内蒙古巴尔哲地区碱性花岗岩的源区性质及其对Zr、REE和Nb等稀有金属元素超常富集的制约

Alkaline granite is not only a widely used lithoprobe, but also an important ore-bearing formation. Therefore, systermatic investigation on the alkaline granitic rocks can be used to reveal the regional geodynamic setting and rare-metals mineralization. Both mineralized (801 giant Zr-REE-Nb deposit) and barren (802 and 803 granite porphyry) alkaline granites of Late Mesozoic were documented in the Baerzhe area, Inner Mongolia. This proposal focus on the Yanshanian alkaline granitic rocks in the Baerzhe area, in terms of petrology, elemental and isotopic geochemistry, to reveal the nature of regional alkaline magma source and the mineralization mechanisms of the rare-metal elements.Systematic study on the mineralogy and petrology of the barren alkaline granitic intrusives can reveal the nature and origin of the primary alkaline magma and the regional geodynamic setting.In addition, the petrography and compositions of the melt inclusions, hosted in early magmatic zircon in both mineralized and barren alkaline granitic intrusives, can help to reveal the primary compositions of the regional alkaline granitic magma and contributions to the super-enrichment of rare-metal elements.Furthermore, comprehensive determination on the typical minerals and hosted melt inclusions, which are formed in different stages during magmatic-hydrothermal evolution, can effectively show the distributions and enrichment patterns of ore-forming elements throughout the alkaline granitic intrusives, and also can reveal the roles of magmatic fractionation in the rare-metals mineralization and the consequence for the formation of giant Zr-REE-Nb deposit in the Baerzhe area.

内蒙古巴尔哲地区除了分布有含超大型Zr-REE-Nb矿床的801碱性花岗岩体之外，还分布有同时期的未成矿802和803碱性花岗斑岩体。本项目以巴尔哲地区燕山期发育的成矿和未成矿的碱性花岗岩体为研究对象，揭示区内碱性岩浆的源区性质，探讨稀有金属元素的超常富集和成矿机制。通过对未成矿碱性花岗岩体进行系统的矿物学和岩石学的研究，示踪碱性花岗岩原始岩浆的化学组成和源区特征，揭示其形成的地球动力学背景。通过成矿和未成矿碱性花岗岩体早期岩浆锆石中的熔体包裹体地球化学组成的对比研究，揭示区内碱性花岗质岩浆的物质组成及其对稀有金属元素超常富集的贡献。通过对不同演化程度碱性花岗岩体中不同演化阶段形成的典型矿物及其内包裹体物质组成的系统研究，揭示成矿稀有金属元素的分布和富集规律，进一步探讨岩浆演化作用对稀有金属元素大规模矿化的影响，阐述巴尔哲地区超大型Zr-REE-Nb矿床的形成机制。

本项目通过对巴尔哲801超大型稀有稀土矿床和中国东部中生代未成矿的碱性花岗岩体进行系统的矿物学、岩石学、元素和同位素地球化学等综合对比研究，特别是通过对成矿和未成矿岩体的岩浆-热液演化和矿物学演化进行综合对比对比研究。项目现已取得的成果包括，可概括为如下三个方面：(1) 碱性花岗质岩石形成的深部地质过程及其地球动力学背景约束；(2) 碱性花岗岩岩浆-热液演化过程中矿物的物理化学性质；(3) 碱性花岗质岩浆的分异和演化及其对稀有金属元素强烈矿化的作用。本项目通过对巴尔哲801矿床和围岩以及其周边未成矿802岩体和碾子山岩体开展系统的的Sr-Nd-Pb同位素和锆石U-Pb年龄或O-Hf同位素的研究，制约其原始岩浆性质、来源和形成的地球动力学背景；通过对比研究中国东部晚中生代未成矿的福建魁歧碱性花岗岩体和河北响山碱性花岗岩体，反演未成矿碱性花岗岩浆系统在其岩浆-热液演化过程中的熔体-流体特征和矿物学演化；通过对801矿床进行详细的矿床解剖和系统的岩石学和矿物学分析，揭示了碱性花岗质岩浆的分异和演化及其对稀有金属元素强烈矿化的作用。项目已取得的三方面成果分别涉及碱性花岗质岩浆的来源、分异和演化、及其岩浆-热液演化过程，这有助于我们对碱性岩浆性质及其稀有金属成矿系统的深刻认识，也有助于相关关键矿产资源的勘探和开发。