Jiama Copper-polymetallic Deposit Miocene Magmatic Tectonic Activity And Diagenetic Mineralization Age

The Jiama copper-polymetallic ore deposit is located in the eastern part of Gangdese metallogenic belt,Tibet.It is in the post-collisional extensional metallogenesis stages.In this stage,the Miocene ore-bearing porphyries emplaced in the Gangdese granite batholith and the Triassic-Cretaceous strata,formed the large scale skarn type-porphyries type deposits.In this paper,a typical profile is selected in the main metallogenic belt of Jiama ore deposit,the ore-bearing porphyries,ore-bearing quartz veins and ore-bearing calcite veins are sampled in the profile,by the methods of geochemical test and analysis,zircon U-Pb isotope dating,ESR dating,the following main achievements are obtained:(1)By the geochemical test and analysis of the eight sample,we got the characteristic content of the following main major elements.Si02:59.66%-67.94%,A1203:14.48%-16.5%,MgO:1.26%-1.87%,etc.The measured porphyries samples have the characteristics of adakites,belong to the C type adakites,which are the products of upper mantle melt in the late continental collisionlate stage.(2)Though the major elements and trace elements characteristic analysis of the eight samples,the high Si,low Mg,partial high Al2O3,K2O,Na2O characteristics are discovered,indicating that the intermediate-acid porphyries in the Jiama mining area belongs to the high K-Ca alkaline series.We infer that the magma is formed by the partial metasomatism of the lower crust or upper mantle material.(3)The six ore-bearing porphyries and skarn were determined by the zircon U-Pb dating.The dating results show that the diagenetic age of the ore-bearing porphyries is around 14-16Ma,the diagenetic age of skarn is about 15-15.8Ma.Hence,we can confirm that the main metallogenic period of Jiama mineral deposit is mainly concentrated in 14-16Ma,which is in the Miocene period.The diagenesis and the metallogenesis of the ore-bearing porphyries are in the same period.(4)The ore-bearing quartz veins and ore-bearing calcite veins were determined by the ESR dating,the quartz veins age is 14-16Ma,the calcite veins age is 5-7Ma.The appearance of the veins is the best record of magmatic-hydrothermal metallogenesis alteration,which can reflect the tectonic activity age and determine the metallogenic period from from another point of view.The age of the calcite veins shows that the calcite veins are the product of the later stage tectonic activity,there are still tectonic activity and magma intrusion in the later stage.Meanwhile,it also shoes that there are still weak metallogenesis in the Jiama deposit during the period of 5-7Ma.In this paper,by the geochemical test and analysis,the diagenetic period dating test,the metallogenic period dating test of the Miocene in the Jiama mineral deposit,the ore-bearing porphyries were analysed and researched from the perspective of adakite.It is confirmed that the magma comes from the mixture of the lower crustal and the upper mantle material,which is the product of post-collisional extensional process,the diagenesis and metallogenesis are in the same period.It shows that the ore-bearing porphyries have an intimate relationship with the metallogenesis in Jiama deposit,indicating the direction for further prospecting.