风化壳型稀土矿床中稀土元素的活化与迁移

Remobilization and transferring of rare earth elements in the formation of regolith-hosted REE deposits

稀土元素(REE)广泛应用于新能源、国防军工等高科技产业中,是一类战略性关键矿产资源。风化壳型稀土矿床是中国最具竞争优势的稀土资源,其供应了全球90%以上的重稀土。阐明这类稀土矿床的成矿机制,可为寻找和高效开采利用该类稀土资源提供理论支持。文章以稀土元素的活化和迁移这两个关键过程为切入点,总结近年来取得的最新研究成果,并对未来的研究方向提出展望。该类矿床主要发育于富稀土花岗岩类的风化壳中,其中稀土配分模式主要受基岩控制。花岗岩类风化壳的形成以化学风化和生物风化作用为主。长石、云母和角闪石等主要造岩矿物风化形成的黏土矿物和铁锰氧化物是该类矿床中离子态稀土的主要赋存载体。而离子态稀土则来源于基岩中易风化和中等抗风化(含)稀土副矿物的风化和分解。此外,微生物分泌的有机酸等代谢产物可以促进难风化的独居石和磷钇矿等副矿物的风化和分解,加速稀土元素活化-迁移。与此同时,微生物作用还会导致轻稀土和重稀土的显著分异,其中革兰氏阳性细菌对重稀土的选择性显著高于轻稀土。在风化淋积过程中,稀土元素的络合离子可能是风化壳中稀土迁移的主要形式,主要受pH值、次生矿物形成和络合环境影响。值得注意的是,除了F^(-)和CO_(3)^(2-)等无机配体,有机质也可以直接与稀土离子络合或螯合,充当有机配体促进稀土的运移。因此,风化壳型稀土矿床中稀土元素的活化和迁移机制主要受化学风化和生物风化过程控制,是无机和有机共同作用的结果,但其对该类矿床形成的贡献尚待定量评估。

[Objective]Rare earth elements(REEs)are indispensable for high-tech industries,such as clean energy,national defense,and military industries,rendering them strategically critical minerals.In China,regolith-hosted REE deposits constitute one of the most important REE resources,supplying over 90%global heavy rare earth elements(HREE).Understanding the formation of such REE deposits can provide a theoretical basis for their efficient utilization.[Methods]This paper summarizes the recent research results on the two key processes of REE remobilization and transferring and puts forward prospects for future research to deepen the knowledge and understanding of the formation of regolith-hosted REE deposit.[Results]These deposits developed primarily in the weathering crusts of REE-rich granitic rocks,with the REE distribution patterns largely reflecting those of the underlying bedrock.The granitoid weathering crusts are primarily developed by chemical and biological weathering.Clay minerals and Fe-Mn(hydr)oxides,resulting from the weathering of major rock-forming minerals,such as feldspar,mica,and amphibole,serve as the primary hosts for REE ions in weathered crusts.These REE ions originate from the weathering and decomposition of REE-bearing accessory minerals in the bedrock,which exhibit varying degrees of susceptibility to weathering.Furthermore,metabolites such as microbial organic acids can breakdown refractory minerals like monazite and xenotime,facilitating REE remobilization.Simultaneously,microbial action can cause significant REE fractionation,and gram-positive bacteria are significantly more selective for HREE than for LREE.During weathering and leaching processes,REE primarily form REE complex ions within weathering crusts and are then transferred by meteoric water or groundwater.This process is primarily controlled by factors such as pH,secondary mineral formation,and the weathering environment.Notably,in addition to inorganic ligands,such as F^(−) and CO_(3)^(2-),organic matter can directly interact with REE,acting as organic ligands that aid in REE transfer.[Conclusion]Consequently,the REE remobilization and transferring mechanisms in regolith-hosted REE deposits are predominantly controlled by chemical and biological weathering processes,which result from interactions between inorganic and organic agents.However,the quantitative impact of these processes on the formation of these deposits requires further evaluation.