Microstructural characteristics of Anchanghe-Guanyinba fault zone in the Yangshan gold deposit of Gansu Province and their geological significance

Abstract：The Yangshan gold deposit in Gansu Province, the largest gold deposit in China, lies in the Shaanxi-Gansu-Sichuan triangle area of the western Qinlin Orogen. The gold deposit was formed in the Lower Jurassic, closely related to the Yanshanian plagioclase granophyre. Based on the field work, the authors carried out systematic microstructural researches on tectonites of Anchanghe-Guanyinba fault zone, in the hope of reflecting the macroscopic structure by means of the microscopic structure. By observing the characteristics of the microscopic structure, the authors have found that brittle and ductile deformation is extensively developed in this area, and the shear orientation is mainly left-lateral, implying that the fault is a sinistral ductile-brittle shear zone. The deformation analysis shows that the metamorphic facies of this fault zone underwent the metamorphism-deformation of high greenschist facies, low greenschist facies and facies lower than the greenschist facies. The observation of the microscopic structure reveals that the structural deformation can be divided into 3 or 4 stages at least, thus providing evidence of multistage structural deformation in the fault zone. Stress analysis and isodensity diagram of stratigraphic attitudes indicate that the regional principal stress is in NNW-NNE direction in this area, synthetically reflecting the principal stress orientations since the Indo-Chinese epoch. Calculated by subgrain method and dynamic recrystallization method, the premineral differential stress was 128.6-95.8 MPa, the differential stress during the mineralization stage was 74.9-69.3 MPa, and the postmineral differential stress was 65.8 MPa. According to the research results, the authors hold that the transition of metamorphic facies in the fault zone was high greenschist facies→low greenschist facies→facies lower than greenschist facies from Middle-Late Triassic, whereas the deformation sequence being in order of ductile→ductile-brittle→brittle. The values of regional principal stress were changed from high to low and the regional principal stress orientation was SN compressional→NE compressional→NW and SN compressional. All these evolutionary characteristics imply that the fault zone was gradually uplifted from the depth to the shallow part.