Genetic Model Of The Gold-bearing VMS Deposit,Bikou Block,China

Only more than 10 gold-rich VMS deposits are discovered in the word.However,the source of gold-bearing fluid,the enrichment and deposition mechanisms of gold and the relationship between good and alkali metals are controversial.Yangba large-scale and Duba small-scale deposit are typical gold-bearing Cu-Au VMS deposits in Bikou Block.And they can provide evidence for the study of genetic model.Based on the detailed geological field work and microscopy-ultramicroscopic observation,by using geochemistry of volcanics and geochronology of diagenesis and mineralization,In-situ microanalytical trace element analysis of metallic minerals,fluid-inclusion and oxygen isotope analyses,this thesis finds two epochs of mineralization process,which is relate to volcanic exhalative sedimentation and regional metamorphism.And it also reveals differences of the source of gold-bearing fluid,the enrichment and deposition mechanisms of gold between the two epochs of mineralization process.It also establishs a genetic model of the deposits and discusses the reason of different reserves in the two deposit.The early Cu-Au mineralization which is relate to volcanic exhalative sedimentation,and the ore body is controlled by Neoproterozoic Bikou Group spilite.The metallogenic age is 804~810 Ma.Base on the study of regionally structural evolution and the trace elements characteristics of simultaneity-metallogenesis volcanic,The deposits form in the island arc setting of the convergent boundary.It is same with the tectonic setting of typical Besshi VMS deposit.It also explains the reason why there are only Cu and Au,but no Pb and Zn in deposits.However,the later Au mineralization forms in the continental collision of Huanan and Huabei plate.The ore body is NW,and controlled by north-west regional fault.The age of the mineralization is 203~220 Ma.And this age is same with the metallogenic age of orogenic gold deposits.The gold is invisible in magnetite and chalcopyrite of the early stage.The fluid property changes from reducibility to oxidbillity,is the reason for gold deposition.The gold is invisible and in the form of electrum inclusion in magnetite and chalcopyrite of the later stage.And there is a few in the form of native gold.Fluid immiscibility causes gold deposition in this stage.The geological feature is similar in this two deposits,only the number and type of quartz veins are different.There are lots of quartz veins with magnetite,chalcopyrite,pyrite,sphalerite and native gold in Yangba deposit.There are a small amount of quartz veins with magnetite,chalcopyrite,and pyrite in Duba deposit.This indicates that there is more metallogenic fluid in Yangba deposit.And this is also the reason why the Yangba deposit has large reserves.