thomas riegler système d'altération et ... - tableau de bord

THÈSE

Pour l'obtention du grade deDOCTEUR DE L'UNIVERSITÉ DE POITIERS

UFR des sciences fondamentales et appliquéesInstitut de chimie des milieux et matériaux de Poitiers - IC2MP

(Diplôme National - Arrêté du 7 août 2006)

École doctorale : Sciences pour l'environnement - Gay Lussac (La Rochelle)Secteur de recherche : Terre solide et enveloppes superficielles

Présentée par :Thomas Riegler

Système d'altération et minéralisation en uranium le long dufaisceau structural Kiggavik-Andrew Lake (Nunavut, Canada) :

modèle génétique et guides d'exploration

Directeur(s) de Thèse :Daniel Beaufort

Soutenue le 10 décembre 2013 devant le jury

Jury :

Président Alain Meunier Professeur des Universités, Université de Poitiers

Rapporteur Michel Cuney Directeur de recherche CNRS, Université de Nancy 1

Rapporteur Michel Jébrak Professeur, Université du Québec à Montréal

Membre Daniel Beaufort Professeur des Universités, Université de Poitiers

Membre Maurice Pagel Professeur des Universités, Université Paris Sud 11, Orsay

Membre David Quirt Senior Geoscientist, AREVA Resources Canada

Membre Thierry Allard Directeur de recherche CNRS, Université Paris 6, Jussieu

Membre Charlie Jefferson GEM uranium Project leader, Geological Survey of Canada

Pour citer cette thèse :Thomas Riegler. Système d'altération et minéralisation en uranium le long du faisceau structural Kiggavik-AndrewLake (Nunavut, Canada) : modèle génétique et guides d'exploration [En ligne]. Thèse Terre solide et enveloppessuperficielles. Poitiers : Université de Poitiers, 2013. Disponible sur Internet <http://theses.univ-poitiers.fr>

false

THESE

Pour l’obtention du Grade de

DOCTEUR DE L’UNIVERSITE DE POITIERS

(Diplôme National - Arrêté du 7 août 2006) UFR Sciences Fondamentales et Appliquées

Ecole Doctorale : Gay-Lussac

Secteur de Recherche : Terre solide et enveloppes superficielles.

Présentée par :

Thomas Riegler

************************

Système d’altération et minéralisation en uranium le long du faisceau structural Kiggavik - Andrew Lake (Nunavut, Canada) : modèle

génétique et guides d’exploration

************************ Directeur de Thèse : M. Daniel Beaufort

************************

Soutenue le 10 décembre 2013

devant la Commission d’Examen

************************

JURY

Rapporteurs: MM. M. Cuney Directeur de recherche CNRS, Nancy I M. Jébrak Professeur, Université du Québec à Montréal

Examinateurs: MM. M. Pagel Professeur, Université Paris Sud XI Orsay D. Quirt Senior Geoscientist, AREVA Resources Canada

C. Jefferson GEM uranium Project Leader, Geological Survey of Canada T. Allard Directeur de recherche CNRS, Paris VI Jussieu A. Meunier Professeur, Université de Poitiers D. Beaufort Professeur, Université de Poitiers

2

Remerciements

Ce travail est le fruit d’une longue aventure commencée dans les prairies de la Saskatchewan,

il y a de cela trois années. Je remercie donc les initiateurs de se projet : Joseph Roux, Dave

Quirt, Jean-Pierre Milesi et Jean-Luc Lescuyer de m’avoir fait confiance pour mener à bien

cette étude. Merci à AREVA Resources Canada, AREVA NC et ERM pour le support financier

et admnistratif de cette thèse, enfin bien sur au laboratoire HydrASA pour son acceuil.

Je tiens également remercier Daniel Beaufort, pour sa patience, ses conseils, et le partage de

ses connaissances. Ce fût un plaisir depuis notre première rencontre à Shea Creek, et j’espère

que cette relation scientifique et amicale durera après le strict cadre de cette thèse.

Bien évidement je suis reconnaissant à Michel Cuney et Michel Jébrak de bien vouloir être les

rapporteurs de ce travail. A Maurice Pagel, Dave Quirt, Charlie Jefferson, Thierry Allard et

Alain Meunier de me faire eux aussi l’honneur de faire partie de mon jury.

De manière plus particulière je voudrais remercier Thierry Allard, Mostafa Fayek et Maurice

Pagel pour leur accueil dans leurs laboratoires respectifs et de leur aide au cours de ce

travail.

Une pensée chaleureuse pour Peter Wollenberg donc la contribution a été essentielle de par

son experience et sa connaissance de la zone de Kiggavik. J’attends avec impatience le livre

du récit de tes aventures arctiques !

J’aurai aussi une pensée pour mes collègues et amis, présents ou passés. De l’Université de

Poitiers ou d’HydrASA ; merci Alain pour tes conseils toujours judicieux, ta disponibilité et

ton humour, à mes voisins de palier Paul et Laurent, Thierry toujours là quand il faut, Abder

bien sûr, et pardon pour tous les autres. Je garderai un excellent souvenir de mon passage ici.

Merci également à Marion, Freddy, Mélissa, Emilie, Antoine et Jo pour votre accueil lors de

mon arrivé à Poitiers. Plus récement à Fabien, Jean-Christophe, Sophie et Valentin pour leur

amitié, avec une mention spéciale pour les pros de la belote et de l’ultracentrifugeuse. Un

pensée aussi pour les amis de l’ESIP, Anne-Laure et Benoit ; du 504 à Orsay, Tony et

Morgane ; ou ceux Dysart Road à U of M, thanks Ryan. Enfin, de Saskatoon, Dwayne, Mario,

et tout particulièrement Nancy, Drew, et Rebecca.

3

Ces trois années à Poitiers ont été riches de nouvelles rencontres. La liste est trop longue,

mais je ne peux pas oublier Lindsay, Mariana, Juan-Pablo, Jamal, Solweig, Camila, Chi-Wei,

Blandine, Emilie, et Gabriel.

Une pensée aux Gaúchos : Amanda, et sa famille ; Edson, Victória.

À Christophe mon camarade depuis maintenant 10 ans. Pour le soutien dans les moments

difficiles et les discussions sur la métallogénie toujours passionnante.

Enfin merci à ma famille ; à Mathias, Maud, et Mathieu pour leur générosité lors de mes

nombreux passages à Paris ; à ma sœur Chloé bien sûr, et tout particulièrement à mes

parents pour leur soutien indéfectible depuis toujours.

4

Aussi audacieux soit-il d’explorer

l’inconnu, il l’est plus encore de remettre

le connu en question.

Kaspar

C’était un ancien basset qui, à force de

travail, d’énergie, d’ambition, de

volonté, de sens civique, avait réussi à

devenir un saint-bernard fort correct.

Pierre Dac

5

Table des matières

1. INTRODUCTION ............................................................................................................................................. 9

1.1. Introduction générale ...................................................................................................................... 9

1.2. Enjeux ............................................................................................................................................ 10

2. PRÉSENTATION DU MÉMOIRE DE THÈSE ............................................................................................................ 11

3. RAPPELS BIBLIOGRAPHIQUES .......................................................................................................................... 14

3.1. Le ou lie Ca adie et les assi s d’âge Pal op ot ozoi ue ..................................................... 14

3.2. Les minéralisations de type discordance ....................................................................................... 19

3.3. Gitologie des i alisatio s u a if es asso i es au dis o da es d’âge pal op ot ozoï ue 21

3.4. Contrôle structural régional et local des minéralisations .............................................................. 25

3.5. Typologie des altérations .............................................................................................................. 26

3.6. Quelques repères chronologiques : âges des minéralisations, événements thermiques (diagenèse

& intrusions) et o te te g od a i ue e lie ave les i alisatio s d’u a iu de l’Atha as a et du

Thelon 30

3.7. G ologie des gise e ts d’u a iu du dist i t de Kiggavik ........................................................... 32

4. RÉFÉRENCES ............................................................................................................................................... 41

5. ECHANTILLONNAGE ...................................................................................................................................... 47

A. LE SYSTEME D’ALTERATION DU FAISCEAU DE KIGGAVIK-ANDREW LAKE ET SES RELATIONS AVEC LES

MINERALISATIONS EN URANIUM ................................................................................................................... 48

1. ALTERATION RELATED TO URANIUM DEPOSITS IN THE KIGGAVIK-ANDREW LAKE STRUCTURAL TREND, NUNAVUT, CANADA;

NEW INSIGHTS FROM PETROGRAPHY AND CLAY MINERALOGY .......................................................................................... 49

1.1. Abstract ......................................................................................................................................... 49

1.2. Introduction ................................................................................................................................... 50

1.3. Geological setting .......................................................................................................................... 53

1.4. Sampling and analytical procedure ............................................................................................... 54

1.5. Petrography and mineralogy......................................................................................................... 57

1.6. Textural properties and crystal structure of phyllosilicates ........................................................... 63

6

1.7. Phyllosilicate crystal chemistry ...................................................................................................... 66

1.8. Discussion ...................................................................................................................................... 69

1.9. Concluding remarks ....................................................................................................................... 77

1.10. References ..................................................................................................................................... 79

2. ETUDE MICROTHERMOMETRIQUE DES INCLUSIONS FLUIDES DANS LES QUARTZ ET DOLOMITE ASSOCIEES AUX

MINERALISATIONS URANIFERES DU FAISCEAU STRUCTURAL KIGGAVIK-ANDREW LAKE .......................................................... 83

2.1. Introduction ................................................................................................................................... 83

2.2. Bong .............................................................................................................................................. 83

2.3. End Grid ......................................................................................................................................... 85

2.4. Interprétations et perspectives...................................................................................................... 91

3. ILLITE & URANINITE GEOCHRONOLOGY ............................................................................................................ 96

3.1. Introduction ................................................................................................................................... 96

3.2. Ar/Ar principle and method ........................................................................................................... 96

3.3. Samples ......................................................................................................................................... 98

3.4. Results ........................................................................................................................................... 99

3.5. Discussion .................................................................................................................................... 103

3.6. References ................................................................................................................................... 107

4. THE BASAL THELON FORMATION AT KIGGAVIK ................................................................................................. 108

4.1. Methods ...................................................................................................................................... 108

4.2. Sandstones regional setting ........................................................................................................ 109

4.3. Bulk-rock chemistry of the Basal Thelon sandstones .................................................................. 112

4.4. Petrography and mineralogy....................................................................................................... 114

4.5. Crystallochemical properties of kaolin minerals.......................................................................... 120

4.6. Microcrystalline quartz cement chemistry and in situ composition of oxygen isotopes ............ 126

4.7. Discussion .................................................................................................................................... 128

4.8. Conclusion ................................................................................................................................... 133

4.9. References ................................................................................................................................... 135

5. CARBONACEOUS MATERIAL OCCURRENCE IN THE KIGGAVIK URANIUM DEPOSITS (THELON, NUNAVUT, CANADA). ......... 139

5.1. Introduction ................................................................................................................................. 139

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5.2. Geological setting and petrography ............................................................................................ 141

5.3. Samples & Methods .................................................................................................................... 144

5.4. Results ......................................................................................................................................... 144

5.5. Discussion .................................................................................................................................... 151

5.6. Conclusion ................................................................................................................................... 157

5.7. References ................................................................................................................................... 157

B. LES MARQUEURS MINÉRALOGIQUES .................................................................................................. 163

1. SPATIAL DISTRIBUTION AND COMPOSITIONAL VARIATION OF APS MINERALS RELATED TO URANIUM DEPOSITS IN THE

KIGGAVIK ANDREW LAKE STRUCTURAL TREND, NUNAVUT, CANADA. ............................................................................. 164

1.1. Abstract ....................................................................................................................................... 164

1.2. Introduction ................................................................................................................................. 165

1.3. Regional geological setting ......................................................................................................... 166

1.4. Sampling and methods ................................................................................................................ 169

1.5. APS minerals and alteration petrography ................................................................................... 172

1.6. Electron microprobe data ............................................................................................................ 179

1.7. Whole rock chemistry and REE distribution................................................................................. 188

1.8. Discussion .................................................................................................................................... 193

1.9. Conclusion ................................................................................................................................... 200

1.10. References ................................................................................................................................... 202

2. NATURE AND STABILITY OF RADIATION INDUCED DEFECTS IN NATURAL ILLITE NEW RESULTS AND IMPLICATIONS FOR ANCIENT

RADIOELEMENT MOBILITY ...................................................................................................................................... 205

2.1. Introduction ................................................................................................................................. 205

2.2. Sampling ...................................................................................................................................... 206

2.3. Methods ...................................................................................................................................... 207

2.4. Annealing Experiments protocol ................................................................................................. 208

2.5. Results ......................................................................................................................................... 209

2.6. Preliminary discussion and concluding remarks .......................................................................... 214

2.7. References ................................................................................................................................... 216

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C. DISCUSSION GÉNÉRALE, CONCLUSIONS ET PERSPECTIVES .................................................................. 217

1. DISCUSSION GENERALE ............................................................................................................................... 218

1.1. Histoi e des v e e ts d’alt atio .......................................................................................... 219

1.2. Evénements précoces Hudsoniens ............................................................................................... 219

1.3. Mise e pla e du p ofil d’alt atio P -Thélon .......................................................................... 223

1.4. Diagenèse et évolution du bassin du Thelon ............................................................................... 224

1.5. Altération hydrothermale et mise en place de la minéralisation ................................................ 225

2. CONCLUSION GENERALES ET PERSPECTIVES ...................................................................................................... 228

3. RÉFÉRENCES ............................................................................................................................................. 229

Introduction

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1. Introduction

1.1. Introduction générale

La région de Kiggavik dans le Territoire du Nunavut est un district majeur pour l’exploration

de l’uranium dans le bouclier Canadien, et l’un des plus actifs au Canada en dehors de

l’Athabasca dans la province de la Saskatchewan. Les premiers travaux d’exploration menés

dans la zone du bassin du Thelon, il y a une trentaine d’années ont fait suite aux découvertes

de minéralisations uranifères à hautes teneurs dans le bassin Paléoprotérozoïque de

l’Athabasca. Dans les deux cas, les reconnaissances radiométriques au sol et aéroportées ont

permis la découverte des minéralisations encaissées dans le socle et dans les formations

gréseuses sus-jacentes. Par la suite, lorsque le modèle de minéralisation de type discordance a

été établi, l’exploration s’est poursuivie plus en profondeur, sur des cibles cachées sous la

couverture sédimentaire et parfois même sous des épaisseurs conséquentes de roches de socle.

Ainsi la géophysique a permis d’identifier les grandes structures régionales tandis que la

pétrographie, la minéralogie, et la géochimie ont fourni les outils pour tracer les systèmes

hydrothermaux potentiellement favorables à la formation de gites uranifères. A Kiggavik les

minéralisations ne sont pour le moment que rattachées par extrapolation à un modèle de type

discordance, de par leur localisation à deux kilomètres des grès de la Formation du Thelon. Il

est alors fondamental de le confirmer par une approche gîtologique, et pétrographique

géochimique, minéralogique afin de mieux cerner le fonctionnement du système d’altération

et de minéralisation et si possible de définir des métallotectes pertinents. Enfin la position des

minéralisations reconnues à plusieurs centaines de mètres sous la surface d’érosion actuelle à

Kiggavik, pose la question de l’extension potentielle en profondeur des minéralisations

encaissées dans le socle dans l’Athabasca. Il y a alors une problématique aux multiples

implications ne se limitant pas aux seules minéralisations du Thelon et qui peuvent s’étendre à

Introduction

10

d’autre district uranifère associées aux discordances, dans le bouclier Canadien ou en

Australie.

1.2. Enjeux

Les enjeux de ce travail sont multiples et recouvrent à la fois la minéralogie, pétrologie, la

géochimie et la gîtologie des minéralisations uranifère de Kiggavik pour tenter de comprendre

les relations entre l’altération et les concentrations en uranium dans une vue prospective. Pour

cela une approche globale, de l’échelle du district à celle du minéral, a été nécessaire afin de

cerner les objets minéralisés dans leur contexte. Il s’est agi de prendre en compte l’ensemble

des éléments favorables à la formation des concentrations en uranium tels que les

discontinuités, les lithologies, les minéraux ou bien encore les paramètres contrôlant l’oxydo-

réduction pour n’en citer que quelques uns. C’est donc par une approche multidisciplinaire,

intégrée, et en ayant recourt à un grand nombre de méthodes chacune pertinente pour la

compréhension d’un élément clef du système géologique si particulier qu’est un gîte

métallique que nous tenterons de répondre aux thématiques ou questions suivantes :

- Quelles sont les paragenèses d’altération associées aux minéralisations uranifère de

Kiggavik ?

- Quelles sont leurs implications pour l’interprétation géologique et quel potentiel

représentent les minéraux argileux produits par ces altérations pour l’exploration.

- Comment s’organisent ces altérations à l’échelle régionale ?

- Quelle est leur contribution au modèle génétique de la minéralisation ?

Présentation du mémoire de thèse

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2. Présentation du mémoire de thèse

Les résultats présentés dans ce mémoire s’organisent autour de β grandes parties, elles-mêmes

subdivisées en chapitres, principalement sous forme d’articles. De manière préliminaire le

cadre géologique régional, local concernant les minéralisations en uranium, sera présenté de

même que l’échantillonnage de la zone d’étude.

Le premier volet de ce travail (partie A) s’attardera sur la caractérisation du système

d’altération et ses relations avec les minéralisations en uranium, tandis que le second (partie

B) sera axé sur les marqueurs minéralogiques s.l. de ce système. Enfin les éléments présentés

dans ces deux grands axes feront l’objet d’une synthèse et seront discutés dans la partie C.

Dans le premier chapitre (article 1), il s’agit de caractériser l’altération en relation avec les

minéralisations uranifère de Kiggavik et pour cela d’identifier la minéralogie et les

paragenèses minérales résultant des différents épisodes d’interactions fluide roches qui se sont

succédés au cours du temps. La minéralogie permet alors de proposer des clefs

d’interprétation des systèmes hydrothermaux de Kiggavik par comparaison avec les

signatures d’altération déjà identifiées autour des gisements de type associés aux discordances

au Canada et en Australie et de proposer des pistes pour les transferts élémentaires lors de

l’altération. Enfin la cristallographie détaillée des minéraux argileux, permet, en plus des

aspects comparatifs, de proposer un nouvel outil cartographique de l’altération. De plus cet

article sera complété d'un deuxième chapitre traitant des aspects paléofluides afin de mieux

cerner les fluides en relations avec les phénomènes d’altération et de minéralisation. Enfin, un

troisième chapitre présentera des éléments de géochronologie, à la fois sur les minéraux

argileux et sur la minéralisation.

Dans un quatrième chapitre (article 2), on s’interessera aux relations entre le socle et la

couverture gréseuse, principalement par l’étude des formations basales du bassin du Thelon. Il


12

s’agira alors de retracer la nature et l’origine des apports sédimentaires puis de suivre

l’évolution diagénétique du bassin grâce entre autres aux minéraux du groupe kaolin.

Enfin en prélude à la conclusion de cette première partie, le cinquième chapitre (article 3)

concernera les matières carbonées, autre composante fréquemment associée aux gîtes de type

discordance. Elles feront l’objet d’une caractérisation poussée afin de proposer leur

intégration comme une composante de la paragenèse d’altération.

La deuxième partie (B) du manuscrit sera consacrée à l’identification de marqueurs

minéralogiques connus pour leur réponse cristallochimique aux variations du pH et des

conditions d’oxydoréduction du milieu ou leur potentiel pour la dosimétrie de la radioactivité

naturelle. Le premier chapitre (article 4), consiste en la compréhension spatiale et temporelle

des phosphates et sulfates d’aluminium formés lors des événements diagénétiques et

hydrothermaux. Il s’agit également de comprendre, à travers le prisme des terres rares, les

transferts et la mobilité des éléments, dont l’uranium, au cours de l’histoire géologique ayant

conduit à la formation des zones minéralisées à Kiggavik. Une approche « bilan de masse »

sera mise en œuvre pour compléter la pétrographie et la cristallochimie. Le deuxième chapitre

aura pour objet la compréhension de l’expression des défauts d’irradiation dans l’illite. Il

s’agit de raffiner la compréhension de la manifestation des différentes composantes

constitutives du signal RPE dans l’illite pour dégager ses caractéristiques fondamentales et

ainsi contribuer à une meilleure interprétation des circulations des radioéléments (pre, syn ou

post minéralisation) dans un système géologique ou les remobilisations de l’uranium sont un

phénomène fréquent (article 5).

Enfin, la troisième dernière partie (C) sera consacrée à une discussion globale de l’ensemble

des résultats des travaux de la thèse dans laquelle l‘accent sera mis sur la proposition d’un


13

modèle métallogénique pour les minéralisations uranifère du faisceau structural de Kiggavik –

Andrew Lake et sur les perspectives scientifiques.

Rappels bibliographiques

14

3. Rappels bibliographiques

La présentation du contexte géologique multi-échelle qui suit a pour objectif de replacer la

zone de Kiggavik dans un cadre lithologique et structural en accordant une attention

particulière à la métallogénie de l’uranium et aux processus d’altération associés. Bien

évidemment compte tenu de la proximité des contextes géologiques, de nombreux éléments

de la littérature qui font référence aux bassins d’âge Méso-Paléoproterozoique de l’Athabasca

(Canada) et de Kombolgie (Australie) seront également mentionnés.

3.1. Le bouclier Canadien et les bassins d’âge Paléoprotérozoique

Figure 3-1 : Carte géologique simplifiée du bouclier canadien, (Corrigan et al. 2007)

Le bouclier canadien, figure 3-1, est un ensemble géologique complexe dont l’organisation

actuelle est liée à l’accrétion de cratons archéens soudés par des ceintures orogéniques lors

des phases Talson - Thelon (2.0 -1.9 Ga) et Trans-Hudsonienne (1.9 - 1.8 Ga) (Hoffman


15

1988; Hoffman 1990). Le supercontinent de la Laurentia était alors composé de blocs

constitués des provinces du Supérieur-Nain, de Churchill-Wyoming et de l’Esclave

respectivement à l’est et l’ouest de la baie d’Hudson. De grandes zone mylonitiques à

l’échelle lithosphérique telle que la Snowbird Tectonic Zone (STZ) séparant les sous

provinces de Rae et de Hearne constituent l’une des discontinuités majeures de la partie ouest

de la province de Chuchill. Elles marquent un événement majeur de dislocation puis de suture

du bloc Churchill-Wyoming avec le bloc de Queen Maude (Hanmer et al. 1995; Hoffman

1988). Au final, la configuration du bouclier résulte à la fois des phases multiples de

structuration des cratons à l’Archéen, et des phases d’extension (rifting) avec mise en place de

séries volcano-sédimentaires archéennes au niveau des marges passives de ces cratons qui

sont à l’origine des ceintures de roches vertes avec les groupes de Murmac Bay, Woodburn,

Prince Albert et Mary River pour la province de Churchill (Hartlaub et al. 2004; Ashton

1988).

L’orogène Trans- Hudsonien est d’une importance majeure du point de vue métallogénique.

On retrouve toutes les étapes de son évolution sur une période allant de 2.45 à 1.95 Ga,

incluant la phase de rifting lors de l’ouverture de l’océan Manikewan (Stauffer 1984) suivie

du dépôt de sédiments sur les marges des cratons archéens ou des bassins intra-cratoniques

(e.g Groupe de Wollaston, Hurwitz, Amer) (Aspler et al. 2001), puis la formation de croûte

océanique, d’arc volcaniques océaniques et continentaux et de bassins d’arrière arc. Cet

enregistrement complet de l’orogène et la variété des environnements géologiques qui le

composent expliquent sa renommée mondiale pour les amas sulfurés volcanogéniques (e.g

Flin-Flon), magmatiques à Ni-Cu-EGP (e.g Thompson, Raglan) et dans une moindre mesure

pour l’or orogénique (e.g Seabee), ainsi que pour les formations de fer rubanées (Corrigan et

al. 2007; Kerrich et al. 2005). A cela s’ajoute des événements tectono-metamorphiques

exprimés dans une tectonique ductile le long de zones de cisaillement, d’écaillage et de


16

plissement ainsi que la recristallisation des roches dans les conditions des faciès schiste vert à

amphibolite supérieur du métamorphisme régional (Aspler et al. 2002).

Vers la transition Paléo -Méso-protérozoique, de grands bassins intra-continentaux de

composition silicoclastique se sont mis en place lors des phases de démantèlement des grands

orogènes Paléoproterozoiques. On en identifie plusieurs au Canada: Il s’agit des bassins

d’Athabasca, du Thelon, d’Hurwitz, ou d’Hornby Bay (Figures 3-1 et 3-3). Parmi ces

bassins, certains possèdent des gisements d’uranium associés à des discordances en cours

d’exploitation et/ou de prospection (Athabasca, Thelon) et les autres présentent de forte

potentialité pour l’exploration (Jefferson et al. 2007a). Enfin les études géodynamiques et les

reconstructions paléogéographiques suggèrent un lien génétique entre le fonctionnement des

grandes orogénèses du Protérozoïque et la formation de grands bassins intra-continentaux via

des phénomènes de tectonique d’échappement et de contraintes à distance, ou de flexuration

thermique d’une croûte épaissie et structurée (Karlstrom et al. 2001; Molnar et al. 1998;

Eriksson et al. 2001; Ramaekers and Catuneanu 2004). Il existe des similitudes entre les

caractéristiques des bassins silicoclasitiques du bouclier Canadien et celles du bouclier nord-

australien, en terme de facies sédimentaires, d’âge, de position paléogéographique et de

métallogénie de l’uranium. Ceci est à rapprocher des reconstitutions paléogéographiques

(Figure 3-2) qui indiquent la proximité des bassins de la Laurentia avec ceux du craton nord

australien vers la fin du Paléoprotérozoique (Betts et al. 2008; Kerrich et al. 2005; Zhao et al.

2004).


17

Figure 3-2 : Reconstitution de l’évolution de l’ensemble Australia-East Antartica et Laurentia entre 1780-1650

Ma. In (Betts et al. 2008) d’après (Bagas 2004; Duebendorfer and Houston 1987; Duebendorfer et al. 2001;

Karlstrom and Bowring 1988)

Les minéralisations d’uranium sont associées à ces bassins intracontinetaux formés d’une

séquence sédimentaire quasi horizontale, dominée par des environnements fluviatiles,

continentaux dans lesquelles des grés très riches en quartz constituent la lithologie dominante.

On retrouve néanmoins des red beds et des siltites que l’on peut rapprocher de facies de plaine

d’inondation ainsi que des facies conglomératiques fluviatiles, comme par exemple ceux qui


18

sont mentionnés à la base de la formation du Thelon (Rainbird et al. 2003; Rainbird and Davis

2007). Ces bassins sont décrits comme de grands lacs intracontinentaux remplis de sables et

de graviers avec un début de sédimentation estimé entre 1730-1740 Ma pour le bassin

d’Athabasca et 17β0-1750 Ma pour le Thelon, soit quelques dizaines de millions d’années

après les derniers événements du métamorphisme enregistrées dans les sphènes des roches de

la ceinture de plis de Wollaston et daté à 1750Ma (Rainbird et al. 2006; Miller et al. 1989;

Orrell et al. 1999).

Les roches du socle situées au niveau de la discordance sont affectées par une intense

hématitisation dont l’épaisseur varie de quelques centimètres à plusieurs centaines de mètres.

L’hématitisation est particulièrement développée à l’aplomb de grandes structures. Elle est

généralement interprétée comme un paléo-profil d’altération liée à la mise en place d’un

régolithe antérieurement à la formation du bassin sur la base d’arguments texturaux ou

minéralogiques (présence de diaspore) ou bien encore de par la nature graduelle du contact

entre les zones à hématite et à chlorite, ou bien par un comportement géochimique des

éléments traces analogue à celui décrit dans d’autres profils d’altération continentale d’âge

précambrien (Macdonald 1980; Hoeve and Quirt 1984). Toutefois des éléments probants de

paléosol (tel que des pisolithes par exemple) reste encore à trouver (Hoeve and Quirt 1984).

Selon une autre hypothèse, l’hématitisation pourrait être la conséquence de l’interaction des

saumures oxydantes issues du bassin avec la partie superficielle des roches réduites du socle

au cours de la diagenèse (Cuney et al. 2003).


19

Figure 3-3 : Localisation actuelle des bassins intracontinentaux MesoProterozoic de la partie West du bouclier

Canadien, d’après Jefferson et al, 2007 et Thomas, 2000.

3.2. Les minéralisations de type discordance

Les gîtes et gisements d’uranium associés aux discordances sont des objets géologiques

uniques formant une classe à part parmi les autres types de minéralisation uranifère de part les

teneurs (avec en moyenne ≈ β % U pour l’Athabasca, et ≈ 0.4 % pour le Thelon et la

Kombolgie) et les tonnages exceptionnels (Gandhi 1995; Ruzicka 1996; Jefferson et al.

2007a). La production issue des mines canadiennes représentait 17 % de l’uranium produit

dans le monde en 2011.


20

La découverte et l‘interprétation des gisements uranifères canadiens est une longue et riche

histoire qui commence par les découvertes de minéralisation de type filoniennes à proximité

du Grand lac de l'Ours (Port Radium) aux alentours de 1930, puis en 1952 avec la mine

Gunnar à Uranium City, exploitée par Eldorado, une compagnie minière qui deviendra

Cameco Corporation dans les années 80. Elle s’est poursuivie par la découverte d’Elliot Lake,

un modèle de minéralisation que l’on pourrait rapprocher de ce qui est connu dans le

Witwatersrand en Afrique du Sud (Hills 1987).

Par la suite, le tournant majeur pour l’exploration de l’uranium au Canada s’est produit à la

fin des années 60, avec la découverte de minéralisations à l’affleurement dans le bassin de

l’Athabasca. Le gisement de Rabbit Lake a été découvert conjointement par Gulf Minerals

Ltd & Uranerz Exploration and Mining limited dans l’Est du bassin d’Athabasca en 1968.

Celui de Cluff Lake a été découvert dans l’ouest du bassin en 1969 par Amok Limited

(Gandhi 2006), Figure 3-4. On peut d’ailleurs noter que les découvertes de Nabarlek et

Ranger en (Territoire du Nord, Australie) ont été faites à la même époque.


21

Figure 3-4 : Panneau d’indication de l’ancien camp d’Amok limited (prédécesseur de COGEMA devenu

AREVA Resources Canada) situé entre rive Sud du lac de Carswell et le site minier de Cluff Lake

(Saskatchewan, Canada).

3.3. Gitologie des minéralisations uranifères associées aux discordances d’âge

paléoprotérozoïque

Les minéralisations sont formées de lentilles massives à semi massives mais aussi de veines à

remplissage quasi exclusif d’uraninite (UO2) et situées au voisinage de la discordance basale

des bassins, entre les grès conglomératiques d’âge Paléo à Mésoproterozoiques et leur socle

métamorphique d’âge Archéen à Paléoproterozoique.Les minéralisations forment des objets

de dimensions relativement limitées mais pour lesquelles les teneurs peuvent atteindre 15 à 20

% U comme dans les mines de Cigar Lake ou de McArthur River Figure 3-5. On peut

également noter que les volumes de minéralisation reconnus pour le bassin du Thelon

représentent le dixième de celle de l’Athabasca et que les teneurs moyennes enregistrées

(environ 0.5 %) sont beaucoup plus faibles que celles mesurées dans l’Athabasca et proches

de celles rencontrées dans les gisements Australiens de l’Alligator River (Battey et al. 1987;

Ruzicka 1993). Ainsi on peut légitiment questionner le potentiel de découverte pour cette

zone encore sous explorée que constitue le Thelon et dans lequel s’inscrit la présente étude.


22

Figure 3-5 : Relation entre teneur et tonnage des différents gisements d'uranium associés à des discordances en

Australie et au Canada (d'après Gandhi 1995; Ruzicka 1996; Jefferson et al. 2007a; Gandhi 2006). Les différents

gisements et prospects de Kiggavik sont représentés en bleu alors que les deux gisements géants de l’Athabasca

que sont McArthur et Cigar sont en rouge. L’encadré donne les teneurs moyennes les ressources et les réserves

ainsi que la production passée pour chacun des bassins Paléoprotérozoiques dans lesquels des minéralisations de

type discordances ont été reconnues. Bien qu’il soit distinct du point de vue génétique, le gisement d’Olympic

Dam est donné pour comparaison, car il constitue le plus gros gisement connu dans lemonde.


23

On peut alors distinguer trois sous-types de minéralisation en fonction de la localisation des

corps minéralisés (Jefferson et al. 2007b): (1) les minéralisations encaissées dans le socle

(gisement de Collins Bay par exemple), (2) les minéralisations distribuées dans des failles ou

des corridors de fractures la long de la discordance basale (gisement de Cigar Lake par

exemple) et (3) les minéralisations situées au dessus de la discordance (minéralisation dite

perchée), Figure 3-6. Les différents sous-types de minéralisation ne sont pas exclusifs. Ils

peuvent être associés dans un même gisement comme c’est le cas à Key Lake ou à Shea

Creek (Andrade 2002; Thomas et al. 2000). Les profondeurs sous la discordance à laquelle se

font les nouvelles découvertes de minéralisation dans le socle sont de plus en plus

importantes. Elles atteignent plus de 300m dans le bassin d’Athabasca (Shea Creek) et plus de

200m dans le gisement de Jabiluka (Australie).

Il est également possible de déterminer deux sous ensembles de minéralisation sur une base

de critères géochimiques : (1) la minéralisation monométallique est essentiellement constituée

de’uraninite. Elle est surtout encaissée dans le socle et est globalement pauvre en terres rares

légères (2) La minéralisation polymétallique est généralement encaissée dans le grès ou

localisée à la discordance. Elle se caractérise par un minerai riche en Ni, Co, Cu, Mo et en

terres rares légères, Figure 3-7.


24

Figure 3-6 : Exemples de gisements illustrant les trois styles de minéralisation dans des gisements d’uranium

associées aux discordances d’après (Andrade 2002; Thomas et al. 2000; Jefferson et al. 2007b).


25

Figure 3-7 : Schéma simplifié des environnements géologiques associées aux gîtes mono- et polymétalliques

dans les systèmes de type discordance d’après (Jefferson et al. 2007b; Sibbald et al. 1976; Hoeve and Sibbald

1978; Hoeve and Quirt 1984; Ruzicka 1996; Thomas et al. 2000; Tourigny et al. 2007).

3.4. Contrôle structural régional et local des minéralisations

Quelles que soient les typologies de minéralisation rencontrées, il existe un fort contrôle

structural de leur mise en place. A l’échelle régionale, ce contrôle s’exerce par de grandes

structures souvent héritées des phases de structuration Archéennes et Paléoprotérozoiques qui

sont exprimées sous la forme de zones mylonitiques graphiteuses (Athabasca, bordure Sud

ouest du Thelon) ou non (Nord Est du Thelon, Kombolgie) (Wilde and Wall 1987). C’est le

cas par exemple de la structure P2 le long de laquelle se distribuent les gisements de Cigare

Lake et McArthur River ou bien celle du « Saskatoon Lake Conductor » qui contrôle les

minéralisations du district de Shea Creek dans les parties est et ouest du bassin de

l’Athabasca respectivement.

Ces zones de déformation majeure du socle ont ensuite été réactivées lors de l’histoire

tectonique post-dépôt sédimentaire. A cela s’ajoute l’ensemble des réseaux de failles

subsidiaires qui forment des zones favorables pour la circulation des fluides et dont le rôle


26

dans le contrôle de la minéralisation à été mis en évidence très tôt sur la mine de Rabbit lake

(Hoeve and Sibbald 1978; Hoeve et al. 1980), à Cigare lake (McGill et al. 1993), à Cluff Lake

(Beaudemont and Fedorowich 1996) ou bien encore à Shea Creek par le biais de « couloirs

brèchiques » (Lorilleux 2001) pour ne citer que quelques exemples.

3.5. Typologie des altérations

La manifestation des interactions fluides roches guidées par les structures et responsables de

la formation des minéralisations, est le témoin d’échanges et de circulations entre le bassin et

les roches de socle sous jacentes. Les conditions nécessaires sont celles décrites par les

auteurs du modèle de type diagénétique-hydrothermal dans lequel les saumures de bassins

sont à la fois les vecteurs de l’altération et de la minéralisation (Hoeve and Sibbald 1978;

Hoeve and Quirt 1984). Ces processus sont mis en œuvre lors des phases de réactivation

permettant la mise en circulation de ces fluides, alors que le bassin est soumis à une diagenèse

poussée (Pagel 1975).

La minéralogie et la géochimie des altérations des roches encaissantes des gisements

d’uranium associées aux discordances ont fait l’objet d’études au Canada, principalement

dans l’Athabasca et dans une moindre mesure dans le Thelon, et en Australie dans l’Alligator

River afin de décrire et comprendre les zonalités des gisements et de les comparer (Hoeve and

Quirt 1984; Miller and LeCheminant 1985; Kotzer and Kyser 1995; Kyser et al. 2000;

Beaufort et al. 2005; Renac et al. 2002; Cuney et al. 2003; Percival and Kodama 1989),

Figure 3-8.

On remarque alors une minéralogie des halos d’altération associés aux minéralisations

uranifères très semblable pour les trois bassins Paléoproterozoiques avec un assemblage à

illite ( interstratifié illite-smectite) sudoite (chlorite Al-Mg) clinochlore (chlorite Mg)

chlorite Fe-Mg phosphate sulfate d’alumium hydratés apatite en différentes


27

proportions selon que l’on considère les grès, la discordance ou bien le socle. On note

également la présence de la dravite parfois en abondance ( tourmaline magnésienne).

Figure 3-8 : Paragenèse minérales associées aux minéralisations de types discordances dans les Bassins de la

Kombolgie, du Thelon et de l’Athabasca in (Jefferson et al. 2007b) d’après (Kyser et al. 2000; Polito et al. 2004;

Polito et al. 2005; Creaser and Stasiuk 2007)

Cette minéralogie des phases d’altération est fortement dominée par l’illite dont les halos

peuvent s’étendre à plusieurs centaines de mètres des minéralisations aussi bien dans le socle

que dans la couverture sédimentaire. Cette altération se manifeste par une forte desilicification

et une déstabilisation des minéraux initialement présents, principalement les aluminosilicates,

et aussi les minéraux accessoires (zircon, monazite, tourmaline, etc). Ces processus induisent

une déstabilisation des minéraux ferromagnésien du socle tels la biotite ou l’amphibole et se

manifeste sous la forme d’un blanchiment des roches ainsi altérées (Hoeve and Quirt 1984).


28

Enfin, le potassium libéré par la dissolution des aluminosilicates du socle peut alors alimenter

l’illitisation des kaolinites présentes dans la couverture gréseuse.

De plus une zonalité apparaît, dans la répartition des polytypes de l’illite autour des corps

minéralisés (Laverret 2002) et la distribution spatiale des différentes chlorites Fe-Mg ou Al-

Mg (sudoite) (Hoeve and Quirt 1984). Cette zonation minéralogique est contrôlée par

plusieurs facteurs tels que l’évolution de la composition du fluide hydrothermal, la durée des

phénomènes d’interaction fluide-roche ou bien encore l’hydrodynamisme du système.

Enfin la distribution spatiale des altérations argileuses s’exprime schématiquement sous la

forme de deux typologies selon que l’expression de l’altération est prédominante dans le socle

ou dans la couverture, Figure 3-9. Ces distributions donnent lieu à une interprétation selon les

modes « egress » ou « ingress » (Quirt 2003). Le mode « egress » se distingue par une large

enveloppe à illite sudoite largement exprimée dans la couverture gréseuse entourant une

zone plus riche en sudoite et un cœur riche en chlorite ferromagnésienne, biotite et sudoite qui

semble s’enraciner sur la structure guidant l’altération. Le mode « ingress » affecte

essentiellement le socle. Il montre une zonalité inverse des altérations avec au cœur un

assemblage à illite sudoite entouré d’un halo à sudoite illite puis d’une zone à chlorite

Fe-Mg, biotite et sudoite. On peut éventuellement considérer que ce dernier assemblage,

dominé par les minéraux métamorphiques du socle est l’expression plus distale de l’altération.

Chacune de ces altérations est considérée comme un marqueur du passage des fluides soit du

socle vers la couverture (egress) soit de la couverture vers le socle (ingress).


29

Figure 3-9 : Schéma des types de minéralisations "egress" et "ingress" in (Jefferson et al. 2007b) d’après (Hoeve

and Quirt 1984; Sibbald 1985; Fayek and Kyser 1997)


30

3.6. Quelques repères chronologiques : âges des minéralisations, événements

thermiques (diagenèse & intrusions) et contexte géodynamique en lien avec les

minéralisations d’uranium de l’Athabasca et du Thelon

Figure 3-10 : Mise en relation des grands événements tectoniques à l’échelle du bouclier avec les âges des

minéralisations uranifère dans le bassin de l’Athabasca, (Alexandre et al. 2009)


31

Les travaux de datation des minéralisations sont toujours complexes de part la forte réactivité

chimique de l’uraninite et sa propension à recristalliser au cours des temps géologiques. Les

différentes études géochronologiques permettent néanmoins de caler l’âge des minéralisations

avec un certain nombre de moments clefs de la géodynamique du craton canadien

(orogenèses, magmatisme, ritfing, diagenèse), Figure 3-10.

Les principaux âges de mise en place des minéralisations uranifères du le bouclier canadien

s’étalent entre 1800 et 900 Ma. Pour les minéralisations de l’Athabasca, ils peuvent être

résumés comme suit: (1) 1780 20 Ma pour les veines de Beaverlodge (Koeppel 1967), (2)

1586 15 (Alexandre et al. 2009) 151922 et 1486 9 Ma U/Pb à McArthur River et

1467 47 à Cigare Lake parmi les plus anciennes mesurées a la sonde ionique dans

l’Athabasca (Fayek et al. 2002), (3) 1329 17 Ma à Key lake (Cumming and Krstic 1992), (4)

1275 22 à Shea Creek, ou bien encore 900 Ma (Fayek et al. 2002). Ces datations

n’excluent pas la possibilité de remobilisations plus tardives d’une partie de la minéralisation

sous la formes de fronts d’oxydo-réduction datés jusqu’à moins de 400 Ma (Mercadier et al.

2010).

A Shea Creek, il existe une bonne concordance entre les âges obtenus sur les illites du halo

d’altération (1453 2, 1330 20 et 1235 Ma ) et ceux obtenus sur les uraninites du gisement

(Laverret et al. 2010).

Comme ces auteurs l’on proposé il est possible de relier ces différents âges d’une part à

l’événement diagénétique ayant affecté l’ensemble du bassin au alentour de 1500 Ma

(contemporain de la fin de l’orogène Mazatzal et des minéralisations les plus anciennes) et

d’autre part aux remobilisations qui ont pu être contemporaines d’autres phases tectoniques

tels que les orogènes Bertoud et Grenville respectivement vers 1400 et 1100 Ma et les dykes

de Mackenzie à 12672 Ma (LeCheminant and Heaman 1989), la formation et dislocation de


32

la Rhodinia ou de la Pangée (1000, 700 et 300 Ma), ou bien enfin des phénomènes d’uplift et

d’érosion pour les fronts redox mis en place vers 400 Ma.

Les données de datations de la littérature sont bien moins abondantes pour les gisements du

bassin du Thelon. On peut toutefois mentionner des âges à 1403 10 Ma U/Pb (Farkas

1984) sur roche totale pour Kiggavik ainsi que des âges K/Ar sur illite à 1386 24 Ma, 1362

21 Ma (Miller and LeCheminant 1985) ou 1073 Ma (Miller 1981). Des âges plus récents à

environ 1200 Ma ont été obtenu sur la minéralisation massive d’End, échantillonné en 2010 et

confiée à M. Brouand pour analyse à la microsonde ionique. On retrouve des gammes d’âges

concordantes entre évènements de minéralisation et d’altération. On peut noter des âges pafois

proches pour les minéralisations en uranium associées à l’Athabasca et au Thelon.

Enfin l’âge de la sédimentation ou du début de la diagenèse à été contraint à 1730-1750 Ma

pour le bassin d’Athabasca par la datation des derniers épisodes métamorphiques (Orrell et al.

1999) et à 17206 Ma et 1667 5 Ma par datation Pb/Pb sur apatite diagénétique en ciment à

la base du Thelon (Miller et al. 1989) et datation in situ U/Pb sur fluorapatite (Davis et al.

2011) respectivement pour le bassin du Thelon .

3.7. Géologie des gisements d’uranium du district de Kiggavik

3.7.1. Localisation et bref historique des travaux d’exploration

Le projet Kiggavik anciennement dénommé Lone Gull est situé à 80 km à l’ouest du hameau

Inuit de Baker Lake, au Nunavut. Les premiers travaux d’exploration de ce gisement qui est

activement prospecté depuis quelques années, remontent à 1974 lors de la découverte de

minéralisations uranifères sub-affleurantes à Kiggavik s.s lors d’une campagne de radiométrie

aéroportée menée par Urangesellschaft Canada (UG). Ceci conduira à la découverte de la

lentille de Main Zone en 1977 avec un premier sondage intersectant 35m de roches altérées

contenant 1% U3O8 (Fuchs et al. 1986). L’actuel camp d’exploration des équipes d’AREVA


33

Resources Canada (ARC) se situe à quelques centaines de mètres à l’Est de cette zone, à

l’emplacement de l’ancien camp Schutlz-Sissons South d’UG, Figure 3-11.

Figure 3-11 : Camp d’exploration de Kiggavik, crédit photo aérienne : AREVA Resources Canada

Le district comprend plusieurs zones minéralisées d’importance économique ainsi que de

nombreux prospects ou zones à fort potentiel en cours de développement ou de test. Ainsi du

Nord Est au Sud Ouest des propriétés formant le projet on retrouve Kiggavik ss., la

découverte initiale (1974), Bong (1986) , End (1987), et enfin Andrew (SW Grid, 1988) puis

Jane (1993), l’ensemble de ces zones minéralisées totalise un contenu de ressources

historiques d’approximativement 58000t d’uranium. Le projet comprend deux grandes zones

non contigües de permis d’exploration composées de St Tropez au Nord et de Kiggavik au

Sud, Figure 3-12 et 3-15.

A l’échelle régionale, les guides de prospection suivis ont été tout d’abord, la géophysique

avec la combinaison de données de résistivité et de gravimétrie permettant de localiser les

structures et les zones altérées (Hasegawa et al. 1990). Par la suite de proche en proche,

l’exploration a été menée en suivant le contact entre les bancs de quarztites massifs (archéen


34

ou Paléoproterozoïques) et les métagraywackes Archéan du Woodburn Lake Group (WLG).

Ce contact étant lui-même souligné par de puissantes zones de brèches à remplissage de

quartz, souvent associées à une forte hématitisation. Ces brèches sont par ailleurs recoupées

en sondages dans l’ensemble des zones minéralisées de Kiggavik ou elles délimitent souvent

deux zones de minéralisations de part et d’autre du couloir de brèche à remplissage de quartz.

Figure 3-12 : Carte des droits miniers pour les projets ARC de la bordure Sud Ouest du bassin du Thelon

(Kiggavik et St Tropez) et localisation du camp d’exploration de Kiggavik (Morisson et al. 2012)


35

3.7.2. Séquence lithostratigraphique de la bordure Sud Est du bassin du

Thelon et position des minéralisations

L’encaissant des minéralisations et principalement formé par des métagraywackes Archéens

(2711 3 Ma) appartenant au groupe de Woodburn, et dans une moindre mesure dans des

métagranodiorites (assignées au méso-Archéen) mis eu contact avec les métagraywackes par

le biais d’une structure mylonitique plate, parallèle à la foliation, Figure 3-13. Les granites et

syénites Hudsonienne ou Nuetlin peuvent être minéralisés plus rarement. Un lien génétique

entre la minéralisation en uranium et les roches intrusives été évoqué dans les travaux

historiques sur le gisement de Kiggavik (Weyer et al. 1987). Cette proximité spatiale entre

roches intrusives et minéralisations uranifères est exprimée à Kiggavik ss, à End Grid et à

Andrew Lake. Les roches intrusives sont majoritairement représentées par des granites et des

syénites hudsoniennes souvent riches en fluorine (1840-1830 Ma) sous la forme de dykes

sécants sur la foliation horizontale ou de sills entourés d’un fin halo de cornéennes (Jeffrey et

al. 2010; Peterson et al. 2002). Les intrusions de l’épisode Nuetlin et leur équivalent

volcanique (Pitz rhyolite) datés à à 1750 Ma sont présents à l’affleurement à Andrew lake et

sont fréquemment recoupées en sondage sous la forme de dykes de granite porphyrique à

texture rapakivi.

Les minéralisations semblent s’enraciner plus en profondeur vers le SW du faisceau

minéralisé. De Kiggavik jusqu’à Andew, la minéralisation est trouvée à l’affleurement, dans

les métagraywackes altérés. A Jane, elle est encaissée dans des orthogneisses sous la forme de

veines à remplissage de pechblende. Les quartzites qui séparent les lentilles supérieures et

inférieures du gisement de Kiggavik sont également rencontrés au dessus de la zone

minéralisée à Bong, Figure 3-14.

On peut enfin noter que les contacts entre les unités lithologiques ou les structures brêchiques

forment souvent des zones favorables au dépôt des lentilles minéralisées. C’est tout


36

particulièrement le cas au contact entre les roches intrusives et les méta-graywackes ou autour

des brèches à remplissage de quartz que l’on peut l’observer à Bong ou End.

Figure 3-13 : Coupe géologique dans une section du gisement d’Andrew Lake (données UG/AREVA Resources

Canada Inc.)


37

Figure 3-14 : Coupe du gisement de Center Zone à Kiggavik montrant la position des minéralisations par rapport

aux bancs massifs de quartzite (Weyer 1992)

3.7.3. Structuration ductile et déformation fragile

Comme il a été présenté précédement, un lien fort est établi entre les structures et les

minéralisations dans les gisements de type discordance. A Kiggavik l’histoire tectono-

métamorphique polyphasée et complexe a contribué au développement des discontinuités qui

sont les drains préférentiels des fluides minéralisateurs. On reprendra rapidement ici les

résultats établis par les travaux de terrains et les analyses structurales de Jean-Louis Feybesse

et Nicolas Flotté réalisés en 2009, 2010 autour de Kiggavik (Flotté 2009; Feybesse 2010).

- La déformation ductile

On peut identifier deux grandes phases tectoniques tardi-archéennes avec (1) la mise en place

de plis à vergence NW et NNW dans le socle méso-Archeén puis (2) la mise en place de

nappes de chevauchement exprimée par une zone mylonitique faiblement pentée à la base des


38

métasédiments du Woodburn group et le développement d’une foliation sub-parallèle au

contact mylonitique que l’on retrouve communément dans la zone de Kiggavik (Beaudemont

1995).

Une troisième phase de déformation est caractérisée par le developpement d’une zone de

cisaillement Nβ70 à linéation d’étirement E-W. Elle est principalement observée à la base des

quartzites et elle constitue une des discontinuités majeures de la zone de Kiggavik avec pour

conséquence l’entrainement des quartzites massifs dont la position dans la pile lithologique

(elles reposent sur les roches du Woodburn Group par le jeu d’une zone mylonitiques

faiblement pentée vers le Nord) fait toujours l’objet de débat. Cette phase de déformation est

attribuée à la phase hudsonienne (Paléoproterozoique)

- Déformation fragile

Plusieurs familles de failles sont identifiées avec à l’échelle régionale. Il s’agit de la faille

du Thelon, Judge Sisson lake ou leur parallèles dont l’expression est visible aussi bien à

l’affleurement qu’en sondage autour des zones End Grid et Bong. On a alors, pour structure

de premier ordre, la faille d’Andrew lake, d’orientation NE-SW et de cinématique interprétée

dextre par la suite réactivée en senestre. Les minéraliations d’Andrew et Jane se trouvent

distribuées le long de cette structure. Le second ordre de faille est d’orientation globale

WNW-ESE (N70 à N100) avec une cinématique dextre. Ces dernières structures sont

soulignées par une forte cataclase et de puissantes zones de brèches à remplissage de quartz.

Elles présentent également une hématitisation intense qui affecte le socle métamorphique, les

intrusifs (hudsonien et Nuetlin) ainsi que les sédiments du Thelon.

A ces failles (et les brêches tectoniques associées) sont reliés un ensemble de structures

dextres redressées d’orientation N1β0-N130, formant parfois des zones de relais avec les


39

structures N70 et un autre ensemble de failles normales Est- Ouest faiblement pentées vers le

Nord.

Enfin une série de failles tardives sont venues modifier la géométrie des éléments structuraux

précédents et affecter les sédiments des bassins de Baker lake et de Wharton. Il s’agit, soit de

failles Nord-Sud, à N30 senestres, soit de failles N160 dextre (Rainbird and Hadlari 2000).

Ces structures tardives ne se limitent pas à la zone de Kiggavik. Elles s’expriment dans

l’ensemble de la province de Rae. Elles ont été observées dans le gisement de Sue C dans la

partie est du bassin d’Athabasca (Flotté et Feybesse, 2008).


40

Figure 3-15 : Carte géologique révisée de la bordure Sud Ouest du Thelon et synthèse stratigraphique des formations Archéennes d’après (Jefferson et al. 2011)

Références

41

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Creaser RA, Stasiuk LD (2007) Depositional age of the Douglas Formation, northern Saskatchewan, determined by Re-Os geochronology In: Jefferson CW (ed) EXTECH IV: Geologyand Uranium EXploration TECHnology of the Proterozoic AthabascaBasin, Saskatchewan and Alberta. Geological Survey of Canada, pp 341-346.

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Jefferson CW, Thomas DJ, Gandhi SS, Ramaekers P, Delaney G, Brisbin D, Cutts C, Portella P, Olson RA (2007a) Unconformity-associeted uranium deposits of the Athabasca Basin, Saskatchewan and Alberta EXTECH IV. pp 23-67.

Jefferson CW, Thomas DJ, Gandhi SS, Ramaekers P, Delaney G, Brisbin D, Cutts C, Quirt D, Portella P, Olson RA (2007b) Unconformity-associated uranium deposits of the Athabasca Basin, Saskatchewan and Alberta In: Goodfellow WD (ed) Mineral Deposits of Canada: A synthesis of Major Deposit-types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods. Geological Association of Canada, Mineral deposits division, pp 273-305.

Jefferson CW, Pehrsson S, Peterson T, Chorlton L, Davis DW, Keating P, Gandhi SS, Fortin R, Buckle JL, Miles W, Rainbird RH, LeCheminant AN, Tschirhart V, Tschirhart P, Morris W, Scott J, Cousens B, McEwan B, Bethune K, Riemer W, Calhoun L, White J, MacIssac D, Leblon B, Lentz D, laRocque A, Shelat Y, Patterson J, Enright A, Stieber C, Riegler T (2011) Northeast Thelon region geoscience framwork-new maps and data for uranium in Nunavut In: Canada NR (ed). Geological Survey of Canada.

Jeffrey S, Peterson T, Jefferson CW, Cousens B (2010) Proterozoic (1.85-1.7 Gaà granitoid rocks and uranium in the Baker Lake -Thelon Basin region, Nunavut GeoCanada2010. Calgary.

Karlstrom KE, Bowring SA (1988) Early Proterozoic assembly of tectonostratigraphic terranes in Southwestern North America. J Geol 96:561-576.

Karlstrom KE, Åhäll K-I, Harlan SS, Williams ML, McLelland J, Geissman JW (2001) Long lived (1.8-1.0 Ga) convergent orogen in southern Laurentia, its extensions to Australia and Baltica, and implications for refining Rodinia. Precambrian Research 111:5-30.

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Kyser K, Hiatt EE, Renac C, Durocher K, Holk G, Deckart K (2000) Diagenetic fluids in Paleo and Meso-proterozoic sedimentary basins and their implications for long protracted fluid histories In: Kyser K (ed) Fluids and Basin Evolution. Mineralogical Association of Canada, pp 225-258.

Laverret E (2002) Evolution temporelles et spatiales des altérations argileuses des gisements d'uranium sous discordance, secteur de Shea Creek (basin d'Athabasca, Canada). Université de Poitiers, pp 192.

Laverret E, Clauer N, Fallick A, Mercadier J, Patrier P, Beaufort D, Bruneton P (2010) K–Ar dating and δ18O–δD tracing of illitization within and outside the Shea Creek uranium prospect, Athabasca Basin, Canada. Applied Geochemistry 25:856-871. doi: http://dx.doi.org/10.1016/j.apgeochem.2010.03.004.

LeCheminant AN, Heaman LM (1989) Mackenzie igneous events, Canada: Middle Proterozoic hotspot magmatism associated with ocean opening. Earth and Planetary Science Letters 96:38-48. doi: http://dx.doi.org/10.1016/0012-821X(89)90122-2.

Lorilleux G (2001) Les brèches associées aux gisements d'uranium de type discordance du bassin d'Athabasca, Saskatchewan, Canada Institut National Polytechnique de Lorraine. Université Henri Poincaré, Vandoeuvre les Nancy.

Macdonald R (1980) Mineralogy and geochemistry of a Precambrian regolith in the Athabasca Basin. University of Saskatchewan, Saskatoon, pp 151.

McGill B, Marlat J, Matthews RB, Sopuck V, Homenuik L, Hubergtse J (1993) The P2 North uranium deposit, Saskatchewan, Canada. Exploration Mining Geology 2:321-331.

Mercadier J, Cuney M, Cathelineau M, Lacorde M (2010) U redox and kaolinisation in basement-hosted unconformity-related U ores of the Athabasca Basin (Canada): late U remobilisation by meteoric fluids. Miner Deposita 46:105-135.

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Miller AR, LeCheminant AN (1985) Geology and uranium metallogeny of Proterozoic supracrustal successions, central District of Keewatin, N.W.T with comparisons to northern Saskatchewan Geology of uranium deposits. Canadian Institute of Mining and Metalurgy, pp 167-185.

Miller AR, Cumming GL, Krstic D (1989) U-Pb, Pb-Pb, and K-Ar isotopic study and petrography of uraniferous phosphate-bearing rocks in the Thelon Formation, Dubawnt Group, Northwest Territories, Canada. Canadian Journal of Earth Sciences 26:867-880.

Molnar P, Houseman GA, Conrad CP (1998) Rayleigh-Taylor instability and convective thinning of mecanically thickened lithosphere:effects of non-linear viscosity decreasing exponentially with depth and horizontal shortening of the layer. Geophys J Int 133:568-584.

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Polito PA, KYSER TK, Marlatt J, Alexandre P, Bajwah Z, Drever G (2004) Significance of Alteration Assemblages for the Origin andEvolution of the Proterozoic Nabarlek Unconformity-Related Uranium Deposit,Northern Territory, Australia. Economic Geology 99:113-139. doi: 10.2113/gsecongeo.99.1.113.

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Echantillonnage

47

5. Echantillonnage

Tableau 5-1 : Description générale de l’échantillonnage de la zone de Kiggavik – St Tropez.

DrillHole Depth

(m) Samples Purpose

W97-4 191.1 18 Regional Alteration background

B1-94-1 224.3 9 "

BSE1 154.5 5 "

W-2 94.2 4 "

Bong51 300.6 4 carbonaceous material

Bong50 386.7 28 Reference in Bong prospect










BongExt2 177.1 9 Alteration without mineralisation

SW8 191.1 17 Reference in Andrew deposit

And10-01 387.0 38 "

End09-11 378.8 15 Reference in End deposit



End09-08A 347.4 18 Reference in End deposit



End09-04 420.0 35 Reference in End deposit/Mineralization



End13 230.7 9 alteration without mineralization

SL9 335.9 5 Sleek Area alteration without Umin

Th18 123.6 7 regional sandstones NE of Kiggavik

Outcrops

Granite Grid

7 basal Thelon Fm conglomeratic sandstones

St Tropez

7 basal Thelon Fm conglomeratic sandstones

Uno

1 Fluorite bearing granite

Andrew&End 10 quartz breccia & porphyitic rhyolite

Unconformity Lake 8 basal Thelon & Woodburn

Echantillonnage

48

A. L E SYSTEME D’ALTERATION DU FAISCEAU DE K IGGAVIK -

ANDREW L AKE ET SES RELATIONS AVEC LES

MINERALISATIONS EN URANIUM

Alteration related to uranium deposits in the Kiggavik-Andrew lake structural trend, Nunavut, Canada; new insights from petrography and clay mineralogy

49

1. Alteration related to uranium deposits in the Kiggavik-Andrew lake structural trend, Nunavut, Canada; new insights from petrography and clay mineralogy

Thomas Riegler*, Jean-Luc Lescuyer***, Peter Wollenberg***

Dave Quirt*** and Daniel Beaufort**

Accepted in Canadian Mineralogist

*IC2MP, HydrASA / ERM, Université de Poitiers, CNRS UMR 7285, Bâtiment

B08, Rue Albert Turpin, 86022 Poitiers cedex, [email protected]

** IC2MP, HydrASA, Université de Poitiers, CNRS UMR 7285, Bâtiment B35,

Rue Michel Brunet, 86022 Poitiers cedex, [email protected]

***AREVA Mines & AREVA Ressources Canada, Tour AREVA, 1 place Jean

Millier 92084 Paris la Défense Cedex

1.1. Abstract

The Kiggavik project, located 70km west of Baker Lake (Nunavut) is a major uranium

exploration project in the Canadian arctic, with three significant basement hosted uranium

deposits (Kiggavik, End and Andrew) which spread along a NE-SW trend a few kilometers to

the south-eastern border of the Thelon Basin. These deposits are closely associated with

alteration zones in which clay minerals are abundant. At the scale of the whole structural

trend, the alteration paragenesis is composed of illite ± sudoite ± hematite ± aluminum

phosphates sulfates minerals (APS). Alteration petrography and mineral paragenesis are

similar to those identified in basement hosted uranium deposits related to Paleoproterozoic

unconformities in the Athabasca Basin (Canada) or the Alligator River (Australia). The


50

alteration haloes are characterized by two similar types of phyllosilicate assemblages

(dioctahedral micas or illite and chlorites) corresponding to a regional retrograde

metamorphic stage that was overprinted by hydrothermal alteration during the mineralization

event. These two assemblages can be distinguished on the basis of crystallographic and

chemical properties and mapping of structural parameters such as the variation of crystallinity

along the c-axis or the polytypes of phyllosilicates can been used as a vector to

mineralization. The crystal-chemistry of the hydrothermal phyllosilicates replacing the

previous metamorphic minerals indicates a release of iron. This last point is fundamental

regarding the occurrence of hematite in alteration zones and points out the potential effects of

iron redox state in the control of uranium precipitation during the hydrothermal event.

1.2. Introduction

The Kiggavik project (previously named Lone Gull), located 70km west of Baker Lake,

Nunavut hosts several significant uranium deposits and very prospective areas, with an overall

uranium content of approximately 50,000t U of historical resources (Jefferson et al. 2007).

The deposits and prospects are structurally controlled along a NE-SW trend and are

exclusively basement hosted in Late Archean metasediments. This rock package consists of

dominant meta-arkose (wacke) and minor quartzite and rhyolites (Miller and LeCheminant

1985) later intruded by a suite of igneous bodies including Hudsonian fluorite-bearing

granites, syenites and lamprophyres (Peterson et al. 2002). The northern end of the trend,

where the Kiggavik deposit is located lies two kilometers south of the unconformity between

the Archean basement the late Paleoproterozoic Thelon Formation sandstones (Miller et al.

1989). Sub-outcropping mineralization was first discovered in 1974 following an airborne

radiometric survey by Urangesellschaft Canada, then followed with ground resistivity and

gravity surveys in further exploration work to detect alteration zones (Fuchs et al. 1986,

Hasegawa et al. 1990). This target identification method has been successfully used and lead


51

to several additional discoveries along the trend indicating that alteration features where in

most cases spatially related to mineralized zones.

Following the trend toward the southwest are found the Bong prospect, the End and

Andrew deposits (Fig. 1-1). The last known mineralization southward of this 18 kilometer

long mineralized trend is the Jane prospect. The main structural feature and regional

pathfinder are kilometer long ENE-WSW faults showing an intense hydrothermal activity

with both hematization and quartz veining. The nature and origin of the hydrothermal

processes responsible for uranium mineralization and its associated alteration haloes are still

unclear at Kiggavik.

Initially, exploration was carried out to find high-grade uranium in a geological context

similar to the basement rocks underlying the late Paleoproterozoic Athabasca Basin in

Northern Saskatchewan. Alternative models favored hydrothermal systems partly related to

heat sources generated by the numerous intrusive bodies identified in the area (Weyer et al.

1987) but geochronology of pitchblende (bulk U/Pb) and illite (whole rock K/Ar) from altered

and mineralized metasediments showed younger but discordant with possible lead loss or late

remobilization ages respectively around 140310 Ma, (Farkas 1984) and 1386±24 Ma K/Ar

(Miller and LeCheminant 1985). Salinity of the basinal brines, stable isotopes on diagenetic

clays and geochronology in the Thelon and Athabasca Basins had highlighted similar

diagenetic histories (Hiatt et al. 2010, Renac et al. 2002). However, uranium deposits within

the Kiggavik-Andrew Lake trend significantly differ from the unconformity-related uranium

deposits in the Athabasca basin by (1) the absence of graphite along the regional faults as well

as in the mineralized structures and (2) a lower average ore grade (around 0.5%).


52

Figure 1-1 : Kiggavik-Andrew Lake simplified geological map, showing deposit & prospects. Black dots for

sampled drillhole locations, simplified geology based on GSC Open file 1839, Schultz Lake GIS map.


53

The aim of the present study is to determine the sequence of mineral crystallization

that occurred in the alteration halo associated with uranium mineralization along the

Kiggavik-Andrew Lake trend. Alteration petrology has been used to refine the spatial

distribution of the mineral assemblages and the chronological relationships between the

secondary mineral phases related to uranium mineralization. The mineralogical characteristics

of the alteration halo have been compared with those of the unconformity-related uranium

deposits of the Athabasca Basin in order to provide new insight on the genesis of uranium

mineralization at Kiggavik. Finally, crystal structure and crystal chemistry of clay minerals

and associated mineral phases have been investigated to identify new vectors to

mineralization at the prospect scale.

1.3. Geological setting

A simplified geological map of the Kiggavik-Andrew lake area is presented in Figure 1-1

(Hadlari et al. 2004). At a regional scale, the litho-structural pile consists of Mesoarchean

granitic, granodioritic and augen gneisses (2866±6 Ma; (Zaleski et al. 2001) tectonically

overlain by a Neoarchean metavolcano-sedimentary package retromorphosed to greenschist

facies. The latter consists in quartzo-feldspatic wackes and minor quartzite with thin

interbedded banded iron formation layers, rare black shales, and locally komatiite and

rhyolite. Geochronology on interbedded volcanics in the wacke give a U-Pb zircon age of

2710±2.1 Ma (Davis and Zaleski 1998). This Archean supracustal package, known as the

Woodburn Lake Group (WLG), belongs to a set of several greenstone belts due to continental

rifting of the Rae Province, which extends over 2000 km from northern Saskatchewan to the

North of Baffin Island (Hartlaub et al. 2004). Both Archean and the unconformably overlying

Early Paleoproterozoic Amer and Ketyet River Groups, underwent tectono-metamorphic

events during Middle Paleoproterozoic times (Hoffman 1990), due to the polyphased tectonic

accretion of the Laurentian Craton during Trans-Hudson (2.0-1.8 Ga) and Taltson-Thelon


54

orogens (2.0-1.9 Ga). Both the Woodburn and Amer-Ketyet Groups were intruded by Hudson

granite and syenite around 1.85-1.79 Ga and by Nueltin granite and associated Pitz rhyolite at

1.75 Ga (Peterson et al. 2002, Van Breemen et al. 2005). Following late Hudsonian

metamorphic events, dated at 1750 Ma in the northern Saskatchewan (Orrell et al. 1999), post

orogenic uplift and erosion generated large volumes of detrital material while regional

thermal and tectonic subsidence accommodated space for deposition of thick flat lying

siliciclastic sequences, forming the Dubwant Supergroup (Rainbird et al. 2003). This Late

Paleoproterozoic non metamorphic rock package comprises the Wharton and Baker Lake

Groups and the uncomformably overlying Barrensland Group which includes the Thelon

Formation sandstones (Rainbird et al. 2003). The siliciclastic sequences accumulated in

widespread intercontinental sag and fault controlled basins. Late thermal events of

Mesoproterozoic age are evidenced by the Kuungmi Formation basalts, of limited extent in

the central part of the Thelon basin, and by the McKenzie dyke swarms (Fahrig 1987).

At the regional and deposit scale uranium mineralization is typically hosted in the Woodburn

Group metagraywackes, in the vicinity of a N080 fault trend, preferably where second order

structures are present. Ore is located within a clay alteration halo centered on faults, itself

surrounded by a hematite pervasive alteration extending over tens of meters. Disseminated

mineralization can also be found in relatively weakly argillized or hematized rocks (e.g. End

deposit).

1.4. Sampling and analytical procedure

Sampling has been carried out extensively along the Kiggavik-Andrew Lake trend in various

basement lithologies to determine the regional alteration background and the specific clay

signature related to uranium ore deposits. A total of 217 samples (selected from 17 diamond


55

drillholes) were collected for detailed petrology and crystal-chemistry. Sampling has been

both focused on altered rocks in mineralized zones (i.e., Bong prospect, End deposit and

Andrew Lake deposit) and on poorly altered to fresh rocks from barren drill holes at various

distances laterally to the main structures controlling the mineralization. Such sampling allows

the lateral variation of mineral paragenesis from fresh rocks to strongly altered fault cores

with or without mineralization. The Bong prospect was chosen for its rock package of

homogeneous metasediments to minimize the influence of the protolith chemistry on the

mineral paragenesis.

Rock samples were gently crushed and put in deionized water to get mineral suspensions

afterwards dispersed ultrasonically during 2 minutes. Clay size fractions < 4µm were

extracted by sedimentation for oriented and randomly oriented powder mounts. No cation

exchange was performed. Chemical composition, morphology and texture of clay mineral

assemblage were studied using a JEOL® 5600 electron microscope equipped with a Bruker

energy dispersive X-ray spectroscopy detector (EDS). Analytical conditions were as follows:

accelerating voltage 15 kV, probe current 1 nA, working distance 17 mm, counting time of

100 s. The analyzed elements were Na, Mg, Al, Si, Mn, Fe, Ti, K, and Ca. The microanalysis

system was calibrated using synthetic and natural oxides and silicates (MnTiO3, hematite,

albite, orthoclase, and diopside) and corrections were made using a ZAF program. The

relative errors on the analyzed values are <1.5% (except Na which is >3%). Total Fe has been

arbitrarily considered as FeO or Fe2O3 according to the nature of the analysed mineral.

All clay preparations were analyzed on a Bruker D8 Advance diffractometer. Diffracted beam

CuKα1+2 radiation was used (40kV, 40mA) and collected by a linxeye detector. Relative

humidity was not controlled during data acquisition. Experimental conditions used for X-ray

diffraction (XRD) data collection are given in Table 1-1.


56

Table 1-1 : Conditions used for X-ray diffraction data collection

Type of preparation Angular range

°βƟ Scanned range

(Å) Step size °βƟCuKα

Counting time (s)

Oriented slides 2-30 44.0-2.98 0.01 1

Clay separation: AD and EG1

Randomly oriented preparations 19-34 4.67-2.64 0.01 5

(polytypes determination)

Randomly oriented preparations 58-65 1.59-1.43 0.01 5 determination of the 060 reflection

1AD: Air dried, EG ethylene glycol solvated 2060 reflection is used to determine the b parameter and hence to distinguish the dioctahedral, di-

trioctahedral and triocataedral phyllosilicates

The crystallinity of illite along the c axis was estimated by measuring the full width at half

maximum intensity (FWHM) of the typical d001 reflection on oriented preparations.

Although more advanced methods have been developed recently (Drits et al., 1997, among

others), this broad method still remains suitable for illite (Guggenheim et al., 2002). Illite

polytypes were identified by XRD on clay separates that were randomly oriented using a

back-loading method as described and recommended by Moore and Reynolds (1989). The

diffraction patterns were recorded using the step-scanning mode from 19 to 34 °2(4.67-2.64

Å) with a step size of 0.01 °2 and a counting time of 5s per step. Illite polytype identification

was based on comparisons with reference data given in Bailey (1980) and (Brindley and

Brown 1980) for 1M and 2M1 polytypes and (Drits et al. 1993) for pure tv-1M (1M polytype

with trans-vacant octahedral cation occupancy) and cv-1M (1M polytype with cis-vacant

octahedral cation occupancy) polytypes. Note that earlier references used the terms “1M” and

“3T” for illite polytypes that are now considered to be tv-1M and cv-1M respectively (Drits et

al. 1993, Reynolds 1993), Table 1-2.


57

Table 1-2 : X-ray diffraction lines characteristic of illite polytypes: tv-1M (octahedral trans vacant), cv-1M

(octahedral cis vacant) and 2M1 polytypes.

tv-1M cv-1M 2M1

Pos Å hkl Pos Å hkl Pos Å hkl Properties

3.655 1 12 3.591 112 Diagnostic peaks of of tv-1M and cv-1M polytypes

3.073 112 3.126 112 Diagnostic peaks of tv-1M and cv-1M polytypes

3.885 3.889 Common to 2M1 and 1Mc polytypes

2.875 2.870 115 Common to 2M1 and 1Mc polytypes

3.735 023 Specific to 2M1 polytype




ArcGIS® and the Geosoft Target® module were used for Kriging interpolation and mapping

of the crystal properties of clay mineral in 2D & 3D with a 12*12*12 meters cell size.

1.5. Petrography and mineralogy

The lithology of fresh rocks and their altered and mineralized equivalents encompasses the

Woodburn Group metasediments, the underlying augen gneiss and highly silicified feldspar

bearing aphanitic rocks thought to be porphyritic rhyolite or mylonitized (micro)granite.

Locally, these rocks have been intruded by late mafic rich gabbro or biotite rich lamprophyre

to more differentiated, coarse to fine grained syenite, and quartz- K feldspar granite and

feldspar porphyry. The dominant ore bearing lithology are the Woodburn Group

metasediments in which alteration is the most widely developed. Macroscopically, fresh rocks

of the metasedimentary package are dark green and dominated by fine to medium grain meta-

arkose to wacke with rare lithic fragments. Pyrite rich pelitic horizons as well as thin layers of

banded iron formation (up to 10 cm thick) are interbedded. The metasediments consists of


58

feldspar (55 %), with dominant K-feldspar and quartz (40 %); accompanied by biotite, white

mica, minor garnet in places and accessory minerals such as tourmaline, zircon, rutile, pyrite

and magnetite. These metasediments were affected by a retrograde metamorphism

(greenschist facies) which consists of pervasive chloritization of biotite, sericitization of

feldspars and occurrence of minor veinlets filled by epidote, alkali feldspars and late

carbonates.

1.5.1. Alteration features

Macroscopically, hydrothermal alteration of the metamorphic basement resulted in color

change of the rocks in response to mineralogical transformation as well as transfer of

chemical elements, especially iron. Several contrasting features have been noted (Figure 1-2):

bleaching related to strong argillization (illitization and chloritization) and desilicification,

reddish coloration related to crystallization of iron oxide (mostly hematite), and late

silicification (secondary quartz). All these features are spatially related to faults from

millimeter to several meters wide fault zones, highlighting the strong structural control of the

alteration. Both fractures and alteration decrease in intensity from the fault core (Figure 1-

2a), to the damage zone and then to the distal surrounding rocks in which only diffuse

alteration is present (Figure 1-2b). The regional N070 fault trends, such as the Thelon and

Judge Sisson North Faults, are characterized by extensive hematization and silicification.

Protracted tectonic and hydrothermal activity is expressed by numerous phases of brecciation

and alteration that led to complex textures of hydraulic quartz breccia (Figure 1-2c)

frequently overprinted by late pervasive silicification, resulting in alternating bands of

hematized and silicified breccia. Strong argillization (bleaching) associated with various

degrees of desilicification occurs along the fault core and within the main mineralized zones.

Although the original foliation is still visible in most of the intensely altered rocks, the

lithology and structure of the protolith is difficult to identify. The thickness of the fault gouge


59

is generally limited to 10s of centimeters whilst the width of the whole mineralized structure

(gouge + damage zone and associated alteration halo) may reach several tens of meters. Fault

gouges are often superimposed on inherited structures such as quartz-carbonate veined

corridors resulting in mixed fragments of sub-angular argilized basement, quartz and

carbonates embedded in an illitic matrix. At greater lateral distance from the uranium deposits

and their associated altered zones, minor faults are expressed as tectonic brecciation corridors

in which the mechanical grinding (attrition) led to a chlorite-rich greenish fault gouge.

Usually, hematization is observed at the transition between the fault damage zone and

surrounding weakly altered basement rocks. These transition zones outlined by redox fronts

are frequently mineralized.

Figure 1-2 A : Totally argilized fault gouge at the footwall of mineralized zone, Bong-043 drillhole; B, Fresh

metagraywacke bleached over 50 cm by diffusive illitization around a centimeter microfault, Bong-045 drillhole.

C, Outcrop along the Sisson North Fault showing intense N080 quartz veining in a strongly hematized

A B

C D

Foliation plan


60

metagraywacke; D, Mineralized rock sample with pitchblende impregnation along the foliation plans Bong-042

drillhole around 400 m below erosion surface.

At the thin section scale, alteration is related to dissolution of preexisting metamorphic

silicates and to crystallization of an abundant clay matrix composed of illite associated with

variable amounts of sudoite (di-trioctahedral Al -Mg chlorite). Illitization occurs both as

pseudomorphic replacement of K-bearing metamorphic minerals (Figure 1-3a) and chlorite,

and as neoformation of fine grained (less than 10m) laths or whiskers in the secondary

porosity of the altered rocks. In altered intrusive rocks, the feldspars phenocrysts are totally

replaced by illite and specular hematite. Sudoite is intimately associated with illite, forming

millimetre size pockets of extremely fine grained flaky or lathy particles aggregates (Figure 1-

3b) or as partial replacement of exfoliated metamorphic phyllosilicates.

In addition iron remobilization from metamorphic phases (phyllosilicates and sulfides) is

shown by textural relashionships. The corrosion of iron bearing minerals is followed the

release of iron and precipitation of hematite in the secondary porosity (Figure 1-3c). Hematite

as well as barite grows after titanium oxides (Figure 1-3d).

Minor amounts of phosphate minerals are closely associated with illite and sudoite. They

consist of aluminum phosphate-sulfate minerals (APS) and apatite. APS minerals occur as

tiny euhedral crystals ranging in size from less than to 10 µm up to 50 µm, and frequently

display features of chemical zoning. Sr and S rich APS (svanbergite) dominate in the external

alteration halo, up to hundreds of meters away from the mineralized zones. With decreasing

distance from orebodies the general trend is a transition from svanbergite to LREE-rich

phosphate (florencite). Locally, The APS minerals can be fractured and partly dissolved with

smoothed edges. Secondary apatite occurs close to and within the mineralized zones. It is

textural association with pitchblende displays features of cogenetic growth, with inclusions of


61

pitchblende inside apatite or coatings of colloform pitchblende around apatite and titanium

oxides.

Figure 1-3 A : Hanging wall of the main mineralized structure with strong alteration and replacement of

metamophic 2M1 micas by laths of tv-1Millite (identified by XRD) showing an exfoliation texture; Bong-042;

B, Sudoite (S) pocket occurring in illitized phengite laths, Bong-008; C Metamorphic chlorite partially altered to

illite with associated crystallization of hematite, End09-11 ; D Illitization of metamorphic Fe-Mg

phyllosilictates leading to precipitation of titanium oxides, hematite spherules and minute crystals of barite in the

secondary porosity, End09-11.

Abbreviations; Brt: barite, Cal: calcite, Chl: chlorite, Coff: coffinite, Gn: galena, Hem: hematite, Ill: illite, Qtz:

quartz, TiO2: titanium oxide, Sud: sudoite, Uran: uraninite

Sud

A B

Ms

Ill

Ill

Ill

Hem Qtz

Brt

TiO2

Qtz

ChlFeMg ChlFeMg

Hem

Qtz C D


62

1.5.2. Ore paragenesis

Uranium mineralization is spatially related to the faults and their alteration haloes. At least

two generations of uranium mineralization have been found. The first consists of pitchblende

coatings on foliation planes and along fracture walls in weakly to strongly bleached host rock

(Figure 1-2d & 1-4a). In this type of primary ore, metamorphic chlorite is normally partially

altered to illite and sudoite and disseminated patches of pitchblende occur in the fault gouge.

Uraninite is epigenetic on iron magnesium chlorite as well as pyrite with boxwork textures

(Figures 1-4b, 1-4c & 1-4d). Uraninite is altered and replaced by coffinite in corrosion pits

latter filled with barite and galena. Pitchblende is also present as fracture filling and smearing

along microfaults planes with or without iron oxides as well as along redox fronts in the fault

damage zone. This last mineralization setting could be interpreted as secondary

remobilization of primary mineralization. Pitchblende does not appear to be associated with

iron oxides, and hematite staining frequently overprints argillized zones that are not

necessarily mineralized.

The primary ore paragenesis observed in thin section is uraninite later replaced by coffinite.

Metallic sulfides are rare, mainly as chalcopyrite and Cu, Co & Ni sulfides. They occur as

minute crystals and colloform products which are cogenetic to pitchblende and clay alteration

minerals. Subhedral to euhedral titanium oxides, apatite and APS disseminated in the clay

matrix often show partial dissolution features, and are frequently rimed by pitchblende

overgrowths. Apatite is particularly abundant within the mineralized zone.


63

Figure 1-4 : A Mineralized rock sample of metasediments showing the pitchblende (colloform uraninite)

precipitation along the foliation plans in which the metamorphic phyllosilicates have been transformed to illite.

The intergranular voids are cemented by pitchblende, Bong-042 drillhole; B Metamorphic phyllosilicate

pseudomorphs and uraninite overgrowths, End09-04 drillhole; C Metamorphic phyllosilicate pseudomorphs and

uraninite overgrowths, postdated coffinite crystallization and calcite cementation, End09-04; D Pyrite and

uraninite (boxwork texture) subsequently altered in coffinite and cemented by late calcite, End09-04.

1.6. Textural properties and crystal structure of phyllosilicates

Two distinctive types of phyllosilicates have been identified on the basis of textural

characteristics. Metamorphic phyllosilicates in large subhexagonal plates ranging from 15 to

50 µm are oriented along foliation planes, whereas hydrothermal phyllosilicates replace all

the minerals of the basement rocks (including the micas) in zones of intense alteration and

consist of fine grained laths to flakey particles of < 5 µm.

A

S

Qtz

Uran

B

Uran

Brt

C

Uran Cal

Coff

Gn D

Coff

Uran

Pyrite

boxwork Cal


64

The XRD data of oriented and randomly oriented powders of the <4 m size fraction of all

samples investigated along the Kiggavik-Andrew Lake trend are dominated by dioctahedral

potassic phyllosilicates (micas and illite) associated with variable but generally small amounts

of chlorite (Figure 1-5). The crystal structure of phyllosilicates was investigated through the

full width at half maximum (FWHM) of the peaks corresponding to the d001 reflections

(close to 10 Å and 14 Å for illite and chlorite respectively) and through the relative amounts

of different polytypes identified from the randomly oriented powders mounts.

Figure 1-5 : X-ray diffraction patterns of oriented preparations of clay separates (< 4m) of representative

samples from the alteration halo surrounding the Bong deposit. (A) sample from the damaged zone, (B) sample

3 8 13 18 23 28

A

B

C

°2θ, Cu Kα

chl/sud chl/sud

chl/sud

chl/sud

Ill

Ill

Qtz

Ill


65

from the fault core hosting the mineralization; (C) sample from a sudoite-rich level. Ill: Illite, Sud/Chl: Sudoite

& Chlorite, Qtz : quartz diffraction peaks.

In fresh to very poorly altered basement rocks, the phyllosilicate assemblage of metamorphic

origin is made of dioctahedral micas (muscovite or phengite) and trioctahedral Fe-chlorite

with weak intensities of odd basal reflections compared to the even ones and d060 ranging

between 1.54 and 1.55Å). The d001 reflections of K-mica and chlorite are characterized by

very low FWHM (less than 0.1° 2) and the XRD patterns of mica has all the index hk

reflections of the 2M1 polytype (Figures 1-6 and Table 2).

Figure 1-6 : XRD diffraction patterns of randomly oriented mount of clay separates illustrating the change in

polytype of the dioctaedral phyllosilicates as a function of their distance from a mineralized structure in the

altered metagraywacke. The index XRD reflections of 2M1 (dotted lines), cv-1M (solid lines) and tv-1M (dashed

line) are from Drits and Tchoubar, (1990). A fault gouge hosting most of the mineralization, B fault damage

zone, C fresh metagraywacke. Note the presence of significant amounts of unaltered feldspar and quartz in the

less than 4 µm size fraction of the fresh metagraywacke. 1 hk reflections of K feldspar, 2 hk reflections of albite.

19 24 29 34

2

1 1

1 1

1

2

2

1Mt 1Mc

2M1

2M1/1Mc 2M1/1Mc

2M1

1Mc

1Mt

C

B

A

°2θ, CuKα


66

In basement rocks affected by hydrothermal alteration, the phyllosilicate assemblage is

largely dominated by illite associated with small amounts of chlorite. Compared to those of

the metamorphic phyllosilicates, the d00l reflections of both hydrothermal illite and chlorite

have distinctive peak profiles and intensity ratios (Figure 1-5). The d00l reflections of ill ite

and chlorite are broad (0.3<FWHM <0.65° 2and >0.15° 2, respectively). XRD patterns of

hydrothermal illite indicate a mixture of tv-1M and cv-1M polytype with a trend to

predominance of tv-1M polytype in the fault gouge that is totally illitized (Figure 1-6). In

addition, the XRD patterns of the hydrothermal chlorite are characteristic of the sudoite. They

differs from that of the trioctahedral chlorite by an intense d003 reflection compared to the

d001 reflection (Figure 1-5) and by a d060 spacing close to 1.51 Å (instead of 1.54 Å or

more for a trioctahedral chlorite) . Comparing the XRD patterns of all rocks samples, it

appears that the FWHM values of the dioctahedral K-phyllosilicates strongly increase and the

amount of 2M1 polytype decreases and then disappears with increasing hydrothermal

alteration (Figure 1-6). As 2M1 polytype and the lowest FWHM values are representative of

the metamorphic micas, such a trend can be a consequence of the alteration of metamorphic

micas and their replacement by illite as observed by SEM investigations (Figure 1-3a).

However, the FWHM values of the hydrothermal illite continue to increase with increasing

alteration, such that in the highly altered zones, no 2M1 micas persist. Increase in FWHM

values of the d00l reflection of illite is correlated with the increasing amount of tv-1M

polytype close to the uranium mineralization.

1.7. Phyllosilicate crystal chemistry

Microprobe analyses of metamorphic micas by EDS and the calculated structural formulas on

the basis of 11 oxygens per half formula unit are fairly homogeneous and are comparable to

muscovite with phengitic substitutions (Si+R2+ = Al IV+AlVI ). This is illustrated by (1) Si


67

contents higher than 3 atoms coupled with amounts of divalent atoms ranging from 3.04 to

3.29 (2) interlayer charges totally satisfied by K and ranging between 0.9 and 1 atom, and (3)

octahedral occupancies close to 2 atoms (Table 3a). The Fe/Fe+Mg ratios (XFe) of these

micas range between 0.6 and 0.8. The structural formula of hydrothermal illite (Table 3b)

differs from those of metamorphic micas by a higher Si content from 3.3 to 3.4 atoms coupled

with a lower interlayer charge ranging from 0.75 to 0.85 atom. Compared to metamorphic

micas (Figure 1-6), the composition of illite is very poor in Fe (<0.04 atom) and richer in Mg

(>0.20 atom). As a consequence, they can be easily distinguished by their very low Fe/Fe+Mg

ratios of < 0.1.

Chemical analyses and structural formulae of metamorphic chlorite indicate an iron-rich

species of trioctahedral chlorite (Table 1-3). Their octahedral occupancy is close to 6 atoms

when the structural formulae were calculated with total Fe in the ferrous state and their XFe is

generally higher than 0.5. The structural formulas of the hydrothermal chlorites are more

heterogeneous than those of the metamorphic chlorites. Most of them are comparable to those

of the sudoite (Table 1-3), which is the di-trioctahedral chlorite species identified by XRD

investigations. The structural formula of sudoite is characterized by an Al-Mg rich

composition and an octahedral occupancy close to 5 atoms. The fact that many analyses of

what was initially thought to be sudoite are in fact mixture of variable amounts of sudoite

with illite is indicative of the very small size of the crystals and their close association with

illite even at a few micrometers scale. Chemical analysis of hydrothermal chlorites has also

permit to identify Mg-rich trioctahedral chlorites in a few altered samples (Table 1-3). These

chlorites sealed late microfractures in some hydrothermally altered samples. The chemical

composition of this Mg-rich chlorite is typical of the clinochlore species. It is characterized by

low iron content (XFe<0.10) and a trioctahedral structure with the presence of a small

amount of octahedral vacancy (octahedral occupancy averaging 5.75 atoms).


68

Table 1-3 : EDS analysis and structural formulas of representative phyllosilicates in the Kiggavik- Andrew Lake

trend

1 2 3 4 5 6 7 8 SiO2 24.01 31.59 33.86 43.96 45.45 42.33 43.00 48.38 TiO2 0.05 0.01 0.06 0.56 0.36 0.34 0.00 0.04 Al 2O3 19.32 15.58 28.78 31.44 33.38 29.35 27.61 27.78 FeO 27.17 4.39 0.20 2.11 2.31 4.28 Fe2O3 0.04 0.45 MnO 0.22 0.00 0.00 0.00 0.10 0.06 0.00 0.03 MgO 11.90 28.33 17.26 1.46 1.32 1.48 1.80 2.63 CaO 0.03 0.19 0.07 0.00 0.03 0.00 0.06 0.14 Na2O 0.26 0.36 0.32 0.39 0.49 0.44 0.21 0.27 K2O 0.03 0.00 0.22 10.23 10.37 10.03 8.60 8.14 Total 83.00 80.45 80.77 90.15 93.83 88.32 81.33 87.85 Si 2.73 3.22 3.27 3.12 3.09 3.11 3.30 3.42 Al IV 1.27 0.78 0.73 0.88 0.91 0.89 0.7 0.58 AlVI 1.32 1.09 2.54 1.74 1.77 1.65 1.80 1.73 Ti 0.00 0.00 0.00 0.03 0.02 0.02 0.00 0.00 Fe 2.58 0.37 0.02 0.12 0.13 0.26 0.00 0.02 Mn 0.02 0.00 0.00 0.00 0.01 0.00 0.00 0.00 Mg 2.02 4.31 2.48 0.15 0.13 0.16 0.21 0.28 Occ 5.94 5.78 5.04 2.05 2.06 2.10 2.01 2.03 XFe 0.56 0.08 0.01 0.45 0.50 0.62 0.01 0.08 Ca 0.00 0.02 0.01 0.00 0.00 0.00 0.00 0.01 Na 0.06 0.07 0.06 0.05 0.06 0.06 0.03 0.04 K 0.00 0.00 0.03 0.93 0.90 0.94 0.84 0.73 Int. Ch. 0.07 0.11 0.10 0.98 0.97 1.00 0.88 0.79

1 Metamorphic chlorites; 2 Hydrothermal clinochlore associated with a dolomite veinlet; 3 Sudoite filling

veinlet; 4-5 Metamorphic micas; 6 Relics of metamorphic micas persisting in a mineralized zone; 7 illite pocket

in a fault gouge; 8 illite in a mineralized zone. Xfe:(Fe/Fe+Mg), Int. Ch : Interlayer Charge, Total: sum of the

oxide Wt%. The fact that the total oxide Wt% of hydrothermal illite and chlorite are slightly lower than expected

for theoretical ones (>90% and 86% respectively) can be explained by the microporosity of the clay material.

Such a deviation did not affect the calculation of the structural formulas.


69

1.8. Discussion

1.8.1. Comparing the alteration pattern of the Kiggavik-Andrew Lake trend

to those of unconformity-related uranium deposits in the Athabasca

or the Kombolgie Basins.

The alteration pattern associated with the uranium deposits of the Kiggavik-Andrew Lake

trend shows a great number of similarities with those of basement hosted unconformity-

related uranium deposits described in both the Athabasca Basin (Cloutier et al. 2009,

Jefferson et al. 2007) and references therein) or the Kombolgie Basin (Beaufort et al. 2005,

Gustafson and Curtis 1983) and references therein). Such similarities include the geometric

shape of the alteration halo as well as the sequence of mineral crystallization and the

mineralogical characteristics of the secondary minerals. They can be summarized as follows:

(1) Similar alteration halos in fracture controlled uranium mineralization in clay-rich rocks

associated with dissolution of silicates and quartz in retro-metamorphosed basement rocks.

(2) Similar crystal chemistry of the secondary mineral phases dominated by illite and both di-

trioctahedral (sudoite) and trioctahedral chlorites with minor amounts of phosphate minerals

(APS, apatite) and hematite.

(3) Similar time-space alteration sequence expressed by zoned alteration around the fracture

network which hosts the uranium mineralization: i.e. illite ± sudoite followed by late

clinochlore in veinlets and lateral transition from APS to apatite toward the mineralized zone.

The occurrences of unconformity-related uranium deposits in Canada and Australia are

usually interpreted on the basis of the widely accepted diagenetic-hydrothermal genetic model

originally proposed by (Hoeve and Quirt 1984, Hoeve et al. 1980). According to this model

and all the refinements proposed thereafter, the uranium ores formed close to the


70

unconformity are the result of a long period of tectonically controlled interaction between

diagenetic oxidizing basinal brines at the base of deep continental basins and hydrothermal

reducing fluids circulating in basement faults (Jefferson et al. 2007, Kyser and Cuney 2008).

The spatial distribution of the alteration haloes associated with unconformity uranium

mineralization in the Athabasca Basin show contrasting patterns interpreted as representative

of discharge (“egress” style) and recharge (“ingress” style) zones (Quirt 1989, 2003). Egress

style is characterized by extensive and zoned alteration pattern in the sub-basinal sandstones

in response to upward fluid flow from basement-rooted faults. Ingress style shows more

limited inverted alteration haloes, mainly within the faulted basement, in response to

downward infiltration of basinal fluids in probable extensional local settings.

From the aforementioned considerations, the alteration pattern associated with the uranium

deposits along the Kiggavik-Andrew Lake area have many of the characteristics associated

with basement hosted unconformity-related uranium deposits. This suggests a close

relationship between the genesis of these ore deposits and both the burial history and tectonic

evolution of the Thelon Basin which has a potential for hidden uranium deposits (Beyer et al.

2011, Miller and LeCheminant 1985, Rainbird et al. 2003). However, compared to the

basement-hosted uranium deposits of the Athabasca Basin such as Shea Creek (Laverret et al.

2006) or Millenium (Cloutier et al. 2009), the alteration pattern associated with the uranium

deposits of the Kiggavik-Andrew Lake differs by the absence of significant amounts of

graphite in the fault system and the absence of dravite in the paragenetic sequence.

1.8.2. Crystal structure of clay minerals as a vector to mineralization in the

Bong prospect


71

Several authors have shown that the crystal structure of phyllosilicates can be used as a vector

to mineralization in the unconformity-related uranium deposits of Australia and Canada

(Beaufort et al. 2005, Laverret et al. 2006). As the full width at half maximum (FWHM) of

phyllosilicates depends on the degree of order-disorder in the layer staking sequence, the d001

XRD reflections of illite or chlorite are broad indicators of P-T conditions in geological

environments (see reviews and critical comments in (Guggenheim et al. 2002). The FWHM

of the d001 reflection of metamorphic white micas (FWHM< 0.10 °βθ) is lower than those of

the hydrothermal illite (0.15 °βθ <FWHM< 0.70 °βθ), and the FWHM values of illite tend to

increase as a function of decreasing distance to the uranium mineralisation (Figure 1-7).

The FWHM values of the d001 reflection of illite have been systematically measured to map

their variation around the Bong deposit.

Figure 1-7 : Change in peak profile of the d001 reflection of the K-bearing dioctahedral phyllosilicates according

to their distance to the uranium mineralization. The FWHM of the peak gradually increases from less than 0.10

8 8.5 9 °2θ, CuKα

I

A

B

C

D


72

°2θ to more than 0.70 °2θ from the unaltered metagraywakes until the mineralized fault core. A, fresh

metagraywacke, B fault damaged zone, C barren fault core, D mineralized fault core.

1.8.2.1. MAPPING THE FWMH OF THE ILLITE/MICA D001 REFLECTION

The extensive dataset of FWHM values obtained from samples from the Bong prospect are

distributed in four categories arbitrarily based on the crystallographic data and the intensity of

clay alteration (Figure 1-8). These ranges corresponds to (1) fresh rocks containing only

metamorphic micas (FWHM< 0.10 βθ), (2) very weakly illitiz ed rocks (FWHM< 0.15 βθ) that

are “fresh” macroscopically; (3) moderately to strongly illitized rocks (FWHM< 0.30 βθ)

which are sill cohesive with a weak to moderate desilicification and that show a typical

mixture of 2M1 and 1M polytypes in the damage and distal alteration zones around the

structure; and (4) totally illitized rocks (FWHM > 0.30 βθ) in which illite is the only potassic

phyllosilicate in the strongly desilicified metasediments and the fault gouges.


73

Figure 1-8 : Interpolation of FWHM values of the d001 XRD reflection of illite/micas using Kreiging method

permits to ouline a zoned distribution of the average crystallinity along the c axis of these minerals around the

uranium orebodies of the Bong prospect. The edge of the mineralized halo in the section is indicated by the black

dotted line.

The spatial populations of samples (in average a point every 10 m in the Z direction and every

20 m on the XY plan) allow to use interpolation tools, in this case Kriging method, to map the

intensity of the alteration related to the mineralization process on the basis of the d001

FWHM of the potassic phyllosilicates. The result of such interpolation methods allows to map

the intensity of the alteration process and to outline the geometry of the hydrothermal haloes

at larger scale (Figure 1-8). Knowing the position of the main structure in the Bong prospect,

this alteration map confirms the location of the uranium orebodies inside the more strongly

illitized part of the alteration halo. Such maps could be also very useful for further

assessment of the altered rock volume around the structures. As a general statement, illite

with the highest FWHM values are closely related to the fault gouge and more specifically to


74

the mineralized zones. Micas with low FWHM values have been found in fresh rocks as well

as in the damage zone where the dioctaedral micas have been only partially transformed

during the alteration processes.

1.8.2.2. MAPPING THE DISTRIBUTION OF SUDOITE

Sudoite occurs proximal to the uranium orebodies in most unconformity-related uranium

deposits (Quirt 1989, 2003). The spatial distribution of this mineral has been mapped in the

alteration halo of the Bong prospect (Figure 1-9).

Figure 1-9: Interpolation of the chlorite-sudoite relative proportions estimated from the diffraction peakratio (

Kcs factor) with a Kreiging method permitting to map the spatial distribution of sudoite in the alteration halo of

the Bong prospect. The edge of the mineralized halo in the section is indicated by the black dotted line.

The relative amount of sudoite in the bulk chlorite material has been estimated based on the

intensity ratio between the d003 and d001 reflections of the chlorite minerals (corresponding


75

to d spacing close to 14Å and 4.75Å respectively) measured after background stripping in the

XRD pattern of oriented clay preparations (referred to Kcs factor hereafter). Trioctahedral

chlorite largely predominates if the Kcs factor is <1, both trioctahedral chlorite and sudoite

are mixed in equal proportions if the Kcs factor is close to 1, and sudoite largely predominates

over trioctahedral chlorite if the Kcs factor >1. It should be mentioned that this method is only

a broad approximation of the amount of sudoite in the altered rocks. A more accurate

quantification could be done using quantitative XRD methods such as decomposition of X-ray

diffraction profile or Rietveld method. However such methods are time consuming and not

really appropriate to the analysis of a great number of heterogeneous samples. The good

correlation that is noted between the obtained Kcs factor and the petrographic observations

give confidence that this parameter highlights the major contrasts in alteration mineralogy.

Nevertheless, the confidence in this factor for mapping hydrothermal alteration has been also

checked on the basis of other structural parameters such as the b parameter of the chlorite

lattice cell (expected from the d 060 reflection of chlorite minerals) and the estimation of

mineral proportions from chemical analysis on clay separates.

Mapping the spatial distribution of sudoite in the Bong prospect shows that this mineral is

present throughout the alteration zone around the main structure where iron rich chlorite tends

to disappear. Thus, sudoite-rich zones are not strictly associated with the mineralization. At a

larger scale it seems distributed along two distinctive structures. The first one, relatively poor

in sudoite, is associated with the mineralization while the other one which occurs above is

richer in sudoite but contains only trace to weak amounts of uranium mineralization.

1.8.3. Relationships between clay alteration and hematization.

The crystal chemistry of phyllosilicates and the chemiographic representation of both the

metamorphic and hydrothermal assemblages in a MR3-2R3-3R2 diagram (Velde 1977, 1985)

outline a close relationship between clay alteration and hematization within the alteration


76

halos (figure 1-10). Dissolution of iron-rich phyllosilicates such as Fe-Mg trioctahedral

chlorite (XFe<0.5) and phengitic micas (0.6<XFe<0.8) and their replacement by illite

(XFe<0.1) and sudoite (XFe<0.02) is a geochemical evidence for a leaching of iron from the

silicate minerals during the hydrothermal process. Such a leaching can be explained by the

oxidation of ferrous iron during the dissolution of metamorphic minerals (silicates, Fe-Ti

oxides and sulfides) by the oxidizing and acidic basinal brines infiltrated along the fracture

network in the basement rocks. As incorporation of ferric iron in phyllosilicates such as

chlorite, di-tri or trioctahedral, is very limited (Billault et al. 2002, Nelson and Guggenheim

1993), most of the ferric iron precipited as cogenetic hematite. Moreover, as ferric iron

mobility is generally very limited due to its very low solubility in hydrothermal solutions, the

iron redistribution is centered on the structures where alteration took place.

Observations on the hydrothermal clay assemblage chemistry as well as the textural evidences

of hematite precipitation in the secondary porosity developed with alteration lead to the

conclusion that hydrothermal hematization was effective along the Kiggavik-Andrew lake

fault trend. Even if this does not mean that all the hematite observed at regional scale is a by-

product of the hydrothermal alteration process, we suggest that the hematite haloes observed

at the field scale are probably good indicators of proximal hydrothermal alteration zones.


77

Figure 1-10 : Chemiographic representation of the phyllosilicate assemblages formed during both retrograde

metamorphic (■) and hydrothermal (◊) stages which successively affected the metasediments hosting uranium

mineralization. Structural formulas of phyllosilicates have been plotted in a MR3-2R3-3R2 diagram (Velde 1977)

in which MR3 represent feldspar compositions (Na+, K+, 2Ca2+ + Al3+) in atomic proportions, 2R3 represents the

dioctahedral pole [1/2 (Fe3+ + Al3+ + Ti4+)-MR3] and 3R2 represents the trioctahedral pole [1/2 (Fe2+ + Mg2+ +

Mn2+)]. The microprobe analyses which fall between the compositional fields of the phyllosilicates phases

labeled in the triangle correspond to two-phases (and more rarely three phases) admixtures.

1.9. Concluding remarks

This study indicates that most of the secondary minerals found within the alteration haloes

associated with uranium mineralization along the Kiggavik-Andrew Lake fault trend are

similar to those produced by diagenetic-hydrothermal alteration in basement hosted


78

unconformity-related uranium deposits (Figure 1-11). Only minor occurrences of propylitic

alteration expressed as epidote and feldspar veinlets give credit to the possible link invoked

between intrusive granitic bodies and a possible early stage of uranium mineralization in the

Kiggavik deposit (Weyer et al. 1987). Nevertheless the extent of such alteration is too limited

to be considered as representative of a major mineralizing event at regional scale.

Figure 1-11: Paragenetic sequence in the Kiggavik-Andrew Lake trend.

Diagenetic-hydrothermal alteration associated with uranium mineralization in the Kiggavik

Andrew Lake trend involves fluids originating from the Late Paleoproterozoic Thelon Basin.

Due to the present level of erosion, only basement hosted mineralization can be found in the

study area and the thickness of basement rocks which has been eroded since the formation of

the ore deposits is unknown. However both alteration and mineralization studied along the


79

Kiggavik Andrew Lake trend are related to steep structures and can be compared to the

basement hosted uranium deposits like Jabiluka in Australia or Shea Creek in the Western

Athabasca Basin of Canada in which mineralization and alteration follow corridors down to

several hundred meters below the unconformity. Nevertheless, contrarily to the Athabasca

basement, basement uranium mineralization in the Thelon Basin area is dominantly hosted by

coarse Late Archean metasediments along faults without graphite. The source of uranium

within the Kiggavik Andrew Lake trend is still conjectural but the very low uranium content

in unaltered metasediments and the evidence of interaction between diagenetic fluids and

basement rocks argue for a potential contribution of uranium from the Thelon Basin or for

mobilization of uranium from fertile basement rock at distance from the structural trend.

A few implications for uranium exploration can be deduced from the findings of this study.

The Kiggavik Andrew Lake trend is interpreted as the deep roots of an unconformity related

uranium deposit system, so the possibility of discovering uranium deposits closer to the

unconformity below the Thelon Basin at appropriate structural locations cannot be discarded.

Perhaps the findings on the mineralogical indicators developed in this study of would permit

to delineate new prospective uranium bearing alteration systems.

Acknowledgment

The authors would like to thank AREVA Mines for financial and technical support of this

study. The authors would like to thanks the reviewers, Pr Kyser and Pr Pagel for their detailed

comments which greatly helped to improve the manuscript.

1.10. References

BEAUFORT, D., PATRIER, P., LAVERRET, E., BRUNETON, P. and MONDY, J. (2005): Clay Alteration Associated with Proterozoic Unconformity-Type Uranium Deposits

in the East Alligator Rivers Uranium Field, Northern Territory, Australia. Economic Geology. v. 100, pp. 515–536.


80

BEYER, S.R., HIATT, E.E., KYSER, K., DALRYMPLE, R.W. and PETTMAN, C. (2011): Hydrogeology, sequence stratigraphy and diagenesis in the Paleoproterozoic western Thelon Basin: Influences on unconformity-related uranium mineralization. Precambrian Research. 187, 293-312. BILLAULT, V., BEAUFORT, D., PATRIER, P. and PETIT, S. (2002): Crystal chemistry of Fe-sudoites rom uranium deposits in the Athabasca basin ( Saskatchewan, Canada). Clays and Clay Minerals. 50, 69-80. BRINDLEY, G.W. and BROWN, G. (1980): Crystal structures of clay minerals and their X-ray identification, Mineralogical Society, London, UK CLOUTIER, J., KYSER, K., OLIVO, G.R., ALEXANDRE, P. and HALABURDA, J. (2009): The Millennium Uranium Deposit, Athabasca Basin, Saskatchewan, Canada: An Atypical Basement-Hosted Unconformity-Related Uranium Deposit. Economic Geology. 104, 815-840. DAVIS, W.J. and ZALESKI, E. (1998): Geochronological investigations of the Woodburn Lake group, western Churchill Province: preliminary results. Current Research 1998-F, Geological Survey Of Canada DRITS, V.A., WEBER, F., SALYN, A.L. and TSIPURKY, S.I. (1993): X-ray identification of one-layer illite varieties: application to the study of illites around uranium deposits of Canda. Clays and Clay Minerals. 41, 389-398. FAHRIG, W.F. (1987): The tectonic settings of continental mafic dyke swarms: Failed arm and early passive margin In Mafic dyke swarms. 34, Geological Association of Canada Special Paper FARKAS, A. (1984): Mineralogy and host rock alteration of the Lone Gull deposit. Internal report, Urangesellschaft FUCHS, H.D., HILGER, W. and PROSSER, E. (1986): Geology and exploration history of the Lone Gull property. Uranium Deposits of Canada, CIM Special Volume 33 GUGGENHEIM, S., BAIN, D.C., BERGAYA, F., BRIGATTI, M.F., DRITS, V.A., EBERL, D.D., FORMOSO, M.L.L., GALAN, E., MERRIMAN, R.J., PEACOR, D.R., STANJEK, H. and TAKASHI, W. (2002): Report of the association internationale pour l'étude des argiles(AIPEA) nomenclature committee for 2011: Order, disorder and crystallinity in phyllosilicates and the use of the "crystallinity index". Clays and Clay Minerals. 50, 406-409. GUSTAFSON, L.B. and CURTIS, L.W. (1983): Post-Kombolgie metasomatism at Jabiluka, Northern Territory, Australia, and it's significance in the formation of high grade uranium mineralization in Lowar Proterozoic rocks. Economic Geology. 78, 26-56. HADLARI, T., RAINBIRD, R.H. and PEHRSSON, S.:(2004) Geology Schultz Lake, Nunavut, open file 1839, scale 1: 250 000. Geological Survey Of Canada. HARTLAUB, R.P., HEAMAN, L.M., ASHTON, K.E. and CHACKO, T. (2004): The Archean Murmac Bay Group: evidence for a giant Archean rift in the Rae Province, Canada. Precambrian Research. 131, 345-372. HASEGAWA, K., DAVIDSON, G.I., WOLLENBERG, P. and YOSHIMASA, I. (1990): Geophysical exploration for unconformity-related uranium deposits in the northeastern part of the Thelon Basin, Northwest Territories, Canada. Mining Geology. 40, 83-95. HIATT, E.E., PALMER, S., E., KYSER, K. and O'CONNOR, T. (2010): Basin evolution, diagenesis and uranium mineralization in the Paleoproterozoic Thelon Basin, Nunavut, Canada. Basin Research. 22, 302-323. HOEVE, J. and QUIRT, D. (1984): Mineralization and host rock alteration in relation to clay mineral diagenesis and evolution of the Middle-Proterozoic Athabasca basin, Northern Saskatchewan, Canada. Saskatchewan Research Concil Technical report, Saskatchewan Reasearch Council


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HOEVE, J., SIBBALD, T.I.I., RAMAEKERS, P. and LEWRY, J.F. (1980): Athabasca basin unconformity-type uranium deposits : a special class of sandstone-type deposits? In Uranium in the Pine Creek Geosyncline. International Atomic Energy Agency HOFFMAN, P.F. (1990): Subdivision of the Churchill Province and extent of the Trans-Hudson orogen In The Early Proterozoic Trans-Hudson Orogen of North Amercia. 37, Geological Survey of Canada Special Paper JEFFERSON, C.W., THOMAS, D.J., GANDHI, S.S., RAMAEKERS, P., DELANEY, G., BRISBIN, D., CUTTS, C., PORTELLA, P. and OLSON, R.A. (2007): Unconformity-associeted uranium deposits of the Athabasca Basin, Saskatchewan and Alberta In EXTECH IV . Geology and Unranium EXploration TECHnology of the Proterozoic Athabasca Basin, Saskatchewan and Alberta, Geological Survey of Canada, Bulletin 588, KYSER, K. and CUNEY, M. (2008): Unconformity-related uranium deposits In Recent and not so recent developments in uranium deposits and implications for exploration. Short Course 39, Mineralogical Association of Canada LAVERRET, E., PATRIER, P., BEAUFORT, D., KISTER, P., QUIRT, D., BRUNETON, P. and CLAUER, N. (2006): Mineralogy and geochemistry of the host-rock alterations associated with the Shea Creek unconformity-type uranium deposits (Athabasca basin, Saskatchewan, Canada) Part1. Spatial variation of illite properties. Clays and Clay Minerals. 54, 275-294. MILLER, A.R., CUMMING, G.L. and KRSTIC, D. (1989): U-Pb, Pb-Pb, and K-Ar isotopic study and petrography of uraniferous phosphate-bearing rocks in the Thelon Formation, Dubawnt Group, Northwest Territories, Canada. Canadian Journal of Earth Sciences. 26, 867-880. MILLER, A.R. and LECHEMINANT, A.N. (1985): Geology and uranium metallogeny of Proterozoic supracrustal successions, central District of Keewatin, N.W.T with comparisons to northern Saskatchewan. NELSON, D.O. and GUGGENHEIM, S. (1993): Inferred limitations to the oxidation of Fe in chlorite : a high-temperature single-crystal X-ray study. American Mineralogist. 78, pp. 1197-1207. ORRELL, S.E., BICKFORD, M.E. and LEWRY, J.F. (1999): Crustal evolution and age of thermotectonic reworking in the western hinterland of the Trans-Hudson Orogen, northern Saskatchewan. Precambrian Research. 95, 187-223. PETERSON, T.D., VAN BREEMEN, O., SANDEMAN, H. and COUSENS, B. (2002): Proterozoic (1.85-1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland. Precambrian Research. 119, 73-100. QUIRT, D. (1989): Host rock alteration at Eagle Point South. R-855-1E-89, Saskatchewan Research Council QUIRT, D. (2003): Athabasca unconformity type uranium deposits: one deposit type with many variations. . Uranium Geochemistry International Conference, Nancy, France RAINBIRD, R.H., HADLARI, T., ASPLER, L.B., DONALDSON, J.A., LECHEMINANT, A.N. and PETERSON, T.D. (2003): Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada. Precambrian Research. 125, 21-53. RENAC, C., KYSER, K., DUROCHER, K., DREAVER, G. and O'CONNOR, T. (2002): Comparison of diagenetic fluids in the Proterozoic Thelon and Athabsca Basins, Canada: implications for protracted fluid histories in stable intracratonic basins. Can. J. Earth Sci. 39, 113-132. REYNOLDS, R.C. (1993): Three-dimentional X-ray powder diffraction form disordered illite: Simulation and interprétation of the diffraction patterns. LECTURES


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VAN BREEMEN, O., PETERSON, T.D. and SANDEMAN, H. (2005): U-Pb zircon geochronology and Nd isotops geochemistry of Proterozoic granitoids in the western Churchill Province: intrusive age pattern and Archean source domains. Canadian Journal of Earth Sciences. 42, 339-377. VELDE, B. (1977): Proposed Phase-Diagram For Illite, Expending Chlorite, Corrensite and Illite-Montmorillonite Mixed Layered Minerals. Clays and Clay Minerals. 25, 264.270. VELDE, B. (1985): Clay minerals: A physico-chemical explanation of their occurence. Developments in Sedimentology, Elsevier WEYER, H.-J., FRIEDRICH, G., BECHTEL, A. and BALLHORN, R.K. (1987): The Lone Gull uranium deposit-New geochemical and petrological data as evidence for the nature of the ore bearing solutions. Metallogenesis of uranium deposits, 542/19, Proceedings of a technical committee meeting, Vienna ZALESKI, E., DAVIS, W.J. and SANDEMAN, H. (2001): Continental rifting, mantle magmas and basement/cover relashionships. 4th International Archean Symposium, Perth, Australia

Etude microthermométrique des inclusions fluides dans les quartz et dolomite associées aux minéralisations uranifères du faisceau structural Kiggavik-Andrew Lake

83

2. Etude microthermométrique des inclusions fluides dans les quartz et dolomite associées aux minéralisations uranifères du faisceau structural Kiggavik-Andrew Lake

2.1. Introduction

Les nombreuses études d’inclusions fluides menées sur les gisements de type discordance

associés au basin de l’Athabasca sont venues étayer les modèles de minéralisations de type

diagénétique-hydrothermal dans lesquels des saumures oxydantes de bassin viennent interagir

avec des fluides et/ou des lithologies réductrices du socle (Pagel 1975, Pagel et Jaffrezic,

1977, Kotzer & Kyser, 1995, Derome et al, 2005, Hoeve & Quirt, 1984).

Une seule étude, limitée à des échantillons d’Andrew Lake a été réalisée sur les cristaux de

quartz associés aux gisements du faisceau de Kiggavik (Pagel et Ahamdach, 1995). Elle a mis

en évidence à la fois la circulation de fluides du bassin dans les roches du socle ainsi qu’une

histoire hydrothermale complexe plus précoce associée à des fluides de plus haute

température.

La présente étude vient donc compléter ces observations en étendant les investigations au

inclusions fluides présentes dans les cristaux de quartz et de dolomites associées au gisement

d’End Grid et Andrew Lake, et aussi au prospect de Bong. Deux échantillons provenant du

gisement d’End Grid seront étudiés. End09-05_13 à 254.9 m dans une veine de dolomite

massive et End09-07_06 à 210.0m dans une brèche hydraulique à remplissage de quartz

laiteux.

2.2. Bong


84

Les brèches hydrauliques à quartz et/ou carbonates sont relativement peu abondantes à

l’échelle de la zone de Bong. Elles sont sont cependant bien représentée dans le sondage

Bong33 situé au NE de la zone explorée. La structure portant la minéralisation a un pendage

vers le WSW. L’échantillon Bong39_04 64.8m est constitué par une veine à quartz et

carbonate de couleur miel clair à rosâtre montrant un jeu normal apparent reactivant

partiellement une microfracture cimentée par de la chlorite, dans un metagraywack très

faiblement altéré, Figure 2-1.

Figure 2-1 : Echantilllon BG39_04 présentant une microfaille à remplissage de quartz et carbonates avec

mouvement normal apparent et traces d’illitisation le long des épontes de la veinule, carotte diamètre NQ. Le

haut de l’image est dirigé vers le haut du sondage


85

2.3. End Grid

2.3.1. Echantillon : End09-05_13 @ 254.9m

Cet échantillon représente une veine de dolomite massive de 60 cm de puissance apparente et

encaissée dans un metagraywacke fortement argilisé au mur d’une zone de faille

plurimétrique.

Les 4 types d’inclusions identifiés dans les échantillons sont :

(1) Des inclusions triphasées constituées d’un liquide, d’une bulle de vapeur (estimée à environ 1% du volume de l’inclusion) et d’un cube de sel (L+V+H). Certaines de ces inclusions présentent également des inclusions solides sous forme de lamelles de nature indéterminée (illite ?).

(2) Des inclusions L+V (1 à10%) dans lesquelles ont observe parfois le mouvement de la bulle de gaz à température ambiante

(3) Des inclusions L+V (80%) (4) Des inclusions monophasées L

Les inclusions triphasées sont de loin les plus nombreuses. Elles sont de grande taille de 20

jusqu'à 50x150 µm et suivent généralement les plans de clivage du cristal de dolomite, figure

2-2.

Les inclusions biphasées présentent une grande hétérogénéité de taille et de morphologie,

certaines sont semblables au type (1) et tandis que d’autres en général de dimension

micrométriques et oblongues se répartissent sous forme d’alignements typiques de fractures

cicatrisées.

Les inclusions de types 3 et 4 sont elles aussi fréquement associées à des fractures secondaires

cicatrisées. Le necking down des inclusions n’étant pas un phénomène isolé il est fort

probable que de nombreuses inclusions des types β, 3, 4 résultent de la striction d’inclusions

triphasées de plus grande taille.


86

Seules les inclusions triphasées L+ V + H et les L + V à petites bulle de vapeur ont été

mesurées.

Figure 2-2 : Microphotographie des inclusions triphasées (L+ V + H) de grande taille dans la dolomite. De

l'échantillon End09-05_13

Les données microthermométriques montrent des températures d’homogénéisation figure 2-3

ainsi que des températures de fusion de la halite (TfNaCl) figure 2-4 très homogènes quelles

que soient la taille et la morphologie des inclusions considérées.

100 µm


87

Figure 2-3 : Histogramme de fréquence des températures d'homogénéisation (en °C) dans l'échantillon End09-

05_13

Figure 2-4 : Temperature de fusion de la glace en fonction des temperatures d'homogénisation, séparation des

inclusions triphasées des carbonates et de quartz

0

1

2

3

4

5

6

7

8

152 154 156 158 160 162 164 166 168 170 172 174 176 178 180 182 184 186 188 190

fré

qu

en

ce

Th en °C

Th Dolomite

70.00

90.00

110.00

130.00

150.00

170.00

190.00

70.00 90.00 110.00 130.00 150.00 170.00 190.00 210.00 230.00

Th

Tfglace °C

carbonates

quartz


88

Les températures de fusion de la glace sont plus homogènes parmis les inclusions triphasées

des cristaux de dolomite tandis que celles mesurées dans les quartz sont relativement

dispersées. De ce fait, il apparaît que certaines inclusions triphasées dans les quartz sont

situées sur des plans de fractures cicatrisées ce qui implique de possible réovertures ou des

piégeages à des instants différents. Au contraire dans les carbonates, les inclusions

apparaissent toutes primaires ou pseudo-primaires. Ainsi soit les carbonates ont piégé un seul

événement ce qui paraît peu probable ou bien les inclusions sont toutes secondaires et

marquent un seul et même événement de réactivation qui a conduit au remplissage des cavités

néoformées avec les saumures de fond de bassin. Cette réactivation est d’autant plus aisée que

les carbonates sont facilement clivables.

La seule température de fusion de la glace mesurée est extrêmement basse à -35.0°C, figure

2-5 et donne des salinités calculées de 14.7 wt% NaCl et 18.9 wt% CaCl2 en utilisant

l’équation de Steele-MacInnis, M., Bodnar, R.J. and Naden, J. (2010). A cette exception près

aucune prise de glace n’a été observée dans les inclusions triphasées malgré un

refroidissement jusqu’à -196°C.


89

Figure 2-5 : Température de fusion de la glace des inclusions triphasées dans la dolomite et biphasées le quartz

2.3.2. Echantillon : End09-07_06 @ 210.0m

Cet échantillon provient d’une brèche hydraulique à remplissage de quartz mise en place dans

un métagraywacke frais. Il se caractérise par une très forte densité d’inclusions fluides qui

rend difficile l’identification des différentes générations d’inclusions.

Les inclusions sont de petites taille avec des diamètres <20 microns. Elles se répartissent dans

la masse de quartz (sans organisation spatiale particulière) et le long de plans entre deux

phases de croissance de quartz ou des fractures cicatrisées.

4 types d’inclusions ont été identifés :

(1) L+V (1-5 %) prédominantes (2) L+V (> 80%) fréquentes (3) L rares (4) L+V+H rares

On note que le long de plans de fractures les inclusions de type (3) à une seule phase liquide

(généralement micrométriques) sont associées à des inclusions biphasées.

0

2

4

6

8

10

12

0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20 -22 -24 -26 -28 -30 -32 -34 -36 -38 -40

fré

qu

en

ce

Tfglace en °C

TfgQuartz

TfgDolomite


90

L’ensemble de ces inclusions a une morphologie ovoïde plus ou moins lobée ou de cristal

négatif de quartz.

Les mesures microthermométriques ont été effectuées sur les inclusions biphasées à bulle de

gaz de petite taille ainsi que sur les inclusions à cube de sel et bulle de gaz, figure 2-4 et 2-5.

Les températures de fusion de la glace mesurées permettent de calculer des salinités

comprises entre 2.8 et 11.2 wt% NaCl. Il est possible de mettre en évidence deux populations

avec une salinitée moyenne de 3.0 wt% NaCl pour l’une et de 9.3 wt% NaCl pour l’autre,

figure2-6. Ces deux familles d’inclusions sont pétrographiquement semblables. On remarque

de plus des températures d’homogénéisation moyenne de 305.5°C pour la population

d’inclusions présentant les salinités les plus élevées ainsi qu’une plus forte dispersion de

température d’homogénéisation, figure 2-7.

Figure 2-6 : Températures d'homogénéisation en fonction de la salinité dans les inclusions biphasées de

l'échantillon End09-07_06

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

500.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Tem

pé

ratu

re d

'ho

mgé

né

isat

ion

en

°C

Salinité calculé en wt% NaCl


91

Figure 2-7 : Histogramme de fréquence des températures d’homogénéisation des inclusions fluides dans les

quartz de l’échantillon End09-07_06

2.4. Interprétations et perspectives

L’ensemble des données microthermométriques acquises a permis de calculer les isochores

figure 2-8.

En prenant l’hypothèse d’une couverture sédimentaire de 5km avec une densité de 2600

kg/m3 et d’un gradient géothermique de 40°C/km (cas idéal purement diffusif) on remarque

que les températures de piégeage données par les isochores sont légèrement plus élevées mais

proches des températures d’homogénéisation mesurées sur les inclusions triphasées à cube de

sel dans la dolomlite, aux environs de 210°C. Dans le cas des inclusions biphasées les

températures sont beaucoup plus variable mais dans l’ensemble plus élevées que celles

calculées sur les inclusions triphasées. Elles pourraient signifier un apport de chaleur local

plus important ou une mise en place à plus grande profondeur.

0

1

2

3

4

5

80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480

fré

qu

en

ce

Th en °C

Th Quartz Fréquence


92

Ces observations sont en accord avec les résultats précédemment acquis sur le gisement

d’Andrew lake (1) avec la mise en évidence de circulation de saumures de bassin à plusieurs

centaines de mètres dans le socle et (2) une histoire des fluides polyphasée avec des saumures

de moyenne température et des fluides peu salés de plus haute temperature, figure 2-9. Il

convient maintenant d’étudier les inclusions des cristaux de quartz et carbonates des autres

gisements du faisceau afin de confirmer ces tendances. Ces mesures pourrant être

éventuellement complétées par de la spectroscopie RAMAN, du LIBS ou encore de l’ablation

laser pour determiner la composition chimique des saumures.


93

Figure 2-8 : Isochores calculées à l’aide de Flincor en utilisant l’équation de Brown & Lamb, 1989

-500

0

500

1000

1500

2000

2500

3000

3500

50 150 250 350 450 550 650 750

TmodelConductif(30°C/km)

TmodelConductif(40°C/km)

END090513_1.1

END090513_1.8

END090513_1.9

END090513_2.1

END090513_2.2

END090513_2.4

END090513_2.15

END090513_4.3

END090513_4.4

END090513_5.1

END090513_5.2

END090513_5.3

END090513_5.4

END090513_6.1

END090513_7.1

END090513_7.2

Phydro(Bar)

Phydro(Bar)

END090706_1.1

END090706_2.2

END090706_2.3

P lihto


94

Figure 2-9 : Histogramme de fréquence des températures d’homogénéisation de l’ensemble des inclusions étudiées le long du faisceau structural de Kiggavik Andrew Lake

0

5

10

15

20

25

30

35

40

0

20

40

60

80

10

0

12

0

14

0

16

0

18

0

20

0

22

0

24

0

26

0

28

0

30

0

32

0

34

0

36

0

38

0

40

0

42

0

44

0

46

0

48

0

Fré

qu

en

ce

Temperature °C

HistoFReq_Th_Carbonates

HistoFReq_Th_Quartz Fréquence


95

Réferences

Derome D, Cathelineau M, Cuney M, Fabre C, Lhomme T, Banks DA (2005) Mixing of Sodic and Calcic Brines and Uranium Deposition at McArthur River, Saskatchewan, Canada: A Raman and Laser-Induced Breakdown Spectroscopic Study of Fluid Inclusions. Economic Geology 100:1529-1545. doi: 10.2113/gsecongeo.100.8.1529.




Pagel M, Jaffrezic H (1977) Analyses chimiques des saumures des inclusionsdu quartz et de la dolomite du gisement d’uranium de Rabbit Lake(Canada). Aspect méthodologique et importance génétique. Comptes Rendus de l'Académie des Sciences 284:113-116.

Pagel M, Ahamdach N (1995) Etude des inclusions fluides dans les quartz des gisements d'uranium de l'Athabasca et du Thelon (Canada). Centre de Recherche sur la Geologie des matières premieres minerales et énérgétiques - CREGU, Vandoeuvre les Nancy, pp 1-10.

http://dx.doi.org/10.1016/0009-2541(94)00114-N

Illite & Uraninite geochronology

96

3. Illite & Uraninite geochronology

3.1. Introduction

Temporal constrains on alteration and mineralization events are of main importance for

geologists who deal with metallogenic interpretation of uranium deposition and more

particularly with the timing of deposition of the present uranium ore deposits. From the

previous study on petrography and mineralogy of both uranium ore bodies and alteration

haloes along the Kiggavik-Andrew Lake structural trend, uraninite and illite seems to be the

best mineral candidates for gaining the radio-chronological data necessary to refine the age

and the temporal relationships between mineralization and uranium alteration processes.

Uranium ore-forming minerals like uraninite are very likely to be remobilized in oxidizing

conditions, (via the oxidation of the U4+ to U6+ which is highly soluble in aqueous solutions).

Then, losses of radiogenic lead or gain of common lead can occur and mislead the

interpretation of the original crystallisation age. However, this could be seen as a great

advantage because uraninite resets may help to record the various remobilisation events since

the primary uranium deposition, either controlled by regional events (e.g. intrusion

emplacements, meteoric water circulation in faults and fractures) either controlled by far field

effects of the continental crust geodynamics (e.g. rifting, diagenesis, collision, uplift,

isostatic rebound).

3.2. Ar/Ar principle and method

The principle of the 40Ar/39Ar geochronology is a variant of the K / Ar method, based on the

fact that potassium rich mineral present elevated 40Ar/36Ar ratios due to the 40K decay

(Aldrich and Nier 1948). Then the amount of radiogenic 40Ar* is proportional to the initial

potassium content in the mineral and time.


97

In nature both stable isotopes (36Ar, 38Ar and 40Ar*) and radioactive ones (37Ar & 39Ar) are

presents. Then artificial irradiation allows the precise quantification of the initial 40K from the

relative abundance between argon isotopes and the argon produced via neutron irradiation

(39Ark), with the relations:

40K = 0.0000125 * 39K, and 39Ark = 40K * J’ with J’, the irradiation parameter.

Then the isotopic ratios and the combination of the different yields can give access to the

40Ar/39Ar age using the equation:

with λ the decay constant and J the irradiation parameter.

The J’ and J parameter constrains the abundance of potassium isotopes, the decay constant,

the neutron fluence, the effective cross section of the K (n, p) reaction, as well as the duration

of irradiation. These parameters are obtained from the co-irradiation of standard of known age

with the samples (Mitchell 1968).

One of the advantages of the Ar/Ar method over the conventional K/Ar is the step heating

degassing (Merrihue and Turner 1966). This is providing information about potential Ar

unrelated to the radioactive decay of potassium, and can possibly be plotted as an isochron

diagram comprising a plot of 40Ar/36Ar vs 39Ar/36Ar. Then one sample can give many ages,

and the presence or absence of plateau is critical for interpretation. The lack of plateau forces

to a more carful interpretation. Then ages resulting from the interpretation of Ar realised must

be taken more carefully. Such assessment and control over the data in not possible with the

K/Ar were the age is representative of the mean value of all the released radiogenic argon.

Then two types of ages can be calculated: (1) an average age comprising several steps that

define a plateau, and (2) a total fusion age, were all the measured steps are used, and then is


98

equivalent to a K/Ar age. Since the several decades the definition of the plateau varied

according to the analytical progress and the authors. As a general guideline, a plateau can be

defined a segment of the age spectrum which consist of 3 steps with overlapping age within

the analytical error (2σ) in which more than 50 % of the argon is released. All samples

were run as conventional furnace step heating analyses on bulk illite mineral separates. All

data are reported at the 1σ uncertainty level, unless noted otherwise. Furnace step heating

analyses produce what is referred to as an apparent age spectrum. The "apparent age" derives

from the fact that ages on an age spectrum plot are calculated assuming that the non-

radiogenic argon (often referred to as trapped, or initial argon) is atmospheric in isotopic

composition (40Ar/36Ar = 295.5).

3.3. Samples

The dataset of our geochronological study consists in 6 mineral Table 3-1 (1) Four of them

consist in illite and sudoite separates which were dated by the Ar/Ar method. Sudoite persists

in the illite concentrate because it was not possible to efficiently separate these two clay

minerals. They constitute a clay assemblage impossible to sort by any existing method of

mineral separation. However, it can be expected that sudoite which does not contain

potassium (or other alkali element) in its crystal structure has not significant influence on the

measured ages. Each clay sample was encapsulated prior to irradiation to avoid Ar losses

(Hess and Lippolt 1986; Foland et al. 1992) and processed in the Nevada Isotope

Geochronology Laboratory at the University of Nevada, Las Vegas, USA. A sudoite-rich,

sample (Bong42_25) was selected in order to test the timing of alteration comparatively with

zones where illite is dominant. The work hypothesis was a possible reset of the illite

geochronometer during the sudoite precipitation. (2) Two semi-massive uraninite samples

from the End Grid deposit were analysed via SIMS using the U/Pb geochronometer at the

CRPG-CNRS facility on a CAMECA IMS 1270 ion-probe. The analytical protocol is given in


99

(Alexandrov et al. 2000) and the ISOPLOT software was used for the isochron calculations

(Ludwig 1993).

Table 3-1 : Main characteristics of minerals samples selected for geochronology

Sample Depth Macroscopic description Illite Sudoite Method

End09_04_32/

9568-38

345.1 uraninite in fracture in a redox front zone,

bleaching

no data U/Pb

SIMS

End09_04_33/

9568-39

355.6 semi-massive pitchblend in a fracture parallel to

the foliation

xxx x “

Bong42_21 309.3 microfault gouge parallel to the foliation xx xx Ar/Ar

Bong42_25 347.7 pervasively altered metagraywacke x xxx “

Bong42_32 388.2 pervasively altered metagraywacke xx x “

Bong42_40 441.0 microfault gouge parallel to the foliation xxx x “

3.4. Results

3.4.1. Ar-Ar datation of illite

All the samples produced discordant age spectrum, while the K/Ca ratios were low and

consistent with illite separate. In addition the high radiogenic yields in agreement indicates

that the samples were not affected by recent alteration. The total gas ages obtained were 1248,

1072, 1033 and 952 Ma (Figure 3-1 and table 3-1). However most of the samples showed a

relatively complex age spectrum, and only one plateau age at 1124 9 Ma can be calculated

from the Bong42_40 sample (figure 3-1d). Nevertheless, some significant steps of the age

spectrum diagrams showing overlaps can cautiously be interpreted, as “pseudo-plateau”. This

age which doesn’t fully agree with the requirements for a plateau definition (in term of age


100

overlap and Ar released) may provide more knowledge on the alteration and mineralization

relationships. From these data, several groups of ages can be identified, near 1300 Ma in the

Bong4221 & 4225 samples and near 1100 Ma in the Bong4232 and Bong4240 samples. A last

an age around 1200 Ma can be also suspected with less confidence.

Figure 3-1 : Age spectrum of illites of the Bong prospect obtained from Ar/Ar datation method. a: Bong42_21;

b: Bong42_25; c: Bong42_32; d: Bong42_40.


101

Table 3-2 Ar/Ar geochronology data

step T

(°C) t

(min.) 36Ar 37Ar 38Ar 39Ar 40Ar %40Ar* % 39Ar rlsd Ca/K 40Ar*/39ArK Age (Ma) 1s.d. Bong42_21, Illite, 0.19 mg, J = 0.00543 ± 0.50%

1 450 12 0.118 0.023 0.192 8.963 1347.69 98.3 9.1 0.08033758 148.094449 1064.6 7.6 2 500 12 0.047 0.022 0.115 6.233 977.83 99.7 6.3 0.110502883 156.763618 1111.0 7.7 3 550 12 0.045 0.017 0.151 9.552 1608.99 99.8 9.7 0.055718043 169.307208 1176.2 8.1 4 600 12 0.050 0.022 0.194 13.161 2222.12 99.8 13.3 0.052332878 169.979488 1179.7 8.0 5 650 12 0.055 0.025 0.269 17.968 3362.74 99.8 18.2 0.043559211 188.752426 1272.8 9.1 6 700 12 0.049 0.028 0.345 22.443 4385.28 99.9 22.7 0.039058566 197.360166 1314.0 8.6 7 750 12 0.031 0.020 0.226 14.783 2902.95 100.0 15.0 0.042355226 197.627568 1315.3 8.9 8 800 12 0.029 0.015 0.062 3.602 814.771 100.0 3.7 0.130376126 223.734354 1434.6 9.3 9 850 12 0.027 0.016 0.021 1.242 323.665 100.0 1.3 0.403350707 248.229084 1539.8 9.8 10 910 12 0.036 0.014 0.016 0.541 153.107 100.0 0.5 0.810337177 250.430085 1549.0 10.2 11 970 12 0.042 0.016 0.014 0.187 58.036 99.9 0.2 2.680684089 167.943639 1169.3 9.5

Cumulative %39Ar rlsd = 100.0 Total gas age = 1248.2 7.3 Bong42_25, Illite, 0.39 mg, J = 0.00472 ± 1.55%

1 450 12 0.596 0.049 2.155 20.042 1901.65 91.0 20.3 0.077876847 87.679309 624.78 9.29 2 500 12 0.161 0.061 1.302 24.770 2048.34 98.3 25.1 0.078443549 81.899919 589.63 8.75 3 540 12 0.073 0.045 0.531 14.914 2595.82 99.6 15.1 0.096111195 174.883789 1085.74 14.15 4 570 12 0.044 0.028 0.323 9.758 2128.38 99.9 9.9 0.091401271 219.612617 1283.18 15.96 5 600 12 0.043 0.029 0.237 8.190 1882.00 99.9 8.3 0.112790305 231.224893 1331.09 16.47 6 640 12 0.222 0.033 0.278 9.022 2122.05 97.5 9.2 0.116511623 231.250191 1331.20 16.40 7 690 12 2.220 0.046 0.711 7.371 2242.50 72.7 7.5 0.198792401 223.065351 1297.56 16.93 8 740 12 0.050 0.014 0.036 1.443 298.336 99.3 1.5 0.309060685 410.064354 1942.77 20.67 9 840 12 0.153 0.023 0.089 2.286 448.787 94.6 2.3 0.320505207 179.954762 1109.24 14.65 10 1000 12 0.189 0.032 0.064 0.706 75.517 42.5 0.7 1.44433639 29.580690 235.79 5.23


1 400 12 0.069 0.014 0.118 2.727 294.022 96.7 5.3 0.164422178 101.868836 710.76 9.55 2 450 12 0.043 0.013 0.117 3.132 398.964 99.5 6.1 0.132933729 124.984666 839.36 10.62 3 480 12 0.031 0.009 0.103 3.076 429.522 99.9 6.0 0.093705459 137.874793 907.29 11.32 4 510 12 0.035 0.014 0.110 4.048 619.357 100.0 7.9 0.110763932 152.413632 980.95 11.96 5 540 12 0.042 0.021 0.126 5.367 852.669 99.8 10.5 0.125314216 158.622804 1011.52 12.25 6 570 12 0.032 0.016 0.128 6.002 995.231 100.0 11.7 0.085375188 166.133281 1047.81 12.65 7 600 12 0.036 0.016 0.135 6.078 1059.91 100.0 11.8 0.084307621 174.877712 1089.17 12.89 8 640 12 0.505 0.014 0.277 8.625 1632.74 91.9 16.8 0.051984331 175.153282 1090.46 13.08 9 690 12 4.233 0.029 1.037 7.700 2565.11 54.2 15.0 0.120620002 182.139769 1122.81 15.47 10 740 12 0.103 0.016 0.091 3.226 613.706 98.4 6.3 0.15884459 183.675258 1129.84 13.29 11 840 12 0.114 0.016 0.067 1.096 254.611 94.8 2.1 0.46758933 205.979005 1229.02 14.66 12 1000 12 0.179 0.016 0.043 0.242 102.156 66.6 0.5 2.118677458 209.431054 1243.90 17.20


1 400 12 0.118 0.015 0.175 4.771 634.887 86.3 3.8 0.102158706 127.854730 869.02 11.06 2 450 12 0.063 0.022 0.195 5.649 780.443 99.0 4.5 0.126545782 136.870450 917.00 11.42 3 480 12 0.038 0.012 0.158 5.432 755.730 99.9 4.3 0.071781289 139.021735 928.27 11.56 4 510 12 0.039 0.013 0.183 7.532 1144.59 99.9 5.9 0.056081659 152.582126 997.69 12.33 5 540 12 0.042 0.012 0.216 10.217 1599.84 99.9 8.1 0.038163082 158.598822 1027.66 12.37 6 570 12 0.042 0.020 0.248 12.560 2017.40 99.9 9.9 0.051740144 161.907354 1043.93 12.58 7 600 12 0.043 0.024 0.248 13.829 2355.68 99.9 10.9 0.056390808 171.838464 1091.91 13.03 8 640 12 0.162 0.029 0.394 20.673 3642.17 99.9 16.3 0.045580701 176.431866 1113.67 13.20 9 690 12 2.722 0.038 1.105 27.837 5656.73 86.7 22.0 0.044355505 178.419659 1123.01 13.60 10 760 12 0.142 0.020 0.297 15.464 2798.37 98.9 12.2 0.042023694 180.891527 1134.55 13.36 11 1000 12 0.334 0.016 0.148 2.685 644.618 88.9 2.1 0.193633545 207.465883 1254.22 14.65

Cumulative %39Ar rlsd = 100.0 Total gas age = 1072.18 6.29

Plateau age = 1123.60 8.98

note: isotope beams in mV, rlsd = released, error in age includes J error, all errors 1 sigma 4 amu discrimination = 1.0598 ± 0.62%, 40/39K = 0.0086 ± 27.68%, 36/37Ca = 0.000256 ± 2.01%, 39/37Ca = 0.000658 ± 1.01% isotope beams in mV, rlsd = released, error in age includes J error, all errors 1 sigma (36Ar through 40Ar are measured beam intensities, corrected for decay for the age calculations)


102

3.4.2. SIMS U/Pb datation of uraninite

Uranium and lead isotopic ratios (206Pb/238U and 207Pb/235U) measured in samples End0904-32

and End0904-33 were plotted on Concordia diagrams, figure 3-2 & 3-3.

Figure 3-2 : U/Pb concordia diagram for End Grid veinlet mineralization

1600

1400

1200

1000

800

600

0.06

0.10

0.14

0.18

0.22

0.26

0.30

0.34

0.5 1.5 2.5 3.5

207Pb/

235U

206P

b/2

38U

Intercepts at

124±25 & 1293.1 ±6.2 [±8.4] Ma

MSWD = 27

KIGGAVIK (End Grid) - 9568-38


103

Figure 3-3 : U/Pb Concordia diagrams for End Grid semi-massive mineralization

Veinlet type mineralization data gave an upper intercept at 12936 Ma and a Mean Square

Weighted Deviation (MSWD) of 27, figure 3-2. The data for the semi-massive mineralization

gave an upper intercept of 118719 Ma and a MSWD of 8.3, Figure 3-3. In both case the

high MSWD indicates relatively scattered values.

3.5. Discussion

Until know most of the available literature on the Kiggavik area were TIMS U/Pb ages on the

mineralization or K/Ar geochronology on illite (c.f introduction). The gathered

geochronological data can be integrated with the literature data especially the recent work

done with in-situ techniques, more suitable to discriminate distinct events of mineralization

and alteration.

1400

1200

1000

800

600

400

200

00.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

0 1 2 3

207Pb/

235U

20

6P

b/2

38U Intercepts at

46 ± 57 & 1187 ± 19 [±20] MaMSWD = 8.3

KIGGAVIK (End Grid) - 9568-39


104

The Ar/Ar data obtained on illite from the Bong deposits can be compared with U/Pb in-situ

data ages on Bong uraninite (Sharpe 2013), as well as the age obtained on End mineralization,

Figure 3-4. Both the identified plateau and pseudo-plateau are within the errors range of the

U/Pb ages on uraninite. In addition, ages around 1300 Ma seems recurrent in all the analysed

samples. This age is in the same time frame as the MacKenzie dykes, so this event could be a

control of the mineralization via the far field tectonics related to Poseidon ocean opening.

They could be indicative of the first mineralization stage, but older ages cannot be totally

excluded. Nevertheless, the step heating doesn’t indicate that this age could be related to a

major reset due to the absence of ages older than 1300 Ma. The only possibility as showed in

the Bong4225 would be the presence of trace of unaltered metamorphic micas, giving a non

significant step of Hudsonian age. These ages are slightly younger than the 1403 Ma U/Pb

TIMS age on uraninite or the K/Ar ages on illite 138624 Ma and 1362 21 Ma at Kiggavik

(Farkas 1984; Miller and LeCheminant 1985). However, the comparison between the bulk

ages and in situ or Ar/Ar via step heating degassing may be not relevant as heterogeneities

and numerous remobilizations affected uraninite. In fact, remoblization could result in

contamination by common lead and apparently older ages. The younger ages around 1100

Ma are possibly connected to a later event of uranium remobilization. They could also be

related to the Grenville orogen.

Finally, it can be noted that similar ages have been obtained for both the uranium

mineralization and the illite crystallization in the associated alteration halo in the Bong and

the End uranium mineralizations. This confirms a close temporal relationship between the two

process, highlighting so the interest of alteration minerals as marker of the paleoconditions of

uranium deposition. The fact that similar ages have been obtained in the two studied areas

confirms also the regional extension of the alteration and mineralization processes. This also

indicates that reactivation of both local and regional faults allowed the fluid circulation.


105

Finally, the ages obtained in the uranium deposits along the Kiggavik-Andrew-lake structural

trend indicate that alteration and mineralization processes were active over a large span of

time (⋍200MA) which is not so different of the span of time reported for alteration

mieralization processes in the Athabasca basin (Jefferson et al., 2007).


106

Figure 3-4 : Summary of new geochronological data on alteration and mineralization at Kiggavik with comparison to tectonic or thermal events affecting the Canadian shield

during the Paleoproterozoic, data from (Fahrig 1987; Rainbird et al. 2003; Davis et al. 2011; Peterson et al. 2002; Turner et al. 2001; Sharpe 2013)


107

3.6. References

Aldrich LT, Nier AO (1948) Argon 40 in Potassium minerals. Phys Rev 74. Alexandrov P, Cheilletz A, Deloule É, Cuney M (2000) 319 ± 7 Ma crystallization age for the Blond

granite (northwest Limousin, French Massif Central) obtained by U/Pb ion-probe dating of zircons. Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary

Science 330:617-622. doi: http://dx.doi.org/10.1016/S1251-8050(00)00201-9. Davis WJ, Gall Q, Jefferson CW, Rainbird RH (2011) Fluorapatite in the Paleoproterozoic Thelon Basin:

Structural-stratigraphic context, in situ ion microprobe U-Pb ages, and fluid-flow history. Geological Society of America Bulletin 123:1056-1073. doi: 10.1130/b30163.1.

Fahrig WF (1987) The tectonic settings of continental mafic dyke swarms: Failed arm and early passive margin In: Halls HC, Fahrig WF (eds) Mafic dyke swarms. Geological Association of Canada Special Paper, St John's, Nfld, pp 331-348.

Farkas A (1984) Mineralogy and host rock alteration of the Lone Gull deposit Internal report. Urangesellschaft, Frankfurt am Main, pp 45.

Foland KA, Hubacher FA, Arehart GB (1992) 40Ar39Ar dating of very fine-grained samples: An encapsulated-vial procedure to overcome the problem of 39Ar recoil loss. Chemical Geology

102:269-276. doi: http://dx.doi.org/10.1016/0009-2541(92)90161-W. Hess JC, Lippolt HJ (1986) 40Ar/39Ar ages of tonstein and tuff sanidines: New calibration points for

the improvement of the Upper Carboniferous time scale. Chemical Geology: Isotope

Geoscience section 59:143-154. doi: http://dx.doi.org/10.1016/0168-9622(86)90066-7. Ludwig KR (1993) Isoplot:excel based program for plotting radiogenic isotopes. U.S. Geological

Survey, pp 1-42. Merrihue C, Turner G (1966) Potassium-argon dating by activation with fast neutrons. Journal of

Geophysical Research 71:2852-2857. doi: 10.1029/JZ071i011p02852. Miller AR, LeCheminant AN (1985) Geology and uranium metallogeny of Proterozoic supracrustal

successions, central District of Keewatin, N.W.T with comparisons to northern Saskatchewan Geology of uranium deposits. Canadian Institute of Mining and Metalurgy, pp 167-185.

Mitchell JG (1968) The argon-40argon-39 method for potassium-argon age determination.

Geochimica et Cosmochimica Acta 32:781-790. doi: http://dx.doi.org/10.1016/0016-7037(68)90012-4.


Rainbird RH, Hadlari T, Aspler LB, Donaldson JA, LeCheminant AN, Peterson TD (2003) Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada. Precambrian Research 125:21-53.

Sharpe R (2013) The geochermistry and geochronolgy of the Bong uranium deposit, Thelon Basin, Nunavut, Canada Department of Geological Sciences. University of Manitoba, Winnipeg, Manitoba.

Turner W, Richards J, Nesbitt B, Muehlenbachs K, Biczok J (2001) Proterozoic low-sulfidation epithermal Au-Ag mineralization in the Mallery Lake area, Nunavut, Canada. Miner Deposita 36:442-457. doi: 10.1007/s001260100181.

http://dx.doi.org/10.1016/S1251-8050(00)00201-9

http://dx.doi.org/10.1016/0009-2541(92)90161-W

http://dx.doi.org/10.1016/0168-9622(86)90066-7

http://dx.doi.org/10.1016/0016-7037(68)90012-4

http://dx.doi.org/10.1016/0016-7037(68)90012-4

The basal Thelon Formation at Kiggavik

108

4. The basal Thelon Formation at Kiggavik

At Kiggavik, Thelon Formation is not widely encountered on actual land ARC land holdings,

and mostly represented by a strip of sandstones outcrops located at the North of the Granite

Grid prospect, close to the Thelon fault escarpment and basal conglomerates outcrops, at the

WNW of Kiggavik camp and in the St Tropez area, south of Schultz Lake. All studied

samples presented here where taken from outcrops in the Granite (labeled as GGx) & St

Tropez areas as well as in the historical Th18 BP drillhole, located 10 kilometers NNW of

Kiggavik.

Nevertheless, as the connection between the Thelon Basin and the mineralization system at

Kiggavik appears more and more preeminent, an overview of local petrographic and

mineralogical properties of these sandstones would provide additional informations for the

metallogenic interpretation of the Kiggavik area.

4.1. Methods

XRD diffraction were performed using a Bruker D8 diffractometer, on the 2- 65° 2θ range

using a 0.02 °2θ step a d se o d ou ti g ti e. Cu a ode X-ray tube set up with a 40kV

acceleration tension and 40mA current, 1° fixed divergence slit.

Transmission Fourier Infrared (FT-IR) spectra were recorded with a Nicolet 760 FT-IR

spectrometer equipped with a Potassium Bromide (KBr) beamsplitter and a DTGS-KBr

detector. Acquisitions were performed in the 400 – 4000 cm-1 (mid infrared range) with a

4cm-1 resolution. Pellets suitable for analysis were made using 1 mg of clay separates, used

for XRD measurement, and grounded in an agate mortar with 150 mg of KBr. The mixture

was therefore pressed in a vacuum die with 10 tons per cm2 of compression. Pellets were dried

24 hours at 120°C prior to measurements to avoid hygroscopic water in KBr. For each

spectrum 100 scans were cumulated after a background acquisition.


109

Electron microprobe measurements on clay minerals were done on a Cameca SX100 using

natural and synthetic standards for calibration. Albite, anortite and orthose, diopside, garnet

and MnTiO3 respectively for Na, Ca, K, Si, Fe, Mn, and Ti, with a column setting with a

15kV acceleration tension and a 10nA beam current.

Quartz oxygen isotopes compositions measurements were performed in situ by secondary ion

mass spectrometry (SIMS) on a CAMECA IMF 7f ion microprobe, at the University of

Manitoba. A ~2 nA primary beam of Cs+ was accelerated at 10kV and focused to a 10 x 15

m spot using a 100 m aperture in the primary column. An offset of 200-volts was used to

eliminate molecular ion interferences (Fayek et al. 2002; Riciputi and Greenwood 1998). Ions

were detected with a Balzers SEV 1217 electron multiplier coupled with an ion-counting

system using an overall deadtime of 52 ns. Two isotopes of oxygen, 16O- and 18O-, were

detected by switching the magnetic field. Analyses comprised 70 cycles and lasted ~10

minutes (Sharpe 2013). All stable-isotope data are presented in the δ-notation relative to the

appropriate standard. Oxygen is reported relative to Vienna Standard Mean Ocean Water (V-

SMOW) in units of per mil (‰) and are calculated using the following equation: δ18O (‰) =

(Rsample / RV-SMOW -1) * 103. Additional informations on SIMS are provided in appendix.

Chemical bulk rock analysis ICP-MS on major and traces elements were performed by the

Service d’Analyses des Roches et Minéraux (SARM) in CRPG, Nancy, France

4.2. Sandstones regional setting

The unconformity is mostly visible along the Thelon fault through the tectonic contact, the

sandstones being generally found on the Northern side of the fault, Figure 4-1a. A significant

vertical displacement, up to 200 meters, has been recognized from drillholes along this fault


110

exhibiting a horst and graben structure (P.Wollenberg, pers. Comm.). Then in one

compartment, both basement rocks at the topographic surface and the unconformity are

eroded while in the other compartment tens or hundreds of meters of sandstones are still

present above the unconformity. This feature reveals that the actual position of the Kiggavik

trend deposits known form to surface up to several hundred meters into the basement rocks

may have been formed deeply and thus are possibly the expression of deep seated

unconformity type uranium deposits. The normal movements with a dextral component

interpreted from the kinematic markers on fault surface, Figure 4-1b, are consistent with the

regional ones and are observed on the both Thelon and judge Sisson faults and are also

marked by significant alteration associated with hydraulic and tectonic breccias. Alteration is

expressed with a zone of intense hematization centered on the fault core which intensity

decrease rapidly in the damage zones in both foot and hanging walls. Into the fault core

hematization zones are forming alternating corridors of silicified / desilificied / hematized

rocks dominantly expressed in the Woodburn metasediments but also in the Archean or

Proterozoic quartzites. Hydraulic and tectonic brecciation is also present in the sandstones

above the unconformity along the Thelon fault zone Figure 4-1f. The lateral change form

strongly altered to fresh rocks is relatively sharp (over less than tens of meters) with

hematization being more disseminated and patchier in a dominantly chloritic metasediments.

Moreover at the vicinity of these regional tectonic structures, the general trend of flat flying

foliation is reworked and vertically reoriented, Figure 4-1c.


111


112

Figure 4-1a : Thelon fault escarpment where the Thelon Formation sandstones are tectonically in contact with

the underlying Woodburn Group metasediments, view looking toward the NNW, outcrop located at the north of

the Granite Grid area on CAMECO corp. ground (checked jointly with R. Hunter, CAMECO & C. Jefferson,

GSC); b, Detail of the previously indicated fault, looking at the handing wall the surface is showing quartzite

fragments dragged into the fault and indicating a normal with a dextral component displacement; c Angular

unconformity between the Thelon sandstones and the locally steep foliation of Woodburn Group metasediments,

strong red-brick hematization, at Unconformity lake (Forum’s ground); d, symmetrical ripple marks develloped

on the Thelon sandstones at Unconformity lake; e, red siltstone facies of the basal Thelon formation; f, hydraulic

fracturation breccias with sub angular sandstones fragments cemented by hydrothermal quartz at 110.9 meter,

with unconformity at 126.4m , BP historic drillcores; g, Polymictic conglomerate at the base of the Thelon

Formation in the St Tropez area, gravel to pebble boulder size elements of red siltite, gneisses and abundant

quartzite in a siliceous and hematitic matrix.

The sedimentary cover overlying the unconformity is formed of gritty to conglomeratic,

moderately to weakly sorted, and locally cross bedded, matrix supported sandstones

containing abundant lithic fragments of basement rocks (various gneisses, quartzite).

Lenticular to isopachous, metric intercalation of red siltstone beds are present within the

sandstones. Finally conglomerates seems to fill paleovalleys Figure 4-1e, d, g. in which

sediments may have been deposited in a relatively high energy, continental environment, at

shallow depth as are indicating from the observed symmetrical ripple marks, characteristic of

the oscillating waves effects Figure 4-1d.

4.3. Bulk-rock chemistry of the Basal Thelon sandstones

Bulk rock chemical analysis from the basal Thelon Formation are representative of quartz

arenite sandstones totally depleted in Na, Mg and Ca elements with only Al as major element

in addition to the strongly predominant Si (table 4-1). In the sandstones samples, the


113

aluminum content ranges from 2 to 8 % and can be essentially assigned to the presence of

kaolinite (in absence of significant amount of K), while iron (less than 1%) is principally

assigned to hematite. The very low potassium amount of sandstones can be related to detrital

micas and locally to traces of illite related to later incipient hydrothermal alteration. The

higher potassium content measured in the siltite sample (GG3a) is representative of the high

content of detrital micas in the sample. All these data are indicative of the absence of

significant amount of feldspar in the sandstones of the basal Thelon formation investigated in

the Kiggavik area.

Table 4-1 Major and traces elements concentration of basal Thelon sandstones (sst) from the NE St Tropez

(NESTP4) and Granite Grid (GG) areas. Red siltites from Granite Grid are represented by the GG3a sample

SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 PF Zr Th U NESTP4 sst 91.50 4.57 0.96 0.01 0.03 < L.D. 0.01 0.41 0.11 0.04 1.51 52.42 5.35 0.70 GG3b sst 93.25 4.36 0.22 0.00 0.02 < L.D. 0.02 0.28 0.13 0.07 1.46 83.88 8.58 0.40 GG3c sst 91.70 4.93 0.24 0.00 0.02 < L.D. 0.01 0.25 0.08 0.04 1.81 51.12 6.07 0.39 GG4 sst 91.46 4.94 0.15 0.00 < L.D. < L.D. 0.01 0.22 0.07 0.03 1.88 50.05 5.47 0.34 GG5 sst 95.01 2.50 0.16 0.00 0.04 < L.D. 0.01 0.61 0.07 0.03 0.58 66.38 5.29 0.34 GG7 sst 87.19 8.06 0.61 0.00 0.02 < L.D. < L.D. 0.33 0.07 0.05 2.79 64.70 5.30 0.44 GG3a siltite 82.74 8.98 1.94 0.00 0.16 0.77 0.02 2.33 0.34 0.28 2.08 68.89 23.40 0.92

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ΣREE 21.08 32.63 3.46 10.60 1.31 0.23 0.77 0.11 0.52 0.09 0.26 0.04 0.32 0.05 71.19 29.04 50.28 5.18 15.88 2.08 0.40 1.28 0.15 0.56 0.07 0.20 0.03 0.24 0.04 105.23 19.57 33.48 3.46 11.10 1.50 0.31 0.85 0.10 0.38 0.06 0.18 0.03 0.24 0.04 71.11 18.49 18.61 3.18 9.76 1.15 0.23 0.52 0.06 0.27 0.04 0.13 0.02 0.18 0.03 52.54 11.96 21.40 2.18 7.09 1.02 0.22 0.64 0.07 0.32 0.05 0.15 0.03 0.20 0.03 45.20 18.72 35.23 3.47 11.30 1.65 0.35 1.16 0.14 0.67 0.11 0.32 0.05 0.37 0.06 73.28 92.91 133.00 19.44 72.43 12.31 2.30 7.17 0.74 2.08 0.21 0.60 0.09 0.73 0.12 343.52

Considering the trace elements, the basal sandstones can be characterized by very low

uranium concentrations (0.3 to 0.9 ppm) and U/Th ratios which range between 7.6 and 21.7.

In addition, sandstones are also characterized by light REE (La to Eu) > to heavy REE (Gd to

Lu) in the chondrite normalized patterns, Figure 4-2. Moreover the (La/Yb)ch ratios are

ranging from 34 to 82. Such fractionation is very common in weathering processes where


114

residual products are enriched in LREE and depleted in HREE (Duddy 1980; Nesbitt 1979,

(Braun et al. 1990).

Figure 4-2: chondrite normalized (Evensen et al. 1978) REE patterns for sandstones and siltites at the base of the

Thelon Formation

4.4. Petrography and mineralogy

According to the available sampling, petrographic and mineralogical study of sandstones

samples is not representative of the whole diagenetic phenomenon throughout the Thelon

basin. Its main goal is to describe in more details the basin / basement relationships at the

vicinity of the N70 trend Thelon fault which is non graphitic and non mineralized, but need to

be accounted for a fault of regional interest for the prospection of uranium mineralization in

the Kiggavik area.

All hand samples present a well developed cementation of the intergranular porosity by very

fine grained “cherty quartz”. Such silica cement trapped diagenetic APS minerals (see the

0.1

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

NESTP4

GG3b

GG3c

GG4

GG5

GG7

GG3a

Siltite

Sandstones


115

paragenetic sequence in chapter 2) as well as large kaolinite vermicules or fragments of

aggregated quartz / kaolinite material of detrital origin.

Three different generations of quartz can be identified in these cemented sandstones: (1)

detrital, angular to sub rounded grains, (2) syntaxial quartz overgrowth forming 10 to 20

micrometer rims around detrital quartz grains, and (3) very fine grained microcrystalline

quartz (Folk and Weaver 1952) sealing all the remaining porosity of the sandstone, figure 4-

3a. The quartz overgrowths display evidence of stopping at the contact with the kaolinite

vermicules, Figure 4-3b, while the microcrystalline quartz cement not only heal the porosity

but show evidences of jigsaw texture suggesting a fluid assisted brecciation. Such hydraulic

fracture together with a matrix formed of a mosaic of tiny quartz crystals, is indicative of a

brutal event of fluid-rock interaction (strong oversaturation of the solution versus quartz)

during which nucleation is favored rather crystal growth.

According to their textural characteristics, three different types of kaolinite minerals have

been distinguished: (1) smalls euhedral crystals (3 * 3m) disseminated in the cemented

porosity (possibly neoformed or fragments of larger size kaolinite), Figure 4-3a, 4-3b; (2)

large vermicular kaolinite up to 0.5 x 0.2 mm which occurred either as individual crystal

either as aggregates, Figure 4-3c; and (3) composite aggregates of detrital quartz grains and

kaolinite, Figure 4-3d.


116

Figure 4-3 : a, epitaxial diagenetic quartz overgrowths (Q2) and (Q3) microcrystalline quartz cement; b, large

kaolinite booklets floating in the cemented porosity with evidence of stopping of the epitaxial growth of

diagenetic quartz overgrowth (Q2), c, aggregate of large, (up to 0.5mm length) kaolinite booklets in the

sandstone porosity, d, composite quartz and kaolinite lithic grain (red dotted circle); e, kaolinite & quartz lithic

grain showing evidence of later illitization; f, detrital quartz grain cross cut by a fracture filled by

microcrystalline quartz.


117

In addition it can be noted that micrometer grained size hematite grains are observed between

the booklets of kaolinite vermicules, as well as disseminated in the porosity in absence of any

trace of associated illitization. These iron oxides, likely hematite, are quite abundant in the

sandstone porosity, as lath and detrital, sub rounded grains, both cemented by the APS

minerals and the microcrystalline quartz, Figure 4-4a & b. It can be noted that a later

generation of hematite is also present in association with illite and APS minerals fillings of

fractures which cross cutt the microcrystalline quartz cement (see part B, chapter 1, on APS

minerals).

Lastly, one centimeter clay filled fracture cross cutting the basal Thelon Formation sandstone

in one of the historical BP drillcore site (located around 10 kilometers at the NW of the

Kiggavik camp, collar coordinates unknown), showed a white clay mineral with a blocky

habit characteristic of diagenetic dickite, figure 4-4c & d &e.

A detailed SEM observation and a chemical mapping of the pore filling material of the

cemented sandstones close to the unconformity (figure 4-5a to d) permitted to identify

fragments of detrital material mostly composed of aluminum-rich minerals associated with

disseminated micrograins of quartz and tiny crystals of cerium oxide. The chemical

composition of the aluminum-rich minerals agree with that of aluminum oxi-hydroxide such

as diaspore or boehmite ( 35.5 % Al, 60.1 % O, 2.8 % Si), Figure 4-4f which contain

significant amount of cerium (Figure 4-4g & h and table 4-2). In addition, some

disseminated cerium oxide (cerianite) are frequently associated with aluminum oxi-

hydroxides minerals, Figure, 4-4f & g.


118


119

Figure 4-4 : a hematite lath cemented by microcrystalline quartz, b hematite grain cemented by APS minerals

and microcrystalline quartz, c kaolinite booklet cemented by microcrystalline quartz (Q3) cementing the

sandstone porosity; d & e blocky dickite (+detail view on the picture) filling a fracture cross cutting the quartz

cemented Thelon sandstones; f, aluminum oxy - hydroxide minerals coating associated with cerium rich grains

on detrital quartz grain later cemented (by Q3), g aluminum hydroxide and kaolinite/dicktite blocky crystals and

associated with cerium oxides (cerianite), h, Ce-rich (over 4%) aluminum hydroxide matrix EDS spectra.

Figure 4-5 : SEM EDS chemical element maps (Si, Al, Ce) of aluminum rich compounds, identified as oxy

hydroxide, a SEM secondary electron image, b, Silicon image, c, Aluminum and silicon composite images, and

d, silicon and cerium composite image.


120

Table 4-2 : Chemical composition of the Al-rich mineral phase identified in the detrital material filling the pore

space of sandstone overlying the basal unconformity between the Thelon sedimentary formation and the

basement rocks (sample GG7).

Al2O3 SiO2 K2O CaO Fe2O3 Ce2O3 total

71,34 6,45 0,53 0,32 0,05 5,12 83,80

65,04 6,91 0,12 0,23 0,07 4,42 76,78

69,75 12,60 0,32 0,26 0,33 2,75 86,00

4.5. Crystallochemical properties of kaolin minerals

The crystal-chemistry of the kaolin group minerals occurring at the base of the Thelon basin

has been done using XRD, mid infra-red spectroscopy and EMPA.

As showed on XRD diffraction patterns, figure 4-6a, kaolin group minerals are well

represented in the basal Thelon sandstones were they have been preserved from subsequent

hydrothermal alteration. In both oriented as well as randomly oriented powder all the kaolin

group minerals are identified by their strong d00l and d002 diffraction peaks at 7.16 Å & 3.57

Å respectively. According to the obtained XRD patterns, both kaolinite and dickite have been

identified in the basal Thelon samples. Kaolinite and dickite polytype has been identified

within all the samples which contain kaolin minerals disseminated within the quartz cemented

porosity (fig 4a). Evidences of a kaolinite/dickite mixing is given by (1) the broadening of the

020 diffraction line resulting from the contribution of both kaolinite (4.47 Å) and dickite (4.44

Å) 020 diffraction lines and (2) specific dickite diffraction peaks in the 35-40 2 theta range,

Figure 4a. Moreover, using the peak intensity ratio between the 132,204 reflections of the

dickite polytype and the 20-2 ,1-31 reflections of the kaolinite polytype (Beaufort 2014) the

proportion of dickite in the bulk kaolin material can be estimated to 35%. In addition to these

estimations, pure dickite polytype has been identified in the late fractures which crosscut the

cemented sandstone (Th18_3), Figure 4-6b.


121

Figure 4-6 : X-ray diffraction patterns for Thelon Formation kaolin group minerals, showing, a a

kaolinite/dickite mixture, and b, a pure dickite fracture. Both diffractograms of kaolinite and dickite display the

diffraction peaks attributed to highly ordered minerals.


122

These results are confirmed by the mid IR technique which appears to be the most effective to

discriminate kaolinite and dickite using the differences in position and relative intensity of

OH stretching bands, figure 4-7. In the GG3 and GG7 samples (disseminated kaolin minerals)

the four bands observed at 3696, 3669, 3653 and 3620 cm-1 are in agreement with data given

in literature for the hydroxyl-stetching bands region of kaolinite (Farmer 1974). In addition

the good distinction between the bands at 3669 and 3653 cm-1, as well as the narrow and

symmetrical shapes of the other bands are characteristic of a rather well ordered kaolinite

microcrystals. It can be noticed that the inflexion at 3598 cm-1 observed on the GG3 spectra is

characteristic of the Al- Fe3+ OH vibration and thus indicate the presence of structural ferric

iron in the kaolinite of certain samples (Petit and Decarreau 1990; Petit et al. 1999).

The 3706, 3654 and 3621 cm-1 bands observed in the FTIR spectrum of the kaolin minerals

from the late fracture which crosscut the cemented sandstone (Th18_3 sample) are typical of a

pure dickite polymorph, figure 4-7.

GG3b (kaolinite)

GG7 (kaolinite)

Th18_3 Fracture (dickite) 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

3550 3600 3650 3700 3750 3800

Wavenumbers (cm-1)

Ab

so

rba

nce

ab

itra

ry u

nits

Figure 4-7 : Mid Infra-red spectra of Thelon formation Kaolin Group minerals in the OH-stretching domain.


123

4.5.1. Crystallographic properties of kaolin group minerals: order /disorder

The structural order of the kaolin group minerals could be investigated using classical

Hinckley index of crystallinity calculation method on XRD diffraction patterns and IR

spectroscopy spectra (Hinckley 1963; Crowley and Vergo 1988). However, this approach

requires the absence of quartz in the analysed kaolin material and the study of samples series,

a major difficulty in our area where most of the sandstones are eroded and in which quartz

microcrystals cannot be separted from kaolin minerals. Nevertheless, the crystallographic

properties giving the level of structural order of kaolinite and dickite can be assessed using

both XRD and IR (Brindley and Brown 1980).

Then the basal Thelon formation kaolinite, are matching with the order criteria of prominent

basal reflections 001 and 002 together with well defined 02l and 11l reflection in the 19-33°

(2θ range (Hayes 1963; Keller et al. 1966). In addition, in the range, 35-40° (2θ), the 13l, 20l

reflections occurs with the 003 basal reflection. These reflections forming two groups of

triplets in the well ordered kaolinites (Brindley and Brown 1980), Figure 4-6a. Lastly, the

tiny shoulder observed on the right sides of diffraction peaks and due to the Kαβ reflection

indicates the presence of highly ordered crystals. Such a high order is moreover coherent with

the presence of dickite diffraction lines.

As in kaolinite, dickite diffraction lines can be used to characterize the level of structural

order. Then even if dickite is generally seen as an highly ordered polymorph of the kaolin

mineral group, some differences in crystallinity have been described in dickite series

(Brindley and Porter 1978). Then, the better ordered samples as the Th18_3 dickite, show all

the X-ray powder diffraction lines (Newnham and Brindley 1956; Newnham 1961), Figure 4-

6b.


124

4.5.2. Chemistry of kaolinite minerals

The electron microprobe data acquired on the kaolinite vermicules confirmed their relatively

high iron content (up to 0.73 % Fe2O3), with a moderate compositional variability in each

single crystal, Table4-3 4-9 & figure4-9. Two distinct populations of kaolinite can be

identified on the basis of their morphology & chemistry. The first one being iron poor with

virtually no iron while the second is iron rich with an average content around 0.5 % wt Fe2O3.

However they are both represented by large booklets and minute crystals in the

microcrystalline quartz matrix.

Table 4-3 : Representative microanalysis of the two kaolinite morphologies present in the basal Thelon

Formation sandstones

Na2O MgO Al 2O3 SiO2 K 2O CaO TiO 2 MnO Fe2O3 H2O total

Large kaolinite Fe 0.01 0.05 38.00 46.99 0.07 0.05 0.00 0.01 0.67 14.00 99.85

Large kaolinite 0.00 0.05 38.30 47.47 0.02 0.07 0.00 0.00 0.06 14.00 99.96

Minute kaolinite 0.01 0.01 38.77 47.16 0.08 0.04 0.00 0.06 0.35 14.00 100.47

Figure 4-8 : plot of the chemical microanalyses of kaolin minerals from the basal Thelon Formation sandstones

in the Al2O3-Fe2O3 cross plot diagram.

y = -0.19x + 38.31

36.0

37.0

38.0

39.0

40.0

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

Al2

O3

%

Fe2O3 %

Basal Thelon Fm sandstones kaolinites


125

Figure 4-9 : Electron microprobe chemical transects in the aggregates of kaolin of the basal Thelon sandstones

of the Kiggavik area. Average distance between the punctual analyses ranges from 10 to 20 micrometers

according to the size of the kaolin grains.

In addition, the kaolinite crystals tends to be chemically homogeneous throughout the grain

with no significant compositional variation near the edges or along the crystallographic c or

ab axis, as shown by the microchemical transects, Figure 4-10 & 4-11.

Figure 4-10 : Location of some of the electron microprobe transects in kaolinite (GG4 sample)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Fe2

O3

wt

%

number of analysis points in the transect

GG4_kaol_Trsct2

GG4_kaol_Trsct3

GG4_kaol_Trsct4

GG4_kaol_Trsct6

GG3b_KaoITrsct4

GG3b_KaolTrsct2


126

4.6. Microcrystalline quartz cement chemistry and in situ composition of

oxygen isotopes

Figure 4-11 : Silica and alumina content measured in detrital and microcrystalline quartz. Note that the

measurements were performed on a SEM-EDS

In addition to their petrographic characteristics, the microcrystalline quartz reveals a peculiar

chemical signature with an increase in aluminum content relatively to the detrital quartz,

figure 4-12. Such variation in Al content has been described in burial diagenesis in successive

quartz overgrowths generations (Tournier 2010). In addition the aluminum content of quartz,

as hydrous Al species, seems to be correlated to the amount of defects and the rate of crystal

growth (Ihinger and Zink 2000). Then, the oxygen isotopes composition has been investigated

to better constrain the paleoconditions at which they crystallized.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

96.00 97.00 98.00 99.00 100.00

Al 2

O3

SiO2

Detrital Quartz

Microcrystalline quartz


127

Figure 4-12 : Histogram of 18O values measured in microcrystalline (Qtz3) and detrital quartz from the basal

Thelon Formation sandstones.

Table 4-4 : SIMS oxygen isotopic data and detrital quartz grain and the microcrystalline quartz (Qtz3).

Mineral 18O/16O FF 1σ 18OV-SMOW (‰) T (°C)

GG4_Qtz3-1 Quartz 1.848399 0.9241131 1.2 32.5 54 GG4_Qtz3-2 Quartz 1.850852 0.92533948 1.3 33.9 48 GG4_Qtz3-3 Quartz 1.857398 0.92861217 1.2 37.5 35 GG4_Qtz3-4 Quartz 1.862655 0.93124043 1.2 40.4 25 GG4_Qtz3-5 Quartz 1.84891 0.92436857 1.2 32.8 53 GG4_Qtz3-6 Quartz 1.847598 0.92371263 1.3 32.0 56 GG4_Qtz3-7 Quartz 1.860585 0.93020553 1.2 39.3 29 GG4_Qtz3-8 Quartz 1.854224 0.92702532 1.3 35.7 41 GG4_Qtz3-9 Quartz 1.857503 0.92866467 1.2 37.6 35 GG4_Qtz3-10 Quartz 1.86143 0.93062799 1.3 39.8 27 GG3_Qtz3-1 Quartz 1.861797 0.93081147 1.2 40.0 27 GG3_Qtz3-2 Quartz 1.860428 0.93012703 1.2 39.2 29 GG3_Qtz3-3 Quartz 1.864177 0.93200136 1.2 41.3 22 GG3_Qtz3-4 Quartz 1.856241 0.92803373 1.2 36.9 37 GG3_Qtz3-5 Quartz 1.855203 0.92751478 1.2 36.3 39 GG4_Detrital-Gr-1 Quartz 1.826909 0.9133691 1.2 20.5 102 GG4_Detrital-Gr-2 Quartz 1.831189 0.9155089 1.2 22.9 85 GG4_Detrital-Gr-3 Quartz 1.826232 0.91303063 1.2 20.1 106 GG4_Detrital-Gr-4 Quartz 1.820232 0.91003091 1.2 16.8 135

0

1

2

3

4

16 18 20 22 24 26 28 30 32 34 36 38 40 42

δ18OV-SMOW (‰)


128

The δ18O values obtained from in situ measurements in the microcrystalline quartz which

cement the porosity of the sandstones range between 31.9 and 41.3 per mil with an error of

1.2 (1σ). The data presents two modes centered on 33 ‰ and 40 ‰, Figure 4-13 & table 4-4.

For comparison the detrital grain were ranging from 16.8 to 22.9 per mil, in a similar range

than other Thelon sandstones detrital quartz grains (Hiatt et al. 2007). Then using the

extrapolation of classical quartz-water fractionation (Clayton et al. 1972) and AlphaDelta

program (Beaudoin and Therrien 2004; Beaudoin and Therrien 2009), the calculated

temperature range from from 1°C up to 35°C and are rather questionable. However, the use of

most recent fractionation equation for low temperature quartz - kaolinite-water system,

between 0 and 130°C (Méheut et al. 2007), provides more realistic temperatures. A water

composition in equilibrium with sea water was chosen for temperature calculation as a more

saline water composition can be assumed in an intracratonic, possibly warm environement,

with shallow water depht. The obtained temperatures show a bimodal distribution, each mode

being centered on 27 °C and 52°C respectively.

4.7. Discussion

Petrographic studies carried on the basal Thelon Formation sandstones around Kiggavik,

showed peculiar mineralogy and textures with (1) the large kaolinite crystals embedded in the

sandstone matrix and (2) the quartz-kaolinite composite grains, (3) the local presence of Al

oxides or oxide-hydroxides associated with cerium oxides, (4) evidences of dickite as fracture

filling material and (5) a sandstone porosity sealed by microcrystalline quartz, figure 4-14.

Another very interesting point is the preservation of most of the basal Thelon formation of the


129

postdating hydrothermal alteration which is strongly developed along the major faults of the

underlying basement rocks. Then, the petrography and the mineralogy of this compartment of

basal Thelon sandstone formation, makes possible an attempt to reconstruct the early history

of the bottom part of the Thelon basin (from sedimentation to the end of diagenetic

evolution).

Figure 4-13 : Paragenetic sequence of minerals determined in the basal Thelon formation. See Part B chapter 2

on APS minerals for more details on the crystal-chemistry of the different aluminium phosphate minerals.

The detrital mineralogy raises questions about the origin of the material leading to such

assemblage as well as the genetic processes involved in there formation prior to erosion

transport and sedimentation. Diagenetic processes and paleoburial conditions have to be

considered at the view of the kaolinite/ dickite transition evidenced from crystallographic

properties in this study, with respect to previous works on kaolin diagenesis in siliciclastic

basins (Beaufort et al. 1998; Shutov et al. 1970; Eckhardt and Von Gaertner 1962; Eckhardt

1965).


130

4.7.1. On the origin of detrital and diagenetic Al bearing minerals in the

basal Thelon Formation

The presence of detrital aluminum minerals with kaolinite, hematite aluminum phosphate

sulfate and local cerium oxide raise the question of the source of the detrital material. Similar

minerals assemblages have been described in various actual or recent lateritic profiles or

bauxites (Valeton 1972; McFarlane 1976; Loughnan and Bayliss 1961; Braun et al. 1990;

Curtis and Spears 1971). In addition the crystallochemical properties of kaolinite with

frequent and relatively high (from 0.3 up to 2% Fe2O3) structural Fe3+ observed in the Thelon

Formation kaolinite is also common in kaolinites formed in laterite under intense weathering

(Herbillon et al. 1976; Malden and Meads 1967; Tardy and Nahon 1985; Cantinolle et al.

1984). Moreover it has been showed that the amount of structural iron is of great influence on

the kaolinite crystallinity, with an increase of disorder positively correlated to the iron content

(Mestdagh et al. 1980; Brindley et al. 1986).

On the contrary in the kaolinite and quartz composite grains, the morphology of kaolinite is

by far more irregular and indicate that the lithic fragments was originally of analogous

composition, and possibly formed by a mechanical mixing of the two minerals. In addition,

the sub rounded shape for such a fragile assemblage also indicates a likely moderate distance

of transport.

As described in many studies, three type of kaolin are widely recognized depending on their

textural properties: (1) as micas replacement, where authigenic kaolinite crystallization takes

place in between expended detrital micas flackes (Nedkvitne and Bjorlykke 1992; Ehrenberg

et al. 1993; Macaulay et al. 1993), (2) as vermicules and (3) as blocky kaolinite (Lanson et al.

2002). The last two types raise numerous questions on the processes responsible for their

crystallization including temperature, pH, H+/K+ ratio and reaction kinetics.


131

Then, the well developed crystals and the pristine shape of kaolinite booklets observed at the

base of the Thelon Formation sandstone is thus in agreement with an in-situ formation in the

sandstone porosity. Moreover petrography showed that the diagenetic quartz overgrowths

(Qtz2) are stopped by the presence of the kaolinite vermicule present in the porosity.

Then it could seem in contradiction with the relatively high crystallographic order of the

kaolinite present in the basal Thelon Formation. However, the evidence of neoformation of

hematite between kaolinite stackes or in place the fine grain hematization observed in the

sealed porosity surrounding the grain might be directly linked to the high iron initially present

in the kaolinite. Such feature, could be explained by diagenetic dissolution recrystallization

processes in which iron is partially taken out from the kaolinite to precipitate as hematite.

Then, it could also indicate that the detrital kaolinites could have been iron richer and that the

vermicules of kaolinite are pro parte recrystallized sedimentary kaolinites or generated

through aluminum oxy-hydroxides silicification processes or neoformation (Trolard and

Tardy 1989).

Nevertheless, our interpretation of such mineral assemblages at the view of present geological

is limited by the two following points:

(1) the atmosphere redox and pH conditions during the Paleoproterozoic paleoweathering

events are not clearly defined but probably developed in a more reducing environment than

today (Partin et al. 2013). Then the condition would have been less oxidizing with

consequences of an higher iron mobility, and a preferential integration of iron in

phyllosilicates rather than in iron oxides or sulfide that would also suggest that sulfur wasn’t

available in the system (2) the persistence of aluminum hydroxides, minerals common in

laterite or bauxite is very rarely found in the sediments and even more in the sandstone

affected by deep burial diagenesis.


132

4.7.2. Sandstone quartz cementation, some contradictory observations

The Churchill province of the Canadian Shield presents several occurrences of silicified

weathering profiles or silicified sediments which were described as silcretes, as in the Thelon

or Hornby Bay Basins (Gall 1994; Ross and Chiarenzelli 1985; Ross 1983). Oxygen isotopes

on the quartz cements are then a critical tool to assess the crystallization conditions but the

comparison with modern cherts and careful use of the actualism principle is necessary. Indeed

some differences in formation environments and forming processes have been established

between Precambrian and Phanerozoic cherts (Maliva et al. 2005). In addition several

isotopic composition of the quartz is then dependent of the crystallization temperature, as well

as the salinity and the isotopic composition of the fluid in equilibrium with the quartz.

Relatively high oxygen isotopes values on quartz are the expression of very low temperature

processes not uncommon in surface environments (Kolodny et al. 2005). In addition such high

silica rich rocks as cherts or silcretes had been studies for a long time and used as indicators

of surface temperature or weathering conditions through geological times (Knauth and

Epstein 1976; Sayin and Jackson 1973).

In addition, the present results are partially in agreement with the petrography and oxygen

isotopes of the quartz cements at the base of the Thelon Formation (Hiatt et al. 2007). These

authors described isopachous microcrystalline quartz rims fringing the detrital quartz grains,

in both eolian facies of Thelon sandstones and the Pitz formations, without any prior

diagenetic overgrowth. In fact, at Kiggavik the microcrystalline is not present as comb

structure and seems to be developed after a first stage of diagenetic overgrowth. However, the

oxygen isotopic composition is quite similar with values between the microcrystalline quartz

at Kiggavik and the quartz cements or even some of the latter quartz generation filling the

porosity described by the same authors. Then two opposite trends are presents, which may

indicate different porosity evolutions throughout the Thelon formation sandstones. In one case


133

the earlier cements have the highest δ18O values up to 33 ‰ with a 26 ‰ mean (Hiatt et al.

2007), while the present study the microcrystalline quartz have a maximum δ18O values value

of 41.3 ‰ with a 37 ‰ mean. These significantly higher values are in agreement with very

low temperatures but contradictory with the petrographic evidence of a rapid precipitation

associated with hydraulic fracturation.

Finally, the sedimentary features as the ripples marks observed at the base of the Thelon

indicates relatively shallow environment. In addition, in similar Paleoproterozoic Basins as

the Athabasca Basin Boron, and Chlorine isotopes studies have proposed an evaporated sea

water as sources for the ore related brines (Richard et al. 2011; Richard et al. 2013). Then, the

position over the Tropics during the Paleoproterozoic (Betts et al. 2008) of these huge

intracratonic basins could be considered as relatively close environment favourable for water

evaporation. This raise numerous question about the water isotopic composition, and might

suggest that the δ18O (H2O) would have been higher. If so, the calculated temperature from

the microcrystalline quartz cement would be necessarily shifter toward higher temperatures.

Then, the temperature would be even more compliant with an hydrothermal origin for the

microcrystalline quartz cement, possibly in relaton with a brutal pressure change.

4.8. Conclusion

Petrography and cystal-chemistry gave a series of arguments to relate the composition of the

clay minerals of the basal Thelon Formation sandstones to a product of paleo weathering

profile dismantling, possibly lateritic or bauxitic, developed during the regolithisation of the

Archean and Paleoproterozoic basement rocks which preceeded the sandstone deposition.

This is also supported by the very specific location of the studies sandstones in the vicinity of


134

an ancient fault system that may have been reactivated with a tectonic control of the

sedimentation, with very little transport of the basin filling material studied. Such settings are

then highly favorable to preserve the fragile clay / quartz composite of the dismantled

weathering profile, or the large kaolinite later recrystallized as massive vermicules in the

porosity during burial diagenesis, as well as the aluminum oxi-hydroxides such as diaspore or

boehmite.

The diagenesis also includes the precipitation of the quartz overgrowths as well as the

recrystallisation of the coarse grained vermicules of ordered kaolinite in the porosity. This

reasonably indicates that significant burial depth may have been reach to generate such a

degree of order in the kaolinite, prior to the microcrystalline quartz cementation. Finally, this

might also be supported by even later fracture controlled precipitation under even higher

pressure condition with the presence of the cross cutting dickite fracture.

Then the basis of the Thelon sandstones, at this particular location are recording the early

basin filling history and the weathering processes active before the Thelon deposition. The

structural control of the sedimentation is giving a good indication on the source material due

to the expected short distance of transport. Finally this preservation of kaolin group mineral

from both the sedimentary and diagenetic histories highlight the fact the sandstone had been

chemically isolated in the Kiggavik area. This could appear contradictory with the occurrence

of authigenic K-feldspar and illite diagenetic cements described in the Thelon basin (Renac et

al. 2002). Such a fact probably indicates that the Thelon Basin was compartmented during its

evolution history as did many others younger basins worldwide.


135

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Carbonaceous material occurrence in the Kiggavik uranium deposits (Thelon, Nunavut, Canada).

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5. Carbonaceous material occurrence in the Kiggavik uranium deposits (Thelon, Nunavut, Canada).

Riegler Thomas & Beaufort Daniel.

Paper in preparation

5.1. Introduction

Carbonaceous materials (CM) in the early earth history are always controversial with two

possible origin, one involving life where bitumen are deriving from classical kerogen thermal

evolution, while the second is the expression of abiotic synthesis of organic compounds in

hydrothermal or magmatic systems e.g. via Fischer-Tropsch type synthesis (FTT) (Shock

1990; Gize 1999; Foustoukos and Seyfried 2004; McCollom and Seewald 2006; Horita 2005;

Ueno et al. 2004; Curiale 1986; McCollom 2013). Both processes are being tracked by

specific geochemical signatures such as molecular markers, carbon and sulfur stable isotopes,

together with petrographic evidences. Such carbonaceous materials, referred as bitumens,

have been frequently identified within the alteration halos surrounding the Mid-Proterozoic

unconformity type uranium deposits (Hoeve and Sibbald 1978; Hoeve et al. 1980; Hoeve and

Quirt 1984; Pagel et al. 1980; Sangély et al. 2007). The reducing potential, the source and the

distribution in brecciated and altered Archean to Paleoproterozoic metamorphic basement or

in the Meso-Proterozoic sedimentary cover of such material have been discussed for uranium

metallogenesis (Leventhal et al. 1987; Kyser et al. 1989; Landais and Dereppe 1985; Yeo and

Potter 2010; Sangély et al. 2007) without strong evidence for ore control, while it was

invoked as ore controlling parameter in gold bearing veins (Mastalerz et al. 1995). On the

contrary in the Paleoproterozoic Oklo mineralization the paragenesis indicates a more obvious


140

link between bitumen and oil occurrences with uranium precipitation, in a petroleum-like

system were faults creates trap and path for oil migration as well as favorable contact between

marine black shale and the bitumen hosting deltaic sandstones (Gauthier-Lafaye and Weber

1989; Nagy et al. 1991).

In previous works in the Athabasca Basins, bitumen is considered to post-date the main

uranium mineralization stage (Wilson et al. 2002; Leventhal et al. 1987) or is sometimes seen

to be contemporaneous to it (Alexandre and Kyser 2006). However the host rock is composed

of a various package of metasediments or meta-igneous rocks devoid of any carbon other than

graphite on faults. Although several lithologies are known as potential sources of oil with

including the Phaneroic blackshales of the Exshaw Formation (Sangély et al. 2007), who

might have migrated downwards trough 1500 meters of Athabasca Group sandstones and

several hundred meters below the unconformity, along the fault and fracture network

controlling the uranium mineralization. The main debate being the in situ formation versus the

migration of the organic compounds.

The aim of this work is to identify and to characterize the chemistry and structure of the

carbonaceous materials associated with the uranium mineralizations at the Southeastern

margin of the Meso-Proterozoic Thelon Basin. Then geochemical, isotopic, petrographic and

crystallographic evidences with comparison with (1) similar type of mineralization in the

Athabasca Basins and (2) laboratory synthesis of organic compounds will gives the basis of

discussion for the genesis and emplacement of such material several hundred meters below

the eroded unconformity at Kiggavik in alteration envelopes surrounding one of the major

uranium ore zones in Canada outside the Athabasca basin. Lastly the new knowledge on the

crystal structure of carbonaceous material in unconformity type uranium mineralization will

give a new perception for metallogenesis and approach in ore deposits studies.


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5.2. Geological setting and petrography

Uranium mineralization (mainly uraninite and coffinite) at Kiggavik is composed of several

ore bodies but one at Bong is particularly rich in carbonaceous material very often easy to

distinguish with the strong cabbage / garlic smells associated with its presence. All the ore

shoots are surrounded by tens of meters large clay alteration envelopes developed in Achean

metagraywackes belonging to the Woodburn Lake Group (Farkas 1984; Miller and

LeCheminant 1985) and controlled by fault and fracturation corridors and slip opening along

the foliation plan. Then all the primary metamorphic silicates (feldspars, muscovite, biotite,

chlorite) are replaced by an alteration paragensis dominated by illite and sudoite (Al-Mg

chlorite) in various proportions displaying similar feature as many other unconformity

associated uranium mineralization in Canada and Australia (Beaufort et al. 2005; Laverret et

al. 2006; Hoeve and Quirt 1984; Polito et al. 2004), as in the present work. Accessory

minerals in the non altered are largely represented by zircon, monazite, titanium oxides,

magnetite and pyrite, that latter being quite abundant in places where the medium to fine grain

graywacke tends to be more pelitic.

Carbonaceous materials are exclusively found within clay alteration zones as fracture filling

up to 1 centimeter thick of massive carbonaceous material, a dark black, stainy, relatively

hard and brittle material showing concoïdal fractures. Dots of similar material are presents

scattered in the argilized host rock. The other expression of carbonaceous material are black

to dark gray fractures and coating (no visible CM aggregate under the SEM) on foliation plan

with a typical cabbage / garlic smell, Figure 5-1a, b. There distribution doesn’t seems to

follow any spatial organization other than alteration but rich zones but most of it is found at

the hanging wall of the alteration envelop in Bong where sudoite is slightly dominant over

illite. In addition there are frequently intersected with several drillholes in the same area


142

roughly over several a tens of meters radius. The CM nodule are ranging from millimetric to

micrometric in diameter and are developed in an illite and sudoite clay matrix showing in

places thin coatings of uraninite are presents as fracture fillings of the CM nodules Figure 5-

1c, d. Uraninite is found dissemintated in the carbonaceous matrix as mint euhedral crystals,

often partially altered to coffinite and organized in pseudo concentric rings forming

alternating bands with the massive carbon carbon in which elevated U and Cu level are

noticeable Figure 5-1e. Finally dissolution pits have been identified at the surface of the

carbonaceous material forming circular to oblate holes of a few micrometers in diameter

Figure 5-1f.


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Figure 5-1 : Carbonaceous materials occurrences at Kiggavik. 1a: carbonaceous material along the foliation plan

and filling millimetric fractures in altered metagraywacke, S : schistosity, BG5101. 1b: detail of fracture filling

material under binocular lense, button of carbonaceous material in the intergrain porosity BG5101. 1c:

carbonaceous nodule in argilized metagrawacke BG4326. 1d: Carbonaceous material nodule in an illite and

sudoite matrix, some microfrature plans are coated with uraninite BG4238. 1e Disseminated uraninite partially

altered to coffinite and chalcopyrite in massive carbonaceous material. 1f Hollow point in massive carbonaceous

material


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5.3. Samples & Methods

Representative samples of each type of carbonaceous material had been chosen. In addition a

particular care had been taken in the sampling itself in taking the samples just after drilling to

avoid contamination as much as possible, as well as alteration of the carbonaceous material by

bacteria.

All the 3 samples from the Bong prospect are representative of the two types of carbonaceous

material found in the alteration halos in association with uranium mineralization. One is

composed of massive carbonaceous material (MCM) (BG4238), at the hanging wall of fault

zone, while the second is set as diffuse impregnations of carbonaceous material (DiffuseCM)

as staining and coating on fractures and foliation plans in a highly argilized rock with the

BG4326 and 5101 samples. Samples were collected respectively at 428, 316 and 286 meters

below the actual erosion surface.

5.4. Results

The both type of material have been analyzed with the most suitable method in order to

extract all the possible information from (1) volatile organic compound disseminations and

(2) the massive carbonaceous material, and tie the two concurrencies in the alteration history.

The two expression of carbon bearing compounds are very distinct in term of petrographic

expression but also in term of carbon content from tenth of per mil to tens from tens of

percent in the massive CM.

5.4.1. GC-MS pyrolysis

Several organic sulfur compounds have been identified in GC-MS flash pyrolysis using both

600 / 400°C as well as low temperature extraction of volatiles at 300°C in dichloromethane

with a HMDS & TMAH treatment. Both methods yield similar results with the identification

of low molecular weight and volatile compounds: carbon disulfide, dimethyldisulfide


145

(DMDS) and dimethyltrisulfide in the black to grayish disseminations in the argilized

metagraywacke. Such compounds haven’t been detected in the massive carbonaceous sample

and the identified alcanes or aromatic are possible contaminations. Sulfur dioxide is also

present in all studied samples and is very likely the expression of sulfides roasting,

chalcopyrite in that case. All these molecules are responsible for the characteristic smell

associated with some areas in the alteration zone, even if in traces. The trace characters of

these compounds make their identification.

5.4.2. X-Ray Diffraction

The massive carbonaceous material 66 % C doesn’t exhibit any reflection of the graphite

diffraction pattern in a whole rock randomly oriented powder mount of the <50 μm size

fraction. This surprising feature raises the question of the type of bonds between the Carbon

atoms and the structure of this material, Figure 5-2. The only minerals identified are

uraninite, coffinite, chalcopyrite, johannite [Cu(UO2)(SO4)2(OH)2.8H2O)] and studtite

[UO4.4H2O] according to XRD data from (Walenta 1974). In addition an oriented slide of the

infra 0.1 μm size fraction extracted by ultracentrifugation haven’t shown any diffraction peak

for graphite either, only a broad baseline indicating either an amorphous material or a

structure without enough layers stacked to induce the X-ray diffraction. Such characteristic

possibly reflect a graphene structure or amorphous graphitic material.

Then in order to probe the structure of C-C bonds and possible spatial organization the Raman

spectroscopy and transmission electron microscopy appears to be the most suitable

techniques.


146

Figure 5-2 : Randomly oriented powder XRD diffractogram of massive carbonaceous material, (5-65° 2theta

scan, 0.02 stepsize and 4 seconds counting time). Abbreviations, Ccp (1): Chalcopyrite; Coff (2): coffinite; Uran

(3) uraninite; Joh (4): johannite and Stu (5): studtite.

5.4.3. Raman spectroscopy

Raman spectra obtained on massive CM show two pronounced bands at about 1340 (D) and

1600 (G) cm-1 as well as a relatively marked fluorescence responsible for the moderate

baseline but also hiding the second order Raman bands (overtone) in the 2000 – 4000 cm-1

range, Figure 5-3. Of these two bands the first one, broad and relatively intense, is attributed

to disordered carbons and edges effects (D band) while the second is the graphite band (G)

linked to the sp2 C-C configuration of the graphene planes (Chu and Li 2006; Jehlička et al.

2003; Tuinstra and Koenig 1970b). The shift toward higher wavelength values of the G band

seems to be indicative of extremely small crystallites (Tuinstra and Koenig 1970a). In

addition, the spectrum decomposition using pseudo Voigt function leads to the identification

of 4 different bands respectively at 1188, 1334, 1510 & 1601 cm-1, and a final fitted profile

(cross) with a reasonable match with the original spectrum Figure 5-3. The graphite G & D

5 15 25 35 45 55 65 2θ

graphite main diffraction line

Coff

2

Ccp 1

Uran

Joh 1 1

1

2

2

2 2 2 2 3

3 2 2 3 3 4 4

2 4

Stu

5 5 5 5 5 5

1

2


147

bands at 1334 & 1601 cm-1 are also well identified in the decomposition, while in addition

the 1188 cm-1 band assigned to disorder in the graphitic lattice and the 1510 cm-1 band

revealing an amorphous component in the signal (Sadezky et al. 2005; Al -Jishi and

Dresselhaus 1982; Dippel and Heintzenberg 1999).

Finally the band intensity ratio ID/IG (0.54) gave an in-plane crystallite size (La), ranging

from 2 to 30 nm using two empirical formula given in literature (Knight and White 1989;

Cancado et al. 2006).

Figure 5-3 : Raman spectra of massive carbonaceous material, a raw signal & b decomposed Raman spectrum

showing 4 components (dotted lines) and fit (cross) after linear background correction.

1000 1200 1400 1600 1800

1188

1334

1510

1601

1000 1200 1400 1600 1800

Raman shift (cm-1)

G D

1338

1601

Spectrum decomposition


148

5.4.4. Transmission electron microscope study of the carbonaceous

material

In addition to RAMAN spectroscopy and XRD, TEM observed were performed in order to

obtain microstructural information. The general view in Figure 5-4a indicates that the

massive carbonaceous materials are in fact formed of carbon veil, best described as graphen

layers. In close association to it, nanometric crystals of size ranging from 5 to 50 nm are

disseminated in the carbonaceous material between the graphen layers. In addition, HRTEM

images (Figure 5-4b) confirm that the structure of the carbonaceous matrix is fully disordered

along the C axis.

Moreover, figure 5-4c & d reported an example of HRTEM image from nanoparticle and the

corresponding Fast Fourier Transform (FFT). The digital diffractogram from the particle

shows spots with fringe spacing of 3.1A and an angle of 70° between the two sets of fringes.

This experimental result agrees with the d{111} planes of the uraninite cubic structure. The

two other spots correspond to fringe spacing of 1.9A and 2.73 of the d{022} and d{002}

respectively showing that the zone axis of the particle is <110>. Lastly, the diffraction pattern

taken over a large area, Figure 5-4e& 5-4f suggests a preferential orientation of the {111}

plans of uraninite.


149

Figure 5-4 : Transmission electron microscope image of MCM and associated nanocrystals; a, disseminated

nano crystals in the carbonaceous material, b, HRTEM images of nano crystals and amorphous carbonaceous


150

material matrix, c, lattice parameter of a euhedral crystal of uraninite, and d, corresponding FFT image, e & f

uraninite crystal in the carbon matrix and electron diffraction over the same area.

5.4.5. Isotopy & geochemistry

Bulk geochemistry gathered on massive and disseminated carbonaceous materiel is given in

the table 5-1. The massive material is almost entirely formed of carbon with more 66% C.

The Very little Si, Al, K, and Mg amounts are coherent with the absence of clay minerals

within the massive material. Nevetheless the material is particularly rich only in Cu and U

while other trace elements, especially those which are sensitive to redox condition like Mo,

Co, and V are negligible.

Table 5-1 : Chemistry and isotope composition of the two types of carbonaceous materials

Sample Massive CM Diffuse CM

SiO2 % 1.94 44.49

Al2O3 % 0.51 27.17

Fe2O3 % 3.44 0.66

K2O % 0.08 8.24

MgO % 0.06 2.06

TiO2 % 0.13 1.53

As (ppm) 139 36.67

Co (ppm) 142.1 31.18

Cu (ppm) 10020 167.9

Mo (ppm) 9.52 15970

Pb (ppm) 2192.82 234.52

U (ppm) 71910 403.6

V (ppm) 31.96 750.7

%C 66.3 0.04

%H 3.9 0.7

%S 4.68 0.5

δC °/00 -39.1 < detection limits

δS °/00 No data + 12.2

13C relative to PDB and 34S to CDT standards


151

5.5. Discussion

Considering the carbonaceous materials present at Kiggavik it appears that the petrographic

and textural relationships with alteration minerals lead us to integrate these materials to the

paragenesis formed during the hydrothermal alteration of the Archean basement and the

associated the transport and deposition of uranium. In Kiggavik as well as in the Athabasca

Basin (Sangély et al. 2007; Landais and Dereppe 1985; Leventhal et al. 1987; Alexandre and

Kyser 2006), the carbonaceous materials are considered to belong to succession of

hydrothermal events responsible for the main uranium deposition. Then, on the basis of this

multi-method characterization we propose to refine the interpretation of the Kiggavik

carbonaceous material in terms of genesis and potential role on uranium precipitation.

5.5.1. Forms of carbon and hydrothermal signatures

To sum-up the two main expressions of carbonaceous materials identified in alteration zone

associated with uranium ore depositions at Kiggavik are: (1) dissemination of short chain

carbon & sulfur compounds, the most common, and (2) more locally as massive, hard and

brittle aggregates or spherules of carbonaceous material. In both cases these carbonaceous

materials were compared with similar products formed by synthesis in laboratory. Then the

comparison of data from literature on experimental synthesis with those collected in natural

system may help to highlight the processes responsible for the genesis of carbonaceous

material and give complementary information to constrain the hydrothermal conditions in

unconformity related uranium deposits.


152

5.5.2. Carbon sulfides a comparison between natural and laboratory created

hydrothermal systems

The first expression of carbonaceous material in the alteration system are the organic sulfur

compounds as short chain carbon sulfides which can be compared to the artificial synthesis of

the same chemical compounds, CS2 as well as thiols, from a FeS/HCl/CO2 or FeS/H2S/CO2

system at 75°C (Heinen and Lauwers 1996) via the 2 following reactions :

7FeS+ 8HCl +CO2 → 4FeCl2+3FeS2+CH3SH+2H2O and 2CH3SH +2H2O → CS2 +CO2+6H2

It also appears in the same study that DMDS and carbon disulfide are indicators of suboptimal

conditions for CO2 reduction, as consequence of a limitation of H2 resulting from an

unfavorable FeS/HCl (H2S) ratio or a low temperature. The formation of DMDS via the 2

methanthiol condensation in addition to short thiols may be significant of H2 limitation. It also

needs to be noted that such reaction produces pyrite. In all these reaction oxidation conditions

are controlled by mineral buffers, which are all present in the unaltered rock (pyrite &

magnetite) prior to alteration and given in (Shock 1990), while a part of H2 may be generated

from water radiolysis:

2 Fe3O4 + H2O = 3 Fe2O3 + H2 and 2 FeS + 4/3 H20 = FeS2 + 1/3 Fe304 + 4/3 H2

Finally thermodynamic studies on similar short chain carbon sulfides with the example of

dimethlysulfide gives another temperature constrain to the hydrothermal condition necessary

to the synthesis of such compounds, Figure 5-5 (Schulte 2010). Then when using the

measured or calculated fluid temperature for the brines involved in the alteration processes

(fluid inclusion and stable isotopes on illite) and ranging from 150 to 220°C at Kiggavik, but

similar to the one in the Athabasca Basins (Pagel and Ahamdach 1995; Kotzer and Kyser

1995; Pagel 1975; Renac et al. 2002), allows us to determine that the reaction is displaced in

the sens of synthesis of carbon sulfide compounds with a LogK just above 0 at 220°C but


153

around 8 at 150°C . Then the hydrothermal conditions related to alteration processes were

favorable for the in situ genesis of carbon sulfide compounds. It may also indicate that these

products were synthesized when temperature started to decrease possibly at the end of an

hydrothermal pulse.

Figure 5-5 : Stability of carbon sulfide coumpounds as a function of Log K and T °C, from

Schulte, 2010

Unfortunaly, the carbon content is too low in the disseminated material to get a carbon isotope

footprint of such compounds.

Then after look at the more volatile compounds the massive carbonaceous materials

5.5.3. Massive carbonaceous materials: solid properties & genesis

The massive carbonaceous materials found at Kiggavik presents similarities in term of

texture, petrographic relashionship with clays within alteration surrounding mineralization

zones as well as low 13C to the one identified as solid bitumen in the Paleoproetrozoic

Athabasca Basins in unconformity related uranium deposits (Leventhal et al. 1987; Sangély et

al. 2007). However the isotopic fractionation alone have been questioned as the efficient tool


154

to identified C fractionation from organic or inorganic processes, while only few literature

refers to the massive carbonaceous material (also named condensed CM) isotopic

fractionation experiments (Horita 2005). Nevertheless, the limited data avaliable quasi

systematic 13C depletion up to -60°/00 and ranging from -5 to – 50 °/00 have been measured on

MCM relatively to CO2 and CH4 between 200°C and 600°C in presence of metallic catalyst

such as Ni produced via FT type reaction (Lancet and Anders 1970; Horita 2001; Kerridge et

al. 1989; Sackett 1995).

As the with the carbon stable isotope fractionation, the solid properties and his structural

characterization have been poorly considered while been of great importance because of the

implications of the types of organic matter precursors, if there is any, in the formation of the

massive carbonaceous materials in hydrothermal environments, or the alteration or chemical

processes responsible for the transformation or neogenesis of such materials (Durand 1980).

5.5.4. Degree of tridimensional organization of massive carbonaceous

material

According to the XRD and RAMAN spectroscopy data the studied massive carbonaceous

materials cannot be called graphite. The lack of diffraction of X-ray indicates no sufficient

coherent carbon layer stacking along the c-axis (or a predominantly amorphous material). In

addition the RAMAN signal (ID/IG band intensity ratio, positions and shapes) share some

similarities with amorphous carbon, activated charcoal, graphite nano-particules or altered

graphite from uranium deposits (Tuinstra and Koenig 1970a; Chu and Li 2006; Wang et al.

1989; Calderon Moreno et al. 2000). The in plane crystallite size ranging between 2 to 30nm

and the images in TEM both indicate that the material is predominantly formed of sp2 bonded

(graphene) carbon mono layers or at best a few layer of graphene with a low degree of spatial

organization. This is also marked by the broad D and G bands in the RAMAN spectra, with an

amorphous component in the signal around 1500cm-1, while the D band at 1350cm-1 signs the


155

graphen layer edges of a disordered graphitic lattice as the interpreted band positions from

spectrum decomposition (Sadezky et al. 2005).

Then if it is not graphite, the origin of such material and the formation processes can be

questioned. In fact graphite resulting for the thermal maturation of organic matter is known to

keep the acquired crystalline order (Luque et al. 1993). Then the low level of order of the

massive carbonaceous material relatively to the regional greenschist to amphibolites

metamorphic facies together with his only presence in the alteration halos seems in favor of a

thermal event and formation processes unrelated with the prograde metamorphism (Pasteris

and Wopenka 1991; Wopenka and Pasteris 1993; Lewry and Sibbald 1980). The hypothesis

of fluid deposited during ore related hydrothermal activity might be given by

crystallochemical properties via the RAMAN signature. From this perspective the massive

carbonaceous from Kiggavik share more similarities with the Neoproterozoic, vein hosted,

brittle solid bitumens or graphitoids described in hydrothermal environments related to

Klecany intrusive complex in Bohemia (Jehlička et al. β003) and also quite similar to shungite

found in Karelia (La of 40 nm; disorder) (Jehlička et al. β005; Wopenka and Pasteris 1993) or

to laboratory produced, fluid deposited graphite (Luque et al. 1998), rather than graphitized

carbonaceous material under greenschist facies or even more to graphite host in higher

metamorphic grade facies (Wopenka and Pasteris 1993). Then all low temperature (< 250°C)

hydrothermally deposited carbonaceous or graphitic material appears to share a small in-plan

crystallite size as well as a high level of disorder.

In addition during experimental precipitation of graphite in a C-H-O system, parameters as

such as lower temperature, and higher fH2 lead to the formation of a poorly organize material

(Pasteris and Chou 1998; Ziegenbein and Johannes 1990; Mastalerz et al. 1995). Moreover,

both nucleation and crystal growth must occur to produce graphite. Finally in unconformity

related uranium deposits system such as Kiggavik the presence of CO2 and H2 have been


156

demonstrated and thought to be produce by water radiolysis (Derome 2002; Dubessy et al.

1988).

5.5.5. Nanoscale graphite and possible implications for uranium

precipitation

Graphite is known as chemically inert and though is role as been debated as a potential

control for uranium mineralization in the Eastern and Western Athabasca Basins and in the

SW Thelon at Boomerang where major deposits are spatially related with regional scale

graphitic fault systems (Davidson and Gandhi 1989; Thomas and Wood 2007; Laverret et al.

2006; Thomas et al. 2000). By contrast with the massive carbonaceous materials studied here,

in these structures, graphite doesn’t show a low level of organization and belong to the

metamorphic history of a metasedimentary rock package which underwent at least a green

schist metamorphic grade (Lewry and Sibbald 1980). In contrast no such graphitic bearing

fault is present at Kiggavik, and the only potential sources of organic compounds are to be

related to black shales recognize at minima 10 km to the East.

In addition a nano-scale association has been identified with the MCM and mint crystals of

uraninite and pyrite. Moreover Cu and U are both elevated in the carbonaceous matrix even if

no mineral phase can be identified with the SEM. But nano crystal of uranium phases seems

to coexist within the carbonaceous material. Such close association between the

carbonaceous material and uraninite could be related to orientation-controlled growth on the

graphene layer. In fact, the epitaxial growth of uraninite would be due to the match between

uraninite lattice, on the {111} plan, and the (0001) hexagonal lattice of graphen.

Then the deposition of the carbonaceous material may be contemporanous to the uraninite

deposition during the ore forming processes (both needing reducing condition to precipitate).

In such a process, the enhanced surface area of the carbonaceous material can act as a


157

potential site for nucleation or uraninite and chalcopyrite. Such process would be adequate to

explain the disseminated low grade mineralization found widely in alteration zones where

carbonaceous materials are present.

5.6. Conclusion

Then regarding the petrography, the isotopic measurement and the crystallographic properties,

we propose that the carbonaceous material in Kiggavik could be the expression of abiotic,

hydrothermal synthesis of carbonaceous compounds, with small chains of carbon sulfide and

also massive carbon. This is supported by laboratory experiments of hydrocarbon synthesis

via the CO2 or CH4 reduction on mineral catalyzes like magnetite or pyrite (Heinen and

Lauwers 1996). In addition the range of temperature used for the synthesis of short chains of

carbon sulfides are similar to the one estimated via fluid inclusion work in unconformity type

uranium deposits around 200°C, (see chapter 3) (Pagel 1975; Derome et al. 2005).

We also suggest that the solid properties may provide a favorable environment for uranium

minerals, and metallic sulfide precipitation.

Acknowledgments:

Authors would like to express their gratitude to Marie-France Beaufort for the HRTEM

charaterisation and to AREVA NC & ERM sponsors of this study.

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B. L ES MARQUEURS MINÉRALOGIQUES

Spatial distribution and compositional variation of APS minerals related to uranium deposits in the Kiggavik Andrew lake structural trend, Nunavut, Canada.

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1. Spatial distribution and compositional variation of APS minerals related to uranium deposits in the Kiggavik Andrew lake structural trend, Nunavut, Canada.

Riegler, Thomas; Quirt, Dave & Daniel Beaufort

In preparation for submission in Mineralium Deposita

1.1. Abstract

The Kiggavik Andrew lake structural trend consists in four mineralized zones, partially

outcropping, lying two kilometers south of the erosional contact with the unmetamorphosed

sandstone and basal conglomerates of the Paleoproterozoic Thelon Formation. The

mineralization is controlled by a major East-West fault system associated with illite and

sudoite alteration halos developed in the Archean metagraywackes of the Woodburn Lake

Group. Aluminum phosphate sulfate minerals (APS) from the alunite group crystallized in

association with the clay minerals in the basement alteration halo as well as in the overlying

sandstones which suffered mostly diagenesis. APS minerals are Sr and S-rich (svanbergite

end-member) in the sedimentary cover overlying the unconformity while they are LREE-rich

(florencite end member) in the altered basement rocks below the unconformity. The

geochemical signature of each group of APS mineral together with the petrography indicates

two distinct generations of APS minerals:(1) a first one related to sedimentation-diagenesis

processes at the bottom of the Thelon sandstones and (2) a second one related to hydrothermal

alteration processes which accompanied the uranium deposition in the basement and partially

overlap the sedimentary-diagenetic mineral parageneses . The primary REE bearing minerals

of the hosts rock were characterized in order to identify the potential sources of REE. The

obtained chemical composition of REE highlights a local re-incorporation of the REE

released from the alteration processes in the APS minerals. The distinctive geochemical

signatures between diagenetic (or sedimentary) and hydrothermal APS minerals suggest a


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different source material in the Thelon basin than the Athabasca basin. Lastly as some of the

primary REE-bearing minerals are potential uranium sources, the potential fertility and the

sources of the uranium can be questioned.

1.2. Introduction

Since several decades hydrated aluminum phosphate and sulfate (APS) minerals of the alunite

supergroup have been found and studied in a broad range of geological environments; form

soils to sedimentary basins, or hydrothermal and volcanic system (Dill, 2001; Stoffregen and

Alpers, 1987). They occur as minute, euhedral crystal with rhombohedral or pseudocubic

habits. Their general structural formula is AB3 (XO4)(OH)6, where A, B, X are the 12, 6 and

4-fold coordinated crystallographic sites. The A site can be occupied by monovalent (H3O+,

K+, Na+, Rb+, NH4+, Ag+, Tl+, etc), divalent (Ca2+, Sr2+, Ba2+, Pb2+) or trivalent (Bi3+,

LREE3+) cations. More rarely tetravalent cation like Th4+ can integrate the APS crystal

structure in this site. The B: 6-fold coordinence site is occupied by Al3+ and Fe3+, and lastly

the X site is commonly occupied by S6+, P5+, and As5+. As the three sites are subject to

numerous substitutions, and thus to complex solid solutions, these crystal-chemical properties

give rise to tens of end-members within the alunite supergroup.

Despite their relative very low abundance as well as their very small size making them

somehow difficult to identify in thin sections, recent studies have highlighted the potential of

APS mineral as geochemical markers of the proximity of orebodies in unconformity related

uranium deposits (Gaboreau et al., 2005; Gaboreau et al., 2007). In fact, two major

characteristics can be pointed out: (1) once formed, they resist to most of the chemical

reactions involved during surface or subsurface alteration processes and (2) their sensitivity to

physic-chemical condition of formation (such as Eh and pH) is recorded in their broad range

of chemical composition, with the ability to trap and concentrate trace elements and

particularly the light rare earth elements (LREE), (Herold, 1987; Stoffregen, 1993).


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Worldwide in the paleoproterozoic unconformity related uranium deposits, APS minerals

seem to be systematically associated with the clay alteration assemblages. They have been

studied in the Kombolgie basin in Australia (Beaufort et al., 2005; Gaboreau et al., 2005) as

well as in the Athabasca and Kombolgie Basins (Gaboreau et al., 2007; (Cloutier et al. 2010)

and both the Thelon and Horny Bay Basins in Canada (Gall and Donaldson, 2006) as well as

in the underlying saprolites (Miller 1983).

The spatial distribution and the compositional variation of these minerals around the uranium

orebodies led some authors to consider them as good indicator of both the redox and pH

paleo-conditions responsible for the development of fronts during the alteration process, and

hence as potential tools for mineral exploration (Gaboreau et al., 2005, 2007).

The aim of this study is to determine the nature and the origin of the APS minerals present

near the bottom of the middle Proterozoic Thelon sandstone formation and in the underlying

archean basement rocks which host the alteration halos of the uranium ore bodies distributed

along the Kiggavik Andrew lake structural trend, Nunavut, Canada . The main goals are (1) to

determine the paragenetic association and the crystal chemistry of APS mineral in the

basement and the overlying Thelon sandstones, (2) to determine the source material for APS

minerals and (3) assign and replace each paragenesis in the basin and the uranium

mineralization history. These results are compared and discussed with respect to those

already obtained from APS in alteration halos associated with the unconformity-type uranium

deposits of the both Australian and Canadian counterparts.

1.3. Regional geological setting

The Kiggavik exploration project is composed of two groups of claims, St Tropez to the

North and Kiggavik down South, going roughly from the Southern Shore of Schultz Lake to

the East of Judge Sisson Lake, about 80km West of the Inuit hamlet of Baker Lake, NU. All


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the deposits, Kiggavik, End and Andrew, and most of the prospects Bong, Granite, Sleek, are

exclusively located in the Kiggavik part of the project, figure 1-1. The overall historical metal

content is estimated to about 50 000t U, with an average 0.5% grade (Jefferson et al., 2007).

Figure 1-1 : Bedrock geology of the NE Thelon Basin margin, modified form Jefferson et al., 2013. Sampling

area are indicated by the following number: 1 Granite Grid area; 2 W94-4 drillhole south of the Judge Sisson

Fault, W2 drillhole; 4 Uno Granite

The study area encompasses the St Tropez and the Kiggavik areas in order to have a more

regional, vision of the geological factors that could be involved to generate a favorable

environment for uranium mineral deposition (basement rocks lithology, structural control of

the fluid flow paths, degree of fluid rock interaction in any place etc.).

4

2

3

1


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The host of the mineralization is the Woodburn Lake Group (WLG), a 2.6-2.7 Ga, neoarchean

supracrustal rock package formed mainly by a sequence of metagraywackes and

metavolcanics, with minor quartzite and iron formation. This sedimentary pile

retromorphosed in a greenschite facies overlies older granitic gneiss of uncertain age and is

part of a series of greenstone belt known from Northern Saskatchewan to Northern Baffin

Island, set during the Rae craton continental riflting (Hartlaub et al., 2004). The archean rock

package had his own tectonometamorphic history. During the paleoproterozoic the archean

and the overlying paleoproterozoic cover where imbricated and structured together in several

stages of deformation related to the Thelon-Taltson (2.0-1.9 Ga) and the Trans-Hudson (2.0-

1.8 Ga) orogens, respectively at the NW and SE during the Laurentia accretion (Hoffman,

1990). These events formed the main structural frame with main foliations and major shield

scale ductile mylonite zones and led to a complex stratigraphic stack not yet completely

understood (Pehrsson et al., 2010). Following the Thelon-Taltson orogeny collapse two major

suites of igneous bodies took place. The older intrusive (1.82-1.85 Ga) belongs to the Hudson

Suite; composed of granite; Martell syenite and the Dubawnt minettes. It was followed

100Ma later by the Nueltin rapakivi granite and its volcanic equivalent, the Pitz rhyolites

which belongs to the Wharton Group which constitute the intermediate and second sequence

of the Dubawnt supergroup (Miller and LeCheminant, 1985; Peterson et al., 2002; Rainbird et

al., 2003). The last sequence is the Barrensland Group which hold the Thelon Formation, a

siliciclastic sedimentary sequence similar the Athabasca Group in term of age and lithology.

As already mentioned in the Athasbasca basin, the upper part the Thelon sandstone have been

locally cemented by apatite during an early diagenesis stage dated between 1720 and 1647

Ma (Miller et al., 1989). Morever in addition to the fluor-apatite cement observed in the

Thelon and the Hornby bay basins, APS cements have been also described in the Thelon

Formation, (Gall and Donaldson, 2006).


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In addition to those already observed in the sandstones, APS minerals where commonly

observed in close association with illite and sudoite (Al-Mg chlorite) within the clay alteration

halos which envelop the uranium deposits along the Kiggavik - Andrew lake structural trend

(Riegler et al. 2013).

1.4. Sampling and methods

A set of several tens of fresh and altered rocks samples from drillcores and outcrops coming

from various locations in the Kiggavik and St Tropez area were selected for a preliminary

petrographic analysis. Such a sampling encompasses the basement rocks with the

metagraywackes and the later intrusive (Hudson Granite and aplite, Martell Syenite, Nueltin

porphyritic rhylolite) as well as the overlying basal sedimentary beds of the Thelon Basin -

conglomerates and conglomeratic sandstones) figure 1-1, Table 1-1. 13 samples were then

selected within the previous extensive set of samples for a detailed analysis of APS minerals

in altered rocks and their parental minerals in the fresh ones. In addition the chemical

compositions of 189 core samples and 50 centimeter long composite samples of altered and

fresh metagraywackes were acquired to investigate the elements transfers, Table 1-1. These

samples are representative of the varying degree of alteration in the alteration halos developed

around the major structures which controlled the deposition of uranium orebodies.


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Table 1-1: Kiggavik-St Tropez APS, REE minerals bearing rocks sampling table

Geochemistry

Sample name Area Drillhole/Outcrop rock type alteration Umin ICP-OES

ICP-MS

Thelon Formation: Sedimentary Cover

GG3a Granite Grid outcrop mudstone no no 1 GG3b,c, GG4, 5, 7,

StTropez " outcrop sandstone no " 6

Basment

GGG Granite Grid outcrop Hudson granite no no

W2-2 Sleek Lake drillhole " no "

W2-03 " " metagraywacke no " 1

SL9_1 to 5 " " Hudson granite yes " 5

UnoGranite St Tropez outcrop " no "

W97-4_10 & 17 Judge

Sisson lake drillhole " no " 1

W97-4_3 " " metagraywacke no "

B1-94-1_08 " " " no " 1

ALCS Andrew

Lake outcrop porphyritic

ryholite no " 1

BSE_05 Bong drillhole metagraywacke no " 1

BG45_13 " " " no " 1

BG42-23 " " metagraywacke yes yes

BG42-21a, b; 25; 30; 40 " " " yes no 5

BG45_28 " " " yes yes 1

BG43_028 " " " yes " 1 BG42 composite

set " " " yes " 133 BG49 composite

set " " " no no 34

Then, for the Sedimentary cover (Thelon Formation): Unaltered mudstones: GG3a as well as

unaltered sandstones GG3b, c; GG4 & GG7 samples have been taken on outcrop along the

Thelon fault North of the Granite Grid prospect, point #1 on the figure 1-2. The GG4 & 7

sampled were taken a few kilometers away to the East. In the basement rocks the main sample

location were:

- Altered and/or mineralized metagraywacke form the Bong prospect with the BG42

and BG49 drillholes (DDH)


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- Unaltered Intrusives: A prophyritic ryholite dyke belonging to the Nueltin Suite

located at the Andrew Lake core storage (ALCS). Hudson granite, part of the Schultz

lake intrusive complex with the W97-4_10 & W2-2 drillholes as well as an outcrop

sample form the Granite Grid and the NE St Tropez areas, respectively the point #2, 3

and 4 on the figure 1-1.

- Altered Hudson granite form the Sleek Lake area SL9_01 distal to mineralization &

SL9_05 in the vicinity of very local and weak mineralization intercepts. Such samples

are representative of barren alteration zones.

1.4.1. Methods

Petrographic study and mineralogical identification were made in thin sections using

observation with a polarizing optical microscope and a scanning electron microscope (SEM,

JEOL 5600LV) equipped with a Brucker Energy Dispersive Spectrometer (EDS) X-ray

analyser at the University of Poitiers (15kV acceleration voltage, 1 nA beam current).

Quantitative chemical analyses were performed using a Cameca SX100 electron microprobe

with wavelength dispersion spectrometers (WDS) at the CAMPARIS micro-analysis facility,

UPMC Jussieu Paris. Chemical microanalysis of Si, Al, Fe, F, P, S, Ca, Ba, Sr, La, Ce, Pr,

Nd, Th and U were obtained at the following analytical conditions: 15kV accelerating

voltage, 4 nA beam current, 2 micrometer spotsize and a counting time ranging from 5 to 20

seconds according to the specific element. The microprobe was calibrated using both natural

and synthetic standards: anorthite, apatite, pyrite, diopside, barite, uraninite, thorite, monazite,

SrSiO3, NdCu and a glass doped in rare earth elements (REE). The relative error on the

elements is below 1 %. The structural formulas of APS minerals were calculated using a 6

cations normalization as in the theoretical mineral formula AB3 (XO4)2(OH)6.

Bulk rock chemistry (including major and 54 traces elements) of representative samples

selected in our study were done using ICP-MS at the Service d'Analyse des Roches et des


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Mineraux - CNRS – CRPG, in Nancy France. These chemical data have been completed by

an exploration chemical data set provided by AREVA Resources Canada (major and traces

elements) analysed at the Saskatchewan Research Council Laboratory in Saskatoon, Canada

using ICP-OES. Due to the lower detection limits and better accuracy provided by ICP-MS,

only the samples analyzed at the SARM will be used to compare the trace elements.

In order to decipher and interpret the geochemical data gathered on the REE bearing phases

we carried a petrographic study to highlight the textural and chronological relationships of

these minerals. Thus the primary, magmatic or sedimentary/diagenetic paragenesis have to be

identified and compared with secondary mineral assemblage linked to the hydrothermal

events in relation with the mineralization processes.

1.5. APS minerals and alteration petrography

In thin sections APS were observed by SEM because of their brightness in backscattered

electron (BSE) mode, due to their high content in heavy elements such as Sr and LREE.

Generally, APS crystals occur as very small euhedral rhombs (2–10 mm in average width,

exceptionally up to 50mm) located in the intergranular porosity of the clay matrix which

constitutes most of the alteration products of both sedimentary and basement rocks or are

associated with secondary quartz and hematite as fracture controlled cement. Locally coarser

grained APS crystals display features of growth zoning characterized by alternation of

concentric bright and light grey thin zones.

Their identification is quite arduous in optical microscopy due to their small size and the

possible confusion with apatite and their relatively scattered distribution in the rock even if

they can form massive aggregate in places (figure 1-2a).


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Figure 1-2 : Aluminum phosphate sulfate minerals (APS) of the Kiggavik area in Thelon Formation ( pictures A

to D) and in the altered basement (E& F). A APS minerals in the sandstone in equilibrium with kaolinite,

remanante porosity filled by microcrystalline quartz (QTZ3) GG7 sample; B Massive APS cement; C

Microcrystalline (QTZ3) and APS minerals fracture cross cutting unaltered red mudstone at the base of the

Thelon Fm, later re-opened and filled with illite and hematite, GG3a sample; D Local ililtization pocket

developed in a large kaolinite booklet with co precipitation of APS, GG4 sample; E Altered granite with Illite


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replacement of feldspar and devellopement of APS minerals, SL9 drillhole, Sleek lake area; F Illitic pocked with

euhedral APS, Andrew Lake, And10-01 drillhole

Abbreviations, Ill: illite; Hm: Hematite; Kao: kaolinite; Qtz: quartz

1.5.1. APS of the basal Thelon Formation

In the sandstones APS (hereafter named and labeled by the number of each generation)

minerals are relatively abundant in the following petrographic settings: (1) as pore filling

aggregates of very fine crystals with a typical pseudocubic habits or as cements (Figure 1-2a

& 2b); (2) as disseminated euhedral crystals coeval of post diagenetic quartz overgrowths

figure 2c and (3) locally associated with illite flakes in the late illitization pockets developed

at the expense of the pore-filling kaolinite, figure 1-2d.

Late reopening the post diagenetic quartz hydraulic breccia is associated with co-precipitation

of illite and anatase followed by specular hematite. The paragenetic sequence determined in

the sandstone is presented figure 1-3.

Figure 1-3 : Mineral paragenesis of APS minerals in the basal Thelon Formation


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According to the petrographic study, the first generation (APS1) is related to a sedimentary-

diagenetic stage which emcompasses the deposition of large sedimentary kaolinite booklets

and then their diagenetic recystallization into dickite ± hematite with increasing burial depth,

contemporaneously of the formation of diagenetic quartz overgrowths (QTZ2). In addition,

these APS1 are associated with a AlO(OH), cerianite, Ce-La oxides, thorianite and hematite

paragenesis in the GG7 sample, figure 1-4. The second generation (APS2) is trapped in a very

fine grained, microcrystalline, quartz filling the remaining porosity. This quartz cementation

heals microbreccia and fractures which affected lithic fragements. The last generation (APS3)

is associated with late illite which replace the previous diagenetic kaolinite during a

postdating hydrothermal event which is well known to be related with the deposition of the

uranium orebodies in the underlying basement rocks (Riegler et al., 2013).

In the sandstone, the crystals of diagenetic APS tend to be relatively homogenous in

composition with no more than one generation of visible overgrowth showing a slightly

brighter core in BSE mode, indicating the incorporation of more heavy elements during the

first crystallization stage. Unfortunately the crystal size is often smaller than the electron spot

size, permitting not to have EMP measurement of the chemical composition on both core and

overgrowth for these crystals.


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Figure 1-4 : a, Oxy-hydroxy aluminium minerals associeted with cerianite grains and exhibiting high Ce levels

up to several percent; b, same AlOOH minerals associated with cerianite, La-Ce oxide and thorianite and related

EDS spectra for these phases:c, c, & e.


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1.5.2. Basement Alteration petrography and mineralogy

Figure 1-5 : Primary REE & Uranium bearing minerals in the Kiggavik & St Tropez area, A Non metamict

unaltered monzite grain, Woodburn Group metagraywacke, W97-4; B Parisite (Ca,Ce, La)CO3 euhedral crystal

with local partial dissolution associated with illite formation, granitoid at the bottom of the W97-4 drillhole; C

Altered allanite, Uno granite, D Uranothorite altered to thorogummite, Andrew lake syenite dyke.

Abbreviations, Aln: allanite, Cal: calcite, Par: Parisite, Py: Pyrite, Ttn:titanite

In the basement rocks, APS minerals are closely associated with the hydrothermal alteration

processes which controlled the uranium ore genesis, Figure1-2e. The alteration of basement

rocks is spatially related to faults at every scale. It results in a strong color change of the

rocks. Two contrasting macroscopic alteration features have been noted: (1) bleaching related

to strong argilization and (2) reddish coloration related to crystallization of iron oxide (mostly

hematite) and local silicification of open fractures (quartz veins). Strong argilization


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(bleaching) associated with various degrees of desilicification is the most common alteration

feature observed in both the fault core and the major mineralized zones. At small scale,

alteration is related to the dissolution of the preexisting metamorphic silicates and the

crystallization of an abundant clay matrix composed of illite associated to variable but

generally small amounts of sudoite (Al-Mg chlorite). In altered intrusive rocks, the feldspars

phenocrysts are totally replaced by illite and specular hematite. Sudoite is always intimatly

associated with illite.

Minor amounts of phosphate minerals are closely associated with illite and sudoite. They

essentially consists in aluminum phosphate-sulfate minerals (APS) and secondary apatite and

they display a marked zoning pattern around the mineralized bodies. APS minerals occur as

tiny pseudocubic crystals which size range from less than to 10 µm up to 50 µm. APS

minerals display frequently features of chemical zoning but the ones observed were quite

homogeneous with only one generation of overgrowth and very little compositional change

(Figure1-2f). Secondary apatite occurs close to and within the mineralized zones.

APS minerals are totally absent in the fresh basement rocks. However, several accessory

minerals of these rocks can be considered as the potential source for the chemical elements

incorporated in the APS minerals observed in the altered zones. According to the nature of the

host rock, they can consist in monazite (Figure 1-5a), mostly in the metagraywackes;

bastnaesite/parasite (Figure1-5b), allanite or REE oxides (Figure1-5c) as accessory phases

in granites, ryholites and syenites. In addition uranothorite altered to thorogummite can be

found in place in ryholite (Figure 1-5d). This last mineral being a potential source for both U

and REE.

To sum up the petrographic observations in both the sedimentary and basement rocks, it

appears that two main generations of APS minerals can be identified with respect to their


179

relationships with clay minerals. In the Thelon sandstones APS1 & 2 coexist with kaolin

minerals and are related to the basin evolution from sedimentation to deep diagenesis as

indicated by the kaolinite to dickite transition (voir chapitre 4). The second generation of APS

is clearly related to the hydrothermal alteration event associated with uranium mineralization

within the underlying basement rocks. The APS3 observed locally with illite near the bottom

of the Thelon sandstone is considered as a signature of the hydrothermal alteration event

which is extensively developed below the unconformity.

Then, the crystal-chemical properties of APS minerals needs to be investigated in more

details to determine if the textural observation can also be linked to a chemical evolution of

the system and how this would fit in the general mineralization model for the uranium

mineralization found along the Kiggavik Andrew lake structural trend. In addition,

complementary EMP measurements have been done on the REE bearing mineral identified in

the fresh rock (monazite, bastnaesite/parasite, allanite, REE oxides). Complementary data on

accessory uranium-bearing mineral is also presented.

1.6. Electron microprobe data

1.6.1. Overall chemistry of APS minerals

Average representative electron microprobe data of each geological environment have been

sum up in the table 1-2. The low and relatively variable sum of oxide weight percent of the

analyses is a result of the water content of the APS minerals as well as a significant

microporosity between the micrograins aggregates of the X-Ray emitting volume. However,

even if the very small size of the APS minerals increases the possibility of contamination of

the microprobe analysis by chemical elements of surrounding clay minerals or quartz, it has

no significant influence on the relative proportions of the APS forming chemical elements (at

the exception of aluminum).


180

Table 1-2 : Representative electron microprobe analyses of APS minerals, B: basement, C, sedimentary cover

B C

Distal Alteration; SL9_01

Intermediate Alteration; SL9_05

Proximal Alteration Zone, BG42_23

GG7_APSCement

GG7_Mtx

GG4 GG3

n = 32 n = 17 n = 17 n = 18 n = 31 n = 9 n = 20

Mean Wt% Std Dv. Mean Wt%

Std Dv.

Mean Wt% Std Dv.

Mean Wt%

Std Dv.

Mean Wt%

Std Dv.

Mean Wt%

Std Dv.

Mean Wt%

Std Dv.

SrO 5.95 1.31 4.61 1.22 3.26 0.76 10.09 0.70 11.31 1.22 7.31 0.95 13.49 1.12

CaO 2.52 0.40 2.31 0.61 1.15 0.28 1.72 0.15 2.18 0.39 3.48 1.04 2.25 0.30

BaO 0.33 0.37 0.31 0.31 0.02 0.03 0.67 0.13 0.72 0.34 0.43 0.31 1.31 0.50

La2O3 6.11 1.63 5.99 1.43 7.71 1.08 5.54 1.04 2.37 1.57 3.00 1.32 1.15 0.28

Ce2O3

6.97 2.06 9.55 1.98 11.60 0.90 2.23 0.72 3.01 0.78 3.69 0.94 1.49 0.42

Pr2O3 0.74 0.21 0.76 0.20 0.97 0.17 0.84 0.13 0.38 0.21 0.67 0.15 0.20 0.07

Nd2O3

2.01 0.76 1.93 0.54 2.27 0.50 2.92 0.48 1.33 0.75 2.92 0.56 0.72 0.28

ThO2 0.36 0.30 0.37 0.43 0.02 0.03 0.30 0.12 0.38 0.13 0.25 0.33 0.36 0.24

Al2O3 29.48 1.42 28.94 1.42 32.78 1.21 31.50 0.55 31.61 1.72 32.70 1.71 31.76 1.00

FeO 0.72 0.66 0.80 0.82 0.08 0.05 1.17 0.50 1.18 0.64 0.54 0.58 1.59 2.28

P2O5 23.65 1.28 24.42 1.56 26.78 2.12 25.75 0.60 23.98 1.84 27.56 2.67 23.31 1.18

SO2 2.35 0.85 1.85 0.87 1.78 1.02 2.10 0.38 3.02 0.74 2.64 0.42 4.73 0.48

Total 81.18 81.83 88.41 84.81 81.48 85.19 82.35

Mean apfu Stdt Dv.

Mean apfu Stdt Dv.

Mean apfu Stdt Dv. Mean apfu

Stdt Dv.

Mean apfu

Stdt Dv.

Mean apfu

Stdt Dv.

Mean apfu

Stdt Dv.

A Sr 0.30 0.06 0.23 0.06 0.15 0.03 0.47 0.03 0.54 0.05 0.33 0.05 0.62 0.04

Ca 0.23 0.03 0.21 0.06 0.10 0.02 0.15 0.01 0.19 0.03 0.29 0.09 0.19 0.02

Ba 0.01 0.01 0.01 0.01 0.00 0.00 0.02 0.00 0.02 0.01 0.01 0.01 0.04 0.02

La 0.20 0.06 0.19 0.05 0.23 0.03 0.17 0.03 0.07 0.05 0.09 0.04 0.03 0.01

Ce 0.22 0.07 0.30 0.06 0.34 0.03 0.07 0.02 0.09 0.02 0.11 0.02 0.04 0.01

Pr 0.02 0.01 0.02 0.01 0.03 0.00 0.02 0.00 0.01 0.01 0.02 0.00 0.01 0.00

Nd 0.06 0.02 0.06 0.02 0.07 0.01 0.08 0.01 0.04 0.02 0.08 0.02 0.02 0.01

Th 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.01 0.01 0.00

B Al 2.99 0.05 2.96 0.06 3.11 0.09 3.01 0.05 3.05 0.09 3.01 0.10 2.99 0.10

Fe 0.05 0.05 0.06 0.06 0.01 0.00 0.08 0.03 0.08 0.04 0.04 0.04 0.10 0.15

X P 1.72 0.04 1.79 0.08 1.83 0.11 1.77 0.04 1.66 0.09 1.82 0.11 1.58 0.07

S 0.19 0.06 0.15 0.07 0.14 0.09 0.16 0.03 0.23 0.06 0.19 0.04 0.35 0.03


181

Table 1-3 : Electron microprobe analyes of primary REE and U bearing minerals in the Kiggavik-St Tropez area, n number of analyses; the low total elements for monazite could be

explained by the fact that Pb, Y, Sm, Ga haven’t been analysed.

Hudson Granite Metagraywackes

Bastnaesite/Parisite Allanite Monazite Thorogummite Thorogummite Monazite

W97-4_10 & 17; W2 Uno W97-4; W2; Uno; Granite Grid

W97-4_17 W97-4_10 W97-4_3

n= 26 n= 5 n= 8 n= 4 n= 3 n= 14

wt% Mean Std Dv. wt% Mean Std Dv. wt% Mean Std Dv. wt% Mean Std Dv. wt% Mean Std Dv. wt% Mean

Std Dv.

F 3.26 0.40 0.20 0.02 0.37 0.04 0.68 0.09 0.34 0.05 0.40 0.06

Al2O3 0.13 0.36 11.76 0.60 0.10 0.11 0.22 0.11 0.39 0.09 0.07 0.04

P2O5 0.00 0.01 0.31 0.26 24.88 2.14 0.25 0.25 1.81 0.39 26.19 1.46

SO2 0.00 0.00 0.00 0.00 0.00 0.01 1.06 1.01 0.46 0.44 0.29 0.25

CaO 17.07 0.70 8.90 0.36 0.55 0.35 1.57 0.14 2.84 0.36 1.16 0.43

FeO 0.36 0.94 12.31 0.98 0.85 1.26 1.95 1.04 2.04 1.46 0.15 0.14

SrO 0.02 0.03 0.28 0.04 0.11 0.13 0.08 0.06 0.05 0.03 0.20 0.11

La2O3 13.63 0.76 4.45 0.73 16.11 2.11 0.21 0.05 0.07 0.05 16.92 1.05

BaO 0.02 0.04 0.56 0.11 0.06 0.08 0.06 0.05 0.36 0.38 0.03 0.05

Pr2O3 2.43 0.21 0.82 0.28 3.22 0.21 0.15 0.09 0.25 0.06 3.32 0.13

Nd2O3 8.70 1.10 2.57 1.32 9.68 1.19 1.41 0.39 0.95 0.29 11.41 0.90

UO2 0.06 0.07 0.11 0.08 0.15 0.15 4.20 2.12 0.90 0.47 0.09 0.08

ThO2 0.23 0.63 1.02 0.22 4.29 2.71 53.32 3.19 56.10 2.63 0.49 0.65

Ce2O3 25.30 1.37 9.33 1.22 30.79 1.53 0.86 0.19 0.84 0.22 31.82 1.20

SiO2 0.15 0.43 28.55 0.84 1.88 1.22 13.45 0.88 13.22 0.20 0.18 0.07

Total 71.36 1.00 81.18 4.07 93.04 2.82 79.47 4.47 80.64 2.32 92.73 1.83


182

Nevertheless it appears that a clear chemical distinction can be made between APS minerals

from the siliciclastic sedimentary cover, (C) and those from the basement (B), table 1-2. APS

minerals from the siliciclastic sedimentary cover are Sr ,Fe and SO2 rich while APS minerals

of the hydrothermal alteration zones are enriched in LREE with a concentration 2 to 3 times

higher than in the APS form the sandstones, and correlatively lower Sr and SO2. These global

consideration leads to the identification of two chemical fields of APS minerals easily

differentiated in a Sr/LREE/S ternary diagram, figure 1-6. The first chemical field which is

next to that of svanbergite, is representative of the sedimentary and diagenetic APS (GG3,

GG7) samples. The second compositional field which is next to the one of florencite is

representative of the hydrothermal APS of the altered basement (SL1&SL5 and BG42).

Figure 1-6 : Ternary plot using Sr, LREE & S composition of APS minerals obtained from electron microprobe

analysis. mineralized metagraywackes ◊ Mineralized zone graywacke BG4β, □ proximal to mineralization in

granite SL9, ■ distal to mineralization SL9, ▲ sandstone GG7, ▽ sandstone GG3, △ sandstone GG4


183

In both chemical fields, the compositional variation of Sr and LREE follows the same

direction with a distribution of analyses between a Sr (svanbergite, goyazite) and a LREE rich

(florencite) end members. Lastly, the GG4 sample represents a group of APS minerals with

intermediate chemical composition between the APS of unaltered sandstones and those of the

illitized basement especially for the Sr and LREE contents.

Figure 1-7 : Ternary plot using La, Pr+Nd & Ce of electron microprobe data on APS minerals form Thelon

formation conglomeratic sandstones and altered as well as mineralized metagraywackes. Mineralized zone

graywacke BG42, □proximal to mineralization in granite SL9, distal to mineralization SL9, ▲ sandstone

GG7, ▽ sandstone GG3, △ sandstone GG4


184

The relative proportion of the LREE incorporated in the APS minerals (La,Ce,Pr,Nd) can be

illustrated using a La/Pr+Nd/Ce ternary diagram, figure 1-7. Over all the APS minerals

analyzed, the greatest variability in LREE relative proportions correspond to the cerium in the

APS minerals from the sandstone and more particularly for the GG7 sample (solid triangle

pointing up) with a broad range LREE proportion, from high Ce in the matrix APS to low in

the massive APS cement. Such a feature of strong variation of cerium content in the APS

minerals while La/Pr+Nd ratio is constant needs to be look at jointly with the presence of

cerianite. Then, cerium might had been fractionated between Ce-oxide and APS minerals

during the crystallization stage of early APS1. In the altered basement the relative

proportions of APS seems broadly constant with a predominance of cerium over the other

LREE elements (figure 1-6).

1.6.2. Element distribution in crystallographic sites

As already demonstrated, (Dill, 2001; Gaboreau et al., 2005; Gaboreau et al., 2007), APS

minerals are subject to numerous and various substitutions in the A, B and X sites forming

solid solution between ten’s of end members of the alunite supergroup. Moreover, the chemo-

sensitive behavior of APS mineral have been identified in the selective incorporation of REE

depending mainly on Eh and to a less extent on pH of the forming environment.

The global composition of the different types of APS minerals identified in various locations

along the Kiggavik Andrew lake structural trend preferentially varies in response to coupled

substitutions of Sr and LREE in their A site and P and S in their X site. In the altered

basement rocks both intra- and inter-sample chemical variations of APS range along a binary

solid solution between Svanbergite [Sr(Al3)(PO4,SO4)(OH)6] (which belongs to the beudantite

group) and Florencite [LREE( Al3)(PO4)2(OH)6] (which belong to the the crandallite group).

Sr-rich APS minerals (closer to the Svanbergite end member) are associated with non-altered

to distal alteration zone while APS minerals with the highest LREE content were analysed


185

within the mineralized zones, and more particularly in the ore grade mineralization zone in

which LREE-rich APS can coexist with secondary apatite.

In the overlying sandstones, the compositional variation of APS cannot be explained only by a

change in svanbergite to florencite solid solution. Indeed, the fact that all the punctual

analyses are anomalously enriched in of Sr versus S (figure 8) is indicative of a significant

contribution the Goyazite end-member [SrAl 3(PO3[(O0.5(OH)0.5]2(OH)6] to the crystal-

chemistry of these minerals.

Figure 1-8 : Cross plot diagram S versus Sr of APS minerals

1.6.3. Chemistry of primary REE bearing minerals

Representative electron microprobe analyses given in % oxydes of primary REE and U, Th

bearing mineral are presented in the table 1-3.

0.00

0.20

0.40

0.60

0.00 0.20 0.40 0.60 0.80

S (

ap

fu)

Sr (apfu)

SL9_01 SL9_05 BG42_23

GG3 GG4 GG7


186

As described in the previous petrographic study, several primary REE-bearing minerals are

found in the unaltered basement rocks in which they belong to magmatic or metamorphic

paragenesis.

Rare earth carbonates are widely encountered in the Hudson granite but their concentration is

to low to permit their characterization by classical X-ray diffraction methods, even after

heavy minerals concentration using sodium polytungstate at a density of 2.9. Nevertheless the

punctual EMP analysis provides valuable information to identify REE carbonates on the basis

of the low sum of oxide weight % (near 70%) and the qualitative analysis of carbon on the

EDS spectra and the overall chemistry. The most common REE carbonates of igneous rock

are: bastnaesite (Ce,La)CO3F, parisite Ca(Ce,La,Nd)(CO3)2F2, and synchysite

Ca(Ce,La)(CO3)2F. The ternary Thorium, calcium and total rare earth plot for the assumed

carbonates, figure 1-9 shows that the REE carbonates are closer to the parisite end member

with relatively high calcium content. More interesting, when plotted in a La/Pr+Nd/Ce ternary

diagram (figure 9), all these minerals have a composition which is superimposed on the

composition field determined for most of the APS minerals of the basement rocks (figure 6).

The second groups of REE bearing minerals are the monazites with higher thorium content in

the Hudson granite monazite than in the metagraywacke ones table 1-3, 1-4. The low sum of

oxide Wt% of the monazite analyses are explained by the fact that HREE have not been

analyzed.

The following simplified structural formulas (without HREE) can be calculated for the

monazite, Table 1-4.


187

Figure 1-9 : Ternary plot using La, Pr+Nd & Ce, of electon microprobe chemical composition on REE bearing

minerals in unaltered Woodbun metagraywackes & Hudson granitoids in the Kiggavik & St Tropez area: ▲

monazite, allanite, REE oxides ? , REE carbonates. The field of composition and the phase

identification for REE carbonates is given in the Ca, Th, REE ternary diagram.

Table 1-4 : Mean simplified structural formulas of monazite analysed in both the metagraywacke and Hudson

granite. n: number of microprobe analysis

Metagraywacke Hudson Granite

n= 14 n= 8

(Ce0.50,La0.27,Pr0.05,Nd0.17,Th0.00)P0.95O4 (Ce0.51,La0.27,Pr0.05,Nd0.16,Th0.05)P0.93O4

Finally, allanite, zircon and uranothoriferous minerals occurs in the Hudson granite.

Uranothorite is frequently altered and replaced by thorogummite with up to 8% UO2, table 1-


188

3. Moreover the Ce, and Nd concentration of the thorogummite (up to 1%) make this mineral

a minor REE source released during the alteration processes.

This result highlights the potential role of the Hudson granite as a regional source of uranium,

and REE while monazite is globally uranium poor but nevertheless a very good candidate as

LREE primary source.

Monazite has not been identified in the sandstone cover probably because of its total

dissolution during the diagenetic processes.

1.7. Whole rock chemistry and REE distribution

The crystal-chemical study of REE and U bearing minerals has been completed by a bulk-

rock chemical analysis of major and trace elements in order to identified the geochemical

behavior of these elements and to to evaluate the elements transfers during the alteration-

mineralization processes.

As the evaluation of a mass transfer requires the assumption of immobile elements, Ti, Al and

Th (or Zr which is not represented in figure 9) have been tested by a comparative study of

their relative proportions in both unaltered and altered rocks of a same metagraywake

protolith. A plot of the relative proportions of these elements from all the unaltered to highly

altered samples of metagraywakes in a ternary diagram (figure 1-10) indicates that the Al and

Ti relative percentages remain constant while elements such as Th or Zr (not represented in

figure 9) slightly vary. Such a geochemical behavior which is enhanced by the factor 1000

used in the ternary plot can be easily explained by bedding heterogeneities in the

metasediments. A similar behavior is observed when Th or Zr have been replaced by the sum

of LREE in the ternary plot. This indicates that Ti and Al can be considered as immobile

elements in the system while minor changes in Th, Zr and LREE, can be reasonably assigned

to the sedimentary control on the amount of detrital heavy minerals (zircon, monazite…).


189

However, the inter-sample variation in Th concentration which is quite low and has probably

no significant impact on the use of Th, Ti or Al for mass balance calculation.

Figure 1-10 : Plot of the chemical composition of all the unaltered to highly altered samples

of metagraywakes in a Ti, Th*1000, Al/10 ternary diagram. (□) altered metagraywacke and

(■) unaltered metagraywacke.

1.7.1. Pearce analysis & Mass balance calculation

Petrography and crystal chemistry lead to the identification of the REE ratio and content on

both fresh and altered rocks. Pearce Element ratio analysis (PER) and mass balance

calculation have been used to approach the chemical transfer during alteration.

At first, local redistribution needs to be assess comparing the samples of the altered zone. In

the figure 1-11a, the Ce/Th and Al/Th molar ratio behavior gives a representation of the


190

redistribution of cerium in APS minerals during the alteration of monazite. It appears that

some samples are depleted in LREE while other are enriched, this being concordant with the

heterogeneities in APS content inside the illite & sudoite alteration halos. In addition the

Ce/Th vs La/Th reveals that very little fractionation between the various REE occurs figure

11b. This is in agreement with the La/Pr+Nd/Ce ternary diagrams where the LREE proportion

between the sources minerals and APS is fairly constant. Then alteration and crystallization of

REE phases seems to be a topochemical processes leading to a redistribution of REE at small

scale inside the alteration halos.

Figure 1-11 : Pearce element ratio diagrams for unaltered and altered metagraywackes representing. a, the

monazite and the florencite, and b evolution of REE ratios relatively to Th

The low mobility of REE can also be noticed in the mass balance calculation, figure 12. That

method gives an estimate of the elemental transfers during alteration processes. The

calculations have been done using both thorium and titanium as reference for immobile

elements. The error on elements loss or gain is given by the relative loss or gain on the

element chosen as immobile. In addition the concentration in the altered rock was corrected as

a function of the rock density using d = 2.70.02 for the unaltered rock and d = 1.70.2 in

0

2

4

6

8

10

12

14

0 500 1000

Ce / T

h

Al / Th

unaltered

altered

a

y = 1.99x - 0.42 R² = 0.98

y = 1.46x + 0.06 R² = 0.99

0

2

4

6

8

10

12

14

0 5 10

Ce /T

h

La / Th

unaltered

altered

b


191

the altered equivalent on the basis of the determination of the density of 4 and 5 representative

samples in laboratory. These values are in agreement with the density estimated from

geophysical methods in the literature (respectively 2.7 and 2.3) (Hasegawa et al. 1990). The

alteration is marked by a relative but moderately significant increase in Al, K, Mg concordant

with the dissolution of the primary minerals (quartz, feldspars, biotite, chlorite and pyrite) and

followed by the crystallization of illite and sudoite.

It can be noted that iron is globally leached out of the system, especially when Th is used as

the immobile element in agreement with the pervasive hematization observed around the main

alteration zones.

The enrichment in phosphorus in altered zone is not only related to formation of APS

minerals because secondary apatite is the most frequent P-bearing mineral of the alteration

halo close to the uranium orebodies.

Considering the trace elements, an important result of the mass balance calculation is the

strong enrichment in U, V and Mo and the substantial gain in Bi, Co, Cr, Ni, W elements with

increasing alteration (expressed by an increase of loss on ignition).


192

Figure 1-12 : Mass balance analysis for metagraywacke Ti & Th as stable elements. Corrected for density change using (average measured densities 2.7 for fresh and 1.7 for

altered metagraywacke)

-500

0

500

1000

1500

2000

2500

3000

3500

4000

4500

As

Ba

Be

Bi

Cd

Ce

Co

Cr

Cs

Cu

Dy

Er

Eu

Ga

Gd

Ge

Hf

Ho

In

La

Lu

Mo

Nb

Nd

Ni

Pb

Pr

Rb

Sc

Sb

Sm

Sn

Sr

Ta

Tb

Th

Tm

U

V

W

Y

Yb

Zn

Zr

SiO

2

Al2

O3

Fe2O

3

MnO

MgO

CaO

Na2O

K2O

TiO

2

P2O

5

Relative Loss/Gain % corrected to densities

Ti as immobile element

Th as immobile element


193

1.8. Discussion

Evidences from petrography and mineral crystal-chemistry support the successive occurrence

of three types of APS minerals in the sedimentary and basement rocks encountered on both

sides of the paleoproterozoic unconformity at which the uranium deposits in the Kiggavik

Andrew lake structural trend are related (Riegler et al., 2013). APS minerals have been

already described in the paleoproterozoic Thelon and Hornby Bay basins (Gall and

Donaldson, 2006). They have been also extensively investigated in both altered basement

rocks and sedimentary cover which constitute the alteration halos of the unconformity related

uranium deposits of the Athabasca and Kombolgie basins (Beaufort et al., 2005; Gaboreau et

al., 2005; Gaboreau et al., 2007).

The paragenetic sequence established from the petrographic study permitted to distinguish

three different populations of APS minerals which can be related respectively to sedimentary

(APS1) and diagenetic (APS2) processes in the sandstones of the basal Thelon formation and

to hydrothermal alteration (APS3) strongly developed in the basement rocks next to the

regional faults and much weakly developed in the overlying Thelon sandstone (Figure 1-3).

The three distinctive generation of APS can be also distinguished on the basis of their crystal-

chemical properties. When plotted in a diagram Ce2O3/La2O3 versus the total amount of the

most abundant light rare earth elements (La2O3 + Ce2O3+ Nd2O3), three distinct compositional

fields can be distinguished (figure 13):

1.8.1. Crystal-chemical signature of sedimentary and diagenetic APS of the

Kiggavik area

The APS related to the sedimentary and the diagenetic history (APS1 and APS2) differ from

the hydrothermal ones (APS3) by their lower content in LREE and a great scattering of their


194

Ce2O3/La2O3 ratio. However this apparent scattering of the APS crystal chemistry within the

basal sandstones can be interpreted in more details on the basis of the petrographic

observations.

The sedimentary APS which are associated with detrital aluminum oxi-hydroxides and cerium

oxides are characterized by a relatively high LREE oxide content (8 to 13%) and a strong

depletion in cerium (Ce2O3/La2O3 ratio less than 0,5) whereas the compositional field of the

diagenetic APS (APS2) characterized by a lower total amount of LREE oxide (from 2 to 7%)

and a variable but quite much higher Ce2O3/La2O3 ratio (from 1 up to 7) which spread over

the composition of APS analysed in upper sandstones elsewhere in the Thelon basin (Gall and

Donaldson, 2006). The variation of the Ce/La and total amount of LREE measured in the APS

formed in sandstones could be related to change in the nature of the sedimentary material. For

instance, the Wharton Group is known to host volcano-sedimentary sequences with rhyolitic

tuffs (Miller and LeCheminant, 1985; Rainbird and Davis, 2007; Rainbird et al., 2003), and

given the highly instable nature of such material and the large volumes possibly involved the

contribution of these lithologies to the LREE budget can be assumed. However we cannot

easily interpreted the APS compositional variations as the result of a chemical change in the

source material because bulk-rock chemical analyses of the sandstone samples do not indicate

any significant variation in total amount of rare earth or in Ce/La ratio.

A better explanation can be found in the possibility of partitioning of rare earth elements

between coexisting mineral phases. Such a phenomenon can be invoked for the chemistry of

the Ce-depleted APS1 minerals which coexist with cerianite and Al-oxi-hydroxides. Indeed,

All these aforementioned minerals as well as kaolinite and iron hematite are well known to

be the typical mineral assemblage of the continental alterites in tropical climatic conditions.

APS can be found in modern laterites (Seghal 1998; Sehnke 1993), and the presence of

persevered Al oxy-hydroxide and CeO2 are also indicative of lateritic environments. In such


195

alterites, cerium, preferentially present in the Ce4+ oxidizing state (Braun et al. 1990), is rather

incorporated in cerianite (CeO2) than in other minerals such as APS in which Ce is

preferentially incorporated in the Ce3+ state. The above considerations lead us to interpret the

specific crystal chemistry of the APS1 as an inheritage of their condition of formation during

the paleoweathering (regolith) of the continental crust prior the formation of the Thelon basin.

In other words, such a geochemical signature confirms that dismantled regolith was involved

in the source material of the sedimentation at the first filling stage of the basin.

The presence of coexisting REE-bearing phases has not been clearly identified in the case of

the diagenetic APS2 which are characterized by low REE content and high Ce/La. Diagenetic

fluorapatite has been identified elsewhere in the Thelon basin (Gall and Donaldson, 2006) but

its REE content remains very low and no information on their Ce/La ratio is available.

However APS related to early diagenetic processes have been already documented in other

siliciclasic basins in which other REE-bearing minerals can be present (Rasmussen, 1996, Pe-

Piper and Dolansky, 2005).

1.8.2. Crystal-chemical signature of the hydrothermal APS of the Kiggavik

area

The hydrothermal APS which coexist with illite and minor sudoite in the altered basement are

characterized by a high LREE oxide content (13 to 26%) and a nearly constant Ce2O3/La2O3

ratio (between 1 and 2) which fits fairly well with the Ce2O3/La2O3 ratio of the parental

REE-bearing minerals which still persist in the unaltered basement rocks (Figure1-13, Table

1-3). This is indicative of the absence of rare earth elements fractioning during the

hydrothermal alteration process and agree with the rather immobile behavior of these

chemical elements which are essentially incorporated in the APS3 minerals which crystallize

close to the site of primary LREE-bearing minerals dissolution (monazite, bastnaesite,


196

parasite …). The first consequence of this phenomenon is the preservation of the relative ratio

of LREE, figure 1-14.

Figure 1-13 : Ce/La oxides ratio verus the sum of REE in APS minerals from the cover and the

basement. Data from Gall & Donaldson, 2006 as a comparison for sandstone APS minerals

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30

Ce2

O3

/La2

O3

LREE (oxide wt%)

APS from altered basement

APS from sandstone cover

APS from Thelon Fm (Gall and Donaldson., 2006)


197

Figure 1-14 : Synthetic plot of composition field REE ratio for all APS minerals in the

Kiggavik area and the primary REE minerals

However, the major compositional variation of APS3 consists in their large range of total

LREE content which variation seems well correlated with the distance to the uranium

mineralization. Such a variation has been already demonstrated in other unconformity related

uranium deposits (Gaboreau et al., 2005, 2007) and is interpreted as a result of Eh-pH

variations in the alteration system. Regarding the previous works made on the Eh/Ph control,

(Dill, 2001; Kolitsch and Ping, 2001), the evolution between the Svanbergite Sr-end member

to the Florencite REE end of APS minerals can be interpreted as an evolution from low pH

oxidizing to more reduced and higher pH conditions (Gaboreau et al., 2005). Then the fact

that LREE-rich APS coexisting with secondary apatite crystallized close to the uranium

orebodies whereas APS with lower LREE content were crystallized outer in the alteration

halo is interpreted as the result of a time-space chemical evolution of the infiltrating fluid


198

which Eh and pH progressively change towards more reducing and more neutral conditions

with increasing interaction with the minerals of the basement rock. Such a geochemical

process could be the cause of the synchronous deposition of uranium mineralization and

LREE-rich APS in both the Athabasca and Kombolgie unconformity type uranium deposits

(Gaboreau et al., 2007, Beaufort et al., 2005 among others).

Finally the mass-balance calculation related to the hydrothermal alteration of basement rocks

lead to a geochemical signature similar to the one found in unconformity type uranium

deposits in the Athabasca Basin (Hoeve and Sibbald 1978; Quirt 1997; Ruzicka 1993) but

also in the Amer Group uranium occurrences, (Miller and LeCheminant 1985) with among

others an increase in Bi, Co, Mo, V, W & U. All these elements been described in the

Athabasca basins either associated to red bed or in lateritic profile to ferric iron hydroxides

and thought to be a possible sources for these metals (Macdonald 1980; Mosser et al. 1985;

Wedepohl 1978).

All the above considerations lead us to interpret the crystal-chemical variations of the

hydrothermal APS related to the uranium deposits in the Kiggavik Andrew lake structural

trend as the consequence of the same geochemical processes than those already identified in

basement-hosted unconformity-related uranium deposits worldwide.

1.8.3. Comparing the overall APS compositional field at Kiggavik with that

of unconformity-related uranium deposits worldwide

First of all one of the striking chemical feature observed previously in both Athabasca and

Kombolgie basin is a fairly continuous range of composition of APS mineral from zone

proximal to mineralization to barren altered and finally the unaltered sedimentary rocks in the

basins (Gaboreau et al, 2005, 2007). The compositional field is then going roughly from the


199

florencite end-member to the svanbergite end-member, disseminated along a steady slope

given by the constant S/Sr ratio, in a Sr-LREE-S ternary diagram figure 1-15.

Figure 1-15 : Comparison of the compositional field of the APS mineral from the Kiggavik area (dotted lines)

with those of APS minerals determined by Gaboreau et al. (2005, 2007) in both the Athabasca & Kombolgie

basins .

On the opposite the in-situ chemical data gathered on APS mineral from the Kiggavik area

shows two groups of minerals with different S/Sr ratio according to the fact that APS mineral

are found in the altered basement or in the un-altered sedimentary cover. The first group of

APS in the altered basement, defines a composition envelop chemically very similar to that

established in the Athabasca and Kombolgie basins. Then, as the conclusions of the first

studies on alteration lead to relate the Kiggavik Andrew lake trend uranium deposits to an

unconformity related type of mineralization, APS minerals had a similar forming history in

these three basins when basement alteration is considered. The second group in the sandstones

defines an envelope of composition which slightly overlaps the Athabasca trend nevertheless


200

with a steeper slope of the composition envelop. This highlights the predominant role of the

S/Sr ratio in the Thelon APS when the Sr/LREE explains more the composition variability in

the Athabasca sandstones. Such chemical differences illustrates the chemical specificity of the

APS minerals of the basal sandstones of the Thelon formation which have been mostly

preserved of the hydrothermal alteration related to the basement hosted uranium

mineralization and highlight their interest as markers of the basin evolution prior the

occurrence of the U-mineralizing hydrothermal event. Contrary to other unconformity-related

uranium deposits, the APS minerals of the sandstones which locally overly the basement

hosted uranium deposits of the Kiggavik area cannot be considered as reliable indicators of

the deep seated mineralizing process because they are inherited minerals from the earlier

basin history which have been preserved from hydrothermal alteration in an unaltered

compartment.

Finally the geochemistry lead to the identification of geochemical signature of alteration

similar to the one found in unconformity type uranium deposits in the Athabasca Basin

(Hoeve and Sibbald 1978; Quirt 1997; Ruzicka 1993) but also in the Amer Group uranium

occurrences, (Miller and LeCheminant 1985) with among others an increase in Bi, Co, Mo, V,

W & U. All these elements been described in the Athabasca basins either associated to red

bed or in lateritic profile to ferric iron hydroxides and thought to be a possible sources for

these metals (Macdonald 1980; Mosser et al. 1985; Wedepohl 1978).

1.9. Conclusion

The concluding remarks of this study are :

(1) The similar trend of composition for hydrothermally related APS minerals in the basement

and locally in the sandstone cover where illitization takes place. The chemical zonation


201

expressed in the LREE enrichment is the same as in the Athabasca and the Kombolgie

unconformity type uranium mineralization occurrences.

(2) To confirm and implement the knowledge of the relationship between the hydrothermal

APS and the ones related to an early diagenetic stage during the basin evolution, similar to the

APS cementation observed in Paleozoic siliciclastic basins, (Pe-Piper and Dolansky 2005).

Moreover the chemical composition of sedimentary cover APS is pro parte in a similar trend

than APS minerals observed in other location within the Thelon. However, new evidence

leads to the identification of pre diagenesis APS minerals. This features, enhanced here by the

pristine conservation of the most early sedimentation stages would be very likely the same in

the Athabasca Basin, in place with no later alteration overprint.

These two facts lead to the conclusions that APS should be observed as markers of a long

lived evolution of the basin some being related to the mineralization and alteration events in a

range of temperature in the range of hydrothermalism, somehow link to thermal peak during

the diagenesis when the others are the relicts of laterites related to the paleoweathering, as

well as witness of the early cementation during diagenesis.

Finally, APS minerals unique mineralogical markers able to record equally the early processes

related to the sedimentation and diagenesis of these Meso-Proterozoic basins. Nevertheless

their complex history doesn’t allow the use of the APS minerals as direct mineralogical

pathfinders to alteration and then mineralization.


202

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Beaufort, D., Patrier, P., Laverret, E., Bruneton, P., and Mondy, J., 2005, Clay Alteration Associated with Proterozoic Unconformity-Type Uranium Depositsin the East Alligator Rivers Uranium Field, Northern Territory, Australia: Economic Geology, v. v. 100, p. pp. 515–536.

Cloutier J, Kyser K, Olivo GR, Alexandre P (2010) Contrasting Patterns of Alteration at the Wheeler

River Area, Athabasca Basin, Saskatchewan, Canada: Insights into the Apparently Uranium-

Barren Zone K Alteration System. Economic Geology 105:303-324. doi:

10.2113/gsecongeo.105.2.303.

Dill, H.G., 2001, The geology of aluminium phosphates and sulphates of the alunite group minerals: a review: Earth-Science Reviews, v. 53, p. 35-93.

Gaboreau, S., Beaufort, D., Vieillard, P., Patrier, P., and Bruneton, P., 2005, Aluminum phosphate- sulfate minerals associated with Proterozoic unconformity-type uranium deposits in the East Alligator River Uranium Field, Northern Territory, Australia: Canadian Mineralogist, v. 43, p. 813-827.

Gaboreau, S., Cuney, M., Quirt, D., BEAUFORT, D., PATRIER, P., and Mathieu, R., 2007, Significance of alumium phosphate-sulfate minerals associeted with U unconformity-type deposits: The Athabasca basin, Canada: American Mineralogist, v. 92, p. 267-280.

Gall, Q., and Donaldson, J.A., 2006, Diagenetic fluorapatite and aluminum phosphate-sulphate in the pPelaoproterozoic Thelon Formation and Hornby Bay Groupe, northwestern Canadian Shield: Canadian Journal of Earth Sciences, v. 43, p. 617-629.

Hartlaub, R.P., Heaman, L.M., Ashton, K.E., and Chacko, T., 2004, The Archean Murmac Bay Group: evidence for a giant Archean rift in the Rae Province, Canada: Precambrian Research, v. 131, p. 345-372.

Hasegawa K, Davidson GI, Wollenberg P, Yoshimasa I (1990) Geophysical exploration for

unconformity-related uranium deposits in the northeastern part of the Thelon Basin,

Northwest Territories, Canada. Mining Geology 40:83-95.

Herold, H., 1987, Zur Kristallchemie und Thermodynamik der Phosphate und Arsenate vom

Crandallit-Typ., Erlangen University.

Hoeve J, Sibbald TII (1978) On the genesis of the Rabbit Lake and other unconformity-type uranium deposits in Northern Saskatchewan, Canada. Economic Geology 73. Hoeve, J., and Quirt, D., 1984, Mineralization and host rock alteration in relation to clay mineral diagenesis and evolution of the Middle-Proterozoic Athabasca basin, Northern Saskatchewan, Canada, Saskatchewan Research Concil Technical report, Volume 197, Saskatchewan Reasearch Council, p. 197.

Hoffman, P.F., 1990, Subdivision of the Churchill Province and extent of the Trans-Hudson orogen, in Lewry, J.F., and Stauffer, M.R., eds., The Early Proterozoic Trans-Hudson Orogen of North Amercia, Volume 37: Special paper, Geological Survey of Canada Special Paper, p. 15-39.

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Jefferson, C.W., Thomas, D.J., Gandhi, S.S., Ramaekers, P., Delaney, G., Brisbin, D., Cutts, C., Portella, P., and Olson, R.A., 2007, Unconformity-associeted uranium deposits of the Athabasca Basin, Saskatchewan and Alberta, EXTECH IV, Volume Geology and Unranium EXploration TECHnology of the Proterozoic Athabasca Basin, Saskatchewan and Alberta, p. 23-67.

Kolitsch, U., and Ping, A., 2001, Crystal chemistry of the crandallite, beudantite and alunite groups: a review and evaluation of the suitability as storage materials for toxic metals: Journal of Mineralogical and Petrological Sciences, p. 67-78.

Macdonald R (1980) Mineralogy and geochemistry of a Precambrian regolith in the Athabasca Basin.

University of Saskatchewan, Saskatoon, pp 151.

Miller AR (1983) A progress report: uranium phosphorus association in the Helikian Thelon Formation and Sub-Thelon saprolite, central district of Keewatin, NWT. Current Research GSC 83-1A:449-456

Miller, A.R., and LeCheminant, A.N., 1985, Geology and uranium metallogeny of Proterozoic supracrustal successions, central District of Keewatin, N.W.T with comparisons to northern Saskatchewan, Geology of uranium deposits, Volume Special Vol. 32, Canadian Institute of Mining and Metalurgy, p. 167-185.

Mosser C, Leprun JC, Blot A (1985) Les éléments traces des fractions < 2 m à kaolinite et smectite formées par altération de roches silicatées acides en Afrique de l'Ouest (Sénégal et Haute-Volta). Chemical Geology 48:165-181. doi: http://dx.doi.org/10.1016/0009-2541(85)90044-0.

Pe-Piper, G., and Dolansky, L.M., 2005, Early diagenetic origin of Al phosphate-sulfate minerals (woodhouseite and crandallite series) in terrestrial sandstones, Nova Scotia, Canada: American Mineralogist, v. 90, p. 1434-1441.

Pehrsson, S., Jefferson, C.W., Peterson, T.D., Scott, J., Chorlton, L., Hillary, B., Patterson, J., Lentz, D., Shelat, Y., and Bethune, K., 2010, Basement to the Thelon Basin, Nunavut- Revisited, GeoCanada2010: Calgary.

Peterson, T.D., Van Breemen, O., Sandeman, H., and Cousens, B., 2002, Proterozoic (1.85-1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland: Precambrian Research, v. 119, p. 73-100.

Quirt D (1997) Athabasca Basin Uranium metallogenic model Thermotectonic and uranium metallogenic evolution of the Wollaston EAGLE project Area. Saskatchewan Research Council, Saskatoon, pp 1-41.

Rainbird, R.H., and Davis, W.J., 2007, U-Pb detrital zircon geochronology and provenance of the late Paleoproterozoic Dubawnt Supergroup: Linking sedimentation with tectonic reworking of the western Churchill Province, Canada: Geological Society of America Bulletin, v. 119, p. 314-328.

Rainbird, R.H., Hadlari, T., Aspler, L.B., Donaldson, J.A., LeCheminant, A.N., and Peterson, T.D., 2003, Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada: Precambrian Research, v. 125, p. 21-53.

Rasmussen, B., 1996. Early-diagenetic REE-phosphate minerals (florencite, gorceixite, crandallite, and xenotime) in marine sandstones: a major sink for oceanic phosphorus. American Journal of Science, 296, 601-632.

Ruzicka VR (1993) Unconformity type uranium deposits In: Kirkham RV, Sinclair WD, Thorpe RI, Duke JM (eds) Mineral Deposit Modeling. Geological Survey of Canada, Ottawa, pp 125-149.

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Nature and stability of radiation induced defects in natural illite new results and implications for ancient radioelement mobility

205

2. Nature and stability of radiation induced defects in natural illite new results and implications for ancient radioelement mobility

Riegler, Thomas; Allard, Thierry; Beaufort, Daniel. in preparation for submission to Physics

and Chemistry of Minerals

2.1. Introduction

Illite is a common clay mineral, widespread in various geological contexts from soils, to

sedimentary basins or hydrothermal systems. It is formed of a tetrahedral-octaedral-tetraedral

layered structure, with a closed interlayer space occupied by potassium ions. Radiation

induced defects (RIDs) spectrum, relatively similar to kaolinite has been identified in illite

subject to artificial or natural irradiation. For the latter similar EPR signal were recorded in

naturally irradiated illite coming from alteration halos associated with Paleo-Proterozoic

uranium deposits in the Athabasca Basin in Canada (Allard et al. 2003; Morichon et al. 2008).

In both cases, in comparison with the extensive data available on kaolinite, the defects in

these illites tend to present a smaller thermal stability. In addition the EPR spectrum showed

very often a different look compared to kaolinite with a broader signal and very unclear

expression of the A, A’ and B centers corresponding to defects in the signal (Allard et al.

2012; Morichon 2008). Then it is critical to have a better understanding of the expression and

properties of radiation induced defects in illite, especially when old and long-lived open

geological systems are considered. In such systems numerous events ( e.g. thermal, multiple

radionuclide migrations) are superimposed over very long and short term periods from billons

years to the actual and illite can be considered as a natural dosimeter. So refinements in the

contribution of the short half life defects such as the A’ & B center in addition to the

permanent A center defects in the total RIDs content would be critical to decipher the radio-

elements migrations over time.


206

The newly acquired data tends to make illite properties more similar to kaolinite than

previously thought, in term of spectra and stability properties. Then, addition to previous

authors work, we propose to revise and implement the knowledge of radiation induced defects

in illite. In this work a naturally irradiated illite formed in an unconformity related deposit

was studied with X & Q-bands hyperfrequencies on natural and annealed samples in order to :

(1) get a better estimation on the thermal stability of RIDs, (2) assess this stability throughout

geological times and (3) refine the spectrum signature, and assign to it the contribution of the

different defects centers. All these properties are critical to refine interpretations of the

radioelement’s mobility and its signature in clay minerals, as around uranium deposits for

both exploration and environmental considerations.

2.2. Sampling

Sample selection have been made in a larger data set of about 60 samples samples from illite

& sudoite rich alteration zones developed in the Woodburn lake Group, Archean volcano-

sedimentary sequence hosting unconformity related uranium mineralization, in the Kiggavik

area, Nunavut, Canada. The age of alteration related to the ore formation is yet relatively

poorly constrained within 1.2-1.4 Ga on historical illite K/Ar geochronological data (Mi ller et

al. 1989).

The majors’ constraints were to obtain a pure illite sample with a high concentration of

radiation defects in which the distinct components of the Electron Paramagnetic Resonance

spectrum are presents. This data screening (XRD followed by X-band EPR) lead to the

selection of the BG814 sample for further characterization of the Electron Paramagnetique

Resonance (EPR) signature of radiation induced defects in illite. The interest of this peculiar

sample is its strong and well defined defect spectra.


207

2.3. Methods

All altered bulk rock samples were dispersed in deionzide water and the clay minerals

extracted by sedimentation in order to obtain the infra 4 micrometer size fraction. Clay

minerals identification and crystallographic properties were determined using X-ray

diffraction (XRD) on oriented slides and randomly oriented powders, following the

preparation methods given in (Brindley and Brown 1980). A D8 Advance Bucker

diffractometer equipped with a Cu anode was used for X-ray diffraction and analytical

conditions were set with a 40kV acceleration tension and 40mA current, and 1° fixed

divergence slit. Data collection scan parameters were 2-30 (oriented slides) or 19-30 (random

mounts) degrees 2theta both with a 0.01° 2theta step and 1 or 5 second counting time. In

addition to XRD, chemical analysis major elements using SEM-EDS on on clay pellets

separates were carried in order to recalculate the mineral proportion during sample selection

phase.

Electron paramagnetic resonance spectra of RID were observed at X- (≈ 9.βGHz) and Q-

bands (≈ 35GHz) and room temperature (20°C) using a Brucker EMXplusTM spectrometer on

both natural and annealed illite. The use of an higher frequency as in the Q-band allows lower

detection limits, and gives better resolved spectra by increasing the gap between spin levels

(Calas and Hawthorne 1988). The acquisition parameters were 1 G amplitude and a 327 ms

time constant and microwave power of 40mW in X-bands.

All spectrums were normalized to sample mass, volume occupied in resonance cavity and

acquisition gain. In addition EPR spectrums were labelled by their effective spectroscopic

factor g values defined by the follow formula: h * = g * β * Ho. The different parameters

being: h the Plack constant; the resonance frequency; g the Landé factor, a tensor with the

eigenvalues: gxx, gyy, gzz calibrated by comparison with a 2,2 diphenyl-1picrylhydrazyl


208

standard of known g value (gDPPH = 2.0036); β the Bohr magneton and Ho the external

magnetic field. The accuracy on magnetic field and g-values measurements were respectively

Ho = 1G, and g 0.001. It must be noted that only the g values can be compared due to

the frequency dependence of the magnitude and magnetic field positions of the resonances.

Concentrations are expressed in arbitrary units’ proportion to the number of spins per gram of

clay.

2.4. Annealing Experiments protocol

With a similar approach to the work carried on kaolinite step annealing was used in order to

differentiate RIDs in illite (Clozel et al. 1994). The isochronal annealing was carried out for 2

hours at 100°C intervals form 100 to 600°C, in order to define the thermal conditions for the

individualization of the different defects center spectra. In addition the lifetime of some of

the most stable defect centers were investigated with isothermal annealing experiments and

were performed at 400°C and 450°C from 30 minutes up to 134 hours. Regarding data on

kaolinite these center are thought to be stable over geological times under the Earth’s surface

thermal regime and then of great interest to trace ancient irradiations. Then illite could be

considered as a natural dosimeter.

A particular attention was given to the samples preparation for experiments in order to get

accurate and reproducible annealing temperatures. Each sample consists in a few 10s of

milligrams of illite, sieved at 50 micrometer and wrapped in aluminum foil and placed in a

regulated furnace equipped with a PID controller. Sampled were sieved a second time in order

to get randomly oriented powder in the Suprasil high purity silica glass capillaries used as

sample holder in the resonance cavity.


209

2.5. Results

2.5.1. EPR parameters of defects in illite

As a first approach natural and annealed illites were studied in X-band in order to identify the

spectroscopic factors characteristic (g values) of the radiation induced defects in illite (A, A’

and B centers), figure 2-1a. Then, spectrums were obtained on natural illite and their

annealed equivalent to be able to identify the characteristic of the A center defects. In the

natural sample, 5 spectroscopic factors were identified respectively at 2.063, 2.050, 2.037,

2.013 and 2.002; while only 4 were present in the annealed illite at 2.063, 2.051, 2.011 and

2.002. It can already be noted that very little shift is present in the g-values positions between

the natural and annealed samples. In addition the intensity ratio of the bands doesn’t seems

affected by heating and the band at 2.063 could be very similar to the N1 defect described in

montmorillonite in term of g-value and annealing temperature around 500°C, figure 2-1b

(Sorieul et al. 2005). Finally the EPR signal for naturally irradiated illite from the Kiggavik

area shows similar spectroscopic parameters with a well expressed A center defect as well as

the presence of less stable ones in the 2.037 region possibly resulting from the superimposed

signal of the A’ and B centers.

Then, in addition to the illite EPR signal characterization, the stability and temperature

dependant kinetics of decay were experimentally tested by both isochronal and isothermal

annealing the latter being used to estimate the half life (t1/2) and the activation energy of Ea

(eV) of the A center defect in illite.


210

Figure 2-1 : EPR spectra of defects in natural (a) and annealed illite (b) at X-Band (9.4GHz). DPPH (2,2

diphenyl-1picrylhydrazyl) is a standard compound with known g factor

2.5.2. Stability of the radiation induced defects in illite

First the stability of the radiation induced defects contents can be assessed using the overall

defect content via double integration of the EPR signal in complement to the defect spectra

evolution across the temperature range selected for annealing, Figure 2a & 2b. The annealing

curve, indicate a sharp decrease in both the defects content as well as a clear diminution of the

2.037 spectroscopic factor at low temperature below 100 °C. From then to around 350°C the


211

defects concentration is increasing by about 30% and is marked by a continuous decrease of

the 2.037 component and an increase of the 2.050, 2.011 and 2.002 g- values. Such behaviour

is similar to the one observed in kaolinite and various materials and due to electron transfers

during the annealing experiments (Griscom 1984; Hennig and Grün 1983). With increasing

annealing temperature the total defects concentration decreases sharply for temperature higher

than 450°C. Finally the total concentration evolution is a balance between at first below 400

°C the healing of the less stable with electron transfers phenomenon’s as a result of thermal

activation and in a second time the healing of the most thermally stable defects, which present

the g values characteristic of the center A type defects. Finally the A center appears to be

stable at until temperatures reaches field of clay mineral deshydoxylation temperature in

which the crystalline structure is affected. Then we propose to investigate the intrinsic

properties of these centers in illite via isothermal annealing experiments.

Figure 2-2 : Isochronal annealing experiments, 2hours from 100 to 600°C


212

Decay processes are generally described using first (Arrhenius) or second order equations

(Furetta 1988). The first order formula can be written: [A] = [A 0]e-Kt , where the different

parameters are: [A] is the instantaneous defect concentration (a.u.), [A0] the intial

concentration , t the time of decay and K the probability of decay per second. This probability

parameters itself is expressed by the following relation K = (t1/2)-1Ln2 = so.e

-Ea/kT, where s0 is

the frequency factor, usually in the range 108 - 1010 s-1 (Marfunin 1979), k the Boltzman

constant (k =8.6 10-5 eV.K-1) and T the temperature (K). Then it appears that the half life of

the defect in only temperature dependant and two isotherms would be required to determine

the activation energy. The second order decay law is more suitable for more complexes

mechanisms (transit stage or retrapping) that may occur during the decay processes, and can

be written as : [A] -1 = K.t+[A0] -1 with K = tgθ = (t1/2 . [A0])-1 = s0.e-Ea/kT or lnK= Ln s0-

(Ea/k)T-1 with θ is the slope of the linear curve. In this case the half life temperature and time

dependant.

Both first and second order equations were used to describe the isothermal annealing of the A-

center in illite. Curves were plotted for the BG814 sample in the figure 2-3, in which the

defect and the A center are particularly abundant. The decay curves, present two distinct

tends with at first a rapid decrease of the defects concentration within the first 8 hours of

annealing followed by a steady decreasing phases over the all annealing time span. Then, this

behaviour might be related to the healing of the least stable components of the defects signal,

as the A’ and B centers. Thereafter this rapid decay stage it can be assumed that the total

defect concentration represent the A center concentration. In both cases at 400 and 450°C

annealing temperatures, the second order law gives the best description of the data, figure 2-

3a. In addition the annealing curves, figures 2-3b, allowed the graphic determination of the

center-A properties with a maximal range of 1.9 to 49 109 years for the half life and activation


213

energies between 1.0 and 1.4 eV when estimation were made using both first and second

order decay laws, Table 2-1. Moreover, as the second order law tends to give the best data

description, the activation energy and half life at 15°C are a little bit lower than the ones

obtained on kaolinite but within similar orders of magnitude (Clozel et al. 1994).

Figure 2-3 : Isothermal annealing experiments at 400 and 450 isotherms. The decay law is described by a second

order kinetics. The fitting of the isotherms in the 1/[A] plot and the slope of the curves provides values of the

temperature dependant decay constant K(h-1). The activation energy and the half life are estimated and deduced

from decay equations.

a

b


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Table 2-1 : A center properties, half life (years) and activation energy (eV) estimated at 15°C for both first and

second orders decay laws.

1st order 2nd order

s0(s-1) t1/2 @ 15°C t1/2 @ 15°C

108 3.9E+09 1.9E+09

1010 4.9E+10 2.9E+10

Ea (eV) 1.0 - 1.2 1.3 - 1.4

2.6. Preliminary discussion and concluding remarks

Additional electron paramagnetic resonance spectroscopy results in Q band couldn’t be

added to this final version of the manuscript. These new data will potentially allow a more

accurate identification of the radiation induced defects acciociated with illite and implement

the current results.

To date, the two major contributions form the present work are (1) the identification of a

singular EPR signal in naturally irradiated illite possibly linked to a type of radiation induced

defect specific to illite, and (2) the determination of the physical properties of such defect.

Thus, the radiation induce defects in illite seem similar to the A center defect, both in term of

EPR spectra and physical properties (e.g. stability), to the ones in kaolinite or

montmorillonite. This similarities could be explained by the nature of the defect and linked to

an electronic vacancy on a Si-O bond. Therefore, the type of sheet assemblage between the

tetrahedral and octaedral layers in the different clay minerals could have no influence on the

defect itself as the control for the decfect is a the atomic bond scale.


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Finally, the identification of defect stable at the scale of geological times (like center A

defect) as well as others less stable (A’, B centers) is critical to attempt to discriminate recent

and ancient radioelement circulations.


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2.7. References

Allard T, Ildefonse P, Del Villar LPr, Sorieul Sp, Pelayo M, Boizot B, Balan E, Calas G (2003) Radiation-induced defects in dickites from the El Berrocal granitic system (Spain): relation with past occurrence of natural radioelements. European Journal of Mineralogy 15:629-640.

Allard T, Balan E, Calas G, Fourdrin C, Morichon E, Sorieul S (2012) Radiation-induced defects in clay minerals: A review. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 277:112-120. doi: http://dx.doi.org/10.1016/j.nimb.2011.12.044.


Calas G, Hawthorne FC (1988) Introduction to spectroscopic methods. Reviews in Mineralogy and Geochemistry 18:1-9.

Clozel B, Allard T, Muller J-P (1994) Nature and stability of radiation-induced defects in natural kaolinites: new resulats and a reappraisal of published works. Clays and Clay Minerals 42:657-666.

Furetta C (1988) New calculations concerning the fading of thermoluminscent materials. Nucl Tracks Radiat Meas 14:413-414.

Griscom DL (1984) Characterization of three E'-center variants in X- and γ-irradiated high purity a-SiO2. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 1:481-488. doi: http://dx.doi.org/10.1016/0168-583X(84)90113-7.

Hennig GJ, Grün R (1983) ESR dating in quaternary geology. Quaternary Science Reviews 2:157-238. doi: http://dx.doi.org/10.1016/0277-3791(83)90006-9.

Marfunin AS (1979) Spectroscopy, Luminescence and Radiation Centers in Minerals. Springer Verlag, Berlin, Heidelberg, New York.

Miller AR, Cumming GL, Krstic D (1989) U-Pb, Pb-Pb, and K-Ar isotopic study and petrography of uraniferous phosphate-bearing rocks in the Thelon Formation, Dubawnt Group, Northwest Territories, Canada. Canadian Journal of Earth Sciences 26:867-880.

Morichon E (2008) Les défauts d'irradiation dans les minéraux argileux: des marqueurs de la mobilité de l'uranium dans le contexte des gisements d'uranium associés à une discordance. Thèse Université de Poitiers, pp 297.

Morichon E, Allard T, Beaufort D, Patrier P (2008) Evidence of native radiation-induced paramagnetic defects in natural illites from unconformity-type uranium deposits. Phys Chem Minerals 35:339-346. doi: 10.1007/s00269-008-0227-5.

Sorieul S, Allard T, Morin G, Boizot B, Calas G (2005) Native and artificial radiation-induced defects in montmorillonite. An EPR study. Phys Chem Minerals 32:1-7. doi: 10.1007/s00269-004-0427-6.

http://dx.doi.org/10.1016/j.nimb.2011.12.044

http://dx.doi.org/10.1016/0168-583X(84)90113-7

http://dx.doi.org/10.1016/0277-3791(83)90006-9


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C. DISCUSSION GÉNÉRALE, CONCLUSIONS ET PERSPECTIVES

Discussion générale

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1. Discussion générale

La genèse d’un gîte métallique implique la succession d’une série d’étapes de pré-

concentration, libération, transport, dépôt et enfin de préservation des minéraux formant le

minerai d’intérêt économique. Les gisements d’uranium associés aux discordances du

Protérozoïque s’inscrivent eux aussi dans ce schéma, depuis les processus de différenciation

magmatique à l’origine des sources primaires de l’uranium, aux divers processus superficiels

qu’ils soient sédimentaires, diagénétiques ou hydrothermaux à l’origine des concentrations

économiques d’uranium (Cuney 2010). L’ensemble des données acquises à l’échelle du

faisceau structural de Kiggavik-Andrew Lake dans ce travail permet d’identifier certaines de

ces étapes et processus.

On peut replacer la formation des minéralisations en uranium de la bordure Sud-Est du bassin

Protérozoïque moyen du Thelon dans un cadre métallogénique plus large à l’échelle du

bouclier Canadien. Il s’agira tout d’abord de retracer l’histoire des altérations ayant

successivement affecté le district de Kiggavik afin de comprendre comment elles ont pu

contribuer, par incrément, à la formation des minéralisations en uranium. Cela amènera à

considérer les conditions physico-chimiques à l’origine des signatures minéralogiques propres

à chaque environnement, en considérant ses implications en termes de source, transport ou de

piège de l’uranium. Enfin l’ensemble sera replacé dans le contexte géodynamique global

ayant conduit à la formation des bassins intracontinentaux hôtes des minéralisations et

contraint du point de vue temporel à l’aide d’éléments de géochronologie.

Au vu des données présentées dans ce mémoire, il est possible de proposer un schéma

retraçant l’évolution géologique de la zone de Kiggavik au cours de la vaste période (1800-

1100Ma) qui couvre l’histoire de cette région, de l’initiation du processus sédimentaire lié à la

mise en place du bassin du Thelon à la fin des épisodes d’altération argileuse associés à la


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mise en place ou au remaniement de la minéralisation uranifère Figure 1-2. Enfin, chacune

des étapes sera mise en parallèle avec les connaissances actuelles des gisements du bassin

Meso Proterozoique de l’Athabasca qui constitue une référence en termes de gisement

d’uranium associé à une discordance Paléoprotérozoique.

1.1. Histoire des événements d’altération

On peut tout d’abord noter que la mise en place des minéralisations s’inscrit principalement

dans le cycle orogénique Hudsonien. On peut alors mettre en parallèle les différents

événements d’altération et les grands événements géologiques ayant affecté la zone de

Kiggavik, depuis les événements tardi-orogéniques Hudsonien jusqu’à la mise en place des

minéralisations uranifères.

1.2. Evénements précoces Hudsoniens

Ils regroupent à la fois les réactivations des grandes structures crustales Archéennes lors de

l’orogenèse Husdonienne ainsi que tous les stades post-orogéniques. Il s’agit à la fois de la

rétromorphose dans le faciès schiste vert des roches du socle métamorphique de la zone de

Kiggavik et de la mise en place d’un système de veines de quartz de haute température

soulignant les failles régionales Est-Ouest. Elles sont identifiées dans la zone d’End Grid dans

cette étude, mais aussi dans le gisement d’Andrew Lake (Pagel and Ahamdach 1995). On peut

aussi rattacher à ces épisodes tardi- métamorphiques les évidences d’altération propylitique

exprimée sous la forme de veinules d’épidotes et/ou d’adulaire localement. Cette

structuration, permet de mettre en place les grandes discontinuités qui pourront par la suite

être réactivées et former les principaux drains empruntés par les fluides minéralisateurs qui

contrôlent la localisation des minéralisations (Annesley et al. 1995; Annesley et al. 1996;

Beaudemont and Fedorowich 1996). De plus la mise en place des corridors brêchiques à

remplissage de quartz, contribue à créer des zones de contraste rhéologiques, qui seront

importantes pour contrôler l’ouverture des discontinuités (zone mylonitique, foliation) ou des


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fractures. Dans le bassin de l’Athabasca de nombreux gisements d’importance économique

sont eux aussi situés aux interfaces entre deux lithologies aux propriétés mécaniques

contrastées. Parmi ceux-ci, on peut citer les dômes de gneiss au contact des roches supra-

crustales de la ceinture de Wollaston (Yeo and Delaney 2007), de même que le contraste

existant entre les quartzites et les autres métasediments, parfois graphitiques, de cette ceinture

de plis et chevauchements.

A cette structuration s’ajoutent les phénomènes propres à l’effondrement de l’orogène

Hudsonien tel que la genèse de granites peralumineux par le biais de la fusion crustale lors de

la phase d’extension post orogénique, puis la mise en place des syénites et des granites

anorogéniques de la suite Nueltin (Van Breemen et al. 2005; Peterson et al. 2002).

Figure 1-4 : Rapport Th/ U des intrusions Hudsoniennes et Nueltin, d’après Van Breemen, β005

Les phases de fusion crustale de roches alumineuses permettent par ailleurs d’aboutir à des

roches plus différenciées et ainsi enrichir la croûte en uranium, comme c’est le cas dans l’Est

du bassin de l’Athabasca (Annesley and Madore 1999). Toutefois il semble que tous les


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granites Hudsonien ne soient pas tous aussi fertiles en uranium (Van Breemen et al. 2005),

Figure 1-1. Le granite de Kiggavik entre dans cette catégorie de granite faiblement fértile

(Weyer et al. 1987). Toutefois, et à contrario de ce qui est observé dans l’Est du bassin de

l’Athabasca, on ne retrouve pas d’évidence de pegmatites à uraninite à Kiggavik. Cette

fertilité moindre peut s’expliquer par une différence de nature des protolithes, les

métasédiments pélitiques du Wollaston étant plus riches en uranium que les séries de l’Amer

ou du Ketyet River Group, principalement constituées de quartzites et d’arkoses. Par ailleurs,

les métagraywackes de la ceinture de roche verte du Woodburn lake Group sont

particulièrement pauvres en U (<3 ppm U).

Dès le stade Hudsonien, il apparaît donc, des différences marquées entre le bassin

d’Athabasca et la bordure Sud Est du bassin du Thelon en ce qui concerne le potentiel

métallogénique des protores pour alimenter des gisements liés à la discordance bassin-socle.

Dans la région de Kiggavik, les sources de l’uranium pourraient être reliées aux tuffs

ryholitiques du groupe de Wharton déposés lors de la mise en place du bassin de Baker Lake.

Ceux-ci pourraient avoir formé alors une source régionale associée ponctuellement à des

sources « plutoniques » plus locales tels que les granitoïdes et les syénites du socle.


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Figure 1-5 : Reconstitution schématique de l’évolution des processus géologiques dans les roches situées au

voisinage de la discordance paleoprotérozoïque dans la zone de Kiggavik.

1.3. Mise en place du profil d’altération Pré-Thélon

L’existence d’un régolithe ayant transformé la partie superficielle des roches du socle avant

la mise en place des premiers sédiments silcoclastiques des bassins de l’Athabasca et du

Thelon a été évoquée par de nombreux auteurs sur la base d’évidence minéralogiques

(kaolinite, diaspore, hematite, phosphate sulfates d’aluminium hydratés) et pétrographiques

(Macdonald 1980; Pagel 1975; Cecile 1973). Les études pétrographiques et minéralogiques

présentées dans ce mémoire démontrent que tous les minéraux index des altérites

continentales de type latérite ou bauxite sont présents dans la matrice sédimentaire des

premiers remplissages de conglomérats et de grès grossiers qui subsistent dans la région de

Kiggavik (kaolinite riches en fer, hématite, oxy-hydroxydes d’aluminium, oxydes de cérium

et de thorium, ainsi que des phosphate-sulfate d’aluminium). Ces observations nous

conduisent à interpréter ces premières formations sédimentaires comme des produits issus de

l’érosion et démantèlement de la surface régolithisée des roches métamorphiques et

magmatiques environnantes. La très faible concentration en uranium des premiers dépôts

sédimentaires qui ont été préservés de l’altération hydrothermale (0,3 à 0,7 ppm) suggère une

mobilité précoce du stock potentiel d’uranium issu des processus de régolithisation des roches

du socle. Dans l’état d’avancement de nos connaissances sur les transferts géochimiques

effectifs pendant la phase de sédimentation précoce du bassin du Thelon à Kiggavik, il semble

prématuré d’établir un lien entre l’uranium mobilisé pendant ce phénomène précoce et les

corps minéralisés actuels. On peut cependant mentionner des résultats préliminaires de

travaux de spectroscopie de résonance paramagnétique électronique sur la kaolinite des

formations sédimentaires basales du bassin du Thelon qui indiquent des concentrations de

défauts d’irradiation élevées dans la région de Kiggavik qui ne sont pas en accord avec les


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concentrations actuelles en radio-éléments. De tels défauts pouvant être la conséquence du

passage de fluides riches en radioélements comme cela a déjà été démontré dans le bassin

d’Athabasca (Morichon 2008; Morichon et al. 2008; Morichon et al. 2010). Des travaux

complémentaires seront nécessaires pour évaluer l’importance de ces migrations qui ont dû se

produire a un stade très précoce de l’histoire du bassin du Thelon, avant la cimentation des

grès et le confinement de l’ensemble des minéraux détritiques par le quartz microcristallin.

1.4. Diagenèse et évolution du bassin du Thelon

Dans le continuum d’évolution des bassins intracratoniques Protérozoïques, l’histoire

diagénétique apparait prolongée pour aboutir à des conditions de diagenèse poussée (Patrier et

al. 2003; Beaufort et al. 2005; Jefferson et al. 2007). La profondeur maximale

d’enfouissement est alors estimée à environ 5km sur la base de données d’inclusions fluides

(Pagel 1975) ou du stade de transformation en dickite des minéraux du groupe kaolin

(Beaufort et al. 1998; Lanson et al. 2002). Cette évolution permet aux saumures de bassins

oxydantes de remobiliser et transporter l’uranium en solution. Les données nouvellement

acquises sur l’ordre/désordre, des kaolinites recristallisées de la base du bassin ainsi que les

fractures à dickite indiquent également que dans la zone de Kiggavik les formations basales

du bassin du Thelon ont subi un enfouissement similaire à ce qui a été évoqué dans le bassin

d’Athabasca ou bien même dans les grès de Kombolgie en Australie (environ 5km ). De plus,

on peut noter que les inclusions fluides triphasées à cube de halite observées dans les veines

de quartz et surtout les dolomites du socle indiquent des paléotempératures aux alentours de

200 °C qui sont en accord avec la transition complète de la kaolinite en dickite dans les roches

gréseuses comme cela a été observée dans les grès de Kombolgie (Patrier et al., 2003) et des

salinités semblables à celles des saumures minéralisatrices connuesdans le bassin de

l’Athabasca (Derome 2002; Pagel and Jaffrezic 1977).


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Certaines caractéristiques des grès telles que les ciments phosphatés (Gall and Donaldson

2006), ainsi que les surcroissances de quartz (Hiatt et al. 2003; Hiatt et al. 2010) ou la

présence de feldspaths potassiques néoformés (Renac et al. 2002) n’ont pas été retrouvées

dans cette étude. On doit néanmoins prendre en compte le fait que ces travaux ne concernent

que les roches sédimentaires de la base des grès du Thelon. Les ciments phosphatés ou les

illites qui ont été mentionnés plus haut dans la série sédiementaire du bassin du Thélon par

ailleurs peuvent avoir été érodés. La préservation de kaolinite détritique non illitisée dans les

grès situés à la base du bassin n’est pas limitée à Kiggavik. En effet, celle-ci a déjà été mise

en évidence à la base du bassin de l’Athabasca, dans la région de Shea Creek (Uri 2012).

Enfin, les températures calculées à partir de la composition isotopique de l’oxygène des illites

néoformées des grès du Thelon par ces mêmes auteurs sont cohérentes avec celles obtenues à

Kiggavik dans le halo d’altération associé à la minéralisation de Bong (Sharpe 2013). Elles se

situent aux alentours de 200°C, en cohérence avec les données microthermométriques de

notre étude. Il est alors envisageable que la partie basale du bassin du Thelon étudiée dans la

zone de Kiggavik appartienne à un compartiment du bassin qui a été préservé de l’altération

hydrothermale à illite et sudoite. Ce type de compartiment se révèle très intéressant pour

recueillir des informations originales sur l’histoire précoce (sédimentaire puis diagénétique)

qui a précédé l’évènement d’altération hydrothermale régionale auquel sont associés les

gisements d’uranium qui sont présents dans les roches de socle.

1.5. Altération hydrothermale et mise en place de la minéralisation

La prise en compte des saumures et de leur température amène naturellement à considérer leur

interaction avec les roches du socle sous la discordance du Thelon, là où sont encaissées les

minéralisations en uranium. De manière générale, ces interactions fluides/roche sont

marquées par la déstabilisation des aluminosilicates et des sulfures du socle avec une

précipitation concomitante de l’illite et de l’uraninite. On comprend alors que les lithologies


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plus riches en phyllosilicates sensibles au redox (donneurs d’électrons comme l’oxydation du

fer des phyllosilicates (chlorite-Fe ou de la biotite) et des sulfures (pyrite) puissent agir en tant

que piège de la minéralisation au cours de leur altération, comme c’est le cas dans les méta-

sédiments du Woodburn Lake Group. Le contrôle de la minéralisation apparaît alors local

guidée par la richesse de l’encaissant en phyllosilicates ferreux ou en pyrite. De plus, les

matières carbonées en relation avec les phénomènes d’altération peuvent localement

consittuer des pièges supplémentaires efficaces. Les teneurs mesurées de plusieurs pour cent

métal dépassant largement la moyenne de celle des gisements de la zone de Kiggavik.

L’étude de l’altération sur l’ensemble des gisements du faisceau structural de Kiggavik-

Andrew Lake a permis de mettre en évidence une séquence paragenétique d’altération

commune à l’ensemble de zones minéralisées. Elle s’exprime de manière analogue à celle des

gisements de type discordance de l’Athabasca ou de la Kombolgie par un mélange d’illite et

de sudoite et la présence de phases accessoires comme les APS ou les matières carbonées. On

notera l’absence de la dravite qui est souvent un minéral index abondant dans de nombreux

gisements associés aux discordances du bassin d’Athabasca et plus accessoire dans les

gisements de Kombolgie. Ces volumes de roches hydrothermalisées forment des halos

d’altération qui sont contrôlés par les discontinuités structurales à toute échelle : les réseaux

de failles et leurs zones d’endommagement, les fractures, mais aussi les plans de foliation des

métasédiments à plus petite échelle. Il apparaît alors un double contrôle à la fois

litholologique et structural pour la précipitation de la minéralisation. On peut aussi remarquer

que la minéralisation peut être également présente dans des secteurs situés en périphérie des

grands drains là où l’altération de la roche encaissante est modérée et où persistent une partie

des minéraux porteurs de fer ferreux (progression des fronts rédox lors des phases de

minéralisations primaire mais aussi des remobilisations). Ainsi, et compte tenu de la position

des minéralisations rencontrées dans le socle à jusqu'à plusieurs centaines de mètres en


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dessous de la surface d’érosion actuelle qui se situe à une profondeur indéterminée sous la

discordance paleoprotérozïque, il semble très probable que les minéralisations du faisceau de

Kiggavik représentent l’extension en profondeur (voir les racines ?) d’un système de

minéralisation en uranium de type discordance tel que ceux déjà connus par ailleurs dans le

monde (Athabasca, Kombolgie…).

Un tel contexte géologique pourrait expliquer à la fois les plus faibles teneurs en uranium,

ainsi que le caractère disséminé de la minéralisation, comparativement à celle connue au

voisinage de la discordance dans le bassin de l’Athabasca et des caractéristiques plus proches

de celles rencontrées dans les gisements australiens plus profonds de Kombolgie tels que celui

de Jabiluka notamment (Jefferson et al. 2007). On ne peut de ce fait pas exclure que des

minéralisations aient été ou soient présentes au niveau de la discordance par ailleurs dans le

bassin du Thelon. Dans un tel schéma, la précipitation de l’uranium en solution est

conditionné par la neutralisation et la reduction progressive du fluide minéralisateur du fait

de son interaction avec les minéraux des roches de socle (Komninou and Sverjensky 1996).

Les derniers progrès réalisés sur l’interprétation des défauts d’irradiation de l’illite sont

primordiaux pour tenter de reconstruire les paléo-circulations des radioéléments au travers des

drains empruntés par les saumures oxydantes. La meilleure connaissance des signaux associés

aux différents types de défauts électroniques devrait permettre de différencier les irradiations

les plus anciennes, de celle plus récentes liées à des remobilisations des minéralisations

actuelles.

Conclusion générales et perspectives

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2. Conclusion générales et perspectives

L’ensemble des travaux menés dans cette thèse ont permis d’affiner la compréhension du

système d’altération associé aux minéralisations en uranium du faisceau structural de

Kiggavik en le reliant à l’expression profonde d’un système de type discordance tel que ceux

déjà connus dans le bassin d’Athabasca au Canada et ceux de Kombolgie en Australie. Par

certains aspects, les minéralisations de socles reconnues à Shea Creek dans le bassin

d’Athabasca plus au sud pourraient constituer un contexte de dépôt proche de celui caractérisé

vers la marge Sud Est du bassin du Thelon. Compte tenu du type de minéralisation la

perspective de découvertes de minéralisations, dans les grès ou à la discordance basale de la

formation du Thelon ne peut pas être exclue. On peut par ailleurs mentionner que l’altération

dans les grès peut apparaître localement, la transition entre les roches altérées ou non pouvant

être réalisée en quelques centaines de mètres. Ainsi l’absence d’altération dans les grès au

dessus de la discordance n’interdit pas que des zones altérées, et donc indicatices d’un

potentiel pour l’exploration soit présente dans le socle sous jacent.

Les relations entre la circulation des fluides minéralisateurs et les structures fertiles sont en

cours d’étude par le biais de l’étude des défauts d’irradiations de l’illite qui est un minéral

ubiquiste dans les roches du socle. Un complément de travail de géologie structurale associée

à de la radiogéochronologie sur les minéraux argileux devraient permettre de mieux

contraindre la succession des événements d’altération et minéralisations.

Par ailleurs, des études de la chimie des inclusions fluides par ablation laser couplée à la

spectrométrie de masse pourraient permettre de donner des indications supplémentaires sur la

composition des saumures et sur leur signature géochimiques.

Références

229

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Segment 1. Saskatchewan Research Council, Saskatoon, pp 132. Annesley I, Madore C, SHi R (1996) Wollastion EAGLE project, Revision mapping/integrated geology

Wollaston EAGLE project:Segment 2. Saskatchewan Research Council, Saskatoon, pp 184. Annesley I, Madore C (1999) Leucogranites and pegmatites of the sub-Athabasca basement,

Saskatchewan: U protore? In: Stanley CJ (ed) Mineral Deposits:Processes to Processing. Balkema, pp 297-30

Beaudemont D, Fedorowich J (1996) Structural control of uranium mineralization at the Dominique-Peter deposit, Saskatchewan, Canada. Economic Geology 91:855-874.

Beaufort D, Cassagnabère A, Petit S, Lanson B, Berger G, Lacharpagne JC, Johansen H (1998) Kaolinite-to-dickite reaction in sandstone reservoirs. Clays minerals 33:297-316.



Cecile MP (1973) Lithofacies analysis of the Proterozoic Thelon Formation, Northwest Territories. Carlton University, pp 119.

Cuney M (2010) Evolution of Uranium Fractionation Processes through Time: Driving the Secular Variation of Uranium Deposit Types. Economic Geology 105:553-569. doi: 10.2113/gsecongeo.105.3.553.

Derome D (2002) Evolution et origines des saumures dans les bassins protérozoiques au voisinage de la discordance socle/couverture. L'exemple de l'environnement des gisements d'uranium associés aux bassins Kombogie (Australie) et Athabasca (Canada). Université Henri Poincaré, Nancy.

Gall Q, Donaldson JA (2006) Diagenetic fluorapatite and aluminum phosphate-sulphate in the Paleoproterozoic Thelon Formation and Hornby Bay Groupe, northwestern Canadian Shield. Canadian Journal of Earth Sciences 43:617-629.

Hiatt EE, Kyser K, Dalrymple RW (2003) Relationships among sedimentology, stratigraphy, and diagenesis in the Proterozoic Thelon Basin, Nunavut, Canada: implications for paleoaquifers and sedimentary-hosted mineral deposits. Journal of Geochemical Exploration 80:221-240.

doi: http://dx.doi.org/10.1016/S0375-6742(03)00192-4. Hiatt EE, Palmer S, E., Kyser K, O'Connor T (2010) Basin evolution, diagenesis and uranium

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Annexe

Méthodologie

Comme précédemment évoquées, de nombreuses méthodes ont pu être mises en œuvre afin

de répondre aux objectifs de ce travail. Afin de rendre la lecture de ce manuscrit plus claire, et

pour donner le supplément de détail qui fait parfois défaut par souci de concision dans les

articles, une rapide revue des méthodes utilisées sera donnée. Il s’agira de présenter pour la

spectroscopie en résonance paramagnétique électronique ou la sonde ionique, l’objet de sa

mise en œuvre en rappelant son grand principe de fonctionnement en faisant un éventuel

rappel à la théorie, et insistant pour certaines d’entre elles sur la méthodologie de traitement

des données acquises.

La caractérisation des milieux géologiques passe par un ensemble de sauts d’ échelles

d’observations et de mesures, de l’affleurement à la roche, agrégat de minéraux, puis aux

minéraux eux-mêmes et enfin leur organisation spatiale ou leur composition isotopique. Ainsi

à la fois chimie et cristallographie sont mobilisées pour identifier, comprendre et interpréter

leur relation texturales et in fine tenter de proposer une interprétation de l’histoire géologique.

A cela s’ajoute la contrainte temporelle afin d’ancrer les successions relatives des événements

dans un l’échelle absolue des temps géologiques.

Spectroscopie de résonance paramagnétique électronique

Généralités

La spectroscopie RPE (Résonance Paramagnétique Electronique) s’apparente à la RMN mais

il s’agit alors de s’intéresser au comportement des électrons non appariés présent dans le

média à analyser, en utilisant les transitions énergétiques entre différents niveaux d’énergie de

spin électronique. Cette méthode de caractérisation d’une grande sensibilité, de l’ordre de la

partie par million (ppm) en concentration s’apparente à de la dosimétrie. Ainsi les défauts

induits par l’irradiation, principalement du aux particules alpha, permettent alors de tracer

les circulations des radioéléments en solution au cours de l’histoire géologique du système

considéré.

Deux appareils ont été utilisés, un spectromètre ESP 300E Bruker mais surtout son équivalent

plus moderne, l’EMXplusTM, offrant à la fois une interface souple sous windows 7 ainsi

qu’une plus grande sensibilité de la cavité de résonance, Figure 1. Le principe de

fonctionnement étant quoi qu’il en soit le même. Les éléments constitutifs sont :

- L’électroaimant pour imposer le champ Ho

- La cavité de résonnance dans laquelle on introduit l’échantillon

- Le pont hyperfréquence (diode de type Gunn) qui génère une onde électromagnétique

de fréquence fixe à 9.4 GHz en bande X

- Un détecteur pour mesure les ondes électromagnétiques

- Ordinateur de contrôle pour paramétrer les conditions analytiques et d’exporter les

spectres acquis

- Système d’échange de chaleur pour le refroidissement du pont et de l’électroaimant.

Figure 1 , Spéctromètre RPE, source de microonde (1) & électroaimant (2) et son alimentation (3), cavité de résonance (4)

Théorie

Les conditions de résonance

Sous l’influence d’un champ magnétique Ho l’électron de moment magnétique µ s’aligne sur

l’axe du champ. Son moment angulaire propre, aussi appelé spin, est lui caractérisé par son

nombre quantique S=1/β. L’énergie d’interaction entre le champ appliqué et le moment

magnétique de l’électron s’écrit alors :

E = g * * Ms * Ho

Avec :

- le magnéton de Bohr, égal à eh/βmc - g le facteur spectroscopique de l’électron libre - Ms la projection du spin sur l’axe Ho

2

1

3

image: www.bruker.com

4

L’effet Zeeman, à pour effet de produire des transitions électroniques de niveau d’énergie

déterminées par les deux orientations possible du spin, donnant un état de basse et de haute

énergie (E= 1/2*g* *Ho), selon que le spin est parallèle ou antiparallèle au champ

magnétique externe.

Les conditions de résonances sont alors obtenues en utilisant une source de micro-ondes

constante (de fréquence =9.4 GHz pour la bande X) dans un champ magnétique variable. Il

est alors possible de mesurer l’absorption induite du quantum d’énergie résultant de la

transition de niveaux d’énergie à la condition de résonnance.

On peut l’exprimer par l’expression suivant :

h * = g * * Ho

Avec :

- h la constante de Planck

- la fréquence des microondes - g le facteur de Landé, spécifique du type de défaut dont les valeurs dépendent des

directions cristallographiques - le magnéton de Bohr - Ho le champ magnétique externe

Les types de défauts d’irradiation et leur genèse

Dès lors que les bases physiques théoriques sont posées, il faut maintenant considérer les

différents types de défauts et les conditions de leur formation et plus important encore leur

persistance dans le temps au sein du minéral.

Type de défaut Nature Stabilité T1/2 années T de (recuit °C) Kaolinite/Illite/Sudoite

Centre A Si-O□ 1012 450 / 600 / 500

Centre A' Si-O□ 103 300

Centre B Al-O□-Al 10-100 200

D’après (Calas et al. 2003; Allard et al. 2012)

Figure 2, type de défaut d'irradiation (Allard et al. 2012)

Les défauts de centre A, les plus stables à l’échelle des temps géologiques, sont ceux

responsables de la majeure partie du signal d’irradiation que l’on peut mesurer en RPE. La

création du défaut peut être induite par tous types de radiation α, , et γ, mais elle nécessite

toutefois la présence d’une substitution (d’un cation trivalent par un cation divalent) créant

un déséquilibre local de charge qui sera par la suite compensé par l’éjection d’un électron lors

de l’interaction matière particule (Jones et al. 1974).

Mise en œuvre et calcul des concentrations en défaut

L’acquisition des données est réalisée sur des extraits argileux purs pour (1) réduire les pertes

de signal par effet de dilution, si par exemple du quartz est présent dans l’échantillon la

quantité de défaut mesurée sera normalisée à 100% d’argile et (2) éviter la présence de

minéraux porteurs de Fe2+/3+ qui peuvent perturber le signal RPE.

Les échantillons sont tamisés (maille 50 microns) afin d’obtenir une orientation aléatoire des

feuillets dans le tube en silice ultra pure qui contient l’échantillon.

Les spectres de défauts sont réalisés sur une gamme de champs magnétiques comprise entre

3100 et 3600 Gauss (soit 310 et 360 mT, 1Tesla = 10 000 Gauss) pour une puissance du pont

hyperfréquence de 40mW. Une phase de post traitement et de correction du signal tenant

compte de la quantité de matière présente dans le tube (en masse, et en hauteur par rapport à

la longueur utile de la cavité) et du gain utilisé pour l’acquisition est alors nécessaire.

Le calcul de la concentration en défaut de l’échantillon passe par le traitement du signal avec

le logiciel Kaleidagraph, il s’agit de corriger le signal de la ligne de base et de procéder à

deux intégrations successives. L’aire sous la courbe de la seconde intégrale donne alors la

quantité de défauts pour un échantillon en unités arbitraires, figure 2. Le passage en unités

absolues est alors aisé après étalonnage de la cavité avec un standard xxx en utilisant la

formule donnée par (Wertz and Bolton 1986) :

Avec :

- (Std) concentration en défaut dans l’étalon

- Sx surface sous la courbe (2nd intégration) pour l’échantillon

- Sstd surface sous la courbe (2nd intégration) pour l’étalon

- Pasx / Passtd pas du spectre de l’échantillon / de étalon

- Gx gain de l’échantillon ; Gstd gain de l’étalon

- gx facteur spectroscopique caractéristique du type de défaut

- gstd facteur spectroscopique du standard

- Px /Pstd puissance de l’échantillon / du standard

Figure 2, séquence schématique du traitement des spectres RPE pou la uantification des défauts d’i adiations

SIMS (Secondary Ion Mass Spectrometry)

Généralités

La spectrométrie de masse à ionisation secondaire permet entre autre l’analyse ponctuelle, in

situ, des rapports isotopiques. Initialement développée à l’Université d’Orsay par G. Slodzian

dans les années 1960 pour l’étude des matériaux, elle s’avère d’une grande utilisé pour les

géosciences ou son utilisation à pris un véritable essor aussi bien pour la datation (U/Pb) que

pour la détermination des rapports isotopiques d’éléments stables (O, H, S). Elle permet de

s’affranchir des problèmes de séparations ou de purifications des différentes phases (parfois

techniquement impossible). Ainsi comme pour la microsonde électronique et la chimie des

minéraux, on accède alors à des données isotopiques que l’on peut mettre en regard

d’information pétrographiques à une échelle plus petites que celle des minéraux (Reed 1990;

Reed 1989). On peut évoquer pour l’exemple l’évolution de la composition isotopique de

l’oxygène des fluides entre plusieurs phases de surcroissances, la mise en évidence d’âges

distincts entre différentes plages de grains de zircons, de monazite ou d’uraninite permettant

ainsi de mieux caler dans le temps l’histoire des cristallisations.

Un photographie ainsi qu’un schéma de l’appareil correspondant est présenté sur la figure 3a

et 3b.

Figure 3, A SIMS CAMECA IMS7f, Université du Manitoba SIMS Facility, B schéma de fonctionnement (CAMECA IMS 6f)

Sources archives CAMECA

A

B

Théorie

La spectrométrie de masse à ionisation secondaire dans le cas de son utilisation en mode

dynamique repose sur le bombardement de la surface de l’échantillon par en faisceau d’ions

primaires focalisés, de haute énergie (16O ou 133Cs) avec une incidence de 20 - 45° pour

optimiser l’émission des ions secondaires. Les collisions atomiques génèrent l’ablation de la

matière minérale en surface et l’éjection de particules neutres ou chargées dont une partie est

ionisée spontanément. Ces ions sont ensuite transférés par un champ électrostatique, de la

surface de l’échantillon vers le spectromètre de masse où ils sont séparés selon leur masse et

charge (m/Q) avant leur introduction dans le détecteur (Ireland 2004). La mesure des isotopes

stables de l’oxygène est la plus problématique du fait de l’usage d’ion primaire Cs+ qui

tendent à s’accumuler à la surface du fait d’une évacuation incomplète par la couche

conductrice déposée en surface (Au). Un canon à électron rentre alors en action pour

compenser l’excès de charge positive sur la surface de l’échantillon et ainsi permettre une

bonne extraction des ions secondaire (Lyon et al. 1994; Ireland 2004; Slodzian et al. 1987).

Un des éléments propres à la technique est le biais induit par le fractionnement des masses lié

à l’ionisation préférentielle des isotopes les plus légers. De ce fait l’enrichissement relatif en

éléments légers du faisceau d’ions secondaire doit être ajusté par rapport aux rapports

isotopiques du standard pour prendre en compte ce phénomène. De plus pour garantir, la

précision et la reproductibilité des mesures et éviter les effets de fractionnement du à la

composition chimique ou la structure du minéral (effets de matrice), le minéral choisi comme

standard doit être le plus proche que possible du minéral à analyser (Ireland 1986).

Mise en œuvre et présentation des données

Les échantillons sont montés sur section polie de diamètre 1ˮ (2.54 cm) puis recouverts d’un

film d’or par évaporation sous vide.

Les résultats sont présentés sous la forme classique :

Réch et Rstd correspondent respectivement aux rapports isotopiques absolus de l’échantillon

analysé et du standard.

Diffractométrie de rayon-X

La diffraction des rayons X gouvernée par la loi de Bragg est la seule technique rendant

possible la connaissance de la position relative de tous les atomes au sein d’un cristal. Il est

alors possible de connaitre la nature et la position des atomes, la présence de symétries,

l’organisation périodique tridimensionnelle (polytypisme, ordre/désordre, cristallinité, ect.)

mais encore de déterminer la taille des domaines cohérents ou les paramètres de maille

cristalline. Elle résulte de l’interaction des rayons X avec les nuages électroniques des atomes

constitutifs du cristal considéré, la nature des atomes (Z) considérés influant sur l’intensité

diffractée (facteur de diffusion atomique).

La loi de Bragg peut alors s’écrire

Avec :

- dhkl, la distance réticulaire caractéristique des plans hkl

- θ l’angle d’incidence des rayons X sur les plans réticulaires

- n l’ordre d’interférence (nombre entier)

- λ la longueur d’onde du rayonnement

La préparation des échantillons s’effectuant selon un protocole tel que donné par (Brindley

and Brown 1980) pour les poudres désorientées lorsque l’identification de toutes les

directions cristallographiques sont nécessaire ou en lames orientées (dépôt filtre ou goutte)

principalement pour les minéraux présentant une forte anisotropie, tel que l’habitus lamellaire

des minéraux argileux.

Réferences

Allard T, Balan E, Calas G, Fourdrin C, Morichon E, Sorieul S (2012) Radiation-induced defects in clay minerals: A review. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 277:112-120. doi: http://dx.doi.org/10.1016/j.nimb.2011.12.044.


Calas G, Allard T, Balan E, Morin G, Sorieul S (2003) Radiation-induced Defects in Nonradioactive Natural Minerals: Mineralogical and Environmental Significance. MRS Online Proceedings Library 792:null-null. doi: doi:10.1557/PROC-792-R2.6.

Ireland TR (1986) Isotope systematics of refractory inclusions from carbonaceous chondrites. The Australian National University.

Ireland TR (2004) Chapter 30 - SIMS Measurement of Stable Isotopes In: Pier AdG (ed) Handbook of Stable Isotope Analytical Techniques. Elsevier, Amsterdam, pp 652-691.

Jones JPE, Angel BR, Hall PL (1974) Electron spin resonance studies of doped synthetic kaolinite II. Clay Minerals 10:257-270.

Lyon IC, Saxton JM, Turner G, Hinton R (1994) Isotopic fractionation in secondary ionization mass spectrometry. Rapid Communications in Mass Spectrometry 8:837-843. doi: 10.1002/rcm.1290081009.

Reed SJB (1989) Ion microprobe analysis-a review of geological applications. Mineralogical Magazine 53:3-24.

Reed SJB (1990) Recent developments in geochemical microanalysis. Chemical Geology 83:1-9. doi: http://dx.doi.org/10.1016/0009-2541(90)90135-T.

Slodzian G, Chainteau MP, Dennebouy RC (1987) SIMS: self-regulated potential at insulating surfaces in presence of strong electrostatic field. CAMECA news.

Wertz J, Bolton J (1986) Electron paramagnetic resonance. Chapman and Hall.

http://dx.doi.org/10.1016/j.nimb.2011.12.044

http://dx.doi.org/10.1016/0009-2541(90)90135-T

Système d’altération et minéralisation en uranium le long du faisceau structural Kiggavik Andrew Lake (Nunavut, Canada) : modèle génétique et guides d’exploration Ce travail présente une étude multi-échelle des relations entre altération et minéralisation en uranium le long de la bordure Sud Est du bassin Meso-Proterozoïque du Thelon, au Nunavut, Canada. Les altérations associées aux minéralisations sont développées dans une série volcano-sédimentaire Archéenne appartenant à la ceinture de roche verte du Woodburn Lake Group (WLG). Elles s’expriment majoritairement par un assemblage à illite (polytypes 1Mcis & 1M trans) sudoite hématite et phosphates sulfates d'aluminium hydratés (APS). De plus des composés carbonés, cogénétiques des minéralisations, ont été identifiés comme des produits des réactions hydrothermales. La signature de l'altération, fortement guidées par les structures Est-Ouest du corridor de Kiggavik-Andrew Lake, apparaît alors très similaire à celle rencontrée dans les roches de socles des parties profondes des autres gisements d'uranium de type discordance du bassin d’Athabasca (Canada) ou de la Kombolgie (Australie). L'étude des marqueurs minéralogiques tels que les APS ont permis de mettre en évidence les transferts élémentaires au cours des processus métallogéniques et de distinguer les caractéristiques pétrographique et chimiques des processus diagénétiques et hydrothermaux. Enfin la compréhension fine de l’expression de marqueurs cristallographiques issus de l’irradiation naturelle des minéraux argileux donne de nouvelles pistes pour le traçage et la compréhension des circulations des radios-éléments à l’échelle géologique. Mots cléfs : Bassin Méso-Protérozoique du Thelon, altération, minéraux argileux, petrographie, cristallochimie et métallogenie de l’uranium. Alteration system and uranium mineralization along the Kiggavik-Andrew Lake structural trend (Nunavut, Canada): genetic model and exploration guides This work presents a multi-scale study of the relationships between alteration and uranium mineralization along the South Eastern margin of the Meso-Proterozoic Thelon Basin, Nunavut, Canada. The ore associated alterations are hosted in an Archean volcano-sedimentary sequence belonging to the Woodburn Lake Group (WLG). Their main expression is a mineral assemblage composed of dominant illite (1Mcis & 1M trans polytypes) together with sudoite hematite and aluminum phosphate sulfate minerals. Moreover carbonaceous materials cogenetic with the uranium mineralization have been identified as potential indicators of the hydrothermal conditions. At a regional scale, alteration is strongly controlled via East-West faults forming the main frame of the Kiggavik-Andrew Lake structural trend. Then from the regional to the mineral scale, alterations signatures at Kiggavik are similar to the ones described in deep basement rocks of unconformity type uranium deposits in both Athabasca (Canada) and Kombolgie (Australia) Paleoproterozoic basins. In addition mineralogical markers studies (APS minerals) lead to the distinction between hydrothermal and diagenetic processes as well as elemental transfers during fluid rock interaction. Finally, detailed studies on radiation induced defects on illite revealed new ways to tracing and better understanding the radio elements mobility in such deep seated natural systems. Keywords : Meso-Proterozoic Thelon Basin, alteration, clay minerals, petrography, crystallochemistry and uranium metallogeny.

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