Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Niobium minerals from weathering crust of carbonatites of the Beloziminskoye deposit

Sokolov S.V., Nechelyustov G.N.

Fedorovsky All-Russia Scientific-Research Institute of Mineral Resources (VIMS),

Moscow, Russia

vims-sokol@mail.ru

At the Beloziminskoye deposit, weathering crust is loose residual formations made up of products of disintegration, leaching and oxidation of carbonatites. Locally it is underlain by their diopside-calcite species, but mostly formed by amphibole-phlogopite-calcite and ankerite carbonatites.

Niobium is most valuable component of crustal ores in this deposit. The highest content of Nb₂O₅ is recorded in the crust, rich in ochres, which accumulate during weathering of ankeritized calcite carbonatites and ankerite carbonatites due to considerable leaching from them of easily soluble compounds in the process of hypergene changes of source rocks.

Mineral-concentrators of niobium are represented by pyrochlore (pyrochlore proper, which considerably predominates, and metamict Ta-U-pyrochlore, transisting in composition to "hatchettolite"), its сolumbitized varieties and сolumbite. They account for nearly 92% niobium present in the ore, and the rest, being in the amount of 5.88-6.16 wt. % Nb₂O_5, is part of rare Nb-rutile and is mainly dispersed in ilmenite and in other minerals.

Pyrochlore occurs in crust as octahedral crystals and their fragments of light grey, creamy, dark brown color; its colourless, translucent varieties were also recorded. The size of crystals did not exceed 2-2.5 mm across.

The results of the electron microprobe analysis (analyser JXA-8100 Superprobe, equipped with an INCA-400 EDS detector) indicate the presence in the ore of two pyrochlore varieties, markedly differing in the chemical composition (Table). The difference stems from the following reasons. First, the varied composition of source pyrochlore associated with calcite and ankerite carbonatites (Pozharitskaya, Samoilov, 1972). Secondly, the change in "hatchettolite" from early calcite carbonatites occurs during later stages of carbonatiteformation by recrystallization and dissolution, as well as substitution by the late pyrochlore. As a consequence, the calcium and sodium content in the newly-formed pyrochlore decreases, as compared with "hatchettolite", whereas the amount of strontium, barium and cerium notably increases (Gaydukova, 1966; Lapin, Kulikova, 1989).

On the other hand, the processes of weathering to some extent level out the chemical diversity of pyrochlores due to their different sustainability in the hypergene environment. For example, "hatchettolites" and Ta, Th, U-enriched pyrochlores are most easily converted (Kapustin, 1973). Apparently, this should account for the absence of samples with high tantalum, uranium and thorium content among the pyrochlore samples we have analyzed.

The table shows that in the Beloziminskoye deposit weathering crust, primary pyrochlore from carbonatites is characterized by high sodium, calcium and fluorine content, whereas secondary pyrochlore from crust is almost completely devoid of fluorine and characterized by changes in the qualitative and quantitative composition of cations in the pyrochlore structural position A. This is reflected in a considerable reduction of calcium and sodium with some increase in the amount of cerium, to a more extent – strontium, but, mainly, in the appearance of barium, the content of which in some specimens reaches 8-10 wt. % BaO.

The elements that make up Group B in the pyrochlore structure behave differently. In both genetic varieties of the mineral, close limit and averages values were set for Nb₂O₅ and TiO₂. Contents of tantalum, thorium, and uranium are subject to significant fluctuations, moreover in many samples they are below the detection limit. In the newly-formed pyrochlore, compared with the primary, these elements accumulate: the average data show 1.6 times increase in the Ta₂O₅ concentration and threefold increase in ThO₂ and UO₃.

Chemical composition (wt. %) of pyrochlore and columbite

* Limit contents/Average value

Formation of the secondary pyrochlore in the hypergene process (in addition to the removal of sodium and calcium simultaneous with replacement by their leached cations by strontium and barium) is accompanied by its hydration, which in total leads to an increase of parameter а_о of the elementary cell. If in pyrochlore of source carbonatites а_о is 10.42±0.01Å, then in minerals from the crust it rises to 10.52-10.53Å. In this case, a number of elements that do not have highest valency in initial carbonatites, undergo oxidation in the process of formation of crustal associations: Fe²⁺→Fe³⁺, Ce³⁺→Ce⁴⁺, U⁴⁺→U⁶⁺ (Kapustin, 1973). It should be noted that the presence of hexavalent uranium as uranyl-ion UO₂²⁺  in some samples of studied pyrochlores is fixed by bright green luminescence that appears under influence of laser radiation with characteristic wavelength λ_exc=337.1 nm (analyst V.A. Rassulov, VIMS).

Another mineral-concentrator of niobium is сolumbite, which forms crystals of various euhedral degrees reaching 2.5-3 mm in size. However, in most cases it occurs in the form of fine-grained aggregates, developing in pyrochlore. Columbitization begins at the endogenous stage, during ankeritization of earlier pyrochlore-bearing carbonatites. This process is particularly intense during crust formation. Columbite, as a rule, develops unevenly on pyrochlore, along fractures, from the periphery of crystals (rims of different thickness appear); the substitution often ends in complete pseudomorphs with preserved octahedral habit typical of pyrochlore crystals.

Most analyses on сolumbite (n=17) show high content of Nb₂O₅ (71.63-75.93 wt. %; average 73.81 wt. %). The concentration of Ta₂O₅ in 5 samples out of 17 varies from 0.70 up to 0.024 wt. %, averaging 0.072 wt. % and the rest is below detection limit. In addition there are сolumbite (3 analyses) with low amount of Nb₂O₅ (65.57-68.94 wt. %), but enriched in tantalum (0.95-3.04 wt. %; average of 2.06 wt. % Ta₂O₅). Note, that all analyzed сolumbite samples are characterized by low amount of manganese and strontium and very low content of rare-earth and radioactive elements (Table).

Crustal rare-metal ores in certain sections of the Beloziminskoye deposit contain elevated amounts of Ta₂O₅ (up to 0.024-0.026 wt. %) and in those cases stand out as an independent tantalum-niobium type. They concentrate tantalum mainly in the studied niobates (84%), and, in total, the Ta content in pyrochlore is higher than in сolumbite (see Table). As tantalum-bearing pyrochlores are often enriched in thorium and uranium, the areas of ores with increased content of tantalum can be identified by radiometric methods.

References

Gaydukova V.S.  About strontiopyrochlore and calcioaeschynite from carbonatites / Geology of the deposits of rare elements. 1966. Issue 30. P. 72-76. (in Rus.).

Kapustin Y. L.  Mineralogy of weathering crust of carbonatites. M.: Nedra, 1973, 200 p. (in Rus.).

Lapin A.V., Kulikova I.M.  Alteration processes in pyrochlore and their products in carbonatite weathering crusts / Proceedings of the Russian Mineralogical Society. 1989. Issue 1. P. 41-49. (in Rus.).

Pozharitskaya L.K., Samoilov V.S. Petrology, mineralogy and geochemistry of the carbonatites of the East Siberia. M.: Nauka, 1972, 267 p. (in Rus.).