Study On The Diagenesis Mechanisms And Metallogenic Model Of Skarn-type Fe-Cu(Mo)polymetallic Deposit In The Lesser Xing'an Range,NE China

The Lesser Xing'an Range is one of the most important nonferrous metal bases in China.There are many kinds of minerals in the study area,such as Fe,Mo,Au,Cu,Pb,Zn,W,Ag and so on,all of which are needed for economic development and national construction.In recent years,mineral prospecting in the Lesser Xing'an Range has made a significant breakthrough,and the study area has become a concentrated area for large-scale mineralization of precious metals and nonferrous metals in the northern China.In order to explore the metallogenic regularity of the skarn-type Fe-Cu?Mo?polymetallic deposits in the Lesser Xing'an Range,this paper has made a systematic research on the geological characteristics of deposits,fluid inclusions,petrogenic and metallogenic geochronology,deposit geochemistry.And achieved the following important results and progress.1.Based on the study of the geological characteristics of the typical skarn-type Fe-Cu?Mo?polymetallic deposits in the study area,we find that the orebodies are mainly hosted in the contact zone between granitic intrusions and Lower Cambrian dolomitic crystalline limestones or skarns.The mineralization is characterized by Fe,Zn,Cu and Mo,followed by W,Pb,Au,Sn,and associated with Ga,In,Cd,Ag and other beneficial components.Ore bodies are mainly lenticular,stratoid,and veins,and along the direction or tendency appears expansion,contraction and branching phenomenon.The wall-rock alteration mainly includes skarnization,epidotization,chloritization,silicification,fluorite,sericite and carbonation.Ore-forming processes can be divided into five stages,according to cross-cutting relationships and mineral assemblages:?I?a prograde skarn stage;?II?a retrograde skarn stage;?III?an oxide stage;?IV?a quartz–sulfide stage;and?V?a carbonate stage.2.The study of fluid inclusion revealed that:?1?The early skarn stage developed W-type and S-type inclusions;The quartz-sulfide stage was mainly consisted of W-type and S-type inclusions,followed by a small amount of C-type inclusions;The late carbonate stage only developed W-type inclusions;?2?The homogeneous temperature of the fluid inclusions from early stage to late stage of the Baoshan Cu-polymetallic deposit are 376.5⁵12.4?,291.2⁴19.5?,and 156.3²47.8?,respectively;And the salinities of fluid inclusions are 4.2⁴9.4% NaCleqv,1.9⁴2.2% NaCleqv and 0.7⁴.5% NaCleqv,respectively;?3?The homogeneous temperature of the fluid inclusions from early stage to late stage of the Cuihongshan Fe-Mo-polymetallic deposit are 332.5⁵25.5 ?,187.3⁴22.1? and 142.3²35.2?;And the salinities of fluid inclusions are 14.8⁴4.1% NaCleqv,1.4⁴4.0% NaCleqv and 0.7⁶.7% NaCleqv,respectively;?4?The homogeneous temperature of the fluid inclusions from early stage to late stage of the Ergu Fe-Zn-polymetallic deposit are 348.9⁵24.5?,231.6³80.4? and 152.9²31.6?;And the salinities of fluid inclusions are 1.9⁴6.84% NaCleqv,1.2³7.4% NaCleqv and 1.4⁴.5% NaCleqv,respectively;?5?The results of laser Raman analysis show that the gas composition of inclusion from the garnet,diopside and epidote in the early skarn stage is H₂ O,and no other components are observed;The gas composition of inclusion from the quartz in the quartz-sulfide stage is CO₂ and CO₂+CH₄,indicated that the fluid has strong reducibility;The gas composition of inclusions in the carbonate stage is mainly H₂ O.?6?H-O isotope results show that the initial ore-forming fluid may be residual magmatic water,and mixed meteoric water or paleo-rainwater during the mineralization progresses.Ore-forming fluid is mainly composed of meteoric water in the late stage of mineralization.The result of Raman component analysis of the fluid inclusions and H-O isotope of quartz further reveals that the initial ore-forming fluid of the skarn-type Fe-Cu?Mo?polymetallic mineralization in the study area are derived from exsolved fluid of residual magma,which is high temperature,high oxygen fugacity and high salinity H₂O-CO₂-CH₄-NaCl fluid system,and mixed meteoric water or paleo-rainwater during the mineralization progresses.The magmatic melt-flow separation and water-rock interaction causes the ore-forming elements to be enriched,and fluid boiling and mixing is the metallogenic mechanism that leads to the final precipitation of mineralization.3.The diagenetic and metallogenic geochronology of the skarn-type Fe-Cu?Mo?polymetallic deposit in the study area shows that:?1?The metallogenic epoch of the Baoshan copper polymetallic deposit is between 245-255Ma?Late Permian-Early Triassic?,skarn-type mineralization is closely related to amphibolite quartz monzonite;?2?At least three magmatic activities in the Cuihongshan mining area,the second stage of the Early Jurassic monzonitic granite magmatism is most closely related to mineralization,and the deposit is formed in 198-204Ma?Late Triassic–Early Jurassic?;?3?The mineralization of the Ergudongshan and Erguxishan ore section occurred in 180-184 Ma,and the mineralization was closely related to the biotite monzogranite,the ore-forming age of the Xiangshui River ore section was 191-194 Ma,which is about 10 Ma earlier than the former.Based on the research results of this paper and predecessors,we believe that there are two metallogenic epochs of skarn type Fe-Cu?Mo?polymetallic deposits in the study area,which are Late Permian-Early Triassic?245-255Ma?and Late Triassic-Early Jurassic?180-205 Ma?,and the latter can be divided into 198-205Ma?represented by Cuihongshan,Daxilin,Qianjindongshan deposit?,191-193Ma?represented by Xiangshui River and Da'anhe deposit?and 180-184Ma?represented by Erguxishan,Ergudongshan,Xulaojiugou deposit?three mineralization stages.4.In the skarn mineralogy study,the garnet and pyroxene of skarn-type Fe-Cu?Mo?polymetallic deposits in the study area is consistent with the world's corresponding mineral skarn deposit.Among them,the garnet in Baoshan and Ergu deposit is mainly andradite,followed by grossular;The Cuihongshan deposit only develops andradite.The pyroxene in the three deposits is dominated by diopside,and only a small amount of hedenbergite appears in the Cuihongshan deposit.These features indicate that the mineralization environment of early skarn stage is acidic,high temperature and high oxygen fugacity.The high oxygen fugacity led to the formation of Fe3+ into the garnet and pyroxene to form andradite and diopside,and evolved into a weakly reducing environment,resulting in a small amount of grossular and hedenbergite.5.The studies of petrology,whole-rock geochemistry,isotope geochemistry and geochronology of ore-forming rock show that the Late Permian amphibole quartz monzonite-porphyritic biotite granite assemblage in the Baoshan deposit is characterized by metaluminous and high-K calc-alkaline;The forming of the Cuihongshan deposit is related to the Early Jurassic monzogranite;The ore-forming rock of the Ergu deposit is biotite monzogranite which is characterized by metaluminous and shoshonite to high-K calc-alkaline.The ore-forming rocks of three skarn-type deposit have high SiO₂,Al2O3,Na2O+K2O and REE,significant differences between LREE and HREE;Enrichment of LREE,Th,U and large ion lithophile elements Rb,K,loss of HREE,Ba and high field strength elements Nb,Ta,P,Ti,etc.;In contrast to the obviously negative Eu anomalies of Cuihongshan and Ergu metallogenic rocks,the Eu negative anomalies of the Baoshan ore-forming rocks is weakly.6.The Sr-Nd and Lu-Hf isotopic studies of the ore-forming rocks show that the amphibole quartz monzonites in the Baoshan deposit were derived from the Mesoproterozoic lower crustal substances;The magma of the alkali-feldspar granites were derived from the partial melting of the old continental crust base,whereas the magma of the monzogranites were derived from the NeoproterozoicMesoproterozoic crust or lithosphere mantle and mixed with a small amount crustal substances during asceny.The magmatic rocks of the Ergu deposits have similar Sr-Nd and Lu-Hf isotopic composition to the Cuihongshan monzogranite,and their magma were derived from the NeoproterozoicMesoproterozoic lower crustal or lithospheric mantle,and mixing with a small amount of ancient crustal component.7.Combined with regional evolution setting of tectonic,the geological process of diagenesis and metallognesis in the study area can be summarized:?1?Late Permian-Early Triassic,with the final closure of the Paleo-Asian Ocean,the continental collision of the North China Craton and Songnen Block caused over thickening and delamination of the lower crust.Adakitic magmas most likely originated from partial melting of delaminated lower crust,with garnet being the main residual mineral and little to no plagioclase.Interactions between adakitic magmas and mantle peridotites occurred during the ascent of adakitic magmas through the mantle,resulting in oxidation of metallic sulfides,and incorporation of chalcophile elements,such as Cu,into the magmas;With the emplacement of the adakitic magmas,hydrothermal fluids exsolved from the magma and caused extensive alteration,forming skarns and associated mineralization in the contact zone between hornblende–quartz monzonite and dolomitic crystalline limestones,and eventually forming the Baoshan skarn Cu-polymetallic deposit.?2?Late Triassic-Early Jurassic,coincided with the Siberian Craton and North China Craton after plate collision orogenic extensional tectonic environment,the regional lithosphere extension caused large-scale asthenosphere upwelling that caused the new basaltic thickened crust partial melting;Ore-bearing magma mixed with a small amount of ancient crust during ascent to the shallow crust.In the late stage of evolution,a large amount of fluid which is characterized by volatile and mineralization elements such as Fe,Mo,and W,were separated;Under the influence of structure and magma,the ore-bearing fluid metasomatismed with the surrounding carbonate formations,and eventually formed the Cuihongshan skarn-type Fe-Mo-polymetallic deposit.?3?During the subduction of the Pacific Plate to the Eurasian continent in the early Jurassic,the early metamorphic lithosphere mantle of the plate is dehydrated to form basaltic magma,and occurred the metasomatism of mantle peridotite,so that the mineralization elements such as Fe and Cu were enriched in the magmatic system;The underplating of basaltic magma caused the partial melting of the lower crust,and mixed with basaltic magma;This crust-mantle mixed magma was contaminated by a small amount of crust during the ascending and evolution process.The metasomatism with the nearby carbonate strata resulted in the large-scale precipitation ore-forming elements and the forming of Ergu skarn-type Fe-Zn-polymetallic deposit.