FERRIFEROUS AND VANADIFEROUS KAOLINITES FROM · PDF fileFERRIFEROUS AND VANADIFEROUS...

Clay Minerals (1996) 31, 291-299

FERRIFEROUS AND V A N A D I F E R O U S KAOLINITES FROM THE H Y D R O T H E R M A L A L T E R A T I O N HALO OF THE

C I G A R L A K E U R A N I U M D E P O S I T ( C A N A D A )

C. M O S S E R , M . B O U D E U L L E * , F. W E B E R AND A . P A C Q U E T t

Centre de G~ochimie de la Surface, UPR 6251 C.N.R.S., 1 rue Blessig, 67084 Strasbourg Cedex, France, *Universit~ Claude Bernard Lyon 1, Laboratoire de Physicochimie des Mat~riaux Luminescents, URA 442 C.N.R.S., 43 Boulevard du 11 novembre 1918, 69622 Villeurbanne Cedex, France, and t Groupe des Sciences de la Terre, COGEMA, Route de

Saint Pardoux, 87640 Razks, France

(Received 1 June 1995; revised 16 February 1996)

A B S T R A C T : The uranium deposit (1350 Ma) of Cigar Lake (Canada) is surrounded by a late hydrothermal alteration halo (330 Ma) containing Fe-illites and kaolinites. Crystallochemical characterization of the kaolinites has been carried on the microscale using XRD, electron microscopy (SEM and TEM) coupled with EDX spectrometry and EPR. The large, well-crystallized particles show large amounts of Fe (0.9-1.8%) and V (0.3-0.5%). According to EPR measurements performed on both random powders and oriented samples, V occurs as the vanadyl ion VO 2§ in substitution within the octahedral sheet of the kaolinite structure in the same way as Fe 3§ Kaolinite growth proceeded through the hydrothermal alteration of anterior phyllosilicates devoid of V, induced by fluids which leached V-rich titano-magnetites in the surrounding sandstones.

R E S U M E : Le gisement d'uranium (1350 Ma) de Cigar Lake (Canada) pr6sente une aur6ole d'alt6ration hydrothermale (330 Ma) contenant des illites ferrif~res et des kaolinites. Nous avons r6alis6 une 6tude cristallochimique des kaolinites h l'6chelle de la particule en combinant la diffraction des RX, la microscopie 61ectronique (MEB et MET) coupl6e h la spectrom6trie RX en dispersion d'6nergie et la r6sonance paramagn6tique 61ectronique (RPE). Les particules, bien form6es et de haute cristaUinit6, montrent un taux 61ev6 de Fe (0.9-1.8%) mais surtout de V (0.3-0.5%). D' apr~s les donn6es RPE, enregistr6es sur des poudres et des 6chantillons orient6s, le V apparait sous forme d'ions vanadyle VO 2+, en substitution, comme Fe 3§ dans la couche octa~drique de la kaolinite. Le ddveloppement des kaolinites r6sulte de l'alt6ration hydrothermale de phyllosilicates ant6rieurs, d6pourvus de V, par des fluides enrichis en cet 616ment lors du lessivage de titanomagn6tites dans les gr~s environnants.

The Canadian uranium deposit at Cigar Lake occurs near the unconformity between the metamorphosed Lower Proterozoic basement and the Middle Proterozoic sedimentary cover. This ore deposit is surrounded by a hydrothermal alteration halo which affects the metamorphosed basement for more than 100 m depth and the Athabasca sandstones for more than 200 m above the unconformity (Fouques et al., 1986; Bruneton, 1987)o

In the sandstones, an argillization trend towards the U mineralization accompanied by changes in mineral species was described by Pacquet & Weber

(1993). The U mineralization contains exclusively ferromagnesian chlorites. The hydrothermally altered sediments close to the mineralization, <5 m thick, contain magnesian sudoRes (Fe, Mg chlorite of lower Fe/Mg ratio than the ferromagnesian chlorites of the mineralization) and hydromuscovites described as hydromuscovites 3T by Pacquet & Weber (1993), which are in fact c - l M illites (Drits et al., 1993). These phyllosilicates are associated with uraninite whose U-Pb age is

1350 Ma (Philippe et aL, 1993). Around that area runs a zone of hydrothermal alteration which

�9 19~6 The Mineralogical Society

292 C. Mosser et al.

'o - l o 4 1 - ~ 2 g

o = \ o

.,4 !

J I I I i I I I i

I0.0 20.0 300 ~0.0

~

FIG. 1. XRD patterns of the <2 fun kaolinite fraction; Go: goyazite; J: jarosite; P: pyrite; Q: quartz. Cu-Ket radiation.

formed Fe-illites and Fe-V-kaolinites associated with pitchblende whose U-Pb age is ~330 Ma (Philippe et al., 1993). These Fe-V-kaolinites were studied using different spectroscopic methods.

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

Most of the bulk samples contain Fe-V kaolinites and Fe-illites in the same granulometric fractions which make separation impossible. The Fe-V- kaolinite sample AP 113 from a perched ore body associated with the 330 Ma pitchblende was the least illite contaminated sample we could find and has, therefore, been chosen for the present study.

The <2 ~tm clay fraction was separated using standard sedimentation procedures without any pre- chemical treatment.

X-ray diffraction (XRD) patterns were performed on powder preparations with a Philips PW 17000 diffractometer equipped with a Cu tube (Cu-Kct =

1.540 ,~), graphite back-monochromator and a 1 ~ divergence slit.

After drying the sample at 110~ Sit2, A1203, MgO, Cat , Fe203, Mn304, Ti t2 and P205 were analysed using spark emission spectrometry; Na20 and K20 by flame spectrometry; V, U, Sr, Ba, Zr, Ni, Co, Cr, Zn, Cu, Sc and Y by inductively coupled plasma (ICP) emission spectrometry; C and S by IR absorption method with a LECO 125 apparatus (Samuel & Rouault, 1983), and loss on ignition (LOI) was measured after 3 h calcination at 1000~ The analytical error is ~5% for the ICP method and 2% for the others.

Microscale analyses were performed using a Jeol JSM-840 scanning electron microscope (SEM) with a Tracor Northern 5400 energy dispersive spectrometer, a Jeol electron microscope 1200 EX TEM- STEM, operating at 120 kV with a Tracor Northern energy dispersive spectrometer, and a CAMEBAX electron microprobe analyser with a wavelength dispersive spectrometer. The samples examined by

Ferriferous and vanadiferous kaolinites 293

SEM were dispersed and diluted in distilled water and evaporated on a refractory vitreous carbon disk on which they were analysed.

The electron paramagnetic resonance measurements were performed at 293 K on an X-band BROCKER ER 200 spectrometer. The EPR spectra were recorded on disoriented powder, first at room temperature and also after heating at 550~ for 20 h. They were also recorded on slightly oriented samples obtained after drying a drop of distilled water containing the kaolinite powder on a flat quartz cell. The EPR spectra were obtained under air-dry conditions with samples in quartz tubes, and under wet conditions, the samples having been soaked in water for 48 h and placed in a flat quartz cell. The EPR spectra were performed simulta- neously on a kaolinite sample from a French Tertiary kaolinite quarry in Charente Maritime, where VO 2+ was surface adsorbed by bathing the sample three times in 0.1 N VOSOa solution, and then washing it repeatedly in water until the pH

approached 7. This sample is referred to as KAOLV.

R E S U L T S

According to XRD, the AP 113 <2 Ixm fraction contains kaolinites, but also quartz, some illite or hydromuscovite and small amounts of pyrite, jarosite, goyazite, anatase (Fig. 1). The ratios of these different minerals are estimated as follows by combining the XRD results with the chemical analysis (Table la): 7 0 - 7 5 % kaolinite, 15% quartz, 8% illite or hydromuscovite, 1.6% goyazite and monazite, 1.5% pyrite and jarosite, 1.3% C, and <0.5% pitchblende and anatase or other Ti oxides. The XRD pattern (Fig. 1) shows well-ordered 1Tk kaolinites with well defined sequences of 020, l i 0 and l l i reflections in the range 2 0 - 3 0 ~ 20 and 20i , 131 and 131 ref lect ions in the range 3 5 - 4 0 ~ 20. The KAOLV contains 600 ppm surface-adsorbed V.

TABLE 1. Chemical analyses.

(a) <2 lun kaolinite fraction by ICP (wt%).

SiO2 A1203 MgO CaO Fe203 Mn304 TiOz PEOs Na20 KzO V U LOI C S

51.8 28.7 0.16 0.2 2.5 0.01 0.4 0.43 0.19 0.75 0.3 0.3 14.85 1.3 0.72

(LOI = loss on ignition)

(b) Hydromuscovites by SEM (atomic %)

Analysis no. I 2 3 4 5

Si 50.4 49.6 49.7 49.6 47.9 AI 37.4 42.1 42.5 41.7 35.5 Mg 1.9 0.7 0.5 0.7 1.5 Fe 0.2 0.8 0.0 0.0 3.0 K 9.8 6.3 6.6 7.4 11.5 Ti 0.3 0.1 0.2 0.0 0.6 V 0.0 0.4 0.5 0.6 0.0

(c) Kaolinites by STEM (atomic %)

Si 50.2 51.0 A! 48.0 46.7 Fe 1.6 1.89 V 0.3 0.5

(d) Kaolinites by SEM (atomic %)

52.1 46.5

0.9 0.5


FI~. 2. Transmission electron photomicrographs: (1) of well-formed, transparent and homogeneous kaolinite crystals; (2) of a kaolinite particle covered with small, dark particles containing Fe, $, Ti, K and U, as analysed

by EDX.


Transmission electron microscope images show well-formed kaolinites of classical hexagonal shape (Fig. 2). They present a good homogeneous electron transparency and the electron diffraction patterns are characteristic of well crystallized kaolinites. Analysis in the STEM mode on transparent and homogeneous zones of kaolinites are given in Table lc. They provide evidence of the presence of both Fe and V in the kaolinite crystals.

Scanning electron microscope analyses were performed on different, almost isolated particles. Representative analyses of kaolinites show the presence of A1, Si, V, Fe and S, with S and part of the Fe being due to minute pyrite particles associated with the kaolinites. Semi-quantitative analysis indicates 41.5% A1203, 54.8% SIO2, 0.6% V203, 1.5% Fe203 and 0.3% SO3. These data, when corrected for pyrite content (Table ld), are very similar to the STEM analyses of Table lc, with almost the same V content. Hydromuscovite in different alteration states as revealed by K loss have been analysed (Table lb). It can be seen from analyses on particles 1 and 5 that fresh hydromuscovite is V free. The other analyses show that V appears when the hydromuscovite is altered and contains less K. This V is probably brought by the hydrothermal fluids which alter the hydromuscovite and transform it into kaolinite, this kaolinization being evidenced by the high A1/Si ratio of the altered hydromuscovites. Some detrital monazites (Pacquet & Weber, 1993) were also analysed and Ce was detected. No V was found in the analysed pyrites but Ti oxide particles show the presence of Fe accompanied by small amounts of Cr, identified by the small Cr-K[3 peaks. Vanadium may be present but this cannot be ascertained because the V-Ket peak is superimposed on the Ti-KI3 peak and the V-KI3 peak coincides with the Cr-K~t peak. As no Ti was detected in the semi-quantitative analyses of kaolinite given in Table ld, it is concluded that the analysed V belongs to the kaolinite structure, as does Fe after pyrite correction. It can be assumed, therefore, that the atomic percentages given in Table lc and ld are indicative of the kaolinite composition which shows noticeable amounts of V (0.3%-0.5%) and Fe (0.9%-1.8%).

Petrographic observations (Pacquet & Weber, 1993) show the replacement of c-lM illites by ferri-kaolinite containing V, this replacement taking place by hyrothermal alteration. Microprobe analysis of these two minerals was given by the

authors, and unlike the kaolinites, no V was detected in c-lM illites.

The EPR disoriented powder spectrum of AP 113 obtained at room temperature (Fig. 3a) shows a resonance near g = 4.4 corresponding to Fe 3+ in an octahedral position in phyllosilicates (Gaite et al., 1993; Jones et al., 1974; Meads & Malden, 1975; Olivier et al., 1975; Hall, 1980; Mestdagh et al., 1980, 1982). The very sharp asymmetric two-line signal at g = 2 is attributable to stable defect centres (Angel & Hall, 1972; Meads & Malden, 1975; Herbillon et al., 1976). Similar signals were induced by X-irradiation (Angel et al., 1974, 1977; Jones et al., 1974; Cuttler, 1980, 1981) whereas Muller & Callas (1989) and Muller et al. (1990, 1992) have shown that natural kaolinites exhibit sharp g = 2 signals related to natural radioactivity. The set of resonances near g = 2 belongs to VO z+ (Pinnavaia et al., 1974; Hall et al., 1974; McBride, 1979; Monsef-Mirzai & McWhinnie, 1982). Symons (1978) and Graham (1987) describe some characteristics of the EPR solid-state spectrum of VO 2§ Vanadium in the vanadyl form, VO 2+, has a d I configuration. The presence of the 02- ion results in a strongly distorted octahedron such that all orbital degeneracy is removed, and anisotropic parameters with g//and g_L are observed. The EPR spectrum of V 4§ is characterized by the prominent octet hyperfine splitting caused by the interaction between theS = 1/2 spin of the unpaired electron and the 51V nucleus with I = 7/2.

The room-temperature spectrum of the disoriented AP 113 powder (Fig. 3a) shows the contribution of the eight hyperfine parallel components (A// = 18 mT) and the eight hyperfine perpendicular components (A_L = 6.7 mT). These values are in good agreement with those given by Graham (1987) for hectorite and Gehring et al. (1993) for kaolinites. We observed, after heating the kaolinite at 550~ like Gehring et al. (1993) after heating at 500~ that the eight hyperfine components disappeared and that a new resonance appeared with g = 1.99 and a line-width of 17.7 mT which is larger than that at 5.12 mT observed by Gehring et al. (1993).

The two oriented AP 113 powder spectra show an evident angular dependence. The eight perpendicular hypeffine components (Fig. 3b) are enhanced when the sample is oriented perpendicular to the magnetic field (A_L = 6.7 mT)) and the eight parallel hyperfine components (Fig. 3e) are enhanced when the sample is oriented parallel to the magnetic tleld (A//= lg mr).


| g=4.4

20mT @ ' ,

20rot

M

20mT I I

g=zo

18mT //components

, , , 6.7mT

_L c o m p o n e n t s

FiG. 3. EPR spectra of the <2 ltrn fraction of AP 113 kaolinite, disoriented (a) and slightly oriented samples, perpendicular (b) and parallel (c) to the magnetic field.

No difference in peak positions between dry and wet samples is noticed.

The EPR disoriented powder spectrum of the untreated KAOLV sample (Fig. 4a) presents, in a similar fashion to AP 113, resonances near g = 4.4 corresponding with Fe 3§ in the octahedral position

and a sharp asymmetric two-line signal near g = 2 related to stable defect centres. The EPR disoriented powder spectra of the VOSO4 treated KAOLV sample (Fig. 4b) shows a supplementary broad line, centred near g = 2, attributed to surface adsorbed VO 2§


g = 4.71

t g = l.,.32

g = 2 . 0

F 50roT

I I

Fic. 4. EPR spectra of the disoriented <2 ~trn fraction of KAOLV, before (a) and after (b) VOSO4 treatement.

D I S C U S S I O N A N D C O N C L U S I O N S

The AP 113 <2 ~tm fraction contains mainly kaolinite, but minor mineral phases are also present, and the question of V and Fe location is raised, as microscale kaolinite analyses indicate V in 0 .3-0 .5 atomic % and Fe in 0 .9-1 .8 atomic % amounts (Table 1).

Microscale analyses and EPR spectroscopy with a resonance near g = 4.4, corresponding with Fe 3+ in the octahedral position in phyllosilicates, confirms the now well known structural position of Fe in kaolinites.

The EPR room-temperature spectra of disoriented AP 113 kaolinite in the <2 I~m fraction material show the presence of vanadyl ions, VO 2§ very clearly. The lack of change of the VO 2§ resonance positions for the EPR spectra of wet and dry samples reveals that the vanadyl unit is not at the

surface of the mineral particles, but is part of the crystal structure. The completely different shape of the resonance peak of surface-adsorbed VO 2§ of the KAOLV sample, as a broad line centred near g = 2, strengthens that conclusion. The angular dependence seen on the two spectra 3b and 3c of oriented samples confirms the structural position of that vanadyl unit in the kaolinite structure. This orientation dependence was interpreted by Muller & Callas (1993) as evidence for vanadyl complexes sorbed as inner-sphere complexes with well defined orientations with respect to the kaolinite surface. We consider that the well resolved EPR resonances are indicative of magnetically diluted V(IV) and inconsistent with the presence of VO 2§ clusters. The disappearance of the hyperfine structure after dehydroxylation of the kaolinite at 550~ which destroys the octahedral sheet, allows us to conclude, like Gehring et al. (1993), that the VO 2§ unit


O hydrothermal fluids at the time when the kaolinites were formed 330 Ma ago. The V enrichment of the hydrothermal fluids is probably due to the alteration of the V-rich titanomagnetites present in quantity in

Q the sandstones (Pacquet & Weber, 1993). The location of V in the kaolinite structure of

these hydrothermally-formed minerals shows that the presence of the vanadyl complex cannot be taken as an unequivocal fingerprint for sedimentary

O kaolins as described by Muller & Callas (1993) and Muller et al (1995).

|

|

Q OZ- O O~-fr~ VO 2+unit

O OH- o At 3+ or Fe 3+

o Si ~'+ �9 V "+

FIG. 5. Proposed structural position of vanadyl ion, VO 2+, in the kaolinite structure (100) projection.

occupies distorted sites in the octahedral sheet. We tentatively suggest the substitution (A13+-OH -) - (V4+-O2-), where OH is an inner hydroxyl of the octahedral sheet. This substitution, taking into accoun t the rad ius of the VO 2+ unit of 1.56-1.59 A (Cotton & Wilkinson, 1986; Sharpe, 1992) or 1.67 A (Jain et al., 1984), actually induces a local distortion of the sheet, without drastic charge compensation problems (Fig. 5).

The location of V in the structure of the kaolinite particles confirms the presence of V in the geochemical environment when kaolinites were formed. On the other hand, there is no V in the c - lM illites associated with the main uraninite mineralization formed 1350 Ma ago. The c - lM illites subsequently have been hydrothermally altered into kaolinites, with V entering octahedral sites~ Vanadium was, therefore, present in the

ACKNOWLEDGMENTS

The authors wish to thank the reviewers for their positive criticisms and suggestions and especially M. McBride for his kind assistance in improving the manuscript.

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