铍在碱性花岗岩型稀有-稀土矿床中的超常富集过程：以巴尔哲矿床为例

Over 65% Be resources are currently being provided by bertrandite and beryl from metaluminous to peraluminous granite-related ore systems, genetic model of which has been studied for decades. Some alkaline granites have recently been expected as another dominant source of Be in a foreseeable future. Extreme enrichment processes and mechanisms of Be in alkaline granite ore systems, however, are still enigmatic. The Baerzhe Zr-Nb-Be-REE deposit, situated in Inner Mongolia (NE China), has an estimated reserve of 1.0 Mt of total REE oxides at an average grade of 0.57 wt%, 2.8 Mt of ZrO2 at an average grade of 2.73 wt%, 300 Kt of Nb2O5 at an average grade of 0.24 wt%, and 48,470 t of BeO at an average grade of 0.05 wt%. Beryllium is extracted from hingganite [(Y,REE,Ca)2(□,Fe2+) Be2[SiO4]2(OH)2] as an important by-product of REE. The Baerzhe deposit is therefore selected in present study aiming at understanding the enrichment processes of Be. Based upon detailed field observation and mapping, mineralogical and microscopic petrography observations will be conducted to identify mineral paragenesis and generations of separated Zr, Nb-Be and REE mineralization. Integrated cathodoluminescence (CL), secondary ion mass spectroscopy (SIMS), and laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) techniques will be applied to ascertain micro-texture, trace-element and isotopic geochemistry, and U-Th-Pb geochronology of quartz and apatite from different generations associated to specific mineralization stages. Such in-situ high-spatial resolution textural, compositional and geochemical analyses on doubly-polished thin sections can greatly prevent the loss of textural context and potential mixing of polygenetic age populations of quartz and apatite that may occur using in methods that require mineral separation and grain mount preparation. The interpreted characteristics and evolution of ore-forming fluids, physicochemical environments, and timing of metal precipitation will help to constrain extreme enrichment processes of Be, and improve our understanding on exploration, development and utilization of Be from alkaline granite-related deposits.

碱性花岗岩型铍矿床可能是未来全球、尤其是我国铍资源的主要来源。然而，铍在碱性花岗岩中的超常富集过程与机制尚不明确，阻碍了对这一类型铍矿床的勘探、开发和利用。巴尔哲Zr-Nb-Be-REE矿床是全球储量最大的碱性花岗岩型铍矿床（BeO为48470t，品位0.05wt%），其Be主要赋存于兴安石，可有效开采和冶炼，是进行铍超常富集和成矿理论研究突破的理想选区。项目在野外地质调研的基础上，拟通过精细的矿物学研究查明Zr、Nb-Be和REE各成矿阶段矿物共生组合和世代，进而借助阴极发光、SIMS和LA-ICP-MS等技术，在探针片开展真正意义的原位分析，对不同世代的石英和磷灰石开展微区结构、元素地球化学、O同位素和U-Th-Pb年代学研究，查明成矿流体性质和演化、成矿物理化学条件和成矿年代，探讨关键金属铍在碱性花岗岩型稀有-稀土矿床中的超常富集过程与机制。

Be在碱性花岗岩型稀有稀土金属矿床中超常富集过程与机制尚未明确。本项目选择全球Be储量最大的巴尔哲碱性花岗岩型Zr-Nb-Be-REE矿床，开展了岩相学、矿物学、地质年代学和地球化学分析，厘定了岩相-蚀变-矿化空间特征，揭示了多阶段矿物世代、共生组合和交代关系，构建了矿床成岩成矿年代学格架，刻画了岩浆-热液演化精细过程，阐明了碱性花岗岩型稀有稀土金属超常富集过程。.取得主要认识如下：.（1）大比例尺岩相-蚀变-矿化填图和显微岩相学特征表明蚀变界熔花岗岩为成矿岩体，主要发育钠化和赤铁矿化蚀变。钠化为Zr成矿标志，赤铁矿化为Nb-Be-REE成矿标志。Be主要以兴安石赋存于赤铁矿化界熔花岗岩，与赤铁矿、钛铁矿、钇易解石、铌铁矿、少量的锌日光榴石和钍石等矿物形成Nb-Be-REE矿物假象。.（2）岩相学、岩石和矿物地球化学、锆石和独居石原位地质年代学揭示了巴尔哲矿床稀有稀土金属元素多期次、多阶段再分配与富集特征：矿床经历了三期五阶段成矿过程，其中Be矿化发生在赤铁矿化期第Ⅲ阶段的钇兴安石-铈兴安石-铌铁矿Nb-Be-REE矿化阶段；Zr矿化发生在约125Ma，Nb-Be-REE成矿稍晚，发生在约123Ma。.（3）岩石和矿物地球化学特征表明巴尔哲矿床Be并不是来自原生锆硅酸盐矿物、钠铁闪石等矿物被交代过程的释放，而是来自不混溶的氟化物熔体和/或晚期岩浆热液流体；富Nb-Be-REE流体出溶交代界熔花岗岩中钠铁闪石是兴安石主要沉淀机制。.项目建立了碱性花岗岩型稀有稀土金属矿床多期次多阶段富集成矿模式，对深化关键金属超常富集过程和成矿机制研究具有重要意义。