The Beishan metallogenic belt in Gansu Province is a significant geological tectonic unit in northwestern China, hosting abundant metal mineral resources. Recently, mineral exploration carried out in the southern part of Beishan identified a marine volcanic-hosted copper-polymetallic deposit for the first time. To achieve a breakthrough in prospecting and effectively discover similar deposits in such areas, GREATEM (grounded electrical source airborne transient electromagnetic) method, which integrates a high-power grounded transmitter with rapid data acquisition via a drone-mounted receiver, was employed to conduct a large-scale exploration in the known mining area and its periphery. A spatially constrained quasi-3D regularized inversion method was adopted to extract the characteristics of the subsurface 3D resistivity structure. The low-resistivity anomalies corresponding to copper-polymetallic ore bodies were confirmed by validation tests along the drilling section. By integrating statistical petrophysical information with observed response characteristics, a synthetic analysis was conducted to further verify that the low-resistivity responses obtained by the GREATEM method in this region were characterized by lower early-time amplitudes and an overall slow decay. The results provide an important basis for identifying prospective target areas for volcanic-hosted copper-polymetallic deposits and demonstrate the practicality and effectiveness of the GREATEM method in such areas. Overall, this work not only highlights the great capability of the GREATEM method for detecting deep concealed deposits in complex mountainous areas, but also provides a novel technology for the exploration of critical strategic mineral resources in the arid desert regions of Northwest China, thereby offering a significant technological and methodological option for the new round of national breakthrough strategic action of exploring mineral resources.