Late Triassic Mineralization Of The Porphyry Copper Deposits In Yidun Arc,Southwest China

Westward subduction of Ganzi-Litang ocean and then formed Yidun arc in the late Triassic.The arc was near north to south.The Zhongdian(Shangri-La)arc at southern part which develops a large number of porphyry intrusion,accompanied by some porphyry-skarn deposits and shows great mineralization potential.The Pulang deposit in the district,it is a rare porphyry deposit that have reduce feature and formed in late Triassic.Pulang is larger than the "oxidizing" deposits in the region,contrary to the conventional understanding of porphyry mineralization.Is the reason that the different source and evolution of magma or evolution and precipitation of fluid?This paper studied the porphyry intrusion and ore-forming fluid in these deposits,and obtains the following understanding:(1)The zircon U-Pb chronology study shows that the weighted average age of quartz porosity porphyry is 223 ± 3.7 Ma and the quartz porphyry is 218 ± 4 Ma,while the granite porphyry is 207 ± 3.9 Ma in East zone.While the quartz diorite porphyrite is 234 ± 8 Ma.The quartz monzonite porphyry is 219 ± 2.6 Ma.The granite porphyry is 218 ± 1.8 Ma in the West zone.Which is consistent with the geological facts.The late Triassic porphyry is formed by the westward subduction of the Ganzi-Litang ocean.The western belt belongs to the subduction front,and the plate is dehydrated and melted to formed porphyry earlier than the east(back)after MASH and AFC process.(2)The Hf-O isotopic composition of the zircon shows that the Hf(t)value of the quartz diorite porphyrite is negative(Pulang:-13.5 to-8.9,Xuejiping: 0.4-2.4),the ?18O(‰)is higher relatively(Pulang: 5.74-6.9,mean=6.11 ± 0.16;Xuejiping: 5.86-6.27,mean 6.02 ± 0.11),while the Hf(t)value is more positive of quartz monzonite porphyry(Pulang:-3.8 to 2.74;Xuejiping: 0.4-2.2),?18O(‰)is relative lower(Pulang: 4.95-6.17,mean= 5.75±0.13;Xuejiping: 5.53-6.42,mean=5.69 ± 0.12),granite porphyry and quartz monzonite porphyry is similar.Means that the source of quartz diorite porphyrite is crust source relatively large,and the mantle-derived matter of quartz monzonite porphyry and granite porphyry is increased.Indicate thatthe latter have more rich ore-forming materials sources.(3)The oxygen fugacity of quartz diorite porphyrite(Ce4+/Ce3+ ratio)is relatively low(Pulang: 0.6-13.9,average 4;Xuejiping: 9.7-142.8,average 74.7).While the quartz monzonite porphyry is more oxidized(Pulang: 5.4-99.6,mean 52.1;Xuejiping: 15.5-533.1,mean 163.7),and the granite porphyry similar to quartz monzonite porphyry.Thus,the latter have more mineralization potentiality,which is consistent with the geological facts.(4)The meteoric waters joined in the later stage of evolution of ore-forming fluids,which causing fluid immiscibility in the Pulang deposit.The meteoric waters bringed organic matter in the surrounding rock strata,then it has good reducing conditions,which leads to precipitation of mineralization effectively.While the Xuejiping or other deposits do not have better reducing components to form a super-large porphyry deposits.