花岗伟晶岩成矿有关的几个问题讨论

Some Issues Relevant to Rare Metal Metallogeny of Granitic Pegmatites

花岗伟晶岩是关键战略金属资源的重要来源之一。对比各种花岗伟晶岩的分类方案,认为以主要成矿(矿化)元素(如Li、Be、Nb、Ta、REE、U和Cs)和/或矿物(锂辉石、绿柱石、铌钽铁矿、独居石、沥青铀矿等)进行伟晶岩分类简单、易行。越来越多的伟晶岩及相关花岗岩的精确定年和同位素组成研究表明,并非所有伟晶岩均由花岗岩演化形成,一些重要的稀有金属成矿伟晶岩与花岗岩有显著时差和源区物质差异,它们与花岗岩无关,自然界存在独立的伟晶岩岩浆。一些大型、超大型稀有金属伟晶岩矿床的成矿作用延续时间较长,经历了岩浆、岩浆-热液过渡、热液和后期成矿叠加。富集Li、Na、B、F、H、O、P和H2O、CO_(3)^(2−)、HCO_(3)^(−)等助溶剂元素或组分是稀有金属伟晶岩的重要特征之一,可作为花岗伟晶岩成矿系统的重要示踪剂。稀有金属伟晶岩源区矿物组成及熔融方式对伟晶岩浆形成与成矿具有重要控制作用。花岗伟晶岩的岩浆演化和热液作用过程中Li同位素组成发生变化,贫锂辉石伟晶岩比含锂辉石伟晶岩具有更高的δ7Li值,即相对富重Li同位素,这可作为锂辉石伟晶岩的成矿、找矿地球化学标志之一。碱性富碳酸盐流体或含水熔体、超临界CO_(2)流体可能存在地球深部更宽的温度压力范围内,具有独特的溶解、萃取和运输能力,可溶解和迁移地幔中许多成矿元素,应针对CO_(2)在稀有金属成矿过程中的作用以及探讨岩浆来源开展更多模拟实验研究。高空间分辨率、高光谱、雷达等遥感技术,也为战略性稀有金属矿产资源调查提供了一种全新的技术手段。

Granitic pegmatite is one of the main sources for strategy critical metals.The classification of pegmatite based on the mineralized rare metals(Li,Be,Nb,Ta,REE,U and Cs)and minerals(spodumene,beryl,columbite,tantalite,monazite,uraninite)is useful and practical.Many granitic pegmatites display temporal decoupling(large gap of forming age)and different sources with their surrounding granites,indicating these pegmatites were not derived from the differentiated granitic melts.A reasonable genetic model is that there exists an anatectic discrete pegmatite magma.Most of the large-supper large rare metal pegmatite deposits have prolonged metallogenic periods,i.e.,from magmatic-,magmatic-hydrothermal-to hydrothermal-stages.One important signature of the rare metal pegmatites is the enriched fluxing elements or components(Li,Na,B,F,H O,P,H2O,CO_(3)^(2−),HCO_(3)^(−)),and this should be an indicator for mineralized pegmatites.Lithium isotopic compositions(δ7Li)might discriminate spodumene-rich with spodumene-poor pegmatites,and can be used as a marker for mineralization and prospecting of granitic pegmatites.Alkaline carbonate-rich fluids or aqueous melts,and supercritical CO_(2) fluid might be important promoting agents for dissolution,extraction and transport of critical metals.More and thorough experimental studies should be carried out for the controls of CO_(2) on mineralization of rare metal pegmatites.High spatial resolution,high spectral and radar remote sensing technologies have provided a brand new technological method for the mineral source survey of strategic critical metals.