Multiscale 3-D imaging of the crustal electrical structure beneath the Caosiyao porphyry Mo deposit, North China

Abstract

SUMMARY We proposed a stepwise procedure to obtain a multiscale 3-D resistivity model beneath the Caosiyao porphyry Mo deposit. Due to the joint inversion of data collected from 302 audio-frequency magnetotelluric and 33 broad-band magnetotelluric sites, this model can simultaneously combine both the shallow resolution and detection depth. The near-surface part of the model reveals the high-resolution structures. Specifically, a high-resistivity body (>500 Ω m) is interpreted as the late Jurassic porphyry intruding into the metamorphic basement. Two NE-trending high-conductivity (<100 Ω m) belts are interpreted as fault zones. The one in the southeast extends deeply (>15 km) is suggested to be a crust-scale weak zone and conduit for transporting ore-forming magmas and fluids. The other one is shallow (<2 km), but coincides with outcropped orebodies and alteration zones, so it may have played an important role in the hydrothermal alteration and mineralization. A predominant high-conductivity block (<30 Ω m) occurs within the lower part (∼15–30 km) of the model and connects with Caosiyao deposit through the inferred crust-scale fault. Its enhanced conductivity may result from the abundant fluids. We determined it had been a mid-lower crustal magma chamber providing space for the mixing of ore-forming magmas and fluids, as well as the element exchange during the mineralization. Combined with a previous magnetotelluric imaging, we suggested the heats and fluids needed for the mineralization might have been transported from a giant upper-mantle basaltic magma reservoir as indicated by a conductor beneath the Datong volcanic area through a lithosphere-scale shear zone. Summarily, the multiscale resistivity model provides us a chance to depict the multiscale metallogenic system for the Caosiyao porphyry Mo deposit from the aspect of electrical resistivity.