Post-magma hydrothermal activity contributed to the generation of Bachi ion-adsorption REE deposits in South China: implications from U-Pb dating and geochemistry of REE-bearing accessory minerals

Abstract

Hydrothermal alteration leading to rare earth elements (REE) mobilization from primary allanite and titanite favors the generation of ion-adsorption REE deposits in South China. However, the timing of regional hydrothermal activity and its relationship with regional tectonic evolution are unknown. This paper provides in-situ geochronology and geochemical data on the REE-rich minerals of titanite, allanite, and zircons from the granites in the giant Bachi ion-adsorption REE deposits. Titanite in the biotite granites is altered with the occurrence of porous ilmenite and synchisite-(Ce) infilled in its fractures, and shows a negative correlation between F and light REE contents. Parts of the titanite were totally altered and were replaced by minor-seized fluorite, apatite, xenotime, rutile, synchysite-(Ce), and unidentified minerals dominantly consisting of Si-Y-Nd-O. Allanite was also altered, and secondary synchisite-(Ce) occurs in the fractures of the altered allanite. The fluid responsible for the alteration was F-rich and REE-deficient, and the hydrothermal alteration caused the generation of more easily weathered REE minerals, including synchisite-(Ce). Zircon and primary allanite U-Pb dating indicate that the granite was intruded in the Late Jurassic (150–153 Ma), and titanite U-Pb dating shows that the hydrothermal alteration occurred in the Early Cretaceous (Ca. 132 Ma). The altered time is consistent with regional large-scale F-rich fluid activity, which also accounts for the generation of widely distributed Late Jurassic to Early Cretaceous fluorite deposits in South China. In this period, F-rich fluids could not only be released from the subducting slab due to the regional tectonic transition from flat to steep subduction, but also resulted from the fluid exsolution in the evolved F-rich magma in the long-term extensional settings. These fluids induced REE mobilization from granites, generating more soluble REE minerals, which favors the generation of ion-adsorption REE deposits in South China. The large-scale fluid activity in South China’s long-term extensional setting may be a key factor for the widespread distribution of ion-adsorption REE deposits.