引用本文：祝向平,陈华安,刘鸿飞,马东方,李光明,张红,刘朝强,卫鲁杰.2015.西藏拿若斑岩铜金矿床成矿斑岩年代学、岩石化学特征及其成矿意义[J].地质学报,89(1):109-128. ZHU Xiangping,CHEN Huaan,LIU Hongfei,MA Dongfang,LI Guangming,ZHANG Hong,LIU Chaoqiang,WEI Lujie.2015.Geochronology and Geochemistry of Porphyries from the Naruo Porphyry Copper Deposit Tibet and Their Metallogenic Significance[J].Acta Geologica Sinica,89(1):109-128.
Abstract:The Naruo deposit, which is located in the north of the Duolong metallogenic district, is the third newly discovered porphyry copper deposit in the middle of the Tibet plateau. Three granodiorite porphyry phases were emplaced in the Naruo porphyry deposit. The former two granodiorite porphyry phases are mineralized porphyries, while the last granodiorite porphyry phase is a late mineral porphyry and barren. An early diorite pluton intruded in the southeast to the Naruo porphyry copper deposit. Zircon U-Pb dating, bulk rock geochemical compositions and Sr-Nd-Hf isotopic compositions of these porphyries and diorite were analyzed. Zircon U-Pb dating results shows that the diorite intruded at 121 Ma and these ganodiorite porphyries were emplaced at 120 Ma. Three granodiorite porphyry phases have similar ages and geochemical features, which might suggest that they were formed in a same magma chamber. These granodiorite porphyry phases are enriched in light rare earth elements and large ion lithophile elements, and depleted in heavy earth elements and high field-strength elements, with weak Eu anomalies, indicating that these granodiorite porphyries resemble arc magmas in composition. These granodiorite porphyries have high SiO2 (63%~65%), Al2O3 (>16%), Na2O (>3%) and low MgO (<2%) content, together with the high Sr and low Y content, showing the adakitic affinities. (87Sr/86Sr)ivalues of the former two granodiorite porphyry phases are 0.7054~0.7058 and 0.7056~0.7057 respectively, with εNd(t) ranges within-3.7~-2.9 and-3.5~-3.2, εHf(t) varies within 3.6~6.7 and 3.6~7.4 respectively, suggesting that the former two granodiorite porphyry phases were derived from juvenile lower crust and some mantle-derived materials were mixed in the source region. The late mineral granodiorite porphyry also shows arc magma affinities in composition, its higher εNd(t)(-0.7~-0.2) and εHf(t)(1.3~12.2) might suggest that late mineral ganodiorite porphyry was also generate from lower crust with more mantle-derived materials, but less middle or upper crust derived materials were involved into the magmas while pooled in the magmas chamber. The diorite also shows arc magma affinities in geochemical composition, and has the lower (87Sr/86Sr)ivalues (0.7052~0.7057) and slightly higher εNd(t) (0.2~3.3) and εHf(t) (1.2~9.5) values, indicating that the diorite was also generated from lower crust but mixed with less crust-derived materials. The diorite and ganodiorite porphyries are fertile and have high copper concentration, which imply that larger amounts of copper might be concentrated in their source region within the lower crust. Comparing with the late mineral granodiorite porphyry phase, mineralized granodiorite porphyry phases contain more crust-derived materials, suggesting that metals were extracted into the magma chamber within the upper or middle crust. The late mineral granodiorite porphyry phase has lower copper concentration, indicating that the late stage of magmas released metals within the magmas chamber and the former magmas were enriched, and multi-stages of magmatic activities is crucial for the formation of porphyry copper deposits.