Microprobe studies of REE-rich accessory minerals from alkaline, appinitic and metamorphic rocks ...

47 Pages·2011·4.91 MB·English

Northern Finland Office
M42/2010/69 2.2.2011
Rovaniemi

Microprobe studies of REE-rich
accessory minerals from alkaline,
appinitic and metamorphic rocks,
Central and Northern Finland
Thair Al-Ani and Olli Sarapää
GEOLOGICAL SURVEY OF FINLAND DOCUMENTATION PAGE
Date / Rec. no.
2.2.2011
Authors Type of report
Thair Al-Ani and Olli Sarapää M42
Commissioned by GTK

Title of report Microprobe studies of REE-rich accessory minerals from alkaline, appinitic and hydrothermal
rocks, Central and Northern Finland
Abstract
Carbonatite, alkaline rocks and appinites, and hydrothermal rocks from 14 geological targets in Finland: Jammi,
Iivaara, Otanmäki (Katajakangas), Korsnäs, Kuusamo (Uuniniemi, Honkilehto), Kortejärvi and Laivajoki,
Mäkärä,Vanttaus, Lehmikari, Suhuvaara, Enontekiö (Palkiskuru, Palovaara) contain one of the most diverse
assemblages of REE - Y -U minerals described thus far from carbonatites and provide an excellent opportunity to
track the evolution of late-stage carbonatite –alkaline rocks and their sub-solidus (secondary) changes. More than
twenty rare earth minerals have been identified and analyzed in detail and compared with the literature analyses.
These minerals include some common to carbonatites, alkaline and hydrothermal rocks e. g. phosphates;
monazite-(Ce), fluor-carbonates; bastnaesite-(Ce), hydrated carbonates; ancylite-(Ce), hydrate aluminium
silicates; allanite, oxides; fergusonite, and U-Pb rich minerals.
The main REE-rich carbonate minerals recognized in the Jammi carbonatite veins are ancylite and bastnaesite.
They are strongly enriched in LREE, P, F, Sr and Ba. Apatite occurs as large and elongated grains, closely
associated with monazite. Allanite-(Ce) and Fergusonite (Y) are the most abundant and widespread REE-bearing
minerals in Katajakangas alkaline rocks at Otanmäki. The levels of REE in allanite are very high. Ce is dominant
over La in all cases, and allanite contains the highest atomic percentage of total LREE (~50 %) and total iron
Fe O (2.5 %) and the lowest aluminium Al O (5.9 %) as well as radioactive elements such as Th (3 %) and
2 3 2 3
uranium (below the detection limit) observed in the present study.

In Korsnäs, the REE-bearing minerals of carbonatite include apatite, monazite, calcio-ancylite and bastnaesite.

In most samples large clusters of monazite grains occur as inclusions within apatite phenocrysts.
In Iivaara the REE-bearing minerals of nepheline-syenite are apatite and allanite with low contents.
In Uuniniemi, hydrothermal rocks are composed mainly of albite, quartz and mica as primary minerals, but they
are also highly enriched with apatite and REE-minerals such as monazite (Ce), Fe-columbite, euxenite (Y) and
zircon. BSE imaging of the samples in Uniniemi show high content of monazite, which characterized by large
grain size with significant porosity and fractures. In the studied arkosic gneisses of Mäkärä in Sodankylä, there
are many varieties of radioactive minerals such as columbite and euxenite. Euxenite occurs as very fine bright
crystals filling the fractures within hosted minerals, whereas columbite is surrounded by euxenite grains. In
Vanttaus, Lehmikari and Suhuvaara appinite rocks the main accessory minerals are apatite and sphene, but the
content of the REE-bearing minerals, allanite and monazite, is generally low.
In the Enontekiö albitites the REE-rich minerals are bastnaesite, monazite, allanite, xenotime as well as U-rich
minerals such as davidite, masuyite and /or sayrite. In Honkilehto, Kuusamo the U-rich minerals are associated
mainly with bastnaesite and allanite. The Kortejärvi carbonatites are characterized by enrichment of apatite with
monazite and allanite. At Laivajoki samples also show highly content of carbonate minerals and apatite with high
concentrated of REE-minerals especially allanite and monazite.
Finally, we conclude that the pattern of evolution of REE, Y, U, Th and Nb in the studied areas was a complex,
multi-stage process, and involved primary magmatic crystallization and late-stage hydrothermal alteration.
Keywords REE-minerals, monazite, ancylite, bastnaesite, allanite, euxenite, fergusonite, columbite, davidite,
masuyite and /or sayrite, U-minerals
Geographical area: Finland, Lapland, Savukoski: Jammi, Rovaniemi: Vanttaus, Lehmikari, Inari: Suhuvaara,
Enontekiö: Palkiskuru, Palovaara, Sodankylä: Mäkärä, Posio: Laivajoki, Pohjois-Pohjanmaa, Kuusamo: Iivaara,
Uuniniemi, Honkilehto, Pudasjärvi: Kortejärvi, Kanuu: Otanmäki: Katajakangas, Korsnäs
Map sheet
Other information
Report serial Archive code
M42 M42/2010/69
Total pages Language English Price Confidentiality
42 English Confidential until 31.12.2011
Unit and section Project code
Northern Finland Office 2141007
Signature/name Signature/name
Thair Al-Ani Olli Sarapää
Contents
1 INTRODUCTION 1
2 SAMPLES AND ANALYTIC METHODS 2
3 MINERALOGY AND MINERAL CHEMISTRY 3
3.1 Jammi carbonatite veins in Sokli complex 4
3.2 Otanmäki 6
3.3 Korsnäs 10
3.4 Kuusamo 12
3.4.1 Uuniniemi 13
3.4.2 Honkilehto 16
3.5 Sodankylä, Mäkärä 18
3.6 Vanttaus 20
3.7 Lehmikari 23
3.8 Suhuvaara 26
3.9 Enontekiö 29
3.9.1 Palovaara 29
3.9.2 Palkiskuru 29
3.10 Kortejärvi 33
3.11 Laivajoki 36
4 CONCLUSIONS 40
5 REFERENCES 42

LITERATURE
1

1 INTRODUCTION
Hi-tech metals, including rare earth elements (REE), lithium, titanium, gallium, germanium, nio-
bium, indium, tellurium and tantalum, have long been known but only recently sought due to
their critical role in high-tech applications and energy efficient technologies. This study summa-
rizes the results from seven reports from Central and Northern Finland (Al-Ani and Sarapää
2009, 2010, Al-Ani and Lauri 2010, Al-Ani et al. 2010). These reports include REE mineralogy
of carbonatite, alkaline rocks and appinites, and hydrothermal rocks from 14 geological targets in
Finland: Jammi, Iivaara, Otanmäki (Katajakangas), Korsnäs, Kuusamo (Uuniniemi, Honkilehto),
Mäkärä,Vanttaus, Lehmikari, Suhuvaara, Enontekiö (Palkiskuru, Palovaara), Kortejärvi and
Laivajoki.

Studies of REE distributions in carbonatites have established that calcite, dolomite, apatite, pyro-
chlore, perovskite and Ca-bearing silicates are strongly enriched in REE, which increase from Lu
to La (Möller et al. 1980, Viladkar and Pawaskar 1989). The REE reside mainly in carbonates,
apatites, Nb-oxides and Ca-silicates, and they substitute with Sr2+ for Ca2+. Specific REE miner-
als such as ancylite, bastnaesite, britholite, monazite, parisite, etc. are commonly, products of the
breakdown of REE- and Sr-rich primary carbonate and apatite during post-magmatic processes
(Andersen, 1986; Lottermoser, 1988, 1990).
The REE-rich accessory minerals bastnaesite, monazite, allanite, fergusonite and xenotime play
a key role in the storage and mobility of geochemically important trace elements (LREE, Y Th
and U) are relatively common accessory minerals in both igneous and metamorphic rocks.

In this study, backscattered electron (BSE) imaging and electron microscope analysis (EDS) are
used to document various mineralogical characteristics and paragenesis of various types of REE
minerals from different localities in Finland (Figure 1). The principal REE- minerals in studied
rocks are phosphates; monazite-(Ce), carbonates; bastnaesite-(Ce), hydrated carbonates;
ancylite-(Ce), hydrate aluminum silicates; allanite, and oxides; fergusonite. Other rare earth min-
erals such as britholite, thorite, carbocernaite and columbite have been reported from some of the
studied samples.

The research was carried out with three objectives: (1) to study the REE, Y, Th and U composi-
tion of major minerals; (2) to learn the nature, abundance, composition, grain-size distribution,
textural relationships and associations of REE, Y, Th, U-rich accessories; (3) to determine the
relative contributions of accessory minerals to REE, Y, Th and U.
2

Figure 1. Location of target areas.
2 SAMPLES AND ANALYTIC METHODS
The samples were collected from the drill cores and outcrops of 14 different targets (Table 1). 57
polished sections were prepared from representative specimens of REE-mineral assemblages and
examined under a polarizing microscope. The polished sections of each sample were studied
with a scanning electron microscopy (SEM) and an electron probe microanalyzer (EPMA). The
mineral compositions were analyzed by a scanning electron microscope (SEM) JEOL JSM 5900
LV, at the Research laboratory of GTK, Espoo.

Mineral chemistry was investigated by a standard electron microprobe techniques on polished
sections, using CAMECASX100/LKP at the Research laboratory GTK, Espoo. The accelerating
voltage was 15keV with beam current of 20nA and 5 micrometers respectively. The spot size
was 1μm in the analyses without fluorine and 5μm when fluorine was determined. Samples and
standards were polished to provide a flat surface for sputtering of secondary ions. After polishing
3

with diamond paste, samples were carbon-coated to prevent charging during electron probe mi-
cro-analyzer EPMA analyses.

Scanning electron microscopy studies were performed on carbon-coated sections using a field-
emission instrument (JEOL JSM 5900 LV), fitted with a back-scattered electron (BSE) detector,
and an energy-dispersive X-ray (EDX) micro-analyzer that was used as an aid in mineral identi-
fication (qualitative analyses). The chemical composition of the REE minerals was determined
using wavelength-dispersion analyses on a CAMECASX100/LKP electron microprobe (EMP).
The acceleration voltage was 20 kV, with a beam current of 40 nA and a beam diameter of 3
mm.

3 MINERALOGY AND MINERAL CHEMISTRY

More than twenty REE-bearing minerals have been identified in the studied samples from differ-
ent target areas in Central and North Finland. The mineralogy and paragenesis of accessory
REE-rich minerals and the associated minerals are summarized in Table (1).

The representative results of the quantitative electron-microprobe analyses of the REE-rich min-
erals demonstrate how Ce predominates over Y in all REE-mineral phases and therefore the
minerals correspond to allanite-(Ce), monazite (Ce), bastnaesite (Ce) and ancylite (Ce). Most of
the target areas are enriched in the light REE, which are less valuable than heavy REE.
Many REE minerals are associated with elevated U and especially Th levels, and therefore some
samples contain U-minerals such as davidite, masuyite, sayrite (Enontekiö and Kuusamo) and
thorite (Uuniniemi and Lehmikari) see Table (1).

Table 1. The main REE-minerals in studied samples.
Locality Rock type REE-mineral phases
Jammi Carbonatite veins F-apatite, Sr-apatite, monazite, bastnaesite, ancylite, strontianite, baryte
Iivaara Nepheline-syenite Apatite, allanite
Otanmäki Alkaline-gneiss Fergusonite(Y), Fergusonite(U), allanite, columbite
Korsnäs Carbonatite Apatite, monazite, carbocernaite, calcio-ancylite, bastnaesite, baryte, barytocalcite
Uuniniemi Carbonatite and albitite Apatite, monazite, euxenite, Fe-columbite, Fe-thorite
Mäkärä Arkosic gneiss Euxenite, columbite, zircon
Vanttaus Appinitic diorite Apatite, allanite, sphene, zircon
Lehmikari Appinite F-apatite, monazite, allanite, ancylite, thorite, zircon, baryte
Palkiskuru Albitite Apatite, bastnaesite, allanite, monazite, ancylite, davidite, masuyite, sayrite, zircon
Palovaara Albite-carbonate-rock Allanite; ancylite, bastnaesite and xenotime
Honkilehto Carbonate mica schist Bastnaesite, allanite, davidite, U-Pb minerals, U-Si minerals
Kortejärvi Carbonatite Apatite, allanite, monazite, bastnaesite, columbite
Laivajoki Silicocarbonatite Apatite, monazite, allanite, bastnaesite
Suhuvaara Diorite Monazite, allanite
4

3.1 Jammi carbonatite veins in Sokli complex

At least 7 minerals containing REE, Sr and/or Ba as major elements have been identified in the
carbonatitite veins (Table 1). The selected drill cores of Jammi carbonatite consist of calcite-
dolomite-apatite with silicate minerals, such as biotite or phlogopite, albite, and aegirine. Alka-
line rocks contain a variety of pyroxene-group minerals, but only aegirine NaFe3+Si O is exclu-
2 6
sive to these rocks. Sulfide and oxide mineral content range from minor amounts to nearly 25
percent and include pyrite, pyrrhotite, chalcopyrite, galena, magnetite, hematite, goethite and
rutile. Typical accessory REE-bearing minerals are monazite, REE-rich Sr-apatite, strontianite,
ancylite-Ce, and bastnaesite. They are strongly enriched in LREE, P, F, Sr and Ba. Apatite oc-
curs as large and elongated grains, closely associated with monazite. Figure 2a shows that Sr-
poor apatite in the core of the grain (dark grey color) has been later transformed to Sr-rich apatite
[(Sr,Ca) (PO ) F] at the rim of the grain (bright color) and there Sr contents reach up to 16 wt%
5 4 3
SrO.

Jammi carbonatite also contains Sr derivatives enriched with LREE, ThO and Y O : REE-rich
2 2 3
Sr-apatite, fluorapatite, (Ca,Sr,Na,Ce) (PO ) F, brabantite CaTh(PO ) and belovite
5 4 3 4 2
(Sr,etc) (PO ) O (Table 2). In all these derivatives, the rare earth element distributions of studied
5 4 3
Sr-apatites are dominated by cerium, with Ce+La+Nd accounting for 25–30 % and also rich in
Th ~9 % and Y ~2 % (Al-Ani and Sarapää 2009). Cerium is therefore an excellent means of dif-
ferentiating between phases of crystallization. Replacement of primary and secondary apatite by
monazite is common in most of the studied samples forming different a circular form as sun
flower center (Fig 2b).

The main REE-rich carbonate minerals recognized in studied Jammi samples are ancylite and
bastnaesite. Ancylite is present as ancylite-Ce forming complete hexagonal prismatic pseudo-
morphs after apatite, and bastnaesite occurs as irregular grains and nodules frequently intergrown
with microcrystalline monazite or other REE–Sr–Ba-bearing minerals or as filling the fractures
within rocks (Fig. 2c, Table 2).
5

Figure 2. Series of backscattered electron images of REE minerals from Jammi carbonatite,
(a) Sub-parallel idiomorphic crystals of apatite with hexagonal cross sections. Sr-poor apatite in
the core of the grain has been altered to Sr-rich apatite at the rim of the grains (bright color),
(b) Apatite as elongated crystals enclosed with Sr-apatite and monazite grains. The monazite
forming different a circular form as sunflower center, (C) Ancylite (Ce) occurs as complete hex-
agonal prismatic pseudomorphs as apatite structure type, the abundance of Sr increases from
cores to rims of grains. In addition the bastnaesite occurs as aggregates with rims of ancylite or
filling the fracture of minerals.
6

Table 2. Electron microprobe analyses (wt %) of apatite, monazite and ancylite from Jammi car-
bonatite.

Mineral Apatite Ce-Monazite Ancylite(Ce)
sample R301( 197.15) R301 (204.9) R301 (209.7)
Oxides Sr-poor Sr-rich grain1 grain2 grain3 grain4 grain1 grain2 grain3 grain4 grain5
SiO2 0,04 0,06 0,4 0,4 0,8 0,6 1,4 0,1 0,1 0,1 0,6
FeO 0,65 1,39 3,4 1,8 1,0 0,7 0,0 0,1 0,1 0,3 0,1
MnO 0,21 1,35 0,0 0,0 0,1 0,0 0,0 0,1 0,0 0,1 0,2
MgO 0,61 0,53 0,1 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0
CaO 45,59 34,71 2,2 2,5 3,1 2,8 2,5 0,7 0,5 0,8 1,2
Na2O 3,70 1,78 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0
SrO 1,93 14,12 3,6 4,2 6,0 6,0 18,2 25,2 23,6 23,9 25,0
BaO 1,98 3,48 0,3 0,2 0,5 0,5 0,4 0,7 0,6 0,4 0,4
Y2O3 0,01 0,00 0,2 0,2 0,1 0,0 0,0 0,0 0,0 0,0 0,0
Nd2O3 0,00 0,00 11,0 10,2 10,1 10,2 9,3 8,3 7,8 8,1 9,1
ThO2 0,04 0,01 1,5 2,3 2,4 2,0 4,9 1,8 2,2 2,5 4,2
U2O3 0,02 0,00 0,0 0,1 0,0 0,0 0,1 0,2 0,1 0,0 0,3
La2O3 0,00 0,03 9,7 10,0 9,3 9,2 11,4 12,6 14,1 14,5 11,0
Ta2O5 0,03 0,01 0,0 0,0 0,0 0,2 0,0 0,0 0,0 0,0 0,0
Nb2O5 0,05 0,00 0,0 0,0 0,0 0,0 0,1 0,0 0,1 0,0 0,0
Ce2O3 0,76 1,38 30,0 29,4 28,6 29,5 33,3 32,5 31,1 33,0 33,9
PbO 0,00 0,02 0,1 0,0 0,1 0,3 0,0 0,0 0,0 0,0 0,0
P2O5 35,20 35,25 25,8 24,9 24,3 24,6 0,0 0,0 0,0 0,1 0,1
SO3 0,01 0,03 0,9 1,5 2,1 2,3 0,0 0,0 0,0 0,0 0,0
F 0,50 1,90 0,7 0,6 0,6 0,8 0,7 0,8 0,7 0,7 0,5
Cl 0,01 0,01 0,0 0,1 0,0 0,1 0,0 0,0 0,0 0,0 0,0
Total 91,34 95,50 90,0 88,4 89,4 89,9 82,3 83,1 81,1 84,7 86,6

3.2 Otanmäki

The studied drill cores are from Otanmäki Katajakangas and include amphibole-biotite gneiss,
quartz-feldspar gneiss and quartz-amphibole-carbonate gneiss and alkaline gneiss.

Allanite-(Ce), which has an idealized formula (Ce,Ca,Y) (Al,Fe3+) (SiO )(OH) and fergusonite
2 3 4
Y(Nb, Ta)O , are the most abundant and widespread REE-bearing minerals in the Otanmäki
4
samples (Al-Ani et al. 2010).

The REE contents of allanite in drill cores of OTA-R19 and OTA-R20 are very high. Ce is do-
minant over La in all cases (Table 3). In addition, it contains the highest atomic percentage of
total REE (~50 %) and total iron (2.5 %) and the lowest aluminum (5.9 %) as well as radioactive
elements such as Th (3%) and uranium (below the detection limit) observed in the present study.
The results of 260 analyses by Čech et al. (1972) represented only five cases where the REE con-
tents are higher than 27.5 wt %, but in the Otanmäki samples the average total REE content is
about 50 %. In the BSE images of the sample OTA-R19-39.65 (Fig. 3a), the allanite crystals dis-
play series of chemically distinct zones that reflect acicular overgrowth of allanite radiating