川西甲基卡花岗伟晶岩的锂铍成矿作用过程——来自308号脉流体包裹体的约束

Metallogenic process of the Jiajika Li-Be deposit in West Sichuan:Constraints from fluid inclusions of No.308 pegmatite

甲基卡位于松潘-甘孜造山带内,为特大型花岗伟晶岩型锂-铍矿床。前人以锂辉石中发育的富子晶包裹体为研究对象,着重剖析了甲基卡锂成矿的物理化学条件。然而,就伟晶岩熔(流)体的演化过程,特别是稀有金属成矿的富集机制和物理化学条件,仍缺乏有效制约。308号脉作为甲基卡出露最大的钠长石型锂-铍伟晶岩脉,具有良好的内部分带,较完整地记录了甲基卡伟晶岩结晶演化过程。本文以308号伟晶岩脉为研究对象,开展了系统的流体包裹体研究工作。308号脉较早阶段的结晶介质以外带绿柱石中富子晶流体包裹体为代表,为高温(492~592℃)、低盐度(0.8%~8.5%NaCleqv)、弱碱性(CO_(3)^(2-)和HCO_(3)^(-))、富挥发分(F、P、B、Li)的硅酸盐水体系,估算成矿压力平均值为400MPa。308脉较晚阶段的结晶介质以内带锂辉石中富子晶流体包裹体为代表,为高温(482~565℃)、低盐度(1.6%~8.5%NaCleqv)、弱碱性、富挥发分(P、Li)的硅酸盐水体系。晚期热液阶段以石英的富CO_(2)流体包裹体为代表,为中高温(291~365℃)、中低盐度(3.9%~13.2%NaCleqv)、弱酸性、富氯的盐水体系。308号脉由铍矿化至锂矿化、最终至热液阶段,是伟晶岩熔(流)体持续分异演化的结果,晚期热液为伟晶岩熔(流)体自身分异形成的。相对封闭的成矿体系、多种挥发组分(F、Li、P、B)的大量聚集和弱碱性的pH环境,是308号脉锂-铍矿化富集与沉淀的主要控制因素。晚期贫F富Cl、弱酸性特征的热液流体有利于锂辉石的保存。

Jiajika giant granitic pegmatite-type rare metal deposit is located in Songpan-Ganzêorogenic belt.Crystal-rich inclusions hosted in spodumene have been selected to analyze the lithium metallogenic process at this deposit in previous work.However,the systematic evolution of pegmatite melt(fluid),especially the enrichment mechanism and physicochemical conditions of rare metal forming process,is still unclear.No.308 Li-Be albite pegmatite dike,as the largest outcrop in Jiajika,records the whole evolution from pegmatitic to hydrothermal stage.In this paper,a systematic study of fluid inclusions in No.308 pegmatite were carried out.Crystal-rich inclusions in beryl,representing the Be mineralization stage,were characterized by high temperature(492~592℃),low salinity(0.8%~8.5%NaCleqv),weak alkali and volatile-rich(F,P,B and Li).Crystal-rich inclusions in spodumene,representing the Li mineralization stage,were characterized by high temperature(482~565℃),low salinity(0.8%~8.5%NaCleqv),weak alkali and volatile-rich(P and Li).The emplacement pressure of the No.308 pegmatite was estimated at 400MPa.CO_(2)-bearing inclusions in quartz,representing the hydrothermal stage,were characterized by medium-low temperature(291~365℃),medium-low salinity(3.9%~13.2%NaCleqv),weak acid and Cl-dominant.The No.308 pegmatite dike in Jiajika has undergone excessive fluid evolution from beryllium mineralization at early pegmatitic stage,to lithium mineralization at late pegmatitic stage,and eventually stepped into hydrothermal stage.This study shows that the closed ore-forming system to accumulate high volatile components and weak alkali conditions are the main factors for the enrichment and precipitation of Li-Be in Jiajika.The weak acid hydrothermal fluid with F-poor and Cl-rich at late stage is beneficial to the preservation of spodumene.