川西甲基卡伟晶岩型锂矿科学钻探(JSD-1)岩芯Li-B-Fe-Nd同位素对稀有金属成矿作用的指示意义

Insights of Li-B-Fe-Nd isotopes in the Jiajika granitic pegmatite type lithium deposit scientific drill core(JSD-1)to the metallogenesis of rare metals in eastern Tibetan Plateau

甲基卡伟晶岩型锂矿位于青藏高原东部松潘-甘孜地体东南部,是我国最大的硬岩型锂矿,其成岩成矿机制至今仍有争议。为了深入认识甲基卡伟晶岩型锂矿床稀有金属超常富集的关键岩浆热液过程,研究团队基于甲基卡一号钻孔(JSD-1)岩芯开展全孔Li-B-Fe-Nd同位素地球化学研究。伟晶岩和二云母花岗岩较低的Nb/Ta、Zr/Hf值以及Li-Nd同位素表明甲基卡伟晶岩可能来源于马颈子二云母花岗岩深成岩体的岩浆结晶分异。JSD-1岩芯花岗岩和伟晶岩中电气石的硼同位素(δ^(11)B)在-9.5‰~-7.1‰之间,与世界上90%的花岗岩和伟晶岩中δ^(11)B变化范围一致。JSD-1岩芯电气石δ^(11)B与全岩Li含量的实验和模型模拟结果表明花岗质岩浆演化过程遵循平衡结晶模型,花岗质岩浆极端结晶分异不能达到熔体中锂辉石过饱和。JSD-1岩芯全岩的Fe同位素(δ^(56)Fe)变化范围为-0.12‰~0.38‰。δ^(56)Fe的显著变化反映了黑云母分离结晶、热液蚀变(电气石化)以及石榴子石堆晶等多阶段岩浆-热液过程的共同结果。全孔Li-B-Fe-Nd同位素综合表明,伴随着广泛流体出溶的岩浆结晶分异过程控制了Li的逐步富集。由于岩浆上升的减压作用,花岗岩岩席穹隆的形成有利于晚期花岗岩岩浆的大量流体出溶,使得浅部形成锂辉石为主的矿体。

The Jiajika pegmatite-type lithium deposit located in the southeastern Songpan-Ganze terrane of the eastern Tibetan Plateau is the largest hard-rock lithium deposit in China,and its paragenetic mechanism is still controversial.To understand the critical magmatic-hydrothermal process that leads to Li concentration of potentially economic levels in the Jiajika pegmatite-type lithium deposit,the association between Li enrichment and granitic magma evolution is investigated using Li-B-Fe-Nd isotopes along the entire 3211-m-depth of the JSD-1 scientific drill core.The data of Li isotope(δ7Li)and Nd isotope(εNd(t)),together with consistently low Nb/Ta,Zr/Hf of the pegmatites and two-mica granites,suggest that the Jiajika pegmatites are more likely from magmatic crystallization of the Majingzi two-mica granite pluton.The boron isotope(δ^(11)B)of tourmalines in granite and pegmatite in JSD-1 scientific drill core vary from 9.5‰to 7.1‰,in line with the range observed in 90%of worldwide granites and pegmatites related to melt-fluid fractionation in fluid exsolution.In comparison of the measured data ofδ^(11)B vs.Li to the modeled curves indicates that the differentiation process of granitic magma follows an equilibrium crystallization model better than a fractional crystallization model and the extreme differentiation of granitic magma alone could not reach the solubility-saturation for spodumene in the pegmatite-forming melt.Theδ^(56)Fe values of bulk rocks in JSD-1 vary from 0.12‰±0.02‰to 0.38‰±0.02‰,and the significant Fe isotope variation reflect the combined results of biotite fractionation,hydrothermal alteration(tourmalinization),and garnet accumulation during multistage magmatic-hydrothermal processes.The compiled evidence of Li-B-Fe-Nd isotopes proves that magma differentiation accompanied by fluid exsolution facilitates the enrichment of Li during granitic magmatic evolution.The formation of the domal structure of granitic sheets due to the decompressing effect of magma ascent favors massive fluid exsolution at the late-stage granitic magma,which contributes to the deposition of spodumene dominated ore body at the shallower depth.