Rare-earth elements in groundwaters from the Osamu Utsumi

Journal of GeochemicalExploration, 45 (1992) 365-387 365 Elsevier Science Publishers B.V., Amsterdam

Rare-earth elements in groundwaters from the Osamu Utsumi mine and Morro do Ferro

analogue study sites, Pogos de Caldas, Brazil

N. Miekeley a, H. Cout inho de Jesus a, C.L. Porto da Silveira ~, P. Linsalata b and R. M o r s e b

aDepartment of Chemistry, Pontifical Catholic University, 22453 Rio de Janeiro, Brazil b New York University Medical Center, Institute of Environmental Medicine, Tuxedo, NY 10987, USA

(Received 17 April 1991; accepted after revision 7 May 1992 )

ABSTRACT

Miekeley, N., Coutinho de Jesus, H., Porto da Silveira, C.L., Linsalata, P. and Morse, R., 1992. Rare- earth elements in groundwaters from the Osamu Utsumi mine and Morro do Ferro analogue study sites, Pogos de Caldas, Brazil. In: N.A. Chapman, I.G. McKinley, M.E. Shea and J.A.T. Smellie (Editors), The Pogos de Caldas Project: Natural Analogues of Processes in a Radioactive Waste Repository. J. Geochem. Explor., 45: 365-387.

Data are presented on rare-earth elements (REE) in prefiltered ( < 450 nm) near-surface and deeper groundwaters and in corresponding particulate matter ( > 450 nm) from the Osamu Utsumi uranium mine and the Morro do Ferro thorium-REE-deposit. Groundwaters from both sites typically contain between 1-50/~g/l of total REE, but can reach values of up to 160/zg/l in the deepest borehole F4 (U- mine: 150-415 m). Even higher REE concentrations of up to 29 mg/l were measured in acidic, sulfate-rich near-surface waters of the same site. The chondrite-norrnalized REE patterns in deeper, more reducing groundwaters and in their corresponding suspended particle fractions are similar to those observed in the bedrock (phonolites), indicating that bedrock leaching and secondary mineral sorption occurred without significant fractionation between these elements, in accordance with the only small variations in the stability constants of the expected REE-sulfate complexes in these waters. Groundwaters from the unsaturated zone of both sites show a very characteristic cerium depletion (less pronounced than that observed in the corresponding suspended particulate fractions), which is most probably related to the oxidation of Ce (III) under the prevailing Eh-conditions of these waters (600 to 800 mV), and to sorption/precipitation reactions of the much less soluble Ce(IV) species. Coarse particulate matter ( > 450 nm), composed mainly of amorphous ferric hydrous oxides, has a strong capacity for sorption of REE. This is shown by its very high REE concentrations, in some boreholes > 8,000 #g/g (total REE), and by the calculated association ratios Ra (ml/g) , which are in the order of 103 to 106. The implications of these findings for the migration behavior of REE in both environments are discussed.

Correspondence to: N. Miekeley, Department of Chemistry, Pontifical Catholic University, 22453 Rio de Janeiro, Brazil.

0375-6742/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

366 N. MIEKtLt } E-~ ~d

1. INTRODUCTION

A considerable amount of data has been published during the last decades on REE in a wide range of rock types and minerals, as these elements are particularly useful as geochemical indicators for solving petrogenetic problems (e.g. Hansen, 1980; M611er, 1983; M611er and Morteani, 1983). How- ever, surprisingly little information is available on the concentrations of these elements in natural waters. This is in great part due to the fact that REE concentrations in waters are generally low and that routine analytical methods with sufficient sensitivity (e.g. ICP-AES, ICP-MS) have become available only quite recently. The published data are concerned mainly with REE in rivers, estuaries and ocean waters (e.g. Hoyle et al., 1984; De Baar et al., 1988; Goldstein and Jacobsen, 1988a, b; Elderfield et al., 1990), and in hydrothermal solutions (e.g. McLennan and Taylor, 1979; Michard and Albar6de, 1986; Michard, 1989; Silveira, 1986; Silveira et al., 1991 ), addressing the geochemical cycling of the REE and their mobilization/fractionation during rock/ore formation and/or alteration. Data on the nature and thermodynamics of inorganic complexes of REE in natural waters and hydrothermal solutions have recently been reviewed by Wood ( 1990a, b ).

From this broad literature it becomes evident that REE can be mobilized by aqueous solutions during many different geochemical processes, such as hydrothermal ore formation, metamorphism, metasomatism, carbonatiza- tion, weathering etc. However, data on the concentrations of REE in low- temperature, chemically well-characterized groundwaters are nearly non- existent. The REE are expected to have a pronounced tendency for hydrolysis in the typical pH-range of natural waters (cf. Wood, 1990a). However, insuf- ficient data are available for natural systems on the role of suspended particles and colloids in the partitioning of REE between solution and particulate phases (e.g. Goldstein and Jacobsen, 1988b). This information is not only relevant to improvement of the general hydrogeochemical understanding of the REE, but is essential in nuclear waste risk assessment, for predicting the migration behavior of REE fission products (e.g. Ce-144, Pm-147, Eu-154) and their chemically analogous actinide elements, for which experimental data are still scarce.

This study summarizes measurements of REE concentrations in 42 near- surface and deeper groundwater samples from the Osamu Utsumi uranium mine and the Morro do Ferro deposit, collected between December 1986 and April 1989. Data on REE in waters from the Osamu Utsumi mine did not exist at the beginning of this study. The thorium-REE occurrence at Morro do Ferro had been investigated with natural analogue and other objectives in mind, some years earlier (e.g. Eisenbud et al., 1982, 1984; Lei, 1984; Lei et al., 1986 ). However, data on REE in waters have pertained mainly to surface

REE IN GROUNDWATERS FROM OSAMU UTSUMI MINE AND MORRO DO FERRO STUDY SITES 367

waters of the South Stream, which represents the major drainage route of water from Morro do Ferro.

Detailed geological and hydrochemical descriptions of both study sites may be found elsewhere in this volume (Waber et al., 1992; Waber, 1992; Nord- strom et al., 1992). Additional information on the geochemistry of natural series nuclides and REE in groundwaters have been published previously (Coutinho de Jesus, 1989; Miekeley et al., 1991a, b).

2. M A T E R I A L S A N D M E T H O D S

2.1. Sampling sites and water characteristics

Deep groundwaters at the Osamu Utsumi uranium mine were sampled mainly from four reference boreholes of intermediate to greater depth (F 1 to F4) and from one shallow borehole (SW03). The location of these boreholes, and their relation to the local geology, hydrology and geochemistry are shown in Figs. 1 and 2.

Most groundwaters from the uranium mine can be classified as " K - F e - SO4" type, i.e. oxidizing and slightly acidic, and are the result of intense leaching of highly weathered K-rich rock masses (Nordstrom et al., 1992). Typical groundwater chemical parameters are (concentrations in rag/l, except for U, Th and the REE, which are in/~g/1): [Si] = 13-15; [K] = 10-13; [ N a ] = 0 . 1 - 1; [Ca]=0 .5-2 ; [Ba]=0.10-0.15; [Sr], [ M g ] = < 0 . 1 ; [Fe(tot) ] and [Fe(II) ] =0.6-2; [Mn] =0.1-0.3; [U] =4-15; [Th] =0 .03- 0.07; [ £ R E E ] = I - 5 0 ; [SO42-1=10-20; [HCO3-]=8-20; [F - ]=0 .3 -2 .4 ; [HPO42- ] = <0.05; [DOC] = 1-3; pH=5.4-6.1 and Eh=200-400 mV. Ad- ditional data may be found in Nordstrom et al. (1991a, b) and in Miekeley et al. (1991a). Some elements of special interest for this study have been included in Tables 1 and 4.

Water sampling at Morro do Ferro was performed in the boreholes MF11, MF10, MF13 and MF12 (Fig. 3) at the South Stream and at one "background" location (North Stream). The boreholes are situated within a magnetite breccia and form a profile which extends from about 50 m below the summit of the hill (MF11 ) to its base (MF12) and is bounded on both sides by massive magnetite dikes. It is believed that this brecciation zone represents the most hydraulically conductive part of the deposit and that the boreholes lie along a distinct groundwater flow path. Hydraulic conductivities typically range between 10-7 to 10-5 m/s for this zone, represented by strongly weathered rocks (laterite, clays), and down to 10-9 m/s for the unweathered bedrocks (phonolites) around MF 12 (Holmes et al., 1992 ).

Information on the groundwater composition at Morro do Ferro is scarcer than for the Osamu Utsumi mine. Borehole MF10, which cuts the region of

368 ~. MIEKf![ [~ '~ [ ~ k i

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[ : ~ M I ~ suEwoletn~ & mir, ot ~ lamie p l~n~i t~, G'~tm'~l i f r~ l r 4.

~ Aby~d & Ir/p~l~.nal [ n ~ nel~t~qin¢ ~ m i t ~ ( i n ~ in 1); fractured i f near 4_

Subvmlt:m,~ p~odolet~t¢ l~on~it¢~ ( i n ~ in 1 and 2); fractured if neat" 4

Subvol~mic conduit ~ locally ~r~ling to e ~ ' m e l y ]mnolithic ultrn-fir~-gr~nc~ fk~t~xurcd phono~it~-

- - ~ o1" major ~ event (formatioa o~ "[wJ~mu¢ rock= ~ d 1J-Mo-Zr-R.EE-p~tat¢ bydtxa~-tm~l mir~ra lmat lons )

RF I,ll,lll , I~ Loeatio~ ~ tmdo~ froou for d~t~ik~l inv~tigatrc~t

E l,II, Ig v ~ ixot i la in ore body 'E'

g I , II Vertical pt'~)fila in or~ body ' B'.

A1 Vertical p~l~le~ ~ o~ body ~'

Fig. 1. Osamu Utsumi mine showing the main geological subdivisions, the borehole locations, the groundwater sampling points and the surface exposure rock sampling profiles.

highest Th and REE mineralization, has significantly lower concentrations of major elements (Na, K, Fe) and potential complexing anions ( F - , HCO3- , SO4 2- ) when compared to MF 12 or uranium mine groundwaters, indicating that this water is representative of the contributions resulting from leaching of the current unsaturated zone of the deposit. The compositions of groundwaters from borehole MF 12 are very similar to those of the uranium mine, reflecting the similarity of the rocks (reduced phonolites). Dissolved organic carbon (DOC) in groundwaters from Morro do Ferro is also low ( 1 to 4 mg/

REE IN GROUNDWATERS FROM OSAMU UTSUM1 MINE AND MORRO DO FERRO STUDY SITES 369

~2

"/

L.Eq~HO r~3LKD~T~ ~ P l E ] ~ E ~,,kHPLIN~ LJOCJ~710HS w i . s . 1 . p

7 r . . . . . . . . . ® . . . . . . . . . . . . [ ]

i - ~ - - i s ® ~ * i

Fig. 2. Cross-section of Osamu Utsumi mine showing borehole locations, redox fronts mineralized zones, groundwater reference sampling locations and general direction of groundwater flow.

l) due to the scarce vegetation in the deposit area, the intense rainfall during the summer season which favours the leaching of humic matter, and its sorption by laterite, clays and other materials, during downward percolation of water. In waters collected from piezometer holes and in shallow percolation waters (mine gallery) much higher concentrations of organic matter, mainly as humic acids, were occasionally observed. These act as efficient complexors/sorbents for the rare-earths and other elements (Miekeley and Kuechler, 1987; Miekeley et al., 1991a, b, 1992b). A compilation of major and trace element concentrations as well as of the corresponding pH-Eh data of waters from Morro do Ferro has been presented previously (Miekeley et al., 1991 a; Nordstrom et al., 1991a). Some of the relevant data have been included in Table 4.

2.2. Water sampling, preconcentration methods and REE-analyses

Water sampling was done in all deeper boreholes using a double packer technique, so that water could be collected from a precisely defined depth interval. Groundwater was sampled with a submersible electric pump and

370 N ~,,IIEK~.I ~! ::~,.i

TABLE I

REE concentrations in prefiltered ( < 450 nm ) near-surface and deeper groundwaters from the Osamu Utsumi mine, also including some other chemical parameters (concentrations for REE and U in ~zg/I: for the other elements in rag/l).

REE SW03 F 1

GW47 GW58 GW66 GW14 GWI5 GWI7 GW22 GW40 GW54

La 13500 567 12515 2,9 2.1 0.45 2.7 15 3.9 Ce 6270 [24 5070 3 2 4.2 0.74 4.9 13 72 Nd 6490 250 6440 1.2 0.16 1.2 7,8 2.1 Sm 764 28.5 734 (/.24 0.14 0.02 0.18 0.9/) 0.24 Eu 206 7.8 200 0.06 0.05 0.01 0.05 0.25 0.076 Gd 585 22.8 555 024 0.13 0.025 0.17 0.73 0.23 Dy 595 23.3 530 0.21 0.14 0.021 0.15 0.75 0.27 Ho 130 5.3 ]15 0.023 0.001 0.004 0.002 0.14 0,036 Er 309 12.2 259 0.048 0.045 0.009 0.039 0,32 0,059 Yb 135 5.5 108 0.019 0.012 0,003 0.015 0,15 0.023 Lu 14 0.59 12 0.002 0.001 0,001 0.003 0,016 0.001

XREE 28998 1047 26538 6.9 8.0 1.4 9.4 39 14

pH 3.57 4.43 3.44 5.39 5.51 5.49 6.05 5.67 5.38 Eh 751 614 800 307 277 U 4500 250 2400 5.3 4.3 3.0 4.9 7.4 4.0 Fe(II) <0.10 <0.10 0.10 1.58 1.62 1.85 1.12 1.20 1.78 Fe(t) 0.12 <0.10 0.29 1,67 1.67 1.87 1.18 1,23 1.80 HCO~ <0.60 <0.60 <0.60 7. I 13 10.7 8.1 11,3 8.21 SO~- 445 8.8 430 17.0 16,4 17.2 18.8 18.1 26,6 F- 10.5 0.45 3.96 0.51 0.59 0.84 0.35 0.40 0.54

REE F2 F3 F4

GW20 GW41 GW43 GW64 GW36 GW65 GW44 GW59 GW67

La 14 8.6 1.6 5.9 12 12 67 4.3 65 Ce 8.4 6.4 1.6 8.4 8.7 11 63 4.5 63 Nd 4.0 4.0 0.62 2,4 2.6 4.7 20 1.3 23 Sm 0.48 0.42 0.060 0.26 0.31 0.74 2.6 0.19 3.0 Eu 0.13 0.097 0.020 0.079 0.10 0.30 0,78 0.072 0.94 Gd 0.41 0.22 0.057 0.24 0.34 1.3 3.1 0.30 3.7 Dy 0.31 0.18 0.079 0.25 0.29 1.4 2,7 0.42 3.8 Ho 0.062 0.036 0.014 0.037 0.054 0.28 0.54 0.088 0.82 Er 0.15 0.087 0.047 0.073 0.13 0.62 1.0 0.20 1.8 Yb 0.072 0.040 0.033 0.031 0.068 0.21 0,26 0.069 0.55 Lu 0.01 0.006 0.004 0.002 0.01 0.025 0.027 0.006 0.056

XREE 28 20 4.1 18 25 33 161 12 166

pH 5.57 5.60 5.72 5.84 5.45 5.10 5.71 6.12 6.01 Eh 559 295 261 420 370 372 233 268 U 10.1 6.4 5.2 6.7 25 4.5 73 21 48 Fe(I1) 0.56 0.98 1.05 1.50 53.4 98.9 4.22 3.55 5.04 Fe(t) 0.58 1.01 1.11 1.52 53.5 99.4 4.24 3.70 5.06 HCO3- 10.7 14,8 13.3 20.0 18.4 11.9 15.8 15.8 18.7 SO 2- 17.4 16,8 16.0 15.0 360 835 78.0 11.7 43.5 F 1.87 2.13 2.18 2.20 4.27 8.49 8.33 7.13 0.32

REE IN GROUNDWATERS FROM OSAMU UTSUMI MINE AND MORRO DO FERRO STUDY SITES

MORRO DO FERRO

371

0m ~ Minemlisatlon ( Th, REE )

_ ~ \ e / \Borehot~HF 11~ \ r-I IN t *

\ - II "

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( D \ 8o~ole ~F 12 2o0~, t0 71 metec5

f i ~ l ~ a t e r sompling ~fl monitoring points.

- ~ Simulated percolntion of rninwoter through the unsaturoted zone

2son, ~ Simutoted groundwoter fLow through the soturoted zone.

Fig. 3. Cross-section of Morro do Ferro illustrating the water sampling locations points, the mineralization zone, and the simulated infiltration and groundwater flows.

passed through an in-line prefilter (Millipore HA, 450 nm, surface area: 154 cm 2) to separate the suspended particle fraction ( > 450 nm) . The filtered water was collected in 20-1itre polyethylene containers to which a sufficient quantity of HCI or HNO3 was added to acidify the samples to pH < 2. The prefilters were air-dried and condit ioned for further analyses. The water containers were transported back to a laboratory at Polos de Caldas, on the day of collection. Volume reduction of the samples to about 500 to 800 ml was performed by evaporation in 5-1itre glass beakers, in a fume hood. The con- centrates, together with precipitated solids, were carefully transferred with deionized and distilled water and small volumes of the same acids into I litre polyethylene bottles and then sent to the analyzing laboratories (NYU and PUC).

The REE were determined by sequential ICP-AES, following two different preconcentration procedures, referred to as the NYU and the PUC methods. The detailed analytical protocols and an intercomparison of results obtained by both methods have been discussed previously (Silveira, 1986; Linsalata et

372 N. MIEKELEY El- Ai_

al., 1987; Miekeley et al., 1991 a); theretore, only a brief description of" both procedures will be presented here.

In the NYU method, the sample concentrate is filtered and the collected insoluble matter (mostly precipitated silica ) dissolved using HNO3/HF, before recombining with the filtrate. Ce-144 is then added as yield tracer and the solution evaporated and wet-ashed to remove organics. The remaining salts are dissolved in 3 M HC1 and the REE (together with Th) coprecipitated with Ca-oxalate at pH 3. The REE and Th are then separated by cation exchange using BioRAD AG 50W X8 resin. Following Ce-144 yield determi- nation (HPGe), the REE are then assayed sequentially by ICP-AES. Only the LREE have been determined by the NYU laboratory. Detection limits (95% confidence interval) range from 0.8 (La and Sm) to 5.0 (Nd) #g/sample.

In the PUC procedure, the water concentrate including the precipitated portion is wet-ashed first with a mixture of HF-HNO3-HC104 and then with H202-HNO3. The residue is then dissolved in 20 to 50 ml of HNO3 ( 1 M) and thorium, together with uranium and some other elements, are extracted with 10 ml of tri-n-octyl phosphine oxide (TOPO) in cyclohexane (0.1 M). Under these conditions, the REE (including cerium, which is now in the ter- valent state), together with other major and trace elements, are retained in the aqueous phase. The organic phase is washed with 10 ml of HNO and the washing solution recombined with the aqueous phase (REE). Uranium and thorium are determined in the organic phase (Miekeley et al., 1991 a). The aqueous solution (REE) is evaporated to dryness, the residue redissolved in 20 ml of HCI ( 10% ) and then slowly adsorbed on an Amberlite IR-120 ( 100- 200 mesh) cation exchange column, previously conditioned with HCI ( 1 M ). ICP-AES interfering elements (e.g. Fe, Ca) are eluted with 400 ml of HC1 ( 1.7 M) and the REE and Y are then etuted with 600 ml of HC1 (4 M). This solution is evaporated to dryness, the residue being redissolved in 10 ml of HCI (10%), which is then submitted for sequential ICP-AES analysis, using an ARL Mod. 35000 spectrometer.

3. RESULTS AND DISCUSSION

3.1. Rare-earth elements in groundwaters (< 450 nm) and suspended particles (> 450 nm) from the Osamu Utsumi mine

Rare-earth element concentrations of 18 water samples from the Osamu Utsumi uranium mine are summarized in Table 1. Very high concentrations of up to 29,000 #g/1 ( ZREE ) were measured in near-surface waters (e.g. sample GW-47, SW03 )) of low pH (3.6) and high sulfate concentrations (445 mg/1). These seem to be the highest REE concentrations ever published for natural waters. Concentrations lower by a factor of about 30 were reported by


Michard (1989) for hydrothermal solutions, also rich in sulfate. According to Wood (1990a), in moderately acidic waters with [SO42- ] in the range of 10 - 4 to 10 -3 M (as observed in SW03, F3 and F4; Table 1 ), the complexation of REE by sulfate, probably as Ln(SO4 ) --species (where Ln represents any REE in the trivalent oxidation state), is favoured.

Concentrations of REE in deeper groundwaters from the Osamu Utsumi mine (F1, F2) are much lower than those measured in near-surface waters (Table 1 ). The lower complexation capacity of these groundwaters and their higher pH (5.3 to 5.8, typically), which results in less effective leaching of REE from the rock and stronger sorption/hydrolysis of the REE (indicated by more pronounced association with particulate matter; Table 2), is the probable reason for the observed differences. For the typical chemical composition of groundwaters from F 1 and F2 one may expect that the REE will exist predominantly as simple ions Ln 3+ and as sulfate complexes (Wood, 1990a). Due to the moderately acidic character of waters from the Osamu Utsumi mine (pH < 6), carbonate complexation should be of minimal importance.The formation of simple, low molecular (weight/size) species is in contradiction, however, with experimental results obtained by ultrafiltration techniques, which indicate that a considerable fraction of the REE in prefiltered (<450 nm) groundwater of both study sites (up to 50% of the total REE concentrations) is associated with colloidal particles in the 1.5 to 450 nm size range (Miekeley et al., 1991b, 1992b). This would indicate that hy- drolytic polymeric species of the REE are of greater importance than believed (Wood, 1990a) and/or that complexation and sorption of these elements by other colloidal phases (e.g. hydrous ferric oxides, humic compounds) deter- mines, at least in part, the aqueous chemistry of these elements. The expected high complexation constants for REE-humic/fulvic complexes (similar to those reported for actinides, e.g. Choppin and Allard, 1985 ) suggest that even at the low DOC-concentrations in groundwater of both sites (1 to 4 mg/l), humic species may be important. For near-surface, organic-rich waters ([DOC] = 13 mg/1) of Morro do Ferro this has been experimentally confirmed for the REE and thorium, the latter element possessing chemical behavior similar to Ce (IV) (Miekeley and Kuechler, 1987; Miekeley et al., 1989, 1992b). About 90% of the REE were associated with colloidal humic compounds, predominantly in the form of humic acids. The association of a significant fraction of REE in prefiltered river water with colloidal particulate matter (inorganic and organic) has been suggested by Goldstein and Jacob- sen (1988b), Elderfield et al. (1990) and others.

Data from borehole F4 (samples GW44 and GW67, Table 1 ) show that in groundwaters from the saturated zone also REE concentrations can occasionally be very high. Both samples have high sulfate concentrations, which should provide the conditions for more aggressive leaching of these elements from

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the rock and more effective complexation, when compared to groundwaters from F l and F2 (see also below).

Figures 4 to 7 show examples of chondrite-normalized REE patterns of groundwaters from the boreholes SW03, F2, F 1 and F4. A very characteristic depletion of cerium (negative anomaly) can be seen in groundwaters from the shallow borehole SW03, which has also been observed in Morro do Ferro

~J

L

r l

U'

1() i

N.. "N-

[23

SW 47

CW~ 58 - - D

ow 66

sp 58

J

Ce Nc EL, Po He ]m L~ [::~ Pr Sm Cd Dy Er "r~

Fig. 4. Chondrite-normalized REE patterns of groundwaters from the shallow borehole SW03 ( 3-12 m), Osamu Utsumi mine. The typical negative Ce-anomaly can be seen in the prefiltered ( < 450 nm) water samples (e.g. GW58 ): it is much less pronounced in the corresponding suspended particle fraction > 450 nm (sample SP58), indicating preferential sorption of Ce 4+ ( E h of these waters 600 to 800 mV; pH: 3.4 to 4.4). REE concentrations of water samples multiplied by 1,000 for better graphical visualiztion.

Z 0 T

EL

L/3

CT~ o

40 f 3.0

2.0

!.0

O0

-1 0

i

- 2 0

~w 20

CW 4'

CW-- 45

CW 64 -L=3

~p 41

Ce Nd Eu Tb Ho Tm Lu L8 Pr Sm Od Dy Er '(b

Fig. 5. Chondrite-normalized REE patterns of groundwaters from borehole F2 (45-60 m), Osamu Utsumi mine, and in corresponding particulate matter. A slight depletion of cerium in these less oxidizing waters is still evident (Eh: 250-550 mV; pH: 5.5-5.8, typically). See also final note in Fig. 4.

3 7 6 N MIEKEiE~ FI * \

4.Cf - I +

zO ~ " ; ~ ~ ..... ' i

IL;

2;C L . . . . . . . . . . . . . . . . . . . . . . Ce Nd Eu Tb k to Tn LLJ

La Pr Sm 0~t D),, Er l b

Fig. 6. Chondrite-normalized REE patterns ofgroundwaters from borehole F1 (96-126 m) and in corresponding particulate matter (Osamu Utsumi mine). Ce-depletion in these deeper groundwaters and in their corresponding particulate matter is not observable any more (except GW14) (Eh: 260-360 mV; pH: 5.3-5.7). See also final note in Fig. 4.

C3 Z o I O b Z£ ~' j ×. ~%.". Lq I . . . . . . "~ cl ! ~ x - -..~

X

! i ! × [ i

:~Jr . . . . . . . . . I ::i;w44+-

J

I ,i Pr %r/~ : }d F,,, ~-, r i

Fig. 7. Chondrite-norrnalized REE patterns ofgroundwaters from the deepest borehole F4 ( 150- 415 m), Osamu Utsumi mine, characterized by higher REE, U, SO 2- and F- concentrations, when compared to F1 and F2 (Eh: 230-370 mV; pH: 5.7-6.1 ). Somewhat flatter REE patterns for the intermediate REE (Sm to Er) are observable (for explanation, see text). See also final note in Fig. 4.

waters from the unsaturated zone (MF11, MFI0 and MFI 3; Fig. 10). Figure 8 shows a pH-Eh diagram, adapted from Braun et al. (1990) , which has been used by those authors to explain Ce-anomalies in lateritic soil profiles, similar to that observed in core sections (MF10) from Morro do Ferro (MacKenzie et al., 1991; Waber, 1991, 1992 ), In the typical pH-Eh range of waters from the unsaturated zone of both study sites (600 -800 mV; Fig. 8 ) firstly, oxidation of Fe (II) with subsequent formation o f goethite should occur, and, sec- ondly, oxidation of Ce (III), followed by sorption onto hydrous ferric oxides

REE IN GROUNDWATERS FROM OSAMU UTSUM! MINE AND MORRO DO FERRO STUDY SITES

E (v)

377

_ o B " - - .

Ce3+ ~ o Cerionite (CeO z tryst )

L

Fe2+ C \" Goethite ((xFeOOH)

",.~ ~ ' . .o t~th-o -. ,'teO ~ ,

.. feC 0 /./~ - . 3,def.;:

I I I I 1

2 4 6 8 10 pH

Fig. 8. pH-Eh diagram adapted from Braun et al. (1990), showing the stability fields of important cerium and iron species for the typical conditions of lateritic environments, such as that observed in both study sites. The pH-Eh ranges of near-surface and groundwaters from both sites are indicated (A = SW01-03; B = MF 10, MF 11, MF 13, F2; C = F 1, F 3, F4, MF 12 ). Ce and Fe concentrations are shown in Tables 1 and 3 (for further explanation see text and orginal publication).

and/or formation of cerianite. The formation of hydrous ferric oxides, visible as surface coatings and coarse/colloidal suspended particles, due to the oxidation of groundwaters with relatively high Fe(II) concentrations, is one of the most characteristic features of the Osamu Utsumi mine environment. Due to the lower solubility of Ce (IV) and its higher affinity for sorption, when compared to the trivalent REE (e.g. Latimer, 1952; M611er, 1983 ), the above- mentioned processes should result in depletion of total Ce in water (as shown in Figs. 4 and 10) and its enrichment in zones of hydrous ferric oxide forma-

, '8 N, M I E K I L t ' I E l < i

' L , , {

I ~ i ~ " ¸ " ~ , ,

• - - . , , ° o0

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C}O t . . . . . . . . . . . . . . . . . . . . Ce Nd E/L~ 7~:, } i,-~ Inn L u

[ o Pr Sr'n {}d D,~, i } f 'b

Fig. 9. Chondrite-normalized REE patterns ofphonolites from the Osamu Utsumi mine, showing that U-mineralization of the rock samples (U-content indicated at the plots) was governed by an increase of Nd to Lu, and a slight decrease of Ce. Samples 34, 43 and 67 represent the non- or only weakly-mineralized phonolites with REE patterns similar to those observed in most of the deeper groundwaters.

tion in soils, rocks and waters. Positive Ce-anomalies have been observed in lateritic soil profiles at Morro do Ferro and in the oxidized parts of phonolites, along redox fronts (Osamu Utsumi mine, MacKenzie et al., 1991, 1992). Groundwater suspended particles ( > 450 nm) from waters of the unsaturated zone, which are composed mainly of hydrous ferric oxides, are heavily enriched in REE (see Tables 2 and 4). Preferential enrichment of Ce in these particles, due to the mentioned processes, is clearly indicated, either by the complete absence of a negative Ce-anomaly (e.g. sample SP58, Fig. 4), or by the less pronounced depletion of this element (e.g. samples SP52 and SP53, Fig. 10), when compared to the corresponding waters. This effect is also observable in the calculated association ratios Ra (representing the normaliza- tion of the REE concentrations in the particulate matter by the corresponding waters ), which are slightly higher for Ce than for the other REE (e.g. sample GW58, Table 2). These data indicate further that Ce can be oxidized in the typical pH-Eh range (600 to 800 mV) of waters from the unsaturated zone.

The deeper, more reducing groundwaters mostly do not show the characteristic negative Ce-anomaly which has been observed in near-surface waters; however, a slight depletion o f this element is indicated in some samples (Figs. 5 to 7 and 11 ). This may show that oxidation of cerium in natural waters is possible at even lower redox potentials. However, the occasional influx of surficial water into the deeper boreholes cannot be excluded, due to the complex hydrology of both sites. Groundwaters from FI, F2 and MF12 have very similar chondrite-normalized REE patterns when compared to the phonolitic


TABLE 3

REE concentrations in prefiltered (<450 nm) near-surface and groundwaters from Morro do Ferro, including also REE data for the South and North Stream (SS,NS) and some other chemical parameters (concentrations for REE and U in #g/l; for the other elements in mg/1)

REE MFIO MFI 1

GW33 GW48 GW52 GW69 GW83 GW35 GW49 GW53 GW70

La 18 32 29 34 43 1.8 6.5 4.1 7.5 Ce 0.93 4.3 3.9 2.9 1.6 0.33 2.6 0.90 1.2 Pr 2.7 4.1 5.9 5.7 5.4 0.27 1.3 1.6 1.2 Nd 6.0 12 12 15 16 0.87 3.6 2.2 4.10 Sm 0.67 1.3 1.2 1.6 1.9 0.080 0.40 0.26 0.44 Eu 0.28 0.34 0.020 0.096 0.059 0.10 Gd 0.64 0.74 0.035 0.22 0.14 0.21 Dy 0.41 0.44 0.020 0.17 0.093 0.13 Ho 0.080 0.085 0.032 0.020 0.019 Er 0.16 0.16 0.059 0.036 0.030 Yb 0.065 0.077 0.003 0.028 0.016 0.023 Lu 0.007 0.007 0.002 0.001 0.002

~REE 29 54 54 62 68 3,5 15 9.5 15

pH 4.6 6.37 5.94 5,91 5.97 5.2 5,12 5,30 5,49 Eh 505 384 420 490 430 590 544 568 U 0.08 1.55 0.70 0.57 0.055 0.13 0.75 0,44 Fe(II) 0.55 2.31 0.62 0.46 0.73 0.19 <0.10 <0.10 <0.10 Fe(t) 0.58 2.32 0.66 0.52 0.76 0.24 <0,10 <0.10 <0.10 HCO3- 12.0 18.9 10.7 8.11 8.1 26.6 12.0 8.84 8.11 SO~- 8.0 <5.0 10.0 <5.0 <5,0 11.2 <5.0 5.8 <5.0 F - 0.07 0.11 <0.10 0.19 <0,10 0.06 <0,10 0.16 0.19

REE MFI3 MFI2

GW73 GW18

Streams

GW30 GW34 GW56 GW71 GW85 SS04 NS05

La 18.2 0.90 0.95 1.9 1.3 0.37 0.23 1.2 1.6 Ce 1.48 1.6 0.46 1.0 1.1 0.94 0.56 1.2 1.0 Pr 0.14 0.39 0.17 0.02 0.29 0.20 Nd 13.0 0.28 0.67 0.49 0.22 0.14 0.36 0.31 Sm 1.51 0.03 0.02 0.08 0.032 0.02 0.01 0.07 0.03 Eu 0.34 0.012 0.009 0.018 0.010 0.017 0.014 Gd 0.56 <0.025 <0.02 0.037 <0.025 0.032 0.026 Dy 0.33 0.017 0.008 0.031 0.015 0.016 0.014 Ho 0.059 0.004 0.002 0.006 0.005 Er 0.10 0.075 0.001 0.003 0.009 0.009 0.005 Yb 0.048 0.004 0.002 0.004 0.005 0.004 0.004 Lu 0.004 0.002 0.0005 0.0005 0.002 0.002

£REE 36 3.1 1.8 4.0 3.0 1.6 1.0 3.2 3.2

pH 5.40 6.21 6.42 5.7 6.13 6.33 6.22 Eh 607 277 265 400 420 U 0.36 0.12 1.00 0.78 0.80 0.35 0.16 Fe(lI) <0.10 2.41 3.04 1.31 1.25 0.57 <0.10 Fe(t) <0.10 2.51 3.05 1.33 1.28 0.58 0.10 HCO~- 24.3 32.9 38.9 35.3 27.8 24.9 31.2 SO~- <5.0 15.4 14.0 13.2 16.2 5.36 16.8 F - 1.56 6.9 5.68 5.95 5.97 7.26 6.48

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( La to Lu): 3.64 _+ 0.06 ], which would not permit elemental fractionation, as opposed to the case for carbonate or fluoride complexation (Wood, 1990a)~.

Figure 9 shows chondritic plots for REE in phonolite samples of increasing U-content. It can be seen that uranium mineralization in these rock samples occurred along with a slight decrease of Ce (probably due to its oxidative depletion in solution, when uranium was mobilized ), but a significant enrichment of the other REE (Nd to Lu), correlating ( r> 0.99 ) with the U-content of the samples (Coutinho de Jesus, 1989). The observed high uranium and total REE concentrations in waters from borehole F4, as well as their less pronounced deficiency of the intermediate to heavy REE (Fig. 7 ), may indicate that pitchblende dissolution is a relevant process which affects the REE patterns of these waters.

Suspended particles ( > 450 nm) of deeper groundwaters, which are composed mainly of amorphous hydrous ferric oxides, have REE concentrations typically higher by a factor of 10,000 than the corresponding waters (Table 2 and 4), confirming the strong affinity of these elements for sorption (also shown by the calculated high association ratios Ra) and the potential importance of particulate matter for the transport of REE (e.g. Goldstein and Ja- cobsen, 1988a, b; Elderfield et al., 1990). The term "sorption" is used here without any distinction of the specific processes involved in the removal of REE from the aqueous phase onto the suspended matter, e.g. polymer adsorp- tion, coprecipitation, surface complexation and cation exchange. In near-surface waters of high complexation capacity, such as the sulfate-rich, acid mine waters, sorption/hydrolysis of the REE is suppressed, as indicated by the con- siderably lower association ratio of sample GW58 (Table 2 ). Whether or not suspended particles are potentially important for the subsurface transport of REE (and of their actinide analogues) depends on the retention capacity of the bedrock for these particles. For the Osamu Utsumi mine environment, the low concentrations of suspended matter ( > 450 nm) in groundwater would cause, by far the major fraction F (%) of the total REE concentrations

[REEl-tot. = [REEl ( > 4 5 0 n m ) + [REEl ( < 450nm)

to be associated with the "solution" phase ( < 450 nm) , as shown in Table 2. Typically, more than 90% of the total REE content of these waters is in the < 450 nm fraction, present in colloidal and "truly" dissolved form (Miekeley et al., 1992b).

3.2. Rare-earth elements in groundwaters (< 450 nm) and suspended particles (> 450 nm) from Morro do Ferro

The composit ion of groundwaters from Morro do Ferro and the aqueous behavior of the REE are qualitatively very similar to those already discussed for the Osamu Utsumi mine. This is a natural consequence of the similarities

REE IN GROUNDWATERS FROM OSAMU UTSUM! MINE AND MORRO DO FERRO STUDY SITES 383

in bedrock compositions and structures, and in the climates of both sites. Dif- ferences, mainly of a quantitative nature, were observed in near-surface waters. Due to the more advanced weathering state of Morro do Ferro, which re- suited in depletion of mobile elements, waters from the strongly weathered zone of the deposit (MF10, MF11, MF13 ) have generally lower contents of dissolved solids (and of potential inorganic complexors such as SO 2- and F - , for the less mobile elements, e.g. REE and Th), when compared to near- surface waters of the Osamu Utsumi mine. The very high concentrations of REE (and Th) measured in shallow waters of the uranium mine were there- fore not observed in Morro do Ferro waters, although the mineralized zone of this deposit is much more enriched in these elements.

Table 3 summarizes REE concentrations of 18 water samples from the relevant sampling locations at Morro do Ferro and of one background location (North Stream). The data indicate an approximate five-fold increase of the REE concentrations [EREE ] between about 35 m below ground level (MF11 ) and 57 m (MF10) and seem to be related to a broad, high concentration REE zone in the bedrock at this depth interval (MacKenzie et al., 1991, 1992). The chondrite-normalized REE patterns of waters (and of their corresponding suspended particle fraction) from this unsaturated zone of the deposit show the typical negative Ce-anomaly absent or only weakly evident in the more reducing groundwaters from MF 12 (Figs. 10 and 11 ). The pronounced tendency of REE for sorption onto particulate matter is also confirmed in groundwaters from this site (Table 4). Groundwater from borehole MF12 at the base of the hill and only about 100 m distant from the highly mineralized REE zone around MF10/MF11 has REE concentrations similar to the re- gional background of these elements in water (Table 3, sample NS05 ).

These data imply that REE are being leached from the highly mineralized and weathered zone of the deposit, but are redeposited before groundwater reappears in borehole MFI2 and at surface in the South Stream (Table 3, sample SS04). The important role of particulate matter in this process may be seen by the extremely high REE concentrations of up to 8,000 #g/g (EREE) in suspended particles of groundwaters from the mineralized zone (Table 4, e.g. samples GW52, GW35, GW53 ). The higher suspended particle load of these waters (e.g. samples GW35 and GW53 ) causes the predominant fraction of the total REE concentrations in water to be associated with coarse particulate matter, as shown by the low F-values in Table 4. However, these panicles seem to be efficiently retained by the porous medium of the highly weathered bedrock (see also Miekeley et al., 1992b).

4. C O N C L U S I O N S

High REE concentrations have been observed in prefiltered ( < 450 nm) groundwaters from the Osamu Utsumi uranium mine and the Morro do Ferro

384 N~ MIEKEI,E'¢ E I , q

thorium-REE deposit and are related to: (a) the abundance of REE mineralizations in both study sites; (b) the slightly acidic character of most of the waters and their moderate to high concentrations of complexing ligands for the REE; (c) the presence of colloidal particulate matter (1.5-450 nm) associated with these elements. REE concentrations in coarse particulate matter ( > 450 nm), composed mainly of hydrous ferric oxides, are higher by orders of magnitude than in the corresponding waters, confirming the pronounced affinity of these elements for sorption onto particles.

Near-surface groundwaters at both sites show a very characteristic depletion of cerium, attributable to the preferential oxidation and sorption of this element and confirmed by its enrichment in the corresponding suspended particle fractions.

This negative Ce-anomaly is not, or is only weakly, in evidence in the chondrite-normalized REE patterns of deeper groundwaters. Furthermore, the REE patterns of deeper groundwaters are also similar to those of the phonolitic bedrock, indicating that these elements are being leached without significant fractionation. Waters from the deepest borehole F4, characterized by high uranium and REE concentrations, have a somewhat less pronounced depletion of the HREE. Enrichments of the intermediate to heavy REE in phonolites of high uranium content (correlating with uranium ) suggest that pitchblende dissolution may be a relevant process which affects the REE concentrations and patterns in groundwaters from the Osamu Utsumi mine.

The results show that low-temperature groundwaters in contact with REE mineralizations may have high REE concentrations. The data from Morro do Ferro suggest, however, that the subsurface transport of these elements (and possibly also of their actinide analogues), either in particulate form or by a "true" solution phase, is strongly limited by sorption reactions. This is best seen by the absence of any significant contribution of the estimated 100,000 tons of REE of this deposit to the concentrations of these elements in prefiltered ground and surface drainage waters collected within only 100 m dis- tance from the mineralization zone.

ACKNOWLEDGEMENTS

This work was financially supported by the Pogos de Caldas Project, by the International Atomic Energy Agency (CRP, Pers. Res. Contr. 3937-BR) and by FINEP and CNPq, Brazil. The scientific and logistic collaborations of J.A.T. Smellie and E. Penna Franca are gratefully acknowledged. The help of R. Frayha, P.H. Rabello, S.S. Filho and L. Barroso in field work and of L.A. Gomiero, W.S. Filho, L.O. Wiikman and M. Dupim in laboratory work is also thankfully recognized. Thanks are also due to C.V. Dutra (GEOSOL, Belo Horizonte, BR) for ICP-AES facilities and to Prof. S.A. Wood for valu-


able suggestions to the manuscript. The authors (PUC team) are especially grateful to our laboratory technician D.J. Santos (in memoriam) for his ded- icated work during more than ten years of collaboration.

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Rare-earth elements in groundwaters from the Osamu Utsumi