Ore Genesis And Tectonic Setting Of 358 Highland Au Deposit In Wandashan Area,Heilongjiang Province

The 358 Highland gold deposit is located in the Wandashan area of Heilongjiang Province.The tectonic location of this area is at the east end of the Xingmeng orogenic belt in the eastern section of the Central Asian orogenic belt To the west,this area is connected to the Jiamusi block divided by the Yuejinshan fault,to the south border,this area is connected with the Xingkai block by the Dunhua-Mishan fault and to the east,this area is adjacent to Sikhote-Alin Terrane of Russia.Based on the regional and mining geology dataanalysis,in this thesis,researches have been carried out in this deposit by ore-forming geological and physicochemical conditions surveys,deposit geology and ore fluid characteristics analysis.Combined with the comparative analysis of typical hydrothermal deposits in eastern Jilin and Heilongjiang Provinces,the genetic forming of this deposit is determined.According to the spatial-temporal and genetic relationship between the moderate to acid intrusions and the gold mineralization in the area,the metallogenic intrusions have been identified,and the diagenesis and metallogenic ages were determined.Based on the geochemical characteristics of the metallogenic intrusions and S isotope composition of the ore,the diagenetic and ore-forming materials were traced,and combined with the research results of this thesis,the latest data and results of regional tectonic evolution,the tectonic setting of the 358Highland gold deposit was determined.The exposed strata in the mining area include Upper Triassic Dajiahe Formation(T₃dj),Lower Cretaceous Muling Formation(K₁m)and Quaternary.The mainly vein-like and strip-like gold ore bodies are are produced in the structural fracture zone of the shallow Early Cretaceous intrusive body,associated with diorite porphyrites.The gold mineralization is obviously controlled by NNE-trending faults and their derived joint fissures.The primary ore structure is mainly block,vein and disseminated.The metal sulfides are mainly pyrrhotite,arsenopyrite,and chalcopyrite.The gold minerals are mainly natrualgold.The metal minerals in the ore are mainly euhedral,subhedraltoanhedral granular,occasionally metasomatic relict texture.Alteration phenomena such as silicification,epidotization,chloritization,kaolinization,and carbonation are commonly developed in ores and surrounding rocks,among which gold mineralization and silicification are most closely related.According to the vein body piercing relationship,paragenetic association of minerals,mineral intergrowth relationship,ore structure and wall rock alteration characteristics,the formation process of the 358 Highland gold deposit is divided into pyrite-quartz stage(early mineralization stage),gold-bearing polymetallic sulfide-quartz stage(main mineralization stage)and sulfide-lean carbonate stage(late mineralization stage).Inclusion studies have shown that gas-liquid two-phase inclusions and a small amount of three-phase inclusions containing CO₂ are mainly developed in gold-bearing polymetallic sulfide-quartz stage quartz;only gas-liquid two-phase inclusions develop in sulfide-lean carbonate stage.The ore-forming fluid in the main metallogenic stage is medium temperature(223.9?372.6?;average 338.4?),medium-low salinity(2.89?10.87 wt.%Na Cl equiv.;average 7.12 wt.%Na Cl equiv.)In the H₂O-Na Cl-CO₂hydrothermal system,the main ore-forming fluid should be magmatic water,and gradually change to a mixture of magmatic water and atmospheric water in the late stage.From begining to the end,the homogenization temperature and salinity generally decreased gradually.Except for the?³⁴S of the 3 chalcopyrite samples which were3.92‰,7.84‰and 8.53‰,the?³⁴S of the 10 metal sulfide samples in the main stage had a narrow range of variation(0.40?2.48‰,average 1.64‰),shows that the source of sulfur from deep-source magma,and may be contaminated by crustal materials.Thepetrogeochemical characteristics show that the ore-forming rock body diorite porphyrite has high Si O₂(58.18%?60.29%),rich Al₂O₃(17.36%?18.07%),rich Na₂O(3.63%?3.73%)and rich Ca O(6.07%?6.67%).The total rare earth elements(?REE)is between 58.22×10^-6 and 75.49×10^-6.The standardized rare earth element distribution curve of chondrites is weakly right-leaning.Among them,the(La/Yb)_N=4.54?5.43.Heavy rare earth elements have no obvious differentiation phenomenon,and light rare earth elements have a strong differentiation phenomenon.The trace elements are generally enriched in incompatible elements(LILE)such as K,U and Th,and depleted in high field strength elements(HFSE)such as Ti,Nb and Ta,The value of Mg^#(45.73?47.53)and Cr(12.02×10^-6?26.78×10^-6)is close to the value of Adakite or Adakite-like rocksformed by partial melting of the subducting oceanic crust.The zircon Lu-Hf isotopic characteristics indicate that the magma source area of diorite porphyrites should mainly originate from the partial melting of the new accretion continental crust from the depleted mantle originated in the Neoproterozoic.The Sr-Nd-Pb isotope results show that the diorite porphyry has a lower initial ⁸⁷Sr/⁸⁶Sr ratio(0.704336?0.704396)and a higher initial ¹⁴³Nd/¹⁴⁴Nd ratio(0.512588?0.512611),the value of?_Nd(t)is low positive(+1.7?+2.2;average+2.0),and the age t _DM2 of the two-stage model is relatively small(729?765Ma).By calculating the value of²⁰⁶Pb/²⁰⁴Pb(18.877?18.947),the value of ²⁰⁷Pb/²⁰⁴Pb(15.605?15.621)and the value of²⁰⁸Pb/²⁰⁴Pb(38.979?39.049),the?value is higher than the normal mantle(8.92)and lower than the orogenic belt(10.87),the Th/U ratio is close to the global upper crust Th/U(3.88)ratio,indicating that the deep source Pb of its low radioactivity origin should be contaminated by the upper crust material,implying that it is originated from the mantle wedge replaced by plate fluid.Based on the results of mineralization geological conditions,ore deposit characteristics,ore-forming fluid sources,ore-forming physical chemical conditions,and diagenetic mineral sources,combined with the comparative study of regional representative hydrothermal gold deposits,it is concluded that the 358 Highland gold deposit is closely related to the diorite porphyrite in time and space.The genetic type should belong to the mesothermal hydrothermal vein deposit,rather than epithermal deposit by the previous researches.The LA-ICP-MS zircon U-Pbweighted average age of zircons from the diorite porphyrite dating by LA-ICP-MS method is 108.4±0.9 Ma,which is closely related to the time and space of gold mineralization genesis.This result limits the metallogenic and diagenetic age of the 358 Highland gold deposit to the Mesozoic(Late Early Cretaceous).According to the geochemical characteristics of the ore-forming rocks and the analysis of regional tectonic evolution,it is believed that the diorite porphyrite and related mineralization of the 358 Highland gold deposit formed inthe background of active continental margin tectonic setting related to the early Cretaceous paleo-Pacific subduction.