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

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 He-4 but no radiogenic Ar-40 and magmatic fluids with mantle-derived He-3 and Ar-40. The Pb-Zn-Ag deposits have a higher contribution of magmatic volatiles than the Sn-polymetallic deposits. Sulfide & delta;S-34 values of - 2.7 to - 0.6 & PTSTHOUSND; in the Pb-Zn-Ag deposits are consistent with a magmatic sulfur source, whereas sulfides with & delta;S-34 values of - 12.2 to - 0.15 & PTSTHOUSND; 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 He-3 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 He-4/Ar-40* ratios (4.8-127). The He-3/heat ratio of the ore fluids from the Sn and Pb-Zn-Ag deposits overlap (0.01-0.76 x 10(-2) cm(3) STP J(-1) (cubic centimeter at standard temperature and pressure per joule) and 0.02-1.08 x 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.