Apatite paragenesis in the Bayan Obo REE-Nb-Fe ore deposit, inner Mongolia, China

Many studies of the Bayan Obo REE-Nb-Fe ore deposit have suggested that the REE mineralization was episodic. To investigate this, mineralized rocks from the main and east orebodies at Bayan Obo were selected for a paragenetic study of apatite. Apatite is abundant at Bayan Obo, displaying a diversity of textures and occurrences, and it is a mineral that is geochemically sensitive to REE environments, and hence, ideal for the purpose. The apatites were examined for their cathodoluminescent (CL) properties, and quantitative electron probe microanalyses were undertaken to determine their compositions. Four distinctive fluorapatite occurrences are recognized: (1) Discrete grains, with minimal trace element substitution, and displaying yellow/green CL. (2) Anhedral, REE-enriched apatites, displaying pink/cream/grey CL. (3) REE-bearing, high-Sr vein apatites with yellow/cream CL. (4) Fluorite-vein associated, large, euhedral, pure apatites, displaying cream/grey CL. Interpretation of the paragenetic sequence indicates a sedimentary host (apatite 1) for the multiple stages of ore deposition (apatites 2 and 3). The source of the hydrothermal/metasomatic ore-fluids is interpreted as carbonatitic, partly from the high Sr-Mn ratios (in apatites 2 and 3). A late event is recorded by type 4 apatites, but its relationship to the main era of REE-ore formation is not clear. (C) 1997 Elsevier Science B.V.