Features Of The Mafic-ultramafic Rocks In Jilin Province And Their Metallogenesis

Jilin Province is located in the area common influence by the Paleo-Asian Ocean tectonic system,Circum-Pacific tectonic system and the Mongolia-Okhotsk tectonic system.The region has experienced a long and complicated geological evolution process.Along with the evolution of geodynamics in different periods,a large number of mafic-ultramafic rocks have been formed,in which a number of Cu-Ni sulfide deposits have been bred,including some large magmatic liquation Cu-Ni sulfide deposits,such as Hongqiling and Chibaisong.The discovery of these large Cu-Ni sulfide deposits has established the status of a large Ni resource province of Jilin Province and made a significant contribution to the national Ni resource security.In recent years,there is no major prospecting breakthrough in the exploration of Cu-Ni sulfide deposits in Jilin Province,which indicates that there is still a gap in the research level of mafic-ultramafic and Cu-Ni sulfide deposits,especially the lack of systematic and in-depth research on the chronological characteristics,geodynamic setting and mineralization of ore bodies have seriously restricted the prospecting work.Based on modern metallogenic theory,this paper combines field exploration and indoor testing analysis to discuss the evolution of geodynamics in different periods,comprehensively analyze typical deposits,and summarize regional metallogenic regularity through regional geological conditions analysis,thereby defining the direction of prospecting and laying a theoretical foundation for the study of Cu-Ni sulfide deposits in Jilin Province.The paper mainly achieved the following understanding:1.This paper systematically summarizes the geodynamic evolution process associated with mafic-ultramafic rocks in Jilin Province,and considers that they have undergone the formation and evolution of the Archean craton basement,the Paleoproterozoic Liaoji ocean tectonic evolution,the Mesoproterozoic cleavage of Colombian supercontinent,the development and evolution of thePaleozoic-Early Mesozoic Paleo-Asian tectonic domain and the transformation of the Mesozoic Pacific Ocean tectonic domain.2.Based on the geological survey and chronological study,the diagenetic events of the mafic-ultramafic rock masses in Jilin Province are divided into five stages:(1)Late New Archean(2589 ~ 2398Ma),the representative rock masses are Rentian No.6 and No.9,and Xiaochenmugou ore-bearing rock masses.It is believed that the arc-continent collision orogeny occurred in the Late New Archean period,and the extension environment after the orogenic closure is the main geodynamic background for the formation of mafic-ultramafic rocks in this period,(2)Middle Paleoproterozoic(2237~1820Ma),the representative rock mass is the Chibaisong No.1 rock mass,formed in the extension environment after the closure of the Liao-Ji Ocean,(3)Middle Mesoproterozoic(~ 1200Ma),the representative rock mass is Hanyanggou rock mass,where the Longgang block in which it was located,was in a strong stretching environment during the Middle Proterozoic period,corresponding to the final cracking time limit of the Colombian supercontinent,(4)Middle and Late Triassic(245~206Ma),the representative rock masses are Piaohechuan No.4,5rock mass,Changren-Zhangxiang No.5,6,and 11 rock mass,Xibeicha No.115 rock mass,and Shirengou ore-bearing rock mass,formed in the extension environment after the closure of the ancient Asian Ocean,(5)Early Jurassic(191~175Ma),the representative rock masses are No.15 and No.26 of the Fudong.The mafic-ultramafic rock mass in this period was the product of the post-arc extension environment of the plate subduction system.3.Through the study of typical copper-nickel sulfide deposits in Jilin,it is concluded that: The primary magma of the Xiaochenmugou Cu-Ni sulfide deposit originates from the depleted mantle affected by crustal contamination or sulfur metasomatism,and the gravity differentiation in the melting phase is obvious.The breccia present in the ore,represents its formation of the turbulent magma environment.Through the chronological study,the deposit is the oldest Cu-Ni sulfide deposit in the country(2589±10 Ma).Based on the analysis of previous research,and study on the genesis of Chibaisong Cu-Ni sulfide deposit which was more controversial in the metallogenic epoch,it is determined that the metallogenic age is the middle Paleoproterozoic(2237±62 Ma),which belongs to magma segregation-injection Cu-Ni sulfide deposit.Based on the comprehensive research of the Hongqiling,Changren-Zhangxiang,Piaohechuan,Erdaogou and Shirengou deposit,it is believed that the metallogenic time of the Cu-Ni sulfide deposit in the eastern section of Xing'anling Mongolian Orogenic Belt(XMOB)should start at 245 Ma and end at 206 Ma.Among them,the geochemical characteristics of the Er'daogou rock mass are characterizedby low silicon,low titanium,high magnesium,low alkali,low ?REE,enriched LILE,loss HFSE,similar to the oceanic basalt(OIB),and the magma source area is depletive mantle of asthenosphere,part of the source areas were intermingled by the enriched mantle.S mainly comes from the upper mantle,and the original magma originates from 10%-20% partial melting of the original mantle.Deep dissociation causes loss of platinum group elements,which was intermingled by a certain degree of crustal material during the ascending process.4.By analyzing the metallogenic potential of the Fudong rock groups in the Early Jurassic,it is believed that the local magmatic melting rate caused by the subduction of the Pacific plate is too small,and the sulfide melts in the source area and can not accumulate in the crust.5.The Cu-Ni sulfide deposit in Jilin Province has strong metallogenetic specialization,which is manifested in the fact that :(1)the ore-bearing rock masses are mainly controlled by the deep fault,(2)fully differentiated mafic-ultramafic complex rocks are beneficial to mineralization,and the pyroxenite facy is the main ore-bearing lithofacies,followed by olivine pyroxenite,harzburgite and norite,the gabbro is generally uncontained,(3)the ore-bearing rocks develop chrysolite and bronzite,Fo?En,m/f value of mafic rocks are 0.5~2,m/f value of ultramafic rocks are between 2~6,which are very favorable for mineralization,(4)ore-bearing facies have high magnesium,low silicon,low calcium,low ?REE,rich in LILE,loss of HFSE,high content of Cr,Co and Ni,(5)high-level partial melting occurs in the mantle source area,reaching the category of high-magnesium basaltic or picritic basaltic magma.6.Based on discriminating ore-bearing rock masses and non-ore-bearing rock masses,and through the establishment of a series of evaluation indicators,this paper makes an objective evaluation of the mineralization and prospecting potential of mafic-ultramafic rock masses in various periods and draw the following conclusions: Middle-Late Triassic is an important metallogenic period of Cu-Ni sulfide deposits in Jilin Province.During this period,the mafic-ultramafic rock masses are large,the degree of rock mass differentiation is high,the lithofacies are complex,and the mineral content is high,with the greatest potential for prospecting;the Paleoproterozoic mafic-ultramafic rocks are mainly distributed in the eastern section of the northern margin of the North China Craton(NCC),spreading from north to south.The tectonic background of the rock mass formation is similar to that of the Middle-Late Triassic rock mass,which was formed in the extended environment after the ocean was closed,and also has great prospecting potential,the Late Neo-Archean mafic-ultramafic rock masses has experienced complex geological development and changes due to the ancient age of rock formation.The requirements for the preservation conditions of the ore bodies are harsh and the prospecting is more difficult,Middle of the mid-proterozoic mafic-ultramafic rockshave poor differentiation and weak mineralization,and there are few Cu-Ni sulfide deposits formed in the same period in China,and the metallogenic potential in this period is unclear.In the exploration,each rock mass should be combined with specific analysis such as rock morphologies,differential degrees,emplacement depths and mineralization characteristics,Early Jurassic mafic-ultra-mafic rocks are scattered in the eastern part of the Xing'anling Mongolian Orogenic Belt(XMOB),with relatively simple lithofacies,most of which are gabbro(vein)and the mineralization of the rock is weak.Because the proportion of partial melting degree in the mantle source area is too small,a large amount of sulfide remains in the mantle and cannot form sulfur-unsaturated primitive magma,rich in metal elements.Even if sufficient sulfide is added in the later period,large-scale deposits cannot be formed,therefore,it has no prospecting potential.