The role of mantle melting in granite-associated hydrothermal systems: He–Ar isotopes in fluids responsible for Sn–Ag–Pb–Zn mineralization in northeast China

• Published in: Mineralium deposita 2023-11, Vol.58 (8), p.1421-1443
• Main Authors: Zhou, Zhenhua, Mao, Jingwen, Stuart, Finlay M., Chen, Xinkai, Wilde, Simon A., Ouyang, Hegen, Gao, Xu, Zhao, Jiaqi
• Format: Article
• Language: English
• Subjects: Asthenosphere; Cassiterite; Conduction heating; Crustal thickness; Degassing; Deposits; Earth and Environmental Science; Earth Sciences; Fluids; Geology; Hydrothermal systems; Isotopes; Lava; Lead; Magma; Melts; Mineral deposits; Mineral Resources; Mineralization; Mineralogy; Rare gases; Residence time; Silver; Sulfides; Sulfur; Sulphides; Sulphur; Tin; Volatiles; Volcanic rocks; Zinc
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-023-01186-8

The relationship between Sn–Ag mineralization and mantle magmatism is a topic of high interest in current ore deposit research. Here, we investigate porphyry-, skarn-, and cassiterite-sulfide type Sn-polymetallic deposits associated with granitoids and vein-type Ag–Pb–Zn deposits hosted in sub-volcanic rocks in the southern Great Xing’an Range (SGXR), northeast China, as a case example. We use He, Ar, and S isotopes and isotopic end-member simulation calculations to determine the contribution of mantle-derived fluids/melts to the ore mineralization. Our He–Ar isotope data demonstrate that the ore-forming fluids are mixtures of shallow crust-derived fluid containing radiogenic 4 He but no radiogenic 40 Ar and magmatic fluids with mantle-derived 3 He and 40 Ar. The Pb–Zn–Ag deposits have a higher contribution of magmatic volatiles than the Sn-polymetallic deposits. Sulfide δ 34 S values of − 2.7 to − 0.6‰ in the Pb–Zn–Ag deposits are consistent with a magmatic sulfur source, whereas sulfides with δ 34 S values of − 12.2 to − 0.15‰ in the Sn-polymetallic deposits signal a possibly bimodal source of sulfur, i.e., crustal light sulfur mixed with magmatic sulfur. The noble gas compositions of the ore fluids are controlled by crustal thickness, high 3 He fluxes (24 to 404 at/s/cm 2 ), and low residence time (1 to 18 Myr) of He in the asthenosphere below the SGXR. Non-equilibrium open-system magma degassing is evidenced by the range of elevated values of 4 He/ 40 Ar * ratios (4.8–127). The 3 He/heat ratio of the ore fluids from the Sn and Pb–Zn–Ag deposits overlap (0.01–0.76 × 10 −2 cm 3 STP J −1 (cubic centimeter at standard temperature and pressure per joule) and 0.02–1.08 × 10 −2 cm 3 STP J −1 , respectively), indicating a consequence of conduction of mantle-derived heat across the magma-hydrothermal interface. Furthermore, an increasing abundance of Sn reserves in the SGXR deposits can be equated with an increase in the mantle-derived He component in the ore fluids. These findings suggest that a continuous flux of mantle-derived fluids/melts plays an essential role in Sn–Ag–Pb–Zn mineralization.