Mantle-Derived Noble Gas Isotopes in the Ore-Forming Fluid of Xingluokeng W-Mo Deposit, Fujian Province

Abstract

China has the largest W reserves in the world, which are mainly concentrated in south China. Although previous studies have been carried out on whether mantle material is incorporated in granites associated with W deposits, the conclusions have been inconsistent. However, rare gas isotopes can be used to study the contribution of mantle-to-W mineralization. In this paper, we investigated the He and Ar isotope compositions of fluid inclusions in pyrite and wolframite from the Xingluokeng ultra-large W-Mo deposit to evaluate the origin of ore-forming fluids and discuss the contribution of the mantle-to-tungsten mineralization. The He-Ar isotopic compositions showed that the 3He/4He ratios of the ore-forming fluid of the Xingluokeng deposit ranged from 0.14 to 1.01 Ra (Ra is the 3He/4He ratio of air, 1 Ra = 1.39 × 10−6), with an average of 0.58 Ra, which is between the 3He/4He ratios of mantle fluids and crustal fluids, suggesting that the mantle-derived He was added to the mineralizing fluid, with a mean of 8.7%. The 40Ar/36Ar ratios of these samples ranged from 361 to 817, with an average of 578, between the atmospheric 40Ar/36Ar and the crustal and/or mantle 40Ar/36Ar. The results of the He-Ar isotopes from Xingluokeng W-Mo deposit showed that the ore-forming fluid of the deposit was not the product of the evolution of pure crustal melt. The upwelling mantle plays an important role in the formation of tungsten deposits.