第四系覆盖区深部热液脉型矿体综合地球物理方法定位预测--内蒙古维拉斯托矿区北侧隐伏矿体勘查例析

摘要

维拉斯托岩浆热液脉型铜多金属矿床位于大兴安岭成矿带南段西坡白音查干,已探明矿石量881.14万吨,平均品位Cu：0.79%、Zn：4.29%、Ag：65~85 g/t。矿体的产出严格受NE向断裂带控制,总体走向NE—NEE,倾向北。维拉斯托矿区北侧第四系覆盖区域是否存在隐伏矿体是当前亟待解决的问题。维拉斯托矿区北侧第四系覆盖区成矿条件与主矿区类似,本文应用重力、磁法、激电中梯和可控源音频大地电磁法对其进行了隐伏铜多金属矿体的探测和研究,构建了覆盖区岩浆热液脉型隐伏矿体定位勘查流程。首先,针对全区的重力方向导数异常圈出三个NE-NNE向的“条带状”重力方向导数高值带,推测为F1、F2和F3三条断裂；同时,在上述“条带状”重力方向导数高值带内,利用磁场化极和上延圈出一“月牙形”高值异常带(含多个串珠状磁异常),推测为含矿岩体；对应“月牙形”串珠状高磁异常带,运用激电中梯视充电率和视电阻率圈定M1和M2两个视电阻率低值、视充电率高值异常区域,推测为矿化岩体；在此基础上,综合分析面积性地球物理探测研究成果,在重力、磁场和激电异常区域进行可控源音频大地电磁法研究中发现2处反演电阻率低值异常点(均位于NW向和NEE向断裂交汇处),推测为富含硫化物矿体引起,据此圈定两处矿体,且推测该区深部可能存在更大规模的隐伏矿体。依据本文研究成果,预测维拉斯托矿区北侧覆盖区存在隐伏矿体。%The Weilasituo magmatic hydrothermal vein-type copper-polymetallic deposit, with proven reserves of 8.811 4 million tons, is located in the western part of the southern section of the Da Hinggan Mountains metallogenic belt. The average grade of Cu, Zn and Ag is 0.79%, 4.29% and 65~85 g/t, respectively. The NE–NEE-trending orebodies are strictly controlled by the NE-trending fault zones and dip northward. The existence or nonexistence of concealed orebody on the northern side of the Weilasituo ore district, which is covered by Quaternary cover, is an urgent problem to be solved. On the northern side of Weilasituo, the areas covered by the Quaternary sediments possess similar metallogenetic conditions to the main ore district. In order to investigate the concealed oresbodies, the authors used integrated geophysical methods such as gravity method, magnetic method, induced polarization in median gradient array method and controlled source audio-frequency magnetotelluric method. Firstly, three NE–NNE trending ribbon-shaped negative residual density zones, which indicate the underlying faults F1, F2 and F3, were detected based on directional derivative of gravity anomaly. In these zones, a crescent-shaped anomaly zone, with high values of magnetic reduction to pole and upward extension, was considered to be the target of ore hosting rocks, including multiple beaded magnetic anomaly. Then, two abnormal areas (M1 and M2) with lower apparent resistivity values and high apparent charging rates were recognized by apparent charging rate and apparent resistivity from induced polarization in median gradient array. These anomalies may be caused by ore-bearing rocks. Finally, controlled source audio-frequency magnetotelluric survey was applied to areas with anomalies of gravity value, magnetic value and induced polarization value, and two locations with low inversion resistivity were found. These may be caused by the sulfide-rich orebodies which are located at the intersection of NW- and NEE-trending faults. On the basis of these results, it is inferred that additional large-tonnage concealed orebodies are existent in the depth. Based on the results obtained, it is held that there exists a concealed orebody on the northern area of the Weilasituo ore district, which is covered by Quaternary sediments.