Hydrothermal alteration of xenotime-(Y) and samarskite-(Y) in highly fractionated granites: Implications for the formation of ion-adsorption HREE deposits

Abstract Highly fractionated granites that serve as protoliths for ion-adsorption HREE deposits commonly exhibit extensive hydrothermal alteration. The structural stability of HREE-rich minerals in protoliths and HREE liberation from their breakdown are crucial to the formation of ion-adsorption HREE deposits. However, the impact of hydrothermal alteration on the structure and composition of HREE-rich minerals and its constraint on supergene HREE mineralization remain poorly understood. In the Dabu muscovite granites of South China, magmatic xenotime-(Y) and samarskite-(Y) have undergone pervasive hydrothermal alteration, as evidenced by significant textural and compositional variations. The alteration of xenotime-(Y) is characterized by randomly distributed pores and tiny thorite and apatite inclusions resulting from fluid-mediated dissolution-reprecipitation processes. Samarskite-(Y) with high U and Th concentrations is more susceptible to fluid-induced alteration due to radiation damage, exhibiting patchy zoning and microfractures as a result of hydrothermal alteration. Raman spectra show a significant degradation of crystallinity in xenotime-(Y) and samarskite-(Y) after hydrothermal alteration. During the alteration processes, HREEs in xenotime-(Y) and samarskite-(Y) were leached out by F-, CO2-, and Ca-rich fluids, with only minor fractions immediately precipitating as weathering-susceptible synchysite-(Y) along the rims of some altered grains. Transmission electron microscopy (TEM) investigations reveal that Th released during xenotime-(Y) dissolution-reprecipitation initially precipitated as metastable nanometric huttonite, which subsequently recrystallized into thorite inclusions. Furthermore, high-resolution TEM shows numerous structural defects in the altered domains of xenotime-(Y) and samarskite-(Y), including lattice dislocations, randomly oriented nanocrystals, and amorphous material. Therefore, hydrothermal fluids can not only leach HREEs from robust HREE minerals to form weathering-susceptible synchysite-(Y), but also compromise their structural integrity, reducing weathering durability. Such alteration processes render the host granites favorable protoliths for ion-adsorption HREE deposits. This study indicates that the robust xenotime-(Y) and samarskite-(Y) altered by hydrothermal fluids can serve as potential sources of ion-exchangeable HREEs in the weathering crusts, thereby expanding the source spectrum for ion-adsorption HREE deposits.