Constraining the genesis of tungsten mineralization in the Jiaoxi deposit, Tibet: A fluid inclusion and H, O, S and Pb isotope investigation

Abstract

The Jiaoxi deposit, located in the western part of the Lhasa terrane, is the first discovered Miocene (ca. 13 Ma) quartz vein-type tungsten deposit in this belt. Ore bodies mainly occur as wolframite-bearing veins hosted in the shales and granite. Three stages were identified for the formation of the Jiaoxi deposit, including the oxide stage (dominated by wolframite and quartz), the sulfide stage (quartz-sulfides veins) and the fluorite stage (late veins comprising fluorite, calcite, and quartz). Here, we investigate the genesis of this deposit based on fluid inclusion and H-O-S-Pb isotope studies. Fluid inclusion microthermometry and oxygen isotope data reveal that the wolframite precipitated from a moderate temperature (340–380 °C), low salinity (2.1–7.5 wt% NaCl equivalent) hydrothermal fluid at a depth of ∼ 6 km. The sulfides from the oxide and sulfide stage have uniform δ34SCDT values (+2.5 to + 3.7‰), and consistent Pb isotope compositions with the coeval granites (206Pb/204Pb: 17.866–18.789; 207Pb/204Pb: 15.555–15.743; 208Pb/204Pb: 37.157–39.335). The δ18OVSMOW value of the hydrothermal fluid decreased from the oxide (δ18OVSMOW = +7.2 to +14.2‰) to the fluorite stage (δ18OVSMOW = +2.1 to +5.7‰), indicating that the exsolved magmatic hydrothermal fluid mixed with meteoric water. This is also suggested by the positive correlation between the salinity and homogenization temperature of the aqueous fluid inclusions. LA-ICP-MS analysis of individual fluid inclusion reveals that the W concentrations of ore-forming fluid in the oxide stage (1.2–70.2 ppm) are much higher than those in the sulfide and fluorite stage (1.1–2.2 ppm) and show positive correlations with the homogenization temperatures and the Cl concentrations. We propose that fluid dilution and cooling due to fluid mixing played an important role during the wolframite precipitation in the Jiaoxi deposit.