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  • 西藏邦铺斑岩钼(铜)多金属矿床侵入岩锆石微量元素特征及其地质意义

西藏邦铺斑岩钼(铜)多金属矿床侵入岩锆石微量元素特征及其地质意义

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  摘 要:西藏邦铺斑岩钼(铜)多金属矿床位于冈底斯成矿带东段,矿区侵入岩期次和类型多样,主要包括成矿前期黑云母二长花岗岩、成矿期闪长玢岩和二长花岗斑岩及成矿后期石英二长斑岩和辉绿玢岩等5种。在系统总结前人研究成果基础上,利用LA-ICP-MS技术获得了矿区闪长玢岩岩体锆石微量元素特征。结果表明:邦铺矿区侵入岩锆石类型主要包括岩浆锆石和热液锆石;5种侵入岩球粒陨石标准化稀土元素配分模式显示具相似的特征,曲线左倾,整体较陡,表现为明显的Ce正异常和Eu负异常,轻稀土元素亏损,重稀土元素富集。通过锆石Ti含量地质温度计计算出侵入岩锆石结晶温度,显示出成矿期和成矿后期侵入岩体岩浆来源于在水近饱和条件下发生的部分熔融。根据锆石中特定微量元素和稀土元素特征变化,各侵入岩锆石均是从陆壳岩浆中结晶分异形成的,与锆石相对应的寄主岩石均形成于陆壳环境。
  关键词:多金属矿床;侵入岩;微量元素;成因类型;锆石Ti含量地质温度计;结晶环境;西藏
  中图分类号:P595 文献标志码:A
  Zircon Trace Element Characteristics of Intrusions in Bangpu Porphyry
  Mo(Cu) Polymetallic Deposit of Tibet and Their Geological Significance
  LI Zhuang1, WANG Li-qiang2, ZHANG Zhong3, LUO Mao-cheng4, LENG Qiu-feng1,
  KANG Hao-ran2, MENG Zhan1
  (1. School of Earth Sciences, Chengdu University of Technology, Chengdu 610059, Sichuan, China;
  2. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;
  3. Tibet Tianyuan Mineral Exploration Co. Ltd., Shigatse 857000, Tibet, China;
  4. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China)
  Abstract: The Bangpu porphyry Mo(Cu) polymetallic deposit is located in the eastern part of Gangdise metallogenic belt. The period and type of intrusive rocks in this area are various, mainly including biotite monzonitic granite during the early mineralization, diorite porphyrite and monzonitic granite during the mineralization, and quartz monzonitic porphyry and allgovite during the late mineralization. On basis of summarizing the previous research, the characteristics of zircon trace elements from diorite porphyrite in Bangpu deposit were studied by the LA-ICP-MS technology. The results show that the types of zircon from intrusive rocks mainly include magmatic and hydrothermal zircons; chondrite-normalized REE patterns of the above five types of intrusive rocks are similar, and the curves are left deviation and steep with the characteristics of positive Ce anomaly, negative Eu anomaly, LREE enrichment and HREE depletion. Crystallization temperatures of zircon from the above five intrusive rocks calculated by the Ti-in-zircon geothermometer show that the magma of intrusive rock during the mineralization and late mineralization are derived from the partial melting under the condition of nearly water saturation. According to the characteristics of special zircon trace element and REE, the zircons of the above five intrusive rocks are crystallized and differentiated from continental crust magma, and the host rocks corresponding to zircon form in the continental crust environment.   邦铺矿区5种不同类型侵入岩中,闪长玢岩Eu异常平均值最小,球粒陨石标准化稀土元素配分模式总体比较相似,曲线较陡[图4(a)~(e)],稀土元素含量从La至Lu之间迅速增加。除闪长玢岩外,其余4种侵入岩体均表现为明显Ce正异常和Eu负异常,轻稀土元素亏损,而重稀土元素富集。不同类型侵入岩球粒陨石标准化稀土元素配分模式平均值[图4(f)]也显示出具典型的Ce正异常和Eu负异常,轻稀土元素亏损,重稀土元素富集。
  5 讨 论
  5.1 锆石成因类型
  锆石w(Th)/w(U)值常被用作判断其成因的标志,以区分岩浆、变质和热液环境。Th4+离子半径与Zr4+离子半径非常接近,电价相同,很容易进入锆石中;又因U4+离子半径比Th4+小,更接近Zr4+,所以锆石中U含量常高于Th[48]。邦铺矿区除闪长玢岩外,其余4种侵入岩体中锆石w(Th)/w(U)值都较高,平均值均大于1。黑云母二长花岗岩中锆石w(Th)/w(U)值为0.27~2.17,平均值为1.16;二长花岗斑岩中锆石w(Th)/w(U)值为1.10~1.34,平均值为1.26;石英二长斑岩中锆石w(Th)/w(U)值为1.09~1.46,平均值为1.29;辉绿玢岩中锆石w(Th)/w(U)值为1.17~1.65,平均值为1.37。不同成因锆石的Th、U含量及其比值差异较大,一般岩浆锆石w(Th)/w(U)值大于0.4,热液锆石w(Th)/w(U)值小于0.1[44,49-51]。据此观点可知,上述4种侵入岩锆石应属典型的岩浆锆石类。然而,Hidaka等研究发现一些典型岩浆锆石也会具有非常低的w(Th)/w(U)值[52],因此,不能仅仅根据锆石的w(Th)/w(U)值来区分锆石的成因类型。此外,赵振华通过研究锆石微量元素地球化学特征也证实:w(Th)/w(U)值主要反映的是母体的特点或形成时局部的甚至包括流体源区的化学环境[43]。因此,当用w(Th)/w(U)值来判定锆石成因时必须谨慎,应当与其他综合地球化学指标相结合最终作出合理判断。
  Ferry等研究发现,利用锆石(Sm/La)N-(Lu/Gd)N图解和δ(Ce)-δ(Eu)图解可以比较准确地区分热液锆石和岩浆锆石[53]。据此方法对邦铺矿区侵入岩锆石作(Sm/La)N-(Lu/Gd)N图解和δ(Ce)-δ(Eu)图解。闪长玢岩锆石在图5(a)中有10个测点落入热液锆石区域,在图5(b)中有9个测点落入热液锆石区域,据此可判断闪长玢岩锆石主要为热液锆石类;二长花岗斑岩锆石在图5(a)中有10个测点落入岩浆锆石区域,在图5(b)中同样有10个测点落入岩浆锆石区域,因此,二长花岗斑岩锆石主要属于岩浆锆石类;黑云母二长花岗岩锆石在图5(a)中有10个测点落入岩浆锆石区域,在图5(b)中有11个测点落入岩浆锆石区域,因此,黑云母二长花岗岩锆石应主要为岩浆锆石类;石英二长斑岩锆石在图5(a)中有9个测点落入岩浆锆石区域,在图5(b)中同样有9个测点落入岩浆锆石区域,据此可推断此类锆石亦主要为岩浆锆石类;基性辉绿玢岩锆石在图5(a)中有10个测点落在岩浆锆石区域,在图5(b)中有9个测点落在岩浆锆石区域,据此可判定辉绿玢岩锆石主要为岩浆锆石类。
  此外,(Sm/La)N-La图解和δ(Ce)-(Sm/La)N图解也可以很好地区分岩浆锆石和热液锆石。图6(a)中黑云母二长花岗岩锆石有8个测点落在岩浆锆石区域,3个测点落在岩浆锆石边界区域内,图6(b)中有8个测点落在岩浆锆石区域,3个测点落在岩浆锆石边界区域,因此,此类锆石主要属于岩浆锆石类;图6(a)中辉绿玢岩锆石有5个测点落在岩浆锆石区域,5个测点落在岩浆锆石边界区域内,图6(b)中有6个测点落在岩浆锆石区域,2个测点落在岩
  底图引自文献[53]
  图5 侵入岩(Sm/La)N-(Lu/Gd)N图解和δ(Ce)-δ(Eu)图解
  Fig.5 Diagrams of (Sm/La)N-(Lu/Gd)N and δ(Ce)-δ(Eu) for Intrusive Rocks
  底图引自文献[54]
  图6 侵入岩(Sm/La)N-La图解和δ(Ce)-(Sm/La)N图解
  Fig.6 Diagrams of (Sm/La)N-La and δ(Ce)-(Sm/La)N for Intrusive Rocks
  浆锆石边界区域, 2个测点靠近岩浆锆石区域,因此,辉绿玢岩锆石主要为岩浆锆石类;二长花岗斑岩锆石在图6(a)中有5个测点落在岩浆锆石区域,4个测点落在岩浆锆石边界区域,在图6(b)中有8个测点落在岩浆锆石区域内,因此,二长花岗斑岩锆石主要属岩浆锆石类;石英二长斑岩锆石在图6(a)中有4个测点落在岩浆锆石区域,2个测点落在过渡区域(更靠近岩浆锆石区域),在图6(b)中有7个测点落入岩浆锆石区域,3个测点落入岩浆锆石边界区域,因此,石英二长斑岩锆石主要属于岩浆锆石类;闪长玢岩锆石在图6(a)中有6个测点落入热液锆石边界区域内,1个测点落在热液锆石区域内, 5个测点靠近热液锆石区域,在图6(b)中有6个测点落在热液锆石区域,2个测点落在热液锆石边界区域,因此,闪长玢岩锆石主要属热液锆石类。
  综上所述,邦铺矿区闪长玢岩锆石主要为热液锆石类,其余4种侵入岩体(包括黑云母二长花岗岩、二长花岗斑岩、辉绿玢岩和石英二长斑岩)中锆石均主要属岩浆锆石类。
  5.2 锆石Ti含量地质温度计
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