Geological-geochemical Characteristics And Metallogenic Background Of Gold Deposits In The South Qinling Orogen

The south Qinling orogen,endowed with more than 100 gold deposits with an estimated gold resource of>2000 t,is one of the most important gold regions in China and has great prospecting potential.Although the gold deposits in the South Qinling Orogen have been mined and studied for decades,the research on key scientific issues such as the nature of ore-forming fluids,the source of ore-forming materials,the relationship between metallogeny and magmatic activity,metallogenic age,and metallogenic dynamics background is still unclear,which leads to disputes on the genesis of gold deposits and restricts the discovery of metallogenic regularity of deposits.The Baguamiao and Maanqiao gold deposits are vein-type gold deposits controlled by brittle-ductile shear zones in the northern metallogenic subzone of the South Qinling Orogen and hosted in Devonian metamorphic clastic rocks,while the Xiajiadian is a micro disseminated type gold deposit controlled by tensor fractures in the Cambrian strata in the southern metallogenic subzone.The Baguamiao,Maanqiao and Xiajiadian gold deposits are the representative of gold deposits in the northern and Southern metallogenic sub-belt of the South Qinling orogen.In this thesis,I present a case study of Baguamiao,Maanqiao,and Xiajiadian gold deposits integrated on geological,mineralogical,fluid inclusions,and isotope geochemical data to figure out the nature of ore-forming fluids,the source of ore-forming materials,the metallogenic mechanism,metallogenic dynamics background,the coupling relationship between gold mineralization and Qinling orogeny,the genetic type of gold deposits,and to establish a genetic model.The main conclusions are as follows:Detailed deposit geological studies suggest that Maanqiao and Baguamiao gold deposits share similar geological characteristics.The orebodies are controlled by the EW-NWW trending brittle-ductile shear zones,hosted by the Devonian greenschist facies metamorphic clastic rocks and occur as auriferous quartz veins and altered phyllite.Ore minerals mainly consist of pyrite and pyrrhotite,gangue minerals consist of quartz,calcite,dolomite,sericite,and chlorite,gold occurs as visible gold,and the ore-related hydrothermal alteration consists of silicification,sulfidation,sericitization chloritization,and carbonation.The ore controlling structures of the Xiajiadian gold deposit are NE trending tensional faults and near EW trending interlayer faults.The ore bodies are hosted by the Lower Cambrian black rock series and Devonian clastic rocks and occur as breccias,altered rock,and quartz veins.Ore minerals mainly consist of pyrite,gangue minerals consist of quartz,calcite,dolomite,sericite,barite,and fluorite,gold occurs as invisible gold,and the ore-related hydrothermal alteration consists of silicification,sulfidation,sericitization,carbonation,baritization,and fluoritization.The systematic isotopic geochemistry(C-H-O-S-Pb)study has determined the source of ore-forming fluids and materials,revealing the gold mineralization process and ore precipitation mechanism.The ore-forming fluid of Baguamiao and Maanqiao gold deposits is sourced from the reduced metamorphic fluids produced during metamorphic devolatilization.Fluid immiscibility and water-rock interaction are the main metallogenic mechanisms.The oreforming fluids of the Xiajiadian gold deposit are mainly formation water and deep circulating atmospheric water,with minor deep magmatic fluid is mixed.The ore-forming materials are mainly sourced from the Cambrian black rock series,water-rock reaction,fluid mixing,and fluid temperature decrease are the main metallogenic mechanisms.Detailed fluid inclusion petrography,microscopic temperature measurement,and laser Raman analysis identified the nature and physical-chemical conditions of ore-forming fluids,providing evidence of fluid inclusions for determining the ore genesis.The initial ore-forming fluid of the Baguamiao and Maanqiao gold deposits is CO2-rich,with a moderate temperature(200～300℃),low salinity(<10 wt.%NaCl equiv.),high pressure(179-397 Mpa)H2O-NaClCO2-CH4-(N2)fluid,which is consistent with those of the typical orogenic gold deposits.The ore-forming fluid of the Xiajiadian gold deposit is a medium-low temperature(160～260℃),medium-low salinity(0.9～18.6 wt.%NaCl equiv.),low pressure(14～21 Mpa)H2O-NaCl fluid,which is similar with those of the epizonal orogenic gold deposits.In combination with geological,geochemical,and chronological evidence,the petrogenic and metallogenic chronology sequence is established,the metallogenic dynamics background is clarified,the spatio-temporal coupling relationship between gold mineralization and Qinling orogeny is revealed,and a genetic model is established.The Ar-Ar plateau ages of sericite from the Maanqiao gold deposit are 120.4±0.9 Ma and 116.8±0.78 Ma respectively,and the zircon U-Pb of granite in the mining area is 242.0±0.8 Ma.The metallogenic age of the Baguamiao gold deposit is about 209 Ma,and the ages of granite in the Fengtai ore concentration area are 231 Ma～213 Ma.The metallogenic age of the Xiajiadian gold deposit is 13 9.6±0.98 Ma.There are two gold metallogenic epochs of Indosinian(T3)and Yanshanian(J3-K1)in the south Qinling orogen.Baguamiao,Maanqiao,and Xiajiadian gold deposits are orogenic gold deposits formed under different orogenic regimes in the south Qinling orogen.The Baguamiao and Maanqiao gold deposits were formed in the transitional tectonic setting,with the transition from syn-collision compression to post-collision extension,and there may be superimposed in the Early Cretaceous.The Xiajiadian gold deposit was formed in the transitional tectonic setting from intracontinental subduction compression to an extension during the Qinling intracontinental orogenic stage.The metallogenic process all experienced the deposition of orebearing formations rich in ore-forming materials during the early extensional period,and the later tectonic-magmatic-hydrothermal process resulted in the mobilization,migration,and precipitation of gold.