Metallogenesis Of Copper-molybdenum-gold Polymetallic In The Duobaoshan Orefield,Heilongjiang Province

Duobaoshan orefield is located in the eastern part of Central Asian Orogenic Belt and the north Xing'an block of NE China,which is the largest porphyry-related Cu-Mo-Au orefield in northeastern Asia.It hosts a number of large-small scale porphyry Cu?PCDs?,epithermal Au and Fe-Cu skarn deposits.Formation ages of these deposits,from the oldest?Ordovician?to youngest?Jurassic?,have spanned across over 300 My.No similar orefields of such huge size and geological complexity are found in NE Asia.Therefore,the metallogenesis,diagenesis,metallogenesis and preservation mechanism of Duobaoshan orefield are of great significance.In this paper,the genesis,geochronology,magmatic genesis and tectonic setting of Duobaoshan and Tongshan PCDs,Sankuanggou Fe-Cu skarn,and Zhengguang porphyry-epithermal Au deposits are discussed.On this basis,the metallogenic models of?a?Caledonian Cu-Mo-?Au?,?b?Indosinian Au-?Cu-Mo?,and?c?Yanshanian Fe-Cu are established.Detailed fluid inclusion and hydrogen-oxygen-sulfur-lead-rhenium isotopic studies have been conducted to constrain the origin,evolution and source of ore-forming material of four typical deposits.Duobaoshan and Tongshan PCDs have liquid-rich two-phase,CO₂-H₂O and daughter-bearing three-phase inclusions.The results of laser Raman spectroscopy also show that the inclusions contain CO₂ and a small amount of N₂.In the early stage of mineralization,the ore-forming fluid is a high temperature and medium-low salinity CO₂-H₂O-NaCl system,and immiscibility occurs,while in the late stage,the ore-forming fluid is a low temperature and low salinity H₂O-NaCl system.The initial ore-forming fluids of Duobaoshan and Tongshan deposits are derived from magmatic water,and there is mixing of meteoric water in the late stage;the ore-forming materials are derived from deep magma and Duobaoshan Fm.,showing a mixed crust-mantle source.The ore-forming fluid of the Sankuanggou Fe-Cu skarn deposit is a high temperature and high salinity H₂O-NaCl system in the early stage,a high-medium temperature and high-medium salinity CO₂-H₂O-NaCl system in the middle stage and a low temperature and low salinity H₂O-NaCl system in the late stage.There are liquid-rich two-phase,CO₂-H₂O and daughter-bearing three-phase fluid inclusions in Sankuanggou deposit.The results of laser Raman spectrum analysis also show that the inclusions contain CO₂ and a small amount of N₂,CH₄ and other components,and the ore-forming fluid has obvious immiscibility.The initial ore-forming fluid of Sankuanggou deposit is magma source,and part of meteoric water is mixed in the late stage;the ore-forming material mainly comes from deep magma,but a small amount of Duobaoshan Fm.is involved,which has the characteristics of a mixed crust-mantle source.There are only liquid-rich two-phase fluid inclusions in the Zhengguang epithermal gold deposit,and the ore-forming fluid is characterized by low temperature and low salinity H₂O-NaCl system.The ore-forming fluid originates from meteoric water and magmatic water,but the former is dominant;the ore-forming material mainly comes from the Duobaoshan Fm.and the deep magma,and has the characteristics of mixed crust-mantle source.In terms of geochronology,we have summarized seven magmatic phases in the orefield?1?Middle-Late Cambrian?506⁴91 Ma?,?2?Early and Middle Ordovician?485⁴71 Ma and⁴62 Ma?,?3?Late Ordovician?450⁴47 Ma?,?4?Early Carboniferous and Late-Carboniferous to Early Permian?351³45 and 323²91 Ma?,?5?Middle-Late Triassic?244²23 Ma?,?6?Early-Middle and Late Jurassic?178¹68 Ma and¹50 Ma?,and?7?Early Cretaceous?¹12 Ma?.The mineralization mainly occurred in?a?Early Ordovician?478⁴74 Ma?,?b?Middle Triassic?246²39 Ma?and?c?Middle Jurassic?174¹73 Ma?and Late Jurassic?probably¹50 Ma?.There are multiple mineralization events in Tongshan and Zhengguang deposits.In terms of whole-rock geochemistry,ore-causative granitoids in the Duobaoshan orefield exhibit adakite-like or adakite-normal arc transitional signatures,but those forming the porphyry-/epithermal-type Cu-Mo-Au mineralization are largely confined to the former.The varying but high Sr/Y,Sm/Yb and La/Yb ratios suggest that the ore-forming magmas were mainly crustal sourced and formed at different depths?clinopyroxene-/amphibole-/garnet-stability fields?.The adakite-like suites may have formed by partial melting of the thickened lower crust at35–40 km?for the Early Ordovician arc?and>40 km?for the Middle-Late Triassic arc?depths.The Early Jurassic Fe-Cu skarn ore-causative granitoids show an adakitic-normal arc transitional geochemical affinity.These granitoids were likely formed by partial melting of the juvenile lower crust?35⁴0 km depth?,and subsequently modified by assimilation and fractional crystallization?AFC?processes.In light of the geological,geochemical and geochronological information,we proposed the following tectono-metallogenic model for the Duobaoshan orefield:In the Paleozoic,the Paleo-Asian Ocean probably subducted NE China in the Middle Cambrian,and formed the PCD-related deposits at Duobaoshan,Tongshan and Zhengguang,and Duobaoshan island arc.Subduction may have ceased in the latest Ordovician to the Late Carboniferous,subduction effect of the Paleo-Asian Ocean weakened,and the regional tectonics passed into long subsidence and extension till the latest Carboniferous.This extensional tectonic regime and the Silurian large-scale terrestrial-shallow marine sedimentation had likely buried and preserved the Ordovician Duobaoshan PCD mineral system.The extensive unmineralized?and probably ore-destructive?Devonian?377³63Ma?and Carboniferous?351³45 Ma and 323²91 Ma?intracontinental rift-related magmatism had likely destroyed much of the Ordovician Duobaoshan Arc and its PCDs outside the orefield.However,this magmatism did not extend into the orefield,thus preserved the most of the Ordovician arc and the PCDs there.The south-dipping Mongol-Okhotsk Ocean subduction from north of the Duobaoshan orefield had generated the Middle-Late Triassic continental arc magmatism and the associated Tongshan PCD and Zhengguang epithermal Au mineralization.In the Late Triassic,the Paleo-Pacific began to subduct in a west oblique direction,and since then,the orefield has been jointly affected by two tectonic systems,namely,Mongol-Okhotsk Ocean?West?and Paleo-Pacific?East?.In the Early Jurassic,the collision of Siberia-Amuria blocks and the subduction of the Paleo-Pacific may make NE China under compression,and formed the granodiorite-tonalite and Fe-Cu skarn deposits at Sankuanggou and Xiaoduobaoshan in the orefield.From the Middle Jurassic,the consecutive accretion of Paleo-Pacific arc terranes?e.g.,Sikhote-Alin and Nadanhada?onto the NE Asian continental margin may have gradually distant the Duobaoshan orefield from the subduction front,and consequently arc-type magmatism and the related mineralization faded.The minor Late Jurassic and Cretaceous unmineralized magmatism in the orefield may have triggered mainly by the far-field extension led by the post-collisional?Siberia-Amuria?gravitational collapse and/or Paleo-Pacific backarc-basin opening.The content of copper and gold in the Ordovician Duobaoshan Fm.is relatively high,which is considered to be the main ore source bed in the Duobaoshan orefield.This understanding has important guiding significance for the exploration of regional polymetallic minerals.In addition,juvenile“wet”magma source,medium crustal thickness,and high oxygen fugacity?fO₂?are also keys for massive mineralization in the orefield.