花岗-伟晶岩型锂矿床围岩变质沉积岩中锂富集的关键因素: 以松潘-甘孜构造带东部可尔因地区为例

The critical factors of lithium enrichment in the metasedimentary wall rocks of granitic pegmatite-type lithium deposit:Insights from the Ke eryin area in the eastern Songpan-Ganzi Belt

变质沉积岩中稀有金属元素的富集通常被认为是形成花岗-伟晶岩型稀有金属矿床的物质基础,但是对于其富集的原因及关键因素却仍不清楚。松潘-甘孜构造带作为我国重要的稀有金属富集区,区内已发现多处锂矿床,是研究花岗-伟晶岩体与变质沉积岩(围岩)成因联系的理想地区。本文详细调查了松潘-甘孜构造带东部可尔因地区距花岗-伟晶岩体不同距离、不同类型的三叠系变质沉积岩。可尔因地区的三叠系变质沉积岩以块状变质砂岩(角岩)和石英/云母片岩为主,其中常夹有泥质千枚岩或云母片岩薄层。显微镜下观察表明不同类型的变质沉积岩中的主要矿物为石英、黑云母和白云母。全岩地球化学分析结果显示,变质砂岩和石英片岩比云母片岩和泥质千枚岩整体具有较高的SiO_(2)含量,较低的Al_(2)O_(3)、K_(2)O、TiO_(2)、MgO和Fe_(2)O T_(3)含量,反映了石英和黑云母含量对全岩成分的控制。这些样品整体具有与平均大陆上地壳相似的微量元素含量,同时表现出不同程度地亏损Sr和Ni-Co等相容元素,以及富集Li(3×10^(-6)~997×10^(-6))和Cs等不相容元素。值得注意的是,个别样品具有异常高的锂含量(>300×10^(-6)),这些样品同时也具有较高的Cs和Sn等元素含量。黑云母成分分析结果显示这些变质沉积岩中的黑云母为富镁黑云母和富铁黑云母,Li、Rb、Cs、Sn、F等稀有金属元素和挥发分元素在这些黑云母中变化较大,而且它们之间具有一定的正相关关系。根据全岩分析结果,该地区变质沉积岩的锂含量中位数为50.4×10^(-6),与周缘造山带内岩浆岩的锂含量相似。结合前人碎屑锆石的研究结果,这反映了物源对沉积岩中锂含量的控制作用。若以此作为花岗质岩浆的源岩,不可能直接通过部分熔融形成含矿熔体,而需要岩浆的高分异演化。变质沉积岩全岩和黑云母中锂的含量与Cs、Sn、F等稀有金属成矿相关元素有正相关性,表明富锂熔-流体对围岩沉积岩的改造是导致围岩变质沉积岩中锂富集的原因。富锂熔体在上升侵位过程中,对围岩中锂的萃取可以忽略不计,与之相反,这些富锂岩浆是导致区域围岩富锂的主要原因。空间上,富锂变质沉积岩与已发现的锂矿点关系密切,或许可以利用这一关系,定位区域内的含锂矿伟晶岩,特别是对于高海拔和高差较大的地区。

The enrichment of rare metal elements in metasedimentary rocks is consider as an essential prerequisite of the formation of granitic pegmatite-type rare metal deposits.However,the causes and critical factors of enrichment of rare metal elements in metasedimentary rocks are still unclear.The Songpan-Ganzi orogenic belt,as an important enrichment zone of rare metals of China with many lithium deposits,is an ideal place to study the genetic relationship between the ore-related granite-pegmatites and metasedimentary rocks(wall-rock).This study investigates various types of Triassic metasedimentary rocks away from the granite-pegmatites at different distances in the Ke eryin area,the eastern part of the Songpan-Ganzi orogenic belt.The Triassic metasedimentary rocks in the Ke eryin area are dominated by metasandstones(include hornfels)and quartz/mica schists,interlayered with thin-bedded of argillaceous phyllite or mica schists.Petrographically,these metasedimentary rocks include abundant quartz,biotite and muscovite.Metasandstone and quartz schist have higher SiO_(2) content and lower Al_(2)O_(3),K_(2)O,TiO_(2),MgO,and Fe_(2)O T_(3) contents than mica schist and argillaceous phyllite,reflecting the control of quartz and biotite on the whole-rock composition.These samples have similar trace element compositions to the average upper continental crust but show various depletion of compatible elements(e.g.,Sr and Ni-Co),and enrichment of incompatible elements,e.g.,Li(3×10^(-6)~997×10^(-6))and Cs.It is noteworthy that some samples have abnormally high lithium content(>300×10^(-6)),together with high Cs and Sn abundances.Mg biotite and Fe biotite are the predominant type of biotite in these samples.Rare metals and volatiles,e.g.,Li,Rb,Cs,Sn and F,vary greatly in these biotites and exhibit positive correlations between them.The median value of lithium in the metasedimentary rocks(50.4×10^(-6))is highly comparable to the average lithium content of the Indosinian intermediate-acid rocks in the surrounding orogens.Combining with previous studies on detrital zircons,it reflects the control of source materials on the Li content of sedimentary rocks.However,formation of ore-bearing melts directly from partial melting of metasedimentary rocks is impossible as it requires high fractionation of magma.The positive correlation between the Li and Cs,Sn,F in the metasedimentary rock and biotite indicates that lithium-rich melt-fluid is likely to be the cause of high Li content in the metasedimentary rocks.During the magma ascent and emplacement,the extraction of lithium from the wall rocks is negligible.On the contrary,it brings the lithium to the wall rocks.Lithium-rich metasedimentary rocks are spatially close related to the lithium deposits.By taking advantage of such a relationship,it may help locate the lithium deposits in specific areas,especially for high altitude and terribly undulate areas.