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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% SO 4 2−), 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−, SO 4 2−, and REEs, and the absence of REE fluoride (REEF 3) and fluorite (CaF 2), suggest that the ore fluids in hydrothermal stage I were a high-temperature, SO 4 2−-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 SO 4 2−, 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 SO 4 2−, Cl−, REEs, CO 2 , 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. Ore fluids in the Mianning–Dechang REE belt contained appreciable chloride (Cl−), sulfate (SO 4 2−), and fluoride (F−), and formed an alkali-, sulfate-, and halogen-rich fluid system, in which chloride REE complexes were predominant over sulfate and fluoride complexes in hydrothermal stage I, whereas sulfate species were predominant in hydrothermal stage II. Fluoride was a depositional ligand in hydrothermal stage II, resulting in formation of REE-rich fluorite (CaF 2) and removal of REEs from the fluids. Due to mixing with meteoric water and further cooling, all ligands became involved in the efficient deposition of REEs by destabilization of complexes and widespread precipitation of barite, fluorite, and bastnäsite-(Ce) in the REE mineralization stage. [Display omitted] • Bulk fluid analysis can effectively quantify carbonatitic fluid compositions. • Ore fluids at MD were enriched in alkalis, sulfate, halogens, and REEs (>5 wt%). • Hydrothermal processes are important for REE mobilization and mineralization. • Cl− and SO 4 2− are the major ligands responsible for hydrothermal REE mobilization. • High ligand, alkali, and CO 2 contents facilitate potential REE mineralization. [ABSTRACT FROM AUTHOR]

• Early- and late-stage pyrite have high concentrations of Au and As at Bangbu. • Sulfur for mineralization was derived from the Greater Himalayan crystalline complex. • Bangbu deposit occurred during or after amphibolite-facies metamorphism during ~50–45 Ma. The Bangbu orogenic gold deposit in the North Himalaya of the southern Tibet contains more than 40 t Au at an average grade of 7.0 g/t. In this deposits, gold-bearing quartz veins were controlled by nearly E-trending Qusong-Cuogu-Zhemulang shear zone and occurred within the secondary faults which crosscut Late Triassic greenschist- facies rocks. To further understand the sulfur source and ore-forming process, we have conducted a compressive study of in-situ SIMS sulfur isotopes and LA-ICP-MS trace element compositions of two stages of pyrite at Bangbu. Early-stage pyrite (Py1) is coarse-grained (mostly 0.2–2 mm) and euhedral, and has gold concentrations of less than 0.3–54 ppm (mean of 20 ppm) and δ34S values of 1.6–5.1‰ (mean of 3.2‰). Late-stage pyrite (Py2) is generally fine-grained (mostly <50 μm to 1 mm) and subhedral to anhedral, and has gold concentrations of 3.6–115 ppm (mean of 30 ppm) and δ34S values of 0.9–5.2‰ (mean of 2.7‰). Gold occurs mainly as invisible refractory within Py1 and Py2, and to a lesser extent as native gold within quartz, pyrite, arsenopyrite and sphalerite. Sulfur for Bangbu gold mineralization was probably sourced from the Greater Himalayan crystalline complex. Release of ore-forming fluids was likely related to amphibolite-facies metamorphism during ~50–45 Ma. Ore fluids deposited Au-rich pyrite during early and late mineralization stage and precipitation of native gold was probably related to fluid boiling and/or remobilization of invisible gold within pyrite. [ABSTRACT FROM AUTHOR]