Mineralogical characteristic of ductile–brittle shear zone in the Baguamiao gold deposit, Shaanxi Province: Implication for gold mineralization

Abstract

The Baguamiao gold deposit in Fengxian County of Shaanxi Province is located in Fengtai polymetallic ore concentration area in the orogenic belt of West Qinling Mountain. It is a typical gold deposit under the control of brittle–ductile shear zone. The progressive deformation of brittle–ductile shear zone has been found to be closely associated with gold enrichment: the gold‐bearing sulphides pyrrhotite and pyrite are distributed along blastobedding S0 in the undeformed area, they are developed along phyllitic foliation S1 (//S0) in the slightly deformed area, along mylonitic foliation (S2#S0), or S–C foliation in the highly deformed area, and they are enriched along rheological quartz vein (S2–L) and joint quartz vein (S3) in the highly deformed and the tectonic fluid area. Researches based on electron microprobe analysis and mineral–structure–geochemistry indicate that the characteristic metamorphic mineral assemblage of the brittle–ductile shear zone is albite–biotite–sericite–chlorite–quartz–ankerite, with diagenetic and metallogenic temperatures and pressures spanning from 350 to 450 °C and from 0.12 to 0.31 GPa, respectively. In addition, greenschist facies in intermediate temperature and intermediate and low‐pressure environment is the metamorphic facies of shear zone in main period. A reconstruction of the palaeotemperature history, based on the biotite and chlorite compositional geothermometer, revealed an intermediate‐temperature phase, a high‐temperature phase, and a high‐temperature critical phase. Biotitization tends to develop towards an oxidation phase, while chloritization is characterized by development towards a reduction phase, indicating large‐scale gold enrichment and mineralization were accompanied by strong redox reactions in the ore‐forming fluid.