川西甲基卡伟晶岩型锂矿的“多层次穹状花岗岩席”控矿新理论——记“川西甲基卡锂矿科学钻探”创新成果

New ore-controlling theory of“multilayered domal granitic sheets”of the Jiajika pegmatite-type lithium deposit:The major discoveries of the“Jiajika Pegmatite-type Lithium Deposit Scientific Drilling Project(JSD)”

川西甲基卡伟晶岩型锂矿是中国大陆最大的硬岩型锂矿床。以揭示伟晶岩型锂矿深部结构和成因机制为目的的甲基卡伟晶岩型锂矿科学钻探工程(JSD)实施了一口3211.21 m(JSD-1)和两口各1000 m(JSD-2和JSD-3)的科学钻探,取得如下的创新性研究成果:(1)首次发现JSD-1中0~900 m深度的晚三叠世浊积岩经历中低压-高温巴罗-巴肯式叠加变质作用和穹状构造特征;(2)揭示JSD-1的100 m深度范围的含锂辉石伟晶岩存在,以及估算了在3211 m深度伟晶岩中的稀有金属丰度及成矿潜力;(3)利用锡石、铌钽矿和独居石的精确定年,确定JSD-1深部伟晶岩形成的两期岩浆-热液事件(210~204 Ma和193~192 Ma);(4)流体包裹体的碱性元素(Li、Na、K、Rb、Cs)和挥发性元素(B、As)的富集指示富锂伟晶岩高度结晶分异。利用JSD-1岩芯Li-B-Fe-Nd同位素示踪了岩浆的演化、流体出熔过程及成矿机制,揭示甲基卡伟晶岩型锂矿床的花岗岩岩浆演化过程中,岩浆结晶分异促进大量流体的出熔,从而在浅部形成钠长石锂辉石伟晶岩为主的矿体。这些结果不支持含锂伟晶岩是地壳深熔作用的直接产物;(5)揭示与甲基卡伟晶岩锂矿有密切成因关系的马颈子S型花岗岩体,并非前人所认为的是一个具有深根的大花岗岩基,而是由若干“无根”的穹状花岗岩席与岩席之间的变质岩及侵入其中的含矿伟晶岩群组成的“多层三明治”结构。由此我们提出了甲基卡伟晶岩型锂矿的“多层次穹状花岗岩席”颠覆性的控矿理论;(6)提出了在松潘甘孜造山带基墨里造山过程中(晚三叠世—早侏罗世)伟晶岩型锂矿的形成过程:经历230~190 Ma以来的深部剪切熔融、岩浆沿裂隙向上注入、在半塑性上地壳域中沿已存在的逆冲断裂(或剪切带)生成花岗岩席岩浆库、伴随岩浆上升减压造成岩席的穹状隆起,以及围岩中伴随巴罗-巴肯式变质作用,造成在多期岩浆-热液的作用下的伟晶岩成矿过程;(7)提出甲基卡锂矿区的下部类似丹巴式深地壳混合岩化基底的深熔作用是甲基卡花岗岩席的成因和岩浆源区的假设;(8)岩芯的低温年代学反演结果表明甲基卡矿区存在三个阶段的剥露速率变化,并估算了甲基卡锂矿区成矿(<200~190 Ma)以来的总体剥蚀厚度为5±1 km,推断新生代以来甲基卡穹隆较低的剥蚀速率为该区锂矿的保存提供了重要条件;(9)提出甲基卡式富锂伟晶岩成矿的6大找矿标志。

The Jiajika pegmatite-type lithium deposit in Western Sichuan is the largest hard-rock lithium deposit in China.The“Jiajika Pegmatite-type Lithium Deposit Scientific Drilling Project”(JSD)was conducted with the aim of revealing the deep structure and ore-forming mechanism of the pegmatite-type lithium deposit.This project involved three scientific drill boreholes:a 3211.21 m borehole(JSD-1)and two 1000 m boreholes(JSD-2 and JSD-3).The following innovative research findings were obtained:①It was discovered that the Late Triassic turbidite within the depth of 0~900 m in JSD-1 experienced medium-to-low pressure and high-temperature Barrovian-Buchan-type metamorphism,as well as deformation related to gneiss dome formation.②Lithium-bearing pegmatites were identified within 0~100 m depth of JSD-1.The rare element abundances and mineralization potential in the pegmatites of the 3211 m deep borehole were estimated.③Precise isotopic dating of cassiterite,columbite-tantalite,and monazite reveals two magmatic-hydrothermal events(ca.210~204 Ma and ca.193~192 Ma)of the pegmatites in JSD-1.④The enrichment of alkaline elements(Li,Na,K,Rb,Cs)and volatile elements(B,As)in fluid inclusions indicates highly fractional crystallization of the pegmatites.Li-B-Fe-Nd isotopes of JSD-1 core samples revealed the magmatic evolution,fluid exsolution process,and ore-forming mechanism.It showed that during the magmatic evolution of the Jiajika pegmatite-type lithium deposit,magma differentiation promoted extensive fluid exsolution,leading to the formation of albite-spodumene pegmatite bodies in shallow depths,refuting the idea that the pegmatites were derived from in-situ melting of sedimentary rocks.⑤The study revealed the Marjinzi S-type granite body closely related to the Jiajika pegmatite lithium deposit is not a deep-seated large granite batholith,contrary to previous assumptions.Instead,it is composed of several“rootless”dome-like granitic sheets separated by multilayered metamorphic rocks and rare-element pegmatites,forming a“multi-layer sandwich”structure.This proposes a groundbreaking ore-controlling theory of“multilayered domal granitic sheets”for the Jiajika pegmatite-type lithium deposit.⑥The formation process of the Jiajika pegmatite-type lithium deposit during the Late-Triassic to-Early-Jurassic Cimmeria Orogeny(230~190 Ma)was outlined.It involved shear-induced melting in deep crust,magmatic ascending,forming of granitic sheets along pre-existing reverse faults(or shear zones),doming of the sheets due to magma ascent and decompression,as well as Barrovian-Buchan-type metamorphism,leading to the mineralization of the pegmatites through multiple magmatic-hydrothermal events.⑦Danba-type migmatization in deep crust was proposed to have provided the magma for the Jiajika granite sheets and the lithium bearing pegmatites.⑧Low-temperature thermochronological results from the borehole samples indicated three stages of exhumations in the Jiajika lithium deposit.The overall amount of exhumation since the mineralization(<200~190 Ma)was estimated to be 5±1 km.The lower exhumation rate of the Jiajika dome since the Cenozoic might have been a favorable condition for the preservation of the lithium deposit.⑨Six exploration indicators for the Jiajika-type lithium-bearing pegmatites were proposed.