Audio magnetotelluric method: Implications for a concealed porphyry-hosted molybdenum-polymetallic deposit in eastern Hebei province

Polymetallic deposits, primarily porphyry-related polymetallic deposits, are hosted in eastern Hebei province in the northern margin of the North China Block. The study area, located in northeastern Chengde, is a typical rhyolite porphyry-hosted molybdenum-polymetallic deposit. Within this area, cryptoexplosive breccias, rhyolites, and the Archean biotite amphibole plagiogneisses are exposed on the surface. Based on previous data, it is speculated that the ore-bearing rhyolite porphyries occur beneath the cryptoexplosive breccias, with a burial depth generally exceeding 500 m. However, the deep-seated molybdenum-polymetallic deposits can barely be accurately located with traditional geophysical methods. Therefore, this paper used the audio magnetotelluric (AMT) method for the first time to locate the concealed porphyry-hosted molybdenum-polymetallic deposits in this area. Based on the low-resistivity characteristic of the molybdenum-polymetallic ores, and with 1000 Ω·m as the upper limit, the range of low-resistivity anomalies was delineated in the deep strata of the study area. Then, combining the geological metallogenic patterns in the study area, this study tried to locate the deep-seated concealed molybdenum-polymetallic deposits based on low-resistivity anomalies. According to the speculated favorable metallogenic positions of molybdenum polymetallic ore bodies, two boreholes were drilled for verification. An industrial-grade molybdenum ore body with a thickness of 504 m was discovered deep in No. 1 borehole, while a molybdenum ore body with a thickness of 185 m and a lead-zinc ore body with a thickness of 200 m were discovered at depth in No. 2 borehole, successfully predicting the positions. Moreover, based on the resistivity distribution revealed by AMT, a three-dimensional geophysical model was established for the exploration of porphyry-hosted molybdenum-polymetallic deposits in this area. This model provides a new geophysical approach for locating deep-seated concealed porphyry-hosted molybdenum-polymetallic deposits.