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1 HydrASA, UMR CNRS 6532, University of Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
2 UMR CNRS 7566 G2R-CREGU, UHP, BP 239, 54506 Vandúuvre-lès-Nancy Cedex, France
3 Saskatchewan Research Council, 15 Innovation Boulevard, Saskatoon, Saskatchewan, S7N 2X8, Canada
4 COGEMA, Business Unit Mines, 2, rue Paul Dautier, BP 4, 78141 Velizy Cedex, France
5 Centre de Géochimie de la Surface, 1 Rue Blessig, 67084 Strasbourg Cedex, France
* E-mail address of corresponding author: patricia.patrier{at}hydrasa.univ-poitiers.fr
Unconformity-related uranium deposits, which represent a significant high-grade uranium resource, are systematically surrounded by a host-rock alteration halo enriched in clay minerals. Illite is often the major clay mineral component of the halo and it displays a variable crystal structure. New data are provided on the crystal structure and the chemistry of illite encountered within and outside of the alteration halo surrounding the Shea Creek deposit. Two illite populations were distinguished using textural and structural criteria: samples rich in the tv-1M polytype display thin (sub-micrometer) and ‘hairy’ shapes, while samples richer in the cv-1M polytype contain illites with rigid lath-like shapes several micrometers wide. In barren ‘regional’ sandstone, the trends with depth of the textural and microstructural properties of illite particles are: (1) an increase of particle size, (2) an evolution to a more isometric form, and (3) a dominance of the cv-1M polytype over the tv-1M polytype. These trends record diagenetic processes under conditions of deep burial and differ from those observed in altered sandstone around the uranium mineralization. The altered sandstone is characterized by enrichment in the tv-1M polytype near the unconformity and/or brittle structural features. This tv-1M illitization took place in response to structurally-controlled infiltration of basement rocks by diagenetic brines which were further recycled after interaction into the overlying basin. Variations of the illite structural and textural properties may result from nucleation/growth kinetics and may be indicative of a change in the flow regime, and/or a change of saturation state of the fluid vs. illite. The tv-1M illite may be favored in environments characterized by a high fluid/rock ratio and a high supersaturation state of the fluids in proximity to mineralization.
Key Words: Athabasca Basin • Crystal Chemistry • Crystal Form • Crystal Structure • Host-rock Alteration • Illite • Kaolin Minerals • Sudoite • Unconformity-type Uranium Deposit
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