Of Qinghai Xitiesan Mining Causes Of Schist-type Pb-zn-bed

In 2005,a kind of rich and thick Pb-Zn ores was found in the deep carbonaceous schist between the Zhongjian ditch and Duanceng ditch of the Xitieshan mine area,breaking through the only exploration idea of encircling the mable.At present,no person has studied the genesis of this deposit,now this paper will give particular expaining about the genesis from the aspects of metallogenic background,geological characters of ore deposits and deposit geochemistry.The author estimate the vocanic rocks of Tanjianshan group situated in an environment of a paleorift trough on the basis of lithological characteristics,petrochemistry and geochemistry,and the Xitieshan carbonaceous schist type Pb-Zn ores occurenced in the stationary phase of the rift trough.the rift trough volcanism supplied the deposits with enough material and energy sources.The carbonaceous schist type Pb-Zn ores mostly occurenced in the carbonaceous fine clastic rock in the middle sedimentary rocks(O₃t^c),according to the horizon between the marble type Pb-Zn ores and the carbonaceous schist type Pb-Zn ores,the Xitieshan Pb-Zn ores shows the characteristics of two-tier structure.the marble type ores formed older than the carbonaceous schist type Pb-Zn ores,and both of their horizon are steady.Researches have made clear that these two ores show the hydrothermal exhalative-sedimentation mineralization structure from silicified alteration zone,to the stringer system mineralization,last to the bedded ore-body.Through reseaches on the geochemical characteristics of the carbonaceous schist type Pb-Zn ores,the author advance some important conclusions as follows:the ore matter origin was composed by Pb-Zn from the lower volcanic rocks and the upper mantle;the sulfur sources of the ore matter origin was composed by the seawater sulfur,volcanic sulfur and the upper mantle sulfur;the mineralizing solution was composed by magrnatic water and seawater;the metallogenic fluid of the Xitieshan carbonaceous schist type Pb-Zn ores was NaCl-H₂O system,the homougeneous temperature was tested to 100～300℃,the average salinity was about 9.8wt%NaCl.eq.,the average density was 0.99g/cm³,and the ore-forming metallogenic pressure was evaluated to 30MPa,its physicochemical enviroment was weak acid and weak reduction.In conclution,the Xitieshan carbonaceous schist type Pb-Zn ores can be classed as the Sedex deposits in the world.