The Mineralogical Characteristics And Genetic Significance Of The Dark Enclaves In The Alkali-rich Porphyry In The Ya Area, Northwestern Yunnan

The Beiya gold-polymetallic deposit is located at Dali prefecture of Yunnan province and western margin of Yangtze plate,also one of the porphyry-skarn depositsthe in the Jinshajiang-Honghe alkaline-rich porphyry blet.The Beiya gold deposit is the very important gold-polymetallic deposit in our country.In addition to gold,the deposit is associated with Fe,Cu,Pb,Zn and other minerals.Based on the previous researchers' works and after systematical field geology investigation researching on this deposit,dark xenoliths in the porphyry are detailedly studied in this paper.Based on the detailed research of electron microprobe analysis,the distribution,type,petrographic characteristics,temperature,pressure and depths of dark xenoliths are systematically studied.The evolution of alkaline-rich magmatic activity and the genetic significance of dark xenoliths are discussed.Then some main views can be obtained as follows:(1)The Beiya alkaline-rich porphyry is a typical representative of the Jinshajiang-Honghe alkaline-rich porphyry blet.The lithology is mainly quartz monzonite porphyry,monzonitic granite-porphyry,and a few biotite monzonitic granite-porphyry,which contains a variety of enclaves.A large number of dark xenoliths have also been found in the Himalaya alkaline-rich porphyry in Beiya adjacent areas.The output of the four regions has a certain direction.The dark xenoliths developed along the intersection part of the NNW Jinshajiang fracture zone and the secondary fracture zone,and influenced by the stress effect.It is obvious that the tectonism dominated by the strike slip shear since the Himalayan period has played a direct control on the Cenozoic magmatic activity in western Yunnan.There are many metamorphic dark xenoliths in Beiya Wandongshan and adjacent areas(Tuan Shan,Ling Dicun,Pu Taishan).The main lithology of the dark xenoliths is amphibolite,biotite plagioclase gneiss,and biotite schist.The number of dark xenoliths in Beiya Wandongshan is less than adjacent area.These dark xenoliths are of great significance for understanding the origin and evolution of the magma and deep geological problems in this area,and provide new information for further understanding of the crustal structure of the Yangtze plate and the metamorphism in the lower crust.(2)Through the study of petrographic characteristics,it shows that the acidity and alkalinity of the alkaline-rich porphyry in the adjacent area are relatively low.Taking the Wandongshan mine area as the center to the outside,the acidity and alkalinity of the alkali rich porphyry tend to decrease.It is conjectured that the evolution degree of the Wandongshan porphyry is higher,and the emplacement time is late.Wandongshan is the center of the alkaline-rich magma evolution in this area.(3)Through the study of electron microprobe analysis of the alkaline-rich porphyry and dark xenoliths in Beiya Wandongshan and adjacent areas(Tuan Shan,Ling Dicun,Pu Taishan),the analysis shows that: the magma of the alkaline-rich porphyry originates below the crust mantle boundary,and is contaminated by the crust material during the process of the magma evolution.Amphibolite facies dark xenoliths in the crystallization temperature is 500 ?-675 ?,the pressure is 7.888Pa-8.238 Pa,the depth is 29.77km-31.10 km.Schist facies dark xenoliths in the crystallization pressure is 6.718 Pa,the depth is 25.4km.They are mainly formed in the middle-lower crust.There is a basement of granulite-amphibolite-schist metamorphic facies series under the research area.(4)Through the analysis of electron microprobe analysis,it is believed that after the formation of the Cenozoic alkaline-rich porphyry magma,as it rises along the deep strike-slip fault,there was a certain degree of assimilation and mixing effect with the metamorphic base.As a result,the metamorphic facies basement rocks from various depths are captured along the way,and the distribution of dark xenoliths is formed in Beiya and adjacent areas.