SOME ND AND SR ISOTOPIC SYSTEMATICS FOR THE REE-ENRICHED DEPOSIT AT BAYAN OBO, CHINA

Sm-Nd and Rb-Sr isotopic systematics have been determined for rare-earth element (REE) rich samples of carbonate, iron ore, and silicate types from the world's largest known REE deposit at Bayan Obo, China. The results yielded neither Rb-Sr nor Sm-Nd isochrons within analytical error. Isochronous relationships may have been disturbed in part by younger events. Modes in isotopic composition suggest two Sr reservoirs; the low Sr-87/Sr-86 ratios of 0.703 and 0.704 measured for two carbonates may be of mantle origin. The Sm-Nd data delineate a trend at approximately 1 Ga; this trend might represent a disturbed fractionation event or mixing of Nd reservoirs. The REE ores could not have formed in a single fractionation event during the igneous activity at 260 Ma. Model ages suggest that the ore REE were isolated from mantle sources between approximately 1.7 and approximately 1.3 Ga. This time period hosts major anorogenic igneous activity, including anorthosites, and a number of other unusual REE iron ores.