矿床地球化学国家重点实验室知识库

40Ar/39Ar stepwise crushing technique has become more important for dating hydrothermal fluids trapped in mineralhosted fluid inclusions. However, understanding the gas release patterns and interpreting the 40Ar/39Ar dating results have remained challenging. Here we use 40Ar/39Ar analyses coupled with quadrupole mass spectrometer (QMS) analyses of gases released by stepwise crushing to reconstruct the hydrothermal activities recorded by fluid inclusions hosted by wolframite, cassiterite, and quartz from the Yaogangxian tungsten deposit in South China. Microscopic observations and laser Raman spectroscopic studies reveal two main groups of fluid inclusions hosted in hydrothermal minerals: small, isolated and CH4- rich primary fluid inclusions (PFIs), and larger, mostly CO2-rich secondary fluid inclusions (SFIs) that distribute along micro-cracks. Formamide and alkane in the vapor phases of large aqueous SFIs are also found. The QMS analyses reveal systematic compositional features for gases released during different crushing stages: (1) gases from the initial crushing stage are H2O-poor, CO2-rich with CO2/CH4 > 1, likely dominated by gases from large vapor-rich SFIs; (2) gases from the final crushing stage are CH4-dominated gas mixtures with CO2/CH4 < 1, likely dominated by gases from small PFIs; while (3) gases from the intermediate stage are rich in H2O, formamide, and ethane, which resemble gases from large liquid-rich SFIs. Combined results from microscopic observations, and Raman and QMS analyses suggest that stepwise crushing extracts gases progressively from large vapor-rich SFIs, large aqueous SFIs to small PFIs. Similarly, gases released during each crushing stage have distinct 40Ar/39Ar compositions and corresponding ages. Gases released during the initial crushing stage show high and correlated 40Ar* and 38ArCl signals with abnormally old 40Ar/39Ar ages, which could be related to the vapor-rich SFIs. Gases released during the final crushing stage yield 40Ar/39Ar ages of 159–154 Ma, which agree well with stepwise heating 40Ar/39Ar age (156 ± 0.9 Ma) of intergrown muscovite and published ore-forming ages. We propose that the 40Ar/39Ar ages of gases released during the final crushing stage are reasonable age estimates for the PFIs. Gases released during the intermediate stage yield 40Ar/39Ar ages of 128–127 Ma, which agree well with stepwise heating 40Ar/39Ar age (129 ± 2.1 Ma) of the cross-cutting K-feldspar vein. This result likely suggests that intermediate stage gases mainly derived from aqueous SFIs that formed during late-stage hydrothermal events. In this study, we demonstrate that gas release patterns revealed by 40Ar/39Ar stepwise crushing are consistent with those revealed by Raman and QMS analyses. Therefore, combining these techniques provides a better picture of the timing and nature of hydrothermal geofluids in general.