The Shizhuyuan ore deposit in southern Hunan Province is a world-class W–Sn–Bi–Mo occurrence hosted by Devonian limestone in the thermal aureole of the Qianlishan granite. Mineralization coincided with intrusion of the granite pluton during Mesozoic crustal extension in South China. We present new He and Ar isotope data for volatiles released from pyrite in the Shizhuyuan deposit. Concentrations of 40Ar range from 0.21 to 2.38 × 10−6 cm3 STP 40Ar/g, and 4He concentrations range from 0.8 to 65.1 × 10−6 cm3 STP 4He/g. 3He/4He ratios vary from 0.06 to 1.66 Ra (where Ra is the 3He/4He ratio of air = 1.39 × 10−6) and 40Ar/36Ar ratios range from 293 to 1072. The isotopic compositions of He and Ar indicate that the ore-forming fluids were mantle-derived, modified by air-saturated crustal fluids. Shallow-level boiling increased 3He concentrations, whereas crustal contamination decreased 3He/4He ratios in the magmatic fluids. The occurrence of mantle-derived components in the magmatic fluid indicates that the associated Qianlishan granite is not a typical S-type pluton that formed entirely by crustal melting. We propose that the mineralization was related to mantle upwelling and Mesozoic lithosphere extension of the South China Block.