Discovery of cyanobacteria fossils in the Bayan Obo Fe-Nb REE deposit and its geological significance

Abstract

<p indent=0mm>The Bayan Obo deposit in China is known for its huge Fe-Nb-REE resource. The genesis of the ore-hosted H8 dolomite is of great significance to the understanding of the Fe-Nb-REE mineralization. Generally, there are two opposite views established: (1) Intense differentiated mantle-derived carbonates as intrusions or submarine volcanics, and (2) sedimentary origin which was metasomatized by extreme REE-enriched mantle fluids sourced from carbonatitic magmas. However, neither of them could well interpret the fuzzy information given by geochemistry and Fe-Nb-REE mineralization, hence a definite sign should be found. This paper reports the newly discovered cyanobacteria fossils in the massive Fe-Nb-REE ores from the western ore deposit and provides detailed petrographic studies of the cyanobacteria and the spatial relationship with the Fe-Nb-REE mineralization and minerals, aiming to give a reasonable interpretation for the genesis of H8 dolomite. The samples are massive Fe-Nb-REE ores, composed mainly of microorganism dolomite fragments, massive Fe-Nb-REE layers, and coarse-grain dolomite veins. The cyanobacteria is distributed in the “dirty” fine-grained dolomites, consisting of a row of ellipsoidal cells, with a maximum diameter of <sc>~131 μm</sc> and a length of <sc>614 μm.</sc> The outline of the fossil is long columnar, without bifurcation, with a cylindrical front end and pointed back end. The morphology of the fossil is similar to the cyanobacterial filaments. The cell structure is clear under the BSE images, with Ca-Si mineralization for the cell wall and Fe-Mg carbonatization for the cavity. Two groups of cyanobacteria could be distinguished based on their different colors under a polarizing microscope. One group shows yellow translucent probably related with the calcite mineralization and the other group is black opacity related with the Fe-Mg carbonatization. Accessory minerals in the cyanobacteria dolomites include calcite, magnetite, hematite, pyrite, REE minerals, barite, and other carbonate and sulfate minerals. Detailed works reveal that the cyanobacteria dolomites and massive Fe-Nb-REE layers are mutually contained and then filled by coarse-grain dolomite veins along the cracks, based on that (1) their host dolomites show very clear BSE boundary with different components, Mg-Ca vs. Fe-Mn, (2) fragments of the Fe-Nb-REE layers could be found in the cyanobacteria dolomites, while cyanobacterial bioclasts are developed in the Fe-Nb-REE layers, and (3) cyanobacteria fossils are included in coarse-grained dolomites by recrystallization of fine-grained dolomites. Therefore, the cyanobacteria dolomites and massive Fe-Nb-REE layers should be syngenetic sedimented in a locally turbulent shallow sea setting. However, we would like to point out that the massive Fe-Nb-REE layers are not the original composition of sedimentary, but the products of modification by igneous carbonatites and the original composition are most likely to be Fe-Mn carbonates. In this paper, we tend to consider that the Bayan Obo H8 dolomite may be composed dominantly of iron-bearing carbonate formation, and is a classic case of microbial dolomite. Furthermore, photosynthesis is needed during the growth of cyanobacteria, therefore, a depth of < <sc>50 m</sc> is the best speculation for the sedimentary of H8 dolomite. The photosynthesis of cyanobacteria was important for the alteration of the paleoenvironment. Extra oxygen produced by the microbial activity could promote the precipitation of Fe-REE minerals within the dolomite. The magmatic emplacement of Nb-REE rich carbonatites and hydrothermal fluid may have facilitated Fe deposition and contributed some iron. Therefore, the life activities of cyanobacteria may indirectly control the Fe-REE migration and mineralization process in Bayan Obo, thus forming the protore of Fe-REE mineralization.