Controls on the Dynamics of Rare Earth Elements During Subtropical Hillslope Processes and Formation of Regolith-Hosted Deposits

Abstract

Subtropical weathering of granitic catchments in South China has led to the formation of numerous giant regolith-hosted rare earth element (REE) deposits that currently account for more than 15% of global REE production and more than 95% of global heavy REE (HREE) production. Understanding the controls on mobilization and redistribution of the REEs during subtropical weathering in these granitic catchments is crucial for efficient exploration for this type of deposit in the world. As exemplified by the Bankeng light REE (LREE) deposit in South China, the key factors controlling the mobilization and redistribution of the REEs, especially the easily exchangeable REEs, are soil pH and primary REE mineralogy. The nature of the primary REE minerals, apatite, monazite-(Ce), and subordinate bastnäsite-(Ce), parisite-(Ce), and xenotime-(Y) places an important control on the behavior of the REEs during incipient weathering. Dissolution of these minerals is slow during incipient weathering, and, therefore, enrichment in REEs in this stage results largely from the removal of major elements during the decomposition of albite, K-feldspar, and biotite. Dissolution of the primary REE minerals higher in the profile liberates the REEs, which are then transported to locations where the soil pH abruptly increases due to water-regolith interaction, such as the pedolith-saprolite interface, and adsorption on kaolinite-group minerals efficiently fixes the REEs in regolith. Geomorphologically, the Bankeng deposit, like most of the other regolith-hosted REE deposits in South China, is located on concave-convex hillslopes, where erosion is prevalent at the ridgetop and decreases in intensity downslope. Results of this study show that strong erosion, coupled with intense chemical weathering at the ridgetop, is responsible for the enrichment in REEs by releasing the REEs, especially the LREEs, from their primary sources and supplying kaolinite and halloysite needed for the REE adsorption by decomposing albite, K-feldspar, and biotite. Decomposition of these major rock-forming minerals also leads to an enrichment of the REEs through the removal of components. The HREEs are lost preferentially to the groundwater and transported downslope, resulting in the enrichment of these elements in the lower part of the weathering crust at the footslope. Significant lateral Ce transport is also probable. A series of oxic fronts were developed at the footslope, with the most persistent one along the saprolite-saprock interface, due to seasonal fluctuations of the groundwater table. Cerium was immobilized there, predominantly through adsorption on Fe-Mn oxyhydroxides, causing enormous accumulation. Therefore, hillslope processes and groundwater flow could redistribute the REEs across the entire catchment, preferentially enriching the LREEs at the ridgetop and the HREEs at the footslope. Also, intense erosion facilitates chemical weathering and the accumulation of REEs, but the development of a thick weathering crust is favored by weak erosion. Repeated periods of high and low erosion rates in South China have enabled the gradual development of thick weathering crusts at the ridgetops that are sufficiently enriched in REEs to now constitute a major resource of these economically important elements.

The genesis of regolith-hosted rare earth element and scandium deposits: Current understanding and outlook to future prospecting

“…一般而 言, 地壳抬升和地表剥蚀之间呈正反馈的关系, 即地壳 抬升的速率愈高、地表剥蚀的速率也愈高 [44,45] . Figure 3 Controls of supergene processes and groundwater on the formation of regolith-hosted REE deposits [34] 而言, 在酸性环境(pH<4), 稀土元素在表生环境主要呈 自由离子, 当遇上高氟和硫酸根浓度的地下水, 部分稀 土元素会以氟和硫酸根络合物的形式迁移 [46] . [7] , 而 pH的升高也有利于稀土元素在黏土矿物上的吸附 [50] , 导致稀土元素在风化壳下部的富集.…”

“…[7] , 而 pH的升高也有利于稀土元素在黏土矿物上的吸附 [50] , 导致稀土元素在风化壳下部的富集. 由于重稀土-碳酸 根络合物的稳定性比轻稀土高 [47] , 重稀土元素会具有 [33,34,55] . 最近Li等人 还原障, 导致在全风化层上部或顶部 [7,9,10,13] 以及在风 化壳下部出现强烈的Ce正异常 [33,34] .…”

“…由于重稀土-碳酸 根络合物的稳定性比轻稀土高 [47] , 重稀土元素会具有 [33,34,55] . 最近Li等人 还原障, 导致在全风化层上部或顶部 [7,9,10,13] 以及在风 化壳下部出现强烈的Ce正异常 [33,34] . 野外调研显示出 现Ce正异常的风化壳层位与地下水潜水面的波动有很 好的相关性 [34] , 而当中地下水潜水面作为一个氧化还 7,10,13,14,31,32,56] , 由此推断稀土元素很可能以微弱 电荷吸引力或其他微弱化学键连结在吸附性强的黏土 矿物上.…”

“…然而, 金属元素在黏土矿物上的 吸附形式会随pH改变, 实验模拟显示在中性到碱性的 pH环境, 稀土元素会更主要以内层络合物的形式吸附 在高岭石和埃洛石上, 并导致强烈的轻-重稀土分 馏 [36,50] . 虽然目前还没有野外证据显示稀土元素会以 内层络合物的形式吸附, 但风化壳下部的风化物pH一 般较高, 呈弱酸性到中性 [7,10,34] , . Li和Zhou [33] 通过进一步的矿物学 研究发现, 风化壳下部的埃洛石主要呈纳米级短管状, 高岭石的结晶度很低; 但是到风化壳上部, 黏土矿物以 微米级的自形高岭石为主 [33] .…”

“…Li和Zhou [33] 通过进一步的矿物学 研究发现, 风化壳下部的埃洛石主要呈纳米级短管状, 高岭石的结晶度很低; 但是到风化壳上部, 黏土矿物以 微米级的自形高岭石为主 [33] . 黏土矿物的种类和形态 的变化指示一个关键的转变过程, 即结晶度低的纳米 级埃洛石和高岭石在风化过程中合并, 并转变成微米 级的自形高岭石 [33,34] . [60] .…”