The Genesis And Zircon U-Pb Age Of Nephrite Placer Deposit,Xiuyan,liaoning

There are two types of nephrite in Xiuyan,Liaoning: primary nephrite and secondary nephrite,which are called Laoyu and Hemoyu.Hemoyu is located in Xiyu gully,Baisha river valley and terrace,Xiuyan,Liaoning.Currently,the Laoyu in Xiyu gully head is regarded as Hemoyu's primary deposit according to location.However,previous researchers have made systematic study about Laoyu,and the mineral composition,geochemical characteristics,genesis and age of Hemoyu is unknown,whether they are consistent with primary nephrite remains to be further studied.This paper researched the petrographic characteristics,major and trace elements' characteristics of whole rocks,hydrogen and oxygen isotope composition,source of ore-forming fluids,age and genesis of Hemoyu through microscope and BSE image,EMPA,XRF,ICP-MS,oxygen isotopic analysis and zircons U-Pb age to confirm whether Xiuyan placer nephrite is from primary nephrite.The results show: 1)The primary minerals of Hemoyu are tremolite,actinolite,minor minerals are diopside,apatite,chlorite,epidote,quartz,titanite,zircon,magnetite,smithsonite and more.The primary mineral of atrovirens Hemoyu is actinolite.There are many metasomatism phenomenons in Hemoyu: tremolite replaced tremolite,diopside,epidote and quartz;chlorite replaced tremolite.The alteration and dissemination are very common in Hemoyu,which may relate to the weathering in the process of crushing,handling and sedimentation of Hemoyu.2)The content of Fe in reseda,yellow white,ash black and atrovirens Hemoyu gradually increases.Fe content is related to its color.3)Whole rock Fe/(Fe +Mg)value and content of Cr,Ni,Co suggest Hemoyu is dolomite-related nephrite.The formation of Hemoyu is related to the contact metasomatic metamorphism between marble and granite.4)Hemoyu's total rare earth element(REE)is low,LREE is relative richand Eu is positive anomaly generally.Hemoyu is depleted in Ba Nb La Ce Pr Sr and Zr,and enriched in U Pb P and Sb.5)Hemoyu Samples' ?18O and ?D isotope compositions are 8.0‰to10.6‰ and-93.78‰ to-75.20‰.?18O and ?D values correspond to fluid are:?18O=8.49‰~11.09‰(330 °C),?D=-73.25‰~-53.50‰(350 °C to 650 °C).These values suggest water in metallogenic fluid mainly from magmatic water.6)The zircons selected from Hemoyu have magma oscillating annule.And it has a high Th/U ratio,which is a magmatic zircon from granite.Zircon U-Pb age is 220.8±7.6 Ma,which suggest placer nephrite formed after this age.The formation of Hemoyu is related to magma intruded in Triassic.Hemoyu is different from Laoyu in bulk-rock REE pattern,hydrogen and oxygen isotope composition,and source of metallogenic fluid and age,which may indicate that the two have different formation mechanisms.