Geochemistry And Ore-bearing Potential Research Of The Indo-porphyries In Zhongdian Arc,SN Yunnan

Zhongdian arc as part of The Tethys-Himalayan Porphyry Metallogenic Re-alms located in the southern of Yidun arc.It is one of key tectonic units in t-he archipelagic arc-basin system in the Sanjiang area.Zhongdian arc formed d-uring Ganzi-Litang oceanic crust westward subduction in the Late Triassic,exp-erienced continental block collision in Yanshanian and convergence orogenics o-f continental margins in Himalayan,accompanied by the formation of a correspo-nding three-stage mineralization events.Among them,supergene porphyrites and porphyries were formed in the process of subduction orogeny.Both porphyry t-ype and skarn type deposits are closely related to magmatic intrusion of porph-yrites and porphyries.This paper based on field investigation and mapping in detail and,focusing on Indo-porphyries in Zhongdian area,analysised the geoch-ronology,geochemistry,petrogenesis and the sources of magma.It also discuss-ed the main difference between productive and nonproductive porphyritic intrus ions in the Zhongdian arc.The LA-ICP-MS zircon U-Pb method is used to date the formational age of the porphyries in the Zhongdian arc.Zircons separated from Xuejiping quartz dioritic porphyry and quartz monzonitic porphyry gave mean ²⁰⁶Pb/²³⁸U ages of 213.2±3.2Ma and 2067±2.9Ma,respectively.Zircons from Chundu quartz dioritic porphyry provided a mean ²⁰⁶Pb/²³⁸U ages of 220.1±4.1Ma.Zircons from Yaza quartz dioritic porphyry had a mean ²⁰⁶Pb/²³⁸U ages of 222.7±3.1Ma.Meanwhile Zircons from Songnuo quartz monzonitic porphyry told a mean ²⁰⁶Pb/²³⁸U ages of 215.8±2.8Ma.The zircon geochronology shows that these Indo-porphyries in the whole Zhongdian arc were predominantly formed at 220?210Ma.In situ Hf isotopic analyses on zircons gave slightly negative?Hf(t)values ranging from-7.5?1.7(with average of-1.0)in Xuejiping porphyries,which are similar to Pulang,suggesting that the primary magmas were mainly derived from a metasomatized mantle,with contamination of crustal rocks during ascending.The productive porphyries of Zhongdian arc are quartz diorite porphyry and quartz monzonitic porphyry.The latter occursmainly in the eastern porphyry belt,while the former exposed dominantly in the western belt.Our study shows that there are significant difference between the productive porphyries and nonproductive porphyries.On the contrary of nonproductive porphyry,the productive porphyry shows the characteristics of low Y and Mn O,high Al₂O₃/(Ca O+Na₂O+K₂O)ratio and oxygen fugacity,zircon Ce⁴⁺/Ce³⁺ratio>300 and ?Eu>0.6.