西藏知不拉矽卡岩矿床成矿母岩的新认识--来自花岗闪长岩岩相学、年代学和Hf同位素的指示

摘要

The Zhibula skarn-type copper polymetallic deposit is a large copper deposit. It is located 2–3 km south of the Qulong porphyry copper deposit, in the middle section of the Gandise metallogenic belt in Tibet. The ores are commonly bedded, stratoid and vein-like hosted in the interformational detachment zone between tuff and marble and in the fracture zone. The granodiorite was discovered lately in the drill holes. The contact zone between granodiorite and tuff or marble is skarnized. The skarn-type ore deposit is closely related to the granodiorite as was demonstrated by the gradual change from the tuff, hornfels, and skarn, to the skarnized granodiorite. LA-ICP-MS zircon U-Pb dating yields a weight average 206Pb/238U age of 16.0±0.4 Ma, which is close to the Re-Os isochron age (16.9±0.6 Ma) of molybdenites from the ores. The granodiorite hasεHf(t) values in range of 3.2–12 and single-stage model ages between 209–563 Ma, which are similar to those of the Miocene intrusives in the Qulong areas. The zircon Hf isotopic compositions of the granodiorite indicate that the magma is likely resulted from partial melting of the juvenile lower crust. The granodiorites are determined as the ore-forming intrusive of the Zhibula skarn-type deposit, and they are derived from the same magma system with those associate with Qulong deposits. Both of them are are of hydrothermal origin.%知不拉矽卡岩铜多金属矿床位于西藏冈底斯成矿带中段驱龙斑岩铜矿南侧2~3 km 处,铜资源量接近大型规模,矽卡岩矿体主要呈似层状、透镜状和大脉状产出,主要受角岩化凝灰岩–大理岩岩性界面和断层破碎带控制。本文以最新勘查工作中钻孔揭露的花岗闪长岩为研究对象,通过岩相学观察、LA-ICP-MS锆石U-Pb定年和Hf同位素分析,对知不拉矿床成矿母岩认识给出新指示。花岗闪长岩与角岩化凝灰岩、大理岩的接触带可见不同程度的矽卡岩化发育,由角岩化凝灰岩至花岗闪长岩有角岩化凝灰岩–矽卡岩化角岩–矽卡岩–矽卡岩化花岗闪长岩–花岗闪长岩的分带规律,岩体内可见长石被石榴子石、绿帘石等矿物交代,指示该岩体与矽卡岩矿体形成关系密切。测试结果显示,花岗闪长岩内锆石15个有效测点给出了206Pb/238U加权平均年龄为16.0±0.4 Ma,该年龄代表花岗闪长岩的结晶年龄,与矽卡岩内成矿年龄16.9±0.6 Ma在误差范围内一致；锆石的176Hf/177Hf(i)值为0.2829~0.2831,εHf(t)为3.2~12,单阶段模式年龄tDM在209~563 Ma 之间,具有与驱龙中新世侵入岩相似的 Hf 同位素地球化学特征,岩浆可能起源于软流圈物质上涌引起的新生下地壳部分熔融。知不拉矽卡岩矿床的成矿母岩为中新世花岗闪长岩,与驱龙斑岩矿床属于同一岩浆房演化出溶岩浆,分别是侵位于向形和背形构造中的热液作用产物。