Multistage F-rich hydrothermal fluids led to a large-scale high-grade fluorite deposit during the Miocene (16Ma) in South China

• Fluorite Sm–Nd dating yields an age of 16.1 ± 3.1 Ma, precisely constraining the youngest known fluorite mineralization event in China. • Cathodoluminescence imaging reveals multi-stage dissolution–reprecipitation textures in fluorite. • A non-magmatic geothermal water system is proposed to drive multi-stage fluorite reactivation and enrichment via dissolution–reprecipitation. South China hosts numerous fluorite deposits of significant economic value. However, the mechanisms responsible for the extraordinary enrichment of fluorine remain uncertain. The Kengxi hydrothermal vein-type fluorite deposit in Zhejiang, South China, is well known for its high grade and large scale. On the basis of detailed geological field investigations, Sm–Nd isochron dating, cathodoluminescence (CL) observations, fluid inclusion studies, and in situ laser ablation inductively coupled plasma mass spectroscopy (LA‒ICP‒MS) trace element and H–O isotope analyses were conducted for fluorite from different mineralization stages. The results indicated that the ore-forming fluids constitute a medium–low-temperature (homogenization temperatures of 120–170 °C), low-salinity (0–5 wt% NaCl eqv.) H 2 O–NaCl system. The δD values of fluorite ranged from –75.4‰ to –47.2‰, and the δ 18 O values ranged from –7.34‰ to –3.72‰. The H–O isotopic compositions suggested that the ore-forming fluids were primarily derived from meteoric water. Fluorite Sm‒Nd isochron dating yielded an age of 16.1 ± 3.1 Ma (MSWD = 0.99), which represents the youngest fluorite mineralization event reported in China to date. Orebody characteristics and ore textures revealed four mineralization stages in the deposit. CL features and in situ LA‒ICP‒MS microanalyses revealed the characteristics of dissolution–reprecipitation of fluorite across the different stages. The fluorite samples exhibited negative εNd (16.1 Ma) values (–8.68 to –8.53), indicating a contribution from crustal materials. In this study, a geothermal water system driven by the geothermal gradient was proposed as the key factor leading to mineralization events. The multistage textures suggest an episodic process of fracture reactivation → fluid convection → mineral precipitation → channel sealing, which constitutes the key mineralization mechanism for the Miocene Kengxi fluorite deposit. We conclude that fluorite deposits controlled by geothermal water systems experienced superimposed mineralization processes. This understanding is crucial for elucidating the genesis of high-grade, compositionally simple fluorite deposits and provides new insights into the reactivation and enrichment processes of fluorite deposits in geothermal water systems.