Acid Mine Drainage in the Iberian Pyrite Belt: an Overview With

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Conferencia 34 Acid Mine Drainage in the Iberian Pyrite Belt: An Overview

El presente trabajo pretende dar un breve repaso sobre la problemática de la generación de aguas ácidas de mina en la Faja Pirítica Ibérica, incluyendo algunos aspectos teóricos básicos sobre los mecanismos de oxidación y disolución de los sulfuros y la lixiviación de metales que tiene lugar en los huecos mineros (pozos, galerías, cortas) tras su abandono, y otros aspectos importantes como la composición química de las aguas ácidas resultantes de esta oxidación disolutiva (incluyendo lixiviados de escombreras y balsas, y lagos formados en cortas), o las fases minerales más relevantes que se generan en estos ambientes, y su papel como control de la composición química de los drenajes ácidos. Las aguas ácidas de la Faja Pirítica presentan importantes variaciones químicas tanto espaciales (composiciones muy distintas en función del foco generador y de la geología local de la mina) como temporales (con mar- cadas variaciones hidrológicas estacionales), y entre las numerosas aguas analizadas hasta la fecha se incluyen casos con valores de pH extremadamente bajos (cercanos a cero en algunas zonas) y contenidos muy altos en sulfato, metales y metaloides disueltos (p.ej., concentraciones de centenares de gramos por litro de sulfato, y de decenas de g/L de Fe, Al, Cu y Zn). También se comentan diver- sos aspectos sobre la mineralogía y quimismo de los precipitados de Fe(III) (schwertmanita, jarosita, goetita, ferrihidrita), Al (hidroba- saluminita, alunita) y sales eflorescentes de Fe(II)-Fe(III)-Mg-Al (melanterita, rozenita, epsomita, hexahidrita, copiapita, halotriquita, coquimbita) presentes en estos ambientes, así como su solubilidad y capacidad de retención y absorción de metales y metaloides traza como Cu, Zn, As o Pb. Finalmente, se proporcionan varios ejemplos de evolución hidrogeoquímica y transporte de metales para ilustrar el tipo y grado de impacto ambiental que generan este tipo de aguas. This work provides a brief overview about the generation of Acid Mine Drainage (AMD) in the Iberian Pyrite Belt (IPB), including some theoretical considerations about the mechanisms of sulphide oxidation and metal leaching that occur in the mining areas, the water chemistry of the resultant AMD discharges and acidic mine pit lakes, and the most relevant mineral phases associated to the acidic mine waters. These acidic solutions vary largely in chemical composition, either spatially (between different mine sites) and seasonally (due to marked hydrologic variations), and include cases with very low pH and very high sulphate and metal contents. The mineralogy and chemistry of the Fe precipitates (e.g., schwertmannite, jarosite, goethite, ferrihydrite), Al phases (e.g., hydrobasaluminite, alunite) and Fe(II)-Fe(III)-Mg-Al efflorescent sulphates (e.g., melanterite, rozenite, epsomite, hexahydrite, copiapite, halotrichite, coquimbite) present in the AMD-generating mine sites, as well as their solubility and trace metal retention capacity, is also discussed. Finally, some examples of hydrogeochemical evolution and metal transport and precipitation are provided to illustrate the environmental impact of AMD in the IPB. Acid Mine Drainage in the Iberian Pyrite Belt: an Overview with Special Emphasis on Generation Mechanisms, Aqueous Composition and Associated Mineral Phases / JAVIER SÁNCHEZ ESPAÑA Instituto Geológico y Minero de España, Rios Rosas, 23, 28003, Madrid, Spain INTRODUCTION The mineralogical and textural characte- 1991; Nelson and Lamothe, 1993; ristics of the IPB ores (with a dominantly Elbaz-Poulichet et al., 1999; Davis et A long history of metalliferous mining in pyritic, fine-grained, usually brecciated al., 2000; Cossa et al., 2001; the IPB mining district has left a legacy and fractured, and highly reactive mine- Braungardt et al., 2003; Olías et al., of abandoned mines, mainly in the pro- ralization, and a considerable lack of 2004; Sánchez-España et al., 2005a, vince of Huelva (SW Spain), and a big carbonates to neutralize acidity), has 2006a). This acidity and metal pollution number of waste piles (n=57, total volu- favoured the oxidation and dissolution has caused the loss of most forms of me of 107 Hm3), tailings impound- of pyrite, and the subsequent formation aquatic life, with the exception of acido- ments (n=10, total volume of 42 Hm3) of AMD. These acid mine waters have philic microorganisms which inhabit and flooded pits (n=22, total volume of caused severe pollution of acidity and these extreme environments (e.g., acidic water of 25 Hm3), which repre- dissolved metals (Fe, Al, Mn, Cu, Zn, López-Archilla and Amils, 1999; López- sents one of the World´s largest accu- Cd, Pb), As and SO4 to the Odiel and Archilla et al., 2001; González-Toril et mulations of mine wastes and AMD. Tinto river systems (Van Geen et al., al., 2003; Aguilera et al., 2006).

palabras clave: Drenaje ácido de mina, oxidación de pirita, trans- key words: Acid mine drainage, pyrite oxidation, metal transport, porte de metales, minerales Fe-Al, Faja Pirítica Ibérica Fe-Al minerals, Iberian Pyrite Belt

Conferencia invitada: Sánchez España, Macla 10 (2008) 34-43 *corresponding author: [email protected]

macla. nº 10. noviembre´08 revista de la sociedad española de mineralogía 35

This paper provides an overview about combination of reactions (2) and (3) is with sulphate as the main product the AMD problem in the IPB district, traditionally known as the “propagation (Schippers and Sand, 1999; Sand et mainly from a geochemical and mineralo- cycle” (Singer and Stumm, 1970), and al., 2001): gical perspective. It firstly reports some along with reaction (1) depicts a model by basic features about the oxidation of pyri- which pyrite oxidation initially starts by +3 FeS2(s) + 6 Fe + 3 H2O → te, the main geochemical process res- reaction (1) with oxygen as the oxidant at S O -2 +7 Fe+2 + 6 H+ (4) ponsible for the generation of AMD, and near-neutral pH conditions, and as pH 2 3 about the microorganisms that catalyze decreases to about 4 the oxidation of -2 +3 S2O3 + 8 Fe + 5 H2O → this process in shafts, galleries, piles, tai- pyrite proceeds through reaction (3). 2 SO -2 + 8 Fe+2 + 10 H+ (5) lings and pits. Secondly, the composition Oxygen will always be required to reple- 4 of AMD typically found in the IPB, as well nish the supply of ferric iron according to Most metal sulphides (including ZnS, as the mineralogy and chemistry of mine- reaction (2), so that the overall rate of CuFeS2 and PbS) are susceptible to a ral phases associated to the acid mine pyrite oxidation in a tailings or waste pile proton acid attack as well as to ferric waters, are also discussed. or in a mine will largely depend on the iron oxidation. They are oxidized through overall rate of oxygen transport by advec- the so-called polysulphide mechanism THEORETICAL CONSIDERATIONS tion and diffusion (Nordstrom and Alpers, (Schippers and Sand, 1999; Sand et al, 1999a; Ritchie, 2003). 2001): Fundamentals of pyrite oxidation The concept of bioleaching: direct vs. MS + Fe+3 + H+ → The oxidative dissolution of pyrite is one indirect mechanism +2 +2 of the most extensively studied geoche- M + 0.5 H2Sn + Fe (n≥2) (6) mical processes on the Earth surface, The mechanisms by which acidophilic +3 althought it is not yet fully understood chemolithotrophic microorganisms 0.5 H2Sn + Fe → +2 + (e.g., Nordstrom and Alpers, 1999a; obtain energy by oxidizing sulphidic 0.125 S8 + Fe + H (7) Ehrlich, 2002). The reaction which clas- minerals have been controversial for sically describes the oxidation of pyrite many years (Ehrlich, 2002). Two basic The produced elemental sulfur can be in the presence of oxygen and water is mechanisms, concerning the relative further oxidized to sulfuric acid: (Singer and Stumm, 1970): relationship between the mineral substrate and the microbial catalyst, the 0.125 S8 + 1.5 O2 + H2O → FeS + 3.5 O + H O → direct and the indirect attack, have -2 + 2(s) 2 2 SO4 + 2 H (8) +2 -2 + been considered (Sand et al., 2001; Fe + 2 SO4 + 2 H (1) Schippers et al., 1996; Schippers and Although microbes do attach to sulphide Sand, 1999). In the direct attack, micro- mineral surfaces and can oxidize them The formed ferrous iron is then usually organisms solubilize mineral sulphides oxidized to Fe+3 by the reaction: (e.g., Edwards et al., 1998), it is generally by attaching to their surface, facilitating accepted that the soluble ferric iron pro- an enzymatic oxidation by channelling 14 Fe+2 + 3.5 O + 14 H+ → duced by microbial oxidation is responsi- 2 electrons from the mineral to an appro- ble for the oxidation of other sulphides +3 14 Fe + 7 H2O (2) priate electron acceptor (respiration). In common in waste piles or tailings. the indirect attack, microorganisms are A critical factor in the oxidation of pyrite and mainly involved in the regeneration of Thus, this model proposes that the princi- the generation of acid mine waters is that ferric iron, a strong oxidant, in the solu- pal microbial function in AMD systems +3 Fe is able to oxidize pyrite under anoxic tion. This soluble ferric iron is responsi- during sulphide bioleaching is to keep the subaqueous conditions at a much faster ble for the oxidation of exposed sulphi- FeIII ions in an oxidized state and to gene- rate than does molecular oxygen, according dic minerals and other reduced com- rate sulphuric acid biologically to supply to the reaction (Garrels and Thompson, pounds. The demonstration that ferric protons for hydrolysis attack. Based on 1960; McKibben and Barnes, 1986): iron present in the cell envelop and/or laboratory experiments, Schippers and the exopolymers of bioleaching microor- Sand (1999) estimated that up to 16% of +3 ganisms is responsible for the electron FeS2(s) + 14 Fe + 8 H2O → the total sulphur released during pyrite +2 -2 + transfer from the sulphidic minerals to 15 Fe + 2 SO4 + 16 H (3) oxidation was elemental sulphur (the rest the electron transport chain (Gehrke et -2 being mainly SO4 ), whereas the oxida- Because at pH<3.5, reaction (2) is al., 1995), has clarified the situation. tion of other sulphides like sphalerite, orders of magnitude slower than reac- Based on this observation, Sand and chalcopyrite or galena can result in the tion (1), the oxidation of Fe+2 by oxygen coworkers proposed that because ferric production of between a 92% and nearly is usually considered the rate-limiting iron is ultimately responsible for the oxi- a 100% of native sulphur, and only trace step in pyrite oxidation (Singer and dation of metal sulphides in both the amounts of sulphate and polythionates direct and the indirect attack, there is -2 -2 Stumm, 1970). However, the presence (S4O6 , S5O6 ). of acidophilic bacteria such as no basic difference between these Acidithiobacillus ferrooxidans and mechanisms. The difference seems to Finally, Schippers et al. (1996) have Leptospirillum ferrooxidans greatly exist at the level of the chemical attack, demonstrated that the bacterial species accelerate (by a factor of around 106) which is dependent on the structure of involved in pyrite oxidation is a determi- the abiotic oxidation rate (Singer and the mineral substrates (Schippers et ning factor as regards to the sulphur Stumm, 1970; Nordstrom and Alpers, al., 1996; Schippers and Sand, 1999; species generated during the cyclic 1999a), thus maintaining a high con- Sand et al., 2001). Some metal sulphi- (indirect) leaching mechanism. Thus, in centration of ferric iron in the system. des, like pyrite (and also MoS2 and the case of pyrite bioleaching by WS2), which can only be oxidized by Leptospirillum ferrooxidans, an orga- The overall process resulting from the ferric iron, undergo oxidation through nism without sulphur-oxidizing capicity, the so-called thiosulfate mechanism,

depósito legal: M-38920-2004 • ISSN: 1885-7264

Conferencia 36 Acid Mine Drainage in the Iberian Pyrite Belt: An Overview

besides the production of tetra- and pentathionate, a detectable accumula- tion of elemental sulphur may occur, whereas in the case of Acidithiobacillus ferrooxidans, only small amounts of elemental sulphur are detectable because of the microorganism’s capacity to oxidize sulphur compounds.

“Iberian” microbes involved in pyrite and Fe(II) oxidation

The microbiological findings carried out until now in the acidic mine waters of the Iberian Pyrite Belt (e.g., López-Archilla and Amils, 1999; López-Archilla et al., 2001; González-Toril et al., 2003; Aguilera et al., 2006) have been mainly centered on the microbial (prokaryotic and eukaryotic) com- munities of the Tinto and (in a lesser extent) the Odiel river systems, which are both highly acidic (average pH values of around 2 and 3.5, respectively) and present metal contents in the order of hun- dreds to a few thousands mg/L. The iron- oxidixing, acidophilic, prokaryotic community of these waters is dominated by Acidithiobacillus ferrooxidans and Leptospirillum ferroxidans, two microorganisms that have been extensively studied in mine drainage environments, along with a minor presence of Acidiphilium spp. These microorganisms have been also found in pit lakes of the IPB such as San Telmo (Sánchez-España et al., 2007c) or Corta Atalaya (D.B. Johnson, University of Bangor, pers. com.). A study conducted in acidic effluents draining several mines (Sánchez- fig 1. Examples of acid mine drainage generation in mines sites of the Iberian Pyrite Belt: (a) Esperanza mine portal (pH 2.6); (b) Lomero mine portal (pH 2.9); (c) Seepage from a waste pile in Aznalcóllar; (d) The acidic Tintillo river (pH 2.7-2.9) after collecting España et al., 2007) has revealed that a number of different acidic leachates draining various waste piles near Corta Atalaya (Riotinto mines); (e) Precipitation of iron these acidophilic microbes oxidize Fe(II) at hydroxysulphates (red) and aluminum hydroxysulphates (white) in the confluence of the Tintillo acidic river and the Odiel river (pH 7-8); (f) Filón Norte pit lake (pH 2.4) in Tharsis mine. Photographs by Enrique López Pamo and Javier Sánchez España. field rates of about 10-7 mol L-1 s-1. evaporative pools and formed crystals of generating mine sites included waste rock +2 Johnson (2006) studied the microbiology of Zn-rich melanterite (Fe SO4·7H2O). The piles (50%), mine adits (30%), tailings an acidic and thixotrophic mine pond in Sao microbiological investigation has revea- impoundments (10%), mine holes (7%), and Domingos (Portugal) and reported an ecos- led a surprisingly high biomass (1.4x106 mine pit lakes (3%) located in some of the ystem composed of L. ferrooxidans, cells mL-1) and an exotic ecosystem com- largest and most important deposits of the Ferrimicrobium spp. and At. Ferrooxidans. posed of acidophilic, Fe-oxidizing IPB (Río Tinto, Tharsis, La Zarza, San Telmo, Rowe et al. (2007) have studied the micro- archaea (mainly Ferroplasma spp., repre- Sotiel-Almagrera) and many others of biology of an acidic effluent (pH 2.5-2.7) drai- senting 52% of the microbial population), medium size (Lomero-Poyatos, San Miguel, ning an old cementation channel in Tharsis and minor numbers of acidophilic bacte- Cueva de la Mora, Aguas Teñidas, mine and found a complex community domi- ria (including Leptospirillum spp. (3.2%), Concepción, San Platón, Poderosa, Tinto- nated by Acidithiobacillus ferrooxidans in the Acidithiobacillus spp. (1.6%), and Sta Rosa). Some examples of mine draina- mine water, and by the heterotrophic acido- Alfaproteobacteria (2.8%), which is only ge systems of the Iberian Pyrite Belt are philic bacteria Acidobacteriacae and comparable to those reported in similar illustrated in Figure 1. Acidiphilium spp. in the streamer growths, waters with pH 0.5-1.0 from Iron along with sulfidogenic bacteria in the lowest Mountain, California (Edwards et al., The physico-chemical data reported for depths of the streamers. 1999, 2000; Druschel et al., 2004). these acidic effluents suggested a highly variable nature as regards to sea- More recently, Sánchez-España et al. (in AQUEOUS CHEMISTRY OF ACID MINE sonal continuity (permanent, seasonal press) have described the microbiologi- DRAINAGE IN THE IBERIAN PYRITE BELT or ephemeral drainage), water volume cal composition of an extremely acidic (0.1-220 L/s), acidity (200-30000 (pH 0.61-0.82) and hypersaline (e.g., Effluents from piles, tailings and mine mg/L CaCO3 eq.), redox conditions -2 134 g/L SO4 , 74 g/L Fe, 7.5 g/L Al, 3 portals (Eh=400-800 mV), electric conductivity g/L Mg, 2 g/L Cu, 1 g/L Zn) leachate (EC=1000-24000 mS/cm), dissolved which seeps from a pyrite pile in San In a previous paper, Sánchez-España et al. O2 content (anoxic to O2-saturated), or Telmo mine (Huelva, SW Spain). This (2005a) studied 64 AMD discharges from dissolved Fe(II) to total iron ratio ultra-concentrated water accumulated in 25 different mines of the IPB. These AMD- (Fe(II)/Fetot=0.1-1).

Conferencia invitada macla. nº 10. noviembre´08 revista de la sociedad española de mineralogía 37

Parameter Units Acid mine drainage Acidic mine pit lakes the order of 265 mg/L Fe, 123 mg/L Al, Average a Extreme case b Average c Extreme case d 34 mg/L Mn, 20 mg/L Cu, and 31 mg/L pH S.U. 2.7 0.82 2,6 1.2 Zn, in addition to around 90 µg/L As, T ºC 21 16.5 23 24 100 µg/L Cd, 1100 µg/L Co, 650 µg/L EC mS/cm 7.9 - 5.3 55.6 Eh m 627 625 782 584 Ni, and 120 µg/L Pb (Table 1). DO mg/L 3.8 - 7,6 0 DO % sat. 42 - 93 0 The lithology of the rock substrate, with abundance of massive sulphide (espe- SO4 mg/L 7.460 134.200 4.000 41.900 Na mg/L 32 37 31 78 cially pyrite) and aluminosilicates, and K mg/L 3 130 2 12 Mg mg/L 414 3.036 357 1.957 scarcity of carbonates, is undoubtedly Ca mg/L 162 137 180 286 determining the water chemistry of the Fet mg/L 1.494 74,215 (1) 332 36.675 Fe(II) mg/L 801 40,600 (2) 15 32.500 IPB mine pit lakes. The high contents of -2 Fe(III) mg/L 476 28,630 (2) 312 3.950 Fe and SO4 dissolved in the waters Al mg/L 386 7.556 123 1.919 result from the oxidative dissolution of Mn mg/L 37 38 34 128 Cu mg/L 64 1.945 20 1.350 pyrite, whereas the rest of metals are Zn mg/L 169 1.096 31 6.670 thought to be the result of the subse- As µg/L 2.123 303 91 158.730 quent dissolution of sphalerite (source Cd µg/L 490 13.759 102 18.020 of Zn, and minor Cd), chalcopyrite (sour- Co µg/L 3.413 2.447 1.100 18.689 Cr µg/L 118 52 38 1.295 ce of Cu and Fe), galena (Pb), arsenopy- Ni µg/L 1.063 3.220 656 5.214 rite (As, Fe), tetrahedrite-tenantite (Fe, Pb µg/L 61 108 122 5.402 Cu, Zn, As, Sb), rhodocrosite (Ca, Mn) Th µg/L 23 376 6 - Tl µg/L 27 2.683 4 - and gangue aluminosilicates like felds- U µg/L 64 981 7 - pars (Al, K, Na, Ca), chlorite (Al, Fe, Mg, V µg/L 65 6.756 27 - Co, Ni) and sericite-muscovite (Al, Na, (a) Average chemical composition of 62 AMD waters from 25 mines of the Iberian Pyrite Belt (from Sánchez- K). In addition to these primary sour- España et al., 2005a); (b) Chemical composition of a pyrite leachate in San Telmo mine (taken from Sánchez- ces, the dissolution of secondary sul- España et al., in press); (c) Average chemical composition of 22 acidic mine pit lakes (AML) from the IPB (exclu- phate salts (see below) must have been ding CA; Sánchez-España et al., 2008); (d) Chemical composition of the Corta Atalaya pit lake (Sánchez-España et al., 2008) another control of the pit lake water (1) Total iron measured by Atomic Absorption Spectrometry (AAS) chemistry, especially during the initial (2) Ferrous and Ferric iron measured on site by reflectance photometry flooding phases. table 1. Chemical composition of acidic mine drainage and acidic mine pit lakes of the Iberian Pyrite Belt (the average value and an extreme case is provided in both cases). The chemical composition of these in press). A liquor found in this mine site MINERAL PHASES ASSOCIATED TO waters is extremely variable, and may showed a pH of 0.6 and extreme sulpha- AMD: ENVIRONMENTAL SIGNIFICANCE include extreme concentrations of dissol- te (134 g/L) and metal concentrations ved SO4 (up to 44 g/L) and metals (up to (e.g., 74 g/L Fe, 7.5 g/L Al, 3 g/L Mg, 1.9 As can be deduced from the above stated 7.7 g/L Fe, 2.6 g/L Al, 2.9 g/L Mg, 1.4 g/L Cu, 1 g/L Zn; Table 1). chemical compositions, iron, aluminum, g/L Zn, 435 mg/L Cu and 440 mg/L Mn). magnesium and calcium are, by far, the Trace elements are also found to be signi- Pit lakes most abundant metals dissolved in AMD ficantly enriched in these acid waters (for (with Fe>Mg≈Al>Ca>>Zn>Cu>Mn), so example, maximum values of 17 mg/L The water chemistry of twenty-two pit that the minerals that are commonly for- As, 8 mg/L Cd, 48 mg/L Co, 17 mg/L Ni, lakes of the IPB has been recently med in these AMD environments are and 725 µg/L Pb). Also, U and Th, with reported in Sánchez-España et al. chiefly Fe(II), Fe(III), Al, Mg and Ca sulpha- peak values around 1100 and 400 µg/L, (2008). These pit lakes show surface tes and (oxy)hydroxysulphates. Some respectively, in a waste pile near Corta areas ranging from less than 1 ha (e.g. Atalaya (Riotinto), were unusually concen- Angostura, Herrerías) up to 28 ha (e.g., trated. An average composition of typical Aznalcóllar), and maximum depths from AMD from the IPB is given in Table 1. around 15 m (e.g., Concepción) to more than 100 m (e.g., San Telmo, Los In general, these compositions are com- Frailes). The pit lakes of the IPB show a prised within the range normally repor- large variety of water compositions, ran- ted for AMD from VMS-type deposits ging from circumneutral and relatively (see Plumlee et al., 1999). The higher low metal contents (e.g., Los Frailes pit SO4 and metal concentrations were lake, with pH 7.2, 0.07 mg/L Fe, 1 usually found in the acid leachates from mg/L Al, 0.01 mg/L Cu, and 30 mg/L the waste piles of Corta Atalaya Zn) to extremely acidic and metal-enri- (Riotinto) and Filón Norte (Tharsis), and ched solutions (e.g., Corta Atalaya pit suggested an important oxidation and lake, with pH 1.2, 36.7 g/L Fe, 1.9 g/L subsequent dissolution of pyrite and Al, 1.3 g/L Cu and 6.7 g/L Zn; Table 1), other sulphides (chalcopyrite, sphaleri- with a general pattern of increasing dis- te, galena, arsenopyrite). solved solids content at decreasing pH. However, with the cited exceptions of Some anomalous exceptions to the repor- Corta Atalaya and Los Frailes, a “typi- ted sulphate and metal concentrations cal” pit lake of the IPB shows a pH can be eventually found in evaporative value around 2.6 (2.2-4.7), total Fe pools of ultra-concentrated water seeping chiefly composed of Fe(III), average fig 2. XRD patterns of selected Fe(III) and Al precipitates found -2 in AMD of the IPB (compiled from Sánchez-España et al., from pyrite piles, as described recently in values of dissolved SO4 around 4 g/L, 2006a,b, with kind permission of Springer Science and San Telmo mine (Sánchez-España et al., and metal contents (average values) in Business Media).

depósito legal: M-38920-2004 • ISSN: 1885-7264

Conferencia 38 Acid Mine Drainage in the Iberian Pyrite Belt: An Overview

Mineral Formula pH range* Occurrence Location Abundance

Soluble iron sulphate salts

+2 Melanterite Fe SO4 · 7H2O <1-1.5 Efflorescences Near pyrite sources Very common +2 Rozenite Fe SO4 · 4H2O - Efflorescences Near pyrite sources Very common +2 Szomolnokite Fe SO4 · H2O - Efflorescences Near pyrite sources Very common +2 +3 Copiapite Fe Fe 4(SO4)6(OH)2·20H2O 1.5-2.5 Efflorescences Mining areas and margins of streams Very common +3 Coquimbite Fe 2(SO4)3 · 9H2O 1.5-2.5 Efflorescences Mining areas and margins of streams Common +3 Rhomboclase (H3O)Fe (SO4)2·3H2O <1 Efflorescences Mining areas, evaporative pools Rare +2 Halotrichite Fe Al2(SO4)4·22H2O 1.5-2.5 Efflorescences Mining areas and margins of streams Very common Other soluble sulphate salts

Epsomite MgSO4 · 7H2O 2-3 Efflorescences Mining areas, margins of streams, pools Very common Hexahydrite MgSO4 · 6H2O - Efflorescences Mining areas, margins of streams, pools Very common Gypsum CaSO4 · 2H2O 1-5 Efflorescences Mining areas, margins of streams, pools Ubiquitous Iron hydroxides/hydroxysulphates

+3 Jarosite KFe 3(SO4)2(OH)6 <2 Efflorescences, precipitates Highly acidic effluents, pools and pit lakes Very common +3 Natrojarosite NaFe 3(SO4)2(OH)6 <2 Efflorescences, precipitates Highly acidic effluents, pools and pit lakes Common +3 Hydronium jarosite (H3O)Fe 3(SO4)2(OH)6 <2 Efflorescences Mining areas Rare +3 Schwertmannite Fe 8O8(SO4)(OH)6 2.5-4 Fine-grained precipitates AMD, AMD-impacted streams, pit lakes Ubiquitous Goethite α-Fe+3O(OH) 2.5-7 Hard crusts AMD, AMD-impacted streams, pit lakes Very common +3 Ferrihydrite Fe 5HO8 · 4H2O 5-8 Fine-grained precipitates Confluences between AMD and streams Common Aluminum hydroxides/hydroxysulphates

Gibbsite γ-Al(OH)3 >4.5-5.0 Fine-grained precipitates Pit lakes, AMD-impacted rivers Rare Alunite KAl3(SO4)2(OH)6 3.5-5.5 Efflorescences, precipitates Mining areas and margins of streams Common Alunogen Al2(SO4)3 · 17H2O <1 Efflorescences, precipitates Mining areas Rare Jurbanite Al(SO4)(OH) · 5H2O <4 Efflorescences, precipitates Mining areas and margins of streams Rare Basaluminite Al4(SO4)(OH)10 · 5H2O >4.5-5.0 Fine-grained precipitates Confluences between AMD and streams Very common Hydro-basaluminite Al4(SO4)(OH)10 · 12-36H2O >4.5-5.0 Fine-grained precipitates Confluences between AMD and streams Very common * Common pH range of the acidic waters associated with these mineral phases table 2. Relevant secondary minerals precipitating from acidic mine drainage and acidic mine pit lakes of the Iberian Pyrite Belt. common Fe, Al, Mg and Ca minerals sely associated with their spatial distribu- acidic effluents, these soluble metal sul- associated to, and precipitating from tion and the pH of the brines from which phates are usually found as botryiodal AMD of the IPB, along with their respecti- these salts are precipitated (Nordstrom (colliform-like) efflorescences of variable ve formulae and usual pH ranges of occu- and Alpers, 1999a,b; Nordstrom et al., colour (turquoise blue to emerald green rrence, are given in Table 2. This table 2000; Jambor et al., 2000; Buckby et al., in the case of melanterite, white in the includes highly crystalline sulphate salts 2003; Velasco et al., 2005; Sánchez- case of szomolnokite, orange to yellow in (e.g., melanterite, rozenite, copiapite, España et al., 2005a; Romero et al., the case of the epsomite-hexahydrite halotrichite) formed by evaporative pro- 2006). Thus, the Fe(II)-sulphates like series; Figure 3a-c), being normally cesses from very acidic brines in small melanterite, rozenite or szomolnokite are zoned from the inner to the outer zones, pools or in the margins of AMD-impacted dominant in isolated and highly concentra- and suggesting a paragenetic sequence streams, as well as nearly amorphous ted pools near the pyrite sources, under of sulphates with distinct solubilities and compounds of Fe(III) (schwertmannite, conditions typical of green, ferrous AMD degrees of dehydration. They are rarely ferrihydrite) and Al (hydrobasaluminite, with very low pH (melanterite is commonly found as monomineralic phases, and basaluminite, gibbsite). Some examples found in close association with waters most commonly consist in mixtures of of typical XRD patterns for some of these with pH below 1, whereas rozenite and Fe(II)-Fe(III)-Al-Mg hydrated sulphates. minerals are provided in Figure 2, and the szomolnokite form by partial dehydration Gypsum is also very frequent as acicular occurrence of several efflorescent sul- of the former). On the other hand, mixed crystals and efflorescences in mine phates and (oxy)hydroxysulphates is illus- Fe(II)-Fe(III), and/or Fe(III)-Al sulphates like adits and waste piles (Figure 3d). trated in Figure 3. For a more comprehen- copiapite, coquimbite or halotrichite, are Stalactites composed of gypsum±copia- sive review about the metal oxides, hydro- common in the margins of rivers impacted pite±jarosite have been observed at xides, sulphates and (oxy)hydroxysulpha- by AMD, where iron has been partially oxi- some AMD discharge points. tes precipitating in AMD environments, dized and the pH is slightly higher (typically the reader is referred to the works of between 1.5 and 3). These sulphates Chemical analyses of mixtures of these Nordstrom and Alpers (1999a), Jambor et have been observed to follow a paragene- sulphates have revealed very high al. (2000), or Bigham and Nordstrom tic sequence of dehydration and mineralo- metal contents (e.g., average values of (2000). A compilation of secondary sul- gical maturation, with melanterite > roze- 2,800 ppm Cu and 9,000 ppm Zn, with phate salts found in the Riotinto mines nite > szomolnokite > copiapite > coquim- Zn values eventually reaching percent and river was reported by Buckby et al. bite > rhomboclase > halotrichite units; Sánchez-España et al., 2005a). (2003), and a list of sulphates and (Nordstrom and Alpers, 1999a,b; Jambor On the other hand, As contents of the (oxy)hydroxysulphates found in mines and et al., 2000; Nordstrom et al., 2000; sulphates are very low (195 ppm on AMD-affected streams of the Odiel fluvial Buckby et al., 2003; Velasco et al., 2005). average), whereas Co, Ni and Cd show system is given in Sánchez-España et al. average values of 240, 155 and 30 (2005a). In the Iberian Pyrite Belt mining district, ppm, respectively. many evaporative sulphate salts have Formation of evaporative sulphate been reported from mine sites and from Because these sulphates are highly salts near the pyrite sources the margins of rivers severely affected by soluble, the first rainstorm events AMD pollution such as the Tinto and taking place in the early autumn usually The formation of efflorescent sulphates Odiel rivers (e.g., García García, 1996; imply the re-dissolution of large from AMD waters occurs throughout all Hudson-Edwards et al., 1999; Buckby et amounts of these salts accumulated the year, although it is especially abun- al., 2003; Velasco et al., 2005; Romero during the spring-summer, and the sub- dant during spring and summer. The mine- et al., 2005; Sánchez-España et al., sequent incorporation of toxic metals to ralogy of these soluble sulphates is clo- 2005a, and in press). In the margins of the rivers. Therefore, seasonal cycles of

Conferencia invitada macla. nº 10. noviembre´08 revista de la sociedad española de mineralogía 39

fig 3. Efflorescent sulphates and iron-aluminum (oxy)hydroxysulphates precipitating from AMD systems of the IPB: (a) Cu-rich melanterite, rozenite and szomolnokite forming from an acidic discharge in the Cantareras mine (Tharsis); (b) Zn-rich melanterite formed by evaporation of extremely acidic (pH 0.6) seepage emerging from a pyrite pile in San Telmo mine; (c) Colliform-like epsomite and hexahydrite near the Lomero mine portal; (d) acicular crystals of gypsum formed in a Ca-rich acidic effluent in Lomero mine; (e) hexahydrite and halotriquite in San Telmo mine; (f) Jarosite deposit (light orange) which is partly transformed into goethite (dark orange to brown); (g) Schwertmannite precipitates in the confluence of an acidic effluent with a pristine stream (Lomero mine); (h) Hydrobasaluminite precipitates deposited on the streambed of the AMD-impacted El Escorial creek (pH 4.5) near Riotinto mines. All photographs by Enrique López Pamo and Javier Sánchez España except (b) which is courtesy of Francisco Velasco Roldán. precipitation/re-dissolution of salts can iron-rich water in the piles. tions reach the solubility product cons- seriously determine the water quality of tant for melanterite, this mineral may AMD-impacted rivers. As an example, a Apparently, the simple dissolution of precipitate from the solution through seasonal pattern of background Cu and melanterite through reaction (10) does reaction (14) until chemical equilibrium Zn values during the summer, followed not produce acid, unless coupled with is attained: by sharp increases in their concentra- oxidation of Fe(II) and hydrolysis/preci- +2 -2 tions during the first rainfalls in autumn, pitation of a ferric iron hydroxide (11): Fe + SO4 + 7H2O → has been reported in rivers draining the FeSO4·7H2O(s) (14) IPB province (e.g., Tinto and Odiel FeSO4·7H2O(s) → rivers; Olías et al., 2004) and in many +2 -2 Fe + SO4 + 7H2O (10) The presence and environmental signifi- other mine districts (e.g., Alpers et al., cance of melanterite has been widely 1994). FeSO4·7H2O(s) + 0.25 O2→ reported in the IPB mining district. For Fe(OH) + SO -2 + 2H+ + 4.5 H O (11) example, Buckby et al. (2003) reported The formation-dissolution-precipitation 3 4 2 the presence of melanterite in the banks cycle of melanterite in pyrite piles of the Tinto river which had apparently However, laboratory experiments carried precipitated from a highly acidic water out by Frau (2000) have demonstrated The formation-dissolution-precipitation (pH≈0.4) draining a pyritic waste pile. that the simple dissolution of melanteri- cycle of melanterite represents an Velasco et al. (2005) have also reported te is a fast acidity-producing mechanism important control of the aqueous com- melanterite and rozenite forming directly without the participation of Fe+2 oxida- position of the acidic leachates draining from pyrite oxidation in a dried pool adja- tion. This author explained the produc- pyrite piles. When oxygen and moisture cent to a pyrite pile in the San Miguel tion of acidity by the coupled reactions of are available, melanterite can be mine, although these authors did not Fe+2 hydrolysis (12) and the SO -2/HSO - directly formed on the pyrite grain surfa- 4 c include water analyses in their study. The buffer system (13): ces by the reaction: role of these sulphate efflorescences in the storage and transport of trace ele- +2 FeS + 3.5 + 8H O → Fe(H2O)6 + H2O ⇔ ments in stream waters polluted by acid 2(s) 2 2 + + Fe(OH)(H2O)5 + H3O (12) FeSO4·7H2O(s)+ H2SO4 (9) mine drainage in Peña del Hierro mine, has been also reported by Romero et al. -2 This mineral can either persist if the SO4 + H2O → (2006). More recently, Sánchez-España - - ambient humidity conditions are HSO4 + OH (13) et al. (in press) have reported the occu- appropriate, or be transformed to less rrence of melanterite crystals directly for- +2 -9.5 hydrated forms of ferrous sulphate The acid Fe(H2O)6 (Ka=10 ) is stronger med in green pools of extremely acidic than the base SO -2 (K =10-12.1; Stumm such as rozenite (FeSO4·4H2O) or szo- 4 b water (pH≈0.6) in San Telmo mine (Figure and Morgan, 1996), so that the dissolu- molnokite (FeSO4·H2O) or even mixed 3b; Table 1). This acidic liquour was Fe+2-Fe+3 salts like copiapite (Fe+2- tion of melanterite is an acidity-generating close to saturation with respect to +3 process. Fe (SO4)(OH)2·20H2O) (Nordstrom melanterite, and its composition was and Alpers, 1999a,b; Frau, 2000), but found to be influenced by dissolution of regardless of the final mineral phase Once the acidic solutions emerge from this mineral. considered, these solids always store the piles to the surface, flow downstre- +2 am and accumulate in pools, the water large amounts of Fe (±Zn±Cu), SO4 Mineral phases controlling the solubi- and acidity in solid form, which are is partly evaporated and therefore all lity of Fe(III) and Al readily released during flushing, thus solutes (including H+) are re-concentra- originating very acidic and ferrous ted. If the Fe2+ and SO4-2 concentra- Although magnesium and calcium are

depósito legal: M-38920-2004 • ISSN: 1885-7264

Conferencia 40 Acid Mine Drainage in the Iberian Pyrite Belt: An Overview

usually very abundant in most acidic effluents and pit lakes of the IPB, these elements are highly soluble and commonly behave conservatively, only precipitating as efflorescent sulphates (mostly as epsomite and gypsum, respectively) in evaporative pools or margins of acidic effluents and AMD-impacted rivers. Conversely, iron and aluminum are much less soluble and precipitate as the pH of the acidic discharges is increased. At pH values below 2, both iron and aluminum are chiefly dissolved, and these metals may only precipitate from highly concentrated brines from evaporative pools, where oversatu- ration of secondary sulphates (e.g., melanterite, copiapite, halotrichite, jarosite, alunogen) is eventually reached. As pH increases (for example, by partial neutralization during mixing with pristine streams), iron is hydrolyzed and precipitates at pH>2.2, whereas Al tends to be hydrolyzed at pH~4.5-5.0 (Nordstrom and Alpers, 1999a; Bigham and fig 4. Examples of iron terraces formed in mine drainage systems of the IPB: (a) Tintillo river (near Corta Atalaya, Riotinto mines); (b) Perrunal creek (near La Zarza mine); (c) acidic effluent in the Filón Norte pit lake (Tharsis mine); (d) Acidic effluent Nordstrom, 2000; Figure 3f-h). The for- in the Lomero mine portal. These terraced iron deposits are mineralogically dominated by schwertmannite, also containing mation of these mineral phases plays minor amounts of other iron minerals (jarosite, goethite), gypsum, as well as algal and bacterial biofilms and plant debris an important role in the geochemical (modified from Sánchez-España et al., 2007b). Photographs by Enrique López Pamo. evolution of the AMD and pit lakes, as minor in relation to schwertmannite or tes; Brake et al., 2002, 2004), having discussed below. ferrihydrite. been also proposed as models for the cycling of iron on Mars (e.g., Iron (oxy)hydroxysulphates The chemical composition of schwert- Fernández-Remolar et al., 2004). mannites analyzed in the IPB district Geochemical, mineralogical, morpho- The solid formed by precipitation of includes minor amounts of SiO2 (0.4- logical and microbiological evidences Fe(III) at pH 2.5-3.5 during mixing and 3.8 wt.%) and Al2O3 (0.7-3.0 wt.%), obtained in the highly acidic and Fe- neutralization of the AMD solutions of which suggests either (1) some silicate rich Tintillo river (Riotinto mines, the IPB is usually schwertmannite (clay) contamination of samples, and/or Huelva; Sánchez-España et al., (Sánchez-España et al., 2005a, (2) some degree of sorption/coprecipi- 2007b; Figures 1d and 4a) suggest 2006a,b, 2007b, 2008). This mineral is tation of Si and Al onto the schwertman- that a number of organic and inorganic a poorly crystallized iron oxyhydroxysul- nite particles. Samples formed by titra- processes appear to control the for- phate whose formation from acid-sul- tion of acid solutions in the laboratory mation and internal arrangement of phate waters is described as follows have shown SiO2 contents of 0.4-1.5%, these iron terraces (Figure 4a). The (Bigham et al., 1994, 1996; Bigham which strengthens the second possibi- photosyntetic production of dissolved and Nordstrom, 2000): lity. Average compositions of the analy- oxygen by eukaryotic microorganisms zed Fe minerals also include significant (green algae, euglenophites and dia- +3 -2 8 Fe + SO4 + 14 H2O ⇔ trace element contents (e.g., 23-12770 toms), and the iron-oxidising metabo- Fe O (SO )(OH) + 22H+ (15) ppm As, 80-4800 ppm Cu, 5-6230 ppm lism of acidophilic prokaryotes are cri- 8 8 4 4 Zn, Sánchez-España et al., 2005a, tical factors for the formation of TIFs, In addition to schwertmannite, some 2006b). This suggests variable sorption whereas abiotic parameters such as other minerals of Fe(III) have been recog- of dissolved trace metals onto the water composition, flow rate and velo- nized, with a close relation between their schwertmannite, ferrihydrite and jarosi- city, or stream channel geometry, also occurrence and the water pH (Table 2). te mineral surfaces. appear to be essential variables. Jarosite is usually favoured to precipitate from very acidic solutions (Figure 3f), nor- Iron terraces forming in AMD effluents Aluminum (oxy)hydroxysulphates mally at pH<2. Schwertmannite precipitates near the discharge points at pH 2.5- An outstanding example of iron preci- If compared with the Fe(III) solids, the 3.5 (Figure 3g), whereas ferrihydrite pitation in AMD systems is the case of aluminum minerals formed at pH~4.5- usually forms in fluvial environments (as the iron terraces, which are unique 5.0 in the IPB mine sites are not so well in the confluences between AMD and geomicrobiological systems which can characterized. These solids are nearly unpolluted rivers) at pH>5. These three provide highly relevant information amorphous to XRD, although two broad minerals are meta-stable with respect to about the interaction between micro- reflections (near 7 and 20 o2θ) are goethite, which is the most stable form of bes and their surrounding aqueous usually recognized by XRD (Figure 2). Fe(III) at low temperature (Bigham et al., environments (Figure 4). These singu- 1996; Bigham and Nordstrom, 2000). lar systems can represent, additio- This diffraction pattern, along with their Goethite can also precipitate from AMD, nally, potential models for the study of chemical composition, suggest that although its presence in the particulate ancient geological formations (e.g., these waite Al precipitates probably fraction of AMD-impacted waters is very banded iron formations, stromatoli- consist of poorly ordered hydroxysulpha-

Conferencia invitada macla. nº 10. noviembre´08 revista de la sociedad española de mineralogía 41

tes with composition intermediate bet- effluent emerged with a pH of ween hydrobasaluminite and basalumi- 2.2, and subsequently converged nite (Figure 3h; see Bigham and with a number of small creeks of Nordstrom (2000) and Sánchez-España unpolluted water, thus provoking et al. (2006b), and references cited the- a slight but progressive pH incre- rein). Alunite and aluminite have been ase from around 2 to values also suggested to precipitate from close to 5. This pH increase allo- waters with high concentrations of Al wed the precipitation of Fe(III) and sulphate (0.1M and 0.4M, respecti- (mostly schwertmannite) during vely), although both minerals are rarely the first 11 km of the AMD cour- found as direct precipitates). se (pH<3), and of Fe(III) and Al compounds (Al as amorphous Hydrobasaluminite is metastable with hydro-basaluminite) during the respect to basaluminite, which forms by next 9 kms of the stream course dehydration of the former. Basaluminite (pH 3-5). The evolution shows is a common Al hydroxysulphate in mine that the precipitation of Fe(III) drainage environments, although it also resulted in a strong decrease of tends to be transformed to alunite the total iron loading to around a during maturation or heating (Bigham tenth of its initial content at only and Nordstrom, 2000). The hydrolysis 10 km from the source. The pre- of Al+3 to form hydrobasaluminite may cipitation of Al compounds in the be written as follows: last reach also accounted for an important loss of the Al loading. +3 -2 4Al + SO4 + 22-46 H2O ⇔ Interestingly, both the Fe and Al + Al4(SO4)(OH)10·12-36H2O + 10 H (16) solids acted as efficient sorbents for a number of trace elements. The formation of this mineral during Arsenic (present mainly as neutralization of AMD can also imply a -2 HAsO4 species, as suggested significant removal (by sorption) of toxic by geochemical modelling with trace elements (e.g., Cu, Cr, Zn, U, etc.) PHREEQC) showed a strong ten- from the aqueous phase (Sánchez- dency to be adsorbed by the España et al., 2006b). ferric iron colloids at low pH (<3), having been totally removed from Natural attenuation of metal concen- the water at less than 10 km trations due to metal precipitation and from the discharge point. Other sorption trace metals such as Cr(VI) - (mainly present as HCrO4 ), was Downstream from the sources, the geo- significantly sorpted onto the Al chemical evolution of AMD is usually compounds formed at pH~4.5-5, controlled by (1) oxidation of Fe(II) to also implying a total scavenging Fe(III), (2) progressive pH increase and of this toxic element at 20 km dilution of metal concentrations by from the source. Finally, divalent mixing with pristine waters, (3) hydroly- metal cations like Cu+2, Zn+2 and sis and precipitation of different metal Mn+2 behaved conservatively cations (e.g., Fe+3, Al+3) as pH increa- until pH~5, and then, they were ses, and (4) sorption of different trace fig 5. Spatial evolution of metal loadings of an acidic mine effluent in only slightly removed by sorption Tharsis mine: (a) Fe and Al; (b) As and Cr; (c) Cu, Zn, and Mn. Metal loa- elements (As, Pb, Cr, Cu, Zn, Mn, Cd) onto the Al minerals during the dings calculated from analyzed aqueous metal concentrations and measured flow rates at distinct sampling stations downstream from the sour- onto the solid surfaces of precipitated final reach. ce point (modified from Sánchez-España et al., 2005b, with kind permis- metal hydroxides/hydroxysulphates. sion of Springer Science and Business Media). The pH-dependent sequences of precipi- Such metal scavenging may imply impor- CONCLUSIONS tation and sorption are very similar and tant water quality improvements at a basin follow the order: scale. For example, it has been estimated The microbially-catalyzed oxidation of pyri- Fe(III) > Pb > Al > Cu > Zn > Fe(II) > Cd that only 1% of the total Fe and As dissol- te and other sulphides present in piles, (Dzombak and Morel, 1990; Nordstrom and ved load and around 20-40% of the total tailings, pits, shafts and galleries of the Alpers, 1999a). The overall result of these presently abandoned mines of the Iberian Al, Mn, Cu, Zn, Cd, Pb and SO4 dissolved processes represents a mechanism of load initially released from the mine sites, Pyrite Belt, generates an important volu- natural attenuation, as has been shown in was transferred from the Odiel river basin me of highly acidic and metal-polluted AMD-impacted rivers of the IPB (Sánchez- (including more than 25 studied mines) to water which drains the mining areas and España et al., 2005b, 2006a, 2007a, the Huelva estuary in 2003 (although this transport this acidity and metal content 2008). still represented very high metal fluxes; to streams, rivers and dumps, thus provo- Sánchez-España et al., 2005a). Most of king and important environmental dama- As an example of this self-mitigating the iron and arsenic load seems to remain ge as regards to the water quality of the capacity, the graphs provided in Figure 5 in solid form near the pyrite sources within water resources. The singular chemical show the spatial evolution of metal loa- the mines, mainly in ochreous Fe(III) preci- composition of the acidic mine waters, dings in an acidic effluent emerging pitates covering the stream channel, whe- dominated by a low pH and high concen- from a waste-pile in Tharsis mine reas the rest of metal cations (Al, Cu, Zn, trations of sulphate and metals (espe- (Sánchez-España et al., 2005b). This Mn) migrate further downstream. cially Fe and Al), gives rise to a number of

depósito legal: M-38920-2004 • ISSN: 1885-7264 Conferencia 42 Acid Mine Drainage in the Iberian Pyrite Belt: An Overview

characteristic mineral phases that preci- other localities. Mineral. Mag., 58, 641- _ , Schrenk, M.O., Hamers, R., Banfield, pitate downstream from the discharge 648. J.F. (1998): Microbial oxidation of pyrite: points as the acidic waters are neutrali- Experiments using microorganisms from an zed by mixing with pristine waters (e.g., _ , Schwertmann, U., Traina, S.J., Winland, extreme acidic environment. Am Mineral, R.L., Wolf, M. (1996): Schwertmannite and 83, 1444-1453. schwertmannite at pH>2.5, hydrobasalu- the chemical modeling of iron in acid sulfa- minite at pH>4.5) and/or by evaporation te waters. Geochim. Cosmochim. Acta, 60, Elbaz-Poulichet, F., Morley, N.H., Cruzado, in the margins of streams or in pools 2111-2121. A., Velasquez, Z., Achterberg, E.P., (e.g., eflorescent sulphate salts like Braungardt, C.B. (1999): Trace metal and melanterite, rozenite, epsomite, hexahy- _ , Nordstrom, D.K. (2000): Iron and nutrient distribution in an extremely low drite, coquimbite, halotrichite, copiapite, Aluminum Hydroxysulfates from Acid pH(2.5) river-estuarine system, the Ria of Sulfate Waters. In Sulfate Minerals: Huelva (South-West Spain). Sci. Total etc.). The iron and aluminum precipitates Crystallography, Geochemistry, and Environ., 227, 73-83. usually sorb trace metals and are thus Environmental Significance (Alpers, C.N., responsible for a certain degree of natu- Jambor, J.L., Nordstrom, D.K. (Eds.). Ehrlich, H.L. (2002): Geomicrobiology. 4th ral attenuation, although this self-mitiga- Reviews in Mineralogy & Geochemistry, 40, Ed. (Marcel Dekker, New York). ting mechanism can be reversible and the 351-403. trace metals may be released to the solu- Fernández-Remolar, D., Gómez-Elvira, J., tion phase as the mineral phases are Braungardt, C.B., Achterberg, E.P., Elbaz- Gómez, F., Sebastian, E., Martiin, J., Poulichet, F., Morley, N.H. (2003): Metal Manfredi, J.A., Torres, J., González Kesler, mineralogically matured (e.g., transfor- geochemistry in a mine-polluted estuarine C., and Amils, R. (2004): The Tinto River, med to more stable phases). The sulpha- system in Spain. Appl. Geochem., 18, an extreme acidic environment under con- te salts are also temporal sinks of acidity 1757-1771. trol of iron, as an analog of the Terra and metals (especially Fe, Al, Cu and Zn), Meridiani hematite site of Mars. Planetary but these elements are readily re-incorpo- Brake, S.S., Hasiotis, S.T., Dannelly, H.K., and Space Science, 52, 239–248. rated to the aqueous phase due to redis- Connors, K.A. (2002): Eukaryotic stromato- solution of these sulphates during rains- lite builders in acid mine drainage: Frau, F. (2000): The formation-dissolution- Implications for Precambrian iron and oxy- precipitation cycle of melanterite at the torm events. The problem of acid mine genation of the atmosphere?. Geology, 30, abandoned pyrite mine of Genna Luas in drainage in the IPB is of regional scale, 599–602. Sardinia, Italy: environmental implications. and any attempt to mitigate or attenuate Mineral Mag, 64, 995-1006. its environmental impact should be affor- _ , Hasiotis, S.T., and Dannelly, H.K. ded by all the government authorities (2004): Diatoms in Acid Mine Drainage and García García, G. (1996): The Rio Tinto involved in decision making and environ- their role in the formation of iron-rich stro- Mines, Huelva, Spain. Mineral Rec., 27, matolites. Geomicrobiology Journal, 21, 275–285. mental quality regulation. 331–340. Garrels, R.M. & Thompson, M.E. (1960): ACKNOWLEDGEMENT Buckby, T., Black, S., Coleman, M.L., Oxidation of pyrite by iron sulphate solu- Hodson, M.E. (2003): Fe-sulphate-rich eva- tions. Am J Sci, 258A, 57-67. I wish to thank Dr. Isabel González for porative mineral precipitates from the Río her kind invitation to present this com- Tinto, southwest Spain. Mineral. Mag., 67, Gehrke, T., Hallmann, R., Sand, W. (1995): munication to the workshop organized 263-278. Importance of exopolymers from Thiobacillus ferrooxidans and Leptospirillum ferrooxidans by the University of Seville. The content Cossa, D., Elbaz-Poulichet, F., Nieto, J. M. for bioleaching. (In T. Vargas, C.A. Jerez, K.V. of this paper has benefitted from data (2001): Mercury in the Tinto-Odiel Wiertz & H. Toledo (Eds.) Biohydrometallurgical partly or totally obtained during the Estuarine System (Gulf of Cádiz, Spain): Processing, vol I, (pp. 1-11), Universidad de course of several research projects fun- Sources and Dispersion. Aquat. Geochem., Chile, Santiago.) ded by IGME. 7, 1–12. González-Toril, E., Llobet-Brossa, E., REFERENCES Davis Jr., R.A., Welty, A.T., Borrego, J., Casamayor, E.O., Amann, R., Amils, R. Morales, J.A., Pendon, J.G., Ryan, J.G. (2003): Microbial ecology of an extreme (2000): Rio Tinto estuary (Spain): 5000 acidic environment, the Tinto River. 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depósito legal: M-38920-2004 • ISSN: 1885-7264