A Study on the Long-Term Exposure of a Tailings Dump, a Product of Processing Sn-Fe-Cu Skarn Ores: Mineralogical Transformations and Impact on Natural Water

Abstract

In the mining industry, one of the principal issues is the management of the waste generated during ore concentration, which represents a potential source of environmental pollution. The most acute issue originates from the mining heritage in the form of dumps formed of mining tailings that were created before the introduction of waste storage standards and may be located in urban areas. This research investigated this problem using the example of the tailings dump “Krasnaya Glinka”, located in a residential area of Pitkäranta (Karelia, Russia) in close proximity to the shoreline of Lake Ladoga. A complex approach, including the investigation of the natural water of the study area and tailings material and an experiment simulating the interaction of this material with atmospheric precipitation, allowed us to obtain the first data on the current status of the tailings dump and its surroundings and to identify environmental pollutants. This research used XRF, XRD, and EPMA analytical methods for assaying the tailings materials obtained from the dump and ion chromatography, potentiometric titration, ICP-MS, and AES for the water samples. The results show the influence of the tailings dump’s materials on the formation of the environmental impact—in the water from the area of the tailings dump, increased concentrations of chalcophilic elements are observed, for example, Zn up to 5028 µg/L. Based on this study of the tailings dump’s materials and the conducted experiment, an attempt is made to connect the chemical compositions shown in the natural water data with the specific mineral phases and processes occurring during supergene transformations in the tailings storage. As a result of the conducted research, it was found that despite more than 100 years of exposure of the tailings materials under natural factors, mostly atmospheric precipitation, equilibrium with the environment has not come. The processes of extracting toxic elements and carcinogenic mineral phases into the environment are continuing. In the process of studying the tailings materials, it was found that they are probably of economic interest as a technogenic source of W and Sn due to the contents of these components exceeding industrially significant values in the exploited fields.

1. Introduction

The history of mining is many centuries old [1,2], and it has impacted the environment for its duration. However, an understanding of the environmental pollution risks that occur with mining dumps has only been developed relatively recently. It is common for human settlements to evolve in mining areas, with some of them continuing to exist even after the mining ceases. During the extraction of minerals, alongside the mines, artificial objects such as ore-dressing plants and tailings dumps appear, which also represent a danger to the environment. In the absence of rules for the handling of waste from mining facilities in the past, such territories often found themselves within city limits and became built up.

The natural water and soil of mining regions are characterized by an increased level of loading from a wide range of chemical elements. The most critical situation has developed in the case of the positioning of tailings dumps within modern residential areas [3,4], which may entail some risks to the health of the population living in that area. The long-term exposure to tailings dumps leads to a cumulative effect of the increasing toxic load.

Control over the exposure to technogenic dumps is required not only from the perspective of environmental protection but also from an economic frame of reference, to assess the feasibility of extracting further valuable chemical elements. With the developments in technology, the extraction of valuable components from the material of historical dumps becomes a question of economic efficiency [5,6].

Pitkäranta, located in the Karelia Republic, Russia, is a mining and industrial city where principally tin and iron were mined from the sulfide-bearing skarns beginning in the early 19th century to the early years of the 20th century. There are many objects of both mining and industrial heritage in the territory of Pitkäranta and its adjacent territories. One such object located within the city limits is the “Krasnaya Glinka” tailings (ash and slag) dump. The dump is interesting because it is located at the water’s edge of Lake Ladoga, near a private residential area. At the same time, there are no traces of work having been undertaken in its preservation and in preventing the pollutants contained in it from entering the groundwater. At its base, there are fragments of skarnified marble with sulfide mineralization and secondary copper hydrocarbonates. Due to the imperfect methods of ore concentration in the 19th and early 20th centuries, there are many useful elements present there and in significant quantities. For example, the chemical analysis data of the bulk sample of the dump materials demonstrated that the WO₃ content in it reached 0.14 wt.%, which is the minimum acceptable value for lean tungsten ores in Russia [7] (the values are given for one sample; testing the dump materials for calculating W resources was not the aim of this work). In addition, studies in recent years [8,9] have shown that tungsten can have adverse effects on the environment and human health; therefore, there is a need for research into relevant industrial heritage objects and the possible use of water-treatment measures. The Sn content in this particular sample was also high, 0.4 wt.%, which is also of industrial interest and is higher than the contents in some of the other deposits in Russia, where it is 0.12 (Tigrinoye, Primorsky Krai) or 0.15 (Sherlovogorskoye, Zabaikalsky Krai), for example [7]; however, part of the Sn is concentrated in the garnet of the andradite–grossular series, which is a factor that complicates its extraction.

The main aim of this work was the investigation of the results of long-term exposure to the tailings dump, primarily from the point of view of mineralogical transformations and the impact on the natural water of the area.

To achieve this goal, a set of studies were conducted, as follows: examination of the phase composition of the tailings dump’s substance, the morphology and history of the formation of these phases, and determination of their elemental composition; experimentation on leaching the material of dumps with water; and comparison of the results obtained with the composition of natural waters.

The presented research is valuable as it addresses the problems of sustainable development in the territory of historical mining areas. In other words, this particular study can serve as an example for conducting larger-scale assessments of environmental pollution in mining areas.

1.1. Historical Geographical Description

The Pitkäranta area is located in the Karelia Republic (Russia), at the northeastern tip of Lake Ladoga, and stretches along the coastline for 50 km (Figure 1) [10]. The climate in this territory is temperate humid continental with signs of a marine climate; the average annual air temperature is 2.7 °C; for seven months of the year, the average temperature is higher than zero (April–October); for three months of the year, the day temperature is higher than +16 °C and approaches the standard values. The Pitkäranta area contains deposits and occurrences: “Karku” uranium unconformity-type deposit [11,12], Ta-Nb ore occurrences [13], Sn-Fe-Cu-Zn-Be skarn, and skarn–greisen deposits and occurrences. Many of the Sn-Fe-Cu-Zn objects were developed in the 19th and early 20th centuries. The ores of these objects have high contents of some critical metals (In, Bi, Be) [14,15]. Currently, only objects for construction materials are being developed in this territory [16,17].

The first discoveries of copper ores in the Pitkäranta area were made at the end of the 18th century, and their development began in 1810 [20]. Over almost 200 years of mining in the Pitkäranta area, the following local ores were extracted: ~60,000 tons of Fe; ~500 tons of Sn; about 7000 tons of Cu; 11 tons of Ag; and 16 kg of Au [21,22]. The peak of the industrial development in Pitkäranta occurred at the end of the 19th century. At that time, mining and metallurgical enterprises were functioning in the vicinity of the city. At the beginning of the 20th century, the mines were closed due to the depletion of the bodies of ore.

More than 20 historical mines previously included in the “Old Ore Field” are now located inside the urban area. In addition, rock dumps, tailings of processing plants, and slag of metal smelters have been identified within the city boundaries. One such object is the “Krasnaya Glinka” tailings dump (“Red clay” in Russian; “clay” in this case having the connotation of “dirt”).

1.2. Object of Study

The object of this research is the “Krasnaya Glinka” tailings dump, which is a product of the industrial enrichment of polymetallic ores using the “wet” method. It is located in the territory of Pitkäranta, in the immediate vicinity of the shoreline of Lake Ladoga (Figure 2). The “Krasnaya Glinka” dump has an irregular shape on the plan, with dimensions of at least 150 × 250 m, a capacity of 0.3 to 9 m, and a volume of at least 3–3.5 × 10⁶ m³ [23]. In modern times, the main part of this tailings dump was divided into two parts by the railway, and it is exposed to water and wind erosion. The dump is partially grass-covered. According to the data [23], this object has a heterogeneous structure both laterally and vertically.

The dump at the observation site is represented by a non-coherent, fine-grained mass of red–pink–brown shades with larger inclusions of mineral grains. The inclusions are unevenly distributed, and often their accumulation has a sub-horizontal orientation. At the base of the dump, there is a coarse-grained material represented by blocks of skarnified rocks with sulfide mineralization and fragments of vitreous slag.

The extraction of elements such as Cu, Ag, and Au must be via the roasting of the primary sulfide ores. The studied tails are probably the products of firing, but this is difficult to definitively assert due to the unsatisfactory documentation of the information.

The dump materials have been in constant interaction with the environment over a period of long-term exposure. First of all, this influence consists of the leaching of the materials via atmospheric precipitation. The atmospheric water draining through the dump probably flows into shallow aquifers, which are used for household purposes in the urban area adjacent to the dump.

1.3. Geological Essay

Sn-Fe-polymetal–rare-metal ore occurrences in the Pitkäranta area are associated with skarns and greisenized skarnified marble of the Sortavala series. The sediments of the Sortavala series are located in the frame of gneiss–granite domes, AR₂-PR₁ (Figure 3).

Ore formation is mainly confined to the introduction of Mesoproterozoic [24,25] granite rocks of the Salmi batholith belonging to anorthosite–rapakivi granite complexes [10].

Historically, the city of Pitkäranta originated around the “Old Mine Field” mines. The ore-dressing waste from these mines is located in the “Krasnaya Glinka” tailings dump. It is not possible to establish a direct connection between the dump materials and specific mines. However, due to the fact that similar ores in both the genesis and set of mineral phases were developed at these mines [10,21,26,27,28], we can omit their primary source from this study.

2. Materials and Methods

2.1. Sampling of Dump Material

The sampling was carried out in September 2022 (Figure 2a). The sample was taken in the upper part of the dump from a depth of 0.5 m (Figure 2b), with a weight of 6 kg. The variability in the dump materials along the strike and into the depth was not investigated; these studies have previously been conducted by other researchers [23].

At the base of the dump is a horizon composed of vitreous slag and blocks of skarnified rocks with a certain visual sulfide mineralization (pyrite, sphalerite, and chalcopyrite). This horizon extends to the edge of the water of Lake Ladoga. Secondary aggregates of copper hydrocarbonates develop on the surface of the blocks of skarnified rocks (Figure 4).

2.2. Sampling of Water

A total of three water samples were taken (Figure 2a): 1—from the underspoil water from a hole (depth about 0.5 m); 2—from a puddle formed on the dump near the vehicular access after rain; 3—from a well located in the private residential area at a distance of about 30 m from the dump. The water samples for elemental analysis and ion chromatography were filtered through polyethersulfone membranes with a 0.45 µm pore size into 15 mL polypropylene vials. To analyze the carbonate system components, the samples were put in 300 mL plastic bottles three times precleaned with the water under study.

2.3. Methodology of Material Research

The materials of the technogenic dump were divided into size fractions: more than 1 mm, 1–0.5, 0.5–0.25, 0.25–0.1, and less than 0.1. The preferential size of fractions was 0.1 mm and <0.1 (dust).

The major elements and the Cr, V, Co, Ni, Cu, Zn, Rb, Sr, Zr, Ba, Pb, As, Cl, Mo, W, Ta, and Sn contents and their different fractions in the samples of materials obtained from the dump were determined using X-ray fluorescence at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry of the Russian Academy of Sciences (IGEM RAS, Moscow, Russia) by Yakushev A.I., using a WD spectrometer (Axios mAX; PANalytical, Almelo, The Netherlands). The spectrometer was equipped with an X-ray tube with a 4 kW Rh anode. The maximum-rated tube voltage of this equipment is 60 kV with a maximum anode current of 160 mA. For the calibration of the spectrometer, industry and state standard samples of the chemical compositions of rocks and ores were used. The quality control of the analytical results was performed on standard samples of rocks from the US Geological Survey (USGS), Denver, CO, USA. All Fe was determined via XRF as Fe₂O₃.

The study of the phase composition of fractions and their mineralogical features was carried out on a Tescan MIRA 3 electron microscope equipped with an X-MAX energy dispersion spectrometer at the Vernadsky Institute of Geochemistry and Analytical Chemistry RAS (GEOKHI RAS), by Lorenz C.A. The analyses were conducted at a working distance of 17 mm with a carbon coating.

Two types of samples were manufactured for research using a scanning electron microscope (SEM) and an electron probe microanalyzer (EPMA): 1—to study the external morphology of grains by pouring various fractions of the material onto carbon tape; 2—to study the internal morphology of grains through impregnation of various fractions of the material with epoxy resin and then opening the internal structure of the grains using polishing.

The fraction less than 0.1 mm was also investigated with the X-ray diffraction method (XRD) for the determination of the mineral phase composition. This analysis was carried out on an X-ray diffractometer (Dron-3m; Burevestnik, Saint Petersburg, Russia). The conditions of analysis were as follows: operating current was 15 mA; operating voltage was 40 kV; X-ray tube with a Co anticathode; Goniometer MiniFlex 300/600; attachment ASC-8; detector d/teX Ultra2; scan mode continuous; scan speed 5 deg/min; step width 0.05 deg, scan axis Theta/2-Theta, scan range 3–64 deg; incident slit 1.25 deg; length limiting slit 10 mm; scatter slit 1–1.25 deg; receiving slit 2–0.3 mm.

The in situ measurements included the determination of pH and Eh values, temperature, and dissolved oxygen content using a PH200 meter (HM Digital, Seoul, Republic of Korea), ORP-200 meter (HM Digital), and portable DO meter (AZ Instruments, Taichung, Taiwan).

The major anions of natural water F⁻, Cl⁻, SO₄⁻², NO₃⁻, and NO₂⁻² were measured using ion chromatography (ICS-6000-AnCat-AS; Dionex; Thermo Fisher Scientific, Sunnyvale, CA, USA). The ion detection method is conductometric, using chemical suppression, which makes it possible to determine individual components at the ppb unit level. The contents of Ca, Mg, Na, K, Fe, and Al were measured with ICP-AES (iCAP 6500 DUO; Thermo Fisher Scientific, Waltham, MA, USA). The concentrations of trace elements of natural water were analyzed using ICP-MS (X-series 2 quadrupole mass spectrometer; Thermo Fisher Scientific). These analytical measurements were carried out at the Vernadsky Institute of Geochemistry and Analytical Chemistry RAS (GEOKHI RAS) by Gromyak I.N., Dogadkin D.N., and Dolgonosov A.A. The content of HCO₃⁻ was determined with potentiometric titration using Expert-001 (Econix-Expert, Moscow, Russia) by Toropov A.S. In this method, the end point of titration is determined by a change in the potential of the indicator electrode depending on the amount of titrant added.

The experimental solutions were measured using ICP-AES (CAP-6500 Duo; Thermo Fisher Scientific) and ICP-MS (X-7 quadrupole mass spectrometer; Thermo Fisher Scientific) at the Institute of Microelectronics Technology and High-Pure Materials, RAS (IMT RAS) by Karandashev V.K.

2.4. Methodology of Experiment

The preparation of samples before the experiment took place in two stages: First, the samples were dried in air in the laboratory room, and then in the drying cabinet, at a temperature of 105 °C. After drying, the samples were thoroughly mixed and quartered. Bi-distilled water saturated with CO₂ (pH 7.4) was used as the liquid phase in the experiment. A rock/water ratio of 0.5 was chosen, according to the method of [29]. The experiment was carried out in plastic containers with a volume of 200 mL. The water (50 mL) was poured into the container, and the waste material (25 g) was filled in. The container was placed in a closed (not fully airproof) box and kept at room temperature (22–24 °C) for a specified time: 1, 2, 4, 8, 12, 16, 20, 24, 28, and 32 days. Thus, the experiment simulated the interaction of the dump materials with atmospheric precipitation in the early days and with the precipitation during evaporation in the later days of exposure (evaporation was significantly slowed down by placing the containers in a closed box). It should be noted that the experiment did not fully recreate the natural conditions, including in the phase ratio. The main objective of the experiments was to obtain an initial conceptualization of the processes of water–rock interaction at the selected location and compare them with natural observations.

The separation of water from the rock after the experiments was carried out through the natural settling of the sediment and further filtration of the solution. A three-component syringe with a single-use nylon filter (0.45 µm pore size) was used to filter water from the dump materials. After filtration, the water was collected in 15 mL polypropylene vials with the acidification of HNO₃ (high purity) in a volume of 0.45 mL for further analysis using ICP-AES and MS.

3. Results

3.1. Mineralogy

The comminuted materials of the dump are represented by coarse fragments of primary minerals of the ore-bearing skarns and newly formed mineral phases.

The composition of the different fractions obtained with XRF is presented in the Supplementary Materials (Table S1).

The data obtained with EPMA are presented in the Supplementary Materials (Table S2).

Fragments of primary minerals are represented by silicate and oxide minerals: quartz, potassium feldspar, vesuvianite, chlorite, actinolite, minerals of the pyroxene group, diopside and a member of the diopside–hedenbergite series, the garnet andradite–grossular series with an admixture of Sn up to 1 wt.%, amphibole actinolite, and the hastingsite–chermakite series, as well as allanite, zircon, cassiterite, scheelite, and stokesite. The composition of the primary mineral fragments in different fractions is identical.

Newly formed minerals are represented by hematite, iron hydroxides (probably goethite–limonite), gypsum, barite (probably), clay minerals, and minerals of the tungstite group. These are present in all fractions; however, their highest concentration is found in the smallest fraction (less than 0.1 mm).

Sulfide minerals were not found in the investigated dump materials; however, in the base of the dumps, as already noted above, there were fragments of sulfide-bearing skarnified marble (Figure 4).

One of the most representative newly formed minerals was gypsum. Gypsum forms elongated crystals, on average from 10 to 50 μm wide and up to 200 μm long (Figure 5).

Barite was probably found as small units, but the insignificant size of the secretions cannot provide reliable proof of the presence of this mineral. Clay minerals are thin leather coats of non-permanent composition; it is impossible to reliably identify them using the available analytical methods. Saponite is noted from the identified compounds. Hematite forms thin lamellar crystals fused in the form of “rosettes” (Figure 6a). Accretions of hematite often form crusts around the cores of coarse-grained silicate minerals (Figure 6b).

Fe-hydroxides are aggregates composed of rounded sinter shapes without clearly defined crystallographic forms (Figure 7a). However, the internal structure of these formations is often quite complex, occurring in several stages. Unfortunately, it was not possible to identify uniform growth patterns for all entities.

For example, the internal structure of such rims composed of Fe-hydroxides around a fragment of a diopside grain is shown in Figure 7b. In this unit, several stages of growth can be distinguished:

Stage I. This is the earliest stage, and it is characterized by large volumes of newly formed substances and visible dissolution voids with depressions of a crystallographic shape (Figure 7c). In this early stage, idiomorphic crystals of the mineral phase were probably formed together with Fe-hydroxides, which were subsequently dissolved.

Stage II. Like the first stage, this is characterized by a large rim volume and has darker areas in the BSE, compared to the first stage. There are no holes present from dissolved phases. Due to the presence of radial cracks, it can be assumed that the cryptocrystalline structure of this formation is radial. Traces of Al, Si (0.17 wt.%), and P (0.11 wt.%) were observed in this zone.

Stage III. This stage consists of several thin edges represented by a radial–fibrous rim. The material chipping also has a radial–needle shape (Figure 7d).

Stage IV. This is the final stage and is represented by slightly lighter areas in the BSE, having rounded sinter shapes, with isometric holes, probably formed as a result of dissolution. In the materials at this stage of growth, there was a slight decrease in the amount of Fe and the presence of an insignificant amount of Si, P, Cu (on average, 0.8 wt.%), and Mn.

In all zones, there was a slight presence of Ca (on average about 0.2 wt.%) and S (on average 0.45 wt.%); the Ca/S ratio was, on average, 0.5.

The minerals of the tungstite group are represented by fine-crystalline aggregates that develop separately from other formations (Figure 8a). These aggregates can have a different size range and also fall into fractions larger than 0.1 mm. Fouling of the tungstite aggregates upon the silicate mineral cores was not observed. Some mineral aggregates of the tungstite group minerals also show a complex, multi-stage history of formation, as do Fe-hydroxides. Unfortunately, it was also not possible to identify a uniform growth pattern for all entities.

These mineral aggregates are not sustained in composition; Cu and Zn contents are observed up to 7 and 6 wt.% (maximum fixed). On average, the content of each of these elements does not exceed 2% by weight. One of the examples of such a mineral aggregate in this section is shown in Figure 8b. Three growth zones are distinguished on this aggregate. From the inner zones to the outer, the W content decreases (from 58 to 40 wt.%) and the Cu content increases (from 0.9 to 5.2 wt.%). The changes in the content of Zn are ambiguous.

3.2. Phase Composition of Less than 0.1 mm Fraction Using XRD

The XRD studies have shown the following mineral composition: smectite with Mg-exchange complex 9%; mica (mainly biotite) 5%; zeolite (mainly laumontite) 5%; chlorite 2%; quartz 2%; K-feldspar 9%; amphibole 9% (mainly riebeckite); gypsum 24%; pyroxene (mainly diopside) 18%; and hematite 17%. The spectra of the analyzed samples are presented in Figure 9. The data obtained with XRD are presented in the Supplementary Materials (Table S3).

3.3. Chemical Composition of Natural Waters

The water samples collected at various points in the research area have different levels of salinity, pH, and Eh. The results of the chemical analysis are presented in the Supplementary Materials (Table S4). The water collected from the well (point 3) was brackish because the salinity value exceeded 1 g/L. The pH value in this water was 7, which corresponds to neutral conditions. According to the Eh measurements in the surface layer of the water in the well, the redox conditions were transitional. The redox potential at depth in the well was not measured. However, based on the dissolved oxygen measurements (3.65 and 1.78 mg/L on the surface and depth, respectively), it can be concluded that the conditions at depth were reducing. The water from a puddle formed after a recent rain shower (point 2) was fresh and slightly alkaline (pH 7.63). The underspoil water (point 1) was fresh and neutral (pH 7). In the surface waters, the measured Eh (100 and 185 mV in the puddle and in underspoil water, respectively) indicated oxidative conditions. Calcium dominated the cationic composition in all samples. There was a greater variety in the anionic composition: the water taken from the puddle was sulfate; from the pit under the dump, sulfate–bicarbonate; and from the well, bicarbonate–sulfate. The names are given from smaller to larger, including ions with a content of at least 25 mg-eq/L.

The highest concentrations of trace elements were found for Zn, Cu, Mn, Ba, and Sr. The accumulation of these elements was most characteristic in the underspoil water. In this sample, the highest concentrations of Zn and Cu were 5028 and 379 µg/L, respectively. In the water taken from the puddle, the concentrations of trace elements were lower, which is probably due to the shorter interaction time of the water with the materials of the dump. In the water taken from the well, sufficiently high concentrations of trace elements such as Mn, Zn, Ni, Sr, and Ba were also observed, which can probably be explained by the drainage of the dump via atmospheric precipitation. In other words, potentially toxic elements are supposed to be leached away from the dump materials via atmospheric precipitation, with their subsequent drainage into shallow underground waters. The salinity of the water in the well was four times higher than that in the other samples. Therefore, it is worth considering the level of accumulation of trace elements relative to the amount of salinity. The degree of concentration, as well as the water migration, can be considered using the special coefficients proposed by A.I. Perelman [30]:

Kx = (m_x × 100)/(a × n_x)

(1)

where Kx is the coefficient of water migration as a dimensionless value; m_x is the content of chemical element x in water (mg/L); a is the sum of minerals dissolved in water (TDS) (mg/l); n_x is the content of element x in rock (%).

According to the proposed classification [30], the elements can be divided into groups according to their degree of mobility. Elements with a very strong migration ability include S, Mn, Zn, Sr, and Ba. The group of elements demonstrating a strong migration ability is also the most numerous—Ca, Na, K, Cl, Ti, V, Co, Ni, Cu, As, Rb, Mo, and Pb. Magnesium, like P, Cu, and Pb, has an average ability to migrate in the well water. A weak degree of mobility is characteristic of Al, Fe, Si, and Sn. It is noteworthy that the distribution spectra of the migration intensity of chemical elements for all samples were very similar (Figure 10). However, the profile of the well water was lower at many points, which indicates a lower concentration of elements in the groundwater of the area relative to their salinity. This was most evident for Al, Cu, Mo, and Pb. Among the elements of ore specialization, there is a noticeably more intensive accumulation in the underspoil water.

3.4. Results of Experiment

The data obtained during the experiment are presented in the Supplementary Materials (Table S5).

There are three types of behavior of the elements in solution, depending on the exposure time:

• A rapid increase in the content of elements in the solution in the first 4 days, followed by a decrease in their content: S and Ca (Figure 11).
• The content of elements in the solution increases rapidly, passes through an extremum, and after 16–20 days of exposure, a decrease in the content is observed: Si, Al, Sr, Mn, Cd, Co, Cu, Zn, and U. Cu, Zn, Cd, and Co are shown in Figure 12.
• The content of elements in the solution reaches a plateau in the first 4 days, and their content either increases slightly until the end of the experiment or increases sharply after the 20th day of exposure: Na, K, Mg, Ba, Pb, and Li (for Li, this behavior is less pronounced) (Figure 13).

Rb had no pronounced tendencies of behavior during the experiment.

During the experiment, a slow decrease in pH was observed, from 6.35 at 1 day to 5.8 at 32 days. The initial pH was 7.7.

4. Discussion

The main elements—pollutants at this facility, capable of mobilization and migration—are chalcophilic elements originally contained in the sulfide minerals of the ores being developed. Sphalerite and chalcopyrite are among the most common sulfide minerals in the skarnified rocks of the “Old Mine Field” [21,26].

However, sulfides were not found in the comminuted materials of the technogenic dump. The traces of sulfides in the original composition of the dump materials are indicated via the presence of newly formed sulfates, as well as the presence of Cu in the newly formed Fe hydroxides, Zn, and Cu in minerals of the tungstite group.

In the event that the ores developed were roasted, then, at some point in time, metals such as Cu and Zn must be in oxide form. Subsequently, the oxides of these metals, when interacting with the atmospheric air and the carbon dioxide contained in it, must be converted into carbonates of these metals. The same pattern is observed in the case of grinding Cu and Zn sulfides; however, this process probably takes a little longer. With a uniform distribution of these components throughout the materials of the dump, since carbonates and bicarbonates form fine-crystalline aggregates, their presence in the materials of the dump in the form of detectable phases is difficult to determine. However, it is probably the carbonate form that is the principal component of these elements.

Copper is observed as an impurity in the outer shells of Fe-hydroxide rims that does not reach a value of 1 wt.%. Due to the increased porosity of these mineral aggregates, the occurrence of Cu in the form of hydrocarbonates, which were observed at the tailings dump, is most likely (Figure 5). The mechanism of the incorporation of Cu into hydroxides was not part of this study; however, studies that have been conducted using various methods [31,32] have indicated that the processes of the coprecipitation and sorption of metals, including Cu, on Fe-hydroxides are the most likely. As noted earlier, in fragments of ore-bearing skarns at the base of dumps with chalcopyrite mineralization, the presence of Cu (malachite, azurite) hydrocarbonates is identified (Figure 4).

The results of this experiment show that the content of chalcophilic elements such as Cu, Zn, Cd, and Co increases in water with exposure time, with a transition through the extremum of about 20 days. The largest concentrations in thousands of µg/L in the experiment were observed for Cu and Zn.

In nature, under the influence of heavy atmospheric precipitation in the upper part of the tailings dump, there will be active washing and removal of the elements. This assumption is confirmed by the authors of [23], who studied the chemical composition of the vertical section of the tailings dump, and the introduction zone of these elements at depth was discovered.

The highest Zn content was observed in the underspoil water (5028 µg/L), and less was observed in the well in a private residential area (881 µg/L). For Cu, the highest content was observed in the underspoil water (379 µg/L), and for the well, the minimum (17 µg/L); meanwhile, in the puddle formed during recent rain, this value was higher (87 µg/L). As can be seen from the data provided, in the natural waters (Supplementary Materials (Table S4)), there is a predominance of Zn content over Cu. Copper does not accumulate in natural water in high concentrations due to precipitation in the form of hydrocarbonates (Figure 4), while secondary zinc minerals are not widely distributed.

In the materials of the dump, Zn is marked only as a minor element in tungstite, the formation of which is limited by the content of W. Presumably, the source of W for the formation of minerals of the tungstite group was scheelite, the fragments of which were found in the materials of the tailings dump. However, based on the presence of scheelite, it can be concluded that even in the upper part of the dump during the entire time of their exposure, the phase equilibrium between tungstite and scheelite has not occurred.

The maximum recorded values of Cu and Zn content in the water during the experiment were 19,658 and 7022 µg/L, respectively. The Zn content in the underspoil water was relatively close to these values; meanwhile, for Cu, the content was 1.5 orders of magnitude lower than the experimentally obtained one. The latter result can be explained via the wider development of secondary copper minerals (malachite and azurite) in natural conditions. The passage of the component contents through the extremum around the 20th day can be explained by the evaporation and an increase in the total mineralization in the experimentally obtained solutions, the reflection of which is conductivity (Supplementary Materials (Table S5)).

Cadmium and Co are also chalcophilic elements—pollutants associated with these ores. The sources of Cd and Co in ores are the minerals, sphalerite and chalcopyrite, respectively, in which these elements are included as isomorphic impurities [15,33,34,35]. In natural waters, the content of Cd and Co was the lowest in the puddle of rainwater; for Cd, it was highest in the underspoil water; and for Co, it was the highest in the well water (Supplementary Materials (Table S4)). In this experiment, the maximum Cd contents observed in nature were already reached during the first day of observation, and at the peak of the extremum, the contents were in excess of that by a little more than twice. The Co content in the well (7.7 µg/L) was higher than the maximum value obtained in this experiment (4.9 µg/L). The reasons for such a high Co content in the well may be due to excellent oxidative reduction conditions compared to other samples and experimental data. When the tailings dump materials are washed with atmospheric precipitation, in conditions rich in oxygen, dilution should occur faster than the duration of the experiment. Therefore, the values of the contents achieved as a result of this experiment seem to be overvalued in natural waters.

The lead in the natural water has the highest concentration in the puddle near the vehicular access (41 µg/L), more than six times higher than the content in the underspoil water (6.2) (Supplementary Materials (Table S4)). In this experiment, the lead content steadily increased by the 32nd day, reaching 4.8 µg/L (Figure 13), which approached the values in the underspoil waters. The lead content in the puddle was probably greatly influenced by the past use (in Russia until 15 November 2002) of tetraethyl lead as a component of gasoline that increases the octane number. The main source of lead in the studied materials of the dump was probably the sulfide mineralization of the ores being developed, where Pb was present in insignificant quantities [15]. Subsequently, lead could form secondary minerals, as evidenced by an increase in its content in the most fine-grained fraction of the materials under study. However, the lead content in the sample is small, and individual mineral phases were not detected. Lead may also be present in small amounts in silicate minerals [36]. At the observation site, the dump materials were not the main source of the high concentration of lead in the water, although lead tends to accumulate during prolonged interactions. However, according to natural observations, motor transport has a greater influence on the lead content in the waters.

Arsenides are developed in the ore skarns of the district: arsenopyrite and loellingite [21,26,27,28,37,38]. Arsenides in the zone of hypergenesis are changed with scorodite. Given the low initial content of arsenides in skarns, it is not possible to detect an impurity of scorodite in porous mineral aggregates like Fe-hydroxides using available analytical methods. However, using the X-ray fluorescence method, arsenic was detected in the bulk sample at 40 ppm (Supplementary Materials (Table S1)), and it was also observed in the natural waters, where it reached maximum values in a sample from the well, 2.7 µg/L (sample 3) (Supplementary Materials (Table S4)). The arsenic content in the experimental solutions was below the detection limit. The highest arsenic content observed in the well water was due to its accumulation in moderately reducing conditions. This is a characteristic behavior of this element, especially in conditions of a stagnant hydrodynamic regime [39,40].

Tin is found in the form of cassiterite and stokesite and also as an impurity in garnet. Garnet tin is difficult to extract, as garnet is extremely resistant to hypergenesis. Cassiterite is also a mineral that accumulates in placers, which suggests its relative physical and chemical inertia. Cassiterite placers of no industrial significance are known in the Pitkäranta area [41]. The tin content in the experimental solutions was found to be below the detection limit.

The highest contents of alkali metals (Na, Mg, K, Ca, Ba, Sr, Li) and sulfur were observed in the well (sample 3), and the highest content of Ba was observed in the underspoil water (sample 1). This behavior of the main cations is probably explained via the stagnation regime and is also associated with human activity through additional input when wells are used for agro-industrial purposes.

The contents of Ca and S showed a strong correlation. The increased content of S and Ca in water is associated with the influence of primary skarnified rocks containing carbonates and sulfides, and also with a secondary developing mineral, for example, sulfur-bearing minerals like gypsum (Supplementary Materials (Table S3)) (Figure 5 and Figure 9). Experimental curves of Ca and S content demonstrate rapid accumulation within the first day and then a gradual decrease, which may probably be due to the crystallization of gypsum. The stable detection of Ca in Fe-hydroxide rims is probably due to the presence of microcrystalline aggregates, for example, gypsum. However, in gypsum, the Ca/S ratio in wt.% is close to 1; therefore, there is an excess of S in the studied object. However, it should be borne in mind that sulfur is a light element, so the determination of sulfur content at low concentrations at a specific point and not in an area can lead to distortion of the results. Ca and S are the elements most rapidly and in large volumes mobilized via precipitation from the tailings dump materials. Thus, it is likely that the materials of the dump will be processed by draining atmospheric precipitation for a long time and through recrystallization of gypsum on porous mineral aggregates.

The other alkali metals (Na, Mg, K, Ba, Li, and Sr) in this experiment showed a different dynamic. Their contents were slowly growing, accelerating after the 20th day of the experiment. The Li content in the experiment on day 32 almost reached the highest natural value. The content of K exceeded it by more than an order of magnitude. The contents of Na and Mg on the 32nd day of the experiment were 2 and 3.5 times lower than the maximum natural values, respectively. The slow increase in the content of Na and Mg can be explained by the extraction of these components from silicates since no secondary silicate-free minerals containing these elements were found.

Barium and strontium are also alkali metals, but their behavior is different. The barium content in the experiment increased slowly; however, despite the exposure time of 32 days, it did not reach the values determined for the puddle (17 and 27 µg/L). This can be explained by the fact that single finds of primers (thin films) of barium sulfate (barite?) were found in the fraction <0.1 mm and most of the barium during the exposure of the tailings dump under atmospheric precipitation was washed away hypsometrically from below. In this case, it can be assumed that the increase in the barium content in the experiment was mainly due to its removal from silicates, such as feldspar.

The maximum strontium content was found in the well (151 µg/L), the lowest in the puddle (79 µg/L), and the average (120 µg/L) in the underspoil water. As a result of this experiment, a sharp increase in the solubility of Sr in water was observed with a maximum value of 668 µg/L on the 20th day, which exceeded the values found in the natural water. Since no strontium minerals were found, it can be assumed that it either forms thin, scattered celestine secretions in the studied sample or is an isomorphic impurity in gypsum, smectite, and laumontite in which it isomorphically replaces Ca. However, the connection of Sr with gypsum seems unlikely, due to the discrepancy in the behavior of the content in this experiment with respect to the contents of Ca and S.

Despite the gradual decrease in pH in the experiment, higher values of pH 7 and above were observed in the natural water. This is due to the climatic conditions in which the region is located—there is no exposure to aridization, and frequent humidification via precipitation occurs. Considering that the average annual temperature is near zero, and nearly 7 months per year are higher than zero degrees Celsius, chemical reactions do not proceed quickly. Temperatures close to standard conditions are observed at the facility only during the daytime in the summer. The consequence of the influence of environmental conditions is the constant dilution of solutions that do not reach equilibrium, formed by the interaction with rock and their runoff to adjacent territories. In other words, there is a constant renewal of water in the system, which, on the one hand, determines the removal of pollutants from the dump into the natural water of the area and, on the other, does not allow high concentrations of chemical elements to accumulate in each portion of water.

In addition to the migration of chemical elements in a dissolved form under the influence of atmospheric precipitation, the “Krasnaya Glinka” tailings dump is also a source of amphibole group minerals forming parallel fibrous–filamentous aggregates [26] called “asbestos” in the metasomatites of the “Old mine Field”.

Amphibole asbestos exhibits increased carcinogenicity, with the greatest danger in small fragments of grains [42,43,44,45,46]. According to the XRD data (Supplementary Materials (Table S3)), in the fraction of less than 0.1 mm, the amphibole group minerals, mainly riebeckite, account for 9%. Small mineral particles from the tailings dump materials, with the help of wind, can be transported within the city limits of Pitkäranta and become a component of urban dust.

Tailings dumps accumulated before the introduction of standards for their storage should be studied to assess the potential harm and the possibility of using the dumps as sources of other previously unextracted components. These dumps are of potential interest as a source of W and Sn, but their economic potential for these elements has yet to be assessed. Given the well-developed infrastructure at this site (the railway passes through this dump), the elimination and removal of a majority of the volume of the tailings can be carried out quickly and at minimal cost.

The optimal way would be to minimize the impact of tailings materials on the environment: the first priority is that while the objects that comprise the materials exist intact, the evolution of the mineral composition should be studied both in area and depth, the feasibility of extracting useful components should be assessed, and a decision should be made on the choice of reclamation method.

5. Conclusions

Despite the long exposure time under the influence of atmospheric precipitation, the materials of the “Krasnaya Glinka” tailings dump are still far from their equilibrium phase, even in the near-surface layers. The process of pollutants entering the external environment continues, so the dump materials remain a source of environmental pollution in the area, especially in the natural water. In accordance with climatic conditions, constant precipitation and its drainage contribute to the removal of pollutants from the dump’s materials. However, the low rate of dissolution of the minerals composing the dump and the constant renewal of water in the system do not allow the accumulation of crucial concentrations of chemical elements in the natural water of the area.

High concentrations of a wide range of trace elements were identified. The highest concentrations of ore elements were determined in the underspoil water: Zn, 5028 µg/L; Cu, 379 µg/L; and Cd, 14 µg/L. High contents of particular elements were also determined in the water of the well in the adjacent, private residential area: Zn, 881 µg/L; Cd, 9.1 µg/L.

The process of dissolution of scheelite, carbonates, and hydrocarbonates of Cu and Zn led to the formation of new mineral phases, such as tungstite, the reprecipitation of Cu and Zn minerals, and the removal of the potentially toxic elements that occur in this dump. Attention should also be paid to the likely release of asbestos material (amphibole group minerals) into the environment under the influence of wind.

For the sustainable development of settlements in the environs of both mining and industrial heritage, it is necessary to study artificial objects for sources of pollutants (chemical compounds and mineral phases). In remediation territories, it is also necessary to examine the materials of the dumps for possible use as raw materials, in this case, for obtaining W and Sn.