| The studied area is located in the alkaline-rich intrusive belt of Sanjiang area,where copper orebodies mainly occur in the porphyry masses and peripheralcontact zones andthe mineralization is closely related to the porphyry mass.This paper takes three typical porphyry copper-polymetal deposits as objects of study,which are Bijiashan,Xiaolongtan,and Fenshuiling deposits.Through the detailed field geological survey,this paper finds out the basic ore-controlling tectonic styles.In this paper,the detailed geochemical,petrographic and chronological studies of the exposed porphyry massesare carried out to determine its formation age,genetic type and the tectonic setting of material source and output.Based on the systematic study of metallogenic chronology and electronic probe analysis,this paper reveals diversified uplifting events,and then speculates erosion degree of the orebodies.Intergrating the field observations and experimental data analysis,this paper discusses the cause of deposits,ore-controlling factors and prospecting clues,and builds the metallogenic model in order to provide certain theoretical guidance for prospecting porphyry copper deposits in Western Yunnan Province.After carrying out the whole rock geochemistry,zircon LA-ICP-MS U-Pb chronology and Hf isotope research into the main intrusive rocks of three typical ore districts of the area studied,this paper obtains the weighted average206Pb/238 Uages of ore-bearing monzoniteporphyries in Bijiashan,Xiaolongtan and Fenshuiling are 35.6± 0.2Ma,36.0 ± 0.2Ma and 36.0 ± 0.2Marespectively,which illustrates that the porphyry masses of the area studiedare the products of the eocene magmatic activities.The porphyry masses belong to calc-alkali-alkaline(a little parlkaline)and metaluminous-peraluminous series,showing large variations in SiO2 and being high in K2 Oand Na2O+K2O.The porphyry masses are rich in large ion lithophile elements such asK,Rb,Ba,La and Nd,lacking in high field-strength elements such as Ta,Nb,Sr,P,Ti,rich in light rare earth elements,lacking in heavy rare earth elements.Although the porphyry deposits are classified as alkali-rich intrusive rocks and A-type granites,they revealsome C-type adakite geochemical characteristics.The values ofzircon's ?Hf(t)in porphyry massesvary greatly,suggesting the Eocene porphyry masses are derived from the partial melting of the lower crustal material with the joining of mantle source materials.Thecrystallizationtemperatures and pressures of the exposed porphyry massesof typical copper-polymetal deposits in Western Yunnan Province obtained by the electronic probe analysis and amphibole-plagioclase thermometerare 833.99?/284.18 MPa,804.72?/163.88 MPa and748.67?/116.70 MPa for Bijiashan,Xiaolongtan,and Fenshuiling deposits respectively.The results show that the emplacementdepths of the porphyry masses in studied area vary greatly with the average depthof 10.53 km,6.07 km and 4.33 km for Bijiashan,Xiaolongtan,and Fenshuiling deposits respectively,which suggests that theemplacement depth is more and more shallow from the SW to the NE.The metallogenic model is summarized as "porphyry emplacement;secondary structure;heat flow circulation;contact metasomatism;supergene enrichment;regenerated mineralization".Combining with the characteristics of regional evolution,this paper considers that the porphyry masses in the studied area are formed in extensional environmentafter the orogenic period.When the stress relaxes after the land-land collision and the magma intrudes upward along the faults and secondary faults,the copper polymetallic deposits come into being. |
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