Intracontinental orogenic structure-altered lithofacies mapping and prospecting model of the Changgou gold deposit in the Hanyin gold orefield, South Qinling

The large-scale tectonic altered lithofacies mapping technology is a new technology gradually formed in the study of the tectonics-physics-chemistry of the orefield and the comprehensive study of the whole survey area. Based on the survey of the Shiquan-Xunyang gold mineraliztion belt in the South Qinling Mountain and the large-scale orefield structure mapping demonstration, it is considered that the 1:25000 tectonic-lithographic mapping implemented at the orefield scale and 1:10000 or 1:5000 structure-altered lithofacies mapping implemented at the mining area scale are feasible and effective. In the past ten years, the gold orefield in the north of Hanyin County, South Qinling, has made great progress in ore prospecting by structure-altered lithofacies mapping and special studies. The Changgou gold deposit in Hanyin County is a new type of altered rock type gold deposit controlled by brittle-ductile shear zones, which was discovered during comprehensive research on ore structure-altered rock facies mapping in recent years. The Changgou gold deposit is located in the DSZ3 brittle-ductile shear zone (RF5), where the lithology is mainly mylonitized biotite-bearing phenocryst sericite quartz schist and garnet bearing sericite quartz schist of Lower Silurian. The alterations of the rock closely related to mineralization are biotitization, silicification, pyrite mineralization, and sericitization. Where biotite morphology is developed and quartz veins are developed densely accompanied by pyrite mineralization and is about 80~150 meters away from the later stage main fracture belt, the gold mineralization grade is relatively high. The ore-forming fluid mainly belongs to the medium-low temperature, medium-low salinity, and low-density NaCl-H₂O-CO₂ system. The ore-forming pressure is from 41.03 MPa to 98.04 MPa and the ore-forming depth is from 1.52 km to 3.63 km. The granodiorite veins and granite aplite related to mineralization measured by U-Pb zircon LA-ICP-MS have ages from 180.2±3.6 Ma to 176.0±1.9 Ma. The ⁴⁰Ar-³⁹Ar age of biotite is 178.44±0.81 Ma, and the reverse isochron age is 178.20±0.76 Ma. It is obvious that the ore-forming epoch of altered rock and gold is early Jurassic, and the ore-forming age belongs to Yanshanian intracontinental orogeny in this orefield. Ore-controlling structures and metallogenic structural planes belong to the brittle-ductile shear zone and its fracture system. On the basis of this study, a prediction model of the intracontinental orogenic hydrothermal altered rock type gold deposit in the Changgou gold mining area was established, and it is inferred that the extensional shear fold superimposed altered rock on the left and the periphery of the northwest of the mining area seems to be a rich ore block. In addition, the prospecting direction in the deep area is pointed out.