Home – Home – Jornals – Russian Geology and Geophysics 2015 number 6

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 km ²) is ~20 km in diameter; it has 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. A weathering crust formed after all the rocks; the thickest crust formed after the carbonatites, which are enriched in phosphates and REE. Four horizons are recognized from the top: kaolinite-crandallite, siderite, goethite, and francolite. The highest-grade ores are observed in the bedded deposit which fills depressions in the “sagging” weathering crust of the carbonatite massif. The ores are thin-bedded and cryptogranular, with high Nb, Y, Sc, and REE contents (on average, 4.5% Nb ₂O ₅, 7-10% REE ₂O ₃, 0.75% Y ₂O ₃, and 0.06% Sc ₂O ₃). The highest-grade ores are natural Nb and REE concentrates. The total REE content of some horizons is >10%. The morphologic features of the highest-grade phosphate ores from the northern part of the Burannyi site were studied. The major ore-forming minerals are minerals of the pyrochlore group, crandallite group (goyazite), and monazite-Ce. The pyrochlore group minerals in the ores 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: The core consists of relics of unaltered pyrochlore, and the rim is replaced. 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 biogenic concentration of REE phosphates.