锆石微量元素示踪锂成矿岩浆-热液演化过程

锂成矿过程中的岩浆- 热液演化过程目前仍不清楚。锆石作为花岗岩和伟晶岩中普遍存在的副矿物,其微量元素成分演化可以记录岩浆演化过程。本文以松潘- 甘孜构造带可尔因地区的花岗岩和伟晶岩中的锆石作为研究对象,对其进行原位微量元素分析。锆石的形态结构特征显示花岗闪长岩与花岗岩中的锆石为岩浆锆石,而伟晶岩中的锆石受到不同程度热液作用影响。元素分析结果显示,从太阳河花岗闪长岩到可尔因二云母花岗岩到无锂辉石伟晶岩到锂辉石伟晶岩,锆石中的稀有金属元素(Li、Sn、Nb、Ta和Hf)和U含量逐渐升高,锆石的Zr/Hf比值逐渐降低。除此之外,太阳河花岗闪长岩和可尔因二云母花岗岩中的锆石具有较低的Fe含量,而无锂辉石伟晶岩和锂辉石伟晶岩中的锆石具有明显升高的Fe含量,且锆石Fe含量与稀有金属元素含量具有正相关关系,反映出热液作用越强,稀有金属含量越高的特征。热液导致伟晶岩中锆石的U- Pb体系受到改造,使得获得的年龄不可靠,而伟晶岩铌钽矿U- Pb年龄(210 Ma)相对可靠。基于锆石的微量元素特征,我们认为太阳河花岗闪长岩与锂成矿伟晶岩没有成因联系,二云母花岗岩与锂成矿伟晶岩具有成因联系,但是二云母花岗岩不是成矿伟晶岩的直接母岩。二云母花岗岩和无锂辉石伟晶岩Li锆石/Li全岩接近甚至超过1,指示其与全岩成分不平衡,表明岩浆演化过程中曾形成富锂熔体,且富锂熔体的形成与液态不混溶作用无关,而是岩浆分异的结果。富锂熔体可能在区域拆离断层作用下从岩浆体系中分离,随后不断演化形成了锂辉石伟晶岩,而残余岩浆逐渐冷却结晶形成了二云母花岗岩和无锂辉石伟晶岩。基于锆石中的微量元素可以有效示踪锂成矿过程,并且利用碎屑锆石的微量元素特征还可以指示区域富锂岩浆的存在及其形成时代,有望成为示踪区域锂成矿的一种新方法。

Tracking the magmatic- hydrothermal evolution during lithium mineralization with zircon trace elements

The understanding of magmatic- hydrothermal evolution during lithium mineralization remains unclear. Zircon, a prevalent accessory mineral in granites and pegmatites, has the potential to provide insights into the magmatic evolutionary process through its trace element composition. In this research paper, we studied the trace element compositions of zircons from granites and pegmatites in the Keeryin area of the Songpan- Ganzi orogenic belt. The morphological and textural features of the zircons indicate that zircons from granodiorite and granite are magmatic in origin, whereas those from pegmatite have been influenced to varying degrees by hydrothermal fluid. Our findings from the trace element analysis reveal a gradual increase in the contents of rare metal elements (Li, Sn, Nb, Ta, and Hf) and U in zircon as the Zr/Hf ratio decreases. This trend is observed from Taiyanghe granodiorite to Keeryin two- mica granite to spodumene- free pegmatite to spodumene- bearing pegmatite. In addition, zircons from Taiyanghe granodiorite and Keeryin two- mica granite exhibit low Fe contents, whereas zircons from spodumene- free pegmatite and spodumene- bearing pegmatite show significantly higher Fe contents. A significant positive correlation exists between Fe and the concentrations of rare metals, indicating a stronger hydrothermal influence leads to a higher rare metal content in zircon. The hydrothermal fluids have impacted the U- Pb system of zircons in pegmatites, resulting in the obtained ages being unreliable. However, the U- Pb age of columbite- tantalite (210 Ma) in pegmatite is more reliable. Based on the trace element composition of zircons, we propose that the Taiyanghe granodiorite is not related to the formation of spodumene pegmatites, whereas the two- mica granite has a genetic relationship with spodumene pegmatites. However, the two- mica granite is not the direct parental rock of the mineralized pegmatites. The Lizircon/Liwhole- rock ratios of the two- mica granite and spodumene- free pegmatite are close to or even exceed 1, indicating that they are not in equilibrium. This suggests that a Li- rich melt was once formed during the magmatic evolution. Furthermore, this Li- rich melt was not a result of melt immiscibility, but rather fractionation. The Li- rich melt could have separated from the magmatic system through a regional detachment fault and subsequently evolved to form spodumene- bearing pegmatite, while the residual magma crystallized to form two- mica granite and spodumene- free pegmatite. Therefore, trace elements in zircon can effectively trace the magmatic- hydrothermal evolution during lithium mineralization. Furthermore, the trace element compositions of detrital zircon can also indicate the presence of lithium- rich magmas, providing information about their formation age and source. This approach may serve as a new method for tracing lithium mineralization.