Mantle-derived and crustal He and Ar in the ore-forming fluids of the Xihuashan granite-associated tungsten ore deposit, South China

Abstract

The Xihuashan tungsten ore deposit in the central Nanling region, South China is a vein-type hydrothermal deposit associated with the Late Jurassic granitoids that were previously thought to be the products of crustal anatexis alone. In this study, we use helium (He) and argon (Ar) isotopes of fluid inclusions entrapped in pyrite and arsenopyrite to determine the origin of the ore-forming fluids. The 3He/4He ratios of the crushed fluid inclusions vary from 0.15 to 1.16 Ra, with a mean of 0.72 Ra (Ra, the atmospheric 3He/4He ratio of 1.4 × 10−6), indicating the presence of mantle-derived noble gasses in the fluids. The amounts of the mantle-derived noble gasses are estimated to be between ∼2 and 17%. The 40Ar/36Ar ratios of the fluid inclusions vary from 306 to 1023, with an average of 440. These ratios are significantly higher than the average crustal value, indicating the presence of air-saturated water in the volatiles released from the mineral separates. The results from this study indicate that the associated, coeval granitoids, which is believed to provide the ore-forming fluids during magma evolution, are not the products of pure crustal melts. Mantle-derived volatiles and heat was also involved in the genesis of the host granite pluton.