西秦岭北缘李子园-太阳寺构造带变形特征与成矿作用关系研究

The West Qinling orogen is the third largest gold producing area in China, with complex types of ore deposits and multiple stages of magmatism and mineralization under subduction, collision, or intracontinental orogeny tectonic settings since the Palaeozoic. Previous studies have revealed that gold deposits at the northern margin of the West Qinling orogen are mainly typical quartz vein type, containing numerous EW-/NW-/NE-trending ore-bearing veins. The veins in different directions are prominently controlled by ductile/brittle shear deformational belts. The structural mechanism, formation age and tectonic background of the shear and brittle deformational belts that controls the ore deposits, however, is still unclear and under debate. This is fundamentally due to the lack of detailed study on the tectonic deformation of the gold district at the northern margin of the West Qinling orogen, and especially the shear deformation process and magmatism closely associated with the metallogenic process. This project is designed to be based on comprehensive tectonic deformation research, including geometric and kinematic characteristics, superposition relations, sequences and ages of the shear deformation belts, and meanwhile, to clarify structural position of the ore bodies, structural surfaces containing mineral intrusions, diagenesis and metallogenic ages, and recover emplacement and migration history of ore-forming fluids. This project aims to establish structural relationships between the deformational characteristics of shear deformation belts in different directions and the multiple stages of metallogenic events in the gold district in the West Qinling orogen, to discuss the tectonic-related ore-controlling process in the gold district, and eventually to understand the regional tectonic evolution since the Palaeozoic.

西秦岭是我国第三大金矿产区，经历了古生代以来的俯冲-碰撞-陆内造山等多期构造岩浆活动及成矿作用，矿床类型复杂。现有研究表明，西秦岭北缘金矿区以石英脉型金矿为主，发育有近东西、北西及北东向等多组方向的含矿脉体，这些不同方向的矿脉明显受韧脆性剪切变形带控制，但是目前对于剪切、脆性变形带控矿的构造机制、时代和背景等方面认识上存在争议。其根本原因在于缺乏对西秦岭北缘金矿区构造变形，尤其是与成矿过程密切相关的剪切变形过程及岩浆作用的解剖。本项目以构造变形研究为基础，通过研究剪切变形带的几何学、运动学特征，以及它们的叠加关系、变形序列和时代，同时查明矿体的构造位置、含矿物质侵入的构造面和成岩成矿时代等特征，恢复剪切变形过程中成矿流体侵位和迁移的过程，从而构建西秦岭金矿区不同方向剪切变形带的变形特征与多期成矿事件之间的构造关系，探讨西秦岭北缘金矿区构造控矿作用过程，以及区域古生代以来的构造演化过程。

造山型金矿通常形成于增生/碰撞造山过程中的挤压向伸展转换构造背景下，多产自变质地体内，由于其巨大的经济价值受到越来越多学者的关注。李子园金矿作为西秦岭造山带内最具代表性的造山型金矿之一，其内包含尖草湾、碎石子等多个矿段，矿体产在寒武-奥陶系李子园群变质火山岩和天子山二长花岗岩中，矿体与区内岩浆岩的分布表现出密切的时空关系。本项目基于详尽的野外调研、巷道与钻孔编录以及室内显微构造分析，识别出区内五期构造活动和两期五阶段成矿作用，两期成矿作用分别对应于第二、第三期构造活动。早期的变质成矿作用包含石英-黄铁矿阶段（阶段I）和条带状石英-多金属硫化物阶段（阶段II），晚期岩浆热液成矿作用叠加在早期矿化体之上，分为石英-钾长石-黄铁矿-辉钼矿阶段（阶段三），石英-多金属硫化物-绿泥石±方解石阶段（阶段IV），以及方解石-石英阶段（阶段V）。地质年代学研究显示，李子园金矿两期成矿事件（220Ma，205-199Ma）刚好晚于区内的两期岩浆活动（天子山岩体，236.1Ma；闪长玢岩与花岗斑岩，212-213Ma）5-15Ma。两期成矿作用流体性质有明显差异，韧性剪切带型矿化成矿流体总体显示富含CO2，成矿温度120-340℃，盐度4.6～6.6wt.%NaCl，H-O同位素显示成矿流体为变质流体+岩浆流体+大气降水组合，S同位素显示深源特征; 晚期成矿流体以单相气相包裹体（VH2O）、单相盐水溶液包裹体（LH2O）两相盐水溶液包裹体（LH2O+VH2O）为主，均一温度110-315℃，盐度4.2～10.6wt.%NaCl，H-O同位素显示岩浆流体和大气降水混合，S同位素同样显示深源特征。综上，我们认为李子园金矿为与岩浆活动有关的造山型金矿，形成于西秦岭造山带由同碰撞向后碰撞伸展阶段转换的构造背景。