新疆阿尔泰将军山天河石花岗岩的岩石成因及其对铷成矿的制约

新疆阿尔泰盛产稀有金属伟晶岩, 但缺乏数量上相当的高分异花岗岩。将军山天河石花岗岩是阿尔泰目前发现的唯一富Rb花岗岩, 但其岩石成因、Rb的赋存状态及其与周边伟晶岩的关系尚不清楚, 从而制约了对Rb富集机理和成矿潜力的认识。据此, 本文对将军山天河石花岗岩和伟晶岩开展了全岩组成、年代学、同位素和矿物学研究。结果显示, 阿尔泰将军山天河石花岗岩为高分异的A型花岗岩, 独居石U-Pb年龄为276.9±2.7Ma, 表明形成于早二叠世。独居石Nd同位素组成(ε_Nd(t)为+4.15~+6.05, 二阶段模式年龄t_DM2为548~708Ma)显示亏损的源区特征, 表明源自新生地壳或有幔源物质的参与。天河石花岗岩与周边的天河石伟晶岩具有空间、时间和成分上的联系, 表明二者具有分异演化上的成因关系。将军山天河石花岗岩类型以及天河石伟晶岩矿物组合指示伟晶岩属于NYF族(富Nb、Y、F)。天河石花岗岩与伟晶岩都显示出Rb的矿化, 矿石矿物主要为天河石和多硅白云母。从花岗岩到伟晶岩, 天河石和云母中的Rb逐步富集。岩浆高度分异是天河石花岗岩Rb富集的主要机制, 但新生地壳物源或幔源物质的加入从源区上限制了Rb的矿化程度。除天河石花岗岩以外, 阿尔泰未来的Rb资源勘查应同时关注稀有金属伟晶岩中的钾长石和云母类矿物。

Petrogenesis of amazonite granite and its constraints on rubidium mineralization in Jiangjunshan,Xinjiang Altai

The Xinjiang Altai is well known for the abundance of rare metal pegmatites, but lacks a considerable number of highly differentiated granites. The Jiangjunshan amazonite granite is the only known Rb-rich granite in Xinjiang Altai. However, its petrogenesis, Rb occurrence and its relationship with the surrounding pegmatites are still unclear, which restricts the understanding on Rb enrichment mechanism and the assessment of ore-forming potential. To address these issues, whole rock composition, chronology, isotope and mineralogy of amazonite granite and pegmatite in Jiangjunshan area were studied. Results demonstrate that the amazonite granite is a highly differentiated A-type granite with monazite U-Pb age of 276.9±2.7Ma, indicating that it was formed in the Early Permian. The monazite Nd isotope composition (εNd(t)=+4.15~+6.05, tDM2=548~708Ma) has the characteristics of the depleted source, indicating the source from juvenile crust or mixed with mantle-derived materials. The amazonite granite is spatially, temporally and compositionally related to the surrounding amazonite pegmatite, indicating that they have a genetic relationship in differentiation and evolution. The geochemical type of the amazonite granite and the mineral assemblage of the amazonite pegmatite both support the NYF (Nb, Y, F-rich) affinity of the amazonite. The Jiangjunshan amazonite granite and the pegmatite both show Rb mineralization, with major ore minerals of amazonite and phengite. From granite to pegmatite, the Rb contents in amazonite and phengite both increase gradually. High degree differentiation of magma is the dominating mechanism for Rb mineralization in the amazonite granite and pegmatite, but the mineralization degree is limited by the source derived from juvenile crust or mixed by mantle materials. Besides amazonite granite, we suggest that potassium feldspar and mica minerals in rare metal pegmatite should be paid attention to the exploration of Rb resources in Xinjiang Altai in the future.