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
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.
PegmatiteMicasShortwave infraredMagmatic evolutionMineral explorationEastern Tianshan Orogenic Belt
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