The ultra-enriched hafnium in the Himalayan rare-metal pegmatite
Zirconium and hafnium are important critical metals and they are mainly found in the zircon.Zircon-hafnon is a series of complete solid solution because Zr and Hf have similar crystallochemical properties,and is divided into four groups:zircon,hafnian zircon,zirconian hafnon and hafnon based on the Hf#(Hf/(Zr+Hf)atomic ratio in crystallochemical formula)of＜0.1,0.1～0.5,0.5～0.9 and＞0.9,respectively.In contrast to zircon,which is the most common accessory mineral in a variety of felsic rocks,natural Hf-rich zircon and hafnon are extremely rare due to the strong Zr and Hf differentiation and the difficulty of achieving ultra-enrichment of Hf.The zirconian hafnon found in China is limited to the Koktokay No.1 granitic pegmatite(LCT-type),Altai.In this study,zirconian hafnon(Hf# up to 0.67)is newly found in the Tsalung elbaite-lepidolite pegmatite,northern Gyirong pluton in the middle of Himalayan orogen.Based on the occurrence and the chemical composition,four kinds of zircon-hafnon in the elbaite-lepidolite pegmatite are identified:porous and mottled core Zrn-1,with the compositions of zircon and hafnian zircon;common homogeneous Zrn-2 with dark CL images,rounded the Zrn-1 and as the core of the grain,with the composition of hafnian zircon;Zrn-3 and Zrn-4 outermost of the grain,having the composition of hafnian zircon and zirconium hafnon,with the higher Y(up to 4.9％Y2O3)and lower Y content(＜1.6％Y2O3),respectively.All the studied zircons all drop into the magmatic and hydrothermal recrystallized fields in the discrimination diagram of rare-earth elements(REEs).Three distinct stages of magmatic-hydrothermal processes are identified:(1)the crystallization of the primary U-rich zircon;(2)late-magmatic to subsolidus dissolution-precipitation by the action of the fluids;and(3)the strong Zr-Hf differentiation and the crystallization of Hf-enriched rim due to the enriched flux elements(Li,B,F,etc.)and the modified melt condition caused by the abundant crystallization of the Li-F minerals(e.g.elbaite and lepidolite).The magmatic-hydrothermal transition is the most significant stage for the ultra-enrichment of hafnium.In addition,hafnian zircon(Hf# up to 0.27)is also present in the Tsalung spodumene pegmatite and beryl pegmatite.Zircon(Hf#＜0.04)is found in the Tsalung tourmaline-muscovite granite and Yingxionggou granite and pegmatite.It is proposed that magmatic fractionation is a key element for the Zr-Hf differentiation in the melt,resulting in an increased Hf content and a decreased Zr/Hf ratio in zircons.Combined with the paragenesis of the global hafnian zircon and zirconian hafnon,the summary of the compositions of the Himalayan zircons,the indicator of the zircon-hafnon and the relative potential researches of ultra-enriched-Hf system are proposed.
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