Structural Evolution And Dike Emplacement Mechanism During Gold Mineralization In The Wulong Gold Mine Filed, Eastern Liaoning

The Wulong gold deposits to the west of Dandong,Liaoning Province are situated in the northeastern North China Craton.The deposits are typical representatives for the Liaodong ore concentration area,and belong to the quartz-vein type ore.A series of NNE-,N-S-and NW-striking dikes,including diorite,granite,granitic pegmatite,lamprophyre and diabase dikes,occur in the gold mine field,and show genetic relation to the gold mineralization.However,emplacement machanism of the dikes remains unclear.Features and evolution of structures developed before and during the gold mineralization were investigated and analyzed in this study.The dikes and plutons were dated with the U-Pb method.Corss-cutting relation of the dikes were observed in the field,their evolution was sumerized,and relationship between the dike emplacement and the gold minerlization was analyzed in this work.These results provide us with inportant imformation on structural-controlling mechanism of the fold deposits and the mineralization time.Our field observation shows that the Wulong pluton of Middle Jurassic was involved in penetrative,ductile deformation as shown by the widespread gneissosity and local ductile shear belts.The shear belts strike NE-SW and dip SE at low angles.Mineral elongation lineation in the deformed rocks plunge from ENE to ESE gently.Macro-and micro-structures in the shear belts suggest NWW-directed thrusting.The microstructures also indicate deformation temperatures of 400~500°C,local >500°C,for the exposed structures.Dating results of the deformed plutons and the dikes constrain the development of the ductile deformation to ca.161 Ma.From comparision with shortening events of Mesozoic in the northern North China Craton,it is suggested that the ductile deformation in the Wulong pluton corresponds to the A episode event of the Yanshan Movement.This event results from low-angle subduction of the Izanagi Plate in the Pacific Basin underneath the East Asia continent.A series of NE-striking brittle faults appear in the Wulong gold mine field.It is demonstrated that the brittle faults originated from sinistral faulting.The study area suffered nearly N-S compression at the beginning of Cretaceous,leading to formation of the NE-striking sinistral faults and associated conjugate fractures.U-Pb ages of igneous rocks cut by the faults suggest 145-131 Ma for the sinistral faulting.Regional tectonic compreison inidcates that the N-S compresion in the Wulong gold mine filed corresponds to the B episode event of the Yanshan Movement.The sinistral faulting of the easliest Cretaceous caused by regional N-S compression resutls from low-angle,high-speed subduction of the Izanagi Plate towards NNW and underneath the East Asia continent.The study area was involved in WNW-ESE extension in the follwing Early Cretaceous(132-113 Ma),leading to formation of NE-to NNE-striking normal faults and reactivity of previous structures.The area experienced emplacement of the Sangouliu pluton(132-120 Ma)in the south,the Wulongbei pluton(127~126 Ma)in the south and a series of diorite,granite,granitic pegmatite,lamprophyre and diabase dikes during the Early Cretaceous extension.The diorite and granite dikes are closely associated with auriferous quartz veins.Our zircon U-Pb dating give 127-113 Ma intrusion time for the diorite and granite dikes.The Early Cretaceous dikes are syn-extensional dikes.The dikes intruded along both the newly-formed,NE-to NNE-striking normal faults and the previous faults or fractures reactivated during the extension,leading to coexistence of the Early Cretaceous dikes with vaired orientations.We propose that the auriferous quartz veins share the similar emplacement mechanism with the synchronous dikes in the Wulong gold mine field.It is suggested therefore that the gold mineralization in the Wulong gold field took place during the period of 127-113 Ma when the North China Craton experienced the peak destruction.However,direct,reliable dating results from ore minerals are needed to constrain the accurate,gold mineralization time in the Wulong gold field.