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 tha t the REE mineralization was episodic. To investigate this, mineralize d rocks from the main and east orebodies at Bayan Obo were selected fo r a paragenetic study of apatite. Apatite is abundant at Bayan Obo, di splaying 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 cathodoluminesce nt (CL) properties, and quantitative electron probe microanalyses were undertaken to determine their compositions. Four distinctive fluorapa tite occurrences are recognized: (1) Discrete grains, with minimal tra ce 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 associ ated, large, euhedral, pure apatites, displaying cream/grey CL. Interp retation of the paragenetic sequence indicates a sedimentary host (apa tite 1) for the multiple stages of ore deposition (apatites 2 and 3). The source of the hydrothermal/metasomatic ore-fluids is interpreted a s 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 t o the main era of REE-ore formation is not clear. (C) 1997 Elsevier Sc ience B.V.