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, email@example.com;
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.
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%)
Research covered by RFBR grant 09-05-12026-ofi_m, NSH-3848.2010.5
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)