Geochemical Characteristics And Ore Genesis Of The Yangzhaiyu Gold Deposit In Xiaoqinling District

The Yangzhaiyu gold deposit is a large lode gold deposit in Xiaoqinling region,located in the southern margin of North China Craton and mainly occurs in the deep metamorphic gneiss series of Archean Group.The deposit can be divided into two metallogenic epochs:hydrothermal metallogenic stage and supergene oxidation stage,in which hydrothermal metallogenic stage can also be divided into?I?pyrite-quartz stage,?II?quartz-pyrite stage,?III?quartz-telluride-polysulfide stage and?IV?quartz-carbonate stage.Acording to the research,the fluid inclusion types of the Yangzhaiyu gold deposit are mainly CO₂-bearing inclusions?C-type?,CO₂-H₂O two-phase inclusions?V+L-type?,three-phase inclusions with minerals?S-type?,pure gas phase inclusions?V-type?and pure liquid inclusions?L-type?.The fluid inclusions have a homogenization temperature range of 216 to 346°C,a salinity of 6%to 8.7%NaCl_eqv,a density of 0.72 to 0.9 g/cm³,a mineralization pressure of 45 to 160 MPa and a mineralization depth of 2.3 to 5.7 km.The ore-forming fluid is characterized by medium to high temperature,low salinity and rich in CO₂,which belongs to CO₂-H₂O-NaCl system.As the mineralization progresses,temperature,salinity and pressure of mineralization show a gradual decrease from stage I to IV.Hydrogen-oxygen isotope analysis shows that ore-forming fluid may be derived from magmatic water,atmospheric precipitation and other sources which are mainly from magmatic water.And the atmospheric precipitation gradually increases in the later stage of mineralization.The sulfur isotope of the ore is characterized by mantle-derived sulfur and is relatively rich in heavy sulfur,which is related to the metamorphic rocks of the Taihua Group and the Cretaceous granites.Lead isotope analysis shows that the lead of ore-forming fluid is derived from the deep-source lead mixture of mantle and crust,which may come from Taihua Group.Helium-argon isotope analysis shows that helium in ore-forming fluid is a mixture of mantle helium and crustal helium,which originated from deep mantle-derived fluids in the early stage and crust-derived fluids were gradually added along with the ore-forming process.The mixing of multiple fluids including deep-source fluids?mantle-derived and crust-derived fluids?and shallow-source fluids?atmospheric precipitation?is the main mechanism for the evolution of ore-forming fluids in Yangzhaiyu gold deposit.The Yangzhaiyu gold deposit belongs to the magmatic hydrothermal gold deposit.The metallogenic model of Yangzhaiyu gold deposit is that the North China Craton was reactivated in the late Mesozoic.And the lithospheric disintegration caused the upwelling of asthenospheric thermal material and partial melting of the upper mantle,which resulted in extensional tectonic movement in the southern margin of the North China Craton.With the tectonic movement,the basement was continuously uplifted,forming a series of shear fracture systems.The deep mantle-derived ore-forming fluids migrate upward under the influence of tectonism.The deep-source fluids and shallow-source fluids mix gradually and water-rock interaction occurs between the deep-source fluids and the surrounding rocks.As the temperature and pressure continue to decrease,the ore-forming materials finally enrich in the appropriate position of the shear fracture zone to form minerals.