Two metasomatic events recorded in apatite from the ore-hosting dolomite marble and implications for genesis of the giant Bayan Obo REE deposit, Inner Mongolia, Northern China

Abstract

In the Bayan Obo REE deposit in Inner Mongolia, Northern China, three major orebodies are hosted in dolomite marble of the Bayan Obo Group. There are carbonatite dikes in the ore district. Apatite is a common accessary mineral in the ore-hosting dolomite marble (DM apatite) and in carbonatite dikes (IC apatite). These two types of apatite are both fluorapatite, and have low SiO2, uniform P2O5, and variable CaO contents. Total REY (REEs + Y) contents are correlated with Na2O contents, indicating that REY of both types of apatite enter lattice via the substitution reaction: Na+ + (REY)3+ = 2Ca2+. These features, combined with high REY (6230–18,906 ppm) and Sr (9653–17,200 ppm) contents of DM apatite, indicate that DM apatite likely had a carbonatite origin. Some DM apatite grains are partially replaced by albite and quartz. Fluid inclusions crosscutting both apatite and albite or quartz indicate that they formed later than quartz and albite replacement. The back-scattered electron images show that DM apatite grains contain many micro-pores (fluid inclusions), and monazite inclusions formed from the fluid inclusions. However, no monazite inclusions are observed within quartz and albite, excluding the possibility that the monazite inclusions were precipitated directly from the fluids. The monazite inclusions were therefore formed during fluid-induced dissolution–reprecipitation processes, where DM apatite served as the source of LREEs. This also explains the depletion of some LREEs in DM apatite. The formation of monazite inclusions in apatite requires fluids with relatively low Na and Si concentrations, different from the fluids responsible for quartz and albite replacement. DM apatite was affected by two stages of fluid activities: the first stage of metasomatism by alkaline fluids that were likely derived from carbonatite magmas when the deposit first formed (represented by quartz and albite replacement), followed by a second stage of modification that caused LREEs depletion and the formation of new REE minerals. Thus, the Bayan Obo REE ore deposit was modified by a significant thermal event after the formation, which provided negligible or only small amounts of REEs.