冈底斯斑岩铜矿(化)带:西藏第二条“玉龙”铜矿带?

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

通过广泛的野外地质调查和岩石地球化学 ,矿床学 ,Re_Os同位素 ,硫、铅同位素的综合研究 ,首次比较系统地论述了雅鲁藏布江北侧冈底斯斑岩型铜矿带含矿斑岩的岩石地球化学特征和矿床的蚀变矿化特征 ,查明了矿化时代和成矿物质来源 ,阐明了该带铜 (钼、金 )多金属成矿作用与冈底斯碰撞造山带发展演化的关系。并通过与玉龙斑岩铜矿带的简要对比 ,指出位于雅鲁藏布江北侧的冈底斯斑岩铜矿带完全有可能成为西藏的第二条“玉龙”铜矿带 ,具有形成世界级铜矿带的巨大潜力。研究表明 ,冈底斯斑岩铜矿带含矿斑岩属钾玄岩至高钾钙碱性岩系。地球化学上以富集大离子不相容元素Rb、Ba、Th、Sr,亏损高场强元素Nb、Ta和重稀土元素Yb为特点 ;稀土元素则为轻、重稀土分馏明显的平滑右倾型式。矿床具有自斑岩体向外由钾化→绢英岩化→青盘岩化的蚀变分带 ;矿化以岩浆期后阶段形成的脉状、网脉状和细脉浸染状矿体为主 ,矿石矿物组合简单。含矿斑岩和硫化物具有一致的硫、铅同位素组成 ,硫同位素具幔源特征 ,铅同位素显示造山带铅特点。由南木矿区 5个辉钼矿样品得出了t =(14.6± 0 .2 0 )Ma的Re_Os等时线年龄 ,说明成矿时代与斑岩体的侵入时代 (2 0～ 14Ma)是一致的。

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).