Comparison Between Lode Gold Deposit And Breccia Pipe-Hosted Gold Deposit In The Southern Margin Of North China Craton,Implication For Regional Metallogeny

The southern North China Craton（NCC）hosts numerous vein-type Au deposits and a small number of breccia pipe-hosted Au deposits,forming one of the largest gold provinces in China.A great progress have been made after being extensively investigated by previous studies.The breccia pipe-hosted Au deposits are generally considered as a magmatic-hydrothermal gold deposit.However,it is still controversial whether the vein-type Au deposits are orogenic gold deposits or decratonization gold deposits.In this thesis,an integrated study of geology,geochronology and geochemistry was presented to probe the connection and distinction between the Dongtongyu Au deposit（Vein-type Au deposit）and the Qiyugou Au deposit（breccia pipe-hosted Au deposit）,allowing us to clarify the genesis of vein-type gold deposit and improve the regional gold metallogenic regularity.The Dongtongyu Au deposit and the Qiyugou Au deposit are located in the Xiaoqinling district and the Xiong’ershan district,respectively.The ore bodies of both deposits are hosted in the Archean amphibolite facies metamorphic rocks of Taihua Group.Gold mineralization in the Dongtongyu Au deposit consists mainly of auriferous quartz veins,which developed preferentially occurred in E-W striking directions and various discontinuities along brittle-ductile to brittle structures.The mineralization in the Qiyugou Au deposit is dominated by breccia-type.Several large gold-bearing breccia pipes are developed in the mining area.The gold mineralization mainly occurred in the cement belonging to breccia,with minor amounts of auriferous quartz veins in the Taihua Group in the proximity of breccia pipe.Besides,a small amount of vein and breccia mineralization have been identified in the altered granite body.Four stage mineral precipitation are recognized based on field and microscopic observations including the quartz-coarse grained pyrite stage（I）,quartz-pyrite stage（II）,quartz-polymetallic sulfide stage（III）and quartz-carbonate stage（IV）in the Dongtongyu Au deposit,and potassium feldspar-quartz-pyrite stage（I）,quartz-pyrite stage（II）,quartz-polymetallic sulfide stage（III）and quartz-carbonate stage（IV）in the breccia ore of the Qiyugou Au deposit,and quartz-magnetite（hematite）-pyrite-molybdenite stage（I）,quartz-pyrite stage（II）,quartz-polymetallic sulfide stage（III）and quartz-carbonate stage（IV）in the altered granite ore of the Qiyugou Au deposit.Moderate to low-temperature alteration is well developed in the Dongtongyu Au deposit,mostly marked by sericitization,pyritization and silicification.Breccia ore and altered granite ore in the Qiyugou Au deposit share similar alteration which consists of moderate to high-temperature alterations like feldspathization,epidotization,sericitization and pyritization.Hydrothermal monazite U-Pb（127.1±2.2 Ma,MSWD=1.3）dating and hydrothermal titanite U-Pb（129.9±1.6 Ma,MSWD=1.3）dating reveal the timing of Au mineralization in the Dongtongyu Au deposit and the Qiyugou Au deposit at early Cretaceous within the error range.Combined with the published reliable geochronology data in this region,the gold deposits in the southern margin of the NCC were mainly formed at～130Ma,synchronously with the large-scale extension and thinning of the North China Craton plate lithosphere caused by the subduction,retention and retraction of paleo-Pacific plates.Petrography observation,electron probe microprobe analysis（EMPA）and LA-ICP-MS analysis were utilized to present the occurrences and compositions of gold minerals.The results show that both invisible and visible gold occurred in the Dongtongyu Au deposit and the Qiyugou Au deposit.Visible gold is recognized in microfractures of pyrite and may be intergrown with galena and chalcopyrite,which is composed of native gold,electrum and Au-bearing telluride,whereas invisible gold mainly occurred as sub-microscopic inclusions within（<1μm）Pyrite.LA-ICP-MS trace element data reveal minor lattice Au occurred in altered granite ore of the Qiyugou Au deposit.Numerous telluride minerals,bismuthide and Bi（Cu,Pb）-sulfosalts have been recognized in the Dongtongyu Au deposit and the Qiyugou Au deposit including petzite,calaverite,hessite,tellurobismuthite,altaite,tetradymite,bismuthinite,cbismuthinite,aikinite,lillianite,and native bismuth.Combined with thermodynamic data and minerals assemblage,we estimate the Log f Te₂of tellurides formed from-12.7 to-6.4and Log f S₂from-11.4 to-6.8.Relative amounts of Bi and Te（S+Se）reveal that telluride and bismuthide formed in Relatively reduced condition.Microthermometric study and sulfides trace element analysis show that hydrothermal fluid for the Dongtongyu Au deposit is a moderate-low temperature（concentrate on 240～300℃）,low salinity（concentrate on 4-8 wt.%Na Cl equiv）and CO₂-rich fluid with Ni,Cu,Zn,Pb,Ag,Te,Au,Bi enriched.Hydrothermal fluid for the breccia ore of the Qiyugou Au deposit is a moderate-high temperature（concentrate on300～360℃）,moderate-low salinity（concentrate on 12～16 wt.%Na Cl equiv）and CO₂-rich fluid with Co,Ni,Cu,Zn,Pb,Ag,Te,Au,Bi enriched.Hydrothermal fluid for the altered granite ore of the Qiyugou Au deposit is a moderate-high temperature（concentrate on 300～360℃）,moderate-low salinity（concentrate on 12～16 wt.%Na Cl equiv）and CO₂-rich fluid with Co,Ni,Cu,Zn,Pb,Ag,Te,Au,Bi enriched in quartz-polymetallic sulfide stage.It is noted that pyrite of the quartz-pyrite stage in the altered granite ore of the Qiyugou Au deposit host anomaly As and Sb,which is explained as relative high oxidation of ore-forming fluid increases the As capacity of pyrite considering the mineral assemblage.Cathodoluminescence（CL）and Scanning electron microscopy combined with cathodoluminescence（CL）revealed that the quartz from the Dongtongyu Au deposit and the Qiyugou breccia pipe-hosted gold deposit shows multi-generation.Combined with the in-situ O isotope analysis and quartz generation indicates that the ore-forming fluid is derived from magmatic water(δ¹⁸O_fluid:6.25-7.60‰).The pyrite in the two deposits also shows multi-generational characteristics under an infrared microscope.Combined with the trace element of pyrite and in-situ isotopic analysis of quartz,it is concluded that the Dongtongyu gold deposit and qiyugou Breccia type gold deposit formed by multiple mineralization stages.Meteoric water was involved in the mineralization process at an early stage,but minor contribution to the precipitation of gold.δ³⁴S of Pyrite from quartz-polymetallic sulfide stage of the Dongtongyu Au deposit and the Qiyugou Au deposit clustered in a narrow range from-2.4～2.4‰implying magmatic S isotope characterstic.²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb,²⁰⁸Pb/²⁰⁴Pb ratios of Pyrite from the Dongtongyu Au deposit and the Qiyugou Au deposit distributed in17.03～17.31,15.36～15.43,37.51～38.00,respectively.The lead isotopes are approximate to the SCLM lead growth curve of the NCC at late Mesozoic and partly overlap with that of mantle-derived mafic dyke with a part of analysis plots approach to the Pb isotope area belonging to the Taihua group.The S-Pb isotopic characters of the Dongtongyu Au deposit and the Qiyugou breccia pipe-hosted Au deposit suggest that the ore-forming materials derived from mantle-derived magma with minor materials of surrounding rock mixed during the minerals precipitated.It is noted that Pb isotope characteristics indicate that the subduction,devolatilization and metasomatism of the Yangtze Craton during Paleozoic and Early Mesozoic may have formed the gold-rich lithospheric mantle in the southern margin of the North China Craton,and provided materials for the large-scale gold mineralization in the Early Cretaceous.Although some different present in mineralization temperature,fluid salinity,sulfide trace element and sulfur isotope,same mineralization time and metallogenic dynamic background,Au occurrence,similar mineralization fluid composition and properties,the same source of mineralization fluid and materials show that the Dongtongyu vein-type Au deposit is magmatic-hydrothermal Au deposit as well as the Qiyugou breccia pipe-hosted deposit.The location and environment of the deposit are the fundamental reason for the difference in mineralization temperature,fluid salinity,sulfide trace element and sulfur isotope between the two deposits.Combined with geological,mineralization and geochronology data of other gold deposits in the region,we believe that the late Mesozoic gold deposits in the southern margin of the North China Craton are magmatic-hydrothermal gold deposits with similar mineralization materials and fluid sources related to decratonization.Thus,we propose the mineralization model:Due to the westward subduction and retraction of the paleo-Pacific plate in the late Mesozoic,the lithospheric mantle of the eastern North China Craton was strongly extended and weakened.Partial melting of the lithospheric mantle led to the formation of a large number of meso-mafic magmatic rocks,which provided heat,fluid and Au mineralization materials for the gold deposits in the southern margin of North China Craton.After the ore-forming materials arrived at the shallow crust with the fluids,different gold mineralized styles like breccia pipe and auriferous quartz vein were formed in different environments.