Sources of Ore-Forming Fluids in the Maoniuping REE Deposit, Sichuan Province, China: Evidence from REE, Radiogenic Sr, Nd, and Stable-Isotope Studies

摘要

The Maoniuping rare-earth-element (REE) deposit, hosted in a carbonatite-syenite complex, is the second largest light REE deposit in China. Fluorite is one of the main gangue minerals. Fluorites with different colors and REE patterns show uniform initial ~(87)Sr/~(86)Sr ratios and epsilon_(Nd) values, similar to carbonatite and syenite, and different from granite. Chondrite-normalized REE patterns of early fluorite and quartz resemble those of carbonatite and syenite, but differ from those of granite and rhyolite. These indicate that the REE ore-forming fluids are closely related to carbonatite and syenite magmas. Because emplacement of the carbonatite was later than that of the syenite, early fluid carrying REE and fluoride mainly evolved from the syenite magma. Mixing of this fluid with a Ca-, carbonatite-derived fluid was the major control of early fluorite deposition. With changes in temperature of the ore-forming fluids, REE mineral deposition occurred as a result of the decreased ligand concentration that accompanied massive later fluorite mineralization. The uniform isotope compositions of fluorite, quartz, and carbonatite show that the REE ore-forming fluids were not contaminated by ground or meteoric water or by country-rock fluids.