四川甲基卡伟晶岩型稀有金属矿床的地质特征及成矿机制概述

Geological characteristics and metallogenic mechanism of Jiajika pegmatite-type rare metal deposit in Sichuan Province

四川甲基卡锂矿床位于松潘-甘孜造山带东南部,是我国已知产量、规模最大的固体锂矿床。甲基卡矿床是花岗伟晶岩型矿床,自内向外依次发育二云母花岗岩、微斜长石型(Ⅰ)、微斜长石-钠长石型(Ⅱ)、钠长石型(Ⅲ)、钠长石-锂辉石型(Ⅳ)、锂-白云母型(Ⅴ)伟晶岩,且二云母花岗岩内发育大量的剥离体。元素地球化学组成上,花岗岩与伟晶岩的稀土、微量元素含量表现出显著的突变关系;各类型伟晶岩中石英和锂辉石内发育的包裹体类型主要有气液两相包裹体、含CO_(2)包裹体、纯CO_(2)包裹体、富子晶包裹体和熔体包裹体。包裹体均一实验表明熔体包裹体的完全均一温度在750~800℃之间,对应的捕获压力为550~700MPa。在花岗岩熔体包裹体均一之前,富水熔体和贫水熔体在同一个体系内共存;石英和锂辉石内含硅酸盐子矿物包裹体与熔体-流体过渡型包裹体明显不同。锂辉石和石英中包裹体的C-H-O同位素测试结果表明成矿流体主要为岩浆水,且其形成演化处于相对密闭的构造环境。全岩Ba-Sr-Nd-Hf同位素组成指示三叠系西康群变沉积岩是花岗岩的重要源岩。全岩Zn和Li同位素则表明在岩浆侵位过程可能发生过显著的不混溶作用。综上,甲基卡矿床形成演化过程中可能经历了岩浆液态不混溶作用。

Jiajika lithium deposit in Sichuan Province,located in the southeast of Songpan-Ganzi orogenic belt,is the largest and most productive solid lithium deposit in China.Two-mica granites,microcline type(I),microcline-albite type(II),albite type(III),albite-spodumene type(IV)and lepidolite-muscovite type(V)pegmatites are developed from the inside to the outside of the Jiajika deposit.A large number of exfoliations are developed in the two-mica granites.For the element geochemical composition,the contents of rare and trace elements in granites and pegmatites show a significant abrupt relationship.There are gas-liquid two-phase inclusions,CO_(2)-bearing inclusions,CO_(2)-pure inclusions,crystal-rich inclusions and melt inclusions in the quartz and spodumene of various types of pegmatites.Microthermometric results show that the trap temperature of the melt inclusions is 750~800℃,and the diagenetic pressure of the pegmatite is 550~700MPa.Before complete homogenization of melt inclusions in granite,water-rich melt and water-poor melt coexist in the same system.The silicate daughter mineral-bearing inclusions in quartz and spodumene are obviously different from the melt-fluid transitional inclusions.The C-H-O isotopic results of the inclusions from the spodumene and quartz indicate that the ore-forming fluid is predominantly magmatic water and was formed in a relatively closed tectonic environment.The Ba-Sr-Nd-Hf isotopic compositions suggest that the Triassic Xikang Group metamorphic sedimentary rocks could be one of the important sources of the granites.The Zn and Li isotopes indicate that melt-melt significant immiscibility could have occurred during the magma emplacement.In summary,the Jiajika deposit may have undergone significant liquid immiscibility of granitic magma.