pb–Zn polymetallic deposit, South China: Constraints from zircon geochronology and geochemistry in skarns

Abstract

The giant Shizhuyuan W–Sn–Mo–Bi–Pb–Zn polymetallic skarn-type deposit, which lies in the world-famous Nanling W–Sn metallogenic belt, has a close affinity with the Qianlishan granite complex. Previous studies focused on the Qianlishan granitoids rather than relevant skarns, where the evolutionary processes of skarn-forming fluids, mineralized fluids and sources of metals are debated. This study carried out systematic EPMA and LA-ICPMS zircon U–Pb dating, major- and trace-element and Lu–Hf isotopic analyses for three types of skarns: ore-free, greisen stockwork and sulfide-bearing skarns at Shizhuyuan. The whole skarn zircons were classified into six age clusters (125–187 Ma, ∼235 Ma, 345–509 Ma, 677–772 Ma 811–921 Ma and >1000 Ma) and three genetic types (detrital, magmatic and hydrothermally-altered). Among them, the majority of the skarn zircons are of detrital origin (345 to 2225 Ma, peaking at ∼410 Ma, ∼710 Ma and ∼905 Ma) and showing a wide range of Th/U ratios (0.05–7.09). They share similar characteristics of zircons hosted by Devonian strata in South China, suggesting the inheritance from wall-rock. The magmatic zircons in skarns can be further subdivided into two clusters: Triassic (∼235 Ma) and Jurassic (∼150 Ma), which are derived from the corresponding granite. Ore-free skarn contains only detrital zircons that have experienced the weakest alteration, indicating the minor participation of granitic fluid. The greisen stockwork skarn consists of both detrital zircons and Jurassic magmatic–hydrothermal zircons, implying that granitic and stratigraphic materials may both contribute greatly to the W–Sn polymetallic mineralization. Sulfide-bearing skarn includes abundant detrital zircon, a few of Triassic zircons but no Jurassic zircons, indicating that the latest sulfide mineralization was dominated by sediments rather than granitic rocks. The two mineralized skarn zircons display elevated concentrations of most trace elements (e.g., U, REE, Ti, Y, P, Nb and Ta) coupled with decreased Si and Zr, which reflects strong fluid-rock reaction. The lowest ratios of Ce/Ce* (1.22–89.78, mean = 24.13) in greisen stockwork skarn zircons reveal the most reducing environment for the main W–Sn mineralizing event. Jurassic zircons are homogeneous in 176Hf/177Hf (0.282352–0.282454) and εHf (t) (−11.58 to −7.27), in well accord with local granitic rocks. Triassic zircons have variable εHf (t) (−9.5 to −1.6) values, suggesting more mantle input of precursor pluton before the Jurassic main mineralization event. Based on the zircon geochronology in skarns, we speculate that concealed Triassic magmatism exists in the deep Shizhuyuan area. Consequently, a three-stage ore-forming model was established for the giant Shizhuyuan deposit. This study further illustrates that zircon can be used as an important indicator for ore genesis in complex metallogenic systems.