Geochemical characteristics of three profiles of main and east ore bodies of Bay-an Obo deposit and its geological significance
In recent years,the debate on the genesis of the H8 rock mass in the Bayan Obo Nb-REE-Fe deposit has gradually subsided,but the mineralization process of the deposit and the mechanism of REE enrichment are still the focus of academic circles.In this paper,systematic rock geochemical work is carried out on three typical pro-files of carbonatite,aegirine alteration zone and slate of Jianshan Fm in Bayan Obo deposit.We found that the trace elements and REE distribution curves of ore-bearing carbonatite,fenite and iron ore are basically consistent.Slate of Jianshan Fm near H8 rock often has higher REE content and similar trace element distribution mode to the carbonatite.The fractionation of REE in various rock is obvious.The variation of LaN/NdN ratio in the whole rock shows that La is more enriched in the interior of H8 rock mass,and Nd is more abundant in the edge.BSE images and electron microprobe analysis show that the REE minerals(mainly monazite,bastnaesite,etc.)in the H8 rock mass can be divided into two groups.One group is relatively rich in La,showing a hypidiomorphic to xenomorphic-granular,star-shaped distribution;the other group is relatively rich in Nd,showing a xenomorphic-granular and distributing as veins.The enrichment characteristics of different elements in the two groups of minerals may represent the change of fluid environment during crystallization.The results of the above experiments combined with geological exploration show that the origin of the initial metallogenic materials of the Bayan Obo deposit is consistent with that of the H8 carbonatite,both of which are derived from Mesoproterozoic carbonatite magmatic activity.Different types of REE minerals correspond to two different REE mineralization stages:dolo-mite carbonatite stage and fluorite mineralization stage.
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