Study On Metallogenesis Of Naozhi Copper-gold Deposit,Yanbian Area

The Naozhi Cu-Au deposit is a intermediate-sulfidation epithermal deposit which is expected to be explored as a medium-size gold deposit.It is located in the Yanbian porphyry-epithermal ore-concentrated area.The study on metallogenesis of this deposit is of great significance to explain the porphyry-epithermal system in Yanbian area.For this reason,this paper has carried out systematic studies on deposit geology,fluid geology,chronology and geochemistry.On the basis of deposit geological research,Naozhi Cu-Au deposit is consisted of 10gold bearing veins,which are stored in NW trending compression-torsional faults,and are mainly hosted in granodiorite of Early Jurassic or andesitic volcanic rocks of Ciweigou Formation.The types of mineralization include gold,silver,pyrite,chalcopyrite,bornite,tetrahedrite,chalcocite,low-iron sphalerite,galena,etc.The alteration of wall rocks include K-silicification,phyllic,kaolinization,propylitization,and a little adularia can be seen in the phyllic zone.The process of metallogenesis can be classified into five stages:quartz-K-feldspar±molybdenite stage(?),beresitization stage(?),chalcopyrite-pyrite-gold-quartz stage(?),quartz-polymetallic sulfide stage(?),quartz-carbonate stage(?).The study of fluid inclusions shows that the inclusions of stage?have the characteristics of porphyry deposits'ore-forming fluid,while the inclusions of stage?-?have the characteristics of epithermal deposits'ore-forming fluid.And the homogeneous temperature from stage?to?are 200?400?,200?360?,160?340?.The temperature is concentrated in the range of 280?320?,reflecting the palaeo-depth of1230?1350m.the salinity of stage?-?are 3.05?16.63wt.%Na Cl,3.85?17.00wt.%Na Cl,1.39?13.55wt.%Na Cl,concentrating in the range of 4?10wt.%Na Cl.The ore-forming fluid of main stages(?-?)was mixed with magmatic water and meteoric water,with reducing and neutral properties.In stage?,the fluid boiling caused by decreasing depth and pressure made the Au(HS)^2-,(Cu Cl)⁰and Cu Cl^-break down to form gold,chalcopyrite,H₂S,CO₂and other gases.In stage?,a massive increase in meteoric water accelerated the process of mineral precipitation.The chronological study shows that the Rb-Sr dating from metal sulfide in this deposit is 124.6±2.7Ma,which is closely related to the volcanism of the Ciweigou stage(125.8±2.5Ma)in Early Cretaceous.And the metallogenesis occurred at 125?123Ma,slightly later than the magmatic intrusion time of the granodiorite porphyry(126.82±0.86Ma)and the granite porphyry(126.01±0.81Ma).Geochemical characteristics reveal that the Early Cretaceous intrusive complexs(130?126Ma),which are closely related to metallogenesis,have island arc and adakitic magma characteristics in the context of oceanic subduction.This set of intrusive complexes originated from the lithospheric mantle or lower crust at Neoproterozoic,and many crustal materials were mixed in it during the magmatic evolution.Mantle-derived magma may provided more Au and Cu elements for the deposit,while crust-derived magma mainly supplied Pb and Zn elements for it.In conclusion,we explain the metallogenesis in view of source-migration-storage.Under the condition of the Palaeo-Pacific Plate subduction in early-Early Cretaceous,the oceanic slab and its overlaying sediments were melting partially to form the adakitic initial magma.At the same time,the subducted fluid modified the mantle wedge,making the latter melt partially to form the mafic magma,and then gabbro diorite was formed.Under the condition of continuous underplating,Island arc calc-alkaline magma from the melting lower crust invaded the earth's surface to form granodiorite.Three types of magma(adakitic magma,mantle-derived magma,crust-derived magma)mixed heterogeneously to form the ore bearing diorite porphyry,granodiorite porphyry and granite porphyry.Around125Ma,the deep residual magma released ore-forming fluid,which intruded along the NW trending secondary faults and enriched to be deposit in shallow.