2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Forms of thorium concentration in rocks of the Kovdor massif

Ermolaeva V.N.*, Chukanov N.V.**, Moiseev M.M.***

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

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

***Fersman Mineralogical Museum RAS, Moscow, Russia

A survey of forms of thorium concentration on different stages of evolution of Kovdor massif, Kola peninsula is given. It is shown, that oxide thorium phases are typical for the magmatic rocks, while on the later stages (pegmatite and hydrothermal ones), as well as at the near-contact zone of the Kovdor massif main concentrators of Th are silicate and phosphate minerals.

As it was shown earlier (Chukanova, 2002), in the course of the evolution of agpaitic complexes, a change of minerals – concentrators of Th takes place (from mainly oxidation forms on the magmatic stage, through water-poor silicates loke steenstrupine-(Ce), thorosteenstrupine, Na,Th-silicates, umbozerite and its Fe- and Mn- analogues on early pegmatitic stage, to different hydrous silicates and phosphates – Na,Th- and Na,Ti,Th hydrosilicates, Th-hydrophosphate etc. on the hydrothermal stage). The data summarized in this work demonstrate that similar tendency is true for the Kovdor complex of alkaline, ultrabasic rocks and carbonatites.

In olivinites, the earliest rocks of the Kovdor massif, the main thorium mineral is thorianite ThO₂ (Ivanyuk et al., 2002). Typical Th-containing accessories of pyroxenites and nepheline pyroxenites of the carbonatite complexes are perovskite (up to 0.3 % ThO₂) and zirkelite (to 7 % ThO₂) respectively (Subbotina et al., 1991). For carbonatites Th-containing oxides also are typical: calzirtite, lueshite, perovskite (up to ca. 3 wt.% ThO₂), pyrochlor (up to 16 wt.% ThO₂), thorianite, zirkonolite (up to 20 wt.% ThO₂). In carbonatites also zircon with »0.1 % ThO₂ has been detected (Kapustin, 1971; Kozyreva et al., 1990; Subbotin, Subbotina, 2000; Ivanyuk et al., 2002).

In alkaline pegmatites and hydrothermalites of the Kovdor massif, the role of accessory minerals – concentrates of Th play mainly silicates, in particular georgechaoite (up to 6.1 wt.% ThO₂), thorite and a number of poor-studied metamict hydrosilicates: Ca-titanosilicate with 2.42 wt.% ThO₂, Th,Ca,Ti-silicate (39.03 wt.% ThO₂), (Ba,Ca),Th-silicate (59.6 wt.% ThO₂), Са,Th-silicate (57 wt.% ThO₂) (Moiseev, Chukanov, 2006). In the near-contact zones of pegmatites and in natrolite veins crossing rocks of the phlogopite complex of the Kovdor massif, also lueshite has been detected, however the later mineral is in all probability a relic phase belonging to an earlier paragenesis.

Thus on all stages of evolution of the Kovdor massif (from magmatic to hydrothermal) Th can concentrate in some mineral phases. Except for thorianite, thorium minerals (thorite, Са,Th-silicate Ca₃Th₅Si₈O₂₉*nH₂O and (Ba,Ca),Th-silicate (Ba,Ca,Sr)₃Th₁₀Si₁₂O₄₇*nH₂O replacing thorite, Са,Th-titanosilicate Ca₅FeTh₅Ti₆Si₈O₄₄, Са,Th-titanoniobate CaTh(Nb,Ti)₂O₉) are confined to the late mineral-forming stages (Moiseev, Chukanov, 2006; our data). In the Kovdor massif, as in alkaline rocks of the Khibiny-Lovozero complex, oxide forms of Th are typical for magmatic rocks, whereas on the later stages (pegmatite and hydrothermal) main concentrators of Th are silicates.

Unlike agpaitic pegmatites of Khibiny and Lovozero massifs, in hydrothermal paragenesis of Kovdor alkaline pegmatites organic substances which could be complex-forming agents for Th⁴⁺ were not found. Hypothetically, it is due to the absence of the mechanism of oxygen bonding in the aegirine form because of low Na activity: instead aegirine, diopside, containing Fe²⁺ was crystallized. Under relatively oxidation conditions carbon formed only carbonate phases. As a result, in the late derivates of the Kovdor massif crystallization of Th minerals are formed mainly not due to thorium transport to the crystallization centers of thorium minerals, but by the mechanism of replacing the earlier minerals of thorium (thorite and others) in situ under the exposure of hydrothermal solutions.

In the near-contact zone of metasomatic transformation (fenitization) of the Kovdor massif, earlier britholite (up to 2.8 wt.% ThO₂) and monazite (about 0.4 wt.% ThO₂) were described (Ivanyuk et al., 2002). Additionally, in a late paragenesis of the fenitization zone, ekanite was detected by us. The latter mineral contains (wt.%, by electron microprobe data): Na₂O 2.02, K₂O 1.96, CaO 7.11, FeO 0.81, Al₂O₃ 2.50, ThO₂ 29.97, TiO₂ 0.30, SiO₂ 48.67, total 93.34; the empirical formula Th_1.05(Ca_1.18Na_0.61K_0.34)_S2.18Fe_0.10Ti_0.03(Si_7.53Al_0.47)_S8.00O_19.73 (see Fifure 1, REM image obtained using scanning electron microscope JSM-Т100, analyst L.A. Pautov). Thus silicate and phosphate forms of the thorium concentration are typical not only for late-magmatic and pegmatitic stages of Kovdor complex evolution, but also for metasomatites related to this complex.

Literature

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