Study On The Mineralization Tectonic Setting And Genesis Of The Dong’an Gold Deposit, Lesser Khingan Range

The Dong’an gold deposit is a large-scale epithermal gold deposit located in thenorthern part of the Lesser Khingan Range–Zhangguangcai Range metallogenic belt,subsequent to the discovery of the Tuanjiegou gold deposit. By now, seven orebodygroups with a total of fifty-eight orebodies have been discovered. The proven goldand silver reserves reach27t and280t respectively, and the prospective gold reservesare more than50t. Among all the discovered orebodies, the scale of NO.5orebody isthe largest, accounting for69%of total resources. The orebodies are located in theEarly Cretaceous volcanic basin, and at the junction of Kuerbin Depression andBaoshan uplift, close to the Baoshan uplift. Formation of the orebodies are closelyrelated to the hypabyssal intrusive facies and subvolcanic facies, and the distributionof these orebodies are controlled by NNE-striking Kuerbin crustal fault. In this paper,we firstly identified the regional geological background, geological features of theDong’an gold deposit, furtherly took geochronology and geochemical research onhost rocks of the deposit and identified the petrogenesis and diagenetic tectonicsetting of the host rocks. Meanwhile, we carried out research on fluid geologicalfeatures and clarified the fluid composition and fluid evolution, furtherly identifiedgenesis of the Dong’an gold deposit and established metallogenic model.The orebodies are majorly located in three groups of NE-, near SN-,NNW-striking faults, orderly yielding characteristics of left-lateral shear, sinistraltranstensional and extensional stress, and they are secondary faults of the Kuerbincrustal fault. Host rocks comprise the Early–Middle Jurassic medium–coarse grainedalkali-feldspar granite and fine-grained alkali granite, intermediate–acid volcanicrocks of the Lower Cretaceous Guanghua Formation. The medium–coarse grained alkali-feldspar granite formed in an extensional tectonic setting caused by subductionof the Pacific plate in the late Early Jurassic (176.3±1.1Ma), constituting the majorhost rock of the Dong’an gold deposit, petrogenesis of which was closely related tothe deep-seated crust-mantle interaction and magma was derived from partial meltingof basaltic materials of the young lower crust accreted from depleted mantle duringthe Neoproterozoic. The medium–coarse grained alkali-feldspar granite yielded closespace-time and genetic relationship with the porphyry and skarn type molybdenumpolymetallic deposits located in the eastern of Heilongjiang and Jilin Provinces.Meanwhile, part of the ore-forming materials in the basement strata were extractedduring the period of magmatic activity. Intermediate–acid volcanic rocks within theore district formed in late Early-Cretaceous, yielding close genetic relationship withthe Dong’an gold deposit. In this paper, we identified age of the trachyandesite(108.0±1.1Ma), which was older than the ore-forming age (105.14±0.70Ma).Petrogenesis of the trachyandesite was closely related to the deep-seated crust-mantleinteraction and magma of which was derived from a mix of partial melting of theMesoproterozoic ancient salic crust and young crust accreted from depleted mantleduring the Mesoproterozoic to Neoproterozoic. The trachyandesite shares unifiedgeodynamic background with mineralization of the Dong’an gold deposit and formedin a continental extension setting caused by the oblique subduction of the Pacificplate.Wall rock alteration types within the mining area mainly consist of silicification,adularization, chalcedonization, fluoritization, chloritization, sericitization,kaolinization and so on, in line with wall rock alteration features of the typical lowsulphidation epithermal gold deposits. Few metallic minerals are existed in ores of theDong’an gold deposit, and the metallic minerals mainly consist of pyrite, chalcopyrite,galena, sphalerite, limonite and so on; The precious metal minerals mainly areelectrum, followed by kustelite, native silver, argentite and so on; Gangue mineralsmainly consist of quartz, adularia, fluorite, sericite, chlorite, kaolinite and so on,showing the same mineral assemblage characteristics with the typical epithermal golddeposit. Ore texture mainly is xenomorphic granular texture, followed by automorphic -hypidiomorphic texture, metasomatic relict texture, pseudomorph texture andcataclastic texture; Ore structures mainly are vein-net vein structure, brecciatedstructure and disseminated structure, followed by comb structure, drusy structure,banded structure, foliaceous structure and so on, yielding textural and structuralcharacteristics of ores of the typical low sulphidation epithermal gold depositsThe microscopic temperature analysis on fluid inclusions in quartz, adularia andfluorite reveal that the gold mineralization temperature ranges from120.3℃to309.1℃, and the main mineralization stage temperature concentrated in150℃–220℃, showing characteristics of low-temperature mineralization. Besides,Ore-forming fluids show low salinity (concentrated in0–8wt%NaCl) and low density(0.76-0.96g/cm3) characteristics; mineralization pressure is low (7.53–18.10Mpa) andthe calculated mineralization depth ranges from0.75–1.81km, showing characteristicsof hypabyssal mineralization. In addition, the hydrogen and oxygen isotopes testresults of the fluid in quartz show that the ore-forming fluid of the Dong’an golddeposit is a mixture of magmatic water and meteoric water, and the major componentis meteoric water. Besides, the laser Raman composition analysis of fluid inclusionsalso prove that major component of the ore-forming fluid is meteoric water. S isotopetest results of the pyrites show that ore-forming materials of the Dong’an gold depositwere derived from a deep source, possibly from the mantle.In conclusion, we consider that the Dong’an gold deposit formed in the lateEarly-Cretaceous, which yielded close genetic relationship with volcanic activitiesoccurred during this period, and the mineralization occurred at the late evolution stageof volcanic basin. Through researches on orebodies, host rocks and ore-forming fluid,we determine the type of the Dong’an gold deposit as low sulphidation epithermalgold deposit, and the deposit formed in a continental extension setting caused by theoblique subduction of the Pacific plate in the late Early-Cretaceous.