Genesis of the carbonatite-syenite complex and REE deposit at Maoniuping, Sichuan Province, China: Evidence from Pb isotope geochemistry

The Maoniuping REE deposit is the second largest light rare earth elements deposit (1.45 million tons of REE2O3) in China, located in the Panxi rift, Sichuan Province. It is a vein-type deposit hosted within, and genetically related to the Himalayan alkalic carbonatite-syenite complex. The initial 206Pb/204Pb and 208Pb/204Pb ratios from the Maoniuping carbonatites are similar to syenites, which, however, show more radiogenic 207Pb/204Pb ratios. This could not only be attributed to contamination by crustal materials. The Pb-Pb isotope plots from carbonatites and syenites define a linear array between EM1 and EM2. It suggests their sources were located in a heterogeneous lithospheric mantle and produced by the mixture of EM1 with EM2. The initial 206Pb/204Pb and 208Pb/204Pb ratios from fluorites, galenas, pyrites and feldspar are similar to carbonatites and syenites, and different from granites. Their 207Pb/204Pb ratios also plot to between syenites and carbonatites. This suggests the REE ore-forming fluids were derived from carbonatite and syenite magmas. The Pb isotopic results are consistent with a model for REE deposition involving mixing of the F-rich, syenite-derived fluid with Ca-, CO2-bearing fluid evolved from carbonatite magma. The early fluorites have higher 207Pb/204Pb ratio than the later ones, which shows that the REE ore-forming fluids were not contaminated by country rocks fluids.