Distribution of trace elements in fracture fillings from the “Mina Fe” uranium deposit (Spain) by sequential leaching: implications for the retention processes

Applied Geochemistry Applied Geochemistry 20 (2005) 487–506 www.elsevier.com/locate/apgeochem

Distribution of trace elements in fracture fillings from the ‘‘Mina Fe’’ uranium deposit (Spain) by sequential leaching: implications for the retention processes A.J. Quejido *, L. Pe´rez del Villar, J.S. Co´zar, M. Ferna´ndez-Dı´az, M.T. Crespo CIEMAT/DIAE/Avda. Complutense 22, 28040 Madrid, Spain Received 6 November 2003; accepted 20 September 2004 Editorial handling by M. Gascoyne

Abstract Sequential leaching methods have been used to determine the mineralogical distribution of some trace elements for environmental purposes, such as radiological contamination of soils and sediments, bioavailability studies and natural analogues of deep geological radwaste disposals. In this context, a 7-step-sequential leaching protocol is applied to Fe(III)–U(VI)-rich fracture filling materials from the oxidised zone of the ‘‘Mina Fe’’ U deposit to identify and evaluate the main sinks of natural nuclides and other analogue trace elements, since it is crucial in the performance assessment of a nuclear waste repository. After a careful characterisation of the samples, the analytical data from each leaching step were statistically analysed and then interpreted in light of the mineralogical and geochemical features of the samples. Precise knowledge of the mineralogical distribution of trace elements by sequential leaching methods is quite complex, mainly due to crosscontamination throughout the different steps of the experiments. Thus, the results obtained suggest that U is retained as U-minerals, mainly oxides, closely associated with crystalline Fe-oxyhydroxides. Though Ce and La also form independent compounds, such as Ce oxides and La–Nd phosphates, they are mainly retained by the amorphous Mn-oxyhydroxides. However, the crystalline Mn-oxyhydroxides are the main sink for Ni and crystalline Fe-oxyhydroxides mainly retain P. Ó 2004 Elsevier Ltd. All rights reserved.

1. Introduction Selective leaching methods applied to geological and soil samples have been used since 1930s, when Tamm (1932) and Morgan (1935) designed two experimental approaches for the dissolution of labile Fe-oxyhydroxides and carbonates, respectively. Ross and Hendricks (1945) used a selective dissolution method to remove *

Corresponding author. Fax: +34 913466121. E-mail address: [email protected] (A.J. Quejido).

amorphous SiO2 and Al2O3 from clayey rocks. Several critical reviews of selective leaching methods for clayey materials were published in 1970s. Among them, that of Schwertmann (1979), which included the methods of Wada and Harward (1974) and Dixon and Weed (1977), is worth mentioning. Leaching methods must be selective enough to avoid the dissolution of other minerals in the sample. However, since it is very difficult to find selective reagents for each mineral or natural compound, this analytical approach is not always feasible and the interpretation

0883-2927/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2004.09.010

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A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

of the results can be a very arduous task. These problems increase with the mineralogical complexity of the samples. In spite of these inconveniences, selective dissolution methods are systematically used to remove soluble salts, carbonates, amorphous SiO2 and Al2O3, Fe(III) and Mn(IV)-oxyhydroxides, allophane-like materials, organic matter, and some clay minerals from geological samples (Alexiades and Jackson, 1966). The main objectives of selective leaching methods are to: (i) remove some compounds hindering the study of major and minor clay minerals in clayey samples; (ii) quantify those minerals undetectable by X-ray diffraction meth-

ods, due either to their low concentrations or amorphous nature. The application of consecutive selective dissolution methods on the same sample led to sequential extraction procedures, which were used to determine the mineralogical distribution of some trace elements for environmental purposes, such as for radiological contamination of soils and sediments (Schultz et al., 1995), bioavailability studies (Tessier et al., 1979; Ure et al., 1993) and for natural analogues of deep geological radwaste disposal (Yanase et al., 1991; Crespo et al., 1996, 2003). The identification and evaluation

Fig. 1. Geographical location of the U-ore deposit of ‘‘Mina Fe’’ and ‘‘Mina D, Hole-01’’, in which investigations were performed. ESPERANZA, ALAMEDA and SAGERAS are other non-mined uraniferous zones in the region.

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

(i)

of sources and sinks of natural nuclides and other trace elements, with similar geochemical behaviour to those present in the spent nuclear fuel (analogue elements), is crucial in the performance assessment of a nuclear waste repository. The sequential leaching methods have, however, additional inconveniences related to cross-contamination throughout the different steps of the experiments, as well as redistribution of the elements by re-adsorption and/or re-precipitation (Raksasataya et al., 1996; Go´mez-Ariza et al., 1999; Ho and Evans, 2000). In the framework of the Matrix Analogue Project, which was mainly focused on the analogies between the natural system and a deep geological radwaste repository and the implications for its performance assessment (Pe´rez del Villar et al., 2001, 2002a; Crespo et al., 2003), the knowledge of the mineralogical distribution of U, LREE, and Ni in fracture fillings from the oxidised zone of the U-ore deposit of ‘‘Mina Fe’’ (Salamanca, Spain) (Both et al., 1994) is relevant because U (IV, VI) are analogues for Pu (IV, V, VI) and Np (IV, V); LREE are analogues for Am, Cm and Pu (III) (Chapman and Smellie, 1986); and Ni is the homologue of the neutron activation products of the steel radwaste container. Furthermore, the mineralogical distribution of these analogue elements is relevant in order to:

In this context, the present work comprises: (a) Determination of the mineralogical, textural and chemical characterisation of the bulk samples of the fracture fillings from the oxidised zone. (b) Application of the sequential extraction method proposed by Pe´rez del Villar et al. (2002b) to fracture fillings sampled at the site. (c) Chemical analysis (major, minor, trace elements and some anionic species) of leachates resulting from the application of the selected method, as well as the statistical analysis of the chemical data, in order to deduce an approximate mineralogical distribution of the trace elements.

UE AG R DA

A DR P PI

R

600

605

E

SM-4

N

I VE

4300

IN

B'

A'

55

0

LT FAU BOA

N-600 N-606

75

618.14

C B

N-63

615

50

610

40

SM-2 SM-3 SM-1

MINE TAILINGS

A

N - 636

EL ANN CH

N-625

Legend

4100

AD RO

635 630

0 62 6

25

Contour lines

N-625 Exploitation level

4900

64 0

640

4700

64 5

(iii)

C' 4300

635

(ii)

4900

4700

630

Establish the role played by secondary minerals, mainly Fe-oxyhydroxides and clay minerals, as sources and sinks of the aforementioned analogue elements. Determine the retention capability of such minerals for these elements. Infer the key water–rock interaction processes controlling the solubility of the investigated elements, taking into account the processes governing the most important physico-chemical variables in the system, such as pH and Eh.

RO AD

625

489

Borehole 4100

Direction and dip

50 m

Fig. 2. Topographic map of ‘‘Mina D, Hole H-01’’ in which the Boa fault, the location of boreholes SM-1 to SM-4 and the orientation of A–A 0 , B–B 0 and C–C 0 cross sections in Fig. 3 are shown.

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2. Geological background The ‘‘Mina Fe’’ is the most important U deposit in the Spanish Iberian Massif. It is located about 10 km NW of Ciudad Rodrigo (Salamanca) (Fig. 1). The U mineralisation either fills open fractures or cements the fault breccia in the metasediments of the Upper Proterozoic–Lower Cambrian Schist–Graywacke Complex, known as ‘‘Complejo Esquisto-Grauva´quico’’ (CEG) (Arribas, 1985). In the ‘‘Mina Fe’’ area the CEG rocks mainly consist of a metamorphosed sequence of carbonaceous pelitic, partially turbiditic, and fine-grained psammitic rocks, which frequently display sedimentary textures. The grade of metamorphism is mainly greenschist facies,

the main rock types being slates, quartzites, conglomerates, sericitic and chloritic phyllites and micaschists, with some interbedded calc-silicate rocks, which represent metamorphosed layers of impure carbonate sediments (Martı´n-Izard, 1989). The U mineralisation is the result of a hydrothermal process in which 3 main phases have been distinguished. The first occurred after a brecciation process that caused chloritisation of the host rocks and breccia fragments, as well as the formation of small ankerite-pyrite bearing veins, with minor sulphide minerals, such as galena, sphalerite and chalcopyrite. The second phase took place after another brecciation process, when most of the U ore was formed. During this phase, adularia, pyrite, pitchblende (UO2 + x), coffinite (USiO4 Æ nH2O) and

Fig. 3. Schematic cross sections of ‘‘Mina D’’, Hole H-01, showing the vertical redox zones, based on fracture filling mineralogy of core-samples from boreholes SM-1, 2, 3 and 4. The location of profiles is shown in Fig. 2.

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

carbonates were formed. The third phase was characterised by the episodic, laminated and repeated precipitation of pyrite, carbonates and collophormic pitchblende. This last phase of U mineralisation filled open fractures and breccia voids (Arribas, 1985). The age of the ‘‘Mina Fe’’ U deposit, determined by the U–Pb method, is 34.8 ± 1.6 Ma; the same age as the Pyrenean tectonic phase of the Alpine orogeny (Both et al., 1994). This tectonic phase is also responsible for the formation of the Ciudad Rodrigo continental basin. Based on fluid inclusion data and the chemical composition of the early chlorite, the temperature of the U mineralisation varies from 230 to 60 °C during the first two phases, while the third phase occurred at a temperature lower than 60 °C (Mangas and Arribas, 1984). The U mineralisation has been intensively eroded and oxidised, as well as overlaid in places with continental Tertiary and Quaternary sediments. The thickness of these sediments in the mining area varies between 5 and 20 m. Numerous secondary U minerals such as yellow gummites, ianthinite, epi-ianthinite, alpha uranotyle, autunite, metaautunite, torbernite, saleeite and uranopilite were also formed as a result of the weathering processes (Arribas, 1962, 1975). The current mineralogical composition of fracture fillings from the oxidised zone of ‘‘Mina Fe’’ is the result of the weathering and leaching of a primary mineral association that mainly consists of clays, sulphides, carbonates, pitchblende and coffinite. Generally, the resulting secondary minerals are Fe-oxyhydroxides, Mn-oxyhydroxides, clays and allophane-like materials, as major components. The main accessory minerals are primary apatite (Ca5(PO4)3(OH, F, Cl)) and monazite ((Ce, La, Th)PO4), and minor secondary autunite (Ca(UO2)2(PO4)2 Æ nH2O), undetermined complex REE-phosphates, Ce oxides, residual carbonates and other partially soluble salts are also present (Pe´rez del Villar et al., 2001).

491

3. Sampling and methodology 3.1. Sampling Four boreholes with continuous core recovery and low contamination risk for groundwaters (SM-1 to SM-4) were drilled to collect fracture-filling samples, among other purposes (Figs. 2 and 3). The available amounts of the infill-materials in the fractures determined their sampling, which was generally done by scraping the fracture surfaces. As fracture fillings from the oxidised zone intersected by the boreholes (SM-1 to SM-4) are mineralogically and geochemically quite similar (Pe´rez del Villar et al., 2001, 2002a; Crespo et al., 2003), only samples from borehole SM-1 were used for this work as the most representative of the site. However, some differences in the accessory minerals were observed. This was taken into consideration during the discussion. 3.2. Methods The X-ray diffraction (XRD) powder method and scanning electron microscopy (SEM) coupled to an energy-dispersive X-ray analytical system (EDX), were used for the mineralogical characterisation of the samples. The XRD technique was used for identification and semi-quantification of major minerals, while SEM was used for detection of minor and trace (accessory) minerals. Some backscattered electron images were taken to show the most relevant accessory minerals in the samples. Chemical analyses of the bulk samples (major, minor and trace elements, except SiO2) were performed using inductively coupled atomic emission spectrometry (ICP-AES), atomic absorption spectrometry (AAS), flame atomic emission spectrometry (FAES), elemental analysis (Leco CS-244) and laser induced kinetic

Table 1 Semiquantitative mineralogical composition of fracture filling bulk samples (XRD) from the oxidised zone (borehole SM-1) Samples

Quartz (%)

Phyllo* (%)

Goethite (%)

Jarosite (%)

Siderite (%)

Chlorite (%)

SM-1-1 SM-1-2 SM-1-5 SM-1-6 SM-1-7 SM-1-8 SM-1-9 SM-1-10 SM-1-11 SM-1-13 SM-1-14 SM-1-16

4 9 5 15 31 8 3 3 24 22 44 31

18 46 33 71 37 59 83 33 24 50 45 43

78 42 62 14 29 31 13 64 52 27 4 26

– – – – 3 – – – – – – –

– – – – – – – – – – 5 –

– 3 – – – 2 1 – – 1 2 –

Phyllo*: Total phyllosilicates except chlorite (biotite, moscovite–sericite–illite, kaolinite and halloysite–metahalloysite).

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phosphorimetry. Differential thermal analyses (DTA) and thermogravimetric analyses (TGA) were also performed on the bulk samples for determining moisture and structural waters, as well as total CO2.

The 7-step-sequential leaching method used in this experiment was proposed by Pe´rez del Villar et al. (2002b). All the steps of the method were carried out in polypropylene tubes with screw caps of the same

Fig. 4. Electron images showing some minerals neoformed in the oxidised zone. (a) Neoformed or inherited U(IV/VI) oxyhydroxide (1) on smectite (2). (b) Neoformed or inherited U(IV/VI) oxyhydroxide on Mn–Fe oxyhydroxides. (c) Subidiomorphic autunite (1) precipitated on inherited biotite (2) from the host rock. (d) Fe oxyhydroxides (goethite) showing stalactitic and botryoidal textures (1), associated with a mixture of illite and smectite (2). (e) Mn-oxyhydroxides with minor Al, Si, Fe, Ca, Ni, Co and Ba (1), closely associated with goethite (2) and coated by smectite (3). (f) Ce oxides (2) precipitated on smectite coated by Fe-oxyhydroxides.

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

material. The agitation at room temperature was performed in an orbital rotary shaker of variable speed, while centrifugation of the extracts was performed by means of a centrifuge, equipped with a rotor suitable to the diameter of the extraction tubes.

493

3.2.1. Step 1: hydrosoluble salts An aliquot of 0.5 g of sample, previously ground, homogenised and sieved to a <60 lm grain size, was treated with 25 ml of sub-boiling doubly distilled water (pH 6.7–6.9). The sample was shaken for 1 h at room

Fig. 5. Electron images showing some minerals neoformed in the oxidised zone. (a) Fibrous-radiating halloysite–metahalloysite formed from allophane-like material by a silication process, under weakly alkaline conditions. (b) Idiomorphic kaolinite showing its typical face to face texture and also formed under weakly acid conditions. (c) Allophane-like material showing its typical colloidal texture, formed under weakly acid conditions. (d) Jarosite (1) coated by goethite (2), with colloformic texture. (e) Spherical secondary rhodochrosite (2) precipitated under weakly alkaline conditions on Mn-oxyhydroxides (1) and closely associated with a mixture of allophane-like material and halloysite–metahalloysite.

494

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

and 16.1 g/l ammonium oxalate, pH 2.85) at room temperature for 4 h in darkness. The extract was separated as in the previous steps. Amorphous or poorly crystalline Fe and Mn-oxyhydroxides were removed in this step. However, some uranyl phosphates such as torbernite (Cu(UO2)2(PO4)2 Æ 12H2O) and autunite and, to a lesser extent microcrystalline apatite, could also be totally or partially removed, due to the low pH of the Tamm
s solution and the presence of a high concentration of complexing agent. Despite the reducing conditions of this step, some pitchblende (UO2 + x) could also be dissolved as observed previously (Pe´rez del Villar et al., 2002b). It should be noted that Tamm
s solution is also used to remove (Al,Fe)-organic matter complexes, amorphous Al-oxyhydroxides and allophane-like products (allophane and imogolite), without any effect on the remaining clay minerals in the sample (Schwertmann, 1979).

temperature, assuring that the solid always remained in suspension. The liquid phase was separated by centrifugation at 10,000 rpm for 30 min. 3.2.2. Step 2: exchangeable cations The residue from step 1 was mixed with 8 ml of 1 M NH4Cl at pH 7.0. The mixture was shaken for 1 h at room temperature, assuring the same precautions as before. The extract was separated by centrifugation at 10,000 rpm for 30 min. 3.2.3. Step 3: carbonates The residue from step 2 was treated with 20 ml of Morgan
s solution (Morgan, 1935) (ammonium acetate 1 M at pH 4.5 adjusted with HNO3), for 4 h at 85 °C and shaken frequently. The extract was separated as in the previous steps. Exchangeable cations not removed in the step 2 will be now extracted. Although this step is specific for carbonates, certain phosphates (some types of microcrystalline apatite) and some U oxides could also be partially dissolved (Crespo et al., 2003).

3.2.5. Step 5: phases soluble in 6 M HCl The residue from the step 4 was treated with 30 ml of 6M HCl and the mixture was left for 2 h at 85 °C. After cooling, the extract was separated as usual. As all the samples are enriched in goethite this treatment was repeated.

3.2.4. Step 4: easily reducible phases The residue from step 3 was mixed with 20 ml of Tamm
s solution (Tamm, 1932) (10.9 g/l oxalic acid

Table 2 Analytical data of fracture filling bulk samples from borehole SM-1

Al2O3 (%) Fe2O3 MgO MnO TiO2 CaO Na2O P2O5 CO2T SO2T H2O
As (ppm) Ba Be Ce Co Cr Cu La Mo Ni Sr V W Y Zn U

SM-1-1

SM-1-2

15.5 55.6 0.55 0.08 0.12 0.48 0.51 2.4 1.17 0.26 3.5

17.5 44.4 0.81 0.02 0.28 0.44 0.88 1.57 1.98 0.18 3.2

155 178 7.6 331 298 289 500 306 2.4 91 223 310 195 95 2460 69

140 326 6.8 464 14 269 389 120 2.3 24 195 235 160 69 1060 64

SM-1-5

SM-1-6

10.6 65.8 0.57 0.02 0.26 0.32 2.61 2.98 0.92 0.20 3.8

35.3 13.1 1.54 0.63 0.59 0.39 0.98 0.11 3.26 0.04 2.9

87 157 7.2 308 3.7 86 450 83 4.9 37 151 195 215 90 680 84

22 590 <5 850 550 455 154 378 2.1 165 92 220 50 33 460 17

SM-1-7

SM-1-8

SM-1-9

17.4 35.7 0.97 0.02 0.45 0.29 1.09 0.85 3.67 0.06 2.7

20.9 37.3 0.77 0.02 0.27 0.29 1.08 0.73 2.02 0.10 3.3

39.5 15.7 1.26 2.4 0.38 0.4 3.09 0.25 1.1 0.04 4.6

21 334 <5 446 4.6 276 276 70 1.3 104 120 263 135 64 7700 33

23 299 <5 566 126 132 496 87 1.8 34 193 132 140 60 1030 32

48 428 <5 980 1830 288 293 874 8.7 568 157 242 58 125 1920 250

SM-1-10

SM-1-11

SM-1-13

SM-1-14

SM-1-16

17.5 45.6 0.62 3.4 0.17 0.26 0.72 0.75 0.88 0.04 4.1

19.5 37.4 0.7 2.5 0.14 0.45 0.36 0.76 0.66 0.04 4.0

18.5 39.1 1.26 0.07 0.52 0.2 0.1 0.54 1.43 0.04 2.5

40.7 7.0 3.34 0.12 0.79 0.24 3.68 4.06 0.77 0.02 1.6

12.3 25.4 1.16 3.1 0.33 0.25 0.09 0.24 0.51 0.04 1.6

265 246 <5 147 1870 25 672 298 3.5 2413 123 212 180 95 1900 395

135 332 9.5 178 337 191 599 128 21 1550 806 258 155 95 1400 450

225 433 5.6 428 45 272 344 59 4.9 144 34 618 170 81 800 1010

24 1600 <5 1480 4.9 339 <2 890 3.8 14 232 270 130 28 1670 188

66 3900 <5 76 154 49 300 55 3.0 270 255 153 110 54 800 765

0.558
0.604 0.685

0.581 0.590 0.749

0.763

0.801


0.653

0.700

0.785


0.637

0.597

0.859

0.730 0.607 0.610


0.825

0.589 0.817 0.590 0.814

Ce Co La Y V U

Mn Ni Zn As Cu Ca Cr Mo Sr

Al

0.558 0.587 0.674

0.655 0.620 0.704 0.694 0.585


0.636
0.892

0.696
0.658 0.773 0.896
0.579 0.918


0.836
0.603 0.826
0.740 0.564
0.828

Fig. 6. Dendrogram depicting the mutual relationships among the chemical variables of original fracture filling samples from borehole SM-1.

By using this treatment most of the clay minerals are dissolved, including Fe-rich chlorites. Other minerals that could be dissolved include phosphates, except for the insoluble ones such as monazite and xenotime (YPO4), and certain Mg oxides (periclase) and oxyhydroxides (brucite).

Fe2O3 MgO Na2O P2O5 TiO2 As Ba Ce Co Cr Cu La Ni Sr V W Y Zn


0.814 0.603

Ba Mg Ti Na Fe W P

0.733

Ni Mo La Cu Cr Co Ba As TiO2 P2O5 Na2O MnO MgO Fe2O3 CaO Al2O3

Table 3 Significant correlation coefficients between elements in the original samples

495

0.959

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

3.2.6. Step 6: oxidizable phases The residue from the step 5 was carefully treated with 5 ml of 8.8 M H2O2, acidified with HNO3 to pH 2. The mixture was kept uncovered at room temperature for 1 h and stirred frequently. Then, the mixture was kept at 85 °C, covered with a watch glass, and manually stirred every 15 min, until the volume was reduced to 1–2 ml through evaporation. After cooling, 5 ml of 8.8 M H2O2 were added to the mixture, keeping it covered at 85 °C for 1 h. Next, with the beaker uncovered, the mixture was heated at 85 °C to reduce the volume to 1–2 ml. Then, 25 ml of 1 M ammonium acetate at pH 2 were added, stirring at room temperature for 16 h. The extract was separated as in the previous steps. The organic matter except graphite is destroyed in this step. However, other remaining minerals, such as uranium dioxide (UO2 + x) inherited from the primary paragenesis and non-dissolved in the previous steps, can be now dissolved and, therefore, those elements associated with these minerals cannot be assigned to organic matter, when such minerals accompany it. 3.2.7. Step 7: insoluble residue The residual material of the previous step was dissolved with a mixture of HF/aqua regia followed by the addition of HClO4 to eliminate excess HF. It is relevant to mention that phyllosilicates were partially dissolved in the previous steps 5 and 6, leaving free SiO2 and TiO2 that precipitated under acid conditions. As a consequence, an excess of SiO2 and TiO2 are found in the final residue. The extracts from the different steps, as well as the final residue, were analysed by the most adequate techniques, based on the characteristics of the elements and the available equipment.

3 1

2 7

Factor 2

nd, Not-detected; Extracted (%): average and standard deviation of extracted elements refers to the total in the bulk samples. Cl
and SO2
4 were not determined in the bulk samples.

26 nd nd 13 21 0.25 0.06 0.03 21 20 1.7 nd nd nd 26 0.08 nd nd 15 28 Al Ca Mg Mn Na U Ce La Cl
SO2
4

10 65 25 nd 27 nd 0.06 0.03 16 135

20 nd nd nd 24 nd nd nd 16 31

nd 30 13 1.0 22 nd nd nd 16 27

2.5 57 13 1.8 28 nd 0.02 nd 19 15

59 45 55 nd 24 0.07 0.03 0.02 13 35

2.5 nd 11 nd 22 nd nd nd 15 10

nd 85 14 1.8 16 0.03 nd 0.03 21 13

nd nd nd 2.5 36 nd nd nd 33 21

18 nd 28 4.7 30 0.03 0.13 0.03 19 31

23 nd nd nd 39 0.07 0.03 nd 24 21

SM-1-16 SM-1-14 SM-1-13 SM-1-11 SM-1-10 SM-1-9 SM-1-8 SM-1-7 SM-1-6 SM-1-5 SM-1-2 SM-1-1

Table 4 Elements extracted (in ppm) in the water-soluble step (step 1)

0.02 ± 0.06 1.5 ± 1.9 0.64 ± 0.76 0.07 ± 0.21 4.5 ± 3.0 0.02 ± 0.06 0.12 ± 0.32 0.24 ± 0.42 – –

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506 Extracted (%)

496

1

0

6

9

11

5

14 10

2 13 16

8 -1

-2 -1.5

-1.0

-.5

0.0

.5

1.0

1.5

2.0

2.5

Factor 1 Fig. 7. Location of samples in the plane defined by the two main rotated factors. Notice that samples SM-1-10 and SM-114 are the richest in NaCl, while sample SM-1-1 is the richest in sulphates.

The anions released in the step 1 were determined by ion chromatography, following the protocol proposed by the United States Environmental Protection Agency (USEPA, 1989; Sa´nchez, 1996). A Dionex 4500i chromatograph, equipped with conductimetric detector was used for the analysis of anions in the extracts. Anions were previously separated in a Dionex AS9 column; the conductivity of the eluant being suppressed with a Dionex ASRS column. Laser induced kinetic phosphorimetry, using a Chemcheck Inst. model KPA-11 phosphorimeter and following a standardised analysis protocol (Sa´nchez and Ferna´ndez, 1998), was used for U determination. Other minor, major and trace elements, except REE, were analysed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), using the combined system Jobin–Yvon JY48 + JY38 VHR. The extracts were analysed after external calibration of the equipment with mono-elemental solutions, without internal standard or matrix matching (Quejido, 1993a). However, the analysis of the major elements in the dissolution of the final residue was carried out using Y as internal standard. Calibration of minor and trace elements was performed using a matrix-matching procedure (Quejido, 1993b). Lanthanum and Ce were analysed by plasma source mass spectrometry (ICPMS), after addition of In as internal standard. The measurements were performed with a Finnigan MAT SOLA instrument. Statistical analysis of the data was performed using both SPSS and Statgraphics statistical software. Chemical results from each extraction step, for all the treated samples, were analysed by multivariate tools (bivariate correlation and factor and cluster analyses). Only those

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

oxyhydroxides, Mn-oxyhydroxides, Fe–Mn-oxyhydroxides and clay minerals. The Fe-oxyhydroxides, with stalactitic and botryoidal textures, are the first secondary minerals formed (Fig. 4(d)). They contain small amounts of Si, Al, Mn and K as detected by EDX, while their U and P contents are always below the detection limit of EDX. The Mn-oxyhydroxides usually have spherulitic texture and are closely associated with Fe-oxyhydroxides. This means that both can be paragenetic. Chemically, they contain minor Al, Si, Fe, K, Ca, Ni, Co and Ba (Fig. 4(e)). Other oxides detected, commonly as trace compounds, are Ce oxide, with minor Ca, P, Mn and Fe (Fig. 4(f)), and cesarolite-like Mn–Pb oxide. The neoformed clay minerals are illite, interstratified illite–smectite, smectite, kaolinite, halloysite–metahalloysite and allophane-like products. Illite and interstratified illite–smectite are scarce. The distribution of smectite in depth is irregular, being abundant in the upper and deeper samples. The distribution of halloysite–metahalloysite and kaolinite also shows a mineralogical zoning in depth. Halloysite–metahalloysite, with its typical radiating fibrous habit (Fig. 5(a)), is relatively abundant down to 15 m depth, where sample SM-1-16 was taken. In contrast, kaolinite (Fig. 5(b)) appears from this depth to the transition between the oxidised and reduced zones, located at 20 m depth (Pe´rez del Villar et al., 2001, 2002a; Crespo et al., 2003). The allophane-like products, with a Si/Al ratio close to that of halloysite and kaolinite and gel textures (Fig. 5(c)), are closely associated with halloysite down to 5.6 m depth. This suggests that halloysite–metahalloysite was formed from such allophane-like products by a resilication process when the alkalinity of the system was restored. Other interesting and neoformed minerals and compounds are jarosite (Fe2O3 = 59.9%, Al2O3 = 6.2%, K2O = 10%, SO = 24%, according to the EDX elemental analysis), which indicates a very acid (pH < 4.5) geochemical environment during its formation, secondary rhodochrosite and La–Nd phosphates. Jarosite was found in sample SM-1-13, showing efflorescent aggregates of microcrystals and being closely associated with

relevant elements detected in the leachates were statistically analysed.

4. Results and discussion 4.1. Mineralogical and geochemical characterisation The semiquantitative mineralogical composition of the samples is summarised in Table 1. These data show that phyllosilicates and goethite are generally the major minerals in all the samples, while chlorite is only present in some of them. Jarosite, a very insoluble double sulphate of Fe(III) and K, and siderite (FeCO3) are present in SM-1-7 and SM-1-14, respectively. Given that the powder XRD method has significant uncertainties concerning: (i) quality and quantity of phyllosilicates; (ii) accessory minerals, whose concentrations are always below the detection limit of the XRD method; (iii) amorphous materials, which are transparent to X-rays, all the samples have also been studied by SEM + EDX, in order to know the quality and textures of phyllosilicates, accessory minerals and amorphous compounds. These minerals and amorphous compounds are relevant scavengers for trace elements. Furthermore, this detailed and complementary mineralogical study has facilitated and improved the interpretation of the analytical results from the sequential leaching method. This study also allowed the discrimination of the inherited minerals, either from the host rock or from the vein fillings, from the neoformed minerals resulting from the oxidation of the primary U paragenesis. Among the inherited minerals from the host rock, biotite, partially altered to chlorite, muscovite, quartz, zircon with minor U, monazite and xenotime, are worth noting. Of the inherited minerals from the primary paragenesis of the filling veins, idiomorphic quartz and occasional small particles of pyrite and molybdenite, all of them as residual minerals, were identified. Furthermore, inherited or neoformed U (IV or VI) oxides and autunite were also observed (Fig. 4(a)–(c)). The most relevant neoformed minerals resulting from the oxidation-precipitation supergene processes are Fe-

Ca

Mg

497

2-

SO 4

Na

Cl

Fig. 8. Dendrogram showing the relationships among species extracted in step 1.

SM-1-16

480 470 42 33 nd 5.7 2.5

SM-1-14

960 510 7.4 33 2.5 5.0 nd

Ca

Sr

Mg

Mn

Na

Ba

Zn

745 216 nd 23 7.2 5.4 1.0

1740 615 57 32 29 14.0 1.0

384 272 1.1 45 21.0 4.0 nd

930 350 3.5 27 9.0 7.5 1.0

2050 800 67 17 4.7 14 1.3

830 345 8.5 35 5.0 6.2 1.3

1450 600 29 29 2.8 11 1.1

730 400 8.5 24 2.0 5.0 0.9

Fig. 9. Dendrogram showing the relationships among species extracted in step 2.

nd, Not-detected.

800 264 2.1 27 10 6.6 1.2 775 240 10 27 16 7.4 nd Ca Mg Mn Na Ba Sr Zn

SM-1-13 SM-1-11 SM-1-10 SM-1-9 SM-1-8 SM-1-7 SM-1-6 SM-1-5 SM-1-2 SM-1-1

Table 5 Elements extracted (in ppm) in the exchangeable step (step 2)

70 ± 15 17 ± 8 1.1 ± 0.9 5.1 ± 3.3 5.5 ± 4.5 30 ± 19 2.7 ± 3.3

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506 Extracted (%)

498

Fe-oxyhydroxides and coated by goethite (Fig. 5(d)). Secondary rhodochrosite was detected only in sample SM-1-6, showing spherulitic texture and precipitated on Fe and Mn-oxyhydroxides (Fig. 5(e)). Secondary La–Nd phosphates were detected in sample SM-1-9, exhibiting spherulitic texture and precipitated on a mixture or interstratified illite–smectite. Table 2 summarises the analytical results for major, minor and trace elements in the samples. Among major elements, the high concentration of total C, expressed as total CO2, is highlighted. This C is almost totally assigned to organic C, as carbonates were not detected by XRD in the samples and the host rocks of the fracture infill materials mainly consist of carbonaceous slates. Though the large number of chemical variables (23) in relation to the number of samples (12) reduces the significance of the bivariate and multivariate statistical analysis, these tools have been used to obtain the geochemical associations among major and trace elements and to deduce the possible minerals that work as sinks for trace elements. The results (Table 3 and Fig. 6) show that phyllosilicates, mainly clay minerals, are responsible for the retention of Ba, as the Mg–Ti–Na–Al–Ba-cluster indicates. The group Fe–W–P indicates that Fe-oxyhydroxides are responsible for the retention of W and P that is consistent with the geochemical affinity among these elements. The cluster constituted by Mn, Ni, Zn, Cu and As seems to indicate that Mn-oxyhydroxides work as sink for Ni, Zn, Cu and As. The cluster formed by Ca, Cr, Mo and Sr is difficult to interpret given that, as explained before, no carbonates have been detected in the samples and Ca is mainly present as an exchangeable cation in the clay minerals. However, though minor but XRD-undetectable carbonates can be present in the samples, this elemental association that includes Cr and Mo cannot be easily interpreted from a geochemical point of view. The cluster formed by Ce, Co, La and Y is also difficult to interpret as Co has no geochemical affinity with REE, and correlation coefficients between Ce– La, Ce–Y and La–Y have no statistical significance either. Finally, V and U cannot be assigned to a determined mineral phase, since uranyl vanadates have not been detected in the samples. However, the geochemical

Extracted (%)

0.19 ± 0.30 13 ± 14 0.03 ± 0.11 6.1 ± 4.5 14 ± 17 2.7 ± 2.7 27 ± 17 5.4 ± 6.1 28 ± 21 5 ± 17 0.3 ± 1.1 36 ± 13 2.7 ± 4.6

SM-1-16

106 nd nd 182 176 22 238 nd 1.3 nd nd 2.4 4.4

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

Ca

Mn

Ce

U

La

102 nd 136 94 46 21 78 nd 1.0 nd nd 0.8 nd 30 nd nd 96 68 7.4 362 nd 1.3 nd nd 1,6 3.0 25 228 nd 160 278 12 90 18 7.1 5.2 nd 0.9 38

4.2. Sequential leaching results The results from the application of the sequential leaching method have been put together according to the different stages, in order to compare the trace elements released from the samples by the extraction reagents. These results are expressed both as ppm of the element extracted from each sample and as the % of total content in the samples. For information purposes, the % is expressed as the average values of all the samples and the standard deviation is related to the mineralogical and chemical variability of the samples, rather than precision of the analytical data.

60 250 nd 178 254 15 172 34 6.8 5.8 14 2.4 29 30 248 nd 110 440 7.4 146 31 6.1 nd nd 3.1 24 30 nd nd 44 17 10 5.2 nd 2.3 nd nd 0.4 nd

Mg

association between U and V is reliable. These clusters are similar to those obtained considering the chemical data of the 40 oxidised samples from boreholes SM-1, SM-2 and SM-3 (Pe´rez del Villar et al., 2001, 2002a).

44 nd nd 60 214 10 5.7 21 11 nd nd 0.9 nd 13 348 nd 94 19 8.9 10 nd 3.8 nd nd 0.6 nd nd, Not-detected.

nd 382 nd 44 9.3 10 2.9 nd 3.2 nd nd 0.2 nd 48 480 3.4 224 480 14 3.6 15 43 116 nd 1.4 15 Al Ca Fe Mg Mn Na U Ba Ce Co Cu La Ni

Na

Fig. 10. Dendrogram showing the relationships among species extracted in step 3.

440 304 35 350 46 46 4.1 20 1.8 nd nd 0.9 nd

SM-1-9 SM-1-8 SM-1-7 SM-1-6 SM-1-5 SM-1-2 SM-1-1

Table 6 Elements extracted (in ppm) in the carbonates step (step 3)

SM-1-10

SM-1-11

SM-1-13

SM-1-14

Al

499

4.2.1. Step 1: water soluble elements Data from Table 4 indicate that elements extracted in most of the samples are Al, Ca, Mg, Mn and Na as cations, and Cl and SO4 as anions. Generally, it shows that significant amounts of trace elements are not detectable in this step, although ultra-traces of U, La and Ce have been detected in some samples. The presence of Al and Mn in this step can only be explained by a probable contamination of the extract by small solid particles from the sample and, therefore, these elements have been excluded in the further statistical treatment. Given that only ionic compounds are soluble in this step, both cations (Ca, Mg and Na) and anions (Cl and SO4) have been expressed as equivalents for statistical analysis. Although non-significant correlation coefficients are observed, there are certain relationships among Ca, Mg and SO4 and between Na and Cl, as confirmed by factor analysis (Fig. 7), in which the two main factors (Factor 1 =
0.391 * Ca
0.373 * Mg + 0.926 * Na + 0.757 * Cl + 0.152 * SO4; Factor 2 = 0.628 * Ca + 0.690 * Mg + 0.027 * Na
0.313 * Cl + 0.855 * SO4) explain 69%

4.6 ± 5.5 1.3 ± 0.9 48 ± 26 7.1 ± 10.0 4.1 ± 2.3 47 ± 24 37 ± 37 49 ± 16 17 ± 19 1900 3260 16,680 108 52 52 106 11 82

Fe

P

Mn Ce

Ni

La

Co

U

640 640 294 nd 5.5 1.6 nd 0.4 nd 1400 2120 11,460 128 17 140 210 2.2 440 4720 3980 14,440 262 26 136 960 3.0 460

600 2640 348 108 55 4.8 10 2.0 10

of total variance. It is also observed in the dendrogram of Fig. 8. This means that minor halite, gypsum and kieserite could be present in the samples, since natural double sulphates of Ca and Mg are not described in the specialised literature. The amounts of these minerals are undetectable by XRD and have not been observed by SEM + EDX, even in the richest halite samples (SM-1-14 and SM-1-10) and in the rich sulphate sample SM-1-1 (see Fig. 7). 4.2.2. Step 2: exchangeable elements The results from Table 5 indicate that Ca, Mg, Na, Mn, Ba and Sr are, in decreasing order of abundances, the elements extracted in this step. Except for Mn, all of them are those expected to be present, even in this order of abundance, as exchangeable cations in clay-rich samples. Calcium extracted represents more than 80% of the total in some samples and the exchangeable Sr also reaches up to 60% in some, and both elements show a strong correlation (Fig. 9). Concerning the Mg–Mn pair, it is suggested that the former is an exchangeable cation, while the latter would be as minor Mn(II)-oxyhydroxides in the samples, given that these compounds are soluble in ammonium chloride. Probably, these compounds would be located inside the phyllosilicate structure, filling interlaminar eye-like voids originated by an adaptation process between two different sized adjacent sheets. Thus, the strong correlation observed between Mg and Mn could be explained (see Fig. 9). Sodium and, to a lesser extent, Ba and Zn are also exchangeable cations. The different absolute values for the exchangeable cations (see Table 5) are explained by the variable phyllosilicate contents of the samples (see Table 1).

nd, Not-detected.

460 2760 39 198 2.0 3.1 nd 0.9 nd Al Fe Mn P U Ce Co La Ni

680 1920 154 90 0.6 67 56 1.2 nd

660 980 17 nd 0.4 2.4 nd 0.2 nd

1420 1220 1280 nd 0.7 59 356 1.0 32

7360 7800 17 192 2.3 1.2 nd 0.5 4.0

600 1080 44 nd 0.4 1.8 nd 0.5 nd

1700 2700 11,240 144 14 67 1100 13 190

SM-1-11 SM-1-9 SM-1-8 SM-1-7 SM-1-6 SM-1-5 SM-1-2 SM-1-1

Table 7 Elements extracted (in ppm) in the easily reducible step (step 4)

Al

Fig. 11. Dendrogram showing the relationships among species extracted in step 4.

SM-1-10

SM-1-13

SM-1-14

Extracted (%)

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

SM-1-16

500

4.2.3. Step 3: carbonates Despite the fact that carbonates have not been detected due to the high detection limit of XRD, significant amounts of the total Ca, Mg, Mn, U, Ce and La were removed in this step (Table 6). Though Ca, Mg and Mn can be explained considering the presence of minor but XRD-undetectable carbonates, some addi-

41 ± 15 96 ± 3 31 ± 16 30 ± 12 6.2 ± 3.8 79 ± 29 13 ± 9 66 ± 19 19 ± 18 18 ± 27 81 ± 30 64 ± 27 26 ± 32 56 ± 38 36 ± 44 7440 162,000 759 1800 33 144 85 468 4.5 15 66 126 nd nd 201 8010 259,000 174 128 16 1260 162 527 nd 17 147 94 nd 288 153 20,300 256,000 449 2348 35 1470 119 343 102 19 270 439 83 93 385

11,700 32,200 5440 321 51 nd 42 102 4.8 nd nd nd nd nd nd

4.2.4. Step 4: easily reducible phases Significant amounts of Al, Fe, Mn, P, U, Ce, Co, La and Ni have been removed in this step (Table 7). Cluster analysis (Fig. 11) shows that Al, Fe and P are closely related, indicating that a probable amorphous Al–Fe compound with P adsorbed was dissolved. Though SiO2 has not been determined in any leachate, it is suggested that the most probable amorphous Al–Fe compound is a Ferich allophane or ferriallophane (Spencer, 1916), since allophane-like products have been detected in the samples. These products are soluble in Tamm
s solution (Schwertmann, 1964; Wada, 1977; Parfitt and Henmi, 1982; Farmer et al., 1983; Parfitt, 1990; Farmer and Russell, 1990) and their dissolution is facilitated by previous reduction of Fe(III). Thus, the existence of ferriallophane would justify the high correlation between Al and Fe. The second cluster shown in Fig. 11 includes Mn, Ce, Ni, La and Co, indicating that amorphous Mn-oxyhydroxides are the sink for LREE, Ni and Co. Uranium behaves independently with respect to the other elements. This could be due to the progressive dissolution of U oxides as the pH of the reagents decreases.

37,500 98,000 852 3854 21 1320 264 86 95 243 81 204 42 21 nd 20,100 275,000 522 72 45 2250 144 21 3.0 nd 291 37 11 41 nd 17,600 226,000 856 52 61 2340 137 24 57 nd 111 81 nd 1.5 nd 24,000 85,300 1723 753 45 750 186 7.5 3.9 41 54 47 nd 16 nd 13,700 347,000 513 50 30 6210 87 59 40 18 162 41 17 68 20 nd, Not-detected.

15,500 295,000 290 34 43 4650 158 52 nd nd 216 17 4.8 75 nd 17,700 329,000 508 106 47 5110 137 57 47 22 252 72 83 144 nd Al Fe Mg Mn Na P Ti U Ba Co Cu Ni Sr V Zn

501

tional considerations must be given to other elements. Thus, the strong correlation observed between Al, Na and Mg (Fig. 10) seems to indicate that some clay minerals, already incipiently altered during the previous step, have been partially dissolved. The second consideration is related to the strong association observed between Mn and Ce (see Fig. 10), suggesting that minor Mn(IV)-oxyhydroxides could have been dissolved leaving free Ce. Besides this, under the physicochemical conditions of this step, the Ce oxides detected in the samples can also be dissolved. The third consideration concerns the behaviour of Ca that is not correlated with any element, and therefore must be assigned to a different mineral phase, i.e., residual calcite in the sample. The presence of minor rhodochrosite and siderite in samples SM-1-6 and SM-1-14, respectively, can partially justify some of the Mg, Mn and Fe and even Ca removed in this step. Finally, U and La can be assigned to U oxides and La–Nd phosphates, also found in the samples. These compounds are also dissolved under these experimental conditions.

23,200 314,000 633 7588 26 1710 129 185 72 321 261 965 nd 54 630

SM-1-13 SM-1-7 SM-1-6 SM-1-5 SM-1-2 SM-1-1

Table 8 Elements extracted (in ppm) in the 6M HCl soluble step (step 5)

SM-1-8

SM-1-9

SM-1-10

SM-1-11

SM-1-14

SM-1-16

Extracted (%)

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

4.2.5. Step 5: phases soluble in HCl 6M Large amounts of Fe, Al, P, Mg, Mn, Ti, Na, U, Cu, Ni, Co, V, Ba and Zn have been extracted in this step (Table 8). In the cluster analysis (Fig. 12) 3 main associations are distinguished. Aluminium and Ti, indicating the partial dissolution of some phyllosilicates or the dissolution of labile clay minerals, form the first association. The second comprises Mn, Co, Ni, Zn and Ba, suggesting that crystalline Mn-oxyhydroxides are the sink for these trace elements, which in turn form a very

502

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

frequent geochemical association. The third group contains Fe, Cu and P and corresponds to the dissolution of the abundant crystalline Fe-oxyhydroxides, mainly goethite. This mineral also is as a very efficient sink for Cu and P. The clusters observed between U–V and Mg–Na have neither geochemical nor statistical significance due to their low correlation coefficients. This means that U is geochemically associated neither with phyllosilicates nor with crystalline Fe-oxyhydroxides. However, given that the highest amounts of Fe and U are removed in this step, a mineralogical association between crystalline Fe-oxyhydroxides and small-sized U minerals, probably U oxides, can be expected. 4.2.6. Step 6: oxidizable phases Moderate amounts of Al and Fe, and minor Na, U, Ce and La have been removed in this step (Table 9). Aluminium, Ce and La are closely related, while Na, U and Fe show a quasi-independent behaviour (Fig. 13). Given the oxidised character of the samples, the main objective of this step is to eliminate the organic matter, expressed in the bulk samples as total CO2 (see Table 2). Consequently, the organic matter should be responsible for the retention, either by adsorption or as organo-complexes, of the extracted elements. In any case, U removed only reaches less than 2% of total U in the samples, while Ce and La represent up to 10% and 40% of the total, respectively. 4.2.7. Step 7: insoluble residue Large amounts of Al, Fe, Ti, Mg, Na, Ca and P as major elements, in decreasing order of absolute abundances, are leached in this step. Among trace elements, important amounts of Ba, V, Zn and Cr are also removed, while Ni, U, Ce and La have almost totally been extracted in the previous steps (Table 10). Although the precise mineral phases remaining in the final residue are not easily deducible by means of cluster analysis (Fig. 14), the dendrogram and the exhaustive sequential leaching protocol applied to the samples suggest that the residue is mainly composed by a mixture of resistant aluminosilicates, mainly phyllosilicates, refractory oxides, such as rutile and ilmenite, and some resistant phosphates, such as monazite and xenotime.

Al

Ti

Fe Cu

P

Mg

5. Conclusions and implications for the retention processes In this research work a comprehensive and exhaustive study on the original fracture filling samples from the oxidised cap of the ‘‘Mina Fe’’, including mineralogical, geochemical and statistical analyses, has been carried out. Then, a specifically designed sequential leaching method has been applied to the samples, which are qualitatively very homogeneous from a mineralogical and geochemical point of view. Finally, exhaustive chemical analyses of the extracts and a statistical study of the results have been performed. In spite of this arduous work, the precise knowledge of the mineralogical distribution of trace elements by sequential leaching methods is quite a complex task, mainly due to the cross-contamination throughout the different steps of the experiments, though a suitable approach to this aim can be inferred. Thus, from data obtained it can be stated:  Among the trace elements of interest for the purposes of this work, U, Ce, La and Ni are highlighted. Uranium, which is not significantly correlated with any other element in the original samples (see Table 3 and Fig. 6), was removed in all the sequential leaching steps, except in steps 1 and 2. However, U extracted in steps 3 and 5 comprises 93% of the total as average. It is assumed that U removed in these steps comes from U minerals in the samples, which are also soluble under these experimental conditions. In this sense, the U phosphates and U oxides detected in the samples seem to corroborate this assumption. The remaining U, 7%, is distributed between the extracts corresponding to the easily reducible phases (5%) and final residue (2%).  Cerium and La, which in the original samples are not significantly correlated with any other element either (see Table 3 and Fig. 6), have been completely removed in steps 3 (28% and 37%), 4 (65% and 45%) and 6 (4% and 14%) that correspond to carbonates, easily reducible and oxidizable phases, respectively. In step 3, no correlation between extracted Ce and La is observed, indicating that Ce and La come from different compounds in the samples. In

Na Mn Co Ni

Zn Ba

U

V

Fig. 12. Dendrogram showing the relationships among species extracted in step 5.

2.1 ± 1.0 0.07 ± 0.08 1.4 ± 1.8 0.38 ± 0.42 3.2 ± 3.3 14 ± 11

503

385 17 5.0 0.16 0.21 0.13 555 62 23.7 2.05 0.32 0.22 645 63 17.1 0.07 0.3 0.41 1025 59 10.4 1.45 0.35 0.52 3750 255 18.5 0.42 0.9 1.5 495 113 nd 0.04 0.25 0.23

2450 110 11.9 0.19 0.54 0.61

1075 43 7.4 0.25 0.22 0.21

935 60 nd 0.06 0.28 0.27

La

Fe

Na

U

this sense, Ce oxides and La–Nd-phosphates, soluble under these conditions, have been detected in the samples. Furthermore, amorphous Mn-oxyhydroxides seem to work as the main sink for Ce and La, as the results obtained in step 4 have demonstrated. Minor amounts of Ce and La seem to be linked to the organic matter in the sample, as the results obtained in step 6 indicate.  Nickel, a homologous element of the neutron activation products of the steel container, has been almost totally removed in steps 4 (20%) and 5 (60%). In both cases, Ni is closely related to Mn-oxyhydroxides as cluster analysis indicates, suggesting that these compounds, mainly the crystalline ones, are responsible for the retention of this homologous element.  Both amorphous and crystalline Fe-oxyhydroxides mainly retain P (10% and 80%, respectively), a minor and interesting element as a ligand of many analogue elements such as U and REE.

nd, Not-detected.

605 31 nd 0.38 0.29 0.27 545 330 nd 0.03 0.21 0.22 Al Fe Na U Ce La

Ce

Fig. 13. Dendrogram showing the relationships among species extracted in step 6.

275 66 11.9 0.29 0.28 0.23

SM-1-16 SM-1-13 SM-1-11 SM-1-10 SM-1-9 SM-1-8 SM-1-7 SM-1-6 SM-1-5 SM-1-2 SM-1-1

Table 9 Elements extracted (in ppm) in the step of the oxidizable phases (step 6)

Al

SM-1-14

Extracted (%)

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506

Summarising, in the oxidised Fe-rich fracture fillings from the ‘‘Mina Fe’’ U deposit, U is mainly retained as U-minerals, mainly oxides, closely associated with crystalline Fe-oxyhydroxides. Though Ce and La also form independent compounds or ‘‘mineraloids’’ such as Ce oxides and La–Nd phosphates, they are mainly retained by amorphous Mn-oxyhydroxides. However, the crystalline Mn-oxyhydroxides are the main sink for Ni, and crystalline Fe-oxyhydroxides mainly retain P. The identification and evaluation of sources and sinks of natural nuclides and other trace elements, with a similar geochemical behaviour to those present in the spent nuclear fuel, is important for the performance assessment of a nuclear waste repository. In the ‘‘Mina Fe’’, the source of natural nuclides is the U-ore, mainly constituted by pitchblende (UO2 + x) (Pe´rez del Villar et al., 2002a), and the role played by the fracture fillings as sinks of U and other analogous trace elements has been identified and partially evaluated. Thus, secondary U oxides and phosphates, amorphous and crystalline Mn-oxyhydroxides and crystalline Fe-oxyhydroxides are the sinks for U, Ce, La, Ni and P. However, all of these minerals can also be sources of these elements if

SM-1-16

21,100 160 1030 1550 43 500 51 1750 1.73 85 15 nd 6.3 24 2.3

SM-1-14

40,700 235 2950 1175 53 7900 27 4120 0.83 128 33 25 13 68 7.3

SM-1-13

24,400 182 4680 1950 36 650 48 1680 2.05 143 19 1.0 4.5 39 18 9500 152 3850 422 10 230 52 315 0.63 23 3.8 4.8 31 4.0 21

Sr

Acknowledgements nd, Not-detected.

SM-1-11 SM-1-10

8350 147 1125 485 12 203 50 470 0.63 34 5.5 3.0 3.8 8.5 7.5 40,725 177 3575 1000 22 300 117 725 0.93 60 11 12 11 14 23

SM-1-9

Zn P

dissolved under strong acid conditions. Nevertheless, these conditions are hardly reached unless important amounts of sulphides subjected to weathering conditions existed in the natural environment. Besides this, Fe-oxyhydroxides deserve special attention since they have usually been considered as strong scavengers of many trace elements, analogous or not. Thus, though in this site it has been found that these oxyhydroxides are very effective for P retention, their effectiveness for U is not clear. As mentioned before, U is not geochemically associated with these oxyhydroxides, but closely mixed with them as U oxides. This suggests that the retention process of U was not by surface sorption but by co-precipitation of both Fe and U-oxyhydroxides, under the same physicochemical conditions. This suggestion is confirmed by natural observations (Pe´rez del Villar et al., 1996, 2002a), laboratory experiments and model calculation (Bruno et al., 1996, 2000). It can then be reasonably stated that U co-precipitation with Fe-oxyhydroxides is a key retention process in the oxidised zone of a geological repository (Pe´rez del Villar et al., 2002a). Finally, it is highlighted that even in extremely perturbed geological media like ‘‘Mina Fe’’, which is intensively fractured and strongly affected by aggressive weathering processes, important amounts of U, Ce, La and Ni are retained by Fe-rich fracture infill materials. Consequently, this fact is relevant for performance assessment of deep geological radwaste disposal, mainly concerning its isolation and retardation of U and other analogue elements towards the biosphere (Crespo et al., 2003).

22,500 177 2375 1100 25 600 52 1220 1.08 103 13 5.3 14 22 11

SM-1-8

Mg U Ba Mn Ti Cr V Na Ni Ca Fe

Fig. 14. Dendrogram showing the relationships among species extracted in step 7.

12,900 160 1080 1100 25 270 55 2200 1.15 113 14 nd 4.5 27 3.8

SM-1-7

Al

61,350 195 4900 2300 45 1920 123 2480 2.33 203 24 18 13 44 18 11,900 165 1650 625 15 208 50 1050 0.63 53 6.5 0.8 3.0 13 7.8 25,200 282 3240 1200 25 1450 55 1080 1.4 113 14 3.3 18 27 14 16,400 190 2350 600 6.0 412 55 445 0.63 48 5.8 3.5 17 12 12

SM-1-6 SM-1-5 SM-1-2 SM-1-1

Al Ca Fe Mg Mn Na P Ti U Ba Cr Ni Sr V Zn

Table 10 Remaining elements (in ppm) in the insoluble residue (step 7)

52 ± 18 15 ± 6 2.1 ± 2.4 43 ± 17 7.6 ± 9.2 78 ± 14 14 ± 28 87 ± 9 2.3 ± 4.0 62 ± 34 58 ± 44 13 ± 28 37 ± 21 44 ± 38 60 ± 45

A.J. Quejido et al. / Applied Geochemistry 20 (2005) 487–506 Extracted (%)

504

This work has been carried out in the framework of the natural analogue programme, particularly in the Matrix Analogue Project, supported by ENRESA. We are grateful to M. Aldea and M.M. Gonza´lez for the chemical analyses of the samples. The authors are grate-

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ful to Dr. M. Gascoyne, Dr. R. Ejeckam and another anonymous reviewer for their useful comments, suggestions and corrections that improved the manuscript.

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