阿尔泰阿斯喀尔特大型Be-Nb-Mo矿床花岗岩-伟晶岩系统的分异演化过程及成矿机制：微区原位矿物学证据

Being regarded as the second largest beryllium deposite in the Chinese Altai, as well as a representative granitic beryllium deposite in China, both of granitic and pegmatitic rare-metal mineralization are developed in the Arskartor large Be-Nb-Mo deposit. Temporal and spatial consistency of Arskartor muscovite-albite granite and rare-metal pegmatite have been revealed by previous studies. Moreover, petrological, mineralogical and petro-geochemical features of granite are consistent with typical parental granite for pegmatite. Thus, genetic relationships between Arskartor granite and pegmatite can be recognized. The understanding on the change of crystallization conditions and geochemical behaviors of rare-elements during internal evolution of the granite-pegmatite system remains elusive or controversial, which caused problems in understanding the internal fractionation process from parental granite to pegmatite, as well as mechanism for granitic and pegmatitic rare-metal mineralization. The Arskartor deposit, with unique petrologic and mineralization associations, provides a natural lab in solving these scientific problems. By adopting analytical techniques including SEM, EMPA and LA-ICP-MS in-situ analysis methods, a systenatic mineralogical study on alkali-feldspar, muscovite, beryl and columbite-group mineral from the Arskartor granite and pegmatite will be carried out. By revealing morphological, internal structural and crystal-chemical features of minerals mentioned above, we try to decipher the detailed internal fractionation process from highly-evolved leucogranite to rare-metal pegmatite, and discuss the mechanism of rare-metal mineralization in both granite and pegmatite.

作为阿尔泰地区第二大铍矿床和国内代表性花岗岩型铍矿床，阿斯喀尔特大型Be-Nb-Mo矿床发育花岗岩型与伟晶岩型两类稀有金属矿化。矿区内白云母钠长花岗岩与稀有金属伟晶岩紧密共生且侵位时代一致，花岗岩的岩石学、矿物学及岩石地球化学特征也显示其与伟晶岩具成因联系。当前对花岗岩-伟晶岩系统演化过程中，体系结晶条件的变化以及稀有金属元素的地球化学行为等的认识不够深入，导致对系统的分异演化过程及稀有金属矿化机制的理解存在问题。阿斯喀尔特矿床独特的岩石和矿化组合为这些问题的深入理解提供了一个绝佳的实例。本申请运用扫描电镜、EMPA及LA-ICP-MS原位分析技术，对花岗岩与伟晶岩各岩相中的碱性长石、白云母、绿柱石和铌铁矿族矿物进行系统的矿物学研究，通过矿物的形貌特征、显微结构和矿物化学的系统变化，来揭示从高分异花岗岩到伟晶岩和稀有金属矿化的分异演化过程，同时探讨花岗岩型和伟晶岩型稀有金属矿化机制。

位于阿尔泰造山带东南部的阿斯喀尔特大型矿床发育独特的Be-Nb-Mo矿化组合，以及少见的稀有金属伟晶岩与其母体花岗岩紧密共生的成岩成矿系统。当前对花岗岩-伟晶岩系统分异演化过程的认识不够精细，稀有金属成矿机制也有待明确。本项目通过对该矿床花岗岩和伟晶岩中各个岩相带中的造岩矿物（碱性长石、白云母）、矿石矿物（绿柱石、铌铁矿族）和副矿物（磷灰石、锆石）开展系统的研究，来解决这两个关键科学问题。贯通性矿物的矿物化学特征显示，花岗岩-伟晶岩系统的分异演化序列为白云母钠长花岗岩→Be矿化花岗岩→细晶岩带→含绿柱石石英-微斜长石带→条带状伟晶岩带→绿柱石-白云母-石英带。指示岩浆演化特征元素的含量及相关比值的连续变化，表明成岩-成矿作用主要发生于淡色花岗质熔体的连续结晶分异。本研究首次发现母体花岗岩中的碱性长石、白云母、绿柱石及锆石均表现出范围较宽的主微量元素组成，部分甚至与稀有金属伟晶岩的分布范围重合，表明形成伟晶岩的熔体在离开母体花岗岩之前就经历一定程度的分异演化。由于在花岗岩到伟晶岩的过渡岩相中不发育的矿物（如锆石）中观察到的明显的成分不连续，表明细晶岩带的形成是导致结晶产物发生巨变的原因。细晶岩带形成之后使得残余熔体在相对封闭的条件下分异演化，从而进而形成伟晶岩的特征结构及岩相分带。由于本矿床绿柱石的演化程度低于典型绿柱石-铌铁矿型伟晶岩，同时花岗岩与伟晶岩中碱性长石和白云母的Be含量并无差异，表明绿柱石的结晶和花岗岩-伟晶岩系统的分异演化关系不大，应为相对富Be的熔体在合适的物化条件下的产物。铌铁矿族矿物发育各种不平衡结构，其成分特征表明其演化程度高于同类型伟晶岩，部分晶体中还观察到了常见于富Li伟晶岩的极端分异行为。这些特征表明Nb-Ta矿化主要形成于花岗岩-伟晶岩系统演化的晚阶段，与富流体相和挥发分的高演化残余熔体有关。综上立项时的研究目标和任务都已完成，相关成果在稀有金属勘查中的应用有待进一步深化。