云母对伟晶岩型关键金属矿床的成因和勘查指示:以东天山镜儿泉伟晶岩型Li-Be-Nb-Ta矿床为例

Indication of mica minerals for the genesis and exploration of critical metal pegmatite deposits:A case study of the Jingerquan Li-Be-Nb-Ta pegmatite ore-field,Eastern Tianshan,Xinjiang

云母是伟晶岩的重要造岩矿物,也是伟晶岩稀有金属成矿作用的重要标志矿物。东天山造山带与举世瞩目的阿尔泰伟晶岩成矿省有很好的对比性,具有很高的伟晶岩型关键金属矿床成矿潜力。本文重点研究了东天山地区目前最大的伟晶岩矿床——镜儿泉Li-Be-Nb-Ta矿床中云母对于矿床成因和找矿勘查的指示作用。镜儿泉矿化伟晶岩中的云母类型均为白云母。电子探针(EPMA)分析表明,矿区白云母具有从多硅白云母到普通白云母的变化趋势。激光剥蚀电感耦合等离子质谱(LA-ICP-MS)分析表明,云母的Rb、Cs含量和K/(Rb+Cs)、Nb/Ta比值能够指示伟晶岩的演化过程;K/(Rb+Cs)指标可以很好区分花岗岩和伟晶岩,Nb/Ta指标可以很好的识别伟晶岩成矿带。云母Li、Be、B、Nb、Ta含量增加能够反映伟晶岩中对应元素富集过程,且云母中Li、Fe、Mn含量的骤降能够反映锂辉石、铌钽铁矿等矿物的沉淀。云母分离结晶可能是导致高分异伟晶岩具有低Nb/Ta值,并发生Nb-Ta矿化的关键;从花岗岩到伟晶岩,云母的分离结晶降低了残余熔体中Ti含量,抑制了具有高Nb、Ta分配系数的含Ti氧化物的生成,为Nb、Ta矿化奠定了基础。云母的Nb、Ta含量变化指示伟晶岩Nb-Ta富集过程解耦,沉淀过程耦合。白云母短波红外光谱2200nm位置的Al-OH特征吸收峰具有良好的指示伟晶岩矿化的能力。镜儿泉伟晶岩成矿带中的白云母具有明显更小的Al-OH特征吸收峰位值(<2202nm)。伟晶岩成矿带中铌钽铁矿分离结晶使与之共生的白云母Fe含量减少是导致Al-OH特征吸收波长偏小的主要原因。

Micas are important rock-forming minerals in pegmatites,and they are also important indicators of rare metals metallogenesis in pegmatites.The Eastern Tianshan Orogenic Belt which has a good contrast with the Altai pegmatite ore-forming province,has a high metallogenic potential of critical metal pegmatite deposits.In this paper,we focus on the significance of mica in Jingerquan Li-Be-Nb-Ta deposit,the largest pegmatite deposit in the Eastern Tianshan Orogenic Belt,as an indicator for the genesis and prospecting of the deposit.All of the micas of the mineralized pegmatites in Jingerquan are white mica.Electron microprobe analysis(EPMA)shows that the white mica in Jingerquan has the evolution trend from phengite to muscovite.Laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)analysis shows that the concentrations of Rb and Cs and the values of K/(Rb+Cs)and Nb/Ta of micas can indicate the evolution of pegmatites.K/(Rb+Cs)values of micas can be used as an index to distinguish pegmatites from granites.Nb/Ta values can identify pegmatite metallogenic belt well.Increase of the concentrations of Li,Be,B,Nb and Ta in mica can reflect the enrichment process of these elements in pegmatites,and the sharp decrease of Li,Fe and Mn concentrations in mica reflects the precipitation of spodumene and niobium-tantalite.Mica crystallization plays a key role in decreasing the Nb/Ta value and Nb-Ta mineralization in highly evolved pegmatites.From granites to pegmatites,the crystallization of micas lowers Ti concentrations in the melt and inhibits the crystallization of Ti-bearing oxides that have high partition coefficient of Nb and Ta,which lays the foundation for Nb and Ta mineralization.The changes of Nb and Ta concentrations in micas indicate that the Nb and Ta enrichment process in pegmatites is decoupled but the segregation behavior is coupled.The Al-OH characteristic absorption peak near 2200nm position of the short wavelength infrared spectrum of white mica has a good application in indicating the metallogenic belt of pegmatites.The white mica in Jingerquan pegmatite metallogenic belt has a significantly lower Al-OH absorption peak value(<2202nm).The decrease of Fe concentrations of white micas associated with niobium-tantalite crystallization in pegmatites metallogenic belt is the main reason for the shorter wavelength of Al-OH characteristic absorption peak position.