Study On The Geological Characteristics And Genesis Of Tongjing Gold (Copper) Deposit,Nanjing City,Jiangsu Province

Tongjing gold(copper)deposit is located in Nanjing City,Jiangsu Province.As a typical gold(copper)deposit in the iron copper metallogenic belt of the middle and lower reaches of Yangtze River,the deposit is located in Ningwu basin,the north of Yangtze plate.The structures of the mining area include mainly NW and NNW trending fault structures and ancient crater structure-Niangniang mountain ancient crater structure.A large number of alkaline series rocks are developed in the mining area.The strata are mainly andesitic and trachyte volcanic rocks of Dawangshan formation in Early Cretaceous,which are mainly distributed around the volcanic mechanism.The lithology of Niangniangshan formation is mainly alkaline volcanic complex,of which the lower section is trachyte pyroclastic rock,the middle section is mainly composed of coarse andesite tuff and pseudoleucite phonolite,and the upper section is hauyne phonolite and trachyte,which are distributed near the ancient crater structure.The zircon U-Pb ages of andesite in dawangshan formation and hauyne phonolite in Niangniangshan formation are 134 ± 1 Ma and 128 ± 1 Ma,respectively.The magmatic rocks are mainly trachyte porphyry,trachyte andesite porphyry,syenite porphyry,diorite porphyry and quartz monzonite,among which trachyte porphyry is closely related to ore body in space.The ore bodies are mainly distributed in the alkaline volcanic complex of Nianniangshan formation,and a small amount in the andesite and trachyte volcanic rocks of Dawangshan formation,which are obviously controlled by the fault structure and the IV ancient crater structure.There are 5 ore sections in the mining area.The ore body is mainly vein and lenticular,which strike concentrated in 310 °~350 °.It inclines to NE above-220 m,and SW below-220 m,with a dip angle of 75 ° ~ 90 °.The ore minerals are mainly pyrite,gold,silver gold and chalcopyrite,and a small amount of bornite,specularite,galena,sphalerite,and limonite and so on.The ore texture mainly includes automorphic texture,allomorphic texture,metasomatic texture,solid solution separation texture and skeletal texture.The ore structure mainly includes vein structure,massive structure,disseminated structure and breccia structure.The wall rock alteration is mainly silicification,baritization,sericitization,kaolinization,adularization and carbonatization,while chloritization is locally developed.The mineralization is closely related to silicification,adularization and sericitization.The ore-forming period mainly includes hydrothermal ore-forming period and supergene oxidation period,in which the hydrothermal period can be divided into three stages: I.quartz pyrite stage;II.Quartzsiderite-gold copper polymetallic sulfide stage;III.Quartz calcite stage.In which the second one is the main ore-forming stage.There are many barite crystals cutted by siderite,which indicate barite is forming earlier than siderite.It makes clear that the ore-forming fluid is from acidic into near neutral.The mineralization is closely related to silicification,adularization and sericitization,which is typical combination of low sulfide epithermal hydrothermal alteration.The studies of fluid inclusions show that the fluid in the main mineralization stage has the characteristics of low temperature(154-182 ℃),low salinity(4.2-7.7 wt% NaCl),and low density(0.82-0.94 g/cm3).The estimated mineralization pressure is 13-26 mpa,and the mineralization depth is 1.3-2.6km,which belongs to hypabyssal and lowtemperature mineralization.The study of sulfur isotopes shows that the ore-forming materials are mainly from the mantle,and a small amount of sulfur contamination occurs in the strata.The study of carbon,hydrogen and oxygen isotopes shows that the ore-forming fluid is dominated by circulating atmospheric precipitation and a small amount of mantle derived primary water.The petrogeochemical characteristics show that the original magma of these alkaline volcanic rocks originated from the enriched lithospheric mantle metasomatized by subduction fluid.The ore body is strictly controlled by the fault structure,which is mainly concentrated in the local extension part with NNW strike and steeper dip,indicating that the ore controlling fault is a right lateral normal fault.The trachyte porphyry is closely related to the ore body in space,and distributed in an intermittent ring near the ore body in the mining area.The analysis shows that the trachyte porphyry provides the heat source and some ore-forming fluids for mineralization.The zircon U-Pb age of trachyte porphyry related to mineralization is 126 ± 1Ma,which was formed in Early Cretaceous,thus the upper limit of metallogenic age is 126 ± 1Ma.In conclusion,the genesis type of Tongjing gold(copper)deposit is low sulfide epithermal gold copper deposit.Based on the analysis above,it is believed that the formation of Tongjing gold copper deposit and a large number of alkaline volcanic complexes were closely related to the subduction mechanism of the oceanic ridge in the early Cretaceous.As a result of the subduction of the young and hot oceanic ridge,the plate is torn apart at the position of the oceanic ridge,forming the plate window structure.Under the subduction of oceanic ridge,strong extension resulted in lithosphere thinning,asthenosphere upwelling,heating the overlying enriched lithospheric mantle,partial melting of oceanic crust under reduced pressure,forming the initial magma rich in gold and copper,which intrudes upward to the vicinity of Niangniang mountain ancient crater,forming a large number of alkaline volcanic complex and hydrothermal vein type gold copper deposits.