藏南努日铜-钨-钼矿床晚白垩世石英闪长岩U-Pb定年及其地球化学特征

藏南努日矿床位于冈底斯成矿带南缘,前人获得的辉钼矿Re-Os同位素年龄为23 Ma,与明则和程巴矿床成矿时代一致,但矿区内至今未发现与矿化有关的成矿斑岩体。本文报道了努日矿区新发现的与矿化关系密切的石英闪长岩的地球化学特征,获得石英闪长岩的LA-ICP-MS锆石U-Pb年龄为93.42±0.76 Ma,与同一成矿带内桑布加拉和克鲁铜金矿成矿时代一致(90~93 Ma),表明矿区可能存在两期成矿事件。石英闪长玢岩的主量微量元素SiO_2含量为57.19%~58.23%,A1_2O_3含量为15.78%~16.03%,MgO含量为4.74%~5.32%,Mg~#指数为65.2~67.3;富集大离子亲石元素(Rb、Sr、Ba、U等)及轻稀土元素,亏损高场强元素,显示出埃达克岩特征。研究表明石英闪长玢岩形成于洋壳俯冲阶段的弧岩浆岩,洋壳熔融形成的母岩浆侵入近地表形成早期铜多金属矽卡岩矿化。晚白垩世成矿事件的发现进一步佐证了研究区存在两期矿化叠加事件,拓展了研究区找矿方向。

LA-ICP-MS Zircon U-Pb Dating and Geochemistry of Late Cretaceous Quartz Diorite in the Nuri Cu-Mo-W Deposit, South Tibet

The Nuri deposit is located at the south margin of the Gangdese metallogenic belt in Tibet. Its mineralization age revealed by Re-Os dating of molybdenite is 23 Ma, which is consistent with that of the Mingze and Chengba deposits. However, the ore-related rocks have not yet been identified. In this study, we report the age and geochemistry of the newly discovered quartz diorite related to mineralization. LA-ICP-MS zircon U-Pb dating indicates that the diorite intruded at 93.42±0.76 Ma. The age is consistent with the ages of Shangbujiala(93 Ma) and Kelu(90 Ma) copper-gold deposits in the same metallogenic belt. These ages and previous ages indicate a two-stage ore-forming process. Major elemental analysis shows that quartz diorites contain 57.19%-58.23% SiO₂, 15.78-16.03% A1₂O₃, and 4.74%-5.32% MgO, with Mg^# indicators of 65.2-67.3. Trace element composition suggests that these rocks are rich in large ion lithophile elements and light rare earth elements, but are poor in high field-strength elements, characteristic of adakite. This study indicates that quartz diorites are arc magmatic rocks formed in the stage of oceanic crust subduction. The primary magma formed by melting of oceanic crust intruded at the shallow crust, resulting in the formation of early skarn copper deposit. The identification of two stages of mineralization in the Nuri deposit expands the prospecting direction of the study area.