湖南香花岭地区厚层状云英岩的成因及其钨锡稀有金属成矿意义

Greisen is one of the important hosts to the W-Sn-rare metal mineralization, occurring on the cupola and in the inner of the granitic body as the mass and the vein, formed by the granite-related hydrothermal process in the course of which feldspars and Fe-micas became instable. The Laiziling and Jianfengling granite stocks were emplaced in the Xianghualing district of Hunan Province. The vertical zonation is well developed within both the stocks. From the lower zone upwards to the peak of the stock, the following zones are successively observed: granites, greisen and pegmatite. The greisens are thicker than the usual, nearly horizontal，specifically zoned on account of granularity and mineral assemblages, providing the obvious signs of magmatism, and host various types of mineralizations. The project will study the particularly layered greisen by the combined researches of field geology, petrology, geochemistry and mineralogy, to define the zones of the greisen based on qualitative or quantitative criteria, to confirm the detailed physicochemical information during the magmatic-hydrothermal stages, and to discuss the formation mechanism responsible for the greisens. It is expected to understand the various mineralizations among the different greisen zones and confirm the causes that attracted the variety by the investigations on the occurrences, the structures and the chemical compositions of the ore elements-bearing minerals. It is also very useful to contrast the greisen with the granite, pegmatite and dykes in the same stocks, in order to explain their association of the chronology, source and origin and the implications of the granitic system to the W-Sn-rare metal mineralizations.

云英岩作为花岗岩顶部和内部常见的热液蚀变产物，通常呈脉状或面型产出。然而，湖南香花岭地区癞子岭和尖峰岭两个具有明显分带特征的花岗岩株，顶部出现了厚达50米的似层状云英岩。它具有与热液作用明显不同的岩石学和矿物学特征，钨锡稀有金属矿物广泛分布，展示出重要的科学研究意义。本项申请，拟选取这类特殊的云英岩作为研究对象，通过综合岩石学、矿物学和地球化学的精细研究，确定云英岩的分带特征，查明云英岩形成过程的物理化学条件，揭示厚层状云英岩在花岗质岩浆-热液演化中所处的阶段，探讨其形成机理。通过研究富含成矿元素矿物的赋存状态和形成阶段，查明不同特征云英岩的钨锡稀有金属成矿作用差异，揭示影响这些差异的主导因素。最后，通过云英岩与岩株中花岗岩、伟晶岩及岩脉的对比，揭示它们在时间、空间、物源、成因上的联系，探寻花岗岩体系对钨锡稀有金属成矿和找矿勘探的意义。

湖南香花岭癞子岭岩体是一个典型的具有分带性的花岗岩体，从底部到顶部分别出露有碱性长石花岗岩和钠长石花岗岩，而顶部有约40m厚的层状云英岩，为了解这种特殊结构和矿物组成的云英岩，本项目开展的野外、岩石学、矿物学、岩石地球化学和地质年代学等方面的工作，同时还与周围的其他类型的云英岩进行了对比，得出了一下的结论：（1）确认了厚层状Li-F云英岩是高演化的岩石，存在于岩浆-热液的过渡阶段，是钠长花岗岩中继续演化的产物，从云母和全岩的Li同位素结果都显示出与后期石英-铁锂云母脉不同的成因，这为理解云英岩的成因模型提供了新概念。（2）癞子岭花岗岩中的W和Nb-Ta成矿作用共生出现，包括了很多标志性的W-Nb-Ta氧化物，如钨铌锰矿、骑田岭矿和富Nb-Ta黑钨矿，晚期的钠长花岗岩和云英岩中出现含Nb-Ta黑钨矿和铌铁矿族矿物，它们的成分变化体现了岩浆中稀有金属元素的原始富集和在富集流体的岩浆环境中结晶，而在热液期形成了贫Nb-Ta黑钨矿。（3）利用锆石和铌铁矿族矿物的U-Pb定年结果，确定了华南最年轻的Nb-Ta成矿作用（~89 Ma），为勾画出华南不同期次的Nb-Ta成矿作用补充了重要信息。（4）综合稀有金属花岗岩的造岩矿物的研究结果，我们认为云母是重要的稀有金属成矿作用的标志性矿物。本项目完善了稀有金属的成矿作用体系，并为找矿勘探提供了理论的基础，一共发表了11篇论文(3篇第一标注)和1篇会议论文，培养两位硕士生。