西藏邦铺铅锌矿区石英二长斑岩锆石U-Pb年龄、Hf同位素和稀土元素特征及其地质意义

摘要

The Bangpu Mo (Cu) polymetallic deposit is a large-size porphyry deposit newly discovered in the Gangdise belt, where there exist many intrusive rocks of different types and stages. The investigation of the Pb-Zn deposit located in the contact zone in the southeast part of the ore district has been very insufficient, in despite of the attention and exploitation paid to it. In this paper, U-Pb dating of LA-ICP-MS zircon and Hf isotopes and trace rnelements of zircon were used to investigate ore-bearing quartz-monzonite porphyry outcropped in the lead-zinc mining area, and the weighted average age of the 23 zircons 206Pb/238U is (15.43 ± 0.12) Ma (MSWD = 2.7, n = 23), suggesting a Miocene magmatic product. After the completion of the zircon dating, part of the dated zircons were analyzed systematically for the in-situ analysis of Hf isotopes. The 175Hf /177Hf ratio varies in the range of 0.28279-0 .282913, and εHf(t) varies in the range of 0.6 - 5.0. The testing results of Hf isotope and the Hf isotope of geochemical reservoirs and depleted mantle are similar, indicating that the source area or the quartz monzonite porphyry in this area mainly came from the depleted mantle, but in the process of magmatic evolution it was contaminated by the lower crust. The analyses of REE show the depletion of light REE and enrichment of heavy REE as well as the obvious existence of positive Ce anomaly and negative Eu anomaly, suggesting zircons of typical magmatic genesis. The results are consistent with the previous study of the Mo (Cu) mining area, and hence it is inferred that lead-zinc ore and molybdenum (copper) ore are products from the same porphyry system. It is shown that this deposit is somewhat identical with Jiama and Qulong large-size porphyry deposits in the region, with unified geodynamic background and deep magmatic activity.%邦铺钼(铜)多金属矿床是冈底斯带新近发现的大型斑岩型矿床,矿区内发育大量不同类型、不同期次的侵入岩体.矿区南东方向接触带的铅锌矿床,尽管最早受到关注并进行开采,但研究程度却仍然相对较低.文章采用LA-ICP-MS锆石U-Pb测年和锆石Hf同位素、微量元素,首次对铅锌矿区内出露的含矿石英二长斑岩进行研究,23颗锆石206Pb/238U加权平均年龄为(15.43±0.12) Ma(MSWD=2.7,n=23),系中新世岩浆产物.在完成锆石测年后,同时对部分所测锆石进行了系统的Hf同位素原位分析,测得176Hf/177Hf值为0.282790～0.282913,εHf(t)变化范围为0.6～5.0,Hf同位素测试结果与亏损地幔地球化学储库的Hf同位素特征较为相似,表明石英二长斑岩的源区具亏损特征,但在岩浆演化过程中遭受了下地壳物质的混染.稀土元素结果显示轻稀土亏损,重稀土富集,明显的正Ce异常,Eu负异常,属典型的岩浆成因锆石.与前人对钼(铜)矿区的研究结果一致,故推断铅锌矿与钼(铜)矿为同一斑岩系统的产物.结合与区域上相邻的甲玛、驱龙矿床,该矿床显示出与甲玛、驱龙等超大型斑岩型矿床具有统一的地球动力学背景及深部岩浆活动.