Mineralogy and environmental relevance of AMD-precipitates from the Tharsis mines, Iberian Pyrite Belt (SW, Spain)

Applied Geochemistry 39 (2013) 11–25

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Mineralogy and environmental relevance of AMD-precipitates from the Tharsis mines, Iberian Pyrite Belt (SW, Spain) T. Valente a,b, J.A. Grande b,⇑, M.L. de la Torre b, M. Santisteban b, J.C. Cerón b a

Centro de Investigação Geológica, Ordenamento e Valorização de Recursos, Departamento de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal Centro de Investigación para la Ingeniería en Minería Sostenible, Escuela Técnica Superior de Ingeniería, Universidad de Huelva, Ctra. Palos de la Frontera, s/n, 21819 Palos de la Frontera, Huelva, Spain b

a r t i c l e

i n f o

Article history: Received 16 April 2013 Accepted 26 September 2013 Available online 4 October 2013 Editorial handling by M. Kersten

a b s t r a c t This paper documents the solid phases associated to acid mine drainage (AMD) at the Tharsis mines (SW Spain). It provides an inventory of the AMD-precipitates, describing their main modes of occurrence and mineral assemblages. Results indicate that iron, aluminum and magnesium sulfates predominate in the assemblages. They occur as efflorescences composed of complex mixtures of metallic salts, and as ochres (jarosite combined with goethite). Also, their distributions illustrate two hydrochemical environments: the open pits, which reflect a proximal secondary paragenesis; and the downstream river banks (Meca River), which represent a more evolved paragenesis, resulting from the evolution of AMD produced throughout the system. These environments can be differentiated by composition and variety of minerals, which is considerably lower along the Meca River. The newly-formed minerals have monitoring significance and proved capable of participating in cycles of retention–liberation of hydrogen ions, sulfate, and metals. In a semi-arid climate, the importance of the AMD-precipitates as environmental indicators is stressed. They may help to understand the response of the system to the episodic rainfall events that occur after prolonged dry periods. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Acid mine drainage (AMD) from abandoned mines is an important area of environmental impact in the world (Gray, 1998; ElbazPoulichet et al., 2001; Sainz et al., 2004; Valente and Leal Gomes, 2007, 2009a; Valente et al. 2011a; Jiménez et al., 2009). In the SW Europe, AMD processes are especially problematic because they affect the environmental quality of watersheds, restricting the use of surface water in regions that are facing water scarcity. The Iberian Pyrite Belt is one of the largest metallogenetic provinces in the world, where AMD is a long-lasting, common problem. Furthermore, the water resources for human use here are degraded by the effects of metallic loads from polluted rivers (Grande et al., 2010; Carro et al., 2011). Consequently, this is an important region for studying AMD and related topics, such as the associated environmental mineralogy. The general process of AMD formation results from a series of interconnected steps that are primarily accomplished by oxidation of sulfides (Lottermoser, 2003). This mechanism can be briefly described by reactions 1 and 2, relative to pyrite. Pyrite is oxidized by oxygen. However, ferric iron also acts as an oxidizing agent rxn (2), in a coupled interplay with microbial activity (Nordstrom and Southam, 1997; Nordstrom and Alpers, 1999). Bacteria isolated ⇑ Corresponding author. Tel.: +34 959217346; fax: +34 959217304. E-mail address: [email protected] (J.A. Grande). 0883-2927/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2013.09.014

from AMD, such as Acidithiobacillus thioooxidans and Acidithiobacillus ferrooxidans, function best in low pH (<4) and aerobic environments. Their activity produces sulfuric acid and the trivalent iron that accelerates the oxidation of sulfides (Lottermoser, 2003). þ FeS2ðsÞ þ 15=4O2ðgÞ þ 7=2H2 OðlÞ ! FeðOHÞ3ðsÞ þ 2SO2 4ðaqÞ þ 4HðaqÞ

ðrxn1Þ

2 2þ þ FeS2ðsÞ þ 14Fe3þ ðaqÞ þ 8H2 OðlÞ ! 15FeðaqÞ þ 2SO4ðaqÞ þ 16HðaqÞ

ðrxn2Þ

The complex chain of biotic and abiotic reactions that involve the oxidative dissolution of pyrite is described in detail in the classical references of McKibben and Barnes (1986), Bhatti et al. (1993), Ritchie (1994), Evangelou and Zhang (1995), Nordstrom and Southam (1997) and Keith and Vaughan (2000). The overall process is accompanied by the development of newly-formed solid phases (generically represented by the ferrichydroxide Fe(OH)3(s) in rxn (1) that form from processes such as evaporation, oxidation, hydrolysis and neutralization (Hammarstrom et al., 2005; Jerz and Rimstidt, 2003). The AMD-precipitates reflect the principal evolutionary steps of an affected system, and, consequently, they represent steps of reaction progress (Valente and Leal Gomes, 2009a,b; Valente et al. 2011b). They are mainly represented by efflorescent sulfate minerals and by ochre products composed of iron oxyhydroxides. The first are worthy of mention because they bear a strong influence on the surficial environments, by releasing metals

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through dissolution by rainfall (e.g., Alpers et al., 1994; Frau, 2000; Harris et al., 2003; Chou et al., 2013). Also, the coupled interplay between dissolution of sulfates and precipitation of iron and aluminum hydroxides contribute to the acidity of the solution (Jambor et al., 2000; Stoffregen et al., 2000). The second group comprises less soluble hosts, which are typically poorly crystalline phases with large surface areas (Bigham and Nordstrom, 2000; Dold, 2003; Lottermoser and Ashley, 2006; Valente et al., 2011a). In addition, some authors present evidences of more evolved mineralogical assemblages, such as iron-rich hardpans (Courtin-Nomade et al., 2003; Gilbert et al., 2003; Pérez-López et al., 2007). There are several works that document the characteristics of the soluble sulfates in AMD environments (e.g. Cravotta, 1994; Hudson-Edwards et al., 1999; Nordstrom and Alpers, 1999; CotterHowells et al., 2000). A detailed review that emphasizes the relevance of metal-sulfates, from a variety of perspectives, is provided by Jambor et al. (2000). In general, these secondary materials are of great relevance in environmental mineralogy, with regard to their role in controlling pollutants in contaminated environments (Cotter-Howells et al., 2000; Hochella, 2002; Vaughan et al., 2002). Also, their ability to indicate, sometimes in an expeditious way, the prevailing conditions in an impacted environment in which they form, enhances their monitoring relevance (Valente and Leal Gomes, 2009a). The Iberian Pyrite Belt is one of the best studied AMD regions in the world in terms of water and solid phases. From the extensive literature, there are many works investigating various aspects of AMD-precipitates in the Iberian Pyrite Belt (e.g., Buckby et al., 2003; Sanchéz-España et al., 2005; Velasco et al., 2005; Acero et al., 2006; Romero et al., 2006; Asta et al., 2010; Alvaro, 2010; Pérez-López et al., 2011; Quental et al., 2011; Maia et al., 2012). Following these studies, further wide-ranging work should be carried out in this exemplary region in order to complete an inventory of AMD-precipitates, as well as to better understand their distribution relative to sources. In this semi-arid climate, a key issue is the investigation of spatial variation of sulfate mineralogy with distance from the sulfide sources. This paper documents the solid phases that form in the AMD setting found at the Tharsis mines (SW Spain). Tharsis is among the largest massive sulfide deposits in the world. Therefore, the present study is a contribution to the knowledge of complex mineralogy and its environmental significance in AMD systems. It has the following primary objectives: (i) to provide an inventory of the AMD-precipitates formed in the open pits and in the nearby aquatic system; (ii) to describe the occurrence, distribution and mineral associations of the precipitates; and (iii) to document the environmental relevance of major sulfate assemblages from the Tharsis mines.

2. Site description Tharsis is one of the historical mining districts from the Iberian Pyrite Belt (IPB), located in SW Spain (Fig. 1). The Pyrite Belt extends from north of Seville – in Spain – to southern of Portugal, constituting a region known for its large massive sulfide ore deposits (Sáez et al., 1999). Here, metal mining has a long tradition. The ore deposits have been exploited for Zn, Cu, Pb, Ag, and Au (Pinedo Vara, 1963) since Pre-Roman times. Therefore, the Pyrite Belt represents a paradigmatic metallogenic region, characterized by its antiquity (more than 5000 years), economic relevance (more than 1700 Mt of sulfide ore), and environmental contamination related to the mining legacy (one of the largest accumulations of pyritic wastes in the World) (e.g., Davis et al., 2000; Sanchéz-España et al., 2005).

Specifically, the Tharsis mining district comprises one of the largest ore bodies in the Iberian Pyrite Belt, with more than 100 Mt of estimated resources (Conde et al., 2009). Presently, the mines are abandoned but the environmental legacy continues. 2.1. Geology and mineralogy From a geologic perspective, the Tharsis region is dominated by a complex system of faults. The geologic map shows the three main directions: E–W; NE–SW, and NNW–SSE (Fig. 1). The massive sulfide deposit is a classic volcanic-hosted massive deposit (VHMS). It consists of several ore lenses located in the lowermost volcanosedimentary complex overlying a thick siliclastic sequence from the Phyllite-Quartzite Group (Tornos et al., 2008; Mantero et al., 2011). In the Tharsis mining district the massive sulfides are distributed among three groups of deposits (Tornos et al., 2008), which were developed by five open pits shown in Fig. 1: the Northern group (Filón Norte and Sierra Bullones), the central group (Filón Centro), and the southern group (Filón Sur and Esperanza). The ore lenses, with up to 130 m thick, are tectonically stacked within the shale. In the footwall there is the carbonate ore, consisting of laminated and brecciated sulfides with siderite and ankerite. As in other VHMS deposits in the Pyrite Belt, there is evidence of strong hydrothermal alteration of felsic volcanic host rocks, positioned just below the masses of sulfides (Sáez et al., 1999; Sanchéz-España et al., 2000; Conde et al., 2009). Chloritization is the most intensive alteration process, transforming the felsic host rocks into chloritic rocks, mainly composed by chlorite + pyrite + quartz + sericite (Sanchéz-España et al., 2000). Additionally, supergene argillization and gossanization promote chemical transformations in the footwall rocks and in the massive sulfides, respectively. This last process is especially evident in Filón Sur, where a late intensively oxidized stockwork presents gossanized sulfides. The sulfides mineralization is mainly represented by massive pyrite (more than 90% of the sulfide mass), followed by chalcopyrite, sphalerite, and galena. Textural characteristics of sulfides are variable, but most of the massive sulfides are monotonous fine grained pyrite, often framboidal and colloform. The grains are usually fractured and brecciated (Tornos et al., 2008), which promote reactivity and enhances dissolution. Gangue includes abundant quartz and aluminum silicates (mainly chlorite), with minor amounts of carbonates. In summary, Table 1 lists the minerals that compose the paragenesis of the Tharsis ore deposit. More detailed information about mineralogy and geochemistry are provided by Garcia de Miguel (1990), Velasco et al. (1998), Sanchéz-España et al., 2000, Sáez et al. (2005), Tornos et al. (2008), Conde et al. (2009) and Mantero et al. (2011). This information is the essential base for understanding mineral–water interactions that will lead to AMD release and to a characteristic newly-formed sulfate paragenesis. Despite the presence of potential neutralizing minerals, such as carbonates, the abundance in sulfides, combined with a fine grained texture, control the overall geochemical balance. It results in strong acidity and in the generation of the typical AMD conditions (Sanchéz-España et al., 2005). 2.2. Climate The study area, located in the Huelva province, has a Mediterranean climate, which can be classified as semi-arid, due to low precipitation rates. Annual precipitation is about 630 mm/year, being mostly concentrated in the wet season from October to May. Monthly precipitation ranges from 3 to 121 mm, corresponding to June and December, respectively. Average annual temperature

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Fig. 1. Study site, with location and identification of samples. Information about pit lakes is provided in the tables. Geological map was adapted from ‘‘Mapa Geológico, Hoja 959- I’’ (ITGE, 1999).

is 17.1 °C, January being the coldest month (mean 9.8 °C), while in the summer, July and August have the highest temperatures (mean 25.7 °C) (Instituto Nacional de Meteorología; unpublished data). July also has higher evapotranspiration (162 mm) than January (19 mm). It should be noted that sampling for this study was performed in July, with temperature and relative humidity readings consistent with the typical mean annual values (temperature up 25 °C and relative humidity around 50%) (Instituto Nacional de Meteorología; unpublished data). Additionally, the field measurements of temperature and relative humidity in the sampling areas gave relatively constant values

throughout at least 3 weeks during sampling. The measured humidity variation, in the interval between 40% and 53%, seems to be controlled by local hydrological conditions instead of seasonal fluctuations. 2.3. Environmental framework Although systematically mined since Roman times, the most intensive mining took place between the 1900s and the 1990s. Five major open pits remain (Fig. 1) with numerous AMD discharges, mainly related to the large waste-dumps. The open pits were

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Table 1 Most typical minerals from the Tharsis massive sulfide deposit. Silicates

Quartz Feldspar Chlorite Plagioclase Sericite Phengite

Carbonates

Siderite Ankerite Calcite

Major sulfides

Pyrite Chalcopyrite Sphalerite Galena

Accessory minerals Pyrrhotite Marcasite Bornite Covelite Calcosite Tetrahedrite Cobaltite Arsenopyrite Hematite Magnetite Goethite Cassiterite Beudantite

abandoned; most of them have been flooded and have now become pit lakes. The geological setting is characterized by the abundance of massive sulfides. These massive sulfides are mainly framboidal and colloform pyrite, which imply the perpetuation of the acid mine drainage. Therefore, environmental impact still persists, mainly in the Meca River watershed (Fig. 1). In the study area there are two major concerns regarding water quality: (i) the discharge of acidic effluent from the waste-dumps, which affect the Meca River, and (ii) the presence of pit lakes with highly contaminated water, which may discharge through galleries or seepages (López-Pamo et al., 2009). Fig. 1 shows the location of the mining facilities in the Tharsis complex: open pits and wastedumps that drain to the Meca River, a tributary of the Odiel River. Currently, four of the five open pits are flooded with very acidic water (pH < 2.5) (López-Pamo et al., 2009 and present work). As hydrological stability was achieved in the pit lakes, they are mainly subject to seasonal level fluctuations, which depend on the rainfall and evaporation rates. Besides the discharge through the galleries, the intense regional fracture system (Fig. 1) had created preferential pathways for water circulation. This permeability was intensified locally by fractures resulting from the use of explosives during mining.

3. Methods 3.1. Mineral sampling and analysis A sampling campaign for newly-formed minerals was performed under strong evaporation and low flow hydrological conditions (July 2012). Two distinctive environments were sampled: (i) the open pits with their outcrops, exposed walls, sulfide piles, seepages and surface runoff, and (ii) the banks of the Meca River. Fig. 1 shows the sampling areas in the open pits and in the Meca River. Inside the open pits, the selection of these areas was controlled by technical issues, especially those concerning accessibility and safety. In the Meca River, sampling was performed downstream of the confluence with the uncontaminated Aserrador Creek (Fig. 1). Inside each sampling area there were a variable number of samples, which were meant to cover the diversity observed in the field on the basis of macroscopic properties. Occurrence modes, color, and texture were used to define this diversity. Samples were collected from outcrops, at various highs in the front walls, in exposed rocks or in fractures and cavities. Also, sampling was focused on the surface of the piles, in drainage channels, and in the dried shores of the pit lakes. Different types of materials were collected, namely efflorescent salts, ochreous-precipitates, rock coatings, and composite hard

crusts. The most powdery efflorescences were collected with plastic spatulas whereas the harder crusts and rock coatings were separated from the substrates with a stainless steel knife. As far as ochreous-precipitates are concerned, the thin films and the muds that compose the iron mixtures were taken from streambeds or collected with a syringe. Air temperature and relative humidity were measured in the field with a portable Hanna Instruments hygrometer, model HI8564. Efflorescent salts were stored in closed plastic vessels and transported to the laboratory soon after being collected, in order to prevent mineralogical changes. The ochreous-precipitates were air-dried at room temperature, sieved to 63 lm and analyzed for mineralogy. Efflorescent salts and crusts were examined for morphology, photographed and sorted by binocular microscopy. Afterwards, samples were lightly ground in preparation for mineralogical analysis. The samples were analyzed by X-ray powder diffraction (XRD) with a Philips X’pert Pro-MPD difractometer, using Cu Ka radiation at 40 kV and 30 mA. The diffractometer was equipped with an automatic divergence slit and graphite monochromator. Morphological and compositional features were analyzed by scanning electron microscopy (on carbon or gold-coated samples) with a LEICA S360 microscope, combined with an energy dispersive system (SEM–EDS, 15 keV). The use of X-ray and standard ZAF corrections allowed for information on the elemental composition of the samples to be obtained. The XRD data were processed with the X’pert Pro-MPD software, which help to identify the most probable phases. However, this type of sample poses particular problems, which imply the use of an iterative procedure to accomplish mineralogical identification. The need for an iterative procedure to refine samples is well described by Jerz and Rimstidt (2003) and Hammarstrom et al. (2005). With respect to the efflorescent salts, it was typically impossible to isolate pure samples for mineralogical analyses. Therefore, binocular microscope was used to obtain subsamples consisting of a lower number of phases. XRD was performed immediately after arriving at the laboratory, and the samples were reanalyzed to evaluate aging effects. Then, SEM allowed verification of the XRD analyses and, in some cases, it aided in the identification of complex assemblages and of poorly crystalline material. Sample preparation procedures and the appropriate XRD and SEM conditions for dealing with low crystallinity, fine grain size, high hydration states, and impurity have been described by Valente and Leal Gomes (2009a). 3.2. Characterization of AMD This work presents the hydrochemical characteristics of samples that represent three end-member environments: surface water from the pit lakes, Meca River water, and surface runoff from one of the open pits (Filón Norte). The water sampling was conducted at the same time as sampling for newly-formed minerals (July 2012). The pH, electrical conductivity (EC), and temperature of the water were measured in the field with a multi-parameter meter (CRISON, MM). Before use, electrodes were calibrated and tested for accuracy, according to the manufacturer’s instructions. Two samples were taken for laboratory analyses using polyethylene bottles (100 mL): one for sulfate and the other for metals. The sample for metals was acidified with HNO3 65% suprapur Merck to prevent metal precipitation. After collection, samples were immediately refrigerated, kept in the dark and stored at 4 °C until analysis. Sulfate was measured by photometry. Metal concentrations (Fe, Al, Zn, As, Ni, Cd, Cu, and Mn) were determined using atomic absorption air-acetylene spectroscopy (AAS), with a Perkin Elmer AAnalyst 800 (Perkin-Elmer, Norwalk), equipped

T. Valente et al. / Applied Geochemistry 39 (2013) 11–25

with graphite furnace and hydride generator. The accuracy of the method was verified with certified reference samples. The measurement precision was greater than 5% RSD, whereas the detection limit was 0.1 lg/L for Cd, As, Sb, and Al and 1 lg/L for the rest of the metals. 3.3. Laboratory dissolution experiments The dissolution of soluble sulfates was simulated in the laboratory, using natural samples. A known mass of each sample (10 or 20 g) was dissolved in ultra-pure water. The experiments were performed in batch reactors, at room temperature (20 °C) with constant agitation (300 rpm). Agitation was performed with a polypropylene propeller in order to avoid heating of the solution. The following protocol was used: – Solution: ultra-pure water from Millipore system, with electrical conductivity of 0.1–0.5 lS/cm (volume of 100 mL). – Mineral:water ratio – 10 g/L and 20 g/L (depending on the availability of sample). – Duration of the experiments – 60 min. – Control parameters: pH was measured each minute; an additional measurement was performed at the first 30 s of the experiments. At the end of the experiments, the supernatants were analyzed for metals (Mn, Fe, Al, Cu, and Zn), arsenic, and sulfate. – Analytical methods: pH was monitored with an ORION meter, model 720A. Sulfate was analyzed by ion chromatography with electrical conductivity suppression, whereas metals were determined by AAS, after filtration through 0.45 lm pore-diameter cellulose ester membrane filters.

Table 2 Inventory of identified AMD-precipitates. EFL – efflorescent salts; OP – ochre products; CRUS – crusts. Mineral

Ideal formula

Occurence

Open pits – close to the primary minerals Rozenite FeSO4 4H2O Szomolnokite FeSO4 H2O Rhomboclase HFe(SO4)2 4(H2O) Copiapite (Fe,Mg)Fe4(SO4)6(OH)2 20H2O Coquimbite Fe2(SO4)3 9H2O Gypsum CaSO4 2H2O Epsomite MgSO4 7H2O Hexahydrite MgSO4 6H2O Starkeyite MgSO4 4H2O Alunogen Al2(SO4)3 17H2O Tamarugite NaAl(SO4)2 6H2O Halotrichite FeAl2(SO4)4 22H2O Pickeringite MgAl2(SO4)4 22H2O Wupatkiite CoAl2(SO4)4 22H2O Apjohnite MnAl2(SO4)4 22H2O Native sulfur S Mallardite MnSO4 7H2O Chalcanthite CuSO4 5H2O Bonattite CuSO4 3H2O Eriochalcite CuCl2 2H2O Jarosite KFe3(SO4)2(OH)6 Goethite FeOOH

EFL; EFL ? CRUS EFL; EFL ? CRUS EFL; EFL ? CRUS EFL; EFL ? CRUS EFL EFL EFL EFL EFL EFL EFL EFL ? CRUS EFL ? CRUS EFL EFL EFL EFL EFL; EFL ? CRUS EFL EFL OP ? CRUS OP ? CRUS

Meca River – distant from primary minerals Gypsum CaSO4 2H2O Tamarugite NaAl(SO4)2 6H2O Hexahydrite MgSO4 6H2O Halotrichite FeAl2(SO4)4 22H2O Pickeringite MgAl2(SO4)4 22H2O Wupatkiite CoAl2(SO4)4 22H2O Jarosite KFe3(SO4)2(OH)6 Goethite FeOOH

EFL EFL EFL EFL EFL EFL OP ? CRUS CRUST

15

4. Results 4.1. Mineralogy of AMD-precipitates Table 2 records the newly-formed minerals identified in the open pits and downstream on the banks of the Meca River (Fig. 1). Mineralogy of these AMD-precipitates is dominated by sulfates and by iron-oxyhydroxides, displaying different occurrence modes. 4.1.1. Composition and occurrence modes The identified minerals (Table 2) show three main occurrence modes: (i) efflorescences (EFL), resulting from evaporative processes, (ii) ochre products (OP), produced by oxidation and neutralization, and (iii) hard crusts (CRUST) formed by progressive dehydration of the precipitates. The efflorescent minerals comprise several soluble salts, mainly sulfates (Table 2). There are sulfate salts of alkali and alkaline earth metals, transition metals and aluminum. Precipitation of calcium and magnesium occurs mainly as gypsum and hexahydrite, respectively. Both minerals are abundant, occurring in the open pits and on the banks of the Meca River. In contrast, epsomite was more rarely found. Regarding iron sulfates, the inventory includes simple hydrated salts with ferrous iron, such as szomolnokite, and more oxidized species represented by the copiapite group. Also, trivalent iron was found in rhomboclase and coquimbite. Aluminum forms a variety of soluble sulfates, including alunogen, tamarugite, and minerals from the halotrichite–pickeringite group. Another divalent metal with a mineralogical expression, albeit rare, is manganese in the heptahydrate mallardite. Finally, copper is often retained by chalcanthite, a triclinic pentahydrate sulfate, and more rarely by bonattite. The ochre-products include relatively insoluble iron(III)-bearing minerals, often as mixtures, displaying ochre colors. The oxyhydroxide goethite and the hydroxysulfate jarosite are abundant phases (Table 2). As sulfates and iron oxyhydroxides form and continue to evolve by oxidation and dehydration, these precipitates may form hard crusts. When well-developed, such encrusting products act like cement for primary and secondary minerals. Minerals, such as gypsum and jarosite are typically included in these rigid crusts. Therefore, it is possible to observe different stages, varying from powdery efflorescences or ochre sludge, to rigid crusts. Fig. 2 shows the different stages, which compose the evolutionary process (EFL ? CRUS and OP ? CRUS). Additionally, field images in Fig. 2 illustrate typical occurrence modes. Soluble salts appear as blooms of fine-grained crystals or botryoidal aggregates (example of copiapite in Fig. 2B) or even with well-developed crystals, as gypsum in Fig. 2C. Desiccation conditions promote dehydration that often leads to impure crusts, such as the salt crust in Fig. 2G. In a similar way, jarosite and goethite may occur as an ochre sludge in the bottom of the drainage surfaces (Fig. 2D) or as hard iron-rich crusts (Fig. 2E and F). 4.1.2. Mineral assemblages and crystal morphology Both efflorescent salts and ochre products were normally observed in complex assemblages and rarely as single phases. Figs. 3–6 show modes of occurrence, crystal morphology and XRD patterns of some common or representative salt assemblages. Chalcanthite is one of the most abundant minerals that occur as efflorescent salt. It is an exception because it may appear as monomineralic aggregates, as verified by XRD and SEM analyses. In such a case, it forms granular, botryoidal and sometimes reticulated bluish globules. However, most of the times, chalcanthite forms

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Fig. 2. Photographs of main occurrence modes. (A) General picture of a small channel inside the ‘‘Filón Norte’’ showing a variety of occurrence modes: blooms of soluble salts near the water, ochre sludge in the bottom of the channel, and dehydrated phases at the edges; (B) yellow–white roses of copiapite at the base of the open pit in ‘‘Filón Centro’’; (C) dendritic gypsum in an exposed rock in the center of the same channel as A; (D) orange flocs of jarosite in the bottom of the channel; (E) red-brown laminated goethite on the streambeds of the channel; (F) iron crusts composed by goethite and jarosite in the banks of the Meca river; (G) salt crusts composed by copiapite and Alsulfates in ‘‘Filón Norte’’. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

assemblages with gypsum, and Al-sulfates (halotrichite and alunogen) (Fig. 3). Efflorescences with chalcanthite are detected on the surface of rock fragments or filling fractures and cavities, generally protected from direct sun exposure. Commonly, these efflorescences appear as greenish-blue or dark to pale-blue silky powder. Nevertheless, in the driest conditions (on the surface of exposed rocks) they may form encrustations, which have a rough and spikey surface. Well-developed crystals of Cu-sulfates are rare in fine powdered efflorescences. Nevertheless, some samples may have

brilliant blue crystals with tabular and prismatic habits, as illustrated in the SEM images from Fig. 3D. Hexahydrite was rarely found without Al-sulfates and gypsum, forming intergrowths that make it nearly impossible to isolate single phases. Field images illustrate different types of efflorescent assemblages containing these minerals (Fig. 4A). These complex mixtures may appear as white slat blooms, as whitish silky powder or as very fine granules, often loosely, over rocks or vegetation. The mineralogy of these intergrowths was

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Fig. 3. Miscellaneous assemblage from ‘‘Corta Esperanza’’ (sample T20; see Fig. 1 for sample location). (A) Field image of powdery efflorescent salts in a fracture on the front wall. The color grading corresponds to a mixture of gray-white Al-sulfates (alunogen and halotrichite) and gypsum plus dark to pale bluish Cu-sulfates (chalcanthite and bonattite). (B) Microscope images of isolated plaques of chalcanthite (above) and of a mixture of acicular Al-sulfates (below). (C) SEM – SE (EDS) image showing oriented crystals of alunogen with associated fine-grained Cu-sulfate, probably chalcanthite. (D) SEM – SE (EDS) image of euhedral prismatic crystals of bonattite (right). AL – Alunogen; CH – Chalcanthite; BO – Bonattite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

confirmed by XRD (Fig. 4B) and SEM study (Fig. 4C). In the assemblages, Al-sulfates are often encapsulated by late hexahydrite. Commonly, oriented alunogen or tamarugite and acicular halotrichite appear associated, as shown in Fig. 4C. Minerals from the halotrichite group as well as hexahydrite include most of the secondary paragenesis, being widely distributed in the Tharsis system. The same happens with gypsum, which is rather common in different environments and constitutes part of several assemblages. Although it may grow as a pure efflorescent mineral, it was often detected in powdery aggregates with Mg and Al-sulfates, or as spherical concretions with copiapite or jarosite. For the iron sulfates, there are assemblages containing the entire sequence: simple hydrated salts of divalent iron, salts that have both divalent and trivalent iron, and those of trivalent iron. Rozenite and szomolnokite were the only ferrous iron sulfates identified. The first occurs with Mg-sulfates and several rare minerals of halotrichite group. This complex assemblage (rozenite + starkeyite + wupatkiite + apjohnite + pickeringite) was only detected in Filón Centro, in an area exposed to south (T3, Fig. 1). Here, it grows in the recently dried shores of the pit lake, leading to thick layers of efflorescences with a variety of colors (white, gray and pink), over jarosite muds. Szomolnokite was found in association with copiapite, coquimbite, and halotrichite (Fig. 5). In fact, copiapite is very abundant in the open pits, in one of the more common assemblages: copiapite + rhomboclase + coquimbite. Typically, this paragenesis develops in the piles, over sulfide-rich

materials (Fig. 6). As illustrated in the field images (Fig. 6B), there are two distinctive layers of yellow-green (copiapite, in direct contact with the sulfides) and grayish-pink colors (rhomboclase and coquimbite, mixed with gray szomolnokite) (above). Trivalent iron was also found in ochre products, represented by goethite (FeOOH) and hydroxysulfates of the jarosite group (K, Na)Fe3(SO4)2(OH)6). Goethite has low to medium crystallinity (as indicated by broad X-ray diffraction peaks), whereas jarosite was always found well crystalline. Both form pure aggregates but more often appear as mixtures (Fig. 7A and B). Typically, jarosite appears as polygonized cracks on dried surfaces, from which efflorescent salts may emerge (Fig. 7A in the center), as was observed by Velasco et al. (2005) at San Miguel mine, also in the Iberian Pyrite Belt. It also forms crusts with gypsum and other sulfates (Fig. 7B), such as epsomite. These crusts are common in evaporating water seepages emerging from the front walls. In summary, these iron-rich materials occur on the exposed rocks as streambed coatings and in effluent channels. As illustrated in Figs. 2 and 7, the main representative occurrences are as follows: – – – –

flocculated materials at the water–air interface, streambed coatings, cracked muds (Fig. 7), and iron-solid crusts.

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Fig. 4. Miscellaneous assemblages with Mg, Ca and Al-sulfates. (A) Field images illustrating different occurrences: in the river banks of the Meca River – silky white blooms with hexyhydrite, gypsum and a mixture of Al-sulfates (above) and very fine-grained white blooms with the same composition (center); in the open pits (Filón Sur), near the water – gray-yellowish blooms composed by epsomite, hexahydrite and gypsum (below); (B) XRD pattern of a selected sample from the Meca River (at least the three most intense peaks of each phase are represented); (C) SEM – SE and respective EDS spectra of the sample presented in B, revealing also the presence of oriented tamarugite and acicular halotrichite. TA – Tamarugite; HA – Halotrichite; HE; He – Hexahydrite; Al – Alunogen; Gy – Gypsum. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.2. Chemical composition of water and salt solutions Table 3 presents the chemical composition of surface waters from the pit lakes and the Meca River. An additional sample is from an acid leachate from Filón Norte (Filón N-CH). The samples have pH values between 1.5 and 2.5, indicating typical AMD. Also, metals and sulfate are present in concentrations expected for typical AMD systems, in accordance with Gray (1996) and Grande (2011). The ability of the soluble salts to produce low pH and metal-rich solutions was verified through the experimental dissolution of typical assemblages. Fig. 8A shows the evolution of pH during the dissolution of three mineral assemblages: one dominated by the Cusulfate chalcanthite, in association with gypsum and Al-sulfates (chalcanthite + gypsum + alunogen + halotrichite), another representing a complex association of Fe and Al-sulfates (szomolnokite + copiapite + halotrichite + coquimbite) and, finally a sample composed by the Fe-sulfates copiapite and rhomboclase. These samples were selected because they are abundant in the open pits and cover different types of occurrences (outcrops, rock fractures and sulfide piles). A low and stable pH value was promptly established for the three samples. However, the chalcanthite-rich sample had lower

acid producing potential, stabilizing at a pH around 4.3. In contrast, lowest pH values were observed for the assemblages with Fe and Al-sulfates, particularly the most complex one (composed by szomolnokite + copiapite + halotrichite + coquimbite). The pH of this sample dropped from 5.5 to 2.1 in the first 30 s. Chemical analysis of the resultant solutions revealed that, as expected, upon dissolution, iron and sulfate are the dominant constituents (Fig. 8B). However, other elements are also released, such as trace metals and arsenic. 5. Discussion It is known from literature that the formation of AMD-precipitates and their mineralogical evolution are influenced by a variety of factors (e.g., Blowes et al., 1998; Johnson, 1998; Nordstrom and Alpers, 1999; Jambor et al., 2000; Brake et al, 2001; Valente and Leal Gomes, 2007, 2009a; Nordstrom, 2011), namely: (i) Geology, related to the nature of the ore deposit and the host rocks. (ii) Climate and topography, related to the abundance and frequency of precipitation and temperature, as well as sun exposure, which control humidity–evaporation cycles.

T. Valente et al. / Applied Geochemistry 39 (2013) 11–25

Fig. 5. XRD pattern of two complex assemblages collected at ‘‘Filón Centro’’ (T11 – above) and ‘‘Filón Sur’’ (T36 – below). See Fig. 1 for samples location. Samples include minerals with divalent iron (szomolnokite – Sz), mixed divalent, trivalent iron (copiapite – Cp), trivalent iron (coquimbite – Cq and rhomboclase – Rh), and halotrichite (Hl). At least the three most intense peaks (from ICDD) of each phase are indicated.

(iii) Geography, which controls the distance to the source, inducing the evolution of sulfate solutions. (iv) Hydrology, which controls the availability of water and influences kinetic factors by imposing variations on AMD composition due to re-dissolution and dilution phenomena. (v) Biogeochemistry, which results from the combination of all the above constrains, in addition to microbiological activity (bacteria, algae and archaea). These create local environments that lead to complex precipitation scenarios, depending on the saturation levels achieved for each mineral species. The present discussion is centered in the control that geographic and geochemical features may have exerted over the distribution, paragenetic relations and pollution potential of AMDprecipitates. 5.1. Paragenetic relations The sampled environments show specific mineralogical assemblages, as summarized in Table 4. Interpretation of the occurrences

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and distribution of these minerals suggests that the open pits and the Meca River define two distinct mineralogical environments. This difference illustrates the control exerted by the distance from the source and by geochemical factors related to AMD. In accordance with the terminology used by Velasco et al. (2005), the open pits and the Meca River represent proximal and distal secondary mineral settings, respectively. The first results from mineral–water interactions that take place in the vicinity of the sulfide-rich parent materials. Consequently, there are assemblages with iron sulfates derived directly from sulfide oxidation. Identically, the abundance of copper sulfates may be reflecting the presence of trace amounts of copper sulfides, such as chalcopyrite. Saturation at the local scale is possible due to the proximity to the source of copper (open pits and sulfide piles). Moreover, the paragenetic relations at each open pit could be influenced by topography and microclimate, although no supporting data are available. In fact, the method used to analyze microclimatic conditions in the field (measurement of temperature and humidity) did not detect significant and consistent differences among settings. However, it is clear that some assemblages tend to appear in moist sites or sheltered areas, protected from dehydration, such as fractures and cavities. This is the case of the assemblages more enriched in chalcanthite. Likewise, other studies revealed similar controls. For example, Valente and Leal Gomes (2009a) established the relation between a gradient of color shades and the sequence of hydrated sulfates that are present around a small creek in the north of Portugal. Similarly, there are several studies that document that transformations of efflorescent salts are accompanied by color changes (Jambor and Traill, 1963; Chou et al., 2013; Durães et al., 2009). Therefore, it is possible that the color gradations observed in the field (as illustrated in Fig. 3A) may be associated with the evolution of these salts, mainly by dehydration. It is also known that chalcanthite (dark blue) may dehydrate to form bonattite (pale blue). Also, the iron-sulfates undergo oxidation, leading to the formation of oxyhydroxides (yellow-ochre) (Nordstrom and Alpers, 1999; Chou et al., 2002; Jerz and Rimstidt, 2003). In contrast to other studies in the Iberian Pyrite Belt, melanterite (FeSO4 7H2O) was not identified in the present work. This mineral has been proposed by several authors as the first phase to form as a result of pyrite oxidation (Jerz and Rimstidt, 2003). In the present case, it is worthy of mention that sampling was performed in July. Therefore, the high temperature and low humidity (T > 30 °C and H < 50%) explain this absence. Under these conditions, rozenite and szomolnokite appear as the stable phases (Chou et al., 2002, 2013). It is possible that they had formed from dehydration of melanterite. Copiapite, coquimbite, and rhomboclase appear in sheltered areas, always in close proximity to pyrite sources. Thus, they should be the subsequent phases, formed by the progressive oxidation of iron. On the other hand, the Meca River paragenesis may result from AMD evolution along the drainage network. The sampling area is located more than 20 km away from the Tharsis mines. Therefore, farther away from the generating sources more oxidized solutions are expected. As observed in several AMD sites around the world (e.g., Jerz and Rimstidt, 2003; Hammarstrom et al., 2005; Velasco et al., 2005), processes such as neutralization, oxidation, and precipitation-re-dissolution cycles should play a role in this evolution. In the present study, the sampling area is located downstream of the confluence with Aserrador Creek (Fig. 1). This stream does not include important sulfide mines in its watershed (Ostale, 2012). Therefore, this input of uncontaminated water may contribute to dilution of the AMD, especially in rainy periods. Such conditions ultimately result in the oxyhydroxides and hydroxysulfates that comprise the ochre products observed on the bed and banks of the Meca River. In accordance with the paragenetic relations

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Fig. 6. Miscellaneous assemblage of Fe-sulfates. (A) SEM–ES (EDS) images of a sample from ‘‘Filón Centro’’ (T11, Fig. 1), illustrating the intergrowth of szolmonokite (SZ), copiapite (CP), and halotrichite (HL). (B) Field and microscope images of a sample from ‘‘Filón Sur’’ (T36). The field images (on the left) illustrate the distinctive yellowishgreen color of copiapite (which grows in direct contact with sulfides) and the above exposed layer composed by coquimbite and rhomboclase. SEM images (on the right) show the typical laminated habit of copiapite (CP – above) and the rose aggregates of coquimbite (CQ – below). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

proposed by Bigham et al. (1996) as well as with observations of Velasco et al. (2005) for a site in the Iberian Pyrite Belt, jarosite and goethite are here the final stages of the evolution of AMD. For the hydrated sulfates, supersaturation with respect to transition-metal phases should be more difficult with increasing distance to the pyrite source. Hence, the diversity of efflorescent salts in the Meca River is much lower than in the open pits and is dominated by magnesium and aluminum sulfates. The relations discussed above justify summarizing the following observations: – The inventory of AMD-precipitates is consistent with typical sulfates from AMD systems, including those found in other studies in the Iberian Pyrite Belt (Buckby et al., 2003; Velasco et al., 2005; Pérez-López et al., 2011; Maia et al., 2012). – Although efflorescent salts from sulfide oxidation are typically dominated by iron minerals, Al and Mg-sulfates were abundant in the present study. – Near the sulfide sources, in the open pits, efflorescent minerals comprise Fe, Cu Al, Mg, and Ca sulfates, which reflect the composition of the oxidizing source materials.

– In the Meca River, distant from the sulfides sources, salt efflorescences generally lack iron sulfates. The current data do not allow for detailed relationships or a precise paragenetic sequence for the AMD minerals to be defined. However, they are consistent with the sulfate paragenesis associated with other massive sulfide deposits (e.g. Velasco et al., 2005; Álvarez-Valero et al., 2008). Also, the occurrences and distribution modes are consistent with the stability conditions proposed for several sulfate phases (Reardon and Beckie, 1987; Chou et al., 2002, 2013; Jerz and Rimstidt, 2003). Fig. 9 represents the probable sequence for AMD-precipitates, deduced from the following observations and from the theoretical stability relations: – Rozenite and szomolnokite carry divalent iron. They occur in outcrops or near acidic seepages, indicating the proximity to its precursors (pyrite). They may also form by sequential dehydration. – Copiapite, coquimbite and rhomboclase result from progressive oxidation of Fe-phases. – Goethite is the final product of evolution by oxidation.

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Fig. 7. Typical assemblages dominated by jarosite. (A) Field images of the main occurrence modes: polygonized coatings composed by natrojarosite and goethite, typically observed in the dry shores of the Meca River (above); fine coatings of jarosite cracked by the extrusion of yellow-greenish copiapite (center) typically observed in the base of the dry pits; rigid crusts of jarosite, including gypsum and epsomite (below) on the front walls. (B) XRD patterns, showing the association of natrojarosite with goethite in the Meca River (above) and with Ca and Mg-sulfates (below) in the open pits. At least the three most intense peaks of each phase are indicated. Jt – Jarosite; Go – Goethite; Ep – Epsomite; Gy – Gypsum; Qz – Quartz; M – Muscovite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 3 Chemical composition of water samples. ND – not determined. Parameter

Filón Centro

Filón Sur

Filón Norte

Filón N-CH

Meca River

pH EC (lS/cm)

2.48 1948 3312

1.80 2100 930

1.79 3470 3216

2.27 2020 3384

2.54 1058 810

700 5.18 0.099 0.241 6.48 12.1 2.38 0.975 0.116 0.336 0.0018

1379 85.8 0.088 0.266 6.22 3.67 8.44 0.912 0.048 1.37 0.0051

2715 87.4 0.281 1.12 7.04 51.5 10.6 2.56 ND 3.54 0.0069

417 3.24 0.052 0.226 6.04 25.6 2.81 1.34 0.558 0.018 0.0012

21.8 9.12 0.318 0.952 4.09 7.06 0.664 0.215 0.521 0.153 0.0080

SO2 4

(mg/L)

Metals (mg/L) Fe Cu Pb Cd Zn Mn Co Ni Al As Sb

– Chalcanthite also precipitates in close proximity to sulfides from copper-rich solutions; it may transform to bonattite by dehydration. These sulfates occur in sheltered areas, where they are protected from dehydration.

– As solutions become depleted in iron, the aluminum and magnesium sulfates may precipitate; this is in accordance with the paragenetic sequence observed by Jerz and Rimstidt (2003) and with the textural relations observed in SEM study (Fig. 4C).

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Fig. 8A shows an abrupt decrease in the pH of the solution. Nevertheless, the three assemblages have different behaviors. Therefore, the experiments give insights into the hosts of major pollutants associated with natural dissolution by rainfall. Reactions (rxn 3), (rxn 4), (rxn 5), (rxn 6), (rxn 7) explain the mechanisms that are responsible for different pollution potentials. In the case of solid phases carrying divalent iron, such as szomolnokite, the pH decreases due to oxidation of ferrous iron and subsequent hydrolysis and precipitation of ferric iron (rxn (3)–(5)). Moreover, Frau (2011) proposed that small amounts of Fe3+ in a divalent sulfate may promote an additional decrease in the solution pH. Similarly, hydrolysis of trivalent ions, such as aluminum from halotrichite (rxn (6)), provides more release of H+ to the solution. In contrast, the pH achieved for the more chalcanthite-rich sample was considerable higher than in the other experiments. This is also supported by the results of metal concentrations, presented in Fig. 8B. This figure shows that the amount of released metals varied with the mineralogy, as it was observed by Jerz and Rimstidt (2003) for efflorescent iron sulfates from a pyrrhotite-dominated massive sulfide deposit. In the present case, the concentration of iron was considerably lower for the chalcanthite sample. On the other hand, this sample, as expected, revealed the higher potential for releasing copper.

Fig. 8. Results of dissolution experiments of selected samples. (A) Evolution of pH during the experimental dissolution; (B) chemical composition of the remnant solutions upon dissolution.

In Fig. 9 there are two phases that were not detected in the present study: melanterite and schwertmannite. However, it is expected to find them under other sampling conditions, as already discussed for melanterite. Schwertmannite has been found to transform into goethite over timescales of weeks to months (Bigham et al., 1996). Valente et al. (2011a) also verified this transformation with natural AMD. In the Tharsis system, schwertmannite may form under conditions that allowed dilution and neutralization. However XRD of ochre mixtures never revealed its presence. Also, its diagnostic morphologies (e.g. tubular aggregates, specular or foam habits) were never observed in SEM.

5.2. Pollution potential 5.2.1. Dissolution behavior The dissolution experiments demonstrated the role of the efflorescent minerals as transient sinks for sulfate, acidity and metals.

2 FeSO4 H2OðsÞ $ Fe2þ ðaqÞ þ SO4ðaqÞ þ H2 OðlÞ

ðrxn3Þ

þ 3þ 4Fe2þ ðaqÞ þ 4HðaqÞ þ O2ðaqÞ ! 4FeðaqÞ þ 2H2 OðlÞ

ðrxn4Þ

þ Fe3þ ðaqÞ þ 3H2 OðlÞ $ FeðOHÞ3ðsÞ þ 3HðaqÞ

ðrxn5Þ

FeAl2 ðSO4 Þ4 22H2 OðsÞ þ 0:25O2ðaqÞ $ FeðOHÞ3ðsÞ þ þ 2AlðOHÞ3ðsÞ þ 13:5H2 OðlÞ þ 4SO2 4ðaqÞ þ 8HðaqÞ

2 CuSO4 5H2 OðsÞ $ Cu2þ ðaqÞ þ SO4ðaqÞ þ 5H2 OðlÞ

ðrxn6Þ

ðrxn7Þ

In the event of rainfall, which occurs generally in October after a prolonged dry period, the highly soluble sulfates will dissolve and affect the receiving drainage network. The environmental relevance of this effect in AMD systems is well documented (e.g. Nordstrom and Alpers, 1999; Valente and Leal Gomes, 2009a). According to Frau (2000), the longer the dry period, the more intense the environmental impact during the subsequent wet period. This is especially relevant in this region of SW of Europe, characterized by a prolonged dry season followed by a torrential rainy period.

Table 4 Most representative mineralogical assemblages (see Fig. 1 for samples location). Values for temperature (T) and air humidity (H) are the average obtained for the set of sampling sites. Site

Typical mineral assemblages

Location

T (°C); H (%)

Filón Centro

Szomolnokite + Copiapite + Halotrichite + (Coquimbite) Chalcanthite + Gypsum + Alunogen + Halotrichite

Outcrops – exposed rocks at the base of the wall (T10–T13) In fractures of rock fragments near the water (T6–T7)

33; 49 30; 52

Corta Esperanza

Copiapite + Rhomboclase + (Chalcanthite + Gypsum) Chalcanthite + Bonattite + Halotrichite–Pickeringite + Gypsum

Along the sides of the sulfide piles (T14–T19) In a big fracture of the wall (1–2 m from the base) (T20–T25)

33; 48 30; 50

Filón Sur

Chalcanthite + Gypsum Hexahydrite + Gypsum + Epsomite Copiapite + Rhomboclase + Coquimbite

Outcrops – in sheltered cavities (T26–T31) Moist shores of the pit lakes (T33) Along the sides of sulfide piles (T32–T38)

30; 50 30; 52 33; 49

Filón Norte

Hexahydrite + Gypsum + Jarosite + (Epsomite)

28; 52

Copiapite + Alunogen + (Tamarugite)

Leachate channel (T42–T45) and in insurgences of the front walls (T46) Stalactites in moist fractures in the pit wall (T50–T53)

Hexahydrite + Gypsum + Alunogen + Halotrichite + (Tamarugite) Jarosite + Goethite + Gypsum

Moist shores, on the rocks and vegetation Along the river banks

28; 52 28; 52

Meca River

30; 51

T. Valente et al. / Applied Geochemistry 39 (2013) 11–25

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Fig. 9. Probable paragenetic sequence for the AMD-precipitates. (A) Evolution of the iron-rich phases controlled by oxidation and dehydration–hydration cycles. (B) Distribution of the sulfates relatively to the distance from the sulfide sources.

5.2.2. Mineral–water interactions It is expected that water properties will reflect the nature of the mineral–water interactions that prevail in sulfide environments. The hydrochemistry of the samples from the open pits (Table 3) may represent the result of water percolation in fractures and intergranular spaces in rocks. This allows for the dissolution of chemical species such as aluminum, which can be solubilized even from more stable minerals such as silicates. Accordingly, the contact with the parent sulfides could explain the low pH of the water that is also very rich in sulfate and metals. The pH of the four samples collected in the open pits (Filón Centro, Filón Sur, Filón Norte, and Filón N-CH) is sufficiently low to expect the acidic waters to be similar in their reactivity. Thus, the differences in metal contents must likely reflect the availability of the respective metal sources in the flow path. The general properties of the samples ‘‘Filón Centro’’, ‘‘Filón Sur’’, and Filón Norte’’ are consistent with other studies that present hydrochemistry data from pit lakes, specifically López-Pamo et al. (2009) and Sanchéz-España (2008). In contrast, the Meca River represents more complex conditions because it collects water from a variety of sources (Fig. 1). As the river flows away from the mining complex, the water should reflect the result of oxidation, hydrolysis of cations and precipitation of iron and other metals (Bigham and Nordstrom, 2000). The effects of these processes have been documented in studies performed in similar AMD streams (Valente and Leal Gomes, 2009a,b; García-Lorenzo et al., 2012). Likewise, Acero et al. (2006) and Asta et al. (2010) noted the contribution of natural attenuation processes, including dilution and neutralization in the Iberian Pyrite Belt. In the Meca River this is noted for sulfate, iron, arsenic, and some metals (Zn, Co, and Ni) that are present at lower levels at this site (Table 3). Thus, the potential to precipitate soluble salts is lower, and, consequently, there is less mineralogical diversity: dominated by Mg–Al sulfates. Consistent with the lower content in iron (Table 3), it was not possible to identify iron sulfates in the Meca River samples. The upstream physical–chemical conditions have caused the oxidation of iron and its removal in the form of goethite and jarosite, as

reported in similar studies in the belt (Velasco et al., 2005; Maia et al., 2012).

6. Conclusion This study shows that iron, aluminum and magnesium sulfates predominate in the acid mine drainage precipitates from the Tharsis mining complex. They occur as efflorescences composed of complex mixtures of metallic salts, and as ochre products (jarosite combined with goethite). Typical mineralogical assemblages were identified for areas in close proximity to pyrite sources and for the respective receiving fluvial system. The sampled areas defined two geochemical environments: the open pits and the Meca River. The first represents the proximal secondary paragenesis, resulting from mineral–water interactions involving the primary minerals. In contrast, the second one represents a more evolved paragenesis, resulting from the geochemical evolution of AMD produced in the broader Tharsis system. These environments can be differentiated by their water composition and their secondary mineral assemblages, which are considerably lower and less diverse in the Meca River. In addition to primary minerals, the relations among secondary minerals seem to be controlled by other factors, such as sun exposure and local hydrological conditions. Together, they influence the AMD composition and create local geochemical environments for mineral precipitation. This may explain the preference of chalcanthite by sheltered areas, where it is protected from dehydration. So, further research should be focused on phase relations, specifically the humidity–temperature conditions for specific mineral assemblages. To accomplish that, seasonal sampling campaigns should be followed on both environments. Also, this work demonstrated the different behavior of particular efflorescences to produce metals and sulfate-rich solutions. Considering that Tharsis is one of the largest ore deposits in the Iberian Pyrite Belt, the compiled inventory of solid phases may

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be of environmental and monitoring relevance. In such a semi-arid climate, the importance of AMD-precipitates should be stressed. They may help to understand the response of the nearby water system, when subjected to strong and episodic rainfall events, from which contamination spikes may occur. Acknowledgements The authors thank António Azevedo for his help in XRD analysis and to Lucia Guise for her assistance with dissolution experiments. CIG-R is supported by the national budget of the Portuguese Republic through FCT – under the Project PEst-OE/CTE/UI0697/2011. Financial support for this research was also provided by DGCICYT National Plan, Project CGL2010-21268-C02-01. The authors are deeply grateful to Associate Editor Robert Seal and to two anonymous reviewers for their valuable comments and suggestions. References Acero, P., Ayora, C., Torrento, C., Nieto, J., 2006. The behavior of trace elements during schwertmannite precipitation and subsequent transformation into goethite and jarosite. Geochim. Cosmochim. Acta 70, 4130–4139. Alpers, C.N., Blowes, D.W., Nordstrom, D.K., Jambor, J.L., 1994. Secondary minerals and acid mine-water chemistry. In: Jambor, J.L., Blowes, D.W. (Eds.), Short Course Handbook on Environmental Geochemistry of Sulfide Mine-wastes. Mineral Assoc. Can., pp. 247–270. Álvarez-Valero, A.M., Pérez-López, R., Matos, J.M.A., Capitán, A., Nieto, J.M., Sáez, R., Delgado, J., Caraball, M., 2008. Potential environmental impact at São Domingos mining district (Iberian Pyrite Belt, SW Iberian Peninsula): evidence from a chemical and mineralogical characterization. Environ. Geol. 55, 1797–1809. Alvaro, G.A., 2010. Mineralogía y geoquímica de sulfatos secundários en ambientes de drenaje ácido de mina. Área minera del yacimiento de san Miguel (Faja Pirítica Ibérica). PhD Thesis, Universidad del País Vasco. Asta, M., Ayora, C., Román-Ross, G., Cama, J., Acero, P., Gault, A., Charnock, J., Bardelli, F., 2010. Natural attenuation of arsenic in the Tinto Santa Rosa acid stream (Iberian Pyritic Belt, SW Spain): the role of iron precipitates. Chem. Geol. 271, 1–12. Bhatti, T.M., Bigham, J.M., Carlson, L., Tuovinen, O.H., 1993. Mineral products of pyrrhotite oxidation by Thiobacillus ferrooxidans. Appl. Environ. Microbiol. 59, 1984–1990. Bigham, J.M., Nordstrom, D.K., 2000. Iron and aluminum hydroxysulfates from acid sulfate waters. In: Alpers, C.N., Jambor, J.L., Nordstrom, D.K. (Eds.), Sulfate Minerals: Crystallography, Geochemistry and Environmental Significance. Rev. Mineral Geochem., vol. 40, pp. 351–403. Bigham, J.M., Schwertmann, U., Traina, S.J., Winland, R.L., Wolf, M., 1996. Schwertmannite and the chemical modeling of iron in acid sulfate waters. Geochim. Cosmochim. Acta 60, 2111–2121. Blowes, D.W., Lortie, G., Gould, W., Jambor, J.L., Hanton-Fong, C.J., 1998. Geochemical, mineralogical and microbiological characterization of sulphidebearing carbonate-rich gold-mine tailings impoundment, Joutel, Québec. Appl. Geochem. 13, 687–705. Brake, S.S., Dannelly, H.K., Connors, K.A., 2001. Controls on the nature and distribution of an alga in coal mine-waste environments and its potential impact on water quality. Environ. Geol. 40, 458–469. Buckby, T., Black, S., Coleman, M.L., Hodson, M.E., 2003. Fe-sulfate-rich evaporative mineral precipitates from the Rio Tinto, southwest Spain. Mineral Mag. 67, 263– 278. Carro, B., Borrego, J., López-González, N., Grande, J.A., Gómez, T., de la Torre, M.L., Valente, T., 2011. Impact of Acid Mine Drainage on the hydrogeochemical characteristics of the Tinto-Odiel Estuary (SW Spain). J. Iberian Geol. 37, 87–96. Chou, M., Seal, R.R., Hemingway, B.S., 2002. Determinantion of rozenite–melanterire and chalcanthite–bonattite equilibria by humidity measurements at 0.1 MPa. Am. Mineral. 87, 108–115. Chou, M., Seal, R.R., Wang, A., 2013. The stability of sulfate and hydrated sulfate minerals near ambient conditions and their significance in environmental and planetary sciences. J. Asian Earth Sci. 62, 734–758. Conde, C., Tornos, F., Large, R., Danyushevsky, L., Solomon, M., 2009. Análisis de elementos traza por la LA-ICPMS en pirita de los sulfuros masivos de Tharsis (FPI). Macla 2009, 63–64. Cotter-Howells, J.D., Campbell, L.S., Valsami-Jones, E., Batchelder, M., 2000. Environmental mineralogy: microbial interactions, anthropogenic influences, contaminated land and waste management. The Mineralogical Society Series 9. Courtin-Nomade, A., Bril, H., Neel, C., Lenain, J., 2003. Arsenic in iron cements developed within tailings of a former metalliferous mine – Enguialès, Aveyron, France. Appl. Geochem. 18, 395–408. Cravotta III C.A., 1994. Secondary iron-sulfate minerals as sources of sulfate and acidity: geochemical evaluation of acidic groundwater at a reclaimed surface coal mine in Pennsylvania. In: Alpers, C.N., Blowes, D.W. (Eds.), Environmental Geochemistry of Sulfide Oxidation. Am. Chem. Soc. Symp. Ser., vol. 550, pp. 345–364.

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