| The Gangdese belt is currently the largest porphyry copper belt in China and contains lots of large and super-large Miocene porphyry copper deposits,such as Qulong,Jiama,and Zhunuo.There are also many Paleocene-Eocene skarn-porphyry Fe-Pb-Zn-Mo-W polymetallic deposits in the northern part of the belt.However,little research was concerned about the oxygen fugacity of the magma systems which was related to those different types of mineralization.In order to discuss this issue,we select Jiaduobule skarn Fe-Cu,Qiagong skarn Fe-Pb-Zn,and Zhunuo porphyry Cu deposits to analyze the trace element compositions of zircons in this study.The trace element compositions of zircons are analyzed by LA-ICP-MS.The results show that the Eu/Eu* values of zircon from Jiaduobule biotite monzogranite related to mineralization range from 0.09 to 0.35(mean 0.24),Ce/Ce* values from 12 to 468(mean 126),zircon Ti temperature from 602 to 864 ?(mean 755 ?),and ?FMQ values calculated from U,Ce and Ti of zircon range from 0.18 to 2.34(mean 1.09).The zircons from Qiagong monzogranite porphyry have Eu/Eu* values of 0.12-0.28(mean 0.20),Ce/Ce* values of 9-234(mean 72),zircon Ti temperature of 708-803 ?(mean 762 ?)and ?FMQ values of-0.78-+0.66(mean0.22).The Eu/Eu* values for monzogranite porphyry zircons from the Zhunuo fertile intrusions from 0.09 to 0.35(mean 0.24),Ce/Ce* values from 2 to 468(mean 105),zircon Ti temperature from 478 to 722 ?(mean 660 ?),and ?FMQ values from-0.03 to 3.21(mean 1.22);Eu/Eu*values based on zircons from monzogranite of 0.28 to 0.56(mean 0.40),Ce/Ce* values of 35 to168(mean 76),zircon Ti temperature of 638 to 691 ?(mean 667 ?),and ?FMQ values of 1.56 to 2.24(mean 1.81);the zircons from post-mineralization granite porphyry have Eu/Eu* values of 0.37 to 0.56(0.45),Ce/Ce* values of 43 to 493(mean 138),zircon Ti temperatures of 516 to756 ?(mean 655 ?)and ?FMQ values of-0.46 to 2.51(mean 0.76);the zircons from pre-mineralization quartz porphyry have Eu/Eu* values of 0.08 to 0.29(mean 0.18),Ce/Ce*values of 20-585(mean 204),zircon Ti temperature of 633-750 ?(mean 671 ?),and ?FMQ values of 0.25-2.97(mean 2.05).Combined with previous data and research results,we suggest that the ore-forming rocks of the Jiaduobule and Qiagong skarn Fe deposits have similar Eu/Eu*,Ce/Ce*,and ?FMQ values to some of the Paleocene-Eocene barren intrusions of the Gangdese belt,suggesting that these indicators alone can not effectively distinguish between iron-forming and iron-poor magmatic rocks.The Zhunuo syn-mineralization monzogranite and monzogranite porphyry(with amphibole)and the post-mineralization granite porphyry(without amphibole)have similar zircon Eu/Eu* and Ce/Ce* values,but the former has higher ?FMQ values and lower zircon U/Yb,demonstrating that a magmatic system with higher oxygen fugacity and less involvement of ancient crustal material favors the formation of porphyry copper deposits.In addition,compared with the intrusions related to mineralization,the Zhunuo pre-mineralization quartz porphyry has a lower Eu/Eu* but similar ?FMQ values,suggesting that there are other factors controlling the fertility of magma in addition to magma oxygen fugacity.Compared with molybdenum-forming magmatic rocks,the iron-forming magmatic rocks in the Paleocene-Eocene Fe-Pb-Zn-Mo polymetallic metallogenic belt that is located in the northern margin of the Gangdese have relatively low zircon Eu/Eu* and ?FMQ,indicating that the intrusion associated with skarn Fe deposit does not need high magma oxygen fugacity. |
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