Origin of <scp> CO ₂ </scp> ‐Bearing Fluids in Hydrothermal Vein‐Type Fluorite Mineralization: Insights From the Dadaogoumen Fluorite Deposit, North China Craton

ABSTRACT The occurrence of CO 2 fluid inclusions in fluorite from the Dadaogoumen deposit (North China Craton) provides critical insights into volatile dynamics in cratonic hydrothermal systems. Although carbonic inclusions are rare in most fluorite deposits, their enrichment indicates a major contribution from CO 2 ‐bearing crustal fluids; however, the underlying processes remain poorly constrained. This study integrates fluid inclusion microthermometry with H‐O and Sm‐Nd isotope analyses to clarify the origin, evolution, and mineralization mechanisms of the hydrothermal fluids. Three distinct fluid inclusion types are identified: aqueous liquid‐rich, aqueous vapour‐rich, and H 2 O‐CO 2 ‐NaCl three‐phase inclusions, which define a two‐stage mineralization sequence. The early stage is characterized by high‐temperature (204°C–360°C) and moderate‐salinity (3.0–8.1 wt.% NaCl eqv.) fluids, whereas the late stage records lower temperatures (105°C–210°C) and salinities (0.3–4.7 wt.% NaCl eqv.). CO 2 ‐bearing primary fluid inclusions likely played a key role in triggering early fluorite precipitation. The occurrence of CO 2 ‐bearing inclusions in this vein‐type fluorite system underscores a complex interplay among multiple crustal volatile sources, potential magmatic inputs, hydrothermal circulation, and structural controls. H‐O isotopic compositions (δD = −121.4 to −98.2‰; δ 18 O = −9.62 to −4.19‰) plot entirely within the meteoric‐water field, indicating the dominance of meteoric fluids. Sm‐Nd isochron data, together with geological evidence, constrain the timing of fluorite mineralization to the Early Cretaceous, and εNd(t) values (−12.3 to −10.8) indicate derivation from ancient, evolved continental crust. The CO 2 ‐bearing inclusions are interpreted to have originated from deeply circulating meteoric waters that interacted with carbonate‐bearing and organic‐rich crustal lithologies, releasing CO 2 into the hydrothermal system and facilitating fluorite precipitation.