Formation of the Xingyuan Fluorite Deposit in Fengning, Hebei Province, China: Geological, Fluid Inclusion, and H‐O Isotopic Evidence

Abstract The Xingyuan large fluorite deposit in Fengning, Hebei Province, China, is a significant deposit in the North Hebei–West Liaoning fluorite mineralization belt. The ore bodies are structurally controlled and occur in fault zones near granitic porphyry veins. Previous studies have focused on the geology and ore‐controlling factors, whereas the general features of the hydrothermal systems that contributed to the formation of the deposit remain unclear. This study investigated the nature, origin, and evolution of mineralizing fluids in the Fengning deposit, based on fluid inclusion and H‐O isotope data. The fluid inclusions in fluorite are mostly H 2 O‐rich, gas–liquid, two‐phase inclusions, along with a few three‐phase inclusions containing halite daughter crystals and CO 2 gas. The ore‐forming fluid was variable in homogenization temperature (108–388°C), salinity (0.2–47.4 wt% NaCl equivalent), and density (0.58–1.11 g/cm 3 ), which indicate it was a H 2 O‐NaCl‐CO 2 system of moderate–low temperature, low salinity, and low density. Fluorite H‐O isotopes ( δ D V‐SMOW = –123.5‰ to –111.8‰; δ 18 O V‐SMOW = –10.3‰ to –6.5‰), temperature data, and fluid compositions indicate the mineralizing fluid was initially dominated by magmatic waters, but then experienced a large influx of meteoric waters. The fluid temperature and salinity decreased and the density increased from the early to late stages of mineralization. The main mechanisms of fluorite precipitation were water–rock reactions and fluid cooling. The Xingyuan fluorite deposit is a post‐magmatic hydrothermal deposit.