Fractionation of the REE during hydrothermal processes: Constraints from the Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China.

The composition of REE minerals from the Bayan Obo Fe-REE-Nb deposit, China, varies widely with paragenesis. The properties of fluid inclusions also vary with paragenesis, indicating changing fluid conditions during the deposits formation. In early disseminated monazite and in the main stage banded ores, the chondrite normalised La/Nd ratio varies from similar to 3 to 7, whilst vein-hosted bastnasite and monazite show ratios in the range similar to 4.8 to 5.8. The La-rich portion of this trend (La/Nd > 4) is inferred to correlate with high X(CO2) in the hydrothermal fluid. Minerals with lower La/Nd ratios are inferred to have formed from dominantly aqueous solutions. This correlation is interpreted as arising from the greater retention of neutral species in low dielectric constant, CO2-rich fluids, relative to the dominantly aqueous fluids. At high T, high ligand number complexes of La are predicted to be more strongly associated than for the other REE, leading to the observed fractionation. The La/Nd ratio of later stage REE-fluorocarbonate minerals ranges from similar to 0.6 to 2. This mineralisation took place at lower T (minimum estimates of 250-150 degrees C from fluid inclusions) from dominantly aqueous fluids. Under these conditions the association constants of REE complexes typically increase with atomic number, leading to greater M- to HREE solubilities in hydrothermal fluids. Late stage alteration of all the earlier paragenetic stages by Ba-bearing fluids lead to the release of La into solution. This process was probably controlled by the relative solubilities of end-member REE minerals as well as speciation in the hydrothermal fluid. The fractionation processes inferred from Bayan Obo are relevant to many other REE-rich hydrothermal systems. In particular La-Nd fractionations are common in REE mineralised carbonatites and could be fully accounted for by the models presented here. Copyright (C) 2000 Elsevier Science Ltd.