Zircon texture and composition fingerprint HREE enrichment in muscovite granite bedrock of the Dabu ion-adsorption REE deposit, South China

Abstract

More than 90% of the global heavy rare earth elements (HREE) supply is currently from the ion-adsorption REE deposits in South China. However, the mechanism of HREE enrichment in granite bedrock of REE mineralized weathering crusts remains poorly understood. In this study, we present a comprehensive investigation on zircon texture and composition as a proxy to unravel the magmatic-hydrothermal processes which resulted in HREE enrichment of the Dabu muscovite granites. Our results show that zircon in the Dabu muscovite granites can be classified into three types based on morphology, internal structure and chemical compositions. The early-magmatic zircon grains (ZrnI) are prismatic crystals with bright oscillatory zoning, and crystallized early in the granitic magma. The late-magmatic zircon grains (Zrn-II) occur as murky euhedral or subhedral crystals or as overgrowth on ZrnI, and crystallized from a volatile-enriched residual melt in the late magmatic stage. The hydrothermal zircon (Zrn-III) is anhedral without oscillatory zoning, possibly formed via intense hydrothermal alteration of Zrn-II in the hydrothermal stage. Raman spectra show a decrease in degree of crystallinity from Zrn-I to Zrn-III. In-situ compositional and isotopic analyses suggest that zircon geochemistry is a good proxy for the magmatic-hydrothermal processes of the host granite. Zircon UPb dating yields similar ages of 155.3 ± 1.4 Ma (2σ) and 154.1 ± 1.0 Ma (2σ) for Zrn-I and Zrn-II, respectively. The decrease of Zr/Hf from Zrn-I to Zrn-III indicates that the Dabu muscovite granites underwent magmatic to hydrothermal evolution. Furthermore, the decreasing δ18O values of Zrn-I to Zrn-III suggest the increasing involvement of meteoric water in the fluids. Significantly, compared with Zrn-I and Zrn-II, Zrn-III has the highest U, Hf and REE, in particular HREE. The LREE/HREE ratios decrease with the increase of REE contents and the decrease of Zr/Hf ratios from early to late zircon of the granite bedrock of ion-adsorption HREE deposits, which is significantly different from those of carbonatite-alkaline related REE deposits. The significant increase of HREE concentrations in early to late zircon should be ascribed to the introduction of HREE-rich fluids during magma evolution. Therefore, we propose that the HREE-rich fluids from magma exsolution metasomatized the granites to achieve further enrichment of HREE, which is an important prerequisite for the formation of ion-adsorption HREE deposits. Accordingly, we argue that zircon composition can be used as a chemical indicator for the REE behaviors in magmatic-hydrothermal systems that are vital to the formation of REE deposits.