Two discrete stages of fenitization in the Lizhuang REE deposit, SW China: Implications for REE mineralization

Abstract

Fenitization is an alkali metasomatism that widely occurs in and around the alkaline-carbonatite complex, and is closely associated with rare earth elements (REE) mineralization. However, the origin and nature of the fenitizing fluids and its role in REE mineralization are poor understood. The Lizhuang REE deposit in the Panxi region, SW China, is hosted in a syenite-carbonatite complex with low- and high-grade fenitization. In this study, we conducted detailed petrographic, compositional and isotopic studies on the fresh/unaltered syenite and low- and high-grade fenites to reveal the origin and nature of the fenitizing fluids and their effects on REE mineralization. Our results show that the whole-rock Na/K ratios gradually increase from the fresh syenite, low-grade fenites to high-grade fenites, as well as mineral replacements, such as K-feldspar is replaced by albite and Ca-rich clinopyroxene is replaced by Na-rich clinopyroxene, indicating that sodic fenitization occurred in the Lizhuang syenite system. In situ U-Pb dating of the unaltered and altered apatite shows that the hydrothermal alteration processes are nearly simultaneous with the emplacement of syenite and carbonatite during 27.4–25.3 Ma, maybe lasting for ~2 Ma. In addition, the altered regions of feldspar, clinopyroxene and apatite have in situ 87Sr/86Sr ratios ranging from 0.70580 to 0.70674, which is consistent in error with those of the unaltered regions and host rock syenite-carbonatite complex (0.70566–0.70666). It is suggested that the fenitizing fluids were exsolved from the syenite-carbonatite complex in a relatively closed system. As the increase of alteration degree, the proportion of hydrothermal minerals (e.g., barite, calcite, fluorite and bastnäsite) in fenite increased gradually, indicating that the fenitizing fluids are enriched in SO42−, CO32− and F−. This is also supported by the elevated F and SO3 contents from the unaltered to altered apatite. The increasing REE concentrations, from the fresh syenite, low-grade fenite to high-grade fenite, indicate that REE minerals such as bastnäsite were precipitated from the fenitizing fluids during fenitization processes. In-situ analyses on the altered clinopyroxene and apatite show variable REE losses compared to the unaltered precursors, which indicates the successive remobilization and transferring of REE by fenitization fluids. Therefore, two discrete stages of fenitization have been identified in the Lizhuang REE deposit, resulted from fluids exsolved from the syenite and carbonatite, respectively. Meanwhile, such fenitization fluids are effective extracting agents of REE in post-magmatic stage, which is crucial to REE remobilization and precipitation to form economic deposit.