Northern Finland Office

24/2011 31.11.2011

Rovaniemi

REE-mineralogy and geochemistry of

selected drillcores from southern side of

Sokli Carbonatite Complex, NE-Finland

Thair Al Ani, Pertti Heikura and Olli Sarapää

GEOLOGICAL SURVEY OF FINLAND DOCUMENTATION PAGE

Date / Rec . no.

31.1 1.2011

Authors Type of report

Thair AI Ani OBi Sarapaa

Commissioned by GTK

Title of report REE-mineralogy and geochemistry of selected dri1lcores from southern side of Sokli carbonatite complex; NE-Finland

Abstract

The Sokli carbonalite complex contains lateritic P-ores, apatite-silicate-P-ores and Nb-Ta pyrochlore ores, which may have economic importance. In addition, late carbonatite dykes contain remarkable amount oj different REE-minerals. The aim oj this work was to study REE-geochemistlY and mineralogy oj carbonatites oj selected drill cores R4, R6, R194, R196, R198, R200, R290, and R70; southern side oj the Sokli Complex. According to mineralogical studies made by SEM-EDS a remarkable suite oj REE-minerals were identified minerals such as allanite, monazite, ancylite (Ce), bastniisite (Ce), barbankite and Nb-Zr minerals (baddeleyite, pyrochlore and zircon). Ancylite-(Ce) is common at Sokli carbonatite samples; bastniisite-(Ce) and monazite-(Ce) are also Jound but are rare. Allanite occurs as acicular habit or aggregates association with veins or jillingJractures, it contains a significant amount oj W (18. 7-26.4 wt %) and is characterized by a high REE content, up to 50 wt%. Characteristic accessory constituents are strontium-rich minerals, calcite, dolomite,jiuorapatite, magnetite, ilmenite, sulphides (pyrrhotite, pyrite), titanite, Zn spinel gahnite.

Geochemically studied samples can be classified mainly as calcio-carbonatile, Jerro-carbonatite and silicocarbonatites with common occurrences oj Jenites. Calcio-carbonatite contains calcite and dolo-mite as the main minerals (-90 %) with apatite, baryte and surly rich in REE-minerals. Ferrocar-bonatite composed mainly from calcite «70 %), magnetite, titanite and dolomite with traces oj apatite, columbite and REE-minerals. Silicocarbonatite is predominant in calcite «60 %) with jiogopite and tremolite. Fenite REE content in studied samples varies Jrom 0.02 % 10 0.5 % and LREEIHREE ratio is high; Ea and Sr are in high level.

Keywords

Sokli complex, carbonatite, allanite, ancylite, bastnasite, pyrochlore and Nb- Zr minerals

Geographical area

Finland, Lapland, Savukoski, Sokli, southern side of the Sokli Complex

Map sheet

4723

Other infoITIlation

Report serial Archive code

24/2011

Total pages Language Price Confidentiality

25 English public

Unit and section Project code

2141007

S;gnaturelnamc 1 . . Thair AI Ani " 1M ~/- Rwt ~~e~~.4'4 .c.;;;. . . . y ""--

Perttl Hetkura lit Sarapiiii

GTK GEOLOGIAN TUTKIMUSKESKUS • GEOLOGISKA FORSKNINGSCENTRALEN • GEOLOGICAL SURVEY OF FINLAND

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Contents

Documentation page

1 INTRODUCTION 1

2 SAMPLES AND METHODS 1

3 RESULTS 3 3.1 Petrography 3 3.2 Geochemistry 6

3.3 Mineralogy and mineral chemistry 16

4 REFERENCES 24

LITERATURE

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1 INTRODUCTION

The aim of this work was to study REE-geochemistry and mineralogy of carbonatites and their

alteration product in the southern side of Sokli. The study was made from drill cores R4, R6,

R194, R196, R198, R200, R290 and R70, which are situated in southwestern part of the Com-

plex (Figure 1).

REE-rich carbonatites of Sokli are typically formed in the late stage of carbonatite emplacement

(Vartiainen 1980). All of the REE minerals in the Sokli carbonatite are strongly light REE en-

riched, which is characteristic for carbonatites (Al Ani and Sarapää 2010, 2011). However, the

light REE-enrichment is particularly strong and suggests a high degree of fractionation. The

REEs occur mainly in the Ca-bearing phases, i.e. carbonates, apatites, Ca, accessory phases such

as pyrochlore and Ca-silicates. The petrography and mineralogy of the carbonatite from Sokli

Complex have been well described in several works (Vartiainen and Paarma, 1979; Vartiainen

and Woolley 1974, 1976; Vartiainen, 1980; Lee and Wyllie, 1996; Lee et al., 2000, 2003, 2004).

2 SAMPLES AND METHODS

This study focuses on REE concentrations in the whole rock samples and studies the REE-

bearing minerals by SEM-EDS. Sample lithological descriptions are listed in Table 1.

Whole rock samples from eight drill holes for chemical analysis were selected for this work

(Figure 1). Slides for thin sections (optical, SEM, and chemical analyses) were cut from 94 sam-

ple (Table 1). After the microscope observations, the thin sections were coated with carbon by

vacuum evaporation to study with a SEM-EDS instrument for characterizing the REE- and U-

bearing phases. Whole-rock analysis were made in Labtium by using alkaline peroxide fusion

and multi-element analysis by ICP-OES upgraded with ICP-MS-analysis for trace with REE, Y,

U or Th and fire assay method using ICP-AES techniques for Au, Pd and Pt analysis (704P).

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Figure 1. Location of studied drill cores in the geological map of the Sokli complex modified from Var-

tiainen 1980 .

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Table 1. Description of the studied boreholes from Sokli complexes

Sample No. Description

R4(53.0) Ultramafic rock , whitish color with green folation, mineralogy may be cal-cite with apatite and black minerals amphibole with biotite

R4(77.35) Ultramafic rock , Ol-Fos, green color (1-3mm) and also found dark green olivine,

white color refer to apatite with 5-10%, pyrokloori, baddelyiitti, calcite and pyrite

R4(84.85) Söv, exactly the same type as above

R4(97.05) Söv, exactly the same style as above, but Fe-content higher

R4(101.05) Tre-fos, medium grain size, green grains are fostorite with reddish grains need to see in microscope,

sample rich in calcite and apatite with 10-15%, Minerals (Tre>Flo>Fem>Ap>Cal)

R4(163.0) Fos, fine-medium grain size, homogenious, Mineralogy (Fos, Apt, flo, cal)

R6(56.0) Tre-fos, coarse grained, mineralogy (Ter>Fem, Flo>Kal, Ap, Flo ) green matrix

R6(73.85) Tre-fos, coarse grained, mineralogy (Ter>Fem, Flo>Kal, Ap, Flo ) green matrix

R6(119.35) same type as above with apatite>pyrite grains, also found Humite minerals

R6(126.15) same type as above with apatite>pyrite grains, also found Humite minerals

R6(146.0) Ter-Söv, fine grained, with ter-volume reduction.

R194(28.15) Söv weathered rock sample of about 80% has become a pale brown, cal-enrichement.

The fragments are quite Fem-Söv

R194(100.30) Söv, homogenius mode, Ter ja Flo modrately concentrated , fos more rich than cal.

R194(153.75) same type as above

R194(211.70) Söv-Fem, mineralogy suhteet: Kal>Fem>Flo>Ter.

R196(68.40) Söv., brown weathered rock, fine fractions, but also yellowish in color with radiation.

R196(119.20) Söv-fen, the stone contain 1-10 cm narrow bands of sov, for which the radiation more intensive than in fen

R196(129.25) Fen, homogeneous, medium dark,mineralogy: Ter, Fem.

R200(70.0) Fine calcite rock

R200(78.4) Carbonatite with grenish matrix of amphibolite or forsterite calcite, apatite very hard rock

R200(83.75) kalsiitt-pirot

R200(99.35) Carbonatite with grenish matrix of amphibolite or forsterite calcite, apatite

R200(104.5) Sokli fenolite, coarse grains with green minerals, and whitish matrix from calcite and apatite

R200(116.0) Fine fractions with greenish color

R200(141.2) Typical Sokli fenolite green and coarse grains with apatite and calcite as whitish color

R200(155.45) Fenolite-pegmatite with ampibole lines or folations

R200(193.25) Carbonatite, with greenish coarse grains of amphibolite and whitish calcite , apatite matrix

3 RESULTS

3.1 Petrography

Calcite is the most common mineral and it ranges in size from extremely fine to coarse-grained.

In porphyritic varieties, the matrix is represented by a mosaic of calcite grains, either subrounded

or subangular, with minor amounts of smaller grains of apatite and barite. The calcite car-

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bonatites are granulated to porphyroblast texture and are coarse to medium grained rocks mainly

composed of calcite, dolomite, apatite, magnetite, mica, amphibole, olivine (forsterite) and

phlogopite. Characteristic accessory constituents are magnetite, fluorapatite, Nb−Zr minerals

(baddeleyite, pyrochlore and zircon), ilmenite, sulphides (pyrrhotite, Pyrite), titanite, Zn spinel

gahnite and rare-earth (fluoro) carbonates. Apatite is present in most studied rock types, and

commonly occurs as one of the major noncarbonated minerals in calcite carbonatites (Fig. 2A).

Olivine, amphibole and phlogopite are the dominant silicate found in most of the studied samples

(Fig 2B, C).

Alkali feldspar also dominates represented by orthoclase and lamellar albite, forming perthitic

texturein fenitic rocks (Fig.2D). Aegirine-augite (AE-AG) also occur in carbonatite samples as

reddish brown grains in a matrix of calcite and quartz associated with phlogopite and minor

rounded (high relief) apatite grains (Fig 2E). Zircon and baddeleyite forms an aggregate of clus-

ters of small zircon crystals set in a matrix of rocks (Fig. 2F).

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Figure 2. Photomicrographs showing mineralogical characteristics of studied drill cores. (A) C1 calcite

carbonatite showing intergrowth texture between calcite and apatite. (B) sub- euhedral olivine grains

(cross, width=2.3 mm). Olivine is the major silicate minerals in studied foscorite. (c) Fibrous amphibole

(tremolite-actionlite). (D) REE-minerals filling fractures or veins within feldspar of fenite-pegmatite. (E)

View in plane-polarized light of aegirine-augite (AE-AG) of reddish brown in a matrix of rounded calcite

and quartz (QZ) with minor rounded (high relief) apatite grains in fenite. (F)Large clusters of small zir-

con and baddeleyite crystals set in a matrix of rocks

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3.2 Geochemistry

The average composition includes many rock types such as calcite-carbonatite, silico-

carbonatite, ferro-carbonatite, fenite-pegmatite, fenite-amphibolite and so on.

The average bulk composition of studied samples in Sokli samples is approximately 27.4% CaO,

15 % SiO2, 14.2 % Fe2O3, 8.2 % MgO, 1.7 % K2O, 3.7 % Al2O3, 0.5 % MnO, 0.6 % P2O5, <1%

for Na2O and TiO2 (Table 2). The high sum of Na20 + K20 at >2 % is abnormal because there

are no or very low feldspars, micas, amphiboles or pyroxenes in the carbonatite. Woolley (1982);

Woolley and Kempe (1989) have classified carbonatite on the basis of their chemical composi-

tions. The Sokli samples in (Fig. 3) were fall in all three fields on Woolley's diagram, but domi-

nantly in the calcio-carbonatite and ferro-carbonatite fields. Calcio-carbonatite bearing was cal-

cite and dolomite as the main minerals (~90 %) with apatite, baryte and surly rich in REE-

minerals. Ferrocarbonatite composed mainly from calcite (<70 %), iron oxide minerals and

dolomite with traces of apatite, columbite and REE-minerals. Silicocarbonatite is predominant in

quartz (>10 %) and calcite (<60 %) with albite, k-feldspar and apatite. According to Woolley

and Kempe (1989), it is possible to distinguish early silicocarbonatites (CI), calciocarbonatite

(CII), and later ferro-carbonatites (C III), show a progressive increase in Fe and Mg from the

oldest C I to the youngest CII carbonatites.

Figure 3. Plots showing the compositional range of the studied rocks at Sokli, which classify as calcio-

and Silico-carbonatites and ferro-carbonatitesbut (after Woolley, 1982; Woolley and Kempe, 1989).

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In the samples of studied area; TiO2, Al2O3, Fe2O3,MgO, K2O show strong to moderate positive

correlations with SiO2, and CaO, P2O5, Sr, Ba, Y and REE strong to moderate negative correla-

tions with SiO2 (Figure 4), whereas the relations of the whole elements with CaO show signifi-

cant difference with SiO2. The whole, SiO2, TiO2, Al2O3, Fe2O3,MgO, K2O show strong to mod-

erate negative correlations with CaO and P2O5, Sr, Ba, Y and REE strong to moderate positive

correlations with CaO (Figure 5).

The carbonatite rocks of Sokli Complex are characterized by unusual high contents of Ba and Sr,

averaging 3006 and 3233 ppm respectively, and in some samples reaching >9770 ppm for Ba

and >6350 ppm for Sr. Barium is mainly found as secondary baryte BaSO4 or barytocalcite

BaCa(CO3)2 in biotite and calcite. Most Sr is located in calcite and apatite to form Sr, Ba, REE-

riche minerals and/or Sr-calcite.

The multi-element variation diagram (Fig. 6), for the Sokli drill cores show relatively high nor-

malized ratios for La, Ce, Nd, Th, U, Sr and Ba but a slightly lower ratio for Ti, K and Y have

smaller ratios as appropriate to elements that occur in minerals commonly fractionated early

from carbonatite magma, i.e. titaniferous magnetite and xenotime. However, electron microprobe

analysis revealed an abundance of monazite-(Ce) (30 wt. % P2O5) in most Sokli studied samples.

The abundance of REE in the studied samples show in the density histogram (Fig. 7A), most

samples will fall into REE content field between (1000-2000 ppm) and few samples in the field

of >5000 ppm. The strong enrichment of LREE shows in boreholes R6 and R198, whereas the

strong enrichment of U and Th show in borehole R196 (~900 ppm) see (Fig. 7B).

Cullers and Graf (1984) pointed out that carbonatites have higher rare-earth element (REE) con-

tents and LREE/HREE ratios than any other igneous rock. The Sokli carbonatites are also rich in

REE, particularly the LREE and have high LaN/YbN (3.0-368 with average 94.4) and CeN/YbN

ratios (2.4-368 with average 63.8) (see Table 2).

The chondrite-normalized REE patterns (Fig. 8) are typical of carbonatites. The Sokli car-

bonatites have no Eu anomalies (a minor negative Eu anomaly is observed in few fenite samples)

and Ce anomalies, a characteristic feature of carbonatites. Carbonates samples from 5 boreholes

in Sokli complexes are remarkably similar in their REE distribution (Fig. 9). They are character-

ized by light-REE enriched patterns and HREE-depleted. The lack of a strong negative Eu

anomaly suggests that silicate minerals may have not play in a more important role for REE en-

richment in Sokli carbonatites rocks, although apatite fractionation may also have tended to off-

set the development of a negative Eu anomaly.

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Table 2. The average bulk composition of Sokli Carbonatite complex in drill cores R4, R6,

&194, R196.

Borehole R4 R6 R194 R196

Sample Min Max Avg Min Max Avg Min Max Avg Min Max Avg

MgO 2.8 15.8 9.2 2.6 16.2 9.4 3.2 13.4 9.2 2.5 8.0 4.9

Al2O3 0.2 3.9 1.4 0.4 2.4 1.0 0.3 2.9 1.1 1.1 14.8 8.6

SiO2 1.2 22.3 8.1 2.1 13.3 5.3 1.2 12.9 7.0 5.8 54.4 33.5

K2O 0.2 2.9 1.3 0.4 2.4 1.1 0.5 2.5 1.2 0.5 3.9 2.0

CaO 6.5 50.1 31.9 20.0 42.1 33.0 10.8 41.4 28.6 4.8 35.7 16.6

TiO2 0.0 2.8 0.9 0.1 1.5 0.4 0.1 1.7 0.7 0.1 1.0 0.4

MnO 0.2 0.6 0.4 0.3 0.7 0.5 0.2 0.7 0.4 0.2 0.9 0.4

Fe2O3 1.9 42.8 15.5 4.8 23.6 9.6 4.9 35.6 15.2 4.5 8.9 6.5

Co 10.7 87.3 40.1 15.6 53.6 30.5 11.0 87.0 37.4 9.3 31.4 19.5

Cu 5.9 137.0 42.8 22.3 214.0 81.4 9.4 133.0 32.8 8.0 65.3 28.4

Pb 20.0 32.4 21.2 20.0 112.0 31.9 20.0 26.4 20.5 20.0 228.0 47.0

Rb 5.0 147.0 29.7 5.5 40.6 14.8 5.0 35.8 14.2 5.0 46.1 24.9

Sb 0.5 2.5 0.6 0.5 1.1 0.6 0.5 0.6 0.5 0.5 12.6 2.1

Se 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

Ta 0.2 49.6 7.2 0.2 52.3 6.2 1.0 42.0 9.5 2.5 43.1 13.6

Zn 50.0 349.0 164.8 64.5 654.0 196.0 50.0 436.0 138.6 61.9 360.0 140.0

Ba 497.0 3410.0 978.2 526.0 9770.0 2846.7 340.0 2140.0 765.5 360.0 7650.0 1654.4

Mo 50.0 127.0 54.4 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0

Ni 50.0 197.0 60.6 50.0 72.3 51.5 50.0 50.0 50.0 50.0 50.0 50.0

S 1790.0 14600.0 5608.0 876.0 21400.0 7264.4 253.0 9200.0 4190.5 200.0 5530.0 1210.2

Sc 20.0 39.3 22.0 20.0 52.8 24.1 20.0 29.7 21.2 20.0 24.6 20.5

Sr 831.0 4850.0 3047.7 2180.0 6150.0 3976.7 1170.0 6350.0 2983.3 512.0 2890.0 1423.4

Th 3.7 71.3 25.2 10.5 145.0 31.5 7.5 185.0 41.6 11.1 405.0 124.1

U 1.0 68.5 9.6 1.2 61.1 12.2 1.2 51.2 14.0 2.8 226.0 38.6

Y 6.7 115.0 55.8 25.0 83.1 53.9 15.0 295.0 72.2 19.7 501.0 124.3

Ce 81.1 802.0 485.2 308.0 2540.0 887.4 217.0 1550.0 492.1 167.0 933.0 372.0

Dy 1.5 25.7 13.4 7.1 24.3 15.0 6.0 79.1 20.8 6.6 103.0 30.4

Er 0.8 10.3 5.4 2.8 8.0 5.7 2.6 25.9 7.9 3.0 67.1 16.4

Eu 1.2 14.9 8.8 5.6 18.2 11.7 3.4 46.1 12.2 3.5 22.8 9.6

Gd 4.1 54.2 31.2 19.4 73.8 43.3 13.3 152.0 42.0 12.1 64.7 30.7

Ge 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

Ho 0.3 4.1 2.1 1.1 3.4 2.2 1.0 11.8 3.3 1.1 22.6 6.0

La 39.4 438.0 243.1 146.0 1540.0 475.9 113.0 663.0 232.9 83.4 478.0 189.9

Lu 0.1 0.9 0.4 0.2 0.5 0.4 0.3 1.4 0.6 0.3 6.0 1.4

Nd 28.9 322.0 198.9 132.0 655.0 315.7 81.0 729.0 213.1 75.7 421.0 158.3

Pr 8.5 87.3 53.7 35.0 223.0 91.6 23.0 177.0 54.4 19.5 111.0 42.1

Sm 3.9 53.2 32.1 21.8 65.3 43.5 11.6 149.0 40.0 12.0 81.8 30.5

Tb 0.4 5.7 3.3 1.9 6.2 4.1 1.4 18.2 4.7 1.4 14.1 5.0

Te 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

Tm 0.1 1.2 0.5 0.2 0.7 0.5 0.3 2.3 0.8 0.3 9.0 2.0

Yb 0.6 6.9 3.2 1.4 3.9 2.9 1.7 12.0 4.5 2.1 48.8 11.0

LaN/YbN 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

LaN/SmN 0.0 0.3 0.1 0.1 0.2 0.1 0.1 0.7 0.2 0.1 1.4 0.4

CeN/YbN 0.0 1.7 0.2 0.0 0.2 0.1 0.0 0.6 0.1 0.0 0.1 0.0

CeN/SmN 0.0 1.0 0.4 0.1 0.6 0.2 0.1 0.9 0.4 0.1 0.2 0.2

EuN/YbN 0.2 0.3 0.3 0.2 0.4 0.3 0.3 0.6 0.3 0.3 0.5 0.4

Sum_REE 181 1807 1086 688 5145 1904 484 3622 1134 412 2176 901

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Continue: drill cores R198, R200, R290, RN70

Borehole R198 R200 R290 RN70

Sample Min Max Avg Min Max Avg Min Max Avg Min Max Avg

MgO 2.2 13.6 7.9 0.4 7.4 2.2 0.6 5.5 3.7 3.2 9.9 6.7

Al2O3 0.5 6.5 2.5 7.3 15.5 12.3 0.5 2.4 1.3 1.1 8.2 4.3

SiO2 2.1 21.2 9.6 30.8 59.3 50.6 5.8 16.1 11.2 6.0 44.1 21.2

K2O 0.5 4.0 1.8 1.4 5.2 3.3 0.6 1.4 0.9 0.3 2.0 0.9

CaO 14.6 43.5 30.6 1.3 12.6 5.2 16.9 27.6 21.5 12.1 26.9 19.8

TiO2 0.1 2.3 0.8 0.2 0.9 0.3 0.2 1.0 0.4 0.3 1.0 0.6

MnO 0.3 0.6 0.4 0.1 0.8 0.3 2.0 7.0 3.5 0.5 2.4 1.4

Fe2O3 1.7 14.6 7.2 3.3 9.3 5.6 19.5 32.6 26.9 10.5 33.3 22.2

Trace

Co 10.4 74.8 32.8 6.2 64.2 18.7 70.2 221.0 104.4 83.6 248.0 167.9

Cu 24.3 230.0 85.0 8.6 136.0 34.0 50.2 171.0 78.3 168.0 749.0 521.7

Pb 20.0 21.0 20.3 20.0 35.1 23.0 26.1 150.0 56.6 20.0 22.9 21.0

Rb 5.0 90.4 37.0 28.9 122.0 61.1 20.2 59.8 37.8 5.0 38.9 16.6

Sb 0.5 0.6 0.5 0.5 1.0 0.5 0.9 2.6 1.5 0.5 0.5 0.5

Se 10.0 11.0 10.3 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

Ta 0.3 20.8 8.0 2.7 45.1 8.4 11.6 32.7 23.6 33.5 61.1 44.8

Zn 50.0 606.0 223.1 70.5 561.0 195.5 211.0 434.0 283.2 236.0 525.0 392.2

Ba 846.0 6240.0 3035.3 1100.0 3620.0 2159.7 338.0 698.0 476.3 832.0 2800.0 1690.7

Mo 50.0 50.0 50.0 50.0 56.4 50.4 82.2 332.0 160.9 50.0 109.0 75.5

Ni 50.0 50.0 50.0 50.0 175.0 58.9 50.0 156.0 75.5 180.0 532.0 299.3

S 413.0 7310.0 3714.0 200.0 4260.0 1288.1 200.0 200.0 200.0 200.0 200.0 200.0

Sc 20.0 42.4 27.2 20.0 24.4 20.2 20.0 20.0 20.0 23.7 70.1 46.0

Sr 2070.0 5560.0 4132.5 440.0 5630.0 1851.0 1360.0 1840.0 1616.2 894.0 1590.0 1186.3

Th 8.2 33.2 19.6 12.6 177.0 57.1 54.3 206.0 103.3 46.6 148.0 102.5

U 2.0 14.7 8.6 1.2 20.5 4.4 20.5 52.7 29.7 43.7 89.1 66.5

Y 21.5 54.2 41.3 10.3 38.2 21.5 167.0 264.0 194.7 46.1 147.0 89.4

REE

Ce 542.0 1710.0 980.5 152.0 2140.0 667.9 887.0 2510.0 1438.8 274.0 1220.0 713.5

Dy 11.8 18.6 15.5 2.7 15.8 6.9 44.6 83.0 55.5 10.4 41.1 24.2

Er 4.5 7.4 6.1 1.0 5.2 2.6 17.3 30.5 21.4 4.2 15.6 9.2

Eu 7.8 13.2 11.2 2.5 15.3 6.2 23.2 47.2 30.2 6.5 28.3 16.3

Gd 26.7 50.5 39.2 7.1 55.1 21.2 72.9 156.0 97.7 19.7 90.6 51.4

Ge 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

Ho 1.8 3.0 2.4 0.4 2.1 1.0 7.1 13.3 9.0 1.7 6.4 3.7

La 270.0 1020.0 534.6 80.2 1430.0 404.8 403.0 1110.0 675.9 126.0 532.0 313.7

Lu 0.3 0.6 0.5 0.1 0.5 0.2 1.2 1.8 1.4 0.3 1.1 0.7

Nd 195.0 482.0 316.3 56.9 534.0 201.4 416.0 1070.0 614.9 133.0 579.0 340.5

Pr 56.4 154.0 94.6 16.6 184.0 64.1 107.0 290.0 165.0 34.1 149.0 87.6

Sm 28.2 52.6 42.1 8.1 58.4 24.0 79.0 172.0 105.4 22.5 96.5 56.7

Tb 2.9 4.5 3.8 0.7 4.7 1.9 10.0 18.2 11.8 2.3 9.8 5.6

Te 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

Tm 0.4 0.8 0.6 0.1 0.5 0.2 1.7 3.0 2.2 0.5 1.5 0.9

Yb 2.4 4.2 3.4 0.7 3.1 1.4 9.3 15.3 11.3 2.6 8.2 5.0

LaN/YbN 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

LaN/SmN 0.1 0.2 0.2 0.0 0.1 0.1 0.4 0.8 0.6 0.1 0.4 0.2

CeN/YbN 0.0 0.2 0.1 0.0 0.1 0.1 0.0 0.0 0.0 0.1 0.1 0.1

CeN/SmN 0.0 0.4 0.2 0.1 0.2 0.1 0.5 0.8 0.7 0.3 0.8 0.5

EuN/YbN 0.3 0.6 0.4 0.3 0.4 0.3 0.3 0.4 0.3 0.3 0.3 0.3

Sum_REE 1155 3518 2055 334 4451 1409 2094 5525 3246 643 2784 1634

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Figure 4. Variation diagrams for selected major and trace elements vs. SiO2 for Sokli studied rocks.

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Figure 5. Variation diagrams for selected major and trace elements vs. CaO for Sokli studied rocks.

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Figure 6. Multi-elements variation diagram illustrating geochemical characteristics of average Sokli

Carbonatite boreholes normalized to the Primitive Mantle values suggested by Wood et al. (1979).

Figure 7. The abundance of REE in the studied samples of Sokli.

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Figure 8. Chondrite-normalized REE pattern of studied samples from Sokli side.

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Figure 9. Chondrite-normalized REE pattern of Sokli side in some selected boreholes.

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Figure 10. Plot of chondrite-normalised light REE ratios to illustrate the degree of light REE enrich-

ment in studied rocks.

All of the REE minerals in the Sokli carbonatite are strongly light REE enriched, which is char-

acteristic of carbonatites. However, the light REE-enrichment is particularly strong and suggests

a high degree of fractionation. This is illustrated by the greater enrichment of La relative to Ce

and Nd in all studied samples in chondrite-normalized REE patterns (Figs.8 and 9). The degree

of light-REE-enrichment is such that only the light REE are detectable by electron microprobe in

most Sokli minerals and thus comparison of the chondrite-normalised REE patterns are a little

difficult. However, a plot of the ratios La/Ce vs. La/Nd ratios (Fig. 10) illustrates the major

points. The strong positive relation between these ratios is provided by the much more light-

REE-enriched minerals such as allanite, burbankite, ancylite and bastnäsite.

The variation of Sr, Ba, Th, U and REE in the studied rocks and their component mineral phases

is depicted in Fig. 11. The enrichment Sr and Ba in Sokli carbonatite is probably important in

stabilizing the early to late carbonatite crystallization that contain high levels of REE, Sr and Ba.

However, the relations between REE vs. Th, U, Sr and Ba show a positive correlation (Fig. 11)

to explain the effect of these elements in the formation of Sr, Ba, REE minerals in Sokli car-

bonatite and fenites.

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Figure 11. Relations between REE vs. Sr, Ba, Th and U in studied rocks.

3.3 Mineralogy and mineral chemistry

Several minerals containing REE, Sr and/or Ba as major elements have been identified in the

Sokli samples (Table 3). The SEM-EDS technique was used to identify some REE minerals in

studied sample. Qualitative chemical analyses of these minerals (normalized to 100 %) are listed

in Table (4).

The main constituents of the studied carbonatite are calcite, strontium calcite, dolomite apatite,

baryte, burbankite and/or carbocernaite (Ca, Na) (Sr, Ce,-Ba) (CO3)2, ancylite-(Ce) and allanite,

plus minor, pyrrhotite, pyrite, sphalerite (Zn, Fe) S and Fe-oxides and. In addition, an assem-

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blage of allanite-(Ce), bastnäsite-(Ce), monazite-(Ce), zircon, baddeleyite and rare columbite and

Th-zircon are present in some samples.

Burbankite is a double carbonate with the general formula A3B3(CO3)5, where the A-site is oc-

cupied by Na and Ca and the B-site contains Sr, REE, Ba and Ca ( Effenberger et al., 1985). We

believe that the analyses presented here are the first detailed SEM_EDS analyses of burbankite

in the Sokli carbonatite complexes. SEM data show a large variation in major oxides: Na

(3.5­7.4 wt. %), Sr (5.1-51, 2 wt. %), Ca (12.1-27.8 wt. %), REE (16.5-40.0 wt. %) and BaO

(10.2­58.2 wt. %) as seen in Table (4). There is no zoning in studied burbankite crystals and the

compositions are also uniform within each rock sample.

Table 3. Minerals containing REE, Nb Sr and/or Ba as major elements identified in Sokli carbonatite

samples.

Mineral Formula

Allanite (Ce,Ca,Y)2(Al,Fe+3

)3(SiO4)3(OH)

Ancylite-Ce SrCe(CO3)2OH.H2O

Bastnäsite-Ce Ce,LaCO3F

Burbankite (Na,Ca)3(Sr,Ba,Ce)3(CO3)5

Monazite-Ce REE.NdPO4

Baryte BaSO4

Barytocalcite BaCa(CO3)2

Columbite Fe+2

Nb2O6

Sr-Calcite SrCaCO3

Apatite Ca5(PO4)3F

Zircon ZrSiO4

Baddeleyite ZrO2

There are two morphological types of burbankite, lamellae prismatic crystals and small drop-like

inclusions within calcite, dolomite, and apatite crystals and along their boundaries (Figs. 10 and

11). The difference in morphology suggests they may have formed by different processes. Long

euhedral prismatic crystals could have crystallized at an early stage of carbonatite formation, and

sometimes large burbankite crystals are observed near the contact of veins that are characterized

by a crustification texture (vein filling) see Figure (10A, B, C). The drop-like burbankite inclu-

sions found mostly in calcite, apatite and pyrite and in some cases associated with barite (Fig 10

E, F) and (Fig 11B). They are more likely to be the result of ‘cotectic’ or overgrowth crystalliza-

tion of burbankite and monazite after the precipitation of the calcite and apatite (Fig. 10B). The

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burbankite is commonly associated with ancylite-(Ce), Sr-calcite and baryte and shows more en-

riched in studied samples of R6 and R196 boreholes.

Ancylite-(Ce) is common at Sokli carbonatite samples; bastnäsite-(Ce) and monazite-(Ce) are

also found but are rare. The composition of ancylite-Ce from Sokli contains between 23.5 and

31.4 wt% Ce, 12.5 and 18.1 wt% La, 5 and 7 wt% Nd and 5.2-51 wt% Sr. The atomic proportion

of Sr and REE in the formula is near 1:2 with a small excess of REE. Ancylite-Ce is relatively

high with Ca 12.6-21.5 wt % and Ba present only as traces in few sample 4.2-8.7 wt %.

Some studied ancylite-(Ce) is characterized by high content of Th between 6.4 and 7.1 wt. % see

Table (4). The ancylite occurs as fibrous crystals overgrowth with allanite (Fig. 11A) or anhedral

aggregates and nodules intergrowth with Sr-calcite (Figs. 11C, E, F). Monazite and bastnäsite

occur as irregular grains and nodules frequently intergroup with REE–Sr–Ba-bearing minerals or

it’s found as filling the fractures within rocks (Fig. 11D).

Table 4. The SEM-EDS for REE-rich minerals in Sokli carbonatite samples.

Mineral Ancylite (Ce) Th-Ancylite Mineral Bastnäsite

(Ce)Spectrum 1 2 3 4 1 2 Spectrum 1 2 3Mg 1,65 8,62 2,67 3,54 F 1,3 1,4 1,3

Ca 19,56 21,48 12,64 13,94 17,41 16,34 Na 3,99 3,1

Fe 1,99 4 10,03 5,39 4,17 Mg 3,8 2,9 2,84

Sr 24,45 22,73 22,08 24,78 16,46 15,77 Si 3,55 2,53 5,54

Ba 7,45 8,69 4,19 P 6,83 11,69

La 18,15 13,23 14,31 16,63 12,56 12,92 Ca 20,42 21,68 13,54

Ce 30,1 24,05 31,39 28,46 23,58 24,2 Fe 4,24 3,68

Nd 5,75 6,47 6,97 6,15 6,87 5,44 Sr 9,01 7,21 33,7

Th 6,37 7,1 Ba 28,43 26,34 23,48

Ce 17,96 19,38 19,6Total 100 95,41 91,39 99,99 90,96 83,03 Total 99,53 99,91 100

Mineral Allanite Mineral Monazite

Spectrum 1 2 3 4 5 6 Spectrum 1 2 3

Na 2,42 5,15 5,41 Si 8,34 8,78 3,84

Al 8,47 9,84 11,02 4,91 9,39 8,49 P 29,42 29,13 19,76

Si 19,41 16,63 21,9 8,47 27,13 25,69 Ca 33,88 32,91 19,27

Fe 1,91 2,04 17,06 2,74 31,15 Fe 7,94 9 4,65

Ca 4,95 4,02 3,5 5,18 4,09 2,03 Y 4,18 6,25

La 16,75 13,65 11,81 13,2 16,09 8,38 La 13,63

Ce 29,42 24,98 22,19 22,04 32,95 18,86 Ce 4,22 4,6829,01

Nd 5,73 5,95 7,62 Nd 8,05

W 19,09 26,43 18,72 20,03 Th 5,22 3

U 4,11 6,25

Total 100 100,01 100,02 96,84 100,01 100,01 Total 97,31 100 98,21

Mineral Burbankite Mineral U-ColumbiteSpectrum 1 2 3 4 5 6 Spectrum 1 2 3Na 6,65 3,52 2,9 7,4 5,7 Si 5,32 18,08

Mg 4,67 4,66 6,07 5,23 6,9 3,2 Ca 11,45 12,43 13,84

Ca 20,41 14,35 13,53 12,11 17,95 27,83 Ti 12,21 5,27 2,57

Fe 6,51 28,7 24,68 3,21 3,63 Fe 5,26 1,55 1,77

Sr 5,15 6,09 14,5 19,7 51,22 21,85 Nb 25,96 33,06 19,41

Ba 58,19 31,42 16,67 19,47 10,22 Ba 12,48 10,67

Ce 11,58 30,32 12,78 15,26 10,12 26,76 Ta 12,55 17,19 21,59

Nd 6,67 6,46 9,71 W 4,03 6,33

U 23,21 18,03 5,75

Total 100 100 95,77 99,35 96,8 99,19 Total 100 100 100

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Figure 12. BSI of burbankite with associated minerals, (A) burbankite and ancylite overgrowth within

apatite and associated with large assemblage of zircon (B) large lamellae aggregates of burbankite with

monazite in apatite, (C) acicular crystals growth of burbankite within Sr-calcite, (D) burbankite with

zircon, (E,F) burbankite intergrowth within apatite.

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Figure 13. BSI of REE in R6 boreholes. (A) intergrowths of ancylite-(Ce) and allanite, (B) inclusions of

burbankite within pyrite, (C) aggregates of ancylite and burbankite within Sr-calcite, (D) clustering of

ancylite, allanite and monazite within dolomite, (E, F) Ancylite associated with calcite and baryite.

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Allanite-(Ce), which has an idealized formula (Ce, Ca, Y) 2(Al, Fe3+) 3(SiO4) (OH) is the most

abundant and widespread REE-bearing minerals in the studied samples. The levels of allanite in

samples of R200 are very high and occur as coarse grains and clustering in many assemblages as

seen in Figures (12 and 13).

The majority of allanite aggregates represent in discordance with the main foliation, but a few

are also present as elongated micro-boudinages with their long axes parallel or subparallel to the

main foliation (Fig. 12). Locally, allanite crystals of acicular habit also are present in close asso-

ciation with veins or filling fractures (Fig 13). Allanite from the study area contains a significant

amount of W (18.7-26.4 wt %) and is characterized by a high REE content, which is evident in

the energy-dispersion spectrum (EDS). Quantitative electron-microprobe analyses yielded light

REE content of up to 50 wt% see Table (4).

Figure 14. Scanning electron micrograph of acicular crystals of allanite in sample R200(141.2) from

Sokli fenite.

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Figure 15. Scanning electron micrograph of acicular crystals of allanite in different clusterin in sample

R200(155.45) from Sokli fenite.

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Columbite (Fe, Mn)(Nb, Ta)2O6 are common accessory minerals in various studied samples, and

incorporate rare earth elements (REEs) as well as radioactive elements such as U. The studied

columbite in Sokli carbonatites are characterized by high U (up to 23 % in some grains) and Ta

(up to 21%) contents see Table (4). Columbite in the Sokli generally has euhedral cubes or octa-

hedral with complex compositional zoning that can be seen in backscattered electron images

(Fig. 14).

Figure 16. Scanning electron micrograph of columbite crystals in different samples of Sokli carbonatite.

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