Fluid inclusion constraints on the genesis of the bayan obo Fe-REE-Nb deposit

摘要

In the Bayan Obo Fe-REE-Nb deposit, fluid inclusions are hosted in many of the main gangue phases as well as in certain REE minerals. Microthermometric measurements on thee inclusions indicate the passage of several different fluid types through the deposit. The initial fluid was probably an alkali chloride brine, which developed high X(CO_2) on interaction with the host dolomite. Initial mineralisation took place at pressures >2-2.5 kbar and temperatures >300-400 deg. Later-stage vein-forming fluids underwent phase separation to produce an aqueous brine and a CO_2-rich fluid. Vein formation took place at 1-1.5 kbar and 250-400 deg. Late-stage modification of the deposit by dominantly aqueous fluids took place at low P and T. All these stages of fluid circulation have important consequences for models of REE mineralisation.