Metallogenesis Of Cu-ni Sulfide Deposits In The Eastern Section Of East Kunlun Orogenic Belt,qinghai Province

The East Kunlun orogenic belt（E-KOB）is located on the northern edge of the Qinghai-Tibet Plateau,is part of the western section of the central orogenic belt of China.Its complex geodynamic evolution,tectonic-magmatic activities,multi-period orogeny,and substantial potential for mineral resources have attracted considerable attention and have been the subjects of several studies.The East Kunlun has experienced two important structural system conversion periods:the early Caledonian period from the passive continental margin to the active continental margin period and the late Indosinian period from the extruded orogenic system to the intra-slab lithospheric extension and thinning system.These two important tectonic events provided favourable geodynamic conditions for large-scale mineralization.Since the Xiarihamu super-large Ni-Cu deposit have been found by Jilin University and Fifth Institute of Geology and Mineral Exploration Association of Qinghai Province in 2011,and invoked upsurge of searching for the magmatic Ni-Cu sulfide deposit.A series of mafic-ultramafic rocks,Akechukesai,Langmuri,Gayahe,Xiwanggou,Kaimuqi deposits and some mineralization anomalies have been discovered in the eastern section of the E-KOB.Nevertheless,the degree of research on copper-nickel sulfide deposits in the eastern section of the E-KOB is still relatively low.The tectonic setting is on debate.Metallogenic epoch unclear,insufficient research on the origin and evolution of magma,the sulfide segregation,metallogenic condition and regularity are lack of systematical study.In this thesis,on the basis of detailed field geological survey;applying modern metallogenic theory;through systematic chronology,petrology,mineralogy and geochemistry;through the comprehensive study of typical ore deposits;analysis regional metallogenic geological conditions,diagenesis and mineralization.Summarize metallogenic regularity and metallogenic potential,establish a metallogenic model,and provide a scientific basis for further work.Further demonstrate the existence of the Wanbaogou oceanic basalt plateau in the Mesoproterozoic,and point out there is a special"soft collision characteristic"during the collage process with the Qaidam block.Therefore,there was no large-scale collision orogeny.The subducting slabs were"break off",the slab fragmentation occurred and the slab window was formed,which triggered large-scale mantle-derived magma activity formed by decompression melting,forming deep-melt granites and related deposits,mantle-derived mafic-ultramafic rocks and related magmatic Cu-Ni sulfide deposits.The mafic-ultramafic intrusions occur in the basement uplift granite belt north of the Middle Kunlun Fault.The wall rocks are mainly Middle Proterozoic Jinshuikou Group formation.Systematic chronology and geochemistry studies show that the diagenetic and metallogenic epoch is Late Caledonian（Langmuri,Gayahe）,Late Hercynian（Xiwanggou）,Late Indosinian（Kaimuqi、Nagengkangqieerxi）.The late Caledonian deposit was formed in the geodynamic background of post-collisional extension,and the deep geodynamic process was related to the break of subduction plates caused the upwelling of the asthenosphere and decompression melting occurred.The Late Hercynian ore spots were formed in the local extensional setting under the subduction environment,and the deep geodynamic process was related to the depleted lithospheric mantle melting.The late Indosinian ore spots were formed in the strong extensional background after land-land collision,and the deep geodynamic process was related to the upwelling of mantle material caused by lithospheric delamination and the induction of enriched lithospheric mantle melting.The main rock types are gabbro,olivine gabbro,pyroxenite,pyroxenite olivine and olivine.The ore bodies are mainly hosted in pyroxenite and olivine facies.The rocks are low Si and high Mg,low alkali and poor Ca,belonging to subalkaline series.The total rare earth elements are low,depleted in high field strength elements such as Nb,Ta,Hf and Zr,and have obvious positive anomalies of Pb,enriched in large ion lithophile elements such as K and Rb.The geochemical characteristics of trace elements in rock mass（such as Ce/Pb,La/Nb,Th/Ta,etc.）,Sr-Nd isotope of whole rock,Lu-Hf isotope of zircon,S isotope of metal sulfide,S and Se content of whole rock indicate that the parent magma source area of ore-bearing rock mass is a depleted mantle,and is contaminated by EMII components.The electron probe analysis shows that the olivine in the main rock-forming minerals is chrysolite,the orthorhombic pyroxene is bronzite,the clinopyroxene is diopside and refractory diopside,the hornblende is magnesium hornblende and magnesium calcium hornblende,the mica in the olivine and pyroxene facies is golden mica,the mica in the gabbro is golden mica and magnesium biotite,and the plagioclase in the gabbro is andesine,labradorite and bytownite.The composition of parental magma was estimated with the highest Fo olivine composition.The Mg O content in Langmuri parental magma was 14.56%,and that in Gayahe parental magma was 13.24%.The Mg O content of parent magma of mafic-ultramafic rocks in late Caledonian is generally high,belonging to high-Mg basalt magma,and a large proportion of melting occurs in the source area.The Mg O content in Xiwanggou parental magma was 10.36%.The Mg O content of parent magma of mafic-ultramafic rocks in late Hercynian is low,and the melting proportion in the source area is low.The Mg O content of Kaimuqi gabbro is 18.64%and the Mg O content of Nagengkangqieerxi gabbro is 8.08%,suggesting that the Mg O content of parent magma is low and the melting ratio of source region is the lowest.Ultramafic rocks account for a small proportion in the late Indosinian mafic-ultramafic complexes,and the existence of ultramafic facies is not found in many complexes.From late Caledonian to late Hercynian and then to late Indosinian,with the change of tectonic background,the melting ratio of mafic-ultramafic rock parent magma source area gradually decreased.The simulation of whole rock Cu/Zr ratio and olivine composition（Fo,Ni）shows that sulfide segregation occurs in each ore-bearing rock mass and complex group,and the higher the degree of early sulfide segregation is,the more favorable it is for mineralization.Magmatic segregation is an important mechanism for the formation of magmatic sulfide deposits,and sulfur saturation in magma is the prerequisite for the occurrence of sulfide segregation.The factors controlling sulfur solubility in magma include temperature,pressure,oxygen fugacity,sulfur fugacity and Fe O content.The geochemical characteristics of mafic-ultramafic rocks in the study area show that magmatic evolution generally experienced crystallization differentiation,and crystallization differentiation and crustal contamination are important mechanisms leading to sulfide segregation.The fluid plays an important role in the mineralization process,and the ore-bearing parts often have a high degree of self-metamorphism.In summary,the mineral assemblage in the study area is chalcopyrite-pentlandite-pyrrhotite,and high magnetism can be used as a prospecting indicator for copper-nickel sulfide magmatic deposits.The presence of high Fo olivine and orthopyroxene indicates favorable metallogenic conditions.The ore-bearing rock mass generally has the characteristics of low silicon,low alkali,low titanium,low calcium and rich magnesium,m/f（2～6.5）shows that it belongs to iron ultrabasic rock and beneficial to Cu,Ni,PGE mineralization.The composition of rare earth elements shows the characteristics of low∑REE.Light rare earth elements and large ion lithophile elements are relatively enriched,and heavy rare earth elements and high field strength elements such as Nb,Ta and Ti are relatively depleted.The content of Cr,Ni and Co is high and uniform.The rock consolidation quality SI>55,and Ti O₂<0.5 has good ore-bearing property.Summarize the metallogenic model is‘deep segregation-in situ segregation-multi-stage pulsating intrusion’.