URANIUM ENRICHMENT IN EUROPEAN PEAT BOGS

M.R. WILSON

Swedish Geological Lulea, Sweden

ABSTRACT

URANIUM ENRICHMENT IN EUROPEAN PEAT BOGS

In Europe peat bogs are extensively developed but high uranium concentrations have been reported from only a relatively few in Sweden, Finland and the United Kingdom. Because of the limited tonnage of uranium ore it is unlikely that they would ever become economically viable The distribution of uramferous peat bogs is governed by enrichments of labile uranium m the source rocks.

1. INTRODUCTION

The only surficial occurrences of uranium m Europe are concentrations in dead organic matter in peat bogs Uranium deposits in coal and brown coal are not described here.

Peat bogs are extensive m Ireland, Sweden and Finland and some data are available from the lattertwo countries and from the United Kingdom.

2. SWEDEN

Sweden has an extensive coverage of post-glacial peat bogs, developed over the last 8 to 1 7 000 yr's. Organic material is an excellent medium for geochemical exploration and therefore some 1 27 000 samples have been taken over an area of 200 000 km2. It is now well-established that some contain considerable enrichments of uranium and other metals. The best documented occurrence is at Masugnsbyn, about 90 km southeast of Kiruna m northern Sweden (Figure 1), where a deposit of some 24 tonnes uranium in peat with an average grade of 0.2 % uranium in ashed material are known While these occurrences are not economic, considered purely as uranium resources, their exploitation for fossil fuel raises two considerations:

a) That the uranium and other metals might be recovered from the ash, adding to the value of the product, or b) That the content of metals may constitute an environmental threat

Content of heavy metals is now one of the factors considered m the evaluation of peatbogs, but it appears that the bogs with the highest uranium contents are not so suitable for peat production because of their high ash content and configuration

The Masugnsbyn uramferous peat bog is described by Armands [1 ]. It was discovered in 1958 during car-borne uranium exploration operated by AB Atomenergi (now Studsvik Energiteknik AB). The peat bog, which lies about 4 5 km northwest of the village of Masugnsbyn was extensively sampled, an area of about 600 by 200 m being sampled on a 20 x 20 m grid. Uranium content in the ashed material ranged from 0.04 to 3 1 %, with an average of 0 2 %. The peat bog has extremely low gamma radioactivity and samples studied indicated a high degree of disequilibrium The highest contents of uranium and radon are connected with the occurrence of radioactive springs m the region. The source of the uranium appears to be the Precambnan (Svecokarelian) bedrock in the area which is slightly enriched in labile uranium and is extensively fractured. Spring waters contain several ppb uranium and water derived from fractures contains 200 to 300 ppb uranium (max 1 800 ppb uranium). Minor uranium mineralizations are also recorded in skarn iron ores in the vicinity.

High uranium contents are also recorded m the county or Jämtland, as a result of uranium prospecting by Swedish Geological, financed by the Swedish Nuclear Fuel Supply Company (SKBF) Uranium contents of up to 3.4 % are recorded from peat bogs near the village of Stugun. Springs in the area are highly radioactive and the bedrock is slightly enriched in labile uranium, there also being some minor uranium mineralizations in the vicinity.

Another region with high uranium contents in peat bogs is situated within the area of the uranium-enriched Jarre granite, about 50 km west of Jokkmokk Within this area practically all springs are radioactive, indicating thatthe uranium m the granite is easily leached. A large number of peat bogs with maximum uranium contents varying from 1.1 to 3.2% have been recorded as a result of a state-financed uranium prospecting campaign. All anomalous peat bogs are related to radioactive springs. Systematic sampling from vertical and horizontal profiles [2], has shown thatthe uranium is very unevenly distributed in the peat material (Figure 2). The strongest uranium enrichment is always found along the drainage route of uranium-enriched spring water through the bogs. It is estimated that the most anomalous part of the bog (11 900 m2) contains 5 64 tonnes of uranium

The above studies, together with investigations of peat bogs carried out in connection with their evaluation for fossil fuel, indicate that the uranium concentration is usually very heterogeneously distributed. High uranium contents m peat bogs is almost always associated with radioactive springs, which in their turn are usually

100 km

GKIRUNA

• MASUGNSBYN

Figure 1

Localities of uraniferous peat bogs in Sweden.

Profilei 120

Legend I I < 1 000 ppm

1 000 - 1 830 ppm 1 830 - 3 200 ppm 3 200 - 31 773 ppm 1 ,2 springs

— • • • Sampling direction

—•— Drainage direction

^~v-^~ Drainage channel

— — — Limit of Forest

——— Limit Swamp-peatbog

£. Q Boulder or gravel

Figure 2

Contour map showing surface and vertical distribution of uranium in a peat bog near Jarre in Sweden.

100km

Figure 3

Localities of uraniferous peat bogs in the United Kingdom.

associated with uranium-enriched granites with labile uranium and fracture zones. Springs associated with the uraniferous black shales of Cambrian age, containing up to 330 ppm uranium, are not conspicuously radioactive, because the uranium in these shales is not labile.

The relationship between uranium and radium concentrations in bedrock, organic material, soils, and surface and groundwaters has been studied [3,4, 5] as part of a wider investigation of radioélément geochemistry sponsored by the Radioactive Waste Program of SKBF (Swedish Nuclear Waste Supply Co.).

3. FINLAND

Uranium concentrations in peat bogs in Finland are described by Yliruokanen [6]. About 1 700 peat samples from 1 31 peat bogs in various parts of Finland were collected and analysed; 21 of these bogs were used for fossil fuel production and had low uranium contents. The uranium content in peat-ash exceeded 1 000 ppm in samples from seven bogs, all of which were on granitic bedrock, and the highest value recorded was 2.4 %. The distribution of uranium is irregular; high uranium contents are found only in a limited area in each case and are restricted to basal peat layers.

4. UNITED KINGDOM 4.1 English Midlands

Within the East Shropshire syncline, overlying the Triassic Keuper Marl, a marshy area adjacent to Chillington Brook (SJ 875 073) contains a shallow peat deposit enriched in uranium [7, 8]. A maximum value of 162 ppm uranium was measured in dry peat, corresponding to about 580 ppm uranium in the ashed sample. High concentrations of several other elements, notably vanadium (up to 8 500 ppm) and also including Ni, Co, Ba, Cu and Zn are present in the peat. Radiometrie readings throughout the uraniferous zone (some 1 300 x 70 m) are generally low (7 to 10 R/h) indicating that the uranium is young in its present environment. The underlying Keuper Marl contains only 3 ppm uranium, while groundwaters in this region contain up to 30 ppb uranium.

4.2 Scottish Highlands

In the Lairg district of Sutherland, areas of high uranium in peat and glacial overburden occur near the southeast margin of the Grudie granite [7, 9, 10]. Enhanced uranium concentrations { 200 ppm) were measured in samples of dried peat derived from an area (about 500 x 160 m) m the outer margins of the granite and over the adjacent Mome schists. The highest uranium value (24 ppm) m rock was recorded from this sector of the stock Attendant cross-cutting galena-fluonte veins m the granite and adjacent schists contribute to associated enrichments of lead (2 000 ppm), molybdenum (100 ppm), copper and zinc m the peaty soils of this zone

High uranium concentrations have also been determined m peat and peaty soils from several other Caledonian granites of northern Scotland.

Uranium levels greater than 3 000 ppm have been measured over parts of the Helmsdale pluton and the Durris (northeast) part of the Forest of Birse granite mass High uranium contents have also been found m peaty soils overlying the Bennachie granite. No systematic studies to elucidate the extent and origin of these enrichments have been undertaken.

ACKNOWLEDGEMENTS

The description of uranium in peat bogs in the United Kingdom was kindly supplied by A G Gunn of the Metalliferous Minerals and Applied Geochemistry Unit of the Institute of Geological Sciences, United Kingdom J Ek of the Geological Survey of Sweden and G. Akerblom of Swedish Geological assisted in the description of the uramferous peat bogs of Sweden

REFERENCES

[1 ] ARMANDS, G., Geochemical prospecting of a uramferous bog deposit at Masugnsbyn, northern Sweden, In, Geochemical Prospecting m Fennoscandia, A. Kvalheim (Ed.), Interscience Pub, New York (1976) 127-154

[2] DUGERDIL Y. Prospection géochemique dans la region de Lovos, Univ. of Genève, Dep of Mmerology (1976)

[3] EK, J-, Bearbetning av uranhaltsmàtnmgar i vatten och backtorv fran backer i Sverige (Interpretation of uranium concentration determination in water and organic stream sediment samples from streams in Sweden), SKBF-KBS Technical Report 1981-11 Stockholm, KBS(1981).

[4] EK, J., EVANS, S., BERGMAN, R , Variations in radioactivity, uranium and radium 226 contents in three radioactive springs and along their outflows, northern Sweden, Studsvik report NW-82/177, Studsvik Energiteknik AB, (1982).

[5] EVANS, S , BERGMAN, R , Uranium and radium in Fmnsjön — an experimental approach for calculation of transfer factors, SKBF-KBS Technical Report 1981-04 Stockholm, KBS(1981).

[6] YLIRUOKANEN, I., The occurrence of uranium in some Finnish peatbogs, Kemia-Kemi, 4(1980) 213-217.

[7] BOWIE, S.H.U., OSTLE, D., CAMPBELL, C.B., Uranium mineralization m northern Scotland, Wales, the Midlands and southwest England, Trans. IMM Sect. 82 (1973) 177-179.

[8] BOWIE, S.H.U , BALL, T K , OSTLE, D , Geochemical methods in the detection of hidden uranium deposits, In, Proc 3rd Int. Geochemical Exploration Symposium, Toronto, CIM Special Vol. II (1971) 103-111 [9] MICHIE, U. McL, GALLAGHER, M.J., SIMPSON, A., Detection of concealed mineralization m northern

Scotland, In, Proc. 4th Int Geochemical Exploration Symposium (1973) 117-130.

[10] GALLAHGER, M J., MICHIE, U McL, SMITH, R.T., HAYNES, L, New evidence of uranium and other mineralization in Scotland, Trans. IMM Sect. B 80 (1971) 150-173.

DEPOTS SUPERFICIELS D'URANIUM EN MAURITANIE

In thé Ain Ben Till region of northern Mauritania, uraniferous calcareous clastic sediments have been discovered at the base of the flat hammada surfaces overlooking the Saharan Reg The groundwaters are phreatic, neutral to slightly alkaline, and have a high content of total dissolved solids, particularly sodium and chloride The source of the uranium in the hammadas is the calc-alkalme granite of the Reguibat basement which contains between 20 and 30 ppm uranium. Precipitation of the epigenetic uranium minerals and calcium carbonate is due to oscillations in the groundwater table and occurred behind local structural barriers

DEPOTS SUPERFICIELS D'URANIUM EN MAURITANIE

Dans la région de Am Ben Till au nord de la Mauritanie, des pediments élastiques calcaires uranifères ont été découverts à la base des surfaces plates d'hammada dominant le Reg saharien Les haffes souterraines sont phréatiques, l'eau est neutre ou légèrement alcaline, et a une grande teneur en solides totalement dissous, en particulier du sodium et des chlorures La source d'uranium dans les hammadas est le granit calco-alcalm de la roche de base du Regubat Ce granit a une teneur en uranium comprise entre 20 et 30 p p m La précipitation des minéreaux d'uranium épigénétiques et du carbonate de calcium est due aux oscillations dans la table des nappes et a eu lieu derrière des barrières structurales locales

Des concentrations uranifères de type calcrete ont été découvertes dans le Nord de la Mauritanie à proximité de la frontière avec le Sahara Occidental et l'Algérie. Elles se localisent dans les hamadas, surfaces tabulaires dominant le reg saharien de quelques mètres à quelques dizaines de mètres Cette région fait partie de la dorsale Reguibat, une des composantes du craton ouest africain Pour l'essentiel, la dorsale est constituée de granité calco-alcalm (type Am Ben Till) associé à des granités gris, des microgranites, des rhyohtes et des filons de quartz, pegmatites et dolentes [1]. Le climat actuel de la région est aride et chaud avec des précipitations moyennes annuelles comprises entre 20 et 50 mm [2] et des températures moyennes annuelles très élevées Le réseau hydrographique est constitué d oueds aboutissant à des zones dépressionnaires ou sebkhas, nom local des lacs salés L'écoulement de ces cours d'eau intermittents s'effectue vers le Sud, conséquence du basculement tertiaire de la dorsale Reguibat.

Les hamadas se présentent comme des plateaux tabulaires à bords pentus qui dominent le reg avoismant d'une hauteur dépassant rarement 15 m [1 ] Ce sont les plus anciennes des formations récentes déposées sur le vieux craton ouest africain II existe deux dépôts hamadiens successifs reposant sur une formation de base conglomératique, appelés hamada basse et hamada haute ou blanche La formation conglomératique basale est complexe (formation d Iguetti) et repose sur le socle granitique arénisé (Figure 1)

L altération des granités est peu poussée, c est une isaltérite à structure grenue reconnaissable et à feldspaths conservés [3] Les granités gris sont beaucoup plus altérés que les granités rosés du type Am Ben Till [1] La présence de fragments de tailles variées provenant probablement d'une croûte ferrugineuse antérieure [4], de galets détritiques siliceux à produits ferrugineux [3] et de traces d'anciens sols ferrallitiques [5] laisse supposer le développement sur ce socle d une histoire pédologique complexe Les formations détritiques les plus récentes auraient pour origine le remaniement d'un puissant manteau d altération [1 ] Sur le socle altéré s'est déposée la formation d'Iguetti, conglomérat dont le matériel détritique constitutif proviendrait de sources proches voire de remaniements m situ [3] Les dépôts hamadiens proprement dit forment un ensemble gréseux mal trié dont une partie seulement a subi un transport 5 %des grains sont façonnés par l'eau, 35 % sont des grains éoliens et 60 % sont des grains non usés [6] Les deux formations détritiques successives ont été, par la suite, affectées de phénomènes appelés hamadisations Celles-ci comportent des carbonatations, des gypsifications et des silicifications [1] Largilisation des dépôts est soit primaire, soit secondaire [1] La fraction argileuse est composée d un mélange de montmorillonite et d'attapulgite, le pourcentage de montmorillonite diminue avec la présence des carbonates et de la silice et disparaît quand les carbonatations et les silicifications sont massives (barre calcaire et silcrete terminal) La sépiolite fait alors sont apparition [1] Les carbonatations sont des cimentations épigénétiques de formations détritiques et semblent liées à la circulation des nappes Une phase terminale de sihcification épigénise certains de ces dépôts carbonates Du gypse et de la célestite ont été également observés La minéralisation exprimée sous forme de carnotite est pour l'essentiel localisée dans la hamada basse mais elle se retrouve aussi dans les autres formations, arène granitique comprise

Les nappes circulant dans les hamadas ont des caractéristiques géochimiques similaires à celles des eaux des chenaux de I Ouest australien [7,8,9] (Figure 2) Les eaux de trois sondages et d'un puits ont été analysées Elles