Geochemistry of neutral mine drainage at sulfide deposits ‒ Example of the „Grot“ Pb -Zn mine, south-eastern Serbia

: This study examines the chemistry of mine waters of the “Grot” Pb -Zn mine and identifies hydrogeochemical factors that influence the formation of mine waters chemical composition. Eleven mine water samples were collected at six locations across the area of Kriva Feja in order to determine their chemical composition. Data analysis revealed that the waters belong to the HCO 3-‒ SO 42-‒ Ca 2+ and SO 42-‒ Ca 2+ water types, with neutral pH values. The metal concentrations in these waters (zinc, lead, barium, copper, chromium) are generally low, and most of the samples meet drinking water quality criteria (USEPA standards). Modeling using the PHREEQC software indicates that the dominant processes in the formation of the chemical composition of these waters are the dissolution of carbonate minerals and the oxidation of sulfide minerals. Carbonate minerals have a scarcer occurrence compared to sulfide minerals, such as galena, sphalerite, and pyrite, which are dominantly distributed. The low intensity of sulfide mineral oxidation is interpreted to result from a rapid water exchange and reduced contact time between the water and the rock. The occurrence of this process is localized only in the ore body zone. This study highlights the importance of kinetics (in terms of the chemical reaction rate) as the main factor influencing the oxidation of sulfide minerals and, subsequently the quality of mine waters.


INTRODUCTION
Negative environmental effects occur when surface water or groundwater comes into contact with primary and secondary minerals from the ore deposit under oxidizing conditions. 1 Deposits containing sulfide minerals tend to generate acid mine drainage (AMD) during the oxidation process of these minerals (e.g., pyrite, marcasite, and pyrrhotite). 2,3,4Acid mine drainage refers to water with low pH values (between 3.5 and 5), high sulfate concentrations, and elevated metal concentrations. 4,5The oxidation of sulfide minerals, primarily pyrite, depends on physicochemical conditions, the type of oxidant (O2 or Fe 3+ ), its concentration, activity, and the presence of microorganisms. 6,7,8According to Lottermoser, several types of pyrite oxidation can be distinguished, including: abiotic oxidation by oxygen, biotic oxidation by oxygen, abiotic oxidation by oxygen and iron, and biotic oxidation by oxygen and iron. 3The formation of acid mine drainage is strongly influenced by the reaction rate (kinetics) and its duration. 9The rate of biotic oxidation is always higher than abiotic, and pyrite oxidation in the presence of Fe 3+ ions can be 2-3 times faster than in the presence of O2 as the oxidant. 5epending on the geological environment in which groundwater circulates (i.e., the type of ore and host rocks), acid mine drainage may not always form.Unlike pyrite, the oxidation of other sulfide minerals such as galena, sphalerite, or arsenopyrite in the presence of oxygen does not directly produce acid mine drainage. 10,11,12This is due to their lower reactivity, resulting from greater crystal structure stability, low concentrations of released iron, and the formation of poorly soluble minerals. 13The absence of sulfide mineral oxidation, in addition to kinetics, can also result from the presence of minerals that neutralize the acidity of mine drainage (carbonates, silicates, and hydroxides). 2,3The neutralization process occurs under the same conditions as the sulfide mineral oxidation process and is independent of the oxygen concentration in the water. 3Neutral mine drainage (NMD) will form in some sulfide deposits due to the previously mentioned processes.Neutral mine drainage refers to waters with a neutral pH value (between 6.5 and 7) and high metal concentrations (most commonly lead, zinc, copper, and cadmium). 3In these waters, sulfates and bicarbonates are the main anions, and calcium, magnesium, and sodium content is elevated compared to acid mine drainage. 3The occurrence of neutral pH and lower metal concentrations in some mine waters can be attributed to kinetic factors, i.e., the slow dissolution and oxidation of sulfide minerals compared to rapid water exchange in mine waters. 2 Additionally, the surfaces of sulfide minerals can be covered with insoluble oxides or carbonates, which limits oxidation. 2Instances of neutral mine drainage have been found in Pb-Zn deposits throughout the United Kingdom and Italy. 14,15,16,17hese examples are characterized by neutral pH and increased concentrations of trace components such as lead, zinc, copper, arsenic, and cadmium (depending on the specific locality where the mine waters are formed).Studies considering these occurrences as the most common reason for the formation of neutral mine drainage cite the neutralization process in the presence of carbonate minerals. 14,15,16,17he objectives of this study are to identify factors that could influence the formation of neutral mine drainage, determine the mineral phases being dissolved, and identify the dominant hydrogeochemical processes that affect the release of trace elements into water.Such an approach could contribute to adapting mineral A c c e p t e d m a n u s c r i p t resource exploitation methods to reduce the negative impact on the environment and water resources.

Study area
The Grot Mine is located in the southeastern part of Serbia, within the municipalities of Bosilegrad and Vranje.The mining area is characterized by mountainous terrain.In the central part of the mining field, there is the ridge of Besna Kobilа, which is also the highest point in the relief (1923 m).Besna Kobilа represents the watershed divide between the Black Sea basin (South Morava River basin) and Aegean Sea basin. 18The river streams are characterized by a snow-rain regime, with peak flows occurring during snowmelt and spring rains.The area has a moderate continental climate with mountainous influences.The average annual air temperature ranges from 5 to 6 °C, with an average annual precipitation exceeding 1000 mm. 19In geological terms, the ore field comprises older Paleozoic crystalline schists that are intruded by older granitoids, younger Tertiary granitoids, and the youngest volcanic rocks (quartz latites) (Figure 1).The ore deposit is hosted in the schists of the Lisina Series, accompanied by quartz-latite, calcschists, and marbles. 18,19The formation of the mineral paragenesis is associated with several stages.The most important stage that led to mineralization (mesothermal phase) is characterized by minerals such as galena, sphalerite, pyrite, chalcopyrite, arsenopyrite, molybdenite, tennantite, quartz, stibnite, rhodochrosite, calcite, manganese siderite, chalcedony, and aragonite. 20During the supergene stage, minerals such as cerussite, smithsonite, pyrolusite, and limonite were formed through transformations. 20From a hydrogeological perspective, the fractured aquifer has a predominant distribution in the exploration area.It is formed within the granitoid and quartzlatite rocks and crystalline schists (Lisina and Jerešnica Series). 19The fracturing of the crystalline schist complex is significant.Groundwater accumulation is related to the fracture and fault systems of crystalline schists, marble, and quartz latite.The permeability of the rocks depends on the type of crystalline schist, with the Jerešnica-age crystalline schists being less permeable than the Lisina-age schists, representing a hydrogeological barrier for groundwater movement. 19The mining works were carried out from 1290 m to 1720 m.Completing the main export adit (MEA) at the lowest mining horizon involved advancing through the entire massif of Besna Kobilа, reducing the inflow of water into the mining works at higher elevations. 18The drainage of mining operations is entirely gravity-based.All the water from the mine is discharged at three levels: VI, VIII and IX horizons.In places where mine waters are discharged, sedimentation ponds have been installed at the outlets of mining horizons. 18The majority of mine waters are drained through the MEA at the IX horizon, where the water flows in three directions (towards the flotation plant, towards the Hajdučki Osoj adit, and the outlet of the IX horizon at the Crna Reka site).Sampling and chemical analysis Mine water samples were collected at eleven locations during the year's low flow period (September-November).During that period, 16 mine water samples were collected, while 11 were further considered in the study.The considered samples were taken at various locations across the exploitation field at discharge points from the adits before the sedimentation ponds, while the samples taken at locations after the ponds were omitted due to the possibility of removing certain components from the water.Mine water sampling was performed according to ISO 5667-1:2006, ISO 5667-3:2012, and ISO 5667-10:1992 standards at adits discharge points. 23,24,25Sampling and chemical analyses were carried out by the Institute of Public Health Vranje.
Determination of the pH was performed potentiometrically according to the ISO 10523:2008 standard. 26The water mineralization was determined by an analytical procedure as the dry residue at 103-105°C using the US EPA 160.3 method. 27Main cation composition (Na + , K + , Mg 2+ , Ca 2+ ) together with metals (zinc, lead, manganese, barium, copper, chromium) were determined by induced coupled plasma with atomic emission spectrometry ICP-AES (according to ISO 11885:2007 standard). 28The content of arsenic in mine waters was determined by atomic absorption spectrophotometry using the hydride technique (ISO 11969:1996). 29The content of ammonium, nitrate, and nitrite ions was determined by spectrophotometric methods (ISO 7150-A c c e p t e d m a n u s c r i p t 1:1984, ISO 7890-3:1988, ISO 6777:1984). 30,31,32The content of sulfate ions was determined by the turbidimetric method (SRPS H.G8.115:1984), while the chloride content was determined volumetrically according to the ISO 9297:1989 standard. 33,34Bicarbonate ions in water were determined by titration, in accordance with ISO 9963-1:1994. 35The content of dissolved oxygen was determined by the volumetric method (ISO 5813:1983), while hydrogen sulfide was determined spectrophotometrically (SZZZ, 1990). 36,37

Analysis and calculations
The degree of saturation of groundwater in relation to a certain mineral can be determined by comparing the product of ionic activity (IAP) of a real water sample and the solubility product of that mineral (Ksp), according to the given formula(1): 38 where SI is the saturation index.
The obtained SI value can indicate oversaturation or undersaturation of the water sample with respect to a given mineral, with the possibility of precipitation (SI>0) or dissolution (SI<0) of the specific mineral. 38aturation indices (SI) for specific mineral phases and dissolved forms of inorganic substances in water were calculated using PHREEQC software (Interactive version 3.7.3-15968). 39This software is widely used in geochemical and mine water research for these types of calculations. 15,40,41,42When calculating SI for carbonate minerals, the MINTEQ database was used, as it covers a wide range of inorganic parameters (metals) and organic compounds. 43The LLNL database was used for calculating SI for sulfur-bearing minerals because it has the most extensive dataset, making it more suitable for such calculations. 38,39For example, it includes the reaction of pyrite with water. 39xcel Workbook was used to determine statistical parameters (minimum, maximum, median) to determine the variations of certain parameters in mine waters.Also, this program was used for the graphical presentation of chemical parameters in the form of Box-Plot diagrams.

Hydrochemistry of neutral mine drainage of the "Grot" Pb-Zn mine
According to their physical properties, the examined mine waters are colourless and odourless.These waters belong to the category of cold waters.The water temperature ranges from 7.1 °C to 14.5 °C, depending on the air temperature.The mineralization (TDS) ranges from 163 to 719 mg L -1 (Tables S1 and I).These waters can be classified as low-mineralized, except for one sample with TDS > 500 mg L -1 .
The pH value of analyzed waters ranges from 7.04 to 7.6, classifying them as neutral.According to the USEPA standards (2018), the pH of drinking water should be within the range of 6.5 to 8.5, which is met in all samples. 44he most dominant cation is calcium (Ca 2+ ), with 25.6 to 56.8 mg L -1 concentrations.Concentrations of magnesium ions (Mg 2+ ) range from 1.2 to 4.2 mg L -1 .The second most abundant cation in these waters is sodium (Na + ), with concentrations ranging from 3.5 to 6.6 mg L -1 .Potassium ion (K + ) concentrations A c c e p t e d m a n u s c r i p t do not vary significantly in analyzed samples (1 to 1.1 mg L -1 ), and potassium has the lowest abundance among all cations in these waters.
The dominant anions in mine waters are bicarbonates and sulfates.Bicarbonates (HCO3 -) in mine waters occur in the range of 61 to 91.5 mg L -1 .The sulfate ions (SO4 2-) concentrations in these waters vary from 37.4 to 177.7 mg L -1 .Chloride ions in these waters occur at low concentrations (4 to 18 mg L -1 ).The content of ammonium (NH4 + ), nitrite (NO2 -), and nitrate (NO3 -) ions in the water is also low.
Concentrations of dissolved oxygen (O2) range from 5.4 to 10.9 mg L -1 , while the concentrations of dissolved hydrogen sulfide (H2S) are relatively low (from 0.006 to 0.351 mg L -1 ).Based on the dominant anions and cations, analyzed mine waters can be classified into two hydrochemical groups: HCO3 --SO4 2--Ca 2+ and SO4 2--Ca 2+ waters (Figure 2).The first group consists of mine waters that are not in direct contact with the rocks from the ore body (water that is pumped out before it reaches the active mining zone) and waters from sites where mining is currently not taking place.The second group consists of mine waters that are in direct contact with the rocks from the deposit, specifically the waters from the VI, VIII and IX horizons.Based on the results obtained from laboratory analysis, all water samples have met the prescribed criteria for wastewater (Table S1). 18A part of the mine water is used for the water supply of the Kriva Feja settlement and the mining complex 19 , indicating that some water samples also meet the criteria for drinking water.

Microelements in mine waters
According to laboratory testing results, the concentrations of metals (zinc, barium, chromium, copper) in the mine waters are generally low (Table S1 and Figure 3). 44These results are not specific to sulfide deposit mine waters, which typically have low pH values and high metal concentrations, defined as acid mine drainage (AMD).Iron concentrations in the tested waters range from 0.11 to 2.37 mg L -1 .Elevated concentrations of iron can be an indicator of the presence of oxidation of sulfide minerals. 2Zinc concentrations in these waters are low (ranging from 0.05 to 0.7 mg L -1 ), while lead concentrations vary from 0.02 to 0.47 mg L -1 .The concentrations of barium in the waters from this area are below 0.05 mg L -1 .Chromium and copper are present in trace amounts in these waters, with concentrations below 0.02 mg L -1 for chromium and below 0.03 mg L -1 for copper.
Manganese concentrations in mine waters range from 0.05 to 0.6 mg L -1 , while arsenic concentrations are below 0.05 mg L -1 .

Speciation modeling and saturation indices
Most trace elements and many major elements in surface water and in groundwater exist in the form of complexes rather than free ions. 45The presence of certain metals in water can indicate reactions such as dissolution and precipitation of mineral phases. 5ccording to the modeling results in PHREEQC software, a list of the most prevalent ionic species in mine waters is provided (Table II).Me 2+ and MeSO4 0 represent the two dominant ionic species for calcium and magnesium, while trivalent iron is most commonly present as Fe(OH)2 + and Fe(OH)3 0 .Metals such as lead and copper are predominantly present as free ions and MeCO3 0 ionic pairs, while manganese is most commonly found as free ions or MnSO4 0 ionic pairs.Both types of mine waters have a similar distribution of ionic species.Only minor differences in the distribution of ionic species of zinc are observed.In both types A c c e p t e d m a n u s c r i p t of water, zinc predominantly occurs as free ions.Additionally, the presence of the MeCO3 0 ionic pair is noticeable in the HCO3 --SO4 2--Ca 2+ water type, while the MeSO4 0 ionic pair is more dominant in the SO4 2--Ca 2+ water type.
Table II.The distribution of ionic species in mine waters (presented in terms of median molalities and the median percentage distribution of ionic species relative to the total molality of metals) Based on the calculated saturation index (SI) values, the analyzed waters are undersaturated (SI<0) with respect to minerals such as calcite, aragonite, cerussite, smithsonite, rhodochrosite, siderite, and pyrolusite (Table III).The origin of Ca 2+ and HCO3 -ions in these waters could be linked to the dissolution of calcite and aragonite.The median SI values for calcite and aragonite are -0.80 and -0.95, respectively.Both minerals have similar saturation index values, implying that they dissolve relatively equally in the investigated area.

A c c e p t e d m a n u s c r i p t
The SI values for minerals such as cerussite, smithsonite, and rhodochrosite range from -1 to -2.9.The presence of Pb 2+ ions in these waters can be attributed to the dissolution of cerussite.Since cerussite has a lower solubility product, this process occurs with lower intensity compared to the dissolution of calciumcarbonates, resulting in lower concentrations of Pb 2+ ions in the water.
The origin of Zn 2+ and Mn 2+ ions in mine waters may be associated with the dissolution of smithsonite and rhodochrosite.The occurrence of iron ions in these waters can be linked to the dissolution of siderite.
On the other hand, the saturation index values for mineral phases galena, sphalerite, pyrite, covellite, and chalcopyrite are very low, indicating that the waters from this research area are undersaturated concerning the given minerals.The median SI value for the mineral phase pyrite is -19.24, while for galena it is around -3.84, and for sphalerite it is around -6.06.Although sulfide minerals dominate over carbonate minerals in the investigated area, it can be assumed that the oxidation process of sulfide minerals occurs to a low extent.
The origin of SO4 2-ions may be from the oxidation processes of pyrite, galena, and sphalerite.This process is specific to the ore deposit zone.Therefore, the origin of lead, zinc, manganese, and iron can be twofold, as these metals can originate from sulfide minerals (such as galena, sphalerite, pyrite, etc.) and carbonate minerals.Most probably, the dissolution of carbonate minerals also takes place during the oxidation process of sulfide minerals.Additionally, the origin of barium may be from galena and sphalerite, where this metal is concentrated. 20The saturation index values for barite in these waters are slightly positive (median SI 0.44), indicating the precipitation of secondary barium sulfate, due to the presence of an abundance of sulphate ions.
All the minerals under consideration have been detected in the ore deposit, which is deposited in specific horizons within the schists. 20ble III  The origin of the mine waters is mainly atmospheric, with significant amounts of rainfall occurring in this area throughout the year.The low intensity of sulfide mineral oxidation may be attributed to the rapid exchange of these waters.Since the process of pyrite oxidation is low, this process is probably not dominant in shaping the chemical composition of these waters.The low concentrations of metals in the water are another indication of the low intensity of this process.Additionally, sulfide minerals' solubility products are much lower than those of carbonate minerals.Therefore, the dissolution of carbonate minerals occurs before the dissolution of sulfide minerals.

CONCLUSIONS
According to the results of chemical analysis, the mine waters of the "Grot" Pb-Zn underground mine have a neutral pH value and low concentrations of trace elements except for manganese and iron.The analyzed mine waters are classified into two groups based on their chemical composition.These differences can be attributed to the fact that the first group of waters of the HCO3 --SO4 2--Ca 2+ type drains before contacting the ore body zone.On the other hand, the second group of waters of the SO4 2--Ca 2+ type is in direct contact with the ore body.However, it does not exhibit characteristics of acid mine drainage, probably due to the short water retention in the system.Modeling in PHREEQC software and the analysis of the obtained saturation index values revealed two dominant processes in shaping the chemical composition of these waters: dissolution of carbonate minerals and oxidation of sulfide minerals.The intensity of sulfide mineral oxidation is low and localized to the ore zone.The lack of intense sulfide mineral oxidation, despite their high occurrence in the study area, is due to the kinetics of this process.Longer rock-water contact is required to initiate this process, while the mine waters in this area are rapidly exchanged and have a short contact with the rock.Therefore, it can be concluded that the kinetic factor plays a crucial role in forming neutral, good-quality mine waters.At the same time, neutralization in the presence of carbonate minerals is not a significant factor.

Fig. 2 .
Fig. 2. Piper diagram of mine waters collected from six different locations within the exploration area.

A c c e p t e d m a n u s c r i p t
SUPPLEMENTARY MATERIALAdditional data are available electronically at the pages of journal website: https://www.shd-pub.org.rs/index.php/JSCS/article/view/12540,or from the corresponding author on request.

Table I .
Physical-chemical parameters and concentrations of macrocomponents in mine water samples from the exploration area . Saturation Index (SI) values for selected minerals