A study of the fine Zr-Nb-Be-REE mineralizing processes of the Baerzhe deposit,China
The Baerzhe alkaline granite-hosted rare-metal deposit represents world-class resources of Zr,Nb,Be,and REE(rare earth elements),which experienced multi-stage ore-forming processes.Zr was concentrated during magmatic stage,manifested by fractionation of primary elpidite in an agpaitic transsolvus granite that always suffered Na metasomatism.In stage I,Zr was redistributed,but not reconcentrated,during the breakdown of sodium zirconosilicates(elpidite)into zircon-quartz pseudomorphs along Na-metasomatism.U-Pb chronology of hydrothermal zircon in zircon-quartz pseudomorphs indicates that the stage Ⅰ mineralization occurred at 123.9±0.6Ma.Nb-Be-REE mineralization occurs as a variety of minerals that are disseminated through all the hematized transsolvus granites and were precipitated in three sequential stages(stages Ⅱ-Ⅳ),the formation of HREE-dominant phases generally preceding LREE-dominant phases.Specifically,stage Ⅱ is characterized mainly by Nb,Be and HREE mineralization accompanied by hematite alteration.The stage Ⅱ mineralization,represented by hingganite-(Y/Ce),aeschynite-(Y)and columbite-(Fe),occurs mainly as quartz-dominant,Nb-Be-REE-rich pseudomorphs,the precursor minerals of which are not Nb-Be-REE-bearing minerals.The following stage Ⅲ is defined by euxenite-group minerals,monazite and bastnäsite crystallization.U-Pb chronology of this hydrothermal monazite indicates 122.8±0.6Ma of stage Ⅲ.Stage Ⅳ is represented by fluocerite and synchysite that partially replaced bastnäsite.The multi-stage hydrothermal redistribution and enrichment of Nb,Be and REE are important factors for the development of a variety of rare-metal mineral associations and high-grade Nb-Be-REE mineralization at Baerzhe.That the Nb-Be-REE-rich pseudomorphs do not contain zircon,and their distribution is independent of the occurrence of snowball quartz,Na metasomatism and zircon-quartz pseudomorphs,in conjunction with the low contents of Nb,Be and REE in amphiboles at Baerzhe,preclude leaching of preexisting sodium zirconosilicates or amphiboles as significant sources of critical metals such as Nb,Be and REE.Based on the changes in amphibole trace-element contents from the various granitic phases and the compositions of silicate melt inclusions,the enrichment of Nb and REE may result from the occurrence of immiscibility of fluoride melts from silicate magma,coupled with hydrothermal leaching of rare metals from both the silicate and fluoride melts.Therefore,it is necessary to conduct detailed research on melt/fluid inclusions in the Baerzhe deposit,with emphasis on the sources of critical metals,especially Be,during the process of magmatic-hydrothermal mineralization.
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维普
