西藏甲玛矿床层状矽卡岩的矿物分带机制研究

The Jiama deposit is a super-large Cu-Mo-Pb-Zn-Au-Ag deposit, and involves mineralization developing in skarn, hornfels and porphyry. The stratiform skarn, hosted in the interlayer structural zones between sial and carbonate layers, is prevailing in the deposit. The minerals zonation of the stratiform skarn differ from the classic skarn, which develop between carbonatite and pluton. Some of characteristics of the minerals zonation can’t be explained through the classic skarn genetic model. Based on previous work, we will log in drill holes, tunnels and pits, and observe the mineral’s characteristics under microscope, and describe the minerals zoning characteristics of stratiform skarn detailedly. The forming mechanism of minerals zonation will be investigated through three aspects. Firstly, the comparative study of petrogeochemistry and lead isotopes characteristics of porphyry, skarns and adjacent rocks in different zones will be done. In this way, we can ascertain the influence of inheritance from adjacent rock, and hydrothermal fluids. Secondly, we will conduct the study of fluid inclusions and mineralogy of typical minerals in different stages, to reveal the influence of fluid evolution. Thirdly, the oxygen isotope characteristics of garnet and wollastonite in different zones will be conducted, to trace the influence of fluid migration direction, path and distance on mineral zoning. This work is of great significance for enriching and developing the skarn metallogenic theory.

甲玛矿床是产于斑岩、矽卡岩、角岩内的超大型铜多金属矿床，其中赋存于硅铝质和碳酸盐岩地层之间的层状矽卡岩规模巨大，其矿物分带特征与受侵入岩-碳酸盐岩接触带控制的矽卡岩明显不同，一些特征难以用典型分带成因模式解释。本项目针对甲玛矿床层状矽卡岩，在已有工作基础上详细观察与编录勘探工程，结合镜下鉴定工作，细化矿物分带结构，从围岩、成岩流体、流体运移路线三方面探究其形成机制：一是通过不同分带的矽卡岩及其邻近围岩的岩石地球化学和铅同位素对比研究，结合成矿斑岩地球化学特征，指示原岩继承和热液改造对矿物分带的影响；二是通过不同阶段、不同分带的典型矿物的流体包裹体和矿物学研究，揭示成岩流体成分和物理化学条件对矿物分带的影响机制；三是通过不同分带的石榴子石和硅灰石氧同位素示踪研究，揭示流体运移方向、路径和距离对矿物分带的影响作用。研究工作对丰富和发展矽卡岩成矿理论具有重要意义。

项目针对甲玛层状矽卡岩的“非典型性”矿物分带现象，通过深化矿物学、地球化学、流体包裹体等研究工作，探讨矿物分带的影响因素和形成机制。甲玛层状矽卡岩在垂向具有矽卡岩化角岩→透辉石-石榴子石矽卡岩→石榴子石硅灰石矽卡岩→硅灰石矽卡岩→矽卡岩化大理岩的分带特征，靠近顶板的石榴子石主要以钙铝榴石为主、靠近底板主要以钙铁榴石为主。矽卡岩在平面上从侵入岩到热液作用远端存在有几方面差异，一是近端发育多层垂向分带而远端发育一套垂向分带组合；二是石榴子石/辉石含量比值从大于20∶1→约10∶1→约5∶1；三是石榴子石颜色由红色-棕色→棕色-绿色→浅绿色-浅黄色转变；四是石榴子石从以钙铝榴石为主向以钙铁榴石为主转变。研究认为，影响层状矽卡岩矿物分带主要因素有围岩条件、元素地球化学差异、岩性圈闭、大规模岩浆热液、构造扩容先于岩浆热液顶破的事件组合。伴随着高温岩浆热液的分异析出和逃逸，由于连通了先成的硅铝质岩石和钙质岩石的层间扩容空间，超临界流体没有像典型斑岩矿床中顶破上覆地层而向上迁移，良好的张性空间和圈闭条件给流体涌入和交代围岩创造了良好条件，超临界流体优先在顶板聚集并从上向下逐渐叠置和交代围岩。热液与硅铝质顶板交代形成的辉石-石榴子石矽卡岩（石榴子石以钙铝榴石为主），与钙质底板交代形成了以硅灰石为主的矽卡岩。热液中Al运移能力弱而Fe、Mg、Ca、Si等元素运移能力强，从中心到外围形成了石榴子石/辉石减少、钙铝榴石/钙铁榴石减少的总体趋势。