Mineralization,Metallogenic Model And Geodynamic Setting Of Skarn Type Au-Fe-Cu Polymetallic Deposits In The Central Of Heilongjiang Province

The central part of Heilongjiang Province is subordinate to Northeast China,including two important metallogenic belts:the Lesser Xingan Range-Zhangguangcailing and the Jiamusi-Xingkai.There are many porphyry-skarn-and epithermal deposits,which provide a large number of important mineral resources such as iron,copper,lead-zinc,gold,silver,tungsten and molybdenum for China's national production and economic development,It has become a concentrated area of large-scale mineralization in Northeast China.For a long time,the problem of endogenous metal mineralization in central Heilongjiang has been concerned by geologists.In order to reveal the metallogenic law and resource potential of endogenous metals in central Heilongjiang and establish the prospecting model and geodynamic background,based on the previous work,this paper selects the important Laozhashan skarn Cu-Au deposit,Daan'he skarn type Au deposit and Ergu skarn type Fe-Cu polymetallic deposit have carried out research on deposit geology,fluid geology,diagenetic and metallogenic chronology and geochemistry.The research results are as follows:1.In terms of geological characteristics of the deposit:There are two periods of skarn type mineralization in the Laozuoshan deposit.The early large-scale skarn type Cu-Au mineralization occurred granodiorite and marble of Mashan Group,forming the main copper gold ore body and a small amount of magnetite ore body in the mining area.The late superimposed small-scale gold mineralization occurred in the skarn belt between diorite/diorite porphyrite and Mashan Group.Da'anhe gold deposit was formed in the skarn belt between gabbro diorite and carbonate rocks of Tumenling formation,and small-medium scale gold deposits were formed in the outer belt near the stratum.The Ergu medium-sized Fe-Cu polymetallic deposits were formed in the skarn belt between the medium grained granodiorite and the marble of the Qianshan formation.The wall rock alteration of the three deposits is mainly skarnization,sericitization,silicification and carbonation.The tremolite of Ergu deposit and Laozuoshan deposit is developed relative to Da'anhe deposit.The three deposits generally develop magnetite mineralization,chalcopyrite mineralization,pyrite mineralization,galena mineralization and sphalerite mineralization.The differences in mineralization characteristics of the three deposits are as follows:the Laozuoshan deposit is developed by chalcopyrite and natural gold mineralization,forming a large copper gold deposit dominated by gold and associated copper;The development of magnetite,sphalerite,chalcopyrite,galena and natural gold mineralization in Da'anhe is weak,forming small and medium-sized gold deposits;The mineralization of magnetite,chalcopyrite and amphibole zinc in Ergu deposit is relatively developed,which can form a relatively independent medium-sized endogenetic deposit.2.The study of fluid inclusions reveals that:The types of fluid inclusions developed in the three deposits are mainly gas-liquid two-phase inclusions(W-type)and daughter bearing three-phase inclusions(S-type),and there are a small amount of pure gas phase(PV)and pure liquid phase(PL)inclusions.The inclusion types of skarn sub stage are mainly S-type and W-type inclusions,S-type inclusions were developed in the early quartz sulfide stage of the late Laozuoshan deposit,and W-type,PV and PL-type inclusions were mainly developed in the other stages.Laozuoshan Cu-Au deposit:The homogenization temperature of fluid inclusions of early Cu-Au mineralization decreased from 469?539?to 228?341?,and the salinity decreased from 10.1?58.6wt.%Na Cleqv to 1.39?4.48wt.%Na Cleqv.The temperature and salinity during the precipitation of Cu and Au were 228?425?and 1.05?11.2wt.%Na Cleqv,respectively.The homogenization temperature of fluid inclusions in late superimposed gold mineralization decreased from 468?513?to 137?235?,and the salinity decreased from 8.54?51.7wt.%Na Cleqv to 0.87?2.89wt.%Na Cleqv.The temperature and salinity during gold formation were 237?406?and 0.87?41.6wt.%Na Cleqv,respectively.Combined with Laser Raman probe analysis and H-O isotopic characteristics,it is further concluded that the early metallogenic fluid is an H₂O-Na Cl-CO₂±CH₄ system containing N₂.The initial metallogenic fluid is a magmatic thermal fluid or a high-temperature supercritical magmatic fluid formed by contact metasomatism between granitic magma and Mashan Group marble.The ore forming fluid of late superimposed gold mineralization is H₂O-Na Cl-CH₄±CO₂ system with less of CO and H₂S.The initial ore-forming fluid is also a thermal fluid of magmatic nature,or a high-temperature supercritical magmatic fluid formed by contact metasomatism between diorite and marble of Mashan Group.Da'anhe Au deposit:The homogenization temperature of fluid inclusions decreased from 420?520?to 137?247?,and the salinity decreased from 11.2?51.0wt.%Na Cleqv to 0.87?3.37wt.%Na Cleqv.The temperature and salinity of gold mineralization were226?366?and 4.17?9.73wt.%Na Cleqv respectively.Combined with the results of Laser Raman analysis and hydrogen and oxygen isotope experiment,it is concluded that the ore-forming fluid is generally a low oxygen fugacity or reduced H₂O-Na Cl-CH₄±CO₂system,and the initial ore-bearing fluid has the property of magmatic thermal fluid,or a high-temperature supercritical magmatic fluid formed by contact metasomatism between gabbro-diorite and marble of Paleozoic tumenling formation.Ergu Fe-Cu polymetallic deposit:the homogenization temperature of fluid inclusions is reduced from 395?525?to 169?263?,and the salinity is reduced from2.89?46.8wt.%Na Cleqv to 1.39?4.48wt.%Na Cleqv.The temperature and salinity of magnetite precipitation are 275?467?and 2.06?40.2wt.%Na Cleqv,respectively.The temperature and salinity of Cu polymetallic ore formation are 193?360?and1.22?7.15wt.%Na Cleqv respectively.Combined with the results of Laser Raman analysis and H-O isotope experiment,it is concluded that the ore-forming fluid is an H₂O-Na Cl-CO₂ system containing CH₄,and the initial ore-forming fluid is a thermal fluid with magmatic properties,or a high-temperature supercritical magmatic fluid formed by the contact and metasomatism between the deep formation process of granodioritic magma and the marble of Paleozoic Qianshan formation.The S-Pb isotopes of sulfides in the three typical deposits have similar characteristics to the ore-related intrusive rocks,but are quite different from the carbonate strata in the mining area,indicating that the ore-forming materials may mainly come from the ore-related intrusive rocks.Based on the above analysis,we believe that the ore-forming material of skarn Au-Fe-Cu polymetallic deposit in the study area comes from the ore-related intrusive rocks,and the initial ore-forming fluid comes from the high-temperature and high salinity ore-bearing hydrothermal solution dissolved by magma.With the occurrence of mineralization,atmospheric water is continuously added to the ore-forming fluid.The precipitation mechanism of magnetite is mainly fluid boiling.The precipitation mechanism of metal minerals(chalcopyrite,pyrite,lead-zinc ore,natural gold,etc.)in the quartz sulfide period are jointly restricted by fluid boiling,fluid mixing and water rock reaction.Throughout the metallogenic process,the ore-forming fluid has experienced multiple boiling,mixing and water rock reaction,Evolution from ore bearing hydrothermal solution with high temperature and medium high salinity to atmospheric precipitation hydrothermal solution with low temperature and low salinity.3.The study of isotopic chronology of diagenesis and mineralization shows that:(1)The zircon U-Pb weighted average ages of granodiorite,diorite and granodiorite porphyry in the Laozuoshan Cu-Au deposit are 264.6±2.6Ma,103.2±1.0Ma and 104.6±1.8Ma,respectively.The reverse isochron age of garnet U-Pb dating in garnet skarn superimposed with gold mineralization in the underground of the east ore belt is 107.4±1.8Ma.Therefore,we believe that the early skarn type Cu-Au mineralization of Laozuoshan deposit was formed in the Late Permian(?265Ma),and the late skarn type gold mineralization was formed in the early Cretaceous(?107Ma).(2)The zircon U-Pb weighted average age of gabbro-diorite related to the Au mineralization of Da'anhe deposit are 185.8±1.3Ma?183.7±1.3Ma.We believe that the diagenesis and mineralization of Da'anhe deposit were formed in the early Jurassic.(3)The zircon U-Pb weighted average ages of granodiorite and porphyry granite in Ergu deposit are 183.8±1.4Ma?181.9±1.6Ma and 182.7±1.0Ma and 182.7±1.4Ma,respectively.Combined with Ouyang et al.(2016)obtained the⁴⁰Ar-³⁹Ar isotopic age of phlogopite was 181.0±4.2Ma.Therefore,we believe that the diagenesis and mineralization of Ergu deposit were formed in early Jurassic.4.In terms of skarn mineralogy:The garnet formed by Laozuoshan Late Permian Cu-Au mineralization and Ergu deposit is mainly composed of andradite(And_63.7⁹3.9Grs_1.15³4.6;And_63.7⁹3.9Grs_1.15³4.6)and pyroxene is mainly composed of diopside(Di52.28?78.17Hd20.64?45.83Jo0.78?1.90;Di81.15?93.67Hd5.30?17.23Jo1.03?1.98),which represents the oxidation weak oxidation environment during the formation of skarn minerals.The garnet formed by Da'anhe deposit and Laozuoshan Early Cretaceous gold mineralization is mainly composed of grossular(Grs_44.93⁷4.76And_21.89⁵2.89;Grs_58.04⁸2.26And_14.02³7.39),the pyroxene of Da'anhe deposit is mainly diopside(Di_65.31⁷0.25Hd_28.67³3.65Jo_0.45¹.12),and the pyroxene formed by Laozuoshan Early Cretaceous gold mineralization is mainly hedenbergite(Hd_51.49⁷5.68Di_41.04⁴5.40Jo_3.05³.44),which represents the reduction environment during the formation of skarn minerals.The composition of garnet and pyroxene in the three deposits is similar to that of the corresponding skarn type Au-Fe-Cu polymetallic deposits in the world.5.The petrographic and geochemical characteristics of mineralization related intrusions reveal:(1)The granodiorite related to the Late Permian large-scale Cu-Au mineralization in Laozuoshan deposit belongs to peraluminous,high-K calc-alkaline adakitic rocks and the initial magma originated from the partial melting of the thickened lower crust(?_Hf(t)=-3.7?-1.7),with the characteristics of hypabyssal emplacement(1.99?2.65km),medium differentiation,weak oxidation and high water content.(2)The diorite/diorite porphyrite related to early Cretaceous small-scale gold mineralization belongs to quasi aluminous,high-K calc-alkaline I-type granite,and the initial magma originated from crust mantle mixing(?_Hf(t)=-0.4?10.7),with medium hypabyssal emplacement(<3km),low differentiation and reduction(?FMQ=+0.24;log(f O₂)=-11.88)and higher water content(6.24 wt.%).(3)The gabbro-diorite related to the mineralization of Da'anhe small-medium scale gold deposit belongs to peraluminous calc-alkaline series,and the initial magma originated from crust-mantle mixed(?_Hf(t)=1.3?9.6;?_Nd(t)=-1.3?-1.2;(⁸⁷Sr/⁸⁶Sr)_i=0.70621?0.70763),with moderate-plutonic emplacement(4.13?4.26km),low differentiation and reduction(?FMQ=+0.17;log(f O₂)=-15.11)and high water content(6.80 wt.%).(4)The granodiorite and porphyry granite of the medium-sized Fe-Cu polymetallic deposit in Ergu belong to quasi aluminous peraluminous and shoshonite high potassium calc-alkaline I-type granite.They are the products of crystallization of homologous magma at different evolution stages.The initial magma originated from the mixed magma formed by the partial melting of the new lower crust and the ancient crust(?_Hf(t)=-0.5?6.2;?_Nd(t)=-1.7?-1.4;(⁸⁷Sr/⁸⁶Sr)_i=0.70599?0.70897),in which the granodiorite magma related to mineralization has medium-plutonic emplacement(3.13?3.55km),medium differentiation and oxidation(?FMQ=+1.82;log(f O₂)=-14.35)and higher water content(5.69 wt.%).Combined with the deposit types and metallogenic scale of each deposit,we conclude that the magmatic p H,evolution degree and origin of metallogenic related intrusive rocks restrict the types of skarn deposits in the study area.The characteristics of high oxidation and high water content of magma are conducive to mineralization,but the most key factor still depends on the emplacement depth of magma.The shallower the emplacement,the larger the deposit scale.6.Based on the formation age and origin of ore-forming magma of the three deposits,the diagenetic and metallogenic model is established,and the uplift and denudation status after the metallogenic period is estimated.From the perspective of geodynamics,it is summarized as follows:In the late late Late Permian,with the gradual closure of the Paleo-Asian Ocean,the Songnen block and the Jiamusi block were collaged,resulting in the transverse shortening of the crust and the increase of the thickness of the crust.Therefore,under the collision compression environment,the crust of the Jiamusi block was thickened,which partially melted the amphibolite facies rich in garnet in the lower crust,Forming adakitic magma rich in minerals.After its formation,the magma rises under the driving of thermal power.During the rising process,it may be mixed with a small amount of ancient crustal materials.When it reaches the shallow crust,the ore bearing fluid is dissolved from adakitic magma.When the ore bearing fluid contacts with the marble of Mashan Group,large-scale contact metasomatism occurs and gold mineralization is formed in the late stage,Finally,Laozuoshan Late Permian skarn type copper gold deposit was formed.In the early Early Jurassic,the study area began to be affected by the westward subduction of the ancient Pacific plate,resulting in the upwelling of asthenosphere materials and the decompression and melting of lithospheric mantle.Some mantle materials were directly emplaced into the middle and deep part of the crust.In this process,ancient crustal materials may be added.When the magma meets the stratum of tumenling formation,double metasomatic reaction occurs,The Da'anhe gold deposit was formed by unloading the ore-forming materials carried by it.In addition,some mantle materials gather at the bottom of the lower crust,and the resulting high temperature makes the crust partially melt to form the original magma chamber.In the process of ascending evolution,the magma successively forms gabbro,granodiorite and granitic magma through crystallization differentiation,and takes place in plutonic,medium hypabyssal positions in turn.When the granodiorite in deep achievement position contacts with the calcareous carbonate stratum of Qianshan formation,Large scale contact double metasomatism occurred,forming magmatic like solutions containing iron,copper,lead and zinc and other useful elements.Then,with the decrease of temperature and pressure,after crystallization in the early skarn stage and wet skarn/degenerate alteration stage,magnetite ore body,pyrite ore body,copper polymetallic ore body and lead-zinc ore body were formed successively,and finally two strands of iron copper polymetallic deposits were formed.In the late Early Cretaceous,the study area was in the extensional environment of back arc extension due to the subduction,rotation and retreat of the Pacific plate.Under this background,the asthenosphere material upwelling and heating the lower crust to make it partially melt.The formed medium acid magma was mixed with mantle derived basic magma,and a small amount of crustal contamination occurred in the process of ascending evolution.The mixed magma moved upward along the regional structure in the study area,and most of them ejected from the surface to form a large amount of calc alkaline volcanic rocks.This magmatic activity produced a series of typical hydrothermal gold deposits in Lesser Xingan Range area,such as Dong'an,Tuanjiegou and Gaosongshan.A large number of calc-alkaline volcanic rocks and a small amount of intrusive rocks were formed in Jiamusi block.Some intrusive rocks dissolved ore bearing fluid from magma when they reached the shallow crust.When the ore bearing fluid contacted the marble of Mashan Group,large-scale contact metasomatism occurred,and gold mineralization was formed in the late stage,finally forming Laozuoshan Early Cretaceous skarn type gold deposit.Combined with the pressure depth calculated by the fluid inclusion data when the deposit is in place,we believe that the Lesser Xingan Range area where the Ergu and Da'anhe deposits are located has a deep denudation depth(?3km)and limited deep prospecting potential,while the Laozuoshan deposit has a relatively shallow denudation degree,deep formation depth and good prospecting prospect.