The Role of Mineral Assemblages in The Environmental Impact of Cu-Sulﬁde Deposits: A Case Study from Norway

: Metallic mineral deposits represent natural geochemical anomalies of economically valu-able commodities but, at the same time, their weathering may have negative environmental implications. Cu-sulﬁde mineral deposits have been recognized as deposits with a particularly large environmental footprint. However, different Cu deposits may result in signiﬁcantly different environmental impacts, mostly depending on weathering conditions, but also on geological characteristics (mineralogy, geochemistry, host-rock lithology) of the Cu mineralization. This study presents new mineral and geochemical data from the Repparfjord Tectonic Window sediment-hosted Cu deposits and the Caledonian volcanogenic massive sulﬁdes (VMS) deposits. The deposits share similar mineral features, with chalcopyrite and bornite as the main ore minerals, but they differ according to their trace element composition, gangue mineralogy, and host lithology. The studied sediment-hosted Cu deposits are depleted in most toxic metals and metalloids like Zn, As, Cd, and Hg, whereas the Røros Caledonian VMS mineralization brings elevated concentrations of Zn, Cd, In, Bi, As, and Cd. The conducted leaching experiments were set to simulate on-land and submarine weathering conditions. A high redox potential was conﬁrmed as the main driving force in the destabilization of Cu-sulﬁdes. Galvanic reactions may also contribute to the destabilization of minerals with low rest potentials, like sphalerite and pyrrhotite, even under near-neutral or slightly alkaline conditions. In addition, the presence of carbonates under near-neutral to slightly alkaline conditions may increase the reactivity of Cu sulﬁdes and mobilize Cu, most likely as CuCO 3 (aq). Author Contributions: Conceptualization, Y.M., S.S.P.; methodology, Y.M. and S.S.P.; validation, Y.M. and S.S.P.; formal analysis, Y.M. and S.S.P.; investigation, Y.M. and S.S.P.; resources, Y.M., S.S.P. and K.K.; data curation, Y.M., S.S.P. and K.K.; writing—original draft preparation, Y.M.; writing—review and editing, Y.M., S.S.P. and K.K.; Y.M. K.K.;


Introduction
Copper is one of the most widely used mineral commodities in modern society, with a particular importance to electronics, electrical power generation, and the renewable energy sector, as well as in electric vehicle technologies [1][2][3]. In nature, Cu can be found in various types of mineral deposits, but in addition to the Cu-porphyry type (e.g., Chuquicamata, Chile [4]; El Teniente, Chile [5]; Ok Tedi, Papua New Guinea [6]), deposits of volcanogenic massive sulfides (VMS) and Cu-sediment hosted types represent the most important sources of Cu. Worldwide, 20 million tons (Mt) of copper was the total mine production in 2020 [7]. This number decreased from 24.5 Mt in 2019 due to COVID-19 lockdowns in April and May [8]. Chile remains the major copper producer (5.7 Mt) followed by Peru (2.2 Mt), China (1.7 Mt), DR Congo (1.3 Mt), and the US (1.2 Mt) [9]. A range from 60% to 75% of copper is mined from porphyry-copper deposits [10], 20% from sediment-hosted Cu deposits [11], and around 6% of Cu is mined from VMS deposits [12].
In this study, sediment-hosted Cu mineralization is represented by samples from the Nussir and Ulveryggen Cu deposits, from the Repparfjord Tectonic Window, while the Røros deposit, located within the Upper Allochthon of Scandinavian Caledonides, was of major ore minerals, and aim to extrapolate the results of the study to apply to produced mine tailings. The reactivity of the tailings with the surrounding environment will be significantly higher due to particle size and an increase in surface energy.
Norway has a long shoreline, and the ore-bearing rocks are often subjected to weathering by seawater. In addition, Norway is one of few countries where subaquatic mine tailings deposition is permitted. In both cases, it is important to understand the role of salinity on weathering of sulfides. Therefore, during the experiments seawater was used together with meteoric water.
The aim of this study is to evaluate the potential environmental impact of the studied Cu mineralization, considering geological characteristics including mineralogy, geochemistry, as well as the main physicochemical features of subaerial and subaquatic disposal sites.

Sediment-Hosted Cu Deposits of Nussir and Ulveryggen, Repparfjord Tectonic Window
The Repparfjord Tectonic Window, Northern Norway, is composed of mafic metavolcanics and carbonate-siliciclastic sequences that were compressed in a SE-NW direction during the Svecofennian Orogeny at ca. 1.84 Ga [48,49]. The rocks are metamorphosed under greenschist to lower amphibolite facies conditions, and [50] determined the age of host volcanics to be about 2.1 Ga, with Nussir mineralization around 1.765 Ga. The Repparfjord Tectonic Window contains numerous sites with Cu mineralization (e.g., [14,51]), of which the Nussir (26.7 Mt at 1.13% Cu) and the Ulveryggen (7.7 Mt at 0.81% Cu) deposits have the greatest implications for the local environment [14,52]. The Nussir deposit is hosted by a thin (no more than 5 m thick) metadolostone layer that can be traced for several kilometres ( Figure 1A,C), intercalated with metasandstone, metasiltsone and metapelites. The metasedimentary complex is overlain by a several hundred meters thick metavolcanic sequence [49,50,53]. Despite the close geographical occurrence of the Nussir and Ulveryggen deposits, they have different lithologies. The Ulveryggen deposit is hosted by arkosic metasandstones, metasiltstones, and metaconglomerates with low carbonate content. Mineralization can be traced for more than 1 km in the northeast direction and is structurally confined to tight folds [48,49,54,55]. Chalcopyrite, bornite, sphalerite, and minor pyrite are the main ore minerals at both deposits.
The Nussir mineralization is confined to a thin (up to 5 m thick) dolomitic marble layer intercalated with metasiltstone, metapelites and metasandstones, localizing the mineralization within quartz-carbonate veins as well as disseminated in mafic metavolcanics [49,50,54,55]. The major ore minerals at the Nussir and Ulveryggen deposits are chalcopyrite, bornite, and chalcocite [49,50,54].  [56] as well as modified after [49,55]. The map of Norway in (C) shows the locations of the Nussir, and Røros areas.

Volcanogenic Massive Sulfide (VMS) Deposit Røros, the Upper Allochthon of Scandinavian Caledonides
The Røros VMS mining area includes several hundreds of mineralizations in the southeastern part of the Upper Allochthon of the Scandinavian Caledonides [57], including the Røros deposit studied herein. The mineralizations are characterized by predominately chalcopyrite, sphalerite, pyrite, and galena hosted by interbedded metatuffite, metagraywacke, and gabbroic sills and dykes [14,58,59].
The Upper Allochthon of Scandinavian Caledonides extends for about 1500 km, from the Stavanger region in southern Norway to the Barents Sea region in northern Norway. This first-order tectonostratigraphic unit is dominated by sedimentary and magmatic rocks derived from the Iapetus Ocean, including ophiolite and island-arc complexes usually associated with VMS type mineralization (Figure 1B,C; [60][61][62]).
The Røros mining area in Trøndelag County, south-eastern Norway, hosts numerous occurrences of the VMS type ( Figure 1B). Among the largest occurrences are found at the Storwartz and Olav mines in the eastern sector, and the Kongens mine in the north-western sector ( Figure 1), with average Cu and Zn contents of about 2.7% and 4.2-5%, respectively [59]. The mineralization is hosted by metagraywacke interbedded with metatuffites, metabasalts and gabbroic sills and dykes [14,58,59], with major ore minerals: chalcopyrite, pyrrhotite, sphalerite, and pyrite.

Materials and Methods
For this study, representative samples of the Cu sulfide mineralization were selected from the Nussir and Ulveryggen sediment-hosted Cu deposits, and from the Røros VMS deposit. Two main types of samples were analyzed: (1) Mineral assemblages composed of ore and gangue minerals and (2) Individual Cu-sulfide minerals.
Twenty-seven polished thin sections of representative mineral assemblages were prepared at the Department of Geosciences at UiT The Arctic University of Norway. Three thin sections represented reference samples for the respective deposits. The reference samples were studied under a reflective polarizing light microscope and a Zeiss Merlin Compact Field Emission Scanning Electron Microscope (FE-SEM) equipped with an energy-dispersive X-ray spectrometer (EDS) at UiT The Arctic University of Norway, to determine mineral and geochemical characteristics of the ore mineral assemblages prior to and after the leaching tests. In order to investigate the primary ore, the SEM was run in a high vacuum regime at 20 kV accelerating voltage, 20 s counting time, and with an aperture of 60 µm.
In order to simulate weathering processes corresponding to the tailings disposal site conditions, a set of leaching experiments were performed on the polished thin sections ( Figure 2). The experiments were designed to simulate (   The average organic content in the natural marine sediments was 0.82 wt.% (Supplementary Table S1), whereas the quartz sand was free of organic matter. To test the influence of redox potential, half of the beakers with quartz sand were doped with 10 wt.% of organic matter and sealed with parafilm tape to prevent oxidation reactions. The other half of the beakers were left open during the entire experiment and refilled with circa 150 mL of meteoric water once per day to ensure oxidative conditions. Pure calcium carbonate was used to buffer relevant solutions. The experiments were run for three months, and in order to accelerate the reactions, the beakers were kept in a water bath at 50 • C.
At the end of the experiments, the samples were carefully removed and investigated under a reflective light microscope. The formation of secondary minerals was studied by Raman spectroscopy, conducted at UiT The Arctic University of Norway in Tromsø. A Renishaw inVia confocal Raman microscope equipped with a 532 nm (green) diode laser was used to identify the mineral phases in the studied ore samples, as well as the degree of weathering after simulation of weathering conditions under on-land and subaquatic conditions. The identification was based on Raman spectra published in the literature [63].
Individual grains of Cu sulfides were handpicked under a binocular microscope, washed in an ultrasonic bath and pulverized in an agate mortar. An amount of 0.5 g was analyzed for bulk trace element composition at the AcmeLabs (Vancouver, B.C. Canada), on an ICP MS instrument following the internal LF202 analysis code. Mineral analyses show that typical ore assemblages from the Nussir and Ulveryggen sediment hosted Cu deposits consist of chalcopyrite, bornite, and chalcocite ( Figure 2B,C). The Røros VMS mineralization is represented by massive sulfide bodies predominantly composed of chalcopyrite, sphalerite, pyrite and pyrrhotite ( Figure 2D).
The Nussir dolomitic marble contains rhomboidal-shaped fragments of carbonates that are up to 5 mm in diameter. The mineralization is confined to crosscutting quartz-carbonate veins with euhedral to subhedral grains of vein carbonate, which are up to 0.1-0.3 mm in diameter. Quartz grains are anhedral and about 0.1 mm in diameter. Chalcopyrite is the dominant Cu mineral together with bornite; the minerals are intergrown and contain abundant inclusions of pyrite and sphalerite ( Figure 2B).
The Ulveryggen arkosic metasandstone contains fragments of quartz and feldspar up to 0.2 mm in size. The ore minerals are disseminated and they have grown interstitially between fragments of quartz and feldspar together with muscovite. The main ore minerals are bornite and chalcopyrite ( Figure 2C), with minor amounts of pyrite and chalcopyrite.

Røros
Since the Røros samples were prepared from pieces of massive ore, only ore minerals were observed under the microscope. The main ore minerals are pyrrhotite, pyrite, chalcopyrite and sphalerite, which show various intergrowth textures. Chalcopyrite, which is the most abundant mineral, occurs as individual grains that are several centimetres in diameter. Pyrite and pyrrhotite crystals also show large grain-sizes (several centimetres in diameter), while sphalerite forms small inclusions of less than 0.1 mm in diameter ( Figure 2D).

Leaching Tests
Experimental conditions #1 (Marine sediments, TOC = 0.82 wt.%; carbonate buffered; infiltrated with seawater ( Figure 3); Eh sed = 239.7 mV, pH sed = 7.64, where Eh sed and pH sed are values of redox potential and pH for pore water in sediments measured after initial stabilization of conditions, i.e. 60 h after the experiment started) did not affect stability of mineral assemblages from the Nussir and Ulveryggen deposits ( Figures 4A and 5A). However, for the sample from the Røros VMS deposit, small grains of sphalerite were weathered while some pyrite grains remained well-preserved ( Figure 6A; Supplementary Figures S3H and S4A,B).
Experimental conditions #5 (quartz sand, TOC = 0 wt.%; carbonate buffered, infiltrated with meteoric water; Eh sed = 222.6 mV, pH sed = 8.76; Figure 3) affected the sulfides from all three studied deposits to different degrees. Chalcopyrite and bornite from Nussir were extensively weathered ( Figure 4E; Supplementary Figure S1G), whereas for the Ulveryggen sample, chalcopyrite and chalcocite were partly weathered while most of the pyrite and bornite remained well-preserved ( Figure 5E; Supplementary Figures S2F-H and S3A). Sulfides from Røros were more oxidized in comparison to sulfides from Nussir and Ulveryggen. In contrast to pyrite and chalcopyrite, which remained well-preserved or only partly oxidized, pyrrhotite and sphalerite were significantly weathered ( Figure 6E  Experimental conditions #6 (quartz sand, TOC = 0 wt.%; no added carbonates; infiltrated with meteoric water; Eh sed = 245.7 mV, pH sed = 7.54; Figure 3) resulted in extensive oxidation of the sulfides from the Nussir and Ulveryggen deposits. Chalcopyrite, bornite and chalcocite obtained a weathered rim around the grains (Figures 4F and 5F; Supplementary Figures S1H and S3B,C). Some grains of chalcopyrite were entirely covered with a thin film of weathering products ( Figure 4F). Pyrite from Ulveryggen remained well-preserved, but might have accelerated the oxidation of bornite (Supplementary Figure S3C). Pyrrhotite and sphalerite from the Røros deposit were significantly weathered, whereas pyrite remained relatively well-preserved with insignificant formation of iron oxides ( Figure 6F; Supplementary Figure S5D). A weathered Cu-containing mineral was also observed within the pyrite (Supplementary Figure S5D).  Experimental conditions #7 (quartz sand, TOC ≈ 10 wt.%, carbonate buffered, infiltrated with meteoric water; initial Eh sed = 191.5 mV, pH sed = 7.86; Figure 3) did not affect the Nussir and Ulveryggen sulfides. The minerals remained well-preserved ( Figure 4G and Figure 5G; Supplementary Figure S3D,E). The Røros ore minerals also remained almost unaffected, however some pyrite grains were slightly tarnished with bright blue secondary covellite and brownish iron hydroxides ( Figure 6G; Supplementary Figure S5E,F).

Raman Spectroscopy
Raman spectra (Figure 7) obtained from chalcopyrite and pyrite from the Nussir, Ulveryggen, and Røros deposits suggest formation of new peaks 450 and 500 cm −1 after exposing samples from the Nussir and Ulveryggen to experimental conditions #2 and #5, respectively ( Figure 7A-D).

Nussir and Ulveryggen
The Nussir chalcopyrite contains 100 ppm of Ni, 85 ppm of Co and 310 ppm of Zn. The content of Bi is 0.3 ppm, while As, Mo, Cd, Sb, Pb, and Hg contents are minor or below the detection limit. The Nussir chalcopyrite also contains Ba (12 ppm), most likely in the form of nearly insoluble inclusions of barite.
Ulveryggen bornite contains 2 ppm of Co and 5 ppm of Mo. Nickel, Zn, As, Sn, Sb, Cd, Pb, and Hg contents are minor or below the detection limit, while the Rb content is 24 ppm. The content of Ba in the Ulveryggen bornite is 527 ppm.

Røros
Chalcopyrite was picked from the crushed Røros sample. The Co content is 535.9 ppm. The chalcopyrite also contains: Ni 7.8 ppm and Zn 2942 ppm. As content is 226.1 ppm, and Sn and Sb contents are 8 and 1.8 ppm respectively. Chalcopyrite contains 30 ppm of Cd and 18 ppm of Ba. The Bi content is 9.9 ppm and Hg content is 4.59 ppm.

Discussion
The Cu mineralization found in the Nussir and Ulveryggen sediment-hosted Cu deposits is characterized by predomination of chalcopyrite, bornite and chalcocite. Mine tailings from these Cu-sulfide deposits are associated with a high risk for the generation of acid mine drainage (AMD) because of the high Fe 2+ /Fe 3+ and S 2− /SO 4 2− ratios in their mineral assemblages. Furthermore, the Ulveryggen deposit has a low carbonate content that additionally increases the risk. In contrast, a low content of potentially toxic elements such as As, Cd, Hg, and Zn in both, reduces their environmental threat [53].
The Røros Cu-Zn VMS deposit, similar to other VMS deposits worldwide [65,66], is characterized by a polymetallic composition ( Table 2). The main ore minerals are sulfides, including chalcopyrite, bornite, pyrite, and pyrrhotite. High Fe 2+ and S 2− contents together with an absence of carbonates from its mineral assemblages point to a high risk for the generation of AMD in this deposit (e.g., [23,27,46,67,68]). In addition, the enrichment in a wide spectrum of potentially toxic metals and metalloids (e.g., As, Bi, Cd, In, and Zn) magnifies the environmental risk associated with mining activities and/or processes of natural weathering in this type of ore deposits (e.g., Rio Tinto VMS deposits, the Iberian Pyrite Belt in Spain [26,28,[69][70][71] and the Britannia Creek VMS deposit, Canada [48]).
Leaching tests are recognized as powerful tools to predict the behavior of sulfides in different conditions (e.g., [20,32,37,39,40]). Such tests play an important role in the initial phases of mining planning, while deciding the potential placement of tailings for future storage. The tests are usually performed in batch reactors specially equipped with sensors controlling temperature, pH, and the amount of dissolved oxygen (e.g., [39,47,72]). Otherwise, the tests can be performed in leaching columns (e.g., [32,73]) or in static conditions (e.g., [46,74]) for periods of several days to several years. The leaching experiments in this study were designed to test the stability of ore mineral parageneses from three different Cu deposits under diverse physicochemical conditions ( Figure 3, Table 1), and predict the behavior of ore-bearing mineral assemblages disposed in on-land and submarine conditions. As expected, the high redox potential was the main driving factor in destabilization of sulfides (e.g., [75] The organic matter content of the natural marine sediments that were used (0.82 wt.%) was sufficient to prevent oxidation of the sulfides in experimental conditions in which the sulfide parageneses were exposed to seawater. In the setups with meteoric water, a different scenario was observed. A lower solubility of oxygen in seawater (4.6 mg/L at 50 • C) compared to meteoric water (5.6 mg/L at 50 • C) was probably one of the controlling factors [76]. When sediments were doped with an additional 10 wt.% of organic matter, the differences between seawater and meteoric water influence were not recorded.
The carbonate buffered conditions were mostly less reactive due to a slightly alkaline pH value of the infiltrating aqueous solutions. However, the samples from the Røros VMS deposit revealed that sphalerite can enter galvanic reactions and be extensively dissolved even in alkaline or near-neutral conditions. The prerequisite is that sphalerite (−0.24 V; [77]) occurs in direct contact with sulfides with greater rest potentials like pyrite (0.63 V; [78]) or chalcopyrite (0.54 V; [79]). In interactions with oxygen-rich solutions, sphalerite will act as an anode: and prevent oxidation of pyrite, and Cu-sulfides reacting with oxygen adsorbed on their surface: This reaction will not affect the pH of the aquifer, but it will promote leaching of Zn. The galvanic reaction may represent a particularly large environmental issue in mineral deposits in which pyrrhotite (−0.24 V; [77]) is intergrown or occurs in direct contact with Cu-sulfides and/or pyrite: The anode reaction will result in dissolution of pyrrhotite, oxidation of ferrous to ferric ions, and consequently in acidification of the system.
In the experimental setups buffered with carbonates, the Cu sulfides showed an increased reactivity ( Figures 4B,E, 5B,E and 6E) whereas pyrite did not show significant changes under these conditions ( Figure 6B). Carbonates are often used for prevention of acid mine drainage (e.g., [80]), but the results of this study show that in near-neutral to slightly alkaline conditions Cu can be mobilized from sulfides, most likely in the form of CuCO 3 (aq) [53,81]. This reaction is more intensive in solutions with a higher redox potential ( Figures 4E, 5E and 6E).
In addition, gangue mineralogy might also play a role in the rate and degree of oxidation. The Nussir mineralogy is hosted by dolomitic marble, while Ulveryggen Cu sulfides are hosted by arkosic sandstone, which retards the oxidation of sulfides [53]. Røros sulfides, studied here, were sampled from a massive ore with an absence of gangue minerals. Therefore, galvanic interaction was favoured to higher degree of oxidation and was not prevented or retarded by gangue mineralogy. Weathering in mine tailings will be accelerated due to higher surface area, however this is also true for gangue mineralogy which in the case of Nussir and Ulveryggen might play a buffering role. On the other hand Røros mineralization being hosted by massive mafic volcanic rocks, which are extremely soluble, will most probably not retard the oxidation reaction or buffer it to limited degree.
Raman spectroscopy confirmed the slight distortion of crystal lattices of pyrite and chalcopyrite (Figure 7), however, the signal was low. Optically, even when the blue tinge was observed in the Nussir and Ulveryggen samples, Raman analyses did not record any changes in crystal structure of sulfides from these two deposits. This is attributed to oxidation occurring in a thin layer of secondary minerals, which tends to be amorphous and is characterized by the absence of a detectable lattice. In addition, the signal from primary minerals is significantly higher.

Conclusions
The mineral assemblages from all three studied deposits point to a high risk for generation of acid mine drainage due their high Fe 2+ /Fe 3+ and S 2− /SO 4 2− ratios. However, different ore-forming conditions of sediment hosted Cu deposits (Nussir and Ulveryggen) from conditions related to the formation of VMS deposits (Røros) resulted in contrasting behavior of the trace elements. As a consequence, the Nussir and Ulveryggen deposits are depleted in most potentially toxic elements, such as As, Cd, Hg, and Zn, whereas the Røros VMS mineralization is enriched in a wide spectrum of potentially toxic metals and metalloids, including As, Bi, Cd, In, and Zn.
The leaching experiments revealed the redox potential as the main factor that controls the stabilities of Cu-sulfides for both on-land as well as submarine conditions. Galvanic reactions may contribute to the destabilization of minerals with low rest potentials, like sphalerite and pyrrhotite, even under near-neutral or slightly alkaline conditions. The destabilization of pyrrhotite can have particularly negative environmental consequences due to the release of ferrous ions to an aquifer and acidification of the system as a result of oxidation of ferrous to ferric ions.
Although carbonates are often used for prevention of acid mine drainage, the presence of carbonates under near-neutral to slightly alkaline conditions may increase the reactivity of Cu sulfides and mobilize Cu, most likely in the form of CuCO 3 (aq).
More complex ore minerals assemblages lead to deeper weathering in given conditions. Thus, Røros chalcopyrite were notably more altered than chalcopyrite from the Ulveryggen and Nussir deposits.