Mineralogy and U–Pb geochronology of REE fluorocarbonates from the Gansha Obo REE deposit, northwest China

Abstract The Gansha Obo deposit is a recently discovered rare earth element (REE) deposit in northwestern China, with REE2O3 reserves of 0.6 Mt at 1.39–1.65 wt%. It is hosted in the Early Cretaceous Gansha Obo alkaline–carbonatite igneous complex. In this study, we present a detailed investigation of the carbonate mineral assemblage in the economic carbonatite veins in the Gansha Obo REE deposit, including the mineral textures, geochemical compositions, and U–Pb ages. The mineral textures and compositions show that bastnasite is the primary ore mineral, while parisite and synchysite are both secondary. In situ U–Pb dating of bastnasite and synchysite yielded ages of 141.8 ± 4.3 Ma (n = 41; MSWD = 0.53) and 53.3 ± 4.4 Ma (n = 85; MSWD = 1.7), respectively. This significant interval of ages cannot be explained by continuous magmatic–hydrothermal activity. The bastnasite U–Pb age reflects the timing of REE mineralization in the deposit. In contrast, the synchysite U–Pb age records a later tectono-thermal event that might have been related to Eocene collision between the Indian and Eurasian plates. Our results show that REE carbonate U–Pb dating is a powerful tool for understanding the precipitation of REE minerals and, potentially, for identifying regional tectono-thermal events.