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川西甲基卡花岗伟晶岩型稀有金属矿床因其巨量锂资源(189万t)而世界闻名(Wang Denghong et al.,2013, 2016;Hao Xuefeng et al.,2015;Liu Shanbao et al.,2019;Fu Xiaofang et al.,2021),前人已在区域内代表性伟晶岩脉的地质背景(Tang Guofan et al.,1984;Li Jiankang et al.,2007;Liu Shanbao et al.,2019;Yang Yueqing et al.,2020)、地球化学特征(Liu Lijun et al., 2017; Li Mingze et al., 2018)、成矿时代(Wang Denghong et al.,2005;Dai Hongzhang et al.,2018)、成矿流体性质(Li et al.,2009, 2013a, 2013b;Li Shenghu et al.,2015;Li et al.,2017)方面开展多年研究并积累了重要资料,但矿物学研究始终相对欠缺(Dai Hongzhang et al.,2018)。308号脉是区内出露面积最大的伟晶岩脉,为一钠长石-锂辉石伟晶岩脉,露头好,分带性相对完善。目前对于308号脉的勘查工作仅达预查程度,综合研究更为薄弱,对伟晶岩脉的成岩成矿及岩浆-热液演化过程还缺乏深入认识。
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已有大量资料显示云母是花岗伟晶岩稀有金属成矿作用的标志性矿物(Zhou Qifeng et al.,2013;Wang Rucheng et al.,2018),电气石也可以用来示踪伟晶岩岩浆-热液演化过程(Henry et al.,1985;Roda et al.,1995;Keller et al.,1999;Novák et al.,1999;Selway et al.,1999;Zhang et al.,2004),同时,磷灰石和锂辉石的化学成分变化都能够指示体系内环境的演变(London et al.,1982a;Charoy et al.,2001;Dias et al.,2019)。本文通过建立这几种矿物的结晶和演变历史,来分析308号伟晶岩脉初始熔体和晚期流体的性质,并探索308号伟晶岩脉的岩浆-热液演化过程,这对于进一步认识甲基卡地区伟晶岩、甚至世界稀有金属(富锂)伟晶岩的结晶分异过程中稀有金属矿物的行为、矿物之间的化学再平衡以及稀有金属伟晶岩的演化过程,都具有重要意义。
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1 地质背景
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甲基卡稀有金属矿床位于四川省西部康定、雅江和道孚三县交界处,地处青藏高原东缘松潘-甘孜造山带主体的东南侧。区内主要出露三叠系西康群砂板岩,经区域变质和接触变质作用后形成一套中浅变质岩系,包括有二云母片岩、黑云母石英片岩以及一系列具有特征变质矿物(矽线石、黑云母、红柱石、十字石、石榴子石等)的云母片岩(Pan Meng,2015)。区内断层、裂隙均发育,甲基卡背斜是区内的主要构造,背斜轴附近发育纵向张裂带、两翼发育共轭性扭裂群,是区内伟晶岩的主要控脉构造(Li Shihong,2019)。马颈子二云母花岗岩是区内出露的唯一岩体,位于甲基卡矿区中南部,侵位于甲基卡背斜轴部南段(图1),随后发生不同程度的热接触变质作用,使地层中形成以侵入体为中心的巴罗氏变质带(Tang Guofan et al.,1984)。甲基卡矿区的各类伟晶岩脉在空间分布上也与马颈子岩体关系密切,在水平和垂直方向上大致围绕岩体呈近同心状带状分布(Tang Yi,2016),具有良好的区域分带性,从内向外依次分布五种类型:微斜长石型(Ⅰ)→微斜长石钠长石型(Ⅱ)→钠长石型(Ⅲ)→锂辉石型(Ⅳ)→锂云母(或白云母)型(Ⅴ)伟晶岩(Tang Guofan et al.,1984;Wang Denghong et al.,2017b)。
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2 308号伟晶岩脉
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308号脉产于马颈子岩体(二云母花岗岩)舌前缘部位(图1),距岩体约1.5km。主要受控于区内一较大的近南北向纵向张裂带(Tang Yi,2016),亦受北东、北西向两组剪裂隙控制,在几组裂隙的交叉处形成膨大体(Dai Hongzhang et al.,2018)。矿脉东侧向东倾斜(倾角76°~89°),西侧向西、南西倾斜(倾角25°~57°),向北逐渐变为上下盘均向西倾斜(倾角60°~80°),于北端尖灭(Dai Hongzhang et al.,2018)。矿床内出露的地层为三叠系上统新都桥组(T3xd),主要由泥岩、粉砂岩和砂岩等经区域变质作用形成的十字石片岩组成,其余为第四系所覆盖。308号伟晶岩脉与围岩接触界线清楚,未见冷凝边,围岩与伟晶岩接触部位发生堇青石化。308号伟晶岩脉勘探程度低,现有资源储量尚不明确。
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308号伟晶岩脉主要可分为五个结构带,本文选取308号脉中部偏南分带性较好的露头位置进行剖面测量(剖面位置见图1),并参照前人(Tang Guofan et al.,1984;Wang Denghong et al.,2017a, 2017b;Dai Hongzhang et al.,2018)资料,依据矿物共生组合及含量将308号伟晶岩脉分为:(Ⅰ)细粒白云母-钠长石-石英带,此带为一薄且不连续的边部带。钠长石约占25%,多为1~5mm,在正交偏光下常显示卡钠复合双晶(图2,Ⅰ带);石英粒径稍粗,多为3~5mm,约占55%;白云母粒径常小于1mm,约占15%;电气石在单偏光下呈灰蓝色—淡紫色,常具有颜色环带,粒径范围0.1~1mm,大部分小于0.5mm,在岩石中的分布不均匀;另有少量副矿物磷灰石、锡石。(Ⅱ)中粗粒钠长石-石英-微斜长石带,微斜长石粒径常大于1cm,最大可达6cm,约占40%,颗粒内部可见少量云母或钠长石出溶;石英粒径多大于5mm,约占40%(图2,Ⅱ带);钠长石粒径多在3mm左右,约占12%;云母较少(3%),一般粒径在0.5mm左右;副矿物以磷灰石为主。(Ⅲ)中粒电气石-钠长石-石英带,黑电气石多见(＜10%),粒径差异大(1~2cm),最大可达4cm(图2,Ⅲ带),正交偏光下多显示灰蓝色;石英粒径5~10mm,约占35%;钠长石粒径2~8mm,约占25%;白云母粒径多小于5mm,含量较少(7%)、分布不均;副矿物绿柱石少量产出,粒径常小于3mm,成分环带不明显;副矿物还包括有铌铁矿族矿物(＜0.1mm)、磷灰石和锡石等。(Ⅳ)中粗粒锂辉石-钠长石-石英带,锂辉石约占15%,颗粒粒径变化大(3~10cm)(图2,Ⅳ带),亦见有10cm以上者,裂隙常发育,其内部石英、云母等包裹体丰富,边缘局部可见次生“毛发状”锂辉石-石英共生体;钠长石多为5~15mm,约占30%;微斜长石约占7%,粒径1~10mm;石英颗粒大部分小于1cm,含量约35%;另有少量副矿物石榴子石、磷灰石和锡石。(Ⅴ)中细粒锂辉石-钠长石-石英带,此带内矿物粒度变化较大,锂辉石5~15mm,含量更高(20%),常具裂隙(图2,Ⅴ带);钠长石5~15mm,约占30%;石英颗粒多为5~10mm,亦见由2mm单个颗粒组成的石英集合体,含量约35%;微斜长石较少(5%);白云母多为3~5mm,含量约10%。带与带之间多呈渐变过渡(图2)。308号伟晶岩脉内部结构分带特征见图1。
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图1 甲基卡矿床地质简图(据Li et al.,2017)
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Fig.1 Simplified geological map showing the distribution of pegmatite in the Jiajika deposit, western Sichuan, China (after Li et al.,2017)
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1 —二云母花岗岩;2—微斜长石伟晶岩;3—微斜长石钠长石伟晶岩;4—钠长石伟晶岩;5—钠长石锂辉石伟晶岩; 6—钠长石锂云母(白云母)伟晶岩;7—三叠系;8—伟晶岩区域分带界线;9—伟晶岩类型分带编号;10—伟晶岩脉号
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1 —Two-mica granite; 2—microcline pegmatite; 3—microcline-albite pegmatite; 4—albite pegmatite; 5—albite-spodumene pegmatite; 6—albite lepidolite pegmatite; 7—Triassic System; 8—boundary line of regional zonation of the pegmatites; 9—serial number of regional zonation of the pegmatites; 10—pegmatite dyke number
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3 样品及测试方法
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野外采集308号伟晶岩脉的6件代表性样品,包括中间带(Ⅳ)样品2件,其余带(Ⅰ、Ⅱ、Ⅲ、Ⅴ)样品各1件。
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伟晶岩样品中矿物的电子探针分析在中国地质科学院矿产资源研究所电子探针实验室完成,采用JXA-8230电子探针和X-max能谱仪,测试条件为加速电压15kV,探针电流20nA,电子束直径为5 μm,极小的矿物使用聚焦电子束。测试使用的标准样品为天然样品和人工合成氧化物,包括石英(Si)、斜长石(Na)、硬玉(Al)、赤铁矿(Fe)、磷灰石(Ca)、钾长石(K)、金云母(F)、石盐(Cl)、金红石(Ti)、镁铝榴石(Mg)、铯榴石(Cs)、天青石(Sr)、铌酸锂(Nb)、钽酸锂(Ta)和MnTiO3(Mn)。Cs、Sr、Nb、Ta特征峰的测定时间设定为20s,其他元素特征峰的测定时间设定为10s,相应的所有元素背景测定时间设定为5s。绝大部分元素在电子探针分析时的检测限为40×10-6~200×10-6。所有数据经过ZAF校正。
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图2 甲基卡308号伟晶岩剖面图及不同结构带代表性样品特征
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Fig.2 Profile of the No.308pegmatite and characteristics of representative samples of different structural zones in the Jiajika deposit
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Ⅰ—细粒白云母-钠长石-石英带;Ⅱ—中粗粒钠长石-石英-微斜长石带;Ⅲ—中粒电气石-钠长石-石英带;Ⅳ—中粗粒锂辉石-钠长石-石英带;Ⅴ—中细粒锂辉石-钠长石-石英带;Ab—钠长石; Mc—微斜长石;Ms—白云母;Qz—石英;Spd—锂辉石;Tur—电气石
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Ⅰ—Fine grained muscovite-albite quartz zone; Ⅱ—medium-coarse grained albite-quartz-microcline zone; Ⅲ—medium grained tourmaline-albite-quartz zone; Ⅳ—medium grained spodumene-albite-quartz zone; Ⅴ—medium-fine grained spodumene-albite-quartz zone; Ab—albite; Mc—microcline; Ms—muscovite; Qz—quartz; Spd—spodumene; Tur—tourmaline
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白云母和锂云母化学式根据24个阴离子计算,Li2O计算方法据Tischendorf et al.(1997,1999),H2O计算方法依据Tindle et al.(1990);磷灰石化学式根据14个阴离子计算;电气石化学式根据31个阴离子计算,假设H2O、B2O3、Li2O的含量都符合化学计量(详见Zhang et al.,2004);锂辉石化学式根据6个氧原子计算,Li2O计算方法据Ca+Na+K+Li=1。
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4 分析结果
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4.1 云母
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云母是甲基卡308号脉中的贯通矿物,以不同比例产出于各分带中。本文对308号脉Ⅰ~Ⅴ带样品进行了详细的电子探针分析,结果表明Ⅰ~Ⅳ带中云母类型以白云母为主,仅在Ⅴ带出现锂云母;从Ⅰ带至Ⅴ带,云母类型和化学成分存在过渡变化。
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4.1.1 云母类型和结构特征
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本文以Neiva(2013)云母分类为参考,根据云母矿物八面体位(Y site)阳离子占有率进行投图,确定云母种类(图4)。结果显示,Ⅰ~Ⅳ带中云母矿物均属于白云母—含锂白云母(图4),其粒度变化较大,从10 μm至厘米级不等(图3a、b),常呈半自形—自形片状产于钠长石中,成分均一,背散射下不显示环带结构;Ⅳ带中白云母与钠长石接触部位发育蠕虫状石英,并有晚期磷灰石沉淀(图3b);Ⅴ带中的云母矿物除白云母-含锂白云母外,常见有白云母的富锂蚀变边(即次生云母矿物),产于其与锂辉石的接触边缘,这种云母在成分上可属于“混合类型”云母或锂云母(图4),其成因可能与富锂流体交代原生白云母相关。这三种类型的云母在背散射图像上容易区分,它们显著的亮度差异主要由铁锰含量的多寡导致:白云母—含锂白云母最暗,锂云母最亮,具有最高含量的FeO和MnO,而“混合类型”云母与锂云母类似,FeO和MnO含量较锂云母稍低,因此在背散射图像上暗于锂云母(图4c、d)。
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4.1.2 云母成分变化
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308号脉不同结构带原生白云母所含有的稀碱元素中:Li2O含量(图5b)在Ⅰ~Ⅲ带中均很低(均值分别为0.07%、0.01%、0.01%),Ⅳ带中略有增高(0.05%),Ⅴ带中原生白云母的Li2O含量具有最大值(0.24%),且变化范围大(0.04%~0.48%);Rb2O(图5d)在I带和II带具有相似的范围和均值(Ⅰ带Rb2O 0.03%~0.35%、均值为0.17%,Ⅱ带Rb2O 0~0.25%、均值为0.15%),Ⅲ带原生白云母中Rb2O含量多低于检出限,Ⅳ带和Ⅴ带则Rb2O含量均有提高(Ⅳ带Rb2O 0~0.53%、均值为0.21%,Ⅴ带Rb2O 0.04%~0.40%、均值为0.25%);Ⅰ~Ⅳ带原生白云母中的Cs2O含量均较低(Ⅱ带Cs2O均值0.23%＞ Ⅰ带Cs2O均值0.14%,但Ⅱ带Cs2O的中值更低,见图5f;Ⅱ带Cs2O均值0.01%;Ⅳ带Cs2O均值0.07%),至Ⅴ带Cs2O均值有了显著提高(0.48%),其变化范围也较大(0~1.42%)。
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图3 甲基卡308号脉各分带云母矿物背散射电子图像
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Fig.3 Backscattered electron images of mica minerals in different zones of the Jiajika No.308pegmatite
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(a)—Ⅰ带白云母,成分均一;(b)—Ⅳ带厘米级白云母,锂辉石与之接触的边缘存在溶解现象;(c)—V带白云母,其内部成分不均一,并具有不规则含锂白云母-锂白云母或锂白云母-锂云母蚀变边;(d)—Ⅴ带白云母,具有不规则锂白云母-锂云母蚀变边; Ab—钠长石;Ap—磷灰石;Lpd—锂云母;Mi-F—混合类型云母;Ms—白云母;Qz—石英;Spd—锂辉石
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(a)—Chemically homogeneous muscovite in zone Ⅰ;(b)—centimeter-scale muscovite in zone Ⅳ, the contacting spodumene shows characteristics of dissolution on its rim; (c)—chemically heterogeneous muscovite in zone Ⅴ, with irregular alteration rims of lithian muscovite-trilithionite or trilithionite-lepidolite; (d)—muscovite in zone Ⅴ, with irregular alteration rims of trilithionite-lepidolite; Ab—albite; Ap—apatite; Lpd—lepidolite; Mi-F—mixed forms; Ms—muscovite; Qz—quartz; Spd—spodumene
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不同结构带原生白云母的主要元素组成中,SiO2(图5a)、Al2O3(图5c)和K2O(图5e)在不同分带中的平均含量呈现波动变化:SiO2在Ⅰ带和Ⅱ带中含量稍低(平均含量分别为46.84%和45.17%),在Ⅲ~Ⅴ带中逐渐增高(平均含量分别为46.88%,47.04%和48.68%);Al2O3则呈现出I~Ⅲ带中增高(平均含量分别36.33%, 37.89%和37.64%)、Ⅳ带和Ⅴ带中降低(平均含量分别36.87%和34.97%)的趋势,不过Ⅴ带中原生白云母的中值略高于Ⅳ带(图5c),且Al2O3含量变化范围显著大(Al2O3含量27.89%~39.32%);K2O在Ⅰ带和Ⅴ带中变化范围大(Ⅰ带6.68%~10.96%,Ⅴ带7.26%~11.09%),Ⅰ带原生白云母的K2O含量相对最低(均值9.06%),Ⅱ带相对最高(均值10.27%),并且Ⅱ~Ⅴ带原生白云母的K2O含量平均值整体呈下降趋势(平均含量分别9.64%,9.15%和9.08%)。其余主量元素MnO、MgO、CaO和Na2O在不同结构带白云母中的含量均较低(＜0.6%),此外FeO含量稍高,其在不同结构带内变化趋势与Rb2O类似(Ⅰ~Ⅴ带白云母FeO含量均值分别为1.28%,0.67%,0.56%,1.72%和1.76%)。Ⅱ带和Ⅲ带中原生白云母主量元素较窄的变化范围可能由样品中白云母含量少导致,但不影响其代表性。
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图4 甲基卡308号脉各分带云母矿物类型投图 (底图据Jolliff et al.,1987;Neiva,2013)
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Fig.4 Classification of micas (after Jolliff et al., 1987; Neiva, 2013) in different zones of the Jiajika No.308pegmatite
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308号脉中仅Ⅴ带发现有次生锂云母,Ⅴ带次生云母的稀碱元素和挥发分含量与其他原生白云母相比显著增高:Rb2O含量0.35%~0.78%、均值0.48%,Cs2O含量0.84%~2.12%、均值1.77%,Li2O含量计算值3.93%~5.27%、均值4.57%,F含量3.53%~8.28%、均值5.97%;SiO2含量增高(范围46.99%~51.65%,均值49.22%),FeO和MnO含量增高(FeO含量范围3.46%~10.65%、均值7.37%, MnO含量范围0.72%~1.71%、均值1.04%);对应以上含量增高元素,Rb+⇌(K+,Na+)、Cs+⇌K+、Li+⇌K+、F⇌OH、2Si4++Li+⇌3Altot3+、Li+VIAl⇌Fe2++Mn等元素替代(Pesquera et al.,1999; Černý et al.,2003; Li Jie et al.,2013; Neiva,2013; Li et al.,2015)使得Al2O3含量(Al2O3含量范围21.03%~32.50%、均值26.46%)和K2O含量(K2O含量范围6.89%~10.16%、均值8.05%)等也发生相应降低。
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图5 甲基卡308号脉各分带原生白云母元素含量箱线图
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Fig.5 Box-whisker plots showing concentration of major and alkali compositions of primary muscovite in each textural zones of the Jiajika No.308pegmatite
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其中,下、中、上短横线和每个框中的叉分别代表全部数据的25%、中位数、75%和平均值;矩形框的上下边缘分别代表90%和10%;空心圆表示最大或最小值,实心圆表示异常值
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The lower, middle, upper lines, and the cross in each box represent 25th percentile, median, 75th percentile, and mean, respectively; the lower and upper whiskers represent the10th and 90th percentiles, respectively; hollow circle represents max or minminum, solid circle represents outliers
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4.2 磷灰石
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甲基卡308号脉中磷灰石在不同分带中均有产出,Ⅰ~Ⅳ带中含量通常 ＜5%,V带中＜2%。磷灰石的产状也具有一定规律:Ⅰ~Ⅳ带中磷灰石通常具有一个氟磷灰石核心和一个羟磷灰石边缘(图6b、c),该边缘以多孔隙、背散射下亮度低于核心为特征;仅Ⅰ带中产出较为均一的氟磷灰石(图6a);Ⅴ带中仅产出羟磷灰石,未观察到氟磷灰石,羟磷灰石可沿岩石裂隙沉淀,也可呈不规则形态产出(图6d)。308号脉中磷灰石的成分相对简单:部分磷灰石有不同程度的Mn替代Ca现象(MnO最高含量为5.75%,但一般不超过3%);由于308号脉中的磷灰石几乎不含Cl(0~0.11%,Ⅰ带次生磷灰石含有最高的Cl含量0.34%),根据简单二分法对磷灰石进行分类,即OH＞F:羟磷灰石;OH＜F:氟磷灰石。氟磷灰石的F含量变化范围在1.84%~4.00%,均值不超过3%,且不同分带之间氟磷灰石的F含量范围相似,变化规律不明显(表2)。
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注: *为计算值。
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4.3 电气石
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甲基卡308号脉中的电气石仅在Ⅰ带和Ⅲ带中产出,部分特征相似:① 手标本上呈黑色,光学显微镜下呈灰蓝色;在背散射图像上均呈现不均匀成分分带,具体表现为相对暗色的幔与浅色核部,在此核幔结构中还可局部出现震荡环带(晶体横截面) (图7)和不规则斑片状分带(晶体柱面)。② 成分上,根据本研究区电气石族矿物成分特征(表3),本文采用三元固溶体分类结合Y位的不同阳离子类型及含量来表征电气石。如图7所示,大部分颗粒具有黑电气石的化学组成,但在背散射图像上的区别仍然显著,背散射下幔部通常具有环带特征。一般来说,此类黑电气石环带往往愈靠近颗粒边缘颜色愈暗(整体上,局部可震荡)、愈富Li和贫Fe。而这种明暗变化主要由Li++Al3+⇌Fe(Mn)2++Mg2+元素替代导致(图10)。不同之处在于:Ⅰ带中电气石含量约5%,粒度通常在50~300 μm;Ⅲ带中电气石含量更高,可达10%,粒度更粗,一般＞200 μm。成分上,Ⅰ带中的电气石常具备黑电气石核和福氏电气石—黑电气石幔的化学组成,幔通常为主体;而Ⅲ带中的电气石主要由黑电气石成分构成主体,幔较窄,幔部可能局部出现更加富锂的黑电气石系列,成分向锂电气石过渡(如图7、9b)。
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续表2
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注: *为计算值。
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图6 甲基卡308号脉各分带磷灰石背散射电子图像
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Fig.6 Backscattered electron images of apatites in different zones of the Jiajika No.308pegmatite
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(a)—Ⅰ带氟磷灰石,成分较为均一;(b)—Ⅰ带羟磷灰石,核部残余氟磷灰石;(c)—Ⅳ带氟磷灰石,背散射图像上的核部亮度不均一主要由CaMn替代导致,相对亮色部分MnO含量高、CaO含量低,边部蚀变成为多孔隙的羟磷灰石;(d)—Ⅴ带羟磷灰石,形态不规则;Ap-(F)—氟磷灰石;Ap-(OH)—羟磷灰石
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(a)—Chemically homogeneous apatite-(F) in zone Ⅰ; (b)—apatite-(OH) with apatite-(F) relict as core in zone Ⅰ; (c)—apatite-(OH) in zone Ⅳ, the heterogeneity of the core is mainly caused by CaMn substitution, where the brighter parts represent higher contents of MnO and less CaO, the rim what altered into porous apatite-(OH); (d)—irregular-shaped apatite-(OH) in zone Ⅴ; Ap-(F)—apatite-(F); Ap-(OH)—apatite-(OH)
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研究区内大部分电气石都显示黑电气石成分,所有电气石均具有低CaO(＜0.61%)、MgO(＜0.49%)、MnO(＜0.64%)、K2O(＜0.08%)和F(＜0.93%)含量的特征。Ⅰ带内电气石相对Ⅲ带内的具有稍高的TiO3、Al2O3、MnO、Na2O、K2O、F平均含量以及更低的SiO2、FeO、MgO、CaO平均含量(表3)。I带内电气石和III带内电气石的B2O3*和Li2O*计算含量范围重合,规律见图8。
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需要注意的是,在Ⅲ带内一颗黑电气石边部见有福氏电气石(表3,探针分析点号308-3-20;图9b),不过其成分并不靠近福氏电气石端元。相比308号脉内的其余碱基型电气石,这颗碱缺位型电气石(X空位组,□代表空位(vacancy))在X位上距平均组成变化较大(图9a);这颗福氏电气石具有比其核部黑电气石显著低的Na和偏低的Fe、Mg(apfu),以及更高的Si、Al、Mn和Licalc(apfu)。
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4.4 锂辉石
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甲基卡308号脉中锂辉石是Ⅳ和Ⅴ带的造岩矿物,具有相似的特征:① 颗粒粗大,所分析样品大部分为厘米级锂辉石颗粒;② 流体交代作用发育,锂辉石颗粒常沿颗粒内部裂隙和颗粒边缘发生蚀变(图11a、b),蚀变产物主要为锂绿泥石(+黏土矿物)和云母矿物,其中锂绿泥石(+黏土矿物)常充填于锂辉石颗粒内裂隙中,白云母即可以充填于裂隙中,也更常见地分布在锂辉石颗粒边缘(图11a~d);③ 锂辉石边缘处常出现不连续分布的蚀变边,也被称为“毛发边”(图11c、e~h),为锂辉石+石英共生体,局部测得透锂长石成分,推测为透锂长石分解所致;④ 背散射下,相比锂辉石颗粒主体,近蚀变边的锂辉石颗粒边缘部分颜色更暗(图11f、h),呈现出成分环带特征。
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注: *为计算值。
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图7 甲基卡308号脉电气石矿物背散射电子图像
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Fig.7 Backscattered electron images of tourmaline in different zones of the Jiajika No.308pegmatite
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(a)—Ⅰ带黑电气石,核部成分落在福氏电气石—黑电气石界限附近,幔为黑电气石;(b)—Ⅲ带黑电气石,边缘局部更富锂,较特殊的福氏电气石也在该颗粒边缘产出;Ft—福氏电气石;Sch—黑电气石;Ft-Sch—福氏电气石-黑电气石;Li-Sch—富锂黑电气石
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(a)—Schorl from zoneⅠ, with a fiotite-schorl core and schorl mantle; (b)—schorl from zone Ⅲ, with higher Li content on part of the edge, and fiotite locally exist; Ft—fiotite; Sch—schorl; Ft-Sch—fiotite-schorl; Li-Sch—Li-schorl
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图8 甲基卡308号脉Ⅰ带和Ⅲ带电气石主要元素含量箱线图
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Fig.8 Box-whisker plots showing concentration of major compositions of tourmaline minerals in textural zone Ⅰ and Ⅲ of the Jiajika No.308pegmatite
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其中,下、中、上短横线和每个框中的叉分别代表全部数据的25%、中位数、75%和平均值;矩形框的上下边缘分别代表90%和10%,该范围之外的数据点以空心圆表示
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The lower, middle, upper lines, and the cross in each box represent 25th percentile, median, 75th percentile, and mean, respectively; the lower and upper whiskers represent the10th and 90th percentiles, respectively; hollow circle below and above the10th and 90th percentile lines represent outliers
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虽然电子探针不能直接测定锂辉石中的Li2O含量,但由于锂辉石颗粒边缘暗色环带较窄,并且暗色部分与母颗粒间具有不规则形态的成分边界,电子探针分析是确定此类锂辉石原位微区成分的理想方法。Ⅳ和Ⅴ带中锂辉石的化学成分非常相似(表4),不过,两结构带中的锂辉石单颗粒均具有相似的成分变化规律:锂辉石边缘的暗色成分环带以及“毛发边”,总是比锂辉石颗粒主体更加富Li2O和贫FeO,这种颗粒外环的成分变化规律十分稳定,即无论锂辉石主体成分高低如何,其外缘总具有比该颗粒主体更高的Li2O和低FeO含量的规律,详见表4。此规律不仅反映出透锂长石分解产生的锂辉石具有更低的杂质含量(FeO)和更高的Li2O含量(Tomas et al.,1994),也可能代表着晚期富Li流体的作用。
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锂辉石蚀变所形成的锂绿泥石和黏土矿物的成分十分不稳定,不同分带中的锂绿泥石和黏土矿物具有相似的成分变化范围,其中主要成分Si2O在36.7%~49.5%之间,Al2O3在34.5%~46.2%之间,代表性锂绿泥石和黏土矿物电子探针分析结果见表5和6。
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图9 甲基卡308号脉电气石成分图解
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Fig.9 Classification of tourmaline minerals of the Jiajika No.308pegmatite
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(a)—Ca-X□-(Na+K)三元图,大部分电气石属于碱基组;(b)—Na/(Na+X□)-YAl/( YAl+Fe)图解
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(a)—Ternary diagram with the apices Ca-X□-(Na+K), showing that most compositions belong to the alkali group; (b)—Na/(Na+X□) vs.YAl/( YAl+Fe) nomenclature diagram
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图10 甲基卡308号脉电气石元素替代矢量图 (底图据London et al.,1995)
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Fig.10 Substitution mechanism of the tourmalines in the Jiajika No.308pegmatite (after London et al.,1995)
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5 讨论
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5.1 矿物化学对熔体和流体性质的限制
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5.1.1 云母矿物演化的指示
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云母矿物对沉淀介质的化学变化敏感,其化学特征可用来示踪花岗岩和伟晶岩的形成和演化过程(Keater et al.,2018),甚至可以指示W、Nb、Ta成矿过程(Morteani et al.,1989;Selway et al.,2005;Li Jie et al.,2013;Yin et al.,2019;Wang Zhen et al.,2019)。甲基卡308号脉云母具有白云母→含锂白云母→锂白云母(包括“混合类型”)→锂云母的变化趋势,其中,不同结构带中的原生云母均落在白云母—含锂白云母区间内(图4),而V带中次生云母,即原生白云母的蚀变边,分布在含锂白云母—锂云母区间内。“混合类型”云母处于二八面体云母到三八面体云母之间的过渡段,其具体结构类型仍然有待确定。Neiva(2013)根据每个配方单位中八面体位(Y site)阳离子数量与Li2O和探针总量(Total)计算结果确定了Namivo地区锂云母伟晶岩中的“混合类型”云母属于三八面体云母;而本次研究中,由于不论采取二八面体还是三八面体云母结构式的计算方式,每个单位结构式中八面体位(Y site)阳离子数量均＞5(基于24个阴离子计算)(参考Rieder et al.,1999),证明本文中“混合类型”云母均属于三八面体云母。因此,核部V带中的原生白云母和次生云母之间显示出成分间隙(gap)和结构上的跳跃(图4),清晰指示出Ⅴ带结晶环境出现突变。值得注意的是,核部Ⅴ带中次生锂白云母和锂云母在成分上靠近铁锂云母成分区间,但仍属于铝质云母。伟晶岩同一结构带内产出的锂云母相对白云母更富集Fe的现象在伟晶岩中较为常见(e.g.Neiva,2013;Zhou Qifeng et al.,2013),不过,甲基卡308号脉内次生锂云母中FeO的富集程度似乎更大(白云母~1.76%,次生云母~7.37%),很可能为体系出溶流体与原生矿物间发生交代作用的结果。
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注: *为计算值。
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续表5
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注: *为计算值。
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图11 甲基卡308号脉锂辉石背散射电子图像
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Fig.11 Backscattered electron images of spodumene in different zones of the Jiajika No.308pegmatite
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(a,b)—Ⅳ带锂辉石,锂绿泥石和黏土矿物沿裂隙穿插交代;(c)—Ⅳ带锂辉石,粒径约1mm,具有不规则“毛发状”蚀变边,其右侧锂辉石颗粒为 (d)—厘米级锂辉石;(e)—Ⅴ带锂辉石,具不规则蚀变边;(f)—(e)的局部放大图,可见靠近蚀变边的锂辉石部分变暗,局部见透锂长石;(g)—Ⅴ带厘米级锂辉石;(h)—(g)的局部放大图,可见蚀变锂辉石与原锂辉石分界截然,与钠长石接触部分具有“毛发状”蚀变边;Ab—钠长石;Ap—磷灰石;Ck—锂绿泥石;Ms—白云母;Pet—透锂长石;Qz—石英;Spd—锂辉石;Spd*—蚀变锂辉石
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(a, b)—Clay minerals and cookeite crosscut spodumene along its fractures in zone Ⅳ; (c)—1mm long spodumene grain in zone Ⅳ, with “fabric” rim, on its right side is (d)—a centimeter-scale spodumene grain; (e)—spodumene in zone Ⅴ with irregular altered rims; (f)—local enlarged image of (e), part of the spodumene gets darker in color near the altered rim, petalite exist locally; (g)—a centimeter-scale spodumene grain in zone Ⅴ, (h)—local enlarged image of (g), a distinct boundary exist between the primary and the altered part of the crystal, and the altered spodumene shows “fabric” rim on the contacting boundary with albite; Ab—albite; Ap—apatite; Ck—cookeite; Ms—muscovite; Pet—petalite; Qz—quartz; Spd—spodumene; Spd*—altered spodumene
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前人研究表明,在伟晶岩浆的演化过程中,云母中的挥发性元素Li、F和不相容元素Rb、Cs会随岩浆演化程度的升高而增加,K/Rb和K/Cs比值发生降低(Foord et al.,1995;Wise,1995;Kile et al.,1998; Roda et al.,2007)。由于Ⅲ带中Rb含量多低于检出限,故未将该数据加入图解中。如图12所示,从Ⅰ→Ⅴ带(除Ⅲ带),原生白云母的Li2O、Cs2O和F均随着K/Rb比值的降低而逐渐增高:在Li2O vs K/Rb图解中(图12b),除Ⅰ带中个别数据点Li2O含量稍高于Ⅱ带,且Ⅳ带显示出较宽的变化范围外,从Ⅰ带至Ⅴ带,原生白云母的Li2O含量均随K/Rb比值的降低而逐渐增高,沿着趋势线(虚线),整体上具有Ⅰ、Ⅱ→Ⅳ→Ⅴ的演化方向;原生白云母中F(图12e)和Cs2O(图12f)含量随K/Rb比值的变化也与Li2O随K/Rb的变化有着相似的特征。Rb2O vs K/Rb可拟合良好的线性关系(图12c),符合正岩浆演化趋势。其中,除Ⅳ带显示很宽的变化范围外,沿着趋势线(虚线),原生白云母的成分演变整体上具有Ⅰ、Ⅱ→Ⅳ、Ⅴ带的演化趋势。以上数据均表明,从Ⅰ带到Ⅴ带,308号伟晶岩浆逐步结晶分异演化。Ⅴ带中原生白云母的不相容元素含量比起其他分带中略有上升,其中,Cs含量增高较明显(图12d),可能暗示着成分跳跃;Ⅴ带次生云母中的挥发分Li和F含量比各带云母矿物中的都显著高(图12a~d),Cs含量则稍显高,程度不及前者。极高的Li、F挥发分对应着云母结构的改变,指示一个极富流体的环境;伟晶岩浆中的Cs因与F结合形成络合物而在残余熔体中富集(Hildreth,1981),比起白云母,Cs通常倾向于进入锂云母晶格中(Wang et al.,2007),并且铯云母被认为形成于低温富流体的环境(Teertstra et al.,1997)。因此,Ⅴ带的含Cs云母很可能由晚期甚至最晚期富Cs流体交代锂云母所形成。虽然Ⅴ带中发育锂云母,但仅以白云母的蚀变边产出,且占比很低,在整个308号脉尺度上,可以说锂云母几乎不产出。一般来说,晚期富水富F环境对锂云母的形成极为有利,并且比起并入电气石,Li、F更易进入锂云母晶格中(Selway et al.,1999)。那么锂云母在308号脉晚期极富挥发分Li、F的环境中为何并不发育?Yin et al.(2020)通过对白龙山锂辉石伟晶岩中云母矿物的研究指出,锂云母仅作为白云母环边产出,代表的是伟晶岩体系具有相对偏低的F浓度/活度(与其他高度演化伟晶岩相比)。所以体系内晚期挥发分富集,但其中F浓度/活度仍然不足是一种可能的解释。此外,Ⅴ带白云母存在交代结构,指示锂云母为热液成因,没有大量锂云母的形成,也可能是由流体出溶量相对较少、自交代作用不是很强烈所致。另有一种可能的解释是Li更倾向于进入锂辉石而不是进入锂云母晶格中,由于锂辉石在酸性条件下更易发生蚀变,而高H、F活度的环境有利于锂云母的形成(London et al.,1982a),故体系内H、F活度的不足有利于锂辉石的保存、不利于锂云母的沉淀。综合以上,认为308号脉中Ⅰ→Ⅴ带分异演化程度逐渐增高,Ⅴ带中原生白云母矿物具有成分跳跃性,并且Ⅴ带处于较富流体的演化阶段,流体具有富挥发分Li、F和携带Cs的特征,但F含量可能不高。
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5.1.2 磷灰石矿物化学的指示
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相比其他稀有金属伟晶岩中的磷灰石,308号脉中原生磷灰石的F含量范围为1.84%~4.00%,平均值一般＜3.0%,均明显偏低(可可托海3号脉2.54%~4.44%,通常＞3.1%,Zhang Hui,2001;南平31号脉3.66%~3.97%,Rao et al.,2017;Lenister钠长石—锂辉石伟晶岩2.72%~4.92%,Barros et al.,2020);与可可托海3号脉相似(初始熔体F含量0.3%~0.4%,Zhang Hui,2001),308号脉中云母类、磷灰石、电气石矿物是主要的含氟矿物,并且这些矿物相对含量较低,因此,推测初始岩浆中F含量亦较低。次生羟磷灰石的出现代表了体系内水的高活度,并且其结构特征(见4.2;图5b~d)提示我们这些羟磷灰石很可能由流体交代原生氟磷灰石所形成。支持此推论的证据包括:① 具有核边结构的磷灰石,其外缘羟磷灰石与核部氟磷灰石具有清晰、截然的界线,代表外部流体作用下自矿物外部向内推进的反应界面(Tsuchiyama,1985),氟磷灰石的溶解和羟磷灰石的再沉淀即在该界面上发生;② 氟磷灰石转变为羟磷灰石过程中,水合反应将使得摩尔体积发生负变化(Ferry,2000),同时产生负应变自由能,从而不仅导致了产物羟磷灰石的多孔隙性,并且能够不断促进该矿物置换反应的进行(Putnis,2002)。这些羟磷灰石具有较低的F含量(~1.0%),说明晚期出溶流体中的F含量不足(London et al.,1982a)。
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5.1.3 电气石成分变化及意义
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前人研究表明,伟晶岩中的电气石化学组成与形成其初始熔体的化学组成以及熔体与围岩相互作用密切相关(Henry et al.,1985;London,1986; Shearer et al.,1986)。308号伟晶岩的化学分析结果显示其(FeO+MgO)＜0.83%(Dai Hongzhang et al.,2018),且其缺乏黑云母等铁镁质矿物,指示形成伟晶岩的初始熔体具有较低的Fe、Mg含量,但如若308伟晶岩的初始熔体具有足够高的B、Al活度,即使在初始熔体FeO+MgO ＜0.83%条件下,在电气石的结晶仍然很有可能发生。实验岩石学模拟结果显示,当电气石在亚固相线下结晶时,体系可以具有更低B2O3浓度和更宽的化学成分范围(Morgan et al.,1989),这表明温度也是电气石结晶的重要控制条件。过铝质岩浆在初始结晶的高温条件下不易结晶出电气石,而后随着温度下降,电气石即可发生结晶。
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图12 甲基卡308号脉各分带云母矿物K/Rb与Li2O(a)(b)、Rb2O(c)、C2Os(d)、F(e)以及F与Rb2O(%)(f)关系图解
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Fig.12 Plots of Li2O vs.K/Rb (a) (b), Rb2O vs.K/Rb (c), Cs2O vs.K/Rb (d), F vs.K/Rb (e) and F vs.Rb2O (f) of micas in different zones of the Jiajika No.308pegmatite
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甲基卡308号脉显示强过铝质特征(Dai Hongzhang et al.,2018),并且自边部带Ⅰ带起即出现原生白云母结晶也指示出伟晶岩初始熔体的过铝质特性。电气石自边部带Ⅰ带开始出现,在Ⅱ带中未发现,而后在Ⅲ带再次出现,且含量更高、粒度更粗。Ⅲ带中相对大量出现的电气石可以用① 初始过铝质熔体中的Fe、Mg含量足以保证电气石的稳定沉淀、② 岩浆演化导致B富集和 ③ 较低温度下电气石更稳定来解释,而Ⅰ带中出现的电气石则可能是受围岩低温影响的产物(Sirbescu et al.,2008;Nabelek et al.,2009),这在一定程度上表明结晶出电气石的初始熔体相对富B(＞2%;Wolf et al.,1997)。电气石在高温下稳定性低(Wolf et al., 1997),故熔体可以携带大量的B而不结晶出电气石;在温度降低至750℃以下后电气石可以晶出,但是熔体中的B饱和度会被F、P等影响Al活度的组分缓冲(Wolf et al.,1997),从而使得熔体必须有更高的B浓度,才能维持电气石的饱和度(Wolf et al.,1994, 1997;Acosta-Vigil et al.,2003)。因此,含F的308号伟晶岩初始熔体需要具有较高的硼含量(大于电气石饱和低限值2%B2O3)才能使得电气石在边部带(Ⅰ带)高温条件下发生沉淀。
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电气石的成分变化可以一定程度上指示体系中是否存在外来流体的混入。通常,接触带由于与富Ca围岩接触将产生内部结构带电气石CaO含量高于外部结构带的现象(Jolliff et al.,1986;Tindle et al.,2002;Selway et al.,2002, 2006;Zhang Aicheng,2006),伟晶岩内部结构带电气石Fe/(Fe+Mg)比值变化也可以用来指示体系的封闭性(Wu Shourong et al.,2015)。也就是说,当伟晶岩中不同结构带的电气石的元素不随岩浆的正向演化而发生规律性变化,尤其是出现异常的成分跳跃(如Mg、Ti、Ca)时,其变化动力很可能来源于外部流体的加入(Tindle et al.,2002);而当电气石遵循正向(岩浆)成分演化时,体系中有外部流体加入的可能性较低,反映的是相对封闭的伟晶岩演化过程(Zhang et al.,2008)。在308号伟晶岩中,从外部带Ⅰ带到中间带Ⅲ带,308号脉中的电气石成分有叠加,看不出明显的演化;但各分带中电气石颗粒内部环带具有从内向外富集Al、Mn、Li、X□以及Fe、Mg、Na减少的趋势。电气石中过剩Al增大可反映伟晶岩温度降低的过程(Jolliff et al.,1986),并且富Li、Al电气石的稳定性高于富Fe、Mg电气石(Wodara et al.,2001),因此,随着伟晶岩向富Li、Al、Mn的方向演化(London,1999),电气石也通过Li++Al3+⇌ Fe(Mn)2++Mg2+元素替代实现黑电气石→锂电气石的连续成分演化,呈现出以上元素变化趋势。显然,308号脉不同结构带中电气石的成分呈连续变化,不存在突变,故认为体系主要处于封闭演化状态。
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不过,Ⅲ带中局部出现的福氏电气石成分是否可作为外部流体注入的证据呢?本文认为答案是否定的。首先,该福氏电气石不能代表成分间隙,前文已述,虽然在类型投图中落入福氏电气石成分范畴,但该电气石贫Fe、Na、Ca且富Li,并不接近福氏电气石组分端元,且Li的增高和Fe、Na、Ca的减少是岩浆正向演化的结果(London,1999;Selway et al.,2002)。其次,这种单颗粒电气石内部的成分变化主要反映的是局部P-T条件的波动(London,1999;Zhang Aicheng,2006),而这种局部环带的形成很可能是流体作用的结果(Norton et al.,2001)。Goerne et al.(2000,2001)的实验研究结果表明,流体中Na和Ca的浓度正比于电气石中的Na和Ca浓度,但体系中的富Na和富Ca矿物相会对电气石中的Na和Ca有缓冲作用,且电气石的结晶温度也可能对不同元素在X位置上的分布有影响。Ⅲ带中较为特殊的福氏电气石,很可能是体系进入钠交代作用之后,大量沉淀的钠长石缓冲了进入电气石结构中Na(Zhang et al.,2004)使得相应形成的电气石具有较低的X位置占有率;同时体系内Li挥发分也随着体系演化而显著增加,从而形成相对富Li、贫Na的福氏电气石。因此,该成分的局部出现反映的是岩浆演化晚阶段,体系出溶的富Li、贫Na和Ca流体对原矿物进行改造的结果。所以,该福氏电气石成分能够指示出此时Ⅲ带内流体的存在,但并不代表外部流体。这种局部流体改造可能是体系进入晶体-熔体-流体共存的岩浆-热液过渡阶段后,出溶流体对原生矿物改造的结果。不过,由于308号脉中电气石的成分环带均较为规则,不存在复杂环带,因此大部分电气石经受的流体改造并不强烈,主要是伟晶岩岩浆分异演化的产物。
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5.1.4 锂成矿的重要影响因素
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锂辉石结晶于岩浆环境中(500~700℃,London,2018),其结晶通常被认为是滞后的:即使是Li饱和熔体,也需要再经历一段演化后(分异结晶导致的矿物沉淀和挥发分持续富集),使Li达到过饱和,才能沉淀出锂辉石(Baker,1998;Maneta et al.,2014;Menta et al.,2015)。308号脉靠近核部的Ⅳ和Ⅴ带有大量锂辉石产出,这很可能说明该伟晶岩初始熔体中具有相对高含量的Li,这样,随着伟晶岩的演化,分异结晶作用下挥发分不断富集,演化至中心分带时体系中方能累积足够量的Li足使之达到过饱和从而大量晶出。因此,除了初始熔体相对富Li外,完善的结晶分异作用对Li的饱和起到了重要作用。
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锂辉石对热液敏感,流体作用下常发生蚀变而导致严重的Li流失(London et al.,1982a;Rao et al.,2017)。308号脉中Ⅳ和Ⅴ带内的锂辉石普遍颗粒粗大,常蚀变产生锂绿泥石+黏土矿物(Bobos et al.,2007)或白云母(London et al.,1982b),蚀变部分的体积约占总锂辉石体积的2%~10%。在蚀变导致Li流失的同时,锂辉石中的其他成分也不可避免被释放进入流体中,如FeO—Ⅳ和Ⅴ带内锂辉石中的FeO含量平均达到0.3%,是锂辉石中含量最高的杂质元素。Fe通过锂辉石蚀变被释放进入流体后,导致流体中Fe浓度增大,白云母在流体改造下形成锂云母环边时,Fe很可能并入锂云母晶格中,使得最晚期锂云母的Fe含量呈现增加趋势。不过,虽然蚀变常常导致锂辉石的破坏,但此处锂辉石仅显示局部蚀变,说明锂辉石的蚀变和改造并不严重,锂辉石很可能遭受了轻度—中度的热液蚀变作用。这种有限规模的Li流失说明锂辉石所遭受的热液蚀变作用程度不强,即308号脉具有完善的岩浆-热液演化过程,但热液阶段持续时间和发育有限,使得此处锂辉石的大部分得以保存。
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5.2 甲基卡308号伟晶岩脉岩浆-热液演化及成矿
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308号伟晶岩脉中边部带I带出现的少量电气石,是由于过铝质、富B的伟晶质熔体就位后,受围岩温度影响,电气石溶解度降低而导致的少量沉淀。此时由于较大的过冷度,结晶出的电气石颗粒也相对较细(London,2009)。电气石结晶消耗了一部分岩浆中的B和Fe,不断增大的F浓度又提高了电气石结晶的B饱和值,于是电气石结晶中止。随后岩浆继续演化,结晶分异作用使得体系内挥发分B继续富集,此时温度持续降低,电气石稳定性下降,于是大量电气石晶出。电气石结晶的结束以接近锂电气石成分的出现为标志,它记录了电气石结晶阶段熔体中Li活度的最高点和Fe活度的最低点。随着岩浆继续演化,挥发分仍不断富集,Li2O在超过饱和点之后迅速以锂铝硅酸盐——锂辉石的形式结晶。锂辉石的结晶贯穿了自锂辉石首次沉淀至晚期形成的结构带,并且由于挥发分的高度富集,锂辉石颗粒十分粗大(Manning,1981;London et al.,1989, 1993, 1997;Morgan et al.,2008)。
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可以确定的是,甲基卡308号脉Ⅴ带已经能够代表一个较富流体的环境;但流体开始出溶的时限,或者岩浆-热液过渡阶段的范围仍有待确定。Zhang Hui et al.(2008)在对可可托海3号伟晶岩脉的研究中指出,岩浆-热液过渡阶段通常只持续很短的时间,常以某个带原生矿物的成分突变为标志,即,岩浆-热液过渡阶段的时长局限于某指示性原生矿物的成分出现突变之间(如绿柱石Cs含量突变),代表着一次流体大量出溶事件;流体出溶后迅速扩散,体系即进入岩浆-热液过渡阶段。这种指示性原生矿物的确定,必须满足于其化学成分对环境改变敏感、贯通各结构带出现的特征,这使得云母成为308号脉理想的指标矿物。从308号脉中原生白云母的化学成分变化可以看出,核部Ⅴ带中的白云母具有明显更高的Li2O、F、Cs2O和Rb2O含量(表1),相比在云母类型和化学组成上的极大重叠的外部带和中间带(图4,10),核部带的这种成分改变显示出了一定的跳跃性,很可能代表着富挥发分的流体出溶。
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此外,308号脉不同分带中磷灰石和锂辉石的广泛交代蚀变表明,流体出溶之后很可能不是局限于某个结构带内,而是广泛渗透和改造了整个伟晶岩,这与Zhang Hui et al.(2008)对热液阶段流体作用范围的研究一致。不过,锂辉石的轻—中度蚀变,说明体系演化至最终较富流体的环境时流体出溶规模并不大/作用时间不长,或锂辉石在流体中仍能够保持稳定。事实上,伟晶岩演化晚期的出溶的酸性富F、P流体不利于锂辉石的保存(London et al.,1982a),所以有限F含量的流体可能是锂辉石蚀变程度不高的原因。最后,由于电气石和云母的成分演化趋势均遵循岩浆正向演化规律,无外部流体的加入的特征,我们认为308号伟晶岩脉形成于岩浆的逐步分异结晶,并于Ⅴ带进入岩浆-热液过渡阶段。
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6 结论
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(1)甲基卡308号脉的初始伟晶岩熔体具有富B、较富Li和贫F的特征,在其岩浆演化过程中很可能不存在外来流体的加入。体系自Ⅴ带发生流体出溶现象,并进入岩浆-热液过渡阶段,出溶流体可能具有富Cs和贫F的性质;普遍蚀变的磷灰石和锂辉石可能代表着流体自出溶后在体系内的广泛渗透。
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(2)甲基卡308号脉的成矿(锂辉石)由岩浆分异结晶作用主导;除初始熔体性之外,伟晶岩浆的结晶分异过程及其随岩浆演化所出溶流体的作用规模和流体性质,对锂辉石的结晶和保存都起到了重要控制作用。
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(3)因露头良好,锂辉石分布集中(Ⅳ与Ⅴ带)、且在岩石中占比高,虽然锂辉石发生一定程度的蚀变,308号脉仍具有较大经济价值。
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摘要
甲基卡稀有金属矿床是亚洲规模最大的伟晶岩型锂矿床,308号脉为其中出露面积最大的伟晶岩脉。308号脉具有完善的分带性:细粒白云母-钠长石-石英带(Ⅰ)、中粗粒钠长石-石英-微斜长石带(Ⅱ)、中粒电气石-钠长石-石英带(Ⅲ)、中粗粒锂辉石-钠长石-石英带(Ⅳ)和中细粒锂辉石-钠长石-石英带(Ⅴ),并且富含稀有金属矿物(如锂辉石、绿柱石、锡石和铌钽氧化物),因此是系统研究伟晶岩演化历史和成岩成矿机制的理想实验对象。308号脉的矿物学研究较为薄弱,其岩浆-热液演化过程及该过程中熔-流体性质的变化规律,以及稀有金属的富集过程和影响因素尚不清楚。本文选取该脉各结构带中的贯通性矿物云母、磷灰石,以及稀有金属矿物锂辉石和特征性矿物电气石,通过详细的电子探针工作,分析它们的产状、结构特征、化学组成及其变化,结果表明:① 308号脉边部Ⅰ带(高温结晶)中电气石的出现(~5%)反映伟晶岩初始熔体具有富B的特征(＞2%);锂辉石在Ⅳ和Ⅴ带内的大量结晶说明初始熔体可能具有富Li的性质;主要含氟矿物电气石、磷灰石和云母类在岩石中的含量均较低,并且原生磷灰石具有相对偏低的F浓度(＜3%),仅能形成于富F环境中的锂云母矿物也十分稀少,这些特征均表明308号脉初始熔体很可能具有较低的F含量;② 伟晶岩体系演化至Ⅴ带时发生流体出溶现象,自此进入岩浆-热液过渡阶段,出溶流体可能具有富Cs和贫F的性质;③ 原生白云母和电气石化学成分的规律性变化说明308号脉的演化过程很可能不存在外来流体的加入;④ 308号伟晶岩的成岩和锂成矿过程主要受分异结晶作用支配,有限的出溶流体规模和伟晶岩分异演化程度(仅演化至岩浆-热液过渡阶段,未充分进入热液阶段)对锂辉石的结晶和保存都起到了重要作用。
Abstract
The Jiajika rare metal deposit is the largest pegmatit-type lithium deposit in Asia, and the No.308 pegmatite is the largest dyke exposed in the area. The No.308 pegmatite is characterized by well zonation (Ⅰ: fine grained muscovite-albite quartz zone; Ⅱ: medium-coarse grained albite-quartz-microcline zone; Ⅲ: medium grained tourmaline-albite-quartz zone; Ⅳ: medium grained spodumene-albite-quartz zone; Ⅴ: medium-fine grained spodumene-albite-quartz zone) and abundant rare-element minerals (including spodumene, beryl, cassiterite and columbite group minerals), thus can be used as an ideal object for our systematic study of the evolution history and diagenesis and mineralization mechanism of pegmatites. At present, the mineralogical study of the No.308 pegmatite is relatively rare, the magmatic-hydrothermal evolution and the characteristic of the initial melt and exsolved fluids of the pegmatite are poorly understood. The process of and influencing factors on the enrichment of rare-elements are still unclear. In this paper, we present EPMA results of mica, apatite, spodumene and tourmaline on their occurrence, textural features and chemical composition variations. The results show that: ① occurence of tourmaline minerals (~5 %) in zone Ⅰ (high temperature crystallization) of No.308 pegmatite indicates that the initial melt of the pegmatite is rich in B (＞2 %); The large amount of spodumene in the Ⅳ and Ⅴ zones shows that the initial melt might be rich in Li; The contents of the main F-bearing minerals (tourmaline, apatite and mica) are relatively low, and the F concentrations of primary fluorapatites are relatively low (＜3%), additionally, the lepidolite which can only be formed in F-rich environment is also very rare. These characteristics indicate that the initial melt of the No.308 pegmatite is likely to have low F content. ② Textural zone Ⅴ probably entered the magmatic-hydrothermal stage in the evolution process of the pegmatite, which is marked by fluid exsolution; these fluids are likely eriched in Cs and depleted in F. ③ The regular changes of chemical composition of primary muscovite and tourmaline indicate that the evolution process of vein 308 probably does not exist the addition of an external fluid. ④ The diagenesis and lithium mineralization of pegmatite No.308 are mainly controlled by fractional crystallization. The limited scale of fluid exsolution and the degree of differentiation evolution of pegmatite (only evolved to the magma—hydrothermal transition stage, not fully entered the hydrothermal stage) significant contribute to the crystallization and preservation of spodumene.
Keywords
pegmatite ； rare metal ； lithium deposit ； magmatic-hydrothermal evolution ； Jiajika
