Genesis of Zhutian ion-adsorption-type rare earth deposit in South China: Constraints from whole-rock and zircon, apatite, monazite geochemistry, and geochronology

Abstract

Ion-adsorption rare earth deposits are special resources that are widely distributed in South China and have become one of the most important sources of rare earth elements (REEs). To illustrate how the host rock affects the formation of these deposits, this study perpetually sampled the weathered crust profile of a typical granite from top to bottom in the Luoyang-Nanmu mining area of Guangdong Province. We collected 7 representative samples to analyze whole-rock major and trace elements and to carry out UPb dating of zircon, apatite, and monazite. Additionally, the relationship between the characteristics of the parent rock and the weathered crust was further studied. (1) The UPb isotope results of zircon, apatite, and monazite show that the parent rocks were formed at 229.1 ± 2.0 Ma, 226.0 ± 1.1 Ma, and 277.4 ± 0.8 Ma, respectively. This indicates, that the weathered crust parent rock is Indosinian biotite monzogranite, which can provide favorable metallogenic conditions for the formation of weathered crust. The major and trace elements data from the whole-rock analysis show that the Chemical Index of Alteration (CIA) values of the weathered crust range from 0.61 to 0.92 gradually decreasing from shallow to deep. As the weathering degree increases, changes in the content and ratios of LREEs to HREEs, (LREEs/HREEs = 12.68–22.97), Ce anomaly (Ce/Ce* = 0.33–1.89) and Eu anomaly (Eu/Eu* =0.56–0.67) in the profile are observed. Combined with the mineral characteristics and REE composition patterns of the minerals, it is evident that the parent rock materials in the weathered crust are controlled by monazite. Monazite and plagioclase are the main minerals responsible for REE enrichment in the weathered crust. Monazite, together with apatite, influences the rare earth distribution mode of the weathered crust (especially the LREEs). Zircon is resistant to weathering, limiting the enrichment of HREEs, and thus, LREEs are more easily enriched in the weathered crust of the parent rock. Furthermore, plagioclase feldspar provides an adsorption medium for ion-REEs as it weathers to form kaolin. (3) The trace elements data characteristics of zircon and apatite in the profile samples can reflect the degree of magma evolution, among which the content of U, Th, and Y in zircon is low while the Th/U ratio is approximately 1, and the ratios of Zr/Hf = 36.3–62.7, indicating that zircon is primarily of magmatic origin and the magma evolution is of moderate degree. The ratios of (La/Yb)_N, (La/Sm)_N, and (Sm/Yb)_N in apatite reflect magma differentiation characteristics: (La/Yb)_N = 1.81–5.29, (La/Sm)_N = 0.48–0.91, and (Sm/Yb)_N = 3.79–6.41, showing a positive correlation with Sr. Additionally, apatite exhibits a strong negative Eu anomaly, indicating plagioclase separation and crystallization during magma differentiation. The evidence presented suggests that the magma that formed the bedrock underwent moderate differentiation, classifying it as moderately differentiated granite. This study provides new reference materials and insights for understanding of the origins of ion-adsorption REE deposits in this area by examining the properties of weathered crust host rocks.