REE mineralization in graphite-bearing albitites and carbonatites from the

Gremiakha-Vyrmes massif of the Kola Peninsula

N. V. Sorokhtina¹, L.N. Kogarko¹, A. K. Shpachenko², V.G .Senin ¹

¹ Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, alkaline@geokhi.ru;

² Geological Institute KSC RAS, Apatity

 

The multiphase alkaline-ultrabasic intrusive massif Gremiakha-Vyrmes is composed of four complexes: 1 basic-ultrabasic (gabbro-norites, gabbro-wehrlites, monzodiorite, anorthosites); 2 alkaline-granites, granosyenites and quartz syenites; 3 alkaline rock (melteigites, ijolites, juvites, nepheline syenites, foyaite, foidolites); 4 carbonatites and various metasomatic rocks - albitites, aegirinites which cross-cut ultrabasic and foidolitic rocks.

REE mineralisation was found in graphite-bearing carbonatites and albitites. There are two parageneses of carbonatites - rare graphite-bearing calcite-rich carbonatites and carbonatites without graphite. The first association, which is essentially alkaline, consists of calcite, aegirine, phlogopite-annite, substantial amount of albite, orthoclase, graphite, fluorapatite, titanite, zircon, prehnite, ilmenite, chlorite, sulphides, rare ferriallanite-(Ce),-(La) and allanite-(Ce) (Sorokhtina et al., 2008).

The similar assemblage of accessory minerals was found in graphite-bearing albitites. The albitites and aegirinites differ from carbonatite consists of Nb-bearing minerals of pyrochlore group (up to 5 vol. % in ore vary of albitites). The association of major pyrochlore with zircon in albitites and aegirinites in alkaline rocks of the Gremiakha massif represents a new type of ore. REE minerals in albitites represented by rare accessory phases: allanite-(Ce), -(La), unidentified REE-rich carbonate-silicate phases (fig.1), which forms interstitial grain, around altered aegirine or rare split crystals (fig. 2b).

 

 

Fig.1 Optical (a) and back-scattered (b) electron images showing relationships between minerals of hydrothermal assemblages in graphite-calcite globules from albitite (sample 99-380). Calc – calcite, Ab – albite, Gr – graphite, REE - unidentified REE-rich carbonate-silicate phases, Al - allanite-(Ce).

 

Fig.2 Backscattered electron images of calcite (Calc), chlorite (Chl), allanite-(Ce) (Al), ilmenite (Ilm), aegirine (Aeg), graphite (Gr), phlogopite-annite (mica) from carbonatite and albitite. (a) Internal composite calcite-I associated with allanite-(Ce) from carbonatite (sample 102-3) and (b) split crystal of allanite-(Ce) from albatite (sample 99-380)

 

REE minerals have been observed in all associations with calcite and graphite. Graphite forms individual spherulites, lamellar or fine-grained aggregates frequently form lenses. Graphite occurs as hexagonal crystals around calcite globules (fig.1). Calcite of carbonatites and albitites is the only carbonate mineral observed and occurs as coarse-grains or isolated microinclusions (up to 5 mkm) in silicate minerals. There are two types chemical calcite compositions in carbonatites and albitites including early Sr-rich calcite-I (up to 5-6 wt%) and late Sr poor calcite-II (up to 1 wt%) (Tab1e). Calcite-I is one of the major constituents of carbonatites and rare accessory mineral of albitites. Calcite-II form thin lamellae in carbonatite calcite-I (fig.2a), inclusions in altered zone of aegirine or mica, grains in graphite-calcite globules which were found in a coarse-grained albitites veins (fig 1). Сalcite-II of graphite-calcite globules including REE minerals of allanite subgroup, not identify silicate-carbonate phases and albite.

The chemical analyses of allanit-subgroup minerals ranges for ΣREE - in albitites from 13.78 to 17.78wt%, in carbonatites from 16.09 to 28.49wt%. The chemical compositions of unidentified REE-rich carbonate-silicate phases are present in table.

According to the distribution of Na and K between albite and orthoclase, the temperature of formation of albitite is close to 550°. According to early investigation the δ13C values in calcite of carbonatites vary from -4,24 to -5,87, in coexisting graphite they vary from -11.51 to -12.93 (Sorokhtina et al., 2010). This shows that the temperatures of formation of graphite-bearing carbonatite are close to 600°C. The isotope composition of carbon in calcite is in the range of mantle carbon. Albitites could have formed during the metasomatism triggered by intrusion of carbonatites into the alkakaline complex of the massif. Formation of graphite-calcite globules with REE phases in albitites and late interstitial individes may reflect late-stage hydrothermal carbonate fluid circulation. The REE minerals were formed in the late stage of carbonatite emplacement from postmagmatic, REE-rich hydrothermal solutions under high activity of SiO₂ in the fluid.

 

Table Microprobe analyses of calcite from graphite-bearing carbonatite (1-4) and albitite (5-7), unidentified inclusions in graphite-calcite globule from albitite (wt%)

N	1	2	3	4	5	6	7	8 (average of 2)	9
Sample	102-3				99-380
mineral	calcite-I grain	calcite-I inclusion in graphite	calcite-I grain	calcite-II lamellae in calcite-I	calcite-I grain	calcite-I grain	calcite-II grain in graphite-calcite globule	unidentified inclusions in calcite-II
CaO	53.00	54.2	53.06	57.49	50.80	52.10	52.10	11.23	28.73
SrO	1.80	2.31	2.46	0.00	2.68	3.06	0.93	0.72	0.54
BaO	nd	0.07	nd	nd	0.10	0.11	0.06	nd	nd
MgO	0.00	0.00	0.00	0.00	0.02	0.00	0.06	nd	nd
MnO	0.40	0.4	0.47	0.04	0.68	0.74	0.02	nd	0.08
FeO	0.20	0.21	0.45	0.04	0.04	0.18	0.72	nd	0.54
Y2O3	-	-	-	-	-	-	-	4.27	0.37
Ce2O3	-	-	-	-	-	-	-	24.92	8.65
La2O3	-	-	-	-	-	-	-	11.86	7.49
Pr2O3	-	-	-	-	-	-	-	2.72	0.96
Nd2O3	-	-	-	-	-	-	-	11.83	3.51
Sm2O3	-	-	-	-	-	-	-	2.12	0.28
SiO2	-	-	-	-	-	-	-	21.59	5.04
F	-	-	-	-	-	-	-	2.60	0.35
Total	55.40	57.19	56.44	57.57	54.32	56.19	53.89	93.84	56.54

 

Research covered by RFBR grant 09-05-12026-ofi_m, NSH-3848.2010.5

 

References:

Sorokhtina N.V., Shpachenko A.K., Kononkova N.N. Ferriallanite--(Се) in carbonatite veins from the

Gremiakha-Vyrmes massif of the Kola Peninsula // Apatity: KSC RAS, 2008. Materials of V FSS Petrology and Mineral genesis of the Kola Peninsula. P. 274-278. (in Russian)

Sorokhtina N.V., Kogarko L.N., Shpachenko A.K. The new mineralogy and geochemistry data of rare metal ore of the Gremiakha-Vyrmes massif // Doklady RAS. 2010 (in press)