The role of sulfate-, alkali-, and halogen-rich fluids in mobilization and mineralization of rare earth elements: Insights from bulk fluid compositions in the Mianning–Dechang carbonatite-related REE belt, southwestern China

Abstract

Carbonatites host the world's most important rare earth element (REE) resources. The origins of REE mineralization in carbonatite-related deposits, particularly the role of hydrothermal fluids in REE mobilization and mineralization, remain enigmatic. The Cenozoic Mianning–Dechang REE belt in eastern Tibet is one of the largest REE production regions worldwide, and is an ideal area for investigating REE mineralization. Geological investigations and fluid inclusion studies suggest that ore fluids in this belt evolved from hydrothermal stage I (fenitization at high temperatures of ~480 °C) to hydrothermal stage II (calcite, quartz, barite, and fluorite crystallization at temperatures of 300–350 °C and salinities of ~20 wt% NaCl equiv.), and then to the REE mineralization stage (temperatures of ~200 °C and low salinities of ~9 wt% NaCl equiv.). The bulk fluid compositions demonstrate that the ore fluids contained significant amounts of alkalis (up to 5 wt% Na + K), halogens (up to 12 wt% Cl; up to 7 wt% F), sulfate (>2 wt% SO42−), Ba (>1123 ppm), Sr (>1120 ppm), and REEs (>5 wt%). Chondrite-normalized REE patterns of these fluids are light REE-enriched and exhibit moderate depletion in Eu ([Eu/Eu⁎]CN = 0.85 ± 0.08), similar to the carbonatites and nordmarkites. These fluid characteristics and plots of Rb/Na vs. K/Na and Mn vs. Na suggest that the ore fluids in the Mianning–Dechang REE belt were derived from a late-stage alkaline–carbonatitic magma. High concentrations of Cl−, F−, SO42−, and REEs, and the absence of REE fluoride (REEF3) and fluorite (CaF2), suggest that the ore fluids in hydrothermal stage I were a high-temperature, SO42−-rich (>2 wt%), and acidic fluid system (pH < 3.5). In this system, chloride REE complexes were predominant over fluoride and sulfate REE complexes, which resulted in efficient transport of REEs. Sulfate species were predominant in hydrothermal stage II at temperatures of 260–350 °C and a pH between 3.5 and 5.2. The higher pH and fluid cooling from hydrothermal stage I to hydrothermal stage II caused an increase in F−, which in turn lowered fluid REE concentrations, owing to the formation of REE-rich fluorite. This suggests that F− was a depositional ligand in hydrothermal stage II. Continued fluid cooling (~200 °C) and increasing pH (~6), combined with the precipitation of barite and fluorite in the REE mineralization stage, destabilized the REE complexes because of the decreasing concentrations of SO42−, Cl−, and F−, which thus led to widespread REE deposition. A review of different-sized deposits in the Mianning–Dechang REE belt indicates that appreciable amounts of SO42−, Cl−, REEs, CO2, and particularly F− and alkalis in fluids, along with a high fluid exsolution temperature, represent the ideal conditions for potential REE mineralization in a carbonatite-related setting.