Lazareva, 7 co-authors, reported in 2015 that the Tomtor massif of Paleozoic ultramafic alkaline rocks and carbonatites is located in the northern part of the Sakha Republic (Yakutia). The massif (its total area is ~250 km2) is ~20 km in diameter, with a rounded shape and a concentrically zoned structure. The core of the massif consists of carbonatites surrounded by a discontinuous ring of ultramafic rocks and foidolites. The outer part is composed of alkali and nepheline syenites. All rocks are weathered and covered with eluvium, which is the thickest after carbonatites enriched in phosphates and REE.
The weathering profile consists of four layers, from the top: kaolinite–crandallite, siderite, goethite, and francolite. The highest-grade ores are observed in the bedded deposit which fills depressions in “sagging” eluvium. The ores are laminated and cryptogranular, with high Nb, Y, Sc, and REE contents (on average, 4.5% Nb2O5, 7–10% REE2O3, 0.75% Y2O3, and 0.06% Sc2O3). The highest-grade ores are natural Nb and REE concentrates. The total REE content in some layers is >10%. The morphologic features of the highest-grade phosphate ores from the northern part of the Burannyi site were studied. The ore-forming minerals belong to the pyrochlore group, crandallite group (goyazite), and monazite-Ce. The pyrochlore group minerals occur mainly as crystals that were completely replaced by barium–strontium pyrochlore and/or plumbopyrochlore but retained the original faces; also, they occur as numerous conchoidal fragments. The grains of the pyrochlore group minerals sometimes have a zonal structure, with an unaltered pyrochlore core and a reaction rim. Goyazite occurs predominantly as colloform grains. According to SEM and TEM data, monazite occurs in the ores as ~50 nm particles, which cover the outer part of halloysite tubes (800–3000 nm long and 300 nm in diameter) as a dense layer and make up peculiar biomorphic aggregates.
The mineralogical data, the occurrence of biomorphic aggregates, and the close association of organic remains with ore minerals suggest that the high-grade ores of the Tomtor deposit, including the Burannyi site, resulted from a hydrothermal–sedimentary process with a presumably important role of bioaccumulation of REE phosphates.
The authors concluded that comprehensive studies have shown that the Tomtor high-grade ores (natural concentrates of Nb, REE, Sc, Th, P, and Ti) precipitated in supergene conditions from thermal waters in a shallow-water lake. The ore minerals underwent short distance or no transport and result mainly from hydrothermal
alteration; most minerals were chemically precipitated from thermal waters, possibly with participation of biota. This origin was inferred from several features: morphology of the ore minerals and particles, including nano- and micrometer ones, free from signatures of deformation or transport in a water flow; preservation of primary crystal habits in both “relict” and authigenic minerals; exotic shapes of mineral aggregates; and laminated structure.
The abnormal concentration of rare metals has at least two causes: (i) accumulation of the heavy fraction on the lake bottom and the concentration of pyrochlore widespread in the Tomtor weathering profile (particularly, in weathered carbonatites), and (ii) precipitation of crandallite group minerals, halloysite, and, partly, monazite from thermal waters. Microbial activity may have been an important agent in ore genesis, which is implicitly evidenced by bacteriomorphic micro- and nanostructures and co-occurrence of organic remains with ores. Of course, many issues of the Tomtor ores still remain unresolved, such as details of their secondary and accessory minerals; the lifespan of hydrothermal systems; the water body type (a spring, a pool, a lake, etc); bedrock hydrothermal alteration; mechanisms of trace element and REE sorption; bioaccumulation rate (in vivo or ex vivo), etc. These issues will be the subject of our future studies, with implications for prediction of microbially-mediated mineralization and for biotechnological recovery and separation of trace and rare-earth elements.
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