F 980009

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

Tiedonanto YST-97 Report YST-97

INVESTIGATION OF ROCK MATRIX POROSITY IN ALTERATION PROFILES

Development of site characterization methodology

Marja Siitari-Kauppi 1, Antero Lindberg2, Karl-Heinz Hellmuth3 and Sakari Lukkarinen4

1 Laboratory of Radiochemistry, 00014 University of Helsinki 2 Geological Survey ofFinland 3 Finnish Centre for Radiation and Nuclear Safety 4 Helsinki University of Technology

Espoo 1997 TIIVISTELMA

Peruskallioon sijoitetuista ydinpolttoaineen Ioppusijoitusvarastoista mahdollisesti vapautuvat radionuklidit kulkeutuvat paaasiassa kalliorakoja pitkin virtaavan veden mukana. Veteen liuenneiden radionuklidien kulkeutumista hidastaa niiden pidattyminen kalliorakojen pintaan ja rakotaytteisiin seka diffuusio kiven mikrorakoihin ja huokoisiin mineraaleihin. Kulkeutumiseen vaikuttavien erilaisten fysikaalisten ja kemiallisten ilmioiden vaikutusten, kuten myos kivimatriisin ominaisuuksien tunteminen on oleellista ennustettaessa ydinjatenuklidien kulkeutumista ja pidattymista.

Tassa tyossa maaritettiin yksityiskohtaisesti kivimatriisin ominaisuudet pohjavetta johtavien kalliorakojen laheisyydessa. Huokoisuusprofiilit mitattiin 14C-polymetyylimetakrylaatti (14C- PMMA) -menetelmalla kolmelle Syyryn alueen kivilajille: tonaliitille, kiillegneissille ja vulkaniitille. Tutkimuksessa kaytetyt 20 - 30 cm:n pituiset kairansydannaytteet impregnoitiin (imeytettiin) 14C-leimatulla metyylimetakrylaatilla (MMA) tyhjiokuivauksen jalkeen. Monomeeri polymeroitiin Co-60-sateilylahteen kentassa. Taman jalkeen naytteet sahattiin ja sahatut pinnat tutkittiin autoradiografisesti. Huokoisuusprofiilit ja huokoisuuden jakaumat maaritettiin autoradiogrammeista digitaalisen kuvienkasittelyn avulla. Naytteiden mineraalikoostumukset ja rakenteet tutkittiin mikroskooppisesti autoradiogrammeissa havaittujen piirteiden selvittamiseksi. Mineraalien muuntumisaste maaritettiin elektronimikroskooppisesti energiadispersiivisella alkuaineanalyysilla (SEM/EDS).

Kaytetylla 14C-PMMA metelmalla saadaan uutta kvalitatiivista ja kvantitatiivista tietoa diffuusiolle kaytettavissa olevan huokostilan leveydesta vetta johtavien ja pidattavien (varastoivien) rakojen laheisyydessa. Huokoisuutta tarkasteltiin kolmena eri alueena: muuntumattoman matriisin (tausta-) huokoisuutena, raon taytteen huokoisuutena ja raon vaikutuspiirissa olevan muuntumisvyohykkeen huokoisuutena. Matriisien taustahuokoisuus oli alhainen ja vaihteli vain vahan (0.05 - 0.2%), raontaytteiden huokoisuus puolestaan vaihteli valilla 1 - 5%. Taytteellisten ja vetta pidattavien rakojen laheisyydessa oleva huokoisuusvyohyke, joka paaosin aiheutuu maasalpien muuttumisesta (serisiittiytymisesta), oli tonaliitissa kapeampi kuin kiillegneississa. Tonaliitissa maasalpien muuttumista havaittiin myos huokoisuusvyohykkeen ulkopuolella. Vulkaniitin huokoisuusvyohykkeet olivat kapeita kuten kiillegneississa ja matriisin taustahuokoisuus oli pieni.

Avainsanat: kiven huokoisuus, muuttuminen, mikrorakoilu, u C-PMMA-menetelma, radioaktiivinen jate, loppusijoitus ABSTRACT

Assessment of bedrock performance for nuclear waste disposal can benefit from a more detailed understanding of the rock matrix properties along actual and potential groundwater flow pathways. The spatial variability along flow paths and the correlation of groundwater flow with rock matrix properties (type of minerals, porosity, internal surface areas) is an input parameter for the quantification of retarding properties and the chemical buffering capacity of the bedrock.

In this investigation tonalite, mica gneiss and volcanite samples from Syyry were studied. The samples were taken from a drill core located near water-conducting, open fractures. The objective of the study was to describe the porosity of different altered and weathered rock matrices next to filled fissures and open fractures.

Petrographic studies were done from several thin sections using optical microscopy complemented by SEM/EDS. The porosity profiles were determined with the l4C- polymethylmethacrylate (14C-PMMA) method. Porosity profile measurement based on autoradiographs produced by 14C-PMMA impregnation provides valuable qualitative and quantitative information on the thickness of the diffusion-accessible pore space of rock samples near water-conducting fractures and water-bearing fissures.

Rock samples from the Syyry site showed that the zones of increased porosity near water­ bearing fissures in tonalites were narrower than comparable zones in mica gneiss. The porosity profile was consistent with plagioclase sericitization in mica gneiss, but not in tonalite samples. The intermediate volcanite samples showed narrow porosity profiles next to laumontite-filled fissures and no consistent correlation with plagioclase sericitization was found. A gradual increase in porosity was observed in mica gneiss, but an abrupt decrease in porosity was found in tonalite samples. Chloritized biotite was found throughout the rock matrix in moderately altered tonalite, but no influence on the increased porosity zones near fissures was found.

Keywords: rock porosity, rock alteration, microfracturing, 14C-PMMA method, nuclear waste disposal CONTENTS

TIIVISTELMA

ABSTRACT

1. INTRODUCTION 1

2. GENERAL GEOLOGY 2

3. EXPERIMENTAL 3 3.1 Materials 3 3.2 14C-PMMA method 5 3.2.1 Infiltration with 14C-methylmethacrylate 5 3.2.2 Digital image processing and analysis 5

4. RESULTS AND DISCUSSION 9 4.1 Porosity profiles 9 4.2 Comparison of porosity profiles 17

5. CONCLUSIONS 20

REFERENCES 21

APPENDICES 1 1. INTRODUCTION

The migration behaviour of radioelements through rock matrices is clearly controlled by a complex combination of matrix diffusion and chemical interactions determined by mineral specific compositional and microstructural factors. Diffusion of radionuclides into the matrix of rocks along water conducting fractures and from fractures into water bearing fissures is one of the major retarding mechanisms ensuring the safety of nuclear waste repositories in deep geological formations. The mineral specific porosity and the breadth of the increased porous zone near water bearing fissures are essential parameters when appraising the ability of different rock types to retard by matrix diffusion.

Assessment of the performance of bedrock can benefit from a more detailed knowledge of the rock matrix properties along actual and potential groundwater flow pathways. The spatial variability along flow paths and the correlation of groundwater flow with rock matrix properties (type of minerals, porosity profiles, internal surface areas) are input parameters for the quantification of retarding properties and the chemical buffering capacity of the geosphere.

In order to increase the predictability of the diffusion-releated properties of rocks in relation to rock type and alteration, tonalite, mica gneiss and volcanite samples were studied. The samples were taken from a drill core located near water-conducting, open fractures. The objective of the study was to describe the porosity of different altered and weathered rock matrices adjacent to filled fissures and open fractures. Because the study was not intended as a comprehensive site-specific investigation, but rather as a contribution to methodological development, it was limited to sections of a single drill core.

Petrographic characterization was done analysing several thin sections with optical microscopy and S EM/EDS. Porosity profiles were determined with 14C- polymethylmethacrylate (14C-PMMA) method. The measured values were fitted to a simple equation to allow comparison of different profiles. An attempt was made to compare the characteristics of different rock types on the basis of porosity profiles near filled fissures. 2 2. GENERAL GEOLOGY

The main rock types of the Syyry area are felsic plutonic rocks, particularly tonalite, granodiorite and quartz diorite, but in the vicinity there are also schists and gneisses of both volcanic and sedimentary origin. Southwest of Syyry there is a large occurrence of volcanic rocks, mainly andesitic and dacitic porphyry, agglomerate and amphipolite, while to the southeast is an area of mica schist. The more basic plutonic rocks (peridotite, gabbro and diorite) grade into quartz diorite, tonalite and granodiorite without sharp contacts. Granite occurs only in small amounts (Anttila et al. 1993, Sail! 1967).

The stratigraphy of the area from oldest to youngest (supracrustal versus plutonic) is as follows: arkose schists with some volcanic material - ultramafic plutonic rocks and gabbrodiorite; greywacke schists - quartz diorite, granodiorite; porphyroblastic mica schists - granites; volcanic and subvolcanic material - various dyke rocks.

The dominant rock type in outcrop as well as in drill core from the Syyry site is tonalite, which contains abundant inclusions (xenolithes) of various schist and plutonic rock types. Aplite, quartz, epidote and pegmatite occur as intersecting dykes and veins. Mica gneiss occurs in the outcrops of the study area only as inclusions, but immediately to the south there is a fairly extensive occurence of greywacke-like mica gneiss which, according to magnetic measurements, continues into the study area (Anttila et al. 1993).

The main folding episode in the area was isoclinal and the schistosity therefore mostly shows steep or vertical dips. The orientations of fold axes undulate gently from the northwest to southeast. The most intensive fracturing is restricted to the regional fracture zones surrounding a block covering 12x25 km in the Syyry study area. The rocks in the area underwent three significant plastic deformation phases followed by three mainly brittle deformation phases (Karki 1991). This study provides information about the last deformation phases and associated weak metamorphic processes in the zeolite facies. 3 3. EXPERIMENTAL

3.1 Materials

The nine rock samples analyzed in this work were sawn from drill core SY7 obtained from Sievi, Syyry (NW-Finland). Bore hole SY-KR7 intersects mainly medium grained, slightly schistose tonalite, which contains some inclusions of fine grained and homogeneous mica gneiss as well intermediate volcanite (tuffite). The bore hole intersected a 60 m thick brittle-ductile shear zone. The central part of the drill core represents a distinct fracture zone with loose rock fragments and abundant clay minerals, while part of the zone comprises different types of my Ionites. Mylonitization was followed by recrystallization, after which the rock was affected by jointing developed in various orientations. As hydrothermal alteration accompanied the mylonitization and late jointing, only the latter phase contains zeolites and some prehnite as secondary minerals. Narrow (10 - 50 mm), sharply bounded alteration zones have developed around fissures (Lindberg and Paananen 1992, Lindberg and Siitari-Kauppi 1998).

The drill core samples examined with the 14C-PMMA method were 56 mm in diameter and ranged from 150 mm to 300 mm in length. Due to the mechanical weakness of fracture fillings drilling can easily open closed fractures and it was often difficult to determine by visual inspection of the core sample, if a fracture was originally open or a drilling artefact. Closed fractures were nevertheless considered as potential flow pathways and included in this study. Moreover, since core loss is common near open fractures, closed fractures and fissures provided a more reliable basis for investigation of the systematics of alteration profiles. The complex pattern of fissures observed at this site required long core samples of up to 30 cm. Three rock types, mica gneiss, tonalite and intermediate volcanite, each containing a variety of altered minerals, were studied. The sample depths, rock types and other essential observations are given in Table 1, while the rock type profile for drill core SY-KR7 is shown in Figure 1. The sample SY7 242 m was separated into two sections along the fracture in the middle of the core after methylmethacrylate impregnation. 4

LEGEND

Soil cover

\\}\ Tonalite AZ\/ - + - Granite /V\/ Mica gneiss

mgn= mica gneiss as inclusions /V\/ = fracture zone/ mylonite

x = 7069.756 y = 2541.500

o B Figure 1. Rock type profile for drill core SY-KR7 (Lindberg and Paananen 1992).

Table 1. Sampling depths, rock types and observations of sample.

sampling rock type special features/sample length (cm) depth (m) SY7 160 mica gneiss strongly altered, abundant fissures/ 17.7 SY7 161 mica gneiss/tonalite fissures cutting both rock types/19.5 SY7 179 tonalite moderately altered, a few narrow fissures/ 17.7 SY7 202 mica gneiss strongly altered, a few narrow fissures/22.3 SY7 205 intermediate tight matrix, a few narrow filled volcanite fissures/26.3 SY7 225 mica a few narrow fissures/ 16.2 gneiss/volcanite SY7 226 mica mylonitized, abundant tight, filled gneiss/volcanite fissures/12.1 SY7 228 mylonite mylonitized, abundant fissures/ 17.4 (mica gneiss) SY7 242 tonalite open fracture, a few narrow filled fissures/15.4+10.5 5 3.2 14C-PMMA method

3.2.1 Infiltration with 14C-methylmethacrylate

The samples were dried at 120°C for about 2 weeks and impregnated under vacuum with 14C labelled methylmethacrylate (14C-MMA) as described previously by Hellmuth et al. (1993,1994). The tracer activity was 925 000 Bq/ml. The infiltration time exceeding 4 weeks to ensure thorough saturation of rock cores with 14C-MMA. After polymerization with gamma radiation from a Co-60 source, core sections were sawed in two along the axis using a diamond saw fitted with a blade 0.6 mm thick. The rock surfaces were exposed on an X-ray film (Kodak X-Omat MA) and a (3-film (Hyperfilm- |3max, Amersham). Exposure times varied from 13 to 32 days.

3.2.2 Digital image processing and analysis

Two parallel systems were used to obtain quantitative porosity results from the autoradiographs. The autoradiographs were digitized by video camera with zoom optics (Quantimet-system) or were scanned with a table scanner (Ricoh FS2 -system). Both methods allow measurement of the intensities of autoradiographs in 256 gray levels. The resolution of the video camera system varies depending on the magnification while the maximum resolution of the scanner is 600 dpi. The range of 150 keV (3 particle is about 90 pm in the matrix, which have the density of 2.7 g/cm 3 and the resolution of 14C-PMMA autoradiographs is about 20 to 30 pm (Hellmuth et al. 1993, 1994, Siitari-Kauppi et al. 1995, Rasilainen et al. 1996).

Calculation of porosity Interpretation of the results is based on digital image analysis of autoradiographs that begins with discretising the autoradiograph into area units called pixels. All the intensities of the subdomains were effectively converted into corresponding optical densities which in turn were converted into activities with the help of measured calibration curves for each exposure. Finally the activities were converted into respective porosities. In principle the interpretation is based on studying the abundance of tracer in each subdomain. 6 The intensity and the optical density Assuming that the response of the image source and the amplifier of the digital image analysator are linear, the digitized gray levels of the film can be treated as intensities. Intensity means here the light intensity penetrating the autoradiographic film. Optical densities which, according to Lambert & Beer’s laws are proportional to concentration, can be derived from intensities

(1) where D is the optical density, I0 is the intensity of the background and I is the intensity of the sample.

The activity and the optical density A conversation function is needed to relate the measured optical densities (grey levels) to corresponding activities. Pure 14C-PMMA standards having specific activities between 462 and 185 000 Bq/ml have been used to establish the calibration function. The following calibration curve was used:

D-Dt (2) where Dmax is the maximum optical density , k is a fitting parameter, and A is the specific activity. Solving A from the Eq. (2) gives:

(3)

The porosity Local porosity £ of the sample was simply obtained from the abundance of the tracer (assuming constant tracer concentration in the PMMA, the higher the abundance of the tracer, the higher the local porosity): e=P(A/A0) z4) where A0 is the speific activity of the tracer used to impregnate the rock matrix, and (3 is 7 the (3-absorption correction factor. Absorption of (3 radiation in a substance depends in practice linearly on the density of the substance . Therefore factor (3 can be approximated from:

P=PVPo (5)

where ps is the density of the sample and p0 is the density of pure PMMA (1.18g/cm3). In the interpretation the sample is assumed to consist of rock materal and pores (containing PMMA), and therefore ps can be expressed as:

PrePo+(l-E)Pr (6)

where p, is the density of mineral grains. In the practice of bulk measurements the average density of the rock sample is used. Using Eqs.(5) and (6) in Eq.(4), the porosity and the activity relationship can be solved:

JPt

e=- Po N Ca Cao 1+(is-1 Po 'AO (7)

The porosity of sample by each individual pixel n from the autoradiograph are calculated according to the equations (3) and (7). The porosity histogram gives the relative frequency of regions of individual porosities. The total porosity is obtained from the porosity distribution by taking the weighted average:

Y,Arean% n 6tot IlArean n (8)

where Arean is the area of pixel n, and en is the local porosity of pixel n. 8

The rock is approximated as a homogeneous mixture of minerals and pore space. This assumption is applicable when the pore apertures are below the limit of lateral resolution of autoradiography, then the major fraction of the emitted beta radiation is attenuated by silicate minerals, and the quantitative determination of porosity from the autoradiographs is possible.

The porosity profile In porosity profile measurement the autoradiographs were scanned using 1-2 mm steps across the fissure or next to the fissure or fracture. The width of the measuring frame was convenient in relation to the grain size, corresponding to the heterogeneity, of the rock matrix studied. Even more narrow steps could be used, but this led to unnecessary scatter of the results. Intensities of each frame were calculated and the porosities were presented as a function of distance. 9 4. RESULTS AND DISCUSSION

4.1 Porosity profiles

Porosity profile measurements were made next to the fissure, while avoiding inclusion of the fissure filling into the calculation. Attempts to classify the profiles according to a simplified model (see Section 4.2) were made based on these profile calculations. Most of the profiles illustrated in Figs. 2-10 were scanned across the whole fissure including the fissure filling; in those calculations the porosity of the filling material could be analyzed. The photographs and the corresponding autoradiographs of sawn surfaces are presented in Appendices 1 - 10.

Sample SY7 160 m

The porosity profile of sample SY7 160 m and the corresponding measured area on the autoradiograph are shown in Fig. 2. This is one of measured profiles, drawn on the autoradiograph shown in Appendix 1.

Sample SY7 160 m consisted mainly of moderately altered fine grained, gray mica gneiss, although medium grained tonalite occurred as a cross-cutting vein. In this sample the fissures were filled and the altered zones next to the fissures were several mm wide, as was evident from the sericitization of plagioclase extending for up to a distance of 5 - 25 mm from the fissure wall. The sericitization of plagioclase grains was also consistent with the measured zone of increased porosity. The porosity was found to be 0.3 - 0.4 % in unaltered regions. The thicknesses of fissure fillings varied from 1 mm to 5 mm and the porosities of the fissure fillings were from 2.0 to 3.0 %. The zone of increased porosity next to the fissures had a rather uniform porosity within the range 0.5 - 0.8 %. 10

distance (mm)

Figure 2. Porosity profile of SY7 160 m (left) and measured area on autoradiograph (right).

Sample SY7 161 m

The porosity profiles of sample SY7 161 m and the measured areas on the autoradiograph are shown in Figs. 3 and 4. These are two of the measured profiles, drawn on the autoradiograph shown in Appendix 2.

The sample SY7 161 m consisted of two rock types, namely tonalite and mica gneiss. The contact between these two rock types is very sharp and tight. Very distinct filled fissures with alteration zones were found to cut both the tonalite and mica gneiss. Fissures several mm wide were filled with zeolites (laumontite, leonhardite) (Lindberg and Siitari-Kauppi 1998). The background porosities were measured in the unaltered matrix of tonalite and mica gneiss as 0.13 % and 0.07 %, respectively. Grain boundary porosity was dominant in the tonalite, but more even porosity distribution was found in mica gneiss due to intraganular porosity present within biotite grains. The increased porosity was observed at distances of up to 3 - 6 mm from the fissure filling in mica gneiss and at distances of 1 - 3 mm from the fissure filling in tonalite. The overall porosity in this alteration zone was determined as 0.4 to 0.5 %. In tonalite the zone of increased porosity next to the fissure was more abrupt than in the mica gneiss, due to the coarser matrix. The plagioclase grains were sericitized to distances of 3 - 6 mm in the mica gneiss part of the sample and was concistent with the observed increased porosity zone. The alteration of plagioclase was also observed in tonalite part of the sample, but without any concomitant increase in porosity. 11 The porosities of fissure fillings were determined, being around 2 - 4 %. However there was one reddish fissure in the tonalite matrix having a porosity at background level.

Figure 3. Porosity profile of SY7 161m, tonalite (left) and measured area on autoradiograph (right).

£ 1-5

distance (mm)

Figure 4. Porosity profile of SY7 161m, mica gneiss (left) and measured area on autoradiograph (right).

Sample SY7 179 m

The porosity profile of sample SY7 179 m and the measured area on the autoradiograph are shown in Fig. 5. This is one of the measured profiles drawn on the autoradiograph shown in Appendix 3.

Sample SY7 179 m was of medium grained, moderately altered tonalite. Plagioclase 12 showed weak sericitization throughout the sample, but approaching the fissure, biotite becomes extensively chloritized. Several zeolite filled fissures cut the core sample. A network of grain boundary pores was found. Numerous of porous, probably chlorite, mineral phases having porosities over 1 % and diameters less than 1 mm were observed in the matrix. The background porosity, which here represents moderately altered tonalite was 0.2 %. The porosity of fissures was 1 - 3 % and no gradual increase in porosity was detected in proximity to filled fissures. A narrow (< 1 mm) fissure filled by calcite cut across the core at a distance of one cm from the top end of the sample (see Appendix 3). The porosity next to this fissure was even lower than the porosity in intact rock matrix.

1.2-r 1 -•

_ 0.8- sS 0.6 - • a 0.4- 0.2 4 0 -t 5 10 15 20 distance (mm)

Figure 5. Porosity profile of SY7 179 m (left) and measured area on autoradiograph (right).

Sample SY7 202 m

The porosity profile for sample SY7 202 m is shown in Fig. 6. This is one of the measured profiles, drawn on the autoradiograph shown in Appendix 4.

Sample SY7 202 m was a strongly mylonitized and altered mica gneiss with a fine to medium grained texture. Fissures filled by laumontite were observed as well as a wider (> 5 mm) calcite filling. The highest porosity (about 5 %) measured from sample SY7 202 m was from both filling types. Fig. 6 shows a profile across a laumontite fissure; porosity dropped to background levels (1 - 2 %) within a few mm of the fissure wall. 13 Anomalous regions not infiltrated by l4C-MMA were found in the matrix (see Appendix 4, autoradiograph) where it consisted mainly of altered mineral phases.

distance (mm)

Figure 6. Porosity profile of SY7 202 m (left) and measured area on autoradiograph (right).

Sample SY7 205 m

The porosity profile of sample SY7 205 m and the measured area on the autoradiograph are shown in Fig. 7. This is the only measured profile drawn on the autoradiograph shown in Appendix 5.

Sample SY7 205 m was an intermediate volcanite consisting plagioclase, quartz, hornblende and biotite. The fine grained and fairly unaltered matrix between fissures was tight and not amenable to impregnation with methylmethacrylate. The porosity was below the detection limit of the PMMA method. Plagioclase was sericitized only near fissures, up to a distance of 3 - 6 mm from the fissure cemented by laumontite. The porosity in the narrow (1-2 mm) closed fissures was around 5 %. The increased porosity extended to a distance of 3 - 5 mm from the fissure being 0.5 - 0.8 %. 14

Figure 7. Porosity profile of SY7 205 m (left) and measured area on autoradiograph (right).

Sample SY7 225 m

The porosity profile of sample SY7 225 m is shown in Fig. 8. This is only one example of two measured profiles, which are drawn on the autoradiograph shown in Appendix 6.

? £

10 12 Distance (mm)

Figure 8. Porosity profile of SY7 225 m, measured next to fissure filling.

Sample SY7 225 m was a mylonitized mica gneiss/volcanite. Biotite and plagioclase were abundant and sporadic hornblende grains were observed. Sericitization of plagioclase grains occurred within a zone 3-10 mm wide surrounding the relatively narrow (0.5 - 2 mm) fissures. The porosity profile shown in Fig. 8 was determined next to the narrow filled fissure crossing the core in the middle part of the sample (see Appendix 6). The porosity increase extended as far as 3 - 5 mm from the fissure filling, in accord with the observed plagioclase sericitization. The fissures were branched, forming a network of migration 15 pathways. The background porosity was 0.05 %.

Sample SY7 226 m

The porosity profile of sample SY7 226 m is shown in Fig. 9. This is one of two measured profiles drawn on the autoradiograph shown in Appendix 7.

Sample SY7 226 m is a fine grained mica gneiss/volcanite, although it is somewhat more mylonitic than sample 225 m. Numerous tight, filled fissures cut the rock sample. The plagioclase alteration in the rock matrix was found to be more intense than in sample SY7 225 m. The visible alteration zone extended for several mm from the fissures, which is consistent with the measured increased porosity zone. The porosity in fissures was 2 - 5%, while the background matrix was tight. The background porosity was 0.1 %.

if •<

distance (mm)

Figure 9. Porosity profile of SY7 205 m (left) and measured area on autoradiograph (right).

Sample SY7 228 m

Sample SY7 228m was a mylonite, probably derived from a mica gneiss, with abundant fissures and microfractures. The total porosity of this sample was high due to the presence of numerous fissures and porosity profiles were not measured due to the complexity of the structure. The porosity measured from the brecciated area was high, around 2 %.

Sample SY7 242 m

The porosity profile for sample SY7 242 m and the measured area on the autoradiograph are shown in Fig. 10. This is one of the measured profiles, drawn on the autoradiographs 16 shown in Appendices 9 and 10, which comprise two pieces of the sample: SY7 242a m and SY7 242b m. The sample was moderately altered tonalite, mineralogically similar to sample SY7 179 m (described above), containing slightly sericitized plagioclase and partially altered hornblende and biotite in the matrix.

Sample SY7 242b m contains two different kinds of fractures; at the end of the sample (see Appendix 10, right end) there is a permeable open fissure with rich calcite and analcime layers on the surface and a visible alteration zone 2 cm wide. The porosity of the filling material was around 2 to 6 % and in the altered zone around 0.5 to 1.5 %. The porosity in the intact matrix was 0.2 %. The porosity profile of the fracture zone is illustrated in Fig. 10. Another fissure is located 11 cm from the right hand end, cemented by laumontite and surrounded by an alteration zone around 10 mm wide. The increased porosity extended to depths of 3 - 7 mm from laumontite fissure walls. A few narrow fissures were observed, mostly filled by laumontite, but no distinct zone of increased porosity were recorded next to the fissures.

Figure 10. Porosity profile of SY7 242b m (left) and measured area on autoradiograph (right). 17 4.2 Comparison of porosity profiles

Different porosity profiles were compared with each other by means of the simple equation:

e=C2eM+Cx (9) where the parameter C, varies between 0.1 to 1.0 and is the background porosity of the rock matrix and C,+C2 is the highest porosity near the fissure. The parameter X represents the shape of the profile.

The porosity profiles for samples SY7 161 m; mica gneiss, SY7 205 m; intermediate volcanite and SY7 242a m; tonalite, are illustrated by means of Equation 9 in Figs. 11,12 and 13. The calculated fit using the function (9) is shown as a solid line.

lamoda =0.4375 d » 0 11 c2 = 1 074 ePM3 *0 04925

Figure 11. Porosity profile for sample SY7 161 m, mica gneiss, fitted by means of Equation 9.

The X-values are summarized in Table 2. An abrupt decrease was observed in porosity in the intermediate volcanite in sample SY7 205 m. The porosity also decreased steeply in tonalite and moderately altered tonalite samples near laumontite filled fissures, in samples SY7 161 m and SY7 242 m. In mica gneiss samples, SY7 160 m and SY7 161 m, the increase in porosity near fissures filled by analcime or laumontite was gradual. 18

lambda = 1 663 cl = -0.001446 c2 = 5 248 eRMS =0 00526

Figure 12. Porosity profile for sample SY7 205 m, intermediate volcanite, fitted by means of Equation 9.

lambda = 1.3 cl =0 1822 c2 » 2 759 eRMS = 0.0257

Distance |nvr>)

Figure 13. Porosity profile for sample SY7 242a m tonalite, fitted by means of Equation 9. Table 2. Parameters from porosity profile measurements. X is shape of profile and C, is background porosity.

sample X c, SY7160m/ ~0M 0.39 mica gneiss 0.13 0.41 0.63 0.48 SY7 161m/ tonalite 0.94 0.12 0.84 0.14

SY7 161m/ mica 0.44 0.11 gneiss 0.54 0.11 SY7 179m/ 0.93 0.15 tonalite 0.64 0.14

SY7 202m/ " mica gneiss SY7 205m/ 1.66 - intermediate 2.03 volcanite SY7 225m/ mica 1.38 0.05 gneiss - volcanite 1.30 0.06 SY7 226m/ mica 0.70 0.11 gneiss - volcanite

SY7 228m/ " mylonite SY7 242mA/ 1.3* 0.18 tonalite SY7 242mB/ 1.13* 0.15 tonalite 0.82 0.20 1.4 0.13 *open fissure 20 5. CONCLUSIONS

The porosity profile measurement based on autoradiographs made by the 14C-PMMA impregnation method is able to provide valuable qualitative and quantitative information on the thickness of the diffusion accessible pore space of rock samples near water­ conducting fractures and water-bearing fissures. The studied rock samples from Syyry showed that the increased porosity zones near water-bearing fissures in tonalites were narrower than the increased porosity zones in mica gneiss. The porosity profile was consistent with plagioclase sericitization in mica gneiss, but not in tonalite samples. The intermediate volcanite samples showed narrow porosity profiles next to laumontite filled fissures and no correlation with degree of plagioclase sericitization was found. A gradual decrease in porosity was observed in mica gneiss, but an abrupt decrease in porosity was found in tonalite samples. Chloritized biotite was found throughout the rock matrix in moderately altered tonalite, but no influence on increased porosity zones near fissures was noted. No significant differences in porosity profiles between open and filled fissures were observed, although only two porosity profiles were measured next to open fissures. 21

REFERENCES

Anttila, P., Kuivamaki, A., Lindberg, A., Kurimo, M., Paananen, M., Front, K., Pitkanen, P. and Karki, A. 1993. The Geology of the Syyry Area, Summary report. Nuclear Waste Commission of Finnish Power Companies, Report-93- 19, 40p.

Hellmuth, K.H., Siitari-Kauppi, M. and Lindberg, A. 1993. Study of Porosity and Migration Pathways in Crystalline Rock by Impregnation with 14C-polymethylmethacrylate. Journal of Contaminant Hydrology, 13: 403-418.

Hellmuth, K.H., Lukkarinen, S. and Siitari-Kauppi, M. 1994. Rock Matrix Studies with Carbon-14-Polymethylmethacrylate (PMMA); Method Development and Applications. Isotopenpraxis Environ. Health Stud., 30: 47-60.

Karki, A. 1991. Structural Interpretation of Syyry in Sievi. Kivitieto Oy. TVO/Site investigations, Work Report 91-06, 53p, in Finnish.

Lindberg, A. and Paananen, M. 1992. Konginkankaan Kivetyn, Sievin Syyryn ja Eurajoen Olkiluodon kallionaytteiden petrografia, geokemia ja geofysiikka. Kairanreiat KI-KR7, SY- KR7 ja OL-KR6. TVO/Paikkatutkimukset, Tyoraportti 92-34, in Finnish.

Lindberg, A. and Siitari-Kauppi, M. 1998. Shear Zone-Related Hydrothermal Alteration in Proterozoic Rocks in Finland, in print: Water-Rock Interaction 9, New Zealand 1998.

Rasilainen, K., Hellmuth, K-H., Kivekas, L., Melamed, A., Ruskeeniemi, T., Siitari-Kauppi, M., Timonen, J. & Valkiainen, M. 1996. An Interlaboratory Comparison of Methods for Measuring Rock Matrix Porosity. Espoo: VTT Energy, VTTRN 1776.

Salli, I. 1967. Pre-quatemary rocks in Lestijarvi and Reisjarvi Map Sheet Areas. Explanation to the Maps of Pre-quatemary Rocks. Map sheets 2341 and 2343. Geological Map of Finland 1:100000. Espoo, Geological Survey of Finland, 37 p., in Finnish, English Summary. 22 Siitari-Kauppi, M., Lukkarinen, S. and Lindberg, A. 1995. Study of Rock Porosity by Impregnation with Carbon-14-Methylmethacrylate, Nuclear Waste Commission of Finnish Power Companies, Report YJT-95-09. 23

APPENDICES

Appendix 1. Photograph and autoradiograph of SY7 160m. Appendix 2. Photograph and autoradiograph of SY7 161m Appendix 3. Photograph and autoradiograph of SY7 179m Appendix 4. Photograph and autoradiograph of SY7 202m Appendix 5. Photograph and autoradiograph of SY7 205m Appendix 6. Photograph and autoradiograph of SY7 225m Appendix 7. Photograph and autoradiograph of SY7 226m Appendix 8. Photograph and autoradiograph of SY7 228m Appendix 9. Photograph and autoradiograph of SY7 242a m Appendix 10. Photograph and autoradiograph of SY7 242b m Appendix 1

Photographand autoradiograph of sample SY7 160m, natural scale. Appendix 2

Photographand autoradiograph of sample SY7 161 m, natural scale. Appendix 3

Photograph and autoradio graph of sample SY7 179 m, natural scale. Appendix 4

Photograph and autoradiograph of sample SY7 202 m, natural scale Appendix 5

Photograph and autoradiograph of sample S Y7 205 m, natural scale. Appendix 6

Photograph and autoradiograph of sample SY7 225 m, natural scale. Appendix 7

■ ■ VsSj .. m . *5 aw> ■ ■

Photograph and autoradiograph of sample SY7 226 m, natural scale. Appendix 8

Photograph and autoradiograph of sample S Y7 228 m, natural scale. Appendix 9

Photograph and autoradiograph of sample S Y7 242a m, natural scale. Appendix 10

Photograph and autoradiograph of sample SY7 242b m, natural scale.