mineralogy, petrography and geochemistry of …tupa.gtk.fi/raportti/arkisto/54_2016.pdfmineralogy,...

GEOLOGIAN TUTKIMUSKESKUS

TIM

Espoo-Rovaniemi

4.7.2016 54/2016

Mineralogy, petrography and geochemistry

of Venejärvi, Tervola, Rytijänkä and

Jaurujoki graphite schists and gneisses in

Northern Finland

Thair Al-Ani, Olli Sarapää and Panu Lintinen

GEOLOGIAN TUTKIMUSKESKUS [54/2016]

4.7.2016

GEOLOGICAL SURVEY OF FINLAND DOCUMENTATION PAGE

Date / Rec. no.

Authors

Thair Al-Ani, Olli Sarapää and Panu Lintinen

Type of report

Archive report

Commissioned by

GTK

Title of report

Mineralogy, petrography and geochemistry of Venejärvi, Tervola, Rytijänkä and Jaurujoki graphite schists and gneisses in

Northern Finland

Abstract

Graphite schist and gneiss samples were collected from Venejärvi, Tervola, Rytijänkä and Jaurujoki drill cores. These target

areas were selected from GTK’s databases from Northern Finland. Geochemical and mineralogical studies were carried out

on selected samples by using optical microscopy, scanning electron microscopy (SEM), X-ray fluorescence (XRF), and in-

ductively coupled plasma mass spectrometer-try (ICP-MS). Graphite occurs mainly as randomly orientation of fine or minute

micro-crystalline flakes within graphite-schist and gneiss rock. The dominating size of the graphite flakes in studied gneiss

rocks is ~30 µm to 100 µm, graphite shows dark grey and lath like with uniform anhedral-subhedral grain shapes. According

to geochemical analysis, the graphite content of the studied rocks varies between 0.2 to 22.6 wt.% with an average of 10.5

wt.%, and the sulphur content varies between 0.1 to 6.5 wt.% with an average of 3.6 wt.%

Keywords

Graphite, Flake graphite, Venejärvi, Tervola, Rytijänkä and Jaurujoki, Northern Finland

Geographical area

Venejärvi, Tervola, Rytijänkä and Jaurujoki, Northern Finland

Map sheet

Other information

Report serial

Archive report

Archive code

Total pages

17

Language

English

Price

Confidentiality

Unit and section

Project code

50402-20048

Signature/name

Thair Al-Ani

Signature/name

Olli Sarapää Panu Lintinen

GEOLOGIAN TUTKIMUSKESKUS [54/2016]

4.7.2016

Contents

Documentation page

1 INTRODUCTION 1

2 METHODS 1

3 PETROGRAPHY AND MINERALOGY 3

4 GRAPHITE MORPHOLOGY AND SIZE 9

5 MINERAL CHEMISTRY 10

6 GEOCHEMISTRY 11

7 REFERENCES 17

GEOLOGIAN TUTKIMUSKESKUS [54/2016] 1

4.7.2016

1 INTRODUCTION

GTK has identified multiple potential areas for graphite exploration in Northern Finland, including the

earlier reported the flake graphite occurrence at Joutsijärvi (Al-Ani and Sarapää, 2016). Graphite -bearing

biotite gneisses and schists are studied from four localities- Venejärvi, Jaurujoki, Rytijänkä and Tervola

(Fig. 1). The specimens were collected from diamond drill cores at the Loppi archive. Four samples were

selected for petrographic studies and six samples were selected for chemical analysis.

2 METHODS

Six drill core samples from Tervola (4 samples) and Venejärvi (2 samples), Northern Finland were se-

lected for whole-rock chemical analysis in Labtium Analytical Laboratories (Kuopio). Major and minor

elements of drill core samples were determined by XRF (Method 175X) and determination of the rare

earth elements and trace elements by ICP-MS (Method + 511P). Whole-rock samples were also analysed

for total sulphur and carbon by using high temperature (LECO) combustion technique with S/C analyser

(Labtium methods 810L and + 811L respectively). Loss-on-ignition (LOI), Labtium methodology (816L)

procedure was used to run the LOI test in the studied samples.

Based on the petrographic and mineralogical studies of 4 thin sections, two samples were selected from

Venejärvi and other two samples from Rytijänkä-Kolari and Jaurujoki-Salla respectively. The petro-

graphic studies were carried out with different thin sections of the graphite-rich rock samples to know the

microstructures, texture, and mineralogy and to some extent paragenesis of the graphite and associated

minerals, by using ore microscope with reflected and transmitted light at different magnifications such as

2.5X, 5X, 10X and 20X objectives. All thin sections were examined by using a Petrographic Microscope

(LEICA DMLP) equipped with a digital camera (Leica).

Mineralogical and detailed petrographic characterizations were analysed by using scanning electron mi-

croscopy (SEM). The secondary electrons (SE) and back scattered electron (BSE) imaging modes was

used to locate graphite at the surface of the thin section and to characterize the composition of the associ-

ated minerals by energy. These analyses were performed at the Mineral Processing Laboratory (GTK) -

Espoo with a JEOL JSM 5900 LV field-emission SEM with a fully automated EDS and BSE detection

system. Standard operating conditions for SEM imaging and ED’s analysis were a 15 kV accelerating

voltage potential and an electron beam current of 0.5 or 1 nA. Energy dispersive X-ray microanalysis on a

SEM using the INCA x-sight EDS detector and INCA energy hardware to provide accurate determination

of the composition of materials, which works for all types of studied samples and conditions.

Whole-rock samples were analysed by Labtium Analytical Laboratories (Kuopio), major and minor ele-

ments of drill core samples were determined by XRF (Method 175X) and determination of the trace ele-

ments by ICP-MS (Method + 511P). Whole-rock samples were also analysed for total sulphur and car-

bon by using high temperature (LECO) combustion technique with S/C analyser (Labtium methods 810L

and + 811L respectively).


4.7.2016

Figure 1. Geological map showing location of studied graphite schists and gneisses. R=drill hole.


4.7.2016

3 PETROGRAPHY AND MINERALOGY

Mineral assemblages in the rock specimens are summarised in Table 1. Four rocks described in detail;

two from Venejärvi R003 (33.10-35.05) and R001 (70.00-70.05), one from Jaurujoki- Salla

M52364396/R303 (42.60-42.65), and one from Kolari, Rytijänkä/R4 (201.95). These samples form the

basis of the present study and will be described in detail. The biotite schist and biotite gneiss are the main

host rocks of the graphite mineralization in the studied area.

Venejärvi/R3 (33.10-35.05), Fine-grained graphite schist- Venejärvi Rovaniemi

Macroscopically: The rock is fine-grained dark grey with well-developed foliation and rusty patches. In

the hand specimen, the rock is dominated by veins and fractures filling by dark materials of graphite and

iron oxide minerals (Fig. 2a).

Microscopically: The studied rocks consisting predominantly of biotite plus graphite lamellae alternate

with quartz plus plagioclase laminae. The mineralogical composition marked by the very fine-grained

matrix of dark biotite, chlorite, Fe-oxide, quartz, graphite and plagioclase (Fig. 2b). The dark materials

are a mixture of graphite, a soft fine-grained platy mineral composed entirely of carbon, and iron oxide

minerals. As seen in microscopic sections of the studied biotite schist is dominated by veins and fractures

filling by coarse grains of biotite, graphite, quartz, amphibole, plagioclase and calcite (Fig. 2b, c, d).

Graphite in the studied sample Venejärvi/R3 (33.10-35.05), occurs as amorphous graphite takes the form

of minute micro-crystalline flakes (Fig. 2e). The dominating size of the graphite flakes in studied gneiss

rocks is ~30 µm to 100 µm, graphite shows dark grey and lath like with uniform anhedral-subhedral grain

shapes (Fig. 2f). Although it is sometimes randomly oriented, it is commonly associated aligned along the

foliation plane with biotite.

Venejärvi R001 (70.00-70.05) Medium-grained Chlorite-garnet-biotite Gneiss- Venejärvi

Rovaniemi

Macroscopically: This rock is strongly foliated of blackish and green colours, due to the presence of

abundant black biotite, and green chlorite and also has brownish bands of garnet and K-feldspar. The

fine- to medium- grained and looks darker, characterised by crystal aggregates of garnet and iron oxides

(Fig.3a).

Microscopically: it is a coarse-grained, strongly foliated biotite schist dominated by garnet poikiloblasts,

biotite, quartz, iron oxides and chlorite with accessory zircon apatite and rutile. Garnet poikiloblasts are

subhedral to anhedral, embayed crystals 300-2000 microns in size and have crenulation fabric trails of

elongate iron oxide inclusions (Fig. 3b, c). The sample is dominated by platy crystals of biotite up to 200

µm in length that define a well-developed schistose fabric. Quartz is present throughout the matrix but is

more abundant in some bands and in particular in the pressure shadows of garnet. Quartz forms granular

masses with a minor grain flattening fabric. Some quartz crystals have minor undulose extinction. Chlo-

rite is present as subhedral crystals up to 250 µm in size, some contain abundant inclusions of quartz and

associated mainly with biotite and garnet (Fig. 3d, f, e). Graphite is rarely observed in the studied sample

Venejärvi R001 (70.00-70.05), interleaved with the biotite. BSI of the studied sample shows that apatite is


4.7.2016

invariably present as individual grains in the sample and associated with biotite, quartz and iron oxides

(Fig. 3g). Apatite crystals are usually elongated and have colorless or pale grey color.

M52364396/R303 (42.60-42.65) Medium-grained Graphite-biotite Gneiss from Jaurujoki- Salla

Macroscopically: This sample is a medium -grained, strongly foliated, dominated by biotite, graphite, al-

bite and quartz with minor titanite, opaque, zircon and garnet. In the hand specimen the darker bands are

rich in mafic minerals such as graphite and/or opaque mineral. But in some parts of the specimen brown-

ish bands dominated of quartz and feldspar (Fig. 4a).

Microscopically: The constituent minerals are biotite, quartz, albite, potassium feldspar and graphite, with

rutile, zircon, apatite, ilmenite, and opaque. Opaque minerals consist mainly of graphite and pyrrhotite,

pyrite, ilmenite and chalcopyrite are rare. Biotite occurs mostly as tabular large subhedral grains, 500-

2000 microns in size, and is always associated with graphite and sulphide (pyrite) grains. It is sometimes

columnar and lathlike, commonly conspicuously restricted to the foliated parts of the rock (Fig. 4a, b).

Some of the biotite shows alteration to chlorite. K- Feldspars which is present in the rock includes both

plagioclase and K- feldspars. K-feldspar is usually perthitic (300-700 µm) and cloudy, subhedral in shape,

colourless and shows straight extinction and some of the grains show alteration to sericite. The style of

graphite mineralization in the studied schist occurs mainly as small size flakes and oriented parallel to

well-foliation of schist rock (Fig. 4c, d). Graphite crystals or flakes are up to 100 μm long and 30 μm

wide; however, the average size of individual crystals is <100 μm and crystals are typically randomly.

Rytijänkä/R4 (201.45-201.50) Porphyroblastic graphite-chlorite gneiss from Rytijänkä Kolari

Macroscopically: The rock fabrics are characterized by equigranular fine-grained aggregates of graphite

and chlorite obviously formed by dynamic recrystallization of coarse grains. These are locally preserved

as porphyroblasts embedded in the fine-grained matrix. Graphite porphyroblasts are subhedral to eye-

shaped and up to 3 mm in size. Quartz and feldspar form thin lighter rims surrounded these graphite blasts

(Fig. 5a). Several microfolds are also present in the hand specimen and oriented perpendicular to the

schistose cleavage plane.

Microscopically: The specimen contains the following minerals: biotite, chlorite, quartz, plagioclase, and

potassium feldspar, with pyrite and pyrrhotite as accessories. This is a strongly foliated, fine-grained, por-

phyroblastic metamorphic dominated by graphite and chlorite poikiloblasts in a fine-grained, embedded

in matrix of quartz, muscovite, biotite and graphite. The porphyroblasts enclosed by black graphite (Fig.

5b). Graphite forms minute flakes (~40-300 µm), that are dark colour and lath-like with uniform anhe-

dral-subhedral grain shape (Fig 5b). Although graphite is sometimes randomly oriented, its commonly

associated aligned along foliation plane with biotite and also graphite associated with sulphide minerals

(Fig 5d, e)


4.7.2016

Figure 2. Photograph illustrating different petrographic features of graphite black schist, (a) Hand specimen pho-

tograph of dark coloured graphite schist sample Venejärvi/R003(33.10-35.05), shows foliation-laminate of Qz/Ab

alternating with Bt/Amph and graphite, (b, c) Veins and fractures filling by Bt, Qz, Amph (XPL, (d) High magnifi-

cation of the veins showed prismatic to acicular crystals of glaucophane formed typically in a highly metamorphic

zone of black schist rocks, (e, f) BSE images show graphite occurs mainly as amorphous or fine flakes associated

mainly with biotite(Bt) and amphibole (Amph).


4.7.2016

Figure 3. Photograph illustrating different petrographic features of chlorite-garnet-biotite gneiss, (a) Hand speci-

men photograph of sample Venejärvi/R001(70.0-70.05), shows alternating with biotite/chlorite (Bt/Chl) and gar-

net/quartz (Grt/Qz), (b) Crossed polarisers (XPL), the parallel mica and hornblende flakes show up in bright col-

ours, garnet (Grt) crystals in the middle and sub rounded iron oxide crystals appear black, (b) Plan polarisers

(PPL), the parallel mica flakes show up in brown colours, and large rounded garnet crystals surrounded by horn-

blende (green colour) and contain inclusions of iron oxides appear black, (c, d, e, f) BSE images showing, the main

minerals are biotite, quartz, garnet and hornblende with accessories of apatite and iron oxides.


4.7.2016

Figure 4. Photograph illustrating different petrographic features of graphite-biotite gneiss, (a) Hand specimen

photograph of sample Jaurujoki/Salla M52364396/R303(42.60-42.65), shows alternating with Bt/Gr and Ab/Qz (b,

c) Crossed polarisers (XPL) showing graphite (Gr) flakes with biotite laths, often restricted to the well floated

parts of the rock the parallel mica and chlorite, (d, e) BSE images showing, the graphite(Gr) flakes oriented paral-

lel to foliation and associated with biotite (Bt), albite (Ab) and quartz (Qz).


4.7.2016

Figure 5. Photograph illustrating different petrographic features of porphyroblastic graphite-chlorite gneiss from

Rytijänkä Kolari, (a) Hand specimen photograph of sample Rytijänkä/R4 (201.45-201.50) (a) Porphyroblasts al-

tered to chlorite (Chl) and sericite (Ser) mainly with their characteristic graphite surrounded these Porphyroblasts,

(c) Aggregate of graphite flakes with matrix of the rock, (d, e) BSE images showing, the graphite (Gr) flakes asso-

ciated mainly with biotite (Bt) sulphides (Py+Po)


4.7.2016

4 GRAPHITE MORPHOLOGY AND SIZE

Aggregate measurements of the graphite flakes from several thin-sections are usually necessary to give a

statistically significant description of the ore. An example of the results of such measurements is shown in

(Fig. 6) which shows aggregate grain morphological measurements of the graphite ore.

The dominating size of the graphite flakes in studied gneiss rocks is ~30 µm to 500 µm and graphite

shows good orientation with flakes frequently in mean length of 300 µm. Most graphite flakes are flaky

shaped, with particularly fibrous, and the ratios between their long and short axes are in the range of 2 to

10 for the majority of the flakes (Fig. 6a, b). Amorphous graphite also occurs in some studied samples, its

form of minute micro-crystalline flakes (Fig. 6c). The dominating size of the graphite flakes in studied

gneiss rocks is ~30 µm to 100 µm, graphite shows dark grey and lath like with uniform anhedral-subhe-

dral grain shapes (Fig. 6d). Although it is sometimes randomly oriented, it is commonly associated

aligned along the foliation plane with biotite. These results are typical for Finnish graphite and are also

characteristic of a coarse, high-quality, flake graphite ore.


4.7.2016

Figure 6. Backscattered electron (BSE) images show orientation and size of graphite flakes in some selected sam-

ples.

5 MINERAL CHEMISTRY

All of the rocks that were analysed are graphitic schist or graphite gneiss, so they contained a significant

amount of graphite (C). The amount of graphite contained in each of the samples is easy to define because

of the large size of the numerous graphite crystals and flakes, and SEM can provide quantitative analyses

for the chemical compositions of the main minerals present in the studied rocks. Spot analyses were com-

pleted for gathering data. In each of the 4 thin sections all minerals found, aside from albite, plagioclase,

K-feldspar, biotite, amphibole, phlogopite, garnet, apatite and rutile are given in Table (1).


4.7.2016

Table 1. The chemical composition of the main and some accessory minerals in the studied rocks.

Oxides Albite plagioclase K-feldspar Biotite Amphibole Phlogopite Garnet Apatite Rutile

SiO2 65.4 62.3 64.7 43.2 52.7 39.1 36.9

TiO2 0.2 2.3 3.5 1.6 94.1

Al2O3 21.5 19.2 16.7 14.8 1.7 18.7 21.2

Fe2O3 1.02 15.2 3.2 5.5 35.8 3.0

MgO 0.3 10.8 21.2 21.8 1.6 1.5

MnO 0.06 0.4 51.7

CaO 2.0 12.2 0.7 6.9 0.8

K2O 0.9 0.8 15.8 9.9 5.7 11.2 1.2

Na2O 9.3 5.1 0.7 3.7

P2O5 44.5

F 1.3 1.2 3.2 3.2

Nb2O3 1.4

Total 100.0 99.6 99.1 97.2 99.8 99.5 99.5 99.4 99.9

6 GEOCHEMISTRY

The chemical analysis (Table 2) of graphite schist and gneiss brings out the following points;

1. The percentage of silica is high, between 33.4 to 56.4 wt. % with average of 46.5 wt. %

2. CaO is not abundant in some deposits, between 0.3 to 8.5 wt. % with average of 3.9 wt. %

3. The percentage of Al2O3 is in excess, between 8.9 to 14.8 wt. % with average of 11.8 wt. %

4. MgO content ranging between 1.5 to 10.8 wt. % with average of 5.4 wt. %, indicating presence of a

magnesium-bearing (chlorite, biotite) and muscovite.

5. High content of Fe2O3 ranging from 4.9 to 38.0 wt. % with average of 15 wt. %. The Fe oxides con-

tents show positive relation with sulfur content (S) seems to suggest that Fe is mainly associated with sul-

phide minerals such as pyrite and pyrrhotite.

The carbon content varies from 5 to 32 wt. % and sulfur content range of 0.1 to 6.5 wt. %. The histograms

in Figures 7 and 8 remarkably large variation in graphite (C wt. %) and sulphur (S wt. %) concentrations

within the studied samples. The studied sample located in Venejärvi VENE/R003 (33.10-35.05) re-

marked the high (22.6 wt. %) carbon content, while the sample VENE/R001 (70.15-71.15) indicated very

low carbon content (0.2 wt. %) (Table. 2 and Fig. 7). Compared to graphite-black schist samples in Ter-

vola, the median C concentrations are rather similar with a few exceptions (9-10.8 wt. %), as shown in

Table (2) and Figure (7).


4.7.2016

The studied rock samples located in Venejärvi R003 (33.10-35.05) and VENE/R001 (70.15-71.15) show

very low content of sulphur content (0.1-1.0 wt. %). Compared to the samples located in Tervola (TER),

the S concentrations are rather higher vary from 2.5 to 6.5 wt. % (Table 2 and Fig.8).

Various authors have reported on the good correlation of organic carbon with the majority of trace metals

(e.g. Vine and Tourtelot 1970, Brumsack 1989, Leventhal 1991). In the studied black schist and gneiss

rock samples investigated in this study, there is generally negative correlation between graphitic carbon

with sulphur and (Fe2O3+MgO+MnO) contents (Fig. 9a, b). At the same time, there is a strong positive

correlation between sulfur content with iron and (Fe2O3+MgO+MnO) contents (Fig. 9c, d), indication the

most Fe and S occur in the form of pyrite and pyrrhotite.


4.7.2016

Table 2. Whole-rock chemical data for selected graphite-rich samples.

Sample VENE

R003(33.10-35.05)

VENE R001(70.15-

71.15)

TER-3 (63.50-64.50)

TER-3 (94.20-95.00)

TER-4 (40.20-42.20)

TER-4 (42.20-44.00)

Na2O 2.4 1.0 1.1 1.2 0.2 0.1

MgO 3.2 10.8 7.8 6.1 1.6 1.5

Al2O3 11.2 8.9 9.8 12.3 14.8 13.9

SiO2 44.3 33.4 41.7 51.0 55.4 56.4

P2O5 0.1 0.2 0.2 0.2 0.1 0.1

K2O 1.6 1.4 2.2 3.0 4.4 4.1

CaO 0.8 5.0 8.5 7.1 0.3 0.3

TiO2 0.9 0.5 0.5 0.7 0.6 0.6

MnO 0.0 0.2 0.1 0.1 0.0 0.0

Fe2O3 5.6 38.0 9.1 4.9 10.0 11.0

S 1.0 0.1 5.6 2.5 6.5 6.3

C 22.6 0.2 10.8 9.5 9.0 9.4

Trace elements (ppm)

As 1.6 1.7 118.2 66.0 112.8 110.4

Be 0.5 0.6 0.8 1.0 0.8 0.8

Bi 1.5 0.1 0.8 0.5 0.7 0.7

Cd 0.0 0.0 3.3 1.6 0.2 0.1

Ce 19.3 24.4 40.8 37.9 94.4 67.1

Pb 1.9 2.1 111.6 34.3 55.8 61.4

Sb 0.1 0.1 5.2 3.1 5.5 5.5

Se 6.1 0.9 9.5 7.3 12.0 11.5

Sn 0.2 0.9 0.3 0.2 0.2 0.2

Te 0.8 0.2 0.4 0.2 0.7 0.6

Th 5.2 2.9 5.0 6.2 6.2 5.1

U 4.2 0.6 5.4 6.0 4.7 4.7

W 0.4 0.2 2.7 1.6 0.6 0.8

B 5.0 5.0 5.0 5.0 5.0 5.0

Ba 50.0 148.0 44.3 49.5 50.8 48.2

Be 0.5 0.5 0.7 1.0 0.8 0.8

Co 57.0 45.8 53.4 32.7 48.4 39.8

Cr 71.4 29.3 45.2 42.9 11.8 11.5

Cu 664.0 44.4 115.0 123.0 91.9 96.8

La 9.3 10.9 20.1 19.2 37.5 27.5

Li 9.9 22.0 30.8 27.2 10.8 10.6

Ni 66.8 184.0 235.0 239.0 254.0 275.0

Sr 6.0 15.5 40.8 43.3 3.5 3.5

Th 10.0 10.0 13.5 15.3 10.0 10.0

V 105.0 1380.0 227.0 148.0 26.0 30.8

Y 7.5 7.5 16.3 15.4 13.5 13.2

Zn 15.7 31.9 282.0 152.0 61.1 29.5


4.7.2016

Figure 7. Histogram showing the concentration of graphite in selected samples.

0.18

22.60

10.80

9.488.95

9.35

0.00

5.00

10.00

15.00

20.00

25.00

Carb

on

e c

on

ten

t (w

t%)

Samples


4.7.2016

Figure 8. Histogram showing the concentration of sulphur in selected samples.

0.09

0.96

5.58

2.49

6.456.31

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

Su

lph

er

co

nte

nt (w

t%)

Samples


4.7.2016

Figure 9. Correlation beteewn (a, b) graphite against S and (Fe2O3+MgO+MnO), (c, d) sulfur against

(Fe2O3+MgO+MnO) and Fe2O3 respectively.


4.7.2016

7 REFERENCES

Thair Al-Ani and Olli Sarapää, 2016. Mineralogy and geochemistry of flake graphite occurrences in Jout-

sijärvi, Northern Finland, GTK Research Report 55/2016, 38p.

Vine, J. D. and Tourtelot, E. B., 1970. Geochemistry of black shale deposits---a summary report: Econ.

Geol. 65, 253-272.

Brumsack, H.-J. 1989. Geochemistry of recent TOC-rich sediments from the Gulf of California and the

Black Sea. Geologische Rundschau 78, 851–882.

Leventhal, J. S. 1991. Comparison of organic geochemistry and metal enrichment in two black shales:

Cambrian Alum Shale of Sweden and Devonian Chattanooga Shale of United States. Mineralium Depos-

ita 2

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