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

Abstract

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.