Geology,Mineralization,and Genesis Of Typical Gold Deposits In The Northern Belt Of The West Qinling Orogen

The West Qinling orogen hosts several dozen gold deposits with a total Au endowment of over 2000 t,forming one of the most important and prospective gold provinces of China.Although most deposits have been investigated surrounding their ore genesis,temporal and spatial distribution characteristics,source of fluid and metal,and genetic type of gold mineralization still remain a matter of debate.In this thesis,four typical gold deposits,including Shuangwang,Liba,Liziyuan,and Ma’anqiao in the northern belt of the West Qinling orogen are selected as research subjects.Based on detailed field investigation and mineralogical observation,geochronological studies are performed to tightly constrain the timing of gold mineralization of the four deposits.Then,systematic geochemical researches are conducted on the representative Shuangwang and Liba gold deposits to ascertain the source of fluid and metal of gold mineralization,further to better understand ore-forming processes and ore genesis.These results,combined with regional geological,geochronological,and geochemical data,allow this thesis to develop a genetic model and reveal gold metallogeny over the West Qinling orogen that have important implications for gold prospecting.The Shuangwang gold deposit is hosted in Late Devonian greenschist facies metamorphosed clastic rocks and carbonates of the Xinghongpu Formation.The deposit is characterized by extreme development of gold-bearing breccia bodies,which define a NW-striking,discontinuous breccia belt.Gold orebodies consist of angular to subangular albitized fragments and mineralized hydrothermal cements,which are crosscut by lamprophyre and granite porphyry dykes.The hydrothermal cements are dominated by dolomite,calcite,pyrite,quartz,albite,and anhydrite,associated with minor to trace amounts of tourmaline,chalcopyrite,arsenopyrite,gersdorffite,enargite,melonite,tetradymite,tsumoite,violarite,clausthalite,and accessory minerals of monazite,brannerite,apatite,xenotime,and parisite.Gold mostly occurs as native grains enclosed in or as stringers filling microfractures of pyrite and dolomite.Hydrothermal monazite U-Pb dating indicates the Shuangwang gold deposit formed at220.9±1.8 Ma（2σ）.This age,within uncertainties,overlaps zircon U-Pb ages of a post-ore lamprophyre dyke of 220.3±1.2 Ma and a granite porphyry dyke of 220.0±1.0 Ma（2σ）in the Shuangwang mine.The Liba gold deposit is located to the northeast of the Late Triassic Zhongchuan granitoid pluton and hosted in Middle Devonian greenschist facies metamorphosed clastic rocks and carbonates of the Liba Group.Numerous pre-ore granite porphyry and quartz syenite porphyry dykes and syn-ore lamprophyre dykes are well developed in the deposit area.Gold mineralization consists mainly of disseminated ores localized within WNW-to EW-striking fracture zones.On the basis of field relationship and mineral assemblages,three auriferous stages are defined,in paragenetic sequence being quartz-pyrite stage（I）,sericite-quartz-pyrite stage（II）,and quartz-polymetallic sulfide stage（III）,followed by the latest barren calcite stage（IV）.Hydrothermal alteration is well developed,mostly marked by silicification,sulfidation,sericitization,chloritization,and carbonatization.Pyrite is the predominant sulfide phase in stage I and II,with arsenopyrite occurring as subordinate minerals.Native gold is present as anhedral grains included in pyrite,quartz,and sericite.In comparison,ore minerals in stage III are dominated by pyrite,arsenopyrite,galena,sphalerite,boulangerite,associated with minor amounts of chalcopyrite and tetrahedrite.Particularly,pyrite contains small stannite,sphalerite,and chalcopyrite inclusions.Native gold occurs as anhedral grains enclosed in,along crystal boundaries of pyrite,quartz,and other ore minerals,or as stringers filling microfractures in pyrite and arsenopyrite.Hydrothermal titanite in stage II and monazite in stage III yield U-Pb dates of 218.8±6.4 Ma and 218.1±4.6 Ma（2σ）,respectively,demonstrating gold mineralization at Liba occurred at Late Triassic.These ages are comparable with age for gold mineralization at Shuangwang.Zircon U-Pb dating shows that magmatism in the mining areas kept active from 221.9±1.0 Ma to218.5±2.3 Ma（2σ）.The Liziyuan gold deposit is mainly hosted in Late Cambrian to Early Ordovician greenschist facies metamorphosed sedimentary-volcanic rocks of the Liziyuan Group.Magmatic rocks in the mining area include the Tianzishan monzogranite pluton,granite porphyry stocks,and diorite porphyry and diorite dykes.Gold mineralization is structurally controlled by NW-trending faults and made up of quartz-pyrite veins with minor calcite-polymetallic sulfide veins.Hydrothermal alteration is well developed on both sides of auriferous veins,with silicification,pyritization,and sericitization being closely related to gold mineralization.Ore minerals are dominated by pyrite,which is variably associated with minor amounts of chalcopyrite,galena,sphalerite,and tetrahedrite.Gold occurs mostly as native gold grains within pyrite and quartz,along their crystal boundaries,or as stringers filling fractures in pyrite.Hydrothermal sericite separates give rise to ⁴⁰Ar/³⁹Ar plateau ages of 200.1±1.2 Ma and 198.8±1.1 Ma（2σ）,indicating that gold mineralization at Liziyuan took place at the Early Jurassic.Zircon U-Pb results reveal prolonged,likely episodic magmatic events in and surrounding the gold mines from 241.3±1.2 Ma to 212.4±0.9 Ma（2σ）.These results suggest that gold mineralization postdates magmatism in the Liziyuan gold deposit at least by about 10million years,thus precluding a possible genetic relation between the two.The Ma’anqiao gold deposit is hosted in Late Ordovician to Early Silurian greenschist facies metamorphosed clastic rocks and carbonates of the Luohansi Group and structurally controlled by the Ma’anqiao shear zone.Gold mineralization consists of quartz-pyrite-pyrrhotite veins and dense pyrite-pyrrhotite disseminations in hydrothermallyalteredrocks,whicharecrosscutbylate K-feldspar-calcite-chlorite±pyrite veins.Ore-related hydrothermal alteration is extensive,with alteration assemblages dominated by quartz,chlorite,sericite,calcite,pyrite,and pyrrhotite.Native gold occurs as anhedral grains within pyrite,pyrrhotite,quartz and calcite,or along crystal boundaries,or as stringers filling microfractures in pyrite.Hydrothermal titanite in disseminated sulfide ores and late K-feldspar-calcite-chlorite veins produce overlapping ages of 121.1±3.1 Ma～120.7±3.5 Ma and 120.8±3.2 Ma～120.3±5.8 Ma（2σ）,respectively,tightly constraining gold mineralization at Ma’anqiao at ca.121-120 Ma.The new geochronological results obtained in this study suggest that the Shuangwang,Liba,and Liziyuan gold deposits were emplaced in the Late Triassic to Early Jurassic,while the Ma’anqiao gold deposit formed in the Early Cretaceous,indicating gold mineralization at Ma’anqiao took place as part of a different metallogenic event and under a distinctive tectonic regime.However,the Ma’anqiao gold deposit have stopped mining,making it impossible for detailed field investigations and systematic sample collections,and to a large extent limiting researches on its genesis.The Shuangwang gold deposit is the only breccia-hosted deposit in the West Qinling orogen.The Liba and Liziyuan gold deposits share similar features on geology,mineralization,and alteration,but the former is much larger in gold resources than the latter.Therefore,the Shuangwang and Liba gold deposits are chosen for further studies of source of ore fluid and metal.Multiple isotopic constraints provide significant new insights into the source of ore fluids and metals of the Shuangwang gold deposit.Dolomite and calcite from the hydrothermal cements haveδ¹³C values of-7.63 to-3.60‰,δ¹⁸O values of 18.70 to7.09‰,and ⁸⁷Sr/⁸⁶Sr ratios of 0.710268 to 0.712715.Dolomite-related quartz displaysδ¹⁸O values ranging from 20.40 to 18.81‰.The oxygen isotopes of quartz and carbonates correspond to calculatedδ¹⁸O values of 12.79 to-2.43‰for the ore fluids.Fluid inclusion extracts from various ore and gangue minerals yield ³He/⁴He values of0.005 to 0.533 Ra.Gold-bearing pyrite showsδ³⁴S values between 4.98 and 17.08‰and radiogenic lead isotopic compositions with ²⁰⁸Pb/²⁰⁴Pb ratios of 38.428 to 40.023,²⁰⁷Pb/²⁰⁴Pb ratios of 15.647 to 15.711,and ²⁰⁶Pb/²⁰⁴Pb ratios of 18.822 to 20.126.These isotopic signatures,combined with unique mineralogical components and pyrite trace element association of Au-As-Cu-Te-Ni-Bi-Co-Se,demonstrate that ore fluids and metals in the Shuangwang gold deposit were most likely derived from metamorphic devolatilization of underlying early Paleozoic carbonaceous sedimentary rocks.Intense pre-ore albitization increased the competence and decreased the permeability of host rocks,promoting the trap of ore fluids.The ore fluids were overpressured during their accumulation,resulting in hydraulic fracturing and hydrothermal brecciation of the wall rocks.Concurrent fluid pressure drop and phase separation triggered precipitation of gold and other hydrothermal cements to form the mineralized breccia bodies.Petrographical observation,microthermometric measurement,and laser raman spectroscopy of fluid inclusions in quartz indicate that gold was deposited from a moderate temperature,low-salinity H₂O-Na Cl-CO₂primary ore fluid in stage I and II,whereas a distinct moderate temperature,moderate-salinity H₂O-Na Cl-CH₄-CO₂system in stage III.Based on arsenopyrite（stage I-III）and chlorite（stage III）mineral geothermometry,the temperature of gold deposition is estimated to be 300-350℃ for stage I,290-370℃ for stage II,280-350℃ for stage III.Quartz separates from stage I and II yield similarδ¹⁸O values of 15.9-18.3‰and 16.3-17.2‰,with calculatedδ¹⁸O values for ore fluids of 9.0-13.0‰and 9.1-12.4‰,respectively,while stage III has lowerδ¹⁸O values of quartz of 14.3-15.8‰and accordingly calculated values of ore fluids of 6.7-10.5‰.Trace element analyses reveal that pyrite in stage I and II contains high concentrations of Au,As,Te,Co,Ni,and Bi,whereas pyrite in stage III shows an enrichment of Cu,Zn,Ag,Sn,Sb,and Pb.Pyrite from stage I and II exhibits comparable ²⁰⁶Pb/²⁰⁴Pb ratios of 18.3575-18.7105 and 18.1381-18.7303,respectively,while lead isotope compositions of pyrite from stage III are more radiogenic with²⁰⁶Pb/²⁰⁴Pb ratios of 18.8600-19.0893.Pyrite from all of the three stages displays approximately equalδ³⁴S values ranging from 4.7 to 10.2‰.These results confirm that the Liba gold deposit were formed from metamorphic ore fluids,with influx of magmatic fluids in stage III.Fluid immiscibility,fluid mixing,and sulphidation were responsible for gold precipitation in auriferous quartz veins and disseminated ores,respectively.A compilation of reliable geochronological data suggests that most gold deposits in the West Qinling orogen were emplaced at ca.250-195 Ma.This time frame overlaps with the subduction of the Mianlue ocean and subsequent continental collision between the North China and Yangtze cratons or post-collisional extension.Available ore-stage sulfide sulfur isotopic compositions show that almost all of the gold deposits in the northern belt of the West Qinling orogen haveδ³⁴S values roughly ranging between 5and 15‰.This implies that ore fluids in these deposits were consistently derived from metamorphic devolatilization of early Paleozoic carbonaceous sedimentary rocks,which was related to crustal thickening during Triassic Qinling orogeny.Moreover,magmatic-hydrothermal fluids exsolved from contemporaneous granitoid plutons may have been variably involved in gold mineralization,as demonstrated in Liba.Gold deposits in the northern belt show distinctly differences on degree of metamorphism of host rocks,mineralization styles,alteration assemblages,and trace element geochemical signatures from those in the southern belt.These differences reflect higher temperature and pressure conditions for gold deposits in the northern belt（mesozonal type）relative to those in the southern belt（epizonal type）,which most likely resulted from contrasting post-mineralization exhumation between the northern and southern belts with the shallow parts of the northern belt having been eroded away.As such,it is reasonable to propose that deep domains covered by Carboniferous to Triassic strata in the western segment of West Qinling orogen,particularly those parts with NW-to NWW-trending thrust faults and higher order fracture zones being well developed,have the potential to host mesozonal gold mineralization.The Ma’anqiao gold deposit（121-120 Ma）provides for the first time substantive evidence for an Early Cretaceous gold mineralization event in the West Qinling orogen.Its formation is attributed to lithospheric thinning and destruction of the North China craton,which was ultimately induced by the westward subduction of the Paleo-Pacific plate during late Mesozoic.The causative elevated geothermal gradients associated with extensive magmatism could have resulted in prograde metamorphism of the Paleozoic sedimentary rocks with the derived fluids ascending along pre-existing crustal-scale regional structures to deposit gold in upper crustal regime.The formation of the Ma’anqiao gold deposit indicates that large areas of the eastern part of the North Qinling terrane,notably where extensive magmatism of Late Jurassic to Early Cretaceous ages is developed,are highly prospective for future gold exploration.