Contribution Of Crust-Mantle Magma Mixing To The Mineralization Of Porphyry Deposits In A Post-Collisional Setting

The Sanjiang orogen is located on the southeastern part of the Tibetan plateau and belongs to the eastern Indo–Asian collision zone.The Sanjiang orogen is one of the most productive and potential regions for metal resources in China and the therein Jinshajiang–Red River metallogenic belt,which lies across the Qiangtang terrane and western Yangtze craton,has exerted primary control on the localisation of Cenozoic magmatism and porphyry–skarn polymetallic deposits formation in western Yunnan.The Machangqing Cu–Mo–?Au?deposit in the central of Jinshajiang–Red River fault belt is one of the representative Cenozoic porphyry Cu and other polymetallic deposits in this belt.Igneous rocks at Machangqing mainly consist of mafic rocks?lamprophyre dykes?and granitic rocks?barren syenite porphyry and ore-bearing monzonite porphyry,granite porphyry and porphyritic granite?.The ore–bearing granitic rocks contain large number of mafic microgranular enclaves?MME?.In this paper,based on the existing research,combined with modern metallogenic theories and using modern analysis testing techniques and methods,this study focuses on the analysis of crust-mantle magma mixing and reveals the mineralization mechanism of the porphyry deposits in western Yunnan in the post-collisional setting.The main achievements are as follows:?1?Macroscopic petrology studies show that the boundary between the MME and the host granitic rock is clear,and there are no condensation and baking edges on both sides of the boundary.The morphology of the MME is characterized by spherical,ellipsoidal or plastic irregular beads,indicating that the diagenesis of the MME originates from the lava enclaves;From the analysis of rock structure,the MME have a fine-grained structure,and the host rocks have a medium-coarse-grained structure,suggesting that the lava enclaves and the host acidic magma constitute a non-equilibrium immiscible mixed magma in the process of consolidation and diagenesis.That is,because the temperature of the basic magma is higher than the temperature of the acidic magma,it is determined that in the same consolidation system,the condensation crystallization rate of the basic lava enclaves is greater than that of the host acidic magma.At the same time,the mineral composition of MME presents a non-standard transitional feature with a basic to neutral composition,resulting in a vague naming of its rock type.?2?Chronology of diagenesis and metallogenesis studies,LA–ICP–MS zircon U–Pb dating and molybdenite Re–Os dating results show that:the time limit for the magmatic activity of the Machangqing ore-bearing magmas is 33.78–35.92Ma,which is basically consistent with the MME diagenetic age and the molybdenite Re–Os model age?34.94±0.38 Ma?.The magmatic diagenesis-mineralization events of the late Paleogene Eocene in the Machangqing area were clearly recorded.Based on the existing data analysis,it is believed that the magmatic evolution sequence of the ore-bearing intrusions of the Machangqing deposit is composed of syenite porphyry?monzonite porphyry?granite porphyry?porphyritic granite.The lamprophyre dykes are formed at the same time or later than the granitic porphyry intrusions and the related metallogenesis is the product of the second peak stage of magmatic activity in the Jinshajiang–Red River fault belt,and it is also an important record of large-scale metallogenic events of Cenozoic magma in western Yunnan in Sanjiang area.?3?In terms of rock geochemistry,Machangqing granitic rocks and mafic rocks belong to the high–K calc–alkaline and shoshonite series and the peraluminous and metaluminous series,all of which have obvious shoshonitic affinity,such as,K₂O+Na₂O>6 wt.%,K₂O/Na₂O>1 and Sr/Y>40;they are enriched in LILE?Rb,Ba,Th,etc.?and LREE,but depleted in HFSE?Nb,Ta,Ti,etc.?.Moreover,both the ore-bearing and barren granitic porphyries of Machangqing have higher Si O₂content?64.67–72.81 wt.%?and Al₂O₃ content?13.43–16.10 wt.%?;their whole rock?Nd?t?value?–6.5––3.3?and(⁸⁷Sr/⁸⁶Sr)_i values?0.7061–0.70776?are basically consistent with the Sr–Nd isotope characteristics of contemporaneous basic rocks in western Yunnan;their zircon Lu–Hf isotope data show the relatively concentrated values of?Hf?t?range from-0.75 to+2.33,mainly positive values,and the corresponding crustal model age(T_DMC)is between 0.9Ga and 1.1Ga,which is corresponding to the formation age of arc magmatic basement in the Meso-Neoproterozoic period.However,the Cr content?18.40 ppm?and Ni content?12.70 ppm?of barren syenite porphyry are significantly lower than the ore-bearing granitic porphyries?Cr:average 44.33 ppm;Ni:average 28.67 ppm?,and the latter also show a wider range of Mg^#values?11–66?.In contrast,lamprophyre has low Si O₂ content?46.50–52.30 wt.%?,high K₂O content?2.63–6.13wt%,average 4.76wt%?,high K₂O\Na₂O ratio?1.05-4.98,average 2.98?,and high Mg O Content?9.84–12.74 wt%?,significantly higher Mg^#value?71–76?and Cr content?average 492ppm?and Ni content?average 195 ppm?,with broad and lower whole rock?Nd?t?values?–7.8––1.8?and the relatively low(⁸⁷Sr/⁸⁶Sr)_i?0.7064–0.7074?values,which show obvious characteristics of super-potassic mantle source;The geochemical characteristics of the MME contained in ore-bearing granitic porphyries are between granitic rocks and lamprophyre rocks,and this transitional geochemical characteristics correspond to the non-standard lithofacies.?4?The zircons of the granitic rocks in Machangqing show obvious HREE enrichment,significant positive Ce anomalies and slight negative Eu anomalies.The total amount of rare earth elements??REE?is between 482-2528 ppm,the zircons crystallization temperature range from 517?to 796?,and these characteristics are same as those of magma zircon;The zircons data points of the Machangqing ore-bearing and barren granitic porphyries are between the FMQ?fayalite-magnetite-quartz?and MH?magnetite-hematite?oxygen buffer lines,indicating that the parent magmas of both are relatively oxidized magmas;compared with the barren syenite porphyry,the zircon Ce⁴⁺/Ce³⁺ratio?average 486?of ore-bearing granitic porphyry is significantly higher,while the Eu/Eu*value?average value 0.55?is significantly lower,indicating that the ore-bearing magmas in Machangqing are more oxidative than the barren magmas.The high oxidation characteristics of ore-bearing magmas are of great significance for the later enrichment and mineralization of copper-gold and other metal elements.?5?According to geochronological,rock geochemistry and isotopic evidence,the granitic rocks of Machangqing have the genetic characteristics of I-type granites,which may have originated from the partial melting of Meso-Neoproterozoic arc magmatic base rocks containing hornblende-eclogite facies.The MME contained in the ore-bearing granitic porphyries are immiscible residual basic lava enclaves formed by the main felsic magma derived from the partial melting of the Meso-Neoproterozoic basement crust rock partially homogenized the mantle magmas?lamprophyre magmas?.The MME are the products of relatively rapid condensation and crystallization and consolidation and diagenesis;Such crust-mantle magma mixing is common in the evolution of the porphyry system in a post-collision setting;Granite rocks contain MME,which is an important sign that the acidic magma is partly homogeneous and immiscible with mantle source magma.?6?The trace elements and Pb?S isotope characteristics of ore sulfides indicate that the ore-forming materials?Cu,Mo,Au,and S?of the Machangqing deposit are closely related to the mafic melt?lamprophyre magma?.The petrological model analysis shows that the Machangqing barren and ore-bearing granitic porphyries magmas are all evolved from the mixture of the partial melting lower crustal materials and lamprophyre magma in the same period,but during the evolution process,the two mixed into different proportions of lamprophyre magmas and lead to the difference of mineralization in the later period.The ore-bearing magmas are relatively mixed into more lamprophyre magmas,and the non-equilibrium?partially?homogeneous promoted by the crust-mantle magma mixing process,which may provide additional H₂O,metal and sulfur sources and other important ore-forming materials to the porphyry system.Meanwhile,it also enhances the oxygen fugacity of the mixed magma,thereby initiating and promoting the production of ore-rich magmas.Therefore,strong crust-mantle magma mixing may be one of the key factors restricting the mineralization of the porphyry system under the post-collision setting.?7?Comprehensive researches suggest that,under the post-collision setting,large-scale lithospheric delamination is the deep geodynamic mechanism that causes regional magmatic activities and mineralization.The deep water-rich mafic ultrapotassic magma may reactivate metal and sulfur elements in the residual sulfide formed by the former arc magmatism to migrate and flow.During the ascending process of mafic magmas,a large amount of volatiles and deep ore-forming materials carried by it were injected into the ascending felsic magma chamber then mixed with it and partly homogeneous.On the western margin of the Yangtze Craton,the process of crust-mantle magma mixing,increasing the metal content and metallogenic potential of the mixed magma,is one of the important mechanisms for the formation of collisional porphyry Cu?Mo-Au?deposits in the Sanjiang tectonic belt.