Fluid inclusion and stable isotope study of the Shagou Ag–Pb–Zn deposit, Luoning, Henan province, China: Implications for the genesis of an orogenic lode Ag–Pb–Zn system

Abstract

The Shagou deposit, located in the eastern Qinling orogenic belt, central China and hosted by the Taihua Supergroup, is a large-sized Ag–Pb–Zn deposit and has proven reserves of 1.54million tons of ore with grades of 767g/t Ag, 13.24% Pb and 4.31% Zn. The ore-forming fluids have been previously interpreted as products of direct exsolution of a crystallizing magma or sudden decompression of the fluid exsolved from magma, defining the Shagou deposit as an intrusion-related system. In this study, the Shagou deposit has been identified as an orogenic type mineralization system. The hydrothermal ore-forming process can be divided into the early quartz–pyrite, middle quartz–polymetallic sulfides and late quartz–carbonate stages. Four types of fluid inclusions in quartz can be distinguished between PC, C, W and PW types. The early and middle stage quartz contains the PC, C and W type fluid inclusions, while only W and PW type fluid inclusions can be observed in the late stage quartz. In the early stage, the W type fluid inclusions homogenized at 173–369°C (concentrating at 200–240°C), with salinities ranging from 5.4 to 8.3wt.% NaCl equiv. But in the middle stage, the C and W type fluid inclusions homogenized at 112–313°C (concentrating at 180–220°C), with salinities ranging from 0.7 to 13.6wt.% NaCl equiv. In the late stage, the W type fluid inclusions homogenized at 108–253°C (concentrating at 160–200°C), with salinities ranging from 2.1 to 8.1wt.% NaCl equiv. The fluids may have experienced a boiling process or fluid mixing in the middle stage. The estimated trapping pressures of fluid inclusions in the middle stage ranged from 33 to 211MPa, corresponding to a mineralization depth from 3.3 to 7.7km. For the early stage fluids, the calculated δ18OH2O and δDH2O values are 7.5–9.1‰, −96.2 to −110.5‰, respectively. For the middle stage fluids, these two values are 1.5–5.3‰, −75.6 to −110.7‰ but change to 1.0‰ and −87‰ for the late stage, suggesting an evolution from metamorphic to meteoric in origin. In general, the Shagou Ag–Pb–Zn deposit is similar in many respects to typical orogenic type mineralization systems.