岩石学报  2021, Vol. 37 Issue (11): 3325-3347, doi: 10.18654/1000-0569/2021.11.06   PDF    
喜马拉雅琼嘉岗超大型伟晶岩锂矿的形成时代、源区特征及分异特征
赵俊兴1, 何畅通1,2, 秦克章1,2, 施睿哲1,2, 刘小驰3, 胡方泱1, 余可龙1,2, 孙政浩1,2     
1. 中国科学院矿产资源研究重点实验室, 中国科学院地质与地球物理研究所, 北京 100029;
2. 中国科学院大学地球与行星科学学院, 北京 100049;
3. 岩石圈演化国家重点实验室, 中国科学院地质与地球物理研究所, 北京 100029
摘要: 喜马拉雅新生代淡色花岗岩带是近年来提出的与高度结晶分异、异地深成淡色花岗岩有关的稀有金属战略远景区,目前其金属组合以铍-铌-钽(-锡-钨)为主。秦克章等(2021a)报道了在高喜马拉雅带珠峰地区发现的琼嘉岗锂矿,是喜马拉雅首例具有工业价值的伟晶岩型锂矿。本次研究重点揭示喜马拉雅琼嘉岗伟晶岩型锂矿的成矿特征、形成时代和源区特征。琼嘉岗矿区矿石矿物主要为锂辉石、铌铁矿-铌锰矿、少量锡石和绿柱石,特征性长柱状锂辉石主要产于块体微斜长石+锂辉石带和分层细晶岩带内。琼嘉岗锂辉石伟晶岩各结构分带的K/Rb含量较为相似,锂含量从边部细粒钠长石带(~100×10-6)到分层细晶岩带(~1000×10-6),再到块体微斜长石+锂辉石带(>3000×10-6)逐渐升高,而Cs含量逐渐降低。独居石和铌钽铁矿族矿物LA-ICPMS定年结果显示,琼嘉岗锂辉石伟晶岩形成于新喜马拉雅阶段早期(25~24Ma),与高喜马拉雅地区淡色花岗岩时代相近。矿物化学和独居石Nd同位素结果显示该稀有金属伟晶岩结晶于高度演化的花岗伟晶岩熔体,源区特征与高喜马拉雅结晶岩系一致。本研究所揭示的琼嘉岗成矿特征、形成时代和源区特征将为高喜马拉雅其它地区找寻大型花岗伟晶岩型锂矿提供重要借鉴意义。
关键词: 琼嘉岗锂矿    伟晶岩结构与成分    独居石和铌钽铁矿U-Pb定年    源区示踪    喜马拉雅淡色花岗岩带区域成矿    
Geochronology, source features and the characteristics of fractional crystallization in pegmatite at the Qongjiagang giant pegmatite-type lithium deposit, Himalaya, Tibet
ZHAO JunXing1, HE ChangTong1,2, QIN KeZhang1,2, SHI RuiZhe1,2, LIU XiaoChi3, HU FangYang1, YU KeLong1,2, SUN ZhengHao1,2     
1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China;
3. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Abstract: Recently, the Himalayan leucogranite belt has been recognized as the potential prospecting area of the rare-metal mineralization related to the newly proposed petrogenesis of highly fractionated granite. Currently, the exploration aim is mainly targeting at the beryllium-niobium-tantalum (tin-tungsten) assemblage, such as Cuonadong deposit. Qin et al. (2021a) reported a discovery of the Qongjiagang giant lithium pegmatite deposit in Himalaya, Tibet, raising more attention to the spodumene pegmatites in the Higher Himalaya Belt. The present work reveals the geological features, age of ore formation and source features of mineralizing pegmatites at Qongjiagang. Our result shows the main ore minerals at Qongjiagang are dominated by spodumene and columbite group minerals, with minor amounts of cassiterite and beryl. The pegmatite zones hosted by a majority of spodumene minerals at Qongjiagang are identified as a massive microcline+spodumene zone and a layered aplite zone. The K/Rb ratios of different rocks in the internal zonation patterns from rim to core remain to be similar, and the lithium concentrations rise from ~100×10-6 at the saccharoidal albite zone, through ~1000×10-6 at the layered aplite zone, to more than 3000×10-6 at the massive microcline+spodumene zone; however, the cesium contents drop, respectively. U-Pb dating results upon monazite and columbite group minerals show the Qongjiagang Li-pegmatite was formed in the initial duration of the Neo-Himalayan stage (25~24Ma), similar to most dating results at the Higher Himalayan leucogranite. Mineral chemistry and monazite Nd isotopic compositions suggest Qongjiagang Li-pegmatite was crystallized in a highly evolved granitic pegmatite melt, with source features consistent with the Higher Himalayan Crystalline. Our research at the geological features, formation age and source characteristics at Qongjiagang lithium pegmatite deposit gives more implication of ore-forming mechanism and exploration targeting for finding large granitic pegmatitic lithium mines in the Higher Himalaya region of China.
Key words: Qongjiagang lithium deposit    Internal zonation patterns of pegmatite    U-Pb dating of monazite and columbite group minerals    Source features    Regional mineralization in the Himalayan leucogranite belt    

喜马拉雅碰撞造山带近2500km长度范围内产出有巨量的新生代淡色花岗岩。空间上,它由北侧的特提斯喜马拉雅淡色花岗岩带与南侧的高喜马拉雅淡色花岗岩带构成(Le Fort et al., 1987; Harrison et al., 1998; Searle et al., 2009; Zhang et al., 2012; 吴福元等,2015)。时间上,其岩浆活动可分为始喜马拉雅阶段(44~26Ma)、新喜马拉雅阶段(26~13Ma)和后喜马拉雅阶段(13~7Ma)三个阶段(吴福元等,2015)。岩性以二云母花岗岩、电气石淡色花岗岩和石榴石淡色花岗岩为主(Guillot and Le Fort, 1995; 吴福元等,2015; Liu et al., 2016),其中北侧特提斯喜马拉雅淡色花岗岩以二云母花岗岩为主,少数有电气石淡色花岗岩和石榴石淡色花岗岩,产状多为独立岩体产出在片麻岩穹隆的核部地区(如雅拉香波地区岩体,曾令森等,2009);南侧高喜马拉雅淡色花岗岩以电气石淡色花岗岩和二云母花岗岩为主,以岩席或岩墙形式沿藏南拆离系(STDS)产出(如Manaslu岩体,Harrison et al., 1999)。传统观点认为喜马拉雅淡色花岗岩是原地侵位的纯地壳来源的低熔花岗岩(Le Fort et al., 1987),不具有稀有金属成矿潜力。近年来,吴福元等(2015)基于淡色花岗岩的基础岩石成因研究,提出喜马拉雅淡色花岗岩为高度结晶分异的花岗岩,为异地深成(而非原地重熔)侵入体。该淡色花岗岩成因理论的创新,预示着喜马拉雅很可能具有稀有金属成矿潜力(王汝成等,2017吴福元等,2017)。

近年来,经过野外不断探索,目前查验出20余处具有稀有金属矿化的淡色花岗岩体和伟晶岩(Wu et al., 2020),稀有金属矿物包括有绿柱石、锡石、铌钽铁矿族矿物、烧绿石-细晶石、重钽铁矿、黑钨矿、锂辉石等(王汝成等,2017秦克章等,2021b)。从产状上看,喜马拉雅稀有金属花岗伟晶岩主要产出于在穹窿中淡色花岗岩周围或变质岩地层中,呈岩脉穿插淡色花岗岩中(王汝成等,2017),或透镜体/囊状体独立产出在地层中(刘志超等,2020),或产出于大理岩、矽卡岩与岩体接触带内(何畅通等,2020)。金属组合以Be-Nb-Ta(W-Sn)为主,大多分布在特提斯喜马拉雅带内,其中以错那洞大型钨-锡-铍矿床为代表,主要为矽卡岩-伟晶岩型铍矿和脉状锡-铍矿(李光明等,2017; Xie et al., 2020; 何畅通等,2020)。喜马拉雅稀有金属伟晶岩总体上呈现“富铍少锂”的矿化特点(秦克章等,2021b),仅在少数高喜马拉雅地区伟晶岩(普士拉与拉隆地区,Liu et al., 2020; Wu et al., 2020)和特提斯喜马拉雅库曲伟晶岩(周起凤等,2021)中报道发现有锂辉石矿物。秦克章等(2021a)报道该团队在喜马拉雅琼嘉岗地区发现数条宽约20~100m的锂辉石伟晶岩,并针对伟晶岩开展科研草测填图(约4km2),针对琼嘉岗锂辉石伟晶岩的地质特征和分带特征开展研究,初步评价锂资源具有大型规模以上,为喜马拉雅首例具有工业价值的伟晶岩型锂矿。

为了更为全面的认识喜马拉雅琼嘉岗伟晶岩型锂矿的成因特征,本文通过对琼嘉岗锂矿中伟晶岩的岩石地球化学、独居石和铌钽铁矿族矿物的矿物成分和年代学,以及独居石单矿物Nd同位素开展系统研究,解析琼嘉岗锂矿的形成时代、源区特征和分异特征。

1 喜马拉雅琼嘉岗伟晶岩型锂矿地质特征

琼嘉岗伟晶岩型锂矿位于高喜马拉雅淡色花岗岩带内的珠峰地区(图 1),藏南拆离系下的高喜马拉雅变质岩系中。区域上,该地区出露有二云母花岗岩、含电气石白云母花岗岩和电气石花岗岩等淡色花岗岩体,且各花岗岩体均有伟晶岩相伴生。伟晶岩或呈囊状体产出在片麻岩和大理岩地层中,或呈岩脉穿插在淡色花岗岩内,或与淡色花岗岩呈渐变过渡关系。填图区以北20~25km为藏南拆离系断层,附近地层和岩体发生糜棱岩化。在填图区域内(图 2),目前共发现有40余条锂辉石伟晶岩呈囊状体、厚板状,产出于高喜马拉雅带震旦系肉切村群大理岩和弱矽卡岩化大理岩中(图 3图 4a-d)。地表出露的伟晶岩体宽度不一,最宽可达百米,共同构成北北东和北东东-东西向、走向延长数百米和近千米的两条主要伟晶岩带。地表部分规模较小伟晶岩脉可能为第四系冰碛物覆盖,怀疑深部相连。其中锂辉石伟晶岩中矿石主要为块状构造,自形粒状结构,主要矿石矿物包括锂辉石,为长柱状,少数为短柱状和粒状,呈淡绿色、黄绿色,少数可见浅黄白色;绿柱石为浅蓝色和浅绿色短柱状;少量铌铁矿-铌锰矿和锡石。脉石矿物包括微斜长石、钠长石、石英、电气石和石榴石等。其中富矿石中锂辉石可达70%以上。填图结果显示区内主要为电气石花岗岩体和岩脉,伟晶岩和电气石花岗岩接触关系包括从电气石花岗岩逐渐渐变过渡到伟晶岩,和以岩脉穿插在电气石花岗岩中(图 4e)。但琼嘉岗锂辉石伟晶岩深部垂向上并未发现有大型花岗岩岩基出露,其母体花岗岩的地质特征尚待查证。

图 1 喜马拉雅淡色花岗岩分布示意图(据潘桂棠等,2004刘志超等,2020) Fig. 1 Distribution map of the Himalayan leucogranite (modified after Pan et al., 2004; Liu et al., 2020)

图 2 喜马拉雅琼嘉岗伟晶岩型锂矿地质简图 Fig. 2 Simplified geological map of the Qongjiagang lithium pegmatite deposit, Himalaya, Tibet

图 3 琼嘉岗锂辉石伟晶岩剖面图及采样位置 Fig. 3 Cross-section of the Qongjiagang lithium pegmatite deposit, Himalaya, Tibet, noted with the sample locations in this study

图 4 喜马拉雅琼嘉岗伟晶岩型锂矿野外露头照片 (a-c)为锂辉石伟晶岩侵入到围岩大理岩和弱矽卡岩化大理岩中;(d)为锂辉石产出于块体微斜长石+锂辉石带内;(e)电气石花岗岩中发育有暗色矿物条带(主要由电气石组成),后期伟晶岩脉穿切电气石花岗岩 Fig. 4 Photographs of the field outcrops at the Qongjiagang lithium pegmatite deposit, Himalaya, Tibet (a-c) Qongjiagang spodumene pegmatite intruded into the marble and skarn marble of the pre-Cambrian Rouqiecun Group; (d) Spodumene mainly occurred in the massive microcline+spodumene zone; (e) barren pegmatite dykes intruded into the tourmaline granite with the dark mineral bands (mainly fine-grained tourmaline)

琼嘉岗锂辉石伟晶岩结构分带(图 5)主要可分为细粒钠长石带(Ⅰ带)、分层细晶岩带(Ⅱ带)和块体微斜长石+锂辉石带(Ⅲ带),少数伟晶岩边部还出现有文象结构带,各结构带之间呈渐变过渡的关系。现简要概述各结构带的矿物组成:

图 5 喜马拉雅琼嘉岗伟晶岩型锂矿各结构带照片 (a、b)为锂辉石伟晶岩的典型分带特征,从边到核为细粒钠长石边部带、分层细晶岩带和含锂辉石主要分带块体微斜长石+锂辉石带;(c)锂辉石伟晶岩边部的分层细晶岩,可见暗色弯曲条带;(d)锂辉石主要产出于块体微斜长石+锂辉石带内(镜头方向近似平行于锂辉石长轴方向) Fig. 5 Photographs of internal zonation patterns of the Qongjiagang spodumene pegmatite (a, b) internal zonation patterns include saccharoidal albite zone, layered aplite zone and massive microcline+spodumene zone from rim to core; (c) the occurrence of the aplitic quartz+tourmaline layers, sometimes with curved bands; (d) spodumene grains mainly occurred in the massive microcline+spodumene zone (camera direction parallel to the long-axis of the spodumene minerals)

(1)Ⅰ带为细粒钠长石带,主要产于伟晶岩边部,与围岩交界处,以含有大量细粒钠长石、电气石和石榴石为特征。主要组成矿物包括:钠长石45%~50%(体积百分数,下同)、粒径为0.5~3mm;石英20%~25%,粒径为0.5~2mm;电气石5%~10%,粒径为0.5~2mm;白云母3%~5%,粒径为0.5~2mm;钾长石5%~10%,粒径为0.5~2mm;和石榴石(< 1%)。副矿物有锆石、锡石、铌钽铁矿族矿物等。

(2)Ⅱ带为分层细晶岩带(图 5a-c),特征为电气石与钠长石组成条带状产出于细粒钠长石带与块体微斜长石+锂辉石带之间,矿物组合为电气石、钠长石、石英、钾长石和少量石榴石,副矿物包括锆石、铌钽铁矿族矿物等,细粒钠长石带和分层细晶岩也可以交替出现在锂辉石伟晶岩边部。

(3)Ⅲ带为块体微斜长石+锂辉石带,是锂辉石伟晶岩的主体部分(图 5d),以块体微斜长石、和块体中的中-粗晶锂辉石、细粒石英和细粒钠长石集合体组成为特征,其中锂辉石边部可见细粒锂辉石和石英交生结构(简称为SQI结构,图 6a, b)。主要矿物包括:微斜长石40%~45%,粒径为5~10cm;青绿色锂辉石25%~30%,粒径通常10~15cm,以长柱状为主,少量短柱状和粒状;石英10%;细粒钠长石10%~15%;和白云母3%~5%。副矿物包括锆石、磷灰石、独居石、铌钽铁矿族矿物等(图 6c, d)。此外,在电气石花岗岩中还穿插有含绿柱石伟晶岩,其主要矿物组合为钾长石、钠长石、石英、白云母、电气石、石榴石和绿柱石。

图 6 琼嘉岗锂辉石伟晶岩中锂辉石及共生稀有金属矿物BSE显微图片 (a、b)由锂辉石与其边部的细粒锂辉石+石英共生结构(细粒锂辉石与石英交生结构,SQI为spodumene-quartz intergrowth缩写);(c)铌铁矿与锆石,其中铌铁矿可见震荡环带,锆石则呈现多孔结构;(d)锡石包裹不规则铌铁矿的矿物包裹体. 矿物缩写:Ab-钠长石;Col-Fe-铌铁矿;Cst-锡石;Kfs-钾长石;Mus-白云母;Qz-石英;Spd-锂辉石;Tur-电气石;Zrn-锆石 Fig. 6 BSE images and coexisted rare-metal minerals in the Qongjiagang spodumene pegmatites (a, b) show spodumene and fine-grained spodumene-quartz intergrowth texture (SQI) in its rim; (c) shows the oscillatory zoned columbite grain and spongy zircon; (d) shows irregular columbite-group minerals enclosed in the cassiterite. Mineral abbreviations: Ab-albite; Col-Fe-columbite; Cst-cassiterite; Kfs-K-feldspar; Mus-muscovite; Qz-quartz; Spd-spodumene; Tur-tourmaline; Zrn-zircon

本次岩石化学、同位素定年和同位素测试样品主要根据实测剖面进行连续取样(图 3),选取包括锂辉石伟晶岩三个结构带的特征性样品,具体样品详见表 1

表 1 琼嘉岗伟晶岩型锂矿样品特征和测试方法 Table 1 Characteristics of the analytical samples and methods in Qongjiagang lithium pegmatite deposit
2 测试方法

样品在粉碎粉末之前,选取体积较大、结构具代表性且矿物分布较为均匀的区域进行切割和粉碎,全样粉碎混合后获取所需测试样品开展研究。全岩主量元素、微量元素和稀土元素在澳实矿物实验室(ALS Minerals)分析获得,主量元素测试误差小于5%,微量元素相对误差优于10%,Li含量优于5%。分析结果见表 2

表 2 琼嘉岗锂辉石伟晶岩全岩主量(wt%)和微量(×10-6)元素成分 Table 2 Major (wt%) and trace (×10-6) element compositions of the Qongjiagang spodumene pegmatite

独居石和铌钽铁矿电子探针分析、独居石U-Pb定年及Nd同位素分析在武汉上谱分析科技有限责任公司完成。独居石和铌铁矿-铌锰矿族矿物成分利用JEOL-JXA8230电子探针完成。其加速电压为20kV,电子束电流2nA (铌铌锰矿)和5nA (独居石),束斑直径为3~5μm,每个元素数据采集时间20~40s。测试使用的标准样品为天然样品和人工合成氧化物,数据采用ZAF方法校正。分析结果见表 3表 4。独居石U-Pb定年和微量元素利用LA-ICPMS同时分析完成。GeolasPro激光剥蚀系统由COMPexPro 102 ArF 193nm准分子激光器和MicroLas光学系统组成,ICP-MS型号为Agilent 7700e。激光剥蚀过程中采用氦气作载气、氩气为补偿气以调节灵敏度,二者在进入ICP之前通过一个T型接头混合,激光剥蚀系统配置有信号平滑装置(Hu et al., 2015)。即使激光脉冲频率低达1Hz,采用该装置后也能获得平滑的分析信号,特别适用于高U含量样品的微区测试(Zong et al., 2015)。本次分析的激光束斑和频率分别为16μm和2Hz,并采用独居石标准物质44069和玻璃标准物质NIST610作外标分别进行同位素和微量元素分馏校正,利用独居石Trebilcock作为检测标样(272±4Ma,Tomascak et al., 1996),本次Trebilcock独居石获得206Pb/238U加权平均年龄为274.9±1.4Ma (n=6)。每个时间分辨分析数据包括大约20~30s空白信号和50s样品信号。分析数据离线处理采用软件ICPMSDataCal (Liu et al., 2008, 2010)完成,独居石样品的U-Pb年龄谐和图绘制和年龄加权平均计算采用Isoplot/Ex_ver3 (Ludwig, 2003) 完成,其中独居石的微量元素采用电子探针获得的Ce元素含量作为内标,分析点位与探针分析点位相同,分析结果见表 5表 6

表 3 琼嘉岗锂辉石伟晶岩中独居石代表性EPMA结果(wt%) Table 3 Representative EPMA results of monazites from the Qongjiagang spodumene pegmatite (wt%)

表 4 琼嘉岗锂辉石伟晶岩中铌钽铁矿族矿物代表性EPMA结果(wt%) Table 4 Representative EPMA results of columbite group minerals from the Qongjiagang spodumene pegmatite (wt%)

表 5 琼嘉岗锂辉石伟晶岩(样品21QJG16-8)中独居石U-Th-Pb同位素定年结果 Table 5 Geochronologic results of monazite U-Th-Pb isotopic compositions from the Qongjiagang spodumene pegmatite (Sample 21QJG16-8)

表 6 琼嘉岗锂辉石伟晶岩(样品21QJG16-8)中独居石LA-ICPMS微量元素(×10-6)测试结果 Table 6 Results of monazite LA-ICPMS trace-element compositions (×10-6) from the Qongjiagang spodumene pegmatite (Sample 21QJG16-8)

独居石微区原位Nd同位素比值测试利用激光剥蚀系统为Geolas HD(Coherent,德国),MC-ICP-MS为Neptune Plus(Thermo Fisher Scientific,德国)。激光剥蚀系统使用氦气作为载气。少量的氮气被加入到ICP以提高Nd同位素的测试信号(Xu et al., 2015)。分析采用单点模式,激光的束斑大小和剥蚀频率根据样品的Nd信号强度调节,激光能量密度固定在~10.0J/cm2,分析过程配备了和定年相同的信号平滑装置。143Nd/144Nd同位素仪器质量分馏校正通过指数法则校正,校正因子利用146Nd/144Nd=0.7219估算获得。144Sm对144Nd的干扰校正通过监控149Sm的信号,并选择144Sm/149Sm=0.2301。144Sm/149Sm的仪器质量分馏校正通过归一化到无干扰的147Sm/149Sm,并选择147Sm/149Sm=1.08680,详细的分析方法校正描述请参考(Xu et al., 2015)。全部分析数据采用专业同位素数据处理软件“Iso-Compass”进行数据处理(Zhang et al., 2020)。一个天然独居石标样GBW44069和一个天然榍石标样MKED1作为未知样品监控微区原位独居石Nd同位素校正方法的可靠性。GBW44069的化学组成和Nd同位素组成参见Xu et al. (2015),本次测试GBW44069和MKED1分别为0.512186±0.000014 (n=10)和0.511635±0.000027(n=12),在推荐值范围内。样品分析结果见表 7

表 7 琼嘉岗锂辉石伟晶岩(样品21QJG16-8)中独居石Sm-Nd同位素组成 Table 7 Monazite Sm-Nd isotopic compositions from the Qongjiagang spodumene pegmatite (Sample 21QJG16-8)

铌钽矿U-Pb同位素分析在北京燕都中实测试技术有限公司利用LA-ICP-MS完成。激光剥蚀系统为NWR193nm Ar-F准分子激光系统,ICP-MS为Analytikjena Plasma Quant MSQ电感耦合等离子质谱仪。铌钽矿U-Pb同位素定年中采用铌钽矿标样ZTa-01(in-house standard,未发表,265Ma)作外标进行同位素分馏校正,并利用Coltan139(Che et al., 2015; Melcher et al., 2015)做监控标样。采用NIST610做外标进行U,Pb含量计算。每分析8~10个样品点,分析一组标样NIST610,ZTa-01,Coltan139。Coltan139是来自马达加斯加的不分带的铌铁矿晶体(Gäbler et al., 2011; Che et al., 20152019),是德国联邦地球科学与自然资源研究所的内部参考物质,TIMS和LA-ICP-MS测试获得Coltan139的U-Pb年龄为506Ma (Melcher et al., 2015),本次测试得到Coltan139的206Pb/238U加权平均年龄为508.7±3.1Ma (n=15)。激光剥蚀过程中采用氦气作载气,由一个T型接头将氦气和氩气混合后进入ICP-MS中。每个采集周期包括大约20s的空白信号和50s的样品信号。测试激光束斑大小为45μm,能量密度1.8J/cm2,剥蚀频率为6Hz。各样品的207Pb校正后加权平均年龄计算及Tera-Wasserburg图件的绘制采用Isoplot(ver 3.0)程序(Ludwig, 2003)。数据结果见表 8

表 8 琼嘉岗锂辉石伟晶岩中铌钽铁矿族矿物U-Th-Pb同位素定年结果 Table 8 Geochronologic dating results of columbite group minerals by U-Th-Pb isotopic compositions from the Qongjiagang spodumene pegmatites
3 测试结果 3.1 岩石地球化学特征

岩石地球化学结果显示(表 2),琼嘉岗伟晶岩属于硅与铝过饱和岩石(SiO2: 66.86%~78.08%; Al2O3: 14.78%~19.53%),贫镁(0.01%~0.05%)、铁(0.03%~0.46%)、钛(< 0.01%)、钙(0.23%~0.71%)等元素,K2O(0.76%~6.95%)和Na2O(2.22%~7.36%)含量变化范围较大,主量元素较大变化可能与不同结构带或样品中钾长石、电气石等矿物含量不同有关。微量元素方面,利用K/Rb含量来比较Li、Cs、Nb和Ta元素的富集情况(图 7),整体K/Rb含量在相对相似的区间。研究显示,锂含量(图 7a)从边部细粒钠长石带(~100×10-6)到分层细晶岩带(~1000×10-6),再到锂辉石主要赋存的块体微斜长石带(>3000×10-6) 逐渐升高,而Cs含量(图 7b)则是从细粒钠长石和分层细晶岩带(>100×10-6),逐渐减少到块体微斜长石+锂辉石带中Cs含量小于100×10-6。研究并没有发现高含量Ta和高Ta/Nb比值在主要含锂辉石矿体样品中被记录下来(图 7c)。从Li、Be、Nb和Ta含量看(表 2),已有伟晶岩剖面样品的Li含量达到最低工业品位(除伟晶岩边部带),Be半数达到边界品位,Nb半数达到工业品位。

图 7 琼嘉岗伟晶岩K/Rb对Li (a)、Cs(b)和Ta/Nb(c)二元图解 Fig. 7 Geochemical variations of K/Rb against Li (a), Cs (b) and Ta/Nb (c) observed in the Qongjiagang pegmatite
3.2 独居石EPMA结果、U-Pb定年和单矿物Sm-Nd同位素

锂辉石伟晶岩(21QJG16-08)中独居石呈浅黄色,半自形到自形,主要呈短柱状,半自形到自形。BSE背散射图像揭示琼嘉岗伟晶岩中部分独居石具有复杂的内部结构,表现出明显的岩浆韵律环带特征。本次电子探针分析、独居石年龄和同位素测试主要挑选BSE图像较为均一区域进行测试。独居石EPMA结果显示(n=29),其具有较低的CaO(0.18%~3.57%)和SiO2含量(0.09%~1.37%);La2O3和Ce2O3的含量在18.50%~30.72%和20.36%~34.79%之间变化;而Y2O3含量最高可达0.81%;独居石的ThO2含量变化较大(1.89%~24.10%)。基于4个氧原子计算方法(Pyle et al., 2001)计算琼嘉岗独居石在2CePO4-CaTh(PO4)2-2ThSiO4体系中(Förster, 1998)端元占比(表 3),结果显示独居石Mo%(2CePO4)占比达63.1%~96.7%,属于Ce-独居石;Brb%(CaTh(PO4)2)占比为14.3%~29.8%;Hu%(2ThSiO4)占比为0.54%~7.13%(图 8a)。而从(U+Th+Si)-(REE +P+Y) (a.p.f.u)的元素替代关系图解可以看出(图 8b):Th和Si替代REE和P的关系为主,U和Si替代REE和P以及Ca和Th替代REE(Ca2++Th4+↔2REE3+)可能同时发生。同时,微量元素结果显示琼嘉岗伟晶岩中独居石大多表现出明显的四分组相应,并具有较强的Eu负异常(图 8c, d),显示独居石结晶于高演化特征的花岗伟晶岩熔体。

图 8 琼嘉岗伟晶岩独居石EPMA矿物地球化学成分图解 (a) 2CePO4-CaTh(PO4)2-2ThSiO4三元图解(底图据Förster, 1998, 根据4个氧原子结果和EPMA结果);(b)(U+Th+Si)-(REE+P+Y) (a.p.f.u)图解(分子式占比据16个氧原子结果和EPMA结果,图中元素替代关系来自Franz et al., 1996);(c)球粒陨石标准化微量元素配分图(标准化值据Sun and McDonough 1989);(d)Eu/Eu*-TE1, 3相关关系图 Fig. 8 Plots of monazite EPMA compositions from the Qongjiagang pegmatites (a) 2CePO4-CaTh(PO4)2-2ThSiO4 plot (base map after Förster, 1998; based on 4 oxygen atom and EPMA results); (b) (U+Th+Si) vs. (REE+P+Y) (a.p.f.u) plot (based on 16 oxygen atom and EPMA results, and elemental substitutions cited from Franz et al. 1996); (c) chondrite-normalization patterns of monazite trace-element compositions (normalizes values refers to Sun and McDonough, 1989); (d) Eu/Eu* vs. TE1, 3 plot

独居石U-Th-Pb同位素定年结果显示,在Tera-Wasserburg图解拟合线上22个测试点年龄下交点年龄24.5±1.1Ma (MSWD=0.66,图 9a),经过207Pb校正后206Pb/238U加权平均年龄为25.3±0.5Ma (MSWD=1.5,图 9b),与下交点年龄误差范围内一致,因此我们认为该年龄代表了琼嘉岗锂辉石伟晶岩的结晶年龄。独居石Nd同位素结果显示相对均一的147Sm/144Nd(0.06027~0.09816)和143Nd/144Nd(0.511873~0.511977)比值(图 10a, b)。我们利用25.3Ma计算出εNd(t)范围集中在-14.6~-12.5和tDM2在1856~2024Ma分布。

图 9 独居石U-Pb同位素Tera-Wasserburg图(a)和经过207Pb校正的206Pb/238U加权平均年龄图(b) Fig. 9 Monazite U-Pb Tera-Wasserburg curve (a) and weighted mean ages of 207Pb-corrected 206Pb/238U ages (b)

图 10 独居石147Sm/144Nd-143Nd/144Nd图解(a)和εNd(t)值直方图(b) Fig. 10 Monazite 143Nd/144Nd vs. 147Sm/144Nd plot (a) and histogram of εNd(t) values (b)
3.3 铌铁矿-铌锰矿矿物EPMA结果和LA-ICPMS U-Pb定年

琼嘉岗锂辉石伟晶岩中铌钽矿物以铌铁矿-铌锰矿为主(图 11),Ta/(Nb+Ta)值相对低,在0.04~0.22之间,Mn/(Fe+Mn)值变化相对较大,在0.29~0.77之间变化。背散射电子图像观察表明本区铌铁矿-铌锰矿具有明显的环带构造特征,且矿物呈现自形-半自形结构,亮暗相间,成分呈现周期性变化,可见环带平行于结晶生长面。而从单个颗粒元素线扫描的结果上看(图 12a, b),铌铁矿-铌锰矿中的Nb与Ta含量发生规律性变化,环带中亮暗对应于富Ta和贫Ta部分,而Fe和Mn含量在晶体生长过程中没有明显的变化。

图 11 琼嘉岗铌铁矿-铌锰矿的Ta/(Nb+Ta)-Mn/(Fe+Mn)图解 Fig. 11 Columbite Ta/(Nb+Ta) vs. Mn/(Fe+Mn) diagram for the Qongjiagang pegmatites

图 12 琼嘉岗铌铁矿-铌锰矿族矿物Fe、Mn、Nb和Ta元素线扫描图及对应BSE背散射图,显示明显的韵律环带结构 Fig. 12 Scan line of Fe, Mn, Nb and Ta elemental contents in columbite minerals from the Qongjiagang pegmatites and their BSE images showing oscillatory zonation

其中锂辉石伟晶岩边部的分层细晶岩样品(21QJG16-02),其铌铁矿-铌锰矿族的U-Pb同位素结果在Tera-Wasserburg图解拟合线上20个测试点年龄下交点年龄24.5±0.2Ma (MSWD=1.05,图 13a),与经过207Pb校正后206Pb/238U加权平均年龄为24.5±0.2Ma (MSWD=1.11,图 13b)在误差范围内一致。琼嘉岗锂辉石伟晶岩边部的钠长石细晶岩样品(21QJG16-03)结果显示,20个测试点年龄下交点年龄25.0±0.2Ma (MSWD=1.5,图 13c)与其207Pb校正后的206Pb/238U加权平均年龄(24.9±0.3Ma,MSWD=1.6,图 13d)。锂辉石伟晶岩(21QJG16-08)中20个测试点年龄下交点年龄24.3±0.2Ma (MSWD=1.17,图 13e)与其207Pb校正后的206Pb/238U加权平均年龄(24.2±0.2Ma,MSWD=1.2,图 13f)相一致。综上所述,琼嘉岗铌铁矿-铌锰矿族矿物U-Pb定年结果揭示琼嘉岗锂辉石伟晶岩稀有金属矿化年龄为在25.0~24.2Ma之间。

图 13 琼嘉岗锂辉石伟晶岩铌铁矿-铌锰矿U-Pb定年Tera-Wasserburg图解(a、c、e)和经过207Pb校正后206Pb/238U加权平均年龄图解(b、d、f) Fig. 13 Columbite +U-Pb Tera-Wasserburg curves(a, c, e) and weighted mean ages of 207Pb-corrected 206Pb/238U ages(b, d, f)for the Qongjiagang pegmatites
4 讨论 4.1 琼嘉岗地区锂辉石伟晶岩形式时代与源区特征 4.1.1 形成时代

伟晶岩中锆石常具高U含量,易发生蜕晶化甚至重结晶作用(图 6cTomaschek et al., 2003; Rayner et al., 2005),因此锆石U-Pb定年并不是稀有金属伟晶岩定年的好选择。而铌钽矿物常具有相对较高的U含量及低的普通Pb含量,且伟晶岩结晶过程中少有继承成因的铌钽矿物(Romer et al., 1996),这些特点使得利用铌钽矿物U-Pb定年获得稀有金属伟晶岩形成年龄成为可能(Romer, 2003; Smith et al., 2004; Melcher et al., 2008, 2015; Che et al., 2015, 2019; Zhou et al., 2021)。加之,独居石作为富含轻稀土副矿物可见于伟晶岩中,具有高U、Th和放射性成因Pb,低普通Pb含量,且其结晶后U-Th-Pb同位素体系基本上处于封闭状态(Parrish, 1990),是理想的U-Th-Pb定年对象。因此,利用独居石和铌钽矿物开展LA-ICP-MS U-Pb定年能够获得精度较高、可靠性较好的稀有金属伟晶岩成岩-成矿年龄。本次独居石、铌铁矿-铌锰矿单矿物U-Pb定年结果显示,琼嘉岗锂辉石伟晶岩形成年龄在25.3±0.5Ma~24.2±0.2Ma之间,说明琼嘉岗伟晶岩型锂矿形成于新喜马拉雅阶段早期(26~13Ma,吴福元等,2015)。该时期是喜马拉雅地区南北两带淡色花岗岩活动的高峰期(Martin et al., 2007; Streule et al., 2010),即琼嘉岗锂辉石伟晶岩的形成时间与大规模淡色花岗岩的岩浆活动同期,形成可能对应于喜马拉雅-青藏高原碰撞造山带的拆沉作用(吴福元等,2015),与典型的LCT型伟晶岩形成于同构造或后构造阶段的构造环境相一致(Černý, 1991; Martin and De Vito, 2005)。

相对于高喜马拉雅地区出露的淡色花岗岩,伟晶岩和稀有金属伟晶岩脉的规模较小,应具有更为快速的冷凝、上升与结晶过程(London, 2008)。本次研究不同结构带中铌铁矿-铌锰矿单矿物U-Pb年龄结果揭示(图 13),锂辉石伟晶岩从边部钠长石细晶岩到中间块体微斜长石伟晶岩,成矿年龄结果在24.9±0.3Ma到24.2±0.2Ma之间,与独居石年龄结果(25.3±0.5Ma)在误差范围内一致。这一结果表明琼嘉岗伟晶岩为快速冷却结晶的产物,与伟晶岩快速结晶模型的认识相一致(Morgan VI and London, 1999; Webber et al., 1999)。

4.1.2 源区特征

对于高分异花岗岩或稀有金属伟晶岩,其岩浆就位后的结晶过程中通常经历晚期熔-流体共存阶段(Černý et al., 2005),常表现在岩石-矿物地球化学组成中(如喜马拉雅带内然巴花岗岩体,刘志超等,2020),或在典型流体出溶结构里(如大兴安岭南段维拉斯托高分异花岗岩中单向固结结构,Shi et al., 2021)。同样的,稀有金属伟晶岩大多经历后期流体交代作用(London, 2018周起凤等,2019),其全岩地球化学组成很难反映花岗伟晶岩质岩浆的源区组成。因此,高分异花岗岩和稀有金属伟晶岩的同位素地球化学组成(如全岩Sr-Nd同位素组成),可能常会受流体作用的干扰影响对其源区的判别。但是,单矿物微区同位素分析技术让我们研究高分异花岗岩和稀有金属伟晶岩的源区特征成为了可能(Ayres and Harris, 1997; 曾令森等,2012; 王春龙等,2015),如锆石Hf同位素、磷灰石、独居石和磷钇矿等Sr-Nd同位素,这些方法为我们探讨高分异花岗岩和稀有金属伟晶岩的源区组成特点提供了重要依据。本次研究中琼嘉岗锂辉石伟晶岩中独居石Nd同位素显示εNd(t)集中在-14.6~-12.5,tDM2在1856~2024Ma,该值与高喜马拉雅结晶岩系的Nd同位素组成一致(总结于吴福元等,2015)。已有研究显示,喜马拉雅淡色花岗岩常具有较大Sr-Nd同位素组成,但在整体上与高喜马拉雅结晶岩系相近(吴福元等,2015)。这说明琼嘉岗锂辉石伟晶岩的源区物质组成与喜马拉雅淡色花岗岩类似,可能为高喜马拉雅的变泥质岩发生部分熔融形成。

通常来说,造山带中的LCT型伟晶岩(指富集Li、Cs、Ta、Be、Ga、Sn等稀有金属伟晶岩类型,Černý, 1991),其源区被认为是太古宙结晶基底以及前寒武微陆块的壳源物质活化而成(Walker et al., 1986),源岩即为变泥质岩及变质浊积岩序列(Meintzer, 1987)。而在变泥质岩中粘土矿物及云母类矿物相对富集Li、Be、Rb、Cs、Sn等亲石元素,碎屑物中的黑云母、绿泥石等矿物可能为Nb、Ta的主要来源(Acosta-Vigil et al., 2003; Martin and De Vito, 2005)。因此,花岗岩-伟晶岩岩浆系统中稀有金属元素含量的高低通常被认为与部分熔融及后续分离结晶过程密切相关(Linnen et al., 2012)。在地壳岩石Li(平均含量为~20×10-6)、Cs(~3×10-6)和Rb(~49×10-6)含量的背景下(Rudnick and Gao, 2014),源区中的云母含量通常控制熔体中的Li、Rb、Cs的含量组成(London, 2005),而源区的不一致熔融过程对Li元素的聚集起到重要作用(London, 2005)。但是角闪石的分解对提高部分熔融熔体中的Li含量贡献有限(Brenan et al., 1998),这可能解释了在北喜马拉雅地区早期~44Ma与角闪岩部分熔融有关的高Sr/Y淡色花岗岩(打拉岩体,谢克家等,2010)中未有含锂矿物的报道这一现象。而富白云母的泥质岩发生含水部分熔融时,由于泥质岩组成与最小熔体组成相差甚远(Icenhower and London, 1995),仅能形成少量熔体,并且源区中水含量有限,使得升温过程中容易发生脱水部分熔融,进而形成大量高Or#的碱性长石。虽然这个过程并不能富集Rb,但是可以形成富Li和Cs的熔体,进而形成LCT型矿床(Simmons et al., 2016)。这样(变)泥质岩部分熔融的模式符合我们本次对琼嘉岗锂辉石伟晶岩源区特征的认识,结合本次琼嘉岗锂辉石伟晶岩(~24Ma)中独居石Nd同位素组成,我们认为,高喜马拉雅地区琼嘉岗伟晶岩型锂矿的岩浆源区可能为前寒武系变泥质岩。

4.2 琼嘉岗地区伟晶岩型锂矿中伟晶岩分异特征

稀有金属伟晶岩的分异特征主要体现在伟晶岩分带结构、各结构带矿物组成及其结构特征、以及岩石-矿物地球化学成分上(Černý, 1991;London, 2005周起凤等, 2013, 2019何畅通等,2020秦克章等,2021b)。

分带特征琼嘉岗锂辉石伟晶岩具有明显的伟晶岩分带特征(图 5a, b),且特征性长柱状锂辉石主要赋存于块体微斜长石带中,但目前该区没有观察到分异程度更高的伟晶岩结构带,锂辉石主要赋存在块体微斜长石带中。通常来说,大多数含锂伟晶岩具有典型的分带结构,如可可托海三号脉中(邹天人和李庆昌,2006)的含锂矿物透锂长石最早出现在石英-白云母带中(Ⅳ号),锂辉石则主要出现在钠长石-锂辉石带(Ⅴ号)和石英-锂辉石带(Ⅵ号)。少数锂辉石伟晶岩则具有较弱的分带特征或无分带,也常缺少单向固结结构、矿物分层或者空间结构与矿物分带(Černý and Ercit, 2005),如美国北卡罗来纳州King's mountain伟晶岩中锂辉石主要产出在细晶岩中的囊状伟晶岩内(Swanson, 2012),爱尔兰Leinster钠长石-锂辉石伟晶岩中锂辉石呈纤维状和板状与石英、斜长石、微斜长石和白云母共生(Barros et al., 2020)和甲基卡细晶-微晶锂辉石赋存在碱长花岗岩中(付小方等,2021)。因此,琼嘉岗锂辉石伟晶岩的分带特征和矿石结构表明这种呈囊状体、板状体赋存于地层中的锂辉石伟晶岩可能与弱分带的钠长石-锂辉石伟晶岩更为相近(Černý and Ercit, 2005)。

矿物组成琼嘉岗独居石具有较高TE1, 3值说明其结晶于分异程度较高的花岗伟晶岩熔体(图 8d)。其铌钽铁矿族矿物的WO3含量与Ta/(Nb+Ta)值呈轻微正相关(图 14a),即随Ta/(Nb+Ta)值增大,WO3含量升高,表明W元素倾向富集于残余熔体。而TiO2含量与Mn/(Fe+Mn)值正相关,即Mn/(Fe+Mn)值轻微升高,导致TiO2含量迅速升高。伴随着伟晶岩演化,铌铁矿-钽铁矿族矿物成分通常向富Ta和富Mn的端元演化(Badanina et al., 2015; Melcher et al., 2015)。其中,铌钽铁矿的Ta/(Nb+Ta)值是伟晶岩重要的分异程度指示标志。岩浆分异程度愈高,铌钽铁矿的Ta/(Nb+Ta)值愈高。而含锂伟晶岩中铌钽铁矿的Mn/(Fe+Mn)值通常较无矿伟晶岩高,如在东秦岭卢氏伟晶岩中含锂伟晶岩Mn/(Fe+Mn)值通常大于0.6,无矿伟晶岩Mn/(Fe+Mn)值则在0.2~0.6之间变化(Zhou et al., 2021)。琼嘉岗锂辉石伟晶岩中铌铁矿和铌锰矿的Ta/(Nb+Ta)普遍小于0.2(图 14a),Mn/(Fe+Mn)值则在0.3~0.8之间变化(图 14b),揭示出锂辉石伟晶岩中铌钽铁矿向着富Mn组分的方向演化。铌钽铁矿的这种低Ta含量特征与伟晶岩全岩地球化学显示的低Ta高Nb的含量特征相一致。而在显微尺度下,琼嘉岗伟晶岩中铌钽铁矿共生矿物主要是微斜长石和钠长石(图 6d),代表花岗伟晶质岩浆的分异程度较弱。

图 14 琼嘉岗锂辉石伟晶岩中铌铁矿-铌锰矿族矿物Ta/(Nb+Ta)-WO3 (a)和TiO2-Mn/(Fe+Mn) (b) Fig. 14 Ta/(Nb+Ta) vs. WO3 (a) and TiO2 vs. Mn/(Fe+Mn) (b) plots of columbite from the Qongjiagang spodumene pegmatite

地球化学成分研究显示,富Li伟晶岩平均Li2O含量为0.5%(Stewart, 1978),且含水花岗质熔体中Li2O含量达到~1.5%时,熔体中开始饱和透锂长石、锂辉石或锂霞石(Maneta et al., 2015)。而在高分异花岗岩和伟晶岩形成过程中,不同矿物的分离结晶对稀有金属元素的富集过程有重要影响。高分异花岗岩往往伴随分离结晶作用不断进行,岩浆中的水、Li、B、F、Cl、P等挥发分组分不断增加,进而出现锂云母、透锂长石、锂辉石、绿柱石、电气石、铌铁矿-铌锰矿族等矿物(London, 2008)。琼嘉岗锂辉石伟晶岩中Li2O含量在0.02%~2.96%之间,满足前人对于熔体分异出含锂矿物和其他稀有金属矿物的Li含量。而LCT型稀有金属伟晶岩通常认为是由低温、富水的花岗质熔体分异形成的(Černý et al., 2005),且往往黑云母和白云母的结晶作用使得熔体容易富集Ta(Raimbault et al., 1995; Kovalenko et al., 1996; Stepanov et al., 2014),独居石和锆石的分离结晶作用使LCT型伟晶岩常具有低Zr含量、低Zr/Hf比值、较低的LREE含量、Nd负异常的REE配分型式(Zaraisky et al., 2009; Stepanov et al., 2012)。从地球化学结果看(表 2),琼嘉岗成矿伟晶岩REE总量为3.36×10-6~10.5×10-6,LREE含量1.99×10-6~7.20×10-6,Zr含量在3×10-6~35×10-6,Zr/Hf在7.50~11.1之间,且锂辉石所在块体微斜长石带的Zr/Hf比值低于边部带(表 2),表明独居石和锆石的分离结晶作用控制熔体组成更为明显;Li含量从边部带到中间带的增减,表明成矿元素随着分离结晶作用不断富集(图 7a);而高Li元素能够增加铌铁矿和钽铁矿的溶解度(Linnen, 1998),并且在高分异花岗岩中对于岩浆分异和Nb-Ta的富集起着重要(Van Lichtervelde et al., 2010),但琼嘉岗的Ta/Nb比值在0.26~0.50(表 2),表明Ta并没有发生明显的富集作用(图 7c)。这可能与源区发生高温深熔作用有关,因为高温熔体中高Ti-低含水量特征使得源区主要富集Nb-Ta矿物为含钛氧化物,使得熔体贫Ta(Ta相对于Nb强烈分配到含钛氧化物中,Nash and Crecraft, 1985; Xiong et al., 2011)。这些都需要进一步开展工作得以确认。

4.3 琼嘉岗伟晶岩型锂矿的研究意义

琼嘉岗伟晶岩型锂矿系喜马拉雅首例具有工业价值的伟晶岩型锂矿,估算规模可望成为继南疆白龙山和川西甲基卡之后的我国第三大锂矿(秦克章等,2021a)。琼嘉岗锂辉石伟晶岩脉(带)与青藏高原西北缘的西昆仑地区白龙山锂矿在矿体产出、分带结构特征和矿体规模有较大不同。白龙山含锂辉石伟晶岩脉带近3.75km长,165m宽(王核等,2017),且巨大的锂辉石晶体(粒径可达数米)主要产出在内部叶钠长石-锂辉石带(Ⅴ带)和石英-锂辉石带(Ⅵ带)(Yin et al., 2020)。但琼嘉岗锂矿主要的长柱状或细粒状锂辉石产出于块体微斜长石+锂辉石带和分层细晶岩带中,串珠状、囊状伟晶岩体组成的伟晶岩带沿走向可延伸近千米。琼嘉岗同世界上典型LCT型伟晶岩的规模和分带特征相比仍具有一定规模。例如,同属钠长石-锂辉石类的美国King's mountain伟晶岩具有产状近乎直立的8条锂辉石伟晶岩脉,且单矿体宽达90m,长度延伸可达千米(Swanson, 2012)。与一些具极度分异特征伟晶岩的规模也具可比性,如加拿大Tanco矿床(透锂长石亚类型)具有较大规模(长1520m,宽106m,厚近百米)、近水平产出的伟晶岩矿体(Stilling et al., 2006),但琼嘉岗的分带性远弱于Tanco矿床,后者从边部到核部具有9个结构分带,中间可到铯榴石和锂云母带。因此,总结喜马拉雅琼嘉岗锂辉石伟晶岩的特点是具弱分带性,以长柱状锂辉石的产出在块体微斜长石+锂辉石带为特征。

从形成时代上,琼嘉岗锂辉石伟晶岩脉群的形成时代(25~24Ma)表明其形成于喜马拉雅-青藏高原碰撞造山带的拆沉环境。由于世界上大多数LCT型伟晶岩产于太古宙或古元古代的碰撞造山带中(Černý, 1991;London, 2005),少数产于年轻的弧-陆碰撞造山带内(意大利Elba伟晶岩形成于6.9Ma,Dini et al., 2002),一般为造山带伸展垮塌阶段之前(Romer et al., 1996)或构造转换阶段(如碰撞-后碰撞向走滑-伸展转换,Zagorsky et al., 2014)。琼嘉岗锂矿形成年代学的厘定为LCT型伟晶岩形成环境和潜在伟晶岩成矿带增加了新的实例,证实喜马拉雅新生代碰撞造山带具有找寻LCT型伟晶岩矿床的潜力。

琼嘉岗锂矿作为喜马拉雅代表性伟晶岩型锂矿,目前研究程度尚浅,仍有如下几个问题需要进一步开展工作,以期对喜马拉雅伟晶岩型锂矿的形成过程进行更深入研究:1)区域藏南拆离系(STDS)在矿区与矿田尺度如何控制琼嘉岗锂辉石伟晶岩脉(带)的侵位、分布及后期改造-破坏;2)锂辉石伟晶岩深部分带特征、是否存在母体花岗岩以及母体花岗岩的性质;3)填图区域内成矿伟晶岩和无矿伟晶岩在矿物组成、岩石化学和结构分带上的差异,建立高喜马拉雅锂辉石伟晶岩含矿性评价标志,并针对赋矿围岩前寒武系肉切村群内的其它高海拔侵位高度的伟晶岩进行调研和评价。

5 结论

本次研究揭示了喜马拉雅淡色花岗岩带中伟晶岩型锂矿的成矿特征、形成时代和源区特征,并取得如下主要结论和认识:

(1) 琼嘉岗矿区矿石矿物主要为锂辉石、铌铁矿-铌锰矿、少量锡石和绿柱石,特征性长柱状锂辉石主要产于块体微斜长石+锂辉石带和分层细晶岩带内;

(2) 琼嘉岗锂辉石伟晶岩各结构分带的K/Rb含量较为相似,锂含量从边部细粒钠长石带(~100×10-6)到分层细晶岩带(~1000×10-6),再到块体微斜长石+锂辉石带(>3000×10-6)逐渐升高,而Cs含量逐渐降低;

(3) 独居石和铌钽铁矿族矿物LA-ICPMS定年结果显示,琼嘉岗锂辉石伟晶岩形成于新喜马拉雅阶段早期(25~24Ma),与高喜马拉雅地区淡色花岗岩时代相近,形成时代早于错那洞等矿化伟晶岩;

(4) 琼嘉岗锂辉石伟晶岩结晶于高度演化的花岗伟晶岩熔体,源区特征与高喜马拉雅结晶岩系一致;

(5) 琼嘉岗锂矿是喜马拉雅首例具有工业价值的伟晶岩型锂矿,矿床类型为简单伟晶岩型,揭示其成矿特征与形成时代将为高喜马拉雅其它地区具有找寻大型花岗伟晶岩型锂矿提供借鉴。

致谢      喜马拉雅地区稀有金属伟晶岩研究工作一直受到吴福元院士的关心与指导;成文过程得到了中国科学院青藏高原研究所李金祥研究员和中山大学刘志超副教授的帮助与启发。审稿人的宝贵修改意见和建议,以及责任编辑的精心修改,使文章得以完善。在此一并致以诚挚的谢意。

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