Mobilization of critical metals by fluorine-bearing hydrothermal alteration: Evidence from the Huajiying Formation in North China Available to Purchase

Fluorine-bearing hydrothermal alteration is an important process that eventually leads to mineralization of certain critical elements, notably rare earth elements (REEs), Nb, W, and Ta. However, due to the difficulty of experimental simulation of F-fluid/melt, which is highly corrosive, the nature of F-bearing fluid and its capability of carrying elements remain poorly understood. The Early Cretaceous period Huajiying Formation in North China provides exceptional natural laboratories for addressing this issue because it features alteration by F-bearing fluid. Our investigation focuses on drill core samples from the Yanshan Scientific Drilling Project (YSDP-3) in the Sichakou Basin. We conducted U-Pb isotopic dating of magmatic and hydrothermal zircons and allanites selected from tuff, in situ major and trace element analyses of these minerals, and fluorite-hosted fluid inclusions. The results reveal that the Huajiying Formation was deposited at 130.0 ± 1.6 Ma and was altered during the early Cenozoic at 63.0 ± 7.3 Ma and 39.7 ± 1.8 Ma. The hydrothermal fluids, enriched in fluorine, along with Na, Al, K, and Rb, were emplaced under epithermal conditions, and profoundly altered the chemical compositions of the bulk rock and its accessory minerals. Such fluid-rock interactions triggered differential migration, selective precipitation, and enrichment of critical metallic elements. This geochemical differentiation sheds light on the metallogenic processes associated with W-Sn-Nb-Ta mineralization.