Paleogeographic and Structural Controls on the Sediment-Hosted Zhaishang Au Deposit, West Qinling Orogen, China

Abstract Structural and paleogeographic controls on sediment-hosted Au deposits are investigated using the giant Zhaishang Au deposit in the west Qinling orogen as an example. Two distinct types of mineralization are identified. The first is fault-controlled mineralization within Devonian Fe-rich calcareous siltstone, deposited in a platform-edge environment. The second is stratabound-replacement mineralization within Permian carbonaceous mudstone. This mudstone, which contains up to 1.46% organic carbon, was deposited in a low-energy, deep-sea environment. Geophysical sections demonstrate that the two types of Au mineralization are bounded by a WNW-ESE–trending fault called F5, which extends to a depth of at least 1 km with secondary mineralized structures converging into it. New structural mapping reveals that deposit-scale, NW-SE–trending locked-up folds and thrust faults developed under north-northeast–south-southwest compression at ca. 279 to 275 Ma according to the sericite Ar-Ar dating. In situ U-Pb dating of Au-related apatite from mineralized veinlets indicates that the WNW- to E-trending reverse faulting, stratabound replacement, and Au mineralization took place at ca. 220 Ma. The fault-controlled Au in the Devonian beds comprises polymetallic sulfides associated with decarbonation with both native gold and invisible Au within pyrite. In contrast, the stratabound-replacement Au orebodies in the Permian beds comprise pyrite and arsenopyrite hosting invisible Au associated with quartz alteration. The contrasting mineralization styles can be attributed to variations in host rocks and their mechanical competency determined by distinct paleogeographic settings and different pathways for fluid-rock interaction.