Metallogenic mechanism of the Qingshan orogenic Au-Pb deposit,northern Qaidam basin:Constraints from ore and pyrite composition
Qingshan Au-Pb deposit,located in Tanjianshan ore concentration area,northern Qaidam basin,is a paragenetic or associated Au-Pb orogenic deposit that is very rare in the world.The ore of the deposit mainly exists in the schist of the Paleoproterozoic Darken Daban Group,whose distribution is controlled by the NW-trending fault.There are two types of ores,i.e.,the altered rock type and the quartz vein type.Sulfides,including pyrite,limonite,galena,chalcopyrite,sphalerite and so on,are well developed in the ores.Among them,the pyrite is the main gold-bearing mineral in the deposit,whose geochemical characteristics not only indicate the existent state of gold but also reveal the properties of ore-forming fluid and the genesis of deposit.Samples of different types of ores and country rocks together with pyrite in them at different depths in boreholes ZK2103 and ZK3701 were selected to analyze their whole rock compositions and ore-forming elements.The chondrite-normalized REE distribution model curves of the measured samples are right-lean-shaped with obvious Eu negative anomaly,but no Ce anomaly,showing that the ore-forming fluid is from metamorphic without any seawater.The ores are relatively enriched Au,Ag,Cu,Pb,Zn,As,Co,Bi,Sb,W,Ni,Sr,Cd and V elements,but depleted in Sn,Mo and Cr elements.The S/Fe values of pyrite in the ore and country rocks are less than 2.0,showing they are depleted in S but enriched in Fe.The Au/Ag value of pyrite is greater than 0.5,and the results of Co-Ni diagram all show that the pyrite is of hydrothermal origin.The gold mainly exists in the form of nanoparticles,with a small amount of gold existing in the form of solid solution.Gold was mainly transported in the form of Au(HS)2-.However,the mixing of fluids destroys the stability of Au(HS)2-,that is most likely the reason for gold precipitation.The sulfur content in the ore-forming fluid is high,as a result,a certain amount of Cu,Pb and Zn will be migrated with sulfur.After the gold precipitates,the reduction of temperature and pressure in the fluid leads to the Pb precipitation.Due to the limited ability of the fluid to transport Pb and Zn,the lead ore of Qingshan deposit developed in small scale.The results of S and Pb isotopes show that the ore-forming material comes from the strata,however,contents of Pb and Zn in the strata of Jinlonggou and Qinglonggou mining areas is low,so there is no lead ore formed in this area.
Characteristics of ore-forming elementsMetallogenic mechanismOrogenic depositQingshan gold-lead depositNorthern Qaidam basin
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