Pyrite-Sericite-Quartz Alteration And Gold Mineralization Mechanism Of The Dayingezhuang-Xiadian Gold-field,Jiaodong Peninsula,China

Jiaodong is the third largest gold ore-concentrated region in the world.And the fracture zone alteration-type gold deposit is the most important gold deposit type of the region.The gold orebody of the alteration-type mainly develops in the pyrite-sericite-quartz alteration zone which is in the footwall of the fault zone.And the intensity of alteration is directly proportional to the strength of mineralization.Therefore,it is the key to reveal the mechanism of gold precipitation and the genesis of the deposit to deeply explore the process and mechanism of the alteration of the pyrite-sericite-quartz alteration.Based on the system geology and geochemical study of structure-alteration-gold mineralization in Dayingezhuang-Xiadian gold field,the main achievements are summarized below:The linglong granite which intruded at 153.0±1.8 Ma-149.3±2.5Ma is the host rocks of the deposit.Subsequently,with the large-scale disassembly and ductile deformation of linglong detachment fault,K-feldspar alteration,pyrite-sericite-quartz alteration developed in the host rocks.K-feldspar occurred in the premineralization,and the Au are released into ore-forming fluid because of the interaction between fluid and wallrock,which laid the foundation for the gold precipitation in the process of pyrite-sericite-quartz alteration.The detachment fault was a ore-forming fluid channel,which controls the spatial distribution of wall rock alteration.The Guojialing granite intruded at 125.8±1.6Ma which provides thermo-motive for large-scale gold mineralization and the process of pyrite-sericite-quartz alteration.At 130.1±4.4Ma-130.8±7.2Ma,with the ductile-brittle transition of detachment fault,small-scale alteration and mineralization occurred in the footwall of main detachment fault.This stage formed in higher temperature?about 280-400??.At this time,it is in an environment of rapid cooling,low oversaturation and low oxygen fugacity.In addition,the permeability of ductile-brittle structure is relatively low,the water-rock reaction in the shear zone is relatively weak,and the feldspar partially altered into sericite and fine grain quartz.The pyrite elongate and quartz dynamic recrystallization are obvious in the ore,which were the characteristics of the ductile-brittle deformation.Large scale brittle deformation superposition occurred at 120±5Ma.The permeability of the fault zone increased dramatically,the deep-source metamorphic fluid poured in,and the water-rock occurred strong reaction,and gold precipitation were all occurred at this time.Which formed in medium and low temperature?300-180??.At this time,the ore-forming fluid has high supersaturation and high oxygen fugacity,and the material supply is sufficient while cooling slowly which benefit to the enrichment and precipitation of ore-forming materials.At the same time,the vulcanization reaction and the fluid phase separation,which lead to the H₂S of the ore-forming fluid was largely released,resulting in the unstable decomposition of Au?HS?^2-complex.Finally,the gold precipitates mainly in the form of fissure gold and a small amount of inclusion gold in pyrite,accompanied by the precipitation of other polymetallic sulfide.The Dyingezhuang-Xiadian gold field have the typical characteristics of Jiaodong-type gold deposits:the gold mineralization is controlled by large scale pyrite-sericite-quartz alteration zone which controlled by the regional fault zone.The process of pyrite-sericite-quartz alteration is closely related to gold mineralization.Both fluid-wallrock interaction and fluid phase separation during the alteration process are the main precipitation mechanism of gold.