An experimental approach to understanding REE patterns in fluorite and application to the Baiyanghe Be-U-Mo deposit

The geochemistry of fluorite associated with critical mineral deposits can provide valuable information on the source(s) of the ore-forming fluids. This is typically done by correlating the REE pattern in fluorite with patterns from accessory minerals in or the bulk rock composition of proximal igneous rocks. This simplified technique is controversial because the REE partitioning between fluorite and fluid has only been experimentally determined by a prior study at 120 °C without consideration of pH, ligands, or fluid Na content. In this study, we synthesized fluorite from H 2 O solutions with 100–500 mg/L of each REE, pH 3–11, 25–150 °C, and < 1 to 4 M NaCl. Our results show that increasing the pH of REE-bearing solutions results in a decrease in the total REE content and a depletion of HREEs. The synthesized fluorite has a nearly identical REE distribution pattern to the fluid in every experiment. Fluorite precipitated from acidic, neutral, and basic solutions yield partition coefficient values ranging 3–69, 6–43, and 1–2100, respectively. Applying these results to fluorite from the Baiyanghe Be–U–Mo deposit, China, indicates fluorite REE patterns must be interpreted with caution. Fluorite from Baiyanghe yields multiple REE patterns, but only a few match the local igneous rocks. Dissimilar patterns could represent incongruent dissolution of accessory minerals in the source rocks, incongruent precipitation of REE minerals, and post-depositional effects.