西藏纳如松多铅锌矿区晚白垩世石英闪长岩形成机制及其地质意义:岩浆锆石证据

Formation mechanism of Late Cretaceous intrusive rocks in Narusongduo Pb-Zn deposit,Tibet:Evidence from magmatic zircon

西藏纳如松多铅锌矿床是冈底斯北侧银铅锌多金属成矿带中的典型矿床,文章利用LA-ICP-MS测试技术,对矿区石英闪长岩开展了详细的锆石U-Pb年代学、锆石微量元素和Hf同位素地球化学特征研究,以此调查其岩浆源区特征,探讨其形成机制并试图探究其成矿潜力。纳如松多矿区石英闪长岩锆石U-Pb年龄为(85.2±1.1)Ma,早于矿区内大规模成矿事件,为晚白垩世岩浆活动的产物。石英闪长岩锆石稀土元素表现为LREE亏损、HREE富集的左倾配分模式,并具有明显的负Eu异常和正Ce异常特征,δEu变化于0.06~0.36之间,δCe变化于5.85~248.22之间。石英闪长岩的锆石176Hf/177Hf分布于0.282 679~0.282 766之间,εHf(t)值变化于-1.44~1.70,单阶段模式年龄TDM集中于679~804 Ma,二阶段模式年龄TDM2值集中于1047~1245 Ma之间。综合分析显示纳如松多矿区石英闪长岩来自于地幔和地壳2个不同的源区,并结晶于陆壳环境。纳如松多铅锌矿区石英闪长岩锆石的Ce(Ⅳ)/Ce(Ⅲ)比值除个别点外均低于100,且δEu集中于0.04~0.22之间,远小于0.4,为还原性岩体,由此推断石英闪长岩还原性的岩浆岩特征可能是其未能具有成矿能力的原因之一。

The Narusongduo lead-zinc deposit in Tibet is one of the typical ore deposits in the Pb-Zn-Ag polymentallic ore belt on the northern side of the Gandise copper ore zone. The deposit is characterized by two types of mineralization, namely the cryptoexplosive breccia type in the eastern part and skarn type in the western part. There are two types of intrusive rocks developed in the deposit, the pre-mineralization quartz diorite and metallogenic granite porphyry. The result of zircon LA-ICP-MS U-Pb dating of quartz diorite in the Narusongduo Pb-Zn deposit is （85.2±1.1） Ma, which is about 20 Ma earlier than that of the metallogenic porphyry. The quartz diorite has the zircon REE patterns with significant negative Eu anomalies and positive Ce anomalies. The 176Hf/177Hf ratios of zircons in quartz diorite are distributed between 0.282 679 and 0.282 766, the εHf（t） values range from -1.44 to 1.70, the values of TDM are concentrated in the range of 679-804 Ma, and values of TDM2 are concentrated in the range of 1047-1245 Ma, suggesting that the magma was derived from the mixture of crustal and mantle materials. The characteristics of zircon trace elements indicate that the quartz diorite crystallized in a continental crust environment. Zircon Ce（Ⅳ）/Ce（Ⅲ）ratios of the quartz diorite are concentrated in the range of 0-100, and values of δEu vary in the range of 0.04-0.22, showing the characteristics of reductive rock. It is inferred that the characteristics of reductive magmatic rock of quartz diorite may be one of the reasons why it can not have similar mineralization capability to the ore-forming porphyry.