Controls On The Metallogenesis Of Wenyu Subaerial Volcanic Rock Type Copper Deposit,Jingdong County,Yunnan Province

Wenyu Copper Deposit is located in the north of Yunxian-Jinghong volcanic arc,where is now the southern section of Sanjiang Metallogenic Belt(SMB).It is the first subaerial volcanic rockhosted copper deposit which has been found in SMB.Based on the detailed geological survey and the accurate test analysis,metallogenic tectonic setting,diagenesis-metallogenesis,relationship between the metallogenesis and volcanic activitie and ore-forming processes had been dissected,and finally metallogenic model of Wenyu Copper deposit had been established,which is so significant for pertecting metallogenic theory of subaerial volcanic rocks and guiding the prospecting.Wenyu Copper Deposit lies in the basic volcanic of Xiaodingxi formation,consisting of bimodal volcanic rocks.The morphology of ore bodies is mostly stratoid,lentoid and vein controlled by high permeability zones and fault fracture zones in volcanics.Within high permeable zones,the ore minerals mainly consist of chalcopyrite and bornite.The structure of ores which are caused by gas-liquid filling-metamorphism in the depsit are stockwork,disseminated,brecciated and amygdaloidal.The textures of ore are controlled by crystallization,exsolution and metasomatism.As for alteration,it's dominated by chloritization with less silicification,pyritization and carbonation.Within fault fracture zones,the ore minerals are mainly chalcocite,bornite and chalcopyrite.The textures of ore are controlled by crystallization and metasomatism.And alteration is dominated by silicification with less chloritization and carbonation.According to mineralization vein,alteration and mineralization characteristics,the metallogenic process can be divided into hydrothermal period and supergene period.Furthermore,the hydrothermal period can be subdivided into four stage: Stage of Pyrite(?),Stage of Chalcopyrite-Bornite(?),Stage of Chalcocite(?)and Stage of GalenaSphalerite(?).The bimodal volcanic rocks of the Xiaodingxi formation are dominated by basalt with a small amount of basaltic volcanic breccia and rhyolite.Chronology had shown that the zircon U-Pb age of rhyolite is 238.4±2.4Ma(MSWD=1.4)and 236.4±3.2Ma(MSWD=1.07).The volcanic activity of the Xiaodingxi formation belongs to the middle Triassic,not the previous study of late Triassic.The geochemical characteristics of the basic rocks indicated that they are of calc-alkaline-high-K calc-alkaline,lowmagnesium and high-alumina.These basic rocks are relatively enriched light rare earth elements and large ion lithophile elements,relatively depleted high field-strength elements and weakly negative anomalies of Eu and negative anomalies of Sr.Acid rocks in this area are also relatively enriched light rare earth elements and large ion lithophile elements,relatively depleted high field-strength elements,strongly negative anomaly of Eu,strongly negative anomaly of Sr and weakly negative anomaly of Ce.These geochemical characteristics of acid rocks in this area indicated that they are of peraluminous high-K calcium-alkaline S-type granite series.The Sr-Nd-Pb isotopes and elements geochemical characteristics showed that the source of mafic volcanic rocks was probably a lithosphere mantle that had been metasomatied by subducted pelagic sediments,and the main source of acid volcanic rocks was partial melting of overlying crustal material.They have the feature of continental intraplate volcanic rocks and arc volcanic rocks,indicated that it was formed in the collisional extensional background between the Baoshan block and the Pu'er block.The characteristics and varying trend of the rare earth and trace elements of metallic sulphides were basically consist with those of the basic volcanic rocks in Xiaodingxi formation,implied that the early ore-forming fluid is mainly derived from the basic volcanic magma of the Xiaodingxi formation,which were in the hightemperature reduction Cl-rich system according to the formation temperature of chlorites.The transformation of temperature and oxidation reduction condition were considered to be the main mechanism of metal elements precipitation.The middle and late stage were mainly derived from atmospheric water by H-O isotopes,with mediumlow temperature,medium salinity and medium-density,which had a strong water-rock reaction with the volcanic magma,fluid decompression boiling was the main mechanism of metal element precipitation.S isotopes have origin from degassed magma,and Pb isotopes composition were consist of those of basic volcanic rocks of Xiaodingxi formation,suggested that the main ore-forming materials were likely to origin from the Xiaodingxi formation basic magma.Based on all the works above,the main control of Wenyu Deposit was volcanic activity under the collisional background between the Baoshan block and the Pu'er block.The basic magma,derived from subduction sediments replacement lithospheric mantle,carried a large amount of metal elements and rose to the deep part of the crust,and then occured degassing.The residual magma continued to rise to the shallow and exsolve volcanic hydrothermal fluid,forming chalcopyrite-bornite orebodies in the volcanic rock in high percolation zone.The downward circulation of atmospheric water was mixed with the residual volcanic magma hydrothermal fluid,and the fluid transported ore-forming material to the fault fracture zone and formed chalcocitebornite orebodies.With the fluid temperature decreasing rapidly,galena orebodies were formed near copper orebodies.The ore-forming process had occured in the dormant of volcanic activity and been terminated by the later long-lasting volcanic activity,but formed the huge thick volcanic rocks played an important role in the preservation of orebodies.Based on the ore-forming process,and compared with the copper deposits related to subaerial volcanism,it was suggested that Wenyu Copper Deposit is a typical subaerial volcanic deposit,which is a new type deposit controlled by subaerial volcanism.Furthermore,It is a new idea of building the metallogenic and prospecting model in this thesis for looking for this typical copper deposit.