Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Rare and rare-earth elements in Late Archaean moderately alkaline intrusive units of Karelia. Examples from the Syargozero  complex

Dmitrieva А.V.^*

*FBGU, KarRC, RAS, IG, Petrozavodsk, Russia

dmitrievaa-v@yandex.ru

Rare and rare-earth elements, such as Ta, Nb, Sr, Y and lanthanoids, are known to be associated with the rocks of alkaline igneous formations and carbonatites. As the application range of these metals is wide, they are in great demand. Rare-earth elements are used in modern high-tech devices and plants; in the production of monocrystals for solid-body lasers, control rods for nuclear reactors, permanent magnets, special radiation-protecting coatings, in medicine, metallurgy, electronics, optics, car production, luminophors, audio systemsа, as catalysts, etc. In the Kola Peninsula, most rare earths are concentrated in the alkaline rocks and apatite-nepheline ores of the Khibiny massif. The Tikshozero massif is an example of such localities in Karelia. Moderately alkaline rocks occupy an intermediate position in igneous series. High-Pt (Kondersky massif in Siberia) and REE deposits are occasionally associated with the ultramafic members of this series.

The high-alkali massifs, formed at final stages in the evolution of Late Archaean greenstone belts and identified as sanukitoids (~2.74-2.7 Ga), e.g. the Panozero, Syargozero, Amindomaoya, Sharavalampi, Elmus, Hautavaara and other intrusive units (Lobach-Zhuchenko et al., 2005), are common in Central Karelia. The Syargozero, Sharavalampi and Torosozero massifs are collectively named the Syargozero moderately alkaline complex (Slyusarev et al., 2001). The Panozero pluton has been studied in most detail (Lobach-Zhuchenko et al., 2007). The intrusives are differentiated from pyroxenites to quartz monzonites and syenites. As the rocks are typically rich in alkalies, P and Ti, Ba, Sr and REE, they are considered to be the possible source of rare and rare-earth elements.

The detailed study of the massifs of the Syargozero complex has shown that early phases are represented by gabbro and pyroxenites and late phases by monzodiorites and syenites. The percentage of SiO₂ varies from 43.5 in pyroxenites to 63.5 % in syenites. Total alkalies increase from 1.7 to 11.4 % (Fig. 1).

As silica increases, the percentages of TiO₂, FeO*, MgO, CaO and P₂O₅ decrease, while Al₂O₃ rises slightly. Associated with gabbro-pyroxenites are low-grade apatite-magnetite and titanite-apatite ores, REE and noble-metal concentrations, which exceed background values (Pt – 0.038 g/t, Pd – 0.074, Au – 0.02; the Pt and Au minerals, such as  keithconnite, moncheite and electrum, were identified). The percentage of TiO₂ in ore zones is as high as 2.17 % and that of P₂O₅ is 2.16 %. The bodies vary in thickness from 40 to 100 m, but high REE, Ba and Sr concentrations in pyroxenites indicated their Ti-P-TR-specialization. The concentrations of these elements are shown in the Table in comparison with alkaline-ultramafic gabbro-carbonatite massifs of Proterozoic age (Eletozero and Tikshozero massifs) in Karelia.

Syargozero rocks are typically rich in REE, especially LREE. In pyroxenites, the concentration of ∑REE is as high as 1262 ppm, that of Ba is 105-439, that of Sr is 546-2749 and that of Zr is 84-264. In gabbro, the concentration of ∑REE is 830 ppm, that of Ba is 1263, that of Sr is 1058 and that of Zr is 167. In  monzodiorites, the concentration of ∑REE 452 is ppm, that of Ba is up to 2144 and those of Sr and Zr are about the same. In syenites, total ∑REE declines to 332 ppm, Ba is 1986, Sr is 1385 and Zr is 143. Based on REE distribution spectra, their concentrations were clearly shown to decrease with a rise in SiO₂ (Fig. 1), which is possible upon the fractionation of a permanent association of the minerals which concentrate these elements, e.g. sphene and apatite. The REE content of Syargozero rocks is slightly higher than that of the Panozero pluton.

Pyroxenites and gabbro (phase I). Early alkaline metasomatism, induced by the intrusion of phase-II monzodiorites, is accompanied by phenites – the appearance of Ba-bearing К-feldspar (up to 2.75 % Bao) and phlogopite in pyroxenites and gabbro; sphene, ilmenite and F-apatite were identified. Sphene is often zonal, which is due to the presence of impurities: 1.48-2.29 % Ce₂O₃ and 2.2% Nd₂O₃. Upon lower-temperature alterations, associated with the intrusion of albitite veins, epidote-group minerals which also contain lanthanoids are formed in these rocks.  Ce-epidote contains 7.3-8.6% Ce₂O₃ and orthite carries 18.68% ∑Ce₂O₃, Nd₂O₃ and La₂O₃. Carbonates, e.g. parasite which contains Ce, La, Nd and Pr, are formed at later stages. Barite with Sr as impurity (SrO 1-10 %) is abundant.

Monzodiorites and syenites (phase II) consist dominantly of feldspars (albite with BaO impurity 0.09-0.81 %, К-feldspar - 0.22-1.46 % BaO). Lanthanoids in these rocks are part of Ce-epidote and orthite (14.4-16.26% ∑La₂O₃, Ce₂O₃, Nd₂O₃). Later barite with Sr impurity (1.64% SrO) is common in both rock types; even celestine (45.70% SrO) appears in syenites at late alteration stages.

Syargozero rocks are generally specialized for P, Ti, Ba and Sr and are enriched in REE. Maximum REE concentrations (up to 1262 ppm) are typical of pyroxenites. Sphene, epidote-group minerals and late F-bearing carbonates are REE-concentrator minerals. Ba accumulates in the feldspars of more felsic differentiates of the complex and associated metasomatic rocks. In late processes, Ba and Sr are concentrated in barite and celestine.

Fig. 1. Percentages of alkalies (%), Ba, Sr and Zr (ppm) in the rocks of the Syargozero complex.

Fig. 2.  REE distribution in the phase-I (a) and phase-II (b) rocks of the Syargozero complex in comparison with the Panozero pluton (after S.B.Lobach-Zhuchenko) and host granodiorites. Normalization for chondrite (Sun & McDonough, 1989). А: 1 – pyroxenites, 2 – gabbro, 3 – mafic complex of the Panozero pluton; B: 1 – monzodiorites, 2 – syenites, 3  -  host granodiorites; monzonites (phases I-III) and quartz monzonites of the Panozero pluton.

 Table. Rare and rare-earth element concentrations in the AR moderately alkaline rocks of the Syargozero complex against those of the Panozero pluton and PR alkaline formation.  

* Total REE (La, Сe, Nd, Sm, Eu, Tb, Yb, Lu), Northern GE data; REE, Y, Sr, Ba, Ta, Nb (ppm), P₂O₅ and TiO₂ (%).

References:

Slyusarev V.D., Kuleshevich L.V., Pavlov G.М., Lavrov М.М., Zemtsov V.А. Subalkaline magmatism of the Syargozero area //Geology and useful minerals of Karelia. Petrozavodsk. 2001. No. 3. P. 66-86.

Lobach-Zhuchenko S.B., Rollinson H., Chekulaev V.P., Guseva N.S., Arestova N.А., Kovalenko А.V. Geology and petrology of the Archaean high-K and high-Mg Panozero massif, Central Karelia // Petrologia. 2007. V.15. No. 5. P. 493-523.

Lobach-Zhuchenko S.B., Rollinson H.R., Chekulaev V.P., Arestova N.A., Kovalenko A.V., Ivanikov V.V. Guseva N.S., Matukov D.I., Jarvis K.E. The Archean sanukitoid series of the Baltic Shield: geological setting, geochemical characteristics and implications for their origin // Lithos. 2005. V. 79. P. 107-128.