REE mineralization in the Bayan Obo deposit, China: Evidence from mineral paragenesis

Preliminary mineralogical and geochemical studies have been carried out on dolomite marble drill cores from the Bayan Obo REE deposit in China. Three types of apatites and four types of monazites have been identified based on textural features: Type 1 apatite occurs as grains with minor monazite (Type 1 monazite) on its border; Type 2 apatite veinlet shows clusters of assemblages with abundant basimasite and parisite at the rim; Type 3 apatite has a linear array associated with fluorite and bastasite veinlets. Type 2 monazite occurs as clusters intergrowing with parisite and fluorite. Type 3 and 4 monazites occur as polymineralic (fluorite and bastnasite) and monomineralic veinlets, respectively. These four types of monazites have similar LREE composition but variable Y content (Y2O3 ranging from below determination limits to 0.7 wt%). The three types of apatites also show different REE content and distribution patterns, ranging from high REE abundance (Sigma REE + Y: 27243-251789 ppm) and strong LREE enrichment [(La/Yb)(CN) similar to 101] in Type 1, less LREE enrichment [(La/Yb)(CN) similar to 8] in Type 2 to relatively low REE abundance (Sigma REE + Y: 4323-11175 ppm) but high REE fractionation [(La/Yb)(CN) similar to 58] in Type 3. The primary apatite has high Sr (5461-6892 ppm) and REE content, implying a carbonatite origin. The late-stage apatites (Types 2 and 3) show different Sr and REE abundances. Significant differences in their Sr composition (6189 +/- 573, 6041 +/- 549 and 3492 +/- 802 for Types 1-3 samples, respectively) and Y/Ho ratio (20.9 +/- 0.11, 19.5 +/- 0.17 and 17.4 +/- 0.37, respectively) indicate that the three types of apatites may have crystallized from different metasomatic fluids. Multi-stage metasomatism resulted in remobilization and redeposition of primary REE minerals to form the Bayan Obo REE deposit.