Structural And Mineralization Characteristics Of Near East-West Trending Veins In Xiaoqinling Gold Field,Shaanxi Province

The Xiaoqinling gold field is located along the southern margin of the North China Plate and is the second largest gold field in China.The study area has undergone multiple superposition of structural deformations,forming the structural features of the Eitai and Xiaohe boundary faults,Taihua complex anticlines,and multiple sets of directional shear zones and faults as grids.It comprises primarily medium-to high-temperature hydrothermal quartz-vein-hosted deposits.The direction of the gold-bearing veins is consistent with the shear zone and can be generally divided into four groups:nearly east-west,north-east,near-south-north and north-west.And the large-size gold deposits are prliminarlly distributed in the near east-west shear zone.Previous studies indicate that near east-west veins have undergone multi-stage structural deformation superposition,and their metallogenic stages can be divided into four phases:pyrite-quartz phase,quartz-pyrite phase,sulfur-polymetallic phase,and carbonate-quartz phase.However,there are still controversies about the ore-controlling characteristics of the ore-forming periods and the metallogenic epoch.The paper selects a number of typical veins?Q507,Q8,Q61,Q886,Q195,Q173,etc.?developed in the three near-east-shear shear deformation belts?Dayueping,Beigaizi-Zhuyu,Sunjiagou-Huaersi,etc.?in Xiaoqinling area,in Shaanxi Province.Detailed structural analysis,metallogenic composition,pyrite typomorphism,fluid inclusions,and isotopic dating have been studied.The results show that:?1?The nearly E-W shear deformation zone has undergone an overall superposition of three structural deformations,such as early left-lateral shear?D1?,medium-term right-lateral thrust?D2?,and late left-lateral normal fault?D3?.At the same time,it also experienced the transformation of the northeast to the leftward thrust deformation.?2?The quartz veins are generally developed with four mineralization stages,such as no ore-bearing quartz veins?I?,pyrite quartz veins?II?,polymetallic sulphides quartz veins?III?and calcite quartz veins?IV?.The gold-bearing minerals are mainly pyrite,chalcopyrite,and galena.The mineralization is distributed along the shear plane.?3?The pyrite typotype is characterized by its proportion of more than 90%of granular particles;the ore-forming fluid is generally a low-salt,CO₂-rich NaCl-H₂O-CO₂system;?4?The Re-Os of pyrite in the Q61 vein dating isochron age is 231 Ma.The comprehensive analysis of the typical vein structure and metallogenic characteristics in the near east-west shear zone indicates that the near-west-western veins in the Xiaoqinling gold ore zone are obviously affected by the pyrite-forming quartz veins?II?and polymetallic sulfides?III?.The right side of the thrust deformation?D2?control,the distribution of ore-forming materials along the shear plane and the shear zone within the shear zone,is generally strip or stripe,the ore-forming fluid has the characteristics of orogenic gold deposits.The mineral age is Indo-Chinese,and may be related to the Indosinian Qinling orogeny.The results of this study further identified the mechanism of ore-controlling ore-forming mechanism of the nearly E-W veins in the Xiaoqinling gold field,and provided important evidence for prospecting prediction in the Xiaoqinling gold deposit and the study of the Mesozoic structural evolution in the Qinling orogenic belt.