西藏冈底斯斑岩铜矿带含矿岩体的相对氧化状态:来自锆石Ce(Ⅳ)/Ce(Ⅲ)比值的约束

RELATIVE OXIDATION STATES OF ORE-BEARING PORPHYRIES INFERRED FROM CE(Ⅳ)/CE(Ⅲ) RATIO IN ZIRCON: APPLICATION TO THE PORPHYRY COPPER BELT AT GANGDESE,TIBET

对西藏冈底斯斑岩铜矿带三个斑岩铜矿床含矿斑岩进行了锆石与全岩的微量元素分析,通过理论计算获得三个矿床含矿斑岩的锆石Ce(Ⅳ)/Ce(Ⅲ)比值和δEu值:拉抗俄矿床为492.25和0.5341,冲江矿床为263.83和0.4353,南木矿床为291.94和0.5273。认为锆石Ce(Ⅳ)/Ce(Ⅲ)比值>260,δEu>0.4是斑岩成矿的地球化学标志。冈底斯斑岩铜矿带的锆石Ce(Ⅳ)/Ce(Ⅲ)比值自东向西呈递减趋势,指示岩浆在上升侵位过程中东部较西部受到更大程度的上部地壳物质的混染。冈底斯斑岩铜矿带的矿化机理可表述为:深部岩浆在上升侵位过程中受到上部地壳物质的混染,使岩浆的氧逸度增加,在随后的结晶分异过程中产生高Cu、Au浓度的岩浆热液流体,并在继续上升的过程中,随着压力、温度的降低,在适当的条件下发生了成岩成矿作用。

Trace-element composition of zircons and whole rocks from ore-bearing porphyries from three porphyry copper deposits of the Gandese porphyry copper belt in Tibet has been analyzed by Laser ablation-inductively coupled plasma-mass spectrometry （LAICPMS）. These data permit the Ce（Ⅳ）/Ce（Ⅲ） ratio within zircon to be calculated using a lattice-strain model for mineral-melt partitioning of Ce（Ⅳ） and Ce（Ⅲ）. The Ce（Ⅳ）/Ce（Ⅲ） ratios of the Lakang＇e deposit, Chongjiang deposit and Nanmu deposit are 492. 25, 263, 83 and 291.94; the δEu values are 0. 5341, 0. 4353 and 0. 5273, respectively. We can draw a conclusion that the porphyry copper mineralization is directly associated only with porphyries with zircon Ce（Ⅳ）/Ce（Ⅲ） 〉 260 and δEu 〉0.4. The decrease of Ce（ Ⅳ）/ Ce（Ⅲ） ratios in zircons from east to west in the Gangdese porphyry copper ore belt suggests that the magma in the east is more contaminated by upper crustal material than that in the west during ascending. Ce（Ⅳ）/Ce（Ⅲ） ratios in zircon can be used to discriminate ore-bearing porphyries from barren ones, which will provide a useful tool for search of porphyry copper deposits. At the end, the paper presents the mineralization mechanism of the Gangdese copper deposit belt.