Progressively released gases from fluid inclusions reveal new insights on W-Sn mineralization of the Yaogangxian tungsten deposit, South China

Abstract

The gentle stepwise crushing technique permits progressive extraction of gases from large, irregular, secondary fluid inclusions (SFIs) along micro-cracks to small, regular, liquid-rich primary fluid inclusions (PFIs). Previous studies have verified this general gas release pattern in different hydrothermal minerals based on the agreements between 40Ar/39Ar ages of the released gases from different crushing stages and the most likely corresponding geological events dated with other methods; and based on the excellent agreements between gas chemistry measured by QMS (quadrupole mass spectrometer) during stepwise crushing and the expected characteristics of SFIs and PFIs in the same samples acquired using Raman spectroscopy. In this study, we applied this QMS-based gentle stepwise crushing technique on ore and gangue minerals from the world-class Yaogangxian tungsten deposit in South China. Gases released from PFIs in cassiterite, wolframite, and quartz from ore-bearing veins show high CH4, N2, C3H8, and low CO2, C4H10 contents, low CO2/CH4 (≪1) and high N2/Ar (327–861 for ore minerals and 60.7–943 for quartzs from ore veins) ratios, along with multiple organic gaseous species. Their compositions resemble those of gases released from PFIs in quartzs from the Yaogangxian granites which represent magmatic-hydrothermal fluids that exsolved from the granitic magma. However, compared with these exsolved magmatic-hydrothermal fluids, ore mineral PFIs have relatively high He contents which indicate contributions from non-magmatic crustal fluids. This is consistent with high Ca2+, low F− concentrations, and low K+/Na+ (0.21–0.32) ratios observed in fluids in the quartz samples, as well as their relatively low water hydrogen isotopes (δD-H2O = −102 to −59‰). These chemical characteristics, coupled with their magmatic sulfur (δ34S-molybdenite and arsenopyrite = −0.2–1.4‰), methane hydrogen (δD-CH4 = −58 to −42‰), and fluid oxygen (δ18O-H2O = 3.9–5.1‰) isotopic compositions, indicate both magmatic and non-magmatic crustal components in the mineralizing fluids. Based on these observations and the low methane carbon isotope values (δ13C-CH4) in the studied quartz samples, we propose that fluid-rock interactions between the exsolved magmatic-hydrothermal fluids and the metasedimentary wall rocks played an important role in promoting W-Sn precipitation in this ore deposit. This study demonstrates the power of combining QMS-based stepwise crushing technique with traditional geochemical techniques in investigating ore-forming processes.