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

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.