Ore Genesis And Metallogenic Mechanism Of The Xiajiadian Gold Deposit In South Qinling Orogen

The Xiajiadian gold deposit is a large gold deposit hosted in the Lower Cambrian black rock series located in the South Qinling orogenic belt.In this paper,we provide a further study on geological features,petrology,and elemental geochemical characteristics of ore-bearing black rock series.We have compared the geochemical characteristics of the black rock series and surrounding rocks in the mining area,which constrain the contribution to mineralization and formation environment of the black rock series.Moreover,by the geochemical study of fluid inclusion,the petrographic characteristics of the fluid inclusions,the physical chemistry characteristics and the evolution of the fluids are discussed.Finally based on further research on the geological characteristics of the deposit and passed the above research,we came up with some understanding of the genesis and metallogenic mechanism of the Xiajiadian gold deposit.The Xiajiadian gold deposit is located at the passive continental northern margin of the Yangtze Block in the Early Paleozoic and within a deep-water to semideep-water retention and hypoxia basin.The orebody is lenticular and layered which is controlled by the strata and NE or nearly EW trending faults.Pyrite,arsenopyrite,chalcopyrite,and limonite are the main metal minerals,the main nonmetallic mineral are quartz and calcite,and there is small amount of barite and fluorite.Major wallrock alteration includes silicification,pyritization,carbonation and limonitization.The mineralization of hydrothermal reformation period can be divided into three stages:I-quartz-minor pyrite stage;II-quartz-multi-metal sulfide stage;III-carbonate-minor sulfide stage.The black rock series of the Xiajiadian gold deposit consists of siliceous rocks,altered siliceous argillaceous rocks,thin-layer carbonaceous-siliceous slate,argillite,and carbonaceous-siliceous argillaceous rock.The fertile black rock series of the Shuigoukou formation in the Xiajiadian gold deposit is likely to be an important source bed,which are super-enriched in Au,Ag,Ni,Cu,Zn,V,Ba,and other elements,which reach several times and even higher than those of crust abundance.The North American shale-normalized REE patterns of the black rock series are left inclined and characterized by weak negative Ce anomalies and negative Eu anomalies to positive Eu anomalies for the siliceous rocks,negative Ce anomalies and negative Eu anomalies for the argillite,and positive Eu anomalies for a minor carboncaceous-siliceous rocks.The enrichment degree of elements of surrounding rocks in mining area is lower and REE characteristics are different from ore,so It is unlikely to provide ore-forming materials.Combining the parameter ratios and discrimination diagrams of major elements,trace elements,and REEs,the results indicate that diagenetic materials of these black rock series had diverse sources;the lower siliceous rocks are mainly products of hydrothermal sedimentation,and with the seawater and terrestrial materials added,they transform from hydrothermal sedimentation to normal sedimentation,thus forming carboncaceous-siliceous rocks and argillite.The black rock series in the Xiajiadian gold deposit was formed in a deep-water to semideep-water retention and hypoxia basin at continental margin,which belongs to the rift deposition area of the passive continental margin of the Yangtze Block.The geochemical study studies of fluid inclusions indicate the ore-forming fluids of hydrothermal reformation period were NaCl-H₂O with low temperature and medium-low salinity.Fluid inclusion types are divided into Type A?liquid phase?and Type B?gas-liquid two-phase?,the main type of fluid inclusion is rich liquid aqueous inclusion?Type LV?.The fluid inclusion with homogenization temperature:160°C²40°C?average at 193°C?,salinity:2 wt.%⁵ wt.%NaCl_eqv(average at 4.53 wt.%NaCl_eqv),density:0.710 g/cm³⁰.984 g/cm³?average 0.893 g/cm³?in major ore-forming stage.Fluid characteristics are similar to those of typical Carlin-type gold deposits.The source of the fluid may be a multi-source fluid dominated by atmospheric precipitation.The metallogenetic geotectonic background,ore-controlling structure,ore type,mineral association,and the fluid property of the Xiajiadian gold deposit are similar to those of the typical Carlin-type gold deposit.The formation of the Xiajiadian gold deposit experienced sedimentary preconcentration and post-structural-hydrothermal reformation.The fertile black rock series of the Shuigoukou formation in the Xiajiadian gold deposit are likely to be an important source bed,which are super-enriched in Au,Ag,Ni,Mo,Cu,Pb,Zn,V,U,As,Sb,Hg and other elements in the early process.and provide a sufficient material source for mineralization,and finally form the Xiajiadian gold deposit through post structural-hydrothermal reformation.