PHYSICAL PROPERTIES OF CARBONATE MARBLE SAMPLES FROM THE KISKO REGION, SOUTHERN FINLAND. by Liisa Kivekäs

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1 Three different types of Svecokarelian granitoids in southeastern Lapland: magnetic... Geological Survey of Finland, Current Research Edited by Sini Autio. Geological Survey of Finland, Special Paper 27, , PHYSICAL PROPERTIES OF CARBONATE MARBLE SAMPLES FROM THE KISKO REGION, SOUTHERN FINLAND by Geological Survey of Finland, P.O. Box 96, FIN ESPOO, FINLAND Key words (GeoRef Thesaurus, AGI): carbonate rocks, marbles, dolostone, limestone, physical properties, electrical properties, Proterozoic, Kisko, Finland Introduction The geological investigations of the Kisko carbonate marble deposit in southern Finland are in progress (Sarapää et al. 1999), and the main purpose of this study is to reveal petrophysical information on the carbonates from this region. These carbonate marble samples provided also an appropriate possibility to continue tests to clarify the abnormal behaviour of carbonate rocks observed in laboratory saturation and resistivity tests in the spring Dolomite rock samples from the Tervola (northern Finland) region had indicated a strange phenomenon: the resistivity values of the samples increased with the water immersion time (Kivekäs 1999). In this study both dolomite and calcite marble samples from the Kisko region, southern Finland, were tested. Besides the saturation and resistivity experiments, the physical properties: density, porosity, seismic P-wave velocity, magnetic susceptibility, natural remanent magnetization (NRM) and electric resistivity were measured at the petrophysical laboratory of the Geological Survey of Finland (GTK). The results of four dolomite and ten calcite marble samples are presented. Samples Drill core samples (20-30 cm) from different depths of drill hole R354/98 were cut for petrophysical measurements of four dolomite and ten calcite marble samples: length 48 mm, diameter 32 mm and volume 38 cm 3. All the samples are very dense (porosity 0.10 ± 0.03 %). Polished thin sections were prepared from both dolomite and calcite marble samples. The dolomite marble is fine-grained ( mm), and composed of dolomite with some calcite, tremolite and quartz. There are also opaque minerals: pyrrhotite, chalcopyrite and pyrite (Fig. 1.). 1). The calcite marble is coarse-grained (2-5 mm), and in addition to calcite there is some tremolite, diopside and phlogopite (Fig. 2). A clear pyrrhotite filled microcrack is identified in the calcite marble sample from a core of depth m. Measurements Bulk density was determined by weighing samples in air and water (Archimedes principle). The weighing accuracy of the balance used in the computerized Petrophysics laboratory at GTK (Puranen et al. 1993) is 0.01 g. Maximum error of density values for the samples, with porosity 0.1% and volume 38 cm 3, is 1 kg/m 3. Porosity was deduced from the mass difference between the water-saturated and oven-dried (105 C) samples (Rasilainen et al. 1996). The accuracy of the balance used to measure the bulk mass of samples during saturation for mass difference curves was g. Seismic P-wave velocity was measured from water-saturated samples by the ultrasonic pulsetransmission method at atmospheric pressure and room temperature. Travel time was determined using two identical ultrasonic (66 khz) P-wave transducers as transmitter and receiver. 151

Fig. 1. Fine-grained dolomite marble R354/98-19.10m; from the Kisko region. Opaque minerals are mainly pyrrhotite.

Coarse-grained calcite marble R354/98-35.50m; from the Kisko region.

2 Fig. 1. Fine-grained dolomite marble R354/ m; from the Kisko region. Opaque minerals are mainly pyrrhotite. Transmitted light, crossed polarizers, field of view is 4mm wide. Photomicrograph by H. Appelqvist. Fig. 2. Coarse-grained calcite marble R354/ m; from the Kisko region. Small fibrous or platy crystals on the grain boundaries of calcite are mainly tremolite. Transmitted light, crossed polarizers, field of view is 4mm wide. Photomicrograph by H. Appelqvist. 152

3 Physical properties of carbonate marble samples from the Kisko region, southern Finland Magnetic susceptibility was measured with an AC bridge apparatus at low frequency (1025 Hz). For isotropic, weakly magnetic (susceptibility < 1000 µsi) samples the standard error of repeated measurements stays generally below 20 µsi (Puranen et al. 1993). For strongly magnetic and anisotropic samples the results measured in different directions can vary by more than 20 %. The values reported here were measured in the direction of the core axis. The intensity of natural remanent magnetization (NRM) was determined with a fluxgate magnetometer inside magnetic shielding. The standard error of repeated remanence measurements is about 10 ma/m. Highly magnetic samples may show variations of more than 10 %. Before measurements Before of electric measurements resistivity, of the electric samples resistivity, are saturated the samples in tap water are saturated for least in tap 2 days, water usually for at for least 3-25 days, days. usually Resistivities for 3 - were 5 days. determined Resistivities at the were frequencies determined of at 10 the and frequencies 500 Hz using of a 10 two-electrode and 500 Hz system using a with two-electrode wet electrodes. system If the with saturating wet electrodes. and If the drying saturating of its surface of a sample before and measure- drying of a sample ment of its are surface repeated before as similarly measurement possible, are repeated then variation as similarly of results as possible, can be then kept variation below 10 of %. results Four dolomite can be kept and below ten calcite 10 %. marble Four samples dolomite from and the ten Kisko calcite carbonate marble samples deposit were from measured the Kisko using carbonate deposit and were results measured are presented using these in this methods paper these methods (Table and results 1). are presented in this paper (Table 1). Porosity, density and P-wave velocity The porosity determination of calcite marble samples was difficult because the samples started to dissolve in water. The samples were saturated with water for four days. The saturation curves (Fig.3) were necessary for porosity estimation. Porosities are calculated from the highest measured mass value of each sample during water saturation. This point was reached one to four hours after the samples were placed in water. Since that time the dissolving mass of calcite samples was bigger than the amount of water the samples could absorb. The time and quantity of full saturation was impossible to determine exactly, but full saturation was probably reached near the lowest resistivity point (between 2-10 hours, see Fig. 4). The calculated porosities are thus under-estimated. The dissolution of dolomite marbles was considerably weaker. The porosity estimates of the Kisko carbonates, for dolomite marble 0.12 ± 0.06 % and calcite marble 0.09 ± 0.02 %, are so low that their grain density, wet bulk density and dry bulk density are very close to each other (Kivekäs 1993), and only the dry bulk density is presented (Table 1). Density values of the dolomite marble samples 2872 ± 7 kg/m 3 and calcite marble samples 2733 ± 6 kg/m 3 are typical for Table 1. Petrophysical properties of carbonate marbles from the Kisko region, southern Finland, drill hole R354/98: porosity [P], dry bulk density [D d ], P-wave velocity [V P ], magnetic susceptibility [K], natural remanent magnetization [NRM] and electric resistivity [R 10Hz ]. Resistivity data are based on measurements after three days saturation in tap water [R w = 55 Ωm], * marked samples in deionised water [R w =120 Ωm]. See text and Figures 4 and 7 for details. Sample Volume Carbonate P D d V P K NRM R 10Hz depth marbles m cm 3 % kg/m 3 m/s µsi ma/m Ωm Dolomite* Dolomite Dolomite* Dolomite Calcite Calcite Calcite Calcite* Calcite Calcite* Calcite Calcite Calcite Calcite Calcite Mean N = 4 Dolomite St Dev Mean N = 10 Calcite St Dev

4 Fig. 3. Mass difference (saturation and dissolving) of Kisko carbonate samples during water saturation. Blue curves = dolomite marble samples from the core depths of m Red curves = calcite marble samples from the core depths of m Open symbols = samples were saturated in deionised water Solid symbols = samples were saturated in tap water Fig. 4. Resistivity at frequency 10 Hz of Kisko carbonate samples versus saturation time. Blue curves = dolomite marble samples from the core depths of m Red curves = calcite marble samples from the core depths of m Open symbols = samples were saturated in deionised water Solid symbols = samples were saturated in tap water. 154

5 Physical properties of carbonate marble samples from the Kisko region, southern Finland Fig. 5. P-wave velocity - density plot of dolomite (blue circles) and calcite (red squares) marbles from the Kisko region. dolomites and calcites. The low values of standard deviations indicate homogeneity. Seismic P-wave velocity of dolomite marble samples is 6800 ± 60 m/s and calcite marble samples 6270 ± 100 m/s. These values correlate positively with density values (Fig. 5). Magnetic properties The magnetic characters of dolomite and calcite marbles differ strongly from each other. In the susceptibility - density plot (Fig. 6) dolomite and calcite marble samples form clearly their own Fig. 6. Density Susceptibility - susceptibility - density diagram plot of of dolomite (blue circles) and calcite (red squares) marbles from the Kisko region. 155

6 groups, which reflect differences in the amount of magnetic minerals. The dolomite marble samples are ferrimagnetic, the susceptibility is ± µsi and the intensity of natural remanent magnetization (NRM) is 950 ± 310 ma/m. The standard deviations are quite large due to the variation of pyrrhotite content. Pyrrhotite is visible in the thin sections of dolomite marble (Fig. 2). The magnetic parameters of the calcite marble samples show much lower values, the susceptibility is 100 ± 130 µsi and the remanent magnetization is 210 ± 60 ma/m. Especially the remanent magnetization suggests that some ferrimagnetic accessories are present. Pyrrhotite is visible in a filled microcrack of the calcite marble sample (56.35), which shows the highest susceptibility value and remanent magnetization of calcite marble samples (Table 1). Electric resistivity The resistivity of four dolomite and four calcite marble samples from the depths of m and m were first monitored during their saturation (Figs.3 and 4) in an attempt to explain the behaviour of water-carbonate interactions. Half of the samples were saturated in tap water and the other half in deionised water. There was 1.25 dm 3 of water in both cases and the combined total volume of the four samples was 152 cm 3. The changes in the properties of saturating waters were recorded. The ph value of deionised water increased in two hours from 5.44 to 8.69 and then remained at a level of 8.16 ± No significant changes were observed in the tap water during the four days of saturation and the ph value was all the time 8.02 ± The temperature of the water was 22.5 ± 0.6 C in both cases. The resistivity of the deionised water decreased during the saturation process from 8550 to 83 Ωm (Fig. 7) and the resistivity of tap water from 66 to 51 Ωm. The use of tap water is thus the best way to saturate samples for resistivity measurements. Any significant differences between the resistivity results of the samples depending on the two different saturation conditions were not noticed, although the resistivities of saturating waters differed strongly from each other (Fig.7). Calcite marble samples dissolved in deionised water slightly more strongly than in tap water (Fig. 3). At first the deionised water was acid (ph = 5.44), but already after one hour of soaking samples the ph-value increased to Any calculations or analyses of anions and cations of dissolved salts have not been done yet. Ions in aqueous solution are affected by the presence of each other. Even apparently simple processes, such as the rate at which calcite (CaCO 3 ) dissolves, have recently been shown to be highly complicated (Bland & Rolls 1998). Fig. 7. Resistivity of saturating waters versus saturation time of carbonate marble samples. 156

7 Physical properties of carbonate marble samples from the Kisko region, southern Finland Table 2. Porosity estimates [P] and minimum resistivity values [R 10HZ ] during water saturation of calcite marble samples from the Kisko region, southern Finland, drill hole R354/98. Samples and were saturated in deionised water (R w = 945Ωm), others in tap water (R w = 64Ωm). See text and Figures 4 and 7 for details. Depth P R 10Hz m % Ωm Mean St Dev The resistivity values at the frequency of 10 Hz in Table 1 have been taken from the data measured after three days of saturation, which is the normal saturation time in routine measurements at GTK. For dolomite samples these resistivity values are correct, but for calcite marble samples the situation is complicated due to their abnormal behaviour during saturation (Fig. 4). Their resistivity values are doubled in three days of saturation from the minimum values (Table 2) between two and ten hours of saturation. The matrix of dolomite and calcite rocks is practically an insulator having resistivity of about Ω m. The conductivity of samples depends thus on the electrical properties of pore water in the interconnected pores, microcracks and at mineral boundaries. Anions and cations can also contribute to the conductivity of rocks by electrochemical interaction with the solid matrix at the fluid-solid interface. The interface acts both as a conductor and capacitor (Schön 1996). Although the porosity range of calcite marble samples is very low and narrow ( %), the cross-correlation plot between the porosity estimates and the lowest measured resistivities (Table 2) indicates a good correlation with the coefficient r = (Fig. 8). The included Archie s equation Fig. 8. Relationship between resistivity and porosity of calcite marble samples. A least-squares fit of Archie s law R = a R Ω -m W Ø -m is is included, where a and m are empirical coefficients, R W Ω = resistivity of pore water, Ø = porosity as a fraction and r = coefficient of correlation. The formula of the fitted curve is based on the values of samples saturated in tap water (R Ω W =64Ωm). The open symbols are values of samples saturated in deionised water (R W Ω =945Ωm), if this resistivity value of saturating water were applied to formula, the points of samples and were out of this figure (R=569000Ωm and Ωm). See text, Table 2 and Figures 4 and 7 for details. 157

8 is relevant only for the rocks with pure electrolytic conductivity. The conditions of pores and electrolytes in the voids were probably changing during the saturation process by the dissolving of calcite, precipitation and recrystallization. Discussion and conclusions The samples of this study are from only one drill hole (R354/98) of the Kisko carbonate marble deposit, and dolomite marble samples from only a very narrow depth interval ( m). So the number of samples is quite small for general conclusions of the physical parameters of this region. The magnetic characters of the studied dolomite and calcite marbles differ strongly from each other. In the susceptibility - density plot (Fig. 6) dolomite and calcite marble samples clearly form their own groups, which reflect differences in the amount of magnetic minerals. The dolomite samples contain visible amounts of pyrrhotite (Fig. 2). Both dolomite and calcite marbles from the drill hole are very dense, porosities are low (0.10 ± 0.03 %) and electric resistivities high (dolomite marble samples ± Ωm and calcite marble samples ± Ωm). The density ( 2872 ± 7 kg/m 3 and 2733 ± 6 kg/m 3 ) and seismic P-wave velocity ( 6800 ± 60 m/s and 6270 ± 100 m/s) values are at the upper level of typical values for dolomites and calcites. In the saturation and resistivity tests (Fig. 4) the dolomite marble samples of the Kisko region behaved normally, as their resistivity decreased by water saturation and stabilised at the resistivity level of full saturation. By contrast, the resistivity values of calcite marble samples started to increase during saturation and the values were doubled or tripled in about four days. In the repeated tests the dolomite samples gave the same results. The calcite samples, instead, did not revert to their former resistivity level but increased and exceeded the measuring range of our equipment (maximum measurable values are Ωm depending on the dimensions of samples). Studies to verify and interpret this abnormal behaviour of carbonate rocks is underway. Suggestions of changes in electrolytes, their paths on mineral boundaries, chemical reactions and carbonate precipitation is under investigation. A key to the solution might be discovered from Davis and Kent s (1990) description of carbonate minerals in aqueous processes. They explain that cation uptake occurs in two steps. A relatively rapid initial step, which reaches completion within one day, is followed by a slow step, whereby the uptake rate appears to be constant over a long period of time (at least several days). The rapid step results from sorption onto a hydrated CaCO 3 layer. The slow step can be interpreted as resulting from recrystallization. A probable explanation for the abnormal behaviour of calcite samples in these laboratory saturation and resistivity tests might be that the paths of electrolytes and electric current can be obstructed. Cooperation between geophysicists, chemists and geologists is necessary to fully understand the physico-chemical conditions in water-carbonate interactions. Acknowledgments Jukka Reinikainen is acknowledged for providing the samples and their petrographical descriptions, Hannu Appelqvist for photographing the thin sections and descriptions of opaque minerals and Satu Vuoriainen for laboratory measurements and assistance. References Bland, W. & Rolls, D Weathering, An introduction to the scientific principles. London: Arnold, 271 p. Davis, J. & Kent, D Surface complexation modeling in aqueous geochemistry. In: Hochella, M.F.,Jr. & White, A.F. (eds.) Mineral-Water Interface Geochemistry. Mineralogical Society of America, Rev. in Mineralogy, Volume 23, Kivekäs, L Density and porosity measurements at the petrophysical laboratory of the Geological Survey of Finland. In: Autio, S. (ed.) Current Research , Geological Survey of Finland, Special Paper 18, Kivekäs, L Abnormal behaviour of carbonate rocks in laboratory saturation and resistivity tests. In: Extended abstract book, EAGE 61st Conference and Technical Exhibition, Helsinki, Finland 7-11 June The Netherlands: European Association of Geoscientists & Engineers p. Puranen, R., Sulkanen,K., Poikonen, A., Nissinen, R., Simelius, P. & Harinen, L User s manual for a computerized petrophysics laboratory. Geological Survey of Finland, upublished report Q19.1/27/93/1, 50 p. Rasilainen, K., Hellmuth, K-H., Kivekäs, L., Melamed, A., Ruskeeniemi, T., Siitari-Kauppi, M., Timonen, J. & Valkiainen, M An interlaboratory comparison of methods for measuring rock matrix porosity. VTT, Espoo, VTT Research Notes 1776, 16 p.+ app. 26 p. Sarapää, O., Kärkkäinen, N., Reinikainen, J., Ahtola, T., Appelqvist, H. & Seppänen, H New results from calcite and ilmenite exploration in Finland. In: Autio, S. (ed.) Current Research , Geological Survey of Finland, Special Paper 27, Schön, J.H Physical properties of rocks: fundamentals and principles of petrophysics. In: Helbig, K. & Treitel, S. (eds.) Handbook of geophysical exploration, Seismic exploration, Volume 18. Pergamon. 583 p. 158

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