Petrography and Mineralogy of REE-bearing minerals of Iivaara, Otanmäki and Korsnäs alkaline ...

25 Pages·2011·3.59 MB·English

Northern Finland Office
M42/2010/15 8.3.2010
Rovaniemi

Petrography and Mineralogy of REE-bearing minerals of
Iivaara, Otanmäki and Korsnäs alkaline rocks

Thair Al-Ani, Olli Sarapää and Akseli Torppa
GEOLOGICAL SURVEY OF FINLAND DOCUMENTATION PAGE
Date / Rec. no.
8.3.2010
Authors Type of report
Thair Al-Ani, Olli Sarapää and Akseli Torppa M42
Commissioned by

GTK
Title of report
Petrography and mineralogy of REE-bearing minerals of Iivaara, Otanmäki and Korsnäs alkaline rocks
Abstract
Thirteen drill core samples from Iivaara, Otanmäki and Korsnäs, middle Finland, were studied in detail to investi-
gate their petrography and concentration of REE in different mineral phases. The studied samples were urtite,
ijolite and melteigite from Iivaara, alkaline gneiss from Otanmäki, and carbonatite from Korsnäs.
Three types of REE- mineralization can be distinguished. The Otanmäki type is represented by slightly enrichment
of allanite-(Ce) and fergusonite with some accessory minerals such as columbite. Fergusonite is characterized by
occurrence of high content of radioactive elements such as Th and U. The carbonatite samples of Korsnäs con-
tains two phases of REE-minerals. First phase is REE-hydrated carbonate namely ancylite-(Ce) and bastnaesite-
(Ce), with traces of monazite. The second phase is marked by enrichment in discrete crystal of apatite with exsolu-
tion-induced domains of monazite grain. The studied samples in Korsnäs show a large cluster of monazite grains
as inclusions within apatite phenocrysts. The occurrence of REE-minerals in the Iivaara samples is lower than in
the studied samples of Otanmäki and Korsnäs.

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. The
occurrence of REE-bearing accessory mineral assemblages is essentially controlled by melt composition, tempera-
ture, and oxygen fugacity.

Keywords
Urtite, ijolite, melteigite, alkaline gneiss, quartz-feldspar gneiss, carbonatite, rare earth minerals, REE, Y, Th, Nb
Geographical area
Middle Finland , Iivaara, Otanmäki, Korsnäs
Map sheet
452308, 343102, 124205
Other information

Report serial Archive code
M42/2010/15
Total pages Language Price Confidentiality
22 English Confidential until 21.12.2011
Unit and section Project code
Northern Finland Office 2141007
Signature/name Signature/name
Thair Al-Ani Olli Sarapää Akseli Torppa
Contents
Documentation page

1 INTRODUCTION 1
2 SAMPLES AND ANALYTIC METHODS 1
3 PETROGRAPHIC DESCRIPTION 3
3.1 Iivaara 3
3.2 Otanmäki 4
3.3 Korsnäs 7
4 MINERALOGY AND MINERAL CHEMISTRY 9
4.1 Iivaara 9
4.2 Otanmäki 11
4.3 Korsnäs 15
5 CONCLUSIONS 22
6 REFERENCES 22

LITERATURE
1

1 INTRODUCTION

High-tech metals, including Rare Earth Elements (REE), lithium, titanium, gallium, germanium,
niobium, 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.

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 miner-
als from three localities in Finland: Iivaara, Otanmäki and Korsnäs (Figure 1). The principal
REE- minerals in studied rocks are phosphates; monazite-(Ce), fluor-carbonates; bastnaesite-
(Ce), hydrated carbonates; ancylite-(Ce), hydrate aluminum silicates; allanite, and oxides; fergu-
sonite. Other rare earth minerals such as britholite, thorite, carbocernaite and columbite have
been reported from some of the studied samples.

2 SAMPLES AND ANALYTIC METHODS
Thirteen samples were selected from the core samples of Iivaara, Otanmäki and Korsnäs alkaline
rocks (Table 1). Petrographic observations and scanning electron microscope (SEM) were per-
formed on JEOL JSM 5900 LV, at the Electron optical laboratory of GTK/Espoo. Our SEM
analyses concentrated on REE, Y and U-rich accessory minerals. The selected samples were also
studied under petrographic microscope. All photographs are taken from thin sections, polished to
.03 mm thickness, in transmitted white light, both under plane polarized light and under crossed
polarizer.

Table 1. Locations and descriptions of samples.
Studied area Drilling core Depth Type of rock
Iivaara
IVA-R1 15.8 Nepheline-syenite
IVA-R3 28.45 Nepheline-syenite
IVA-R4 43.35 Clinopyroxenites
Otanmäki
OTA-R19 39.65 Quartz-feldspar gneiss
OTA-R19 79.4 Amphibole-quartz-feldspar gneiss
OTA-R20 122.05 Quartz-feldspar gneiss
OTA-R20 151.95 Carbonatite-gneiss rocks
OTA-R142 52.8 Quartz-feldspar gneiss
OTA-R142 283.8 Amphibole-quartz-feldspar gneiss
OTA-R142 448.0 Quartz-feldspar gneiss
Korsnäs
SÖ-66 13.6 Marble
SÖ-66 22.8 Calc-silicate gneiss
SÖ-104 5.05 Calc-silicate gneiss
2

Figure 1. Location of studied areas.
3

3 PETROGRAPHIC DESCRIPTION

A total of 13 samples were studied under petrographic microscope. All photographs are of thin
sections polished to .03 mm thickness, in transmitted white light, under plane polarized light and
under crossed polarizer.

3.1 Iivaara
Iivaara alkaline complex belongs to the Devonian alkaline province, 373- 363 Ma, and consists
of nepheline –clinopyroxene rocks, urtite (nepheline>70 modal %), ijolite (30-70 modal %), and
melteigite (nepheline <30 modal %) (O’Brien et al. 2005).

Petrological study indicates that, two families of alkaline rocks have been distinguished:
1. Ijolite with large (several mm) euhedral, slightly zoned, aegirine–augite crystals sur-
rounded by subhedral nepheline associated with magnetite and rutile. Locally, large sub-
hedral nepheline grains are granulated to small subgrains that grade into the fine-grained
matrix made of nepheline and cancrinite surrounded aegirine–augite and pyroxene crys-
tals (Fig. 2a, b). Alkali feldspar occurs as recrystallized grains characterized by deformed
albite twin lamellae, with most grains composed of nearly pure albite, but grains with
perthitic texture, diagnostic of orthoclase, also occur (Fig. 2f).
2. Melteigite display typical cumulate texture, varying from orthocumulate to mesocumu-
late. The essential cumulus mineral is pyroxene with subsidiary nepheline, olivine, and
apatite. The interstitial space is filled with anhedral, locally poikilitic phlogopite, magnet-
ite, nepheline/cancrinite, and calcite (Fig. 2c, d).

Alkali feldspar also dominates represented by orthoclase and lamellar albite, forming perthitic
texture (Fig.1f). Apatite (up to >1 mm) is present in both types of Iivaara rocks as subhedral
grains located either in the interstitial space between the aegirine–augite grains or in the fine-
grained matrix. Apatite is locally in equilibrium within anhedral nepheline (Fig. 2e); elsewhere it
occurs as polygonal grains concentrated in segregation pockets filling fractures.
4

Figure 2. Photographs showing textures of alkaline rocks from the Iivaara locality, on the left, in plane
polarized light, but on the right, photographs under crossed polarizer. (a, b) Sample IVA-R1-15.8; (c, d)
Sample IVA-R3-28.45; (e, f) Sample IVA-R4-43.35; (Ap: Apatite, Ae-Au: Aegirine–augite, Ne: Nepheline,
Mg-Ru: Magnetite-rutile, Py: pyroxene, Ab: Albite, Cn: nepheline/cancrinite, Bi-Qz: Biotite-quartz, Srp:
serpentine; Fe: Feldspar).
3.2 Otanmäki

Petrographic description of seven samples from the Otanmäki Katajakangas area included am-
phibole-biotite gneiss, quartz-feldspar gneiss and quartz-amphibole-carbonate rocks, and alka-
line-gneiss which are the main rock types in the studied drill cores. The cores were drilled by
Rautaruukki Oy in 1983 during the study of the Katajakangas Nb-lanthanide mineralization.
5

Quartz-feldspar gneiss: This rock is composed mainly of quartz, K-feldspar, albite, hornblende,
biotite, colorless mica, garnet and chlorite. Grain sizes of the constituent minerals vary between
50-300 µm, except for biotite and quartz porphyroblasts which are up to 1-3 mm in diameter.
Elongated hornblende grains with lobed grain boundaries poikiloblastically enclose quartz, apa-
tite and feldspar. Opaque minerals are magnetite, ilmenite and hematite. Prehnite occurs in stud-
ied quartz-feldspar gneiss as secondary or hydrothermal mineral in veins and cavities. Prehnite is
a calcium aluminum silicate hydroxide mineral Ca Al (AlSi O ) (OH) . The prehnite aggre-
2 3 10 2
gates in studied samples are a probably a product of low-grade or hydrothermal alteration. Figure
(3) illustrates occurrence of prehnite in veins and cavities, indicating hydrothermal alteration of
the studied area. Sphene, iron oxide, and apatite are common abundant accessories, whereas zir-
con is rather rare.

Amphibole-biotite-quartz gneiss: This rock includes amphibole ±biotite, quartz, feldspar, pla-
gioclase ± opaques, zircon, calcite, and apatite. In the studied rock, the iron-rich alkaline amphi-
bole is segregated as veins or distributed interstitially (Fig. 3a, b and Fig. 4 c, e). The matrix of
the rock comprises quartz, muscovite, calcite, and chlorite, and also aggregates of magnetite
grains are common.

Alkaline-gneiss is represented by sample OTA-R20-151.95, consisting mainly of calcite, biotite,
amphibole, and variable amounts of alkali feldspar and nepheline (Fig. 4a, b). The calcite typi-
cally occurs as subhedral to euhedral megacrysts in a foliated matrix of fine-grained calcite, bio-
tite, and amphibole (Fig. 3g, h). Selected minerals in the alkaline-gneiss were analyzed using the
SEM, where major element contents were determined using Energy-dispersive spectrometry
(EDS). Calcite compositions show little compositional variation with CaO contents in the range
between 49–54 wt%, negligible MgO contents, significant but variable SrO contents (0–1.4 wt
%), and low concentrations of MnO and FeO. FeO contents are highly variable, up to c. 1 wt%,
whereas the MnO contents are less variable and are generally c. 0.45 wt%.
6

Figure 3. Photographs showing textures of gneiss rocks from the Otanmäki locality, on the left, in plane
polarized light, but on the right, photographs under crossed polarizers. (a, b) Sample OTA-R19-39.65; (c,
d) Sample OTA-R19-79.74; (e, f) Sample OTA-R20-122.05; (g, h) OTA-R20-151.95 alkaline-gneiss.
7

Figure 4. Photographs showing textures of alkaline-gneiss and amphibole-biotite-quartz gneisses from
the Otanmäki locality, on the left, in plane polarized light, and on the right, photographs under crossed
polarizers. (a, b) Sample OTA-R20-151.9; (c, d) Sample OTA-R142-283.8 amphibole-biotite- quartz
gneiss; (e, f) Sample OTA-R142-448.0.
3.3 Korsnäs

Calcite-marble is composed mainly of calcite and as accessory minerals quartz, pyroxene and
traces of REE- bearing minerals. Calcite forms nearly equidimensional grains that are usually
white coloured and translucent in thin section. Calcite has an average grain size of 3 mm and the
grains are euhedral-subhedral (Fig 5a).