*Revised Manuscript STUDY OF ARSENOPYRITE WEATHERING PRODUCTS IN MINE WASTES 1 2 FROM ABANDONED TUNGSTEN AND TIN EXPLOITATIONS 3 4 Murciego A.a, Álvarez-Ayuso E.b,*, Pellitero E.a, Rodríguez Mª.A.c, García-Sánchez 5 b d d d e 6 A. , Tamayo A. , Rubio J. , Rubio F. and Rubin J. 7 a 8 Department of Geology. Plza. de los Caídos s/n. Salamanca University, 37008 9 10 Salamanca (Spain). 11 b 12 Department of Environmental Geochemistry. IRNASA (CSIC). Apdo. 257, 37071 13 14 Salamanca (Spain). 15 c 16 Faculty of Sciences, Crystallography and Mineralogy Area, Avd. Elvas s/n. 17 18 Extremadura University, 06071 Badajoz (Spain). 19 d 20 Ceramic and Glass Institute (CSIC). c/ Kelsen, 5, 28049 Cantoblanco, Madrid (Spain). 21 e 22 Material Science Institute of Aragón, CSIC-Zaragoza University. c/ María de Luna 3, 23 24 50009 Zaragoza (Spain). 25 26 *Corresponding author. Fax number: +34 923219609. E-mail: [email protected] 27 28 29 30 ABSTRACT 31 32 Arsenopyrite-rich wastes from abandoned tungsten and tin exploitations were studied to 33 34 determine the composition and characteristics of the secondary phases formed under 35 natural weathering conditions so as to assess their potential environmental risk. 36 37 Representative weathered arsenopyrite-bearing rock wastes collected from the mine 38 39 dumps were analysed using the following techniques: X-ray powder diffraction (XRD) 40 41 analysis, polarizing microscopy analysis, electron microprobe analysis (EMPA) and 42 43 microRaman and Mössbauer spectroscopies. Scorodite, pharmacosiderite and 44 45 amorphous ferric arsenates (AFA) with Fe/As molar ratios in the range 1.2-2.5 were 46 identified as secondary arsenic products. The former showed to be the most abundant 47 48 and present inAccepted the different studied mining areas Manuscript. Its chemical composition showed to 49 50 vary in function of the original surrounding rock mineralogy in such a way that 51 52 phosphoscorodite was found as the mineral variety present in apatite-containing 53 54 geoenvirons. Other ever-present weathering phases were goethite and hydrous ferric 55 56 oxides (HFO), displaying, respectively, As retained amounts about 1 and 20% 57 (expressed as As O ). The low solubility of scorodite, the relatively low content of AFA 58 2 5 59 and the formation of compounds of variable charge, mostly of amorphous nature, with 60 61 62 63 1 64 Page 1 of 30 65 high capacity to adsorb As attenuate importantly the dispersion of this element into the 1 2 environment from these arsenopyrite-bearing wastes. 3 4 5 6 Keywords: Arsenopyrite, arsenic, mine wastes, secondary or weathering products, 7 8 environmental risk assessment. 9 10 11 12 1. INTRODUCTION 13 14 Arsenic is currently regarded as one of the most toxic inorganic pollutants, being 15 16 responsible for severe environmental and health impacts. Contamination with As is a 17 18 global concern due to its worldwide distribution. In nature As occurs as a constituent of 19 different minerals, but its release has been promoted by anthropogenic activities such as 20 21 mining. More than 300 As minerals are known, with approximately 60% being 22 23 arsenates, 20% sulphides and sulphosalts and the remaining 20% including arsenides, 24 25 arsenites, oxides and elemental As [1,2]. Sulphide deposits are the main mineral source 26 27 of As, in which this element can be present in high concentrations. Particularly, 28 29 arsenopyrite (FeAsS) is the most common As-bearing mineral, being found in a range 30 of ore deposits. Thus, arsenopyrite is the ubiquitous As-bearing mineral in a variety of 31 32 hydrothermal environments, from high-temperature magmatic-hydrothermal porphyry- 33 34 style Sn-W and Cu(±Au) deposits to mesothermal polymetallic Cu-Pb-Zn-Ag and gold 35 36 deposits [3]. These deposits have been extensively exploited for the economic ore 37 38 minerals, inducing a wide legacy of As in mine spoils. 39 40 Arsenopyrite is stable under reducing conditions, and mine tailings containing 41 42 arsenopyrite should be chemically stable during long-term storage provided they are 43 kept water-saturated and moderately reduced [4]. Conversely, under oxidising 44 45 conditions the mobilisation and migration of As into the environment can take place. 46 47 All over the world, there are many historic derelict mines that have generated a large 48 Accepted Manuscript 49 amount of arsenopyrite-bearing waste rocks and tailings. These wastes have been poorly 50 51 managed, allowing oxidation processes to occur. Arsenopyrite can be easily oxidised by 52 3+ 53 both O2 and Fe , this process being promoted by micro-organisms, especially by 54 acidophilic Fe- and S-oxidising bacteria [5]. 55 56 Secondary As compounds such as scorodite (FeAsO ·2H O), pharmacosiderite 57 4 2 58 (KFe4(AsO4)3(OH)4·6-7H2O), arsenolite (As2O3), arseniosiderite 59 60 (Ca2Fe3(AsO4)3O2·3H2O) and amorphous ferric arsenate-(sulphate) have been reported 61 62 63 2 64 Page 2 of 30 65 as the most common products of arsenopyrite weathering in natural environments, 1 2 together with other secondary compounds such as goethite (FeO(OH)), ferrihydrite 3 (Fe O ·0.5H O) or amorphous hydrous ferric oxides (HFO) [6-11]. The genesis of these 4 2 3 2 5 secondary products is dependent on the original ore mineralogy and on the processes 6 7 that have occurred during their formation (mainly redox and pH changes) [2]. The 8 9 release of As from mine wastes is controlled by precipitation-dissolution and 10 11 adsorption-desorption reactions involving the secondary compounds generated during 12 13 the weathering of arsenopyrite [12]. Therefore, their identification and characterisation 14 are crucial for the risk assessment and environmental control of the historic 15 16 arsenopyrite-bearing mine spoils. Whereas many works studying wastes from gold 17 18 mining areas have been performed [4,7,8,11,13-17], only some efforts have been 19 20 devoted to study wastes from tungsten- and tin-mining districts [18-20]. 21 22 The aim of this work is to study the mine wastes from abandoned tungsten and tin 23 24 exploitations in order to identify and characterise the secondary products generated 25 26 from arsenopyrite weathering and to assess the potential environmental risk of such 27 wastes. 28 29 30 31 32 2. MATERIALS AND METHODS 33 34 2.1. Study areas 35 36 Two abandoned mine exploitations were considered in this study, the Barruecopardo 37 38 and Terrubias mines, located, respectively, in the north-west and centre of the 39 Salamanca province, Spain (Figure 1). The Barruecopardo mine corresponds to the most 40 41 important tungsten deposit in Spain, exploited extensively from 1912 to 1983. This is 42 43 constituted by subvertical quartz veins disposed in parallel intragranitic bands. The 44 45 main minerals present are scheelite (CaWO4) and wolframite ((Fe,Mn)WO4), which 46 47 constitute the ore, and arsenopyrite, pyrite (FeS2), chalcopyrite (CuFeS2) and ilmenite 48 (FeTiO3) as primaryAccepted minerals. These minerals appearManuscript in the veins and on the borders of 49 50 granite in which apatite (Ca (PO ) (OH,F,Cl,Br) ) appears as an accessory mineral 51 10 4 6 2 52 [21]. The Terrubias mine corresponds to a tungsten- and tin-rich deposit worked 53 54 intermittently during the last century, and whose exploitation ceased more than three 55 56 decades ago. Two Sn-W mineralisation types are present in this mining area: stratiform, 57 58 in calcosilicate bands, and in subvertical veins of NNE-NE and N100-130E directions, 59 60 cutting the scheelite-rich calcosilicate bands. The paragenesis of these veins is 61 62 63 3 64 Page 3 of 30 65 dominated by quartz, mica, tourmaline, wolframite, scheelite, cassiterite (SnO2) and 1 2 arsenopyrite [22]. 3 4 2.2. Characterisation of weathered arsenopyrite-bearing wastes 5 6 About 25 samples of weathered arsenopyrite were collected from the Barruecopardo 7 8 and Terrubias mining areas. All these samples were surface samples. In the studied 9 mining areas wastes were dispersed on surface or accumulated in very small dumps 10 11 where the existence of weathering profiles was not observed. The weathering products 12 13 show yellow-greenish colours, graduating towards orange-reddish tonalities in the 14 15 sample borders, where these display numerous holes (Figs. 2a and 3a). Such weathered 16 17 arsenopyrite samples were studied by X-ray powder diffraction (XRD) analysis, 18 19 polarizing microscopy analysis, electron microprobe analysis (EMPA) and microRaman 20 and Mössbauer spectroscopies. X-ray diffraction analyses were performed on a D8 21 22 Advance Bruker diffractometer equipped with a diffracted-beam graphite 23 24 monochromator, using the CuK radiation (λKα =1.54Å). Solids were scanned as 25 26 unoriented powder samples from 4 to 60º 2 with a 0.04 2 step and a 1 s per step 27 28 counting time. The analyses by polarizing microscopy were carried out on polished thin 29 30 sections using a Leica DMLP microscope in both transmitted and reflected light. 31 32 Electron microprobe analyses for chemical analyses and for electron backscatter images 33 were conducted on carbon-coated polished thin sections in wavelength-dispersive mode 34 35 using a JEOL Superprobe JXA-8900 M electron probe microanalyser. The analysed 36 37 elements were Al, Mn, P, Ba, As, Si, Fe, S, K, Cu, Pb, Ca and Zn. Operating conditions 38 39 were an accelerating voltage of 15 kV and a beam current of 2 nA with a spot size of 1- 40 41 5 µm and spot analysis and background collection times of 10 and 5 s, respectively. 42 43 Calibration was carried out using the following standards: sillimanite for Al, almandine 44 for Fe, pyrolusite for Mn, apatite for P, witherite for Ba, gallium arsenide for As, albite 45 46 for Si, galena for S and Pb, microcline for K, chalcopyrite for Cu, kaersutite for Ca and 47 48 gahnite for Zn.Accepted The microRaman spectra of polishedManuscript thin sections of samples were 49 50 recorded with a Via Renishaw spectrometer equipped with a Charge Coupled Device 51 52 (CCD) detector.