Geological,Geochemical Characteristics And Genesis Of The Xiaoxigong Gold Deposit In The Beishan Area Of Gansu Province

The Xiaoxigong gold deposit is located in the south of Beishan area of Gansu province,in the north margin of Dunhuang block of Tarim Craton.Since the discovery of the deposit,some exploration and geological studies have been carried out.However,due to the lack of systematic research and complicated geological background,there are great controversies on the ore-forming fluid property,source of metallogenic materials and genesis of the deposit,and the research on the relationship between magmatite and mineralization in the mining area is also weak,which restricts the prospecting process.On the basis of systematic field work,this paper clarifies the relationship between magmatic rocks and mineralization,finds out the property of ore-forming fluid and the source of mineralization material,and further deepens the understanding of the genesis and ore-forming process by means of petrogeochemistry,fluid inclusion and stable isotope,etc.The ore body of Xiaoxigong gold deposit is controlled by NWW trending secondary faults,and the host surrounding rocks are mainly metamorphic rocks of Xijianshan group of middle Proterozoic and Neoproterozoic syenogranite.The mineralization process can be divided into three stages:quartz-pyrite stage?early stage?,quartz-polymetallic sulfide stage?main metallogenic stage?and quartz-carbonate stage?late stage?.The ore types include quartz vein type and altered rock type,and the content of metallic minerals is low,which is dominated by pyrite and arsenopyrite,followed by pyrrhotite,sphalerite,galena,chalcopyrite and native gold.The quartz syenite porphyry in the center of Xiaoxigong gold deposit has the characteristics of type A granite with a weighted mean ²⁰⁶Pb/²³⁸U age 246.7±0.86 Ma,belonging to the Triassic intrusion.It was formed in a relatively stretched post-tectonic background and by the partial melting of the overlying crust caused by the bottom intrusion of mantle magma in asthenosphere.The rock mass does not have any deformation and is post-metallogenic that has no direct relationship with metallogenesis.Studies on fluid inclusions,quartz trace element and stable isotopes indicate that the ore-forming fluid in the main metallogenic stage is the CO₂-H₂O-NaCl±CH₄system with medium-high temperature?194³97??,low salinity(2.2⁸.9%NaCl_eq)and medium-low density?0.63⁰.98 g/cm³?,and has the characteristics of relative reduction,formed in the mesozonal-hypozonal environment.the ore-forming fluids are mainly metamorphic water,and the immiscibility of the fluids is an important mechanism of gold enrichment and ore-forming.Metallogenic materials mainly come from the upper crust and are closely related to orogeny.Based on the comprehensive study of ore deposit geology,fluid inclusion,quartz composition standard and stable isotope,and the comparison with typical orogenic gold deposits,it is concluded that Xiaoxigong gold deposit belongs to orogenic gold deposit.