西藏甲玛矿床则古朗北矿段花岗斑岩U-Pb年龄及地球化学特征

Zircon U-Pb age and geochemistry of the granite porphyry in northern Zegulang of Jiama deposit,Tibet

则古朗北位于甲玛矿区东北部,是甲玛矿区新发现的与Ⅱ号复式岩体有关的重要矿段。地质调查和钻孔编录揭示Ⅱ号复式岩体包含多种岩石类型,如花岗斑岩、花岗闪长斑岩、二长花岗斑岩以及石英闪长玢岩等,发育厚大的铜钼矿体。为查明Ⅱ号复式岩体的成岩时代和岩石成因,本文对其中的花岗斑岩进行锆石LA-ICP-MS U-Pb年代学和主、微量地球化学分析。结果显示:花岗斑岩锆石U-Pb年龄为15.7±0.3 Ma(n=22,MSWD=3.3),与主矿段含矿斑岩侵位时代一致。同时,花岗斑岩具有高硅、高钾、富铝的特征,明显富集轻稀土和大离子亲石元素,Eu异常不明显,有弱负Ce异常,分异指数较高,是后碰撞环境下壳源砂质沉积岩受热部分熔融的产物。此外,作为最新发现的矿段,地表的大面积"火烧皮"、深部厚大斑岩型矿体以及角岩型矿体的揭露,均暗示Ⅱ号复式岩体具有独立形成大型-超大型规模矿床的潜力,应该作为后续勘查评价的重要靶区。

Thenewly discovered ore blocks in the Jiama mining area related to the No. Ⅱ complex massif are located in northern Zegulang that lies in the northeast of the Jiama mining area. Geological surveys and drilling data reveal thatthe No. Ⅱ complex massif includes many kinds of rocks such as granite porphyry, granodiorite-porphyry, monzoniticgraniteporphyry, quartz diorite porphyrite etc., with thick copper and molybdenum ores. To obtainthe diagenetic time and petrogenesis of the No. Ⅱcomplex massif, zircon LA-ICP-MS U-Pb chronology and primary and trace geochemical analyses of the granite porphyry are carried out in this paper. The result show that the zircon U-Pb age of granite porphyry is 15.7±0.3 Ma(n=22, MSWD=3.3) which is consistent with theemplacement age of the ore-bearing porphyry in the main ore blocks. Meanwhile, thegranite porphyrythat is the product formed by melting of crust-derived sandy sedimentary rocks in the post-collision environment, has the characteristics of high silicon, high potassium, and high aluminum, and is obviously enriched in light rare earth and large ion lithophile elements with inconspicuous Eu anomaly, abnormal weak and negative Ce, and high differentiation indexes. In addition, as a newly discovered ore block, the No. Ⅱ complex massif has the potential to be a large-to-super-large-scale deposit and is considered as an important target area for the follow-up exploration evaluation because of uncovering of surficial "limonite coverage" with large area, deep thick porphyry ore bodies and hornstone ore bodies.