Geochemical exploration methods for niobium and rare earth elements at Sæteråsen, Vestfold volcanic area, Oslo Graben

PER RYGHAUG

Ryghaug, P.: Geochemicai exploration methods for niobium and rare earth elements at Sætcråscn. Vestfold volcanic area, Oslo Graben. Norsk Geologisk Tidsskrift. Vol. 63, pp. 1-13. Oslo 1983. ISSN 0029-196X. Geochemical �xploration methods for niobium and rare earth elements in the Oslo Graben have been studied using the occurrence at Sæteråsen, Vestfold as a test area. The deposit consists of two lens-shaped bodies of aphyric trachytic lava within a sequence of porphyritic latites and trachytes. The extrusive rocks are intruded by ncrdmarkite and larvikite. Samples of various types of stream sediment, strcam moss, soil and rock have been analyzed for the total contents of Ba, Ce, La, Nb, Rb, Sr, Th, U, Y and Zr by an X­ray fluorescence method. The relationships between the concentrations and ionic potentials of trace elements in stream-moss samples and different types of sediment and rock samples are discussed. Stream­moss s�mples gave significant La-and U-anomalies jn the area, white no distinct stream-sediment anomahes were encountered.

P. Ryghaug, Norges geologiske undersøkelse, Box 3006, N-7001 Trondheim, Norway

The Sæteråsen Nb-deposit is situated in the Vest­fold volcanic area (Oftedahl 1952, 1978) in the southem part of the Oslo Graben, 25 km north of the town of Larvik (Fig. 1). The deposit was registered as a radiometric anomaly in 1961 and examined more closely by Hysingjord (1971, 1974). It consists of two lens-shaped radioactive bodies (250--600 m long, 50-100 m wide) of aphyric trachytic lava, and has recently been mapped in detail by Jorde (1978) and Ihlen (1982) as part of a prospecting campaign by the Geological Survey of Norway. The surrounding (barren) rocks are less radioactive. They are dominated by nordmarkite, Iarvikite, Iatite por­phyry and trachyte porphyry. The investigated area has mainly a thin overburden. Geochemical studies, including sampling of bedrock, soil, inor­ganic stream sediments, stream moss (aquatic bryophytes), moss-trapped stream sediments and heavy minerals, were carried out during the 1978 and 1979 field seasons (Ryghaug 1983), with the intention of finding suitable exploration methods for this kind of mineralization. The area sur­veyed (Fig. l) was 28 km2•

Sampling and analytical methods Drainage samples were collected at stream/road intersections, except for streams draining the

l- Geologisk Tidsskr. 1/83

Sæteråsen hillock itself, along which the samples were collected at intervals of approximately 200 m. All stream sediments were wet-sieved in the field to a coarse fraction ( <0.6mm >0.18mm) and a fine fraction (<0.18mm). The sieving was repeated in the laboratory after drying of the sample. Stream moss (mainly Scapania undulata and Fontinalis antipyretica) was collected from boulders in the streams, and washed free of sedi­ments with stream water. These moss-trapped sediments were collected for analyses. Heavy­mineral concentrates from stream sediments were obtained by panning. Heavy Iiquids and magnetic separation were used for further treat­ment of the panned concentrates.

In some scattered locations outside the miner­alized area soil samples were collected from the B and C horizons of thicker Iayers of till and glacio-fluvial sediments. After drying, the soil samples were sieved into a coarse ( <0.6mm > 0.18mm) and a fine fraction (< 0.18 mm) in the laboratory. Composite samples of the trachytic lava and the surrounding barren rocks were also collected.

Trace elements in all types of samples, and major elements in the rock samples were deter­mined by X-ray fluorescence analysis (Faye & Ødegård 1975). The moss samples were ashed at 420°C ovemight before analysis and the results were converted to dry matter. A neutron acti-

2 P. Ryghaug

D D

B 1 SVARSTAD

NORSK GEOLOGISK TIDSSKRIFT l (1983)

APHYRIC TRACHYTE ( SATE R ÅSEN Nb-DEPOSI Tl

TRACHYTE PORPHYRY

LATITE PORPHYRY

NORDMARK!TE

LARVIKITE

" HO�EN

TØNSBERG

Fig. l. Geological sketch map of the Sæteråsen deposit. After Jorde (1978) and Ihlen (pers. co mm.).

vation method was used on some of the rock samples for determination of the contents of rare earth elements (REE).

Diffractometer (X-ray Debye-Scherrer cam­era) and microscope were used to identify the minerals.

Results The average content of 57. 9 percent Si02 in the aphyric trachyte at Sæteråsen (Table l) is less than that of trachytes in general, which has been estimated to be approximately 64. 0 percent (Brøgger 1933, Ramberg & Larsen 1978, Ofte­dahl & Petersen 1978). The mean contents of Zr, Nb, Ce, La, Rb, Y, Th and U are more than 10 times the average concentration of these ele­ments in plutonic rocks of the Oslo Region (Neu­mann et al. 1977, Raade 1978). Pyrochlore is the dominant niobium mineral. The Th/U ratio (17. 9 ± 6. 5) is high compared to other volcanic

and igneous rocks (3. 80 ± 0.40) in the Oslo Gra­ben (Raade 1973, 1978).

In the aphyric trachyte, a chondrite-normal­ized REE pattem (Haskin et al. 1968) shows a strong negative Sm and Eu anomaly (Fig. 2). The La/Lu ratio (71) of the aphyric trachyte is rela­tively low when compared to those of the por­phyric trachyte (103) and the nordmarkite (148). There is a similarity in REE pattem between the aphyric trachyte and silicic peralkaline nordmar­kites, ekerites and Drammen granites, and the pattem deviates from that of less silicic rocks (Finstad 1972, Neumann et al. 1977).

The elements with low ionic potential have higher concentration in the stream moss than in the stream sediments (Table 2). Elements with high ionic potential show the opposite trend. The elements with the lowest ionic potential have higher concentrations in the coarse fraction of the stream sediments than in the fine fraction, while the opposite situation appears with increas­ing ionic potential.

NORSK GEOLOGISK TIDSSKRIFT l (1983)

Table l. Average chemical composition of 8 composite samples of aphyric trachyte lava from Sæteråsen, Vestfold, Southern Norway.

Si02 no2 Al203 Fe203 Mn O MgO Ca O Na20 K20 P20s Zr02 Nb205 Ce02 La203 Rb20 Y203 Th02 u�

Sum

Contents in weight percent

57.89 0.95

14.37 8.52 0.89 0.77 1.60 6.1 4. 61 0.08 1.89 0.43 0.20 0.11 0.09 0.08 0.07 0.003

98.65

The concentrations of Nb in stream sediments and soil from the Sæteråsen area correspond with those in the various types of barren rock (Fig 3a). In stream moss the content of Nb is low. La, however, is enriched to concentrations similar to those of the aphyric trachyte (Fig 3b ) . The geo­chemical use of stream moss has been reported

1000 UJ t:: 500 a: o z o I LJ

UJ 100 _, a_ :L <( 50 Vl

10

Nb and REE at Sæteråsen 3

·--•

·�

��0�0.0 o,o • Aphyric Trochyte

o Porphyric Trochyte o Nordmorkite

Lo Ce Sm Eu Tb Yb Lu Fig. 2. Chondrite-normalized patterns of REE (Haskin et al. 1968) in some plutonic and volcanic rocks from Sæteråsen, Vestfold, Southern Norway.

by Bengtsson & Lithner (1981), Burton & Peter­son (1979) and Whitehead & Brooks (1969) among others.

The heavy-mineral concentrates, which are dominated by Ti-minerals, Zr-minerals and pyr­ochlore (Table 3), show anomalously high con­tents of Nb, Zr, Th and U in the heavy fraction

Table 2. Contents of trace elements in various types of drainage samples from 21 sarnple sites at the Sæteråsen area, Oslo Graben, Vestfold, Southern Norway. The elements are arranged according to their ionic potential (IP), (Goldschmidt 1937).

Element IP Stream moss Stream Moss Stream Moss Comments sediment sediment sediment sediment

As hes Fine Fine Coarse Coarse >- greater than (A) fraction fraction fraction fraction � - greater than

(B) (C) (BC) (CC) or equal to :X V x V x V x V x V

Rb 0.7 229 (0.34) 90 (0.16) 81 (0.20) 121 (0.17) 109 (0.28) A>BC,CC>B,C Ba 1.5 867 (0.34) 545 (0.21) 550 (0.20) 635 (0.31) 669 (0.32) A>BC,CC>B,C Sr 1.8 218 (0.32) 187 (0.17) 176 (0.28) 198 (0.24) 192 (0.37) A> BC, CC� B �C La 2.6 933 (1.41) 247 (0.63) 301 (0.82) 126 (1.05) 291 (2.06) A> C, CC> B > BC

Ce 2.6

986 (0.97) 415 (0.51) 486 (0.70) 238 (1.00) 376 (1.35) A>C>B>CC>BC (4.3)

y 3.3 175 (0.99) 107 (0.38) 104 (0.44) 62 (0.31) 66 (0.48) A>B, C>BC, CC u 3.8 38 (1.24) 17 (0.45) 19 (0.85) 12 (0.38) 14 (0.96) A>B, C �BC, CC Th 3.9 18 (0.69) 29 (0.43) 26 (0.40) 18 (0.26) 18 (0.32) B,C>BC,CC,A Zr 5.1 639 (0.76) 1984 (0.50) 1546 (0.62) 1354 (0.82) 970 (0.76) B>C>BC>CC>A Nb 7.5 79 (0.68) 203 (0.48) 162 (0.61) 217 (0.88) 166 (0.83) BC, B>CC, C>A

No of samples 20 21 18 21 20

X: Arithmetical mean (ppm). V: Coefficient of variation.

a

b

4 P. Ryghaug NORSK GEOLOGISK TIDSSKRIFT l (1983)

Nb STREAM MOSS STREAM SOlL TRACHYTE PORPHYRY

NORDMARK BASALT l C - ORG SUBST.

SEDIMENT SAMPLES LARVIKITE ROCK p pm --AS HES ( APHYRIC}

3000 IX 2000

1000 t-BOO 1-600

400

fx T x

f, l

200

l l

x

j· "' -

100

BO-60- IX 40- -

< l ppm

N= 40 42 32 B 7 10 4

La STREAM MOSS STREAM SOlL TRACHYTE PORPHYRY

NORDMARK. BASAL Tl C

p pm - ORG SUBST. SEDIMENT SAMPLES ( APHYRICJ LARVIKITE ROCK

--ASHES 560� ppm

1' l

1000 l fx 800 +xash 600 l

-t-400 -

X

Ix 200

x X

100 t- X org

80 Jx 60

40

20

N= 40 42 32 B 7 10 4

Fig. 3trb. Ranges of contents of a) Nb and b) La in plutonic and volcanic rocks, soil, stream sediments and stream moss at Sæteråsen, Vestfold. Oslo Graben. N =No. of samples, x = Årithm. mean.

NORSK GEOLOGISK TIDSSKRIFf l (1983)

Table 3. Contents in ppm of La, Ce, Y, U, Th, Zr and Nb in two fractions of heavy-mineral concentrates from the Sæter­åsen area, Oslo Graben, Vestfold, Southern Norway. The elements are given in order of increasing ionic potential. N =No. of samples.

Medium fraction Heavy fraction (S.G. 2.96-3.31) (S.G. > 3.31) N=20 N= 19

Symbol Min Max x Min Max x

La 182 1300 517 129 2400 849 Ce 387 2500 1048 279 3800 1314 y 202 680 326 140 865 398 u < 30 30 101 54 Th <30 54 84 354 172 Zr 285 2200 1273 11400 44400 20715 Nb 34 388 215 1800 5300 3357

(specific gravity > 3.31). The REE concentration in this fraction is approximately the same as in the medium fraction (S. G. 2.96-3.31), which is dominated by pyroxene, amphibole and apatite. Other results show that a large part of the Nb content in the drainage sediments is located in the non-magnetic fraction (> 0.8 Amp).

This fraction is dominated by zircon, rutile and trace minerals such as pyrochlore, låvenite, bad­deleyite, anatase, apatite and brookite. Signifi­cant Nb concentrations are also present in the magnetic fractions. These are dominated by il­menite, probably originating from the plutonic rocks of the area, since only traces of ilmenite were found in the Sæteråsen aphyric trachyte.

In the heavy fraction of the heavy mineral concentrates, La, Ce, Y, U, Th and Zr are posi­tively correlated one with the other, while Nb is negatively correlated with these elements (Table 4). Except for Nb the correlation coefficients increase with increasing similarity of ionic poten­tials. In the medium fraction La, Ce and Y are positively correlated one with the other, while Zr and Nb both show weakly negative correlation with these elements. Nb shows a positive correla­tion with Zr.

Nb and REE at Sæteråsen 5

Tab/e 4. Correlation coefficients between contents of La, Ce, Y, U, Th, Zr, and Nb in two fractions (S.G. 2.96-3.31 and S.G. > 3.31) of heavy-mineral concentrates from Sæteråsen area, Oslo Graben, Vestfold, Southern Norway. The elements are given in order of increasing ionic potential. The number of samples involved are 18 to 20.

Increasing ionic potential •

La Ce y

La 0.73**

Ce "'"00.74**

u Th

DL DL

DL DL 'u y 0.70** 0.75**

"'-"�(hDL DL

o-v u 0.57 0.60** 0.80** DL

Th 0.69** 0.71** 0.84** 0.92**/t"�vr,.

Zr

-0.13

-0.13

-0.26

DL

DL 71� Zr 0.38 0.42 0.70** 0.91** 0.90**()'10-v Nb -0.36 -0.37 -0.52 -0.12 -0.20 -0.31

DL: below detection limit. •• Level of significance p< 0.005

Nb

-0.36

-0.34

-0.02

DL

DL

0.77**

In stream moss, coarse fractions of stream sediment and soil samples, no correlation occurs between elements with large differences in ionic potential (Table 5). However, a positive inter­correlation of elements seems to increase with increasing similarity of ionic potential. In the fine fraction of the stream sediments, in barren rock and in the aphyric trachyte, positive correlations occur in all pairs of elements, except for La/U in the aphyric trachyte which show a negative cor­relation. The number of samples included in the correlation matrix varies since extreme values in relation to the mean population are excluded.

Scattergrams for La versus Ce indicate the significant positive correlation between these ele­ments both in rock and drainage samples (Fig. 4a-d).

The scattergram for La versus Nb (Fig. 5a) indicates that in barren rock two populations may be present. Good positive correlation is pre­sent only at low concentrations. At high concen-

6 P. Ryghaug NORSK GEOLOGISK TIDSSKRIFT l (1983)

Table 5. Correlation coefficients between pairs of trace elements in different types of samples, from the Sæteråsen area, Oslo Gra­ben, Vestfold, Southern Norway

lncreasing similarity in ionic potential

Sample Nb/Zr Nb/Th Nb/Ce

Stream moss (dry matter) 0.93** 0.59** 0.13 Stream sed. fine fraction 0.86** 0.82** 0.69** Stream sed. coarse fraction 0.91** 0.35 0.34

Soil (B- and C-horizon) 0.77** 0.33 0.13

Aphyric trachyte o.ss•• 0.83** 0.55

Barren rocks 1 0.91** 0.75** 0.86**

1 Rock types in accordance with fig. Sa. Leve) of significance: •• p= <0.005 DL = Too many analyses below detection limit

BARREN ROCK a :E ::l z < I 1-z < _J

STREAM SEDIMENT (FLne frl C !i

Nb/La La/Zr

-O .JO -0.11

0.85** 0.73**

0.30 0.34

0.18 0.65**

0.42 0.41

o.ss•• 0.75**

22 b!i .Sl .G

i2 .!l&

z < I 1-z < _J

d!i z <

....

Th/La

0.09

0.78**

0.70**

0.74**

0.52

DL

z < I 1-z < _J

p= ( .(11; I 1011 1-

200 • .

rJffo

2111. 1011. eoo.

o . o o

···�····:·· o ·

lill. lill. CERIUM

z < _J 5IXf ........... ... , ... .

2111

tOll.

La/U La/Y La/Ce

0.60** o.so•• 0.90**

0.60** 0.81** 0.98**

DL 0.73** 0.97**

DL 0.75** 0.95**

-0.44 0.76 0.93**

DL 0.86** 0.97**

TRACHYTE

CERIUM

STREAM MOSS o o

o ... · - -�- -

2111. ""· CERIUM

No of samples

38-40

40--42

40--42

30-32

7-8

17-22

.. -

N• 10 R• .lli p=< -

Fig. 4a-d. Scatter diagrams of La and Ce in barren rock, trachyte, stream sediments and stream moss from the Sæteråsen area, Vestfold, Oslo Graben. All values are in ppm. N = No. of samples. R = Corrclation coefficients. p = Leve) of significance.

NORSK GEOLOGISK TIDSSKRIFT l (1983)

a � 1(1)

z < I 1-z < !!IlD _J

""·

'""

c� z � ..., 1-z < _J

..... ; .

•

. t '

'""·

""·

BARREN ROCK

. . .

""· ""'·

o o

.. Q ...

1(1).

N = Z2 R = .19 p= c .Dto

iOO. SIXI. NIOBIUM

i2 ·" .æ;

dE � z < I 1-z < _J

Nb and REE at Sæteråsen 7

TRACHYTE

o

�����---.æoo�.--���.--�moo�.---.��.--� NIOBIUM

STREAM MOSS

200 --�·-· �

o

� o oO o

100 .o� .. 0 ... o . . . �--

�- Ooo

"iJ � lo">

o. Ill.

o

IQ], S), NIOBIUM

.il

iCI -.10

Fig. 5a-d. Scatter diagrams of La and Nb in samples of barren rock, trachyte, stream sediments and stream moss from the Sæteråsen area, Vestfold, Oslo Graben. All values are given in ppm. N = No. of samples. R = Correlation coefficients. p = Level of significance.

trations the correlation is poor or perhaps nega­tive. A positive correlation seems to be present between these elements in the aphyric trachyte (Fig. 5b). A significant positive correlation oc­curs in stream sediments (Fig. Se), but not in stream moss (Fig. 5d).

Nb is not anomalous in the drainage samples from the Sæteråsen mineralization (Figs. 6, 7 and 8). Nor is there any significant REE anomaly in

the fine fractions of the stream sediments (shown by La in Fig. 9). A REE anomaly is, however, found in the coarse sediment fraction and in heavy-mineral separates in the Sæteråsen area (shown by La in Fig. 10), but sparse sampling in the surrounding barren rock area may have ac­centuated this picture.

La in stream moss (Fig. 11) shows significant anomalies over the Sæteråsen mineralization.

8 P. Ryghaug

......... \

\ \ \ \

1km

NORSK GEOLOGISK TIDSSKRIFT l (1983)

SETERAASEN NHEPOSlT

STREAM SED l H. l -o. t8f9t J

PPM NB

.Sli • 100 • ISO

.250 eæo

.6&)

e I(XXJ e) 1001

Fig. 6. Concentrations of Nb in stream sediments (fine fraction). Rock boundaries are dashed. The number given at each symbol indicates the upper limit of the concentration interval.

Discussion and conclusions The contents of Nb, Th and REE in the Sæter­åsen aphyric trachyte lava are more than lO times higher than the average concentration of these elements in the surrounding barren rocks. This distinctive lava is not the most acid type of rock in the area and the contents of trace elements seem to be higher than normal magmatic differ­entiation would indicate.

No significant Nb, La, Ce etc. anomalies occur in sediment samples downstream from the Sæter­åsen Nb-deposit. The explanation for this seems to be the dominance of trace minerals coming from the more easily weathered alkaline plutonic rocks. The aphyric trachyte is very fine grained and resistant to weathering and does not seem to contribute significantly to the composition of the stream sediments. It is possible, however, that the investigated area could be anomalous as a whole. Regional geochemical mapping in neigh-

NORSK GEOLOGISK TIDSSKR!Ff l (1983)

bouring areas shows that the average concentra­tions for these elements are 10 times less than in the Sæteråsen area (unpubl. data).

La and, to some extent, U in stream moss show significant anomalies in streams draining the Sæteråsen deposit, and stream moss sampling would seem to be an interesting geochemical method of exploration for mineralizations of the Sæteråsen type.

The contents of trace elements in stream moss are apparently dominated by ionic exchange, and are therefore dependent upon the solubility and mobility of the respective elements. Elements with low ionic potentials (La and U) would therefore be more easily concentrated in the

........... \

\ \ \ \

--t

Nb and REE at Sæteråsen 9

moss samples. Differences in species and age of the stream moss do not seem to overshadow the anomalies, though large pH variations must be taken into consideration.

The strong positive correlation between Nb and Th in the alkaline rocks indicates that radio­metric methods are suitable for this type of de­posit.

Other similar mineralizations might occur in this part of the Vestfold volcanic area. A region­al exploration programme should include sam­pling of stream sediment and stream moss and analyses of the samples for Ce, La, Nb, Th, U, Y and Zr.

SETERAASEN NHJEP0S l T

STREAH-MOSS

PPM NB

• 6.9 • 8.� • 10.0 • 16.0

.26.0

• i59.0 _6&.0

-100.0

• ) 100.0

Fig. 7. Concentrations of Nb in stream moss (dry matter) . Rock boundaries are dashed. The number given at each symbol indicates the upper limit of the concentration interval.

10 P. �yghaug

.......... \

\ \ \ \

-

1km

NORSK GEOLOGISK TIDSSKRIFT l (1983)

SETE.RMSEN 1'6-(EPOS l T HEAVY HINERN... (.>�.50

PPM NB

• t6Cll • 2500 .� .6600

• ) 6000

Fig. 8. Concentrations of Nb in heavy mineral concentrates (S.G. > 3.31). Rock boundaries are dashed. The number given at each symbol indicates the upper limit of the concentration interval.

Acknowledgements. My sincere thanks go to J. Hysingjord of the Geological Survey of Norway, for pointing out the anoma­lous area and proposing this geochemical study. The analytical work was carried out at the Survey's laboratories and at the Institute for Energy Technology, Kjeller. I wish to thank G. Faye, B. Nilsen, P. E. Kvalø. B. Horgmo and B. l. Denstad for the analytical work at the Survey, L. Holiløkk and G. Grønli for the illustrations and drawings, and Å. Minde for typing the

manuscript, G. Juve and J. Hysingjord for their support during the exploration work, P. M. Ihlen, R. Sinding-Larsen and B. Bølviken for their critical reviews of the manuscript and R. Boyd and T. Ryghaug for correcting the English text.

Manuscript received November 1981. Revised J anuary 1983.

NORSK GEOLOGISK TIDSSKRIFT l (1983)

.......... \

\ \ \ \

1km

Nb and REE at Sæteråsen 1 1

SETERMSEN �IT

STREAM SED l M. l -o. 19N1 J

PPH LA

.Sl; • 100 • 160

.250 eæo

.6&) -lem

e· ICXXl

Fig. 9. Concentrations of La in stream sediments (fine fraction). Rock boundaries are dashed. The number given at each symbol indicates the upper limit of the concentration interval.

· References Bengtsson, Å. & Lithner, G. 1981: Vattenmossa (Fontinalis)

som matare på metallfiirorening. Statens naturvdrdsverk, Sverige. Rapport PM 1391.

Brøgger, W. C. 1933: Die Eruptivgesteine des Oslogebietes. VII. Die chemische Zusammensetzung der Eruptivgesteine des Oslogebietes. Skr. Nor. Vidensk. Akad. i Oslo, I. Mat.­Natur-vidensk. Kl., 1933, No. l, 147 pp.

Burton, M. A. S. & Peterson, P. J. 1979: Metal accumulation by aquatic bryophytes from polluted mine streams. Environ. Pol/ut. 19, 39-46.

Faye, G. C., Ødegård, M. 1975: Determination of major and trace elements in rocks employing optical emission spectros­copy and X-ray fluorescence. Nor. geo/. unders. 322, 35-53.

12 P. Ryghaug

........ '\

\ \ \ \

l l -

l J

/ /

1km

NORSK GEOLOGISK TIDSSKRIFT l (1983)

SETERAf\SEN I'IHE'OS l T HEAVY MINERAL

PPM LA

• 2Q) .390 .6&)

• 1000

• tG(X)

.2600

e·Z500

( .)�.�·

Fig. JO. Concentrations of La in heavy mineral concentrates (S.G. > 3.31). Rock boundaries are dashed. The number given at each symbol indicates the upper limit of the concentration interval.

Finstad, K. G. 1972: En undersøkelse av urvalgte edelmetaller og sjeldne jordartselementer i noen norske, hovedsakelig ba­siske og ultrabasiske bergarter. Unpubl. cand.real.thesis, Univ. of Oslo, Norway.

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NORSK GEOLOGISK TIDSSKRIFT l (1983)

......... \

\ \ \ \

1km

Nb and REE at Sæteråsen 13

SETERAASEN f\S-!E'OS l T STREAH-MOSS

PPM LA

.æ

.æ • 100

.,60

.260

.Bl

e, 5!ll

Fig. Il. Concentrations of La in stream moss (dry matter). Rock boundaries are dashed. The number given at each symbol indicates the upper limit of the concentration interval.

Oftedahl, C. & Petersen, J. S. 1978: Southern part of the Oslo Rifts. In Dons, J. A. & Larsen, B. T. (eds.), The Oslo Paleorift. Nor. geo/. unders. 337, 163--182.

Ramberg, l. B. , Larsen, B. T. 1978: Tectonomagmatic Evolu­tion. In Dons J. A. & Larsen, B. T. (eds.), The Oslo Paleorift. Nor. geo/. unders. 337, 55-73.

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