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Is Gangdese Porphyry Copper Belt the Second "Yulong" Copper Belt?

Qu Xiaoming 1, Hou Zengqian 1, HuangWei 2 (1 Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China; 2 No.6 Geological Party of Xizang Bureau of Geology and Mineral Resources, Lasa 851400, Tibet, China)  
Based on extensive geological field investigation and comprehensive researches on petro_geochemistry, geology and Re_Os, S, Pb isotope data of the ore deposits, the authors systematically elucidate petro_geochemical characteristics of the ore_bearing porphyries and the alternation_mineralization features of the copper polymetallic ore deposits in the Gangdese porphyry copper belt along the northern side of the Yarlung Zangbo River, clarify the ore_forming epoch and the ore sources, and discuss the relationship of the Cu (Mo, Au) polymetallic mineralization to the evolution of Gangdese collision_orogenic belt. Comparing it with the Yulong porphyry copper belt, the paper points out that the Gangdese porphyry copper belt located on the northern side of the Yarlung Zangbo River is most likely to become the second "Yulong"copper belt in Xizang Region. Its ore_forming potential is expected to form one of the greatest copper belt in the world. Researches indicate that the ore_bearing porphyries in the Gangdese copper belt are characterized petrochemically by high K content (K 2O=2.97%~ 8.56%) and can be classified between shoshonite and high_k calc_alkaline series. In geochemistry they show high enrichment of Rb, Ba, Th, Sr and obvious depletion of Nb, Ta and Yb. Their rare earth elements assume clear fractionation of LREE from HREE [(La/Yb) n=19.91~53.21] and smooth right_dipping C 1_chondrite normalized patterns without any Eu anomaly. All these features are in high consistency with those of the Yulong ore_bearing porphyries. The orebodies occur in both endo_ and exo_ contact zones between porphyries and wall rocks and have the essential alternation_mineralization characteristics of porphyry Cu deposits. The alternation zonation can be established in sequence of potassic alternation (k_feldsparization and biotitization) to silicification+sericitization to propylitization from the inner part of ore_bearing porphyries to outer granites. The mineralization is dominated by veinlet, network and vein_disseminated ores formed in post_magma hydrothermal stages and has a simple ore mineral association consisting of pyrite, chalcopyrite, bornite, sphalerite and molybdenite. In oxidation zone the primary ores form the secondary enrichment zone through weathering and leaching, which often contain rich industrial orebodies.The S, Pb isotopic analyses of the ore_bearing porphyries and sulphides from Jama, Nanmu and Tinggong deposit areas demonstrate that the orebodies and the porphyries have identical S, Pb isotopic compositions. Their δ 34 S (‰) values are in the range of -2.7~1.2 and assume mantle sulfur signature. In addition, their Pb isotopic compositions are not different from each other and vary from 18.532 to 18.765 for 206 Pb/ 204 Pb , from 15.572 to 15.683 for 207 Pb/ 204 Pb, and from 38.596 to 39.058 for 208 Pb/ 204 Pb. In Pb isotopic diagrams they are all plotted on or near the orogenic belt evolution line and obliquely intersect it, consistent with the tectonic environment of their formation. Five molybdenite samples from the Nanmu Cu, Mo deposit area yield a Re_Os isochon age of (14.67±0.20) Ma, suggesting a close relationship of the Cu, Mo mineralization to porphyry intrusions (20~14 Ma).
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