花岗伟晶岩型稀有金属矿床石英标型特征及成矿指示意义——以可可托海3号脉为例

Quartz occurs in all internal zones of granitic pegmatites and records the important information of rare-metal mineralization process. The previous studies of quartz from granitic pegmatites are mostly on indicators of degree of magma fractionation, but with less attention paid on its genetic and metallogenic significance, leading to slow development of typomorphic characteristics of quartz. Thus, this proposal takes quartz from different zones of the Koktokay No.3 pegmatite as the research object and is on the basis of the feature of the Koktokay No.3 pegmatite that it consists of nine internal zones and has different rare-metal mineralization types during different evolution stages. We plan to perform in situ analysis of trace element compositions and water components of quartz by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and Fourier transform infrared spectroscopy (FTIR), combined with oxygen isotopic analysis by Secondary Ion Mass Spectrometry (SIMS) and lithium isotopic analysis by Multi-Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS). Through these researches, the evolution degree and temperature of the medium in which the assemblages of rare-metal minerals and quartz formed will be constrained, and the evolution of water activities during rare-metal mineralization process will be revealed, respectively. Based on these studies, the typomorphic characteristics of quartz from pegmatites will be established in three aspects, including rare-metal types, temperature of rare-metal deposition, and major fluid exsolution, respectively. This work will complete and advance the metallogenic theory of pegmatites and also be good for the further exploration of rare-metal mineralized pegmatites in the region.

石英是花岗伟晶岩最重要的贯通矿物之一，也是记录稀有金属成矿过程的有效矿物。以往集中于石英的岩浆分异程度指示标志研究，缺乏对其蕴含的丰富成因信息和成矿指示意义的深入挖掘，制约了稀有金属矿床石英标型特征的研究与应用。本申请以阿尔泰可可托海3号脉伟晶岩型稀有金属矿床为研究对象，基于其发育完整内部结构分带（9个带）和多阶段演化-多种稀有金属组合矿化（Be-Nb-Ta、Li-Be-Nb-Ta和Li-Cs-Ta-Hf）的特点，运用激光剥蚀等离子体质谱和傅立叶变换红外光谱分析各结构带石英微量元素组成和水含量，辅以离子探针氧同位素和多接收等离子体质谱锂同位素分析，查明3号脉不同矿化组合结晶介质的分异演化程度和温度条件，厘清稀有金属成矿过程的水含量演化，建立指示稀有金属矿化类型、稀有金属沉淀温度和流体大量出溶的石英标型特征。该研究将丰富伟晶岩成矿理论，对促进区域稀有金属勘查也有重要意义。

石英是花岗伟晶岩的重要贯通矿物，记录了伟晶岩形成过程中的关键信息，其标型特征研究对伟晶岩型稀有金属成矿具有重要的指示意义。然而，以往研究集中于石英的分异演化程度指标研究，对于其成因信息和稀有金属成矿指示意义的挖掘较少，制约了花岗伟晶岩的石英标型特征研究与应用。可可托海3号脉是阿尔泰造山带规模最大、分异演化程度最高和矿化元素最多的花岗伟晶岩型稀有金属矿床，发育9个内部结构分带（I-IX带），其多阶段演化-多种稀有金属组合矿化的特色和较高的研究程度为建立指示稀有金属成矿的矿物标型特征提供了可供解剖的典型实例。通过详细的地质观察查明3号脉的石英产状与共生矿物组合，运用阴极发光（CL）厘清不同结构带石英的显微结构特征，在此基础上，利用激光剥蚀等离子质谱（LA-ICP-MS）、傅里叶变换红外光谱（FTIR）和二次离子质谱（SIMS）等微区原位分析手段，获得石英的微量元素组成、水含量和氧-锂同位素组成。CL图像显示以均一原生石英为主，偶见次生石英边。由外向内，石英的Li、Al、Ge含量升高，Ti含量降低，Ge/Ti值和Al/Ti值升高，岩浆分异演化程度不断加大。石英Ge含量、Ge/Ti值和Al/Ti值可作为指示不同稀有金属矿化组合（Be-Nb-Ta、Li-Be-Nb-Ta和Li-Ta-Cs-Hf）的标志，Sb和Cs含量可作为Li-Cs-Ta-Hf矿化的指示标志。3号脉稀有金属成矿过程中，随着结晶分异作用，熔体阶段（I-IV带，Be-Nb-Ta）内，石英水含量升高，δ18O值和δ7Li值降低；流体大量出溶，熔流体阶段（V-VII带，Li-Be-Nb-Ta）中，石英水含量、δ18O值和δ7Li值变化范围加大；流体阶段（VIII-IX带，Li-Ta-Cs-Hf）中，石英水含量明显升高，δ18O值和δ7Li值升高。石英水含量、氧同位素和锂同位素能够联合示踪稀有金属成矿过程和熔流体演化过程，可作为流体大量出溶的指示标志。本研究初步建立了能够指示稀有金属成矿（矿化组合和控制机制）的石英标型特征，加深了对稀有金属成矿过程的理解和认识，并对区域上不同类型稀有金属矿化伟晶岩的勘查具有重要借鉴意义。