特提斯喜马拉雅铅锌锑金成矿带含炭质岩石中热液脉型矿床的综合电法勘探——以西藏扎西康铅锌锑多金属矿为例

Comprehensive electrical prospecting of hydrothermal vein type deposits in carbonaceous rocks in the Tethyan Himalayan Pb-Zn-Sb-Au metallogenic belt: A case study of the Zhaxikang Pb-Zn-Sb polymetallic deposit in southern Tibet

关键词:  含炭质岩石  热液脉型矿床  电法勘探  特提斯喜马拉雅  扎西康铅锌锑多金属矿

Key words:  carbonaceous rock  hydrothermal vein type deposit  electric prospecting  Tethyan Himalayas  Zhaxikang Pb-Zn-Sb polymetallic deposit

摘要:

扎西康铅锌锑多金属矿床产出于特提斯喜马拉雅炭质板岩的断裂带内，是特提斯喜马拉雅铅锌锑金成矿带内典型的热液脉型矿床。由于含炭质岩石和金属硫化物都呈现出相似的低阻高极化电性特征，加之热液脉型矿床的矿体普遍较小，使得在含炭质岩石中对金属硫化物矿体进行电法勘探存在较大困难。本文通过对扎西康矿床已知矿体的音频大地电磁测深和激电中梯测量，发现矿区的炭质板岩呈现低电阻率（10^-0.4~10⁰Ω·m）和高极化率（9%~20%）特征，而矿体呈现出高电阻率（10²~10³Ω·m）和低极化率（1%~5%）的特征。经研究分析，认为造成这种现象的原因有两方面：（1）扎西康炭质板岩中的炭质物质量分数平均为0.79%，变质温度约在300±25℃~340±25℃，炭质物电阻率为6.1×10^-5~6.8×10^-4Ω·m，显示极好的导电性；此外，炭质板岩中存在大量黏土矿物，黏土矿物的吸水性促进了炭质物的连通性，因此炭质物高导电性与连通性的耦合使得炭质板岩呈现低阻高极化电性特征；（2）扎西康矿床的脉型矿体除包含金属硫化物外，还产出大量的脉石矿物，脉石矿物普遍具有高阻低极化电性特征，是造成整个矿体在炭质板岩中呈现高阻低极化异常的根本原因。据此，本文提出在炭质板岩中通过识别脉石矿物引起的高阻低极化异常带间接找矿的新思路，相应的技术方法组合为：利用激电中梯测量定位高阻低极化带的平面位置，再利用音频大地电磁测深探测其深部产状。

Abstract:

Zhaxikang Pb-Zn-Sb polymetallic deposit, a typical hydrothermal vein type deposit in the Tethyan Himalayan Pb-Zn-Sb-Au metallogenic belt, occurs in the fault zone of carbonaceous slate. Because carbonaceous rocks and metal-bearing sulfides are usually with similar resistivity and polarization, and the ore bodies of hydrothermal vein type deposits are generally small, it is difficult to conduct electrical prospecting for sulfide ore bodies in carbonaceous rocks. The AMT and IP ladder measurements of known ore bodies in the Zhaxikang deposit show that carbonaceous slate in the area has low resistivity (10^-0.4~0Ω·m) and high polarizability (9%~20%), while the ore bodies are characterized by high resistivity (10^2~3Ω·m) and low polarizability (1%~5%). Based on following facts，an indirect geophysical prospecting method is proposed in this paper: (1) The average content of organic carbon in carbonaceous slate is 0.79% in Zhaxikang mining area; (2) the metamorphic temperature is about 300 ±25 ℃ to 340 ±25 ℃; (3) the resistivity of organic carbon is (6.1×10^-5~6.8×10^-4Ω·m) and shows excellent conductivity; (4) a lot of clay minerals are contained in the carbonaceous slate, and the water absorption of clay minerals promotes the connectivity of organic carbon. The coupling of high conductivity and connectivity of organic carbon makes carbonaceous slate present low resistivity and high polarization; (5) vein-type ore bodies in the Zhaxikang deposit contain not only metal sulfides, but also a large number of gangue minerals, and gangue minerals are generally with high resistivity and low polarization, it is suggested that locating ore bodies in carbonaceous slate by identifying high resistivity and low polarization anomalies caused by gangue minerals is effective and useful. In addition, a suitable combination of prospecting methods is proposed, in which the planar position of the high resistance and low polarization zone can be located by IP measurement, and its deep occurrence can be detected by AMT measurement.