Secondary copper iodide in till in the Löytösuo area, Ylikiiminki, Finland

Applied Geochemistry, Vol, 5, pp. 347-355, 1990 0883-2927/90 $3.00+ .00 Printed in Great Britain Pergamon Press plc

Secondary copper iodide in till in the Lrytrsuo area, Ylikiiminki, Finland

RISTO AARIO a n d VESA PEURANIEMI Department of Geology, Oulu University, 90570 Oulu, Finland

(Received 28 March 1989; accepted in revised form 8 September 1989)

Abstract--Till geochemical studies were carried out in the Ylikiiminki area in northern Finland while prospecting for tungsten and sulphide ores. The area forms part of the Proterozoic schist belt of northern Ostrobothnia. Lodgement till was deposited on the bedrock surface in an earlier ice flow phase, followed during the later deglaciation by looser till types overlaid in succession by beach gravels and sands. A special postglacial type of Cu anomaly was found at Lfyt6suo, the nature of which was studied using partial extractability analyses and mineralogical investigations. Copper was found to be present in the form of both primary and secondary sulphid~esan'd CuI (marshite) was found as a rare mode of occurrence. The source of the I is possibly related to enrichment in algae in the Litorina phase of the Baltic Sea; in general the more iodine-rich soils occur within the sphere of this brackish water stage.

INTRODUCTION

LARGE-scale investigations into till geochemistry were carried out in the Ylikiiminki area of nor thern Finland (Fig. 1) while prospecting for tungsten and sulphide ores, resulting in the discovery of several Cu anomalies. One of these, the L6yt rsuo anomaly proved in later chemical and mineralogical evalu- ations to be quite exceptional and interesting, because it included an I mineral which had not been found earl ier in Finland. This led to further investigations at the site.

FIELD AND LABORATORY INVESTIGATIONS

The bedrock data used here are based on earlier mapping by HONrAMO (1985). NO new field data were collected. The glacial geology of the area is based on aerial photographs with field control. The whole area was covered by infra-red diapositive films at a scale of 1:60,000 (Kodak Infrared Aero Film, type 8443). The fieldwork was carried out in connection with the Soma project of the Finnish Academy, which studied the origin and properties of morainic landforms in northern Finland (AAmo, 1984). Samples for examination of the regional till geochemistry were taken at 50 m intervals along N-S oriented traverses located 500 m apart. The traverse spacing was then reduced to 100 m in the anomalous areas. One till sample was taken from as deep a position as possible at each sampling site using light-weight drilling equipment (Partner) and the sampling depth varied from 0.5 to 9 m. The structure, texture, orientation and sedimentological and geochemical properties of the till were studied using test pits made with tractor-mounted exca- vator.

The till samples were sieved into three fractions: <0.06 mm, 0.06--0.25 mm and >0.25 mm. The fine fraction (<0.06 ram) was dissolved totally in a mixture of HF, HNO 3 and HC10 4 and Cu, Co, Zn, Pb, Mo and W were analyzed by atomic absorption spectrometry. Cold extraction analyses were performed from the same fraction using citric acid and ascorbic acid-hydrogen peroxide as solvents in order to decipher the mode of occurrence of metals. The sand fraction (0.06--0.25 mm) was divided into light and heavy

fractions using tetrabromoethane (d = 2.96 g/cm3). The mineralogy and mineral chemistry of the heavy fraction was studied by optical microscopy, scanning electron microscopy (JEOL JSM-35), microprobe analysis (JEOL SUPERPROBE 733) and X-ray diffraction.

GEOLOGICAL SETTING

Bedrock

The area forms part of the Proterozoic schist belt of northern Ostrobothnia in northern Finland. The main rock types are greywackes, mica schists, black schists, dolomitic limestones, skarn rocks and mafic metavolcanics. The bedrock surface has been weathered in places, especially in areas occupied by black schists.

Quaternary environment

The area is located close to the centre of the Pleistocene Fennoscandian ice sheet and consequently has also experienced pronounced isostatic uplift in late-glacial and post-glacial times. The loca- tion of the area is shown on the map in Fig. 1, which also indicates the main glacial e lements (AARIO, 1984).

During the earl ier flow phases of the Weichselian ice sheet, more densely packed lodgement till was deposited onto the bedrock (e.g. cover moraine, large parallel till landforms, drumlins and related features). As the deglaciation proceeded looser till types were then deposited, which are now often markedly oxidized and stained with Fe compounds (e.g. some cover moraine and hummocky disinte- gration moraine) . The ice was found to have flowed from NW to SE during the earlier deglacial ice flow phases, with a younger flow phase from W N W to

s:3-g 347

348 Risto Aario and Vesa Peuraniemi

[] []

0 5 10 km I

E Z cover moraine

drumlins

r , ; ~ para l le l or sinuous -~ morainic r idge ( l a r g e )

~ para l l e l morainic r idges ( less prominent )

minor para l le l f e a t u r e s

~ i r o g e n - r i d g e s

' morain ic hummocks

Z ~ 9o~/E

D N(

r - ,

4"~.

" ~ train of morainic hummocks

prominent morainic landform." of i r regular shape (of ten end mora ines or

in te r loba te mora ines )

lake and river N

" : ~ glac io f luv ia l landforms

e s c a r p m e n t or slope

~ . ~ c h a n n e l

[ ~ Loytosuo area

NORWAYI/'~ j

./~'/~ k

,NO)

FIG. 1. The area studied, including the L6ytOsuo site, located about 40 km east of Oulu. The glacial morphology of the area is characterized by active-ice features, usually veneered by looser till types from an ice-margin environment. The area is below the highest shoreline and, consequently, there are often beach deposits lying on top of the sequence. The youngest ice flow direction WNW-ESE is clearly visible as the

orientation of the glacially sculptured terrain.

ESE. Being situated in the centre of the schist belt, the area does not feature any significant variations in till l ithology due to these differences in ice flow directions.

The ice margin bordered by the Ancylus Lake phase of the Baltic Sea, retreated from the area about 9000 a B.P. , the corresponding Ancylus level being at about 210 m above sea level. This means that the area was left submerged beneath about 130 m of water upon the disappearance of the ice (ERONEN and H A I L A , 1981). There were no appreciable m o v e m e n t s

of water within the sublacustrine till, and therefore the ti l l-water system at the bottom of the Ancylus Lake formed an environment which was generally reducing in its geochemical character.

The brackish water influence of the Litorina Sea spread to the area about 70(0)-7400 a B.P. , when it was still covered by about 15 m of water. The Litorina limit is about 95 m above sea level (ERONEN, 1974). Owing to the isostatic uplift (present uplift is about 8 mrn/a) the area emerged from the sea about 5800 a B.P. , a date interpolated from values quoted for

Secondary CuI in till, Finland 349

adjacent areas by EaONEN (1974, 1983) and SAAR- NISTO (1981). During the shoreline stage the waves and the sea ice eroded and sorted some of the earlier glacial sediments and in many places gravels and sands were deposited on top of the till layers. Infil- tration water and ground water flow caused differences in redox and temperature conditions between the layers, and the environment was no longer so obviously reducing. Paludification soon followed, resulting in a peat layer on top of the sequence, and the geochemical environment again shifted in the direction of reducing conditions.

TILL GEOCHEMISTRY AND MINERALOGY

The results of the laboratory tests on samples from the L0yt0suo site are presented in Figs 2-13. Figure 2 shows the frequency distribution of Cu content. The distribution is positively skewed, showing anomalous Cu concentrations.

Beyond the Lrytrsuo site there are also anomalies of other metals, but no other metals correlated with Cu in the LOytOsuo anomaly itself (Fig. 3), the one selected for closer examination.

The Cu anomaly forms a NW-SE oriented train

%

80

60

40

20 j 'E~% 50=53 p p m

80=82 p p m

9 0 = 1 2 0 p p m 9 5 = 1 4 0 p p m

9 8 = 1 6 0 DPm

60 120 180 2 4 0 3 0 0 3 6 0

Fie. 2. Distribution of Cu in the till at Lrytrsuo.

Cu

p p m

u~ ~N

oO (O

~'7202.7

4- + ÷

÷ + +

+ + t- + + +

+ + +

4- + +

!;? ; +

; ; + . • • + 4-

+ 4. + : 4 . 4. 4 ` • r + +

• 4- 4. + + +

• • + • + + • 4" 4" • 4- + 4. • 4- •

+ 4. + + 4- + 4 1 8 4 . 4"

+ + + + • • + 4. ,4- 4- • • + 0 ; • + 4- + + + 181 +

+ + +

4. + 4- 4- + +

t- + •

4. 4. 4.

C o p p e r in t i l l

a t L O y t O s u o

p p m

+ < 8 2

• 8 2 - 1 2 0

• 1 2 1 - 1 4 0

• 1 4 1 - 1 6 0

• ~ 1 6 0 1 8 1

o I i

200m I

FiG. 3. The Cu anomaly train in the till at Lrytrsuo.


(Fig. 3). The bedrock configuration and the direction of ice flow have the same orientation. The highest measured Cu content in the till was 418 ppm. Partial extraction analyses performed on seven till samples are shown in Figs 4, 5 in which the total amount present forms the abscissa. The portion of Cu soluble in citric acid is relatively low (5-16%), whilst much more (34-73%), dissolved in the mixture of ascorbic acid and hydrogen peroxide. This suggests that Cu is predominantly present in the form of sulphides (PEA- CHEY and ALLEN, 1977). The secondary Mn-oxides, which may also dissolve by this method and be responsible for the copper content detected, were not found in the unweathered till in this area. The maxi- mum solubilities in the partial extraction analyses (16% and 73%, respectively) are found in a sample with 418 ppm Cu which was also carefully studied as to its mineralogy.

The mineralogical examinations were concerned with determining the mode of occurrence of the Cu in till. Copper is present both as a primary constituent, i.e. chalcopyrite introduced by glacial erosion, and as a secondary constituent, i.e. copper sulphides created in the Quaternary deposits. The secondary chalcopyrite can be identified on the basis of its general appearance, composition and textural properties, being usually somewhat darker in colour than the primary chaicopyrite.

As a first mode of occurrence, the secondary chalcopyrite can cover the primary chalcopyrite grains in concentric layers, within which there are also silicate minerals from the till matrix (Fig. 6 and Table 1). The inner part of the secondary chalcopyrite rim has a fine, concentric lamination. The outer part of the rim is poorer in Cu and richer in Fe than the inner part (cf. analyses 3, 4 and 5 in Table 1). The laminae of the rim conform strictly to the outlines of the primary grain.

Another mode of occurrence is that it may fill pyrite cracks and coat the grains (Fig. 7 and Table 2). In places the secondary chalcopyrite seems to have altered to a hydrous Fe-oxide, or goethite (analysis 4 in Fig. 7 and Table 2).

100 C u c i t r %

Cu t o t

50

• • • C~totpOm

0 i i I i

2 0 0 4 0 0

Fro. 4. (Citric acid soluble Cu in the ti l l at E~ytSsuo.

1 O0

50

C u s %

Cu tot

• g o #

C u tot p p m

0 I I i

2 0 0 4 0 0

Fro. 5. Ascorbic acid-hydrogen peroxide soluble Cu in the till at Lryt0suo.

The third mode of occurrence of the secondary chalcopyrite is as ooids with a colloform texture (Fig. 8 and Table 3). There is no consistent difference in Cu, Fe or S contents between the lighter and darker laminae, and the colour difference may be due to impurities not shown by the analysis or to grain size differences. The ooids are roundish and have a pitted surface (Figs 9, 10). Similar ooids have been described by BINDA et al. (1985) in a Proterozoic arenite bed in Alberta, Canada. Scott and TAYLOR (1987) describe a corresponding type of colloform texture in coronadite (Pbl_2Mns016 • xH20 ) included in gossan, and GLASSON et al. (1988) observed a similar colloform texture in secondary Fe hydroxides and Au in laterite.

Table 1. Electron microprobe analyses of the grain shown in Fig. 6

Primary Secondary chalcopyrite chalcopyrite

% 1 2 3 4 5

S 34.23 33.14 29.24 27.78 29.88 Fe 29.89 30.27 29.10 27.35 25.94 Cu 33.84 31.81 27.35 30.15 34.17

Total 97.96 95.22 85.89 85.28 89.99


Secondary Pyrite chalcopyrite Goethite

% I 2 3 4

S 53.24 33.56 30.93 - - Fe 46.66 28.30 30.38 61.05 Cu - - 35.22 32.16 - -

Total 99 .90 97.08 93.47 61.05

Secondary Cul in till, Finland 351

FIG. 6. A primary chalcopyrite grain, in the centre, covered by a coating of secondary chalcopyrite, which forms a matrix between the till particles, dark quartz and amphibole. SEM, secondary electron image.

Numbers 1-5 refer to analyses in Table 1.

Fie. 7. Grain composed of pyrite, secondary chalcopyrite and goethite. SEM, backscattered elcctron image. Numbers 1-4 refer to analyses in Table 2.

FK;. 8. Colloform-textured chalcopyrite ooid. SEM, backscattered electron image. Numbcrs 1-8 rcfcr to analyses in Table 3.

FIG. 9. A dimple-surfaced chalcopyrite grain. SEM, secondary electron image.

Table 3. Electron microprobe analyses of thc grain shown in Fig. 8

Secondary chalcopyritc

% 1 2 3 4 5 6 7 8

S 31.79 33.66 31.69 31.83 31.53 32.80 32.77 29.75 Fe 28.76 2 7 . 8 1 27.04 29.96 27.40 28.42 2 8 . 3 5 27.04 Cu 30.72 31.63 31.50 31.30 2 9 . 6 1 3(I .05 30.98 29.97

Total 91.27 93.10 90.23 93.09 88.54 91.27 92.10 86.76


As a fourth mode of occurrence, the secondary chalcopyrite occurs in composite sulphide grains (Fig. 11 and Table 4), where it is exceptionally rich in Cu, its real formula being from CUl.2Feo.9S 2 to C U 1 . 5 F e 0 . 6 S 2 •

It is characteristic of the composition of the secondary chalcopyrite as opposed to the primary one, that there is great variation in Cu, Fe and S contents and a considerable deficit in the analysis sum (Tables 1-4), which could be due to the presence of water in the mineral structure. Other secondary Cu sulphides present in the sample are bornite and covellite (Figs 11, 12 and Tables 4, 5). Of the other sulphide minerals, only pyrite is present.

A very rare secondary Cu mineral, CuI, was present in two places. In one case it was found to form a

coating surrounding coveilite (Figs 12, 13 and Table 5), and in the other it formed a discontinuous margin around a composite sulphidic grain (Fig. 11 and Table 4). Because of the lack of an I standard, the I content indicated in the electron microprobe analyses of the CuI (Tables 4, 5) is not accurate. A composition analysis of the CuI coating surrounding the coveilite was also performed by an energy dispersive X-ray spectrometer of a scanning electron micro- scope. The result obtained, Cu 34.30% and I 65.85%, is very close to the stoichiometric composition of marshite, CuI (Cu 33.36%, I 66.64%). There is also a darker inner margin to the CuI coating (Fig. 12), which contains 6.6% S in addition to Cu and I (Table 5). A mineral species of this kind containing Cu, I and S has not been described pre-

FlO. 10. A close-up of Fig. 9. SEM, secondary electron image.

Flo. l 1. Grain composed of pyrite (1), covellite (2), chalcopyrite (3.4), bornite (5) and marshite (6). SEM, backscattered electron image. Numbers 14) refer to analyses in Table 4.

FIG. 12. Grain with a dark core of covellite and a light rim of marshite. SEM. secondary electron image. Numbers 1-3 refer to analyses in Table 5.

Secondary CuI in till, Finland 353


Secondary Pyrite Covellite chalcopyrite Bornite Marshite

% 1 2 3 4 5 6

S 53.99 31.73 32.50 31.73 32.23 - - Fe 46.67 2.98 25.14 17.73 6.16 1.20 Cu - - 66.21 40.55 47.67 60.96 31.36 I . . . . . 72.41

Total 100.66 100.92 98.19 97.13 99.35 104.97


Cu-I-S Covellite mineral Marshite

% 1 2 3

S 33.95 6.64 0.27 F e - - - - - -

Cu 66.80 42.49 33.32 I - - 55.24 75.71

Total 100 .75 104 .37 109.30

viously in the literature. Minerals of the marshite- miersite group (miersite and cuprian miersite) are described by CHn'AYEVA et al. (1971) in the supergene zone of a Cu ore. A silver iodide, iodyrite, has been described by BURGESS (1911) in the weathered parts of sulphide veins in Nevada.

The mode of occurrence of the CuI is quite similar to that of the secondary chalcopyrite. To judge from their texture and composition, the secondary Cu compounds at L6yt6suo were evidently precipitated in the till from groundwater under reducing conditions (LETr and FLETCHER, 1980).

The L6yt6suo sample was also studied for other minerals, and was found to contain amphibole, quartz, chlorite, plagioclase, epidote, haematite, ilmenite, titanite, fluorapatite, zircon and magnetite.

SUMMARY AND DISCUSSION

The present survey of the till geochemistry of an area in Ylikiiminki, northern Finland, led to the discovery of a Cu anomaly at LtiytOsuo. The maxi- mum Cu content of the fine till fraction was 418 ppm. According to the partial extraction analyses, the Cu occurs mainly in the form of sulphides. Mineralogical investigations indicated that Cu occurs in the form of both primary and secondary chalcopyrite, covellite, bornite and CuI (marshite). The modes of occur-- rence of the secondary chalcopyrite, as ooids, coat- ings and crack fills, and also its compositional variations, suggest that it originated as a result of supergene activity in the till. Groundwater containing Cu ions came into contact with H2S and Fe ions in a reducing environment, creating circumstances in which both chalcopyrite and also covellite and bornite could be precipitated.

The question of the origin of the 1 is an interesting one and it is worth discussing its geochemistry a little further. The whole lithosphere contains an average of 0.3 ppm I (RANKAMA and SAHAMA, 1950), its incidence being from 0.2 to 0.5 ppm in igneous rocks, 0.8--2.7 ppm in sedimentary rocks and 0.04-2.3 ppm in metamorphic rocks (RANKAMA and SAnAMA, 1950; Geochemistry of Iodine, 1956; BREHkER and Fuc,~, 1978; FurF et al., 1986). The overburden includes several times as much I as the bedrock, commonly 2- 10 ppm (Geochemistry of Iodine, 1956). Bccause of its chalcophile nature, it is also enriched in sulphide

6083-4798 Marshite

8

7

6

0 C3

4

3

2

1 j

1 2 3 4 5 6 7 9

Enerqy in key

FIG. 13. Energy-dispersive X-ray spectrum for marshite in Fig 12

~ 0


minerals to concentrat ions of 6 ppm (CHITAYEVA e t

al., 1971; ANDREWS et al., 1984). Iodine can be enriched from a few ppm up to some

thousand ppm in the weathered crusts of sulphide ores (CHITAYEVA et al., 1971). It can also be enriched in the topmost humus-rich horizon in various types of soil profiles (BREHLER and FUGE, 1978). The I level in the glacial overburden is very low owing to its relatively young age (GOLDSCHMIDT, 1954).

There are not many iodine minerals. Sometimes when I-rich solutions come into contact with heavy metals non-soluble metal iodides can be precipitated (RANKAMA and SAHAMA, 1950), of which one might ment ion iodargyrite (AgI), marshite (CuI) and miersite (4AgI. CuI).

The Finnish bedrock is mostly composed of igneous and metamorphic rocks of Archaean and Proterozoic age; it is overlaid by sediments mostly derived from the last glaciation, followed by later postglacial sediments. The I content of the Finnish bedrock and overburden is accordingly quite low, and we should also look for o ther possible sources.

Ocean water includes an average of 0.05 ppm of I, which accounts for about one fourth of all the I on the Earth (RANKAMA and SAHAMA, 1950). The overburden of coastal areas also contains higher than average amounts of l, 8-19 ppm (Geochemistry o f Iodine, 1956), due to the marine influence. Fur ther data can be obtained from medical research, in that it is well known that a lack of I in the diet leads to goitre. The incidence of goitre used to be greater in the inland areas of Finland than in the coastal areas, which were influenced by the Litorina Sea. Thus the soils in the coastal areas may be richer in I than those in inland areas (VILKKI, 1956). F rom 1949 onward, however , I has been added to table salt to prevent goitre.

Because of its biophile nature, I in the sea is concentrated in organic matter , especially marine algae. The brown algae are especially well known I accumulators, containing 55-8800 ppm and averag- ing 2488 ppm, while the red algae contain 200-565 ppm 1, the average being 381 ppm. Terrestrial plants contain 4--5 ppm I, as do freshwater green algae (SHACKLETrE and CUTHBERT, 1967).

As stated above, the bedrock and Quaternary deposits of Finland are rather poor in I, but there is an enrichment in sea water. It would be reasonable, therefore, to consider an earlier stage of the Baltic as a possible source of I at LOyt6suo. Organic activity in the Litorina Sea was high, and when this area emerged about 5800 a B.P. l-rich Fucus algae were evidently accumulated in the beach gravels and sands. In the case of the sequence studied here, the beach deposits were lying upon tills, and after the organic mat ter had decomposed the I passed into the soil water and the groundwater circulation. As paludification of the area had led to a reducing environment, Cu and l could well have been precipitated as marshite, Accordingly, algal concentrat ions could be

suggested as the source of I at L6ytOsuo and local I enrichments could also be anticipated within the whole sphere of the Litorina Sea. The modes of occurrence of Cu observed here can just as well be found in the overburden of non-glaciated areas.

Iodine has been used as an indicator e lement when prospecting for hydrothermal sulphide ores (CHI- TAYEVA et al., 1971; KRYLOVA, 1978; ANDREWS et al., 1984; FUGE et al., 1986) and its content in deepseated groundwater has been used in oil prospecting (KUDELSKIY, 1978), because it is a mobile e lement and forms large dispersion halos. The present mode of occurrence at LrytOsuo is not really so strongly promising for ei ther of these purposes, however. In any case, this phase should be taken into account by workers studying the geochemical cycle of 1, and care is required in interpreting explorat ion data in these kinds of environments.

Acknowledgements--The fieldwork was carried out under the auspices of the Rautaruukki Company, and the Soma project financed by the Finnish Academy. The elemental analyses were performed in the Raahe Research Center of the Rautaruukki Company. The microprobe analyses and scanning electron microscopy were performed at the De- partment of Electron Optics of Oulu University. The figures were drawn by Mrs. Kristiina Karjalainen and the English of the manuscript was checked by Mr. Malcolm Hicks. Thc authors wish to extend their gratitude to all the institutions and persons involved.

Editorial handling: Brian Hitchon.

REFERENCES

AAR10 R. (1984) The origin and properties of morainic landforms in northern Finland, with special reference to their potential range of use. Miiiiriiaikainen loppuraportti Suomen Akatemian tutkimusprojektista, 04-114. Oulu (in Finnish).

ANDREWS M. J., B1BBY J. M., FUGE R. and JOHNSON C. C. (1984) The distribution of iodine and chlorine in soils over lead-zinc mineralisations east of Glogfawr. mid-Wales. J. geochem. Explor. 20.19-32.

BINDA P. L., KOOPMAN H. J. and SCHWANN P. L. (1985) Sulphide ooids from the Proterozoic Siyeh Formation of Alberta, Canada. Mineral. Deposita 20, 43-49.

BREHLER B. and FUGE R. (1978) Iodine. In Handbook o[ Geochemistry (ed. K. H. WEDEPOHL), VO[. 11.4, Chap. 53. Springer-Verlag.

BURGESS J. A. (1911) The halogen salts of silver and associated minerals at Tonopah. Nevada. Econ. Geol. 6, 13-21.

CHITAYEVA N. A.. MILLER A. D., GROSSE YU. 1. and CHRISTYAKOVA N. I. (197l) Iodine distribution in the supergene zone of the Gay chalcopyrite deposit. Geo- chem. lnternat. 8,426~436.

ERONEN M. (1974) The history of the Litorina Sea and associated holocene events. Soc. Sci. Fennica. Comm. Phys.-Math. 44, 79-195.

ERONEN M. (1983) Late Weichselian and Holocene shore displacement in Finland. In Shorelines and lsostasy (eds. S. E. Sr~ITH and A. G. DAWSON), Institute of British Geographers, Spec. Pub. 16, Academic Press.

ERONEN U. and HAILA H. (1981) The highest shore-line of the Baltic in Finland. Striae 14, 157-158.

FUGE R., ANDREWS M. J. and JOHNSON C. C. (1986) Chlorine and iodine, potential pathfinder elements in exploration geochemistry. Appl. Geochem. 1, 111-116.

Secondary Cul in till, Finland 355

Geochemistry of Iodine (1956) Bibliography 1825-1954. Chilean Iodine Educational Bureau.

GOLDSCHMIDT V. M. (1954) Geochemistry. Clarendon. GLASSON M. J., KEHNE R. W. and WOLLMER F. W. (1988)

Gold exploration in the CaUion Area, eastern Goldfields, Western Australia. J. geochem. Explor. 31, 1-19.

HONKAMO M. (1985) On the Proterozoic metasedimentary rocks of the Northern Pohjanmaa schist area, Finland. Geol. Surv. Finland Bull. 331,117-129.

KRYLOVA L. YA (1978) Exploration significance of iodine distribution in the bedrock and weathering crust of the Maykain deposit. Int. Geol. Rev. 20, 357-361.

KUDELSKIY A. V. (1978) Forecasting petroleum prospects on the basis of amount of iodine in subsurface waters. Int. Geol. Rev. 20, 362-366.

LEar R, E. W. and FLETCHER W. K. (1980) Syngenetic sulphide minerals in a copper-rich bog. Mineral. Deposita 15, 61-67.

PEACHEY D. and ALLEN B. P. (1977) An investigation into the selective dissolution of sulphide phases from stream sediments and soils. J. geochem. Explor. 8, 571-577.

RANKAMA K. and SAHAMA Trt. G. (1950) Geochemistry. University of Chicago Press.

SAARNISTO M. (1981) Holocene emergence history and stratigraphy in the area north of the Gulf of Bothnia. Ann. Acad. Sci. Fennicae A 111 130, 1-42.

Scoax K. M. and TAYLOR G. F. (1987) The oxidized profile of BIF-associated Pb -Zn-mineralization: PegmenL northwest Queensland, Australia. J. geochem. Explor. 27,103-124.

SHACKLETTE H. J. and CUTHBERT M. E. (1967) Iodine content of plant groups as influenced by variation in rock and soil type. Geol. Soc. Am. Spec. Paper 90, 31-46.

VILKKI P. (1956) Iodine in the Finnish diet and its relation to goitre incidence. Ann. Acad. Sci. Fennicae A, 11 Chemica 71, 1-61.

Documents

Secondary copper iodide in till in the Löytösuo area, Ylikiiminki, Finland