Characteristics of fluid inclusions in the sandstone‐hosted Qianjiadian uranium deposit, southwest Songliao Basin, northeastern China: Implications for the nature and evolution of ore‐forming fluids

The Qianjiadian deposit is a typical sandstone‐hosted uranium deposit that is hosted mainly in sandstone and siltstone of the Lower Cretaceous Yaojia Formation, located within the transition between the Kailu Depression and Jiamatu Uplift in the Songliao Basin, northeastern China. We studied the geological characteristics of this deposit, and analysed the mineralized sandstone by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) to identify the host minerals of fluid inclusions associated with uranium mineralization and describe their petrographic characteristics. In addition, this research investigated the origin of ore‐forming fluids and the relationship between petroleum fluids and uranium mineralization, based on the following findings. (1) EPMA and SEM data show that uranium minerals are hosted mainly in hydrothermal quartz (HQ) within sandstone cement, which indicates that uranium minerals co‐precipitated with HQ. (2) The fluid inclusions in the HQ show low homogenization temperatures (102.5–169.5°C) and low salinities (1.7–6.1 wt%). In contrast, aqueous inclusions in quartz overgrowths show lower temperatures (60.5–117°C) and higher salinities (4.2–8.7 wt%). (3) Petroleum fluids trapped in HQ homogenize fluid and those along healed microfractures in quartz overgrowth or plagioclase overgrowths at ~70–80°C, those trapped in present‐day organic inclusions at ~85–95°C, and at the boundary of detrital mineral grains show higher homogenization temperatures (~135–145°C). These results indicate that the hydrothermal quartz cement is associated with uranium mineralization, and the ore‐forming fluid of Qianjiadian sandstone‐hosted uranium deposit are characterized by low‐temperature, low‐salinity hydrothermal fluids, and the changes of temperature and salinity of hydrothermal fluids may not be intimately associated with uranium mineralization.