Geological Characteristics And Genesis Of Jirimai Gold Deposit,Qinghai

Jirimai Gold deposit is located at the junction of Dulan County in Haixi Mongolian Tibetan Autonomous Prefecture and Mado County in Guoluo Tibetan Autonomous Prefecture.The mine is located in the East Kunlun orogenic belt uplift granite belt of Eastern Kunlun base in northern Kunlun fault.The strata of Jinshuikou Group of Upper Archaean to Lower Proterozoic in the mining area are widely exposed,accounting for more than 85%of the mining area.A small amount of tuff of Orashan Formation can be seen in the southeastern part of the mining area.The main faults in the mining area are faulted structures,and small interlayer folds are widely developed in common strata.The magmatic rocks in this area are mainly Indosinian monzonitic granite,distributed in the northern part of the mining area,and diabase dikes developed in the southeastern part.There are three orebodies in the area,one gold orebody in VI fracture zone and one gold orebody in V fracture zone.The orebodies are mostly vein-like and lenticular in shape.The grade of gold is between 1.15×10^-6-8.68×10^-6.The orebodies are strictly controlled by the mineralized structural alteration zone.The orebodies are all located in the altered fracture zone.The ore types in this mining area are mainly fragmented silicified quartz vein type ores.The ore minerals include pyrite,pyrrhotite,chalcopyrite,magnetite,hematite,galena,sphalerite,fibrinite and goethite,and natural gold.The wall rocks of gold deposits are amphibolite and monzonitic granite.The rocks have strong alteration phenomena,which are mainly silicification,kaolinization,carbonation and chloritization.The main mineralizations are pyritization and limonitization.Among them,silicification and pyritization are closely related to gold mineralization.The diabase in the mining area was formed in the late Hercynian period?250+1Ma?,characterized by crust-mantle mixing and active continental margin in the late subduction period?before collision?.Characteristics of fluid inclusions:Gas-liquid two-phase inclusions?type a?,carbon dioxide three-phase inclusions?type b?and pure carbon dioxide inclusions?type c?.Among them,the homogenization temperature of type a inclusions is 189.3-234.5?,salinity is 2.06%-9.34%,and density is 0.88-0.92 g/cm3.The homogenization temperature and density of carbon dioxide in inclusions of type B and C are 25.3-29.1?and 0.71-0.81 g/cm³.In the quartz-sulphide-poor quartz veins,only type a inclusions are found,of which type a inclusions are homogeneous at 170.2-232.2?,salinity is4.17-7.44%,density is 0.86-0.94g/cm3.The ore-forming pressure is 65-90 MPa and the corresponding ore-forming depth is 6.5-7.7 km.The values of delta?¹⁸O_H2O in quartz-polymetallic sulphide stage and quartz-sulphide-poor stage are-2.5%⁵.3%.Its?¹⁸O_H2O value is lower than that of magmatic water?5.5%⁹.5%?.The main ore-forming fluids in the main metallogenic period are mainly mantle-derived fluids.The fluids evolve to the later stage,and the meteoric precipitation dominates.The sulfur isotope of pyrite is 3.0%⁶.4%,which is positive.The sulfur in ore-forming fluids may mainly come from deep mantle or magmatic sulfur related to mantle.In conclusion,the Jirimai gold deposit belongs to orogenic gold deposits and its metallogenic period is later than that of the early Indosinian period,possibly in the late Indosinian period.Owing to the upwelling of asthenosphere caused by lithospheric detachment,the C-H-O fluids from mantle source upwelled to the upper mantle source of gold,enriched gold and other mineralizing materials,temperature and pressure conditions changed instantaneously,resulting in a large amount of CO₂ escape in gold-rich ore-forming fluids,changing the original chemical properties of fluids,destroying the stability of gold complexes,and making a large number of gold precipitated in the tectonic favorable areas.