He and Ar Isotopes of Fluid Inclusions in the Taoxikeng Tungsten Deposit, South China

Abstract

China accounts for about 60 % tungsten reserves of the world. More than 90% of the Chinese tungsten resources occur in the Nanling region as tungsten polymetallic deposits. The Nanling region locates at the central west Cathaysia Block in South China where abundant Jurassic granitic intrusions are distributed. Previous studies have significantly advanced our understanding of the tungsten ore formation in the Nanling region. These deposits are usually of the quartz-wolframite vein type. They have ages of about 150-160 Ma, and are spatially and temporally related to the Jurassic granites, believed to be produced by crustal anatexis. The ore-forming fluids were considered to be mixtures of meteoric fluid with magmatic hydrothermal fluid derived from the W-associated granitic magma. However, it has been poorly constrained whether or not mantle components were involved in the genesis of the deposits.The Taoxikeng tungsten deposit, one of the important tungsten deposits in the Nanling region, is located in southern Jiangxi provinde. The deposit consists of numerous ore veins. Most ore veins occur along the contact zone between the granite and the sedimentary strata, and crosscut both lithologies. Generally, the veins increase in thickness with depth, but the number of veins decreases. Ore minerals in these veins are mainly wolframite, cassiterite, molybdenite, scheelite, chalcopyrite, sphalerite, arsenopyrite, pyrite and bismuthinite. The main gangue minerals include quartz, calcite, muscovite, sericite, fluorite, chlorite, lithionite, topaz, and tourmaline. The molybdenite Re-Os dating of the veins indicate that tungsten mineralization took place at 154 Ma (Chen et al., 2006), which well corresponds to the age of tungsten-bearing granite in the Taoxikeng mining district 157~158Ma (Guo et al., 2007). Samples of arsenopyrite and pyrite from the ore veins were analyzed for He and Ar abundances and isotopes.

Characteristics and Mechanism of the Ore-Forming Fluids in the Shimensi Tungsten Polymetallic Deposit in Southeastern China

“…The H and O isotopic compositions of the Shimensi tungsten polymetallic deposit are mainly derived from magmatic fluids that were mixed with atmospheric precipitation during evolution of fluids, which is consistent with most current studies on the genesis of mineralizing fluids for tungsten deposits [7,43,51].…”

“…But quartz contains almost no hydrogen atoms, so the exchange effect on the hydrogen isotopic compositions of the fluid inclusions is minimal [50]. The surrounding rocks of the mine are biotite granodiorite, which also contains hydrogen-bearing minerals, but if the water-rock exchange reaction occurs, the hydrogen isotopic composition of the fluid will not change greatly after the exchange, i.e., this change is negligible [51]. Therefore, the hydrogen isotopic composition represents the composition of the original solution.…”

Characteristics and Mechanism of the Ore-Forming Fluids in the Shimensi Tungsten Polymetallic Deposit in Southeastern China

“…The H and O isotopic compositions of the Shimenshi tungsten polymetallic deposit are mainly derived from magmatic fluids that were mixed with atmospheric precipitation during and evolved, which is consistent with most current studies on the genesis of mineralizing fluids for tungsten deposits [7,42,50].…”

“…But quartz contains almost no hydrogen atoms, so the exchange effect on the hydrogen isotopic compositions of the fluid inclusions is minimal [49]. The surrounding rocks of the mine are biotite granodiorite, which also contains hydrogen-bearing minerals, but if the water-rock exchange reaction occurs, the hydrogen isotopic composition of the fluid will not change greatly after the exchange, i.e., this change is negligible [50]. Therefore, the hydrogen isotopic composition represents the composition of the original solution.…”