Carboniferous compounds in pegmatites of different genetic types: comparative study

V.N. Ermolaeva*, N.V. Chukanov**, I.V. Pekov***

*Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia; e-mail: cvera@mail.ru

**Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia;

***Lomonosov Moscow State University, Moscow, Russia

 

Organic compounds are known in pegmatites of different types. In peralkaline pegmatites, the content of organic compounds is high as compared with parent magmatic rocks and strongly increases from early-pegmatitic to late, hydrothermal stages [1, 2]. Similar behaviour is typical for a number of rare non-coherent elements (Th, U, REE, Zr, Hf, Nb, Ta, W, Sn, Ti): both in alkaline and granitic formations, their contents increase to late-magmatic and postmagmatic stages. In particular, źthucholite╗ (carboniferous substance containing U and/or Th) from granitic pegmatites of Canada, where it was described for the first time, is associated with uraninite, zircon (cyrtolite), titanite, allanite, samarskite [3, 4]. Carboniferous substance, containing oxides of U, Th, Y, Nb, Zr, was found in parageneses with uraninite, titanite, allanite, cyrtolite, xenotime, monazite in granitic pegmatites of North Karelia and described as źcarburan╗ [5, 6]. Bituminous substances from agpaitic pegmatites studied by us earlier, are also associated with minerals of rare elements including silicates and phosphates of REE, Th, U and show ability to selective accumulation of all above-mentioned elements [7, 8]. Thus one can suppose the existence of a genetic connection between carboniferous substances and a specific group of źbitumophilic╗ elements: U, Th, REE, Zr, Hf, Nb, Ta, W, Sn, Ti.

In this paper, we present the comparative study of carboniferous substances from different types of pegmatites. In rare-metal pegmatites (Viitaniemi, Finland) [9], spherical inclusions of thucholite up to 1 cm in diameter occur in sugar-like and lamellar albite in close association with zirkon, tantalite and pyrochlore. In amazonite pegmatites related to alkaline granites (Ploskaya mountain, Western Keivy, Kola Peninsula) dispersed organic compounds was discovered by IR-spectroscopy method in thorite from late-pegmatitic [10] albite aggregate. In mica-type granitic pegmatite (Lopatova Guba, North Karelia), carboniferous substance forms pseudomorphs after cubic crystals of uraninite and is accompanied by rare-element minerals (monazite-(Ce), xenotime-(Y), zircon). In hyperagpaitic pegmatite (Koashva mountain, Khibiny massif, Kola Peninsula) large (up to 4 cm) spherical segregations of bituminous substance occur in aegirine-natrolite aggregates in hydrothermally altered core and are associated with titanosilicates (astrophyllite, lorenzenite, titanite, lemmleinite-K), zirconosilicate (catapleiite), and chlorbartonite.

Electron microprobe analyses and BSE images have been obtained using the digital scanning electron microscope CamScan MV2300 (analysts A.N. Nekrasov, A.A. Muhanova, K.V. Van).

All studied samples are inhomogeneous being thin intergrowths of carboniferous substance with minerals of specific rare elements. By morphological criteria [11], both known mechanisms (namely breakdown of metal-organic complexes and substitution of uranium minerals by organic substances) can be realized during the formation of carboniferous substances in pegmatites (Figs. 1, 2). As a result, microscopic crystals of mineral phases, that are randomly distributed in an organic matrix, were formed. These crystals, in their turn, contain submicroscopic inclusions of organic matter.

 

Figure 1. Left ľ disintegration growths of thorium-calcium silicophosphate (Th,Ca,Na)4(Mn,Ti,Nb)1-2(SiO4)4(PO4nH2O in bituminous substance, Lovozero. Back-scattered electron image. Right ľ rasvumite (1) with Sr-containing fluorite rims (2) and growths of thorium niobosilicate (Ca,Na)2REETh4(Nb,Ti)1+xSi7-8(O,OH)y ×nH2O (3) in high-sulphur bituminous substance (4), Khibiny. Back-scattered electron image.

 

 

 

 

 

 

 

Figure 2. Congestions of carbon substance (1) in aggregate of hydroxide (carbonate?) of U and Pb (2) from carburane, Lopatova Guba deposit. Back-scattered electron image.

 

In thucholite from the zone of lamellar albite in the Viitaniemi pegmatite, electron microprobe analyses show presence of very small (less than 1 mm) equant particles of a phase containing only uranium as a detectable element (presumably, uraninite). In thucholite from sugar-like albite zone in the Viitaniemi pegmatite, uranium is distributed between carboniferous phase and spherical inclusions of autunite, or meta-autunite, aggregates (Ca0.89Fe0.25Sr0.11Sb0.07Zr0.04Pb0.02)1.39[U2.08Ox][PO4]2 whereas thorium is present only in a mineral of the brockite-grayite series with the empirical formula (Сa0.48Th0.39Al0.19Pb0.16Fe0.06U0.04Sr0.03Y0.03Ti0.01Zr0.01La0.01Ce0.01)1.42[PO4]S0.03. In contrast to above-described samples carburan from Lopatova Guba deposit shows obvious features of the replacement of a mineral phase (uraninite) by a carboniferous substance. Accumulations of the latter being depleted by mineral inclusions are present in the polymineral aggregate forming pseudomorphs after cubic crystal of uraninite (Fig. 2). Mineral phases of carburan are oxides and/or carbonates of U and Pb (U:Pb=3:1 in atomic proportions), uranium phosphosilicate (U0.69Pb0.15Th0.14Dy0.03)1.01[Si0.61P0.35As0.05]1.01Ox×nH2O, and anglesite.

According to IR-spectroscopic data, all samples of carboniferous substances studied in the present work are characterized by high contents of H2O and/or OH groups (3100-3600 sm-1). Bitumens from peralkaline pegmatites of the Khibiny-Lovozero complex are characterized by high contents of both aliphatic hydro-carbone groups (С-Н-stretching vibrations, the range 2800-3000 cm-1), and unsaturated organic compounds (a series of bands in the range 1200-1680 cm-1). IR spectrum of Viitaniemi thucholite shows lower contents of aliphatic hydrocarbon groups, but higher contents of aromatic compounds. The phenomenon of entrapping of organic substances by growing crystals of thorium minerals at low temperatures known in peralkaline pegmatites [7] was detected also in case of thorite from amazonite pegmatite.

The above data demonstrates the general tendency to co-existence of carboniferous substances with minerals concentrating specific uncoherent rare elements in pegmatites of different genetic types. Usual presence of minerals of the above high-valence and high-force-strength transition elements in close association with organic substances in objects under consideration can be explained by their well-known ability to catalyze many reactions of synthesis and transformations of organic substances. Thus, we can ascertain in the existence of a group of źbitumophilic╗ elements including U, Th, Y, Ln, Zr, Hf, Nb, Ta, Ti, W, Sn. It is remarkable, that this set of elements is very close to the totality of elements concentrating in solid kaustobiolithes, as it was demonstrated on the basis of very large and representative statistical material [12, 13]. In granitic pegmatites the temperature of the beginning of carburan formation is approximately 700ťС that corresponds to the stage of crystallization of titanite, cyrtolite, xenotime-(Y), monazite-(Се) and uraninite [5]. Unlike granitic pegmatites, in agpaitic pegmatites the parageneses of carboniferous substances with rare-element minerals are formed under significantly lower temperatures, corresponding to the hydrothermal stage.

This study was supported by RFBR grant No. 07-05-00130-а, grant of President of Russain Federation No. 863.2008.5 and grant of the Russian Science Support Foundation (I.V.P.).

 

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