Natural Radioactivity of Intrusive-Metamorphic and Sedimentary Rocks of the Balkan Mountain Range ( Serbia , Stara Planina )

Stara Planina (also known as the Balkan mountain range) is known for numerous occurrences and deposits of uranium and associated radionuclides. It is also famous for its geodiversity. The geologic framework is highly complex. The mountain is situated between the latitudes of 43◦ and 44◦ N and the longitudes from 22◦16′ to 23◦00′ E. Uranium exploration and radioactivity testing on Stara Planina began back in 1948. Uranium has also been mined in the zone of Kalna, within the Janja granite intrusive. The naturally radioactive geologic units of Stara Planina are presented in detail in this paper. The main sources of radioactivity on Stara Planina can be classified as: 1. Granitic endogenous—syngenetic–epigenetic deposits and occurrences; 2. Metamorphogenic—syngenetic; and 3. Sedimentary, including occurrences of uranium deposition and fluctuation caused by water in different types of sedimentary rocks formed in a continental setting, which could be classified under epigenetic types. The area of Stara Planina with increased radioactivity (higher than 200 cps), measured by airborne gamma spectrometry, is about 380 square kilometers. The highest values of measured radioactivity and uranium grade were obtained from a sample taken from the Mezdreja uranium mine tailing dump, where 226Ra measures 2600 ± 100 Bq/kg and the uranium grade is from 76.54 to 77.65 ppm U. The highest uranium (and lead) concentration, among all samples, is measured in graphitic schist with high concentrations of organic (graphitic) material from the Inovska Series—99.47 ppm U and 107.69 ppm Pb. Thorium related radioactivity is the highest in granite samples from the Janja granite in the vicinity of the Mezdreja granite mine and the Gabrovnica granite mine tailing dump, and it is the same—250 ± 10 Bq/kg for 232Th, while the thorium grade varies from 30.82 to 60.27 ppm Th. In gray siltstones with a small amount of organic material, the highest radioactivity is related to potassium—2080 ± 90 Bq/kg for 40K.


Introduction
The objective of this paper is to provide an overview of the naturally radioactive geologic units in known areas of Stara Planina (also known as the Balkan mountain range).The studied localities include Mezdreja and Gabrovnica (abandoned mines in the Janja granites), graphitic schists of the Paleozoic "Inovo Series", and early Triassic sedimentary units in the Dojkinci-Jelovica area.These occurrences are known and have been described in papers, reports and books, largely from the perspective of Stara Planina is not clearly defined and can be followed on the Zaječar and Bor maps along Cretaceous formations and intrusions over a length of about 30 km northward and farther via Brusnik and Brestovac to Negotin, but this is not the subject of the present paper.
In 1997, Stara Planina was designated a nature park, where there are a number of unique examples of geodiversity.
The following genetic types of uranium deposits and occurrences can be distinguished on Stara Planina after Gertik [11] and Kovacevic [3]: 1. Uranium mineralization in pegmatites; 2. Uranium mineralization related to auto-metasomatic hydrothermal processes in granite; 3. Sedimentary infiltration deposits and occurrences; and 4. Metamorphogenic occurrences.The map in Figure 1 includes a schematic representation of the geology and distribution of radioactivity on Stara Planina.In general terms and based on Geoinstitute's activities [3], two levels of radioactivity have been identified: (1) about 200 cps, divided into three zones whose total surface area is 307.5 km 2 ; and (2) greater than 200 and up to 500 cps (mostly greater than 200 cps but rarely Generally speaking, Stara Planina Mt. is a complex geologic system, built up of different geologic units (with regard to the composition, characteristics and origin).From the north, where the geologic units are separated by structures and where granite and granodiorite intrusions begin, the area is defined by faults that separate the Late Jurassic in the south from the Early Cretaceous in the north.Towards the south, there is a complex geotectonic assemblage made up of the Janja (Figure 1), Radičevo and Ravno Bučja granites; the Zaglavak gabbro massif; Paleozoic metamorphic rocks (Proterozoic-Cambrian, Silurian-Devonian, the Inovo Series and others); and Permian red sandstones and conglomerates.To the south, there are Mesozoic formations: Triassic (Kopren-Gostuša-Dojkinci), Jurassic (Basara, Odorovci), and Early Cretaceous (Visočka Ržana, Dimitrovgrad).In the southern part of the Mesozoic block the Jurassic and the Cretaceous are intersected by structures running from the northwest to the southeast.The northern boundary of Stara Planina is not clearly defined and can be followed on the Zaječar and Bor maps along Cretaceous formations and intrusions over a length of about 30 km northward and farther via Brusnik and Brestovac to Negotin, but this is not the subject of the present paper.
In 1997, Stara Planina was designated a nature park, where there are a number of unique examples of geodiversity.
The following genetic types of uranium deposits and occurrences can be distinguished on Stara Planina after Gertik [11] and Kovacevic [3]: 1. Uranium mineralization in pegmatites; 2. Uranium mineralization related to auto-metasomatic hydrothermal processes in granite; 3. Sedimentary infiltration deposits and occurrences; and 4. Metamorphogenic occurrences.
The map in Figure 1 includes a schematic representation of the geology and distribution of radioactivity on Stara Planina.In general terms and based on Geoinstitute's activities [3], two levels of radioactivity have been identified: (1) about 200 cps, divided into three zones whose total surface area is 307.5 km 2 ; and (2) greater than 200 and up to 500 cps (mostly greater than 200 cps but rarely 500 cps), divided into four zones whose surface area is 70.84 km 2 .The zones were explored by airborne gamma spectrometry during 1981.The methodology of airborne gamma survey consisted of the following: flights was carried out along the straight-line profiles that were at a distance of 250 m from each other.The flight height of a plane was between 100 and 200 m above the surface of the terrain.Detectors used in the airborne gamma survey were crystals of NaJ "GR-800D" activated by a thallium, dimension of crystals was 10 cm × 10 cm × 40 cm [12].
In essence, the main sources of radioactivity on Stara Planina can be classified as: 1. Granitic endogenous, syngenetic-epigenetic deposits and occurrences; 2. Metamorphogenic, syngenetic; and 3. Sedimentary, including occurrences of uranium deposition and fluctuation caused by water in different types of sedimentary rocks formed in a continental setting, which could be classified under epigenetic types.

Geological Characteristics of the Janja Granite
The Janja granite massif has intruded into Late Proterozoic and Cambrian crystalline schists.The crystalline schists feature thermo-contact and metasomatic alterations.The primary structural elements include fractures filled with aplite, pegmatite and quartz veins.In places, the fractures trending NW-SE, concentrated on the fringes of the massif, exhibit white mica alteration.
In its north-western part, the massif bifurcates into two masses separated by amphibolites, amphibole schists, gneisses and mica gneisses.Peripherally there are alteration zones developed around the contacts characterized by silification, biotitization and local deposition of feldspar [8].The Janja granite is overall calc-alkaline in composition, but with variations such as monzonite-akerite fringes, and an alkaline granite core.Deformations and secondary alterations have been noted in the entire massif, particularly on the fringes.They are represented by a schistose texture, crushed minerals and crystallization of secondary minerals.The primary components are quartz, oligoclase, K-feldspar (microcline, rarely orthoclase), and biotite.The accessory components are sphene, apatite, zircon and magnetite, and the secondary components sericite, chlorite, epidote, calcite, limonite and a clayey substance.There are syenite diorites in the periphery of the Janja granite and on its fringes.Their origin is attributed to contact metasomatic processes in the syenite diorites.
Pegmatite veins are made up of quartz, plagioclase (albite-oligoclase) enriched with U [13], microcline, biotite and muscovite.The accessory components are apatite, zircon, alanite and metallic minerals.There are quartz veins inside and around the massif.Their thickness is up to several meters and they are up to 200 m long.In addition to quartz, they contain tourmaline and metallic minerals.
Typical naturally-radioactive geologic representatives were tested within the Janja granite.Samples of granite, the host rock at Mezdreja and Gabrovnica, were of primary interest (Figure 2A,B).In addition to granite, tailing dump samples from Mezdreja (clayey material with cataclazed granite fragments), silicified batches with limonite stains and fragments of contact gabbroid with coarse (1.5 cm) K-feldspar were examined.
with occurrences of ore impregnation and limonitization.Radioactivity measured 320 cps and 0.182 µSv/h.3. The mine dump contained crushed granite material, clayey, kaolinized and chloritized (Figure 2F).Radioactivity measured up to 1250 cps and 0.421 µSv/h.4. A sample with large pink K-feldspar was collected from the point of contact between granitic and gabbroid rocks.Radioactivity measured 120 cps and 0.192 µSv/h.The K-feldspar shown in Figure 2E was separately tested by XRF.The results are shown further below.Tables 1 and 2 show radioactivity concentrations (Table 1) and grades of U, Th, Pb and Sr (Table 2), in samples from the Mezdreja mine area.

Geological Characteristics of Graphitic Schists from the Inovo Series
The Inovo Series transgresses the south-western part of the Janja granite-metamorphic system.Metaconglomerates and metasandstones are the base of the metamorphic-sediment domain.They are overlain by metasandstones containing argillophyllites, with schists in the upper part.The sequence is built up of chlorite-phyllite schists, green schists, amphibolites, graphitic schists, greywackes and conglomerates [8].
The Inovo Series graphite-bearing schist is located in relative proximity to the Gabrovnica mine, 2.5 km southeast and 1.3 km southwest of the closest mapped point of the Janja granite (Figure 1).
The Inovska River occurrence is situated in the river (on the riverbanks), developed in fractured, altered and brecciated metasandstones with interbeds and lenses of black graphitic schists (Figure 3A-C), from 50 to 70 m thick.
The upper part of the domain contains layers of coarse-grained and fine-grained metasandstones with black clayey schist intercalations.In the lower part there are brecciated arkose metasandstones.The package dips to the northeast at an angle of 70 • -80 • .sampling site measured 650 cps and 0.279 µSv/h.
With regard to the thin sections, the lighter and harder variety had more petrogenic minerals and less graphitic material, and also contained limonitic stains (Figure 3B).
Since these two samples were collected in relative proximity and their alterations varied at a decimeter level, radioactivity was measured at the Vinča Institute lab, using homogenized samples of the two varieties of graphitic schists of the Inovo Series.Tables 5 and 6 show radioactivity concentrations (Table 5) and grades of U, Th, Pb and Sr (Table 6) in graphite schist samples from the Inovska Reka occurrence.Uranium mineralization is in the form of elongated lenses in the direction of the dip, following layers of metasediments between coarse-grained metasandstones and black clayey schists [11].The lens-like ore body runs along the dip to about 30 m. Ore bodies are built up of carbonitized, pyritized, chloritized and sericitized microconglomerates to arkose sandstones with traces of chalcopyrite and galenite.

Geological Characteristics of the Multicolored Series
The observation points within the Multicolored Series are located in an area defined as Early Triassic.Known places where elevated uranium concentrations have been detected are in the part of the terrain called Dojkinci-Jelovica (Figure 1).The area is known as that of "clastic rocks of Stara Planina", a formation that features clearly defined continuous sedimentation in continental warm and humid climate conditions.The geology is represented by continental formations built up of fragments of crystalline schists and granites, light-red quartz conglomerates, red and gray sandstones, and gray to grayish-pink siltstones [9].The upper part of the Multicolored Series includes Middle Triassic marls and sandy limestones.Uranium mineralization is usually found in the form of pitchblende [3].The color of the sandstones varies from red (usually; Figure 4A,B) to light green-gray (rarely).All siltstones exhibited elevated concentrations of uranium.From the north, the Early Triassic (Permo-Triassic) sediments of the Multicolored Series are in contact with Ripheo-Cambrian schists.
biotite, turning into an iron substance and chlorite.Rare metallic minerals and fragments of coaly-organic material (Figure 4D) were also found.The amounts of the gray siltstone were not large and they were likely created from the sediments of small local lakes and wetlands.The sample of gray siltstone was collected at the redox contact between the red siltstone and reddish sandstone (Figure 4C), where the grain sizes of the fragments increased from the sampling point to the substrate (road level).Radioactivity in the redox zone measured 280 cps and 0.429 µSv/h.Tables 7 and 8 show radioactivity concentrations (Table 7) and grades of U, Th, Pb and Sr (Table 8) in graphite schist samples from "Multicolored Series" sediments.

1.
The samples were collected at pre-determined locations.The goal was to find representative samples of naturally radioactive rocks.The sampling points are shown in the UTM system, zone 34N, ellipsoid WGS84.The weight of the samples was from 2 to 2.5 kg.

2.
Thin sections were made from granite and schist samples, and where ore minerals were detected polished sections were also made.The samples were viewed macroscopically and microscopically on a Leitz Ortholux Pol 2 microscope (Wetzlar, Germany), at the Geological Survey of Serbia.The structure of the samples was examined using a Bresser binocular magnifier (Rhede, Germany).3.
On the ground, the radioactivity of the terrain and at the observation points was measured by a Radiation Detector Explouranium 110 (Mississauga, ON, Canada) in cps and Gamma-Scout (Newark, DE, USA) in µSv/h.The results are included in the descriptions of the tested samples.The values were recorded after a period of ten minutes, when they stabilized on the display and when there were no ±10% fluctuations.The data is presented in the paper in intervals characteristic of the tested location.4.
The samples were ground to 70 µm.Chemical analyses of powder were performed on an XRF Thermo Fisher Scientific Niton Xl3t Goldd+ analyzer (Waltham, MA, USA), at the University of Belgrade, Faculty of Civil Engineering.Each sample was tested two or three times, for about 180-190 s in the Soil mode, and checked by Test Allgeo.Some of the samples were analyzed in their solid state, for example schist; assays were performed on plate and schistosity in resection.

5.
The radioactivity of the samples ( 226 Ra, 232 Th, 40 K and 137 Cs) was measured at Vinča Nuclear Institute.Homogenized samples were dried in an oven at 105 • C to constant weight, placed in plastic Marinelli beakers, sealed and left for four weeks to reach radioactive equilibrium [18].Each prepared sample was placed in an HPGe detector and measured for 90,000 s.Gamma background in the laboratory was determined prior to testing, by measuring an empty Marinelli baker under identical conditions.The counting time for background measurement was 240,000 s.It was later subtracted from the measured gamma spectra of each sample.
The activity of the samples was measured using a high-resolution coaxial semiconductor detector with high-purity germanium crystal HPGe ORTEC GEM 50 and 50% relative efficiency at 1332 keV.The detector was shielded by lead in order to achieve the lowest possible background level.
Energy and efficiency calibration was undertaken before measurement.The calibration source used was a commercially available gamma standard, with mixed radionuclides-type MBSS 2 in Marinelli geometry of 0.5 L, developed by the Inspectorate for Ionizing Radiation of the Czech Metrological Institute, with the isotopes: 241 Am, 109 Cd, 57 Co, 139 Ce, 113 Sn, 85 Sr, 137 Cs, 88 Y, 203 Hg, and 60 Co.The energy of gamma lines of these radionuclides is highly suitable for calibration and covers the region of interest, i.e., from 30 to 3000 keV.Quality assurance of the measurements was carried out by daily efficiency and energy calibration, repeating each sample measurement.
Correction for radioactive decay and background, as well as analysis of the results, were conducted using dedicated software ORTEC Gamma Vision-32 Model A66-B32 Version 6.01 (Oak Ridge, TN, USA).
The 226 Ra activity was determined by its decay products: 214 Pb (295.22 keV, 351.93 keV) and 214 Bi (609.31keV, 1120.29 keV).In the case of 232 Th, two photopeaks of 228 Ac (911.20 and 698.97 keV) were used.The activities of 40 K and 137 Cs were derived from 1460.83 keV and 661.66 keV gamma lines, respectively.

Geological Observations, Geochemical and Radiometric Results
The main characteristics and results of geological observations, measured radioactivity of typical petrologic representatives and the concentrations of U, Th, Pb and Sr are presented below in a from-to-form, given that measurements were repeated several times to obtain concentration ranges of powdered and solid samples and to also check for any large variations.

Mezdreja Mine Area
Mezdreja is located in the southern part of the Janja igneous metamorphic complex.It is defined by two fault zones, so-called zones 0 and 1 [11].
Fault zone 0 trending NW-SE is 1300 m long and has vertically been explored from 200 to 600 m.The following zonality has been noted in the vertical profile: kaolinized, white mica altered and chloritized zones in the upper parts, and silification, pyritization and hematitization in the lower parts.The ore is developed in the form of lenses that locally form columns. Uranium mineralization is finely dispersed in crushed and hydrothermally altered granite or in the form of veinlets, coatings and stains of pitchblende visible to the eye.Non-uranium-bearing parts of the fault zone are filled with sericite and chalcopyrite.Fault zone 1 is developed adjacent to metagabbroid rocks (Figure 2E).The form and extent are similar to those of Zone 0. Mineralization is of the vein/lens type at the point of contract with the metagabbros.Along Zones 0 and 1 there are biotite granites and metamorphic gabbros.Uranium is mineralized in the form of impregnations of veinlets and stains, and is represented by pitchblende and secondary pitchblende.
Four samples were acquired from the Mezdreja site: 1.
A granite sample was collected near the pit and radioactivity measured by GR110 in situ was 520 cps at the sampling point and 420 cps and 0.322 µSv/h at the mine portal.The pit has Minerals 2018, 8, 6 9 of 17 caved and could not be accessed.The rock is partially fractured.The predominant components are pink K-feldspar and white plagioclase, both fractured with white mica and kaolinized to different degrees.The dimensions were up to 0.5-1.5 cm (Figure 2A).There was also chloritization, occasionally accompanied by magnetite grains developed at a later date during short hydrothermal or auto-metasomatic episodes.

2.
In the area between the mine portal and dump, there were small outcrops of silicified material with occurrences of ore impregnation and limonitization.Radioactivity measured 320 cps and 0.182 µSv/h.
Radioactivity measured up to 1250 cps and 0.421 µSv/h.

4.
A sample with large pink K-feldspar was collected from the point of contact between granitic and gabbroid rocks.Radioactivity measured 120 cps and 0.192 µSv/h.The K-feldspar shown in Figure 2E was separately tested by XRF.The results are shown further below.
Tables 1 and 2 show radioactivity concentrations (Table 1) and grades of U, Th, Pb and Sr (Table 2), in samples from the Mezdreja mine area.

Gabrovnica Mine Area
The origin of the deposits is similar to that of Mezdreja.The only difference is that there are eight fault zones that can be divided into two groups: (1) diabase dikes in granites and milky white quartz; and (2) crushed granites.Uranium ore is developed in chloritized phyllonites and crushed granites.Solid and non-tectonized domains are barren to weakly mineralized.The fault zones are highly tectonized by post-ore tectonics.The origin of the uranium is similar to that at Mezdreja.Fresh granite near the pit and granite (Figure 2B) from the mine dump were sampled as representative of the locality.Radioactivity measured 240 cps and 0.210 µSv/h at the mine portal and 360 cps and 0.248 µSv/h at the mine dump.
Tables 3 and 4 show radioactivity concentrations (Table 3) and grades of U, Th, Pb and Sr (Table 4) in samples from the Gabrovnica mine area.

Inovska Reka Occurrence
A number of samples were collected from the site (Figure 3A).Those shown here contain small and large amounts of graphitic material, respectively.Niton XRF Goldd+ analyses showed that the sample with more graphitic material carried a larger amount of uranium.Also, that sample was brittle, with limonitic stains (Figure 3C,D) along the directions of shearing.Radioactivity at the sampling site measured 650 cps and 0.279 µSv/h.
With regard to the thin sections, the lighter and harder variety had more petrogenic minerals and less graphitic material, and also contained limonitic stains (Figure 3B).
Since these two samples were collected in relative proximity and their alterations varied at a decimeter level, radioactivity was measured at the Vinča Institute lab, using homogenized samples of the two varieties of graphitic schists of the Inovo Series.
Tables 5 and 6 show radioactivity concentrations (Table 5) and grades of U, Th, Pb and Sr (Table 6) in graphite schist samples from the Inovska Reka occurrence.

Dojkinci and Jelovica Area
A typical sample of the Multicolored Series sandstone was dark "hematite" red to pink-gray (Figure 4A), built up of mainly quartz (Figure 4B), orthoclase, plagioclase, muscovite, biotite, apatite, epidote, chlorite and fragments of metamorphic rocks.The grains were several tens to 100 and exceptionally 300 µm.The grains were highly altered into clay minerals and white mica.The plagioclases are white mica altered as well as calcitized.The quartz, orthoclase and plagioclase are angular.The micas exhibited linear orientation and locally built nest-like forms.The apatite was rounded.Zircon was noted locally in the quartz.The epidote was developed as independent entities.The cement is of the iron-carbonate type.The sample was collected as a "representative" from the direction of contact with metamorphic rocks, before the gray siltstones.The sampling point measured 120 cps and 0.172 µSv/h.
The sample of the gray siltstone was pelitic, fine-grained and compact.It was built up mainly of clay-sericitic material.There was also fine sharp-edged quartz with thin plates of muscovite and biotite, turning into an iron substance and chlorite.Rare metallic minerals and fragments of coaly-organic material (Figure 4D) were also found.The amounts of the gray siltstone were not large and they were likely created from the sediments of small local lakes and wetlands.The sample of gray siltstone was collected at the redox contact between the red siltstone and reddish sandstone (Figure 4C), where the grain sizes of the fragments increased from the sampling point to the substrate (road level).Radioactivity in the redox zone measured 280 cps and 0.429 µSv/h.
Tables 7 and 8 show radioactivity concentrations (Table 7) and grades of U, Th, Pb and Sr (Table 8) in graphite schist samples from "Multicolored Series" sediments.

Discussion
The occurrences described in this paper can be grouped into two geological-structural blocks [19,20] (Figure 1): A. an intrusive metamorphic block, and B. a sedimentary block.
A. In the intrusive metamorphic block, the elevated radioactivity is associated with the Janja granite and Aldinac grandiorite porphyritic rocks, as well as graphitic schists of the Inovo Series, over a surface area of about 195 km 2 .
B. The radioactivity in the sedimentary block is associated with both secondary deposition of minerals and reddish Fe-rich cement.Elevated radioactivity has particularly been noted at points of contact between gray and red siltstones, where the gray parts have been deposited as lenses of different sizes in the basal reddish mass of continental sediments.The surface area is approx.97 km 2 .
Figure 5 shows graphics of activity concentrations of 226 Ra, 232 Th and 40 K and of U, Th and Pb grades.They refer to the text that follows.
Assays of coarse-grained pink K-feldspar sampled at Mezdreja, from the point of contact between the gabbroids and granites, showed that the concentrations of uranium were low (from 6.5 to 6.99 ppm), while those of Th varied from 4.56 to 9.72 ppm.The sample was a 2-cm solid grain, tested at the basis and section.The concentrations of uranium in the clayey and kaolinized material from the Mezdreja mine tailings were higher than in fresh granites, but still lower than in the graphite-rich schist.The values ranged from 76.54 to 77.65 ppm of U. In this case there was less Th than U; Th concentrations were from 43.17 to 52.91 ppm.The concentrations of lead were nearly double those in granites, from 97.48The concentrations of uranium in the granite samples from Mezdreja and Gabrovnica were always lower than those of thorium by a factor of 3-4.At Mezdreja, the uranium concentrations varied from 12.21 to 14.39 ppm and those of thorium from 36.63 to 60.27 ppm.Radioactivity ( 226 Ra in Bq/kg) of the granites near the Mezdreja mine and at the mine portal measured 116 ± 5 and 142 ± 7, while 232 Th was 230 ± 10 to 250 ± 10 Bq/kg.At Gabrovnica, the concentrations of uranium were from 11.61 to 21.09 ppm and of thorium from 30.82 to 38.66 ppm.Here the radioactivity ( 226 Ra) of the granite samples from the mine portal area were 58 ± 3 and of the granite samples from the mine dump 206 ± 9. The measured 232 Th radioactivity of the mine portal granite was 163 ± 8 Bq/kg and of the mine dump granite 250 ± 10 Bq/kg.Hence, in both cases (Mezdreja and Gabrovnica), the thorium concentrations and radioactivity were higher than those of uranium.It should be noted that the concentrations of Pb at Gabrovnica were somewhat higher (from 64.27 to 74.66 ppm) than at Mezdreja (from 42.39 to 54.15 ppm).The grades of U and Th in the granite samples from Mezdreja and Gabrovnica were higher than in samples from volcanic islands of the Southern Tyrrhenian Sea, where U varied from 8.2 to 9.8 ppm and Th from 20.6 to 27.8 ppm [21].It should be noted that Stromboli, Salina, Filicuidi and Panarea are Late Quarternary and the Janja granite is Silurian.In addition, in eastern Mongolia (Berkh region) uranium anomaly No. 246 is alkaline biotite granite porphyry of middle-to-late Jurassic age, with 67 ppm U and 23 ppm Th [22], including silicified and kaolin altered parts like in the Janja granite deposits.
Assays of coarse-grained pink K-feldspar sampled at Mezdreja, from the point of contact between the gabbroids and granites, showed that the concentrations of uranium were low (from 6.5 to 6.99 ppm), while those of Th varied from 4.56 to 9.72 ppm.The sample was a 2-cm solid grain, tested at the basis and section.
The concentrations of uranium in the clayey and kaolinized material from the Mezdreja mine tailings were higher than in fresh granites, but still lower than in the graphite-rich schist.The values ranged from 76.54 to 77.65 ppm of U. In this case there was less Th than U; Th concentrations were from 43.17 to 52.91 ppm.The concentrations of lead were nearly double those in granites, from 97.48 to 103.06 ppm.With regard to radioactivity, this material measured the highest equivalent values for uranium 226 Ra-2600 ± 100 Bq/kg.The radioactivity of 232 Th was 169 ± 2 Bq/kg and correlated with the values measured in the Mezdreje and Gabrovnica granites.This particular sample exhibited the highest radioactivity of 137 Cs (10.8 ± 0.5 Bq/kg), compared to all the other samples tested in the research.In the granites, 137 Cs measured 2.3 ± 0.1 at the pit and 4.8 ± 0.3 Bq/kg at the mine dump.The values of 137 Cs at Mezdreja were low (less than 0.6 Bq/kg).In clay minerals, U enrichments are in illite-bearing uranium ore from Baiyanghe [23] and the uranium mineralization is located near the fracture zone, which represents the center of hydrothermal fluid activity or mineralization, similar to the Mezdreja mine.
No uranium (LOD) was found in a sample of silicified-limonitic material from Mezdreja (slcf/lim on Figure 5B).The concentration of thorium was from 28.42 to 30.25 ppm, and of lead from 94.31 to 104.22 (i.e., higher than in the other endogenous products tested).It is interesting to note that this sample measured the highest concentrations of cadmium (311.72-352.69ppm).The radioactivity of 226 Ra was 400 ± 20 Bq/kg and of 232 Th 188 ± 9.
A highly-silicified sample from Mezdreja measured the highest concentration of strontium-from 1850.9 to 1962.7 ppm.In the samples from Mezdreja and of the monomineral pink K-feldspar, the concentrations were from 634.69 to 840.98 ppm and from 655.11 to 665.36 ppm, respectively.The Sr concentrations in the Gabrovnica granites were lower and ranged from 186.94 to 308.1 ppm.
In the endogenous group, specifically in the case of the Mezdreja and Gabrovnica granites and ore deposits, feldspar and mica minerals are the most important in terms of uranium concentrations.Given that the concentrations of uranium in the main petrogenic minerals are rather low, and the total amounts of such minerals and their spread large, they represent sources from which "hot" granites leach considerable concentrations of uranium.Uranium occurs as U 4+ in biotite, muscovite and minerals from the feldspar group.If these minerals have been altered under the influence of oxygen-rich hydrothermal or meteoric waters [24], uranium in the form of U 6+ (as the U 6+ O 2 ion) might be present in them, as well as in accessory minerals: sphene, zircon, monazite, ortite, xenotime, apatite, tourmaline, apatite [25] and others.
Since uranium is remobilized from the primary granite at Mezdreja and Gabrovnica, it should be noted that in the presence of water and the H + ion: (1) kaolinite, the K + ion, U 4+ uranium ion and orthosilicilic acid are created from the primary "uranium-bearing" K-feldspar; and (2) sericite, K + ion, UO 2 2+ uranyl ion and again orthosilicilic acid might be formed.White mica (sericite) alteration has been observed in petrologic samples from Mezdreja and Gabrovnica (Figure 2C,D).In the samples collected as part of the present research, sericitization was more pronounced at Mezdreja than at Gabrovnica.Biotite, another primary "uranium-bearing" mineral in granite, can also be transformed in two ways: (1) when the negative OH − ion is present, creating sericite, silicon dioxide, water, aluminum silicate (which may occur as andalusite, kyanite or sillimanite), K + ion and U 4+ uranium ion; and (2) in the presence of water and the H + ion creating sericite, aluminosilicate, K + ion and UO 2 2+ uranyl ion.
The highest concentration of uranium in all samples was measured in the graphitic schist of the Inovo Series, rich in graphitic (organic) material (Figure 3C,D), and it amounted to 99.47 ppm, as opposed to the schist samples from the same sequence that were richer in silicate material (Figure 5B), which measured less uranium by a factor of nearly 4 (20.99-27.31ppm).The concentration of Th was generally lower than that of U. The graphite-rich samples had nearly half the Th.
The samples that contained more silicate material had Th concentrations varying from the ratio 1:2 in favor of uranium to the same ratio in favor of thorium.The highest measured concentration of Th was 42.01 ppm in a graphite-rich sample.The concentration of lead varied from 47.33 to 51.55 ppm and was similar to the lead concentrations in the granites, particularly at Mezdreja where 107.69 ppm of Pb was exceptionally measured in graphitic schists (which exhibited the highest uranium concentrations).Radioactivity was measured in both cases.In the graphite-rich schist 226 Ra was 380 ± 20 and in the graphite-poor schist 220 ± 10 Bq/kg, which correlated with the concentrations of U measured by XRF.The radioactivity of 232 Th in the graphite-rich schist was 169 ± 8 and in the graphite-poor schist 141 ± 7 Bq/kg.The radioactivity of 137 Cs was 3.7 ± 0.2 in the graphite-poor sample.
It should be noted that a household water well is located approximately 200 m from the observation point, in the vicinity of a river, so that the water should be tested for uranium as carried out in Montana [26].
The highest Sr concentrations were noted in the graphitic schists of the Inovo Series with less organic material and the grade was 244.78 ppm.
Elevated concentrations of uranium in graphite schist are a result of redistribution of ore components under dynamic-thermal metamorphism conditions.
All the gray siltstones of the Multicolored Series measured uranium concentrations from 49.36 to 54.43 ppm.In this case the concentrations of thorium were lower by a factor of about 4 (from 11.83 to 13.13 ppm).Lead concentrations were similar to those of Th and ranged from 11.59 to 18.18 ppm.These samples exhibited somewhat elevated concentrations of Ba (from 590.14 to 603.42 ppm).The radioactivity of 226 Ra was relatively low (102 ± 5 Bq/kg) and 232 Th measured 97 ± 5 Bq/kg.The radioactivity of 40 K was relatively high (2080 ± 90 Bq/kg).The values of 40 K of the other samples were lower and ranged from 900 ± 40 to 1700 ± 80 Bq/kg (the lowest in the case of a highly silicified sample with limonite stains collected near Mezdreja).
The red sandstones of the Multicolored Series measured relatively low concentrations of uranium (Figure 5B), from 7.78 to 12.64 ppm, and of thorium from 3.42 to 9.25 ppm.Lead concentrations varied from 8.42 to 11.81 ppm.These samples measured the lowest radioactivity of 226 Ra-28 ± 1, and 232 Th was 52 ± 3 Bq/kg.
In all sedimentary units the concentrations of strontium were much lower (in relation to granite and graphite schist samples) and generally varied from 90 to 120 ppm.
By way of an explanation of the uranium concentrations in the red members (mainly sandstone) of the Multicolored Series, it should be noted that the reduction of mobile uranium (U 6+ ) to insoluble uranium (U 4+ ), such as uraninite, takes place when the fugasity of oxygen in solution drops.This reaction occurs on account of iron or sulfur oxidation.When the solution (water) is rich in oxygen, bivalent iron will oxidize into trivalent iron, but if there is an excess of Fe 2+ relative to the oxygen, the oxygen will be spent and the uranyl ion is the one to convert bivalent iron into trivalent, or sulfides to sulfates, and is itself precipitated as uraninite.It is known in geology that iron oxidation in nature can result in uranium-bearing hematite [27].The typical red color is attributed to Fe hydroxides, largely deposited as cement.Their widespread presence suggests considerable incoming Fe from the Zaglavak gabbroids (granites carry a much smaller amount of Fe).
Uranium concentrations in the case of "Multicolored Series" sediments (gray siltstone) are attributable to adsorption to clays and organic-coaly material (well described in Wang et al. [28]).This process takes place in the presence of specific adsorbents such as clay minerals, limonite, carbonate sediments or organic substances, humic acid or caustobiolites, with which the uranium-bearing solution comes into contact.Muto et al. [29] tested uranium adsorption to the clays commonly found in nature: montmorillonite, haloizite and kaolin.The results they reported show that uranium is fixed most efficiently at pH from 6.1 to 6.2.
In the Dojkinci area, pH measured in the river was about seven, locally 7.4.In addition, in the Dojkinci-Jelovica area uranium migrativity and a reduction environment have been noted [30].Uranium is being deposited in a reduction environment, after the change of redox conditions.Organic substances are the major reducers of uranium, followed by iron compounds and clay minerals [31].In the zone of the geochemical barrier in Permo-Triassic sediments, pH levels of water recently measured about seven and Eh about 140 mV.

Conclusions
One of the major causes of elevation of naturally occurring radionuclide material concentrations on the Earth's surface is mining [32].All the above-mentioned occurrences can conditionally be deemed natural.Still, the Mezdreja and Gabrovnica mine dumps carried non-processed material.It is also a fact that these areas are relatively large and that the radioactive material of the described samples is continuously drained into watercourses.According to Dragović et al. [33], the total gamma dose rate in the areas of Mezdreja and Gabrovnica is double the world average.
The samples shown in Figure 4A,C were collected next to a road.Peng et al. [34] show that the groundwater from an oxidizing aquifer with a high dissolved oxygen concentration (O 2 ), like in the case of the Jelovička and Dojkinčka rivers is enriched in U.The material from this and similar outcrops is regularly washed out by runoff after heavy rainfall or snowmelt, into the Jelovička River (a tributary of the Dojkinačka).
There is widespread contamination of the environment due to natural and anthropogenic enrichment of radionuclides in the world.In soil samples and alluvial sediments from Gabrovnica and Mezdreja mines, an increased concentration of uranium in relation to natural background levels was noted [35].In the Niger Delta [36], for example, the highest activity concentration in all fish species of gamma emitting radionuclides was observed for 40 K, followed by 238 U, 232 Th and 226 Ra.Exploring the Gawib River floodplain in Namibia, Abive and Shaduka 2017 concluded that the radioactive contaminants can spread into the deeper aquifer system through major structures such as joints and faults [37].
The study area in Serbia used to be well-known for livestock breeding and the production of cheese and meat, especially between the two world wars.However, since the 1970s the population has been migrating to industrial centres.Now, some of the villages are completely abandoned and the average age of the sparse population is above 60.There has never been any systematic monitoring of the impact of naturally elevated radioactivity on human and animal health, such that no data has been compiled.

Figure 1 .
Figure 1.Schematic geological map of Stara Planina Mt. with areas of elevated radioactivity (pink).

Figure 1 .
Figure 1.Schematic geological map of Stara Planina Mt. with areas of elevated radioactivity (pink).

Figure 2 .
Figure 2. Samples from Mezdreja mine and Gabrovnica mine areas.(A) Granite from the vicinity of the Mezdreja mine portal; (B) granite form the Gabrovnica mine dump; (C) white mica developed in a granite sample from Mezdreja; (D) white mica developed in a granite sample from Ganbrovnica; (E) gabbroid with large pink K feldspar; (F) material from the Mezdeja mine dump.

Figure 2 .
Figure 2. Samples from Mezdreja mine and Gabrovnica mine areas.(A) Granite from the vicinity of the Mezdreja mine portal; (B) granite form the Gabrovnica mine dump; (C) white mica developed in a granite sample from Mezdreja; (D) white mica developed in a granite sample from Ganbrovnica; (E) gabbroid with large pink K feldspar; (F) material from the Mezdeja mine dump.

Figure 3 .
Figure 3. (A) Outcrop of graphitic schist from the Inovska Reka occurrence; (B) variety with more petrogenic minerals; (C) variety with more graphitic material; (D) thin section of the sample shown in (C).

Figure 3 .
Figure 3. (A) Outcrop of graphitic schist from the Inovska Reka occurrence; (B) variety with more petrogenic minerals; (C) variety with more graphitic material; (D) thin section of the sample shown in (C).

Table 5 .
Radioactivity of characteristic samples from the Inovska Reka occurrence.

Table 1 .
Radioactivity of characteristic samples from the Mezdreja mine area.

Table 2 .
Assays from the Mezdreja mine area (values are in ppm).

Table 3 .
Radioactivity of characteristic samples from the Gabrovnica mine area.

Table 4 .
Assays from Gabrovnica mine area (values are in ppm).

Table 5 .
Radioactivity of characteristic samples from the Inovska Reka occurrence.

Table 7 .
Radioactivity of characteristic samples from "Multicolored Series" sediments.

Table 8 .
Assays related to "Multicolored Series" sediments (values are in ppm).