Investigation of Trace and Critical Elements ( Including Actinides ) in Flotation Sulphide Concentrates of Kassandra Mines ( Chalkidiki , Greece )

Pyrite/arsenopyrite (Py-AsPy), galena (PbS), and sphalerite (ZnS) concentrates from the flotation plants of Olympias and Stratoni (Kassandra mines, Chalkidiki, N. Greece) were investigated for their major, trace, minor, and critical element contents, including actinides associated to natural radioactivity. It is revealed that in addition to the Pb, Zn, Ag, and Au being exploited by Hellas Gold S.A., there are also significant concentrations of Sb and Ga (Sb: >0.2 wt.% in PbS concentrate; Ga:25 ppm in ZnS concentrate), but no considerable contents of Bi, Co, V, or REE. Concerning other elements, As was found in elevated concentrations (>1 wt.% in Py-(As)Py-AsPy Olympias concentrate and almost 1 wt.% in Stratoni PbS and ZnS concentrates) together with Cd (specifically in ZnS concentrate). However, actinides occurred in very low concentrations (U < 2 ppm and Th < 0.5 ppm in all examined concentrates), limiting the possibility of natural radioactivity in the Hellas Gold S.A. products. The concentrations of the natural radionuclides (238U, 232Th, and 40K) are much lower than those of commercial granitic rocks, and thus the associated radioactive dose is insignificant.


Kassandra Mines Flotation Concentrates
The Au-Cu and Au-Ag-Pb-Zn-Cu Kassandra deposits are spread across several mines located in the Chalkidiki peninsula, Northern Greece.Presently, production is being held in the Olympias and Stratoni mines comprising two deposits: Madem Lakkos (currently not in production) and Mavres Petres.Currently, the Kassandra mines are operated by Hellenic Gold S.A. [1] and produce PbS (galena), ZnS (sphalerite), and Fe-As-S (pyrite/arsenopyrite) concentrates in two flotation plants, constructed during the 1970s, at Stratoni and Olympias areas.Of course, the concentrates are not pure sulfides as is expected from the global hydrometallurgical practice (purity varies between 80-90%).It has to be noted that the Olympias mine recently restarted (2018) the production of concentrates containing Pb, Zn, Ag, and Au after about 10 years of pause.
In the flotation plants, for the concentration of PbS, pH value is set at 9-10 by using Ca(OH) 2 and other mineral phases are depressed by the addition of NaCN solution.Galena flotation is carried out with the addition of solutions of foaming and collector agents.For the concentration of ZnS, the pH value is set at 10-11 by using Ca(OH) 2 .For sphalerite activation, CuSO 4 •5H 2 O solution is added and the flotation is carried out with the addition of foaming and collector agent solutions.Finally, for the concentration of pyrites, the pH value is set at 6.5-7 by the addition of H 2 SO 4 .For the activation of the pyrites mixture, CuSO 4 •5H 2 O solution is added and the flotation is carried out with the addition of foaming and collector agent solutions.

Kassandra Deposits and Previous Work on the Mineralogy and Geochemistry of the Concentrates
The Kassandra mining district contains porphyry Au-Cu and Au-Ag-Pb-Zn-Cu carbonate replacement deposits that are associated with Oligocene-Miocene intrusions emplaced into poly-deformed metamorphic basement rocks belonging to the Permo-Carboniferous to Late Jurassic Kerdilion unit and the Ordovician-Silurian Vertiskos unit.Regional extensional tectonics active since the middle Eocene resulted in the development of widespread normal and transtensional faults, including the Stratoni fault zone that hosts carbonate replacement sulfide ore bodies [2].More particularly, Stratoni (Madem Lakkos, Mavres Petres) and Olympias are the two main carbonate-replacement massive sulfide Pb-Zn (Ag-Au) deposits of the district.They are located on the footwall of the Tertiary Stratoni-Varvara fault.Both deposits are interpreted to form the proximal and distal parts of a fault-controlled exoskarn-type ore system triggered by nearby small-scale intrusions close to the fault system [3].Sulfide mineralization occurs within amphibolite-grade metamorphic rocks (including marbles) of the Kerdylion assemblage.The assemblage represents a metamorphosed marine sedimentary-volcanic sequence of probable Mesozoic or older age.Eocene and Oligocene age granitic and granodioritic intrusions occur throughout the Kerdylion unit, mainly as pegmatite and granite dykes of several generations that range from syn-to post-metamorphic in age.The sequence is affected by syn-peak metamorphic penetrative deformation that is manifested by a dominant shallow dipping layer-parallel foliation.At least two other foliation-forming events affect the sequence with progressively less strain, as well as significant late extensional faulting.
Previous works [4][5][6][7][8] have interpreted the area to lie at the southwestern margin of the Rhodope complex, and that the shallow dipping foliations which are present formed in response to tertiary unroofing of the Rhodope complex as a metamorphic core complex.Other interpretations suggest that the fabrics are contractional and that the fault may remobilize a major reverse structure that superimposed the Vertiskos unit against the Kerdylion.Geological relationships suggest that the metamorphic fabrics represent contractional rather than extensional fabrics, and the Stratoni fault as is currently manifested is dominantly a later, lower greenschist-grade extensional structure that is superimposed onto the amphibolite-grade fabrics.
Mineralization at Olympias and Stratoni (Madem Lakkos-Mavres Petres) is considered (e.g., [2,3]) as carbonate replacement, and is associated with a marble horizon.Mineralization occurred as a late structural event and is superimposed on the metamorphic fabrics of the area.It is associated with an extensional, brittle to semi-brittle fault network that was likely active coevally with the ore-hosting Stratoni fault to the south.

Scope of the Present Study
Although there are many studies regarding the untreated ores, to the best of our knowledge there are no published works about the mineralogy and geochemistry of the flotation concentrates produced by Kassandra mines since the 1970s.Additionally, there are no published data about trace or critical elements in these hydrometallurgy (flotation) products.Critical elements, which are usually reported as Critical Raw Materials (CRMs), are those elements that are economically and strategically important for the economy but have a high risk associated with their supply.It is important to note that these materials are not classified as "critical" because they are considered scarce; rather, they are classified as such because they have a significant economic importance for key sectors in the economy, such as consumer electronics, environmental technologies, automotive, aerospace, defense, health, and steel.They have a high supply risk due to the very-high import dependence and high level of concentration of set CRMs in particular countries and there is a lack of (viable) substitutes due to the very unique and reliable properties of these materials for existing as well as future applications.
Thus, the scope of the present study was to report new results concerning: (i) the mineral chemistry and formulae of the sulfide minerals in the concentrates; (ii) the minor, trace, and critical element content, specifically REE, Sb, Bi, Ge, V, Ga, and Co; (iii) the actinide element content (U, Th) and their natural radioactivity.Radioisotopes present in the environment can be classified as naturally occurring, and are components of the Earth's crust since its formation (e.g., 238 U, 235 U, 232 Th, and their decay products as well as 40 K), cosmogenic radioisotopes (radioisotopes that are produced by the interaction between cosmic radiation and the atmosphere (e.g., 14 C, 10 Be, 44 Ti, and 22 Na), and finally artificially produced radionuclides that are produced in nuclear reactors (e.g., 90 Sr and 137 Cs).Natural radionuclides can be found in soil, rocks, water, air, food, building materials, etc.The study of the natural radioactivity present in geological materials and ores is an important subject in environmental radiological protection as it provides the possibility to assess any associated health hazard.In this paper, the products of the Kassandra mines are studied for their natural radioactivity.Moreover, the results are explained by the bulk geochemistry of the samples.

Samples
The studied composite samples were provided by the mining company.The three representative composite pyrite/arsenopyrite (Py-AsPy from the Olympias flotation plant), galena (PbS from the Stratoni flotation plant), and sphalerite (ZnS from the Stratoni flotation plant) concentrates-in powdered form-were supplied by Hellas Gold S.A. (Kassandra mines, Chalkidiki, N. Greece).

Point Analyses
Scanning electron microscopy (SEM) images of free mineral grains and microprobe analyses of polished mineral grains (after examination in an optical microscope, see Figure 1) were obtained at the Earth Sciences Department of the University of Milan, using a JEOL 8200 (Tokyo, Japan) electron probe micro-analyzer (EPMA) equipped with a wavelength dispersive spectrometer (WDS).Analytical conditions were: 15 kV accelerating voltage, 15 nA beam current, and 2 µm beam diameter with a counting time of 20 s on the peaks and 10 s on the background.The approximate detection limit was 0.01 wt.% for each element.

Bulk Analyses
Major and trace elements in the powdered concentrates were analyzed using a Perkin Elmer ICP-OES (Waltham, MA, USA) and a Perkin Elmer Sciex Elan 9000 ICP-MS (Waltham, MA, USA) following a LiBO2/LiB4O7 fusion and HNO3 digestion of the fused solid sample, in both Hellas Gold S.A. and external collaborating laboratories (for QA/QC details see Supplementary Materials Table S1).

Gamma-Ray Spectroscopy
Samples were measured in the laboratories of the Department of Physics (Aristotle University of Thessaloniki).After being oven-dried at 60 • C to constant weight, the samples were measured using two high-resolution gamma ray spectrometry systems.The first one consisted of a high-purity Ge (HPGe) coaxial detector with 42% efficiency and 2.0 keV resolution at 1.33 MeV photons, shielded by 4" Pb, 1 mm Cd, and 1 mm Cu and the second one consisted of a low-energy Ge (LEGe) planar detector with 0.7 keV resolution at 122 keV photons, shielded by 3.3" Fe-Pb, 1 mm Cd, and 1 mm Cu.The first spectrometry system with the HPGe detector was used to measure the majority of the natural radionuclides examined in this study, except 238 U.The second one with the low-energy planar Ge detector was used to determine only the concentration of 238 U, considering the low-energy γ-ray of 63 keV emitted by its daughter 234 Th.
The 40 K content was obtained using its 1461 keV γ-ray.The 232 Th content was calculated as the weighted mean value of 228 Ra concentration (measured as 228 Ac, using 911, 968, and 338 keV γ-rays) and 228 Th concentration (measured as decay products in equilibrium, i.e., 212 Pb, using 238 and 300 keV γ-rays; 212 Bi, using 727 keV γ-rays; and 208 Tl, using 2614, 583, and 860 keV γ-rays).The determination of the 226 Ra content was based on the measurement of 222 Rn decay products being in equilibrium.The measurement of 226 Ra from its own γ-ray at 186.25 keV introduced some problems because of the adjacent photo peak of 235 U at 185.75 keV, so the isotopic ratio between 235 U and 238 U was considered as the natural one (i.e., 0.0072) and secular equilibrium between 238 U and 226 Ra had to be assumed.Accuracy in the measurements of 226 Ra concentrations by 222 Rn decay products depended on the integral trapping of radon gas in the sample volume, so a small addition (~2%) of charcoal in powder form (less than 400 µm in size) was mixed with the sample before sealing it hermetically and storing it in a freezer during the 222 Rn in-growth period [21].The efficiency calibration of the gamma spectrometry systems was performed with the radionuclide specific efficiency method in order to avoid any uncertainty in gamma ray intensities as well as the influence of coincidence summation and self-absorption effects of the emitting gamma photons.A set of high-quality certified reference materials (RGU-1, RGTh-1, RGK-1) [22] was used, with densities similar to the average beach sands measured after pulverization.Cylindrical geometry was used assuming that the radioactivity was homogenously distributed in the measuring samples.The samples were measured up to 200,000 s in order to achieve a minimum detectable activity of 12 Bq•kg −1 for 40 K, 4 Bq•kg −1 for 232 Th, 2 Bq•kg −1 for 228 Th, 2 Bq•kg −1 for 226 Ra, and 21 Bq•kg −1 for 238 U, with 33% uncertainty.The total uncertainty of the radioactivity levels was calculated by propagation of the systematic and random errors of measurements.The systematic errors in the efficiency calibration ranged from 0.3-2% and the random errors of the radioactivity measurements extended up to 19%, except in the 238 U measurement, where the error extended up to 50% for activities measured lower 10 Bq•kg −1 .

Mineral Chemistry
The SEM and EPMA data concerning the mineral chemistry of the sulfide minerals (major phases) in the concentrates from the flotation plants of Stratoni and Olympias mines are given in Figures 2-4 and Tables 1-3.

Geochemistry
The bulk chemical composition (ICP-OES/MS) of the studied Kassandra mines concentrates are given in Table 4.It is obvious that basic and noble metals (Pb, Zn, Ag, and Au) being exploited by Hellas Gold S.A. showed high concentrations, as well as Sb and Ga (Sb: >0.2 wt.% in PbS concentrate; Ga:25 ppm in ZnS concentrate).On the other hand, there were no considerable contents of Bi, Co, V, and REE.Considering other elements, as was found in elevated concentrations (>1 wt.% in Py-AsPy Olympias concentrate and almost 1 wt.% in Stratoni PbS and ZnS concentrates) along with Cd (specifically in ZnS concentrate).Moreover, actinides occurred in very low concentrations (U < 2 ppm and Th < 0.5 ppm in all concentrates).

Actinide Elements and Natural Radioactivity
The concentrations of the natural radionuclides detected by gamma-ray spectroscopy are given in Table 5.All radionuclides activity concentrations determined ( 238 U, 232 Th-series, and 40 K) were relatively low and the results were in good agreement with the bulk chemical composition of the samples in actinides determined by ICP-OES/MS.

Discussion
All sulfide minerals studied were found to exhibit typical/expected chemical compositions in major elements."Invisible" gold was found in relevant high concentrations in pyrite and As-pyrite from the Olympias mine and this is part of ongoing research on these samples targeting the characterization of noble metals-preliminary results have been published elsewhere [19,23].
The enrichment and depletion of the studied elements can be revealed from the normalization to the upper continental crust (UCC) (Figure 5).REEs and other elements like Cs, Rb, Co, Ni, Ba, and V were depleted.As expected, major elements like Pb, Zn, and Cu were enriched, as were other trace elements like Mo, As, Sb, Se, Sn, Cd, Hg, Rb, and U. Note that the enrichment in these trace elements, relative to UCC, except for its geochemical significance, may also imply element enrichment or depletion related to practical mining and metallurgical issues.For instance, if we consider U, the bulk natural radioactivity of the samples was negligible.
The latter was confirmed by the concentrations of the radionuclides of 238 U, 232 Th-series, and 40 K, which were small and close to the detection limit of gamma-ray spectroscopy.These small concentrations are mainly due to the small ability of the chemical components of the sulfides to be substituted by the measured radionuclides.Moreover, low concentrations of these radionuclides have been detected in the Stratoni granitic bodies [24].Similar conclusions on the U content of sulfides were previously reported in References [25,26].However, the previous researchers mention that high U concentrations may be present in the late accessory mineral phases deposited in micro fissures.These values are far lower than a typical granitic rock used as building material [26].Therefore, the radioactive dose to humans from these materials is insignificant.

Figure 4 .
Figure 4. BSE image of free sphalerite grain with galena veins in Stratoni ZnS concentrate.

Figure 4 .
Figure 4. BSE image of free sphalerite grain with galena veins in Stratoni ZnS concentrate.

Table 2 .
EPMA analyses of mineral phases present in Stratoni PbS concentrate (bdl: below detection limit).

Table 4 .
Trace and critical element concentrations of the studied Kassandra mines concentrates.
Spider diagram of sample/upper continental crust (UCC).