Mesozoic Porphyry Copper Mineralization In The Eastern Continental Margin Of Jilin And Heilongjiang Province

Porphyry copper deposits are the main source of copper resources in the world,and they have been concerned and studied by economic geologists worldwide for a long time because of their considerable non-ferrous precious metal mineral resources such as molybdenum,gold,silver and PGE.The eastern of Jilin and Heilongjiang Provinceis is a part of the magmatic arc on the continental margin of the Western Pacific Ocean.In order to deeply reveal the metallogenic potential of porphyry copper deposits,on the basis of previous work,this paper selected typical Mesozoic representative Yanghuidongzi,Xiaoxinancha,Nongping and Ermi porphyry copper deposits in the metallogenic belt as the main research objects,and systematically carried out the research on mineralization.The genesis and metallogenic mechanism of the deposit have been thoroughly demonstrated through the evidence of deposit geology,fluid geology,fine chronology,element and isotope geochemistry.The mineralization model is established from the perspective of magmatic fluid evolution and unloading of ore-forming elements.The research results are as follows:1.The geological study of the deposit reveals:The mineralization types of Yanghuidongzi,Xiaoxinnancha and Ermi porphyry copper deposits are veinlet disseminated type-sulfide quartz vein type-breccia type.The mineralization types of Nongping porphyry copper deposit are mainly veinlet disseminated type and sulfide quartz vein type.These porphyry deposits are all developed with different degrees of potasic,silicific,serictic,propylitic and carbonatization,and shows certain zonation.The main mineralization stages are dominated by chalcopyrite,pyritization,molybdenite and pyrrhotite.The difference is that the Yanghuidongzi deposit obviously lacks magnetite,hematite.According to the vein cutting relationship,the hydrothermal mineralization of each deposit can be roughly divided into five mineralization stages:（I）K-feldspar-biotite-magnetite,（II）quartz-pyrite-pyrrhotite,（III）quartz-molybdenite-pyrite-chalcopyrite,（IV）quartz-polymetallic sulfide and（V）quartz-carbonate.2.The micropetrography and temperature measurement of fluid inclusions,H-O of mineral fluid inclusions and S-Pb isotope geochemistry of ore minerals reveal:Liquid rich phase inclusions,gas rich phase inclusions and three-phase inclusions containing daughter crystals are widely developed in all deposits,and a small amount of pure liquid phase inclusions and pure gas phase inclusions are developed.The ore-forming fluid of the deposit comes from magmatic melting,and the ore-forming material mainly comes from magma.The initial ore-forming fluids of Xiaoxinancha,Nongping and Ermi deposits are high-temperature,high salinity and strong oxidizing Na Cl-H₂O system.The initial ore-forming fluids of Yanghuidongzi deposit is the Na Cl-H₂O system with medium high temperature,high salinity,reducibility（or weak oxidation）.In the process of fluid evolution,the crystallization of magnetite and hematite in the early stage of mineralization of Xiaoxinancha,Nongping and Ermi deposits provided the reductive S^2–.In the early stage of mineralization of Yanghuidongzi deposit,there may be the addition of reducing components from the overlying strata.Extensive fluid boiling occurred in all four deposits,and the main mechanism of ore-forming material precipitation was the decrease of temperature,pressure and the mixing of atmospheric precipitation.3.Detailed isotopic chronological studies show:The metallogenic granodiorite porphyry of Yanghuidongzi deposit was formed in the early Jurassic（195.3±1.4Ma,194.1±1.9Ma）.The granodiorite and diorite porphyrite closely related to the mineralization of Xiaoxinancha deposit were formed in the late Early Cretaceous（107.4±1.2Ma,107.1±1.1Ma）.The granodiorite porphyry related to Nongping mineralization was formed in the early Late Cretaceous（96.6±0.9Ma,95.8±0.8Ma）.The quartz diorite,granodiorite,granodiorite porphyry and granite porphyry closely related to mineralization of Ermi deposit,were all formed in the early Late Cretaceous（95.0±1.0Ma,94.2±1.4Ma,93.9±1.2Ma and 92.0±1.2Ma,respectively）.The sericite ⁴⁰Ar-³⁹Ar age of Nongping deposit is 94.80±0.86Ma.The Re-Os isochron age of molybdenite in Yanghuidongzi deposit obtained by previous studies is 194.8±2.1 Ma.Re-Os isochron age of molybdenite in Xiaoxinancha deposit is 110.3±1.5 Ma,and K-Ar age of cryptoexplosive breccia in Ermi deposit is 95 Ma.The paper determines that the metallogenic belt developed three periods of magmatic and porphyry copper mineralization in the early Jurassic（190～200Ma）,late Early Cretaceous（100～115Ma）and early Late Cretaceous（90～100Ma）.4.The geochemical characteristics of elements and Sr Nd Hf isotopes of magmatism closely related to mineralization reveal:The early Jurassic（190～200Ma）metallogenic porphyry is characterized by high Sr/Y adakite,which is the product of partial melting of the thickened lower crust of the Cenozoic continent.The late Early Cretaceous（100～115Ma）ore-forming magma is the product of mixing of adakite magma partially melted by subducting oceanic plate with calc-alkaline magma formed by crustal remelting and crystallization of magma chamber.The early Late Cretaceous（90～100Ma）metallogenic magma has two sources:the ore-forming magma occurring in the Xingmeng orogenic belt is characterized by adakitic magma produced by partial melting of subducted oceanic plates mixed with a small amount of mantle derived magma;the ore-forming magma occurring in the North China plate is the product of magmatic fractional crystallization produced by partial melting of Proterozoic lower crust materials（with the addition of a small amount of enriched lithospheric mantle end member materials）.5.The geochemical characteristics of rocks and minerals closely related to mineralization reveal:（1）The crystallization temperature of granodiorite porphyry in the Early Jurassic Yanghuidongzi deposit is 679～781℃,the pressure is 1.46～1.74 kbar,and the corresponding rock emplacement depth is 5.5～6.6 km.The crystallization pressure of the granodiorite in the late Early Cretaceous（100～115Ma）Xiaoxinancha deposit is 1.22～1.76 kbar,and the emplacement depth is 4.6～6.6 km.The crystallization temperature of granodiorite porphyry of Nongping deposit in Xingmeng orogenic belt is 711～729℃,the crystallization pressure of biotite is 1.63～2.26 kbar,and the emplacement depth is 6.1～7.9km.The crystallization temperature of quartz diorite,granodiorite,granodiorite porphyry and granite porphyry in the Ermi deposit is 635～789℃,the crystallization temperature decreases with the evolution of magma.The crystallization pressure of biotite and amphibole in each rock is 0.51～1.38 kbar,and the emplacement depth is 1.9～5.2km.（2）The granodiorite porphyry of Yanghuidongzi deposit in the Early Jurassic has a low Fe₂O₃/Fe O ratio（0.23～0.47,with an average value of 0.35）.It is a ilmenite series granite with a low oxygen fugacity.The oxygen fugacity（ΔFMQ）calculated by amphibole,biotite and zircon is-0.92,-0.48 and-3.71,respectively.Xiaoxinnancha granodiorite,Nongping granodiorite porphyry and Ermi granite porphyry have relatively high Fe₂O₃/Fe O（average values are 0.76,0.54 and 0.62 respectively）,which are all magnetite series granites.TheΔFMQ of biotite from xiaoxinnancha granodiorite ranged from 1.08 to 1.86 with an average of 1.26.TheΔFMQ of biotite and zircon in granodiorite porphyry of Nongping deposit ranged from-2.86 to 5.05 with an average of1.85.The oxygen fugacity of the Ermi metallogenic complex gradually increases with the evolution of magma from neutral to acidic:TheΔFMQ of hornblende,biotite and zircon in quartz diorite ranged from-3.06 to 3.21,with an average of-0.04;theΔFMQ of biotite and zircon in granodiorite ranged from-2.73 to 3.83,with an average of 1.36;theΔFMQ of zircon in granodiorite porphyry ranged from-2.98 to 5.51,with an average of2.51;theΔFMQ of biotite and zircon in granite porphyry ranged from 0.25 to 4.00,with an average of 1.19.（3）In terms of volatile component content,the ore-forming magma of the above porphyry copper deposits has high Al₂O₃/Ti O₂and V/Sc,showing water rich characteristics.The water content of Yanghuidongzi granodiorite porphyry magma is 5.7～6.5%,and that of the Ermi quartz diorite magma is 5.0～5.6%.Combining the above characteristics with the characteristics of mineralization alteration and ore-forming fluid,it is determined that the ore-forming magma and ore-forming fluid of the Early Jurassic Yanghuidongzi deposit are porphyry copper deposit that are relatively reduced or"reduced"（relative oxidation type）,while the Early Cretaceous Xiaoxinancha and Late Cretaceous Nongping and Ermi deposits are relatively typical"oxidation type"porphyry copper deposits.6.Based on the dynamic background,crustal evolution and mantle crust magmatism,from the perspective of the metallogenic characteristics of the representative deposits formed,the magmatic processes and corresponding mineralization of the Mesozoic porphyry copper deposits in the metallogenic belt are divided into four types:（1）Early Jurassic paleo Pacific plate subduction+Thickening lower Crust partial melting-Magma evolution-Ore forming fluid exsolution and metallogenesis,（2）Early Cretaceous paleo Pacific plate retreating+Partial melting of subducted oceanic crust+Crustal remelting-Magma mixing and evolution-Fluid exsolution and mineralization,（3）Early Late Cretaceous rapid retreat of paleo Pacific plate+Partial melting of subducted oceanic crust-Magmatic evolution-Fluid exsolution and mineralization and（4）Early Late Cretaceous rapid retreat of paleo Pacific plate+Crustal thinning-Underplating of mantle derived basaltic magma-Partial melting of lower crust-Magmatic evolution-Fluid exsolution and mineralization.