Comparative mineralogy of carbonatite complexes belonging to different formations

Yu. A. Bagdasarov

The Institute of Mineralogy, Geochemistry and Crystallochemistry of Rare Elements, Moscow, Russia

Carbonatites, like some other formations are very deficient in silica,š silicate-magnetite-apatite, apatitolites, calcitic magnetitolites, fluorite calcite, in individual caseš apatite iolite-urtites (Khibiny) are formed at final stages of complex massifs with alkaline rocks of subcrustal, mantle origin.

I. Within well known formation of ultrabasic-alkaline rocks within carbonatites (UAC), the main rare metal ore components are Nb, Zr, REE_Ce, rarely Sr, Ba, U, Ta, Th, as well as phosphorus in apatite, in zones of hypergenesis – minerals of crandallite group and others; fluorite, Fe in magnetite, rarely hematite and its hydroxides, sulfides of non-ferrous metals – Cu, Zn and others. Under specific conditions of metasomatic formation of rare metal carbonatite facies (stages) the peculiaritiesš of acid-basic interaction of the mineral forming medium and substrate result in relative separation of niobates, usually of pyrochlore group. In these cases they actively vary according to the value of Nb/Ta ratio; zones of commercial areasš of Ta-U mineralization (Nb/Ta ratio from 1.2 to 4-6), blocks of increased ratio (10-30) and areas of totally pyrochloreš mineralization (ratio of these elements 40-60 and more). In the first case (individual massifs of this formation in Russia), absolute Ta₂O₅ content in rocks is more than 0.025-0.03 mass%, of uranium oxide - 0.5-0.8 of this value.

The majority of these complexes is characterized by Na-type chemistry and their early silicate varieties are usually represented by the rocks of jacupirangite-melteigite-iolite row, nepheline and alkaline syenites, melilites, picrites and others. The rocks of K-types alkaline rocks with leucite and others are more rare and, as a whole, depleted of the above mentioned rare elements.

2. Massifs ofš alkaline-gabbroid-carbonatite formational type in Russia are actually represented only by individual units, mainly on the Kola Peninsula. Large carbonatites bodies with significant amounts of pyrochlore and some silicate minerals of Nb, Ta, Ba and others, occur only within the complex Tikshiosero. Nb₂O₅ content in ores usually exceeds 0.07-0.1 mass%; in other complexes it is even less.

3. Formation of so called linear (linear-fractured) bodies of carbonatites and fenites, is more rarely than intrusive silicate rocks of syenite composition. They are known as ”near fault zone carbonatite metasomatites” and others.š This formation was first identified by theš author in 1979. Now they include the known carbonatites bearing complexes Ilmen-Vishnevogorsk, Chernigovsky in Ukranian Priasov’e, Dubravinsky in the Belgorod region, Tatarka on the Yenisei Ridge, Siilin’jarvi in Finland, perhaps, some occurrences of carbonatites on the Timansk Rildge, individual units in Arctic Norway, India, Australia. Part of these occurrences has no silicate intrusive rocks that preceded carbonatites; in contrast, Ilmen-Vishnevogorsk zone is characterized byš predominant miaskite nepheline and alkaline syenites, which many times exceed carbonatites mass. The latter, as a whole, are characterized by decreasedš Nb and Ta content in all comparable temperature facies with similar amounts of Sr, Ba, and phosphorus and absence of high REE concentrations, typical of some UAC carbonatite facies. Attentionš is drawn to drastic increase in amount of elements, which are not typical of carbonatites elements – Cr, and Ni (10-20 times increase comparing to UAC formation). Distribution of Nb and Taš and associated U and Th in niobates is defined by the same factors, i.e. in the facies of increased temperatures uraniferousš tantaloniobates occur with Nb/Ta ratio in mineralsš equal from 3-5 to 10-20 (Chernigov and Vishnevogorsk); at low T-parameters, niobates with insignificant Ta and U amounts are typical (Tatarka, Yenisei Ridge); in some cases niobates are absolutely absent (Dubravinsk zone). Carbonatites of this formation were as a rule generated at great depth and under sharply reducing conditions, which is confirmed by existence of weight graphite amounts (Chernigov zone), decreased contents of magnetite and Fe₂O₃ component in any other minerals as a whole.

4. Several assemblages of carbonatite complexes are distinguished, which are associated with silicate rocks of moderate alkalinity, sometimes with expressed K specificš chemistry, in other combined Na-K chemistry, i.e. with nepheline and leucite. Earlier (Ginsburg A.I. and Samoilov V.S., 1983), massifs of Mongolia and Mountain Pass in the USA with rich REE deposits were assigned to them. These types are characterized by somewhat indefinite geologic-petrochemical features; perhaps now, they can include nepheline-leucite complex Murun with abnormally high Sr and Ba concentrations in the composition of some carbonatites; this massif is similar to massif Mushugai-Khuduk in Mongolia with high REE contents in apatite and other.

The special place is taken by carbonatites, the association of which with low alkaline rocks (granosyenites) and sometimes with typical granitoids is not clear. These assemblages are also characterized by increased concentrations of cerium REE, Sr, Ba, fluorite and Fe minerals, siderite and oxides, sometimes in commercial concentrations. These are carbonatites of Khalutinsk-Arshansk zone in Pribaikal’e, Karasug and many small occurrences of this type in Tuva. Definite features of similarity are demonstrated by the largest in the world REE depositš Bojun-Oboš in China, some occurrencesš of carbonatites-like rocks in Vietnam and other. It is worth noting that in absence of nepheline or its negligible amount in early alkaline rocks, later carbonatites are eliminated in niobates, within all above discussed formational types of these rocks, including UAC (massif Palabora in South African Republic). As a whole, the bulk of carbonatites associated with low alkaline rocks, except the above listed, contain low concentrations of Sr, Ba, sometimes REE, negligible concentration of niobates and other.

5. In Muisk region (Vitim basin) small carbonatites bodiesš have recentlyš became known, which yet didn’t reveal genetic associations with any intrusive silicate rocks. They contain not high amounts of niobates and zirconates with notable addition of strontium, rare earth carbonates. A drastic increase in chromium content, up to occurrence of spinellides, close to chromite, and weight amounts of graphite can point to closeness of these occurrences to linear carbonatites bodies formation (G.S. Ripp and oth., 2000s).

šIn future, this type of carbonatite occurrences, distinctly belonging to the “fund” of hardly discovered deposits, will be very likely discovered.