Metallogenic Model Of The Zhengchong Gold Deposit,jiangnan Orogen

Jiangnan Orogen,is the third largest gold producing area with more than 250 gold deposits and?970 t gold reserves.However,the understanding of metallogenic model of gold deposits in this belt is still controversial.Zhengchong gold deposit,a hydrothermal vein-type gold deposit hosted in the metamorphic sedimentary rocks,is located in the Changsha-Pingjiang gold metallogenic belt in the middle part of orogenic belt.NNE-NE-and NW-trendings are the main ore-controlling faults in the deposit.Neoproterozoic rocks are the main host rock in the deposit,and a little NNE-NE ore veins cut the early granites.The ore types are mainly pyrite-(arsenopyrite)-sericite-quartz mineralized rock,gold-bearing sulfide-quartz vein and structural breccia type.Based on structure-alteration-mineralization field mapping,mineral assemblage and microstructure,this paper studies the sources of ore-forming materials and fluids,gold precipitation mechanism,coupled structure-fluid mechanism and metallogenic age,and further determines the metallogenic model of Zhengchong gold deposit.The main achievements are summarized below:(1)The ore-controlling structures in Zhengchong are characterized by two stages of activation.Before the mineralization,a large number of NNE-NE trending lamination cleavage of slate were preserved in the deposit.These structures were formed straight NNE-NE trending right reverse faults and filled the barren quartz veins under the near E-W compression in the early stage of mineralization.The block rotates horizontally counterclockwise between the reverse faults,and formed NW trending structural fracture zone.The both secondary extensional veins developed in the footwall and hanging-wall and geometries of the NW-and NNE-NE-trending orebodies indicate that the faults developed normal faulting activities under the background of local extensional.The barren quartz vein was ruptured and filled with a large number of fine sulfide-quartz veins.(2)It is pointed out that the metallogenic material and fluid of the vein-type gold deposits,represented by Zhengchong gold deposit in this area,are origined from the metamorphic fluid.Gold-bearing pyrite and arsenopyrite sulfur,lead isotopes,quartz hydrogen,oxygen isotopes in the main metallogenic stage show that the ore-forming materials and fluid in the main metallogenic stage of Zhengchong deposit may origin from deep metamorphic rocks.The metamorphic fluid was generated during the transformation from greenschist facies to amphibolite facies by devolatilization,which produced a large number of H₂O,CO₂,S and Au.(3)Fluid-rock reaction and phase separation are the important gold precipitation mechanism in Zhengchong deposit.In the early mineralization stage,the temperature of mineralization is322?397°C,and sulfur fugacity is 10^-10.5?10^-6.7,constrained by mineral thermometers.In the main metallogenic stage,the mineralization temperature is 258?274°C,and high-grade banded NNE-NE-trending sulfide-quartz veins are formed by fluid pressure fluctuations,in which there is obvious Au-As elemental zoning of pyrite characteristics.In the NW trending fracture zone,a large amount of slate breccia is produced by hydraulic fracturing and participates in the efficient fluid-rock reaction.The sudden drop in fluid pressure leads to the instability of Au(HS)₂^-complex,which precipitates gold in a relatively reducing environment.(4)The Zhengchong gold deposit was formed during the collision of the South China and North China plates in Triassic.The ⁴⁰Ar-³⁹Ar age of hydrothermal muscovite closely related to gold mineralization is limited to 211 Ma.The early stage of mineralization started from 220 Ma to211 Ma.The long-distance effect of the collision of South China and North China plates caused the near E-W compression in the region,which leads the NNE-NE-trending foliation of slate initially occurred the dextral-thrust faults and formed the NW-trending fractured zone in the reverse fault.The main mineralization occurred that NW trending high-grade orebody and NNE-NE trending ore vein were formed under the fluid phase separation and local extension background at about 211 Ma.According to the geological characteristics and geochemical evidence of Zhengchong gold deposit,the deposit is classified as an orogenic gold deposit hosted in metamorphic sedimentary rocks.