Crustal electrical resistivity structure beneath the sediment-hosted stratiform Cu (SSC) deposits: A case study from Wuding area on the western margin of Yangtze Craton

• High-resolution imaging of SSC deposits using multi-scale magnetotelluric soundings. • High-conductivity fault system and high-resistivity diabase intrusions being imaged. • Cu-bearing strata formed under Mesoproterozoic rifting controlled by the fault system. • Evidence for enrichment and depositing of Cu due to superposed hydrothermal reworks. The Zhujiaba and Xiaoxinchang sediment-hosted stratiform Cu (SSC) deposits in Wuding area, located on the western margin of Yangtze Craton, are mainly hosted in the Mesoproterozoic strata and have also undergone multi-stage overprinted post-sedimentary hydrothermal reworks. There has been considerable debate over the metallogenic model. This research utilized a combined broad-band magnetotelluric (BBMT) and audio-frequency magnetotelluric (AMT) data acquisition method, which balances both detection depth and resolution. A NE-SW-oriented MT and AMT profile crossing both the Zhujiaba and Xiaoxinchang deposits was used to obtain a resistivity model of the upper crust to a depth of 10 km. Below these two deposits, two near-vertical high-conductivity belt coincidently exist beneath the known ore deposits, which may represent the main faults, extending deeper than 10 km. The upper part of them branches into several narrower conductive bands with slightly higher resistivity, possibly representing the secondary faults that cut through the higher-resistivity layers of Mesoproterozoic strata in the upper crust, dividing them into smaller blocks. These features may jointly make up an overall ‘finger-shaped’ intra-crustal fault system and a corresponding graben-type basin, which have played a key role in Cu mineralization in this region. Specifically, the determined main faults are likely old, deep faults associated with plate boundary suture zones that existed prior to the Cu metallogenesis. During the Mesoproterozoic sedimentary- diagenesis stage, these faults became active under the back-arc extensional tectonic environment, resulting in rifting and the formation of an ocean to the east of the Zhujiaba deposit. Marine sedimentation occurred, and seawater entered into the graben-type basin, resulting in a Cu ore-bearing layer between the Luoxue and underlying Yinmin strata. Following the formation of the sedimentary orebody, during the closure of oceanic basin, the fault system provided pathways for mantle-derived basic magma to ascend to the shallow Mesoproterozoic strata during the hydrothermal overprinting stage. This can be determined from two shallow (<2 km) ‘droplet-shaped’ high-resistivity bodies near the secondary faults beneath the known Cu deposits which are interpreted as diabase intrusions. As a result of the corresponding hydrothermal circulation caused, highly saline hot water raised along the breccia zones at the periphery of salt diapirs where deep faults existed, leading to the concentration of Cu elements and final mineralization.