Geological, Geochemical Characteristics And Ore Genesis Of Chahansala Gold Deposit In West Tianshan, Xinjiang Province

Chahansala,a newly discovered gold deposit in Western Tianshan,locates in the west of Yilianhabi'erga arc-trench belt of Later Paleozoic,which closely related with mid-Variscan granite magmatic activity and faults.Chahansala gold deposit was formed during the late orogenic tectonic -magmatic-hydrothermal process,Early stage of tensile structure-magmation and late stage of sustained tensile structures-hydrothermalization play important role of gold activation.In mining area,nearly east-westward Diorites body invaded to upper rocks of Qiergustao formation,and was fragmentated serious.The Ore body mainly occurs in a breccia zone of fine-grain diorite which intruded in the tuffaceous siltstone of Upper Carboniferous system,some occurs in contacted zone about the wall-rock,With the strike NW and dip NNE,the incline to southwest is the main trend of these Diorites body.Gold bodys are metasomatic vains,and was generated by diorite and surrounding rock which suffered multi-stage breccia,silicification and sericite hydrothermal alteration.Mineralization types are quartz vein and altered rock.The sulfide minerals is mainly pyrite,and some of pyrrhotite,chalcopyrite and galena.Gold present as native gold and electrum,and it was observed in the form of gold-sulfide(Te),Au-U minerals.Distribution of gold in ores are uneven,gold with relatively fine grain occurs as fracture gold and encapsulated gold in pyrite,less comonly as intergranular gold(between pyrite and vein mineral),or rarely as encapsulated gold in pyrrhotite,The size of Gold minerals are sub-microscopic and micro gold.The gold deposit experienced three epochs,represented as the hydrothermal -mineraliztion,structure-rupture and supergene leaching-enrichment.The study of C,H and O isotopes indicate that the ore-forming fluid of chahansala gold deposit is mainly composed of magmatic water and formational water,and the study of the fluid inclusions suggest there has two types in the mineralization,as follows:â‘ two-phase liquid-vapor inclusions andâ‘¡three-phase CO₂-rich inclusions.The fluid inclusions was characterized by low-meso temperatures and low salinity,and was CO₂-H₂O-NaCl system.The ore-forming depth was about 1.0km.The change of ore-forming stress caused fluid immiscibility, and led to gold precipitation possibly.The range of ore sulfidesδ³⁴S are -9.8‰～-7.3‰,average is -8.28‰,loss of ³⁴S obversely.Ore and Qiergusitao formation have the character of low levels of sulfide, lacking of sulfate minerals,respectively,which suggest that H₂S resulted from crust. The sulfides show a narrow Pb-isotope variation for ²⁰⁶Pb/²⁰⁴Pb(18.0359～18.1731), ²⁰⁷Pb/²⁰⁴Pb(15.5357～15.6115),and ²⁰⁸Pb/²⁰⁴Pb(37.9396～38.0974),the result show that the Pb isotopic data suggest a mixing between of upper mantle or lower crust and upper crust in different proportion.On the basis of these studies,combined with regional geological environment, Chahansala gold deposit belong to structure-alteration-type,which formed by structure-magma-hydrothermal processes during an extension at the end of orogenic movement.In the end,we summarized exploration marks and constructed mineralizat-ion model.