Genesis of the Daxigou Fluorite Deposit, Harqin Banner, Inner Mongolia, China: Constraints from Rare Earth Elements, Fluid Inclusions and H-O Isotopes

Fluorite is a strategic mineral that plays an important role in maintaining national resource security and stable economic development. The Harqin Banner in Inner Mongolia is located at the northern margin of the North China Craton, which is an important production area for fluorite ore. In this study, rare earth element (REE), fluid inclusion and H-O isotope analyses of the Daxigou fluorite deposit of the Harqin Banner were carried out to investigate the nature of its ore-forming fluids and the genesis of the deposit. The rare earth element content of the early-stage fluorite in Daxigou is higher than the late-stage fluorite, and the Y/Ho ratios of the early and late-stage fluorspar are similar, indicating that the mineralizing fluids are homologous. In the Tb/La-Tb/Ca diagram, all the samples fall within the range of hydrothermal deposits, indicating that the deposits are hydrothermal in origin. The early- and late-stage fluorites exhibit negative Eu anomalies and weak negative Ce anomalies, indicative of their formation in a reducing environment. The fluid inclusions trapped in fluorite are dominated by liquid-rich gas‒liquid two-phase inclusions, with the homogeneous temperatures concentrated at 160～190 ℃, the salinities ranging from 0.18% to 2.57%NaCl_eqv, the densities concentrated at 0.88～0.94 g/cm³. The H-O isotope data from regional fluorite deposits indicated that the H-O isotope compositions of the independent fluorites show a significant latitudinal effect, which suggests the ore-forming fluids were dominated by meteoric precipitation. The fluorspar deposit in Daxigou is a medium-low temperature hydrothermal vein type deposit. According to the crosscutting relationships, it is inferred that the Daxigou fluorite deposit formed during the late Yanshan period (<120 Ma) , a large number of faults and fissures formed by ductile tectonic deformation during the two phases of the Late Jurassic and the Early Cretaceous provided a channel for the transport and aggregation of ore-forming fluids, and the fluid-rock interactions were the main reason for fluorite precipitation.