Fluid inclusions and He[sbnd]Ar isotopes in pyrite from the Yinjiagou deposit in the southern margin of the North China Craton: A mantle connection for poly-metallic mineralization

Abstract

The Yinjiagou poly-metallic deposit is located at the southern margin of the North China Craton (SMNCC). Pyrite, the most economically important mineral, is associated with Cu, Mo, Au, Pb, Zn and Ag mineralization. Two main mineralization stages have been identified and studied: a porphyry stage, including the quartz–molybdenite and quartz–sericite–pyrite (QSP) veins, and a vein stage, including the early and the late (economically most important) pyrite stages.Three types of fluid inclusions are distinguished in quartz and pyrite, including liquid-rich, vapor-rich biphase (LV) and halite-bearing inclusions. The transmitted and infrared microthermometry produced several results. (1) Each of the three types of fluid inclusions is trapped in the quartz of the porphyry stage, with homogenization temperatures of 335–419°C and salinities of 4.7–49.6wt.% NaCl equiv. The coexistence of halite-bearing inclusions and low density, vapor-rich biphase (LV) inclusions indicates that the fluid was boiling. The boiling elevated the Fe and S levels in the hyper-saline fluid, as reflected by the development of pyrite, chalcopyrite and S daughter minerals in the halite-bearing inclusions. (2) The fluid in the early pyrite also displays features characteristic of boiling, with homogenization temperatures of 352–>400°C and salinities of 3.7–42.4wt.% NaCl equiv, whereas the late pyrite only contains liquid-rich biphase (LV) inclusions, with homogenization temperatures of 263–354°C and salinities of 6.0–21.3wt.% NaCl equiv. The relationship between the decreasing salinity and the corresponding decreasing homogenization temperature clearly indicates extensive mixing and dilution. (3) Finally, the infrared calibration indicates that the impact of infrared light intensity on the microthermometric results for the Yinjiagou pyrite-hosted fluid inclusions is limited.The HeAr isotopic analysis of the fluid inclusions in the pyrite indicates two different fluid sources that involve abundant mantle input. The 3He/4He ratios in the porphyry stage are relatively uniform, ranging from 1.39 to 1.78 Ra (Ra=1.39×10−6 for air), corresponding to 16–22% mantle 4He contribution. This relatively constant range represents the actual helium isotopic composition of the fluids emanating from the cooling intrusions at depth. The range of 3He/4He ratios in the vein stage is wide, varying from 0.80 to 5.26Ra, corresponding to 9–65% mantle 4He. The significant variation in the 3He/4He ratios in the vein stage reflects mixing between two fluids.These data indicate that there are two different fluid sources and separate evolutionary processes for the porphyry and vein stages and support a multi-stage mineralization model. The veinlet molybdenite mineralization was induced by the boiling and associated cooling of a magma-sourced fluid that emanated from intrusions at depth. The QSP veins are likely to be pressure release zones and late fluid flow channels. The early pyrite mineralization may be attributed to the influx of a dominantly mantle-derived fluid with high 3He/4He ratio (5.26Ra), which is most likely related to a later, more mafic magmatic event. The precipitation of the most economically important pyrite resulted from the mixing and diluting of this mantle-derived fluid with a surface-derived fluid. Additionally, the high 3He/4He ratios indicate a strongly extensional setting, which is most likely related to the Late Jurassic to Early Cretaceous lithospheric modification and thinning of the SMNCC.