Geochronology, source features and the characteristics of fractional crystallization in pegmatite at the Qongjiagang giant pegmatite-type lithium deposit, Himalaya, Tibet

Abstract

喜马拉雅新生代淡色花岗岩带是近年来提出的与高度结晶分异、异地深成淡色花岗岩有关的稀有金属战略远景区，目前其金属组合以铍-铌-钽（-锡-钨）为主。秦克章等（2021a）报道了在高喜马拉雅带珠峰地区发现的琼嘉岗锂矿，是喜马拉雅首例具有工业价值的伟晶岩型锂矿。本次研究重点揭示喜马拉雅琼嘉岗伟晶岩型锂矿的成矿特征、形成时代和源区特征。琼嘉岗矿区矿石矿物主要为锂辉石、铌铁矿-铌锰矿、少量锡石和绿柱石，特征性长柱状锂辉石主要产于块体微斜长石+锂辉石带和分层细晶岩带内。琼嘉岗锂辉石伟晶岩各结构分带的K/Rb含量较为相似，锂含量从边部细粒钠长石带（~100×10^-6）到分层细晶岩带（~1000×10^-6），再到块体微斜长石+锂辉石带（>3000×10^-6）逐渐升高，而Cs含量逐渐降低。独居石和铌钽铁矿族矿物LA-ICPMS定年结果显示，琼嘉岗锂辉石伟晶岩形成于新喜马拉雅阶段早期（25~24Ma），与高喜马拉雅地区淡色花岗岩时代相近。矿物化学和独居石Nd同位素结果显示该稀有金属伟晶岩结晶于高度演化的花岗伟晶岩熔体，源区特征与高喜马拉雅结晶岩系一致。本研究所揭示的琼嘉岗成矿特征、形成时代和源区特征将为高喜马拉雅其它地区找寻大型花岗伟晶岩型锂矿提供重要借鉴意义。;Recently, the Himalayan leucogranite belt has been recognized as the potential prospecting area of the rare-metal mineralization related to the newly proposed petrogenesis of highly fractionated granite. Currently, the exploration aim is mainly targeting at the beryllium-niobium-tantalum (tin-tungsten) assemblage, such as Cuonadong deposit. Qin et al. (2021a) reported a discovery of the Qongjiagang giant lithium pegmatite deposit in Himalaya, Tibet, raising more attention to the spodumene pegmatites in the Higher Himalaya Belt. The present work reveals the geological features, age of ore formation and source features of mineralizing pegmatites at Qongjiagang. Our result shows the main ore minerals at Qongjiagang are dominated by spodumene and columbite group minerals, with minor amounts of cassiterite and beryl. The pegmatite zones hosted by a majority of spodumene minerals at Qongjiagang are identified as a massive microcline+spodumene zone and a layered aplite zone. The K/Rb ratios of different rocks in the internal zonation patterns from rim to core remain to be similar, and the lithium concentrations rise from ~100×10^-6 at the saccharoidal albite zone, through ~1000×10^-6 at the layered aplite zone, to more than 3000×10^-6 at the massive microcline+spodumene zone; however, the cesium contents drop, respectively. U-Pb dating results upon monazite and columbite group minerals show the Qongjiagang Li-pegmatite was formed in the initial duration of the Neo-Himalayan stage (25~24Ma), similar to most dating results at the Higher Himalayan leucogranite. Mineral chemistry and monazite Nd isotopic compositions suggest Qongjiagang Li-pegmatite was crystallized in a highly evolved granitic pegmatite melt, with source features consistent with the Higher Himalayan Crystalline. Our research at the geological features, formation age and source characteristics at Qongjiagang lithium pegmatite deposit gives more implication of ore-forming mechanism and exploration targeting for finding large granitic pegmatitic lithium mines in the Higher Himalaya region of China.