Geological,Geochemical Characteristics And Genetic Mechanism Of Baishan Molybdenum Deposit,Eastern Tianshan,NW China

The Baishan Mo deposit is located in the northeastern part of the Kanggur-Huangshan ductile shear zone in Eastern Tianshan,Xinjiang,and is hostedin the Lower Carboniferous Gandun Formation.The ore body is vein like,and the vein group is distributed in the east-west direction,about 10km long and 400-700m wide.The mo grade of the ore body is about0.06%.The ore is mainly vein,net vein and disseminated structure,scaly,leaf,self semiself other and metasomatic structure.The mineralization stage of the Baishan mo deposit can be divided into:?I?pyrite-coarse-grained potassium feldspar quartz vein stage;?II?pyrite-fluorite-medium-grained quartz potassium feldspar vein stage;?III?polymetallic sulfide-calcite-medium potassium Feldspar quartz vein stage;?IV?molybdenite-fine grain calcite quartz vein stage.Among them,the polymetallic sulfide-calcite-medium-potassium feldspar quartz vein stage and the molybdenite-fine grain calcite quartz vein stage are the main mineralization stages.Within the area of Baishan molybdenum mine,granite body is developed and closely related to molybdenum ore space.The U-Pb geochronology of zircon shows that the emplacement ages of the biotite monzogranite in the west of gandun,the monzogranite porphyry in Baishan and the deep granite porphyry are 211.9 Ma?Late Triassic?,259.1 Ma?Late Permian?and 227 Ma?Late Triassic?,respectively.The geochemical characteristics of the Late Triassic gandunxi biotite monzogranite and the deep granite porphyry indicate that the magma was formed in the extensional tectonic environment after collisional orogeny;The geochemical characteristics of the Late Permian erchang granite porphyry indicate that it was formed in the compressional tectonic environment in the late collisional orogeny.According to the metallogenic age of Baishan molybdenum ore?225-229Ma?published by predecessors,it is believed that the mineralization of Baishan molybdenum ore is closely related to the deep granitic porphyry,which was formed in the extensional tectonic background after the collisional orogeny.The fluid inclusion study shows that the ore-forming fluid of Baishan mo deposit is characterized by low-middletemperature?98.9??394.3??,low-middle salinity?0.7?24.8wt%Na Cl?and low density?0.82?1.05g/cm3?.The ore-forming depth is 0.7?2.1km.The composition of hydrogen and oxygen isotopes(?D=-100.5‰?-91.1‰;?¹⁸O_H2O=-7.29‰?2.74‰)of ore-forming fluid shows that the ore-forming fluid of Baishan mo mine has the characteristics of mixing magmatic water and atmospheric precipitation,and is greatly affected by the addition of atmospheric water;In situ sulfur isotopes(?³⁴S_py=-1.44‰?1.49‰;?³⁴S_mo=-1.32‰?-0.63‰)of sulfide indicate that the sulfur in ore-forming fluid is mainly from magma.Compared with typical porphyry mo and quartz vein mo,Baishan mo deposit is obviously different from typical porphyry mo in ore form,ore structure,ore-forming fluid property and ore-forming material source.However,it is similar to quartz vein type mo deposit,so Baishan mo deposit may be quartz vein type mo deposit.