Ore-forming Fluid Evolution And Metallogenic Progress Of The Yangshan Gold Deposit,Gansu Province

The Yangshan gold deposit is a superlarge gold deposit in Gansu province,China.Based on the field observations and previous studies,the Yangshan gold deposit is systematically studied to determine the metallogenic stage and the metallogenic age,and carried out microthermometry and composition analysis of fluid inclusions,and H-O isotope to constrain the evolution and metallogenic progress of the deposit.1,According to the field investigation and previous work,we put forward the view that the Yangshan gold deposit has multi-period mineralization and can be divided into sedimentary diagenestic period,metamorphic hydrothermal period,magmatic hydrothermal period and supergene oxidation period,Every hydrothermal periods can be divided into different stages.Metamorphic hydrothermal stage can be divided into quartz sericite pyrite stage,pyrite arsenopyrite quartz stage,arsenopyrite pyrite quartz stage and quartz carbonate stage;magmatic hydrothermal period can be divided into quartz pyrite stage,natural gold stibnite quartz stage,and quartz carbonate stage.2,The ore-forming fluids of two mineralization periods are characterized by medium-low temperature(110°C~380°C),low salinity(<8wt.% NaCl eqv)and low density.The ore-forming fluid in the metamorphic hydrothermal period is rich in CO2,and the homogenization temperature and salinity are concentrated at 220°C~320°C,and 2.0~6.6wt.% NaCl eqv,respectively.The ore-forming fluid in the magmatic hydrothermal period is rich in organic gas,the homogenization temperature and salinity are concentrated at 120°C~220°C,and 2.0~6.0wt.% NaCl eqv,respectively.3,The H-O isotopic result of ore-forming fluids show the characteristics of ore-forming fluids.In combination with the characteristics of fluid inclusion above,H-O isotopic results indicate that the early metamorphic hydrothermal fluids were derived from the metamorphic fluids of the lower crust.In the late magmatic period,the fluids maybe the mixture of magmatic hydrothermal and meteoric water.4,Considering the results of fluid inclusions and H-O isotope study,we thought that decline of the pressure of the fluid,lead to a large amount of gas such as CO2 escape due to the fluid boiling,the decline of the temperature of the fluid and the rising of the salinity and the absorbtion of Au in the surface of pyrite,which ultimately resulted in the precipitation of gold and pyrite in the metamorphic hydrothermal period.During magmatic hydrothermal period,the balance of the hydrosulphide complexes was destroyed due to the input of organic gas and water-rock interation,making a large amount of the precipitation of sulfur hydrogen complexes.