A. M. Álvarez-Valero (&) R. Pérez-López. J. Matos Æ M. A. Capitán Æ J. M. Nieto Æ R. Sáez Æ J. Delgado Æ M. Caraballo

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1 Environ Geol (2008) 55: DOI /s x ORIGINAL ARTICLE Potential environmental impact at São Domingos mining district (Iberian Pyrite Belt, SW Iberian Peninsula): evidence from a chemical and mineralogical characterization A. M. Álvarez-Valero Æ R. Pérez-López Æ J. Matos Æ M. A. Capitán Æ J. M. Nieto Æ R. Sáez Æ J. Delgado Æ M. Caraballo Received: 24 April 2007 / Accepted: 12 November 2007 / Published online: 28 November 2007 Ó Springer-Verlag 2007 Abstract São Domingos like other long-term activity mines of the Iberian Pyrite Belt (IPB) dating back to pre- Roman times, is supposed to produce considerable amounts of mining wastes which cause significant downstream negative environment impact related to the acid mine drainage (AMD) production and high content of potentially toxic metals and metalloids in Chanza and Guadiana Rivers. The AMD production of a given mining waste depends on the ratio of its acid production to neutralizing phases. In this work, a chemical and mineralogical characterization of the sulphide-rich wastes from São Domingos has been developed to discriminate which residues are the main sources of AMD generation. A total of 47 representative samples of the different residue types were collected to estimate their possible contamination hazards through detailed studies of (1) for a mineralogical characterization: reflected-light optical microscope, scanning electron microscope (SEM) and XRD analysis; and (2) for a chemical characterization: bulk-rock analysis. AMD prediction by the standard acid-base accounting method (ABA) was used in order to determine the acidification potential of each residue type. This study also offers an estimation of the contribution of toxic elements to the A. M. Álvarez-Valero (&) R. Pérez-López M. A. Capitán J. M. Nieto R. Sáez J. Delgado M. Caraballo Department of Geology, Faculty of Experimental Sciences, University of Huelva, Campus El Carmen, Avda. Fuerzas Armadas, s/n, 21071, Huelva, Spain antoniomiguel.alvarez@dgeo.uhu.es J. Matos INETI, R. Frei Amador Arrais 39 rc, Ap , Beja, Portugal environment, being thus, a base for future remediation actions at São Domingos and other abandoned massive sulphide mines within the IPB. Keywords Iberian Pyrite Belt Mining wastes Acid mine drainage São Domingos mine Environmental impact Introduction The exploitation of mineral resources has been one of the essential activities for the development of the humanity. However, the human intrusion in the environment produces adverse alterations that always finish getting into debt to the man with the nature. A clear example of this is the Iberian Pyrite Belt (IPB), located in the southwest part of the Iberian Peninsula. It is one of the largest metallogenetic provinces of massive sulphides in the world with original reserves over 1,700 Mt (Sáez et al. 1999). The miningmetallurgical wealth of the region was the economic support of numerous civilizations seated from prehistoric times (Nocete et al. 2005). The intense mining activity produced considerable amount of residues, which have caused the environmental deterioration of the zone in all its meanings: soil degradation, water resources pollution, biodiversity decrease, and even, atmospheric pollution in some moments of the history. The most important environmental problem derives from the sulphide (mainly pyrite) oxidation contained in the aforementioned residues. This process produces an extremely acid leachate with high contents of sulphate, metals and metalloids known as acid mine drainage (AMD) (Lowson 1982; Parker and Robertson 1999; Younger et al. 2002). The AMD is the main pollution source of natural watercourses in mining

2 1798 Environ Geol (2008) 55: environments of the IPB (Guadiana, Tinto and Odiel rivers) (Olías et al. 2006; Nieto et al. 2007). In particular, the São Domingos mine is one of the most emblematic Portuguese massive sulphide deposits. The mining district is located in the northern sector of the IPB, about 5 km from the Spanish border (Fig. 1). Although mining activity has ceased at present, the large-scale exploitation of this deposit between 1857 and 1966 favoured the production of enormous waste dumps, where oxidation of pyrite and associated sulphides is resulting in the AMD production. The final acid discharge with high contents of metals from São Domingos reaches the Chanza river, main effluent of Guadiana river, causing its partial pollution. The field-bearing secondary minerals within waste deposits indicate that these materials are reactive. Jarosite and other secondary low crystalline minerals as oxides, oxhydroxides, iron hydroxy-sulphates are important for the environment as they play a crucial role in the solubility of the potentially toxic chemical elements. For this reason the chemical and mineralogical detailed studies at the intensively contaminated area of São Domingos are fundamental to know the processes that influence liberation, transport, retention and likely later remobilization of freemetals after sulphide-oxidation. A variety of proposals for restoration have been suggested to reduce the environmental impact associated with the AMD generation on the river basins of the IPB (Serrano et al. 1995). The different technologies applied were basically the passive treatment of AMD by means of anaerobic compost wetlands and anoxic limestone drains (Viñas and López Fernández 1994). Nevertheless, the Fig. 1 Geographic location of the Iberian Pyrite Belt and related main mining districts treatment of the acidic waters generated in the IPB is a short-term solution to the problem. The extreme acidity and high concentration of metals (mainly Fe) of these drainages saturate rapidly the treatment systems. Summarizing, the sulphide oxidation processes at São Domingos inevitably produces AMD leachates rich in sulphates, iron and other heavy metals and metalloids which are an important source of pollution, and hence a threat to the ecologic equilibrium. The SW Iberian Peninsula climate favours the AMD generation process at São Domingos and adjacent mining districts within the IPB. The main objective of this work is the chemical and mineralogical characterization of all types of mining and smelting wastes in the São Domingos mining district, as a base for the assessment of their environmental impact. This will be the foot to design future remediation actions in the area. For instance, recent investigations have been focused on direct treatment of the mine residues, as sources of this type of drainages. The experiments in laboratory of Pérez- López et al. (2007a, b) show how the addition of a strongly alkaline substance as fly ash material to a pyrite-rich mining residue, favours the acid neutralization, metal retention in AMD, and therefore, the improvement of the quality of the leachates produced. Only to carry out a treatment of this type in the field, the preliminary step is to characterize and identify which are the potentially acid producing wastes. A brief geological setting The IPB is located in the Sud-Portuguese Zone (ZSP) of the Hesperic Range (Fig. 1). It contains a complex stratigraphic succession composed by volcanic and sedimentary rocks ranging in age from Devonian to Carboniferous (Schermerhörn 1971). The middle series known as volcanicsedimentary complex (VSC) comprise essentially felsic volcanic and subvolcanic rocks, with local basic flows and sills, intercalated in a volcanoclastic sequence including some marker horizons of purple slates and jasper. The VSC host the massive sulphide deposits in the IPB. The São Domingos pyrite orebody is located near the top of the VSC. It is a single subvertical body of massive sulphides, which main features can be summarised as follows: (1) lense shaped 537 m long and m thick; (2) mineral assemblage composed by pyrite, sphalerite, chalcopyrite, galena, arsenopyrite and sulphosalts; (3) reserves 27 Mt at 1.25% Cu, 1% Pb, 2% Zn and 45 48% S (Leistel et al. 1998); (4) products as pyrite, roasted pyrite, sulphur and copper. Although the São Domingos orebody was mined since the Calcolithic age, the main activity began in Roman times (Custódio 1996). Afterwards, the two more important events of mining exploitation occurred in the last decades

3 Environ Geol (2008) 55: of the XIX century, and within the 1930s 1960s. Different residue types were generated as a consequence of this historical mining activity. Methodology Sampling and Mapping Forty-seven samples (approx. 2 kg each) were collected in the whole mining area, in the range of 3 7 samples of each residue type, using a polypropylene shovel, and subsequently transferred to clean polypropylene bags in April The identification and sampling of the wastes in the field were coeval to the cartographic activity. The waste impact cartographic map at scale of 1:2,500 was compelled with two main objectives: (1) to make a detailed map of the abandoned open pit, mainly of the orebody hydrothermal system and its structural control; (2) to possess a two dimensional overview of the mining infrastructures and detailed distribution of the wastes (Matos 2004). The mapping was also supported by aerial photograph and multispectral remote sensing data (GIS- ArcView 3.2 Ó and CAD software) from INETI MINEO project. A global understanding of the mining site was always present during the mapping development, mainly the location of the most important mine infrastructures (e.g. open pit, tunnels, railway, ore mills, leaching and cementation tanks, sulphur plants, acid water dams and channels). Additionally, a theoretical reconstruction of the volume occupied by the different residue types was performed considering the surface values, dump body shapes and respective thickness (estimated from some old boreholes drilled into the dumps; Table 1). microanalysis (JEOL JSM 5410), and in some occasions with reflected and transmitted light optical microscope. Bulk rock composition of the samples was analyzed by Acme Analytical Laboratories Ltd (Vancouver, Canada), accredited under ISO 9002, through its Italian affiliate (ERS Srl, Napoli). The major (Si, Al, Fe, Mg, Ca, Na, K, Ti and P) and minor (Ba, Co, Rb, Sn, Sr, Ta, Th, U, V, W and Zr) elements were subjected to LiBO 2 fusion/dilute nitric digestion and analyzed by ICP-AES (inductively coupled plasma-atomic emission spectroscopy) and ICP-MS (inductively coupled plasma-mass spectrometry), respectively. The heavy metals and metalloids (Mo, Cu, Pb, Zn, Ni, As, Cd, Sb, Bi, Ag, Au, Hg, Tl and Se) were determined by leaching of 0.5 g of each sample with aqua regia extracts (3:1 HCl HNO 3 ) at 95 C for 1 h, follow by ICP-AES analysis. Volatile phases were calculated by loss on ignition (LOI, 550 C), and total carbon and sulphur by Leco. A Sobek acid base accounting (ABA) test (Sobek et al. 1978) was also used to calculate the AMD generation capacity (acidic potential or AP) and the AMD neutralizing capacity (neutralization potential or NP) in all residues through the modified ABA procedure proposed by Lawrence and Wang (1997). AP was calculated following the equation: AP = % total S (expressed in kg H 2 SO 4 /t equivalent) and the NP was determined by titration methods (expressed in kg H 2 SO 4 /t equivalent). The difference between the NP and AP values represent the net neutralization potential (NNP) of the sample, i.e. NP - AP = NNP. Thus, if the sample shows a NNP value lower than -20 kg H 2 SO 4 /t, it is considered AMD generating, while a NNP value higher than +20 kg H 2 SO 4 /t indicates that the sample is non-amd generating. If NNP values are between -20 and 20 kg H 2 SO 4 /t, the sample is within an uncertainty zone (Lawrence and Wang 1997). Mineralogy and chemistry The chemical analyses and the mineralogical characterization were performed on the 47 samples of each residue types. Samples were collected and immediately air-dried, weakly rolled to break the aggregates and passed through a 2 mm sieve. The mineralogical characterization of all samples was carried out by X-ray diffraction (XRD, powder method) using a Bruker diffractometer (model D8 Advanced). Working conditions were slit fixed at 12 mm, Cu Ka monochromatic radiation, 20 ma and 40 kv. Samples were run at a speed of 0.3 2h/min (5 60 ). Semiquantitative mineralogical determination was estimated with DIFFRACplus software. In order to complete the mineralogical characterization, samples were also observed by means of a scanning electron microscopy equipped with an energy dispersive system (SEM-EDS) for the chemical Results and discussion Mapping Figure 2 shows the cartographic map of the different residue types in the São Domingos mining area. Two main groups of mining residues can be recognized: (1) industrial wastes derived from the ore processing operations, including: Roman and modern slags, iron oxides, smelting ashes, pyrite-rich waste dumps, leaching tanks refuses, and industrial landfills; and (2) mine wastes heaped as dumps, including gossan and sulphide disseminated country rocks (host volcanic rocks and shales). Slags are the residues from Cu ore (mainly chalcopyrite) smelted in furnaces (Fig. 3a, b). Smelting ashes (Fig. 3c) derive from the cleaning of high temperature condensers during the smelting process. However, the preferred method for extracting copper was

4 1800 Environ Geol (2008) 55: Table 1 Waste types and origin of the São Domingos mining area extracted from the geological and mining map of Fig. 2 Rock type Mining waste Surface (m 2 ) Volume (m 3 ) Predominant location Waste origin Roman period XIX century 1930s 1960s Ore processing wastes Modern slag 109, ,736 Open pit, Moitinha, Achada Roman slag 28,173(a) 225,384 Open pit Smelting 27,968 27,968 Moitinha, Achada ashes Pyrite-rich samples 8,035 64,280 Open pit, Moitinha, Achada Iron oxides 3,606 3,606 Achada Leaching 152,306? 1,218,448 Moitinha tanks Industrial landfills 501,265 4,010,120 São Domingos valley Residues from Cu ore, smelted in furnaces cleaning of HT condensers during the smelting process products from Cu ore roasting process remains of pyrite-rich wastes compacted and spread as containment barriers adjacent material heaped as dumps Ore extracting wastes Gossan wastes Country rocks 209,832 1,678,656 Open pit, Village, Moitinha, Achada 334,214 2,673,712 Open pit Surface covered by wastes [255,300 (b) 2,963,900 (c) Total m 3 = 10,779,910 25,753,600 (d) From open pit to Chumbeiro Classification as a function of the ore origin. Surface data obtained by CAD geological and mining mapping. Volume values calculated after the surface and thickness data observation. (a) Inferred surface; (b) total surface and volume affected by the mining activity including; (c) mining wastes areas and volumes; (d) area under village houses and contaminated urban landfill (not considered for the study. See details in the text) cementation, where low-grade Cu ores were roasted in piles and washed with acidic water to extract the soluble Cu that was later precipitated onto Fe sheets. Iron oxides are the waste products from this Cu ore roasting process. The pyrite-rich waste dumps (i.e. samples of brittle pyrite and pyrite blocks) are the input material for this roasting process, previously crushed in the case of brittle pyrite. Leaching tank refuses (Fig. 3d) correspond to remains of pyrite-rich wastes in which the leaching process is in such an advanced state that pyrite is practically dissolved. Finally, industrial landfills refer to those residues compacted and spread in layers used as containment barriers. From a chronological point of view, the three main groups of wastes are: (1) Roman slags related to gossan and supergene enrichment of exploitation zone; (2) gossan + host rocks + local reworked Roman slags + roasted ore related to XIX century open pit/underground exploitation; and (3) ore related to XX century mine exploitation as modern slags + sulphide roasted ashes + brittle pyrite ore + coarse blocks of pyrite + smelting ashes + dumps. Figure 3e shows the gossan wastes field appearance. The extremely final acid drainage after the residues leaching (Fig. 3f) achieves the Chanza dam, main reservoir supplying potable water to the Huelva province, although the contamination is relatively attenuated. The surface and volume of the waste dumps, and a rief description of the wastes origin is presented in Table 1. The total area affected by mining activities in São Domingos is around 3,200,000 m 2, where ca. 544,000 m 2 is occupied by waste material from the ore extraction and around 831,000 m 2 by residues from the ore processing including industrial landfills and leaching tank refuses. The rest of the affected area, ca. 1,610,000 m 2, is occupied by material under the village houses, never sampled including, e.g. acid drainage related with evaporation process; acid waters lagoons/dams; leached clay materials; and cementation tanks. Taking into account the different thickness of each residue, a total of 25 Mm 3 of mining wastes can be estimated in the total area of the mine (Table 1). However, around 14 Mm 3 of this total is under the village houses whereas the total of the ore extracting wastes and the ore processing residue types are around 11 Mm 3 (ca. 32 Mt of mining wastes). Mineralogical characterization of the wastes The mineralogical composition of the wastes from São Domingos is summarized in Table 2. The typical mineral association of the wastes is as follows: quartz, goethite,

5 Environ Geol (2008) 55: Fig. 2 Geological and mining map of the São Domingos area, including the cartography of the main type of wastes. Geology: (1) Quaternary alluvium sediments; (2) Palaeozoic Basement (South Portuguese Zone): Mértola Fm. (Upper Viséan); Volcano Sedimentary Complex (Late Famennian Late Viséan); Phyllite-Quartzite Group (Frasnian Late Famennian); Represa Fm. (Late Famennian); Barranco do Homem Fm. (Famennian?); Gafo Fm. (Lower Frasnian). Mining wastes: Tailings: E1 Modern slag; E2 Roman slag; E3 Gossan; E4 Volcanics + shales; E5 Shales; E6 Brittle pyrite ore; E7 Roasted pyrite ore (sulphur factories ashes); E8 Iron oxides (hematite roasted pyrite). Landfills and infrastructures: F Leached materials in seasonal flooded areas; L Mine landfill; U Urban contaminated landfills; (AMD) unvegetated area affected by extreme acid mine drainage; (LF) Pyrite ore leaching plateau; (3) Acid water dam/lagoon; (4) Clean water dam; (5) Cu cementation tank; (6) Orkla sulphur factories; (7) Railway station; (8) Power plant; (9) Abandoned mine railway; (10) Mine channel; (11) Stream hematite, jarosite and mica. The sulphides presence/ absence within the wastes (Table 2) determines their main subdivision from an AMD-origin point of view. The climate of the area is of a Mediterranean type, which alternate long warm-dry periods and short but intense rainy periods. The oxidation of pyrite and other minor metallic sulphides in pyrite-bearing wastes (modern and Roman slags, smelting ashes and pyrite-rich samples) produce direct AMD generation during the entire year. The large AMD production became a huge problem for carrying out the labours during mining activity periods. For this reason, most of pyrite-absence wastes (mainly the most volumetric ones, i.e. leaching tanks and industrial landfill) were strategically collocated for blocking the acidic discharges, and later loose them by evaporation in warm periods. This favours the precipitation of evaporitic soluble salts in these residues, besides in direct AMD producing residues.

6 1802 Environ Geol (2008) 55: Fig. 3 Field occurrence of: a modern and b Roman slags; c smelting ashes; d leaching tank refuses; e gossan wastes; and f final AMD discharge São Domingos processing ore wastes As mentioned in Mapping, Fig. 2 and Table 1 show the processing ore wastes: modern and Roman slags, smelting ashes, iron oxides, pyrite-rich waste dumps (coarse blocks and brittle pyrite), leaching tanks refuses and industrial landfills. Both, Roman and modern slags typically show quartz (20 35%), olivine-group minerals (mainly fayalite) in the range of 15 25%, magnetite up to 40%, Fe oxides (10 15% of hematite and goethite), glass, sulphides up to 15% (pyrite and chalcopyrite), and minor secondary phases formed by sulphide weathering (e.g., jarosite). Two immiscible glass types (quenched melt) also occur as evidence of the base-metal smelting process: (1) the original melt from the mixing of the processing material doped with a flux; and (2) the metallic alloy segregated from the original melt (Fig. 3c). Fine grain-sized smelting ashes mainly comprise of silicate minerals (quartz and feldspar), relatively high contents of gypsum (around 20 wt%) and graphite (5 10 wt%), and minor proportions of pyrite (1 5 wt%). The pyrite-rich samples are mainly composed of quartz (25 40%), pyrite (15 40%), and minor goethite, hematite, jarosite and rare micas (Fig. 3d, e). Iron oxides from piles of roasted pyrite ore are typically composed of hematite/ goethite (Fig. 3f). Leaching tanks refuses have a mineral composition of quartz (around 80%) and minor gypsum and sulphide oxidation phases, like hematite, goethite, jarosite. Industrial landfill material show similar mineralogy to the leaching tanks refuses. Quartz is present up to 90%, hematite, goethite, jarosite also occur as minor phases, and gypsum is absent. SEM images of Fig. 4 show the pyrite-bearing residues involving minor metallic sulphide phases, not detectable by XRD owing to low proportions, as galena, sphalerite and arsenopyrite. These minerals are the original source for the metallic elements as Pb, Zn, As, inherited by the residues. The percolation of air and water into the pyrite-bearing dumps controls the depth of pyrite oxidation. The residues are highly porous media favouring thus the O 2 diffusion (in water or air) and hence the oxidation of pyrite and other minor sulphides (Ritchie 1994; Lefebvre et al. 2001). In the pyritebearing residues, the pyrite grains present features evidencing the oxidation process, and hence, the AMD production in time. Figure 4a, for instance, shows pyrite grains rounded by secondary oxidation minerals as jarosite (image from pyrite-rich samples) and Fig. 4b presents a pyrite grain totally corroded (image from smelting ashes). São Domingos extraction ore wastes The residues of this group (see Table 2) are: gossan wastes (gossan coarse blocks, gossan brittle, gossanized volcanics), and country rocks (host volcanics with shales, and shales). The mineralogy of the oxidized gossan wastes is mainly characterized by quartz (70 85%), minor micas and typical mineral assemblages of sulphide oxidation covers (i.e. hematite and goethite, jarosite,

7 Environ Geol (2008) 55: Table 2 Mineralogical composition of the São Domingos residues Mining waste Primary minerals (wt%) Secondary minerals (wt%) Fayalite Graphite Halloysite Kaolinite K-feldspar Magnetite Muscovite Py + Cpy Quartz Beudantite Goethite Gypsum Hematite Jarosite Modern slag Roman slag Smelting ashes Pyrite-rich samples Iron oxides 5 95 Leaching tanks Industrial landfills Gossan wastes Country rocks beudantite and Fe Cu metals hydrated sulphates as copiapite and poitevinite). The country rocks waste group presents a similar mineralogy to the gossan wastes. They include quartz ranging from 75 to 85%, and minor micas and K-feldspar (Fig. 3b). This group also contains sulphide oxidation minerals including hematite, goethite and also jarosite. Efflorescent salts can be observed in situ in both residue types of São Domingos (Fig. 5). They are lowcrystalline metallic hydrated sulphate minerals and hence not identified by XRD within the host residue. Figure 5 shows examples of copiapite (Fe 2+ Fe 3+ 4 (SO 4 ) 6 (OH) 2 20(H 2 O)) and poitevinite (Cu,Fe 2+,Zn)SO 4 (H 2 O)). Buckby et al. (2003) andsánchez-españa etal.(2005) described these salts within the IPB as epsomite, hexahydrite, copiapite, halotrichite, rozenite, coquimbite, among others. Those from the processing residues occur after pyrite samples oxidation, whereas the salts related to the extraction process residues precipitated after water evaporation during warm seasons. In fact, recent data from Smuda et al. (2007) suggest that AMD formation is strongly controlled by the local climate. During the normal warmhot season from May to October in the SW Iberian Peninsula, high evaporation of porous solutions outcrops and subsequently precipitation of efflorescent salts may provide a heavy metal-enrichment at the base of the waste rock dumps (Fig. 5). The rain-waters (during rainy seasons) and the acid waters from the mining (fluxes addressed by the miner), may cause both the dissolution of most of efflorescent salts, and remove the enrichment at the base of the waste heap, leading to a washout of acid solutions rich in heavy metal elements. The dissolved elements are further precipitated within the sulphate mineral during the water evaporation. In fact, the residual dumps at São Domingos are the contamination source of the adjacent areas, especially to the catchments area of the Chanza and Guadiana Rivers, supplying acidic waters with high amounts of metals in solution. These stream water samples, flowing from the dumps, show ph values ranging from 2 to 2.8 (Delgado et al. 2006). Hence, even lower ph values probably occur at those stream waters directly leachated from the dumps. In fact, recent works (e.g., Freitas et al. 2004; Gerhardtetal.2005; GadanhoandSampaio2006) evidence the AMD toxic effects on plants and diverse water-soils kinds of life in the São Domingos mining district. Geochemical characterization of the wastes Major elements composition of the different residue types are presented in Table 3 and minor and trace element compositions are graphically shown in Fig. 6.

8 1804 Environ Geol (2008) 55: Fig. 4 SEM views of: a, b corroded (oxidized) pyrite crystals within the from pyriterich samples and smelting ashes, respectively. The jarosite presence (secondary) evidences the pyrite oxidation process, whereas galena is a primary phase as pyrite; c Roman slag with fayalite crystals, quartz, and both immiscible melt types: one original melt from the processing material, and another from the metallic alloy segregated from the original melt; d iron-oxide residue Fig. 5 Photos of efflorescent salts precipitated during warm periods: a copiapite and b poitevinite. c XRD patterns of these evaporitic salts. d SEM view of the same copiapite São Domingos processing ore wastes Slags are hugely variable in Zn and As contents. Modern slags show average values of Zn around 8,500 ppm and As ca. 215 ppm, whereas Roman slags have Zn ca. 210 ppm and As around 760 ppm. Modern slags have the highest values of Ca, around 4.5 CaO wt%. ABA test calculations indicate an average positive acid potential for both slag types (90 170), and hence a negative NNP (Fig. 7). The sequential extraction study of Piatak et al. (2004) in slags

9 Environ Geol (2008) 55: Table 3 Geochemical composition and NNP of the São Domingos wastes in major elements, LOI, and total content of C and S Minning waste SiO 2 Al 2 O 3 Fe 2 O 3 MgO CaO Na 2 K 2 O TiO 2 P 2 O 5 LOI Total/C Total/S Sum Modern slag Mean (wt%) (NNP a = -170 ± 42) SD Roman slag Mean (wt%) (NNP = -90 ± 33) SD Smelting ashes Mean (wt%) (NNP = -35 ± 17) SD Pyrite-rich samples Mean (wt%) (NNP = -250 ± 26) SD Iron oxides Mean (wt%) (NNP = no value) SD Leaching tanks Mean (wt%) (NNP = 15 ± 12) SD Industrial landfills Mean (wt%) (NNP = no value) SD Gossan wastes Mean (wt%) (NNP = no value) SD Country rocks Mean (wt%) (NNP = no value) SD no value indicates that NP = 0 and AP = 0. However, AP cannot be estimated since pyrite is absent, and the AMD generating capacity derives from dissolution of evaporitic soluble salts (which cannot be calculated) a Net neutralization potential (kg H 2 SO 4 /t) shows the importance of the slags as source of toxic metals in abandoned mine sites, and how the existence of jarosite, which only occurs at ph \ 4.2, evidences the acidic condition of leaching. Smelting ashes are characterized by high contents of numerous elements as Pb, As, Sn, Sb, Cd, Mo, Se and Cu (ca. 5,900, 840, 485, 135, 1.5, 14, 55, 2,200 ppm, respectively). The NNP typically gives values within the uncertainty zone (Fig. 7). According to the studies from Dudka et al. (1995) and Adamo et al. (1996), this residue could emits and release large quantities of S, Zn and Pb into the environment when their metallic sulphides hosts are oxidized. Similar circumstances have been mentioned in other mining wastes by e.g., Lefebvre and Gélinas (1995) and Romero et al. (2006). Pyrite-rich samples contain high contents of Pb (ca. 4,700 ppm) and As (ca. 1,700 ppm). They have the highest acid potential values in the whole São Domingos mining district (around 250 kg H 2 SO 4 /t), and therefore the highest negative NNP (Fig. 7). The iron oxides samples show high contents of Pb (around 7,900 ppm) and Sb (ca. 290 ppm), and a NNP within the uncertainty zone. They have been described as intensely mine-impacted soils, when internal drainage (endorheic drain) occurs (Velasco et al. 2005). The potential pollution of this common residue at the IPB has been shown by, e.g. Pauwels et al. (2002), Sáinz et al. (2003), Romero et al. (2006), who describe how the spoils of this treatment caused many environmental problems due to the release of SO 2 to the atmosphere, and further replacement by a variant of the extraction process which consisted on the extraction by irrigating with acidic water in open-air for more than 2 years. Leaching tank refuses are characterized by high concentrations of As, Pb, Se, and Zn (ca. 1,750; 4,700; 55; 2,750 ppm, respectively). Their NNP values are within the uncertainty zone (Fig. 7). The industrial landfill material shows high Pb (around 8,400 ppm), As (ca. 3,700 ppm) and Se (ca. 75 ppm) concentrations, and the NNP are within the uncertainty zone. São Domingos extraction ore wastes The gossan wastes and country rocks typically contain high amounts of Pb (ca. 5,700 and 2,700 ppm, respectively) and As (around 3,550 and 1,250, respectively), and do not present acid potential due to the absence of pyrite or any other sulphide phase. Hence, the potential toxicity of gossan wastes and country rocks is based on the dissolution of their secondary soluble sulphates, e.g. beudantite and jarosite (precipitated during warm seasons) and soluble salts, which release their important amounts of heavy metals (e.g., As and Pb) to the environment.

10 1806 Environ Geol (2008) 55: Fig. 6 Selected hazardous minor and trace elements composition of the nine different residue types Fig. 7 Values of the neutralization, acidification and net neutralizing potentials (NP, AP, NNP) from the ABA test calculations. AP calculated base on S content within sulphide minerals; note that a no AP value can produce acidity on dissolving sulphates as jarosite. Note that the absent wastes have NP and AP values equal to zero. The Concluding remarks The interpretation of the results presented in the previous section lead to the following main conclusions: AMD generation from the sulphide-absent residues is from the soluble salts precipitation. (I) Acidification zone, (II) Uncertainty zone, (III) Neutralization zone. (LT: leaching tank refuses; PS: pyrite samples; SA: smelting ashes; RS: Roman slags; MS: modern slags) 1. In São Domingos, the potential pollution and contamination dynamics of the residues are continuous since the dissolution process of the heavy-metal fraction during the rainy period is cyclical. The presence of

11 Environ Geol (2008) 55: Table 4 Amount (tons) of metals contained in the mining wastes calculated base on mass/volume ratio estimation, and total acidity in tons of H 2 SO 4 Mining waste Volume (m 3 ) Mass (Mt) Elements (t) Acidity (in H 2 SO 4 ) As Cd Cu Fe Ni Pb Sb Sn V Zn Modern slag 1,097, , ,777 1,948, , ,696 Roman slag 84, , , , Smelting ashes 55, , , Pyrite-rich 88, , , , samples Iron oxides 7, , Leaching tanks 1,523, ,349 6, , , , , ,756 Industrial landfills 5,012, , ,813 1,713, ,951 1,457 4, ,389 Gossan wastes 1,783, , , , , Country rocks 2,840, , , , , Total impact 12,493, ,654 73, ,659 5,995, ,501 2,441 10, ,295 sulphides (mainly pyrite) within numerous waste types (Table 2) favours AMD production during the whole year. In warm periods, one part of the AMD-pollutants is retained by precipitation of evaporitic salts (Fig. 6) that is further re-dissolved in rainy periods. Hence, as the São Domingos environment presents pyrite, this process is repeated every year and the pollution is assured. Although iron oxides, leaching tank refuses, industrial landfill, gossan wastes and country rocks are pyrite-absent residues, the circulation of AMD through them favour the precipitation of these soluble metallic salts, activating thus their pollution potential. 2. In order to have an idea of the polluting potential of all residues it is important to consider: (1) the amount of contaminants (Table 3; Fig. 6) and (2) the residue mass/volume rate, e.g. industrial landfill is one of the residues with the lowest total metals content but the most volumetric one. As a result, the maximum amount of contaminants present in each waste can be estimated taking both factors into account (Table 4). Thus, if the absolute quantity of metals is summed, the hazard order of residues is: modern slag [ industrial landfills [ country rocks [ leaching tank refuses [ gossan wastes [ Roman slag [ pyrite-rich samples [ iron oxides [ smelting ashes. The total quantity of metals at São Domingos is under-valuated as there are very significant residue volumes beneath the village houses that cannot be considered. 3. The AP calculations for the São Domingos waste dumps indicate that the main sources of AMD are the slags, smelting ashes and pyrite-rich samples in spite of their reduced volume which represent ca. 20% of the total volume of residues. The huge heavy metal content and highest acid potential of these residues (Fig. 7; Table 4) is mainly due to: (1) their richness in sulphides (mainly pyrite) and other secondary sulphate phases as jarosite after pyrite oxidation; (2) the absence of neutralizing minerals as carbonates, and (3) their wide exposed distribution (Figs. 2, 3). 4. Despite São Domingos is considered as a relatively medium-sized mine (approximately 32 Mt of exposed residues), the average thick of their dumps/landfills assure a significant waste exposition volume (around 25 Mm 3, see Table 1), and the large total acidification potential is nowadays an alert concerning the importance of pollution environmental impact. 5. The AMD process at São Domingos, as confirm the studied textures of the pyrite oxidation and dissolution (Figs. 4, 5), is still active. 6. The main intention and recommendation of this work is the application of the São Domingos characterization methodology and database for future restoration plans on this abandoned mine and other mines of the IPB, like Caveira, Lousal and Aljustrel in Portugal (e.g., Oliveira 1997; Oliveira et al. 2002; Matos and Martins 2006), and Rio Tinto, Tharsis, La Zarza, Peña del Hierro in Spain (e.g., Braungardt et al. 2003; Sánchez-España et al. 2005; Romero et al. 2006), where similar mining wastes are present with large amounts of residues and may be as São Domingos being still active from the contamination point of view. A next interesting study to continue the contamination assessment at São Domingos is to know the amount of bio-available contaminants that would be contained in each residue.

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