Abstract: The Yama fluorite deposit, situated east of the Tataleng granitic basement within the southern Qilian orogenic belt, Qinghai Province, is a recently identified, large-scale, low-temperature hydrothermal deposit of the quartz-fluorite vein type. Its principal ore mineral assemblage is fluorite-quartz. A significant M82 fluorite vein-type ore body was recently been delineated in the southeastern part of the mining concession. However, its metallogenic age and genesis remain poorly constrained. In this paper, we present ICP-MS trace element and rare earth element concentration analyses of fluorite separates from the M82 fluorite vein, along with Sm-Nd isochron dating, to investigate the metallogenic age and formation mechanism of the rich orebody. The isochron dating yields an age of (155±30) Ma, indicating that mineralization occurred in the Late Jurassic. The orebody displays enrichment in heavy rare earth elements (HREE) and Y, coupled with negative Eu anomalies, consistent Y/Ho ratios, and elevated normalized (La/Yb)N values. These geochemical features suggest a low-oxygen-fugacity environment for the ore-forming fluid. Tb/Ca versus Tb/La ratios support a hydrothermal origin. Synthesizing new and existing data, we propose that Late Jurassic meteoric water, characterized by low temperature, low salinity, reducing conditions, and fluorine-rich composition, served as the primary ore-forming fluid in the Yama region. This fluorine-rich fluid predominantly precipitated fluorite-quartz veins via fracture-filling within granitic country rock. In the southeastern Yama area, the fluid mainly formed fluorite veins by fracture-filling in porphyritic coarse-grained monzonite, suggesting that the deposit is genetically analogous to mesothermal to low-temperature hydrothermal fluorite deposits of the Qiquanba type.

Key words: fluorite deposit, trace elements, rare earth element, Sm-Nd dating, Yama fluorite deposit