The Super-large Zhunuo Porphyry Cu Deposit in the Gangdese Belt,Southern Tibet:Magmatism and Mineralization

Author:Wu Song

Supervisor:Zheng Youye


Degree Year:2016





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 H2Omelt(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,εNd(t)between-8.0 and-5.5,and zirconεHf(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,εNd(t)between-13.6 and-9.0,andεHf(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,εNd(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 byεNd(t)>5 andεHf(t)=010,whereas the western one isεNd(t)<5andεHf(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;H2Omelt: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.