Metallogenic Conditions Of Eclogite-type Rutile Deposit From The North Qaidam UHP Metamorphic Belt And Their Implications For Mineral Exploration

A series of super-large eclogite-type rutile deposits have been discovered from the Northern Qaidam ultra-high pressure metamorphic belt in Qinghai Province.The metallogenic conditions,the genesis of these deposits,and the mechanism of the effect of melts/fluids on the metallogenesis of these deposits are still controversial.On the basis of detailed field geological survey of Yuka and Dulan eclogite-type rutile deposits in Northern Qaidam UHP metamorphic belt,detailed geochemical and geochronological studies of quartz veins and feldspathic veins in various types of eclogites were carried out to determine the protolith age,genesis,and tectonic setting.Combined with the previous studies on the P-T path of eclogite,the origin of eclogite-type rutile deposit and its later metamorphic process are further discussed.The main achievements and understandings of this paper are as follows:（1）Part of the rutile-bearing eclogite protoliths in Yuka-Laofengpo area originated from Paleozoic oceanic crust slice.Zircon U-Pb ages and Hf isotopes of rutile-bearing eclogites from Yuka-Luofengpo area were studied to explore their protolith setting.According to zircon morphology and CL image,it can be divided into two categories:the first category has a black core and gray rim,and the second category has a gray core and bright rim.The chondrite-normalized pattern of the black core is steep and the Th/U ratio is high,which indicates that they are the magmatic source.LA-ICP-MS dating of fresh eclogite yields the lower intercept age of⁵51-529 Ma,which is consistent with the age of the formation of regional oceanic crust in the Paleozoic and can represent the age of the formation of protoliths.The second type grey zircon core has a flat REE curve and a low Th/U ratio,ranging from 440 to 432 Ma,which is consistent with the age of eclogite-facies metamorphism in the North Qaidam UHP metamorphic belt,while the bright rim has a relatively steep REE curve relative to the grey core and yield a lower intercept age of⁴19 Ma,which may represent the age of retrograde metamorphism.Theε_Hf（t）value of the first type of magmatic zircon core is-0.8-+10.9,which indicates that it was replaced by crustal fluid or continental crustal material in the magmatic source.Therefore,we consider that some of the protoliths of rutile-bearing eclogites in Yuka area are Paleozoic oceanic crust mafic rocks,which are located on the active continental margin during the Paleozoic oceanic formation-subduction process.Deep subduction took place along with the Qaidam plate and underwent ultra-high pressure metamorphism to form eclogite-type rutile deposits.（2）The rutile-bearing eclogite protoliths in Dulan area have Paleozoic oceanic crust remnants and Neoproterozoic continental basalts.This study investigates ore and mineralization bodies belonging to the two types by way of detailed field work and geochronological,geochemical and Sr-Nd-Hf isotopic compositional analysis,taking the Dulan eclogite-type rutile deposit as its case study.Type-1 rutile-bearing eclogites enclosed in granitic gneisses are found to have Neoproterozoic protolith ages of 845±4Ma and an eclogite-facies metamorphic age of ca.433 Ma.Their high TiO₂,TFe₂O₃,and P2O5 contents cannot have been produced by rutile-formation during UHP metamorphism but could have arisen through the accumulation of ilmenite and apatite in high-Ti magmatism.They have ocean-island basalt（OIB）-like trace element patterns,with enrichment in light rare-earth elements（LREEs）and high field strength elements（HFSEs）（Nb,Ta,Zr,and Hf）.The Hf isotopic compositions of the residual magmatic zircon cores in Type-1 rutile-bearing eclogites(ε_Hf（845 Ma）=+1.4–+6.0,T_DM1=1.2–1.4 Ga,T_DM2=1.3–1.6 Ga),indicate that their parental magmas were derived from the Proterozoic depleted mantle.However,they show initial ⁸⁷Sr/⁸⁶Sr values of 0.71059–0.71326 andε_Nd（845）values of-1.5 to-2.5,suggesting significant crustal contamination.Thus,the Type-1rutile-bearing eclogites may have originated through partial melting of an OIB-type mantle plume source during the Neoproterozoic supercontinent Rodinia rift event.In contrast,the Type-2 rutile-bearing eclogites,which are hosted in a mafic-ultramafic ophiolitic complex,have protolith ages of 449±3 Ma and eclogite-facies metamorphic ages of ca.435–433 Ma.Correlations between whole rock MgO and major and trace elements suggest they are derived from cumulates with alternating olivine and pyroxene-rich and ilmenite-rich layers.They have mid-ocean-ridge basalt（MORB）and OIB-like trace element patterns,indicating extensive fractional crystallization and the possibility that there were multiple injections of magma during the formation of the protolith.Meanwhile,the positiveε_Nd（449 Ma）andε_Hf（449 Ma）values of the Type-2 rutile-bearing eclogites indicate that the protoliths may have been generated by partial melting of a depleted mantle source metasomatized by slab-derived components.The short interval（ca.14 m.y）between magma generation and eclogite-facies metamorphism,in combination with the type of rock assemblage within which they are found,indicates that the Type-2 rutile-bearing eclogites originated in a supra-subduction zone（SSZ）ophiolite.Type-1 and Type-2 ores have similar eclogite-facies metamorphic ages of ca.435–433Ma,indicating they were metamorphosed during the same metamorphic event.Considering the tectonic evolution of the region,the two stages of magmatism revealed by these rutile-bearing eclogites may record the full Wilson cycle of continental rifting,ocean floor spreading,oceanic subduction,and continental-arc collision from Neoproterozoic to Paleozoic.The predominantly continental materials and Type-1protolith in the lower plate and the SSZ-related Type-2 protolith from the upper plate were dragged into the subduction channel to undergo eclogite-facies metamorphism via subduction of the Qaidam Block,eventually forming eclogite-type rutile deposits.（3）The formation mechanism of quartz veins enclosed in eclogite in Yuka-Luofeng area is discussed.High-pressure（HP）or ultrahigh-pressure（UHP）rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,we investigated the major and trace elements of Ti-rich minerals.Additionally,U–Pb dating,trace element contents,and Lu-Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt,Yuka terrane.The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings,and show low Th/U ratios,steep chondrite-normalized heavy rare earth elements（HREEs）patterns,and insignificant negative Eu anomalies,indicating their growth in metamorphic fluids.These zircon grains formed in 431±3 Ma,which is consistent with the 432±2 Ma age of the host eclogite.As for the zircons in rutile-quartz veins,they showed steep HREE patterns on one hand,and were different from the zircons present in the host eclogite on the other.This demonstrates that their formations might have been related to the breakdown of the early stage of garnet,which corresponds to the fluid activities during the early exhumation stage.The Zr-in-rutile temperature of host eclogite in Yuka quartz vein is 609-669°C,and the temperature obtained by Zr-in-rutile thermometer in quartz vein is 664-682°C,which represents the crystallization temperature of rutile in early exhumation stage.The Zr-in-titanite temperature of the edge of titanite is 550-592°C,which which is basically consistent with the amphibolite-facies temperature.The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain;the rutile core has higher Nb,Ta,W,and Zr contents and lower Nb/Ta ratios than the rim,indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim.The significant enrichment of high field strength elements（HFSEs）in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs.Furthermore,variations in Nb versus Cr in rutile indicate a connection of rutile to mafic protolith.The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions,indicating that the vein-forming fluids are internally derived from the host eclogite.These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.Finally,this reveals a significant fluid source to interact with the overlying mantle wedge and triggers crust-mantle interaction.（4）The formation mechanism of feldspathic veins enclosed by eclogite in Yuka-Luofeng area is discussed.Zircon U-Pb dating,trace element content and Lu-Hf isotope and rutile geochemical characteristics are used to discuss the formation of rutile-bearing feldspathic veins in Luofengpo area.CL analysis of in-situ zircons in feldspathic veins shows obvious core-rim structure.The core is black,the rim is bright,and the weighted average age of black core is⁴36 Ma,which is consistent with the metamorphic age of eclogite-facies（440-430 Ma）obtained in Yuka-Luofengpo area.This indicates that feldspathic melts crystallized during the metamorphic stage of the eclogite-facies stage in Luofengpo area.The weighted average age of the bright zircon rim is 413-412 Ma,representing the lower time limit for the melt to be modified.Combined with the characteristics of Hf isotopes of melts,the values ofε_Hf（t）in the black core are between-10.79 and-9.85,and the values ofε_Hf（t）in the white rim are between-8.66 and-2.52,representing two different stages of melt activities.The initial Hf isotope ratios of zircons in felsic veins are between the host granitic gneiss and eclogites.These data are consistent with zircon growth from a mixed melts/fluids generated internally within the gneisses and the eclogite.U-Pb dating of rutile in feldspathic veins yields an age of⁴40 Ma,which is consistent with the age of eclogite-facies metamorphism.It may represent the crystallization age of rutile.Some rutile grows in multiple stages.Rutile edges are enriched with Nb,V,W,Sb,Zr,Nb/Ta,and Zr/Hf ratios,indicating that the temperature at the edge is higher.Therefore,the core may be formed in the stage of subduction to the peak stage of ultra-high pressure metamorphism.The rim is formed in the early exhumation stage.The Zr-in-rutile temperature of rutile in Luofengpo eclogite is 635-675°C,and the Zr-in-rutile temperature of rutile core in feldspathic vein is 632-647°C,which is consistent with that of host eclogite.Before the peak stage of eclogite facies,the crystallization temperature of rutile rim in feldspathic vein is 657-670°C,which represents the crystallization temperature of rutile in feldspathic vein and forms in the early exhumation stage.The Zr thermometer of the titanite rim is 553-604°C,which is basically consistent with the amphibolite-facies temperature.（5）According to metallographic study,there are five main occurrence states of rutile in eclogite.The rutiles occur as mineral inclusions,intergranular structure in garnet and omphacite,mineral aggregate intergrowth with quartz,ilmenite or titanite in rutile rim as residual structure and silk thread-shaped exsolution structure in fluid and the ore grades are better when rutile occur as the intergranular and mineral aggregate structure.There are ilmenite and titanite at the edge of rutile.The mineral formation sequence is rutile-ilmenite-titanite.The formation of ilmenite may be related to the heating of the exhumation stage.According to the rutile occurrence in fluids/melts,it can be concluded that the formation and preservation of rutile will be affected by the activity of fluids/melts.The early（before the early exhumation stage）activity of fluids/melts is beneficial to the mineralization of rutile.The small-scale activity of fluids/melts between mineral grains in eclogite can promote the further enrichment of rutile into a series of beaded rutile and increase the size of ore grains.Late melts/fluids activity（amphibolite-facies stage）resulted in rutile retrograde to ilmenite and titanite.The stronger the activity of melts/fluids,the more unfavorable to the preservation of rutile deposits.（6）The ore-controlling factors,metallogenic regularity and prospecting criteria of eclogite-type rutile deposits are summarized.Rutile deposits are controlled by factors such as the content of TiO₂ in the protolith and the size and shape of rutile,which affect the preservation of such deposit;（1）the Ti-rich mafic protoliths from fertile mantle source,with high degree of evolution and low contamination of the continental crust are the essential condition for formation of eclogite-type rutile deposits.（2）The high-pressure（ultra-high pressure）metamorphism promotes the formation of rutile.（3）The influence of melts/fluid activity on rutile mineralization is different during the early exhumation and late exhumation stages;（4）Exhumation rate of UHP metamorphic terrane,late brittle-ductile shear deformation,magmatic activities will influence rutile mineralization.The Yuka and Dulan eclogite-type rutile deposits are similar in the morphology of orebodies,ore fabric and occurrence state of rutile.The difference is that the Dulan eclogite-type rutile deposit is mainly oceanic eclogite,which is thicker but lower TiO₂ content than continental eclogite.The distribution of eclogite is inhomogeneous and its shape is mainly influenced by the thickness of original volcanic rocks,fold structure before or during subduction,ductile shear and superimposed transformation of brittle-ductile deformation during late exhumation.The grade change has stratabound characteristics.Searching for fresh eclogites with high titanium content in ultrahigh metamorphic belts is a typical prospecting indicator for this type of deposit.（7）According to the characteristics and principles of eclogite-type rutile mineralization,two rutile A-type metallogenic prospecting areas,one rutile B-type metallogenic prospecting area and one rutile C-type metallogenic prospecting area are divided in the North Qaidam UHP metamorphic belt:Yuka-Kaipinggou rutile metallogenic prospecting area（A1）,the North Dulan eclogite-type rutile prospecting area（A2）,and South Dulan-Shaliuhe eclogite-type rutile prospecting area（C1）,and Xitieshan-Quanjihe eclogite-type rutile prospecting area（B1）.