Terrestrial radioisotopes in black shale hosted Mn-carbonate deposit (Úrkút, Hungary)

Acta Geophysica vol. 61, No. 4, Aug. 2013, pp. 831-847

DOI: 10.2478/s11600-013-0124-2

________________________________________________ © 2013 Institute of Geophysics, Polish Academy of Sciences

Terrestrial Radioisotopes in Black Shale Hosted Mn-Carbonate Deposit (Úrkút, Hungary)

Tamás VIGH1, Tibor KOVÁCS2, János SOMLAI2, Norbert KÁVÁSI3, Márta POLGÁRI4, and Lóránt BÍRÓ5

1Mangan Mining and Processing Ltd., Úrkút, Hungary e-mail: [email protected]

2Institute of Radiochemistry and Radioecology, University of Pannonia, Veszprém, Hungary

3National Institute for Radiological Sciences, Chiba, Japan 4Research Center for Astronomy and Geosciences, Institute for Geology and Geochemistry, Hungarian Academy of Sciences, Budapest, Hungary

5Department of Mineralogy and Petrology, University of Szeged, Szeged, Hungary

A b s t r a c t

Previously, little attention has been paid to terrestrial radioisotopes (U, Th, 40K) occurring in manganese ores, despite the fact that the bio-geochemical relationship between Mn and U is versatile. Occurrence of terrestrial radioisotopes in great amounts during mining on a long-term causes significant radiation exposure. It is important to inspect black shale-hosted manganese ores from this aspect, as black shales are typi-cally potential U-rich formations. Despite the increased radon concentra-tion in the mine, based on the detailed major elements, trace elements and gamma spectroscopy inspection of the rock types of deposit, the U, Th enrichment was undetectable. However, the U and Th content of about average terrestrial abundance of the great ore amount may be in the background of the increased radon concentration level. This Mn-carbon-ate ore deposit in spite of the low U content exhibit potential radon dan-ger for miners, which can be eliminated with intensive air change only.

Key words: manganese; black shale; trace elements; radioactivity; ura-nium.

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1. INTRODUCTION Terrestrial radioisotopes (U, Th isotopes, and 40K) occurring in manganese ores as trace elements were described by Grasselly and Pantó (1988), Roy (1988), and Laznicka (1992), or as isotopes by Segev (1992) and Abel-Ghany (2010). The occurrence of terrestrial radioisotopes in great amounts during mining on a long-term cause significant radiation (radon) exposure (Darby et al. 1995, Tirmarche et al. 1993, Cote and Townsend 1981). It is especially important to investigate black shale-hosted manganese ores from this aspect as black shales are typically potential U-rich formations.

Underground exploitation of Jurassic black shale-hosted Mn-carbonate deposit (Úrkút, Hungary) showed radon danger for miners according to de-tailed radon concentration measurements in the mine. The radon (222Rn) con-tent (originating from the decay chain of 238U) in the mine air has continu-ously been inspected since 2002. The initial results have already shown that during the operation of the air change system (mine ventilation), the average radon concentration of the mine air stays around 600 Bq m–3, while during the time of out of the ventilation system (after working hours) it rises to 2000-3000 Bq m–3, and in caverns isolated from the airing for a longer period it may even reach extreme values (over 10 000 Bq m–3). Similarly, the con-centration of the thoron gas (220Rn) generated during the decay of 232Th is typically 200-300 Bq m–3, which may even be higher than 1000 Bq m–3 (Kávási et al. 2007, 2009). This phenomenon – beyond the importance of work protection measures concerning ventilation – drew attention to the im-portance of researching the source rock of radon (i.e., U) and thoron (i.e., Th) as well.

From the terrestrial radioisotopes, as considerable radiation sources (in-fluencing the living organisms), the isotopes of decay chain of uranium (238U) and thorium (232Th) are important. The 40K are used to be taken into consideration as gamma radiation source due to the relatively high terrestrial abundance of potassium. It is generally known that in some geo-phases (magmatic, metamorphic, sedimentary), the concentration levels of K, U, and Th show typical distribution. In a sedimentary environment, besides the primary nature of the eroding surface, the relief relations of the generated deposit (in case of placers), its physical structure, aquiferous capability, Eh-pH relations, its adsorption capacity (in case of caustobiolithes), the molecu-lar structure of the mineralization and the radii of the participant nuclides de-termine the possibilities of U, Th enrichment. It is a global experience, con-sidered generally, that:

with the increase of the Corg-content of the sediment, the U, Th concentra-tion level usually increases, while the K-content decreases;

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in phosphatic sediments and in sediments containing phosphorus, the U, Th content is anomalously high;

the natural radioisotope concentration of clay deposits is typically higher than that of the carbonate or siliceous deposits. The above observations are widely used during the research of uranium

ore, and during the initial (theoretical) delimitation of potential resources. The anomalous U, Th content in organic deposits (caustobiolithes) is

a general phenomenon. Black shale is also a significant U-carrier rock (Wignall 1994). The “Red Book” of OECD of the year 2007 analyzing the U-market identifies lignite and black shales as unconventional U resources (OECD/NEA-IAEA 2008). The U content of black shale occurs clearly bind-ing to organic material, its concentration changes depending on the organic material content, but it may also reach 400 ppm (e.g., Ranstad, Sweden). Re-search work is being carried out for surveying the U resources bound to black shale and their economic exploitation (e.g., USA, Cuney and Kyser 2008).

The global average of the U content of sedimentary rocks with high phosphorus content (phosphorites, phosphatites, apatite-hosted, etc.) varies between 50 and 200 ppm, and it may reach even 600 ppm depending on the grade of organic matter contribution (Cuney and Kyser 2008).

The cause of the U anomaly in apatite-bearing sediments is often ob-served relating to fossil bone material. Apatite grains found in marine sedi-ments are typically small fish-teeth and pieces of fish-bone (Polgári et al. 2003). Extraordinary U concentration of 3210 ppm was measured in fossil bone materials (Trueman and Tuross 2002). Despite the point-like nature of apatite-rich fish remnants, they may cause an increase in the radiation expo-sure in the closed vicinity (mine).

The anomalies of the natural radioisotope concentration levels of clay-bearing rock are in relation with the high K content, mainly typical of clay minerals, but U, Th accumulation may also occur in clays according to the sedimentation mechanism of the rock (e.g., minerals from magmatic rocks, volcanic glass, organic material may even bear U and Th-isotopes besides a high K content; Breitner et al. 2008, Cuney and Kyser 2008, Kelly 2010).

Syngenetic palaeoenvironmental conditions of the black shale-hosted Mn-carbonate deposit were oxidative supported by microbial Mn(II) oxida-tion as main process of sequestration of metal from ore forming fluids, and Corg-rich character is the result of high productivity and high accumulation rate (Polgári et al. 2012). The oxidative palaeoenvironment is not favorable for primary U accumulation, as biogeochemical properties of U and Mn in geological processes are bound, in U reducing environment the element cu-mulates in U(IV) form. In appropriate situation this form can be oxidized to

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U(VI). In metallurgical processes, agents containing Fe or Mn are used as oxidizers (e.g., MnO2, KMnO4). Oxidation of U can be catalyzed by micro-organisms, as heterotrophic and autotrophic organisms (e.g., Thiobacillus ferrooxidans and T. thiooxidans). This effect has been used by uranium ore mining for a long time. During the direct oxidation of U(IV) in acidic medi-um, U is oxidized via microbially mediated processes (T. ferroroxidans), through intermediate U(VI) and uranyl ion (UO2+) is generated. Energy re-leased during the reaction is utilized by the organism, while the generated uranyl ion is water soluble, mobile (Szabó 1989).

Under marine, oxidative conditions (neutral or slightly alkaline medium), uranyl forms a stable carbonate complex, while in anoxic (acidic – pH < 7) seawater it segregates as a fluoride complex or binds to humic acidic organic material. The latter causes the positive correlation between the sediments rich in organic materials and U. The investigation of recent anoxic phenom-ena shows that non-humic acidic organic sediments are also capable of bind-ing U, and its degree increases with increasing intensity, period of time, and covering rate of anoxia (Wignall 1994).

The aim of the study was the determination of potential radon source in the black shale-hosted Mn carbonate deposit via main and trace element dis-tribution, geochemical characterisation, and alpha and gamma spectrometry measurements.

2. GEOLOGICAL BACKGROUND The Jurassic (Upper Lias) black shale-hosted Mn-carbonate deposit (Úrkút Manganese Formation – UMF) is a huge deposit with more than 100 Mt of ore reservoir (geological reservoir before erosion could reach 300 Mt). Char-acteristics of the deposit and ore types are summarized in Fig. 1, and by Pol-gári et al. (2012).

3. MATERIALS AND METHODS 3.1 Sampling In the frame of the investigation, the early diagenetic Mn carbonate ore types from shaft No. III area (50 samples, green-, brown-black-, grey-ore types from main bed; green-grey-ore type from bed No. 2), black shale (claystone, 31 samples), primary cherty Fe-rich Mn oxide ore (21 samples, Csárda-hegy), contact Mn oxide ore (5 samples, shaft No. III), and secondary Mn oxide ore types and black shale (57 samples, shaft No. III and Kislőd area) were studied. The locations of sampling are given in Fig. 1.

Main- and trace element determinations were held (wet chemical anal-yses, atomic absorption spectroscopy, instrumental neutron activation anal-yses, and inductively coupled plasma) as routinely used methods (Polgári et


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Fig. 1. Locality map (A), geological sketch map (B), general section (C) of the Mn carbonate ore bed (Úrkút Mine, Shaft No. III, +180 m), and ore type series (D).

al. 2000, 2012). Chemical data base was set up, according to which the char-acteristics of the different ore types and locality distributions could be inves-tigated.

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The collection of the series of samples representing the series of layers of the UMF for gamma spectroscopic investigation was done in 2007. The se-ries of samples cover the manganese carbonate ore beds (main bed, bed No. 2), footwall limestone and black shale, and embedded black shale for-mations (between the two ore beds); one sample represents the contact Mn-oxide ore occurring in the contact zone of footwall black shale and the man-ganese carbonate main ore bed.

3.2 Measurement of uranium Microwave digestion method and anion-exchange resin process with ura-nium tracer were applied for sample preparation. The purified uranium frac-tions were electro-deposited onto stainless steel plates. The activity of uranium of these sample plates was measured from 40 000 to 80 000 seconds in vacuum with an Oxford Tennelec alpha chamber, Eurisys 19 keV resolu-tion PIPS detector, Silena-9302 multi-channel analyzer, and EMCA data col-lector software.

3.3 Measurement of thorium and potassium The samples were dried and stored for 30 days in airtight aluminum 600 cm3 Marinelli beakers, to reach the radioactive equilibrium between 228Th and 208Tl.

The activity of potassium (40K) was determined by the 1461 keV gamma rays, the thorium (232Th) by the 911 keV gamma rays of 228Ac, and the 2614 keV gamma rays of 208Tl.

Eurisys EGNC 20-190-R n-type HPGe detector with an efficiency of 20% and an energy resolution of 1.8 keV at 1332.5 keV was used in this measurement.

The gamma-spectrums were recorded by a Tennelec PCA-MR 8192-channel analyzer. The data collection time varied from 60 000 to 80 000 se-conds. The system was calibrated using an etalon certified by the Hungarian National Authority of Measures.

4. RESULTS AND DISCUSSION 4.1 Elemental distribution in the UMF (Mn-carbonate ore types

and black shale) Archive data according to ore types and accompanying black shale are sum-marized in Table 1 (Polgári et al. 2000, 2012). The values of average terres-trial abundance (Mason 1958) are also given, and for a better transparency the trends of difference from the terrestrial abundance are also indicated, so archived data in Table 1 give an average trend.


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Table 1 Average main- and trace-element content of different Mn carbonate ore types

and black shale (number of samples in brackets)

Explanations: N – no difference from clark value, bs – black shale.

Data in Table 1 are considered as average values based on a limited number of samples, from which there may be significant local differences. For example, in case of Mn the extremities of the series of samples listing all

Rock type →

element [wt. %] ↓

Main bedbrown-black

rodochrolite

Main bedgreen

rodochrolite

Bed No. 2grey

rodochrolite

Black shale

Average terrestrial abundance

[wt. %, ppm]

Trend

Si 8.66 (15) 9.79 (14) 8.21 (10) 21.2 (14) 27.7 ↓ Al 1.93 (15) 1.24 (14) 1.55 (10) 4.14 (14) 8.1 ↓ Fe 8.88 (15) 7.94 (14) 13.8 (10) 5.96 (14) 5 ↑ Mn 22.9 (15) 19.4 (14) 12.8 (10) 3.12 (14) 0.1 ↑↑ Ca 2.70 (15) 1.71 (14) 7.91 (10) 5.25 (14) 3.6 ↓↑ Mg 2.73 (15) 1.25 (14) 1.76 (10) 1.34 (14) 2 N K 1.05 (15) 1.77 (14) 0.73 (10) 1.26 (14) 2.6 ↓ Na 0.21 (15) 0.17 (14) 0.23 (10) 0.44 (14) 2.8 ↓ P 0.25 (15) 0.20 (14) 1.18 (10) 0.18 (14) 0.12 ↑ CO2 17.5 (12) 15.8 (14) 22.2 (4) 6.87 (13) – – Ba ↓ [ppm] 473 (15) 170 (14) 263 (10) 335 (16) 400 N Sr 131 (15) 93 (14) 608 (10) 265 (16) 450 ↓↑ Co 269 (15) 217 (14) 141 (10) 131 (16) 23 ↑↑ Cu 37 (9) 33 (10) 32 (9) 100 (15) 45 N, bs ↑ Ni 44 (15) 24 (8) 28 (10) 58 (16) 80 ↓ Zn 52 (15) 65 (14) 36 (10) 88 (16) 65 N Pb 34 (15) 29 (14) 23 (10) 21 (16) 15 ↑ Sc 4 (14) 4 (13) 4 (8) 10 (14) 5 N, bs ↑ Cr 2 (14) 22 (13) 29 (10) 74 (14) 200 ↓ As 48 (13) 11 (11) 63 (10) 53 (14) 2 ↑↑ Hf 1.2 (11) 1.1 (13) 1.0 (4) 2.5 (13) 5 ↓ Th 3.3 (11) 2.8 (13) 3.5 (4) 8.8 (14) 8-12 ↓, bs N U 0.83 (11) 0.86 (13) 1.22 (4) 1.94 (14) 2-3 ↓, bs N Rb 42 (8) 77 (11) 33 (2) 79 (11) 120 ↓ La 46 (14) 47 (13) 46 (10) 39 (14) 18 ↑ Ce 153 (14) 143 (13) 119 (10) 145 (14) 46 ↑ V 45 (3) – 60 (6) 101 (1) 110 ↓, bs N Mo 3 (3) – 4 (6) 4 (1) 1 ↑ Corg [wt. %] 0 – 0 2.94 (6) – –

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types of Mn-carbonate ores of the main deposit analyzed by wet chemical analysis for production purposes are varied between 13 wt. % and 32 wt. % (the average Mn content of the mine production was 26.3 wt. % in 2008).

The average values of U, Th, and K-content, as the source of the 40K radioisotope, are highlighted in grey in Table 1. The elements of terrestrial natural radio nuclides do not show any anomalous enrichment at all; on average, they show a decrease. The U, Th concentration levels in the case of black shale reach the average terrestrial abundance. However, black shale (bs) shows relative enrichment (with respect to main bed rodochrolite, main bed green rodochrolite, and bed No. 2 grey rodochrolite) in Si, Al, K (however, leaving main bed green rodochrolite), Th and U; but not in Fe, Mn, P, and CO2. Th is 8.8 ppm, U is 1.94 ppm, and K is 1.26 wt. % in black shale, and these values are close to average terrestrial abundance. From this perspective, black shale’ samples (less enriched in Mn and Fe more in U, Th, and K) get attention for checking radioactive minerals content in them in de-tail. Archived data given in Table 1 did not have 226Ra, which is essential for radon hazard study as uranium behaves like an open system (open means migration).

As a next step, the distribution of natural radioisotope occurrence for UMF Mn-carbonate ore types and black shale were studied in more detail according to different Mn-carbonate ore types (Table 2, Fig. 2). The U and Th histograms show logarithmic distribution, K histograms show normal (Gauss) distribution with 2 modes (more effects influence the distribution).

The K content of the green Mn-carbonate ore – knowing the composition of celadonite – is not unexpected, but U and Th do not show any enrichment bound to this type of formation.

Table 2 Average K, Th, U concentrations of different Mn-carbonate ore types

and black shale host (number of samples in brackets)

Rock type K [wt. %]

Th [ppm]

U [ppm]

Green and grey Mn-carbonate ore, bed No. 2 (4) 1.10 3.4 1.4 Grey Mn-carbonate ore, main bed (8) 0.58 2.2 1.5 Green, celadonite-bearing Mn-carbonate ore, main bed (12)

2.29 1.8 0.6

Brown and black Mn-carbonate ore, main bed (11) 1.03 3.2 0.8 Green-grey Mn-carbonate ore, main bed (7) 1.73 3.9 1.3 Clay, clayey marl (8) 1.21 8.4 1.9


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(a)

(b)

(c)

Fig. 2. Histograms of Mn-carbonate sam-ples: (a) U content, (b) Th content, and (c) K content.

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Based on Table 2 it can be stated that there is no considerable Th or U anomaly within the series of Mn-carbonate ores, and in the black shale it is appropriate to the average crustal abundance. K in the green type of ore is higher.

The identification of the green clay mineral coloring, the laminae of the Mn-carbonate ore proved to be an important issue from ore genetic aspects; therefore, detailed investigations have been carried out from this phase (Kaeding et al. 1983, Varentsov et al. 1988, Polgári 1993, Polgári et al. 2000). The investigations proved local syngenetic formation of celadonite (Weiszburg et al. 2004). The K content of celadonite indicates that the origin of gamma radiation of K can be related to this phase. However, data of Th and U do not give more information related to the previous data (Table 3).

Table 3 K, U, Th content of celadonite-rich ore types and separates

Sample K [wt. %]

Th [ppm]

U [ppm]

Green rodochrolite without treatment 7.01 4.0 2.2 Green clay 6.95 2.3 0.6 Separated green phase from green rodochrolite 4.09 n.a. n.a.

Explanation: n.a. – no data.

Taking into consideration that the celadonite content of the green Mn-carbonate ore in the upper third of the main ore bed is high (its facies thick-ness varies between 1-3 m), it can be concerned as the source of gamma ra-diation of K origin. However, the significant amount of celadonite is typical of the whole carbonate ore bed. The color of the ore is variable because of its goethite content (brown). So, the gamma radiation of K origin is more com-plex.

Based on the geochemical data of the series of Mn-carbonate ore and black shale, the distribution of the gamma resource elements, especially that of K can be well delimited to sources, however, the delimitation of the U and Th resource is impossible due to the low concentration levels.

4.2 Investigation of secondary Mn oxide ore types Since no significant anomaly was discovered during the investigation of the Mn-carbonate series, Mn-oxide ore type was also investigated in detail.

The average values resulting from the database are summarized in Table 4 and Fig. 3. It can be stated that:


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The K content in the formations remains significantly under the terrestrial abundance.

Based on the U, Th data the oxidation of the black shale resulted in the increase of the U and Th concentration levels. In Table 4, it is seen that black shale in Mn-carbonate environment shows 9.43 ppm of Th and 2.30 ppm of U (locally higher than the average given in Table 1) and black shale in Mn-oxide environment shows 12.66 ppm of Th and 7.28 ppm of U (also locally much higher than in Table 1 average). Thus black shale consideration assumes significance with respect to radiation hazard.

The decrease of the Th/U ratio shows, that during the oxidation of the Mn-carbonate series (in both the cases of Mn-carbonate ore and black shale) the U concentration level increased in a greater ratio than the Th-concentration level.

Table 4 Data of archive chemical analyses (based on Polgári et al. 2000)

Rock type Sample No.

K [wt. %]

Th [ppm]

U [ppm]

Th/U÷

Ua [ppm]

Mn carbonate series with black shales

58 1.17 3.96 1.13 3.51 0

Mn carbonate without black shale

35 1.20 2.91 0.86 3.39 0

Black shale in Mn-carbonate environment

11 1.25 9.43 2.30 4.10 0

Black shale in Mn-oxide environment

9 2.12 12.66 7.28 1.74 3.06

Secondary Mn-oxide ore 57 1.60 3.67 3.47 1.06 2.25 Primary, cherty, Fe-rich Mn-oxide ore (Csárda-hegy) without phosphorite concretion

21 0.94 3.00 4.23 0.71 3.23

Contact Mn-oxide ore 5 2.00 2.20 1.04 2.12 0.31 Average terrestrial abundance 2.60 10.00 2.50 4.00 –

4.3 Investigation of cherty, Fe-rich Mn oxide ore type (Csárda-hegy) The “uranium-like” nature of samples from Csárda-hegy is striking, espe-cially knowing that, related to the other parts of the deposit, a genetic differ-ence also exists here (Polgári et al. 2012). In primary cherty Fe-rich Mn oxide ore type, U and Th, on the average, should be low (oxidation environ-ment) but due to phosphorite concretion (one sample) gives 181 ppm of U. Even without phosphorite concretion (given in Table 4), the average U con-tent is 4.23 ppm, higher than the value in black shale in Mn-carbonate envi-

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Fig. 3. Box-plot diagram of samples (Table 4): 1 – average Mn-carbonate ore (with black shale), 2 – average Mn-carbonate ore (without black shale), 3 – average black shale (Mn-carbonate environment), 4 – average black shale (Mn-oxide environ-ment), 5 – average secondary Mn-oxide ore, 6 – average primary Mn-oxide ore (without phosphorite lens), and 7 – Contact Mn-oxide ore.

ronment. This anomalous phenomenon happened. Due to its anomalous value it was not taken into account for the average calculation given in Table 4.

The U content of the primary cherty, Fe-rich Mn-oxide ore exceeds the Th content, which, compared to the terrestrial Th/U ratio, is an anomalous phenomenon, in spite of the low concentration level.

4.4 Investigation of contact Mn-oxide ore types (Table 4, Fig. 3) Concerning the Th/U ratio aspect, the data of the contact Mn-oxide ore sam-ples (concretions collected in contact zone of footwall limestone and Mn-carbonate main ore bed) are located between the original Mn-carbonate ore and its oxidized version, but the number of samples was only 5.

The amount of authigenic uranium (Ua) was calculated using the method published by Wignall (1994). According to this, during the geochemical cal-


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culation of the U content of black shale, the U content of authigenic (Ua) and detrital (Ud) origin of the formation must be separated, which can be esti-mated by the following formula:

a totalU = U Th/3 .−

The formula assumes that Th has detrital origin and its average rate re-lated to U is: Udet = Th/3.

In those cases when the calculation resulted in a negative value, zero was taken into account. It is apparent that Wignall’s (1994) method is a rough es-timation, however it emphasizes the result that each rock type of UMF be-haves in a different way, related to U and Th. The results had a positive Ua value for formations having gone through a secondary chemical transfor-mation, while it was not positive for the primary formation of the same rock types. This shows on the one hand that U was enriched together with Mn during the secondary transformation, and on the other hand it shows that Wignall’s (1994) method can be applied with caution, and the possibility of secondary transformation must be taken into account.

Using Wignall’s method for primary formations, like the Csárda-hegy type primary cherty Fe-rich Mn-oxide ore, considerable authigenic U accu-mulation is determined, which may indicate bacterial activity. This can be verified after clarifying the chemical form of U. According to the proposed model of Csárda-hegy type ore formation, alternating redox conditions were characteristic (oxidative microbial Mn(II) oxidation), and sulphide for-mation, and mixing with reactive manganate, Fe(II), and organic matter (Aller and Rude 1988), which were realized in microbially mediated reduc-tion and oxidation processes (Polgári et al. 2012). These can be responsible for the special behavior of U in Csárda-hegy ore section, where U could pre-serve lower valent solid accumulation as a result of very quick mineraliza-tion and accumulation.

4.5 Gamma spectroscopy inspection results The gamma spectroscopy inspection does not provide direct results on the U content; however, via the measurement of the 226Ra isotope, the 238U concen-tration level and the tendencies among the formations can be concluded. Since natural uranium occurs in the form of 238U in more than 99%, this es-timation is acceptable. So, gamma spectroscopy result (Table 5) giving 226Ra is the most important in this kind of study, as the authors are concerned with radiation exposure due to radon. Table 5 shows that the embedded black shale has 28.5 Bq/m–3 of 226Ra and 38.4 Bq/m–3 of 234Th. Assuming 234Th be-ing produced from 238U, the embedded black shale shows 238U > 226Ra; i.e., parent favoured disequilibrium in U-series. This is reverse in case of contact

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Mn-oxide ore (44.9 Bq/m–3 226Ra and 20.96 Bq/m–3 238U, only one sample, however). It indicates that U has leached out leaving behind radium from which radon is produced. Underlayer black shale (3 samples) also shows that parent has leached out (Fig. 4).

Table 5 Activity concentration measured by gamma spectrometry

(ordering to sedimentary sequence, number of samples are in brackets)

Rock type Average activity concentration of isotopes

[Bq m–3] 232Th 234Th 226Ra 40K

Green-grey Mn-carbonate ore (bed No. 2) (3)

12.5 0 8.13 234

Embedded black shale (5) 24 38.4 28.5 512 Grey, pyritic Mn-carbonate ore (2) 2.65 15.7 18.6 70 Green, celadonite-bearing Mn-carbonate ore (4)

5.75 0 3.06 1369

Black Mn-carbonate ore (4) 10.4 0 2.76 247 Grey Mn-carbonate ore (2) 11.7 10.8 10.7 414 Brown-green Mn-carbonate ore (1) 14.7 2.3 10.3 1130 Contact Mn-oxide ore (1) 4.04 20.96 44.9 84.2 Footwall black shale (3) 40.2 26.5 36.1 1028 Footwall limestone (2) 6 0 3 101

Fig. 4. Activity concentration of 232Th and 226Ra in the different rock types (accord-ing to sedimentary sequence).


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5. CONCLUSIONS In case of black shale-hosted manganese ore deposits, the presence of anomalous terrestrial radioisotopes must be suspected, which is a useful tool in genetic investigation; furthermore, it may have consequences of even eco-nomical, or health protection significance.

Concerning the Úrkút Manganese Formation (UMF), complex character-istics and processes influenced the behavior of terrestrial radioisotopes, like: (i) contribution of Corg in the formation of black shale and Mn-carbonate ore (Polgári 1993); (ii) P-rich layers represented by fish remnants in hetero-genous distribution (P2O5 content up to 16 wt. %; Polgári et al. 2000, 2003); (iii) clay-rich composition of the ore deposit (celadonite occurring in Mn-carbonate ore, containing K), moreover, the adsorption characteristics of clay minerals should also be taken into account concerning the binding of radio nuclides; (iv) the microbial mediation in formation processes of manganese mineralization, and the marine-geochemical relationship of U also preferred in microbial processes, and that of the two elements may also cause U anomaly in the deposit; and last but not least (v) the effect of secondary oxi-dizing processes causing chemical transformations.

In spite of the low U content of the deposit, radon danger occurs during underground exploitation, as a huge mass of the ore (ore types and black shale) is in contact with mine air.

In conclusion it can be said that: (i) grey, pyrite Mn-carbonate ore, (ii) embedded black shale, (iii) underlayer black shale, and (iv) contact Mn-oxide ore, all contribute to radon in mine environment, representing the source of geo hazard.

Acknowledgemen t s . We are grateful to the anonym referees for their useful contributions.

R e f e r e n c e s

Abel-Ghany, H.A. (2010), Natural activities of 238U, 232Th, and 40K in manganese ore, Am. J. Environ. Sci. 6, 1, 90-94, DOI: 10.3844/ajessp.2010.90.94.

Aller, R.C., and P.D. Rude (1988), Complete oxidation of solid phase sulphides by manganese and bacteria in anoxic marine sediments, Geochim. Cosmochim. Acta 52, 3, 751-765, DOI: 10.1016/0016-7037(88)90335-3.

Breitner, D., T. Turtiainen, H. Arvela, P. Vesterbacka, B. Johanson, M. Lehtonen, K.-H. Hellmuth, and C. Szabó (2008), Multidisciplinary analysis of Finnish esker sediment in radon source identification, Sci. Total. Environ. 405, 1-3, 129-139, DOI: 10.1016/j.scitotenv.2008.06.015.

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Cote, P., and M.G. Townsend (1981), Mixtures of radon and thoron daughters in un-derground atmospheres, Health Phys. 40, 1, 5-17, DOI: 10.1097/00004032-198101000-00002.

Cuney, M., and K. Kyser (2008), Recent and Not-so-recent Developments in Ura-nium Deposits and Implications for Exploration, Mineralogical Association of Canada, Short Course Series, Vol. 39.

Darby, S.C., E. Whitely, G.R. Howe, S.J. Hutchings, R.A. Kusiak, J.H. Lubin, H.I. Morrison, M. Tirmarche, L. Tomásek, E.P. Radford, R.J. Roscoe, J.M. Samet, and S.X. Yao (1995), Radon and cancers other than lung can-cer in underground miners: a collaborative analysis of 11 studies, J. Natl. Cancer Inst. 87, 5, 378-384, DOI: 10.1093/jnci/87.5.378.

Grasselly, G., and G. Pantó (1988), Rare Earth elements in the manganese deposit of Úrkút (Bakony Mountains, Hungary), Ore Geol. Rev. 4, 1-2, 115-124, DOI: 10.1016/0169-1368(88)90007-8.

Kaeding, L, O. Brockamp, and H. Harder (1983), Submarin-hydrothermale Entste-hung der Sedimentären Mangan-Lagerstätte Úrkút (Ungarn), Chem. Geol. 40, 3-4, 251-268, 10.1016/0009-2541(83)90032-3 (in German).

Kávási, N., C. Németh, T. Kovács, S. Tokonami, V. Jobbágy, A. Várhegyi, Z. Gor-jánácz, T. Vigh, and J. Somlai (2007), Radon and thoron parallel measure-ments in Hungary, Radiat. Prot. Dosim. 123, 2, 250-253, DOI: 10.1093/ rpd/ncl102.

Kávási, N., J. Somlai, T. Vigh, S. Tokonami, T. Ishikawa, A. Sorimachi, and T. Kovács (2009), Difficulties in the dose estimate of workers originated from radon and radon progeny in a manganese mine, Radiat. Meas. 44, 3, 300-305, DOI: 10.1016/j.radmeas.2009.03.014.

Kelly, S.D. (2010), Uranium chemistry in soils and sediments. In: A. Hartemink and A. McBratney (eds.), Developments in Soil Science, Vol. 34, Elsevier Press, Amsterdam, 411-466, DOI: 10.1016/S0166-2481(10)34014-1.

Laznicka, P. (1992), Manganese deposits in the global lithogenetic system: Quantita-tive approach, Ore Geol. Rev. 7, 4, 279-356, DOI: 10.1016/0169-1368(92) 90013-B.

Mason, B.H. (1958), Principles of Geochemistry, 2nd ed., John Wiley and Sons, New York.

OECD/NEA-IAEA (2008), Uranium 2007: Resources, Production and Demand, Red Book, OECD, Paris.

Polgári, M. (1993), Manganese Geochemistry Reflected by Black Shale Formation and Diagenetic Processes: Model of Formation of the Carbonatic Manga-nese Ore of Úrkút, Special Series of Hungarian Geological Institute, Kar-pati Publish House, Ushgorod, 211 pp.

Polgári, M., Z. Szabó, and T. Szederkényi (eds.) (2000), Manganese ores in Hun-gary. In Commemoration of Professor Gyula Grasselly, Juhász Publishing House, Szeged, 675 pp.


847

Polgári, M., M. Szabó-Drubina, and J.R. Hein (2003), Phosphogenesis in Jurassic black shale-hosted Mn-carbonate deposits, Úrkút and Eplény, Hungary, In-vestigations on archive sample drillcore Úrkút-136, Bull. Hung. Geol. Surv. 133, 1, 37-48.

Polgári, M., J.R. Hein, T. Vigh, M. Szabó-Drubina, I. Fórizs, L. Bíró, A. Müller, and A.L. Tóth (2012), Microbial processes and the origin of the Úrkút manga-nese deposit, Hungary, Ore Geol. Rev. 47, 87-109, DOI: 10.1016/ j.oregeorev.2011.10.001.

Roy, S. (1988), Manganese metallogenesis: A review, Ore Geol. Rev. 4, 1-2, 155-170, DOI: 10.1016/0169-1368(88)90011-X.

Segev, A. (1992), Remobilization of uranium and associated metals through karstifi-cation processes: A case study from the Timna Formation (Cambrian), Southern Israel, Ore Geol. Rev. 7, 2, 135-148, DOI: 10.1016/0169-1368 (92)90009-A.

Szabó, I.M. (1989), Microbiology of Biosphere, Akadémiai Kiadó, Budapest (in Hungarian).

Tirmarche, M., A. Raphalen, F. Allin, J. Chameaud, and P. Bredon (1993), Mortality of a cohort of French uranium miners exposed to relatively low radon con-centrations, Br. J. Cancer. 67, 5, 1090-1097, DOI: 10.1038/bjc.1993.200.

Trueman, C.N., and N. Tuross (2002), Trace elements in recent and fossil bone apa-tite, Rev. Mineral Geochem. 48, 1, 489-521, DOI: 10.2138/rmg.2002.48.13.

Varentsov, I.M., G., Grasselly, and Z. Szabó (1988), Ore-formation in the early-Jurassic basin of Central-Europe: Aspects of mineralogy, geochemistry, and genesis of the Úrkút manganese deposit, Hungary, Chem. Erde – Geochem. 48, 257-304.

Weiszburg, T.G., E. Tóth, and A. Beran (2004), Celadonite, the 10-Å green clay mineral of the manganese carbonate ore, Úrkút, Hungary, Acta Mineral. Petrogr. 45, 1, 65-80.

Wignall, P.B. (1994), Black Shales, Clarendon Press, Oxford.

Received 30 July 2012 Received in revised form 21 November 2012

Accepted 13 December 2012

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Terrestrial radioisotopes in black shale hosted Mn-carbonate deposit (Úrkút, Hungary)