He, Ar, and S isotopic compositions and origin of giant porphyry Mo deposits in the Lesser Xing’an Range–Zhangguangcai Range metallogenic belt, northeast China

Abstract The Lesser Xing’an Range–Zhangguangcai Range metallogenic belt, located at the eastern segment of the Central Asian metallogenic domain, hosts two giant Mo porphyry deposits, the Early Jurassic Luming deposit and the Middle Jurassic Daheishan deposit. In this study, the He–Ar and S isotopic compositions of sulfides from different hydrothermal stages of the Daheishan and Luming deposits were determined. Fluid inclusions hosted in pyrites shown that 3 He/ 4 He ratios of the Luming and Daheishan deposit range from 0.02 to 0.32 Ra, and 0.7 to 2.5 Ra, respectively. 40 Ar/ 36 Ar ratios of the Luming and Daheishan deposit range from 313.3 to 535.1, and 309.9 to 360.6, respectively. He and Ar isotopic compositions suggest that the ore-forming fluids of the Luming deposit are mainly crustal-derived fluids, with near atmospheric Ar and crustal He values. Mantle-derived fluids are a minor component. In contrast, the ore-forming fluids of the Daheishan deposit are a mixture between a crust-derived fluid and a mantle-derived fluid. The δ 34 S values of the sulfide samples from the Luming deposit (4.98–7.14‰) and the Daheishan deposit (1.83–2.88‰) indicate that the sulfur in the ore-forming fluids was primarily derived from granitic magma. The isotopic compositions across the stages within one deposit have no variability. The similar He–Ar and S isotopic compositions of the sulfides from the different Mo-bearing hydrothermal stages of these two giant Mo deposits indicate that the overprinting of the multiple stages of Mo-bearing hydrothermal fluids may be a key factor in the formation of giant porphyry Mo deposits. Meanwhile, the westward subduction of the Paleo-Pacific plate might have played an important role for the formation of the giant Mo deposits in the Lesser Xing’an Range–Zhangguangcai Range metallogenic belt.