Geological Features and Metallogenic Model of the Jiama Copper-Polymetallic Deposit in Tibet

The Jiama copper-polymetallic ore deposit is a superlarge ore deposit in the eastern part of central Gangdise.Its discovery is a breakthrough in the exploration work in that skarn-type Cu,Mo,Pb,Zn,Au and Ag and hornfels-type Cu-Mo in this ore deposit all reach large-size reserves.Geological features of the ore bodies,geochemistry of magmatic rocks and chronology of diagenesis and mineralization indicate that Jiama is a typical skarn-hornfels type copper polymetallic ore deposit related to porphyry.The skarn-type ore bodies are distributed in the expansion space between Late Jurassic Duodigou Formation and Early Cretaceous Linbuzong Formation,whereas the hornfels-type ore bodies occur in hornfels.There exist six ore types,twenty-nine metallic minerals and four periods of wall rock alteration in this ore deposit,characterized by wall rock alteration and ores of the porphyry metallogenic system.From top to bottom,metallogenic element zoning is in order of Pb+Zn(Au+Ag)→Pb+Zn(Cu+Au+Ag)→Cu(Mo+Au+Ag)→Cu+Mo(+Au+Ag)→Mo,constituting a complete met-allogenic element zoning and ore mineral zoning related to magmatism.The ore-bearing magmatic rocks contain 59.58%~73.16% SiO2 and have peraluminous nature;in addition,they are enriched with K2O,F(0.08% on average)and Cl(0.02% on average),but is poor in Mg.These data indicate that the ore-bearing magmatic rocks are per-aluminous high-K calc-alkaline rocks and regular calc-alkaline rocks.The ore-bearing magmatic rocks also have a total of rare earth elements between 70.35×10-6 and 175.01×10-6(116.47×10-6 on average),higher Sr,lower Y and Yb,with an obvious positive Sr anomaly and evident negative anomalies of Nb,Ta and Ti,and also have relatively abundant Rb,Ba and Sr.In the diagram of Y-Sr/Y,these magmatic rocks belong to the ore-bearing porphyry rocks of Gangdise,somewhat with the features of high-K adakitic rocks,like C-type adakite.The magmatic evolution indicates the evolutionary trend of regular high-K calc-alkaline rocks characterized by the rock series of diorite porphyrite and granite porphyry and features of certain magma mixing.The basic magma mixing was in favor of mineralization of copper polymetallic ore deposit,especially the mineralization of associated high-content Au and Ag.The U-Pb ages of barren porphyry dykes or peripheral wall rocks dated by LA-ICP-MS of zircons are 16.27±0.31 Ma(MSWD=1.9)~15.99±0.34 Ma(MSWD=2.5)indicating that the intrusive activity took place before mineralization.The SHRIMP U-Pb ages of zircons from the ore-bearing porphyry dyke are 14.2±0.2 Ma and 14.1±0.3 Ma,implying that the intrusive activity was slightly later than major mineralization.Re-Os isotopic isochron age of molybdenite from porphyry-type ore is 14.78±0.33 Ma,that of hornfels-type ore is 14.67±0.19 Ma,and that of skarn-type ore is also about 15 Ma(14.5~15 Ma).These data suggest that the major mineralization pe-riod should be Miocene Langhian Stage.Therefore,this study presents the ore-forming control based on the nappe-slipping structure,and also puts forward the model of the ore deposit which holds that magma provided mineral sources after the intrusion.Therefore,this study gives further guide for regional prospecting.