Sulphur Isotopes, Trace Elements and Mineral ...

Water Air Soil Pollut (2008) 192:85–103 DOI 10.1007/s11270-008-9637-8

Sulphur Isotopes, Trace Elements and Mineral Stability Diagrams of Waters from the Abandoned Fe–Cu Mines of Libiola and Vigonzano (Northern Apennines, Italy) Gianni Cortecci & Tiziano Boschetti & Enrico Dinelli & Roberto Cabella

Received: 5 November 2007 / Accepted: 27 January 2008 / Published online: 17 February 2008 # Springer Science + Business Media B.V. 2008

Abstract The geochemical characteristics of rills draining pyrite-chalcopyrite tailings impoundments and of bordering streams were investigated at the ophiolite-hosted Libiola and Vigonzano abandoned massive sulphide mines, northern Apennines Italy. G. Cortecci (*) Istituto di Geoscienze e Georisorse, Area della Ricerca-CNR, Via Moruzzi 1, I-56124 Pisa, Italy e-mail: [email protected] T. Boschetti Dipartimento di Scienze della Terra, Università di Parma, Viale Usberti 157a, I-43100 Parma, Italy E. Dinelli Centro Interdipartimentale di Ricerca per le Scienze Ambientali, Alma Mater Studiorum-Università di Bologna, Centro di Ravenna, Via Sant’Alberto 163, I-48100 Ravenna, Italy E. Dinelli Dipartimento di Scienze della Terra e Geologico-Ambientali, Alma Mater Studiorum-Università di Bologna, Piazza Porta San Donato1, I-40126 Bologna, Italy R. Cabella Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Corso Europa 26, I-16132 Genova, Italy

Water samples were analysed for major and trace chemical composition, hydrogen and oxygen isotope composition, and sulphur isotope composition of aqueous sulphate. Sulphur isotope composition was determined also for some samples of ore sulphides. At Libiola, the newly acquired chemical results on waters corroborate those from previous investigations, thus providing additional support to existing geochemical models in terms of metal distribution, solid phases precipitation, reaction path modelling and mixing reaction paths, and environmental problems. At Vigonzano, the chemical characteristics of waters are similar to those at Libiola. In both localities, solution-secondary phase equilibria estimated using an updated thermodynamic dataset account for mineralogy in the field, including poorly crystalline phases like jurbanite and hydrowoodwardite. The hydrogen and oxygen isotope composition of waters at Libiola and Vigonzano agrees with their meteoric origin. Acid to neutral mine waters do not show any significant isotope shift with respect to the initial water, in spite of the oxidation of even large amounts of pyrite/chalcopyrite ore. The sulphur isotope composition of aqueous sulphate in mine rills at Libiola (δ34S=5.6 to 8.5‰; mean 6.5‰) matches that of massive sulphide ore (δ34S=−0.5 to 6.7‰; mean 5.8‰), in keeping with the supergenic origin of the sulphate and related isotope effects in the sulphide oxidation process. Sulphate in mine waters at Vigonzano displays lower δ34S values in the range 0.6 to

86

1.5‰. The δ34S signature of massive ore specimens is within the range reported for most volcanic-hosted massive sulphide deposits, including Cyprus-type deposits. Keywords Apennine ophiolites . mine water . sulphidic mine tailings . sulphur isotopes . water isotopes

1 Introduction Massive sulphide ore deposits have great potential environmental concerns, which include human health risks, ecosystem risks and physical hazards (Seal et al. 2002; Seal and Hammarstrom 2003). The ecosystem risks are related to the acid mine drainage (AMD) especially in abandoned mine sites. AMD is originated by the oxidation of sulphides, mostly pyrite, and leads to the mobilization of metals. This process has been recognized as a major environmental issue, and geochemical, pollution, health and management issues have been summarized in several important reference works dealing with geoenvironmental models of mineral deposits or mine wastes (Alpers et al. 1994; Plumlee and Logsdon 1999; Filipek and Plumlee 1999; Jambor et al. 2003; Lottermoser 2003). The study sites are located in the northern Apennines (Italy). In particular the Libiola mine is located in the drainage basin of the Gromolo River, about 8 km NE of the town of Sestri Levante on the Ligurian Sea coast (Fig. 1). The ore deposit is stratabound, and occurs in pillow lavas and pillow breccias of the Internal Ligurides (Eastern Liguria province) at the floor of the sedimentary cover (late Jurassic-Cretaceous) (Ferrario and Garuti 1980, and references therein). The lavas are partially overthrusted by serpentinites with local intercalations of rodingitized gabbros. The deposit consists mostly of massive lenses of pyrite and chalcopyrite (and minor sphalerite) nearly concordant with the pillow layering. Gangue minerals are scarce and include mainly carbonates and some quartz (Bertolani 1952). Ore also exists as disseminations (small aggregates) in pillows and host volcanic breccia, and as stockwork discordant with respect to the lava beds. No primary sulphate minerals were identified in the deposit (Bertolani 1952; Ferrario 1973). The presence in the ore of strongly deformed and fractured chalcopyrite crystals testifies

Water Air Soil Pollut (2008) 192:85–103

for the Alpine tectonics undergone by the deposit (Bertolani 1952). A submarine hydrothermal origin was suggested by Spooner et al. (1974) and Bonatti et al. (1976) for all the three types of ore occurrences, whereas some doubts were raised by Ferrario and Garuti (1980) for the massive and disseminated ones which they thought may be magmatic, that is closely related to the volcanic products. The economic exploitation of the Libiola mine started in the XVII century and ended in 1965. The mine tailings, located at 335 and 230 m a.s.l., extend over about 0.5 km2 and are crossed by rills that convey in the Gromolo, Rio Boeno and Rio Cattan creeks (see Fig. 1). The latter two are tributaries of the Gromolo creek. Previous hydrochemical studies in the Libiola mine area were carried out by Cortecci et al. (2001), Dinelli et al. (2001) and Marini et al. (2003) on acid to near neutral waters from the tailings and polluted and unpolluted bordering stream waters. To be also mentioned the geochemical modelling by Accornero et al. (2005), who investigated the fate of major and trace elements in acid water from the Libiola mine after mixing with the Gromolo stream freshwater. These papers deal with sampling campaigns carried out in different seasons, i.e. November 1996, April 1997, May 1997, December 1997, May 1998, October 1998 and May 2000. Additional water samples from acidic discharges and the Gromolo stream were analysed in July to October 2003 and April 2004 (Accornero et al. 2005). The ore deposit of the derelict Vigonzano mine is located at 750–800 m a.s.l. in the vicinity of the homonymous village in the Piacenza province (EmiliaRomagna region), about 30 km inland from the Libiola mine, on the other side of the Apennine divide. As the latter, it consists mostly of massive pyrite and chalcopyrite, hosted within serpentinites and basalt breccias (External Ligurides), at the boundary between ophiolitic rocks and pelitic sediments (Dinelli et al. 1996, and references therein); it should be also related to Jurassic-Cretaceous submarine volcanism, and then underwent intense tectonic deformation during the Alpine orogeny. The mine tailings at Vigonzano cover a surface of about 0.3 km2 and are crossed by rills, which convey in the Vigonzano creek. Water sampling were carried out in November 1997 from three sites along an


87

Fig. 1 The Libiola and Vigonzano mine tailings impoundments within the Apennine context and sampling location of selected stream and mine waters (see Tables 2, 3 and 4). Water samples were analysed for chemical and isotopic (H, O and S) compositions (complete), for chemical and isotopic (H and O; no S isotopes) compositions, or only for chemical composition (no isotopes)

ephemeral rill issuing from an elevated exploratory pit, and in June 1998 from only the uppermost site fed by a quite small water flow. In June 1998, a blue deposit was observed along the uppermost part of the rill. The Rio Vigonzano was also sampled just downstream of the rill both in November 1997 and June 1998. In the present work, water samples (Libiola: December 1997, May and October 1998; Vigonzano:

November 1997, June 1998) were analysed for major and trace chemical elements, and for hydrogen and oxygen isotopes of water and sulphur isotopes of dissolved sulphate (Libiola: December 1997; Vigonzano: Novembre 1997). In addition, ore samples and vein sulphides in the rocks were also analysed for the sulphur isotope composition. The present work was aimed to (1) complement the extended abstract by Cortecci et al. (2001) on the Libiola mine with a more

88

complete presentation and a deeper discussion of the isotopic data dealing with water and ore samples, (2) integrate and extend geochemical modelling of acid mine waters and their evolution, (3) compare the original data on the Vigonzano mine with those on the Libiola mine, the ore deposits being nearly coeval and both hosted in basalts, (4) use the hydrogen and oxygen isotopes of water as natural tracers of hydrologic processes in acid mine drainage systems, (5) verify the sulphur isotope fractionation involved in the sulphide mineral supergenic oxidation, and (6) provide constraints on the origin of the ore deposits.

2 Previous Studies Waters from the Libiola mine tailings include acid terms from oxidative drainage of pyrite and chalcopyrite ore. These waters are “red” in the most acidic sites (pH 2.4–2.8), and their colour is due to very fine suspended Fe-particulate. They are oversaturated with respect to jurbanite and close to saturation with respect to ferrihydrite (Marini et al. 2003). The red waters neutralize to “blue” waters (pH 6.5–7.5) through water–rock interaction (Dinelli et al. 2001), reaching saturation/oversaturation with respect to gibbsite and basaluminite, and saturation relative to antlerite, brochantite and alunite. Both red and blue waters are slightly undersaturated with respect to gypsum (Marini et al. 2003). However, identified minerals in red waters were amorphous silica, Fe(III)oxhydroxides, schwertmannite and jarosite (Dinelli et al. 1998; Derron 1999; Dinelli and Tateo 2002), and probably basaluminite, gibbsite and a Cu-Al sulphate hydrate (azure-blue coloured) are the precipitates in blue waters (Derron 1999). Depending on the water discharge, the sediments may change seasonally their colour. In these cases, the water-sediment pairs are located at higher elevation in the mine area and are affected by considerable fluctuations of the water table. The colour of sediments was found to change from ochre (Fe-rich) to blue-green (Cu-rich), following a pH variation of the water from acid to near neutral (Dinelli and Tateo 2002). Red waters issue from exploratory pits located in the bottom of the Gromolo River valley (Ida and Castagna tunnels, at 106 and 72 m a.s.l., respectively), whereas blue waters are discharged by a higher


tunnel near to the Rio Cattan stream (Margherita tunnel; 209 m a.s.l.), a tributary of the Gromolo River. All these sites display a rather constant water flow rate over the year, testifying that the discharge point is close to the intersection of the water table with the topographic surface (Dinelli et al. 2001), and the sediments maintain their colour all over the year. According to Marini et al. (2003), the main process governing the chemical composition of all the mine waters of Libiola (red, blue and intermediate) is their interaction to variable extents with the waste material. Mixing between acid mine waters (red to orange) and local groundwater should be insignificant, that is all mine waters at Libiola derive from the interaction of percolating meteoric water with mine tailings or with ore. In both red and blue waters, sulphate is the dominant anion; calcium and magnesium are the main cations, reflecting the acid alteration of local rocks (Dinelli et al. 2001; Marini et al. 2003). Mg is particularly rich in the acid waters, which greatly alter the mafic silicates present in the tailings and country rocks. At times, iron is the prevailing cation in the red water, due to an appropriate combination of ore oxidation and pH of solution (Dinelli et al. 2001). Finally, red waters are richer in metals (Al, Fe, Cu, Zn and Mn) than blue waters. Water from the Gromolo River and Rio Boeno upstream of the mined area is Mg (± Ca)-HCO3 and comparatively very diluted for both major and trace constituents (Dinelli et al. 2001; Marini et al. 2003). The chemical composition of these surface waters match that of spring water issuing from serpentinites in Liguria and Emilia Romagna regions, its chemical composition varying from Ca-HCO3 to Mg-HCO3 to Ca–(Na)–OH (Bruni et al. 2002; Boschetti and Toscani 2008). At Vigonzano, previous studies dealt with the geochemical characterization of the waste rock piles from the mining works (Dinelli et al. 1996), the transfer of metals to vegetation (Dinelli and Lombini 1996) and the element mobility in the surrounding area (Dinelli and Tateo 2001a, b) with particular reference to the role played by fine-grained authigenic particulate (Dinelli et al. 1998). Some data on surface waters (Dinelli and Tateo 2001a) indicate that there is acidic water circulation, just limited to the mine waste area and not affecting consistently the metal chemistry of stream waters.


89

3 Sampling and Methods The sampling sites at Libiola in December 1997 to October 1998 are shown in Fig. 1. Water samples were collected from rills draining the waste dumps

and the streams bordering the mining area (Gromolo River and Rio Boeno). Temperature, pH, Eh, EC and alkalinity were measured in the field. Water was filtered through 0.45 μm membranes of cellulose acetate, and portions stored for analyses. The portion

Table 1 Thermodynamic data of aqueous species and mineral phases added to the thermo.com.V8.R6 dataset Aqueous species and mineral phases

ΔG0f kJ=mol ΔH0f kJ=mol LogKa (25°C)

logKa (0°C)

Reference

AlðOHÞ 4 AlðSO4 Þ 2 AlSOþ 4 Al2 ðOHÞ4þ 2 AlðOHÞ03 Al13 O4 ðOHÞ7þ 24 Al3 ðOHÞ5þ 4 CuSO04 CuOH+

– –

– –

– – – – −691.76 −126.16

– – – – −838.81 –

−22.1567 −4.9029 −3.0096 7.67 16.17 98.71 13.86 −2.3607 7.9500

−25.4631 −4.7131 −2.9442 9 18.44 125.19 17.64 −2.2789 –

CuHCOþ 3 CuðOHÞ02

−535.45 −315.75

– –

−2.41 16.30

– –

Cu2 ðOHÞ2þ 2

−248.06

–

10.36

–

thermo.dat thermo.dat thermo.dat thermo.dat thermo.dat thermo.dat thermo.dat Vieillard 1988 Vieillard 1988; Powell et al. 2007 Vieillard 1988 Vielliard 1988; Powell et al. 2007 Vieillard 1988

Antlerite – Bayerite Basaluminite

Cu3SO4(OH)4(H2O)2 Cu3SO4(OH)4 Al(OH)3 amorphous α–Al(OH)3 Al4(OH)10SO4

−1919.6 −1445.0 −1137.63 −1149.8 −4937.199b

−2303.8 −1726.7 −1285.04 −1282.71 −5516.73

8.9718 9.0130 10.78 8.6497 22.7

10.9561 11.0842 12.5599 10.4647 29.1909

Bonattite

CuSO4(H2O)3

−1399.36

−1675.73

−1.67

−1.42

Copiapite Coquimbite – Ferricopiapite Halotrichite Hydrowoodwardite

Fe5(SO4)6(OH)2(H2O)20 Fe1.47Al0.53(SO4)3(H2O)9.65 Fe2(SO4)3(H2O)5.03 Fe4.78(SO4)6(OH)2.34(H2O)20.71 FeAl2(SO4)4(H2O)22 Cu0.83Al0.17(OH)2(SO4)0.085 (H2O)1.1 Al(OH)SO4 MgAl2(SO4)4(H2O)22 (H3O)Fe(SO4)2(H2O)3 Fe8O8(OH)4.4(SO4)1.8 FeSO4(H2O)5 Cu(OH)2

−1510.75 −9971 −3499.7 −3499.7 −9036.9 −786.91

−5738.4 −11824 −4115.8 −4115.8 −11041 −951.64

−22.04 −6.94 −6.8 −12.89 −8.13 7.0252

−18.63 −4.4 −4.4 −9.08 −7.25 7.8073

Pollard et al. 1992 Pollard et al. 1992 wateq4f.dat Verdes et al. 1992 wateq4f.dat; Singh 1980; this study Vieillard 1988; Chou et al. 2002 Hemingway et al. 2002 Majzlan et al. 2006 Majzlan et al. 2006 Majzlan et al. 2006 Hemingway et al. 2002 this work

−1487.692 b −9674.88 −2688 −4378.07 b −2033.9 −359.35

−1635.22 – −3201.1 −5052.85 −2424.3 −443.10

−3.23 −8.73 −2.05 7.17 −2.11 8.66

−1.6489 – −0.76 16.05 −2 9.6697

wateq4f.dat; this study Reardon 1988 Majzlan et al. 2006 Accornero et al. 2005 Hemingway et al. 2002 Vieillard 1988

Aqueous species

Mineral phases

Jurbanite Pickeringite Rhomboclase Schwertmannite Siderotil Spertiniite

Last version of the original thermo.com.V8.R6 and thermo.dat files are available in Hydrogeology Program, Department of Geology, University of Illinois (retrieved January 11, 2008, from http://www.geology.uiuc.edu/Hydrogeology/hydro_thermo.htm). See text for details a

Note that in phreeqc’s datasets the solution species are in the right side of reaction, therefore the sign must be inverted.

b

Calculated from logK(25°C).

90


for determination of metals was added with suprapure HNO3. Within a few days after collection, all water samples were analysed for Cl (Idrimetric-Kits by Carlo Erba), SO4 (turbidimetry/spectrophotometry; HACH reagent and instrument), Na, K, Ca, Mg, Fe, and Mn (AAS), Zn, Cu, Cd, Co, Ni, Cr, Ba and Pb (GF-AAS), and Al (turbidimetry/spectrophotometry; HACH reagent and instrument). Precisions were of 3 to 5% for major elements and 5–10% for minor and trace elements. Detection limits were: 1 μg/L for Cd and Pb, 2 μ/L for Cu, Zn, Cr and Ba, 5 μg/L for Ni and Co, and 10 μg/L for Al, Fe and Mn. In the colourless waters at Libiola (see Table 3), Cr and Cd were determined after preconcentration by solvent extraction technique (Danielsonn et al. 1978).

Aqueous sulphate for the sulphur isotope analysis was separated as BaSO4, which was thermally decomposed to SO3, and then reduced to SO2 for the spectrometric measurement, using a method similar to that of Holt and Engelkemeir (1970). Ore samples were checked for main mineralogy by XRD. Total ore samples and mineral separates were analysed, the latter being not pure phases, but mixtures in variable proportions of pyrite and chalcopyrite (± sphalerite). Before combustion in a stream of pure oxygen to produce SO2 for the mass spectrometric analysis (Thode et al. 1961), ore samples were treated with diluted HCl in an ultrasonic bath to remove alteration products, carbonates and oxides. The results are reported in δ34S unit, in per mil, relative to the CDT

Table 2 Chemical and isotopic composition data on coloured waters from the mine tailings at Libiola Sampling site

Colour T°C pH EC (μS/cm) Eh (mV) HCO3 (mg/L) Cl SO4 Na K Ca Mg Fe Zn Cu Al Mn Cd (μg/L) Ni Co Pb Ba Cr δ34S(SO4) δ18O(H2O) δ2H(H2O)

18 (1)

18 (2)

18 (3)

22 (2)

22 (3)

16 (1)

16 (2)

16 (3)

17 (1)

17 (2)

17 (3)

Red 12 2.5 7670 466 nd nd 5100 24.9 1.4 340.2 306 891 34 221 265 9.9 130 6400 4140 7