The Beishan metallogenic belt in Gansu Province is a significant geological tectonic unit in northwestern China, hosting abundant metal mineral resources. Recently, the mineral exploration work carried out in the southern part of Beishan has identified a marine volcanic-hosted copper-polymetallic deposit for the first time. To achieve a breakthrough in prospecting and effectively discover similar mineral deposits in such areas, the grounded electrical source airborne transient electromagnetic (GREATEM) method, which integrates a high-power grounded transmitter with rapid data acquisition via a drone-mounted receiver, was employed to realize a large-scale exploration in known mineral and peripheral areas. In this paper, a spatially constrained quasi-3D regularized inversion method was adopted to extract the characteristics of the subsurface 3D resistivity structure. Several relatively low-resistivity anomalies associated with the copper-polymetallic ore bodies were confirmed by validation tests based on the drilling section. By integrating statistical petrophysical information with observed response characteristics, one synthetic analysis was conducted to further verify that the low-resistivity responses of the GREATEM responses in such region are characterized by lower early-time amplitudes and an overall slow decay. The result provides an important basis for identifying prospective target areas for volcanic-hosted copper-polymetallic deposits and demonstrates the practicality and effectiveness of the GREATEM method in such areas. To sum up, 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.