Research On Metallogenesis Of Ag Polymetallic Deposits In The East Segment Of The East Kunlun Orogenic Belt,Qinghai Province

The Dulan area in the eastern part of East Kunlun is one of the most important mineral concentration areas in Qinghai Province,where a large number of endogenous metal deposits such as Au,Fe,Mo,Cu,Pb and Zn are developed.In recent years,with the successive discoveries of a series of silver polymetallics(such as the Harizha large silver polymetallic mine)and independent silver deposits(such as the Naganqier super-large silver mine and the Naganqier North small silver mine),It shows that the Dulan area is also an important ore concentration area of silver,with huge silver metallogenic potential.At present,the research level of silver deposits in this area is relatively low,which restricts the thinking and direction of prospecting.The research on silver metallogenesis and metallogenic regularity is urgently needed.Based on the above,this paper focuses on silver polymetallic deposit in Dulan area.Through field geological investigation and indoor comprehensive analysis and research,three typical silver(polymetallic)deposits are studied in detail in terms of deposit geological characteristics,metallogenic material sources,metallogenic fluid evolution and geochronology.The following main understandings are obtained:The study area is located in the Central Kunlun granite uplift Belt(CKB),which is an important part of the East Kunlun Orogenic Belt and has undergone complex tectonic evolution.During Early Caledonian the study area entered the evolution stage of the Proto-Tethys Ocean tectonic domain.In the late Early Paleozoic,the Proto-Tethys Oceanic crust began to subduct to the Qaidam Block along the current Central Kunlun Fault,and a tectonic regime transition occurred.The southern margin of Qaidam Block has changed from passive continental margin to active continental margin,forming a trench at the location of the present Central Kunlun Fault,evolving into an active continental margin arc setting in the CKB between the present Central Kunlun Fault and the North Kunlun Fault,and causing a back arc rifting in the North Kunlun Belt.After the Oceanic Basalt Plateau(OBP)collage and collision during Late Caledonian,the oceanic subduction in central Kunlun has not disappeared,and migrated southward.With the closure of the Proto-Tethys Ocean in the East Kunlun region,the huge thick OBP hindered the further subduction,and the deep oceanic crust continued to subduct,resulting in the subduction plates being pulled apart to form a "plate window" and the upwelling of mantle-derived materials.Then,the tectonic regime of the whole East Kunlun region has changed from collision to regional extension.The 430.7 ± 1.9 Ma Harizha gabbro is formed in an island arc environment,indicating that the Wanbaogou Oceanic Basalt Plateau has not been combined with the CKB at this time;The 417.5 ± 1.5 Ma porphyry monzogranite and416.7 ± 4.8 Ma syenite from Harizha are the products of the extension background after closure,indicating that the Wanbaogou Oceanic Basalt Plateau has been spliced to the CKB,and the whole region has turned to extension.After the Caledonian movement,as the Wanbaogou oceanic basalt plateau amalgamated to the Central Kunlun belt along the Central Kunlun Fault,the subduction can no longer occur along the CKB.The subduction belt migrates southward,forming a new subduction belt along the current South Kunlun Fault,along which the Paleo-Tethys Oceanic crust subducted northward.The 259.8 ± 0.89 Ma monzogranite porphyry and 246.3 ± 0.9 Ma granodiorite in Nagengkangqieer are the products of Andean active continental margin,indicating that the East Kunlun entered the Andean orogenics from Late Hercynian to Early Indosinian.The East Kunlun Orogenic Belt entered the post-collisional extension stage during Late Indosinian.We systematically studied the volcanic rocks of the Elashan Formation,which are the host rocks of the silver polymetallic deposits.The Harizha 227.9 ± 0.4 Ma dacite and 225.0 ± 0.4 Ma crystalline tuff have the characteristics of high Mg and low Sr/Y,which are suggested to be derived from the partial melting of the mafic lower crust and formed in the lithospheric extension environment caused by the delamination of the thickened lower crust.Together with a large number of adakitic granites in the region,they reflect the extension background of the delamination of a thickened lower crust at ～230 – 223 Ma.The rhyolites in the Nagengkangqieer and Nagengkangqieer North were formed at 221.3 ± 1.1 Ma and 221.9 ±0.6 Ma respectively,which are similar to the characteristics of A-type granite and are sourced from the re-melting of the upper crust.The andesite and basaltic andesite in the Nagengkangqieer and Nagengkangqieer North areas were formed at 221.1～217.9 Ma,and originated from the decompression melting of the ancient mafic lower crust.Based on the genesis of coeval magmatic rocks,we believe that the study area was in a post-collisional extension environment exhibiting continuous lithosphere delamination,asthenosphere upwelling and continuous crustal thinning during Late Triassic(～222 – 201 Ma).We have studied three typical deposits in Dulan area.The main characteristics are as follows:(1)the host rocks of Harizha silver polymetallic deposit are mainly Early Devonian porphyry monzogranite,Late Triassic granodiorite and crystalline tuff in Elashan Formation,the ore body is controlled by tensile fracture.The Ag mineralization of the deposit can be divided into two stages: silver polymetallic mineralization and independent silver mineralization.The Pb-Zn-Ag ore bodies formed during the silver polymetallic metallogenic period are controlled by NW trending faults,strike NW,tend to SW.The wall rock alteration is mainly silicification,kaolinization,potassic and carbonatization,and locally developed chlorite,epidotization and sericitization,among which silicification and sericitization are closely related to Ag-Pb-Zn mineralization;The metal minerals are mainly arsenopyrite,pyrite,sphalerite,chalcopyrite,galena and crimson silver,and a small amount of pyrrhotite,marcasite,and(silver)tetrahedrite are developed;The ore-forming fluid has the characteristics of medium temperature,medium to low salinity and medium to low density.The ore-forming fluid is mainly sourced from mantle-derived fluid,which is added by atmospheric precipitation in the late stage.The ore-forming material is derived from deep source,and the ore-forming age is about ～223Ma.In the independent silver metallogenic period,the ore body strikes near the SN,dips to the south,and the dip angle is steep.The wall rock is altered into low-temperature silicification(chalcedony developed),and the metal minerals are mainly silver-bearing sulfide and natural silver.The ore-forming fluid has the characteristics of low temperature,low salinity and medium to low density.The source of ore-forming fluid is atmospheric precipitation.The ore-forming age is consistent with that of Nagenkangcheer and nagenkangcheer North silver deposits,at about ～217 Ma.We believe that the Harizha silver polymetallic deposit is a mesothermal hydrothermal vein-type Pb-Zn deposit and superimposes the later epithermal Ag mineralization.(2)The ore body of Nagenkangqier silver deposit is controlled by a near EW trending thrust nappe structure and its secondary fault systems.The host rocks are Paleoproterozoic Jinshuikou Group and Late Trassic Elashan formation.The wall rock alteration mainly includes silicification,sericitization,kaolinization,carbonatization,chloritization,epidotization,glaciation and fluorization,of which silicification is closely related to silver mineralization;The main metal minerals include pyrite,arsenopyrite,anhydrite and pyrolusite,then a small amount of pyrite,chalcopyrite,sphalerite,galena and marcasite;Silver bearing minerals include sliver,pyrargyrite,argentite,miargyrite,pyrostilpmite,and andorite,etc;The fluid is characterized by low temperature,low salinity and medium to low density.The ore-forming fluid is mainly meteoric water.The early mineralization was affected by mantle-derived fluid.The ore-forming material mainly sourced from the mantle.The ore-forming age is ca.217 Ma.The ore deposit was a low sulfide epithermal Ag deposit.(3)The ore body of Nagenkangcheer North silver deposit is controlled by the nearly SN trending fault structure and a series of secondary extensional fault structures;the host rock is the rhyolite of the Elashan formation.The wall rock alteration mainly includes kaolinization,sericitization,fluorization,carbonation and silicification,of which silicification is the most closely related to mineralization;The ore minerals are mainly pyrite,arsenopyrite,sphalerite and limonite,and the main silver bearing minerals are sliver,argentite,tetrahedrite,melanargyrit,pyrargyrite,etc;The ore-forming fluid shows the characteristics of low temperature,low salinity and low density.The ore-forming fluid is mainly affected by atmospheric precipitation.The ore-forming material sourced from the deep mantle.The ore-forming age determined by the zircon U-Pb age of ore-bearing hydrothermal quartz vein dated at 217.6 Ma.The ore deposit is an epithermal Ag deposit.The thrust nappe structure in the study area is an important ore controlling structure for independent silver mineralization.This structure should have regional contribution and is of great significance to future prospecting and exploration.The silver metallogenic mechanism of the eastern Kunlun orogenic belt is as follows:During late Hercynian to early Indosinian,the Bayan-Kara Ocean continued to subduct northward,and the subducted oceanic plate transported water,metal sulfur,large ion lithophile elements(LILE),halogen and other components into the upper mantle,forming an upper Ag-rich mantle source.It also provides a fluid basis for the formation of mantle-derived C-H-O fluid rich in Ag ore-forming materials,and the regional deep faults formed during this period provide a channel for fluid rising;During the late Indosinian,the Eastern Kunlun Orogenic Belt entered an intracontinental extension environment,the fluid in the mantle wedge was separated from the mantle in the subsequent post collision extension process,and then the mantle-sourced silver was migrated to the shallow crustt to form a mesothermal hydrothermal vein type deposits;With the continuous thinning of the lithosphere and the decrease of the pressure,when the fluid continued to migrate to the shallow,with the participation of atmospheric precipitation,later epithermal Ag deposits was finally formed.The East Kunlun region experienced a compression from the North Qiangtang block to the northern Kunlun block caused by the rapid expansion of the Middle-Tethys Ocean,forming a regional thrust nappe event.In the nagenkangqier region,it is shown as a nearly EW trending thrust nappe structure dipping to SW.The development of this structure led to a series of secondary tensile fault systems distributed in the near SN direction in the study area.The thrust nappe structure in the relaxation to extension stage and the SN trending tensile faults in provide excellent ore channel and hosting conditions for the formation of Late Triassic silver polymetallic deposits in this area.The main metallogenic period of silver in this area is the Late Indosinian(～223-217Ma),and silver polymetallic deposits are mainly distributed in the eastern part of the East Kunlun Orogenic Belt.Due to the regional uplift during the late Yanshanian to Himalayan period,the eastern and western segments of the East Kunlun Orogenic belt experienced differential uplift and denudation due to uneven uplifting.The metal deposits in the western part of the East Kunlun Orogenic Belt are relatively large in uplift,strong in denudation,and difficult to preserve in shallow depth.However,the eastern segment of this belt has relatively small uplift and denudation.Thus,the Late Triassic volcanic rocks and the epithermal silver deposits can be preserved.Therefore,the Dulan area in the eastern part of the East Kunlun Orogenic Belt has the metallogenic potential and preservation conditions for silver polymetallic mineralization,and there is in great potential and prospect for silver exploration in the future.The summary of regional metallogenic regularity and the establishment of regional metallogenic model are expected to provide effective theory for regional metallogenic prospecting.