A Discussion On Genesis Of Yuqia Gold Deposit In Da Chaidan Of Qinghai Province

Yuqia gold deposit is located in the area of Lvliang mountain and Shuang kou mountain in northern qaidam basin.The wall rock is variable clastic rock group of Tanjian mountain.Wall rock alteration of silicification and pyritization are most closely related to gold mineralization.It can be used as direct ore-prospecting criteria.Ore types are dominated by quartz vein type and followed by tectonic altered rock type.The hydrothermal mineralization period can be divided into four stages:quartz —epidote stage,quartz—polymetallic sulfide stage and pyrite—quartz stage,carbonate rock stage.Deposit is mainly located in the secondary fault in ductile-brittle structure transformation of upper plate location of Lvliang mountain large ductile shear zone.The entire mineralized zone and the single ore body is characterized by the brittle deformation more intense in the north side,but the ductile deformation characteristics is more significant in the sorth side,may indicate mineralization to the laws of south.Homogenization temperature of fluid inclusion is 188.7 ~ 188.7 ?,the salinity is 1.22 ~ 11.10%,density is 0.77 ~ 0.95 g/cm3,pressure is 45.91 ~ 76.57 MPa.The metallogenic depth is 5.34 ~ 7.12 Km with mean depth of 6.23 Km.These parameters have a growing tendency from south part to north part in the ore body which reverse demonstrates the characteristics of the ore body to the south side.The gas phase compositions of inclusions are CO2 and small amount of CH4.The ore-forming fluid is low temperature,salinity,density,and full of CO2.Hydrogen and oxygen isotopes shows that ?D of the main metallogenic stage changes from 58.5 ‰ to 41.6 ‰,?18OH2O is between 5.20‰ and 5.20 ‰.It indicates that the fluid mainly comes from the metamorphic water.The sulfur isotopic values of pyrite is 0.5~7.4‰ with mean value of 4.43%,which changes relatively large.The sources of sulfur mainly come from magma of the deep source and may mix the shell source material in different degrees.The Pb isotopic compositions of Yuqia deposit are 206pb/204pb=18.238 ~ 18.62(mean value: 18.3133),207pb/204pb=15.59 ~ 15.618(mean value: 15.604),208pb/204 pb =38.039 ~ 38.775(mean value: 38.1697).It is relatively enrichment Pb of uranium and a slight loss in thorium Pb.The ? of Pb is 9.45 ~ 9.45,average is 9.48,and the ? value is 35.64 to 35.64,average is 35.97.On the growth curve diagram,the Pb isotopic data are on and near the evolution of orogenic belt line.On the tectonic discrimination diagrams,the Pb isotopic data are within the scope of orogenic belt.It is believed that the source of Pb in Yuqia gold deposit is mainly from orogenic belt,and also mix shell source and curtain Pb inordinately.The gold of Yuqia Gold deposit mainly exsits in the form of independent minerals and enters the gold-bearing mineral in the form of solid solution.The main gold-bearing minerals of this deposit is pyrite.The diagenetic age of gabbro was 387.5±7.2Ma.The gabbro is commonly mineralized,alteration and ductile deformation,which suggests that formation age of Yuqia gold deposit is later than the diagenetic age of gabbro.That is,the invasion of gabbro is not very important in the Mineralization of Yuqia gold deposit.Yuqia gold deposit has universal characteristics of typical orogenic gold deposits and It belongs to the orogenic gold deposit.