Gold Metallogeny And Prospecting In The Nenjiang-Heihe Tectonic Mélange Zone,Heilongjiang Province

Nenjiang-Heihe tectonic melange is located in the junction of Xing'an and Songnen blocks hosting several middle-large scale Au depositsdiscovered duringthe recent years.The melange is characterized by its specific location,complex rock types,and multiple stages of petrogenesis deformation and metamorphism.Based on the systematic study of mineralogy,petrology,whole rock and deposit geochemistryof the samples from the Yongxin,Mengdehe and Keluo Au deposits,the author discussed the deposits genesis,metallogenic models,and established the structural ore-controlling models.In addition,on the basis of the ore body parameters and typomorphic mineral characteristics,the author indicated the prospecting target areas of ore depth and periphery.The main conclusions are summarized as follows.(1)There is a relationship exists between deformation,spatial-temporal distribution,and origin of Au deposit.Yongxin Au deposit is located in the edge of the ductile shear belt,and its metallogenesis shows insignificant relation with deformation.Both Mengdehe and Kekuo Au deposits are extended in the ductile shear zone,and their metallogenic ages are simultaneous or slightly later than the peak age of deformation.Au element mobilization is significantly triggered by the deformation.(2)We presented the main metallic mineral assemblages of the Au deposits.The metallic minerals is dominated by pyrite,with relatively few other sulphides.The Yongxin gold deposit and the firstly discovered natural gold and telluride(tetradymite,hessite,altaite and petzite)are common in ores.The Mengdehe gold deposit with a small amount of fine grain electrum,and telluride,while the Keluo gold deposit Au mainly occurs as natural gold and electrum.(3)We studied the source of the metallogenic material and the characteristics of ore-forming fluid.The metallogenic material of the Yongxin and Keluo gold deposit is sourced form the mantle with minor contribution of crustal material.The low temperature and low salinity ore-forming fluid,originated from a deep source hydrothermal system.A small amount of meteoric water also involved in the ore-forming process.The metallogenic material of the Mengdehe gold deposit was derived from the crust,and the ore-forming fluid wasmainly the metamorphic hydrothermal fluid.(4)Igneous petrogenesis of the deposits was also carried out.Whole rock Sr-Nd and Zircon Hf isotope indicate that volcanic rocks and vein rocks in the Yongxin Au deposit were mostly derived from the enriched mantle,and they show similar ?Hf(t)values.The protolith of mylonitic schist and mylonite is also considered to be derived from depleted mantle,with minor ancient crustal contamination.(5)The diagenetic and metallogenic ages were determined,based on which the deposit genesis was discussed.The Yongxin pyrite Rb-Sr isotopic age is 114.6±1.2Ma,Zircon U-Pbages of intermediate-acid vein and volcanic rocks vary from 113.7 to 115.2.The granite was intruded during late Carboniferous-early Permian while the mylonites show multi-stage features.Combined with the geochemical and isotopic characteristics,the author concluded that the Yongxin Au deposit is a typical epithermal Au deposit that formed in the early Cretaceous.Pyrite from Mengdehe Au deposit shows Rb-Sr age of 209.6±3.1Ma,while zircons from vein rock and wall rock in the Mengdehe Au deposit show U-Pb ages of 211.5Ma and 245.8 Ma respectively,indicating that the Mengdehe Au deposit is a firstly discovered ductile shear zone-type gold deposit formed during thelate Triassic in the Heilongjiang Province.In the Keluo deposit,vein rock and felsic mylonite exhibit zircon U-Pb ages of103.6 Ma and 214 Ma,respectively,and the deposit shows characteristics of two stagehydrothermal activity,resulted in mainly hydrothermal gold mineralization.(6)The metallogenic geodynamic model of the studied deposits were established.In the late Triassic,the Mongol-Okhotsk Ocean closed from west to east in a scissor-like way.Combined with the westward subduction of Paleo-Pacific,which led to the regional dextral shear,and triggered the formation of Mengdehe Au deposit and early mineralization stage of Keluo Au deposit.During the late Jurassic-early Cretaceous,the transition of Paleo-Pacific Ocean subduction ward and post-orogenic extension environment,due to the final closure of Mongol-Okhotsk Ocean,resulted in the obvious left-lateral shear and the formation of Daxintun Au deposit.In the last stage,the roll-back of Paleo-Pacific oceanic slab led to the extension and detachment,triggering the generation of Yongxin and Keluo Au deposits.(7)The prospecting target area of the concealed ore-body was predicted.According to the contour projection maps of the ore-body bottom elevation,the thickness of the ore-body,the average grade of ore-body,pyrite thermoelectric coefficients and the thickness of hydrothermal alteration,we considered thatthere are good ore prospecting potential within an altitude of 10 m of explore line No.205,within an altitude of 40 m of explore line No.215 and within an altitude of 0m of explore line No.225 in the Yongxin gold deposit.In the Mengdehe gold deposit,good prospecting potential in northeast part of explore line No.168,southwest part within an altitude of 440-390 m of explore line No.7,within an altitude of 370 m of explore line No.96,within an altitude of 420-370 m of explore line No.112-136,northeastern part within an altitude of 480-450 m of explore line No.160 is suggested.