Geochemstry And Genesis Of The Xiadian Gold Deposit In Zhaoyuan,Jiaodong

Jiaodong Peninsula,located in the eastern margin of the North China Craton,is an important gold mining district in China.And it belongs to the circum-Pacific metallogenic domain.The deposits mainly have been classified as quartz-vein type(Linglong-type)and fracture-belt alterad rock type(Jiaojia-type)gold deposits.A great deal of research work had been done concerning the genesis of the Jiaodong gold deposit,and the metallogenic theory has been deepened continuously.The genetic type of Jiaodong gold deposits evolved from the earlier post-magmatic hydrothermal type and the greenstone type to the orogenic type and the inner continental non-orogenic type,until the nearest Craton destruction type.Inspite of some researches taken,there is no consensus on the genesis of gold deposits,especially the sources of gold ore-forming fluids and ore-forming materials.There is still a great debate about whether the ore-forming fluid of gold deposit is dominated by crust-derived magmatic water or mantle-derived magmatic water or mantle fluid The main view is that Jiaodong gold deposits have multi-source of ore-forming materials.Gold comes from the surrounding granite,metamorphic rock and mantle-derived basic rock veins,but which is the dominant one is still uncertain.The Xiadian gold deposit is a large Jiaojia-type gold deposit in Jiaodong district.Its gold resource reserves are more than 200t and the grade of gold ranges from 5?8g/t Compared with gold deposits such as Linglong and Jiaojia,the research degree of Xiadian gold deposit is relatively weak.Previous studies have been carried out on the mineralization age,sulfur isotope,fluid inclusions and H-O isotopes of the Xiadian deposit.On the basis of detailed petrographic and mineralogical studies,we carried out the H-O isotopes and the first He-Ar and Sr-Nd-Pb isotope tracing studies of the Xiadian gold deposit,which furnish the new constrains on the source of ore-forming fluids and materials.It is of great scientific significance to further understand the genesis of gold deposits in Jiaodong gold deposit district.Major results obtained in this study are as follows:(1)The hydrothermal alteration types of the Xiadian gold deposit were further determined.The hydrothermal alterations from early to late are:1)Potassic alteration;2)(pyrite)-sericite-quartz alteration;3)silication;4)carbonation and chlorite.Pyrite-sericite-quartz alteration is manly related with the gold mineralization.The gold ores of Xiadian gold deposit are a variety of pyrite-sericite-quartz altered rocks.(2)The Late Jurassic Linglong granite which exhibits affinities with the adakite is characterized by high Sr(89.40?38.32ppm),low Yb(0.32?0.39ppm)and Y(3.28?5.20ppm)contents,high Sr/Y(143.65?214.95)and La/Yb(54.29?119.69)ratio.The source area of Linglong granite takes garnet + amphibole-Plagioclase as the residual phase,and its origin depth is more than 50km.The subduction of the Paleo-Pacific plate from the early Jurassic period resulted in the thickening of the crust of the north China continent,and the tension from the retreating subduction plate in the late Jurassic period resulted in the partial melting of the thickened lower crust of the North China Craton and the formation of Linglong granite.(3)The Early Cretaceous lamprophyre in the Xiadian gold deposit area belongs to the low-Ti type and originates from the partial melting of the enriched lithospheric mantle.The appearance of the lamprophyre shows that the lithosphere of the North China Craton has undergone rapid thinning at?120 Ma.(4)H-O isotopes of fluid inclusions in quartz and He-Ar isotopes of fluid inclusions in pyrite reveal that the ore-forming fluids is mainly composed of enriched mantle-derived fluid with involved a little meteoric water.(5)The Sr-Nd-Pb isotopes characteristics of the ore and altered rocks in the Xiadian gold deposit indicate that the ore-forming materials are mainly derived from the enriched lithospheric mantle,and may be extracted from the metamorphic bansement rocks during during the ascending of the ore-forming fluid.(6)The Paleo-Pacific plate began to subduct under the North China plate at?177Ma,leading to the thickening of the lower crust of the North China Craton.The subducted oceanic plate began to retreat at?160Ma,and the post-arc tension caused the upwelling of mantle material.Partial melting of the thickened lower crust forms Linglong granite with adakite characteristics(?158Ma).Continuous plate retreat and post-arc tension resulted in rapid thinning of the lithosphere in the North China Craton.Decompression and melting of the enriched lithospheric mantle at?120Ma,forming a series of basic rocks such as the Xiadian lamprophyre.The basic magma from enriched lithospheric mantle carried a huge amount of ore-forming materials for the Xiadian gold deposit.The underplating of the lithospheric mantle induced partial melting of crust,resulting in strong crust-mantle interactions,leading to large-scale gold mineralization events in Jiaodong area.