川西甲基卡区域伟晶岩分异演化及锂成矿过程:来自白云母矿物学的约束

Muscovite as a potential tool for magmatic differentiation and identifying Li mineralization in the Jiajika regional pegmatites,West Sichuan

甲基卡位于松潘-甘孜造山带内,为我国超大型伟晶岩型锂矿床之一,具有较大的经济价值。甲基卡伟晶岩在空间上具有良好的分带,以二云母花岗岩为中心,向外依次为微斜长石伟晶岩带(Ⅰ带)→微斜长石-钠长石伟晶岩带(Ⅱ带)→钠长石伟晶岩带(Ⅲ带)→锂辉石伟晶岩带(Ⅳ带)→白云母伟晶岩带(Ⅴ带)。为了研究甲基卡区域伟晶岩脉空间演化和稀有金属富集规律,本文对各分带伟晶岩的白云母进行了主量、微量元素研究。根据矿物内部结构和化学成分,区域伟晶岩存在两阶段演化:早阶段在Ⅰ带至Ⅳ带形成均一结构的原生白云母;晚阶段Ⅴ带形成具有成分分带的白云母,二者在成分上Li、Rb、Cs含量和K/Rb、K/Cs比值呈现明显差异,表明演化程度明显加大,流体组分比例升高,表明体系由以熔体为主的阶段进入以熔流体为主相对不稳定的阶段。从Ⅰ带至Ⅳ带,原生白云母的K/Rb、Kb/Cs比值降低有限,微量元素Li、Rb、Cs、Ta含量总体略微升高,表明甲基卡区域伟晶岩脉经历了中等程度的结晶分异演化。V带云母的主微量成分呈振荡变化,该现象主要受熔体不混溶过程的控制。总体上,原生白云母均有具有高Li、Cs、B含量的特征,表明初始熔体极具成矿潜力。白云母中K/Rb比值小于等于20或Cs含量大于等于400×10^(-6)可以作为评价Li-Cs-Ta(LCT)伟晶岩发生锂辉石矿化的指标。

Jiajika is of great economic value as one of the super-large pegmatite lithium deposits in China.The Jiajika pegmatite exhibits spatial zoning around the two-mica granite,which from proximal to distal regions are(Ⅰ)microcline pegmatite zone,(Ⅱ)microcline-albite pegmatite zone,(Ⅲ)albite pegmatite zone,(Ⅳ)spodumene pegmatite zone and(Ⅴ)muscovite(lepidolite)pegmatite zone.In order to study the regional evolution of pegmatite and the enrichment mechanism of rare element,the muscovite in pegmatites have been studied by EPMA and LA-ICP-MS.Based on the internal structure and chemical composition,two-stage evolution of regional pegmatites could be recognized:homogeneous primary muscovite in zonesⅠ-Ⅳand muscovite with compositional zoning in zoneⅤ.They show significant differences in the composition of Li,Rb,Cs,K/Rb and K/Cs,indicating a significant increase in the degree of evolution and an increase in the proportion of fluid components.This indicates that the system transitioned from a stage dominated by melt to a relatively unstable stage dominated by fluid.From zone I to zone IV primary muscovite shows a limited decreasing trend in K/Rb and Kb/Cs ratios but slightly relatively increases in Li,Rb,Cs and Ta contents,indicating regional pegmatites experience moderate degree differentiation.The oscillatory changes of muscovite in zone V are mainly controlled by the melt-fluid immiscibility.Overall,primary muscovite at Jiajika has high contents in Li,Cs and B,indicating that the initial melt has great ore-forming potential.The K/Rb ratio≤20 or the Cs content≥400×10^(-6)in muscovite,which can be a geochemical indicator of lithium mineralization in Li-Cs-Ta(LCT)pegmatites.