陕西小秦岭华阳川断裂变形特征及其对成矿的制约

The Huayangchuan fault zone is located in the Xiaoqinling orogenic belt at the southern margin of the North China Block. This fault zone plays a crucial role in controlling the regional ore deposits and the formation and evolution of Xiaoqinling orogenic belt. The Huayangchuan fault is the main ore-controlling fault in the Huayangchuan U-Nb-Pb deposit which strictly control the distribution of mineralization vein rocks in the district. The fault zone experiences multiphases of activities and conversions of mechanical properties. Previous studies have roughly divided the activities of Huayangchuan fault into three stages: early compression (with torsion), middle tension (with torsion) and late compression (with torsion. Due to the weak studies on the structural deformation and chronology of the Huayangchuan fault zone, this work aims at unraveling the structural geometry, kinematics and microstructures of the Huayangchuan fault zone and figuring out the ore-controlling law of the fault zone, transforming sequences of the tectonic stress and the and the evolution of stress field. The fission track dating and the Ar-Ar dating of mica minerals in the of fault tectonites are applied to determine the age of the fault movement. The relationship between the fault zone and the minerogenesis was discussed in the end. This study provides key evidences from structural deformation for further understanding of the mineralization process of the Huayangchuan deposit and the formation and evolution Xiaoqinling orogenic belt.

华阳川断裂破碎带位于华北陆块南缘小秦岭陆内造山带，对区域矿床及小秦岭的形成演化起到至关重要的控制作用。华阳川断裂是华阳川铀铌铅矿床最主要的控矿断裂，严格控制区内含矿脉岩的展布。断裂具有多期活动性和力学性质多次转换特点。已有研究工作仅大致将华阳川断裂分为早期压（扭）性，中期张（扭）性，后期压扭性三期三个阶段。对断裂带系统的构造变形特征及活动时间的研究尚为薄弱。本研究在前期工作的基础上重点开展华阳川断裂带构造几何学、动力学、显微构造研究，查明该区构造应力转化序列、应力场演化和断裂控矿规律；利用构造岩中云母类矿物的Ar-Ar定年及裂变径迹测年方法，系统揭示NW走向华阳川断裂的活动时间，分析其成因，探讨断裂构造对成矿的制约关系。对深入理解华阳川矿床成因模式及其与区域构造形成与演化的时空耦合关系提供了来自构造变形方面的关键证据。

本次研究通过岩石学、矿物学等方法对华阳川铀多金属矿床的成岩成矿作用进行较系统的研究，将华阳川U-Nb-REE主体成矿划分为两期（碳酸岩晶质铀矿LA-ICP-MS定年213±3.7和133±1.8Ma），铀成矿作用和规模以第一期为主。查明了该矿床稀土的赋存特征，认为该矿床稀土以独立矿物（褐帘石、磷铈镧矿、磷钇矿、氟碳铈镧矿、褐钇铌矿）与类质同象（磷灰石、贝塔石、榍石等矿物）两种形式存在，稀土资源以La、Ce轻稀土元素为主，并富含重稀土元素Y。采用构造解析，对华阳川脆-韧性剪切断裂带及其上下盘开展地质剖面和构造要素的测量与统计（包括糜棱面理、节理、擦痕、拉伸线理等），确认华阳川地区断裂带的空间展布、发育规模及其与矿体的关系等几何学特征。将地区断裂发展历史分为4个时期，即成矿前断裂（韧性剪切带）、成矿早期断裂（脆韧或韧脆性剪切带）、主成矿期断裂（脆性断裂）和成矿后期断裂（脆性断裂）。认为北西向华阳川断裂是华阳川地区主要控矿和含矿断裂，属浅层次构造变形，具有多期构造-热液活动特征；经历了早期为压（扭）性，中期为张（扭）性，后期压扭性特征。中期成矿期断裂及其次级裂隙为含矿流体的运移提供了良好的通道，张性断裂形成的空间为矿质的沉淀提供了充足的场所。通过低温热年代学磷灰石裂变径迹（AFT）测试，显示了华阳川地区相对一致的年龄（52-35Ma）和单峰式封闭径迹特征。热史模拟表明华阳川地区经历了始新世相对快速隆升剥蚀、然后在一段时间保持稳定以及到现今的隆升至地表过程。热史模拟显示研究区北西侧的剥蚀量多（6.46-17.67km），大于研究区中部（5.81-12.86km），研究区南东侧的剥蚀量较少（2.95-3.20km）。结合铀矿成矿年龄和温压条件，认为华阳川南东侧具有较高的资源勘查潜力。