Adoption of a mineral system model in successful deep exploration at Erdaogou, China's deepest gold mine, on the northeastern margin of the North China Craton

The Erdaogou orogenic gold deposit, in the deepest gold mine in China, is situated within a dilational jog in the curvilinear Jiapigou shear zone, which broadly follows the margin of a Neoarchean granite intrusion within Neoarchean high-grade metamorphic basement on the northeastern margin of the North China Craton. Uranium-Pb dating of hydrothermal gold-related monazite provides a robust gold mineralization age of 151.8 ± 6.3 Ma. In contrast, zircon from the ore-hosting metamorphic rock has an age of 2514 ± 18 Ma and that from felsic dykes have ages from 216 to 231 Ma. The mineralization age is much younger than regional metamorphism and magmatism, indicating a subcrustal origin. Within the broad dilational NW-trending jog in the Jiapigou shear zone, various orogenic gold deposits display variable orientations relative to that of the controlling shear zone, implicating district-scale heterogeneous stress. As the strike of the subsidiary ore-controlling Erdaogou shear zone, changes from a N-S trend to a broadly NW-trend, the orebodies exhibit variations in thickness and gold grade. At upper mine levels, the moderately east-dipping Erdaogou shear zone displays reverse-slip movement, implicating a local more-extensional regime, but becomes near-vertical, implicating a more local compressional regime at intermediate depth levels, with a change to steep westerly dips at deeper levels. Based on this structural geometry, a 3D reverse-slip spherical rotation model, involving torsional stresses in a heterogeneous stress regime, is proposed for the ore-controlling Erdaogou shear zone with the vertical segment at intermediate mine levels situated at the maximum rotation point of the torsional motion. Importantly, the average T_h and salinity of fluid inclusions in main ore-forming stage quartz from the 1365 m to 1500 m mining levels indicates limited pressure gradients, implicating a deeper continuation of the gold orebody. Based on this structural model, a local more-extensional regime was predicted for the zone where the orientation of the shear zone changes from a vertical to westerly dip. Drilling of this structural target in 2020 successfully located the high-grade new orebodies No. N1 and N2.