Indosinian Magmatic Evolution And Copper Polymetallic Mineralization In The Erashan Area,Qinghai Province

As the junction of three orogenic belt-Qinling,Qilian,and Kunlun Mountains,Elashan area is located in the easternmost part of the east Kunlun orogenic belt,next to the west of Qinling orogenic belt,and immediately contiguous to the south of Qilian orogenic belt from its north.The widely exposed Indosinian magma in the area is strictly controlled by north-north-west large scale strike-slip faults(Wahongshan—Wenquan Fault Zone).Based on material collection and field geological study,this study uses petrography,lithogeochemistry,geochemistry of mineral deposits and isotope geochemistry and other ways as study method to analyze the relationship between evolutionary process of Indosinian tectonic-magma in metallogenic belt of Elashan-Shiduolong-Saishitang area,and copper polymetallic ore,summarizes the laws of regional mineralization and conducts potential evaluation.The feature of the magmatic rock distribution in Elashan area appears on diminishing scale from north to the south – the northern area from Daheba is the most widespread,Shiduolong-Elashan area is the second scale,and Tongyugou-Saishitang area is the least.The northern area from Elashan belongs to East Kunlun Mountain Unit,and the rock mass spread as north-north-west.The southern area from Elashan is Kuhai-Saishitang ophiolitic mélange area,and most rock mass are single sporadic rock mass.The main rock types are granodiorite and quartz diorite,followed by diorite and moyite.Volcanic is exposed widely,which is mainly intermediate-acid Elashan continental volcanic rocks.Through dating of zircon U-Pb,age of a batch of intrusive rock and volcanic rock(246Ma-216Ma)has been confirmed.After studying the collection of researching achievements from other scholar on Elashan,the results show that the time span of Indosinian magma is large(252-215Ma),the age span is about 37 Ma,in which the peak age focused on 243 Ma and 224 Ma,and middle Triassic epoch and the early phase of late Triassic epoch(-230Ma)saw relatively subdued magmation.The age feature of the intrusive rock appears on diminishing age from north to the south,while the volcanic rock appears on the contrary.Shiduolong granodiorite(242.6±1.9 Ma)is CNK middle-high-k calc-alkaline granodiorite,formed by part of lower crust melting during Mesoproterozoic,and mixed with mantle compositions at the same time,showing the feature of are magma.The volcanic rock of Elashan pass(246-242Ma)is mainly mixed with andesitic and rhyolitic continental pyroclastic rock,which belongs to aluminum to prealuminum high-k calc-alkaline rock,formed in an environment of volcanic arc collision – under partly extensional tectonic and decompression melting of mafic rock in lower crust.Solagou moyite(233.0 ±1.2 Ma)is an NKC high-k calc-alkaline high-differentiation type I granite,which is formed by the upwelling of the asbestos in the post-collision extension environment.The Huda complex massif(229~224Ma)is composed of diorite and quartz diorite containing dark inclusions.The inclusion body is a homogenous heap crystal formed by pressure unloading quenching;diorite and quartz diorite are formed by the new lower crust melting of the East Kunlun orogenic belt,and then followed by different lithologies formed by crystallization differentiation.Boheqin granitic diorite(219 Ma)is an Adakite with high La/Yb and Sr/Y ratios.The rock masses in the Huda and Boheqin area are closely related to the demolition of the lower crust.In the Indosinian period,the Elashan area experienced three stages: the northward dip-collision of Animaqing ocean transition stage(243~237 Ma),the same collision(237~230 Ma),and the post-collision extension(230~215 Ma).It is consistent with the tectonic evolution of the Indosinian period of the Central Orogenic System.The formation of magmatic rocks in the early Indosinian(about 243 Ma)is related to the background of the dip-collision transition stage(such as the Shiduolong and Elashan volcanic rocks)and the late Indosinian(about 224 Ma)granite(represented by Huda rock mass)is formed in a geodynamic background by the demolition of the lithosphere after the crustal thickening of the central orogenic belt.The main deposits(points)in the Elashan area include the Shiduolong lead-zinc deposit,the Solagou copper-lead-zinc polymetallic deposit,the Elashan copper polymetallic deposit,the Saishang copper deposit,the Tongyugou copper deposit,and the Rilonggou tin polymetallic deposits,etc.The deposits can be roughly divided into two categories.One type is the shallow magmatic hydrothermal copper-lead-zinc mines such as the Shiduolong,Saishtang and Elashan mining areas.There may be porphyry mineralization in the deep,and the other type is a sedimentary-metamorphic copper polymetallic ore produced in sandstone,siltstone,labile sandstone and skarn(Solagou,Tongyuogou mining area).The timeline age of the Eilashankou copper polymetallic ore blende Rb-Sr is 246.6±2.6 Ma,the timeline age of pyrite Re-Os is 239.9 ± 4.9 Ma,and the mean age is 240.5±3.3 Ma.The results from these two methods are consistent with the age of the ore-bearing rhyolite porphyry(243.3±1.7 Ma)in the error range,the age of molybdenite Re-Os in the Tongyuogou mining area is 213.5±2.7 Ma.After analyzing the ore-forming age statistics of adjacent mining areas,it can be found that large-scale hydrothermal polymetallic ore-forming events occurred in the early and late periods(~238 Ma,~225 Ma)of the Indosinian area,which can be compared with the eastern section of Qimantage and east Kunlun.Indosinian magmatism provides an important material source,heat source and power for ore-forming in the area.Mainly a group of hypabyssal rock or subvolcanic rock are directly related to mineralization,such as rhyolite porphyry,granite porphyry and quartz diorite porphyrite,which formed a series of hypabyssal magmatic hydrothermal copper polymetallic deposits and formed porphyry mineralization in the deep.Sedimentary-metamorphic copper polymetallic ore deposits are formed during the intermittent and post-collisional extension phases of magma activity.The source of C in the ore-forming fluids of the Elashan metallogenic belt should be closely related to the magmatism.The low-temperature alteration plays an important role in the mineralization of lead and zinc.The H-O isotope shows in the early stage of mineralization the magmatic water played important part,and in the later stage atmospheric precipitation were added.In the Tongyugou mining area metamorphic water were added.The composition of sulfur isotope is more complicated.The sulfur in the north mining area of the Elashan is mainly magmatic sulfur.The Tongyugou-Saishitang ore field in the south is dominated by magma sulfur,while the west is mainly sedimentary sulfur,mixed with a small amount of metamorphic sulfur.The Pb isotope means that the formation of the deposit is closely related to the orogenic environment,and the ore-forming material may be derived from the subduction Pb of the upper crust and mantle(related to magmatism).The Re content in molybdenite shows that the ore-forming materials in the early Indosinian period were crust-mantle mixing sources,and the materials in the late-stage were mainly crust-source.After a comprehensive analysis of eleven 1:50,000 geophysico/gchemical prospect data,a total of 13 latticed fault structures in the northwest and northeast directions can be inferred,and a total of 24 high magnetic bodies can be inferred,most of which were surface or deep concealed rocks.There are 35 circled geochemical exploration comprehensive anomaly,and three NW-NNW anomaly zone arranged in a beaded.The main ore-forming elements have a distinct zoning pattern in the NEE direction,and have a regular change of Cu polymetallic and Au polymetallic from southwest to northeast.Combining the above results with the actual work in the field,four ore prospective areas can be circled,including Jiamuger south.