新疆可可托海式伟晶岩型稀有金属矿床成矿机制  

Metallogenic mechanism of the Koktokay pegmatite-type rare-metal deposit,Northwest China

作　　者：张辉[1] 李国胜 ZHANG Hui;LI GuoSheng(State Key Laboratory for Critical Mineral Research and Exploration,Institute of Geochemistry,Chinese Academy Sciences,Guiyang 550081,China;University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院地球化学研究所,关键矿产成矿与预测重点实验室,贵阳550081 [2]中国科学院大学,北京100049

出　　处：《岩石学报》2024年第9期2769-2785,共17页Acta Petrologica Sinica

基　　金：第三次新疆综合科学考察(2022xjkk1302);国家自然科学基金项目(41873030、91962222)联合资助.

摘　　要：可可托海3号脉形成于晚三叠世(~220Ma),侵入于3号脉的白云母钠长石花岗岩形成于早侏罗世,而同时代的阿拉尔花岗岩不是稀有金属花岗岩。因此,3号脉不可能是花岗岩分异的产物,而是变质深熔成因。可可托海3号脉具有低的锆石ε_(Hf)(t)(+1.25~+2.39)、δ^(18)O (7.57‰~8.85‰)等特征,指示物源为新生地壳与古老地壳的混合物。可可托海3号脉经历了岩浆(Ⅰ-Ⅳ带)、岩浆-热液过渡(Ⅴ-Ⅷ带)和热液(Ⅸ带)阶段,伟晶岩经历了70%的分离结晶后进入晶体、熔体、流体共存的岩浆-热液过渡阶段。高分异伟晶岩以显著内部结构分带为特征,而结构单元也是伟晶岩的组成部分,与岩浆过冷条件下不平衡结晶或热液蚀变有关。Be矿化主要在3号脉岩钟体部分中的Ⅰ、Ⅱ、Ⅳ带,Li矿化主要产于Ⅴ、Ⅵ、Ⅷ带,指示3号脉铍、锂矿体分别形成于岩浆阶段和岩浆-热液过渡阶段。3号脉Ⅱ带中板状绿柱石与骸晶磷灰石、石榴子石和少量白云母共生,磷灰石结晶前后熔体体系的ASI显著变化,是导致3号脉Ⅱ带中大量绿柱石饱和结晶的成矿机制;从早期到晚期结构带,碱性长石和云母类矿物中K/Rb比值随Li、Rb、Cs含量增大而减小以及锆石中Zr/Hf比值随Hf含量增大而降低的分异趋势,指示岩浆分离结晶作用是伟晶岩型锂矿床中Li超常富集的主要机制。The Koktokay No.3 pegmatite dike was formed in the Late Triassic(~220Ma),and the muscovite-albite granite intruded into the Koktokay No.3 pegmatite was formed in the Early Jurassic,while the Aral granite which was contemporaneous to the Koktokay No.3 pegmatite dike was not a rare-metal parent granite.Therefore,the Koktokay No.3 pegmatite dike can not be the product of granite differentiation,but from anatexis of metasedimentary rocks.The Koktokay No.3 pegmatite dike is characterized by low zirconsε_(Hf)(t)(+1.25~+2.39)and δ^(18)O(7.57‰~8.85‰)values,indicating that its source is a mixture of juvenile crust and ancient crust.The Koktokay No.3 dike has undergone magmatic stage(I-IV zones),magmatic-hydrothermal transition stage(V-VIII zones)and hydrothermal stage(IX zone),in which the magmatic-hydrothermal transition stage was characterized by coexistence of crystal,melt and fluid after 70%fractional crystallization of pegmatite-forming magma.The highly differentiated pegmatites are characterized by obvious internal textural zonation,in which the textural units were components of the pegmatites and associated to the disequilibrated crystallization under liquidus undercooling and hydrothermal alteration.The Be mineralization occurs mainly in the zones of I,II and IV,while the Li mineralization occurs mainly in the zones of V,VI and VIII,indicating that beryllium and lithium ore bodies in the Koktokay No.3 pegmatite dike were formed during the magmatic stage and the magmatic-hydrothermal transition stage,respectively.As there is mineral paragenesis of plate-like beryl with skeletal crystal apatite,garnet and a small amount of muscovite,metallogenic mechanism of Be for beryl saturated crystallization in the II zone of the Koktokay No.3 pegmatite dike was attributed to ASI changes in melts before and after apatite crystallization.From early to late textural zones,the K/Rb ratios in alkaline feldspars and muscovites decrease as the Li,Rb and Cs contents increase,and the Zr/Hf ratio in zircon decreases as Hf contents

关 键 词：源区特征 岩石成因 成矿时代 成矿机制 可可托海3号脉 

分 类 号：P588.131[天文地球—岩石学] P611.12[天文地球—地质学] P618.7