陕西凤太矿集区多金属成矿作用的构造控制

陕西凤县-太白(简称凤太)矿集区铅、锌、金、银、铜多金属资源丰富,已发现二十余个大中小型矿床。在大地构造位置上,凤太矿集区位于南秦岭造山带北缘,紧邻商丹缝合带。以往的工作缺乏对矿集区整体的构造研究,本次工作通过比较系统的构造测量和解析,提出在南秦岭晚三叠世碰撞造山过程中,凤太矿集区南北两条边界断裂带的左行走滑运动导致在区内衍生了NNE向主压应力场,从而形成了NWW向复式褶皱、脆韧性剪切带、断裂和节理(纵向破裂)、B型线理,以及NNE向断裂和节理(横向破裂)、劈理、张裂隙等一系列构造组合,所有构造形迹都是在统一构造应力场下随着构造层次不断抬升,脆韧性和脆性递进变形叠加的产物,共同构成了一个大型压扭性走滑双重构造变形系统。在构造几何学上,凤太矿集区整体上表现为一个隔档式复式褶皱,由一组NWW向紧闭复背斜和一组相对宽缓复向斜组成。区内的多金属成矿作用、岩浆活动、动力变质变形作用的同位素年龄数据集中于230~190Ma。综合地质演化和成矿作用的研究成果,提出在南秦岭碰撞造山过程中引发的动力变质变形作用和岩浆活动提供了成矿元素和成矿流体,在温压梯度以及浮力效应的驱动下向上运移至走滑双重构造变形系统中的有利扩容空间中发生充填型和交代型矿化,即凤太矿集区多金属矿床是区域大规模变形变质-岩浆活动-流体作用的产物,是在构造作用这一主导因素控制下形成的一个多金属后生热液成矿系统。

Tectonic controls on polymetallic mineralization in the Fengxian-Taibai ore cluster zone, Western Qinling, Shannxi Province

The Fengxian-Taibai (abbr. Fengtai) polymetallic ore cluster zone in the Shaanxi Province, located in the Western Qinling orogen, Central China, is rich in mineral resources of zinc, lead, gold, silver, copper, etc. More than twenty zinc-lead deposits and gold deposits have been discovered in this zone, and the proved reserve of zinc-lead and gold exceeds 5Mt and 200t, respectively. The Qinling orogen, resulted from the convergence and collision of the North China craton and the Yangtze craton, is a multi-stage composite orogen with two suture zones and three tectonic units. The northern suture zone is the Early Devonian Shangdan suture, and the southern one is the Triassic Mianlue suture. The Fengtai zone is located to the southern margin of the Shangdan suture, which belongs to the South Qinling belt. Detailed structural measurement and systematic analysis indicate that the maximum principal stress (σ₁) is NNE-trending in the Fengtai polymetallic ore cluster zone, and we suggest that the structural style is a large strike-slip duplex resulted from regional sinistral strike-slip faulting in Late Triassic, which is mainly composed of NWW-trending complex folds and shear zones, and NNE-trending faults. Meanwhile, abundant associated fractures, joints, lineations and cleavages are well developed within the deformational system. The structural geometry of the whole zone is a complex fold with wide spaced anticlines, i.e. a set of NWW-trending closed anticlines and broad synclines. Previous age data and our new data of Zn-Pb and Au mineralization, magmatic activity, dynamic metamorphism, and structural deformation range from 230Ma to 190Ma, i.e. Late Triassic to Early Jurassic. Most previous studies suggested that the Zn-Pb-Ag (Cu) deposits in the Fengtai zone are of Sedex (Devonian) or Sedex-reformed type, and some researchers thought that the Au deposits are of Carlin, Carlin-like or Sedex-reformed type. By integrating the structural analysis data with previous studies of regional tectonic evolution and ploymetallic mineralization, we argue that deposits in the Fengtai zone are the products of epigenetic hydrothermal processes. The Late Triassic collision of the Yangtze craton and the South Qinling belt along the Mianlue suture zone triggered regional deformation, dynamic metamorphism and magmatism within the obduction block of South Qinling belt. In the Fengtai zone, the sinistral strike-slip faulting of the two boundary faults results in the formation of strike-slip duplex, the Devonian basin sediments of littoral-neritic facies are partially epimetamorphosed, and plutons and numerous dikes are well developed filling the NWW- and NNE-trending faults. The ore-forming fluids and metals are mainly derived from dynamic metamorphism and magmatism, and the shear deformation system provides fluid pathways and sites of ore deposition (filling and replacement) to form an epigenetic hydrothermal polymetallic system.