Paleogene hydrothermal reworking of a cretaceous fluorite system: multi-stage fluorine hyper-enrichment at the Hushan Deposit, Southeastern China

The Hushan fluorite deposit in Suichang, Zhejiang Province, is a large, classic vein-type hydrothermal deposit within the giant South China fluorite metallogenic belt. This study reappraises its age and genesis by integrating fs -LA-ICP-MS U-Pb geochronology, TIMS Sm-Nd dating, fluid inclusion microthermometry, C-H-O isotopes, and in-situ LA-ICP-MS trace-element profiling. Fluorite U-Pb and Sm-Nd dating yield ages of 39.1 ± 1.1 Ma (MSWD = 0.8) and 32.7 ± 2.6 Ma (MSWD = 15), respectively; together, they constrain a Paleogene hydrothermal interval of ca. 39–33 Ma, approximately 40 Myr younger than the previously accepted Late Cretaceous age of 70–80 Ma. The dated Paleogene overprint is expressed by two paragenetic fluorite stages: Stage 1 green fluorite is enriched in light rare earth elements, whereas Stage 2 colorless fluorite displays pronounced heavy rare earth elements enrichment. Both stages are characterized by negative Eu anomalies. The geochemical evolution from Stage 1 to Stage 2, together with cross-cutting and overgrowth textures, indicates remobilization and recrystallization during late hydrothermal activity. Fluid inclusions in fluorite homogenize mainly at 100–160 °C and have low salinities of approximately 1–4 wt% NaCl equiv., indicating a low-temperature, low-salinity NaCl-H 2 O hydrothermal system. Measured δD (mean –83.9‰), δ 18 O (mean –8.5‰), and δ 13 C (mean –12.0‰) values are inconsistent with a dominant magmatic-hydrothermal fluid and instead indicate a meteoric-water-dominated source. Strongly negative εNd(t) values (–7.3 to –6.9) and old two-stage Nd model ages (average T DM2 = 1426 Ma) further indicate that the ore-forming materials were derived mainly from the continental crust. We therefore argue that the Hushan deposit is not the product of a single Cretaceous event, but instead records at least two stages of evolution: initial Late Cretaceous fluorite mineralization followed by a Paleogene overprint that drove substantial enrichment. During the Paleogene (39–33 Ma), deeply circulated, heated meteoric water dissolved and reprecipitated pre-existing Cretaceous fluorite along reactivated fault arrays, resulting in fluorine hyper-enrichment. This recognition provides a critical basis for reinterpreting the multi-phase, multi-stage metallogenic architecture of the giant South China fluorite belt.