GEOLOGIAN TUTKIMUSKESKUS GEOLOGICAL SURVEY OF FINLAND Ydinjätteiden sijoitustutkimukset Nuclear Waste Disposal Research

Report Y ST-78

THE PALMOTTU ANALOGUE PRO JECT

Progress Report 1991 The behaviour of natural radionuclides in and around

uranium deposits, Nr. 5

Edited by

Juhani Suksi', Lasse Ahonen2, and Heikki Niini3

University of Helsinki, Department of Radiochemistry Geological Survey of Finland Helsinki University of Technology, Laboratory of Engineering Geology and Geophysics

Study financed by the Ministry of Trade and Industry, Department of Energy, and from the beginning of 1992 also by Finnish Centre for Radiation and Nuclear Safety

Espoo 1992

Suksi, Juhani, Lase Ahonen, and Heikki Mini {editors), 1992. The Palmotiu Airalogue Project, Pmgress Report 1991. The behavwur of ttatuml dwnuclides in and atvund umnium deposirs, Nr 5. Geological Survey of Finland, NucIear W m e Disposal Research, Repon YST- 78, 138 pages. ISBN 951 -690468-8. ISSN 033-3555.

?he reporr presenbs a comprehemive summary of $he resulfs of investigabiom carried our dun'ng 1988 - 1991 ar tha Palmottu wural analogue sndy site, a small U-7% mineralization in Nummi-Purula, southwesrem Finland. In tke stuiies i ~ u ~ ~ o n Imas been soughr concerning rodionuclide mobilimion and rransport phemmna analogour to those assumed to app& to repository near- and far-ficld phenomenu. A major efforr has k e n directed towards the geolog ica L and hydrogeological churacrerimzanon of rhe Pdmm area. Deiailed informarion on [he regiorlal and local structural geology in three dimett- siom has been obtained. The project has improved the bwwledge cf t?ze occurrence and areal distribubion of groundwater in fracrured crystalline bedrock. Various gmndwater rypes were detecred ciose to $he uraniwn bearing horizon, mainly dependant on lvcal hydrological regimes. Aho in a single water phasé, diflerent components with difererit origins could be discerned. The srudies on urcrniwn tmnrport have improved the u&r- standing of migrarion processes of uranium. Observdons concerning uranim so ption and rock mmrix dimion have been made. Detailed infomrarion &OM uruniwn fixananon mechanisms has been obtained. rendering cemin interprerarion aboui uranium mobil i~ ar Palrnottu. Fraemre-conrrolled alterarion of rock is a conwnon phenumemn which is seen to a$rect u raniwn distri bution near individual fractu res. High uranim concenrrations hnve been found near the fraciures where the most inrenre alrerarion has occurred. Elernenc mobility hm BIsa been shrdied wing the muItielement profile a p p m h . Studies for finding and chracteriung groudwater colloids have been pe@mnedd Srrcdies dealing with the biosphen'c trarispoi7 of rudionuclih have also been initiaed for establishing background &ta.

As a comlusion of this 4-year study period an unulysis of progress rhrough the vanous stages of rhe sr@ is also given. The nnalysis clearly indicares #he usemness of the study attd encuurages researchers to proceed in the direction established.

Juhani Suksi bsse Ahonen University of Helsinki Geological Survey Department of Radio- of Finland chemi~try SF-02 150 Espoo SF-00 170 Helsinki

Heikki Niini Helsinki University of Technology , 'lIWC-V SF-02150 ESF

This study is a part of the govemmentai research programme concerning the d i s p d of radioactive wastes in geologid formations, financed by the Ministry oC Trade and Industry, Department of Energy. The project was initiated by the Geological Survey of Finland (GSF) and the Department of Radiochernistry at the University of Helsinki (UHDR) in 1987 as a co-operational research project and was enlarged to include the hboratory oE Engineering Geology and Geophysics at the Helsinki University of Technology (HUT) in 1988. At the present phase, contributions from the following laboratories of Tec hnical Research Centre are also included: Reactor Laboratory ; Road, Traffic and Geotec hnicd Labratory ; Nuclear Engindng Laboratory .

The main responsibilities of the overlapping study activities were shared between the CO- operating research organisations in the following way:

- The Nuclear Waste Disposal Research Group at the Geological Survey of Finland was resporisible for the primary exploration and ore W y inventorial results of the Paimottu uranium deposit. The research in terest is devo ted to characterize the hydrogeological and stnictural features of the study site as well as to characterize waterlrock interaction processes and phenomena.

- The Department of Radiochemistry at the University of Helsinki was respon- sible for the activities conceming the distnbution and rnigration of natural radionuclides in the bedrock, groundwater, and superficid deposits.

- The Laboratory of Engineering Gwlogy and Geophysics at the Helsinki University of Technology was respnsible for the autoradiographical, petrographical-mineralogicd, and microanal ytical determinations of drill-core rock sarnples and groundwater particulates. The laboratory is also involved in the hydrogeological studies.

This publication is a comprehensive summary of the activities of the Palrnottu analogue study up to the end of 1991. It consists of a short review article of the present status of the Palniottu Analogue Study Project and of several short artides describing the ongoing subprojects. The writers of each article are primarily responsible for their presentatioris whereas the editorial work is mainly restricted to technical and CO-ordinational features of the articles.

(ONYINIJ JO A3AtlftS lV31E)0103~ AB 3OVW) SdVW 3113NI)VW JO SNOIlV13tldä31NI

3Hl ONV (8861 N3NYSlYY) V3UV nllOWlVd 3Hl JO dVW 31N013313Hl NO 03SVB SI

dVW 3H1'1661 IWIWVAInX OWlV AE 0311dW03 SI dVW lV3190l030 AtlVNIWll3Ud 3H1

LEGEND

granite

mica gneiss

ultrabaaic rocks

overburden

- - fault plane

C? magnetite

6 . pyrite

Mo molybdenite

p pyrrhotite

chalcopyrite

U uraninite 0 50 m 8 s

Fig. 2. Cross section of the bedrock at the Palmottu U-mineralization along one of the

drill hole profiles.

Depth (cm)

Figure 3. Total 21?b content versus depth in the sediment profile from Lake Palmottu in 1990. Water depth at the sampling site was 5 m.

Depth (cm)

Figure 4. Age-depth curve based on C.R.S. (Constant Rate of Supply) model (Appleby et al. 1978) of 21?b dating for the sediment profile of Lake Palmottu in 1990.

Table 1. The state of the project at the end of 1991. Categories of interest: A = identification of basic phenomena or factors, B = quantification of spatial properties and their mutual connections, C = determination of time-dependence of factors, D = relative influence on migration phenomena.

Explanations: Columns A B and C 0 = not et investi ated- kuming underway 1 = stuJes initiate%, re&L preiirninary 2 = roughly assessed 3 = essentially established, results available

/ Column D 1 - not relevant / * modest / + reasonable / + + strong

D. Influ- ence

-

+ + *

++

+

++

+

+

++

++

++

+

- matrix 2 2 1 diffusion

Biospheric 1 1 0 migration

Geological framework

- Formation evolution

- Tectonic structures

- Country-rock .......... ~!etr.c!!o=

- E E 3 0 P Y

Deposit structures

- Fracture distribution

- Fracture ........... minera!om

- U/Th alteration /mobility

.......... ~ s . ~ . ~ o ~ ! Z ? ! . O ~

- Surficial .......... h~b.rog~!og-)i

- Drillhole .......... ~ Y ~ . E O ~ _ ! C ! ~ P

- GW-flow

Hydrogeo- ......... .&~hta

- groundwater conditions

- UI''@ in GW .......... /.!Po!?.!!.!tr.

- Colloids and organics

Radionuclide

A. Identifi- cation

3

3

2

3

3

3

2

3

2

2

3

2

1

B. Spatial conn.

................................................................................................................................................................................. 3

................................................................................................................................................................................. 2

................................................................................................................................................................................. 2

................................................................................................................................................. 2

................................................................................................................................................................................. 2

................................................................................................................................................................................. 2

............................................................................................................................................. 1

...................................................................................................................................... 2

....................................................................................................................................... 1

........................................................................................................................................ 2

................................................................................................................................................ 2

................................................................................................................................................................................. 1

................................................................................................................................................. 1

C. Time d e ~ .

3

2

1

3

1

1

1

1

1

1

1

1

0

INTRODUCTION

Since the inception of the Palmottu Analogue Project, studies of the chemistry and the

behaviour of uranium in groundwater has been one of the main focuses of research.

This is due to the fact that groundwater chemistry, including the physico-chemical

conditions in groundwater, and the behaviour of radionuclides in the rock-water system

are strongly interrelated, as dissolution and precipitation of various minerals, speciation

on radionuclide bearing phases, and their mobilization, transport and retardation.

The target of the project work is the small uneconomic U-Th deposit of Palmottu in

Nummi-Pusula, S-W Finland. The deposit is located within the central part of the

Precambrian Fennoscandian Shield which is composed of crystalline rocks as granitoids

and gneisses. The local bedrock as well as the U-Th deposit was studied in connection

with the explorational activities in the late 70's and early 80's. The results of these

activities and the regional geological map is presented in the work of Räisänen (1986),

the same map is reproduced in the work of Suutarinen et al. (1991). Recently a local

geological map of the Palmottu study area and a lithological map of the surroundings

of the ore body were compiled (Kuivamäki et al. 1990).

Fig. 1. Lithological map of the surroundings of the Palmottu U-Th deposit (Kuivamäki et al. 1990). The drill holes studied are located along drilling profile 100.00.

The study area is part of a Proterozoic schist belt of supracrustal volcanic and sedirnentary rocks that extends from southern Finland to middle Sweden. The schist belt was metamorphosed during the Svecokarelian orogeny 1.9 - 1.8 Ga ago, when also

a major part of the associated plutonism and volcanism tmk place. The Palmottu mineralizations are likely to be related to the Iatest phase of the orogenic events 1.8 -

1.7 Ga ago; that is supported by dating results of the nearby Hyrkköla deposit which gave an uraninite age of 1744 Ma vouvo 1983). The main rock type of the Palmottu study site is an inhomogeneous, gamet, hypersthene, and often also cordierite bearing mica gneiss that is strongly migmatized by granitic and granite pegmatitic veins (Fig. 1). The width of the veins varies from some miilimetres to tens of metms (Räisänen 1986).

The Palmottu uranium deposit wntains 1 million tons of ore with an average grade of 0.1 % uranium. The orebmiy is discontinuous and has a total length of 400 rn. The

thichess of h e orebody varies from 1 to 15 metres and it is has been recognized by

drilling down to 300 metres. The main uranium bearing mineral, uraninite, occurs as disseminated grains associatsd with coarse-grained feldspar-quartz-biotik pegmatites or sheared granitic veins rich in quartz and biotite (Ráisänen 1986). The uranium concentrations in the ucaninite behng veins are on average 1000 - 2000 pprn, and thorium concentrations vary from negligibie to 1000 ppm (see Fig. 2 in Suutarinen et

d. 1991).

Preliminaq resul ts and conclusions on groundwater composition and condi tions, radiochemical studies, uranium and thorium bearing minerals as well as fracture mineralogy ase presented in h e annual reports of the project (Blomqvist et al. 1987,

1991; Jaakkola et al. 1989; Niini et al. 1990. Additionally, mineralogical results are given by Ruskeeniemi et ai. (1989) and Ruskeenierni and Vesterinen (1991), hydraulic rneasurements and groundwater sampling are descnbed by Ahonen et al. (1990) and

Ahonen and Paananen (1991), hydrochemistry is presented by Blomqvist et d. (1991)

and uranium in groundwater is discussed by Suutarinen et al. (1991). Processes affecting water-rock interaction phenomena are discussed by Ahonen et al. (1992), and a model for hydrochemical evolution of the gmundwaters in drill hole 346 is presented

by Pitkänen et al. (1991).

The present work focuses on groundwater chernistry and the physico-chernicd

conditions in the bedrock around the Palmottu U-Th deposit, as well & on uraniurn

wricentrations and radioactivity ratios in groundwater . Factors affecting distribution and mixing of the various groundwater types and uranium concentrations and activity

ratios are analyzed and discussed. The discussion on the mobility of uranium is based on mineralogical observations and thermd ynamic dcuhtions. The stud y is based on

five drill holes all of which are located within the sarne vertid cross-section of the

orebudy. Three of the drill holes (324, 346, and 357) are studied in detail.

SAMPLING AND METHODS

In samp1ing for groundwater, or lithological and fracture mineral studies, drill holes (dieter 46 mm) or drill core matenal (diameter 32 mm) received during the

explorational phase of study target were used. The drilling m k place between 198 1

and 1983, and water from the nearby Lake Palmottu was used as the drilling fiuid.

Since that time the drill holes have stayed Open and been intact with respect to human activity. As the sampling was done 7 to 8 years since drilling, the major part of the contamination caused during driIling is iikel y to have been vanished. However , certain precaution is needed during interpretation, especially the possible effects of the open- hole-situation upon groundwater flow must be considered carefully.

A comprehensive fracture mineral sampling was done and fracture infiilings were

studied using optical microscopy , a-autoradiography , X-ray diffractometry , and

scanning electron microscup y . The composition and alteration of the main radioactive

mineral uraninite was studied using scanning electron microscope and energy-dispersive

spectrometry .

l n groundwater sampling two techniques were used. The tube sampling technique

(Nurmi and Kukkonen 1986) was used in the preceding reconnaissance phase of hydrogmhemistry studies and a recently developed prtable double packer sampling

technique (hakoharju et al. 199 1) was used when representative groundwater samples

from bedrock fractures wer; required. That was p r d e d by a short-time hydraulic

testing to lmte zones of high hydraulic conductivity for groundwater sampling With

the tube sampling technique one water sample is usually received for every 25 or 50 m of drill hole length whereas with the packer sampling method, groundwakr samples are

taken whenever water is received from a fracture mne. A flow of water exceeding 100 mUmin is, however, rquired for this to be effective. The purnped water was directed

via a flow-through cell where a continuous monitonng of electric conductivity, Ph, Eh,

and dissolved oxygen was taking place. A standard calomel electrode was used in redox-poten tial measurements. Samples for groundwater chemistty were taken only

after constant values were reached in the flow-through d s . Filtering and preservation of samples, when needed, were done immediately after the samples were received on

sampling site. A filter with a pore diameter of 0.45 pm was used, and the preservation

was done by acidifying the sample to pH 1 with ultrapure HNQ. The practical

procedures in tube sampling and the subsequent field measurements are documented by

Nurmi and Kukkonen (1986).

Chemical composition of groundwater samples were analyzed at the Geological Survey

of Finland (GSF) using standard analytical methods (Table 1). 14C activity and 613C determinations of groundwater were made at the Radiocarbon Laboratory and the

Isotope Geology Unit of the GSF, respectively. Tritium values of groundwater samples

were measured at the Department of Radiochemistry, University of Helsinki, using a

multiparameter low-level liquid scintillation counting system. The concentrations of

uranium and the 234U/238U radioactivity ratios were analyzed at the Department of

Radiochemistry at the University of Helsinki with a alphaspectrometric method

(Suutarinen et al. 1991). Both filtered groundwater and the particulate fraction (0 > 0.45 pm) of groundwater were analyzed.

Table 1. Analytical methods and detection limits (mgll, in parentheses) of chemical constituents analysed in groundwater samples in 1987 - 1988 and from 1989 onwards, respectively .

K Fe Mn, Zn SO.4 Si02 Si Al B, La, Li, Mo, V Ba, Sr HCO, F C1 Br

FAAS (0.5) ICP-OES (1)* FAAS (0.5) ICP-OES (5)* FAAS (0.05) ICP-OES (0.5)*

FAAS (0.05) FAAS (0.5) FAAS (0.5) FAAS (0.05) FAAS (0.05) FAAS (0.02) FAAS (0.02) FAAS, indirect (1) ICHGR (1) Spectrophotometry (0,2) - - ICP-OES (1)* ICP-OES (0.1)* ICP-OES (0.1)* ICP-OES (0.02)* ICP-OES (0.02)* ICP-OES (0.01)* ICP-OES (0.01)* Titrimetry (5) Titrimetry (5) Ionselective electrode (0.1) Ionsel. electrode (0.1) Titrimetry', ICHGR (1)' ICHGR (1) ICHGR (0.1)'* Titrimetry2 (0.1) ICHGR (1) Spectrophotometry2 (Br < 0.1 mgll: 0.005)

* At high salinities the detection limits are 10 - 100 times higher.

FAAS Flame atomic absorption spectrophotometry. ICP-OES Inductively coupled plasma - optical emission spectrometry ICHGR Ionchrornatography 1) in 1987 2) in 1988

The equilibrium thermodynamic calculations were made using the code PHREEQE

(Parkhurst et al. 1980). In the termodynamic calculations for uranium data of Grenthe

et al. (1992) and the MINTEQ2A-Database, version A.201, (U.S. Environmental

Protection Agency 1989) was used.

Bedmck lithology and minemlogy

Based on drill core mapping performed during the explorational phase of the ore

deposit and a re-evaluation of the geological structures, a geological map of the studied

cross-section (drilling profile 100.00) of the U-Th deposit is given (Fig. 2). The

geological interpretation includes 2 semi-horizontal fault zones the existence of which

is still unverified. Accordingly, also other structural interpretations can be put forward

(see Kuivamäki et al. 1990).

- - tault plsne

dl mapnetite

A pyrtie Mo mdybdsnlte

? pyrrhotlte

9 chatcopyrlte 0 50 m u uraninlte I

Fig. 2. Geological cross-section 100.00 of Palmottu study site and location of the studied drill holes and the sampling sites for groundwater. The lithology is based on E. Räisänen, and the structural interpretations on A. Lindberg and P. Vuorela, Geological Survey of Finland. Sampling methods: 1. Fracture water sampling, interval 6 m; 2. Fracture water sampling, interval 25 m; 3. Drill hole water sampling, interval 25 m.

The longest of the drill holes (Dh 357) penetraks the migmatized mica gneiss sequence

for a Iength of 320 m. It cross-cuts the steeply dipping U-nch m k horimn at a depth of 200 metres. The next drill hole @Ii 346) penetrates the uranium-bearing horizon approximately 40 metres closer to surface than the previous one, and the third one @h

324) at a depth from 80 to 90 metres. Sdphide minerals, mainly pyrrhotite, pyrite, chalcopyrite, and mol ybdenite are frquenti y assaciated with t he uranium-bearing

horiuin. Pyrrhotite often bears indications of corrosion (see Fig. 3). The major part of uranium and thorium are found within the surface-dose part of he ore deposit . In the

vicinity of the drillholes 302, 304, and 325 the average wncentrations of uranium and thorium are 1000 - 3000 ppm and 200 - 500 ppm, respectively. Deeper down at drill

holes 346 and 357 practicaily no thorium is present, but uranium wncentrations (500 - 1000 ppm) are still relatively high (see Fig. 2 in Suutarinen et d. 1991).

Umnium and thoniim rninemls

The main radioactive minerals identified so far are uraninite, monazite, zircon, and

apatite. Practicaily all Th is incorporated in uraninite or monazite, and the majority of U is in uraninite or ib aIteration pmiucts Wuskeeniemi et al. 1989; Jaakkola et al. 1989, Niini et al. 1990). Accordingly, with respect to uranium mobility , the behaviour

of uraninite and its alteration products in the rocklwater system is of utmost

importance.

Uraninite has an average grain size of less than 0.3 mm. The chernical composition of uraninite is relatively unifom with U, Th, and Pb as the predorninant elements. Their

mutual relations vary to some extent, but on average the ratio Th/U is 1:8, and the

thorium and lead concentrations ranges from 7 to 10 5% and 10 - 18 %, respectively (Table 2). Uraninite grains are usually strongly alterd along their borders and fractures (Fig. 31, in ultimate cases to such an extent that no primary uraninite is

preserved, The rims of uraninite grains are replaced by amorphous or metamict phases

mrnposed of heterogenic mixtures of mainly of U, Si, Th, Pb, P, Ca, Al, Ti, Fe, K,

and probably also light dements (e.g H, 0, F). Usually U, Si, Th and Pb are,

however, the most frequent elernen ts of the altered phase (Table 2). Based on el emen ta1 concentrations and a g d correlation between U, Si, and Th in the alteration rim of

the uraninite (Fig. 31, an U-silicate phase, probably coffinite WiO,) , seerns likely. Compared to unalterd uraninite the coffinite has rnainly received an addition of Si, and minor amounts of other elements e.g. Ca and P. A recalculation of U, Th, and Pb

concentrations of wffinite into uraninite, points to the fact that lead is the only element

that is distinctly depleted from uraninite during the coffinitization prwss .

Table 2. Spot microprobe analyses for 3 uraninite grains (1 - 3) and the coffinitic alteration rim (4 and 5) surrounding the uraninite in weight-%. All samples from Dh 304170.70 m, the coffinite analyses from the uraninite-coffinite grain of Fig. 3. Each uraninite analyze is based on three spot measurements of a single grain. For comparison, coffinite analyses from Oklo (6), (Janeczek and Ewing 1991) and Witwatersrand (7 and 8, Smits 1989) are presented.

uo2 Th02 PbO Si02 Ti02 A1203 FezO3 MgO CaO Na20 K20 p 2 0 5

Total 91.7 95.7 96.9 85.1 77.8 86.43 87.3g2) %@

') as FeO, 2, including 0.39 % SO,, 1.38% Y203, and 0.18 % ZrO,, 3, including 1.13 % S03 and 1.22% Y203

The release of lead during the uraninite alteration is a common feature as pointed out

by Janeczek and Ewing (1991). In Palmottu it is verified by galena (PbS) being in close

connection with the coffinitic alteration rim of the uraninite grains (Fig. 3). Despite the

release of lead during the coffinitization process, an appreciable amount of lead is still

dispersed among the coffinitized part of the uraninite grains. It is likely that an

considerable part of this lead is created by radioactive decay since the coffinite

formation; in that case there might also be a chance to roughly estimate the age of the

coffinitization. Based on the formation of the sulphide mineral galena, the

coffinitization must have taken place under reducing conditions. Additionally, a high

silica concentration in groundwater would have been needed to transform uraninite into

coffinite, at least 28 mg1L (see Langmuir 1978). That is much higher than the present

Si concentrations in groundwater at Palmottu (3 - 5 mgll). Altogether, the observations

point to an old event for coffinitization, possibly a hydrothermal event.

The monazite-bearing samples dominate in the upper part of the orebody close to

bedrock surface, in good accordance with the high thorium concentrations detected at

Fig. 3. Scanning electron photomicrograph of an aitered uraninite grain (a) and variation of the chemicai composition along the alteration rim (b). Note the good correlation between U and Si in the aiteration rim, which indicates the presence of an U-silicate phase, most likely coffinite. As each element has its own scaie, the concentrations are not comparable. Sample from drill hole 304, 70.70 m. Ur = uraninite, Ga = galena, Py = Pyrrhotite, Alt = coffinite.

that part of the orebody. Monazite occurs as euhedral crystals of variable size with

uranium and thorium concentrations of 0.3 - 1.1 % and 8 - 15 %, respectively. As the

monazite grains show no sign of secondary alteration, the assiociated radionuclides are

not likely to have to be involved in recent remobilization prosesses of radionuclides.

Fmcture minemls

The fracture minerals in three drill holes 324, 346 and 357 have been studied in detail.

Calcite is the most dominant fracture filling mineral in these drill holes. Kaolinite is

often associated with calcite, and pyrite is rather frequent in two of the studied drill

holes, 324 and 346 (Fig. 4). The almost total lack of pyrite in the fractures of drill

hole 357 is a striking feature. That is, however, compatible with the scanty

observations of sulphides in the bedrock penetrated by that drill hole.

In drillholes 324 distinct signs of corroded calcite crystals have been detected from an

Open water-conducting fracture at 101 m of drill hole 324 (Ahonen et al. 1992). The

calcite is associated with and partly coated by ferric oxyhydroxide. These observations

O v e r b u r d e n O G r o n i t e 1 P e g m o t 8 t e O Mkca g n e i s s

Fig. 4. Distribution of calcite-bearing and pyrite-bearing fractures (left-hand and right- hand parts of the frequency histograms, respectively) in drill holes 324, 346, and 357.

are in good agreement with hydrogeochemical results, which display distinctly corrosive groundwaters within the fractures of that drill hole, as is discussed later. Based on Fig. 4 a onIy a small amount of calcite bearing fractures can be detected in the upper part of bedruck surrounding drill hole 324. As, however, the fracture density is low in that part of the rock, it is difficult to conclude whether calcite dissolution could be a wmmon feature in the fractures of drill hole 324. Nevertheless, as high uranium concentrations have been reported in calcites of Palmottu (Suksi et al, 1991),

dissolution of calcite rnight have a definite irnpact upon local concentrations and

activity ratios of uraniurn in groundwater.

Alfa-au toradiog raphy revealed that migration o f radionuclides has occurred along minor fractures and grain boundaries. The identification of the radionuclide bearing phases of

the fracture infillings have in most c m been unsuccessful, the tendency to be incorpratd with tiny iron oxide and hydroxide grains of the fracture infillings is,

however, striking. In some cases i t has been possibly to deduce from semiquantitative microanalyses that coffinite (uranium silicate) is present as a fracture infilling material. In these cases no signs of Th and Pb have been detected, possibly indicating a younger

age for this coffinite cornpared to the coffinite surrounding the uraninite grains.

Hydmgeo ch emistry

Chemical composition and Eh-pH wnditions of groundwater has k n determined in several drill holes and a wide variety of dilute or siightly saline groundwater types with

total dissolved solids (TDS) from 0.2 to 1.5 gll, and bicarbonate, chloride, or sulphate as the dominant anion and have been detectd. The groundwater types in the drilling profile perpendicular to the uranium bearing ore horizon are as follows: The fracture groundwater samples in the deeper parts of the sulphide and uraninite-bearing horizon

of drill holes 346 and 357 have increased sulphate concentrations (up to 750 rngfl) with elevated C1 concentrations (up to 250 mgll). Within the same drill holes further from the uranium bearing horizon, HC0,-rich groundwaters are found in the upper part of the bedrock. The Eh vary from -35 to - 125 mV, and pH from 8 to 9, respectively

(Table 3) . The bicarbonate groundwaters have high tritium values, often of the order of present precipitation, which based on five samples of the local lake waters is 30.6

TCJ. The sulphate and chloride dominant groundwaters have lower tritium values,

usually close to detsction limits.

Fracture groundwater samples in one of the drill holes (324) cross-cutting the uranium- &ng ore horizon ase of exceptional type having positive Eh vdues (SS - 100 mV),

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slightly acidic pH values (6.6 - 7.01, and high tritium values (17 - 29). Also, the iron

and manganese concentrations are exceptionally high in fiftered groundwater of the

upper part of the drill hole (1 -2 - 2.8 mgll and 1 -0 - 1.1 mgll, respectively), which are seen as midish brown colour of wakr. All these features are typicai for less mature surficial groundwaters. Additionaiiy Che groundwaters of drillhole 324 diverge strongly from the other bedrock groundwaters of the study site. Accordingly , the groundwaters of driii hole 324 seem to represent surficial waters with a high recharge speed.

To study the internal relationships of the various groundwater types, the molar ratios

of the main cations and anions, respectively , have been plotted on triangular diagrams

(Fig. 5). In the cation diagram (Ca-Na-Mg) the groundwaters from drilI holes 304,

324, 346, and 357 plot on a slightly bending curve starting from the Ca corner and ending at the Na corner of the diagram. The samples from drilIholes 324 and 346

approach the Na comer of the diagram with increasing salinity whereas the

groundwater in drill hole 357 are Na dorninated throughout. In the diagram for HCQ- Cl-SO, the sampled groundwaters form 3 end groups: HCO,, Cl, and S04 groups. Accordingly 4 major groundwater types seem to be detected: Ca and Na dorninant

diIute HC03 groundwaters, and dightly Aine Na-CI and Na-SO, groundwaters. Additionally a variety of intervening samples are found, partly due to mixing, partly to

on-going evolu tionary processes.

The various groundwater types of the three neighbouring drill holes are also shown as semilogari thmic Schoeller-type diagrams (Fig , 6). Each of the three diagrams presents

a different groundwater type. The most dilute one, found in drill hoie 324 and the

upper part of 346, is a Ca-HCO, groundwater resembling typical surficial waters (part A}. The deepest sarnpIe of drill hole 324 displays slightly increased Na and SO, values,

resembling in that respect the slightly saline Na- SO, dominant groundwaters of the

deeper part of drill hole 346 @art B). The fracture groundwaters from drill hole 357 form a uniform series of Na dominant groundwaters @art C) where a continuous increase in salinity with depth is visible. The various groundwaters of drill hole 357 could be produced by progressively adding more Na, Cl, and S (and minor Ca and

Mg) in the water. Accordingly, groundwaters of drill hole 357 seem to represent

various stages of an evolutionary process, where leaching of bedrock minerals takes place and increases the salinity of the groundwaters as propsed by P geochemical modelling studies (Pitkänen et al. 199 1; Ahonen et al. 1992). An evolutionary trend

could also be interpreted for the groundwaters of drill hole 324, where an increase in

Na and SO, concentrations at hole bottom could represent the first stage towards the

Na-SO, groundwaters of drill hole 346.

A. Drill hole 357 Drill hole 346

8 175-200 rn Drill hole 304

80 rn 225-250 rn

Drill hole 324 1990 65-71 rn

1991 O 60-66 m 240-246 rn

Drill hole 357

1990 @ 110-116 rn

@ 145-151 rn

Drill

Drill

199 1

HCO 3

Drill hole 346

El 265-271 rn A 125-150 rn

h:'e hole 2?:0, 324 60, 8 1 @ [ (/ 1990 8 225-250 175-200 65-71 m rn rn

o 60-66 m 240-246 rn

0 95-101 m y Ocean

Fig. 5. Ca-Na-Mg (A) and HC0,-C1-SO, (B) trends of sampled fracture waters (molar ratios). Numbers close to symbols indicate the order of the sample within a certain sampling set. The samples from 304 represent the water in the drill hole.

mmol

mmol

0.01 1 1 1 , 1 - Mg2 + Ca2+ Ca2+ M g 2 + N a + K + CI- HC03- C032- S042-

0.1 I I 1 , i Mg2 + Ca2+ C a 2 + M g 2 + N a + K + CI- HC03- C032- S042-

O l C i I -- Mg2 + Ca2+ C a 2 + M g 2 + N a + K + CI- HC03 C032- S042-

Fig. 6. Semilogarithmic diagrams for groundwaters of drill holes 324, 346, and 357.

The results on tritium, 613C, and I4C are given in Table 3.The tritium values recorded in the upper parts of the driii holes are generaily high indicating that these waters

consist to an appreciable amounts of recent surficiai waters (usually 50 to 100 %), In

bottom part of drill hole 346, low tritium values prevail in the saline fracture water

(see ako Suutarinen et al. 1991). The tritium values within driil hole 304 are also low

whereas dnll hole 324 have high tritium values throughout. To obtain an idea of the

average residence times of the water Mies in the bedrock, 4 samples for 14C age measurements were taken, one from drill hole 346, two from drill hole 357, and one from drill hole 324 (Table 3). The received ''C activity ratios for the samples were

from 16.0 to 60.4 % of modem precipitation. Tf carbon in groundwater could be considered a closed system, these activities would indicate residence times from 5 000

to 15 000 a. That is, however, not the case, as indicated by high tritium values in two

of the samples. Accordingly, both mixing of various groundwater sources as well as water-rock interaction processes must be taken in account.

Based on thermdynamic calculations (Pitkänen et al. 1991; Ahonen et al. 1992), groundwaters from the upper parts of drill holes 346 and 357 display quilibrium wjth

atmospheric CO, and calcite, whereas the deeper, slightly saline groundwaters are

undersaturated with respect to the atmospheric CO, (about a decade) but display

equilibrium between calcite and water. The groundwater in driH hole 324, on the other

hand, is distinctly undersaturated with respect to carbonate minerals, and supersaturated with respct to atmospheric CQ. These Iatter calculations are also supported by dirsct

mineralogical observations, according to which calcite is dissolving within the fracture zone of 10 1 rn of drill hole 324 (see Ahonen et al. 1992).

Applied to drill hole 324, the above data would indicate that part of the carbon is

ancient being dissolved from calcites in the fractures, and the rest, based on high 3H- values, is likely to represent carbon associated with the recharging surficial waters. The

bi-modal nature of carbon is supported by high 3H values and simultaneous relatively low 14C activities. The groundwater samples from the slightly saline parts of drill holes 346 and 357 have tritium values below detection limits and are in equilibrium or slightl y oversaturated with respect to calcite and undersaturated with respect to atmospheric COZ. The carbon of these groundwaters is likely to have remained rattier

persi sten t, and the 14C ac tivities wuld be regarded as reasonable represen tative ones,

indicating that the mean residence time for these groundwaters could be of the order of 12 000 and 15 000 a, respectively . However, as the figures point to a glacial-covered

time, the received values will probably not represent the age of a specific groundwater in the Mmk, but preferable the m m residence tirne for a mixture of groundwaters in the bedrck.

The 613C values of ihe sarnpled groundwaters fall wiihin a narrow range. The fairly low 613c indicates that biogenic carbon or organic &n have affected the sy stem, as

is also the case in the Stripa groundwaters F i tz et al. (1989).

Uranium concentrations and 234U(U8~ radioactivity ratios were analyzed in filtered fracture waters (0.45 pm) and the particulate fractions of drill holes 324, 346, and 357.

The results are presented in Tables 4 and 5 and in Fig. 7. Additional results from groundwaters received with the tube sampling technique are presented from driII holes 302, 304, and 357 (Tables 6 and 7). The uranium results for each of these drill holes varies sy stematically , as do also the respctive groundwater compositions.

The- groundwaters in drill hole 324 are fresh and wrrosive with slightly positive Eh

values. High uranium concentrations (3 1 - 71 ppb) and low 2MUp8U radioactivity ratios close to equilibrium (1.15 - 1.30) characterize these waters, indicating that chemical dissolution of uraniurn is taking place in these waters. The proportion of uranium in the parhculate fraction is low, except at drillhote bottom, where a high

amount of evidently fenic ox yhydroxide particles are dekcted in the water. The highest uranium concentrations are deteckd in the deeper part of the drill hole, contrary to the observations in drill hole 346 (see next chapter). Within the fracture zone 95 - 101 m a dissolution of calcite crystals is going on (Ahonen et d. 1992).

The fracture water of the uppr part of drill hole 346 is slightly ducing and high uraniurn concen trations (10 1 ppb) and U 4 ~ p 8 ~ radioactivity ratios close to equilibrium are detected in these waters (see also Suutarinen et al. 199 1). In the deeper, more reducing part of the drill hole, the concentration of uranium is lower (32 ppb) and the m ~ / U s U radioactivity ratio is higher (1.56) than in the upper part of the drill hole. Companng to the results of previous sarnpling set from the deeper part of the drill hole (Suutarinen et al. 1991), the precent result is, however, distinctly higher in concentration and lower in 234~P8U radioactivity ratio. That is interpreted to have k e n caused b y large volume-pumping during sampling which reflected in more surface-close groundwaters to be incorprated within the sampling interval at 240 - 246 m.

Table 4. Uranium concentrations ahd 2 " ~ / 2 3 8 ~ radioactivity ratios from filtered fracture waters in drill holes 324, 346, and 357. Standard deviation of radioassay (+ la) is indicated.

Dnll hole number 2)8U U a4Ul*U 95 of UI&* and length (m) (mBq/l) @pb)

* Utu is a sum of U concentration in filtered water and the particulate fraction separated with filtering through 0.45 pm Millipore filter.

Table 5. Uranium concentrations and 234U/238U radioactivity ratios from particulate fractions of fracture waters in drill holes 324, 346, and 357. Standard deviation of radioassay (+ la) is indicated.

Drill hole number Zj8U U "4U12)8U % of UI&* ...

and length (m) (mBq/l) ( P P ~ )

* U, is a sum of U concentration in filtered water and the particulate fraction separated with filtenng through 0.45 pm Millipore filter.

Table 6. Uranium concentrations and mU/238U radioactivity ratios from filtered groundwaters in drill holes 302, 304, and 357. Standard deviation of radioassay (+ lu) is indicated.

Drill hole number 238U U "4U/238U % of U,*' and length (m) 1 date (mB¶/l) bpb)

U, is a sum of U concentration in filtered water and the particulate fraction separated with filtering through 0.45 jtm Millipore filter.

Table 7. Uranium concentrations and 234U/238U radioactivity ratios from particulate fractions of groundwaters in drill holes 302, 304, and 357. Standard deviation of radioassay (+ lu) is indicated.

Drill hole number 238U U mU/238U % of U,' and length (m) / date (mBq/l) b ~ b )

U, is a sum of U concentration in filtered water and the particulate fraction separated with filtering through 0.45 jtm Millipore filter.

53

compared to the groundwater from which the uranium was received to the particles.

The general implication of the above discussion is, that the uranium in the particles is

mainly derived from the present surrounding groundwater.

FILTERED WATER PARTICULATE FRACTION REDOX CONDITIONS

U concentration (mBq/l) U conccntration (mBq/l) Eh (mV)

FILTERED WATER PARTICULATE FRACTION REDOX CONDITIONS

Fig. 7. Uranium concentrations and 2MU/L38~ radioactivity ratios in filtered fracture waters and particulate fraction of waters with Eh values in drill holes 324 (o), 346 (+ in 1989; A in 1990), and 357 (e). Results from drill hole 346 in 1989 according to Suutarinen et al. (199 1).

distribution of the various groundwater types. Accordingly, recharge of surficial waters

takes place in the middle part of the studied drilling profile. Around drill hole 324 relatively fast recharge channels seem to be available. Based on overflowing drill

holes, discharge of groundwater is taking place along the Palmottu stream valley.

3.18/85 U activity ratio/ U concentration (mBq/l)

Fig. 8. Distribution of groundwater types, 234U/238U activity ratios and U concentrations in groundwater in cross-section 100.00 of the Palmottu U-Th ore deposit. Slightly saline sulphate and chloride waters with low 3H values, low 14C activities (of modern) and high activity ratios of 234U/238U characterize the bedrock close to the U-Th ore deposit. Bicarbonate waters (without overprinting) with high 3H values indicative of recent recharge predominate further. The bedrock surrounding drill hole 324 displays a fast recharge channel. Uranium data for 346 partly from Suutarinen et al. (1991).

Fig. 9. Relative stability of different uranium oxide phases at 25 "C compared to the measured Eh and pH values obtained during sampling of fracture groundwaters in drill holes 324, 346, and 357 of Palmottu.

Fig. 10. Calculated stability fields of uraninite and uranium silicate minerals at 25 "C compared to the measured Eh and pH values of fracture groundwaters in drill holes 324, 346, and 357 of Palmottu. Assumed activities for dissolved species correspond to typical groundwater composition at Palmottu: Si = 1.3x104, Ca = 5x10".

able to control the uranium mobilization. Another possibility is that the analyzed

uranium of the groundwater samples was only partly in solution, and partly as tiny

particles below the filter pore size 0.45 pm. With respect to that, the role of colloids

should be carefully examined. Generally, however, despite intersting implications based

on a relatively vast information it is still difficult to present an comprehensive

interpretation of the behaviour of dissolved uranium in groundwater at Palmottu.

Besides a solid interpretation of hydrogeoly, additional information on fracture

infillings and the solubility-controlling solid uranium phases as well as aqueous

speciation of uranium are needed.

Fig. 11. Measured Ehlph values plotted on the phase diagram for the system U-Si02- CO,-S0,-H,O at 25 OC. Assumed activities for dissolved species are: Si = 1 .3x104, total carbonic acid 2x103, SO, = 10-2, U = 1x10-', alternatively 1 ~ 1 0 - ~ (dashed line).

Depth (ml

Fig. 12. Saturation indices for solid uranium phases as a function of depth in groundwaters from drill holes 324 (i), 346 ( ) and 357 (A). SI =

10g&icAchityProduct/&uilibnd.

Table 8. Thermodynamic data used in the caiculations. AH-vaiues in kJ/mol. Data according to Grenthe et al. (1992), except those indicated by asterisk (*) where MINTEQA2 - Database, Version A2.0 1 (U. S . Environmental Protection Agency 1989), was the source.

1 0 g ~ AH ...................................................................... u4+ + 2H20 = uo2*+ + 4H+ + 2e- -9 .04 3 4 . 3

Fig. 3. Cross section of the studied drill hole profile, along with a simplified lithological profile. Distribution of ferric oxyhydroxide (on the right) and pyrite (on the left) bearing fractures are shown. Horizontai scale is only approximate.

3.2 Water composition, evolution and mixing

The composition of fracture waters vary from very dilute fresh water due to exchange

with recent surface water to slightly saline types (TDS about 1 - 2 gll). More than

about 95 percent of the totai dissolved solids in the waters can be expressed in terms

of four salt-components (Fig 2): Ca(HCO,),, NaHCO,, Na,S04, and NaCl. The slightly

acidic, fresh water in drill hole 324 represents a quite pure Ca-bicarbonate type,

whereas the upper-most waters in the other two drill holes contain appreciable amounts

(30 - 50 %) of Na-bicarbonate component, as well. The increase in the salinity of the

waters is due to the increase in the Na', C1-, and S04= concentrations, as Ca2+- and

HCO,' concentrations remain low. Highest salinities are observed in sulphate waters.

It is evident that different processes control the evolution of sulphate-dominated, and

chloride-dominated water types. Sulphate water prevails in the lower part of DH346,

in which iron sulfides, especially pyrite, are observed frequently as fracture fillings.

Chloride-dominated waters were typically observed in fractures in mica gneiss, where

the low hydraulic conductivity of the adjoining bedrock also seems to be a controlling

factor. A long-term leaching process increasing the chlorine concentration can thus be

established. The gradual change of chloride/sulphate ratio in fracture waters from drill

hole 346 is interpreted to be a result of mixing.

The chemical characteristics of the water do not support the idea that ancient sea water

has made a significant contribution to the total salinity. During the disintegration of

silicates sodium is released into solution, as the other components are incorporated into

the fracture minerals (Pitkänen et al. 1991).

NaCI

Fig. 4. Chemical composition of fracture water sarnples in terms of four major salt components.

76

4. FRACTURE WATER / FRACTURE MIIXEXAL INTERACTION

4.1 Carbonate minerals

Saturation indices for calcite and atmospheric CO, are plotted as a function of depth in

fig. 5. Calculations indicate that the water samples from drill holes 346 and 357 are

approximately in equilibrium with calcite. Water in the upper parts of the drill holes

display equilibrium between atmospheric CO, and CaCO,, whereas below a depth of

about 140 m only CaCO,/water equilibrium is attained.

Water in drill hole 324 differs significantly from the other two drill holes, being

distinctly undersaturated with respect to the carbonate minerals, and supersaturated

witl~ respect to the atmospheric CO,. High CO2 partial pressure may be due to

biological respiration taking place in oxygenated surface waters. Another possible

mechanism for increasing the ground water P,,, is the acid producing oxidation of

pyrite, and concomitant partial neutralization of acid by the carbonate mineral dissolution.

Depth (m)

Fig. 5. Saturation indices for calcite (solid symbols) and atmospheric CO, (open symbols). Squares represent values for samples from DH324, circles from DH346, triangles from DH357.

4.2 Clay minerals

The analyzed ground waters are typically 3 - 4 orders of magnitude supersaturated with

respect to kaolinite. In the sequence of hydrolytical precipitation of alumina and silica,

the first possible solubility controlling phases are the pure amorphous precipitates.

Calculations indicate, however, that the water samples are, on average about one order

of magnitude undersaturated with respect to amorphous Al(0Hh and amorphous silica.

The cryptocrystalline phases show also on average a slight undersaturation. Neither

pure alumina nor pure silica minerals are observed in the fracture mineral study, but

the constituents are more likely to be incorporated into some type of clay mineral

precursor phase. Paks (1978) proposed, that a cryptocrystalline or amorphous

aluminosilicate controls the composition of natural waters. The K,, of the compound

was derived from solubilities of amorphous alumina and silica assuming an idea1 solid

solution behaviour. The saturation indices for this aluminosilicate are plotted in Fig. 6

together with those of halloysite, which must also be considered as a possible precursor

of kaolinite at high water activities (Hem et al. 1973). Scattering in the results is

partially due to the low aluminium concentrations, in many cases only slightly

exceeding the detection limit (0.1 mg/l).

Depth (ml

Fig. 6. Saturation indices (SI) for metastable aluminosilicate (PaEes (1978) marked by solid symbols, and saturation indices for halloysite (open symbols). Squares represent values for samples from DH324, circles from DH346, triangles from DH357.

4.3 Iron minerals

The plot of measured Eh-pH-values indicate, that ferric hydroxide is an effective

EhIpH buffer at lower pH-values (Fig 7). The results indicate that although pyrite is

partially oxidized and sulphate is formed, the pyritelsulphate equilibrium redox-

potential is not attained. In most of the water samples soluble oxygen concentration

was below the detection limit (0.1 mgll). Sulphate reduction without microbial catalysis

is sluggish at low temperatures. It is therefore quite clear that pyrite oxidation in the

fractures is limited by the restricted supply of effective oxidant, but the question still

remains, why the Eh-pH values cluster as observed. Goldhaber (1983) observed that

oxidation of pyrite in basic solutions produced appreciable amounts of thiosulphate,

which remained unoxidized for prolonged periods.

We have calculated some thermodynamic equilibrium relations between iron minerals

and metastable sulphoxy-anions, the results of which are presented graphically in Fig.

7. So far we have made no attempt to analyze reduced sulphur species from the water

samples, the presence of hydrogen sulphide was, however, occasionally observed

during the sampling.

Fig. 7. The measured Eh-pH values compared with some thermodynamic relationships between iron and unstable sulfoxy anions. Phase boundaries (solid lines) are drawn to correspond to the activity 106 of ferrous iron and predominant soluble sulfoxy species. Dashed lines separate the predominance fields of sulfoxy anions taken into consideration. Dotted line approximates the pyritelsulphate equilibrium conditions.

which is less aggressive than TAMM's oxalate. Thus, the major part of the uranium

(61-76 % of the total U) could be removed without substantial dissolution of iron

oxyh ydroxides.

A considerable part of the uranium seems to be in an easily accessible part of the rock.

This indicates that recent (in the geological sense) uranium deposition has taken place

at the site. Although the UIFe ratio in both phases is totally different, the two

distribution patterns of UIFe vs. depth are remarkably similar, see Figure 3. This

suggests that both extractants dissolve the same iron phase that has adsorbed uranium.

Distance from fracture [mm]

Fig. 2. The relative easy of dissolution of iron and uranium in core sample 1031R346. A = Ammonium acetate (NH40Ac) extractable phase, and T = TAMM's oxalate extractable phase. Data used has been taken from Suksi and Ruskeeniemi (1991).

In the core samples a significant proportion of 232Th was found in the phase extracted

with TAMM's oxalate. The concentrations of thorium in the extract varied between 10

and 33 pglg, being of the same order of magnitude as in the residual phase. Thorium

concentration in the ammonium acetate extractable phase was below the determination

limit, so its distribution cannot be related to that of uranium found in this phase. The

major part of the uranium series thorium (23%) was found in the ammonium acetate

extractable phase. ThIU mass ratios in the extractant and in the residual phase were

0.5-1.2 and 0.9-3.0, respectively. The presence of mobilized 23% may be associated

with weathering of Th bearing minerals, possibly monazite which is an accessory

CORE SAMPLE 1 03/R3 46 CORE SAMPLE 2 1 1 /R3 25

Distance from fracture surface [mm] Distance from fracture surface [mm]

Fig. 3. Uranium distribution, 2"U123sU and 23?13PMU activity ratio profiles in core samples 103lR346 and 2111R325. A = Ammonium acetate extractable phase, T = TAMM's oxalate extractable phase and R = residual phase. Radioactive equilibrium is indicated by the straight line.

When the activity ratio diagrams in Fig. 4 are compared the different histories of

uranium deposition and removal in the two drill core samples can be inferred. The

interpretation is based on the approach presented by Thiel et al. (1983) where they

defined areas in the 2 3 4 ~ / 2 3 8 ~ and 239h238U activity ratio diagram to demonstrate

various cases of uranium deposition and removal. The approach was later developed by

Alexander et al. (1990). In Figure 5 a simplified graphical representation of Thiel et al. and Alexander et al. is presented.

Fig. 4. The 2 3 4 ~ / 2 3 8 ~ and 23?hP38~ activity ratio diagrams. A) Core sample 103JR346 (see text on page 87) and B) Core sample 2 11JR325. Open symbols refer to ammonium acetate extraction and filled symbols to TAMM's oxalate extraction.

FORBIDDEN FOR CONTINUOUS SINGLE PROCESS

1.50

FORBIDDEN FOR ANY SINGLE PROCESS

. . .

URANIUM REMOVAL

0.50 1 .OO 1.50

Fig. 5. A simplified 234U/238U and 239h/238U activity ratio diagram for demonstrating various cases of uranium deposition and removal. Redrawn after Thiel et al. (1983) and Alexander et al. (1990).

5. GEOLOGICAL CONSTRAINS

The effect of fracture type. Joints are formed in an extensional stress field where

surfaces separate apart, thus giving rise to rough surfaces with large specific area. The

exposed fracture surfaces are fresh i.e. free from alteration products related to the

opening of the fracture. A contrasting fracture type, known as a shear fracture, is

formed in conditions where bedrock blocks move relative to each other. Shearing

causes disintegration of the primary minerals and subjacent formation of secondary,

commonl y hydrothermal phases, including chlorite, biotite, clay minerals, epidote, and

calcite. The stable mineral assemblage depends on the primary composition of the wall

rock.

Different formation mechanisms may give rise to differences between h e two fracture types relative to matrix diffusion . If the rock to be fractured by shearing has a mineral

compsition such that phyllosilicates may fom, the prevailing stress field will cause

micas to crystallise in a preferd orientation. This sort of structure, where mica flakes overlap each olher is relatively impermeable compared to a fresh tensional joint surface and may prevent or inhibit diffusion through the fracture surfaces into the bedrock. Althoug h rnicas are effective sorbents, this tightly packed and polished surface has only lirnited sorbing capacity and it wilI become saturated quickly . In addition the shearing induced formation of secondary minerals close to the fracture surfaces will further close the connections between pores. In Palmottu shear fractures oC this type are common in rnica gneiss layers, but rare in granitic rocks due to the lack of proper

precursor minerals. In pegmatite shearing may generate polished fracture surfaces withou t major alteration phenomena. However, recrystailization of quartz grains near

the fracture may lead to a seding-effect similar to that of micas in gneisses.

The significance of fracture type is supported by the observations of autoradiographic

studies, which fail to show evidence for rnatrix diffusion in samples with shear fractures , even though corresponding fracture fillings have elevated uranium concentrations (see ch. 2.1). It is also evident that diffusion of uranium into mica gneiss is insignificant regardless of the fracture type. This is partly explained by the

high surption ability of biotite, which in turn is available in large amounts at the very edge of tension fractures. Another factor is the weak permeability of gneissose rocks. In this context the anisotropy of foliated texture may have some significance especially

in gneisses, which are differentiated into mafic and felsic mineral bands, because the

texture is more Open parallel to the foliation than perpendicular to it.

Microfractures. The permeability of rock matrix is attributed to the presence of interconnected pores. In crysdline rocks microfractures and grain boundarics are chiefly respnsible for the totai porosity. Zn addition cleavage planes in certain minerals, such as plagioclase and mica, may serve as routes for diffusing solutes.

The response of rock to an applied stress depends on its mineral composition and texture. At Palmottu the mica gneisses contain appreciable amount of biotite, which is a flexible and even elastic minerai. It follows that mica gneiss tolerate stress to a

certain yield point until failure takes place and can recover when the stress is removed.

The behaviour of the other major constituents of gneisses and pegmatites during low- temperahire deformation is brittle. Hence, the occurrence of fissures in rigid quartz- feldspardominating rock types is more probable than in mica gneiss. This is evidenced at Palmottu by the frequent occurrence of subparallel s w m s of both intragranular and

intergranular microfractures in granitic rock rich in quartz. The lmsening of m k matrix along grain boundaries dso seems probable. At higher temperatures or when the

duration of the applied stress is prolonged, quartz may deform plastidly. This is usually the case in the mica gneiss, in which quartz aggregates frequently exhibit recrystailisation and flow textures. This deformation style may be correlated with the early Proterozuic, Svecofennian regional folding md metarnorphisrn.

The mineral cornposition of the rwk matrix. It is a well established observation that certain minerals, such as ferrous silicates, fmic oxyhydroxide and clays, are more effective sorbents than others, either due to their physico-chemical surface properties or their large specific area. The observations from Palmottu indicate that the altemtion of plagiodase is essential for uraniurn retardation. Two different alteration types are detected. The first, prribably of hydrothermal origin, is characterized by minor sericitization. The second stage has been much more aggressive. It is typically an in situ- p m s s which has caused the replacement of plagioclase grains by clay rninerals and with a rninor amount of ferric oxyhydroxides. The aiteration has often precsded along cleavage planes into the host mineral . Usually the dteration has not been uni form even in a single grain, but has influencsd certain parts more strongly than others.

Partly this is a question of access, but more important is due to the charachteristic heterogeneity of plagioclase. Piagioclase is a mixture of two components, a Na-end member and a Ca-end member, which often define a distinct cornpositional zoning

pattern. The latter alters more readily and hence, the distribution of these phases control the alteration of the whole grain. The alteration is commonly controlled by

fractures indicating that the alteration may be due to weathering. The aikmtion zone is indicated by the light colour of plagioclase grains and is usually visible up to few

centirnetres on both sides of the fractures. This has increased the sorption capacity of the rock mass but it seems that it has had only a limited impact on the permeability of the matrix, because the alteration has involved only more or less isolated plagioclase grains while the rest of the rock matrix has rernained untouched.

6. DTSCUSSION AND CONCLUDING REMARKS

When comparing the results of the two core samples, the significance of selective subsampling is ernphasized. If bulk samples are usd, complications may arise in measuring concentrations due to the heterogeneity of natural materids. BuIk samples suffer serious and irregular dilution when he total matrix is crushed and analyzed. The effects can be m i l y seen in core sample 1031R346 where uraniurn concentrations vary

in an erratic way through the core sample. The heterogeneity of natural materia1 does

not effect ttie activity ratios in this case because the effect is rnasked by &he high amount of the deposited (easil y dissolvable) seandary uranium .

Uranium seems to have been enriched in a limikd alkration zone near the fracture in both core samples. Aftered plagiwlase crystals (containing clays and iron ox yhydroxides) and s h d biotite flakes seem to have effectively s o M uranium: the highest uranium concentrations are near the Fractures where the most intense alteration has occurred. Most of the uranium could be removed easily which indicates that uranium m u r s in easily accessible parts of the rock. This is an important finding because the easily removable phase can be inkrpreted as representing potentially mobile uranium. The 2 3 4 ~ f 3 8 ~ activity ratio in the residual phase indicates uranium deposition in the wre samples after the iron mineral phase has formed. Distinct

diffusion pathways along the altered plagioclase grains and within biotite were observed in both samples. In sample 2 1 1 lR325, the obsewd preferential Ioss of 2MU along the

whole studied length (25 mm) of the core sample indicates waterlrock inkraction, and, more importantly, also radionuclide transport in drill core matrix.

A different uranium depositionlremoval history for the two wre samples seems possible from the activity ratios (see Figures 4 and 5). The disturbance affecting core sarnple 1031R346 may have murred considerably earl ier than that of 2 1 11R325. In 103/R346 the 23Th/234U activity ratio profile suggests a long (several haif-lives of 2%) undisturbed history. The data base for the interpretations is incomplete however, and the radionuciides n6Ra and 2 3 ' ~ a have not been analyzed for these samples, It seems obvious that two different disturbance scenarios, or the same scenario but starting at different times, are responsible for the uranium distributions in the two core

samples.

Tn order to assess the role of matrix diffusion numerous factors have to be taken into account: 1 ) the distribution and proportions of different rock types, 2) the number of

fractures considered 10 be water conducting, 3) the ability of groundwater to transport uranium 41, the nature of fracture surface, its permeability and coating rnaterials, 5) the

occurrence of interconnected pores iri the matrix close to the fracture, 6) physicai conditions in and properties of the pore waters, and finally 7) the retardation capacity of the bedrcck, depending on the amount of active sorbents. An important issue in this

context is also h e question of reversibility of the processes. The radionuciides which

are captured on the fracture surface are more susceptible to desorption p r m s e s when

the fracture is reactivated or when the chemical properties of the ground water are

changed. On the other hand, the mobilization of radionuclides from rock matrix is delayed by the same factors that tend to impede movement into the rock.

Table 1. Radiochemical data on core sample 1031R346.

Distance Activity Activity ratios f rom [ PPm 1 CBq/kgl

Specimen fracture [mm] U Th 2 3 4 ~ =%h 2Mu/23gu ')oTh/'%

21f2 14f 1 15'2 12'1 15I2 10I1 llf 1 11'1

*) Successive extraction with NH,OAc (=45A), TAMM'S oxalate (=45B) and total dissolution of the residual phase (=45C)

- ) Below determination limit

Table 2. Radiochemical data on core sample 2 11lR325.

Distance Activity Activity ratios f rom [ PPm 1 [Bq/kgl

Specimen fracture * 1 [ml u ~h 2 3 4 ~ 9 h u~h/zYu

*) Successive extraction with NH,OAc (=47A), TAMM'S oxalate (=47B) and total dissolution of the residual phase (=47C)

-) Below determination limit nd Not determined

ACKNO WLEDGEMENT. The study was carried out as a part of the publicly financed Nuclear Waste Managernent Research Pmgramme in Finland funded by the Ministry

of Trade and Industry .

Alexander, R., MacKenzie, A.B., Scott, R.D. and McKinley, I .G., 1990. Natural analogue studies in crystalline mk: The influence of water-Mng fractures on radionuclide immobilization in a granitic rock r e p i tory , NAGRA, Technical Report 8748.

Bashman, Z.R., 1981. Some applications of autoradiographs in textural analysis of uranium-bearing samples - A discussion. &on. Geol. 76, pp. 974-982.

Bradbury, M. H. and Stephen, I.G., 1986. Diffusion and pemeability based sorption measurements in intact rwk samples, Scientific Basis for Nuclear Waste Management IX, Stockholm, L.Werme (4). Mater. Ra. Soc. Proc. 50, Pittsburgh 1986, pp. 73-80.

Ivanovich , M. and Harrnon, R., 1982. Uranium series disequilibrium: Applications to environmentai problems. Clarendon Press, Oxford 1982, p. 571.

Ivanovich, M., 199 1. Aspects of UraniumlThorium Series Disequilibrium Applications to Radionuclide Migration Studies. Radiochim. Acta 52/53, 257-268.

Jaakkola, T., Suksi, J . , Suutarinen, R., Niini, H., Ruskeeniemi, T., Söderholm, B., Vesterinen, M., Blomqvist, R. , Halonen, S., Lindberg, A. , 1989. The behaviour of natural radionuclides in and around uranium deposits. 2. Results of investigations at the Palmottu analogue study site, SW Finland, Geological Survey of Finland, Nuclear Waste Dispsai Research, Report Y ST-64, pp. 60.

Kamineni, D.C. and Dugal, J.B., 1982. A study of rock alteration in the Eye-Dashwa Lakes Pluton, Atikokan, Northwestern Ontario, Canada. Chem. Geol. 3, 35-57.

Neretnieks, I . , 1980. Diffusion in the rock matrix: An important factor in radionuclide retardation ? J. Geuphys. Res. 88, 4379-4397.

Ohnuki, T., Watanabe, S., and Murakami, T., 1991. Study ori role of % in uranium series nuclides migration . P m . Symposium on Scientific Basis for Nuclear Waste Management XII, (Mat. Res. Sm. Symp. Proc. Vo1.212), 733-740.

Rosholt, J.N., 1983. Isotopic composition of uranium and thonum in crystalline rocks. J . Geophys. Res. j@, 7315-7330.

Ruskeeniemi, T., Niini, H., Söderholm, B., Vesterinen, M., Blomqvist, R., Halonen, S . , Lindberg, A. , Jaakkola, T., Suksi, J. , and Suutarinen, R., 1989. l'he Palmottu U- Th deposit in S W Finland as a natural analogue to h e behaviour of spent nuclear fuel in bedrock: A prelirninary report. Proc. the 6th International Symposium on Water- Rock interaction, Malvern 3-8 August 1989.

Schwarz, H,P., Gascoyne, M. and Ford, D. C., 1982. Umium-mies disequilibnum studies of granitic rocks. Chem. Geol. 3, 87-102.

Skagius, K. and Neretnieks, I . , 1986. Diffusivities in crystalline rock materials, In: Scientific Basis for Nuclear Waste Management TX, Stockholm, L.O. Weme (ed). Mater. Res. Soc. Proc. 50, Pittsburgh 1986, 73-80.

Smellie, J. A .T., MacKenzie, A.B. and Scott, R.D., 1986. An analogue validation study of natural radionuclide migration in crystalline m k using uranium-series disequilibrium studies. Chem. Geol. 3, 233-255.

Suksi, J. and Ruskeeniemi, T., 1991. Matrix diffusion in siiu in Palmottu. In: The Palrnottu Analogue Project , Progress Report 1990, (eds. R. Blomqvist, T. Jaakkola and L. Ahonen. Geologicai Survey of Finland, Nuclear Waste Disposal Research, Report YST-73, 121-137.

Suksi, J . , Ruskeeniemi, T., Lindberg, A. and Jaakkola, T., 1991. The distribution of natural radionuclides on fracture surfaces in Palmottu analogue study site in SW Finland. Radiochim. Acta 52/53, 367-372.

Thiel, K., Vonverk, R., Saager, R. and Stupp, D.H., 1983. ='U fission tracks and series disequilibria as a means to study recent rnubiIization of uranium in Archean

pyritic conglomerotes. Earth Planet Sci. Lett. 65, pp 249-262.

The Palmottu Analogue Project, Progress Report 1991

ELEMENTAL MOBILITY IN CRYSTALLINE ROCK AROUND OPEN FUCTURES AT PALMOTTU

Kumpulainen, H.'), Melamed, A.2), Pitkänen, P .~ ) , Valkiainen, M.'), Manninen, PV1' 1) Technical Research Centre of Finland, Reactor Laboratory 2) Technical Research Centre of Finland, Road, Traffic and Geotechnical Laboratory

Abstract. Rock specimens adjacens to nuo water conducting fractures m a depth of about 205 m frum fhe Palmotru uraniiun deposit were studied in order ta obtain i n f o w i o n on element mobility. me drill core wm sawn in such a way that a series of specirnenr perpendicular to rhe wmer conducting Jwcture were obtained for each of the fructures. Concentration profiles for a nwnber of elernents were detemined. Uranium series disequilibrium shldies m well as petrographic studies and porosity deteminarionr were also pe f o m d . Hordly any rnobilizarion was observed for [he lower fracture, bur for the upper fructure sewral elements had been mobilited, while many remuined immobile, Elernental concewration daba atwlysis pmvided also for the alrerm-on depbh about 25 mm. The conditions have been reducing for long times.

An understanding of the processes of water-rmk interaction is essential for the safe long-term disposal of radioactive wastes. Almost dl of the prwesxs that could be

effective in the transporktion of radionuclides in groundwater can dso be found opetating in riatural environments. Uranium and thorium are clearly the best analogues

of themselves, whereas rare earth elements (REE) such as La and Nd, are considered to be apprapriate andogues of Am and Cm (Airey, 1986). Uranium series disequilibrium studies provide information on recent uranium mobility (in the

geochemical sense), while REE variations may be used to distinguish between

hydrothermal and low temperature alteration (Latham et ai., 1989).

In this work element mobility in crysiailine rock around two Open fractures at Palmottu

was studied from eIement profiles. In addition to element anal y ses, petrographic studies, porosity measurements and urani um series disequilibrium measuremen ts were performed. This paper provides a brief sumrnary of the work, which will be published elsewhere in detail (Kumpulainen et al., 1992).

2. ROCK SPECIMENS

Rock specimens were obtained from drill hole 346, from which a complete sectiori of wre between depths of 205.328 m and 205.995 m was selected. The section was situated between two water conducting fractures occurring a few metres away from uranium mineralization. At similar depths (140 - 212 m) groundwater is of slightly saiine Na-Cl-SO,-HCO, -type (390- 130- 120-80 mgll respectively) according to Jaakkola et al. (1989). Suutarinen et al. (1991) have measured characteristic values of O, = 1 mgll, Eh - -300 mV and pH - 9 at a depth of approximately 200 m, indicating that

conditions were reducing .

A series of samples as a function of distance perpendicular from the fracture were obtained using a thin diamond saw . A part of these 1.5 mm thick sarnples (every third)

was used for neutron activation anal y sis (NAA) and x-ray fluorescence anal ysis (XRF) , a part for petrograp hic investigations, a part for porosity studies and a part for uranium isotope disequilibriurn studies by alpha spectrometry . A series of specimens were taken from both fractures. The upper one (series 2), at 205.328 m trending downwards began with the code number 2 and the lower one (series 102), at 205.995 m trending upwards, with the code nurnber 102. Petrographic and porosity studies as well as uranium isotope disequilibrium measurements were performsd for the series 102. Thin sections were also prqared for analysis by optical microscope.

Tn all the thin sections examined, the major primary minerals were quartz, plagioclase, biotite and garnet. K-feldspar was an additional major component in the series 2, as

was gamet in the series 102 specimens. Zircon, apatite, and pyroxene (hypersthene) were present in minor amounts in the series 102 sections. Secondary minerals included sericite, chlorite, and carbonate (calcite), the Iatter usually representing alteration products of plagiaclase and hypersthene, and to a minor degree, of biotite and garnet.

Complete alteration of hypersthene was only evident in thin section close to the fracture surface for mies 102. This section dso showed the highest overall total alteration and secondary crystallization (8 % ). These obsemations correlakd well with the porosity curve, which showed the highest vdue (0.38 %) for the part of the rock sarnple taken

from the vicinity (3 mm) of the macrofracture. The porosity decreased to < 0.2 % at

distances > 6 mm from the fracture surface and to as low as 0.03 % at a distance of

14 mm. In the series 2 specimens the alteration of hypersthene was stronger adjacent to the fracture surface. No clear evidence for low ternperature alteration of the specimens could be seen, which seems to be in accordance with the obtained low prosity values.

3. ELEMENT PROFILES

The variations in chemical composition across each profile reflect the specirnen

mineralogy , the nature of each alteration process and the extent of element dissolution or accumulation. By using neutron activation analysis (NAA) and x-ray fluorescence analysis (XRF) a multitude of elements such as Fe, Sc, Br, Na, K, Rb, Cs, Ca, Ba, Al, Si, U, Th, Mn, Cu, Co, S, Au and Ti were detemined. Furthermore the REE La, Ce, Nd , Sm, Eu, Tb, Yb and Lu were determined.

The concentrations were normalized against the concentraton of tibnium, based on the assumption of gmhemical immobility of titanium in most alteration processes (Kamineni, 1986). The elemen t profiles showed signi ficant d teration for the upper fracture and only very slight alteration for the lower one. In order to form an appropriate basis for the more convenient estimation of the intensity of alteration processes a few of the tiranium normalized element profiles were also normai id against concentrations in the deeper "parent wk". Figure 1 shows the curves for Na, Ca, Al, Si, Fe, Mn, U and Th. The growth at the fracture surface for series 2 was

strong, whereas for series 102 changes have been very rnodest. For series 2, concentration increased 800 % for Na and some 200 percent for Ca, Al and U. For Si,

Mn and Th the increases were around 70 %. On ttie other hand there were hardly any changes in Fe concentration due to the fracture. The changes in the U and Th profiles near the fracture surface for the series 102 are very maiest.

The vhances of eIement concentrations were ala assessed using the chemometry computer program SIRIUS (Kvaltieirn et al., 1987). Only the results for the upper series 2 are presented, since the treatment of series 102 met with difficulties due to the nature of the data. Separate samples were classified according to the distance from the fracture. Tt was evident that the signi ficance of the point (ie. the specimen) correlated directly with proximity to the water conducting fracture. Furthermore, the results also

clearly showed that the effect of the fracture extended to a depth of about 25 mm. One

could also refer to the work of Suksi et al., (1991) cuncerning the Palmottu investigation site where they found, on the basis of mU/23aU -activity ratio profiles, that the minimum extent of interconnected porosity was 25 - 30 mm. Similar depths for

interconnected porosi t y have aiso been obtained in other studies.

Ufii -ratio profile

" T

- Deplh 205.320 m - 205.995 m

M s t a m fmrn W u r a (mm)

T M I -ratlo profile

601

Dlstanca fmrn fracturo (mm)

Feili -ratlo profile

4 0 1

Distanco (mm hadura (mm)

Mn/li -ratlo profile

-40 l

Distnnw fmrn haetun (mm)

N a i -ratio profile

oistum fmm fndura (mm)

Ca/il ratio profiie

Dlslanca fmm frncluro (mm)

AUTI -ratio profile

-40

Dlslanco fmrn fmciura (mm)

4 1

Dlstanw fmm haetun (mm)

- Depih 205.995 m, m Figure 1. Titanium and "parent rock" normalized concentration profiles for selected elements.

Figure 2. Element analysis results for series 2 samples according to different elements by means of a chemometry computer program. Origin indicated by cross. Abscissa represents the effect of the determining variable 1 (the fracture) and ordinate the determining variable 2.

The elements are classified in Figure 2. Elements situated furthest to the right of the

origin (cross) have been most enriched due to the fracture surface. These are Na, Ca,

U, Th, La, Sm, Al and Si. In contrast, elements located in the vicinity of Ti, the

"immobile element", have not been dissolved, namely Au, Fe, Sc, Co, Cu and S.

Lanthanum, Sm and Th in particular are very similar in their behaviour, since in

Figure 3 they are clustered together and their elemental profiles resemble one another.

It could therefore be expected that they would be good analogues for each other in the

processes in question. The elements Fe, Co and Sc as well as Rb and Cs also form

similar groups. It is also interesting to note the difference between Fe and Mn. The

latter shows an obvious enrichment effect, whereas the former does not.

The results are in quite gmd accordance with the fracture fillings and the results of interpretation of water-rock interaction at this depth. Ruskeeniemi et al. (1991)

identified calcite, kaolinite, py rite and illite as the main fracture mineral s. Plagiwlase dissolution and calcite and kaolinite precipitation with simultaneous release of sodium into groundwater have been interpreted as the dominant processes affecting groundwater composition at this depth (Pitkänen et af., 199 1). Although sulphate

concentration is high, pyrite has not been altered (Ruskeeniemi et al., 1991) reflecting reducing conditions for iron and sulphide. Their immobility is aiso evident in Figure 2. The mobilization of Ca, Na, Si and Al close to the fracture is consistent with

plagioclase dissolution. However, sodium enrichment in the vicinity of the fracture surface suggests that the element mobilization has taken place under conditions different to those prevailing at present. These might have been metamorphic, when if for instance hypersthene dtered to amphibole and sericite near the frachire surface.

U/Th ratio profiles have b m drawn in Figure 3a. The series 2 and 102 vaiues for the specimens are presented as a function of distance from the upper fracture (at 205.328

m depth). The line is broken between the two series, since the distance from lower fmcture has been calculated from the sepafation of the fractures (667 mm) minus the perpendicular distance from the lower fracture. In the vicinity of the upper fracture, vaiues for the more mobile uranium are lower (mean ThlU = 21.4 for the series 2) pssibly as a result ofearlier mobilization, than near the lower fracture ( m m ThlU =

7.7 for the series 102). Thorium is generaily considered to be a relatively insoluble element in an aquatic environment (Ivanovich et al., 1982). The enrichment of uranium adjacent to the upper fiacture leads to a fall in the ratio within a distance of some 3

centimeters from the fracture. Thorium enrichment may also have taken place, but to a lesser extent than for uranium. The 'depleted' uranium region extends for about 20 centimeters into the parent rock, as seen in Figure 3b. As discussed later in the text

uranium mies disequilibrium studies were performed for the series 102 sarnples, but

not for series 2. Presumably the latter would have provided helpful confirmation of the

conclusions. No uranium mobilization could be observd for the series 102 specimens in the vicinity of the lower fracture.

Depth 205.328 m, downwards

35.00

0.00 4 0 1 00 200 300 400 500 600

Distance from the upper fracture (mm)

Depth 205.328 m, downwards

Distance from the upper fracture (mm)

Figure 3. ThIU concentration ratio a) and U concentration b) as a function of distance from the upper fracture.

The REE are unique because they all have very similar chemical properties and in

general exhibit rather uniform geochemical behaviour. Except for Ce and Eu, these

elements always occur in the trivalent state. Evaluating the fractionation history of the

REE is complicated (Fig. 4). The decrease of the negative Eu anomaly and

concomitant Eu enrichment on the fracture surface is obvious. Slight leaching of the

other REE is possible compared to the other fracture series 102 (Fig. 4). The physico-

chemical conditions for alteration are not clear. They could have been hydrothermai

(low grade metamorphic) in origin connected to major mineral alteration, quartz

recrystallization with fluid inclusion capture, and possibly U-mineralization. Also the

REE fractionation may be a result of both hydrothermal and low temperature alteration.

Rare eanh element panem at depth 205.328 m

Rare ea<lh elerneni panem al depIh 205.328 m

-C 0.8 mm fmm rn

-- 0.8 mm horn lfaclm - 16.05mnhorn

Rare eaRh elemerd pafiem al depth 205.328 rn

Rare eaih eiemeni pafiem at depth 205.328 m

Figure 4. The chondrite normalized REE pattern for series 2 at different distances from the fracture surface. REE patterns nearest to the fracture have been added for easier comparison.

The leaching of REE is suggested as taking place under oxidizing redox conditions with

the ligands in solution, especially as carbonate, phosphate, and fluoride, but dso as

chloride (Leroy et al., 1 988). Precipitation occurs under reducing conditions, under which Eu is preferentially enriched with respect to the other REE. According to Figure

3, uranium has clearly been depletd in seies 2, reflecting oxidizing conditions around the fracture during U mineralization. The REE and Th are also slightly depleted in series 2 compared to the other fracture. The later enrichment of U and Eu (Figs. 3 and 4) in proximity to the frachrre surface may be a resuit of prevailing reducing conditions, indicated by sulphide stability in low temperature.

Uranium series disequilibnum studies were initially used in environmentd appliations, but they have been recognissd as valuable in natural analogue studies relating to

radioactive waste disposal as well (Alexander at al., 1990; Airey et al., 1987). For crystalline rocks ihe uranium series system is a sensitive indicator of recent uranium mobility, the time span for w ~ 1 2 3 8 ~ -ratio extending back some 1 million years.

In oxidizing conditions uranium has a valency state 6 + , so that it dissolves readily in

groundwater. It occurs mainly as anionic complexes of h e uranyl ion, for example as [UQ(C0,),]2'. However, under reducing conditions uranium is tatravalent and less soluble, so that it is easily precipitated or adsorbed on rock surfaces. Thus redox

conditions strongly control uranium mobility . By means of uranium series activity ratios it should be pssibte to discover recent (in the geochemical sense) uranium

dissolu tion or accumulation.

By using specimen dissolution, chemical separation and cr-spectrometry the uranium isotope ratio "UIUsu was measured for the senes 102, and the resdts are presented in Figure 5 as a function of specimen distance from the fracture surface. One sigma standard deviations are al so indicatsd. No clear disequilibriurn is discemible after taking the f 2u error limits into consideration. A probable interpretation of the results

could be that uranium has not been mobile during the last 1 million years. This is in

accordance with the U concentration profile, in which there has been no obvious U enrichment close to the fracture. However series 2 exhibits distinctive behaviour with estimations from elernent profiles indicating evident U accumulation close to the fracture. Unfortunately the U series disequilibrium measurements were not available for this U-profile.

0 -1 I

0 50 1 00 1 50

Distance from fracture (mm)

Figure 5. Uranium isotope ratio 2"U/238U as a function of distance from the fracture surface for series 102.

5. CONCLUSIONS

The two Open fractures studied clearly have different characteristics. In the vicinity of

the upper fracture the effects of element mobilization could clearly be seen. On the

other hand the lower fracture did not show any significant element mobility, not even

for uranium. Near the upper fracture the U concentration was low, but very close to

the fracture (< 30 mm distances) U turned out to be enriched. Several elements such

as Na, Ca, Al, Si, and U were enriched adjacent to the upper fracture. On the other

hand Fe, Sc, Co, S , and Cu had been immobile. For example La, Sm and Th were

very similar to each other in their behaviour. Therefore they were excellent analogies

for each other. The depth of alteration was 25 mm. Groundwater conditions have been

reducing for a long time. The multielement profile approach applied in this work could

be used to prove immobility, depletion and enrichment as well as also other relevant

information .

Acknowledgement. The drill core sample has been kindly selected for this pupose by Antero Lindberg of the Geological Suwey of Finland.

6. REFERENCES

Airey, P.L., 1986. Geochemical analogues of high-levd radioactive waste repositories, Chem. Geol. 55, 203 - 213.

Alexander, W.R., MacKenzie, A.B., Scott, R.D., McKinley, I.G., 1990. Naturai andogue studies in crystalline rock: The influence of water-bearing fractures on radionuclide immobilisation in a granitic repository . Nagra Technical Report 87-08.

Ivanovich, M., Harrnon, R.S . (Eds.), 1982, Uraniurn Series Disequilibrium: Applications to Environmental Problems, Oxford University Press, Oxford.

Jaakkola, T., Suksi, J . , Suutarinen, R. , Niini, H., Ruskeeniemi, T., Söderholm, B., Vesterinen, M., Blomqvist, R., Halonen, S., Lindberg, A. , 1989. The Behaviour of Natural Radionuclides in and around Uranium Deposits. 2. Results of Investigations at the Palmottu Analogue Study Site, SW Finland, Geologid Survey of Finland, Report Y ST-64.

Kamineni, D.C., 1986. Distnbution of uranium, thorium and rare-earth elements in the Eye-Dashwa Lakes pluton - a study of some analogue elements, Chem. Gml. 55, 314, 361 - 373.

Kumpulainen, H., Melamed, A. , Pitkinen, P., Valkiainen, M., Manninen, P . , 1992. Elementai mobili ty in cry stalli ne rock around Open fractures, Technical Research Centre of Finland Hesearch Notes (to be published).

Kvalheim, O.M., Karstang, T.V., 1987. Muftivariate data analysis prograrn SIRIUS, Chernom. Intell. Lab. Syst. 2, 235 - 237.

Latham, A.G., Schwartz, H.P., 1989. Review of the modelling of radionuclide transport from U-series diquilibria and of its use in assessing the safe disposal of nuclear waste in crystalline rock, Appl. Geochem. 4, 527 - 537.

Leroy, J. L., Turpin, L. , 1988. REE, Th and U behaviour during hydrothermal and Supergene processes in a granitic environment. Chemical Geolog y 68, 239 -25 1 .

Pitkänen, P., kino-Forsman, H. , Ahonen, L., Ollila, K . , 199 1 . Hydrogeochemical interaction in Pdmottu natural analogue site based on the results of borehole 346. Geologicai Survey of Finland, Nuclear Waste Disposal Research, Report Y ST-77, 39p.

Ruskeeniemi, T., Vesterinen, M., 1990. Fractures and fracture minerals in drill core 346, In: Palmottu Analogue Project Progress Report 1990, Geological Survey of Finland, Report Y ST-73, Espoo 199 1, 47 - 58,

Smellie, J . A.T., MacKenzie, A.B., Scott, R.D., 1986. An analogue validatiori study of natural radionucl ide migration in cry stalline rock using uraniu m series disequilibrium studies. Svensk Kämbninsle hantering Ab, SKB Technical Reprt 86-01.

Suksi, J . , Ruskeeniemi, T., 1991. Matrix diffusion in situ in Palmottu, in: Paimottu Analogue Project Progress Report 1990, Gmlogical Survey of Finland, Report Y ST- 73, 47 - 58,

Suutarinen, R., Blomqvist, R., Halonen, S., Jaakkola, T., 1991. Uranium in groundwater in Palmottu analogue study site in Finland, Radiochirn. Acta 52153, 373 - 380.

The Palrnottu Analogue Project , Progress Report 1991

MODELLING OF URANTUM SERIES DATA AT PALMOTTU

K. Rasilainenl and J. Suksi2

'Technid Researc h Centre of Finland, Nuclear Engineering hboratory

Vniversity of Helsinki, Departmen t of Radiochernistry

Abstmt. Compurer code URSE has been iieveloped for rhe qwi ta t ive inteyrefabion of uraniwn series disequilibnwn d m . lk c& is useful in asessing the time period needed for an observed disequilibriwn State to & velop. From pe@omuuace assessmenb poim of view two importaru daring applicatiom of the code can be seen: mrix d i p i o n projiles and pseud~colloids.

INTRODUCTION

The U-Th deposit at Palrnottu is being studied as a migration analogue for a nuclear waste repository in granitic Wrock, Migration and related processes have been studied using natural uranium as a tracer by investigating its behaviour, occurrence and

distribution in rock-groundwater system. During these studies numemus uranium series disequilibrium determinations have k n performed for water and m k samples (Suksi

et al. 1991; Suutarinen et ai. 199 1; Blomqvist et al. 1992; Suksi and Ruskeeniemi 1992).

Due to the importance of migration processes in periomane assessments and the need to validate the used models it was wnsidered necessary to start to develop the numerical interpretation of uranium series data. In this report a short description of the developed code and of its use in testing a simplistic conceptual model is given. The test

case consists of an effort to interpret the matrix diffusion profiles presented in the work of Suksi and Ruskeeniemi (199 1).

A SHORT BACKGROUND FOR MODELLING URANUM S E R B DATA

Disequilibrium in naturai radioacuve decay series paU, '"u and U2~h) always

indicates some kind of disturbances where due to the different geochemical behaviour

some nuclides are partly separated from the rest of the decay chain. By wrnparing the a-activities of decay series nuclides the time the system was discurbed can be estimated. I k a y series disequilibia have been widely used for various dating purposes and to s tud y geochemical processes in surficid and subsurficial environments (see e. g . Ivanovich and Harmon 1982). The applications in nuclear waste disposai study has

been first described in detail by Schwan et d. (1982) and recently considered by

Ivanovich ( 199 1 ) .

In the modelling prmess the first task is the characterization of ?he system: geulogical and ph y sical characterization , radionuclide concentrations, radionuclide distribution and the State of radioactive equilibrium in decay series. In the next stage the initial conditions and the degree of closure of the system will be postulated. Finally hypotheses concerning the mobility of radionuclides will be made, qualitative (e.g.

immobile Th, and mobile U and Ra) and later of course quantitative. The cr-activity

ratios of nuclides are then calculated as a function of time for the seleckd scenarios of uranium accumulation and leaching, see e.g. tatham and Schwan (1989) and

Alexander et al. (199 1). The calculations are made applying the Bateman equatiuns for the natural decay series e.g. 4N+2, 4N+3.

COMPUTER CODE URSE

The devloped computer code URSE &aniurn sries disequiiibria) is an extended

modiftcation of the equations reported in the literature, e.g. Alexander et al. (1991). URSE soIves anaiytidly a 4-member radioactive decay chain (e.g. 23aU-234U-23?h-

226~a) in four basic cases:

- sudden addition of material - sudden removal of materia1 - continuous addition of material - continuous removal of material

The rate of the continuous addition or removal of material can be constant or relative to the concentration. The rate of addition/removal for any nuclide is independent of

that of other members of the decay chain. The number of basic cases forming a disturbance scenario is not limited. The basic cases can be combined into continuous

disturbance scenarios. A more detailed description of the code, including the equations,

and wider disscussion on its applicability will be presented elsewhere (Rasilainen and

Suksi 1992).

URSE SIMULATION

MQtrix diffusion. The following simplified conceptual closed system model was tested

as a "zeroth order" numerical interpretation of the distribution of loosely bound

uranium in two rock profiles:

- the decay chain 4N + 2 was simulated - uranium diffused into the rock matrix, thorium did not, d l

observed thorium and radium were produced via chain decay - no nuclides were in the matrix originally (t < 0) - diffusion was approximated with a step function, i.e.,

sudden addition; after addition the system was assumed closed - the same disturbance (sudden addition) was applied to all

points in the profile, the only difference being that deeper in the matrix the disturbance started to evolve later.

The input data was taken from the profile 21 1/R325 (Suksi and Ruskeeniemi 1992) by

extrapolating the concentration of the loosely bound (IW,OAc extractable) uranium to

the fracture surface (see Fig. 1.). The W O A c extractable uranium was interpreted to

be the migrated uranium phase. The data for the simulation by URSE are presented in

Table 1, and the results of simulation in Figure 2.

Distance from fracture tmml Diatance from fracture [mm1

Fig. 1. Distribution of loosely bound uranium (NH,,OAc extractable) , 234U/U8U and 23@Th/mU activity ratios in profiles 103/R346 and 2111R325 (after Suksi and Ruskeeniemi 1992, in this report).

Table 1. Input data for URSE simulation.

Nuclide Initial concentration (t = 0) [BqJkgl

The simulation results in Fig. 2 refer to the evolution of concentrations at a given point

as a function of time. The observed concentration and activity ratio profiles, however,

refer to the evolution of concentrations at a given time as a function of distance from

the fracture face, making the comparison difficult.

To convert the simulation results in time to profiles in depth one must use the same

point of time as a reference time for the surface (depth = 0). For each pint towards

the matrix one must move backwards in time on the graph a certain time step, since the disturbance starts later in the matrix. The time difference between consecutive points

in space can be readily assessed by the diffusion theory (as an approximation, the

corresponding difference in time between spatiai points in profile is proportionai to the

difference of the respective depths squared). This approximation is not completely

satisfactory, however, because it can not take radioactive chain decay into account.

As profiles for the whole simulated decay chain are not yet available, the comparison

between simulations and observations is incomplete and qualitative. In the following the

observed profiles are studied against the background of the simulated ones. The

absolute activity levels of uranium should show a decreasing trend towards the matrix,

which is roughly what they do in both profiles. The activity levels of thorium and

radium should show strongly decreasing trends; the results available for thorium,

however, indicate an ambiguous behaviour.

The activity ratios of 23"T'h/234U should show a strongly decreasing trend towards the

matrix. However, no such trend is observed in either profile. The activity ratios of 234U/238U should show a slightly decreasing trend, which again is not observed: .for

211lR325 it is almost constant and for 103lR346 the trend is rising!

Palmottu: Scen 1, Activities

LEGEND 0 = U-238 0 = U-234

= Th-230 + = Ra-226

0.0 1.0 2.0 3.0 4.0

Time (a) *jo5

Palmottu: Scen 1, Activity ratios ui "! 7

0 3 7

LEGEND 0 = RAT 1

. 2 - i-" o = R A T 2 0 0 L = RAT 3 X C . - + = RAT 4 .? 0 - 'c = RAT 5 2 o

ui "! 0

0

8 0.0 1.0 2.0 3.0 4.0

Time (a) *lo5

Fig.2. Results of URSE-simulations. a) absolute concentrations and b) radioactivity ratios of nuclides: RAT 1 = 234U/238U, RAT 2 = U%/238U, RAT 3 = 2 3 9 h / 2 3 4 ~ , RAT 4 = n6Ra/234U and RAT 5 = 226Ra/23qh.

amount of groundwater for colloid characterization that can be collected at a time is

about 50 1 or 20 1. The collected groundwater can then be either taken to the laboratory

or, depending on the local facilities, be further treated in the field.

Fig. 1. shows the drilling profile at Palmottu where the drill holes sampled for colloid

characterization have been made. At the end of September 1990, the first colloid

samples were obtained in late September 1990 from drill hole R357, which is about

320 m long and reaches a depth of about 270 m. The drill hole penetrates the uranium

mineralization between about 228 m and 255 m. Colloid samples were collected from

two sections; the upper range was from 165 m down to 171 m and the deeper one

between 265 m and 27 1 m. In July 1991, two samples were taken from drill hole R324,

which is about 205 m long and reaches a depth of about 150 m. This drill hole

penetrates the uranium rnineralization at several points below 100 m. Colloid samples

from this drill hole were also collected from two sections, 95 - 101 m and 175 - 200 m.

The long packer space (25 m) in the last sampled range was due to the very low water

inflow in the bottom part of the drill hole. About 50 1 of groundwater was collected

from each sarnpled section.

Fig. 1. Cross-section of bedrock at the Palmottu U-rnineralization of the drilling profile of colloid sampling (Blomqvist et al. 1990).

Imrnediately after collection, the groundwater samples were taken to the laboratory for

Fig. 2. EDS spectra of colloids. a) Mineral colloids from R357, 265m-271m and b) iron-containing colloids from R324, 95m-101m.

CONCLUSIONS AND DISCUSSION

These first data has provided evidence of different types of colloids existing in the

groundwater at Palmottu. Evidence for the association of uranium and thorium with

colloids has also been obtained. Additional data on colloid concentrations and on

isotope ratios is being processed. To date characterization of only inorganic colloids has

been performed, although indications of the presence of some organic material of

colloidal size have been gained. At this stage the available results are not

Table 1. Mineral precipitates in R3241175-200 and R324195-101 groundwaters predicted by EQ6 reaction path modelling to exist in equilibrium.

kaolinite [A&Si,O,(OH) J (not predicted to precipitate

in R324195- 10 1 groundwater)

nontronite-ca [Ca-Fe-Al-Si mineral]

pyrite FeSJ

SiO,

thorianite [ThOJ

uranophane [Ca(UO,),Si,O,- 6H20]

The theoretical solubilities for uranium in R3241175-200 groundwater given by EQ6

reaction path calculations and EQ3NR solubility calculations assurning uranophane to

be the solubility-lirniting solid phase, are many orders of magnitude lower than the

measured concentration of uranium in true solution. This can be seen from Figure 1, in

which the calculated solubilities have been plotted in the reducing and slightly oxidizing

redox regime (Eh= -0.4...+0.2 V). In contrast, the theoretical and measured solubilities

would seem to be in agreement when uraninite (UO,) or coffinite (USiO,) is controlling

solubility.

For the limiting mineral thonanite, the theoretical solubility of Th (EQ3NR) in

R324195-101 groundwater is 1.10-l4 mol/l, two orders of magnitude lower than the

measured concentration of thorium in true solution, 4.10-" mol/l. If the solid oxide

phase ThO,, is the controlling solid, the theoretical solubility is higher, at 3.10-l0 mol/l.

The aqueous speciation of uranium is dominated by the anionic and uncharged carbon-

ate and hydroxide complexes in the slightly oxidizing redox regime, while in the

reducing redox regime uranium exists as U(OH),, (Fig. 2). The dominant thorium

species in this case is the uncharged Th(OH),.

The solubilities or speciation for both elements in R324195-101 groundwater do not

differ markedly.

- uraninite - cofflnite - uranophane

I I ' analyzed (Eh= 0.110 V)

Fig. 1. Solubilities of uranium in R3241175-200 groundwater as a function of redox potential.

water

Fig. 2. Speciation of uranium in R3241175-200 groundwater as a function of Eh.