甘肃阳山金矿区安昌河—观音坝断裂带显微构造特征及地质意义

甘肃阳山金矿是我国最大的金矿床,位于西秦岭造山带的陕甘川"金三角"地区。金矿成矿时代为早侏罗世,与燕山期斜长花岗斑岩有密切的成因联系。基于野外地质调查,本文对安昌河—观音坝断裂带构造岩进行了细致的显微构造研究,以期通过微观构造特征认识宏观断裂构造的活动规律。镜下观察表明断裂带内兼具大量的脆性与塑性显微变形,主要发育左行剪切,暗示该断裂为左行韧-脆性剪切带。断裂带内构造岩经历了高绿片岩相、低绿片岩相及低于绿片岩相的变质-变形过程,且断裂带内至少存在过三到四期构造变形,为断裂带曾发育"多期构造变形"提供依据。显微构造应力分析及岩层产状等密度图显示区域主压应力方向为NNW-NNE,是对印支期以来多期主应力方位的综合反映。据亚颗粒法及动态重结晶法计算的成矿前古应力差值为128.6～95.8 MPa,成矿期古应力差值为74.9～69.3 MPa,成矿后古应力差值为65.8 MPa。综合分析认为中—晚三叠世以来安昌河—观音坝断裂带变质相相变为高绿片岩相→低绿片岩相→低于绿片岩相,变形序列为韧性→韧-脆性→脆性,区域主应力大小发生了大→小的转变,主应力方位经历了SN向挤压→NE向挤压→NW及SN向挤压的转换。安昌河—音坝断裂带构造演化特征反映其经历了从深部到浅部逐渐抬升的过程。

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

The Yangshan gold deposit in Gansu Province,the largest gold deposit in China,lies in the ShaanxiGansu-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.