A synthesis of mineralization styles with an integrated genetic model of carbonatite-syenite-hosted REE deposits in the Cenozoic Mianning-Dechang REE metallogenic belt, the eastern Tibetan Plateau, southwestern China

Abstract

The Cenozoic Mianning–Dechang (MD) rare earth element (REE) belt in eastern Tibet is an important source of light REE in southwest China. The belt is 270km long and 15km wide. The total REE resources are >3Mt of light rare earth oxides (REO), including 3.17Mt of REO at Maoniuping (average grade=2.95wt.%), 81,556t at Dalucao (average grade=5.21wt.%), 0.1Mt at Muluozhai (average grade=3.97wt.%), and 5764t of REO at Lizhuang (average grade=2.38wt.%). Recent results from detailed geological surveys, and studies of petrographic features, ore-forming ages, ore forming conditions, and wallrock alteration are synthesized in this paper. REE mineralization within this belt is associated with carbonatite–syenite complexes, with syenites occurring as stocks intruded by carbonatitic sills or dikes. The mineralization is present as complex vein systems that contain veinlet, stringer, stockwork, and brecciated pipe type mineralization. Carbonatites in these carbonatite-related REE deposits (CARDs) are extremely rich in light REEs, Sr (>5000ppm), and Ba (>1000ppm), and have low Sr/Ba and high Ba/Th ratios, and radiogenic Sr–Nd isotopic compositions. These fertile magmas, which may lead to the formation of REE deposits, were generated by the partial melting of sub-continental lithospheric mantle (SCLM) that was metasomatized by REE- and CO2-rich fluids derived from subducted marine sediments. We suggest that this refertilization occurred along cratonic margins and, in particular, at a convergent margin where small-volume carbonatitic melts ascended along trans-lithospheric faults and transported REEs into the overlying crust, leading to the formation of the CARDs. The formation of fertile carbonatites requires a thick lithosphere and/or high pressures (>25kbar), a metasomatized and enriched mantle source, and favorable pathways for magma to ascend into the overlying crust where REE-rich fluids exsolve from cooling magma. The optimal combination of these three factors only occurs along the margins of a craton with a continental root, rather than in modern subduction zones where the lithosphere is relatively thin.U–Pb zircon dating indicates that the Maoniuping, Lizhuang, and Muluozhai alkali igneous complexes in the northern part of the belt formed at 27–22Ma, whereas the Dalucao complex in the southern part of the belt formed at 12–11Ma. Biotite and arfvedsonite in Lizhuang and Maoniuping REE deposit have 40Ar/39Ar ages of 30.8±0.4Ma (MSWD=0.98) and 27.6±2.0Ma (MSWD=0.06), respectively. Biotitaion alteration in syenite and fenitization caused by the relatively amount of carbonatite on syenite and host rocks is the main alteration along the whole belt.Initial Sr (0.7059–0.7079), 143Nd/144Nd (0.5123–0.5127), and 207Pb/204Pb (15.601–15.628) and 208Pb/204Pb (38.422–38.604) isotopic compositions of fluorite, barite, celestite, and calcite in the MD belt are similar to those of the associated syenite and carbonatite. Given the relatively high contents of Cl, F, SO42−, and CO2 in the rocks of the complexes, it is likely that the REEs were transported by these ligands within hydrothermal fluids, and the presence of bastnäsite indicates that the REEs were precipitated as fluorocarbonates. Petrographic, fluid inclusion, and field studies of the ores indicate that bastnäsite and other REE minerals formed during the final stages (<300°C) of the evolution of magmatic–hydrothermal systems in the belt.The mineralization formed from magmatic and meteoric fluids containing CO2 derived from the decarbonation of carbonatite, as indicated by C–O isotopic values of hydrothermal calcite and bastnäsite (δ13C=−4.8 to −8.7 and δ18O=5.8 to 12.5‰) and O–H isotopic values of quartz (330°C) and arfvedsonite (260°C), which correspond to fluid isotope compositions of δ18O=0.3–9.8‰ and δD=−70.0 to −152.8‰ in the belt. This study indicates that formation the largest REE deposits are related to voluminous carbonatite–syenite complexes, compositionally similar ore-forming fluids, extensive alteration, multiple stages of REE mineralization, and tectonic setting.