Characteristics And Genetic Relationship Of Indosinian Ore-bearing Porphyry In Zhongdian Area,Northwestern Yunnan

Zhongdian area in Northwestern Yunnan is located in the northeast of Himalaya and belongs to the southern Yidun Arc belt.The east and south area of zhongdian is the junction zone of the Ganzi-Litang plate,and the west is the Gezan fault.There are a large number of Indosinian porphyry and porphyry-skarn deposits in the study area.The lithology of the porphyry is mainly quartz diorite porphyry and quartz monzonite porphyry.There are different views about the genetic relationship between the two types of porphyry.Based on the existing research,this thesis discusses the genetic relationship between the two types of porphyry from the aspects of petrology mineralogy and geochemistry of porphyries in Zhongdian area,so as to provide valuable information for the study of porphyry copper deposits in this area.The whole-rock geochemical analysis shows that the quartz diorite porphyry is mainly characterized by relatively enrichment of sodium,while the quartz monzonite porphyry is characterized by relatively enrichment of potassium.Both of them are high-K calc-alkaline shoshonite and peraluminous,and have certain similarities in the concentration of trace elements and rare earth elements.They are enriched in large ion lithophile elements and depleted in high field strength elements.The results of microscopic measurement and geochemical analysis of plagioclase show that the plagioclase of these two porphyries is oligoclase-andesine,the average plagioclase An of quartz diorite porphyry is 39,while the average plagioclase An of quartz monzonite porphyry is 36.A lower An value represents a higher degree of evolution and the results of plagioclase temperature calculation show that the crystallization temperature of the latter is relatively low,which indicates that they may be magmatic evolution..Chronological studies have shown that the overall peaks of quartz diorite porphyry and quartz monzonite porphyry are both around 216 Ma.Some quartz diorite porphyry are older.The peaks coincide with the regionally tectonic events.Therefore,they are the products of the same magmatism that happend after the subduction.In the mass ratio analysis,the plots of K₂O are close to the mass ratio line.Besides,there is no correlation between LOI and various elements,so the quartz monzonite porphyry was not generated by the alteration of quartz diorite porphyry.Petrological,mineralogical and geochemisty of two porphyries intrusions from different mining areas in Zhongdian area show that the two porphyries have certain inherited mineralogies,similar geochemical characteristics and evolutionary relationships.Their geochemical compositions do not show alteration characteristics,indicating that the quartz monzonite porphyry was not formed by the alteration of quartz diorite porphyry.They are products of homologous magma that generated in different stages of evolution towards high Si and K,low Al,Na,Ca and Mg as the magma cools,they are I-type granites formed in volcanic arc environment.By distinguishing the water content and oxygen fugacity of the two rock bodies,it is found that the water content and oxygen fugacity of the quartz monzonite porphyry are higher than that of the quartz diorite porphyry,which is relatively more conducive to mineralization.