Conceptualizing Rare Earth Element Mass Transfer in Hydrothermal Mineral Systems

Rare earth element and yttrium (REEY) signatures in hydrothermal gangue carbonates are among the most widely used proxies for studying hydrothermal mineralization. The observed patterns essentially provide information on REEY fractionating processes operating along the trajectory of the hydro-thermal fluid. This study adopts a source-sink approach to synthesize the current understanding of REEY mass transfer in aqueous hydrothermal fluids. REEY are initially inherited from the primary fluid reservoirs, i.e., seawater or magmatic fluids. REEY are then selectively added or removed by mineral breakdown reactions, wall-rock sorption and Rayleigh fractionation of saturated mineral phases along the fluid’s trajectory. Simultaneously, complexation with fluorine and chlorine selectively maintains LREE in the fluid, while comple-xation with carbonates or hydroxides preferentially maintains HREE in the fluid. Finally, the REEY are redistributed between coprecipitating mineral phases and stable complexes at the mineralization site. The observed pattern shapes result from superposition of these processes, summarized in a flow chart. All of these processes should be considered, taking into account constraints provided by other methods.status: publishe