The Super-large Zhunuo Porphyry Cu Deposit In The Gangdese Belt,Southern Tibet:Magmatism And Mineralization

The Zhunuo super-large deposit in the western segment of Gangdese Porphyry Copper Belt in Tibet,one typical porphyry Cu deposit formed in the post-collisional setting,is located at the Yamu,Ang'ren County.The magmatism in the Zhunuo ore district is widely developed,including pre-ore?⁵0Ma?magmatic rocks,ore-forming complex bodies in the main mineralization epoch?13¹5Ma?,and barren intrusions in the later-mineralization epoch?10¹2Ma?.The pre-ore magmatic rocks are characteristed by low Sr/Y,La/Yb ratios,and high La/Sm ratios,indicative of the fractionation of plagioclase,magnetite and pyroxene from relatively dry magmas.They has high magmatic oxygen fugacity with zircon Ce⁴⁺/Ce³⁺ratios of 328 to 1014.They have relatively deplete Sr–Nd(⁸⁷Sr/⁸⁶Sr_?i?=0.704626 to 0.706787,eNd?t?=–3.8 to–1.7)and zircon Hf?eHf?t?=–0.3 to+4.1?isotopic compositions,suggesting the partial melting of underplated basaltic lower crustal melts.The basaltic material was sourced from a mantle wedge that was metasomatized by Neo-Tethyan slab-derived fluids and oceanic sediments through dehydration.The main mineralization related intrusions are characteristed by high Sr/Y,La/Yb and Sm/Yb ratios,indicative of the early-crystallized hornblende and late-crystallized plagioclase from hydrous melts,and the residual mineral phases are dominted by garnet and amphibole.Based on the thermobarometric formulations of amphibole,the estimated magmatic oxygen fugacity are?NNO=+2.2 to+2.8,logfO₂=-13.6 to-11.0,magmatic water is H₂O_melt?wt.%?=+3.1 to+3.9,and relatively high magmatic oxygen fugacity with zircon Ce⁴⁺/Ce³⁺ratios of 45 to 363.They have a limited range of isotopic compositions,with⁸⁷Sr/⁸⁶Sr_?i?ratios between 0.707194 and 0.707901,?Nd?t?between-8.0 and-5.5,and zircon?Hf?t?values of-4.2 to-0.7,indicating the remelting of previous juvenile lower crust.Their enriched isotopic features are related to the involvement of mantle derived melts into the lower crust,which have been metasomatized by subducted Indian slab through prograde metamorphism.Later-mineralization related rocks are characteristed by low Sr/Y and high La/Yb ratios,indicative of barren magmatic water.They have ⁷Sr/⁸⁶Sr_?i?ratios between 0.709435 and0.720919,?Nd?t?between-13.6 and-9.0,and?Hf?t?values of-7.8 to-0.9.The simulated calculation shows that the source has been contaminated by Indian continental material?45⁷5%?.In addition,they has low magmatic oxygen fugacity with zircon Ce⁴⁺/Ce³⁺ratios of39 to 196.The potassic and ultrapotassic magmatism were also occurred in the Zhunuo area represented by lamprophyre,high Mg diorite and dioritic enclave that were developed in the entire ore-forming processes.These rocks are high Cr,Ni and Mg^# contents,enriched in LILE,depleted in HFSE,with ⁷Sr/⁸⁶Sr_?i?ratios between 0.709764 and 0.731343,?Nd?t?values of-14.3 to-10.5.These features are similar to the post-collisional potassic and ultrapotassic in the southern Tibet,reflecting the high-pressure melting of lithospheric mantle metasomatized by abundant Indian continental material.There is something different about the porphyry Cu mineralization between the western and eastern segments of Gangdese.For example,the ore-forming magma in the eastern Gangdese is characteristed by?Nd_?t?>5 and?Hf?t?=0¹0,whereas the western one is?Nd_?t?<5and?Hf?t?=-10⁰.Moreover,the depth of magmatic emplacement in eastern Qulong porphyry deposit is 4.8¹0.0Km and magmatic water is 3.8⁴.6%,which are higher than the western Zhunuo deposit(depth:1.2⁴.4Km;H₂O_melt:3.1³.9%).Finnally,we proposed that previous subduction metasomatism and the inheritance of magmatic source,the development of complex bodies during ore-forming process,as well as the additional water recharged by potassic and ultrapotassic rocks,are the key coupling condition of Zhunuo deposit.In view of extensive distribution of ultrapotassic rocks and Linzizong volcanics in the western Gangdese,there is still the potential of porphyry mineralization in this area.