Geological Characteristics And Genesis Of The Sefu Primary Gold Deposit,Qushui Country,Tibet,China

Epithermal gold deposits play an important role in all types of primary gold deposits,but currently there are few discoveries and reports of this type of deposits in the Gangdese metallogenic belt.The Sefu primary gold deposit,as a new discovery epithermal gold deposit,is located in the eastern part of the southern edge of the Gangdese.The research on the geological characteristics and genesis of the Sefu primary gold deposit not only enriches the research content of Epithermal gold deposits on the Gangdese metallogenic belt,but also further perfects the metallogenic series during continental collision between Indian and Asian plate,and enhance the discovery of this primary gold deposit on the Gangdese metallogenic belt in the subsequent research.The copper ore and gold ore bodies of the Sefu primary gold deposit are lenticular or stratiform in metamorphosed pyroclastic rocks in the south of Gangdese,constrained by the approximately EW and SN trending normal faults,respectively,and gold mineralization significantly postdated copper mineralization.The main deposit types are altered cataclastic rock type.The copper mineralization is mainly related to silicification and chlorite,and the gold mineralization is mainly related to low-temperature altered mineral assemblage of beresitization,siallitization and carbonation.The petrology,micro-thermometry and micro-laser Raman analysis of the fluid inclusions in the ore show significant differences between the gold-forming and copper-forming fluids.Fluid inclusions associated with copper mineralization are mainly AC species with higher density of CO₂,and a small amount of AV species with gas-liquid two-phase solution.Homogenization temperature of AC species and AV species concentrate at 287³06?and 181³47?,respectively,with the salinity of1.63³.76wt%NaCl.eqv and 4.49¹2.51wt%NaCl.eqv.Ore-forming fluids are characterized by the mixing of high-temperature low-salinity H₂O-NaCl-CO₂ system fluids,medium-low-temperature medium-salinity H₂O-NaCl system fluids and high-temperature medium-high-salinity H₂O-NaCl system fluids.Fluid inclusions associated with gold mineralization also are mainly AC species with lower density of CO₂,and a small amount of AV species with gas-liquid two-phase solution.Homogenization temperature of AC species and AV species concentrate at 184²73?and 226³33?,respectively,with the salinity of 7.14¹9.41wt%NaCl.eqv and0.70⁶.59wt%NaCl.eqv.Ore-forming fluids are characterized by the mixing of high-temperature low-salinity H₂O-NaCl system fluids and medium-low-temperature medium-salinity H₂O-NaCl-CO₂±CH₄±N₂ system fluids.H-O isotopic data suggest that fluids associated with copper mineralization show the inheritance of fluids associated with molybdenum mineralization,as a result of the mixing of magmatic hydrothermal fluids with meteoric water,and which may experience more water-rock reaction with H isotope exchanges.The fluids associated with gold mineralization originate from the mixed fluids composed by meteoric water,magmatic water and metamorphic water,and are significantly different from those associated with ductile shear zone.S isotopic data shows that the ore-forming materials of the gold ore body and the copper ore body are mainly derived from the products of magmatic degassing.However,the?³⁴S value of the gold ore body is lower,indicating a product of magmatic degassing under lower pressure conditions.The geological and geochemical characteristics indicate that the Sefu primary gold deposit is a low-sulfidation Epithermal gold deposit,and it is a product of the post-collision extension period after 21Ma.This deposit formation is driven by the deep heat sources,the gold-bearing magmatic water rising into the shallow surface,and then mixing with metamorphic water and meteoric water during reacting with the surrounding rock in a water-rock reaction.Finally,the minerals are precipitated by in the favorable structural parts.