花岗岩风化区离子相稀土元素多尺度空间变化规律及富集分异机制研究

To enhance understanding of the metallogenic regularity and genetic mechanism of ion- adsorption type REE deposit in south China, a typical ore deposit from Guangxi is selected for study on the enrichment of ionic rare earth elements (iREEs) by chemical weathering of granite from multi-scale perspective.Firstly, from the weathering landscape scale, spatial variation of ore distribution with weathering landscape type are analyzed, and the ore-forming factors controlling on the development of weathering crust and its ore-bearing property is revealed by the bedrock-geomorphology-hydrogeology surveys. Secondly, from the weathering profile scale, variation of ore body location with weathering crust depth was analyzed, and the influence of redox environment on the migration and enrichment of iREEs was discussed using Fe isotope technique. Thirdly, from the weathering fabric scale, variation of REE-enriched patches with weathering microstructures was analyzed, and the process of REEs speciation transformation is discussed using mineralogical information of water and rock reaction. This study is a new attempt to deeply discuss the macro and micro metallogenic rules and metallogenic mechanism of ion-adsorbent type REE deposits from multi-scale perspectives, which will support the mineralization theory of the ion-adsorption type REE deposits of "endogenous-source and exogenous-sink" nature, as well as exploration of the HREE-type ion-adsorption REE ores.

查明风化过程中稀土元素地球化学行为及其离子相分量的形成演化是理解离子吸附型稀土矿床成因的关键科学问题。本申请以广西某离子吸附型稀土矿区为案例，通过重稀土矿段和轻稀土矿段对比，重点探讨花岗岩风化区离子相稀土元素多尺度空间变化规律及其控制因素：（1）从风化景观尺度研究矿体分布随风化景观类型的变化规律，结合母岩-地貌-水文环境地质综合调查，揭示内外生控矿因素对风化壳发育及其含矿性的影响；（2）从风化剖面尺度分析矿石品位随风化壳深度的变化规律，利用铁同位素示踪等技术，查明氧化还原环境对稀土元素迁移富集行为的影响；（3）从风化组构尺度探讨矿质聚集斑块随风化微结构体类型的变化规律，结合矿物学水岩反应信息，阐明离子相稀土元素富集分异的微观过程。这种基于多尺度联合视角下的典型矿床成矿规律和成因解剖研究，有望为增进离子吸附型稀土矿床“内源外成”成因认识、指导广西离子吸附型重稀土找矿突破提供科学依据。

查明稀土元素在风化过程中的地球化学行为及其离子相分量富集分异是理解离子吸附型稀土矿床成因的关键科学问题。本申请以广西佳平轻重稀土共生型稀土矿区为案例，通过宏观-中观-微观三个尺度，采用野外地质调查、表生地球化学和精细矿物学等综合分析技术手段，重点探讨花岗岩风化区离子相稀土元素多尺度空间变化规律及其控制因素。结果表明，研究区赋矿风化剖面中同时出现轻稀土和重稀土两类矿层，形成“上轻下重”垂向二元矿化结构。成矿母岩具有低度轻稀土配分属性（1＜∑LREE/∑HREY＜5）且含有丰富易风化稀土副矿物，是主导轻/重稀土矿化类型空间分异的首要因素。磷灰石、榍石等HREE配分属性含稀土副矿物提供了风化壳离子相HREE的主要物质来源。风化壳稀土成矿类型随地貌类型及地形部位发生变化，重稀土矿化主要发育在起伏山丘区的山麓、山脊以及岛状矮丘区的弧形山顶部位。基于高密度电法地形断面水文探测、古水文土壤学调查以及Fe同位素等证据，指示稀土矿层主要发育在包气带土壤水作用水文环境。土壤水淋滤作用驱动了稀土元素次生富集，而轻重稀土分异则与淋滤程度和淋滤距离有关。在厚层风化壳中，随着土壤水淋滤路程加长，活跃的重稀土比轻稀土更易往风化壳下部迁移，轻重稀土比值下降趋势就越明显，这是导致“上轻下重“二元矿化结构发育的原因。在微观尺度上风化壳中稀土元素的次生富集与纳米微粒关系密切，有机大分子、粘土矿物微粒、锰氧化物微粒以及独立REE矿物微粒等对于REE的富集具有协同作用。风化壳中纳米微粒的粒径、类型和分布还影响了轻重稀土的分异。本研究加深了离子吸附型稀土矿床中轻重稀土共生亚类的成因认识，所提出的成矿模式和找矿标志，可为广西乃至中国南方同类矿床的找矿勘查参考借鉴。