GEOLOGY AND MINERALOGICAL COMPOSITION OF EPIGENETICALLY ALTERED
ORES OF WEATHERING CARBONATITE CRUSTS
A.V. Tolstov and N.P. Pokhilenko
Institute of Geology and Mineralogy SB RAS (Novosibirsk, Russia), tolstov@igm.nsc.ru
A.V. Lapin
Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements (Moscow, Russia)
ABSTRACT
Epigenetic deposits were originally found in 1986 in the Tomtor supergene complex (the north Siberian
platform). The Tomtor massif, consisting of ultramafic alkaline rocks and carbonatites, is a ring complex
with a diameter of 20 km and an area of 250 km2. The massif contains 3 zones: the oval core consists of
carbonatites (40 km2), the intermediate zone is composed of nepheline-pyroxene rocks (jakupirangiteijolite), its width is 3 km and finally the peripheral zone contains alkaline and nepheline syenites with a
width of 6 km. Dikes and veins of apatite-magnetite ores and picrites intrude the massif rocks. The age of
most of the rocks of the massif is Lower-Middle Paleozoic and also the age of some complexes varies from
Vendian to Permian. The carbonatites are characterized by high concentrations of Nb, trace elements (TR)
and phosphorus.
Lateritic weathering of the crusts of up to 300 m is commonly found in the carbonatite ores. The hypergene
complex contains the following layers (from top downward): kaolinite-crandallite, siderite, goethite, and
francolite. The epigenetically altered horizons (kaolinite-crandallite and siderite) are characterized by the
highest amount of Nb and TR. The total resources of the Tomtor deposit are colossal: Nb 2О5 – 73.6 Mt,
TR2О3 – 153.7 Mt, P2О5 – 2 Gt. The richest ores form a layer in the upper part of the epigenesis area and
fill the lower area consisting of Permian coal-bearing strata. The ores are slightly lithified, thinly laminated,
cryptomerous, and contain anomalously high concentrations of Nb, Y, Sc, and TR (Nb 2O5 – 4–15 %,
TR2O3 – 7–25 %, Y2O3 – 0.5–2.0 %, Sc2O3 – 0.05–0.15 %).
The mineral composition of the epigenetically altered ores is represented by pyrochlore, monazite, and
crandallite. The mineral-concentrators for niobium are Ba-Sr pyrochlore and columbite. The mineralconcentrators for TR are monazite (hydromonazite, rhabdophane), and churchite. The crandallite group is
represented by calcium species and also isomorphous solid solutions of strontium (goyazite), rare earths
(florencite), and barium (gorceixite). The ore minerals in total occupy 20-80 % of the volume of the ore.
Due to the unusual composition and the extremely high concentrations, the epigenetically altered deposit is
the natural concentrator, processing of which is possible without ore preconcentration.
KEYWORDS
Tomtor supergene complex, weathered carbonatite, geology, mineralogy, niobium, rare earths, yttrium,
scandium