基底构造对矿床定位的控制机制:焦家金矿带构造应力转移模拟

大型-超大型金矿床具有群聚产出的空间分布规律,这种空间产出规律与断裂带的渗透性结构有关。通过应力转移模拟,计算库伦破裂应力(CFS)的变化值,获取断裂带渗透性结构可以为成矿流体通量及其中金属沉淀成矿概率提供半定量约束。胶西北焦家金矿田是我国第一个千吨级金矿田,其内金矿化严格受NE-NNE向的焦家断裂带构造控制,而其深部隐伏的近EW向基底构造带对成矿的贡献尚不明确。为此,我们根据地球物理资料解译的基底构造带空间形态,通过三维有限元模拟计算基底构造带对理想模型和焦家断裂带内库伦破裂应力变化的影响,探讨基底构造对矿床定位的控制机理。研究表明,EW向基底构造带对区域库伦破裂应力的变化有明显影响,模拟结果图像可视作该深度上NE-NNE向浅表断层和EW向基底构造带分别引起的库伦破裂应力变化之和。成矿深度上某一点因基底构造带活动引起的库伦破裂应力变化的值与基底构造带埋深与成矿深度间距的平方呈反比。当基底构造带和浅表断层运动方向指向同一象限时,基底构造带和浅表断层交汇部位的库伦破裂应力变化值相对减小;而基底构造带和浅表断层运动方向指向不同一象限时,基底构造带和浅表断层交汇部位的库伦破裂应力变化值相对增大。焦家金矿带内基底构造带在成矿期发生右行为主的走滑活动,滑动侧伏角不大于30°,滑动位移量略大于浅表的焦家断裂带。基底构造带在成矿期的再活动导致在其与浅表断裂交汇部位形成构造节点(如新城、焦家),引起该处断层破裂的传播受到阻滞,库伦破裂应力增大,而岩石破碎,有利于高渗透性的裂隙-网脉系统的发育和大型-超大型金矿床的产出;在远离焦家主断裂带的前孙家、洼孙家等部位,浅表断裂引起的库伦破裂应力变化不明显,而基底构造带引起的库伦破裂应力变化占主导,发育高渗透性裂隙-网脉系统的发育和中-小型金矿床(点)的产出。基底构造带的空间展布及其埋深与成矿深度的间距可作为评估区域成矿潜力的重要因素,EW向基底构造带与NE-NNE向浅表断层的交汇部位是重点靶区,且基底构造埋深与成矿深度的间距越小则发育金矿床的概率和规模越大。

Effect of basement structure on the spatial distribution of gold deposits: Structure stress transfer modeling of Jiaojia fault

The giant, super-giant gold deposits always occur in clusters, whose spatial distribution is related to the permeability structures of fault zones. By conducting stress transfer modeling, the change of Coulomb Failure Stress (CFS) is calculated, from which the permeability structures of fault zones can be obtained, and the metallogenic fluid flux and metallogenic probability of metal precipitation can be semi-quantitative constrained. The Jiaojia gold belt is the first place where the kiloton class gold ore field in China was found. In this belt, the gold deposits are structurally controlled by the NE-NEE trending Jiaojia fault. However, the contribution of nearly EW trending basement structure beneath the Jiaojia fault to gold mineralization remains unclear. Therefore, according to the spatial morphology of basement structures interpreted from geophysical data, the influence of basement structures on the CFS was calculated in three-dimensional finite element simulation to reveal the control mechanism of basement structures to deposit locations. The research shows that the EW trending basement structures have apparent influence on the change of regional CFS, and the simulation results imagine can be regard as the sum of the change of CFS caused by NE-NNE trending shallow faults and EW trending basement structures respectively. The variation value of the CFS caused by the activity of basement structures at a certain point in the metallogenic depth is inversely proportional to the square of the distance between the buried depth and the metallogenic depth. When the movement direction of the basement structure and the shallow fault points to same quadrant, the change of CFS near the intersection of the basement structure and the shallow fault decrease relatively. On the contrary, when the movement direction of basement structure and shallow fault points to different quadrants, the change of CFS increases relatively near the intersection of basement structures and shallow faults. The basement structures beneath Jiaojia fault zone had dextral strike-slip during mineralization, the plunge angle of movement is less than 30°, and the slip displacement was slightly larger than that of shallow Jiaojia fault. The tectonic reactivity of the basement structures leads to the formation jogs at the intersections of shallow faults and basement structures, such as the places of Xincheng and Jiaojia. At these places the propagation of fault rupture was blocked, and the CFS increased, resulting in rock fragmentation and the formation of high permeability fracture-vein system giant, super-giant gold deposits. At the places far away from the main fault zone, such as Qiansunjia and Wasunjia, where the CFS change caused by the Jiaojia fault is not obvious, i.e., the CFS change caused by the basement structures was dominant, the highly high permeability fracture-vein system can be developed and medium-small scale gold deposits were formed. The spatial distribution of basement structures and the differences between the buried depth and the metallogenic depth can be regarded as an important factor to evaluate the regional metallogenic potential. The intersection of the EW trending basement structures and the shallow NE-NNE trending faults is the key target area. Meanwhile, the smaller the distance between the buried depth of basement structures and the metallogenic depth, the greater the probability of mineralization and scale of gold deposits develop.