Research On Metallogenesis Of Polymetal Deposits In The Phanerozoic Continental Volcanic Rocks Areas,the Periphery Of Qaidam Block,Qinghai Province

Periphery of Qaidam Block,which is located in the northern margin of the Qinghai-Tibet Plateau and mainly composed of the composite orogens of Eastern Kunlun and Qilian,is an important part of the Central Orogenic Belt in China.Its unique tectonic location,complex tectonic environment,frequent magmatic activity and different degrees of metamorphism recorded the tectono-magmatic-mineralization orogenic cycle process,which not only has produced extremely rich in mineral resources in the region,which not only creates extremely rich mineral resources in the region,but also is an ideal place to explore the spatiotemporal structure of continental lithosphere and the interaction of different spheres,and reveal the Phanerozoic geodynamic evolution.This paper selects six representative deposits in continental volcanic rock area discovered in recent years as the research object,emphasizes the field investigation of geological phenomena,combined with detailed indoor observation and analysis,systematically summarizes the geological characteristics and metallogenic conditions of the deposit,and accurately determines the genetic type of the deposit.The volcanic rocks and intermediate-acid intrusive rocks in the mining area have been comprehensively studied in petrology,zircon LA-ICP-MS,whole rock geochemistry and zircon Hf isotope.Combined with mineralogy,fluid inclusion,H-O isotope,the following main achievements have been achieved:The crystallization ages of the Maoniushan formation acid volcanic rocks in the northern margin of Qaidam block orogenic belt are 407Ma,378Ma and 377Ma.Combined with the previous research data in this period,the dynamic evolution events of collision and post-collision in the Caledonian and Variscan are systematically summarized.The time point of 410Ma is an important tectonic system transition time from syn-collision to post-collision.At this time,the slab break-off event occurred in the northern margin of Qaidam Block,and the whole environment changed from compressional orogenic environment to extensional environment.As the crust continued to thicken,lithosphere delamination occurred in～380Ma,and a large number of mantle derived magma upwelling.The crystallization ages are 240Ma,232Ma,230Ma for intermediate acid intrusive rocks of late Variscan and middle Indosinian in the northern margin of Qaidam Block.After the Caledonian orogeny,the Qaidam block and the Qilian block have been put together.This study suggests that no new ocean basin was formed in this period,but was affected by the northward subduction of Bayan-Har ocean,experience the same dynamic background with the Eastern Kunlun orogenic belt.The detailed geochronology of Mesozoic volcanic rocks in the East Kunlun orogenic belt shows that there were two stages of volcanic eruption events which show significant different lithology,age and tectonic setting,rather than the Elashan formation as previously viewpoint.Based on the above geological facts,this paper proposes to disintegrate the Elashan Formation and establish the Xiahe Formation,which is different from the traditional volcanic facies of the Elashan Formation.The age of XIahe Formation is early Indosinian.Geochemical and zircon Hf isotopic characteristics show that the source was derived from the mixed magma formed by melted mafic crust and enriched mantle formed by dehydration and metasomatism of subducted slab,and formed in the setting of active continental margin of the Bayan-Har ocean subducted northward under the Qaidam margin blocks.The age of the traditional volcanic rocks of the Elashan formation is late Indosinian.The source area is characterized by strong crust and mantle mixed magma,which was formed in the post-collisional extensional setting that relation to the intense lithospheric detachment after continental collision.Therefore,there are three continental volcanic rocks in the Phanerozoic of the the Qaidam Block,rather than two.In this paper,detailed field and laboratory work has been carried out for six typical ore deposits.The Dakenwulashan polymetallic deposit is hydrothermal vein deposit,not a VMS type.The surface show obvious features of surface alteration including propylitization altered zone and argillization zone.Kongquegou-habuqige Mo（Cu）polymetallic deposit is characterized by typical surface alteration,which is a porphyry deposit.Although the research degree is relatively low,it shows great prospecting potential.The Xiahe copper polymetallic deposit is a high sulfide epithermal deposit,while the Elashankou Pb-Zn deposit,Harizha silver polymetallic deposit and Nagengkangqieer silver polymetallic deposit are epithermal vein of middle-low temperature deposits in the Eastern Kunlun orogenic belt.Among them,Xiahe,Elashankou and Harizha are not porphyry deposits considered by predecessors.The fluid inclusions in the Elashankou Pb-Zn deposit are mainly composed of gas-liquid two-phase and CO₂ containing three-phase system,belonging to the system of H₂O-Na Cl-CO₂.H-O isotope shows that the ore-forming fluid came from the mixture of magmatic and meteoric water,and the sulfur isotope shows diversity,which is affected by acid magma and strata.The Xiahe copper polymetallic deposit is mainly composed of gas-liquid two-phase and CO₂ containing three-phase.H-O isotope shows that the ore-forming fluid is of deep origin.In the late stage of evolution,a lot of meteoric water participated in the mineralization,and the sulfur isotope came from the intermediate acid magmatism.The fluid inclusions in the Hariza and Nagengkangqieer deposits are mainly composed of CO₂containing three-phase and gas-liquid two-phase.The C-H-O-S-Pb isotope shows that the ore-forming fluid has the characteristics of mantle derived primary water,the Pb comes from the mixture of mantle and crust source,and the sulfur isotope shows the characteristics of mantle derived.Inaddition,tellurides were first discovered at the Nagengkangqieer mine area.All kinds of features reflect the control of deep geological processes on silver polymetallic deposits.Based on the above research,the coupling relationship between regional mineralization and geodynamic background is summarized.During the northward subduction of Bayan-Har ocean during the late Variscan and Indosinian,the Eastern Kunlun orogenic belt transported a large amount of water and metal sulfur,hydrophilic large ion lithophile elements（LILE）,halogens elements and other components into the upper mantle,which provided the basis for the formation of mantle derived C-H-O fluid rich in Ag and Au.At the same time,a series of regional large faults,large shear belts and secondary folds and fault ore controlling were formed.During this period,the underplating of mantle derived magma led to partial melting of the lower crust and formed mixed magma,which carried ore-forming materials along the fault.During the rising process,the physical and chemical conditions changed,resulting in the formation of sulfide deposits,such as the Elashankou and Xiahe deposits.After the closure of ocean in the late indosinian,the region experienced a strong tectonic regime transformation.A large number of mantle derived C-H-O fluids rich in Ag,Au and other metal elements in the upper mantle moved along the deep faults to the shallow crust.During the migration of the ore-forming fluid,the ore-forming materials in the surrounding rocks were also continuously extracted.At the shallow crust,the ore-forming materials finally precipitated to form silver polymetallic deposits with the participation of meteoric fluid.The prospecting direction of the ore deposits in the continental volcanic rock area around the Qaidam basin has been defined.That is to say,we should explore the ore deposits that forming at the shallow depths,such as porphyry deposits,epithermal hydrothermal deposits and some hydrothermal vein deposits.Due to the reason that in Mesozoic the northern Qaidam basin is far away from the subduction zone,leading that the ore-forming ability of the East Kunlun orogenic belt is obviously stronger than that of the northern Qaidam basin.Furthermore,the denudation depth of the Eastern Kunlun orogenic belt,this paper considers that the continental volcanic rock area is the next key target area to search for such Ag polymetallic deposits.In this paper,we combine theoretical research with case analysis with a new perspective,and put forward some laws of prospective exploration and practical experience.The coupling relationship between mineralization and geodynamics in the periphery of Qaidam Block is further clarified.In terms of views,methods,process and conclusion,we are grateful to peer experts for correction.