Petrogenesis Of Mafic Microgranular Enclaves Hosted In Cretaceous Granitic Intrusions And Implication For The Large-scale Gold Mineralization In The Jiaodong Gold Province,North China

The Jiaodong gold province,located on the southeast edge of the North China Craton（NCC）,is the largest gold accumulation area in China.For a long time,the annual production of gold in this area accounts for 1/4 of the total output of the country.Since the 1980s,many studies have been carried out to better understand the geneses of the gold deposits within this province.The gold mineralization was attributed to post magmatic mesothermal vein gold deposit in the 1980s,ductile-brittle shear zone gold deposit in the1990s,granite-greenstone belt gold deposit/orogenic gold deposit in more than ten years in the early 21st century,and craton destructive gold deposit since 2015.Recently,the genetic type of the gold mineralization within Jiaodong:orogenic gold type or craton destructive type,is still one of the hot issues.Among them,the migration of mantle-derived ore-forming materials to the shallow crust is a focus of debate.According to the genetic model of orogenic gold deposit,mantle-derived ore-forming metals combined with mantle fluids passed the crust through the deep and large fault zones（e.g.,Tanlu deep fault）,cutting through the lithosphere,and then entered into the shallow second and third-order faults to precipitate the gold mineralization.The genetic model of craton destructive gold deposit holds that mantle-derived ore-forming materials were brought into the crust through the emplacement of mafic magma sourced from partial melting of the metasomatic mantle,and then the magma differentiated and crystallized.After the magmatic event,the hydrothermal fluids transferred the ore-forming materials into shallow faults,dominated by magmatic-hydrothermal fluids in the syn-gold mineralization stage.The gold was precipitated through fluid mixing,water-rock interaction,and/or boiling.Therefore,the migration channels of the ore-forming materials from the lithospheric mantle to the upper crust is critical for the genesis of gold deposits in this area.The Jiaodong peninsula was characterized by intense Early Cretaceous magmatic activity and crust-mantle mixing.Mafic microgranular enclaves（MME）are common in granitic plutons and may provide the possibility to trace the properties of mantle-derived magma and the deep magmatism processes.This thesis took the granitic plutons and their MMEs that were emplaced before（Congjia pluton and its MME of Guojialing super unit）,during（Gushan pluton and its MME）,and after（Yashan pluton and its MME）the large-scale gold mineralization as the research object,and investigated the properties of mantle-derived magma before,during,and after the large-scale mineralization in this area.Through systematic petrological,isotopic geochemistry,and geochronological studies,the properties of mantle-derived magma before,during,and after the large-scale mineralization in this area were investigated,and the processes of deep magmatism and its relationship with large-scale gold mineralization were discussed.The main results and conclusions of this thesis are as follows:1.The rock types and petrochemical characteristics indicate that the pre-ore Congjia pluton mainly consists of granodiorite,and the hosted mafic microgranular enclaves（CJ-MMEs）are mainly diorite.The mafic magma forming CJ-MMEs has low temperature and water and volatile contents compared to GS-and YS-MME.The syn-gold mineralization Gushan pluton is mainly granite,and the hosted mafic microgranular enclaves（GS-MMEs）in the rock body are diorite monzonitic.The mafic magma forming GS-MMEs is potassic with high temperature,water,and volatile contents.The post-mineralization Yashan pluton is mainly composed of monzogranite and granodiorite.The hosted mafic microgranular enclaves（YS-MMEs）are diorite.The mafic magma forming YS-MMEs has high temperature but low water and volatile contents.2.The zircon U-Pb ages of the pre-gold mineralization Congjia granodiorite and CJ-MMEs are 127 Ma and 127.4±1.2 Ma,respectively.The syn-gold mineralization Gushan granite yields zircon U-Pb ages of 118.1±0.5 Ma.The hosted GS-MMEs and YS-MMEs yield two types of zircons.GS-MME Type I yields zircon U-Pb ages 119.8±0.8 Ma,similar to type II 120.7±1.0 Ma.The zircon U-Pb age of post-gold mineralization Yanshan granite is 116.6±0.4 Ma.Zircon U-Pb age of YS MMEs type I and type II are116.2±0.4 Ma and 117.5±0.8 Ma,respectively.In general,the ages of each pluton and its MMEs are consistent,indicating that the granitoid plutons and their MMEs were formed concurrently.3.Geochronology and Nd-Sr-Hf isotopes show that Congjia granodiorite formed by partial melting of thickened continental lower crust at≤40 km,Gushan and Yashan plutons formed by partial melting of thinned continental crust at a depth of about 20 km.CJ-MMEs were generated by an intense mixing of mantle-derived magma and mafic lower crust-derived magma in a deep hot zone of the crust-mantle transition at≤40 km.The late mafic enclaves（GS-and YS-MMEs）originated from the partial melting of the lithospheric mantle with little crustal input.4.The results of geochronological studies of quartz-molybdenite veins and Gushan granite and its MMEs,gold mineralized Xiejia diorite,and some large gold deposit samples such as Jinchiling,Taishang,Linglong gold deposits…are consistent,indicating that the hydrothermal event,gold mineralization,and emplacement of Weideshan intrusions（i.e.,Gushan granite and Xiejia diorite）are the result of a single geological event rather than multiple events.Obviously,the age of 120±3 Ma is the most important time for the occurrence of the Weideshan intrusions,the magmatic-hydrothermal event,and the large-scale gold mineralization.5.The results of zircon U-Pb and molybdenite Re-Os dating show that the magmatic-hydrothermal event in the potassic Gushan granite occurred at ca.120 Ma at the same time as the gold mineralization in Jiaodong Peninsula.GS-MME pyrites show a wide range of Co/Ni（0.004-33 ppm;average=7.08 ppm）ratios,with most of the values>1and high Co and Ni concentrations andδ³⁴S values ranging from 4.14‰to 6.73‰,suggesting that pyrite in GS-MME is of magmatic origin and precipitated during or shortly after magma mixing.LA-ICP-MS analysis shows that the metal distribution and abundance of elements（i.e.,Co,Cu,Ni,Zn,As,Ag,Au,Pb,Bi）in GS-MME pyrites are identical to those of Xiejia diorite.Similar pyrite trace element distribution and sulfur isotope indicate a common source between GS-MME and the Xiejia diorite.These observations claim that the mafic-intermediate magma formed by partial melting of the metasomatized lithospheric mantle around 120±3 Ma is the carrier of Au and other metallogenic components from the deep mantle to the shallow crust.6.The high La/Y ratios of the pre-ore Congjia pluton compared to the lower La/Y ratios of the plutons from the Late-stage magmas indicate that the early magmatic events are related to crustal growth,and the later magmatic events are related to rapid lithospheric thinning,which is caused by the gradual rollback of the ancient Pacific plate subducted beneath the NCC.The metasomatism fluids and ore-forming materials could have been concentrated and formed a metasomatized layer in the lithosphere.The delamination of the lithospheric mantle prompted rapid decompression melting and quick upwelling of the asthenospheric mantle.The rapid thinning of the lithosphere（<100 km）lowered the solidus of the metasomatized layer,which underwent decompression melting.The injection of pulses of high-temperature and water-rich mafic magma into the cool felsic magma chambers could have prompted a reaction of devolatilization of the enriched mafic magma,which released a tremendous quantity of volatiles.The cool felsic magma from the host rock played the role of stopper that favored the liberation of volatiles and ore materials through favorable structures where the gold precipitated.7.The characteristics of the syn-gold mineralization Gushan mafic microgranular enclaves have important implications for the large-scale gold mineralization:（1）Areas with strong magmatic activity around 120 Ma,including granites with MME formed by crustal and mantle magma mixing and widely distributed mafic dikes,are favorable areas for future gold exploration.（2）Areas with deep-seated faulting with first or second-order faults are probable locales for gold precipitation.（3）It is recommended to use 2000～4000m deep drilling and some applicable geophysical methods in the Jiaodong gold district as more conceal gold mineralized mafic intrusives may be discovered.（4）Areas with extensive arc-like mafic rocks that occur accompanied by preferable faults might be noticeable for deep gold exploration.