Preliminary chemical and mineralogical characterization of tailings from base metal sulfide deposits in Serbia and North Macedonia

Tailings of old mines often contain metals, which were not of economic interest or could not be recovered with the existing technology at the time of active mining. This is especially true for metals that often occur as by-products in Cu-Pb-Zn-(Ag-Au) ores as Sb, Mo, Ge, and In. A fundamental characterization of some tailings is presented in terms of their mineralogy and content of valuable metals which could be extracted to finance a possible remediation and improve the supply of the EU with critical metals. Tailings from active and abandoned mines in Serbia (Bor, porphyry Cu/Au; Krivelj, porphyry Cu/Au; Blagodat, hydrothermal Pb-Zn; Lece, epithermal Au; Rudnik, hydrothermal/skarn Pb-Zn) and North Macedonia (Sasa, Pb-Zn; Probištip, Pb-Zn; Bučim, porphyry Cu; Lojane, fault-bound vein-type low-temperature As, Sb, Cr at the contact of rhyolite and serpentinite) were studied. Analysis for major and trace elements used a multi-method ap-proach (lithium borate fusion and ICP-MS/OES analysis, gravimetric analysis, instrumental neu- tron activation analysis, total digestion ICP-OES, infrared spectroscopy) with mineral identifica tion by scanning electron microscopy. Concentrations of the major commodity elements (Cu, Pb, Zn, Au, Ag) varies within several orders of magnitude depending on mineralogy and ore type. Critical metals (Co, Ga, Ge, Sb) contents are low with some exceptions. Some tailings contain moderate to elevated potentially toxic element levels (As, Cd, Pb, Tl). For the sample from Probištip which yielded the highest valuable metal concentrations (>5000 ppm Pb, 4020 ppm Zn), a heavy mineral concentrate of the sand size fraction (0.06 mm to 0.5 mm) was produced and analyzed by SEM and LA-ICP-MS for additional rare phases and trace elements. In all tail ings studied, additional milling would be needed to separate ore from gangue minerals. Increasing metal prices might facilitate feasibility studies for some of the localities in the future, despite the limited quantitative information about the characterized tailings.


INTRODUCTION
Tailings are defined by LOTTERMOSER (2010) as "the processing waste from a mill, washery or concentrator that removed the economic metals, minerals, mineral fuels or coal from the mined resource". Tailings of abandoned mines often contain significant amounts of metals, which could not be recovered with the existing technology at the time of mining, were not of economic interest, or were not analyzed at that time. The latter holds true for ronment. This mineral surface was mostly "fresh", i.e., unweathered at the time of mining. This can present a hazard for the local environment, as leaching of toxic minerals and acid mine drainage can be greatly enhanced. This long ongoing weathering is also a threat to possible mining of the tailings as it may have changed the original ores significantly. Alternatively, large tailings can also represent an opportunity for recovering valuable metals needed for high-tech applications.

STUDY SITES
Material from the tailings dams of the Bor, Krivelj, Grot/Blagodat, Lece, Rudnik (all located in Serbia), and Sasa, Probištip, Bučim and Lojane (located in North Macedonia) mines were collected in order to gain an initial overview of their chemical and mineralogical composition and to estimate the potential for future metal recovery. Locations are shown in Fig. 1, while coordinates of the sampled locations, deposit status, the surface of the tailings and the quantity stored in the tailings are listed in Table 1. Table 2 presents basic information on the ore deposit type, host rocks and reserves of the primary deposits related to the sampled tailing dams as provided in the literature. Concerning environmental hazards, the tailings of Probištip (acid mine drainage) and Lojane (high concentrations of As and Sb in the tailings) must be noted. The localities Bor and Krivelj are both part of the same deposit.

Bor and Krivelj
The Bor deposit is part of the Bor Metallogenetic Zone and formed as a porphyry copper deposit (JELENKOVIĆ et al. 2016).
It constitutes the type location of the Bor Metallogenetic Zone which is part of the Tethyan Eurasian Metallogenetic Belt (JANKOVIĆ 1977). The deposit formed in the Upper Cretaceous due to subduction related magmatism as part of the Banatitic Magmatic Belt (JELENKOVIĆ et al. 2016 and references therein). Typical ore minerals are pyrite, enargite, tetrahedrite, bornite, and chalcopyrite. The reserves of the deposit are 130.5 million tons at 0.56 % Cu, 0.24 g/t Au, and 1.66 g/t Ag; the inferred resources are 450.8 million tons at 0.49 % Cu, 0.1 g/t Au, and 1.72 g/t Ag (JELENKOVIĆ et al. 2008). The tailings of Bor (Fig. 2 a

Lece
The Lece mine is situated in the Lece volcanic complex in SE Serbia at the border of the Eastern Vardar Ophiolite zone and the Serbo-Macedonian unit (VELOJIĆ et al. 2018). The deposit is a high and low sulphidation epithermal gold mineralization hosted by andesites and quartz veins (JELENKOVIĆ et al. 2008), which formed during Oligocene-Miocene magmatism (VELOJIĆ et al. 2018). Ore minerals are galena, sphalerite, pyrite, and gold (JELENKOVIĆ et al. 2008). Reserves are 3.4 million tons at 1.8% Pb, 3.5% Zn, and 3.3 g/t Au (JELENKOVIĆ et al. 2008). The tailings of Lece (Fig. 3 a) cover an area of 23 ha (GEOLOGICAL SURVEY OF SLOVENIA 2022).

Rudnik
The Rudnik deposit is a hydrothermal skarn Pb-Zn deposit hosted by limestones, flysch and quartz latites (JELENKOVIĆ et al. 2008). The deposit is part of the Šumadija metallogenetic zone in the Vardar zone and was formed by Oligocene-Miocene magmatism (STOJAVONIĆ et al. 2006) from fluids of magmatic origin

Probištip
The Probištip tailings dam comprises tailings of the Zletovo subvolcanic hydrothermal Pb-Zn deposit related to Cenozoic volcanism (SERAFIMOVSKI et al. 2006). The deposit has been mined from medieval times until recently (SPASOVSKI & SPASOVSKI 2013). An apparently long ongoing problem is acid mine drainage as the river into which the mine dewaters (SPASOVSKI & SPASOVSKI 2013) is called Kiselnica, which can be literally translated to "acid". It is unknown, if this name refers to natural 2008). The tailings of Rudnik (Fig. 3 b) cover an area of 41 ha (GEOLOGICAL SURVEY OF SLOVENIA 2022).

Lojane
Lojane is a former As, Sb, and Cr mine in Northern Macedonia, which was in operation from 1923-1979(TASEV et al. 2017. Veins formed at the contact of rhyolite and podiform chromite bearing ophiolite (KOLITSCH et al. 2018). The main ore minerals are chromite and As and Sb phases: orpiment, realgar, and stibnite; additional phases include bravoite, vaesite, cattierite, pyrite, marcasite, molybdenite, and pitchblende (TASEV et al. 2017). See KOLITSCH et al. (2018) for an exhaustive list of primary and secondary minerals. The reserves in the mine are estimated at 0.3 million tons (CASSARD et al. 2015). The completely unprotected tailings hold 3.5 million tons of which 15 000 tons are 50 % As concentrate (TASEV et al. 2017). Currently, the acid mine drainage potential is low, but the waste material is distributed by wind, surface water and groundwater (UIBLEIN 2021). The tailings of Lojane ( Fig. 4 b) cover an area of 3 ha (GEOLOG-ICAL SURVEY OF SLOVENIA 2022).

METHODS
The 10 kg samples were collected by the Geological Survey of Slovenia from flotation tailings, which were already affected by remnants of vegetation. Representative samples were obtained by taking composite samples from the surface, trenches, and handdrilled holes. At the facilities of the Geological Survey of Slovenia, the samples were dried at 40 °C, crushed and homogenized. The resulting fine sand-size material was embedded in resin and polished for examination by reflected light microscopy and scanning-electron microscopy (SEM-EDX) employing a Zeiss Evo MA 10 with Bruker energy-dispersive analysis system. Additionally, for sample Pr01a from Probištip, a heavy mineral concentrate was produced by sieving the sample to a grain size between 60 µm and 500 µm, and separation with bromoform (density of 2.89 g/cm³). After washing and drying, the sample was embedded in resin and polished for examination by SEM-EDX. Subsequently, sphalerite identified in sample Pr01a was examined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), utilizing an ESI Nd:YAG laser ablation system coupled to a PerkinElmer NexION 2000 ICP-MS. LA-ICP-MS analyses were carried out at the Department of Applied Geosciences and Geophysics, Montanuniversität Leoben in Austria. Helium was used as carrier gas (flow rate of 0.75 L/min) and mixed with Ar during transport into the ICP-MS. A spot size of 50 μm was used for all analyses with an on-mineral fluency of 2-3 J/cm 2 . Individual spot analyses are composed of 30s background collection (pre-ablation) followed by 60s of ablation and data collection. Thirty seconds of washout followed each laser ablation cycle.  2002) was used for these elements. To ensure the quality of the analyses, the MASS-1 was used for quality control. The Iolite 4 software (PATON et al. 2011) was utilized for data reduction of the measured count signals into concentrations and detection limits. Detection limits were calculated using the 'Howell et al. 2010' method found in the Iolite 4 software. Instrumental drift correction was done by repeatedly measuring the reference materials. Time-resolved count signals of the different isotopes were scrutinized during data reduction to identify possible inclusions within the ablated sphalerite.

RESULTS
All samples showed traces of organic material (mainly plant roots) and signs of advanced weathering (mostly reddish colours and the typical smell of weathered sulfides) and have a grain size in the silt to fine-sand range (0.002 mm to 0.2 mm). Many grains had weathered rims and showed fractures probably related to processing. The weathered rusty iron-rich rims are indicated as FeOx as the determination of the water/OH content is not possible using SEM analysis and a full characterization of those phases was not relevant for this study. Table 3 presents the minerals identified by SEM in each sample. Mineral abundances were estimated qualitatively based on the SEM analyses. Rare minerals were identified a few times (less than 10) per sample and accessory minerals were only identified once or twice per sample. Mineral formulae of uncommon minerals are given in the Appendix.
In all cases except for Grot/Blagodat and Lojane, the samples were dominated by large silicate grains, mainly quartz and feldspars. In the case of Grot/Blagodat, the sample was dominated by large grains of quartz and pyrite. The sample from Lojane was dominated by iron oxides, olivine, quartz, and realgar. In Figs. 5 -7, representative mineral associations of the tailings samples are illustrated using back-scattered electron images (BSE).
In the Bor tailings, the sulfides are mostly intergrown with quartz. In the Krivelj tailing, part of the sulfides occur as free grains. In both tailings, the sulfides have a grain size of 1 µm to 50 µm.

Grot/Blagodat
The sample from Grot (Fig. 5c) shows a polymodal size distribution with 100 µm to 300 µm, 25 µm to 50 µm, and < 10 µm size classes. The two larger size classes are dominated by quartz and pyrite, which forms liberated grains of up to 50 µm and also occurs as inclusions in quartz. Abundant minerals in the two smaller size classes are biotite, FeOx, calcite, siderite, rutile, and jacobsite. Rare minerals are baryte, apatite, allanite, haematite, magnetite, phlogopite, xenotime, and zircon. The sulfides reach a grain size of 10 µm to 30 µm and may be present as liberated grains and as inclusions in quartz.

Lece
The sample from Lece (Fig. 5d) shows a bimodal grain size distribution with the majority (by volume) of grains in the 50 -200 µm size range and a fine fraction of < 5 µm. The main minerals are quartz, plagioclase, and orthoclase along with FeOx as an abundant phase. Baryte, galena, sphalerite, kaolinite, rutile, titanomagnetite, and zircon are rare minerals. Haematite, Fe-sulfates, and tetrahedrite-(Zn) are present as accessories. The sulfides attain a grain size of <5 µm to 30 µm and are mainly present as inclusions in quartz.

Rudnik
The sample from Rudnik (Fig. 6a) shows a bimodal grain size distribution with the majority (by volume) of grains in the 30 µm to 200 µm size range and a fine fraction with a size of ~ 5 µm. The main minerals are amphibole and quartz. Abundant minerals are andradite, pyroxene, chlorite, FeOx, orthoclase, and titanite. Rare minerals are galena, ilmenite, pyrrhotite, sphalerite, and zircon. The sulfides have a grain size of 5 µm to 30 µm and are mainly present as inclusions in quartz.

Sasa
The sample from Sasa (Fig. 6b) shows a bimodal grain size distribution with the majority of grains in the 30 -200 µm size range and a fine fraction with a size of ~ 5 µm. The main minerals are kaolinite and quartz. Abundant minerals are chlorite, orthopy-  roxene, clinopyroxene (e.g., augite), orthoclase, muscovite, epidote, FeOx, galena, andradite garnet, Mn-calcite, and pyrite. Rare minerals are apatite, rutile, siderite, pyrrhotite, and sphalerite. Accessories are monazite, rhodonite, and zircon. The grain size of sulfides that are either liberated or intergrown with silicates, ranges from 5 µm to 200 µm.

Probištip
The grain size of the sample from Probištip (Fig. 6c, d)

Lojane
The sample from Lojane (Fig. 7b) shows a bimodal size distribution with 50 -200 µm and 5 -20 µm size classes. The main minerals are FeOx, olivine, quartz, orpiment and realgar. Abundant minerals are albite, ankerite, dimorphite, dolomite, magnesite, orthoclase, plagioclase, pyrite, and siderite. Rare minerals are angelellite, cervantite, chromite, enstatite, muscovite, phlogopite, stibnite, and wakabayashilite. The sulfides are mostly free and present in all size classes. Table 4 presents the analytical whole-rock data of the samples. In the following, the measured concentrations of the major commodity metals Cu, Zn, Pb, Ag and Au are briefly described, along with the levels of critical metals to the EU (Co, Ga, Ge, In, Sb) (LØVIK et al. 2018) and potentially toxic trace elements (As, Cd, Tl).

BULK CHEMICAL ANALYSES
The Bor and Krivelj tailings show expected similarities as they originate from the same deposit. However, the sample from Krivelj shows a higher amount of Cu (560 ppm), whereas S concentrations are higher in the Bor sample, indicating a higher proportion of pyrite. Based on the SEM analyses, Cu is mainly hosted by chalcopyrite. Concentrations of Au are higher in Bor (0.3 ppm) than in Krivelj (0.04 ppm). Compared to more massive Cu-Zn-Pb sulfide ores, the concentrations of critical metals are generally low with 3 ppm Co, 15 ppm Ga, 2 ppm Ge, 0.4 ppm Sb, < 0.2 ppm In for Krivelj, with the values for Bor even being slightly lower.
The Grot/Blagodat tailings contain Pb (3870 ppm) and Zn (2980 ppm), Cu (96 ppm) and REE (60 ppm Ce). Lead and Zn are hosted by galena and sphalerite, respectively. Cu bearing minerals were not observed. Critical element concentrations are low, but potentially toxic elements are slightly elevated (101 ppm As, 16 ppm Cd, 7 ppm Tl). The concentrations of Au (0.08 ppm) and Ag (<1 ppm) are too low to regard these elements as potential bonuses for a case for reprocessing.
The Lece tailings contain 455 ppm Zn, 1680 ppm Pb, 0.5 ppm Au, but also 722 ppm As and 9.3 ppm Tl. Zinc is present in sphalerite, Pb in galena, and Ba (600 ppm) in baryte. No As-bearing minerals were discovered by SEM analysis, suggesting that As is hosted by Fe oxide or other secondary phases. The low S concentrations (0.2%) indicate that pyrite is only of very minor importance. Among the critical metals, only Sb is of importance, showing concentrations of 75 ppm.
The Rudnik tailings contain 160 ppm Cr, 230 ppm Cu, 540 ppm Zn, and 1710 ppm Pb. No Cr or Cu bearing minerals were found, but as serpentinites are present in the deposit , such Cr contents can be expected. It remains unclear though, which mineral phase hosts the Cr (probably chromite or Cr-bearing chlorite). Zinc is hosted by sphalerite; Pb is mainly hosted by galena, and additionally by anglesite. Precious metal concentrations are low (5.7 ppm Ag, <2 ppb Au), as well as high-tech metals. Among the potentially toxic elements, As has a concentration of 67 ppm, whereas Cd and Tl are below 1 ppm. The Rudnik sample carries the highest concentration of Bi (11 ppm) analyzed in this study.
The Sasa tailings contain 4540 ppm Pb, 1900 Zn ppm, 90 ppb Au, 240 ppm Cu, and 520 ppm Ba. Zinc is present as sphalerite, Pb as galena, and Ce (54 ppm) mainly in monazite. From the literature (STRIMIĆ PALINKAS et al. 2013) it can be expected that Cu is present in chalcopyrite and Ba in baryte. Among the high-tech metals, Ge attains the highest value recorded in this  Table 4. Chemical analysis data. DL = detection limit; methods: FUS-ICP = lithium borate fusion and ICP-MS/OES analysis; GRAV = gravimetric; INAA = instrumental neutron activation analysis; TD-ICP = total digestion ICP-OES; CO2 = infrared spectroscopy. Copper is hosted by chalcopyrite, Pb by galena (especially its alteration product cerussite), and Zn is hosted by sphalerite. Silver concentrations (17 ppm) are the highest reported from the selection of samples analyzed in this study. As shown later, Ag is partly hosted by sphalerite. The Probištip sample carries moderate As (306 ppm), Cd (18 ppm) and Tl (8.5 ppm), and low concentrations of high-tech metals except for In (9.1 ppm) which reaches the maximum concentration in the sample set studied.
The Bučim tailings contain some Cu (360 ppm) and Ba (723 ppm). Cu is hosted by chalcopyrite and Ba is likely to be hosted by baryte. Weak Au concentrations (0.09 ppm) do not increase the economic value of the material. Concentrations of toxic elements are low (<50 ppm As), as are those of the high-tech metals.
The Lojane tailings contain very high amounts of As (>2000 ppm) and Sb (>10000 ppm), as well as Cr (1710 ppm). Arsenic is primarily hosted by realgar, and antimony by stibnite, but both are also present in numerous sulfides, arsenates and oxides. Cr is hosted by chromite and Ni (1090 ppm) is likely hosted by bravoite and vaesite. The analyzed sample reveals 8.3 % S, 60 ppm Co, 79 ppm Mo, and 45 ppm W. Copper is low (30 ppm), and further high-tech elements (Ga, Ge, In) are close to or below detection limit. The Lojane tailings differ from all the above as they do not provide any options to recover Cu, Pb, Au and Ag. The potential is clearly dominated by Sb and As with minor Ni and Co.

SPHALERITE TRACE ELEMENT DATA
Chemical analysis of the tailings sample from Probištip indicated elevated concentrations of combined Zn and Pb, at 2.7 % S. The polished section contained a variety of ore minerals, including galena, sphalerite and chalcopyrite. Sphalerite is often an important host of minor and trace elements. Therefore, it was decided to perform preliminary trace element analyses on sphalerite using LA-ICP-MS. Data presented in Table 5 indicate low median concentrations of Fe (0.47%), with some higher concentrations reaching 6.8%, and moderate Cd (0.41%) reaching more than 1% in cases. Element spectra over the time of analysis in the majority of cases do not show significant spikes that would point to invisible inclusions. In most sphalerite particles, Cu contents are elevated, most probably due to fine exsolution of and intergrowth with chalcopyrite. The median contents of typical by-product elements in sphalerite are 3.7 ppm Ge, 2.5 ppm Ga and 7.4 ppm In. Compared with the large dataset of COOK et al. (2009) of trace elements in sphalerites, Ga and Ge are in the expected range, while In is clearly below the median. The median content of Ag is 13 ppm but ranges up to 700 ppm in single particles.

DISCUSSION AND CONCLUSIONS
Besides mineralogical and chemical differences, the analyzed tailings from nine sites in southeastern Europe have some features in common: First, their predominant grain-size is in the sand-silt range (0.002 mm to 0.5 mm), many show a bimodal size distribution and, their composition is quite variable as expected from the literature. The first point is positive in the context of the possible recovery of the metals since mining fine-grained material needs little specialized equipment, and even the processing is more energy/cost-efficient as most of the necessary preparation steps as crushing or milling have been done previously during former operations. However, as shown in Table 3, most of the sulfides have a small grain size compared to the gangue minerals and are often intergrown with or present as inclusions in the latter, respectively.
There are several possible explanations for the bimodal size distribution. For tailings of flotation plants, a fairly uniform grain size should be expected; this would be the larger size class observed. The smaller size class in the range of < 10 µm could represent dust that was not removed prior to flotation, abrasion prod- ucts originating from the flotation and subsequent processes, dust that was removed before flotation but mixed before or during dumping in the tailings dam, or in situ formed weathering products including clay minerals. As a final possibility, analytical bias may have been introduced during crushing, sample preparation, or an unfavourable cutting plane of the polished section could be responsible for the observed bimodal size distribution. As most samples with a bimodal size distribution show a different mineralogy in the various size classes, an unfavourable cutting plane or an effect of crushing is unlikely. Given the attempts to gather and prepare representative samples during all steps from sampling to preparation, it can be assumed, that the bimodal size distribution is really present in the sampled tailings. The high spatial variability poses problems for mining and processing, some of which may be tackled using geophysical methods to investigate the shape of the tailings bodies which in some cases were just dumped to fill valleys. More problems are likely to be caused by the chemical variability, even at the sample scale, as the tailings are far from homogeneous. All the available reports on these tailings draw attention to their enormous inhomogeneity (KOLITSCH et al. 2018). In an extreme case, for the Lojane tailings, there are even reports of "numerous heaps of black and white powder" consisting of stibnite concentrate and arsenolite (As 2 O 3 ) thought to be remnants of 2500 wooden barrels stored at the smelter (TASEV et al. 2017). At the same place a tailings pile and a concentrate storage pond are reported, the latter holding approximately 15 000 tons of 50 % As concentrate (TASEV et al. 2017).
From an economic point of view, the metal contents analyzed in the tailings samples do not indicate a high potential for mining because they are comparatively low. However, given the easy access and likely low mining costs, some of the tailings may become economic if metal prices increase due to foreseen future shortages for base and critical high-tech metals. In any case, a possible and in many cases ecologically worthwhile remediation could at least partly be financed by the recovery of some valuable metals. In Table 6, we provide a summary of the potential identified using whole-rock chemical and in-situ mineralogical and mineral chemical analyses.
The highest total metal concentrations were found in the sample from the Probištip tailings with 0.4% Zn, >0.5% ppm Pb and 300 ppm Cu; however, those grades are still low compared to commercial mines. Unfortunately, the sulfide phases such as sphalerite do not carry elevated trace element concentrations that would economically support re-mining and processing the material. The base metal sulfides are often locked in pyrite which is mostly present as free grains. This poses challenges for reprocessing, but it should be kept in mind, that modern processing plants are significantly more powerful in every aspect than in the past.
Additionally, it should be considered worthwhile to process the Lojane tailings dam as this would not only reveal considerable amounts of As and Sb, but also protect the local environment from further toxification. Increasing metal prices might facilitate feasibility studies for some of the localities in the near future.