伟晶岩锂同位素研究进展及其对锂成矿过程的指示  

Progress in lithium isotopic study of pegmatites and its application in lithium deposi

作　　者：陈洁 李建康[1] CHEN Jie;LI Jiankang(MNR Key Laboratory of Metallogeny and Mineral Resource Assessment,Institute of Mineral Resource,Chinese Academy of Geological Sciences,Beijing 100037,China;MOE Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]自然资源部成矿作用与资源评价重点实验室,中国地质科学院矿产资源研究所,北京100037 [2]北京大学地球与空间科学院,造山带与地壳演化教育部重点实验室,北京100871

出　　处：《地质学报》2024年第5期1600-1614,共15页Acta Geologica Sinica

基　　金：国家自然科学基金重点项目(编号42330806);中国地质科学基本业务费项目(编号JKYZD202315,JKYQN202327);中国地质大调查项目(编号DD20243484,DD20230289)联合资助的成果。

摘　　要：花岗伟晶岩是最重要的硬岩型锂资源来源,近年来前人应用锂同位素对伟晶岩成岩成矿过程开展较多的研究工作。本文在综述伟晶岩锂同位素研究进展的基础上,重点分析了伟晶岩的锂同位素在指示伟晶岩熔体源区、指示伟晶岩熔体分离、结晶和流体出溶中的研究思路,为揭示伟晶岩的锂成矿过程提供了启示。前人研究表明,伟晶岩熔体结晶分异和流体出溶过程均会产生较明显的锂同位素分馏:(1)结晶分异使残余伟晶岩熔体富集^(7)Li;(2)流体出溶使熔体富集^(6)Li;(3)扩散分馏使快速迁移的^(6)Li累积在熔体远端。全球伟晶岩(包括甲基卡伟晶岩)的Li含量与δ^(7)Li之间均有明显的负相关关系,甲基卡伟晶岩分异程度的增高与δ^(7)Li呈负相关关系。前人研究表明,高分异伟晶岩体系中锂同位素变轻可能是云母和锂辉石等富^(6)Li矿物的大量结晶、富^(7)Li流体出溶丢失或动力分馏作用等原因造成的。另一方面,不平衡结晶分异作用会产生显著的动力分馏,热液蚀变作用会产生强烈的低温分馏,这些过程产生的同位素分馏效应会模糊锂的来源信息,给同位素示踪带来困难。因此,伟晶岩锂同位素组成可能对于指示锂的来源作用有限,但可以较好地指示熔体结晶分异和流体出溶等地质过程,有助于揭示伟晶岩熔体-流体的演化过程,指示锂矿化机制。Granite pegmatites are the most important source of lithium within hard-rock lithium resources.In recent years,numerous studies have investigated the diagenesis and mineralization processes in pegmatites using lithium isotopes.This review synthesizes the advancements in lithium isotope applications for pegmatites,focusing on their ability to:①trace the source region of pegmatite melts;②delineate melt separation,crystallization,and fluid exsolution events;and③provide insights into lithium metallogenesis.Researchers have demonstrated that crystallization differentiation and fluid exsolution processes in pegmatite melts cause significant lithium isotope fractionation.This fractionation manifests in several ways:①crystallization enriches the melt in heavier lithium isotopes(δ^(7)Li);②fluid exsolution depletes the melt in heavier lithium isotopes;and③diffusion fractionation leads to the accumulation of ^(6)Li at the distal end of the melt.A significant negative correlation exists between Li content and δ^(7)Li in global pegmatite occurrences,including the Jiajika pegmatite,where increasing differentiation is negatively correlated with δ^(7)Li.This isotopic lightening in differentiated pegmatite systems is likely attributable to several factors:extensive crystallization of ^(6)Li-rich minerals like mica and spodumene,fluid exsolution,or dynamic fractionation,as previously documented.In addition,unbalanced crystal differentiation and hydrothermal alteration can induce significant isotope fractionation,thereby complicating the identification of lithium sources and hampering isotope tracing efforts.The lithium isotopic composition of pegmatites may not consistently reflect the origin of lithium sources.Nonetheless,it provides valuable insights into geological processes such as melt crystallization differentiation and fluid exsolution,contributing to a more comprehensive understanding of the melt-fluid evolution and lithium mineralization mechanisms within pegmatite systems.

关 键 词：锂同位素 伟晶岩 成矿作用 结晶分异 流体出溶 

分 类 号：P618.71[天文地球—矿床学]